Carrier Recombination in A-SI:H P-I-N Devices Studied by PL and EL Spectroscopies
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INTRODUCTION PL has been used to study the recombination mechanism and the energy distribution of localized states in a-Si:H films. [1] At low temperature, T < 50 K, and with band-gap excitation the radiative tail-to-tail transition gives a main-band luminescence peak at 1.3-1.4 eV. A weak tail-to-defect radiative transition gives a defect-band luminescence peak at 0.8-0.9 eV, which can be observed at T > 150 K or at high defect density. With increasing temperature, the PL
efficiency is gradually thermally quenched. At room temperature, the PL efficiency is reduced by about four orders of magnitude. It was found that the PL efficiency, IpL(T), decreases with increasing temperature in the range of 80 K < T < 300 K and follows the relation [2] (1)
IPL(T) = IpLoexp(-T/TL)
where TL = To/ln(v0otr) =-T0 /23 with V0 = 1013 s-1 and an average radiative lifetime tr = 10-3 s. To is the characteristic temperature of the exponentially distributed band tail states, g(E) = goexp(-AE/kTo). In intrinsic a-Si:H TL is 20 - 25 K, corresponding to To of 460-575 K. In this model only tail-to-tail transitions were taken into account. This is the case in low defect a-Si:H films excited by band-gap light. EL, on the other hand, has been used to study recombination in p-i-n structures.13-8] Under forward bias, electrons and holes are injected into the i-layer from opposite sides of the diode. EL is the result of excess electron-hole recombination in the i-layer, including the interfaces. In the early studies of EL in a-Si:H, the same temperature dependence and energy spectrum as that of PL were reported [3-5], except that the EL efficiency was two orders of magnitude lower than that of the PL. [3-5] It has been found, in fact, [6] that the EL efficiency is as high as the PL efficiency when one uses a correct expression for the generation rate, g: (2)
g = (JF/ed)(T/to) = CV/(edto) 281 Mat. Res. Soc. Symp. Proc. Vol. 377 ©1995 Materials Research Society
where JF, e, d, r, to, C, and V are the density of forward bias current, the electron charge, the ilayer thickness, the carrier's average lifetime, the average transit time of carriers, the sample capacitance, and the applied voltage respectively. Hence, high EL efficiency can be obtained under constant voltage conditions at varied temperature.[6] Furthermore, we have observed previously [7,8] that the EL spectrum line shape depends on several parameters, such as the ilayer thickness, the applied voltage, and the diode structure with or without a buffer layer. Based on the observation of the thickness dependence of EL spectrum[7], it was suggested that in p-i-n structures the main-band luminescence is dominated by bulk recombination, while the defect-band luminescence is dominated by interface recombination. On the other hand, the PL spectrum in undoped a-Si:H films has suggested[l] that both the main-band and defect-band luminescence are due to the bulk recombination. In order to explore what recombination mechanisms are responsible for the dependence of EL spectra on the diode structu
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