Graded-gap Layers Research Articles

Properties of MIS structures based on graded-gap HgCdTe grown by molecular beam epitaxy

The effect of near-surface graded-gap layers on the electrical characteristics of MIS structures fabricated based on heteroepitaxial Hg1 − x Cd x Te films grown by molecular beam epitaxy with a two-layer SiO2/Si3N4 insulator and anodic oxide film is studied experimentally. It is shown that a larger modulation of capacitance (depth and width of the valley) is observed compared with the structures without the graded-gap layer. The field dependences of photovoltage of MIS structures with the graded-gap layers had a classical form and were characterized by a drop only in the enrichment region. For the structures without the graded-gap layer with x = 0.22, a drop in the voltage dependence of the photocurrent is observed in the region of pronounced inversion. This drop is governed by limitation of the space charge region by processes of tunneling generation via deep levels. The properties of the HgCdTe-insulator interfaces are studied.

Capacitance-voltage characteristics of the p-Cd0.27Hg0.73Te-based structures with a wide-gap graded-gap surface layer

Capacitance-voltage characteristics of the structure In-SiO2-(graded-gap layer Cd0.71–0.27Hg0.29–0.73Te)-p-Cd0.27Hg0.73Te-GaAs are investigated at temperatures of 80 K and higher. The characteristics have the hysteresis, specifically, the characteristic is similar to a forward portion of the usual high-frequency characteristic (from enrichment to inversion), while the reverse portion has an extended plateau, in which the capacitance of the space-charge region is larger by a factor of approximately 2. To interpret the capacitance-voltage characteristic, the effect of partial screening of the graded-gap part of the space-charge region from the electric field of the test signal, as well as the effect of formation of the potential electron well near the surface due to the recharging of donor levels are considered.

Optically pumped semiconductor disk laser with graded and step indices

Results of a study of different gain section designs in nonresonantly optically pumped vertically emitting semiconductor disk lasers (SCDLs) are presented. Clear superiority of structures with barriers based on graded-gap layers is demonstrated. This finding is assigned to the lack of absorption saturation within the barriers caused by the efficient carrier collection capabilities of graded structures compared to ungraded ones. Transient photoluminescence experiments providing direct access to the carrier transfer from the barrier to the quantum-well active region confirm this explanation. Consequently, the authors propose graded designs for SCDLs.

Blocking of photoconductor surfaces by thin graded‐gap layers as technique to serious improvement of excess charge carrier excitation efficiency

Excitation efficiency (η) of excess electron-hole pairs in semiconductor body is one of key parameters determining performance of semiconductor radiation detectors. Designers and producers of semiconductor radiation detectors are interested in effective affordable techniques that improve excitation efficiency seriously. We present general consideration of perspective technique based on model structure consisting of homogenous absorber layer with fixed value of energy gap and two thin graded-gap layers blocking absorber layer from front and back sides. Calculations of η were performed with assumptions: interfaces are electrically neutral; density of recombination centres at interfaces is negligible and value of graded-gap field at interfaces is changed abruptly to zero. It was shown that η could be sufficiently high even at infinite surface recombination rate on outer surfaces of the structure, it is important to note that big gradient of energy gap in graded-gap layers is not required in this case. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Acoustostimulated expansion of the short-wavelength sensitivity range of AlGaAs/GaAs Solar cells

The effect of ultrasonic waves on the spectral sensitivity of solar energy converters based on AlGaAs/GaAs heterostructures has been studied. Ultrasonic treatment of a zinc-doped graded-gap AlxGa1−xAs film leads to the formation of a surface layer sensitive to electromagnetic radiation in the wavelength range λ < 0.551 μm. It is established that this layer is formed as a result of the acoustostimulated inward diffusion of zinc from the surface to the bulk of the graded-gap layer. The observed expansion of the short-wavelength sensitivity range and an increase in the efficiency of nonequilibrium charge carrier collection in AlGaAs/GaAs solar cells are due to improvement of the crystal defect structure and the dopant redistribution under the action of ultrasound.

A graded-gap detector of ionizing radiation

The current response of AlxGa1−x As graded-gap layers to optical and X-ray radiation was studied. A graded-gap electric field in the 15-µm-thick AlxGa1−x As layers, with x varying from 0 to 0.4, ensures the complete collection of charges generated by ionizing radiation and makes it possible to attain the value of 0.25 A/W for the current-power sensitivity of AlxGa1−x As. In the layers with a lowered doping level of the narrow-gap region of the graded-gap AlxGa1−x As layer, the voltage-power sensitivity to X-ray radiation with energy lower than 15 keV is as high as 1.6×103 V/W in the photovoltaic mode.

Optical response of the graded-gap Al xGa 1− xAs X-ray detector

It has been observed that only a part of the Al x Ga 1− x As graded-gap layer with the energy gap gradient g>20 eV/cm is active as a X-ray luminescence source. The thickness of the active layer is 30–50 μm. To increase the detectors optical response efficiency, the following methods are proposed and tested: 1. Reflection of the X-ray luminescence light, generated in the bulk Al x Ga 1− x As layer, inside the total reflection angle θ; 2. Optical stimulation of the electron–hole radiative recombination.

Pressure sensors based on AlGaAs doped with Te and Sn

Resistivity and Hall concentration in AlGaAs layers have been measured as a function of pressure up to 25 kbar and of temperature from 77 K to 400 K. The layers were grown by LPE on GaAs and doped with Te and Sn. We first studied the properties of AlxGal1-x As with Al composition x from 0.1 to 0.3. Then we measured the graded-gap layers with x varying e.g., from 10% to 40%. Such graded-gap layers can be considered as the parallel connection of layers with uniform composition. They reveal strong pressure sensitivity and flat temperature variation of resistance. Tin-doped layers are good candidates for “laboratory” pressure sensors operating up to 30 kbar in the 77-300 K temperature range. Tellurium-doped layers reveal metastable properties below 120K but they can be useful for pressure sensing in the high-temperature range. The properties of graded-gap AlGaAs layers are superior to those of manganin and InSb sensors.

Study of the effect of graded gap epilayers on the performance of CdxHg1−x Te photodiodes

The characteristics of the photodiodes based on CdxHg1−xTe solid solutions with graded gap layers were calculated in the context of a one-dimensional diffusion-drift model. The parameters of the photodiodes were shown to be improved when the p-n junction was located in the near-surface graded gap region rather than in the central homogeneous section of the structure. The photodiodes with the n-type layer adjacent to the substrate were found to offer an advantage over the diodes with the p-layer close to the substrate in the case of illumination from the substrate side.

Photoconductivity spectra of CdHgTe crystals with photoactive inclusions

This paper analyzes how both wide-and narrow-band-gap inclusions in CdHgTe host material affect the generation-recombination behavior of the latter. It is found that the shape of the photoconductivity (PC) spectral characteristic is sensitive to the type of inclusion present in the host: wide-band-gap inclusions lead to additional maxima in the spectral region near the fundamental absorption edge, while narrow-band-gap inclusions wash out the edge at long wavelengths. It is found that the shape of the PC spectra of these nonuniform crystals depends on the magnitude and polarity of the applied bias voltage. A photovoltage which alternates in sign as a function of wavelength and which is similar to the photosensitivity spectra of opposing barriers in graded-gap layers, is observed.

Influence of dislocations on the performance of Hg 1−xCd xTe graded gap photoresistors

Numerical calculations of detectivity of photoresistors made of graded gap n-type Hg 1− x Cd x Te structures have been carried out. The model involves a generalized approach, taking into account the position-dependent and wavelength-dependent material parameters, absorption coefficient, band-to-band recombination mechanism and recombination mechanism associated with misfit dislocations present in the graded gap layers. The last-mentioned recombination process reduces considerably the photoresistor detectivity. This is a probable reason why failure to fabricate high performance photoresistors from graded gap Hg 1− x Cd x Te structures has been experienced.

The use of linearly graded composition AlGaAs injectors for intervalley transfer in GaAs: theory and experiment

The use of semiconductor multilayers for hot electron injection and intervalley transfer have recently been demonstrated. In this study graded composition AlGaAs layers are used to generate a non-equilibrium distribution of electrons, in order to spatially control the transfer of electrons from the Γ to L valleys. The consequent growth of charge and field instabilities has been investigated both experimentally and by Monte-Carlo simulation. Particular emphasis has been placed on the exploitation of these hot electrons in transferred electron devices. Gunn diodes consisting of such graded gap layers, and transit regions of 1 μm have been fabricated by molecular beam epitaxy. They have been observed to oscillate at a frequency of 95 GHz. Experimental results on these structures, together with their Monte-Carlo simulations, are discussed.

Effect of reradiation on the luminescence of graded-gap semiconductors

Recombination radiation carries definite information on the parameters of semiconductors, which enables the use of luminescence, excited by light (photoluminescence) or an electron beam (cathodoluminescence) for monitoring the parameters of semiconducting graded-gap structures. In this work the authors develop a theory which can be used in the practice of photo- and cathodoluminescence measurements of the parameters of graded-gap semiconducting structures. The recombination radiation used is assumed to be isotropic. The practical method of luminescence analysis are based on the analysis of spectrometric measurements in accordance with the theory of transfer of the primary excitation through the bulk of the crystal as a result of diffusion and drift of non-equilibrium charge carriers (NCC) in the quasielectric field in the graded-gap layer.

Electrical and photoelectric properties of graded-gap epitaxial CdxHg1−xTe layers

Abstract Three-component mixed CdxHg1−xTe semiconductor crystals have been the subject of intensitive investigation recently. Epitaxial CdxHg1−xTe layers with a gradient of band parameters such as the energy gap, the effective mass of carriers etc. (so-called quasi-heterojunctions) are of particular interest. The technology of epitaxial graded-gap CdxHg1−xTe layers is described and some results concerning properties that result from gradients both of the energy gap and of the effective mass of carriers existing in the layers are presented. In the discussion of electrical properties special consideration is given to a method of investigating the energy level diagram of a layer with carrier concentration and energy gap gradients, and to a new non-linear electrical effect related to the effective-mass gradient of the electrons: when an external electric field is applied perpendicularly to the effective-mass gradient a double-frequency voltage is generated parallel to the gradient; this effect has been found for the first time in epitaxial CdxHg1−xTe graded-gap layers. The discussion of photoelectric properties is devoted chiefly to the results obtained from measurements of the high efficiency photovoltaic effect at a p-n junction made in a graded-gap epitaxial CdxHg1−xTe layer. From measurements of the electrical transport and of the transmission and reflection coefficients, with a calculation of the Fermil level energy, we present a schematic energy level diagram of the p-n junction. This, in turn, makes it possible to explain the narrow and high peaks of the photoresponse of the photovoltaic effect qualitatively.