半導體奈米結構電性傳輸與奈米元件接點電阻之研究

Abstract

過去數十年間，藉由物理與化學的方法各式各樣半導體奈米線被成功的製造。透過由上而下或由下而上的組裝方法，這些半導體奈米線已被廣泛地應用於研究單電子電晶體、場效電晶體或光電元件等領域中。但在大多數的研究中由於奈米電子元件多具有兩點(Two-probe)的電極結構，因此透過該兩點電極結構，系統性地徹底掌握本質奈米線電性傳輸特性是一項相當重要的研究議題。在過去的文獻中雖然有許多半導體奈米線電性的研究被報導，但對於所量測到的電性傳輸行為究竟是本質奈米線所主導或由於接點電阻所造成的效應並沒有一個深入的探討。本篇論文將透過兩點電極結構深入研究接點電阻在半導體奈米線元件中所扮演的角色，並探求本質氧化鋅(ZnO)、磷化銦(InP)、磷化鎵(GaP)半導體奈米線與聚苯胺(polyaniline)奈米纖維的傳輸特性。 本實驗主要分成三大部分，在第一部分的研究裡，我們利用氧化鋅(ZnO)奈米線製造出多組具有兩點電極的奈米線元件，並系統性地研究接點電阻在奈米元件中的影響。我們發現依據與溫度相依的電流-電壓關係曲線圖與電阻的分析，兩點氧化鋅奈米線元件可區分成雙邊歐姆接觸(two Ohmic)、單邊蕭特基接觸(one Schottky)與背對背蕭特基接觸(back-to-back Schottky)等三種類型。發現由接點電阻所主導的背對背蕭特基奈米電子元件，其溫度與電阻關係曲線圖可藉由Mott變程跳躍傳輸模型(variable range hopping) 來加以說明。在接點電阻所主導的樣品中，Mott變程跳躍傳輸模型的指數函數p值，隨著室溫接點電阻率的上升逐漸由2上升到4，意味著變程跳躍傳輸模型在接點系統中，由低維度擴展至高維度的變程跳躍傳輸機制。更進一步地，藉由了解接點電阻在奈米線元件中所扮演的角色，我們可以成功地探究本質氧化鋅(ZnO)、磷化銦(InP)與磷化鎵(GaP)半導體奈米線電性傳輸特性。 由於了解到接點電阻在奈米線元件中所具有的影響。因此，第二部分的研究裡，我們針對磷化銦(InP)奈米線元件進行研究，探求接點電阻與本質奈米線電阻所主導的電子元件在光與氣體的環境中其反應是否具有差異化。實驗過程中，我們採用標準電子束微影的方式製作出多組磷化銦(InP)奈米線電子元件，藉由分析其變溫電阻變化曲線圖，我們可以成功地將樣品區分為奈米線所主導(nanowire-dominated)與接點電阻所主導(contact-dominated)的兩類型元件。在本質奈米線所主導的磷化銦(InP)電子元件中，其高、低溫部分的溫度-電阻關係圖分別可以使用熱活化傳輸理論與三維變程式跳躍傳輸理論來加以說明。更進一步地，將磷化銦奈米線電子元件曝照於光與氧氣的環境下，我們發現無論是奈米線或接點電阻所主導的奈米元件皆具有曝照前/後電阻的變化。令人訝異的是接點電阻所主導的奈米線電子元件無論在曝照光或氣體的反應下皆具有較高的電阻變化率出現。 第三部分的研究中，我們探討有機半導體奈米材料-聚苯胺(polyaniline)奈米纖維的電性傳輸性質。實驗中，藉由介面聚合的化學方法，聚苯胺(polyaniline)奈米纖維成功的被合成。在酸、鹼溶液的摻雜(doping)與去摻雜(dedoping)化學反應中，我們發現聚苯胺奈米纖維之薄膜樣品，其與溫度相依的電阻皆依循著 關係式變化。另一方面，為了探究單根聚苯胺(polyaniline)奈米纖維本質電性傳輸機制，我們首次利用介電泳動法成功地製作出多組具單根纖維之奈米元件。透過系統化的分析與研究，其電性傳輸實驗結果可用charge-energy-limited-tunneling理論來加定量說明之。

Various semiconductor nanowires have been synthesized by using either physical or chemical growth methods in the past decades. These nanowires applied to single electron transistors, field-effect transistors, optoelectronics, and nanoelectronics through top-down or bottom-up assembling approaches has been immediately demonstrated. Because of most nanoscale electronics having two-probe configuration as the source and drain electrodes, identification and determination of intrinsic electrical properties of nanowires and the contribution of nanocontact through a systematic procedure of this two-probe approach become very important. Although a lot of two-probe nanoelectronics and applications have been demonstrated in previous reports, the electrical properties bringing either from the nanocontacts or from the nanowires have not been uncovered clear yet. In this work, a two-probe technique was adopted to explore electrical properties of ZnO, InP, GaP nanowires and polyaniline nanofibers. The first portion of this work is to utilize high quality ZnO nanowires to fabricate two-probe nanodevices and to survey the impact of nanocontact on nanowire based nanoelectronics. According to temperature behaviors of current-voltage curves and resistances, the devices could be grouped into three types, including two Ohmic contacts, one Ohmic and one Schottky contacts, and two back-to-back Schottky contacts. The nanocontact could be treated as disordered system and be explained by Mott variable range hopping model for electrons of the form . The exponential parameters of Mott variable range hopping theory rises from 2 to 4 with an increase of specific contact resistivity at room temperature, implying a change from one- to three-dimensional hopping. Moreover, after understanding how to distinguish the nanowire- and contact-dominated nanodevices, we demonstrate that the two-probe measurement can be applied to the exploration of the intrinsic properties of semiconductor nanowires. This two-probe measurement approach also works on highly resistive nanowires without an Ohmic contact issue. By using this method, electron transport behaviors, resistivity, and carrier concentrations of ZnO, InP, and GaP semiconductor nanowires have been investigated. The interface problems in nanowire-based electronics play important roles due to the reason that the reduced contact area in nanoelectronics multiplies enormously the contribution of electrical contact properties. The second portion is to illustrate the sensitivity difference in response to light and oxygen gas between the nanowire- and contact-dominated InP nanowire devices. By using a standard electron-beam lithography technique, two-probe InP nanowire devices were fabricated. Although the InP were picked up from the same source sample and the dimensions of the nanowires and nanodevices were also kept the same, the room-temperature resistance of these devices varied considerably. It was conjectured the difference of room-temperature comes from the contribution of contact resistance. According to the temperature behaviors, the nanowire devices can be categorized into nanowire- and contact-dominated ones. The temperature dependent resistances follow the thermally activated and three-dimensional Mott variable range hopping transport at high and low temperatures, respectively. Both nanowire- and contact-dominated devices were exposed to light and oxygen gas to see any difference. In comparison with the nanowire-dominated devices, the contact-dominated InP nanowire devices always exhibit a much higher ratio of resistance changes in response to either light or oxygen gas exposures. The last portion of this work is to study the electrical transport of polyaniline nanofibers. Polyaniline nanofibers were synthesized by using polymerization at the interface of immiscible solvents. They exhibit a uniform nanoscale morphology rather than agglomeration with granular structures as that produced via conventional chemical oxidation. The as-synthesized polyaniline nanofibers are doped (dedoped) with an HCl acid (NH3 base) and their temperature behaviors of resistances all follow an exponential function with an exponent of T-1/2. To achieve the measurement of conduction mechanism in a single nanofiber, the dielectrophoresis technique is implemented to position nanofibers on top of and across two Ti/Au electrodes patterned by electron-beam with a nanogap of 100-200 nm. Their temperature behaviors and electric field dependences are unveiled and the experimental results agree well with the theoretical model of charge-energy-limited-tunneling. Through fitting to this transport model, the size of conductive grain, the separation distance between two-grains, and the charging energy per grain in a single polyaniline nanofiber are estimated to be about 5 nm, 3 nm, and 78 meV, respectively. This nanotechnological approach has been applied to determination of mesoscopic charge transport in the polyaniline conducting polymer.