Michael J. Dinezza

Growth, steady-state, and time-resolved photoluminescence study of CdTe/MgCdTe double heterostructures on InSb substrates using molecular beam epitaxy

CdTe/MgCdTe double heterostructures (DHs) are grown on InSb substrates using molecular beam epitaxy and reveal strong photoluminescence with over double the intensity of a GaAs/AlGaAs DH with an identical layer structure design grown on GaAs. Time-resolved photoluminescence of the CdTe/MgCdTe DH gives a Shockley-Read-Hall recombination lifetime of 86 ns, which is more than one order of magnitude longer than that of typical polycrystalline CdTe films. These findings indicate that monocrystalline CdTe/MgCdTe DHs effectively reduce surface recombination, have limited nonradiative interface recombination, and are promising for solar cells that could reach power conversion efficiencies similar to that of GaAs.

Determination of CdTe bulk carrier lifetime and interface recombination velocity of CdTe/MgCdTe double heterostructures grown by molecular beam epitaxy

The bulk Shockley-Read-Hall carrier lifetime of CdTe and interface recombination velocity at the CdTe/Mg0.24Cd0.76Te heterointerface are estimated to be around 0.5 μs and (4.7 ± 0.4) × 102 cm/s, respectively, using time-resolved photoluminescence (PL) measurements. Four CdTe/MgCdTe double heterostructures (DHs) with varying CdTe layer thicknesses were grown on nearly lattice-matched InSb (001) substrates using molecular beam epitaxy. The longest lifetime of 179 ns is observed in the DH with a 2 μm thick CdTe layer. It is also shown that the photon recycling effect has a strong influence on the bulk radiative lifetime, and the reabsorption process affects the measured PL spectrum shape and intensity.

Time-resolved and excitation-dependent photoluminescence study of CdTe/MgCdTe double heterostructures grown by molecular beam epitaxy

This Letter reports the optical properties of CdTe/MgCdTe double heterostructures grown by molecular beam epitaxy. Low-temperature photoluminescence shows strong band-to-band emission and very weak defect related peaks, indicating low defect densities. The measured Shockley–Read–Hall lifetimes range from 57 to 86 ns at room temperature for samples grown under different conditions. The material radiative recombination coefficient B in the recombination rate defined as R=AΔn+(1−γ)BΔn2+CΔn3 [Wang et al., Phys. Status Solidi B 244, 2740 (2007)] is evaluated to be 4.3 ± 0.5 × 10−9 cm3·s−1 with a photon recycling factor γ of 0.85 calculated based on the geometric structure of the samples.

Minority carrier lifetime of lattice-matched CdZnTe alloy grown on InSb substrates using molecular beam epitaxy

A CdZnTe/MgCdTe double-heterostructure (DH) consisting of a 3 μm thick Cd0.9946Zn0.0054Te middle layer that is lattice-matched to an InSb substrate has been grown using molecular beam epitaxy. A long carrier lifetime of 3.4 × 102 ns has been demonstrated at room temperature, which is approximately three times as long as that of a CdTe/MgCdTe DH with identical layer thickness. This substantial improvement is due to the reduction in misfit dislocation density in the CdZnTe alloy. In contrast, a CdTe/MgCdTe DH with 3 μm thick CdTe layer grown on an InSb substrate exhibits a strain relaxation of ∼30%, which leads to a wider x-ray diffraction peak, a weaker integrated photoluminescence intensity, and a shorter minority carrier lifetime of 1.0 × 102 ns. These findings indicate that CdZnTe lattice-matched to InSb has great potential as applied to high-efficiency solar cells as well as virtual substrates for high-performance large-area HgCdTe focal plane arrays.

CdTe vs. GaAs solar cells — A modeling case study with preliminary experimental results

A modeling study has been undertaken to compare CdTe versus GaAs solar cells in order to explore and forecast the possible evolution of CdTe solar cell performance while using GaAs solar cells as a benchmark. A complementary experimental comparison study of two almost identical MgCdTe/CdTe/MgCdTe and AlGaAs/GaAs/AlGaAs double-heterostructure (DH) samples grown by MBE guided the solar cell modeling. Both DH samples give strong photoluminescence at room temperature (RT) while time-resolved photoluminescence measurements reveal a 50 ns carrier lifetime at RT for the MgCdTe/CdTe/MgCdTe DH sample. The modeling results based on these experimental data indicate that CdTe solar cells can achieve efficiency in the range of 24-27%, depending on the carrier lifetime, under the AM1.5G spectrum.

Optically-addressed two-terminal multicolor photodetector

A two-terminal multicolor photodetector that is most advantageous for greater than two bands is proposed. This two-terminal design is particularly significant for focal plane arrays as it maximizes the fill factor and simplifies the readout integrated circuits. Individual color detection is realized with appropriate optical biasing. This concept is demonstrated experimentally using a three-color photodetector and biasing light emitting diodes. The measured linear dynamic range is greater than four orders of magnitude, making it a practical device for a broad range of applications.

Probing carrier lifetimes at dislocations in epitaxial CdTe

Dark line defects arising from dislocations in epitaxial CdTe films are known to strongly limit the overall performance of optoelectronic devices. However, their effect on carrier diffusion length and lifetime in the material immediately surrounding dislocations is not well quantified. We apply a photoluminescence imaging technique to directly measure these parameters in a CdTe/MgCdTe double heterostructure. Radiative recombination is reduced by up to 85% within 5 µm of the dislocation. Additionally, the carrier diffusion length and lifetime decrease by ~50 and ~80%, respectively.

InAs/InAsSb Type-II superlattice: a promising material for mid-wavelength and long-wavelength infrared applications

Optical and structural properties of InAs/InAsSb type-II superlattices (T2SL) and their feasibility for mid- and longwavelength infrared (MWIR and LWIR) photodetector applications are investigated. The InAs/InAsSb T2SL structures with a broad bandgap range covering 4 μm to 12 μm are grown by molecular beam epitaxy and characterized by highresolution x-ray diffraction and photoluminescence (PL) spectroscopy. All of the samples have excellent structural properties and strong PL signal intensities of the same order of magnitude, indicating that non-radiative recombination is not dominant and the material system is promising for high performance MWIR and LWIR detectors and multiband FPAs.

Determination of Mean Inner Potential and Inelastic Mean Free Path of ZnTe Using Off-Axis Electron Holography and Dynamical Effects Affecting Phase Determination

The mean inner potential (MIP) and inelastic mean free path (IMFP) of undoped ZnTe are determined using a combination of off-axis electron holography and convergent beam electron diffraction. The ZnTe MIP is measured to be 13.7±0.6 V, agreeing with previously reported simulations, and the IMFP at 200 keV is determined to be 46±2 nm for a collection angle of 0.75 mrad. Dynamical effects affecting holographic phase imaging as a function of incident beam direction for several common semiconductors are systematically studied and compared using Bloch wave simulations. These simulation results emphasize the need for careful choice of specimen orientation when carrying out quantitative electron holography studies in order to avoid erroneous phase measurements.

Temperature-dependent time-resolved photoluminescence study of monocrystalline CdTe/MgCdTe double heterostructures with low defect density

Confocal photoluminescence scans of monocrystalline CdTe/MgCdTe double heterostructures epitaxially grown on lattice-matched InSb substrates reveal very low twin defect density, below 1 × 10⁵ cm^-2. Room-temperature Shockley-Read-Hall (SRH) lifetimes of these samples are determined in the range of 35 ns to 86 ns using time-resolved photoluminescence (TRPL) measurements. Temperature-dependent TRPL measurements show that the carrier lifetime reaches a peak of 910 ns at 200 K. Excitation-dependent PL measurements reveal the radiative recombination coefficient of CdTe to be 4.3 × 10^-9 cm³·s^-1.