DLTS study of the influence of annealing on deep level defects induced in xenon ions implanted n-type 4H-SiC

In this study, nitrogen-doped 4H-SiC samples were bombarded with 167 MeV xenon ions to a fluence of 1?×?10⁸ cm^?2 at 300 K prior to the fabrication of Schottky barrier diodes. The implanted samples were annealed at approximately 900 °C for 1 h before the resistive evaporation of nickel Schottky barrier diodes. In comparing the current–voltage results of the implanted devices with as-deposited ones, generation-recombination took place in the implanted Schottky barrier diodes. Four defects (100, 120, 170, and 650 meV) were present in as-deposited Schottky barrier diodes when characterized by deep level transient spectroscopy (DLTS). In addition to the defects observed in the as-deposited samples, two additional defects with activation energies of 400 and 700 meV below the conduction band minimum were induced by Xe ions implantation. The two deep level defects present have signatures similar to defects present after irradiated by MeV electron. The two defects present after irradiation disappeared after annealing at 400 °C which indicate instability of the defects after annealing implanted samples.

     

Thermally stable and low-resistance W/Ti/Au contacts to n-type GaN

We report on the formation of thermally stable and low-resistance Ti/Au-based ohmic contacts to n-type GaN (4.0 × 10¹⁸ cm⁻³) by using a W barrier layer. It is shown that the electrical characteristic of the sample is considerably improved upon annealing at 900 °C for 1 min in a N₂ ambient. The contacts produce the specific contact resistance as low as 6.7 × 10⁻⁶ Ω cm² after annealing. The Norde and current–voltage methods are used to determine the effective Schottky barrier heights (SBHs). It is shown that annealing results in a reduction in the SBHs as compared to that of the as-deposited sample. Auger electron spectroscopy (AES), scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD) examinations show that nitride and gallide phases are formed at the contact/GaN interface. Based on the AES, STEM and XRD results, a possible ohmic formation mechanism is described and discussed.     

Development of refractory ohmic contact materials for gallium arsenide compound semiconductors

Recent strong demands for optoelectronic communication and portable telephones have encouraged engineers to develop optoelectronic devices, microwave devices, and high-speed devices using hetero-structural GaAs-based compound semiconductors. Although the GaAs crystal growth techniques had reached a level to control the compositional stoichiometry and crystal defects on a nearly atomic scale by the advanced techniques such as molecular beam epitaxy and metal organic chemical vapor deposition techniques, development of ohmic contact materials (which play a key role to inject external electric current from the metals to the semiconductors) was still on a trial-and-error basis.Our research efforts have been focused to develop low resistance, refractory ohmic contact materials to n-type GaAs using the deposition and annealing techniques, and it was found the growth of homo-or hetero-epitaxial intermediate semiconductor layers (ISL) on the GaAs surface was essential for the low resistance ohmic contact formation. In this paper, two typical examples of ohmic contact materials developed by forming ISL were given. The one was refractory NiGe-based ohmic contact material, which was developed by forming the homo-epitaxial ISL doped heavily with donors. This heavily doped ISL was discovered to be formed through the regrowth mechanism of GaAs layers at the NiGe/GaAs interfaces during annealing at elevated temperatures. To reduce the contact resistance further down to a value required by the device designers, an addition of small amounts of third elements to NiGe, which have strong binding energy with Ga, was found to be essential. These third elements contributed to increase the carrier concentration in ISL. The low resistance ohmic contact materials developed by forming homo-epitaxial ISL were Ni/M/Ge where a slash ‘/’ denotes the deposition sequence and M is an extremely thin (∼5 nm) layer of Au, Ag, Pd, Pt or In. The other was refractory In_xGa_1−xAs-based ohmic contact materials which were developed by forming the hetero-epitaxial ISL with low Schottky barrier to the contacting metals by growing the In_xGa_1−xAs layers on the GaAs substrate by sputter-depositing In_xGa_1−xAs targets and subsequently annealing at elevated temperatures. To reduce the contact resistance, it was found that this In_xGa_1−xAs (ISL) layer had to have In compositional gradient normal to the GaAs surface: the In concentration being rich at the metal/In_xGa_1−xAs interface and poor close to the In_xGa_1−xAs/GaAs interface. This concentration graded ISL reduced both the barrier heights at the metal/ISL and ISL/GaAs interfaces and reduced the contact resistance. The ohmic contact materials developed by forming hetero-epitaxial ISL was In_0.7Ga_0.3As/Ni/WN₂/W. These contact materials formed refractory compounds at the interfaces, which was also found to be essential to improve thermal stability of ohmic contacts used in the GaAs devices.     

Non-alloy Cr/Au Ohmic contacts in the technology of planar beam-lead GaAs <Emphasis Type="Italic">p-i-n</Emphasis> diodes

The characteristics of non-alloy Cr/Au ohmic contacts in planar beam-lead GaAs p-i-n diodes have been studied. The room-temperature reduced contact resistance in the structures studied was 2 × 10⁻⁶ Ω cm². The obtained parameters of p-i-n diodes allow these devices to be used as limiters in radar protection systems.     

Thermal stability of AuPt/n−-GaAs Schottky contacts

The possibility of optimization and the thermal stability of AuPt Schottky contacts on n^–-GaAs epitaxial layer has been investigated. The thermal treatment has been carried out in an RTA apparatus for 100 s. The contacts have been found to remain thermally stable up to 350 °C, the optimal annealing temperature is in the range 330–340 °C. The structure, which was annealed at 438 °C, has ohmic character with a contact resistivity of 3.70×10^-5cm², and a thickness of the modified subcontact layer of 160 nm. The reaction between the metallization and GaAs is negligible in the case of the optimal annealing. Au and Pt react very strongly with GaAs during annealing, when the metallization is converted into an ohmic contact structure. Arsenic, as a volatile element, leaks from the structure.     

Electrical characterization of 4H–SiC Schottky diodes with a RuO2 and a RuWO x Schottky contacts

Two types of Schottky diodes were prepared on n-type silicon carbide (4H–SiC) substrates by deposition of ruthenium oxide (RuO₂) Schottky contacts or ruthenium tungsten oxide (RuWO _x ) Schottky contacts. The RuO₂/4H–SiC and RuWO _x /4H–SiC Schottky barrier diodes were examined first by current–voltage (I-V) measurements, which confirmed symmetry of the I-V characteristics. The ideality factor (n) is rather high (～1.28/～1.15) at the temperature 300 K, the current of saturation is I _S ～10 pA/～7 pA and the Schottky barrier height is ～1.13 eV/～1.11 eV. After this diagnostic step, the samples were analysed by C-V and standard DLTS methods in the temperature range from 83 K to 450 K. In measured DLTS spectra were identified five deep levels ET1–ET5 (0.27, 0.45, 0.56, 0.58 and 0.85 eV) in RuO₂/4H–SiC Schottky barrier diodes and three deep levels E1–E3 (0.36, 0.38 and 0.69 eV) in RuWO _x /4H–SiC Schottky barrier diodes.     

Effect of thermal annealing on Ni/Au contact to a-InGaZnO films deposited by dc sputtering

We deposited Ni (15 nm)/Au (30 nm) layers on a-InGaZnO in order to produce low-resistance ohmic contacts by using a dc sputtering method. The samples were annealed at various temperatures for 5 min in Ar ambient. The electrical and the structural properties of the Ni/Au contact to a-InGaZnO were investigated. According to the current-voltage measurements, both the as-deposited and low-temperature annealed samples showed an ohmic behavior. The specific contact resistance of the as-deposited sample was 4.1 × 10^− 5 Ω cm², which was the lowest value. Further increasing the temperature above 400 °C led to an increase in the specific contact resistance. This is due to the chemical intermixing and formation of the oxide in the contact interface caused by the post-growth thermal annealing.     

Determination of the laterally homogeneous barrier height of metal/p-InP Schottky barrier diodes

We have reported a study of a number of metal/p-type InP (Cu, Au, Al, Sn, Pb, Ti, Zn) Schottky barrier diodes (SBDs). Each one diode has been identically prepared on p-InP under vacuum conditions with metal deposition. In Schottky diodes, the current transport occurs by thermionic emission over the Schottky barrier. The current–voltage characteristics of Schottky contacts are described by two fitting parameters such as effective barrier height and the ideality factor. Due to lateral inhomogeneities of the barrier height, both characteristic diode parameters differ from one diode to another. We have determined the lateral homogeneous barrier height of the SBDs from the linear relationship between experimental barrier heights and ideality factors that can be explained by lateral inhomogeneity of the barrier height. Furthermore, the barrier heights of metal–semiconductor contacts have been explained by the continuum of metal-induced gap states (MIGS). It has been seen that the laterally homogeneous barrier heights obtained from the experimental data of the metal/p-type InP Schottky contacts quantitatively confirm the predictions of the combination of the physical MIGS and the chemical electronegativity.     

Charge transport mechanism and photovoltaic behaviour of undoped and I2 doped tris (1,10 phenanthroline) iron (II) complex (TPFe) thin film devices

Tris (1,10 phenanthroline) iron (II) or Fe (Phen)²⁺ ₃, a metal-to-ligand charge transfer (MLCT) type complex (TPFe), was employed in the form of thin films, for the fabrication of Schottky diodes, Al/ TPFe/ITO, where ITO is indium tin oxide. The effect of iodine doping on the electrical behaviour has been emphasized. The diodes exhibit a rectification effect which improves on iodine doping. The diodes can be classified as MIS Schottky diodes with a graded dopant profile. The current-voltage (J-V ), and capacitance-voltage (C-V ) characteristics, the photoaction spectra of the devices and the absorption spectra of the complex, reveal that both doped and undoped complexes behave as a p-type organic semiconductor which form a Schottky barrier with Al and an ohmic contact with ITO. Various electrical and photovoltaic parameters were determined from the detailed analysis of J-V and C-V characteristics and these are discussed in detail. The effect of I_{_2} doping on the rectification and photovoltaic properties is also discussed.     

Effects of low-energy ion beam glancing angle nitridation on nGaAs surface and Co-nGaAs Schottky contact properties

In this research, effects of low-energy nitrogen and argon ion beam irradiation at a glancing angle on chemical composition and morphology of the GaAs surface as well as electrical properties of Co-nGaAs Schottky contact are studied. Substantial reduction of the effective barrier height was observed. This reduction was explained by formation of the irradiation-induced thin n⁺ layer. This revealed that low effective barrier height and low noise Co-nGaAs Schottky contacts can be fabricated on GaAs by low-energy nitrogen ion beam irradiation of surface. On the other hand, GaAs surface irradiation by low-energy noble gas (argon) ion beam resulted in substantial increase of the low-frequency noise at liquid nitrogen temperature. Formation of a thin GaN layer as a result of the GaAs surface irradiation by low-energy nitrogen ion beam was observed by XPS. Experimentally registered decrease of the low-frequency noise (in the case of nitrogen irradiation) was explained in terms of the screening of irradiation-induced defects, passivation of dangling bonds at the GaAs surface nGaAs and increased surface and Schottky contact homogeneity due to the ion beam nitridation.     

Electrical, structural and morphological characteristics of rapidly annealed Pd/n-InP (100) Schottky structure

The electrical and structural properties of the Pd/InP (100) Schottky barrier diodes have been investigated as a function of annealing temperature by current–voltage (I–V), capacitance–voltage (C–V) and X-ray diffraction (XRD) measurements. The Schottky barrier height of the as-deposited, 100 and 200°C annealed contacts determined from the I–V and C–V measurements are 0.56 and 0.81 eV, 0.57 and 0.81 eV, and 0.58 and 0.82 eV, respectively. However, both the measurements showed that the Schottky barrier height of the Pd/n-InP Schottky contact is increased to 0.59 eV (I–V) and 0.83 eV (C–V) when the contact is annealed at 300°C for 1 min in nitrogen atmosphere. Further Schottky barrier height decreases to 0.57 eV (I–V), 0.71 eV (C–V) and 0.53 eV (I–V), 0.67 eV (C–V) after annealing at 400 and 500°C samples. The result shows that the optimum annealing temperature for the Pd/InP Schottky diode is 300°C. Norde method is also used to determine the barrier height of Pd Schottky contacts and the values are 0.56 eV for the as-deposited contact, 0.57, 0.57, 0.58, 0.57 and 0.54 eV for contacts annealed at 100, 200, 300, 400 and 500°C which are consistent with the values obtained by the I–V measurements. From the atomic force microscopy results, it is evident that the overall surface morphology of the Pd/InP Schottky diode is fairly smooth. Based on the XRD results, the formation of phosphorus-oxygen compounds at the interface may be responsible for the variation in barrier heights observed in Pd/InP Schottky contacts with annealing temperature.     

Peculiarities of the formation and thermal stability of barrier contacts in high-sensitivity silicon carbide detector diodes

The effect of rapid thermal treatment at T=1000°C on the formation of \textTiB_x - n-\textSiC6H(00\text1^-- ){\text{TiB}}_x - n--{\text{SiC6H(00}}\mathop {\text{1}}\limits^-- ) barrier contacts and Ni-n-SiC6H(0001) ohmic contacts was studied. In the former case, thermal treatment neither disturbs the layer structure nor reduces the thermal stability of the barrier contacts. The rapid annealing of an Ni-n-SiC6H(0001) structure results in the formation of a stable ohmic contact. At the same time, this treatment does not change the parameters of the static current-voltage characteristics of the \textAu - \textTiB_x - n-\textSiC6H(00\text1^-- ){\text{Au}} - {\text{TiB}}_x - n--{\text{SiC6H(00}}\mathop {\text{1}}\limits^-- ) Schottky diodes. These thermally stable diodes are characterized by a sensitivity of ∼3300–3500 mV/mW at an incident radiation power of 10–7 W and are capable of operating at a microwave power of up to 1 W. The dynamic range of a linear portion of the conversion characteristic reaches up to 50 dB.     

Scanned electron beam irradiation of Ti Schottky contacts to n-GaAs

Ti Schottky contacts were formed on n-GaAs surfaces and irradiated using a low energy scanned electron beam at various fluence levels from 10¹⁵ to 10¹⁸ cm^–2. For fluence levels up to 10¹⁷ cm^–2, the Schottky contacts were found to exhibit a reduction in their leakage currents and increased barrier voltages. For fluence levels in excess of 10¹⁷ cm^–2, the Schottky diodes were found to exhibit significantly increased leakage currents and barrier voltages. The changes in leakage currents were consistent with the changes in their respective interface state density (D _it) values. However, the electron beam irradiation had little or no effect on the diode ideality factorn.     

Schottky barrier characteristics of Pt contacts to all sputtering-made n-type GaN and MOS diodes

All sputtering-made Pt/n-GaN [metal–semiconductor (MS)] and Pt/SiO₂/n-GaN [metal–oxide–semiconductor (MOS)] diodes were investigated before and after annealing at 500 °C. n-GaN, Pt, and SiO₂ films were all fabricated by the cost-effective radio-frequency sputtering technique. A cermet target was used for depositing GaN. The Schottky barrier heights (SBHs) of both MS and MOS Schottky diodes have been investigated by the current–voltage (I–V) measurements. The results showed that SBHs increased after annealing at 500 °C for 20 min in N₂ ambient, compared to the as-deposited at 400 °C. By using Cheung’s and Norde methods, the highest SBHs of MOS Schottky diodes were respectively found to be 0.79 and 0.91 eV for the as-deposited and had reached to 0.81 and 0.94 eV after annealing. The annealed Schottky diode had showed the higher SBH, lower leakage current, smaller ideality factor, and denser microstructure.     

Effect of annealing temperature on electrical properties of Au/polyvinyl alcohol/n-InP Schottky barrier structure

In the present work, thin film of polyvinyl alcohol (PVA) is fabricated on n-type InP substrate as an interfacial layer for electronic modification of Au/n-InP Schottky contact. The electrical characteristics of Au/PVA/n-InP Schottky diode are determined at annealing temperature in the range of 100-300 °C by current-voltage (I-V) and capacitance-voltage (C-V) methods. The Schottky barrier height and ideality factor (n) values of the as-deposited Au/PVA/n-InP diode are obtained at room temperature as 0.66 eV (I-V), 0.82 eV (C-V) and 1.32, respectively. Upon annealing at 200 °C in nitrogen atmosphere for 1 min, the barrier height value increases to 0.81 eV (I-V), 0.99 eV (C-V) and ideality factor decreases to 1.18. When the contact is annealed at 300 °C, the barrier height value decreases to 0.77 eV (I-V), 0.96 eV (C-V) and ideality factor increases to 1.22. It is observed that the interfacial layer of PVA increases the barrier height by the influence of the space charge region of the Au/n-InP Schottky junction. The discrepancy between Schottky barrier heights calculated from I-V and C-V measurements is also explained. Further, Cheung's functions are used to extract the series resistance of Au/PVA/n-InP Schottky diode. The interface state density as determined by Terman's method is found to be 1.04 × 10¹² and 0.59 × 10¹² cm^− 2 eV^− 1 for the as-deposited and 200 °C annealed Au/PVA/n-InP Schottky diodes. Finally, it is seen that the Schottky diode parameters changed with increase in the annealing temperature.     

Evaluation of diffusion barrier and electrical properties of tantalum oxynitride thin films for silver metallization

The thermal stability and the diffusion barrier properties of DC reactively sputtered tantalum oxynitride (Ta-O-N) thin films, between silver (Ag) and silicon (Si) p⁺n diodes were investigated. Both materials characterization (X-ray diffraction analysis, Rutherford backscattering spectrometry (RBS), Auger depth profiling) and electrical measurements (reverse-biased junction leakage current-density) were used to evaluate diffusion barrier properties of the thin films. The leakage current density of p⁺n diodes with the barrier (Ta-O-N) was approximately four orders of magnitude lower than those without barriers after a 30 min, 400 °C back contact anneal. The Ta-O-N barriers were stable up to 500 °C, 30 min anneals. However, this was not the case for the 600 °C anneal. RBS spectra and cross-sectional transmission electron microscopy of as-deposited and vacuum annealed samples of Ag/barrier (Ta-O-N)/Si indicate the absence of any interfacial interaction between the barrier and substrate (silicon). The failure of the Ta-O-N barriers has been attributed to thermally induced stresses, which cause the thin film to crack at elevated temperatures.     

Barrier height enhancement in WSi x /GaAs Schottky diodes by rapid thermal annealing

The influence of rapid thermal annealing on WSi₂/GaAs and WSi_0.6/GaAs Schottky diodes was studied by means of Rutherford backscattering spectroscopy (RBS), particle induced X-ray emission spectroscopy (PIXE), X-ray diffraction (XRD) analysis and current-voltage electrical measurement. Despite the amorphicity of the layers remaining after annealing and only relatively minor changes in RBS and PIXE spectra, large barrier enhancements were registered, especially for WSi₂/GaAs diodes. For an explanation of this effect, the barrier height inhomogeneity concept is used.     

Metallic contacts to nitrogen and boron doped diamond-like carbon films

Hydrogenated diamond-like carbon (DLC) was deposited using a radio-frequency plasma-enhanced chemical vapor deposition method. Electrical properties of Al, Au, Ti, and Zr contacts to nitrogen and boron doped DLC films have been studied, and mechanisms of the observed current-voltage (I-V) characteristics are investigated. Linear I-V characteristics were observed for Au, Ti, and Zr contacts to both nitrogen and boron doped DLC films. A band structure model for metal-DLC contact is proposed to explain the observed ohmic contacts. Fermi level shifting at the surface of DLC films produces an ohmic resistive layer instead of a Schottky barrier for metal-DLC contacts. Al contacts to both nitrogen and boron doped DLC films show nonlinear I-V characteristics, which are attributed to a dielectric layer of carbide (Al₄C₃) instead of a Schottky barrier suggested by other groups. Inert elements such as Au and Pt, and transition metals such as Ti, Zr and W, which form conductive carbides, are considered good contacting metals for electrical studies of DLC films.     

Conductivity of amorphous hydrogenated silicon in the planar and sandwich configurations

The temperature dependence of the conductivity of amorphous hydrogenated silicon in planar and sandwich configurations prepared under identical conditions is measured. The planar conductivity is measured on heat-dried samples; the sandwich conductivity is obtained from (i) the ohmic series resistance of forward-biased Schottky diodes and (ii) the ohmic conductivity of n⁺/n/n⁺ structures. We find that the conductivities for the two configurations compare favourably, thus ruling out any appreciable effect of space charge layers on the conductivity of the planar samples.     

Reconfigurable Diodes Based on Vertical WSe2 Transistors with van der Waals Bonded Contacts

New device concepts can increase the functionality of scaled electronic devices, with reconfigurable diodes allowing the design of more compact logic gates being one of the examples. In recent years, there has been significant interest in creating reconfigurable diodes based on ultrathin transition metal dichalcogenide crystals due to their unique combination of gate‐tunable charge carriers, high mobility, and sizeable band gap. Thanks to their large surface areas, these devices are constructed under planar geometry and the device characteristics are controlled by electrostatic gating through rather complex two independent local gates or ionic‐liquid gating. In this work, similar reconfigurable diode action is demonstrated in a WSe₂ transistor by only utilizing van der Waals bonded graphene and Co/h‐BN contacts. Toward this, first the charge injection efficiencies into WSe₂ by graphene and Co/h‐BN contacts are characterized. While Co/h‐BN contact results in nearly Schottky‐barrier‐free charge injection, graphene/WSe₂ interface has an average barrier height of ≈80 meV. By taking the advantage of the electrostatic transparency of graphene and the different work‐function values of graphene and Co/h‐BN, vertical devices are constructed where different gate‐tunable diode actions are demonstrated. This architecture reveals the opportunities for exploring new device concepts.