ZnMgO is extensively being used in UV based optoelectronic applications owing to its high bandgap and excitonic emissions at room temperature. In many applications, UV-Vis detection is required such as spectroscopy, medical applications and plasmonics. ZnMgO film and silicon can work in parallel to give a broad UV-Vis response. We fabricated ZnMgO/Si UV-Vis photo detector. UV-Ozone annealing can be used to increase the responsivity due to an increase in photoconductive gain. Here in this report, we have shown the effect of UV-Ozone on the transient behavior of the photodetector. The calculated responsivity values for 300 nm, 400 nm and white light were 3 A/W, 95 A/W and 50 A/W respectively. After UV-Ozone corresponding responsivity increased to 5 A/W, 180 A/W and 105 A/W respectively. The transient response of the photodetector was measured for asfabricated and UV-Ozone annealing. The transient behavior was fitted into two time constant of rise and fall time and switching parameters at 300 nm, 400 nm and white light are compared. The lesser time constant indicates carrier generation and recombination and the larger time constant is related with traps capture and release process. Rise time for 300 nm, 400 nm and white light was estimated to be 100, 10 and 8 ms respectively and the corresponding fall times were 140, 45 and 40 ms. Rise time for 300 nm illumination decreased from 100 to 70 ms after UV-Ozone.
Zinc oxide is a well-known wide bandgap semiconductor material. High exciton energy at room temperature makes it useful in optoelectronic applications. ZnO can be made available in bulk, film and nanostructured form as well. Nanostructures of ZnO have been proven to have augmented optical properties and extensively being used in optoelectronics and sensing applications. Here we have shown the effect of seed layer thickness on the chemical growth of ZnO nanorods. Two samples A and B has been prepared by hydrothermal bath synthesis having seed layer thicknesses of 120 nm and 350 nm, respectively. FEG-SEM images of nanorods revealed longer and high aspect ratio for sample B while sample A showed lower aspect ratio nanorods. XRD results indicate a dominant c-axis oriented (002) plane at around 33.5°. The calculated grain size of Sample A and B were 31.5 nm and 34.4 nm, respectively. Calculated strain values show a compressive strain in the crystal lattice. The Band edge PL peak emission for sample A and B was found to be at 368.74 nm and 368.38 nm at 18 K.
ZnO thin films and nanorods are being used for UV optoelectronic applications such as UV-detectors, UVLEDs and Laser diodes because of its large bandgap (3.37 eV) and high excitonic binding energy at room temperature (60 meV). However native point defects in as grown ZnO films need to be suppressed before its device application. Here in this work, a comparative study on the effect of UV-Ozone annealing on the optical properties of ZnO thin films and nanorods have been carried out. Thin films were deposited using RF sputter system and hydrothermal route was used for nanorods growth, followed by UVO annealing for 50 min. Field emission gun scanning electron microscopy (FEG-SEM) confirmed formation of high density nanorods. High resolution X-ray diffraction (HRXRD) results exhibited (002) crystal orientation as the dominant peak for all samples. Calculated grain size for as-grown thin films and nanorods were 27 nm and 37 nm respectively. After UVO annealing it increased to 35 and 47 nm respectively. Room temperature photoluminescence showed enhancement in near band emissions (NBE) for both thin films and nanorods. Maximum enhancement in NBE as compared to as-grown for thin films and nanorods were found to be 6.6 and 3.6 times, respectively. Maximum NBE to DBE integrated area ratio for thin films and nanorods were 0.17 and 0.70 respectively.
Zinc magnesium oxide has emerged as a potential candidate for sensing and optoelectronic applications due to its structural advantages over ZnO. However, especially for UV-optoelectronic device applications, suppression of defects bound emissions in thin films and nanorods are a challenging task. In this report, we show comparison of enhancement in near-band emission and suppression in the defects-band emissions in ZnMgO thin film and nanorods using UV-Ozone (UVO) annealing. Thin films were deposited using RF sputter system and hydrothermal route was used to grow nanorods on the rapid thermal annealed ZnMgO seed layer (as-grown) followed by UVO annealing for 10, 30, 50, 70 and 90 min. Field-emission gun scanning electron microscopy confirmed growth of high density nanorods. High-resolution x-ray diffraction pattern exhibited <002> peak for all samples and a gradual increase in grain size. Room temperature photoluminescence (PL) spectra showed highest NBE emission for 10 min in thin films and 50 min for nanorods. Calculated NBE to DBE integrated area ratio increased to 1.2 for 10 min in thin films and 7.8 times for 50 min nanorods sample as compared to respective as-grown samples. Activation energy calculated from NBE integrated area of nanorods temperature-dependent PL confirmed that 50 min annealed sample showed the highest activation energy. Authors would like to acknowledge DST, India and IITBNF.
A futuristic thin-film transistor based on a double-well heterostructure exploiting the band-gap tailoring property of Zinc Oxide (ZnO) has been proposed. Effects of carrier confinement in the MgZnO/CdZnO heterojunction have been studied by employing an additional MgZnO barrier in the CdZnO channel to create two potential wells. Carrier transport and device operation have been explained with the help of energy band diagrams extracted at different operating voltages. The optimised double-well structure yields an unprecedented ION/IOFF=1015, simultaneously achieving a sub-threshold swing=74mV/decade, thereby indicating high switching speed. A high value of field-effect mobility, (μFE, max=32cm2 /V-s) over a wide range of gate bias, manifests its ability to overcome the carrier scattering problem due to confinement, making it a promising candidate for high resolution and fast response optical display applications.
ZnO, a wide band gap material, has interesting features like high electron mobility, high optical transparency and costeffectiveness. The major drawback of using ZnO as the channel layer is the presence of point defects which affects the carrier concentration. Carrier concentration (n) and interface states of the thin film greatly influence its threshold voltage, thus controlling the intrinsic defects of the film and passivation of interface states is crucial to reduce the threshold voltage variation. UV-Ozone (UV-O) treatment can be used to suppress these defects by supplying reactive oxygen to the film. In this report, we have studied grain boundary, carrier concentration and interface charge effects separately to emulate the effect of UV-O treatment. We have incorporated two distinct defect levels in our model for acceptor-like and donor-like defects in the grain boundary. The electrical parameter extraction was carried out in Silvaco using the TCAD simulator. We have observed an enhancement in corresponding electron mobility from 142.9 to 149.9 cm2 /Vs, along with the positive shift in threshold voltage from -0.40 to -0.25 V, with the decrease in number of grain boundaries. With change in carrier concentration, mobility of the device was increased from 44.2 to 150 cm2 /Vs. Passivation of interface charge density from 1 x 1012 to 1 x 1010 cm-2 in our model resulted in a significant change in the threshold voltage from 1.25 V to -0.4 V. Authors would like to acknowledge Department of Science and Technology (DST), India and IIT Bombay.
In this paper, the authors have propounded a pragmatic solution to circumvent the problem of inherent n-type conductivity of Zinc Oxide (ZnO), which remains the major obstacle to superior device performance. The proposed method employs the concept of carrier confinement in heterojunction thin film transistors that offers a prospective alternative to intruding into lesser known materials and their associated complexities. Carrier confinement is achieved in the low band-gap Cadmium Zinc Oxide (CdZnO) channel shielded by high band-gap Magnesium Zinc Oxide (MgZnO) on one side and gate dielectric SiO2 on the other, which has been further corroborated by Energy Band diagram of the confined region that manifests the formation of two dimensional electron gas (2-DEG) at the CdZnO-SiO2 interface. The device exhibits almost ideal transfer characteristics with a very narrow transition region between the ON and OFF states. The sub-threshold region is characterized by a high ION/IOFF ratio (1011) and near ideal sub-threshold swing (74mV/decade). Although a slight compromise in field effect mobility is incurred owing to the carrier transport mechanisms in confined regions, the benefits of carrier confinement in the low potential well far outweigh its detriments to emerge as a promising method for improving device performance.
Zinc magnesium oxide (ZnMgO) thin films has emerged as a promising material for optoelectronic applications in last decade. Its high electron mobility makes it a good candidate for thin film transistors applications used in active matrix of LCDs (AMLCDs). ZnMgO thin film have inherent n-type conductivity due to oxygen vacancies, oxygen interstitials and zinc vacancies. For thin film transistors (TFTs), control of doping and defects is very important to maintain carrier concentration and proper threshold voltage of device. Threshold voltage of device is greatly influenced by carrier concentrations and stoichiometry of the film. So defects need to be minimized/controlled for achieving best device performances. These defects can be controlled using reactive oxygen supplied by UV-Ozone treatment by varying the rate and time of oxygen supply. In this study we report effect of UV-Ozone treatment on bottom gate enhancement type ZnMgO TFT using air as source of oxygen for Ozone. ZnMgO thin film was deposited using RF sputter technique using ZnMgO target. Titanium (Ti)/Gold (Au) was deposited to fabricate source and drain contacts. The fabricated TFT was characterized for input characteristics. Post UVO treatment, an increase of two times in drain current is measured along with a decrease in the threshold voltage by 5.8V. The On-OFF (ION/IOFF) ratio for as-deposited film was 7.5 × 103 which increased to 2 × 104 for UVO annealed TFT.
Over the past few years, zinc oxide nanorods (ZnO NRs) have started emerging as a promising candidate in the area of optoelectronics and various sensor applications due to their structural advantages over the thin film. Enhancing near band edge emission (NBE) with suppression in the defect state emission (DBE) is a challenging problem for utilizing hydrothermally grown ZnO NRs in device application. In this work, we are reporting improvement in NBE and decrement in the DBE peak intensities with post growth UV-Ozone (UV-O) treatment. Hydrothermal bath process was used to fabricate nanorods on the annealed ZnO seed layer followed by UV-O treatment for 20 minutes. Growth of the nanorods was confirmed using field emission gun scanning electron microscopy (FEGSEM). Room temperature photoluminescence (PL) spectra of sample B shows 1.6 times enhancement of NBE/DBE ratio as compared to as grown sample A. Reduction in the oxygen vacancies was confirmed using high resolution x-ray photoelectron spectroscopy (HRXPS), where it was observed to reduce from 25% for as-deposited sample to 7% for UVO annealed sample, leading to increase of NBE/DBE ratio, as observed from PL spectra. High resolution x-ray diffraction (HRXRD) pattern exhibited dominant (002) peak from both samples. A slight right shift was observed in HRXRD peak which suggest improvement in stoichiometric ratio for UV-O treated sample.
Owing to wide band gap and high exciton energy of ZnMgO can be used in UV based applications like laser diodes, LED and as transparent conducting oxide in solar cells. In present work, we study the effect of UVOzone (UVO) annealing on RF sputtered ZnMgO thin films. Here, we have deposited ZnMgO thin films on Si <100< substrate followed by UVO annealing treatment for 30 min. The as-deposited and UVO treated films were characterized using various optical, structural and elemental characterization techniques and compared with as-deposited ZnMgO thin films. Room Temperature PL results for as-deposited film exhibited dominant defects band emission (DBE) peak intensities in visible region with negligible emission from near band emission (NBE) peak. An increase in NBE emission peak at around ~ 360 nm (3.44eV) with minimal defect states emission was observed for UVO treated sample. NBE to DBE Intensity ratio (INBE/IDefect) of 1.7 was observed from sample B which was almost zero for as-grown film. High resolution X-ray diffraction (HRXRD) results exhibited (002), (220) and (311) crystal orientation peak in as-deposited sample. Post UVO (002) peak shifted to higher angle side. X-ray photoelectron spectroscopy results of Zn-2p and O-1s shows increase in metal-oxide bonds and decrease in oxygen vacancies. Atomic force microscopy results show increase in film roughness from 2.3 to 2.46 nm for as-deposited and UVO treated sample respectively. Authors would like to acknowledge IITBNF for all its facilities at IIT Bombay.
UV photodetector have been successfully implemented in various applications like ozone sensing, communication, astronomy and flame detection etc. Zn(Mg)O is a wide band gap material with high excitonic binding energy at room temperature thus making it a promising material in optoelectronic industries. In the present work, we are achieving increased photocurrent and high responsivity in UV-A regime with post growth UV-ozone treatment from photodetector fabricated using RF sputtered ZnMgO thin film. The thin film was deposited on semi-insulating silicon wafer at 400C for 20 min followed by 70 min UV-Ozone treatment. The final step was to fabricate an interdigitated electrode on the processed samples. More than two times enhancement of photocurrent was observed after UVO treatment. Noteworthy responsivity values of 22 A/W and 67 A/W were measured from as-deposited and UVO annealed photodetector, respectively at 380 nm with an applied bias of -5 V bias. However, the measured detectivity values for as grown and UV-O annealed sample was 1.3 × 1013 and 2.7 × 1013, respectively. Noise equivalent power in as-deposited sample and UVO treated sample was estimated to be 2.4 × 10-12 W/√Hz and 1.1 × 10-12 W/√Hz respectively. Photo detector fabricated with UV-O annealing exhibited good switching behaviors with 37 ms and 30 ms rise and fall time, respectively.
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