Surface Space Charge Research Articles

Surface waves and space charge layers in a spatially inhomogeneous plasma

A theory of surface waves in a layer of a spatially inhomogeneous cold electron plasma is presented. Four types of surface waves are revealed, and the conditions under which they can exist are determined. Complex frequency spectra are obtained, and the mechanisms for wave damping by plasma inhomogeneity are discussed.

In adlayers onc-plane InN surfaces: A polarity-dependent study by x-ray photoemission spectroscopy

The surfaces of In- and N-polarity InN grown by molecular-beam epitaxy have been investigated using core-level and valence band x-ray photoemission spectroscopy (XPS). From the ratio of the In and N core-level XPS signal intensities, the clean InN surfaces have been found to be terminated by In adlayers, in agreement with the predictions of previous first-principles calculations. The In- and N-polarity surfaces are terminated by $\ensuremath{\sim}3.4$ and $\ensuremath{\sim}2.0\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ of In, in each case 1 ML more than for Ga on GaN surfaces under metal-rich growth conditions. The valence band XPS indicates a valence band maximum to surface Fermi level separation of $1.32\ifmmode\pm\else\textpm\fi{}0.10\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for both In- and N-polarity InN surfaces, indicating that the surface space charge is independent of polarity and, unlike for GaN, is not significantly affected by spontaneous polarization. This is due to the In coverage on the surface of InN of both polarities being sufficient to efficiently screen the polarization charge.

Ion beam generation from sheath field of grid electrode and its application to surface treatment

As plasma immersion ion implantation using a conducting grid is very useful to reduce the effect of capacitance and charging in surface modification, it is broadly applied in the fields of insulating material implantation. On the other hand, there have been only few attempts to analyze the sheath dynamics including the conducting grid. If the bias voltage applied to the conducting grid is in the range of hundreds of volts for sputtering, or the energy for local restructuring of the atomic arrangement is much lower than voltage for implantation, the surface charge and space charge substantially affect the incident ion energy and ion current to the surface. The formation of the space and surface charges during one bias pulse period are thus analyzed through a one-dimensional and a two-dimensional particle-in-cell simulation. Experiment with the optimally designed grid on the basis of the simulation results is conducted, and the results of both cases with grid and without grid are compared. The improvement of adhesion is yielded by increasing surface roughness and chemical change in the polyurethane treatment using the conducting grid.

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Electrical Conductivity of Epitaxial SrTiO3 Thin Films as a Function of Oxygen Partial Pressure and Temperature

SrTiO3 (100) epitaxial films with thicknesses of 3, 1 μm, and 250 nm were prepared on MgO (100) substrates by pulsed‐laser deposition. The electrical conductivities of the thin films were systematically investigated as a function of temperature and ambient oxygen partial pressure. This was made possible by using a specially designed measurement setup, allowing the reliable determination of resistances of up to 25 GΩ in the temperature range of 600°–1000°C under continuously adjustable oxygen partial pressures ranging from 10−20 to 1 bar. The capabilities of the measurement setup were tested thoroughly by measuring a SrTiO3 single crystal. The well‐known characteristics, e.g., the decline of the conductivity with a slope of –1/4 under reducing conditions and the opposite +1/4 behavior in oxidizing atmospheres, are found in the log(σ)–log(pO2) profiles of the epitaxial films. However, the p‐type conductivity decreases, and the n‐type conductivity increases with decreasing film thickness. This phenomenon is attributed to the charge carrier redistribution in the surface space charge layers. Owing to the high surface‐to‐volume ratio, the space charge layers play an important role in thin films.

Effect of nanocrystalline alumina on ionic conductivity of rubidium nitrate solid electrolyte system

Complex impedance spectroscopic studies were carried out on RbNO 3 single crystal and polycrystalline samples in the temperature range between 50 and 300 °C. The ionic conductivity in polycrystalline sample is higher, about one order of magnitude, than that of single crystals. The increase in the value of conductivity in pellets could be due to increase in the concentration of defects at the grain boundaries, which can arise normally due to surface space charge and surface conduction of the sample. RbNO 3–Al 2O 3 composite solid electrolyte system has been characterized by DSC. Maximum enhancement of conductivity observed was more than two orders of magnitude at 140 °C in the RbNO 3–40 m/o Al 2O 3 system as against that in pure system. The enhanced conductivity in dispersed solid electrolyte system (DSES) was attributed to the formation of a space charge layer between the host material and the dispersoid.

Total photoyield experiments on hydrogen terminated n-type diamond

Total photoyield spectroscopy (TPYS) is applied to characterize the surface electronic properties of n-type chemical-vapour-deposited (CVD) (111) homoepitaxial diamond with hydrogen termination (H-termination). To discuss the negative electron affinity of n-type diamond surfaces in a general way, TPYS is also applied on p-type diamond such as a high-pressure and high-temperature synthetic (HPHT) IIb (001) and a homoepitaxially grown CVD (111) film with H-termination. In case of IIb sample, a temperature dependent experiment was also applied. The onset of electron emission at 4.4 eV on p-, and n-type diamond indicates that Fermi levels do not affect the sub-band spectra. The sub-band spectra in the energy regime 4.5–5.2 eV shows nearly no temperature dependence between RT and 300 °C, than the TPY spectra above 5.2 and below 4.4 eV. Obviously, direct excitation of valence-band electrons into the vacuum takes place in the close vicinity of the surface with temperature independent optical cross sections. It is interesting to note that the expected rise of photoyield due to NEA in the regime hν > 5.47 eV is missing in n-type diamond. We attribute this to a surface space charge region arising from ionized phosphorous atoms.

Negative electron affinity on hydrogen terminated diamond

AbstractTotal photoyield spectroscopy (TPYS) is applied to characterize the surface electronic properties of p‐ and n‐type chemical‐vapour‐deposited (111) homoepitaxial diamond films with hydrogen terminated (H‐terminated) surfaces. All experiments on H‐terminated high quality diamond show the same threshold energy of 4.4 eV and comparable spectra in the regime 4.4–5.5 eV. A H‐terminated intrinsic IIa (100) sample also show the same result. The onset of electron emission at 4.4 eV on p‐, intrinsic and n‐type diamond indicates that Fermi levels do not affect the sub‐band spectra. Obviously, direct excitation of valence‐band electrons into the vacuum takes place in the close vicinity of the surface. If we assume a transition from valence band maximum the threshold energy of 4.4 eV gives a negative electron affinity (NEA) of –1.07 eV (Eg ∼ 5.47 eV). TPYS therefore allows a precise and direct detection of the NEA of H‐terminated diamond. It is interesting that the expected rise of photoyield as NEA in the regime hv > 5.47 eV is missing in n‐type diamond. We attribute this to a surface space charge region due to ionized phosphorus atoms. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth, structure and electrical properties of tungsten oxide nanorods

Tungsten trioxide has shown good sensing properties towards various gases. Recently thin nanostructured WO 3 films have been tested. Due to their large surface area to volume ratio they exhibit good sensitivity depending on the grain size. However in conventional WO 3 thin films the average grain size exceeds the thickness of the surface space charge layer, so the electrical conduction is mainly controlled by the carriers transport across the grain boundaries. An alternative way seems to be in a monocrystalline material with nanometric dimensions. Our objective is to fabricate nanosized tungsten oxide rods and to test their sensing properties under gas adsorption. In this work, we focus on the growth, the structure and the electrical properties of tungsten nanorods. The tungsten oxide nanorods were grown by vapour transport from a WO 3 layer onto a substrate (Mica). The nanorods growth was controlled by the temperature gradient between the WO 3 layer and the substrate. Their morphology was investigated by AFM and their structure by TED and TEM. We have investigated the conductivity of the WO 3 nanorods with a technique derived from Atomic Force Microscopy operating in contact mode with a conductive tip (Resiscope).

금속/copper(Ⅱ)-phthalocyanine 계면에서의 Space Charge 연구

We report the space charge and the surface potential of the interface between metal and copper(Ⅱ)-phthalocyanine(CuPc) thin films by measuring the microwave reflection coefficients S/sub 11/ of thin films using a near-field scanning microwave microscope(NSMM). CuPc thin films were prepared on Au and Al thin films using a thermal evaporation method. Two kinds of CuPc thin films were prepared by different substrate heating conditions; one was deposited on preheated substrate at 150。C and the other was annealed after deposition. The microwave reflection coefficients S/sub 11/ of CuPc thin films were changed by the dependence on grain alignment due to heat treatment conditions and depended on thickness of CuPc thin films. Electrical conductivity of interface between metal and organic CuPc was changed by the space charge of the interface. By comparing reflection coefficient S/sub 11/ we observed the electrical conductivity changes of CuPc thin films by the changes of surface potential and space charge at the interface.

Studies of GaAs metal–insulator–semiconductor structures by the admittance spectroscopy method

The Au/Pd/Ti–SiO 2–GaAs structures with p- and n-GaAs have been investigated. The measurements of the capacitance–voltage characteristics at the different frequencies have been performed as well as the frequency dependence of MIS capacitance and conductance at fixed gate voltages. The complex frequency behaviour indicating the existence of a broad spectrum of the time constants has been observed. The equivalent circuit which allows to describe the obtained characteristics in a simple way has been presented. It contains two parallel branches including the constant phase elements (CPE 1 and CPE 2 in series with resistors R 1 and R 2) in parallel with the space charge layer capacitance, all of which is in series with an insulator capacitance and the series resistance. The parameters of that circuit have been estimated at different gate voltages as well as the characteristic time constants which describe the electron processes resulting in the observed frequency behaviour of the admittance characteristics. The CPE 1 with R 1 are attributed to the electron processes evoked by GaAs–SiO 2 interface states while CPE 2 with R 2 describe the contribution of the localized electron states in the surface space charge region, which have been observed only in p-GaAs structures.

Dependence of GaN photoluminescence on the excitation intensity

The dependence of the edge photoluminescence (PL) intensity on the excitation intensity in (0001) HVPE-grown GaN samples has been studied. The specific behavior found is that, at a low excitation level, the dependence is markedly superlinear, namely, superquadratic, and at high excitation levels it is nearly linear. A model accounting for the observed superquadratic behavior is proposed which is based on the identity of the recombination processes in the surface space charge region (SCR) under optical excitation with those in the SCR of the Schottky barrier or a p-n junction under current flow conditions. The superquadratic dependence is obtained analytically under the assumption that the nonradiative recombination channel is associated with multiple-hopping tunneling along a dislocation, which is formed by a chain of carrier localization centers and crosses the SCR. The experimental dependence of the PL intensity on the excitation intensity is a power-law function. The distance between the neighboring localization centers, i.e., the period of the potential along the dislocation, is determined as ∼4.1 nm.

Surface photovoltage measurements in μc-Si:H: Manifestation of the bottom space charge region

We discuss results of surface photovoltage (U) measurements for d=10 μm thick layers of undoped hydrogenated microcrystalline silicon (μc-Si:H). By applying excitation with low energetic photons (down to 1.1 eV), i.e., with small absorption coefficient α and a large penetration depth α−1, a photovoltage peak appears on a curve U=U(α). This peak is located at α≳d−1 and its occurrence depends critically on the substrate material. The peak is present in a μc-Si:H film grown on p+ crystalline silicon (c-Si), on the other hand it is missing in the μc-Si:H samples grown on n+ c-Si or ZnO film. We present a mathematical model that enables us to link this peak to photocharge separation in the bottom space charge region at the interface μc-Si:H/substrate. Besides the magnitude of the ambipolar carrier diffusion length L, a parameter particularly critical for the occurrence of the peak turns out to be the ratio n of reverse saturation current densities of the two diodes representing surface and bottom space charge regions. The peak can be observed only when |n| is below a certain threshold value |nth|. In the simplified case when d/L≫1 and when the thicknesses of the upper and lower depletion layers can be neglected, we have found |nth|≈0.135 or 1, depending on the orientation of the top and bottom barriers to each other. However, the magnitude of the peak increases exponentially with further lowering of |n|. Therefore, the surface photovoltage method is suitable not only for evaluating the minority carrier diffusion length L, but also for detecting the occurrence and properties of the bottom space charge region in thin film solar cells.

Observation of subpicosecond transient driving force for coherent phonons in GaAs by double-pulse excitation

Temporal variation in the driving force of coherent phonon was studied in n-type GaAs by a double-pulse pump–probe experiment by changing the pulse separation Δ t. Coherent phonons induced by the second pulse corresponding to the LO-phonon mode and the LO-phonon–plasmon coupled mode rapidly decreased in amplitude to their minima in ∼5 ps, then slowly recovered. This attenuation was explained by a screening of the surface space–charge field due to photo-excited carriers, while the recovery was attributed to a weakening of the screening through recombination and diffusion of photo-excited carriers.

Determination of Surface Space Charge Density on Semiconductor from Displacement Current-Voltage Curve using a Scanning Vibrating Probe

AbstractA determination method of the local surface space charge density in thin films on semiconductor surfaces is described. In this method, the displacement current and the tunneling current flow periodically in accordance with the perpendicular vibration of the probe of the scanning tunneling microscopy (STM). The external circuit current is separated into the mean displacement current and the mean tunneling current by using the two-phase lock-in amplifier. The displacement current-probe voltage curve of a p-type silicon with a native oxide layer was measured in a vacuum (>3×10−9 torr). The displacement current-voltage curve has been analyzed taking into account surface space charge. The theoretical displacement current-voltage curve has been fitted to the measurement in order to obtain both the local surface space charge density and the surface potential difference of thin films, and the theory is in good agreement with the measurement in both voltage regions where the majority carriers are accumulated and depleted. We also demonstrate a simple determination method of both the surface space charge density and the surface potential difference of thin films from flat-band conditions in the displacement current-voltage curve.

The development of Fresnel contrast analysis, and the interpretation of mean inner potential profiles at interfaces

This paper provides a summary of recent published and unpublished research on the development of Fresnel contrast analysis, a transmission electron microscopy technique for measuring the mean inner potential profile across an interface or a narrow layer. An algorithm for finding a best-fitting potential profile is described, energy-filtered experimental data are analyzed and contributions to Fresnel contrast from surface grooves and space charge are assessed. Many of the conclusions drawn are equally relevant for the interpretation of phases measured using off-axis electron holography.

Nongeometric field enhancement in semiconducting cold cathodes and in metal–insulator–semiconductor structures

We extend the usual one-dimensional equilibrium theory of the surface space charge region that screens a semiconductor from an external electric field in order to admit perturbations in three dimensions and time. We identify a class of perturbations of the one-dimensional equilibrium that grow exponentially in time at least until our first-order perturbation theory fails. The resulting spontaneous field enhancement may explain enhancement factors observed in electron emission from semiconducting cathodes and may similarly contribute to gate leakage in metal–oxide–semiconductor field effect transistors.

The use of a scanning tunneling microscope (STM) for investigation of local photoconductivity of quantum-dimensional semiconductor structures

The possibility is demonstrated of using the STM for recording spectra of photoconductivity of quantum-dimensional semiconductor structures with a high spatial resolution. Studies are made into the local photoconductivity of GaAs/InxGa1−x As based quantum wells and quantum points as a function of the depth of location of the quantum-dimensional structure relative to the surface space charge region. For quantum points in the vicinity of the sample surface, spectra are obtained which are characterized by features associated with the individual energy spectrum of those points.

The effect of surface space-charge on the shape of the potential barrier

A linear perturbation theory is used to solve the Poisson–Boltzmann equation giving analytical solutions in one-, two- and three-dimension for the band bending. Compared to previous results (Monch, 1993. Semiconductor Surface and Interfaces, Springer-Verlag), a more accurate relation between the surface space charge and the potential barrier is obtained. The result shows that the profile of the potential barrier decays quasi-exponentially but is not of Schottky's form. A comparison between numerical, analytical solutions and Schottky barrier is also presented.