E A Johnson

Subband dependent mobilities and carrier saturation mechanisms in thin Si doping layers in GaAs in the high density limit

Shubnikov-de Haas and persistent photoconductivity measurements have been performed as a function of hydrostatic pressure to study the saturation of the free electron concentration and the mobilities of the individual subbands at high doping densities in very thin sheets (2, 5, 10 nm) of silicon donors in MBE GaAs. The samples were grown at very low temperature (400 degrees C) in order to limit dopant diffusion, and silicon concentrations were close to the solubility limit at this temperature. As has been shown previously with spike-doped GaAs(Si), the relative occupancies and the mobilities of the lower subbands are very sensitive to the spreading of the dopant distribution. A routine was developed for the analysis of the Fourier transforms of the complex pattern of Shubnikov-de Haas peaks in order to provide quantitative values for the mobilities of the individual subbands. The results of this analysis are compared with values deduced from the magnetic field dependence of the resistivity and Hall effect. On applying hydrostatic pressures of the order of 15 kbar in the dark, a decrease of the free electron concentration of a factor of two was observed. This was accompanied by an increase in the mobility of all the subbands due to the change in the charge state of the silicon donors in the doping slab. With the two thinnest slabs the mobility at ambient pressure is so low in the i=0 subband that Shubnikov-de Haas peaks from this subband could not be detected at fields up to 15 T, although strong peaks could be observed from the higher order subbands. After illumination of the thinnest sample at high pressure the measured free electron concentration is not restored to the zero pressure value. One possibility is that the missing electrons occupy a localized non-metastable Si state resonant with the conduction band rather than a DX centre. The pressure coefficient of the carrier density yields an extrapolated position for the energy level for the Si localized state of 270+or-10 meV above the Gamma -conduction band edge at ambient pressure.

Electronic properties of spike-doped InSb

Electronic properties of spike-doped quantum wells in InSb are calculated and the effects of a uniform background p-doping are studied. The calculation is self-consistent and includes non-parabolic corrections. In this many-sub-band system, electrons in high- and low-energy sub-bands are found to respond differently to the overall transfer of charge from the well to acceptor states.

GaAs asymmetrically doped n‐i‐p‐i‐ superlattices for 10 μm infrared subband detector and modulator applications

A theoretical framework for the design of asymmetrically doped n‐type n‐i‐p‐i‐ superlattices for subband detector applications in the 10 μm spectral range is described. Excellent agreement is found with the subband absorption spectra measured in a series of GaAs n‐type n‐i‐p‐i‐ samples and oscillator strengths, transition energies, and dipole matrix elements comparable with conventional quantum‐well heterostructure detectors are found. Pronounced IR‐absorption modulation by optical pumping with band‐gap radiation is seen, due to the enhanced interband recombination time resulting from the type‐II n‐i‐p‐i‐ potential. The prospective advantages of n‐type n‐i‐p‐i‐ devices for the detection and modulation of 10 μm radiation are discussed.

Resonant interband tunnelling in homoepitaxial InSb structures with inversion barriers

Resonant interband tunnelling through doping barriers in homoepitaxial InSb is observed. The band structures for the chosen doping profiles are calculated self-consistently including non-parabolicity effects. A p+- delta p+-i-n+-i- delta p+-p+ doping structure is designed to create a two-dimensional triangular inversion well between two degenerate p-type slab layers. Holes in the p-type layer then tunnel resonantly through the two-dimensional occupied electron states of the n+-induced well into the collector on the other side. These structures exhibit symmetric negative differential resistance (NDR) with peak-to-valley ratios of 1.6 and peak current densities of 120 A cm-2 at 4.2 K.

Shubnikov–de Haas effect and persistent photoconductivity in In0.52Al0.48As

The Shubnikov–de Haas effect in InAlAs measured using pulsed magnetic fields up to 50 T is reported. The InAlAs samples were grown by molecular beam epitaxy (MBE) and were either δ or slab doped with silicon at densities up to 7×1012 cm−2. Comparison of experimental subband densities with those calculated self-consistently shows that spreading of Si occurs by surface segregation at growth temperatures of ∼520 °C, similar to its behavior in MBE-grown InGaAs. In contrast to InGaAs, the InAlAs exhibits persistent photoconductivity which appears to be caused by a bulk defect rather than DX(Si) states.

Damage-induced changes in the electronic properties of InSb(100): implications for surface preparation

A combination of surface-sensitive techniques and electron transport measurements have been used to characterize the effect of argon ion bombardment and annealing on a series of InSb(100) samples. Ex situ electrical conductivity and magnetoresistance measurements at 4.2 K, and in situ high-resolution electron energy loss spectroscopy (HREELS) carried out at 300 K, indicate that all the samples studied exhibit enhanced n-type behaviour after the surface cleaning procedure. This effect is most pronounced after annealing to between 450 and 500 K and arises from the formation of a high-density electron gas with a sheet carrier concentration of approximately (6.5-9.0)*1012 cm-2. The carrier concentration is significantly reduced on annealing to higher temperatures up to a maximum of 700 K. Electron-energy-dependent HREELS measurements of the plasmon energy and intensity, in conjunction with model calculations based on dielectric theory, indicate that the n-type layer is approximately 500 AA thick and located approximately 175 AA below a surface depletion layer. The occupancy of the electronic subbands has been obtained by Shubnikov-de Haas measurements and self-consistent calculations. These show that the positive charge which confines the electrons is spread over approximately 300 AA with a best fit being provided by a Gaussian-like potential profile. The calculations demonstrate that the corresponding wavefunction spread for the i=0 subband, which contains approximately 40% of the total carriers induced, has a spatial dimension of approximately 500 AA in good agreement with the HREELS results.

Charge depletion of n+-In0.53Ga0.47As potential wells by background acceptor doping

Charge depletion from 20 monolayers of n+-In0.53Ga0.47As, uniformly doped with Si donors and embedded within Be-doped In0.53Ga0.47As, has been studied at 1.2 K by magnetotransport measurements. Electron subband energies and densities associated with the n+-In0.53Ga0.47As potential well prove sensitive to the presence of the acceptors at concentrations up to 3×1016 cm−3. Agreement between the experimental data and the electronic subband structure calculated self-consistently by solving the one-dimensional Schrodinger and Poisson equations is excellent. The results suggest that intentional background acceptor doping could be a useful mechanism for tuning subband fillings and energies in potential wells formed by highly confined donors.

Observation of Intersubband Transitions in Asymmetric δ-Doped GaAs, InSb and InAs Structures

Optical measurements in the mid infrared band on asymmetric δ-doped GaAs and asymmetrically slab-doped InSb and InAs doping superlattices (a-nipi’s) at 10K showed strong intersubband absorption features. We also report, for the first time, the observation of narrow (FWHM ~85cm-1) intersubband absorption in 10nm wide δ-doped GaAs/AlGaAs quantum wells at wavelengths near 10μm. The observed subband spectra are compared with a multi-layer optical matrix model in which the bare subband splitting energies are calculated self-consistently and the well is taken to be an anisotropic oscillator with an absorption resonance blue shifted by depolarisation effects. Good agreement is found between the calculated and the measured transition energies for the δ-doped GaAs/AlGaAs QW. In the δ-doped GaAs a-nipi transition energies are only weakly broadened by impurity disorder and FWHM line widths of <300cm-1 are found. The long recombination time produced by the electrostatic potentials in the a-nipi samples allows us to modulate the subband electron concentrations by optical excitation. Modulation of the intersubband absorption spectra was seen for optical pump densities as low as 20mW cm-2 for the GaAs superlattices and the induced subband spectra yield new information about the subband structure.

Conduction-band universality in GaAs-based systems

The non-parabolicity of the conduction band is predicted by a Kane four-band calculation to be approximately universal in direct-gap GaAs under varying pressure, and in direct-gap AlxGa1-xAs for various compositions x, when the band dispersion is plotted in natural units of effective Rydbergs and effective lattice constants. The predicted universality is expected to be a better approximation than the Kane approximation itself.

The utilization of DX centres in high-pressure studies of low-dimensional doping structures in GaAs

High-pressure experiments with thin slabs of silicon donors in GaAs are employed to search for the correlation effects expected from the formation of D+D- pairs expected from the negative-U model of DX centres. The mobilities in the individual subbands are very dependent on subband energy but a preliminary analysis does not require the existence of such pairs.