Gap State Distribution and Interface States in a-Si:H and a-SiGe:H by Modulated Photocurrent
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GAP STATE DISTRIBUTION AND INTERFACE STATES IN a-Si:H AND a-SiGe:H BY MODULATED PHOTOCURRENT
G. SCHUMM*, K. NITSCH**, M.B. SCHUBERT* AND G.H. BAUER* *UniversitAt Stuttgart, Institut fur Physikalische Elektronik, Pfaffenwaldring 47, D-7000 Stuttgart 80, Federal Republic of Germany **Instytut Technologii Elektronowej, Politechniki Wroclawskiej, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland ABSTRACT The energy distribution of localized states above the Fermi level in undoped a-Si:H and a-SiGe:H has been determined by phase shift analysis of modulated photocurrents. (1)A peak in the DOS with 0.56 - 0.65 eV activation energy has been found, reflecting the D--state of isolated dangling bonds. A second peak with 0.35 eV activation energy has been detected which is attributed to the T3 +-state of T3+-Tj--pairs or to the antibonding state of weak Si-Si bonds, respectively. Strong illumination raises the lower peak and quenches the upper one supporting a shallow state - deep state conversion model. (2) From temperature-dependent measurements a shift of the dominant electron transport path into the tail with decreasing temperature - associated with tail state hopping - has been obtained. INTRODUCTION
Trapping, reemission and recombination processes in the gap of a-Si:H are frequently studied by transient experiments in the time-domain (Timeof-Flight, Transient Photocurrents) where excess charge carriers are generated instantaneously and their return to equilibrium is monitored as a function of time. We have taken the complementary approach with sinusoidal excitation of carriers and recording the generated signal as a function of frequency - with some differences: (1)Typical fast decays of signals in the time-domain (exponential or power law) are difficult to record over long times, especially in view of the inevitable dark currents involved, yielding only a limited range for evaluation of gap state distributions, typically tail states down to 200 meV below the dominant transport path for charge carriers. In comparison measurements of signals in the frequency-domain are limited to a maximum frequency given by the band width of the photocurrent amplifier, yielding the gap state distribution for deep states (Fermi level up to 200 meV below the transport path). (2) Use of lock-in techniques in the frequency-domain with improved signal-to-noise ratios allow by orders of magnitude smaller excitations of the samples and thus lower deviations from dark thermal equilibrium. Possible structural changes by light during the measurements (Staebler-Wronski effect) can also be held at a minimum. Using the same assumptions for the physical processes as for Time-ofFlight or Transient Photocurrent measurements - trap limited conduction by electrons (holes) at a transport path close to the band edge with appropriate transition rates (generation, capture and reemission, recombination) - we can set up the usual rate equations and solve for the frequency-dependent phase shift W(w) as a function of the gap state distribution N(E) [1]: W + wkT
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