Ivan A. Larkin

Noncontact scanning probe microscope potentiometry of surface charge patches: Origin and interpretation of time-dependent signals

We have modeled the generation of a voltage signal in noncontact potentiometric scanning force microscopy for a locally enhanced potential due to an isolated patch of electrostatic charge on an insulating surface. Both the distribution of the charge and the dielectric nature of the insulator is taken into account. When the charge is assumed to disperse ohmically on the surface, a complex time-dependent decay in the signal occurs. The profile of the decay predicted by this assumption has a shape similar to that observed experimentally in submicron scale contact charging experiments.

Modeling the patterned two‐dimensional electron gas: Electrostatics

We present analytical calculations of the potential in a two‐dimensional electron gas (2DEG) generated by patterned polygon gates on the surface of a heterostructure. They give the bare and screened potentials and reveal the effect of different boundary conditions on the surface. The formulas for the bare electrostatic potential from patterned gates are simple enough to be plotted in spreadsheets; they give threshold voltages, estimates of the region occupied by the 2DEG, and the energies of some collective infra‐red excitations. We also consider the screened potentials in linear response, where no part of the 2DEG is fully depleted, which can again be found within an electrostatic approximation. The behavior of the exposed surface between the gates affects the potential strongly. Surface states provide perfect pinning of the Fermi energy in the ‘‘equipotential’’ model, the usual assumption, but this requires charge to move to the surface from the 2DEG. The charge on the surface is held fixed in response ...

Anisotropic piezoelectric effect in lateral surface superlattices

We have studied the potential induced by lateral surface superlattices deposited on a GaAs/AlGaAs heterostructure as a function of bias and orientation of the gates. By using the gates to null the total potential, we extracted the contribution to this potential in the absence of gate bias. Its angular dependence shows that it is dominated by strain from the gates coupled to the electrons by the piezoelectric effect.

Anharmonic periodic modulation in lateral surface superlattices

Abstract We have measured the longitudinal magnetoresistance of a lateral surface superlattice with a period of 270 nm where the electrons are only 28 nm deep. The commensurability oscillations have a strong second harmonic content. This reflects a non-sinusoidal potential in the two-dimensional electron gas, a consequence of the shallow structure. The shape of the potential cannot be explained by a pinned GaAs surface and indicates that the surface charge is frozen or that the electrons feel an elastic strain field from the metal gates.

Single electron charging at temperatures above 4 K in ultrasmall lateral quantum dots patterned on shallow GaAs/AlGaAs heterostructures

We demonstrate single electron charging in fully controllable nanoscale quantum devices at temperatures above 4 K. Hitherto, single electron devices operating at ‘‘high’’ temperatures have been two‐terminal, having no control electrode, whereas fully tunable structures such as quantum dots have only shown charging effects at temperatures of 4 K or less. We have fabricated ultrasmall quantum dots on modulation doped heterostructures where the two‐dimensional electron gas is less than 30 nm from the surface. Dots with lithographic diameter 150 nm show Coulomb oscillations up to temperatures of 7 K. Higher temperature operation allows potential applications to be considered without the need, for example, of a dilution fridge.

The effect of growth temperature, δ-doping and barrier composition on mobilities in shallow AlGaAsGaAs two-dimensional electron gases

Abstract A series of two-dimensional electron gas (2DEG) structures have been grown with the 2DEG only 28 nm from the surface. The effects of growth temperature and δ-doping density have been investigated, and a comparison has been made between AlAs and Al 0.3 Ga 0.7 As barriers. A mobility of 340,000 cm 2 V −1 s −1 at 4 K has been measured for a shallow 2DEG with an Al 0.3 Ga 0.7 As barrier, which is the highest reported for such a structure.

Potential modulation under lateral surface superlattices

We have used the commensurability oscillations to make a systematic analysis of the origin and form of the potential under lateral surface superlattices with periods down to 100 nm. This potential arises from a combination of mechanical strain and electrostatics, and we have determined its dependence on the gate bias, the period and mark-space ratio of the gate array, and the depth of the electrons.

Edge of the two-dimensional electron gas in a gated heterostructure

We describe a quantitative electrostatic theory of a two-dimensional electron gas (2DEG) confined by a semi-infinite gate. It includes the finite depth of the 2DEG below the surface of the heterostructure, which enables us to treat the whole range of gate voltages. Two models are used for the boundary condition on the free surface, with either pinned potential or frozen charge. Our results predict the position and width of the strips of incompressible liquid in the quantum Hall regime, and are in good agreement with recent experiments.

Spectroscopy of quantum dots in the few electron limit

We have fabricated quantum dots with diameters of around 150 nm which contain less than 10 electrons. The conductance as a function of d.c. bias shows that the energy for adding successive electrons is around 3 meV, in good agreement with numerical modelling.

Resonant tunneling through a pseudo-quantum well in single-barrier heterostructure

Resonant tunneling through virtual states in a wide pseudo-quantum well has been observed at liquid He temperature. This well is formed in the spacer layer region (n - -GaAs) of the single barrier heterostructure n + -GaAs/n - -GaAs/AlAs/n - -GaAs/n + -GaAs due to electron reflection from the main barrier at one side and from the smooth potential drop through the n - /n + junction in GaAs at the other side.