Vol. 35, No. 1 Journal of Semiconductors January 2014 A p-silicon nanowire/n-ZnO thin film heterojunction diode prepared by thermal evaporation Purnima Hazra  and S. Jit Department of Electronics Engineering, Indian Institute of Technology (BHU), Varanasi-221005, India Abstract: This paper represents the electrical and optical characteristics of a SiNW/ZnO heterojunction diode and subsequent studies on the photodetection properties of the diode in the ultraviolet (UV) wavelength region. In this work, silicon nanowire arrays were prepared on p-type (100)-oriented Si substrate by an electroless metal deposi- tion and etching method with the help of ultrasonication. After that, catalyst-free deposition of zinc oxide (ZnO) nanowires on a silicon nanowire (SiNW) array substrate was done by utilizing a simple and cost-effective thermal evaporation technique without using a buffer layer. The SEM and XRD techniques are used to show the quality of the as-grown ZnO nanowire film. The junction properties of the diode are evaluated by measuring current–voltage and capacitance–voltage characteristics. The diode has a well-defined rectifying behavior with a rectification ratio of 190 at ˙2 V, turn-on voltage of 0.5 V, and barrier height is 0.727 eV at room temperature under dark conditions. The photodetection parameters of the diode are investigated in the bias voltage range of ˙2 V. The diode shows responsivity of 0.8 A/W at a bias voltage of 2 V under UV illumination (wavelength D 365 nm). The characteristics of the device indicate that it can be used for UV detection applications in nano-optoelectronic and photonic devices. Key words: silicon nanowire; ZnO nanowire; heterojunction diode; interface; barrier height; photo responsivity DOI: 10.1088/1674-4926/35/1/014001 EEACC: 2520 1. Introduction ZnO is an inherently n-type transparent semiconductor with a direct band gap of approximately 3.7 eV Œ1 . It is a low cost and widely available material with unique electrical and optical properties Œ13 . ZnO nanostructure-based thin films have found wide applications in nanoelectronics and optoelec- tronics areas, including in solar cells, gas sensors, memory de- vices, and ultraviolet photodetectors Œ13 . Although many re- search groups have done an extensive amount of research on the synthesis and characterization of ZnO nanostructures over the last decades Œ117 , the material is still at a research stage as ZnO can form a wide range of nanostructures depending on the experimental conditions Œ36 , such as type of substrate Œ4; 5 , deposition method Œ6 , type of seed layer Œ5 , and deposition tem- perature Œ4; 7 . Therefore, a lot of research is still necessary for ZnO to be used in commercial applications. Many researchers have fabricated ZnO-based devices on various substrates, like quartz Œ4 , glass Œ1; 4 and bulk silicon Œ2; 3 to investigate the sub- strate effect on electrical and optical properties of the device. However, there are very few literatures in which the deposition of ZnO on a SiNW (Si nanowire) substrate is reported Œ816 . Researchers have deposited ZnO on SiNW substrates by dif- ferent techniques, like atomic layer deposition (ALD) Œ810 , chemical vapor deposition Œ11 , solution synthesis Œ12 , and RF sputtering Œ13; 14 to fabricate SiNW/ZnO device prototypes. Choi et al. Œ13 have investigated the electrical properties of their SiNW/ZnO device prepared by using the RF sputtering method and reported a very poor ideality factor and turn-on voltage of 4.5 and 2.6 V respectively. Um et al. Œ8 have stud- ied the optical properties of their 2 m and 6 m long core- shell SiNW/ZnO heterojunction diodes prepared by the ALD method and reported a responsivity of 0.42 and 0.63 A/W re- spectively in forward bias voltage for UV light at 365 nm wave- length. Kang et al. Œ8 have also studied the optical properties of core-shell SiNW/ZnO heterojunction diodes prepared by us- ing the ALD method and reported a responsivity of 0.24 A/W in forward bias voltage for UV light of 365 nm wavelength. However, to the best of our knowledge, until now no report is presented in the literature where deposition of ZnO nano- structures was done on a SiNW substrate by using the thermal evaporation technique to investigate the electronic as well as the optical properties of a SiNW/ZnO heterojunction device prototype. Since the TE technique is a simple and cost effec- tive method as compared to above-mentioned methods for the catalyst-free deposition of ZnO with excellent uniformity over a large-area Œ2 , the technique has been explored in the present work to successfully fabricate n-ZnO/p-SiNW heterojunction diodes. In this work, the p-SiNW arrays were first developed on a p-Si substrate by using the electroless metal deposition and etching method as this method is more reliable for prepar- ing highly ordered, large-area, wafer-scale SiNW arrays with desirable crystallographic orientations by selecting Si wafers with corresponding crystallographic orientations Œ9 . After the fabrication of SiNW arrays, ZnO was deposited on SiNW ar- rays by using the thermal evaporation method. Ohmic contacts were deposited on both sides of the SiNW/ZnO heterostructure film to fabricate the device prototypes. The surface morphol- ogy, electronic and optoelectronic properties of the SiNW/ZnO nanowire based heterojunction devices have been studied in de- tail to demonstrate their possible applications in UV photode- tection applications. † Corresponding author. Email: phazra.rs.ece@itbhu.ac.in Received 28 June 2013, revised manuscript received 14 August 2013 © 2014 Chinese Institute of Electronics 014001-1