ZnO nanostructures for optoelectronics: Material properties and device applications

Zinc oxide (ZnO), with its excellent luminescent properties and the ease of growth of its nanostructures, holds promise for the development of photonic devices. The recent advances in growth of ZnO nanorods are discussed. Results from both low temperature and high temperature growth approaches are presented. The techniques which are presented include metal?organic chemical vapour deposition (MOCVD), vapour phase epitaxy (VPE), pulse laser deposition (PLD), vapour?liquid?solid (VLS), aqueous chemical growth (ACG) and finally the electrodeposition technique as an example of a selective growth approach. Results from structural as well as optical properties of a variety of ZnO nanorods are shown and analysed using different techniques, including high resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), photoluminescence (PL) and cathodoluminescence (CL), for both room temperature and for low temperature performance. These results indicate that the grown ZnO nanorods possess reproducible and interesting optical properties. Results on obtaining p-type doping in ZnO micro-?and nanorods are also demonstrated using PLD. Three independent indications were found for p-type conducting, phosphorus-doped ZnO nanorods: first, acceptor-related CL peaks, second, opposite transfer characteristics of back-gate field effect transistors using undoped and phosphorus doped wire channels, and finally, rectifying I?V characteristics of ZnO:P nanowire/ZnO:Ga p?n junctions. Then light emitting diodes (LEDs) based on n-ZnO nanorods combined with different technologies (hybrid technologies) are suggested and the recent electrical, as well as electro-optical, characteristics of these LEDs are shown and discussed. The hybrid LEDs reviewed and discussed here are mainly presented for two groups: those based on n-ZnO nanorods and p-type crystalline substrates, and those based on n-ZnO nanorods and p-type amorphous substrates. Promising electroluminescence characteristics aimed at the development of white LEDs are demonstrated. Although some of the presented LEDs show visible emission for applied biases in excess of 10 V, optimized structures are expected to provide the same emission at much lower voltage. Finally, lasing from ZnO nanorods is briefly reviewed. An example of a recent whispering gallery mode (WGM) lasing from ZnO is demonstrated as a way to enhance the stimulated emission from small size structures.

https://iopscience.iop.org/article/10.1088/0957-4484/20/33/332001/pdf

Zinc oxide nanorod based photonic devices: recent progress in growth, light emitting diodes and lasers.

ZnO is a material which is of great interest for a variety of applications due to its unique properties and the availability of a variety of growth methods resulting in a number of different morphologies and a wide range of material properties of synthesized nanostructures. In this review, we will discuss recent advances in important and/or controversial issues concerning ZnO properties and its applications. We will also discuss areas where further improvements are needed, and in particular discuss the issues related to the environmental stability of ZnO and its implications on reproducibility of measurements and the toxicity of ZnO nanomaterials.

ZnO nanostructures: growth, properties and applications

Nanomaterials and the environment: A review for the biennium 2008-2010

We demonstrate the piezoelectric effect on the responsivity of a metalsemiconductormetal ZnO micro-/nanowire photodetector. The responsivity of the photodetector is respectively enhanced by 530%, 190%, 9%, and 15% upon 4.1 pW, 120.0 pW, 4.1 nW, and 180.4 nW UV light illumination onto the wire by introducing a0.36% compressive strain in the wire, which effectively tuned the Schottky barrier height at the contact by the produced local piezopotential. After a systematic study on the Schottky barrier height change with tuning of the strain and the excitation light intensity, an in-depth understanding is provided about the physical mechanism of the coupling of piezoelectric, optical, and semiconducting properties. Our results show that the piezo-phototronic effect can enhance the detection sensitivity more than 5-fold for pW levels of light detection.

http://xugroup.eng.ucsd.edu/wp-content/uploads/2015/09/18_acs_nano.pdf

Enhancing sensitivity of a single ZnO micro-/nanowire photodetector by piezo-phototronic effect

The optical properties of high-quality GaN co-doped with silicon and zinc are investigated by using temperature-dependent continuous-wave and time-resolved photoluminescence measurements. The blue luminescence band is related to the ZnGa acceptor in GaN:Si,Zn, which exhibits an exceptionally high absolute internal quantum efficiency (IQE). An IQE above 90% was calculated for several samples having different concentrations of Zn. Accurate and reliable values of the IQE were obtained by using several approaches based on rate equations. The concentrations of the ZnGa acceptors and free electrons were also estimated from the photoluminescence measurements.

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Determination of the absolute internal quantum efficiency of photoluminescence in GaN co-doped with Si and Zn

Self-organized ZnO nanowires were grown by metal-organic chemical vapor deposition on sapphire substrate. Steady-state photoluminescence (PL) from the samples with different densities of the nanowires was studied in wide range of temperatures and excitation intensities. At 13K the PL spectrum consisted of sharp exciton lines at 3.354, 3.357, and 3.363eV and a weak red band with a maximum at ∼1.75eV. The peak intensity of the red band was four orders of magnitude lower than that of the strongest exciton line. The authors investigated also the effect of ambient on PL at room temperature. In vacuum the PL intensity increased linearly with the excitation power density in the range from 3×10−4to0.3W∕cm2, whereas in air the PL intensity increased superlinearly. Such behavior is attributed to photoinduced adsorption of species from air which increases the near-surface barrier and depletion region.

Photoluminescence from ZnO nanowires

Reliableandreproduciblep-typedopingisthemain challengeforfabricatinghighlyefficientZnO-basedlight-emitting diodes. During the last few years, the lack of reliable p-type conductivity in ZnO has initiated research concerning the combination of ZnO and other semiconductors, which can then bedopedp-type.OneoftheseconceptsisthecombinationofZnO with GaN heterostructures aiming at the fabrication of hybrid LEDs. We discuss the problems as well as potential benefits from a combinationof ZnO andGaN hybrid heterostructures in a single device. We also present our recent results on ZnO-GaN hybrid LEDs using an inverted LED concept. The hybrid LEDs have an external quantum efficiency of more than 35%.

ZnO-GaN Hybrid Heterostructures as Potential Cost-Efficient LED Technology Low-temperature and low-cost chemical growth, using a combination of nanorods and conducting material, was used to fabricate efficient ZnO-based light-emitting diodes.

Reliable and reproducible p-type doping is the main challenge for fabricating highly efficient ZnO-based light-emitting diodes. During the last few years, the lack of reliable p-type conductivity in ZnO has initiated research concerning the combination of ZnO and other semiconductors, which can then be doped p-type. One of these concepts is the combination of ZnO with GaN heterostructures aiming at the fabrication of hybrid LEDs. We discuss the problems as well as potential benefits from a combination of ZnO and GaN hybrid heterostructures in a single device. We also present our recent results on ZnO-GaN hybrid LEDs using an inverted LED concept. The hybrid LEDs have an external quantum efficiency of more than 35%.

Arne Behrends

Papers

Zinc oxide nanorod based photonic devices: recent progress in growth, light emitting diodes and lasers.

Determination of the absolute internal quantum efficiency of photoluminescence in GaN co-doped with Si and Zn

Photoluminescence from ZnO nanowires

ZnO-GaN Hybrid Heterostructures as Potential Cost-Efficient LED Technology Low-temperature and low-cost chemical growth, using a combination of nanorods and conducting material, was used to fabricate efficient ZnO-based light-emitting diodes.

ZnO-GaN Hybrid Heterostructures as Potential Cost-Efficient LED Technology