Cyan electroluminescence from n-ZnO/ i -CdZnO/ p -Si heterojunction diodes
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1201-H01-09
Cyan electroluminescence from n-ZnO/i-CdZnO/p-Si heterojunction diodes Lin Li, Zheng Yang and Jianlin Liu Quantum Structures Laboratory, Department of Electrical Engineering, University of California, Riverside, California 92521, USA ABSTRACT n-ZnO/i-CdZnO thin film was grown on p-type Si substrate by plasma-assisted molecular-beam epitaxy (MBE). Rectifying I-V curves show typical diode characteristics. Cyan electroluminescence emissions at around 473 nm were observed when the diodes were forwardbiased at room temperature. The emission intensity increases with the increase of the injection current. Room temperature photoluminescence verifies the electroluminescence emissions come from CdZnO layer. INTRODUCTION ZnO is a promising optoelectronic material for light emitting devices in ultraviolet and visible wavelength [1] because of its large wide direct band gap of 3.3 eV at room temperature and large exciton binding energy of 60meV. The bandgap of ZnO can be extended wider by alloying with MgO [2], while it can also be adjusted narrower to 1.8 eV by alloying with CdO [3-6]. This property makes ZnO based light sources emitting light from ultraviolet region to green band or even further. However, reliable p-type ZnO with high carrier concentration has always been an issue for ZnO light emitting devices application. Here, we demonstrate ZnO based double heterojunction LED devices using p-type Si as p-layer, with inserted CdZnO as active layer. Visible cyan emissions were observed and studied. EXPERIMENT ZnO based heterojunction diodes were grown by plasma-assisted MBE on p-type (1– 10Ωcm) Si (100) substrates. Elemental Zn (6N), Cd (6N), Ga (6N) were heated by effusion cells and used as metal sources. Oxygen (5N) plasma generated by a radiofrequency plasma generator was used as the oxygen source. Fig. 1 shows the structure of the sample. A 100 nm CdZnO layer was first deposited at 150 ˚C on p-type Si(100) substrate, followed by 800 ˚C in situ annealing for 5 min under vacuum, then a 350 nm Ga-doped ZnO layer was deposited at 500 ˚C as n-type layer. Room temperature (RT) PL measurements were carried out using a home-built PL system, with a 325-nm He-Cd laser as excitation source and a photomultiplier tube behind the monochrometer as detector. Samples were etched by diluted hydrochloride acid to different depth to investigate the PL emission from different layers. Heterojunction diodes were fabricated by standard photolithography techniques. Etching was done using diluted hydrochloride acid to reach down to the substrate. Mesas with size of 800µm×800µm were formed on the samples. Metal contacts were deposited by E-beam evaporator. Au/Ti contacts with thicknesses of 200/10nm were used for contacts of both Ga-doped ZnO and p-type Si. The contacts were subjected to rapid thermal annealing (RTA) under nitrogen ambient at 600 ˚C for 60 seconds to form ohmic contacts. Current-voltage (I-V) characteristics were measured using Agilent 4155C
semiconductor parameter analyzer. EL measurements were carried out in the s
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