M.S. Han

The effect of hydrogenation in HgxCd1-xTe thin films grown on p-CdTe (211) B substrates

Fourier transform infrared (FTIR) transmission, Hall effect, and nuclear resonance reaction measurements have been carried out to investigate the effect of hydrogenation on the deep levels and the hydrogen depth profiling in nominally undoped HgxCd1 −xTe layers grown on undoped p-CdTe (211) B-orientation substrates by molecular beam epitaxy. After hydrogenation, the FTIR spectra showed that the transmittance intensity increased in comparison to that of the as-grown HgxCd1 −xTe and that the absorption edge shifted to the short wavelength range. Hall effect measurements showed that the carrier concentration decreased and the mobility increased after hydrogenation. After hydrogenation, p-type HgxCd1 −xTe is converted to n-type HgxCd1 −xTe with high resistivity. Nuclear resonance raaction measurements show that the concentration and the penetration depth of the hydrogen atom in n-Hg0.77Cd0.23Te are 3.5% and 640A, respectively. The areal density of the hydrogen-containing layer at the surface of the hydrogenated n-Hg0.77Cd0.23Te film is 4.39 × 1015atoms/cm2. These results indicate that hydrogen atoms not only effectively passivate impurities or defects in the HgxCd1 −xTe film but also change the carrier type of p-HgxCd1 −xTe.

Rapid Thermal Annealing Effects in CdTe (111) Thin Films Grown on GaAs (100) Substrates

Photoluminescence (PL) and double crystal X-ray rocking curve (DCRC) measurements have been carried out to investigate the rapid thermal annealing (RTA) effects in CdTe (111) epilayers grown on GaAs (100) by molecular beam epitaxy. The result of the X-ray diffraction showed that the orientation of the grown CdTe films was the (111) orientation. The full width at half maximum (FWHM) of the DCRC for as-grown CdTe layer was 400 arcs. When RTA was performed at 550° C, the FWHM of the DCRC for the CdTe layer decreased 265 arcs. The results of the PL spectra showed that the luminescence intensity of a bound exciton ( D0, X) for the CdTe/GaAs annealed at 550° C was raised by as much as about 53 times in comparison to the as-grown CdTe. The relative intensity ratio between ( D0, X) and the luminescence related to the defects for the as-grown and the annealed CdTe decreased by a factor of 7.8. These results indicate that the crystallinity of the CdTe epilayers grown on GaAs (100) is improved by RTA and that the RTA CdTe films grown on Si can be used for applications as buffer layers for the growth of Hgx Cd1-x Te.

STRUCTURAL, ELECTRICAL, AND OPTICAL PROPERTY STUDIES OF INDIUM-DOPED HG0.8CD0.2TE/CD0.96ZN0.04TE HETEROSTRUCTURES

Transmission electron microsopy (TEM), Hall effect, and Fourier transform infrared (FTIR) transmission measurements were performed to investigate the structural, electrical, and optical properties of indium-doped Hg 0.8 Cd 0.2 Te epitaxial layers grown on Cd 0.96 Zn 0.04 Te (211) B substrates by molecular-beam epitaxy. The TEM measurements showed that high-quality Hg 0.8 Cd 0.2 Te epitaxial layers with interfacial abruptnesses were grown on the Cd 0.96 Zn 0.04 Te substrates. The Van der Pauw Hall effect measurements on typical indium-doped Hg 0.8 Cd 0.2 Te/Cd 0.96 Zn 0.04 Te heterostructures with a doping concentration of 6 × 10 16 cm −3 at 10 K in a magnetic field of 0.5 T yielded a carrier density and a mobility of 2.2 × 10 16 cm −3 and 40,000 cm 2 /V s, respectively. The FTIR spectra showed that the absorption edges of the indium-doped Hg 0.8 Cd 0.2 Te/Cd 0.96 Zn 0.04 Te heterostructures shifted to a shorter wavelength range than those of the undoped samples, which was caused by the Burstein–Moss effect. The FTIR spectra also showed that the transmittance intensities of the indium-doped Hg 0.8 Cd 0.2 Te/Cd 0.96 Zn 0.04 Te heterostructures increased compared with those of the undoped heterostructures, which is due to the compensation of the Hg vacancy defects by the indium atoms. These results indicate that the indium-doped Hg 0.8 Cd 0.2 Te epitaxial layers were high-quality n -type layers and that p -Hg x Cd 1− x Te epilayers can be grown on indium-doped Hg 0.8 Cd 0.2 Te/Cd 0.96 Zn 0.04 Te heterostructures for the fabrication of Hg x Cd 1− x Te photoconductors and photodiodes.

Effect of thermal annealing on the structural and electrical properties of HgTe/CdTe superlattices

Abstract Rapid thermal annealing effects on the structural and electrical properties of the HgTe/CdTe superlattices grown on the Cd0.96Zn0.04Te substrates by molecular beam epitaxy have been investigated by double-crystal X-ray rocking curve (DCRC) and Van der Pauw Hall effect measurements. The sharp satellite peaks of the DCRC spectra measured on as-grown HgTe/CdTe superlattices show a periodic arrangement of the superlattice with high-quality interfaces. As the annealing time increases, the peak intensities corresponding to m=±1 of the DCRC spectra decrease dramatically. The average diffusion coefficient of the Hg in a HgTe/CdTe superlattice annealed at 220°C is approximately 10−18 cm2/s. After annealing, the results of the Hall effect measurements show that the carrier concentration and the mobility of the HgTe/CdTe superlattice are changed resulting from the formation of Hg vacancies and that the n-type HgTe/CdTe superlattice has converted to the p-type HgTe/CdTe superlattice with a high carrier concentration. These results indicate that the heterointerface of the HgTe/CdTe superlattice annealed at 220°C is significantly intermixed and that the monotonous variation of the intensity for the satellite peak in annealed HgTe/CdTe superlattice is attributed to a linear diffusion behavior.

Effect of thermal annealing on the structural and optical properties of CdTe (111)/GaAs (100) heterostructures

Abstract Reflection high-energy electron diffraction (RHEED), double-crystal X-ray rocking curve (DCRC), and photoluminescence (PL) measurements were performed to investigate the effect of thermal annealing on the structural and the optical properties of CdTe (111) epilayers grown on GaAs (100) substrates by molecular beam epitaxy (MBE) at low temperature. The results of the RHEED patterns showed that the oxidized layer on the GaAs substrate was removed in a Te atmosphere, and that the 20-A CdTe layer was grown by three-dimensional process. When the rapid thermal annealing (RTA) was performed at 500°C for 14 s, the FWHM of the DCRC for the CdTe layer had the smallest value. After the RTA process, the luminescence intensity of the exciton remarkably increased, and the peak at 1.476 eV was dominant. As the RTA temperature increased, the luminescence intensity of the exciton peak related to neutral acceptors (A°, X) increased. The temperature dependence of the spectra showed that the (A°, X) peak originated from the recombination of the excitons bound in high-density defects. The excitation power intensity dependence of the PL spectra showed that the peaks of the transitions due to donor-acceptor pairs shifted to larger energies. These results indicate that the structural and the optical properties of the CdTe epilayers grown on GaAs (100) are improved by RTA, and that the RTA process is very useful for the growth of HgxCd1−xTe on CdTe/GaAs heterostructures.

Deep levels in In-doped CdTe epitaxial films grown on p-CdTe substrates

Abstract Deep-level transient spectroscopy measurements have been carried out to investigate the behavior of the deep levels existing in In-doped n-CdTe grown by molecular beam epitaxy on nominally undoped p-CdTe (211) B-orientation substrates. One electron-trap of the as-grown In-doped CdTe epilayer was observed, and the trap originated from complexes of Cd vacancies and In impurities. After the In-doped CdTe epilayer was annealed, one new trap at E c − 0.49 eV was observed, and the deep level was related to the Te vacancies or the Cd interstitials. These results indicate that the electron deep trap in In-doped CdTe epilayers is affected remarkably by annealing and that the variation of the position of the deep level due to thermal treatment is a significant problem for electronic devices.

The dependence of the structural and optical properties on the CdTe epitaxial layer thicknesses in CdTe (111)/GaAs (100) heterostructures

X-ray diffraction (XRD), transmission electron microscopy (TEM), and spectroscopic ellipsometry measurements on the CdTe (111) epitaxial layers grown on the GaAs (100) substrates by molecular beam epitaxy were carried out to investigate the dependence of structural and optical properties on the CdTe epitaxial layer thickness. TEM measurements show that there is a lattice mismatch between the CdTe layer and the GaAs substrate and that the CdTe layers are twinned due to the strain. The spectroscopic ellipsometry spectra show that the critical-point energy shifts monotonically with decreasing the thickness of the CdTe epilayer. The results of the XRD curves show that compressive strains exist in CdTe layers thinner than 1.9 μm. As the strain calculated from the XRD spectra increases, the values of the critical-point energy-shifts obtained from the spectroscopic ellipsometry measurements increase linearly. These results indicate that the strains in the CdTe layers grown on GaAs substrates decrease with increasing the CdTe layer thickness.

Rapid thermal annealing effects in CdTe (111) thin films grown on Si (100) substrates by molecular beam epitaxy

Abstract Photoluminescence (PL) measurements have been carried out to investigate rapid thermal annealing (RTA) effects in CdTe (111) epilayers grown on Si (100) by molecular beam epitaxy, The full width at half maximum (FWHM) of the bound exciton peak of the PL spectrum for the as-grown CdTe layer was 20 meV. When RTA was performed at 400°C, the bound exciton peak was resolved into two bound exciton peaks due to neutral acceptors ( A°, X) and to neutral donors (D°, X) and one free exciton peak; the FWHM of the ( A°, X) peak was as small as 3.7 meV. When RTA was carried out at 600°C, the relative intensity ratio between the luminescences related to the defects and (A°, X) in the as-grown and the annealed CdTe layers decreased by a factor of 15. The activation energy of the ( A°, X) peak as obtained from temperature-dependent PL measurements was 18.7 meV. The bound exciton peak for the CdTe, which was rapidly thermally annealed at 400°C, shifted by 7.5 meV in comparison with that for the CdTe bulk, and the strain obtained from the PL peak shift was −0.0286, These results indicate that the crystallinity of the CdTe epilayers grown on Si is improved by RTA and that the RTA CdTe films grown on Si can be used for applications as buffer layers for the growth of Hg x Td 1−x Te.

The dependence of the structural properties on the period numbers and the CdTe thicknesses in HgTe/CdTe superlattices

Double-crystal X-ray rocking curve (DCRC) measurements were performed on HgTe/CdTe superlattices grown by molecular beam epitaxy (MBE) in order to investigate the dependence of the structural properties on the superlattice period number and the CdTe layer thickness. The results of the DCRC measurements showed that the full width at half maximum (FWHM) value of the principal peak decreased with increasing superlattice period number. As the CdTe layer thickness approached the HgTe layer thickness, the FWHM value corresponding to the principal peak decreased, and the optical absorption decreased. As the CdTe layer thickness changed, the detectable range of the HgTe/CdTe superlattice related to the value of the cut-off wavelength varied. These results indicate that HgTe/CdTe superlattices with various superlattice periods and CdTe layer thicknesses hold promise for potential applications in infrared photodetectors operating in a wavelength region between 4.3 and 12.5 μm.

Improvement of the crystallinity of CdTe epitaxial film grown on Si substrates by molecular beam epitaxy using the two-step growth method

Abstract Molecular beam epitaxy growth of CdTe epitaxial layers on Si (100) substrates using the two-step growth method was performed to produce high-quality CdTe thin layers. The reflection high-energy electron diffraction patterns were streaky with clear Kikuchi lines, which is direct evidence for layer-by-layer two-dimensional growth of CdTe on Si. From the X-ray diffraction analysis, the grown layer was found to be a CdTe (111) epitaxial film,regardless of the film thickness. Photoluminescence (PL) measurements at 12 K showed that the defect density of the CdTe film grownon Si using two-step growth decreased in comparison with that grownusing direct growth. The bound exciton appearing in the PL measurements shifted to the low energy side as the thickness of the CdTe increased. When the CdTe thickness increased from 1 to 1.8 μm, the peak position of the bound exciton shifted by 7.2 meV, and the stress obtained from the exciton peak shift was - 12.405 kbar. These results indicate that high quality CdTe films grown by two-step growth hold promise for applications as buffer layers for the subsequent growth of Hg x Cd 1− x Te.