Pressure sintering of GaSb

Mat. Res. Bull. Vol. 2, pp. 91-100, 1967. P e r g a m o n P r e s s , Inc. P r i n t e d in the United States.

PRESSURE SINTERING OF GaSb

Peter R. Sahm RCA Laboratories~ Princeton~ New Jersey

(Received November 23, 1966; Communicated by J. H. Schulman) ABSTRACT GaSb with nearly theoretical densities has been hot pressed at 25~000 psi and 690°C. The compaction mechanism of coarse GaSb particle matrices (20-500~) is largely one of plastic flow. However~ GaSb powders with average particle diameters below 4~ compact according to a mechanism that involves the surface dissociation of GaSb resulting in the presence of liquid Ga. The liquid phase leads to a high compaction rate and grain regrowth.

Introduction Although semiconductor applications usually involve the preparation of high purity single crystals~ in certain cases cheaper polycrystalline aggregates can be utilized equally well (for instanc% electric modules (2)).

lasers(l) or thermo-

A simple method of preparing such materials is pressure

sintering~ but relatively little is known about the basic parameters controlling the compaction of semiconductors.

The present work is concerned with the

influence of powder particle size on the densification behavior of III-V compounds.

One interesting feature is their surface dissociation at elevated

temperatures~

thereby providing a liquid phase that will affect the densi-

fication process.

It is found that a critical particle size exists which

marks a line of division between the mechanism of densification in the presence of a liquid phase and densification by plastic deformation.

91

The

92

PRESSURE SINTERING G'aSb

experiments

Vol. 2, No. 1

described here have been carried out with GaSb s but the results

should be applicable

to most s if not ali s III-V and other similar compounds

that dissociate upon heating with the formation of a liquid phase. Experimental Two vacuum chambers have been constructed and used to hot press GaSb. In preparing

i x i cm diameter discs to determine sintering conditions s a

die assembly enclosed in a glass envelope was used (Fig.

I).

The preparation

of larger samples s 0.7 x 2.5 cm diameter s as required for complete characterizationj was carried out in a similar die assembly which was enclosed in a water-cooled

steel vessel.

The hot pressing of thin and large diameter

rather than thick and small diameter discs was preferred for purposes of more uniform pressure and temperature distribution. The hot pressing was carried out in a vacuum of about I x 10 -4 Torr. The temperature was monitored by a tungsten/tungsten-rhenium

(26%)

thermo-

couple which proved to be stable in the antimony vapor encountered during the hot pressing. The powders were prepared from cast s undoped and doped GaSb. of five narrow particle size fractions through selective sieving of material Three smaller size fractions

A series

ranging from 500 to 20~ was obtained crushed with steel mortar and pestle.

in the one micron and submicron range were

obtained by grinding precrushed

fractions

in such a way that three differently

in a jet mill~ which was modified

sized portions

Coulter counter analysis was used to establish

could be separated.

the particle size distribution.

The hot pressing operation consisted of heating

the specimens at a rate

of 22 deg/min to 690°C at a constant precompaction pressure of 5000 psi; was followed by a compression with 25~000 psi for 3 hrs at 690 ~ 50C. rate of densification was measured by the displacement which is proportional

this

The

of the press platen

to the reduction of the powder column height in the die.

At the end of the run the pressure was released first and then the temperature lowered. The evaluation of the compacted structures minations according mlcro-hardness

to the Archimedes

consisted of density deter-

principle s optical microscopy s and

measurements° Results and Discussion

Table I gives all densification

characteristics

of the sintered compacts.

1.7x104

2.3xi04

Powder

Compacted Specimen

0.56

399

Height to width Ratio of Grains h/d

Vickers Microhardn. 200 g load (kg/mm2) 386

O. 63

1.5xlO 5

2.8xi05

5.603

Density after Compression (g/cm3)

Number of !Grains in i (cm -3)

5.613

0

weight Loss after Compression (wt.%~

190

180-200

485

450 -520

Average Particle Size by Volume (G)

Powder Particle Size Range (~)

nominally

Doping

A4

A5

Sample Number

105.5

0.44 a t top 0.63 at bottom 397

i. 6xlO 6

I. 6xlO 6

5.617

0

r

[ e d

5~617

44.5

43-46

A6

378

0.67

5.0xlO 7

2o3XI07

undop 104-107

A3

410

0.50

i. 2xlO 8

i. 7x108

5.6195

22.5

20-25

A7

i

I

I

482

0.86

5.2xi0 II

l. SxlO 13

5.6305

6.95

0.5

0.2-5.5

A9

Characterization of Pressure Sintered GaSb

TABLE i

I

527

0.92

6.7x10 II

4.5xlO 13

5. 641

I0.26

0.35

0.1-4.0

AIO

464

0.89

i. 9xlO II

4x10 II

5.633

5.0

1.7

79 4""

5. 609

0

325

149 -500

Zn-doped 1-15

A2

~O

o

0

94

PRESSURE SINTERING GaSb

Vol. 2, No. 1

PIll[

MOLYB

VACUUM PW~'

COl

FI,

FIG.

i

Air Cooled Hot Pressing Chamber

I

I

I

I

/

80

485/~ 7O >- 6 0 I-

o.35/~

t

~ 50 W

~ J w

4O

~

30.

CONDITIONS: 5000 psi 22 deg / m in

FLOW

l

TEMPERATURE

20

I

200

I

I

300

400

I

500

I

600

TEMPERATURE ( ° C ) FIG.

2

Densification of GaSh Powders at Rising Temperature and Constant Pressure

700

Vol. 2, No. 1

PRESSURE SINTERING GaSb

95

Densification Mechanisms The data presented in Table I suggests that two basic densification mechanisms were operativ%

depending upon the powder particle size.

This was

demonstrated first by the degree of deformation~ secondly by the number or size of grains before and after densification~ and finally by the packing density as a function of temperature. The degree of deformation was estimated by the ratio of grain thicknessj h~ to width~ d~ when looked at in a plane parallel and perpendicular to the direction of pressure propagation.

It was found that the ratio was approxi-

mately equal to unity for fine powders, indicating little or no deformation of individual grains~ but much lower than one for the coarse material~ signifying that considerable deformation had taken place (see Table I).

The

second criterion for distinguishing the sintering mechanisms was taken to be the number or size of grains before and after compaction (Table I).

The

analysis showed no changes in the coarse~ but appreciable grain regrowth in the fine material. temperature.

Finally Fig. 2 shows the densification as a function of While the coarse powders have a higher initial packing density

but no appreciable flow before the final pressure is applied~ the fine powders are loosely packed and display a distinct flow temperature which is associated with GaSb dissociation delivering liquid Ga that acts as a lubricant during particle rearrangement and as a catalyst during grain regrowtho

Thus the

densification of fine powders is governed by the presence of a liquid phase~ whereas the densification of coarse powders occurs predominantly by plastic flow of the individual grains. Compaction of Coarse Powders A model for the packing of both the undeformed powder column and the densified specimen can be proposed for the data in Table I.

Initially~

the

particles were packed very nearly like spheres in a combination of an hcp-and fcc-arrangement~ approaching the theoretical porosity of 26% (Fig. 2). A dense array of hexagonal discs was approximated after densification with the observed h/d ratios close to the theoretical 0.605 (Table I).

The large

deviation in the top portion of specimen A 3 (Table I) is due to enhanced plastic deformation caused by a crack in the graphite liner which allowed the upper portion of the sample to flow more freely than the remainder.* * An estimate of the flow rate yielded 9.73 x 10 -5 cm/s~ which corresponds to a viscosity of 8.8 x I0 II g/cm-s (690°Cj 25 000 psi).

96

PRESSURE SINTERING GaSb Detailed metallographlc

polygonized (Fig. 3).

Vol. 2, No. 1

investigation showed that all the grains were

Furthermore~

the coarse grained and lightly deformed

material displayed lines apparently comprised of closely spaced dislocations~ possibly slip traces(3) or twins (4) (Fig. 3j Sample A5).

The two central

photographs in Fig. 3 show the effect of the amount of deformation on equal grain size material.

In the heavily deformed specimeh (Fig. 3~ Sample A3) a

pronounced cell structure exists~ but no sllp traces and/or twins are visible; in the lightly deformed specimen (Fig. 3~ Sample AS) a faintly outlined cell structure coexists with well pronounced systems of parallel lines.

The

compaction process is thus characterized by the processes of slip and/or twinning and polygonization.

Recrystallizationj

formed high angle grain boundariesj

i.e. the appearance of newly

is only given in Sample A 3 where high

angle grain boundaries are possibly present in the heavily flown upper section of the sample (Fig. 3). In accordance with the above observationsj

microhardness

tests also

indicated that no work hardening occurred in the large grain specimens. Polygonization seems to have released the pre-existing strain within the cast raw material whose hardness the compressed specimens

(410 kg/mm 2) was found to be higher than that of

(Table I).

The increase of hardness for grain sizes

below 44.5~ is explained by grain boundary interferenc% size range of the diamond microhardness

since the absolute

indentations lay between 20 and 30~

and thus was comparable to the smallest grain sizes involved. Compaction of Fine Powders It was noted earlier that the pressure sintered structure of fine powders showed little evidence of plastic deformation.

Instead~ extensive

grain growth related to the dissociation of GaSb was observed.

Dissociation

was inferred from the fact that a sudden compression was measured between 560°C and 590°C (Fig. 2) coinciding with the dissociation temperature of GaSh reported by Haneman et al (5).

Another indirect evidence for the dissociation

effect is given by the measured densities.

If the values are compared to the

theoretical 5.619 g/cm 3 then densities in excess of 100% are found for small particle sizes (Table I).

These are explained by the presence of free Ga and

Sb in the microstructure.

Both are more dense than the compound.

Since the

GaSb-powder was found to be a single phase of stoichiometrlc compositionj dissociation must have occurred during the compaction cycle.

This was most

evident for small particles where the surface to volume ratio is greatest. Also weight losses were observed in increasing amounts with use of smaller particle sizes (Table I).

If one assumes both complete immiscibility of

A5

Microstructure

FIG.

3

I00/~

I

Of Pressure Sintered GaSb with Different Grain Sizes

I

A8

A3

I0/~

Aio

o"

0

L'zJ

0

0

98

PRESSURE SINTERING GaSb

Vol. 2, No. 1

solid Ga and Sb outside the composition of the compound and zero porosity in the mixture~

then the highest density measured for hot pressed material~

p = 5.641 g/cm3~ is compatible only with a GaSb specimen with approximately 2 wt.% free Sb or 6 wt.% free Ga~ or a combination of these.

Despite the

drastic differences in vapor pressures at the sintering temperature (Psb = 3 x i0 -I mm and PGa = 1.5 x 10 -9 mm)~ it is believed that both Sb and Ga were partially retained in the compact.

Then dissociated Ga and Sb

must also have been lost from the compacted sample in roughly corresponding amounts.

Indeed~

traces of liquid Ga outside the die and an Sb-rich deposit

on the chamber walls indicated that both components were extruded from the pressform at 690°C where they are liquid. the flow temperatur%

The excessive densities as well as

150 degrees below the melting pointj exclude the

possibility that the liquid phase was generated by pressure induced melting point depression (6). The large specific surface area of the fine powders generated sufficient liquid Ga to cause the observed sudden compaction through "lubricated particle rearrangement."

Simultaneously~

the liquid Ga-skin probably intiated

regrowth of grains through the following reactions: I°

DISSOCIATION

(GaSb)

T~60°C

(Ga)£ + (Sb)g,~

2.

SOLUTION

(GaSb) s + (Ga)~ ~ (Ga-GaSb)~

3o DEPOSITION (Ga-GaSb)£ + (GaSb) s 2(GaSb)s + (Ga)£ where s~ %~ and g indicate solid~ liquidj and gaseous phases respectively.

During a certain stage the growth mechanism may possibly be likened to VLS-crystallization

(7j8).

The typical appearance of a regrown structure is shown in Fig. 3~ Sample AIO.

The largest grains encountered correspond to the largest powder

particles used.

The lack of grain growth beyond a critical size suggests that

a third step usually encountered in sintering in the presence of a liquid phase(9)~ namely that of gradual final densificat{on by boundary migration~ was not active. The particle size below which grain growth and thus densification through liquid phase hot pressing will occur for the given conditions can be estimated from the data in Table Ij using the measured powder particle size distribution shown in Fig. 4.

For Sample A2~ A9~ and AI0 the percentages "recrystallized"

are 51.3~ 96.8~ and 98.5%.

As pointed out a b o v %

dissociation and regrowth

occurs first among the smallest and then proceeds to increasingly larger particles.

One can assume that the percentage crystallized is equal to the

percentage of the smaller particles

that have been removed by the solution and

Vol. 2, No. 1

PRESSURE SINTERING GaSb

redeposition process.

99

Referring to Fig. 4~ this implies that~ in each case~

the regrowth process consumed particles up to a size of roughly 4 ~

which is

thereby considered the critical particle size. The critical particle size is essentially a function of the particle size distributio%

the time spent above the dissociation temperature while

porosity is still present~ i.e. the heating and pressurization r a t % temperatures and pressures employed.

and the

Thus~ the empirical relationship between

the original average powder particle s i z %

do~ and the resulting average

grain size~ dl~ which can be formulated from the data in Table I~ d I = 1.2 + 0.52 do ~ also strongly depends on the critical particle size.

In addition~

the two

constants are probably functions of the impurity level inhibiting grain growth beyond a certain size.

0.01

I

I

I

I

I

I

0.1

x A2 WN

0 A9

~

=

i

°

_

• AIO

z

2

o~3

~o~\ ~

-

x_

°

80 90

-

x

L

\\\

99 , 99"90

x

2

4 15 PARTICLE

8 SIZE

I0 (/u.)

--

I 12

FIG. 4 Powder ParticLe Size Distributions of GaSb

14

I00

PRESSURE SINTERING G'aSb

Vol. 2, No. 1

Conclusion GaSb with densities close to theoretical can be reproducibly hot pressed at 25 000 psi and 690°C.

Fine particle powders also yield dense specimens,

however, with an overall composition that is slightly off stolchiometry. In both cases good mechanical properties are maintained, fine grained material showing higher hardness values.

Thermal and electrical properties

are reported in a separate paper (I0). Acknowledgement The research reported in this paper was sponsored by the United States Atomic Energy Commission, New York Operations, under Contract No. AT(30-I)3500 and RCA Laboratories, Princeton, N. J. The author is indebted to T. V. Pruss who prepared the powders and performed the density and most of the electrical measurements.

Helpful

suggestions in regard to the construction of the steel pressing chamber were made by H. Moss and W. P. Stollar. References i.

S. E. Hatch, W. E. Parsons, and R. J. Weagerly, Appl. Phys. Letters 5~ 153 (1964).

2.

K. Langrod and F. R. Bennett~ Ceram. Age, 1962, p. 48.

3.

G. L. Pearson and F. ~. Vogel, Progr. Semicond. 6_~3 (1962).

4.

A. T. Churchman, G. A. Geach, and J. Winston, Proc. Roy. Soc. 238, 194 (1956/7).

5.

D. Hanemann, G. J. Russel~ and H. K. Ipj Physique de Semiconducteurs~ Proc. 7th Intern. Conf., Paris 1964~ p. 1141.

6.

F. P. Bundy, J. Chem. Phys. ~

7.

R. S. Wagener and W. C. Ellis, Trans. AIME 223, 1053 (1965).

8.

R. L. Barns and W. C. Ellis, J. Appl. Phys. 36. 2296 (1965).

9.

H. Fischmeister and E. Exnerj Metallwiss. Techn. ~

i0.

P. R. Sahm, to be published.

3809 (1964).

941 (1965).