JP4494567B2 - Method of forming electrode on n-type gallium nitride compound semiconductor layer - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming an electrode of a semiconductor device using an n-type gallium nitride compound semiconductor.
[0002]
[Prior art]
Gallium nitride (GaN) -based compound semiconductors such as GaN, GaAlN, InGaN, and InGaAlN can change the band gap over a wide range by selecting the composition. Since wavelength emission can be obtained, application to light-emitting elements such as semiconductor light-emitting diodes and semiconductor lasers has been studied. In addition, gallium nitride-based compound semiconductors have excellent composition stability at high temperatures, and are expected as semiconductor materials such as transistors that can operate at high temperatures.
[0003]
The n-type electrode of the semiconductor element using these gallium nitride-based materials has a relatively low contact resistance due to the multilayer electrode or alloy electrode and the n-type GaN contact layer. Examples of the electrode include an alloy of Ti and Al or a multilayer film of Ti and Al. FIG. 3 shows a sectional view of the electrode structure of Conventional Example 1. The n-type electrode has a structure in which a Ti layer 2 and an Al layer 3 are sequentially laminated on the surface of the n-type GaN contact layer 1. In this electrode structure, good ohmic characteristics are obtained by heat treatment at a temperature of 400 ° C. or higher after electrode deposition.
[0004]
Further, as an electrode structure obtained by improving the electrode of the conventional example 1, an electrode structure in which a Ti layer 2 and an Al layer 3 are sequentially laminated on the surface of the n-type GaN contact layer 1 and then a metal having a higher melting point than Al is laminated is proposed. ing. Examples of metals having higher melting points than Al include Au, Ti, Ni, Pt, W, Mo, Cr, and Cu, and Au, Ti, and Ni are particularly preferable.
[0005]
FIG. 4 shows the structure of the n-type electrode of Conventional Example 2. In FIG. 4, the n-type electrode is composed of a Ti layer 2, an Al layer 3, a Ni layer 4, and an Au layer 5 that are sequentially stacked on the surface of the n-type GaN contact layer 1. In this case, as in the case of Conventional Example 1, good ohmic characteristics can be obtained by heat treatment at a temperature of 400 ° C. or higher.
[0006]
[Problems to be solved by the invention]
The results of our experiment are described below. FIG. 6 shows a cross-sectional view of the n-type electrode structure. On the surface of the n-type GaN contact layer 1, electrodes were laminated in the order of the Ti layer 2, the Al layer 3, and the Au layer 5. Thereafter, heat treatment was performed at 400 ° C. or higher. The results of measuring contact characteristics (current I-voltage V characteristics) are shown in FIG. Thus, it was difficult to obtain ohmic characteristics.
[0007]
[Means for Solving the Problems]
In the conventional electrode forming method, a Ti layer 2 is first deposited on the GaN contact layer 1. Thereafter, the Al layer 3 and the Au layer 5 are deposited in this order. The following were considered as the reasons why the ohmic characteristics were not obtained with good reproducibility in our experimental results.
[0008]
Pretreatment is performed for the purpose of removing the oxide film, and vapor deposition is performed in a vacuum state. However, a certain amount of Ti oxide is generated at the interface between the surface of the GaN contact layer 1 and the Ti layer 2. Ti oxide remains at this interface after the heat treatment step, which makes it difficult to obtain ohmic characteristics with good reproducibility. FIG. 5 shows the relationship between the absolute value of the standard free energy of oxide formation | ΔG | and the temperature (Richard A. Swalin “Thermodynamics of Solid”, Corona (1965)). When the absolute values of the standard free energy of formation of Ti oxide and Al oxide are compared, the free oxide of Ti is smaller in the temperature range of 0 ° C. to 1000 ° C. Therefore, Ti oxide is easier to produce. That is, when the first layer in contact with the surface of the n-type GaN contact layer 1 is the Al layer 3, the oxide remaining at the interface is reduced.
[0009]
Since Ti and Al are alloyed by heat treatment, conventionally, the order of stacking during electrode deposition has not been a problem. This time, from the above consideration, we paid attention to the order of stacking during electrode deposition. That is, in the conventional example, the Ti layer 2 is deposited on the surface of the GaN contact layer 1, but in the present invention, the Al layer 3 is first deposited. Thereafter, the Ti layer 2 and the Au layer 5 were deposited in this order.
[0010]
In the method for forming an n-type electrode according to the present invention, an Al layer 3, a Ti layer 2, and an Au layer 5 are laminated in this order on an n-type contact layer of a gallium nitride compound semiconductor in an electrode vapor deposition step before heat treatment , and the electrode After forming the metal, heat treatment is performed . Also, the heat treatment temperature is, and less than 9 00 ° C.. The 9 00 ° C. or higher, crystallinity of the GaN-based compound semiconductor film is deteriorated, adversely affect the properties after creation semiconductor device.
[0012]
In the present invention, when the electrode metal is deposited and heat treated, the electrode metal is deposited and then heat treated. When the heat treatment is performed, the first stage is maintained at a temperature of 250 ° C. to 400 ° C., and the second stage is 400 ° C. to The temperature is kept at 900 ° C., and the second stage is always higher in temperature than the first stage. The heat treatment time is sufficient within 15 minutes for the first stage and within 15 minutes for the second stage. In the first heat treatment, the Al oxide slightly present at the interface between the surface of the n-type GaN contact layer 1 and the Al layer 3 is decomposed, Al is combined with nitrogen, and the oxide is present at the interface. Disappear. Oxygen generated by decomposition diffuses outside the electrode. At this stage, a Ti—Al alloy is formed on the surface of the GaN contact layer. In the second heat treatment, an Au—Ti—Al alloy is formed at the interface between Au and the Ti—Al alloy, and these metals are blended.
[0014]
In the two-stage heat treatment method, first, Al and Ti are vapor-deposited, then the first-stage heat treatment is performed, and then, after the Au layer 5 is vapor-deposited, the second-stage heat treatment may be performed. By adopting such a configuration, good ohmic characteristics can be obtained with good reproducibility, and the object of the present invention can be achieved.
[0015]
As another problem, when heat treatment is performed at a high temperature, there is no particular problem in the chip shape (several mm square level), but if the wafer shape has a diameter of 2 inches or more, the wafer may be cracked or warped. If the wafer warp is large, troubles such as the fact that the wafer cannot be vacuum-sucked occur in the subsequent semiconductor element fabrication process. According to the present invention, the heat treatment temperature can be lowered to 400 ° C. or lower, which is effective for the above problem.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
In the embodiment of the method for forming an electrode on the n-type gallium nitride compound semiconductor layer in the present invention, the surface of the n-type GaN contact layer 1 and the Al layer 3 are in contact before the start of heat treatment. Furthermore, in the embodiment of the method for forming an electrode on the n-type gallium nitride compound semiconductor layer in the present invention, good ohmic characteristics can be obtained with good reproducibility even in a wide temperature range of 20 ° C. to 900 ° C.
[0017]
【Example】
[ Reference Example 1] Reference Example 1 of the embodiment of the present invention will be described with reference to FIG. FIG. 1 shows a cross-sectional view of an n-type electrode structure according to an embodiment of the present invention and a reference example . First, an Al layer 3 is vacuum-deposited on the GaN contact layer 1 by 20 nm. Next, a Ti layer 2 is vacuum-deposited at 70 nm. Further thereon, an Au layer 5 was sputter deposited by 1000 nm. Next, heat treatment was performed at 400 ° C. for 15 minutes. The characteristics of the n contact layer were measured using the electrode thus prepared. The results are shown in FIG. Good ohmic characteristics are obtained with good reproducibility, the contact resistance is 1 × 10 ^- was ⁷ [Omega] cm ² and sufficiently low.
[0018]
[ Reference Example 2] Reference Example 2 will be described below. The process up to the n-type electrode deposition is the same as in Reference Example 1 above. The heat treatment was performed at 100 ° C. for 30 minutes. As a result, good ohmic characteristics were obtained with good reproducibility.
[0019]
For Example 1 of an embodiment of Example 1 the present invention will be described with reference to FIG. The process up to electrode deposition is the same as in Reference Example 1 above. FIG. 2 shows a two-stage heat treatment method. The first stage temperature was 400 ° C. for 15 minutes, and the second stage temperature was 850 ° C. for 5 minutes. The characteristics of the n contact layer were measured using the electrode thus prepared. The results are shown in FIG. Good ohmic characteristics are obtained with good reproducibility, the contact resistance 1 × 10 ^- was ⁷ [Omega] cm ² and sufficiently low.
[0020]
[Embodiment 2 ] FIG. 10 shows an electrode formation process of a GaN field effect transistor (FET). First, crystal growth of GaN was performed using a gas source molecular beam epitaxial growth method. An ultra-high vacuum apparatus having a growth chamber and a patterning chamber is used.
[0021]
First, in the growth chamber, on an insulating sapphire substrate 6, radicalized nitrogen (3 × 10 ^{- 6} Torr) and Ga ^- by molecular beam epitaxy using (5 × 10 ⁷ Torr), at a growth temperature 640 ° C. thickness of 50 nm ⁿ - to form a GaN buffer layer 7, further Ga thereon (1 × 10 ^{- 6} Torr) and ammonia ⁽⁵ × 10 ^- 5 Torr) using a undoped GaN undoped at a growth temperature 850 ° C. Layer 8 was grown 1000 nm. Next thereon, Ga using a ^{- - (5 Torr 5 × 10} ), Si as a dopant (1 × 10 ⁷ Torr) and ammonia ^- with ^{(1 × 10 9 Torr),} GaN n at a growth temperature of 850 ° C. Layer 9 is formed to 200 nm. The carrier concentration of this layer is 2 × 10 ¹⁷ cm ^- was set by using a ³ become as previously hole measurement. Next thereon ^{Ga (1 × 10 - 7 Torr} ) and ammonia ^{(5 × 10 - 5 Torr)} , Si - using (5 × 10 ⁸ Torr), the n-type GaN contact layer 1. The carrier concentration of this time 5 × 10 ¹⁸ cm ^- was ^three.
[0022]
Next, an electrode creation process using the above-described GaN epitaxial film will be described. The S _i O ₂ as a protective film GaN epi entire surface marked by a thermal chemical deposition method. Thereafter, patterning was performed using photolithography, and an opening was formed using hydrofluoric acid in a portion to be an electrode. Next, the Al layer 3 and the Ti layer 2 to be electrodes are sequentially deposited by a vacuum deposition apparatus. First, the Al layer 3 is deposited by 20 nm. Next, a Ti layer 2 is deposited by 70 nm. Further, an Au layer 5 is sputter deposited by 1000 nm thereon. Next, heat treatment was performed at 400 ° C. for 15 minutes to form a Ti—Al based alloy layer 13. Furthermore, it heated up to 850 degreeC and heat-processed for 5 minutes, and the Au-Ti-Al type alloy layer 14 was formed. A two-stage heat treatment method is shown in FIG. The source electrode 11 and the drain electrode 12 thus prepared had good ohmic characteristics. Further, the contact resistance between the GaN contact layer 1 and these electrodes 11 and 12 was measured. Consequently 1 × 10 ^- was ⁷ [Omega] cm ² and sufficiently low contact resistance.
[0023]
Metal organic chemical vapor deposition (MOCVD) may be used for the growth of GaN. For forming the GaN film, dimethylhydrazine, monomethylhydrazine, ammonia or the like is used as a nitrogen source. As the Ga source, an organic metal gas such as triethylgallium or trimethylgallium is used. Moreover, monosilane, disilane, etc. are used as an n-type dopant.
[0024]
Further, the same effect can be obtained with the Au layer 5 which is the third-layer electrode even when Ni, Pt, W, Mo, Cr and Cu are used.
[0025]
In addition to GaN, the same effect can be obtained in gallium nitride semiconductors such as GaAlN, InGaN, and InGaAlN.
[0026]
Moreover, although the GaN field effect transistor was shown in Example 2 , the same effect is acquired also in the n-type electrode utilized with a light emitting diode, a laser diode, etc.
[0027]
[Comparative example]
A comparative example of the present invention will be described with reference to FIG. FIG. 6 shows a cross-sectional view of a comparative n-type electrode structure. On the surface of the GaN contact layer 1, a Ti layer 2 is first vacuum-deposited by 20 nm. Next, the Al layer 3 is vacuum-deposited by 70 nm. Further thereon, an Au layer 5 was sputter deposited by 1000 nm. Next, heat treatment was performed at 400 ° C. for 15 minutes. The characteristics of the n contact layer were measured using the electrode thus prepared. The results are shown in FIG. Good ohmic characteristics could not be obtained.
[0028]
【The invention's effect】
As apparent from the above description, in the method for forming an electrode on the surface of the n-type gallium nitride compound semiconductor layer according to claims 1 to 5 of the present invention, the oxidation remaining at the interface between the n-type GaN contact layer and the electrode Things are suppressed, good ohmic characteristics can be obtained with good reproducibility, and a sufficiently small value of contact resistance can be obtained.
[0029]
In addition, when electrodes are formed on a wafer having a diameter of 2 inches or more, the heat treatment temperature can be set particularly in a low temperature region, and troubles such as cracking and warping of the wafer can be prevented.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing an n-type electrode structure according to Reference Examples 1 and 2 of the present invention.
FIG. 2 is a two-stage heat treatment method according to Examples 1 and 2 of the present invention.
3 is a cross-sectional view showing an n-type electrode structure of Conventional Example 1. FIG.
4 is a cross-sectional view showing an n-type electrode structure of Conventional Example 2. FIG.
FIG. 5 is a cross-sectional view showing an n-type electrode structure of a comparative example.
7 is a graph showing current-voltage characteristics of an electrode according to Reference Example 1 of the present invention. FIG.
FIG. 8 is a diagram showing current-voltage characteristics of an electrode according to Reference Example 2 of the present invention.
9 is a graph showing current-voltage characteristics of an electrode according to Comparative Example 1. FIG.
FIG. 10 is a diagram showing an electrode formation process of a GaN FET.
[Explanation of symbols]
1 GaN contact layer 2 Ti layer 3 Al layer 4 Ni layer 5 Au layer 6 Sapphire substrate 7 GaN buffer layer 8 GaN undoped layer 9 GaN n layer 10 Gate electrode 11 Source electrode 12 Drain electrode 13 Ti—Au alloy 14 Au—Ti -Al alloy

Claims (1)

In the method of forming an electrode on the surface of the n-type gallium nitride compound semiconductor layer, the electrode is sequentially formed from the side in contact with the n-type gallium nitride compound semiconductor layer, aluminum as the first thin film, titanium as the second thin film, As a third thin film, an n-type gallium nitride compound semiconductor layer in which a metal having a melting point higher than that of aluminum is laminated is held at a temperature of 250 ° C. to 400 ° C. as a first stage, and then 400 as a second stage. A method of forming an electrode on an n-type gallium nitride compound semiconductor layer, characterized in that the second stage is always kept at a temperature higher than that of the first stage, and the temperature is always higher than that of the first stage .

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