Thermal Equilibrium in a-Si:H N + -P-P + and P + -N-N + Structures
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THERMAL EQUILIBRIUM IN a-Si:H N+-P-P+ AND P+-N-N+ STRUCTURES
YI SANG LEE, JUNG MOKJUN AND CHOOCHON LEE* JIN JANG, YOUNG KUEN LEE, Dept. of Physics, Kyung [lee University, Dongdaemoon-ku, Seoul 131, Korea *Dept. of Physics, Korea Advanced Institute of Science & Technology, P.O. Box 150 Chungyang, Seoul, Korea ABSTRACT Thermal equilibration processes in diode structures of doped hydrogenated amorphous silicon (a-Si:H) have been studied. The fast cooling from above the thermal equilibrium temperature (TE) results in an increase in dark reverse current as well as in forward current. The reverse leakage currrent and the diode quality factor increase with quenching temperature at above TE. It is concluded that the dangling bond density increases upon fast cooling from above TE, even though the amount of the increase is small compared with that for the active dopants. On the other hand, the drift mobility changes little after fast cooling. We propose a new model to explain the experimental results. INTRODUCTION When doped hydrogenated amorphous silicon (a-Si:H) films are rapidly quenched from above the thermal equilibrium temperature, a nonequilibrium electronic and atomic structure is frozen in, which slowly relaxes to the annealed (very slow-cooled) state. Thermal equilibrium temperature (Tb) is the temperature at which the defect structure comes into equilibrium within 0 a few minutes and is ',130 C for n-type and is '800C for p-type a-Si:H [1]. When doped a-Si:H films are rapidly quenched from above the TE, the room temperature dark conductivity is higher than the annealed value and slowly relaxes to the annealed one at room temperature [2]. This higher conductivity can arise either from the increase of active donors (or acceptors for p-type) or from the decrease of dangling bonds or a combination of the two effects. Hack et al. [3] investigated equilibrium changes in n-type a-Si:H by means of capacitance-voltage measurements at varying frequencies and concluded that the density of dangling bonds appears to remain approximately constant, while thedopant density is temperature dependent. Stutzmann [41 has investigated the response of doped a-Si:H to thermal quenching with electron-spin resonance techniques. He explained the metastable increase of shallow state electrons by rapid quenching as the reversible decrease of the doping-induced, compensating dangling bond density. Street et al. [1] also insisted that the dangling bond density might be reduced by fast cooling through the measurements of photoluminescence. On the other hand, it is believed that the dangling bond density increases by fast cooling from above the TE for undoped a-Si:H [5], even though the TE is much higher than that for doped a-Si:H [6]. It is counterintuitive that undoped and doped a-Si:H films behave oppositely when quenched from above TE: In this paper we investigated the changes in the current-voltage TCO/p-p+ a-Si:H/Al, TCO/p+-n-n+ characteristics for TCO/n+-p-p+ a-Si:H/Al, a-Si:H/Al structures and in thin film transistor characteristics usin
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