J. H. Leem

Optical and magnetic measurements of p-type GaN epilayers implanted with Mn+ ions

The p-type GaN epilayers were prepared by metalorganic chemical vapor deposition and subsequently Mn+ ions implanted. The properties of Mn+ ions-implanted GaN epilayers were investigated by optical and magnetic measurements. The results of photoluminescence measurement show that optical transitions related to Mn apparently appear at 2.5 eV and around 3.0 eV. It is confirmed that the photoluminescence peak at 2.5 eV is a donor–Mn acceptor transition. Ferromagnetic hysteresis loop was observed, and the temperature-dependent magnetization displayed a ferromagnetic behavior persisting up to ∼270 K.

Surface passivation by sulfur treatment of undoped p-CdTe(100)

The effect of surface passivation of undoped p-CdTe(100) by (NH4)2Sx treatment was investigated by using photoluminescence (PL), photoconductivity (PC), and x-ray photoelectron spectroscopy (XPS). After sulfur treatment for 2 min, the acceptor bound exciton (A0, X) peak increases greatly in the PL spectrum, and the minority-carrier lifetime of CdTe becomes the longest value in the PC measurement. The XPS spectrum for untreated CdTe shows the additional peaks on the right side of two main Te peaks corresponding to the Te 3d core levels, and these additional peaks are related to TeO3 with binding energies of 576.2 and 586.5 eV. After sulfur treatment, while the intensities of the Te 3d core levels decreased gradually, those of the TeO3 peaks disappear. In addition, the S 2p core-level spectra for sulfur-treated CdTe show the peaks at the 161.7 and 162.8 eV, which are attributed to a CdS formation at the surface of CdTe. These results indicate the sulfur effectively dissociates the native oxides from and neu...

Strain effects in CdTe/Si heterostructures

Photoluminescence (PL) and spectroscopic ellipsometry measurements on CdTe/Si strained heterostructures grown by molecular beam epitaxy were carried out to investigate the effect of the strain and the dependence of the strain on the Si tilted substrates. The results of the PL spectra showed that the relative intensity ratio between the peak at 1.452 eV and the bound-exciton peak for the CdTe epilayer grown on the Si (100) 1° tilted substrate had a minimum value and that the strain for the CdTe epilayer grown on the Si (100) 8° tilted substrate had a minimum value. When rapid thermal annealing (RTA) was performed at 55 °C, the PL spectra showed that the relative intensity ratio between the peak at 1.452 eV and that at 1.574 eV for the CdTe epilayer grown on the Si (100) 8° tilted substrate had a minimum value and that the strain for the CdTe epilayer grown on the Si (100) 1° tilted substrate had a minimum value. Spectroscopic ellipsometry measurements showed that the spectrum of the dielectric constant of ...

Effects of hydrogenation and annealing on the shallow donor-band recombination in In-doped CdTe epitaxial layers grown on p-CdTe (211) substrates

Photoluminescence (PL) measurements have been performed on as-grown, annealed, and hydrogenated In-doped CdTe epitaxial films grown on p-CdTe (211) substrates by using molecular beam epitaxy in order to investigate the annealing and the hydrogenation effects on the behavior of the shallow donor-band recombination (D,h) peak. The smaller full width at half maximum (FWHM) values of the (D,h) emission lines in the annealed and the hydrogenated epilayers show that the crystallinity of the In-doped CdTe epilayers has been improved due to the annealing and the hydrogenation treatments. The inhomogeneous broadening of the FWHM values of the (D,h) peaks in the In-doped CdTe epilayers might be related to a change from correlated distributions of the electrons and the holes at low temperatures to random distributions at high temperatures. The concentrations of the shallow donors and the compensation coefficients were estimated from the dependence of the FWHM of the (D,h) lines on the temperature by using PL spectra. These results indicate that the crystallinity and the electrical properties of the In-doped CdTe epilayers are affected significantly by the annealing and the hydrogenation treatments.

HYDROGENATION AND ANNEALING EFFECTS ON THE TRAPPING TIMES OF THE MINORITY CARRIERS IN IN-DOPED CDTE EPITAXIAL LAYERS GROWN ON P-CDTE (211) SUBSTRATES

Photoconductivity (PC) measurements on as-grown, annealed, hydrogenated, and hydrogenated and annealed In-doped CdTe epitaxial films grown on p-CdTe (211) substrates by molecular beam epitaxy have been performed in order to investigate the behavior of the trapping times of minority carriers in In-doped CdTe films due to annealing and hydrogenation. The results of the PC decay curve showed a slow component with a time constant of a few milliseconds and this behavior was related to the existence of deep trap levels corresponding to minority carriers. The activation energies of the traps, as determined from the temperature dependence of the PC decay times, were (Ev+0.35) and (Ev+0.43) eV for the as-grown and hydrogenated In-doped CdTe epilayers, respectively. The trapping times of the minority carriers were significantly reduced by the hydrogenation treatment.

Optical properties of Mn-doped GaAs layers grown on (100) GaAs substrate

Mn-doped Ga1−xMnxAs epilayers grown on semi-insulating (100) GaAs substrates using the liquid phase epitaxy technique were investigated using photoluminescence and Hall effect measurements from 20 to 300 K. Transitions involving shallow Mn acceptors were identified through photoluminescence measurements and the ionization energy of Mn acceptor was determined to be 104.6 meV which is in good agreement with values calculated from a hydrogenic hole model including sp–d exchange contribution. Also, it was concluded that Ga1−xMnxAs alloys were obtained from samples with low Mn concentration (below x≈1%) and these layers have a simple impurity band merging to the valence band.

Effects of thermal annealing on the strains and structures of CdTe epilayers grown on Si(100) substrates for various substrate tilt angles

Photoluminescence (PL) measurements on epitaxial films grown on Si substrates by using molecular beam epitaxy were carried out at various substrate tilt angles to investigate the effect of annealing on the optical and the structural properties of CdTe epilayers. The strains for the as-grown and the annealed CdTe epitaxial layers on Si substrates as functions of the substrate tilt angle were obtained from the acceptor bound-exciton peak of the PL spectra. Possible primitive unit cells of the as-grown and the annealed CdTe epilayers are presented.

Effect of lattice mismatch and thermal expansion on the strain of CdTe/GaAs heterostructures

Photoluminescence (PL) measurements on CdTe/GaAs heterostructures grown by molecular beam epitaxy (MBE) were carried out to investigate the effect of the lattice mismatch and the thermal expansion on the strain due to the CdTe epitaxial layer thickness in CdTe/GaAs heterostructures. The PL peak of the acceptor bound exciton shifts toward the higher-energy side with increasing CdTe film thickness. A new theoretical equation obtained from the strain on the lattice-mismatched heterostructure is proposed. The values of the strains determined from the PL measurements were in reasonable agreement with those determined from the new theoretical calculations taking into account the lattice mismatch together with the thermal expansion difference between the CdTe epilayers and the GaAs substrates. These results can help improve the understanding of the structural properties of CdTe/GaAs heterostructures.

Diffusion mechanisms in intermixed HgTe/CdTe superlattices

The interdiffusion behaviour of Hg in annealed HgTe/CdTe superlattices due to rapid thermal annealing (RTA) has been investigated by double-crystal x-ray rocking curve (DCRC) and secondary-ion mass spectroscopy (SIMS) measurements. The sharp satellite peaks of the DCRC measurements 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 of the DCRC spectra decrease dramatically. The negative direction of the entropy change obtained from the diffusion coefficients as a function of the reciprocal of the temperature after RTA indicates that the Hg diffusion for the annealed HgTe/CdTe superlattices is due to an interstitial mechanism. The SIMS profiles of the Cd and the Hg concentration near the annealed HgTe/CdTe superlattice interfaces show a nonlinear behaviour for the Hg, due to an interstitial diffusion mechanism for the Hg composition. As the thickness of the CdTe barrier decreases, the diffusion coefficient increases. These results indicate that nonlinear interdiffusion behaviour is dominant for HgTe/CdTe superlattices annealed at and that the Hg and the Cd can more easily intermix in HgTe/CdTe superlattices with thinner CdTe barrier layers due to the thermal treatment.