A novel technique has been proposed for determining the density‐of‐state (DOS) distribution in the energy gap of highly resistive amorphous semiconductors, using amorphous/crystalline heterojunction structures. The technique has been tested and applied to undoped hydrogenated amorphous silicon (a‐Si:H) films and silicon‐germanium (a‐Si1−xGex: H ) alloy films, covering the optical gap (E0) range of 1.3–1.7 eV. For undoped a‐Si:H with E0=1.7 eV, the peak of the midgap DOS distribution has been located at 0.84 eV below the conduction‐band mobility edge, EC, with a value of 2×1015 cm−3 eV−1. For undoped a‐Si1−xGex: H (E0=1.44 eV), the same has been obtained at 0.70 eV below EC, with a magnitude of 7×1016 cm−3 eV−1. Those midgap DOS have been found to be correlated with singly occupied dangling bonds, representative of a homogeneous bulk property of the material, unaffected by interface states.

A novel method for determining the gap‐state profile and its application to amorphous Si1−xGex: H films

Experimental studies by means of a constant photocurrent method (CPM) have been carried out on the deep defect states in undoped hydrogenated amorphous silicon (a‐Si:H). Assuming Gaussian energy distributions, two types of defect states (ST1 and ST2) have been found from careful analysis of CPM spectra; one of these states (ST1) is a neutral Si dangling‐bond (Si D0) state, and the other (ST2) is a negatively charged dangling‐bond‐like defect state located in the lower gap. ST2 lies deeper in energy by 0.1 eV, and has a narrower full width at half‐maximum (FWHM) when compared with ST1. Possible candidates for ST2 have been discussed by referring to the previous models of defects in a‐Si:H. The FWHM of ST1 (the Si D0 state) does not always depend on the overall structural disorder estimated from Raman spectra. The energy position of the Si D0 state measured from the valence‐band edge is almost independent of the optical band gap. Long exposure to light increases the density of ST1 (the Si D0 state) and decr...

Deep defect states in hydrogenated amorphous silicon studied by a constant photocurrent method

We propose an explanation for large discrepancies in the absorption spectrum of a-Si:H between measurements by DC and AC constant photocurrent methods (CPM). DC measurement gives a consistently higher value for the absorption coefficient α at low photon energies. A small-signal analysis of the photoconductive response to modulated sub-gap illumination reveals low frequency poles associated with thermal emission processes, which explain this discrepancy. We demonstrate with computer simulations that while DC CPM, which includes these transitions, gives a more accurate value for absorption, AC CPM provides a more accurate means of determining the distribution of occupied gap-states. Further, we show that combining DC and AC methods allows determination of the distribution of deep unoccupied gap-states. These concepts are applied to experimental results for several undoped a-Si:H films.

Comparison of AC and DC constant photocurrent methods for determination of defect densities

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. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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

This work investigates the steady-state photoconductivity in hydrogenated amorphous silicon taking into account an exponential distribution of conduction-and valence-band tails and the dangling-bond defect states. Thermal quenching of photoconductivity is also explained; its activation energy at different light intensities is related to the electron quasi-Fermi level. The calculation presented here gives a broad picture of the overall photoconductivity characteristics and also allows the energy location of the doubly occupied dangling-bond state to be obtained.

Temperature dependence of steady-state photoconductivity in hydrogenated amorphous silicon

The energetic distribution and nature of dangling-bond states in undoped a-Si: H have been investigated by a newly developed gap-state spectroscopy which is based on the frequency-resolved spectrum of modulated photocurrent with below-gap photoexcitation. The energy scale is directly specified by the energy of the below-gap light populating the particular gap states. The result has led to the conclusion that the peak of the gap-state distribution associated with doubly occupied dangling bonds (D-) is located about 0•5 eV below the conduction band edge. The analysis for the pre-exponential factor of the thermal emission rate of electron from the D- centre suggests that the emission occurs in two stages through excited states.

Gap states in undoped amorphous silicon studied by below-gap modulated photocurrent spectroscopy

Frequency-resolved modulated photocurrent spectroscopy has been introduced to investigate the gap states distribution in undoped a-Si:H The result shows that the D− centre has a near-Guassian distribution centered at about 05 eV below the conduction band edge The deduce DOS exhibits characteristics structures including the D− centre over a wide range of the energy band gap

Energetic distribution of dangling bond states in undoped amorphous silicon

A new technique for the quantitative characterization of the gap states in a‐Si p‐i‐n junction solar cells has been developed. The measurement is directly correlated with electronic activities of dangling bond states in the i‐layer of solar cells, namely by below‐gap modulated and transient photocurrent processes. Energy levels and trapped charge densities of the dangling bond center are clearly identified. By use of this technique, effect of light soaking on carrier trapping and distribution of the dangling bond states has been studied. A series of technical data and model of related phenomena are presented and discussed.

K. Abe

Papers

Gap states in undoped amorphous silicon studied by below-gap modulated photocurrent spectroscopy

Energetic distribution of dangling bond states in undoped amorphous silicon

Changes in the localized states and their trapped carrier distributions by light soaking in a-Si p-i-n junction solar cells