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Three-fold symmetric doping mechanism in GaAs nanowires Mohammad Hadi Tavakoli Dastjerdi, Elisabetta Maria Fiordaliso, Egor Leshchenko, Azadeh AkhtariZavareh, Takeshi Kasama, Martin Aagesen, Vladimir G. Dubrovskii, and Ray Robert LaPierre Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00794 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017
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Three-fold symmetric doping mechanism in GaAs nanowires M.H.T. Dastjerdi1, E.M. Fiordaliso2, E.D. Leshchenko3, A. Akhtari-Zavareh1, T. Kasama2, M. Aagesen4,5, V.G. Dubrovskii3,6,7, R.R. LaPierre1,3,* 1
Department of Engineering Physics, Centre for Emerging Device Technologies, McMaster University, Hamilton, ON, Canada, L8S 4L7 2 Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark 3 ITMO University, Kronverkskiy pr. 49, 197101 St. Petersburg, Russia 4 Gasp Solar ApS, Gregersensvej 7, DK-2630 Taastrup, Denmark 5 Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark 6 St. Petersburg Academic University, Khlopina 8/3, 194021 St. Petersburg, Russia 7 Ioffe Physical Technical Institute of the Russian Academy of Sciences, Politekhnicheskaya 26, 194021 St. Petersburg, Russia
* Corresponding author:
[email protected] Keywords: GaAs, gallium arsenide, nanowires, self-assisted, molecular beam epitaxy, doping, beryllium.
Abstract A new dopant incorporation mechanism in Ga-assisted GaAs nanowires grown by molecular beam epitaxy is reported. Off-axis electron holography revealed that p-type Be dopants introduced in situ during molecular beam epitaxy growth of the nanowires were distributed inhomogeneously in the nanowire cross-section, perpendicular to the growth direction. The active dopants showed a remarkable azimuthal distribution along the (111)B flat top of the nanowires, which is attributed to preferred incorporation along three-fold symmetric truncated facets under the Ga droplet. A diffusion model is presented to explain the unique radial and azimuthal variation of the active dopants in the GaAs nanowires. 1 ACS Paragon Plus Environment
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Text Semiconductor nanowires (NWs) are structures with length typically on the order of microns and diameter on the order of tens to hundreds of nanometers. III-V NWs impart various benefits to optoelectronic and electronic devices, as compared to their thin film counterparts, including the ability to accommodate large lattice mismatch strain that enables new heterostructure material combinations and monolithic growth on Si [1, 2]. Group III-V semiconductor NWs are therefore being investigated for various device applications including solar cells, photodetectors, and sensors [1]. The most common method of NW growth involves the vapor-liquid-solid (VLS) method [1, 2]. In VLS growth, a seed particle such as Au, or a metallic element of the NW itself, acts as a collector of the growth species during material deposition (for example, during molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy (MOVPE)). The seed particle becomes supersaturated in the growth species, resulting in localized nucleation of the NWs. One of the critical aspects of NW growth is the intentional incorporation of impurity dopants, which is critical for the functionality of optoelectronic and electronic devices, especially for the formation of p-n junctions. However, the controlled incorporation of impurity dopants into NWs is not straightforward due to the unique growth mechanism of NWs. The doping in semiconductor NWs has been assessed by a variety of techniques, including photoluminescence [3], Hall effect [4-6], terahertz photoconductivity [7], fourpoint probe resistivity [8], field effect transistors [9], off-axis electron holography [1013], Raman spectroscopy [14-16], x-ray photoemission spectroscopy [17], secondary ion mass spectrometry [18], Kelvin probe force microscopy (KPFM) [19, 20], capacitancevoltage measurements [21], and atom probe tomography (APT) [22-24]. A review on the doping of semiconductor NWs is available in Ref. [25]. 2 ACS Paragon Plus Environment
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Among these techniques, off-axis electron holography is perhaps one of the most powerful methods for dopant assessment in nanostructures [10-13]. Electron holography is a transmission electron microscopy (TEM) technique that measures a spatially-resolved phase difference, ∆φ, by interference between electrons that pass through the specimen (object wave) and electrons that pass through vacuum (reference wave). ∆φ is related to the crystal potential, or mean inner potential (MIP) V(x,y,z), according to:
Δφ = , ,
(1)
where CE is a microscope acceleration voltage-dependent constant and t is the specimen thickness. For example, electron holography has been used to assess the doping in axial p-n junctions in Au-assisted VLS Si NWs [10, 11] and GaAs NWs [12]. Similarly, the doping in Au-assisted GaP core-shell NW p-n junctions has been assessed by electron holography [13]. Electron holographic tomography has provided three-dimensional mapping of electrostatic potentials of Au-assisted GaAs/AlGaAs core-shell structures [26, 27]. Until recently [28], electron holography has not been applied to NWs grown by the self-assisted VLS process, and the application of holography to III-V NWs in general is rather limited. In Ref. [28], the low open-circuit voltage in a Ga-assisted GaAs coreshell p-i-n NW photovoltaic device was attributed to a low built-in potential as measured by electron holography. In principle, the built-in potential, Vbi, in a p-n junction can be determined from the phase contrast, in the absence of other charges, and related to the electrically active dopant concentrations according to the well-known expression [29]: Vbi = (kT/e)⋅ln(NA- ND+/ni2)
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(2)
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where kT/e is the thermal voltage, NA- is the ionized acceptor concentration, ND+ is the ionized dopant concentration, and ni is the intrinsic carrier concentration (ni = 2.1x106 cm3
for GaAs). In the present paper, electron holography was used to further assess the p-type Be
dopant incorporation in Ga-assisted GaAs NW arrays grown by gas source molecular beam epitaxy (GS-MBE). We hypothesize that the Be dopant incorporation is limited by diffusion from three-fold symmetric facets of the NWs, resulting in a remarkable radial and azimuthal distribution of active dopants. NWs were grown via the Ga-assisted VLS method using a GS-MBE system, where group V hydride sources are cracked to supply group V dimers (As2 and P2). The NW growth is similar to that reported previously for a core-shell p-i-n photovoltaic device [28]. A 300 µm thick, boron-doped Si (111) substrate with
