Method of producing GaAs single crystals doped with boron

GaAs single crystals doped with boron and having a lowered dislocation density are grown from a GaAs melt covered with B.sub.2 O.sub.3 melt as a liquid encapsulant. The method comprises using a crucible made of a material selected from the group consisting of PBN, AlN and Al.sub.2 O.sub.3 as a crucible for holding the GaAs melt, adding 0.25 to 0.95 atomic percent of boron to the GaAs melt under conditions such that the residual oxygen quantity is at most 5.times.10.sup.-2 mole percent to the GaAs melt, and thereby adjusting the concentration of boron in the grown crystal to 2.times.10.sup.18 to 1.times.10.sup.19 atoms per cm.sup.3. The method is applied to an LE-VB method and an LE-VGF method as well as an LEC method.

BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
This invention relates to a method of producing GaAs single crystal doped 
with boron (B) and more particularly, it is concerned with a method of 
growing GaAs crystal doped with boron from GaAs melt covered with B.sub.2 
O.sub.3 melt as a liquid encapsulant, thus lowering the dislocation 
density (etch pit density) in the grown crystal. 
2. DESCRIPTION OF THE PRIOR ART 
The important technical problem of GaAs single crystals (whose use has 
lately been developed as basic material for semiconductor lasers, GaAs IC 
or optoelectronic GaAs IC) is that it is difficult to obtain a large-sized 
dislocation-free crystal such as silicon (Si). 
GaAs has a smaller mechanical strength at a high temperature than Si. Thus, 
it is considerably difficult to produce a single crystal with a cross 
section of 5 cm.sup.2 or more and a dislocation density (EPD).ltoreq.2,000 
cm.sup.-2 on a commercial scale (see Japanese Patent application OPI No. 
18471/1976 and 18472/1976). A large-sized single crystal of the so-called 
dislocation-free (DF) class with EPD.ltoreq.100 cm.sup.-2 can be obtained 
in only GaAs crystal doped with Si in a quantity of 1.5.times.10.sup.18 to 
5.5.times.10.sup.18 atoms per cm.sup.3 using the three-temperature-zone 
horizontal Bridgman (HB) method (Japanese Patent application OPI No. 
62200/1977). 
Since the range of its use as GaAs IC is increasing of late, a large-sized, 
circular low-dislocation GaAs single crystal has eagerly been demanded and 
it is thus required to realize a low dislocation by a technique such as a 
liquid encapsulated Czochralski method (LEC method), not by the prior art 
boat growth method (HB method, etc.). 
As such a technique for realizing a low dislocation of GaAs by LEC method, 
in particular, the so-called "impurity hardening method" has been thought 
promising and is disclosed in Japanese Patent Application OPI No. 
63065/1977. This describes that a low dislocation of an object single 
crystal can be realized by incorporating in the crystal one or more 
impurities selected in such a manner that the bond energy (single bond 
energy) between the added impurity atoms and the constitutional atoms of 
the object single crystal is larger than the bond energy of the object 
crystal in a total concentration of 1.times.10.sup.-3 atom % or more. In 
the case of GaAs, 1.times.10.sup.-3 atom % corresponds to about 
4.4.times.10.sup.17 atoms per cm.sup.3. It is further described therein 
that the above described method is available for not only the LEC method 
but also other crystal growth methods such as three-temperature-zone 
method and HB method. Furthermore, phosphorus (P), aluminum (Al), oxygen 
(O) nitrogen (N), boron (B), sulfur (S) and zinc (Zn) are proposed as 
examples of the impurity to be added to GaAs. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of producing 
GaAs single crystals doped with boron. 
It is another object of the present invention to provide a process for the 
growth of GaAs single crystal with a decreased dislocation density. 
It is a further object of the present invention to provide a method of 
growing GaAs single crystal doped with boron from GaAs melt covered with 
B.sub.2 O.sub.3 melt. 
These objects can be attained by a method of producing GaAs single crystals 
doped with boron comprising growing GaAs crystal doped with boron from 
GaAs melt covered with B.sub.2 O.sub.3 melt as a liquid encapsulant and 
thus lowering the dislocation density in the grown crystal, characterized 
by using a crucible made of BN, AlN or Al.sub.2 O.sub.3 as a crucible for 
holding the GaAs melt, adding 0.25 to 0.95 atom % of boron under 
conditions such that the residual oxygen quantity is 5.times.10.sup.-2 mol 
% or less to the GaAs melt and thereby adjusting the concentration of 
boron in the grown crystal to 2.times.10.sup.18 to 1.times.10.sup.19 atoms 
per cm.sup.3.

DETAILED DESCRIPTION OF THE INVENTION 
The inventor has found that of various impurities proposed in the prior 
art, boron is particularly effective for obtaining GaAs with a low 
dislocation density by LEC method. Aluminum reacts with B.sub.2 O.sub.3 as 
a liquid encapsulant to release B as follows: 
EQU 2Al+B.sub.2 O.sub.3 =Al.sub.2 O.sub.3 +2B, (1) 
and so addition of Al is substantially equivalent to that of B. However, 
the Al.sub.2 O.sub.3 tends to be incorporated in the grown crystal and 
accordingly, it is preferable to add B. The other impurities do not have 
such an effect or have little effect since uniform doping thereof is very 
difficult, or cannot give a low dislocation density without increase of 
micro defects such as precipitates. 
Japanese Patent Application OPI No. 63065/1977 describes that a single 
crystal with EPD of 0-10 cm.sup.-2 and resistivity of 10.sup.8 .OMEGA.cm 
is obtained by growing from a GaAs polycrystal melt to which 1 atom % of B 
and 0.36 atom % of chromium (Cr) are added by the LEC method. However, 
there is no disclosure as to the material of the crucible used in the LEC 
method and the diameter of the pulled crystal is approximately 20 mm. Up 
to a diameter of about 15 mm (i.e. cross-sectional area=1.8 cm.sup.2), a 
low dislocation GaAs can be obtained without doping by the so-called 
necking-in technique. 
The inventor has hitherto proposed a Cr-doped semiinsulating GaAs crystal 
with a diameter of 50 mm and EPD.mu.10.sup.3 cm.sup.-2 at the center of 
wafer, which is doped with a suitable quantity of B using a quartz 
crucible (Japanese Patent Application OPI No. 100410/1981). However, it 
has been desired to improve the B-doping method using a crucible material 
hardly reactive with B such as PBN (e.g. pyrolytic boron nitride), AlN or 
Al.sub.2 O.sub.3 in addition to quartz. 
The added B is dissolved in the GaAs melt, reacted with oxygen (O) from the 
water (H.sub.2 O, OH) in raw materials or B.sub.2 O.sub.3 and removed as 
B.sub.2 O.sub.3 until the following reaction is equilibrated: 
EQU 2B (in GaAs melt)+30 (in GaAs melt)=B.sub.2 O.sub.3 (liquid). (2) 
Therefore, the doping quantity of B varies with the quantity of oxygen 
incorporated from raw materials and the water in the liquid encapsulant 
B.sub.2 O.sub.3, which will hereinafter be referred to as "residual oxygen 
quantity". It is further found that an effective concentration of B for 
realizing a low dislocation of GaAs crystal with a practical size, i.e. a 
diameter of at least 50 mm, is within a range of about 2.times.10.sup.18 
to 1.times.10.sup.19 atoms per cm.sup.3. The above described Japanese 
Patent Application OPI No. 63065/1977 is silent as to the effective 
segregation coefficient of B, so it is not clear how much quantity of B 
should practically be added to the raw materials. 
FIG. 1 sums up results of a series of doping tests of B in an LEC method 
using a crucible made of PBN (Pyrolytic Boron Nitride), in which Curve I 
shows the concentration of B in a grown GaAs crystal where doping of B is 
carried out using B.sub.2 O.sub.3 containing 1000 wt ppm or more of water 
due to insufficient removal of water. It is found that the mean residual 
oxygen quantity is about 5.times.10.sup.-2 mol % to GaAs raw material and 
if the residual oxygen quantity is more than this, doping of B in a 
desired value with reproducibility is difficult. On the other hand, when 
doping of B is carried out using a low water content B.sub.2 O.sub.3 
containing about 150 to 200 wt ppm of water, the B.sub.2 O.sub.3 having 
been subjected to a vacuum baking treatment at a high temperature for the 
purpose of removing water, the concentration of B in a grown GaAs crystal 
is as shown by Curve II. In this case, the mean residual oxygen quantity 
is about 6.times.10.sup. -3 mol %. Even if the baking treatment of B.sub.2 
O.sub.3 is further carried out, a doping relationship substantially 
similar to Curve II is obtained where the concentration of B in a grown 
crystal is within the aimed range of 2.times.10.sup.18 to 
1.times.10.sup.19 atoms per cm.sup.3. 
According to a series of doping tests as described above, it is found that 
in order to control the concentration of B in a crystal within a range of 
2.times.10.sup.18 -1.times.10.sup.19 atoms per cm.sup.3 with good 
reproducibility, 0.25 to 0.95 atom % of B should be added under conditions 
such that the residual oxygen quantity is 5 .times.10.sup.-2 mol % or less 
to GaAs melt. If the quantity of added B is less than 0.2 atom %, a large 
effect to realize a low dislocation of GaAs crystal with a diameter of at 
least 50 mm is not expected, while if it is more than 1 atom %, there 
occurs often constitutional supercooling and the so-called striation 
phenomenon so that a high concentration of B is locally incorporated, 
resulting in a local misfit of crystal lattice being rather a cause of 
dislocation. 
The doping relationship as shown in FIG. 1 is held similar even if a 
crucible made of a material other than PBN is used, the other material 
being a material hardly reactive with B, e.g. AlN. Similar results can 
also be obtained in the case of using a crucible of Al.sub.2 O.sub.3 
although there is some problem as to the mechanical strength. 
In a GaAs crystal containing B in the optimum concentration of 
2.times.10.sup.18 to 1.times.10.sup.19 atoms per cm.sup.3, it depends on 
the growth conditions how the dislocation is lowered. For example, it is 
described in Japanese Patent Application OPI No. 63065/1977 that the 
temperature gradient near the solid-liquid interface in LEC method should 
be 100 .degree. C./cm or less, but it is not always easy to adjust the 
temperature gradient to 100 .degree. C./cm or less in a high pressure LEC 
method. 
When the pressure of compressing nitrogen gas is about 20 atm, for example, 
a temperature gradient of 20.degree.-80 .degree.C./cm can be obtained only 
by holding the thickness of B.sub.2 O.sub.3 considerably large, i.e. 2 to 
5 cm before the growth. In this case, a grown crystal is protected by the 
thick B.sub.2 O.sub.3 melt and simultaneously, release of arsenic from the 
surface thereof through pyrolysis is prevented. For obtaining a smaller 
temperature gradient the liquid encapsulated vertical Bridgman method 
(LE-VB method) and the liquid encapsulated vertical gradient freezing 
method (LE-VGF method) are effective and, not the LEC method. According to 
these methods, a temperature gradient of 5.degree. to 20.degree. C. can be 
realized. 
In summary, the present invention provides a method of producing GaAs 
single crystals doped with boron comprising growing GaAs crystal doped 
with boron from GaAs melt covered with B.sub.2 O.sub.3 melt as a liquid 
encapsulant and thus lowering the dislocation density in the grown 
crystal, characterized by using a crucible made of a material hardly 
reactive with boron, such as BN, AlN or Al.sub.2 O.sub.3 as a crucible for 
holding the GaAs melt, adding 0.25 to 0.95 atom % of boron under 
conditions such that oxygen quantity is 5.times.10.sup.-2 mol % or less to 
the GaAs melt and thereby adjusting the concentration of boron in the 
grown crystal to 2.times.10.sup.18 to 1.times.10.sup.19 atoms per 
cm.sup.3. As additives of boron, there can be used not only elementary B 
but also boron compounds such as BAs, Ga.sub.1-x B.sub.x As (0&lt;.times.&lt;1), 
previously B-added GaAs polycrystals, and the like. The dislocation 
density in the grown GaAs crystal is in the range of 2.times.10.sup.2 to 
2.times.10.sup.3 cm.sup.2 -. 
In the present invention, in particular, the use of the liquid encapsulated 
Czochralski method (LEC method) is more effective when the thickness of 
B.sub.2 O.sub.3 melt is chosen in the range of 2-5 cm before start of the 
growth and the use of the liquid encapsulated vertical Bridgman method 
(LE-VB method) or the liquid encapsulated vertical gradient freezing 
method (LE-VGF method) results in decrease of the temperature gradient at 
the growth boundary to a great extent and in low dislocation. 
The following examples are given in order to illustrate the present 
invention in detail without limiting the same. 
EXAMPLE 1 
FIG. 2 is a cross-sectional view of a high pressure single crystal pulling 
apparatus for practicing a method of doping boron by LEC method according 
to the present invention. 
Referring to FIG. 2, in pressure vessel 1 filled with high pressure 
nitrogen (N.sub.2) gas of about 10 atm, carbon heater 2 is provided and 
carbon crucible 3 and PBN crucible 4 are mounted on lower driving shaft 5. 
Lower driving shaft 5 is movable vertically and rotatable to control 
crucibles 3 and 4 with respect to heater 2 so that an optimum temperature 
gradient may be obtained. About 2 kg of high purity GaAs polycrystal and 
0.55 atom % of B with a purity of 99.999 % were changed in PBN crucible 4. 
Adequately dehydrated low water content B.sub.2 O.sub.3 was used and 450 g 
of the B.sub.2 O.sub.3 was changed therein. In this case, the thickness of 
B.sub.2 O.sub.3 melt was about 3 cm before start of the growth. Under this 
condition, the temperature gradient measured was about 35.degree. C./cm 
near the solid-liquid interface. 
The crucible was heated at 1270.degree. C. by carbon heater 2 to form GaAs 
melt 7 under B.sub.2 O.sub.3 melt 6. Then, the temperature was gradually 
lowered to about 1250.degree. C., while single crystal seed 9 with an 
orientation &lt;100&gt;, fitted to upper driving shaft 8, was lowered with 
rotation, brought into contact with GaAs melt 7 through the layer of 
B.sub.2 O.sub.3 melt 6 and pulled at a rate of about 4-10 mm/hr with 
rotating at 3-15 times/min, thus obtaining GaAs single crystal 10 with a 
diameter of about 50 mm. 
Mass spectrometry showed that the thus resulting gallium arsenide crystal 
contained 5-6.times.10.sup.18 atoms per cm.sup.3 of boron, 
1.times.10.sup.16 atoms per cm.sup.3 or less of oxygen (less than 
detecting limit), 1.times.10.sup.15 atoms per cm.sup.3 or less of silicon 
and 5.times.10.sup.14 atoms per cm.sup.3 of chromium. The resistivity of 
the crystal was 2.times.10.sup.7 .OMEGA..cm at 300.degree. K. and after a 
heat treatment at 800.degree. C. in hydrogen gas for 30 minutes, it was 
1.times.10.sup.7 .OMEGA..cm or more. 
When (100) wafer was cut out of the substantially central part of the 
single crystal and subjected to examination of the dislocation density by 
etching using a KOH solution, the dislocation density was 2.times.10.sup.2 
to 5.times.10.sup.2 cm.sup.-2 at the center of the wafer and about 
3.times.10.sup.3 cm.sup.-2 even at the circumference of 5 mm, thus on the 
average amounting to 1400 cm.sup.-2 and the average value except 5 mm from 
the periphery being about 3.times.10.sup.2 cm.sup.-2. 
Since the doping quantity of B is largely affected by the quantity of water 
in B.sub.2 0.sub.3, doping should be carried out under such a condition 
that the residual oxygen quantity be at most 5.times.10.sup.-2 mol %, as 
illustrated above referring to FIG. 1. In addition, it is, as described 
above, desirable to hold the optimum concentration of B to be added to a 
raw material at 0.25 to 0.95 atom % and the B concentration in GaAs 
crystal grown under this condition at about 2.times.10.sup.18 to 1.times. 
.sup.19 atoms per cm.sup.3. If the B concentration is less than 2.times.10 
.sup.18 atoms per cm.sup.3, not only the effect of realizing low 
dislocation decreased but also the B concentration is varied to a great 
extent with the residual oxygen quantity and the reproducibility is 
largely deteriorated (Cf. FIG. 1). If the concentration of B to be added 
to raw material GaAs exceeds about 1 atom %, there occurs constitutional 
supercooling and the so-called striation phenomenon that a high 
concentration of B is locally incorporated resulting in a local misfit of 
crystal lattice being rather a cause of dislocation. In an extreme case, 
defects such as lineage occur in GaAs crystal, leading to formation of 
polycrystal. 
This Example is concerned with only the high resistivity GaAs doped with B 
alone, but it is obvious to those skilled in the art that the present 
invention is applicable to semi-insulating GaAs doped simultaneously with 
B and Cr, p-type GaAs doped simultaneously with B and Zn, and n-type GaAs 
doped simultaneously with B and S. If B and O are simultaneously doped, 
however, the effects of each are cancelled. 
A suitable pressure range of a nitrogen atmosphere is 2 to 20 atm in LEC 
method. 
EXAMPLE 2 
This Example is one embodiment of the present invention according to the 
liquid encapsulated vertical Bridgman method (LE-VB method). 
FIG. 3 is a cross-sectional view of a high pressure single crystal growing 
apparatus for practicing LE-VB method. The apparatus of FIG. 3 is operable 
by two modes. One (LE-VB mode) is a method wherein the whole body of a 
crucible is gradually pulled down while holding a constant temperature 
distribution and the other (LE-VGF mode) is a method wherein the whole 
temperature is gradually lowered while forming a temperature gradient 
throughout the melt. In either method, there are basically obtained 
similar effects. 
Referring to FIG. 3, in pressure vessel 11 filled with high pressure 
nitrogen gas of about 20 atm, carbon heater 12 is provided and carbon 
crucible 13 and PBN crucible 14 are mounted on lower driving shaft 15. The 
lower driving shaft is movable vertically and rotatable. About 2 kg of 
high purity GaAs polycrystal and 0.35 atom % of B with a purity of 99.999 
% were charged in PBN crucible 14. Low water content B.sub.2 O.sub.3 was 
used and about 35 g of the B.sub.2 O.sub.3 was changed therein. The 
thickness of B.sub.2 O.sub.3 16 was about 1 cm under melted state. Under 
this condition, the temperature gradient measured in GaAs melt 17 was 
5.degree.-20.degree. C./cm near insulator 18. The temperature gradient 
could be controlled by the positional relationship of crucible 14 to 
heater 12 or by the heater temperature. GaAs single crystal was grown by 
the vertical Bridgman method comprising melting the raw material in such a 
manner that upper surface 22 of GaAs single crystal seed 19 with 
crystallographic orientation &lt;111&gt; B (i.e. &lt;111&gt; As) set by BN seed holder 
21 was not melted, controlling the position of crucible 14 to melt the 
upper surface of seed 19 and pulling down the whole crucible at a rate of 
about 4 mm/hour in the direction of arrow 23. In this figure, GaAs crystal 
20 was grown upward according to PBN crucible 14. 
Mass spectrometry showed that the resulting gallium arsenide crystal 
contained 2.5-4 .times.10.sup.18 atoms per cm.sup.3 of boron, 
8.times.10.sup.14 atoms per cm.sup.3 of chromium and 
.ltoreq.1.times.10.sup.15 atoms per cm.sup.3 of silicon. The quantity of 
oxygen was less than detecting limit. Similarly to Example 1, chromium and 
silicon were unintensional impurities. The resistivity at 300.degree. K. 
was 3.times.10.sup.7 .OMEGA..cm and even after a heat treatment at 
800.degree. C. in hydrogen gas for 30 minutes, it was 1.times.10.sup.7 
.OMEGA..cm or more. 
When (111) wafer was cut out of the central part of the single crystal as 
in Example 1 and subjected to examination of the dislocation density by 
etching using an H.sub.2 SO.sub.4 /H.sub.2 O.sub.2 /H.sub.2 O solution, 
the dislocation was substantially uniformly distributed in the wafer with 
a measured value of 2.times.10.sup.2 to 1.times.10.sup.3 cm.sup.-2. 
A suitable pressure range of a nitrogen atmosphere is 2 to 60 atm B in the 
LE-VB method or the LE-VGF method . When an undoped GaAs crystal with a 
diameter of 50 mm is grown using B.sub.2 O.sub.3 melt with an ordinary 
thickness, i.e. 10-15 mm in nitrogen gas at about 10 to 20 atm by the LEC 
method, the mean dislocation density is 2.times.10.sup.4 to 
1.times.10.sup.5 cm.sup.-2 and the temperature gradient near the 
solid-liquid interface is 90.degree. 120.degree. C./cm. The dislocation 
density lowers sometimes to 5.times.10.sup.3 -1.times.10.sup.4 cm.sup.-2 
at the front portion of the single crystal and the central portion of the 
wafer, but amounts to about 1.times.10.sup.5 cm.sup.-2 in the peripheral 
region of the wafer. 
The method of the present invention is not intended to be limited to the 
scope of the foregoing Examples. 
For example, this method can be applied to another LEC method wherein GaAs 
melt is directly synthesized from elemental Ga and As as disclosed by 
AuCoin et al., "Liquid Encapsulated Compounding and Czochralski Growth of 
Semi-Insulating Gallium Arsenide", Solid State Technology, January, 1979, 
pages 59-62. This method is called the in-situ compounding LEC method or 
the direct synthesis LEC method. Therefore, this invention can be applied 
to the direct synthesis LEC method comprising subjecting raw materials 
gallium and arsenic with boron to direct synthesis of GaAs melt at a 
nitrogen gas pressure of 60 atm or more and lowering the pressure of 
nitrogen gas to a desired value of 5-30 atm and then growing GaAs crystal 
doped with boron. Moreover, if the bottom of the PBN crucible is made 
porous and arsenic is fed into melt 7 from another chamber holding 
arsenic, as shown in Akai et al., U.S. Pat. No. 3,902,860, GaAs doped with 
B can be grown with controlling the vapor pressure of arsenic. 
As apparant from the foregoing detailed description, the present invention 
provides a method of producing GaAs crystals having a large size, e.g. at 
least a diameter of 50 mm and fewer crystalline defects, e.g. dislocation 
and precipitates, and doped with boron in an optimum concentration, 
whereby it is made possible to produce an inexpensive and high quality 
single crystal substrate for semiconductor lasers, GaAs IC or new type 
GaAs IC for optoelectronics (OE) requiring a large-sized, circular low 
dislocation GaAs single crystal.