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G-Doping-Based Metal-Semiconductor Junction coatings Article G-Doping-Based Metal-Semiconductor Junction Avtandil Tavkhelidze 1,* , Larissa Jangidze 1,2, Zaza Taliashvili 2 and Nima E. Gorji 3 1 Center of Nanotechnology for Renewable Energy, Ilia State University, Cholokashvili Ave. 3/5, Tbilisi 0162, Georgia; [email protected] 2 Institute of Micro and Nano Electronics, Chavchavadze Ave. 13, Tbilisi 0179, Georgia; [email protected] 3 School of Physics, Dublin City University, Dublin, Ireland; [email protected] * Correspondence: [email protected]; Tel.: +995-59958-7316 Abstract: Geometry-induced doping (G-doping) has been realized in semiconductors nanograting layers. G-doping-based p-p(v) junction has been fabricated and demonstrated with extremely low forward voltage and reduced reverse current. The formation mechanism of p-p(v) junction has been proposed. To obtain G-doping, the surfaces of p-type and p+-type silicon substrates were patterned with nanograting indents of depth d = 30 nm. The Ti/Ag contacts were deposited on top of G-doped layers to form metal-semiconductor junctions. The two-probe method has been used to record the I–V characteristics and the four-probe method has been deployed to exclude the contribution of metal-semiconductor interface. The collected data show a considerably lower reverse current in p-type substrates with nanograting pattern. In the case of p+-type substrate, nanograting reduced the reverse current dramatically (by 1–2 orders of magnitude). However, the forward currents are not affected in both substrates. We explained these unusual I–V characteristics with G-doping theory and p-p(v) junction formation mechanism. The decrease of reverse current is explained by the drop of carrier generation rate which resulted from reduced density of quantum states within the G-doped region. Analysis of energy-band diagrams suggested that the magnitude of reverse current reduction depends on the relationship between G-doping depth and depletion width. Citation: Tavkhelidze, A.; Jangidze, L.; Taliashvili, Z.; Gorji, N.E. Keywords: G-Doping-Based nanograting; G-doping; metal-semiconductor junction; reverse current Metal-Semiconductor Junction. Coatings 2021, 11, 945. https:// doi.org/10.3390/coatings11080945 1. Introduction Academic Editor: Maja Miˇceti´c Current developments in nanotechnology have enabled patterning the surface of semiconductor layers by nanoscale gratings with periodical arrays of width smaller than Received: 9 July 2021 1 µm [1–4]. Nanograting (NG) patterns have been shown to dramatically change the Accepted: 4 August 2021 electronic [5–7], magnetic [8,9], optical [10–14], and electron emission [15,16] properties Published: 7 August 2021 of the semiconductor substrate when the grating depth becomes comparable with de Broglie wavelength of electrons. This can be attributed to the special boundary conditions Publisher’s Note: MDPI stays neutral enforced by the NG on the wave function. Solution of time-independent Schrodinger with regard to jurisdictional claims in equation has to satisfy additional boundary conditions. Eigenfunctions are modified and published maps and institutional affil- the probability of finding electrons in the proximity of NG reduces. NG partly forbids some iations. quantum states within the patterned region and reduces the density of quantum states (DOS) [17]. In this case, the rejected electrons have to occupy empty quantum states with higher energy. As a result, the Fermi energy rises under the patterned region because the electron concentration, n, in the conduction band increases. We call this geometry-induced Copyright: © 2021 by the authors. electron doping (G-doping) [5]. Within this concept, both n and Fermi energy increase Licensee MDPI, Basel, Switzerland. without any ionized external impurities, which promises an interesting doping approach This article is an open access article for hard-doping materials such as GaN or for impurity-sensitive materials such as for distributed under the terms and thermotunnelling and solar cell applications, especially for large-production commercial conditions of the Creative Commons lines. Attribution (CC BY) license (https:// Various phenomena related to G-doping were already reported for different configu- creativecommons.org/licenses/by/ rations of other periodic structures. They have been reported in disordered nanostructures 4.0/). Coatings 2021, 11, 945. https://doi.org/10.3390/coatings11080945 https://www.mdpi.com/journal/coatings Coatings 2021, 11, x FOR PEER REVIEW 2 of 9 Coatings 2021, 11, 945 2 of 9 Various phenomena related to G-doping were already reported for different config- urations of other periodic structures. They have been reported in disordered nanostruc- tures obtainedobtained by wet-etching by wet-etching of p-Si [18], of p-Si as well [18], as as in well nearly as in periodic nearly periodic nanostructures nanostructures made made by laser byradiation laser radiation interaction interaction with surfaces with surfaces of Si, Ge, of Si,and Ge, SiGe and crystals SiGe crystals [19], in [ 19in-], in indium tin dium tin oxideoxide [20] and [20] graphene and graphene oxide oxide [21] layers. [21] layers. A dramatic A dramatic increase increase of conductivity of conductivity (n-type (n-type conductivity)conductivity) was observed was observed in the ZnO in the crystal ZnO after crystal the after formation the formation of nanoparticles of nanoparticles on its on its surface [11].surface We [ 11have]. We shown have that shown NG that changes NG changes the conductivity the conductivity of p-type of p-typesilicon siliconto to n+-type n+-type in thinin device thin device (SOI) (SOI)layer layer[6]. Temperature [6]. Temperature dependences dependences of resistivity of resistivity and and Hall Hall coefficient coefficient of SOIof SOI device device layers layers show show meta metallicllic behavior behavior and and the the ellipsometry ellipsometry measure- measurements indicate ments indicatethat that the the dielectric dielectric function function is is metallic metallic type type [10 [10,13].,13]. Strong Strong photoluminescence photolumines- spectra were cence spectra recordedwere recorded for SOI for nanograting SOI nanograting surfaces surfaces [10,14 ][10,14] and for and the for periodic the periodic nanocones [19]. It is nanocones [19].remarkable It is remarkable that the that photoluminescence the photoluminescence phenomenon phenomenon is observed is observed in indirect in band gap indirect band materials.gap materials. In our In measurementsour measurements on NG on samples, NG samples, we observed we observed additional addi- periodic peaks tional periodicin peaks the photoluminescence in the photoluminesce spectrumnce spectrum and the and peak the positions peak positions were found were to be in agree- found to be in agreementment with with G-doping G-doping theory theo [10ry,14 [10,14].]. Giant Giant negative negative magnetoresistance magnetoresistance was found in SOI was found in SOInanograting nanograting samples samples [8] and [8] and other other periodic periodic nanostructures nanostructures [9]. Work-function[9]. Work- reduction function reductionwas investigatedwas investigated in Si in nanograting Si nanograting devices devices [22] [22] and and a strong a strong field field emission emis- was observed sion was observedin Si in periodic Si period nanoconeic nanocone structures structures [15]. [15]. A general energy-bandA general diagram energy-band of an diagramintrinsic bulk of an semiconductor intrinsic bulk semiconductor with NG patterns with NG patterns on the surface onis shown the surface in Figure is shown 1a. Some in Figure of the1 a.energy Some levels of the in energythe valence levels band in the are valence band forbidden due areto reduced forbidden quantum due to states reduced (resulting quantum from states NG patterning), (resulting from and the NG rejected patterning), and the electrons mustrejected occupy the electrons higher mustenergy occupy levels,the which higher in turn, energy increases levels, the which electron in turn,con- increases the centration in conductionelectron concentration band (G-doping). in conduction Figure 1b bandrepresents (G-doping). an energy-band Figure1b diagram represents an energy- of a p-type semiconductorband diagram substrate of a p-type with semiconductor NG patterns substrate on the surface. with NG In patterns this case, on thethe surface. In this rejected electronscase, will, the first, rejected move electrons to the top will, of the first, valence move toband the and top ofcompensate the valence the band holes, and compensate + which raises thethe Fermi holes, energy which level. raises Figure the Fermi 1c is the energy diagram level. of Figurea p+-type1c issemiconductor the diagram of a p -type with NG patternssemiconductor on the surface, with which NG patternsonly quantitatively on the surface, differs which from onlythat quantitativelyof the p-type differs from that of the p-type semiconductor. semiconductor. Figure 1. BasicFigure energy-band 1. Basic energy-band diagrams of diagrams NG layers of fabricated NG layers on fabricated the surface on ofthe (a )surface an intrinsic of (a) semiconductor,an intrinsic sem- (b) a p-type, and (c) a p+-type.iconductor, The green (b) a linesp-type, refer and to energy(c) a p+-type. levels occupiedThe green by lines electrons, refer to and energy red lines levels refer occupied to energy by levels elec- forbidden trons, and red lines refer to energy levels forbidden by NG patterns. by NG patterns. We
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