Schottky Metal Deposition Research Articles

Overcoming the impact of post-annealing on uniformity of diamond (100) Schottky barrier diodes through corrosion-resistant nanocarbon ohmic contacts

Diamond-based Schottky barrier diodes (SBDs) are involved in many technological applications. In a conventional SBD fabrication process that involves interface carbide forming ohmic contacts, a post-annealing step is necessary for ohmic contacts to achieve their operational efficiency. However, this step deteriorates the essential oxygen coverage at the diamond surface which in turn affects SBDs uniformity. So, an additional oxygen termination step is necessary prior to Schottky metal deposition. In this study, a non-conventional fabrication method is introduced using corrosion-resistant nanocarbon ohmic contacts fabricated by coaxial arc plasma deposition. As a result, The SBD parameters including ideality factors and barrier heights exhibited high uniformity with a very small standard deviation for the proposed fabrication process flow when compared with process flow including a post-annealing step. Furthermore, the contact behavior of nanocarbon ohmic electrodes is investigated on a heavily boron-doped diamond film using circular transmission line model theory and a specific contact resistance of ∼10−5 Ωcm2 is obtained, suggesting the practical application of nanocarbon ohmic contacts for diamond-based electronic devices.

Effects of microwave plasma treatment on β-Ga2O3 Schottky barrier diodes

In this work, the effects of O2 or N2 microwave plasma treatment on β-Ga2O3 surface prior to Schottky metal deposition are reported. The device uniformity of Schottky barrier diodes is improved significantly by the microwave plasma treatments without any degradation such as ideality factor (near 1.0), and on-state resistance (R on ∼3 mΩ cm2). The standard deviation of Schottky barrier height (SBH, φB) is as small as less than 10 m eV. Kelvin probe force microscope analysis shows that the surface electrostatic potential after O2 microwave plasma treatment is lower than that of the N2 microwave plasma treatment, which is consistent with the change of SBH obtained by capacitance–voltage (C–V) and current–voltage (I–V) measurements. The relatively low SBH with O2 microwave-plasma treatment corresponds to the high reverse leakage current. The oxygen related adsorption at metal/Ga2O3 interface by O2 microwave plasma treatment confirmed by x-ray photoelectron spectroscopy can be attributed to the SBH and surface potential lowing.

Effect of Downstream Plasma Exposure on Schottky Diodes Fabricated on β-Ga2O3

The effects of downstream plasma exposure on Ga2O3 Schottky diode forward and reverse IV characteristics were investigated. Ultra-wide bandgap semiconductor β-Ga2O3 has attracted increasing attention because of the prospects for use in next generation high-power electronics. However, the Ga2O3 surface has been reported to be particularly sensitive to plasma-induced damage, which could lead to device performance degradation[1]. In our study, the plasma treatments were performed using a PIE Scientific Tergeo Plasma Cleaner with O2, N2 or CF4 discharges. The system can be operated either in immersion mode or downstream mode. The samples were exposed to fluxes of electrons, ions and neutral and an rf power of 50 W was used to generate the plasma at a pressure of 400 mTorr, with a fixed treatment time of 1 min. Ti/Au and Ni/Au metallization were employed as the Ohmic and Schottky contacts, respectively, on the test diodes. Prior to the Schottky metal deposition, the samples were treated to ozone plasma/dilute HCl solution/ozone treatments to remove surface contamination. A Schottky barrier height of 1.1 eV was obtained for the reference sample without plasma treatment, and an ideality factor of 1.001 was achieved. From the forward I-V characteristic, as illustrated in Figure 1, the diodes exposed to CF4 showed a 0.25V shift from the I-V of the reference sample due to a Schottky barrier height lowering around 13.9%, as show in Table 1. The diodes exposed with N2 and O2 plasmas showed a decrease of Schottky barrier height of 2.5 and 6.5 % with O2 or N2 treatments, respectively, as shown in Figure 2. The effect of plasma exposure on the ideality factor of diodes treated with these plasmas was minimal; 0.2% for O2 and N2, 0.3% for CF4, respectively, as shown in Figure 2. The reverse leakage currents were 0.0012, 0.0022 and 0.0048 mA/cm2 for the diodes treated with O2, and CF4, and N2 respectively, as shown in Figure 3. The effect of downstream plasma treatment on diode on-resistance and on-off ratio were also very minimal as shown in Figure 4 and 5.[1] Jiancheng Yang, Zachary Sparks, Fan Ren, Stephen J. Pearton, and Marko Tadjer, J. Vac. Sci. & Technol. B36, 061201-1-9 (2018). Figure 1

Leakage Current Reduction in β-Ga2O3 Schottky Barrier Diodes by CF4 Plasma Treatment

This letter reports on the suppression of reverse leakage current (Ir) in β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Schottky barrier diodes (SBDs) through Schottky barrier modification by a low power CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -plasma treatment prior to Schottky metal deposition. Revealed by an X-ray photoelectron spectroscopy (XPS) analysis, the fluorine-plasma treatment brought an incorporation of fluorine ions and depletion of silicon donors in the near surface region of the β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , and thus raised its surface potential by around 0.14 eV. Furthermore, insulating GaFx was likely created at the Schottky interface. Attributed to the fluorine-plasma modified Schottky barrier, a reduced Ir by around four orders of magnitude and enhanced blocking voltage (Vblock) from 150 V to 470 V at Ir = 100μ A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> have been achieved without degrading the forward characteristics. Different from the untreated device whose Ir was purely governed by the thermionic field emission (TFE), the fluorine-plasma-treated SBD showed a greatly suppressed TFE-current until a space-chargelimited current (SCLC) started to dominate at around -500 V.

Effects of fluorine-based plasma treatment and thermal annealing on high-Al content AlGaN Schottky contact

Fluorine plasma treatment was used prior to the Schottky metal deposition on the undoped Al0.45Ga0.55N, which aimed at the solar-blind wavelength. After fluorine plasma treatment and before depositing the Ni/Au Schottky, the samples were thermal annealed in the N2 gas at 400 °C. The reverse leakage current density of Al0.45Ga0.55N Schottky diode was reduced by 2 orders of magnitude at −10 V. The reverse leakage current density was reduced by 3 orders of magnitude after thermal annealing. Further capacitance–frequency analysis revealed that the fluorine-based plasma treatment reduces the surface states of AlGaN by one order of magnitude at different surface state energies. The capacitance–frequency analysis also proved that the concentration of carriers in AlGaN top is reduced through fluorine plasma treatment.

Formation of Ni Diffusion-Induced Surface Traps in GaN/Al x Ga1−x N/GaN Heterostructures on Silicon Substrate During Gate Metal Deposition

The diffusion of the Schottky metal (Ni) in GaN is known to occur at elevated temperatures and as a result of prolonged electric field-driven stress. This leads to device degradation and reliability issues in AlxGa1−xN/GaN-on-silicon high electron mobility transistors (HEMTs). In this study, we have investigated the formation of Ni-induced deep level traps across the Ni-GaN interface in AlxGa1−xN/GaN HEMT-based Schottky diodes on Si substrates during the Schottky metal deposition process prior to any gate/Schottky metal annealing step. Two deep level traps were detected at 0.14 eV and 0.54 eV using Fourier deep level transient spectroscopy, which correlated well with nitrogen vacancies and nitrogen antisite defects, respectively. Our results are further supported by transmission electron microscopy-based energy dispersive x-ray analysis and electron energy loss spectroscopy measurements which confirm the interdiffusion of Ga and Ni across the Ni/GaN interface on the HEMT structure. Understanding the nature of such defects may help to employ suitable␣growth or passivation schemes for development of improved GaN-based electronic devices.

Leakage current and sub-bandgap photo-response of oxygen-plasma treated GaN Schottky barrier diodes

The effect of oxygen plasma treatment on the performance of GaN Schottky barrier diodes is studied. The GaN surface is intentionally exposed to oxygen plasma generated in an inductively coupled plasma etching system before Schottky metal deposition. The reverse leakage current of the treated diodes is suppressed in low bias range with enhanced diode ideality factor and series resistance. However, in high bias range the treated diodes exhibit higher reverse leakage current and corresponding lower breakdown voltage. The X-ray photoelectron spectroscopy analysis reveals the growth of a thin GaOx layer on GaN surface during oxygen plasma treatment. Under sub-bandgap light illumination, the plasma-treated diodes show larger photovoltaic response compared with that of untreated diodes, suggesting that additional defect states at GaN surface are induced by the oxygen plasma treatment.

Analysis of current–voltage–temperature characteristics and T0 anomaly in Cr/ n-GaAs Schottky diodes fabricated by magnetron sputtering technique

We have fabricated two groups of Cr/ n-GaAs Schottky diodes (SDs) by magnetron sputtering technique to determine whether T 0 anomaly varies in similarly fabricated SDs or not. Firstly, the first group diodes were inserted into a vacuum chamber to form the Schottky contacts, then the second group diodes which are held in the clean room medium for 3 h before Schottky metal deposition. The current–voltage ( I– V) characteristics of three diodes (the dots of the sample CrD1) from the first group and two diodes (the dots of the sample CrD2) from the second group were measured in temperature range of 60–320 K. The barrier heights increased with increasing temperature in range of 60–160 K, and did not changed in range of 160–320 K. Ideality factory value decreased with increasing temperature in range of 60–160 K and changed between 1.05 and 1.10 in range of 160–320 K. T 0 anomaly values were calculated from straight lines fitted to nT′– T plots. The fits to the experimental values of nT– T plots are parallel to the ideal Schottky contact line, especially for the dots (Schottky diodes) of the sample CrD1. T 0 anomaly values for the dots of the sample CrD1 were obtained as 13.9, 11.20 and 13.31 K; and the values of 19.74 and 19.20 K was obtained for the dots of the sample CrD2. It has been concluded that the T 0 anomaly values for the similarly fabricated diodes (the dots of the sample CrD1 or the CrD2) are almost very close to each other within the margins of experimental error.

Electrical characteristics of AlxGa1−xN Schottky diodes prepared by a two-step surface treatment

Near-ideal Schottky barrier contacts to n-type Al0.22Ga0.78N have been developed by a two-step surface treatment technique. Plasma etching of the AlxGa1−xN surface prior to Schottky metal deposition, combined with sequential chemical treatment of the etched surface, holds promise for developing high quality low-leakage Schottky contacts for low noise applications and for recessed gate high electron mobility transistors. In this work, the effect of postetch chemical treatment of the n-type Al0.22Ga0.78N surface on the performance of the Ni∕Au based Schottky contact has been investigated. Three different types of chemical treatment: viz, reactive ion etching, reactive ion etching plus dipping in hot aqua regia, and reactive ion etching plus dipping in hot KOH, are studied. Detailed current-voltage studies of three different surface treated diodes and a comparison with as-deposited diodes reveal significant improvement in the diode characteristics. The latter surface treatment yields Ni∕Au Schottky diodes with very low reverse leakage currents, breakdown voltages greater than 44V, and an ideality factor as low as 1.14.

Probing bulk and surface damage in widegap semiconductors

Plasma etching of the widegap materials SiC and GaN is veryimportant as there are only limited wet etches available. However, this isknown to introduce defects into surface layers under typical etch conditionsfor widegap materials. Bulk defects are also present in these materials. Wehave conducted a series of experiments to compare the effects on electricaltransport of surface and bulk damage. It is known that reactive ion etching asurface prior to Schottky metal deposition leads to degraded junctionbehaviour and our experiments confirm this. We have also irradiated thesematerials in a relativistic proton beam to induce additional bulk defects. ForSchottky metal deposition after irradiation the usual effects of significantsurface damage are observed. For deposition prior to irradiation the staticI-V characteristics show only some degradation. Raman spectroscopy, which isusually sensitive to structural changes in the material subsurface region,shows, surprisingly, little change due to the high-energy proton bombardment.We conclude that even though there may be significant levels of bulk defectsin widegap materials, transport is most sensitive to surface damage and it isvery important to minimize the effects of any dry etch damage to obtain thebest possible device performance.

Enhanced Schottky barrier on InGaAs for high performance photodetector application

A low temperature (LT=77 K) processing technique was used for Schottky metal deposition on InGaAs. The Schottky barrier height of Ag/InGaAs contact processed at low temperature was found to be 0.64 eV. This value is more than two times higher than that of the barrier height of 0.30 eV obtained from the sample processed at room temperature (RT). The enhanced Schottky contacts are superior for the application on high performance semiconductor photodetectors. The current–voltage–temperature (I–V–T) measurements were conducted to study the current transport mechanism. Atomic force microscope was used to study the surface morphology correlating with the electrical properties. The LT metal showed cracked structure, which combined by islands, while the RT metal surface appeared to be more uniform with some sharp dots.

High performance Schottky contacts on Se-doped Al xGa 1− xAs by cryogenic processing

Al x Ga 1− x As samples were grown on Se-doped Al x Ga 1− x As layer above GaAs substrate by metalorganic chemical vapor deposition (MOCVD). It was reported that the crystal quality of the re-growth Al x Ga 1− x As layer can be improved significantly when an Se-doped Al x Ga 1 − x As layer was used. The surface carrier concentration, however, was increased as a result of the Se-doping. Therefore, it could be difficult to obtain a high performance Schottky contacts on such a structure for practical devices application. In this work, Schottky contacts to Al x Ga 1− x As/Se-doped Al x Ga 1− x As were formed by cryogenic processing. In such a processing, the Schottky metal deposition was carried out with the sample cooled to a low temperature (LT) of 77 K. The Schottky barrier height was increased significantly in a Au/Al x Ga 1− x As/Se-doped Al x Ga 1− x As contact. The leakage current, for the LT contact, was nearly 4 orders lower than that of contact made at room temperature. A current transport mechanism dominated by thermionic emission for the LT contacts was found from current-voltage-temperature ( I- V- T) measurement.

Comparative characteristics of plasma etched GaAs and InP

Plasma etching was performed on GaAs and InP to prepare the surface before Schottky metal deposition. A comparative study was carried out for the two materials to study the influence of CF4 plasma etching on material properties and device characteristics. Photoreflectance (PR) and electroreflectance (ER) spectroscopy measurements were conducted to study the effect of plasma etching on the material surface. The PR and ER spectra showed the introduction of considerable radiation damage by dry etching. Current–voltage (I–V) and deep level transient spectroscopy (DLTS) were used for device characterization and defect level identification. Rapid thermal annealing (RTA) effectively removed the damage which was evident by all test techniques. The Schottky diodes on the GaAs control wafer, the plasma etched wafer, and the wafer after RTA showed barrier heights φB of 0.82, 0.65, and 0.86 eV, respectively. For InP, the plasma etched sample showed such serious damage that no Schottky contact was achieved. For the control wafer and the plasma etched wafer after RTA, the barrier heights obtained were 0.50 and 0.49 eV. The DLTS results showed two trap levels in GaAs with activation energy below the conduction band of 0.634 and 0.397 eV introduced by plasma etching. For both GaAs and InP, the control wafer and plasma etched sample after RTA showed very similar DLTS spectra which indicated the effectiveness of the RTA treatment.

Interface-state density analysis of thermally and electron-beam-annealed GaAs surfaces

Schottky diodes were formed on n-GaAs surfaces that had undergone furnace and scanned electron beam (SEB) heat treatments that are similar to those typically used for Ohmic contact formation. The diode parameters of heat-treated samples were inferior to those that were not heat treated. The interface-state density Dit, determined from Tseng and Wu’s interfacial layer model, was found to exhibit a strong correlation with the heat treatments, with furnace-annealed samples exhibiting progressively larger Dit values than SEB annealed samples. The presence of a spin-on glass capping layer during heat treatment resulted in reduced Dit values. Photochemical passivation of the GaAs surface before Schottky metal deposition resulted in a reduction in the interface-state density. Controlled etching of a heat-treated GaAs surface revealed that the surface damage caused by heat treatments extends to about 500 nm. Elevated thermal aging studies of Schottky contacts show a monotonic increase in Dit with aging time, irrespective of surface preparation.

Electrical modelling of Ion- Damaged GaAs schottky barrier interfaces

Electrical measurements and related computer modelling have been carried out for Al-GaAs Schottky diodes fabricated on ion-etched GaAs surfaces. Diode changes are seen at all ion energies, with a salient feature being reduction or total loss of the EL2-DLTS peak. A chemical etch before Schottky metal deposition results in diode parameter and EL2-DLTS recovery after about 100 and 1000A removal for 0.5 and 3 keV ion energies, respectively. The resulting equivalent circuit consists of lumped R and C components which represent the diode space-charge, a damaged, partly amorphous top layer, and bulk damage. Computer generated DLTS spectra and low frequency dispersion agree with experimental results.

Introduction of defect levels in resistive-evaporated n-Si Schottky barrier diodes

Defect levels in n-type Si Schottky barrier diodes made by resistive evaporation have been investigated by deep level transient spectroscopy. Three defect levels are observed at 0.16, 0.14, and 0.12 eV below the conduction band. The concentrations of the defect levels exponentially decrease into the substrate. The defects are introduced during etching process rather than evaporation process. The concentration of the defects increase with the thickness of the layer removed by etching before Schottky metal deposition, and decrease with the etching rate. This suggests that the defect levels are produced near the surface and are driven into the substrate during etching processes.

Electrical Modelling of Ion-Damaged GaAs Schottky Barrier Interfaces

Electrical measurements and related computer modelling have been carried out for Al-GaAs Schottky diodes fabricated on ion-etched GaAs surfaces. Diode changes are seen at all ion energies, with a salient feature being reduction or total loss of the EL2-DLTS peak. A chemical etch before Schottky metal deposition results in diode parameter and EL2-DLTS recovery after about 100 and 1000A removal for 0.5 and 3 keV ion energies, respectively. The resulting equivalent circuit consists of lumped R and C components which represent the diode space-charge, a damaged, partly amorphous top layer, and bulk damage. Computer generated DLTS spectra and low frequency dispersion agree with experimental results.