Highly c-axis oriented zinc oxide (ZnO) thin films with a wurtzite structure have been grown on glass substrates by metal organic chemical vapor deposition. The influence of growth parameters on the luminescent properties of the ZnO layers is investigated. It is shown that the growth temperature and VI/II ratio strongly influence the luminescent properties of ZnO thin films. For the films grown at low temperatures (250–325°C) a broad violet emission band at about 3.1eV has been observed. As the growth temperature increases, ultraviolet emission dominates the spectra and deep level emission is suppressed. It is suggested that the violet emission depends on grain size and hence the width of the free-carrier depletion region at the particle surface; the narrower the depletion width compared to the grain size, the higher the intensity. The result suggests that the mechanism responsible for the violet emission is recombination of delocalized electrons close to the conduction band with holes trapped in neutral zinc vacancy (VZn0) centers. For films grown under different VI/II ratios, the spectra are increasingly dominated by deep level emission as the VI/II ratio is increased. This broad emission is resolved into three bands at ∼2.0, ∼2.3, and ∼2.5eV. Based on the growth conditions, possible origins are proposed and discussed.
The electrical properties of zinc oxide (ZnO) thin films of various thicknesses (0.3-4.4 µm) grown by metalorganic chemical vapour deposition on glass substrates have been studied by using temperature-dependent Hall-effect (TDH) measurements in the 18-300 K range. The high quality of the layers has been confirmed with x-ray diffraction, transmission electron microscopy, scanning electron microscopy and photoluminescence techniques. TDH measurements indicate the presence of a degenerate layer which significantly influences the low-temperature data. It is found that the measured mobility generally increases with increasing layer thickness, reaching a value of 120 cm 2 V −1 s −1 at room temperature for the 4.4 µm thick sample. The lateral grain size of the layers is also found to increase with thickness indicating a clear correlation between the size of the surface grains and the electrical properties of corresponding films. Theoretical fits to the Hall data suggest that the bulk conduction of the layers is dominated by a weakly compensated donor with activation energy in the 33-41 meV range and concentration of the order of 10 17 cm −3 , as well as a total acceptor concentration of mid-10 15 cm −3 . Grain boundary scattering is found to be an important limiting factor of the mobility throughout the temperature range considered.
Micron-scale mapping has been employed to study a contacted InGaN/GaN LED using combined electroluminescence (EL), cathodoluminescence (CL), and electron beam induced current (EBIC). Correlations between parameters, such as the EBIC and CL intensity, were studied as a function of applied bias. The CL and EBIC maps reveal small areas, 2–10 μm in size, which have increased nonradiative recombination rate and/or a lower conductivity. The CL emission from these spots is blue shifted, by 30–40 meV. Increasing the reverse bias causes the size of the spots to decrease, due to competition between in-plane diffusion and drift in the growth direction. EL mapping shows large bright areas (∼100 μm) which also have increased EBIC, indicating domains of increased conductivity in the p and/or n-GaN.
In this paper we present a combined cathodoluminescence and electron beam induced current study of the optical and electrical properties of InGaN LEDs grown using different active region growth methods. In one device, both the quantum wells and quantum barriers were deposited at their optimum temperatures (2T) whereas in the other device, each barrier was grown in a two step process, with the first few nanometers at a lower temperature (Q2T). It was found that, in the Q2T sample, small micron scale domains of lower emission intensity correlate strongly to a lower EBIC signal, whereas in the 2T sample which has a more uniform emission pattern and an anti-correlation exists between CL emission intensity and EBIC signal
Nanogap sensors have a wide range of applications as they can provide accurate direct detection of biomolecules through impedimetric or amperometric signals. Signal response from nanogap sensors is dependent on both the electrode spacing and surface area. However, creating large surface area nanogap sensors presents several challenges during fabrication. We show two different approaches to achieve both horizontal and vertical coplanar nanogap geometries. In the first method we use electron-beam lithography (EBL) to pattern an 11 mm long serpentine nanogap (215 nm) between two electrodes. For the second method we use inductively-coupled plasma (ICP) reactive ion etching (RIE) to create a channel in a silicon substrate, optically pattern a buried 1.0 mm × 1.5 mm electrode before anodically bonding a second identical electrode, patterned on glass, directly above. The devices have a wide range of applicability in different sensing techniques with the large area nanogaps presenting advantages over other devices of the same family. As a case study we explore the detection of peptide nucleic acid (PNA)−DNA binding events using dielectric spectroscopy with the horizontal coplanar device.
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