Growth and characterization of GaN and AlN films on (1 1 1) and (0 0 1) Si substrates

GaN and AlN films were grown on (1 1 1) and (0 0 1) Si substrates by separate admittances of trimethylgallium (or trimethylaluminum) and ammonia (NH₃) at 1000°C. A high temperature (HT) or low temperature (LT) grown AlN thin layer was employed as the buffer layer between HT GaN (or HT AlN) film and Si substrate. Experimental results show that HT AlN and HT GaN films grown on the HT AlN-coated Si substrates exhibit better crystalline quality than those deposited on the LT AlN-coated Si substrates. Transmission electron microscopy (TEM) of the HT GaN/HT AlN buffer layer/(1 1 1)Si samples shows a particular orientation relationship between the (0 0 0 1) planes of GaN film and the (1 1 1) planes of Si substrate. High quality HT GaN films were achieved on (1 1 1) Si substrates using a 200 Å thick HT AlN buffer layer. Room temperature photoluminescence spectra of the high quality HT GaN films show strong near band edge luminescence at 3.41 eV with an emission linewidth of ∼110 meV and weak yellow luminescence.     

Geometric modeling of homoepitaxial CVD diamond growth: I. The {1 0 0}{1 1 1}{1 1 0}{1 1 3} system

Plasma-assisted CVD homoepitaxial diamond growth is a process that must satisfy many stringent requirements to meet industrial applications, particularly in high-power electronics. Purity control and crystalline quality of the obtained samples are of paramount importance and their optimization is a subject of active research. In the process of such studies, we have obtained high purity CVD diamond monocrystals with unusual morphologies, namely with apparent {1 1 3} stable faces. This phenomenon has led us to examine the process of CVD diamond growth and build up a 3D geometrical model, presented here, describing the film growth as a function of time. The model has been able to successfully describe the morphology of our obtained crystals and can be used as a predictive tool to predetermine the shape and size of a diamond crystal grown in a given process configuration. This renders accessible control of desirable properties such as largest usable diamond surface area and/or film thickness, before the cutting and polishing manufacture steps take place. The two latter steps are more sensitive to the geometry of the growth sectors, which will be addressed in a companion paper.Our model, applicable to the growth of any cubic lattice material, establishes a complete mapping of the final morphology state of growing diamond, as a function of the growth rates of the crystalline planes considered, namely {1 0 0}, {1 1 1}, {1 1 0}, and {1 1 3} planes, all of which have been observed experimentally in diamond films. The model makes no claim as to the stability of the obtained faces, such as the occurrence of non-epitaxial crystallites or twinning. It is also possible to deduce transient behavior of the crystal morphology as growth time is increased. The model conclusions are presented in the form of a series of diagrams, which trace the existence (and dominance) boundaries of each face type, in presence of the others, and where each boundary crossing represent a topology change in terms of number of faces, edges and vertices. We validate the model by matching it against crystals published in the literature and illustrate its predictive value by suggesting ways to increase usable surface area of the diamond film.     

Polarities of AlN films and underlying 3C-SiC intermediate layers grown on (1 1 1) Si substrates

The hydride vapor phase epitaxy (HVPE) of {0 0 0 1} AlN films on {1 1 1} Si substrates covered with epitaxial {1 1 1} cubic SiC (3C-SiC intermediate layers) was carried out. 3C-SiC intermediate layers are essential to obtain high-quality AlN films on Si substrates, because specular AlN films are obtained with 3C-SiC intermediate layers, whereas rough AlN films are obtained without 3C-SiC intermediate layers. We determined the polarities of AlN films and the underlying 3C-SiC intermediate layers by convergent beam electron diffraction (CBED) using transmission electron microscopy. For the first time, the polarities of the AlN films and the 3C-SiC intermediate layers were determined as Al and Si polarities, respectively. The AlN films were hardly etched by aqueous KOH solution, thereby indicating Al polarity. This supports the results obtained by CBED. The result is also consistent with electrostatic arguments. An interfacial structure was proposed. The 3C-SiC intermediate layers are promising for the HVPE of AlN films on Si substrates.     

Synthesis and characterization of TM-doped CuO (TM = Fe,Ni)

Polycrystalline Cu_1?xTM_xO samples (x = 0 and 0.06; TM = Ni²⁺ and Fe³⁺) were grown using a co-precipitation method. The structural and magnetic properties were investigated by means of temperature dependent magnetic susceptibility and room temperature X-ray powder diffraction (XRPD). The XRPD analyses of the samples reveal the formation of single phase with structure isomorphous to the CuO. Interestingly, T-dependent magnetization shows the reduction of Néel temperature, T_N, from 213 K in the copper oxide to 70 K in the Fe-doped sample (x = 0.06). Because in the Ni-doped samples T_N seems to be unaffected, this decrease in T_N is believed to be due to the different electronic structure of the dopant. The ferromagnetic behavior observed at room temperature in all samples can be related to both the level of oxygen (excess or vacancy) of our samples and to the difference in the magnetic structure of the dopant.     

Thickness-dependent structural transition in GaAs nanocrystals grown on Si (1 1 1) surface

Structural stabilities in GaAs nanocrystals grown on the Si (1 1 1) substrate have been studied by transmission electron microscopy in order to see the structure and growth mechanism. The GaAs nanocrystals grown epitaxially on the Si (1 1 1) surface kept at 573 K have thin shapes consisting of a flat surface which is parallel to the Si (1 1 1) surface. The crystalline structure of the initial growth layer approximately below 5 nm in thickness is the zincblend structure, but with increasing thickness the structure changes to the wurtzite structure by formation of orderly-arranged stacking faults. The small difference in the driving force between the wurtzite structure and the zincblende structure could bring about a situation, where the kinetic rate of nucleus formation is high for the wurtzite structure than for the zincblende structure. It would highly increase the probability that the wurtzite structure is formed as a non-equilibrium state.     

N-type doping of GaN/Si(1 1 1) using Al0.2Ga0.8N/ALN composite buffer layer and Al0.2Ga0.8N/GaN superlattice

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al_0.2Ga_0.8N/AlN and superlattice of Al_0.2Ga_0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×10⁹ and 1.32×10⁹ cm⁻² for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.     

Etch pits of precipitates in CdZnTe crystals on (1 1 1) B surface

The etch pits of Te-rich and Cd-rich precipitates of the Cd_0.94Zn_0.04Te crystals situated on (1 1 1) B surface were distinguished from the dislocation etch pits and evaluated by using Everson Etching for the first time. By using an IR transmission microscope and a scanning electron microscope (SEM) with an energy-dispersive x-ray spectroscopy (EDX) analytical system, the characteristics of the precipitate etch pits were studied. The etch pit of the Te-rich precipitate which would influence the etch rate of its ambient materials shows the shape of triangle or hexagon with a rough surface different from the cone-shaped triangle etch pit as the dislocation etch pit. The etch pit of the Cd-rich precipitate is always enwrapped by large quantities of dislocation etch pits, which indicates the existence of the damaged layers around the Cd-rich precipitate. Base on the characteristics of the precipitate etch pits, the areal densities of the precipitates with the size larger than 5 μm on the substrate surfaces can be measured by the optical microscope with a magnification of 50 and under Nomarski mode. Compared with the conventional measurement method based on the IR transmission microscope (IRTM), the method of the etch pit observation (EPO) is more practical for measuring the areal densities of the precipitates of CdZnTe substrates.     

Effects of vicinal angles from (0 0 1) surface on the Boron-doping features of high-quality homoepitaxial diamond films grown by the high-power microwave plasma chemical-vapor-deposition method

Vicinal surface effects on homoepitaxial growth and boron-doping processes have been studied in case of single-crystalline diamond (0 0 1) surfaces grown using the high-power microwave plasma chemical-vapor-deposition (MWPCVD) method. The off-angles inclined from the on-orientation (0 0 1) surfaces ranged to 5° along the [1 1 0] or [1 0 0] direction, while the concentration of doping B(CH₃)₃ gas was kept constant with a B/C ratio of 50 ppm. Although a number of square-like growth hillocks often appeared, depending substantially on the crystalline quality of the high-pressure/high-temperature-synthesized (HPHT) Ib diamond substrates employed, the number and shape of the hillocks changed significantly with the increasing off-angle. For the vicinal surfaces with off-angles of ≈3° inclined along the [1 1 0] direction, macroscopically flat surfaces were obtained, compared with the other off-angle cases examined. Furthermore, the growth rate and acceptor density of substitutional boron atoms in the homoepitaxial layers were found to substantially increase with the increasing off-angle. These indicate that the step density can play important roles not only in the homoepitaxial growth but also in the boron-incorporation process during the high-power MWPCVD growth.     

Growth and characterization of GaSb/AlGaSb multi-quantum well structures on Si (0 0 1) substrates

GaSb/AlGaSb multi-quantum well (MQW) structures with an AlSb initiation layer and a relatively thick GaSb buffer layer grown on Si (0 0 1) substrates were prepared by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM) images and high-resolution X-ray diffraction (XRD) patterns indicated definite MQW structures. The photoluminescence (PL) emission around 1.55 μm wavelength was observed for 10.34 nm GaSb/30 nm Al_0.6Ga_0.4Sb MQW structure at room temperature. Dependence of PL emission energy on GaSb well width was well explained by finite square well potential model.     

Characterization of chromium silicide thin layer formed on amorphous silicon films

A detailed investigation of the compositional, optical and electrical properties of a chromium silicide layer grown at room temperature on top of doped amorphous silicon films is presented. The formation of the layer is promoted only when phosphorous atoms are present in the film. The deposition of a very thin n-type doped layer (around 5 nm) on top of a p-type doped film has allowed us to achieve the chromium silicide formation also on p-type material without changing its doping properties. Angle resolved X-ray photoelectron spectroscopy measurements demonstrate the presence of chromium-oxide, chromium silicide and metallic chromium in similar percentages for both p- and n-type doped layers. From the ellipsometric analysis, the refractive index spectra have been extracted, and the layer thickness has been estimated to be 5 nm for both p- and n-type doped layers. From planar conductivity measurements, we have found that the chromium silicide promotes an activation energy reduction from 0.24 eV down to 0.017 eV for the n-type layer and from 0.36 eV down to 0.14 eV for the p-type film.     

Improving the performance of amorphous and crystalline silicon heterojunction solar cells by monitoring surface passivation

The influence of thermal annealing on the crystalline silicon surface passivating properties of selected amorphous silicon containing layer stacks (including intrinsic and doped films), as well as the correlation with silicon heterojunction solar cell performance has been investigated. All samples have been isochronally annealed for 1 h in an N₂ ambient at temperatures between 150 °C and 300 °C in incremental steps of 15 °C. For intrinsic films and intrinsic/n-type stacks, an improvement in passivation quality is observed up to 255 °C and 270 °C, respectively, and a deterioration at higher temperatures. For intrinsic/n-type a-Si:H layer stacks, a maximum minority carrier lifetime of 13.3 ms at an injection level of 10¹⁵ cm^? 3 has been measured. In contrast, for intrinsic/p-type a-Si:H layer stacks, a deterioration in passivation is observed upon annealing over the whole temperature range. Comparing the lifetime values and trends for the different layer stacks to the performance of the corresponding cells, it is inferred that the intrinsic/p-layer stack is limiting device performance. Furthermore, thermal annealing of p-type layers should be avoided entirely. We therefore propose an adapted processing sequence, leading to a substantial improvement in efficiency to 16.7%, well above the efficiency of 15.8% obtained with the ‘standard’ processing sequence.     

Ternary chalcogenides LiBC2 (B = In, Ga; C = S, Se, Te) for mid-IR nonlinear optics

We review on the growth improvement and optical characterization of a new family of ternary lithium-based chalcogenide crystals of generic formula ${\text{LiB}}^{\text{III}}{\text{C}}_2^{\text{VI}}$ (B = In, Ga; C = S, Se, Te) which displays improved thermo-mechanical properties for mid-IR nonlinear optical applications. Some of these compounds are now produced in sufficiently large size, single-domain quality to allow their implementation in optical parametric oscillators.     

Characteristics of p-type nanocrystalline silicon thin films developed for window layer of solar cells

Different rf-power and chamber pressures have been used to deposit boron doped hydrogenated silicon films by the PECVD method. The optoelectronic and structural properties of the silicon films have been investigated. With the increase of power and pressure the crystallinity of the films increases while the absorption decreases. As a very thin p-layer is needed in p–i–n thin film solar cells the variation of properties with film thickness has been studied. The fraction of crystallinity and thus dark conductivity vary also with the thickness of the film. Conductivity as high as 2.46 S cm⁻¹ has been achieved for 400 Å thin film while for 3000 Å thick film it is 21 S cm⁻¹. Characterization of these films by XRD, Raman Spectroscopy, TEM and SEM indicate that the grain size, crystalline volume fraction as well as the surface morphology of p-layers depend on the deposition conditions as well as on the thickness of the film. Optical band gap varies from 2.19 eV to 2.63 eV. The thin p-type crystalline silicon film with high conductivity and wide band gap prepared under high power and pressure is suitable for application as window layer for Silicon thin film solar cells.     

Undoped and arsenic-doped low temperature (∼165 °C) microcrystalline silicon for electronic devices process

Microcrystalline silicon films are deposited at 165 °C by plasma enhanced chemical vapor deposition (PECVD) from silane, highly diluted in hydrogen–argon mixtures. Ar addition during the deposition allows to increase the crystallinity from 24% to 58% for 20 nm thick films. The final crystallinity for 350 nm thick films reaches 72% with an increase in the grain size. A further increase, still 80%, is provided by substrate pre-treatment using hydrogen plasma before the deposition process. Arsenic doped μc-Si films, deposited on previous optimized (5 W power and 1.33 mbar pressure) undoped films without stopping the plasma between the deposition of both layers, show high electrical conductivity up to 20 S cm⁻¹.     

Silicon nanowire solar cells grown by PECVD

Silicon nanowires offer an opportunity to improve light trapping in low-cost silicon photovoltaic cells. We have grown radial junctions of hydrogenated amorphous silicon over p-doped crystalline silicon nanowires in a single pump-down plasma enhanced chemical vapor deposition process on glass substrates. By using Sn catalysts and boosting p-type doping in the nanowires, the open-circuit voltage of the devices increased from 200 to 800 mV. Light trapping was optimized by extending the length of nanowires in these devices from 1 to 3 μm, producing currents in excess of – 13 mA cm^? 2 and energy conversion efficiencies of 5.6%. The advantages of using thinner window layers to increase blue spectral response were also assessed.     

Synthesis and luminescent properties of novel RENbO4:Ln3+ (RE = Y, Gd, Lu; Ln = Eu, Tb) micro-crystalline phosphors

Using rare earth coordination polymers with aromatic carboxylic acids as the precursors of rare earth oxide components, with polyethylene glycol (PEG) as the dispersing media, micro-crystalline phosphors RENbO₄:Ln³⁺ (RE = Y, Gd, Lu; Ln = Eu, Tb) have been synthesized by an in situ co-precipitation method. Both X-ray diffraction and scanning electron microscopy have shown that the resultant samples present are crystalline with ‘rice glue ball’ micro-morphology and crystalline grain sizes in the range of 1–2 μm. The luminescent properties of these phosphors have been studied, which show that the best photoluminescent performance is achieved for GdNbO₄:Tb³⁺ or Eu³⁺. This was because Gd³⁺ plays an important role to enhance the luminescence of Tb³⁺ or Eu³⁺ in an energy transfer process. In addition, the influence of the doping concentration on the fluorescence behaviors has been examined. With increase of the doping concentration from 1 mol% to 5 mol%, both the red emission intensity of Eu³⁺ and the green emission intensity of Tb³⁺ increase.     

Heteroepitaxy of InSb films grown on a Si(0 0 1) substrate with AlSb buffer layer

The growth of InSb films on a Si(0 0 1) substrate with AlSb buffer layer was performed in an ultra high vacuum chamber (UHV) by a co-evaporation of elemental Indium (In) and antimony (Sb) sources. The samples were characterized by Auger electron spectroscopy (AES), X-ray diffraction (XRD) and atomic force microscopy (AFM). The surface morphology and the crystal quality of the grown films strongly depend on the flux ratio of Sb/In. It is found that the optimized flux ratio for the one-step growth procedure is about 2.9 to obtain the InSb films with smooth surface and good crystal quality, for the growth temperature of 300 °C. The two-step growth procedure was also used to further improve the crystal quality of the films.