GaN layers grown on sapphire substrates were characterized using high resolution optical reflectivity and absorption spectroscopy in the region of ground and excited exciton states. The main exciton parameters are deduced from calculations of reflectivity contours for A and B exciton S-states. The parameters of the Γ5 state of the A-exciton as well as those of the Γ5 and Γ1 states of the B-exciton are determined from a comparative analysis of reflectivity and absorption spectra in thin layers. The influence of strains inherent to MOCVD-grown GaN layers on the exciton parameters including effective masses and longitudinal-transverse splitting is discussed. Electron transitions from the three (Γ9, Γ7, Γ7) upper valence bands to the second Ec2 conduction band of Γ3 symmetry were evidenced.

High-performance solar-blind AlGaN-based Schottky photodiodes have been demonstrated. The detectors were fabricated on MOCVD-grown AlGaN/GaN hetero-structures using a microwave-compatible fabrication process. Current-voltage, spectral responsivity, noise, and high-speed characteristics of the detectors were measured and analyzed. Dark currents lower than 1 pA at bias voltages as high as 30 V were obtained. True solar-blind detection was achieved with a cut-off wavelength lower than 266 nm. A peak device responsivity of 78 mA/W at 250 nm was measured under 15 V reverse bias. A visible rejection of more than 4 orders of magnitude was observed. The solar-blind photodiodes exhibited noise densities below the measurement setup noise floor of 3×10−29 A2/Hz around 10 KHz. High-speed measurements at the solar-blind wavelength of 267 nm resulted in 3-dB bandwidths as high as 870 MHz.

In this review paper we address one of the most rapidly developing new domains of semiconductor optics: light-matter coupling in semiconductor microcavities. Using the non-local dielectric response theory and transfer matrix technique, we show how two-dimensional confinement of a photonic mode coupled to an exciton resonance results in the appearance of two branches of exciton-polaritons, quasi-particles combining properties of photons and excitons. We obtain the dispersion relations of polaritons in microcavities and derive a condition for strong-weak coupling threshold. We show that being bosons, exciton-polaritons are subject to Bose-condensation which might result in emission of a coherent and monochromatic light in the strong coupling regime. A source of such coherent light is referred to as a polariton laser. We show that polariton lasers have theoretically no threshold and require essentially new basic physics as compared to conventional lasers described by Einstein theory. We give examples of model polariton laser structures expected to work at room temperature and overview the main difficulties on the way to producing these new opto-electronic devices.

We have calculated the thermal boundary resistance at the GaN/SiC, GaN/sapphire and GaN/AlN interfaces in the diffuse mismatch approximation. The obtained values were then used to examine the effect of the thermal boundary resistance on heat diffusion in AlGaN/GaN heterostructure field-effect transistors. The results show that the thermal boundary resistance at the device layer interfaces can strongly influence the temperature rise in the device channel.

Semi-insulating undoped GaN films were grown based on controlling the size of the nucleation sites through a special two-step growth method: First, 16 nm LT-GaN was annealed at 950 ° with a ramping time of 4 min, then the GaN was grown at this temperature for 1 min. Second, the growth temperature was increased to 1020° with a ramping time of 2 min and the GaN layer finally grown at 1020 ° for 40 min. The film grown by this sequence exhibited sheet resistance of up to 109 Ω/sq with mirror-like surface morphology. By slow ramping to 950° in the initial phase of growth, smaller grain sizes and higher nuclei densities were formed and the columnar growth mode along the c direction was dominant. The observation of higher resistance in two-step growth is believed due to the increased misorientation of nuclei when the growth proceeds during temperature ramping to 1020°. The fabricated saw filter on semi-insulating GaN exhibited a high velocity of 5342 m/s at center frequencies of 133.57 MHz and an electromechanical coupling coefficient(k2 ) of about 0.763 %, which was enhanced due to the improvement of surface morphology with high sheet resistance by the two- step ramping technique.

Non-polar a-plane GaN films doped with Si or Mg were grown by plasma-assisted molecular-beam epitaxy on r-plane sapphire substrates. The (110) orientation of the GaN epilayers was confirmed by x-ray diffraction. The layers were further characterized by atomic force microscopy, Hall effect, and photoluminescence measurements. The Mg-doped layers showed p-type conductivity, with a maximum hole concentration of 6×1017 cm−3 (μ = 2 cm2/Vs). Comparison with Mg-doping of N-polar c-plane GaN suggests the Mg sticking coefficient may be higher on the GaN (110) surface compared to the GaN (000) surface. The electron mobility obtained for a-plane GaN:Si (18 cm2/Vs for n = 1×1018 cm−3) was low compared to that of typical c-plane epilayers. The lower electron mobility is attributed to the higher density of structural defects in a-plane GaN.

Reactive Ion Etching (RIE) and Photo-Assisted RIE (PA-RIE) induced damage in GaN using simple Schottky structures and a BCl3/Cl2/N2gas mixture have been investigated. Schottky diode I-V characteristics following different RF powers and exposure times show significant changes caused by damage. This damage results in a reduction of the reverse breakdown voltage VB in n-type GaN and an increase in VB for p-type GaN. Our preliminary data on the PA-RIE process points to much reduced damage levels compared to conventional RIE. This result may be due to a change in surface chemistry or to a photo-enhanced diffusion of defects into the GaN layer, leaving a cleaner near-surface region.

We have designed, fabricated and tested resonant cavity enhanced visible-blind AlGaN-based Schottky photodiodes. The bottom mirror of the resonant cavity was formed with a 20 pair AlN/Al0.2Ga0.8N Bragg mirror. The devices were fabricated using a microwave compatible fabrication process. Au and indium-tin-oxide (ITO) thin films were used for Schottky contact formation. ITO and Au-Schottky devices exhibited resonant peaks with 0.153 A/W and 0.046 A/W responsivity values at 337 nm and 350 nm respectively. Temporal high-speed measurements at 357 nm resulted in fast pulse responses with pulse widths as short as 77 ps. The fastest UV detector had a 3-dB bandwidth of 780 MHz.