Calli M. Campbell

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.

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.

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.

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.

Band alignment at the CdTe/InSb (001) heterointerface

CdTe/InSb heterojunctions have attracted considerable attention because of its almost perfect lattice match and the presence of nonoctal interface bonding. This heterojunction is a model heterovalent system to describe band offsets. In this research, molecular beam epitaxy was used to deposit a ∼5 nm epitaxial CdTe (001) layer on an InSb (001) surface. Monochromatic x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy were used to characterize the electronic states of clean InSb and CdTe surfaces and CdTe/InSb (001) heterostructures. A room temperature remote hydrogen-plasma process was used to clean the surfaces prior to characterization. The results indicate a valence band offset of 0.89 eV and a type-I (straddling gap) alignment for the CdTe/InSb (001) heterostructure interface. In addition, In-Te bonding was observed at the interface. Downward band bending of the InSb is attributed to excess electrons introduced by nonoctal In-Te interface bonding.

Monocrystalline CdTe/MgCdTe double-heterostructure solar cells with a ZnTe hole-contact and passivation layer

Monocrystalline p-ZnTe/i-MgCdTe/n-CdTe/n-MgCdTe double-heterostructure (DH) solar cells are grown through a combination of MBE and MOCVD deposition techniques using several different dopants. The adverse effects (high interface recombination velocity) of the lattice mismatched ZnTe/CdTe hetero-interface is suppressed by the use of a DH with an intrinsic MgCdTe top barrier layer that functions as a passivation layer. The steady-state photoluminescence intensity is used to compare the potential device performance with previous ZnTe/CdTe and a-Si/i-MgCdTe/CdTe device structures while quantum efficiency measurements demonstrate the benefit of using ZnTe over previously demonstrated contact layers.

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 CdTe/MgCdTe Double-Heterostructure Solar Cells With ZnTe Hole Contacts

Monocrystalline p-ZnTe/i-MgCdTe/n-CdTe/n-MgCdTe double-heterostructure solar cells are grown through a combination of molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) deposition techniques using two different dopants within the ZnTe contact layer. The recombination at the ZnTe/CdTe heterointerface is believed to be suppressed by the use of a double heterostructure with an intrinsic MgCdTe passivation layer. A comparison of the steady-state photoluminescence intensity of these cells with record-Voc monocrystalline CdTe solar cells indicates the performance potential of devices with ZnTe contacts, while increases in the internal quantum efficiency demonstrate the benefit of using ZnTe over these previously demonstrated contacts. Solar cells utilizing a copper-doped ZnTe hole contact show promise in terms of built-in voltage but do not realize that potential in terms of Voc with a power conversion efficiency of 9.4% and a Voc of 819 mV. Solar cells utilizing an arsenic-doped ZnTe hole contact exhibit the highest power conversion efficiency, reaching 14.08% with an open-circuit voltage of 867 mV.

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.

CdTe nBn photodetectors with ZnTe barrier layer grown on InSb substrates

We have demonstrated an 820 nm cutoff CdTe nBn photodetector with ZnTe barrier layer grown on an InSb substrate. At room temperature, under a bias of −0.1 V, the photodetector shows Johnson and shot noise limited specific detectivity (D*) of 3 × 1013 cm Hz1/2/W at a wavelength of 800 nm and 2 × 1012 cm Hz1/2/W at 200 nm. The D* is optimized by using a top contact design of ITO/undoped-CdTe. This device not only possesses nBn advantageous characteristics, such as generation-recombination dark current suppression and voltage-bias-addressed two-color photodetection, but also offers features including responsivity enhancements by deep-depletion and by using a heterostructure ZnTe barrier layer. In addition, this device provides a platform to study nBn device physics at room temperature, which will help us to understand more sophisticated properties of infrared nBn photodetectors that may possess a large band-to-band tunneling current at a high voltage bias, because this current is greatly suppressed in the la...