Hydrogen and hot electron defect creation at the Si(100)/SiO2interface of metal-oxide-semiconductor field effect transistors

We explore the hydrogen-related microstructures involved in hot electron defect creation at the Si(100)–SiO₂interface of metal-oxide-semiconductor field effect transistors. With ab initio density functional calculations, the energetics and defect levels have been calculated for hydrogen in bulk silicon, bulk silicon dioxide and at their interface. We relate these calculations to several experiments and suggest a microscopic model for hydrogen-related hot electron degradation.     

Nanoporous silicon doped by Cu for gas-sensing applications

The effect of hydrogen sulphide on the current–voltage characteristics of metal–insulator–semiconductor (MIS) structures based on nanoporous silicon (Si_nanopor) under copper doping has been investigated. Scanning electron (SEM), atomic force (AFM) and optic microscopes and/or secondary ion mass spectroscopy (SIMS) were used to obtain detailed characterisation of copper cluster distribution present at the surface and pores, respectively. SIMS spectra reveal that finite gradient in copper distribution along the pores and oxidation of nanoporous silicon simultaneously can be obtained successfully under electroless deposition process. It was also shown that the doping of nanoporous silicon by Cu leads to enhanced hydrogen sulphide sensitivity of MIS structures even without catalytic active top electrodes (for example, Pd) at room temperature. Furthermore, for different types of familiar MIS structures based on nanoporous silicon, e.g., MIS structures doped or undoped by copper and by using Pd metal electrodes, the hydrogen sulphide detection at room temperature mainly depends on the modification in the height of barrier of hetero- (Al–Cu–Si_nanopor–c-Si) or Schottky-like (Pd–Cu–Si_nanopor–c-Si) structures resulting the chemical interaction of molecular H₂S gas with copper clusters at the surface and in the pores. It is demonstrated that MIS structures based on the nanoporous silicon with copper doping are more sensitive to H₂S action at room temperature. In addition, the physical mechanism explaining the observed phenomena is also discussed.     

Hydrogen sensitive gas sensor based on porous silicon/TiO2−x structure

Porous silicon (PS) layer was formed by electrochemical anodization on a p-type Si surface. Thereafter, n-type TiO_2−x thin film was deposited onto the PS surface by electron-beam evaporation. Pt catalytic layer and Au electrical contacts for further measurements were deposited onto the PS/TiO_2−x structure by ion-beam sputtering. Current–voltage characteristic, sensitivity to different concentration of hydrogen and resistance change of obtained structures versus time were examined. Results of measurements have shown that the current–voltage characteristics of structures are similar to that of diode. High sensitivity to hydrogen of obtained structures was also detected. Note that all measurements were carried out at room temperature.     

Electron paramagnetic resonance of hydrogen in silicon

A review of the investigations by means of electron paramagnetic resonance (EPR) of hydrogen and hydrogen-related defects in crystalline silicon is presented. The main features of the EPR center Si-AA9 (bond-centered hydrogen), which is known as the hydrogenic analogue of the anomalous muonium (Mu^*) in silicon, are discussed. It was found that the process of annealing the AA9 center is characterized by an activation energy, E = 0.48 ± 0.04 eV with a second-order pre-exponential factor, K₀ = (1.25 ± 2.5) × 10^-7 cm³/s.

A detailed investigation by EPR of the defect (Si-AA1), which we identify as the hydrogen-related shallow donor in a positive charge state, is also presented. In particular it is shown that the H-related shallow donor is a helium-like center and its wave function has C_2v symmetry. Moreover, the main features of the series of EPR spectra in silicon characteristic for the implantation of hydrogen are presented.     

Electrical behavior of size-controlled Si nanocrystals arranged as single layers

A metal–oxide–semiconductor structure containing a single layer of size-controlled silicon nanocrystals embedded into gate oxide was fabricated. Size control for the silicon nanocrystals was realized by using a SiO₂/SiO/SiO₂ layer structure with the embedded SiO layer having the thickness of the desired Si nanocrystals and using a high-temperature annealing for forming the silicon nanocrystals. Current–voltage, capacitance–voltage, and conductance–voltage characteristics were measured for a sample containing 4-nm-sized crystals. From the Fowler–Nordheim plot an effective barrier height of 1.6 eV is estimated for our silicon nanocrystals. Electron trapping, storing, and de-trapping in silicon nanocrystals were observed by capacitance–voltage and conductance–voltage measurements. The charge density was measured to be 1.6×10¹² /cm², which is nearly identical to the silicon-nanocrystal density measured approximately via a transmission electron microscopy image. Conductance measurements reveal a very low interface charge of our structure. PACS 72.80.Sk; 73.63.Bd; 73.40.Qv     

Photoluminescence and growth mechanism of amorphous silica nanowires by vapor phase transport

Amorphous silica [SiO_x (1<x<2)] nanowires were fabricated on silicon substrate in an acidic environment by heating the mixture of ZnCl₂, and VO₂ powders at 1100 °C. The length of SiO_x nanowires ranges from micrometers to centimeters, with uniform diameters of 10–500 nm depending on substrate temperature. Room-temperature photoluminescence spectra of the SiO_x nanowires showed two strong luminescence peaks in the red and green region, respectively. The photoluminescence was suggested to originate from nonbridging oxygen hole center (red band), and hydrogen-related species in the structure of SiO_x (green band). The study on chemical reactions and growth of the SiO_x nanowires revealed the formation process of silica nanowires in acidic environment was closely related to the vapor–solid–liquid mechanism.     

Effect of neutral scatterers on electron mobility in silicon (100) inversion layers

The electron mobility inn-type inversion layers at (100) silicon surface is studied at low temperature. Using the lowest subband calculated numerically, and taking into account the neutral impurity distribution in the vicinity of the Si–SiO₂ interface, the process of carrier scattering is investigated by the Green's function formalism. Experimental data for electron mobility are in good agreement with the results from Dyson's equation in Born approximation in the case of shallow and steep profiles of impurities.     

Direct role of hydrogen in the Staebler-Wronski effect in hydrogenated amorphous silicon

We report a hydrogen-related defect that establishes the direct role of hydrogen in stabilizing the silicon dangling bonds created in the Staebler-Wronski effect in hydrogenated amorphous silicon. A specific NMR signal due to paired hydrogen atoms occurs only after optical excitation, exists at an intensity that is consistent with the density of optically induced silicon dangling bonds, and anneals at temperatures that are consistent with the annealing of the optically induced silicon dangling bonds. At this defect the hydrogen atoms are 2.3+/-0.2 A apart.     

Nanostructuring Si surface and Si/SiO2 interface using porous-alumina-on-Si template technology. Electrical characterization of Si/SiO2 interface

In this work, anodic porous alumina thin films with pores in the nanometer range are grown on silicon by electrochemistry and are used as masking material for the nanopatterning of the silicon substrate. The pore diameter and density are controlled by the electrochemical process. Through the pores of the alumina film chemical oxidation of the silicon substrate is performed, leading to the formation of regular arrays of well-separated stoichiometric silicon dioxide nanodots on silicon, with a density following the alumina pores density and a diameter adjustable by adjusting the chemical oxidation time. The alumina film is dissolved chemically after the SiO₂ nanodots growth, revealing the arrays of silicon dioxide dots on silicon. In a next step, the nanodots are also removed, leaving a nanopatterned bare silicon surface with regular arrays of nanopits at the footprint of each nanodot. This silicon surface structuring finds interesting applications in nanoelectronics. One such application is in silicon nanocrystals memories, where the structuring of the oxidized silicon surface leads to the growth of discrete silicon nanocrystals of uniform size. In this work, we examine the electrical quality of the Si/SiO₂ interface of a nanostructured oxidized silicon surface fabricated as above and we find that it is appropriate for electronic applications (an interface trap density below 1–3×10¹⁰ eV⁻¹ cm⁻² is obtained, indicative of the high quality of the thermal silicon oxide).     

Formation of interface bubbles in bonded silicon wafers: A thermodynamic model

A thermodynamic model of the formation of unbonded areas or bubbles generated at the interface of bonded silicon wafers in the temperature range of 200–800°C is presented. Within this model it is assumed that the desorption of hydrocarbon contamination at the silicon wafer surfaces leads to small hydrocarbon molecules which are mobile at the bonding interface. When the vapor pressure generated by these molecules overcomes the interface bonding strength, interface bubbles are nucleated. These bubbles grow by incorporating further hydrocarbon and also possible hydrogen molecules. The model semiquantitatively explains all the essential features of interface bubble formation observed experimentally.     

Molecular dynamics simulation comparison of atomic scale intermixing at the amorphous Al2O3/semiconductor interface for a-Al2O3/Ge, a-Al2O3/InGaAs, and a-Al2O3/InAlAs/InGaAs

The structural properties of a-Al₂O₃/Ge, a-Al₂O₃/In_0.5Ga_0.5As and a-Al₂O₃/In_0.5Al_0.5As/InGaAs interfaces were investigated by density-functional theory (DFT) molecular dynamics (MD) simulations. Realistic a-Al₂O₃ samples were generated using a hybrid classical-DFT MD “melt and quench” approach. The interfaces were formed by annealing at 700 K/800 K and 1100 K with subsequent cooling and relaxation. The a-Al₂O₃/Ge interface demonstrates pronounced interface intermixing and interface bonding exclusively through Al–O–Ge bonds generating high interface polarity. In contrast, the a-Al₂O₃/InGaAs interface has no intermixing, Al–As and O–In/Ga bonding, low interface polarity due to nearly compensating interface dipoles, and low substrate deformation. The a-Al₂O₃/InAlAs interface demonstrated mild intermixing with some substrate Al atoms being adsorbed into the oxide, mixed Al–As/O and O–Al/In bonding, medium interface polarity, and medium substrate deformation. The simulated results demonstrate strong correlation to experimental measurements and illustrate the role of weak bonding in generating an unpinned interface for metal oxide/semiconductor interfaces.     

Kinetics of Ni2Si growth from pure Ni and Ni(V) films on (111) and (100) Si

The kinetics of Ni₂Si growth from pure Ni and from Ni_0.93V_0.07 films on (111) and (100) silicon has been studied by the combination of He⁺ backscattering, x-ray diffraction, Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) techniques. The activation energies are 1.5 and 1.0 eV for pure Ni and Ni(V) films, respectively while the pre-exponential factors in Ni(V) are 4–5 orders of magnitude smaller than in the pure Ni case. The variations in the measured rates are related to the different grain size of the growing suicide layers. The vanadium is rejected from the silicide layer and piles up at the metalsilicide interface.     

Investigation on preparation and physical properties of nanocrystalline Si/SiO2 superlattices for Si-based light-emitting devices

Structures containing silicon nanocrystals (nc-Si) are very promising for Si-based light-emitting devices. Using a technology compatible with that of silicon, a broader wavelength range of the emitted photoluminescence (PL) was obtained with nc-Si/SiO₂ multilayer structures. The main characteristic of these structures is that both layers are light emitters. In this study we report results on a series of nc-Si/SiO₂ multilayer periods deposited on 200 nm thermal oxide SiO₂/Si substrate. Each period contains around 10 nm silicon thin films obtained by low-pressure chemical vapour deposition at T=625°C and 100 nmSiO₂ obtained by atmospheric pressure chemical vapour deposition T=400°C. Optical and microstructural properties of the multilayer structures have been studied by spectroscopic ellipsometry (using the Bruggemann effective medium approximation model for multilayer and multicomponent films), FTIR and UV–visible reflectance spectroscopy. IR spectroscopy revealed the presence of SiO_x structural entities in each nc-Si/SiO₂ interface. Investigation of the PL spectra (using continuous wave-CW 325 nm and pulsed 266 nm laser excitation) has shown several peaks at 1.7, 2, 2.3, 2.7, 3.2 and 3.7 eV, associated with the PL centres in SiO₂, nc-Si and Si–SiO₂ interface. Their contribution to the PL spectra depends on the number of layers in the stack.     

Thermodynamic and kinetic stability with respect to hole trapping. of silanic bonds at the Si–SiO<Subscript>2</Subscript> interface

The thermodynamic and kinetic stabilities of the Si–H bonds at the Si–SiO₂ interface are studied on the basis of high-level quantum-mechanical calculations in the framework of density-functional theory. In the absence of an applied electric field, the silanic bond is shown to be stable with respect to both hole capture from the top of the silicon valence band and electron loss to the bottom of the silicon conduction band, but unstable with respect to hole capture from the top of the SiO₂ conduction band. The positively charged hydrogen does not shift spontaneously to protonate a neighbouring siloxanic bridge unless it contains one adsorbed water molecule at least. The protonated siloxanic site thus formed may restore the original silanic site (via simultaneous electron capture from the conduction band and hydron shift to silicon) but also evolve spontaneously to a hydrogen atom via simple electron capture. Received: 2 November 2001 / Accepted: 6 January 2002 / Published online: 20 March 2002 / Published online: 20 March 2002 RID="*" ID="*"Also at: STMicroelectronics, 20041 Agrate MI, Italy (E-mail: gianfranco.cerofolini@st.com)     

Effect of hydrogen on modulation-doped AlGaAs/InGaAs/GaAs heterostructures: a photoluminescence study

The effect of hydrogen on donors and interface defects in silicon modulation doped Al_xGa_1−xAs/In_yGa_1−yAs/GaAs heterostructures has been investigated by photoluminescence (PL). Hydrogenation was carried out on two sets of samples, one set consists of high quality pseudomorphic heterostructures and another set having partially lattice relaxed structures prone to the defects. On exposure of high quality pseudomorphic structures to hydrogen plasma above 150 °C, a significant blue shift in the PL peak positions as well as bandwidth narrowing is observed. This indicates, the reduction in two-dimensional electron gas in the In_yGa_1−yAs quantum well due to hydrogen passivation of silicon donors in the Al_xGa_1−xAs supply layer. The reactivation of the donors is observed upon annealing the hydrogenated sample for 1 h at 250 °C under hydrogen ambient. Another interesting feature is a significant improvement in the PL of lattice-relaxed structures upon hydrogenation of the samples above 250 °C, which is attributed to the hydrogen passivation of interface defects due to the misfit dislocations.     

Tunable wavelength light emission from longitudinally biased p-GaAs/n-Ga1−xAlxAs junction containing GaAs quantum wells: non-linear dynamics

We report on the operation and non-linear dynamics of a hot electron device that emits light with wavelength tunablity. The device consists of p-GaAs/n-Ga_1−xAl_xAs heterojunction containing an inversion layer on the p-side, and GaAs quantum wells on the n-side. It is referred to as HELLISH-2 (Hot Electron Light Emitting and Lasing in Semiconductor Heterostructure – Type 2). The device utilises hot electron longitudinal transport and the light emission is independent of the polarity of applied voltage. The wavelength of the emitted light can be tuned with applied bias from 1.50 to 1.61 eV. The operation of the device requires only two diffused in point contacts. Theoretical modelling of the device operation has been carried out and compared with the experimental results.     

Time Dependence of the Capacitance of a MOS Silicon Tunnel Diode under the Influence of Hydrogen

The results of theoretical and experimental investigations into the time dependence of the capacitance of MOS silicon diodes with a thin tunnel dielectric and a palladium field electrode under the influence of a gas hydrogen-containing mixture are presented. Analytical expressions are derived that describe the time dependence of the capacitance of the space charge region (SCR) of the MOS tunnel diode operating in depletion and enrichment regimes. It is demonstrated that the time dependence of the SCR capacitance of the MOS diode placed in the gas mixture is caused by diffusion of hydrogen atoms from the field electrode to the SiO _x –n-Si interface. The relaxation time of hydrogen atom accumulation at the interface is 47 s for the diode with a SiO _x layer thickness of 3.7 nm placed in the gas mixture with 0.3 vol.% of H₂. It has been established that the flat band voltage and the SCR capacitance of the MOS diode change under the influence of the gas mixture due to the decreased density of surface acceptor states at the SiO _x –n-Si interface, the increased positive charge density in the dielectric, and the decreased contact potential difference.     

Formation of silicon oxide nanowires directly from Au/Si and Pd–Au/Si substrates

Amorphous silicon oxide (SiO_x) nanowires were directly grown by thermal processing of Si substrates. Au and Pd–Au thin films with thicknesses of 3 nm deposited on Si (0 0 1) substrates were used as catalysts for the growth of nanowires. High-yield synthesis of SiO_x nanowires was achieved by a simple heating process (1000–1150 °C) in an Ar ambient atmosphere without introducing any additional Si source materials. The as-synthesized products were characterized by field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy measurements. The SiO_x nanowires with lengths of a few and tens of micrometers had an amorphous crystal structure. The solid–liquid–solid model of nanowire formation was shown to be valid.     

Theory of hydrogen-related metastability in disordered silicon

Density functional electronic structure calculations are employed to examine hydrogen for a variety of configurations in silicon. A novel complex is found for hydrogen in amorphous silicon. The complex involves the breaking of a weak silicon bond to form two Si-H bonds with both hydrogens in between the original silicon atoms. This complex provides a microscopic model for new metastable complexes observed in amorphous silicon. Mechanisms for hydrogen-related metastability are discussed for amorphous and polycrystalline silicon.     

AES and factor analysis study of silicide growth at the Pd/c-Si interface

We have measured the evolution of a palladium/silicon interface under consecutive annealing periods, performed at 200°C in UHV conditions. The interface was analyzed by means of Auger electron spectroscopy combined with factor analysis applied in a sequential way. We found that silicide appears only after annealing and evolves until all the palladium is consumed. A silicon compound different from silicide, identified as Pd_xSi with x<2 is found at the interface Pd/Si and Pd₂Si/Si, before and after annealing respectively.