Silicon DOI 10.1007/s12633-015-9374-8 ORIGINAL PAPER Effect of Pressure on Bonding Environment and Carrier Transport of a-Si:H Thin Films Deposited Using 27.12 MHz Assisted PECVD Process Deepika Chaudhary 1,2 · Mansi Sharma 1,2 · S. Sudhakar 1 · Sushil Kumar 1,2 Received: 25 March 2015 / Accepted: 13 October 2015 © Springer Science+Business Media Dordrecht 2016 Abstract Investigation of carrier transport in hydrogenated amorphous silicon (a-Si:H) thin films deposited at various pressures (0.03 - 0.53 Torr) using 27.12 MHz assisted high frequency Plasma Enhanced Chemical vapor Deposition (PECVD) process is presented. From results of Steady State Photocarrier Grating (SSPG) the carrier diffusion length was found to vary from 0.098 - 0.189 μm. Moreover a direct influence of ambipolar diffusion length was observed with the transport mechanism for deposition pressure in the range (0.13 - 0.53 Torr). There was a correlation observed for photosensitivity and microstructure parameter with mobil- ity lifetime (μτ ) product and diffusion length of carriers. Diffusion length and μτ product were observed to be max- imum (0.189 μm and 0.471 x 10 −8 cm 2 V −1 ) for the film having high photosensitivity (7.2x10 3 ) deposited at a rate ∼1.39 ˚ A/s at 0.53 Torr deposition pressure. In addition to electrical transport properties, the effect of deposition pres- sure on structural and optical properties was also studied using various characterization tools such as Raman, UV-Vis and infrared spectroscopy.  Sushil Kumar skumar@nplindia.org 1 Network of Institutes for Solar Energy (CSIR-NISE), Physics of Energy Harvesting Division, CSIR – National Physical Laboratory, New Delhi, India 2 Academy of Scientific and Innovative Research (AcSIR), CSIR-NPL Campus, Dr. K.S. Krishnan Marg, New Delhi, 110012, India Keywords 27.12 MHz assisted PECVD process · Microstructure parameter · Photosensitivity · Ambipolar diffusion length 1 Introduction Hydrogenated amorphous silicon (a-Si:H) is a promising material for low cost solar cell applications owing to its large absorption coefficient and tunable band gap in the vis- ible range of the energy spectrum, and ability to fabricated at a large area. The most common method of depositing a- Si:H thin films is the Plasma Enhanced Chemical Vapour Deposition (PECVD) process, based on plasma decomposi- tion of silane gas (SiH 4 ), with a conventional frequency of 13.56 MHz. Recently the material has also been found to be a suitable candidate for the fabrication of Heterojunction with intrinsic layer (HIT) solar cells in which the intrinsic layer is used as a passivation layer [1, 2] whereas in the p- i-n configuration it is the active absorber layer of the cell in which the carriers are generated. Thus the device operation is directly influenced by the material properties of the intrin- sic layer. For sufficient light absorption the typical thickness of the i layer more than 3000 ˚ A is required and hence high deposition rates are desirable. However, the a-Si:H prepared by 13.56 MHz frequency shows a low deposition rate (typi- cally less than 1 ˚ A/sec), this increases the process time and hence manufacturing cost [3]. To overcome the low deposi- tion rate, a high plasma excitation frequency has been used. Higher frequency leads to an increase of electron density, together with an increase in the occupation of the elevated energy tail states of the electron energy density function (EEDF) [4, 5]. Hence deposition rate increases as an out- come of enhanced dissociations due to gas electron impact.