Ga-rich Growth Conditions Research Articles

Point Defects in Silicon-Doped β-Ga2O3: Hybrid-DFT Calculations.

In this work, hybrid density functional theory calculations are used to evaluate the structural and electronic properties and formation energies of Si-doped β-Ga2O3. Overall, eight interstitial (Sii) and two substitutional (SiGa) positions are considered. In general, our results indicate that the formation energy of such systems is significantly influenced by the charge state of the defect. It is confirmed that it is energetically more favorable for the substitution process to proceed under Ga-poor growth conditions than under Ga-rich growth conditions. Furthermore, it is confirmed that the formation of SiGaI with a tetrahedral coordination geometry is more favorable than the formation of SiGaII with an octahedral one. Out of all considered interstitial positions, due to the negative formation energy of the Si +3 charge state at i8 and i9 interstitial positions over the wide range of Fermi energy, this type of defect can be spontaneously stable. Finally, due to a local distortion caused by the presence of the interstitial atom as well as its charge state, these systems obtain a spin-polarized ground state with a noticeable magnetic moment.

Blue luminescence origin and Mg acceptor saturation in highly doped zinc-blende GaN with Mg

We study the incorporation of Mg at degenerated doping concentration in zinc-blende GaN and its effect on the optical and electrical properties. The study starts with a theoretical analysis by Density Functional Theory (DFT) and then supported by an experimental methodology employing X-Ray Diffractometry (XRD), Atomic Force Microscopy (AFM), Photoluminescence (PL), Hall effect, Secondary Ion-Mass Spectroscopy (SIMS), and X-Ray Photoelectron Spectroscopy (XPS). Employing Ga-rich growth conditions allow providing the amount of Mg able to obtain an Mg-acceptor concentration limit above 2 × 1019 cm−3. Moreover, the blue band emission centered at 2.85 eV is discussed and used as a systematic tool to detect the Mg incorporation into an undesired site. To the best of our knowledge, the blue emission mechanism is consistently explained in zb-GaN:Mg for the first time by theory and experimentation.

First-principles study of defects at [formula omitted] grain boundaries in CuGaSe[formula omitted

We present a first-principles computational study of cation–Se Σ3 (112) grain boundaries in CuGaSe2. We discuss the structure of these grain boundaries, as well as the effect of native defects and Na impurities on their electronic properties. The formation energies show that the defects will tend to form preferentially at the grain boundaries, rather than in the grain interiors. We find that in Ga-rich growth conditions Cu vacancies as well as Ga at Cu and Cu at Ga antisites are mainly responsible for having the equilibrium Fermi level pinned toward the middle of the gap, resulting in carrier depletion. The Na at Cu impurity in its +1 charge state contributes to this. In Ga-poor growth conditions, on the other hand, the formation energies of Cu vacancies and Ga at Cu antisites are comparatively too high for any significant influence on carrier density or on the equilibrium Fermi level position. Thus, under these conditions, the Cu at Ga antisites give rise to a p-type grain boundary. Also, their formation energy is lower than the formation energy of Na at Cu impurities. Thus, the latter will fail to act as a hole barrier preventing recombination at the grain boundary, in contrast to what occurs in CuInSe2 grain boundaries. We also discuss the effect of the defects on the electronic properties of bulk CuGaSe2, which we assume reflect the properties of the grain interiors.

Crystalline growth and alloying of In[formula omitted]Ga[formula omitted]Sb films by electrodeposition onto liquid metal electrodes

Incorporating indium (In) atoms into the GaSb lattice provides broad tunability of the optical bandgap within the infrared spectral region. This research documents the first instance in the literature of electrochemical liquid–liquid–solid (ec-LLS) growth of a ternary semiconductor alloy. Indium content of deposited InxGa1−xSb samples was regulated by controlling the Ga/In ratio of the liquid metal electrode. All depositions were performed using a single-step growth method under ambient pressure at 90 ± 5 °C. X-ray diffraction analysis showed shifts of the (111) diffraction peaks toward the peak locations of cubic InSb for the x=0.50, 0.60, and 0.70 samples, indicating a consistent increase in lattice parameter. However, the x=0.28 and 0.41 samples displayed unexpected shifts toward larger angles, possibly due to increased presence of Ga vacancies and antisites caused by Ga-rich growth conditions, as well as insufficient mixing of the liquid metal electrode for x< 0.50. Optical bandgaps ranging from 0.594 to 0.707 eV were determined via the Tauc method and Kubelka–Munk theory applied to diffuse reflectance data, and showed general agreement with measured lattice constants. Data obtained in this research supports In incorporation into the GaSb lattice for samples with x≥ 0.50, but with significantly lower In compositions relative to the liquid metal electrode. No general trend was observed for average crystallite size as a function of In composition, likely due to inconsistencies in the film harvesting process. This research provides further support for ec-LLS as a semiconductor growth technique capable of crystalline growth and alloying of III-V semiconductors.

Theoretical study of the structure and magnetism of Ga1−xVxSb compounds for spintronic applications

In this paper, the structural, electronic, and magnetic properties of Ga1−xVxSb compounds with various vanadium (V) contents (x) ranging from dilute doping to the extreme doping limit were systematically investigated by first-principles calculations. The results show that V atoms prefer to substitute for gallium (Ga) atoms, and the formation energy is lower under Sb-rich growth conditions than under Ga-rich growth conditions. Meanwhile, the SbGa antisite defects effectively decrease the energy barrier of the substitution process from 0.85 to 0.53 eV. The diffusion of V atoms in the GaSb lattice occurs through metastable interstitial sites with an energy barrier of 0.6 eV. At a low V concentration (x = 0.0625), V atoms prefer a homogeneous distribution with antiferromagnetic coupling among the V atoms. However, when x increases above 0.5, the magnetic coupling among V atoms changes to ferromagnetic coupling due to the enhanced superexchange interaction between the eg and t2g states of neighboring V atoms. At the extreme doping limit of x = 1.00, zinc blende VSb along with its analogs VAs and VP is an intrinsic ferromagnetic semiconductor that exhibits a large change in light absorption at the Curie temperature. The results indicate that Ga1−xVxSb compounds provide a platform to design next-generation electronic, spintronic, and optoelectronic devices.

Effects of N₂ Partial Pressure on Growth, Structure, and Optical Properties of GaN Nanorods Deposited by Liquid-Target Reactive Magnetron Sputter Epitaxy.

GaN nanorods, essentially free from crystal defects and exhibiting very sharp band-edge luminescence, have been grown by reactive direct-current magnetron sputter epitaxy onto Si (111) substrates at a low working pressure of 5 mTorr. Upon diluting the reactive N2 working gas with a small amount of Ar (0.5 mTorr), we observed an increase in the nanorod aspect ratio from 8 to ~35, a decrease in the average diameter from 74 to 35 nm, and a two-fold increase in nanorod density. With further dilution (Ar = 2.5 mTorr), the aspect ratio decreased to 14, while the diameter increased to 60 nm and the nanorod density increased to a maximum of 2.4 × 109 cm−2. Yet, lower N2 partial pressures eventually led to the growth of continuous GaN films. The observed morphological dependence on N2 partial pressure is explained by a change from N-rich to Ga-rich growth conditions, combined with reduced GaN-poisoning of the Ga-target as the N2 gas pressure is reduced. Nanorods grown at 2.5 mTorr N2 partial pressure exhibited a high intensity 4 K photoluminescence neutral donor bound exciton transitions (D0XA) peak at ~3.479 eV with a full-width-at-half-maximum of 1.7 meV. High-resolution transmission electron microscopy corroborated the excellent crystalline quality of the nanorods.

Structural, electronic and magnetic properties of the MnGa(111)-1 × 2 and 2 × 2 reconstructions: Spin polarized first principles total energy calculations

Using first principles total energy calculations within the periodic spin polarized density functional theory, we have investigated the structural, electronic, and magnetic properties of manganese gallium (MnGa) alloys. Specifically, we explore the MnGa(111)-1×2 and 2×2 reconstructions. The surface formation energies reveal that selected substitutions occur under Mn (Ga) rich growth conditions. Structures with top layers missing all Mn (Ga) atoms and two layers deep substitutions are also investigated. However, the formation energy shows that these structures are less favorable. For the stable structures, the magnetic properties per layer are proportional to the Mn:Ga ratio. Also, the density of states shows that the MnGa surfaces are metallic. The projected density of states shows that the electronic states in the vicinity of the Fermi level are due mainly to the manganese 3d orbitals. However charge density plots reveal that Mn 3d electrons are closer to the nucleus than Ga sp electrons. Consequently, experimental scanning tunneling microscopy images reveal periodically-arranged bright features, corresponding to the Ga atoms.

Preparation and structure of ultra-thin GaN (0001) layers on In0.11Ga0.89N-single quantum wells

Multiple surface reconstructions have been observed on ultra-thin GaN (0001) layers of 1–10nm thickness, covering a 3nm thick In0.11Ga0.89N single quantum well in a GaN matrix. Low energy electron diffraction patterns show (2×2) and (√3×√3)-R30° symmetries for samples annealed in nitrogen plasma, and (2×2), (3×3), (4×4), and (6×6) symmetries for samples overgrown with an additional monolayer-thin GaN film by molecular beam epitaxy under Ga-rich growth conditions. Photoelectron spectroscopy shows that the InGaN quantum wells and capping layers are stable for growth temperatures up to 760°C, and do not show formation of indium or gallium droplets on the surface. The photoluminescence emission from the buried InGaN SQWs remains unchanged by the preparation process, demonstrating that the SQWs do not undergo any significant modification.

Droplet-mediated formation of embedded GaAs nanowires in MBE GaAs1−xBix films

We have examined the morphology and composition of embedded nanowires that can be formed during molecular beam epitaxy of GaAs1−xBix using high angle annular dark field (‘Z-contrast’) imaging in an aberration-corrected scanning transmission electron microscope. Samples were grown in Ga-rich growth conditions on a stationary GaAs substrate. Ga-rich droplets are observed on the surface with lateral trails extending from the droplet in the [110] direction. Cross-sectional scanning transmission electron microscopy of the film reveals epitaxial nanowire structures of composition ∼GaAs embedded in the GaAs1−xBix epitaxial layers. These nanowires extend from a surface droplet to the substrate at a shallow angle of inclination (∼4°). They typically are 4 μm long and have a lens-shaped cross section with major and minor axes dimensions of 800 and 120 nm. The top surface of the nanowires exhibits a linear trace in longitudinal cross-section, across which the composition change from ∼GaAs to GaAs1−xBix appears abrupt. The bottom surfaces of the nanowires appear wavy and the composition change appears to be graded over ∼25 nm. The droplets have phase separated into Ga- and Bi-rich components. A qualitative model is proposed in which Bi is gettered into Ga droplets, leaving Bi depleted nanowires in the wakes of the droplets as they migrate in one direction across the surface during GaAs1−xBix film growth.

Effect of Mg Doping on the Photoluminescence of GaN:Mg Films by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

We investigate undoped GaN and Mg-doped GaN grown by rf plasma-assisted molecular beam epitaxy (MBE) with different Mg concentrations by photoluminescence (PL) at low temperature, Hall-effect and XRD measurements. In the PL spectra of lightly Mg-doped GaN films, a low intensity near band edge (NBE) emission and strong donor-acceptor pair (DAP) emission with its phonon replicas are observed. As the Mg concentration is increased, the DAP and NBE bands become weaker and a red shift of these bands is observed in the PL spectra. Yellow luminescence (YL) is observed in heavily Mg-doped GaN. The x-ray diffraction is employed to study the structure of the films. Hall measurement shows that there is a maximum value (3.9 × 1018 cm−3) of hole concentration with increasing Mg source temperature for compensation effect. PL spectra of undoped GaN are also studied under N-rich and Ga-rich growth conditions. Yellow luminescences of undoped Ga-rich GaN and heavily Mg-doped GaN are compared, indicating the different origins of the YL bands.

Role of strain in growth kinetics of AlGaN layers during plasma-assisted molecular beam epitaxy

Comparative study of growth kinetics of the AlxGa1−xN (x=0–1) layers grown by plasma-assisted molecular beam epitaxy (PA MBE) under different growth conditions (group III to activated nitrogen and Al to Ga flux ratios) is presented. The strong influence of elastic stress on the surface morphology, growth rate and Al content in AlxGa1−xN (x=0–1) layers is most pronounced for the AlxGa1−xN films with high Al-content grown atop of the 2D-AlN buffer layer at near the unity flux ratio FIII/FN∼1. The use of strong Ga-rich growth conditions with FIII/FN∼1.6–2 for the growth of AlxGa1−xN/2D-AlN with high Al-content (x>0.25) allows one to reduce the strain effect as well as provide smooth surface morphology and precise control of Al content in the AlxGa1−xN (x=0–1) layers by employing a simple ratio x=FAl/FN.

Structural characterization of cubic and hexagonal GaN thin films grown by IBA–MBE on SiC/Si

Buried ion beam synthesized 3C-SiC layers were revealed to the surface of silicon wafers to provide lattice matched substrates for GaN thin film epitaxy. Both epitaxial SiC/Si(1 1 1) and SiC/Si(0 0 1) growth surfaces with either six- or four-fold crystal symmetry, respectively, were formed. GaN thin film growth was achieved by ion beam assisted molecular beam epitaxy (IBA–MBE) which – owing to the energy and momentum transfer of the ions – allows to deposit epitaxial thin films at particularly low growth temperatures where both the stable hexagonal and the metastable cubic polytype of GaN can be formed. It is shown by X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (XTEM) that using appropriate fluxes of Ga atoms both purely hexagonal and purely cubic GaN films can be grown epitaxially on SiC/Si(1 1 1) and SiC/Si(0 0 1), respectively. Thereby Ga rich growth conditions seem to stabilize the formation of the cubic polytype. It is obvious from XTEM studies that the high density of crystal defects in the SiC layer is not transferred onto the growing GaN films and that the crystalline quality of GaN films improves with increasing film thickness. The influence of surface roughness and wettability, interfacial cavities and the nucleation of twin-type defects at the GaN/SiC interface on the crystalline quality of the GaN thin films is discussed.

Structure of AlSb(001) and GaSb(001) surfaces under extreme Sb-rich conditions

We use density-functional theory to study the structure of AlSb(001) and GaSb(001) surfaces. Based on a variety of reconstruction models, we construct surface stability diagrams for AlSb and GaSb under different growth conditions. For AlSb(001), the predictions are in excellent agreement with experimentally observed reconstructions. For GaSb(001), we show that the previously proposed model accounts for the experimentally observed reconstructions under Ga-rich growth conditions but fails to explain the experimental observations under Sb-rich conditions. We propose a model that has a substantially lower surface energy than all $(n\ifmmode\times\else\texttimes\fi{}5)$-like reconstructions proposed previously and that, in addition, leads to a simulated scanning tunneling microscopy image in better agreement with experiment than existing models. However, this model has higher surface energy than some of $(4\ifmmode\times\else\texttimes\fi{}3)$-like reconstructions, models with periodicity that has not been observed. Hence, we conclude that the experimentally observed $(1\ifmmode\times\else\texttimes\fi{}5)$ and $(2\ifmmode\times\else\texttimes\fi{}5)$ structures on GaSb(001) are kinetically limited rather than at the ground state.

Gallium desorption kinetics on (0001) GaN surface during the growth of GaN by molecular-beam epitaxy

Gallium (Ga) surface desorption behavior was investigated using reflection high-energy electron diffraction during the GaN growth. It was found that the desorption of Ga atoms from the (0001) GaN surfaces under different III-V ratio dependents on the coverage of adsorbed atoms. Doing so led to desorption energies of 2.76 eV for Ga droplets, 1.24–1.89 eV for Ga under Ga-rich growth conditions, and 0.82 eV – 0.94 eV for Ga under stoichiometric growth conditions. Moreover, the variation of the GaN surface morphology under different III-V ratios on porous templates supports the conclusion that Ga desorption energy depends on the coverage, and the III/V ratio dominates the growth mode.

Growth and formation of InGaN and GaN nano-structures studied by STM

Growth and morphology of metal organic vapour phase epitaxy (MOVPE) deposited InGaN nano-islands and molecular beam epitaxy (MBE) grown GaN films on GaN(0001) template layers on sapphire substrates have been investigated using scanning tunneling microscopy. For MOVPE InGaN growth, the nucleation of self-organized nano-structures can be achieved by a careful choice of the growth temperature, the In partial pressure, the growth rate and V/III flux ratio. For growth at 650°C, large spiral disc-like islands are found, preferentially nucleating at GaN substrate defects. At 600°C, islands of smaller average size are observed. Lowering the In flux at this temperature, a homogeneous nucleation of small quantum dot like islands with a density of 1012/cm2 is found. For homoepitaxial MBE growth of thin GaN layers on GaN templates, a layer-by-layer growth mode is observed for Ga rich growth conditions. For growth at 750°C, an atomically resolved 4×4 surface reconstruction with a high defect density is found in the initial growth stage. However, subsequent growth at 790°C leads to the formation of one dimensional nanoclusters of about 3 nm lateral spacing. For GaN growth at a lower Ga-flux, a rougher surface morphology and three dimensional growth is observed. Independent on the Ga flux, one-dimensional nanostructures appear after prolonged growth at higher temperature, which are attribute to the impact of ions emerging from the N-plasma. [DOI: 10.1380/ejssnt.2006.90]

Large-scale synthesis and optical properties of hexagonal GaN micropyramid/nanowirehomostructures

Free-standing, hexagonal GaN micropyramid/nanowire homostructures have been successfullysynthesized on a large scale by a facile carbon-assisted thermal reduction route. The product wasinvestigated using SEM, XRD, TEM, EDS, SAED, Raman and PL. The pyramids have lengths of5–8 µm,and the maximallateral size of 2–3 µm, exhibiting the growth characteristic of a hexagonal crystal along thec-axis. A single nanowire is grown out from the end tip of the pyramid along the growth direction, indicating its epitaxial relationship with the pyramid. Raman studiesreveal the characteristics of the hexagonal structure and high crystallinity of the product.The room temperature PL spectrum displays strong band-edge emission centred at 372 nm.The slight redshift originates from N vacancies caused by Ga-rich growth conditions. Theformation mechanism is also discussed. The unique micro/nano-structures are good entitiesfor the investigation of fundamental physical phenomena and also as building blocks forfunctional nanodevices.

Mn doping and p-type conductivity in zinc-blende GaMnN layers grown by molecular beam epitaxy

Mn-doped cubic GaN films were grown by plasma-assisted molecular beam epitaxy on GaAs (001) substrates. There was a strong influence of the Ga:N ratio on the Mn incorporation and a dramatic increase in the Mn concentration was observed for the layers grown under N-rich conditions. Hall-effect measurements unambiguously showed that the GaMnN samples had strong p-type conductivity. For both N-rich and Ga-rich growth conditions a bulk hole density of about 1018cm−3 was achieved with a corresponding mobility of &amp;gt;300cm2V−1s−1. This compares very favorably with other values reported for p doping of cubic GaN doped with C, suggesting that Mn may be a very suitable dopant for GaN-based electronic structures. The apparent values of ionization energy for Mn were around 50 meV, which is much shallower than for C or Mg in cubic GaN. In addition to a small high temperature ferromagnetic signal, a ferromagnetic correlation was detected among the remaining Mn ions at low temperatures, which is assigned to the onset of hole-mediated ferromagnetism in cubic GaMnN.

High-density self-assembled GaN nanoislands on SiC (0001) by molecular-beam epitaxy

We report the fabrication of self-assembled coherent GaN nanoislands on Ga-adsorbed √3×√3R30° 6H–SiC(0001) surfaces via Stranski–Krastanov (SK) growth mode by molecular-beam epitaxy at 760°C. The initial GaN deposition reveals a stable two-dimensional growth (2D) mode under Ga-rich growth conditions (Ga∕N⪢1), as evidenced by the variation of in-plane misfit strain (Δa∕a0). We show that above ∼2.2 monolayer (ML) surface coverage, the 2D layer spontaneously transformed into SK islands by the accommodation of elastic strain in the epitaxial lattice misfit under a vacuum. The SK islands density is found to be ∼4×1011cm−2 and the mean height and diameter is 2±0.5nm and 20±5nm for 3.2 ML surface coverage, respectively.

Continuous evolution of Ga adlayer coverages during plasma-assisted molecular-beam epitaxy of (0001) GaN

We present a study of the evolution of the Ga adlayer during plasma-assisted molecular-beam epitaxy of (0001) GaN as a function of both Ga flux and growth temperature. In situ quadrupole mass spectrometry was used to quantitatively determine the adsorbed Ga coverage by monitoring its subsequent desorption after GaN growth. Independent of the growth time, the Ga adlayer was found to form steady-state coverages that increase continuously from 0 to 2.5 monolayers when raising the Ga flux from N-rich to moderate Ga-rich growth conditions. At higher Ga fluxes or lower growth temperatures, macroscopic Ga droplets form on top of the Ga adlayer (Ga droplet regime). Based on the temperature dependency for the transition between the Ga adlayer and Ga droplet regime, we determined an apparent activation energy of 3.4eV, which is discussed with respect to previously reported values.

Carbon doping of GaN with CBr4 in radio-frequency plasma-assisted molecular beam epitaxy

Carbon tetrabromide (CBr4) was studied as an intentional dopant during rf plasma molecular beam epitaxy of GaN. Secondary ion mass spectroscopy was used to quantify incorporation behavior. Carbon was found to readily incorporate under Ga-rich and N-rich growth conditions with no detectable bromine incorporation. The carbon incorporation [C] was found to be linearly related to the incident CBr4 flux. Reflection high-energy electron diffraction, atomic force microscopy and x-ray diffraction were used to characterize the structural quality of the film’s postgrowth. No deterioration of structural quality was observed for [C] from mid 1017 to ∼1019 cm−3. The growth rate was also unaffected by carbon doping with CBr4. The electrical and optical behavior of carbon doping was studied by co-doping carbon with silicon. Carbon was found to compensate the silicon although an exact compensation factor was difficult to extract from the data. Photoluminescence was performed to examine the optical performance of the films. Carbon doping was seen to monotonically decrease the band edge emission. Properties of carbon-doped GaN are interpreted to be consistent with recent theoretical work describing incorporation of carbon as function of Fermi level conditions during growth.