JPS60239388A - Growth of low heat conductivity semiconductor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is useful as a substrate for optical elements such as infrared transparent windows or lenses, gamma-ray and x-ray spectrometers, electro-optic and acousto-optic modulators, solar cells and infrared detection arrays. A manufacturing method for growing large diameter pool semiconductors such as cadmium telluride. The major hurdles in developing large area infrared detector arrays based on mercury-cadmium telluride, epilayer technology are the need for sufficient area and crystal quality, and especially for the large quantities planned to meet the requirements of future defense systems. There is a constant shortage of cadmium telluride substrates with large diameter, round cross-sections compatible with production silicon and other processing methods. Typical commercially available cadmium telluride substrates are conventionally obtained by cutting monooriented grains (generally up to 1-3 ci in size) from polycrystalline pools of cadmium telluride produced by vertical Bridgman growth. . The resulting substrates have poor crystal quality, small area and irregular shape in contrast to silicon and gallium arsenide. Such conventional methods of manufacturing substrates have always limited the number of substrates to a relatively small number, have high manufacturing costs, and have resulted in considerable waste of material.

A well-known method for producing crystals of compound semiconductors is the liquid encapsulation Czochralski (LEC) method, which has been described by R.N. Thomas, H.M.・Article co-authored by Elle Barreto and T.T. Blasogin [Growth and properties of large diameter undoped semi-insulating GaAs for tie-left ion implant FE'r technology" Soli,
Sodo State Electronics Magazine Vol. 4 No. 687
It has been used to produce large crystals of IJI Lv group materials such as gallium arsenide, as described in more detail in Page (1981). Growing large crystals of cadmium telluride from the melt presents certain special problems due to the low thermal conductivity of this semiconductor. That is, due to the extremely low value of the thermal conductivity, the radial and axial heat transfer within the solid cadmium telluride is extremely poor, especially near the melt interface. As shown in Figure 9, the heat flow at the interface of cadmium telluride is extremely significantly reduced compared to silicon or gallium arsenide, making it difficult to grow this semiconductor from the melt under the conventional LEC method. This has become a serious obstacle.

Accordingly, a principal object of the present invention is to provide an Ic method which overcomes the problems caused by the low thermal conductivity of cadmium telluride and which allows the growth of large crystals of this material. .

To achieve this objective, the present invention covers the semiconductor melt in a refractory crucible with a liquid encapsulation layer, and draws crystals from the melt while maintaining a high pressure inert gas on the liquid encapsulation layer. In a method for manufacturing low thermal conductivity semiconductors in large diameter pools by the liquid-filled Czochralski method, heat is applied to the growth interface and removed through the crystal, and this heat input is transferred from the interface through the crystal. wherein the heat input is sufficient to maintain a thick axial warm gradient across the encapsulation layer/melt interface during crystal growth; There is a way to do it.

The manufacturing method of the present invention can also be used with other low thermal conductivity semiconductors such as zinc selenide.

Next, the present invention will be described by way of example of one embodiment with reference to the accompanying drawings.

The attached drawing shows cadmium telluride CdT in a large diameter pool.
An improved method for producing I is shown, particularly in FIG.
It clearly shows the growth shape of JC.

As shown in FIG. 1, the sealed crucible 10 has a one kilogram melt of cadmium telluride 12 at its bottom.
On top of this cadmium telluride is a relatively thin layer of liquid encapsulation layer 14 made of boron oxide, and on top of this liquid encapsulation layer 14 of boron oxide is a high pressure material having high thermal conductivity such as helium at 75 atmospheres. Inert gas is maintained.

A heating device 16 is provided near the base of the Lutsuho 10, and a heat shield 18 is provided around the heater 16. A tubular drive device 20 for rotating the crucible during the crystal pulling operation is connected to the base 10a of the crucible, and this tubular drive device 20 supplies the coolant close to the base of the crucible, as will be described later. Can be introduced.

Seed crystal drive device 2 through upper sealing means (not shown)
2 is rotatably sealed, and this driving device supports the seed crystal being pulled from the melt while being rotated synchronously with the crucible.

A seed crystal 24 is located at the end of the crystal puller 22 and a boule % of cadmium telluride is formed from the melt and liquid inclusions. This cadmium telluride melt is
Thermal conductivity is extremely low (0.013 w/k at melting point)
). Figure 2 shows a comparison of the thermal conductivities of compound semiconductors according to the formula 1, where the data point marked with a square on line 30 is at 300 ok, and the data point marked with a circle on line 1 and line 2 is at 300 ok. The data points are at the melting points of these materials.

To explain the LKC method of the present invention, the main sources of heat entering the interface during IJC growth of CdTe and the modes of conduction heat released from the interface are schematically shown in FIG. Heat input into the crystal by conduction and latent heat from the melt is of course the main source of heat input, and there is also radiant heat input from the gases entering the crystal originating from the melt interface.

The output heat sources mainly include conduction heat from boron oxide to the liquid enclosure, radiant heat to the liquid enclosure, and conduction heat and radiant heat from the crystal to the gas.

In the case of cadmium telluride, due to the low thermal conductivity of this material, approximately 98% of the heat output is due to heat radiation from and conduction from the boron oxide liquid inclusion, whereas the heat output It has been found that only about 2% of the amount of heat is due to conduction and radiation to the surrounding air. To maintain crystal growth, the heat input to the growth interface must be balanced with the heat output released from the growth interface through the crystal. Because of the low thermal conductivity of cadmium telluride, almost all of the heat at the melt interface must be dissipated by conduction to the boron oxide inclusions and radiation. In contrast, heat dissipation occurs as the gallium arsenide grows by splitting the dissipated heat equally between the inclusion and the surrounding gas.

FIG. 9 shows a comparison of calculated interface heat flow (in watts) and crystal length marks for six different crystalline materials grown under conventional LEC conditions, such as those used in GaAS growth. The interface between the boron oxide inclusion and the inert atmosphere of argon at 5 psi is shown as a dotted line at a crystal length of 2 cm or an inclusion layer thickness of 2Q rtr, rn. The top curve is for a high thermal conductivity material (silicon) with K = O-21 W/cmk, and the middle curve is for a somewhat lower thermal conductivity material (gallium arsenide) with K = 0.07 w/cmk. If the bottom curve is −0,0
This is the case with a material (cadmium telluride) that has an extremely low thermal conductivity of 1W/(1)k. This figure shows that the conventional LEC method cannot grow CdTe due to insufficient heat removal at the interface.

FIG. 4 shows the calculated change in the heat flow QI removed from the crystal during growth and the change in the heat flow Qm input from the melt to the interface as a function of crystal length.

If growth conditions are not carefully controlled, thermal imbalances can occur at the growth interface that can pull the crystals apart from the melt. To maintain crystal growth, it is desirable to maximize the saturation level of QI and minimize the heat input Qm. To overcome the problems associated with the low thermal conductivity of cadmium telluride and continuously balance the heat flow at the growing crystal/melt interface, the conventional IJC method was modified as follows: was found to be effective.

It has been found effective to maintain a large axial temperature gradient across the boron oxide/melt interface, with an axial gradient of approximately 450° C. per cm. As shown in Figure 1, the heat shield is placed high within the heating zone near the top edge of the graphite refractory heater, and the end of the heater is located at the interface between the melt and the inclusion body. let High pressure, high purity helium gas at approximately 75 atmospheres is maintained above the encapsulation layer. The reason for using this high pressure helium gas is to maximize the thermal conductivity of the surrounding gas. The high axial temperature gradient that must be maintained for CdTe crystal growth is shown in FIG. The data in Figure 5 is the temperature display (in degrees Celsius) from the thermocouple moved along the driving axis of the seed crystal, and the horizontal axis indicates the distance of the thermocouple from the interface between boron oxide and helium. Take the surface as the zero point. The positive distance is in millimeters above the interface in the helium atmosphere, the negative distance is the distance below the inclusion and gas interface, and the dotted line indicates the inclusion and molten acid interface. The axial slope or slope of the data line is approximately 450°C per cm, or more precisely 446°C per cm. It has been found that a large axial gradient is required to promote the growth of cadmium telluride in large diameter pools. Approximately 50 m, y
A pool of several inches in diameter grew. Due to the large axial gradient, the heat flux leaving the melt interface is lower than the heat flux entering the interface, preventing premature separation of the crystals from the melt. In contrast, in arsenic When growing gallium, axial gradients of 100 to 2008 C per G are commonly used.

Since the boron oxide inclusion is the primary means of removing heat from the crystal, it is necessary to optimize the thickness of the inclusion layer to control the removal of heat from the growing crystal. Figure 6 shows the interface heat flow variation with inclusion thickness as a function of crystal length. This figure shows that the thickness of the boron oxide inclusion layer between 10 and 20 mm is This indicates that it corresponds to the maximum value of the saturation level. The curve in Figure 6 is 5fJJTL under an overpressure helium of 75 atmospheres.
A state in which CdTe with a diameter of Tn is grown is simulated. The top curve is obtained when the thickness L of the boron oxide layer is 1α, the second curve is obtained when the thickness L of the boron oxide layer is L−2, and the final curve is obtained when the thickness L of the boron oxide layer is L−2.
= 5 cm. The vertical dotted line at the peak of each curve indicates the point at which each crystal arises from the inclusion bodies. These calculations indicate that to maintain optimal crystal growth, the boron oxide should be maintained between 10 and 20 W 1 m, but typically the thickness is 15 m, ya.

As the crystal is pulled from the melt, the temperature of the crucible is lowered to offset the increased amount of heat injected into the growth interface as the melt decreases. , FIG. 8 shows the variation of QI and Qm as a function of melt depth LM (Cm) corresponding to the optimal reduction in crucible temperature with respect to time, keeping Qm constant and not exceeding QI. show. Regarding the crystal pulling speed and the crystal and crucible rotation speed implemented in the present invention, 1. Experience has shown that approximately -15°C per hour is the optimum power increase rate. During the crystal growth process, the seed crystal and the grown crystal are rotated in the same rotational direction as the crucible, but their relative rotational speeds are synchronous and can be manipulated to direct the growth interface from the melt. It is possible to control the amount of heat injected into the Crystal and crucible rotational speeds of 12 and 15 revolutions per minute, respectively, have been found to be optimal for practicing the invention. In order to control the generation of latent heat at the growth interface and to minimize the heat component injected into the growth interface, crystal pulling rates of 1-6 m7M/hour have been used.

It has been found preferable to use crucible materials with isotropic thermal properties and low thermal conductivity, such as fused silica, especially for producing pulled crystals of the highest purity. High purity synthetic quartz is preferred.

It is also preferable to carry out local cooling of the melt, which can be carried out by introducing coolant through the tubular melt drive. By local cooling of the base,
The center point temperature of the inclusion body/cadmium telluride melt interface is lowered, resulting in a radial temperature profile that is flat or more concave to the melt, which controls the crystal diameter and It is preferable for the structure to be of high quality.

7th line shows the radial temperature gradient of the cadmium telluride melt along the radial direction, where curve 36 shows the slope in the absence of coolant or coolant introduced via the cold finger; Curve 68, on the other hand, shows the slope when the cold finger or coolant is introduced via the tubular crucible drive. This coolant - cools a local central region of the crucible, half a radius (R/2) from the center point of the melt;
The temperature in the radial direction of the melt is flattened, ie, constant, over the center of the melt.

The improved liquid inclusion Czochralski method of the present invention has allowed the growth of large diameter cadmium telluride boules of up to 5 Qm, m diameter, several centimeter groups, and weights of about 1.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an LEC growth apparatus utilized in practicing the present invention; FIG. 2 includes cadmium telluride shown as 1 as a function of a scaling parameter proportional to the cube of the Debye temperature of the solid; A graph comparing the thermal conductivity of several types of compound semiconductors. Figure 5 is a schematic diagram showing the heat source of heat input and output when growing cadmium telluride by LEC. Figure 4 shows the crystal length of cadmium telluride. FIG. 5 is a graph showing the axial temperature gradient maintained in carrying out the present invention; FIG. Graph showing changes in interfacial heat flow with respect to body thickness, seventh
The figure shows the radial gradient of the cadmium telluride melt (curve A
curve B shows the radial slope in the absence of cold finger cooling and curve B shows the radial slope when cold finger cooling is applied to the crucible), Figure 8 is shown as a function of melt depth. A graph showing the effect of temperature TO on the heat flow Qm of an interface melt.
2 is a graph showing the heat flow (watts) at the interface with respect to the change in crystal length (cm) when using a material with low thermal conductivity, namely cadmium telluride, and a low thermal conductivity material, ie, cadmium telluride. 10... Crucible, 10a... Crucible base, 12...
- Melt, 14... Liquid sealing layer, 16... Heating device, 18... Heat shield, 2o... Drive device, 22...
- Seed crystal driving device, 24...seed crystal. 1 FIG, 7 Jitsui ka-) 111 翫 i6 Yasameo, (Wara V) FIG, 8 Ll(-

Claims (1)

[Claims] (1) Liquid encapsulation in which a molten semiconductor in a refractory crucible is covered with a liquid encapsulation layer, and a crystal is pulled from the melt while a high pressure inert gas is maintained on the liquid encapsulation layer. In the Czochralski method (Czochralski method) for manufacturing large-diameter pool low thermal conductivity semiconductors, heat is applied to the growing interface and removed through the crystal, and this heat input and interface force The application of heat is controlled to balance the heat removed, and the heat input is sufficient to maintain a large axial temperature gradient across the encapsulation layer/melt interface during crystal growth. Method. +21. The method according to claim 1, characterized in that the semiconductor is cadmium telluride. (Maintaining an axial temperature gradient of about 450°C per 311c++)
The method described in section. 7. A method according to claim 1, 2 or 6, characterized in that a liquid confinement layer is provided with a thickness of between +41 and 10. (5) Crystal pulling is characterized by controlling the heat input from the melt to the crystal growth interface by reducing the heat input to the crucible so that it is a function of the amount of decrease in the melt with respect to the growth of the medium crystal. A method according to any one of claims 1 to 4. (6) A claim characterized in that the local central region of the melt is cooled during crystal growth so that the radial temperature gradient of the melt across the center of the melt remains constant. The method according to any one of items 1 to 5. (Claims 1 to 6) wherein the heat input from the melt to the growth interface is made equal by rotating the crystal and the melt in the same direction at synchronous speeds.
The method described in any of the paragraphs. (8) The method according to any one of claims 1 to 7, wherein the crystal pulling rate is about 6 nrn per hour.