Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Havin growth from molten state
Reexamination Certificate
1999-11-30
2002-01-01
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Havin growth from molten state
C117S083000
Reexamination Certificate
active
06334897
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for producing a compound semiconductor single crystal, for example, to a useful technique applied to a vertical gradient freeze method (VGF) or vertical Bridgman (VB) method for growing a single crystal in a vertical direction by cooling raw material melt of a compound semiconductor, such as InP.
BACKGROUND ART
Previously, for producing industrially a compound semiconductor single crystal ingot of group III-V, such as GaAs, InP or the like, for example, a liquid encapsulated Czochralski (LEC) method or a horizontal Bridgman (HB) method has been generally applied.
While the LEC method has advantages that crystal orientation is controllable by seeding, a wafer having a large diameter and a circular shape in sectional form can be obtained, and a high purity crystal can be easily obtained by using a liquid encapsulating material (B
2
O
3
), the method has disadvantage that a large temperature gradient in a direction of crystal growth makes thermal stress to the crystal large, so that etch pit density (EPD) in the crystal becomes high. Accordingly, in a case of making an optical device by using the crystal, a problem that a property deteriorates due to a crystal defect is occurred.
On the other hand, while the HB method has an advantage that a crystal having low etch pit density can be obtained due to a small temperature gradient in the direction of crystal growth, the method has disadvantages that making the diameter of wafer large is difficult and further, the only wafer having a shape depending a shape of a crucible (for example, a vault-like shape) can be obtained, because the raw material melt is solidified in the crucible (boat).
As a method for producing a single crystal, which complements the respective disadvantages of the HB method and the LEC method and has the respective advantages thereof, the vertical gradient freeze (VGF) method and the vertical Bridgman (VB) method have been developed.
With the VGF method and the VB method, a temperature of a crystal growth furnace is lowered to gradually cool compound semiconductor raw material melt contained in a refractory crucible, thereby the compound semiconductor single crystal is grown. Therefore, these methods have advantages that the thermal stress is small and the etch pit density is small, because the temperature gradient in the melt during the crystal growth is tens to several ° C./cm, which is one digit smaller than the LEC method.
In a case of the VGF method, a prior crystal growth apparatus will be explained based on
FIGS. 8 and 9
.
FIG. 8
is a sectional view of a crystal growth apparatus used for the prior VGF method, and
FIG. 9
is a schematically sectional view of a lower portion of another crystal growth apparatus.
The crystal growth apparatus shown in
FIG. 8
has a hot zone Z surrounded by a cylindrical refractory wall
113
and a top cover
114
in a high pressure container
110
. In the hot zone Z, heaters
111
, for example, graphite heaters are disposed along inner peripheries thereof. An inner container
105
is set on an inside of the heaters
111
with being supported by a lower shaft
112
. A refractory crucible
101
, such as a pBN crucible is disposed in an inside of the inner container
105
, with being supported by a crucible supporting base
106
. A seed crystal set portion
102
having a bottom and a cylindrical shape is formed at a bottom of the crucible
101
. The inner container
105
containing the crucible supporting base
106
in the bottom portion and the lower shaft
112
supporting the inner container
105
constitute a crucible supporting means. A thermocouple
107
for measuring a temperature of a seed crystal is provided in the crucible supporting base
106
, in the vicinity of the seed crystal set portion. A reference numeral
108
denotes a cover made of quartz and sealing the inner container
105
with fitting.
A seed crystal S is contained in the seed crystal set portion
102
. A compound semiconductor raw material
103
and an encapsulating material
104
are contained in the crucible
101
.
In the crystal growth apparatus, the compound semiconductor single crystal is grown by heating the hot zone Z, melting the compound semiconductor raw material
103
in the crucible
101
to be raw material melt, and by gradually cooling the raw material melt of the compound semiconductor so that a single crystal will grow upward from a lower portion of the crucible
101
.
On the other hand, with the crystal growth apparatus shown in
FIG. 9
, a cylindrical refractory wall
153
is set in a high pressure container
151
and heaters
154
are buried in the cylindrical refractory wall
153
. In an inside of the cylindrical refractory wall
153
, a furnace tube
152
is provided, and in an interior thereof, a crucible
156
is fixed and supported by a susceptor
155
as a crucible supporting means. The susceptor
155
has a crucible supporting portion
155
a
and a lower shaft portion
155
b.
In this example, the interior of the furnace tube
152
constitutes a hot zone Z. In the crystal growth apparatus, the susceptor
155
is heated by the heaters
154
, and the crucible
156
is heated by the heated susceptor
155
to melt the compound semiconductor raw material in the crucible to be the raw material melt. Then, the raw material melt of the compound semiconductor is gradually cooled so that the single crystal will grow upward from a lower portion of the crucible
156
, thereby the compound semiconductor single crystal is grown.
According to the VGF method and the VB method, the wafer having the circular shape can be obtained by using the crucible having the bottom and the cylindrical shape. Further, the compound semiconductor raw material melt contained in the crucible is gradually cooled to grow the compound semiconductor single crystal, with lowering the temperature of the crystal growth furnace, thereby the temperature gradient in the direction of the crystal growth becomes small, so that making the etch pit density of the grown crystal low can be easily achieved.
However, the VGF method and the VB method are easy influenced by a slight change of the temperature in the reaction furnace, by an unevenness on an inner wall of the crucible or by contaminants attached to the inner wall. Thus, crystal defect portion, such as a twin or polycrystal is generated in a crystal diameter increasing portion which is from an initial point of the crystal growth in the crucible to a body portion thereof. These are main causes to lower a yield of the single crystal production.
Among these defects, the polycrystal generation due to the influence of the unevenness of the crucible or the contaminants attached thereto comes to be prevented by using the liquid encapsulating material (B
2
O
3
) during the crystal growth. On the other hand, the slight change of the temperature in the furnace comes to be solved by recent progresses of the temperature control technology. However, a method for effectively preventing the twin generation in the crystal diameter increasing portion from the initial point of the crystal growth to the body portion has not been developed.
In a case of growing a compound semiconductor single crystal of zinc-blende structure, such as GaAs or InP by using a seed crystal, it has shown that there is a closely relation between a tilt angle of the diameter increasing portion from the seed crystal to the body portion and probability of generation of the twin.
That is, in a case of growing a (100)-oriented crystal, a (111) facet plane appears in the diameter increasing portion and the twin generates from the facet plane. This phenomenon is verified by the experiments carried out by the present inventors. According to the experiments by the presest inventors, all twins generate along the facet plane of the crystal in which the twins generate.
An angle between the (111) facet plane and (100) orientation is 54.7°. Generally, in order to prevent appearance of the (111) facet plane, the tilt angle of the diameter increasing portion of the cruc
Asahi Toshiaki
Kainosho Keiji
Nozaki Tatsuya
Sato Kenji
Hiteshew Felisa
Japan Energy Corporation
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