Mat . R e s . Bul l . Vol . 8, pp. 1Z57-1Z68, 1973. P e r g a m o n P r e s s , Inc. P r i n t e d in the Uni ted S t a t e s .

PHASE DIAGRAM AND PROPERTIES OF Cu^SbSe 4

AND OTHER A31BVc4VI COMPOUNDS .3

Walter Scott and J. R. Kench Honeywell Corporate Research Center

10701 Lyndale Avenue South Bloomington, Minnesota 55420

(Received A u g u s t ZI, 1973; Communicated by R. J. Stokes)

ABSTRACT The phase diagram of Cu3SbSe. indicates that this material is not

4 a congruently melting compound, but that it forms via a peritectic reaction at about 390°C. The material grown either by a Bridgman or quenching process is p-type, with carrier concentrations of about 2 x 1019/cm 3 and 3 x 1018/cm 3, respectively. The addition of Zn produces n-type crystals with a carrier concentration of 3 x i017/cm3. The room temperature energy gap of Cu3SbSe 4 is 0.25 ± .05 eV as determined by optical absorption. Cu3AsSe 4 was also prepared but was not congruently melting, while Cu3AsTe 4 and Cu3SbTe 4 apparently do not form.

Introduction

The first reports of the preparation of Cu3SbSe 4 were made by Wernick

(i) and Wernick and Benson (2) who reported a single phase structure with

a melting point of 425°C and an energy gap of 0.2 eV. Recent investigations

by Nakaniski et al. (3) have indicated that Cu3SbSe 4 is a semiconductor with

a bandgap of 0.13 eV. The energy gap was determined from room temperature

transmission measurements in powdered samples dispersed in a KBr plate.

Their material appeared to be degenerate p-type with a carrier concentration

of about 1019 holes/cm 3. Also only a brief mention was made of the fact

that a second phase might be present in their material which they could not

detect by x-ray analysis.

This work was partially supported by the Office of Naval Research

IZ57

IZ58 A31BVc4 VI COMPOUNDS Vol. 8, No. I0

properties of other A31BVc4 VI compounds are reviewed by Berger and The

Prochukhan (4). They list the properties of Cu3AsS4, Cu3SbS4, Cu3AsSe 4 in

addition to Cu3SbSe4, but made no mention of any compounds with tellurium.

The energy gaps listed for those compounds were 1.24 eV for Cu3AsS4, 0.65 eV

for Cu3AsSe 4 and 0.31 eV for Cu3SbSe 4.

Since small bandgap semiconductors are of interest for various device

applications, we undertook a study of the growth and properties of some

A31BVc4VI compounds. We attempted to extend the work of Nakanishi et al.

(3), but found that the material prepared by a Bridgman process was not

single phase. As a result, a study of the phase diagram along the Cu.75Sb.25

to Se section was carried out to determine a growth technique capable of

producing single phase material. The method finally used to produce single

phase material was a rapid quench of a stoichiometric melt. Electrical

measurements of both Bridgman grown and quenched Cu3SbSe 4 have been made

in addition to room temperature optical absorption measurements. Three other

combinations from this class of compounds have also been prepared.

Experimental Procedure

The procedures used to investigate Cu3SbSe 4 can be divided into three

categories; differential thermal analysis to determine the phase diagram;

crystal growths using both a modified Bridgman and a rapid quench method;

and the evaluation of the properties of the crystals. Only one attempt

was made to prepare Cu3AsSe4, Cu3AsTe 4 and Cu3SbTe 4 using the differential

thermal analysis procedures.

For the phase diagram determination the elementfi were sealed in a quartz

ampoule with a re-entrant dimple to provide good thermal contact with the

reactants. The samples were heated to about 800°C and agitated before being

cooled, typically at 50°C/hour for 5 gram samples. The phase transforma-

tions were recorded on cool-down only for all runs. An equal weight of

indium sealed in a similar quartz ampoule was used both as a reference and

calibration sample for each run. The measured freezing point of indium was

used as an indication of the accuracy of the measured temperatures. Typi-

cally the measured indium melting point was within one or two degrees of

the listed value. Temperature differences were kept to a minimum in the

furnace by using the center portion with a thick walled Ni liner. Typical

temperature differences along the length of the ampoules were less than I°C.

Vol. 8, No. I0 A31BVc4 VI COMPOUNDS 1259

Also a 50°C/hour cool-down rate appeared to be slow enough to eliminate any

temperature gradients throughout the sample and reference ampoules even for

the relatively large samples used in these measurements. The zero offset on

differential temperature was less than 1.5°C at this cool-down rate.

For crystal growth, 99.9999% pure starting materials were used. In

the modified Bridgman method the samples were sealed in quartz tubes and

heated to about 800°C and agitated to promote mixing. The samples were

kept in a temperature gradient, so the crystals were grown by slowly cooling

the furnace to give an equivalent growth rate of about 2 mm/hour. In the

quench method, samples about i gm in total weight were loaded into thin

walled quartz tubes. These were kept at 650°C for about 16 hours to insure

homogenization and then dropped straight into ice water. In some cases Zn

was added to these crystals as a foreign atom impurity.

The ingots were all examined microscopically to determine the number

of phases present. Each phase was identified by electron beam microprobe

x-ray analysis. This method appeared to be quantitatively quite good on

the elemental and binary phases, but was not better than about 5 atomic

percent for each constituent in the ternary phases. As a result, the com-

position of the ternary phases were only nominally identified as the desired

A3IBVc4VI compounds. Electrical conductivity and Hall effect were measured

by the standard dc technique using a i0 kG magnetic field. Optical absorp-

tion measurements were also done in a standard manner using samples lapped

and polished to i0 to 50 ~m.

Results

Phase Diagram and Crystal Growth of Cu3SbSe 4

A vertical section of the ternary phase diagram along the Cu.75Sb.25to

Se line is shown in Figure i. Five compositions were used along this section,

with the first phase boundary encountered on cooling indicated with a cross.

The other phase boundaries encountered on continued cooling are indicated

by the circles.

Solidification from a stoichiometric Cu3SbSe 4 melt produces four phases;

Cu2Se , Cu3SbSe4, Sb2Se 3 and Se. The microstructure of a stoichiometric ingot

produced by a fairly rapid growth is shown in Figure 2. The darkest regions

have been :identified as Cu2Se which appears as a dendritic structure in a

background of Cu3SbSe 4. The light regions are Sb2Se 3 along with Se in a

few places.

IZ60 A3IBVc4 VI COMPOUNDS Vol. 8, No. 10

7oo \ x

600 // \ L, / \ L2

Cu2Se A 500 / LI + L2 \ ? + /

LI x / \ e~ / x

0 ~ ~ . . . . X X 1 ~

400

1 t r ) ~ 0 . . . . ~ " " - ' -

O.

" ' / ~- 300 Cu3SbSe4 + CuzSe + L2

o- / 20C o~

100

0 I I I I [ .3 ,4 .5 .6 .7 .8 .9 I.O

(Se) ATOMIC FRACTION Se ALONG THE Cu.75 Sb.z5'LSe LINE

FIG. 1

Phase diagram of Cu-Sb-Se along the Cu.75Sb.25-Se section.

Relatively single phase crystals were produced from the stoichiometric

melt by the Bridgman growths, but in general, these crystals had small

inclusions of Sb2Se 3 and Se which accounted for a few percent of the total

sample volume. These crystals were later used for the electrical and

optical measurements even though they were not entirely single phase.

The Se rich section of the phase diagram was investigated to determine

if growth from an Se solution would be possible. Samples loaded with more

than 50 atomic percent Se always segregated into two distinct regions

suggesting the presence of two immiscible liquids. The top region was

Se with a small amount of Sb2Se 3 scattered throughout. Along the bottom

of the ingots, and clearly separated from the Se phase, was a region of

Cu3SbSe 4 with entrapped globules of Se. Also scattered throughout the

Cu3SbSe 4 was a small amount of Cu2Se. The presence of both the Cu2Se and

Sb2Se 3 indicates that the equilibrium compositions for Se rich melts do not

Vol. 8, No. 10 A31BVc4 VI COMPOUNDS 1261

FIG. 2

Microstructure of slowly cooled Cu3SbSe 4 (440x)

lie along the Cu 75Sb.25 to Se vertical section. The fact that there is

a clear separation of the two regions in the ingot, in addition to the fact

that the Se entrapped in the Cu3SbSe 4 is predominantly spherically shaped,

were the main factors identifying the presence of two immiscible liquids.

For each of the compositions used in the differential thermal analysis

runs, a large reaction was observed at 410 ± 5°C. The material had a tenden-

cy to supercool, so there was a temperature jump at this point giving rise

to the ± 5°C temperature uncertainty in the phase boundary. As a result, it

is uncertain whether or not the bottom of the two-liquid region is isothermal,

but has been drawn so nevertheless. Another phase boundary was observed on

cooling below 410°C and this has been tentatively identified as a three phase

region of Cu3SbSe4, Cu2Se and L 2. The extent of this region was not clearly

established, so is only dotted in the diagram. The actual stability limit

of Cu3SbSe 4 is therefore only about 390°C, at which temperature it begins

decomposing into the three p~ases. A further indication that Cu3SbSe 4 forms

via a peritectic reaction was obtained from the run on the liquid of composi-

tion Cu.43Sb.14Se.43. The microstructure obtained in this ingot is shown in

Figure 3. The dark regions are dendritic Cu2Se, which is the first phase to

form. Surrounding the Cu2Se is a cored type of structure indicative of a

peritectic reaction. Because of the fine structure in the background region,

A31BVc4VI COMPOUNDS Vol. 8, No. I0 IZ6Z

no positive identification of these phases could be made to clearly establish

the sequence of events on freezing. However, the structures observed both

on this composition and on the Cu3SbSe 4 samples are consistent with the

interpretation that the compound forms via a peritectic reaction.

FIG. 3

Microstructure of Cu.43Sb.14Se.43 showing cored

structure surrounding Cu2Se (256x).

The main fact obtained from the phase diagram determination is that

Cu3SbSe 4 is not a congruently melting compound, and that growth of single

crystals is not likely from an Se solution because of the two-liquid region.

By quenching the samples from a stoichiometric melt it was hoped that the

peritectic reaction could be suppressed so that homogeneous Cu3SbSe 4 could

be produced. The samples grown this way were never entirely single phase,

but large enough single phase regions could be found for electrical and

optical measurements. A micrograph of one of the quenched ingots is shown

in Figure 4.

Comparing the structure of this crystal with results of a slower cool-

down (Figure 3) clea~ly demonstrates that single phase material can be produc-

ed by a rapid quench. Generally the outside skin of the ingot was single

phase with the interior having the same structure as in the slowly cooled

samples. This single phase skin varied from about 50 ~m to i mm in thickness

for the various crystals. The limiting factor in the quenching is believed

Vol. 8, No. 10 A3IB VC4 vI COMPOUNDS 1Z63

FIG. 4

Photomicrograph (440X) of quenched Cu3SbSe 4 surface, demonstrating homogeneity.

to be the quartz tube itself. Decreasing the wall thickness or cracking the

tube during the quench are possible ways to increase the volume of single

phase material.

X-ray analysis of both the Bridgman grown and quenched material indicate

essentially a cubic structure with lattice constant of 5.63 A. A slight

splitting in the higher angle lines was; observed indicating that there is

some distortion of the cubic structure.

Electrical and Optical Properties of Cu3SbSe 4

The resistivity and Hall coefficient were measured at 300 ° and 77°K for

all of the samples and in a few cases as a function of temperature over this

range. The Hall coefficient was found to be independent of temperature in

all the samples studied down to low temperatures. The carrier concentrations

1019 l 1018 ranged from about 2 x holes/cm 3 in the Bridgman grown samples to 3 x

holes/cm 3 in the undoped quenched samples. The addition of 0.5 wt% Zn to the

1017 melt produced n-type conductivity with a carrier concentration of 3 x ~Icm 3.

A reduction of the excess Zn to .05 wt% produced p-type conductivity. The

room temperature mobility was approximately 50 cm2/volt sec in the Bridgman

grown samples, which agrees with the results of Nakanishi et al.(3) for this

carrier concentration. At a hole concentration of 3 x lol8/cm 3 the room

1264 A31BVC4 vI COMPOUNDS Vol. 8, No. I0

temperature mobility was 90 cm2/volt sec. In the n-type samples the room

temperature mobility was as high as 280 cm2/volt sec at a carrier concentra-

tion of 3 x 1017/cm 3, but had decreased to 128 cm2/volt sec at 77°K. The

difference in mobility between the Bridgman grown crystals and the quenched

crystals is believed to be due to the difference in carrier concentration

rather than due to the growth method. As a result the two methods appear

to produce material with similar electrical properties, except that the

carrier concentration has been made lower in the quenched ingots.

iO 4

'E

t.- QUENCHED z w n - T Y P E - - - - - - - - - - - ~ t'

L,~ LI-

0 0

Z _o

n -

O

rn 103 <Z p - T Y P E

4 0 0 I I I I I

.10 .15 .20 .25 .30 .35 .40

ENERGY (eV)

FIG. S

Absorption coefficient in Cu3SbSe 4 at room temperature for p-type carrier

concentrations of 1 x lolg/cm 3 (Bridgman) and 3 x i018/cm3 (quenched), and

an n-type carrier concentration of 3 x 1017/cm 3.

The optical absorption coefficient for two quenched samples and a Bridg-

man grown sample are shown in Figure 5. The carrier concentration in these

samples is 2 x lol9/cm 3 for the Bridgman, 3 x 1018 holes/cm 3 and 3 x 1017

electrons/cm 3 for the p-type and n-type quenched samples. The absorption

edge for the p-type samples is quite broad and is shifted to lower energies

Vol. 8, No. 10 A31BVc4 VI COMPOUNDS IZ65

in the one with the lower carrier concentration. The absorption edge is -i

fairly sharp in the n-type sample, but only decreases to 800 cm The

reason for such a high residual absorption is not known, but it does not

appear to be free carrier absorption. The reflectivity was measured in a

quenched sample relative to A~ as a standard in order to calculate the

absorption coefficient. The reflectivity was essentially wavelength inde-

pendent over the range 3 ~m to 2 ~m, with a value of 0.34.

Other A31BVc4 VI Crystal Growths

Other compounds from this class that were investigated are Cu3AsSe 4,

Cu3AsTe 4 and Cu3SbTe 4. In each case, one small ingot was grown for differ-

ential thermal analysis measurements and analyzed as described previously.

The results of these growths are listed in Table i. In each case the resul-

tant phases from the stoichiometric growths are listed.

TABLE i

Results of A31BVc4 VI Crystal Growths

Starting Composition Results

Cu3AsSe4 Cu2Se , Cu3AsSe4, As2Se3, Se

Cu3SbSe4 Cu2Se , Cu3Se4, Sb2Se3, Se

Cu3AsTe4 Cu2Te , As2Te3, Te, Possible

traces of Cu3AsTe 4

Cu3SbTe 4 Cu2Te , Sb2Te3, Cu4Te 3 or CuTe, Te

Cu3AsSe 4 appears to be similar to Cu3SbSe 4 in the growth process. The

first constituent to freeze at 490°C from a stoichiometric melt is Cu2Se.

At 440°C a large reaction was observed, probably due to the formation of

Cu3AsSe 4 via a peritectic reaction and then the formation of As2Se 3 and Se.

The ternary phase Cu3AsSe 4 made up the bulk of the crystal in the same way

IZ66 A31BVc4 vl COMPOUNDS Vol. 8, No. I0

that CusSbSe 4 was the predominant phase in those crystals. In the case of

the tellurides, no trace of Cu3SbTe 4 was found, and only small areas of a

phase tentatively identified as Cu3AsTe 4 were found in that crystal. In

both tellurldes the binary compounds made up the bulk of the volume as

opposed to the ternary phase which dominated in the selenides.

Discussion

It appears that previous authors have overlooked the fact that Cu3SbSe 4

is not a congruently melting cpmpound, but that it forms via a peritectic

reaction at about $90°C. The material appears to be a semiconductor with

an energy gap of 0.25 ± .05 eV at room temperature. This energy gap was

determined from the absorption coefficient in an n-type sample shown in

Figure 5. An extrapolation of the absorption coefficient in this sample -i

below i000 cm would asymptote approximately .25 ± .05 eV. The effects

of band filling have not been considered, but the fact that both the n-type

and p-type samples show some sort of an absorption edge at about the same

energy indicates that this is a reasonable estimate of the energy gap. This

value is somewhat in agreement with the value of 0.2 eV quoted by Wernick

(i), but disagrees with the value of 0.13 eV measured by Nakanishi et al.(3).

The reason for such a large discrepancy is not known at this time. The

electrical properties we measure do agree with those presented by Nakanishi

et al.

As far as crystal growth of these compounds is concerned, it appears

as if single phase material cannot be grown from a melt in the case of either

CusSbSe 4 or Cu3AsSe 4. Rapid quenching of a stolchiometric liquid was partly

successful in producing single phase Cu3SbSe4, but improvements in the tech-

nique used here are necessary in order to increase the relative amount of

single phase material in the crystal. In the case of the tellurides, there

is a possibility that neither exist as ternary compounds. Cu3SbTe 4 definitely

does not exist, and there is doubt as to whether the phase identified as

Cu3AsTe 4 is, in fact, this compound.

Acknowledgements

We would llke to thank Messers R. Camp, A. Contolatis, and J. Sjerven

for the preparation and analysis of the materials used in this investigation.

We would also like to thank Mr. C. Knudson for the x-ray analysis and Ms. S.

Marquardt for the electron beam microprobe analysis on these materials.

Vol. 8, No. i0 A31~Vc4VI COMPOUNDS IZ67

References

i. J. H. Wernick, U. S. Patent 2,882,470, April 14, 1959.

2. J. H. Wernick and K. E. Benson, J. Phys. Chem. Solids 3, 157 (1957).

3. H. Nakanishi, S. Endo and T. Irie, Jap. J. Appl. Phys. 8, 443 (1969).

4. L. I. Berger and V. D. Prochukhan, Ternary Diamond-Like Semiconductors. Consultants Bureau, New York {1969).