Geometry Controlled Growth of Single Crystalline CdS Nanopillar Arrays and the Investigation of Their Electrical Properties

Recently, one dimensional semiconductor nanomaterials have been extensively explored as building blocks for nanoscale electronic devices such as field-effect transistors (FETs), optoelectronic devices such as solar cells, light emitting diodes, and lasers. Apart from their advantageous small dimensions for device miniaturization, semiconductor nanowires (NWs) and nanopillars (NPLs) have shown intriguing properties that promise high device performance. Notably, many of these intriguing properties have shown strong geometry dependence, in terms of length, diameter, pitch, cross-section shape etc. Past studies have shown that well organized nanostructure arrays exhibit unique photon management ability, which further resulted in the potency to demonstrate light absorption exceeding conventional limits. Therefore, the capability to program and engineer the shape and morphology of NWs and NPLs is of paramount importance and in urgent need. However, conventional bottom-up growth of NWs in free space using a catalytic process lacks precise control over NW length and diameter. Template-assisted electrochemical deposition can produce NWs/NPLs with designed geometry. Nevertheless, the typically low crystalline quality of its products places them into disadvantageous situation for high performance electronic and opto-electronic devices. Herein, we utilize a template-assisted, Vapor-Liquid-Solid (VLS) growth process for the fabrication of regular and single-crystalline NW/NPL arrays with tunable shapes. Self-organized and perfectly ordered Anodized Alumina Membranes (AAMs) have been fabricated by properly controlling the anodization conditions in combination with the utilization of nanoimprint technique. With these nanoengineered AAM as templates, a variety of nanostructures, including nanopillar arrays, nanotower arrays, and nanocone arrays, have been successfully fabricated. It is worth noting that due to the capability of programmable structural design and fabrication of the AAM templates, the semiconductor arrays can be well defined as desired. ZnO nanostructures were also prepared with the same method. This is to demonstrate that the process is applied for the growth of CdS NPLs, but can be easily extended to other materials. All in all, the process reported here presented a generic platform toward the controlled growth of nanostructures with tunable shape and geometry. 

In addition, we have investigated the electrical properties of the CdS NPLs from AAM assisted growth. Vertical NPL array devices and horizontal individual NPL field-effect transistors have been fabricated and characterized. The intrinsic CdS NPLs showed poor conductivity and weak gate dependent performance, which is not desirable for device applications. Thus, Indium was chosen as dopant to tailor the electrical and optical properties of CdS NPLs. Specifically, in-situ Indium doping was conducted with 80 mg of Indium metal pellet. The measurements showed that the location of the Indium doping source significantly affected carrier concentration, conductivity and field-effect mobility of the prepared CdS NPLs. Particularly, it was found that conductivity could be improved by 4 orders of magnitude and field-effect mobility could be enhanced up to 50 cm2/Vs via proper doping control. These results enable further applications of CdS nanopillars for nano-optoelectronic applications such as photodetection and photovoltaics in the future.