A broadband GaN-based semiconductor saturable absorber mirror (SESAM) with a dielectric SiO2/Si3N4 distributed Bragg reflector (DBR) operating at wavelength around 415 nm was fabricated. Serious oscillation fringes due to the light interference were observed in the SESAM's reflectance spectrum. Such oscillation in reflectivity can impede the function of the saturable absorber. Simulations showed that by removing the sapphire substrate and thinning the GaN buffer layer, oscillation fringes could be significantly reduced. Experiments were carried out and the results agreed well with the simulation prediction.

Ultraviolet light-emitting diodes (UV LEDs) with AlxGa1−xN/AlyGa1−yN multiple quantum well (MQW) active regions, doped in the barriers with different Si doping level, show a sharp near band-edge emission (UV luminescence). Besides the near band-edge emission, some samples also show parasitic emissions with a broad peak centered at about 520 nm (green luminescence). The EL intensities of the UV emission line and the green emission line are studied. The UV luminescence peak intensity increases superlinearly with the injection current, following a power law with an exponent of about 2.0. In contrast, the green luminescence peak intensity increases linearly with the injection current, with an exponent of about 1.0. A theoretical model is proposed to explain the relationship between the peak intensities and the injection current. The results obtained from the model are in excellent agreement with the experimental results. The model provides a method to evaluate the dominant recombination process by measuring the exponent of the power-law dependence.

Sublimation growth of AlN was performed on Si-face, 8 ° off-axis 4H-SiC (0001) and 3.5 ° off-axis 6H-SiC (0001) seeds. AlN layers 500 - 900 μm thick and 20 mm in diameter were grown at 1830 °C by consecutive growths and continuous growth. The c-axis growth rate was approximately 8 - 18 μm/hr. On both the 8 ° and 3.5° off-axis SiC substrates, “step” features formed on the AlN surface with uniform terrace width and step density. The step heights and terrace widths increased as the AlN grew thicker. In addition, a single facet of 9 mm × 5 mm formed on the top of the layer grown on the 3.5 ° off-axis SiC. High resolution x-ray diffraction showed the AlN (00.2) 2θ value shifted from pure AlN toward SiC. Approximately 3 – 4 at% of SiC was detected at the surface of the AlN by XPS. Molten KOH/NaOH etching revealed that both samples had Al-polar surface with dislocation densities on the order of 106-107 cm−2. The cross-section etching showed the re-nucleation layer and void defects at the interfaces of the consecutive growths.

We discuss origin of ferromagnetism in dilute magnetic semiconductors (DMS) and propose accurate calculation method for Curie temperature from first-principles. For more realistic materials design, we simulate inhomogeneous impurity distribution due to the spinodal decomposition in DMS from first-principles. Combining these techniques, we discuss super-paramagnetism and blocking phenomena in DMS and propose fabrication method of DMS with high blocking temperature.

Different types of phase separation in thick InGaN layers were studied using photoluminescence (PL) and x-ray diffraction (XRD). InGaN films of 100 nm in thickness were grown on 2 μm GaN templates with an In molar fraction ranging from 0% to 20% by metal organic chemical phase deposition (MOCVD). It is shown that suppression of the phase separation in InGaN can be made possible by increasing the TMIn flow rate, decreasing the layer thickness and decreasing the growth rate. Based on the results, two types of phase separation, microscopic quantum dots and macroscopic domains, are proposed accordingly. The influence of the growth conditions on each type is summarized respectively in this paper.

Silicon and zinc were doped to cubic boron nitride (cBN) thin films by sputtering bulk dopant sources during the film deposition. Concentrations of these two elements showed different in-plane distributions, presumably caused by the difference of the sticking probability on the growing surface. Silicon impurity did not behave as an active dopant in cBN. However, zinc doping enhanced the electric conductivity from 10−8 to 10−2 (Ω−1cm−1) as the zinc concentration increased from undoped to approximately 2 %.

We report the growth and characterization of a new dilute nitride, InNAsSb/InAs, by solid source molecular beam epitaxy. Optimizing growth conditions for nitrogen incorporation has resulted in high-quality InNAsSb epilayers without any structural degradation, as confirmed by high-resolution x-ray diffraction. Optical properties were investigated by temperature dependent and excitation power dependent photoluminescence. We obtained mid-infrared luminescence around 4 mm at low temperature, which reveals strong carrier localization behavior at low temperature induced by nitrogen interacted with antimony. The band alignment of InNAsSb/InAs can be type-I and instead of conventional type-II, InAsSb/InAs, and a conduction band offset, Ec,of ∼102meV was obtained.

Cubic InN films were grown on top of a c-GaN buffer layer by rf-plasma assisted MBE at different growth temperatures. X-Ray diffraction investigations show that the c-InN layers consist of a nearly phase-pure zinc blende (cubic) structure with a small fraction of the wurtzite (hexagonal) phase grown on the (111) facets of the cubic layer. The content of hexagonal inclusions is decreasing with decreasing growth temperature. The full-width at half-maximum (FWHM) of c-InN (002) rocking curve is about 50 arcmin. Low temperature photoluminescence measurements reveal a band gap of about 0.61eV for cubic InN.

Growth of AlN on 4H-SiC (11-20) substrates by plasma-assisted molecular-beam epitaxy is presented. Very high-quality AlN can be grown under a slightly Al-rich condition. Transmission electron microscopy revealed that the AlN layer has 4H crystalline structure with high-phase purity (stacking-fault density is 5×106cm−1) and the density of threading dislocations is as small as 8×107cm−2.

Aluminum nitride (AlN) films are being investigated for piezoelectric and high temperature applications, but the piezoelectric response is still much lower than that of more common piezoelectric materials such as lead zirconate titanate or zinc oxide. A method of maximizing the piezoelectric response of aluminum nitride has been explored by depositing stack structures composed of aluminum nitride and platinum. These stack structures were created by depositing a thin, ∼50nm, metal layer in between thicker, ∼150-350nm, layers of the piezoelectric film. Platinum was chosen as the metal interlayer due to the tendency of AlN to become highly c-oriented when deposited on Pt. An electric field was applied across the structure and displacements were measured using a Laser Doppler Vibrometer. A maximum piezoelectric coefficient d33 was found to be over two times larger than the theoretical value for AlN (3.9pm/V). However, some of the stack structures were found to be conductive when measuring the displacement. I-V measurements as well as Fowler-Nordheim theory and plots were applied to investigate tunneling due to high electric fields in the structures.

We studied spin-orbit coupling in wurtzite AlxGa1−xN/AlN/GaN heterostructures with different Al concentrations using weak antilocalization measurements at 1.6 K. Using the persistent photoconductivity effect we change the carrier density in controllable manner. We find that the electron spin splitting energies does not scale linearly with the Fermi wavevector at high carrier densities. From this deviation, for the first time, we are able to extract the cubic spin-orbit parameter for this material system.

In this work, electro-luminescence (EL) of a AlGaN p-i-n diode have been investigated in both avalanche and injection modes. The active i-region of the diode consists of Al0.1Ga0.9N/Al0.15Ga0.85N MQWs. Strong interband luminescence from the Al0.1Ga0.9N active layers was observed when operating the device in both avalanche and injection modes. The threshold voltage for avalanche breakdown is as low as 9 V. This indicates that the impact ionization coefficient of electrons is greatly enhanced in these Al0.1Ga0.9N/Al0.15Ga0.85N MQWs comparing to AlGaN bulk materials. Polarization-induced electric fields in the Al0.1Ga0.9N well layers are believed to be responsible for the enhancement of the ionization coefficient. In a control sample that has higher defect density, the electroluminescence was dominated by long-wavelength emissions, which results from impact ionizations of the defect levels.

Growth and polarity control of GaN and AlN on carbon-face SiC (C-SiC) by metalorganic vapor phase epitaxy (MOVPE) are reported. The polarities of GaN and AlN layers were found to be strongly dependent on the pre-growth treatment of C-SiC substrates. A pre-flow of trimethyaluminum (TMAl) or a very low NH3/TMAl ratio results in Al(Ga)-polarity layers on C-SiC. Otherwise, N-polarity layers resulted. The polarities of AlN and GaN layers were conveniently determined by their etching rate in KOH or H3PO4, a method reported earlier. We suggest that the Al adatoms, which have a high sticking coefficient on SiC, form several Al adlayers on C-SiC and change the incorporation sequence of Ga(Al) and N leading to metal polarity surface. In addition, the hexagonal pyramids, typical on N-polarity GaN surface, are absent on N-polarity GaN on off-axis C-SiC owing to high density of terraces on off-axis C-SiC. The properties of GaN layers grown on C-SiC are studied by X-ray diffraction.

Exciton-Exciton coupling in hybrid persovskites conjugated to GaN Semiconductors system has been studied. Excitons are strongly coupled in (C6H5C2H4NH3)2PbI4[bis(phenethyl-ammonium)tetraiodoplumbatel] (PEPI) is a hybrid inorganic-organic layered, with a perovskite structure quantum well (QW). The inorganic PbI4 monolayer is sandwiched between organic layers. The interaction between electron and hole forming the bound exciton is significantly stronger due to dielectric confinement. The lowest exciton binding energy is 220 meV, which is 20 times in comparison to GaAs. Gallium nitride (GaN) semiconductor has defect bound excitons with absorption band, which overlaps with the emission spectra of the PEPI system. This facilitates resonant energy transfer (RET) from the GaN defect bound exciton states to the excitons confined in the PEPI layer. We investigated the interaction by photoluminescence (PL) and found that 1s exciton in the PEPI layer strongly couples with the GaN defect level exciton, which is pronounced at lower temperature (< ∼ 100 K).

Single crystalline AlN epitaxial layers have been grown on and (0001) sapphire and 6H-SiC substrates by MOVPE technique at high temperatures in the range of 1340-1500°C. The structural qualities of the high temperature grown AlN layers were found to be good as evidenced by X-ray diffraction analyses results. By transmission electron microscopic analysis, dislocation densities of the layers were found to be 6.2 × 107 cm−2 or lower and the formation of dislocation loops was confirmed. High temperature bridge layers of AlN and AlxGa1−xN layers were grown on linear-groove patterned sapphire based AlN templates and 6H-SiC substrates. AlxGa1−xN bridge layers exhibited different growth behaviours depending on the direction of groove patterns on the sub-strates.

We compared the photoluminescence (PL) mappings of epitaxial wafers for light emitting diodes (LEDs) by using a He-Cd laser (325 nm line) and a laser diode (LD) with peak wavelength of 405 nm as excitation sources and the electroluminescence (EL) mappings of the same wafers. The samples were epitaxial wafers for blue and green InGaN-LEDs obtained in commercial. The wafers were fabricated into LEDs with a Ni/Au transparent p-type electrode and a Ti/Ni n-type electrode after the PL mapping measurements. The He-Cd laser performed the band to band excitation of (Al)GaN cladding and contact layers (non-selective excitation). Because the photo-excited carriers at the cladding and contact layers diffused into the multi-quantum wells (MQWs) and contributed the PL emission by radiative recombination in the MQWs, the PL mapping under the influence of the (Al)GaN cladding and contact layers was obtained. On the other hand, the LD (405 nm) enable us to obtain the PL mapping under selective excitation of the MQWs without the influence of the cladding and contact layers. The PL mapping measurements were carried out at room temperature (RT) at the excitation power density of 310 W/cm2 under non-selective excitation (by the He-Cd laser) and at that of 11.5 W/cm2 under selective excitation (by the LD). The EL mapping was measured at a forward current of 20 mA at RT. The area of the wafer with high EL intensity was coincident with the area with the high PL intensity under selective excitation. Therefore, the PL mapping measurement under selective excitation of MQWs is recommended to characterize the epitaxial wafers and to estimate the device performance of InGaN-LEDs.

Nanoparticle-loaded encapsulants provide unique optical and material properties for the enhancement of light extraction efficiency in light-emitting diodes (LEDs). We report on the uniform dispersion of TiO2 nanoparticles with average diameter of 40 nm in epoxy, and the demonstration of a refractive index (n)of 1.68 at 400 nm wavelength, higher than that of pure epoxy (n = 1.53). It is found that proper chemical surfactants and nanoparticle preparation are critical to eliminate agglomeration of nanoparticles. Theoretical analysis of optical scattering in nanoparticle-loaded encapsulation materials reveals that although the size and loading factor of nanoparticles greatly influence scattering, specular transparency of the encapsulant film occurs if the thicknesses of the films are kept below the optical scattering length. Furthermore, the encapsulants benefit from an optimized scattering coefficient as demonstrated by three-dimensional ray-tracing simulations showing light extraction efficiency enhancements greater than 50%.

We report a two-step growth method to obtain uniformly coalesced epitaxial lateral overgrown a-plane GaN by metalorganic chemical vapor deposition (MOCVD). By obtaining a large wing height to width aspect ratio in the first step followed by enhanced lateral growth in the second step via controlling the growth temperature, we reduced the tilt angle between the advancing Ga-polar and N-polar wings for improved properties. Transmission electron microscopy (TEM) showed that the threading dislocation density in the wing area was 1.0×108cm−2, more than two orders of magnitude lower than that in the window area (4.2×1010cm−2). However, a high density of basal stacking faults, 1.2×104cm−1, was still observed in the wing area. Near field scanning optical microscopy (NSOM) at room temperature revealed that the luminescence was mainly from the wing regions with very little contribution from the windows and meeting fronts. These observations suggest that due to significant reduction of threading dislocations radiative recombination is enhanced in the wings.

Two sets of AlN/AlxGa1−xN quantum wells (QW) have been grown by metalorganic chemical vapor deposition (MOCVD). The first set consists of five samples of AlN/AlxGa1−xN QWs with (x ∼ 0.65) with well width, Lw, varying from 1 to 3 nm. The second set consists of four samples of AlN/AlxGa1−xN with (Lw = 1.5 nm) with Al composition, x, varying from 0.70 to 0.85. Low temperature photoluminescence (PL) spectroscopy has been employed to study the Lw dependence of the PL spectral peak position, emission efficiency, and line width. Our results have shown that these AlN/AlGaN QW structures exhibit polarization fields of ∼ 4 MV/cm. Due to effects of quantum confinement and polarization fields, AlN/AlGaN QWs with Lw between 2 and 2.5 nm exhibit the highest quantum efficiency. The dependence of the emission linewidth on Lw yielded a linear relationship. The implications of our results on deep ultraviolet (UV) optoelectronic device applications are also discussed.

Light emitting diodes (LEDs) are normally operated at high current levels resulting in substantial junction heating. However, the junction temperature cannot be measured directly. In the study reported here, it was attempted to estimate junction temperature in LEDs with peak emission 400 nm. Temperature was estimated from current-voltage (I-V) measurements as a function temperature and peak-shift of optical emission spectra with increasing temperature. For a diode operated at a current-level of 100 mA, a temperature of 100 °C was estimate from current voltage measurement and 160 °C from the peak-shift in the optical emission spectra. The difference in temperature estimated using the two different technique is not understood at this point.