Evolution of Charged Gap Statesin a - Si:H Under Light Exposure
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Evolution of Charged Gap States in a-Si:H Under Light Exposure M. Zeman1, V. Nádaždy1, R.A.C.M.M. van Swaaij1, R. Durný2, J.W. Metselaar1 1 Delft University of Technology – DIMES, P.O. Box 5053, 2600 GB Delft, The Netherlands 2 Slovak University of Technology, Ilkovičova 3, 812 19 Bratislava, Slovakia ABSTRACT The charge deep-level transient spectroscopy (Q-DLTS) experiments on undoped hydrogenated amorphous silicon (a-Si:H) demonstrate that during light soaking the states in the upper part of the gap disappear, while additional states around and below midgap are created. Since no direct correlation is observed in light-induced changes of the three groups of states that we identify from the Q-DLTS signal, we believe that we deal with three different types of defects. Positively charged states above midgap are related to a complex formed by a hydrogen molecule and a dangling bond. Negatively charged states below midgap are attributed to floating bonds. Various trends in the evolution of dark conductivity due to light soaking indicate that the kinetics of light-induced changes of the three gap-state components depend on their initial energy distributions and on the spectrum and intensity of light during exposure. INTRODUCTION Inherent to hydrogenated amorphous silicon (a-Si:H) are the reversible changes in electronic properties of a-Si:H under light exposure. This is known today as the Staebler-Wronski effect (SWE) [1]. Since the observation of the SWE, a large effort has been put into obtaining understanding of the processes that cause the structural and opto-electronic light-induced changes in a-Si:H. It is generally accepted that light soaking leads to the creation of additional dangling-bond defects [2]. However, many unresolved issues regarding the SWE still remain, such as the exact role of hydrogen, the influence of local bonding configurations, and the presence of more than one metastable defect. We present experimental results from the charge deep-level transient spectroscopy (Q-DLTS) [3] that reveal a surprising behavior of the gap states in a-Si:H during light soaking. The results of the Q-DLTS experiment show that prior to the creation of the defect states around midgap there is an initial decrease of the density of gap states located above midgap. This observation indicates that in the early stage of light soaking, annihilation of some type of defects takes place. Combining our results from the Q-DLTS experiments with recent results from the nuclear magnetic resonance (NMR) experiments [4], ab initio pseudopotential calculations [5,6], and molecular dynamics simulations (MDS) [7,8] we propose microscopic atomic configurations that introduce charged gap states in a-Si:H. Q-DLTS TECHNIQUE The DLTS technique has proved to be a powerful and straightforward method to analyze gap states in a-Si:H [9,10]. The advantage of the DLTS technique in comparison to other techniques that are used to study the gap states in a-Si:H, such as electron-spin resonance or the constant photocurrent method is that the ra
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