GB2205824A

GB2205824A - Semi-insulating GaAs single crystal

Info

Publication number: GB2205824A
Application number: GB08810936A
Authority: GB
Inventors: Toshio Kikuta; Yoshio Nakamura
Original assignee: Furukawa Electric Co Ltd
Current assignee: Furukawa Electric Co Ltd
Priority date: 1987-05-08
Filing date: 1988-05-09
Publication date: 1988-12-21
Anticipated expiration: 2008-05-09
Also published as: GB2205824B; DE3815575A1; GB8810936D0; JPS6452700A; DE3815575C2

Description

2205824 SEMI-INSULATIG GaAs-SINGLE CRYSTAL AND METHOD FOR ITS PREPARATION.

The present invention relates to a high-quality semi insulating GaAs single crystal with concentration of inpurities controlled and a method of preparing the same.

The GaAs single crystal has a high elextron mobility among the III-V compounds semiconductors and is widely used as substrates for elements of ultrahigh-speed ICs, light-electron integrated circuits, and the like GaAs as single crystals are widely used because the specific resistance of high-quality GaAs single crystal is over 107n.cm showing its semi-insulating property, defects in the crystal are reduced permitting a crystal with uniform distribution to be obtained, and the manufacture of a large-sized wafer is readily possible.

As a method of preparing GaAs single crystal satisfying such requirements, the lifting method through liquid encapsulant (Liquid Encapsulated Czochralski method) (hereinafter abbreviated as LEC method) is usually adopted. By this method, the GaAs single cry stal can be synthesized directly, wherein, as shown in Fig. 7, a rotating support axis (4) is provided underneath a crucible (3), the upper end thereof being opened and the outer circumference thereof being covered with support members (2) such as carbon material etc., placed in a highpressure vessel (1) to support said crucible (3) so that it can rotate and move up and down, and a lieater (5) is provided around the crucible (3) to heat it to a fixed temperature and to keep at that temperature. To the upper portion of the crucible (3), a lifting bar (7), the seed crystal being attached to the lower end thereof and the up and down move ment being possible together with rotation, is provided.

In said apparatus, fixed amounts of Ga and As are placed in the crucible (3), respectively, further B203 is placed as a liquid encapsulant"the crucible (3) is set up in the highpressure vessel (3). After an inert gas such as Ar, N2 or the like was introdu- ced to said vessel (1) to pressurize, the raw materials for crystal and the encapsulant in the crucible (3) are allowed to melt at a temperature higher than the melting temperature of said raw mate rials by the heater (5). When the raw materials melted complete ly and a molten layer of B203 (8) as a liquid encapsulant and a layer of GaAs -melt (9) were formed at the upper portion and the lower portion of crucible (3), respectively, the lifting bar (7) is allowed to descend to contact the seed crystal (6) with the 2 layer of GaAs melt and then the seed crystal (6) is rotated at a fixed speed to grow the GaAs single crystal (10) while it is lif ted.

The inventors have confirmed experimentally that, into the GaAs single crystal thus obtained, carbon is admixed from the constitutive members etc. of apparatus as an impurity (T. Kikuta, H. Emori, T. Fukuda and K. Ishida, Journal of Crystal.Growth,.76 (1986) 517). Moreover, the inventors have found that this carbon works as a main acceptor in the GaAs single crystal, but, when the concentration of carbon (hereinafter abbreviated as CAs) becomes over 6 x 1015 cm-3, said crystal becomes instable thermally and a phenomenon so-called the thermal conversion, in which the resisti vity and the mobility (hereinafter abbreviated as /0 and U, respec tively) decrease when the heat treatment was given, is observed on the surface of wafer resulting in the collapse of the semi insulating characteristic of GaAs single crystal. If the crystal, the semi-insulating characteristic being collapsed in such way, is used as a substrate to form the device, it becomes difficult to realize the integrated circuit uniform in the electric chara cteristics and the element characteristics in good reproducibility.

It is said that the thermal conversion phenomenon as mentioned above occurs because of that the lattice defect inherent in GaAs (so-called EL2) which governs the semi-insulating property decreases less than the concentration of carbon (D.E. Holmes, K.R. Elliott, R.T. Chen and C.G. Kurkpatric, Semi-Insulating III- 3 V Materials, Evian, p.19 (1982)). Therefore, in order not to cause the thermal conversion, the lower the concentration of carbon in single crystal, the more advantageous. Furthermore, there is a study that CAs is preferable to be not more than 5 x 10 15 cm- 3 from the characteristics of device, too (R.T. Chen, D.E. Holmes and P.M. Asbeck, Appl. Phys. Lett., 45,459 (1984)). There, based on the various data obtained by the inventors, the optimum crystal characteristics were established as follows:

Fig. 2 shows the calculated value of P as a function of the difference between the concentration of acceptor (N A) and the con centration of donor (N D) (i.e. the concentration of electrons, holes, etc. which cause the electric conduction) in an undoped GaAs single crystal, which was performed by the inventors. On the other hand, the semi-insulating property is defined usually as a case when the values of /0 satisfy the formula (1).

P:- 1 x 107 n-cm (1) Thus, from Fig. 2, the values of N A- N D satisfying the formula (1) are to satisfy a formula (2).

N A - N D > 3 x 10 14 cm- 3 (2) Also, Fig. 3 shows a relationship between C A; and N A- N D accor- ding to the study by the inventors (Y. Nakamura, Y. Ohtsuki, Y.

Itoh, J. Kikawa and Y. Kashiwayanagi, Proceeding of 12th Inter national Symposium on GaAs and Related Compounds, Karuizawa, 1985).

From the thermal conversion of single crystal and the characte- ristics of d6vice, C is neede to satisfy a formula (3) as shown As above.

4 C < 5 x 10 15 cm- 3 (3) As = Accordingly, the inventors have found that NA - ND:5 6 x 10 14 cm- 3 holds good further from Fig. 3, that the optimum condition of N A - ND becomes a formula (4) by combining with the formula (2), and also that CAS and /0 corresponding thereto become a formula (5) and a formula (6), respectively.

3 x 1014 cm- 3 N - N 6 x 10 14 cm-3 (4) A D = 2.5 x 10 15 cm-3 < C: 5 x 10 15 cm- 3 (5) = As - 1 x 10 7 _L-cm < /0 < 2 x 10 7_a cm (6) As a result, the inventors have known that, by preparing the GaAs single crystal satisfying the formula (5) and the formula (6), the semiconductor elements which have semi-insulating pro- perty and in which the thermal conversion phenomenon does not occur can be obrained.

The probability, the impurity being taken from the melt into the crystal (segregation coefficient: abbreviated as k), is constant through the substances and is shown by the formula (10).

CS = k-COQ - g) k-1 (10) Here, CS and CO are the concentrations of impurity in the crystal and the initial melt, respectively, and g is the frac tion solidified. The segregation coefficient of carbon in GaAs is said to be 1.44. If using this value, C As decreases from head portion toward tail portion of the GaAs single crystal lifted by LEC method and there occurs a difference in concentration amo unting about two times between g 0.1 and g = 0.8. Further, CAs I fluctuates depending on the quantity of carbon being the consti tutive material of apparatus by LEC method as mentioned above, the degree of vacuum, etc. and yet, because of extending over whole crystal, it is very difficult to realize the concentration of carbon in the ingot of GaAs single crystal by LEC method in good reproducibility so as to satisfy the formula (5).

Moreover,,/0 also varys similarly between head portion and tail portion of single crystal accompanying with the variation of C As and yet it is common that there is a fluctuation amounting about two times even across the wafer diameter cut out from this single crystal. Accordingly, even when the concentration of carbon is controlled to satisfy the formula (3), the occurrence of such wafer thatP becomes under 1 x 10 7 a-cm is anticipated. For cop ing with this, it has been known to add Cr into the single crystal.

When adding Cr as above, 4 can be increased without varying C but, if over 0.5 wt.ppm, the deposition of Cr takes place or As' the crystal is insufficient also to the thermal conversion resul ting in the generation of problem from the point of manufacture of device. On the other hand, if the addition level of Cr is made to a concentration as low as under 0.05 wt.ppm, the uniformity of melt comes not to be kept. For this reason, a method of the mag netic field applied to the melt of GaAs is proposed in order to enhance the uniformity of melt (Japanese Patent Application No.

sho 58-078899). However, with this magnetic field applied method, the apparatus is extremely expensive and this produces a cause to raise the cost of product leading to the problem.

6 From the formula (5) and the formuia (6) derived form Fig. 2 and Fig. 3, respectively, when controlling CAs in the undoped GaAs single crystal to not more than 5 x 10 15 cm-3 becomes 7 only 2 x 10 ZL-cm even at the highest. However, if the scatter- ing in/0 across the wafer diameter is taken into account, a value exceeding 2 x 101-()-.cm is necessary as a design target for /0 of the single crystal.

As a result of extensive investigations in view of this situa- tion, a semi-insulating GaAs single crystal satisfying simulta neously both the condition to prevent the thermal conversion of -3 GaAs single crystal (CAs 5 x 10 cm) and the condition to eliminate the influence of the scattering in /0 of said single cry stal ( 2 x 10 7 R-cm) and a method of preparing the same have been developed by the invention.

The essence of the inventionlies in the semi-insulating GaAs sin- gle crystal characterized in that, at a position of the fraction solidified of GaAs single crystal being 0.8, the concentration of carbon in said single crystal is not more than 5 x 10 15 cm- 3 and the content of Cr is 0.035 to 0.05 wt.ppm, resulting in the achie vement of anticipated purpose by making so.

Moreover, the invention provides a mehtod for the preparation permitting it possible to make the uniform crystal even at such low concentration without using the magnetic field applied method.

That is, the method of preparing the semi-insulating GaAs single crystal characterized in that--, at a position of the fraction 7 solidified of GaAs single crystal being 0.8, the concentration of carbon and the content of Cr are controlled to not more than 5 x -3 cm and 0.035 to 0.05 wt.ppm, respectively, as the con- centration of impurities in said crystal is also another gist.

Reference is made to the accompanying drawings wherein:

Fig. 1 is a measured diagram showing the relationship between the fraction solidified and the resistivity of GaAs single crystal, Fig. 2 is a correlation diagram showing the relationship between the resistivity and N A - N D in GaAs single crystal, Fig. 3 is a correlation diagram showing the relationship between the concent ration of carbon and N A - N D in GaAs single crystal, Fig. 4 is a correlation diagram showing the relationship between the mobility and the concentration of-Cr in GaAs single crystal, Fig. 5 is a correlation diagram showing the relationship between the depth from the surface and the concentration of Cr in GaAs single cry stal, Fig. 6 is a correlation diagram showing the relationship between the resistivity and the concentration of Cr in GaAs sin gle crystal, and Fig. 7 is a sectioned diagram in side view showing the apparatus for the preparation of single crystal by LEC method.

In putting the method of the invention in practice, the method of preparing semi-insulating GaAs single crystal according to

Claims

Claim 3 characterized in that the seed crystal of single crystal of GaAs

is contacted with the mixed melt of Ga with As in the crucible pressurized with inert gas and the method of preparing semi-insulating GaAs single c-rystal according to Claim 4 chara- 8 cterized in that the GaAs single crystal is prepared by rotating the seed crystal while lifting can both be adopted advantageous ly.

The addition of Cr is made for the reason of increasing without increasing CAs as mentioned above. Moreover, the reason why the content of Cr was made to be 0.035 to 0.05 wt.ppm is due to the calculation shown below.

Fig. 4 is a result calculated the dependency of mobility (y) in the GaAs single crystal on the concentration of Cr putting the 7 resistivity on 1 x 10 _(L. cm which was obtained in the case of un- doped crystal. According tp-this, if the lowest critical value of mobility is put as p = 5000 cm 2./V_ sec, the concentration of Cr(Ncr) must be Ncr < 1.0 wt.ppm in order to make 5000 cm 2/V.

sec. Furthermore, Fig. 5 shows the variation of Ncr depending on the depth from the surface when the GaAs single crystal containing a trace of Cr was subjected to the heat treatment fot 15 minutes at 855 C. According to this, it can be seen that Ncr in the surface area is about 20 times as high as that in the bulk.

Therefore, in order to secure Ner 1.0 wt.ppm in the surface area, too, Ncr in the bulk should be regulated like a formulci (7).

Ncr 0.05 wt.ppm (7) Moreover, Fig. 6 is a result calculated the dependency of the resistivity (/0) of GaAs single crystal on the concentration of Cr putting the resistivity on 1 x 107fL.cm which was obtained in the case of undoped crystal. According to this, in order to maintainP > 2 x 10711-cm, which is a design target for making 9 the single crystal, Ner a 0.007 wt.ppm is necessary. However, the segregation coefficient of Cr in GaAs single crystal is 5.9 x 10- 4 and the product tolerable for use among the single crystals lifted by LEC method is within a range of the fraction solidified (g) of 0.1 to 0.8. Nevertheless, since Ncr increases from a por tion of g = 0.1 toward a portion of g = 0.8 and the concentration in the portion of g = 0.8 becomes about five times as high as that in the portion of g = 0.1, by establishing Ncr like a formula (8) in the portion of g = 0.8, Ncr > 0.007 wt.ppm can be secured also in the portion of g = 0.1.

Ncr 0.035 wt.ppm (8) Accordingly, from the formula (7) and the formula (8), the concentration of Cr has only to become like a formula (9) at the fraction solidified of 0.8.

0.035 wt.ppm Ncr 0.05 wt.ppm (9) Though the explanation has been made about LEC method as above, this is possible to apply to GaAs made by HB method, GF method, VGF method or the like.

Example The GaAs single crystal to which Cr was added so that Ncr 0.05 wt.ppm was realized at g = 0.8 was prepared by LEC method under the condition of CAs:5 5 x 1015 cm-3 and the resistivity thereof was measured as a function of the fraction solidified, the results of which are shown in Fig. 1 (polygonal line A in diagram). Besides, for comparison, the GaAs single crystal with out Cr added was prepared by LEC method under the'condition of - 10 C As < 5 x 10 15 cm-3 and the resistivity thereof was measured simi larly as a function of the fraction solidified, the results of which are shown simultaneously in Fig. 1 (polygonal line B in dia gram).

Besides, in Fig. 1, the resistivity of GaAs (A) with Cr added at the position of g = 0.8 was 7 x 1014-1-cm on the average and the resistivity of GaAs (B) without Cr added was 3 x 105SI-cm on the average.

As evident from Fig. 1, the resistivity of GaAs with Cr added shows > 3 x 107-CL-cm at any fraction solidified and it can be seen that the resistivity (not less than 1 x 10 7 -0--cm) indispen sable for the semi-insulating substrate is reproduced even at a low concentration of carbon (not more than 5 x 10 15 cm-3 Whereas, the resistivity /0 of GaAs wihtout Cr added becomes small with increasing the fraction solidified, that is, with going to ward the lower portion of the crystal lifted and ti can be under stood that the crystal cannot be used in the range of g > 0.2 as a semiinsulating substrate.

As described, according to the invention, it is possible to provide the crystal excellent in the resistivity and uniform, further, to supply the substrate for IC having high mobility (not less than 7000 cm2/V_sec) and high activation efficiency (not less than 70 %) due to low concentration of carbon, and so on. Therefore, the invention exerts remarkable effects industri ally.

CLAIMS (1) A semi-insulating GaAs single crystal characterized in that, at a position of the solidification rate of GaAs single cry stal being 0.8 to the mixed molten liquor of Ga with As, the concentration of carbon in said single crystal is not more than 5 x 10 15 cm- 3 and the content of Cr is 0.035 to 0.05 wt.

ppm.

(2) A method of preparing semi-insulating GaAs single crystal characterized in that, at a position of the solidification rate of GaAs single crystal being 0.8 to the mixed molten liquor of Ga with As, the concentration of carbon and the content of Cr are controlled to not more than 5 x 10 15 cm- 3 and 0.035 to 0.05 wt.ppm, respectively, as the concentrations of impurities in said crystal.

(3) The method of preparing semi-insulating GaAs single crystal characterized in that, in the mehtod of preparing semi-insu lating GaAs single crystal according to Claim 2, the seed crystal of single crystal of GaAs is contacted with the mixed molten liquor of Ga with As in the crucible pressurized with inert gas.

(4) The method of preparing semi-insulating GaAs single crystal according to Claim 3 characterized in that the GaAs single crystal is prepared by rotating the seed crystal while lifting.

12 PuLlished 1988 at The Patent Office. State House. 66! High 11-Tolborn. London WCIR 4TP nirther c,Dies may be obtained from The Patent Office, Sales Brg-nch. St MaTy Cray, Orpington. Ktnt BR3 3RD- Printed by Multiplex techniques ltd. St Mary Cray, Kent. Con. D87.