Structural, optical and vibrational properties of self-assembled Pbn+1(Ti1-x Fex)nO(3n+1)-δ Ruddlesden-Popper superstructures.

Bulk crystals and thin films of PbTi(1-x)FexO3(-δ) (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pbn+1(Ti(1-x)Fex)nO3(n+1)-δ forms spontaneously during pulsed laser deposition of PTFO on LaAlO3 substrates. High-resolution transmission electron microscopy, x-ray diffraction and x-ray photoemission spectroscopy were utilised to perform a structural and compositional analysis, demonstrating that n ≃ 8 and x ≃ 0.5. The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes and the observation of second harmonic generation establishes a non-centrosymmetric point group for Pbn+1(Ti0.5Fe0.5)nO3(n+1)-δ, a prerequisite for (but not proof of) ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO3 and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

Giant reduction of InN surface electron accumulation: compensation of surface donors by Mg dopants.

Extreme electron accumulation with sheet density greater than 10(13) cm(-2) is almost universally present at the surface of indium nitride (InN). Here, x-ray photoemission spectroscopy and secondary ion mass spectrometry are used to show that the surface Fermi level decreases as the Mg concentration increases, with the sheet electron density falling to below 10(8) cm(-2). Surface space-charge calculations indicate that the lowering of the surface Fermi level increases the density of unoccupied donor-type surface states and that these are largely compensated by Mg acceptors in the near-surface hole depletion region rather than by accumulated electrons. This is a significant step towards the realization of InN-based optoelectronic devices.

Thickness dependence of the strain, band gap and transport properties of epitaxial In2O3 thin films grown on Y-stabilised ZrO2(111).

Epitaxial films of In(2)O(3) have been grown on Y-stabilised ZrO(2)(111) substrates by molecular beam epitaxy over a range of thicknesses between 35 and 420 nm. The thinnest films are strained, but display a 'cross-hatch' morphology associated with a network of misfit dislocations which allow partial accommodation of the lattice mismatch. With increasing thickness a 'dewetting' process occurs and the films break up into micron sized mesas, which coalesce into continuous films at the highest coverages. The changes in morphology are accompanied by a progressive release of strain and an increase in carrier mobility to a maximum value of 73 cm(2) V(-1) s(-1). The optical band gap in strained ultrathin films is found to be smaller than for thicker films. Modelling of the system, using a combination of classical pair-wise potentials and ab initio density functional theory, provides a microscopic description of the elastic contributions to the strained epitaxial growth, as well as the electronic effects that give rise to the observed band gap changes. The band gap increase induced by the uniaxial compression is offset by the band gap reduction associated with the epitaxial tensile strain.

Surface band-gap narrowing in quantized electron accumulation layers.

An energy gap between the valence and the conduction band is the defining property of a semiconductor, and the gap size plays a crucial role in the design of semiconductor devices. We show that the presence of a two-dimensional electron gas near to the surface of a semiconductor can significantly alter the size of its band gap through many-body effects caused by its high electron density, resulting in a surface band gap that is much smaller than that in the bulk. Apart from reconciling a number of disparate previous experimental findings, the results suggest an entirely new route to spatially inhomogeneous band-gap engineering.

Accurate ultra-low-energy secondary ion mass spectrometry analysis of wide bandgap GaN/In(x)Ga(1-x)N structures using optical conductivity enhancement.

Ultra-low-energy secondary ion mass spectrometry has been used to undertake a structural analysis of GaN-In(x)Ga(1-x)N (x approximately 0.25) quantum wells used in optoelectronic devices. The high resistivity of intrinsic GaN-In(x)Ga(1-x)N restricts the necessary electrical path between the analyzed area and the instrument ground potential resulting in surface charge accumulation. Consequently, unstable and unrepresentative depth profiles tend to be produced. A technique known as optical conductivity enhancement (OCE) has been used during depth profiling to reduce the material resistivity. This creates an electrical path between the sample and holder, eliminating charge build up and resulting in accurate depth profiles.

The donor nature of muonium in undoped, heavily n-type and p-type InAs.

The charge state of muonium has been investigated in p-type doped, nominally undoped (low n-type) and heavily n-type doped InAs. The donor Mu(+) state is shown to be the dominant defect in all cases. Consequently, muonium does not simply counteract the prevailing conductivity in this material. This is consistent with the charge neutrality level lying above the conduction band minimum in InAs.

Unintentional conductivity of indium nitride: transport modelling and microscopic origins.

A three-region model for the high n-type conductivity in InN, including contributions from the bulk, surface and buffer layer interface of the sample, is considered. In particular, a parallel conduction analysis is used to show that this model can account for the carrier concentration and mobility variation with film thickness that has previously been determined from single-field Hall effect measurements. Microscopic origins for the donors in each region are considered, and the overriding tendency towards n-type conductivity is discussed in terms of the bulk band structure of InN.

Surface electron accumulation and the charge neutrality level in In2O3.

High-resolution x-ray photoemission spectroscopy, infrared reflectivity and Hall effect measurements, combined with surface space-charge calculations, are used to show that electron accumulation occurs at the surface of undoped single-crystalline In2O3. From a combination of measurements performed on undoped and heavily Sn-doped samples, the charge neutrality level is shown to lie approximately 0.4 eV above the conduction band minimum in In2O3, explaining the electron accumulation at the surface of undoped material, the propensity for n-type conductivity, and the ease of n-type doping in In2O3, and hence its use as a transparent conducting oxide material.

Quantized electron accumulation states in indium nitride studied by angle-resolved photoemission spectroscopy.

Electron accumulation states in InN have been measured using high resolution angle-resolved photoemission spectroscopy (ARPES). The electrons in the accumulation layer have been discovered to reside in quantum well states. ARPES was also used to measure the Fermi surface of these quantum well states, as well as their constant binding energy contours below the Fermi level E(F). The energy of the Fermi level and the size of the Fermi surface for these quantum well states could be controlled by varying the method of surface preparation. This is the first unambiguous observation that electrons in the InN accumulation layer are quantized and the first time the Fermi surface associated with such states has been measured.

Negative band gaps in dilute InNxSb1-x alloys.

A thin layer of InNSb has been fabricated by low energy nitrogen implantation in the near-surface region of InSb. X-ray photoelectron spectroscopy indicates that nitrogen occupies approximately 6% of the anion lattice sites. High-resolution electron-energy-loss spectroscopy of the conduction band electron plasma reveals the absence of a depletion layer for this alloy, thus indicating that the Fermi level is located below the valence band maximum (VBM). The plasma frequency for this alloy combined with the semiconductor statistics indicates that the Fermi level is located above the conduction band minimum (CBM). Consequently, the CBM is located below the VBM, indicating a negative band gap material has been formed. These measurements are consistent with k.p calculations for InN0.06Sb0.94 that predict a semimetallic band structure.

Intrinsic electron accumulation at clean InN surfaces.

The electronic structure of clean InN(0001) surfaces has been investigated by high-resolution electron-energy-loss spectroscopy of the conduction band electron plasmon excitations. An intrinsic surface electron accumulation layer is found to exist and is explained in terms of a particularly low Gamma-point conduction band minimum in wurtzite InN. As a result, surface Fermi level pinning high in the conduction band in the vicinity of the Gamma point, but near the average midgap energy, produces charged donor-type surface states with associated downward band bending. Semiclassical dielectric theory simulations of the energy-loss spectra and charge-profile calculations indicate a surface state density of 2.5 (+/-0.2)x10(13) cm(-2) and a surface Fermi level of 1.64+/-0.10 eV above the valence band maximum.