Tunable n‐Type Conductivity and Transport Properties of Ga‐doped ZnO Nanowire Arrays

Abstract

As an important II-VI semiconductor, ZnO has attracted increasing interests owing to its unique properties such as wide band-gap (3.37 eV) and large exciton binding energy (60 meV).[1] ZnO has shown great potential in optoelectronic devices such as light emitting diodes (LED) and laser diodes (LDs) operating in the short-wavelength or UV region.[2]Compared to their thin-film counterparts, [3][4] nanoscale devices assembled on free-standing nanowires [5][6][7][8][9][10] could enable new functions, high efficiency, enhanced performance, and diverse applications. [11][12][13][14][15][16][17][18][19][20] As in thin-film devices, the success of nanodevices similarly relies on the capability of controlling the transport and electrical properties of the selected materials. Doping via introducing electron donor or acceptor elements into the host crystal is a successful approach in thin-film or planar electronic/optoelectronic devices. However, such doping approach remains a challenge for nanostructured materials. To date, while n-and p-type dopings have been achieved in Si, [11][12] InP, [13] CdS, [14] and GaN [15] nanowires/ nanoribbons, many issues of doping, such as control of doping type and conductivity, remain largely untapped or unresolved. For ZnO nanostructures, group III elements (Al, Ga, or In) are commonly used to substitute Zn to induce n-type conductivity. The success of doping is often accompanied and characterized by changes in optical, electrical, and/or structural properties of ZnO nanostructures. For example, Al-doped ZnO nanowires exhibited a blue shift from 3.29 to 3.34 eV in the cathodoluminescent (CL) spectra.[21] Ga 2 O 3 was also employed to dope n-type ZnO nanofibers grown in a vaporphase transport process. [22] however switched to n-type after two months storage in an ambient environment. Despite the considerable efforts, rational synthesis of ZnO nanostructures with tunable n-type conductivity is not available. The as-synthesized ZnO nanostructures are often randomly oriented, and thus have limited applications in optoelectronic devices. Therefore, it is necessary to have a better understanding of the doping efficiency and transport properties of ZnO nanostructures. Herein, we report a controlled growth and doping process of well-aligned ZnO nanowire (NW) arrays via thermal evaporation. The growth direction of ZnO NWs was found to depend on the dopant content, and NW conductivity could be varied over two orders of magnitude. The electrical properties of ZnO NWs were characterized using single-nanowire field-effect transistors (FETs). Figure 1 shows the electron microscopy images of ZnO NW arrays synthesized on a-plane sapphire substrates. The content of Ga 2 O 3 in the source mixtures was varied from 0 to 1 at %. The representative NWs synthesized with 0, 0.2, and 1 at % of Ga 2 O 3 are denoted as samples A, B, and C, respectively. Both the undoped (Fig. 1a and b) and Ga-doped (Fig. 1c) ZnO NWs are aligned vertically on the substrates, and uniform over a large area. The NWs have a uniform dia...

Electrothermal phenomena in zinc oxide nanowires and contacts

“…Electrothermal transport data for relevant materials are examined to assess the impact of relative heating on an array of nanowire devices proposed for energy conversion applications. Figure 4.2 shows the relative heating versus specific contact resistivity for silicon [36,[44][45][46][47][48], gallium nitride [38,[49][50][51][52], and zinc oxide [2,[53][54][55]. Using data reported by previous studies, the relative heat generation rate is determined assuming typical metal-nanowire contact length, nanowire length, and device current of 1 µm, 10 µm, and 1 µA, respectively.…”

Carbon-assisted nucleation and vertical growth of high-quality ZnO nanowire arrays

“…The field-effect mobility of present NWs is about three times higher than that of high-quality ZnO NWs reported previously (10.2 cm 2 V -1 s -1 in ref. 10 and 12 cm 2 V -1 s -1 in ref. 5), in which the ZnO NWs were grown by the traditional carbon thermal method using Au as the catalysts.…”

“…4 For the formation of highly-aligned ZnO NW arrays, metal catalysts are often employed to control the NW nucleation sites and diameters. 1 The alignment of ZnO NWs is mainly determined by the single crystalline substrates such as GaN, 9 sapphire 10 and SiC 11 which are nearly lattice matched to ZnO NWs. The main disadvantage of using metal catalysts is that the catalysts, such as Au may contaminate the NWs.…”

“…For the growth of ZnO NWs using Au patterned films, randomly oriented ZnO NWs are always resulted on these substrates. 10 To understand the function of the a-C film on the growth of ZnO NWs, we investigated the interfaces between the roots of the NWs and the a-C film by cross-sectional HRTEM. Figure 3(d) and 3(e) reveal that ZnO crystals directly nucleate on the flat a-C film with their c-axis perpendicular to the surface of the a-C film.…”

Effect of Mg Doping on Structural and Optical Properties of Al-Mg Co-doped ZnO Thin Films Deposited via Sol-gel Technique

“…In most cases, the purpose of the doping is to modulate the optical and or electrical properties of ZnO thin films. Dopants such as Al, Ga, In, B, Ag, and Cl were all expected to enhance the native n type conductivity of ZnO [16][17][18][19][20][21][22] . Dopants like Cd, Mg have been shown to modify the optical properties of ZnO by changing its band gap [23] .…”