Improvement of GaInNAsSb films fabricated by atomic hydrogen-assisted molecular beam epitaxy

The effect of incorporation of antimony in GaInNAs films grown by atomic hydrogen-assisted molecular beam epitaxy (MBE) has been investigated. We show that the rate of incorporation of N and In forming GaInNAs do not depend on the Sb beam flux. However, the incorporation of Sb is strongly dependent on the Sb/As₂ flux ratio. Introducing a small amount of Sb (<∼1%) significantly improves the photoluminescence (PL) emission efficiency of GaInNAs, but Sb concentration of >1% rapidly degrades the PL intensity, though a large redshift can still be achieved. Therefore, there is an optimum amount of Sb for the growth of low-strained GaInNAs films to improve the overall optical quality.     

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.     

Growth and annealing effect of high-quality ZnSe:N/ZnSe by MOCVD

The ZnSe : N epitaxial layers were grown on (1 1 0) ZnSe substrates in a low-pressure metalorganic chemical vapor deposition (MOCVD) system using hydrogen as a carrier gas, and using ammonia as a dopant source. In order to obtain highly doped ZnSe : N epitaxial layers, the optimum growth and doping conditions were determined by studying the photoluminescence (PL) spectra from the ZnSe epitaxial layers grown at different ammonia flux and VI/II flux ratio. Furthermore, in order to enhance the concentration of active nitrogen in ZnSe epitaxial layer, a rapid thermal anneal technique was used for post-heat-treating. The results show that the annealing temperature of over 1023 K is necessary. Beside, a novel treatment method to obtain a smooth substrate surface for growing high quality ZnSe epitaxial layers is also described.     

Measurement of nitrogen atomic flux for RF-MBE growth of GaN and AlN on Si substrates

Production and measurement of active nitrogen atoms (N+N*), which consist of ground state nitrogen atoms N and excited state nitrogen atoms N*, in an inductively coupled radio frequency discharge for the growth of group III nitrides and their alloys using a molecular beam epitaxy (MBE) were studied. Two discharge modes of the low brightness (LB) and the high brightness (HB) used in this study to produce excited nitrogen molecules (N₂*) and dissociated active nitrogen atoms (N+N*). The flux of (N+N*) was measured by a Langumuir-like electrode due to the self-ionization of adsorbed (N+N*) on a negatively biased electrode. The self-ionization, which emits electrons from (N+N*), forms an atom current and is confirmed using different electrodes such as Pt and CuBe and different electrode area. The atom current was calibrated by the grown GaN thickness in a VG80H MBE machine. The calibrated flux of (N+N*) per atom current in the VG80H machine is 5.5×10⁻⁴ ML/s/nA, where ML is monolayer. The atom current is useful to monitor the flux of chemically active nitrogen atoms N+N* for growth of group III nitrides and their alloys. Activity modulation migration enhanced epitaxial growth (AM-MEE) was demonstrated as an application of the measurement of atom current for the growth of the group III nitrides.     

Carbon reduction and antimony incorporation in InGaAsSb films grown by metalorganic molecular beam epitaxy using tris-dimethylaminoantimony

InGaAsSb layers nearly lattice-matched to InP were grown by metalorganic molecular beam epitaxy using tris-dimethylaminoantimony (TDMASb). Secondary-ion mass spectroscopy measurements revealed that TDMASb is useful not only as an Sb source but also as an additive that reduces the incorporation of C into the film from group-III metalorganic sources. In the room-temperature photoluminescence spectrum, the incorporation of Sb into InGaAs shifted the peak wavelength from 1.66 to 1.75 μm and, simultaneously, the peak intensity of InGaAsSb became more than twice that of InGaAs.     

Growth and properties of Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals directly from melt

Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT91/9) single crystals were grown by a modified Bridgman method directly from melt using an allomeric Pb[(Mg_1/3Nb_2/3)_0.69Ti_0.31]O₃ (PMNT69/31) single crystal as a seed. X-ray diffraction (XRD) measurement confirmed that the as-grown PZNT91/9 single crystals are of pure perovskite structure. Electrical properties and thermal stabilization of PZNT91/9 crystals grown directly from melt exhibit different characters from those of PZNT91/9 crystals grown from flux, although segregation and the variation of chemical composition are not seriously confirmed by X-ray fluorescence analysis (XPS). The [0 0 1]-oriented PZNT91/9 crystals cut from the middle part of the as-grown crystal boules exhibit broad dielectric-response peaks at around 105 °C, accompanied by apparent frequency dispersion. The values of piezoelectric constant d₃₃, remnant polarization P_r, and induced strain are about 1800–2200 pC/N, 38.8 μC/cm², and 0.3%, respectively, indicating that the quality of PZNT crystals grown directly from melt can be comparable to those of PZNT91/9 single crystals grown from flux. However, further work deserves attention to improve the dielectric properties of PZNT crystals grown directly from melt. Such unusual characterizations of dielectric properties of PZNT crystals grown directly from melt are considered as correlating with defects, microinhomogeneities, and polar regions.     

The incorporation behavior of arsenic and antimony in GaAsSb/GaAs grown by solid source molecular beam epitaxy

GaAsSb ternary epitaxial layers were grown on GaAs (0 0 1) substrate in various Sb₄/As₂ flux ratios by solid source molecular beam epitaxy. The alloy compositions of GaAs_1−ySb_y were inferred using high-resolution X-ray symmetric (0 0 4) and asymmetric (2 2 4) glance exit diffraction. The non-equilibrium thermodynamic model is used to explain the different incorporation behavior between the Sb₄ and As₂ under the assumption that one incident Sb₄ molecule produces one active Sb₂ molecule. It is inferred that the activation energy of Sb₄ dissociation is about 0.46 eV. The calculated results for the incorporation efficiency of group V are in good agreement with the experimental data.     

Growth and energy band gap of CaSeS thin films prepared by hot-wall epitaxy

This paper investigates preparation of CaSeS thin films using hot-wall epitaxy. These films can be grown epitaxially on cleaved BaF₂(1 1 1) at a substrate temperature of 873 K by tailoring the VI/II flux ratio vaporized from Ca and SeS resources. The optical absorption edge of these films thus tailored can be observed clearly, shifting toward higher photon energy with increasing S content. In particular, the energy band gap of CaSe_0.66S_0.34, capable of lattice-matching to InP was found to be 4.69 eV, producing considerably large band gap difference of 3.34 eV between the CaSe_0.66S_0.34 and InP.     

Growth and morphology of ruby crystals with unusual chromium concentration

Single crystals of ruby have been obtained from fluxed melts based on the systems Li₂O–MoO₃, Li₂O–WO₃, Na₂O–WO₃, 2PbO–3V₂O₅, PbO–V₂O₅–WO₃, PbF₂–Bi₂O₃ and Na₃AlF₆ by both the TSSG method and spontaneous crystallization at the temperatures 1330–900 °C. Al₂O₃ solubility has been measured for the flux composition of 2Bi₂O₃–5PbF₂ in the temperature range 1200–1000 °C and dissolution enthalpy has been defined as 29.4 KJ/Mol. The composition of grown crystals was studied by electron microprobe analysis. The synthetic ruby contains from 0.51 to 6.38 at% of chromium admixture depending on the crystal growth conditions. Experimental results on growth conditions, composition and morphology of grown crystals are presented for each flux and temperature interval.     

Synthesis of AlGaAs-based strained separately confined heterostructure laser diodes by low temperature liquid-phase epitaxy

AlGaAs-based strained separately confined heterostructure laser diodes operating in the 800–900 nm wavelength range have been fabricated from the structures grown using low temperature liquid-phase epitaxy (LPE). Addition of P/Sb to the active region introduces tensile/compressive strain in the structure and is found useful in tuning the emission wavelength. Details of the growth experiments, strain measurements, fabrication and characterization of the laser diodes are described.     

Influence of Be on N composition in Be-doped InGaAsN grown by RF plasma-assisted molecular beam epitaxy

Undoped and Be-doped InGaAsN layers were grown on GaAs substrates under the same growth conditions by radio frequency plasma-assisted molecular beam epitaxy. Increased tensile strain (Δa/a=3×10⁻³) was observed for Be-doped InGaAsN layers, compared to undoped InGaAsN layers. The strain is shown to originate from the increase in N composition related to Be incorporation, rather than solely from Be atoms substituting Ga atom sites (Be_Ga). A possible reason is the high Be–N bond strength, which inhibits the loss of N from the growth surface during epitaxial growth, thereby increasing the N composition in the Be-doped InGaAsN layer.     

Incorporation of group III,IV and V elements in 3C–SiC(1 1 1) layers grown by the vapour–liquid–solid mechanism

We report on a comparative investigation of the incorporation of group III, IV and V impurities in 3C–SiC heteroepitaxial layers grown by the vapour–liquid–solid (VLS) mechanism on on-axis α-SiC substrates. To this end, various Si-based melts have been used with addition of Al, Ga, Ge and Sn species. Homoepitaxial α-SiC layers grown using Al-based melts were used for comparison purposed for Al incorporation. Nitrogen incorporation depth profile systematically displays an overshoot at the substrate/epilayer interface for all the layers. Ga and Al incorporations follow the same distribution shape as N whereas this is not the case for the isoelectronic impurities Ge and Sn. This suggests some interaction between Ga/Al and N coming from the high bonding energy between the group III and V elements, which does not exist with Ge and Sn. This is why both incorporate as a cluster. A model of incorporation is proposed taking into account metal-N and metal-C bonding energies together with the solid solubility of the corresponding nitrides.     

Flux growth of PbMg1/3Ta2/3O3 single crystals

Single crystals of PbMg_1/3Ta_2/3O₃ (PMT) were grown by the flux method. The PbO–Pb₃O₄–B₂O₃ system was used as a solvent. Transparent and light yellow PMT single crystals of rectangular shape and dimensions up to 10×6×4 mm³ were obtained. For the applied growth conditions only, the crystals of the perovskite structure were grown. X-ray diffraction tests showed that at room temperature PMT exhibits cubic symmetry with lattice parameter a=4.042(1) Å. Dielectric studies pointed to relaxor properties of PMT. The characteristic broad and frequency-dependent maximum of dielectric permittivity was observed at 179.7 K (1 kHz).     

Emission studies of InGaN layers and LEDs grown by plasma-assisted MBE

We prepared InGaN layers on GaN/sapphire substrates using rf-MBE. Photoluminescence (PL) from these layers, grown at different temperatures T_S, shows that there is a strong tendency of GaN to form a separate phase as T_S is increased from 600°C to 650°C. Concomitant with the phase separation, the PL from the InGaN phase broadens, which indicates that indium composition in this phase becomes increasingly non-uniform. Indium compositions measured by Rutherford backscattering (RBS) are consistent with these results. We also observed an increase in PL intensity for InGaN layers grown at higher temperatures. In this paper, we also report on preparing a top-contact InGaN/GaN light emitting diode. The device was operated at 447 nm and had the emission line width of 37 nm with no observable impurity related features. The turn-on voltage was 3.0 V. The output power was 20 μW at 60 mA drive current.     

Local atomic and electronic structures of equiatomic liquid alloy KSb from 923 to 1773 K

The local atomic and electronic structures of equiatomic liquid alloy KSb have been investigated using ab initio molecular dynamics simulations. There is a small peak located at about 62° in the covalent SbSb bond-angle distribution function. The height of 62° peak increases with temperature. The further analysis show that this peak attributes to the triangular structures with each Sb atom covalently bonded to the other two in the liquid KSb and their number increases with temperature. Therefore, there exist isolated Sb atoms, covalently bonded Sb dimers, triangles with three covalent SbSb bonds and short covalently bonded Sb chains in the liquid KSb. Upon temperature increasing the covalent Sb chains tend to break into short pieces so that the isolated Sb atoms, covalently bonded Sb dimers and triangles with three covalent SbSb bonds increase in numbers. The obtained total electronic densities of states at different temperatures give a reasonable explanation of the electronic conductivity increasing with temperature.     

The effect of temperature and ammonia flux on the surface morphology and composition of InxAl1−xN epitaxial layers

An interesting recent development in the Group III nitrides is the growth of InAlN lattice matched to GaN, with applications in distributed Bragg reflectors (DBRs), high electron mobility transistors (HEMTs) and as etch-layers. This work presents a systematic study of the effects of changing the key growth conditions of ammonia flux and growth temperature in InAlN growth by metal-organic vapour phase epitaxy (MOPVE) and describes our current optimised parameter set. We also particularly concentrate on the details of surface morphology assessed by atomic force microscopy (AFM). The nanoscale surfaces are characterised by low hillocks and dislocation pits, while at a larger scale microscopic indium droplets are also present. However, these droplets are eliminated when the layers are capped with GaN. Other trends observed are that increasing the growth temperature will lower the indium incorporation approximately linearly at a rate of approximately 0.25% per °C, and that increasing the ammonia flux from 44.6 to 178.6 mmol min⁻¹ increased the indium incorporation, but further increases to 446 mmol min⁻¹ did not result in any further increase.     

Growth of InAs on GaAs(1 0 0) by low-pressure metalorganic chemical vapor deposition employing in situ generated arsine radicals

InAs was grown by low-pressure metalorganic chemical vapor deposition on vicinal GaAs(1 0 0) substrates misoriented by 2° toward [0 0 1]. We observed InAs crystal growth, at substrate temperatures down to 300°C, employing in situ plasma-generated arsine radicals as the arsenic source. The in situ generated arsine was produced by placing solid arsenic downstream of a microwave driven hydrogen plasma. Trimethylindium (TMIn) feedstock carried by hydrogen gas was used as the indium source. The Arrhenius plot of InAs growth rate vs. reciprocal substrate temperature displayed an activation energy of 46.1 kcal/mol in the temperature range of 300–350°C. This measured activation energy value is very close to the energy necessary to remove the first methyl radical from the TMIn molecule, which has never been reported in prior InAs growth to the best of authors’ knowledge. The film growth mechanism is discussed. The crystallinity, infrared spectrum, electrical properties and impurity levels of grown InAs are also presented.     

MgO films grown on yttria-stabilized zirconia by molecular beam epitaxy

MgO films were grown on (0 0 1) yttria-stabilized zirconia (YSZ) substrates by molecular beam epitaxy (MBE). The crystalline structures of these films were investigated using X-ray diffraction and transmission electron microscopy. Growth temperature was varied from 350 to 550 °C, with crystalline quality being improved at higher temperatures. The MgO films had a domain structure: (1 1 1)[1 1 2¯]_MgO(0 0 1)[1 0 0]_YSZ with four twin variants related by a 90° in-plane rotation about the [1 1 1]_MgO axis. The observed epitaxial orientation was compared to previous reports of films grown by pulsed laser deposition and sputtering and explained as resulting in the lowest interface energy.     

The role of Sb in the molecular beam epitaxy growth of 1.30–1.55 μm wavelength GaInNAs/GaAs quantum well with high indium content

Sb-assisted GaInNAs/GaAs quantum wells (QWs) with high (42.5%) indium content were investigated systematically. Transmission electron microscopy, reflection high-energy electron diffraction and photoluminescence (PL) measurements reveal that Sb acts as a surfactant to suppress three-dimensional growth. The improvement in the 1.55 μm range is much more apparent than that in the 1.3 μm range, which can be attributed to the difference in N composition. The PL intensity and the full-width at half maximum of the 1.55 μm single-QW were comparable with that of the 1.3 μm QWs.     

Evidence of free carrier concentration gradient along the c-axis for undoped GaN single crystals

Results of measurements of infrared reflectivity and micro-Raman scattering on the undoped GaN high pressure grown single crystals are reported. These crystals have usually a high electron concentration due to unintentional doping by oxygen. We show, by the shift of the plasma edge (infrared reflectivity measurements), that the free electron concentration is always higher on the (0 0 0  )N face of the GaN single crystal than on the (0 0 0 1)Ga face. In order to determine the profile of the free carrier concentration, we performed transverse micro-Raman scattering measurements along the (0 0 0 1) c-axis of the crystal with spatial resolution of 1 μm. Micro-Raman experiments give a quantitative information on the free carrier concentration via the longitudinal optical phonon–plasmon (LPP) coupling modes. Thus, by studying the behavior of the LPP mode along the c-axis, we found the presence of a gradient of free electrons. We suppose that this gradient of electrons is due to the gradient of the main electron donor, in undoped GaN single crystals, i.e. oxygen impurity. We propose a growth model which explains qualitatively the incorporation of oxygen during the growth of GaN crystal under high pressure of nitrogen.