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

It has been demonstrated that the well known modulated photocurrent technique can be modified to escape imperfect data in intrinsic parameters of amorphous and microcrystalline silicon films and to simplify measurements of the density of localized states distribution in the band gaps of these semiconductors. The information on the density-of-states distribution can be extracted from temperature dependence measurements of the constant and modulated components of the photoconductivity in the film illuminated by the light modulated with some selected frequencies. The modified method has been applied to microcrystalline hydrogenated silicon films with n- and p-type conduction. The study has demonstrated that the tail of the density-of-states distribution near the valence band of microcrystalline hydrogenated silicon is less steep than that near the conduction band.     

Transient photoconductivity in amorphous Se70Sb20Ag10 thin films

Se₇₀Sb₂₀Ag₁₀ bulk material was obtained using a conventional melt quenching technique. Thin films of a-Se₇₀Sb₂₀Ag₁₀ were prepared by vacuum evaporation technique in a base pressure of 10⁻⁴ mbar onto well-cleaned glass substrates. Dark, light conductivity and transient photoconductivity have been studied. The photoconductivity increases initially and then saturates with time when illuminated with low intensity light (<400 Lux); however, when illuminated with higher intensity light (>600 Lux) the photoconductivity increases initially, attains a maximum and then decreases with time. This behavior of photoconductivity has been studied at various temperatures. The results have been explained on the basis of Dember voltage and the interaction between photoexcited holes and the trapped electrons on the surface.     

Characteristic of the photoconductivity of thallium monoselenide

Systematic investigations on thallium monoselenide single crystal photoconductivity were reported at 300 K as a function of excitation intensity, side illumination, and wavelength. The samples were prepared using Bridgman technique. The photoconductivity as a function of the modulation frequency of the incident light was plotted at different values of light intensity. The effect of light intensity on the photocarrier lifetime was studied. The relation between the photocurrent and applied voltage at different values of light intensity was represented graphically. Side illumination effect in all these relations was checked. The spectral distribution of the photoconductivity of TlSe in the spectral region 1.24 ev to 1.77 ev was measured, and the energy gap was evaluated. All results were discussed and analyzed. Studies for such samples are very important and necessary in order to understand their use as photoconductor.     

a-Si:H transport parameters from experiments based on photoconductivity

In this paper we review some of the techniques based on the photoconductivity property of hydrogenated amorphous silicon (a-Si:H) from which it is possible to extract transport parameters as well as density of states (DOS) spectroscopies. We also present a new experiment based on the steady state photocarrier grating technique. We show that combined with simple steady state photoconductivity it gives information on the DOS. The comparison of these results with those of other techniques used for DOS measurements theoretically allows determination of transport parameters in a-Si:H.     

Improvement of photoconductivity in Silicon Tin (SiSn) thin films

Hydrogenated-amorphous silicon tin alloy (a-SiSn (C:H)) films have been prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique. We present investigations of the absorption and photoconductivity characteristics of a-SiSn(C:H) thin films as a function of the tin concentration, which could be a pivotal feature for the bottom solar cell development. We demonstrate that the photoconductivity is significantly improved by the film growth under enhanced hydrogen dilution. The results suggest that the hydrogen dilution plays an important role in development of high quality a-SiSn(C:H) photosensitive thin films.     

Growth and characterization of 2-amino-5-chlorobenzophenone (2A-5CB) single crystals

Organic nonlinear optical single crystals of 2-amino-5-chlorobenzophenone (2A-5CB) were grown in ethanol by slow solvent evaporation technique. The grown crystals were characterized by single-crystal XRD, FTIR, FT-Raman and UV–vis–NIR techniques. The UV–vis–NIR spectrum ascertains the cut-off wavelength of the sample as 390 nm. The powder second harmonic generation (SHG) technique reveals that 2A-5CB crystal has its SHG efficiency nearly three times that of KDP. The dielectric response of the sample was studied in the frequency region of 50 Hz–1 MHz at varying temperatures. The photoconductivity studies indicate that the 2A-5CB crystal exhibits negative photoconductivity. TGA–DTA studies confirm the melting point of the sample as 101.5 °C.     

Growth and characterization of diaquatetrakis (thiocyanato) cobalt (II) mercury (II) N-methyl-2-pyrolidone (CMTWMP) single crystals

Single crystals of diaquatetrakis (thiocyanato) cobalt (II) mercury (II) N-methyl-2-pyrolidone, (CoHg(SCN)₄·(H₂O)₂·2(C₃H₆CONCH₃) (abbreviated as CMTWMP) were grown using slow solvent evaporation technique. The structure of the grown crystals was confirmed by single-crystal X-ray diffraction (XRD) technique. The optical properties of the crystals were investigated by Fourier transform infrared (FTIR) and UV–Vis–NIR transmission spectra. The grown crystals of CMTWMP were also subjected to dielectric, photoconductivity, thermal and microhardness studies. The photoconductivity study of CMTWMP confirms the electrochromism behaviour in the sample. The SHG efficiency of the sample was measured by Kurtz and Perry powder technique and its value is almost comparable with KDP.     

Study of anomalous behavior of steady state photoconductivity in highly crystallized undoped microcrystalline Si films

The photoconductivity (σ_ph) of highly crystallized dense undoped hydrogenated microcrystalline silicon (μc-Si:H) films was measured as a function of light illumination over a wide temperature range (∼15–325 K). A thermal quenching behavior in σ_ph was observed at ∼240 K. The photoconductivity exponent (γ) was found to be sublinear with γ as low as 0.13. A density of states (DOS) profile having a steep conduction band tail, and valence band tail with two distinct distributions was found to be necessary to understand the electronic transport behavior in the inherently heterogeneous μc-Si:H films.     

Influence of plastic deformation on the photoconductivity of CdTe

Investigation of photoconductivity have been performed on p-type phosphorus doped CdTe (5 × 10¹⁵ cm⁻³) deformed by compression at room temperature. Using a technique of axial illumination, the contribution of the surface to the photoconductivity is suppressed. In the investigated energy range (0.08–1.54 eV), and at room temperature, a maximum of the signal of the non-deformed material is of found at 1.312 ± 0.009 eV, with a half width of 0.100 eV, whereas the 0.3% deformed material presents a maximum at 1.348 ± 0.009 eV with the same half width. At 223 K these peaks are shifted to higher energies, the half width remaining unchanged, and the amplitudes being ∼ 30 times higher.     

Laser crystallization of compensated hydrogenated amorphous silicon thin films

Raman backscattering and hydrogen effusion measurements were performed on compensated, highly P- and B-doped laser crystallized polycrystalline silicon. From hydrogen effusion spectra the hydrogen chemical potential, μ_H, is determined as a function of hydrogen concentration, which can be related to the hydrogen density-of-states distribution. Interestingly, hydrogen bonding is affected by doping of the amorphous starting material. Below the hydrogen transport states, four peaks are observed in the hydrogen density-of-states at 2.0, 2.2, 2.5 and 2.8 eV. The latest peak is not observed in B-doped samples. The hydrogen effusion results will be correlated with the results obtained from Raman backscattering measurements.     

Photoconductivity of amorphous CdS films

The photoconductivity of amorphous CdS films, vacuum evaporated onto cooled substrates, has been investigated. The conditions and possible reasons for the appearance of a low temperature maximum in the photoconductivity temperature dependence of chalcogen rich samples have been found. The presence of a slow recombination centre related to the lone pair level of the one-fold coordinated negative chalcogen has been assumed. The energy necessary for both the electron (2 eV) and hole (0.4 eV) escape from this centre has been determined. Photoinduced changes in photoconductivity have been examined as well.     

Time-of-flight measurements in inhomogeneous electric fields

Time-of-flight (TOF) measurements are commonly accepted as one of the most reliable experimental technique to determine the mobility and density-of-states (DOS) distribution in disordered organic and inorganic materials. However, interpretation of TOF data is bound to several a priori assumptions one of which is the assumption of an essentially constant and homogeneous field distribution in the bulk of a sample. This condition can be fulfilled in a wide-band-gap intrinsic semiconductor supplied with blocking contacts. Dark charge injection from contacts and, notably, background doping immediately invalidate this assumption. In the present work we analyze the effects of inhomogeneous field distribution on the TOF photocurrent transients. In particular, it will be shown that the post-transit photocurrent analysis of the data, obtained with a steeply decreasing field, can yield an overestimated value of the carrier mobility and an apparent DOS distribution strongly deviating from the true one.     

Long-lifetime photoconductivity in Cz n- and p-Si

Long-lifetime photoconductivity is observed in Czochralski-grown oxygen-rich heattreated n- and p-type silicon crystals. It appears due to a slow electron (in Cz n-Si) and hole (in Cz p-Si) recombination via two kinds of oxygen-induced recombination centres. Dependences of the long-lifetime photoconductivity on the oxygen content, excitation intensity and temperature are presented. Possible models to explain an appearance of the long-lifetime photoconductivity in Cz n- and p-Si are discussed. The data presented are important for understanding the long-time photoeffects in semiconductors and for elucidating the origin and structure of oxygen-induced recombination centres in silicon.     

Die Phasenübergänge und die Störstellenphotoleitung von Sb2S3

The influence of the Sb₂S₃ single crystals growing conditions on the mikrowave dielectric permittivity and the photoconductivity has been investigated. An improper photoconductivity arises on the ferroelectric plane (001) in the crystals with a high dielectric permittivity.     

Gap states in a-Si:H by photoconductivity and absorption

The spectral dependence of photoconductivity in a series of a-Si:H samples has been analyzed in order to derive the absorption coefficient α. It has been found that the exponent β which relates photoconductivity and generation rate varies with photon energy and chopping frequency. Its critical influence upon the value of α is established. The Urbach tail in the region 1.4–1.8 eV has been studied and found to be independent of extrinsic gap states and hydrogen content. Finally an inverse correlation has been determined between the photoconductivity lifetime and the extrinsic gap-state density, as measured from the corresponding integrated area of the absorption coefficient.     

Electronic structure of amorphous materials under laser irradiation

The electronic structure of amorphous materials under laser irradiation is investigated in the one-electron approximation. It is shown that in this approximation the modification of the density-of-states is much less important than in the crystal case. Many-electron effects are briefly discussed.     

Influence of compositional and structural changes on hydrogen bonding in silicon and silicon–germanium alloys

Hydrogen bonding in amorphous silicon and silicon–germanium alloys is investigated using Raman and gas effusion measurements. Special emphasis is given to the influence of structural changes on H bonding that occur in the cause of hydrogen effusion measurements. The data show that the analysis of the effusion spectra in terms of a hydrogen density-of-states distribution delivers reliable results for amorphous semiconductors.     

Stark effect and photoconductivity of fullerene С<Subscript>70</Subscript> measured on samples with a submicron electrode grating and sandwich geometry

The electroabsorption (Stark effect) and photoconductivity of polycrystalline fullerene C₇₀ in thinfilm samples with radically different geometries (sample 1 with interdigital Cr electrodes and sandwichlike sample 2 with SnO₂ and Al electrodes) have been investigated. It is noteworthy that sample 1 is a submicron grating with 0.88-µm gaps between electrodes. When measuring electroabsorption and photoconductivity, the external voltage does not exceed 4 V. It was assumed that the small gap size may significantly affect the polycrystalline film morphology and surface effects and, as a consequence, the electroabsorption and charge carrier mobility. However, the results of the Stark effect measurements on samples with different geometries differ by only 25%, whereas the spectral photoconductivity values differ by factors of 1–2.5. The increase in photoconductivity may be related to the decrease in the carrier mobility in the sandwich sample or the increase in the number of carriers under the influence of atmospheric oxygen in the surface layer of C₇₀ sample with interdigital electrodes.     

Growth and characterization of 4‐(2‐hydroxyphenylamino)‐pent‐3‐en‐2‐one single crystals

4‐(2‐hydroxyphenylamino)‐pent‐3‐en‐2‐one (HPAP) was synthesized and single crystals were grown by the solution growth technique using methanol as a solvent. The crystals having orthorhombic symmetry were characterized by single crystal XRD, FTIR spectroscopy, NMR spectroscopy, TGA, DSC and dielectric studies. Very less variation in the value of dielectric constant is found for different frequencies of applied field. The crystals were exhibiting positive photoconductivity and poor NLO responses. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A systematic investigation of the role of material parameters in metastability of hydrogenated amorphous silicon

In this work, the role of structural, electronic and optical parameters of as-deposited amorphous silicon films in photoconductivity decay during light soaking was systematically investigated. Deposition temperature was varied in the range 130–270°C, in order to obtain samples with different structural, optical and electronic properties. As a result, two degradation regimes were identified. At short illumination times (within a few days in typical samples, and within a few hours in the low quality samples), the material showed different tendency to degradation depending on the content of the SiH bond clusters. At long illumination times, in all the light soaked samples the photoconductivity decay followed the t^−1/3 law. The measured photoconductivity degradation was simulated starting from the bond-breaking model. The observed correlation between the material structural parameters and the different tendencies to degradation is explained in terms of variations of the Staebler-Wronski susceptibility.