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Vacancy-Induced Nanoscale Wire Structure in Gallium Selenide Layers Print

Low-dimensional materials have gained much attention not only because of the nonstop march toward miniaturization in the electronics industry but also for the exotic properties that are inherent in their small size. One approach for creating low-dimensional structures is to exploit the nanoscale or atomic-scale features that exist naturally in the three-dimensional (bulk) form of materials. By this means, a group from the University of Washington has demonstrated a new way of creating one-dimensional nanoscale structures (nanowires) in the compound gallium selenide. In short, ordered lines of structural vacancies in the material stimulate the growth of "one-dimensional" structures less than 1 nanometer in width.

Accepting What Nature Offers

We live in a three-dimensional world, or so it would appear. But exploring what happens when one or more of the dimensions shrinks to the size of a few atoms (one nanometer on down) and finding ways to exploit the novel properties that result are frontier areas of today's solid-state physics and materials science. However, before exploring and exploiting comes making. So developing techniques to controllably fabricate structures at the nanoscale is the necessary foundation for studying so-called low-dimensional materials, such as two-dimensional, nanometer-thick layers (a technology now well in hand) and one-dimensional, nanometer-wide wires (a skill just beginning to be acquired).

Many of the approaches to growing nanowires rely on naturally occurring structures in bulkier materials to provide a kind of one-dimensional template. Ohta et al. have taken a related tack by taking advantage of atomic defects called vacancies that occur in crystalline materials (most metals and semiconductors, for example). In the case of the compound semiconductor gallium selenide, the vacancies line up in rows along specific directions in the crystal lattice, and these rows guide the growth of wire-like structures less than one nanometer thick. Understanding how this occurs opens new opportunities for controlled growth or self-assembling process of nanoscale structures in other materials containing structural vacancies.

A structural vacancy is a vacant atomic position in the crystal lattice. Introduction of chemical impurities (doping) can result in vacancy generation as the crystal seeks an equilibrium between local charge balance, bonding coordination, and valency around the impurity, and thermal energy can create vacancies even in ultrapure materials at temperatures above absolute zero. Structural vacancies are a source of numerous interesting structural, electronic, and optical properties, and materials scientists often rely them as an important building block for constructing the desired behavior.

Crystal structure of Ga2Se3. One-third of the gallium sites are vacant along the direction of the zinc blende lattice.

As a semiconductor formed from group III (gallium) and VI (selenium) elements, gallium selenide (Ga2Se3) exhibits still another way to generate vacancies. So-called III–V semiconductors consisting of group III and group V elements or II–VI semiconductors consisting of group II and group VI elements typically exhibit tetrahedral sp3 bonding to accommodate the eight electrons per cation–anion pair and crystallize in either zinc blende or wurzite structures. But gallium selenide (Ga2Se3) has nine valence electrons per cation–anion pair, resulting in a competition between the desire to retain tetrahedral sp3 bonding and the need to accommodate the extra electron. So, the compound stabilizes itself by incorporating structural vacancies.

In particular, Ga2Se3 crystallizes in a "defected" zinc blende (ZB) structure with one-third of the cation (gallium) sites vacant. In β-Ga2Se3, these sites are ordered in one dimension. These ordered vacancy arrays yield anisotropic optical transmission and photoluminescence and are predicted by theory to yield nondispersing electronic bands (energy is flat in momentum space), which is a signature of one-dimensional materials.

Scanning tunneling microscopy (STM) of the growth morphology of 0.9-nm-thick Ga2Se3 on a Si(001) substrate. Left: 100 x 100 nm2 three-dimensional image and cross section of surface height along the line between the white arrows. Right: Higher-magnification 20 x 20 nm2 image and cross section of surface height between the white arrows.

The Washington researchers studied a heteroepitaxial layer of Ga2Se3 on a silicon (001) substrate covered by a single monolayer of arsenic by means of scanning tunneling microscopy (STM), core-level photoemission spectroscopy (PES), and photoemission diffraction (PED). They used STM to observe surface morphology and PES and PED to probe the chemical bonding and the bonding configuration. All PES and PED measurements were performed at the Electronic Structure Factory endstation of ALS Beamline 7.0.1.

STM of Ga2Se3 films revealed nanoscale, wire-like structures less than 1 nm wide and up to 30 nm long covering the surface in parallel lines. Core-level PES and PED showed the local structure of gallium and selenium atoms to reflect that of bulk Ga2Se3. The direction of the wire structure also coincided with the ordered vacancy direction usually seen in β-Ga2Se3. The researchers attributed the anisotropic growth of Ga2Se3 to passivation of the side walls of the wire structure by fully occupied lone-pair (nonbonding) orbitals of selenium atoms that arise due to the extra electron, so that adsorbing atoms during the growth process (adatoms) attach at the ends of the wires rather than on their chemically inert sides. Another way of looking at the observed anisotropic morphology is a coalescence of the vacancies at the surface.

Photoemission spectroscopy and diffraction of 0.8-nm-thick Ga2Se3 film reflect that of bulk Ga2Se3. Top: Selenium 3d and gallium 3d spectra (photon energy hν = 246 eV, normal emission geometry). Bottom: Selenium 3d and gallium 3d scanned-angle stereographic projections of photoemission intensities (magnesium Kα x-ray source, hν = 1253 eV) and characteristic angles for the first-nearest and second-nearest-neighbor directions in a zinc-blende lattice.

The formation of wire structures results from the strongly anisotropic growth of Ga2Se3, driven by intrinsic structural vacancies. To the group's knowledge, this mechanism has not been previously observed, and its understanding opens new opportunities for controlled growth or self-assembling processes of nanoscale structures in other materials containing structural vacancies.

Research conducted by T. Ohta, D.A. Schmidt, S. Meng, A. Klust, A. Bostwick, Q. Yu, M.A. Olmstead, and F.S. Ohuchi (University of Washington).

Research funding: Research funding: National Science Foundation, M.J. Murdock Charitable Trust, University of Washington, UW-PNNL Joint Institute for Nanoscience, and Alexander von Humboldt Foundation. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: T. Ohta, D.A. Schmidt, S. Meng, A. Klust, A. Bostwick, Q. Yu, M.A. Olmstead, and F.S. Ohuchi, "Intrinsic vacancy-induced nanoscale wire structure in heteroepitaxial Ga2Se3/Si(001)," Phys. Rev. Lett. 94, 116102 (2005).