An Investigation of Spin and Charge States Associated with Metastable Defects in N-Type Hydrogenated Amorphous Silicon
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AN INVESTIGATION OF SPIN AND CHARGE STATES ASSOCIATED WITH METASTABLE DEFECTS IN N-TYPE HYDROGENATED AMORPHOUS SILICON
R. J. RASMUSSEN5 , J. D. COHEN*, and J. M. ESSICK* *Department of Physics, University of Oregon, Eugene, OR 97403 **Department of Physics, Occidental College, Los Angeles, CA 90041
ABSTRACT Depletion-width-modulated (DWM) ESR spectroscopy and junction capacitance techniques were employed to investigate deep-defect states in various metastable states of 10 and 80 Vppm PH3 -doped a-Si:H. From the joint application of these methods, we enumerate the change in total spin and charge associated with emission of carriers from states within the mobility gap resulting from a modulated bias voltage. We conclude that the magnitude of the modulated spin-to-charge ratio is nearly unity, consistent with earlier similar investigations of n-type a-Si:H, where the D'/D 0 transition was shown to dominate. Furthermore, we present results of a previously unreported "quadrature" DWM-ESR signal of large magnitude under a wide range of experimental conditions. Similar related effects are also reported from spin- and capacitance-transient studies. These results clearly demonstrate that the changes in spin significantly lag corresponding changes in charge.
INTRODUCTION Various experiments, including photocapacitance [1], space charge limited currents [2] as well as a variety of other techniques, have identified a major defect band in a-Si:H at approximately 0.9 to 1.0 eV below the conduction-band mobility edge. It is generally believed that this deep defect (D) is associated with the isolated silicon dangling bond [3]. For intrinsic material, the dangling bond is singly occupied (DO) and gives rise to an ESR signal with g = 2.0055 [4]. Consistently, a positive effective correlation energy (Ueff) of roughly 0.3 eV is estimated for this deep defect [1,5,6,7]. For phosphorous-doped n-type a-Si:H, this defect is doubly occupied (D-) and presumably diamagnetic, as evidenced by a decrease in the ESR line at g = 2.0055 with increasing phosphorus doping [6]. Since ESR is capable of distinguishing between various types of defects, it is often advantageous to combine ESR with other experimental techniques. For example, spindependent photoconductivity (SDPC) and optically detected magnetic resonance (ODMR) have revealed several recombination processes in intrinsic a-Si:H [8,9]. In the present investigation we employ depletion-width-modulated ESR (DWM-ESR) spectroscopy to monitor changes in spin associated with thermally removing defect electrons within the depletion region of a Schottky-diode device structure. The technique was first applied to study deep defects in PH 3-doped a-Si:H, where it was possible to identify the D-/D 0 transition from a band near Ec -0.9 eV, and, furthermore, that Uff and for this defect band is significantly positive (ca. 0.2 eV). Strikingly different results were obtained on intrinsic a-Si:H, where the dominant transition involved D°/D' with Ueff = 0 [10]. In the present work, we employ DWM-ESR and jun
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