Light-induced Creation of Defects and Lifetime Distribution of Photoluminescence in a-Si:H Based Films
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Light-induced Creation of Defects and Lifetime Distribution of Photoluminescence in a-Si:H Based Films Chisato Ogihara, Hitoshi Takemura and Kazuo Morigaki1 Dept of Applied Science, Yamaguchi Univ, 2-16-1 Tokiwadai, Ube 755-8611, JAPAN 1 Dept of Electrical and Digital-System Engineering, Hiroshima Institute of Technology, 2-1-1 Miyake, Saeki-ku, Hiroshima 731-5193, JAPAN
ABSTRACT We have studied the lifetime distributions of photoluminescence (PL) at 10K after the pulsed excitation for a-Si:H based films. Effects of light-induced creation of defects on the lifetime distributions have been studied. The lifetime distributions of PL in a-Si:H based films have a distinct component at about 10 ns together with a longer lifetime component seen in microsecond region. The PL in a-Si:H decreases in intensity after the illumination of visible light. The decreasing of the nanosecond component is slower than that of microsecond component. The decreasing of the PL intensity and increasing of the defect density have also been observed in a-Si:H after illumination of sub-bandgap light, although the absorption coefficient is much smaller than that of visible light. The quenching of PL is discussed with distribution of non-radiative lifetime calculated by assuming random distribution of the DBs. INTRODUCTION Recently techniques of lifetime resolved measurements have been established for the PL studies in a-Si:H based films. Frequency resolved spectroscopy (FRS) is such a technique and has been widely used. It is rigorously proved that the lifetime distribution is approximately represented by the imaginary part of the Fourier transform of the PL decay plotted as a function of the logarithm of the frequency [1]. The FRS technique, which uses CW laser with acousto-optic modulator, is suitable for obtaining the lifetime distribution in microsecond and millisecond region. However, pulsed excitation is preferred to obtain the lifetime distribution in nanosecond region. Since the digital recording of signal of PL decay after pulsed excitation has become possible with the resolution of 1 ns, the calculation of the Fourier transform can be done accurately and easily. We previously reported the lifetime resolved PL in nanosecond region done in such a way [2]. In this paper we report our recent measurements of lifetime distribution of PL in a-Si:H based films and application of this new technique to the study of PL fatigue. PL fatigue after prolonged illumination is known in a-Si:H and understood as a result of light-induced creation of DBs [3,4]. The non-radiative recombination of carriers at native DBs is negligible in good quality Si:H films. However the illumination of intense pulsed light effectively creates DBs of high density in the Si:H films. We expect considerable changes in the PL intensity and lifetime distribution caused by the increasing of probability of non-radiative recombination of carriers at photocreated DBs, i.e. decreasing of non-radiative lifetime of the carriers due to the photocreated DBs. The study of the non-radiati
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