E. J. Tarsa

Bio: E. J. Tarsa is an academic researcher from University of California, Berkeley. The author has contributed to research in topics: Metalorganic vapour phase epitaxy & Thin film. The author has an hindex of 12, co-authored 16 publications receiving 1919 citations.

Homoepitaxial growth of GaN under Ga-stable and N-stable conditions by plasma-assisted molecular beam epitaxy

Abstract: The structure, morphology, and optical properties of homoepitaxial GaN layers grown by molecular beam epitaxy on metalorganic chemical vapor deposition (MOCVD)-grown GaN “template” layers were investigated as a function of the group III/group V flux ratio during growth GaN layers grown with a low III/V ratio (N-stable growth) displayed a faceted surface morphology and a tilted columnar structure with a high density of stacking faults In contrast, films grown with a high III/V ratio (Ga-stable growth) displayed comparable structure to the underlying MOCVD-grown template The transition from N-stable to Ga-stable growth modes was found to occur over a narrow range of Ga fluxes at a growth temperature of 650 °C Evidence of Ga accumulation and step-flow growth was observed for films grown under Ga-stable conditions, leading to the formation of spiral growth features at the surface termination of mixed edge/screw dislocations Photoluminescence measurements indicate that the deep-level (∼550 nm) emission is

Dislocation mediated surface morphology of GaN

Abstract: The surfaces of GaN films grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were studied using atomic force microscopy (AFM). Due to the high dislocation densities in the films (108 cm−2), the typical surface morphologies of layers grown by both techniques were dominated by three dislocation mediated surface structures—pinned steps, spiral hillocks, and surface depressions. The characteristics of these surface structures were found to depend on growth technique (MOCVD vs MBE) and the group-III to group-V ratio used in the growth of MBE GaN films. Pinned steps, created by the intersections of mixed character dislocations with the free surface, were found on all GaN films. The pinned steps were observed to be predominantly straight on the MOCVD GaN and curved into spiral hillock formations on the MBE GaN. Spiral growth hillocks form when pinned steps grow outward and around the dislocation under step-flow growth conditions. The tightness of the spiral hillocks on MBE G...

High-performance (Al,Ga)N-based solar-blind ultraviolet p–i–n detectors on laterally epitaxially overgrown GaN

Abstract: Solar-blind ultraviolet photodiodes with a band-edge wavelength of 285 nm were fabricated on laterally epitaxially overgrown GaN grown by metalorganic chemical vapor deposition. Current–voltage measurements of the diodes exhibited dark current densities as low as 10 nA/cm2 at −5 V. Spectral response measurements revealed peak responsivities of up to 0.05 A/W. Response times for these diodes were measured to be as low as 4.5 ns for 90%-to-10% fall time. For comparison, diodes were fabricated using the same p–i–n structure deposited on dislocated GaN. These diodes had dark current densities many orders of magnitude higher, as well as a less sharp cutoff, and a significant slow tail under impulse excitation.

Scanning capacitance microscopy imaging of threading dislocations in GaN films grown on (0001) sapphire by metalorganic chemical vapor deposition

Abstract: A combination of atomic force microscopy and scanning capacitance microscopy was used to investigate the relationship between the surface morphology and the near-surface electrical properties of GaN films grown on c-axis sapphire substrates by metalorganic chemical vapor deposition. Local regions surrounding the surface termination of threading dislocations displayed a reduced change in capacitance with applied voltage relative to regions that contained no dislocations. Capacitance–voltage characteristics obtained from these regions indicated the presence of negative charge in the vicinity of dislocations.

Nucleation layer evolution in metal‐organic chemical vapor deposition grown GaN

Abstract: The structure and morphology of low growth temperature GaN nucleation layers have been studied using atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED), and transmission electron microscopy (TEM). The nucleation layers were grown at 600 °C by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) on c‐plane sapphire. The layers consist of predominantly cubic GaN (c‐GaN) with a high density of stacking faults and twins parallel to the film/substrate interface. The average grain size increases with increasing layer thickness and during the transition from low temperature (600 °C) to the high temperatures (1080 °C) necessary for the growth of device quality GaN. Upon heating to 1080 °C the nucleation layer partially converts to hexagonal GaN (h‐GaN) while retaining a high stacking fault density. The mixed cubic‐hexagonal character of the nucleation layer region is sustained after subsequent high‐temperature GaN growth.

First-principles calculations for defects and impurities: Applications to III-nitrides

Abstract: First-principles calculations have evolved from mere aids in explaining and supporting experiments to powerful tools for predicting new materials and their properties. In the first part of this review we describe the state-of-the-art computational methodology for calculating the structure and energetics of point defects and impurities in semiconductors. We will pay particular attention to computational aspects which are unique to defects or impurities, such as how to deal with charge states and how to describe and interpret transition levels. In the second part of the review we will illustrate these capabilities with examples for defects and impurities in nitride semiconductors. Point defects have traditionally been considered to play a major role in wide-band-gap semiconductors, and first-principles calculations have been particularly helpful in elucidating the issues. Specifically, calculations have shown that the unintentional n-type conductivity that has often been observed in as-grown GaN cannot be a...

Luminescence properties of defects in GaN

Abstract: Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

The Roles of Structural Imperfections in InGaN-Based Blue Light-Emitting Diodes and Laser Diodes

Abstract: REVIEW High-efficiency light-emitting diodes emitting amber, green, blue, and ultraviolet light have been obtained through the use of an InGaN active layer instead of a GaN active layer. The localized energy states caused by In composition fluctuation in the InGaN active layer are related to the high efficiency of the InGaN-based emitting devices. The blue and green InGaN quantum-well structure light-emitting diodes with luminous efficiencies of 5 and 30 lumens per watt, respectively, can be made despite the large number of threading dislocations (1 x 10(8) to 1 x 10(12) cm-2). Epitaxially laterally overgrown GaN on sapphire reduces the number of threading dislocations originating from the interface of the GaN epilayer with the sapphire substrate. InGaN multi-quantum-well structure laser diodes formed on the GaN layer above the SiO2 mask area can have a lifetime of more than 10,000 hours. Dislocations increase the threshold current density of the laser diodes.

Ferroelectric thin films: Review of materials, properties, and applications

Abstract: An overview of the state of art in ferroelectric thin films is presented. First, we review applications: microsystems' applications, applications in high frequency electronics, and memories based on ferroelectric materials. The second section deals with materials, structure (domains, in particular), and size effects. Properties of thin films that are important for applications are then addressed: polarization reversal and properties related to the reliability of ferroelectric memories, piezoelectric nonlinearity of ferroelectric films which is relevant to microsystems' applications, and permittivity and loss in ferroelectric films-important in all applications and essential in high frequency devices. In the context of properties we also discuss nanoscale probing of ferroelectrics. Finally, we comment on two important emerging topics: multiferroic materials and ferroelectric one-dimensional nanostructures. (c) 2006 American Institute of Physics.

Growth and applications of Group III-nitrides

Abstract: Recent research results pertaining to InN, GaN and AlN are reviewed, focusing on the different growth techniques of Group III-nitride crystals and epitaxial films, heterostructures and devices. The chemical and thermal stability of epitaxial nitride films is discussed in relation to the problems of deposition processes and the advantages for applications in high-power and high-temperature devices. The development of growth methods like metalorganic chemical vapour deposition and plasma-induced molecular beam epitaxy has resulted in remarkable improvements in the structural, optical and electrical properties. New developments in precursor chemistry, plasma-based nitrogen sources, substrates, the growth of nucleation layers and selective growth are covered. Deposition conditions and methods used to grow alloys for optical bandgap and lattice engineering are introduced. The review is concluded with a description of recent Group III-nitride semiconductor devices such as bright blue and white light-emitting diodes, the first blue-emitting laser, high-power transistors, and a discussion of further applications in surface acoustic wave devices and sensors.