Metal-semiconductor Barrier Research Articles

The through-film electrical properties of sputtered GaAs films with aluminum contacts

Results are presented for a.c. and d.c. electrical measurements made on Al-GaAs-Al sandwich structures. The d.c. current-voltage characteristics were found to be strongly temperature dependent; this is indicative either of Schottky emission over the metal-semiconductor interfacial barrier or of Poole-Frenkel conduction in the bulk. The a.c. measurements of impedance, magnitude and phase angle indicate that a Schottky barrier is present at the interface and that the small signal contact and bulk resistance are of comparable magnitudes. An analysis of Schottky emission over the interfacial potential barrier when back-to-back Schottky barriers are present is given and it is shown that both Schottky emission and Poole-Frenkel conduction are present and can be separated.

New phenomena in Schottky barrier formation on III–V compounds

The possibility of a defect mechanism is proposed to explain the Fermi-level pinning in Schottky barriers on III–V semiconductor surfaces. This suggestion comes from the results of photoemission spectroscopy applied to the study of formation of metal–semiconductor barrier heights. Changes in the electronic structure and composition of the interface are studied. For Au metal overlayers on the (110) surfaces of GaAs, GaSb, and InP, the Fermi-level pinning occurs at 0.1 or less of a monolayer coverage. Furthermore, the pinning position is roughly independent of the choice of adatom: Cs, Al, Au, or O. The partial yield structure disappears at about monolayer coverage of Au. The Au valence band had the characteristic shape for atomiclike Au, indicating that the metal was dispersed homogeneously on the surface without the formation of thick islands. Deposition of Au onto the GaSb surface causes the compound to decompose. The antimony segregates to the surface and leaves behind a nonstoichiometric interface. The results for GaSb are compared to those for GaAs(110) and InP(110), where removal takes place of both semiconductor constituents in the interface.

A note on the evaluation of Schottky diode parameters in the presence of an interfacial layer

Metal-semiconductor barrier height and semiconductor doping concentration evaluated from room temperature current-voltage and capacitance-voltage data of Schottky barrier diodes exhibit significant error in the presence of an interfacial oxide layer of appreciable thickness (≥50 A). The effective Richardson constant obtained from an activation energy plot of saturation current is shown to provide a sensitive indication of the presence of the interfacial layer.

The Schottky effect in punch-through diodes

The thermionic emission of holes from a metal into a depleted n-type semiconductor is studied in a metal–n-p silicon structure commonly known as a punch-through or Baritt diode. The experimental conditions are such that the current is controlled by image-force lowering of the metal-semiconductor barrier. It is found that the increase of current with electric field obeys the Schottky law. A small oscillatory deviation from this law due to a quantum-mechanical interference has also been found. The barrier height, as determined from these measurements, agrees well with the barrier height for electrons measured on single Schottky-barrier diodes.

Ionicity and the theory of Schottky barriers

We have investigated the role of ionicity in metal-semiconductor Schottky barriers by examining interfaces of increasing semiconductor ionicity. The electronic structure of four separate interfaces consisting of jellium (of A1 density) in contact with the (111) surface of Si and the (110) surfaces of GaAs, ZnSe, and ZnS is investigated through the use of a self-consistent pseudopotential method. The barrier height and the surface density of states in the semiconductor band gap are determined. The phenomenological index of interface behavior $S$ (studied by Kurtin, McGill, and Mead for semiconductors of different ionicity) is discussed in terms of a simple model involving metal-induced states in the semiconductor gap.

An Integrated Photodetector Using the Partially Metal-Clad-Dielectric-Slab Waveguide Structure

A hybrid-type integrated photodetector which uses metal-semiconductor Schottky barrier diode incorporated in the partially metal-clad-dielectric-slab waveguide (PMC line) structure is proposed, analysed and experimentally studied. The PMC line serves not only as electrodes but also as a two-dimensionally confining structure for a guided optical wave. The wave is coupled from the PMC line into the depletion layer of the diode through a coupling layer and a thin metal film; this scheme avoids the reflection loss inherent to conventional detectors. The analysis indicates that the present photodetector using Si-Schottky barrier diode can be designed so that the quantum efficiency of 92% and cut-off frequency of 6.1 GHz are obtainable at 0.63 µm wavelength.

Exciton diffusion and motion of electron-hole drops in Ge

The motion of electron-hole drops in $n\ensuremath{-}\mathrm{G}\mathrm{e}$ at 2 K is investigated by measuring the time delay between an excitation pulse and the detection of drops in a reverse-biased metal-semiconductor barrier contact as a function of distance between the excitation spot and the contact. The interpretation includes exciton diffusion as well as real drop motion. The electron-hole-drop (EHD) motion is characterized by a free flight rather than thermal diffusion and velocities in the range 200-2000 cm/sec are observed. The observed motion in the presence of an electric field indicates that the EHD are uncharged, and it is suggested that the drops are ejected from the nucleation region with the above velocities by a strong free-exciton density gradient.

The effect of pressure on the high electric field instabilities in n-type GaAs

The high electric field properties of n-type epitaxial GaAs diodes have been studied to 40 kbar. In contrast to previous results, the authors find that the threshold field increases to 15 kbar. This is explained by the dominant effect of the increases of effective mass in the high mobility Gamma 1c minimum with pressure. The constraints these results place on the intervalley coupling constants to be used in theoretical calculations are indicated. Results from a series of diodes suggest that the variation of ET with pressure depends sensitively on contact effects. The data are discussed qualitatively in terms of possible pressure effects on metal-semiconductor barriers.

Computer techniques for solving metal-semiconductor barrier height. Application to Au - CdS thin-film Schottky diodes

Computer programs are developed for numerical determination of the Schottky-barrier heights from the differential capacitance method, the current/voltage characteristic method under forward bias and the Fowler photocurrent method. The basic theory of these methods is briefly described. As a practical application, the programs have been used to investigate metal-semiconductor contact properties.

New method of determination of a metal-semiconductor barrier height

By using a Pb–Al2O3–Al–n–Ge structure, it is demonstrated that the Schottky barrier height eVB at the Al–n–Ge contact can be determined through a study of the tunneling currents in this device at 1.2°K. One obtains a value for eVB = 0.49 eV.

Granular metal-semiconductor Schottky barriers

Granular metal-semiconductor barriers were investigated on n-type Si, GaAs, and CdSe single crystals. The barriers were formed by cosputtering metals, Au or Ni, and insulators, SiO2 or Al2O3, onto the semiconductor surface. By varying the relative volume fraction of the insulator over the range 0–90%, the structure of the granular metal changed from that of a continuous metal to that of isolated small ([inverted lazy s]20 Å ) metallic particles embedded in the insulator. Even with as little as 10% by volume of the metal, the granular metal contacts were found to behave as metal-semiconductor Schottky barriers. In the composition range 60–90% insulator in the granular metal, the barrier heights were independent of composition and their values were 0.57, 0.58, and 0.65 eV on Si, GaAs, and CdSe, respectively. In the case of Si and GaAs, the barriers are significantly lower than those observed for the pure metals; in the case of CdSe no such difference is found.

Wavelength modulated photoresponse spectra in metal-semiconductor Schottky barrier

The wavelength modulated photoresponse measurement in a metal-semiconductor Schottky barrier is demonstrated as a new tool of modulation spectroscopy, available at low temperatures and covering a wide spectral region. Experimental results on the direct and indirect excitons of GaP and their analysis are presented, as compared with the differential absorption spectra of bulk GaP. We also discuss the possibility of the quantitative analysis of the differential absorption spectra.

Diode characteristics and edge effects of metal-semiconductor diodes

When investigating metal-semiconductor barriers, measured I– V-characteristics are often used for deducing barrier height, origin of current components etc. This paper discusses under what conditions errors may be caused by edge-generated currents, and the influence of guard rings is investigated. The system studied was gold on chemically prepared silicon. Investigating reverse characteristics, it was shown that, for diodes having diameters of a few mm, edge effects are important when the silicon resistivity is ≳ 100 Ω cm , but negligible for lower ϱ. Diodes with no guard rings (NGR) had currents several times those surrounded by rings held at the same voltage (balanced guard ring, BGR), at low voltages, and the excess increased with voltage. Studies of activation curves for the current revealed that the current in the BGR case essentially obeyed a Schottky barrier expression (barrier height 0·75–0·80 V) while NGR curves showed appreciably smaller activation energies, indicating contribution of surface- and volume-generated currents at the edge. For diodes of low resistivity avalanche effects are important at high voltages. The main features of the current variation with voltage for higher resistivity BGR diodes (up to 30 kΩ cm ) are in agreement with the image-force lowering effect and the influence of bulk generation current.

Small bandgap semiconductor infrared detectors

Abstract The development of small bandgap Pb 1− x Sn x Te and Hg 1− x Cd x Te semiconductor materials has brought about major advances in the area of infrared detection, particularly in the 8–14 μm wavelength region. The ability to change the energy gap by adjusting the composition x has made possible the fabrication of detectors with different cutoff wavelengths in the approximate range of 4–30 μm for Pb 1− x Sn x Te and 0.8–30 μm for Hg 1− x Cd x Te, thus permitting operation at the maximum possible temperature. Because of its more favorable metallurgical properties, Pb 1− x Sn x Te crystals can be grown from a vapour or from a liquid by a wide variety of techniques. Considerable progress has recently been also made in the crystal growth of Hg 1− x Cd x Te in spite of severe difficulties inherent in this alloy system. Although photoconductive detectors, particularly those of Hg 1− x Cd x Te have proven to be quite useful, recent trends in detector technology have been toward photovoltaic devices, both for applications in thermal detection and for 10.6 μm laser detection where high speed of response is needed. Devices which have been investigated include p-n junctions produced by diffusion, proton bombardment or ion implantation, and metal-semiconductor Schottky barrier devices. Response at frequencies up to 1 GHz has been observed in diodes designed for wide-band heterodyne detection at 10.6 μm, and room temperature background-noise limited operation has been achieved in thermal detectors operated at 77°K and lower temperatures.

Platinum silicide-aluminum Schottky diode characteristics

The effects of heat treatment on platinum silicide Schottky diodes with aluminum metal contacts have been investigated. It has been found that the aluminum reacts with the silicide causing a change in the effective metal-semiconductor barrier height over a range of approximately 0.25 eV. Schottky diodes with initial platinum silicide characteristics convert to devices with barrier heights characteristic of aluminum after extended heat treatments.

Capacitance-Voltage Characteristics of Metal Barriers on p PbTe and p InAs: Effects of the Inversion Layer

The dependence of the capacitance C on the voltage is derived for metal-semiconductor barriers in which there is a strong inversion layer at the metal-semiconductor interface, a case of interest in the study of surface effects and in applications for laser structures. Experimental results for evaporated lead and tin barriers on p PbTe at 77 °K and for evaporated gold barriers on p InAs at 77 and 4.2 °K are presented and compared to calculations performed with the inversion layer model. Both the theory and experiment show an essentially linear dependence of C−2 with voltage, as in the usual Schottky barrier, but a voltage intercept which is surprisingly dependent on band-structure parameters, bulk carrier concentration, and temperature. Reasonable quantitative agreement between the theory and experiment is obtained. In all cases the presence of a strong inversion layer can be clearly established. The analysis of the capacitance-voltage data, however, gives only a lower limit to the amount of inversion present.

Some investigations on evaporated AuCdS-type metal-semiconductor barriers

Experimental thin film AuCdSAl diodes have been fabricated utilizing vacuum evaporation techniques. On the basis of the Crowell and Sze theory, the AuCdS barrier height has been determined from temperature-dependence measurements of the I– V characteristics and voltage-dependence measurements of the barrier capacitance. The estimated value, about 0·67 V, has been confirmed by photoresponse measurements made for comparison. It is shown that the barrier formed by deposition of polycrystalline CdS on Au can be described under the assumptions of the basic diffusion theory.

Specific contact resistance of metal-semiconductor barriers

The specific contact resistance at zero bias, R c , serves as a measure of the ohmic or rectifying behavior of a metal-semiconductor barrier under operating conditions. It is thus an important design parameter for semiconductor devices. The values of R c have been calculated for Metal-Si and metal-GaAs barriers on p-type and n-type samples. The theoretical calculation is based on the generalized transport study of metal-semiconductor systems. The results, which are presented graphically, show the dependence of R c on temperature over the range 50°K–500°K, the barrier height from 0.2 to 1.0 eV, and the ionized impurity concentration from 10 14 to 10 21 cm −3. Generally R c decreases exponentially with increasing temperature and with decreasing barrier height. For samples with lower dopings where the thermionic emission dominates, R c is essentially independent of doping; for higher dopings where the tunneling dominates, R c decreases rapidly with increasing doping. The experimental results of R c for various metals on silicon samples are in good agreement with the predictions.

Tunneling in Pb/n-GaAs junctions under hydrostatic pressure

It is pointed out that Metal-Semiconductor barriers exhibit very good mechanical properties under pressure, at least up to 17 kbars. Tunneling spectroscopy carried out on Pb-GaAs junctions measures the pressure coefficients of some physical quantities such as the phonon energies or the superconducting gap.

The effect of injecting contacts on avalanche diode performance

Metal-semiconductor contact injection on the junction side of diffused-mesa avalanche diodes has been found to have a significant effect on the performance of these diodes as oscillators. A minority carrier injection ratio of 6 percent reduces the efficiency of what would be 9 percent efficient diodes to less than 1 percent and increases the FM noise by a factor of 2 as tested in a 6-GHz oscillator circuit. The dependence of the minority carrier injection ratio of the metal-semiconductor barrier upon current density has been measured and quantitatively modeled. Calculated values of diode admittance, including the effects of injection at the contact, are shown to be in agreement with measured values of both small-signal diode admittance versus frequency and large-signal diode admittance versus RF voltage. Germanium avalanche diodes with low-minority carrier injection contacts have demonstrated CW oscillation efficiencies greater than 9 percent at 6 GHz. The realization of low-injection contacts is shown to be a requirement for achievement of high-efficiency avalanche oscillation.