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

I nci i ,Ill Journal of Purc & Arplicd 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. Mithihai 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): reviscd 10 February 2000

Crystal growth of undopcd and Te-doped InSh single crystals has heen achieved hy thc modified vertical directional solidification (YDS) technique. The grown crystnls of undoped growth are /Hype with carrier concentration and mohility of the order of2.4 x IOlll 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' cm², Ys al20 K. Te doped cryslab

;II"L'II-type with electron dcnsity (.'i.2 x 10¹⁷ cm-') virtually remaining unchanged with temperature up to 20 K. While mohilily has increased with dccre;lsc intcmpcraturc (3 x IO.l em²,ys at 2() K). Growth morrhology reve,ils the eutectic micmstructurc\

hut heavi Iy Te-dorcd growth showed strimion. hand formation and preci pitation. The quality of I nSh crystals have heen improved hy the optimi7.ed growth conditions.

1 Introduction

I/l-V compound semiconductor devices I and circuits are becoming extensively important in optoelectronics applications". Fabrication of devices needs large size single crystals '. InSb has highest electron mobilit/ among the ITI-V compounds. Growth of bulk JnSb single crystal was done hy the modified Czocharalski tech- nique" and low defect crystals of carrier concentration

I.') x 1()'7 CI11-' have heen grown. Experiments were carried out in microgravit/ and using centrifuge tech- niques^7, undoped crystals with mobility 2.5 x IO.l

CI112/VS and Te doped crystals of mobility 3.7 x 10' cm²/Vs had also been grown. InSb crystals were also grown hy horizontal Bridgeman^x• vertical Bridgeman, vertical gradient freeze technique^'4 and submerged heater methoc\') hut crystals were seen with striations, ridges and void:-,. Growth ITlorphologylO and electrical propert ics II of the crysta Is grown hy above methods were studied. These properties were influenced by the presence or gravit/~ and solfdification or InSb from the melt I '. Further, undoped and doped crystals 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 Ie' an Inter ace play major roles. We have modified 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 crystals has been performed using VDS techniques. Detailed experi- mental procedure is explained else.where^l7 Thl' 1ll,Iin features of the growth are briefly given here. Indium and antimony in stoichiometric 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 growth axis of thickness

son

~lm. Suhstrates were lapped hy carborundum power and polished by alumina abrasive.

The ohmic contacts using indium were made at four corners of the substrates for Hall measurement. The substrates have also been used for the fabrication of P-II junction diode, Schottky diode and MOS devices.

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

In this paper, we report the growth morphology ob- served through Nomarski optical microscope. Philips X' pert single crystal di ffractometer has been used for the single crystal ~1I1~dvsis. Resistivitv was measured

using four point probe set up and Hall voltage l1leasure- ments were made using four probe CTI closed cycle liquid He-cryostat and magnetic field set up at 2.7':. kG.

800 700 600

U 500

0

w ₄₀₀

a:::

:J

r-

<I: 300

a:::

w 0...

200 2 w

r-

100

800'C CRYSTAL GROWTH PROFILE

Lower ing rot.

10mm/hr Rolotion rol.

10 rpm

575'

InSb- 16

. lI[ 530

liquid -Solid --- - -- - 17 boundorv

:nz:: Crystal Growth

>-

f-

8

Lowering role 3mm/hr Rotolion role

10rpm

16

1

24 32

250'C

3ZI

40 48 56 64

GROWTH TIM E ( hI's)

ORIENTATrON PATTERN (X'pert Philips Diffractom~ter)

PollnSb 135/0 Omeoo 16.980 (220) 2The,o 39.258

Phi 14.000 P.17.350

XO.OO YO.OO

10000

8750

7500

InSbP5-4(growth no.J6) FWHM: 72 arc sec Mobility: (Tc300K)

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1250

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Fig. 1- (;,) Optimized t!rCJwth cyclogram ane! the crystal growth process usee! in growth no. 16 ane! (b) X-ray diffraction pallerns for (22()) rclkclion shows the single phase growth without seed i.e. self-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

All the ingots came out of ampoule very easily after gently tapping and the surfaces had a dull appearance but the conical regions of ingots were more shiny and smooth. There was no sign of striations, voids and ridges on the ingots grown by the optimized growth conditions.

Ingots in this growth showed contraction in diameter, which reveals that the melt was not sticking to the wall and the diameter of the ingots are smaller than the internal diameter of the quartz ampoule. The ampoules had sufficient space above the melt. Therefore, vertical stress on solidifying crystals would be absent. The de- wetting was observed similar to that in Ref. 6. Contrac- tion in the diameter reveals the absence of contact between ingot and ampoule wall during solidification, which reduce the radial heat flow. Therefore, the tem- perature gradient was maintained steady at crystal-liq- uid interface. Further, it reduces thermal stress of the ampoule on the ingot surface. This reveals the condition of three phase boundary (TPB) i.e. the melt-crystal-am- poule wall was critically satisfied in VDS technique.

This results in the growth of nearly perfect single crys- tals of JnSb. The rotation of ampoules assisted in aver- aging out the temperature asymmetries. Further conical angle less that 20° promoted single nucleation with steady liquid-solid intelt·ace. The inert argon pressure inside the ampoule prevented Sb loss. Quartz tube growth chamber with air channel helped in stabilising liquid-crystal interface shape (plane or convex) near the melt. Therefore, thermal stress was curbed and radially outer part was last freezing region in that layer. Appar- ently. it has resulted in the no sticking of melt to walls of the ampoules and no entrapping of bubbles. The single crystal growth could be possible by these opti- mized growth conditions in VDS technique.

.1.1.2 X-mr rli/fi'{/l'Iio/l

FWHM of X-ray pattern of (220) retleetion for a good qual ity InSb ingot no. 16 was

o .m

degree as compared to 0.3° for low quality ingot no. 8. The powder pattern of diffraction peaks is in agreement with star marked ASTM data and JCPDS card no. 6-20S. Single crystal growth was confirmed also by Philips X'pert diffrac- tometer with FWHM 72 arc sec. This reveals that self- orientation growth without seed is possible by this tech- nique. see Fig. I(b). The Laue diffraction photograph also revealed the single crystal nature of the undoped and Te-doped [nSb crystals.

3.1.3 Ulldoped growth

Polished substrates of undoped growth were etched in CP₄ and modified CP4 (HNO,:HF:CH1COOH:H20: :5:3:3: 10 for 7-10 s). Etched samples revealed the eu- tectic microstructure as shown in Fig. 2(a). These fea- tures represent growth of two solid phases of decomposition in homogeneous layer as probed by EDAX. Sb-rich regions are non-stoichiometric growth regions with decorative microstructures and the ex- tended regions of it reduced their size away from the main eutectic regions. These are the volume defect regions formed by constitutional supercooling and mi- crophotograph exhibits the eutectic embedded features.

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

cm²) revealing the clusters of point defects and site of dislocation. Further, undoped low quality crystal growth is seen with intrinsic crystallographic (twin boundary and grain boundary) microstructures. By the optimized growth condition, the eutectic microstructures and ex- tended defects are eliminated.

3.1.4 Te-doped gmwth

Etching procedure is similar to the undoped growth.

Heavily Te-doped growth showed stacking fau,lts ancl striations. The magnified version of band formation is shown in Fig. 2(b). Interestingly, the eutectic micro- structure and conical etch pits are absent in this growth. Heavily Te-doped growth reveals that the former micro- structures are restricted but resulted in impurity band formation. However, the heavi Iy Te-doped growth was also seen with Te-precipitate and micro-cracks. These features (heavily) reveal that the doping concentration (I Olt) cm') varied significantly in ingot and resulted into compositional 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- cipitates were observed in heavily Te-doped (1.7 x IOlt) cm-~) ingots but absent in lightly Te-doped (5.2 x 10¹⁷ cm-~) growth. The high degree of crystall inity growth or undoped and Te-doped InSb crystals resulted into like microstructures, such as low angled crystallographic defects and conical etch pits. The crystal in growth no. 16 (see Fig. I) had drastic reduction in microstructures.

However, eutectic microstructures were dependent on the ampoule lowing velocity and vertical position. This indicates that if the ampoule lowering velocity becomes equal to the eutectic growth rate then the decomposition in the melt is favoured at high temperature.

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

J.2 Electrkal properties 3. ~.I 111Ir/o/!('r/ CITSI(fls

The Hall-Van clef' Pauw measurements have been made ill T = ~()() K. The H;d I measurements at 20 K of these inlloh are shown ill Table I. The low quality ingol showed high impurity concentration (1.6 x 10¹⁷ cm-\ low mobility (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 higher mobility (5.6 x 10-1 cm²/Vs) for optimized growth conditions with improvement in crystallinity. The car- rier concentration (2.4 x 10^1(, cm-^{I )} is slightly higher than the intrinsic carrier concentration (2 x 10^1(, cm-') at

'Ioo~m

.

Fi),!. 1- 1:1) The eutectic micl'(lstrllclurc phl)\u),!raph and (h) ilc:ll'ily Te-doped sa111ples arlO seen with h:11ld rormation and stack in),! 1':11111

Tahle I - Hall measuremenl or undoped and Te-dopecl samples al 20 K

Crystal M()~ility Resistivity Carric:r H:tli cm-/V s ohm-e111 Densit

,

y ^" cperl

NI-Il'IIl'leu\.

em

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

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

InShp2-12 977 0.15 4_0X x 10¹(, 154

InSh1l2-4 28X40 3.7 x IO-l S.X x 10¹⁷ -IOJ)7

InShn2-12 29000 3.85 x 10-1 5.19x 1CP -lUX

T= 300 K. Therefore, mobility has be n decreased with decrease in concentration of carrier~

1

see Fig. J( h)

I,

a;.

. II)

the complex boundary IS closer to the above values . The Hall coefficient as a function of temperature is represented in Fig. ~(a). The undoped materials showed net p-type conduction in low telllperature range (e.xtrin- sic range) whi Ie cOlllplex conduction of carriers are seen after the inversion temperature up to maximum negative value of RH. The value of RH decreases slowly with increase in temperature, which reveals the degenerate mode of conduction. In this investigation the R" Illeas- urement has a shift towards high telllperature side. Simi- larly, the RH

= ()

^(inver.~ion telllperarure) has been observed to the higher telllperature :.;ide. These two results are new and contradicts the existing result. Our results are seen as i Illprovements in crysta II i n ity and higher Illobility of InSb crystals grown by VDS tech- nique. Below 100 K the salllple InSb P2-12 showed the illlpurity conduction by increase in RH with decrease in telllperature but with further decrease in telllperature RII is also decreased which showed the impurity ionization below ~O K. The results of carrier concentration versus Illobility and RH at 300 K is shown in Fig. 3(b). TIll' transport properties of InSb crystals are reported else- where I~. It is observed that the InSbP2-4 samples showed higher Illobility of holes at 20 K and lower concentration of holes, see Table I. Radial colllPositional variation.~

were absent in undoped growth as confirllled by the resistance Illeasurements.

3.2.2 TI'-doIWd ('rrs/III.I·

The substrate for Hall Illeasurelllents had been pre- pared by the procedure laid down for the undoped wafers and the Hallllleasurements are shown in Fig. 3(b). The impurity distribution along the growth axis showed un i- form distribution of single irnpurity. There is no large

GADKARI & ARORA CHARACTERIZATION OF InSh CRYSTALS GROWN BY YDS TECHNIQUE 241

variation of Hall measurements along the length of the ingots; which reveals that the growth conditions resulted . in high degree of crystall inity of the doped material. The electrical characteristics of the Te-doped InSb crystals at T= 20 K are represented in Table I. These properties are seen uniformly distributed at low temperature. The measurements were taken 011 two samples which were obtained 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 impurity in undoped and Te-doped inSb crystals al'e related to the rotation of the ampoule. in the Bridgeman- Stockbarger method there is no relative motion of the crystals and ampoule. The rotation might h<lve assisted

1000

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;:l o 100 u r<J '-

E

u I 0:

~

^{o o . \ \ \ I • \ I I}

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in stirring the melt as well as uniform radial temper<lture during solidification of melt. This effect will maintain the shape of liquid-solid interface steady throughout the crystal growth. The resistivity measurements reveallhc uniformity in composition axi<llly as well as radially in Te-doped growth.

4 Conclusions

The main aim of the modification in the VDS tech- nique is to develop a method for growth of bulk crystals.

The experimental results showed that by the VDS tech- nique good quality InSb crystals could be grown reli- ably. The properties of InSb crystals were confirmed by the X-ray, Hall, Four point probe measurements ancl chemical etching. The VDS technique has been pro-

.,

I'

2

1. In Sb PI -7 3

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

10~ __ ^~ ____ - L ____ ~ ____ ~ __ ~ ____ ~~~~ __

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CARRIER CONCENTRATION (cm3)

Fig. :1- (a) The gr;lph oi" HH versus tcmpcraturc givcs the I?hlllax. HH = () (inversion temperature). which arE' shiflccltllwards the hi!.!hcr side ni"thc tempcJ"<lIurc i"or the high quality crystals. This is in conlrast with existing result. The graph represcl1ls (h) thc c1ITicr ' CllllCcnlration against the mohility and Hall coelTicient (HH) for the undoped growth (!'-type) and Te-doped (/1-type)lnSh crystals ;It :100 I,

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

posed for the growth of elemental and mixed compound semiconductors.

Acknowledgement

The authors would like to express thanks to Dr A J Singh (Chemistry Department, BARC, Mumbai) and Prof K S Chandrasekar (UGC Emeritus), fordiscussions and valuable suggestions. Thanks are also expressed to Prof S B Patel. (Head), Department of Physics, Univer·- sity of Mumbai for the facility extended during the crystal growth. One of the authors (DBG) would like to thank the Principal. Mithibai College, Mumbai.

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