Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Recent advances in Schottky barrier concepts

The formation of the Schottky barrier height (SBH) is a complex problem because of the dependence of the SBH on the atomic structure of the metal-semiconductor (MS) interface. Existing models of the SBH are too simple to realistically treat the chemistry exhibited at MS interfaces. This article points out, through examination of available experimental and theoretical results, that a comprehensive, quantum-mechanics-based picture of SBH formation can already be constructed, although no simple equations can emerge, which are applicable for all MS interfaces. Important concepts and principles in physics and chemistry that govern the formation of the SBH are described in detail, from which the experimental and theoretical results for individual MS interfaces can be understood. Strategies used and results obtained from recent investigations to systematically modify the SBH are also examined from the perspective of the physical and chemical principles of the MS interface.

The physics and chemistry of the Schottky barrier height

Recent advances in nonsilica fiber technology have prompted the development of suitable materials for devices operating beyond 155 mu m The III-V ternaries and quaternaries (AlGaIn)(AsSb) lattice matched to GaSb seem to be the obvious choice and have turned out to be promising candidates for high speed electronic and long wavelength photonic devices Consequently, there has been tremendous upthrust in research activities of GaSb-based systems As a matter of fact, this compound has proved to be an interesting material for both basic and applied research At present, GaSb technology is in its infancy and considerable research has to be carried out before it can be employed for large scale device fabrication This article presents an up to date comprehensive account of research carried out hitherto It explores in detail the material aspects of GaSb starting from crystal growth in bulk and epitaxial form, post growth material processing to device feasibility An overview of the lattice, electronic, transport, optical and device related properties is presented Some of the current areas of research and development have been critically reviewed and their significance for both understanding the basic physics as well as for device applications are addressed These include the role of defects and impurities on the structural, optical and electrical properties of the material, various techniques employed for surface and bulk defect passivation and their effect on the device characteristics, development of novel device structures, etc Several avenues where further work is required in order to upgrade this III-V compound for optoelectronic devices are listed It is concluded that the present day knowledge in this material system is sufficient to understand the basic properties and what should be more vigorously pursued is their implementation for device fabrication (C) 1997 American Institute of Physics

The physics and technology of gallium antimonide: An emerging optoelectronic material

Objectives. To assess the dose-related effects of inhaled nitric oxide (I-NO) as a specific adjunct to early conventional therapy for term infants with persistent pulmonary hypertension (PPHN), with regard to neonatal outcome, oxygenation, and safety. Methods. Randomized, placebo-controlled, double-masked, dose-response, clinical trial at 25 tertiary centers from April 1994 to June 1996. The primary endpoint was the PPHN Major Sequelae Index ([MSI], including the incidence of death, extracorporeal membrane oxygenation (ECMO), neurologic injury, or bronchopulmonary dysplasia [BPD]). Patients required a fraction of inspired oxygen [Fio2] of 1.0, a mean airway pressure ≥10 cm H2O on a conventional ventilator, and echocardiographic evidence of PPHN. Exogenous surfactant, concomitant high-frequency ventilation, and lung hypoplasia were exclusion factors. Control (0 ppm) or nitric oxide (NO) (5, 20, or 80 ppm) treatments were administered until success or failure criteria were met. Due to slowing recruitment, the trial was stopped at N = 155 (320 planned). Results. The baseline oxygenation index (OI) was 24 ± 9 at 25 ± 17 hours old (mean ± SD). Efficacy results were similar among NO doses. By 30 minutes (no ventilator changes) the Pao2 for only the NO groups increased significantly from 64 ± 39 to 109 ± 78 Torr (pooled) and systemic arterial pressure remained unchanged. The baseline adjusted time-weighted OI was also significantly reduced in the NO groups (-5 ± 8) for the first 24 hours of treatment. The MSI rate was 59% for the control and 50% for the NO doses (P = .36). The ECMO rate was 34% for control and 22% for the NO doses (P = .12). Elevated methemoglobin (>7%) and nitrogen dioxide (NO2) (>3 ppm) were observed only in the 80 ppm NO group, otherwise no adverse events could be attributed to I-NO, including BPD. Conclusion. For term infants with PPHN, early I-NO as the sole adjunct to conventional management produced an acute and sustained improvement in oxygenation for 24 hours without short-term side effects (5 and 20 ppm doses), and the suggestion that ECMO use may be reduced.

Inhaled Nitric Oxide for the Early Treatment of Persistent Pulmonary Hypertension of the Term Newborn: A Randomized, Double-Masked, Placebo-Controlled, Dose-Response, Multicenter Study

The physical properties of GaSb are briefly presented and the device implications reviewed. GaSb is a direct gap semiconductor (0.72 eV) capable of being doped either p or n type with good mobilities and it has significant electro-optical potential in the near IR range. As a substrate, or active layer, GaSb can be employed in conjunction with many semiconductors such as (AlGa)Sb or In(AsSb) and has interesting heterojunction potential for detectors and lasers and quantum well structures.

Gallium antimonide device related properties

The electrical properties of Schottky diodes and p-n junctions on GaSb ((100), doped with 2 × 10 17 cm −3 tellurium) were examined. Capacitance-voltage measurements at 300 K show barrier heights of0.65 eV (Al), 0.6 eV (Au, In, Pd), 0.5 eV (Ga) and 0.42 eV (Sb). The barrier heights tend to track the band gap increase as the temperature is lowered. Forward I-V characteristics of the GaSb Schottky diodes at 300 K have ideality factors in the range 1.9–2.4, indicating that generation-recombination current from a near midgap center is dominating the behavior. At room temperature and below, the activation energy for J s in the expression J=J s exp (qV/nkT) is 0.24 eV. However, at higher temperatures the activation energy becomes more nearly equal to the barrier height as deduced from C-V measurements and the n value decreases below two. Deep level transient spectroscopy (DLTS) measurements reveal the presence of one electron trap with a concentration around 10 15 cm −3 at 0.25 eV below the conduction band edge. Electron beam induced current (EBIC) measurements give an electron diffusion length of 1.3 μm. In GaSb p-n + junctions grown by molecular beam epitaxy, DLTS measurements show the presence of a generation-recombination center at E v +0.33 eV with a concentration of 5×10 14 cm −3 . The effective lifetime inferred from the saturation current value is around 2×10 −9 s. For reverse bias, the mechanism of breakdown at large voltages in p-n + structures includes tunneling via local centers with an energy of around 0.3 eV. Forward and reverse characteristics can be improved by (NH 4 ) 2 S treatments for both Schottky barriers and pn + junctions.

Electrical properties of GaSb Schottky diodes and p-n junctions

It is shown by spreading resistance and capacitance–voltage measurements that atomic hydrogen passivates shallow acceptors and donors in GaSb. Deep level passivation by hydrogen also occurs, as revealed by deep level transient spectroscopy measurements on Schottky diode structures. Effective diffusion coefficients for hydrogen were determined for both n+ and p+ GaSb; in the former case the diffusion is thermally activated with the relationship DH=3.4×10−5e−0.55 eV/kT, whereas in p+ material DH=1.5×10−6e−0.45 eV/kT over the temperature range 100–250 °C. Reactivation of passivated shallow and deep levels occurs for temperatures of 250–300 °C.

Hydrogen treatment effect on shallow and deep centers in GaSb

It is shown that when grown by molecular‐beam epitaxy at high temperatures around 700 °C, undoped GaAs layers display high resistivity. Preliminary results seem to imply that Ga‐related inclusions that produce internal Schottky diodes might be responsible for this effect rather than the presence of deep centers. It is proposed that Ga‐related inclusions that produce internal Schottky diodes might possibly be responsible for this effect rather than the presence of deep centers.

High‐resistivity GaAs grown by high‐temperature molecular‐beam epitaxy

Schottky barriers of Au, Al, and Sb on n‐ and p‐type layers of Al0.5Ga0.5As0.05Sb0.95 have been studied. The Schottky barriers are high for Au (1.3 eV) and Al (1.2 eV) deposited on n‐type material and very low for these metals on p‐type layers. The behavior of Sb is unique with the barrier heights being 0.6–0.7 eV for both n‐ and p‐type AlGaAsSb. The reason for the surface Fermi‐level pinning for Au and Al could be related to a predominance of Ga‐antisite–type native acceptors at the surface, which is not the case for Sb. Sulfur treatment of the surface is shown to decrease the barrier height for Au and to increase greatly the photosensitivity of Au Schottky diodes. The same effect is observed after treatment in a hydrogen plasma. In the latter case, changes in the Schottky barrier height are correlated with passivation of native acceptors in the bulk of the Al0.5Ga0.5As0.05Sb0.95.

Schottky barriers of various metals on Al0.5Ga0.5As0.05Sb0.95 and the influence of hydrogen and sulfur treatments on their properties

GaSb undoped layers grown by molecular‐beam epitaxy on GaSb or on semi‐insulating GaAs substrates at temperatures between 600 and 630 °C are shown to have carrier concentrations in the low 1013 cm−3 range, corresponding to almost intrinsic conditions. The materials have been characterized using current‐voltage, capacitance‐voltage, Hall effect, photoluminescence, thermally stimulated current, and secondary‐ion mass spectrometry. Bulk GaSb (n type) is also found to have converted to high‐resistance p type after a heat treatment at 630 °C. Speculations are offered for the responsible mechanism, but a definitive explanation does not exist at this time.

M. Stam

Papers

Electrical properties of GaSb Schottky diodes and p-n junctions

Hydrogen treatment effect on shallow and deep centers in GaSb

High‐resistivity GaAs grown by high‐temperature molecular‐beam epitaxy

Schottky barriers of various metals on Al0.5Ga0.5As0.05Sb0.95 and the influence of hydrogen and sulfur treatments on their properties

High‐resistivity GaSb grown by molecular‐beam epitaxy