Showing papers on "Photoconductivity published in 1990"

Explanation of the apparent sublinear photoconductivity of photorefractive barium titanate.

Abstract: We explain the apparent sublinear intensity dependence of photoconductivity in barium titanate. In our model shallow acceptors act as a reservoir for charges optically excited from the donors. As this reservoir fills, the fraction of occupied donors changes appreciably, changing the lifetime of the free carriers. We identify two types of barium titanate crystals having quite different photorefractive characteristics, depending on their relative density of donors and acceptors, and we find that the depth of the shallow acceptor level is \ensuremath{\sim}0.4\ifmmode\pm\else\textpm\fi{}0.1 eV in both types of crystals.

Thermal generation currents in hydrogenated amorphous silicon p - i - n structures

Abstract: Dark conductivity in amorphous silicon p‐i‐n devices arising from thermal generation through bulk defect states is explored. The current decays slowly after a voltage is applied, due to depletion of charge from the undoped layer, and is voltage dependent due to a field‐enhanced generation rate. Creation of metastable bulk defects by light soaking reversibly increases the current. The steady‐state generation current is dervied from the measured relaxation time and depletion charge.

Properties of Indium Phosphide

Abstract: Designed for those who study, process or use indium phosphate in their work, this volume contains 133 specialized surveys of research and development on the substance. It includes material on resistivity, photoconductivity, band structure, optical functions, diffusion and more.

Novel Si1−xGex/Si heterojunction internal photoemission long‐wavelength infrared detectors

Abstract: A new approach to the design of a Si‐based infrared detector is demonstrated, based on internal photoemission over a Si1−xGex/Si heterojunction barrier. The heterojunction internal photoemission device structure is grown by molecular beam epitaxy (MBE). The detector requires a degenerately doped p+‐Si1−xGex layer for strong infrared absorption and photoresponse. Doping concentrations to 1020 cm−3 are achieved using boron from a HBO2 source during MBE growth of the Si1−xGex layers. The photoresponse of this device is tailorable, and most significantly, can be extended into the long‐wavelength infrared regime by varying the Ge ratio x in the Si1−xGex layers. Results are obtained with x=0.2, 0.28, 0.3, and 0.4 on patterned Si (100) substrates. Photoresponse at wavelengths ranging from 2 to 10 μm is obtained with quantum efficiencies above ∼1% in these nonoptimized structures.

Structural, optical, and electrical characterization of improved amorphous hydrogenated germanium

Abstract: High‐quality amorphous hydrogenated germanium has been deposited using the diode rf glow discharge method out of a gas plasma of GeH4 and H2. The optical, electrical, and structural properties of this material have been extensively characterized. The optical and electrical properties are all consistent with material containing a low density of defect related states in the energy gap. In particular, this material has an ημτ=3.2×10−7 cm2/V, ratio of photocurrent to dark current of 1.3×10−1, and flux dependence of the photocurrent with γ=0.79 at 1.25 eV measured using photoconductivity, a μτ=4×10−8 cm2/V measured using time of flight, an Urbach energy of 51 meV and α at 0.7 eV of 8.3 cm−1 measured using photothermal deflection spectroscopy, a dangling bond spin density of 5×1016 cm−3 measured using electron spin resonance, photoluminescence with a peak energy position of 0.81 eV and full width at half maximum of 0.19 eV, an activation energy of 0.52 eV and σ0 of 6.1×103 (Ω cm)−1 measured using dark conductiv...

Excitonic electroabsorption in extremely shallow quantum wells

Abstract: We report the remarkable observation of strong room‐temperature excitonic features in the absorption spectra of GaAs‐AlxGa1−xAs quantum wells (QWs) for values of x as low as 0.02. This has important implications for high‐power modulators, since saturation intensities have been shown to be higher in QW modulators with low barriers. In addition, very shallow QWs have enhanced electroabsorption at small biases because of ease of ionization. In our p‐i(multi‐QW)‐n device with x=0.02, we obtain a transmission change from 29% to 47% for a voltage change from +1 to −3 V.

Persistent photoconductivity and two-band effects in GaAs/AlxGa1-xAs heterojunctions.

Abstract: Mesure de la mobilite et des densites electroniques des sous-bandes individuelles de ces heterojonctions presentant deux occupations de sous-bandes en fonction de la densite du gaz d'electrons bidimensionnel. On fait varier la densite au moyen de la photoconductivite persistante en utilisant les rayonnements IR et rouge

Infrared optical characterization of InAs/Ga1−xInxSb superlattices

Abstract: InAs/Ga1−xInxSb superlattices have been examined by photoluminescence, photoconductivity, and infrared optical transmission. Samples display clear photoconductive thresholds at energies in agreement with band gaps derived from photoluminescence. Far‐infrared energy gaps (8–14 μm and beyond) are obtained for InAs/Ga0.75In0.25Sb superlattices with periods <75 A, in good agreement with gaps calculated from a simple two‐band model. An absorption coefficient of ∼2000 cm−1 at 10 μm is measured in a superlattice with an energy gap of 11.4 μm. The magnitude and shape of this absorption edge is comparable to that of bulk Hg1−xCdxTe, suggesting that infrared detectors based on InAs/Ga1−xInxSb superlattices may be competitive in the 8–14 μm range and beyond.

Electronic and optical properties of a-Si1-xCx films prepared from a H2-diluted mixture of SiH4 and CH4

Abstract: This paper deals with the electronic and optical properties of a-Si1-xCx films deposited by glow-discharge decomposition of SiH4 and CH4 mixtures, in most cases with H2 dilution. These results complement the structural and compositional data on the same specimens reported in the preceding paper. A systematic investigation of optical absorption, dark conductivity, photoconductivity and H evolution has been carried out as a function of the carbon content of specimens prepared with and without H2 dilution. It is shown that the former leads to a significant increase in the photosensitivity of specimens having optical gaps of up to 2.1 eV, in agreement with the work of Matsuda and co-workers. Further support for the improvement in electronic properties produced by H, dilution is obtained from density-of-states measurements, using the space-charge-limited current technique, and from photothermal deflection spectroscopy results, both of which show a significant decrease in the density of deep defect sta...

Photoconductive bismuth sulphide thin films by chemical deposition

Abstract: A chemical bath deposition technique for obtaining photoconductive grade bismuth sulphide (Bi2S3) thin films of thickness approximately=0.05 to 0.3 mu m is described. These films show a photo-current to dark-current ratio of 100-500 and photoconductivity of approximately=1 Omega -1 cm-1 at 25 degrees C, under 1200 W m-2 tungsten-halogen illumination. Air annealing in the 100-250 degrees C range can drastically affect the photocurrent response of these films. The dark conductivity of the films can be increased by a factor of approximately=107 during such an annealing process. This suggests the possibility of post-deposition processing of the films for specific optical and optoelectronic applications.

Excimer‐laser‐induced crystallization of hydrogenated amorphous silicon

Abstract: The electronic transport properties and structural morphology of fast‐pulse excimer‐laser‐ crystallized hydrogenated amorphous silicon (a‐Si:H) thin films have been measured. The room‐temperature dark dc conductivities and Hall mobilities increase by several orders of magnitude at well‐defined laser energy density thresholds which decrease as the impurity concentration in the films increases. The structural morphology of the films suggests an impurity‐induced reduction of the a‐Si:H melt temperature as the origin of this behavior.

Photofluxonic detection: A new mechanism for infrared detection in superconducting thin films

Abstract: A new model is proposed in which a photon of energy E=hf is absorbed by a superconducting film to create a pair of equal and opposite fluxons (or vortices), each with quantized flux Φ0=h/2e. An applied current sweeps these fluxons to opposite edges of the film, causing a voltage pulse with time‐integrated magnitude Φ0, and leading to a time‐averaged voltage responsivity Rv = Φ0/E = 1/(2ef). This is directly analogous to photoconductive detection in a semiconductor via creation of electron‐hole pairs. Data on an ultrathin granular NbN film are presented which indicate a responsivity of 6000 V/W in red light, in agreement with the model. This is promising for the development of a sensitive, high‐speed infrared detector using thin films of either low or high Tc superconductors.

Electro-optical multistability in GaAs/AlAs superlattices at room temperature

Abstract: We have studied the optical absorption properties of a GaAs/AlAs short‐period superlattice at room temperature in an electric field perpendicular to the layers. Several pronounced optical transitions related to Wannier–Stark localization are observed indicating a coherence length of at least five superlattice periods. These transitions produce multiple regions of negative differential photoconductivity which are used to realize a multistable self‐electro‐optic effect device.

Amorphous silicon phototransistors

Abstract: An amorphous silicon field‐effect phototransistor is fabricated using a processing technology compatible with conventional amorphous silicon‐silicon nitride thin‐film transistors. The phototransistor has an offset structure between the source and gate electrodes, where light is absorbed to produce a photocurrent. In an electron accumulation mode, the photocurrent is greater than the dark current by three orders of magnitude. In addition, the phototransistor is found to have output characteristics showing good saturation. Typical photoconductive gain of this saturation current is 17.

10 μm infrared hot‐electron transistors

Abstract: A new hot‐electron transistor for 10 μm infrared radiation detection is presented and discussed. The device utilizes an infrared sensitive GaAs/AlGaAs multiple quantum well structure as emitter, a wide quantum well as base, and a thick quantum barrier placed in front of the collector as an electron energy high pass filter. The energy filter selectively permits the higher energy photocurrent to pass to the collector; the lower energy dark current is rejected by the filter, and is drained through the base. The device detectivity, as noted by the collector photocurrent measurements, is much enhanced in comparison with companion infrared photoconductive devices.

Conductivity and Light-Induced Absorption in BaTiO3

Abstract: A charge transport model including deep and shallow traps explains both the nonlinear relation between photoconductivity and light intensity and the light-induced absorption in BaTiO3. A correlation between measurements of photoconductivity and light-induced absorption as a function of temperature yields parameters for the shallow center, among them thermal activation energy and generation rate.

Reversible photoinduced change of photoconductivity in amorphous chalcogenide films.

Abstract: Prolonged exposure to strongly absorbed light decreases the photoconductivity of well-annealed amorphous chalcogenide (Se, ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$, and ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$) films, similar to the behavior observed in hydrogenated amorphous silicon and organic amorphous polysilanes. This change is removed by annealing near the glass transition temperature. The reversible change in photoconductivity appears to be an intrinsic effect in amorphous semiconductors. The optically induced defect-creation reactions responsible for this, and for light-induced changes in ac conductivity, are discussed in detail.

Dependence of the saturated light‐induced defect density on macroscopic properties of hydrogenated amorphous silicon

Abstract: We report a study of the saturated light‐induced defect density Ns,sat in 37 hydrogenated (and in part fluorinated) amorphous silicon [a‐Si:H(F)] films grown in six different reactors under widely different conditions. Ns,sat was attained by exposing the films to light from a krypton ion laser (λ=647.1 nm). Ns,sat is determined by the constant photocurrent method and lies between 5×1016 and 2×1017 cm−3. Ns,sat drops with decreasing optical gap Eopt and hydrogen content cH, but is not correlated with the initial defect density Ns,ann or with the Urbach tail energy Eu. We discuss our results within the framework of existing models for light‐induced defects.

Effect of low‐level boron doping and its implication to the nature of gap states in hydrogenated amorphous silicon

Abstract: Large simultaneous changes in ambipolar diffusion length (Ld) and photoconductivity (σph) were observed with boron doping below 1 ppm. The results can be explained satisfactorily by postulating that electrons and holes interchange their roles as majority or minority carriers at ∼0.4 ppm. The μτ products for both carriers are determined as a function of doping. The light intensity dependences of Ld and σph present new evidence for the existence of the hole trapping centers in a‐Si:H and show that doping enhances the sensitizing effect due to these centers.

A.c. photoconductivity and optical properties of bulk polycrystalline and amorphous In x Se1?x thin films

Abstract: The a.c. photoconductivity of bulk polycrystalline In x Se1−x and the optical properties of amorphous In x Se1−x thin films were investigated. The effect of heat treatment on the absorption coefficient of InSe thin films was also studied. The spectral response of the photoconductivity of polycrystalline In x Se1−x related to the near-edge absorption spectrum, shows a well-defined maximum corresponding to the width of the forbidden gap. Polycrystalline In x Se1−x realizes the behaviour of photoconductors which are sensitive in a spectral range between 400 and 1700 nm. Optical absorption of amorphous In x Se1−x thin films was recorded as a function of photon energy and the data were used to deduce the value of optical energy gap of the films. It was found that the optical energy gap decreased with increasing indium content in both bulk polycrystalline and amorphous thin films. The effect of heat treatment on the optical gap of the film has been interpreted in terms of the density of states model of Mott and Davis.

Stretched exponential illumination time dependence of positive charge and spin generation in amorphous silicon nitride

Abstract: We report for the first time on the stretched exponential time dependence of positive charge and spin generated by subband‐gap ultraviolet illumination in gate‐quality nitrogen‐rich amorphous silicon nitride films. We have found that a stretched exponential function, which characterizes dispersive charge transport in silicon nitride, gives the best description of our experimental results. We also propose a mechanism which we believe is responsible for the creation of the positive charge and spin in the amorphous silicon nitride films.

Inter-band transitions in Zn3P2

Abstract: Optical methods such as measurements of transmissivity, reflectivity and photoconductivity were used to investigate inter-band transitions in Zn3P2 at room temperature. The optical measurements were performed within the 1.4-5 eV energy range, for light polarised both parallel and perpendicular to the c axis of the crystal; with reflectivity, the measurements extended up to 11 eV, but for unpolarised light only. The birefringence of Zn3P2 crystal was also measured and was discussed in terms of band-to-band transitions. Transitions observed within the 4.5-11 eV energy range were interpreted on the basis of the pseudo-cubic energy-band structure model. However, this model could not be adopted to explain low-energy transitions. The band-structure model at the Gamma point was suggested based on the results of measurements for polarised light and applying selection rules obtained from group-theory analysis. The direct energy gap of Zn3P2 at the Gamma point was estimated in this model as 1.6 eV at 300 K, whereas crystal-field and spin-orbit splittings of the valence band were equal to 0.03 eV and 0.11 eV, respectively.

Growth and characterisation of CuAlxIn1-xSe2 mixed crystals

Abstract: New chalcopyrite quaternary compound CuAlxIn1-xSe2 (0

Reduced light‐induced changes of photoconductivity in duterated amorphous silicon

Abstract: The light‐induced decrease of the photoconductivity in deuterated amorphous silicon is a factor of 3 less even though the defect density increase is greater than in hydrogenated material having equivalent as‐deposited properties. Consequent changes in the average recombination cross section of the defects is illustrated. Since the differences in the light soaking behavior upon isotopic substitution has been found to disappear in films deposited at low temperatures, the changes are thought to arise from differences in the silicon network occurring during growth.

Surface Chemistries and Photoelectrochemistries of Thin Film Molecular Semiconductor Materials

Abstract: Thin films of vacuum deposited trivalent and tetravalent metal phthalocyanines (Pc), and derivatives of perylenes (Pe), have been explored by a combination of ultrahigh vacuum deposition, surface spectroscopic characterization techniques, and measurement of photoconductivity and photocurrent spectral responses involving a) interdigitated array microcircuits (MC), and b) photoelectrochemical cells. Of particular interest has been the way adsorbed molecules such as O 2 and NH 3 affect the photoconductivity of these thin film materials, and the effect on photoactivity of the formation of a sharp interface between a phthalocyanine and a perylene. Multiple chemisorption sites are implicated for molecules such as O 2 and NH 3 on trivalent metal Pc thin film surfaces. These sites may either increase or decrease photoconductivity, depending on the partial pressure of the adsorbant, and the coverage of molecules competing for the same chemisorption sites. Photoelectrochemical modification of the Pc surface introduces submonolayer coverages of metals like Ag, providing a chemisorption site for molecules such as NH 3 , thereby improving the performance of these photoconductivity-based chemical sensors. Photoelectrochemical techniques can be used to characterize Pc/Pe bilayers, since the electrolyte provides a noncorrosive, optically transparent electrical contact. Photocurrent yield spectra, obtained from two different illumination directions, have been used to estimate the width of the interfacial region which is most active for exciton dissociation in the Pc/Pe bilayer. Transient photocurrent yields in Pc/Pe multilayers (up to 16 total layers), show that the photocurrent increases linearly with the number of Pc/Pe interfaces. These increases occur up to the point where the width of the individual layers becomes narrower than the interface resolution that our present vacuum deposition technologies can provide

Optically induced femtosecond electromagnetic pulses from GaSb/AlSb strained‐layer superlattices

Abstract: The strain generated electric field in a 〈111〉 oriented misfit superlattice provides a suitable bias for generating optically induced femtosecond electromagnetic radiation. We have measured the electromagnetic radiation from GaSb/AlSb strained‐layer superlattices and GaSb thin films; extremely fast electromagnetic pulses from 〈111〉 oriented superlattices have been observed. Because the quantum well structure in the superlattice samples limits the pulse duration of the transient photocurrent, it is possible to generate electromagnetic radiation having a pulse duration comparable with the photocarrier transit time across the quantum well.

Effect of hydrogen content on the light induced defect generation in direct current magnetron reactively sputtered hydrogenated amorphous silicon thin films

Abstract: The kinetics of light induced defect generation or the Staebler–Wronski effect have been investigated on device quality hydrogenated amorphous silicon films which were deposited by dc magnetron reactive sputtering. The total hydrogen content (CH) of the films, which varied from ∼10 to 28 at. %, had a strong influence on the defect generation. Low CH (10%–15%) films had a high initial density of defect states (∼7 to 10×1015 cm−3 ) compared to the high CH (≥17 at. %) films with a density of ∼3×1015 cm−3. However, light exposure increased the defect density more slowly on the low CH films, such that after about 1 h of light exposure their defect density was lower. A high‐quality glow discharge produced film was also measured, and behaved similarly to the high CH sputtered films. The greater stability of the low CH films was also reflected in a slower decrease of the electron photoconductivity relative to the other samples. For exposure times (t) up to 1000 h, the total density of defect states of the films i...

Transient decay of persistent photoconductivity in Al0.3Ga0.7As

Abstract: Transient decay of persistent photoconductivity (TDPPC) measurements were performed and analyzed in terms of models in which the TDPPC is associated with thermally activated electron capture into DX and a modification of the ionized impurity density, and hence the mobility, concomitant with electron capture. Quantitative agreement between theory and experiment was possible when Chadi and Chang’s model for DX [Phys. Rev. Lett. 61, 873 (1988); Phys. Rev. B 39, 10063 (1989)] was employed in conjunction with a photo‐induced shallow donor.