In this study, the growth and properties of LiAlO2 material and a nonpolar GaN-based light-emitting-diode (LED) structure on LiAlO2 have been investigated. The LiAlO2 material is grown by the Czochralski pulling technique and is used as a substrate for nonpolar nitride growth. An improved surface roughness can be obtained by a four-step polishing process. With subsequent nitridation treatment, a pure M-plane (101̱0) GaN can be obtained. An electron microscope shows an abundance of cracks that are oriented parallel to the (001) and (100) planes of the LiAlO2 substrate on the rear surface of GaN. The absence of the polarization-induced electric field of a GaN-based LED structure on LiAlO2 was shown by using photoluminescence measurements. Therefore, this approach is promising to further increase the luminescence performance of GaN-based LEDs.
The optical properties of InxAlyGa1−x−yN quaternary alloys were investigated by photoconductivity (PC), persistent photoconductivity (PPC), photoluminescence (PL), and photoluminescence excitation (PLE) measurements. Quite interestingly, persistent photoconductivity was observed. Through the combination of our optical studies, we show that the PPC effect arises from composition fluctuations in InxAlyGa1−x−yN quaternary alloys. From the analysis of the decay kinetics, the localization depth caused by composition fluctuations was determined. A comparison between the PL, PLE, and PC measurements gives a direct access to the Stokes’ shift. The Stokes’ shift can be explained in terms of localization due to the existence of nanoscale clusters, and it is consistent with the PPC result. The results shown here provide concrete evidence to support our previously proposed model that the existence of InGaN-like clusters is responsible for the strong luminescence in InxAlyGa1−x−yN quaternary alloys.
Magnetic-field-induced phase transitions were studied with a two-dimensional electron Al-GaAs/GaAs system. The temperature-driven flow diagram shows the features of the Γ(2) modular symmetry, which includes distorted flowlines and shiftted critical point. The deviation of the critical conductivities is attributed to a small but resolved spin splitting, which reduces the symmetry in Landau quantization. [B. P. Dolan, Phys. Rev. B 62, 10278.] Universal scaling is found under the reduction of the modular symmetry. It is also shown that the Hall conductivity could still be governed by the scaling law when the semicircle law and the scaling on the longitudinal conductivity are invalid. *corresponding author:yhchang@phys.ntu.edu.tw PACS numbers: 1 Magnetic-field-induced phase transitions in two-dimensional electron systems (2DESs) have been an active research topic since the discovery of the quantum Hall effect. [1-18] The law of corresponding states proposed by Kivelson, Lee, and Zhang (KLZ) [2], which was based on the effective field Maxwell-Chern-Simon theory, provides a powerful method for classifying quantum Hall states and the transitions between them. According to the law of corresponding states, all the magnetic-field-induced phase transitions are of an equivalent class. In the integer quantum Hall effect (IQHE), the equivalence is established by the Landau-level addition transformation [1,2]. Magnetic-field-induced phase transitions are believed to be good examples of quantum phase transitions. [1,19] Universal properties suchas the reflection symmetry [3], universality of critical conductivities [1,4], and the universal scaling with same critical exponent [5,6] are expected and in addition, it is also expected that the temperature-driven flow lines [1,7,8] are governed by the semicircle law.Because of the existence of the law of corresponding states, the phase diagram of the QHE has a symmetry equivalent to the Γ 0 (2) symmetry group, which is a subgroup of the modular group. [8][9][10][20][21][22][23][24] The universal properties mentioned above can be taken as the manifestations of the Γ 0 (2) modular symmetry. [8-10] However, this symmetry relies on the assumption that all the Landau bands are equally spaced in energy, a condition satisfied
The fabrication and properties of n-ZnO nanowires/p-CuO coaxial heterojunction (CH) with a photoresist (PR) blocking layer are reported. In our study, c-plane wurtzite ZnO nanowires were grown by aqueous chemical method, and monoclinic CuO (111) was then coated on the ZnO nanowires by electrochemical deposition to form CH. To improve the device performance, a PR layer was inserted between the ZnO buffer layer and the CuO film to serve as a blocking layer to block the leakage current. Structural investigations of the CH indicate that the sample has good crystalline quality. It was found that our refined structure possesses a better rectifying ratio and smaller reverse leakage current. As there is a large on/off ratio between light on and off and the major light response is centered at around 424 nm, the experimental results suggest that the PR-inserted ZnO/CuO CH can be used as a good narrow-band blue light detector.
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