Butcher, Chen and Downes (Appl. Phys. Lett. 100, 011913 (2012)) measured an activation energy of 0.4±0.10.4 \pm 0.1 eV for the thermally activated removal of excess nitrogen - attributed to interstitial nitrogen, NiN_i - from nitrogen-rich indium nitride (InN) grown by remote plasma enhanced chemical vapor deposition, and argued that this low barrier supports a low formation energy for the same native defect, reachable only under non-equilibrium plasma conditions. The microscopic basis of this process remains undetermined: it is not known whether nitrogen loss is limited by diffusion of NiN_i to the surface or by recombination of two NiN_i into an N2N_2 molecule that then desorbs; and the atomic configuration and charge state of the mobile defect, its electrical activity and quantitative contribution to InN's bulk conductivity and surface electron accumulation, and the composition of the residual defects surviving above 400 °C are all open. These questions remain unanswered in the 2012-2026 literature, so the problem - a microscopic, quantitative account of the mobile nitrogen-rich defect in plasma-grown InN - remains open.

Background

Indium nitride (InN) is a wurtzite-structure group-III nitride semiconductor with a narrow band gap of roughly 0.65 eV. Because thermal cracking of nitrogen molecules is inefficient at the low substrate temperatures this material requires, InN is grown by plasma-assisted methods - remote plasma enhanced chemical vapor deposition (RPECVD), plasma-assisted molecular beam epitaxy (PA-MBE), and sputtering - in which excited nitrogen species drive film growth. Elastic recoil detection (ERD), an ion-beam technique measuring film composition and depth profiles, repeatedly showed that InN films grown at low temperatures contain large excess nitrogen, with N/In ratios up to about 1.7, attributed to nitrogen interstitials (NiN_i, nitrogen atoms occupying non-lattice positions between regular sites) and related nitrogen-rich point defects (Butcher et al. 2007). This posed a theoretical puzzle: early equilibrium first-principles calculations predicted very large NiN_i formation energies (of order 6 eV, which would make nitrogen-rich defects negligible), while later calculations for wurtzite InN reported much lower values, about 2.3-2.9 eV under nitrogen-rich, high-Fermi-level conditions (Duan and Stampfl 2009; Zhao, Shi and Yang 2010). Butcher, Chen and Downes (2012) measured an Arrhenius activation energy of 0.4±0.10.4 \pm 0.1 eV for thermally activated removal of the excess nitrogen (from ERD stoichiometry of films grown for 10 h at substrate temperatures of 200-400 °C) and argued that such a low removal barrier supports a low NiN_i formation energy that is reachable only because energetic plasma species - e.g. the metastable N2∗(A3Σu+)N_2^*(A^3\Sigma_u^+) precursor carrying roughly 4.9 eV of available excess energy - push defect incorporation away from thermodynamic equilibrium. A 'split interstitial' (two nitrogen atoms sharing one lattice site) and an octahedral-site interstitial (nitrogen in the large interstice of the wurtzite lattice) are the candidate configurations; 'hybrid functional' calculations, a first-principles density-functional scheme that mixes exact exchange into the standard approximation to fix the notorious band-gap underestimate, are the modern reference for defect formation energies and charge-state transition levels. Separately, InN has long been known for an intrinsic tendency to accumulate electrons at its polar surfaces: photoemission showed that even atomically clean surfaces pin the Fermi level above the conduction-band minimum, producing a surface electron accumulation layer (a two-dimensional electron gas) that, together with bulk native defects and impurities, dominates transport in this unintentionally n-type material (Mahboob et al. 2004; Segev and Van de Walle 2006). Whether the mobile nitrogen-rich defects contribute measurably to this accumulation, or to bulk conductivity, has remained speculative.

Problem Statement

Give a microscopic, quantitative account of the mobile nitrogen-rich defect in plasma-grown indium nitride - its identity, its removal kinetics, and its electrical role - resolving the questions left open by Butcher, Chen and Downes (2012). Concretely: (1) determine the rate-limiting microscopic mechanism for thermally activated loss of excess nitrogen - whether diffusion of nitrogen interstitials (NiN_i) to the surface, or recombination of two NiN_i into an N2N_2 molecule followed by desorption - together with the elementary activation energies of these channels, consistent with the measured overall Arrhenius barrier of 0.4±0.10.4 \pm 0.1 eV. (2) Identify the atomic configuration of the mobile nitrogen-rich defect (split interstitial, octahedral-site interstitial, or another configuration) and its charge state, and how these determine its mobility. (3) Establish the electrical activity of these defects: their donor or acceptor character, and their quantitative contribution to bulk conductivity and, if such a contribution exists, to the surface electron accumulation layer of InN. (4) Identify the residual nitrogen-rich defects that survive growth temperatures above 400 °C (which may include indium vacancies). These sub-questions jointly support one overarching objective - a self-consistent microscopic account of the excess-nitrogen defect, its kinetics, and its electrical consequences - and an answer is complete when the defect is identified (configuration and charge state) together with how that configuration and charge state determine its mobility, its dominant removal mechanism is assigned with elementary activation energies reconciled with the measured overall barrier of 0.4±0.10.4 \pm 0.1 eV, its electrical contribution is quantified, and the residual nitrogen-rich defects that survive growth temperatures above 400 °C are identified.

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