C. M. Knoedler

Tunneling hot-electron transfer amplifier: a hot-electron GaAs device with current gain

Tunneling hot‐electron transfer amplifier (THETA) devices, based on GaAs‐AlGaAs heterojunctions, were fabricated and tested. Hot‐electron transfer (α) through a 1100‐A base in excess of 70% was found at 4.2 K. This resulted in a corresponding current gain ( β) in a common emitter configuration of about 2.3. In the temperature range of 4.2–80 K and under constant biasing conditions, α was nearly temperature independent. Electron energy distributions for motion normal to the layers and electron total energy loss while traversing the device were estimated. Typical widths of the energy distributions were less than 200 meV, and both widths and energy peak positions were only slightly dependent on temperature and initial injection energy.

dc performance of ballistic tunneling hot‐electron transfer amplifiers

We present new experimental results of ballistic electron transport through thin n+‐GaAs layers. Measurements were done on tunneling hot‐electron transfer amplifier devices composed of GaAs and AlGaAs layers. In devices with GaAs active regions (bases) of 300 and 800 A, collisionless or ballistic transport was observed. By performing hot‐electron energy spectroscopy we found that the collected ballistic distributions were similar in shape but differed in magnitude. This suggests the existence of a strong scattering mechanism which randomizes the otherwise ballistic electrons. The maximum differential current transfer ratio α was 0.9 in devices for which about 75% of the injected current traversed the base ballistically. The presence of ballistic transport has also allowed the measurement of the AlGaAs barrier height through observation of the onset of current collection in the devices. Barrier heights higher than those recently reported have been measured. In addition we show the effects of grading the co...

High-gain pseudomorphic InGaAs base ballistic hot-electron device

A high-gain ballistic hot-electron device is described. The GaAs-AlGaAs heterostructure device, with a 21-mm-thick pseudomorphic In/sub 0.12/Ga/sub 0.88/As base, had a current gain of 27 at 77 K and 41 at 4.2 K. As characteristically seen in ballistic devices, transfer into the L valley limited the maximum gain. The Gamma -L valley separation in the strained In/sub 0.12/Ga/sub 0.88/As was estimated to be about 380 meV.<<ETX>>

Gated, asymmetric rings as tunable electron interferometers

Abstract We have fabricated gated, asymmetric rings which, in principle, enable interference between electron waves to be varied with a gate voltage. Although close to the minimum dimensions currently achievable, the results are far from clear-cut, and imply that there are many problems to be overcome before useful devices based on quantum interference can be demonstrated, even supposing that the geometries proposed will actually work. We describe the methods used to analyse our data to show any consistent interference effects caused by the gate voltage variation. Narrower, higher-mobility devices are required in order to produce more easily-resolved gate voltage-controlled interference effects.

Sidewall damage in n+‐GaAs quantum wires from reactive ion etching

Electron cyclotron resonance and radio frequency reactive ion etching have been used to fabricate narrow n+‐GaAs wires employing CCl2F2/He as the etch gas. A comparison of the induced sidewall damage is made using room‐temperature conductivity measurements of the etched structures and the effect of overetching is investigated. In addition, preliminary analysis of low‐temperature transport reveals that the amplitude of universal conductance fluctuations is extremely sensitive to sidewall damage.

Pseudomorphic InGaAs base ballistic hot‐electron device

We report the first successful incorporation of a pseudomorphic InGaAs base in a ballistic hot‐electron device. The device, with a 28‐nm‐thick In0.15Ga0.85As base, had a collector‐base breakdown voltage of 0.55 V and a maximum current transfer ratio of 0.89 at 4.2 K, considerably higher than the 0.75 in a comparable GaAs‐base device. Electron energy spectroscopy measurements revealed that at least 30% of the injected electrons traversed the InGaAs base ballistically, causing a strong modulation in the injected currents into the quantized base. The Γ‐L valley separation in the strained In0.15Ga0.85As was estimated to be about 410 meV.

Fractional states in few‐electron systems

We have observed fractional quantization of very few electrons confined in a semiconductor quantum dot using capacitance spectroscopy. The number of electrons per dot varies from 0 to about 40 as a function of bias on the quantum capacitors. The capacitance spectra have clear minima at gate voltages and magnetic fields, where the filling factors are 1/3 and 2/3. These measurements may allow direct comparison with few‐particle calculations.

Inert gas reactive ion etching damage to GaAs using inverted heterojunctions

Selectively doped inverted heterojunctions containing a two‐dimensional electron gas were used as a sensitive vehicle for monitoring dry processing damage. We found that the electron sheet concentration, strongly dependent on the total number of carriers in the GaAs cap layer, and the mobilities were significantly depressed even for very short exposures to low‐voltage helium plasmas. Argon, which caused less degradation than helium, was found to increase the sheet carrier concentration and hence the mobility after prolonged exposure. The damage mechanism responsible for the carrier loss in both cases is most likely the production of traps. The subsequent carrier increase seen for the argon case is probably attributable to the creation of a very thin donorlike damage layer on the surface of the GaAs cap layer.

The hall effect in ballistic junctions

Abstract In narrow high-mobility conductors the predominant source of scattering is reflection of carriers off the confining potential. We demonstrate that by changing the geometry of the intersection of the Hall probes with the conductor, the Hall resistance can be quenched, negative or enhanced. More complex junction geometries can lead to one of these phenomena for one field polarity and to another for the other field polarity. At liquid helium temperatures these results can be explained by following trajectories. In the milli-Kelvin range fluctuations are superimposed. At high fields strong resonant depressions of the Hall resistance are found which may be associated with bound states in the region of the cross.

Submicron trenching of semiconductor nanostructures

We have devised and demonstrated a novel technique for fabricating structures with nanometer scale features in semiconductor heterostructures. The technique is based on definition of nanometer scale patterns by submicron trenches in a GaAs‐AlGaAs heterostructure. The depletion of free carriers below the trenches gives rise to very strong electrostatic confinement. This technique avoids the complications associated with the use of negative resist materials and lift‐off techniques while minimizing the time required to expose densely packed patterns. Aharonov–Bohm rings fabricated using this technique exhibit interference oscillation larger than any reported previously.