Metal-rich Conditions Research Articles

First-principles study of intrinsic point defects in hexagonal barium titanate

Density functional theory (DFT) calculations have been used to study the nature of intrinsic defects in the hexagonal polymorph of barium titanate. Defect formation energies are derived for multiple charge states, and due consideration is given to finite-size effects (elastic and electrostatic) and the band gap error in defective cells. Correct treatment of the chemical potential of atomic oxygen means that it is possible to circumvent the usual errors associated with the inaccuracy of DFT calculations on the oxygen dimer. Results confirm that both mono- and di-vacancies exist in their nominal charge states over the majority of the band gap. Oxygen vacancies are found to dominate the system in metal-rich conditions with face sharing oxygen vacancies being preferred over corner sharing oxygen vacancies. In oxygen-rich conditions, the dominant vacancy found depends on the Fermi level. Binding energies also show the preference for metal-oxygen di-vacancy formation. Calculated equilibrium concentrations of vacancies in the system are presented for numerous temperatures. Comparisons are drawn with the cubic polymorph as well as with previous potential-based simulations and experimental results.

Magnetic properties of Cu-doped GaN grown by molecular beam epitaxy

Cu-doped GaN films were epitaxially grown by molecular beam epitaxy on $C$-plane sapphire substrates with an AlN buffer layer. Growth under metal-rich and nitrogen-rich conditions was investigated for different Cu-to-Ga beam-equivalent-pressure ratios ${x}_{\mathrm{BEP}}$. The samples were characterized by scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction. Films within a narrow range of ${x}_{\mathrm{BEP}}$ around 1$%$ exhibit ferromagnetic behavior with a Curie temperature higher than 400 K. For higher ${x}_{\mathrm{BEP}}g1%$, islands of a Cu-Ga compound are predominantly formed at the surface as nonferromagnetic precipitates. Our detailed study shows that the saturation magnetization of ferromagnetic films with ${x}_{\mathrm{BEP}}\ensuremath{\approx}1%$ decreases with increasing film thickness. This suggests that the ferromagnetism arises from defects, such as threading dislocations, created by the incorporation of Cu into the GaN.

Realization of two‐dimensional electron gas in AlGaN/GaN HEMT structure grown on Si(111) by PA‐MBE

AbstractWe report on the growth and characterization of AlN, GaN and AlGaN/GaN HEMT structure on 100‐mm Si (111) by PA‐MBE. AlN and GaN layers were grown as function of metal fluxes at a constant active nitrogen supply. Smooth surface morphology was achieved with metal rich growth conditions. Electrical properties of GaN were investigated as a function of Ga‐flux. For the first time, the presence of 2DEG is demonstrated in AlGaN/GaN HEMT structure on Si (111) substrate by PA‐MBE. A high electron mobility of 1100 cm2/V.sec is obtained with a sheet carrier density of 9 × 1012 cm‐2 (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Application of Droplet Elimination Process by Radical-Beam Irradiation to InGaN Growth and Fabrication of InN/InGaN Periodic Structure

A growth method, named droplet elimination by radical-beam irradiation (DERI), consisting of a metal-rich growth process (MRGP) and a droplet elimination process (DEP), was developed for the growth of InGaN ternary alloys. In the MRGP using the growth of InGaN under a metal-rich condition [N*/(In+Ga)<1], Ga was preferentially incorporated into the growing InGaN and In was forced out to the surface. This In was transformed to epitaxial InN on top of the InGaN underlayer in the DEP using a subsequent nitrogen radical-beam irradiation. By repeating the MRGP and DEP, an InN/InGaN periodic structure was successfully fabricated.

Indium Gallium Nitride on Germanium by Molecular Beam Epitaxy

ABSTRACTIndium containing III-Nitride layers are predominantly grown by heteroepitaxy on foreign substrates, most often Al2O3, SiC and Si. We have investigated the epitaxial growth of InxGa1-xN (InGaN) alloys on Ge substrates. First we looked at the influence of buffer layers between the InGaN and Ge substrate. When applying a high temperature (850 °C) GaN buffer, the InGaN showed superior crystal quality. Furthermore the influence of growth parameters on the structural quality and composition of InGaN layers has been looked into. For a fixed gallium and nitrogen supply, the indium beam flux was increased incrementally. For both nitrogen- as well as for metal (Ga + In) rich growth conditions, the In incorporation increases for increasing In flux. However, for metal rich growth conditions, segregation of metallic In is observed. An optimum in crystal quality is obtained for a metal:nitrogen flux ratio close to unity. The XRD FWHM of the GaN (0002) reflection increases significantly after InGaN growth. Apparently the presence of indium deteriorates the GaN buffer during InGaN growth. The mechanism of the effect is not known yet.

Surface Characterization of Ga-doped ZnO layers

ABSTRACTEpitaxial ZnO layers heavily doped with Ga (GZO) were grown at 400 °C under metaland oxygen-rich conditions in terms of metal-to-reactive oxygen ratio by plasma-assisted molecular beam epitaxy (MBE). Several atomic force microscopy (AFM) techniques were used to characterize the surface morphology and electrical properties of these GZO films in ambient conditions. Local I-V spectra indicate that layers grown under both O-rich and metal-rich conditions are highly resistive until a relatively high voltage sweep (±12 V) is used. After removal of an insulating surface layer, conduction is possible at lower voltages, but eventually the film resistivity increases and it again becomes insulating. In addition to local I-V spectra, local charge injection and subsequent surface potential measurements were used to probe surface charging characteristics. For charge injection experiments, a reverse-bias voltage is applied to the sample while scanning in contact mode with a metallized tip. The resultant change in surface potential due to trapped charge is subsequently observed using scanning Kelvin probe microscopy (SKPM). The layers deposited in a metal-rich environment demonstrate the expected behavior, but the O-rich layers show anomalous negative and positive charging. Finally, surface photovoltage (SPV) measurements using above-bandgap UV illumination were performed. The GZO layers produce SPV values of 0.4 to 0.5 eV, where the films deposited in an O-rich environment have slightly higher SPV values and faster restoration.

Growth and characterization of InGaN by RF-MBE

In this contribution we report on the growth of coherently strained, pseudomorphic InGaN layers on GaN templates by RF-MBE. Structural properties characterized by HR-XRD exhibit good crystalline quality and homogeneity of the grown layers. The incorporation efficiency of In into the InGaN layer was estimated and compared to the literature. Characterization by AFM and in situ RHEED observation showed differences in surface morphology between nitrogen rich and metal rich growth conditions. Further investigations of the optical properties were carried out by PL and spectroscopic ellipsometry measurements.

Interfacial structure of semipolar AlN grown on m -plane sapphire by MBE

The interfacial properties of epilayers grown on m-plane sapphire by plasma-assisted molecular-beam epitaxy (PAMBE) were investigated in the context of achieving high-quality semipolar GaN/AlN heterostructures with reduced internal polarization. Epilayers grown under metal-rich conditions with and without nitridation pre-treatment of the sapphire surface were compared using transmission electron microscopy (TEM). In addition to the dominant semipolar orientation, the films exhibited an interfacial zone comprising AlN nanocrystals with the m-plane orientation. The size of the nonpolar nanocrystals increased significantly with sapphire nitridation. A good epitaxial relationship with the substrate was attained by both the semipolar as well as the nonpolar AlN. However, the interfacial structure was perturbed by interfacial roughness and pit formation. The surface nanofacets were correlated to the increased introduction of nanocrystals with m-plane orientation.

Effect of Growth Conditions on Structural and Electrical Properties of Ga-doped ZnO Films Grown by Plasma-assisted MBE

AbstractZnO has recently attracted a great deal of attention as a material for transparent contacts in light emitters and adsorbers. ZnO films heavily doped with Ga (carrier concentration in the range of 1020 - 1021 cm-3) were grown on a-plane sapphire substrates by RF plasma-assisted molecular beam epitaxy. Oxygen pressure during growth (i.e. metal (Zn+Ga)–to–oxygen ratio) was found to have a crucial effect on structural, electrical, and optical properties of the ZnO:Ga films. As-grown layers prepared under metal-rich conditions exhibited resistivities below 3×10-4 Ω-cm and an optical transparency exceeding 90% in the visible spectral range. In contrast, the films grown under the oxygen-rich conditions required thermal activation and showed inferior structural, electrical, and optical characteristics even after annealing.

AlGaN quantum well structures for deep-UV LEDs grown by plasma-assisted MBE using sub-monolayer digital-alloying technique

Al-rich AlGaN layers and quantum well (QW) structures with a reasonable structural quality have been grown by plasma-assisted molecular beam epitaxy (PA-MBE) under metal-rich stoichiometric conditions directly on c-sapphire. The bowing parameter of 1.1 eV for the compositional dependence of AlGaN band gap energy has been determined using an electron-probe microanalysis, Raman spectroscopy and optical transmission spectroscopy. A successful application of a sub-monolayer digital-alloying (SMDA) technique in PA-MBE for fabrication of AlGaN-based multiple QW structures, including deep-UV MQW light-emitting diodes, has been demonstrated for the first time. These structures exhibit both photo- and electroluminescence in the spectral range of 300–320 nm at room temperature.

Closed‐loop MBE growth of droplet‐free GaN with very metal rich conditions using Metal Modulated Epitaxy with Mg and In

AbstractImprovements to the Metal Modulated Epitaxy (MME) technique are achieved through computer controlled shutter transitions based on feedback from RHEED transients (S. D. Burnham and W. A. Doolittle, J. Vac. Sci. Technol. B 24, 2100 (2006) [1]), thus creating a closed‐loop control system for nitride MBE, the first of its kind. A high‐sensitivity 22 bit camera is used to determine when RHEED transients have subsided, upon which a shutter transition is initiated allowing the efficient buildup and depletion of the metal bilayer, which improves surface morphology and growth rate compared to the standard MME technique. RMS surface roughness was reduced by 41% by using this “Smart Shuttering” improvement to MME. A substantially higher peak concentration of Mg, approaching 2% atomic concentration, was achieved using the MME technique. As expected, a negligible amount of In was incorporated into the very Ga‐rich films. Using the new closed loop control system developed for MBE, the surface morphology was improved for GaN films while highly repeatable holes concentrations as high as 4.7×1018 cm–3 with 1.07 cm2/V‐sec mobility was obtained. This approach offers great promise for improved reproducibility and improved material quality. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Thermodynamics of mono- and di-vacancies in barium titanate

The thermodynamic and kinetic properties of mono- and di-vacancy defects in cubic (para-electric) barium titanate BaTiO3 are studied by means of density-functional theory calculations. It is determined which vacancy types prevail for given thermodynamic boundary conditions. The calculations confirm the established picture that vacancies occur in their nominal charge states almost over the entire band gap. For the dominating range of the band gap the di-vacancy binding energies are constant and negative. The system, therefore, strives to achieve a state in which, under metal-rich (oxygen-rich) conditions, all metal (oxygen) vacancies are bound in di-vacancy clusters. The migration barriers are calculated for mono-vacancies in different charge states. As oxygen vacancies are found to readily migrate at typical growth temperatures, di-vacancies can be formed at ease. The key results of the present study with respect to the thermodynamic behavior of mono- and di-vacancies influence the initial defect distribution in the ferroelectric phases and therefore the conditions for aging.

In adlayers onc-plane InN surfaces: A polarity-dependent study by x-ray photoemission spectroscopy

The surfaces of In- and N-polarity InN grown by molecular-beam epitaxy have been investigated using core-level and valence band x-ray photoemission spectroscopy (XPS). From the ratio of the In and N core-level XPS signal intensities, the clean InN surfaces have been found to be terminated by In adlayers, in agreement with the predictions of previous first-principles calculations. The In- and N-polarity surfaces are terminated by $\ensuremath{\sim}3.4$ and $\ensuremath{\sim}2.0\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ of In, in each case 1 ML more than for Ga on GaN surfaces under metal-rich growth conditions. The valence band XPS indicates a valence band maximum to surface Fermi level separation of $1.32\ifmmode\pm\else\textpm\fi{}0.10\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for both In- and N-polarity InN surfaces, indicating that the surface space charge is independent of polarity and, unlike for GaN, is not significantly affected by spontaneous polarization. This is due to the In coverage on the surface of InN of both polarities being sufficient to efficiently screen the polarization charge.

Theoretical Analysis for Surface Reconstruction of AlN and InN in the Presence of Hydrogen

We have investigated the surface structures of AlN and InN using ab initio calculations based on the density functional theory within generalized gradient approximation. We studied the surface energies obtained from total energy calculations for various (2×2) geometries of cation-terminated (0001) surfaces and anion-terminated (0001) surfaces of AlN and InN, with hydrogen in a carrier gas. It was found that the structures with N–H bonds were favorable under hydrogen ambient, while in the absence of hydrogen, the structures with metal adatoms and N adatoms tended to be stable under metal-rich conditions and N-rich conditions, respectively.

Thermodynamic analysis of cation incorporation during molecular beam epitaxy of nitride films using metal-rich growth conditions

The conventional approach to growth of the nitride films GaN, AlN, InN, and their alloys by rf plasma molecular beam epitaxy uses metal-rich surface conditions due to improved material quality compared to nitrogen-rich conditions. The surface metal may incorporate into the growing film, act as a surfactant, and/or react with the underlying film or substrate. Using a simple chemical exchange reaction model and tabulated thermodynamic data at molecular beam epitaxy growth temperatures the predicted preferential incorporation series on the column III site under metal-rich conditions is found to be Al&amp;gt;B,Be,Si, Mg&amp;gt;Ga&amp;gt;In,Fe. This series is consistent with the observed ternary growth behavior and surfactant order. The series is also consistent with silicon migration in AlN but not GaN, sharper beryllium transitions in GaN than AlN, the significant migration of iron in GaN, and the reactivity of AlN nucleation layers with SiC surfaces. The model is used to predict boron incorporation under metal-rich conditions in BGaN and BAlN and should prove useful as a tool in predicting the incorporation behavior of other cations during metal-rich epitaxial growth of nitride films and possibly other materials.

Role of dislocation-free GaN substrates in the growth of indium containing optoelectronic structures by plasma-assisted MBE

Plasma-assisted molecular beam epitaxy (PAMBE) has recently emerged as a viable tool for production of nitride blue-violet laser diodes operating at room temperature in continuous wave mode and high output powers [C. Skierbiszewski, P. Wisniewski, M. Siekacz, P. Perlin, A. Feduniewicz-Zmuda, G. Nowak, I. Grzegory, M. Leszczynski, S. Porowski, Appl. Phys. Lett. 88 (2006) 221108]. The present work reviews the current state of the art in this program as well as discusses its future directions. Two elements are given particular attention: (1) the epitaxial growth in metal-rich conditions, which enables effective lateral diffusion of N adatoms at low growth temperatures and (2) the role of threading dislocations in destabilizing the growth front. Low-temperature growth by PAMBE on dislocation-free GaN substrates is instrumental in achieving high performance of optoelectronic structures. The inherent to this process capability of sustaining two-dimensional step-flow growth mode (with straight and parallel atomic steps) at low growth temperatures opens up the way to the growth of strained multilayer structures with no compositional fluctuations and with flat interfaces.

Strain relaxation in the growth of planar InAs

In this work, we look at the strain relaxation of InAs growth on GaAs under metal rich conditions. InAs layers, grown under these conditions, have been found to maintain a planar morphology for thicknesses well beyond a Matthews-Blakeslee critical thickness of ∼2 ML. We employed reflection high-energy electron diffraction, in situ spectroscopic ellipsometry and atomic force microscopy to follow the InAs growth under In stabilized conditions. The critical thickness was found to increase with a reduction in growth temperature. A region of growth was seen in which three-dimensional diffraction was not visible in the reflection high energy electron diffraction pattern before annealing. For thicknesses less than the critical thickness observed by atomic force microscope, the planar morphology has been found to be stable with annealing. These results indicate that strain relaxation via the formation of Lomer-type dislocations is essential to stabilizing planar morphology during growth.

Room temperature ferromagnetism in CrGaN: Dependence on growth conditions in rf N-plasma molecular beam epitaxy

Here, we report the influence of the growth conditions for CrGaN grown with the N-polarity on sapphire (0 0 0 1) using radio frequency N plasma-assisted molecular beam epitaxy. Samples are grown under different Ga/N flux ratio of 0.65–1.0 at substrate temperatures of 650 and 700 ∘ C . The Cr/Ga flux ratio is set to either 3% or 5%. These growth parameters allow to vary over a range of growth conditions from N-rich to Ga-rich. It is shown that the surface condition during growth influences the surface morphology and magnetic properties of CrGaN. In particular, we show that N-rich and metal-rich growth conditions result in room temperature ferromagnetism. Also discussed are the influence of Cr on the surface reconstruction and also the influence of annealing on the properties of the CrGaN film.

Planar InAs growth on GaAs(001) and subsequent quantum dot formation by a surface induced morphological instability

InAs growth on GaAs has been studied extensively and most recently for the self-assembly of quantum dots on the surface. In this work, we have studied the growth conditions to form planar InAs, at an In beam-equivalent pressure of 3.1×10−8 Torr, under the metal rich (4×2) reconstruction and the subsequent formation of quantum dots (QDs) condensed from this layer by a rapid change in the As4 pressure. Atomic force microscopy (AFM) studied the surface morphology of InAs resulting from a nominal 12 ML growth with various As4 growth pressures while maintaining a metal rich condition. It was found that the surface roughness was dependent on the As4 pressure. At the higher As4 pressures studied, the growth was extremely rough and much smoother at the lower As4 pressures. For a beam-equivalent pressure of 1×10−7 Torr for As4 and 3.1×10−8 Torr for In, growth was seen to remain planar for a deposition thickness up to 3 ML. AFM analysis showed that a fingered morphology was formed with a roughness of ∼1 ML. When the As4 pressure incident on this surface was quickly changed, the reflection high-energy electron diffraction showed this surface roughened. Subsequent AFM analysis showed that QDs formed in long chains oriented down the [110]. We propose a model in which the fingered surface is segmented due to an underlying dislocation array in the formation of the chained QDs.

Heteroepitaxial evolution of AlN on GaN Grown by metal-organic chemical vapor deposition

We have investigated the morphological evolution during heteroepitaxial growth of AlN on GaN by metal-organic chemical vapor deposition at different V/III ratios. Two-dimensional layer–by–layer and step flow growth modes, combined with strain-induced cracking, are observed at low and intermediate V/III ratios, while nitrogen-rich conditions yield three-dimensional domain-like growth due to limited Al adatom diffusion. Samples grown at the metal-rich conditions exhibit a crosshatch pattern of surface undulations possibly related to the presence of misfit dislocations that form at the early stages of nucleation. Our observations show that the local stoichiometry at the vapor-solid interface strongly influences the adatom kinetics during the growth, thereby affecting the nature of strain relaxation and growth mode.