M. Moldovan

Controlled oxygen doping of GaN using plasma assisted molecular-beam epitaxy

High-quality (0001) and (0001)-GaN films were grown by plasma-assisted molecular-beam epitaxy to study the dependence of oxygen incorporation on polarity and oxygen partial pressure. Oxygen incorporates at a rate ten times faster on nitrogen-polar GaN than on the Ga polarity. Oxygen doping is controllable, reproducible, and produces low compensation material up to concentrations of at least 1018 cm−3 with higher levels showing significant compensation. Layers containing oxygen at levels above 1022 cm−3 exhibit severe cracking while oxygen concentrations less than 1021 cm−3 do not introduce significant strain. The oxygen incorporation rate has a weak dependence on Ga overpressure during Ga-stable growth but dramatically increases for conditions approaching N-stable growth.

Observation of singly ionized selenium vacancies in ZnSe grown by molecular beam epitaxy

Electron paramagnetic resonance (EPR) has been used to investigate singly ionized selenium vacancy VSe+ centers in ZnSe epilayers grown by molecular beam epitaxy (MBE). The study included undoped and nitrogen-doped films. Spectra taken at 8 K and 9.45 GHz, as the magnetic field was rotated in the plane from [100] to [010], showed an isotropic signal at g=2.0027±0.0004 with a linewidth of 5.8 G. In the two samples where this signal was observed, estimates of concentration were approximately 1.1×1017 and 6.3×1017 cm−3. The appearance of the EPR signal correlated with an increase in the Zn/Se beam equivalent pressure ratio (during growth) in undoped films and with an increase in the nitrogen concentration in doped films. We conclude that the singly ionized selenium vacancy may be a dominant point defect in many MBE-grown ZnSe layers and that these defects may play a role in the compensation mechanisms in heavily nitrogen-doped ZnSe thin films.

Optical and EPR characterization of point defects in bismuth-doped CdWO4 crystals

Abstract We suggest that the 550-nm “yellow” emission in CdWO4 is due to Bi3+ ions substituting for Cd2+ ions. The absorption band corresponding to this emission has a peak near 350 nm. This yellow emission was only observed in crystals that contained bismuth impurities. Electron paramagnetic resonace has identified two new centers in X-ray irradiated bismuth-doped CdWO4. One center is electron-like and is suggested to be a Bi2+ ion located on a Cd2+ site. The other center is assigned to a hole trapped on an oxygen ion adjacent to a Nb5+ ion substituting for a W6+ ion.

Compensating defects in heavily nitrogen-doped zinc selenide: A photoluminescence study

Photoluminescence (PL) from a heavily nitrogen-doped ZnSe epilayer grown by molecular beam epitaxy was studied as a function of excitation wavelength, power density, and temperature. Also, the time decay of the PL emission was measured. Detailed analysis of the PL data indicates that the deep broad emission is composed of three distinct recombination processes, two are dominant at low power and a third can be detected at higher excitation power. These three bands are labeled NI, NII, and NIII with corresponding peak energies at 2.54, ∼2.58, and 2.65 eV. The NI band is accompanied by phonon replicas of energy 69±3 meV. The behaviors of the NI, NII, and NIII bands are consistent with intracenter recombination, donor–acceptor pair recombination, and electron–acceptor recombination, respectively.

Broad-band photoluminescence from ZnGeP2

The near-infrared photoluminescence from single crystals of bulk ZnGeP2 was studied as a function of excitation power, excitation wavelength, sample temperature, and polarization. The nature of this broad-band luminescence from large single crystals grown for nonlinear optical applications is established. Two distinct bands with quite different polarization, power, and temperature behaviors were resolved. At 5 K, these broad bands have peaks in intensity near 1.58 and 1.36 eV. The 1.58 eV band is partially polarized perpendicular to the crystal’s c axis, has a relatively small thermal quenching activation energy (45 meV), and excitation spectra show a resonance in intensity associated with a shallow level approximately 90 meV below the minimum conduction band. The 1.36 eV band is partially polarized parallel to c, has a much larger quenching activation energy (220 meV), and its excitation spectrum includes two weak resonances corresponding to the A′ and B′ n=1 excitons. The high-energy band was enhanced i...

Photoluminescence excitation study of nitrogen-doped zinc selenide epilayers

Photoluminescence excitation (PLE) spectroscopy at liquid-helium temperature is used to study six ZnSe:N epilayers grown by molecular beam epitaxy. These samples represent nitrogen-doping levels ranging from less than 1017 to 3×1019 cm−3. The luminescence emission bands from the heavily doped samples exhibit peak energies varying from 2.45 to 2.61 eV. The energy range over which the PLE signal intensity decays and the energy difference between the onset of this decay and the PL peak energy are essentially the same for all the heavy-doped samples. A model is proposed to explain the PL and PLE results for a semiconductor in the presence of potential fluctuations.

Investigation of donor-acceptor pair luminescence from ZnSe:N epilayers

Low-temperature photoluminescence (PL) due to donor-acceptor pair recombination was monitored in a series of nitrogen-doped ZnSe epilayers grown by molecular beam epitaxy on GaAs substrates. Emission energies were measured for incident power densities ranging from 10−4 to 10 W/cm2 in samples having different nitrogen doping levels. An ionization energy of 50 meV for the “deep” donor in a lightly doped ZnSe:N sample is determined using power dependence data. Heavily nitrogen-doped samples (⩾8×1018 cm−3) provided evidence for a second deeper donor with an ionization energy greater than 100 meV. In addition, we show the importance of accounting for interference effects when identifying the emission peaks in the PL spectra from heavily doped ZnSe:N.

A Comparison of Magnesium and Beryllium Acceptors in GaN Grown by rf-Plasma Assisted Molecular Beam Epitaxy

Step-doped structures of both magnesium and beryllium were grown in GaN and analyzed using secondary ion mass spectrometry. Dopant incorporation was studied as a function of substrate temperature and dopant flux for Ga-polarity and N-polarity GaN. Incorporation is different for each polarity, with Mg incorporating by up to a factor of 20 times more (30 times more with atomic hydrogen) on the Ga-face, while Be incorporates more readily on the N-face. The effect of atomic hydrogen on the incorporation kinetics of both Mg and Be is also discussed. Mg and Be both undergo surface segregation during growth. Photoluminescence measurements suggest that Be is a p-type dopant with an optical activation energy of approximately 100 meV.