Surface Conductivity Changes Research Articles

Role Of Deep Level Trapping On Surface Photovoltage Of semiinsulating GaAs

AbstractDual beam (bias and probe) transient Surface Photovoltage (SPV) measurements were made on undoped Semi-Insulating (SI) GaAs over an extended temperature range. Above 270 K, SPV recovery transients following a bias pulse were shown to reflect near surface conductivity changes; these are in turn controlled by surface/interface state thermal emission. Owing to the absence of a strong surface electric field in this material, the emitted carriers are not immediately removed from the near surface region. The recapturing of the emitted carriers is shown to be responsible for non-exponential conductivity and reciprocal-SPV transients. This behavior is considered to be characteristic of relaxation-type semiconductors with near-surface ungated structures. Below 150 K, the photoinduced transition of EL2 from its ground to metastable state EL2& was shown to change the effective electron and hole mobilities and augment the SPV signals immediately following the bias pulse. Thermally induced EL2& recovery above 120 K decreases the SPV signal from its maximum. This decay transient was analyzed and the decay rate fitted to a single exponential. An activation energy of 0.32 eV and a pre-exponential constant of 1. 9× 1012 s−1 were obtained, and attributed to the thermal recovery rate for EL2&.

Chemical Sensors for Acid‐Base‐Active Gases: Applications to CO2 and NH3

AbstractIt is shown how acid‐base interactions of complex gases such as CO2, NH3 or H2O with solids can be used to measure the partial pressures of these gases. Two examples developed in our laboratory are presented: The interactions of acid‐base‐active gases such as NH3 (Lewis base) or BF3 (Lewis acid) with the ionic conductors AgCl or CaF2 cause ionic space charge regions giving rise to a change of the ionic surface conductivity. The response time can be significantly improved by applying a microelectrode impedance method. The use of a potentiometric arrangement the cell reaction of which is a pure acid‐base reaction, leads to a fast, precise and drift‐free CO2‐sensor. Since the reference electrode is not affected by CO2 itself, the entire cell can be exposed to the CO2 and O2 containing atmosphere without the necessity of sealing the reference electrode. The chemical potential of oxygen even though important for defining the component activities at each electrode cancels in the overall reaction of our cells. The performance of two cells is presented. While the measuring electrode consists of Na2CO3, the reference electrodes discussed are made of Na2Ti6O13/TiO2 or Na2Ti6O13/Na2Ti3O7 two‐phase mixtures, both being stable with respect to a reaction with CO2. The sensors are very fast, response times are of the order of seconds.

A mechanism for the production of electromagnetic radiation during fracture of brittle materials

The feasibility of the charge separation model as the source of electromagnetic radiation received from the fracture of brittle materials is demonstrated. An analytical expression and a finite difference method enable the calculation of currents flowing around the tip of a crack. Results from the model compare favourably with fracture events recorded from ice samples. Changes in material conductivity affect pulse shape although changes in crack surface conductance and not crack apex conductance cause most variation in the received signal.

Changes in abscisic acid concentration, surface conductance, and water content of developing kiwifruit

Seasonal changes in abscisic acid (ABA) concentration, surface conductance to water vapour and water content, were measured in developing fruit from the upper and lower canopies of 8 year old kiwifruit vines ( Actinidia deliciosa var. deliciosa). Three stages were apparent for the accumulation of ABA by the developing fruit. The first stage was characterised by a high initial concentration of ABA in the skin, coupled with high surface conductance of the fruit, suggesting that ABA accumulation during the first 1–2 weeks after anthesis was via the transpiration stream. A marked reduction in concentration of ABA during the second stage of accumulation was consistent with dilution by fruit growth. The constancy in the concentration of ABA during stage three (from Week 12 until harvest) suggested that ABA was, in some way, linked to photosynthate loading into the fruit. The larger quantity of ABA in fruit from the upper canopy, which had greater dry matter gains, is consistent with this conclusion. The positive relationship between ABA concentration and rapid fruit growth suggests that ABA may have a role in the allocation of assimilates to fruit. Changes in water content of kiwifruit followed closely the stages of fruit growth. A close relationship between the water content and shape of the developing fruit and published data for glucose in kiwifruit, is consistent with fruit enlargement being osmotically driven. The reduction in percent water content during the latter stages of growth, especially for fruit from the upper canopy, could largely be accounted by the large build-up of starch during that period. The surface conductance of the fruit declined rapidly during the first 9 weeks after anthesis. The conductance then remained relatively constant at low levels until harvest. Compared with apple, the surface conductance of kiwifruit (0.1–0.8 mmol H 2O m −2 s −1) was up to ten times greater during the early stages of growth.

PIMOD processing and properties of a LiNbO3-based MFSFET for nonvolatile memories

Abstract The photo-induced metallo-organic decomposition (PIMOD) process has been successfully used to deposit a lithium niobate thin film acting as the gate oxide of the conventional MFSFET structure. The use of the low-temperature PIMOD process for thin film deposition has increased the device yields of the molybdenum liftoff for small area isolation. The electronic alteration of the properties of the ferroelectric gate transistor was previously shown to be caused by charges in the semiconductor being injected into the ferroelectric film. To prevent this problem, a thin SiO2 buffer layer was thermally grown on the silicon substrate immediately before lithium niobate deposition. The silicon-lithium niobate interface was stabilized and the charge injection effect was eliminated due to the formation of the buffer layer. The channel current was shown to be greatly altered by the application of voltage pulses between the gate of the device and the substrate. Upon switching, the change in surface conductivity...

Acoustic devices for simultaneous determination of adsorbed amount and surface-conductivity changes by gas adsorption

Acoustic devices for simultaneous determination of adsorbed amount and surface-conductivity changes by gas adsorption

Polarization effects of poled ferroelectric substrates upon surface conductivity changes in phthalocyanine and SnO2 films following gas adsorption

Organic semiconductors of phthalocyanines (H2Pc and CuPc) have been deposited as thin films of 15–150 nm on the polarized surfaces of ferroelectric LiNbO3, and the surface conductive behaviour with the adsorption of gases has been studied. Upon the adsorption of NO2, the conductivity increases in H2Pc films as thick as 150 nm were similar between the (+) and (–) polar substrates, where the (–) polar substrate gave rise to a larger conductivity increase than did the (+) polar substrate with decreasing thickness. Differences between oppositely polarized substrates became prominent for gases with larger electron affinity, i.e. NO2 > NO > O2. The X-ray diffraction patterns of the thin films showed the growth of crystal morphology with α form, independent of the polarization direction of the substrates. The effect of polarized surfaces is accounted for in terms of a band-bending model. Conductivity changes due to the polar substrates were also demonstrated in the thin films of SnO2 deposited on LiNbO3 for the adsorption of C2H5OH and in a poled ferroelectric semiconductor, lead strontium zirconate titanate, for the adsorption of H2O.

Surface conductivity changes during the electrochemical adsorption of UPD layers on silver and gold

The surface conductivity changes for UPD and UPS of Cu, Ag, Tl, Cd, Pb and Bi on gold and of Tl, Cd, Pb and Bi on silver were measured. At low coverages the relative resistance change, ΔR R depends linearly upon the adatom coverage, θ, obeying Linde's rule for the surface. At intermediate coverages ΔR R goes through a maximum and then decreases. The values of ΔR R at the maximum and at θ≅1 follows the sequence of the atomic radius of the adsorbate. The change of the specularity parameter, Δ p, at θ≅1 indicates that the more dense the surface adlayer is, the more specularly the electrons are reflected. The shape of the ΔR R versus θ curve is explained qualitatively in terms of the structure of the adlayer and the distance between the adatoms.

Temperature dependence of the surface conductivity of YBa 2Cu 3O 7−δ accompanying the change in the concentration of dimerized oxygen holes

The intensity of inelastically scattered electrons measured by electron energy loss spectroscopy has been employed to monitor the surface conductivity of YBa 2Cu 3O 6.9 as a function of temperature. The study shows a drastic change in surface conductivity precedes the superconducting transition at 90K. The increase in surface conductivity is accompanied by the formation of dimerized holes in the oxygen derived p-band. This phenomenon is not observed in the non-superconducting YBa 2Cu 3O 6.2.

Surface electronic structure of the high-T c oxides

Electron energy loss spectroscopy (EELS) has been employed to monitor surface conductivity changes in YBa 2Cu 3O 7 as a function of temperature. Concomitant use of x-ray photoelectron spectroscopy (XPS) establishes that the formation of oxygen dimers with lowering of temperature is accompanied by a simultaneous increase of surface conductivity.

Surface conductivity changes in SnO 2(110): Effects of oxygen

Sheet conductivity measurements have been used to monitor the effects of temperature and exposure to oxygen on the (110) surface of tin dioxide. In these experiments three surface structures, characterized by LEED and other techniques, were studied using a retractable four-point conductivity probe under UHV conditions and at oxygen pressures up to 10 Torr. The data presented show that this electronic property is a sensitive measure of changes in composition and structure at and just below the surface of SnO2. The observed conductivity changes are discussed in the context of the distribution and concentration of oxygen in the space-charge layer, and are correlated with other surface measurements.

Surface conductivity changes in SnO2(110): effects of oxygen

Surface conductivity changes in SnO2(110): effects of oxygen

Chemisorption of H 2 and CO on stoichiometric and defective TiO 2(110)

Chemisorption of H 2 and CO was studied on stoichiometric and defective TiO 2(110) surfaces by means of changes in surface conductivity, work function, XPS, AES, LEED and ELS. Defective surfaces were prepared by thermal pretreatment under thermodynamically controlled conditions and by evaporation of excess Ti. The results are discussed in a modified charge transfer model in terms of partial charges and dipole moments formally attributed to adsorption complexes.

Chemisorption of H 2 and CO on stoichiometric and defective TiO 2(110)

Chemisorption of H 2 and CO was studied on stoichiometric and defective TiO 2(110) surfaces by means of changes in surface conductivity, work function, XPS, AES, LEED and ELS. Defective surfaces were prepared by thermal pretreatment under thermodynamically controlled conditions and by evaporation of excess Ti. The results are discussed in a modified charge transfer model in terms of partial charges and dipole moments formally attributed to adsorption complexes.

Barrier height and surface states at cleaned InSb(110) surfaces

The electronic properties of p-InSb(110) surfaces which were cleaned by the ion bombardment annealing technique are investigated by measurements of the integral and differential field effects. The field-induced changes in surface conductivity imply inversion layers with electron conduction in the space charge layer. The analysis of the experimental results yields the barrier height at the surface and the term spectrum of surface states. The general shape of the surface state density per energy interval shows a disorder-dependent continuous distribution with a minimum about midgap and increasing tails towards the bulk band edges. Near the conduction band minimum a peaking distribution of donor-type surface states is superimposed on the continuous distribution depending on the ion bombardment annealing conditions. The nature of the spectrum of surface states is discussed on the basis of intrinsic and extrinsic states.

Gap states at the GaAs-natural oxide interface

The relaxation of surface conductivity changes during the application and after the removal of external electric fields was investigated for n-GaAs surfaces covered with natural oxide. The relaxation processes in the heterojunction system of the oxidized surface are governed by various excitation mechanisms between the depletion layer in the bulk and fast surface states at the GaAs-oxide interface. Analysis of the experimental results yields new data on the energetic distribution of the fast surface states which show that there is a set of empty states in the upper half of the bulk energy gap and a set of occupied states near the middle of the bulk energy gap.

Charge transfer between ZnO crystals and dye layers

The interaction between crystal and adsorbed dye molecules has been studied under well defined conditions by measurements of field effect and spectrally sensitized photoconductivity. The (101̄0) surfaces of n-type ZnO crystals (band gap 3.3 eV) are cleaned in ultrahigh vacuum. A pretreatment with atomic hydrogen produces an accumulation layer. Merocyanine (polymethine) dye molecules are deposited by sublimation in the same vacuum (coverage (1–2000) × 10 14 cm −2. Optical excitation of the dye causes a sensitized photoconductivity in the ZnO crystal close to the surface. The spectra distribution resembles the absorption spectrum of the dye with a maximum at 2.3 eV. An electric field applied perpendicular to the dye covered surface induces charge carriers in the crystal and changes the surface conductivity (field effect). Additional excitation of the dye by light causes a slow relaxation of the field-induced change of surface conductivity. This relaxation is observed for both signs of the field. Furthermore a memory of the dye covered crystals has been found. It can be programmed by field and light, read out via the surface conductivity and quenched by light. A phenomenological model for relaxation and memory is refined by kinetic equations and by considerations about charge transport within the dye layer. The observations can only be explained by a charge transfer between crystal and dye operating in both directions. From these results the following conclusion is drawn for the mechanism of spectrally sensitized photoconductivity of the present system: An electron transfer between dye molecules and crystal represents the decisive step rather than an energy transfer.

Phase-sensitive detection of electron-beam-induced desorption from ZnO

We report a method of phase-sensitive detection of desorption from surfaces bombarded by a properly modulated electron beam. Mass-spectrometer signals detected by this method assure a high sensitivity and selectivity of the results. This method is used here for studying surfaces of ZnO powders, and it found that CO2 is the only desorbed species during electron bombardment of such surfaces. Following the CO2 desorption kinetics simultaneously with the associated surface conductivity change, we find one-to-one correspondence between both quantitites. Furthermore, the rates of the electron-induced CO2 desportion and the surface conductivity are in a very good agreement with a theoretical model, previously developed for photodesorption. This model predicts a slowing of the desorption rate due to the buildup of an electron accumulation layer and a corresponding band bending which hinder irradiation-generated holes from reaching the surface and neutralizing CO−2 species chemisorbed on it. Our observations seem to support this model together with pointing out the similarities between electron- and photon-induced desorption in this case.

Surface layers in heat-treated GaAs

2014 Short-time heat treatments of undoped high-resistivity and Se-doped GaAs samples have been performed in an H2 flow at temperatures from 750 °C up to 850 °C. Surface conductivity changes have been determined by Schottky barrier measurements. After annealings, undoped samples showed a n-p double layer at the surface, while in Se-doped samples only new acceptors were detected. REVUE DE PHYSIQUE APPLIQUEE TOME 13, JUIN 1978, Classification Physics Abstracts 73 . 40N

Conductivity of powdered ZnO with chemisorbed oxygen during photodesorption

We have investigated the connection between changes in surface conductivity of ZnO with chemisorbed oxygen, and the number of gas molecules photodesorbed with band-gap illumination. Experiments under UHV conditions with powdered samples show that the initial stages of photodesorption are not described by a simple electron transfer mechanism, while later stages can be reasonably well explained in that way.