The preparation and properties of ZnSiAs2, ZnGeP2 and CdGeP2 semiconducting compounds

J. Phys. Chem. Solids.

THE ZnGeP,

Pergamon

Press 1966. Vol. 27, pp. 1939-1947.

PREPARATION AND CdGeP,

AND PROPERTIES SEMICONDUCTING

K. h%ASUMOTO, National Research

Printed in Great Britain.

S. ISOMURA

OF ZnSiAs,, COMPOUNDS*

and W. GOT0

Institute for Metals, Meguro-ku,

Tokyo, Japan

(Receiwed 28 February 1966) Abstract-Good crystals of ZnSiAs2, ZnGePa and CdGePz of the group of Ar1BKVC2”compounds have been grown by either vertical Bridgman method or slow cooling. Preparation of the phosphides was carried out with an internal heating high-pressure resistance furnace. The following physical and electronic properties of these compounds were measured: the melting point, lattice constants, microhardness, Seebeck coefficient and thermal conductivity at room temperature, and the temperature dependences of resistivity and Hall coefficient. ZnSi& and ZnGePz are P-type, and CdGePa is n-type. ZnSiAsa is regarded as a similar material to the “semi-insulating” GaAs from the temperature dependence of electrical properties. It was found that the melting ooints of ZnSiAs,. ZnGePz and CdGePl lie at 1096”. 1025” and 800°C. I respectively, and ZnGePi undergoes a solidstate transformation at 952%. CdGePa was determined to have the chalcopyrite structure with the lattice constants of a = 5.740 +O*OOl A and c/u = I.876 &-0.001 A. All the compounds show total thermal conductivities of 0.1-0.2 W/cm-deg, lower than most binary semiconductors A rrrB v. It can be said that on the AnB1”CaV compounds, a general linear trend such as found in the AIBnrCaV1 compounds of the same type exists between the shortest interatomic distance and the hardness or the energy gap, and between the tetragonality and the ordering factor, Aycm(Ay, = the differences in the ionic radii of cations, AX = the electronegativity difference of the constituents).

INTBODUCTION preparation and semiconducting properties of single crystals of ZnSnAsa, which is one of the group of A1lBIVC,V compounds, was previously reported by two of the authors.‘1*2) Further investigations on other compounds of the same group having the chalcopyrite structure established the preparation and properties of ZnSiAs,, ZnGePa, and CdGeP, semiconducting compounds. Energy gaps for these compounds have been estimated from optical absorption measurement,c3) but no data on the preparation and electronic properties of these compounds have been published. For ZnGeP, only, the lattice parameters have been reported.(4*5) During our experiments, however, the values at room temperature of the physical and electrical properties for ZnSiAsa were reported only as a part of a related paper of VAIPOLIN et al.@) THE

* The above paper was presented at the Fall Meeting

of the Japan Institute of Metals in Toyama, 1964.

October 4,

It seems that the major reasons for little being studied on these compounds so far are the technical difficulties in preparing these crystals, especially phosphides. The problem of high vapour pressure of certain constituents at high temperature was overcome by using argon at high pressure surrounding the sealed ampoule containing the starting materials. We were able to prepare crystals of ZnSiAs,, ZnGeP, and CdGePa and measure some of their physical and semiconducting properties. In this paper we shall describe our preparation methods and report on the physical, thermal and electrical properties of these compounds. PREPARATION METHODS The preparation of ZnSiAs, crystals The synthesis of ZnSiAsa was carried out by direct melting of zinc (99.9998%), p-type silicon (-500 Q-cm) and arsenic (99.999%) in stoichiometric quantities in a fused quartz tube having thick wall and a point at the tip. The inside surface of the tube was carbon-coated by the pyrolysis of benzene.

1939

1940

K. MASUMOTO,

S. ISOMURA

and

W.

GOTO

After the air in the tube had been completely substituted with pure argon, it was sealed off at an argon pressure of approximately one atmosphere. Two different crystallization techniques were used: vertical Bridgman and slow cooling. The ampoule in a vertical Bridgman resistance furnace was heated at a rate of 75”C/hr until its contents were completely molten, and was held at less than 40°C above the melting point of ZnSiAs, for 1 hr. The ampoule was vibrated mechanically at 100 c/s from about 400°C to the maximum temperature to remove blow holes and voids and insure complete mixing. The melt was lowered out of the furnace at a rate of 3 mm/hr until room temperature was reached. Polycrystalline ingots, containing a few cracks and sizeable single crystalline regions, were prepared by this procedure. A number of ingots were prepared by slowcooling in order to perform thermal analysis for determination of the melting point of ZnSiAs,. Namely, after the melting process was done under the same condition as mentioned above, the ampoule was cooled to room temperature at a rate of 25”C/hr. Polycrystalline ingots with tine cracks were prepared by this procedure. From these it was possible to isolate crack-free samples for electronic measurements. In both cases, there was no loss of arsenic by condensation on the upper part of the ampoule.

follows: the sealed ampoule was heated at the rates of about 200”, 10” and 20”C/hr, respectively, in the temperature ranges of room temperature to 5OO”C, 500”--800°C and 800”-1060°C in a vertical Bridgman resistance furnace and was cooled to room temperature at a rate of 75”C/hr after it was kept at a uniform temperature of 1060°C for 1 hr. In this case also, a mechanical vibration of 100 c/s was given to it from a low temperature to above the melting point of ZnGeP,. The polycrystalline ingots prepared in this manner contained a few blow holes and cracks. For CdGeP,, it was not possible to avoid explosion of the ampoule under the same melting process as in the case of ZnGeP,. Therefore, after keeping the ampoule at 370°C for 13 hr, in expectation of a quantitative reaction between the molten cadmium and the red phosphorus,* it was gradually heated at a rate of lO”C/hr. However, it exploded at about 600°C. It is thought that the crystal formation velocity of CdGeP, is less than that of ZnGeP,. Thus, the preliminary test for the preparation of CdGeP, failed. After considering the possibilities of penetration of the melt into the graphite crucible, and contamination by diffusion of carbon atoms or impurities in the graphite into the melt during a long heating time in the above methods for both phosphides, the following preparation was made.

The preparation of ZnGeP, and CdGeP, crystals 1. Preliminary tests. In preparing the phosphide

2. Preparation with a high-pressure resistance furnace. Preparation of the phosphide was carried

directly from elements in a sealed ampoule an explosion arises occasionally because of the high vapour pressure of red phosphorus at elevated temperatures. Thus, the preparation has to be done by quantitative reactions between the contents of the ampoulc at sufficiently slow heating rates; however, if the heating time is too long, a reaction may occur between the melt and the ampoule. TO overcome those effects, we used a thick fused quartz tube of 21 mm in o.d. and 4 mm wall, in which a high-purity graphite crucible with 2 mm wall was closely fitted. Stoichiometric quantities of cadmium (99.9999%) or zinc (99*9998%), ptype germanium (N 50 Q-cm) and red phosphorus (99.9999% or 98%) were loaded into the crucible and sealed in the ampoule under the same condition as the case of ZnSiAs,. The crystallization technique for ZnGeP, is as

out in a fused quartz ampoule, of the same size and sealed off under the same conditions as in the case of ZnSiA,, with a Gakei internal heating highpressure graphite resistance furnace which was reconstructed as a vertical Bridgman furnace from a high-pressure Tammann furnace. The heating process was done at rates corresponding to reaction velocities between the elements, with compensating for high vapour pressure of the elements in the ampoule at elevated temperatures with external argon pressures of up to 150 atm. For the synthesis of ZnGeP,, the temperature of the ampoule was raised during the first hour of ~--*After this experiment, HAACKE and CASTELLION"' reported that a reaction between the elements occurs at about SOO”C, but CdsP2 forms a layer on the cadmium surface preventing more phosphorus from reacting with the cadmium.

THE PREPARATION

AND

PROPERTIES

synthesis to SOo”C, and then under the same condi-

tions as in the above preliminary test. During the heating process, the ampoule was subjected to external argon pressures of 7, 13, 45, 80, 100, 125 and 150 atm, respectively, in the temperature ranges of room temperature to 4OO”C, 400”-SOO”C, 500”-SWC, 580”-63O”C, 630”-68O”C, 680”-88O”C, and 880”-106O”C, in consideration of the vapour pressure of phosphorus in it, its durability for pressure and the situation of reactions between the elements. After the ampoule reached 106O”C, rotation and vibration were given to it during the first fr hr. The temperature was held another 4 hr and then the Bridgman process was performed at a lowering rate of 10 mm/hr until the melt reached room temperature. Polycrystalline ingots with a few fine cracks were prepared by this procedure. For the direct synthesis of CdGeP,, the ampoule has to be heated more slowly than in the case of ZnGeP, and kept below 600°C for a proper period, as known from the above preliminary test. We devised the following process: after heating the ampoule to 500°C at the same rate as for ZnGeP,, it was heated to 980°C (above the melting point of germanium) at a rate of lO”C/hr. During the heating process, its temperature was 580°C for 12 hr under an external argon pressure of 60 atm. After a period of + hr at 98O”C, the temperature was brought down to 830°C just above the melting point of CdGeP,, at which temperature the ampoule was subjected to rotation and vibration for i hr, and then the Bridgman process was done at the same rate as for ZnGeP,. In this case the ingots obtained were also polycrystalline with a few fine cracks. PHYSICAL PROPERTIES

X-ray radiation

powder diffraction with copper for the obtained ZnSiAs,, ZnGeP,

K, and

OF ZnSiAsa,

ZnSiAsa ZnGePz CdGePz

AND

CdGePa

1941

CdGeP, crystals confirmed the tetragonal chalcopyrite structure with a single phase. The lattice constants at room temperature are given in Table 1. Values reported here represent the results of a number of determinations. The lattice constants for ZnSiAs,, a = 5~612+0~001 A and c = 10.878 + 0.002 A, are roughly equal in magnitude to those reported by VAIPOLIN et al. (@ For ZnGeP,, a and c are in good agreement with values reported by PFISTER.‘~) For CdGeP,, we have found that the lattice constants are a = 5.740 + 0.001 A,

and

c = 10.773 5 o-002 A, c/a = 1*876&O-001

Differential thermal analyses (DTA) were made on crushed samples. A pure graphite crucible containing the sample was sealed in a fused quartz tube under the same condition used for making the compounds to prevent the attack of quartz by molten sample in repeated analyses. Both the graphite crucible and the quartz tube with concentric thermocouple wells were placed in a massive nickel cell. The DTA apparatus was operated at temperatures up to 1200°C. The vertica1 Kanthal resistance furnace, which had an automatic programme control, provided essentially linear heating rates of 6-lO”C/min. The temperature difference (AT) between a sample and the thermally inert reference material (A1203) was continuously recorded vs. the temperature of the reference material. As an example, the thermal arrests on heating and cooling ZnGeP, are shown in Fig. 1. Thermometry of the apparatus was calibrated by measuring the melting points of high-purity zinc, aluminium, silver and copper. The melting points of the compounds assigned to the arrests are given in Table 1. The measured melting point of ZnSiAs,, 1096”C, is in substantial disagreement with the value of 1038°C reported by VAIPOLIN et al@)

Table I. Physical properties of ZnSiAsz,

Compound

ZnGePa

ZnGePz

and CdGeP,

crystals

Melting point (“C)

Lattice constant a(&

Lattice constant c(A)

Microhardness Hv(kg/mma)

1096 1025 800

5.612 kO.001 5.466 f O-001 s-740 f 0.001

10.878 +0*002 10.722 f0.002 10.773 +0.002

480 + 20 660 * 20 460 + 20

1942

K. 2

MASUMOTO,

S.

ISOMURA

and

W.

GOTO

0.15 - - -

2

Heating Cooling

G”C/min YC/min

8 0.10 E e G?! z r f E t 2 c”

0.05 -

_’ OS20

A \

/

/’

\___*-

I

I

,

940

960

980

1000 Temperature,

1020 “C

1040

1060

IO60

1100

FIG. 1. Typical differential thermal analysis heating and cooling curves showing temperature difference vs. temperature for ZnGePs.

It has been found that the melting points of ZnGeP, and CdGePa lie at 1025°C and gOO”C, respectively, and ZnGeP, undergoes a solid-solid phase transformation at 952°C. The average Vickers microhardness of ZnSiAs,, ZnGePa and CdGePa were also measured as shown in Table 1. The value for ZnSiAs, is considerably lower than the value of 920+20 kg/mm2 reported by VAIPOLIN et al.@) Table 2 was made to show more systematic correlation between the hardness and the structural parameter, d, which represents the distance between the closest neighbours in A. Table 2 shows a general linear trend, though very crude: an increase in the shortest interatomic distance produces a decrease in the hardness and the energy gap.

THERMAL

PROPERTIES

Most of the thermal measurements were made at room temperature for blocks of the compounds 4 x 4 mm2 in cross section and 10 mm in length.

The Seebeck coefficients were determined from the slope of a linear relation between the temperature difference (within SC) and thermal e.m.f. measured while heating one end of the sample with a small heater in vacuum. The total thermal conductivities were measured by the conventiona steady state linear heat flow absolute method on a well polished face of the sample. The sample and the copper block containing a heater were enclosed in a chamber with silver plated inside wall, so that the radiation did not contribute to the heat loss. Contacts were made by using silver paste. The measured value for germanium above 50 Q-cm, 0.56 W/cm-deg, which is in good agreement with that reported by GLASSBRENNER and SLACK,(~) served as a check on the calibration. A summary of the thermal data is given in Table 3. The Seebeck coefficients for ZnGeP, and CdGeP, were obtained at 98” and 108”C, respectively, because it was difficult to measure such low thermal e.m.f. at room temperature. The total thermal conductivities for all the compounds

Table 2. Physical properties of the ArlBrVCZV compounds (d is the distance between closest neighbours) Compound ZnSiPs ZnGePs ZnSiAsz ZnGeAsz ZnSnAsz CdGePz CdGeAsz

Lattice constant a(A) 5.398’4’ 5.466 5.612 5~670’~’ 5-851’2’ 5.740 5.942’5’

d(A) 2.34 2-37 2.43 2.45 2-53 2.49 2.57

Microhardness Hv(kg/mm2) 1100’6’ 660 480 640’s’ 460 460 471(e)

Energy gap %(eV) 2.5’8’ 2.2’3’ 1.76’s’ -0.6’3’ 0*59’s’ 1.8’3’ 0.53’6’

THE

PREPARATION

AND

PROPERTIES

are lower than those for most of binary conducting compounds, A1’*BV.

semi-

Table 3. Thermal properties of ZnSiAs,, ZnGeP, and CdGePs crystals at room temperature Seebeck coefficient Compound ZnSiAsa ZnGePa

No. 5 No. 11

CdGePz

No. 6

S(pV/deg)

Thermal conductivity K(W/cm-deg)

+1100 +1200*

0.14 0.18

N -1200t

0.11 -

* At 98°C. f At 1OVC. ELECTRICAL PROPERTIES The measurements of resistivity, p, and Hall coefficient, RH, for ZnSiAs,, ZnGeP, and CdGeP, were performed by the conventional d.c. potentiometric method over the temperature range from the temperature of liquid nitrogen to about 700°C. The dimension of the sample is 1 x 1 mm2 in cross .section and about 7 mm in length. As the attempts to solder electrodes were not always successful, for most of the samples electrolytic spot-coating with nickel and for some high-resistivity samples electroless coating with nickel were applied. The electrodes consisting of platinum probes of 0.05 mm in dia. were soldered to the nickel coat; they were held in contact with the coat by spring tension at temperatures above the melting point of the solder. Measurements below room temperature were made Table 4. Electrical properties of ZnSiAs,,

Compound

Type

OF

ZnSiAss,

1943

CdGePs

ZnGeP, and CdGePs crystals at room temperature (ZnSiAs, No. 4 is a single crystal)

Resistivity .~(a-cm)

Hall coefficient &f(cms/C)

P P P

1+3x10s 1.9x10” 4.5 x 102

+1.9 x 103 +7+3 x 103 +1*1 x 104

ZnGePa

No. 9 No. 12

P P

1.3 6.5 x lOa

Low Hall voltage Low Hall voltage

CdGePs

No. 5 No. 6

n n

6.5 x10-r 4.8 x 10’

-6x10-a Low Hall voltage

-

AND

in vacuum, while those above room temperature were made in argon at a pressure of about 2 atm in order to suppress evaporation from the sample of the elements. A summary of the electrical data on some ingots of ZnSiAss, ZnGeP, and CdGeP, at room temperature is given in Table 4. The carrier concentration, n, is assumed to be given by n = I/R,e, and the Hall mobility, p, is defined as RJp. In Table 4, ZnGePa No. 9 and CdGeP, No. 5, which were prepared by using less pure phosphorus (98%), showed very low resistivities in comparison with ZnGePa No. 12 and CdGePs No. 6 using high-purity phosphorus. It seems that the difference between both high and low resistivities is due to impurities in phosphorus. As a result it can be said that because of very high resistivities of the phosphides and probably very low mobilities of the current carriers, it was not possible to measure the Hall coefficients. For ZnSiAs,, all samples show the very low carrier concentrations of approximately 1014/cm3. These low values are generally reasonable only for the semiconducting compounds prepared by floating-zone-refining etc. Therefore, it seems likely that in ZnSiAs,, the donors and acceptors present were compensating each other automatically. The higher mobility of ZnSiAss No. 4 compared to that of other ZnSiAs, must be because No. 4 is a single crystal. The electrical properties at room temperature of No. 5 not single crystal are very close in magnitude to the values obtained by VAIPOLIN et aZ.@) The temperature dependence of the electrical properties of these compounds is described below:

No. 1 No. 4 No. 5

ZnSiAsa

ZnGePs

--

Carrier concentration n(cm- 3,

Hall mobility r(cms/V-set)

3.2 x lOI5 8.0 x 1Or4 5.6 x 1Or4

-1

x 1020

11 41 25

-1

x10-1

1944

K. MASUMOTO,

S. ISOMURA

Fig. 2 shows the temperature dependence of the resistivity for three samples of ZnSiAs, given in Table 4. The interesting feature of the result for ZnSiAs,, which is one of the cross substitutional

and W. GOTO

Fig. 2. However, the impurities or defects responsible for this ladder of levels have not yet been identified. Typical temperature dependence of the resistivity of ZnSiAs, (No. 7) over the temperature

FIG. 2. Temperature dependence of the resistivity for three samples of ZnSiAsz given in Table 4. derivatives of AIIIBV compounds, is that the dependence is quite similar to that observed in highresistivity GaAs (it has been called “semi-insulating” material(lO*ll)). Since our ZnSiAs, also shows a relatively high resistivity, and some process of automatic compensation may be taking place in it as mentioned above, this compound is regarded as a material similar to GaAs. Two activation energies, which may be interpreted in terms of two acceptor levels located above the top of the valence band, were calculated from the graph of

range from about 90°C to about 520°C is shown in Fig. 3. The resistivity of this sample rises with increasing temperature beyond a minimum at about 480°C and the property expected of intrinsic material is not revealed in the temperature range measured. Above about 53O”C, ZnSiAs, dissociated. The temperature dependence of the Hall mobility for three samples of ZnSiAsa given in Table 4 is illustrated in Fig. 4. The decrease of mobility in low temperature range studied indicates that scattering due to impurities, defects, etc.

THE

PREPARATION

AND PROPERTIES

contributes to a great extent as the temperature is decreased. Figure 5 shows the temperature dependence of the resistivity for ZnGePs No. 12 and CdGePs No.

6 prepared by using high-purity

Temperature

103/T.

phosphorus.

Both

OF ZnSiAss,

ZnGePa

AND CdGePz

1945

purity phosphorus, The temperature dependence of the resistivity is characteristic of a degenerate semiconductor which exhibits the usual variations with impurity concentration and crystal perfection. After carrying out the above work, a related paper of VAIPOLIN et al. on “Studies on ZnSiPs, CdSiPs and ZnSiAss Crystals” appeared. The temperature dependences of resistivity and Hall coefficient of ZnSiAs, in the temperature range measured by them are similar to those presented in our paper, however their values of melting point and microhardness are in substantial disagreement with our data.

OK-’

Typical temperature dependence of the resist&$ of ZnSiAss (No. 7) in the higher temperature range up to about 520°C.

FIG. 3.

FIG. 4.

Temperature dependence of the Hall mobility for three samples of ZnSiAsz given in Table 4.

resistivities fall considerably with increasing temperature in the range of room temperature to about 5Oo”C, but do not show the intrinsic conduction. ZnGePa dissociated above about 53O”C, too, and CdGeP, dissociated above about 490°C. CdGeP, No. 5 obtained by using less pure phosphorus shows smaller resistivity and greater electron concentration than No. 6 using highY

u

I

15

I

I

2.5 Temperature 103/T, “K-’ 20

I

30

5

FIG. 5. Temperature dependence of the resistivity for ZnGeP, (No. 12) and CdGePs (No. 6) prepared by using 99.9999 o/Ored phosphorus.

1946

K.

MASUMOTO,

S. ISOMURA

and

W.

GOT6

I

0

f-2 Orr&p FIG. 6. Dependence

cationic

ORDERING

ordering

AND TETRAGONALITY AnBI”Czv COMPOUNDS

factor

5.

Tetragonality

factor Ay,Ax compounds.

IN THE

6, ionic radius

ZR

of the tetragonality

It is considered that the ordering may have a large effect on the electronic properties of multicomponent semiconductors (in particular, on charge carrier mobility). In ternary semiconducting compounds with covalent type bonds, if we call “anions” the atoms in such a compound that have the highest value of electronegativity, the atoms of one constituent (anion) differ considerably in their chemical properties from the atoms of the other two constituents (cations). As a result, the cation-anion order in the lattice of a covalent crystal is preserved at very high temperatures up to the melting point of the Table

Hi

04?

for

8 on the interthe

AnBrvC2v

crystal. For this reason, the ordering in the cationic sublattice is regarded as that in the ternary compounds. As a measure of the ordering, we take the tetragonality of the lattice of the compounds as proposed by FOLBERTH and PFISTER.“~) By the term “tetragonality” we mean the quantity S = a-c/2

(1)

where a and c are the lattice constants, The tetragonalities in several A1lBIVCaV compounds containing the data presented in our paper are given in Table 5. A’Y,, the difference in the ionic radii of cations in the compounds, was calculated from the ionic radii given by BELOV and BoKII.(~~) It is quite obvious from Table 5 that

difference Aye and electranegativity .i~r’B~~Cs~ compounds

difference

Ax

of the

Lattice constant

Ordering factor

Compound

Source

a(A)

6 (A)

ZnSiP, ZnSiAsl ZnGePa ZnGeAsz ZnSnAsz

(4) Authors Authors (5) Authors

5.398 5.612 5.466 5.670 5.851

0.18 0.17 0.11 0.09 0.00

0.44 0.44 o-39 0.39 0.16

0.45 0.35 0.45 0.35 0.35

0.20 0.15 0.18 0.14 0.06

(6)

5.678 5.740 5.942 6.093

0.46 0.35 0.33 0.13

0.60 0.55 0.55 0.32

0.45 0.45 0.35 0.35

0.27 0.25 0.19 0.11

CdSiP, CdGePz CdGe& CdSnAsz

--I_

Authors

(5) (14) __--.-_____

._____

AYE

- -----:

-

THE PREPARATION

AND PROPERTIES

there is a very close correspondence between Ay, and 6 of the compounds (as an example, see the ZnS+,-ZnGeAs,series of compounds ZnSnAs,). Despite the predominantly covalent bonds in semiconducting compound crystals, the relatively small fraction of ionic bonds, namely, the ionicity (defined as the percentage of the ionic component in mixed covalent-ionic bonds) is always present in them and responsible for the ordering in their cationic sublattice. It is logical to assume that the larger the ionicity in a covalent compound, the greater the effect of the ionic size of cations in the compound. The former is determined from the electronegativity difference of the constituents, fi.(15*16) Then the tetragonality is proportional to two quantities:

(2) The quantity Ay,m, which describes the energy of the intercationic ordering in a covalent crystal with an ionicity, has been called the ordering factor by PALATNIK et a1.(17) In Table 5, a and the ordering factors in several A1rB1VC2V compounds are also shown. Figure 6 shows the dependence of 6 on the ordering factor for the compounds, and approximately the general form of the function (2). Thus, it is considered that the factor Arc= determines the tetragonality for this group of compounds. Acknowledgements-The authors would like to express their thanks to staffs of the Gakei Electric Works Co., Ltd. (3108, Senju-Sakuragicho, Adachi-ku, Tokyo,

OF ZnSiAsa,

ZnGePz,

AND CdGePz

1947

Japan) for the preparations of the compounds, and to Dr. U. HASHIMOTO,Director of the Institute, for permission to publish this paper. REFERENCES 1. MASUMOTOK. and ISOMIJRAS., J. Japan Inst. Metals 28,663 (1964). 2. MASUMOTOK. and ISOMURAS., J. Phys. Chem. Solids 26, 163 (1965). 3. GOODMANC. H. L., Nature, bnd. 179,828 (1957). 4. FOLBERTH 0. G. and PFISTER H., Semiconductors and Phosphors, p. 474. Friedr. Vieweg, Braunschweig (1958). 11, 221(1958). 5. PFISTER H., Acta Crystallogr. 6. VAIPOLIN A. A., GA~HIMZADEF. M., GORYIJNOVA N. A., -AMANLY F. P., NASLBDOVD.N., OSMANOV E. 0. and RUD Yu V., Izv. Akad. Nauk SSSR, Ser. Fiz. 28. 1085 (19641. G. and CA&EL&N G. A., J. Appl. Phys. 7. HAACKB 35,2484 (1964). 8. VOITSEKHIVS’KII0. V., Ukr. Fiz. Zh. 9,796 (1964). 9. GLA~~BRENNERC. J. and SLACK G. A., Phys. Rev.

134,Al058 (1964).

IO. BLANC J., BUBE R. H. and MACDONALD H. E., J. AppL Phys. 32, 1666 (1961). Vol. 9, p. 137 Il. HILSUM C., Prog. in Semicond. (1965). 12. VAIPOLINA. A., GORYUNOVAN. A., OSMANOVE. O., RUD Yu. V. and TRETIAKOVD. N., Dokl. Akad. Nauk SSSR 154,1116 (1964). 13. FOLBERTH 0. G. and PFISTER H., Acta Crystallogr. 13, 199 (1960). 14. GASSON D. B., HOLMES P. J., JENNINGS I. C., MARATHEB. R. and PARROTTJ. E., J. Phys. Chem. Solids 23, 1291 (1962). 15. BOKII G. B., CrystaZChemistry(in Russian) Izd. MGU (1960). 16. GORDY W. and THOM.U W. J. O., J. Chem. Phys. 24, 439 (1956). 17. PALATNIK L. S., KOSHKIN V. M. and GAL’CHINETSKII L. P., Sow. Phys. Solid St. 4,1732 (1963).