The modulated photocurrent technique: a powerful tool to investigate band gap states in silicon based thin films

Abstract

The Modulated PhotoCurrent (MPC) technique is a very simple experiment that provides insight into localized gap states in semiconductors. It is particularly well suited to disordered silicon based thin films. We here give a review of the technique, emphasizing the effect of experimental parameters (frequency, temperature, dc generation rate) and the reconstruction of the density of states (DOS) in the two regimes: the widely used high frequency regime and the recently developed low frequency regime. We discuss the effect of different trap species and propose a new way to determine capture cross sections from the matching of the reconstructed DOS in both regimes. Illustrations are given theoretically with the help of a numerical modelling, and experimentally from data obtained on polymorphous silicon-germanium alloys.

The role of the recombination centers on the modulated photocurrent: Determination of the gap state parameters of semiconductors

“…This method is applicable exclusively for the MPC data in the HF regime. Various attempts have been made to extract information about the DOS and to determine the usually unknown ratio of the capture coefficient to the mobility of the predominated carriers, c/µ, by means of the MPC data of lower ω [9,[19][20][21][22][23][24][25][26][27][28][29][30]. However, the general F o r P e e r R e v i e w O n l y 3 analytical expressions originally derived by Longeaud and Kleider [19] in 1992 and Hattori et al [20] in 1994 are very complicated to extract a simple relation between the MPC of lower ω and the DOS.…”

Density-of-states in microcrystalline silicon from thermally-stimulated conductivity

“…Despite the largely different experimental parameters, the DOS data from the three methods agree well. High-frequency MPC and LF-MPC have been matched to determine values for c n and mobility in [9]. In Fig.…”

“…The DOS profile was also determined from low-frequency modulated photoconductivity (LF-MPC) [9] and steady-state photocarrier grating (SSPG) [10]. In addition, the photogeneration rate analysis of SSPC for the majority carriers with free-carrier mobility l n allows to determine the DOS at the electron quasi-Fermi level E fn according to [11] …”

Modulated photoconductivity method for investigation of band gap states distribution in silicon-based thin films

“…In the MPC experiment two frequency regions can be clearly distinguished: the high-frequency region and the low-frequency region [2,5]. According to Oheda [2] for ntype semiconductors these frequency regions differ by relative positions of the electrons quasi-Fermi level E Fn and the energy E x at which the thermal emission rate of a trapped electron coincides with the modulation frequency x.…”

“…In particular, information on the energy distribution of the localized states in the band gap is essentially important. The measurement of spectral dependence of absorption coefficient in the sub-band gap region [1] and modulated photocurrent (MPC) method [2][3][4][5] are the most widely used among different methods of obtaining the information on energy distribution of the localized states in the band gap of amorphous (a-Si:H) and microcrystalline (lc-Si:H) hydrogenated silicon. The MPC technique allows one to obtain information on localized states, which participate in nonequilibrium charge carrier trapping.…”