JP2005203544A - Nitride semiconductor device and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a nitride semiconductor device which maintains a threshold voltage to a negative voltage and enables a high power operation at high breakdown strength. <P>SOLUTION: This nitride semiconductor device comprises a substrate 1, a channel layer 2 formed on the substrate 1 and composed of a nitride semiconductor, a barrier layer 3 formed on the channel layer 2 and composed of the nitride semiconductor having larger band gap energy than the channel layer 2, a gate electrode 5 formed on the barrier layer 3, and a source electrode 4 and a drain electrode 6 formed at opposing positions via the gate electrode 5 on the barrier layer 3, respectively. The thickness of the layer of the region of at least a part of the barrier layer 3 between the drain electrode 6 and the gate electrode 5 is thinner than that of the barrier layer 3 immediately below the gate electrode 5. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a nitride semiconductor device and a manufacturing method thereof, and more particularly to a nitride semiconductor device having a high breakdown voltage and a high output operation while maintaining a threshold voltage at a negative voltage and a manufacturing method thereof.

  Nitride semiconductors, especially gallium nitride semiconductors, have high dielectric breakdown field strength, high thermal conductivity, and high electron saturation speed, and are expected as constituent materials for high-frequency, high-power, high-electron mobility transistors. In particular, a high electron mobility transistor having an aluminum gallium nitride (AlGaN) / gallium nitride (GaN) heterojunction structure (AlGaN / GaN High Electron Mobility Transistor (HEMT)) has electrons near the heterojunction interface between AlGaN and GaN. A so-called two-dimensional electron gas that accumulates at a high concentration is formed, but since the two-dimensional electron gas exists spatially separated from donor impurities added to AlGaN, high electron mobility can be realized. It can operate at high frequency. Furthermore, the two-dimensional electrons in the AlGaN / GaN heterostructure have an electron velocity more than twice in the high electric field region compared with the AlGaAs / GaAs system that is currently popular as HEMT, and switching operation at high frequency. It is expected to be applied to a HEMT with a high output that is possible.

  In a conventional high electron mobility transistor (AlGaN / GaN High Electron Mobility Transistor (HEMT)) using a nitride semiconductor, for example, as shown in FIG. Constant between drain electrodes. That is, the thickness of the AlGaN barrier layer immediately below the gate electrode is equal to the thickness of the AlGaN barrier layer between the gate / drain electrodes. Note that the element cross-sectional view of FIG. 1B in the first embodiment described later is almost the same as the element structure according to the prior art.

Ando and others, “110W output AlGaN / GaN hetero FET on thin sapphire substrate”, IEICE technical report ED2001-185, pp. 7-12

When using an AlGaN / GaN HEMT as a power amplifier, it is desirable that the output current (drain current) be as large as possible. In order to increase the drain current, it is necessary to increase the input voltage (gate voltage). However, if the gate voltage is increased too much to change from a negative voltage (minus voltage) to a positive voltage (plus voltage), a positive voltage is applied to the Schottky barrier in the gate electrode. In this case, the gate current increases rapidly, which is not desirable for transistor operation. This is because an increase in the gate current causes a deterioration of the Schottky barrier, leading to deterioration of device characteristics, particularly reliability. Therefore, in the AlGaN / GaN HEMT, the threshold voltage _Vth is desired to be a negative voltage equal to or less than a minus number V.

However, in the conventional AlGaN / GaN HEMT, the breakdown voltage between the gate and drain electrodes (hereinafter, simply referred to as “breakdown breakdown voltage”) with the threshold voltage _Vth held at a negative number V (for example, −5 V or less). ) Is difficult to improve. This is because if the thickness of the AlGaN barrier layer is reduced for the purpose of improving the breakdown voltage, the threshold voltage _Vth is shifted to the positive bias side.

As described above, in the conventional element structure, there is a trade-off relationship between the threshold voltage _Vth and the breakdown voltage, so that the threshold voltage _Vth is maintained at a negative voltage of about minus several V, and There is a problem that it is difficult to improve the breakdown voltage.

The present invention has been made in order to solve the above-described problems. The threshold voltage _Vth is maintained at a negative voltage of about minus several V, and the breakdown voltage is high and high output operation is possible. It is an object to obtain a nitride semiconductor device and to provide a manufacturing method thereof.

  The nitride semiconductor device according to the present invention includes a substrate, a channel layer formed on the substrate and made of a nitride semiconductor, and a barrier made of a nitride semiconductor formed on the channel layer and having a larger band gap energy than the channel layer. A gate electrode formed on the barrier layer, and a source electrode and a drain electrode formed on the barrier layer at positions facing each other via the gate electrode, the drain electrode and the gate The layer thickness of at least a part of the barrier layer between the electrodes is smaller than the layer thickness of the barrier layer immediately below the gate electrode.

In the nitride semiconductor device according to the present invention, since the above-described configuration is applied, the trade-off between the threshold voltage _Vth and the breakdown voltage is alleviated and the threshold voltage _Vth is maintained at a negative voltage and High breakdown voltage enables high output operation.

Embodiment 1 FIG.
FIG. 1A shows an element cross-sectional view of a nitride semiconductor device according to Embodiment 1 of the present invention, more specifically, an AlGaN / GaN HEMT. A GaN channel layer 2 and an AlGaN barrier layer 3 are formed on the substrate 1. The substrate 1 may be made of a material such as sapphire (Al ₂ O ₃ ), silicon carbide (SiC), GaN, silicon (Si), etc., but a silicon carbide substrate is most preferable.

  On the AlGaN barrier layer 3, a source electrode 4, a gate electrode 5, and a drain electrode 6 are formed. The source electrode 4 and the drain electrode 6 are provided at positions facing each other through the gate electrode 5.

  A characteristic feature of the nitride semiconductor device of the present embodiment over the conventional nitride semiconductor device is that the thickness of the AlGaN barrier layer 3 between the source electrode 4 and the gate electrode 5 and between the gate electrode 5 and the drain electrode 6 is the gate. The thickness is smaller than the thickness of the AlGaN barrier layer directly under the electrode 5. That is, the layer thickness of at least a part of the AlGaN barrier layer 3 between the drain electrode 6 and the gate electrode 5 and between the source electrode 4 and the gate electrode 5 is smaller than the layer thickness of the AlGaN barrier layer 3 immediately below the gate electrode 5. Yes. Specifically, an etching groove 7 is formed in the AlGaN barrier layer 3 in such a region. For comparison, FIG. 1B shows an element cross-sectional view of a nitride semiconductor device having an AlGaN barrier layer having a conventional uniform layer thickness without such a feature.

Hereinafter, characteristics of element characteristics realized by the nitride semiconductor device of the present embodiment will be described. FIG. 2A shows the relationship between the threshold voltage _Vth and the layer thickness of the AlGaN barrier layer 3 immediately below the gate electrode 5. As can be seen from FIG. 2A, the threshold voltage _Vth increases in proportion to the positive voltage side as the thickness of the AlGaN barrier layer 3 immediately below the gate electrode 5 decreases. This is because as the layer thickness of the AlGaN barrier layer 3 is reduced, the gate voltage required to deplete the entire GaN channel layer 2 below the gate electrode 5 becomes smaller. Hereinafter, the relationship between the threshold voltage _Vth and the layer thickness of the AlGaN barrier layer 3 will be further described.

When the threshold voltage of AlGaN / GaN HEMT is _Vth ,

V _th = φ−ΔEc−qd ₁ N ² / (2ε) −qPd ₂ / ε (1)

It becomes. In the equation (1), φ is the Schottky barrier height, ΔEc is the band discontinuity on the conduction band side, q is the charge amount, N is the doping concentration of the AlGaN barrier layer, and d ₁ is the doping film in the AlGaN barrier layer. Thickness, ε is dielectric constant, P is polarizability, and d ₂ is the thickness of the AlGaN barrier layer. In the formula (1), the third term represents a term due to doping of the AlGaN barrier layer, and the fourth term represents a term due to polarization. The third term is a value common to the GaAs semiconductor material, but the term due to the polarization of the fourth term is a large value peculiar to the nitride semiconductor material such as GaN. Therefore, in the AlGaN / GaN HEMT, the third term contributes little to the threshold voltage _Vth, while the contribution from the fourth term occupies the majority. Accordingly, as a factor of AlGaN / GaN HEMT at the threshold voltage _{V th} control, thickness _{d 2} of the AlGaN barrier layer is very important. In short, the threshold voltage V _th strongly depends on the layer thickness d ₂ of the AlGaN barrier layer.

  On the other hand, with respect to the breakdown voltage, as can be seen from the relationship between the breakdown voltage in FIG. 2B and the thickness of the AlGaN barrier layer 3 between the gate / drain electrodes 5, 6, the AlGaN between the gate / drain electrodes 5, 6. As the layer thickness of the barrier layer 3 decreases, the breakdown voltage increases rapidly, that is, improves. This is because the degree of extension of the depletion layer from the gate electrode 5 toward the drain electrode 6 increases as the thickness of the AlGaN barrier layer 3 decreases.

From FIG. 2, it is difficult to fabricate an AlGaN / GaN HEMT having a threshold voltage _Vth of −5 V or less and a breakdown voltage exceeding 150 V in the conventional structure (FIG. 1B) in which the thickness of the AlGaN barrier layer 3 is uniform. It can be seen that it is. That is, in order to make the threshold voltage _Vth -5 V or less from FIG. 2A, it is necessary to set the layer thickness of the AlGaN barrier layer 3 to 25 nm or more, but according to FIG. This is because the layer thickness of the AlGaN barrier layer 3 must be set to 20 nm or less in order to set it to 150 V or more.

On the other hand, like the AlGaN / GaN HEMT according to the present embodiment, while maintaining the layer thickness of the AlGaN barrier layer 3 under the gate electrode 5 and setting the threshold voltage _Vth to −5 V, for example, the gate / drain If the thickness of the AlGaN barrier layer between the electrodes 5 and 6 is reduced to 20 nm or less, the breakdown voltage can be maintained at 150 V or more. In short, the threshold voltage _Vth and the breakdown voltage can be controlled independently within a predetermined range.

  From the above description, in the AlGaN / GaN HEMT according to the present embodiment, the thickness of the AlGaN barrier layer 3 between the source / gate electrodes 4 and 5 and between the gate / drain electrodes 5 and 6 is the AlGaN barrier immediately below the gate electrode 5. It can be seen that how thin the layer thickness is is extremely important. The thickness of the thinned AlGaN barrier layer 3 is preferably in the range of 20% or more and 80% or less, and more preferably in the range of 35% or more and 65% or less with respect to the thickness of the AlGaN barrier layer immediately below the gate electrode 5. It is. Note that the lower limit of the above range is determined from the condition that the piezoelectric effect needs to be sufficiently developed at the AlGaN / GaN interface.

  In the AlGaN / GaN HEMT according to the present embodiment, the thicknesses of the GaN channel layer 2 and the AlGaN barrier layer 3 are preferably about 0.5 to 3 μm and about 5 to 50 nm, respectively. The Al composition ratio of the AlGaN barrier layer 3 is preferably about 0.1 to 0.5.

  Note that the channel layer 2 and the barrier layer 3 can be easily replaced with other materials as long as they are made of a nitride semiconductor exhibiting a piezo effect and have different band gap energies. For example, the above-described GaN channel layer 2 and AlGaN barrier layer 3 can be replaced with a combination of materials such as InGaN and AlGaN, InGaAlN and AlGaN, and AlGaN and AlN, respectively.

As described above, in the nitride semiconductor device according to the present embodiment, since the above-described element structure is applied, a nitride capable of high output operation with a high breakdown voltage while maintaining threshold voltage _Vth at a negative voltage. A semiconductor device can be easily provided.

Next, a method for manufacturing the nitride semiconductor device of the present embodiment will be described with reference to FIG.
First, a GaN channel layer 2 and an AlGaN barrier layer 3 are epitaxially grown on the substrate 1 in order. As such a crystal growth method, a chemical vapor deposition method (MOCVD: Metalorganic Chemical Vapor Deposition) using an organic metal and a molecular beam epitaxy (MBE) method are suitable, but other crystal growth methods can be used. It may be used (FIG. 3A).

  Subsequently, source and drain electrodes 4 and 6 are formed on the AlGaN barrier layer 3. The formation method of each electrode is as follows. The portions other than the source and drain electrodes 4 and 6 are covered with a resist film, and the metal constituting the source and drain electrodes 4 and 6, that is, the metal capable of obtaining ohmic characteristics is formed by vacuum deposition or sputtering. . Examples of such metal materials include combinations of Ti / Al, Ti / Al / Ti / Mo / Au, Ti / WSi, and the like. The resist film is patterned by a known lithography technique. Although the film thickness of the source and drain electrodes 4 and 6 depends on the material, a range of 2 to 1000 nm is preferable for each metal material. If a known so-called lift-off method is used, a metal pattern is easily formed in the source / drain regions.

  In order to obtain good ohmic characteristics in the source / drain electrodes 4 and 6, heat treatment is usually performed after lift-off. Heat treatment is performed using a lamp, a furnace, or the like, and is performed in an inert gas such as nitrogen or argon, or in a hydrogen or oxygen atmosphere at 500 to 1000 ° C. for about 10 to 300 seconds. This secures a current path from the source electrode 4 to the GaN channel layer 2 immediately below the AlGaN barrier layer 3 and to the drain electrode 6.

  Subsequently, a metal film for forming the gate electrode 5 is formed on the AlGaN barrier layer 3 by the lift-off method similar to the case where the source / drain electrodes 4 and 6 are formed. That is, after a pattern is formed in a region other than the gate region using a resist film, a gate metal is deposited and the resist film is removed. As the gate electrode 5, a metal exhibiting Schottky characteristics, for example, Ni, Pt, Ir, Pd, PtSi, or the like is applied. The metal is deposited to a thickness of about 1 to 500 nm. Further, gold (Au) or aluminum (Al) may be further stacked on the gate metal. A cross-sectional view of the device after the formation of the gate electrode 5 is shown in FIG.

  Further, a resist film 8 is formed on the wafer on which the above-described electrodes are formed, and an opening is provided only at a site where the groove 7 is formed in the AlGaN barrier layer 3 by a known lithography technique (FIG. 3C). Through this opening, the AlGaN barrier layer 3 is removed by etching to a desired depth (FIG. 3D). As an etching method, in the case of dry etching, for example, plasma of a mixed gas of chlorine gas and inert gas (argon, nitrogen) can be used as the etching gas. Further, in the case of wet etching, it is possible to perform etching in a potassium hydroxide (KOH) solution while irradiating light.

  In the above manufacturing method, the AlGaN barrier layer 3 is etched using the resist film 8 as an etching mask. However, other manufacturing methods, for example, the outermost layer of the source electrode 4, the gate electrode 5 and the drain electrode 6 are resistant to etching. It is also possible to form the groove 7 by using a metal having a property (in this case, Au) and using the electrode as an etching mask.

By applying such a manufacturing method, a nitride semiconductor device capable of high output operation with a high breakdown voltage while maintaining the threshold voltage _Vth at a negative voltage can be easily obtained.

Embodiment 2. FIG.
FIG. 4 shows an element cross-sectional view of the nitride semiconductor device according to the second embodiment of the present invention, specifically, an AlGaN / GaN HEMT. In the nitride semiconductor device of the first embodiment, the trench 7 is formed by etching the AlGaN barrier layer 3 between the source / gate electrodes 4 and 5 and between the gate / drain electrodes 5 and 6. However, the breakdown voltage is almost determined by the AlGaN barrier layer 3 between the gate / drain electrodes 5 and 6. This is because the depletion layer generated at the gate electrode 5 mainly extends to the drain electrode 6 side. Therefore, the groove 7 may be formed only in the AlGaN barrier layer 3 between the gate electrode 5 and the drain electrode 6 as shown in FIG. Such an element structure can be realized only by providing the opening of the resist film 8 at a desired position between the gate / drain electrodes 5 and 6 in the etching process in the first embodiment.

  FIG. 5 shows the relationship between the recess width, that is, the width of the groove 7 between the gate / drain electrodes 5 and 6 measured from the gate electrode 5 and the breakdown voltage. It is possible to improve the breakdown voltage by adjusting the etching amount, that is, the etching depth and the recess width. According to FIG. 5, when the groove 7 having a depth of 5 nm is formed by etching with respect to the layer thickness 25 nm before the groove formation by etching of the AlGaN barrier layer 3, the recess width is set to 500 nm or more before the groove formation. It can be seen that the breakdown voltage can be improved by 20 V or more with respect to the breakdown voltage of the element. It can also be seen that when the etching amount is further increased, a desired breakdown voltage can be realized even if the recess width is small. The recess width is preferably in the range of 500 nm to 2000 nm when the distance between the gate / drain electrodes 5 and 6 is 2000 nm. More generally, the ratio of the recess width to the distance between the gate / drain electrodes 5 and 6 is preferably 25% or more and 100% or less.

  Furthermore, as shown in FIG. 6, the groove 7 may be formed so as not to contact the AlGaN barrier layer 3 in the vicinity of the ends of the gate electrode 5 and the drain electrode 6. Such an element structure can be realized simply by providing an opening in the resist film at a desired position between the gate / drain electrodes 5 and 6. This is because non-uniformity of the groove depth due to the influence of the end portions of the respective electrodes when the groove 7 is formed is alleviated.

According to the nitride semiconductor device of the second embodiment, since trench 7 is formed only in AlGaN barrier layer 3 between the gate / drain electrodes, the breakdown voltage is high while maintaining threshold voltage _Vth at a negative voltage. Thus, a nitride semiconductor device capable of high output operation can be manufactured more easily.

(A) is element sectional drawing of the nitride semiconductor device of Embodiment 1, (b) is element sectional drawing of the conventional nitride semiconductor device. (A) is a figure which shows the relationship between threshold voltage and the layer thickness of the AlGaN barrier layer right under a gate electrode, (b) is a figure which shows the relationship between a breakdown voltage and the layer thickness of the AlGaN barrier layer between gate / drain electrodes. It is. 3 is a diagram showing a method for manufacturing the nitride semiconductor device of the first embodiment. FIG. FIG. 6 is an element cross-sectional view of the nitride semiconductor device of the second embodiment. It is a figure which shows the relationship between a breakdown voltage and a recess width. FIG. 12 is an element cross-sectional view of another nitride semiconductor device in the second embodiment.

Explanation of symbols

1 substrate, 2 GaN channel layer, 3 AlGaN barrier layer, 4 source electrode, 5 gate electrode, 6 drain electrode, 7 groove, 8 resist film.

Claims (7)

A substrate,
A channel layer formed on the substrate and made of a nitride semiconductor;
A barrier layer made of a nitride semiconductor formed on the channel layer and having a larger band gap energy than the channel layer;
A gate electrode formed on the barrier layer;
A source electrode and a drain electrode respectively formed at positions facing each other through the gate electrode on the barrier layer,
The nitride semiconductor device, wherein a thickness of at least a part of the barrier layer between the drain electrode and the gate electrode is thinner than a thickness of the barrier layer immediately below the gate electrode. 2. The nitride semiconductor device according to claim 1, wherein a layer thickness of at least a part of the barrier layer between the source electrode and the gate electrode is smaller than a layer thickness of the barrier layer immediately below the gate electrode. 3. The nitride semiconductor device according to claim 1, wherein the thin region of the barrier layer is a groove formed by etching. The nitride semiconductor device according to claim 1, wherein the channel layer is made of gallium nitride (GaN), and the barrier layer is made of aluminum gallium nitride (AlGaN). 2. The nitride semiconductor device according to claim 1, wherein a thickness of a thin portion of the barrier layer is 20% to 80% of a thickness of the barrier layer immediately below the gate electrode. 2. The nitride semiconductor device according to claim 1, wherein a ratio of a width of a thin portion of the barrier layer to a distance between the gate electrode and the drain electrode is 25% or more. A method for manufacturing a nitride semiconductor device according to claim 1,
Epitaxially growing a channel layer and a barrier layer on a substrate;
Forming a gate electrode, a source electrode and a drain electrode on the barrier layer,
Forming a metal film or a resist film on the barrier layer including the gate electrode;
A step of providing an opening in the metal film or resist film on a region corresponding to a portion where the barrier layer is thinned by a lithography technique;
Etching away the barrier layer from the opening to a predetermined layer thickness;
Removing the metal film or resist film;
A method for manufacturing a nitride semiconductor device comprising:

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