Carl O. Bozler

A technique for producing epitaxial films on reuseable substrates

Using a new technique, which we have named the CLEFT process (cleavage of lateral epitaxial films for transfer), we have grown single‐crystal GaAs films by vapor phase epitaxy (VPE) on reusable GaAs substrates. A carbonized photoresist mask with narrow, widely spaced stripe openings is first deposited on a (110) GaAs substrate. Epitaxial growth initiated within the openings, followed by lateral growth over the mask, produces a continuous single‐crystal GaAs film. The upper surface of the film is bonded to a glass substrate, and the film is then cleaved from the GaAs substrate, leaving the surface of the latter in condition for repeating the procedure. The same GaAs substrate has been used for successive growth of four GaAs films, each about 4 cm2, ranging in thickness from 5 to 10 μm. The electrical properties of a CLEFT film were found comparable to those of conventional VPE layers. The CLEFT process should also be applicable to other semiconductors.

Arrays of gated field‐emitter cones having 0.32 μm tip‐to‐tip spacing

We have reduced the gate voltage required to achieve a given emission current density in field‐emitter arrays by scaling down the gate‐to‐tip and tip‐to‐tip spacing to the unprecedented levels of 0.08 and 0.32 μm, respectively. The submicrometer features of our arrays are patterned using interferometric lithography. Electrical tests of arrays we have fabricated have shown a record low turn‐on voltage of 8 V for cesiated molybdenum emitters. Emission current densities of 1600 A/cm2 have been obtained, which is also a record for such structures. These arrays provide large advantages for applications such as flat panel displays and microwave devices.

MEMS microswitches for reconfigurable microwave circuitry

The performance is reported for a new microelectromechanical structure (MEMS) cantilever microswitch. We report on both dc- and capacitively-contacted microswitches. The dc-contacted microswitches have contact resistance of less than 1 /spl Omega/, and the RF loss of the switch up to 40 GHz in the closed position is 0.1-0.2 dB. Capacitively-contacted switches have an impedance ratio of 141:1 from the open to closed state and in the closed position have a series capacitance of 1.2 pF. The capacitively-contacted switches have been measured up to 40 GHz with S/sub 22/ less than -0.7 dB across the 5-40 GHz band.

Low‐dislocation‐density GaAs epilayers grown on Ge‐coated Si substrates by means of lateral epitaxial overgrowth

Single‐crystal GaAs layers have been obtained by means of lateral epitaxial overgrowth seeded within stripe openings in a SiO2 mask over GaAs layers grown on Ge‐coated Si substrates. Transmission electron microscope and scanning cathodoluminescence studies indicate that the laterally overgrown GaAs layers have a dislocation density of less than 104 cm−2, compared to 107–108 cm−2 for the GaAs layers grown directly on the Ge/Si substrates. Initial experiments indicate that the electrical properties of the laterally overgrown layers are comparable to those of conventional GaAs epilayers grown on single‐crystal GaAs substrates.

Electroabsorption in GaAs and its application to waveguide detectors and modulators

The electroabsorption coefficient of GaAs has been measured in uniform electric fields at wavelengths from 0.91 to 0.93 μm. These measurements were made using Schottky barrier contacts on low‐loss GaAs waveguides consisting of high‐purity epitaxial GaAs grown on heavily doped GaAs substrates. The experimental results are in good agreement with theoretical calculations of the Franz‐Keldysh effect. Electroabsorption detectors with subnanosecond response time and 100% internal quantum efficiency have been integrated into these waveguides. Small values of avalanche gain have been obtained without any intentional guard‐ring structure. Integrated electroabsorption modulators with greater than 20‐dB depth of modulation were also fabricated.

Lateral epitaxial overgrowth of GaAs by organometallic chemical vapor deposition

Lateral epitaxial overgrowth of GaAs by organometallic chemical vapor deposition has been demonstrated. Pyrolytic decomposition of trimethylgallium and arsine, without the use of HCl, was used to deposit GaAs on substrates prepared by coating (110) GaAs wafers with SiO2, then using photolithography to open narrow stripes in the oxide. Lateral overgrowth was seeded by epitaxial deposits formed on the GaAs surfaces exposed by the stripe openings. The extent of lateral overgrowth was investigated as a function of stripe orientation and growth temperature. Ratios of lateral to vertical growth rates greater than 5 have been obtained. The lateral growth is due to surface‐kinetic control for the two‐dimensional growth geometry studied. A continuous epitaxial GaAs layer 3 μm thick has been grown over a patterned mask on a GaAs substrate and then cleaved from the substrate.

Simplified fabrication of GaAs homojunction solar cells with increased conversion efficiencies

Conversion efficiencies as high as 20% of AM1 have been obtained for single‐crystal GaAs shallow‐homojunction solar cells without Ga1−xAlxAs layers. These cells, which are fabricated by a simplified technique that does not require any vacuum processing steps, utilize an n+/p/p+ structure with an antireflection coating prepared by anodic oxidation of the n+ layer.

MEMS microswitch arrays for reconfigurable distributed microwave components

A revolutionary device technology and circuit concept is introduced for a new class of reconfigurable microwave circuits and antennas. The underlying mechanism is a compact MEMs cantilever microswitch that is arrayed in two-dimensions. The switches have the ability to be individually actuated. By constructing distributed circuit components from an array, the individual addressability of the microswitch provides the means to reconfigure the circuit trace and, thus, provides the ability to either fine-tune or completely reconfigure the circuit elements behavior. Device performance can be reconfigured over a decade in bandwidth in the nominal frequency range of 1 to 100 GHz. In addition, other circuit-element attributes can be reconfigured such as instantaneous bandwidth, impedance, and polarization (for antennas). This will enable the development of next-generation communication, radar and surveillance systems with agility to reconfigure operation for diverse operating bands, modes, power levels, and waveforms.A novel MEMS switch design for use in microwave circuits is presented. The microswitch is capable of being configured in a multi-element X-Y array for use in tunable distributed circuits. Circuit design concepts using microswitch arrays and measurements of single microswitch performance are given.

Submicrometer patterning by projected excimer‐laser‐beam induced chemistry

Projection imaging with the deep‐UV (193 nm) and vacuum ultraviolet (VUV) (157 nm) output of an excimer laser has been applied to submicrometer patterning of thin films by injected‐defect, surface‐chemical, and solid‐transformation processing. The methods have been designed to take advantage of the short‐wavelength, high‐peak‐intensity pulsed radiation from these sources. Examples are described of pattern definition by exposure of multilayer organic resists, by maskless etching and doping of solids in reactive vapors, and by solid‐state chemical transformations in inorganic Al/O films. Well‐resolved 0.4‐μm lines and spaces have been achieved. Required doses, between 0.04 and 1 J/cm2, are compatible with single‐ or multipulse step‐and‐repeat projection patterning with a small excimer laser.

GaAs p+n−n+ directional‐coupler switch

GaAs p+n−n+ electro‐optic directional‐coupler switches have been successfully fabricated and evaluated at 1.06 μm for use as components in integrated optical circuits. The devices were fabricated from a pair of closely spaced low‐loss [α≈1 cm−1 (4.3 dB/cm) at 1.06 μm] single‐mode p+n−n+ channel‐stop strip guides. They are operable both as passive directional couplers and as electro‐optic switches. The couplers have exhibited 98% power transfer and have an attenuation only about 0.1 dB/cm greater than that of a single guide having the same dimensions as one of the coupled guides. The switch performance was found to depend on the crystallographic direction chosen for light propagation. All devices were in the {100} plane, and 17‐dB (98%) power isolation in both the switched and unswitched states with constant total power output (≲0.2‐dB variation) was achieved for propagation along a [011] direction. These switches were 7.2 mm long, and had an optimum switching voltage of 43 V and a calculated power‐bandwid...