E. D. Beebe

Ultranarrow conducting channels defined in GaAs-AlGaAs by low-energy ion damage

We have laterally patterned the narrowest conducting wires of two-dimensional electron gas (2DEG) material reported to date. The depletion induced by low-energy ion etching of GaAs-AlGaAs 2DEG structures was used to define narrow conducting channels. We employed high voltage electron beam lithography to create a range of channel geometries with widths as small as 75 nm. Using ion beam assisted etching by Cl2 gas and Ar ions with energies as low as 150 eV, conducting channels were defined by etching only through the thin GaAs cap layer. This slight etching is sufficient to entirely deplete the underlying material without necessitating exposure of the sidewalls that results in long lateral depletion lengths. At 4.2 K, without illumination, our narrowest wires retain a carrier density and mobility at least as high as that of the bulk 2DEG and exhibit quantized Hall effects. Aharonov–Bohm oscillations are seen in rings defined by this controlled etch-damage patterning. This patterning technique holds promise for creating one-dimensional conducting wires of even smaller sizes.

Gallium Arsenide and Aluminum Gallium Arsenide Reactive Ion Etching in Boron Trichloride-Argon Mixtures

GaAs and Al0.3Ga0.7As were reactive ion etched in a mixture of boron trichloride and argon. The effects of the independent variables such as the time, power, pressure, and gas composition on the etch depth as well as the quality of the resulting etched surfaces were analyzed through a multiple linear regression approach. This provided second‐order equations with an excellent ability to describe experimental results. The etching conditions for GaAs and AlGaAs were compared and optimum parameters for equal rate etching of GaAs/Al0.3Ga0.7As layers with straight walls were obtained. These were then sucessfully applied to the high‐resolution structuring of multiple quantum well layers, and structures as small as 40 nm were etched. The addition of oxygen to the etch gas produced a high‐quality etch for GaAs with >5:1 rate selectivity over Al0.3Ga0.7As.

Fluoride etch masks for high‐resolution pattern transfer

We use thermally evaporated fluoride films as dry etch masks to fabricate sub‐100 nm wide features into III–V heterostructures. Fluorides are compatible with lift‐off processing and display excellent resistance to halide etch gas mixtures. Among the advantages which these materials offer over conventional metal masks are their thermal stability, good adhesion, high resistivity, the avoidance of ‘‘flags’’ arising from angle evaporation, and the ease with which they can be removed from the surface. By adjusting the composition of these fluorides and thereby controlling their grain size, we can optimize the resolution and erosion rates of our masks and fabricate structures with lateral sizes below 20 nm.

Lithographic fabrication of transmission electron microscopy cross sections in III-V materials

A procedure for forming specifically located transmission electron microscopy (TEM) cross section specimens of III–V semiconductor materials is described. By electron‐beam lithography in a TEM/STEM followed by a lift‐off technique, 40–100 nm wide metal lines were formed on the sample surface. Anisotropic reactive ion etching in a BCl3 and Ar plasma removed the material unprotected by metal lines to leave uniform thickness vertical sections of material. Details of the procedure and the TEM analysis of a sectioned GaAs/AlAs superlattice structure will be described. The ability to form a cross section from a specifically desired region of the sample and the ability to form long uniform sections makes this technique particularly applicable to analysis of devices in brittle semiconductor materials.

High Reflectivity Dielectric Mirror Deposition by Reactive Magnetron Sputtering

We use room temperature reactive sputter deposition from a silicon target to deposit Si, Si3N4, and SiO2 layers. The resulting amorphous layers are grown to form highly reflecting dielectric mirrors and used for coating end facets of layers. Among the advantages of our computer controlled sputter deposition process are the reproducibly excellent optical quality, good adhesion properties together with high chemical and thermal stability of the deposited films. We use Si/SiO2 mirrors for broadband infrared distributed Bragg reflectors and Si3N4/SiO2 multilayers in the visible wavelength range. The high (≳99%) reflectivities which are obtained from these mirrors make them suitable for use in vertical‐cavity surface emitting and low‐threshold edge‐emitting lasers.

1.5 μm InGaAsP/InP vertically coupled semiconductor optical pre‐amplifier

Vertical coupling of light output from a stripe InGaAsP/InP laser amplifier to the bottom of the substrate for detection by means of a 45° mirror has been demonstrated. The composite‐cavity amplifier structure is shown to have an inherent low facet reflectivity and unidirectional amplification property. With a fiber‐to‐detector gain of 15.4 dB and a nearly diffraction‐limited output spot size, the device is suitable for compact integration with a photodetector as an optical pre‐amplifier in a high bit‐rate direct detection system.

Transport in ultra-narrow quantum well wires

We have fabricated the narrowest conducting channels reported to date using hihg mobility GaAs/AlGaAs heterojunction material. In our narrowest wires at low temperatures and for small magnetic fields, where the lD subband splitting exceed both κBTand hωC, we observe striking departures from the 2D Hall effect. An unexpected Hall plateau at low magnetic field is observed which systematically moves to higher field and higher Hall resistance, RH, as the wire width is decreased. In samples having widths less than ≈ 200 nm, this is acompanied by a precipitous, complete suppression of RH within a region about zero field. We believe these to be unambiguous manifestations of one-dimensional electrical transport; they appear to provide a direct measure of the number of quantum conduction channels that participate [1].

Electron Waveguide Junctions: Scattering from a Microfabrication-Imposed Potential

In quantum wires, ultranarrow conduction channels having very low disorder, electrons can scatter from potentials which are radically different from those encountered in the bulk. Entirely new types of scattering potentials can be created by microfabrication, since ballistic devices having overall dimensions much smaller than the transport mean free path, and widths only slightly larger than one electron wavelength, can now be fabricated.1