Transport and growth of GaSe from the vapour phase

Mat. Res. Bull. Vol. 9, pp. 763-774, in the United States.

1974.

Pergamon

Press, Inc.

Printed

TRANSPORT AND GROWTH OF GaSe FROM THE VAPOUR PHASE G.E. van Egmond and R.M.A. Lieth Chem.Phys.Division, Dept of Physics Technological University, Eindhoven The Netherlands

(Received M a r c h

25, 1974; C o m m u n i c a t e d

by S. Amelinckx)

ABSTRACT A comparative study of the sublimation and chemical transport of the compound GaSe is reported. The parameters investigated in the closed system are the iodine concentration, the tube geometry, and undercooling. The rate of transport in case of sublimation is diffusioncontrolled, in the iodine-assisted transport process both diffusion and convection play a role. If the sublimation technique is used, the crystals are not exclusively plate-like, the majority of the crystals exhibit needle-like and ribbon-like habit. With growth in the length direction, the growth axis is parallel to the c-axis, it takes place around screw dislocations and two crystallization mechanisms are assumed to take place simultaneously.

Introduction In a previous paper the growth and habit of vapour phase grown single crystals of GaS has been discussed (I). In this report analogous experiments are described for the compound GaSe. For the growth of single crystals of the semiconducting III-Vl compound GaSe, from the vapour phase in closed systems, use is made of the chemical transport technique and the sublimation method (1,2). In each case the closed system consists of an evacuated silica ampoule, placed in suitable temperature gradient in a horizontal tube furnace. In previous work (2) the differences in the produced crystals were mentioned; chemical transport grown crystals (C.T.) exhibit n-type conductivity and are exclusively plate-like, sublimation grown material (S.G.) are p-type and are needle-like, ribbon-like or in the shape 763

764

GROWTH

OF

GaSe

Vol. 9, No. 6

TABLE ! Cr,rstal h~bit as a function of temperature crystal growth forms in S.G.

needles (not seldom branched)

growth temperature

840 to 870 ~

O,1_Imm.~

ribbons

800 to 890

(straight,kinked and large) M

rods (not seldom branched)

0

840 to 870

~section.

platelets

"rose" (leaves lay not in one plane)

~4L~

'

o-~-----~-~

800 to 890

800 to 890

(°C)

Vol. 9, No.

6

GROWTH

OF

GaSe

765

of small plates (table I) all dependent on growth temperature and rate. Two growth techniques will be compared in this report and the vapour composition transport rate and crystal habit as function of crystallization temperature and ampoule geometry will be discussed. An attempt is made to explain the differences in growth mechanisms and morphology. Experimental procedure The employed silica tubes are 180 mm in length with inside diameters ranging from 8 to 30 mm, dependent on the kind of experiment. Handling the ampoules, the employed furnaces and the temperature control have been discussed in an earlier report on GaS (1). In all experiments the source region temperature (TH) was 920°C, the temperature of the growth region (TL~ was 800 ° < T L <

890°C. Very pure GaSe powder x) was used, the amount per cm

ampoule volume was 0,066 gr. C.T. experiments took from 17 to ]91 hr., S.G. experiments took from 56 to 187 hr. In C.T. experiments the ampoules were removed from the furnace with the source region first to avoid condensation of gallium iodide vapour on the deposited crystals. Composition of the Vapour The vapour phase composition differs for the C.T. and S.G. processes. Similar to the case of GaS, the vapour of GaSe in the sublimation process consists of molecules

(5) and dissociation into its components is negligible

as long as T L > 800°C. In fig. I a plot of PGaSe versus reciprocal temperature is presented. At lower values of T L free gallium is found in the coolest part of the ampoule in the form of a gallium deposit. The formation of Ga2Se3, however, has never been observed under the conditions employed in these experiments. In C.T. the vapour consists of several species; analogous to the situation in the GaS-I 2 system (I) it is assumed that in the reaction between solid GaSe with Iodine - followed by condensation of GaSe in the growth region - the intermediate gaseous products are selenium, iodine and a mixture of gallium mono-and tri-iodides. This has also been proposed by Gadzhiev (6) and is confirmed for GaS by the work of Nishinaga (7). In the source region TH, Se 2 and gallium iodides are found, followed by transport of these species to TL. In the exothermic disproportionation reaction at TL, Gal 3 is formed

Purity data of GaSe are presented in reference 4.

766

GROWTH

OF

GaSe

Vol. 9, No. 6

50

"

"-r

E

t

~%

a

I 5o

0..

I

10: &0 A

10 20

" A

1

I 0.83

I 0.84

A ~ "

FIG.1 A plot of log. PGaSe versus~ in the

, ' J- ' ~

I

0

i

~,/ I

I

I

L

I

I

100 150 200 ~ r 2. FIG.2 Rate of transported GaSe as a function of the square radius of the reaction ampoule. Tube length= 180ram; TH:920°C ; TL:850°C; I2concentration= 4 mg/cm3.

range 910° - 940°C

u~

50

ul

,7_

A

bi

• a

I

I

6'

I

I lli'~!

1

t

I

L [qLtl!

C.I.

l0

I

I

I Ill

If

Io"

FIG.3 -'- m g I z / c m 3 Rate of transported GaSe versus the iodine concentration for t w o sorts of ampoules. Curve(a) is for tubes with 11ram i.d.,Curve(b) for tubes with 20ram i.d.,tube length= 180mm; TH:920°C; TL:BS0°C.

I

800

[

1

820

I

I

840

I

I

860

I

i

880

i

I

900 ~-- "fL (°C)

FIG.4 Rate of transported GaSe as a function of undercooling. TH--920°C.;TL-,-growth temperature; (a):sub[imation growth; lb)= chemical transport.

Vol. 9, No. 6

GROWTH

OF

GaSe

767

and Gage crystals are formed. Transport of Gal 3 back to the TH-region completes the cycle. According to (8) 12 and Gage starts to react at about 300°C and above 700°C the vapour consists of GaSe and Se 2 which is transported to T L. This leads to the overall reaction: Gage(s ) + I/2gal3(v) ~

3/2Gal(v ) + I/2Se(v )

(I)

As reported before (7) other reactions of importance are GaSe(s ) + 1 / 2 1 2 ( v ) ~ G a l ( v and

GaSe(s ) + 3/212(v)'~-~Ga%(v)

) + I/2Se2(v)

(2)

+ I/2Se2(v)

(3)

For S.G. (and probably also in C,T. in the region of low iodine concentrations) the following reaction may be important: GaSe(s )

. GaSe(v )

(4) Transport Rate

The rate of transport was studied for both C.T. and S.G. as a function of ampoule geometry (fig.2) for C.T. as a function of the iodine concentration (fig.3) and for both processes as a function of undercooling AT (fig.4). The dependence of the transport rate in mg/h on the square radius of the ampoule, as depicted in fig.2, shows the strong increase of curve (A), indicating the increasing influence of convection in the case of iodine assisted transport. In sublimation experiments,

curve (B) is at first linear, with a

strong increase at large diameters. No efforts were made to control nucleation in this stage of the experiments.

In the larger tubes (20 mm i.d.) the

deposited material in C.T. was rather intergrown, with a large amount of the yield in microcrystalline form. Material transported in S.G. is always for more than 90% microcrystalline,

condensed in the form of a crust on the

ampoule wall. In 20 mm ampoules the ribbons, plates and needles grow on the crust; in wider ampoules only an entangled mass of twinned ribbons is obtained. As a consequence of these results we employed in all later C.T. and S.G. experiments 20 mm ampoules only. In this case we may assume that S.G. always is diffusion controlled (linear part of curve B). The strong increase in the case of C.T. experiments

(curve A) indicates the strong influence of convection in

all kinds of ampoules. In fig.3 the influence of the iodine concentration on the rate of transport is shown in a plot of the flux (mol/cm2.sec) of solid Gage collected in the growth region, versus the amount of initially added iodine (mg/cm 3) and

768

G R O W T H OF GaSe

Vol. 9, No. 6

this for ampoules with an internal diameter of I0 and 20 mm. It shows a minimum at respectively 2 mg/cm 3 and 0.5 mg/cm 3. The increase of the rate towards the left suggests that in the range of low iodine concentrations, diffusion is rate determining, while at higher concentrations convection predominates, although both transport mechanisms must be assumed to be simultaneously operative. Referring to the ease of GaS (7) we assume two different reactions; to the left of the minimum reaction (4) governs the transport rate, to the right a combination of (I), (2) and (3) play the main role. In fig.4 the transport rate is presented as a function of the undercooling AT or rather as a function of the growth temperature. The flux is for S.G. material about equal to that of C.T. when an undercooling AT of 120°C is maintained. The difference increases strongly with decreasing AT. In C.T. for the growth of good quality crystals AT never exceeds 70°C. Higher values of AT resulted in much intergrown material. At a AT

of

70°C (TL= 850) the rate for S.G. is about twice that of C.T., and the yield of microcrystalline deposited material in S.G. is strikingly larger than in C.T. An indication of the supersaturation ~ in S.G. experiments can be obtained from the slope of PGaSe versus I/T (fig.l). At an undercooling of 15°C the supersaturation is about 140%. Apart from such factors like transport rate and saturation the whole process of nucleation and growth affects the quality of the deposited GaSe, and important steps are like for GaS: a. the disproportionation of Gal into Gal 3 and gallium (as taking place in reactions (I) and (2); b. the formation of nuclei; c. The subsequent diffusion and trapping of building units (either Ga and Se or GaSe) on the growing surface. Step a) is not relevant for S.G., step b) and c) are probably affected by the composition of the vapour. The formation of only a few nuclei and, the subsequent lateral outgrow into large single crystals is far more favoured in the halogen-assisted process than in the sublimation process. Cr~stal Habit The crystal habit of C.T. and S.G. produced material is.described earlier (1,2). In contrast to GaS, C.T. experiments on GaSe with decreasing iodine con-

Vol. 9, No. 6

GROWTH OF GaSe

769

centrations do not result in an increasing amount of needle- and ribbon-like crystals below a certain iodine concentration; needles and rods could only be obtained by S.G° and under such conditions that the transport rate is not too high and not too low. In this case it means that ampoules should be used having diameters ranging between |5 and 24 mm, and furthermore an undercooling AT in the range between 50 ° and 80°C. An analogous difference in conductivity as for GaS is found for GaSe; the conductivity is p-type for S.G. products and n-type for C.T. material. Employing more than 0.5 mg/cm 3 iodine, the first deposited plate crystals in C.T. grow flat along the ampoule wall and are rather small; in a later stage of the process, crystals grow out in all directions, not exclusively radially, becoming large and thick, Using less than 0.5 mg/cm 3 iodine, the first deposited plate crystals still grow flat along the ampoule wall, but the number is less and the crystals are thinner. The variety in habit as observed in sublimation experiments is not seen here. S.G. crystals show different forms as a response to the temperatures in the growth region. They are found in the form of needles and rods (fig.5), thin platelets and ribbons

~, ~

in various forms; straight or kinked, narrow or wide, round- or straight edged,

-

single or branched, but all bendable and very thin with colours ranging from green to blue and red,

lj ~

However, most of the S.G. material (>90%) deposits as a mlcrocrystalline crust on the ampoule-wall. Microscopic examination shows this crust to consist of what may be called

FIG.5 GaSe needles and rod grown in sublimation process

a dense field of "roses" tfig.6) grown close against each other. Each "rose" consists of microscopic small GaSe platelets intergrown in various directions, like the leaves of a flower. Each plate has the characteristic triangular- or hexagonal form, the transparant dark red brown colour and shows the triangular or hexagonal growth layer terraces. In a later stage a few of the crystallites grow out in the form of narrow ribbons, thin triangular needles or rods ~ d small plates, all dependent on T L (see Table 2) and on the ampoule geometry

770

GROWTH

O F GaSe

I D

~

~ ~

i

J~

L

Vol. 9, No. 6

~

~

radially inwards. However, plates with dimensions comparable to those obtained in C.T. are rare, in most cases they will be smaller and are often found intergrownwith needles or show the clear

~

•

b

I~ P

marks of the onset of needle-growth. ~

[

'i ~

i q

FIG.6 Sublimation grown GaSe "roses"

(see Table 3) The direction of growth is mostly

~

Ribbons as well as needles and rods never grow directly on the ampoule-wall, but on the first formed substrate. The majority of the straight or branched needles don't show the smooth, well developed prism faces and pencil sharp tips found with GaS, but have irregular shaped prism faces (fig.7). If pressure is exerted on a side face normal to the growth axis, GaSe needles deform and slip occurs resulting in a collection of thin plates. For the needles a V.L.S. growth mechanism can be excluded since droplet formatlon is absent. As reported earlier (2) the top parts of the needles are pure rhombohedral (y-modi-

Voi.

9, N o .

6

GROWTH

fication of GaSe), is pure hexagonal ficatio~;

OF GaSe

771

the base (g-modi-

~

the y - m o d i f i c a -

tion is an ordered polytype, the g - m o d i f i c a t i o n

a dis-

ordered polytype. The c o n c e n t r a t i o n incorporated C.T.

of

iodine in the

grown crystals detected

w i t h the aid of neutron a c t i v a t i o n data m was < 5 weight p.p.m.

FIG. 7 S u b l i m a t i o n - g r o w n rod with irregularly formed prism faces and one branch changing into a needle Growth M e c h a n i s m TABLE The H a r t m a n

(9)

enabled

theory

Crystal habit as a function of undercooling

to explain AT(°C)

GaS habit differences

(I).

In this theory chains of strong bands running through

30

are

50

taken to govern crystal The three

kinds of faces in this theory are flat- or Ffaces containing

slice of thickness

in a d,

stepped- or S-faces

ribbons

needles

+++

++++

+

oo

ooo

+++

+++++

and rods

+

O0

000

++

+++++

+++

oo

ooo

oooo

++++

+++++

+++

oo

oooo

oooo

+++

++++

+

OO

OOOO

70 80 100

+++

+++++

+++

OO

OOOO

OOOO

++ oo

++++ oooo

+

++

++++

+

OO

OOOO

120

with

only one chain per slice, and kinked- or K-faces containing

60

two or

more of such chains

platelets

40

the structure

morphology.

2

no chain.

The GaSe structure

'" lquantity + ++ +++ ++++ +++++

= = = = =

dimension nothing very few few much very much

o oo ooo oooo ooooo

= = = = =

m i c r o s c o p i c ( < 0,5mm) very small(0,5-2mm) small (2-5mm) large (5-10 am) very large(10-15mm)

X

This was carried out at the reactor plant of Reactor C e n t r u m Nederland Petten, The Netherlands.

at

77Z

GROWTH

OF

GaSe

Vol.

TABLE 3

is described

Crystal habit as a function of ampoule radius r

2

16 25 45,5

platelets

ribbons

needles

++

+++

+

oo

ooo

++

++++

O0

000

72,25 81

100 121 144

0000

the basal plane or (0001)

and rods

face is analogous

to that

to three

+

chains.

+++++

+++

prism

oo

oooo

ooo

continuous

++

+++

++++

[0001] direction.

++

+++

++++

sequently,

OO

OOO

OOOO

OOO

OOOO

++

The side or faces show no chains in the Con-

lateral o u t -

++++

grow of the (0001) face like in C.T.

oo

ooo

oooo

++

+++++

++++

O0

000

0000

is quite

understandable.

Initia-

J

++

+++++

++++

tion of new layers on

O0 ++

000 +++++

0000 +++

these smooth

oo +

ooo ++

oooo +

169 +

(0001) F-

faces - a growth in thickness of the platelet

OO

225

stacking of slices and

being parallel

+++++

++ 90,25

as a

of GaS (I), an F-face,

+++

OO

++

+

-

is however difficult.

oo

The presence of one or

quantity + = nothing ++ = very few

dimension o = microscopic(<0,5mm) oo = very small(0,5-2nml)

several screw disloca-

+++

ooo

the growth normal to

= few

++++ = much +++++ = very much

= small

tions, however,

(2-5mm)

oooo = large (5-]0mm) ooooo = very large (10-15mm)

growth spirals

observed

show an abundancy of such triangular

shapes found in S.G. crystals

nucleation mechanism and pronounced

in the needles

is explained

leads one to assume a twogrowth in the [0001] direction

in an analogous way as is done for GaS

(I). The tip part of our GaSe needles dislocation,

step created

(fig.8).

The different dimensional

C.T. plate crystals

enables

such an F-face due to the everpresent

by this dislocation.

are of the y-type containing

one screw

the bottom part is of the e-type, having several dislocations.

This change in type going from tip to base is explained by a variation growth conditions

due to a change in supersaturation

ribbons and platelets

6

+

O0

+++ 56,5

9, No.

in

(2). The fact that

have been found, which do not show - even under the

largest possible magnification

- any sign of screw dislocations

(O00l) face (1,I0)

leads us to believe

been the mechanism

in these ribbons

that two-dimensional

on the basal

nucleation has

and plates during a period of large

Vol. 9, No. 6

GROWTH OF GaSe

supersaturation.

773

In this

connection the faint traces of heterogenous nucleation and limited outgrow on the

[li

silica ampoule wall presented in fig.9 are of interest. It shows a kind of fishbone-like dendritic growth in three directions

............

starting from a centre, which has already platelike appearance.

It is

not possible to see where the dendrites begin, it

FIG.8 A GaSe platelet grown in the sublimation process showing straight grown layer edges

is clear that they make 120 ° angles with each other, while the picture of the familiar hexagon is almost completed by the arms sprouting from the three main "dendrites".

FIG. 9 Fishbone-like dendrite growth (sublimation-grown)

774

GROWTH

OF GaSe

Vol. 9, No. 6

It remains a question whether plate crystals can be obtained by a process of filling up of the space between the arms. In such a case this "dendritic skeleton" merely represents a stage in the growth process which could be due to a very large supersaturation. Conclusions Large n-type single crystals can be grown with iodine concentrations~0.5mg/cm 3 and transport is convection-determined. At lower concentrations diffusion becomes important, here crystals are smaller and thinner. C.T. growth seems to be favourable for the outgrowth of a few crystallites into larger plates. The S.G. technique is unfavourable for the growth of large plates. This technique is of interest because needle-like crystals can be produced. Transport rate in S.G. is higher than in C.T. and completely diffusion-controlled. The platelets in S.G. experiments have p-type conductivity. Habit differences have been observed and have been explained by the Hartman theory and the assumption of the simultaneous occurrence of two crystallization mechanisms. References I. R.M.A. Lieth, phys.stat.sol. (a) 12, 399 (1972). 2. J.C.J.M. Terhell and R.M.A. Lieth, J.of Crystal Growth 16, 54 (1972). 3. R.M.A. Lieth, C.W.M.v.d. Heijden and J.W.M. Kessel, J.Crystal Growth ~, 251 (1969). 4. J.C.J.M. Terhell and R.M.A. Lieth, phys.stat.sol. (a) ~, 719 (1971). 5. Unpublished results. 6. A.M. Gadzhiev, R.G. Bakhyshov, D.M. Suleimanov and A.A. Kuliev, Russiana Phys.Chem., 45, 10 (1971). 7. T. Nishinaga and R.M.A. Lieth, Submitted to Journal of Crystal Growth. 8. P.G. Rustamov and B.A. Geidarov, Paper given at Conf.Crystal Growth and epitaxy from the Vapour Phase, ZUrich (1970). 9. P. Hartman and W.G. Perdok, Acta Cryst. 8, 49 (;955); ~, 521 (;955); Z.Krist. I19, 65 (1963). I0. C. Paorici and G. Zuccalli, J.Crystal Growth 15, 240 (1972).