Growth kinetics and electronic properties of unintentionally doped semi-insulating GaN on SiC and high-resistivity GaN on sapphire grown by ammonia molecular-beam epitaxy

Growth of unintentionally doped (UID) semi-insulating GaN on SiC and highly resistive GaN on sapphire using the ammonia molecular-beam epitaxy technique is reported. The semi-insulating UID GaN on SiC shows room temperature (RT) resistivity of 10(11) W cm and well defined activation energy of 1.0 eV. The balance of compensation of unintentional donors and acceptors is such that the Fermi level is lowered to midgap, and controlled by a 1.0 eV deep level defect, which is thought to be related to the nitrogen antisite N(Ga), similar to the \u201cEL2\u201d center (arsenic antisite) in unintentionally doped semi-insulating GaAs. The highly resistive GaN on sapphire shows RT resistivity in range of 10(6)\u201310(9) W cm and activation energy varying from 0.25 to 0.9 eV. In this case, the compensation of shallow donors is incomplete, and the Fermi level is controlled by levels shallower than the 1.0 eV deep centers. The growth mechanisms for the resistive UID GaN materials were investigated by experimental studies of the surface kinetics during growth. The required growth regime involves a moderate growth temperature range of 740\u2013780 \ub0C, and a high ammonia flux (beam equivalent pressure of 1x10 124 Torr), which ensures supersaturated coverage of surface adsorption sites with NHx radicals. Such highly nitrogen rich growth conditions lead to two-dimensional layer by layer growth and reduced oxygen incorporation.Peer reviewed: YesNRC publication: Ye

Watching semiconductor circuitry work

Semiconductor devices that are not generally thought of as light sources do emit radiation in the visible and the near infrared as they operate. Observation of this electroluminescence furnishes insight into the operation of the devices and of the circuitry that they constitute but it requires an extremely sensitive light detector and picosecond time resolution. This can be achieved using a Mepsicron™ photodetector system that enables single photon counting time-correlated imaging with a spatial resolution of about 1 μm and a time resolution approaching 10 ps. Information extracted from the time-resolved imagery can be compared with circuit layout and topology and individual device structures and with electrical measurements that are performed concurrently. These time-correlated measurements allow signal waveforms to be determined optically, much as the waveforms measured electronically with an oscilloscope and microprobing, but with the advantage that the acquisition is entirely non-invasive. Images can be dissected in both space and time to provide information for individual components of a circuit or regions of a device. This imaging equipment has been used in our laboratory for measurements on Si, GaAsP and GaN technologies and analyses will be presented

Strong potential profile fluctuations and effective localization process in InGaN∕GaN multiple quantum wells grown on {10‐1m} faceted surface GaN template.

International audienceCathodoluminescence(CL) and time-resolved photoluminescence(TRPL) spectroscopy were used to investigate the relation between the surface morphology and emission efficiency in 10× InGaN(3nm)∕GaN(4nm)quantum wells(QWs) deposited by plasma-assisted molecular beam epitaxy(MBE). For this study, two QWs with peak emission around 405nm but grown on different surface morphologies have been investigated. A strong increase in the emission efficiency was observed in the QWs grown on {10‐1m} faceted surfaceGaN template (m>2) as compared to those grown on an atomically smooth template. CL mapping and temperature-dependent PL studies revealed that the QWs grown on the faceted surfaceGaN epilayer exhibit much stronger in-plane indium content fluctuations and larger width PL peak in the temperature range of 8–300K. We found that the use of {10‐1m} faceted surfaceGaN template resulted in strong potential profile fluctuations (PPFs) inducing different localization centers at different energy levels. We found that the deeper the corresponding fluctuation of the energy level, the weaker the decrease of the PL intensity with increasing temperature, the higher the PL decay time (τPL) in the whole temperature range and the slower the collapse of τPL. Our results demonstrate that the use of {10‐1m} faceted surface morphologyGaN template is an amplifying process of the PPFs which favors a regime dominated by the recombination of localized carriers

Energy transfer upconversion determination by thermal-lens and Z-scan techniques in Nd(3+)-doped laser materials

The Z-scan and thermal-lens techniques have been used to obtain the energy transfer upconversion parameter in Nd(3+)-doped materials. A comparison between these methods is done, showing that they are independent and provide similar results. Moreover, the advantages and applicability of each one are also discussed. The results point to these approaches as valuable alternative methods because of their sensitivity, which allows measurements to be performed in a pump-power regime without causing damage to the investigated material. (C) 2009 Optical Society of AmericaFAPESPCoordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)FAPEALCNP

A HPLC method to evaluate the influence of photostabilizers on cosmetic formulations containing UV-filters and vitamins A and E

This paper reports a simple and reliable HPLC method to evaluate the influence of two currently available photostabilizers on cosmetic formulations containing combined UV-filters and vitamins A and E. Vitamins and UV-filters, widely encountered in products of daily use have to be routinely evaluated since photoinstability can lead to reductions in their efficacy and safety. UV-irradiated formulation samples were submitted to a procedure that included a reliable, precise and specific HPLC method employing a C18 column and detection at 325 and 235 nm. Methanol, isopropanol and water were the mobile phases in gradient elution. The method precision was between 0.28 and 5.07. The photostabilizers studied [diethylhexyl 2,6-naphthalate (DEHN) and benzotriazolyl dodecyl p-cresol (BTDC)], influenced the stability of octyl methoxycinnamate (OMC) associated with vitamins A and E. BTDC was considered the best photostabilizer to vitamins and OMC when the UV-filters were combined with both vitamins A and E. (C) 2010 Elsevier B.V. All rights reserved.Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP

Windowed growth of AlGaN/GaN heterostructures on Silicon 〈111〉 substrates for future MOS integration

A novel windowed growth technique that allows for the exposure of a smooth silicon surface for MOS circuitry between AlGaN/GaN HEMT layers has been developed. A sacrificial oxide based dielectric stack is used to protect regions of the silicon surface before growth by ammonia-MBE. SIMS analysis shows elevated oxygen impurity incorporation in the stress-relief layer, but controllable levels in the C-doped GaN buffer. A Hall mobility of 1.37 × 10 3 cm 2/Vs is measured in the AlGaN/GaN regions. AFM results show a RMS roughness of 0.12 nm on the silicon surface