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Direct Synthesis of Hyperdoped Germanium Nanowires Michael S. Seifner, Masiar Sistani, Fabrizio Porrati, Giorgia Di Prima, Patrik Pertl, Michael Huth, Alois Lugstein, and Sven Barth ACS Nano, Just Accepted Manuscript • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018
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Direct Synthesis of Hyperdoped Germanium Nanowires Michael S. Seifner,¥,‡ Masiar Sistani,ǁ,‡ Fabrizio Porrati,⊥ Giorgia Di Prima,⊥ Patrik Pertl,¥ Michael Huth,⊥ Alois Lugstein,ǁ Sven Barth*,¥ ¥
TU Wien, Institute of Materials Chemistry, Getreidemarkt 9, 1060 Vienna, Austria.
ǁ
TU Wien, Institute of Solid State Electronics, Floragasse 7, 1040 Vienna, Austria.
⊥
Goethe-Universität, Physikalisches Institut, Max-von-Laue-Street 1, 60438 Frankfurt am Main,
Germany.
*Corresponding author: S. Barth, E-mail:
[email protected]; Fax: +43 158801 165 99; Tel: +43 158801 165 207
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ABSTRACT.
A low temperature chemical vapor deposition (CVD) growth of Ge nanowires using Ga as seed material is demonstrated. The structural and chemical analysis reveals the homogeneous incorporation of ~3.5 at% Ga in the Ge nanowires. The Ga-containing Ge nanowires behave like metallic conductors with a resistivity of about ~300 µΩcm due to Ga hyperdoping with electronic contributions of one third of the incorporated Ga atoms. This is the highest conduction value observed by in situ doping of group IV nanowires reported to date. This work demonstrates that Ga is both an efficient seed material at low temperatures for Ge nanowire growth and an effective dopant rendering the semiconductor into a metal-like conductor.
KEYWORDS. germanium; nanowires; hyperdoping; gallium; quasi-metallic; semiconductor
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Anisotropic Ge nanostructures have been used as active components for different applications including field effect transistors,1 lithium ion batteries,2 solar cells3 and humidity sensors.4 Ge nanowires (NWs) have been successfully prepared by different methods in bottom-up and topdown approaches.5 The most popular synthesis approach is the use of metal growth promoters in bottom-up processes including the vapor-liquid-solid (VLS),6 supercritical-fluid-liquid-solid (SCFLS)7 and solution-liquid-solid (SLS)8 as well as the growth by solid metal seeds.9 Many metallic growth seeds have been described in literature to result in highly crystalline Ge NWs.9-11 For some of the above mentioned applications doping of the NWs is prerequisite, which can be achieved either by external sources during crystal growth12-14 or the incorporation of atoms from metal seeds15-18 used for the realization of anisotropic crystal habitus. The incorporation of dopants in the Ge matrix has recently been in the focus of several studies and rather effective doping with heavy group III atoms has been observed in low temperature growth of Ge NWs using In as seeding material19 and also for Bi in Ge nanoparticles.20 The electrical properties of the In-containing Ge NWs have not been investigated, which might be related to pronounced twinning of the NWs derived by that approach. Therefore the actual activity and nature of the incorporated In atoms on the electronic properties is unknown. In contrast, Bi-containing Ge nanoparticles exhibit an increased charge carrier density when compared to undoped Ge crystals prepared by the same method.20 Even though Ga is known to be an excellent p-dopant for Ge, to the best of our knowledge, Ga has not been used for vapor-based growth strategies of Ge nanostructures in the past. The electrodeposition using Ga as nucleation sites for the growth of Ge microwires, the so called electrochemical liquid-liquid-growth mode, is the only exception where Ga was identified as an effective growth promoter.21-22 This type of growth using Ga as an electrode and seed leads to
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protuberances along the microwires. The Ga seeded microwires typically showed highly pronounced tapering and incorporation of 8-10 at% Ga in the Ge matrix. This is accompanied by p-type behavior in the electronic properties.21 However, the dopant activation was poor and the carrier concentration several orders of magnitude lower than the actual Ga concentration.22 First indications for suitable growth conditions of Ga mediated Ge NW growth can be deduced from the binary Ge-Ga phase diagram. The Ga/Ge eutectic is very close to the melting point of Ga (29.8 °C) with only 0.006 at% Ge (
