Nanowire-Induced Wurtzite InAs Thin Film on Zinc-Blende InAs Substrate

2009 WILEY-VCH Verlag Gmb Synthesis of materials with a desired crystal structure is a major challenge in materials engineering. Single-crystal thin films grown by epitaxy typically adopt the same crystal structure as that of their substrates. Here, we report on the observation of a wurtzite InAs thin-film structure on a zinc-blende InAs substrate. Electron-backscatter diffraction (EBSD) and transmission electron microscopy (TEM) confirm the wurtzite crystal structure. The bandgap of wurtzite InAs, obtained by low-temperature photoluminescence, is found to be 20% higher than that of zinc-blende InAs, in good agreement with band-structure calculations. Microscopy studies suggest that the wurtzite InAs thin film is the result of step flow along the surface from the base of wurtzite InAs nanowires synthesized by chemical beam epitaxy on a zinc-blende InAs substrate, leading to layer-by-layer lateral expansion. Although the conditions for the controlled growth of wurtzite InAs films need to be investigated, our observations suggest a new approach to creating thin films with nanowire-induced crystal structure. The creation and integration of a material with different crystal structures, such as polymorphism and the associated heterocrystalline heterostructures, could open up new opportunities in bandgap engineering and related device applications. Polymorphism or polytypism has been a fascinating subject becausematerials with different crystal structures can exhibit very distinct electronic and optical properties, although they are chemically identical. Typically, there is only one stable structure for a bulk material under ambient condition, and the synthesis and preservation of a crystal with a different structure require high pressure and extreme temperatures. It has been observed that size and dimensionality can have a strong impact on the crystal structure of a material. For example, III–V (except for nitrides) and group IV bulk semiconductors naturally take zinc-blende or diamond structure, respectively, but chemically synthesized whiskers or nanowires often exhibit a wurtzite structure. Recent advances in nanowire synthesis have achieved controlled growth of single-crystal wurtzite nanowires, single-crystal zinc-blende nanowires, as well as nanowires with periodically alternating structures between wurtzite and zinc-blende. Despite alternative crystal structures generated in nanowires, materials in the form of single-crystal thin film or two-dimensional structures are still desirable in many applications, due to their ease of device fabrication and integration. However, it is a challenge to grow thin films with a structure different from that of the bulk substrate, because single-crystal thin films obtained by epitaxial growth, such as molecular beam epitaxy or chemical vapor deposition, usually have the same crystal structure as the substrate. InAs is a semiconductor with high electron mobility related to its low bandgap. As such, InAs, especially in nanostructures such as quantum dots and nanowires, is being explored for applications in optoelectronic and high-speed electronic devices. Like other III–V nanowires, InAs nanowires frequently exhibit wurtzite crystal structures. Experimental studies of these wurtzite structures, as well as theory, have indicated that the wurtzite InAs has a higher bandgap than zinc-blende InAs. This indirect experimental evidence includes photoluminescence of InAs/InP core/shell nanowires, the g factor of InAs nanowire quantum dots in InP nanowires, and photocurrent measurements of InAsP nanowires. However a direct measurement of the bandgap of wurtzite InAs has not yet been reported. Figure 1 shows scanning electronmicroscopy (SEM) images of the sample. The InAs nanowires grow vertically from the substrate. Due to the conditions for the formation of the nanowires, the wurtzite crystal structure is preferred, in contrast to the zinc-blende crystal structure that is energetically favorable for bulk InAs. As part of the nanowire fabrication process, a pyramid-like base forms at the root of each InAs nanowire. Broader-area platelets form between the pyramid