PECVD grown hydrogenated polymorphous silicon studied using current transient spectroscopies in PIN Diodes
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A18.2.1
PECVD grown hydrogenated polymorphous silicon studied using current transient spectroscopies in PIN Diodes Vibha Tripathi1, Y. N. Mohapatra1 and P. Roca i Cabarrocas2 1 Department of Physics, Indian Institute of Technology, Kanpur, India 2 LPICM (UMR 7647 CNRS), Ecole Polytechnique, 91128 Palaiseau Cedex France ABSTRACT Hydrogenated polymorphous silicon (pm-Si:H) has steadily emerged as a potential replacement of hydrogenated amorphous silicon. Possible changes in the density of gap states due to the presence of crystallites is of central importance in understanding steady state and dynamic characteristics of devices using these materials. We have studied a-Si:H and pm-Si:H grown by PECVD at optimized conditions through the measurement of the steady state reverse current and their transients in PIN devices. The transients are analyzed using isothermal spectroscopic techniques such as Time Analyzed Transient Spectroscopy (TATS), and high resolution Laplace DLTS as a function of temperature. In case of a-Si:H, we obtain the expected signature of emission from a broad density of states in the form of stretched exponentials. In contrast the corresponding spectra for pm-Si:H are dominated by nearly exponential fast current decay processes with discrete energies between 0.20 and 0.26 eV. It is shown that the study of the density of states by dynamic methods such as transient techniques reveal features not accessible to steady state measurements.
INTRODUCTION Polymorphous Silicon (pm-Si:H) thin films have emerged as an alternative to conventional a-Si:H due to their better transport properties and improved stability. These improvements are attributed to the presence of nano-crystalline inclusions in the amorphous matrix when grown under controlled conditions by PECVD. The presence of crystallites in the amorphous matrix of polymorphous silicon is related to improved medium range order. [1] Moreover, improved electrical and optical properties, such as higher hole mobility and ambipolar diffusion length have been reported. [2,3 ] A comparison of amorphous and polymorphous silicon samples has shown polymorphous silicon to have a much lower density of states. [4 ] Effort is currently focused on translating changes in the material property to better devices.[5,6 ] Due to the very nature of the material, we expect to observe novel phenomenon such as room temperature photoluminescence which are not present in the amorphous counterpart. [7] Additionally, new deep states might manifest in the band-gap due to crystallites. However, a coherent understanding of the role that embedded nano-crystallites play in controlling electrical properties is yet to emerge. Investigations involving transport or steady state characteristics of devices fabricated using these methods fail to distinguish between specific contributions of nanocrystallites from that of the surrounding matrix. In this paper, we focus on studying the electrical properties of this heterogeneous material through the analysis of reverse current transients at differe
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