CN1347581A - Semiconductor structures having strain compensated layer and method of fabrication - Google Patents

Abstract

The present invention provides a semiconductor structure which includes a strain compensated superlattice layer comprising a plurality of pairs of constituent layers, with the first constituent layer comprising a material under tensile stress, and the second constituent layer comprising a material under compressive stress, such that the stresses of the adjacent layer compensate one another and lead to reduced defect generation. Appropriate selection of materials provides increased band gap and optical confinement in at least some implementations. The structure is particularly suited to the construction of laser diodes, photodiodes, phototransistors, and heterojunction field effect and bipolar transistors.

Description

Semiconductor structure with strain compensation layer and method of making same

Field of Invention

The present invention relates to semiconductor structures and methods of making them, and in particular to methods of using strain compensating layers in Group III nitride material systems and minimizing the occurrence of lattice defects.

Background of the Invention

The successful development of blue laser sources heralded the next generation of high-density optical devices, including disk storage, DVD, etc. Fig. 1 shows a cross-sectional view of a prior art semiconductor laser device (S. Nakamura, MRS BULLETIN, Vol. 23, No. 5, pp. 37-43, 1998). On the sapphire substrate 5, a gallium nitride (GaN) buffer layer 10 is formed, then an n-type GaN layer 15 is formed, and a 0.1 mm thick silicon dioxide (SiO ₂ ) layer 20 is patterned so that 4 mm wide windows 25 are formed with a periodicity of 12 mm in the GaN<1-100> direction. Then, form n-type GaN layer 30, n-type indium gallium nitride (In _0.1 Ga _0.9 N) layer 35, n-type aluminum gallium nitride (Al _0.14 Ga _0.86 N)/GaN MD-SLS ((modulated doped Modulation Doped Stained-Layer Superlattices) cladding layer 40 , and n-type GaN cladding layer 45 . Next, form an In _0.02 Ga _0.98 N/In _0.15 Ga _0.85 NMQW (Multiple Quantum Well) active layer 50, followed by a p-type Al _0.2 Ga _0.8 N cladding layer 55, a p-type GaN cladding layer 60 , p-type Al _0.14 Ga _0.86 N/GaNMD-SLS cladding layer 65, and p-type GaN cladding layer 70. A ridge strip structure is formed in the p-type Al _0.14 Ga _0.86 N/GaNMD-SLS cladding layer 55 , so as to confine the optical field propagating in the lateral ridge waveguide structure. Electrodes are formed on the p-type GaN cladding layer 70 and the n-type GaN layer 30 to provide current injection.

In the structure shown in FIG. 1, the n-type GaN cladding layer 45 and the p-type GaN cladding layer 60 are optical guiding layers. The n-type Al _0.14 Ga _0.86 N/GaN MD-SLS cladding layer 40 and the p-type Al _0.14 Ga _0.86 N/GaN MD-SLS cladding layer 65 function as cladding layers for confinement from the InGaN MQW layer 50 Carriers and light emitted by the active region. The n-type In _0.1 Ga _0.9 N layer 35 functions as a buffer layer for thick AlGaN film growth in order to prevent cracking.

By utilizing the structure shown in FIG. 1 , carriers are injected into the InGaN MQW active layer 50 through the electrodes, so that light emission is performed in the 400 nm wavelength range. Since the effective refractive index under the ridge-stripe region is greater than that outside the ridge-stripe region, the optical field is confined in the lateral active layer, which is due to the p-type Al _0.14 Ga _0.86 N/GaN MD- This is due to the ridge waveguide structure formed in the SLS cladding layer 65. On the other hand, since the active layer has a higher refractive index than the n-type GaN cladding layer 45 and the p-type GaN cladding layer 60, the n-type Al _0.14 Ga _0.86 N/GaNMD-SLS cladding layer 40 and the p-type Al _0.14 The refractive index of Ga _0.86 N/GaN MD-SLS cladding layer 60, therefore, through n-type GaN cladding layer 45, n-type Al _0.14 Ga _0.86 N/GaN MD-SLS cladding layer 40, p-type GaN cladding layer 40, p-type GaN cladding layer The cladding layer 60, and the p-type Al _0.14 Ga _0.86 N/GaN MD-SLS cladding layer 55, laterally confine the optical field in the active layer. Thus, substantially transverse mode operation is obtained.

However, with the structure shown in Fig. 1, it is difficult to reduce the defect density below 10 ⁸ cm ^-2 because the lattice constants of AlGaN, InGaN and GaN are completely different from each other, when the n-type In _0.1 Ga _0.9 N layer 35, In _0.02 Ga _0.98 N/In _0.15 Ga _0.85 N MQW active layer 50, n-type Al _0.14 Ga _0.86 N/GaN MD-SLS cladding layer 40, p-type Al _0.14 Ga _0.86 N/GaNMD-SLS cladding Layer 65, and p-type Al _0.2 Ga _0.8 N cladding layer 55 beyond a critical thickness will create defects in the structure as a means of releasing strain energy. Defects are caused by phase separation and function as laser absorption centers, which will cause a decrease in light radiation efficiency and increase threshold current. The consequence is that the operating current becomes larger, which in turn compromises reliability.

Furthermore, in the structure shown in FIG. 1, a ternary alloy system of InGaN is used as the active layer. In this case, the bandgap energy varies between 1.9eV (InN) and 3.5eV (GaN). Therefore, ultraviolet light with an energy value higher than 3.5 eV cannot be obtained by using an InGaN active layer. This presents some problems since ultraviolet light is attractive as a light source for optical pickup devices in eg higher density optical disk storage systems and other devices.

To better understand the defects caused by phase separation in conventional ternary material systems, the mismatch of lattice constants between InN, GaN and AlN must be understood. The lattice mismatches between InN and GaN, InN and AlN, and GaN and AlN are 11.3%, 13.9%, and 2.3%, respectively. Therefore, even though the equivalent lattice constant is the same as that of the base material, internal strain energy will accumulate in the InGaN layer since the equivalent bonding length is different from each other among InN, GaN, and AlN. In order to reduce the internal strain energy, in the InGaN lattice-mismatched material system, there is a composition range where phase separation occurs, where In atoms, Ga atoms, and Al atoms are not uniformly distributed in the layer. As a result of the phase separation, the In atoms, Ga atoms, and Al atoms in the InGaN layer will not be uniformly distributed according to the mole fraction of atoms in each constituent layer. This means that the bandgap energy distribution of any layer including phase separation will also become non-uniform. The bandgap region of the phase-separated portion disproportionately acts as an optical absorption center, or optically scatteres light from the waveguide. As mentioned above, a typical prior art solution to these problems is to increase the drive current, thus reducing the lifetime of the semiconductor device.

Another conventional approach to achieve low defect density laser diodes using GaN material systems is to use only GaN in the cladding layer. However, this approach has the disadvantage that the optical confinement in the active layer will be lower than with the AlGaN cladding layer, because the refractive index jump between the active layer and the GaN cladding layer is smaller than if the cladding layer The refractive index jump when using AlGaN in . Therefore, the light field is distributed laterally. Optical confinement in the active layer requires an increased threshold current to obtain the same brightness. In addition, for the GaN cladding layer, its potential barrier is smaller than that of the AlGaN cladding layer; this allows carriers to easily overflow the active layer, which will again increase the threshold current. Therefore, as the operating current increases, reliability, and statistically, lifetime decreases. Therefore, the AlGaN cladding layer is widely used although the cladding layer will generate defects.

Therefore, there has long been a need for a semiconductor structure that reduces lattice defects and that can be used to obtain laser diodes, transistors or other devices, with low threshold current and long-term reliability.

Invention Summary

The present invention substantially overcomes the limitations of the prior art and provides semiconductor structures with low defect density and thus improved reliability. The invention can be used to fabricate blue light and other laser diodes, heterojunction field effect transistors, heterojunction bipolar transistors, and photodiodes, among other devices.

Briefly, the present invention provides a semiconductor structure having a substrate on which a first cladding layer of a first conductivity type is formed. A first superlattice layer of the first conductivity type is then formed on the first cladding layer, wherein the superlattice layer has properties discussed further below. Then, an active layer is formed on the superlattice layer, and subsequently, a second superlattice layer of the second conductivity type is formed. Finally, a second cladding layer of the second conductivity type is formed. In addition, it is also possible to use conductive layers on both sides immediately following the active layer. The electrodes are formed in a conventional manner.

The superlattice layers each form a cladding layer consisting of many layers of alternating ternary and quaternary materials such as AlGaN and InGaN, or InGaAlN material in different molar fractions, each cladding layer having a thickness between below its critical thickness. In an exemplary embodiment, the superlattice layer may comprise: about 200 layer pairs. For a superlattice, if a ternary system is used, such as AlGaN and InGaN, the AlGaN layer will be under tensile stress and the InGaN layer will be under compressive stress. By alternating these layers, the stress is compensated at the AlGaN/InGaN layer interface, The result is fewer defects within the layers and increased reliability. The superlattice layers have layers of opposite conductivity types sandwiching quantum well active layers, which can be implemented as single or multiple wells. By proper selection of mole fractions, the lattice constant of the AlGaN layer can be arranged lower than that of the adjacent GaN layer, and the lattice constant of the InGaN layer can be arranged higher than that of the adjacent GaN layer. constant. The end result is the formation of a superlattice layer with balanced stress substantially balanced to the lattice constant of the adjacent GaN layer, thus greatly reducing the formation of stress-induced defects.

In a first embodiment of the present invention, the semiconductor structure - which may be, for example, a laser diode - comprises the following steps: forming a GaN first cladding layer of the first conductivity type on GaN or other substrates, and then forming a first cladding layer with the first cladding layer A first superlattice layer of the same conductivity type. The first superlattice layer, which can be considered the second cladding layer, can consist of many layer pairs, usually AlGaN and InGaN, or InGaN and InAlN. A conductive layer is then formed, typically of InGaN material, and of the same conductivity type as the first cladding layer, followed by a quantum well active layer, typically of InGaN material. The active layer can be formed using a single quantum well or multiple (eg triple pair) quantum well designs. Another conductive layer of InGaN is usually formed on the active layer, but of the opposite conductivity type to the first cladding layer.

A second superlattice layer is then formed on the conductive layer, the lattice layer functioning as a third cladding layer and having the opposite conductivity type to the first cladding layer. When using a first superlattice layer, the second superlattice layer is usually composed of many layers, for example AlGaN combined with InGaN, or InGaN combined with InAlN. The superlattice layers may each contain about 200 pairs of complementary material layers, although the exact number is not critical. A GaN fourth cladding layer is typically formed on the superlattice third cladding layer. The electrodes are formed in a conventional manner.

As mentioned above, the superlattice material pair may be selected from the following material pairs: Al _xal Ga _1-xal N/In _xi Ga _1-xi N and In _xay Ga _1-xay N/In _xn Al _1-xn N. With the stated structure, in the first superlattice layer, the Al _xal Ga _1-xal N layer is under tensile stress and the In _xi Ga _1-xi N layer is under compressive stress, as a result, at the interface of the respective constituent layers The stresses can compensate each other. Likewise, in the second superlattice layer, the Al _xal Ga _1-xal N layer is under tensile stress, while the In _xi Ga _1-xi N layer is under compressive stress, with the result that, in this superlattice, The stresses at their interfaces can also compensate each other. The operation is the same if the In _xay Ga _1-xay N/In _xn Al _1-xn N material pair is selected.

In addition, Al _xal Ga _1-xal N/In _xi Ga _1-xi N and In _xay Ga _1-xay N/In _xn Al _1-xn N superlattice layers can be designed so that the optical field within the active layer Confinement is better than if GaN for the cladding layer is used alone. By increasing the optical confinement within the active layer in the lateral direction, the threshold current of the device can be decreased. In addition, the design of Al _xal Ga _1-xal N/In _xi Ga _1-xi N and In _xay Ga _1-xay N/In _xn Al _1-xn N superlattice layers will allow the absorption of laser light from the active layer least. Therefore, a laser diode with low threshold current and low defect density is obtained.

The first embodiment can be accomplished by choice of materials for the superlattice and active layer, and can include various alternatives for the substrate and outer cladding. In particular, the equipment of the first embodiment may include: sapphire substrates, silicon carbide, GaN, and the like. The superlattice layer may comprise Al _xal Ga _1-xal N and In _xi Ga _1-xi N, where xal is about 0.2 and xi is about 0.04 up to 0.2; or may comprise In _xay Ga _1-xay N and In _xn Al _1-xn N, where xay is about 0.04 and xn is about 0.13. Additionally, the active layer may comprise single or multiple quantum wells of _{InxaGa1 -xaN} material. Preferably, the relationship between the variables xi and xa is xa>xi.

In a second embodiment of the invention, a semiconductor structure based on a ternary material system is again implemented. A second arrangement in which a laser diode is again an exemplary device comprises: a suitable substrate together with a first cladding layer of GaN or similar of the first conductivity type, a superlattice second cladding layer of the first conductivity type, and a quantum well active layer of In _xa Ga _1-xa N material, which may be single or multiple quantum wells. Additionally, conductive layers may also be formed immediately on each side of the active layer to aid in optical field confinement, but they are not required in all embodiments. When using the first superlattice layer, the superlattice second cladding layer can be Al _xal Ga _1-xal N/In _xi Ga _1-xi N, In _xay Ga _1-xay N/In _xn Al _1-xn N or its equivalent.

Then, a superlattice third cladding layer of the opposite conductivity type to the first cladding layer is formed, but in this embodiment comprising only 14 to 50 _{AlxalGa1 -xalN} / _{InxiGa1 -xiN} , layer pairs of In _xay Ga _1-xay N/In _xn Al _1-xn N or equivalent materials. Then, a current blocking layer is formed on the superlattice third cladding layer, and a window is formed in the current blocking layer, the blocking layer exposes a part of the superlattice third cladding layer. Then, a superlattice fourth cladding layer is formed on the current blocking layer, and may be about 200 layer pairs. A window in the current blocking layer provides an interface between the superlattice fourth cladding layer and the superlattice third cladding layer. The fourth cladding layer of the superlattice has the same conductivity type as the third cladding layer of the superlattice. Like the first embodiment, xal determines the mole fraction of AlN (using this material as an example), xi and xa determine the mole fraction of InN, and xi and xa are related by xa>xi. Finally, a fifth cladding layer such as GaN is formed on the fourth cladding layer and electrodes are formed in a conventional manner.

Similar to the first embodiment, the lattice constant of AlGaN (or equivalent) in the superlattice layer is smaller than that of the GaN cladding layer, while the lattice constant of InGaN in the superlattice layer is larger than that of the GaN cladding layer The lattice constant of . In this case, the AlGaN layer will be under tensile stress and the InGaN layer will be under compressive stress, which in turn will allow the stresses in the complementary layer to compensate each other at the AlGaN/InGaN layer interface. Likewise, the AlGaN/InGaN superlattice layer will provide better confinement of the optical field within the active layer than if GaN were used for the cladding layer. Additionally, improved optical confinement within the active layer in the lateral direction will lead to a reduced threshold current. Since the mole fraction xa of InN is greater than the mole fraction xi of InN, a reduced threshold current is also possible because the AlGaN/InGaN superlattice layer does not absorb laser light from the InGaN single quantum well active layer. This will make the bandgap energy of InGaN in the AlGaN/InGaN superlattice layer larger than the bandgap energy of the InGaN single quantum well active layer. The end result is a semiconductor structure with low threshold current and low defect density.

As can be appreciated from the foregoing, the main difference between the first and second embodiments is the addition of a current blocking layer which, in the exemplary embodiment described above, is sandwiched between a smaller superlattice layer and a larger supercrystalline between layers. In the above exemplary arrangement, the semiconductor structure has an _{AlxbGa1 -xbN} current blocking layer with a third cladding through which the _{AlxalGa1 -xalN} / _{InxiGa1 -xiN} superlattice is formed layer, where the current blocking layer has the opposite conductivity type to the Al _xal Ga _1-xal N/In _xi Ga _1-xi N superlattice layer, where xb determines the mole fraction of AlN, and xb and xal The relationship is xb>xal. By utilizing said current blocking layer to form a window region in the superlattice layer, the effective refractive index in the window region will be greater than the refractive index outside the window region. This helps to laterally confine the light field to the activity under the window area. Since the molar fraction xb of AlN is greater than the molar fraction xal of the superlattice layer outside the window region, the effective refractive index in the refractive index window region will increase. In addition, since the conductivity type of the AlGaN current blocking layer is different from that of the AlGaN/InGaN superlattice cladding layer, the injection current will be limited to the window region. This will make the injected current density in the active layer in the window area sufficiently large to obtain laser oscillation. Thus, using said current blocking layer with windows into the superlattice layer, it will be possible to obtain laser diodes operating in single transverse mode.

The third embodiment of the present invention is similar in structure to the first embodiment, but is implemented using a quaternary material system instead of the ternary material system described above. In such embodiments, a cladding layer of In _1-x1-y1 Ga _{x1 Aly1} N material of the first conductivity type is formed on GaN or other substrate. Then, as the second cladding layer, a first superlattice layer of the first conductivity type is formed, which layer includes In _1-x2-y2 Ga _{x2 Aly2} N and In _1-x3-y3 Ga _{x3 Aly3} N materials. In the cladding layer, the lattice constant of the In _1-x2-y2 Ga _x2 Al _y2 N material is selected to be larger than that of the In _{1-x1-y1 Ga x1} Al _y1 N material, while the In _1-x3- The lattice constant of _{the y3Gax3Aly3N} material is chosen to _be larger than the lattice constant of the In1 _{-x1- y1Gax1Aly1N material} . Then form an active layer of quantum wells such as InGaN material, the quantum wells can be single or multiple quantum wells; then form a second superlattice layer of the opposite conductivity type. The second superlattice layer may for example comprise: In _1-x4-y4 Ga _{x4 Aly4} N and In _1-x5-y5 Ga _{x5 Aly5} N, wherein the lattice of In _1-x4-y4 Ga _{x4 Aly4} N The constant is larger than the lattice constant of the In _1-x1-y1 Ga _x1 Al _y1 N material, while the lattice constant of the In _1-x5-y5 Ga _x5 Al _y5 N is smaller than the In _1-x1-y1 Ga _x1 Al _y1 N material The lattice constant of . The second superlattice layer functions as a third cladding layer. Then, a fourth cladding layer of opposite conductivity type to the first cladding layer is formed, and its material is usually In _1-x6-y6 Ga _x6 Al _y6 N material. The values of x1, x2, x3, x4, x5, and x6 determine the mole fraction of GaN and y1, y2, y3, y4, y5, and y6 determine the mole fraction of AlN. As with the first and second embodiments, a conductive layer may be supplemented in some embodiments to help confine the light field and, if supplemented, be formed directly on either side of the active layer.

In the third embodiment, as in the first and second embodiments, in the first superlattice layer, the In _1-x2-y2 Ga _x2 Al _y2 N layer is under tensile stress, and the In _1-x3-y3 The Ga _x3 Al _y3 N layer is under compressive stress, and as a result, the stresses can compensate each other at the interface of the In _1-x2-y2 Ga _x2 Al _y2 N layer and the In _1-x3-y3 Ga _x3 Al _y3 N layer. Likewise, in the second lattice layer, the In _1-x4-y4 Ga _{x4 Aly4} N layer is under tensile stress, while the In _1-x5-y5 Ga _{x5 Aly5} N layer is under compressive stress, as a result, in At the interface of the In _1-x4-y4 Ga _x4 Al _y4 N layer and the In _1-x5-y5 Ga _x5 Al _y5 N layer, the stresses can compensate each other.

The InGaN superlattice layer is designed to confine the optical field within the active layer better than GaN alone for the cladding layer. By increasing the optical confinement within the active layer in the lateral direction, the threshold current of the device can be reduced. It is also preferred that the InGaAlN superlattice layer is also designed not to absorb laser light from the active layer. Therefore, a laser diode with low threshold current and low defect density is obtained.

The fourth embodiment of the present invention basically comprises: the quaternary material of the third embodiment, and the overall structure of the second embodiment, i.e., the use of superlattice third and fourth cladding layers on either side of the current blocking layer , will allow interfaces between these superlattice layers by means of windows in the current blocking layer.

The foregoing aspects outlining the invention will be more readily understood in light of the following detailed description of the invention, together with the accompanying drawings shown below.

Overview of drawings

Figure 1 shows a prior art laser diode.

Figure 2 shows a simplified cross-sectional view of the invention.

Fig. 3 shows a simplified cross-sectional view of the semiconductor device of the first embodiment.

4A-4C show a simplified series of steps for fabricating a semiconductor structure according to a first embodiment.

Fig. 5 shows the relationship between excess stress and In content in the superlattice cladding layer.

Fig. 6 shows the output power depending on the injected current density of the first embodiment.

Fig. 7 shows the dependence of the output power on the injected current density of the third embodiment.

Fig. 8 shows the relationship between excess stress and In content in the superlattice cladding layer.

FIG. 9 shows the relationship between the excess stress and the In content of the InAlN layer in the superlattice cladding layer.

Fig. 10 shows a simplified cross-sectional view of a semiconductor device according to a second embodiment.

11A-C show a series of simplified steps for fabricating a semiconductor laser diode according to a second embodiment.

Figure 12 shows the relationship between the effective refractive index difference (Dn) between the inside and outside of the window region and the third cladding thickness (dp).

Fig. 13 shows the dependence of the output power on the injected current density of the second embodiment.

Figure 14 shows the relationship between the effective refractive index difference (Dn) between the inside and outside of the window region and the third cladding thickness (dp).

Fig. 15 shows the dependence of the output power on the injection current density of the fourth embodiment.

Fig. 16 shows a simplified cross-sectional view of a semiconductor device according to a third embodiment.

Fig. 17 shows a simplified cross-sectional view of a semiconductor device according to a fourth embodiment.

Figure 18 shows a heterojunction field effect transistor constructed in accordance with the present invention.

Figure 19 shows a heterojunction bipolar transistor constructed in accordance with the present invention.

Figure 20 shows a photodiode constructed in accordance with the present invention.

Figure 21 shows a phototransistor constructed in accordance with the present invention.

Detailed description of the invention

Referring first to FIG. 2, there is shown a semiconductor structure in accordance with the present invention in its general form. The first cladding layer 105 is formed on the substrate 100, which may be GaN, sapphire, silicon carbide or other suitable substrates. The first cladding layer is generally of the same conductivity type as the substrate. Then a second cladding layer 110 is formed on the first cladding layer 105 , wherein the conductivity type of the second cladding layer is the same as that of the first cladding layer.

The second cladding layer 110 is composed of a number of layer pairs 115, each layer pair having a thickness less than its critical thickness, but which together form a superlattice. The use of a superlattice layer overcomes the relatively small lattice constant of AlGaN (and similar materials) relative to GaN, while providing the benefits of the relatively large bandgap of AlGaN (and similar materials) relative to GaN, and also provides Smaller refractive index. Typically, the constituent layers of the superlattice are chosen such that one of the layers has a larger lattice constant than the active layer, eg a GaN layer, and the other constituent layer has a smaller lattice constant than the active layer. This relationship between the lattice constants of the superlattice constituent layers can be simply expressed as SL1>GaN>SL2. For group III nitrides, the relationship of lattice constants is InN>GaN>AlN, which also means that the lattice constants of InGaN, GaN, and AlGaN are: InGaN>GaN>AlGaN. Likewise, by properly selecting the atomic content, the relationship of lattice constants of other materials can be set as follows: InAlN>GaN>AlGaN, and InGaN>GaN>InAlN. These relationships are discussed in more detail below.

The constituent layers 15 of the superlattice second cladding layer 110 are under resistive stress, so that a first layer is under tensile stress and an adjacent layer is under compressive stress. For the material, each layer is below its critical thickness, and therefore cracking inside the material is avoided. The number of layer pairs comprising the superlattice can vary greatly from 20 or less to more than 200, where layers of increasing thickness will provide greater optical confinement but require increased electrical and thermal resistance, Hence the need for increased heating.

After the superlattice layer 110 is fabricated, an active layer 120 is added on the superlattice layer 110 , and a third cladding layer 125 of superlattice that is complementary to the conductivity type of the superlattice layer 110 is added. Additionally, as will also be discussed in detail below, a conductive layer may also be provided over the superlattice layer 110 . In this case, a second conductive layer is added on top of the active layer, and then a third cladding layer 125 of superlattice is added. The conducting layer will have the same conductivity type as its adjacent superlattice layer.

After the provision of the superlattice layer 125, a fourth cladding layer 130 is formed, the conductivity type of which is the same as layer 125. Electrode pairs 135 and 140 are then formed in a conventional manner, for example on the underside of substrate 100 and on top of fourth cladding layer 130 .

By proper selection of materials for the superlattice layers, tensile and compressive stresses can be balanced in these layers so that defect density is minimized. In addition, since the refractive index difference between the superlattice cladding layer and the active layer is larger than that between conventional GaN and the active layer, it is possible to transfer light to The field is better confined within the active layer.

Reference is next made to FIGS. 3 and 4A-4C, which illustrate in detail a first embodiment of a semiconductor structure according to the present invention. For the sake of simplicity, when illustrating the first embodiment of the present invention, a laser diode is chosen as an exemplary semiconductor structure and is shown in a simplified cross-sectional view. An n-type GaN first cladding layer 155 is formed on the n-type GaN substrate 150 to a thickness of about 0.5 microns. A superlattice second cladding layer 160 of n-type material is then formed. For the exemplary device of the first embodiment, the number of layer pairs may be about 200. The material used for the superlattice layer can be any of several combinations of suitable lattice constants, conductivity types, and the like. As will be discussed in detail below, exemplary materials are Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N, or Al _0.2 Ga _0.8 N/In _0.2 Al _0.8 N or In _0.04 Ga _0.96 N/In _0.13 Al _0.87 N. The typical thickness of each of the superlattice constituent layers is about 20 angstroms, but the exact thickness may vary within reasonable tolerances as long as the critical thickness for dislocation generation is not exceeded.

After the superlattice second cladding layer 160 is prepared, an n-type conduction layer 165 of In _0.02 Ga _0.98 N material to a thickness of about 35 Angstroms can be formed, if desired, but in at least some embodiments, a conduction layer need not be used. Then, an active layer 170 of quantum wells, which may be single or multiple quantum wells, is formed. If multiple quantum wells are used, a three-pair configuration has been found to be desirable, although the exact configuration may vary depending on the application. For a single well device, the layer 170 may comprise: In _0.15 Ga _0.85 N material about 35 Angstroms thick. If multiple well quantum layers are preferred, layer 170 may comprise: three pairs of In _0.15 Ga _0.85 N/In _0.03 Ga _0.98 N (35 Angstroms thick) material. If a conductive layer is used, for example, a second conductive layer of In _0.03 Ga _0.97 N material with a conductivity type opposite to that of the first conductive layer is formed about 35 Angstroms thick. Then, a superlattice third cladding layer 180 of p-type material is formed. Like layer 160, superlattice layer 180 may comprise: 200 pairs of In _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N material, typically 20 Angstroms thick, or may be Al _0.2 Ga _0.8 N/In _0.2 Al _0.8 N or material In _0.04 Ga _0.96 N/In _0.13 Al _0.87 N. Finally, a p-type GaN fourth cladding layer 185 is formed, the thickness of which is usually about 0.5 microns. Electrode pairs (not shown) are formed in a conventional manner.

In order to emit blue light having a wavelength of 450 nm from the active layer 170, the mole fraction of InN in the active layer 170 is set at about 0.15. Carriers are mainly injected from n-type substrate 150 and p-type GaN fourth cladding layer 185 and recombine in active layer 170, which will result in blue light emission.

The superlattice layers 160 and 180 play the role of confining the optical field in the lateral direction, which will be better than conventional GaN cladding layers, because the refractive index difference between active layer and cladding layer is larger than active layer and conventional GaN layer difference in refractive index between them. Strong optical confinement in the active layer will result in a low threshold current laser diode.

To minimize defect generation, strain compensating superlattice layers 160 and 180 are used for cladding instead of conventional AlGaN cladding or simple GaN cladding. In the superlattice structure of the present invention, the thickness of the constituent layers comprising the superlattice layer is maintained below the critical thickness, or typically about 20 Angstroms. This will greatly reduce the stress in the cladding layer and thus minimize the defect density in this layer. For each material pair used in the superlattice layer, one layer of material, such as Al _0.2 Ga _0.8 N, is under tensile stress, while the other layer, such as In _0.04 Ga _0.96 N, is under compressive stress. Usually, one material (such as Al _0.2 Ga _0.8 N) is selected for its lattice constant smaller than GaN, while the other material (such as In _0.04 Ga _0.96 N) has a lattice constant larger than GaN, at the interface between layers , the stress can be compensated. This prevents stress buildup and reduces defect density relative to conventional GaN cladding.

As mentioned previously, combinations of several materials can also be used for the superlattice layer. Each exemplary combination of materials— _{Al 0.2} Ga _0.8 N/In _0.04 Ga _0.96 N/In _0.2 Al _0.8 N, or In _0.04 Ga _0.96 N/In _0.13 Al _0.87 N—is discussed below.

Figure 5 shows the relationship between excess stress in the waveguide structure and the In content of the InGaN layer in the AlGaN/InGaN superlattice cladding layer if the Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N combination is used. Except for the In content of the InGaN layer in the AlGaN/InGaN superlattice cladding layer, other structural parameters are fixed. For this purpose, excess stress was determined as the difference between the maximum stress in the epilayer of the waveguide and the effective stress associated with the dislocation lines in the absence of dislocations in the waveguide structure. If the excess stress is positive, then the strain energy becomes smaller when dislocations are generated in the waveguide structure than when dislocations are not generated in the waveguide structure. This means that when dislocations are generated in a waveguide structure, the structure will be more stable than when they are not.

However, if the excess stress becomes negative, the opposite will happen: when dislocations are not generated in the waveguide structure, the strain energy will become smaller than when dislocations are generated in the waveguide structure. This means that the structure will be more stable when dislocations are not generated in the waveguide structure compared to when dislocations are generated in the waveguide structure. As shown in Fig. 5, when the In content is equal to 0.04, the excessive stress becomes the minimum. Therefore, in the structure of the embodiment shown in FIG. 3, the molar fraction of AlN in the AlGaN layer in the superlattice cladding layer and the molar fraction of InN in the InGaN layer in the superlattice cladding layer are set at 0.2 and 0.4, respectively.

Furthermore, if the Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N superlattice layer is used as the cladding layer, in the lateral direction, the optical confinement in the active layer will be greater than that of using only the GaN cladding layer as the cladding material. This is because the average refractive index of the Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N superlattice cladding layer is smaller than the average refractive index of the GaN cladding layer, which will result in a better gap between the cladding layer and the active layer. large refractive index difference.

Fig. 6 shows photo-current characteristics of the laser diode of the first embodiment in which the superlattice material is Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N and a single quantum well is used. The laser diode was driven with a pulsed current with a duty cycle of 1%. The threshold current density is 5.2 kA/cm ² , which is about half of the threshold current density of a laser diode with a cladding made only of GaN. Figure 7 shows the photo-current characteristics of a laser diode constructed according to the first embodiment, but using a multiple quantum well design. The laser diode is driven with a pulsed current with a duty factor of 1%. The threshold current density is 4.2 kA/cm ² , which is also about half of the threshold current density of a multi-quantum well laser diode that uses only GaN for its cladding.

A second exemplary combination of materials for superlattice layers ₁₆₀ and 180 _{is Al0.2Ga0.8N} / _In0.2Al0.8N . When Al _0.2 Ga _0.8 N/In _0.2 Al _0.8 N is used for the superlattice layer, the stress balance is slightly different. Figure 8 shows the relationship between excess stress in the waveguide structure and the In content of the InAlN layer in the AlGaN/InAlN superlattice cladding. Except for the In content of the InAlN layer in the AlGaN/InAlN superlattice cladding layer, other structural parameters are fixed. As shown in Fig. 8, in the case of In content equal to 0.2, excessive stress becomes minimum. Therefore, if Al _0.2 Ga _0.8 N/In _0.2 Al _0.8 N is used for the superlattice layer, the molar fraction of AlN in the AlGaN layer in the superlattice cladding layer and the molar fraction of InN in the InAlN layer in the superlattice cladding layer The values are set at 0.2 and 0.2, respectively, in order to ensure that the strain is compensated by the adjacent constituent layers.

Also, for the case where Al _0.2 Ga _0.8 N/In _0.2 Al _0.8 N is used as the superlattice layer, in the lateral direction, the confinement of the optical field within the active layer will be better than that of the GaN cladding alone, because The average refractive index of the Al _0.2 Ga _0.8 N/In _0.2 Al _0.8 N superlattice layer is smaller than that of the GaN cladding layer.

A third exemplary combination of materials for superlattice layers 160 and 180 is In _0.04 Ga _0.96 N/In _0.13 Al _0.87 N. In this combination, the In _0.13 Al _0.87 N layer is under tensile stress, while the In _0.04 Ga _0.96 N layer is under compressive stress. Therefore, stress is compensated at the interface between the In _0.13 Al _0.87 N layer and the In _0.04 Ga _0.96 N layer. The relationship of the lattice constant is In _0.04 Ga _0.96 N>GaN>In _0.13 Al _0.87 N.

As with the previous material combinations, the stress balance will change due to the material. Figure 9 shows the relationship between excess stress in the waveguide structure and the In content of the InAlN layer in the InGaN/InAlN superlattice cladding. Except for the In content of the InAlN layer in the InGaN/InAlN superlattice cladding layer, other structural parameters are fixed at the above-mentioned values. As shown in FIG. 9, in the case of an In content of 0.13 in the InAlN layer, excess stress becomes minimum. Therefore, in the structure of the ninth embodiment shown in FIG. 9, in order to compensate for the strain, the molar fraction of InN in the InGaN layer in the superlattice cladding layer and the molar fraction of AlN in the InAlN layer in the superlattice cladding layer are respectively Set at 0.04 and 0.87.

In addition, if In _0.04 Ga _0.96 N > GaN > In _0.13 Al _0.87 N is used for the superlattice cladding layer, in the lateral direction, the optical field confinement in the active layer will be better than the cladding layer using only GaN. In _0.04 Ga _0.96 N>GaN>In _0.13 Al _0.87 N The average refractive index of the superlattice cladding layer is smaller than that of the GaN cladding layer, which will obtain a larger refractive index between the cladding layer and the active layer Poor, this happens when only GaN cladding is used.

Referring next to Figure 10 and Figures 11A-11C, a second embodiment of the present invention may be better understood. As with the embodiment of FIG. 3 , FIG. 10 is a simplified cross-sectional view of a second embodiment semiconductor laser diode, while FIGS. 11A-11C show simplified variations of the fabrication steps leading to the structure of FIG. 10 . On the n-type GaN substrate 300, a first cladding layer 305 of n-type GaN is formed with a thickness of about 0.5 microns, followed by a second cladding layer 310 of n-type superlattice with about 200 constituent layer pairs. Then, an In _0.02 Ga _0.98 N conductive layer about 35 Angstroms thick is formed, followed by a quantum well active layer 320 . The quantum well active layer (approximately 35 Angstroms) can be single or multiple quantum wells. If a single quantum well design is used, then the active layer typically comprises In _0.15 Ga _0.85 N. If the MQW design is used, then three pairs of In _0.15 Ga _0.85 N/In _0.03 Ga _0.98 N MQWs can be used to prepare the active layer, each layer having a thickness of about 35 angstroms. Then, in some embodiments, an In _0.03 Ga _0.97 N conductive layer 325 may be formed to a thickness of about 35 Angstroms.

Then, an obvious difference from the first embodiment is that a p-type superlattice third cladding layer 330 is formed. However, the layer 330 typically includes only about 25 pairs of constituent layers, each layer having a thickness of about 20 angstroms. Then, a p-type Al _0.22 Ga _0.78 N current blocking layer 335 is formed to a thickness of about 100 angstroms. Then, strip windows 340 are formed in the current blocking layer 335 so as to expose a portion of the third cladding layer 330 . A p-type fourth superlattice cladding layer 345 is then formed, typically comprising about 200 pairs of constituent layers. Finally, a fifth cladding layer 350 of P-type GaN is formed with a thickness of about 0.5 microns. Electrodes can be formed in a conventional manner.

As with the first embodiment, several different combinations of materials can be used to make superlattice layers 310, 330, and 345, which can include: Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N, Al _0.2 Ga _0.8 N/In _0.2 Al _0.8 N, or In _0.04 Ga _0.96 N/In _0.13 Al _0.87 N. The manipulation of these materials is the same as discussed in connection with the first embodiment, except that manipulation of the current blocking layer is discussed in detail below. Therefore, the remainder of the discussion of the second embodiment will use Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N as example.

In order to emit blue light with a wavelength of 450 nm from the active layer 320 , the mole fraction of InN in the active layer 320 is set at 0.15. In order to obtain the manipulation of the basic transverse form, the window width is set to 2mm.

In order to have single lateral mode oscillation, the AlN mole fraction of the current blocking layer 335 is set to be greater than that of the p-type Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N superlattice fourth cladding layer 350 . When the AlN molar fraction of the current blocking layer 335 is the same as that of the fourth cladding layer, the refractive index within the stripe will be lowered due to the plasmon effect and a waveguide will be formed, resulting in no one-sided mode oscillation. When the mole fraction of AlN in the current blocking layer 335 is lower than the fourth cladding layer 345 of the p-type Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N superlattice, the lateral mode oscillation becomes unstable. In this case, the AlN mole fraction of the current blocking layer 335 is set at 0.22, which is higher than the Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N superlattice fourth cladding layer 345 . In addition, the thickness (dp) of the third cladding layer 330 will also affect the effective refractive index difference (Δn) inside and outside the window region. When the value of dp is large, Δn will become small. On the other hand, when the value of dp is small, Δn will become large. In the case of a large value of Δn, the confinement of the light field in the lateral direction will be more intense, which will cause the spatial holes to burn, so that the light field will be deformed. The deformation of the light field will be a decisive issue for utilizing said means for an optical pick-up system. If Δn is small, the light field will be distributed laterally into the active layer outside the window area. In this case, the active layer outside the window region will be greatly activated by the injected carriers, and as a result, the optical field will suffer optical losses, which will increase the threshold current.

Fig. 12 shows the relationship between Δn and dp. As shown in Figure 12, when dp becomes larger, Δn will become smaller. In order to properly confine the light field within the window region in the lateral direction, the value of Δn is set at about 6×10 ⁻³ . In the second embodiment, in order to obtain a Δn value of 6×10 ⁻³ , dp is set to 0.1 mm.

According to the structure of the second embodiment shown in FIG. 10 , the current injected through the fourth cladding layer 345 of the p-type Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N superlattice is confined in the window 340 and is located at the window Laser oscillation with a bandwidth of 450 nm is generated in the underlying quantum well active layer 320 . As mentioned earlier, in the superlattice layer, the use of AlGaN to form the layer will help to strongly confine the optical field in the lateral direction. Strong optical confinement in the active layer will result in a laser diode with low threshold current.

Figure 13 shows the photo-current characteristics of a laser diode constructed according to the second embodiment with a single quantum well. The laser diode is driven with a pulsed current with a duty factor of 1%. The threshold current density was 4.0 kA/cm ² , which is about half of the threshold current density of a laser diode with a cladding made only of GaN.

In the case of using multiple quantum well active layers, the relationship between Δn and dp will slightly change. As in Figure 2, the relationship shown in Figure 14 shows that Δn will become smaller as dp becomes larger. In order to properly confine the light field laterally within the window region, the value of Δn is set at about 6×10 ⁻³ . In the second embodiment, in order to obtain a Δn value of 6×10 ⁻³ , dp is set to 0.08 mm.

Fig. 15 shows the photo-current characteristics of the laser diode of the second embodiment, but using a multiple quantum well active layer. This device will result in improved (ie increased) optical confinement in the active layer in the lateral direction beyond what can be achieved with a single quantum well active layer. Therefore, the multiple quantum well device can further reduce the threshold current. Fig. 15 shows a laser diode driven by a pulsed current with a duty factor of 1%. The threshold current density is about 3.8kA/cm ² , which is also about half of the threshold current density of a laser diode with a GaN cladding layer.

Referring now to FIG. 16, a third embodiment of the present invention may be better understood. The embodiment shown in FIG. 16 differs from the embodiment shown in FIG. 3 in that it uses a quaternary material system instead of the ternary system of FIG. 3 . Therefore, in a third embodiment of the present invention, the semiconductor structure - which may be a laser diode for example - comprises the following: on a GaN or other substrate 400, forming an In _1-x1-y1 Ga _x1 Al _y1 N material first Conductive cladding layer 405 . Then, the second cladding layer 410 of the superlattice of the first conductivity type is formed, and the layer includes: In _1-x2-y2 Ga _x2 Al _y2 N and In _1-x3-y3 Ga _x3 Al _y3 N materials. In the cladding layer, the lattice constant of the In _1-x2-y2 Ga _x2 Al _y2 N material is selected to be larger than the lattice constant of the In _{1-x1-y1 Ga x1} Al _y1 N material, and the In _1-x3- The lattice constant of the _y3 Ga _x3 Al _y3 N material is selected to be smaller than that of the In _1-x1-y1 Ga _x1 Al _y1 N material. In certain embodiments, conductive layer 415 of any suitable material may be formed on superlattice layer 410 . Then, the quantum well active layer 420 is formed, and the quantum well active layer can be designed with single well or multiple wells. Then, in at least some embodiments, a conductive layer 425 is formed. A superlattice third cladding layer 430 of the opposite conductivity type is then formed. The third cladding layer of the superlattice may include, for example: In _1-x4-y4 Ga _x4 Al _y4 N and In _1-x5-y5 Ga _x5 Al _y5 N materials, wherein In _1-x4-y4 Ga _x4 Al The lattice constant of _{the y4} N material is larger than that of the In _1-x1-y1 Ga _x1 Al _y1 N material, while the lattice constant of the In _1-x5-y5 Ga _x5 Al _y5 N material is smaller than that of the In _1-x1-y1 Ga Lattice constants of _x1 Al _y1 N materials. A fourth cladding layer 435 is then formed, typically of In _1-x6-y6 Ga _{x6 Aly6} material, of the opposite conductivity type to the first cladding layer. x1, x2, x3, x4, x5, and x6 define the mole fraction of GaN, while y1, y2, y3, y4, y5, and y6 define the mole fraction of AlN.

With the stated structure, in the superlattice layer, the In _1-x2-y2 Ga _{x2 Aly2} N layer is under compressive stress and the In _1-x3-y3 Ga _{x3 Aly3} N layer is under tensile stress, as a result, in At the interface of the In _1-x2-y2 Ga _x2 Al _y2 N layer and the In _1-x3-y3 Ga _x3 Al _y3 N layer, the stresses will be mutually compensated. Likewise, in the second superlattice layer, the In _1-x4-y4 Ga _{x4 Aly4} N layer is under compressive stress and the In _1-x5-y5 Ga _{x5 Aly5} N layer is under tensile stress, as a result, at At the interface of the In _1-x4-y4 Ga _x4 Al _y4 N layer and the In _1-x5-y5 Ga _x5 Al _y5 N layer, stresses can be compensated for each other.

As with previous embodiments, the InGaAlN superlattice layer is designed to confine the optical field in the active layer better than if GaN were used for the cladding layer. By increasing the optical confinement within the lateral active layer, the threshold current of the device can be reduced. In addition, the InGaAlN superlattice layer can be designed so as not to absorb laser light from the active layer. Therefore, a low threshold current and a low defect density are obtained.

Referring to Figure 17, a fourth embodiment of the present invention can be better understood. The fourth embodiment uses the quaternary material system of the third embodiment, but utilizes the structure of the second embodiment shown in Fig. 10 and Figs. 11A-11C. On the GaN or other substrate 500, a first conductivity type cladding layer 505 of In _1-x1-y1 Ga _{x1 Aly1} N material is formed. Then, the second cladding layer 510 of the superlattice of the first conductivity type is formed, and the layer includes: In _1-x2-y2 Ga _x2 Al _y2 N and In _1-x3-y3 Ga _x3 Al _y3 N materials. In the cladding layer, the lattice constant of the In _1-x2-y2 Ga _x2 Al _y2 N material is selected to be larger than the lattice constant of the In _{1-x1-y1 Ga x1} Al _y1 N material, and the In _1-x3- The lattice constant of the _y3 Ga _x3 Al _y3 N material is selected to be smaller than that of the In _1-x1-y1 Ga _x1 Al _y1 N material. In certain embodiments, a conductive layer 515 of any suitable material may be formed on the superlattice layer 510 . Then, the quantum well active layer 520 is formed, and the quantum well active layer can be designed with single well or multiple wells. Then, as with the previous embodiments, in at least some embodiments, another conduction layer 525 of the opposite conductivity type is formed. A superlattice third cladding layer 530 of the opposite conductivity type is then formed. The third cladding layer of the superlattice may include, for example: In _1-x4-y4 Ga _x4 Al _y4 N and In _1-x5-y5 Ga _x5 Al _y5 N materials, wherein In _1-x4-y4 Ga _x4 Al The lattice constant of _{the y4} N material is larger than that of the In _1-x1-y1 Ga _x1 Al _y1 N material, while the lattice constant of the In _1-x5-y5 Ga _x5 Al _y5 N material is smaller than that of the In _1-x1-y1 Ga Lattice constants of _x1 Al _y1 N materials. The superlattice layer 530 may be about 25 pairs of constituent layers. Then a current blocking layer of p-type Al _0.22 Ga _0.78 N material is formed with a thickness of about 100 angstroms. Strip windows are then formed in the current blocking layer 532 to expose the superlattice layer 530 . A fourth cladding layer 535 of superlattice of the same material as superlattice layer 330 but with about 20 pairs of constituent layers is then formed. Finally, a fifth cladding layer was formed in the same manner as described above. Likewise, electrode pairs may be formed in a conventional manner.

Reference is next made to Figure 18, which illustrates a heterojunction field effect transistor formed by the methods and structures of the present invention. Firstly, an i-GaN cladding layer 605 with a thickness of about 0.5 μm is formed on the GaN substrate 600, and then an n-type channel layer 610 with a thickness of about 100 angstroms is formed thereon. Then form about five pairs of superlattice layers 615 on it, the thickness of each layer is about 20 angstroms, and its material is Al _0.2 Ga _0.8 N (6 layers)/n-type In _0.04 Ga _0.96 N (5 layers) . Then, source, drain and gate electrodes 620 , 625 and 630 are formed on the superlattice layer 615 . Group III nitride materials, especially GaN and AlN, are desirable materials for high-energy electronic devices capable of operating at high power and high temperatures due to their wider bandgaps (GaN is 3.5eV, AlN is 6.2eV), therefore, will produce higher breakdown electric field, and higher saturation velocity. This is compared to AlAs, GaAs, and Si (2.16eV, 1.42eV, and 1.12eV), respectively. Therefore, field-effect transistors (FETs) utilizing AlGaN/GaN materials are widely explored for use in the field of microwave power transistors.

Referring now to Figure 19, there is shown a heterojunction bipolar transistor formed in accordance with the present invention. GaN substrate 650 provides the basis upon which superlattice collector layer 655 is formed. A p-type GaN base layer 660 is then formed, followed by a superlattice emitter layer 665 . Collector, base and emitter electrodes 670, 675 and 680 are then formed. Figure 19 shows an embodiment of a heterojunction bipolar transistor (HBT). First, one hundred pairs of n-type, 20 Å thick Al _0.2 Ga _0.8 N (101 layers)/n-type, 20 Å thick In _0.04 Ga _0.96 N (100 layers) superlattice sets are formed on the GaN substrate 650 The electrode layer is then formed with a 50nm thick p-type GaN base layer. Then, as the emitter, about 80 pairs of n-type, 20 Å thick Al _0.2 Ga _0.8 N (81 layers)/n-type, 20 Å thick In _0.04 Ga _0.96 N (80 layers) superlattice layers were formed . The stress between the AlGaN layer and the InGaN layer will be mutually compensated at the interface, so that the generated defects are reduced, which will produce a high-quality heterojunction AlGaN/GaN. The band gap of the AlGaN/GaN superlattice emitter layer is larger than the band gap of the GaN base layer, so that the holes generated in the p-type base layer will be well confined in the base layer, which is Because, compared to the valence band of a GaN homojunction bipolar transistor, there is a larger discontinuous valence band between GaN and the AlGaN/InGaN superlattice layer. Therefore, a large current amplification is obtained between the base current and the collector current. In addition, as described above, the bandgap of the AlGaN/InGaN superlattice layer and GaN is large so that the transistor can be used as a high temperature transistor. It will also be appreciated that although the embodiments described above use superlattice layers for both the emitter and collector, in all instances it is not necessary for both layers to be of the superlattice type and act as collector or emitter pole, a single superlattice layer can be used.

An embodiment of the present invention implemented as a photodiode can be better understood with reference now to FIG. 20 . First, an n-type GaN first cladding layer 705 with a thickness of about 0.5 μm is formed on the n-type GaN substrate 700, and then an n-type superlattice second cladding layer 710 of about 200 pairs of constituent layers is formed. Subsequently, an In _0.02 Ga _0.98 N conductive layer 715 is formed approximately 35 Angstroms thick, followed by a quantum well active layer 720 . The active layer typically contains In _0.15 Ga _0.85 N. Then, in some embodiments, a conductive layer 325 of In _0.03 Ga _0.97 N, about 35 Angstroms thick, may be supplemented.

Then, in a marked difference from the first embodiment, a p-type superlattice third cladding layer is formed. However, layer 330 typically comprises about 25 pairs of constituent layers, each layer being about 20 Angstroms thick. Then, a p-type Al _0.22 Ga _0.78 N current blocking layer 335 is formed to a thickness of about 100 angstroms. Strip windows 340 are then formed in the current blocking layer 335 to expose a portion of the third cladding layer 330 . Electrodes are formed in a conventional manner.

As with the first embodiment, several different combinations of materials can be used to make superlattice layers 710 and 730, and can include Al _0.2 Ga _0.8 N/In _0.04 Ga _0.96 N, Al _0.2 Ga _0.8 N/In _0.2 Al _0.8 N, or In _0.04 Ga _0.96 N/In _0.13 Al _0.87 N. The operation of these materials is as described in the second embodiment, except that the upper cladding layer and the third superlattice layer are removed. In a preferred arrangement of the invention, the window 340 may be shaped in the form of a small outer ring.

Referring to FIG. 21, there is shown an embodiment of a semiconductor device of the present invention implemented as a heterojunction phototransistor. The device is particularly well suited for manipulation in the ultraviolet (UV) range, although other frequencies, including blue light, can also be detected with slight modification. Since GaN and AlGaN have wide band gaps (corresponding to a wavelength of 200 nm for GaN3.5eV and 350 nm for AlN6.2eV), they are used as photodetectors in the ultraviolet (UV) range is attractive. Due to the direct bandgap and efficacy of AlGaN in the whole AlN alloy composition range, the AlGaN/GaN based UV photodetector will have high quantum efficiency and high cut-off wavelength tunability. However, like the foregoing embodiments, AlGaN has a different lattice constant from GaN, and therefore, defects tend to be generated, which will cause leakage.

As mentioned above, by compensating the stress at the interface of the AlGaN and InGaN layers while making the effective bandgap of the superlattice layer larger than that of GaN itself, the strain-compensated superlattice structure of the present invention can reduce the defects in . Still referring to the heterojunction phototransistor (HPT) shown in FIG. 21 , a superlattice collector layer 805 comprising about 120 pairs of n-type, 20 Angstrom thick Al _0.2 Ga is first formed on a GaN substrate 800. _0.8 N (101 layers) and n-type, 20 Angstroms thick In _0.04 Ga _0.96 N (100 layers) constitute layers 805A and 805B. Next, a p-type GaN base layer 820 of about 200 nm thick is formed, followed by a superlattice emitter layer 825 comprising about 80 pairs of n-type, 20 Angstrom thick Al _0.2 Ga _0.8 N (81 layers) and Constituent layers of n-type, 2 nm thick In _0.04 Ga _0.96 N (80 layers). As with the superlattice layers of each embodiment, in the case of AlGaN layers and InGaN layers, the stresses between the constituent layers will compensate each other at their interfaces. These strain-compensated layers significantly reduce the generation of defects, resulting in high-quality heterojunction AlGaN/GaN. Electrodes 830 and 835 are formed in a conventional manner.

As mentioned above, the bandgap of the AlGaN/InGaN superlattice layer is larger than the bandgap of the GaN base layer. When manipulating, the light shines from the emitter side. If the photon energy of the irradiating light is greater than the bandgap energy of the GaN base layer but less than the bandgap energy of the AlGaN/InGaN superlattice emitter layer, the irradiating light will be transmitted to the emitter layer so that the light is absorbed in the GaN base layer inside and generate electron and hole pairs. Since there is a larger valence band between the GaN layer and the AlGaN/InGaN superlattice layer than exists in a GaN homojunction phototransistor, by the Holes generated by light absorption will be better confined in the base layer. This in turn will result in a larger emitter current and better compensation in the base region than is the case with conventional homojunction phototransistors. Therefore, UV photodetectors with high quantum efficiency and high sensitivity can be obtained, which means high conversion efficiency from input light to collector current. In cases where it is desired to detect other frequencies, such as blue light, the GaN base layer can simply be replaced by InGaN.

Having thus fully described several embodiments of this invention, it will be apparent to those of ordinary skill in the art that there are many alternatives and equivalents that exist without departing from the invention. Accordingly, the scope of the invention is not limited by the description described, but is only defined by the appended claims.

Claims (13)

1. A semiconductor structure comprising: a substrate of the first conductivity type, and a first superlattice layer comprising a plurality of pairs of first and second constituent layers; said first constituent layers comprising material under tensile stress and said second constituent layers comprising material under compressive stress; at the interface thereof Compressive stress and tensile stress compensate each other. 2. The semiconductor structure of claim 1 comprising: an active layer formed on a first superlattice layer having a first conductivity type, and a second superlattice layer complementary to the first conductivity type comprising a plurality of third and fourth constituent layers, the third constituent layers comprising material under tensile stress and the fourth constituent layers comprising material under compressive stress, The compressive stress and tensile stress compensate each other at the interface. 3. The semiconductor structure according to claim 1, wherein the substrate is GaN, the semiconductor structure further comprising: a first i-GaN cladding layer formed between the substrate and the first superlattice layer, and An n-GaN channel layer is formed between the i-GaN cladding layer and the first superlattice layer. 4. The semiconductor structure of claim 1, further comprising a base layer and an emitter layer, and wherein the first superlattice layer comprises a collector layer. 5. The semiconductor structure according to claim 1, further comprising: a first cladding layer formed thereon and of the same conductivity type as the substrate, a second cladding layer of the same conductivity type as the substrate, active layer, and wherein the first superlattice layer forms a third cladding layer and has a conductivity type complementary to that of the substrate, A barrier layer formed over the superlattice third cladding layer and having windows therein exposing portions of the superlattice third cladding layer. 6. The semiconductor structure of claim 1, wherein the substrate is GaN and the first superlattice layer forms a collector layer, further comprising a base layer and a superlattice emitter layer, which are controlled by a number of corresponding strains Compensation is composed of constituent layers. 7. A method for preparing a semiconductor structure, comprising: providing a substrate of the first conductivity type, forming a first constituent layer less than a critical thickness, the first layer being under a tensile stress of a predetermined magnitude, A second constituent layer is formed on the first constituent layer, the second constituent layer being under a compressive stress of the same magnitude as the tensile stress of the first constituent layer, with the result that the tensile and compressive stresses compensate each other in the constituent layer. 8. A transistor device comprising: On the semi-insulating layer of the In _1-x1-y1 Ga _x1 Al _y1 N layer, sequentially form: n-type In _1-x1-y1 Ga _x1 Al _y1 N conductive channel layer, composed of In _1-x2-y2 Ga _x2 Al An n-type superlattice layer composed of _y2 N and In _1-x3-y3 Ga _x3 Al _y3 N, wherein the lattice constant of the In _1-x2-y 2Ga _x2 Al _y2 N is greater than that of the In _{1-x1 -y1} Ga _x1 Al _y1 N, and the lattice constant of In _1-x3-y3 Ga _x3 Al _y3 N is smaller than the lattice constant of In _1-x1-y1 Ga _x1 Al _y1 N, where x1, x2, and x3 define the mole fraction of GaN, while y1, y2, and y3 define the mole fraction of AlN, and the effective bandgap of the superlattice layer is greater than that of the In _1-x1-y1 Ga _x1 Al _y1 N layer effective bandgap. 9. A transistor device comprising: The first conductivity type superlattice collector layer composed of In _1-x1-y1 Ga _x1 Al _y1 N and In _1-x2- y2 Ga _x2 Al _y2 N; the opposite conductivity type In _1-x3-y3 Ga _x3 Al _y3 N base layer; In _1-x1-y1 Ga _x1 Al _y1 N and In _{1-x2-y2 Ga x2} Al _y2 N emitter layers of the first conductivity type; wherein, the In _1-x2-y2 Ga _x2 The lattice constant of Al _y2 N is larger than that of the In _1-x3-y3 Ga _x3 Al _y3 N, and the lattice constant of In _1-x2y2 Ga _x2 Al _y2 N is smaller than that of the In _1-x3-y3 Ga Lattice constant of _x3 Al _y3 N, all layers formed sequentially, where In _1-x3-y3 Ga _x3 Al _y3 N has a smaller bandgap than In _{1-x1-y1 Ga x1 Al y1} N and In _1-x2-y2 The effective bandgap of the Ga _x2 Al _y2 N superlattice layer, and x1, x2, and x3 define the mole fraction of GaN, and y1, y2, and y3 define the mole fraction of AlN. 10. A semiconductor laser diode, comprising: On In _1-x1-y1 Ga _x1 Al _y1 N of a certain conductivity type, sequentially formed: a certain conductivity type composed of In _1-x2-y2 Ga _x2 Al _y2 N and In _1-x3-y3 Ga _x3 Al _y3 N superlattice layer, wherein the lattice constant of the In _1-x2-y2 Ga _x2 Al _y2 N is larger than the lattice constant of the In _1-x1-y1 Ga _x1 Al _y1 N, and the In _1-x3- The lattice constant of _y3 Ga _x3 Al _y3 N is smaller than that of In _1-x1-y1 Ga _x1 Al _y1 N; by In _1-x4-y4 Ga _x4 Al _y4 N and In _1-x5-y5 Ga _x5 A superlattice layer of the opposite conductivity type composed of Al _y5 N, wherein the lattice constant of the In _1-x4-y4 Ga _x4 Al _y4 N is greater than that of material 1, and the In _1-x5-y5 The lattice constant of Ga _x5 Al _y5 N is smaller than that of In _1-x1-y1 Ga _x1 Al _y1 N; the opposite conductivity type In _1-x6-y6 Ga _x6 Al _y6 N; where x1, x2, x3, x4 , x5, and x6 define the mole fraction of GaN, while y1, y2, y3, y4, y5, and y6 define the mole fraction of AlN. 11. A semiconductor laser diode, comprising: GaN first cladding layer of a certain conductivity type, Al _xal Ga _1-xal N/ln _xi Ga _1-xi N superlattice second cladding layer of a certain conductivity type, Al _xa Ga _1-xa N single quantum Well active layer, Al _xal Ga _1-xal N/In _xi Ga _1-xi N superlattice cladding layer of opposite conductivity type, GaN fourth cladding layer of opposite conductivity type, all layers are formed sequentially, wherein , xal defines the mole fraction of AlN, xi and xa define the mole fraction of InN, and the relationship between xi and xa is xa>xi. 12. The semiconductor laser diode according to claim 2, wherein Al _xb Ga with windows is formed in the Al _xal Ga _1-xal N/In _xi Ga _1-xi N superlattice third cladding layer _1-xb N current blocking layer, and the third cladding layer of the Al _xal Ga _1-xal N/In _xi Ga _1-xi N superlattice has the opposite conductivity type, and this layer contains the Al _xb Ga _{1- xb} N current blocking layer, wherein, xb defines the mole fraction of AlN, and the relationship between xb and xal is xb>xal. 13. A semiconductor laser diode, comprising: The first cladding layer of GaN of a certain conductivity type, the second layer of Al _xal Ga _1-xal N/In _xi Ga _1-xi N superlattice of a certain conductivity type, the active layer of InGaN multiple quantum wells, and the opposite conductivity type Al _xal Ga _1-xal N/In _xi Ga _1-xi N superlattice third layer, opposite conductivity type GaN fourth cladding layer, all layers are formed sequentially, where xal defines the mole fraction of AlN, xi The mole fraction of InN is defined.

Images