Electronic properties of wurtzite GaAs: A correlated structural, optical, and theoretical analysis of the same polytypic GaAs nanowire

Alexander Senichev,Steffen Breuer,Manfred Ramsteiner,Lutz Geelhaar,Pierre Corfdir,Peter Werner,Jörg Schilling,Oliver Brandt

doi:10.1007/s12274-018-2053-5

Abstract

III-V compound semiconductor nanowires are generally characterized by the coexistence of zincblende and wurtzite structures. So far, this polytypism has impeded the determination of the electronic properties of the metastable wurtzite phase of GaAs, which thus remain highly controversial. In an effort to obtain new insights into this topic, we cross-correlate nanoscale spectral imaging by near-field scanning optical microscopy with a transmission electron microscopy analysis of the very same polytypic GaAs nanowire dispersed onto a Si wafer. Thus, spatially resolved photoluminescence spectra could be unambiguously assigned to nanowire segments whose structure is known with lattice-resolved accuracy. An emission energy of 1.528 eV was observed from extended zincblende segments, revealing that the dispersed nanowire was under uniaxial strain presumably due to interaction with its supporting substrate. These crucial information and the emission energy obtained for extended pure wurtzite segments were used to perform envelope function calculations of zincblende quantum disks in a wurtzite matrix as well as the inverse structure. In these calculations, we varied the fundamental bandgap, the electron mass, and the band offset between zincblende and wurtzite GaAs. From this multi-parameter comparison with the experimental data, we deduced that the bandgap between the Γ8 conduction and A valence band ranges from 1.532 to 1.539 eV in strain-free wurtzite GaAs, and estimated values of 1.507 to 1.514 eV for the Γ7–A bandgap.

Highlights

When III-V compound semiconductors are grown in the form of nanowires, their crystal lattice may adopt a wurtzite (WZ) structure, their thermodynamically stable crystal phase has, with the exception of nitrides, a zincblende (ZB) structure [1]
By combining the PL energies measured for the WZ and ZB segments as well as that from 3 ML thick ZB disks in a WZ GaAs matrix with a simulation of the exciton energy in polytype nanowire segments, we obtained that the energy of the Γ8–A free exciton in strain-free WZ GaAs at 10 K is 1.526 eV, and that the Γ8–A bandgap energy ranges between 1.532 and 1.539 eV
Using the strain dependence reported in Ref. [27], we estimated that the Γ7–A bandgap energy expected in strain-free nanowires is 1.507 to 1.514 eV

Summary

Introduction

When III-V compound semiconductors are grown in the form of nanowires, their crystal lattice may adopt a wurtzite (WZ) structure, their thermodynamically stable crystal phase has, with the exception of nitrides, a zincblende (ZB) structure [1]. Bulk crystals and planar thin films of these materials invariably crystallize in the zincblende structure. Changes of the stacking sequence are, introduced, and such nanowires represent axial polytype heterostructures. Crystal-phase quantum structures may be exploited as the very basis for fundamental investigations and applications since they are structurally perfect by nature. Such heterostructures exhibit interfaces that are defined by crystal stacking and are atomically abrupt. Crystal-phase heterostructures are free of any alloy disorder, in contrast to conventional heterostructures based on changes in chemical composition that often involve ternary materials

Methods

Results

Conclusion