Surface Dimer Engineering and Properties of GaAs(N)(Bi) Alloys

Abstract

Due to the significant bandgap narrowing induced by dilute fractions of N and Bi in III-V semiconductors, emerging dilute nitride-bismide semiconductor alloys are of significant interest for long-wavelength applications ranging from temperature-insensitive laser diodes to ultra-high efficiency multijunction photovoltaic cells. However, both dilute nitride and dilute bismide devices have exhibited significant sensitivity to the local atomic environments of N or Bi solute atoms, while their incorporation mechanisms are not well understood. In this work, we investigate the role of the surface reconstruction on doping, alloy formation, and electronic and optical properties of GaAs(N)(Bi) alloys. For GaAs(Bi), we examine the influence of surface reconstruction on silicon dopant incorporation and electronic properties. Si incorporation into GaAs(Bi) with an (nx3) surface reconstruction leads to n-type conductivity, while growth with a (2x1) reconstruction leads to p-type conductivity. We hypothesize that the presence or absence of surface arsenic dimers prevents or enables dopant incorporation into arsenic lattice sites. We consider the influence of bismuth anions on arsenic-dimer mediated dopant incorporation and the resulting electronic transport properties, demonstrating the applicability of this mechanism to mixed anion semiconductor alloys. For GaAsNBi alloys, we examine the influence of Bi and N fluxes on N and Bi incorporation. The incorporation of Bi is found to be independent of N flux, while the total N incorporation and the fraction of N atoms occupying non-substitutional lattice sites increase with increasing Bi flux. A comparison of channeling nuclear reaction analysis with Monte Carlo – molecular dynamics simulations indicates that the non-substitutional N primarily incorporate as (N-As) interstitial complexes. We discuss the influence of Bi adatoms on the formation of arsenic-terminated [110]-oriented step edges with a (1x3) surface reconstruction and the resulting enhancement in total N incorporation via the formation of additional (N-As). We also consider the influence of Bi as an incorporating surfactant on chemical ordering in GaAsN:Bi alloys. While epitaxy with a (2x1) reconstruction leads to random GaAsN formation, the introduction of a Bi flux induces long-range chemical ordering of the {111} planes. We propose a mechanism in which Bi enhances the formation of dimer rows aligned along the [110] direction in the (2x1) surface reconstruction, facilitating N incorporation beneath surface dimers and Bi incorporation between dimer rows to form alternating N-rich and Bi-rich {111} planes. These findings suggest a route to tailoring the local atomic environment of N and Bi atoms in a wide range of emerging dilute nitride-bismide alloys. Finally, we have examined the alloy composition dependence of the energy bandgap and electronic states in GaAsNBi alloys. Using direct measurements of N and Bi mole fractions, via ion beam analysis, in conjunction with direct measurements of the out-of-plane misfit via x-ray rocking curves, we determine a new "magic ratio" for lattice-matching of GaAsNBi alloys with GaAs substrates. In addition, using a combination of photoreflectance and photoluminescence spectroscopy, we determine a new map of the composition- and misfit-dependence of the energy bandgaps, along with revealing the energetic position of Bi-related states at approximately 0.18 eV above the valence band maximum.