Growth and characterization of bismuth and antimony thin films

ELSEVIER Journal of Crystal Growth 174 (1997) 845 850

, . . . . . . . . C R Y S T A L G R O W T H

Growth and characterization of bismuth and antimony thin films

A. Martinez*, R. Collazo, A.R. Berrios, G.O. Ducoudray Physics Department and Materials Research Center, UniversiO, o/'Puerto Rico, Rio Piedras 00931-3343, Puerto Rico

Abstract

A modified hot-wall epitaxy evaporator was used to deposit Bi and Sb thin films onto (1 1 1)-oriented Si substrates. Auger electron spectra were taken to establish the purity of the films. X-ray diffraction measurements indicate that the films were polycrystalline and displayed a preferential orientation about the (1 1 l) axis in a direction perpendicular to the film plane depending on the substrate temperature during growth.

PACS: 81.15.El; 81.05.Bx

Keywords: Hot-wall epitaxy; Bismuth; Antimony; Thin films

1. Introduction

Bismuth and antimony, as well as solid solutions of these materials, have been widely studied due to their interesting physical properties and their tech- nologically important characteristics [1-7]. Quan- tum size effect induced semimetal-semiconductor transitions have been observed in bismuth and antimony thin films [1-3]. This transition is caused by the disappearance of the overlap between the L-point light electron band and the T-point heavy hole bands with the emergence of a band gap for films with sufficiently small thickness (t < 2dea~oglie).

* Corresponding author.

Takaoka and Murase [3] proposed such mecha- nism to explain the optical behavior of thin bismuth films. In addition, the solid solution system, BixSbl-x, also exhibits a semimetal-semiconductor-semimetal transition with changes in the mole fraction x, as reported by Subotowicz and Mikolaj- czak [4]. Of particular interest is the L-point band inversion occurring for x = 0.06. Due to this behavior, band inverted multilayer structures of Bi/BixSb~_x, have been suggested as a probable system to search for a novel type of interracial electron states [5]. In addition, theoretical work by Agassi has predicted the existence of excitonic superconductivity in this system [6].

Hot wall vapor deposition has been used in the past to grow a number of materials with excellent results [8-14]. IV VI semiconductors,

0022-0248/97/$17.00 Copyright '~i~ 1997 Elsevier Science B.V. All rights reserved PII $0022-0248197)00042-0

846 A. Martinez et al. /Journal o f C~. stal Growth 174 (1997) 845-850

heterostructures, and superlattices of these materials have been successfully grown using hot-wall epitaxy [8-10]. The fabrication of infrared quan- tum well based diode lasers based on IV-VI multilayer structures grown by hot-wall epitaxy attests its capabilities [11]. Though optimally suited to deposit materials which sublimate in the molecular form, the basic hot wall evaporator has been modified to deposit materials which do not exhibit such behavior, e.g. ZnSe [12], Pbl-xEuxS [13], and more recently GaN [14]. Whereas hot-wall epitaxy lacks some of the capabilities of more widely used techniques such as organometallic vapor phase deposition or molecular beam epitaxy, it is apparent that the hot wall technique remains a viable, and relatively inexpensive technique to deposit high quality thin films.

2. Growth apparatus and procedure

The evaporator used to grow the samples in the work being reported here was a modified hot-wall system featuring two furnaces similar to the one used by Clemens et al. [10]. Each stack consisted of three independently heated regions: the source oven, the hot wall, and the substrate holder. The heaters for the source and the hot wall zones consisted of nichrome wire wound around machined ce- ramic cylinders with grooves to prevent the sagging of the wire during heating. The substrate was heated by a 600 W halogen lamp located behind the substrates. The substrate pre-heater stations implemented in the evaporator were also heated via the same halogen lamps. The temperature of the three different regions for both stacks in the evaporator was controlled by means of six PID temperature controllers. Two concentric cylindrical stainless steel heat shields around each hot wall avoided thermal cross-talk between the two evap- oration stacks. A schematic figure of the hot wall evaporator is shown in Fig. 1. The vacuum system consisted of a 440 I/s ion pump assisted by a tita- nium sublimation pump with liquid nitrogen cryoshroud. The high vacuum chamber was en- closed by a water cooled stainless steel bell-jar. The chamber was roughed using a diaphragm mechanical pump followed by dual sorption pumps. The

Substrate Heater

Substrate

Turntable

U - -

Substrate Preheater - - -

Quartz Source Tube L

I I

Fig. 1. Hot wall evaporator schematic diagram.

typical base pressure in the system was 6 x 10 .7 Torr; during growth the pressure rose to 3 x 10 -6 Torr.

The growth procedure began with the loading of (1 1 1)-oriented Si substrates onto the substrate holders. The Si wafers were cut into 1 cm 2 squares and loaded into the system with no chemical pre- treatment. Each substrate holder held four Si square substrates yielding a total of eight samples per run. The source ovens of each stack were loaded with Bi and Sb shots 99.999% pure. Thus each growth run produced four Bi and four Sb films. The substrates laid on a ledge in the substrate holder and stainless steel tablets were placed behind them to ensure a uniform temperature profile across the substrates. One thermocouple for each substrate holder was placed in a hole machined into one of the stainless steel tablets at a distance of approximately 1 mm from the back of the substrate. Each substrate holder was placed on a turntable which allowed rotations between the two growth stacks and the two pre-heater stations. Routinely, Bi and Sb films were deposited in the same growth run. The substrate holders were positioned initially over the pre-heaters until the desired growth substrate temperature was achieved. After control within _+ 1 °C was achieved in all the zones, the substrates were rotated over the evaporator stacks and the deposition began. The temperatures used for the different zones are summarized in Table 1. Growth rates of 0.2-0.8 gm/h were obtained for the two materials evaporated. The film thickness was controlled by

,4. Martinez et al. /Journal ql'C~ystal Growth 174 (1997) 845 850

Table 1 Temperatures for the different zones in the hot-wall evaporator

847

Material Source temperature (C) Hot wall temperature (C) Substrate temperature (C)

Bi 480 400 150 300 Sb 560 415 150 300

==~

J= 4 ,z

I M

7 0031

5 . . . . .

• 11o t1

1 - - z - - -

o ~.~[ ._~ 20

I I I i . I ,

. . . . Antzmonlr, film J I i i

. = . t t . . u ~ . . . . . .J . . . . . . . . . - , . . . . . . . . . . ~. . . . . . . . . . ~ . . . . . . . . . . I - - " i I =

. . . . T , ~ = 150 C I i I i

T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . m -I r T . . . . . . . . . .

, i ..I. . . . . . . . . . J . . . . . . . . . . I . . . . . . . . . . L . . . . . . . . . . . . . . . . . . . t !

," [o~1 ~', "r . . . . . . . . . q . . . . . . . . t ~ . . . . . . . . . . r . . . . . . . . . . . . . . . . . . .

* I , ,

I I i i i I

.:., . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . ,,- . . . . . . . . .

i I ,

i f , T . . . . . . . . . . . . . . . . . . . I . . . . . . . . . . r . . . . . . . . .

i i ,

, , (024} ~091 . . . . __)U . . . . L ; . . . .

31] 4D 50 60 70 80 2-Theta Angle (Degrees)

. - -- 5

.=

• 4

. 1 Z

t ~ 4 1

'__ ;., . ~ .L .

iii!:i:: 0 20

, ' Antimon) film . . I , . . . . . . . . . . i . . . . . . . . . . z.. . . . . . . . . . ~ . . . . . . . . . a . . . . . . . . .

,=.,,> ! T ,.=ilso c i * L ,

" r . . . . . . . . . . . . . . . . . . . . i - . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

i i

I ! • I i I

" v . . . . . . . . . . . . . . . . . . . r . . . . . . . . . . . . . . . . . . . . . . . . . . . .

i I i i

" . . . . . . . . . . . . . . . . . . . f ' i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

i i I , i i i I i ,

, = , , ,

{. i | t ,

30 4Q 5D 60 70 80 2-Theta Angle (Degrees)

Fig. 2. () 20 scan X-ray diffraction patterns of antimony films.

the time the substrates were positioned over the evaporator stacks. Once the growth run was com- pleted, the substrate holders were rotated back over the pre-heater stations, whose power had been

turned off after the preheating stage. The films obtained had featureless mirror-like surfaces. The film thickness was determined using a mechanical profilometer.

848 A. Martinez et al. / Journal o [C~s ta l Growth 174 (1997) 845 850

3. Characterization

3.1. X-ray diffraction

X-ray diffraction measurements were performed on the as-grown samples using a SIEMENS GAADS diffractometer outfitted with an area detector and a standard powder diffractometer SIE- MENS D5000. The data were obtained using the radiation from a Cu X-ray tube with a graphite monochromator and a 0.5 mm collimator for the area detector data. The X-ray diffraction data all of the Bi films grown for the range of substrate temperatures considered in our study, 150"C < Tsubs < 300'C, showed texture, with a preferential orientation in the direction perpendicular to the film plane along the [0 0 3] crystallographic direction. Lower temperatures were unachievable using the scheme of temperature zones of Table 1 due to thermal loading of the substrates by the hot wall and the source heaters. For the case of Sb, seen in Fig. 2, it was determined that although there is evidence of orientation in both samples, complete orientation in the direction perpendicular to the plane of the film was observed for samples grown

with Tsubs > 190°C. The polycrystallinity of the samples was determined from the analysis of the X-ray diffraction patterns obtained with the diffractometer outfitted with the area detector. Typical data for a (0 0 3)-oriented Sb sample is shown on Fig. 3. The data were taken with a goniometer angle Z = 90'-~, and a detector angle covering the range of 10 < 20 < 80. The line markers are De- bye-Scherrer rings corresponding to bulk Sb. The segments of arc along the marked rings are indica- tive of the polycrystalline nature of the film while the absence of diffracted X-ray intensity in the 20 direction other than those corresponding to the [0 0 6] and [0 0 9] reflection verify the results obtained by the 0-20 scan of Fig. 2b. Similar findings about the appearance of texture in Bi and Sb films with varying temperatures, grown on quartz substrates, have been reported by Gladyszewsky et al. [15]. Their study also found that different orienta- tions in the films could be obtained with changes in temperature. Our studies did not indicate such behavior.

4. Auger electron spectroscopy (AES)

Fig. 3. X-Ray diffraction pattern of Sb thin film grown on Si(1 1 1); Tsubs = 275C. The dashed lines correspond to the Debye Scherrer rings for bulk antimony, see text for further details.

Figs. 4 and 5 show the typical results of ex situ Auger survey scans on Bi and Sb films after a 1 rain light sputtering was performed. The scans were obtained using a Physical Electronics 660 scanning Auger microprobe with a 5-keV, 1-/aA electron beam. The sputtering was achieved using a 5-keV, 10-~tA argon beam. The sputtering rates were not calibrated for this specific system but a calibration with a Cr/Ni multilayer standard yields a sputtering rate for these Ar beam parameters of approximately 1 ~,/s. The as-grown films exhibited the usual contaminants, i.e. C and O, associated with the handling of the samples outside the vacuum system disappeared after the first few monolayers were eroded. The AES survey scans show no indication of contaminants, featuring only Auger peaks associated with the materials being deposited, i.e. Bi and Sb. It is also important to point out that no cross-contamination between sources was detected in the films, within the sensitivity of the Auger technique.

A. Martinez et al. /Journal o/CJystal Growth 174 (1997) 845 850 849

u.I

ILl

z

'°t[9 ~--" Bi ' ' ' ' ' ' ' Bi ' - - ~

iI . . . . . . . . . . . . . . . 7 .............

't t 0 I I I I I I I ! I 500 1000 1500 2000

Kinetic Energy (eV)

Fig. 4. AES survey of Bi film after 1 rain light sputtering, See text for delails.

UJ

z "o

1 ~ I i I I I .....

8 Sb - *

4

500 1000

I I J I I

af ter 1 minute sputtering 5 keY 1 0 ~.tA Ar b e a m

I I I I I

1500 2000

Kinetic Energy (eV)

Fig. 5. AES survey after 1 rain light sputtering. See text for details.

5. Summary

We have grown for the first time (to our know- ledge} Bi and Sb films using hot-wall epitaxy onto (1 1 1)-oriented Si substrates. X-ray diffraction studies reveal that the films are polycrystalline. In the case of the Bi films it was observed that for substrate temperatures T~ub~ > 150°C, preferential orientation always occurred in the [0 0 3] direction. For the Sb films, this behavior was observed for Tsubs > 190°C, while for lower temperatures the samples displayed some texture but were not totally

oriented. Auger electron spectroscopy measurements performed on the Bi and Sb films did not reveal the presence of impurities in the samples.

Acknowledgements

The authors wish to thank Michael Joyner and Rudy Rivera for invaluable help with the fabrication of the evaporator. This work was supported by NCRADA-NSWCDD-92-01 and by the NSF-EP- SCoR program.

850 A. Martinez et al. / Journal o f Crystal Growth 174 (1997) 845-850

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Growth and characterization of bismuth and antimony thin films