Influence of quantum confinement on the photoemission from superlattices of optoelectronic materials

We study the photoemission from quantum wire and quantum dot superlattices with graded interfaces of optoelectronic materials on the basis of newly formulated electron dispersion relations in the presence of external photo-excitation. Besides, the influence of a magnetic field on the photoemission from the aforementioned superlattices together with quantum well superlattices in the presence of a quantizing magnetic field has also been studied in this context. It has been observed taking into account HgTe/Hg_1−xCd_xTe and In_xGa_1−xAs/InP that the photoemission from these nanostructures increases with increasing photon energy in quantized steps and exhibits oscillatory dependences with the increase in carrier concentration. Besides, the photoemission decreases with increasing light intensity and wavelength, together with the fact that said emission decreases with increasing thickness exhibiting oscillatory spikes. The strong dependences of the photoemission on the light intensity reflects the direct signature of light waves on the carrier energy spectra. The content of this paper finds six applications in the fields of low dimensional systems in general.     

Optimization of absorption in InAs/InxGa1-xSb superlattices for long-wavelength infrared detection

The linear absorption coefficient of InAs/In_xGa_1-xSb superlattices is optimized based on an 8×8 envelope-function approximation (EFA) model. The effect of layer widths, indium content, buffer choice, substrate orientation, interface type, layer growth order, piezoelectricity, and layer width variations on the cutoff wavelength and the linear absorption coefficient is investigated. We propose specific superlattice parameters that optimize absorption for superlattices grown on GaSb at three cutoff wavelengths.     

Numerical analysis of long wavelength infrared HgCdTe photodiodes

We present a detailed investigation of the performance limiting factors of long and very long wavelength infrared (LWIR and VLWIR) p on n Hg_1−xCd_xTe detectors through numerical simulations at 77 K incorporating all considerable generation-recombination (G-R) mechanisms including trap assisted tunneling (TAT), Shockley-Read-Hall (SRH), Auger and radiative processes. The results identify the relative strengths of the dark current generation mechanisms by numerically extracting the contribution of each G-R mechanism to the detector characteristics with various cut-off wavelengths (λ_c) and practically achievable material parameters.The results show that the dominant sensitivity degrading trap level depends on the detector cut-off wavelength being ∼0.7E_g for LWIR HgCdTe sensors (λ_c = ∼10 μm) instead of 0.5E_g which is generally believed to be the most efficient R-G level. TAT related 1/f noise dominates the sensor noise even under small reverse bias voltages at a trap density as low as 1 × 10¹⁴ cm⁻³ for sensors with λ_c > 11 μm. Considering the fact that trap densities below this level are rarely reported for HgCdTe material, exceptionally trap-free material is required to achieve desirable imaging performance with these sensors. Simulation results show that Auger mechanism has twofold effect on the sensitivity of the sensor by increasing the dark current and decreasing the photo current of the detector.     

Hopping mobility in semiconductor superlattices

We present results on the electrical transport perpendicular to interfaces in GaAs/Al_xGa_1?xAs superlattices. We have measured the current-voltage characteristics on a series of superlattices. This has been simulated numerically, the superlattice being replaced by an effective medium. Using this model we obtain the values of the effective mobility as a function of the superlattice period. Our data are in good agreement with a theory of phonon-assisted hopping transport between localized states, rather than the theory of phonon-limited band transport of Bloch waves.     

窄带隙超晶格中载流子俄歇寿命和碰撞电离率的第一性原理研究

通过第一性原理的完整形式,基于全势能线性化增广平面波方法确定的精确能带结构和波函数,推算了技术上极为重要的窄带隙半导体超晶格中载流子俄歇复合时间.少数载流子的俄歇寿命由两种相关的方法来确定:1)由Fermi-金规则直接估算,2)联系俄歇复合和其相反过程碰撞电离,建立细致平衡公式,在一个统一的结构中进行间接估算.在n掺杂HgTe/CdTe和InAs/In_xGa_1－xSb超晶格中,由直接和间接的方法确定的寿命与一些实验结果相当一致.这说明该计算模式可以作为一种精确的手段用于窄带隙超晶格材料的性能优化. 关键词： 第一性原理 俄歇复合 碰撞电离 半导体超晶格     

Resonant tunneling in double superlattice barrier heterostructures

We have investigated resonant tunneling in double barrier heterostructures in which the tunnel barriers have been replaced by short period superlattices, and have shown for the first time quantum well confinement in a single quantum well bounded by superlattices. These results also demonstrate the first utilization of short period binary superlattices as effective tunnel barriers to replace the conventional Al_xGa_1−xAs barriers. The superlattice structure does not exhibit the asymmetry around zero bias in the electrical characteristics normally observed in the conventional Al_xGa_1−xAs barrier structures, suggestive of reduced roughness at the inverted interface by superlattice smoothing. The superlattice barrier also exhibits an anomalously low barrier height. The performance of this symmetric superlattice structure is compared with an intentionally constructed asymmetric double barrier superlattice structure, which exhibits pronounced asymmetry in the electrical characteristics. The observed behavior supports the view that resonant enhancement occurs in the quantum well.     

Existence of two types of perfect Bi<Subscript>2</Subscript>Sr<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>6+δ</Subscript> single crystals

It is found that perfect Bi₂Sr_2?x La _x CuO_6+δ single crystals with the same concentrations of lanthanum x = 0.64 and excess oxygen δ = 0.237 exist in two types. Single crystals of the first type are obtained by slow cooling (the synthesis time is 90–105 h). They have a monoclinic superlattice and exhibit no superconducting transition down to 2 K. Crystals of the second type are obtained by rapid cooling (the synthesis time is 30–40 h) and are characterized by a orthorhombic superlattice and T _c = 18 K. Thus, the superconducting transition temperature is determined not only by the concentration of carriers but also by the configuration of defects. A rhombic superlattice prevails in single crystals obtained by slow cooling in the lanthanum concentration range x = 0.3–0.5, while a monoclinic superlattice dominates in the range x = 0.75–0.85. This fact explains the high values of T _c at optimal doping (x = 0.4) and the absence of high-T _c superconductivity at p < 0.10.     

Raman scattering from GaAs-InxGa1−xAs strained-layer superllatices

Measurements of Raman scattering were performed on GaAs-In_xGa_1?xAs strained-layer superlattices, grown by molecular beam epitaxy, with lattice periods ranging from 30 ～ 250 Å and In concentrations x, 0.22 and 0.37. Only one GaAs-like longitudinal optical phonon peak was observed in each strained-layer superlattice, in contrast to the well-known result that two peaks were observed in GaAs-Al_xGa_1?xAs superlattices. The GaAs-like phonon frequencies shifted from those of bulk GaAs to those of bulk In_xGa_1?xAs alloys as the ratio of the one-layer thickness of In_xGa_1?xAs to the lattice period increases from zero to one. We conclude that the GaAs-like phonon mode is a uniform mode of the whole strained-layer superlattice and the phonon frequency is determined by the averaged In concentration.     

IR optical absorption and reflection spectra in narrow gap semiconductors

Light transmission T(λ) and reflection R(λ) spectra were measured for the crystals Cd_xHg_1−xTe (x = 0.26). InSb and Mn_xHg_1−xTe (x = 0.12) with low impurity content at T = 300 K in the wavelength range 2.5 μm ⩽ λ ⩽40 μm. From the measured T(λ) and R(λ) data a dependence of the absorption coefficient x(λ) for the free-carriers light absorption (FCLA) was derived, which is caused by the intrabund electron and hole transitions and the intcrband transitions of the heavy holes from the band V₁ to the band V₂ of the light holes: x(λ) = x_intraband(λ) + x_interband(λ).It is concluded from the quantum mechanical approach that the intraband FCLA is directly related with the carriers scattering mechanisms. Calculations of x_intraband(λ) within the framework of this approach demonstrated that under room temperature the major contribution was from the scattering by polar optical phonons. In the case of less perfect Mn_xHg_1−xTe however, one should take into account the scattering by the short-range potentials of the defects.     

Spectral sensitivity dependencies of backside illuminated planar MCT photodiodes

Spectral sensitivity dependencies of Hg_1−xCd_xTe (0.20 ⩽ x ⩽ 0.25) backside illuminated planar photodiodes were investigated at T = 80 K to study their longwavelength edge features. It was shown that the longwavelength part of these spectral dependencies is mainly formed by the exponential wavelength dependence of the optical transitions. Empirical dependencies of cut-off wavelengths at different values (λ_max, λ_0.5, λ_0.9) were obtained. The influence of the epitaxial layer thickness on the maximum sensitivity position was also studied.     

Vertical transport in GaAs/Ga1-xAlxAs superlattices observed by photoluminescence

Photoluminescence experiments on GaAs/Ga_1-xAl_xAs small period superlattices in which enlarged wells have been purposely introduced reveal a transfer of photoexcited carriers from the superlattice to the enlarged well localized levels. The transfer efficiency characterized by the relative intensities of luminescence peaks increases when the superlattice period decreases. Within a simple model, ionized impurity scattering and well size fluctuations account for this carrier transfer.     

Tenseur de mobilite d'un super-reseau en presence de desordre d'interface

Mobility tensor, limited by island-like interface disorder, in type-I superlattices, is studied within a simple model (Kronig-Penney and plane waves). Computed results are presented, for GaAs-Ga_1?xAl_xAs superlattices, versus variations of temperature, superlattice period and defect parameters such as localisation, width and thickness. This interface scattering process has a significant influence on near equilibrium transport at low temperature.     

Optimal properties of semimagnetic compound Pb1−xGdxTe with x=0.01 and 0.025

Reflectivity and transmission spectra of a new semimagnetic compound Pb_1−xGd_xTe with x=0.01 in the 300-77K temperature range are reported.From reflectivity data the high dielectric constant ϵ(∞) is deduced. Lyddane-Sachs-Teller relation gives the static dielectric constant ϵ(0). The conductivity effective mass m¹_c measured from the plasma frequency agrees well with the one deduced from absorption measurements α vs the wavelength in the short wavelength range.The energy gap as well as its temperature dependence is given by absorption data. The introduction of low concentration magnetic ion Gd does not affect drastically the optical properties of the material comparatively to the optical PbTe ones.     

Electrical resistivity and the phase transition temperatures of Ti1−xHfxSe2 mixed crystals

The onset temperature of the superlattice T_c developed in the mixed system Ti_1-xHf_xSe₂ is determined from measurements of the electrical resistivity. As x is increased, the onset temperature T_c reaches the maximum around x?0.07 before the superlattice formation is completely suppressed around x?0.35. The functional dependence of T_c(x) can be interpreted by considering TiSe₂ as an excitonic insulator.     

McMillan-Rowell tunneling spectroscopy in the Nb-Zr system

We report the first full application of tunneling spectroscopy to a superconducting transition metal alloy: Nb_1?xZr_x at x = 0.25, corresponding to the maximum T_c in the Nb-Zr system. The spectral function α²F(ω) and related parameters, when compared to those for the Nb, confirm that the increase in T_c from 9.22 K (x = 0) to 10.8 K(x = 0.25) arises largely by softening of the effective phonon spectrum.     

The noise equivalent temperature difference performance of HgCdTe photodiode array

The noise equivalent temperature difference (NETD) of Hg_1−xCd_xTe photodiode line arrays, used for the detection of radiation in the 8–12 μm spectral region is calculated. The dependence of the photodiode dark-current, signal and background photo-currents upon the photodiode cut-off wavelength and operating temperature, is presented with the material properties as parameters, and the NETD dependence on these parameters is calculated. The NETD is optimized with respect to the Hg_1−xCd_xTe molar composition, with excess carrier life-time and bulk doping level as parameters. The optimal cut-off wavelength and temperature for operating the photodiodes are derived. For current technology and focal plane temperature of 77 K, the optimal cut-off wavelength is shown to be, for an example system, in the range of 9.5–10.5 μm.     

High resolution x-ray diffractometry of the structural characteristics of a semiconducting (InGa)As/GaAs superlattice

A statistical approach is used to construct a kinematic theory of x-ray diffraction on a semiconducting superlattice with a two layer period. This theory takes two types of structural deformations into account: crystal lattice defects caused by microdefects distributed chaotically over the thickness of the superlattice, and periodicity defects of an additional superlattice potential owing to random deviations in the thicknesses of the layers of its period from specified values. Numerical simulation is used to illustrate the effect of structural defects on the development of the diffraction reflection curve. The theory is used to analyze experimental x-ray diffraction spectra of semiconducting In_xGa_1−x As/GaAs superlattices. Zh. Tekh. Fiz. 69, 44–53 (February 1999)     

On the nature of a lattice vibrational mode at a frequency of 135 cm−1 in Hg1 − x Cd x Te alloys

The vibrational spectrum of a cadmium impurity atom in the HgTe crystal has been calculated using the microscopic theory of lattice dynamics in the approximation of a low impurity concentration. Within this theory, the behavior of the local and quasi-local modes induced upon substitution of the lighter Cd atom for the Hg atom in the region of the zero or very low one-phonon density of states in the HgTe crystal has been considered. It has been found that, apart from the local mode at a frequency of 155 cm^?1, the calculated vibrational spectra exhibit a weak (but clearly pronounced) feature at a frequency of 134 cm^?1, which coincides with the experimentally observed vibrational mode (the “minicluster” mode) at a frequency of 135 cm^?1 in the Hg_{1 ? x} Cd _x Te (x = 0.2–0.3) alloys at 80 K.     

Combinatorial synthesis and optical characterization of alloy and superlattice films based on SrTiO3 and LaAlO3

We have grown alloy and superlattice films consisting of SrTiO₃ (STO) and LaAlO₃ (LAO) by pulsed laser deposition using composition-spread technique. All the (STO)_x(LAO)_1−x (0 ≤ x ≤ 1) alloy and superlattice films exhibited a single-phase perovskite structure. The optical properties of these films were characterized by absorption spectroscopy at room temperature. The spectra show a broad absorption due to O 2p-Ti 3d(t_2g) transition in an ultraviolet region. We found that absorption edges of both alloy and superlattice films systematically shifted to higher energy with increasing LAO composition. Clear difference was observed in the composition dependence of the indirect and a direct band edges.     

Intrinsic radiative recombination from quantum states in GaAs-AℓxGa1−xAs multi-quantum well structures

From absorption, emission, luminescence excitation and electron spin orientation studies of undoped GaAs-A?_xGa_1?xAs superlattices we demonstrate the $\text{intrinsic}$ nature of the radiative recombination process. This is in direct contrast to recombination observed in similar purity thick GaAs material. Moreover, our results do not support a recent suggestion that enhanced LO phonon-electron coupling should occur in such superlattice structures.