JP2017199711A - Light receiving element - Google Patents

Abstract

In a light receiving element using an InP-based compound semiconductor, light having a wavelength of 1.8 μm or more can be detected with high light receiving sensitivity in a state where generation of crystal defects or the like is suppressed. A light absorption layer includes a compressive strain layer made of InGaAsSb and having a compressive strain with respect to InP, and a tension layer made of InGaAsSb and having a tensile strain with respect to InP. Strain layers are alternately stacked in multiple layers, and the cutoff wavelength is 1.8 μm or more. For example, in the compressive strain layer, the Sb composition ratio in the group V may be 0.03 or more and 0.3 or less, and the lattice mismatch with respect to InP may be 1.0% or more. Moreover, the tensile strain layer should just be Sb composition ratio in V group being 0.03 or more. [Selection] Figure 1

Description

  The present invention relates to a light receiving element composed of a compound semiconductor.

  The present age is based on an excessive environmental load, and immediate measures for environmental protection are required. In order to reduce this environmental load, it is important to reduce the amount of gas discharged from factories, power plants, garbage incineration facilities, automobiles, and the like. In order to reduce gas emissions, highly accurate measurement of gas concentrations in the gas emission source and the atmosphere is required. Gas measurement using light absorption by a gas absorption line is characterized in that it can be measured in real time, remotely, and even gas isotopes can be specified, and is applied to various gas measurement systems.

FIG. 8 shows the types of gas having absorption lines in the wavelength range from 1.8 μm to 2.4 μm and their intensities. In this wavelength range, not only gas species such as CO ₂ and N ₂ O, which have a large impact on the global greenhouse effect, but also emission standards from factories and incineration facilities are established to control air pollution. There are absorption lines of gaseous species such as HCl, NH ₃ , CO. For this reason, the wavelength region is one of wavelength regions important in gas measurement. In FIG. 8, a wavelength region having a large absorption line intensity for each gas type is indicated by a band-like line, and a plurality of isolated absorption lines are included in the band-like line.

  By the way, a light receiving element is used as an optical detecting element. For example, as a light receiving element on an InP substrate, a light receiving element using InGaAs having an In composition of 0.53 lattice-matched to InP as a light absorption layer is widely used.

  FIG. 9 is a characteristic diagram showing a light absorption spectrum of InGaAs lattice-matched to InP. InGaAs hardly absorbs light having a wavelength exceeding 1.8 μm under the condition of lattice matching with InP. For this reason, the cutoff wavelength of a light receiving element using InGaAs that is lattice-matched to InP as a light absorption layer is 1.8 μm or less. For receiving light having a wavelength longer than 1.8 μm, an extended type light receiving element having an InGaAs absorption layer having an In composition larger than 0.53 and lattice-matched to InP is used.

K. Makita et al., "Ga1-yInyAs / InAsxP1-x (y> 0.53, x> 0) pin photodiodes for long wavelength regions (l> 2μm) grown by hydride vapor phase epitaxy", Electronics Letters, Vol.24, No.7, pp.379-380, 1988. M. Ogasawara et al., "Influence of net strain, strain type, and temperature on the critical thickness of In (Ga) AsP-strained multi quantum wells", Journal of Applied Physics, Vol.84, No.9, pp. 4775-4780, 1998. T. Tsuchiya et al., "Investigation of effect of strain-compensated structure and compensation limit in strained-layer multiple quantum wells", Journal of Crystal Growth, Vol.145, pp.371-375, 1994. M. Mitsuhara et al., "Effect of strain in the barrier layer on structural and optical properties of highly strained In0.77Ga0.23As / InGaAs multiple quantum wells", Journal of Crystal Growth, Vol.210, pp.463-470, 2000. J. Dries et al., "Strain compensated In1-xGaxAs (x <0.47) quantum well photodiodes for extended wavelength operation", Applied Physics Letters, Vol.73, No.16, pp.2263-2265, 1998. J. Harmand et al., "GaInAs / GaAs quantum-well growth assisted by Sb surfactant: Toward 1.3 μm emission", Applied Physics Letters, Vol.84, No.20, pp.3981-3983, 2004. I. Markov, "Kinetics of surfactant-mediated epitaxial growth", Physical Review B, Vol.50, No.15, pp.11271-11274, 1994.

  The above-described extended type light receiving element is a layer on the premise that a metamorphic buffer layer made of InAsP is grown before growing InGaAs constituting the light absorption layer, and the lattice constant is increased in the metamorphic buffer layer. It has a configuration. However, some of the dislocations generated in the metamorphic buffer layer propagate to the light absorption layer. For this reason, the expansion type light receiving element has a problem that it is difficult to reduce the dark current (see, for example, Non-Patent Document 1).

  In order to increase the wavelength that can be absorbed by a light receiving element using a light absorbing layer made of InGaAs, an InGaAs layer having a large In composition is grown thickly and used as a light absorbing layer, as in the case of an extended type light receiving element. In addition to the method, a structure in which a multi-layered structure is used as a light absorption layer while sandwiching a layer with tensile strain between thin InGaAs layers with large In composition and compressive strain is considered. This structure is called a distortion compensation structure.

The strain compensation structure is often applied to a multiple quantum well (MQW) structure used for an active layer of a laser. FIG. 10 is a cross-sectional view schematically showing a strain compensation structure. When the layer thickness of the layer 401 subjected to compressive strain shown in FIG. 10 is L _A , the strain amount ε _A for InP, and the layer thickness of the layer 402 subjected to tensile strain is L _B , the strain amount ε _B for InP. The effective strain ε ^* applied to the entire strain compensation structure is expressed by the following equation (1).

On the other hand, the light absorption layer of the light receiving element requires a large light absorption coefficient and layer thickness in order to sufficiently absorb incident light. Specifically, in the light absorbing layer of the light receiving element, a material having an absorption coefficient for incident light of 5000 cm ⁻¹ or more is almost always used. When the absorption coefficient of the light absorption layer with respect to incident light is 5000 cm ⁻¹ , a layer thickness of about 500 nm is required to absorb this 20% or more of light.

In the strain compensation structure shown in FIG. 10, when the number of periods is N, the total layer thickness of the entire strain compensation structure is N × (L _A + L _B ). When the absolute value of the effective strain ε ^* is 0.2 or less in the formula (1), it is known that no crystal defect occurs even when the total layer thickness is 500 nm (for example, Non-Patent Document 2). See). Therefore, if the effective strain is 0.2 or less using the strain compensation structure described with reference to FIG. 10, a light absorption layer with few crystal defects can be grown and applied to a light receiving element.

  However, it is known that when a strain compensation structure is produced using an InGaAs layer in which compressive strain is applied to InP on an InP substrate, crystal defects are generated even though the effective strain is small ( For example, see Non-Patent Document 3.)

  FIG. 11 shows the relationship between the compressive strain of the well layer and the PL emission intensity in the strain compensated MQW structure with InGaAs / InGaAs including the well layer composed of InGaAs with the compressive strain reported in Non-Patent Document 3. FIG. In the strain compensation MQW structure, the layer thickness and the amount of strain of the well layer and the barrier layer are adjusted so that the effective strain becomes zero. FIG. 11 shows the case where the number of wells is 5 cycles and 15 cycles.

  As shown in FIG. 11, when PL light emission is increased from 1% to 1.5% in the compressive strain of the well layer, it does not decrease greatly when the number of wells is 5, but it is almost not when the number of wells is 15. It turns out that it stops emitting light. One of the factors is that when the number of wells is increased using a strain compensation structure for a well layer made of InGaAs to which a large compressive strain is applied, the well layer and the barrier layer are increased as the number of wells increases. This is because the crystal interface fluctuates greatly, and as a result, a crystal defect that becomes a non-radiative recombination center occurs (see, for example, Non-Patent Document 4). As described above, when InGaAs containing compressive strain is included, even if a strain compensation structure is used, it is difficult to suppress the generation of crystal defects because the interface fluctuates.

  FIG. 12 is a cross-sectional view schematically showing the state of interface fluctuation in the strain-compensated InGaAs / InGaAs MQW structure reported in Non-Patent Document 4. A plurality of well layers 501 made of InGaAs in which compressive strain is applied to InP and barrier layers 502 made of InGaAs in which tensile strain is applied to InP are alternately stacked. In this report, not only the well layer 501 but also the thickness of the barrier layer 502 locally increases and decreases, and the amplitude of the increase and decrease increases with the number of wells. As a result, the strain stress locally applied to the crystal lattice is large, and crystal defects 521 are generated.

  As described above, when a strain compensation structure including an InGaAs layer subjected to compression strain is grown, it is difficult to obtain good crystallinity because local increase / decrease in the layer thickness occurs in each layer of the strain compensation structure. . One effective means of solving this problem is to insert a layer for flattening the interface between the well layer and the barrier layer. As a material for the layer for flattening the interface, InP which is also used for flat embedding of a diffraction grating in a distributed feedback (DFB) laser is useful.

  Non-Patent Document 5 reports a light receiving element using a strain compensation MQW in which a layer of InP is inserted between a well layer of InGaAs to which compressive strain is added and a barrier layer to which tensile strain is added as a light absorption layer. FIG. 13 is a cross-sectional view schematically showing a light absorption layer of a light receiving element studied in Non-Patent Document 5. On the semiconductor layer 601, a stacked structure of a barrier layer 602, a semiconductor layer 603, a semiconductor layer 604, a well layer 605, a semiconductor layer 606, and a semiconductor layer 607 is stacked 50 periods to form a light absorption layer.

  The semiconductor layer 601 is made of InGaAsP lattice-matched to InP. The barrier layer 602 is made of InGaP in which a tensile strain of 1.2% is applied to InP, and has a layer thickness of 7 nm. The semiconductor layer 603 is made of InP. The semiconductor layer 604 is made of InGaAsP lattice-matched to InP. The well layer 605 is made of InGaAs with 2% compressive strain applied to InP, and has a layer thickness of 7 nm. The semiconductor layer 606 is made of InGaAsP lattice-matched to InP. The semiconductor layer 607 is made of InP. The light absorption layer laminated in this way has a strain compensation MQW structure.

  In the light receiving element using this strain compensation MQW structure for the light absorption layer, generation of crystal defects is not recognized. On the other hand, it has been reported that the fabricated light receiving element has low light receiving sensitivity and it is necessary to increase the bias voltage to be applied. This is because the band discontinuity in the valence band of the well layer and the barrier layer is large, and it is difficult to extract holes generated by photoexcitation from the well layer. Since the light receiving sensitivity is low and it is difficult to drive with a low bias voltage, a light receiving element including the light absorption layer having the distortion compensation structure described with reference to FIG. 13 has not been studied at present.

  The present invention has been made to solve the above-described problems, and is a light receiving element using an InP-based compound semiconductor, and has a wavelength of 1.8 μm or more in a state where generation of crystal defects and the like is suppressed. The purpose is to enable detection with high light receiving sensitivity.

  A light receiving element according to the present invention includes a substrate made of InP, a first semiconductor layer of a first conductivity type formed on the substrate, a light absorption layer formed on the first semiconductor layer, and a light absorption layer A second semiconductor layer of the second conductivity type formed on the substrate, and the light absorption layer is made of InGaAsSb and is made of a compressive strain layer made to have a compressive strain with respect to InP, and InGaAsSb Thus, tensile strain layers having a tensile strain with respect to InP are alternately stacked, and the cutoff wavelength is set to 1.8 μm or more.

  In the light receiving element, the compressive strain layer may have an Sb composition ratio in the group V of 0.03 or more and 0.3 or less and a lattice mismatch with respect to InP of 1.0% or more.

  In the light receiving element, the tensile strain layer may have an Sb composition ratio in the group V of 0.03 or more.

  In the above light receiving element, the compressive strain layer is expressed by ε = (ε1 × h1 + ε2 × h2) / (h1 + h2) where h1 is the thickness of the compressive strain layer and ε1 is the strain amount, h2 is the thickness of the tensile strain layer and ε2 is the strain amount. In other words, the effective strain ε of the entire light absorption layer may be 0.2 or less.

  As described above, according to the present invention, since a compressive strain layer and a tensile strain layer composed of InGaAsSb are alternately laminated to form a light absorption layer, a light receiving element using an InP-based compound semiconductor is It is possible to obtain an excellent effect that light having a wavelength of 1.8 μm or more can be detected with high light receiving sensitivity in a state where generation of defects and the like is suppressed.

FIG. 1 is a cross-sectional view showing a configuration of a light receiving element according to an embodiment of the present invention. FIG. 2 is an explanatory diagram for explaining a state of an interface between a layer to which compressive strain is applied and a layer to which tensile strain is applied. FIG. 3 is a cross-sectional view showing the layer structure of the sample manufactured in the embodiment. FIG. 4 is a characteristic diagram comparing the experimental result (a) and the simulation result (b) of the X-ray diffraction pattern of the sample prepared in the embodiment. FIG. 5 is a characteristic diagram showing a photoluminescence emission spectrum at room temperature of a sample having a period number of 30 in the embodiment. FIG. 6 is a characteristic diagram obtained by calculation of the change in the band gap wavelength when the Sb composition ratio is changed for the compressive strain layer to which the compressive strain on InP is applied. FIG. 7 is a characteristic diagram showing a light absorption spectrum at room temperature of a sample in which the number of periods is 30 in the embodiment. FIG. 8 is a characteristic diagram showing a gas type having an absorption line in the wavelength region from 1.8 μm to 2.4 μm and its intensity. FIG. 9 is a characteristic diagram showing a light absorption spectrum of InGaAs lattice-matched to InP. FIG. 10 is a cross-sectional view schematically showing a strain compensation structure. FIG. 11 shows the relationship between the compressive strain of the well layer and the PL emission intensity in the strain compensated MQW structure with InGaAs / InGaAs including the well layer composed of InGaAs with the compressive strain reported in Non-Patent Document 3. FIG. FIG. 12 is a cross-sectional view schematically showing the state of interface fluctuation in the strain-compensated InGaAs / InGaAs MQW structure reported in Non-Patent Document 4. FIG. 13 is a cross-sectional view schematically showing a light absorption layer of a light receiving element studied in Non-Patent Document 5.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing a configuration of a light receiving element according to an embodiment of the present invention. The light receiving element includes a substrate 101 made of semi-insulating InP, a first semiconductor layer 102 of a first conductivity type formed on the substrate 101, and a light absorption layer formed on the first semiconductor layer 102. 103 and a second conductivity type second semiconductor layer 104 formed on the light absorption layer 103.

  Here, the light absorption layer 103 is composed of InGaAsSb and has a compressive strain with respect to InP, and the tensile strain is composed of InGaAsSb and has a tensile strain with respect to InP. The layers are alternately stacked, and the cutoff wavelength is 1.8 μm or more. For example, in the compressive strain layer, the Sb composition ratio in the group V may be 0.03 or more and 0.3 or less, and the lattice mismatch with respect to InP may be 1.0% or more. Moreover, the tensile strain layer should just be Sb composition ratio in V group being 0.03 or more.

  Here, the compressive strain layer is represented by ε = (ε1 × h1 + ε2 × h2) / (h1 + h2) where h1 is the thickness of the compressive strain layer and ε1 is the strain amount, h2 is the thickness of the tensile strain layer and ε2 is the strain amount. It is only necessary that the effective strain ε of the entire light absorption layer 103 is 0.2 or less.

  In the embodiment, the first conductivity type first semiconductor layer 102 is formed over the substrate 101 with the buffer layer 105 interposed therebetween. A second conductivity type cap layer 106 is formed on the second semiconductor layer 104. The first semiconductor layer 102, the light absorption layer 103, the second semiconductor layer 104, and the cap layer 106 are mesas having a smaller area than the substrate 101 and the buffer layer 105. A first electrode 111 is formed on the cap layer 106. A second electrode 112 is formed on the buffer layer 105 around the mesa. In addition, an antireflection layer 107 is formed on the back surface side of the substrate 101 which is the light incident side. The side surfaces of the mesa are covered with a passivation layer 108 made of an insulating material such as SiN.

  According to the light receiving element in the above-described embodiment, a strain compensation structure that has been difficult to apply to InGaAs having a compressive strain exceeding 1% so far can be easily applied to the light absorption layer 103. The light absorption layer 103 has a structure in which a compression strain layer made of InGaAsSb to which compressive strain is applied and a tensile strain layer made of InGaAsSb to which tensile strain is applied are alternately laminated, so that the light absorption wavelength is lengthened. Generation of crystal defects can be suppressed. As a result, a light receiving element having a cut-off wavelength of 1.8 μm or more and a small dark current can be realized on the substrate 101 made of InP without using a metamorphic buffer layer.

  Details will be described below. When a strain compensation structure is formed by alternately laminating an InGaAs layer (compressive strain layer) with a large compressive strain and an InGaAs layer (tensile strain layer) with a tensile strain, as described above, a well layer and a barrier Since the thickness of the layer locally increases and decreases, crystal defects are generated. In order to suppress this local increase / decrease in film thickness, the movement (migration) of group III atoms on the crystal growth surface may be reduced.

  It is known that the presence of Sb on the crystal growth surface can reduce the movement of group III atoms (see, for example, Non-Patent Document 6). Sb acts as a kind of surfactant and is called a surfactant. The mechanism by which Sb suppresses the movement of the group III atom is that a part of Sb is attached on the group V atom at the step end of crystal growth, and is replaced with Sb when the group III atom passes through this step end. (For example, refer nonpatent literature 7).

  Until now, Sb as a surfactant has been used to suppress the occurrence of three-dimensional growth in a well layer of InGaAs to which compressive strain has been applied. In this case, Sb is not actively mixed in the crystal, and the Sb composition ratio in the group V element is almost 0.02 or less. In addition, no study has been conducted to suppress the occurrence of three-dimensional growth using Sb for a layer subjected to tensile strain.

  If the mechanism by which Sb suppresses the movement of group III atoms is also effective for a layer having an Sb composition ratio of 0.03 or more or a layer to which tensile strain is applied, it is considered that the occurrence of three-dimensional growth can be suppressed. . In this case, if the three-dimensional growth can be suppressed, the increase and decrease of the film thickness can be suppressed, and a flat interface can be obtained. FIG. 2 schematically shows this state.

  As shown in FIG. 2A, a plurality of well layers 501 made of InGaAs in which compressive strain is applied to InP and barrier layers 502 made of InGaAs in which tensile strain is applied to InP are alternately stacked. In the strain compensation structure, a large fluctuation occurs at the interface. On the other hand, as shown in FIG. 2B, a compressive strain layer 131 made of InGaAsSb in which compressive strain is applied to InP, and a tensile strain layer 132 made of InGaAsSb in which tensile strain is applied to InP, However, in the light absorption layer 103 having a strain compensation structure in which a plurality of layers are alternately stacked, fluctuation of the interface is suppressed, and a flat interface is obtained.

  Hereinafter, as shown in FIG. 10B, even if the generation of crystal defects becomes a problem in the strain compensation structure including InGaAs having a compressive strain exceeding 1%, as shown in FIG. An explanation will be given of the fact that it is possible to grow a crystal and avoid this problem.

Below, the result of actually producing and examining a sample will be described. FIG. 3 is a cross-sectional view showing the layer structure of the manufactured sample. In the preparation of the sample, each layer made of InGaAsSb was epitaxially grown on a substrate 301 made of InP by a well-known metalorganic molecular beam epitaxy method. In the growth, trimethylindium (TMIn) and triethylgallium (TEGa) were used as the group III source gas, and phosphine (PH ₃ ), arsine (AsH ₃ ), and trisdimethylaminoantimony (TDMASb) were used as the group V source gas.

  Specifically, a buffer layer 302 made of undoped InP and having a layer thickness of 0.2 μm is grown on the substrate 301, and subsequently a tensile strain layer 303 made of InGaAsSb to which tensile strain is applied and an InGaAsSb to which compressive strain is applied. The compression-strained layers 304 are alternately laminated for 16 periods. Finally, a cap layer 305 made of undoped InP and having a layer thickness of 0.05 μm is grown. The tensile strain of the tensile strained layer 303 to which tensile strain is applied is adjusted so that the thickness of only the lowermost layer and the uppermost layer is half that of the other tensile strained layers 303. The Sb composition ratio in the V group in both the compressive strain layer 304 to which the compressive strain is applied and the tensile strain layer 303 to which the tensile strain is applied is 0.1.

  FIG. 4 is a characteristic diagram comparing the experimental result (a) and the simulation result (b) of the X-ray diffraction pattern of the manufactured sample. From the comparison between the experimental results and the simulation results, the strain amount of the compressive strain layer 304 to which the compressive strain is applied is + 1.2%, the layer thickness is 18.5 nm, and the strain amount of the tensile strain layer 303 to which the tensile strain is applied. Was −0.7%, the layer thickness was 37 nm, and the effective strain of the strain compensation structure was found to be 0.07% or less. As shown in FIG. 4, the experimental result and the simulation result are in good agreement.

  In periodic structures with poor interface flatness, X-ray diffraction pattern peaks generally show significant broadening, but the prepared sample does not show any significant broadening, so it has a flat interface. I found out that When InGaAs is used, as described with reference to FIG. 11, when a strain compensation structure having a period of 15 cycles with an applied compressive strain of 1.0% or more is grown, a crystal defect that significantly reduces the PL emission intensity. Occurs. On the other hand, in the strain compensation structure of the sample using InGaAsSb, even when the layer including the layer having a compressive strain of 1.2% as described above is stacked for 16 cycles or more, no remarkable crystal defects are generated. It was confirmed from the X-ray diffraction pattern and PL emission described later.

  In the structure of FIG. 3, a layer that absorbs light having a wavelength exceeding 1.8 μm is a compressive strain layer 304 to which compressive strain is added. When the number of periods is 16, the total thickness of the compressive strain layer 304 is about 0.3 μm. On the other hand, in the light absorption layer of the light receiving element, a layer thickness of 0.5 μm or more is generally used in order to increase the light receiving sensitivity. In order to increase the amount of light absorption, a sample with the number of periods increased from 16 to 30 was prepared. Even if the number of periods was increased from 16 to 30, it was confirmed from the X-ray diffraction pattern that there was no lattice relaxation due to the small effective strain. FIG. 5 is a characteristic diagram showing a photoluminescence emission spectrum of a sample having a period number of 30 at room temperature. The emission peak wavelength was 2.1 μm, and a large emission intensity was obtained.

  FIG. 6 is a characteristic diagram obtained by calculation of the change in the band gap wavelength when the Sb composition ratio is changed in the compression strain layer to which the compression strain on InP (substrate) is applied. In the calculation results shown in FIG. 6, since the band gap wavelength of InGaAsSb having a compressive strain of 1.2% and an Sb composition ratio of 0.1 is 2.1 μm, the emission peak shown in FIG. It was found that this was due to light emission from the compression strain layer.

FIG. 7 is a characteristic diagram showing a light absorption spectrum of a sample having a period number of 30 at room temperature. Increase in absorption coefficient from the vicinity of a wavelength of 2.1μm was observed, approximately 3000 cm ^-1 in a wavelength 2.0 .mu.m, the absorption coefficient of about 9000 cm ^-1 in a wavelength 1.8μm was obtained. From this light absorption spectrum, if a light-receiving element using a light absorption layer in which a compressive strain layer to which a compressive strain is applied and a tensile strain layer to which a tensile strain is applied is periodically stacked is used, InGaAs lattice-matched to InP. Thus, it can be seen that light of 1.8 μm or more which is difficult to absorb can be easily absorbed. As a result, the cutoff wavelength in the light receiving element can be set to 1.8 μm or more.

  In the embodiment, the case where the Sb composition ratio in the group V element in the compressive strain layer made of InGaAsSb to which compressive strain is applied and the tensile strain layer made of InGaAsSb to which tensile strain is applied is 0.1 is shown. The Sb composition ratio is not necessarily equal. Specifically, if the Sb composition ratio in the group V element of InGaAsSb is 0.03 or more, many Sb atoms exist on the growth surface. It is possible to suppress the movement of the surface.

  There is a desirable upper limit to the Sb composition ratio of InGaAsSb to which compressive strain is applied. As shown in FIG. 6, when Sb is added to InGaAs, the band gap wavelength monotonously increases until the Sb composition ratio is around 0.3. On the other hand, when the Sb composition ratio exceeds 0.3, the composition becomes unstable, for example, the composition is locally separated and the composition is modulated. From these facts, it is desirable that the Sb composition ratio in the group V element of the InGaAsSb layer to which compressive strain is applied is 0.03 or more and 0.3 or less.

  In the above description, the case where the metalorganic molecular beam epitaxy method is used as the crystal growth method has been described. However, the present invention is not limited to this, and any growth method capable of periodically laminating InGaAsSb layers having different compositions may be used. Needless to say, it is also effective when other growth methods such as metal organic vapor phase epitaxy and molecular beam epitaxy are used.

  By the way, the compressive strain layer and the tensile strain layer constituting the light absorption layer 103 have a large layer thickness and almost no quantum size effect, but even when a small layer thickness with a quantum size effect is used, the number of periods It is possible to increase the amount of light absorption simply by increasing. For this reason, it goes without saying that the light absorption layer 103 is effective even in a structure in which a compressive strain layer and a tensile strain layer having a small layer thickness in which a quantum size effect is seen are laminated.

  When the composition ratio of Sb is increased, the energy level at the valence band edge in the compressive strain layer is increased, and when the In composition ratio is increased, the energy level at the conduction band edge is decreased. In this configuration, when lattice strain is applied, the valence band edge is separated into heavy holes and light holes, and each energy level changes. In a thin compressive strained layer in which a quantum size effect is observed, the quantum size effect becomes significant when the band discontinuity with the tensile strained layer is large. When the quantum size effect is obtained, the strain compensation structure of the embodiment is such that the compressive strain layer serving as the quantum well has a lower energy level at the conduction band edge than the tensile strain layer serving as the barrier, and the valence electrons. A Type-I superlattice structure having a high energy level at the band edge is preferable.

  Next, the configuration of the light receiving element described with reference to FIG. 1 will be described in more detail. The substrate 101 is made of, for example, semi-insulating InP that is made high resistance by doping Fe. The buffer layer 105 is made of InP (n-InP) doped with n-type impurities to be n-type, and the first semiconductor layer 102 is made of InGaAsP (n-type impurities doped with n-type impurities). n-InGaAsP). The second semiconductor layer 104 is made of p-type InGaAsP (p-InGaAsP) doped with p-type impurities, and the cap layer 106 is made of p-type InGaAs (p-type impurities doped with p-type impurities). p-InGaAs).

  In addition, the compressive strain layer of the light absorption layer 103 is in a state where 1.0% compressive strain is applied to InP, and the layer thickness is 20 nm. In addition, the tensile strain layer of the light absorption layer 103 is in a state where a tensile strain of 0.5% is applied to InP, and the layer thickness is 40 nm. In addition, the light absorption layer 103 is configured by alternately stacking 30 layers of compression strain layers and tensile strain layers. Moreover, the Sb composition in InGaAsSb is 0.15 in both the compressive strain layer and the tensile strain layer. The band gap wavelength of the compression strain layer is about 2.08 μm.

  Next, a method for manufacturing the light receiving element in the embodiment will be described. First, the aforementioned organic metal molecular beam epitaxy method is used for the growth of each semiconductor layer. Thus, the light receiving element shown in FIG. 1 is manufactured using the epitaxial wafer on which the layers of the respective semiconductor materials are grown. Specifically, after forming a circular mesa having a diameter of 10 μm, silicon nitride is deposited on the entire surface of the wafer by plasma CVD, and then a region other than a necessary region is removed to form a passivation layer 108.

  On the cap layer 106 patterned on the mesa, the second electrode 112 is formed on the circular first electrode 111 having a diameter of 8 μm in plan view and on the buffer layer 105 outside the passivation layer 108 around the mesa. For example, it may be formed by evaporating a metal material to be an electrode. After vapor deposition, an ohmic connection is established by heat treatment. Thereafter, the antireflection layer 107 is formed on the back surface of the substrate 101 by vapor deposition using an electron beam vapor deposition method. Signal light is incident from the antireflection layer 107 side. The signal light that has entered and passed through the light absorption layer 103 is reflected by the first electrode 111 and passes through the light absorption layer 103 again.

  In the light receiving element in the above-described embodiment, the wavelength of incident light at which the photocurrent becomes maximum at an operating temperature of 20 ° C. and a bias voltage of −2 V is 1.9 μm, and the light receiving sensitivity is 0.6 A / W. The light receiving sensitivity when the wavelength of incident light is 2.0 μm is 0.2 A / W. The dark current is 70 ± 8 nA at an operating temperature of 20 ° C. and a bias voltage of −2 V, which is lower than a general extended InGaAs light receiving element. This is because the light receiving element in the embodiment does not require a metamorphic buffer layer unlike the extended type InGaAs light receiving element, and the crystal defect density in the light absorbing layer 103 is small.

  In the above description, the case where a semi-insulating InP substrate is used as the substrate 101 has been described. However, a conductive substrate such as an n-type InP substrate or a p-type InP substrate is used, and the first electrode and the second electrode are provided on the front and back surfaces. It is clear that the same effect can be obtained even when the structure taken from is used.

  As described above, according to the present invention, since the compressive strain layer and the tensile strain layer made of InGaAsSb are alternately stacked to form a light absorption layer, the light receiving element using the InP-based compound semiconductor is used. Light with a wavelength of 1.8 μm or more can be detected with high light receiving sensitivity in a state where generation of crystal defects and the like is suppressed.

  According to the light receiving element of the present invention, it is possible to receive light having a long wavelength, which is difficult with a light receiving element having an InGaAs layer lattice-matched to InP as a light absorption layer. In addition, since the light receiving element of the present invention does not require a metamorphic buffer layer unlike the extended type light receiving element, a light receiving element with a small dark current can be realized. By using the light receiving element of the present invention described above, it can be applied to a highly sensitive gas measurement system using a gas absorption line.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious.

  DESCRIPTION OF SYMBOLS 101 ... Board | substrate, 102 ... 1st semiconductor layer, 103 ... Light absorption layer, 104 ... 2nd semiconductor layer, 105 ... Buffer layer, 106 ... Cap layer, 107 ... Antireflection layer, 108 ... Passivation layer, 111 ... 1st electrode 112 ... Second electrode.

Claims (4)

A substrate made of InP;
A first semiconductor layer of a first conductivity type formed on the substrate;
A light absorption layer formed on the first semiconductor layer;
A second conductivity type second semiconductor layer formed on the light absorption layer,
The light absorption layer is composed of InGaAsSb and a compressive strain layer that has a compressive strain with respect to InP, and a tensile strain layer that includes InGaAsSb and has a tensile strain with respect to InP. And a cut-off wavelength of 1.8 μm or more. The light receiving element according to claim 1,
The light-receiving element, wherein the compressive strain layer has an Sb composition ratio in the group V of 0.03 or more and 0.3 or less and a lattice mismatch with respect to InP of 1.0% or more. The light receiving element according to claim 1,
The light-receiving element, wherein the tensile strain layer has an Sb composition ratio in the group V of 0.03 or more. In the light receiving element according to any one of claims 1 to 3,
The compressive strain layer is expressed by ε = (ε1 × h1 + ε2 × h2) / (h1 + h2), where h1 is the thickness of the compressive strain layer and ε1 is the strain amount, and h2 is the thickness of the tensile strain layer and ε2 is the strain amount. A light receiving element, wherein the effective strain ε of the entire light absorbing layer is 0.2 or less.