Stimulated emission and ultrafast carrier relaxation in InGaN multiple quantum wells

Stimulated emission (SE) was measured from two InGaN multiple quantum well (MQW) laser structures with different In compositions. SE threshold power densities (I_th) increased with increasing QW depth (x). Time-resolved differential transmission measurements mapped the carrier relaxation mechanisms and explained the dependence of I_th on x. Carriers are captured from the barriers to the QWs in < 1 ps, while carrier recombination rates increased with increasing x. For excitation above I_th an additional, fast relaxation mechanism appears due to the loss of carriers in the barriers through a cascaded refilling of the QW state undergoing SE. The increased material inhomogeneity with increasing x provides additional relaxation channels outside the cascaded refilling process, removing carriers from the SE process and increasing I_th.Comment: submitted to Appl. Phys. Let

Interfacial N Vacancies in GaN/(Al,Ga)N/GaN Heterostructures

We show that N-polar GaN/(Al, Ga)N/GaN heterostructures exhibit significant N deficiency at the bottom (Al, Ga)N/GaN interface, and that these N vacancies are responsible for the trapping of holes observed in unoptimized N-polar GaN/(Al, Ga)N/GaN high electron mobility transistors. We arrive at this conclusion by performing positron annihilation experiments on GaN/(Al, Ga)N/GaN heterostructures of both N and Ga polarity, as well as state-of-the-art theoretical calculations of the positron states and positron-electron annihilation signals. We suggest that the occurrence of high interfacial N vacancy concentrations is a universal property of nitride semiconductor heterostructures at net negative polarization interfaces.Peer reviewe

Lateral Confinement of Electrons in Vicinal N-polar AlGaN/GaN Heterostructure

We studied orientation dependent transport in vicinal N-polar AlGaN/GaN heterostructures. We observed significant anisotropy in the current carrying charge parallel and perpendicular to the miscut direction. A quantitative estimate of the charge anisotropy was made based on gated TLM and Hall measurements. The formation of electro-statically confined one-dimensional channels is hypothesized to explain charge anisotropy. A mathematical model was used to verify that polarization charges distributed on miscut structure can create lateral one-dimensional confinement in vicinal substrates. This polarization-engineered electrostatic confinement observed is promising for new research on low-dimensional physics and devices besides providing a template for manufacturable one-dimensional devices

Charge control and mobility in AlGaN/GaN transistors: Experimental and theoretical studies

In this article we report on two dimensional sheet charge and mobility in GaN/AlGaN heterostructure field effect transistors. Both experimental and theoretical results are presented. Experimental results are reported on samples grown by metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Theoretical studies are done to examine how spontaneous polarization and piezoelectric effect control the sheet charge density. The studies also focus on how interface roughness, aluminum mole fraction in the barrier and phonon scattering influence mobility. We find that interface roughness is a dominant source of scattering in the samples reported. Due to the variation in growth techniques we find that the MBE samples have a smoother interface compared to the MOCVD samples. By carefully fitting the experimental data we present results on interface roughness parameters for MBE and MOCVD samples. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70927/2/JAPIAU-87-11-7981-1.pd

Polarization effects in AlGaN/GaN and GaN/AlGaN/GaN heterostructures

The influence of AlGaN and GaN cap layer thickness on Hall sheet carrier density and mobility was investigated for Al0.32Ga0.68N/GaN and GaN/Al0.32Ga0.68N/GaN heterostructures deposited on sapphire substrates. The sheet carrier density was found to increase and saturate with the AlGaN layer thickness, while for the GaN-capped structures it decreased and saturated with the GaN cap layer thickness. A relatively close fit was achieved between the measured data and two-dimensional electron gas densities predicted from simulations of the band diagrams. The simulations also indicated the presence of a two-dimensional hole gas at the upper interface of GaN/AlGaN/GaN structures with sufficiently thick GaN cap layers. A surface Fermi-level pinning position of 1.7 eV for AlGaN and 0.9-1.0 eV for GaN, and an interface polarization charge density of 1.6x10(13)-1.7x10(13) cm(-2), were extracted from the simulations. (C) 2003 American Institute of Physics

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Preface: phys. stat. sol. (c) 7/10

The 36th International Symposium on Compound Semiconductors (ISCS 2009) was held from August 30 to September 2, 2009, in Santa Barbara, California. As in previous symposia, ISCS united researchers investigating III–V and II–IV compound semiconductors as well as SiGe, SiC and carbon related materials and devices. The ISCS series looks upon a long history, which begun in 1966 with the first “International Symposium on GaAs and Related Compounds” in Reading, England. Since then, the conference has been held annually, rotating between Asia, Europe and North America. The conference selects eminent scientists for three prestigious awards, namely the Welker Award, the Quantum Device Award and the Young Scientist Award. The 36th symposium in Santa Barbara was attended by 215 scientists from 19 countries, with 28% from Asia, 16% from Europe, 2% from Australia, and 54% from America. The agenda consisted of 4 plenary talks, 16 invited talks, and 96 contributing talks that were presented together with 65 posters. The plenary talks highlighted advances in various areas: Hideo Ohno from Tohoku University spoke on “Ferromagnetism in III–V Semiconductors”, Peidong Yang from the University of California Berkeley on “Semiconductor Nanowires for Photonics and Energy Conversion”, Hari Manoharan from Stanford University discussed “Spin-Atom Spintronics of Graphene”, and John Bowers from the University of California Santa Barbara “Ge/Si PIN and APD Photodetectors.” The 37th symposium, held from May 31 to June 4, 2010 in Takamatsu, Japan, is organized by Prof. Yoshiro Hirayama from Tohoku University. We would like to thank Yasuhiko Arakawa, Oliver Ambacher and Pallab Bhattacharya, who served as Regional Program Chairs and the Technical Program Committee for reviewing the abstracts and generating the program. In addition we would like to thank Ben Streetman who served as the chair of the Awards Committee, the session chairs, and all those who helped with arrangements (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78053/1/2349_ftp.pd