C. H. Swartz

Radiative and interfacial recombination in CdTe heterostructures

Double heterostructures (DH) were produced consisting of a CdTe film between two wide band gap barriers of CdMgTe alloy. A combined method was developed to quantify radiative and non-radiative recombination rates by examining the dependence of photoluminescence (PL) on both excitation intensity and time. The measured PL characteristics, and the interface state density extracted by modeling, indicate that the radiative efficiency of CdMgTe/CdTe DHs is comparable to that of AlGaAs/GaAs DHs, with interface state densities in the low 1010u2009cm−2 and carrier lifetimes as long as 240u2009ns. The radiative recombination coefficient of CdTe is found to be near 10−10 cm3s−1. CdTe film growth on bulk CdTe substrates resulted in a homoepitaxial interface layer with a high non-radiative recombination rate.

Impact of extended defects on recombination in CdTe heterostructures grown by molecular beam epitaxy

Heterostructures with CdTe and CdTe1-xSex (xu2009∼u20090.01) absorbers between two wider-band-gap Cd1-xMgxTe barriers (xu2009∼u20090.25–0.3) were grown by molecular beam epitaxy to study carrier generation and recombination in bulk materials with passivated interfaces. Using a combination of confocal photoluminescence (PL), time-resolved PL, and low-temperature PL emission spectroscopy, two extended defect types were identified and the impact of these defects on charge-carrier recombination was analyzed. The dominant defects identified by confocal PL were dislocations in samples grown on (211)B CdTe substrates and crystallographic twinning-related defects in samples on (100)-oriented InSb substrates. Low-temperature PL shows that twin-related defects have a zero-phonon energy of 1.460u2009eV and a Huang-Rhys factor of 1.50, while dislocation-dominated samples have a 1.473-eV zero-phonon energy and a Huang-Rhys factor of 1.22. The charge carrier diffusion length near both types of defects is ∼6u2009μm, suggesting that recombinati...

Factors influencing photoluminescence and photocarrier lifetime in CdSeTe/CdMgTe double heterostructures

CdSeTe/CdMgTe double heterostructures were produced with both n-type and unintentionally doped absorber layers. Measurements of the dependence of photoluminescence intensity on excitation intensity were carried out, as well as measurements of time-resolved photoluminescence decay after an excitation pulse. It was found that decay times under very low photon injection conditions are dominated by a non-radiative Shockley-Read-Hall process described using a recombination center with an asymmetric capture cross section, where the cross section for holes is larger than that for electrons. As a result of the asymmetry, the center effectively extends photoluminescence decay by a hole trapping phenomenon. A reduction in electron capture cross section appeared at doping densities over 1016cm−3. An analysis of the excitation intensity dependence of room temperature photoluminescence revealed a strong relationship with doping concentration. This allows estimates of the carrier concentration to be made through a non-...

Effect of free-carrier concentration and optical injection on carrier lifetimes in undoped and iodine doped CdMgTe/CdSeTe double heterostructures grown by molecular beam epitaxy

Time-resolved and time integrated photoluminescence (PL) studies are reported for undoped and doped CdMgTe/CdSeTe double heterostructures (DHs) grown by molecular beam epitaxy. Undoped DHs are studied with absorber layer thickness varying from 0.5 to 2.5 µm. The n-type free-carrier concentration is varied ~7 × 1015, 8.4 × 1016, and 8.4 × 1017 cm−3 using iodine as a dopant in different absorber layer thicknesses (0.25–2.0 µm). Optical injection is varied from 1 × 1010 to 3 × 1011 photons/pulse/cm2, corresponding to the initial injection of photo-carriers up to ~8 × 1015 cm−3, to examine the effects of excess carrier concentration on the PL lifetimes. Undoped DHs exhibit an initial rapid decay followed by a slower dependence with carrier lifetimes up to ~485 ns. The dependence of carrier lifetimes on the thickness of the absorber layers (0.5–2.5 µm) suggests interface recombination velocities () ~ 1288 and 238 cm s−1 in the initial and later decay times, respectively, corresponding to high and low photo-carrier concentrations. The Shockley–Read–Hall model is used to describe the results in which variations are observed in for undoped DHs. The lifetimes of doped DHs show a consistent trend with thickness. The ~ 80–200 cm s−1 is estimated for doping n ~ 7 × 1015 cm−3 and 240–410 cm s−1 for n ~ 8.4 × 1016 cm−3. The observed decrease in carrier lifetimes with increasing n is consistent with growing importance of the radiative recombination rate due to the excess carrier concentration. The effect of carrier concentration on the PL spectrum is also discussed.

Comment on I. Lashkevych, O. Yu. Titov, and Yu. G. Gurevich, “Ohm’s Law for a Bipolar Semiconductor: The Role of Carrier Concentration and Energy Nonequilibria” [J. Electron. Mater., 46, 585 (2017)]

In a recent publication [J. Electron. Mater., 46, 585 (2017)], a number of formulae are presented for the effective conductivity of a bipolar semiconductor sandwiched between two metal contacts. However, the results are shown to be nonphysical, and the explanation is traced to errors appearing in previous literature on the subject.

Determining and Controlling the Magnesium Composition in CdTe/CdMgTe Heterostructures

The relationships between Mg composition, band gap, and lattice characteristics are investigated for Cd1−xMgxTe barrier layers using a combination of cathodoluminescence, energy dispersive x-ray spectroscopy, variable angle spectral ellipsometry, and atom probe tomography. The use of a simplified, yet accurate, variable angle spectral ellipsometry analysis is shown to be appropriate for fast determination of composition in thin Cd1−xMgxTe layers. The validity of using high-resolution x-ray diffraction for CdTe/Cd1−xMgxTe double heterostructures is discussed. The stability of CdTe/Cd1−xMgxTe heterostructures are investigated with respect to thermal processing.

Study of defects in CdTe heterostructures using imaging confocal photoluminescence and photoluminescence intensity measurements

Confocal photoluminescence is shown to be a powerful tool for analyzing defect structure in epitaxial CdTe appropriate for photovoltaic applications. Non-radiative defects such as dislocations are easily mapped and quantified. Photoluminescence intensity measurements are shown to be a valuable tool for quantifying interface state density. Very low dislocation density and twin content can be achieved for epitaxial CdTe, and low interface state densities result from using CdMgTe barriers.

Electrical and optical characterization of CdTe solar cells with CdS and CdSe buffers—A comparative study

A study is reported comparing the electrical and optical properties of CdTe solar cells, prepared using CdS and CdSe buffer layers, to investigate defects in the bulk and interface, carrier transport, and recombination. Temperature dependent capacitance–voltage measurement and admittance spectroscopy were used to extract carrier concentration, resistivity, charge carrier mobility, and their temperature dependence. The authors identify the presence of two defect signatures corresponding to carrier freeze-out and the formation of a Schottky back-contact barrier. The back-contact barrier height (≈300u2009meV) extracted from the temperature dependent current density–voltage (JVT) experiment was confirmed by conventional admittance spectroscopy. The activation energies of mobility (resistivity) are 101.2u2009±u20092.5u2009meV (92.6u2009±u20092.3u2009meV) and 84.7u2009±u20092.7u2009meV (77.6u2009±u20094.5u2009meV) for CdS and CdSe buffer layers, respectively. Intensity dependent photoluminescence analysis demonstrates that the CdSe/CdTe device exhibits lower radiative efficiency than the CdS/CdTe device. This confirms the presence of higher defects in the CdSe/CdTe device corroborated by temperature dependent VOC analysis. The comparative electrical and optical analysis provides insight into improving the performance of CdTe solar cell device by selenization.A study is reported comparing the electrical and optical properties of CdTe solar cells, prepared using CdS and CdSe buffer layers, to investigate defects in the bulk and interface, carrier transport, and recombination. Temperature dependent capacitance–voltage measurement and admittance spectroscopy were used to extract carrier concentration, resistivity, charge carrier mobility, and their temperature dependence. The authors identify the presence of two defect signatures corresponding to carrier freeze-out and the formation of a Schottky back-contact barrier. The back-contact barrier height (≈300u2009meV) extracted from the temperature dependent current density–voltage (JVT) experiment was confirmed by conventional admittance spectroscopy. The activation energies of mobility (resistivity) are 101.2u2009±u20092.5u2009meV (92.6u2009±u20092.3u2009meV) and 84.7u2009±u20092.7u2009meV (77.6u2009±u20094.5u2009meV) for CdS and CdSe buffer layers, respectively. Intensity dependent photoluminescence analysis demonstrates that the CdSe/CdTe device exhibits lower rad...

Iodine Doping of CdTe and CdMgTe for Photovoltaic Applications

Iodine-doped CdTe and Cd1−xMgxTe layers were grown by molecular beam epitaxy. Secondary ion mass spectrometry characterization was used to measure dopant concentration, while Hall measurement was used for determining carrier concentration. Photoluminescence intensity and time-resolved photoluminescence techniques were used for optical characterization. Maximum n-type carrier concentrations of 7.4xa0×xa01018xa0cm−3 for CdTe and 3xa0×xa01017xa0cm−3 for Cd0.65Mg0.35Te were achieved. Studies suggest that electrically active doping with iodine is limited with dopant concentration much above these values. Dopant activation of about 80% was observed in most of the CdTe samples. The estimated activation energy is about 6xa0meV for CdTe and the value for Cd0.65Mg0.35Te is about 58xa0meV. Iodine-doped samples exhibit long lifetimes with no evidence of photoluminescence degradation with doping as high as 2xa0×xa01018xa0cm−3, while indium shows substantial non-radiative recombination at carrier concentrations above 5xa0×xa01016xa0cm−3. Iodine was shown to be thermally stable in CdTe at temperatures up to 600°C. Results suggest iodine may be a preferred n-type dopant compared to indium in achieving heavily doped n-type CdTe.

Carrier Lifetimes of Iodine-Doped CdMgTe/CdSeTe Double Heterostructures Grown by Molecular Beam Epitaxy

Iodine-doped CdMgTe/CdSeTe double heterostructures (DHs) have been grown by molecular beam epitaxy and studied using time-resolved photoluminescence (PL), focusing on absorber layer thickness of 2xa0μm. The n-type free carrier concentration was varied to ∼7xa0×xa01015xa0cm−3, 8.4xa0×xa01016xa0cm−3, and 8.4xa0×xa01017xa0cm−3 using iodine as dopant in DHs. Optical injection at 1xa0×xa01010xa0photons/pulse/cm2 to 3xa0×xa01011xa0photons/pulse/cm2, corresponding to initial injection of photocarriers up to ∼8xa0×xa01015xa0cm−3, was applied to examine the effects of excess carrier concentration on the PL lifetimes. Iodine-doped DHs exhibited an initial rapid decay followed by a slower decay at free carrier concentration of 7xa0×xa01015xa0cm−3 and 8.4xa0×xa01016xa0cm−3. The optical injection dependence of the carrier lifetimes for DHs was interpreted based on the Shockley–Read–Hall model. The observed decrease in lifetime with increasing n is consistent with growing importance of radiative recombination.