Single-Photon Ionization, in situ Optical Diagnostic of Molecular Beam Epitaxial Growth of GaAs
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ADINA K. OTT, SEAN M. CASEY*, APRIL L. ALSTRIN**, AND STEPHEN R. LEONE*** JILA, National Institute of Standards and Technology and University of Colorado, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 803090440 * National Research Council Postdoctoral Fellow, National Institute of Standards and Technology "**Presentaddress, Quantum, 2270 S. 88th St., Louisville, CO 80028 ***Staff member, Quantum Physics Division, National Institute of Standards and Technology
ABSTRACT Single-photon ionization time-of-flight mass spectrometry (SPI-TOFMS) is used in situ to monitor desorbing species and surface reactions during molecular beam epitaxy (MBE) of GaAs. In this method, the 1064 nm fundamental output of a Nd:YAG laser is tripled twice to produce 118 nm (10.5 eV) photons. The pulsed light is passed in front of a growing substrate, giving gaseous scattered molecules sufficient energy to ionize, but not fragment, them. Ionized species are detected with time-of-flight mass spectrometry. Arrangement of the experiment also allows for simultaneous real time monitoring with reflection high-energy electron diffraction (RHEED). Mass spectra are examined and analyzed to quantify fluxes and relative ionization cross sections of growth species. The real time behavior of arsenic and gallium mass signals during epitaxy is presented as a function of substrate temperature and incident gallium flux. Surface reactions are proposed to elucidate mechanisms of arsenic incorporation and compared to measured RHEED results.
INTRODUCTION In situ diagnostics of molecular beam epitaxy (MBE) are a valuable adjunct to semiconductor process control. The ability to monitor specifically the composition of the molecular beams during deposition of III-V and silicon materials could lead to better control over growth processes and, ultimately, higher device quality. Many such diagnostics have been introduced with the hope of yielding a record of the fluxes used in MBE, but there are drawbacks to some of these methods. For instance, nude ionization gauges and quartz crystal microbalances can quantify the flux of molecular beams, but they are not species specific. Hollow cathode discharge lamps and laserinduced fluorescence can detect a specific molecule, but these methods can monitor only one species at a time. Reflection mass spectrometry (REMS)' 2 can simultaneously monitor multiple specific molecules. However, the parent molecules can be fragmented by electron impact ionization in the mass spectrometer yielding complex interpretation of the spectra. A widely used in situ monitor of MBE is reflection high-energy electron diffraction (RHEED) which monitors the topography of the surface. Surface reconstructions can be inferred from the 101 Mat. Res. Soc. Symp. Proc. Vol. 406 01996 Materials Research Society
diffraction pattern, while the smoothness of the surface is directly related to the intensity of the specular beam. RHEED has been used to monitor the layer-by-layer deposition of Ill-V materials, 3 to investiga
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