Influence of the synthesis conditions of silicon nanodots in an industrial low pressure chemical vapor deposition reactor

Experiments conducted in an industrial tubular low pressure chemical vapor deposition (LPCVD) reactor have demonstrated the reproducibility and spatial uniformity of silicon nanodots (NDs) area density and mean radius. The wafer to wafer uniformity was satisfactory (density and radius standard deviations <10%) for the whole conditions tested except for low silane flow rates, high silane partial pressures and short run durations(<20 s). Original synthesis conditions have then been searched to reach both excellent wafer to wafer uniformities along the industrial load of wafers and high NDs densities. From previous results, it was deduced that the key was to markedly increase run duration in decreasing temperature and in increasing silane pressure. At 773 K, run durations as long as 180 and 240 s have thus allowed to reach NDs densities respectively equal to 9 1011 and 6.5 1011 NDs/cm2 for the two highest silane pressures tested in the range 60–150 Pa

Development of an original model for the synthesis of silicon nanodots by Low Pressure Chemical Vapor Deposition

Using the Computational Fluid Dynamics code Fluent, a simulation model of an industrial Low Pressure Chemical Vapor Deposition reactor has been developed for the synthesis of silicon nanodots from silane SiH4 on silicon dioxide SiO2 substrates. A comparison between experimental and simulated deposition rates has shown that classical kinetic laws largely over-estimated these deposits. So, an original heterogeneous kinetic model is proposed as a first attempt to quantify the temporal evolution of deposition rates and of surface site numbers, as a function of operating conditions and of the chemical nature of substrate sites, for the early stages of silicon deposition. Contributions of silane and of the homogeneously born silylene SiH2 to nucleation and growth have been considered on different surface sites, silanol Si–OH, siloxane Si–O–Si and fresh silicon bonds. Simulations have revealed that for the conditions tested, the classical heterogeneous kinetic laws over-estimate, by more than 60%, silicon deposition during the first stages. The assumption that silylene and more largely all the unsaturated species formed in the gas phase contribute in priority to nucleation has been validated. Nucleation appears as a mandatory step to form the first fresh Si sites to allow deposition to occur from silane via growth phenomena

LPCVD synthesis of silicon nanodots from silane and for flash mamory devices

The increase of microelectronic device potentialities essentially derives from the reduction of feature size down to the nanometer scale. Multinanocrystals memories are one illustration of this trend. A multi-field study is reported, aiming to better understand phenomena involved in silicon nanocrystals elaboration. A first objective is to better control the density and the size of silicon nanocrystals formed during Low Pressure Chemical Vapor Deposition (LPCVD). Some experimental results will be presented, aiming a reliable estimation of densities and sizes of nanocrystals using several techniques: ellipsometry measurements, SEM and TEM image analysis. These experimental data have been simulated at the reactor scale using the Computational Fluid Dynamics (CFD) code FLUENT. A new kinetic scheme has been developed, which considers the various chemisorption sites existing on SiO2 substrates. This new modelling approach allows to correlate in transient mode the local composition of the gas phase near the substrate with the various sites number and with the density and the size of nanocrystals

Multi-scale modelling of silicon nanocrystal synthesis by Low Pressure Chemical Vapor Deposition.

A multi-scale model has been developed in order to represent the nucleation and growth phenomena taking place during silicon nanocrystal (NC) synthesis on SiO2 substrates by Low Pressure Chemical Vapor Deposition from pure silane SiH4. Intrinsic sticking coefficients and H2 desorption kinetic parameters were established by ab initio modelling for the first three stages of silicon chemisorption on SiO2 sites, i.e. silanol Si―OH bonds and siloxane Si―O―Si bridges. This ab initio study has revealed that silane cannot directly chemisorb on SiO2 sites, the first silicon chemisorption proceeds from homogeneously born unsaturated species like silylene SiH2. These kinetic data were implemented into the Computational Fluid Dynamics Fluent code at the industrial reactor scale, by activating its system of surface site control in transient conditions. NC area densities and radii deduced from Fluent calculations were validated by comparison with experimental data. Information about the deposition mechanisms was then obtained. In particular, hydrogen desorption has been identified as the main limiting step of NC nucleation and growth, and the NC growth rate highly increases with run duration due to the autocatalytic nature of deposition

Towards multiscale modeling of Si nanocrystals LPCVD deposition on SiO2: From ab initio calculations to reactor scale simulations

A modeling study is presented involving calculations at continuum and atomistic (DFT, Density Functional Theory) levels so as to better understand mechanisms leading to silicon nanocrystals (NC) nucleation and growth on SiO2 silicon dioxide surface, by Low Pressure Chemical Vapor Deposition (LPCVD) from silane SiH4. Calculations at the industrial reactor scale show that a promising way to improve reproducibility and uniformity of NC deposition at short term could be to increase deposition time by highly diluting silane in a carrier gas. This dilution leads to a decrease of silane deposition rate and to a marked increase of the contribution to deposition of unsaturated species such as silylene SiH2. This result gives importance to our DFT calculations since they reveal that only silylene (and probably other unsaturated species) are involved in the very first steps of nucleation i.e. silicon chemisorption on silanol Si–OH or siloxane Si–O–Si bonds present on SiO2 substrates. Saturated molecules such as silane could only contribute to NC growth, i.e. chemisorption on already deposited silicon bonds, since their decomposition activation barriers on SiO2 surface are as high as 3 eV

Diamonds's Temperature: Unruh effect for bounded trajectories and thermal time hypothesis

We study the Unruh effect for an observer with a finite lifetime, using the thermal time hypothesis. The thermal time hypothesis maintains that: (i) time is the physical quantity determined by the flow defined by a state over an observable algebra, and (ii) when this flow is proportional to a geometric flow in spacetime, temperature is the ratio between flow parameter and proper time. An eternal accelerated Unruh observer has access to the local algebra associated to a Rindler wedge. The flow defined by the Minkowski vacuum of a field theory over this algebra is proportional to a flow in spacetime and the associated temperature is the Unruh temperature. An observer with a finite lifetime has access to the local observable algebra associated to a finite spacetime region called a "diamond". The flow defined by the Minkowski vacuum of a (four dimensional, conformally invariant) quantum field theory over this algebra is also proportional to a flow in spacetime. The associated temperature generalizes the Unruh temperature to finite lifetime observers. Furthermore, this temperature does not vanish even in the limit in which the acceleration is zero. The temperature associated to an inertial observer with lifetime T, which we denote as "diamond's temperature", is 2hbar/(pi k_b T).This temperature is related to the fact that a finite lifetime observer does not have access to all the degrees of freedom of the quantum field theory.Comment: One reference correcte

Manufacturing and characterization of III-V on silicon multijunction solar cells

Tandem GaInP/GaAs//Si(inactive) solar cells were manufactured by direct wafer bonding under vacuum. At this early stage, an inactive silicon substrate was used (i.e. n+ Si substrate instead of an active n-p Si junction). Bonded devices presented an Sshaped J-V curve with a kink close to Voc caused by a built-in potential barrier at the III-V//Si interface that reduces the fill factor and therefore the efficiency of the device by 7% compared to the stand-alone GaInP/GaAs tandem cells. Nevertheless, losses in Jsc and Voc caused by the bonding process, account for less than 10%. AlGaAs single junction cells, designed to be bonded on a silicon cell for low concentrator photovoltaics (LCPV), were also manufactured reaching an efficiency of 15.9% under one sun AM1.5G spectrum for a 2 cm² cell