Hot wall epitaxy of II–VI compounds: CdS and CdTe

Thin Solid Films, 90 (1982) 101-105 PREPARATION

101

AND CHARACTERIZATION

HOT WALL EPITAXY OF II-VI COMPOUNDS:

CdS AND CdTe*

J. HUMENBERGER,

H. SITTER, W. HUBER, N. C. SHARMA AND A. LOPEZ-OTERO

Experitnentalphysik,

Vniversitiit Linz, A-4040 Linz (Austria)

(Received August 9,198l;

accepted

September

23,198l)

CdS and CdTe films were grown by hot wall epitaxy on single-crystal CdTe, BaF, and SrF, substrates. The films grow epitaxially and do not show any misorientations under usual X-ray investigations. N-type films were grown by coevaporation of indium. Typical electron concentrations of up to 2 x 10” cmm3 in CdTe and 3 x lo’* crn3 in CdS were obtained. P-type CdTe layers were also obtained using antimony as a dopant, with hole concentrations up to lo’*10” cme3. Room temperature values of the electron mobility up to 600 cm2 V-’ s-i for CdTe and 230 cm2 V-’ s-l for CdS were obtained. Minority carrier diffusion lengths larger than 1 pm were measured in n-CdTe layers grown on pCdTe substrates. Deep level transient spectroscopy was used to characterize carrier traps in the CdTe films. The concentration of the traps was a function of the growth conditions and could be drastically reduced with annealing of the samples.

1. INTRODUCTION

The successes achieved by the method of hot wall epitaxy in the growth of IVVI compounds’ was one of the reasons for the application of this growth technique to the preparation of thin films of the II-VI compounds. These materials are well behaved, i.e. although they dissociate on sublimation they evaporate congruently and the elements of the compound have relatively high partial pressures at the temperatures of interest. In this work we present some of the main results obtained in our laboratories on the growth and characterization of CdS and CdTe films. This is part of a project on the preparation of solar cells with CdTe as the main absorber2. 2.

EXPERIMENTAL

PROCEDURE

The type of hot wall apparatus used in the present work was similar to that used in the preparation of films of the IV-VI compounds’. Most of the layers were * Paper presented 21-25.1981.

at the Fifth International

0040~6090/82/0000-oBOO/ooo/$o2.75

Thin Films Congress,

Herzlia-on-Sea,

0 Elsevier Sequoia/Printed

Israel,

September

in The Netherlands

102

J. HUMENBERGER

et cd.

deposited in granular polycrystalline form on air-cleaved BaF,(l 11) and SrF,(l 11) substrates using a single source. Some of the CdTe films were grown on PbTe buffer layers previously deposited onto BaF,. Others were grown on CdTe bulk material which had been polished and etched before introduction into the vacuum system. The electrical characterization of the films was made by means of the Hall effect in the standard Van der Pauw geometry. Optical measurements were made using a Cary spectrophotometer. Deep level transient spectroscopy (DLTS) measurements were also carried out on Au/CdTe Schottky diodes and on PbTe/CdTe heterojunctions using an experimental set-up similar to that proposed by Lang3. 3.

RESULTS

AND DISCUSSION

The typical ranges of temperatures used for the growth of the CdTe and CdS epitaxial films are given in Table I. X-ray measurements (Bragg diffractometry and Laue photographs) revealed that the CdTe and CdS films obtained had the cubic and wurtzite structures respectively. We were unable to detect any polymorphs. The layers grew epitaxially, i.e. with the (111) direction (CdTe) and c axis (CdS) perpendicular to the (111) plane of the substrate. Deviations from epitaxy began to be observed for growth rates (30-50 urn h-l) much larger than those given in Table I, the threshold for epitaxy being also a function of the substrate temperature. TABLE

I

TYPICAL

GROWTH

Material

CONDITIONS

FOR

CdTe

AND

CdS

FILMS

Temperature (“C)

Growth rate (wh-‘)

n-CdTe p-CdTe CdS

Substrate

Material source

Dopant source

Cd source

480 480 550

580 580 630

590 (In) 500 (Sb) 550 (In)

260 260

3 3 1.5

Morphological studies made with a Nomarski interference microscope revealed that many films, although epitaxial, had a grained structure usually in the form of pyramids (CdTe) or hexagonal hillocks (CdS) with defects acting as centres of growth (spiral growth). Careful adjustment of the initial growth conditions4 resulted in two-dimensional growth and films with very smooth surfaces, although their reproducibility was rather poor. The surface of a typical CdS film on SrF, is shown in Fig. 1, After numerous trials using many different sets of growth conditions we could only confirm the results of Dlweritz’ for the growth of thin films of CdTe. According to calculations by this author the range of growth conditions for the preparation of defect-free (no twins) CdTe epitaxial layers is extremely narrow. This is compounded by the use of substrates with a relatively poor lattice match to CdTe (a = 6.481 A) such as BaF, (a = 6.200 A). The quality of the films improved when a very thin buffer layer (around 200 A) of PbTe (a = 6.439 A) was deposited onto the BaF, substrate prior to the evaporation of the CdTe. Even better results were obtained by growing on CdTe bulk crystals.

HOT WALL

EPITAXY OF

Fig. 1. Surface

CdS

AND

CdTe

103

of a typical CdS film deposited onto an SrF, substrate. (Magnification, 238x .)

Both CdTe and CdS films had to be doped with indium to make them low ohmic. In this way, electron concentrations up to 2 x 1O1’ crnm3 in CdTe and 3 x lo’* cmm3 in CdS could be achieved. When the cadmium partial pressure is above the value required to obtain stoichiometric compounds, carrier concentrations of the order of only 1O14-1O’5cmp3 resulted. P-type CdTe material with carrier concentrations in the range 10’*-1019 cmw3 was obtained by doping with antimony. The preparation conditions, however, were very critical. Small deviations ( f 5 to f 10 “C) from the substrate and antimony temperatures given in Table I resulted in either material with metallic inclusions or material with a high resistivity. Although the electron mobilities obtained by us are the largest reported for CdTe films (600 cm2 V-’ s-l at 300 K, deposited onto bulk CdTe) we were unable until now to reach the maximum of about 1000 cm2 V-’ s-l at 300 K obtained in single-crystal bulk material. The results for layers deposited onto bulk CdTe with surfaces other than the (11 l), however, are very promising. The attainment of CdS (a = 4.136 A, c = 6.716 A) films with high electron mobilities on poorly matched substrates seems to be less critical. Mobilities in the neighbourhood of the bulk values were obtained for CdS films deposited onto SrF, (a = 5.800 A). All our samples could be divided into two groups: (a) those where grain boundaries dominate the scattering mechanisms in the whole range of temperatures studied (300-15 K); (b) those where the grain boundaries play no role at all or determine the mobility in only a part of the temperature interval mentioned above. To the first group belong all the CdTe films grown on BaF,. In many of the films grown on PbTe buffer layers or on high resistivity CdTe bulk material, however, the mobility is determined by impurity and phonon scattering at low and high temperatures respectively. Grain boundaries also determine the mobilities of most of the CdS films. For films of sufficiently large grains, however, the mobility at high temperatures is dominated by phonon scattering. Figure 2 shows the dependence of the electron mobility on the inverse of the temperature for a CdS film deposited onto BaF,. The thickness of the film was 5.2 urn and its carrier concentration was 9.0 x 10” cmm3. The curves in the figures represent the theoretical values of the contributions of the different scattering mechanisms to the total mobility6. Figure 3 shows the transmission curves for two typical CdTe (Eo = 1.51 eV)

104

J. HUMENBERGER

et al.

and CdS (I& = 2.42 eV) films. The sharpness of the transmission edges at about A = 0.850 pm and A = 0.514 pm is an indication of the good quality and stoichiometry of the samples, free from, among other defects, metallic inclusions. 300

100

50

10

20

T

0.4

0.5

0.6

103/T [K-l]

0.6

0.9

WAVELENGTH

0.7

bm]

1.0

1.1

1.2

Fig. 2. Dependence of the electron mobility on the inverse of the temperature for a CdS film deposited onto BaF,. pr,,, pi and p,, represent the contributions to the mobility from scattering by phonons, impurities and grain boundaries respectively. p is the total mobility. Fig. 3. Transmission

as a function

bf wavelength

for typical

(4

@I DEPTH C urn 1

CdTe (d = 2.7 pm) and CdS (d = 3.8 urn)

h

0

.l

.2 3 DEPTH C u ;I

Fig. 4. Dependence of defect concentration profiles on growth conditions for CdTe films (CdTe/PbTe heterojunction; 0, E2; A, E,; 0, E,; 0, Es): (a) substrate temperature T, = 500°C; (b) T, = 400 “C. Fig. 5. Variation ofdefect concentration profiles with annealing in a typical CdTe film (A, E,; 0, EZ): (a) as-grown material;(b) after 1 h at 360 K;(c) after 15 h at 390 K.

HOT WALL EPITAXY OF

CdS AND CdTe

105

The hot wall apparatus was also used to prepare p-n junctions by growing nCdTe layers on p-type CdTe bulk crystals. Diode reverse saturation currents of 10-10-10-9 A cme2 and open-circuit voltages in the neighbourhood of 0.62 V under simulated air mass 1.5 sunlight were measured in these junctions. Electronbeam-induced current measurements of the hole diffusion length gave values close to 1.2 urn 2. DLTS measurements gave six different levels in the upper half of the forbidden gap’ (E, = 0.20eV, E, = 0.36eV, E, = 0.34eV, E, = 0.24eV, E, = 0.46eV, E, = 0.64eV). The capture cross section was determined separately for the energy levels E, and E, and gave the values cr3 = 6 x lo-‘* cm2 and c5 = 1.5 x 10-l’ cm2 respectively. It was determined that the concentration of the levels and their distribution with depth away from the interface in the diode are a function of the growth conditions (Fig. 4). We were able to reduce the concentration of the defects and even to eliminate some of them, to the limit of our detection capabilities, using mild annealing (Fig. 5). 4. CONCLUSION The results reported in this work for as-grown CdTe and CdS thin films deposited onto different types of substrates confirm the usefulness of hot wall epitaxy for the growth of good quality films of these compounds. For CdTe, however, growth closer to equilibrium conditions cannot avoid the formation of defects resulting from deposition onto poorly matched substrates. Growth on CdTe single crystals looks very promising. The preparation of high mobility CdS films, in contrast, is much less critical. The possibility of doping the materials during growth was also demonstrated. ACKNOWLEDGMENT

This work was supported in part by Fonds zur Fijrderung der wissenschaftlichen Forschung, Austria. REFERENCES 1 A. Lopez-Otero, Thin SolidFilms, 49 (1978) 3. W. Huber, A. Lopez-Otero, C. Fortmann, A. L. Fahrenbruch and R. H. Bube, Proc. 15th Photovoltaic Specialists’ Conf., Orlando, FL, 1981, IEEE, New York, 1981. 3 D. V. Lang, J. Appl. Phys., 45 (1974) 3014. 4 A. Lopez-Otero and W. Huber, Surf Sci., 86 (1979) 167. 5 L. Daweritz, Krisr. Tech., 7(1972) 167. 6 J. Humenberger, Thesis, Linz University, Linz, February 1981. 7 H. Sitter, W. Huber and A. Lopez-Otero, Proc. Inr. Con/. on Defects and Radiation E&cts in Semiconductors, Oiso, September 1980, Institute of Physics, London, 1981, p. 377. 2