H-terminated Si Surfaces Research Articles

(Invited) Area-Selective Atomic Layer Deposition: Role of Surface Chemistry

Area-selective atomic layer deposition (AS-ALD) is being considered as the next paradigm shift in future device fabrication. Given the chemical nature of the deposition technique, ALD is very sensitive to surface functional groups and deposition conditions. This characteristic often leads to different nucleation delays when growing a material on one surface (growth surface) as opposed to another (non-growth surface). This feature has been exploited by several research groups to obtain area-selective growth of various materials.1,2 The approaches that have been developed for achieving AS-ALD can be categorized as either area-activation1 or area-deactivation3. When no subtractive steps are employed, area-activation is referred to as direct-write (i.e. maskless) ALD. In this contribution, results will be presented based on recently developed direct-write ALD approaches for two relevant metal oxides: In2O3:H, and ZnO. The method exploits the difference in nucleation delay on OH- and H-terminated Si surfaces. As shown in Figure 1, direct-write ALD can be achieved by locally activating an H-terminated Si starting surface using an O2-fed µ-plasma printer or by electron beam induced deposition (EBID) of a SiO2 seed layer. This allows to define OH-rich regions with dimensions ranging from a few microns (µ-plasma) down to tens of nanometers (EBID). Therefore, high-quality materials provided by ALD can be combined with the patterning capabilities of direct-write techniques to obtain patterns in a bottom-up approach. The area-selective characteristics for these two different processes was probed by X-ray photoelectron spectroscopy and electron dispersive X-ray spectroscopy for micro- and nanoscale patterns, respectively. As shown in Figure 2, in-situ spectroscopic ellipsometry was used to investigate the nucleation behaviours of the ALD processes on the growth and non-growth surfaces. Finally, the key reasons for the selectivity have been unveiled using density functional theory (DFT) simulations.

Potential changes and chemical bonding features for Si-MOS structure as evaluated from HAXPES analysis

An Au/SiO2/Si MOS structure was fabricated by the thermal oxidation of H-terminated Si(100) surface and the electrode formation, and hard X-ray photo-emission spectroscopy (HAXPES) measurements of MOS structure under the different applied bias conditions were performed systematically to investigate the potential distribution such as the band bending in the MOS structure and the chemical bonding features in the region near the Au/SiO2 interface. And, the measured energy position of HAXPES signals originating from the Au electrode, SiO2, and Si substrate were compared with the calculation results of potential change for ideal MOS structure in consideration of SiO2 thickness and impurity concentration of Si substrate.

Rapid Surface Functionalization of Hydrogen-Terminated Silicon by Alkyl Silanols.

Surface functionalization of inorganic semiconductor substrates, particularly silicon, has focused attention toward many technologically important applications, involving photovoltaic energy, biosensing and catalysis. For such modification processes, oxide-free (H-terminated) silicon surfaces are highly required, and different chemical approaches have been described in the past decades. However, their reactivity is often poor, requiring long reaction times (2–18 h) or the use of UV light (10–30 min). Here, we report a simple and rapid surface functionalization for H-terminated Si(111) surfaces using alkyl silanols. This catalyst-free surface reaction is fast (15 min at room temperature) and can be accelerated with UV light irradiation, reducing the reaction time to 1–2 min. This grafting procedure leads to densely packed organic monolayers that are hydrolytically stable (even up to 30 days at pH 3 or 11) and can display excellent antifouling behavior against a range of organic polymers.

Atomic Mechanism of Arsenic Monolayer Doping on oxide-free Silicon(111)

AbstractThe reaction pathway for shallow arsenic doping of silicon by methylarsenic acid molecules directly grafted on oxide-free, H-terminated Si(111) surfaces is unraveled combining Infrared absorption spectroscopy, X-ray Photoelectron Spectroscopy, Low Energy Ion Scattering and ab initio Molecular Dynamics simulations. The overall driving force is identified as a thermodynamic instability of As+5 in contact with silicon, which initiates a self-decomposition of chemisorbed methylarsenic molecules at ∼600 K. As the temperature is increased, the As-C bond breaks -- the weakest link of the adsorbed molecule -- with release of the organic ligand and a rearrangement from a monodentate to a bidentate bonding configuration. In this process, oxygen atoms evolve by partial desorption as H2O and partial incorporation into the surface Si atom backbonds. At ∼1050 K, diffusion of As into the sub-surface region of silicon is observed. There is no evidence for As desorption and no remaining C contamination.

(Invited) Atomically Controlled Processing for Si and Ge CVD Epitaxial Growth

Atomically controlled processing has become indispensable for the fabrication of Si-based ultrasmall devices and heterodevices for ultralarge scale integration. Our concept of atomically controlled processing is based on atomic-order surface reaction control [1-3]. The final goal is the generalization of atomic-order surface reaction processes and the creation of new properties in Si-based ultimate small structures. In the stage of nanometer-scale device fabrication, preparing atomic-order controlled surfaces and interfaces and the introduction of Ge into the Si process are required. In this work, we describe thesegregation of dopants to the surface and surface roughness generation observed during in-situ doping and their suppression by applying an atomic layer doping approach. Furthermore results of nm-order thick Ge epitaxial growth on Si(100) using an ultraclean low-pressure CVD system are presented. Figure 1 shows the P depth profile in a Si epitaxial film. Even after switching off the PH3 gas supply, P atoms are incorporated into the Si layer during Si epitaxial growth. In the steady state, it is estimated that P concentration saturates at around 3x1019cm-3 and P coverage on the Si surface is about 0.1 monolayer (6.8x1013cm-2). These values are calculated from the PH3 partial pressure and Si growth rate shown in Fig. 1 based on the surface reaction model shown in Ref. 3. The P incorporation rate, that is the product of the Si growth rate (0.8nm/min) and P concentration (about 1x1019cm-3), is about 8x1011cm-2/min. If it is assumed that the desorption of P atoms on the Si surface can be neglected [4], 6.8x1013cm-2 P atoms on the surface are incorporated into the Si epitaxial layer with 8x1011cm-2/min even after the PH3 supply has been stopped. The increase of surface roughness from RMS 0.1nm to 0.8nm during in-situ doping can be explained by the suppression of the SiH4reaction caused by the P atoms which are present on the surface. Suppression of P segregation and surface roughening are achieved using P atomic layer doping in Si. This includes the optimization of the capping Si layer deposition conditions and the P atomic amount [5] as shown in Fig. 2. This may result from low temperature Si epitaxial growth on P atoms with high growth rate. That could be demonstrated for P atomic layer doping during Ge epitaxial growth too [6]. In a second part of the paper results for nm-order-thick Ge high quality epitaxial growth on Si (100) are described. Because pure Ge and Si oxide are formed on Si substrate even at 300oC by the reaction between vaporized Ge oxide and Si substrate [7], the suppression of Ge oxide vaporization in the CVD reactor is very important. It was proposed that Ge island growth in an initial growth stage is caused by the suppression of adsorption and/or decomposition of GeH4 on the H-terminated Si surface [8]. Even on the initially H-free Si surface, Si atoms are H-terminated by the decomposition of GeH4. In order to decrease the hydrogen desorption temperature from the Si site, nm-thick Si1-xGex buffer layer on Si (100) was proposed [9,10]. The suppression of surface roughening was improved by nm-order-thick Ge epitaxial growth on nm-order-thick Si1-xGex/Si(100). The surface segregation of Si through nm-order-thick Ge layer from the substrate was observed at Ge growth temperature of 350oC. These results demonstrate the capability of atomically controlled processing for nm-order thick Si and Ge CVD epitaxial growth for ultralarge scale integration. This work is partially supported by the JSPS Core-to-Core Program, “International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration”.

Silicon epitaxy on H-terminated Si (100) surfaces at 250 °C

Low temperature Si epitaxy has become increasingly important due to its critical role in the encapsulation and performance of buried nanoscale dopant devices. We demonstrate epitaxial growth up to nominally 25nm, at 250°C, with analysis at successive growth steps using STM and cross section TEM to reveal the nature and quality of the epitaxial growth. STM images indicate that growth morphology of both Si on Si and Si on H-terminated Si (H: Si) is epitaxial in nature at temperatures as low as 250°C. For Si on Si growth at 250°C, we show that the Si epitaxial growth front maintains a constant morphology after reaching a specific thickness threshold. Although the in-plane mobility of silicon is affected on the H: Si surface due to the presence of H atoms during initial sub-monolayer growth, STM images reveal long range order and demonstrate that growth proceeds by epitaxial island growth albeit with noticeable surface roughening.

Ammonia modification of oxide-free Si(111) surfaces

Amination of surfaces is useful in a variety of fields, ranging from device manufacturing to biological applications. Previous studies of ammonia reaction on silicon surfaces have concentrated on vapor phase rather than wet chemical processes, and mostly on clean Si surfaces. In this work, the interaction of liquid and vapor-phase ammonia is examined on three types of oxide-free surfaces – passivated by hydrogen, fluorine (1/3 monolayer) or chlorine – combining infrared absorption spectroscopy, X-ray photoelectron spectroscopy, and first-principles calculations. The resulting chemical composition highly depends on the starting surface; there is a stronger reaction on both F- and Cl-terminated than on the H-terminated Si surfaces, as evidenced by the formation of Si-NH2. Side reactions can also occur, such as solvent reaction with surfaces, formation of ammonium salt by-products (in the case of 0.2M ammonia in dioxane solution), and nitridation of silicon (in the case of neat and gas-phase ammonia reactions for instance). Unexpectedly, there is formation of Si-H bonds on hydrogen-free Cl-terminated Si(111) surfaces in all cases, whether vapor phase of neat liquid ammonia is used. The first-principles modeling of this complex system suggests that step-edge surface defects may play a key role in enabling the reaction under certain circumstances, despite the endothermic nature for Si-H bond formation.

水素終端及び清浄Si(001)面の酸化と酸素の脱離

水素終端及び清浄Si(001)面の酸化と酸素の脱離

Role of the Deposition Precursor Molecules in Defining Oxidation State of Deposited Copper in Surface Reduction Reactions on H-Terminated Si(111) Surface.

Surface-limited deposition reactions leading to the formation of copper nanoparticles on H-terminated Si(111) surface can serve as a model for understanding the role of structure of the deposition precursor molecules in determining the oxidation state of the metal deposited. This study compares three different precursor molecules: Cu(acac)2 (Cu(II) acetylacetonate), Cu(hfac)2, and Cu(hfac)VTMS (Cu(I)-(hexafluoroacetylacetonato)-vinyltrimethylsilane) as copper deposition sources in a process with a controlled oxidation state of copper. X-ray photoelectron spectroscopy suggests that single-electron reduction governs the deposition of Cu(I) from the first two precursor molecules and that the last of the precursors studied yields predominantly metallic copper. Time-of-fight secondary ion mass spectrometry (ToF-SIMS) and infrared spectroscopy are utilized to interrogate surface species produced. Atomic force microscopy is used to quantify the deposition process and to follow the size distribution of the deposited copper containing nanoparticles. A plausible explanation supported by density functional theory calculations is offered on the basis of the difference in the reaction pathways for Cu(I) and Cu(II) precursors.

Effect of α-Heteroatoms on the Formation of Alkene-Derived Monolayers on H-Si(111): A Combined Experimental and Theoretical Study.

We investigate herein whether the reactivity and surface coverage of 1-alkenes toward hydrogen-terminated Si(111) surfaces [H-Si(111)] can be improved by introducing heteroatoms such as oxygen and sulfur at the α-position next to the alkene functional group. To this end, the reactivity of 1-pentene, 1-pentyne, vinyl ethyl ether, and vinyl ethyl sulfide toward H-Si(111) and the surface coverage of the resulting monolayers were studied and compared. All modified surfaces were characterized by static water contact angle measurements, ellipsometry, X-ray photoelectron spectroscopy (XPS), and infrared absorption reflection spectroscopy (IRRAS). Quantum chemical calculations were performed to calculate the activation barriers and driving forces for monolayer formation at the M11-L/6-311G(d,p) level of theory. Both experiments and theory indicate that the presence of α-heteroatoms next to the alkene function improved both the reactivity and surface coverage on H-terminated Si(111) surfaces.

Atomic Layer Deposition of Cobalt Silicide Thin Films Studied by in Situ Infrared Spectroscopy

Atomic layer deposition of cobalt silicide (CoSi2) thin films on H-terminated Si(111) surfaces, using the cobalt-based precursor tertiarybutylallylcobalttricarbonyl (tBu-AllylCo(CO)3) and trisilane, is investigated by in situ Fourier transform infrared spectroscopy (FTIR) and ex situ X-ray photoelectron spectroscopy (XPS) to uncover the film growth mechanisms. The strong reactivity of tBu-AllylCo(CO)3 with H-terminated silicon surfaces and inertness with silicon oxide surfaces, as previously determined by IR spectroscopy [Chem. Mater. 2012, 24, 1025], opens the door for selective deposition. Deposition of CoSi2 is observed after a brief nucleation period (∼3 cycles), during which the stabilization of the cobalt precursor takes place, as evidenced by a shift of the stretch frequency of the carbonyl groups bonded to the Co center from 2010 to 1980 cm–1. This shift is evidence for completion of the catalytic reaction and leads to a surface termination and configuration that is favorable for subsequent ligand...

Nanopatterning on H-Terminated Si(111) Explained as Dynamic Equilibrium of the Chemical Reaction with Methanol

Here the microscopic mechanism that leads to the surprising formation of a nanopattern upon methanol reacting with a H-terminated Si(111) surface [Michalak et al., Nat. Mater. 2010, 9, 266–271] is reinvestigated from both theory and experiment. First-principles calculations determine the fully OCH3-terminated Si(111) surface as the thermodynamic ground state in the presence of methanol, seemingly in contrast to the experimentally found nanopattern. At 65 °C the presence of H2 initiates desorption of the methoxy groups and thereby leads to a dynamic equilibrium of the reaction of methanol with the H-terminated Si(111) surface, which is a 1/3 OCH3-terminated and 2/3 H-terminated Si(111) surface, with all OCH3 groups standing as NNNs, corresponding to the nanopattern observed earlier. Investigating fluorine group termination of Si(111) as a test system the present study demonstrates that the combination of theoretical and experimental techniques used here is in fact sufficiently sensitive to resolve complex molecular systems even with lateral resolution.

Controlling the polarity of metalorganic vapor phase epitaxy-grown GaP on Si(111) for subsequent III-V nanowire growth

Nanowire growth on heteroepitaxial GaP/Si(111) by metalorganic vapor phase epitaxy requires the [-1-1-1] face, i.e., GaP(111) material with B-type polarity. Low-energy electron diffraction (LEED) allows us to identify the polarity of GaP grown on Si(111), since (2×2) and (1×1) surface reconstructions are associated with GaP(111)A and GaP(111)B, respectively. In dependence on the pre-growth treatment of the Si(111) substrates, we were able to control the polarity of the GaP buffers. GaP films grown on the H-terminated Si(111) surface exhibited A-type polarity, while GaP grown on Si surfaces terminated with arsenic exhibited a (1×1) LEED pattern, indicating B-type polarity. We obtained vertical GaAs nanowire growth on heteroepitaxial GaP with (1×1) surface reconstruction only, in agreement with growth experiments on homoepitaxially grown GaP(111).

Water wettability of Si(1 1 1) and (0 0 1) surfaces prepared to be reconstructed, atomic-hydrogen terminated and thinly oxidized in an ultrahigh vacuum chamber

Water contact angles on three types of Si(111) and (001) surfaces were measured in nitrogen gas atmosphere without exposing them to air; the surfaces were prepared to be (I) cleaned and reconstructed, subsequently (II) atomic hydrogen (H) terminated, or (III) thinly oxidized in an ultrahigh vacuum (UHV) chamber. The surfaces were characterized by Auger electron spectroscopy and atomic force microscopy. The H-terminated Si surfaces showed as less hydrophilic with a water contact angle of about 37° on the (111) and about 60° on the (001) surface, respectively. The clean and oxidized surfaces showed as super-hydrophilic with the angles less than 6°. To quantitatively evaluate the contact angles on the super-hydrophilic surfaces, an optical interference fringe method was used under an optical microscope in air. The most super-hydrophilic surface was the oxidized Si(111) surface. The reactivity of the Si surfaces and their degree of hydroxyl group termination to the Si surfaces responsible for water wettability were discussed.

Phase diagram of fractional quantum Hall effect of composite fermions in multicomponent systems

While the integer quantum Hall effect of composite fermions manifests as the prominent fractional quantum Hall effect (FQHE) of electrons, the FQHE of composite fermions produces further, more delicate states, arising from a weak residual interaction between composite fermions. We study the spin phase diagram of these states, motivated by the recent experimental observation by Liu {\em et al.} \cite{Liu14a,Liu14b} of several spin-polarization transitions at 4/5, 5/7, 6/5, 9/7, 7/9, 8/11 and 10/13 in GaAs systems. We show that the FQHE of composite fermions is much more prevalent in multicomponent systems, and consider the feasibility of such states for systems with ${\cal N}$ components for an SU(${\cal N}$) symmetric interaction. Our results apply to GaAs quantum wells, wherein electrons have two components, to AlAs quantum wells and graphene, wherein electrons have four components (two spins and two valleys), and to an H-terminated Si(111) surface, which can have six components. The aim of this article is to provide a fairly comprehensive list of possible incompressible fractional quantum Hall states of composite fermions, their SU(${\cal N}$) spin content, their energies, and their phase diagram as a function of the generalized "Zeeman" energy. We obtain results at three levels of approximation: from ground state wave functions of the composite fermion theory, from composite fermion diagonalization, and, whenever possible, from exact diagonalization. Effects of finite quantum well thickness and Landau level mixing are neglected in this study. We compare our theoretical results with the experiments of Liu {\em et al.} \cite{Liu14a,Liu14b} as well as of Yeh {\em et al.} \cite{Yeh99} for a two component system.

Use of a Purged FOUP to Improve H-Terminated Silicon Surface Stability Prior to Epitaxial Growth

H-terminated Si surfaces serve as the starting surface for hetero-epitaxial growth. The presence of oxygen on the wafer surface can significantly reduce the epitaxial layer quality. The oxygen present on the surface is, among others, determined by the re-oxidation of the surface during the queuing between cleaning and epitaxy. In this paper we report on the use of a N2-purged FOUP to reduce the re-oxidation of Si surfaces.

Use of a Purged FOUP to Improve H-Terminated Silicon Surface Stability Prior to Epitaxial Growth

A H-terminated Si surface serves as the starting surface for hetero-epitaxial growth. The presence of oxygen on the wafer surface caused by surface re-oxidation can significantly reduce the epitaxial quality. In this paper we evaluated the capabilities of an Entegris EBM (Entegris Barrier Material) purged FOUP (Front-Opening-Unified-Pod) to reduce this re-oxidation. The surface cleanliness is one of the factors affecting (hetero-) epitaxial quality. Clean starting surfaces can be obtained by a high temperature H2-bake (T > 800 °C). [1] Surface reflow during these high temperature bakes can significantly impact the integrity of small structures, e.g. causing corner rounding of FINFET structures. [2] As the clean starting surface can no longer be obtained by the H2-bake at relative high temperatures, more stringent requirements are put on the pre-epi clean and on the impact of queue time between wet-chemical clean and epitaxial growth. We have evaluated the re-oxidation of H-terminated 300 mm Si surfaces in a non-purged FOUP compared to an EBM purged FOUP while the FOUP is open and docked to the factory interface.Re-oxidation or degradation of the H-terminated Si surface has been evaluated by capping the surface with an epitaxial-grown 80nm Si-cap/80nm Si0.8Ge0.2 bilayer. The H-terminated surface was created in a separate step before the epitaxial growth by applying a thermal treatment in H2 in a conventional ASM Epsilon3200 epi reactor. The Si wafer was unloaded in between the thermal treatment and the Si-cap/SiGe epitaxial growth step. This procedure offers the most-controllable scheme for creating a H-passivated surface but the learnings are also applicable to wet-chemical pre-epi treatments. The Si0.8Ge0.2 / Si substrate interfacial cleanliness was evaluated at the center of the wafer by SIMS (Fig. 1). The EBM purged FOUP shows a lower interfacial degradation compared to the standard FOUP. The uniformity of the surface degradation has been investigated by surface light scattering (data not shown). While the light-scattering intensity and its range increased significantly after queuing for 1hr in a non-purged FOUP, this increase was suppressed when using the N2-purged FOUP. The light scattering measurements also show that the epitaxial quality is better in a N2-purged FOUP compared to a non-purged FOUP but degraded compared to the loadlock reference.In this paper we evaluated the impact of queue time of an H-terminated Si surface in a non-purged FOUP compared to an EBM N2-purged FOUP. The interfacial degradation as measured by light scattering and SIMS for 1hr of queue time is shown to be less when the EBM N2-purged FOUP is used. However the epitaxial quality degradation was shown not to be completely suppressed.

Interfacial Layer Engineering Using Thulium Silicate/Germanate for High-k/Metal Gate MOSFETs

State-of-the-art CMOS technology employs high-k dielectrics in place of the conventional SiO2 and SiON gate dielectrics, due to the ability to achieve high capacitance density while guaranteeing sufficiently low leakage current density. Hf-based dielectrics are the most widely used high-k dielectrics in current CMOS technology and are usually deposited on top of an interfacial layer (IL), whose primary role is to provide good electrical quality of the interface with Si [1].A sub-nm chemical oxide (SiOx) or oxynitride (SiON) layer is typically employed as IL, although its relatively low dielectric constant hinders the long term scalability of the stack. It has however been shown that reducing or eliminating the IL has strong negative effects on device performance, therefore the IL thickness remains subject to a strict trade-off [2].An alternative approach consists in the integration of high-k interfacial layers, which can provide good electrical quality of the interface with the semiconductor while achieving very low IL EOT [3]. We have recently demonstrated a process module for integration of thulium silicate (TmxSiyO, “TmSiO” hereafter) as replacement for chemical oxide interfacial layer in a generic gate stack, providing interfacial layer EOT of 0.2±0.1 nm and interface state density lower than 2·1011 cm-2eV-1 at a total CET of >1.5 nm [4]. In this work TmSiO was integrated with HfO2/TiN gate stack to achieve a total EOT<1 nm.To evaluate the TmSiO/HfO2/TiN gate stack long channel P and NMOSFETs were fabricated using a gate last scheme with predefined dopant activated source and drain areas. The thermal budget was 400 °C after gate stack formation. Prior to gate formation the wafers were cleaned in H2SO4:H2O2 (3:1) and dipped in 5% HF followed by ALD Tm2O3 deposition at 250 °C using TmCp3 and H2O vapour as precursors [5]. A post deposition anneal at 550 °C for 60 s in N2 was employed to form a ~0.8 nm thick TmSiO layer. Unreacted Tm2O3 was stripped in H2SO4 with a selectivity of 23:1 towards TmSiO. HfO2 was deposited by ALD at 350 °C, using HfD04 and H2O vapor as precursors followed by post deposition anneal in ozone. Fig. 1 shows (a) schematically the TmSiO IL process and (b) a cross-section TEM of the complete gate stack. PVD TiN/TiW 15/100 nm was used as gate electrode. Ti/TiW/Al metallization and FGA (10% H2 in N2) ended the MOSFET fabrication.Excellent electron (~230 cm2/Vs) and hole mobility (~75 cm2/Vs) at EOT=0.8 nm and Ninv=1013 cm-2 was measured. These values represent a ~20 % improvement compared to SiOx/HfO2 gate stacks [2],[6]. The higher mobility possibly originates from the thicker physical thickness of the interfacial layer compared to SiOx interfacial layer for the same total EOT. Fig. 2 shows well behaved ID-VG curves for both P and NFETs with subthreshold slopes in the range 65-85 mV/dec.Given the excellent results of TmSiO interfacial layer on Si channel P and NFETs using HfO2/TiN as high-k/metal gate stacks, we investigate the potential for passivation of Ge MOSFETs using the same concept with the aim of forming thulium germanate (TmGeO) as an interfacial layer between the Ge channel and the high-k dielectric. Potentially the TmGeO interfacial layer can improve the scalability of GeO2 interfacial layer based gate stacks such as GeO2/Al2O3/HfO2 [7]. The formation of TmGeO is however not as straightforward as in the Si case, where the ALD growth of Tm2O3 was initiated on H-terminated Si surface. For Ge channel MOSFETs the Ge surface is not H terminated after HF etching but instead a relatively stable monoxide (GeO) exists on the Ge surface. Since GeO is insoluble in H2O it is challenging to completely remove it before ALD growth of Tm2O3. In our initial experiments we have found that annealing of Tm2O3 layers grown by ALD on HF-last Ge surfaces leads to inhibited formation of TmGeO. In this work we will report on various cleaning and pre-ALD treatments and their influence on the formation of TmGeO.REFERENCES[1] M. M. Frank, ESSDERC, 25-33(2011).[2] L.-A. Ragnarssson et al., Microelectron. Eng., vol. 88, no 7, 1317-1322, 2011.[3] T. Kawanago et al., IEEE TED, vol 59, no 2, 269-276, 2012[4] E. Dentoni Litta et al., IEEE TED, vol. 60, no 10, 3271-3276, 2013[5] E. Dentoni Litta et al., J. Electrochem. Soc., vol. 160, no 11, D538-D542, 2013[6] L.-A. Ragnarsson et al., VLSI_TSA, 2011[7] R. Zhang et al., IEEE TED, vol. 60, no 3, 927-934, 2013

Activation of silicon surfaces for H2 evolution by electrografting of pyridine molecules

Abstract In this work we describe an electrografting procedure for the preparation of pyridine-functionalized Si(111) surfaces, and we demonstrate that this modified surface reduces the overpotential for the hydrogen evolution reaction by about 400 mV compared to the H-terminated Si surface. Thus, evidence is presented that the functionalized Si surface has a catalytic activity for an inner sphere electrochemical reaction and might be an attractive alternative to metal or metal oxide cluster-covered semiconductors in photoelectrochemical devices converting solar energy into chemical energy.

Lowering the density of electronic defects on organic-functionalized Si(100) surfaces

The electrical quality of functionalized, oxide-free silicon surfaces is critical for chemical sensing, photovoltaics, and molecular electronics applications. In contrast to Si/SiO2 interfaces, the density of interface states (Dit) cannot be reduced by high temperature annealing because organic layers decompose above 300 °C. While a reasonable Dit is achieved on functionalized atomically flat Si(111) surfaces, it has been challenging to develop successful chemical treatments for the technologically relevant Si(100) surfaces. We demonstrate here that recent advances in the chemical preparation of quasi-atomically-flat, H-terminated Si(100) surfaces lead to a marked suppression of electronic states of functionalized surfaces. Using a non-invasive conductance-voltage method to study functionalized Si(100) surfaces with varying roughness, a Dit as low as 2.5 × 1011 cm−2eV−1 is obtained for the quasi-atomically-flat surfaces, in contrast to &amp;gt;7 × 1011 cm−2eV−1 on atomically rough Si(100) surfaces. The interfacial quality of the organic/quasi-atomically-flat Si(100) interface is very close to that obtained on organic/atomically flat Si(111) surfaces, opening the door to applications previously thought to be restricted to Si(111).