Tom Oder

III-Nitride Blue and Ultraviolet Photonic Crystal Light Emitting Diodes

We present results on enhancement of 460 nm blue and 340 nm UV optical power output in III-nitride light emitting diodes (LEDs) using photonic crystals (PCs) under current injection. Triangular arrays of the PCs with diameter/periodicity of 300/700 nm were patterned using electron-beam lithography and inductively coupled plasma dry etching. The total power at 20 mA of 300×300 μm2 unpackaged LED chips revealed an increase by 63% and 95% for the blue and UV LEDs, respectively, as a result of the PC formation. Possible ways for further improving enhancement of light extraction using PCs are discussed.

III-nitride photonic crystals

We report the achievement of nanofabrication and characterization of a triangular lattice array of photonic crystals (PCs) with diameter/periodicity as small as 100/180 nm on an InGaN/GaN multiple quantum well using electron-beam lithography and inductively coupled plasma dry etching. Optical measurements of the PCs performed using near-field scanning optical microscopy showed a 60° periodic variation with the angle between the propagation direction of emission light and the PCs lattice. An unprecedented maximum enhancement factor of 20 was obtained for the emission light intensity at wavelengths as short as 475 nm at room temperature with emission light parallel to the Γ–K direction of the PCs lattice. The implications of these results to nitride-based optoelectronic devices, particularly in improving the light extraction efficiency in light-emitting diodes both for blue/green as well as UV emitters, are discussed.

Optical and electrical properties of Mg-doped p-type AlxGa1−xN

Mg-doped AlxGa1−xN epilayers with Al content up to 0.27 were grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD). p-type conduction in these alloys has been achieved, as confirmed by variable temperature Hall-effect measurements. Emission lines of band-to-impurity transitions of free electrons with neutral Mg acceptors as well as localized excitons have been observed in the p-type AlxGa1−xN alloys. The Mg acceptor activation energies EA were deduced from photoluminescence spectra and were found to increase with Al content and agreed very well with those obtained by Hall measurements. From the measured activation energy as a function of the Al content, EA versus x the resistivity of AlxGa1−xN alloys with high Al contents can be deduced. Our results thus indicated that alternative methods for acceptor activation in AlGaN alloys with high Al contents must be developed. Our results have also shown that PL measurements provide direct means of obtaining EA, especially where this canno...

Nitride microlens arrays for blue and ultraviolet wavelength applications

Nitride microlens arrays with sizes as small as 10 μm in diameter have been fabricated on GaN and AlN epilayers using the method of photoresist reflow and inductively coupled plasma dry etching. The focal lengths of the microlenses varied from 7–30 μm as determined by theoretical fitting as well as by the near-field scanning optical microscopy measurement. Scanning electron and atomic force microscopies were used to obtain the surface profile of the microlenses which were found to match very well with hemispherical fitting and a surface roughness value around 1 nm was obtained. Nitride microlens arrays would be naturally chosen for green/blue to deep ultraviolet wavelength applications. In addition, nitride microlenses offer the possibility of integrating nitride-based microsize photonic devices as well as of coupling light into, out of, and between arrays of III-nitride emitters for other applications, such as spatially resolved fluorescence spectroscopy studies of biological and medical systems and optical links, thereby further expanding the applications of III nitrides.

Photoresponsivity of ultraviolet detectors based on InxAlyGa1−x−yN quaternary alloys

We describe the growth, fabrication, and characterization of an ultraviolet (UV) photoconductive detector based on InxAlyGa1−x−yN quaternary alloy that is lattice matched to GaN. The detector consisted of 0.1 μm InxAlyGa1−x−yN alloy grown on 0.5–1.0 μm GaN epilayer by metalorganic chemical vapor deposition. With varying indium concentration, the cut-off wavelength of the InxAlyGa1−x−yN detectors could be varied to the deep UV range. The most important and intriguing result is that the responsivity of the InxAlyGa1−x−yN quaternary alloy exceeded that of AlGaN alloy of a comparable cutoff wavelength by a factor of five. This makes the nitride quaternary alloy very important material for solar blind UV detectors applications particularly in the deep UV range where Al rich AlGaN alloys have problems with low quantum efficiency and cracks due in part to lattice mismatch with GaN. The advantages of InxAlyGa1−x−yN quaternary over AlGaN ternary alloys for UV detector applications are also discussed.

Fabrication and optical studies of AlGaN/GaN quantum-well waveguides

We report the successful fabrication and optical study of submicron waveguide structures based on AlGaN/GaN multiple-quantum wells (MQWs). The MQW structures were grown by metalorganic chemical vapor deposition on sapphire substrates and the waveguides were fabricated by electron-beam lithography and inductively coupled plasma dry etching. The waveguides were patterned with a fixed width of 0.5 μm but with orientations varying from −30° to 60° relative to the a axis of GaN. Optical emission from these structures was studied by photoluminescence spectroscopy. The peak position and linewidth of the emission peak were found to vary systematically with the orientations of the waveguides and followed the sixfold symmetry of a wurtzite structure. This is most likely related to the anisotropy of the exciton/carrier diffusion coefficient along the different crystal orientations in the quasione-dimensional case. The implication from the results is that in proper designs of photonic and electronic devices where sub...

Thermally stable Schottky contacts on n-type GaN using ZrB2

The electrical properties and thermal stability of ZrB2 Schottky contacts deposited on n-type GaN have been studied. As-deposited contacts had a barrier height of 0.80eV, which decreased to 0.7eV after annealing at 300°C, and to 0.6eV after additional annealing at 400°C in nitrogen for 20min. However, the barrier height remained at about 0.6eV even when the diodes were annealed at 600°C for 20min. The Rutherford backscattering spectra of annealed contacts showed no reaction at the ZrB2∕GaN interface. These results make ZrB2∕GaN Schottky contacts attractive for high temperature device applications.

Propagation properties of light in AlGaN/GaN quantum-well waveguides

The dynamic properties of light propagation in AlGaN/GaN-based multiple-quantum-well waveguides have been investigated by time-resolved photoluminescence (PL) spectroscopy. The waveguides were patterned with a fixed width of 0.5 μm and length 500 μm using electron-beam lithography and inductively coupled plasma dry etching. Our results reveal a remarkable decrease in the PL intensity as well as an increase in time delay of the temporal response as the location of the laser excitation spot on the waveguide is varied. These results can be understood in terms of polariton propagation in the waveguides. From the time delay of the temporal response, it has been determined that the speed of generated polaritons, with energy corresponding to the well transitions in the waveguides, is approximately (1.26±0.16)×107 m/s. The implications of these results to waveguiding in optical devices based on the group III-nitride semiconductors are discussed.

III-nitride blue and UV photonic-crystal light-emitting diodes

We report on the successful nano-fabrication and characterization of III-nitride blue and ultraviolet (UV) photonic crystal light emitting diodes (PC-LEDs) using electron beam lithography and inductively coupled plasma dry etching. Triangular arrays of holes with different diameters/periodicities were etched on the LEDs. Optical measurements on the photonic crystals (PCs) performed using near-field scanning optical microscopy (NSOM) showed a 60° periodic variation with the angle between the propagation direction of emission light and the PCs lattice. Under optical pumping, an unprecedented enhancement factor of 20 in emission light intensity of wavelength 475 nm was achieved at room temperature with emission light parallel to the Γ-K direction of the PCs lattice. Guided by the optical pumping results, new design geometry of LEDs with PCs has been employed to optimize the light extraction. Enhancement in optical power of current injected blue and UV PC-LEDs over conventional LEDs is discussed. It was observed that the optical enhancement factor depends strongly on the PC lattice constant and hole size. The achievement of nitride photonic crystal emitters with enhanced light extraction efficiency is expected to benefit many new applications of III-nitrides including solid-state lighting for general illumination and photonic integrated circuits operating in the visible and UV spectral regions.

Elevated temperature dependent transport properties of phosphorus and arsenic doped zinc oxide thin films

Elevated temperature dependent Hall effect measurements were performed in a wide temperature range from 80 to 800 K to study transport properties of zinc oxide (ZnO) thin films heavily doped with phosphorus (P) and arsenic (As), and grown on sapphire substrates by RF magnetron sputtering. Double thermal activation processes in both P- and As-doped ZnO thin films with small activation energy of ∼0.04 eV and large activation energy of ∼0.8 eV were observed from variable temperature Hall effect measurements. The samples exhibited n-type conductivities throughout the temperature range. Based on photoluminescence measurements at 11 K and theoretical results, the large activation energy observed in the temperature dependent Hall effect measurement has been assigned to a deep donor level, which could be related to oxygen vacancy (VO) in the doped ZnO thin films.