CA1228525A

CA1228525A - Method for growing gaas single crystal by a floating zone technique

Info

Publication number: CA1228525A
Application number: CA000434887A
Authority: CA
Inventors: Jun-Ichi Nishizawa
Original assignee: Zaidan Hojin Handotai Kenkyu Shinkokai
Current assignee: Zaidan Hojin Handotai Kenkyu Shinkokai
Priority date: 1982-08-27
Filing date: 1983-08-18
Publication date: 1987-10-27
Also published as: EP0102054B1; JPS6041036B2; EP0102054A1; DE3372383D1; JPS5939797A; US4619811A

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention provides a method for fabricating a GaAs single crystal by relying on a floating zone technique in a closed type cylinder charged with a GaAs polycrystal and an adjacently located GaAs single seed crystal, in which: an As container charged with As and communicating with the interior of said cylinder supplies pressurized As vapor into said cylinder, said As container is heated to a temperature sufficient to ensure that an optimum As vapor pressure of PAS ? 2.6 x 106 exp (-1.05eV/kT) Torr is applied to a melt region located at a boundary between said GaAs single seed crystal and said GaAs polycrystal in said cylinder to satisfy stoichiometry of the crystal to be grown, said melt region is heated to a temperature set at or somewhat higher that the melting point of GaAs, a continuous temperature gradient is established in a region of said cylinder leading from said As container to said GaAs polycrystal and to said GaAs single seed crystal.

Description

The present invention relates to a method and an Papa-fetus for growing, by utilizing a floating zone, a Gays compound semiconductor single crystal which contains As having a high vapor pressure as a constituent component of the crystal.

As is well known, a Gays single crystal which is a come pound semiconductor is used as a substrate in fabricating a device such as a field effect transistor, an SUE (static induct talon transistor), ICY millimeter ware diode, Twenty diode, lo liyht-ernltting diode, or a laser diode, and concurrently researchers are making a steady progress with a great expectation for the development of Good Gays single crystals to serve as a material for fabricating such semiconductor devices as transit-ions and semiconductors integrated circuits which are operated at high speeds, from the viewpoint that the electron mobility of a Gays compound semiconductor is 8500 cm2/v.sec which is more than five times greater than that of silicon.

As a matter of course, as the Gays single crystal which is used as the substrate for the fabrication of such semiconduc-ion devices as mentioned above, the crystal is desired to have an excellent crystal quality.

In the past, Gays single crystals lo. use as substrates have been obtained by a single crystal growth method such as the horizontal Bridyman method or the LEO (Liquid Encapsulated Czochralski) method which uses a B203 layer for encapsulation of the growth zoner. The former method has disadvantages such that, because it uses a quart tube for the growth, it is difficult to obtain a single crystal of a large diameter, and that, slice it is usual to row a crystal in the (ill) orientation, there are the inconveniences -that (1) in order to obtain a suhstratr-3 having a (100) surface which is frequently used in semiconductor devices the crystal has to be cut not perpendicular to the (I
or:Lr-~ntation but obliquely relative to the surface ox the crystal;
and (2) although it is usual to obtain a semi-insulatlng or highly resistive substrate by an inclusion of an impurity which is Or or O or both, it is necessary that such an impurity or impurities must be introduced at a high temperature ox 800C or higher, so that the inclusion of such a impurity cannot be accom-polished in a s-table manner. The latter method can produce a crystal of a large diameter in the (100) surface. However, since this method uses a Sue layer as a sealing material, the tempera-lure gradient between the Gays melt and the seed crystal is steep, being 100-300C/crm, and accordingly there is the disadvan-tare that the crystal which is grown easily tends to have a very large etch pits density (END) and develop slush defects as lattice dislocation. Other than those conventional methods which have been rrlentioned above, there is performed Czochralski's pulling method. This known method also has similar drawbacks to those lo mentioned above.

There are, in fact, more important problems as will be described hereunder.

Unlike a crystal such as silicon crystal, a Gays come pound semiconductor is a crystal which is produced by compounding two different elements. Besides, the III column element Go has a vapor pressure which is substantially different from the vapor pressure of the V column element As. The vapor pressure of As is by far greater than that of Gay Accordingly, the grown Gays crystal exhibits a considerably deviation from stoichiometory.
It has been usual in the growth of a Gays crystal by conventional manners that AS atoms show a shortage within the crystal so that Lo such defects as As vacancies or lattice dislocation, or the defect representing a combination of AS vacancies and impurity atoms occur.

US

Recent research has revealed that the deviation from stoichiometry of the grown crystal imparts a very great effect upon the characteristics or the life of the devices formed with such crystals having a deviation of s-toichiometry. The present inventor, in order to solve the above-mentioned problems in the technical field of epitaxial growth, earlier proposed in Japanese Patent Application No. So 57-118916 a method for supplying As vapor throughout the growth process, with successful results.
however, in the field of growing a single crystal in the above-said earlier proposed method, -there was no-t given a consideration to a growth apparatus which is constructed to avoid such Della-lion from stoichiometry of the grown crystal.

French patent No. 1,3 70, 63 8, US. patent No. 3,2G0,573 and US. patent No. 3,235,339 relate to the floating zone tech-unique of the Gays crystal and have shown various improvements in order to prevent the dissociation of As from the Gays crystals.
However, there was not given a consideration -to the pressure of V
column element and the stoichiometry composition of the Gays Crystal.

Therefore, the present invention provides a method for growing a Gays single crystal having a minimized deviation from stoichiometry and having a goad crystal habit by utilizing a floating zone, which eliminates the above said drawbacks ox -the conventional apparatus for growing a Gays single crystal.

According to the present invention there is provided a method for fabricating a Gays single crystal by relying on a ~:I.oat:Lng zone -technique in a closed type cylinder charged with a Lo Gays po].ycrystal and an adjacently located Gays single seed cry-tall, in which: an As container charged with As and communicating with the interior of said cylinder supplies pressurized As vapor into said cylinder, said As container is heated to a temperature sufficient to ensure that an optimum As vapor pressure of PUS

2.6 x owe exp (-evict) Torn is applied to a melt region located at a boundary between said Gays single seed crystal and said Gays polycrystal in said cylinder to satisfy stoichiometry of -the crystal to be grown, said melt region is heated to a them-portray set at or somewhat higher than the m21ting point of Gays, a continuous temperature gradient is established in a region of said cylinder leading from said As container to said Gays polycrystal and to said Gays single seed crystal. Suitably said heating is performed by radio frequency induction or nests-lance heating with radio frequency. Preferably a radio frequerlcy induction coil type heater is used which has a coil portion for heating a part of said Gays crystals charged in said cylinder, swilled coil portion having a wider upper turns of coil than its lower turns. Desirably a part of said Gays crystals charged in that cylinder is heated by a coil portion, said coil portion con-slating ox a first radio frequency induction heating coil and a second radio frequency induction heating coil provided below said first heating coil, both coil portions having same size turns.
Preferably sass Gays polycrystal and said Gays single crystal are housed in a housing made with at least one of -the materials selected from the group consisting of quartz, carbon and sapphire, said housing having joints which are ground-and-fit in I

its fabrication to provide a hermetic sealing at such joints.

The present invention also provides an apparatus for growing Gays single crystals using a floating zone method, said apparatus comprising: a closed cylinder charged with a Gays polyp crystal and an adjacently located Gays single seed crystal, an us vessel charged with As and communicating, via a narrow passage, with said closed cylinder for supplying As vapor into said closed cylinder, first heating means for heating said Gays polycrystal lo and establishing a Gays melted portion, first temperature Coil troller for novena said Gays melted portion a-t a temperature set at or Just above the melting point of Gays, second heating means for holding said As vessel at a substantially constant tempera-lure sufficient to impart a Gays single crystal being grown an optimum us vapor pressure to cause said Gays single crystal being grown to be at least substantially devoid ox deviation from the s-toichiome-tric composition, second temperature controller for keeping said us vessel a-t constant, third heating means for heat-in said closed cylinder to prevent said us vapor from depositing on the inside of said closed cylinder, first rotating and Verdi-gaily moving means for said Gays polycrystal connected to a rod rotated by a chuck, second rotating and vertically moving means for said Gays single seed crystal connected to a rod rotated by a chuck. Suitably said Gays poly~rystal and said Gays single crystal are housed in a housing made with at least one of the materials selected from the group consisting ox quartz, carbon and sapphire said housing having joints which are ground-in-fit ill Lots fabrication to provide a hermetic sealing at such joints.
Preferably said - pa first heating means is a radio frecluency coil heater connected to a radio frequency power supply. Desirably each of said heating means is a resistance heater connected to a power supply.

The present invention will hereunder be described with respect to the preferred embodiments by referring to the accomp-awning drawings, in which:-Fig. 1 is a schematic diagram of an embociiment of the apparatus structure for growing a Gays single crystal by utilize-lay a floating zone, according to the present invention;

Fig. 2 is a schematic diagram showing an example of a working coil used in the apparatus of Fig. 1 for performing an induction heating to locally melt a Gays rod, wherein the golf has different diameters in its upper part relative to the lower part;

Fig. 3 is a schematic diagram showing another example of working coil used in the apparatus of Fig. 1 for performing an induction heating to locally melt a Gays rod, wherein two coils are provicied so that the radio frequency of the electric power supplied to these golfs are varied; and Fig. 4 is a schematic diagram of another embodiment of the apparatus structure for growing a Gays crystal by utilizing a floating zone, according to the present invention.

Fig. 1 shows an embodiment of the Gays single crystal growing apparatus which reties on the floating zone method according to the present irlventi.on. Reference numeral 1 repro-sets a quartz cylinder; 3 a Gays seed crystal; 4 a Gays single crystal rod; 5 a Gays melt region;
6 a Gays polycrystal rod; 8 a supporting rod for the Gays polycrystal rod 6; 7 a chuclc for establishing a connection of the Gays polycrys-tal 6 with the supporting foal 8; 9 a connector for the Gays seed crystal 3 and its supporting rod 10; 11 a quartz vessel for continuing As; 12 a narrow quartz '3 tube for connecting the As Grow 11 with the quartz cylinder 1 of the apparatus for communication there between;
lo an As element of a high purity; 14 and lo stainless IO plclteS or seal:ingly supporting an open end of the quartz cylinder 1, respectively; 16 and 17 sealing members made of, four example, Wrings or gaskets, respectively 20 a working coil serving as a radio frequency (hereinafter to be -lo I

I

referred to as RF) induction heater for heating the Gays polycrystal 6 to form its melt 5; 21 a power supply using, for example, RF oscillator; 22 a working coil serving as an RF induction heater to heat As 13; 23 a power supply of a high frequency such as, similar to 21, an RF oscillator; 24 a thermometer such as a pyrometer for measuring the temperature of the melt region S; 26 a thermometer such as a pyrometer for controlling the vapor pressure of As 13; 25 and 27 temperature controllers for controlling the RF power supplies 21 and 23, respectively; and 2B a window for observing the temperature.
As stated above, the crystal growth according to the present invention is performed by relying on the floating zone method. To begin with, the polycrystal located at -the bottom end of the polycrystal rod is heated by the induction heater to the meltinc3 point of Gays or somewhat higher than that to Abbott 1240C to lo to l render it to await state huh will hereunder be re~errecl to as a melted portion to which is connected the seed crystal 3. After an appropriate necking is established there between, the supporting foals lo and 8 are rotated while gently moving these supporting rods lo and 8 downwardly, and as a result a single crystal its grown.
One owe the features of the present invention it, yin the crystal grittily method using said gloating Zulu technique an us continuer Err supplying As vapor is connected to the quartz cylinder 1 which is a closed type container, to supply As vapor in-to the sealed space wherein -the polycrystal and -the single crystal are charged. Using the seed crystal 3 as the seed for -the growth, the temperature of As in the As 11 is set in such a way that an optimum vapor pressure P pi of As is applied to the melt region 5 throughout -the growth process of crystal. The optimum As vapor pressure is provided at a value of Put 2.6 x 10 exp (-evict) (Torn). The -temperature of -the As region, TASK may be g s Tess/ Gays PAs/PGaAs i he do meter -the pipe 12 for connecting the As vessel 11 with the quartz cylinder 1 is small. Here, PUS represents the vapor pressure produced by the As 13 in the As region 11;
TGaA represents the temperature of the Malta region, and PGaAS represents the vapor pressure of As at the melted region, and this vapor pressure may be considered as being a value close to Pout in order to control the vaporization of As from the melted region of the Gays crystal. Accordingly, if TGaAS is assumed to be 1240C

Gays , TAX As y be set so as to establish the formula: auspice ~G-a-As/PGaAs Since PUS is determined by the value of TASK it it only necessary to determine the temperature TASK
Lo order to secure a desirable stoichiometry of the Gays single crystal, the temperature of As reloan has to `5~5i be controlled with precision, i.e. it is necessary to hold the stability of the temperature to fall within the range of tempera-lure fluctuation less than okay The second feature of the present invention is to establish a continuous temperature gradient in the region between the As vessel if and the Gays melt region 5 in order to insure that the above said As vapor supply is carried out under the per-fect:Ly controlled state. Especially, the temperature gradient in lo the vicinity of the boundary between the melted region 5 and the po:Lycrystal region 6, or between the melted region 5 and the single crystal region I, has to be made gentle in order to irlhJ.bit the development of lattice dislocations. In order to materialize this continuous temperature distribution, there may be employed not only the RF heating as mentioned in the embody-mint, but also such a heating technique as resistances heating, a lamp or a laser. Furthermore, the temperature of the region leading from the As vessel if to the melted region 5 ought to be held always higher than the temperature of As 13 to prevent us vapor from depositing, in the form owe solids, onto the wall of the quartz cylinder l.

In order to effect a growth of a Gays single crystal having a good crystal perfection, sufficient car has to be taken with respect to the precise controlling of the As vapor pressure which is applied, especially to a precise temperature controlling of the Gays crystal rod centering around the melted region, and also to the speed of rotation as well as the speed of movement of the crystal rods. The measurement of -the temperatures may be made not only through the window, but also by providing quartz fibers at the As region and -the boundary region between the melt region and -the single crystal so as to measure the temperature thereby.
The controlling of the diameter of the grown crystal may be made by observing, through the window I the conditions of the melted region 5 of the crystal rod and of the single crystal 4. It is needless to say that this controlling of the diameter of the crystal to be grown may be made in an automatic fashion by the use of an appropriate automatic controlling system.
As will be understood from the above-mentioned value of P to the vapor pressure of As in the As region 11 for the purpose of obtaining a Gays single crystal having a satisfactory stoichiometric composition is almost close to 1 elm of atmospheric pressure, ~ccordin~ly, it is not so difficult technically to establish a sealed state of the quart cylinder 1 in the growth apparatus shown in the abo~e-described e~l~odiment. It will be needless to say that the apparatus may have such an arrangement that this growth apparatus as a whole is enclosed in a pressure-resistant outer cylinder in order to prevent any leak of the As vapor from the quartz cylinder 1 to the outside thereof.
In such case, there may be employed such an inert gas as nitrogen, argon and helium for the purpose of establishing an applicable pressure, or -to use a mixed gas consisting of such inert gas and As vapor or the same purpose. Furthermore, though not shown in Fig. 1, i-t will be needless to say that there may be provided a mechanism for cooling any particular region where a rise in the temperature is not desirable.
Figs. 2 and 3 are further embodiments of the present invention, showing modified structures in the vicinity of the melted region of~GaAs crystal. These embodiments also employ the floating zone technique of the present invention as in the case of the preceding embodiment. However, these latter embodiments are invariably intended to obtain a Gays single crystal of a large diameter. In -the embodiment of Fig. 2, the diameter of the upper turns of the working coil 20 intended to produce a melted region 5 is designed larger than that of its lower turns. By winding the workirlg coil in the "upper flared type" as shown, there will be developed a lotting power buoyancy in the material charged in the melting region due to the inter-action between the magnetic field produced by the current flowing through the coil and the induction current flowing through the material, so that a single crystal having a large diameter is obtained.

Fig. 3 shows an arrangement that there are provided two working coils AYE and 20B intended to melt the material Gays polycrystal. The coil 20B is provided below the coil AYE. The frequency f2 of the power supply 21B for the lower coil 20B is set lower than -the frequency if of the power supply AYE for the upper coil AYE. By this arrangement, it is possible to develop a floating force as in the case of Fig. 2. Thus, there is obtained a single crystal having a large diameter.

Fig. 4 shows still another embodiment of the apparatus of the present invention. In this embodiment, in order to improve the temperature distribution in the region leading from the position where there is provided a melt region 5 to the us vessel]. 11, the close container accommodating a Gays rod is con-strutted with a carbon cylinder 41, a quartz cylinder 40 and a cap 42 provided with an As vessel 11. The quartz cylinder 40 and the cap 42 may be replaced by sapphire cylinder and cap as required. reference numerals 43,44 and 45 represent round and fit adjoining portions, respectively. Other parts are similar to those shown in Fig. 1, so that their description is omitted. In order to avoid the development that the supplied A becomes a solid and is deposited on the walls of the container, thy temper-azure of the region leading from the As vessel 11 and the melt region 5 Lo hold higher than that of the As vessel 11 with a con-tenuous gradient there between. By using a carbon container 41 having a high thermal conductivity, -the abovesald improvement of temperature distribution is made feasible. It is necessary, how-ever, to give attention so that, in order to avoid the mingling of -the gas flown out of the carbon material of the container Into the crystal as an impurity, the carbon material has to be selected from a highly pure carbon and to be sufficiently deprived of its outgoing gas by baking it in a vacuum.

As described above, the apparatus according to the pro-sent invention for growing a Gays single crystal is designed so that, in the floating zone -technique, an optimum As vapor pros-sure is applied to -the melted portion of the Gays rod so as to satisfy the stoichiometric composition of the grown crystal.
Thus, as compared with the conventional horizontal regimen method or liquid encapsulation Czochralski ALEC) method, the apparatus of the present invention features a greatly reduced mingling of undesirable impurities into the grown crystal. More-over, the crystal which is grown has very few lattice dislike-talons, and it is possible to obtain a perfect Gays single crystal ox a large diameter having a high degree of purity. Thus, ..22~

-the apparatus of the present invention can give a great contribution industrially since it is possible to obtain an extremely superior single semiconductor crystal to be used as a substrate for various kinds of microwave devices such as high-speed transistors and high-speed semiconductor integrated circuits.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for fabricating a GaAs single crystal by relying on a floating zone technique in a closed type cylinder charged with a GaAs polycrystal and an adjacently located GaAs single seed crystal, in which: an As container charged with As and communicating with the interior of said cylinder supplies pressurized As vapor into said cylinder, said As container is heated to a temperature sufficient to ensure that an optimum As vapor pressure of PAS ? 2.6 x 106 exp (1.05eV/kT) Torr is applied to a melt region located at a boundary between said GaAs single seed crystal and said GaAs polycrystal in said cylinder to satisfy stoichiometry of the crystal to be grown, said melt region is heated to a temperature set at or somewhat higher than the melting point of GaAs, a continuous temperature gradient is established in a region of said cylinder leading from said As container to said GaAs polycrystal and to said GaAs single seed crystal.

2. A method according to claim 1, in which: said heat-ing is performed by radio frequency induction or resistance heat-ing with radio frequency.

3. A method according to claim 2, in which: a radio frequency induction coil type heater is used which has a coil portion for heating a part of said GaAs crystals charged in said cylinder, said coil portion having a winder upper turns of coil than its lower turns.

4. A method according to claim 3, in which: a part of said GaAs crystals charged in that cylinder is heated by a coil portion, said coil portion consisting of a first radio frequency induction heating coil and a second radio frequency induction heating coil provided below said first heating coil, both coil portions having same size turns.

heating coil provided below said first heating coil, both coil portions having same size turns.

5. A method according to claim 1, 2 or 3, in which:
said GaAs polycrystal and said GaAs single crystal are housed in a housing made with at least one of the materials selected from the group consisting of quartz, carbon and sapphire, said housing having joints which are ground-and-fit in its fabrication to pro-vide a hermetic sealing at such joints.

6. A method according to claim 4, in which: said GaAs polycrystal and said GaAs single crystal are housed in a housing made with at least one of the materials selected from the group consisting of quartz, carbon and sapphire, said housing having joints which are ground-and-fit in its fabrication to provide a hermetic sealing at such joints.