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Notes: Channeled ions offer a means of patterning a wafer with a two-dimensional electron gas (2DEG) buried deep below the surface. The implantation of 60–140 keV Si++ ions into a 580-nm-deep 2DEG formed at a GaAs/AlGaAs heterojunction has been characterized, with respect to ion energy, dose, and angle of incidence. Transverse electron focusing measurements have been used to investigate the roughness of the implanted boundary, leading to measured values for the specularity coefficient of about 0.5 at 1.7 and 4.2 K. Similar measurements at lower temperatures (120 mK) have shown fine structure in the magnetoresistance, in addition to the classical transverse electron focusing effect. The fine structure is attributed to electron interference effects at, or close to, the boundary. Channeled ion implantation has been used to define an in-plane-gated transistor which, at 300 mK, shows clear evidence of ballistic electron transport.

Notes: We have investigated the growth of three-dimensional Ag particles at atomic steps on the surface of highly oriented pyrolytic graphite using a scanning electron microscope. By controlling the growth parameters the cluster growth was confined to the steps avoiding terrace nucleation. In this way quasi-one-dimensional chains of Ag nanoclusters of approximately 10 nm diam were produced. The results suggest the viability of an important new route to the creation of controlled nanoscale structures. A comprehensive surface study indicates that cluster mobility and coalescence play an important role in the growth mechanism on the steps. Evidence was also found that the graphite surface has several different types of surface steps. A quantitative analysis of the cluster distribution on the steps was performed, to investigate the nucleation and growth processes at temperatures from 50 to 205 °C. © 1996 American Institute of Physics.

Notes: The magnetoresistance anomalies that are observed in multiprobe quantum wires (such as quenching of the Hall effect and negative bend resistance) have been investigated using a semiclassical billiard-ball model that includes the effects of diffuse boundary scattering. This modeling predicts that two peaks are expected in the magnetoresistance of a quantum wire in which there is a significant amount of diffuse boundary scattering. One peak is due to diffuse boundary scattering in the wire and the other due to specular boundary scattering in the junctions at either end of the wire. The modeling also predicts that the well-known quenching of the Hall effect and negative bend resistance anomalies are both expected to be enhanced by diffuse boundary scattering. This is explained in terms of the way in which diffuse boundary scattering affects the angular distribution of the electrons entering the junctions in the multiprobe wires. "Diffuse collimation'' of the electron distribution occurs, increasing the probability for direct transmission of the electrons across the junctions. Experiments performed on wires fabricated in GaAs/AlxGa1−xAs high-mobility heterostructure material, using implanted p-type gates to provide the lateral confinement, have confirmed the twin-peak structure in the magnetoresistance. Although the diffuse boundary scattering magnetoresistance peak has been observed often before, this is the first unambiguous observation of the junction scattering peak. Other device geometries are investigated using the semiclassical model, and a prediction is made for negative longitudinal resistance in a multiprobe wire in which the voltage probes are shadowed from either the current source or the drain. This phenomenon was experimentally verified with devices fabricated in GaAs/AlxGa1−xAs high-mobility heterostructure material using surface Schottky gates or wet etching to provide the lateral confinement. Thus, the trio of negative resistance effects in multiprobe quantum wires has been completed; in addition to the negative Hall resistance and the negative bend resistance a negative longitudinal resistance has now been measured. © 1995 American Institute of Physics.

Notes: Variable-area resonant tunneling diodes have been fabricated using a process in which the lateral confinement is produced by an in-plane implanted gate. The basic operation of such devices is discussed, and the lateral confinement shown by both measurements and numerical modeling to be very nearly symmetrical about the resonant tunneling diode (RTD) barriers. Fine structure has been observed near threshold for relatively large area devices and this has been attributed to single-electron or few-electron tunneling through donor states in the quantum well. Additional fine structure seen in the valley current of small-area devices has been shown to be consistent with the effects of lateral quantization in the system. Finally, results are presented for strip devices in which the resonant tunneling peak breaks up into numerous subpeaks on application of the gate voltage, and this is attributed to pinning of the confining potential by donors close to the RTD barriers.

Notes: A fabrication process for free-standing single-crystal silicon wires of submicrometer cross-sectional dimensions and lengths in excess of 40 μm is reported. The starting material is silicon-on-insulator that has been recrystallized using a dual electron beam recrystallizing system. The wires are then fabrictaed in the recrystallized layer by a combination of electron beam lithography and plasma etching. Electrical measurements have been performed and the fabrication limits of the process are discussed.

Notes: Double barrier heterostructure materials containing very thick, low-doped layers adjacent to the barriers have been studied. In particular, charge build-up in the quantum well has been investigated along with the effects on this of a magnetic field perpendicular to the barriers. Phonon-assisted tunneling has also been observed from both localized and delocalized states in the emitter.

Notes: GaAs microdisk lasers with holes pierced through the disk surface are investigated for their threshold characteristics. Disks are fabricated with either a single hole or two diametrically opposite holes at various distances from the disk outer edge. Even though the disk area is reduced by only 1%, we find that the lasing threshold for a disk with one hole is reduced by up to 50% compared to a disk with no hole. We attribute this reduction to the perturbation of nonlasing modes, which decreases the amplification of spontaneous emission in these modes and makes more carriers available to contribute to lasing. © 1999 American Institute of Physics.

Notes: Practical nonalloyed ohmic contacts on δ-doped GaAs have been compared for AuGeNi (88:12:5) /and Cr metallizations to show the importance of metallization type for minimizing the contact resistance. They are shown to have low contact resistances even at 4.2 K and for contact sizes down to 240 nm diam. The effect of heating AuGeNi contacts to 270 °C is shown to be beneficial for large-area contacts but not for submicron contacts, implying that nonuniformity is introduced. © 1996 American Institute of Physics.

Notes: Coulomb blockade has been observed in the current-voltage characteristics of structures fabricated in silicon germanium δ-doped material at temperatures up to 50 K. This is consistent with the estimated effective tunnel capacitance of 10 aF which is significantly smaller than the reported capacitances of tunnel junctions made from Al or GaAs/AlGaAs heterostructures.

Notes: An in-plane gate field-effect transistor is characterized by ultrafast electro-optic sampling. The transistor is monolithically integrated with photoconductive switches in coplanar waveguide and 〈0.5 ps measurement time resolution is achieved. The gate-drain capacitance of the transistor is obtained as 1.8 fF at zero drain voltage from displacement current transients. The gate-drain capacitance is dominated by parasitic capacitance and the intrinsic gate-drain capacitance is estimated as less than 0.2 fF. © 1995 American Institute of Physics.