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.     

Chemical lift-off of (11–22) semipolar GaN using periodic triangular cavities

Chemical lift-off of (11–22) semipolar GaN using triangular cavities was investigated. The (11–22) semipolar GaN was grown using epitaxial lateral overgrowth by metal-organic chemical vapor deposition on m-plane sapphire, in such a way as to keep N terminated surface of c-plane GaN exposed in the cavities. After regrowing 300 μm thick (11–22) semipolar GaN by hydride vapor phase epitaxy for a free-standing (11–22) semipolar GaN substrate, the triangular cavities of the templates were chemically etched in molten KOH. The (000-2) plane in the triangular cavities can be etched in the [0002] direction with the high lateral etching rate of 196 μm/min. The resulting free-standing (11–22) semipolar GaN substrate was confirmed to be strain-free by the Raman analysis.     

N‐GaN column arrays on the vertical InGaN/GaN blue LEDs formed by maskless dry etching

The InGaN/GaN blue light emitting diodes (LED) structures have been grown on sapphire substrates with a hexagonal array of hemispherical patterns by low‐pressure metalorganic chemical vapor deposition. Vertical LED structures on Cu carriers are fabricated using electroplating and KrF laser lift‐off techniques. After removal of the patterned sapphire substrate, an inductively coupled plasma etching is carried out to expose the n‐GaN layer for n‐metal contact. It is observed that GaN columns are formed at the center of the concave hemispheres after dry etching processes. Cathodoluminescence and wet chemical etching investigations reveal that a high density of dislocations is found to be generated at these specific positions. The possible mechanism for these observations is attributed to the defect distribution and defect‐dependent selective etching of the GaN.     

The K2S2O8–KOH photoetching system for GaN

A recently developed photoetching system for n-type GaN, a KOH solution containing the strong oxidizing agent potassium peroxydisulphate (K₂S₂O₈), was studied in detail. By careful selection of the etching parameters, such as the ratio of components and the hydrodynamics, two distinct modes were defined: defect-selective etching (denoted by KSO-D) and polishing (KSO-P). Both photoetching methods can be used under open-circuit (electroless) conditions. Well-defined dislocation-related etch whiskers are formed during KSO-D etching. All types of dislocations are revealed, and this was confirmed by cross-sectional TEM examination of the etched samples. Extended electrically active defects are also clearly revealed. The known relationship between etch rate and carrier concentration for photoetching of GaN in KOH solutions was confirmed for KSO-D etch using Raman measurements. It is shown that during KSO-P etching diffusion is the rate-limiting step, i.e. this etch is suitable for polishing of GaN. Some constraints of the KSO etching system for GaN are discussed and peculiar etch features, so far not understood, are described.     

Growth and etching of calcite crystals

Single crystals of calcite (CaCO₃) have been grown by the method employed by GRUZENSKY , using an aquoeus solution of CaCl₂ and a solid (NH₄)₂SO₃, The chemical reaction takes place according to the following equation: CaCl₂ + (NH₄)₂SO₃ CaCO₃ + 2 NH₄Cl The crystals grown by this method are about 0.2 to 0.8 mm in edge dimensions. Synthetic calcite crystals have been cleaved along (100) planes and the cleavage surfaces have been studied by multiple beam interferometry. The interferograms have revealed that the cleavages are quite flat. The cleavage faces have also been chemically etched and the etch patterns studied optically. By etching a cleavage successively for three different periods it was found that the bottoms of the point-bottomed pits follow a linear etch path. By etching a cleavage pair, one face in one etchant and the other face in a different etchant and by comparing the etch patterns produced, before and after polishing a cleavage face it has been shown that the etch pits nucleate at the sites of dislocations in the crystal. The etch patterns have also been compared with those produced on the cleavage faces of natural crystals. The density of dislocations in the syntheitc calctie crystals was generally less than the density of dislocations in the natural calcite crystals. The implications have been discussed.     

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.     

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.     

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.     

Method for modulating the wafer bow of free-standing GaN substrates via inductively coupled plasma etching

The bowing curvature of the free-standing GaN substrate significantly decreased almost linearly from 0.67 to 0.056 m⁻¹ (i.e. the bowing radius increased from 1.5 to 17.8 m) with increase in inductively coupled plasma (ICP) etching time at the N-polar face, and eventually changed the bowing direction from convex to concave. Furthermore, the influences of the bowing curvature on the measured full width at half maximum (FWHM) of high-resolution X-ray diffraction (HRXRD) in (0 0 2) reflection were also deduced, which reduced from 176.8 to 88.8 arcsec with increase in ICP etching time. Decrease in the nonhomogeneous distribution of threading dislocations and point defects as well as V_Ga–O_N complex defects on removing the GaN layer from N-polar face, which removed large amount of defects, was one of the reasons that improved the bowing of the free-standing GaN substrate. Another reason was the high aspect ratio of needle-like GaN that appeared at the N-polar face after ICP etching, which released the compressive strain of the free-standing GaN substrate. By doing so, crack-free and extremely flat free-standing GaN substrates with a bowing radius of 17.8 m could be obtained.     

Studies of helical dislocations,polygonization and pile-ups of dislocations in magnesium orthosilicate single crystals

(010) cleavages of magnesium orthosilicate crystals have been etched in concentrated hydrochloric acid and in its vapour at room temperature and also in the melts of sodium and potassium hydroxides at 400 °. Evidences of helical dislocations with their axes along [100] and [001] directions as revealed by etching is reported. Observations of polygonizations and pile-ups of dislocations are described and the implications are discussed.     

Investigation of characteristics of laterally overgrown GaN on striped sapphire substrates patterned by wet chemical etching

The new developed maskless lateral-epitaxial-overgrowth technique, in which the striped substrates are patterned by wet chemical etching, is systematically investigated using scanning electron microscopy, X-ray diffraction, and atomic force microscopy (AFM). Wing tilt is measured for the GaN films on patterned substrates with a range of “fill factor” (ratio of groove width to stripe period) and for the GaN in different growth time. It is found that changes in these parameters have a significant effect on the extent and distribution of wing tilt in the laterally overgrown regions relative to the GaN directly on the sapphire substrate. Increasing the thickness of GaN films is benefit to reduce wing tilt. The tilt is avoided in the GaN films with 4.5 μm thickness and fill factor for 0.46. The full-width at half-maximum of X-ray rocking curves of the asymmetric diffraction peaks and the image of AFM both show that the threading dislocations in the developed lateral epitaxial overgrowth of GaN films are reduced sharply. The GaN template could be used as an excellent substrate to fabricate high-performance optoelectronic devices.     

Revealing extended defects in HVPE-grown GaN

In this communication we will summarize the results of a complementary study of structural and chemical non-homogeneities that are present in thick HVPE-grown GaN layers. It will be shown that complex extended defects are formed during HVPE growth, and are clearly visible after photo-etching on both Ga-polar surface and on any non-polar cleavage or section planes. Large chemical (electrically active) defects, such as growth striations, overgrown or empty pits (pinholes) and clustered irregular inclusions, are accompanied by non-uniform distribution of crystallographic defects (dislocations). Possible reasons of formation of these complex structures are discussed. The nature of defects revealed by selective etching was subsequently confirmed using TEM, orthodox etching and compared with the CL method. The non-homogeneities were studied in GaN crystals grown in different laboratories showing markedly different morphological characteristics.     

Selective etching of dislocations in GaN grown by low-pressure solution growth

This work presents an experimental study on the identification and quantification of different types of dislocations in GaN grown by low-pressure solution growth. A reliable defect selective etching procedure in a NaOH-KOH melt is developed and validated using transmission electron microscopy that permits to define groups of etch pits that belong each to dislocations with a specific Burgers vector. This way a comparably fast method is provided for determining the total, the specific dislocation densities and the type of dislocation in a statistically representative way. The results for the solution grown samples are compared to those obtained for MOCVD GaN.     

Overgrowth of GaN on GaN nanowires produced by mask-less etching

We report on the generation of GaN nanowires (NWs) using mask-less reactive ion etching (RIE). The NWs are believed to be the result of a high etching rate in regions where a high dislocation density is present in the GaN films grown on sapphire substrates. We have studied the effect of defect densities in the original GaN films and its relation to the generation of these NWs. We show that defect reduction in the overgrown GaN is related to the presence of a network of embedded voids generated between these nanowires during the regrowth on the etched nanowires. We show that further reduction in dislocation density can be achieved by repeating the process of nanowire generation and overgrowth. Also we report on the residual strain and curvature in GaN after the first and second embedded voids approach (EVA).     

In situ mesa etching and immediate regrowth in a HVPE reactor for buried heterostructure device fabrication

Mesa etching in a hydride vapour-phase epitaxy (HVPE) reactor has been studied. Etched depth, underetching and shape of the mesas have been analysed as a function of partial pressures of active gases (HCl, PH₃ and InCl), stripe orientation and etching temperature. The experimental results show that the depth and undercut can be etched independently. We propose qualitative mechanisms for etching each of the emerging crystallographic planes ((0 0 1), (1 1 0) and {1 1 1}). In situ mesa etching with immediate regrowth was applied to the fabrication of buried heterostructure Fabry–Perot lasers. No surface contamination due to exposure to ambient and low process time are advantages of this technique.     

Photoacoustic Spectroscopy of Defect States in Etched and Air-annealed CuInSe2 Single Crystals

Photoacoustic spectra of cleaved, polished and etched, and air-annealed n-type CuInSe₂ single crystals are measured at different frequencies between 30 and 312 Hz. The spectra related to the bulk of the crystals exhibit five structures due to defects that are also present in p-type crystals. Polishing and etching as well as subsequent air annealing at 100, 200 and 300 °C reveal rather complex changes of the defect equilibrium in the near-surface region of the crystals which include both relative concentration changes of existing defects and creation of new defects. The results for polished and etched crystals correspond to trends expected from etching induced local modifications of the composition and structure as revealed by electron spectroscopies and ion channeling. Air annealing is found to affect all existing defects and to create up to five new defects which cannot be explained in terms of the related point defect model proposed by CAHEN and NOUFI.     

Studies on twin boundaries in calcite crystals

Natural single crystals of calcite have been cleaved along (100) planes and cleavage faces have been etched in 2% and 3% citric acid solutions. Etching produces twin boundaries oriented in 〈010〉 directions. The etch pits on the two sides of the twin boundary are oppositely oriented. It has been conjectured that the rows of pits might have been formed due to etching of dislocations on twin boundaries. One to one correspondence of twin boundaries has been established on matched cleavage faces. This is further confirmed by studying the induced twin regions produced on a (100) cleavage plane by indenting that plane itself. The implications are discussed.     

表面处理对蓝宝石衬底的影响

蓝宝石衬底是目前最为普遍的一种衬底材料,是生长GaN、ZnO材料最常用的衬底.本文用光学显微镜、原子力显微镜(AFM)和高分辨X射线双晶洐射对蓝宝石衬底进行了分析测试,系统研究了经过机械抛光、化学机械抛光、化学腐蚀等表面处理对蓝宝石衬底表面性能的影响.结果表明经过化学机械抛光随后再经腐蚀后的蓝宝石衬底的表面性能最好.     

Kinetics of dissolution and calcium sulphate growth on {111} faces of calcium fluoride crystals etched in sulphuric acid

The effect of temperature (20–100°C) and concentration of H₂SO₄ etching solutions on the etching behaviour of {111} faces of calcium fluoride crystals is investigated. Whisker growth of calcium sulphate is observed. Temperature of the etchant also has an effect on their formation. The kinetics of dissolution at the sites of dislocations and general dissolution have been studied. Studies on induction period and its dependence on the concentration of the acid and temperature are also described. It is observed that: (i) at low and high acid concentrations, the dissolution is diffusion controlled while it is predominantly reaction-rate controlled in the intermediate concentration range and (ii) the growth rate of calcium sulphate whiskers decreases with time and their induction period decreases with increase in the temperature of the solution. The implications are discussed.     

On the kinetics of nitrogen incorporation in GaP LPE layers using NH3 vapour doping

The partial substitution of N for P in GaP has a considerable effect on the luminescence quantum efficiency of GaP epitactic layers. The incorporation of N in GaP liquid phase epitactic layers using NH₃ vapour doping has been studied at different H₂ partial pressures in argon gas. It is shown that the incorporation of N in different ambients is conveniently described by just one parameter. The reaction of NH₃ with Ga is found to be an equilibrium process. The N concentration in the layer is directly proportional to the GaN concentration in the melt. From the correlation of the dissociation of NH₃ with the formation of GaN it is concluded that the NH₃ pressure, as measured in the exit gas, is not representative of the NH₃ pressure at the melt. The dissociation of NH₃ is found to be strongly retarded by H₂ owing to adsorption of H₂ on the surface of reactor materials which catalyse the dissociation.