JPH0788277B2 - Semi-insulating gallium arsenide single crystal - Google Patents

Description

TECHNICAL FIELD The present invention relates to a semi-insulating gallium arsenide single crystal.

[Conventional technology]

In the production of a semi-insulating gallium arsenide (GaAs) single crystal by the liquid-encapsulated Czochralski method, conventional techniques include, for example, crystal pulling using a PBN crucible without addition, or PBN crucible or Chromium (C
r) was added to pull up the crystal. In the method, carbon is the main chemical residual impurity, forming a shallow acceptor level. The C concentration is about 5 × 10 ¹⁴ to 1 × 10 ¹⁶ cm ^-3 .
This shallow acceptor forms a deep donor-type level
A semi-insulation is realized by being compensated by EL2 and by having the Fermi level near the center of the forbidden band. In the method, carbon and silicon (Si) are the main chemical residual impurities, carbon forms a shallow acceptor type level, and Si forms a shallow donor type level, as described above. When a quartz crucible is used, the Si concentration is 1 × 10 ¹⁴ to 1 × 10 ^16.
It contains about cm ^-3 . The Si concentration when using the PBN crucible is 10 ¹⁵ cm ^-3 or less. The impurities forming these shallow levels are EL2, which forms a deep donor type level as described above,
And it is compensated by an appropriate combination of Cr forming a deep acceptor type level, and the semi-insulation is realized by the Fermi level being near the center of the forbidden band.

In the conventional method of this type, for example, in the non-doped semi-insulating GaAs crystal, the carbon concentration has a distribution in the ingot growth axis direction such that the carbon concentration is large in the crystal front part and small in the crystal tail part, but the EL2 concentration is axial. Since the distribution has an almost constant distribution in the direction, the compensation ratio changes greatly in the axial direction, and as a result, the position of the Fermi level changes, and the resistivity of the crystal (ρ), mobility (μn), etc. There is a problem that the electrical characteristics change greatly in the axial direction. In addition, for example, in a Cr-added semi-insulating GaAs crystal, since the segregation coefficient of Cr itself as the compensation center is smaller than 1, the Cr concentration is small in the crystal front part and becomes large in the crystal tail part in the axial distribution. Therefore, there is a problem that the electrical characteristics of the crystal have a large distribution in the growth axis direction for the same reason as described above.

In addition, in the crystal having a Cr concentration of 1 × 10 ¹⁶ cm ^-3 or more,
When devices were created by ion implantation, the surface diffusion of Cr (a so-called pile-up phenomenon) was apt to cause thermal transformation.

[Problems to be solved by the invention]

The present invention solves the above-mentioned problems in a semi-insulating gallium arsenide single crystal containing carbon, and provides a semi-insulating gallium arsenide single crystal having excellent uniformity of electrical characteristics in the growth axis direction. It is what

[Means for solving problems]

The present invention is directed to a gallium arsenide crystal produced by the liquid-encapsulated Czochralski method in which the carbon concentration decreases from the seed portion to the tail portion of the crystal in the range of 5 × 10 ¹⁴ to 1 × 10 ¹⁶ cm ^-3. , A semi-insulating gallium arsenide single crystal containing chromium or zinc as an impurity forming an acceptor type level in a concentration range of 5 × 10 ¹⁴ to 1 × 10 ¹⁶ cm ⁻³ .

Note that Cr is an impurity that forms the acceptor type level.
Alternatively, Zn can be used, and B ₂ O ₃ can be used as the liquid sealant. As the crystal raw material, a GaAs melt may be formed by directly synthesizing metal gallium and metal arsenic in a crucible, or a GaAs polycrystal prepared beforehand may be used.

The pressure of the crystal growth atmosphere is preferably adjusted to 2 to 50 atmospheres.

[Action]

The mechanism of semi-insulation in the undoped semi-insulating GaAs crystal is as follows. Carbon (C) mixed in the GaAs melt during crystal growth is taken into the crystal when the crystal is solidified. This ratio is usually expressed by a segregation coefficient, but the segregation coefficient of C is larger than 1 because the atomic radius of C is small. That is, the C concentration in the solidified crystal tends to be high at the crystal front portion and low at the crystal tail portion. As a pollution source of C,
It is most probable that both the heater for heating and the heat insulating material for heat shield are made of graphite, but no conspicuous pollution source is known. C is mixed in the GaAs melt through a liquid sealant such as B ₂ O ₃ . As a method of controlling the degree of mixing, there are H ₂ O amount in B ₂ O _{3 and} inert gas pressure during crystal growth. In any case, if the conventional method is used, the C concentration of the crystal front part is 5 × 10 ¹⁴ to 1 × 10
¹⁶ cm ^-3 can be obtained, facing the crystal tail,
It tends to decrease. As described above, C forms a shallow acceptor level in the band gap. This shallow acceptor level is compensated by a deep donor level called EL2, and semi-insulation is achieved. The identity of EL2 is unclear, but it is true that it is not a chemical element but an intrinsic defect. The concentration is 1-2 x 10 ¹⁶ cm ^-3
There is a certain degree, and there is no large distribution from the crystal front part to the tail part, and it is almost constant. Where compensation ratio β ≡
Consider [EL2] / [C]. [EL2] and [C] represent the concentrations of EL2 and C, respectively. Although the shallow donor level concentration (N _SD ) is ignored here, β ≡ [EL2] / ([C] -N _SD ) may be used when considering this. In normal pulling, [C] >> N _SD . Now,
The electrical properties of the crystal are determined by the Fermi level E _F. When the compensation ratio β increases, the Fermi level E _F shifts to the conduction band side, and as a result, the specific resistance ρ decreases and the Hall mobility μH increases. On the contrary, when the compensation ratio β becomes small, the Fermi level E _F shifts to the valence band side, and as a result, the specific resistance ρ becomes large and the hole mobility μH becomes small. Now, considering the case of a normal non-doped semi-insulating crystal, while [EL2] is almost constant in the axial direction of the crystal as described above, [C] (or [C] -N _SD But good) is high at the front and low at the tail. That is, the compensation ratio β is small in the front part and large in the tail part. For example, in terms of resistivity, this is high in the front part and low in the tail part.

The problem is that C forming a shallow acceptor level has a large distribution in the crystal growth axis direction. That is, it is high at the front part and low at the tail part. Therefore, as a means for solving this, if an impurity element forming an acceptor level, which has a distribution tendency opposite to that of C, that is, an element having a segregation coefficient of less than 1, is intentionally added in a small amount, I know it's good. In this way, as the total amount of impurities forming the acceptor level, the difference between the crystal front part and the tail part is relaxed, and the direction is made uniform. Of course, in order to maintain semi-insulating property, C concentration,
It is necessary that the total concentration of the second acceptor levels intentionally added is smaller than the EL2 concentration. It goes without saying that the acceptor level of the second additional element has the same function whether it is shallow or deep.

〔Example〕

Comparative Example Ga-free by additive-encapsulated Czochralski method
As single crystal was pulled up. As the starting material, a GaAs polycrystal prepared by the HB method was used. The temperature of the As portion is controlled so that the composition ratio of Ga and As is 1: 1. This GaAs poly crystal 6k
g was pretreated by aqua regia etching and stored in a 6-inch crucible made of PBN. Poron oxide (B ₂ O ₃ ) having a water content of 100 wt ppm or less was used as the liquid sealant. This was placed in a high-pressure container, heated to melt, and a single crystal was pulled up using a GaAs seed crystal. Crystal rotation speed is 5rp
m and the crucible rotation speed were 20 rpm, and they were rotated in opposite directions. The pulling speed was 8 mm / H, and a crystal of 84 ± 4 mmφ was prepared using an automatic diameter control device. The obtained crystal is a single crystal except for a part of the tail part, and the length of the straight body part is 170 m.
It was m. The EL2 concentration, C concentration, and specific resistance of this crystal were measured. The EL2 concentration was determined by an infrared absorption method with a wavelength of 1.0 μm, the C concentration was determined by a far infrared absorption method (FTIR method), and the specific resistance was determined by a Van der Pauw type four-terminal method. The results are shown in FIGS. 3 and 4. The EL2 concentration is approximately 1.3 × 10 ¹⁶ cm ^-3 over the entire length of the ingot. The C concentration is high at the crystal front portion and is 7 × 10 ¹⁵ cm ⁻³ , whereas it is 2 × 10 ¹⁵ cm ⁻³ at the crystal tail portion. As a result, the electrical resistivity is 6.0 × 10 ^{7 in the} axial direction of the ingot as shown in FIG.
→ It can be seen that there is a large change of 1.5 × 10 ⁷ Ω · cm.
The concentration of Si was determined by SIMS method, but all were below the detection limit.

Example Under these growth conditions, Cr was doped as the second acceptor type impurity. The concentration is 2 at the front of the crystal.
It was designed to be × 10 ¹⁵ cm ^-3 . Other crystal growth conditions are
All were the same as the comparative example. The obtained crystals were evaluated in the same manner as the comparative example. The Cr concentration was determined by the GFA method.
The results are shown in FIGS. 1 and 2. The segregation coefficient of Cr in the GaAs crystal is 5.9 × 10 ^-4 , which is very small compared to 1, so it is almost 1 / (1
Increase in proportion to -g). Here, g represents the solidification rate. It was found that the chromium concentration in the crystal obtained by the GFA method was almost as designed as shown in FIG. FIG. 2 shows the distribution of electrical resistivity in the axial direction, but it can be seen that it is almost constant in the axial direction.

Example of use Since the distribution of electrical characteristics in the ingot growth axis direction is made uniform, it is easily expected that the axial distribution of characteristics of various devices made using this substrate will also be made uniform. The following experiments were conducted to demonstrate this. As an example that the substrate property is best reflected in the device characteristics,
There is Vth (threshold voltage) of FET (field effect transistor) created by the ion implantation method. This is because the active layer is directly formed inside the substrate surface by the ion implantation method, so that the substrate characteristics directly influence. The above-mentioned two semi-insulating GaAs crystals, that is, one is a crystal prepared without addition, and the other one is a crystal prepared by intentionally doping a small amount of Cr. Create a FET with
The th was measured. Ion implantation conditions are E = 120 Kev, Φ = 1.5
²⁸ Si was implanted at × 10 ¹² cm ^-2 . Annealing for activation was performed at 820 ° C. for 15 minutes in an arsine atmosphere with the surfaces facing each other. The FET was formed on the entire surface of the wafer with a pitch of 200 μm. The gate length is Lg = to prevent the short channel effect.
The distance was 2 μm, and the distance between the source and drain was 5 μm. Results are shown in FIG. In the case of additive-free semi-insulating crystal,
It can be seen that Vth shifts to the negative side (ON shift) because the C concentration decreases from the front to the tail. On the other hand, in a semi-insulating crystal with a small amount of Cr added, there is no such shift in Vth, and it shifts slightly to the positive side from the front to the tail (OFF shift), but the shift is extremely larger than in the case without addition. Can be said to be small. Converting the Vth difference between the front and tail ΔVth per wafer, it is 4 mV / wafer when there is no additive, whereas it is 0.7 mV / wafer when a small amount of Cr is doped. . By the way, one of the preconditions for GaAs ICs to be produced on an industrial scale is that ΔVth is required to be 1 mV / piece or less, but it is clear that the results this time pass this criterion.

On the other hand, the variation of Vth within the wafer surface is an important factor in determining the scale of GaAs IC integration. When the variation of Vth is expressed by σVth (standard deviation), σVth is about 25 to 100 mV in the non-added crystal, whereas it is very small.
When Cr was doped, σVth was 10 to 30 mV. GaAs
As one guideline for realizing LSI, "σVth is 30 mV
It is said that it is smaller than this ", but this time it is clear that this result clears that standard. However,
At present, it is not clear what causes σVth to improve.

〔The invention's effect〕

The present invention, by adopting the above configuration, with respect to the carbon-containing semi-insulating gallium arsenide single crystal, by doping a small amount of Cr or the like, it is possible to improve the uniformity of the electrical characteristics in the axial direction, We have found that it is effective when used as a substrate for GaAs ICs.

[Brief description of drawings]

Figure 1 shows EL2, carbon (C), chromium (C) in the growth axis direction in a semi-insulating GaAs crystal prepared by adding a trace amount of Cr.
r) Concentration distribution graph, Fig. 2 is a graph showing the electrical resistivity (ρ) of the crystal shown in Fig. 1 in the growth axis direction, and Fig. 3 is a conventional semi-insulating material prepared without any addition. Graph of concentration distribution of EL2 and carbon (C) in the growth axis direction in GaAs crystal, FIG. 4 is graph of distribution of electric resistivity (ρ) of crystal shown in FIG. 3 in growth axis direction, FIG. FIG. 4 is a graph of the Vth (threshold voltage) distribution of the FET in the growth axis direction produced by the ion implantation method.

Claims (1)

[Claims] 1. A gallium arsenide crystal produced by the liquid-encapsulated Czochralski method, in which the carbon concentration decreases in the range of 5 × 10 ¹⁴ to 1 × 10 ¹⁶ cm ^-3 from the seed portion to the tail portion of the crystal. On the other hand, a semi-insulating gallium arsenide single crystal containing chromium or zinc as an impurity forming an acceptor type level in a concentration range of 5 × 10 ¹⁴ to 1 × 10 ¹⁶ cm ^-3 .