Investigation of the Schottky barriers in aluminum — vitreous semiconductor contacts

In the present paper, results of investigations of the Schottky barriers based on aluminum — chalcogenide vitreous semiconductor contacts are given. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 73–76, October, 2005.     

The kinetics of Schottky barrier formation: Al on low-temperature GaAs(110)

The deposition of Al on both low-temperature and room-temperature cleaved GaAs(110) substrates results in Al-Ga exchange and in some Al clusterization. In contrast, the kinetics of the Shottky barrier formation process is dramatically different for low- and room-temperature substrates. This requires substantial modifications of the barrier formation model deduced from room-temperature experiments.     

Mo/GaAs Schottky barriers prepared by d.c. sputtering

Summary Mo/GaAs Schottky barriers have been prepared by d.c. sputtering, for different values of the sputtering voltage. Current-voltage and capacitance-voltage measurements show that these barriers can have very good properties (near one ideality factor, very low inverse saturation current) if suitable preparation conditions are chosen. A detailed study of the current-voltage characteristics as a function of the temperature allows us to analyse the carrier transport mechanisms and to correlate them to the preparation conditions. The experimental results show that the behaviour of the Mo/GaAs Schottky barriers, prepared by d.c. sputtering, can be successfully explained on the basis of the unified defect model proposed for the GaAs schottky barriers.

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Riassunto In questo lavoro vengono presentati risultati concernenti barriere Schottky Mo/GaAs preparate mediante ?sputtering? in continua, per diversi valori delle tensioni di ?sputtering?. Le misure di caratteristiche corrente-tensione e capacità-tensione mostrano che è possibile ottenere barriere con buone caratteristiche elettriche (fattore di idealità prossimo ad uno, corrente di saturazione inversa molto bassa), purché siano scelte le giuste condizioni di deposizione. è stato anche condotto uno studio dettagliato delle ricavare informazioni sul meccanismo di trasporto dei portatori di carica. I risultati sperimentali mostrano che il comportamento di questo tipo di barriere è perfettamente spiegabile nell'ambito del modello unificato dei difetti nelle barriere Schottky nel GaAs.

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Резюме Барьеры Шоттки Mo/GaAs приготовлены с помощью распыления при различных значениях напряЗения распыления. Измерения зависимостей тока от напряЗения и емкости от напряЗения показывают, что зти барьеры могут иметь очень хорощие свойства, если выбираются соответствующие условия приготовления. Подробное исследование вольт-амперных характеристик в зависимости от температуры позволяет проанализировать механизмы переноса заряда и связать их с усломиями приготовления. Зкспериментальые результаты показывают, что поведение барьеров Шоттки Mo/GaAs, приготовленных с помощьюDC распыления, моЗно успешно объяснить на основе единой модели дефектов, предлоЗенной для барьеров Шоттки GaAs.

     

Phonon-assisted tunneling in Schottky barriers

Experimental current-voltage characteristics and their temperature dependencies for AlGaAs Schottky barrier structures are shown to be in agreement with the recent theory of phonon-assisted tunneling.     

Nonideality of Au/Si and Au/GaAs Schottky barriers due to process-induced defects

A mechanism of local lowering of the Schottky barrier height (SBH) is proposed, which causes nonideality in nearly ideal Au/n-Si and Au/n-GaAs Schottky barriers. Positively ionized defects generated by the process very close to the interface induce electrons in the metal-induced gap states (MIGS) and lower the SBH locally. The spatial density distribution of the ionized defects obtained from the SBH distribution is determined by the unique interaction with the MIGS. The defects are considered to have the negative-U property and are neutralized at very close positions to the MIGS. The potential distributions close to the interface have a considerable potential drop due to the large defect density. These inhomogeneous potentials are coincident with the energy level scheme of the defect identified as the defect causing the nonideality. This defect is Si self-interstitial in Au/Si SB, and As antisite in Au/n-GaAs SB. This MIGS with process-induced defect model supersedes the previously proposed two major Fermi level pinning models. The mystery of the T₀ effect is solved. The thermionic-field emission current taking place in the strong electric field has influence on the I-V characteristics at low temperatures. Regarding the C-V characteristics of Au/Si SB, the observed extra capacitance under the forward bias is an experimental evidence in accordance with the proposed model.     

Schottky barriers in carbon nanotube heterojunctions

Electronic properties of heterojunctions between metallic and semiconducting single-wall carbon nanotubes are investigated. Ineffective screening of the long-range Coulomb interaction in one-dimensional nanotube systems drastically modifies the charge transfer phenomena compared to conventional semiconductor heterostructures. The length of depletion region varies over a wide range sensitively depending on the doping strength. The Schottky barrier gives rise to an asymmetry of the I-V characteristics of heterojunctions, in agreement with recent experimental results by Yao et al. and Fuhrer et al. Dynamic charge buildup near the junction results in a steplike growth of the current at reverse bias.     

Barrier height engineering of Ag/GaAs(100) Schottky contacts by a thin organic interlayer

Thin films of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) were used as an interlayer for the electronic modification of Ag/n-GaAs(100) Schottky contacts. The electronic properties were investigated recording in situ current–voltage (I–V) and capacitance–voltage (C–V) characteristics. For H-plasma treated substrates the effective barrier height decreases from 0.81 to 0.64 eV as a function of the PTCDA layer thickness (d_PTCDA). In the case of the sulphur passivated GaAs the effective barrier height first increases and then decreases, the overall range being 0.54–0.73 eV. The substrate treatment leads to a different alignment between the band edges of the GaAs and the molecular orbitals of the PTCDA, making it possible to determine the energy position of the LUMO transport level.     

High reliability sputtered Schottky diodes on GaAs

We describe developments to improve reliability and power handling for high frequency applications in whisker-contacted Schottky barrier diode mixers, detectors and multipliers fabricated using sputtered refractory metals and silicides. The lift-off process has been used to fabricate diodes on GaAs with ideality factors of 1.18 and 1.06 for W/GaAs and TiSi_x/GaAs contacts respectively.     

Electrical properties of metal-GaAs Schottky barriers

Schottky barrier contacts have been fabricated by vacuum evaporation of thin films of six different metals (Mg, In, Al, Au, Bi and Sb) onto chemically cleaned n-type GaAs substrates. Bi and Sb contacts on GaAs have been reported for the first time. The contacts were examined for their C-V and I-V characteristics. The diodes exhibit near ideal characteristics for all the metals with values of the ideality factor n ranging from 1.06 to 1.1. The values of the barrier height obtained from C-V measurements could be brought into agreement with those obtained from I-V measurements only when the ideality factor was also included in the expression of the saturation current. By employing the most recent and reliable values of metal work function φ_M, the dependence of barrier height on φ_M has been investigated and an estimate of the surface state density and the Fermi level at the GaAs surface has been made. The surface state density obtained from this analysis is significantly larger than that reported by previous workers.     

Investigation of ruthenium Schottky contacts to n-GaAs

Ruthenium was evaporated on n-GaAs to form Schottky contacts. Initial electrical measurements revealed a near ideal Schottky behaviour with low leakage currents. The Schottky diodes exhibit good stability upon thermal aging at elevated temperatures up to 300° C. However, the diode parameters rapidly deteriorate after aging at temperatures in excess of 400° C. The room temperature (300 K) median life of the diodes, based on a failure criterion of a tenfold increase in the diode saturation current, J _riv ^s , from reverse bias current-voltage (I–V) data, was of the order of 10⁴ h.     

Amorphous and crystalline IrSi Schottky barriers on silicon

The electro-optical properties of the films are studied by measuring the infrared optical absorption and the yield of hole photoemission across the Schottky barrier to the p-Si substrate. The Schottky barrier height of the amorphous a-IrSi to the valence band of Si is 0.17–0.18 eV. The barrier height of the polycrystalline c-IrSi depends on the degree of texturing of the silicide; the Schottky barrier height increases from 0.14 eV to 0.18 eV when the substrate temperature is increased from room temperature to 400 °C during the Ir metal deposition prior to the 500 °C silicidation. The non-linearity of the photoemission yield observed for the amorphous a-IrSi in the Fowler plot is due to an energy-dependent photon absorption of these amorphous films. An evaluation of the scattering events in the metal films shows that the mean free path for inelastic scattering is λ_in=500 nm in the amorphous a-IrSi which is one order of magnitude larger than that in the crystalline c-IrSi. The mean free path for quasi-elastic scattering λ_qe=0.3 nm, of the amorphous a-IrSi is in the order of the interatomic distance, indicating a localization of the electronic states. Received: 14 November 1997/Accepted: 12 January 1998     

PEDOT:PSS Schottky contacts on annealed ZnO films

Polycrystalline ZnO and ITO films on SiO₂ substrates are prepared by radio frequency (RF) reactive magnetron sputtering. Schottky contacts are fabricated on ZnO films by spin coating with a high conducting polymer, poly(3, 4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as the metal electrodes. The current-voltage measurements for samples on unannealed ZnO films exhibit rectifying behaviours with a barrier height of 0.72 eV (n=1.93). The current for the sample is improved by two orders of magnitude at 1 V after annealing ZnO film at 850 ℃, whose barrier height is 0.75 eV with an ideality factor of 1.12. X-ray diffraction, atomic force microscopy and scanning electron microscopy are used to study the properties of the PEDOT:PSS/ZnO/ITO/SiO₂. The results are useful for applications such as metal-semiconductor field-effect transistors and UV photodetectors.     

Direct probing of Schottky barriers in Si nanowire Schottky barrier field effect transistors

This work elucidates the role of the Schottky junction in the electronic transport of nanometer-scale transistors. In the example of Schottky barrier silicon nanowire field effect transistors, an electrical scanning probe technique is applied to examine the charge transport effects of a nanometer-scale local top gate during operation. The results prove experimentally that Schottky barriers control the charge carrier transport in these devices. In addition, a proof of concept for a reprogrammable nonvolatile memory device based on band bending at the Schottky barriers will be shown.     

A simple model for calculating the height of Schottky barriers at contacts of transition metals with silicon carbide polytypes

The heights of Schottky barriers at contacts of Ag, Au, Pd, Pt, Ti, Ru, Ni, Cr, Al, Mg, and Mn metals with different polytypes of silicon carbide SiC are self-consistently calculated in the framework of a simple model proposed earlier. The results of calculations performed for contacts of transition metals with silicon carbide polytypes are in quite reasonable agreement with experimental data under the assumption that silicon vacancies with an energy E_d=E_V+2.1 eV make a dominant contribution to the Schottky barrier height.     

C-V Technique on Schottky contacts — Limitation of implanted profiles

Some limiting effects for the application of theC-V technique on Schottky contacts for measuring ion-implanted impurity distributions in semiconductors are studied. The basic demand for this technique lies in the possibility to measure the profile with sufficient accuracy within a wide range of depths. The depth of the space charge region for zero bias determines the minimum measurable distance. In greater depths the avalanche breakdown of the metal-semiconductor junction is a limiting factor. The region of applicable implanted doses and energies, in which it is possible to measure the profile within the vicinity of the maximum concentration, is determined by a numerical computation. At the same time the errors resulting from the application of the assumption of a Debye lenght equal to zero (depletion approximation) are examined. The regions in which it is possible to measure the impurity profile in the vicinity of the maximum with an error less than 5% are shown by the dose vs. implanted energy dependence. The computations are performed for As, B, P ions implanted into Si, and for Si, Te, Se ions implanted into GaAs with energies ranging from 0 to 200 keV.     

Polytypism of silicon carbide and Schottky barriers

The results obtained in our previous work [4] are revised taking into account the dependence of the electron affinity on the polytype of silicon carbide SiC. The dependence of the energy level of vacancies in a polytype of silicon carbide on the band gap is determined from the data on the Schottky barrier height and is explained in the framework of a simple two-band model.     

Schottky barriers on diamond (1 1 1)

We have measured Schottky barrier heights Ø_B = 1.3 eV for Au and Ø_B = 1.5 eV for Al on (p-type) diamond(1 1 1)?(1 × 1) using photoelectron spectroscopy with synchroton radiation. These barrier heights yield a barrier index of S = 0.2, which is closer to the values for Si and Ge (S ～ 0.1) than to the value S = 0.4 calculated for jellium on an ideal diamond(1 1 1) surface. After reacting Al with the diamond surface by annealing to 800° C, we find that Ø_B decreases by 0.24 to 1.25 eV.