Ultrathin GeSn p-channel MOSFETs grown directly on Si(111) substrate using solid phase epitaxy

Ultrathin GeSn layers with a thickness of 5.5 nm are fabricated on a Si(111) substrate by solid phase epitaxy (SPE) of amorphous GeSn layers with Sn concentrations up to 6.7%. We demonstrate well-behaved depletion-mode operation of GeSn p-channel metal–oxide–semiconductor field-effect transistors (pMOSFETs) with an on/off ratio of more than 1000 owing to the ultrathin GeSn channel layer (5.5 nm). It is found that the on current increases significantly with increasing Sn concentration at the same gate overdrive, attributed to an increasing substitutional Sn incorporation in Ge. The GeSn (6.7%) layer sample shows approximately 90% enhancement in hole mobility in comparison with a pure Ge channel on Si.status: publishe

Multimodal switching of a redox-active macrocycle

Molecules that can switch between multiple stable states in response to stimuli are promising for many applications, but are challenging to construct. Here, the authors design a resorcinarene switching manifold with multiple oxidation states and coupled charge-transfer states, which can access up to five distinct switch-states with unique optical outputs

Nanomolecular singlet oxygen photosensitizers based on hemiquinonoid-resorcinarenes, the fuchsonarenes

Singlet oxygen sensitization involving a class of hemiquinonoid-substituted resorcinarenes prepared from the corresponding 3,5-di-t-butyl-4-hydroxyphenyl-substituted resorcinarenes is reported. Based on variation in the molecular structures, quantum yields comparable with that of the well-known photosensitizing compound meso-tetraphenylporphyrin were obtained for the octabenzyloxy-substituted double hemiquinonoid resorcinarene reported herein. The following classes of compounds were studied: benzyloxy-substituted resorcinarenes, acetyloxy-substituted resorcinarenes and acetyloxy-substituted pyrogallarenes. Single crystal X-ray crystallographic analyses revealed structural variations in the compounds with conformation (i.e., rctt, rccc, rcct) having some influence on the identity of hemiquinonoid product available. Multiplicity of hemiquinonoid group affects singlet oxygen quantum yield with those doubly substituted being more active than those containing a single hemiquinone. Compounds reported here lacking hemiquinonoid groups are inactive as photosensitizers. The term ‘fuchsonarene’ (fuchson + arene of resorcinarene) is proposed for use to classify the compounds

Marimo-Bead-Supported Core-Shell Nanocomposites of Titanium Nitride and Chromium-Doped Titanium Dioxide as a Highly Efficient Water-Floatable Green Photocatalyst

The release of untreated industrial wastewater creates a hazardous impact on the environment. In this regard, the development of an environmentally friendly catalyst is of paramount importance. Here, we report a highly efficient and reusable core-shell TiN/SiO2/Cr-TiO2 (TSCT) photocatalyst that is composed of SiO2-cladded titanium nitride (TiN) nanoparticles (NPs) decorated with Cr-doped TiO2 NPs for the removal of organic contaminants from water. The TiN NPs serve as the main light absorber component with excellent visible-light absorption along with Cr-TiO2 NPs. The TSCT shows remarkable improvement in the photodecomposition of methylene blue (MB) over Cr-TiO2 and TiO2 NPs. An efficient structural design is proposed by adopting calcium alginate beads (P-Marimo beads) as a transparent scaffold for supporting our TSCT, which floats nature on the water surface and realizes easy handling as well as excellent reusability for multipurpose water purification. Surprisingly, our TSCT is found to keep its catalytic activity even after the illumination is turned off. Our proposed P-Marimo-encapsulated TSCT can be utilized as an excellent green photocatalyst with high photocatalytic performance, good recyclability, and easy handling

Junctionless GeSn pMOSFETs on Si (111) by solid phase epitaxy

1. Introduction GeSn has been predicted to exhibit carrier mobilities exceeding both that of Ge and Si, which makes GeSn suitable as alternative channel material in high-speed Si-CMOS technology. In addition, GeSn exhibits a direct band gap for Sn concentration of ±8%, creating the possibility of optical applications. While previous GeSn channel transistors were predominately on Ge substrates [1-3], integration into Si is preferred for CMOS compatibility. Epitaxial growth of GeSn on Si substrates poses several challenges: the limited solubility of Sn in Ge (0.5%), compositional fluctuations, Sn segregation and large lattice mismatch (>4%). It is critical to suppress these effects for obtaining high performance devices with GeSn layers. Recently, we demonstrate single crystalline GeSn layers on Si(111) substrates by solid phase epitaxy (SPE) of amorphous GeSn layers with excellent structural quality [4]. This technique has advantage to realize ultrathin GeSn layer directly on Si, which have high potential to make depletion-mode pMOSFET owing to the decent valence band offset between Si and GeSn. As a bulk transport is dominant in depletion-mode devices [5, 6], excellent GeSn bulk mobility can be fully utilized. In this work, employing ultrathin single crystalline GeSn layer directly on Si substrates, we fabricate depletion-type GeSn pMOSFETs with TaN/Al2O3 gate stack and NiGeSn metal S/D. We observe well-behaved junctionless GeSn pMOSFET operation thanks to ultrathin GeSn channel layer. 2. GeSn on Si by SPE We have deposited 40 nm amorphous GeSn (4.5% Sn) on insulating Si substrates with (111) orientation at RT. We limit the adatom surface mobility during deposition with the introduction of inert gas species to improve the quality of SPE. Subsequent annealing at 600Ë C for 1 minute in N2 ambient is carried out to transform the amorphous GeSn layer into single crystalline GeSn. XRD Ï /2θ scan of the as-deposited layer does not show diffraction of GeSn, confirming that the GeSn does not grow epitaxially with the growth conditions that were applied, see Figure 1 (black, solid line with squares). Subsequent annealing at 600 Ë C for 1 minute transforms the amorphous GeSn into a single crystalline layer by SPE. An intense and narrow GeSn(111) diffraction peak is observed, indicating high crystal quality, as observed from Ï /2θ XRD scan, see Figure 1 (red, solid line). The (111) crystal planes of the GeSn layer are parallel to the (111) Si surface. The presence of Pendellösung fringes indicates a smooth GeSn surface and interface between GeSn and Si. Hall mobility of p-type GeSn layer is 128 cm2/Vs with a hole concentration of 1.6 x 1018 cm-3, which is higher than that of Si. 3. Device fabrication Prior to the device fabrication, we reduced GeSn layer thickness using RIE etching for 2 ~ 4 sec. Then, we fabricated GeSn pMOSFETs with ALD Al2O3 dielectrics and self-aligned NiGeSn S/D. After surface cleaning, a 100-cycle Al2O3 layer was deposited as a gate insulator by ALD and PDA carried out at 400ºC. Next, TaN metal gate was formed by sputtering and dry etching. Subsequently, 25nm-thick Ni was deposited on the GeSn layer in order to form the NiGeSn metal S/D. Self-aligned Ni-based metal S/D for GeSn was fabricated by annealing at 350ºC for 1 min, followed by selective wet etching using a HCl solution to remove the unreacted Ni. 3. Electrical properties Idâ Vg characteristics of GeSn p-MOSFETs with various GeSn thicknesses at Vd of -50mV are shown in Fig. 2. The devices show good transfer characteristics. By decreasing the channel thickness from 35 nm to around 10 nm we reduce the off current significantly so as to deplete majority carriers in the doped channel for the off-state. Figure 3 shows the Idâ Vg characteristics of a 10nm-thick GeSn pMOSFET on Si with the gate length of 10μm. We can successfully control Id with On/Off ratio of 84 by reducing the Ge thickness to thinner than its maximum depletion layer width. These results indicate the first successful operation of depletion-mode junctionless GeSn pMOSFET on Si. While thinner GeSn layer improve gate control to the channel, we observe that the hole mobility decrease as the GeSn thickness reduce presumably because of the surface roughening by RIE etching and the carrier scattering at the GeSn/Si heterointerface. 4. Conclusion In conclusion, we have demonstrated depletion-mode operation of GeSn pMOSFETs on Si (111) using SPE, Ni-based metal S/D processes, and TaN/Al2O3 metal-gate/high-k gate stacks. We present good transfer characteristics with on/off ratio of 84 as a result of ultrathin GeSn layer on Si. Further improvement in depletion-mode junctionless GeSn pMOSFET can be obtained to control the structural quality of thin film GeSn. Acknowledgment A part of the device fabrication was carried out at AIST-NPF. R.R.L acknowledges support as Research Fellow of the Research Foundation â Flanders (FWO). References [1] S. Gupta et al., IEDM 398 (2011). [2] G. Han et al., IEDM, 402 (2011). [3] G. Han et al., VLSI symp., 97 (2012).status: publishe

Hot Electron Excitation from Titanium Nitride Using Visible Light

One major strategy that has been used to inject carriers into wide-band-gap materials involves exciting hot carriers from a nanostructured metal using low-energy photons. Here, we demonstrate that titanium nitride (a conductive ceramic) can be used as an alternative for photoexciting hot carriers. Planar samples that form titanium nitride/zinc oxide/titanium nitride trilayers are fabricated, and the generation of photocurrent using visible light is confirmed. The photocurrent obtained by using titanium nitride is much larger than that obtained by using gold in a similar structure. Our results will therefore facilitate the use of titanium nitride, which is robust and inexpensive, in harvesting the visible region of the solar spectrum in photovoltaics and photocatalysis

Tensile strained GeSn on Si by solid phase epitaxy for high mobility FET devices

GeSn has been predicted to exhibit carrier mobilities exceeding both that of Ge and Si, which makes GeSn suitable as alternative channel material in high-speed Si-CMOS technology. In addition, GeSn exhibits a direct band gap for Sn concentration of ±8%, creating the potential for optical applications. Epitaxial growth of GeSn on Si substrates poses several challenges: the limited solubility of Sn in Ge (0.5%), compositional fluctuations, Sn segregation and large lattice mismatch (>4%). It is critical to suppress these effects for obtaining high performance devices with GeSn layers. In this work, we investigate the deposition of amorphous GeSn layers and subsequent crystallization on Si substrates, without Ge buffer layer. We limit the adatom surface mobility during deposition with the introduction of inert gas species to improve the quality of solid phase epitaxy. Using this approach we demonstrate single crystalline GeSn layers on Si substrates with excellent crystal quality and smooth surface with up to 6.1% of Sn. The GeSn layers show tensile strain (up to +0.34%), which lowers the difference between direct and indirect band transition and makes this approach promising for obtaining direct band gap group IV layers. GeSn with 4.5% Sn shows an optical band gap of 0.52 eV. We have fabricated and characterized GeSn(111) pMOSFETs with TaN/Al2O3 gate stack. We observe well-behaved depletion mode GeSn pMOSFET operation on Si with an On/Off ratio of 66 at VD = 1V.status: publishe

Surface-Enhanced Raman Spectroscopy of Ammonium Nitrate Using Al Structures, Fabricated by Laser Processing of AlN Ceramic

This work presents results on laser-induced surface structuring of AlN ceramic and its application in Surface-Enhanced Raman Spectroscopy (SERS). The laser processing is performed by nanosecond pulses in air and vacuum. Depending on the processing conditions, different surface morphology can be obtained. The ablation process is realized by ceramic decomposition as the formation of an aluminium layer is detected. The efficiency of the fabricated structures as active substrates in SERS is estimated by the ability of the detection of ammonium nitrate (NH4NO3). It is conducted for Raman spectrometer systems that operate at wavelengths of 514 and 785 nm where the most common commercial systems work. The obtained structures contribute to enhancement of the Raman signal at both wavelengths, as the efficiency is higher for excitation at 514 nm. The limit of detection (LOD) of ammonium nitrate is estimated to be below the maximum allowed value in drinking water. The analysis of the obtained results was based on the calculations of the near field enhancement at different conditions based on Finite Difference Time Domain simulation and the extinction spectra calculations based on Generalized Mie scattering theory. The structures considered in these simulations were taken from the SEM images of the real samples. The oxidation issue of the ablated surface was studied by X-ray photoelectron spectroscopy. The presented results indicated that laser structuring of AlN ceramics is a way for fabrication of Al structures with specific near-field properties that can be used for the detection of substances with high social impact

Ultrathin GeSn p-channel MOSFETs grown directly on Si(111) substrate using solid phase epitaxy

Ultrathin GeSn layers with a thickness of 5.5 nm are fabricated on a Si(111) substrate by solid phase epitaxy (SPE) of amorphous GeSn layers with Sn concentrations up to 6.7%. We demonstrate well-behaved depletion-mode operation of GeSn p-channel metal–oxide–semiconductor field-effect transistors (pMOSFETs) with an on/off ratio of more than 1000 owing to the ultrathin GeSn channel layer (5.5 nm). It is found that the on current increases significantly with increasing Sn concentration at the same gate overdrive, attributed to an increasing substitutional Sn incorporation in Ge. The GeSn (6.7%) layer sample shows approximately 90% enhancement in hole mobility in comparison with a pure Ge channel on Si.status: publishe

Initial Processes of Atomic Layer Deposition of Al2O3 on InGaAs: Interface Formation Mechanisms and Impact on Metal-Insulator-Semiconductor Device Performance

material