JP2000312000A - Compound semiconductor wafer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress leak current in a buffer layer by differentiating the lattice constant of a plurality of epitaxial layers laminated on a substrate partially or entirely from that of the substrate and providing a buffer layer doped with oxygen in the plurality of epitaxial layers. SOLUTION: A buffer layer 2 of undoped InAlAs is grown eptaxially on a substrate 1 by increasing the value of y in the undoped InyAl1-yAs gradually while doping and a channel layer 3 is formed thereon. A spacer layer 4 of AlAs is formed further thereon and followed by formation of a carrier supply layer 5 of Si doped n-type In0.5GaAs, a cap layer 6 of undoped In0.5AlAs, and a contact layer 7 of n-type In0.5GaAs doped with Si at concentration of 1×1019/cm3. Since the buffer layer 2 between the substrate 1 and the channel layer 3 is doped with oxygen, leak current of buffer layer can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a compound semiconductor wafer, and more particularly, to a compound semiconductor wafer having good transistor characteristics.

[0002]

2. Description of the Related Art Since a compound semiconductor has a different bandgap depending on the material constituting the compound semiconductor, a number of device devices have been proposed which utilize hetero-physical properties which appear by combining various materials.

[0003] Compound semiconductor wafers are made of semi-insulating GaAs.
s substrate by growing a plurality of epitaxial layers by liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), metal organic vapor phase epitaxy (MOVPE), etc. A heterojunction is formed.

[0004] Since the compounds constituting the epitaxial layer have different lattice constants, a stress is generated due to the mismatch of the lattice constants in the process of epitaxial growth, thereby causing warpage or distortion of the crystal lattice and deteriorating the characteristics. It is known to cause. The mismatch of the lattice constants is a cause of a failure mode that foggs the surface of the epitaxial crystal, in addition to the characteristic deterioration.

The only known hetero system free from such a problem is a combination of GaAs / AlGaAs. The constant difference (Δa /
a) is as extremely small as 0.14% at the maximum, and is being used over the entire range of the mixed crystal ratio.

[0006] Other than the above, a material such as GaAs or InP, which is epitaxially grown only with a mixed crystal crystal lattice-matched to the crystal of the substrate, is used. Is
It is in a state where restrictions are large in terms of function and cost.

Various attempts have been made to expand the range of application of mixed crystal combinations. For example, as a countermeasure using a lattice matching system, a G layer using InGaAs having excellent electron transport characteristics in a channel layer (active layer) is used.
In the hetero system of aAs / InGaAs / AlGaAs, the In composition is set to 0.2 or less and the thickness is set to 15n.
In practice, a method of forming a so-called pseudomorphic structure in which the InGaAs layer is in a state in which the lattice is distorted but does not cause lattice relaxation is set to about m or less.

However, according to this method, there is a problem that materials, compositions and film thicknesses are extremely limited in order to produce a good device.

As a conventional measure for applying to a hetero system in which the lattice constant of the crystal of the substrate and the lattice constant of the channel layer are largely different,
There is known a metal-morphic configuration in which a buffer layer is interposed between a substrate and a channel layer, and the composition of the buffer layer is gradually changed from the substrate toward the channel layer.

[0010] Specifically, for example, on a GaAs substrate, an In _y was gradually or stepwise changing the y value
This is a configuration in which a buffer layer of Al _1-y As is provided. By providing such a buffer layer, the effect of lattice mismatch between the substrate and the channel layer is reduced, and an active layer having a lattice constant matching the buffer layer is formed on the buffer layer. The purpose is to secure good electron transporting properties by forming.

[0011]

However, according to the conventional compound semiconductor wafer having this configuration, the occurrence of dislocations due to lattice distortion and the reduction of the band gap increase the intrinsic carrier concentration, thereby increasing the leakage current of the buffer layer. May increase, which may degrade the transistor characteristics. In a field-effect transistor, which is a mainstream of electronic devices, the magnitude of the leak current of the buffer layer is an important factor that affects the quality of the element.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a compound semiconductor wafer having excellent transistor characteristics in which a leakage current of a buffer layer is suppressed.

[0013]

According to the present invention, in order to achieve the above object, a plurality of epitaxial layers are stacked on a substrate, and a part or all of the plurality of epitaxial layers have a lattice constant of the substrate. Different from the lattice constant, the present invention provides a compound semiconductor wafer having a buffer layer among the plurality of epitaxial layers, wherein the buffer layer is doped with oxygen.

The effect of the present invention becomes more apparent as the difference between the lattice constants of the epitaxial layer and the substrate becomes larger. The difference is 0.
As described above, 14% is utilized, and therefore, in many cases, the present invention is applied when the lattice constant is 0.15% or more.

The doping amount of oxygen to the buffer layer is 3
It is preferable to set within the range of E16 cm ^{−3 to} 1E21 cm ⁻³ , and when the doping amount is less than this range, it is difficult to obtain a sufficient result for the effect of suppressing the leakage current due to the doping. The doping of oxygen is to give the buffer layer a predetermined level of resistance. In this sense, the doping amount exceeding 1E21 cm ^-3 becomes excessive.

As a method for doping oxygen, for example, there is a method in which a mixture of oxygen gas and argon gas is caused to act by a gas cracking cell, or a method in which a compound having insufficient oxygen removal is used as a raw material.

It is preferable that the buffer layer has a composition in which the composition gradually changes from the substrate to the channel layer. Specifically, a configuration in which the composition changes gradually and continuously, or a configuration in which a plurality of layers having different compositions are stacked can be considered.

In the latter case, the smaller the degree of change in the composition between the layers, the better. However, if the degree of change is too small, the buffer layer becomes thicker, which leads to an increase in cost and a decrease in specific resistance. It is desirable to set the composition of each layer so that the difference in lattice constant between adjacent layers is 1% or less.

[0019]

Next, an embodiment of a compound semiconductor wafer according to the present invention will be described. FIG. 1 shows a semi-insulating Ga.
The structure in which the composition is matched and an oxygen-doped InAlAs buffer layer 2 is formed on an As substrate 1 is shown. The buffer layer 2 was formed as follows.

In the MBE apparatus, the temperature of the substrate 1
Set to 400 ° C and undoped In _yAl_1-yAs y
Values are 0, 0.12, 0.23, 0.3, 0.35, 0.
Gradually increased to 4, 0.45 and 0.5, during which time
3E18cm^-3While doping oxygen on the substrate 1
By undoped InAlAs with a total thickness of 1000 nm
The buffer layer 2 was epitaxially grown.

When the sheet resistance of the grown buffer layer 2 was measured, it was 100 KΩ or more. In addition, doping of oxygen is performed using a normal gas cracking cell,
A mixed gas of oxygen and argon was used as a dope material.

FIG. 2 shows the structure of a wafer formed by growing each epitaxial layer on the base of the epitaxial wafer of FIG. 1 by MBE.
In the figure, reference numeral 3 denotes a channel layer formed on the buffer layer 2 and has a thickness of 30 nm, In _0.5 GaAs.
It is constituted by.

4 is a 2 nm thick In._0.5By AlAs
The spacer layer 5, which has a thickness of 10 nm,
5 × 10 Si¹⁸/ Cm^ThreeN-type doped at a concentration of
In _0.5A carrier supply layer composed of AlAs
Show.

6 is an undoped In film having a thickness of 20 nm._0.5A
The cap layer 7 is made of 1As,
10¹⁹/ Cm^ThreeN-type doped to a concentration of 50 nm
In _0.52 shows a contact layer formed of GaAs.
You.

FIG. 3 shows a device structure of a high electron mobility transistor (HEMT) formed by using the above epitaxial wafer.
Is etched to form a Pt gate electrode 8, a source electrode 9, and a drain electrode 10.

In the device structure configured as described above, when the voltage applied to the gate electrode 8 is set to a pinch-off voltage or less, the source electrode 9 and the drain electrode 1
When the leak current flowing between zero was measured, an excellent result of 1 pA was obtained. Obviously, this low current level does not adversely affect the transistor characteristics and guarantees the required characteristics.

In the present embodiment, the oxygen doping for growing the buffer layer 2 is performed at a doping amount of 3E18 cm ^−3.
However, when a desired resistance is obtained in the buffer layer 2, the smaller the doping amount, the better. Also,
In the present embodiment, the modulation doping structure, that is, the so-called HEMT structure is used. However, a good result can be obtained in the case of application to another field effect transistor structure having a doped channel layer.

[0028]

As described above, according to the compound semiconductor wafer of the present invention, since the buffer layer formed between the substrate and the channel layer is doped with oxygen, the leak current of the buffer layer can be suppressed. Therefore, a compound semiconductor wafer having excellent transistor characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a structure of a substrate and a buffer layer grown thereon in the embodiment of the compound semiconductor wafer according to the present invention.

FIG. 2 is an explanatory view showing a configuration of a compound semiconductor wafer according to an embodiment of the present invention in which a predetermined epitaxial layer is grown thereon based on the configuration of FIG.

FIG. 3 is an explanatory view showing a device structure of a HEMT configured using the compound semiconductor wafer of FIG. 2;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Buffer layer 3 Channel layer 4 Spacer layer 5 Carrier supply layer 6 Cap layer 7 Contact layer 8 Pt gate electrode 9 Source electrode 10 Drain electrode

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5F052 DA05 DB06 GC04 JA10 KA01 KA05 5F102 FA00 GB01 GC01 GD01 GJ05 GK04 GL04 GM04 GN04 GQ01 GR04 GR09 GT01 HC01

Claims (5)

[Claims] A plurality of epitaxial layers stacked on a substrate, wherein a lattice constant of some or all of the plurality of epitaxial layers is different from a lattice constant of the substrate, and a buffer is provided between the substrate and a channel layer. A compound semiconductor wafer comprising a layer, wherein the buffer layer is doped with oxygen. 2. A method according to claim 1, wherein said plurality of epitaxial layers have a lattice constant which is partially or entirely equal to the lattice constant of said substrate.
2. The method according to claim 1, wherein the difference is not less than 15%.
Item 10. The compound semiconductor wafer according to item 8. 3. The buffer layer according to claim 2, wherein oxygen is 3E16 cm ^-3.
2. The compound semiconductor wafer according to claim 1, wherein the compound semiconductor wafer is doped in a range of 1 to 21E21 cm- ³ . 4. A method according to claim 1, wherein said buffer layer is composed of a plurality of layers whose compositions are changed stepwise, and wherein said plurality of layers have a difference in lattice constant between adjacent layers set to 1% or less. The compound semiconductor wafer according to claim 1, wherein 5. The compound according to claim 1, wherein said substrate is made of semi-insulating GaAs, said buffer layer is made of undoped InAlAs, and said channel layer is made of InGaAs. Semiconductor wafer.