Optoelectronic characterisation of AlGaN based Schottky barrier diodes

Recent advances in growth techniques have lead to the production of high quality GaN and this has played a vital role in the improvement of GaN based devices. A number of device types can be produced from GaN. Spectrally selective devices can be produced by creating ternary or quaternary material systems by partially substituting either Al or In for Ga in GaN. This allows a wide spectral range that can be achieved ranging from the visible to the ultraviolet. The applications of detectors based on these material systems are vast and include areas such as biological, military, environmental, industrial and scientific spheres. In front illuminated Schottky barrier photodetectors, two major factors influencing the sensitivity of the device are the reverse leakage current and the transparency of the Schottky contact. In order to reduce the reverse current of semiconductor based devices, increase the barrier height, and enhance the adhesion of a metal on a semiconductor it is important to subject the contact to annealing. Annealing studies have been performed on AlGaN based photodiodes to investigate the evolution of the optical and electrical properties. In this study, the electrical and optical characteristics of AlGaN based Ni/Au and Ni/Ir/Au Schottky photodiodes were investigated. The electrical properties of the photodiodes were optimised by annealing in an Ar ambient. An increase in the Schottky barrier height and a decrease in the reverse leakage current were observed with increasing annealing temperature up to 500 oC. This effect was observed for both the Ni/Au and Ni/Ir/Au photodiodes. The optical characteristics of the photodiodes, which include the responsivity and the quantum efficiency, were also investigated. UV/visible rejection ratios of as high as 103 were obtained. The transmittance of Ni/Au and Ni/Ir/Au metal layers deposited on a quartz substrate were optimised by annealing. This was under the same ambient conditions as the Schottky photodiode. The transmittance increased with annealing temperature for the Ni/Au metal layer whereas it decreased at higher temperatures for the Ni/Ir/Au layer. The transmittance of the Ni/Au metal layer reached as high as 85 % after 500 oC annealing. The transmittance of the Ni/Ir/Au only reached a high of 41 % after 400 oC annealing.Dissertation (MSc)--University of Pretoria, 2013.Physicsunrestricte

Response of Ni/4H-SiC Schottky barrier diodes to alpha-particle irradiation at different fluences

Irradiation experiments have been carried out on 1.9 × 1016 cm–3 nitrogen-doped 4H-SiC at room temperature using 5.4 MeV alpha-particle irradiation over a fluence ranges from 2.6 × 1010 to 9.2 × 1011 cm–2. Current-voltage (I-V), capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements have been carried out to study the change in characteristics of the devices and free carrier removal rate due to alpha-particle irradiation, respectively. As radiation fluence increases, the ideality factors increased from 1.20 to 1.85 but the Schottky barrier height (SBHI-V) decreased from 1.47 to 1.34 eV. Free carrier concentration, Nd decreased with increasing fluence from 1.7 × 1016 to 1.1 × 1016 cm–2 at approximately 0.70 μm depth. The reduction in Nd shows that defects were induced during the irradiation and have effect on compensating the free carrier. The free carrier removal rate was estimated to be 6480±70 cm–1. Alpha-particle irradiation introduced two electron traps (E0.39 and E0.62), with activation energies of 0.39±0.03 eV and 0.62±0.08 eV, respectively. The E0.39 as attribute related to silicon or carbon vacancy, while the E0.62 has the attribute of Z1/Z2.National Research Foundation (NRF) of South African (Grant specific unique reference number (UID) 78838).http://www.elsevier.com/locate/physb2017-01-31hb201

Electrical characterization of defects introduced in n-type N-doped 4H-SiC during electron beam exposure

Deep level transient spectroscopy (DLTS) was used to characterize the defects introduced in n-type, N-doped, 4H-SiC while being exposed to electron beam evaporation conditions. This was done by heating a tungsten source using an electron beam current of 100 mA, which was not sufficient to evaporate tungsten. Two new defects were introduced during the exposure of 4H-SiC samples to electron beam deposition conditions (without metal deposition) after resistively evaporated nickel Schottky contacts. We established the identity of these defects by comparing their signatures to those of high energy particle irradiation induced defects of the same materials. The defect E0.42 had acceptor-like behaviour and could be attributed to be a silicon or carbon vacancy. The E0.71 had intrinsic nature and was linked to a carbon vacancy and/or carbon interstials.The National Research Foundation (NRF) of South African (Grant specific unique reference number (UID) 78838).https://www.scientific.net/SSPhb2017Physic

Implementation of an AlGaN-based solar-blind UV four-quadrant detector

Please read abstract in the article.Technology Innovation Agency (TIA) and the National Research Foundation (NRF)http://www.elsevier.com/locate/physbhb201

DLTS characterization of defects in GaN induced by electron beam exposure

The deep level transient spectroscopy (DLTS) technique was used to investigate the effects of electron beam exposure (EBE) on n-GaN. A defect with activation energy of 0.12 eV and capture cross section of 8.0 × 10-16 cm2 was induced by the exposure. The defect was similar to defects induced by other irradiation techniques such as proton, electron, and gamma irradiation. In comparison to GaN, the EBE induced defects in other materials such as Si and SiC are similar to those induced by other irradiation methods.The South African National Research Foundation (NRF)http://www.elsevier.com/locate/mssp2018-06-30hb2017Physic

Optical and electrical characterization of AlGaN based Schottky photodiodes after annealing at different temperatures

In this study a comparison is made between the optical and electrical properties of Ni/Au and Ni/Ir/Au Schottky photodiodes based on Al0.35Ga0.65N. The effects of inserting Ir between Ni and Au are of particular interest. The comparison in the properties is done after annealing the photodiodes at different temperatures in an argon gas ambient. The reverse current decreased with annealing temperature up to 400 oC for the Ni/Au Schottky photodiode and up to 500 oC for the Ni/Ir/Au photodiode. The Schottky barrier heights increased with increasing annealing temperature. The responsivity of the Ni/Au photodiode was higher than that of the Ni/Ir/Au photodiode. The transmission of the Ni/Au metal layer improved with increasing annealing temperature up to 500 oC and the best transmission of the Ni/Ir/Au metal layer was after 400 oC annealing.http://www.elsevier.com/locate/physbhb201

Optoelectronic characterization of Au/Ni/n-AIGaN photodiodes after annealing at different temperatures

Please read abstract in the article.http://www.elsevier.com/locate/physbnf201

Electrical characterization of defects introduced during electron beam deposition of W Schottky contacts on n-type 4H-SiC

We have studied the defects introduced in n-type 4H-SiC during electron beam deposition (EBD) of tungsten by deep-level transient spectroscopy (DLTS). The results from currentvoltage and capacitance-voltage measurements showed deviations from ideality due to damage, but were still well suited to a DLTS study. We compared the electrical properties of six electrically active defects observed in EBD Schottky barrier diodes with those introduced in resistively evaporated material on the same material, as-grown, as well as after high energy electron irradiation (HEEI). We observed that EBD introduced two electrically active defects with energies EC – 0.42 and EC – 0.70 eV in the 4H-SiC at and near the interface with the tungsten. The defects introduced by EBD had properties similar to defect attributed to the silicon or carbon vacancy, introduced during HEEI of 4H-SiC. EBD was also responsible for the increase in concentration of a defect attributed to nitrogen impurities (EC – 0.10) as well as a defect linked to the carbon vacancy (EC – 0.67). Annealing at 400 °C in Ar ambient removed these two defects introduced during the EBD.In part by the National Research Foundation (NRF) of South African (Grant specific unique reference number (UID) 78838).http://www.elsevier.com/locate/mssp2017-08-31hb2016Physic

Chemical and electrical characteristics of annealed Ni/Au and Ni/Ir/Au contacts on AlGaN

The evolution of Ni/Au and Ni/Ir/Au metal contacts deposited on AlGaN was investigated at different annealing temperatures. The samples were studied with electrical and chemical composition techniques. I-V characteristics of the Schottky diodes were optimum after 500 and 600 ºC annealing for Ni/Au and Ni/Ir/Au based diodes, respectively. The depth profiles of the contacts were measured by x-ray photoelectron spectroscopy and time of flight secondary ion mass spectroscopy. These chemical composition techniques were used to examine the evolution of the metal contacts in order to verify the influence the metals have on the electrical properties of the diodes. The insertion of Ir as a diffusion barrier between Ni and Au effected the electrical properties, improving the stability of the contacts at high temperatures. Gold diffuses into the AlGaN film, degrading the electrical properties of the Ni/Au diode. At 500 ºC, the insertion of Ir, however, prevented the in-diffusion of Au into the AlGaN substrate.The National Research Foundation of South Africa (Grant specific unique reference number (UID) 87352).http://www.journals.elsevier.com/physica-b-condensed-matterhb2017Physic

Effects of 5.4 MeV alpha-particle irradiation on the electrical properties of nickel Schottky diodes on 4H-SiC

Current–voltage, capacitance–voltage and conventional deep level transient spectroscopy at temperature ranges from 40 to 300 K have been employed to study the influence of alpha-particle irradiation from an 241Am source on Ni/4H–SiC Schottky contacts. The nickel Schottky barrier diodes were resistively evaporated on n-type 4H–SiC samples of doping density of 7.1 1015 cm 3. It was observed that radiation damage caused an increase in ideality factors of the samples from 1.04 to 1.07, an increase in Schottky barrier height from 1.25 to 1.31 eV, an increase in series resistance from 48 to 270 X but a decrease in saturation current density from 55 to 9 10 12Am 2 from I–V plots at 300 K. The free carrier concentration of the sample decreased slightly after irradiation. Conventional DLTS showed peaks due to four deep levels for as-grown and five deep levels after irradiation. The Richardson constant, as determined from a modified Richardson plot assuming a Gaussian distribution of barrier heights for the as-grown and irradiated samples were 133 and 151 A cm 2 K 2, respectively. These values are similar to literature values.National Research Foundation (NRF) of South African (Grant specific unique reference number (UID) 78838).http://www.elsevier.com/locate/nimb2016-12-31hb201