Aberration-corrected scanning transmission electron microscopy images of the In0.15Ga0.85N active region of a blue light-emitting diode, acquired at ~0.1% of the electron dose known to cause electron beam damage, show no lateral compositional fluctuations, but do exhibit one to four atomic plane steps in the active layer’s upper boundary. The area imaged was measured to be 2.9 nm thick using position averaged convergent beam electron diffraction, ensuring the sample was thin enough to capture compositional variation if it was present. A focused ion beam prepared sample with a very large thin area provides the possibility to directly observe large fluctuations in the active layer thickness that constrict the active layer at an average lateral length scale of 430 nm.

ZnO has recently attracted a great deal of attention as a material for transparent contacts in light emitters and adsorbers. ZnO films heavily doped with Ga (carrier concentration in the range of 1020 - 1021 cm-3) were grown on a-plane sapphire substrates by RF plasma-assisted molecular beam epitaxy. Oxygen pressure during growth (i.e. metal (Zn+Ga)–to–oxygen ratio) was found to have a crucial effect on structural, electrical, and optical properties of the ZnO:Ga films. As-grown layers prepared under metal-rich conditions exhibited resistivities below 3×10-4 Ω-cm and an optical transparency exceeding 90% in the visible spectral range. In contrast, the films grown under the oxygen-rich conditions required thermal activation and showed inferior structural, electrical, and optical characteristics even after annealing.

For light emitting diodes (LEDs) to be used for general lighting, high efficiencies would need to be retained at high injection levels to meet the intensity and efficiency requirements. In this regard, it is imperative to overcome the observed drop in LED efficiency at high injection levels beyond that would be expected from junction temperature. The suggested genesis of efficiency degradation includes electron overflow or spillover, also suggested to be aided by polarization induced electric field, Auger recombination, current crowding, and elevated junction temperature. Setting the junction temperature aside, the degree to which or even whether each of these mechanisms plays a role is still under debate. We have undertaken a series of experiments to isolate, whenever possible, the aforementioned processes in an effort to determine the causes of efficiency loss at high injection levels. By using 1μs pulsed electrical injection with 0.1% duty cycle, we were able to minimize the effect of the junction temperature. By changing the design of the multiple quantum well region as well as by employing or not employing electron blocking layers, we demonstrated the important role that electron overflow plays on efficiency. Furthermore, by also exploring the same on non polar surfaces and observing any lack of dispersion in terms of the effect of the electron blocking layer we can conclude that the polarization induced field does not seem to play a major role. LEDs on non polar surface with no notable efficiency degradation, up to current densities of about 2250 Acm-2 used for measurements, have been obtained which seems to imply that Auger recombination up to these injection levels is not of major importance, at least in the structures investigated. The effect of current crowding on efficiency droop was investigated by comparing semitransparent Ni/Au p-contacts and transparent conducting oxide contacts (Ga-doped ZnO). Because the latter showed notably reduced efficiency degradation at high injection levels, we can conclude that current crowding plays a role as well.

We observed strong shifts of the blue and yellow luminescence bands with variation of excitation intensity in ZnO films grown on sapphire by MBE using hydrogen peroxide as a source of reactive oxygen. The blue band, having a maximum in the range from 2.85 to 3.15 eV in different samples and different excitation intensities at 10 K, is attributed to diagonal transitions from the conduction band (or shallow donors) to the valence band in realm of potential fluctuations caused by random distribution of charged point defects in a compensated semiconductor. The yellow band is related to an unidentified deep acceptor.

The effect of growth conditions on the luminescence properties of ZnO films grown on a-Al2O3/GaN(0001)/c-Al2O3 templates by plasma-assisted molecular beam epitaxy has been investigated. We observed that the deflecting of the ions produced by the RF oxygen plasma away from substrate results in improved excitonic emission and modification of the defect-related PL spectrum. The intensity of the near-band-edge lines in photoluminescence spectra from the layers grown with the ion deflector deflection was found to increase as compared to the controls grown without the ion deflector. The yellow-green spectral range was dominated by different defect bands in the films grown with and without ion deflection. The effect of RF power on peak positions of the defect band was studied for the films grown without ion deflection.

Pb(ZrxTi1−x)O3 (PZT) films were grown by molecular beam epitaxy using hydrogen peroxide as a source of reactive oxygen. Phase composition as well as structural and electrical properties of the films were studied by x-ray diffractometry, scanning and transmission electron microscopy, conductive atomic force microscopy, and electrical (I-V and polarization-field) measurements. The hydrogen peroxide pressure was found to control the phase composition of the films. Excess peroxide leads to PbO inclusions in PZT layers, whereas deficiency results in the TiO2 or the ZrO2 phase. The second-phase inclusions can be responsible for high leakage current in the films. Precise control over the peroxide pressure is imperative for single-phase PZT films with good ferroelectric properties.

The growth of Pb(ZrxTi1-x)O3 (PZT) films by molecular beam epitaxy was demonstrated. Single-crystal, single-phase PZT films were grown on (001) SrTiO3 substrates at a growth temperature of 600°C. In situ monitoring of the growth process by reflection high-energy electron diffraction revealed two dimensional growth for the PZT constituent ternaries, namely, PbTiO3 and PbZrO3, and three-dimensional growth for PZT films of intermediate compositions. Layer-by-layer growth of PZT films, however, was achieved by using a PbTiO3 buffer layer between the SrTiO3 substrate and PZT films. Optical properties of the films of the end ternaries were investigated by spectroscopic ellipsometry. Refractive index at 633 nm was found to be 2.66 for PbTiO3 and 2.40 for PbZrO3. Band gap energies of PbTiO3 and PbZrO3 were determined as 3.81 and 3.86 eV, in good agreement with theoretically calculated values. The P-E hysteresis loop of a 70-nm-thick PZT film was well saturated and had a square shape. The remanent polarization and the coercive field were 83 μC/cm2 and 77 kV/cm, respectively, which are respectable.

Epitaxial growth of ZrO2 has been achieved on MOCVD grown GaN template by oxides MBE using a reactive H2O2 oxygen source. Using the low temperature buffer followed by high temperature in-situ annealing and high-temperature growth, monoclinic, (100)-oriented ZrO2 thin film with decent structural quality and smooth surface morphology was achieved. The full width at half maximum of ZrO2 (100) rocking curve is 0.4 arc degree and the rms roughness for a 5μm by 5 μm AFM scan is 4.1Å. The use of epitaxially grown ZrO2 in the AlGaN/GaN HFET structure as a gate dielectric layer have resulted in the increase of the saturation-current density and pinch-off voltage as well as showed near symmetrical gate-drain I-V behavior.

ZnO nanorods were grown by catalyst-assisted vapor phase transport on Si(001), GaN(0001)/c-Al2O3, and bulk ZnO(0001) substrates. Morphology studies showed that ZnO nanorods grew mostly perpendicularly to the GaN substrate surface, whereas a more random directional distribution was found for nanorods on Si. Optical properties of fabricated nanorods were studied by steady-state photoluminescence and time-resolved photoluminescence. Stimulated emission was observed from ZnO nanorods on GaN substrates. Raman spectroscopy revealed biaxial strain in the nanorod samples grown on Si. Conductive atomic force microscopy was applied to study I-V spectra of individual nanorods.

Lead titanate (PbTiO3), a ferroelectric material with perovskite structure, has received a great deal of attention owing to a unique combination of its piezoelectric, pyroelectric, dielectric, electo- and acousto-optic properties [1–3]. PbTiO3 is a very attractive material for the use in a wide variety of fields, including ultrasonic sensors, infrared detectors, electro-optic modulators, and ferroelectric random access memories. To harness the intrinsic properties of PbTiO3 for device applications, however, high-quality epitaxial films are required. PbTiO3 thin films have been prepared by various methods such as chemical vapor deposition, rf magnetron sputtering, pulsed laser deposition, hydrothermal method, and sol-gel technique. Surprisingly, molecular beam epitaxy (MBE), with its precise control over composition, has not been widely applied, except of a few reports on MBE growth with the use of ozone as an oxidizing agent [4]. In the present work, high-quality single-crystal PbTiO3 layers were grown on (001) SrTiO3 substrates by MBE with the use of hydrogen peroxide as an oxidant [5]. Phase composition as well as structural and optical properties of the films were examined as a function of growth parameters by high-resolution x-ray diffractometry, spectroscopic ellipsometry, and photoluminescence. It was found that single-phase PbTiO3 films grew epitaxially at substrate temperatures above 600°C, whereas layers grown at lower temperature contained lead oxide inclusions. All the PbTiO3 layers were c-axis oriented with the epitaxial relationship PbTiO3(100)//SrTiO3(100) and PbTiO3[001]//SrTiO3[001]. No evidence of a-domains was found. Full width at half maximum of (001) rocking curves for 50–60 nm thick PbTiO3 layers are as low as 6–8 arcmin, indicating their good crystal quality. Pseudodielectric function of PbTiO3 was measured using variable angle spectroscopic ellipsometry at room temperature. Refractive index was found to be 2.605 at 633 nm (1.96 eV), which is consistent with the literature data. With the help of the standard critical point (SCP) lineshape analysis the band gap energy was calculated to be (3.778±0.005) eV.

Epitaxial growth of PbO, TiO2 and ZrO2 has been achieved on MOCVD grown GaN template using oxides MBE with a reactive H2O2 oxygen source. In situ RHEED was used to monitor the growth in-situ. AFM was used to characterize the surface morphology of the thin PbO and ZrO2, which show streaky, 2-D RHEED patterns. XRD pattern indicates that the growth orientation of these oxides are PbO [111]//GaN [0002], ZrO2[100]//GaN [0002] and TiO2[200]//GaN[0002].