JP2701560B2 - GaAs single crystal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a semi-insulating GaAs single crystal, and more particularly to a semi-insulating GaAs wafer having a reduced leakage current.

[0002]

2. Description of the Related Art Generally, a GaAs high-speed device is GaAs.
formed on the s wafer. The active layer of the device may be formed by directly implanting ions into the wafer or by an epitaxial method.

In the case of the ion implantation method, since excessive implantation causes deterioration of crystallinity, the carrier concentration of the active layer is limited and cannot be made too high. It is desirable for the high frequency characteristics that the carrier concentration changes sharply at the interface between the active layer and the substrate. However, in the ion implantation method, there is a limit to the sharpness of the interface in principle. In this regard, the epitaxial method is advantageous, and an epitaxial growth method such as MOVPE (organic metal vapor phase epitaxy) or MBE (molecular beam epitaxy) is used to improve the performance of the device.

As a substrate for epitaxial growth, a semi-insulating GaAs wafer is usually used in order to facilitate separation between elements constituting a device. There are mainly the following two methods for producing a semi-insulating crystal for obtaining this wafer.

[0005] One is C, which forms a deep acceptor level in the crystal.
By adding r and O ₂ for forming a deep donor level, these deep levels compensate for the shallow donor level and the shallow acceptor level, respectively, to make them semi-insulating. It is represented by the horizontal Bridgman method.

Another method is to make the crystal composition excessive in As and to add a small amount of a shallow acceptor. If the crystal composition is made excess As, a deep donor called EL2 is produced. EL2 an excess As is said that a lattice defect involved, its concentration is usually 2 × 10 ¹⁶ cm ~ about ^3.
The shallow acceptor in the crystal is compensated by the shallow acceptor, and the shallow acceptor is compensated by the deep donor EL2, so that the crystal becomes semi-insulating. This crystal is usually manufactured by a liquid-sealed Czochralski method (LEC method). In the LEC method, since carbon is used for the heater, a small amount of C is mixed in the crystal. Since C becomes a shallow acceptor, a semi-insulating crystal can be obtained by the LEC method without particularly intentionally adding a shallow acceptor impurity, that is, without adding an impurity as in the horizontal Bridgman method. . For this reason, the LEC method is widely used as a method for producing a semi-insulating crystal, and the present invention is also directed to the LEC method.

[0007]

When a conventional semi-insulating substrate wafer to which Cr is not added is used, the biggest factor that deteriorates the device characteristics is the substrate leakage current caused by the wafer. The electron concentration in the substrate is certainly very low at 10 ⁷ cm ³ or less. However, during operation of the device, a strong electric field is applied to the substrate and the active layer, so that a small amount of electrons leaks from the active layer to the substrate. These electrons are
If the device element is an FET, it flows into the drain electrode or an adjacent element, which becomes noise and degrades the device characteristics. The substrate originally has a low carrier concentration,
Since there are few impurities, the mobility of electrons is very high (7000 cm ² / V · s). From this, the leaked electrons move easily.

In addition, Cr produced by the horizontal Bridgman method
In the case of an added substrate, in addition to the above-described leakage charge, Cr has a large diffusion coefficient, so that it may diffuse into the epitaxial layer and deteriorate device performance.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a novel GaAs wafer which is semi-insulating and has a small electron mobility.

[0010]

SUMMARY OF THE INVENTION The gist of the present invention is that a donor impurity forming a shallow level and an acceptor impurity forming a shallow level have a concentration of 10 ¹⁷ cm- ³ to reduce electron mobility. Add so that it becomes above. That is, the sum of the shallow impurity concentrations is intentionally set to 2 × 10 ¹⁷ or more. In addition, in order to obtain semi-insulating properties, the shallow acceptor impurities are more than the shallow donor impurities, and the difference between them is EL.
It is to be added so as to have a concentration lower than 2 concentrations.
This greatly reduces the leakage current.

More specifically, it contains one or more kinds of impurities that form a shallow donor level and one or more kinds of impurities that form a shallow acceptor level, and has an electron mobility of 1000 cm ² / V.
· S to less the sum of the impurity concentration for forming these shallow impurity level of the ^{NSD + NSA ≧ 2 × 10 17} cm ~ 3, and, in order to semi-insulating, the difference between the impurity concentration A GaAs single crystal, wherein NSD-NSA <NDD.

However, NSD is the sum of one or more shallow donor impurity concentrations, NSA is the sum of one or more shallow acceptor impurity concentrations, and NDD is the deep donor concentration.

Here, the one or more kinds of impurities serving as shallow donors may be any of Si, S, Sn, Se and Te, and the one or more kinds of impurities serving as shallow acceptors may be C, Mn, Be, Mg, Li, Zn or Cu
Either may be used.

[0014]

In the case of a semi-insulating crystal, scattering by ionized impurities has the greatest influence on electron mobility. All shallow levels are ionized, and the mobility decreases as the total amount increases. It is the concentration of the ionized impurities that determines the mobility in this way, and both the ionized shallow donor and the acceptor contribute to the mobility.

On the other hand, the high frequency characteristic is that the mobility is 1000 c
It is expected that if it is less than m ² / V · s, it will be dramatically improved.

Incidentally, the relationship between the impurity concentration and the mobility in consideration of ionic impurity scattering in GaAs has been theoretically studied, and there is a report by W. Walukiewicz et al. (JA
ppl.Phys. 53 (1982) P769). According to this, the mobility is 10
The ionic impurity concentration needs to be 2.2 × 10 ¹⁷ cm to ³ or more in order to be 00 cm ² / V · s or less. This is the total amount of acceptor impurities and donor impurities. The difference between the two is that in order to make the GaAs single crystal semi-insulating, EL2
It must be below the concentration. EL2 concentration is usually 2 × 1
0 ¹⁶ or less. For these reasons, the concentration of the shallow impurity is required to be 1 × 10 ¹⁷ cm to ³ or more for both the donor and the acceptor.

In order to make the GaAs single crystal semi-insulating, in addition to NDD> NSA-NSD as in the present invention, NDA> NSD-NSA NDA: the acceptor is controlled so as to have a deep acceptor impurity concentration. Although it is possible to use a mold, it is necessary to add a large amount of deep acceptor impurities, which is not preferable because problems such as precipitation occur.

If a donor-dominated type is used as in the present invention, E
With only L2 concentration (NDD ≥ EL2), ND> NSA-NSD
It can be.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

A GaAs single crystal of this embodiment was manufactured by the LEC method by adding Si as a donor for forming a shallow level and C as an acceptor for forming a shallow level. as a result,
A semi-insulating crystal having a mobility of 1000 cm ² / V · s or less was obtained. This will be described in detail.

In 6.1 kg of the GaAs melt, 0.4 g of Si as a shallow donor impurity and 0.5 g of C (carbon) as a shallow acceptor impurity were mixed. A 5 kg GaAs single crystal having a diameter of about 60 mm was obtained by the LEC method. A wafer for evaluation is cut out from this crystal, and the C concentration, the Si concentration,
EL2 concentration, specific resistance, carrier concentration, and mobility were measured. The C concentration was determined by measuring the absorptance near 580 cm- ¹ due to the localized vibration of C by using a Fourier spectrophotometer (FTIR) and converting it to a concentration. Conversion factor is 8
× 10 ¹⁵ cm ~ ¹ was used. The Si concentration was measured using glow discharge mass spectrometry (GDMS). The EL2 concentration was measured by a 1.0 μm light absorption method. The specific resistance is Van der
The Pau method, carrier concentration, and mobility were determined by Hall measurement.

FIGS. 1 and 2 show the solidification rate dependence of the C concentration, the Si concentration, and the specific resistance. Seed side of crystal (solidification rate 0 side)
Shows p-type semiconductivity and n-type semiconductivity on the tail side (solidification rate 1.0 side) (FIG. 1). Although the specific resistance between the seed side and the tail side is low, the crystal becomes
^It shows semi-insulating properties with a value of ⁸ Ω-cm or more (Fig. 2).

Further, as shown in FIG. 1, the carbon concentration in the portion having a solidification rate of 0.5 is 1.8 × 10 ¹⁷ cm ³ , and the Si concentration is 1.0.
7 is a × 10 ¹⁷ cm ~ ^3. EL2 concentration is 2 × 1 of normal value
It was 0 ¹⁶ cm ~ ^3. In Hall carrier concentration by measuring the 1.5 × 10 ¹⁷ cm ~ ^3, the mobility is very low 840
cm ² / V · s. This crystal shows n-type, and therefore, the Hall mobility can be considered to be almost the electron mobility, and a low-mobility semi-insulating crystal could be obtained.

As described above, according to the present embodiment, since the mobility of electrons is very low and the leaked electrons cannot be easily moved, a strong electric field is applied to the active layer during device operation. Even if a small amount of electrons leaks from the active layer to the substrate, these electrons do not flow to the drain electrode or to an adjacent element when the device is an FET, generating noise and degrading device characteristics. Deterioration can be effectively prevented. Further, since the insulation resistance of the GaAs wafer is increased to achieve stability at a deep level, the effect is great if applied to a high-speed device wafer by epitaxial growth.

Further, the LEC method is employed as in this embodiment, and only a small amount of shallow impurities need to be doped. As in the case of the horizontal Bridgman method, it is necessary to add many deep acceptor impurities having a large diffusion coefficient. As a result, device performance is not degraded.

[0026]

According to the present invention, a GaAs single crystal having reduced electron mobility can be obtained by forming a GaAs single crystal having a total shallow impurity concentration of 2 × 10 ¹⁷ cm ³ . Therefore, by using a wafer cut from the GaAs single crystal, the substrate leakage current of a high-frequency device formed by an epitaxial method can be significantly reduced. As a result, a short channel phenomenon caused by an unexpected extra leakage current flowing between the electrodes through the substrate, a side gate effect is greatly improved without particularly improving the device manufacturing method, and noise is reduced. Device characteristics such as gain can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing the solidification rate dependence of carbon concentration and Si concentration in an example of the crystal of the present invention.

FIG. 2 is a characteristic diagram showing the solidification rate dependence of each specific resistance of C and Si in an example of the crystal of the present invention.

Claims (4)

(57) [Claims] Claims 1. One or more kinds of impurities that form a shallow donor level and one or more kinds of impurities that form a shallow acceptor level, so that the electron mobility is 1000 cm ² / V · s or less. The sum of the impurity concentrations forming shallow impurity levels is NSD + NSA ≧ 2 × 10 ¹⁷ cm ³ , and the difference between these impurity concentrations is NSD−NSA <NDD in order to achieve semi-insulating properties. Characteristic GaAs single crystal. Here, NSD is the sum of one or more shallow donor impurity concentrations, NSA is the sum of one or more shallow acceptor impurity concentrations, and NDD is the deep donor concentration. 2. The GaAs single crystal according to claim 1, wherein the one or more kinds of impurities serving as shallow donors are Si, S,
GaAs characterized by being Sn, Se or Te
Single crystal. 3. The GaAs single crystal according to claim 1, wherein the impurity serving as one or more kinds of shallow acceptors is C, Mn, Be, Mg, Li, Zn, or Cu. crystal. 4. The Ga according to claim 1, wherein
A GaAs single crystal, wherein the deep donor is EL2 in the As single crystal.