Stable Schottky Contacts Research Articles

Metal–diamond semiconductor interface and photodiode application

Carrier transport mechanism at p-diamond/metal interfaces are studied by analyzing dependencies of specific contact resistance ( ρ c) on measurement temperature and acceptor concentration ( N A). A variety of metals, such as Ti, Mo, Cr (carbide-forming metals), Pd, and Co (carbon-soluble metals), are deposited on boron-doped polycrystalline diamond layers, and the ρ c values are measured by a transmission line method. Thermal annealing which produces metallurgical reactions between diamond and metal reduces Schottky barrier heights of the contact metals to a constant value. It is found that use of a metal compound which does not react with diamond at elevated temperatures is the key to develop the thermally stable Schottky contact material for p-diamond. Along this guideline, we test the suitability of tungsten carbide (WC) and hafnium nitride (HfN) as thermally stable Schottky contacts to develop a thermally stable, deep-ultraviolet (DUV) photodiode using a boron-doped homoepitaxial p-diamond epilayer. Thermal annealing at 500 °C improves the rectifying current–voltage characteristics of the photodiode, resulting in the excellent thermal stability. The discrimination ratio between DUV and visible light is measured to be as large as 10 6 at a reverse bias voltage as small as 2 V, and it remains almost constant after annealing at 500 °C for 5 h. Metal carbide and nitride contacts for diamond are thus useful for developing a thermally stable diamond DUV photodetector.

Effect of Cryogenic Temperature Deposition of Various Metal Contacts to Bulk, Single-Crystal n-type ZnO

ABSTRACTThe development of reliable and thermally stable Ohmic and Schottky contacts to ZnO is one of the critical issues related to the fabrication of ZnO-based UV light emitters/detectors and field effect transistors. To date, a number of different metallization schemes and surface cleaning procedures prior to metal deposition have been examined for rectifying contacts on n-ZnO. While these reports have shown that low reactive metals such as Au, Ag and Pd form rectifying contacts with Schottky barrier heights in the 0.6-0.8 eV range, the thermal stability of these contacts is usually extremely poor, with degradation occurring even at 60°C for Au/n-ZnO. One approach to achieving increased barrier heights that has proven successful for GaAs, InP, InGaAs and other compound semiconductors is the use of cryogenic deposition temperatures. In this context, we report in this work on the effect of cryogenic temperature metal deposition on the contact properties of Pd, Pt, Ti, and Ni on single-crystal n-type ZnO. Deposition at both room and low temperature produced contacts with Ohmic characteristics for Ti and Ni metallizations. In comparison, both Pd and Pt contacts showed rectifying characteristics after deposition. All rectifying contacts exhibited barrier heights around 1-2 eV and idealities between 1 and 2. Low temperature deposition gave higher resistances in comparison to room temperature deposition for all cases. Larger contacts also corresponded to an increase in resistance. Changes in contact behavior were measured on Pd to anneal temperatures of ∼300°C, showing an increase in barrier height along with a decrease in ideality with increasing temperature. This difference with annealing temperature is in sharp contrast to previous results for Au contacts to ZnO. There were no differences in near-surface stoichiometry for the different deposition temperatures; however low temperature contacts demonstrated some cracking in Pt and Pd, probably due to surface stress.

Thermally stable visible-blind diamond photodiode using tungsten carbide Schottky contact

We have developed a thermally stable, deep-ultraviolet (DUV) photodiode using tungsten carbide (WC) Schottky and Ti/WC ohmic contacts for a boron-doped homoepitaxial p-diamond epilayer. Effects of thermal annealing in an argon ambient on the electrical and photoresponse properties were investigated. Annealing at temperatures up to 550°C improves the rectifying current-voltage characteristics, resulting in a dramatic enhancement of DUV responsivity at 220nm by a factor of 4×103. A blind ratio as large as 105 between DUV and visible light has been achieved at a reverse bias as small as 1V. Development of the thermally stable WC-based Schottky and ohmic contacts provides a route for stable operation of a diamond photodetector at high temperatures.

GaN MSM Ultraviolet Photodetectors with Transparent and Thermally Stable RuO2 and IrO2 Schottky Contacts

Ruthenium oxide and iridium oxide were used as the Schottky barrier materials of GaN metal-semiconductor-metal (MSM) ultraviolet photodetector. After annealing Ru and Ir contacts at 500°C under ambient, the reverse leakage current density at −5 V drastically decreased by four orders of magnitude. Furthermore, the Schottky barrier height and optical transmittance at 360 nm simultaneously increased to 1.43 eV and 71.5% for the Ru, and to 1.48 eV and 74.8% for the Ir contact. The dramatic improvements originated from the formation of thermally stable oxide phases, and by the annealing, resulting in the increase in the responsivity of the GaN MSM photodetector by one order of magnitude in comparison with the photodetector with Pt Schottky contact. © 2004 The Electrochemical Society. All rights reserved.

Characteristics of Schottky contacts on n-type 4H–SiC using IrO2 and RuO2

Thermally stable Schottky contacts on n-type 4H–SiC with high Schottky barrier height were demonstrated by annealing the rare earth metal contacts (Ir and Ru) under O2 ambient. The formation of rare earth metal oxides (IrO2 and RuO2) after O2 annealing led to the increase of Schottky barrier height (>1.9 eV) and a low reverse leakage current (∼10−9 A/cm2). Synchrotron radiation photoemission spectroscopy showed that the work function of IrO2 is higher about 0.23 eV than that of Ir and the binding energies of Si 2p and C 1s shifted toward lower binding energies by 0.12 eV in both O2 and N2 annealed samples. The oxidation annealing caused predominant Si outdiffusion to the IrO2 (RuO2), leaving Si vacancies behind, leading to the shift of surface Fermi level to the energy level of Si vacancy. Both the formation of oxide and the Fermi level movement played a role in forming the Schottky contact with high barrier height and excellent thermally stability.

IrO 2 Schottky contact on n-type 4H-SiC

A thermally stable IrO2 Schottky contact on n-type 4H-SiC was achieved by annealing an Ir contact under O2 ambient. The IrO2 contact exhibited a high Schottky barrier height of 2.22 eV and low reverse leakage current. Little degradation in Schottky barrier height was observed even after annealing at 450 °C for 24 h under atmospheric air. The oxidation annealing transformed the Ir layer into IrO2, resulting in the increase in the work function of the contact layer. Simultaneously, Si atoms diffused out, leaving the Si vacancy below the contact. These played a role in forming a thermally stable Schottky contact with a high Schottky barrier height.

Thermally stable Ir Schottky contact on AlGaN/GaN heterostructure

We report thermally stable Ir Schottky contacts on AlGaN/GaN heterostructure. The Schottky barrier height was increased from 0.68 to 1.07 eV, and the reverse leakage current dramatically decreased after annealing at 500 °C under O2 ambient. No degradation in Schottky barrier height was observed after annealing at 500 °C for 24 h. The oxidation annealing caused predominant Ga outdiffusion to the surface, leading to the shift of surface Fermi level to the energy level of Ga vacancy. This played a role in forming the Schottky contact with large barrier height and excellent thermal stability.

Fabrication of T‐Gate AlGaN/GaN Heterostructure Field Effect Transistor with Thermally Stable Schottky Contact

The effective Schottky barrier height of an Ir contact was increased on annealing up to 500 °C in O2 ambient. The barrier height was increased by about 0.3 eV compared to that of an as-deposited Ir contact and reverse leakage current dramatically decreased after annealing. The IrO2 formed by annealing played a role in increasing the effective Schottky barrier height and prevented interdiffusion of Ir atoms to the AlGaN layer, and led to a thermally stable Schottky contact for the gate electrode of an AlGaN/GaN heterostructure field effect transistor. The fabricated device had Imax = 628 mA/mm and Gm,max = 245 mS/mm, with a T-shaped gate of Lg = 0.8 μm and Wg = 25 × 2 μm.

Chemically dependent traps and polytypes at Pt/Ti contacts to 4H and 6H–SiC

We have used low energy electron-excited nanoluminescence (LEEN) spectroscopy and x-ray photoemission spectroscopy (XPS) to probe deep level defect states at interfaces of 4H and 6H–SiC with Ti/Pt metallization. These studies aim to identify process conditions under which thermally stable ohmic and Schottky contacts can be obtained on SiC while minimizing the formation of deep level electronic states. Depth-dependent LEEN measurements establish the presence of localized states and their spatial distribution on a nanometer scale. Spectra from the near interface region of 6H–SiC indicate the existence of a SiC polytype with a higher band gap of ∼3.4 eV. Excitation of the intimate metal–SiC interface reveals a process-dependent discrete state deep within the SiC band gap. XPS measurements reveal consistent differences in the C 1s chemical bonding changes with specific process steps. Analogous chemical treatments of 4H–SiC also produce a lower band gap SiC polytype with ∼2.5 eV energy extending tens of nanometers beyond the interface—confirmed by transmission electron microscopy. This work is the first to show the effect of metal–semiconductor interactions not only on localized states but also on the lattice structure of the semiconductor near the interface.

Cu3Ge Schottky contacts on n-GaN

The electrical properties and thermal stability of \(\varepsilon _1 - Cu_3 Ge\) (hereafter referred to as \(Cu_3 Ge\)) Schottky contacts to n-GaN as a function of the Ge concentration and annealing temperature are studied. Upon rapid thermal annealing the \(Cu_3 Ge\)/n-GaN Schottky diode is formed at 300 °C and is stable up to 650 °C. At 700 °C the agglomeration occurs in the \(Cu_3 Ge\) films. For the Cu-25 at % Ge films the ideality factor, n, barrier height, \(\phi _{B0}\), and flat band barrier height, \(\phi _{BF}\), of the \(Cu_3 Ge\)/n-GaN diodes are in the ranges of 1.22–1.36, 0.53–0.72 eV, and 0.65–0.97 eV, respectively, being decreased with the annealing temperature. Higher Ge concentration in the Cu-35 at % Ge films results in larger n, \(\phi _{B0}\), and \(\phi _{BF}\) of \(Cu_3 Ge\)/n-GaN Schottky diodes. The \(Cu_3 Ge\) film is expected to be a suitable candidate for stable Schottky contacts to n-GaN at elevated temperatures.

Approaches to designing thermally stable Schottky contacts to n-GaN

The barrier heights of PdIn, Ni/Ga/Ni and Re Schottky contacts to n-GaN were investigated by current-voltage and capacitance-voltage measurements. Elemental Pd and Ni contacts were also investigated for comparison. In each case, the barrier heights were determined as a function of annealing temperature. It was shown that stoichiometric PdIn contacts were more stable than Pd-only contacts. Similarly, Ni/Ga/Ni diodes were found to be more stable than Ni diodes. Both the Ni/Ga/Ni contact and the elemental Re contact were stable on short-term annealing up to 700 °C.

High Temperature Stable Titanium Carbide Ohmic and Schottky Contacts to SiC

ABSTRACTEpitaxial titanium carbide is investigated as a low resistivity, high temperature stable Ohmic and Schottky contact to 4H and 6H SiC. The titanium carbide films were deposited at 500°C using co-evaporation of titanium (e-beam) and C60(Knudsen cell) in a UHV system. Schottky diodes and TLM structures were fabricated on low and high doped SiC material respectively. The samples were annealed at 700 °C in a vacuum furnace, and electrical measurements were performed up to 300 °C.

Technology towards GaAs MESFET-based IC for high temperature applications

A GaAs MESFET technology for the fabrication of devices, specially developed for continuous, reliable operation at high temperatures is presented. The technology is based on highly stable ohmic and Schottky contacts containing WSiN diffusion barriers and is optimized towards minimum temperature induced leakage currents across the substrate or along the semiconductor surface. The isolation of individual devices on chip is accomplished by multiple implantations of O + -ions at different energies. Drain/source leakage currents at high temperatures are significantly reduced due to the use of GaAs epitaxial MESFET wafers with an AlAs/GaAs superlattice structure followed by a p −-buffer. MESFETs, fabricated by using this technology, have been optimized to match the requirements for continuous operation at high temperatures up to 300 °C and have been successfully implemented in high temperature MMICs. Microwave circuits such as mixers, buffer amplifiers and voltage controlled oscillators aswell as operational amplifiers have been realized and characterized at temperatures up to 300 °C.

Thermally stable InGaP/GaAs Schottky contacts using low N content double layer WSiN

This article describes thermally stable Schottky contacts using low N content double layered WSiN for InGaP/(In)GaAs metal–semiconductor field-effect transistors with self-aligned ion-implanted n+ layers. Transmission electron microscopy observations show that the microscopic interfacial reaction between WSiN and GaAs is completely suppressed when N content in the WSiN is 10% or less. As a result, a WSiN/GaAs-cap/InGaP Schottky contact shows high thermal stability after annealing at 800 °C for 100 min, even though the GaAs cap is as thin as 25 Å. Moreover, the double-layer WSiN structure suppresses reduction in the carrier concentration in the channel during activation annealing. The carrier concentration of the GaAs-cap (40 Å)/InGaP (100 Å)/ InGaAs-channel (100 Å) film, 6.5×1018 cm−3, decreases to less then 2×1018 cm−3 with low N content WSiN (N=10%, 4000 Å) after annealing at 900 °C for 0.1 s, but it remains 3.8×1018 cm−3 with the double layered WSiN (N=37%, 3800 Å/N=10%, 200 Å).

Metal contacts to gallium arsenide

In this paper, some aspects that determine the properties of Schottky and ohmic contacts to GaAs are discussed. For Schottky barrier diodes (SBD), we present results of a comprehensive study involving 41 different metals. We pay special attention to Ru and show that its thermal and chemical stability makes it ideal for use in devices operating above room temperature and for experiments involving annealing. Further, we discuss the effect of different metallization methods on SBD properties and show that methods which use energetic particles, such as electron beam deposition and sputter deposition, often result in inferior SBD properties—the consequence of electrically active defects introduced by the energetic particles at and close to the semiconductor surface. The advantages of using Ru as contact material to GaAs are that it forms high quality, thermally stable Schottky contacts to n-GaAs and thermally stable ohmic contacts with low specific contact resistance to p-GaAs. The versatile applicability of Ru contacts makes them extremely important for future use in devices such as heterojunction bipolar transistors and solid state lasers.

Chemical Vapor Deposition of Tungsten Schottky Diodes to 6H‐SiC

Thermally stable tungsten Schottky contacts to low doped 6H‐silicon carbide (SiC) were fabricated via chemical vapor deposition at 670 K followed by a reactive ion etch to pattern the contacts. Physical characterization by x‐ray diffraction, x‐ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and transmission electron microscopy verified a distinct W/6H‐SiC interface after a 2 h vacuum anneal at 1073 K. Current‐voltage (I‐V) and capacitance‐voltage (C‐V) measurements were performed at temperatures ranging from 295 to 773 K and revealed a low n‐type Schottky barrier Φ = 0.79 eV, ideal for low ohmic applications and a high p‐type Schottky barrier ΦBp773 = 1.89 eV. At 473 K a current rectification ratio of 107 at ±10 V was observed for the p‐type contact.

Excimer laser-induced metallization for in situ processing on Si and GaAs

We have investigated both the large area excimer laser-induced deposition of W and its silicides on GaAs to form Thermally stable Schottky contacts, and the reduction of a Cu(I) compound for the deposition of Cu interconnects for Si microelectronics. Using a KrF excimer laser at 25 mJ / cm 2 and a mixture of WF 6, SiH 4 and Ar, metallic W is deposited with an average growth rate of 1 Å/pulse. For Cu deposition, the reduction by H 2 of the precursor Cu(hfac)(TMVS) under a KrF excimer laser illumination of 9 mJ/cm 2 gives metallic Cu with a Cu/C ratio of 4.35. For both processes, possible deposition mechanisms are discussed in terms of gas phase and surface reactions.

Comparison between ruthenium-based and other ohmic contact systems to p-type GaAs

A conventional furnace annealed Ru/Au ohmic contact system on p-GaAs has been investigated. Electrical and morphological characteristics of this contact system were compared with other systems such as Cr/Au, Ti/Pt, and Mn/Au. The Ru/Au contact system has been shown to have superior surface morphology and a comparable specific contact resistance value, even after annealing at 485 °C. The advantages of utilizing Ru as contact material to GaAs are that it forms high quality, thermally stable Schottky contacts to n-GaAs and thermally stable ohmic contacts with low specific contact resistance to p-GaAs. This dual nature of Ru contacts to GaAs makes them extremely important for future use in devices such as heterojunction bipolar transistors (HBTs) and solid state lasers.

Thermal stabilities and microstructures of tungsten compound contacts on n-GaAs fabricated by low-energy ion-beam-assisted deposition

A low-energy ion-beam-assisted deposition (IBAD) technique has been developed to fabricate W-Si-N Schottky contacts of enhanced thermal stabilities on n-GaAs. By implementing remote sputtering and glancing angle low-energy N+ ion irradiation, the thermal stabilities of the W-Si-N/n-GaAs Schottky contacts were enhanced to be stable up to 850 °C, while keeping the Schottky barrier heights to the best values obtained with conventional sputtering. Thermal stabilities of the IBAD refractory metals/n-GaAs interfaces were investigated also for various W-Si-N compositions by examining the microstructure and Schottky diode characteristics. WSi0.3N0.4 showed the most stable Schottky contacts on n-GaAs of which the barrier height changed only 20 meV after thermal annealing between 700 and 850 °C, and that is proposed to be due to the stable metallurgical microstructure compared to the element tungsten and tungsten nitride.

Effect of etch treatment prior to Schottky contact fabrication on In0.05Ga0.95As

The Schottky barrier characteristics of aluminum contacts to In0.05Ga0.95As/GaAs strain-relieved material were investigated. Acidic and alkaline wet chemical etching experiments were performed to determine the importance of semiconductor surface preparation prior to the deposition of the contact metal. Acidic etch treatments utilizing HCl and HF resulted in a range of ideality factors significantly greater than 1 (1.90–2.91 for HCl, 1.35–2.46 for HF), variable reverse bias leakage currents (0.038–0.440 μA/cm2 for HCl, 23.0–110 μA/cm2 for HF), and a range of barrier heights (0.88–0.93 eV for HCl, 0.63–0.77 eV for HF). In addition, the HCl treated devices exhibit time-dependent I-V behavior with continuous measurements, as well as changes in characteristics with both annealing and room-temperature storage. The 10 s NH4OH:H2O (1:1) alkaline etch surface preparation produced stable Schottky contacts with a 0.79 eV barrier height, a 1.15±0.02 ideality factor, and ≊10 μA/cm2 leakage current at −1 V bias.