Grace D. Chern

In-polar InN grown by plasma-assisted molecular beam epitaxy

We study the effect of different deposition conditions on the properties of In-polar InN grown by plasma-assisted molecular beam epitaxy. GaN buffer layers grown in the Ga-droplet regime prior to the InN deposition significantly improved the surface morphology of InN films grown with excess In flux. Using this approach, In-polar InN films have been realized with room temperature electron mobilities as high as 2250cm2∕Vs. We correlate electron concentrations in our InN films with the unintentionally incorporated impurities, oxygen and hydrogen. A surface electron accumulation layer of 5.11×1013cm−2 is measured for In-polar InN. Analysis of optical absorption data provides a band gap energy of ∼0.65eV for the thickest InN films.

Optimization of the surface and structural quality of N-face InN grown by molecular beam epitaxy

The authors demonstrate the impact of growth kinetics on the surface and structural properties of N-face InN grown by molecular beam epitaxy. Superior surface morphology with step-flow growth features is achieved consistently under In-rich conditions in a low-temperature region of 500-540 degrees C. Remarkably, off-axis x-ray rocking curve (omega scans) widths are found to be independent of the growth conditions. The band gap determined from optical absorption measurements of optimized InN is 0.651 eV, while photoluminescence peak emission occurs at even lower energies of similar to 0.626 eV. Hall measurements show room temperature peak electron mobilities as high as 2370 cm(2)/V s at a carrier concentration in the low 10(17) cm(-3) region. Analysis of the thickness dependence of the carrier concentration demonstrates a n-type surface accumulation layer with a sheet carrier concentration of similar to 3x10(13) cm(-2). (c) 2006 American Institute of Physics.

Excitation wavelength dependence of terahertz emission from InN and InAs

The authors report on the excitation wavelength dependence of terahertz emission from n-InN and bulk p-InAs pumped with femtosecond pulses tunable from 800to1500nm. The terahertz amplitude, normalized to pump and probe power, from both narrow band gap semiconductors remains relatively constant over the excitation wavelength range. In addition, terahertz radiation from In- and N-face InN samples with bulk carrier concentrations ranging from 1017to1019cm−3 is also investigated, showing a strong dependence of terahertz emission on bulk carrier concentration. The experimental results agree well with calculations based on drift-diffusion equations incorporating momentum conservation and relaxation.

Wavelength and intensity switching in directly coupled semiconductor microdisk lasers.

We demonstrate output wavelength and intensity switching in a three-element directly coupled microdisk device consisting of one spiral microdisk coupled to two semicircle microdisks. The gapless coupling mechanism used allows individual elements to achieve lasing while achieving optimal transfer of optical power between adjacent microdisks. By controlling the transparency of the center element via injection current, the edge elements can be allowed to exchange their amplified spontaneous emission. In this manner, on-off-on switching of the output intensity, as well as discontinuous shifts in the output wavelength, can be achieved as a function of increasing injection current.

Terahertz Emission from Indium Nitride Multiple Quantum Wells

We report enhanced terahertz emission from N-face InN/InGaN multiple quantum wells relative to that from bulk N-face InN when excited by 800 nm femtosecond optical pulses with low pump intensities.

INDIUM NITRIDE: A NEW MATERIAL FOR HIGH EFFICIENCY, COMPACT, 1550nm LASER-BASED TERAHERTZ SOURCES IN CHEMICAL AND BIOLOGICAL DETECTION

Indium nitride (InN) is identified as a promising terahertz (THz) emitter based on the optical and electronic properties of high quality In- and N-face samples. Time domain THz spectroscopy has been employed to measure the pump wavelength and background carrier concentration dependence of THz emission from InN. There is no discernable difference between the In- and N-face InN samples, as expected for the improved crystalline quality and concomitant low background electron density and high mobility for both polarities. While there is only a weak dependence of THz signal on pump wavelength from 800 nm to 1500 nm, there is a strong dependence on background electron density. Modeling shows that the dominant mechanism for THz generation in bulk InN is the current associated with the diffusion of the photo-generated electrons at elevated electron temperature (photo-Dember effect) and the redistribution of the background electrons under drift, with larger screening from the higher mobility electrons as compared to holes. Compensation or p-type doping in conjunction with manipulation of the large internal electric fields in InN/InGaN nanostructures should lead to significant improvements in THz emitters.

Varying the Overlap of Direct-Coupling between Spiral and Semicircle Semiconductor Microdisk Lasers

We present the lasing characteristics of a directly coupled spiral-shaped and semicircle-shaped 2-d microdisk device using a single quantum well AlGaAs heterostructure. Different degrees of coupling, as well as mode selection are achieved by altering the length along which the spiral and semicircle cavities are coupled and the curvature of the coupling junction. Because of the spirals chirality, the high quality factor and low threshold of the spiral microdisk are preserved when coupled to a semicircle microdisk. A three-element device consisting of a spiral microdisk coupled to two semicircle microdisks is also considered and it is found to control the amount of gain to the spiral output.

Excitation Wavelength Dependence of Terahertz Emission from Indium Nitride Multiple Quantum Wells

We report the excitation wavelength dependence of terahertz emission from N-face InN/InGaN multiple quantum wells relative to that from bulk N-face InN when excited by femtosecond optical pulses tunable from 800 nm to 1700 nm.

Excitation Wavelength Dependence Of Terahertz Emission From Indium Nitride Thin Films

We report on terahertz emission from n‐type indium nitride excited with femtosecond pulses tunable between 800 nm and 1500 nm. The terahertz amplitude, normalized to pump and probe power, from the semiconductor remains relatively constant over the excitation wavelength range. The experimental results agree with calculations based on drift‐diffusion equations incorporating momentum conservation and relaxation.