Observations of crystal orientation effects in tin-telluride thin films

Observations of Crystal O r i e n t a t i o n Effects in T i n - T e l l u r l d e Thin Films S

R

SHORTES

and D

P

MILLER

Texas Instruments Incorporated, Dallas, Texas (Received 2 March 1961, accepted 7 December 1961) Tm-tellurMe thin films may be cr),stallographtcally orwnted to certam preJet red directions either during evaporation on to glass substrates or during subsequent anneals X-ray dtff'ractlon measurements reveal the effects oJ ambient gas and ambient gas pressure, and anneahng ambient Esther {110} or {111} planes show preJerred orientation when hehum 1~ used as evaporant ambient, whde {100} planes show preferred orientation m argon Anneahng m vacuum produces {111} orwntatton whde anneahng m tellurium vapor produces {110} orientation Crystalhte size measurements reveal preferences related to growth o f larger crystalhtes oJ preferred crystal direction normal to substrate as compared to unpreferred crystalhtes during evaporations of fdms Size measurements indicate regrowth o f preferred crystalhtes larger tn plane o f film during anneahng Two mechamsms are suggested to explain orientation effects in evaporation, scattering o f evaporant atoms by the ambient gas and adsorption/ occlusion o f ambtent gas into the film The mcluslon o f tellurmm m regrowth ts suggested as an explanation of the different regrowth habits m vacuum and tellurmm vapor

Introduction T m - t e l l u n d e h a s been of interest recently for its possible use as a carrier alloy in contacts to l n t e r m e t a l h c semiconductors T h e application of such c o n t a c t s as thin films has p r o v e n a useful m e t h o d of device f a b r i c a t i o n for h i g h frequency apphcat~ons It was in the course o f such m v e s h gations t h a t film orientations was first o b s e r v e d Effects of the pressure o f two a m b i e n t gases, h e h u m a n d argon, u p o n the film o r i e n t a t i o n a n d the effects of a t m e a l m e n t in v a c u u m or in t e l l u r m m v a p o r h a v e been studied

O r i e n t a t i o n of thin films o n crystal substrates has been a subject of interest, as described In the recent p a p e r by D A B r i n e a n d R A Y o u n g of the E n g i n e e r i n g E x p e r i m e n t Station, G e o r g i a Institute o f T e c h n o l o g y 1 R e l a h v e l y few o b s e r v a t i o n s h a v e been m a d e of orientations o f t h i n films o n a m o r p h o u s substrates 2-5 While the m e c h a n i s m s involved In such orientations are n o t k n o w n , it is of interest to study the effects o f various p a r a m e t e r s o f the system used to lay d o w n the films

~

~

Y

R

E

X

SLIDE

- - S LATTERSHIELD

FILAMENTTEMPERATURE1750°C EVAPORANTFEED.25 mg/sec FIG 1 Evaporatmn arrangement

282

Observations of Crystal Orlentatmn Effects m Tm-Tellunde Thin Fdms

Evaporation procedure Pyrex glass slides were thoroughly cleaned in acid and washed in " i o n - e x c h a n g e " water before being placed In the evaporator They were further cleaned by ion bombardment during pump down The tln-tellurlde was prepared fresh and powdered under liquid nitrogen just before being placed m the evaporator The vacuum system was first pumped to 1 × 10-6 Torr by a diffusion pump whereupon a Metssner trap was employed to condense most of the remaining vapors out of the system atmosphere The resulting pressure was about 1 × 10 -7 Torr Helium or argon, dried by a passage through a molecular sieve and cleansed of oxygen by passage through Deoxo was bled Into the vacuum chamber by a controlled leak The pressure was measured by a Kinney mmzatlon gauge and was controlled w~thIn :kl × 10-s Torr of the desired pressure during evaporations Some of the mcreased pressure was undoubtedly due to scattered tin and tellurmm so that the actual ambient gas pressure was lowered during evaporation The ambient gas pressure was not independently measured Because tellurium is a gas at the temperatures necessary to evaporate tln-tellunde, a flash evaporation techmque was employed The powdered evaporant was fed (Fig 1) from a constricted shaker tube on to a hot molybdenum ribbon The ribbon was maintained at such a temperature that the evaporant powder particles vaporized without wetting the filament The slotted mask could be moved laterally, allowing evaporations at several different ambient pressures to be laid down on the same glass shde Films were evaporated on four shdes m separate runs for each ambient gas used

Measurement of film orientatmn A Norelco X-ray dlffractometer was used for the orientation measurements Copper K~ radiation was employed, and detected by a Norelco Proportional Counter Pulse height discrimination was used together with enough Nickel foil filters that the K~ diffractions were not dlscermble above background The P H D effectively blocked out high energy background and harmomc wavelength diffraction contributions A diffraction stage attachment was constructed which allowed the glass slides to be traversed in the plane of the film under the incident X-ray beam Measurements could then be made on the several films apphed to each glass slide The traverse arrangement also allowed each film to be examined for variation in orientation A shde could also be cut, rotated 90 °, and examined for orientation varIatmn This was necessary since the diffractometer employed a hne configuration m o d e n t beam The diffraction techmque just described observed only those crystalhtes in the film which were oriented with particular {hkl) planes parallel to the glass substrate The d~ffractometer rate meter recorder traces thus measured the number of the crystalhtes of each orientation observed It must be remarked that several other factors beside the number of oriented crystalhtes also affect diffraction

283

peak height The structure factor of each reflection and the multiplicity of the (hkl} planes, making up the symmetry associated set of {hkl} planes, influence the intensity of each different (hkl) Intensity measured However, these factors are constant for each {hkl} reflection Crystalhte size and lnhomogeneous strain hne broadening may also affect peak intensity Increased crystalhte size would reduce the observed hne halfwldth and increase peak intensity Part of the effects observed was due to changes in crystalhte size as will be shown Since all films were evaporated m reasonably equivalent conditions, other than ambient pressure or annealment ambient gas, strain line broadening effect upon changes m hne peak Intensity was assumed negligible Two complete scans were run on each film with the beam incident on different areas of the film and the traces compared for reproducibility Peak intensities varied less than 5 per cent among traces indicating only shght varmt~ons in orientation throughout each film Occasional checks on mtensmes measured with films rotated 90 ° confirmed th~s uniformity Line halfw~dths were measured with the same diffraction equipment Instrument broadening was determined by measuring halfwldth with successively smaller beam spht systems The measurements were then extrapolated to zero beam width Jones's6 corrections were apphed for Ks doublet broademng Ekstem and Slegel's7 corrections were apphed for spectral broadening The corrected halfwldths were then used in the Scherrer8 9 equation to calculate crystalhte sizes Values obtained are estimated to be accurate to w~thm 4 5 per cent

Effects of ambient pressure When helium was used as the ambient atmosphere during deposition the resulting films exhibited two preferred orientations (Fig 2) At a pressure of 5 × 10-5 Torr a strong {111) orientation was observed while at 1 × 10-3 Torr a (110) preference occurred It must be remarked that In Fig 2, and the subsequent figures of diffraction traces, the traces measured of different films are offset m the ordinate (intensity) direction for clarity of presentation When argon was used as the ambient atmosphere, a strong (100} preferred orientation (Fig 3) occurred when the film was deposited at 1 x 10 -4 Torr Crystalhte size determinations (Table 1) (by diffraction line halfwldth measurements) were made for the three orientations of crystalhtes measured These values indicate the average thickness of each type of crystalhte normal to the film surface TABLE ]

C r y s t a l h t e sizes ( × 10-s cm)

Argon pressure (Torr) Crystal onentatmn (lO0} {110} {111}

1 × 10-5

1 )< 10-4

950 570 580

1000 270 220

1x

10.--3

650 370 230

S

284

R

SHORTES AND D

P

MILLER

U laJ

u) 0 7 5

222

f,n k-

o f,.)

T i n - T e l l u r l d e (SnTe) E v a p o r a t e d From S t o c h l o m e t r , c Alloy Powder ~n H e l i u m Onto P y r e x Gloss

o o o

Evaporation Distance 6 in Filament Temperature 17500C

z

Film

Thickness

3 microns

I

~5xlO-

s mm Hg

220

~'050 I-

~lxlO'3mmHg z

hi hZ Z

o I,.-

~025


200

m

400

2,

0

15 o

i0 o

25 °

20 ° BRAGG

30 °

ANGLE

FiG 2 Effect o f evaporation m hehum ambient 200

15-t.3 Ld ¢ao

Iz o (.)

Tin--Tellurlde (SnTe) Evaporated From Stochlometric Alloy Powder Of Tin-Tellurlde in A r g o n O n t o P y r e x Glass

I.C - -

o o o

Evaporation

Distance 6in Filament Temperature 1750 ° C F~lm Thickness 3 microns

--

Iz ILl

I x 10-4 m m

5--

Z Z m

o t--

-_

u_ 121

~

X I0 -3 m m

~

220

222

400

--

0

100

I 15 °

m

~ 25 °

20 ° BRAGG

ANGLE

FIG 3 Effect o f e v a p o r a t i o n m a r g o n a m b i e n t

30 °

Observatxons of Crystal Orientat=on Effects m Tm-Telluride Thin Fdms 075

285

-222

Tin--Telluride (SnTe) Annealed ~n V a c u u m

Film

o3 %.

Film

Thickness

5 M,crons

lz

2 hr

S

o

At 4000C

In Vacuum

0050-o o >l-

z w Fz -025-z 0 I.-¢o ,,:::[ rr L~ I.L

200

~ .

220

A__

t ~=#-O r ,g n a I F = l m

I i0 o

I

I 200

15 °

BRAGG

30*

25 ° ANGLE

FIG 4 Effectof vacuum anneal

075-T=n-Tellur,de (SnTe} Annealed ,n T e l l u r i u m

ILl O3

F,Im Vapor 220

O3

Iz D O o 0050-o o

r~2 hr

at 450°C

,n T e V a p o r

>-

tO3 Z I.I 1"-

z z o

025

--

Io

condensed film

]l•TellurJum onto

ur,um

O0

iY i.l_ El.

L

(:3

L

O

0 o

15"

20 °

BRAGG ANGLE

FIG 5 Effectof anneal m tellurium vapor

Condensed

Onto

Film

22

~ 25"

Or,g,nal

F,Im

J 30"

286

S R SHORTES AND D

Effects of annealment atmosphere Three films each were a n n e a l e d in v a c u u m a n d in tellurium vapor Q u a r t z a m p o u l e s were p r e p a r e d a n d flamed o u t under vacuum T h e films o n the pyrex slides were then sealed m the a m p o u l e s u n d e r v a c u u m I f tellurium v a p o r was desired d u r i n g a n n e a l m e n t , e n o u g h t e l l u r m m was a d d e d at o n e e n d o f the a m p o u l e to m a i n t a i n the necessary v a p o r pressure at the a n n e a l m e n t t e m p e r a t u r e The ampoules were t h e n placed in a n lnconel t u b e inside a furnace for a n n e a l m e n t Samples were a n n e a l e d at 400 °C m v a c u u m for periods o f 2-52 h r a n d at 4 5 0 ° C in tellurium v a p o r (0 15 Torr) for 2 h r A n n e a l e d in v a c u u m the films tend to orient with the {111 } parallel to the s u b s t r a t e (Fig 4) A n n e a l e d in tellurium v a p o r the films tend to orient with the {110} parallel to the substrate (Fig 5) T h e crystalhtes grow d u r i n g the a n n e a h n g such t h a t after a n n e a l m e n t all crystal o r i e n t a t i o n s n o r m a l to the s u b s t r a t e exhibit crystalhte dimensions o f 1100-1300 × 10 -8 c m thickness

Discussion Effects o f ambient pressure m film ortentatton T h e o b s e r v e d o r i e n t a t i o n s o f the films in h e l i u m a n d a r g o n suggest two possible m e c h a n i s m s influencing crystalhte growth, a scattering o f e v a p o r a n t b y the a m b m n t gas a n d an a d s o r p t i o n o f a m b i e n t gas o n t o the s u b s t r a t e or occlusion into the film T h e scattering m e c h a m s m ~s suggested m the v a r i a t i o n of o r i e n t a t i o n with pressure I n a r g o n at 1 × 10 -5 m m the film resembles a true p o w d e r in t h a t its d~ffractlon trace correlates with the p o w d e r diffraction d a t a as p u b l i s h e d by A S T M Also the crystalhte sizes (Table I) suggest a r e a s o n a b l y u n i f o r m crystalhte distribution A t 1 × 10-3 m m , however, the film exhibits all diffractions at low intensities a n d the crystalhte size is considerably reduced, suggesting scattering o f e v a p o r a n t a t o m s F i l m o r i e n t a t i o n effects are also observed in e v a p o r a t i o n s in h e h u m a m b i e n t A s would be expected, since h e l i u m h a s a b o u t o n e - t e n t h the mass of argon, a n o r i e n t a t i o n effect occurs at a higher (1 × 10 -3 m m ) pressure t h a n m a r g o n (1 × 1 0 - 4 r a m ) The orientation

P MILLER

effect o b s e r v e d at 5 × 1 0 - 5 r a m in helium, {111}, suggests t h a t a similar effect m a y be observed in a r g o n at 5 × 10-6 m m P a l a t n l k a n d K o m n l k 5 h a v e p r o p o s e d t h a t at sufficiently low pressures, a t o m s are deposited o n a n a m o r p h o u s surface to f o r m the crystal face h a v i n g the highest reticular density F o r tln-tellurlde this is the {111) p l a n e However, this e v a p o r a t i o n was n o t a t t e m p t e d since the a m b i e n t pressure c o u l d n o t be so closely m a i n t a i n e d d u r i n g e v a p o r a t i o n T h e role of a d s o r b e d or occluded a m b i e n t gases IS suggested b y the difference in o r m n t a t l o n h a b i t of the l × 10 -4 m m a r g o n film a n d the 1 × l 0 -3 m m h e h u m film Effects o f annealment atmosphere on f i l m orwntatton T h e anneals b o t h m v a c u u m a n d m tellurium v a p o r were carried o n at t e m p e r a t u r e s at which recrystalhzat~on s h o u l d be expected to o c c u r This is verified b y the increase m crystalhte sizes after a n n e a l B u t the crystalhte s~zes show t h a t if only g r o w t h of crystalhtes n o r m a l to t h e substrate is considered, n o preferred o r i e n t a t i o n s h o u l d b e observed T h e g r o w t h of preferred crystalhtes m u s t therefore take place primarily m the p l a n e o f the s u b s t r a t e T h e g r o w t h of differently oriented crystalhtes, in v a c u u m or m tellurium vapor, suggests t h a t d e p a r t u r e f r o m stolChlometry m a y play a role m the r e g r o w t h h a b i t In a c o n d i t i o n of stoichIometry or tellurium shortage ( v a c u u m anneal) the film recrystalhzes so as to close pack parallel to the substrate However, ff excess t e l l u r m m is present d u r i n g regrowth, close packing in the s u b s t r a t e p l a n e is lrlhlblted R e g i o w t h of the {110) crystalhtes would allow inclusion of tellurium In the a n n e a l e d film

References D A Brine and R A Young, National Symposium on Vacuum Technology (1960) 2 L G Schulz , J Chem Phys 17, 1153 (1949) D M Evans and H Wdman, Acta Cryst, Camb 5, 731 (1952) 4 S A Semdetov, Krtstallografta 4, 629 (1959) L S Palatmk and Y F Kammk , Krtstallografia 5, 738 (1961) F W Jones, Proc Roy Soc A 166, 16 (1938) 7 H Ekstem and S Siegel, Acta Cryst, Camb 2 99 (1949) P Scherrer, Gotttnger Nachrwhten 2, 98 (1918) 9 H P Klug and L E Alexander, X-ra~ Dlffract~on Procedure~, Chap 9 p 512 Wiley. New York (1954)