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.

https://iopscience.iop.org/article/10.1088/0268-1242/6/6/023/pdf

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

We have used secondary ion mass spectrometry, local vibrational mode infrared absorption, and electrical characterization to study the incorporation of Si delta‐doped planes in GaAs grown by molecular beam epitaxy at 400 °C, in the concentration range 0.01–0.5 monolayers. A correspondence is observed between the density of SiGa donors, the free electron concentration and the total Si coverage, up to a coverage of ∼1013 cm−2; however, above this value, the electron density falls, while [SiGa] remains constant up to a coverage of ∼1014 cm−2, and then falls below the detection limit at 0.5 monolayer coverage. These effects have been interpreted in terms of a model which takes account of Si migration and aggregation on the delta‐doped plane during deposition.

The lattice locations of silicon atoms in delta‐doped layers in GaAs

We present a theoretical study of the subband structure of both single and periodically -doped GaAs layers. We will discuss the influence of the -doping concentration and the -layer spacing on the confinement properties of such structures. A self-consistent analysis is made on these -doping systems in two different cases: (i) in the presence of a uniform electric field and inside an infinite potential barrier and (ii) under a linearly varying built-in electric field and within a finite potential barrier. Two features have been observed for the Stark effect in the first case: (i) when the electric field increases, the energies of the electronic subbands become lower and simultaneously a second quantum well appears and (ii) for larger electric fields, the subband structure is dramatically changed and the main role of confining charge carriers is reversed to the new quantum well. In the second case, the Stark effect is studied by taking, as a finite barrier, the Schottky potential of a -GaAs diode. One main effect was observed: the distortion of the V-shaped potential being more or less accentuated, depending on the applied bias voltage being reverse or direct respectively. Photocurrent (PC) data of a Si -doped GaAs structure have been taken as experimental support to validate our computation.

http://iopscience.iop.org/article/10.1088/0268-1242/12/11/010/pdf

Stark effect studied in -doped GaAs structures

Comparison of Si δ-doping with homogeneous doping in GaAs

The authors report on temperature-dependent Hall effect measurements on Si delta-doped GaAs samples grown by MBE at 480 °C, 530 °C and 620 °C. In the best sample grown at 480 °C the mobility is 6760 cm2 V-1 s-1 at 4.2 K. To the authors' knowledge this is the highest mobility ever reported in a delta-doped structure. From subband population measurements the spreading of the donors in the samples grown at low temperature is determined to be 20 A. On these high-mobility samples they were able to perform the first reported cyclotron resonance measurements. The electron effective mass is found to be considerably higher than that at the Gamma-conduction band minimum in GaAs.

/pdf/observation-of-high-mobility-and-cyclotron-resonance-in-20-a-2jwlqzr7fl.pdf

Observation of high mobility and cyclotron resonance in 20 Å silicon delta-doped GaAs grown by MBE at 480 °C

The saturation of the free-electron concentration at high doping densities in delta -doped GaAs is studied by Shubnikov-de Haas measurements under hydrostatic pressure. Saturation occurs as soon as the depth of the Hartree potential is large enough to make the energy of the DX centre line up with the Fermi energy. The ultimate limit of the low-temperature equilibrium free-electron concentration in truly delta -doped GaAs is estimated to be 5.5*1012 cm-2 for Si donors. For realistic samples grown at Ts=500 degrees C the electron concentration is saturated at 7*1012 cm-2 due to the occupation of DX centres in a doping sheet, of finite thickness ( approximately=30 AA). For samples grown at higher temperatures (Ts=600 degrees C) autocompensation can occur prior to the population of the DX centre. Increases in mobility with increasing pressure observed in the Shubnikov-de Haas measurements for the i=0 sub-band provide additional evidence for a movement of the donors away from the original doping plane.

http://iopscience.iop.org/article/10.1088/0268-1242/3/12/009/pdf

R L Williams

Papers

Saturation of the free-electron concentration in delta -doped GaAs: the DX centre in two dimensions