V. P. LaBella

Enhancing the Student‐Instructor Interaction Frequency

A 100-fold increase in the frequency of student–teacher interaction has been achieved in a large-enrollment classroom. Students answer in-class questions using personalized hand-held transmitters. Outside the classroom, personalized homework sets are generated and collected via the Internet.

Observation of crystallite formation in ferromagnetic Mn-implanted Si

Mn-implanted Si was investigated using transmission electron microscopy to gain insight into the structure of the implanted region. Diffraction contrast images, selected area diffraction patterns, and high resolution images of the samples were acquired before and after postimplant annealing at 800°C. The images of the annealed samples revealed the formation of nanometer size precipitates distributed throughout the implanted region. Analysis of the selected area diffraction pattern determined that the most prominent lattice spacing of the crystallites is 2.15A. This spacing indicates that the most probable phase of the crystallites is MnSi1.7 and this is consistent with the Mn:Si binary phase diagram. This phase is paramagnetic at room temperature with a Curie temperature of 47K and cannot readily account for the high Curie temperature of the material.Mn-implanted Si was investigated using transmission electron microscopy to gain insight into the structure of the implanted region. Diffraction contrast images, selected area diffraction patterns, and high resolution images of the samples were acquired before and after postimplant annealing at 800°C. The images of the annealed samples revealed the formation of nanometer size precipitates distributed throughout the implanted region. Analysis of the selected area diffraction pattern determined that the most prominent lattice spacing of the crystallites is 2.15A. This spacing indicates that the most probable phase of the crystallites is MnSi1.7 and this is consistent with the Mn:Si binary phase diagram. This phase is paramagnetic at room temperature with a Curie temperature of 47K and cannot readily account for the high Curie temperature of the material.

Annealing temperature effects on the structure of ferromagnetic Mn-implanted Si

The dependence of the magnetization of Mn-implanted Si on the postimplant annealing temperature is studied. p-type Si wafers were implanted with 300keV Mn+ ions at 350°C to a fluence of 1×1016cm−2 and then annealed at 500–900°C for 5min. Ferromagnetic hysteresis loops were obtained at 10K using a superconducting quantum interference device magnetometer. The saturation magnetization increases with the postimplant annealing temperature, reaching an optimum field strength of 0.2emu∕g at 800°C. An out diffusion of Mn is observed at higher temperatures that coincides with a decrease in the saturation magnetization. The calculated point-defect profile that was generated by the implantation process peaks around the Mn-depleted region, suggesting that the residual implant damage may play a role in the ferromagnetic behavior of Mn-implanted Si.

Activation energy for Ga diffusion on the GaAs(0 0 1)-(2×4) surface: an MBE-STM study

The pure migration of individual Ga atoms on the technologically important GaAs(0 0 1)-(2]4) reconstructed surface has been studied as a function of substrate temperature using a combined molecular beam epitaxy and scanning tunneling microscopy (STM) ultra-high vacuum, multi-chamber facility. We have successfully deposited 1 10 of a plane of Ga atoms onto a pristine GaAs surface under a constant As 4 beam equivalent pressure of 10~6 Torr, at various substrate temperatures. After deposition the substrate was quenched to room temperature and transferred to the surface analysis chamber for STM imaging. A plot of the number density of islands formed as a function of deposition temperature follows an Arrhenius relationship. Assuming either a pure one-dimensional di!usion model or a pure isotropic two-dimensional di!usion model, the activation energy for di!usion is 2.3 or 1.7 eV, respectively. ( 1999 Elsevier Science B.V. All rights reserved.

Combined molecular beam epitaxy low temperature scanning tunneling microscopy system: Enabling atomic scale characterization of semiconductor surfaces and interfaces

A molecular beam epitaxy and low temperature scanning tunneling microscopy chamber have been integrated to characterize both compound and elemental semiconductor surfaces and interfaces. The integration of these two commercially available systems has been achieved using a custom designed sample transfer mechanism. The MBE growth chamber is equipped with electron diffraction and provides substrate temperature measurements and control by means of band-edge thermometry accurate to within ±0.5°C. In addition, the microscope can operate at temperatures as low as 4K and perform ballistic electron emission microscopy measurements. The chamber that houses the microscope includes a preparation chamber with an evaporation source for metals. The entire STM chamber also rests on an active vibration isolation table, while still maintaining an all ultrahigh vacuum connection to the MBE system.

Monte Carlo derived diffusion parameters for Ga on the GaAs(001)- (2×4) surface: A molecular beam epitaxy–scanning tunneling microscopy study

The migration of individual Ga atoms on the technologically important GaAs(001)-(2×4) reconstructed surface has been studied as a function of substrate temperature and As4 pressure using a combined molecular beam epitaxy and scanning tunneling microscope ultrahigh vacuum multichamber facility. We have deposited 10% of a plane of Ga onto a GaAs(001) surface with a low defect density ( 0.5 μm) to avoid the influence of surface defects like step edges and vacancies. Both the island number density and the geometry are measured and compared to Monte Carlo solid-on-solid simulations. Basic diffusion parameters, such as the activation energy, directional hopping-rate ratio, directional sticking-probability ratio, etc., are reported.

Role of As4 in Ga diffusion on the GaAs(001)-(2×4) surface: A molecular beam epitaxy-scanning tunneling microscopy study

The role of As4 molecules in Ga diffusion on the GaAs(001)-(2×4) reconstructed surface has been studied using a combined molecular beam epitaxy and scanning tunneling microscopy multichamber facility. We deposited 10% of a plane of Ga atoms onto an otherwise pristine surface, while exposed to two separate As4 beam equivalent pressures of 10−5 and 10−6 Torr. The higher As4 flux resulted in the production of fewer and larger islands, indicating that increasing the As4 flux increases the total interrogation area available to the Ga atoms before forming islands.

Reflection high-energy electron diffraction and scanning tunneling microscopy study of InP(001) surface reconstructions

The reconstructions of the InP(001) surface prepared by molecular beam epitaxy have been studied with in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). The growth chamber contains a highly accurate temperature measurement system and uses a solid-source, cracked phosphorus, valved effusion cell. Five InP(001) reconstructions are observed with RHEED by analyzing patterns in three principal directions. Under a fixed P2 flux, decreasing the substrate temperature gives the following reconstructions: c(2×8), (2×4), (2×1), (2×2), and c(4×4). In situ STM images reveal that only two of these reconstructions yields long-range periodicity in real space. InP(001) does not form the metal rich (4×2) reconstruction, which is surprising because the (4×2) reconstruction has been coined the universal surface reconstruction since all III–V(001) surfaces were thought to favor its formation.

Measurement of the hot electron attenuation length of copper

Ballistic electron emission microscopy is utilized to investigate the hot-electron scattering properties of Cu through Cu/Si(001) Schottky diodes. A Schottky barrier height of 0.64±0.02 eV and a hot-electron attenuation length of 33.4±2.9 nm are measured at a tip bias of 1.0 eV and a temperature of 80 K. The dependence of the attenuation length with tip bias is fit to a Fermi liquid model that allows extraction of the inelastic and elastic scattering components. This modeling indicates that elastic scattering due to defects, grain boundaries, and interfaces is the dominant scattering mechanism in this energy range.

Enabling electron diffraction as a tool for determining substrate temperature and surface morphology

The reconstruction transitions for the GaAs(001) surface have been identified as a function of the band gap-derived substrate temperature and As4 beam equivalent pressure. Surface morphology measurements using in situ scanning tunneling microscopy reveal that the surface spontaneously forms a random distribution of two-dimensional islands. The onset of island formation is coincident with the reflected high-energy electron diffraction pattern changing from the β to α subphase of the (2×4) reconstruction. An electron diffraction-based method for determining the substrate temperature and engineering the surface morphology with a desired amount of roughness is presented.