In situ surface composition and structure of InGaN and GaN thin films by time-of-flight mass spectroscopy of recoiled ions

Abstract

Time-of-flight mass spectroscopy of recoiled ions (TOF–MSRI) is used to determine the surface chemical composition and termination of GaN and InGaN thin films grown by gas source and electron cyclotron resonance molecular beam epitaxy (GS–MBE and ECR–MBE). We show that using TOF–MSRI all the critical growth steps, the nitridation, the buffer layer and the epilayers can be optimized in real time. In the case of GS–MBE, the ammonia pressure can be, reproducibly and easily, adjusted to achieve the highest N surface composition at the minimum corrosive ammonia flow. For InGaN the total TOF–MSRI ion counts drop with increasing In content. Such an observation can be applied to evaluate the thin film surface morphology in addition to its surface composition. Finally, we present preliminary data showing the use of TOF–MSRI for in situ GaN surface structure determination. We also show that by using a reflectron ion analyzer, much higher ion counts and better resolution can be achieved than a conventional electrostatic sector system. With such a modification, dopant level sensitivities should be achievable and data rates compatible with closed loop process control algorithms become possible.