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.
- thin films
- chemical physics