Growth and characterization of ferromagnetic shape memory alloy Co50Ni20FeGa29 single crystals

Single crystals of Co₅₀Ni₂₀FeGa₂₉ with B2 phase have been obtained in a deep supercooling condition. The interface-facets and the segregation effect lead to the formation of ordered defects that store a directional internal stress. These defects give to a large energy barrier that leads to a very sharp martensitic transformation within a temperature window of only 2 K. The single crystals show good shape memory effect and superelasticity, which are anisotropic between the growth direction [0 0 1] and its equivalent direction [0 1 0]. The anisotropic behaviors are attributed to the directional internal stress caused by the ordered defects.     

Ambient-condition growth of superconducting YBa2Cu4O8 single crystals using KOH flux

YBa₂Cu₄O₈ is a stoichiometric oxide superconductor of T_c80 K. Unlike YBa₂Cu₃O_7−δ, this compound is free from oxygen vacancy or twin formation and does not have any microscopic disorder in the crystal. Doping with Ca raises its T_c to 90 K. The compound is a promising superconductor for technological application. Up to now, single crystals have not been grown without using specialized apparatus with extremely high oxygen pressure up to 3000 bar and at over 1100 °C due to the limited range of reaction kinetics of the compound. This fact has delayed the progress in the study of its physical properties and potential applications. We present here a simple growth method using KOH as flux that acts effectively for obtaining high-quality single crystals in air/oxygen at the temperature as low as 550 °C. As-grown crystals can readily be separated from the flux and exhibit a perfect orthorhombic morphology with sizes up to 0.7×0.4×0.2 mm³. Our results are reproducible and suggest that the crystals can be grown using a conventional flux method under ambient condition.     

Inversion domains and parallel growth in ammonothermally grown GaN crystals

Macro-defects such as twins, inversion domains, crevices, and columnar growth occasionally appear in ammonothermally grown GaN crystals. Twinning mechanisms and parallel growth are proposed to explain the formation of these defects. As a polar crystal with wurtzite structure, GaN can have several different kinds of twins depending on the polarity arrangement of each individual twin. Inversion domains are formed in one of the twinning mechanisms. Parallel growth is used to explain the formation of pits on the nitrogen face and the columnar growth on the gallium face. Etching in hot H₃PO₄ is used to reveal the polarities and defects of GaN crystals when they are indistinguishable. Optical microscopy, scanning electron microscope, and cathodoluminescence are also employed to study these defects. In addition, seed quality, avoidance of macro-defect formation, and impurity effects are also discussed.     

Extended defects in bulk GaN and III-nitrides grown on this substrate

A short review of the structural perfection of high-pressure grown bulk crystals is given. As-grown undoped and Mg-doped crystals are described. The dependence of defect arrangement and quality of the surface on growth polarity is described. A high perfection of homoepitaxial layers grown on these substrates is shown. However, growth of thick layers by HVPE may lead to the formation of differently arranged dislocations and the formation of low angle grain boundaries associated with cracks. It is shown that the introduction of dopant or growth of mismatched layers on undoped high-pressure substrates may lead to the formation of additional defects.     

Hydride vapor phase epitaxy of GaN boules using high growth rates

The boule-like growth of GaN in a vertical AIXTRON HVPE reactor was studied. Extrinsic factors like properties of the starting substrate and fundamental growth parameters especially the vapor gas composition at the surface have crucial impact on the formation of inverse pyramidal defects. The partial pressure of GaCl strongly affects defect formation, in-plane strain, and crystalline quality. Optimized growth conditions resulted in growth rates of 300–500 μm/h. GaN layers with thicknesses of 2.6 and of 5.8 mm were grown at rates above 300 μm/h. The threading dislocation density reduces with an inverse proportionality to the GaN layer thickness. Thus, it is demonstrated that growth rates above 300 μm/h are promising for GaN boule growth.     

Twin-plane reentrant edge growth of rhombohedra boron suboxide platelets

Large quantities of rhombohedra and elongated rhombohedra boron suboxide platelets with flat (0 0 1) surface have been synthesized through conventional solid state reaction. Detailed structural investigations by selected area electron diffraction (SAED) and high-resolution electron microscopy (HRTEM) of these platelets are presented. We present the direct experimental observation of extensive lateral (0 0 1) microtwins in rhombohedra platelets and they give rise to the fractional diffractions spots. It is believed that the growth of these rhombohedra platelets is prompted by the twin-plane reentrant edge (TPRE) mechanism. The transition from rhombohedra platelets to elongated rhombohedra platelets in morphology is probably the result of catalytic growth at the apexes of the platelets. This proposed growth model can be representative of various platelets with low defects formation energy, especially in twinned crystals having a rhombohedra structure. Besides, the presence of extensive microtwins will yield interesting physical properties and probably results in the broadening of photoluminescence (PL) spectra from the rhombohedra and elongated rhombohedra platelets.     

Blue emission from arsenic doped gallium nitride

We have observed strong blue emission at room temperature from arsenic doped GaN samples grown by molecular beam epitaxy. Similar results were obtained for samples doped with both arsenic dimers and tetramers. The origin of this blue emission is discussed and a growth model proposed to account for our observations. We propose that arsenic doped GaN may be a suitable replacement for (InGa)N as the active region for blue light emitting devices.     

Correlations between photoluminescence and Hall mobility in GaN/sapphire grown by metalorganic chemical vapor deposition

We have investigated photoluminescence (PL) and electron Hall mobility for unintentionally doped GaN epitaxial layers grown by low-pressure metalorganic chemical vapor deposition on c-plane Al₂O₃ substrates. Four GaN films having identical dislocation density but remarkably different electron Hall mobility were exploited. At low temperature (12 K), a PL line associated with a bound exciton was observed and strong correlations were found between the Hall mobility and the PL intensity of the exciton transition. That is, relative PL intensity of the bound exciton to a donor-bound exciton monotonously increased with decreasing the electron mobility of the GaN films. This correlation was interpreted in terms of electrical compensation. Efforts to find the chemical origin of the PL line led to the conclusion that the BE line originated neither from threading dislocations nor from extrinsic point defects. Intrinsic acceptors such as Ga vacancy and GaN anti-site were suspected as plausible origin.     

The crystal growth and perfection of 2,4,6-trinitrotoluene

Large crystals of TNT were grown from ethyl acetate solution by both temperature lowering and solvent evaporation. The perfection of crystals grown from seeds under carefully controlled conditions was generally higher than those prepared by uncontrolled solvent evaporation. Examination by X-ray topography revealed the crystals to have a characteristic growth induced defect structure comprising growth sectors and boundaries, growth banding, solvent inclusions and dislocations. Twins and stacking faults (SF) were also observed. Many of the defects noted in the topographs can be attributed to impurities. The influence of the highly anisotropic crystal structure on the nature of growth defects is discussed. A structural model proposed to explain twinning and SF formation is partially supported by topographic evidence.     

The effect of oxygen pressure on the microstructures of Co-doped rutile TiO2 thin films grown by pulsed laser deposition

Comprehensive microstructures of 7% cobalt-doped rutile TiO₂ thin films grown on c-plane sapphire by pulsed laser deposition were characterized using transmission electron microscopy (TEM). The effects of oxygen pressure during growth on the Co distribution inside the films were investigated, and the detailed growth mechanism of both TiO₂ and TiO₂+Co was discussed. The similar oxygen sublattices and low mismatch between (1 0 0) rutile and c-plane sapphire favors the rutile phase. However, the three-fold symmetry of the substrate surface resulted in three rutile domain orientation variants, and they grow adjacent to each other. Cobalt was found to precipitate out as nanocrystals inside the TiO₂ matrix as the growth pressure of oxygen was decreased. At 0.05 mTorr oxygen pressure, almost all of the Co segregates into crystallographically aligned nanocrystals with a particle size of 4.4±0.15 nm. All the samples have magnetic coercivity at room temperature. The magnetic moment per Co atom increased with decreased oxygen pressure, suggesting that the Co that replaced the Ti²⁺ in the TiO₂ lattice does not have a large magnetic moment.     

The effect of dopants on the microstructure of polycrystalline silicon thin film grown by MILC method

The effect of dopants on the crystal growth and the microstructure of poly-crystalline silicon (poly-Si) thin film grown by metal induced lateral crystallization (MILC) method was intensively investigated. PH₃ and B₂H₆ were used as source gases in ion mass doping (IMD) process to make n-type and p-type semiconductor respectively. It was revealed that the microstructure of MILC region varies significantly as the doping type of the samples varied from intrinsic to n-type and p-type, which was investigated by field emission (FE)-SEM. The microstructure of MILC region of the intrinsic was bi-directional needle network structure whose crystal structure has a (1 1 0) preferred orientation. For p-type doped sample, the microstructure of MILC region was revealed to become unidirectional parallel growth structure more and more as MILC growth proceed, which was led by unidirectional division of needlelike grain at the front of MILC region. And for n-type doped sample, the microstructure was random-directional needlelike growth structure. These phenomena can be explained by an original model of Ni ion and Ni vacancy hopping in the NiSi₂ phase and its interface at the front of MILC region.     

Pattern formation mechanism of a periodically faceted interface during crystallization ofSi

We investigated the pattern formation mechanism of a periodically faceted crystal–melt interface during the crystallization of Si by in situ observation. It was directly proved that spacing between the reentrants of adjacent zigzag facets increases with the unification of adjacent facets when a facet with a higher growth velocity catches up with the one with a lower growth velocity. The spacing becomes stable after unification, and the stable spacing was found to increase with increase in growth velocity. The experimental results was discussed by taking the negative temperature gradient in front of the growth interface into account.     

Growth of single crystals of B28 at high pressures and high temperatures

A method of the high-pressure high-temperature synthesis of single crystals of orthorhombic high-pressure boron B₂₈ from metal solutions is presented. The method is based on the high-pressure multi-anvil technique. The feasibility of single-crystal growth was demonstrated in a number of experiments conducted at various pressure–temperature conditions with various precursors including β-boron of 99.99% purity and various metals (Cu, Au, and Pt) used as fluxes and capsule materials. It was found that after dissolution in metals at high pressures and high temperatures, boron crystallizes in the form of single crystals at low temperature. The process is accompanied by chemical reactions resulting in the formation of borides. The maximum length of the B₂₈ crystals achieved is ∼100 μm.     

Growth and optical properties of high-density InN nanodots

High density InN/GaN nanodots were grown by pulsed mode (PM) metal–organic chemical vapor deposition (MOCVD). InN nanodots density of up to ∼5×10¹⁰ cm⁻² at a growth temperature of 550 °C was achieved. The high diffusion activation energy of 2.65 eV due to high NH₃ flow rate generated more reactive nitrogen adatoms on the growth surface, and is believed to be the main reason for the growth of high density InN nanodots. In addition, an anomalous temperature dependence of the PL peak energy was observed for high density InN nanodots. The high carrier concentration, due to high In vacancy (V_In) in the InN nanodots, thermally agitated to the conduction band. As the measurement temperature increased, the increase of Fermi energy resulted in blue-shifted PL peak energy. From the Arrhenius plot of integrated PL intensity, the thermal activation energy for the PM grown InN nanodots was estimated to be E_a∼51 meV, indicating strong localization of carriers in the high density InN nanodots.     

The influences of O/Zn ratio and growth temperature on carbon impurity incorporation in ZnO grown by metal-organic chemical vapor deposition

The strong correlations between the O/Zn ratio and carbon impurity incorporation have been observed on the ZnO films grown using N₂O or O₂ as oxygen source in metal-organic chemical vapor deposition (MOCVD). From in-situ mass spectrometric measurements, the O/Zn ratio in the MOCVD reactor is found to decrease to a minimum value as the growth temperature increased till a critical growth temperature T_c, and then increased above T_c due to different dissociation rates of the oxygen and Zn sources. The strongest D and G modes, which are ascribed to carbon clusters sp² related modes, have been observed in Raman scattering spectroscopy for the ZnO samples grown at T_c, indicating the highest incorporation rate of carbon impurity in the samples grown at T_c. Compared with O₂, N₂O has a low dissociation rate and that leads to a lower value of O/Zn ratio, resulting in much stronger D and G modes and higher incorporation rate of carbon impurities in the samples grown at T_c. It is interesting to note that the lowest specific resistances from Hall effect measurements were also obtained on the samples grown at T_c, indicating possible electrical contributions from the formation of carbon clusters, which should be highly conductive regions in ZnO. Furthermore, ionization or addition of H₂ in the case of N₂O can significantly enhance the dissociation of N₂O, with film quality improved significantly. This study shows that a high O/Zn ratio is critical to suppress carbon impurity incorporation and to grow high quality ZnO by MOCVD, especially at low growth temperature.     

Elucidation of factors obstructing quality improvement of MOVPE-grown InN

A systematic study of the crystallographic and electrical/optical properties of MOVPE-grown InN was performed, and the factors that restrict the quality of MOVPE InN were elucidated. The quality of grown InN is highly dependant on the thermal decomposition of NH₃ as a nitrogen source. At a lower growth temperature (~550 °C) a shortage of active nitrogen, due to a lower decomposition rate of NH₃, causes the formation of N vacancies in the grown InN. With increasing growth temperature, a more stoichiometric crystal is grown and the electrical/optical properties improve. At temperatures above 600 °C, however, deterioration occurs at the N-face of In-polar InN near the substrate interface. This deterioration results in the formation of a porous layer during high temperature (~650 °C) growth. There are a few evidences that show that the hydrogen produced by NH₃ decomposition causes this degradation. Thus, improving the quality of MOVPE-grown InN by changing the growth temperature can be difficult. However, a short growth time at a high growth rate and a relatively high temperature is one effective way to solve this dilemma, and one can achieve carrier concentrations as low as 4×10¹⁸ cm⁻³ by growth at 650 °C for 30 min.     

The transition from As-doped GaN, showing blue emission, to GaNAs alloys in films grown by molecular beam epitaxy

We have studied the transition from As-doped GaN showing strong blue emission (2.6 eV) at room temperature to the formation of GaN_1−xAs_x alloys for films grown by plasma-assisted molecular beam epitaxy. We have demonstrated that with increasing N-to-Ga ratio there is first an increase in the intensity of blue emission at about 2.6 eV and then a transition to the growth of GaN_1−xAs_x alloy films. We present a model based on thermodynamic considerations, which can explain how this might occur.     

Growth of hexagonal and cubic InN nanowires using MOCVD with different growth temperatures

We have performed a detailed investigation of the metal-organic chemical vapor deposition (MOCVD) growth and characterization of InN nanowires formed on Si(1 1 1) substrates under nitrogen rich conditions. The growth of InN nanowires has been demonstrated by using an ion beam sputtered (∼10 nm) Au seeding layer prior to the initiation of growth. We tried to vary the growth temperature and pressure in order to obtain an optimum growth condition for InN nanowires. The InN nanowires were grown on the Au+In solid solution droplets caused by annealing in a nitrogen ambient at 700 °C. By applying this technique, we have achieved the formation of InN nanowires that are relatively free of dislocations and stacking faults. Scanning electron microscopy (SEM) showed wires with diameters of 90–200 nm and lengths varying between 3 and 5 μm. Hexagonal and cubic structure is verified by high resolution X-ray diffraction (HR-XRD) spectrum. Raman measurements show that these wurtzite InN nanowires have sharp peaks E₂ (high) at 491 cm⁻¹ and A₁ (LO) at 591 cm⁻¹.     

Catalytic growth of In2O3 nanobelts by vapor transport

Indium oxide (In₂O₃) nanobelts have been fabricated by thermal evaporation of metallic indium powders with the assistance of Au catalysts. The as-synthesized nanobelts are single-crystalline In₂O₃ with cubic structure, and usually tens of nanometers in thickness, tens to hundreds of nanometers in width, and several hundreds of micrometers in length. The room temperature photoluminescence spectrum of In₂O₃ nanobelts features a broad emission band at 620 nm, which could be attributed to oxygen deficiencies in the as-synthesized belts. The formation of In₂O₃ nanobelts follows a catalyst-assistant vapor—liquid–-solid growth mechanism, which enables the controlled growth of individual belts on predetermined sites.     

Revealing extended defects in HVPE-grown GaN

In this communication we will summarize the results of a complementary study of structural and chemical non-homogeneities that are present in thick HVPE-grown GaN layers. It will be shown that complex extended defects are formed during HVPE growth, and are clearly visible after photo-etching on both Ga-polar surface and on any non-polar cleavage or section planes. Large chemical (electrically active) defects, such as growth striations, overgrown or empty pits (pinholes) and clustered irregular inclusions, are accompanied by non-uniform distribution of crystallographic defects (dislocations). Possible reasons of formation of these complex structures are discussed. The nature of defects revealed by selective etching was subsequently confirmed using TEM, orthodox etching and compared with the CL method. The non-homogeneities were studied in GaN crystals grown in different laboratories showing markedly different morphological characteristics.