Xin Hao Zhao

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.

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.

Carrier lifetimes and interface recombination velocities in CdTe/MgxCd1−xTe double heterostructures with different Mg compositions grown by molecular beam epitaxy

The interface recombination velocities of CdTe/MgxCd1−xTe double heterostructure (DH) samples with different CdTe layer thicknesses and Mg compositions are studied using time-resolved photoluminescence measurements. A lowest interface recombination velocity of 30 ± 10 cm/s has been measured for the CdTe/Mg0.46Cd0.54Te interface, and a longest carrier lifetime of 0.83 μs has been observed for the studied DHs. These values are very close to the best reported numbers for GaAs/AlGaAs DHs. The impact of carrier escape through thermionic emission over the MgCdTe barrier on the recombination process in the DHs is also studied.

Molecular beam epitaxy using bismuth as a constituent in InAs and a surfactant in InAs/InAsSb superlattices

Alloying bismuth with InAs provides a ternary material system near the 6.1 A lattice constant, which covers the technologically important mid- and long-wavelength infrared region. One challenge for this material system is that it is not straightforward to incorporate bismuth into the bulk InAs lattice, since bismuth has a tendency to surface-segregate and form droplets during growth. In this work, the conditions for InAsBi growth using molecular beam epitaxy are explored. A growth window is identified (temperatures ⪞ 270 °C, V/III flux ratios 0.98 ⪝ As/In ⪝ 1.02, and Bi/In ≅ 0.065) for droplet-free, high-quality crystalline material, where InAsBi layers with compositions of up to 5.8% bismuth (nearly lattice-matched to GaSb) are attained. The structural quality of InAsBi bulk and quantum well samples is evaluated using x-ray diffraction and transmission electron microscopy. The optical quality is assessed using photoluminescence, which is observed from quantum well structures up to room temperature and fr...

Electrical and Optical Properties of n-Type Indium-Doped CdTe/Mg 0.46 Cd 0.54 Te Double Heterostructures

CdTe/Mg_0.46Cd_0.54Te double heterostructures with n-type In doping concentrations, varied from 1 × 10¹⁶ to 7 × 10¹⁸ cm^-3, have been grown on InSb substrates using molecular beam epitaxy. Secondary ion mass spectroscopy measurements show strong diffusion of In from the InSb substrate to the CdTe buffer layer, while the In concentration is constant in the CdTe layer between the two Mg_0.46Cd_0.54Te barrier layers. Capacitance-voltage measurements show that the dopants are 100% ionized for the doping concentration range from 1 × 10¹⁶ to 1 × 10¹⁸ cm ^-3. The carrier lifetime decreases with increasing doping concentration (from 0.73 μs for an unintentionally doped sample to 0.74 ns for a 1 × 10¹⁸ cm^-3 doped sample) due to the decrease of both radiative and nonradiative lifetimes. Decent carrier lifetimes are achieved (~100 ns) between 1 × 10¹⁶ and 1 × 10¹⁷ cm^-3 doping levels, which is beneficial for developing n-type monocrystalline CdTe solar cells, photodetectors, and other optoelectronic devices. The strongest photoluminescence intensity is observed when the doping concentration is 1 × 10¹⁷ cm ^-3, which corresponds to the highest internal quantum efficiency.

Ultralow interface recombination velocity (∼1 cm/s) in CdTe/Mg x Cd 1−x Te double-heterostructures

CdTe/MgxCd1−xTe double heterostructures (DHs) grown on InSb (001) substrates using molecular beam epitaxy have demonstrated very long carrier lifetime and low interface recombination velocity (IRV) due to the effective carrier confinement and surface passivation provided by MgxCd1−xTe. However, both thermionic emission and tunneling effects can cause carrier loss over or through the MgxCd1−xTe barriers when the barrier potential is low or when the barrier is thin. Thus carrier lifetime measurement can only give an effective IRV, which consists of the actual IRV that is purely due to recombination through interface trap states, and carrier loss due to thermionic emission and tunneling. By conducting temperature dependent carrier lifetime measurements, the thermionic emission induced interface recombination can be distinguished. Also by comparing samples with different barrier layer thicknesses, the contribution to effective IRV from tunneling effect can be quantified. When both thermionic emission and tunneling effects are eliminated, the actual IRV is measured to be ∼1 cm/s and a very long carrier lifetime of 3.6 μs is observed.CdTe/MgxCd1-xTe double heterostructures (DHs) grown on InSb (001) substrates using molecular beam epitaxy have demonstrated very long carrier lifetime and low interface recombination velocity (IRV) due to the effective carrier confinement and surface passivation provided by MgxCd1-xTe. However, both thermionic emission and tunneling effects can cause carrier loss over or through the MgxCd1-xTe barriers when the barrier potential is low or when the barrier is thin. Thus carrier lifetime measurement can only give an effective IRV, which consists of the actual IRV that is purely due to recombination through interface trap states, and carrier loss due to thermionic emission and tunneling. By conducting temperature dependent carrier lifetime measurements, the thermionic emission induced interface recombination can be distinguished. Also by comparing samples with different barrier layer thicknesses, the contribution to effective IRV from tunneling effect can be quantified. When both thermionic emission and tunneling effects are eliminated, the actual IRV is measured to be ~1 cm/s and a very long carrier lifetime of 3.6 μs is observed.

Monocrystalline ZnTe/CdTe/MgCdTe double heterostructure solar cells grown on InSb substrates

Monocrystalline p-ZnTe/p-CdTe/n-CdTe/n-MgCdTe double-heterostructure (DH) solar cells are designed and demonstrated with a maximum efficiency of 10.9 %, an open-circuit voltage (VOC) of 759 mV, a short-circuit current density (JSC) of 21.2 mA/cm2 and a fill factor (FF) of 67.4 %. The low efficiency is mainly due to the combination of the low VOC and FF, which are attributed to high interface recombination at the ZnTe/CdTe, and p-CdTe/n-CdTe interfaces. Activation energies (EA) of 1.54 eV and 1.25 eV are obtained from temperature dependent light-IV measurements, indicating that the dominant recombination mechanism changes from interface recombination for non-annealed devices to bulk recombination for devices annealed at 450 °C.

1.7 eV MgCdTe double-heterostructure solar cells for tandem device applications

MgxCdi-xTe/Si tandem cells have the potential to reach a conversion efficiency greater than 40%. MgxCd1-xTe /MgyCd1-yTe (y>x) double heterostructures (DHs) grown by molecular beam epitaxy exhibit ~1.7 eV bandgaps and very high absorption coefficients, as measured using photoluminescence (PL) and spectroscopic ellipsometry. Indium-doped n-type MgxCd1-xTe (x ~ 13% Mg mole fraction) with a ~1.7 eV bandgap shows strong PL, comparable to that from high-quality CdTe/MgCdTe double heterostructures. Devices consisting of an n-type MgxCd1-xTe DH absorber, a p-type hydrogenated amorphous silicon (a-Si:H) hole contact layer and an indium tin oxide (ITO) top electrode are demonstrated with promising performance.