Is the Mg-related GaN blue luminescence deep-level an MgO surface state?

TL;DR

This study investigates the origin of Mg-related blue luminescence in GaN, revealing it likely stems from an MgO surface state formed by oxidized Mg, with experimental evidence from surface photovoltage spectroscopy and MgO powder luminescence.

Contribution

The paper identifies a new MgO surface state as the source of blue luminescence in Mg-doped GaN, linking surface chemistry to optical properties and surface states.

Findings

Optical transitions indicate a deep trap at 0.49 eV and a related transition at 2.84 eV.

MgO powder alone exhibits the same blue luminescence.

Etching with HCl affects the photovoltage related to the luminescence.

Abstract

Mg is currently the only p-type dopant in technological use in GaN. Its incorporation into the GaN lattice is difficult. It requires a thermal treatment that only partially activates the Mg. To achieve moderate p-type doping requires high doses of Mg that mostly remain inactive. High p-type doping is thus typically achieved at the cost of certain lattice distortion and the creation of defects. Using low-temperature surface photovoltage spectroscopy, we obtain a wide spectrum of optical transitions within the bandgap of Mg-doped GaN. The results reveal an optical transition from the valence band into a deep trap around 0.49 eV above the valence band, along with what appears to be a complimentary transition from the same trap into the conduction band observed at 2.84 eV (coinciding with the energy of the famous Mg-related GaN blue luminescence). The similar shape of the spectra, their complimentary energies within the GaN gap and their opposite nature (hole vs. electron trap) appear to be more than a coincidence suggesting that this is an Mg-related surface state. The density of charge we calculate for this surface state is about 2x10^12 cm-2. We suggest that these small amounts of surface-segregated Mg partially oxidize during the growth and further oxidize during the consecutive Mg-activation heat treatment. This minute quantity of oxidized surface Mg should be about enough to form an Mg-related surface state. Etching the GaN with HCl is shown to affect the photovoltage at the blue-luminescence-related energy. Finally, we show that pure MgO powder produces the same blue luminescence even at the absolute absence of GaN.