Stacking fault density in evaporated thin films of germanium

Short Communications Stacking fault density in evaporated thin films of germanium

Previous studies of structural defects, including stacking faults, in Ge and Si epitaxial thin films have been reviewed by Mendelsonl. The results communicated here were obtained during an investigation of Ge auto-epitaxy’, and concern the variation of stacking fault density at the surface of deposited films with changes in the heat treatment of the substrate prior to deposition, the deposition temperature, and the vacuum conditions. Details of substrate preparation relevant to the resulting surface condition are described briefly. Wafers were cut parallel to (111) from intrinsic single-crystal Ge having a dislocation (etch pit) density of 1000-1500 cm-‘, were lapped, mechanically polished, and etched in boiling N/40 KOBr solution. The resulting surface contained shallow troughs remaining from polishing scratches, but these merely gave rise to corresponding depressions at the film surface, and were not active in initiating faults. Glancing angle reflection electron diffraction (50 kV) from the substrates showed that the surface was relatively flat and of a single orientation (symmetrically elongated Laue spots), and that the lattice at the surface was relatively undistorted (sharp Kikuchi lines). Further, no polycrystalline impurities were detected (absence of rings), and any amorphous layer, e.g. oxide, cannot have been thicker than a few Angstrom units (low background intensity). The evaporant was zone-refined polycrystalIine Ge, melted on a watercooled hearth by electron bombardment, so that the evaporation of contaminants was largely avoided. The films described here were deposited for 5 min at an average rate of 50 &see to a thickness of approx. 15000 A. The stacking faults, which had propagated from points at, or near, the substrate-film interface along inclined { 11I} planes up to the surface, were revealed by etching for 10 set in boiling “ferricyanide” etchant (6 g KOH, 4 g K,Fe(CN),, 50 g water), and counted under an optical microscope. Faults originating from the same point TABLE

I

CONDITIONS

IN THE VACUUM

SYSTEMS

Type of system

Pressure (torr) and main constituents before evaporation Pressure (torr) and main constituents during evaporation

HV

UHV

Oil diffusion pump, liquid air trap, Viton seals, unbaked 3 x 10-1

Sputter ion pump, metal seals, baked at 250 “C 5 x lo-”

H, H,O

H,

CO+N,

0,

5-10x10-‘0

5 x10-6

_

He CO+N,

Thin Solid Films, 2 (1968) 173-175-Elsevier,

COSN,

CO, Lausanne-Printed

H,

CO+N,

in the Netherlands

174

SHORT COMMUNICATIONS

TABLE

II

V A L U E S OF E X P E R I M E N T A L V A R I A B L E S A N D R E S U L T S

Run No.

H V or U H V

Tp f C )

t (rain)

Td (C)

F ( c m '-~

1 2 3 4 5 6 7

HV HV HV HV UHV UHV UHV

570 670 770 870 570 770 540

10 10 10 10 10 10 400

570 670 770 870 570 570 540

3 2 2~ 3 2. I . 2

10: 10'; 10 ~ 101 10: I0 ~ 106

were counted once only, so that a density of fault origins was actually obtained. The experimental variables were the temperature of the substrate, Tp, during the preheat period immediately before deposition, the preheat time, t, the substrate temperature during deposition, Td, and the vacuum conditions. The latter, denoted by high vacuum (HV) and ultra-high vacuum (UHV), are summarised in Table I. The values of Tp, t, and Td used for tile various runs are shown in Table II, together with the resulting stacking fault densities, F (cm-2). The values of Td were in the range for which single-crystal films were obtained. For Runs 1 te 4 a remarkable decrease in F was observed with increasing Tp and Td. In Fig. 1 log F is plotted against Td, and a parabola of the form log F = - a T 2 + b T d + C is fitted to the results. When T d is in C , a - 3.87 x 10 -5 d e c a d e / ( C ) 2, b = 3.53 x l 0 - 2 decade/CC, and (fortuitously) c = 0. In this equation F has a maximum, Fn..... of 1.1 x l0 s c m - 2 at 456 °C. In practice densities greater than 108 c m - z are difficult to count because of overlapping o f faults at the film thickness used, and so it is uncertain whether this limit is exceeded. A point at Td = 420 °C

109[ 8V ] max I

L 106 t

°7

~105 5

°6

"

~ 104

~ 4

~10 a ~102 1 0400

500

600

700

Deposition temperature, Td (°C) Fig. 1. Stacking fault density vs. deposition temperature. Thin S o l i d Films, 2 (1968) 173 175

800

900

175

SHORT COMMUNICATIONS

(F = 8 × 10 7 c m - 2 ) from another series of runs, not strictly comparable to the above, is included in Fig. 1 to demonstrate that densities near the limit Fmax are obtainable. At this stage it could not be decided whether the decrease in F was a result of the increase in Tp, in Td, or in both. Naturally, omission of the preheat period would have resolved the question, but this was impossible because a finite time was required to stabilise the substrate temperature before deposition. The remaining results, although rather few in number, served to clarify the situation. Firstly, Run 5, a repeat of Run 1 in UHV, showed that F is not altered significantly by changing from HV to UHV conditions, at least at 570 °C. Next, Run 6 showed that increasing Tp independently of Td decreases F, but not to the same extent as when both are increased (as in Run 3). Thus F is apparently determined by both Tp and Td. Lastly, Run 7 may be regarded as a repeat of Run 5 with a much longer preheat time (although the temperatures, owing to experimental difficulties, were 30 °C lower). The result showed that an increase in t also reduces F, but that the effect is smallcompared to that of changing the temperature. No theoretical basis for the relation between F and Td for Runs 1 to 4 has yet been found, but a tentative explanation for the reduction of F with increasing Tp, t, and Td may be put forward. This assumes that as a result of substrate surface preparation there is a high density, Fo, of centres for the potential initiation of stacking faults. This value is reduced by a factor fp (Tp, t) after preheating, and the fraction of the remaining centres active in fault formation, which depends on Td, is denoted by fa (Td). Thus F (Tp, Td, t) = F o. fp(Tp, t). fd(Td). Using the extrapolated value Fmax as an estimate for F o, it may be shown 2 that for Run 3, for example, fp ~ f d ~ 10-2" The fault initiation centres on the substrate surface may be situated at the edges of patches or particles of GeO2, in which case the reduction in their density as a result of preheating might be due to removal of oxide by the reaction Ge (s) + GeOz(s) = 2GeO(g). The factor fd may be regarded as the probability of fault formation from a given density of centres. Its reduction with increasing Td may be due to increased selfdiffusion of Ge atoms which could have the effect, during the early stages of deposition, of allowing increased accommodation of mismatches between various parts of the growing lattice, with the formation of fewer stacking faults.

Institut fiir technische Physik, Eidoen6ssische Technische Hochschule, Ziirich (Switzerland)

T . P . WOODMAN

1 S. MENDELSON, Mater. Sci. Eng., 1 (1966) 42. 2 T . P . WOODMAN, Dissertation, Eidgen6ssische Technische Hochschule, Ztirich, 1968.

Received February 22, 1968 Thin Solid Fihns, 2 (1968) 173-175