Orthodox etching of HVPE-grown GaN in molten eutectic of KOH + NaOH (E etch) and in hot sulfuric and phosphoric acids (HH etch) is discussed in detail. Three size grades of pits are formed by the preferential E etching at the outcrops of threading dislocations on the Ga-polar surface of GaN. Using transmission electron microscopy (TEM) as the calibration tool it is shown that the largest pits are formed on screw, intermediate on mixed and the smallest on edge dislocations. This sequence of size does not follow the sequence of the Burgers values (and thus the magnitude of the elastic energy) of corresponding dislocations. This discrepancy is explained taking into account the effect of decoration of dislocations, the degree of which is expected to be different depending on the lattice deformation around the dislocations, i.e. on the edge component of the Burgers vector. It is argued that the large scatter of optimal etching temperatures required for revealing all three types of dislocations in HVPE-grown samples from different sources also depends upon the energetic status of dislocations. The role of kinetics for reliability of etching in both etches is discussed and the way of optimization of the etching parameters is shown.
In this communication it is shown that both "orthodox" etching in molten KOH -NaOH eutectic (E etch and its modification) and photo-etching in aqueous KOH solution (PEC method) permit quick assessment of density, distribution and, after appropriate calibration, type of defects in GaN single crystals and epitaxial layers. Characteristic features of both etching methods are briefly discussed. It is shown that dislocations and micro-defects can be revealed in the form of etch pits (in "orthodox" E etch) and as etch hillocks (PEC method) on both Ga-and N-polar surfaces. The reliability of both methods has been confirmed by direct TEM calibration and by using indentation method. Very low dislocation density (EPD ≤ 2 × 10 2 cm −2 ) in the undoped GaN single crystals and MOCVD-grown epitaxial layers has been confirmed by this study.Introduction High-pressure grown GaN single crystals constitute an attractive substrate material for manufacturing lattice matched (i.e. low defect-density) high performance III-V opto-electronic device structures [1][2][3]. At the early stage of the development of sizable GaN single crystals and homo-epitaxial layers, the structural quality of the material was determined using X-ray diffraction [4-6] and transmission electron microscopy (TEM) [7][8][9]. These techniques demonstrated superior quality of the GaN single crystals and the layers grown on them over hetero-epitaxial layers, but failed in precise determining of the density of dislocations due to the intrinsic limits of the methods.More recently, the structural quality of GaN crystals and epitaxial layers has been examined by defectselective etching [10][11][12]. The delay in using simple and quick etching vs. X-ray and TEM, has been caused by the very high chemical resistance of nitrides. From the recent studies it follows however, that two approaches are the most promising in revealing and analysing defects in different types of GaN, namely photo-electrochemical (PEC or electroless) [12][13][14] and molten bases [11,15] etching. In the present communication these two techniques are briefly characterized and some representative examples are shown on the use of these methods for studying defects in GaN single crystals and epitaxial layers.
Potentiostatic experiments in KOH solution were used to investigate the photoetching of the Ga-polar face of heteroepitaxial GaN layers grown on sapphire. Different etching regimes are identified; these depend on the applied potential, KOH concentration, light intensity and electron concentration. In particular, the importance of the relative rates of transport of photogenerated holes and OH À ions to the surface for the etching kinetics and morphology is demonstrated. Consequently, the hydrodynamics of the etching system are important. These results form the basis for a comparison with a more widely used approach: photoenhanced open-circuit etching with a counter electrode. r
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.
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