Electron-molecular Beam Research Articles

Buried heterostructure laser diodes fabricated using in situ processing

An in situ process which includes electron cyclotron resonance plasma etching and molecular beam epitaxial regrowth is applied to the fabrication of buried heterostructure vertical cavity surface-emitting laser (SEL) diodes and edge-emitting laser (EEL) diodes. The buried SEL with a 7.5 μm diameter has a pulsed laser threshold current of 1 mA, and a threshold voltage of 4 V with a peak power of 0.9 mW. The buried EEL with 2.5 μm stripe width and 800 μm cavity length has a threshold current density of 500 A/cm2.

Spectroscopy and dynamics of jet-cooled hydrazines and ammonia. II. Electron-impact dissociative ionization

Dissociative-ionization cross sections, fragment appearance potentials, and fragment kinetic energies were measured for electron-impact excitation of jet-cooled NH3, hydrazine (N2H4), and monomethyl hydrazine (MMH) over an energy range of 10–270 eV. A data base of 35 parent and fragment ions is reported. All measurements were made in a crossed electron–molecular beam apparatus using pulsed extraction and time-of-flight mass detection to ensure field-free excitation and high collection efficiency for energetic ions. Cross sections for NH3 ionization are in good agreement with previous measurements except for ions with high kinetic energy (KE). These discrepancies are attributed to instrument-dependent KE detection efficiencies in the previous results. Cross section data have not been previously reported for N2H4 and MMH. The measured cross sections for total ionization at 70 eV are 2.35 Å2 (NH3), 3.76 Å2 (N2H4), and 4.20 Å2 (MMH). KE distributions were measured by an ion deflection method and gave results consistent with time-of-flight peak-shape analysis. Mean KE values <εt≳ are reported for all fragment ions studied. For 170-eV excitation of NH3, <εt≳ varied from 0.026 eV (NH2+) to 1.4 eV (H+). The kinetic energies for N2H4 and MMH fragment ions at similar excitation energies are typically much lower than for fragment ions from NH3, conforming to statistical arguments based on density of internal states. High resolution mass spectra were recorded for MMH in order to distinguish different fragment ions of the same unit mass. Substantial rearrangement is evident for N2H4 and MMH dissociative ionization based on the appearance of ions such as NH3+ and NH4+ (the latter for MMH ionization only) and the magnitude of <εt≳ for certain ions. The role of electronic structure and geometry on dissociation is explored using a molecular orbital analysis to predict product correlations for the excited states of N2H4+.

Elastic scattering of low-energy electrons from ammonia

The authors report absolute differential cross-section measurements for vibrationally elastic electron scattering from NH3 at incident energies from 2-30 eV. The present results, from a crossed electron-molecular beam apparatus, represent the first comprehensive experimental attempt to quantify the elastic electron-NH3 scattering process. At each energy studied they have integrated their differential cross-section data to generate total elastic and elastic momentum transfer cross sections and a critical comparison of both their differential and integral cross sections against previous experiment and theory is provided. They also report their observation of a strong Feshbach resonance in the elastic channel at energy of 5.59+or-0.05 eV.

Fabrication of sub-50 nm finger spacing and width high-speed metal–semiconductor–metal photodetectors using high-resolution electron beam lithography and molecular beam epitaxy

Using high-resolution electron beam lithography, we have fabricated metal–semiconductor–metal photodetectors with sub-50 nm finger spacing and finger width on GaAs grown by molecular beam epitaxy, which are, to our knowledge, the smallest ever reported. dc measurements showed that they have low dark current and high sensitivity. Proper scaling of the detectors to reduce the finger resistance and detector capacitance and to increase detector speed was studied. The resistances of thin metal lines with various widths were measured and compared with the value calculated from resistivity for bulk metal. Monte Carlo simulation demonstrates that for the photodetectors with 30 nm finger spacing and width, the response time is below picosecond and the cut-off frequency is over 1 THz.

Dissociative ionisation of O2 and CO2 by electron impact: Energy and angular distributions of fragments

The mass spectrometric apparatus and the method of studying the processes of dissociative ionisation of atmospheric molecules under collisions with electrons in crossed electron and gas dynamical molecular beams are described. The energy spectra and the angular distributions of O+(O2, CO2) and CO+(CO2) ionised fragments were measured over the incident electron energy range 45-150 eV. On the basis of the known potential curves and the appearance potentials using the measured energy and angular distributions the possible mechanisms of ionised fragment production are discussed. It is shown that, besides the main process of CO2+ predissociation, the fraction of ionised fragments is produced in the process of direct dissociation.

Engineering lateral quantum interference devices using electron beam lithography and molecular beam epitaxy

Various new lateral quantum interference devices (QIDs) have been facbricated using high resolution electron beam lithography and molecular beam epitaxy. These QIDs utilize various nanometer scale electrodes to introduce electrostatic potential barriers in the electron gas at an AlGaAs/GaAs heterojunction. At low temperatures, as the electron mean free path approaches the dimensions of the gate electrode, quantum interference of the electron wave with the barriers and wells creates quasibound states that result in conductance oscillations as the gate voltage is scanned. High resolution electron beam lithography is used to define the nanometer gates, readily allowing the definition and alignment of different electrodes on the same wafer. A planar implantation isolation step is used to facilitate the liftoff of the Ti/Au gates patterned in polymethyl methacrylate (PMMA). Several novel QIDs will be described; emphasis will be on a new lateral double barrier resonant tunneling diode formed by two 80 nm gates separated by 100 nm. At 4.2 K, structure in the drain current is seen as the gate voltage is scanned for drain voltages less than 5 mV.

RHEED study of superstructures of submonolayer lead films on silicon (111) surfaces

Coverage and temperature dependences of Pb/Si(111) surface structure have been investigated using reflection high energy electron diffraction (RHEED) and molecular beam deposition techniques. With the deposition of Pb on a clean Si(111) surface in the temperature range between 30 and 500 ° C, sequential changes in the RHEED pattern were observed in situ and thus a phase diagram of superstructures of submonolayer lead films on Si(111) surface could be established.

RHEED observation of Pb/Si (111) surface superstructures.

Pb/Si (111) surface superstructures have been studied using reflection high energy electron diffraction (RHEED) and molecular beam deposition techniques. The dependences of Pb/Si (111) surface structures on the Pb coverage and on the substrate temperature were elucidated on the basis of the RHEED observation in the coverage range from 0ML to 2ML and in the temperature range from 30°C to 500°C.In addition, we have measured changes in the RHEED intensities during Pb deposition. The results of the RHEED intensity measurements show that the saturation coverage for √3×√3 (α) and √3×√3 (β) superstructures are 4/3ML and 1/3ML, respectively.

The adsorption and decomposition of N 2O on nickel (100)

The adsorption and decomposition of N 2O on clean and oxygen covered Ni(100) surfaces has been studied using a combination of Auger electron spectroscopy (AES) and molecular beam relaxation spectroscopy (MBRS) techniques. As observed in a previous study of this reaction on the Ni(110) surface, N 2O decomposes to yield N 2 gas and adsorbed O at temperatures between 200 and 800 K. Measurements at temperatures below 200 K led to the identification of two weakly adsorbed precursor species, one on clean surfaces and the other on surfaces covered with 0.25 ML of adsorbed O. The adsorption rate constants measured for these two species are consistent with values inferred indirectly in the previous study.

Decomposition of N2O on Ni (110)

The catalytic decomposition of N2O to yield N2 gas and adsorbed oxygen has been studied on the Ni (110) surface over the temperature range from 323 to 873 K. The combination of Auger electron spectroscopy and molecular beam mass spectrometry has been used to determine the overall rates of production of adsorbed oxygen and N2 gas, and the scattering behavior of unreacted N2O. These data have been analyzed to determine the single collision reaction probability as a function of surface temperature and adsorbed oxygen coverage. The reaction procedes from a zero‐coverage reaction probability of from 0.97 to 0.52 depending on temperature, to zero reaction probability at an adsorbed oxygen coverage of 1/3 ML. The behavior at partial oxygen coverage is strongly influenced by the presence of a molecular precursor species, especially at low temperatures.

Comparison of Thai's theory with experimental boron doping profiles in silicon, diffused from boron nitride sources: K. P. Frohmader and L. Baumbauer. Solid-St. Electron.23, 1263 (1980)

Dissociative electron attachment (DEA) to perfluorohexane (C6F14), perfluoro(methylcyclo-hexane) (C6F11CF3) and perfluorocyclohexanecarbonyl chloride (C6F11C(O)Cl) is studied in a crossed electron-molecular beam experiment with mass spectrometric detection of the fragment anions. We find that all molecules undergo effective decomposition in the subexcitation energy range (0–3 eV) yielding ions that arise from direct bond cleavages creating a fragment anion and its neutral counterpart but also from more complicated reactions due to substantial rearrangement in the transient anion formed upon resonant electron capture. All three compounds preferentially decompose into closed shell anions of the form CnF2n+1−. While in perfluorocyclohexanecarbonyl chloride (C6F11C(O)Cl) DEA into Cl− represents the most intense reaction, formation of F− is in any case comparatively weak. The resonance positions for F− formation show specific features for the three compounds which may allow a correlation between the location of the DEA resonance and the site from which the F− ion originates.

Diffusion of boron into silicon from doped oxide source: P. M. Prasad and V. P. Sundarsingh. Microelectron. J.11 (6), 21 (1980)

Dissociative electron attachment (DEA) to perfluorohexane (C6F14), perfluoro(methylcyclo-hexane) (C6F11CF3) and perfluorocyclohexanecarbonyl chloride (C6F11C(O)Cl) is studied in a crossed electron-molecular beam experiment with mass spectrometric detection of the fragment anions. We find that all molecules undergo effective decomposition in the subexcitation energy range (0–3 eV) yielding ions that arise from direct bond cleavages creating a fragment anion and its neutral counterpart but also from more complicated reactions due to substantial rearrangement in the transient anion formed upon resonant electron capture. All three compounds preferentially decompose into closed shell anions of the form CnF2n+1−. While in perfluorocyclohexanecarbonyl chloride (C6F11C(O)Cl) DEA into Cl− represents the most intense reaction, formation of F− is in any case comparatively weak. The resonance positions for F− formation show specific features for the three compounds which may allow a correlation between the location of the DEA resonance and the site from which the F− ion originates.

Final-state effects in the excitation spectra of deep impurities in semiconductors: Sokrates T. Pantelides and Hermann G. Grimmeiss. Solid St. Commun.35, 653 (1980)

Dissociative electron attachment (DEA) to perfluorohexane (C6F14), perfluoro(methylcyclo-hexane) (C6F11CF3) and perfluorocyclohexanecarbonyl chloride (C6F11C(O)Cl) is studied in a crossed electron-molecular beam experiment with mass spectrometric detection of the fragment anions. We find that all molecules undergo effective decomposition in the subexcitation energy range (0–3 eV) yielding ions that arise from direct bond cleavages creating a fragment anion and its neutral counterpart but also from more complicated reactions due to substantial rearrangement in the transient anion formed upon resonant electron capture. All three compounds preferentially decompose into closed shell anions of the form CnF2n+1−. While in perfluorocyclohexanecarbonyl chloride (C6F11C(O)Cl) DEA into Cl− represents the most intense reaction, formation of F− is in any case comparatively weak. The resonance positions for F− formation show specific features for the three compounds which may allow a correlation between the location of the DEA resonance and the site from which the F− ion originates.

The oxidation of graphitic monolayers on Ni(110)

The reaction between oxygen gas and a graphitic carbon monolayer formed on an initially clean Ni(110) surface has been studied by a combination of Auger electron spectrometric and molecular beam relaxation spectrometric techniques. The only gaseous reaction product observed is CO. The kinetics of the reaction at 873 K are well described by a model involving oxygen adsorption at defects in the graphitic monolayer followed by reaction to produce CO at these points and the subsequent growth of holes in the carbon layer as reaction proceeds.

Hydrogen and Nitrogen Desorption Phenomena Associated with a Stainless Steel 304 Low Energy Electron Diffraction (LEED) and Molecular Beam Assembly

The rates of hydrogen desorption from the walls of a standard LEED stainless steel vacuum chamber isolated from the ion pump have been measured using an oxygen-free copper orifice of known conductance, which acts as the final focussing slit in a molecular beam assembly, and found unacceptably high for the maintenance of a clean metal single crystal surface. The causes are discussed and the rates of desorption are presented after the stainless steel apparatus had been subjected to vacuum degassing at 850–950 °C in a vacuum furnace at pressures ≤10−5 Torr. Desorption rates are decreased ∼130 times. Rates of desorption of nitrogen from Varian post-acceleration type two and three grid LEED optics screens (P-4 CR 422 phosphor) have been measured. Possible displacement reactions with residual hydrogen, small quantities of which are adsorbed during nitrogen desorption, are discussed. It is concluded that the two processes are independent of each other. Contamination of the single crystal samples with the nitrogen desorbed for the LEED screen is also discussed. Brief reference is made to the adsorption properties of stainless steel 304 with special reference to hydrogen and nitrogen.

Thermospheric Temperature Measurement Technique

A method for measurement of temperature in the earth’s lower thermosphere from a high-velocity probe is described. All undisturbed atmospheric sample is admitted to the instrument by means of a free molecular flow inlet system of skimmers which avoids surface collisions of the molecules prior to detection. Measurement of the time-of-flight (TOF) distribution of an initially well-localized group of nitrogen metastable molecular states produced in an open, crossed electron-molecular beam source, yields information on the atmospheric temperature. It is shown that for high vehicle velocities, the time-of-flight distribution of the metastable flux is a sensitive indicator of atmospheric temperature. The temperature measurement precision should be greater than 94% at the 99% confidence level over the range of altitudes from 120–170 km. These precision and altitude range estimates are based on the statistical consideration of the counting rates achieved with a multichannel analyzer using realistic values for system parameters.