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ANNEALING OF ION-IMPLANTED HYDROGENATED AMORPHOUS SILICON: STABLE AND REMOVABLE DAMAGE A. J. M. BERNTSEN*, P. A. STOLK*, W. F. VAN DER WEG* and F. W. SARIS**

*Debye Institute, Utrecht University, P.O.Box 80000, 3508 TA Utrecht, The Netherlands **FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands

ABSTRACT Hydrogenated amorphous silicon (a-Si:H) films were irradiated with 1-MeV Si+ ions. The accumulation and annealing of ion damage was investigated by Raman scattering, optical reflection and transmission, and conductivity measurements. For damage levels up to 0.003 displacements per atom, electrical defects are created with no measurable effect on the structural properties. These defects can be completely annealed out at 1800C. Further irradiation results in an increase in the average bond-angle variation in the films. This structural disorder causes a decrease of the optical band gap with 0.46 eV. The structural changes caused by high-dose implantation can not be reversed by annealing at 1800C, which results in the formation of anneal-stable electrical defects.

INTRODUCTION It is well established that electrical defects are created in hydrogenated amorphous silicon (a-Si:H) when the samples are subjected to light soaking and current injection [1]. These defects are metastable and can be annealed out at 150-200'C. However, in ion implantation studies of a-Si:H it has been observed that high-dose ion irradiation produces anneal-stable defects [2, 3, 4]. The reason for this defect stabilization is presently not clear. Recent Raman scattering experiments have shown that for pure, non hydrogenated amorphous silicon (a-Si) the average bond-angle variation AE) is strongly influenced by ion implantation [5]. Here, silicon atoms are displaced through violent collisions with the implanted ions, and these displacement collisions lead to an increase in AE). It is expected that ion implantation will influence AG in a-Si:H as well, which may put severe limits on the annealing of ion damage in a-Si:H. Understanding of the irradiation-induced structural changes in a-Si:H is therefore important, both from a fundamental and an applied point of view. In this paper, we investigate the relation between the electrical, structural, and optical properties of ion-implanted a-Si:H. We use photo- and dark conductivity, Raman scattering, and optical reflection and transmission measurements to characterize the samples. We observe a strong correlation between ion-induced structural disorder in a-Si:H and the formation of anneal-stable defects.

EXPERIMENTAL The a-Si:H films were prepared by decomposition of SiH 4 in an ultra-high-vacuum rf glow discharge at a substrate temperature of 2000C. For Raman scattering, optical, and conductivity measurements we deposited films of different thickness, ranging between 60 and 220 nm, onto Corning 7059 glass substrates. In addition, a 2-/1 m-thick a-Si:H film was deposited on a crystalline silicon (c-Si) substrate for infrared spectroscopy measurements.2 The