Albert Savage

Epitaxial structures with alternate‐atomic‐layer composition modulation

Epitaxial structures grown by alternate monolayer depositions of GaAs and AlAs are reported. As many as 104 alternate (100) layers of GaAs and AlAs as thin as 1.0±0.1 and 1.0±0.1 monolayers, respectively, were deposited by sequential molecular beam epitaxial growth on a (100)GaAs substrate. Transmission electron microscopy showed the resultant films to be perfectly epitaxial with layered composition modulation of the expected periodicity. Their electronic properties were studied by optical absorption and luminescence.

TEMPERATURE DEPENDENCE OF THE OPTICAL PROPERTIES OF FERROELECTRIC LiNbO3 AND LiTaO3

Linear and nonlinear optical investigations of LiNbO3 and LiTaO3 from room temperature to temperatures above their ferroelectric phase transitions are described. The measurements include studies of second harmonic generation with an Nd laser (1.06 μ) and determinations of the refractive indices as well as the birefringence at 0.6328 μ. All the optical data exhibit distinct changes at the Curie temperatures, Tc = 1195°C ± 15°C for LiNbO3, and Tc = 610°C ± 10°C for LiTaO3. In each case the phase change has the characteristics of a second‐order ferroelectric transition. The uniaxial paraelectric phases are concluded to be centrosymmetric — probably space group R3c.

Crystal growth kinetics in (GaAs)n−(AlAs)m superlattices deposited by molecular beam epitaxy: I. Growth on singular (100)GaAs substrates

Abstract A new technique for the study of crystal growth processes on an atomic scale is presented. The technique comprises epitaxial growth of layered crystals by alternate molecular beam depositions of monolayer quantities of GaAs and AlAs, followed by examination of cross section of the crystals by transmission electron microscopy. Epitaxial films of (GaAs) n −(AlAs) m consisting of n monolayers of GaAs deposited alternately with m monolayers of AlAs (monolayers thickness ≊2.83 A) have been studied by this technique. Effects of both chemical composition (0.9 ⩽ n ⩽ 4 and 0.98 ⩽ m ⩽5) and growth temperature T S (510 ⩽ T S ⩽ 630°C are reported. Both imaging and diffraction modes of cross sections of films are examined and are used to study the process of crystal growth. Critical temperatures, T c , above which surface roughening effects prevent the formation of an ordered layered structure are found with T c ≊ 610°C at n = m = 1 , T c being a function of n and m .

Motion of 180° Domain Walls in Metal Electroded Barium Titanate Crystals as a Function of Electric Field and Sample Thickness

The domain dynamics of polarization reversal in metal electroded BaTiO3 single crystals have not been adequately described in the literature. The present paper contains quantitative measurements of the velocity of the sidewise motion of 180° domain‐walls in BaTiO3 crystals, most of which were electroded with 200‐A thick films of Au. The wall velocity has been measured as a function of several variables including the applied electric field, the crystal thickness and the impurity content added to the crystal growth melts.The wall velocity as a function of the field E is described by v∞ exp (−δ/E) where δ and v∞ are constants over several decades of velocity. The quantities δ and v∞ are the activation field and extrapolated wall velocity for E=∞, respectively. This field dependence of the wall velocity is the same as that reported previously for liquid electroded specimens. The thickness dependence of the wall velocity, which is through δ, is very pronounced and similar to that observed in measurements of ot...

Asymmetric Hysteresis Loops and the Pyroelectric Effect in Barium Titanate

Dynamic pyroelectric techniques have been used to study single crystal BaTiO3 hysteresis loops which have been reported in several recent investigations to be asymmetric with respect to both the polarization and electric field axes. It is shown that an apparent polarization bias occurs because of electrode‐edge effects and that this bias may be shifted with an appropriate application of a dc field which serves to alter the direction of the polarization in fringe regions which contribute to the observed pyroelectric signal. The pyroelectric hysteresis loops are symmetric with respect to the polarization when electrode‐edge effects are eliminated. These data strongly suggest that asymmetric remanent polarizations reported in several piezoelectric and pyroelectric investigations may have been due to edge effects. The pyroelectric hysteresis loops are found to be biased with respect to the electric field axis in the same manner as that reported in a recent piezoelectric study. Small residual pyroelectric sign...

Pyroelectricity and Spontaneous Polarization in LiNbO3

Measurements have been made concerning the magnitude of the pyroelectric coefficient dPs/dT in LiNbO3 over the temperature range from 25° to 450°C. The pyroelectric coefficient was found to be −4×10−9 C/cm2/°C at 25°C. The spontaneous polarization was determined to be a decreasing function of temperature in the crystals measured which were pulled from the melt. Over the temperature range measured Ps was found to decrease by 3.7 μC/cm2. From these pyroelectric measurements and preliminary data on the temperature dependence of the nonlinear optical coefficient d33 in LiNbO3, for d33 ∝ Ps, the magnitude of Ps at room temperature is estimated to be at least of the order of 50 μC/cm2.

Molecular beam epitaxy of Ge and Ga1−xAlxAs ultra thin film superlattices

Abstract Superlattices of alternate ultra-thin layers of Ge and Ga1−xAlxAs have been deposited epitaxially on GaAs substrates by molecular beam epitaxy. The structural perfection, interface roughness and crystal growth processes for these superlattices have been analysed by transmission electron microscopy. Deposition periods of n monolayers of Ga1−xAlx As and 2m monolayers of Ge with n,m ⪖ 1 and 0 ⪕ x ⪕ 1 were examined for growth temperatures in the range of 25 to 620°C. A columnar growth regime produced by nucleation of GaAs over GaAs regions predominated in the monolayer superlattices for low x and high growth temperatures, producing epitaxial columnar structures. Smooth interfaces were produced for x = 1 down to n = m = 7 at growth temperatures below 500°C, although stacking faults were present. This behavior is interpreted in terms of atomic bond geometries, equilibrium phase diagrams and the non-equilibrium characteristics of MBE crystal growth.

Diffusion of Aluminum in Single Crystal Silicon

The diffusion of aluminum into silicon has been reinvestigated to clear up certain discrepancies in the existing aluminum data. The diffusion constants found in this work are in good agreement with those determined by Fuller and Ditzenberger and an order of magnitude less than those reported in another investigation. Aluminum solid solubilities in the neighborhood of 1019 atoms/cm3 have been found over the 1200 to 1400°C temperature range. These concentrations are several orders of magnitude larger than those reported in other aluminum diffusion investigations. The p‐n junction penetrations and transition capacitances of the diffused samples were used to determine diffusivities and solid solubilities.

Low threshold pulsed and continuous laser oscillation from AlGaAs/GaAs double heterostructures grown by metalorganic chemical vapor deposition on Si substrates

Using graded refractive index, single quantum well GaAs lasers grown by metalorganic chemical vapor deposition on Si substrates, we have obtained injection lasers with broad area pulsed thresholds of 3.5 kA/cm2 at 300 K. With the current flow restricted to 5‐μm‐wide stripes, pulsed thresholds of 130 mA and differential quantum efficiencies of 70% were obtained at 300 K. Pulsed oscillation has been obtained up to 375 K and continuous oscillation up to 160 K. The rapid degradation of the lasers suggests the growth of nonradiative regions in the active layer.

High‐speed distributed feedback lasers grown by hydride epitaxy

We have used hydride vapor phase epitaxy to prepare re‐entrant mesa buried‐heterostructure distributed feedback lasers. The grating is overgrown uniformly with minimal amplitude reduction. A semi‐insulating Fe‐InP blocking layer around the etched mesas is grown very reproducibly with little dependence upon the details of mesa etching. Laser diodes show cw thresholds of ∼25 mA. Single‐mode operation has been shown at power levels as high as 18 mW and temperatures up to 100 °C. Modulation bandwidth in excess of ∼8 GHz has been demonstrated with simple and reliable full surface Ohmic contacts.