SiC籽晶表面状态对晶体质量的影响

为了研究表面状态对晶体质量的影响,本文分别采用腐蚀后抛光与未抛光的晶片作为籽晶,利用金相显微镜对所得晶体进行了显微观察,结果表明:经过腐蚀后抛光的籽晶生长出的晶体质量高于未抛光籽晶所得晶体;生长前端的位错尺寸以及数量小于未抛光籽晶,说明抛光去除了部分表面浅的缺陷腐蚀坑,同时减小深腐蚀坑的尺寸,使得籽晶生长表面的缺陷密度和尺寸大大降低,有助于减少后期生长的晶体中的缺陷密度,提高结晶质量.     

The morphology of Zn3P2 single crystals grown from the vapour phase

The morphology of Zn₃P₂ single crystals, grown from the vapour phase by sublimation of bulk Zn₃P₂ in a sealed quartz tube pulled through a high temperature gradient, has been investigated. X-ray diffraction examination showed that the main direction of crystal growth with a tetragonal unit cell is the [021] direction which corresponds to the [111] direction in the pseudocubic system. Transverse and longitudinal cross sections of the single crystals were mechanically polished and chemically etched. Optical examination of the cross sections indicated that the growth of Zn₃P₂ begins most frequently in the (011) plane. Striae which appeared in a cross section after chemical etching are attributed to the existence of strain fields generated in the sample as a result of temperature fluctuations or as a result of crystal structure defects in the core region.     

Wet KOH etching of freestanding AlN single crystals

We investigated defect-selective wet chemical etching of freestanding aluminum nitride (AlN) single crystals and polished cuts in a molten NaOH–KOH eutectic at temperatures ranging from 240 to 400 °C. Due to the strong anisotropy of the AlN wurtzite structure, different AlN faces get etched at very different etching rates. On as-grown rhombohedral and prismatic facets, defect-related etching features could not be traced, as etching these facets was found to mainly emphasize features present already on the un-etched surface. On nitrogen polar basal planes, hexagonal pyramids/hillocks exceeding 100 μm in diameter may form within seconds of etching at 240 °C. They sometimes are arranged in lines and clusters, thus we attribute them to defects on the surface, presumably originating in the bulk material. On aluminum polar basal planes, the etch pit density which saturates after approx. 2–3 min of total etching time at 350 °C equals the density of a certain type of dislocations (presumably screw dislocations) threading the surface. Smaller etch pits form around annealed indentations, in the vicinity of some bigger etch pits after repeated etching, and sometimes also isolated on the surface area. Although alternate explanations exist, we attribute these etch pits to threading mixed and edge dislocations. This paper features etching parameters optimized for different planes and models on the formation of etching features especially on the polar faces. Finally, the issue of reliability and reproducibility of defect detection and evaluation by wet chemical etching is addressed.     

Micromorphology of pure and PbI2‐doped CdI2 dendritic single crystals

Undoped and PbI₂‐doped dendritic single crystals were grown by vapour growth technique. The basal surfaces of the as grown crystals were examined by optical and electron microscopy to observe wide variety of growth and defect features. Apart from typical features of dendritic growth, features of overgrowth, slip bands, growth steps and their bunching, etc. were observed. The basal surfaces of the crystals were then etched by controlled condensation of water vapour, after optimizing the etching condition, and the microscopic studies were repeated. Etch pits of hexagonal and triangular shape, both symmetric and asymmetric, and of different density, were observed in the case of undoped and doped crystals, respectively. In some cases, crystallographic hillocks were also observed. The crystals were also examined by X‐ray diffraction for their polytypism and related behaviour. The results are analyzed to elicit information on the correlation of structure, defects and surface features of the crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reduction of domain wall width in VTE treated near stoichiometric LiNbO3 crystals

LiNbO₃ is a ferroelectric crystal and grows with multi domains. Different domains are separated by boundaries which are known as domain boundaries. Domain walls for congruent and VTE (Vapor Transport Equilibration) treated near stoichiometric lithium niobate samples were visualised in different crystallographic directions using chemical etching technique. The sample is etched in the mixture of HF and HNO₃ (in 1:2 volume ratios) for 10 minutes at boiling temperature. Measured domain wall width was found approximately 15‐20 µm for congruent (CLN) and it reduces to 1‐3 µm for VTE treated near stoichiometric (SLN). Activation energies were also measured by two‐probe method and found to be increasing in stoichiometric sample. This activation energy is related to defect density in the crystals. Activation energy is higher for less defective crystals. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Beiträge zum Ätzen von Galliumarsenid (I) Strukturtheoretische Diskussion der erzeugten Ätzmorphologien

{111}-, {110}- and {100}-plates of GaAs have been etched in various etching solutions followed by comparing the obtained etching structures. The fundamentals of the discussion is the existing correlation between the morphology of etch pits and crystal structure. It is shown that a qualitative interpretation of the manifold etching phenomena on the above faces in a structure characteristic for GaAs can be given by including 1. the presence of primary defects as a constitutive property of real crystals and 2. the consideration of recordable influences of the surroundings.     

Probing the local structural changes across the amorphous to crystalline transition in iron phosphate glass: Positron annihilation and micro Raman studies

Iron phosphate glasses have been investigated using positron lifetime and micro Raman spectroscopy to study the crystallization kinetics and defect evolution across the glass and crystallization transitions. Positron lifetime parameters show distinct changes at temperatures corresponding to both the transitions. These changes have been understood as the signature of the evolution of open volume defects and their clustering across the amorphous to crystalline interfaces. Micro Raman measurements on annealed samples have identified the crystalline phases as Fe₃(P₂O₇)₂ and FePO₄. Ab-initio positron lifetime computations reveal that the positron probes the Fe/P sites in defect free crystals. The nature of open volume defects produced upon annealing is identified by computing positron lifetimes for various possible defect clusters. A combination of both experimental and theoretical techniques has provided insights in understanding the evolution of defects across the phase transition in these glasses.     

Modification of the CuInSe2 Crystal Surface during Polishing and Annealing

The RBS/channelling technique was used to study the near-surface damage characteristics of CuInSe₂ crystals after polishing with 0.05 μm grade alumina and subsequent annealing up to temperatures of 600 °C. A comparative RUMP and damage density depth profile analysis of the channelling spectra revealed a polish-induced near-surface disordered layer with a thickness close to 40 nm. Up to annealing temperatures of about 400 °C a gradual overall decrease of the defect density in the damaged layer is observed without detectable changes in its thickness. An indium-rich surface layer is formed after vacuum annealing at 600 °C.     

Luminescence and Photoconductivity Caused by Antisite Defects in CdIn2S4 Single Crystals

Results of a study of the influence of nonstoichiometry as well as of annealing and argon ion implantation on photoluminescence and photoconductivity spectra of cadmium thioindate single crystals are presented. The energy positions of levels of antisite In_cd and Cd_In defects are found. The role of antisite defects in the process of conductivity compensation is analysed.     

Defect-related emission characteristics of nonpolar m-plane GaN revealed by selective etching

A comprehensive study of the morphology and luminescence characteristics of nonpolar m-plane GaN etched in hot acids was presented. It was found that many four-sided pyramidal pits were distributed on the etched GaN surface with the long side perpendicular to the [1 1 2? 0] direction, corresponding to the threading dislocations. When compared to the as-grown GaN, DAP emission intensity and its LO-phonon coupling phenomenon in the etched GaN were greatly attenuated, whereas the intensity of BSF-related band almost kept constant due to its immunity to chemical etching. Especially, a new PSF-related emission at 3.32 eV emerged in CL spectra of etched GaN. Simultaneously, partial relaxation of compressive stress happened for the etched GaN epilayer according to the red shift of NBE emission in photoluminescence (PL) and E₂(high) phonon peak in the Raman spectra. Contrary, the DAP peak in etched GaN was blueshifted, likely due to the reduced impurity level fluctuation by etching. In addition, the different behaviors were discussed for NBE and defect-related transitions in the etched GaN, characterized by excitation power- and temperature-dependent PL.     

Dislocation studies by chemical etching on solution-grown NaSbF4 and Na3Sb4F15 single crystals

An attempt has been made to study the etch pit dislocations of isolated as well as matched pairs of cleavage planes of solution grown NaSbF₄ and Na₃Sb₄F₁₅ single crystals when etched with analar grade methyl alcohol and ethyl alcohol. On etching of cleavage faces the pits nucleate at the intersection sites of dislocations with the cleavage face. On successive etching of a cleavage face, arrays of triangular shape etch pits are observed. After prolonged etching some of the etch pits disappear. These pits are curved and terminated indicating that they are crystallographically non-oriented. One-to-one correspondence with respect to number, shape, position and structure of the pits has been established by etching of a matched cleavage face. The etch pit densities of both NaSbF₄ and Na₃Sb₄F₁₅ crystals are found to be 10³ per cm².     

The effect of HF/NH4F etching on the morphology of surface fractures on fused silica

The effects of HF/NH₄F, wet chemical etching on the morphology of individual surface fractures (indentations, scratches) and of an ensemble of surface fractures (ground surfaces) on fused silica glass has been characterized. For the individual surface fractures, a series of static or dynamic (sliding) Vickers and Brinnell indenters were used to create radial, lateral, Hertzian cone and trailing indentation fractures on a set of polished fused silica substrates which were subsequently etched. After short etch times, the visibility of both surface and subsurface cracks is significantly enhanced when observed by optical microscopy. This is attributed to the increased width of the cracks following etching, allowing for greater optical scatter at the fracture interface. The removal of material during etching was found to be isotropic except in areas where the etchant has difficulty penetrating or in areas that exhibit significant plastic deformation/densification. Isolated fractures continue to etch, but will never be completely removed since the bottom and top of the crack both etch at the same rate. The etching behavior of ensembles of closely spaced cracks, such as those produced during grinding, has also been characterized. This was done using a second set of fused silica samples that were ground using either fixed or loose abrasives. The resulting samples were etched and both the etch rate and the morphology of the surfaces were monitored as a function of time. Etching results in the formation of a series of open cracks or cusps, each corresponding to the individual fractures originally on the surface of the substrate. During extended etching, the individual cusps coalesce with one another, providing a means of reducing the depth of subsurface damage and the peak-to-valley roughness. In addition, the material removal rate of the ground surfaces was found to scale with the surface area of the cracks as a function of etch time. The initial removal rate for the ground surface was typically 3.5× the bulk etch rate. The evolving morphology of ground surfaces during etching was simulated using an isotropic finite difference model. This model illustrates the importance that the initial distributions of fracture sizes and spatial locations have on the evolution of roughness and the rate at which material is removed during the etching process. The etching of ground surfaces can be used during optical fabrication to convert subsurface damage into surface roughness thereby reducing the time required to produce polished surfaces that are free of subsurface damage.     

Influence of proton implantation on the properties of CuInSe2 single crystals (I). Ion channeling study of lattice damage

The lattice damage of p-type CuInSe₂ single crystals implanted with 10 keV protons in the fluence range from 2.5 · 10¹⁴ to 8 · 10¹⁵ cm⁻² was investigated using the Rutherford backscattering/channeling technique. At proton fluences up to about 2 · 10¹⁵ cm⁻² radiation annealing of the defects is observed which is ascribed to very high defect concentrations in the unimplanted samples. At higher fluences radiation damage occurs but the concentration of radiation induced defects ramains low. There are indications that selenium interstitials or defect complexes with selenium interstitials involved are stable defects at room temperature.     

Characterization of the carrot defect in 4H-SiC epitaxial layers

Characterization of the epitaxial defect known as the carrot defect was performed in thick 4H-SiC epilayers. A large number of carrot defects have been studied using different experimental techniques such as Nomarski optical microscopy, KOH etching, cathodoluminescence and synchrotron white beam X-ray topography. This has revealed that carrot defects appear in many different shapes and structures in the epilayers. Our results support the previous assignment of the carrot defect as related to a prismatic stacking fault. However, we have observed carrot defects with and without a visible threading dislocation related etch pit in the head region, after KOH etching. Polishing of epilayers in a few μm steps in combination with etching in molten KOH and imaging using Nomarski optical microscope has been used to find the geometry and origin of the carrot defects in different epilayers. The defects were found to originate both at the epi-substrate interface and during the epitaxial growth. Different sources of the carrot defect have been observed at the epi-substrate interface, which result in different structures and surfaces appearance of the defect in the epilayer. Furthermore, termination of the carrot defect inside the epilayer and the influence of substrate surface damage and growth conditions on the density of carrot defects are studied.     

Colour centre studies in MoO3 films

The growth of defects in vacuum-evaporated MoO₃ films has been studied by optical absorption in the present work. The as-grown films were subjected to different kinds of treatment such as thermal annealing, UV and X-ray irradiation, and their optical absorption spectra were recorded between 300 and 1500 nm at room temperature as well as at liquid N₂ temperature The films were found to be amorphous from X-ray and electron-microscopic studies. From these studies it is observed that in addition to point defects, defect aggregates similar to the colloidal centres in silver and alkali halides, are present in these films.     

Spectroscopic investigation of porous GaAs prepared by laser-induced etching

We have investigated the etching mechanism in Cr–O doped GaAs wafer under super- and sub-bandgap photon illumination. A comparison of the etching rate and properties of nanostructures from two samples which are etched with argon-ion laser (2.41 eV) and Nd:YAG laser (1.16 eV), are presented here. The etching mechanism is found different for these different illuminations, which play the key role in the formation of defects. It is observed that the etching process starts vigorously under sub-bandgap photon illumination through the mediation of intermediate defect states. SEM micrograph shows the formation of distinct GaAs nanostructures in sample etched by Nd:YAG laser. Porous structure produced by Nd:YAG laser shows strong room temperature luminescence in the red region. The size and size distribution of the nanocrystals are investigated by non-destructive Raman and photoluminescence spectroscopies. The data are analyzed within the framework of quantum confinement models.     

Zur Erholung des elektrischen Widerstandes einer Ni3Fe-Legierung nach plastischer Deformation

Starting from simple kinetical considerations about ordering during the annealing of deformation induced lattice defects an interpretation of isochronal resistivity annealing of different deformed Ni₃Fe alloys is given. The proposed method for separation of contributions from lattice defects and order to the resistivity changes allows the calculation of specific defect resistivities.     

Hydrogen etching on the surface of GaN for producing patterned structures

The morphology of GaN etched in hydrogen atmosphere is investigated. It is found that GaN surfaces have different profiles after being etched at different pressures. The profile resembles a surface that has been decorated with columns or mooring posts at high pressure and with deep cavities at low pressure. Etch pit density (EPD) experiment shows all dislocations have been etched to form cavities at low pressure, but not all the cavities result from etched dislocations. A model has been developed to explain the mechanism of H₂ etching. Patterned structure with a flat surface and porous inside has been produced by two-step etching which is designed according to the model.     

Investigation of radiation defects in electron irradiated Hg1−xCdxTe crystals using positron annihilation

The electron characteristics of defects in the initial and electron irradiated Hg_1−xCd_xTe (2–3 MeV, 10¹⁸ cm⁻², 300 K) crystals using the positron annihilation method have been investigated. The data of electric measurements are confirmed on connection of p-type conductivity with vacancy defects of metal sublattice initial crystals Hg_1−xCd_xTe. An analysis of correlation curves of irradiated crystals has shown a possibility of formation of associations of initial defects and radiation damages of vacancy type during radiation process. The presence of narrow component on correlation curves in the region of small angles is associated with formation of positronium states localized in the region of radiation defect complex of vacancy type. Identification of positron-sensitive defects with electrically active radiation induced ones has been carried out according to the results of isochronal annealing of irradiated crystals.     

Climb mechanisms and critical temperature in off-stoichiometric V3Si

A dislocation climb model is proposed for both vanadium and silicon rich V₃Si single crystals. Just the type of constitutional point defects is needed solely which dominates in the sample under consideration. The climb mechanism involves a conversion of one defect type into another and so may be responsible for the changes in critical temperature of the superconductor as observed after plastic deformation at elevated temperatures.