Thermal and Photoinduced Decomposition Pathways of Arsine on GaAs(100)
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THERMAL AND PHOTOINDUCED DECOMPOSITION PATHWAYS OF ARSINE ON GaAs(100) X.-Y. Zhu, M. Wolf, T. Huett, and J. M. White. Department of Chemistry, University of Texas, Austin,TX 78712. ABSTRACT We have studied the thermal and photoinduced dissociation pathway of AsH 3 on the Ga-rich GaAs(100) surface. Arsine adsorbs molecularly at 115 K and dissociates upon either heating to above 140 K or upon irradiation with 3.5 - 6.4 eV photons. The decomposition of arsine is accompanied by the formation of surface Ga-H species, which are thermally and photochemically more stable than surface AsHx. A comparison of the wavelength dependence for adsorbed and gas phase arsine reveals that the excitation mechanism of the AsH 3 surface photochemistry is substrate mediated, which probably involves a charge transfer between surface states and the adsorbate. I. INTRODUCTION The last few years have seen an increasing effort to use laser-assisted organometallic chemical vapor deposition (OMCVD) and atomic-layer-epitaxy (ALE) to enhance growth rate, to lower the growth temperature, to realize selective growth, and to control dimensions with atomic accuracy [1,2,3]. Obviously a mechanistic understanding of the surface chemistry of these processes is one key for the future development of these laser assisted technologies. However even the thermal chemistry of arsine, which is the most widely used As-precursor molecule for GaAs growth, is far from being understood [4,5]. We have therefore studied one of the fundamental reactions in ALE and laser-assisted GaAs growth: the thermal and photoinduced decomposition of AsH 3 on GaAs(100). While the details will be published elsewhere [6,7], we give here a brief report of our results. II. EXPERIMENTAL The experiments were performed in a UHV chamber, equipped with a high resolution electron energy loss spectrometer (HREELS), a quadrupole mass spectrometer for thermal desorption spectroscopy (TDS), a hemispherical energy analyzer and an X-ray source for X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) optics, an ion gun for sputter-cleaning, and a pin-hole collimated molecular doser [8]. For the photochemical studies, pulsed UV light of an excimer laser with photon energies of 3.5 eV (XeF), 5 eV (KrF) and 6.4 eV(ArF) was used. The laser pulse energy was kept below 1 mJ/cm 2 to prevent desorption due to thermal heating. The samples were cut from GaAs(100) wafers (10151018 /cm 3 Si-doped) and mounted on the edges by two Ta clips spot-welded to the sample holder. The sample could be cooled by liquid nitrogen to - 115 K and resistively heated to 900 K via a 4000 A thick Ta film sputtered onto the backside. Cleaning was achieved by Ar ion sputtering, annealing (773 K), and flashing (900 K) cycles. Surface cleanliness was verified by XPS and LEED. The latter showed a (4x6) Mat. Res. Soc. Symp. Proc. Vol. 236. @1992 Materials Research Society
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LEED pattern, as discussed previously [9,10,11]. Arsine was dosed at 115 K with the doser tube terminated at about 1-2 mm from the GaAs
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