The effects of annealing on non-polar (1 1 2¯ 0) a-plane GaN films

Non-polar a-plane (1 1 2¯ 0) GaN films were grown on r-plane sapphire by metal–organic vapor phase epitaxy and were subsequently annealed for 90 min at 1070 °C. Most dislocations were partial dislocations, which terminated basal plane stacking faults. Prior to annealing, these dislocations were randomly distributed. After annealing, these dislocations moved into arrays oriented along the [0 0 0 1] direction and aligned perpendicular to the film–substrate interface throughout their length, although the total dislocation density remained unchanged. These changes were accompanied by broadening of the symmetric X-ray diffraction 1 1 2¯ 0 ω-scan widths. The mechanism of movement was identified as dislocation glide, occurring due to highly anisotropic stresses (confirmed by X-ray diffraction lattice parameter measurements) and evidenced by macroscopic slip bands observed on the sample surface. There was also an increase in the density of unintentionally n-type doped electrically conductive inclined features present at the film–substrate interface (as observed in cross-section using scanning capacitance microscopy), suggesting out-diffusion of impurities from the substrate along with prismatic stacking faults. These data suggest that annealing processes performed close to film growth temperatures can affect both the microstructure and the electrical properties of non-polar GaN films.     

Influence of deposition conditions on nanocrystalline InN layers synthesized on Si(1 1 1) and GaN templates by RF sputtering

We present a detailed investigation on the influence of deposition conditions on morphological, structural and optical properties of InN films deposited on Si(1 1 1) and GaN-on-sapphire templates by reactive radio-frequency (RF) sputtering. The deposition parameters under study are nitrogen content in the sputtering gas, substrate–target distance, substrate temperature and RF power. X-ray diffraction measurements confirm the (0 0 0 1) preferred growth orientation and the wurtzite crystallographic structure of the material. For optimized deposition conditions, InN on Si(1 1 1) substrates presents smooth surface with root-mean-square roughness ∼1 nm. Surface quality of the InN films can be further improved by deposition on GaN-on-sapphire templates, achieving root-mean-square roughness as low as ∼0.4 nm, comparable to that of the underlying substrate. The room-temperature absorption edge is located at 1.70 eV. Intense low-temperature photoluminescence peaking at 1.60 eV is observed.     

Growth of epitaxial ZnO films on Si (1 1 1) substrates with Cr compound buffer layer by plasma-assisted molecular beam epitaxy

Single crystalline ZnO film was grown on (1 1 1) Si substrate through employing an oxidized CrN buffer layer by plasma-assisted molecular beam epitaxy. Single crystalline characteristics were confirmed from in-situ reflection high energy electron diffraction, X-ray pole figure measurement, and transmission electron diffraction pattern, consistently. Epitaxial relationship between ZnO film and Si substrate is determined to be (0 0 0 1)_ZnO‖(1 1 1)_Si and [1 1 2¯ 0]_ZnO‖[0 1 1]_Si. Full-width at half-maximums (FWHMs) of (0 0 0 2) and (1 0 1¯ 1) X-ray rocking curves (XRCs) were 1.379° and 3.634°, respectively, which were significantly smaller than the FWHMs (4.532° and 32.8°, respectively) of the ZnO film grown directly on Si (1 1 1) substrate without any buffer. Total dislocation density in the top region of film was estimated to be ∼5×10⁹ cm⁻². Most of dislocations have a screw type component, which is different from the general cases of ZnO films with the major threading dislocations with an edge component.     

Growth of GaN based structures on Si(1 1 0) by molecular beam epitaxy

The growth of GaN based structures on Si(1 1 0) substrates by molecular beam epitaxy using ammonia as the nitrogen precursor is reported. The structural, optical and electrical properties of such structures are assessed and are quite similar to the ones obtained on Si(1 1 1) in-spite of the very different substrate surface symmetry. A threading dislocation density of 3.7×10⁹ cm⁻² is evaluated by transmission electron microscopy, which is in the low range of typical densities obtained on up to 2 μm thick GaN structures grown on Si(1 1 1). To assess the potential of such structure for device realization, AlGaN/GaN high electron mobility transistor and InGaN/GaN light emitting diode heterostructures were grown and their properties are compared with the ones obtained on Si(1 1 1).     

Epitaxial growth of ZnO films on thin FeO(1 1 1) layers

Thin FeO(1 1 1) buffer layers prepared on Mo(1 1 0) substrate were used to grow ordered ZnO films under ultrahigh vacuum condition, and were in situ characterized by various surface analytical techniques. A chemical interaction between Zn (or ZnO) and FeO(1 1 1) can effectively lower the interfacial energy, which is in favor of an epitaxial growth of ZnO on FeO layers. Compared with the MgO(1 1 1) buffer layer used for the growth of ZnO(0 0 0 1) on sapphire (0 0 0 1) surface, the FeO(1 1 1) thin films might be a better one because it is more thermally stable. Our experimental results provide constructive information on the growth mechanism of ZnO-based materials, which is helpful for further understanding the growth mechanism of related oxide materials.     

Epitaxial growth of tin oxide film on TiO2(1 1 0) using molecular beam epitaxy

Growth of tin oxide thin films using molecular beam epitaxy in a pyrolyzed nitrogen dioxide atmosphere on a titanium dioxide (1 1 0) substrate was investigated using X-ray photoelectron spectroscopy (XPS), electron diffraction, and atomic force microscopy (AFM). Properties of deposited films were studied for their dependence on substrate temperature and oxidation gas pressure. Analyses using XPS data revealed that tin atoms were fully oxidized to Sn⁴⁺ and SnO₂ films were grown epitaxially in deposition conditions of substrate temperatures of 627 K or higher and NO₂ pressure greater than 3×10⁻³ Pa. At a substrate temperature of 773 K, a smooth surface with atomic steps was visible in the SnO₂ films, but above or below this temperature, fine grains with crystal facets or porous structures appeared. At pressures of 8×10⁻⁴ to 3×10⁻⁴ Pa, the randomly oriented SnO phase was dominantly grown. Further decreasing the pressure, the Sn metal phase, which was epitaxially crystallized at less than 500 K, was also grown.     

HVPE growth of semi-polar (1 1 2¯ 2)GaN on GaN template (1 1 3)Si substrate

The hydride-vapour-phase-epitaxial (HVPE) growth of semi-polar (1 1 2¯ 2)GaN is attempted on a GaN template layer grown on a patterned (1 1 3) Si substrate. It is found that the chemical reaction between the GaN grown layer and the Si substrate during the growth is suppressed substantially by lowering the growth temperatures no higher than 900 °C. And the surface morphology is improved by decreasing the V/III ratio. It is shown that a 230-μm-thick (1 1 2¯ 2)GaN with smooth surface is obtained at a growth temperature of 870 °C with V/III of 14.     

Epitaxial NiO (1 0 0) and NiO (1 1 1) films grown by atomic layer deposition

Epitaxial NiO (1 1 1) and NiO (1 0 0) films have been grown by atomic layer deposition on both MgO (1 0 0) and α-Al₂O₃ (0 0 l) substrates at temperatures as low as 200 °C by using bis(2,2,6,6-tetramethyl-3,5-heptanedionato)Ni(II) and water as precursors. The films grown on the MgO (1 0 0) substrate show the expected cube on cube growth while the NiO (1 1 1) films grow with a twin rotated 180° on the α-Al₂O₃ (0 0 l) substrate surface. The films had columnar microstructures on both substrate types. The single grains were running throughout the whole film thickness and were significantly smaller in the direction parallel to the surface. Thin NiO (1 1 1) films can be grown with high crystal quality with a FWHM of 0.02–0.05° in the rocking curve measurements.     

MBE growth of PbSe thin film with a 9×10 cm etch-pits density on patterned (1 1 1)-oriented Si substrate

PbSe thin film was grown on a patterned Si substrate with (1 1 1)-orientation by molecular-beam epitaxy (MBE). On the mesa, a low dislocation density of 9×10⁵ cm⁻² was confirmed by the etch-pits density (EPD) wet-etching technique. The photoluminescence (PL) intensity at room temperature from the low dislocation PbSe film was much higher than that from the PbSe film grown on the planar area, which further indicated the high-quality of PbSe thin film grown on patterned Si substrate.     

Growth of embedded ErAs nanorods on (4 1 1)A and (4 1 1)B GaAs by molecular beam epitaxy

The low solubility of Er in GaAs results in the formation of ErAs nanostructures when GaAs is grown with 5–6 at% Er/Ga ratio by molecular beam epitaxy on GaAs surfaces. For growth on the (4 1 1)A GaAs surface, cross-sectional scanning transmission electron microscopy images show the presence of ErAs nanorods embedded in a GaAs matrix extending along the [2 1 1] direction with a spacing of roughly 7 nm and a diameter of roughly 2 nm. Growth on the GaAs (4 1 1)B surface resulted in only nanoparticle formation. Variation of the polarized optical absorption with in-plane polarization angle is consistent with coupling to surface plasmon resonances of the semimetallic nanostructures.     

In-situ visualization of a super-accelerated synthesis of zinc oxide nanostructures through CO2 laser heating

A simple growth technique capable of growing a variety of zinc oxide (ZnO) nanostructures with record growth rates of 25 μm/s is demonstrated. Visible lengths of ZnO nanowires, nanotubes, comb-like and pencil-like nanostructures could be grown by employing a focused CO₂ laser-assisted heating of a sintered ZnO rod in ambient air, in few seconds. For the first time, the growth process of nanowires was videographed, in-situ, on an optical microscope. It showed that ZnO was evaporated and presumably decomposed into Zn and oxygen by laser heating, reforming ZnO nanostructures at places with suitable growth temperatures. Analysis on the representative nanowires shows a rectangular cross-section, with a [0 0 0 1] growth direction. With CO₂ laser heating replacing furnace heating used conventionally, and using different reactants and forming gases, this method could be easily adopted for other semiconducting inorganic nanostructures in addition to ZnO.     

Low-temperature growth of epitaxial (1 0 0) silicon based on silane and disilane in a 300 mm UHV/CVD cold-wall reactor

Epitaxial (1 0 0) silicon layers were grown at temperatures ranging from 500 to 800 °C in a commercial cold-wall type UHV/CVD reactor at pressures less than 7×10⁻⁵ Torr. The substrates were 300 mm SIMOX SOI wafers and spectroscopic ellipsometry was used to assess growth rates and deposition uniformities. High-resolution atomic force microscopy (AFM) was employed to verify the atomic terrace configuration that resulted from epitaxial step-flow growth. Deposition from disilane exhibited a nearly perfect reaction limit for low temperatures and high precursor flow rates (partial pressures) with measured activation energies of ≈2.0 eV, while a linear dependence of growth rate on precursor gas flow was found for the massflow-controlled regime. A similar behavior was observed in the case of silane with substantially reduced deposition rates in the massflow-limited regime and nearly a factor of 2 reduced growth rates deep in the reaction limited regime. High growth rates of up to 50 μm/h and non-uniformities as low as 1σ=1.45% were obtained in the massflow-limited deposition regime. Silicon layers as thin as 0.6 nm (4.5 atomic layers ) were deposited continuously as determined using a unique wet chemical etching technique as well as cross-sectional high-resolution transmission electron microscopy (HRTEM). In contrast, epitaxial silicon deposited in RPCVD at 10 Torr using disilane within the same temperature range showed imperfect reaction limitation. While activation energies similar to that of UHV/CVD were found, no partial pressure limitation could be observed. Furthermore, layers deposited using disilane in RPCVD exhibited a large number of defects that appeared to form randomly during growth. We attribute this effect to gas phase reactions that create precursor fragments and radicals—an effect that is negligible in UHV/CVD.     

Atomic ordering in GaAsSb (0 0 1) grown by metalorganic vapor phase epitaxy

Epitaxial GaAsSb (0 0 1) semiconductor alloys grown by metalorganic vapor phase epitaxy exhibit several spontaneously ordered structures. A superlattice structure with three-fold ordering in the [1 1 0] direction has been previously observed by different groups. CuAu structures with (1 0 0) and (0 1 0) ordering planes have also been reported. The physical origin of CuAu ordering in III–V semiconductors has not yet been explained. In this work we report the effect of growth conditions on CuAu ordering in GaAsSb, including miscut from (0 0 1), growth rate, bismuth surfactant concentration, and growth temperature. These data point to a surface kinetic mechanism not based on dimer strain, but possibly due to one-dimensional ordering at step edges.     

Effect of oxygen partial pressure on the growth of zinc micro and nanostructures

Zinc micro and nanostructures were synthesized in vacuum by condensing evaporated zinc on Si substrate at different gas pressures. The morphology of the grown Zn structures was found to be dependent on the oxygen partial pressure. Depending on oxygen partial pressure it varied from two-dimensional microdisks to one-dimensional nanowire. The morphology and structural properties of the grown micro and nanostructures were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Transmission electron microscopy (TEM) studies on the grown Zn nanowires have shown that they exhibit core/shell-like structures, where a thin ZnO layer forms the shell. A possible growth mechanism behind the formation of different micro and nanostructures has been proposed. In addition, we have synthesized ZnO nanocanal-like structures by annealing Zn nanowires in vacuum at 350 °C for 30 min.     

Reduction of the dislocation density in molecular beam epitaxial CdTe(2 1 1)B on Ge(2 1 1)

The high dislocation density (2×10⁷/cm² for a thickness of 7 μm) in CdTe(2 1 1)B on Ge(2 1 1) has become a roadblock for the technological exploitation of this material. We present a systematic study of in situ and post-growth annealing cycles aimed at reducing it. An etch pit density of 2×10⁶/cm² was achieved by optimizing the growth conditions and annealing the samples in situ. This finding was corroborated by high-resolution X-ray diffraction, atomic force microscopy, photoluminescence and ellipsometry measurements.     

Structural,electrical and optical properties of SnO2 films deposited on Y-stabilized ZrO2 (1 0 0) substrates by MOCVD

SnO₂ films have been deposited on Y-stabilized ZrO₂ (YSZ) (1 0 0) substrates at different substrate temperatures (500–800 °C) by metalorganic chemical vapor deposition (MOCVD). Structural, electrical and optical properties of the films have been investigated. The films deposited at 500 and 600 °C are epitaxial SnO₂ films with orthorhombic columbite structure, and the HRTEM analysis shows a clear epitaxial relationship of columbite SnO₂(1 0 0)||YSZ(1 0 0). The films deposited at 700 and 800 °C have mixed-phase structures of rutile and columbite SnO₂. The carrier concentration of the films is in the range from 1.15×10¹⁹ to 2.68×10¹⁹ cm⁻³, and the resistivity is from 2.48×10⁻² to 1.16×10⁻² Ω cm. The absolute average transmittance of the films in the visible range exceeds 90%. The band gap of the obtained SnO₂ films is about 3.75–3.87 eV.     

Investigation of nonpolar (1 1 2¯ 0) a-plane ZnO films grown under various Zn/O ratios by plasma-assisted molecular beam epitaxy

Structural and optical properties of nonpolar a-plane ZnO films grown with different II/VI ratios on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy were investigated. Even by increasing the II/VI ratio across the stoichiometric flux condition a consistent surface morphology of striated stripes along the ZnO 〈0 0 0 1〉 direction without any pit formation was observed, which is contrary to polar c-plane ZnO films. Root mean square surface roughness, full width at half maximum values of X-ray rocking curves, defect densities, and photoluminescence were changed with the II/VI ratio. The sample grown with stoichiometric flux condition showed the lowest value of rms roughness, the smallest threading dislocation and stacking fault densities of ∼4.7×10⁸ cm⁻² and ∼9.5×10⁴ cm⁻¹, respectively, and the highest intensity of D^oX peak. These results imply that the stoichiometric flux growth condition is suitable to obtain superior structural and optical properties compared to other flux conditions.     

Decisive role of Au layer on the oriented growth of ZnO nanorod arrays via a simple aqueous solution method

Vertically aligned arrays of ZnO nanorod were synthesized on the Au/SiO₂/Si(1 0 0) substrate by a simple aqueous solution growth process, without pre-prepared ZnO seed layer. For comparison, glass and SiO₂/Si were also used as substrates, and the results show that the Au layer plays a decisive role in orienting the growth of the ZnO nanorod. The effects of other growth parameters, including Zn²⁺ concentration and growth time, on morphology, density, and orientation of the ZnO nanostructure were also studied and with longer reaction time, a new structure namely ZnO nanotip was obtained. Moreover, the growth mechanism of ZnO nanorod arrays grown on the Au/SiO₂/Si substrate was proposed.     

Reduction of dislocations in a (1 1 2¯ 2)GaN grown by selective MOVPE on (1 1 3)Si

Two-step selective epitaxy (SAG/ELO) of (1 1 2¯ 2)GaN on (1 1 3)Si substrate is studied to reduce the defect density in the epitaxial lateral overgrowth. The first SAG/ELO is to prepare a (1 1 2¯ 2)GaN template on a (1 1 3)Si and the second SAG/ELO is to get a uniform (1 1 2¯ 2)GaN. It is found that the reduction of the defect density is improved by optimizing the mask configuration in the second SAG/ELO. The minimum dark spot density obtained is 3×10⁷/cm², which is two orders of magnitude lower than that found in a (0 0 0 1)GaN grown on (1 1 1)Si.     

Formation of GaN nanodots on Si (1 1 1) by droplet nitridation

GaN nanodots (NDs) are obtained by Ga metallic droplet formation on Si (1 1 1) substrates followed by their nitridation. The size and density of Ga droplets and GaN NDs can be controlled by varying the growth temperature within the range 514–640 °C. Atomic force microscopy (AFM) investigation of Ga droplets shows an increase in the average diameter with temperature. The average diameter of GaN NDs increases with growth temperature while their density decreases more than one order of magnitude. In addition, the formation of a GaN crystallite rough layer on Si, in-between NDs, indicates that a spreading mechanism takes place during the nitridation process. High-resolution transmission electron microscopy (HRTEM) is used for the investigation of shape, crystalline quality and surface distribution of GaN dots. X-ray photoelectron spectroscopy (XPS) results confirm that Ga droplets that are transformed into GaN NDs spread over the sample surface during nitridation.