The electronic properties of clean InN(0001) surfaces have been investigated by high-resolution electron-energy-loss spectroscopy of the conduction band electron plasmon excitations and valence band x-ray photoemission spectroscopy. An intrinsic surface electron accumulation layer is found to exist, with the associated downward band bending occurring to neutralize charged donor-type surface states. Semiclassical dielectric theory simulations of the energy-loss spectra and charge profile calculations indicate a surface state density of 2.5(±0.2)×1013 cm−2 and a surface Fermi level of 1.64±0.10 eV above the valence band maximum and 0.89±0.10 eV above the conduction band minimum. This location of the surface Fermi level is in agreement with the valence band photoemission spectra. These observations are compared with recent evidence of electron accumulation at InN surfaces provided by capacitance–voltage profiling, Hall measurements, and synchrotron-radiation photoemission spectroscopy.

Indium nitride: Evidence of electron accumulation

The temperature dependence of the electron accumulation at clean $\mathrm{InN}(0001)\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ surfaces has been investigated by high-resolution electron-energy-loss spectroscopy. Semiclassical dielectric theory simulations of the energy-loss spectra at $295\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $565\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ along with solutions to the Poisson equation enable carrier profiles of the near-surface region to be determined. These measurements reveal similar electron accumulation for both temperatures. The surface-state density, ${N}_{\mathrm{ss}}$, and the surface Fermi level, ${E}_{\mathrm{FS}}$, were found to be independent of temperature, with ${N}_{\mathrm{ss}}\ensuremath{\sim}2.4\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ and ${E}_{\mathrm{FS}}\ensuremath{\sim}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the valence-band maximum. The slight difference in the carrier profiles between the two temperatures can be accounted for by the change in the electron screening length. This is a consequence of the reduction in the band gap that results in a decrease in the electron effective mass with increasing temperature.

Temperature invariance of InN electron accumulation

We demonstrate a novel doping mechanism of silicon, namely $n$-type transfer doping by adsorbed organic cobaltocene (${\mathrm{CoCp}}_{2}*$) molecules. The amount of transferred charge as a function of coverage is monitored by following the ensuing band bending via surface sensitive core-level photoelectron spectroscopy. The concomitant loss of electrons in the ${\mathrm{CoCp}}_{2}*$ adlayer is quantified by the relative intensities of chemically shifted Co2p components in core-level photoelectron spectroscopy which correspond to charged and neutral molecules. Using a previously developed model for transfer doping, the evolution in relative intensities of the two components as a function of coverage has been reproduced successfully. A single, molecule-specific parameter, the negative donor energy of $\ensuremath{-}(0.50\ifmmode\pm\else\textpm\fi{}0.15)\text{ }\text{ }\mathrm{eV}$ suffices to describe the self-limiting doping process with a maximum areal density of transferred electrons of $2\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$ in agreement with the measured downward band bending. The advantage of this doping mechanism over conventional doping for nanostructures is addressed.

Charge Transfer Doping of Silicon

Electron tunnelling spectroscopy has been used to investigate quantized levels in electron accumulation layers at InGaN surfaces. The tunnelling spectra exhibit a plateau in the normalized conductance which widens with increasing Ga-content, corresponding to the band gap of InGaN. The measured In x Ga 1-x N band gaps (between ∼0.65 eV for x = 1 and 1.8 eV for x = 0.43) are consistent with the band gaps determined by previous optical absorption and cathodoluminescence spectroscopy. Additional structures in the spectra reflect the two-dimensional electronic subbands in the surface quantum well. The subband energies depend on Ga-content, bulk doping level and the resultant shape of the surface potential well. The tunnelling spectra are compared with calculations of the potential well, the charge-profile and the subband energies.

Scanning tunnelling spectroscopy of quantized electron accumulation at InxGa1−xN surfaces

A modified Thomas-Fermi approximation (TFA) developed originally for inversion layers is used to calculate electron-density profiles, the spatial dependences of the potential, surface potentials, and the energies of bound states of depletion and accumulation layers in n-type GaAs. Remarkable agreement with recently published results of self-consistent calculations of Ehlers and Mills [Phys. Rev. B 34, 3939 (1986)] is achieved. All deficiencies of the conventional TFA discussed there are removed. Methodological advantages of the modified TFA are discussed.

Modified Thomas-Fermi theory for depletion and accumulation layers in n-type GaAs.

A modified Thomas-Fermi method for the calculation of the electronic structure of accumulation layers in MIS-systems is presented which in spite of the simplicity of a “classical” method yields results in good agreement with extensive self-consistent quantum mechanical calculations. It is shown that the modified Thomas-Fermi method is suitable to describe the characteristics of bound and mobile carriers in accumulation layers. The method is used to calculate accumulation layers in n- and p-type GaAs and to discuss the influence of the quantum effects at the interface on the dependence of the total charge in the accumulation layer on the surface band bending. Comparison is made with the classical results.



Eine modifizierte Thomas-Fermi-Methode fur die Berechnung der elektronischen Struktur von Akkumulationsschichten in MIS-Systemen wird vorgestellt, die trotz der Einfachheit einer „klassischen” Methode Resultate liefert, die gut mit denen quantenmechanischer selbstkonsistenter Rechnungen ubereinstimmen. Es wird gezeigt, das die modifizierte Thomas-Fermi-Methode sowohl in der Akkumulationsschicht gebundene als auch im gesamten Kristall frei bewegliche Ladungstrager beschreibt. Die Methode wird verwendet zur Berechnung der elektronischen Struktur von Akkumulationsschichten in n- und p-GaAs und zur Untersuchung des Einflusses der Quantisierungseffekte an der Halbleitergrenzflache auf die Abhangigkeit der Gesamtladung im Halbleiter von der Oberflachenbandverbiegung. Die Resultate werden mit denen klassischer Rechnungen verglichen.

Modified Thomas-Fermi Approximation for Accumulation Layers in MIS Structures

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Modified Thomas-Fermi Approximation for Accumulation Layers in MIS Structures