J. M. Hong

Tamm states in superlattices

Abstract We present experimental results on the localized “surface” states, known as Tamm states, in superlattices. A terminating layer of Al y Ga 1− y As on GaAs/Al x Ga 1− x As As superlattices produced Tamm states at the states at the interface, which are observed by interaband optical transitions. Results clearly show the presence of localized Tamm states, either above or below the miniband states depending on the composition of the terminating layer, and their interaction with other extended superlattice states.

Studies of exchange-induced properties of CdTe/Cd1-xMnxTe superlattices

Abstract We report on some magneto-photoluminescence experiments on CdTe⧸Cd 1− x Mn x Te superlattices, which allow the observation of the type-I → type-II transition induced by the giant Zeeman effect of the semimagnetic barrier. The study of the magnetic field, for which the transition occurs, as a function of temperature leads to a determination of the band offset. In addition, several observations about defect luminescence are reported and several specific properties of CdTe⧸Cd 1− x Mn x Te structures are discussed: strain effects on the band offset, exciton binding energy, spin-flip scattering mechanism.

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.

Temperature dependence of the electronic coherence of GaAs‐GaAlAs superlattices

We have shown that the coherence length of electrons in a 55‐A‐period GaAs‐GaAlAs superlattice does not depend strongly on temperature in the range 5–292 K, varying from 17 periods at 5 K to a minimum of nine periods at room temperature. The quantum coherence was determined by photocurrent spectroscopy experiments that exploit the formation of Stark ladders in superlattices under electric fields.

Correlations between Aharonov-Bohm effects and one-dimensional subband populations in GaAs/AlxGa1-xAs rings

The Aharonov-Bohm (AB) interference patterns in ring-shaped conductors are usually dominated by random features. The amplitude of the oscillations is random from sample to sample and from point to point on the magnetic field axis owing to random scattering of the electron trajectories by impurities within the wires. We report experiments on devices made wet etching and global gates, which have shown major progress towards removing the random features. In loops that exhibit ballistic conductance plateaus and cyclotron orbit trapping at 4.2 K, the random pattern of AB oscillations (observed for T<0.1 K) can be replaced by a much more ordered one-especially if only a few transverse modes are populated in the ring

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.

Electron transport through one quantum dot and through a string of quantum dots

We have built strings of quantum dots consisting of 1 up to 4 dots. Transport measurements of these devices show sharp resonant conductance peaks around threshold. Our results are discussed in terms of Coulomb charging and of quantum confinement effects, since both effects are expected to be of the same order of magnitude for these devices.

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.

Novel properties of a 0.1- mu m-long split-gate MODFET

A MODFET with two 30-nm-long gates (separated by 40 nm) has been fabricated using ultrahigh-resolution electron-beam lithography. The proximity of the two gate fingers along with the ability to independently bias them results in the following features: (a) tunability of the threshold voltage, (b) enhancement of the transconductance, especially at low current levels, (c) reduction in short-channel effects, and (d) high-voltage gain and cutoff frequency.<<ETX>>

Cyclotron trapping, mode spectroscopy, and mass enhancement in small GaAs/AlxGa1-xAs rings

A new electronic trapping effect has been observed in a small GaAs/Al x Ga 1-x As ring, which takes place when the cyclotron orbits match the loop geometry. The trapping causes peaks in the magnetoresistance R(B) at low fields, and the peak positions were used to study the dispersion relation E n (k) of the one-dimensional magnetoelectric subbands. This allows the ring to be used as a mesoscopic mass spectrometer. The electron effective mass increases with magnetic field by a factor of 50 at field of 0.5 T. General agreement obtains between the experiment and the subband calculations for one-dimensional channels, indicating that true solid-state interferometers are possible