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Second, I will discuss the synthesis of tapered nanowire structures and their electrical and optical characterization. By finely tuning growth temperature, precursor partial pressure, and catalyst size, detailed control of the nanowire tapering angle can be achieved. Moreover, tapered core/shell nanowires can be configured into devices with highly-localized electrical and optical functionalities. I show that control of the tapering angle plays an important role in determining the electrical and optical properties of nanowires.
Semiconductor nanowires are nanostructures with lengths up to few microns and small cross sections (10ths of nanometers). In the recent years the development in the field of III-N nanowire technology has been spectacular. In particular they are consider as promising building in nanoscale electronics and optoelectronics devices; such as photodetectors, transistors, biosensors, light source, solar cells, etc. In this work, we present fabrication and the characterization of photodetector and light emitter based devices on III-N nanowires. First we present a study of a visible blind photodetector based on p-i-n GaN nanowires ensembles grown on Si (111). We show that these devices exhibit a high responsivity exceeding that of thin film counterparts. We also demonstrate UV photodetectors based on single nanowires containing GaN/AlN multi-axial quantum discs in the intrinsic region of the nanowires. Photoluminescence and cathodoluminescence spectroscopy show spectral contributions above and below the GaN bandgap according to the variation of the discs thickness. The photocurrent spectra show a sub-band-gap peak related to the interband absorption between the confined states in the large Qdiscs. Finally we present a study of photodetectors and light emitters based on radial InGaN/GaN MQW embedded in GaN wires. The wires used as photodetectors showed a contribution below the GaN bandgap. OBIC measurements demonstrate that, this signal is exclusively generated in the InGaN MQW region. We showed that LEDs based on this structure show a electroluminescence emission and a red shift when the In content present in the QWs increases which is in good agreement with photoluminescence and cathodoluminescence results.
Certain current methods of nanostructure device fabrication such as wet or in solution nanostructure synthesis and photo-resist electrode patterning, expose the surface of the micro and nanostructure to contaminants.
Abstract: Over the past 15 years, nanowires (NWs) and nanotubes have drawn great attention since the application of VLS growth mechanism into the synthesis of one dimensional structures. Semiconductor nanowires exhibit novel electrical and optical properties. With a broad selection of composition and band structures, these one-dimensional semiconductor nanostructures are considered to be the critical components in a wide range of potential nanoscale device applications. To fully exploit these one-dimensional nanostructures, current research has focused on synthetic control of one-dimensional nanoscale building blocks, characterization of their novel properties, device fabrication based on nanowire building blocks, and integration of nanowire elements into complex functional architectures. Progress has been made in past two decades. However, there are still challenges in NWs growth controls, such as size, shape, position, stoichiometry and defects. Due to the dimensionality and possible quantum confinement effects of nanowires, there are also challenges in characterization and device fabrication. A systematic study of controlled growth of nanowires has been conducted in this dissertation. The first part of this dissertation presents various synthesis techniques of semiconductor nanowires via metal catalyzed vapor-liquid-solid (VLS) growth mechanism. Pulse laser deposition (PLD) with arsenic over pressure method has been successfully utilized for GaAs nanowires. Challenges such as uniformity issue commonly seen in MOCVD and MBE systems, morphology and stoichiometry issues commonly seen in conventional PLD systems have been overcome. Si nanowires fabrication via ultrahigh vacuum magnetron sputtering has reported for the first time, which also provides an alternate route for Si nanowires synthesis. The second part of this dissertation discusses optical properties of ensemble direct band gap nanowires. Photoluminescence spectra have been measured on an ensemble of random orientated InP nanowires. Polarization anisotropy has been explored on ensemble nanowires and oxide-coated nanowires. Our calculation for randomly oriented nanowires agrees well with experimental results. The control of polarization anisotropy of nanowires is realized by coating nanowires with an oxide layer composed of matching dielectric constant media. This opens a path to optical spin injection and detection on direct band gap nanowires.
One dimensional nanostructure materials such as nanowires have drawn many interests among the scientific community for a wide range of applications such as field-effect transistors [1], [2], inverters[3], light-emitting diode [1], lasers [4], nanosensors [5], [6], and photodetectors [7]... Comparing with the characterization of nanowire arrays, characterizing a single nanowire will definitely provide a better understanding on new nanowire properties due to simplified behaviors of devices. Although promising theories could be drawn from those results, fabrication of test structure for single nanowire measurements cannot be easily processed using standard microfabrication techniques. Therefore, electron beam lithography integrated with photolithography technique has been used to manipulate the connection; which provides I-V characteristics, of single horizontal nanowire with a specific device. Single Si nanowire characterization could be extended to various materials for further studies. In addition to single horizontal nanowire device, single vertical nanowire structure has been fabricated. Electron beam lithography technique is mainly used to pattern well-defined nanostructures where single ZnO nanowire is grown. Optical measurement, photoluminescence, is conducted to verify ZnO nanowires. This thesis also emphasizes on fabrication process to pattern various structures such as lines, rings, and circles with different sizes from 1um to sub 100nm... They could be potential candidate to create nanodisk antenna (rings), fishnet structure (lines), and base to grown single nanowire (circle).