Zinc oxide (ZnO) is a strong luminescent material, as are several polymers. These two materials have distinct drawbacks and advantages, and they can be combined to form nanostructures with many important applications, e.g., large-area white lighting. This paper discusses the origin of visible emission centers in ZnO nanorods grown with different approaches. White light emitting diodes (LEDs) were fabricated by combining n- ZnO nanorods and hollow nanotubes with different p-type materials to form heterojunctions. The p-type component of the hybrids includes p-SiC, p-GaN, and polymers. We conclude by analyzing the electroluminescence of the different light emitting diodes we fabricated. The observed optical, electrical, and electro-optical characteristics of these LEDs are discussed with an emphasis on the deep level centers that cause the emission.

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Luminescence from Zinc Oxide Nanostructures and Polymers and their Hybrid Devices

We report on the fabrication of zinc oxide (ZnO) nanorods on p-type silicon (p-Si) photodiodes. The nanorods are prepared by low-temperature hydrothermal processing. The fabricated photodiodes exhibit an excellent rectifying ratio of 370 at 10 V. The responsivity to ultraviolet (UV) photons is stable at 0.29 A/W up to 300 nm, with a peak value of 0.38 A/W at 360 nm. Furthermore, the prepared photodiodes demonstrate visible blind behavior, indicating that ZnO nanorods grown on p-Si substrates can be used as UV photodiodes with visible blind responses.

A wide-band UV photodiode based on n-ZnO/p-Si heterojunctions

There is no signature of the trap filled limit voltage (VTFL) in the J‐V characteristics of a sample containing exponentially distributed traps. We show that VTFL and the voltage at which V2 dependence sets in (VMott) can be determined accurately. These voltages are independent of the energy distributions of the traps and depend strongly on the trap density Hb. Contrary to the literature results, it turns out that VTFL is significantly smaller than the VMott. In a specific case with Hb=1.6×1018cm−3 and for a 5% accuracy in the current the value of VMott is about 400V whereas VTFL is 13.5V. Universal J‐V curves in reduced units are derived and plotted. The reduced value of VTFL is 0.5. These curves are valid for all organics and inorganic semiconductors and for all energy distributions of traps. It is shown mathematically that all J‐V curves approached Mott’s V2 law asymptotically as V increases to infinity. To validate the theory, the experimental J‐V curves in polycrystalline undoped and Al doped ZnO thin films are made. The experimental results show good agreement with the theory. In the undoped ZnO films the traps are exponentially distributed and the trap concentration is calculated to be 1.7×1017cm−3. The trap distribution in Al doped ZnO films is found to be discrete at a single level with trap concentration of 8×1016cm−3.

Trap filled limit voltage (VTFL) and V2 law in space charge limited currents

ZnO nanorods with excellent optical and electro-optical emission characteristics were grown using high- and low-temperature techniques on solid and soft substrate materials. The solid crystalline substrates included p-4H-SiC and p-GaN, while the soft amorphous substrates included p-type polymers deposited on glass and flexible plastic. Two different growth approaches were used to produce these samples. We used the vapor–liquid–solid (VLS) technique (high temperature) and aqueous chemical growth (ACG), which is a low-temperature technique. These ZnO nanorod samples were characterized by room temperature photoluminescence (PL) and processed to fabricate light-emitting diodes (LEDs). The LED characteristics were further investigated by I–V and electroluminescence (EL). As observed by PL measurements, all samples revealed a sharp narrow ultraviolet (UV) peak due to band-edge emission, indicating the good crystalline quality of the grown ZnO nanorods. The origin of the different peaks within the visible region was correlated to different deep level defects reported earlier for ZnO. All fabricated LEDs showed EL providing a wide band extended through the whole visible spectrum and hence produced clear white light observable to the naked eye. The emitted color quality investigation showed that superior color quality was manifested in a high color rendering index and stable color under current variation, indicating that these heterojunction and hybrid LEDs have potential for the development of future light sources. The ZnO nanorod-based LEDs grown by low-temperature ACG on glass and flexible plastic can, after further development, be candidates for future large-area white-light sources.

/pdf/zinc-oxide-nanorod-based-heterostructures-on-solid-and-soft-uiwd79hm4n.pdf

Zinc oxide nanorod-based heterostructures on solid and soft substrates for white-light-emitting diode applications

This work investigates the effects of postannealing on the electrical properties of the n-ZnO nanorods/p-Si heterojunction. Well-aligned ZnO nanorods are grown on the p-Si substrate with a ZnO seed layer through a hydrothermal method. The as-grown ZnO nanorods are grown along the preferential [0001] direction with high single crystalline quality. The rectifying characteristics of the heterojunction are effectively improved and the rectification ratio is increased tens of times after annealing in air. The leakage current is decreased by two orders of magnitude and the forward-bias injection efficiency is largely enhanced after annealing. The photoluminescence and x-ray photoelectron spectroscopy results show that annealing in the air ambient can effectively reduce the loosely adsorbed oxygen on the surface of the ZnO nanorods so that the bound free electrons can be released, which can result in larger carrier concentration and improve the injection efficiency of the n-ZnO nanorods/p-Si structure in the for...

The effect of postannealing on the electrical properties of well-aligned n-ZnO nanorods/p-Si heterojunction

Rectifying undoped and nitrogen-doped ZnO∕p-Si heterojunctions were fabricated by plasma immersion ion implantation and deposition. The undoped and nitrogen-doped ZnO films were n type (n∼1019cm−3) and highly resistive (resistivity ∼105Ωcm), respectively. While forward biasing the undoped-ZnO∕p-Si, the current follows Ohmic behavior if the applied bias Vforward is larger than ∼0.4V. However, for the nitrogen-doped-ZnO∕p-Si sample, the current is Ohmic for Vforward 2.5V. The transport properties of the undoped-ZnO∕p-Si and the N-doped-ZnO∕p-Si diodes were explained in terms of the Anderson model and the space charge limited current model, respectively.

C. C. Ling

Papers

Current transport studies of ZnO/p-Si heterostructures grown by plasma immersion ion implantation and deposition