I nci i ,Ill Journal of Purc & Arp li cd Physics Vol. JX. April 2(X)O. pp 2:17-242

Characterization of p and n-type bulk InSb crystals grown by vertical directional solidification technique

DB Gadkari

Dcpartmcnt of Physics. Mithiha i College. Mumhai 4(X) O.'i6

and

B M Arora

CM and MS. Tata Institute of Fundamental Research. Mumhai 400 OO.'i

Received 2.'i Novemher IlJl)l): rev iscd 10 February 2000

Crystal growth of undopcd and Te-doped InSh sing le crysta ls has heen achieved hy thc modified vert ica l directi onal

solidification (YDS) technique. T he grown crystnls of undoped growth are /Hype with carri er concentration and mohility of the

order of2.4 x IOl ll cm-·1

and .'i.e) x IO.l cm:: IVs at JOO K and 1.32 x 1016 cm- .1 and 3.4 x 10' cm 2, Ys al20 K. Te doped cryslab

;II"L' II -type wi th electron dcnsity (.'i.2 x 1017

cm- ' ) vi rtuall y remaining unchanged with temperature up to 20 K. Whil e mohilil y

has increased with dccre; lsc intcmpcraturc (3 x IO.l em2,ys at 2() K ). Growth morrhology reve,il s the eutectic micmstructurc\

hut heavi Iy Te-dorcd growth showed strimion. hand formation and preci pitation . T he quality of I nSh crysta ls have heen improved

hy the optim i7.ed growth conditi ons.

1 Introduction I/l -V compound semi conductor devices I and circuits

are becoming extensi vely important in optoelectronics applications". Fabrication of devices needs large size si ngle crystals ' . InSb has hi ghest electron mobilit/ among the ITI- V compound s. Growth of bulk JnSb single crysta l was done hy the modifi ed Czocharalski tec h­nique" and low defect crystal s of ca rrier co ncentration I.') x 1() '7 CI11- ' have heen grown. Experiments were carried out in microgravit/ and usi ng centrifuge tech­

niques 7, und oped crysta ls with mobility 2.5 x IO.l

CI11 2/VS and Te doped crystals of mobility 3.7 x 10' cm2/Vs had also been grow n. InSb crystals were also grown hy hori zontal Bridgemanx

• vertical Bridgeman , verti ca l gradient freeze tec hnique ' 4 and submerged heater methoc\') hut crystal s were seen with striations, ridges and void :-,. Growth ITlorphology lO and electri ca l propert ics I I of the crys ta Is grown hy above methods were stud ied. These properties were influenced by the presence or grav it/~ and so lfdifica ti on or InSb from the melt I '. Further, undoped and doped crystal s showed mobility 6 x I O.l cm2/Vs and7 x 10 \ cm2/Vs respectively at 30() K. In the growth process heat transfer coeffi-

. 1.1 t' f"I I " d' f III clent , urnace temperature pro I e ' an Inter ace play major roles. We have mod ified the VDS technique and growth parameters were optimized for high degree

of crystallinity of InSb, InGaSb, InTeSb. GaAISb and GalnSb. In the present investigation. we report the growth of InSb and InTeSb crystals by VDS technique.

2 Experimental Procedure Growth of undoped and Te-doped InSb crysta ls has

been performed using VDS techniques . Detailed experi ­mental procedure is ex plained else.w here l 7 Thl' 1ll,Iin features of the growth are briefly given here. Indium and antimony in stoichiomet ri c proportion were kept in quartz tube conical at one end . Doped crystals were grown by mixing Te into In and Sb charge before sealing. The quartz ampoule with charge was evacuated ( 10-<; torr) and back fi lied alternately lOti meso then ampoules were sealed by refilling of argon (350 torr) . A typical growth cyclogram is shown in Pig. I (a). The growth of InSb crystals were performed by lowering ampoule from the hot zone at the rate of ~ mm/hr ,1I1e1 rotational speed at 10 rpm .

The ingots were slieed perpendicular to the growt h

ax is of thickness son ~lm . Suhstrates were lapped hy carborundum power and poli shed by alumina abrasive. The ohmic contacts using indium were made at fo ur corners of the substrates for Hall measurement. The substrates have also been used for the fabrication of P-II junction di ode, Schottky di ode and MOS dev ices .

INDIA ' .1 PURE & APPL PHYS. VOL ]X, APRI L :W()()

In thi s paper, we report the growth morphology ob­served through Nomarsk i optical microscope. Phili ps X' pert single crystal di ffrac tometer has been used for the single crystal ~1I1 ~ d vs i s. Resisti vitv was measured

using four poin t probe set up and Hall vo ltage l1leasure­ments were made using fo ur probe CTI c losed cyc le liquid He-cryostat and magnetic fi e ld set up at 2.7':. kG .

800

700

600

U 500 0

w 400 a::: :J r-<I: 3 00 a::: w 0... 200 2 w r-

100

800' C CRYSTAL GROWTH PROFI LE

Lower ing rot.

10mm/ hr Rolot ion rol.

10 rpm

575'

In Sb - 16

. lI[ 530 liquid - Sol id - - ---- - 17 bou ndorv

:nz:: Crystal Growth

>­f-

8

Lower ing rol e 3 mm / hr

Rotol ion ro le

10 rpm

16 1

24 32

250'C

3ZI

40 48 56 64

GROWTH TIM E ( hI's)

ORIE NTATrON PATTERN

(X'pert Philips Diffractom~ter) PollnSb 135/0 Omeoo 16 .980

(22 0) 2The,o 3 9 .2 5 8 Phi 14.0 0 0

P.1 7.350 XO.OO

YO.OO

10000

8750

7 5 00

InSbP5-4(growth no.J6)

FWHM : 72 arc sec

Mobility: (T c 300 K)

fL : 5 ·7x I04cm2/ V.Sec.

6250

(j) 5000

Z

~ 3750

Z 2 500

125 0

°1~6.4;:7;;;5-~---:-:-!~-~--'-_...J 17.485

OMEGA / 2THETA(deQ)

\

'.,W \ \ \

-.L.J 72 80

Fig. 1- (; ,) Optim ized t!rCJwth cyc logram ane! the crysta l growth process usee! in growth no. 16 ane! (b) X-ray di f fraction pallerns for (22()) rclkc l ion shows the si ngle phase growth wi thout seed i .e. sel f-seeded growth and FWHM 72 arc sec

GADKARI & ARORA: CHARACTERIZATION OF InSh CRYSTALS GROWN BY VDS TECHNIQUE 2)1)

3 Results and Discussion

3.1 (;rowth morphology

3. 1.1 As grown crrswls

A ll the ingots came out of ampoule very eas ily afte r gentl y tapping and the surfaces had a dull appearance

but the conical regions of ingots were more shiny and smooth . There was no sign of stri at io ns, voids and ridges

on the ingots grown by the optimi zed growth conditions.

Ingots in thi s growth showed contraction in diameter,

which revea ls that the me lt was not sti cking to the wa ll

and the di amete r of the ingots are sma lle r than the in te rna l d iamete r of the quart z ampoule. The ampoules had suffi c ient space above the melt. The refore, verti ca l

stress on solid ifying crys ta ls would be absent. The de­wetti ng was observed simil ar to that in Ref. 6 . Contrac­ti on in the di amete r revea ls the absence of contact between in got and ampoule wall durin g solidificati on, whi ch red uce the radi a l heat fl ow . The refore, the te m­

pe ratu re grad ient was maintai ned steady a t c rysta l- liq­uid in te rface. Furthe r, it reduces the rmal stress o f the

ampoule on the in got surface. T hi s re vea ls the conditio n of three phase bounda ry (TPB) i.e. the me lt-crystal-am­poul e wa ll was criti ca ll y sati sf ied in VDS technique.

Th is results in the growth of nea rl y pe rfect s ingle c rys­

ta ls of JnSb. The rotati on of ampoules ass isted in aver­agin g out the temperature asy mmetries. Furthe r coni cal

angle less that 20° promoted sing le nucleati on w ith steady liquid -solid inte lt·ace. The ine rt argon pressure

ins ide the ampoul e prevented Sb loss . Quartz tu be growth chamber with a ir channe l he lped in stabili s ing

li quid-crystal inte rface shape (pl ane or convex) near the

melt. The refore, the rmal st ress was curbed and radia ll y outer pa rt was last freezin g reg ion in that layer. Appar­entl y. it has resulted in the no sti cking of me lt to wa ll s

of the am poul es and no e ntrapping of bubbles. The sing le crysta l growth could be possibl e by these opti ­mi zed growth conditi ons in V DS technique.

.1. 1.2 X-mr rl i/fi '{/ l'I io/l

FWHM of X-ray patte rn of (220) re tl ee tion for a good qua l ity InSb ingot no . 16 was o.m degree as compared

to 0 .3° for low quality ingot no . 8. The powder patte rn of diffrac ti on peaks is in agreement w ith star marked AST M data and JC PDS card no. 6-20S. S ing le c rysta l growth was confirmed a lso by Philips X ' pe rt diffrac­to meter w ith FWHM 72 arc sec. Thi s reveals that se lf­ori entat ion growth wi thout seed is possible by thi s tech­

niq ue. see Fig . I(b). The Laue d iffraction pho tograph a lso revea led the sing le c rystal nature of the undoped and Te-doped [nSb c rysta ls .

3.1.3 Ulldoped growth

Poli shed substrates of und oped growth were etched

in CP4 and modified CP4 (HNO,: HF:C H1COO H:H20 : :5:3:3: 10 for 7-1 0 s) . Etched samples revealed the eu­

tecti c mic rostructure as shown in Fi g. 2(a). These fea­

tures re prese nt g row th o f two so lid ph as es o f

decomposition in homogeneous layer as probed by

EDAX. Sb-rich regions are non-sto ichi ometri c growth

regions with decorati ve mi crostructures and the ex­

tended regions of it reduced the ir s ize away from the

ma in eutectic regions. These are the vo lu me de fect

regions formed by constituti ona l supercooling and mi­

crophotograph ex hibits the eutect ic embedded fea tures.

Undoped growth showed conical etch pits (EPD 2 x 10"

c m2) revealing the c lu sters of point defects and site of

di slocation. Further, undoped low qua lity c rysta l growt h

is seen with intrinsic crysta ll ographic (twin boundary

and gra in boundary) microstructures . By the optimized

growth condition, the eutec ti c mic rostructures and ex ­

tended defects are e liminated .

3.1.4 Te-doped gm wth

Etching procedure is s imilar to the undoped growt h.

Heav il y Te-doped growth showed stacking fau,l ts ancl

striations. The magnified version of band formati on is

shown in Fig. 2(b). Interestingly, the eutecti c micro­

structure and conical etch pits are absent in thi s growth .

Heavil y Te-doped growth reveals that the fo rmer mi cro­

structures are restri cted but resulted in impurity band

formation. However, the heavi Iy Te-doped growth was

a lso seen with Te-precipitate and micro-c racks. T hese

features (heav il y) reveal that the doping concent rati on

( I Olt) cm' ) varied significantl y in ingot and resulted into

compos itional stresses in the form of microstructures .

Excess Te may be precipitated due to non-availability

of space to occupy within the In-Te-Sb structure. Pre­

c ipitates were observed in heavily Te-doped ( 1.7 x IOlt)

cm-~ ) ingots but absent in li ghtl y Te-doped (5 .2 x 10 17

cm-~ ) growth . The high degree of c rysta ll inity growth o r

undoped and Te-doped In Sb c rysta ls resulted into lik e

mi crostructures, such as low angled crys ta ll ograph ic

defec ts and conical e tch pits. The c rysta l in g rowth no .

16 (see Fig. I) had drasti c reductio n in mi c rostructures.

However, eutectic microstruc tures were dependent on

the ampoule lowing ve loc ity and vertical pos iti on. T hi s

indicates that if the ampo ule lowering ve loc ity becomes

equa l to the eutecti c growth rate then the decompositi on

in the melt is favoured at hi gh temperature .

INDIAN J PURE & APPL PI-IYS, VOL 38, APRIL 2000

J.2 Electrka l properties

3. ~ . I 111Ir/o/!('r/ CITSI(fls

The Hall-Van cle f' Pauw measurements have been made ill T = ~()() K. The H;d I measurements at 20 K of these inlloh are show n i ll Tabl e I. The low quality ingol

showed hi gh impurity concentrati on ( 1.6 x 10 17 cm- \

low mobi lity (4 x to; cm2/Vs ) and the variation in concentration along the ingots. This reveals that there are different Iypes of impurity. Growth no 16 showed

hi gher mobility (5 .6 x 10-1 cm2/Vs) for optimized growth conditi ons with improvement in crystallinity. The car­ri er concentration (2.4 x 101

(, cm-I ) is slight ly hi gher

than the intrinsic carrier concentrati on (2 x 10 1

(, cm-') at

'Ioo~m .

Fi),! . 1 - 1:1) The eutectic micl'(lstrll clurc phl)\u),! raph and

(h) ilc: l l' il y Te-doped sa 111ples arlO seen with h:11ld rormation and

stack in ),! 1':11111

Tahle I - Hall measuremenl or undoped and T e-dopecl samples

al 20 K

Crystal M()~ ili t y Resisti vity Carri c: r H:tl i cm-/V s ohm- e111 Densit y cperl , "

NI-I l'IIl 'leu\. em

InShpl - 7 524 O. I X () .6:1 x I OIr' 1) 5

InShp2-4 ~40() 0.14 1.:11 )( I OIr. 47()

InShp2- 12 977 0. 15 4_0X x 10 1(, 154

InSh1l2-4 28X40 3.7 x IO-l S.X x 10 17 - IOJ)7

InShn2- 12 29000 3. 85 x 10-1 5.19 x 1CP - l UX

T= 300 K. Therefore, mobili ty has be n decreased with decrease in concentrati on of carri er~ 1 see Fig. J( h) I, a;.

. II) the compl ex boundary IS closer to the above va lues .

The Hall coeffici ent as a functi on of temperature is represented in Fig. ~(a) . The undoped material s showed net p-type conduction in low telllperature ran ge (e.xtrin­sic range) whi Ie cOlllpl ex conduction of carr iers are see n after the inversion temperature up to max imum negat ive va lue of RH. The va lue of RH decreases slowly with increase in temperature, which reveals the degenera te mode of conducti on. In this in vest igat ion the R" Illeas­urement has a shift towards hi gh te lllperature side. Simi­larl y, the RH = () ( inve r.~ion te lllperarure) has bee n observed to the hi gher telll perature :.; ide. These two results are new and con tradicts the existing result. Our results are seen as i Illprovements in crysta II i n ity and hi gher Illob ility of InSb crystals grow n by VDS tech­nique. Below 100 K the salllple InSbP2- 12 showed the illlpurity conduction by increase in RH with decrease in te lllperature but with further decrease in te lllperature RII

is a lso decreased wh ich showed the im purity ioni zati on below ~O K. The result s of carrie r concentrati on versus Illob ility and RH at 300 K is shown in Fig. 3(b) . TIll' transport properties of InSb crys tal s are reported e lse­where I ~ . It is observed that the InSbP2-4 samples showed hi gher Illobility of holes at 20 K and lower concentration of holes, see Table I. Radi al colllPos itional variation .~

were absent in undoped growth as confirllled by the res istance Illeasurements.

3.2.2 TI' -doIWd (' rrs/III.I·

The substrate fo r Hall Illeasurelllents had bee n pre­pared by the procedure laid down for the undoped wafers and the Hallllleasurements are shown in Fig. 3( b) . The impurity distributi on al ong the growth axis showed un i­form distributi on of sin gle irnpurity. There is no large

GADKAR I & ARORA CHA RACTERIZATION OF InSh CRYSTALS GROWN BY YDS TECHN IQU E 24 1

variation of Hall measurements along the length of the ingots; which revea ls that the growth conditions resulted

. in high degree of crystall inity of the doped materi al. The elect rica l characteristics of the Te-doped InSb crysta ls at T= 20 K are represented in Table I . These properties are seen uniforml y di stributed at low temperature. The measurements were taken 0 11 two samples which were obta ined from the different sections of the ingot. Hall measurements on undoped and Te doped samples are in agreement with Ref. 19. The uniform distribution of impu rity in undoped and Te-doped inSb crystals al'e re lated to the rotation of the ampoule. in the Bridgeman­Stock barger method there is no relati ve motion of the crys tals and ampoule. The rotation mi ght h<lve assi sted

1000

.D

E o ;:l o 100 u '-

r<J

E u

I 0:

~, o \ • I

o . \ \ I \ I

in stirring the melt as we ll as uniform radial temper<ltu re during solidi ficat ion of melt. This effect will maintain the shape of liquid-solid interface steady throughout the crystal growth. The resist ivity measurements reveallhc uniformity in compositi on ax i<l ll y as wel l as radiall y in Te-doped growth.

4 Conclusions The main aim of the modificat ion in the VDS tec h­

nique is to develop a method for growth of bulk crystals. The experimental results showed that by the VDS tech­nique good quality InSb crystals could be grown reli ­ably. The propert ies of InSb crystal s were confirmed by the X-ray , Hall, Four point probe measurement s ancl chemi ca l etching. The VDS technique has been pro-

., I'

2

1. In Sb PI -7

3

2 . In Sb P2-4 3 . InSbP2-12

1 0 ~ __ ~ ____ -L ____ ~ ____ ~ __ ~ ____ ~~~~ __

3 4 5 6 7 8 9 10

1000 k-I T

106

10 3 102

( b ) Mobilit y :; o p- ln Sb e • n-lnSb

",'-E

u Hall coeff u

Q)

105 r-

(f) • p -InSb Id 10' .Ll

Z > .0 CJ) W

"- CJ) ~ S! N

E oS Ii. '" Ii. U

'" ~ w >-

a. - 0 4

,., c u

f- lO Id ~ I -' ..J -' iIi « 0 :r: :2'

103

I 0 . 1 10'6 1020

CARRIER CONCENTRATION (cm3)

Fig. :1 - (a) The gr;lph oi" HH versus tcmpcraturc givcs the I?hlllax. HH = () ( invers ion temperature). which arE' shi fl ccltllwa rds the hi !.!hcr side ni"thc tempcJ"<lIurc i"or the high quality crysta ls. This is in conlrast with ex isting result . T he graph represcl1l s (h) thc c 1ITicr '

CllllCcnlration again st the mohility and Hall coelTic ient (HH ) for the undoped growth (!'- type) and Te-doped ( /1- type) lnSh crystal s ;It :100 I,

242 INDI AN J PURE & APPL PHYS. VOL 38, APRIL 2000

posed for the growth of e leme nta l and mixed compound semiconductors.

Acknowledgement The authors would like to express thanks to Dr A J

S ingh (C hemi stry Department , BARC, Mumbai) and

Prof K S C handrasekar (UGC Emeritus), fordi scuss io ns

and va luable suggesti ons. Thanks are a lso expressed to Prof S B Pate l. (Head), Department of Physics, Univer·­

sity of Mumbai fo r the fac ility extended durin g the

crysta l growth . One of the authors (DBG) would like to

thank the Principal. Mithiba i College, Mumbai .

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