Formation and Evolution of InAs Nanowires on an InP(001) Surface
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surface energy, i.e. changing the surface reconstruction EXPERIMENT The experiment was carried out in a combined ultra high vacuum (UHV), MBE-STM system. The MBE chamber has solid sources of arsenic and phosphorous that are equipped with valves and shutters to reduce intermixing of arsenic and phosphorous. The substrate temperature is measured using optical transmission thermometry within ±2 'C 17]. After loading an n-type InP (001) wafer into the MBE chamber, the oxide layer on the wafer is removed by heating the wafer above 480'C under a cracked phosphorous (P2) beam equivalent pressure of 10-5 Torr. The resultant oxide-free surface yields a 2x4 pattern in reflection high energy electron diffraction (RHEED), indicating a P-terminated surface. A 0.3 4m thick buffer layer of InP is then grown on the substrate at 470'C after oxide removal. Before InAs deposition or exposure to an arsenic flux, the valve for the P2 flux is closed and the residual phosphorous is pumped out for about 3 min. The 2x4 RHEED pattern is preserved during annealing without phosphorous flux. The substrate is then either exposed to an arsenic flux or InAs is deposited for a desired thickness of about 3ML. The sample is then immediately (or after annealing) cooled to room temperature, and transferred to the STM chamber through the UHV
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Mat. Res. Soc. Symp. Proc. Vol. 618 © 2000 Materials Research Society
modutrack. STM images are taken for filled states (-3V on the sample) with a tunneling current around 100 pA. RESULTS Startine surface and surface reconstruction of 2x4 and 4x2 An InP (001) surface at 480 0 C, under no phosphorous flux, yields a 2x4 RHEED pattern similar to that of a GaAs(001)-2x4 surface. Figure 1 shows STM images of the InP (001) starting surface, which is prepared at 4800 C just before deposition of InAs or exposure to an arsenic flux. They show terraces, steps with one monolayer high, and a 2x4 surface reconstruction with the dimer rows resolved. Figure 1, STM images of InP(001) starting surface. The high-resolution image (right lower one) shows trenches. The separation between trenches is four surface lattice spacing of InP(001)2x4 reconstruction, i.e. 1.660nm.
Application of arsenic flux on this surface at 480'C is observed to make the RHEED pattern blurry, suggesting a surface reaction induced by the arsenic flux. Depending on the amount of the flux, the surface reconstruction of the substrate is observed to change. With a low arsenic beam equivalent pressure (BEP) of 1.5x10 7 Torr on the starting InP surface, the RHEED pattern changes from 2x4 to 4x2. If the As pressure is then increased tolxlO5 Torr the reconstruction changes back to 2x4. The change is similar to the manner in which reconstruction on a InAs(001) surface depends on the level of arsenic flux[8]. For example, the InAs(001) surface has a 4x2 surface reconstruction when arsenic flux is low and 4x2 when high. The similar dependence observed here suggests that the exposure to arsenic has produced an InAs layer on top of the smooth InP(001)-2x4 surface.
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