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Nano-buffer controlled electron tunneling to regulate heterojunctional interface emission
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Abstract
Interface emission from heterojunction is a shortcoming for electroluminescent devices. A buffer layer introduced in the heterojunctional interfaces is a potential solution for the challenge. However, the dynamics for carrier tunneling to control the interface emission is still a mystery. Herein, the low-refractive HfO2 with a proper energy band configuration is employed as the buffer layer in achieving ZnO-microwire/HfO2/GaN heterojunctional light-emitting diodes (LEDs). The optically pumped lasing threshold and lifetime of the ZnO microwire are reduced with the introduced HfO2 layer. As a result, the interface emission is of blue-shift from visible wavelengths to 394 nm whereas the ultraviolet (UV) emission is enhanced. To regulate the interface recombination between electrons in the conduction band of ZnO and holes in the valence band of GaN, the tunneling electrons with higher conduction band are employed to produce a higher tunneling current through regulation of thin HfO2 film causing blue shift and interface emission enhancement. Our results provide a method to control the tunneling electrons in heterojunction for high-performance LEDs. -
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References
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Figure 1.
The diagram of device fabrication.
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Figure 2.
(a) A schematic diagram for ZnO/GaN LEDs. (b) The Gaussian decomposition of EL spectra for LED and the inset depicts the PL spectra of the GaN film as well as ZnO MWs. (c−f) The height of the step-like HfO2 film with different sputtering time (c) 2 min, (d) 3 min, (e) 5 min and (f) 7 min. (g, h) AFM image for the GaN surface (g) before and (h) after bonding HfO2 film with thickness of 5.03 nm.
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Figure 3.
(a) Schematic diagram of ZnO/HfO2/GaN for in-situ optical test. (b) Lasing emission intensity for ZnO MW versus the excitation power density on different substrates. (c−f) the corresponding PL spectra under different excitation power densities for ZnO MW with different thickness of HfO2 films: (c) 0 nm, (d) 5.03 nm, (e) 8.79 nm and (f) 12.55 nm.
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Figure 4.
The lifetime of ZnO MW on various substrates with excitation power of 8 μW.
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Figure 5.
(a) I-V characteristics of ITO/ZnO/HfO2/GaN LEDs. The inset is a schematic diagram for ITO/ZnO/HfO2/GaN LEDs. (b) Normalized EL spectra for LEDs under an excitation current of 1 mA. (c−f) EL intensity of ZnO/HfO2/GaN LEDs with HfO2 films of different thickness and the inset is the light pictures: (c) 0 nm, (d) 5.03 nm, (e) 8.79 nm, (f) 12.55 nm. (g) Chromaticity coordinates of the spectra in (b). (h) EL peak intensities of the LEDs from (c) to (f).
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Figure 6.
Gaussian conversion of Fig. 5(c)−5(f) at a current of 1 mA: (a) 0 nm, (b) 5.03 nm, (c) 8.79 nm, (d) 12.55 nm, and the inset images depict the contents of UV and visible lights. (e) The peak positions of emissions and (f) FWHM of emissions for these LEDs.
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Figure 7.
(a) Energy band of ZnO/HfO2/GaN LEDs based on theoretical simulation. (b) The distribution of electronical current density in ZnO/HfO2/GaN LEDs. The inset picture displays a structure diagram of simulation.
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Figure 8.
Schematic band structures of (a) ZnO/GaN, (b) ZnO /thin HfO2/ GaN and (c) ZnO /thick HfO2/ GaN.
