Highly mismatched crystalline and amorphous GaN1−xAsx alloys in the whole composition range

Abstract

Alloying is a commonly accepted method to tailor properties of semiconductor materials for specific applications. Only a limited number of semiconductor alloys can be easily synthesized in the full composition range. Such alloys are, in general formed of component elements that are well matched in terms of ionicity, atom size and electronegativity. In contrast there is a broad class of potential semiconductor alloys formed of component materials with distinctly different properties. In most instances these mismatched alloys are immiscible under standard growth conditions. Here we report on the properties of GaN 1-x As x a highly mismatched, immiscible alloy system that was successfuly synthesized in the whole composition range using a non-equilibrium low temperature molecular beam epitaxy technique. The alloys are amorphous in the composition range of 0.17<x<0.75 and crystalline outside this region. The amorphous films have smooth morphology, homogeneous composition and sharp, well defined optical absorption edges. The bandgap energy varies in a broad energy range from ~3.4 eV in GaN to~0.8 eV at x~0.85. The reduction of the band gap can be attributed primarily to the downward movement of the conduction band for alloys with x>0.2, and to the upward movement of the valence band for alloys with x<0.2. The unique features of the band structure offers an opportunity of using GaN 1-x As x alloys for various types of solar power conversion devices. 3 1. The Ga-N-As Alloy system Dilute alloysSemiconductor alloys formed through substitution of atoms with very different electronegativity and/or size have been known as highly mismatched alloys (HMAs).Because of large miscibility gaps, typically HMAs consist of a semiconductor matrix (elemental or compound) with the substitution of a small amount (up to several percents) of distinctly different isovalent atoms. The HMAs show large band gap reduction and their electronic structure is drastically different from their host materials. This is in contrast to conventional semiconductor alloys such as AlGaAs and GaAsP whose physical, electronic and optical properties can be deduced from a linear interpolation between their corresponding end point compounds with only a slight deviation, as predicted by the virtual crystal approximation (VCA). A most notable example of HMAs is the As-rich GaN x As 1-x in which strong band gap reduction by as much as 180 meV per mole percent of N has been observed [1,2,3,4]. The strong dependence of the band gap on the N content has made these diluted III-V nitrides important materials for a variety of applications, including long wavelength optoelectronic devices [5,6] and high efficiency hybrid solar cells [7,8].In contrast to the very extensively studied As-rich GaNAs alloys much less work has been devoted to HMAs on the N-rich side of this alloy system. It was found that GaN doped with As at low, impurity like levels shows a characteristic blue emission at room temperature. At higher As doping levels an abrupt decrease in the band gap of...

Highly mismatched GaN1−xSbxalloys: synthesis, structure and electronic properties

“…6(a). The ring pattern for this sample is consistent with the XRD result that cubic polycrystalline phase of GaN1-xAsx was formed for As-rich alloys [39]. TEM micrograph of the crystalline 2% As film (not shown) reveals a columnar crystalline film with high density of planar defects.…”

“…Using highly non-equilibrium LT-MBE with growth temperatures as low as 200°C we have synthesized GaN1-xAsx across the entire composition range [39,76,77]. The resulting alloys are amorphous in the intermediate composition range (0.10 <x< 0.75), with the transition between crystalline to amorphous structure depending on the growth substrate.…”

“…In the first approximation it was proposed that the BAC model can be extended to the whole composition range by compositional weighting of the BAC results obtained in the dilute limits [29]. This approach has been developed for GaN1-xAsx [39]. As will be discussed later in section 3.3 in a study of the composition dependence of the bandgap of GaN1-xAsx over the entire composition range at dilute composition limits (dilute-N GaAs and dilute-As GaN), the calculated band gap is in a reasonably good agreement with the experimentally determined optical band gaps [11,29,39] (Fig.…”

“…Most of these previous studies focused on the structural properties of GaN synthesized at low temperatures (<300 o C) by radio frequency sputtering. In MBE high quality GaN thin films for optoelectronic applications are typically grown at a growth temperature 800 o C. Recently as part of our investigation on the low temperature synthesis of highly mismatched GaN1-xAsx [39] and GaN1-xSbx [41,70] alloys, we explored LT-MBE synthesis of GaN thin films grown at temperatures ranging from ~80 to 500 o C under a wide range of Ga:N flux ratios [71].…”

“…Thus reducing the growth temperature allowed synthesis of group III-N-V HMAs over almost the entire composition range. The unusual evolution of the band gap and the conduction and valence band energies generated interest in potential applications of these HMAs for solar power conversion applications [39]. This review focuses on the GaNxSb1-x HMA which has been suggested as a potential material for solar driven water dissociation [40,41].…”