Radical-beam gettering epitaxy of GaN layers on nitrogen-ion-implanted GaAs substrates

Thin GaN films have been grown on N⁺-ion-implanted single-crystal GaAs(111) substrates by radical-beam gettering epitaxy, and their structural perfection has been assessed by high-resolution x-ray diffraction. At growth temperatures from 770 to 970 K, the layers consist of hexagonal GaN and have mirror-smooth surfaces. Nitrogen-ion implantation into the substrate favors the formation of a sharp film/substrate interface owing to radiation-enhanced gallium diffusion. Analysis of the GaN/GaAs structures by Auger electron and x-ray photoelectron spectroscopies in combination with depth profiling indicates that the GaN layer is enriched in gallium. The N: Ga atomic ratio in the films is 0.98: 1, which is attributable to radiation-enhanced gallium diffusion.     

Collective effects in the system of structural defects in homoepitaxial GaN grown on porous substrates

Results of experiments on the homoepitaxial growth of gallium nitride (GaN) on porous GaN substrates with nanostructured volume are reported. A mechanism that can be used to exclude the dislocations of substrate from the sources of threading dislocations in homoepitaxial layers is described for the first time.     

Retroreflecting ellipsometer for measuring the birefringence of optical disk substrates

A retroreflecting ellipsometer has been constructed for measuring the birefringence of optical disk substrates. In contrast to conventional ellipsometers with two mechanical arms, this system has only one arm along which both the incident and reflected beams travel. This construction eliminates the mechanical limitations of conventional ellipsometers, thereby permitting normal incidence on the sample. In addition, the single arm is adjustable in two dimensions, with the polar incident angle, θ(inc), varying from 0° to 70°, and the azimuthal incident angle, Φ(inc), varying from 0° to 360°. The condition of normal incidence permits accurate measurement of in-plane birefringence. The adjustability of both θ(inc) and Φ(inc) is necessary for the measurement of possible tilts of the index ellipsoid, and also for the variation of birefringence through the substrate thickness. Measurement results showing the useful features of the equipment are presented. The optics of the hemispherical assembly used for retroreflection as well as for the elimination of undesirable refractions are also studied by use of the ZEMAX lens design program.     

The effect of illumination on the parameters of the polymer layer deposited from solution onto a semiconductor substrate

The effect of illumination on the thickness and roughness of monolayers of polycationic molecules of polyethyleneimine deposited from solution onto a silicon substrate was discovered and investigated. The super-bandgap illumination of the substrate during polyethyleneimine adsorption causes a decrease in both the roughness and integral thickness of the organic layer on n- and p-Si substrates.     

Solidification contact angles of molten droplets deposited on solid surfaces

Droplet impact and equilibrium contact angle have been extensively studied. However, solidification contact angle, which is the final contact angle formed by molten droplets impacting on cold surfaces, has never been a study focus. The formation of this type of contact angle was investigated by experimentally studying the deposition of micro-size droplets (∼39 μm in diameter) of molten wax ink on cold solid surfaces. Scanning Electron Microscope (SEM) was used to visualize dots formed by droplets impacted under various impact conditions, and parameters varied included droplet initial temperature, substrate temperature, flight distance of droplet, and type of substrate surface. It was found that the solidification contact angle was not single-valued for given droplet and substrate materials and substrate temperature, but was strongly dependent on the impact history of droplet. The angle decreased with increasing substrate and droplet temperatures. Smaller angles were formed on the surface with high wettability, and this wetting effect increased with increasing substrate temperature. Applying oil lubricant to solid surfaces could change solidification contact angle by affecting the local fluid dynamics near the contact line of spreading droplets. Assuming final shape as hemispheres did not give correct data of contact angles, since the final shape of deposited droplets significantly differs from a hemispherical shape.     

GaN nanostructures by hot dense and extremely non-equilibrium plasma and their characterizations

High temperature, high density, and extremely non-equilibrium argon plasma produced in modified dense plasma focus device is used to generate ions from pellet of gallium nitride (GaN) fixed on the top of anode and deposited on glass, quartz, and silicon substrates. Atomic force microscope (AFM) study on glass substrate shows nearly spherical nanostructures. AFM and scanning electron microscope studies on quartz substrate show nearly spherical nanostructures and on silicon substrate show comet-like nanostructures. Energy-dispersive X-ray scattering does not show any peak of impurities and there is deficiency of nitrogen in GaN. X-ray diffraction spectra show amorphous nature on glass substrate, whereas nanocrystalline GaN is observed on quartz and silicon substrates. Photoluminescence spectra show peaks of band edge emission, red luminescence, blue luminescence, and UV luminescence bands. Raman study on silicon substrate show peaks of E2 (high), A1, and E1 modes.     

Highly flexible frontside-illuminated dye-sensitized solar cells using three-dimensional network TiO2 nanowires

Here we report a highly flexible frontside-illuminated dye-sensitized solar cell (FIDSSC) using Ti foils as the substrates. The laser-drilled microhole arrays (LDMAs) on Ti foil substrates as the photoanode provided an efficient pathway for the diffusion of liquid electrolyte, which would be particularly favorable for frontside illumination of FIDSSC designs. The three-dimensional (3D) network TiO2 nanowires (NWs) were directly grown on the Ti substrate with LDMAs via a simple hydrothermal method. Platinized Ti sheet was used as the counter electrode in the FIDSSC. The mechanical properties of the novel structured device were measured. It was shown that as-synthesized large-scale 3D network TiO2 NWs with a diameter of about 20-30 nm and a length of about 6 microm can prevent crack from generating efficiently when bended to an extreme angle of 120 degree. Furthermore, we demonstrated that the effects of the different bending angles on the performance of the 3D network TiO2 NWs-based FIDSSCs were slight, indicating NWs preferable advantages for the fabrication of flexible DSSCs. The results showed that the FIDSSC achieved an efficiency of 0.72% under front illumination of AM 1.5 simulated one sun light (100 mWcm(-2)).     

Passivating gallium arsenide surface by gallium chalcogenide

We have studied the protective properties of thin films of gallium selenide formed by the method of heterovalent substitution on the surface of GaAs substrates. The data of transmission (Hitachi H-800) and scanning (JEOL JSM-638 OLV) electron microscopy showed that GaAs substrates treated with selenium vapor produced a more pronounced orienting action on the subsequent deposition of GaAs as compared to the substrates covered with a natural oxide. The processing of a GaAs substrate in selenium vapor followed by the removal of the resulting Ga₂Se₃ layer increases the degree of smoothness of the substrate surface on the atomic level.     

Copper indium gallium diselenide thin films for sun angle detectors in space applications

This work reports on processing, analysis and characterization of copper indium gallium diselenide (CIGS) used as a photosensitive layer for sensors such as sun angle detectors in space applications. CIGS-based solar cell devices with different CIGS layer thicknesses and the pn-junction located on the opposite side of the incidence of light were illuminated through their ultra-thin transparent molybdenum back contacts. The results from the current density versus voltage and quantum efficiency measurement indicate that the CIGS absorber layer may not exceed 750 nm at backside illumination, due to the limited CIGS diffusion length.     

Cubic GaN layers grown by metalorganic chemical vapor deposition on GaN templates obtained by nitridation of GaAs

Cubic gallium nitride epitaxial layers were grown by metalorganic chemical vapor deposition on GaN templates obtained by nitridation of GaAs substrates. The GaN structure can be peeled off the GaAs substrate and it can be handled separately. X-ray diffraction, Raman and photoluminescence measurements show that the epitaxial layers are cubic and monocrystalline.     

Effect of polarization on a solid immersion lens of arbitrary thickness

A solid immersion lens can be applied for high-resolution subsurface analysis of integrated circuits and other physical systems. We present a thorough analysis of the focal field distribution of a solid immersion lens system of arbitrary thickness. Cases of linearly and radially polarized illumination are examined and accurate expressions derived for the electric field in the image space. The effect of the spherical interface on both transverse and axial intensity profiles is analyzed. The performance and practicality of configurations deviating from the hemispherical and aplanatic cases are studied. The results show that optimal resolution is obtained at focal positions between the hemispherical and aplanatic points when radially polarized illumination is applied.     

A low temperature processable ternary gallium alloy for via filling application in microelectronic packaging

A low temperature processable ternary gallium alloy is formulated using an appropriate mixture of liquid gallium metal with nickel and copper powders. The processability of this alloy for room temperature via filling application is demonstrated using a stencil print process. A test vehicle is fabricated using a 0.2 mm thick and 300 mm×300 mm stainless steel (SS) panel with laser-drilled vias. Parylene N is deposited on the SS panel and around the inside via walls in order to electrically isolate via filling material from the body of the SS substrate and also making the SS surface non-conductive. Filled vias were examined for electrical isolation from neighboring vias and for electrical continuity in the thickness direction. Results show that the ternary gallium alloy is a good via filling material and can be applied to vias as small as 0.25 mm. It is believed that this novel alloy can also be used for thin film process on related MCM-L (multichip module-laminate) substrates.     

Substrate facet replication by epitaxial magnetic garnet films

Epitaxial ferrimagnetic garnet films were deposited on Czochralski grown single crystal gadolinium gallium garnet substrates containing faceted regions. Films grown by both chemical vapor deposition (CVD) and liquid phase epitaxy (LPE) were studied. Lattice parameter distributions were determined by the method of X-ray double crystal topography with rocking curve analysis. The demagnetized domain strip width, magnetization, and characteristic length were measured in a CVD film in regions inside and outside the substrate facets. It was determined that replication of the substrate facets by the epitaxial film is accomplished by a difference in film stress. This stress difference arises from the lattice parameter difference between the faceted and unfaceted regions of the substrate. These results lead to the establishment of a criterion for allowable lattice parameter variation in substrates to be used for magnetic bubble domain films with stress induced anisotropy.     

Computational simulation of metal ion propagation from plasma to substrates with uneven surfaces

The influence of the unevenness of substrates immersed into plasma on plasma–surface interactions was studied by the combination of multidimensional fluid modelling and particle simulation connected together in iterative hybrid code. The fluid part of our model consisted of continuity equations for all charged species and Poisson's equation. The non self-consistent particle simulation technique was used both for the calculation of local electron energy distribution function and for the derivation of quantities characterising plasma–surface interaction. This approach enabled us to study in detail the structure of the plasma near metal substrates with complex geometries, i.e. planar substrates covered by a variety of hemispherical protrusions. The main attention was devoted to the influence of substrate unevenness in both macroscopic and microscopic spatial scales on both metal ions and ions of background gas propagation to the substrate.     

New perspective on nano-porous gallium nitride formation on p-type silicon with plasma focus device

For the first time, the formation of nano-porous (NP) gallium nitride (GaN) on p-type silicon (Si) (100) substrate with the plasma focus device was observed. Four and six main plasma focus shots with GaN as the target on the Si substrate were applied. Subsequently, two shots of nitrogen plasma without the GaN target were applied to both of the Si surfaces. Then the substrates were gone through a 1050°C nitrogen treatment by a thermal chemical vapor deposition (TCVD) device. Field emission scanning electron microscopy and atomic force microscopy indicated NP structures on the surface of the substrates. With the increase in the amount of shots, thicker and rougher NP structures were formed. Additionally, X-ray diffraction shows GaN polycrystalline formation on the surface with the energy-dispersive X-ray spectroscopy, indicating that there was nitrogen deficiency on the Si substrates.     

Doping gradients in layers of gallium phosphide grown by liquid epitaxy

Layers of epitaxial gallium phosphide doped with either tellurium, sulphur or zinc over the range 10¹⁶ to 10¹⁸ cm⁻³ have been grown from gallium solution using a vertical dipping system. These layers of thickness 60 to 80 μm have been produced on the (100) and (111)B faces of gallium phosphide single crystal substrates at high growth rates. Doping gradients have been investigated by a Schottky barrier technique over an angle lapped region of the grown slice, measuring the capacitance voltage characteristics of the individual barriers. Significant changes in doping level have been observed throughout the thickness of the layers and these are related to the variation of distribution coefficient, the losses of impurity from the system and the presence of competing background impurity systems.     

Epitaxy of semipolar GaN on a Si(001) substrate with a SiC buffer layer

A new method of synthesis of semipolar gallium nitride on a silicon substrate using the technology of solid-phase epitaxy of 3C-SiC nanocrystals has been suggested. It has been demonstrated that application of buffer layers of 3C-SiC and AlN enables one to form epitaxial layers of semipolar gallium nitride with layer deviation from the polar position of the c axis of a wurtzite crystal by an angle of 48°–51° at the minimal half-width of the X-ray diffraction rocking curve (ω_θ) ～ 24′. The observed bend of a cylindrical character in the structure of GaN/AlN/3C-SiC(001) is explained by the anisotropic deformation of semipolar GaN on silicon.     

Design of low-haze holographic notch filters

We have studied the optical quality of holographic notch filters and have identified that stray illumination during the exposure stage and substrate scatter are the limiting factors. We propose and analyze a novel holographic recording scheme in which the substrate is rotated during exposure. Using this method, we have produced high-uniformity, flare-free dichromated gelatin notch filters with optical density greater than 5 and with scatter comparable with that from polished substrates.     

蒸发源位于半球面正下方膜厚分布理论研究

对蒸发源位于非平面基底一半球面正下方时膜厚均匀性进行了理论研究。通过建立无量纲模型计算了此种几何配置下,半球表面在两种常见理想蒸发源下各位置的膜厚公式以及膜厚分布方程。选择基底与蒸发源问较大的距离,可获得更大的可镀膜区域,同时该距离对基底上镀制的膜层厚度分布影响也较大。最后对实用蒸发源的发射系数,对该几何配置下半球面膜厚分布影响进行了理论研究。     

Liquid phase epitaxial growth of pure and doped GaSb layers: morphological evolution and native defects

Undoped and Te-doped gallium antimonide (GaSb) layers have been grown on GaSb bulk substrates by the liquid phase epitaxial technique from Ga-rich and Sb-rich melts. The nucleation morphology of the grown layers has been studied as a function of growth temperature and substrate orientation. MOS structures have been fabricated on the epilayers to evaluate the native defect content in the grown layers from theC-V characteristics. Layers grown from antimony rich melts always exhibitp-type conductivity. In contrast, a type conversion fromp- ton- was observed in layers grown from gallium rich melts below 400 C. The electron mobility of undopedn-type layers grown from Ga-rich melts and tellurium doped layers grown from Sb- and Ga-rich solutions has been evaluated. Paper presented at the poster session of MRSI AGM VI. Kharagpur. 1995