Investigation of defects and surface polarity in GaN using hot wet etching together with microscopy and diffraction techniques

Abstract

The availability of reliable and quick methods to determine defect density and polarity in GaN films is of great interest. We have used photo-electrochemical (PEC) and hot wet etching using H 3 PO 4 and molten KOH to estimate the defect density in GaN films grown by hydride vapor phase epitaxy (HVPE) and molecular beam epitaxy (MBE). Free-standing whiskers and hexagonal etch pits are formed by PEC and wet etching respectively. Using Atomic Force Microscopy (AFM), we found the whisker density to be similar to etch pit densities for samples etched under precise conditions. densities obtained by etching which increased our confidence in the consistency of methods used. Additionally Transmission Electron Microscopy (TEM) observations confirmed dislocation Hot wet etching was used also to investigate the polarity of GaN films together with ConvergentBeam Electron Diffraction (CBED) and AFM imaging. We found that hot H 3 PO 4 etches N-polarity GaN films very quickly resulting in the complete removal or drastic change of surface morphology as revealed by AFM or optical microscopy. On the contrary, the acid attacks only defect sites in Gapolarity films producing nanometer-scale pits but leaving the defect-free GaN intact and the morphology unchanged. Additionally, the polarity assignments were related to the as-grown morphology and to the growth conditions of the buffer layer and the subsequent GaN layer.

Nanopipes in GaN: photo-etching and TEM study

“…Amongst these methods photoelectrochemical (PEC) etching [10] seems to be very promising. The reliability of PEC method in revealing dislocations in GaN has been confirmed by direct TEM calibration [10,13] and compared with the results of "orthodox" etching [14]. In addition it was shown that the method is capable of revealing IDs in the form of tiny whiskers similar to those formed on dislocations or in the form of deep craters, depending upon their diameter [15,16].…”

Orthodox etching of HVPE-grown GaN

“…Clear example of this phenomenon is shown in Fig. 8: etching in HH at 160°C (temperature recommended in [31]) did not reveal any dislocations while etching at higher temperature resulted in formation of shallow and therefore overlapping pits (Fig. 8a), with inclination angles of the side walls well below 10°.…”

“…This refers not only to etching in molten salts (KOH, KOH+NaOH eutectic) but also more specifically to etching in hot acids (H 2 SO 4 , H 3 PO 4 ). In the latter etches, in addition to the use of the nonoptimized temperature [31], the kinetic factor may result under-estimation of EPD (see section 3.2.2); -in order to find the optimal conditions of etching of a new HVPE-grown GaN sample it is recommended performing sequent etching starting at low temperature, e.g. 380°C and, depending on the result, gradually increase temperature and/or time of subsequent etching steps.…”

Dislocation density assessment via X‐ray GaN rocking curve scans

“…Assessment of the dislocation type densities via etching in H 3 PO 4 and counting from AFM images is hampered by a high sensitivity to etching parameters and etch pit coalescence. The etch pit densities scatter over an order of magnitude [11] in the ranges given in Table 1. X-ray diffraction measurements were performed with a Philips X'Pert PRO four-circle diffractometer equipped with a Cu Kα 1 source conditioned by an incident beam hybrid monochromator consisting of a parabolic X-ray mirror and a Ge(220) channel-cut crystal.…”