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In inelastic light scattering experiments we observe for the first time clearly resolved one dimensional (1D) intersubband spin-density excitations. Together with new structure at energies close to the 2D intersubband transitions, these observations display the formation of 1D subbands. The depolarization shift (W dep ) and the excitonic shift (W xc ) can be deduced approximately from our experiments. These shifts are of special interest because they are related to the direct and exchange-correlation terms of the electron-electron interaction. We find ratios of the shifts (W xc /W dep ) of up to 55%

We have recently developed a spectroscopic technique which allows direct measurement of quantum energy levels. The method is based on observation of the capacitance signal resulting from single electrons tunneling into discrete quantum levels. The electrons tunnel between a metallic layer and confined states of a microscopic capacitor fabricated in GaAs. Charge transfer occurs only for bias voltages at which a quantum level resonates with the Fermi energy of the metallic layer. This creates a sequence of distinct capacitance peaks whose bias positions directly reflect the electronic spectrum of the confined structure. Using this “single-electron capacitance spectroscopy”, we map the magnetic field dependence of the ground state energies of a single quantum dot containing from 0 to 50 electrons. Along with a spectroscopic measurement of the dot's ground states, we probe tunneling rates of electrons to individual quantum states. The experimental spectra reproduce many features of a noninteracting electron model with an added fixed charging energy. However, in detailed observations deviations are apparent: exchange induces a two-electron singlet-triplet transition, self-consistency of the confinement potential causes the dot to assume a quasi two-dimensional character, and features develop which are suggestive of the fractional quantum Hall effect.

This paper presents a technique which permits a quantitative spectroscopy of discrete quantum levels in structures containing as few as one electron. The ground state energy in a quantum dot can be measured for an arbitrary number of electrons and followed as a function of magnetic field. The method involves monitoring the capacitance signal resulting from the tunneling of single electrons. In a microscopic capacitor fabricated in GaAs we study the confined states of a single 1 μm disk to which electrons can tunnel from a nearby metallic layer. Charge transfer occurs only for bias voltages at which a quantum level is resonant with the Fermi energy of the metallic layer. This creates a sequence of distinct capacitance peaks whose bias positions can ve directly converted to an energy scale to determine the electronic spectrum of the confined structure. The evolution of the spectrum in magnetic field allows deduction of the nature of the bound states.

We present a technique which permits a quantitative spectroscopy of discrete quantum levels in semiconductor microstructures. The method involves monitoring the capacitance signal resulting from single-electron tunneling. In a microscopic capacitor fabricated in GaAs we study the confined states of a single 1 μm disk to which electrons can tunnel from a nearby metallic layer. Charge transfer occurs only for bias voltages at which a quantum level is resonant with the Fermi energy of the metallic layer. This creates a sequence of distinct capacitance peaks whose bias positions can be directly converted to an energy scale to determine the electronic spectrum of the confined structure. The evolution of the spectrum in magnetic fields allows deduction of the nature of the bound states.

The optical properties of a one-dimensional (1D) electron gas with only one or two occupied subbands have been studied in semiconductor quantum wires, obtained by electron beam lithography and subsequent low-energy ion bombardment of modulation-doped GaAs quantum wells. Large optical singularities have been observed at the Fermi level, both in optical absorption and emission. They disappear at temperatures comparable to the Fermi energy. The 1D singularities are much larger and sharper than in 2D systems due to the lack of certain hole recoil effects in 1D. The experimental data are in qualitative agreement with theoretical results based on the exact diagonalization of finite chains.

We demonstrate that ballistic electrons in ultra-high mobility two-dimensional electron systems can be controlled in ways analogous to the manipulation of photons in optical systems. Among the electron-optical structures we demonstrate are emitters, detectors, absorbers, and refractors. These advances may enable the design of new types of electronic circuits.

Modulation-doped quantum wire structures have been fabricated in the extreme quantum limit in which only the lowest one-dimensional (1D) subband is occupied by electrons. A pronounced Fermi edge singularity is observed for the first time in the absorption and emission spectra of the 1D electron gas. It has a strong temperature dependence determined by the Fermi energy and is much sharper than in two dimensions. The experimental results agree qualitatively with exact diagonalization studies of finite Hubbard chains.

Recent progress of GaAs-on-Si technology is reviewed and the importance of initial nucleation is emphasized. Growth initiation with a gallium prelayer at a suitable low temperature combined with migration enhanced epitaxy and in-situ thermal annealing may be able to give substantially improved material quality. During the initial growth, use of the minimum necessary As4:Ga flux ration is found to be critical. Patterned growth or post-growth patterning releases tensile stress only if the growth edges are free. GaAs-on-Si was almost completely relaxed from stress by post-growth patterning to 5 μm × 5 μm size patterns. Steps on the surface owing to substrate misorientation do not affect the two-dimensional electron gas (2DEG) transport properties in AlGaAs/GaAs selectively doped heterojunction transistor (SDHT) structures. For a sheet density of 1012 cm−2, a 2DEG mobility greater than 50 000 cm2 V−1 s−1 at 77 K was obtained on silicon substrates, which was found to be adequate for the fabrication of the state-of-the-art devices. For 1 μm-gate-length SDHTs, maximum transconductances of 220 and 365 mS mm−1 were measured at 300 K and 77 K, respectively. A minimum propagation delay time of 28 ps stage−1 was measured at 300 K for direct coupled field effect transistor logic (DCFL) ring oscillators with a power dissipation of 1.1 mW stage−1. The propagation delay time reduced to 17.6 ps stage−1 at 77 K. These results are comparable to the SDHT technology on GaAs substrates.