G. Cui

Catalytic growth of group III-nitride nanowires and nanostructures by metalorganic chemical vapor deposition

We report flexible synthesis of group III–nitride nanowires and nanostructures by metalorganic chemical vapor deposition (MOCVD) via a catalytic vapor-liquid-solid (VLS) growth mechanism. Supersaturation and surface stoichiometry strongly influence the stability of liquid droplets and growth selectivity. To facilitate and sustain the VLS growth, indium catalyst is introduced based on thermodynamic consideration. The employment of mesoporous molecular sieves (MCM-41) helps to prevent the coalescence of catalyst droplets and to promote nucleation statistics. Both GaN and AlN nanowires have been synthesized using MOCVD. Three-dimensional AlN∕GaN trunk-branch nanostructures are reported to illustrate the versatility of incorporating the VLS mechanism into MOCVD process.

Ultraviolet light-emitting diodes operating in the 340nm wavelength range and application to time-resolved fluorescence spectroscopy

We report on the development of UV light-emitting diodes in the 340nm wavelength range, based on quaternary AlGaInN quantum-well active media. Output powers up to 1mW from small area devices (<100μm diameter) directly off a planar chip have been achieved. The devices have been operated as subnanosecond pulsed sources to demonstrate their applicability to compact time-resolved fluorescence spectroscopy.

Investigation of Mg doping in high-Al content p-type AlxGa1−xN (0.3<x<0.5)

A study of Mg doping of AlxGa1−xN up to x∼50% using microstructural and electrical probes is reported. The viability of effective p-type doping is defined by a minimum concentration of Mg required to offset the background impurities and, more importantly, a maximum limit above which inversion domains and structural defects start to nucleate, accompanied by a rapid degradation of electrical transport. Resistivity of 10 Ω cm and free hole concentrations above 1017cm−3 are achieved for AlxGa1−xN up to x∼50% within an optimum window of Mg incorporation.

Heteroepitaxial evolution of AlN on GaN Grown by metal-organic chemical vapor deposition

We have investigated the morphological evolution during heteroepitaxial growth of AlN on GaN by metal-organic chemical vapor deposition at different V/III ratios. Two-dimensional layer–by–layer and step flow growth modes, combined with strain-induced cracking, are observed at low and intermediate V/III ratios, while nitrogen-rich conditions yield three-dimensional domain-like growth due to limited Al adatom diffusion. Samples grown at the metal-rich conditions exhibit a crosshatch pattern of surface undulations possibly related to the presence of misfit dislocations that form at the early stages of nucleation. Our observations show that the local stoichiometry at the vapor-solid interface strongly influences the adatom kinetics during the growth, thereby affecting the nature of strain relaxation and growth mode.

Growth of AlGaN nanowires by metalorganic chemical vapor deposition

Growth of ternary AlGaN nanowires using metalorganic chemical vapor deposition is investigated. Structural, chemical, and optical characterization at nanoscopic scale is carried out by high resolution transmission electron microscopy, x-ray energy dispersive spectroscopy, and spatially resolved cathodoluminescence. Spontaneous formation of Al(Ga)N∕GaN coaxial nanowires with distinct emission at 370 nm is observed. It is identified that the interplay between surface kinetics and thermodynamics facilitates the catalytic growth of GaN core while a limited surface diffusion of Al adatoms leads to nonselective, vapor-solid growth of Al(Ga)N sheath. The observation points to a fundamental difference in nanosynthesis using near-equilibrium and nonequilibrium techniques.

Optically pumped ultraviolet AlGaInN quantum well laser at 340 nm wavelength

We have demonstrated and characterized an optically pumped room temperature pulsed laser at 340 nm, based on a separate confinement AlGaInN multiple quantum well heterostructure design. The lasing was achieved at threshold peak power of ∼800 kW/cm2. Gain spectroscopy showed modest peak modal gain on the order of 5 cm−1, suggesting that the combination of weak optical confinement and nonradiative recombination would lead to high threshold currents for comparably constructed diode lasers.

Droplet heteroepitaxy of GaN quantum dots by metal-organic chemical vapor deposition

Optically active GaN quantum dots on conductive AlGaN templates are synthesized by droplet heteroepitaxy, whereby the Ga droplets are converted to GaN islands in the presence of ammonia at 600°C. We have investigated the evolution of metallic Ga layers on AlGaN, obtaining the optimal surface densities and size distribution of the Ga droplets. The stability of GaN islands is influenced by the surface kinetics and the initial droplet size; the condition of Ga deposition and subsequent nitrogen exposure is identified, which preserves the initial density of the Ga droplets. A nitrogen-rich environment is identified as a necessary condition for maintaining the optimal GaN morphology by suppressing the Ga surface diffusion and preventing two-dimensional layer growth.

High Performance AlGaInN Ultraviolet Light-Emitting Diode at the 340 nm Wavelength

We report on high output power from the quaternary AlGaInN multiple quantum well (MQW) ultraviolet light emitting diodes (UV LEDs) in the 340 nm wavelength range. The output power up to 1.5 mW from a 100 µm diameter device with bare-chip configuration was measured under room temperature cw operation. The internal quantum efficiency was estimated to be between 7 and 10%. In addition, the output power and external quantum efficiency for fully packaged 1×1 mm2 large area device were as high as 54.6 mW and 1.45%, respectively, at the injection current of 200 A/cm2 under pulsed operation. The devices were incorporated into prototype system for fluorescence based bio-sensing as excitation source.

Growth, Characterization, and Application of High Al-content AlGaN and High Power III-Nitride Ultraviolet Emitters

A study of Si-doped and Mg-doped Al x Ga 1-x N up to × ∼ 50 % and the characteristics of ultraviolet (UV) light emitting diodes (LEDs) with emission wavelengths at 340 nm and 290 nm are reported. By using grading super-lattices (SLs) before n-type AlGaN growth, surface roughness is much improved. Resistivity of 2.9×10- 2 Ωcm and free electron concentrations of 2.9×10 cm- are achieved for n-type Al 0.45 Ga 0.55 N. The viability of effective p-type doping is defined by a minimum concentration of Mg required to offset the background impurities and, more importantly, a maximum limit above which inversion domains and structural defects start to nucleate, accompanied by a rapid degradation of electrical transport. Resistivity of 10 Ωcm and free hole concentrations above 10 17 cm −3 are achieved for Al x Ga 1-x N up to × ∼ 50 % within an optimum window of Mg incorporation. Output powers up to 1.5 mW from small area 340 nm LEDs ( 2 device area and 0.52 mm 2 mesa area), output power of 79 mW from 340 nm LEDs and 8.5 mW from 290 nm LEDs are achieved under pulse mode (1kHz, 2% duty factor).

Performance and application of high power ultraviolet AlGaInN light emitting diodes

We report on high output power from the quaternary AlGaInN multiple quantum well (MQW) ultraviolet light emitting diodes (UV LEDs) in the 340 nm and 280 nm wavelength range. The output power up to 1.5 mW from a 100 μm diameter device with bare-chip configuration was measured under room temperature cw operation. The internal quantum efficiency was estimated to be between 7 and 10%. In addition, the output power and external quantum efficiency for fully packaged 1x1mm2 large area device were as high as 54.6 mW and 1.45%, respectively, at the injection current of 200 A/cm2 under pulsed operation. The devices were incorporated into prototype system for fluorescence based bio-sensing. We also report the performance of 285 nm UV LEDs.