K. Pakuła

Lattice parameters of gallium nitride

Lattice parameters of gallium nitride were measured using high‐resolution x‐ray diffraction. The following samples were examined: (i) single crystals grown at pressure of about 15 kbar, (ii) homoepitaxial layers, (iii) heteroepitaxial layers (wurtzite structure) on silicon carbide, on sapphire, and on gallium arsenide, (iv) cubic gallium nitride layers on gallium arsenide. The differences between the samples are discussed in terms of their concentrations of free electrons and structural defects.

Luminescence and reflectivity in the exciton region of homoepitaxial GaN layers grown on GaN substrates

Abstract In this work we report results of photoluminescence (PL) and reflectivity measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapour deposition on GaN substrates. At low temperature (4.2K), very narrow (FWHM = 1.0meV) PL lines related to excitons bound to neutral acceptor (3.4666eV) and neutral donor (3.4719eV) were observed. The energies of free excitons from reflectivity and PL measurements were found to be: E A = 3.4780eV, E B = 3.4835eV and E C = 3.502eV.

Electron effective mass in hexagonal GaN

Far-infrared magneto-optical investigations of shallow donors in epitaxial GaN layers on sapphire were carried out by means of Fourier transform spectrometry up to 23 T. From the splitting of the donor p states in a magnetic field, the cyclotron effective mass for conduction electrons was found to be m*=0.222 m0. A precise determination of the mass was made possible by the high quality of the spectra and by taking into account high magnetic field data above 12 T.

Structural and Optical Properties of Homoepitaxial GaN Layers

The review of structural and optical properties of homoepitaxial layers grown by MOVCD on single crystals GaN substrates is presented. The TEM technique is used to characterise the structural properties of epi-layers. It is found that the structural properties of GaN homoepitaxial layers are determined by the polarity of the substrate surface on which the growth takes place. It is shown that threading dislocations are present only in the layers grown on the [0001] “smooth” surface. On the other hand the layers grown on the [0001] “rough” surface are free from vertical defects. The characteristic feature of the growth on the “rough” surface are pinholes. The optical properties of homoepitaxial layers are predominantly determined by the growth polarity as well. It is shown also that the reflectivity measurement is the most precise way to determine the exciton energies and that emissions due to free excitons are strongly affected by polariton effects.

Localized vibrational modes of carbon-hydrogen complexes in GaN

Localized vibrational modes of carbon-hydrogen complexes in metalorganic chemical vapor deposition grown GaN on sapphire were studied using a Fourier-transform infrared spectroscopy technique. Three distinctive localized vibrational modes were observed around 2850, 2922, and 2959 cm−1 for undoped, Si-, and Mg-doped samples. These peaks are related to CH, CH2, and CH3 defect complexes, respectively. However, the localized vibrational modes were not observed in some undoped samples, which is indicative of high quality grown epitaxial layers. It is also observed that the frequencies and intensities of the localized vibrational modes are sample dependent.

Impurity-Related Luminescence of Homoepitaxial GaN Studied with High Magnetic Fields

Recent magnetoluminescence results obtained for homoepitaxial GaN layers are presented. The neutral impurity-bound excitons and donor–acceptor pair emission lines have been studied in magnetic fields up to 27 T. Low-temperature luminescence spectra have been measured with the magnetic field parallel and perpendicular to the hexagonal c-axis of the GaN layers. Experimental results allowed us to evaluate diamagnetic shifts, effective g-factors of electrons and holes involved in neutral donor and neutral acceptor complexes as well as the electron–hole exchange constant for close donor–acceptor pairs. Both the fine structure of the neutral acceptor-bound exciton emission and the specific properties of donor–acceptor pair spectra observed in magnetoluminescence experiments are tentatively attributed to the internal structure of the acceptor state.

Convergent Beam Electron Diffraction and Transmission Electron Microscopy Study of Interfacial Defects in Gallium Nitride Homoepitaxial Films

Convergent Beam Electron Diffraction and Transmission Electron Microscopy Study of Interfacial Defects in Gallium Nitride Homoepitaxial Films

Effects of defect scattering on the photoluminescence of exciton-polaritons in n-GaN

Temperature dependent studies of the resonant and phonon-assisted radiative recombination of free excitons (FEs) in GaN are performed and are analyzed within the polariton concept. A strong impurity scattering of exciton-polaritons is proposed to be responsible for the revealed unusual behavior of the free A exciton in GaN, i.e. an enhanced intensity of the resonant FE emission in comparison with its longitudinal optical (LO) phonon replicas at low temperatures, as well as the narrow line shape of the 1-LO assisted transitions.

Magnetospectroscopy of free exciton in CdFeSe semimagnetic semiconductor

The magnetospectroscopy data of free exciton in hexagonal Cd1−xFexSe are presented for different magnetic field orientation. Combining exciton splitting data with magnetic susceptibility data we estimate p-d exchange integral for CdFeSe valence band.

Homoepitaxial layers of gallium nitride grown by metalorganic vapour phase epitaxy

Gallium nitride epitaxial layers were grown by metalorganic vapour phase epitaxy (MOVPE) on bulk GaN crystals. These substrates were grown at high temperature (about 1800 K) and high pressure (about 15 kbar). MOVPE was carried out in a vertical growth chamber at atmospheric pressure. was used as the nitrogen precursor. The layers were examined using x-ray diffraction and optical photoluminescence. It was found that most of the layers exhibit smaller lattice constants than the substrates, which indicates a smaller concentration of point defects and free electrons. The photoluminescence spectra consist of sharp (down to 1.2 meV) near-band-edge peaks attributed to the excitons bound to neutral donors and broad yellow peaks centred at 2.3 eV.