bluuebunny/crossref_metadata_2025 huggingface dataset:
The Importance of Accreditation in Materials Education
Authors: Mikkola D.E.
Published: August 2016

The strength, vitality, and development of the materials professions are determined largely by the quality of education of their new members. While this truism applies to any profession, it is critically important that it be recognized in a rapidly changing and evolving area such as the materials field, which includes activities in materials engineering, ceramic engineering, polymer engineering, and metallurgical engineering. The recent development of tools and techniques that allow relating details of atomic and electronic structure to material properties, the discovery of new classes of materials, many of which are complex integrations of metals, ceramics, and/or polymers, along with the rich diversity of new processing methods that yields improved properties and new materials concepts, are examples that emphasize the dynamic and exciting nature of the field.

The primary responsibility for monitoring, evaluating, and certifying the quality of engineering and engineering-related education in colleges and universities in the United States lies with ABET, the Accreditation Board for Engineering and Technology. (For a discussion of accreditation in chemistry, see “Chemistry Education Programs Relevant to Materials Science” by P. Lykos in this issue.) ABET, working with the engineering professional societies, develops accreditation policies and criteria and conducts a comprehensive program of evaluation of first professional engineering degree programs. Programs meeting the prescribed criteria are granted accredited status.
The Metamict State
Authors: Ewing R.C. , Chakoumakos B.C. , Lumpkin G.R. , Murakami T.
Published: August 2016

Recently, the words “metamict” or “metamictization” have been increasingly used instead of “amorphous” or “amorphization” in acknowledgment of the fact that the first recognized example of the transition from the crystalline to the aperiodic state was in natural materials—minerals. The term (originally “metamikte” from Greek for “mix otherwise” because of their complex compositions) was first defined by Broegger in a Danish encyclopedia as one of three classes of amorphous substances: porodine (= colloidal), hyaline, and metamikte. Minerals considered to be metamict were judged amorphous because of their conchoidal fracture and isotropic optical properties; however, well-developed crystal faces evidenced the prior crystalline state.

Hamberg was the first to suggest that metamictization is a radiation-induced, periodic-to-aperiodic phase transition caused by alpha particles which originate from constituent radionuclides in the uranium and thorium decay series. Rinne and Vegard confirmed by x-ray diffraction studies that metamict minerals were either amorphous or finely crystalline. Later work supported the idea of a radiation-induced transformation. The work of Stackelberg and Rottenbach established that the decrease in density, refractive indices, and birefringence correlated with the breakdown of the structure with increasing alpha-decay dose. Stackelberg and Rottenback tried to test this hypothesis directly by bombarding a thin slab of zircon with alpha particles. The results were inconclusive because the slab fractured, but this must have been one of the first experiments in which an “ion beam” was used to “modify” a material.
Intercalation In Layered Materials
Authors: Dresselhaus M.S.
Published: August 2016

There is great current interest in layered anisotropic materials. Much of the interesi relates to the possibility of an approximate decoupling of 3-dimensional (3D) behavioi into ID properties perpendicular to the layer planes and 2D properties associated with the layer planes. Since this decoupling in layered materials is enhanced by intercalation, much attention has been given to intercalation compounds in recenl years as a means to study interesting questions of low dimensional physics. Since many deliberately structured materials also are prepared in layered sequences, there are many close connections between the properties of intercalated layered materials and deliberately structured materials. This article will focus on several important connections between intercalated layered materials and multilayered superlattice structures prepared by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD).

Interest in intercalation compounds has led to three lively MRS symposia on this topic (Fall 1982, Fall 1984, Fall 1986) and a number of summer schools and workshops, one of which was held in July 1986 at Erice, Sicily, under the auspices of NATO and the European Physical Society. The published lecture notes1 from this summer school provide a useful introduction and reference for students and others interested in this field.
Ion Beam Mixing: Amorphous, Crystalline, and Quasicrystalline Phases
Authors: Lilienfeld D.A. , Hung L.S. , Mayer J.W.
Published: August 2016

In the last quarter of a century, modification of the near-surface region of materials has become of major technological importance. The principal surface modification technique utilized in integrated circuit technology is ion implantation, a technique which has more recently been applied in the metal-processing industry as well. The very high doses required for applications such as increasing the hardness of steel or forming buried oxide layers in silicon have pushed ion implantation to its limits. Ion beam mixing, the intermixing of surface layers by the penetration of energetic ions through them, was developed to overcome these limits. Additionally, ion beam mixing has been able to produce new phases, amorphous and crystalline, which have technologically and scientifically interesting properties.

Ion beam mixing was studied extensively in silicide forming systems, due partly to applications to electrical contacts for silicon devices. In intermetallic alloy systems, research has concentrated on determining the interplay between the formation of amorphous and crystalline structures and that between equilibrium and metastable phases. Although over 50 alloy systems have been studied, this article will concentrate on the Al-based alloys. These alloys, particularly the near-noble-metal alloys, demonstrate nearly all the features associated with ion-induced phase formation. Further, Al-rich refractory metal alloys form quasicrystalline icosahe-dral alloys. Ion-beam mixing results parallel those of splat-quenching, the technique first used to produce the fivefold symmetric structure.
Ion-Beam-Assisted Deposition and Synthesis
Authors: Rossnagel S.M. , Cuomo J.J.
Published: August 2016

Concurrent energetic particle bombardment during film deposition can strongly modify the structural and chemical properties of the resulting thin film. The interest in this technique, ion-assisted deposition, comes about because it can be used to produce thin films with properties not achievable by conventional deposition. Bombardment by low energy ions occurs during almost all plasma-based thin film deposition techniques. Bombardment of a growing film, particularly by accelerated ions, can also be combined with non-plasma-based deposition techniques, such as evaporation, to simulate some of the effects observed with sputtering. The bombarding particle flux is usually controllable so that the arrival rate, energy, and species can be independently varied from the depositing flux. Thus, a basic aspect of ion-beam-based deposition techniques is the “control” often absent in plasma-based techniques. In plasmas, the voltage, current, and pressure are all interdependent. The energetic bombardment at the substrate-film interface depends on the various properties of the plasma, as does the deposition rate. It is often difficult, or even impossible, to decouple these processes. With ion-beam-based deposition techniques, the ion bombardment is essentially independent of the deposition process, and both can be more easily controlled.

The incident energetic particle contributes some of its energy or momentum to irreversibly change the dynamics of the film surface. The incident particle may also be incorporated into the growing film, changing the film's chemical nature. The changes induced by particle bombardment during deposition are often not characteristic of equilibrium thermodynamics because the incident particle's energy is often many times the local adsorption or binding energy.
The effects of indium concentration and well-thickness on the mechanisms of radiative recombination in InxGa1−xN quantum wells
Authors: Shapiro N. A. , Perlin Piotr , Kisielowski Christian , Mattos L. S. , Yang J. W. , Weber Eicke R.
Published: August 2016

A correlation of the local indium concentration measured on an atomic scale with luminescence properties of InxGa1−xN quantum wells reveals two different types of recombination mechanisms. A piezoelectric-field based mechanism is shown to dominate in samples with thick wells (L > 3 nm) of low indium concentration (x < 0.15−0.20). Spatial indium concentration fluctuations dominate luminescence properties in samples of higher indium concentrations in thinner wells. Quantum confinement is shown to have a major effect on the radiative recombination energy. A model is presented that relates the experimentally measured nano scale structural and chemical properties of quantum wells to the characteristics of the luminescence.
Preparation of Sapphire for High Quality III-Nitride Growth
Authors: Cui J. , Sun A. , Reshichkov M. , Yun F. , Baski A. , Morkoç H.
Published: August 2016

We developed a unique preparation technique to eliminate surface damage on the c-plane of sapphire and render it atomically flat. AFM images of c-plane sapphire annealed at 1380 °C for 1hour show terrace-like features with about 0.2 μm long terraces. The GaN layers grown by MBE on annealed sapphire have [0 0 2] symmetric and [1 0 4] asymmetric full width at half maximum (FWHM) of about 60 and 132 arcsec, respectively. This compares with 408 and 600 arcsec, respectively, for GaN grown on sapphire having gone through conventional chemical cleaning.
Influence of the Mg precursor on the incorporation of Mg in MOVPE grown GaN.
Authors: de Mierry P. , Beaumont B. , Feltin E. , Schenk H.P.D. , Gibart Pierre , Jomard F. , Rushworth S. , Smith L. , Odedra R.
Published: August 2016

Incorporation of Mg in metalorganic vapour phase epitaxy (MOVPE) GaN has been investigated, using two different Mg precursors: bis-methylcyclopentadienyl magnesium [(MeCp)2Mg] and Solution bis-cyclopentadienyl magnesium [Solution Cp2Mg]. SIMS analysis reveals an increased (two fold) efficiency of Mg incorporation for Solution Cp2Mg as compared to (MeCp)2Mg. These results are attributed to the stronger interaction of (MeCp)2Mg with NH3, leading to the formation of alkylmagnesium amine adducts, and a reduced effective Mg surface concentration. A decreased GaN growth rate with increasing Mg fluxes is also reported for both precursors. This effect is more pronounced for Solution Cp2Mg indicating that incorporation of Mg in the lattice proceeds via the capture of Mg into group III sites, and that the supply of Mg from the surface is reduced in the case when (MeCp)2Mg is used.
Solar-Blind AlGaN Heterostructure Photodiodes
Authors: Brown J.D. , Li Jizhong , Srinivasan P. , Matthews J. , Schetzina J.F.
Published: August 2016

A backside-illuminated solar-blind UV detector based on an AlGaN p-i-n heterostructure has been successfully synthesized, fabricated and tested. The p-i-n photodiode structure consists of a 1.0 μm n-type Al0.64Ga0.36N:Si layer grown by MOVPE onto a low temperature AlN buffer layer on a polished sapphire substrate. On top of this base layer is a 0.2 μm undoped Al0.47Ga0.53N active layer and a 0.5 μm p-type Al0.47Ga0.53N:Mg top layer. Square mesas of area A = 4 × 10−4 cm2 were obtained by reactive ion etching using BCl3. The solar-blind photodiode exhibits a very narrow UV spectral responsivity band peaked at 273 nm with a FWHM = 21 nm. Maximum responsivity R = 0.051 A/W at 273 nm, corresponding to an internal quantum efficiency of 27%. R0A values up to 8 × 107Ω-cm2 were obtained, corresponding to D* = 3.5 × 1012 cm Hz1/2W−1 at 273 nm.
Low Temperature Nitridation Combined With High Temperature Buffer Annealing for High Quality GaN Grown by Plasma-Assisted MBE
Authors: Namkoong Gon , Alan Doolittle W. , Kang Sangbeom , Sa Huang , Brown April S. , Stock Stuart R.
Published: August 2016

The effect of the initial nitridation of the sapphire substrate on the GaN crystal quality as a function of substrate temperature was studied. GaN layers were grown by plasma-assisted molecular beam epitaxy (MBE) on sapphire substrates nitridated at different substrate temperatures. A strong improvement in the GaN crystal quality was observed at 100 °C nitridation temperature. Symmetric (0004) and asymmetric (10-5) full widths at half maximum (FWHM) of the x-ray rocking curves were 136 and 261 arcsec, respectively. This compares to an x-ray rocking curve full width at half maximum of 818 arcsec (0004) for conventional MBE buffer conditions. For our conventional buffer conditions, sapphire substrates were exposed to a N plasma at temperatures above 500 °C for 10min and then 25~50nm buffers were deposited without annealing at high temperature. The low temperature nitridation also shows an enhancement of the lateral growth of the GaN, resulting in larger grain sizes. The largest grain size achieved was approximately 2.8μm, while the average grain size was approximately 2.4μm at 100 °C nitridation temperature.
Review of polarity determination and control of GaN
Authors: Sumiya M. , Fuke S.
Published: August 2016

Polarity issues affecting III-V nitride semiconductors are reviewed with respect to their determination and control. A set of conditions crucial to the polarity control of GaN is provided for each of the following growth techniques; molecular beam epitaxy (MBE), pulsed laser deposition (PLD) and hydride vapor phase epitaxy (HVPE). Although GaN films might have been deposited by identical growth methods using the same buffer layer technologies, there is often a conflict between the resulting polarities achieved by different research groups. In this paper, we present the implications of the conditions used in each of the processes used for two-step metalorganic chemical vapor deposition (MOCVD), demonstrating systematic control of the polarity of GaN films on sapphire substrates. The potential for confusion in polarity control will be explained, taking into account the implications clarified in our studies. The correlation between the polarity and the growth conditions will be discussed in order to provide a mechanism for the determination and control of the crystal polarity during the growth of GaN films.
Low dislocation density, high power InGaN laser diodes
Authors: Perlin Piotr , Leszczyñski M. , Prystawko P. , Wisniewski P. , Czernetzki R. , Skierbiszewski C. , Nowak G. , Purgal W. , Weyher J. L. , Kamler G. , Borysiuk J. , Krysko M. , Sarzynski M. , Suski T. , Litwin-Staszewska E. , Dmowski L. , Franssen G. , Grzanka S. , Swietlik T. , Grzegory I. , Bockowski M. , Lucznik B. , Porowski S. , Gorczyca L , Bering A. , Krupczynski W. , Makarowa I. , Wisniewska R. , Libura A.
Published: August 2016

We used single crystals of GaN, obtained from high-pressure synthesis, as substrates for Metalorganics Vapor Phase Epitaxy growth of violet and UV laser diodes. The use of high-quality bulk GaN leads to the decrease of the dislocation density to the low level of 105 cm−2, i.e. two orders of magnitude better than typical for the Epitaxial Lateral Overgrowth laser structures fabricated on sapphire. The low density and homogeneous distribution of defects in our structures enables the realization of broad stripe laser diodes. We demonstrate that our laser diodes, having 15 μm wide stripes, are able to emit 1.3-1.9 W per facet (50% reflectivity) in 30 ns long pulses. This result, which is among the best ever reported for nitride lasers, opens the path for the development of a new generation of high power laser diodes.
Aluminum Nitride-Silicon Carbide Alloy Crystals Grown on SiC Substrates by Sublimation
Authors: Gu Z. , Du L. , Edgar J.H. , Payzant E.A. , Walker L. , Liu R. , Engelhard M.H.
Published: August 2016

AlN-SiC alloy crystals, with a thickness greater than 500 µm, were grown on 4H- and 6H-SiC substrates from a mixture of AlN and SiC powders by the sublimation-recondensation method at 1860-1990 °C. On-axis SiC substrates produced a rough surface covered with hexagonal grains, while 6H- and 4H- off-axis SiC substrates with different miscut angles (8° or 3.68°) formed a relatively smooth surface with terraces and steps. The substrate misorientation ensured that the AlN-SiC alloy crystals grew two dimensionally as identified by scanning electron microscopy (SEM). X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed that the AlN-SiC alloys had the wurtzite structure. Electron probe microanalysis (EPMA) and x-ray photoelectron spectroscopy (XPS) demonstrated that the resultant alloy crystals had non-stoichiometric ratios of Al:N and Si:C and a uniform composition throughout the alloy crystal from the interface to the surface. The composition ratio of Al:Si of the alloy crystals changed with the growth temperature, and differed from the original source composition, which was consistent with the results predicted by thermodynamic calculation of the solid-vapor distribution of each element. XPS detected the bonding between Si-C, Si-N, Si-O for the Si 2p spectra. The dislocation density decreased with the growth, which was lower than 106 cm−2 at the alloy surface, more than two orders of magnitude lower compared to regions close to the crystal/substrate interface, as determined by TEM.
Characteristic of InGaN/GaN Laser Diode Grown by a Multi-Wafer MOCVD System
Authors: Park Y. , Kim B.j. , Lee J. W. , Nam O. H. , Sone C. , Park H. , Eunsoon Oh , Shin H. , Chae S. , Cho J. , Kim Ig-Hyeon , Khim J.S. , Cho S. , Kim T.I.
Published: August 2016

InGaN/GaN multi-quantum well (MQW) laser diodes (LDs) were grown on c-plane sapphire substrates using a multi-wafer MOCVD system. The threshold current for pulsed lasing was 1.6 A for a gain-guided laser diode with a stripe of 10 × 800 μm2. The threshold current density was 20.3 kA cm−2 and the threshold voltage was 16.5 V. The optical power ratio of transverse electric mode to transverse magnetic mode was found to be greater than 50. The characteristic temperature measured from the plot of threshold current versus measurement temperature was between 130 and 150K.
Optical phonons and free-carrier effects in MOVPE grown AlxGa1−xN measured by Infrared Ellipsometry
Authors: Schubert M. , Kasic A. , Tiwald T.E. , Off J. , Kuhn B. , Scholz Ferdinand
Published: August 2016

We report on the application of infrared spectroscopic ellipsometry (IR-SE) for wavenumbers from 333cm−1to 1200cm−1as a novel approach to non-destructive optical characterization of free-carrier and optical phonon properties of group III-nitride heterostructures. Undopedα-GaN,α-AlN,α-AlxGa1−xN (x= 0.17, 0.28, 0.5), andn-type silicon (Si) dopedα-GaN layers were grown by metal-organic vapor phase epitaxy (MOVPE) onc-plane sapphire (α-Al2O3). The four-parameter semi-quantum (FPSQ) dielectric lattice-dispersion model and the Drude model for free-carrier response are employed for analysis of the IR-SE data. Model calculations for the ordinary (∈⊥) and extraordinary (∈||) dielectric functions of the heterostructure components provide sensitivity to IR-active phonon frequencies and free-carrier parameters. We observe that theα-AlxGa1−xN layers are unintentionally doped with a back ground free-carrier concentration of 1–4 1018cm−3. The ternary compounds reveal a two-mode behavior in ∈⊥, whereas a one-mode behavior is sufficient to explain the optical response for ∈||. We further provide a precise set of model parameters for calculation of the sapphire infrared dielectric functions which are prerequisites for analysis of infrared spectra of III-nitride heterostructures grown onα-Al2O3.
Photoluminescence, Magnetospectroscopy, and Resonant Electronic Raman Studies of Heteroepitaxial Gallium Nitride
Authors: Skromme B.J.
Published: August 2016

Optical spectroscopy, including low and room temperature photoluminescence (PL), reflectance, PL measurements in high magnetic fields up to 12 T, and resonantly-enhanced electronic Raman scattering (RERS) in zero and high magnetic field, has been used to investigate exciton and impurity states and surface recombination in high quality heteroepitaxial GaN grown on sapphire and SiC. Theoretical finite-difference calculations of the donor states as a function of magnetic field have been carried out for comparison, including the effects of anisotropy in the effective mass and dielectric constant. Up to six residual donor species are observed in material grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) from their n=2 and n=3 two-electron satellites observed in PL and by RERS. The donor-related nature of the relevant transitions is confirmed from their magnetic field dependence, and the spectral resolution is improved at high fields. The Si donor level is determined to have a binding energy of about 21 meV from observation of its two-electron satellite in lightly Si-doped HVPE material. The free exciton binding energy is shown to be about 26.4 meV, independent of strain, based on observations of the n=2 free exciton. The room temperature band-edge PL peak is confirmed to be free excitonic in nature, based on its linewidth and on comparison with simple reflectance measurements. Reflectance from the edge of a thick HVPE layer shows clear evidence of A, B, and C excitons obeying the relevant selection rules at both low and room temperature. Surface chemical treatments are shown to have substantial effects on room temperature PL efficiency. Passivation with ammonium or sodium sulfide solutions, in particular, yields increases in PL efficiency by a factor of five to seven over air-exposed surfaces. The passivation effect is stable in air, lasting at least one month.
Radiative recombination in In0.15Ga0.85N/GaN multiple quantum well structures
Authors: Monemar B. , Bergman J. P. , Dalfors J. , Pozina G. , Sernelius B.E. , Holtz P.O. , Amano H. , Akasaki I.
Published: August 2016

We present a study of the radiative recombination in In0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature (< 800 °C) and donor doping. The photoluminescence peak position varies strongly (over a range as large as 0.3 eV) with delay time after pulsed excitation, but also with donor doping and with excitation intensity. The peak position is mainly determined by the Stark effect induced by the piezoelectric field. In addition potential fluctuations, originating from segregation effects in the InGaN material, from interface roughness, and the strain fluctuations related to these phenomena, play an important role, and largely determine the width of the emission. These potential fluctuations may be as large as 0.2 eV in the present samples, and appear to be important for all studied growth temperatures for the InGaN layers. Screening effects from donor electrons and excited electron-hole pairs are important, and account for a large part of the spectral shift with donor doping (an upward shift of the photoluminescence peak up to 0.2 eV is observed for a Si donor density of 2 × 1018cm−3in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). We suggest a two-dimensional model for electron- and donor screening in this case, which is in reasonable agreement with the observed data, if rather strong localization potentials of short range (of the order 100 Å) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in the rather strong lateral potential fluctuations present at this In composition is discussed
Electron Overflow to the AlGaN p-Cladding Layer in InGaN/GaN/AlGaN MQW Laser Diodes
Authors: Domen Kay , Soejima Reiko , Kuramata Akito , Tanahashi Toshiyuki
Published: August 2016

Current flow through an InGaN/GaN/AlGaN multi-quantum well (MQW) laser diode is simulated. We found that electron overflow to the AlGaN p-cladding layer is very large, which prevents the current injection into the MQW layers. We clarified that the electron overflow occurs easily in nitride lasers because of three intrinsic reasons; poor hole injection due to the small hole mobility and thermal velocity, the small conduction band offset for InGaN/GaN, and the high threshold carrier density. We show that the Al composition and the p-doping of the AlGaN p-cladding layer is of critical importance to obtain laser oscillation by current injection.
Study of the Epitaxial Lateral Overgrowth (ELO) Process for GaN on Sapphire Using Scanning Electron Microscopy and Monochromatic Cathodoluminescence
Authors: Yu Zhonghai , Johnson M.A.L. , Mcnulty T. , Brown J.D. , Cook J.W. , Schetzina J.F.
Published: August 2016

Growth of GaN by MOVPE on mismatched substrates such as sapphire or SiC produces a columnar material consisting of many hexagonal grains ~1 μm across. In contrast, the epitaxial-lateral-overgrowth (ELO) process creates a new material — single-crystal GaN. We have studied the ELO process using GaN/sapphire layers patterned with SiO2 stripes. SEM images show that the (0001) GaN surface remains very flat as the ELO progresses. Cathodoluminescence images at 590 nm reveal spotty yellow-green emission from the columnar GaN as it emerges from the window areas. Very bright 590 nm emission occurs as the ELO process begins. We associate this deep-level cathodoluminescence with the strain field that accompanies the conversion of columnar GaN into single-crystal GaN via the ELO process. As the ELO process continues across the SiO2 stripes, the 590 nm emission disappears and is replaced with pure band edge cathodoluminescence at 365 nm which is maintained until coalescence of adjacent ELO layers occurs near the centers of the SiO2 stripes.
Surface Reconstruction during Molecular Beam Epitaxial Growth of GaN (0001)
Authors: Smith A. R. , Ramachandran V. , Feenstra R. M. , Greve D. W. , Ptak A. , Myers T. , Sarney W. , Salamanca-Riba L. , Shin M. , Skowronski M.
Published: August 2016

Surface reconstructions during homoepitaxial growth of GaN (0001) are studied using reflection high-energy electron diffraction and scanning tunneling microscopy. In agreement with previous workers, a distinct transition from rough to smooth morphology is seen as a function of Ga to N ratio during growth. However, in contrast to some prior reports, no evidence for a 2×2 reconstruction during GaN growth is observed. Observations have been made using four different nitrogen plasma sources, with similar results in each case. A 2×2 structure of the surface can be obtained, but only during nitridation of the surface in the absence of a Ga flux.
Surface Morphology of MBE-grown GaN on GaAs(001) as Function of the N/Ga-ratio
Authors: Zsebök O. , Thordson J.V. , Andersson T.G.
Published: August 2016

Molecular beam epitaxy growth utilising an RF-plasma nitrogen source was used to study surface reconstruction and surface morphology of GaN on GaAs (001) at 580 °C. While both the nitrogen flow and plasma excitation power were constant, the grown layers were characterised as a function of Ga-flux. In the initial growth stage a (3×3) surface reconstruction was observed. This surface periodicity only lasted up to a maximum thickness of 2.5 ML, followed by a transition to the unreconstructed surface. Samples grown under N-rich, Ga-rich and stoichiometric conditions were characterised by high-resolution scanning electron microscopy and atomic force microscopy. We found that the smoothest surfaces were provided by the N/Ga-ratio giving the thickest layer at the (3×3)=>(1×1) transition. The defect formation at the GaN/GaAs interface also depended on the N/Ga-flux ratio.
Luminescence of Be-doped GaN layers grown by molecular beam epitaxy on Si (111).
Authors: Sánchez F. J. , Calle F. , Sanchez-Garcia M.A. , Calleja E. , Muñoz E. , Molloy C. H. , Somerford D. J. , Koschnick F. K. , Michael K. , Spaeth J.-M.
Published: August 2016

Low temperature photoluminescence spectra of Be-doped layers grown on Si (111) by molecular beam epitaxy have been analyzed. Emissions at 3.466 eV and 3.384 eV, and a broad band centered at 2.4-2.5 eV are observed. Their evolution with temperature and excitation power, and time resolved PL measurements ascribe an excitonic character for the luminescence at 3.466 eV, whereas the emission at 3.384 eV is associated with a donor-acceptor pair transition. This recombination involves residual donors and Be-related acceptors, which are located around 90meV above the valence band, confirming Be as the shallowest acceptor reported in GaN. The intensity of the band at 2.4-2.5 eV increases with the Be content. This emission involves a band of deep acceptors generated by Be complex defects, as suggested by the parameter g = 2.008 ± 0.003 obtained by photoluminescence-detected electron paramagnetic resonance.
Mg-enhanced lateral overgrowth of GaN on patterned GaN/sapphire substrate by selective Metal Organic Vapor Phase Epitaxy
Authors: Beaumont B. , Vaille M. , Nataf G. , Bouillé A. , Guillaume J.-C. , Vénnègues P. , Haffouz S. , Gibart Pierre
Published: August 2016

Selective and lateral overgrowth by Metal Organics Vapour Phase Epitaxy (MOVPE) was carried out until coalescence to produce smooth and optically flat thick GaN layers. A GaN epitaxial layer is first grown using atmospheric pressure Metalorganic Vapour Phase Epitaxy on a {0001} Al2O3. substrate. Then a 30Å silicon nitride dielectric film is deposited in-situ by reaction of silane and ammonia to form a selective mask. Afterwards, the openings and the figures in the dielectric films are achieved using standard photolithographic technology. Stripes openings in the mask, revealing free GaN surface, are aligned in the 〈100〉 direction. Typical stripes spacing and width are 10 µm and 5 µm respectively. These patterned layers are further on used for epitaxial regrowth of GaN by MOVPE. The growth anisotropy and therefore the coalescence process is achieved by introducing (MeCp)2Mg in the vapour phase. A two-step process is reported which allows a dramatic reduction of threading dislocations density not only above the masked areas but also above the windows opened in the mask. With this process, very sharp bound exciton luminescence peaks are measured at low temperature in the overgrown GaN.
Analysis of the Visible and UV Electroluminescence in Homojunction GaN LED's
Authors: Calle F. , Monroy E. , Sánchez F. J. , Muñoz E. , Beaumont B. , Haffouz S. , Leroux M. , Gibart Pierre
Published: August 2016

The electrical and electroluminescent properties of MOVPE GaN p-n homojunctions have been analyzed as a function of temperature and bias. Electroluminescence is observed for V>3 V under dc and ac conditions. The main emission at low T is a donor-acceptor transition involving shallow acceptors, though it disappears at higher T due to the ionization of the acceptors and compensation by ionized donors. Room temperature dc and ac electroluminescence spectra evolve under increasing bias from a blue-shifting visible band involving deep states at the p-type side of the p-n junction, to a band-to-band UV recombination at high bias. In agreement, the superlinear dependence of light-current characteristics at low current injection becomes linear when the defects are saturated. Time analysis of the spectra vs pulse duration and duty cycle allows the determination of the visible radiative recombination and relaxation times associated to the Mg-related deep states, which are found to behave as acceptors lying 0.55 eV above the valence band. A simple 3-level model is able to explain the visible emission, which involves the conduction band (or shallow donor) and those deep acceptors in the p-layer. Optimum UV/visible ratio emission requires intense and relatively long pulses, with a high duty cycle to impede visible recombination.
Thermodynamic properties of group-III nitrides and related species
Authors: Przhevalskii I. N. , Karpov S. Yu. , Makarov Yu. N.
Published: August 2016

A database for thermodynamic properties of group-III nitrides and relevant species involved into growth of these materials is developed in this paper. Standard formation enthalpies of materials and coefficients of polynomial approximations of the reduced Gibbs free energies are collected in the tables. They allow one to determine the Gibbs free energy, enthalpy, entropy and specific heat of a species as a function of temperature. The database covers solid and gaseous group-III nitrides, elemental species, gaseous metal-organic compounds, chlorides and hydrides of group-III elements, nitrogen containing precursors and organic byproducts of various chemical reactions proceeding during growth processes. Thermodynamic properties of adducts which can be formed in the vapor phase while mixing ammonia and metal-organic compounds are presented in the database as well. Much of the data given in this paper is presented for the first time. All the data are checked for self-consistency and therefore can be used for thermodynamic calculations.
Properties of GaN epilayers grown on misoriented sapphire substrates
Authors: Trager-Cowan Carol , McArthur S. , Middleton P. G. , O'Donnell K. P. , Zubia D. , Hersee S. D.
Published: August 2016

Three silicon-doped 3 µm thick GaN epilayers were grown simultaneously by metalorganic chemical vapour deposition on (0001) sapphire substrates misorientated by 0°, 4° and 10° toward the m-plane (100). A comparative study of these epilayers was undertaken using photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), cathodoluminescence (CL) imaging, CL spectroscopy and Hall effect measurements. Low temperature PL of the 0° and 4° epilayers shows donor bound exciton (BE) emission between 3.47 and 3.48 eV and a low level of yellow band emission. The peak intensities of both emission bands are a factor of 2 higher for the 4° layer. In the 10° epilayer, the BE band is 3x stronger than in the 0° epilayer but there is no discernible yellow band. However, a number of additional bands appear at 3.459, 3.417, 3.362, 3.345, 3.309, and 3.285 eV. These bands, some of which are acceptor related, may be attributed to the presence of structural defects in this epilayer, pointing to an abrupt degradation of its structural quality compared to the others. This degradation is confirmed by AFM studies. On a 20 µm x 20 µm image the 0° and 4° epilayers exhibit smooth surface morphologies, while the 10° epilayer shows a high density of hexagonal pits. Finally, SEM images reveal the surface of the 10° epilayer to be “streaked” and pitted. Low temperature CL images at 3.48 eV (bound exciton region) show random spotty emission, while those at 3.28 eV and 3.41 eV exhibit a streaky appearance similar to the SEM image. This suggests that these luminescence bands are indeed associated with structural defects.
Pinholes, Dislocations and Strain Relaxation in InGaN
Authors: Jahnen B. , Albrecht M. , Dorsch W. , Christiansen S. , Strunk H. P. , Hanser D. , Davis Robert F.
Published: August 2016

We analyse by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM) the strain relaxation mechanisms in InGaN layers on GaN as dependent on the In content. At the experimentally given thickness of 100 nm, the layers remain coherently strained, up to an In concentration of 14 %. We show that part of the strain is reduced elastically by formation of hexagonally facetted pinholes. First misfit dislocations are observed to form at pinholes that reach the InGaN/GaN interface. We discuss these results in the framework of the Matthews-Blakeslee model for the critical thickness considering the Peierls force for glide of threading dislocations in the different slip systems of the wurtzite lattice.
Macro- and microstrains in MOCVD-grown GaN
Authors: Usikov A. , Ratnikov V.V. , Kyutt R. , Lundin W. V. , Pushnyi B. , Shmidt N. M. , Scheglov M.P.
Published: August 2016

Undoped and Si-doped GaN films were grown by low pressure MOCVD on (0001) sapphire substrates. The angular distribution of the X-ray diffraction corresponding to the (0002), (0004), (100), (200), and (114) reflections has been measured by means of double- and triple -crystal diffractometry with Mo Kα1 and Cu Kα1 radiation under conditions of symmetrical and asymmetrical Bragg- and Laue-geometry. In our experiments a non-coplanar geometry was also applied. On the basis of the performed studies, five independent components of the tensor of microdistortion were evaluated and the average grain-size in two directions was determined. The type, position, and density of dislocations were established as well. The role of dislocations in strain relaxation and their influence on the optical and electrical properties are discussed.
Nucleation of AlN on the (7×7) Reconstructed Silicon (1 1 1) Surface
Authors: Hellman E. S. , Buchanan D. N. E. , Chen C. H.
Published: August 2016

The (7×7) reconstructed (1 1 1) surface of silicon is found to be an excellent surface for the nucleation of epitaxial aluminum nitride, despite the +23.4% misfit in the AlN/Si system. AlN nucleated above the (7×7) to (1×1) transition temperature (830°C) is found to contain 30° misoriented grains, while films nucleated below the transition temperature are single orientation. Optimized aluminum nitride films grown on (7×7) silicon surfaces make excellent substrates for GaN heteroepitaxy.
GaN Single Crystal Habits and Their Relation to GaN Growth Under High Pressure of Nitrogen
Authors: Prywer Jolanta , Krukowski S.
Published: August 2016

In the growth of GaN from nitrogen dissolved in Ga under high N2 pressure, two main habits are observed: plate-like and needle-like. The plate-like crystals can be divided into those having (0001), (000) and {100} faces and those with the additional {101} and {102} faces. The needle-like crystals belong to three classes: with or without (0001) faces and a third with unusual, star-like needles. The plate-like and needle-like habits and transformation between these habits are discussed in greater detail. It is shown that it is possible to evaluate the relative growth rates corresponding to such transitions.
Current status of GaN crystal growth by sublimation sandwich technique
Authors: Baranov P. G. , Mokhov E. N. , Ostroumov A. O. , Ramm M. G. , Ramm M. S. , Ratnikov V. V. , Roenkov A. D. , Vodakov Yu. A. , Wolfson A. A. , Saparin G. V. , Karpov S. Yu. , Zimina D. V. , Makarov Yu. N. , Juergensen Holger
Published: August 2016

The current status of GaN crystal growth using the Sublimation Sandwich Technique is discussed in the paper. We use modeling to analyze gas dynamics in the reactor and the supply of the main gaseous species into the growth cell under growth conditions used in experiments. Important features of growth process — non-equilibrium cracking of ammonia, partial sticking of ammonia at the growing surface and kinetic limitation of GaN thermal decomposition — are taken into account in the model. Growth is carried out on sapphire and 6H-SiC substrates in ammonia atmosphere using a Ga/GaN mixture as the group-III element source. Single crystals of GaN of size 15×15 mm and up to 0.5 mm thick are normally grown with the optimized growth rates of 0.25-0.35 mm/h. The GaN crystals are characterized by photoluminescence, by the Color Cathodoluminescence Scanning Electron Microscopy technique, by differential double-crystal and triple-crystal X-ray diffractometry, and by electron paramagnetic resonance. Mechanisms of sublimation growth of GaN and physical limitations of the growth process are discussed.
Temperature-Composition Dependence of the Bandgap and Possible Non-complanar Structures in GaN-AlN, GaN-InN and InN-AlN Mixed Crystals
Authors: Kalashnikov E. V. , Nikolaev V. I.
Published: August 2016

The virtual crystal approximation has been used to determine the temperature-composition dependence of the GaN-AlN, GaN-InN, and InN-AlN band gap energies. Also, the thermodynamic instability states in the mixed crystals were studied. The expression for the band gap of mixed A-B crystals has been derived: EgAB = (1−x)EgA + xEgB − bSxx, where EgA and EgB are the direct gaps for compounds A and B, respectively, and x is the alloy concentration. The term Sxx ~ T0/(∂2G/∂x2) where G is the thermodynamic potential of the mixed crystal, b is a bowing parameter, and T0 has the meaning of a growth temperature.
Improved optical activation of ion-implanted Zn acceptors in GaN by annealing under N2 overpressure
Authors: Pelzmann A. , Strite S. , Dommann A. , Kirchner C. , Kamp Markus , Ebeling K. J. , Nazzal A.
Published: August 2016

We investigated the properties of ion-implanted GaN:Zn annealed under various conditions using photoluminescence (PL) and high resolution x-ray diffraction (HRXRD). Epitaxial GaN/sapphire of high optical quality was ion-implanted with a 1013 cm−2 dose of Zn+ ions at 200 keV. The sample was capped with 200 Å of SiNx and then diced into numerous pieces which were annealed under varied conditions in an attempt to optically activate the Zn. Annealing was performed in a tube furnace under flowing N2, an atmospheric pressure MOCVD reactor under flowing NH3 or N2, and under an N2 overpressure of 190 atm. The observed improvement in the optical quality of GaN:Zn annealed under N2 overpressure yields further insights into the trade-off between defect annealing and N loss from the GaN crystal.
Avalanche Breakdown Luminescence of InGaN/AlGaN/GaN Heterostructures
Authors: Manyakhin F. , Kovalev A. , Kudryashov V.E. , Turkin A.N. , Yunovich A.E.
Published: August 2016

Luminescence spectra of InGaN/AlGaN/GaN p-n-heterostructures were studied at reverse bias sufficient for impact ionization. There is a high electric field in the active InGaN-layer, and the tunnel component of the current dominates at the low reverse bias. Avalanche breakdown begins at |Vth|> 8⋄10 V, i.e. ≈3 Eg/e. Radiation spectra have a short wavelength edge 3.40 eV, and maxima in the range 2.60⋄2.80 eV corresponding to the injection spectra. Mechanisms of the hot plasma recombination in p-n-heterojunctions are discussed.
GaInN/GaN-Heterostructures and Quantum Wells Grown by Metalorganic Vapor-Phase Epitaxy
Authors: Sohmer A. , Off J. , Bolay H. , Härle V. , Syganow V. , Seo Im Jin , Wagner V. , Adler F. , Hangleiter Andreas , Dörnen A. , Scholz Ferdinand , Brunner D. , Ambacher O. , Lakner H.
Published: August 2016

The dependence of the In-incorporation efficiency and the optical properties of MOVPE-grown GaInN/GaN-heterostructures on various growth parameters has been investigated. A significant improvement of the In-incorporation rate could be obtained by increasing the growth rate and reducing the H2-partial pressure in the MOVPE reactor. However, GaInN layers with a high In-content typically show an additional low energy photoluminescence peak, whose distance to the band-edge increases with increasing In-content. For GaInN/GaN quantum wells with an In-content of approximately 12%, an increase of the well thickness is accompanied by a significant line broadening and a large increase of the Stokes shift between the emission peak and the band edge determined by photothermal deflection spectroscopy. With a further increase of the thickness of the GaInN layer, a second GaInN-correlated emission peak emerges. To elucidate the nature of these optical transitions, power-dependent as well as time-resolved photoluminescence measurements have been performed and compared to the results of scanning transmission electron microscopy.
Yellow luminescence in Mg-doped GaN
Authors: Sánchez F. J. , Calle F. , Basak D. , Tijero J. M. G. , Sánchez-García M. A. , Monroy E. , Calleja E. , Muñoz E. , Beaumont B. , Gibart Pierre , Serrano J.J. , Blanco J.M.
Published: August 2016

Optical thresholds, that correspond to a level located at 1 eV above the valence band, are observed by photocapacitance techniques in n-type Mg-doped GaN. In undoped GaN, this level has been previously related to the yellow emission detected by photoluminescence. In Mg-doped GaN, this yellow luminescence is only observed for excitation energies below the Mg-related band (2.9 - 3 eV). This result evidences that Mg-doping may reduce but not avoid the formation of the yellow band related defects in n-type and semiinsulating Mg-doped samples. The fact that the yellow luminescence is not observed for excitation energies above the bandgap may be justified by a higher efficiency of the Mg-related recombination path.
Properties of InGaN deposited on Glass at Low Temperature
Authors: Beierlein Tilman , Strite S. , Dommann A. , Smith D. J.
Published: August 2016

We have investigated the properties of InGaN grown at low temperature on glass substrates by a plasma enhanced MBE process. The goal of this study was to evaluate the potential of InGaN as an oxide-free, transparent conductor material which could be deposited at or slightly above room temperature with minimal interaction or damage to the underlying material. InxGa1−xN films deposited on glass, even without substrate heating, are highly crystalline, but the crystallinity as measured by x-ray degrades at x < 0.5. The microstructure observed by TEM of InGaN films deposited on unheated substrates is highly columnar, with typical column widths of ~10 nm. The optical absorption spectra of InGaN/glass have a distinct absorption edge at the bandgap, but also high background absorption in the bandgap. InxGa1−xN grown on glass (x > 0.5) is conductive due to its high electron concentration. InN electron Hall mobilities > 20 cm2/Vs when grown at 400°C, and ~ 7 cm2/Vs on unheated substrates were obtained. The addition of GaN degraded the electrical properties of the films to a greater extent than it improved the transparency. As a result, the best transparent conductor films were pure InN which, when deposited at 400°C, were half as transparent in the green as an indium tin oxide film having the same sheet resistance.
GaN based LED's with different recombination zones
Authors: Schauler M. , Kirchner C. , Mayer M. , Pelzmann A. , Eberhard F. , Kamp Markus , Unger P. , Ebeling K. J.
Published: August 2016

GaN based homo- and heterotype LED's have been fabricated and characterized which emit in the blue and ultra-violet part of the spectral range. Complete epitaxial LED layer sequences with different recombination zones have been grown using MOVPE as well as MBE. Subsequent to the material growth, chemically-assisted ion-beam etching and contact metallization are utilized to achieve full LED devices. MBE-grown homotype LED's reveal a peak in the output light spectrum at a wavelength of 372 nm with a linewidth being as narrow as 12 nm. GaN/InGaN LED's grown by MOVPE show visible single peak emission with linewidths of 23 nm. The optical output power as measured in a calibrated Ulbricht sphere is in the 1 μW regime.