Cevdet Coskun

Temperature dependence of reverse bias capacitance–voltage characteristics of Sn/p-GaTe Schottky diodes

A Schottky barrier diode on unintentionally doped p-type GaTe grown by the directional freezing method was obtained and characterized by the capacitance–voltage technique as a function of temperature (100–300 K). Using vacuum evaporated Sn as the Schottky barrier contact and In for the ohmic contact, high-quality diodes were produced. The discrepancy between Schottky barrier heights (BHs) obtained from current–voltage–temperature and capacitance–voltage–temperature measurements is explained by the introduction of a spatial distribution of BHs due to barrier height inhomogeneities that prevail at the metal/GaTe interface. The deviations of apparent BHs were investigated by considering the microstructure of the metal/GaTe interface. It was found that the dispersion of this distribution across the contact area grew increasingly larger at lower temperatures and was responsible for the increasing difference between apparent BHs obtained from the two techniques.

High-temperature Schottky diode characteristics of bulk ZnO

Current?voltage (I?V) measurements of Ag/n-ZnO have been carried out at temperatures of 200?500?K in order to understand the temperature dependence of the diode characteristics. Forward-bias I?V analysis results in a Schottky barrier height of 0.82?eV and an ideality factor of 1.55 at room temperature. The barrier height of 0.74?eV and Richardson constant of 0.248?A?K?2?cm?2 were also calculated from the Richardson plot, which shows nearly linear characteristics in the temperature range 240?440?K. From the nkbT/q versus kbT/q graph, where n is ideality factor, kb the Boltzmann constant, T the temperature and q the electronic charge we deduce that thermionic field emission (TFE) is dominant in the charge transport mechanism. At higher sample temperatures (>440?K), a trap-assisted tunnelling mechanism is proposed due to the existence of a deep donor situated at Ec?0.62?eV with 3.3 ? 10?15?cm2 capture cross section observed by both deep-level transient spectroscopy (DLTS) and lnI0 versus 1/kbT plots. The ideality factor almost remains constant in the temperature range 240?400?K, which shows the stability of the Schottky contact in this temperature range.

Electrochemical growth of n-ZnO onto the p-type GaN substrate: p-n heterojunction characteristics

n-ZnO thin films were deposited electrochemically onto the p-GaN/Al2O3 substrate in order to form hetero p-n junction. X-ray diffraction measurement has been showed clearly (0002) c-axis orientation of grown ZnO thin film. Absorption measurements were carried out before and after growth process indicating both sharp absorption edges of GaN and ZnO thin films. Photoluminescence measurement shows n-ZnO film grown on p-GaN has a dominant emission at 2.8 eV. I-V characteristic of n-ZnO/p-GaN/Al2O3 heterojunction showed that almost five order of rectification has been achieved. Turn on voltages of the p-n heterojunction is found to be 1.12 V.n-ZnO thin films were deposited electrochemically onto the p-GaN/Al2O3 substrate in order to form hetero p-n junction. X-ray diffraction measurement has been showed clearly (0002) c-axis orientation of grown ZnO thin film. Absorption measurements were carried out before and after growth process indicating both sharp absorption edges of GaN and ZnO thin films. Photoluminescence measurement shows n-ZnO film grown on p-GaN has a dominant emission at 2.8 eV. I-V characteristic of n-ZnO/p-GaN/Al2O3 heterojunction showed that almost five order of rectification has been achieved. Turn on voltages of the p-n heterojunction is found to be 1.12 V.

The effect of high-energy electron irradiation on ZnO-based ohmic and Schottky contacts

A systematic study of radiation effects on the major parameters of ohmic and Schottky contacts based on n-ZnO is introduced. Al and Au metals were used as contact elements in order to fabricate the ohmic and Schottky structures, respectively. The transmission line method (TLM) measurements on Al/n-ZnO have revealed that high-energy (6, 9, 12 MeV) and relatively low-dose (3 × 1012 e− cm−2) electron irradiation produced lower specific ohmic contact resistivity values as compared with the reference sample. The current–voltage (I–V) and capacitance–voltage (C–V) measurements on the Au/n-ZnO structures are shown to increase in ideality and to decrease in the Schottky barrier heights with increasing electron energy. These findings have been interpreted based on the assumption that the atoms of the contact elements diffused into the semiconductor material, thus turning the rectifying character to ohmic behaviour with the influence of radiation–matter interaction and subsequent annealing effects.

High energy electron irradiation effects on electrical properties of Au/n-ZnO Schottky diodes

High energy electron irradiation (HEEI) was performed on Au/n-ZnO Schottky diodes (SDs) and the effects of irradiation were compared with a reference SD. Current–voltage and capacitance–voltage measurements revealed that the barrier height and donor concentration decrease from 0.746 to 0.665 eV and from 4.55 × 1014 cm−3 to 1.76 × 1014 cm−3, respectively, while the ideality factor increases from 1.61 to 3.95 after irradiation. Ionization temperatures of traps were observed by means of thermally stimulated capacitance measurements at temperatures 307 K, 365 K and 332 K, 385 K and 477 K for the irradiated and the reference SDs, respectively. Deep level transient spectroscopy measurements revealed a defect level at 870 meV and capture cross sections of 0.88 × 1012 cm2 for the reference SD and two HEEI induced defects at energies 670 and 780 meV and capture cross sections of 29.6 × 10−12 cm2 and 3.08 × 10−12 cm2 for the irradiated SD, respectively.

Formation of low and stable ohmic contacts to GaTe layered crystal

In this paper, we report on a systematic study of the formation of ohmic contacts to a GaTe layered crystal grown by the directional freezing method. In this study, the transmission line method (TLM) was used for the measurement of specific contact resistance of ohmic contacts to GaTe. We used In, Au, Al and Ag metals and Au–In eutectic alloy as contact elements. A ladder pattern was formed directly on the GaTe surface by evaporation of metals through a pre-patterned shadow mask. The lowest ohmic contact resistance, 2.5 ± 1.4 × 10−5 ohm cm2, was achieved by annealing In at 200 °C for 2.5 min. Ohmic contacts fabricated by this process remained very stable up to six months after the anneal, although In contacts on some other samples, processed at 175–250 °C for 2.5–14 min, and having higher contact resistance, were unstable. The other elements used in this study showed rectification behaviour after annealing at 175–400 °C for 5 min. X-ray diffraction measurements showed that InGaTe2 formation at the In/GaTe interface was minimized for the sample annealed with the optimum process. We found that the formation of InTe was essential for the successful production of ohmic contacts, and that the quality of the contacts was determined by the competition between InTe and InGaTe2.

Oxygen effects on radiation hardness of ZnO thin films

Six ZnO thin films were grown under different oxygen flow rates by electrochemical deposition onto commercial indium tin oxide substrates. X-ray diffraction (XRD), optical absorption, and photoluminescence (PL) measurements were performed on all films. XRD measurements showed that films are highly (0002) c-axis oriented. It has been observed that the growth rates of the films are highly dependent on the oxygen flow rates. High growth rate is obtained for the midoxygen flow rates in the cell. Calculated crystallite size values have an increasing trend as the oxygen flow rate increases. Absorption measurements have revealed that the band gap energy of ZnO thin films is about 3.4eV. PL measurements showed that two emissions are observed in all films: free exciton emission at about 3.37eV and so-called blue emission at 2.66eV in ZnO. Relatively low dose (5×1012e−∕cm2) and high-energy electron-irradiation (HEEI) (12MeV) experiments were performed on all films. Their effects on the optical and structural charac...

An Experimental Set-Up for In Situ Hall Measurements Under High-Energy Electron Irradiation for Wide-Bandgap Materials

A dc Hall-effect apparatus, based on conventional van der Pauw specimen geometry, has been developed for in situ measurements under van de Graaff electron irradiation (0.7–2.2 MeV). An associated cryostat permits ambient temperatures of 95–300 K. A key element is a flat permanent magnet of field strength 3.55 kG. Initial test measurements have been performed on wide-bandgap semiconductor materials, including an HVPE-grown AlGaN/GaN heterostructure field-effect transistor structure.

A Simple Method Producing Shadow Masks Used in Electrical Characterization Techniques

In this paper, a novel technique is introduced to make a shadow mask (SM) that is used in various electrical characterization techniques such as Hall-resistivity, magnetoresistance, and TLM (Transmission Line Method) ohmic contact measurements. By using this technology, a film thickness of about 0.25–0.30 μm was obtained after photoprocessing. We reached an SM thickness of 15–20 μm by electrodeposition, which is required for a sufficiently high toughness. It was observed that the sharpness of the masks was also fairly good. The SMs obtained using this technology have been successfully used in various applications.

In situ Hall investigation of the electron-irradiated and annealed AlGaN/GaN HFETs

A comparative study is introduced of the electrical behaviours of high-energy electron-irradiated (E=1 and 1.5 MeV, F=5×1016 cm−2 at room temperature (RT) and annealed (T=250, 300, 350, and 400 °C for 10 min) AlGaN/GaN heterostructure field effect transistors (HFETs), which have been grown by three different techniques: MBE, HVPE, and MOCVD. The electrical measurements are carried out between 100–290 K temperature range by using an in situ Hall system. From the point of view of mobility degradation, the most radiation-resistant structure to the electron-beam irradiation is found to be the HVPE-grown AlGaN/GaN HFET. Electron mobility is shown to increase after first electron irradiation for HVPE-grown AlGaN/GaN HFETs, whereas, for both MBE and MOCVD-grown structures, mobility is reduced remarkably. Two-dimensional carrier concentration is increased considerably after each irradiation for all samples. The highest mobility (6200 cm2/Vs at 100 K) is obtained from 1 MeV electron-irradiated HVPE-grown AlGaN/GaN HFETs, and the highest sheet carrier concentration values (3.7×1013 cm−2 at RT) are obtained from MBE-grown AlGaN/GaN HFETs; both irradiated at 1 MeV. It is also found that thermal processes carried out on these samples do not result in any improvement; and once electron-beam disorder has been produced in these structures, it is extremely difficult to remove by annealing.