Xin Ou

Carrier Profiling of Individual Si Nanowires by Scanning Spreading Resistance Microscopy

Individual silicon nanowires (NWs) doped either by ion implantation or by in situ dopant incorporation during NW growth were investigated by scanning spreading resistance microscopy (SSRM). The carrier profiles across the axial cross sections of the NWs are derived from the measured spreading resistance values and calibrated by the known carrier concentrations of the connected Si substrate or epi-layer. In the case of the phosphorus ion-implanted and subsequently annealed NWs, the SSRM profiles revealed a radial core-shell distribution of the activated dopants. The carrier concentration close to the surface of a phosphorus-doped NW is found to be by a factor of 6-7 higher than the value in the core and on average only 25% of the implanted phosphorus is electrically active. In contrast, for the in situ boron-doped NW the activation rate of the boron atoms is significantly higher than for phosphorus atoms. Some specific features of SSRM experiments of Si NWs are discussed including the possibility of three-dimensional measurements.

Key concepts behind forming-free resistive switching incorporated with rectifying transport properties

This work reports the effect of Ti diffusion on the bipolar resistive switching in Au/BiFeO3/Pt/Ti capacitor-like structures. Polycrystalline BiFeO3 thin films are deposited by pulsed laser deposition at different temperatures on Pt/Ti/SiO2/Si substrates. From the energy filtered transmission electron microscopy and Rutherford backscattering spectrometry it is observed that Ti diffusion occurs if the deposition temperature is above 600°C. The current-voltage (I–V) curves indicate that resistive switching can only be achieved in Au/BiFeO3/Pt/Ti capacitor-like structures where this Ti diffusion occurs. The effect of Ti diffusion is confirmed by the BiFeO3 thin films deposited on Pt/sapphire and Pt/Ti/sapphire substrates. The resistive switching needs no electroforming process, and is incorporated with rectifying properties which is potentially useful to suppress the sneak current in a crossbar architecture. Those specific features open a promising alternative concept for nonvolatile memory devices as well as for other memristive devices like synapses in neuromorphic circuits.

Substrate effect on the resistive switching in BiFeO3 thin films

BiFeO3 thin films have been deposited on Pt/sapphire and Pt/Ti/SiO2/Si substrates with pulsed laser deposition using the same growth conditions. Au was sputtered as the top electrode. The microscopic structure of the thin film varies by changing the underlying substrate. Thin films on Pt/sapphire are not resistively switchable due to the formation of Schottky contacts at both the top and the bottom interfaces. However, thin films on Pt/Ti/SiO2/Si exhibit an obvious resistive switching behavior under forward bias. The conduction mechanisms in BiFeO3 thin films on Pt/sapphire and Pt/Ti/SiO2/Si substrates are discussed to understand the different resistive switching behaviors.

Forming-Free Resistive Switching in Multiferroic BiFeO3 thin Films with Enhanced Nanoscale Shunts

A controlled shunting of polycrystalline oxide thin films on the nanometer length scale opens the door to significantly modify their transport properties. In this paper, the low energy Ar(+) irradiation induced shunting effect of forming-free, non-volatile resistive switching in polycrystalline BiFeO3 thin film capacitor-like structures with macroscopic bottom and top contacts was investigated. Oxygen atoms at the BiFeO3 surface are preferentially sputtered by Ar(+) ion irradiation and oxygen vacancies and a metallic Bi phase are formed at the surface of the BiFeO3 thin film before deposition of the top contacts. A phenomenological model is that of nanoscale shunt resistors formed in parallel to the actual BiFeO3 thin film capacitor-like structure. This model fits the noticeable increase of the retention stability and current density after irradiation. The formation of stable and conductive shunts is further evidenced by conductive atomic force microscopy measurements.

Hysteretic anomalous Hall effect in a ferromagnetic, Mn-rich Ge:Mn nanonet

Ferromagnetic Ge:Mn has been fabricated by Mn implantation in intrinsic Ge wafers and by pulsed laser annealing with a pulse duration of 300 ns. Due to a segregation instability during laser annealing, Mn segregates at the liquid-solid interface and an approximately 40 nm thick Ge:Mn surface layer is strongly enriched with Mn. Plan-view images reveal a percolating Mn-rich nanonet. Hysteretic anomalous Hall effect has been observed up to 30 K, but it vanishes after etching away the 40 nm thick Mn-rich Ge:Mn surface layer. The nanonet seems to support the correlation between magnetization and hysteretic Hall resistance. Intrinsic scattering in the threads or vertices of this nanonet may lead to the observed anomalous Hall effect.

The use of nanocavities for the fabrication of ultrathin buried oxide layers

A continuous buried oxide layer with a thickness of only 58 nm is formed in silicon by oxygen implantation at 185 keV with a very low ion fluence of 1×1017 cm−2 and subsequent He implantation. Due to the implanted He efficient oxygen gettering occurs at the implantation induced damage and results in the accumulation of the implanted oxygen as well as of oxygen indiffused from the annealing atmosphere. The morphology and the resistivity of the resulting silicon-on-insulator structure are analyzed by cross section transmission electron microscopy and by cross section scanning spreading resistance microscopy.

The origin of the energy-dose window in separation-by-implanted-oxygen materials processing

The energy-dose (ED) window (so called Izumi window) for the formation of a perfect planar and homogeneous buried oxide layer in silicon using ion implantation is controlled by the interaction of excess radiation defects and the local oxygen concentration. The ED window is defined by an appropriate correlation between the distribution of implantation-induced excess defects and the position of the finally formed oxide layer. A quantitative relation is established on the basis of collisional computer simulations. The findings are discussed in terms of oxide precipitation under the influence of defects.

The role of helium implantation induced vacancy defect on hardening of tungsten

Vacancy-type defects created by helium implantation in tungsten and their impact on the nano-hardness characteristics were investigated by correlating the results from the positron annihilation spectroscopy and the nano-indentation technique. Helium implantation was performed at room temperature (RT) and at an elevated temperate of 600 °C. Also, the effect of post-annealing of the RT implanted sample was studied. The S parameter characterizing the open volume in the material was found to increase after helium irradiation and is significantly enhanced for the samples thermally treated at 600 °C either by irradiation at high temperature or by post-annealing. Two types of helium-vacancy defects were detected after helium irradiation; small defects with high helium-to-vacancy ratio (low S parameter) for RT irradiation and large defects with low helium-to-vacancy ratio (high S parameter) for thermally treated tungsten. The hardness of the heat treated tungsten coincides with the S parameter, and hence is controlled by the large helium-vacancy defects. The hardness of tungsten irradiated at RT without thermal treatment is dominated by manufacturing related defects such as dislocation loops and impurity clusters and additionally by trapped He atoms from irradiation effects, which enhance hardness. He-stabilized dislocation loops mainly cause the very high hardness values in RT irradiated samples without post-annealing.

Three-Dimensional Carrier Profiling of Individual Si Nanowires by Scanning Spreading Resistance Microscopy

Individual silicon nanowires (Si NWs) grown by molecular beam epitaxy and in situ doped with boron were investigated by scanning spreading resistance microscopy (SSRM), a technique based on conductive atomic force microscopy. The carrier profi les of the NWs were derived from the measured spreading resistance values and calibrated with the known carrier concentration of the underlying epilayer. The 3D-SSRM profi le of a NW was obtained by measuring the NW cross sections at different depths along the radial direction. Scanning the same NW with a controlled force on the SSRM tip can abrade material from the cross-sectional surface and the tip moves deeper into the volume of the NW after each image is scanned. Repeated stripping of the material from the NW results in a “thinning” of the remaining NW segment and a corresponding increase of its resistance, which can be addressed by an appropriate data correction. The achieved three-dimensional carrier profi le reveals a multishell structure of the carrier distribution across the NW diameter, which consists of a lower doped core region, a higher doped shell region, and a carrier depleted subsurface region. Si nanowires are considered promising candidates for incorporation into the next generation of electronic devices. [ 1 , 2 ] Vertical Si NWs on Si substrate are commonly grown based on the so-called vapor-liquid-solid (VLS) [ 3 , 4 ] method by different techniques such as chemical vapor deposition (CVD) [ 5 ] and molecular beam epitaxy (MBE). [ 6 ] Doping of Si NWs has already been demonstrated using in situ [ 5 , 7–9 ] and ex situ [ 10 , 11 ] techniques. In order to precisely control the doping process for device fabrication it is imperative to understand the electrical properties of the doped Si NWs, especially the spatial distribution of the dopants in the NWs. Due to their large surfaceto-volume ratio, a strong surface segregation of dopants in NWs was theoretically predicted by ab intio calculation [ 12 , 13 ]

Rectifying filamentary resistive switching in ion-exfoliated LiNbO3 thin films

In this letter, we report the resistive switching properties of ion-exfoliated LiNbO3 thin films. After annealing in Ar or in vacuum, electro-forming has been observed on the thin films, and the oxygen gas bubbles can be eliminated by tuning the annealing conditions in order to prevent the destruction of top electrodes. The thin films show rectifying filamentary resistive switching after forming, which is interpreted by a simplified model that the local filament does not penetrate throughout the LiNbO3 thin film, resulting in asymmetric contact barriers at the two interfaces. The well controlled electro-forming step and the highly reproducible switching properties are attributed to the more homogeneous distribution of defects in single crystalline materials and the specific geometry of filament.