J.L. Weyher

Morphological and structural characteristics of homoepitaxial GaN grown by metalorganic chemical vapour deposition (MOCVD)

MOCVD-grown GaN on the N-polar surface of GaN substrates has been found to exhibit gross hexagonal pyramidal features (typically 10}50 lm in size depending on layer thickness). The evolution of the pyramidal defects is dominated by the growth rate of an emergent core of inversion domain (typically 100 nm in size). The inversion domains nucleate at a thin band of oxygen containing amorphous material (2}5 nm in thickness), being remnant contamination from the mechano-chemical polishing technique used to prepare the substrates prior to growth. Apart from pyramidal hillocks, the #at-topped hillocks are also formed. The arguments are presented on the association between these features and the core dislocations, which constitute the source of the growth steps. Improvement in the substrate polishing procedures allowed the e!ective elimination of these surface hillocks. ( 1999 Elsevier Science B.V. All rights reserved.

Carrier recombination at single dislocations in GaN measured by cathodoluminescence in a transmission electron microscope

We study radiative and nonradiative recombination at individual dislocations in GaN by cathodoluminescence performed in a transmission electron microscope. The dislocations are produced by indentation of dislocation free single crystals and have a-type Burgers vectors (b=1/3〈1120〉). They are aligned along 〈1120〉 directions in the basal plane. Our direct correlation between structural and optical properties on a microscopic scale yields two main results: (i) 60°-basal plane dislocations show radiative recombination at 2.9 eV; (ii) screw-type basal plane dislocations act as nonradiative recombination centers. We explain the nonradiative recombination by splitting this dislocation into 30° partials that have dangling bonds in the core. The dissociation width of these dislocations is <2 nm.

Detection of Hepatitis B virus antigen from human blood: SERS immunoassay in a microfluidic system

A highly sensitive immunoassay utilizing surface-enhanced Raman scattering (SERS) has been developed with a new Raman reporter and a unique SERS-active substrate incorporated into a microfluidic device. An appropriately designed Raman reporter, basic fuchsin (FC), gives strong SERS enhancement and has the ability to bind both the antibody and gold nanostructures. The fuchsin-labeled immuno-Au nanoflowers can form a sandwich structure with the antigen and the antibody immobilized on the SERS-active substrate based on Au-Ag coated GaN. Our experimental results indicate that this SERS-active substrate with its strong surface-enhancement factor, high stability and reproducibility plays a crucial role in improving the efficiency of SERS immunoassay. This SERS assay was applied to the detection of Hepatitis B virus antigen (HBsAg) in human blood plasma. A calibration curve was obtained by plotting the intensity of SERS signal of FC band at 1178cm(-1) versus the concentration of antigen. The low detection limit for Hepatitis B virus antigen was estimated to be 0.01IU/mL. The average relative standard deviation (RSD) of this method is less than 10%. This SERS immunoassay gives exact results over a broad linear range, reflecting clinically relevant HBsAg concentrations. It also exhibits high biological specificity for the detection of Hepatitis B virus antigen.

Nonradiative recombination at threading dislocations in n-type GaN: Studied by cathodoluminescence and defect selective etching

Dislocations in GaN single crystal were studied by means of spectral cathodoluminescence (CL) mapping and defect selective etching. We show that the c-type screw dislocations are not recombination active. The recombination strength of the a- and (a+c)-type dislocations is influenced by impurity gettering. While fresh dislocations exhibit a CL contrast of 0.01–0.05 in accordance with intrinsic dislocation states, grown in dislocations show a contrast of 0.25. From the analysis of spectral CL maps, we find that impurities such as oxygen and silicon are depleted in the surrounding of the dislocations. We explain the increased contrast by a reduced screening of the electrical field of the dislocation.

Selective photoetching and transmission electron microscopy studies of defects in heteroepitaxial GaN

Photoelectrochemical (PEC) etching has been used to study defects in heteroepitaxial GaN layers. In Ga-polar layers PEC etching reveals only dislocations in the form of filamentary etch features (whiskers). Transmission electron microscopy (TEM) confirmed a one-to-one correspondence between the whiskers and straight threading dislocations, which are mainly of edge and mixed type. In N-polar layers, apart from dislocations, inversion domains (IDs) also give rise to the formation of more complex etch features that also have been confirmed by TEM. IDs of nanometer diameter result in formation of whiskers similar to the dislocation-related ones. However, when the diameter of IDs exceeds a critical size (about 100 nm), crater-like deep etch features are formed during PEC etching. Based on the mechanism of PEC etching of GaN in aqueous KOH solutions, it is argued that inversion domain boundaries are electrically active defects.

Highly reproducible, stable and multiply regenerated surface-enhanced Raman scattering substrate for biomedical applications

We fabricated a Surface Enhanced Raman Scattering (SERS)-active surface based on photo-etched and Au-coated GaN. The highest enhancement factor (EF) in SERS and high reproducibility of spectra were obtained from surfaces covered with bunched nanopillars which were produced by relatively long defect-selective photo-etching. The surfaces exhibited SERS enhancements of the order of 2.8 × 106 for malachite green isothiocyanate (MGITC) and 2 × 106 for p-mercaptobenzoic acid (PMBA). These SERS enhancement factors were comparable to those of conventional SERS substrates, while the EF for MGITC was two orders of magnitude larger than the corresponding one reported for the SERS platform made on porous GaN. The standard deviation of the relative intensity of the 1180 cm−1 mode of MGITC was less than 5% for 100 randomly distributed locations across a single platform and less than 10% between different platforms. The SERS signal of MGITC at our GaN/Au surface (kept under ambient conditions) was extremely stable. We could not detect any peak shift or appreciable change of intensity even after three months. We used these surfaces to detect biological molecules such as amino acids and bovine serum albumin (BSA) at low concentration and with short detection time. We developed simple and effective cleaning procedures for our substrates. After cleaning, the same substrate could be used multiple times retaining the SERS activity. We are not aware of any other multiply regenerated SERS substrate which provides simultaneously such high stability with high enhancement, good uniformity, and high reproducibility.

Study of individual grown-in and indentation-induced dislocations in GaN by defect-selective etching and transmission electron microscopy

Abstract Vickers diamond indentation at 370°C has been employed to introduce dislocations into the plate-like (000-1) N-polar GaN single crystals. It has been established that using standard Vickers diamond indenter, well-defined ‘rosettes’ of defects are formed under 1.5–2 N load applied for 10 min. The resolved patterns of dislocation-related etch pits are formed using molten KOH–NaOH eutectic (E) at 200°C for 1.5–2 min. Individual grown-in dislocations are revealed by this E etch in the GaN matrix. Transmission Electron Microscopy confirmed the correlation of etch pits to individual dislocations emerging at the surface. Nano-crystalline material was found in the highly deformed central region of the indentation rosette. The structure of these nano-crystals was analyzed using electron diffraction. Speculative explanation on a phase transition induced by high local pressure is briefly discussed.

Preparation of Free-Standing GaN Substrates from Thick GaN Layers Crystallized by Hydride Vapor Phase Epitaxy on Ammonothermally Grown GaN Seeds

Crystallization of GaN by hydride vapor phase epitaxy (HVPE) on ammonothermally grown GaN seed crystals is described. The initial growth conditions for HVPE are determined and applied for further bulk growth. Smooth GaN layers up to 1.1 mm thick and of excellent crystalline quality, without cracks, and with low dislocation density are obtained. Preparation of the free-standing HVPE-GaN crystal by slicing and structural and optical quality of the resulting wafer are presented.

Reduction of dislocation density in epitaxial GaN layers by overgrowth of defect-related etch pits

GaN templates grown by the metal organic chemical vapor deposition method were etched in a defect-selective molten salts eutectic and were subsequently overgrown by a GaN layer using the hydride vapor phase epitaxy (HVPE) method. Optimized conditions of etching and of HVPE growth processes resulted in a significant reduction of the dislocations density (DD). Local areas virtually free of dislocations were obtained on ∼50% of the surface, while the average DD was reduced from 3×109 cm−2 in the template to about 2×107 cm−2 in the HVPE-grown GaN layer. A model has been developed to explain the mechanism of reduction of the DD during the overgrowth process. The model was confirmed by the photoetching of cleaved layers.

Photoetching Mechanisms of GaN in Alkaline S2O8 2 − Solution

2� solution was used to explain the mechanism of photoetching of the semiconductor under open-circuit conditions. The observed enhancement of the photoetch rate as a result of platinum either directly on or in electrical contact with the semiconductor is shown to be mainly a photogalvanic effect. The factors determining the etching kinetics and surface morphology are elucidated.