Robert C. Wetzel

Electron impact ionization cross sections of SiF2

Absolute cross sections are measured for electron impact ionization and dissociative ionization of SiF2 from threshold to 200 eV. A fast (3 keV) neutral beam of SiF2 is formed by charge transfer neutralization of SiF+2 with Xe; it is primarily in the ground electronic state with about 10% in the metastable first excited electronic state (a 3B1). The absolute cross section for ionization of the ground state by 70 eV electrons to the parent SiF+2 is 1.38±0.18 A2. Formation of SiF+ is the major process with a cross section at 70 eV of 2.32±0.30 A2. The cross section at 70 eV for formation of the Si fragment ion is 0.48±0.08 A2. Ion pair production contributes a significant fraction of the positively charged fragment ions.

Absolute cross sections for electron-impact ionization and dissociative ionization of the SiF free radical

Absolute cross sections for electron‐impact ionization of the SiF free radical from threshold to 200 eV are presented for formation of the parent SiF+ ion and the fragment Si+ and F+ ions. A fast beam of SiF is prepared by charge transfer neutralization of an SiF+ beam. The radicals form in the ground electronic state and predominantly in their ground vibrational state, as shown by agreement of the measured ionization threshold with the ionization potential. The absolute cross section for SiF→SiF+ at 70 eV is 3.90±0.32 A2. The ratio of cross sections for formation of Si+ to that for SiF+ at 70 eV is 0.528±0.024; the ratio for formation of F+ to that of SiF+ is 0.060±0.008. The observed threshold energy for Si+ formation indicates the importance of ion pair formation SiF→Si++F−. Breaks in the cross section at 14.3 and 17 eV are assigned as dissociative ionization thresholds.

Electron‐impact ionization cross sections of the SiF3 free radical

Absolute cross sections for electron‐impact ionization of the SiF3 free radical from threshold to 200 eV are presented for formation of the parent SiF+3 ion and the fragment SiF+2, SiF+, and Si+ ions. A 3 keV beam of SiF3 is prepared by near‐resonant charge transfer of SiF+3 with 1,3,5‐trimethylbenzene. The beam contains only ground electronic state neutral radicals, but with as much as 1.5 eV of vibrational energy. The absolute cross section for formation of the parent ion at 70 eV is 0.67±0.09 A2. At 70 eV the formation of SiF+2 is the major process, having a cross section 2.51±0.02 times larger than that of the parent ion, while the SiF+ fragment has a cross section 1.47±0.08 times larger than the parent. Threshold measurements show that ion pair dissociation processes make a significant contribution to the formation of positively charged fragment ions.

Vertical-cavity surface-emitting laser diodes fabricated by in situ dry etching and molecular beam epitaxial regrowth

The authors report the first buried active region vertical-cavity surface-emitting laser diodes fabricated using in situ dry etching and molecular beam epitaxial regrowth. The laser emissions of the etched/regrown devices persist over a greater current range and exhibit maximum output powers larger than air-post lasers. The lasers are anisotropically etched into the lower monolithic distributed Bragg reflector using an electron cyclotron resonance SiCl/sub 4/ plasma etch. After transfer in ultra-high vacuum, epitaxial AlGaAs current blocking layers are regrown around the etched mesas. Polycrystalline deposition on the SiO/sub 2/ mask is removed by reactive ion etching to allow electrical contact and top surface emission. The etched/regrown laser characteristics demonstrate efficient current confinement and low thermal impedance. The vacuum integrated processing described offers the prospect of further device performance enhancements and greater functionality.<<ETX>>

Electron cyclotron resonance plasma preparation of GaAs substrates for molecular beam epitaxy

We report the preparation of GaAs substrates, using only electron cyclotron resonance plasma techniques, and the subsequent growth of GaAs by molecular beam epitaxy, accomplished within an integrated processing facility. Exposure of the substrates to a hydrogen plasma with an ion current density less than 3 mA/cm2 for 1 h removes the native GaAs oxide leaving a smooth crystalline surface as revealed by streaky (1×1) reflection high energy electron diffraction patterns. At a greater ion current density a spotty diffraction pattern is obtained; a subsequent SiCl4 plasma etch restores a streaky (1×1) diffraction pattern. After plasma processing, evidence of surface reconstruction is observed at substrate temperatures greater than 400 °C in an As overpressure and during GaAs overgrowth. Impurity concentrations at the epilayer/substrate interfaces of plasma‐prepared samples are found to be comparable to those of chemically prepared wet etched substrates. This vacuum substrate preparation scheme is a first step...

Hydrogen plasma processing of GaAs and AlGaAs

Hydrogen plasma processing of GaAs and AlGaAs using an electron cyclotron resonance plasma reactor, which is vacuum‐linked to a molecular‐beam epitaxial (MBE) growth chamber is reported. Native oxide removal and surface cleaning of GaAs is characterized using hydrogen plasma processing, subsequent thermal Cl2 etching, and vacuum annealing. It is shown that surface reconstruction and excellent GaAs/GaAs interfaces can be achieved using these dry vacuum procedures. It is also shown that AlxGa1−xAs native oxides can be removed for 0≤x≤1 using hydrogen plasma processing before MBE overgrowth. The best AlGaAs/AlGaAs interfaces are obtained using low microwave power during hydrogen plasma processing. O and C impurities detected at these interfaces increase with higher Al composition; Si interface impurities tend to increase with higher microwave power. In general, hydrogen plasma processing is judged effective for surface preparation before MBE growth for the complete range of AlGaAs alloys.

Electron cyclotron resonance plasma etching using downstream magnetic confinement

We demonstrate that electron cyclotron resonance plasma etching with a magnetically confined plasma in the region of the sample produces an enhanced etch rate, an anisotropic etch profile, and low self‐bias voltage. Results are presented for GaAs etch rates, etch profiles, and macroscopic etch uniformity using a SiCl4 plasma, comparing the effects of a confining magnetic field and a diverging magnetic field in the reactor. The etch rates and saturated ion current density to the sample are found to be correlated. An anisotropic near vertical etch profile with smooth‐etched surfaces is obtained with a negative self‐bias voltage of typically 5–25 V for electrically floating samples in a magnetically confined plasma. When the magnetic field lines are perpendicular to the sample surface, the measured macroscopic etch uniformity is ±6% across a 5 cm diam wafer.

In-situ process for AlGaAs compound semiconductor: materials science and device fabrication

Processing of III-V compound semiconductor devices in an ultra-high vacuum or a controlled environment has received much attention during the past few years. Major advantages ofn- situ processing include the preservation of pristine material surface, improved device performance, and fabrication of novel devices. This paper reviews anin- situ process compatible with molecular beam epitaxy (MBE) with emphasis on the removal of oxides and surface contaminants from air-exposed GaAs and AIGaAs. We have characterized deep-etched and MBE regrown AIGaAs with the etching achieved using electron cyclotron resonance plasma treatment. A buried heterostructure vertical-cavity surface emitting laser diode fabricated using thisin- situ process is presented.

Vertical-cavity surface-emitting lasers fabricated by vacuum integrated processing

The authors report on the fabrication of vertical-cavity surface-emitting lasers (VCSELs) using vacuum processing techniques. The upper monolithic distributed Bragg reflector around the laser cavity is dry etched down to the top of the active region, followed by in situ contact deposition on the mesa sidewall, providing a short current path through the p-type mirror. These etched VCSELs exhibit lower series resistance, lower threshold voltage, greater thermal dissipation, and higher maximum output power than conventional planar VCSELs made from the same material.<<ETX>>

In situ deposition of Au on plasma-prepared GaAs substrates

In situ deposition of single crystal epitaxial and textured polycrystalline gold films on plasma-cleaned or plasma-etched GaAs substrates is accomplished in an ultrahigh vacuum integrated processing facility. Au/GaAs samples are characterized using reflection high energy electron diffraction, Auger electron spectroscopy, and ion channeling. Au crystallinity in films deposited at 100° C is shown to strongly depend on the GaAs surface cleanliness after plasma processing. Heating the substrate to 250° C after plasma processing subsequently yields epitaxial Au films; omitting the heating procedure results in polycrystalline Au films. The substrate thermal treatment removes residual physisorbed gas molecules and reaction products from the GaAs surface. Epitaxial Au films contain significantly less Ga and As on the free surface of Au than polycrystalline films, and no interaction between epitaxial Au and GaAs is observed.