CdTe crystal or CdZnTe crystal and method for preparing the...

Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth with a subsequent step acting on the...

Reexamination Certificate

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C117S003000, C117S081000, C117S082000, C117S083000, C117S956000, C117S957000, C423S099000, C423S508000

Reexamination Certificate

active

06299680

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a compound semiconductor single crystal and a process for producing the same. In particular, the present invention relates to an effective producing technique which is applied to a CdTe crystal or a CdZnTe crystal.
BACKGROUND ON THE INVENTION
Compound semiconductor single crystals, such as cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe) and the like, have advantages that a light emission efficiency thereof is higher than that of silicon (Si) and that a heterojunction can be applied thereto. The compound semiconductor single crystals are expected to be applied to a light emitting element, a photodetector, a low-noise amplifier, or the like.
In particular, a CdTe crystal has a zinc blende structure, has a property that an energy gap thereof is 1.5 V and is a material which can have both a p-type conductivity and an n-type conductivity. Therefore, the CdTe crystal is used for semiconductor devices, such as a substrate for an epitaxial growth of an infrared detector, a solar battery, a visible light sensor, an infrared sensor, a radiation detector for &ggr;-ray or for X-ray, a non-destructive detector and an HgCdTe mixed crystal epitaxy (a far infrared detector), or the like.
Because many properties of these devices depend on a purity of a compound semiconductor single crystal for forming a substrate, it is desired to obtain a high purity compound semiconductor single crystal in order to improve performance of the devices.
Conventionally, a CdTe crystal or a CdZnTe crystal is produced by the Bridgman method (Bridgman Method), the gradient freezing method (Gradient Freezing Method: GF method), the vertical gradient freezing method (Vertical Gradient Freezing Method: VGF method) or the like. However, there is a problem that a grown crystal includes many deposits having Cd or Te, which cause the properties of these devices to be lower (for example, a deterioration in performance to detect infrared ray).
The method for reducing deposits having Cd or Te has been studied. For example, the reducing method is reported by H. R. VYDYANATH et al. Journal of Electronic Materials, Vol. 22, No 8, 1993 (p. 1073).
The above method for reducing deposits having Cd or Te was that a grown bulk CdTe crystal or CdZnTe crystal was cut in a wafer form to carry out a predetermined heat treatment.
However, when the above heat treatment was carried out to the crystal, there was a problem that a full-width-half-maximum (FWHM value) of a double crystal X-ray rocking curve became higher than that of the crystal to which the heat treatment was not carried out.
When the FWHM value of the double crystal X-ray rocking curve thereof was high, after the HgCdTe mixed crystal epitaxy, the FWHM value of the double crystal X-ray rocking curve of the surface of the grown HgCdTe mixed crystal became high. There was a problem that the property of a photodiode manufactured by using the epitaxy was deteriorated.
Further, there was a problem that an etch pit density (EPD) of the CdTe crystal or that of the CdZnTe crystal became high after the heat treatment.
That is, for example, an experience in which the heat treatment was carried out to the CdTe substrate or the CdZnTe substrate under Cd pressure atmosphere, at 850° C. and for 20 hours was attempted. It was found that the EPD value which was 4×10
4
cm
−2
before the heat treatment, increased to 1.2×10
5
cm
−2
after the heat treatment.
The inventors studied about the above-described situations. As a result, the present invention was completed. An object of the present invention is to reduce the etch pit density (EPD) and the full-width-half-maximum (FWHM) value of the double crystal X-ray rocking curve, and to provide a CdTe crystal or a CdZnTe crystal which does not include deposits having Cd or Te and the process for producing the same.
DISCLOSURE OF THE INVENTION
In order to accomplish the above-described object, the process for producing a CdTe crystal or a CdZnTe crystal comprises the steps of; growing a CdTe crystal or a CdZnTe crystal, keeping a temperature of the grown crystal from 700 to 1050° C., adjusting a Cd pressure so as to keep a stoichiometry of the crystal at the temperature, leaving the crystal for time t which is determined so that each of a diameter L(r) of the crystal and a length L(z) thereof satisfies equation 1, and cooling the left crystal so that the temperature of the crystal is decreased within a range in which the temperature of the crystal and that of a Cd reservoir satisfy equation 2;
{L
(
r
),(
L
(
z
))}/2<{4exp(−1.15
/kT

t}
½
  equation 1
where
k is Boltzmann's constant,
T is an absolute temperature,
t is time (second), and
{4exp(−1.15/kT)×t}
½
is a chemical diffusion distance (cm), and
−288+1.68×
T
Cd
<T
CdTe
<402+0.76×
T
Cd
  equation 2
where
T
CdTe
is the temperature of the crystal, and
T
cd
is the temperature of the Cd reservoir.
The step of growing the CdTe crystal or the CdZnTe crystal may be carried out by a vapor pressure (Cd pressure) control method, a VGF method or an HB method.
It is preferable to provide a CdTe crystal or a CdZnTe crystal including no deposits having Te or Cd, which have a particle diameter of not less than 1 &mgr;m, wherein an etch pit density (EPD) of the crystal is not more than 5×10
4
cm
−2
, and a full-width-half-maximum (FWHM) of a double crystal X-ray rocking curve of the crystal is not more than 20S.
The crystal may be an n-type of CdTe crystal or an n-type of CdZnTe crystal, or may be a p-type of CdTe crystal or a p-type of CdZnTe crystal.
Hereinafter, the consideration for accomplishing the present invention and the research therefor will be summarized by the inventors.
The inventors have studied on the basis of the reasoning in which the deposits having Te or Cd can be reduced not by carrying out the heat treatment after the grown CdTe crystal or the grown CdZnTe crystal is taken out from a reactor and is cut in a wafer form like an earlier technology, but by contriving heat treatment condition in a state that the crystal was grown, that is, in a state that the crystal was set to the reactor.
With respect to the above-described method, the process for producing a II-VI group compound semiconductor crystal has been proposed by Asahi et al (See Japanese Patent Application No. Tokukai-Hei 3-126693).
The producing process was as follows. While a vapor pressure of the component element of the grown crystal was applied to the grown crystal after the crystal growth was finished, the diffusion of the component element was promoted at a predetermined heat treatment temperature which was lower than the melting point of the crystal. The crystal was held at the predetermined heat treatment temperature for a predetermined time. The heat treatment was carried out in order to restrain the growth of the deposits in the grown crystal. The grown crystal was slowly cooled to a room temperature after the heat treatment step was finished.
However, although the inventors repeatedly carried out a precise experience with respect to the case of producing the CdTe crystal by using the above method, it was found that contrary to the inventors' expectations, the density of the deposits having Te or Cd increased according to the condition even though the heat treatment was carried out to the grown crystal under the vapor pressure of the component element and the grown crystal was slowly cooled.
In the Japanese Patent Application No. Tokukai-Sho 63-79797, Miyazaki discloses the following technical idea. In the process for producing a II-VI group compound semiconductor single crystal, when a reactor is slowly cooled to a room temperature after the growth of the single crystal was finished, the temperature of the vapor pressure controlling part is decreased. Because the amount of evaporation is reduced in the vapor pressure controlling part and the vapor pressure drops in the reactor, inert gas i

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