JPH0722614A - Compound semiconductor epitaxial wafer - Google Patents

Abstract

PURPOSE:To remarkably enhance characteristics of an HEMT in a low current region by improving a buffer layer. CONSTITUTION:An undoped GaAs buffer layer B is formed on a semiinsulative GaAs substrate 11. The buffer layer B consists of an undoped GaAs buffer layer 12, a delta doping layer 13 formed on the layer 12, and an undoped GaAs spacer layer 14. An undoped GaAs carrier transit layer 15, an undoped AlGaAs spacer layer 16, and an N-type AlGaAs carrier supply layer 17 are formed on the buffer layer B. It is preferable that the doping element of the delta doping layer 13 formed in the buffer layer B is C (carbon).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compound semiconductor epitaxial wafer, and more particularly to a device structure for improving HEMT characteristics.

[0002]

2. Description of the Related Art The basic structure of a general compound semiconductor epitaxial wafer having a HEMT structure is shown in FIG. Undoped GaAs 52 having a film thickness of 0.1 to 1 μm is provided on a semi-insulating GaAs substrate 51, and undoped GaAs is formed thereon.
A carrier transit layer 53 is provided, and an n-type AlGaAs carrier supply layer 5 having a film thickness of 30 to 50 nm is further provided thereon with an undoped AlGaAs spacer layer 54 of 0 to 2 nm interposed therebetween.
5 is provided.

The two-dimensional electron gas that characterizes HEMT is
It accumulates in the undoped InGaAs carrier transit layer 53. Depending on the concentration of electrons accumulated in the carrier transit layer 53 and the mobility thereof, major performances such as the amplification factor and noise characteristics of the HEMT device are significantly changed. Generally, the higher the electron concentration and the higher its mobility, the better the performance.
Pseudo morphic HEMT (Pseudo morphic HEMT) is one of the methods to increase the mobility of electrons in the carrier transit layer.
A HEMT called an electron mobility transistor) is known. Pseudomorphic is GaAs and InGaA
Two types of semiconductors with different lattice constants, such as s,
It is a term used for a state in which the interface is distorted and bonded so that the in-plane lattice constants are matched at the interface. When the grown film thickness is smaller than the critical film thickness, the lattice is distorted even if the lattice constants are different, so that a junction in which dislocation does not occur can be obtained. A state in which the lattice is distorted and no lattice defect occurs at the interface is called a pseudomorphic state.

The basic structure of such a compound semiconductor epitaxial wafer having a pseudomorphic HEMT structure is shown in FIG. An undoped GaAs buffer layer 62 having a film thickness of 0.5 to 1.0 μm is provided on a semi-insulating GaAs substrate 61, an undoped InGaAs carrier transit layer 63 is provided thereon in a pseudomorphic state, and a film thickness of 0 is further provided thereon. N-type AlGaA with a film thickness of 30 to 50 nm through an undoped AlGaAs spacer layer 64 of up to 2 nm
An s carrier supply layer 65 is provided.

On the other hand, a method called the delta doping method has been studied as one of the methods for increasing the concentration of electrons accumulated in the carrier transit layer (References 1 and 2 below).

Reference 1: Japanese Journal Of Applied Physics VOL 3
0, No.6, June, 1991, pp.1158-1163 Reference 2: Japanese Journal Of Applied Physics VOL 7
2, No. 1, July, 1992, pp. 276-278 The basic structure of HEMT in which a carrier supply layer is formed by this method called delta doping method is as shown in FIG. The concentration distribution of the dopant element serving as the carrier generation source in the carrier supply layers 74, 75 and 76 is different from that in FIGS. 5 and 6, the dopant element is distributed throughout the carrier supply layers 55 and 65,
In the delta doping method of FIG. 7, the dopant element is a very thin layer portion 75 of a part of the carrier supply layers 74, 75 and 76.
It is distributed in a very high concentration only in (called a delta doping layer). In this method, the delta doping layer 7
By optimizing the position of 5 and the forming method thereof, the concentration of electrons accumulated in the carrier transit layer 73 can be increased to some extent as compared with the structures of FIGS.

A HEMT having a structure in which both pseudomorphic and delta doping are combined has also been proposed. As shown in FIG. 8, an undoped InGaAs carrier transit layer 83 is provided in a pseudomorphic state on the undoped GaAs buffer layer 82, and an undoped AlGaAs spacer layer 84 serving as a carrier supply layer and a dopant element are polar. The delta doping layer 85 and the undoped AlGaAs carrier supply layer 86, which are distributed at a high concentration, are provided. According to this, both the concentration of electrons accumulated in the carrier transit layer and the mobility thereof can be increased, so that further improvement in performance can be expected.

[0008]

The main characteristics such as the amplification factor and noise characteristics of the HEMT device are largely controlled by the concentration of electrons accumulated in the carrier transit layer and the mobility thereof. For this reason, conventionally, only the structures of the carrier transit layer and the carrier supply layer have been focused on. Regarding the buffer layer structure below the carrier transit layer, the crystallinity of the layer grown on the buffer layer and the semi-insulating property of the layer itself have to be considered. However, the structure itself has not been studied so much.

Therefore, as the buffer layer structure, normally, undoped GaAs or AlGaAs as shown in FIGS. 5 to 8 is used, or these are combined in multiple layers to form a semi-insulating layer. Stop at. And
In most cases, a dopant is not intentionally used and the growth conditions are changed to form a p-type semi-insulating property (p-minus type) or an n-type semi-insulating property (n-minus type).

Recently, however, the demand for lower power consumption of HEMTs has increased, and the characteristics at low operating current have become important. In this low operating current region, the carrier concentration in the carrier transit layer is low and tends to be widely distributed to the buffer layer side. Therefore, optimization of the buffer layer structure is extremely important for improving HEMT characteristics in the low operating current region.

Therefore, an object of the present invention is to improve the buffer layer structure to solve the above-mentioned problems of the prior art and to significantly improve the characteristics of HEMT in a low current region. To provide a wafer.

[0012]

A compound semiconductor epitaxial wafer of the present invention is a compound semiconductor having a HEMT structure in which a buffer layer is provided on a semi-insulating substrate, and a carrier transit layer and a carrier supply layer are provided thereon. In the epitaxial wafer, the structure of the buffer layer is composed of an undoped layer and a delta doping layer formed in the layer. The present invention relates to the structure of the buffer layer, and thus does not limit the position of the delta doping layer in the buffer layer or the method of forming the delta doping layer. However, as the dopant element, C (carbon), which hardly diffuses during crystal growth, is considered to be most suitable.

The optimum carrier concentration is about 5 × 10 ¹⁷ cm ^-3 or less, although the optimum concentration varies depending on the n-type carrier concentration on the carrier supply layer side and the operating current range.

[0014]

When the delta doping layer is provided in the buffer layer in the epitaxial wafer having the HEMT structure as in the present invention, the carrier distribution of the channel required for the FET operation is sharpened, so that in the low current region. HEMT characteristics are improved. In particular, the HEMT characteristics in the low current region can be significantly improved by optimizing the position of the delta doping layer and its carrier concentration.

The formation of the delta doping layer is usually carried out by stopping the supply of the group III element (thus stopping the growth of the layer) and sufficiently supplying the dopant atoms. Therefore, it is considered that the concentration of the dopant atom is a saturated concentration under the condition and the layer thickness is about 1 to 2 atomic layers.

[0016]

EXAMPLES Examples in which the compound semiconductor epitaxial wafer of the present invention is applied to an n-type AlGaAs / GaAs HEMT structure will be described below. Figure 1 shows the HEM
2 shows a cross-sectional structure of a T-structure epitaxial wafer. In this embodiment, MOVPE (metalorganic vapor phase epitaxy method) capable of controlling crystal growth at the atomic level was adopted as a means for epitaxial growth.

An undoped GaAs buffer layer B was provided on the semi-insulating GaAs substrate 11. As the buffer layer B, first, an undoped GaAs buffer layer 12 having a thickness of 0.4 μm was provided. Arsine gas (AsH ₃ ) as raw material
And trimethylgallium (TMG) are used to optimize the V / III ratio to obtain undoped semi-insulating GaAs. The substrate temperature is 650 ° C. Undoped GaAs
When the growth of the buffer layer 12 is completed, the supply of TMG is temporarily stopped, and CCl ₄ (carbon tetrachloride) is supplied for 20 seconds in the arsine gas atmosphere to obtain the delta doping layer 1.
Formation of 3 was performed.

By supplying CCl ₄ , a delta doping layer 12 of C (carbon) is formed. The practical carrier concentration at that time can be controlled with good reproducibility by the supply time and the supply concentration of CCl ₄ . In this example, the carrier concentration was 1 × 10 ¹⁶ cm ⁻³ .

After forming the delta doping layer 13, CCl
The supply of ₄ was stopped, and after 15 seconds, the supply of TMG was restarted to form an undoped GaAs spacer layer 14 of the same quality as the undoped GaAs buffer layer 12.

In this way, the structure of the buffer layer B was composed of the undoped layers 12 and 14 and the delta doping layer 13 formed in the layers. On the buffer layer B, Ga of the same quality as the undoped GaAs buffer layer 12 is formed.
The As carrier running layer 15 was formed. The total thickness of the spacer layer 14 and the carrier traveling layer 15 is 25 nm.

In this embodiment, the undoped buffer layer 1
2, 14 and undoped carrier transit layer 15 are grown under exactly the same growth conditions. When the composition and growth conditions of the buffer layer and the carrier transit layer are different, the spacer layer 14 of the buffer layer is about 10 nm, and the carrier transit layer 15 is 15 nm.
In many cases, a high performance HEMT can be obtained.

On top of that, undoped Al having a thickness of 6 nm is formed.
An n-type AlGaAs carrier supply layer 17 having a thickness of 35 nm is provided via the GaAs spacer layer 16 to complete the HEMT structure epitaxial wafer. The Al composition X of both the spacer layer 16 and the carrier supply layer 17 is 0.3, and the carrier concentration of the carrier supply layer 17 is 2 × 10 ¹⁸ cm ⁻³ . In addition to arsine and TMG, trimethyl aluminum (T
MA) and disilane (Si ₂ H ₆ ) for n-type dopant were used.

The evaluation is performed by using the epitaxial wafer of this embodiment having a structure in which the buffer layer has the delta doping layer 13.
A HEM having a gate length of 0.3 μm and a gate width of 200 μm was formed on each of the epitaxial wafers of the comparative examples having the structure without the delta doping layer 13 prepared under the same conditions as described above.
This was done by making T and comparing its true transconductance. As a result, the true transconductance at an operating current of 1 mm is 15 ms for the wafer of this embodiment.
/ 200 μm, comparative example has 11 ms / 200 μm and 3
It was higher than 0%. As the operating current further decreased, this ratio increased, and a large improvement in characteristics was observed in the low operating current region.

The present embodiment relates to the structure of the buffer layer, and does not limit the structure or composition of the carrier transit layer or the carrier supply layer. Therefore, the structure of the buffer layers 22 to 24, 32 to 34, and 42 to 44 of FIGS. 2 to 4 can be considered for the HEMT structure of FIGS. Especially in the watershed, a large improvement in the true transconductance characteristic can be obtained.

[0025]

(1) According to the invention described in claim 1,
Since the delta doping layer is formed in the buffer layer, H
The true transconductance, which is one of the indicators of the EMT characteristic, can be significantly improved. Further, as the operating current decreases, the improvement effect increases, and the HE of the small current type is
The characteristics of MT are significantly improved.

(2) According to the second aspect of the present invention, since the dopant element is C (carbon), which hardly diffuses during crystal growth, the characteristics of the HEMT are significantly improved.

[Brief description of drawings]

FIG. 1H shows an embodiment having a buffer layer structure of the present invention
Sectional drawing of an EMT structure epitaxial wafer.

FIG. 2 is a pseudomorphic H showing a modification of this embodiment.
Sectional drawing of an EMT structure epitaxial wafer.

FIG. 3 is a cross-sectional structure diagram of a HEMT structure epitaxial wafer showing another modification of the present embodiment.

FIG. 4 is a sectional structural view of a HEMT structure epitaxial wafer showing still another modified example of the present embodiment.

FIG. 5 is a typical cross-sectional structural diagram of a HEMT structure epitaxial wafer having a conventional buffer layer structure.

FIG. 6 is a basic cross-sectional configuration diagram of an epitaxial wafer having a conventional pseudomorphic HEMT structure.

FIG. 7 is a typical sectional structure view of a HEMT structure epitaxial wafer having a conventional buffer layer structure.

FIG. 8 is a basic cross-sectional structural diagram of an epitaxial wafer having a pseudomorphic HEMT structure using a delta doping method for a conventional carrier supply layer.

[Explanation of symbols]

 11 semi-insulating GaAs substrate 12 undoped GaAs buffer layer 13 delta doping layer 14 undoped GaAs spacer layer 15 undoped GaAs carrier transit layer 16 undoped AlGaAs spacer layer 17 n-type AlGaAs carrier supply layer B buffer layer

Claims (2)

[Claims] 1. A compound semiconductor epitaxial wafer having a HEMT structure in which a buffer layer is provided on a semi-insulating compound semiconductor substrate, and a carrier transit layer and a carrier supply layer are provided thereon, and the structure of the buffer layer is undoped. And a delta-doped layer formed in the layer of the compound semiconductor epitaxial wafer. 2. The compound semiconductor epitaxial wafer according to claim 1, wherein the doping element of the delta doping layer formed in the buffer layer is C (carbon).