Ideal Schottky Contact Research Articles

Slope parameters of the barrier heights of metal-organic contacts

The continuum of metal-induced gap states (MIGS) determines the barrier heights of ideal metal-semiconductor or Schottky contacts. The charge transfer across such interfaces may be attributed to the partial ionic character of the covalent bonds between the metal and the semiconductor atoms right at the interface. Consequently, the barrier heights are split up into a zero-charge-transfer term, which equals the energy separation between the MIGSs branch point and the majority-carrier band edge, and an electric-dipole term, which varies proportional to the difference of the metal and the semiconductor electronegativities. For Schottky contacts on inorganic semiconductors, the respective slope parameters were found to depend on the square (ϵ∞−1)2 of the optical susceptibility of the semiconductors. It is demonstrated that experimental as well as theoretical slope parameters reported for metal contacts to organic semiconductors follow the same relationship which was observed earlier with Schottky contacts of inorganic semiconductors. This finding is not surprising as the MIGS originate from the quantum-mechanical tunnel effect of the bulk metal electrons in the energy range from the highest occupied electronic energy level of the semiconductor up to the Fermi level, irrespective of whether inorganic or organic semiconductors are considered.

Potassium reaction on sulfur-passivated GaAs(100)

It has been confirmed by high-resolution x-ray photoelectron spectroscopy that the Fermi level of K-contacted sulfur-passivated GaAs(100) is not located at the Fermi level of the ideal Schottky contact and is pinned near the midgap, even though the initial S passivation removes the high density gap states. With K deposition the Fermi level moves to 0.5 eV above the valence band maximum for both n- and p-type surfaces. The interfacial chemical reaction shows the top As element preferentially reacts with deposited K, and the interface still has dominant Ga–S bondings. These results imply that although the reactive alkali metal such as K does not break Ga–S bondings, it induces the metallic state which pins the Fermi level at 0.5 eV above the valence band maximum.

Barrier heights of real Schottky contacts explained by metal-induced gap states and lateral inhomogeneities

Most metal–semiconductor contacts are rectifying. For moderately doped semiconductors, the current transport across such Schottky contacts occurs by thermionic emission over the Schottky barrier. The current–voltage characteristics of real Schottky contacts are described by two fitting parameters that are the effective barrier heights ΦBeff and the ideality factors n. Due to lateral inhomogeneities of the barrier height, both parameters differ from one diode to another. However, their variations are correlated in that ΦBeff becomes smaller with increasing n. Extrapolations of such ΦBeff-versus-n plots to the corresponding image-force-controlled ideality factors nif give the barrier heights of laterally homogeneous contacts. They are then compared with the theoretical predictions for ideal Schottky contacts. Data of Si, GaN, GaAs, and CdTe Schottky contacts reveal that the continuum of metal-induced gap states is the fundamental mechanism that determines the barrier heights. However, there are additional but then secondary mechanisms. As an example, contacts with (7×7)i-reconstructed interfaces have smaller barrier heights than diodes with (1×1)i-unreconstructed interfaces. This lowering of the Schottky barrier is caused by the electric dipole associated with the stacking fault in one of the triangular halves of the (7×7) unit mesh.

Schottky barriers on GaAs: Screened pinning at defect levels

Schottky barrier heights on n-type GaAs can be made to vary from 0.30 to 1.20 eV by adjusting the conditions under which the Schottky contact is fabricated. To explain the observed trends it is asserted (a) that there are several defect pinning points associated with GaAs surfaces, (b) that an appropriate interfacial permittivity may be defined to predict the barrier height from the metal work function and the position of the appropriate pinning point, and (c) the pinning point is sensitive to the metal-GaAs reaction chemistry. In a thermodynamic context the present analysis is an electronegativity model. The work function of a polycrystalline metal and the charge neutrality level of the semiconductor are interpreted as the bulk chemical potentials of the two materials. When the two materials are brought into contact electronegativity equalization takes place as charge is transferred across the interface in the formation of metal-semiconductor bonds. Proper derivation of the equilibrium bond charge dipole requires a knowledge of the chemical ``softness'' of the two surfaces being brought into contact as well as the electronegativity difference of the two materials. The equilibrium dipole determines the barrier height of an ``ideal interface Schottky contact.'' A description is given of how the softness of metal and semiconductor surfaces may be defined and how near surface defects in the semiconductor modify the interfacial dipole. The strength of the model is in its descriptive power as it can identify the specific native defects at which pinning is observed in GaAs Schottky diodes. It provides a framework for the development of a predictive model based on identifying the particular native defects produced by the reaction of specific metals with a semiconductor as a function of the process history.

Barrier heights of GaN Schottky contacts

Silver and lead contacts prepared by evaporation onto clean n-GaN(0001) surfaces are rectifying. Their zero-bias barrier heights and ideality factors were determined from the current-voltage characteristics. The observed linear correlation between the barrier heights and the ideality factors is attributed to nonuniform distributions of barrier heights along the interfaces. The barrier heights of ideal Schottky contacts depend on the applied voltage due to the image-force lowering only and their ideally factors n if are approximately 1.01. By extrapolation of our experimental data to n = 1.01, we obtain barrier heights of 0.82 eV and 0.73 eV for uniform Ag- and Pb/n-GaN(0001) contacts, respectively. By applying the idea of metal-induced gap states (MIGS), the barrier heights of ideal Schottky contacts have been predicted to vary linearly as a function of the difference of the metal and the semiconductor electronegativities. The zero-charge-transfer barrier height and slope parameter are characteristic of the respective semiconductor. The zero-charge-transfer barrier heights have been calculated using an empirical tight-binding approach and the slope parameters are given by the optical dielectric constants. The experimental barrier heights of GaN Schottky contacts confirm the predictions of the MIGS-and-electronegativity model.

Metal Contacts on a-GaN

The Schottky barrier heights of silver and lead contacts on n-type GaN (0001) epilayers were determined from current-voltage characteristics. The zero-bias barrier heights and the ideality factors were found to be linearly correlated. Similar observations were previously reported for metal contacts on Si (111) and GaAs (110) surfaces. The barrier heights of ideal Schottky contacts are characterized by image force lowering of the barrier only. This gives an ideality factor of 1.01. From our data we obtain barrier heights of 0.82 eV and 0.73eV for ideal Ag and Pb contacts on GaN, respectively. The metal-induced gap states (MIGS) model predicts the barrier heights of ideal Schottky contacts on a given semiconductor to be linearly correlated with the electronegativities of the metals. The two important parameters of this MIGS-and-electronegativity model are the charge neutrality level (CNL) of the MIGS and a slope parameter. The CNL may be calculated from the dielectric band gap and using the empirical tight-binding method. The slope parameters are given by the optical dielectric constant of the respective semiconductor. The predictions of the MIGS model for metal/GaN contacts are confirmed by the results presented here and by barrier heights previously reported by others for Au, Ti, Pt, and Pd contacts on GaN.

Molecular solid of zinc tetraphenylporphyrin as a model organic semiconductor with a well-defined depletion layer

Capacitances ( C) of the molecular solids of zinc tetraphenylporphyrin (ZnTPP) are measured as functions of an applied voltage ( V), the frequency, and the thickness of the ZnTPP film. The analysis of the 1 C 2 vs V plots for the frequencies far below 0.1 Hz reveals that the ZnTPP solids behave well as a semiconductor without any complication due to surface states, etc. and thus form an almost ideal Schottky contact with Al. A space charge concentration is evaluated to be 6 × 10 16cm −3. At higher frequencies, Al/ZnTPP/Au cells exhibit voltage-independent capacitances, which are inversely proportional to the thickness of the ZnTPP layer in accord with the parallel-plate capacitor model. This finding in the high frequency region is explained in terms of an extremely slow kinetics of space charges. In addition, a change in photocurrent with the film thickness of ZnTPP is explained by the formation of a depletion layer with 90 nm-width.

Metal-semiconductor contacts: electronic properties

Rectification in metal-semiconductor contacts was first described by Braun in 1874. We owe the explanation of this observation to Schottky. He demonstrated that depletion layers exist on the semiconductor side of such interfaces. The current transport across such contacts is determined by their barrier heights, i.e., the respective energy difference between the Fermi level and the edge of the majority-carrier band. Since Schottky had published his pioneering work in 1938 the mechanisms, which determine the barrier heights of metal-semiconductor contacts, have remained under discussion. In 1947, Bardeen attributed the failure of the early Schottky-Mott rule to the neglect of electronic interface states. The foundations for a microscopic description of interface states in ideal Schottky contacts was laid by Heine in 1965. He demonstrated that a continuum of metal-induced gap states (MIGS), as they were called later, derives from the virtual gap states of the complex semiconductor band-structure. Neither this MIGS model nor any of the many other monocausal approaches, the most prominent is Spicer's Unified Defect Model, can explain the experimental data. In 1987, Mönch concluded that the continuum of MIG states represents the primary mechanism, which determines the barrier heights in ideal, i.e., intimate, abrupt, and homogeneous metal-semiconductor contacts. He attributed deviations from what is predicted by the MIGS model to other and then secondary mechanisms. In this respect, interface defects, structure-related interface dipoles. interface strain, interface compound formation, and interface intermixing, to name a few examples, were considered.

The p-Inp semiconductor / electrolyte contact under depletion conditions: Impedance, reverse currents and photopotentials

Current—voltage and impedance spectroscopic measurements were made at the p-InP < 100 >H 2SO 4 contact. Small reverse saturation current densities (ca. −1 nA cm −2) at low doped samples allowed measurement of the space-charge capacity C sc over more than seven frequency decades (20 mHz–30 kHz). An extremely low and potential-independent frequency dispersion of C sc of 0.4% per frequency decade was obtained (20 Hz–20 kHz). Results obtained from capacitance data for the semiconductor—surface energy barrier height φ B with and without platinum coating and for different doping densities are compared with data from photentials. Larger variations of φ B (0.7–1.0 eV) than those typically achieved at solid state p-Inp—metal contacts are verified. Flatband potentials and photopotentials are used to estimate the band-bending under illumination which, at bare and Pt-coated p-InP—electrolyte contacts, turns out to be 70–240 mV lower than at ideal Schottky contacts.

An in situ characterization of the p-indium phosphide-indium/electrolyte contact formed by cathodic corrosion of the semiconductor

Hydrogen evolution at illuminated p-InP/electrolyte contacts is accompanied by a corrosion reaction leading to indium formation at the semiconductor surface. After this process the electrode response is analyzed in 1 M H{sub 2}SO{sub 4} using current-voltage curves and Mott-Schottky as well as impedance spectroscopic measurements. Mott-Schottky data indicate a shift of the flat-band potential due to In formation whereas barrier heights at the semiconductor surface remain almost constant ({Phi}{beta} {approx} 1 eV). Impedance data under photocurrent flow reveal a nearly ideal Schottky contact after separation of the electrochemical charge transfer at the In surface. 17 refs., 4 figs., 1 tab.

Effects of hydrogen on the barrier height of a titanium Schottky diode on p-type silicon.

Hydrogen is shown to play an important role in chemically prepared semiconductor surfaces covered by metals. The stability in air of a hydrogen-terminated silicon surface is such that one should be able to form an ideal Schottky contact after metal evaporation. An ideal Ti Schottky contact on chemically prepared p-type silicon is observed after hydrogen is drifted away from the metal-semiconductor interface by reverse-bias low-temperature annealing. It is argued that the observed degraded electrical properties of chemically prepared Schottky contacts result from the effects of the entry of hydrogen into the silicon and hydrogen passivation (deactivation) of dopant atoms.

Electrical Properties of Amorphous Si-P/P-Type Si Schottky Barrier Contact

Electrical properties of a contact between amorphous silicon-phosphorous solid solution film (a-Si-P) and p-type Si are described. This contact is basically a Schottky contact because a-Si-P is one of the Fermi glasses. Its current-voltage (I-V) and capacitance-voltage (C-V) characteristics, however, exhibit anomalous features differing from those of an ideal Schottky contact. In the I-V curves, the forward current saturates at around 0.4 V, followed by an exponential reincrease in current above 0.5 V. This is interpreted as a charge exchange between the interface states and a space-charge region formed in the a-Si-P electrode. In the C-V curves, an excess capacitance is observed even for reverse bias. The origin of this capacitance is also discussed.