Process for Homogenizing Compound Semiconductor Single Crystal in Properties

This invention relates to a process for homogenizing a compound semiconductor single crystal in various properties such as electrical, electronic and optical properties.

Semiconductor substrate crystals produced by a liquid encapsulated Gzochralski (LEC) method or a horizontal Bridgman method usually have various properties such as electrical properties heterogenously distributed in (100) plane used in integrated circuits. In order to homogenize these properties, there have been proposed various processes.

For example, in the case of a gallium arsenide (GaAs) single crystal, since arsenic (As) is released from the surface of an ingot or wafer, a heat treatment is carried out in an arsenic atmosphere in order to prevent the release of the arsenic (e.g. Japanese Journal of Applied Physics vol. 8, No. 8, p. 1000, 1969). But, according to this process, it is difficult to control the arsenic pressure and to handle poisons.

It is an object of the present invention to provide a process for homogenizing various properties of a compound semiconductor single crystal effectively by an easy heat treatment.

The present invention provide a process for homogenizing a compound semiconductor single crystal in properties which comprises

covering a compound semiconductor single crystal with a powder which reacts with the compound semiconductor single crystal upon heating and forms a film of the reaction product on the surface of the compound semiconductor single crystal, and

heating the powder-covered compound semi-conductor single crystal, followed by cooling.

FIGS. 1 and 2 are plane views, partially cross-sectional, of quartz ampules illustrating the process of the present invention.

FIG. 3 is a cross-sectional view of an apparatus used for the process of the present invention.

As the compound semiconductor single crystal, there can be used a single crystal of GaAs, GaP, InP, CdTe, or the like, obtained by a conventional method such as a liquid encapsulated Czochralski method, a horizontal Bridgman method, etc.

As the powder which reacts with the compound semiconductor single crystal upon heating and forms a film of the reaction product on the surface of the compound semiconductor single crystal, there can be used metal oxide powders. From the viewpoint of preventing contamination of the compound semiconductor single crystal with impurities, the use of oxides of the same elements as those constituting the compound semiconductor single crystal.

For examples, there can be used powders of Ga.sub.2 O.sub.3, SiO.sub.2, etc. in the case of single crystals of GaP and InP; powders of SiO.sub.2, CdO, ZnO, etc. in the case of single crystal of CdTe; and Ga.sub.2 O.sub.3, SiO.sub.2, Al.sub.2 O.sub.3, etc. in the case of single crystal of GaAs.

It is preferable to use metal oxide powders having a small particle size, such as 50 to 0.5 .mu.m. These metal oxide powders are used so as to have good contact with the compound semiconductor single crystal in the form of wafer or ingot with a sufficient amount to cover the wafer or ingot and to make contact with air as small as possible. As a result, release of arsenic (As) or the like can be prevented.

The heat treatment of compound semiconductor single crystal can preferably be carried out under vacuum. It is possible to carry out the heat treatment in a stream of air, an inert gas such as orgon or nitrogen gas, etc., or under a reduced pressure (e.g. 1 to 10 pascals).

The heat treatment of heating and cooling can be carried out repeating heating and cooling several times. The heating temperature is usually 1200.degree. to 240.degree. C., preferably 1100.degree. to 500.degree. C., more preferably 900.degree. to 650.degree. C., and selected properly depending on purposes.

Heating time is usually 1 to 72 hours and selected properly depending on purposes.

In the heat treatment, it is sufficient when the compound semiconductor single crystal is covered with a powder at least in the temperature range wherein vaporization of single crystal-constituting elements takes place. Covering of the single crystal can be attained, for example, by inserting a single crystal in a powder or by forming a pellet by compressing a powder-covered wafer or ingot.

The process of the present invention is explained referring to the drawings. In FIG. 1, a single crystal wafer 3 is placed in a quartz ampule 1, which is then filled with a metal oxide powder 2 and vacuum sealed. The ampule is subjected to a heat treatment of heating and rapid cooling. In FIG. 2, a pellet 4 obtained by compressing a whole of a wafer 3 covered with a metal oxide is placed in a quartz ampule 1, which is further filled with a metal oxide powder 2 and vacuum sealed. By this, contact of the wafer with gas phase (air, Ar, N.sub.2, etc.) can further be prevented.

The effect of the present invention can further be improved by making the volume of gas phase in the ampule as small as possible.

It is possible to use a single crystal ingot in place of the wafer. This is advantageous from the industrial point of view.

FIG. 3 shows another example for carrying out the present invention. In FIG. 3, a single crystal ingot 6 is placed in a quartz petri dish 5 together with a metal oxide powder 7 so as to cover the ingot. Then, a lid 8 made of quartz is placed thereon. The resulting quartz petri dish is placed in a quartz tube 9 (inner diameter 10 cm) and subjected to heat treatment under a reduced pressure (e.g. 1 to 10 pascals). Numeral 10 denotes an electric furnace for heating.

In the present invention, it seems that some surface reaction takes place during the heat treatment by covering the compound semiconductor single crystal with a special powder to cover the single crystal with a thin film, which results in preventing the single crystal from release of arsenic, etc.

This invention is illustrated by way of the following Examples.

A whole of GaAs single crystal wafer cut into 6 mm long, 6 mm wide and 0.5 mm thick was covered with a Ga.sub.2 O.sub.3 powder (average particle size 1.2 .mu.m). The wafer had been etched on the surface in about 10 .mu.m with an etchant of H.sub.2 SO.sub.4 H.sub.2 O.sub.2 H.sub.2 O=3:1:1. The powder-covered wafer was placed in a quartz ampule as shown in FIG. 1 and vacuum sealed. After heating at 677.degree. C. for 46 hours, the ampule was rapidly cooled. The wafer was taken out of the ampule. The wafer was covered with a thin (several .mu.m) amorphous film and gallium oxide powder was retained therearound. The amorphous film was composed of gallium oxide and an amorphous substance, when measured by an X-ray.

After etching the heat treated wafer in 50 .mu.m using the same etchant as mentioned above, electrical properties were measured. By the heat treatment, the resistivity was changed from 3.4.times.10.sup.8 .OMEGA..cm to 2.1.times.10.sup.8 .OMEGA..cm, the carrier concentration was changed from 6.times.10.sup.6 cm.sup.-3 to 1.times.10.sup.7 cm.sup.-3, and the mobility was retained at 3200 cm.sup.2 /v s. (v.s.=volt. sec.).

For comparison, when the wafer was not covered with the powder, the resistivity became 1.times.10.sup.4 .omega..cm, the carrier concentration became 2.times.10.sup.14 cm.sup.-3, and the mobility became 2 cm.sup.2 /v.s. This means that arsenic is clearly released from the wafer surface.

Therefore, the present invention is effective for preventing the release of As from the single crystal surface.

A whole of GaAs single crystal wafer cut into 6 mm long, 6 mm wide and 0.7 mm thick was covered with silicon oxide powder (average particle size 1 .mu.m). The wafer had been etched on the surface in about 10 .mu.m with an etchant of H.sub.2 SO.sub.4 : H.sub.2 O.sub.2 : H.sub.2 O=3:1:1. The powder-covered wafer was placed in a quartz ampule as shown in FIG. 1 and vacuum sealed. After heating at 677.degree. C. for 24 hours, the ampule was rapidly cooled. The wafer was taken out of the ampule. The wafer was covered with a thin (several .mu.m) amorphous film and silicon oxide powder was retained therearound. The amorphous film was composed of gallium oxide and an amorphous substance, when measured by an X-ray diffractometry.

After the heat treatment, the wafer was washed sufficiently and subjected to measurement of electrical properties. By the heat treatment, the resistivity was changed from 3.4.times.10.sup.8 .OMEGA..cm to 2.5.times.10.sup.8 .omega..cm; the carrier concentration was changed from 6.times.10.sup.8 cm.sup.-3 to 1.times.10.sup.7 cm.sup.-3, and the mobility was changed from 3200 to 2600 cm.sup.2 /v.s.

For comparison, when the wafer was not covered with the powder, the resistivity became 1.times.10.sup.4 .OMEGA..cm, the carrier concentration became 2.times.10.sup.14 cm.sup.-3, and the mobility became 2 cm.sup.2 /v.s. This means that arsenic is clearly released from the wafer surface.

Therefore, the present invention is effective for preventing the release of arsenic from the single crystal surface.

A whole of GaAs single crystal cut into 6 mm long, 6 mm wide and 0.7 mm thick was covered with aluminum oxide powder (average particle size 5 .mu.m). The wafer had been etched on the surface in about 10 .mu.m with an etchant of H.sub.2 SO.sub.4 : H.sub.2 O.sub.2 : H.sup.2 O=3:1:1. The powder-covered wafer was placed in a quartz ampule as shown in FIG. 1 and vacuum sealed. After heating at 677.degree. C. for 24 hours, the ampule was rapidly cooled. The wafer was taken out of the ampule. The wafer was covered with a thin film (several .mu.m) amorphous film and aluminum oxide powder was retained therearound. The amorphous film was composed of gallium oxide and an amorphous substance, when measured by an X-ray diffractometry.

After the heat treatment, the wafer was washed sufficiently and subjected to measurement of electrical properties. By the heat treatment, the resistivity was changed from 3.4.times.10.sup.8 .OMEGA..cm to 1.times.10.sup.6 .OMEGA..cm; the carrier concentration was changed from 6.times.10 cm.sup.-3 to 8.times.10.sup.10 cm.sup.-3, and the mobility was changed from 3200 to 80 cm.sup.2 /v.s.

For comparison, when the wafer was not covered with the powder, the resistivity became 1.times.10.sup.4 .OMEGA..cm, the carrier concentration became 2.times.10.sup.14 cm.sup.-3, and the mobility became 2 cm.sup.2 /v s. This means that arsenic is clearly released from the wafer surface.

Therefore, the present invention is effective for preventing the release of arsenic from the single crystal surface.

As mentioned above, various properties of compound semiconductor single crystals, e.g. electrical, electronic and optical peoperties, can be homogenized effectively by an easy procedure.