Semiconductor Space Charge Region Research Articles

Redox behavior of hemin at p-GaAs(1 0 0) electrode

Electrochemical behavior of hemin on p-GaAs(1 0 0) electrodes was examined by cyclic voltammetry (CV) and impedance spectroscopy (EIS) in phosphate buffer solutions (PBS) at pH 7.45. CV investigations in 0.6 mM hemin in PBS revealed a pair of reversible peaks at −0.44 and −0.32 V vs. SCE resulting in stable adsorbed species. EIS spectra analysis pointed out that these adsorbed species bring significant changes in the semiconductor surface state population and the potential drop distribution between the semiconductor space charge region and the Helmholtz layer.

Protein adsorption on titanium dioxide: Effects on double layer and semiconductor space charge region studied by EIS

The semiconductor properties of an n-type TiO 2 oxide surface and its modification by protein adsorption using electrochemical impedance spectroscopy ( EIS ) were studied. Impedance spectra under steady state conditions were obtained as a function of electrode potential and human serum albumin (HSA) concentration in solution. The effect of the adsorption potential on the EIS response was also analyzed ( E ads −0.70, −0.50 and −0.080 V vs. saturated calomel electrode). The impedance spectra were modeled using different equivalent circuits, and data analysis were performed by data fitting in the whole frequency range, as well as in the low and high frequency ranges. The electric representation that better fit experimental data consisted of two equivalent subcircuits composed by a combination of constant phase elements ( cpe 1 and cpe 2 ) and resistances ( R s and R 1 ). The cpe 1 element represented the distributed capacity in the semiconductor oxide. The cpe 2 element was associated with diffusional processes. We have analyzed EIS data based on a theoretical calculation of interface apparent capacitance ( C app ) from the cpe 1 parameters ( Q 1 and ϕ 1 ) and resistance elements. The changes in capacitance, produced by the presence of HSA, were associated to changes in the space charge layer capacitance of the semiconductor, as a result of the energy band bending near the surface. The values obtained for the R 1 element from the theoretical fitting procedures (in the high and whole frequency range) were correlated to the electric resistance inherent to the semiconductor properties having a value of 6.10 3 Ω cm 2 .

Electrochemical Sensing of Membrane Potential and Enzyme Function Using Gallium Arsenide Electrodes Functionalized with Supported Membranes

We deposit phospholipid monolayers on highly doped p-GaAs electrodes that are precoated with methyl-mercaptobiphenyl monolayers and operate such a biofunctional electrolyte-insulator-semiconductor (EIS) setup as an analogue of a metal-oxide-semiconductor setup. Electrochemical impedance spectra measured over a wide frequency range demonstrate that the presence of a lipid monolayer remarkably slows down the diffusion of ions so that the membrane-functionalized GaAs can be subjected to electrochemical investigations for more than 3 days with no sign of degradation. The biofunctional EIS setup enables us to translate changes in the surface charge density Q and bias potentials Ubias into the change in the interface capacitance Cp. Since Cp is governed by the capacitance of semiconductor space charge region CSC, the linear relationships obtained for 1/Cp2 vs Q and 1/Cp2 vs Ubias suggests that Cp can be used to detect the surface charges with a high sensitivity (1 charge per 18 nm2). Furthermore, the kinetics of phospholipids degradation by phospholipase A2 can also be monitored by a significant decrease in diffusion coefficients through the membrane by a factor of 104. Thus, the operation of GaAs membrane composites established here allows for electrochemical sensing of surface potential and barrier capability of biological membranes in a quantitative manner.

Effect of stationary ionization of traps near the midgap on the spectrum of thermally stimulated capacitance of semiconductor devices

An algorithm for calculating the parameters of deep-level centers in the space-charge region of semiconductor devices on the basis of an analysis of the curves of thermally stimulated capacitance in the presence of recombination centers with the levels located near the midgap is suggested. The method is tested using the commercial AL-107 GaAs-based light-emitting diodes. The results of calculations of the deep-center parameters in the materials under study are consistent with the results of other methods of analysis.

Energetics and Kinetics of Interfacial Electron-Transfer Processes at Chemically Modified InP/Liquid Junctions

The electrochemical behavior of freshly etched (111)B-oriented InP surfaces was compared to that of (111)B-oriented InP surfaces that had been chemically modified by reaction with p-BrCH2C6H4CF3. Differential capacitance versus potential and current density versus potential techniques were used to measure the energetics and kinetics of interfacial electron-transfer reactions in contact with a 70:30 (v:v) mixture of CH3CN−tetrahydrofuran that contained either 1,1‘-dimethylferrrocene+/0 or decamethylferrocene+/0. For both the etched and modified (111)B InP contacts, plots of differential capacitance versus potential measurements indicated a linear dependence of the equilibrium voltage drop (Vbi) in the semiconductor space-charge region, as a function of the redox potential (E(A/A-)) of the solution, with the slope of Vbi vs E(A/A-) ≈ 1.0, as expected for ideal behavior of a semiconductor/liquid junction. The barrier heights calculated for the chemically modified InP/liquid junctions were 100 ± 20 mV higher ...

Near-field surface photovoltage

A phenomenon called near-field surface photovoltage is presented. It is based on inducing photovoltage only at a semiconductor space-charge region using near-field illumination. The photovoltage is obtained by measuring the contact potential difference between an optical near-field force sensor and a semiconductor surface under illumination. It is shown that the near-field illumination induces photovoltage at the surface which is principally different from photovoltage induced by far-field illumination. The mechanisms that govern the different far-field and near-field photovoltage response are discussed.

Real-time imaging of semiconductor space-charge regions using high-spatial resolution evanescent microwave microscope

A high-resolution evanescent microwave probe (EMP) was used to detect and image depletion regions in solar cell p-n junctions in real time. The EMP uses a microwave resonator operating around 10 GHz that is coupled to a thin wire probe. Unable to travel beyond the waveguide discontinuity, the microwave fields set up evanescent fields in the tip’s vicinity. When coupled to an object nearby, these evanescent fields are modified and change the resonant characteristics of the resonator. The microwave conductivity of the nearby object affects the extent of the modification of the probe’s output which is monitored as the probe is scanned over the object. Using these EMP scans, steady-state and transient expansions/contractions of the p-n junction’s depletion region under dc and pulsed reverse/forward biases are mapped. These experimental data along with the conductivity calibration of the EMP were then used to quantitatively calculate doping concentrations, diffusion lengths, and carrier recombination lifetimes in the junction. Junctions are one of the most crucial building blocks of semiconductor devices and these studies clearly show the ability of the EMP in quantitative and nondestructive evaluations of electronic devices and circuits.

Measurement of Interfacial Charge-Transfer Rate Constants at n-Type InP/CH3OH Junctions

Steady-state current density vs potential methods have been used to measure interfacial electron-transfer rate constants at n-type indium phosphide/liquid junctions. n-InP/CH3OH-1,1‘-dimethylferrocene+/0, n-InP/CH3OH-ferrocene+/0, n-InP/CH3OH-tetrahydrofuran-decamethylferrocene+/0, and n-InP/CH3OH-1,1‘-diphenyl-4,4‘-dipyridinium2+/+• contacts displayed bimolecular kinetic behavior in which the observed current density was first order in the concentration of electrons at the semiconductor surface and in the concentration of acceptors in the solution. Differential capacitance potential measurements were used to determine the energetics for the charge-transfer process as well as to determine the concentration of electrons at the semiconductor surface as a function of applied potential. These measurements indicated that the voltage dropped across the semiconductor space charge region varied linearly with changes in the Nernst potential of the solution, as expected for an ideally behaving semiconductor/liquid junction. The measured charge-transfer rate constants, ket, for these systems were ≈10-16 cm4 s-1, in excellent agreement with previous theoretical predictions.

Modeling electrostatic scanning force microscopy of semiconductors

A model is presented for the electrostatic interaction between the tip of a scanning force microscope and the surface of a semiconductor in condition of depletion. By representing the tip as a conducting sphere and suitably approximating the semiconductor space-charge region, analytical expressions are derived for the boundary of this region and the tip-sample capacitance and force. Operation in an immersion medium with high dielectric constant is found to enhance the doping sensitivity of the electrostatic force. As a function of the tip-sample distance, this force exhibits a zero point, if the permittivity of the immersion medium exceeds that of the semiconductor.

Electro-optical structure with high speed and high reflectivity modulation

A voltage controlled, multilayer electro-optic structure is proposed. It is constructed from repeated Schottky-like modules, which consist of layers of semiconductor-dielectric-semiconductor. The device reflectivity, in the range of incidence angles corresponding to surface plasmon excitation, is determined by the combined thickness and permittivity of the semiconductor space charge regions of all modules. A structure consisting of many modules allows for a large effective thickness of the voltage controlled space charge region which yields a high reflectivity modulation. Calculations show that fast (response time ∼ 10−10 s) and low-voltage structures with reflectivity modulation depth of more than 80% can be constructed.

Space-charge layer, metallization, and collective excitations of the Bi/GaAs(110) interface.

The collective excitations and the metallization of the Bi/GaAs(110) interface grown at room temperature, up to the completion of a few tens of a monolayer, have been studied by means of the high-resolution electron-energy-loss spectroscopy (HREELS). Through analysis of the HREELS data, also by means of an appropriate semiclassical dielectric model, the modifications experienced by the substrate-related loss structures (Fuchs-Kliewer phonon and dopant-induced free-carrier plasmon) and by the quasielastic peak are related to changes in the dielectric response of the overlayer and in the semiconductor space-charge region. The influence of bismuth is effective in enlarging the depletion layer thickness, leaving the interface semiconducting at the monolayer-coverage scale. A band bending value of 0.56 eV is obtained at the coverage of one monolayer on the highly n-type doped sample (n\ensuremath{\sim}2.7\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$); a determination free from any possible surface photovoltaic effect. At coverages greater than two monolayers, which corresponds to a structural transition, the intermediate structural phase becomes metallic, thus marking a clear semiconductor-metal transition. This intermediate metallic stage further develops towards the formation of actual semimetallic crystalline bismuth layers oriented with the basal plane parallel to the substrate surface.

Surface second harmonic studies of Si(111)/electrolyte and Si(111)/SiO 2/electrolyte interfaces

The optical second harmonic (SH) response from Si(111) electrode surfaces has been studied and has been found to be highly potential dependent. For both H-terminated Si(111) surfaces in NH 4F, and oxide-covered surfaces biased in H 2SO 4, the phase and the amplitude display a potential dependence which are attributed to field effects within the semiconductor space—charge region and at the surface of the Si(111) electrode. These studies are the first to demonstrate that for Si(111)/oxide samples, the potential dependence in the SH phase can be correlated with oxide thickness, as demonstrated by examining samples of 0–40 Å thickness.

Coulomb Energy of Traps in Semiconductor Space Charge Regions

Large Coulomb barriers exceeding ΔE≊250 meV are estimated for capture and emission rates of trap centers in semiconductor space‐charge regions. Depending on the charge state of the trap, the capture rate or both the capture and emission rates are activated or deactivated, respectively. The Coulomb energy raises the equilibrium energy state of a trap center that is repulsively charged when occupied. Quantitative agreement of the calculated Coulomb energy is obtained with trapping rates for single individual interface traps in metal‐oxide‐semiconductor (MOS) structures measured by random telegraph signals. The Coulomb barrier is reduced in MOS capacitors by partial screening due to mobile charge carriers in the inversion channel. The Coulomb energy can be externally controlled in MOS structures by the gate bias voltage.

Role of ionic defects at semiconductor-insulator interfaces: Spectroscopic results on CaF2/InP(001) and SrF2/InP(001) structures.

Epitaxial ${\mathrm{CaF}}_{2}$ and ${\mathrm{SrF}}_{2}$ layers were prepared by molecular-beam epitaxy onto clean, (4\ifmmode\times\else\texttimes\fi{}2)-reconstructed InP(001) surfaces. The geometric and electronic structures of the layers and the interfaces were investigated by low-energy electron diffraction (LEED), Auger-electron spectroscopy (AES), and x-ray and ultraviolet photoelectron spectroscopies (XPS,UPS). Both fluorides grow at the substrate in a (001) orientation in their bulk cubic structures and form atomically rough, (111)-faceted surfaces as deduced from LEED. The XPS core-level intensity attenuations and the AES depth profiles indicate the formation of sharp interfaces. No interface bonds between InP and overlayer atoms were observed with photoelectron spectroscopy, and the Fermi-level position at the InP surface does not shift after fluoride deposition. The core levels and valence-band maxima of the fluoride overlayers shift to higher binding energies with increasing thickness. Up to 0.5-nm coverage, this is explained by additional screening from the InP substrate. Above 0.5-nm coverage, the shifts are caused by electrostatic-potential differences extending 10 nm away from the interfaces into the fluoride overlayers. The potential differences decrease the binding energies of all electronic levels in the fluoride layers near the interfaces, analogous to a band bending in semiconductors at semiconductor surfaces or semiconductor-metal interfaces. These potential differences are generated by mobile ionic defects in the fluoride layers. A model is presented for this space-charge region caused by ionic point defects in insulating overlayers. Fluoride ions are chemisorbed at the InP surface, thus creating a negative interface charge, which is compensated by excess positively charged fluoride vacancies in the space-charge region. The analogy to space-charge regions in semiconductors due to electronic surface states is discussed. Furthermore, the electronic interface schemes with the band discontinuities, the work functions, and the electron affinities of thick ${\mathrm{CaF}}_{2}$ and ${\mathrm{SrF}}_{2}$ overlayers were deduced from the measurements.

Energy oscillations in electron transport across a triangular barrier

Carrier transport across the semiconductor space-charge region of a silicon triangular barrier diode was investigated by a Monte Carlo simulation. Oscillations of the electron mean kinetic energy are observed as a function of position along the uphill slope of the barrier under bias. At a given point on the uphill slope, the energy distribution function shows an oscillatory behavior, with a periodicity corresponding to the optical phonon energy. These oscillations are shown to be due to the nonequilibrium dynamics of the electron interaction with optical phonons in the situation when other inelastic electron scattering processes are negligible. The energy oscillations are superimposed on a smooth cooling of the distribution in the transport toward the top of the barrier, as current flows through the system. A comparison with the thermionic theory quantifies the importance of nonequilibrium effects in short-range electronic transport.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A DLTS Study of InAs MIS Structures

Deep level transient spectroscopy (DLTS), capacitance–voltage (C–U) measurement, and thermally stimulated polarization current (TSPC) methods are used to study InAs MIS-structures in the temperature region from 5 to 400K. It is found that above 200K the measured DLTS signal is caused by charge accumulation in the insulator layer. The parameters of two electrically active defect centers in the semiconductor space charge region (SCR) Ec – 0.06 eV and Ec – (0.15–0.20) eV are determined. These levelsapear in the energy spectrum of the interface states density as two separate peaks. The DLTS data are complemented by and compared with the C–U data. [Russian Text Ignored].

A DLTS study of InAs mis structures

Deep level transient spectroscopy (DLTS), capacitance–voltage (C–U) measurement, and thermally stimulated polarization current (TSPC) methods are used to study InAs MIS-structures in the temperature region from 5 to 400K. It is found that above 200K the measured DLTS signal is caused by charge accumulation in the insulator layer. The parameters of two electrically active defect centers in the semiconductor space charge region (SCR) Ec – 0.06 eV and Ec – (0.15–0.20) eV are determined. These levelsapear in the energy spectrum of the interface states density as two separate peaks. The DLTS data are complemented by and compared with the C–U data. [Russian Text Ignored].

Photoluminescent properties of n-GaAs electrodes: Simultaneous determination of depletion widths and surface hole-capture velocities in photoelectrochemical cells

Steady-state photoluminescence measurements performed on n-GaAs electrodes used in photoelectrochemical cells (PEC’s) employing a stabilizing, aqueous telluride electrolyte yield values for the electrode’s depletion width W and surface hole-capture velocity S. Between −1.0 V (a potential near short circuit) and −1.5 V vs an SCE reference electrode (a potential near open circuit at the photon flux of 1×1015 photons/s cm2 employed), the interface behaves ideally: virtually all of the applied potential appears in the semiconductor space-charge region. Over this potential regime S is determined to be constant to within 10% and has a value, using literature values for hole lifetime and diffusion length, of approximately 2×105 cm/s for n-GaAs electrodes having carrier concentrations of (1–4)×1017 cm−3. Similar values of S obtained in air and in the PEC suggest a common rate-limiting mechanism for hole consumption in the two media.

On electron tunneling in the metal-insulator-semiconductor systems including various electron effective masses

It is shown that in some cases of the metal-insulator-semiconductor systems the electron tunnel current may be expressed in the form of one-dimensional integral with modified ‘‘supply function’’ even if the effective mass is not the same in each region of the system. The proposed formula together with description of the semiconductor space charge region (which is not considered in the paper) may be used for modeling of the electron tunnel current-voltage characteristics, e.g., of the metal–SiO2–Si〈100〉 tunnel diodes. The considerations are based on the electron total energy and transverse wave-vector conservation assumptions.

Effects of interfacial charge on the electron affinity, work function, and electrical characteristics of thinly oxidized semiconductor-insulator-semiconductor and metal-insulator-semiconductor devices

A phenomenological theory involving interfacial charge is presented to account for the result that the diode constant n at low forward biases might be temperature dependent in some thinly oxidized (&amp;lt; 20 Å) semiconductor-insulator-semiconductor and metal-insulator-semiconductor devices, resulting in parallel I-V curves. Neither multistep tunneling through the semiconductor space-charge region, nor conventional thermionic emission over the barrier, can explain the low-voltage portion of the I-V characteristics. In this paper it is shown that an interfacial charge redistribution occurs with changes in temperature, resulting in changes in the electron affinity of the base semiconductor and in the work function of the contact material. This results in a linear dependence of n on reciprocal temperature. An explanation is presented to account for the large temperature coefficient that has been observed for both the diffusion potential VD and open-circuit photovoltage Voc in these devices. And an explanation of the I-V characteristics in terms of shunt currents is also given.