Y. W. Lam

Evidence for deviations from a single-exponential distribution of conduction band tail states in hydrogenated amorphous silicon: A transient photocurrent analysis

We employ transient photocurrent decay measurements to determine the distribution of conduction band tail states in hydrogenated amorphous silicon. It is found that these experimental results suggest the possibility of deviations from a single-exponential functional form. This is consistent with other more recent experimental determinations of the distribution of conduction band tail states in hydrogenated amorphous silicon.

Effects of bandwidth limitations on the localized state distribution calculated from transient photoconductivity data

The possible effects of experimental bandwidth limitation on the accuracy of the energy distribution of the density of localized states (DOS) calculated from transient photoconductivity data by the Fourier transform method is examined. An argument concerning the size of missing contributions to the numerical Fourier integrals is developed. It is shown that the degree of distortion is not necessarily large even for relatively small experimental bandwidths. The density of states calculated from transient photodecay measurements in amorphous arsenic triselenide is validated by comparing with modulated photocurrent data. It is pointed out that DOS distributions calculated from transient photoconductivity data at a high photoexcitation density are valid under certain conditions. This argument is used to probe the conduction band tail in undoped a-Si:H to energies shallower than 0.1 eV below the mobility edge. It is concluded that there is a deviation in the DOS from exponential at about 0.15 eV below the mobil...

The modification of electrophotographic and mechanical properties of organic photoconductors by ultra-violet irradiation

Experiments have shown that UV treatment has a hardening effect on the surface of a drum-based organic photoconductor (OPC) and improves the retentivity. The dark decay and the photoinduced discharge rates were reduced linearly with the increase of time of UV irradiation. The dark decay rate and photoinduced discharge rates were reduced to 60% and more than 70% of the initial rate, respectively, while there was an increase of the activation energy of the charge transport layer. An initial increase in the residual potential was also observed from 20 to about 70 V. After 40 min of UV irradiation, the residual potential was seen to drop to 50 V again, and at the same time a slight increase of the hardness of the photoreceptor surface was detected from 13 to 16 in Vickers scale (Hv). The phenomena is most likely explained by a photochemical change which reduces the density of charge transport sites, builds up the number of deep traps and changes the molecular distribution. The decrease in charge transport site density and buildup of deep traps reduces the conductivity and hence the dark decay and photoinduced discharge rates. The fragmentation of charge transport molecules may result in a change of activation energy as well as the increase in the residue potential. Moreover, the redistribution of molecules leads to the change in molecular density and hardness.

Sign reversal in transient photoconductivity in the presence of optical bias in undoped homogeneous a-Si:H

An undershoot or sign reversal in the transient photoconductive response to pulse illumination in the presence of optical bias has been observed in homogeneous films of undoped amorphous silicon. This report is the first of experimental observation, to our knowledge. The undershoot is seen in the regime of linear response to the photoexcitation pulse, when the background steady state generation rate, G ) 10 18 cm y3 s y1 . The time, t , at which sign reversal occurs varies ss us . inversely and sublinearly with generation rate. When the quasi-Fermi level is maintained constant at E y E s 0.24 eV, Fn F t is thermally activated with energy 0.36 eV. Direct application of theory to the Fourier transform of the time resolved us transient photoconductivity data yields a figure of 1.5 = 10 y8 cm 3 s y1 for the recombination coefficient. Numerical simulation not including dangling bonds can only be induced to exhibit an undershoot by adopting capture coefficients such that recombination occurs by the path of electron capture by trapped holes. q 1998 Elsevier Science B.V. All rights reserved.

Gap states and stability of rapidly deposited hydrogenated amorphous silicon films

Abstract The sub-band gap absorption spectrum α(hv) in a-Si:H films prepared at high deposition rates was measured by the constant photo-current method (CPM) for photon energy ranging from 0.8 to 1.7 eV in a thermally-annealed state and light-soaked state. The Simmons–Taylor theory and occupation statistics of correlated defects are used to model the distribution of band tail and gap states. It is found that the density of gap states increases after light soaking, however, there is no evident change found in the density and distribution of band tail states. Measurements of the light-induced changes find that the photoconductivity decreases by less than one order of magnitude after long time of light illumination for the high rate deposited a-Si:H. This demonstrates that the high rate deposited samples have relatively high stability compared with conventionally deposited a-Si:H.

Investigation of mis-estimation of structure of amorphous silicon films in ellipsometric modeling

Abstract Dielectric functions for thin films of hydrogenated amorphous silicon (a-Si 1− x :H x ) with varying hydrogen content x , void concentration and surface roughness may be constructed from measured dielectric functions for amorphous silicon (a-Si) by use of the tetrahedron model, scaling procedures, dielectric formulation, and the effective medium approximation (EMA). The measured dielectric functions may correspond to relaxed or unrelaxed (ion-implanted) a-Si. Ellipsometric measurements on thin films of a-Si 1− x :H x are often fitted with such dielectric functions to obtain film parameters such as void concentration and surface roughness, with the fit quality being assessed from the value of the unbiased estimator σ . Due to the strong preparation dependence of the a-Si 1− x :H x lattice structure, it may not be clear whether it is relaxed or unrelaxed dielectric functions which are appropriate for the fit. In this work, dielectric functions are constructed for a-Si 1− x :H x thin films using unrelaxed a-Si dielectric functions, and fitted using relaxed a-Si 1− x :H x dielectric functions and Levenberg–Marquardt non-linear regression. It is demonstrated that a small value of σ may be obtained despite the incorrect choice of relaxation state. Comparison of the input and output void concentration and surface roughness shows significant mis-estimation in the fits.

Analysis of the infrared transmission data of hydrogenated amorphous silicon film fabricated by high rate PECVD

The optical properties of hydrogenated amorphous silicon thin films prepared by a new thermocatalytic plasma enhanced chemical vapor deposition (PECVD) method are here reported for the first time. The transmission spectrum of the film, deposited at a rate of 1.5 nm/s, was measured between 500 and 1100 nm. The envelopes of the transmission spectrum interference maxima and minima were analyzed to reveal the absorption coefficient α(λ@#@), the refractive indexn(λ), the average thickness of the film (791 nm) and the variation of the thickness (11.4 nm), using an analysis which takes into account film inhomogeneity. The modified Newtons method of numerical analysis was used to obtain the optical parameters. The optical band gap ε0} was determined to be 1.69 eV from the absorption coefficient spectrum, commensurate with values quoted for lower deposition rate PECVD films. The value for ε0}, the small variation of the film thickness, and a value for the defect density of 3.7 x 1015}cm-3} determined for similar material in other work indicate that the thermocatalytic PECVD method can produce acceptable quality films at a high deposition rate.

Reliability in Modeling of Spectroscopic Ellipsometry

Spectroscopic ellipsometry (SE) [1, 2] is a powerful non-destructive technique to determine the structure of silicon-based materials such as amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si :H). In analyzing the SE spectra, the Bruggeman effective medium approximation (EMA) [3, 4] is usually used to model a thin film as a mixture of amorphous silicon and voids. In these models, however, precise knowledge of the dielectric functions of the various components is required in order to obtain reliable estimation of the structural parameters. This requirement is particular critical to the amorphous component because the structure of amorphous silicon is known to depend considerably on the preparation procedures [5]. So far, the dielectric functions used for modeling a-Si1 x :Hx have been based on certain dielectric functions without consideration of the preparation method. It may not be clear, particularly for thin films of amorphous silicon prepared by novel method, whether it is relaxed or unrelaxed dielectric functions which are appropriate for the modeling. In the present work the scope of the problem is assessed by constructing dielectric functions for relaxed and unrelaxed a-Si1 x :Hx thin films using measured a-Si dielectric functions, and fitting, respectively, using simulated dielectric functions for bulk unrelaxed and relaxed a-Si1 x :Hx on the surface roughness and void concentration parameters. The tetrahedron model [3, 4] is applied to construct the complex dielectric functions for relaxed (e 1⁄4 e1r þ ie2r) and unrelaxed (e 1⁄4 e1ur þ ie2ur) a-Si1 x :Hx, by using the dielectric functions for relaxed and unrelaxed a-Si [5], respectively. For details we refer to the work of Mui and Smith [3], [4]. The complex dielectric functions constructed for relaxed and unrelaxed a-Si1 x :Hx components with various hydrogen concentrations are shown in Fig. 1. It is seen that the relaxed and unrelaxed spectra exhibit different peak position and amplitude. Moreover, as the hydrogen content increases, the complex dielectric functions shift to higher photon energy with lower peak values. The complex dielectric functions for the relaxed (unrelaxed) a-Si1–x :Hx film are constructed using a three-layer structure: the first layer is a surface roughness layer, modeled using the EMA, consisting of 50% voids, 50% layer 2. Layer 2 is modeled using the EMA, consisting of a mixture of relaxed (unrelaxed) a-Si1 x :Hx and voids. The substrate is smooth silicon dioxide (SiO2). Fractional void concentrations in layer 2 are varied from 0.01 to 0.26 in the simulations. The thickness of layer 1 is set to be 0.001 mm, while the film thickness (layer 2) is fixed at 1 mm. The incidence angle is set to be 75 . The constructed complex dielectric functions for relaxed a-Si1 x :Hx films are then fitted using unrelaxed a-Si :H spectra, applying a nonlinear regression based on the Levenberg-Marquardt method [6], and vice-versa. In order to avoid interference caused by the interface, the fits are done above 1.75 eV. We denote the void concentration and the thickness of the surface roughness by Cvoid and TR respectively. The fitting error of the void concentration and the thickness of surface roughness are denoted by DCvoid and DTR, respectively, i.e. Cvoid 1⁄4 Cconstructed void Cfit void , similarly for DTR. In order to reveal the quality of the modeling, we define

Depth profiling of vacancy-type defects in homogenous and multilayered a-Si films by positron beam Doppler broadening

Positron annihilation measurements have been carried out on a-Si:H thin films deposited by plasma enhanced chemical vapour deposition at a high rate and on pin/pin double-junction diodes prepared conventionally by means of the variable-energy positron beam Doppler-broadening technique. The depth profiles of microvoids in films grown under different conditions have been determined. We found a smaller void fraction in the surface region of all films compared to the interior. The depth profiles of microvoid-like defects in the a-Si:H films were extracted by use of the VEPFIT programme. Variable-energy positron-annihilation spectroscopy results on the diode show that the interfaces are of good quality except for the Au/n interface, consistent with the diode characteristic.

Experimental determination of the distribution of tail states of hydrogenated amorphous silicon: A transient photocurrent analysis

Recent experimental developments have cast doubt on the validity of the common assumption that the distribution of tail states of hydrogenated amorphous silicon exhibits a single exponential functional form. The authors employ transient photocurrent decay measurements to determine this distribution of tail states. In their approach, however, they determine the distribution of tail states directly from the experimental data, without assuming, a priori, a specific functional form. It is found that these experimental results are consistent with other more recent experimental determinations of the distribution of tail states, suggesting the possibility of deviations from a single exponential distribution of tail states in hydrogenated amorphous silicon.