JP2001274376A - Low-resistant garium nitride buffer layer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a GaN buffer layer which is high purity but low resistance. SOLUTION: The buffer layer 2 has heterojunction comprising at least two thin layers comprised of nitride III-V compound semiconductor with Ga as an essential component, for example, AlGaN 2A/GaN 2B/AlGaN 2C. A 2-dimension electron gas layer formed at the heterojunction is positively utilized for low resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-resistance GaN-based buffer layer formed on a substrate by an epitaxial growth method, and more particularly to a method of manufacturing various GaN-based semiconductor devices such as FETs and MESFETs. By doing
The present invention relates to a low-resistance GaN-based buffer layer capable of forming an operation electrode on the back surface of a substrate of the semiconductor device.

[0002]

2. Description of the Related Art For example, when an FET is manufactured from a GaN-based material, a Ga having a predetermined composition is formed on a substrate by an epitaxial growth method such as MOCVD or MBE.
It is necessary to form an FET layer structure by sequentially stacking N-type crystal layers. In this case, there is no substrate material whose lattice constant matches that of the GaN-based material, and thus a substrate made of a different material is used as a substrate for crystal growth. Usually, a sapphire substrate is used.

However, since the lattice mismatch between the sapphire substrate and the GaN crystal that grows is 20% or more, the lattice mismatch between the GaN crystal and the grown GaN crystal is reduced, and
For the purpose of minimizing crystal defects in a crystal, a buffer layer is once formed on a sapphire substrate. Usually, this buffer layer is formed in a non-doped state, and an n-type G layer having a desired film thickness is formed thereon.
By forming an aN crystal layer as an n-type active layer, Ga
An N-based FET layer structure is formed.

The following two-stage growth method has conventionally been applied to the formation of the buffer layer described above. The first method uses MOCVD, for example, using triethylaluminum (TEA) and ammonia (NH ₃ ).
Using hydrogen as a carrier gas, an ultra-thin AlN having a thickness of about 5 nm is temporarily formed on a sapphire substrate at 800 ° C.
The layer is deposited as a lower buffer layer and then grown at a temperature of 110
This is a method in which the temperature is raised to 0 ° C. and a thick non-doped GaN crystal layer is formed as an upper buffer layer on the AlN layer using trimethylgallium (TMG) and ammonia (NH ₃ ).

The following method is applied as a second method. That is, an amorphous GaN layer having a thickness of about 1 to 2 nm is formed as a lower buffer layer at a low temperature of 500 to 600 ° C. by using, for example, TMG and NH ₃ , using hydrogen as a carrier gas by MOCVD, Then, the temperature was increased to 1100 ° C. to perform epitaxial growth, and a thick non-doped GaN was formed on the amorphous GaN layer.
In this method, a crystal layer is formed as an upper buffer layer.

The conventional GaN-based buffer layer formed as described above has a problem that it has many crystal defects, and at the same time, a thick non-doped GaN crystal layer formed as an upper buffer layer has high resistance. As a result, there is a problem that the overall resistance of the buffer layer is high.

[0007]

When a vertical GaN-based FET is to be manufactured, for example, a substrate made of a conductive material is used as the substrate, and the upper and lower surfaces of the manufactured device (ie, the back surface of the substrate). ) Requires the formation of working electrodes. However, in the FET structure formed by the above-described conventional method, since the buffer layer located on the substrate has a high resistance, even if the working electrode is formed on the back surface of the substrate, no electrode operation is exhibited. Therefore, when attempting to manufacture a vertical GaN-based FET, it is necessary to make the buffer layer non-doped and have low resistance.

An object of the present invention is to provide a GaN-based buffer layer which can satisfy the above-mentioned demands and has a high purity and a low resistance.

[0009]

In order to achieve the above-mentioned object, the present inventor has repeatedly studied and found that a heterojunction interface between mutually pure compound semiconductors, as typified by HEMT, has been found. Attention was paid to the fact that a two-dimensional electron gas layer was formed, and the electrons in that region had high mobility. Even in the case of the GaN-based buffer layer described above, if a heterojunction structure is formed using a plurality of high-purity GaN-based materials, a two-dimensional electron gas layer is formed at the heterojunction interface between each other. Thus, the idea that it becomes possible to reduce the resistance of the buffer layer as a whole, and further studies based on the idea, as a result, confirmed that the idea is valid, and to develop the present invention Reached.

That is, the low-resistance GaN buffer layer of the present invention has a layer structure in which at least two thin layers made of a nitride III-V compound semiconductor containing Ga as an essential component are heterojuncted. Features. And, as such a layer structure, AlGaN / GaN / AlGa
N, GaN / AlGaN / GaN, GaN / InGaN
/ GaN, InGaN / GaN / InGaN, InAl
GaN / GaN / InAlGaN (however, InAlG
aN has a composition in which the band gap energy is larger than that of GaN), GaN / InAlGaN / Ga
N, AlGaNP / GaNP / AlGaNP, AlGa
NAs / GaNAs / AlGaNAs, AlGaNP /
InGaNP / AlGaNP, AlGaNAs / InG
It is preferable that any one of aNAs / AlGaNAs is used.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a buffer layer according to the present invention will be described with reference to the drawings. FIG. 1 shows that a high-purity buffer layer 2 to be described later is formed on a substrate 1 and further a Si-doped G layer is formed thereon.
A cross-sectional structure is shown in which a similarly high-purity buffer layer 2 such as an aN crystal layer is formed, and a high-purity n-type active layer 3 such as a Si-doped GaN crystal layer is further formed thereon.

Here, for example, Si, S
If a substrate made of a conductive material such as a layer structure composed of a heterojunction of a substrate such as iC, GaAs, GaP, or InP is used, an upper electrode is formed on the n-type active layer 3 from this material.
By forming the lower electrodes on the back surface of the substrate 1 respectively, a vertical FET can be manufactured. Buffer layer 2
Has a three-layer structure composed of a first buffer layer 2A, a second buffer layer 2B, and a third buffer layer 2C as a whole, and the junction interface of each partial buffer layer is a hetero junction interface.

The buffer layers 2A, 2B, 2C
Is made of a nitride III-V compound semiconductor containing Ga as an essential component. In the case of the material shown in FIG. 1, specifically, the first buffer layer 2A is made of AlGaN (Al: 15 atomic%) crystal. The second buffer layer 2B is a GaN crystal layer, and the third buffer layer 2C is an AlGaN (Al: 20 atomic%) crystal layer.

[0014] In this case of the buffer layer 2, a first buffer layer 2A in the second buffer layer 2B side at a joint interface of the second buffer layer 2B 2-dimensional electron gas layer g ₁ is formed, and the second buffer layer 2
B and second buffer layer 2B at the bonding interface between third buffer layer 2C
So that the two-dimensional electron gas layer g ₂ in the side is formed.
That is, two two-dimensional electron gas layers are formed above and below the second buffer layer 2B corresponding to the two bonding interfaces.

Therefore, by forming the second buffer layer 2B thinly, the above-described two-dimensional electron gas layers g ₁ and g _{2 are formed.}
With this operation, the resistance of the second buffer layer 2B in the vertical direction can be reduced. Specifically, the second buffer layer 2
If the thickness of B is set to 10 nm or less, conductivity in the vertical direction can be exhibited. Each of the first buffer layer 2A and the third buffer layer 2C is formed only to form a two-dimensional electron gas layer at the bonding interface with the second buffer layer 2B. Therefore, as the material,
A composition having a band gap energy larger than the band gap energy of the material in the second buffer layer 2B is selected. The thicknesses of these layers 2A and 2C do not need to be large, and are sufficient as long as they are necessary for forming the above-described two-dimensional electron gas layer. Rather, layers 2A, 2
Increasing the thickness of C increases the resistance of the entire buffer layer in the vertical direction. Therefore, at the time of film formation, layer 2
30 to 5 in consideration of making A and 2C flat surfaces
It may be set to about 0 nm.

The layer structure of the buffer layer 2 exhibiting the above-mentioned functions and effects can be expressed as a first buffer layer 2A / second buffer layer 2B / third buffer layer 3C. The structure can be given as a preferred example. That is, Al
GaN / GaN / AlGaN, GaN / AlGaN / G
aN, GaN / InGaN / GaN, InGaN / Ga
N / InGaN, InAlGaN / GaN / InAlG
aN (however, InAlGaN has a composition in which the band gap energy is larger than that of GaN), Ga
N / InAlGaN / GaN, AlGaNP / GaNP
/ AlGaNP, AlGaNAs / GaNAs / AlG
aNAs, AlGaNP / InGaNP / AlGaN
P, AlGaNAs / InGaNAs / AlGaNAs
Is one of

[0017]

EXAMPLE 1 The material shown in FIG. 1 was produced by the MBE method as follows. At a growth temperature of 750 ° C. on a conductive Si substrate 1,
First, one atomic layer of Al was deposited.

Next, the plasma-converted nitrogen (3 × 10 ⁻⁶⁾
Torr), metal Ga (5 × 10 ⁻⁷ Torr), metal Al (1 ×
10 ^-7 Torr) and metal Si (1 × 10 ^-9 Torr) at a growth temperature of 800 ° C. and a thickness of 3 nm with Si-added AlGaN.
A (Al: 15 atomic%) crystal layer was formed as the first buffer layer 2A. When the streak pattern on the surface of this layer was observed with a high-speed electron beam diffractometer (RHEED), it was confirmed that the layer was flat.

Then, the plasma nitrogen (3 × 10 ⁻⁶⁾
Torr) and a metal Ga (5 × 10 ⁻⁷ Torr) alone to form a 30-nm thick GaN crystal layer on the first buffer layer 2A.
A film was formed as the buffer layer 2B. According to RHEED observation, the surface of this layer was also flat. Further, metal A is used as the gas source.
1 (2 × 10 ⁻⁷ Torr) and the MBE method is performed, and a 3 nm thick AlGaN (Al: 20: 20) is formed on the second buffer layer 2B.
(Atomic%) A crystal layer was formed as the third buffer layer 2C. RH
According to EED observation, the surface of this layer was also flat.

Then, the plasma-converted nitrogen (5 × 10 ⁻⁵⁾
Torr) and metal Ga (8 × 10 ⁻⁷ Torr), metal Si (5 × 10 ⁻⁸ Torr) as an n-type dopant, and a growth temperature of 850 ° C. and a thickness of 30 nm on the third buffer layer 2C. A Si-doped GaN crystal layer was formed as an n-type active layer 3. The obtained material was subjected to CV measurement to examine the carrier concentration near the heterojunction with the buffer layer.
The carrier concentration at the interface between the buffer layer 2 and the n-type active layer 3 is 3 × 10 ¹⁸ cm ^-3, it was confirmed that sufficiently has a vertical conductivity. This is based on the fact that the two-dimensional electron gas layer is formed on the buffer layer.

Example 2 A growth temperature of 7 was formed on a conductive Si substrate 1 by MBE.
At 00 ° C., one atomic layer of Ga was first deposited. Next, plasma-converted nitrogen (3 × 10 ⁻⁶ Torr), metal Ga
(5 × 10 ⁻⁷ Torr) and metallic Si (1 × 10 ⁻⁹ Torr)
r), a 3 nm-thick Si-added G at a growth temperature of 800 ° C.
An aN crystal layer was formed as the first buffer layer 2A. The streak pattern on the surface of this layer is converted to a high-speed electron beam diffractometer (RH
Observation by EED) confirmed that the surface was flat.

Next, the plasma-converted nitrogen (3 × 10 ⁻⁶⁾
Torr), metal Ga (8 × 10 ⁻⁷ Torr) and metal Al
An AlGaN (Al: 15 atomic%) crystal layer having a thickness of 30 nm was formed as a second buffer layer 2B on the first buffer layer 2A using (1 × 10 ⁻⁷ Torr). According to RHEED observation, the surface of this layer was also flat. Then, the supply of metal Al was cut off to perform the MBE method, and a GaN crystal layer having a thickness of 3 nm was formed as a third buffer layer 2C on the second buffer layer 2B. According to RHEED observation, the surface of this layer was also flat.

Then, plasma-converted nitrogen (5 × 10 ⁻⁵⁾
Torr) and metal Ga (8 × 10 ⁻⁷ Torr), metal Si (5 × 10 ⁻⁸ Torr) as an n-type dopant, and a growth temperature of 850 ° C. and a thickness of 30 nm on the third buffer layer 2C. A Si-doped GaN crystal layer was formed as an n-type active layer 3. The obtained material was subjected to CV measurement to check the carrier concentration near the buffer layer. As a result, the carrier concentration at the interface between the buffer layer 2 and the n-type active layer 3 was 5 × 10 ¹⁸ cm ^−3.
It was confirmed that the film had sufficient vertical conductivity.

In the above embodiment, radicalized nitrogen or ammonia may be used as a nitrogen source for forming the GaN crystal layer, and a Ga source such as trimethylgallium (TMG) or triethylgallium (TEG) may be used. An organometallic gas may be used, and an organometallic gas such as trimethylaluminum (TMA) or triethylaluminum (TEA) may be used as an Al source, and silane gas may be used instead of Si as an impurity.

Further, in the above embodiment, the MBE method is adopted as the epitaxial growth method. However, the same result can be obtained by applying the MOCVD method using the above-mentioned organometallic gas.

[0026]

As is apparent from the above description, the low-resistance GaN-based buffer layer of the present invention utilizes a two-dimensional electron gas layer formed at the heterojunction interface of a high-purity compound semiconductor. It is. Therefore, by using this buffer layer, a vertical GaN-based FET or a GaN-based MESFE
T can be manufactured, and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a layer structure using a buffer layer of the present invention.

[Explanation of symbols]

 Reference Signs List 1 Crystal growth substrate (conductive Si substrate) 2 Buffer layer 2A First buffer layer (AlGaN) 2B Second buffer layer (GaN) 2C Third buffer layer (AlGaN) 3 n-type active layer (Si-doped GaN)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (reference) H01L 33/00 H01S 5/32 F term (reference) 4G077 AA03 BE11 BE15 BE42 BE45 DA05 DB08 ED06 EF03 EF04 HA06 5F041 AA40 CA34 CA40 5F073 CA07 CA17 CB04 CB07 DA05 DA06 5F102 GB01 GC01 GD01 GJ03 GJ04 GJ05 GJ06 GK08 GQ01 HC01 5F103 AA04 DD01 GG01 HH03 HH04 JJ03 KK01 LL08 RR05

Claims (2)

[Claims] 1. A nitride III containing Ga as an essential component
-A low-resistance GaN-based buffer layer having a layer structure in which at least two thin layers made of a group V compound semiconductor are hetero-joined. 2. The method according to claim 1, wherein the layer structure is AlGaN / GaN / A.
lGaN, GaN / AlGaN / GaN, GaN / In
GaN / GaN, InGaN / GaN / InGaN, I
nAlGaN / GaN / InAlGaN (where In
AlGaN has a composition with a larger band gap energy than GaN), GaN / InAlGaN
/ GaN, AlGaNP / GaNP / AlGaNP, A
lGaNAs / GaNAs / AlGaNAs, AlGa
NP / InGaNP / AlGaNP, AlGaNAs /
2. The low-resistance GaN-based buffer layer according to claim 1, which is one of InGaNAs / AlGaNAs.