Solid State Communications, Vol. 80, No. 8, pp. 563-566, 1991. Printed in Great Britain. 0038-1098/91$3.00+.00 Pergamon Press plc zyxwvut TAIL STATES IN HYDROGENATED AMORPHOUS SILICON STUDIED BY THERMOLUMINESCENCE Sharad Nikum, Milan Banerjee, P.B.Vidyasagar and S.V.Bhoraskar Department of Physics, University of Poona, Pune-411 007, India. Suvarna Babras School of Energy Studies, Department of Physics, University of Poona, Pune-411 007, India. (Received 15th April zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJI 1991 by J. Taut) Technique of Thermoluminescence was employed to study the nature of defects introduced in hydrogenated amorphous silicon (a-Si:H) by various external treatments like light soaking, y - irradiation, hydrogen removal and rehydrogenation. Defect introduced in each case was identified to be similar and found to exhibit an activation energy of 0.2 eV, which was correlated to the defects produced by bond angle variation in the samples. Defects were also characterised by a much higher order of kinetics compared to those found in conventional thermoluminescence peaks. The results indicated that these structural defects are intrinsic to hydrogenated amorphous silicon samples. The continuous nature of defect distribution in mobility edges is confirmed by higher value of the order of kinetics. I. Introduction A direct consequence of structural disorder in a-Si:H is the presence of mobility edges near the conduction and valance band and a large number of energy levels within the mobilit due to disorder induced defects’, Y gap which arise . Various experiments have shown that not only the deep defect levels but the energy levels within mobility edges also play important role in governing various properties of the materia13‘5. They limit the performance of the device, reduce or enhance the luminescence yield and affect the condution process as wel16v7. Considerable effort has gone into understanding the role of defects in transport and recombination process. Various techniques have been developed to gain information about their distribution and nature*-“. However, yet it is not very clear as to how the mobility edges affect various processes. Another point of debate is the identification of the defects usually present within the mobility edges. Theoretically, defects due to bond angle variations are known to lie in mobility edges”. In a-Si:H, bond angle variation to different extent can be brou ht out by various means such as hydro rem0vaF2~‘3, en ?.4 high energy particle e.g. electrons , y irradiation etc. In such cases therefore, the observed energy levels within the mobility edges could be correlated, unambiguously, to defects due to bond angle variation. In view of the fact, we have irradiated a-Si:H samples by gamma and light radiations and tried to measure the defect levels introduced by them. The technique of thermoluminescence (TL) has been used to probe the energy of defect levels. The technique has been successfully a insulating materials by various workers ps71’,ed to In this technique, one necessarily studies the TL glow, observed at various temperatures. Important parameters such as activation energy of the defects and defect assisted recombination processes can be easily studied through this technique. The temperature at which TL glow peaks appear, provide the activation energy of the defects and the intensity variation of the glow peaks give idea about the order of kinetics of the recombination processes. It is known that as a result, extensive non- radiative recombinations, the yield of TL spectra are usually very low at higher temperatures. However at low temperatures sufficient yield of thermoluminiscence makes this technique more sensitive to the shallow level defects. Mazumdar et al have developed and applied a model for the calculation of activation energies for general order glow peaks”. In the present paper we have used the same model to calculate the activation energies and the order of kinetics of the defects in a-Si:H films. The defects that are created by irradiation are characterised by their order of kinetics. This 563