April L. Alstrin

Chemistry of arsenic incorporation during GaAs/GaAs(100) molecular beam epitaxy probed by simultaneous laser flux monitoring and reflection high-energy electron diffraction

Arsenic incorporation during GaAs/GaAs(100) molecular beam epitaxy is studied in situ with laser single‐photon ionization time‐of‐flight mass spectrometry and reflection high‐energy electron diffraction (RHEED). Incident and scattered fluxes of Ga and Asn species in front of the growing GaAs wafer are ionized repetitively by a pulsed laser beam of 118 nm (10.5 eV) photons. The methods to obtain and interpret time‐of‐flight mass spectra and the simultaneous RHEED measurements are described. The real time behaviors of incident Ga and desorbing As2 and As4, obtained without mass spectral cracking, are studied during growth of GaAs layers with As4 and when growth is arrested as a function of substrate temperature and Ga/As4 flux ratio. During growth only with As4, both As2 and As4 are desorbed or scattered in varying amounts depending on flux and substrate temperature conditions. Without an incident gallium flux, desorbing As4 decreases while desorbing As2 increases with increasing surface temperature. During...

Direct detection of atomic arsenic desorption from Si(100)

Application of the 118 nm single photon laser ionization technique to a molecular beam epitaxy machine is used for the first time to demonstrate direct desorption of As atoms from Si(100). Both As2 and As are the desorbing species from 1 ML of arsenic on silicon above 1000 K. This is in contrast to previously reported models that considered only dimer desorption. With a continuous flux of As4, the scattered and desorbing arsenic species from Si(100) are examined as a function of surface temperature (650–1200 K). Atomic desorption is large, 75%±19%, above 1000 K, and complete conversion of As4 to As2 and As occurs at 1200 K. The species selectivity of laser ionization time‐of‐flight mass spectroscopy has broader implications for GaAs growth.

Single-photon laser ionization time-of-flight mass spectroscopy detection in molecular-beam epitaxy: application to As4, As2, and Ga

Single-photon laser ionization time-of-flight mass spectroscopy (TOF-MS) is used to monitor fluxes of As(4), As(2), and Ga, species that are important in molecular-beam epitaxy of GaAs. With this technique, fluxes of multiple chemical species above a substrate can be measured noninvasively and in real time during conventional molecular-beam epitaxy. Additionally, the geometry of the single-photon ionization TOF-MS permits simultaneous film-growth monitoring by using techniques such as reflection highenergyelectron diffraction (RHEED). Here gas-phase arsenic and gallium beams are ionized by a single 118-nm (10.5-eV) photon and detected with a TOF-MS. The 118-nm photons are produced by frequency tripling 355-nm light from a pulsed Nd:YAG laser in Xe. With single-photon ionization, less than 0.4% ofthe As(4)(+) signal fragments to As(2)(+). Neither As(4)(+) nor As(2)(+) fragments to As(+) at 118 nm. The relative ionization probability of As(4)/As(2) at 118 nm is approximately 4:1. This technique promises to be a powerful tool for analyzing most III-V and II-VI molecular-beam epitaxy growth species.

Vibrational distributions of As2 in the cracking of As4 on Si(100) and Si(111)

The desorption dynamics of arsenic from Si(100) and Si(111) are studied by measuring the vibrational population distributions of desorbed As2 using laser‐induced fluorescence. In these measurements a steady state flux of desorbing As2 is produced by continuously dosing a heated Si surface with a beam of As4 from a conventional molecular beam epitaxy oven. Measurements of the fluxes of As2 as a function of surface temperature suggest that the As2 may be kinetically formed in two distinct steps: The As4 first chemisorbs to form atoms on the surface which then recombine to desorb as dimers. However, there may also be direct dissociation of As4 at the hot surface. The vibrational populations of As2 (v‘=0–3) desorbed from Si(100) are Boltzmann and indicate a vibrational temperature Tv that is nearly 350 K lower than the surface temperature Ts (Tv=790±100 K vs Ts=1140 K). The vibrational populations of As2 desorbed from Si(111) are not Boltzmann, but also have an effective vibrational temperature 400 K colder t...

Pulsed laser irradiation at 532 nm of In and Ga adsorbed on Si(100): Desportion, incorporation, and damage

Laser‐induced desorption (LID) of In and Ga from Si(100) under ultrahigh vacuum conditions is investigated. The frequency‐doubled 532 nm, 2–6 ns output of a Nd:YAG laser is focused to 0.14±0.03 J/cm2 on the Si surface to induce desorption. Desorbed In or Ga atoms are detected by laser‐induced fluorescence initiated by a second pulsed laser propagating in front of the surface. LID occurs by thermal desorption with approximate desorption energies and preexponential factors in agreement with the literature values obtained previously by isothermal desorption measurements. Experiments at higher coverages suggest that desorption occurs predominantly from the two‐dimensional (2D) adlayer with little desorption occurring from the adsorbate islands directly into the vacuum. The 2D layer is resupplied by either diffusion out of adsorbate islands or by diffusion of incorporated adsorbate out of the bulk. Adsorbate‐enhanced laser‐induced surface damage is also observed; only 0.2 monolayer of In reduces the number of ...

Single-photon ionization and detection of Ga, In, and Asn species in GaAs growth

In this paper, single photon ionization time-of-flight mass spectroscopy (SPI-TOFMS) is used to monitor chemical fluxes of In, Ga, and Asn, relevant in molecular beam epitaxy of GaAs. With single photon ionization at 118 nm (10.5 eV), the photon energy is large enough to ionize the species, but not sufficient to ionize and fragment. The lack of molecular dissociation of species such as As2 and As4 greatly simplifies the interpretation of mass spectra. SPI-TOFMS provides the ability to measure densities, and hence fluxes, of multiple chemical species above a substrate noninvasively and in real time during conventional molecular beam epitaxy. The relative ionization efficiencies of Ga and the Asn species at 118 nm are determined. Additionally, this laser probing technique is used to study the isothermal and temperature programmed desorption of arsenic from Si(100). The catalytic cracking of As4 on Si is also examined and discussed. This technique promises to be a valuable in-situ optical diagnostic for III-V and II-VI molecular beam epitaxy.

Laser vacuum ultraviolet single photon ionization probing of III-V semiconductor growth

Epitaxial growth of III-V semiconductor materials is probed in a molecular beam epitaxy reactor by single photon ionization of the gaseous fluxes using vacuum ultraviolet (VUV) laser radiation. The ninth harmonic of the Nd:YAG laser is produced by frequency tripling the output to 355 nm and then to 118 nm in a Xe/Ar mixture. Together with a time-of-flight mass spectrometer, this radiation is used to selectively probe the gaseous fluxes of Ga, As, As2, and As4 during molecular beam epitaxy of III-V materials. The essential aspects of the method and details of calibration procedures to obtain relative fluxes are described. Cracking of the arsenic species does not occur in the laser/mass spectrometer, making relative species concentration measurements very reliable. Rapid data acquisition provides real time measurements of the fluxes of incident and scattered or desorbed materials during growth. Several basic examples are considered, including the thermal cracking of As4 on silicon and the desorption of arsenic and gallium species from GaAs during epitaxial growth. Recent work to correlate the flux determinations with reflection high energy electron diffraction (RHEED) oscillations during GaAs epitaxial growth is discussed.

In-situ monitoring of molecular beam epitaxial growth using single photon ionization

Single photon laser ionization time-of-flight mass spectroscopy is used to monitor the gaseous fluxes of Ga, As2, and As4, which are relevant in molecular beam epitaxy (MBE) of GaAs. This noninvasive and real-time technique measures densities, and hence fluxes, of multiple chemical species impinging on or scattered from a substrate during conventional MBE. With single photon ionization at 118 nm (10.5 eV), the energy is sufficient to ionize the species, but insufficient to ionize and fragment. The lack of molecular fragmentation greatly simplifies the interpretation of mass spectra. Additionally, the probe geometry permits simultaneous film growth monitoring using RHEED. Results will be presented on the probing of scattering and desorption of III-V MBE species during GaAs growth. This technique promises to be a valuable in-situ diagnostic for III-V and II-VI MBE.

Single photon ionization, laser optical probe technique for semiconductor growth

Single Photon Ionization Time-of-Flight Mass Spectroscopy (SPI-TOFMS) is used as an in situ optical characterization technique to monitor chemical reactions occurring at semiconductor surfaces during molecular beam epitaxial (MBE) growth be detecting gaseous species. In this approach, 118 nm (10.5 eV) laser photons are generated and passed on front of a semiconductor substrate in the ultra-high vacuum (UHV) chamber. Here, the photons ionize the gaseous scattered and desorbed growth species which are detected by time-of-flight mass spectroscopy. The photons are produced by frequency tripling the fundamental Nd:YAG output to 355 nm and tripling again in a static cell of Xe/Ar to 118 nm. The 10.5 eV photons have sufficient energy to ionize III-V species of interest, but not fragment them, allowing simple interpretation of mass spectra. Gated boxcars allow for rapid data acquisition of growth species in real time. SPI-TOFMS has been used to study Asn/Si(100) desorption kinetics and, more recently, MBE growth of GaAs. Results are presented on the real-time monitoring of Gan and Asn growth species. Simultaneous monitoring of growth with Reflection High-Energy Electron Diffraction (RHEED) is also discussed. Future work includes SPI_TOFMS studies of Si delta-doping in GaAs and surfactant-enhanced epitaxy of Ge on Si. SPI-TOFMS is an in situ UHV optical probe used to study the growth chemistry of semiconductor surfaces. This noninstrusive, species-specific real-time monitor of growth can be applied to increase the quality of device manufacturing.

Single Photon Laser Ionization as an In-Situ Diagnostic for MBE Growth

Single photon laser ionization time-of-flight mass spectroscopy (SPI-TOFMS) is used to monitor the gaseous fluxes of Ga and As n , during molecular beam epitaxy of GaAs. This noninvasive and real-time probe measures densities, and hence fluxes, of multiple chemical species impinging on or scattered from a substrate during conventional MBE. With single photon ionization at 118 nm (10.5 eV, ninth harmonic of Nd:YAG laser), the photon energy is large enough to ionize the species, but insufficient to both ionize and fragment. The lack of molecular dissociation of As 2 and As 4 greatly simplifies the interpretation of mass spectra. Additionally, the geometry of the single photon ionization TOFMS permits simultaneous film growth monitoring using RHEED. Results will be presented on the probing of scattering and desorption of III-V MBE species during GaAs growth. This technique promises to be a valuable in-situ diagnostic for III-V and II-VI MBE.