Semiconductivity of CdS as a function of s-vapor pressure during heat treatment between 500° and 700° C

Mat. Res. Bull. Vol. 4, pp. 153-160, 1969. in the United States.

SEMICONDUCTIVITY

Pergamon

Press, Inc. Printed

OF CdS AS A FUNCTION OF S-VAPOR

PRESSURE DURING HEAT TREATMENT B E T W E E N 500 ° AND 700 ° C *

K. W. BSer and W. J. N a l e s n i k * * Physics Department U n i v e r s i t y of Delaware Newark, Delaware

(Received D e c e m b e r 6, 1968 and in final form January 15, 1969; Refereed)

ABSTRACT The dark conductivity of CdS as a function of the sulfur vapor pressure is investigated during heat treatment in a temperature range 500 ° < ~j< 700 ° C. The pressure dependence follows j N ps_~/m with m = 2x, being the average number of sulfur atoms per sulfur molecule in the gas phase. For 500 ° < T < 525 ° C, x = 3, for 525 ° < T < 630 ° C, x = I and for 630 ° < T < 700 ° C, x = 3. These results are explained by thermodynamic disorder and a Cd-rich n o n s t o i c h i o m e t r i c equilibrium below 525 ° C. S c h o t t k y - W a g n e r disorder most probably is dominant above 525 ° C. The thermal energy of doubly ionizing a sulfur v a c a n c y is Ec-~TS=0°~Sv eV.

Introduction Some of the most direct structure

of CdS can be obtained by heat treatment

S-vapor and an analysis properties

information about the native defect

of changes

of the crystal.

in the electrical

However,

very difficult.

and optical

the high temperatures

essary for such heat treatment make direct the treatment

in Cd- or

Therefore

observations

nec-

during

early investigations

* Supported in part by a NASA grant, Goddard Space Flight Center, by a contract from U.S. Army, Aberdeen, Md., and by a grant of the Office of Naval Research, Washington, D.C. ** Presently, Department vania, Philadelphia, Pa.

of Astronomy,

153

U n i v e r s i t y of Pennsyl-

154

SEIkIICONDUCTIVITY

were made at room temperature to freeze-in (I)

the equilibrium

(near 800 ° C).

Vol. 4, No. 3

after rapid

cooling

state achieved

These measurements

some investigations treatment

O F CdS

at high temperatures

have been supplemented

of the semiconductivity

at slightly

in an attempt

lower temperatures

by

of CdS during heat

(up to about

600 ° C)*

(2,3). Recent (4,5) have

diffusion

of the conductivity

to slightly higher

temperatures

more information

disorder

using a direct

shown that marked diffusion

an extension

taining

measurements

CdS single

treatments,

were employed.

wire point treatment

used.

used:

type of thermodynamic

(10ma)

The results

temperature evaporated a treatment

500 ° C.

with Au point

Au layers

the current

oven the temperature sulfur was adjusted.

results

above Au

up to

with a current

Again four point probing

contacts

agree

a two vessel,

obtained

period

1 was used.

The upper vessel

with

was not subjected

in

In the lower

the vapor pressure contained

to

of time.

two oven arrangement

and consequently

was

in an overlapping

well with the results

in Fig.

elec-

and therefore

"forming"

650 ° C for an extended

as given

the

For lower temper-

At temperatures

as long as the crystal

For the treatment alignment

probing.

of 700 ° C after

range reasonably above

between

start to deteriorate

at about

at

size was approximately

were used which gave useful

temperatures

by subli-

Au and Au covered with Pt electrodes

for potential

electrodes

obtained

l0 x 3 x 0.I mm 3.

Two small electrodes

contacts

(6)

vertical

for ob-

in an N 2 - H2S atmosphere

The crystal

evaporated

were applied

600 ° C these

pulse

treatment

Method

crystal platelets,

of ultra pure CdS powder

same for all crystals

trodes

during

seemed very attractive

on the predominant

I000 ° C, were used.

ature

600 ° C, thus

in CdS.

"Undoped"

about

about

measurements

Experimental

mation

starts

tracer analysis

of the

the CdS platelet

* Difficulties arise at higher temperatures because of the fact that most of the electrodes do not remain unchanged during heat treatment in the corrosive atmosphere or start to diffuse into the CdS-lattice.

Vol. 4, No. 3

SEMICONDUCTIVITY

O F CdS

155

and was connected by a 2mm diameter capillary with the lower vessel.

~

~,~

Both quartz vessels were thermally

~

_~_~

shielded with Fibrefax from each #f

other.

The temperature was measured

2

inside each vessel with a Pt/Pt-Rh 5-

thermocouple enclosed in a quartz capillary.

The crystal was mounted

on a quartz plate with the aid of quartz springs and Au wire.

The wire

feedthrough was initially sealed with Torrseal and during the measurements permanently sulfur.

sealed with distilled

55~I

I~6

This sulfur seal has in

equilibrium automatically a surface temperature T s equal to the temperature in the sulfur vessel. The sulfur used was 99.999 o/o pure and carbohydrate-free, distilled in the vessel.

vacuum-

Before

sealing the vessel,

it was outgassed -6 and evacuated to I0 torr. The current through the crystal and both probe potentials were continuously recorded. One and a half or six volts were applied to the crystal. Due to difficulties

with elec-

trodes only a few high temperature treatments could be made per crystal, As soon as lower temperature

conduc-

tivities became non-reproducible

FIG. i Treatment vessels and furnace. i. stainless steel tube 2. asbestos disk 3. Mullite tube with Kantal winding 4. asbestos board with Kantal winding 5. Fibrefax insulation 6. asbestos box 7. CdS 8. Si02-spring 9. thermocouples I0. sulfur Ii. electrical feedthrough 12. 0-ring seal.

or

the potential probes indicated the build-up of excessive barriers, the crystal was discarded and the measurement

continued with

a new crystal. Twelve undoped crystals were used for the measurements reported here~ all of them showed a surprisingly similar behavior.

156

S E A i I C O N D U C T I V I T Y O F CdS

Vol. 4, No. 3

Experimental Results The CdS crystal was heated to the desired treatment temperature TCd S and the sulfur vapor pressure PS adjusted to about lO torr by choosing the proper TS.*

After reaching a stationary

dark current, T S was increased to provide the next sulfur vapor pressure point etc.

Stationarity was reached after 30 min. to

several hours depending on the treatment temperature. reaching the highest S-vapor pressure

After

(always T S < TCdS) , T S was

lowered again and the dark current measured with decreasing PS" A slight hysteresis was sometimes observed.

If lower temperature

currents did not agree within 5 percent with previous values obtained with increasing PS" this indicated electrode difficulties and the crystal was discarded

(concurrently usually also a steep

increase in the potential drop at the electrodes,

i.e. a more

pronounced Schottky barrier was observed). Fig. 2 shows a typical /0"

set of current vs. sulfur vapor pressure curves obtained at different crystal -e~°-t~T~

/0-4

645 °

,~-,___,__~__,_.629 ° ~ - - o ~

-~

lO !

575e

"---®~o----_.___ ,

I(~

At low crys-

tal temperatures in many crystals the current does not change with sulfur vapor

"e~ i0 -5

temperatures.

529 o ,

pressure below about lO 2 tort

•

Probably impurities

103

present in the crystal mask

{t~'r)

the influence of the sulfur

FIG. 2 Conductance at treatment temperatures TCd S as function of the sulfur vapor pressure (typical example measured at one crystal,

vapor treatment. At sulfur vapor pressures above lO 2 tort a decrease of the semiconductance AX = AJ/AV with increasing PS according to ~Z ~ PS -l/m

(I)

* For S-sealing, as described above, a short initial "treatment" at high S-pressure (about 1 atm.) was done.

Vol. 4, No. 3

SEMICONDUCTIVITY

has always been observed. sulfur pressure atm.),

it changes

At treatment

quite

Near

increased

much

tures between

X=3 251

=!

and was

~

larger with

i

'

I

~'\

I

15 I ~ | ~

tempera-

I

530 ° and 630 ° C.

At temperatures

above

again relatively with m = 15 were

5

630 ° C

one

=3

served between

i

CdS crystals

electrodes

in Section 2. CdS treatment

~

~

oe®o ® ,

,

I

FIG.

and

or dif-

|

600

ob-

the m-values

i e1"~ I~" ~

500

small slopes obtained.

There were no correlations

tals

I

i

!

ferent

(about

525 ° C the slope

m = 5 at intermediate

different

used

with temperature.

I/m was

small with

drastically

consistently

pressures

for a wide

temperatures

below 520 ° C the slope

m = 25.

drastically

157

is constant

range up to the highest

however

in general

The exponent

O F CdS

700

Tcds~oC) 3

m (see Equation i) as function of treatment temperature (for all crystals investigated).

as described

Fig.

3 shows

temperature

the m-values

as obtained

as a function

from twelve

of the

different

crys-

in 28 runs. Discussion An analysis

treatment tures

experiments

slightly

above

in CdS platelets will assume periments

of diffusion (2,3)

kinetics

indicates

500 ° C diffusion

for the bulk within

that the observed

are proportional

changes

to changes

in earlier

sulfur vapor

that already

at tempera-

equilibrium "can be reached a few hours.

Therefore,

in conductance

we

in our ex-

in bulk electron

density

(3). Then a simple mass reaction equilibrium

between

analysis,

assuming

the sulfur vapor and a certain

type of elec-

trically

active

defect

vacancy,

acting

as a donor and being progressively

annihilated

PS seems

a relationship

with increasing

in the bulk of the crystal,

thermodynamic

appropriate. n - ps~l/~x

This yields

e.g.

a sulfur

(2)

158

SEIVI[CONDUCTIVITY

between

the electron

2 or 3 for relatively kinetic

The results temperature

Vol. 4, No. 3

density n and the average

atoms ~ of a sulfur molecule

reaction

O F CdS

in the gas phase,

simple processes

equilibrium

of sulfur

x being

and reflecting

I, 3/2, the type of

within the crystal•

shown in Fig.

be divided

number

3 suggest

into three

that the treatment

ranges:

500 ° < T I < 525 ° C, 525 ° < T 2 < 630 ° C and 630 ° < T 3 < 700 ° C, with a respective

change

of x in these

x = I, and back to x = 3. average about

This

indicates

size of a sulfur molecule

5 at 700 ° C, values,

for the superheated surfaces

three

ranges

from x = 3 to

a reduction

from about

8 atoms

of the at 500 ° C to

which are slightly higher than given

sulfur vapor

at T S at the electric

( 7- 9).

Nearby

feedthrough

liquid

sulfur

seals may account

for

this discrepancy. For further native dence

defects

discussions

contribute

it will be assumed,

to the observed

of the semiconductivity.

purity

crystal

platelets

This

which

to crystal•

the semiconductivity

of these

tivity

lower temperatures

ates

crystals.

The heat treatment crease

the density

Moreover

crystals

or decrease

pressure

(marked

the diffusion

at these

sulfur vacancies

incorporation

to the change

in semiconductivity).

neutral

Schottky-Wagner

quasi-neutrality,

native

the proper

seen that already

semiconduc-

of defect

in principle Cadmium

moreover,

ininter-

sulfur does probthe sulfur

(5) and do not contribute

disorder

with singly

and self-compensation

set of mass •

associ-

low temperatures.

with increased

or Frenkel

defects

at

the same for all

decrease

temperatures;

are electrically

ionizable

for high

of sulfur interstitials

interstitials

and doubly

relatively

in sulfur vapor can,

ably occur at somewhat higher

Assuming

errors

depen-

of reproduci-

is determined

of Cadmium vacancies,

stitials

reasonable it has been

experimental

Finally,

should be negligible

seems

only by self-activated

(I0, ii), being within

investigated

sulfur pressure

show a high degree

bility from crystal

somewhat

that only single

action X

!

for

law equations

involving the defects VS x, V S , VS" , VCd , VCd , VCd , cdix , Cd i" o, and Cd~ in the usual notation (12) allows for the observed

Vol. 4, No. 3

SEMICONDUCTIVITY

sulfur-pressure

(Eq. 2 )

dependence

OF CdS

159

and the p r o p e r

sequence

x

(3,1,3)

in o n l y two ways*:

I

•

and V C d : Cd i

predominant V S

!

of

,,

o.

or Cd i

for x : 3

(

rsp. V C d = V S" f o r x = I

f o r more d e t a i l

see

ref. 1% ). ,.

In o t h e r words,

for Frenkel

disorder

one o b t a i n s

a V~ (3

..

•

()di , Cd i

sequence

UT S .. s e q u e n c e temperature

and f o r S c h o t t k y - W a g n e r

for the p r e d o m i n a n t ranges.

On the b a s i s

here b o t h

sequences

formation

obtained from heat

direct

diffusion

the

further

between

s u l f u r vacancy:

in-

(3) and f r o m

that S c h o t t k y - W a g n e r

at t e m p e r a t u r e s

temperature

renorted

in C d - v a p o r * *

= 630 ° C, one can then e s t i m a t e ionizing

in the t h r e e

However,

(5) i n d i c a t e s

•

a Cd i , V<~. ,

of the e x p e r i m e n t s

treatment

measurements

F r o m the t r a n s i t i o n

of d o u b l y

defects

are e q u a l l y p o s s i b l e °

disorder probably predominates

and 3, T~

lattice

disorder

, ..

above

539 ° !.

temperature the energy,

At T~

r'in£e 2 E!U_'),

the ~ l o w i n £

-/,3

e q u a l i t y must hold:

V S " (T2,3)

the E ( V S ) : EF + kTgnq, Usirg

the m e a s u r e d

reeff = 0.2 m._ one o b t a i n s

stantiate years more ~]d ~•

this model. and more

sin~]~_.~ d e f e c t s ,

high mobility

(T9,3)o

This

w i t h E F the F e r m i - l e v e l

j (630 ° C), •

It c e r t a i n l y n e e d s

= VS

z( VS" )

further However

at ~

an e s t i m a t e d ~ = 50

Ec

resuizs

~ i r e a d v at r e l a t i v e l y

~w

£.

(cmq/Y~ ''/ and

evidence

it is r e m a r k a b l e

such as v a c a n c i e ~

,7~'~_~

£ . 4 8 eV.

experimental

experimental

requires

that

to ~ t in recent

seem tc i n d i c a t e

,or i n t e r s t i t i a l s ~

~

"~

that

iq

have

~,:i

....

* A s s u m i n z that o n l y one d e n s i t y is o r e d o m i n a n t cn b' th ~,~/4c .... Df the o u a s i - n e u t r a l i t y c o n d i t i o n and this will not change ..... .~ h c h a n g i n g PS ( E r o u w e r - d i a g r a m - ref. 13), since h e r e w i t K elf i? ~ e ~ L t l v ~ small changes in d e n s i t i e s w i ~ l be induced. ** ~'he r e s u l t s of ~oynj G o e d e and K u s c h n e r u s (3) l n d i c a Z e zhaS with C d - t r e a t m e n t m o r e Cd i n t e r s t i t i a l s are i n t r o d u c e d one ~n.= t e m p e r a t u r e range T I is w i d e n e d The c o n d u c t i v i t y is : n c ~ .......... , the F e r m i - l e v e l s h i f t e d t o w a r d s the c o n d u c t i o n b a n d and the c r o s s o v e r for Cd i to Cd i to b e c o m e ~he most i m p o r t a n t defec~z o.

(for the d i f f e r e n t c o n c l u s i o n VCd + £di'" ~ Cdcd , is used)

in ref. 3 a too s i m p l i f i e d model, in the n e u t r a l i t y c o n d i t i o n is ~Iso

s h i f t e d ~owards s l i g h t l y h i g h e r t e m p e r a t u r e s ( - 500 <~ C in ref. o to < DOO C f o r S - t r e a t m e n t s r e p o r t e d here)o Cd-interstitials d e t e r m i n e the "low t e m p e r a t u r e r a n g e " of Cd- surplus• Sulfur v a c a n c i e s must p r e d o m i n a t e at h i g h e r t e m p e r a t u r e s .

3

160

SEIvIICONDUCTIVITY O F CdS

markedly through the lattice. defect diffusion seems impeded.

Vol. 4, No. 3

At higher temperature some single Defect associate-formation prob-

ably is one cause of a lower than expected diffusion above 700 ° C (i).

Cd-divacancies are one example of such associates which are

expected to be relatively stable (12) and are probably formed only at higher temperatures at the expense of VCd. At lower temperature this associate formation may be hindered because of Coulomb-repulsive forces between equally charged defects. References I.

F . A . KrSger, V. J. Vink and J. Van den Boomgaard~ Chem. 203, I (1954).

2.

K . W . BSer, R. Boyn and 0. Goede, Phys. Stat. Sol. 3_, 1684 (1963).

3.

R. Boyn, 0o Goede and S. Kuschnerus, 57 (1965).

4.

H.H.

5.

~. H. Wqodbury and R. B. Hall, Phys. Rev. 157, 641 (1967).

6.

I.T.

Woodbury,

Z. Phys.

Phys. Stat. Sol. 123

Phys. Rev. 134, A492 (1964).

Sihvonen and D. R. Boyd, J. Appl. Phys. 29, 1143

(1958). 7.

A . N . Nesmeyanov, Vapor Pressure of the Chemical Elements, P. 319, Elsevier Publishing Co., New York, N. Y. (1963).

8.

W . N . Tuller, The Sulphur Data Book, p. 29, McGraw-Hill Book Co., Inc., New York, N. Y. (1954).

9.

J. Berkowitz and J. P. Marquart, J. Chem. Phys. 38, 275 (1963).

I0.

K. W. B8er, Phys. Stat. Sol. ~, 1684 (1963).

ii.

K. W. BSer and R. Boyn, Monatsber. D. Akad. Wiss. Berlin ~, 396 (1959).

12.

F. A. KrSger, The Chemistry of Imperfect Crystals° Holland Publ. Co., Amsterdam (1964).

13.

G. Brouwer,

14.

K. W. BSer and W. J. Nalesnik, Technical Report No. 12 NASA NsG-573, Goddard Space Flight Center, Greenbelt, Maryland (1967)

15.

E. Spenke, Elektronische Halbleiter, 2nd Edition~ p. 451, Springer Verlag, Berlin-Heidelberg-New York (1965).

North

Philips Res. Rep. 9_, 366 (1954).