Photocurrents in p-n Si diodes under high intensity (pulsed-laser) illumination: Quantum yields and kinetic evaluation

The photoelectric response of p-n Si photodiodes under pulsed laser illumination (half width 10 ns) at 532 nm was studied as a function of dose which was varied over 6 orders of magnitude. The photocurrent transients are dominated by a plateau-like feature due to the build up of space charge at the intensities used. Increasing bias voltage increases the height of the plateau and decreases its length. In the low-dose range the length of the transient increases linearly with dose and the collected charge (integrated current) reaches a constant value. At high doses (above 10^–5 J/pulse · cm² or 2.7×10¹³ quanta/pulse · cm²) considerable charge loss (decrease in quantum yields) is accompanied by a less than proportional increase of the transient lifetime. From model calculations the dose and voltage dependence of the quantum yield of charge collection is shown to be the result of competition between current flow and first and higher order recombination. The model calculations are consistent with experimental results. Rate constants have been obtained by fitting.     

Determination of photoexcited carrier concentration and mobility in GaAs doping superlattices by hall effect measurements

The photo-Hall effect in a new type of periodicp-n doping multilayer structures (superlattices) of GaAs grown by molecular beam epitaxy has been investigated. In these space charge systems electrons and holes are separated in real space. As a consequence, large deviations from thermal equilibrium become quasi-stable. Carrier generation by optical absorption occurs in these doping superlattices even at photon energies far below the gap of the homogeneous semiconductor material. The photoexcitation results in a strong enhancement of the conductivityparallel to the layers and in a substantial photovoltaic response. An increase in carrierconcentration as well as an increase in carriermobility both contribute to the observed enhancement of the conductivity under excitation. The absolute values of changes in free-carrier concentration are very large due to the manyfold active layers of the structure. The measured free-carrier mobilities depend on the population of the multilayer system. A reduction in mobility as compared to bulk material is found to be more pronounced in weakly populated systems. This finding is caused by the larger weight of the boundary regions of the total active layers where the free-carrier density is lower than the density of ionized impurities resulting in an enhanced impurity scattering.     

Optoelectronic characteristics of close-packed HgTe nanoparticles in the infrared range

Absorption, photoluminescence (PL), photoresponse, and I-V measurements were made for a close-packed HgTe nanoparticle film without organic capping materials to investigate its optoelectronic characteristics in the infrared (IR) range. In the absorption and PL spectra taken for the close-packed nanoparticle film, the wavelength of exciton peak was red-shifted, compared with 1-thioglycerol capped HgTe nanoparticles dispersed in solution. For the HgTe nanoparticle film, dark current was below several pA level, current was increased by about three orders of magnitude at a biased voltage of 3 V under the illumination, and photoresponse was very rapid compared with 1-thioglycerol capped HgTe nanoparticles. These optoelectronic characteristics illustrate that HgTe nanoparticles are one of promising materials for the photodetector in the IR range. Finally, the origin for the increase of photocurrent with increasing temperature observed in this study will be discussed.     

Optical-Phonon Deformation Potentials in Diamond-structure crystals: ab initio LMTO-ASA Calculation

The optical-phonon deformation potentials of C, Si, Ge and Sn are calculated by LMTO-ASA approach within the frozen-phonon approximation. The comparisons between our results and those of several present theoretical calculations and experiments demonstrate that our calculation model is reasonable     

Performance Improvement of GaN Based Schottky Barrier Ultraviolet Photodetector by Adding a Thin A1GaN Window Layer

We propose a new structure of GaN based Schottky barrier ultraviolet photodetector, in which a thin n-type A1GaN window layer is added on the conventional n^--GaN/n^＋-GaN device structure. The performance of the Schottky barrier ultraviolet photodetector is found to be improved by the new structure. The simulation result shows that the new structure can reduce the negative effect of surface states on the performance of Schottky barrier GaN photodetectors, improving the quantum efficiency and decreasing the dark current. The investigations suggest that the new photodetector can exhibit a better responsivity by choosing a suitably high carrier concentration and thin thickness for the A1GaN window layer.     

Single-Photon Detection at Telecom Wavelengths

A single-photon detector based on an InGaAs avalanche photodiode has been developed for use at telecom wavelengths. A suitable delay and sampling gate modulation circuit are used to prevent positive and negative transient pulses from influencing the detection of true photon induced avalanches. A monostable trigger circuit eliminates the influence of avalanche peak jitter, and a dead time modulation feedback control circuit decreases the afterpulsing. From performance tests we lind that at the optimum operation point, the quantum efficiency is 12% and the dark count rate 1.5 × 10^-6 ns^-1, with a detection rate of 500 kHz.     

A new method of analysis of DLTS-spectra

We propose a new algorithm to obtain the energy and the capture cross section of a deep trap in the band gap of a semiconductor from deep-level transient spectroscopy (DLTS) measurements. This numerical method requires only a single temperature cycle with a fixed rate window for data acquisition. It is capable to resolve DLTS signals with a shoulder, generated by two trap levels. Experiments with Schottky barrier diodes onn-GaAs demonstrate the contribution of a second trap to the EL6 level in GaAs and the superior reliability in cases of non-negligible resistivity of the back-contacts compared to conventional Arrhenius plot method.     

Luminescence properties of a new Mn-activated red long-afterglow phosphor

A new phosphor, CaZnGe₂O₆:Mn²⁺, which emits red long-lasting phosphorescence centered at 648 nm upon UV light excitation, is prepared by the conventional high-temperature solid-state method and its luminescent properties are systematically investigated in this paper. XRD, photoluminescence, thermoluminescence spectra and afterglow decay curve are used to characterize the synthesized phosphor. This phosphor is well crystallized by calcination at 1150 °C for 3 h and possesses excellent performance. The color coordinate values of this phosphor are x=0.64, y=0.26 under 250 nm UV light excitation. Under 250-nm UV light irradiation, this phosphor shows obvious long-lasting phosphorescence that can be seen with the naked eye in the dark clearly after the irradiation source has been removed for more than 3 h. The possible mechanism of this red-light-emitting long-afterglow phosphor is also investigated based on the experiment results.     

Theory of electron escape from the surface of liquid helium

We have studied the properties of an electron bubble close to the surface of liquid³ He, by using a Density Functional approach. We find that up to an electron-surface distanced ₀ 23 Åthe bubble is stable, while at smaller distances it becomes unstable and bursts. A potential energy barrier /K _B 38°K for the thermal emission of electrons is obtained from our results, in agreement with experiments. Even when the electron-surface distance is larger thand ₀, however, tunneling through the surface layer dominates the electron escape probability. Large deviations of the electron potential energy from its ideal value are found close to the surface. These deviations have a profound effect on the calculated decay rates of the tunneling curent, which are much smaller than those obtained previously and in semi-quantitative agreement with experiments.     

Charge carriers and mobility of Cu-Ti ferrite

The concentration and drift mobility of charge carriers in Cu_1–x Ti _x Fe₂O₄ ferrite are calculated, over a wide range of temperatures (300–773 K), employing d.c. conductivity and thermoelectric power data. With increasing temperature the concentration of charge carriers decreases whilst the drift mobility exhibits an exponential increase. Over the above-mentioned temperature range, the obtained density of charge carriers varies between 10²¹ and 10²² cm^–3 whereas the drift mobility has values between 10^–8 and 10^–4 cm²/V s. The results are discussed on the basis of a small-polaron hopping conduction. The activation of the d.c. conductivity has been attributed to the thermal activation of the mobility.     

Leakage current study of Si1−xCx embedded source/drain junctions

This work investigates the impact of various processing parameters on the leakage current of in situ P-doped Si_1−xC_x embedded source/drain (S/D) junctions, i.e., the carbon content x (%) and the thermal budget used either before or after the selective epitaxial deposition. It is shown that while the area leakage current density, generated by defects in the depletion region is not affected by the epitaxial process or the strain in the substrate, the perimeter leakage current density (J_P) increases with x. From the stronger reverse bias dependence of J_P, it is derived that a higher electric field exists along the junction periphery. This is confirmed by capacitance-voltage (C-V) measurements, demonstrating a higher p-well B doping density for increasing x. It is believed that this originates from the strain dependence of the B diffusivity in the p-well region. No evidence of electrically active extended defect formation was found, so it is expected that the off-state current of embedded Si_1−xC_x S/D nMOSFETs will not be adversely affected by the selective epitaxial deposition.     

Low field DC investigation of hot carrier trapping in silicon dioxide films

The experimental method used in this work is based upon the idea of nonavalanche injection of carriers heated by direct electric field. The structure consisted of an n-channel MOS transistor and two p-n junctions. The process of charge injection in this structure was investigated by studying the dependence of gate current on heating voltage. The trapping properties of the SiO₂ film were studied by monitoring the charging of the film during injection of electrons. The capture cross-sections, the trap centre concentrations and the dependence of the capture cross section on the electric field for fields between 1 MV/cm and 2.5 MV/cm were determined.     

Photoconductance‐calibrated photoluminescence lifetime imaging of crystalline silicon

We use photoluminescence (PL) measurements by a silicon charge‐coupled device camera to generate high‐resolution lifetime images of multicrystalline silicon wafers. Absolute values of the excess carrier density are determined by calibrating the PL image by means of contactless photoconductance measurements. The photoconductance setup is integrated in the camera‐based PL setup and therefore identical measurement conditions are realised. We demonstrate the validity of this method by comparison with microwave‐detected photoconductance decay measurements. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theoretical approaches to nonlinear and chaotic dynamics of generation-recombination processes in semiconductors

We present an overview of recent progress in the theoretical modelling of nonlinear and chaotic dynamics induced by generation and recombination processes of charge carriers. Impact ionisation of impurities is the autocatalytic, i.e. destabilizing, step of three different physical mechanisms for spontaneous, self-sustained oscillations of the carrier density. The restoring force is furnished by one of the following three processes: (i) dielectric relaxation of the internal electric field, (ii) energy relaxation of the hot carriers, and (iii) trapping at impurities, where the discrete nature of the individual generationrecombination processes is taken into account.     

An Innovative Gas Sensor with On-Chip Reference Using Monolithic Twin Laser

An innovative gas sensor with on-chip reference using a monolithic twin laser is proposed. In this sensor a monolithic twin laser generates two closer laser beams with slight different wavelengths alternatively, one photodiode is used to catch both absorption and reference signals by time division multiplexing. The detection of nitrous oxide adopting this scheme using a 2.1μm antimonide laser and an InGaAs photodiode has been demonstrated experimentally with detection limit below 1ppm. Using this on chip reference scheme the fluctuations from the optical path and devices can be compensated effectively; the sensor system is simplified distinctly.     

Electronic band gap of SrSe at high pressure

The electronic band gap of SrSe, in the CsCl-stuctured phase, was measured to 42 GPa via optical absorption studies. The indirect electronic band gap was found to close monotonically with pressure for the range of pressures studied. The change in band gap with respect to pressure, dE_gap/dP, was determined to be −6.1(5)×10⁻³ eV/GPa. By extrapolation of our line fit, we estimate band gap closure to occur at 180(20) GPa.     

Characterization of X-ray photo- and inverse photoelectron spectra of orthorhombic III–VII compounds in terms of norm-conserving pseudopotential calculation: Application to TII

Energy distribution curves for occupied and virtual states in orthorhombic thallium iodide through angle-integrated photoelectron spectroscopy are compared with the result of the total density of states calculation performed without any fitting parameters. The main spectral features are analyzed via charge density maps and generalized for other compounds of the TII-type structure.     

Experimental analysis and modeling of the IV characteristics of photovoltaic solar cells under solar spectrum spot illumination

In this paper, some models that have been put forward to explain the characteristics of a photovoltaic solar cell device under solar spot-illumination are investigated. In the experimental procedure, small areas of the cell were selected and illuminated at different solar intensities. The solar cell open circuit voltage (V_oc) and short circuit current (I_sc) obtained at different illumination intensities was used to determine the solar cell ideality factor. By varying the illuminated area on the solar cell, changes in the ideality factor were studied. The ideality factor obtained increases with decreasing illumination surface ratio. The photo-generated current at the illuminated part of the cell is assumed to act as a dc source that injects charge carriers into the p-n junction of the whole solar cell while the dark region of the solar cell operates in a low space charge recombination regime with small diffusion currents. From this analysis, a different model of a spot illuminated cell that uses the variation of ideality factor with the illuminated area is proposed.     

Contribution ofd-like states to magnesium,aluminium and phosphorusL-emission bands of MgO,Al2O3 and AlPO4

We have measured the MgL-, AlL- and PL-emission bands of MgO, -Al₂O₃ and AlPO₄, respectively. For MgO and Al₂O₃ the electronic structure and the X-ray emission bands have been calculated. In spite of different crystal structure and chemical composition of these compounds the cationL-emission bands are very similar. We have found that for the interpretation of theL-emission bands of these compounds the cation 3d-like electrons are crucial.     

The origin of reduced fill factors of emitter‐wrap‐through‐solar cells

Low fill factors generally limit the efficiency of emitter‐wrap‐through (EWT) solar cells. Until now, a conventional series resistance limitation along the laser‐drilled EWT vias has usually been assumed to be responsible for this effect. We demonstrate that the characteristic fill factor loss is caused by a crucial change in the diffusion currents inside the base, which are influenced by the conductivity along the laser‐drilled EWT vias. In addition, we show that the EWT via conductivity influences the fill factor loss caused by an iron contaminated base. This result affects the proposition that the EWT design is suitable for multicrystalline silicon in which interstitial iron is known to be the main contaminant. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)