Wavelength-dispersive x-ray microanalysis as a novel method for studying magnesium doping in gallium nitride epitaxial films

Magnesium doping is critically important in GaN device technology, since it provides the only viable method of producing layers with p-type conductivity. Electron probe microanalysis with wavelength dispersive x-ray spectrometry (WDX-EPMA) was used to measure magnesium atom concentrations in doped GaN films grown by metal organic vapour phase epitaxy (MOVPE). Our study compared the behaviour of a widely used magnesium source in MOVPE, bis(cyclopentadienyl) magnesium, when vaporized as a solid and as a proprietary two-phase source, Solution Cp2Mg™. The WDX-EPMA technique was capable of measuring [Mg] values in GaN layers at practically useful concentrations of 1019 cm−3 upwards. Excellent agreement in [Mg] values was obtained between [Mg] values measured by WDX-EPMA and the more widely used technique of secondary ion mass spectrometry (SIMS). A set of 12 GaN:Mg samples was studied by WDX-EPMA to investigate the dependence of [Mg] on the flow rate of the magnesium source into the MOVPE reactor, with other conditions held constant, including a growth set-point temperature of 1130 °C. These measurements suggested a solid solubility limit at ~1020 cm−3, consistent with previous studies. Up to a value of about half the saturation limit, [Mg] values were proportional to the magnesium source flow, and indicated magnesium atom incorporation from the gas phase with ~11% of the efficiency of gallium atoms. No systematic differences were seen between the behaviour of solid magnesocene and Solution Cp2Mg™. A more limited study of the temperature dependence of magnesium incorporation showed a reduction in incorporation of ~40% as the growth temperature was reduced from 1130 to 1090 °C, consistent with kinetic control. Selected GaN:Mg samples were studied by Hall measurements and high-resolution x-ray diffraction. This work showed no systematic structural degradation of GaN:Mg close to the magnesium solubility limit. Our most conductive sample had a hole concentration of 4.4 × 1017 cm−3, consistent with the expected generation of acceptors from only a small fraction of the magnesium atoms. We also discuss the relative capabilities of SIMS and WDX-EPMA in the context of analysing GaN:Mg samples. SIMS offers superior depth profiling capability and detection limits, whilst WDX-EPMA offers superior spatial resolution, non-destructive analysis, plus simultaneous imaging and cathodoluminescence spectroscopy.