Study On The Growth Of â…¢-V Semiconductor Nanowires By MBE And Their Physical Properties

III-V semiconductor nanowires have drawn extensive attentions in recentdecades because of their distinctive physical properties and hence potentialapplications in the nanoelectronics and optoelectronics. The growth technique ofnanowires is one of the issues studied most by researchers. In this thesis, the growthof GaAs-based III-V semiconductor nanowires, including GaAs nanowires,GaAs-AlGaAs core-shell nanowires and InAs nanowires, has been carried out byAu-catalyzed molecular beam epitaxy according to the vapor-liquid-solidmechanism. Following the growth is the study on the morphology, crystallographicstructure and optical properties of nanowires. The details are as follows:1. By exploring detailedly the impact of growth parameters on themorphology and microstructure of epitaxial GaAs nanowires grown by molecularbeam epitaxy, we have firstly found the growth window for GaAs nanowires of highquality. It is found that GaAs nanowires are of wurtzite structure, and are defect-freewhen grown at low growth temperature. As the growth temperature increases, thenumber of stacking faults in nanowires rises gradually. Compared to growthtemperature, V/III flux ratio affects more greatly the morphology than the crystalstructure of nanowires. GaAs nanowires, perpendicular to the substrate and with theuniform growth orientation, are obtained under the high V/III flux ratio. Byinvestigating the photoluminescence of the single wurtzite GaAs nanowire withdifferent excitation powers and temperatures, we have found that the electron–phonon interaction in wurtzite GaAs is weaker than that in zinc-blende GaAs andthe extrinsic neutral-donor-bound exciton peak appears under the strong excitationpower. The Raman spectra at different positions along the single GaAs nanowireshow the good homogeneity of crystal structure along the nanowire since the twocharacteristic peaks, E2H(TO) and E1(TO), shift hardly along the nanowires. 2. The epitaxial GaAs-AlGaAs core-shell nanowires have been grown bymolecular beam epitaxy. The results of morphology and crystal structure show thatGaAs-AlGaAs core-shell nanowires present a “double-tapered” shape, and they arealso of wurtzite structure. The EDS results reveal that the lateral distribution of Alcomposition in the AlGaAs shell layer is uniform, whereas the axial distributionshows a gradual decrease of Al composition near the top of nanowires. According tothis result, a possible growth mechanism of GaAs-AlGaAs core-shell nanowires hasbeen proposed. By investigating the photoluminescence of GaAs-AlGaAs core-shellnanowires, we have found both the (heavy-hole) free exciton emission (1.51eV) andthe light-hole free exciton emission (1.57eV). These two emissions are caused bythe special energy band structure of wurtzite GaAs and the diminution of surfacestates in GaAs nanowires by the AlGaAs shell layer. Because of the passivation bythe AlGaAs shell, the free exciton lifetime of GaAs-AlGaAs core-shell nanowireshas increased to the order of magnitude of nanoseconds, comparable with the bulkGaAs, which has provided the possibility of applications in optoelectronic devices.3. The crystallographic structure of GaAs nanowires has been tuned bybismuth. It is found that under the growth conditions that favor the formation ofwurtzite structure, the introduction of bismuth to the ambient can change the crystalstructure of GaAs nanowires from wurtzite to zinc blende and a zinc-blende GaAssegment forms; after the removal of bismuth, the zinc-blende GaAs segmentconverts to a4H polytypism segment before it turns back finally to the wurtzitestructure. The density of twin defects in the zinc-blende GaAs segment decreases asthe content of bismuth in the ambient increases. The plausible reason for thistransformation of crystal structure is that the introduced bismuth atoms becomereacting surfactants on the surface of GaAs substrates, which can reduce the surfacediffusion length of adatoms, and in turn decrease the concentration of Ga in theliquid Au catalysts. As a result, the supersaturation becomes smaller, resulting in theappearance of zinc-blende GaAs segment.4. The hetero-epitaxial InAs nanowires have been grown successfully bymolecular beam epitaxy, and the window of growth conditions has been obtained. It is found that the window of growth temperature for InAs nanowires on GaAs(111)Bsubstrates is330°C≤Tgrowth≤430°C, during which InAs nanowires are grownperpendicular to the substrate and the length of nanowires increases with growthtemperature; the window of V/III flux ratio is15≤V/III≤40, where the verticalInAs nanowires become dominant. We have also found that the crystallographicstructure of InAs nanowires is also wurtzite, same as GaAs naowires, and thedominant growth orientation is [111]Beven in the non-(111)B substrate. Finally wehave found that the crystal structure of InAs nanowires can be controlled by the sizeof Au catalyst droplet: small Au catalysts favor the formation of defect-free InAsnanowires, while large Au catalysts tend to induce InAs nanowires with many planardefects.