Image Force Lowering Research Articles

Short Channel GaAs MESFET의 채널전하분포와 채널전하에 의한 전위장벽의 변화

In this paper, the gate leakage current is first calculated using the experimental method between gate and drain by opening source electrode. the gate to drain current has been obtained with ground source. The difference between two currents has been tested and proves that the electric field generated by channel charge effect against the image force lowering.

Schottky barrier transport in InSb/AlInSb quantum well field effect transistor structures

Gate leakage currents in high performance InSb/AlInSb quantum well field effect transistors are presented. Thermionic emission theory for Schottky contacts, including the Schottky effect (image force lowering), is successfully used to explain the form of the gate leakage current as a function of gate voltage (I(Vg)) at room temperature. A value for the unlowered Schottky barrier height is extracted from this analysis (∼210 meV). The measurement of barrier height is seen to be in good agreement with the assertion that the Al0.15In0.85Sb material system exhibits mid-bandgap pinning at the metal–semiconductor interface. Gate current as a function of temperature is also used to extract barrier heights, using Arrhenius style plots, and they are shown to be consistent with thermionic emission over an image force lowered barrier. Low temperature I(Vg) data are also presented whereby the gate current is seen to increase below ∼30 K. This effect is unexpected and is attributed to possible movement of the pinning level at the metal–semiconductor interface.

Epitaxial Bi∕GaAs diodes via electrodeposition

Epitaxial Bi∕GaAs diodes have been formed by electrodeposition from bismuth nitrate and ammonium sulfate ((NH4)2SO4) aqueous solutions. Bi grows (0001) oriented on both GaAs (111)B and (001) substrates while it tilts 16° to a (011̱8) surface orientation for (011) GaAs. The metal orients in all cases with its {112̱0} planes parallel the GaAs {110} planes. Diodes prepared on (001), (111)B, and (011) wafers have current-voltage barrier heights ΦBIV that vary from 0.74, to 0.76, to 0.83eV (n=1.01–1.11), respectively. These barrier heights straggle values from earlier reports for polycrystalline Bi deposited by ultrahigh vacuum techniques or electrodeposition. Barrier heights measured from high frequency, capacitance-voltage characteristics are higher than the ΦBIV results, 0.06–1.5eV, as a function of the GaAs orientation, increasing in value in order of (011), (001), to (111)B. This is explained by a combination of image force lowering and field emission corrections, and interface state/dipoles that are likely dependent on the GaAs orientation and on the degree of (0001) Bi alignment. These results are supported by cross-sectional transmission electron microscopy investigations indicating abrupt Bi∕GaAs interfaces without evidence of a significant interfacial oxide or reacted layer.

Direct Measurement of Quantum Confinement Effects at Metal to Quantum-Well Nanocontacts

Model metal-semiconductor nanostructure Schottky nanocontacts were made on cleaved heterostructures containing GaAs quantum wells (QWs) of varying width and were locally probed by ballistic electron emission microscopy. The local Schottky barrier was found to increase by approximately 0.140 eV as the QW width was systematically decreased from 15 to 1 nm, due mostly to a large (approximately 0.200 eV) quantum-confinement increase to the QW conduction band. The measured barrier increase over the full 1 to 15 nm QW range was quantitatively explained when local "interface pinning" and image force lowering effects are also considered.

On the Current-Voltage Characteristic of an Ideal Metal-Semiconductor Schottky-Barrier Contact

The model of an ideal (without an intermediate layer and electron surface states) metal-semiconductor Schottky-barrier contact is numerically analyzed with allowance for the effect of image-force barrier-height lowering. It is shown that the nonlinear dependence of barrier height on the bias voltage inherent in this contact causes not only the deviation of the current-voltage (I-V) characteristic from an ideal one but also the so-called “low-temperature anomaly” — an increase in the I-V-characteristic ideality factor n and a decrease in the barrier height ϕbm measured from the saturation current as the temperature is decreased. A more exact equation for the I-V characteristic of the ideal contact is theoretically substantiated using parameters n and ϕbm in a wide temperature range.

Contact mechanisms and design principles for alloyed Ohmic contacts to p-type GaN

Contact mechanisms and design principles of alloyed Ohmic contacts to p-type GaN (p-GaN) are studied. Illustrative studies include bilayer, trilayer and quadrilayer Ohmic contacts. Almost all contacts appear to follow the proposed design principles. The removal of the surface insulating layer, preferably by plasma etching, leads to metal/semiconductor barrier lowering. This, together with thermionic emission, plays a crucial role for yielding low-resistance metal/p-GaN contacts. Band-gap narrowing and/or image force lowering due to heavy doping also contribute to the low contact resistivity. A judicious choice of the layer thicknesses of appropriate metal combination, rapid thermal annealing (RTA) time, RTA temperature and RTA ambient can produce large-work-function alloy(s) in contact with the p-GaN epitaxial layer, creating a robust low-resistivity thermionic-emission-induced Ohmic contact. The fundamental physics of contact mechanisms and design principles proposed in the study is useful for making other contacts. These are general enough to be extended to other III–V nitride materials, at the least.

Barrier heights of organic modified Schottky contacts: theory and experiment

The impact of thin organic films on the barrier heights of Ag/n-GaAs(0 0 1) Schottky contacts is investigated using complementary experimental and theoretical techniques, namely current–voltage ( I– V) measurements, impedance spectroscopy, and internal photoemisson spectroscopy, as well as drift diffusion simulations. The in situ current–voltage characteristics show that the effective barrier height decreases by ∼200 meV as a function of the 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) layer thickness. The effects responsible for the decrease in barrier height are derived from calculations using a two-dimensional drift-diffusion simulator. The simulations show that image force lowering is responsible for the decrease in barrier height. Upon air exposure barrier heights of organic modified contacts are found to increase reaching values which are higher than the ones for the unmodified contacts. Internal photoemission spectroscopy investigations show that the properties of organic/GaAs(1 0 0) interface is not modified by the exposure to air. Therefore, the increase in barrier height is due the contamination of the PTCDA/Ag interfaces resulting in interface states. Due to these interface states the Fermi level shifts by about 0.2 eV.

Contact mechanisms and design principles for alloyed ohmic contacts to n-GaN

Contact mechanisms and design principles of alloyed ohmic contacts to n-GaN are investigated. For the investigation, both tunnel contacts and thermionic contacts are considered. While the tunnel contacts include the Ti/Al/Ti/Au, Ti/Al/Ni/Au, Ti/Al/Pd/Au, (Ta,Ti)/Ni/Au, Ti/Al/Mo/Au, and Ti/Au/Pd/Au contacts, the thermionic contacts include the Ni/Au contacts. The proposed design principles correctly dictate the characteristics of all these contacts. At present, tunneling is believed to be the primary mechanism for low resistivity of the tunnel contacts. The present study demonstrates that both tunneling and thermionic emission are equally important for the low resistivity of these contacts. Band-gap narrowing and/or image force lowering due to heavy doping also contribute to the resistivity reduction of these contacts. An exciting feature of the present study is the observation of a very low work function intermetallic alloy formed during annealing of an appropriate combination of large work function metals. If the annealing conditions are optimized, the contacts become very robust, thermally stable, and lowly resistive with thermionic emission as the primary mechanism for electron transport. The observation is very promising and has potential to open up avenues for different types of thermionic contacts. The fundamental physics underlying the design principles are discussed. These principles are general enough to be applicable to other III–V nitrides, at the least.

Contact mechanisms and design principles for nonalloyed ohmic contacts to n-GaN

The contact mechanism and design principles for nonalloyed ohmic contacts are investigated. Illustrative studies of various contacts show wide validity of the design principles. Experimental measurement suggests that reactive ion etching (RIE) of the n-GaN surface creates extra electrons. There is an optimum etching time that produces the highest electron concentration. RIE creates disturbances in the N–Ga bonds. This, together with metal deposition such as Ti/Al/Ni/Au and Ti/Au/Pd/Au depositions on RIE-treated samples, promotes the formation of TiN. N vacancies, thus created in the GaN subsurface, act as donor atoms. There occurs structural, atomic, and/or bonding reorganization at the metal/semiconductor (M/S) interface during postdeposition cooling yielding the M/S barrier height lower than the Schottky–Mott barrier height. Band gap narrowing and/or image force lowering resulting from heavy doping lead to further reduction of the barrier height. As a result thermionic emission, rather than tunneling, may be the primary mechanism for the low resistivity of many contacts.

Reply to “Comment on ‘Unified compact theory of tunneling gate current in metal-oxide-semiconductor structures: Quantum and image force barrier lowering’ [J. Appl. Phys. 92, 3724 (2002)

Our transparency factor, Peff using the effective triangular approximation is shown to practically match PWKB given in the comment. It indicates that Peff can model the effect of image force lowering (if it exists) on the gate current (Ig) in the Fowler–Nordheim regime as accurately as the Wentzel–Kramers–Brillouin method. The experimentally extracted value of α≈0 indicates that PWKB incorporating the image potential still overestimates Ig. Additionally, physics underlying the image force lowering in metal-oxide-silicon systems is briefly discussed in light of the wave nature of electrons and other reported experimental data.

Comment on “Unified compact theory of tunneling gate current in metal–oxide–semiconductor structures: Quantum and image force barrier lowering” [J. Appl. Phys. 92, 3724 (2002)

Quan, Kim, and Cho [J. Appl. Phys. 92, 3724 (2002)] have proposed a compact model to evaluate the tunneling current across the insulator of metal–oxide–semiconductor structures and have claimed to have “shown conclusively that image force lowering of the oxide barrier is negligible for the oxide as thin as 1 nm.” Unfortunately, the model is based on a questionable approximation for the “transparency factor.” Most importantly, the argument brought forward to explain the negligible effect of the image-induced barrier-lowering ignores simple concepts of electrostatics.

Barrier characteristics of Cd/p-GaTe Schottky diodes based on I–V–T measurements

The current–voltage (I–V) characteristics of Cd/p-GaTe Schottky barrier diodes were measured in the temperature range 90–330 K. The apparent barrier height and the ideality factor derived by using thermionic emission (TE) theory were found to be strongly temperature dependent. Evaluating forward I–V data reveals a decrease of zero-bias barrier height (Φb0) but an increase of ideality factor (n) with decrease in temperature, and these changes are more pronounced below 150 K. The conventional Richardson plot exhibits nonlinearity below 150 K with the linear portion corresponding to an activation energy of 0.52 eV. The value of effective Richardson constant (A*) turns out to be 6.74 × 10−2 A K−2 cm−2 against the theoretical value of 119.4 A K−2 cm−2. It is demonstrated that the findings cannot be explained on the basis of tunnelling and image force lowering effects. Also, the concept of the flat-band barrier height (Φfb) fails to account for the temperature dependence of the diode parameters. Finally, it is demonstrated that these anomalies result due to the barrier height inhomogeneities prevailing at the metal–semiconductor interface. The inhomogeneities are considered to have Gaussian distribution with a mean barrier height of = 0.886 eV and a standard deviation of σs0 = 0.091 eV at zero bias. Furthermore, the mean barrier height and the Richardson constant values were obtained as 0.875 eV and 62.2 A K−2 cm−2, respectively, by means of the modified Richardson plot, ln(J0/T2) − (q2σ2s0/2k2T2) versus 1000/T. Hence, it has been concluded that the temperature dependence of the I–V characteristics of the Schottky barrier on p-type GaTe can be successfully explained on the basis of TE mechanism with Gaussian distribution of the barrier heights.

Unified compact theory of tunneling gate current in metal–oxide–semiconductor structures: Quantum and image force barrier lowering

A compact model of gate current due to Fowler–Nordheim tunneling is presented, which agrees closely with the self-consistent numerical analyses of the surface inversion region of metal–oxide–semiconductor field-effect transistors (MOSFETs). The model can quantify the measured data with the accuracy practically identical to the time consuming numerical simulation. It is also shown conclusively that image force lowering of the oxide barrier height is negligible for the oxide as thin as 1 nm. The quantum barrier lowering resulting from subband splitting is rigorously incorporated, including the effect of two-dimensional electrons inverted at the higher lying subbands. Finally, it is pointed out that the compact model can be readily generalized to include the direct tunneling in deep submicron MOSFETs.

Double-Junction Schottky Diodes Using a Ti/Si/Al Structure

Ti/Si/Al structures are studied theoretically and experimentally. The effect of the reverse-biased junction on the current–voltage characteristic of the entire structure is revealed. It is noted that the behavior of the curve is determined by the image-force lowering of the potential barrier. The current–voltage characteristic is shown to obey a power law if the voltage is low enough. It is established that the capacitance of the structure decreases with increasing voltage if the latter is positive. This phenomenon is modeled using two capacitors in a series.

Current transport at the p-InP|poly(pyrrole) interface

The interface between the inorganic semiconductor p-type InP and the conjugated polymer poly(pyrrole) exhibits the electrical characteristics of a Schottky diode. Capacitance–voltage measurements yield an average barrier height of 0.62 ± 0.01 eV at temperature T =298 K. At the same temperature, the empirical quality factor, extracted from current–voltage measurements, is near unity. However, the current–voltage measurements show a deviation from thermionic emission theory as the temperature is reduced, as witnessed by the increase of the quality factor and the curvature in the Richardson plot. Such deviation is best explained by the barrier inhomogeneity model, in which the barrier becomes voltage dependent due to the interaction of a small low-barrier region with a higher surrounding potential, termed the “pinch-off” effect. Traditional current–voltage models, including image force lowering or an interfacial layer, cannot predict the temperature dependence of the current–voltage data, although thermionic field emission may facilitate current transport in the interfaces with a higher doped InP substrate. Furthermore, the probability of sufficiently energetic incident charge carriers crossing the interface, termed the transmission coefficient, is smaller than that observed in metal Schottky diodes.

Hot carrier transport effects in Al2O3-based metal-oxide-semiconductor structures

Over the barrier, hot electron transport across 8 nm thick amorphous Al2O3 layers embedded in metal-oxide-semiconductor (MOS) structures was investigated with ballistic electron emission microscopy (BEEM). The oxide field dependence of the BEEM threshold voltage Vth, which corresponds to the potential maximum of the barrier, was found to be dominated by image force and charge trapping/detrapping effects. The static barrier height at the W–Al2O3 interface ΦB=3.90±0.03 eV and the dynamic dielectric constant εif=1.86±0.1, which reflects the strong image force lowering of the barrier observed at both interfaces. A band offset between the Al2O3 and Si conduction bands of 2.78±0.06 eV was deduced. Electron trap levels at energies overlapping the Si band gap and of densities in the 1012 cm−2 range were deduced to lie in the oxide near the Al2O3–Si interface. Their occupancy is determined by the position of the interface Fermi level. For p-type substrates the traps were empty (filled) for positive (negative) applied bias. Local, electrical stressing increased the interface trap charge for n-type substrates, but had negligible consequences on p-type substrates. The Al2O3 was readily stressed to failure upon injecting sub-nano-Coulomb of charge at electron kinetic energies in the 4–6 eV range.

Ion-bombardment effects on PtSi/n-Si Schottky contacts studied by ballistic electron emission microscopy

Ballistic electron emission microscopy has been used to study physical damage effects on PtSi/n-Si Schottky contacts. The physical damages are introduced into Si substrates by ion bombardment with well-defined energies in an ion-milling process. Schottky barrier height (SBH) distribution is measured on the subsequently formed PtSi/n-Si Schottky diodes. The results show that mean SBH decreases with ion energy in a square-root relation. A simple SBH model is developed to consider image-force lowering effect for a semiconductor with a step-function distribution of donor concentration. The model is successfully used to explain quantitatively the experimental relation between SBH and ion energy.

Thermally activated injection limited conduction in single layer N,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′-biphenyl-4,4′-diamine light emitting diodes

Impedance, current–voltage–luminosity and spectral measurements have been carried out on indium-tin-oxide/N,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′-biphenyl-4,4′-diamine (TPD)/Al light emitting diodes. The devices have a blue/violet emission with a spectrum peaked at 404 nm. Capacitance–voltage measurements show that at zero bias the devices are fully depleted. The impedance measurements show that the devices can be modeled on a single, frequency-independent parallel resistor-capacitor RPCP circuit with a small series resistance RS. RP changes with applied bias and temperature, while CP remains constant. The values of CP give εr=3.0±0.3. Analysis of the current–voltage (J–V) characteristics show that the dominant conduction mechanism cannot be either ohmic, trap free space charge limited, or tunneling injection. The temperature and thickness dependence indicate that it must be either thermionic emission or thermally assisted tunneling, the carrier density varying from about 1010/1011 to 3×1013 cm−3 over the measured bias range. The EL efficiency increases 20 fold upon cooling but shows little variation with bias at all temperatures, indicating the same mechanism is responsible for the injection of both holes and electrons. Modeling the results with thermionic emission suggests that image force lowering is responsible for the variation of the current with applied bias, but the calculated injection barrier height and Richardson constant are much smaller than expected. This cannot be explained by models based on a backflowing surface recombination current due to the high carrier mobility found in TPD.

Properties of Regioregular Poly(3-alkylthiophene) Schottky Diodes

An investigation into the effect of carrier trapping in polymer Schottky diodes is presented. The diodes have been fabricated using regioregular poly(3-octylthiophene) as the semiconductor and aluminium and indium doped tin-oxide as the rectifying and ohmic contacts respectively. The diodes show rectification ratios up to 3–4 orders of magnitude. Evidence for image-force lowering of the Schottky barrier is observed. The frequency dependence of the capacitance, conductance and dielectric loss is analysed, from which detailed information on the metal/semiconductor interface and the doping level is obtained. An equivalent circuit is proposed.

Device model for single carrier organic diodes

We present a unified device model for single layer organic light emitting diodes (LEDs) which includes charge injection, transport, and space charge effects in the organic material. The model can describe both injection limited and space charge limited current flow and the transition between them. We specifically considered cases in which the energy barrier to injection of electrons is much larger than that for holes so that holes dominate the current flow in the device. Charge injection into the organic material occurs by thermionic emission and by tunneling. For Schottky energy barriers less than about 0.3–0.4 eV, for typical organic LED device parameters, the current flow is space charge limited and the electric field in the structure is highly nonuniform. For larger energy barriers the current flow is injection limited. In the injection limited regime, the net injected charge is relatively small, the electric field is nearly uniform, and space charge effects are not important. At smaller bias in the injection limited regime, thermionic emission is the dominant injection mechanism. For this case the thermionic emission injection current and a backward flowing interface recombination current, which is the time reversed process of thermionic emission, combine to establish a quasi-equilibrium carrier density. The quasi-equilibrium density is bias dependent because of image force lowering of the injection barrier. The net device current is determined by the drift of these carriers in the nearly constant electric field. The net device current is much smaller than either the thermionic emission or interface recombination current which nearly cancel. At higher bias, injection is dominated by tunneling. The bias at which tunneling exceeds thermionic emission depends on the size of the Schottky energy barrier. When tunneling is the dominant injection mechanism, a combination of tunneling injection current and the backflowing interface recombination current combine to establish the carrier density. We compare the model results with experimental measurements on devices fabricated using the electroluminescent conjugated polymer poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] which by changing the contacts can show either injection limited behavior or space charge limited behavior.