Process for producing single-crystal bulk zinc selenide

When producing a single-crystal bulk ZnSe from a melt by a high-pressure melt technique in a vertical Bridgman (VB) furnace or a vertical gradient freezing (VGF) furnace, preliminarily grown polycrystalline ZnSe (which may be a crystal solely composed of twins) is used as a seed and, after melting the starting ZnSe material and part of the seed, a twin-free ZnSe bulk crystal is grown on the seed; alternatively, polycrystalline ZnSe is grown at the tip of the growing crystal and part of it is melted, followed by growing a single crystal on that polycrystal to produce a twin-free, high-purity ZnSe bulk crystal. In either way, the process assures that twin-free single crystals of bulk ZnSe can be produced with good reproducibility without adding dopants or using any materials that are difficult to obtain.

BACKGROUND OF THE INVENTION 
This invention relates to a process for producing twin-free single-crystal 
bulk zinc selenide (ZnSe) suitable for use as a substrate for fabricating 
a blue-emitting semiconductor laser or LED by epitaxial growth of ZnSe 
compound in thin film. 
Various methods are currently used to produce single-crystal bulk ZnSe from 
a melt and representative examples include: (1) an ordinary high-pressure 
melt process that uses a crucible having a fine tube in the growth start 
region as described in J. Cryst. Growth, 41 (1977) 103-108; (2) a process 
that performs growth in a Zn atmosphere as described in J. Cryst. Growth, 
98 (1989) 302-308; (3) a high-pressure melt process that uses a crucible 
having a cone angle of 30 degrees as-described in J. Cryst. Growth, 117 
(1992) 75-79; (4) a method that uses a "sealed hot-walled system" and a 
seed crystal as described in J. Cryst. Growth, 117 (1992) 80-84; and (5) a 
process that performs doping as described in J. Cryst. Growth, 86 (1988) 
132-137. 
In the production of single-crystal bulk ZnSe, the avoidance of twin 
formation is one of the major objectives to be attained. However, 
processes (1) and (2) do not take the formation of twins into account and 
only processes (3), (4) and (5) have succeeded in the growth of twin-free 
single-crystal bulk ZnSe. Nevertheless, process (3) is not very different 
from the ordinary high-pressure process and, what is more, the 
reproducibility of results is not taken into account. In process (4), a 
twin-free single crystal (having a smaller diameter than the 
constant-diameter portion of the growing crystal) is used as a seed; on 
the other hand, manganese (Mn) which may be considered as an impurity is 
added in process (5). Thus, the conventional processes under consideration 
have suffered from the following drawbacks: twins are formed in processes 
(1) and (2); process (3) does not have any means to prevent twin formation 
and the reproducibility of its results is not clearly established; (4) 
process involves great difficulty in obtaining a twin-free single crystal 
useful as a seed; and the effect that would be caused by the dopant Mn on 
the substrate characteristics is not clearly known with process (5). 
SUMMARY OF THE INVENTION 
An object, therefore, of the present invention is to provide a process by 
which twin-free single-crystal bulk ZnSe can be produced from a melt with 
good reproducibility using an easily available material instead of using a 
difficult-to-obtain twin-free single crystal or adding dopants. 
With a view to attaining this object, the present inventors repeated many 
experiments to examine closely the process of ZnSe crystal growth. As a 
result, they found the following phenomena: if the crystal of bulk ZnSe is 
to be produced from a melt and when a polycrystal exists that has the same 
diameter as the crystal to be grown, a twin-free single crystal in bulk 
form is obtained on said polycrystal; and even in the case where a crystal 
that is grown by the ordinary high-pressure melt process and which is 
solely composed of twins is used as a seed, the seed is converted to a 
polycrystalline form in the process of temperature elevation for melting 
the starting material and, thereafter, a twin-free single crystal in bulk 
form is obtained on that polycrystal. The present invention has been 
accomplished on the basis of these findings. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates generally to a process for producing a 
single-crystal bulk ZnSe from a melt by a high-pressure melt technique in 
a vertical Bridgman (VB) furnace or a vertical gradient freezing furnace. 
In its first aspect, the invention provides such a process that is 
characterized by using polycrystaliline ZnSe as a seed and growing a 
single crystal on said seed. In its second aspect, the invention provides 
such a process that is characterized by using polycrystalline ZnSe as a 
seed, melting the starting material ,and part of the seed, and growing a 
single crystal on said seed. In its third aspect, the invention provides a 
process according to the first or second aspect, wherein said seed is a 
ZnSe crystal solely composed of twins. In its fourth aspect, the invention 
provides a process according to any one of the first to the third aspect, 
wherein the seed has the same diameter as the single crystal to be grown. 
In its fifth aspect, the present invention provides a process for 
producing a single-crystal bulk ZnSe from a melt by a high-pressure melt 
technique in a VF or VGF furnace, characterized in that polycrystalline 
ZnSe is grown from the ZnSe melt and, after temporary interruption of the 
growth, part of the polycrystalline ZnSe is melted again, followed by the 
growth of a single crystal on that polycrystal. In its sixth aspect, the 
invention provides a process according to the fifth aspect, wherein the 
polycrystalline ZnSe has the same diameter as the single crystal to be 
grown. 
The process of the present invention is classified as one of two categories 
according to the conditions of crystal growth. In the first approach, 
preliminarily grown polycrystalline ZnSe (which may be a crystal solely 
composed of twins) is placed as a seed on the bottom of a crucible; a 
starting ZnSe material is placed on top of the seed; the crucible is put 
in a VB or VGF furnace and heated to a predetermined temperature higher 
than the melting point of ZnSe in an insert gas atmosphere such as 
nitrogen so that the starting material or part of the seed will melt; 
thereafter, the crucible is moved to the lower-temperature region (i.e., 
downward) at a predetermined rate so that a single crystal is grown on the 
seed. This method may be described as a "two-stage process". 
In the second approach, a crucible solely charged with a starting ZnSe 
material is put in a VB or VGF furnace and heated to a predetermined 
temperature higher than the melting point of ZnSe in an inert gas 
atmosphere such as nitrogen so that the starting material will melt; 
thereafter, the crucible is lowered at a comparatively fast rate so that 
polycrystalline ZnSe is grown only at the tip of the growing crystal; the 
growth is interrupted and part of the polycrystal is melted again; 
thereafter, the crucible is lowered at a predetermined rate so as to grow 
a single crystal. This method may be described as a "one-stage process". 
Whichever of these methods is used, a twin-free single crystal of bulk ZnSe 
can be grown on the seed or polycrystalline ZnSe with good reproducibility 
.

The following examples are provided for the purpose of further illustrating 
the present invention but are in no way to be taken as limiting. 
EXAMPLE 1 
An experiment of producing a single-crystal bulk ZnSe was conducted using 
the following starting material and apparatus. The starting material was a 
mass of high-purity (99.999%) ZnSe that was produced by the vaporphase 
process at the Central Research Laboratory of DOWA MINING CO., LTD. The 
furnace was a VB furnace (Model GHPA-202; a high-temperature and pressure 
atmosphere furnace manufactured by GAKEI DENKI SEISAKUSHO). The crucible 
was made of boron nitride and it consisted of a constant-diameter zone 
having an inside diameter of about 30 mm and a taper zone having a cone 
angle of 38' and a length of 35mm. 
Single-crystal ZnSe was produced by the following procedure. Preliminarily 
grown polycrystalline ZnSe having a constant-diameter portion longer than 
about 5 mm and a taper portion was placed as a seed on the bottom of the 
crucible and the starting material was put on top of it. The crucible was 
mounted on the shaft in the VB furnace and placed in position. After 
nitrogen purge, the furnace was supplied with nitrogen to a pressure of 30 
kgf/cm.sup.2 (nitrogen may be replaced by another inert gas). The 
temperature in the furnace was raised to 1,550.degree. C. (maximum design 
temperature), which was maintained for 5 hours to melt the starting 
material and part of the seed (extending to about 5 mm below the top end 
of the constant-diameter portion) in the crucible. Thereafter, the 
crucible was lowered at a rate of 3-5 mm/h to achieve crystal growth on 
top of the seed. The thermal gradient near the melting point of ZnSe was 
about 22.degree. C./cm. When all of the molten starting material fully 
solidified, the movement of the crucible was stopped and the temperature 
in the crucible was lowered to room temperature for crystal recovery. As a 
result, a twin-free ZnSe single crystal was obtained on the seed. It had a 
diameter of about 28 mm and a length of about 15 mm. 
EXAMPLE 2 
The procedure of Example 1 was repeated except that the seed was a ZnSe 
crystal solely composed of twins. The seed was converted to a 
polycrystalline form, on top of which a twin-free ZnSe single crystal 
could be grown. The diameter and length of the single crystal were about 
28 mm and 15 mm, respectively. 
EXAMPLE 3 
A crucible of the same type as used in Example 1 was charged with the same 
starting material as used in Example 1. The crucible was mounted on the 
shaft in a VB furnace of the same model as used in Example 1 and placed in 
position. 
After nitrogen purge, the furnace was supplied with nitrogen to a pressure 
of 30 kgf/cm.sup.2 (nitrogen may be replaced by another inert gas). The 
temperature in the furnace was raised to 1,630.degree. C. (maximum design 
temperature), which was maintained for 5 hours to melt the starting 
material in the crucible. Thereafter, the crucible was lowered at a 
comparative fast rate higher than 20 mm/cm to achieve polycrystalline 
growth solely at the tip of the growing crystal. When a polycrystal having 
a constant-diameter portion about 5 mm long had grown, the growth process 
was stopped and part of the polycrystal (extending to about 5 mm below the 
top end of the constant-diameter portion)was melted again; subsequently, 
the crucible was lowered at a rate of 3-5 mm/h for single-crystal growth. 
The thermal gradient near the melting point of ZnSe was about 42.degree. 
C./cm. When all of the molten starting material fully solidified, the 
movement of the crucible was stopped and the temperature in the crucible 
was lowered to room temperature for crystal recovery. As a result, a 
twin-free ZnSe single-crystal was obtained on the polycrystalline tip. It 
had a diameter of about 20 mm and length of about 10 mm. 
As will be obvious to the skilled artisan, the process of the present 
invention is applicable even if a VGF furnace is substituted for the VB 
furnace used in Examples 1-3 and similar results are obtained. 
Comparative Example 
As typically shown in J. Cryst. Growth, 75 (1986) 609-612, the crystal 
produced by the ordinary high-pressure melt process using no seeds has in 
all cases a number of twins in the direction [111]. 
In the prior art, twin-free single crystals that are extremely difficult to 
obtain must be used as seeds or it is necessary to add dopants that may 
potentially affect the substrate characteristics and, hence, it has been 
difficult to produce twin-free ZnSe single crystals on an industrial 
scale. In contrast, the process of the present invention is capable of 
producing high-purity and twin-free single-crystal bulk ZnSe with good 
reproducibility using or without using seeds that comprise either 
polycrystals or crystals solely composed of twins which are easy to grow.