Ion Bombarment Effect on the Growth of Microcrystalline Germanium
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ION BOMBAR[MENT EFFECT ON THE GRIiWTH OF MICROCRYSTALLINE GER•MANIUM B. DREVILLON, C. GODET- AND A.M. ANTOINE Equipe Synthnse de Couches Minces pour 1'Energ6tique (ER 258), Laboratoire de Physique Nuclkaire des Hautes Energies, Ecole Polytechnique, 91128 Palaiseau (France). * also at : Laboratoire de Physico-Chimie des Rayonnements (UA 75), Universit6 Paris-Sud, 91405 Orsay (France). ABSTRACT The influence of surface mobility on the growth of Ge films is studied as a function of preparation conditions. The positive ion bombardment during film deposition on chromium substrates is analyzed using an electrostatic analyzer. The dielectric functions S(E) of the films are measured over the range 1.7-4.5 eV using in-situ spectroscopic phase modulated ellipsometry (SPME). The S(E) spectra of microcrystalline germanium (pc-Ge) present a shoulder near 4.2 eV which corresponds to the E. optical transition observed in single crystal germanium. For substrate temperatures greater than 150°C, a transition from a-Ge:H to pc-Ge appears when the ion kinetic energy exceeds a threshold energy Eth, around 120 eV. Eth decreases as a function of substrate temperature. The Eth value is found to be higher than the threshold value corresponding to the opposite transition, giving evidence of a substrate memory effect on the growth of Pc-Ge. Kinetic ellipsometry measurements of the early stage of the pc-Ge deposition on chromium are accurately, modelled by microcrystalline nucleation at a 50 A level. After 200 A thickness, pc-Ge grows with an overlayer. The description of pc-Ge as a mixture of c-Ge, a-Ge:H and voids is discussed. INTRODUCTION The enhancement of the reactive species surface mobility due to an increase of ion bombardment or substrate temperature has been shown to produce a film densification of a-Si:H and a-Ge:H films deposited by RF glow-discharge [1]. With increasing dilution and discharge power, a structural transition from a-Si:H to pc-Si has been reported [2,3]. Spectroscopic ellipsometry has been used to check the microcrystalline character of Si [4,5,6] and Ge [5,7] films which present a remnant E2 optical transition near 4.2 eV. The description of the optical properties of microcrystalline films as a mixture of three reference phases has been performed using an effective medium approximation. The dependence upon the reference dielectric functions of the three-phase description has been pointed out [6]. Real-time spectroscopic phase modulated ellipsometry (SPME)[8] is a powerful technique to study in situ the growth processes as a function of film thickness. It has revealed structural modifications during pc-Si growth. A similar analysis performed on pc-Ge films deposited in carefully controlled ion bombardment and temperature conditions is reported here. The insight in pc-Ge growth processes given by kinetic ellipsometry is used in order to simulate the early stage of the growth [9, 10, 11].
Mat. Res. Soc. Symp. Proc. Vol. 75.
1987 Materials Research Society
342
EXPERIUMNTAL DETAILS Samples are prepared by RF (13.5
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