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Title: Controlled doping in CdSe nanowires : synthesis, characterization and devices applications
Other Titles: Xi hua ge na mi xian de ke kong chan za : he cheng, biao zheng ji qi jian de yan jiu
硒化鎘納米綫的可控摻雜 : 合成 , 表徵及器件的研究
Authors: He, Zhubing (何祝兵)
Department: Department of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2009
Publisher: City University of Hong Kong
Subjects: Nanowires.
Cadmium selenide.
Nanostructured materials.
Notes: CityU Call Number: TK7874.85 .H4 2009
xix, 131 leaves : ill. (some col.) 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2009.
Includes bibliographical references.
Type: thesis
Abstract: CdSe is an important group II-VI direct band gap (visible) semiconductor material with attractive electronic, spintronics and optoelectronic properties. It has shown great potential in the applications, such as biosensing/bioimaging, light emitting diode, and photodetectors. Although CdSe Nanowires (NWs) are very promising building blocks for the applications mentioned above, some inevitable obstacles remain for the practical application of CdSe NWs based nano-optoelectronic devices. First, controllable and uniform doping in CdSe NWs is the critical prerequisite in semiconducting applications. Second, how to define these NWs to a desired position with reasonable reliability and reproducibility, or how to construct these building blocks into the desired structure, is a technique to be developed. Third, the influences of NWs surface, contact between NWs and electrodes, interface between NWs and package films on the electronic transportation properties are not well understood yet. Last but not the least, how to enhance the performance of the whole device based on these nanomaterials is predominant in devices applications. In this thesis, controlled doping in CdSe NWs and optimization of devices based on them are systematically studied. Indium doping in CdSe NWs via two approaches, ie., i) in-situ co-evaporation of both CdSe and indium powder source in a thermal transport system, ii) doping via post annealing of CdSe NWs in indium vapor, were successfully demonstrated for the first time. The methods were found to be effective and the transport properties of CdSe NWs were tuned to vary over a wide range. The conductivity of CdSe NWs was increased by nearly five orders of the magnitude from ~10-4 to tens Scm-1 by the doping, and the carrier concentration as high as ~1019 cm-3 was achieved for the heaviest doping. The doped CdSe NWs showed a high sensitivity to light irradiation. Prolonged decay edges for the heavily doped CdSe NWs were observed, which were attributed to increased trapping centers arising from an increasing indium concentration. Although we achieved uniform CdSe NWs with controlled doping concentration, the gate effect of transistors based on single CdSe:In NWs did not fully meet the expectation, owing to the inherent disadvantage of the structure of these field effect transistors (FETs). Herein, high-performance CdSe:In nanowire FETs using high k Al2O3 as the gate insulator is reported. A simple technique that involved rapid thermal annealing the prepared Al metallic gate was employed to fabricate the ultrathin Al2O3 gate layer. In contrast to the devices constructed on conventional SiO2/Si substrate, the CdSe:In nanowire FETs with Al2O3 gate show considerate improvement in device performances, such as large transconductance and enhanced current switching characteristics. In addition, this developed method has been proven to be compatible with flexible substrates which thus open the opportunities for the applications of nano-FETs in flexible electronics. Moreover, the performance gain of transistor based on single doped CdSe NWs is also augmented by substituting the planar back gate by the top gate geometry with another high k dielectric material, Si3N4, as discussed in chapter 4. A systematic study of the gate performance of the top gate and back gate FET based on the same single indium doped CdSe nanowire (NW), using Si3N4 and SiO2 as gate dielectric materials, respectively was conducted. The Ion/Ioff ratio and field effect mobility of the top gate transistor reached over 105 and 166 cm2/V s, respectively. The threshold voltage and the subthreshold swing were reduced to -1.7 V and 508 mV/dec. The performance was the most exceptional ever reported for CdSe and even II-VI semiconductor nanomaterial based devices. Both the high κ gate material, Si3N4, and the gate geometry contribute most to a significant enhancement of the performance of FET devices. Based on the good performance of these top gate transistors alone, two basic logic gates, ‘AND’ and ‘OR’, were fabricated and a rapid and stable switch effect was shown. These logic gates can also be functionalized by common white lamp light owing to the good photoconductivity of CdSe. We also studied electronics properties of heretrojuncts between n- type CdSe:In NWs and p- type silicon nanoribbon (NRs). Patterned p- type Si NRs were fabricated using reactive ion etching (RIE) of SOI substrate. By positioning the doped n- type CdSe NWs just crossing each as-etched silicon NR by electrical field, heterjunctions array with these n-type CdSe NWs and p- type silicon nanoribbon (NRs) were formed. These heterojunctions exhibit multi-functionalities, including p-n diode with small turn-on voltage (0.5~2 V) and large breakdown voltage (over 30V), Junction FETs (JFETs) and possible light emission diode (LED). These JFETs demonstrate superior performance to single CdSe:In NW based back gate metal oxide semiconductor FETs (MOSFETs) in transconductance, Ion/Ioff ratio, threshold voltage, substhreshold swing and so on. The conductance of the CdSe:In NW channel can be changed by a factor of more than 103 with only -2 to -1V variation in the p-n junction gate. We attribute the high sensitivity of the JFETs to outstanding channel tuning effects of the new type nano p-n junction gate diode. For LED, it is believed that this work would serve as a catalyst to construct red pixels, as well as study of light emission mechanisms at the CdSe and Si interface. Additionally, the controllable indium in-situ doping can be utilized in other II-VI semiconductors nanostructures such as ZnSe NWs. The doping concentration variation and derived morphology evolution of ZnSe:In NWs are discussed. For unintentionally doped ZnSe NWs, the surface of NWs is smooth and the cross-section is rectangular. In contrast, the surface changes coarsely and even into hierarchy structures, and the cross-section evolves from a square to circular and eventually to irregular polygon, which are attributed to the indium vapor in the growth system. The indium concentration varies from undetectable to near 10 at% in a single NW electron diffraction X-ray spectrum. The result proves that this in-situ doping method can be a general method for doping of II-VI semiconductor 1D nanomaterials. The series of efforts mentioned above were dedicated to study some basic electronic and optoelectronic properties of doped CdSe NWs, and also to explore some exciting and promising performance of nano- devices based on them at an incipient stage. We believe that these works would serve as a catalyst for future research on CdSe and relative II-VI semiconductor nanomaterials based nano- optoelectronic and electronic devices.
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