Electronic structure near the band gap of heavily nitrogen doped GaAs and GaP
- PDF / 194,150 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 5 Downloads / 214 Views
Electronic structure near the band gap of heavily nitrogen doped GaAs and GaP Yong Zhang*, B. Fluegel, M. Hanna, A. Duda, and A. Mascarenhas National Renewable Energy Laboratory, 1617 Cole Boulevard Golden, CO 80401, USA *[email protected] ABSTRACT Isoelectronic impurity nitrogen atoms have been found to generate a series of localized states in GaP and GaAs. These states can be either bound (within the band gap) or resonant (above the band gap) when in the dilute doping limit (roughly < 1019 cm-3 for GaP and < 1018 cm-3 for GaAs). With increasing nitrogen doping level, a shift of the absorption edge from the binary band gap has been observed for the so-called GaPN or GaAsN alloy. We discuss the similarity and dissimilarity between the two systems in the following aspects: (1) How does the nitrogen doping perturb the host band structure? (2) How do the nitrogen bound states evolve with increasing nitrogen doping level? (3) What are the dominant contributors to the band edge absorption? And (4) does a universal model exist for GaPN and GaAsN? Other issues that will be discussed are: how does one define the band gap for these materials, and what is the relevance of various theoretical band structure calculations to the experimentally measured parameters.
INTRODUCTION Large band gap reductions, along with many other modifications to the band structure, have been observed in heavily nitrogen doped GaAs and GaP for nearly a decade [1,2]. Two recent reviews [3,4] have related the newer research activities in this area to a field that is more than thirty-year old, that is, isoelectronic impurities in semiconductors. These materials have frequently been referred to as dilute nitride alloys. Since GaN has a much larger band gap than either of the hosts, the observed large band gap reduction has been portrayed as “giant” bowing, using the terminology for describing conventional alloys. However, if one notes that the band alignment for GaP/GaN or GaAs/GaN is type II with the conduction band edge of the GaN lower than that of GaP by ~ 560 meV or GaAs by ~ 200 meV, then the large band gap reduction is less surprising [3]. Nitrogen is one of a very distinct group of isoelectronic impurities in III-V semiconductors. Long before the finding of the large band gap reduction it was known that the impurity states associated with an isolated nitrogen and various nitrogen pairs have progressively lower energy levels in GaP [5] and GaAs [6,7]. This trend had actually already hinted at the type II band alignment between GaP or GaAs and GaN. Considering the bowing for each individual band edge instead of the entire band gap, one will find the bowing coefficient to be rather different from that given in the literature for the band gap of GaAsN or GaPN. However, the microscopic origin for the band gap reduction, i.e., its relationship with the host band structure or the nitrogen impurity states, has been an intensively debated issue in recent years. A phenomenological model, the so-called “band anti-crossing” model, suggests that the primary ef
Data Loading...
