Henry P. Lee

Photoluminescence and polarized photodetection of single ZnO nanowires

Single crystal ZnOnanowires are synthesized and configured as field-effect transistors.Photoluminescence and photoconductivity measurements show defect-related deep electronic states giving rise to green-red emission and absorption. Photocurrent temporal response shows that current decay time is significantly prolonged in vacuum due to a slower oxygen chemisorption process. The photoconductivity of ZnOnanowires is strongly polarization dependent. Collectively, these results demonstrate that ZnOnanowire is a remarkable optoelectronic material for nanoscale device applications.

High-performance GaN-based light-emitting diode using high-transparency Ni/Au/Al-doped ZnO composite contacts

We report on a high-transparency low-resistance composite contact structure on p-GaN for light-emitting diode applications. The structure consists of a thin Ni(5nm)∕Au(5nm) layer overcoated with a sputtered Al-doped ZnO (170nm) layer. Enhancement in light emission intensity as high as 74% at 40mA and forward operating voltages in the range of 3.36–3.48V at 20mA are obtained for these devices using a two-step thermal annealing process.

DIFFUSE REFLECTANCE SPECTROSCOPY MEASUREMENT OF SUBSTRATE TEMPERATURE AND TEMPERATURE TRANSIENT DURING MOLECULAR BEAM EPITAXY AND IMPLICATIONS FOR LOW -TEMPERATURE III-V EPITAXY

We report diffuse reflectance spectroscopy (DRS) measurement of Knudsen cell induced radiative heating of the sample during molecular beam epitaxy of GaAs at substrate temperatures between 200 and 600 °C. The temperature rises, as large as 12 °C, were observed for In-bonded samples at a substrate temperature of 200 °C. As-grown GaAs layers deposited between 200 and 300 °C are characterized using double crystal x-ray diffraction. The onset of a distinct x-ray peak associated with the low-temperature grown GaAs layer is identified, at a DRS measured temperature between 260 and 270 °C.

Physical modeling of pyrometric interferometry during molecular beam epitaxial growth of III–V layered structures

A detailed physical model of pyrometric interferometry during molecular beam epitaxy growth of III–V layers is presented. The pyrometric radiation intensity is expressed as spatial convolution of the spontaneous emission rates and the propagation response of the sample. The new formalism, together with empirical modeling of background signal provide a generic framework for implementing a model‐reference closed‐loop control system for epitaxial growth of multilayer structures.

Metamorphic InAsyP1−y (y=0.30–0.75) and AlδIn1−δAsyP1−y buffer layers on InP substrates

The authors report the growth and characterization of InAsyP1−y and AlδIn1−δAsyP1−y buffer layers on InP by metal-organic vapor phase epitaxy. Under optimized growth conditions, they achieved sheet resistances of 2.8×105 and 4.8×104Ω∕sq for single layer InAs0.44P0.56 (0.5μm) and step-graded InAs0.75P0.25∕InAs0.42P0.58 (0.075∕0.5μm) layers, respectively. A bowing parameter for InAsyP1−y of −0.22eV is found based on photoreflectance measurement. When 0.5μm thick Al0.11In0.89As.62P.38 is grown, they obtain sheet resistance and sheet carrier concentration of 7.76×105Ω∕sq and 7.92×109cm−2, respectively. This opens interesting possibilities for realizing high-performance metamorphic field-effect transistors based on strained InAs or InAsyP1−y (0.5

Simultaneous in situ measurement of substrate temperature and layer thickness using diffuse reflectance spectroscopy (DRS) during molecular beam epitaxy

Abstract Diffuse reflectance spectroscopy (DRS) is used for in situ measurements of temperature transient and temperature drift during MBE growth of AlAs GaAs and InGaAs GaAs layered structures. The experimental results are compared together with pyrometric interferometry (PI) measurement taken concurrently. Our results show that DRS is a viable method for in situ real-time monitoring of both substrate temperature and layer thickness.

UNIFORM AND DELTA DOPING OF CARBON IN GAAS BY SOLID-SOURCE MOLECULAR BEAM EPITAXY USING ELECTRON BEAM EVAPORATION : EVIDENCE FOR ATOMIC PAIRING

Uniform and delta‐doped GaAs:C layers were grown by solid source molecular beam epitaxy (MBE) using electron‐beam evaporation of the dopant. Uniform doping concentrations of up to 6×1019 cm−3 were measured and degradation of optical properties was noted when the hole concentration exceeded the mid‐1018 cm−3 range. Delta‐doping studies were carried out under various deposition conditions and an extremely high sheet carrier concentration of 9.6×1012 cm−2 was obtained under modulated beam condition. In contrast to carbon doping by resistively heated filament sources, all doping results were obtained with C1/C3 isomer ratios greater than unity. These findings show strong evidence for the identification of atomic carbon pairing as the dominant defect formation mechanism in heavily doped GaAs:C films prepared by solid source MBE.

Virtual control simulator for closed-loop epitaxial growth

Abstract We report the use of a virtual control simulator for analyzing the behavior of a closed-loop molecular beam epitaxy system based on in situ narrow-band pyrometric interferometry and single-wavelength laser reflectance monitoring. Specific case studies include thickness control of GaAs AlAs distributed Bragg reflectors and continuous growth rate estimation of GaAs AlAs calibration structures. In both cases, the control simulator provides valuable insight on the design of the control system prior to its actual implementation.

A Compact All-Fiber LPG-AOTF Frequency Shifter on Single-Mode Fiber and Its Application to Vibration Measurement

We demonstrate a compact, all-fiber frequency shifter by cascading a CO2-written long-period grating with a flexural acoustic-wave-induced microbending grating on a single-mode fiber. Carrier and image sideband suppressions of 30.0 and 28.1 dB, respectively, are measured. An all-fiber vibrometer based on this frequency shifter capable of nanometer resolution at frequencies up to megahertz is also demonstrated.

Physical origins of temperature variation and background radiation associated with pyrometric interferometry measurement during III–V molecular‐beam‐epitaxy growth

The physical origins of temperature variation and background radiation associated with in situ pyrometric interferometry (PI) monitoring during III–V molecular‐beam epitaxy are examined. By means of complementary PI and diffused reflectance spectroscopy measurements on GaAs and GaAs/AlAs structures grown at different temperature, the temperature variations due to the changes of sample emissivity and radiative heating from the Knudsen cells are identified and characterized. The studies will lead to more accurate modeling of PI signal and better control of substrate temperature.