JP2008205029A - Nitride semiconductor device - Google Patents

Abstract

A nitride semiconductor device with less gate leakage current and current collapse is provided.
A GaN layer formed on a main surface of a substrate, a first AlGaN layer formed on the GaN layer and supplying electrons to the GaN layer, a first AlGaN layer formed on the first AlGaN layer, and hydrogen Semiconductor layer 15 containing 0.5 to 30 atoms%, gate electrode 16 formed on nitride semiconductor layer 15, and gate electrode 16 sandwiched between nitride semiconductor layer 15 in the gate length direction. The formed source electrode 17 and drain electrode 18 are provided. The dangling bonds in the vicinity of the surface of the nitride semiconductor layer 15 are terminated with hydrogen to suppress leakage between electrodes due to surface defects.
[Selection] Figure 1

Description

  The present invention relates to a nitride semiconductor device.

A high-power transistor using a nitride semiconductor is known as a high-frequency transistor used in satellite broadcasting equipment, mobile communication equipment, and the like.
However, transistor characteristics using nitride semiconductors have device characteristics due to gate leakage current and current collapse (a phenomenon in which current output decreases due to high-frequency high-power operation) due to surface defects and grain boundaries in the nitride semiconductor layer. Becomes unstable and has low reliability (for example, refer to Patent Document 1 or Patent Document 2).

  The nitride semiconductor device disclosed in Patent Document 1 includes a GaN channel layer, an AlGaN electron supply layer, and an AlGaN cap layer having an Al composition ratio larger than that of the electron supply layer, and a gate electrode is formed on the cap layer. A part of the cap layer is removed, and source / drain electrodes are formed on the electron supply layer.

  AlGaN, which has a large Al composition ratio, has a large band gap energy and a high equilibrium vapor pressure of nitrogen. Therefore, by increasing the barrier height between the gate electrode and the interface and making it difficult to generate nitrogen vacancies, the gate leakage current is suppressed. ing.

The nitride semiconductor device disclosed in Patent Document 2 includes a GaN channel layer, an AlGaN electron supply layer, and a GaN thin film layer, and a gate electrode and source / drain electrodes are formed on the GaN thin film layer.
Furthermore, a SiN protective film having a hydrogen content of 15% or less is provided on the GaN cap layer to passivate the surface of the n-type GaN thin film layer.

  As a result, the change in the state of the nitride semiconductor surface and the change in the charge state of the surface defect level caused by the presence of hydrogen in the SiN protective film are suppressed, and the gate leakage current and current collapse are suppressed.

However, the nitride semiconductor device disclosed in Patent Document 1 or Patent Document 2 has a problem that it is difficult to stably suppress the current coplus and the gate leakage current.
JP 2004-260114 A JP-A-2005-286135

  The present invention provides a nitride semiconductor device with low gate leakage current and current collapse.

  The nitride semiconductor device of one embodiment of the present invention includes a GaN layer, a first AlGaN layer that is formed on the GaN layer, supplies electrons to the GaN layer, and is formed on the first AlGaN layer. A nitride semiconductor layer containing 5 to 30 atoms%, a gate electrode formed on the nitride semiconductor layer, and a source electrode formed on the nitride semiconductor layer so as to sandwich the gate electrode in the gate length direction And a drain electrode.

  According to the present invention, a nitride semiconductor device with little gate leakage current and current collapse can be obtained.

  Embodiments of the present invention will be described below with reference to the drawings.

  A nitride semiconductor device according to Example 1 of the present invention will be described with reference to FIG. FIG. 1 is a cross-sectional view showing the structure of a nitride semiconductor device.

As shown in FIG. 1, the nitride semiconductor device 10 of the present embodiment is formed on a substrate 11, for example, a sapphire substrate, through a buffer layer 12, for example, an undoped AlN layer having a thickness of 300 nm. An undoped GaN layer 13 and an undoped first Al _x Ga _(1-x) N layer (hereinafter referred to as a first AlGaN layer) 14 formed on the GaN layer 13 and having an Al composition x of 0.2 and a film thickness of 30 nm, for example. A nitride semiconductor layer formed on the first AlGaN layer 14, for example, an undoped second Al _y Ga _(1-y) having an Al composition y of 0.25, a film thickness of 2.5 nm, and a hydrogen content of about 3 atoms%. N layer (hereinafter referred to as a second AlGaN layer) 15.

  Furthermore, a gate electrode 16 formed on the second AlGaN layer 15 and a source electrode 17 and a drain electrode 18 formed on the second AlGaN layer 15 so as to sandwich the gate electrode 16 in the gate length direction are provided.

  The nitride semiconductor device 10 is a well-known modulation-doped field effect transistor. In the modulation-doped field effect transistor, electrons are supplied from the first AlGaN layer 14 which is an electron supply layer to the GaN layer 13 having a small band gap and a high electron affinity, and a two-dimensional electron gas layer 19 is generated.

The gate electrode 16 is formed so as to obtain a Schottky contact with the first AlGaN layer 14 which is an electron supply layer.
The source electrode 17 and the drain electrode 18 are formed so as to obtain ohmic contact with the two-dimensional electron gas layer 19.

  At this time, two depletion layers are formed in the first AlGaN layer 14 which is an electron supply layer. One is a depletion layer of a Schottky junction, and the other is a depletion layer extending from the heterointerface side accompanying the formation of a two-dimensional electron gas.

  The thickness of the first AlGaN layer 14 serving as the electron supply layer is set to such an extent that the two depletion layers described above are in contact with each other. By applying a voltage to the gate electrode 16, the thickness of the two depletion layers is changed, and the concentration of the two-dimensional electron gas is controlled by the field effect.

  When the source electrode 17 is grounded, a negative potential is applied to the gate electrode 16 and a positive potential is applied to the drain electrode 17, electrons injected from the source electrode 17 are contained in the second AlGaN layer 15 and the first AlGaN layer 14 which is an electron supply layer. And is injected into the two-dimensional electron gas layer 19 of the GaN layer 13.

  The injected electrons travel in the two-dimensional electron gas layer 19 along a potential gradient formed by a bias applied to the gate electrode 16 and the drain electrode 18, and the first AlGaN layer 14 and the second AlGaN layer which are electron supply layers. 15 drifts into the drain electrode 18.

  In such a bias application state, when a high-density defect level due to dangling bonds exists on the surface of the second AlGaN layer 15, electrons in the gate electrode 16 are trapped in the surface defect level of the second AlGaN layer 15. The leakage current generated by the transport of electric charges along the interface between the 2AlGaN layer 15 and the first AlGaN layer 14 which is the electron supply layer, and the surface level serves as an apparent source from the gate electrode 16 to the channel. A leak current or the like generated by electron injection occurs.

  Although the cause of current collapse has not yet been specified, it is considered to be a phenomenon related to electron capture levels caused by defects on the surface of the nitride semiconductor.

By terminating the dangling bonds on the surface of the second AlGaN layer 15 with hydrogen, the surface defects are inactivated and the electron trap level density is reduced.
Therefore, it is possible to suppress the gate leakage current and current collapse by containing a large amount of hydrogen in the second AlGaN layer 15 and finding the optimum value of the hydrogen content.

If the hydrogen content is too small, the dangling bond termination effect on the surface of the second AlGaN layer 15 is insufficient, so that a sufficient leakage current reduction effect between the electrodes cannot be obtained.
If the hydrogen content is too large, the crystallinity of the second AlGaN layer 15 is impaired, and the generation of crystal defects may cause an increase in gate leakage current or the like.

  As a result of various studies on the hydrogen content, it has been found that the hydrogen content is preferably 0.5 to 30 atoms%, more preferably 1 to 10 atoms%.

Next, a method for manufacturing the nitride semiconductor device 10 will be described with reference to FIG.
As shown in FIG. 2, a nitride semiconductor layer is grown on the (0001) C surface of a substrate 11 made of sapphire by a well-known MOCVD (Metal Organic Chemical Vapor Deposition) method.

Here, trimethylaluminum (TMA), trimethylgallium (TMG), and trimethylindium (TMI), which are organometallic compounds of Al, Ga, and In, are used as the group III source gas, and ammonia gas (NH) as the group V source gas. ₃ ) is used.
Hydrogen or a mixed gas of hydrogen and nitrogen is used as the carrier gas. Furthermore, silane gas (SiH ₄ ) is used as an n-type dopant.

First, as shown in FIG. 2A, after the sapphire substrate 11 is set in a reaction vessel of the MOCVD apparatus, and the inside of the vessel is sufficiently replaced with hydrogen, the temperature of the sapphire substrate 11 is about 1050 ° C. while flowing hydrogen. The sapphire substrate 11 is thermally cleaned.
Next, the temperature of the sapphire substrate 11 is lowered to about 600 ° C., and an undoped AlN layer is formed as the buffer layer 12 to about 300 nm.

  Next, as shown in FIG. 2B, the temperature of the sapphire substrate 11 is raised to about 900 ° C. to form an undoped GaN layer 13 of about 3 μm.

  Next, as shown in FIG. 2C, the temperature of the sapphire substrate 11 is raised to about 1000 ° C. to form an undoped first AlGaN layer 14 having an Al composition of about 0.2 nm.

Next, as shown in FIG. 2 (d), the flow rate of hydrogen is increased while maintaining the flow ratio of the process gas such as TMA, TMG, NH ₃ and the carrier gas, and the hydrogen partial pressure is increased. An undoped second AlGaN layer 15 having an Al composition of 0.25 is formed to about 2.5 nm.

  In general, the temperature range in which AlGaN grows is 800 ° C. to 1200 ° C., and a growth temperature of 900 ° C. or higher at which AlGaN is likely to grow two-dimensionally to 1150 ° C. or lower at which thermal decomposition of AlGaN becomes intense is preferred.

Usually, it is known that a nitride semiconductor layer formed by the MOCVD method contains about 1E18 to 1E20 cm ⁻³ (<0.1 atoms%) of hydrogen by SIMS (Secondary Ion Mass Spectroscopy) analysis. ing.

In the MOCVD method in which the hydrogen partial pressure was increased in this example, there was a tendency that hydrogen was easily taken into the nitride semiconductor layer.
In addition to the hydrogen partial pressure, the rate at which hydrogen was taken in varied depending on the growth temperature, and it was observed that the lower the growth temperature, the easier the uptake.

  Next, the supply of process gas is stopped, and the temperature of the sapphire substrate 11 is lowered. When the temperature of the sapphire substrate 11 approaches room temperature, the sapphire substrate 11 is taken out from the MOCVD apparatus.

  Next, when hydrogen in the second AlGaN layer 15 was analyzed by, for example, RBS (Rutherford Back Scattering) method, it was confirmed that hydrogen was contained in the second AlGaN layer 15 in% order.

Next, a Ni / Au film is deposited on the second AlGaN layer 15 by, for example, a sputtering method and patterned by a photolithography method to form a gate electrode 16 having a gate length of about 1 μm.
Next, a source electrode 17 and a drain electrode 18 are formed so as to sandwich the gate electrode 16 by depositing a Ti / Al film, for example, by sputtering, patterning by photolithography, and heat treatment.

  Thereby, the nitride semiconductor device 10 provided with the second AlGaN layer 15 containing about 3% of hydrogen shown in FIG. 1 is obtained.

When the source electrode 17 was set to the ground potential, a negative potential was applied to the gate electrode 16, a positive potential was applied to the drain electrode 17, and the gate leakage current of the nitride semiconductor device 10 was measured while changing the potential of the gate electrode 16. It was confirmed that the gate leakage current was suppressed to ½ or less as compared with the nitride semiconductor device having the second AlGaN layer of 0.1 atoms% or less.
Further, when the drain current was measured while the potential of the gate electrode 16 was kept constant and the potential of the drain electrode 17 was changed, it was confirmed that the decrease amount of the drain current was suppressed to ½ or less.

  As described above, in this embodiment, the second AlGaN layer 15 containing 3 atoms% of hydrogen is formed on the first AlGaN layer 14 serving as the electron supply layer.

As a result, dangling bonds on the surface of the second AlGaN layer 15 are terminated with hydrogen, and surface defects can be suppressed.
Furthermore, the interface trap density between the first AlGaN layer 14 and the second AlGaN layer 15 can be suppressed by containing a large amount of hydrogen inside the second AlGaN layer 15.

  Therefore, a nitride semiconductor device with little inter-electrode leakage current such as gate leakage current or current collapse due to surface or interface defects can be obtained.

  Although the case where the substrate 11 is sapphire has been described here, a substrate on which another nitride semiconductor film can be formed, for example, SiC, GaN, Si, or the like may be used.

  Although the case where the nitride semiconductor layer is the second AlGaN layer 15 has been described, other nitride semiconductor layers such as an InGaN layer and a GaN layer may be used.

The method of adding hydrogen to the second AlGaN layer 15 has been described with respect to the case where the partial pressure of hydrogen of the carrier gas is increased by the MOCVD method, but it can also be performed by the HVPE (Hydride Vapor Phase Epitaxy) method.
After the second AlGaN layer 15 is formed, it can also be performed by a hydrogen annealing method. Moreover, you may carry out using both together.

  FIG. 3 is a sectional view showing a nitride semiconductor device according to Example 2 of the present invention. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, description thereof will be omitted, and different portions will be described. The present embodiment is different from the first embodiment in that a gate electrode is formed on the nitride semiconductor layer through an insulating film.

  That is, as shown in FIG. 3, the nitride semiconductor device 30 of this example includes a silicon nitride film having a thickness of about 50 nm formed by, for example, plasma CVD on the second AlGaN layer 15 containing about 3 atoms% of hydrogen. This is a MIS (Metal Insulator Semiconductor) type modulation-doped field effect transistor in which a gate electrode 16 is formed via an (insulating film) 31.

Since the second AlGaN layer 15 is passivated by the silicon nitride film 31, the surface of the second AlGaN layer 15 having Al which is easily oxidized is not oxidized.
As a result, it is possible to suppress electrode leakage current, current collapse phenomenon, and the like caused by Al being oxidized.

As described above, in the nitride semiconductor device 30 of this embodiment, the gate electrode is formed on the second AlGaN layer 15 via the silicon nitride film 31.
As a result, there is an advantage that gate leakage current, current collapse phenomenon, and the like due to oxidation of the surface of the second AlGaN layer 15 are suppressed.

Sectional drawing which shows the nitride semiconductor substrate which concerns on Example 1 of this invention. Sectional drawing which shows the manufacturing process of the nitride semiconductor device which concerns on Example 1 of this invention in order. Sectional drawing which shows the nitride semiconductor substrate which concerns on Example 2 of this invention.

Explanation of symbols

10, 30 Nitride semiconductor device 11 Substrate 12 Buffer layer 13 GaN layer 14 First AlGaN layer 15 Second AlGaN layer (nitride semiconductor layer)
16 Gate electrode 17 Source electrode 18 Drain electrode 19 Two-dimensional electron gas layer 31 Silicon nitride film (insulating film)

Claims (3)

A GaN layer;
A first AlGaN layer formed on the GaN layer and supplying electrons to the GaN layer;
A nitride semiconductor layer formed on the first AlGaN layer and containing 0.5 to 30 atoms% hydrogen;
A gate electrode formed on the nitride semiconductor layer;
A source electrode and a drain electrode formed on the nitride semiconductor layer so as to sandwich the gate electrode in the gate length direction;
A nitride semiconductor device comprising:   2. The nitride semiconductor device according to claim 1, wherein the nitride semiconductor layer is one of a second AlGaN layer, an InGaN layer, and a GaN layer having an Al composition larger than that of the first AlGaN layer.   The nitride semiconductor device according to claim 1, wherein the gate electrode is formed on the nitride semiconductor layer via an insulating film.

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