Modulated Photocurrent Spectroscopy Research Articles

Electrochemical and optical studies of silicon dissolution in ammonium fluoride solutions

The mechanisms involved in the (photo)dissolution of silicon in fluoride solutions are discussed. Several methods, including intensity modulated photocurrent spectroscopy, in situ ir spectroscopy, rotating ring disc voltammetry and ellipsometry, have been used to examine the anodic dissolution process. The valence state of the dissolving silicon has been determined under different conditions, and the optical properties of the surface have also been characterized. The generation of hydrogen has been linked in the case of n-Si to intensity dependent photocurrent multiplication, and in the case of p-Si it has been related to different etching regimes. The current oscillations observed during the anodic dissolution of p-Si are also discussed.

Miniband and Wannier-Stark states in GaAsAlAs superlattices investigated by differential photocurrent spectroscopy

We used a new modulation technique, the wavelength modulated photocurrent spectroscopy, to study the field dependent transition from the miniband to the Wannier-Stark regime in GaAsAlAs-superlattices. In the zero field limit the edges of the joint miniband density of states between electrons and heavy as well as light holes create characteristic features in our modulated spectra. Applying small electric fields bulk-like Franz-Keldysh-Oscillations appear at the lower and upper miniband edges. At large electric fields the well known Wannier-Stark-Levels are observable.

A simple microcomputer-based modulated photocurrent spectroscopy system for the measurement of localized-state distributions in amorphous semiconductors

The construction of a simple microcomputer-based modulated photocurrent spectroscopy (MPS) system is described. The laborious and time-consuming measurement with a conventional set-up is avoided by the novel use of the combination of a simple and inexpensive computer-controlled pulse generator and a light-emitting diode. Data of localized-state distributions obtained with this system are presented for a amorphous chalcogenide semiconductor, amorphous As2Se3. The thermally created defect states are found and the defect creation energy is estimated to be 0.64 eV.

Photocurrent doubling during the oxidation of formic acid at n-CdS: an investigation by intensity modulated photocurrent spectroscopy

The oxidation of formic acid at illuminated single crystal n-CdS electrodes has been investigated using intensity modulated photocurrent spectroscopy (IMPS). The quantum yield for the process is 1.8, indicating that hole capture is followed by electron injection (“current doubling”). The IMPS response in the saturation photocurrent region provides evidence that electron injection involves a short-lived intermediate with a first-order lifetime of 2 × 10 −5s. It is suggested that electron injection occurs indirectly via a surface state located about 0.5 eV below the conduction band of CdS. It has been shown that Fe IIIscavenges the intermediate, leading to a progressive reduction of the quantum yield as the concentration is increased. The rate constant for the scavenging reaction was found to be 10 7 dm 3 mol −1 s −1.

Light-induced changes in the deep-states of undoped a-Si:H

A systematic investigation has been carried out on the light-induced modification of deep-lying defect states in undoped a-Si:H by means of modulated photocurrent spectroscopy (MPCS). The results lead us to a conclusion that non-equilibrium carriers play a central role in establishing metastable defect configurations.

Measurement of gap states in undoped a-SiGe: H alloys by modulated photocurrent spectroscopy

Abstract Modulated photocurrent spectroscopy has been used to investigate the energy location and nature of dangling bonds in undoped a-SiGe: H alloys, with particular emphasis on the doubly occupied dangling-bond (D−) centre. It is found that the Ge D− centre lies about 0·1 eV below the Si D− centre, while a sharp switch of the predominant deep centres occurs from Si dangling bonds to Ge dangling bonds as the Ge concentration approaches about 35 at.%, which corresponds to an optical bandgap energy of around 1·5 eV. The analysis for the pre-exponential factor of the thermal emission rate of electrons from the D− centre suggests that different electron transition processes are operative for Si and Ge dangling bonds.

Band Tailing and Transport in a-SiGe:H-Alloys

ABSTRACTOptical and transport studies of both cb- and vb-tail states in a-Si1−xGex:H such as subband absorption (PDS), instationary photocurrent experiments (TOF, PTS) for electrons and holes, Modulated Photocurrent Spectroscopy (MPS), and Raman scattering have been performed. The main consequences of Ge-alloying into the a-Si:H network are i) an increase in cb-tail state density at the conduction band edge and in the exponential cb- tail even for small x (O<x<0.3), accompanied by ii) a rise in deep cb-tail and midgap states which to some extent can be reduced by appropriate deposition methods; iii) at the valence band side up to x≈0.3 the tail seems not to be affected at all and for 0.3<x<0.9 the vb-tail obviously can be kept similar to that of a-Si:H (Evo≈(50–60) meV). Halfwidths of Raman TO-like modes point to the existence of a rigid Si-network in O<x<0.3 in which Ge is incorporated and to a transition at x>0.35 into a Si-Ge compound structure with maximum disorder at x≈0.5.

Energetic distribution of dangling bond states in undoped amorphous silicon

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 0.5 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.