Theodore Moustakas

Approved fes probate *« & f 2LC QUALITY iwSESCIBa) ■ This work was supported in part by the Office of Naval Research under Qrant Number ONR: ri00014-97-l-0055. The United States Government has a royalty-free license throughout the world in all copyrightable material contained herein. Single article reprints from this publication are available through

Hydrogenated microcrystalline silicon is a mixed-phase material consisting of submicron size silicon crystallites embedded into an amorphous matrix1. This new phase material has received increased attention because of its potential application in thin film devices2,3. Such material was first prepared by Veprek and his coworkers4–6 by hydrogen-plasma assisted silicon transport. In this method, the silicon charge, kept at relatively low temperatures, reacts with atomic hydrogen and forms volatile hydrides, which are transported and decomposed on a substrate, kept at higher temperatures. More recently hydrogenated microcrystalline silicon has been prepared by high power glow discharge decomposition of silane, diluted in hydrogen7,8 Material produced by these two methods has been investigated by a number of groups and models regarding the kinetics of growth have been proposed5,8. Limited progress has also been made in the deposition of such material by the method of sputtering3,9,10,11. However, the mechanism of thin film growth and crystallization has only been briefly addressed11.

We report on a temperature-dependent photoluminescence study of disorder effects on the optical properties of Al0.2Ga0.8N/GaN multiple quantum wells as a function of the well width. It is found that the disorder-induced inhomogeneous broadening of the excitonic density of states increases with decreasing well width. A nonmonotonic temperature variation of the photoluminescence peak energy is observed, and interpreted as a crossover from a thermal to a nonthermal (trapped) distribution of recombining excitons amongst the band-tail states. The luminescence is quenched by two thermally activated mechanisms, and the dependence of the activation energies with well width is accounted for.

We use Raman scattering to obtain a stress map of lateral epitaxy overgrown GaN. ... The overgrowth region is under slightly smaller stress, about 0.15 GPa. We attribute marked variations in the phonon intensity to spatial variations in the free carrier concentration. ...

Lattice constants of gallium nitride (wurzite structure) have been measured at temperatures 294–753 K. The measurements were performed by using x‐ray diffractometry. Two kinds of samples were used: (1) bulk monocrystal grown at pressure of 15 kbar, (2) epitaxial layer grown on ...

We investigated the pressure behavior of yellow luminescence in bulk crystals and epitaxial layers of GaN. This photoluminescence band exhibits a blueshift of 30±2 meV/GPa for pressures up to about 20 GPa. For higher pressure we observe the saturation of the position of this ...

Abstract: The interface between n-type GaN and V-based contacts was characterized by soft x-ray spectroscopy. We have investigated the chemical interface structure before and after a rapid thermal annealing (RTA) step, which is crucial for the formation of an Ohmic contact. X-ray photoelectron and x-ray excited Auger electron spectra suggest that RTA induces an accumulation of metallic Ga at the surface. Using x-ray emission spectroscopy, we find that the probed nitrogen atoms are in a VN-like environment, indicating that vanadium interacts with nitrogen atoms from the GaN to form VN.

This paper describes the effect of a number of deposition parameters on the photovoltaic properties of intrinsic amorphous silicon films produced by RF sputtering. The authors find that the argon pressure and the power in the discharge affect the photovoltaic properties through the modification of the film's microstructure. Hydrogen incorporation and small levels of phosphorus and boron impurities affect the photovoltaic properties through reduction of residual dangling bond related defects and modification of their occupation. These optimization studies lead to intrinsic films, which when incorporated in P-I-N solar cell structures, generate external currents up to 13 mA/cm/sup 2/ and open circuit voltages of between 0.85 to 0.95 volts. The efficiency of these devices, 5.5%, is limited by the low FF, typically less than 50%.

This paper reviews progress in the heteroepitaxial growth of Ill-Nitride semiconductors. The growth of wurtzite and zinc-blende allotropic forms of GaN on various substrates with hexagonal and cubic symmetry respectively were discussed. In particular we addressed the growth on the various faces of sapphire, 6H-SiC and (001) Si. It has been shown that the kinetics of growth by plasma-MBE or ammonia-MBE are different. Specifically, in plasma-assisted MBE smooth films are obtained under group-III rich conditions of growth. On the other hand in ammonia-MBE smooth films are obtained under nitrogen rich conditions of growth. High quality films were obtained on 6H-SiC without the employment of any buffer. The various nucleation steps used to improve the two dimensional growth as well as to control the film polarity were discussed. The n- and p-doping of GaN were addressed. The concept of increasing the solubility of Mg in GaN by simultaneously bombarding the surface of the growing film with a flux of electrons (co-doping GaN with Mg and electrons) was discussed. The influence of the strength of Al-N, Ga-N and In-N bonds on the kinetics of growth of nitride alloys was pointed out. Specifically, it was shown that in both the nitrogen-rich and group-III rich growth regimes, the incorporation probability of aluminum is unity for the investigated temperature range of 750-800° C. On the other hand the incorporation probability of gallium is constant but less than unity only in the nitrogen-rich regime of growth. In the group-III regime the incorporation probability of gallium decreases monotonically with the total group-III flux, due to the competition with aluminum for the available active nitrogen. Alloy phenomena such as phase separation and atomic ordering and the influence of these phenomena to the optical properties were addressed. InGaN alloys are thermodynamically unstable against phase separation. At compositions above 30% they tend to undergo partial phase separation. Furthermore, InGaN alloys were found to undergo 1x1 monolayer cation ordering. AlGaN alloys do not show evidence of phase separation but they were found to undergo multiple type of superlattice ordering. Under nitrogen-rich growth conditions they show one monolayer periodicity, while under group-III rich growth it was found that the structure is a superposition of a seven monolayer and twelve monolayer superlattices. Finally, the growth of heterostructures and MQWs and the use of the MBE method for the fabrication of optical, electronic and electromechanical devices were discussed.

MQWS based on quaternary III-nitride alloys are likely to be the active region of UV emitters as well as other optical devices like modulators. The majority of reported work so far is based on GaN/AlGaN MQWs, grown heteroepitaxially along the polar direction [0001] on a variety of substrates. There is also some recent work on growing such MQWs heteroepitaxially along

ABSTRACTPhotoconductive detectors were fabricated on autodoped n-GaN films, with resistivity varying from 10 Ohm-cm to 107Ohm-cm, by molecular beam epitaxy. The mobility-lifetime product, determined from the measurement of photoconductive gain, was found to decrease monotonically from 10−2 cm2/V to 10−7 cm2/V as the dark resistivity was increased. This variation in the ντ products is attributed to changes in photocarrier lifetimes. In order to understand the recombination mechanisms responsible for this photoconductive behavior, the dependence of photoconductivity on excitation intensity (δμσ fλ) was investigated. The exponent λ was found to vary from 0.5 to 1.0, as the dark resistivity of the films increased. These results indicate the presence of exponential band tails extending from the conduction band edge. Furthermore, the dependence of photoconductivity on dark resistivity indicates that the photoconductive response is governed primarily by the location of the dark fermi level. A model accounting for these observations is presented.

ABSTRACTWide band gap semiconductors are attractive for developing high power switching devices because of their ability to operate at both higher temperatures as well as higher frequencies than conventional Si. In this paper we report on the growth and fabrication of GaN/SiC np heterojunction diodes by depositing Si doped n-GaN films by plasma-assisted molecular beam epitaxy directly on SiC without the use of GaN or AlN buffers.Careful ex-situ and in-situ preparation of the Si terminated 6H- SiC surface was necessary to produce high quality diodes. Vertical circular diodes were fabricated with sizes varying from 200 microns to 1mm in diameter. Mesas were formed by ICP etching of the MBE deposited n-GaN layer using Cl2. A Ti/Al/Ni/Au metal stack was employed as an n-ohmic contact to the GaN layer and an Al/Ti/Au metal stack was employed as a backside p-ohmic contact to the 6H- SiC layer. The diodes were characterized by I-V and C-V measurements. The 1 mm diameter diodes exhibited almost ideal behavior under forward bias with an ideality factor of 1.6, and a reverse saturation current of 10–19 A/cm2. Under reverse bias, these devices were driven up to 1000 V with a measured leakage current of 5×10–7 A. and a dynamic resistance varying from 1010 to 109 ohms with increasing reverse bias. The built-in potential in these n-p heterojunctions was determined from C-V measurements to be 2.25 V. From these values we determined that the heterojunction is of Type II with conduction and valence bands offsets calculated to be 0.65 and 1.1 eV respectively.

Plasmon-enhanced near-green light emission from InGaN/GaN quantum wells is demonstrated using periodic arrays of silver nanoparticles. Particularly large (nearly fivefold) enhancements are obtained when the array dimensions are near the onset of the diffractive regime.

ABSTRACTA new technique is presented that employs luminescence downconversion using an ultrashort gating pulse to enable the characterization of UV light emission from III-nitride semiconductors with subpicosecond temporal resolution. This technique also allows one to measure PL rise times and fast components of multiple decays in the subsequent time evolution of the PL intensity. Comparison of luminescence emission intensity and lifetime in GaN and AlGaN with ∼0.1 Al content grown homoepitaxially on GaN templates with the same quantities measured in heteroepitaxial layers grown on sapphire indicate significant improvement in the homoepitaxial layers due to reduction in dislocation density. Fast (<15 ps) initial decays in the AlGaN are attributed to localization associated with alloy fluctuations and subsequent recombination through gap states.

We performed optical‐absorption studies of the energy gap in various GaN samples in the temperature range from 10 up to 600 K. We investigated both bulk single crystals of GaN and an epitaxial layer grown on a sapphire substrate. The observed positions of the absorption edge vary for different samples of GaN (from 3.45 to 3.6 eV at T=20 K). We attribute this effect to different free‐electron concentrations (Burstein–Moss effect) characterizing the employed samples. For the sample for which the Burstein shift is zero (low free‐electron concentration) we could deduce the value of the energy gap as equal to 3.427 eV at 20 K. Samples with a different free‐electron concentration exhibit differences in the temperature dependence of the absorption edge. We explain the origin of these differences by the temperature dependence of the Burstein–Moss effect.

The plasmonic excitations of square-periodic arrays of silver nanocylinders are used to increase the radiative efficiency of nearby InGaN/GaN quantum wells emitting at near-green wavelengths. These nanostructures allow for an accurate control of the plasmonic ...

Optically pumped pulsed emission of short-wave infrared radiation based on intersubband transitions in GaN/AlN quantum wells is demonstrated. Nanosecond-scale pump pulses are used to resonantly excite electrons from the ground states to the second-excited subbands, followed by ...

Dr. Moustakas' research contributions cover a broad spectrum of topics in optoelectronic materials and devices, including nitride semiconductors, amorphous semiconductors, III-V compounds, diamond thin films and metallic multi-layers. He is the co-editor of eight books, including Gallium Nitride I (Academic Press, 1998) and Gallium Nitride II (academic Press, 1999), the author of chapters in eight books and 362 papers in technical journals (Google citations more than 17,500, h index 67). He served as a special editor of the Journal of Electronic Materials and the Journal of Vacuum Science and Technology. He presented 138 invited, keynote, and plenary talks in national and international conferences. He has been granted 39 U.S. patents and several are pending in the fields of nitride semiconductors, amorphous silicon and diamond materials. Intellectual property that resulted from his work has been licensed to more than 40 companies, including major manufacturers and users of blue and UV LEDs and lasers (

This paper reviews progress in ultraviolet (UV) optoelectronic devices based on AlGaN films and their quantum wells (QWs), grown by plasma-assisted molecular beam epitaxy. A growth mode, leading to band-structure potential fluctuations and resulting in AlGaN multiple QWs with internal quantum efficiency as high as 68%, is discussed. Atomic ordering in these alloys, which is different from that observed in traditional III-V alloys, and its effect on device performance is also addressed. Finally, progress in UV-light-emitting diodes, UV lasers, UV detectors, electroabsorption modulators, and distributed Bragg reflectors is presented.

The unique electronic band structure of indium nitride InN, part of the industrially significant III−N class of semiconductors, offers charge transport properties with great application potential due to its robust n-type conductivity. Here, we explore the water sensing mechanism of InN thin films. Using angle-resolved photoemission spectroscopy, core level spectroscopy, and theory, we derive the charge carrier density and electrical potential of a two-dimensional electron gas, 2DEG, at the InN surface and monitor its electronic properties upon in situ modulation of adsorbed water. An electric dipole layer formed by water molecules raises the surface potential and accumulates charge in the 2DEG, enhancing surface conductivity. Our intuitive model provides a novel route toward understanding the water sensing mechanism in InN and, more generally, for understanding sensing material systems beyond InN.

This paper reviews the device physics and technology of optoelectronic devices based on semiconductors of the GaN family, operating in the spectral regions from deep UV to Terahertz. Such devices include LEDs, lasers, detectors, electroabsorption modulators and devices based on intersubband transitions in AlGaN quantum wells (QWs). After a brief history of the development of the field, we describe how the unique crystal structure, chemical bonding, and resulting spontaneous and piezoelectric polarizations in heterostructures affect the design, fabrication and performance of devices based on these materials. The heteroepitaxial growth and the formation and role of extended defects are addressed. The role of the chemical bonding in the formation of metallic contacts to this class of materials is also addressed. A detailed discussion is then presented on potential origins of the high performance of blue LEDs and poorer performance of green LEDs (green gap), as well as of the efficiency reduction of both blue and green LEDs at high injection current (efficiency droop). The relatively poor performance of deep-UV LEDs based on AlGaN alloys and methods to address the materials issues responsible are similarly addressed. Other devices whose state-of-the-art performance and materials-related issues are reviewed include violet-blue lasers, 'visible blind' and 'solar blind' detectors based on photoconductive and photovoltaic designs, and electroabsorption modulators based on bulk GaN or GaN/AlGaN QWs. Finally, we describe the basic physics of intersubband transitions in AlGaN QWs, and their applications to near-infrared and terahertz devices.