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Realizing a SnO2-based ultraviolet light-emitting diode via breaking the dipole-forbidden rule

Yongfeng Li, Wanjian Yin, Rui Deng, Rui Chen, Jing Chen, Qingyu Yan, Bin Yao, Handong Sun, Su-Huai Wei and Tom Wu

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Figure 1 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Figure 1.

Conceptual illustration of breaking of the dipole-forbidden transition rule in SnO2 surfaces and nanocrystals. (a) Band structure and the corresponding optical absorption spectra of bulk SnO2 calculated using generalized gradient approximation. (b) Crystal structure (left), partial charge density mappings of valence-band maximum (VBM) (middle) and conduction-band minimum (CBM) (right) of the SnO2 (101) surface. (c) Band structure and the corresponding optical absorption spectrum of the SnO2 (101) surface. (d) Partial charge densities of lowest unoccupied molecular orbital (left) and highest occupied molecular orbital (right) of a quantum dot (QD) with diameter of 1.5nm. (e) Energy levels and the corresponding optical absorption spectrum of SnO2 QD. For the ideal bulk SnO2 with infinite dimensions, the absence of optical transition between VBM and CBM (corresponding to the fundamental bandgap) indicates that the band-edge light emission is dipole forbidden as a result of the symmetry of the band-edge wavefunctions. For the (101) surface and the QD with defects, the fundamental bandgap becomes dipole transition allowed, so the optical gap is largely reduced. As a consequence, the dipole-forbidden transition rule breaks down in SnO2 surfaces and QDs.

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Figure 2.

Structural characterizations of nanostructure-engineered SnO2 thin films. (a) X-ray diffraction patterns of as-grown, 400, 600 and 800°C annealed SnO2 thin films. The appearance of (110), (101) and (211) diffraction peaks after annealing indicates that the as-grown amorphous SnO2 thin films have been gradually crystallized. The reference powder diffraction data is also presented. (b) Raman spectra of the as-grown and the annealed SnO2 thin films. The A1g mode is enhanced and shifts to higher wave numbers with the increasing annealing temperature, suggesting enhanced crystallization. (c) High-resolution transmission electron microscope image of the SnO2 film annealed at 400°C, showing nanocrystals embedded in the amorphous SnO2 matrix. (d) Magnified image of the nanocrystal marked in (c). (e) Corresponding selected area electron diffraction pattern taken on the 400°C annealed sample.

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Figure 3.

Optical properties of nanostructure-engineered SnO2 thin films. (a) Room temperature (RT) optical absorption spectra of the SnO2 thin films. A strong absorption tail was observed in the 400°C annealed sample. With increasing annealing temperatures, the absorption edges significantly blueshift and approach the bandgap of bulk SnO2 as a result of enhanced crystallization. As a reference sample, the bulk-like crystalline SnO2 film was deposited directly at 600°C. (b) RT photoluminescence (PL) spectra of the SnO2 thin films. The ultraviolet (UV) emission is much sharper in the 400°C annealed samples compared with the others. As the annealing temperature increases, the UV emission becomes weaker, and eventually it is replaced by a broad band in the visible region, which is characteristic of the bulk SnO2. The inset shows the PL spectrum with fine structures of the 400°C annealed sample measured at 10K.

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Figure 4.

Performance of SnO2-based heterojunction light-emitting diodes (LEDs). (a) IV curves of the SnO2/p-GaN and the SnO2/MgO/p-GaN heterojunctions. The turn-on voltages are about 6 and 9V for SnO2/p-GaN and SnO2/MgO/p-GaN, respectively. Inset in (a) shows the ohmic behavior of the n-n and the p-p contacts in the LED device. (b) Normalized room temperature (RT) photoluminescence spectra of the p-GaN substrate, the SnO2/p-GaN and the SnO2/MgO/p-GaN heterojunctions. (c and d) RT electroluminescence (EL) spectra of the SnO2/p-GaN and the SnO2/MgO/p-GaN heterojunction LEDs under various forward excitation currents. Inset in (d) shows the photograph of the SnO2/MgO/p-GaN LEDs biased under a forward current of 6mA. (e and f) Schematics illustrating the band alignment of the SnO2/p-GaN and the SnO2/MgO/p-GaN heterojunctions under a forward bias. In the SnO2/p-GaN junction, the charge leakage makes the GaN-related emission dominate the spectra. On the other hand, in the SnO2/MgO/p-GaN junction, electrons in the SnO2 layer are blocked by the high barrier at the SnO2/MgO interface, giving rise to the efficient SnO2-related EL in the ultraviolet regime.

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