Investigation of Amorphous-Crystalline Silicon Interface Via Capacitance Techniques
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INVESTIGATION OF AMORPHOUS-CRYSTALLINE SILICON INTERFACE VIA CAPACITANCE TECHNIQUES J.M. ESSICK AND J.D. COHEN Department of Physics, University of Oregon, Eugene,OR 97403
ABSTRACT Amorphous-crystalline heterostructures composed of sub-micron thick undoped amorphous hydrogenated silicon deposited on lightly doped n-type crystalline silicon substrates have been studied by capacitance techniques. Capacitance vs. temperature scans along with model calculations on these samples are used to investigate the amorphous-crystalline silicon interface region. We deduce the existence of an anomalously defective a-Si:H region extending roughly 1000 Angstroms from the growth interface. We also deduce considerable fluctuations in the interface potential which indicate lateral variations of ±20% in the defect distritribution over a 1000 Angstrom scale.
INTRODUCTION Hydrogenated amorphous silicon near an amorphous-crystalline interface is expected to differ from bulk a-Si:H due to nucleation and transfer doping effects that take place uiwing sample growth. Previous studies of the defect structure and extent of this interface region have been hampered by the effects of band bending in this region[l]. In this paper, we present a sample geometry that exploits the interface region band bending, enabling us to probe this region via junction capacitance techniques. Junction capacitance measurements have provided a clear picture of the gap states in a-Si:H. In an AC capacitance measurement, a small modulated voltage is applied to a reverse biased Schottic barrier, producing an AC charge distribution m the depeletion region. The resulting junction capacitance is then C = eA/ , where e is tile static dielectric constant, A is the cross sectional area of the barrier and is the first moment of the AC charge distribution[2]. By varying the applied reverse bias or temperature, varies and thus information a out the energy and spatial distribution of gap states can be gleaned. Our heterostructure samples are composed of a palladium metal Schottky barrier evaporated on top of a sub-micron thick undoped a-Si:H layer which has been deposited on a lightly doped n-t crystalline silicon substrate. The amporphous film thickness was chosen so that for a values of reverse bias applied to the metal contact, the depletion region extends throughout the entire amorphous layer, penetrating into the crystal. Band bending calculations reveal a voltage plateau forms in the amorphous region near the interface. The height of this plateau is (1)highly dependent on the density and energy depth of defects in the interface region and (2) varies only by a few tenths of a volt when the applied reverse bias is varied over many volts. Capacitance-temperature-frequency scans, after an initial crystalline substrate dielectric turn-on at 20-30 K, reveal a capacitance step in the 100-250 K range, temperatures far below that required for the activation of deep defects in undoped a-Si:H. This step can be moved to higher temperatures by increasing the applied reverse bias. We a
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