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Développement de méthodes ex-situ de dopage de nanofils semiconducteurs IV

Abstract : This thesis aims at studying the ex-situ doping of semiconducting nanowires (NWs) for applications in electronics, spintronics or thermoelectricity. Two widely used techniques have been envisaged: ion beam implantation and Spin-On-Doping (SOD).The ion beam implantation of Mn ions has been tested in Ge NWs in an attempt to form a 1D diluted magnetic semiconductor structure. A Mn concentration of few percents can be achieved without amorphization of the nanowire nor clustering, what is very promizing. During implantation done in situ in a transmission electron microscope, a strong enhancement of the sputtering under electron irradiation has been observed.The doping by SOD results from the thermal diffusion of p-type or n-type impurities contained in a HSQ (Hydrogen silsesquioxane) resist in which the NWs are embedded. The curing of the HSQ resist (doped or not) leads to a structural modification of nanowires (while SOD is generally assumed to be non-destructive). As a result of the annealing, a partial transformation of the 3C diamond phase towards the 2H hexagonal phase is observed in Si and Ge nanowires, above 500 et 400°C respectively. The main parameters of that phase transformation are the shear stress due to the HSQ densification and the thermal budget. Ge NWs are found to turn to amorphous above 650°C, what renders SOD practically unusable for Ge NWs. Two methods are currently used for the fabrication of nanowires, the VLS (vapor-liquid-solid) growth and reactive ion etching of (111) Si wafers. For practical reasons, etched NWs were used for the study of their electrical properties.The electrical characterizations were done on arrays of Si NWs embedded in a HSQ matrix or on single NWs. For contacting single NWs in the NW-FET(nanowire field effect transistors) configuration , a process based on electron beam lithography has been developed. The issues to be solved were related to the low length of NWs. Various measurement techniques were used: I-V in two tips or TLM (Transient Linear Measurement) arrangement, SSRM (scanning spreading resistance microscopy), EBIC (electron beam induced current). Collective measurements were done on arrays of p-type NWs embedded in a HSQ resist, doped or not. It was firstly observed that an annealing is needed to observe a noticeable current in the structure. Above an annealing temperature of 600°C, for a negative bias applied to the substrate, the observed behavior can be described by the SCLC (space charge limited current) mechanism expected for poorly doped NWs in an isolating matrix, while a positive bias applied to the substrate results in an ohmic characteristic and in a current density up to 500 times higher in the NWs than in the substrate. This unexpectedly high intensity in direct bias may be attributed to electrically active surface states resulting from the etching process. This hypothesis is conforted by the fact that an annealing at 900°C (without extra doping) the measured intensity can be explained by assuming the same doping level in NWS than in the substrate. In addition, an interfacial conductive between resist and nanowires can be observed by SSRM. These interfacial states can be also involved for understanding the measurements done on n-type NWS. The SCLC mechanism of transport has been also observed for single NWs contacted by tips or by lithographied contacts. These measurements were not able to evidence the effect of the phase transformation on the electrical properties.P-type and n-type doping by SOD becomes effective after annealing at 900°C. After doping, ohmic or rectifying behaviours on p-type substrate are observed as expected. That renders more easy the interpretation of results, by assuming doping levels in the NWs of 3×10¹⁶ cm⁻³ and 2×10¹⁶ cm⁻³ for p-type and n-type respectively. These values as deduced from resistivities are probably very underestimated as the mobilities in NW are probably much lower than in the bulk.
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Mariam Fakhfakh. Développement de méthodes ex-situ de dopage de nanofils semiconducteurs IV. Science des matériaux [cond-mat.mtrl-sci]. Université Paris Saclay (COmUE), 2018. Français. ⟨NNT : 2018SACLS005⟩. ⟨tel-01737874⟩

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