Quantized Accumulation Layers Research Articles

A model for the quantized accumulation layer in metal-insulator-semiconductor structures

A model is proposed for the quantized accumulation layer based on the union of a two-dimensional electron gas contained in several energy subbands and a three-dimensional electron-gas distributed in a continuum of energy levels. The model is valid for both low and high temperatures and is formulated to allow the incorporation of quantum effects in a simulation based on classical models. It therefore permits the study of the different operation regions of a metal-insulator-semiconductor structure with continuity between them. Equations of the model are detailed as well as the solution procedure. The validity of the model is discussed and results obtained from both the quantum and classical model are compared. Capacitance curves obtained with both models are also compared to experimental ones.

Persistent photoenhancement of hydrogen-implanted quantized accumulation layers on ZnO surfaces

Strong hydrogen-implanted accumulation layers on ZnO surfaces, produced and maintained at 80 K, can be further enhanced by illumination with visible light. The attainable enhancement is surprisingly large, resulting in an enormous surface electron density of up to 6 × 10 14cm −2, and persists indefinitely after the light is switched off. The photoenhanced layer is practically identical in width (10–20 Å) and transport characteristics to the implanted layer upon which it is based. Thus, it constitutes, just as the initial hydrogen-implanted layer, a two-dimensional electron accumulation layer, by far the strongest ever attained on any semiconductor surface. The photoenhancement effect is not fully understood. Various aspects of this effect are studied in an attempt to gain some insight into the processes involved. On the basis of the results presented we tentatively suggest that latent centers consisting of H + 2 species are introduced by the hydrogen implantation, in addition to the fully ionized proton donors responsible for the implanted accumulation layer. Illumination dissociates these species and shifts the protons so produced into sites in which they become fully ionized, thus augmenting the surface electron density of the implanted surface.

Large photoenhancement of hydrogen-implanted quantized accumulation layers on ZnO surfaces

We report in this letter on an interesting and unexpected effect which is not fully understood yet. Strong hydrogen-implanted accumulation layers on ZnO surfaces, produced and maintained at 80 K, can be further enhanced by illumination with visible light. The enhancement attained is surprisingly large, amounting to a twofold increase in surface electron density, up to 6×1014 cm−2. The photoenhanced layer is less than 10–20 Å in width, thus constituting a two-dimensional electron system, which is by far the strongest ever attained on any semiconductor surface. Measurements of the electron transport characteristics and of the spectral response of the photoenhancement process are presented.

Space charge and surface state characteristics of the silicon/electrolyte interface

The essentially blocking nature of the silicon/electrolyte (S/E) interface enables charge to be induced electrostatically at the interface by an applied bias. The use of pulsed rather than DC biases provides a fairly detailed picture of the silicon interface. The results reported here concern the silicon space-charge layer, localized states at the S/E interface and charge leakage across the interface. As to the first, evidence is presented that strong, quantized accumulation layers of excess surface-electron densities as high as 10 14 cm −2 can be induced at the Si surface, an order of magnitude larger that can be attained in Si inversion layers in MOS structures. The localized states are of total density of about 10 12 cm −2 and of capture cross sections around 5 × 10 −18 cm 2. The nature of these states is not known; they are probably fast surface states at the silicon surface. The charge leakage occurs under strong accumulation conditions, very likely by electron tunneling from the silicon electrode into the electrolyte. It takes place practically instantaneously, but the leaked charge remains stored near the interface for a considerable time. Some suggestions concerning this unexpected behavior are put forward.

Magnetotunneling from accumulation layers in AlxGa1-xAs capacitors.

Current-voltage (I-V), ac capacitance-voltage (C-V), and conductance-voltage (G-V) measurements have been made of the effect of magnetic fields up to 15 T on tunneling at 1.6 K from accumulation layers on n-type GaAs. The samples used are ${n}^{\mathrm{\ensuremath{-}}}$-GaAs--${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As--n $^{+}$-GaAs capacitors in which the GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As barrier height is low enough, \ensuremath{\sim}0.3 eV, and the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As is thin enough, \ensuremath{\sim}20 nm, that direct tunneling of electrons occurs. Magnetic freezeout of carriers in ${n}^{\mathrm{\ensuremath{-}}}$-GaAs for H\ensuremath{\gtrsim}4 T can be determined from C-V curves. For ${n}^{+}$-GaAs biased positive, ${V}_{G}$>0, a quantized accumulation layer forms at the ${n}^{\mathrm{\ensuremath{-}}}$-GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As interface. Structure is observed in I-V, C-V, and G-V curves due to tunneling from Landau levels in the accumulation layer. Two samples are compared in detail. In one sample the Landau-level structure is relatively simple. In the other, spin splitting is observed in the first three Landau levels. The dependence of the surface concentration of electrons on ${V}_{G}$ is determined from tunneling currents at fixed ${V}_{G}$ and varying magnetic field. A second subband in the accumulation layer begins to be populated when the surface concentration \ensuremath{\gtrsim}7\ifmmode\times\else\texttimes\fi{}${10}^{11}$/${\mathrm{cm}}^{2}$. This is the first example of a system in which it is possible to use tunneling to examine the formation of an accumulation layer in a semiconductor, going from depletion through flat-band into accumulation, by applying a bias voltage.

Plasmas on Quantized Accumulation Layers on ZnO Surfaces

Low-energy---electron-loss spectroscopy is used to study collective surface excitations on accumulation layers on ZnO. A prominent loss peak is observed which shifts in energy with electron concentration. The results are well accounted for theoretically and provide direct evidence for the existence of two-dimensional plasmons.

Mobility of Electrons in a Quantized Accumulation Layer on ZnO Limited by Phonon Scattering

Mobility of Electrons in a Quantized Accumulation Layer on ZnO Limited by Phonon Scattering

ELS studies of quantized accumulation layers on ZnO surfaces

ELS studies of quantized accumulation layers on ZnO surfaces

ELS studies of quantized accumulation layers on ZnO surfaces

Electron energy loss spectroscopy studies of clean ZnO surfaces under extreme accumulation layer conditions are presented. The experimental results are analyzed in terms of excitations due to coupled surface phonons and two-dimensional surface plasmons. The fair agreement between the experiment and theory strongly suggests that part of the losses are due to two-dimensional surface plasmons.

Field-effect mobility in quantized accumulation layers on ZnO surfaces

Measurements are reported of the field-effect mobility ${\ensuremath{\mu}}_{\mathrm{FE}}$ in strong accumulation layers on the polar surfaces of ZnO crystals. Results are presented of ${\ensuremath{\mu}}_{\mathrm{FE}}$ as a function of surface electron density $\ensuremath{\Delta}N$ (in the range ${10}^{12}$-${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}2}$) and of temperature (2-300 K). By a suitable integration procedure it has been possible to derive from the field-effect data the ordinary conductivity mobility $\ensuremath{\mu}$ as a function of $\ensuremath{\Delta}N$ and temperature. At a fixed temperature both ${\ensuremath{\mu}}_{\mathrm{FE}}$ and $\ensuremath{\mu}$ initially rise with increasing $\ensuremath{\Delta}N$, reach a maximum at $\ensuremath{\Delta}N\ensuremath{\approx}(2\ensuremath{-}5)\ifmmode\times\else\texttimes\fi{}{10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}2}$, and then gradually decrease with a further increase in $\ensuremath{\Delta}N$. For high $\ensuremath{\Delta}N$ (\ensuremath{\gtrsim} ${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}2}$) ${\ensuremath{\mu}}_{\mathrm{FE}}$ and $\ensuremath{\mu}$ are practically temperature independent; for low $\ensuremath{\Delta}N$ (\ensuremath{\lesssim} 3 \ifmmode\times\else\texttimes\fi{} ${10}^{12}$) they decrease strongly with decreasing temperature, indicating carrier localization at low temperatures. The results of the conductivity mobility $\ensuremath{\mu}$ agree well with those of the Hall mobility ${\ensuremath{\mu}}_{H}$ reported earlier, and provide further support for the model proposed there of charged scattering centers consisting of large conglomerates of surface ions.

Negative magnetoresistance in quantized accumulation layers on ZnO surfaces

Results are presented of magnetoresistance in quantized accumulation layers on the ($000\overline{1}$) surface of ZnO. The measurements were performed at low temperatures $T(2\ensuremath{-}80 \mathrm{K})$ and very high surface electron densities [(0.7-7) \ifmmode\times\else\texttimes\fi{} ${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}2}$]. The magnetoresistance was found to be negative over the entire studied range of magnetic induction $B$ (up to 60 kG). Its absolute magnitude increases quadratically with $\frac{B}{T}$ at low values of $\frac{B}{T}$ and tends to saturation at high $\frac{B}{T}$. A spin scattering model adapted to surface channels accounts very well for the experimental data. The main conclusion drawn is that scattering centers with giant magnetic moments (around 100 Bohr magnetons) must be present on the ZnO surface. Such giant moments may originate from ionic clusters in combination with the free electrons localized at and/or screening the charged clusters. A quantitative analysis of the data reveals also the presence of centers with much lower magnetic moments (around 10 Bohr magnetons).

Hall mobility of electrons in quantized accumulation layers on ZnO surfaces

Hall-effect measurements were performed on the ($000\overline{1}$) face of ZnO single crystals under extreme accumulation layer conditions with surface electron densities ranging from \ensuremath{\sim}${10}^{12}$ up to \ensuremath{\sim}${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}2}$. The measurements were performed as a function of the surface-electron density and temperatures in the temperature range of 2-300 K. The electron density in the accumulation layer was found to be essentially temperature independent over the whole temperature range. The electron mobility, on the other hand, is temperature dependent for surface-electron densities up to \ensuremath{\sim}${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}2}$, decreases with decreasing temperatures and at low temperatures carrier localization occurs. As a function of the surface-electron density (at a fixed temperature) the mobility initially increases, reaches a maximum, and then slowly decreases. The results suggest scattering and/or localization by charged scattering centers which apparently conglomerate into large clusters.

Density-of-states mass for quantized accumulation layers measured from C-V characteristics

Density-of-states mass for quantized accumulation layers measured from C-V characteristics

Negative Magnetoresistance in Very Strong Accumulation Layers on ZnO Surfaces

Negative transverse magnetoresistance was observed in quantized accumulation layers on the ($000\overline{1}$) face of ZnO single crystals. The results are interpreted in terms of surface scattering center with giant magnetic moments, possibly due to the formation of charged clusters. Good quantitative agreement is found between theory and experiment.