Haoning Wang

Improved and color tunable electroluminescence from n-ZnO/HfO2/p-GaN heterojunction light emitting diodes

n-ZnO/HfO2/p-GaN based heterojunction light emitting diodes were fabricated using a radio frequency magnetron sputtering system. The electroluminescence measurements revealed that dominant violet emissions centered at around 415 nm were emitted and improved performances were observed for the devices with the HfO2 intermediate layer; the color of the devices could be tuned from violet (0.18, 0.10) to cold white (0.22, 0.20) by varying the Ar/O2 flow ratio during the deposition of HfO2, which are probably ascribed to the deep level emission bands in ZnO. The results were studied by peak-deconvolution with Gaussian functions and were discussed in terms of band diagram of the heterojunctions.

A ZnO/ZnMgO Multiple-Quantum-Well Ultraviolet Random Laser Diode

A ZnO/ZnMgO multiple-quantum-well ultraviolet (UV) random laser diode was fabricated on a commercially available n-type GaN wafer using a radio frequency magnetron sputtering system. The electroluminescence measurements revealed that the diode exhibited fairly pure UV random lasing centered at ~370 nm under sufficient forward bias at room temperature. The full-widths at half-maximum of the sharp lasing peaks are less than 0.4 nm. The device has a very low threshold current density of 4.7 A/cm2 and extremely weak visible emission.

Improved Subthreshold Swing and Gate-Bias Stressing Stability of p-Type

p-Type Cu₂O thin films and HfO₂ high-<i>k</i> gate dielectrics are deposited by pulsed laser ablation. p-Type Cu₂O metal-oxide-semiconductor capacitors and thin-film transistors (TFTs) are then fabricated and investigated. Experimental results show that a HfO₂/SiO₂-stacked gate dielectric can effectively improve interface properties and decrease gate-leakage current when compared with a SiO₂ gate dielectric. Thus, increased mobility, a decreased subthreshold swing, and enhanced gate-bias-voltage stressing stability have been achieved for the relevant Cu₂O TFTs. Bottom-gate and top-source/drain-contact p-channel Cu₂O TFTs (<i>W</i>/<i>L</i>= 500/20 μm) with the HfO₂/SiO₂-stacked gate dielectric exhibit superior performance with a saturation-carrier-mobility value of 2.7 cm²/V·s, an on-off current ratio of 1.5×10⁶, a subthreshold swing of 137 mV/dec, and a threshold-voltage shift of 1.4 V after gate-bias stress at 10 V for 3600 s.

\hbox{Cu}_{2}\hbox{O}

Efficient organic solar cells (OSCs) based on regioregular of poly (3-hexylthiophene):fullerene derivative [6,6]-phenyl-C61butyric acid methyl ester composites have been fabricated on indium tin oxide (ITO) coated glass substrates by using a sputtered sulfur-doped molybdenum oxide (S-MoO3) film as anode interface layer (AIL). With the help of X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy, we find that oxygen flow ratio control can modulate the amount of sulfur doping into MoO3, then further tune the Mo(+4)/Mo(+5)/Mo(+6) composition ratios, Fermi level, electron affinity, valence band ionization energy and band gap of MoO3. A partially occupied Mo 4d-bands of Mo(5+) and Mo(4+) states modulated by sulfur doping are the main factor which influences the valence electronic structure of S-MoO3.These orbitals overlap interrelation push the valence band close to S-MoO3s Fermi level, thus make it into a p-type semiconductor. S-MoO3 with smaller ionization energy and electron affinity is better suitable as an efficient AIL. On the basis of these AILs, a photovoltaic power conversion efficiency up to 3.69% has been achieved, which is 12% higher than that in pure MoO3 AIL case. The result thus shows that sulfur doping is a useful method to modify anode interface layer for improving the hole-transport properties of MoO3, which can improve the device performances.

Thin-Film Transistors Using a

n-ZnO/p-Si heterojunction light emitting devices with and without a NiO intermediate layer were fabricated using a radio frequency magnetron sputtering system. Electroluminescence measurements revealed that the device with the NiO intermediate layer exhibits a sharper and stronger orange emission peaks at ∼670 nm compared with that of the device without the NiO layer. And the light output-current characteristic of the n-ZnO/NiO/p-Si heterojunction device follows a nearly linear relationship (L ∝ I) rather than the superlinear relationship (L ∝ I1.5) for the n-ZnO/p-Si heterojunction device. This work indicates that the NiO intermediate layer could effectively improve the performance of the n-ZnO/p-Si heterojunction light emitting device.

\hbox{HfO}_{2}

In this paper, layered ZnO nanowall networks were directly grown on Al substrates using a hydrothermal method without predepositing seed layers. The individual ZnO nanowalls with a thickness of several nanometers and a size of several hundred nanometers were (002) surface dominated, in which the preferential growth direction of ZnO was suppressed. White electroluminescence devices were fabricated based on Au/polymethylmethacrylate/ZnO-nanowall (metal-insulator-semiconductor) structures. The chromaticity coordinate of the electroluminescence spectrum for the optimal device was calculated as (0.27, 0.34), which is close to (0.33, 0.33) of standard white light.

High-

We fabricate an ultraviolet (UV)/orange bicolor light emitting diode (LED) based on an n-ZnO/n-GaN isotype heterojunction, which presents a sharp ultraviolet emission centered at 367 nm and a broad orange emission centered at 640 nm under forward and reverse biases, respectively. Time dependence electroluminescence (EL) measurements reveal that this device shows good stability. The electroluminescence mechanism of the bicolor light emitting diode is discussed in terms of the material properties of the interfacial layer and the luminescence properties of the device in this work.

k

Light-emitting diodes (LEDs) with MgZnO/ZnO/MgZnO double heterojunction structure have been fabricated and the room temperature electroluminescence (EL) spectra have been studied. With the help of double heterostructure, LEDs show better visible EL performance than that of LED with ordinary p-i-n structure. By replacing ZnO film with ZnO nanorod arrays in this double heterostructure, strong ultraviolet EL emission around 380 nm was achieved. The ZnO-nanorod-based double heterostructured light-emitting diode exhibits superior stability with an intensity degradation of less than 3% over 8 h. The EL mechanisms were discussed in terms of carrier confinement and carrier transport based on semiconductor heterojunction theory.

Gate Dielectric Grown on a

We demonstrated the capability of realizing enhanced ZnO-related UV emissions by using the low-cost and solution-processable ZnO quantum dots (QDs) with the help of a high-k HfO2 electron blocking layer (EBL) for the ZnO QDs/p-GaN light-emitting diodes (LEDs). Full-width at half maximum of the LED devices was greatly decreased from ∼110 to ∼54 nm, and recombinations related to nonradiative centers were significantly suppressed with inserting HfO2 EBL. The electroluminescence of the ZnO QDs/HfO2/p-GaN LEDs demonstrated an interesting spectral narrowing effect with increasing HfO2 thickness. The Gaussian fitting revealed that the great enhancement of the Zni-related emission at ∼414 nm whereas the deep suppression of the interfacial recombination at ∼477 nm should be the main reason for the spectral narrowing effect.

\hbox{SiO}_{2}/\hbox{Si}

Diodes based on NiO/GaN and NiO/MgO/GaN heterojunctions were fabricated on commercially available n-GaN/u-GaN/Sapphire substrates using a radio frequency magnetron sputtering system. Electroluminescence (EL) measurements revealed that the diode with the MgO layer exhibited fairly pure ultraviolet emission peaking at ~370 nm with a full-width at half maximum of about 9 nm, while no EL was detected from the diode without the MgO layer. By choosing a proper thickness of the insulator MgO layer, the EL performance of the devices could be greatly improved. The results were discussed in terms of the I–V characteristics and the band diagrams of the heterojunctions.