J. H. Davies

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.

Random telegraph signal amplitudes in sub 100 nm (decanano) MOSFETs: a 3D 'Atomistic' simulation study

In this paper we use 3D simulations to study the amplitudes of random telegraph signals (RTS) associated with the trapping of a single carrier in interface states in the channel of sub 100 nm (decanano) MOSFETs. Both simulations using continuous doping charge and random discrete dopants in the active region of the MOSFETs are presented. We have studied the dependence of the RTS amplitudes on the position of the trapped charge in the channel and on the device design parameters. We have observed a significant increase in the maximum RTS amplitude when discrete random dopants are employed in the simulations.

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.

A simple model for the characteristics of GaAs/AlGaAs modulation-doped devices

The authors have derived a simple model based on electrostatics for predicting the density and threshold voltages of the two-dimensional electron gas in a GaAs-AlGaAs heterostructure, and tested it against measurements of the capacitance-voltage characteristics. The model correctly accounts for the large decrease in magnitude of the threshold voltage on cooling from room to liquid helium temperatures, caused by the freezing of the DX centres below 150 K. It also shows that the concentration of electrons is almost independent of the concentration of donors, due to the layer of frozen DX centres. The authors have confirmed the model by performing an experiment in which the sample is cooled under a large negative gate bias, which keeps the DX centres empty. The threshold voltage at low temperatures now retains its high-temperature value. Subsequent annealing causes the threshold voltage to move towards its initial low-temperature value as the DX centres refill, before returning to the high-temperature value again. The measured values of the capacitance are also explained by the model.

Ab-initio Coulomb Scattering in Atomistic Device Simulation

As devices shrink to dimensions below 0.1 μm, it becomes essential to treat impurities and carriers as individual charges. We describe some approaches to ionised impurity scattering where the potential of the impurities is included directly in the dynamics of a Monto Carlo type simulation rather than as a scattering rate. The divergence in the Coulomb potential creates difficulties for mesh-based solutions of Poissons equation, which we have compared with more accurate Ewald summation. However, we find that the mesh does not introduce significant errors, and reproduces well the expected mobility as a function of doping. Highly accurate integration of the equation of motion is needed for free carriers, and the initial distribution is problematic. In contrast, a simple treatment of phonon scattering by Brownian dynamics is more tolerant of errors because it tends to restore the system to equilibrium.

Modulation of Landau levels by a one-dimensional periodic potential

Abstract We have studied the magnetoresistance of short-period one-dimensional lateral superlattices with strong modulation at low temperatures. Striking interference was seen between commensurability oscillations and the Shubnikov–de Haas effect. This cannot be explained within the usual semiclassical theory and a quantum mechanical explanation is proposed.

LARGE PERIODIC POTENTIAL UNDER LATERAL SURFACE SUPERLATTICES FABRICATED FROM HETEROEPITAXIAL STRESSOR LAYERS

We have fabricated lateral surface superlattices by etching a strained layer of In0.2Ga0.8As near the surface of a heterostructure. This provides strong modulation of the electron gas while retaining a high electron mobility. The potential arises mainly from strain and the piezoelectric effect, which depends on orientation, and from the change in the surface profile. The fundamental components of these two contributions cancel in one orientation to leave a dominant second harmonic. This effectively halves the period of the superlattice from its lithographic value and provides a promising technique for creating potentials with a period comparable to the Fermi wavelength.

Electron transport in shallow heterostructures with AlGaAs and AlAs barriers

Two series of shallow GaAs heterostructures, with AlGaAs and AlAs barriers respectively and both delta -doped with around 4*1016 m-2 Si donors, have been studied using low-temperature magnetotransport techniques. The electrons in these structures were confined against interfaces 28 nm from the surface. The AlGaAs barrier samples depleted at a uniform rate with bias under a Schottky gate, but the carrier mobility was considerably greater at all biases than predicted assuming randomly positioned donors. The depletion and mobility data for the AlAs barrier samples could only be explained by postulating the existence of a pool of electrons around the doping plane, which screened the donors to produce high mobilities in ungated samples but which could be removed by the application of gate bias. Bias cooling experiments on the AlGaAs samples showed that a proportion of the donor centres were occupied when samples with as few as 4*1016 m-2 donors were cooled without bias. The mobility data from such samples are discussed assuming correlations between the positions of these occupied donors.

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.