GaN is a wide band gap semiconductor with many optoelectronic applications. High density of threading dislocations is present in heteroepitaxially grown layers, reducing the device performance. The influence of dislocation on layer properties and especially on device electrical properties depends on the type of dislocation. It necessitates a method to determine densities of any specified dislocation type. Transmission electron microscopy (TEM) is an accurate but time-consuming technique. Chemical selective etching delineates dislocations by revealing etch pits, however a special approach have to be used to determine the densities for various dislocation types.

In this work GaN epitaxial layers grown by metal-organic vapour phase epitaxy on sapphire substrates, with total threading dislocation density of 2.4x10⁹ cm^-2 ± 0.2x10⁹ cm^-2 (as determined by TEM), were subjected to wet chemical etching in hot phosphoric acid, with various etching times and temperatures.

A new statistical approach, i.e. the method based on size-distribution of revealed etch pits, was used for revealing densities of various types of dislocations. Etched samples were examined in scanning electron microscope (SEM) and afterwards the obtained images were digitally processed to determine the etch-pit size distribution. If appropriate etching conditions are applied, the distribution is multimodal. TEM results serve for an attribution of the peaks to the types of dislocations and as the values that should be achieved from the hot-acid etching method, which allows to determine optimal etching conditions. Hence, SEM (or Atomic Force Microscopy, AFM) technique is suitable for determining densities of various types of threading dislocations in GaN layers.

Thus, a simple and low time-consuming method, with an application of selective etching, SEM (or AFM) observations and size-distribution approach, is presented and calibrated by analyses based on TEM.

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