Graded Gap Structures Research Articles

Development and Creation of a New Class of Graded-Gap Structures Based on Silicon with the Participation of Zn and Se Atoms

Development and Creation of a New Class of Graded-Gap Structures Based on Silicon with the Participation of Zn and Se Atoms

Impact of the Graded-Gap Layer on the Admittance of MIS Structures Based on MBE-Grown n-Hg1 – xCd x Te (x = 0.22–0.23) with the Al2O3 Insulator

The impact of the presence of the near-surface graded-gap layers with an increased content of CdTe on the admittance of MIS structures based on MBE-grown n-Hg1–xCdxTe (x = 0.22–0.23) with the Al2O3 insulating coating has been experimentally studied. It has been shown that the structures with a gradedgap layer are characterized by a deeper and wider capacitance dip in the low-frequency capacitance–voltage (CV) characteristic and by higher values of the differential resistance of the space-charge region than the structures without such a layer. It has been found that the main features of the hysteresis of capacitance dependences typical of the graded-gap structures with SiO2/Si3N4 are also characteristic of the MIS structures with the Al2O3 insulator. The factors that cause an increase in the CV characteristic hysteresis upon formation of the graded-gap layer in structures with SiO2/Si3N4 or Al2O3 are still debatable, although it may be assumed that oxygen plays a certain role in formation of this hysteresis.

Engineering steps for optimizing high temperature LWIR HgCdTe photodiodes

The authors report on energy gap engineering solutions to improve the high-temperature performance of long-wave infrared (LWIR) HgCdTe photodiodes. Metalorganic chemical vapour deposition (MOCVD) technology with awide range of composition and donor/acceptor doping and without ex-situ post grown annealing seems to be an excellent tool for HgCdTe heterostructure epitaxial growth. The heterojunction HgCdTe photovoltaic device based on epitaxial graded gap structures integrated with Auger-suppression is a magnificent solution for high operating temperature (HOT) infrared detectors. The thickness, composition and doping of HgCdTe heterostructure were optimized with respect to photoelectrical parameters like dark current, the responsivity and the response time. In this paper we focus on graded interface abruptness in the progressive optimization.

Electroluminescence of Graded-gap Structures with Blocking and Ohmic Contacts

Special features of the electroluminescence of uniformly doped graded-gap structures with blocking and ohmic contacts are studied theoretically. Analytical expressions for the spectral and integrated electroluminescence intensities are derived and analyzed in the case of a constant gradient of the band gap and a strong absorption of light. It is shown that the presence of the blocking contact on the wide-gap face of the graded-gap structure leads to an increase in the efficiency of the negative and, especially, positive electroluminescence compared to the case of a graded-gap structure with two ohmic contacts. The recombination radiation of a graded-gap structure with the blocking contact at the narrow-gap face features in the majority of cases a sign-alternating spectral dependence; a broad band of negative electroluminescence is prevalent in this dependence in the region of high currents. absorption.

Electro- and Photoluminescence in Graded-Gap Structures with Double Injection

The electro- and photoluminescence processes in graded gap structures operating in the double injection mode are discussed. The intensities of band-to-band radiative recombination and radiative recombination via centers for low and high injection levels are calculated. Both analytical and graphic dependencies of the intensities on the current density are also given.

Characteristic Zeeman patterns in novel graded gap II–VI quantum well structures

Novel methods of growing graded gap structures are described. In order to provide a check that the structures are indeed graded in the correct manner, magnetic ions in the structure have been employed. The good agreement between the predicted and observed Zeeman splitting of the corresponding light- and heavy-hole exciton transitions, together with the magnetic field dependence of the Zeeman splittings and intensity of a forbidden heavy-hole exciton transition, is taken as direct evidence that graded gap structures having the correct composition have indeed been fabricated.

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.

Comparison of the performance of uncooled CdHgTe PC and PEM 10.6 μm detectors

The ultimate performance of uncooled CdHgTe photon detectors for 10.6 μm radiation has been analysed. PC and PEM detectors based on CdHgTe homogeneous epitaxial layers or graded-gap structures are considered. The performance has been calculated numerically using a generalized model of PC and PEM effects in heterogeneous structures. Relatively high ultimate voltage responsivity and normalized detectivity for the detectors mentioned have been predicted. It has been shown that these detectors are better than other types of uncooled detectors as far as high-frequency modulation and the 10.6 μm wavelength range are concerned.

Photoelectromagnetic effect in Cd xHg 1−xTe graded-gap structures

Numerical calculations of voltage responsivity of PEM detectors made of graded-gap Cd x Hg 1− x Te structures have been carried out. The model involves a generalized approach, taking into account the x-value and wavelength-dependent material parameters, absorption coefficient and band-to-band recombination time—including Auger-1, Auger-7 and radiative mechanisms.

Transport of photocarriers in Cd xHg 1−xTe graded-gap structures

A theoretical and experimental study has been made on the transport of light injected excess charge carriers in Cd xHg 1−xTe variable band gap materials. A theoretical analysis is given of the ambipolar motion of the charge carriers in terms of diffusion and drift in the electric fields associated with the gradients of the band gap, lifetime, effective masses and doping level. The photoelectromagnetic effect (P.E.M.) has been calculated for two limiting cases: when the electric field is predominantly associated with a gradient of band gap (ideal case), and when the net electric field is zero (pseudohomogeneous case). It is shown that the two situations can be distinguished by analysing the variation of the P.E.M. effect in a magnetic field. Two different experimental studies have been made: a study of the optical transmission spectra which leads to the determination of the profiles of the structures and the spatial distribution of the absorbed photons, and a study of the P.E.M. effect and the photoconductivity as a function of several parameters: magnetic field, wavelength and temperature. The interpretation of these results shows that the structures studied can be characterized as pseudohomogeneous and an estimation of the variation of the diffusion length (1·5–18·5 μ) and the lifetime (5·10 −11–2·10 −8 sec) of the electrons in the composition range 0·15 < x < 0·25 has been made.