JPS6122855B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an impurity diffusion method that can improve diffusion depth, controllability and selectivity of diffusion layer concentration when impurities are diffused into a material to be diffused.

Diffusion of impurities is important in the semiconductor industry and is an important process that forms many manufacturing steps. In other words, currently, many of the p-n junctions in semiconductor devices and integrated circuits are made using an impurity diffusion process, and the diffusion process is said to be the technology that made planar structures possible and promised the development of integrated circuits. It is no exaggeration.

However, the diffusion process always carries the risk of thermally deteriorating the material to be diffused. For example, GaAs, GaP, InP, etc. or their mixed crystals
When diffusing impurities into compound semiconductor materials such as AlGaAs and InGaP in an impurity vapor atmosphere,
If the partial pressure of the constituent elements of the base material necessary to maintain thermal equilibrium with the base material of the diffused material is not added to the atmosphere surrounding these materials to be diffused, some of the constituent elements of the base material will be covered by the material to be diffused. It comes out from the surface of the diffusion material,
A large amount of so-called stoichiometric defects, ie, atomic vacancies, etc., occur in the material to be diffused. However, in many cases, these stoichiometric defects significantly reduce carrier mobility, which is a fundamental property of the material to be diffused, so even in devices fabricated using such materials, stoichiometric defects This not only significantly deteriorates the performance of the device, but also causes a reduction in the life of the device. For example, it is now clear that stoichiometric defects in GaP crystals are a significant factor that significantly reduces the efficiency of luminescence from GaP crystals. The fact that stoichiometric defects are an important factor that determines the properties of materials has recently begun to attract attention in the field of compound semiconductors. , ZnS, and other - group compounds have been studied for a long time.
Due to its basic properties, GaAs is a material that is being used to produce many practical devices such as infrared light emitting diodes, pn junction injection lasers, Gunn diodes, field effect transistors, and light receiving devices. However, since it is a compound semiconductor, it is also a material in which stoichiometric defects can be introduced very easily through careless heat treatment processes. Therefore, when explaining this invention, GaAs will be used as the material to be diffused,
Zinc will be taken as an example of an impurity element serving as a diffusion source.

An extremely common and practically important representative example of the heat treatment process for GaAs is the diffusion of zinc (hereinafter referred to as Zn), which is a diffusion source for P-type impurities. Zn diffusion is essential for manufacturing GaAs light-emitting diodes and planar striped double heterojunction laser diodes.

When diffusing Zn into GaAs, the diffusion material
The atmosphere surrounding GaAs is basically Ga, As,
If this is not carried out in a state of thermal equilibrium with the GaAs crystal, which is composed of a Zn system, Ga and As, which are constituent elements of the base material of the material to be diffused, will scatter from the GaAs surface of the material to be diffused, causing stoichiometric defects near the surface. A new area is generated. As mentioned above, when Zn is diffused into GaAs under a vapor pressure that is out of thermal equilibrium with GaAs,
It is well known that even the surface condition of the material is significantly damaged and the flatness of the diffusion front becomes poor.

As a method of Zn diffusion into GaAs with good reproducibility, conventionally, as a Zn diffusion source, for example, Ga5%,
This was achieved by using an alloy synthesized with a composition of 45% Zn and 50% As. When this alloy is used as a Zn diffusion source, the alloy will be in thermal equilibrium with GaAs.
Since As and Ga are supplied together with Zn into the atmosphere surrounding GaAs, it is possible to prevent the occurrence of large surface conditions of GaAs and stoichiometric defects within GaAs, and to perform well-controlled Zn diffusion. Ta. However, the above-mentioned ternary diffusion source alloy consisting of GaAs and Zn is synthesized in a sealed tube at high temperature (approximately 1100°C) and under high pressure, which poses risks and contamination from the sealed tube material and crucible material. It is difficult to be a good diffusion source from the viewpoint of purity. Also, this
When the above-mentioned alloy consisting of Ga, As, and Zn is used as a diffusion source, the partial pressure of As at the equilibrium vapor pressure during diffusion is high, and the Zn concentration in the diffusion layer becomes extremely high. Using this ternary alloy as a diffusion source,
When Zn is diffused into GaAs, the Zn concentration on the GaAs surface is approximately 2.4×10 ²⁰ cm ^-3 . Also, this
Zn concentration is 1.6×10 ²⁰ cm ^-3 at 650℃ and 1.2 at 600℃
×10 ²⁰ cm ^-3 , which lowers the diffusion temperature and maintains an extremely high surface concentration. On the other hand, in order to perform Zn diffusion with a Zn concentration of 10 ¹⁰ cm ^-3 or lower, a system consisting of Zn and Ga is usually used as a diffusion source. However, in closed tube diffusion using a system consisting of Zn and Ga as a diffusion source, extremely accurate
It is necessary to weigh a small amount of Ga and Zn to match the volume inside the closed tube.
The temperature is below 30°C, the diffusion source is easily contaminated with oxygen in the air, and handling is extremely inconvenient. Furthermore, since quartz, which is commonly used as a sealing tube material, is easily compatible with and reacts with Ga alloys at high temperatures, contamination from the sealing tube material occurs during the heat treatment process. Also Ga and Zn
In the case of diffusion using a diffusion source consisting of
A high temperature of ℃ or higher is required. Therefore, by diffusion using a diffusion source made of Ga and Zn, it is difficult to perform Zn diffusion with good reproducibility in terms of surface condition, diffusion depth, diffusion concentration, etc. after diffusion.

Another method conventionally used for Zn diffusion into GaAs is to form a Zn-doped SiO ₂ film on GaAs, which is _the material to be diffused.
There is also a way to spread it. However, the diffusion temperature is about 900
In addition to requiring high temperatures of ℃ or higher, there is no Ga in the SiO ₂ film.
diffuses, resulting in the generation of stoichiometric defects in GaAs. In addition, SiO ₂ film can be used to remove Zn vapor from Zn vapor.
There is a method of diffusing Zn into GaAs, but it is difficult to perform well-controlled diffusion because factors such as the SiO ₂ film thickness are involved in determining the diffusion depth and concentration. There is also a method of using pure Zn metal as a diffusion source. However, when pure Zn metal is used as a diffusion source, the surface of GaAs, which is the material to be diffused, is exposed to Zn.
Due to the reaction with steam, the surface condition is severely damaged. In order to prevent this change in the surface state, the usual method is to prepare powdered GaAs in addition to pure Zn metal as a Zn diffusion source. By placing powdered GaAs in a quartz ampoule, the surface area is much larger than that of the GaAs material to be diffused.
The surface of GaAs is made on the diffusion source side, and the vapor pressure of As and Ga is released from the powdered GaAs when the diffusion temperature rises.
In addition, since Zn vapor pressure can be supplied from molten Zn to powdered GaAs, GaAs, which is the material to be diffused,
Can prevent surface changes. but powdery
To prepare GaAs, it is necessary to crush it by grinding it in a mortar or the like, which is very dangerous as it creates dust. Furthermore, when preparing powdered GaAs in a quartz ampoule, extremely careful handling is required since GaAs powder may adhere to the walls of the quartz tube. Further, when the particle size of the powdered GaAs powder is increased, it is not possible to prevent the surface of GaAs, which is a material to be diffused, from being disturbed without increasing the amount of powdered GaAs. Furthermore, by turning GaAs into powder, a decrease in purity will inevitably occur, and it is thought that the surface of powdered GaAs will adsorb a huge amount of air gas, etc. Moreover, since the atmospheric gas pressure of Zn, As, and Ga is determined independently from the time of temperature rise until thermal equilibrium is reached with atmospheric gas vapor inside the ampoule, controllability of diffusion is not sufficient. Furthermore, Zn diffusion in an open tube requires precise control of the Zn vapor pressure on the surface of the material to be diffused, and
In order to prevent thermal deterioration of GaAs,
It is difficult to achieve reproducible Zn diffusion because it requires precise control of the Ga and As vapor pressures above, and the vapors of Ga, As, and Zn must be continuously flowed into the reaction tube during Zn diffusion. It is. In addition, this type of open tube method produces a large amount of Ga, As, and
This is not always a preferable method from the standpoint of preventing pollution since Zn vapor is emitted.

An object of the present invention is to provide a diffusion method for diffusing impurities in a manner that eliminates the above-mentioned drawbacks of conventional methods and prevents stoichiometric defects from occurring.

The present invention deals with the diffusion of impurities into a compound semiconductor material from an impurity element or a compound of an impurity element formed on the same base material as the material to be diffused, as a diffusion source placed separately from the compound semiconductor material to be diffused. In a closed system impurity diffusion method using a multi-layer or single-layer thin film containing at least a layer of This is an impurity diffusion method characterized in that the amount is set to obtain a vapor pressure lower than the thermal equilibrium vapor pressure of the element in the coexistence state of the element. This method prevents thermal deterioration of the material to be diffused, has extremely good reproducibility of the diffusion depth and concentration, and allows the diffusion concentration to be arbitrarily selected within a wide range. It is possible to achieve diffusion of impurities without destroying the stoichiometric ratio.

Hereinafter, a method for diffusing Zn into GaAs will be described in detail as an embodiment of the present invention using the drawings.

Figure 1 shows the diffusion of GaAs, which is the material to be diffused in the present invention.
FIG. 3 is a cross-sectional view of a diffusion source when applied to Zn diffusion. 11 is a GaAs crystal wafer, and 12 is a Zn thin film layer formed by vapor deposition or the like. GaAs crystal wafer 11 (in this case, an undoped single crystal
GaAs was used. ) was mirror-polished with diamond paste and then etched for 30 seconds at 90° C. with a solution containing H ₂ SO ₄ , H ₂ O ₂ and H ₂ O in a volume ratio of 3:1:1. The thickness of the GaAs crystal wafer 11 was set to 400 μm. next
The GaAs wafer 11 was placed in a vapor deposition apparatus, and Zn was vapor-deposited to a thickness of about 1 μm. Thus, a diffusion source can be easily made with the structure shown in FIG.

Next, as is generally done, GaAs, which is the material to be diffused, and the diffusion source shown in FIG. 1 are sealed in a closed cylindrical quartz ampoule in a vacuum. FIG. 2 is a cross-sectional view of the quartz closed tube ampoule made in this way, where 21 is a quartz ampoule, 22 is a diffusion source housing part of the quartz ampoule, 23 is an enclosure part when the quartz ampoule is sealed in vacuum, and 24 is a diffusion source. , 25 indicates GaAs which is the material to be diffused. The internal volume of the quartz ampoule 21 shown in FIG. 2 is approximately 5 ml. The quartz ampoule 21 manufactured in this way as shown in Figure 2 was inserted into an electric furnace with a temperature uniformity of about ±1°C along the length of the ampoule (approximately 80 mm), and Zn was diffused at 566°C. , the effective diffusion coefficient D varies depending on the amount of the diffusion source 24 as shown in FIG. In Fig. 3, the horizontal axis is the grain size of the 400 μm thick GaAs crystal wafer 11 on which Zn is deposited, that is, the amount W of the diffusion source.
(in milligrams), and the vertical axis represents the effective diffusion coefficient D. The effective diffusion coefficient D is the diffusion depth
If Xj (units of microns) and diffusion time are expressed by t (units of time), it is defined as Xj2/t. According to FIG. 3, it can be seen that the effective diffusion coefficient D changes depending on the amount of the diffusion source. FIG. 4 shows changes in the surface concentration Ns of the diffusion layer formed in the material to be diffused with respect to the amount W (milligrams) of the diffusion source corresponding to FIG.
That is, by changing the amount W of the diffusion source,
The surface concentration Ns can be varied from 10 ¹⁸ cm ^-3 to 3×10 ¹⁹ cm ^-3 . When the Zn thin film layer 12 formed in close contact with the GaAs crystal wafer 11 as shown in FIG. and As are melted, and the vapor pressure of Zn, Ga, and As in the ampoule is mainly supplied from the Zn thin film layer 12 in which Ga and As are melted. Fused Zn with Ga and As
The decomposition pressure of the solution is lower than that of GaAs, so the
The thermal equilibrium vapor pressure of Ga, As, and Zn is mostly due to diffusion source 2
GaAs2, which is the material to be diffused, is supplied from 4.
Zn diffusion is performed without damaging the surface of 5. By preparing the diffusion source 24 so that even if the thickness of the Zn vapor deposited film 12 is varied from 0.2 to 3 μm, the total amount of Zn in an ampoule with an internal volume of 5 ml is less than about 5 μg at a diffusion temperature of 566° C. Diffusion rate and concentration can be precisely controlled. Zn
The thickness of the thin film layer 12 is desirably 10 μm or less in order for Ga and As to melt quickly into the Zn thin film layer 12. In addition, the amount of diffusion source is
It is necessary that the total amount of Zn in the thin film layer 12 is such that a vapor pressure less than the saturated vapor pressure P of pure Zn can be obtained at the diffusion temperature. That is, if the diffusion temperature is T and the saturated vapor pressure of Zn at this temperature is P atmospheric pressure, then the upper limit Mmax of the total amount m (grams) of Zn is determined by the following equation.

Mmax=PVM/RT where V is the ampoule internal volume, M is the atomic weight of Zn, and R is the gas constant 8.2×10 ^-2・atom/
It is degomol. The saturated vapor pressure of pure Zn is
The metallurgical thermometer of Kubasiewski et al.
Chemistry (Pergamon Press) 4th edition was used as a reference. Even if Au is further applied by vapor deposition to a thickness of about 0.3 μm on the Zn thin film 12 of the diffusion source 24 to prevent oxidation of Zn, Zn diffusion is performed in the same way regardless of whether Au is present or not.

The embodiments of this invention have been described above for the case of Zn diffusion into GaAs, but as mentioned earlier,
Similar effects can be achieved for a wide range of diffused materials such as compound semiconductors such as GaP and InGaP, compounds such as alkali halides, and - group compounds by providing a diffusion source suitable for the diffused material. .

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining an embodiment of the present invention.
FIG. 2 is a cross-sectional view of a diffusion source used when diffusing Zn into GaAs, and FIG. 2 is a cross-sectional view of a quartz ampoule showing the configuration of a closed tube ampoule used during diffusion. 11
12 is a GaAs wafer, 12 is a Zn formed by vapor deposition, etc.
layer, 21 is a quartz ampoule, 22 is a diffusion source storage part,
23 is a quartz ampoule enclosure, 24 is a diffusion source, 25
is the diffused material GaAs. In both Figures 3 and 4, 400 μm is plotted on the horizontal axis.
Figure 3 shows the effective diffusion coefficient D (=Xj2/t) on the vertical axis, and the vertical axis in Figure 4 shows the amount of diffusion source obtained by depositing 1 μm of Zn on a GaAs crystal wafer. The Zn concentration Ns on the surface of the formed diffusion layer is shown.

Claims (1)

[Claims] 1 As a method for diffusing impurities into a compound semiconductor material, at least a layer consisting of an impurity element or a compound of an impurity element formed on the same base material as the material to be diffused is used as a diffusion source placed separately from the compound semiconductor material to be diffused. In a closed system impurity diffusion method using a multi-layered or single-layered thin film, the amount of the impurity element that is the diffusion source is determined at the temperature of the diffusion source, and the coexistence state of only the element and the material to be diffused is determined. A method for diffusing impurities, characterized in that the amount is such that a vapor pressure less than the thermal equilibrium vapor of the element is obtained.