New optical spectroscopy approach reveals electronic properties of doped gallium nitride

To meet a need for p-type gallium nitride (GaN) in the optoelectronics industry, investigators have searched for a suitable dopant that can efficiently introduce holes. Zinc (Zn) atoms were the first to be tried as a dopant in GaN, but the resulting materials were semi-insulating, rather than the expected p-type conductive. In Applied Physics Letters, investigators solve this mystery and report an approach to determine the electrical properties of Zn-doped GaN in epitaxial structures using contactless electroreflectance (CER) spectroscopy.

The authors grew specially designed GaN/GaN:Zn structures with metal-organic vapor deposition, with an undoped layer on top of a layer of the same material intentionally doped with Zn. They also constructed a reference system by the same epitaxial technique that used Si as the dopant. CER allowed them to determine the electric field distribution in the samples, in addition to the photoluminescence (PL) measurements made.

By applying a large external electric field, the authors are able to move carriers back and forth, which allows the modulation of near-surface band bending. This is then sensed by a CER measurement. Coupling the CER experimental results with theoretical calculations, they determined that Zn in GaN creates an acceptor level with an energy of 1.1 eV above the valence band maximum.

The PL studies showed that this level is created by a doubly charged Zn atom on a nitrogen site — in other words, a Zn_N²⁺ level — as opposed to a Ga site. Measurements also confirmed that the surface Fermi level of structures is located in the middle of the band gap, which was previously shown for similar structures doped with magnesium and manganese.

Source: “Zn acceptor position in GaN:Zn probed by contactless electroreflectance spectroscopy,” by Łukasz Janicki, Matin Sadat Mohajerani, Jana Hartmann, Ewelina Zdanowicz, Hergo-Heinrich Wehmann, Andreas Waag, and Robert Kudrawiec, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5040941 .