Room-temperature violet luminescence and ultraviolet photodetection of Sb-doped ZnO/Al-doped ZnO homojunction array
© Chen et al.; licensee Springer. 2013
Received: 6 May 2013
Accepted: 27 June 2013
Published: 5 July 2013
A Sb-doped ZnO microrod array was fabricated on an Al-doped ZnO thin film by electrodeposition. Strong violet luminescence, originated from free electron-to-acceptor level transitions, was identified by temperature-dependent photoluminescence measurements. This acceptor-related transition was attributed to substitution of Sb dopants for Zn sites, instead of O sites, to form a complex with two Zn vacancies (VZn), the SbZn-2VZn complex. This SbZn-2VZn complex has a lower formation energy and acts as a shallow acceptor which can induce the observed strong violet luminescence. The photoresponsivity of our ZnO p-n homojunction device under a negative bias demonstrated a nearly 40-fold current gain, illustrating that our device is potentially an excellent candidate for photodetector applications in the ultraviolet wavelength region.
ZnO has gained considerable attention as a material for short-wavelength optoelectronic devices, such as light-emitting diodes , photodetectors , and laser diodes , because of its large bandgap (3.37 eV) and exciton binding energy (60 meV) [4, 5]. As-grown ZnO is usually an n-type semiconductor because of the existence of oxygen vacancies. To enhance n-type conduction, Ga, In, or Sn can be used as extrinsic dopants. While n-doped ZnO can be readily prepared, it should be noted that p-type doping is essential for functional device applications based on ZnO. The p-type doping of ZnO is made using group V elements such as N, P, As, and Sb as dopants. Compared with n-type ZnO, the p-type ZnO is rather difficult to prepare due to the electronegative O 2p character of valence band maxima and the presence of n-type intrinsic defects, oxygen and Zn interstitial . Therefore, the fabrication of a durable and reproducible p-type ZnO-based nanostructure remains a challenging task.
The growth of ZnO nanorod arrays has been reported using different growth methods such as pulsed laser deposition , thermal evaporation , metal-organic vapor-phase epitaxy , physical vapor deposition into porous anodic aluminum templates , or template-assisted vapor-liquid-solid and hydrothermal synthesis . Recently, it was reported that ZnO nanocolumns can be grown by a low-temperature solution method , and arrays of vertical ZnO nanorods and nanowires grown by electrodeposition using anodic alumina  or polycarbonate porous membranes as templates have also been reported .
In this study, a Sb-doped ZnO microrod array was successfully grown on an Al-doped n-type ZnO thin film by electrodeposition. Strong violet luminescence, originated from free electron-to-acceptor level transitions, was identified by temperature-dependent photoluminescence measurements. This acceptor-related transition was attributed to substitution of Sb dopants for Zn sites, instead of O sites, to form a complex with two Zn vacancies (VZn), the SbZn-2VZn complex. This SbZn-2VZn complex has a lower formation energy and acts as a shallow acceptor which can induce a strong violet luminescence. ZnO homojunction diode made with Sb-doped ZnO and Al-doped ZnO exhibits the expected p-n diode characteristic on the current-voltage (I-V) measurement and confirms that the Sb-doped ZnO microrod array is p-type and can be fabricated successfully using the electrodeposition method. Finally, the photoresponse of the ZnO p-n diode operating at an increased reverse bias shows a good optical response and high photocurrent gain, indicating that it can be a good candidate for use as an ultraviolet photodetector.
A Sb-doped ZnO microrod array was electrodeposited on a patterned Al-doped n-type ZnO thin film at 98°C for 1 h . The array pattern was fabricated on the ZnO thin film by optical lithography method. The reaction solution for the growth of the microrod array was a mixture of 0.05 M zinc nitrate (Zn(NO3)2·6H2O), 0.05 M hexamethylenetetramine (C6H12N4), and 0.05 g antimony acetate (Sb(CH3COO)3). The conditions for electrodeposition were optimized at I = 10 mA and V = 3.1 V. Ohmic contacts on the n-type ZnO thin film and on the Sb-doped ZnO microrod array were fabricated by depositing aluminum and gold antimony (AuSb), respectively, for the electrical characterization. The surface morphology and the crystalline structure of the sample were examined using a scanning electron microscope (SEM; HITACHI S-2400, Chiyoda-ku, Japan) and by X-ray diffraction (XRD; PANalytical X'Pert PRO, Almelo, The Netherlands), respectively. The chemical and elemental identification on the surface was carried out by X-ray photoelectron spectroscopy (XPS). A He-Cd laser at the wavelength of 325 nm was used for the photoluminescence (PL) measurement. Keithley 236 and 4200-SCS (Cleveland, OH, USA) were used for the characteristic I-V measurement. To study the photoresponse of our ZnO homojunction device, we employed a xenon arc lamp (LHX150 08002, Glasgow, UK) as a variable-wavelength light source. The monochromatic light was selected using an iHR-320 monochromator (HORIBA Scientific, Albany, NY, USA) and irradiated on the sample. The beam line of 365 nm was selected in the photocurrent measurement.
Results and discussion
In this work, a high-quality Sb-doped ZnO microrod array was synthesized by electrodeposition. In Sb-doped ZnO, the shift of the XRD peak from that of the intrinsic ZnO was attributed to the increase of the lattice constant due to the replacement of a Zn atom by the Sb atom. In the case of the Sb-doped ZnO microrod array, the PL measurement indicated an acceptor-related photoemission. Strong violet luminescence at room temperature was observed since the Sb dopants would substitute Zn sites, instead of O sites, (SbZn) to form a complex with two VZn, which is the SbZn-2VZn complex. This SbZn-2VZn complex has lower formation energy and acts as a shallow acceptor, which can induce a strong violet luminescence. In the I-V measurement, the diode-like behavior of the ZnO homojunction device indicated the successful integration of antimony atoms by electrodeposition. The nearly 40-fold current gain of the photoresponsivity of the ZnO homojunction device, acting like a p-n diode, indicates a potential application in photodetectors operating at the ultraviolet wavelength region.
This work was funded by the NSC, Taiwan (grant no. NSC 100-2112-M-002-017-MY3).
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