Colin E. C. Wood

Re‐evaporation effects and optical properties of molecular‐beam‐epitaxial AlGaAs/GaAs/AlGaAs wells

Elemental materials that condense on surfaces near effusion cells can be reevaporated toward substrates when heated by radiation from effusion furnaces. There they accumulate as unwanted impurities at interfaces and distribute throughout epitaxial films during growth. This effect is greatly increased when shutters are closed. Re‐evaporated aluminum is shown to degrade minority‐carrier properties of Al0.3Ga0.7As/GaAs double heterostructures. Modified temperature schedules and hardware to reduce re‐evaporation effects are suggested.

Dopant-surface migration and interactions from reflection high-energy electron diffraction dynamics

The effect of silicon and beryllium atom coverage on the intensity of electrons specularly reflected from vicinal (100) GaAs is reported. Intensities typically drop to minima when concentrations approach gallium step‐site densities, and increase to broad maxima associated with changes in dominant reconstruction order. Transient relaxation effects are also reported together with possible applications to surface migration kinetics, dopant flux calibrations, and determination of misorientation angles.

GaAs-AlGaAs QW diluted waveguide laser with low-loss, alignment-tolerant coupling to a single-mode fiber

We report on low-loss, alignment-tolerant coupling between a single-quantum-well (SQW) GaAs-AlGaAs laser and a cleaved single-mode fiber. The laser was fabricated using conventional growth and processing techniques, and did not involve any regrowth. The mode size in the transverse direction was expanded by using a diluted waveguide structure. A butt-coupling efficiency of 66% was achieved against a theoretically possible 78% (which does not correct for Fresnel losses). In addition, large alignment tolerances for a 1-dB excess loss of /spl plusmn/2.0 pm and /spl plusmn/1.0 /spl mu/m were measured in the lateral and the transverse directions, respectively.

Differential photo-voltage spectroscopy for characterizing epitaxial multilayered and quantum well structures

An improved photo-voltage spectroscopy (PVS) technique, capable of accurately characterizing multilayered and quantum well structures, is presented. The technique is developed by noise reduction and differentiation of the photovoltage signal which improves the accuracy and sensitivity of the standard photo-voltage spectroscopy dramatically. The resulting differential-photo-voltage spectroscopy is capable of measuring the energy gap (Eg) and hence the composition, of ternary and quaternary compounds in multilayered structures at room temperature, with a substantially higher degree of confidence than the standard PVS technique. Several structures have been examined successfully. Two representative AlGaAs/GaAs structures are reported here to establish the capabilities of this technique. These structures were found to provide values for Eg and Al mole fraction, in excellent agreement with both the targeted values based on reflective high-energy electron diffraction oscillations during molecular beam epitaxial growth, and the measured values from variable angle ellipsometry. Numerous exciton transitions from quantum wells are clearly resolved and shown to be in excellent agreement with the theoretically predicted ones.

Phosphorus cracking efficiency and flux transients from a valved effusion cell

The transient behavior of the flux from a valved effusion cell containing red phosphorus is shown to result from a low elemental sublimation coefficient (α=1.3×10−7) and not from the presence of white phosphorus in the sublimator region. The criteria for generating white phosphorus in the sublimator are given. The conversion from tetramers to dimers is modeled as a function of temperature and pressure. At higher pressures, higher temperatures are needed to convert P4 to P2. Experimental results are shown to be in qualitative agreement with results from thermodynamic equilibrium arguments.

Molecular beam flux measurements and equilibria for As4, P4, As2, P2, and AsP

The dynamic equilibrium between P2, As2, and AsP at 1000 °C has been studied by mass spectrometry of fluxes from a double effusion cell. From simple ion gauge measurements of heated and unheated fluxes we determined the ionization efficiency for the dimers P2 and As2 relative to the tetramers, P4 and As4. Simultaneous mass spectrometer and ion gauge measurements of the effusion fluxes allowed the relative ionization sensitivities for P2, As2, and AsP to be determined. From the mass spectrometry data it was possible to determine the equilibrium constant 1.9±0.4 and free energy −0.55 eV molecule−1 for the process As2+P2■2 AsP.

Wavelength conversion using a T-gate laser

Wavelength conversion from 830 nm to 837 nm of a 250 Mb/s NRZ optical signal has been demonstrated at 10/sup -9/ bit-error rate and a detector sensitivity of -29 dBm using a semiconductor power amplifier monolithically integrated at 90/spl deg/ with a GaAs-AlGaAs SQW semiconductor laser. This device offers a high degree of isolation between the input and the output and has the potential for high-speed operation with a wide continuous-wavelength conversion range.

Two-dimensional molecular beam epitaxy of {001} Cdte on Cd and Zn terminated {001} GaAs

Amorphous layers of CdTe deposited on Cd or Zn terminated GaAs {001} surfaces can be recrystallized above ∼200°C. Subsequent molecular beam epitaxy of CdTe proceeds in a two-dimensional mode and leads to layers which are specular and single domain {0011}. Threading dislocation density in these layers was 1–2 x 105 cm−2. Values of full width at half maximum for x-ray rocking curves were as low as 80 arc-s.

Crystallized amorphous deposits for relaxed epitaxy: CdTe(001) on GaAs(001)

When annealed, thin amorphous deposits of highly lattice mismatched materials provide specular crystalline surfaces for epitaxy. Mismatch strain is predominantly relieved by misfit dislocations propagating in the plane of the interface, so that resulting films have low threading dislocation densities. We demonstrate the application of this concept to the growth of (001) oriented CdTe epitaxy on (001) GaAs.

Gallium acceptor incorporation in molecular beam epitaxial germanium on GaAs

Conditions for ≤1021 cm−3 gallium doped p‐type germanium films on vicinal (100) GaAs substrates are reported. Steady state gallium concentrations are linearly related to incident fluxes. Between 470 and 650 °C, incorporation is by thermodynamic distribution between accumulated surface concentrations and growing films. The distribution function favors higher incorporation with increasing temperature, however surface concentrations are significantly reduced by desorption above 550 °C. Kinetic limitations produce a retrograde temperature dependence of the distribution below ∼460 °C.