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

Nano Research https://doi.org/10.1007/s12274-018-2053-5 Electronic properties of wurtzite GaAs: A correlated structural, optical, and theoretical analysis of the same polytypic GaAs nanowire Alexander Senichev1,†,§ (), Pierre Corfdir2,‡,§, Oliver Brandt2, Manfred Ramsteiner2, Steffen Breuer2,||, Jörg Schilling3, Lutz Geelhaar2, and Peter Werner1 1 Max-Planck-Institut für Mikrostrukturphysik, Halle 06120, Germany Paul-Drude-Institut für Festkörperelektronik, Berlin 10117, Germany 3 Centre for Innovation Competence SiLi-nano, Martin-Luther-Universität, Halle 06120, Germany † Present address: Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA ‡ Present address: Fraunhofer HHI, Berlin 10587, Germany ǁ Present address: ABB Corporate Research, Baden-Dättwil 5405, Switzerland § Alexander Senichev and Pierre Corfdir contributed equally to this work. 2 Received: 5 August 2017 ABSTRACT Revised: 15 March 2018 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. Accepted: 17 March 2018 © The author(s) 2018. This article is published with open access at link.Springer.com KEYWORDS nanowires, crystal-phase quantum structures, wurtzite GaAs, strain, near-field scanning optical microscopy, photoluminescence Address correspondence to 2 1 Nano Res. Introduction When III-V compound semiconductors are grown in the form of nanowires, their crystal lattice may adopt a wurtzite (WZ) structure, although 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. However, for nanowires extending along the〈111〉direction, the formation probability of the different crystal phases is generally similar, and the preferential formation of a specific phase during the nucleation of each atomic layer sensitively depends on several energetic and kinetic factors, such as the supersaturation of the catalyst, interfacial energies, and the nucleation site [2–5]. Changes of the stacking sequence are, hence, easily introduced, and such nanowires consequently represent axial polytype heterostructures. Since the different polytypes have different electronic band structures [6], the resulting polytype heterostructures give rise to complex luminescence spectra [7–9] and can affect charge transport [10, 11]. On the one hand, such phenomena may deteriorate the performance of nanowire devices. On the other hand, crystal-phase quantum structures may also be exploited as the very basis for fundamental investigations and applications since they are structurally perfect by nature. More specifically, such heterostructures exhibit interfaces that are defined by crystal stacking and are hence atomically abrupt. Furthermore, 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. Impressive progress has been achieved with respect to the understanding and control of the formation of crystal polytypes in nanowires [5, 12, 13]. Equally important is the understanding of the electronic structure of crystal-phase quantum structures. This objective primarily requires the determination of the WZ band structure, since the properties of the ZB phase are very well known. Most studies in this direction have been carried out on the polytypic III–V compound semiconductor GaAs. Despite considerable efforts, the results from the vast body of literature on this subject are entirely inconsistent [9, 14–24]. For example, the reported values for the bandgap of WZ GaAs scatter from 20 meV below [14] to 100 meV above [17] the bandgap of ZB GaAs. To the best of our knowledge, the results of all reported studies to date are summarized in Fig. S1 in the Electronic Supplementary Material (ESM). This overview reveals that even among recent studies, there is no consensus on the band structure of WZ GaAs. Its experimental analysis is made challenging by the fact that bulk material is essentially not available, as reported by McMahon and Nelmes [25]; moreover, in nanowires, typically both phases occur on a nanometer length scale, leading to difficulties in assigning the optical transitions to specific structural configurations. The diffusion length in GaAs nanowires is on the order of 1 μm [26], which makes it complicated to characterize WZ GaAs nanowires by standard luminescence spectroscopy techniques. This requires either WZ GaAs nanowires with a stacking faults/twins density much lower than 1 per micrometer, which in most cases poses a challenge, or highly spatially-resolved spectroscopic techniques. In the present study, we characterized one and the same dispersed GaAs/(Al,Ga)As core/shell nanowire by both near-field scanning optical microscopy (NSOM) and transmission electron microscopy (TEM). Thus, we established a cross-correlation between photoluminescence bands and crystal structure with extreme spatial resolution. From these experiments, we succeeded at extracting the emission energy at 10 K of extended ZB and WZ segments, WZ quantum disks in a ZB matrix, and ZB quantum disks in a WZ matrix. The results for the extended ZB segment revealed that the nanowire was actually under a compressive uniaxial strain along the nanowire axis, presumably due to its interaction with the supporting substrate. A homogeneous uniaxial strain naturally affects the elec (...truncated)