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PHYSICAL REVIEW B VOLUME 62 NUMBER 12 15 SEPTEMBER 2000 II Reflectance difference spectroscopy of mixed arsenic rich phases of gallium arsenide 001 M J Begarney 1 L Li 2 C H Li 1 D C Law 1 Q Fu 1 and R F Hicks1 1 2 Chemical Engineering Department University of California Los Angeles California 90095 Department of Physics and Laboratory for Surface Study University of Wisconsin Milwaukee Wisconsin 53201 Received 12 May 2000 The relationship between the reflectance difference spectra and the atomic structure of arsenic rich reconstructions of GaAs 001 has been investigated Scanning tunneling micrographs reveal that a roughening process occurs as the surface structure changes with decreasing arsenic coverage from 1 75 to 0 75 monolayers ML At 1 65 ML As small pits one bilayer in depth and having the same c 4 4 reconstruction as the top layer form in the terraces At the same time gallium atoms are liberated to the surface disrupting the c 4 4 ordering At about 1 4 ML As 2 4 domains nucleate and grow on top of the c 4 4 Further desorption of arsenic causes the underlying layer to gradually decompose into a metastable 2 n phase n 2 3 or 4 and finally into the 2 4 In the reflectance difference spectra negative peaks at 2 25 and 2 8 eV correlate with the c 4 4 type arsenic dimers However the intensity of the latter feature strongly depends on the presence of adsorbates such as alkyl groups and gallium adatoms By contrast the intensity of the positive peak at 2 9 eV is directly proportional to the density of 2 4 type dimers I INTRODUCTION Reflectance difference spectroscopy RDS is an effective in situ probe of the surface reconstructions of compound semiconductors during growth by molecular beam epitaxy chemical beam epitaxy and metalorganic vapor phase epitaxy 1 4 The technique determines the relative difference in the near normal reflectance of light polarized along the two principle axes of the surface and since the bulk crystals are isotropic the spectra quantify the optical anisotropy of the first few atomic layers The reconstructions of gallium arsenide 001 have been widely studied using this technique 5 7 While the line shapes of the c 4 4 2 4 c 2 8 and 4 2 c 8 2 surfaces have been correlated with electron diffraction patterns reflection highenergy electron diffraction and low energy electron diffraction 1 the physical origin of the reflectance anisotropy has not been conclusively determined Uncertainty remains as to whether the reflectance anisotropy is a result of bulksurface transitions 8 12 or by transitions among the molecular orbitals of the surface dimers 13 14 Early efforts to account for the reflectance anisotropy involved the calculation of the surface dielectric function of simplified GaAs surfaces 13 14 In these studies 2 1 and 1 2 dimerized surfaces were used to approximate the arsenic rich 2 4 and gallium rich 4 2 reconstructions The authors identified transitions within the dimer structures as the source of the RDS spectra and obtained rough qualitative agreement with the experimental data Based on more recent first principles calculations other researchers have concluded that transitions between bulk valence states and unoccupied surface states are primarily responsible for the reflectance anisotropy 9 12 While these results more closely match the RDS data discrepancies in the energies and magnitudes of spectral features remain The best agreement between theory and experiment has been achieved through tight binding calculations of the dimer structures 8 The influence of both discrete dimer structures and surface modified 0163 1829 2000 62 12 8092 6 15 00 PRB 62 bulk wave functions were cited as contributing factors to the observed anisotropy In this paper we report on the reflectance difference spectra of a series of gallium arsenide 001 reconstructions at arsenic coverages ranging from 1 75 to 0 75 monolayers ML The main structural features on these surfaces are arsenic dimers which are bonded to either a sublayer of As or Ga atoms These two kinds of arsenic dimers are referred to as c 4 4 type and 2 4 type respectively To aid in the interpretation of the spectra a direct comparison has been made between the RDS line shapes and the atomic structures as seen by scanning tunneling microscopy STM We have found that surface roughening occurs as a result of the nucleation and growth of the 2 4 phase on top of the c 4 4 phase This process is accompanied by gallium out diffusion into the c 4 4 layer Furthermore we have discovered that the reflectance anisotropy is affected not only by the types of arsenic dimers but also by the presence of transitional structures such as a new 2 n phase and by adsorbates including alkyl species and gallium adatoms II EXPERIMENTAL METHODS Gallium arsenide films approximately 1 m in thickness were grown on nominally flat GaAs 001 substrates in a horizontal flow metalorganic vapor phase epitaxy MOVPE reactor The substrates were doped n type with 1 1017 Si atoms cm3 The wafer temperature during growth was 550 25 C and the organometallic reagents triisobutylgallium TIBGa and tertiarybutylarsine TBAs were used at concentrations of 5 and 50 ppm respectively Palladium diffused hydrogen was the carrier gas and the total reactor pressure was 20 torr The wafers were cooled immediately following growth with the TBAs and H2 supplies maintained until room temperature was reached This ensured that the surface would have the maximum arsenic coverage possible The samples were transferred to an ultrahigh vacuum cluster tool via a turbo pumped interface cham8092 2000 The American Physical Society PRB 62 REFLECTANCE DIFFERENCE SPECTROSCOPY OF 8093 was between 0 0005 and 0 0010 The pressure in the chamber did not exceed 2 10 9 torr during heating or cooling All spectra reported in the paper were taken after the sample had cooled to 20 C III RESULTS FIG 1 Reflectance difference spectra of GaAs 001 surfaces with arsenic coverages of a 1 75 ML b 1 75 ML c 1 65 ML d 1 4 ML e 1 2 ML and f 0 75 ML The spectra corresponding to the 1 2 c 4 4 and 2 4 surfaces are indicated ber Details of this system have been described previously 15 A commercially available ISA J Y Nisel reflectance difference spectrometer was used for our measurements of the GaAs surfaces The spectral range varied from 1 5 to 5 2 eV with increments of 0 025 eV and integration times of 1000 ms Three scans were recorded consecutively and then averaged to minimize the noise Real space images of the


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