Lattice Relaxation of Deep Defects in Light-Soaked N-Type Hydrogenated Amorphous Silicon
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LATTICE RELAXATION OF DEEP DEFECTS IN LIGHT-SOAKED N-TYPE HYDROGENATED AMORPHOUS SILICON THOMAS M. LEEN and J. DAVID COHEN Department of Physics, University of Oregon, Eugene, OR 97403. ABSTRACT We report evidence for a deep defect exhibiting very large lattice relaxation in n-type a-Si:H. In light-soaked partially-annealed samples with low phosphorus doping levels one obtains large controlled variation of the Fermi energy position in the mobility gap. Examination by photocapacitance spectroscopy of a sample having a Fermi energy near the minimum in the density of states shows dramatic change in the shape of the photocapacitance spectra. We interpret this as strong evidence for lattice relaxation of deep defects. INTRODUCTION It has been well established that the defect band located near mid-gap in ntype doped a-Si:H is a doubly-occupied (D-) state of the dominant deep defect, D [1,2,3]. While a defect band is also observed near midgap in intrinsic samples, it is now generally believed that this represents the singly-occupied defect (DO) in such samples. Assuming that the D defect possesses a significantly positive correlation energy between its charge states as has been reported [4,5], this implies that the Dand Do sub-bands must lie approximately 0.3 eV deeper in n-type than in intrinsic samples. A variety of mechanisms have been proposed for this difference in defect energies [6,7,8]. A few unsuccessful attempts have been made to observe the actual shift in D defect energies as the doping is varied [9]. However, it is rather difficult to obtain a sample with the Fermi level E at a position truly intermediate between a doped and intrinsic sample. We recently demonstrated that, in addition to the commonly observed initial creation of deep defects, light soaking and partial-anneal treatment of n-type a-Si:H also appears to change the amount of phosphorus dopant activation [10,11]. Thus, by light soaking and partial-annealing in the dark we have found that it is usually possible to vary E. (identified as the activation energy of dark conductivity) from roughly [11]t. E - 0.7 eV foEC-0.25 eV without greatly changing the number of
deep defects
In this paper we examine the deep defect distribution in detail as EF is swept through such a range of energies in the gap. For EF in the vicinity of 0.5 to 0.6 eV below Ec in one sample, we have found direct evidence of a deep defect exhibiting unusually large lattice relaxation properties. We suggest that this likely represents configurational switching of the D defect itself. Such evidence for a configurational change in the defect suggests a natural explanation for the different energy distributions of this defect in n-type and intrinsic a-Si:H. EXPERIMENTAL METHODS Sample preparation. We deposit n-type hydrogenated amorphous silicon film on p+ crystalline silicon substrates by plasma glow discharge decomposition of silane/phosphine/argon gas mixtures. The range of gas-phase phosphine doping levels that have been investigated includes 10 to 300 Vppm PH 3 . A semi-transpare
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