Thermoionic Emission Research Articles

Temperature dependence of the specific resistance in Ti∕Al∕Ni∕Au contacts on n-type GaN

The temperature dependence of the specific resistance ρc in annealed Ti∕Al∕Ni∕Au contacts on n-type GaN was monitored, obtaining information on the current transport mechanisms. After annealing at 600°C, the contacts exhibited a rectifying behavior and became Ohmic only after high temperature processes (>700°C), with ρc in the low 10−5Ωcm2 range. The results demonstrated that the current transport is ruled by two different mechanisms: thermoionic field emission occurs in the contacts annealed at 600°C, whereas field emission dominates after higher temperature annealing. The significant physical parameters related to the current transport, i.e., the Schottky barrier height and the carrier concentration under the contact, could be determined. In particular, a reduction of the Schottky barrier from 1.21eV after annealing at 600°Cto0.81eV at 800°C was determined, accompanied by a strong increase of the carrier concentration, i.e., from 2×1018cm−3 in the as-prepared sample to 4.6×1019cm−3 in the annealed contacts. The electrical properties were correlated to the microstructure of the interfacial region, providing a scenario to explain the transition from Schottky to Ohmic behavior in annealed Ti∕Al∕Ni∕Au contacts.

Electrical and optical study of Cu(In, Ga)Se 2 co-evaporated thin films

Co-evaporation technique from three sources was used to prepare Cu(In, Ga)Se 2 polycrystalline thin films for photovoltaic conversion. Their conductivity was studied in the range 20–300 K. The grain boundary scattering mechanism is mainly responsible for the diffusion process in the latter materials. In the low temperature region, we interpret the data in terms of Mott law and the analysis is very consistent with the variable range hopping. However, thermoionic emission is predominant at high temperatures. When the conductivity deviates from the classical grain boundary conduction models, inhomogeneity is then considered and parameters such as the standard deviation and the mean potential barrier height are derived. Transmittance measurements yielded band gap values of 1.07 and 1.64 eV for CuInSe 2 and CuGaSe 2, respectively.

Intrinsic 4H-SiC parameters study by temperature behaviour analysis of Schottky diodes

The main intrinsic parameters in silicon carbide material are not exhaustively studied up to now. An experimental study of the behaviour of electrical parameters of Schottky barrier diodes (SBD) fabricated on the Si face of 4H-SiC epitaxial layers with respect to temperature is shown. Analysed devices present an electrical behaviour in accordance to thermoionic emission and a good metal–semiconductor interface uniformly confirmed by capacitance/voltage measurements ( C– V). Current/voltage measurements ( I– V) have been performed in a large temperature range and lead to an evaluation of the Richardson constant.

Wafer level reliability and leakage current modeling of PZT capacitors

Over the last few years, thin films of PbZr x Ti 1− x O 3 (PZT) have been the focus of extensive researches for high-k capacitor applications. However, some electrical properties such as leakage current conduction and degradation mechanisms need to be better understood. From Constant Voltage Stress experiments, we identified two distinct failure mechanisms depending on the applied voltage levels. The existence of these two failure mechanisms makes it impossible to extrapolate lifetime results from high voltage to low voltage. Since the typical operating voltage for decoupling capacitors is around 3 V the reliability study has to be focused on the low voltage breakdown. A good opportunity to learn about the low voltage failure mechanisms is to investigate the characteristics of leakage current. The time evolution of leakage current is mainly controlled by the resistance degradation phenomenon. A quantitative analytical model has already been developed to account for this effect. We propose a more complete model that also includes dielectric relaxation and trapping effects. The resulting model is combined to a charge-influenced thermoionic emission model that fits fairly well the voltage and temperature dependence of leakage current. The static and dynamic parts of our model are found to be consistent, especially in terms of barrier lowering effect induced by the resistance degradation process. We believe that the low voltage breakdown is related to a trapping-induced creation of defects in the film.

Electrical characterization of InGaN/GaN light emitting diodes grown by molecular beam epitaxy

We studied the electrical behavior of multiple InGaN/GaN quantum well based light emitting diodes grown by molecular beam epitaxy and we determined three different domains of current-voltage dependence. We then described the charge carrier transport mechanisms for these three domains. The first domain, corresponding to leakage currents (V<1.5 V), takes place in the mesa side wall, in a semi-insulating material formed by air contamination, after the etching process. Below room temperature, electrical transport occurs by hopping between localized states and above room temperature by the Poole–Frenkel mechanism. The second domain, that of injection currents (1.5 V<V<3.5 V), corresponds to a hole injection from the valence band of p-GaN to the first InGaN quantum well. At low temperature (T<250 K) this hole injection is due to tunnel transfer and above 250 K to thermoionic emission. In the third domain, which corresponds to series resistor (V>3.5 V), the current is limited by the p-GaN zone. In this zone, the density of the free holes is controlled by the combined effects of the temperature and the applied forward bias. The results obtained enables the electronic states resulting from the magnesium doping to be localized at 190 meV above the valence band maximum.

Output power and its limitation in ridge-waveguide 1.3 µm wavelength quantum-dot lasers

Low-threshold (3.7 mA) high-power (150 mW) ridge-waveguide lasers based on 1.3 µm self-organized quantum dots were fabricated and studied at and above room temperature. The output power spectrum of these lasers consists of two components, which correspond to the ground-state (∼1.29 µm) and the excited-state (∼1.22 µm) optical transitions. The ground-state component of the lasing mode saturates with increase in the drive current and then persists. Further growth of the total output power is due to the excited-state component. Design criteria of quantum-dot lasers, which maximize the ground-state output power, are considered by solving the carrier-photon rate equations. Current-induced ground-to-excited-state lasing transition is due to a combination of slow carrier capture/relaxation to and efficient thermoionic emission from the ground-state level.

Effect of O2 and H2 plasma treatments on the properties of CuInSe2 polycrystalline thin films

In this work, the effects of O2 and H2 plasmas on the properties of grain bulk and the grain boundaries in polycrystalline CuInSe2 (CIS) thin films are investigated. The electrical behaviour of such material is dominated by the defects; in particular grain boundaries, which control charge transport and carrier recombination. CuInSe2 thin films obey to two mechanisms of carrier transport, which operate simultaneously: The thermoionic emission through the intergrain barrier at high temperatures, and the "Variable Range Hopping" (VRH) in the forbidden band at low temperatures. Characterization including Conductivity, Impedance and Thermally Stimulated Current (TSC) measurements was done before and after O2 and H2 treatments. Conductivity versus temperature characteristics were studied in the range of 80-400K while Impedance measurements for different frequencies were realized at high temperature. These measurements showed that O2 and H2 plasmas passivate grain boundary defects in p and n type samples, respectively and have doping effects on the bulk grain: p type films are doped p+ by oxygen while n type ones are n+ doped by hydrogen.

Ablation of molybdenum and niobium with a HyBrID copper laser

A hydrogen bromide in discharge (HyBrID) copper laser with 12 W of maximum average power, 16 kHz repetition rate and 35 ns pulse duration, emitting simultaneously in 512 and 578 nm, was used to study molybdenum and niobium ablation. A modified Balzer QMG-311 mass spectrometer was used for monitoring presence of metal vapor and a pair of polarized flat electrodes placed parallel to the metal vapor stream was used in order to observe electric charges formation. Thin films on BK-7 optical-quality flats allowed the estimation of deposition rate. The measured ablation thresholds of 6.9 and 5.4 J/cm 2 for Mo and Nb, respectively, agreed well with theoretical estimations. Charge formation was observed for laser fluencies below the vapor formation thresholds and this was attributed to thermoionic emission. The maximum film growth rate in our experimental facility was measured to be about 16 Å/min for Mo and 11 Å/min for Nb.

Model for arc cathode regionin a wide pressure range

A one-dimensional model for the near-cathode region of electric arcsis applied to define the current, heat and mass transfer mechanisms in theelectrontemperature range of 10-40 kK and the pressure range of 104-107 Pa.The model considers details of the space charge zone, the ionization zone andevaporation of the cathode material.A copper-tungsten cathode is investigated, and it is found thatevaporation is an important cooling mechanism of the cathode and shouldnot be neglected. The plasma near the cathode is constituted ofevaporated contact material in the main part of the investigatedpressure-temperature domain.The results show that Schottky-enhanced thermoionic emission, togetherwith the present model for the near-cathode layer, explains the currenttransfer mechanism for a wide range of pressures and temperatures and forrefractory and non-refractory cathode materials.

Current-Voltage Characteristic and Schottky Barrier Height of the GaAlAsSb(p)/GaSb(n+) Heterostructure

The GaAlAsSb/GaSb is an interesting system for optoelectronic device technology and particularly for the preparation of lasers and photodetectors. The current-voltage characteristic of Ga 0.53 Al 0.47 As 0.04 Sb 0.96 (p)/GaSb(n + ) heterojunctions, obtained by liquid phase epitaxy on GaSb substrate, is studied. The current of the heterojunction, which is essentially a thermoionic emission of holes, has a behavior identical to that of the Schottky diode, allowing to derive the barrier height V b = 685 meV for this heterostructure. The Richardson constant of the system is calculated to be 49.1 A cm -2 K -2 . The band offsets at the interface are determined as ΔE c = 460 meV, ΔE v = 128 meV, in agreement with other works. The variation of the valence band offset with Al composition shows a good agreement with the theoretical models proposed by Jaros and Mujica.

Temperature and excitation power dependencies of the photoluminescence of planar and vertically self-organized Si 0.70Ge 0.30/Si strained superlattices

We have studied the correlation between the morphological characteristics and the photoluminescence (PL) behavior of nominally 20-nm Si/7-nm Si 0.70Ge 0.30 superlattices (SLs) showing planar SiGe layers (#1) and vertical self-aligned undulations (#2). At 8 K and low PL excitation density, no-phonon (NP) peaks were centered at 919 and 934 meV, for SLs #1 and #2, respectively. Increasing excitation power resulted in a high-energy side broadening and a strong blue-shift of sample #2 peaks, which were interpreted by in-plane localization and spreading of the hole wave function within the thickness undulations. Up to intermediate temperature values (∼120 K), carrier diffusion and localization at the crest (∼11 nm) of the undulations enhanced PL efficiency. Room temperature SiGe PL was observed for both samples with an intensity exponential decay above 150 K. Activation energies of 260 meV (#1) and 172 meV (#2) were derived from Arrhenius plots. The energy difference can be explained by the lateral channel formed by the troughs (∼2 nm) in the long scale (∼120 nm) waviness of sample #2 that assists hole thermoionic emission in the Si barriers.

Effect of steric hinderance on junction properties of poly (3-alkylthiophene)s based schottky diodes

-hexylthiophene), and metals, such as, In, Ag, Al, Sn, etc. The polymers were synthesised chemically using ferric chloride as catalyst. The electrical properties of the devices have been studied by current-voltage (I-V) and capacitance-voltage (C-V) measurements. Junction parameters such as ideality factor (n) and barrier height (χ) have been calculated on the basis of thermoionic emission theory. Better performance of P3cHT/metal diode, compared to P3nHT was attributed to the steric effects produced by cyclohexyl unit present in the polymer.

Suppression of Self-Sustained Field Domain Oscillation in GaAs/AlAs Superlattice by Hydrostatic Pressure at Room Temperature

The behavior of room temperature self-sustained current oscillations resulting from sequential resonance tunneling in a doped weakly-coupled GaAs/AlAs superlattice (SL) is investigated under hydrostatic pressure. From atmosphere pressure to 6.5 kbar, oscillations exist in the whole plateau of the I-V curve and oscillating characteristics are affected by the pressure. When hydrostatic pressure is higher than 6.5 kbar, the current oscillations are completely suppressed although a current plateau still can be seen in the I-V curve. The plateau disappears when the pressure is close to 13.5 kbar. As the main effect of hydrostatic pressure is to lower the X point valley with respect to Γ point valley, the disappearance of oscillation and the plateau shrinkage before Γ - X resonance takes place are attributed to the increases of thermoionic emission and nonresonant tunneling components determined by the lowest Γ - X barrier height in GaAs/AlAs SL structure.

Bayard–Alpert vacuum gauge with microtips

The Bayard–Alpert ionization gauge is one of the most commonly used gauges for the measurement and control of high and ultrahigh vacua (UHV). The electrons that ionize the residual gas are produced by a hot filament. The replacement of this source by field-effect microtips would overcome many problems caused by thermoionic emission, such as degassing, high power consumption, and mechanical weakness. The results of the experiments involving the replacement of the tungsten filament by an array of microtips fabricated at LETI in a classical Bayard–Alpert gauge are presented. They show that microtips could be a good substitute for filaments in this application, and also they improve some characteristics such as the sensitivity of the gauge and the width of the working scale. Moreover, new options such as fast pulsed operation or fast security cutout modes are possible due to the very short time needed to stabilize the electronic emission. However, an improvement of the tip’s material is necessary if the gauge is intended to work continuously at pressures above 10−5 mbar.

Low-temperature behaviour of high- Tc superconductor/semiconductor heterojunctions

The transport mechanisms of p-type YBa 2Cu 3O 7− δ / n-type Si heterojunctions were determined in the temperature range 10 K ≤ T ≤ 300 K by current-voltage measurements. The knowledge of these mechanisms is of great importance for studies and applications of superconductor/semiconductor contacts. For T > T c, the junction behaves as a Schottky contact with several transport mechanisms occurring simultaneously. The main mechanisms in the temperature ranges 200 K ≤ T ≤ 300 K and T c ≤ T ≤ 200 K are thermoionic field emission and the multistep-tunneling-mechanism, respectively. Tunneling of Copper pairs besides quasiparticles dominates in the temperature range T < T c. The contact resistance measures to about 10 3 Ω cm 2 at room temperature. The investigated contacts show a better diode characteristic for T < T c than for T > T c.

Leakage current behaviors in rapid thermal annealed Bi4Ti3O12 thin films

Bi4Ti3O12 thin films have been grown on indium-tin-oxide coated glass by pulsed laser deposition. Films are rapidly thermal annealed at 650 °C in three kinds of atmospheres such as O2, N2, and air. The annealing atmosphere is found to be an important growth parameter which determines the crystallization, microstructures, and the leakage current behaviors. The film annealed in oxygen has a columnar grain structure with an amorphous phase, and its leakage current behavior is in agreement with the prediction of the space-charge-limited conduction model. The film annealed in nitrogen has polycrystalline porous structure, and its high field conduction is well explained by the thermoionic emission model, called the Poole–Frenkel emission. On the other hand, the film annealed in air has both the columnar and porous structures, and its electrical behavior shows characteristics of both models.

Grain boundary conductivity in different polycrystalline MoSe2 thin films

It has been shown early that curved plots for the conductivity of polycrystalline diselenide molybdenum were systematically obtained whatever the technique used for the obtention of the layers. In the general case of polycrystalline semiconductors these deviations from the simple thermoionic emission across the grain boundaries have been recently attributed to potential fluctuations at the grain boundaries. Here a good agreement between the experimental results and this new theory is obtained. The result deduced from theoretical propositions is conforted by the scanning electron micrographs of the layers and other preceeding experimental studies. In the light of the discussion of the MoSe 2 layers, a quality factor Q is proposed for photovoltaic thin films. This estimation shows that the films obtained by solid state reaction and substitution are the best. Mo and Te thin films are sequentially deposited. Then an annealing under Se and Te pressure at 770 K for 24 h gives crystallized and stoichiometric MoSe 2 thin films. These films have the higher quality factor among the films studied here (Q=0.75). This is in close agreement with the good electrical and optical properties of these layers

Temperature and field effects on the conduction mechanisms in semi - insulating polycrystalline silicon

We have studied the temperature dependence of electrical conductivity in semi-insulating polycrystalline silicon (SIPOS) layers deposited on SiO 2 with and without transverse electric fields for oxygen concentrations ranging from 2 to 30 at. % o. At 30 at. % O, for transverse fields of the order of 106 V/cm, the room temperature conductance increases by a factor ?300 with respect to weak transverse field conditions. The results are interpreted by assuming that carrier transport is controlled by thermoionic field emission and by Frenkel generation of carriers at the grain boundaries. The comparison of the carrier transport fitting parameters with the transmission electron microscopy data on grain size supports a model of SIPOS structure in which each silicon grain is surrounded by a SiO 2 shell which becomes continuous at large oxygen contents.

Electrical Characterization of the Silicon‐Electrolyte Interface in the Conditions of Porous Silicon Formation

A detailed experimental investigation of the electrical behavior of the silicon‐electrolyte interface is presented, which leads to the conclusion that electrochemical dissolution of p‐type silicon (doping range ) during porous silicon formation is mainly determined by the charge exchange at the silicon surface over the Schottky barrier formed at the interface through a thermoionic emission process. Impedance characterization of the interface allows the determination of the potential barrier formed between p‐silicon and hydrofluoric acid solutions, and analysis of I(V) characteristics shows that the silicon‐electrolyte junction behaves like a Schottky diode, with a particular dependence of the anodization potential vs. silicon doping which results from the voltage drop in the Helmholtz layer.

A STUDY OF ELECTRONIC CHARACTERIZATION IN MOLECULAR BEAM EPITAXIALLY GROWN GaAs ON Si

The electronic characteristics of GaAs grown on Si by molecular beam epitaxy(MBE) has been examined by studying the diode characteristics and deep level transient spectroscopy (DLTS) of Schottky barriers. l-V characteristics of samples show the existence large leakage current. Post-growth rapid thermal annealing(RTA)has been found to be able to significantly improve the diode behaviour. The reverse current in the as-grown material shows a very weak temperature dependence, indicating that its origin is not thermoionic emission or generation-recombination currents. We think that a large part of this current is due to defect-assisted tunneling, which is reduced by RTA. DLTS spectra show that Ec-0.41eV and Ec-0.57eV electron traps are observable. The former is located near the GaAs-Si interface probably, the latter relates to the well-known electron trap M5 typical of MBE GaAs.