Chemical Vapor Deposition Of SiH4 Research Articles

Self-assembling mechanism of Si-QDs on thermally grown SiO2

The self-assembling formation of Si quantum dots (Si-QDs) on as-grown SiO2 layers was shown by controlling the early stages of low-pressure chemical vapor deposition of SiH4. The QD height and radius distributions assessed by atomic force microscopy and scanning electron microscopy images revealed that the Si-QDs become hemispherical due to them being rate-limited by aggregation, which reduces the surface energy at substrate temperatures above ∼580 °C. Moreover, at temperatures below ∼580 °C, semi-ellipsoidal shaped Si-QDs are formed because the precursor supply is a dominant factor.

Phase-controlled synthesis of nickel silicide nanostructures

Dense single-crystalline Ni2Si nanowires, Ni3Si2 nanowires and nanorods, and NiSi nanowires have been successfully synthesized in large-scale on the surface of Ni foam via a simple chemical vapor deposition of SiH4 at 450–600°C. It is indicated that the reaction temperature and the flow rate of SiH4 play an important role in the control of phases and morphologies of nickel silicide nanostructures. The temperature controls the balance of Ni-diffusion and the degree of supersaturation of Si in Ni, thus producing different nickel silicide phases, whereas the different flow rates of SiH4 lead to the different nickel silicide morphologies by controlling the size and the quality of the nucleation centers. We expect that our synthesis approach and results will provide valuable information for the fabrication and application of other metal silicide nanostructures.

Kinetics-driven high power Li-ion battery with a-Si/NiSix core-shell nanowire anodes

We report amorphous-Si nanowire shell anodes, supported by NiSixnanowire cores, by catalyst-free two-step SiH4 chemical vapor deposition, where the metallic core acts as a mechanical supporter and a kinetically unlimited charge supplier. We have achieved highly reversible capacitance of over 3000 mAh g−1 even at a 2C rate, with stable cyclic retention which stems from the altered electrochemical reactions with relatively small volume expansion routes by a kinetic effect.

Effect of Delta Doping in a-SiN:H Thin Film on the Mobility of Thin Film Transistors

Amorphous silicon modulation-doped field effect transistors using the heterostructure of phosphorous-contained a-SiN:H and intrinsic a-Si:H films have been fabricated by plasma enhanced chemical vapor deposition of SiH4, PH3, Ar and NH3 gases. The characteristics of the devices have been investigated and compared with conventional amorphous silicon thin film transistors. Various studies have been carried out on the properties of the materials of a-SiN:H films that were deposited with a varied delta-doping thickness and it was found that the Vth shifted from 2.5 to 4.5 V with an increase in the delta-doping thickness from 0 to 200 Å. The μn changed also from 0.19 to 0.38 cm2/V·sec.

Charge Storage and Optical Response of Hybrid Nanodots Floating Gate For Functional Memories

AbstractWe have proposed and fabricated a hybrid nanodots floating gate (FG), in which Si quantum dots (QDs) as charge injection/emission nodes and NiSi nanodots as charge storage nodes are stacked with an ultrathin SiO2 interlayer, to satisfy both large memory window and multivalued capability. In this study, Si-QDs with an areal density of ˜3×1011cm-2 were formed on ultrathin SiO2 layer by controlling SiH4 chemical vapor deposition (CVD) and NiSi nanodots were prepared by full-silicidation of Si-QDs promoted with remote H2-plasma exposure after Ni evaporation. From capacitance-voltage(C-V) characteristics of MOS capacitors with a NiSi nanodots/Si-QDs hybrid FG, stable storage of many charges in the deep potential well of each NiSi nanodot was confirmed. Also, by applying pulsed gate biases, stepwise charge injection to and emission from NiSi nanodots through discrete energy states in Si-QDs were demonstrated. In addition, by 1310nm (˜0.95eV) light irradiation, a distinct optical response in C-V characteristics was detected, which can be interpreted in terms of the shift of charge centroid in the hybrid FG stack due to transfer of photoexcited electrons from NiSi-nanodots to the Si-QDs.

Analysis of Si1–xGex:H thin films with graded composition and structure by real time spectroscopic ellipsometry

AbstractSilicon‐germanium (Si1−x Gex:H) thin films have been prepared by plasma enhanced chemical vapor deposition of SiH4 and GeH4 and measured during growth using real time spectroscopic ellipsometry. A two‐layer virtual interface analysis has been applied to study the structural evolution of Si1−x Gex:H films that initially nucleate in the amorphous (a) phase, but evolve to the microcrystalline (μc) phase with accumulated thickness. The compositional evolution of alloy‐graded a‐Si1−x Gex:H has been studied as well using similar methods. Both types of films are of interest for Si‐based photovoltaic devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Role of NiOx Overlayers on Spontaneous Growth of NiSix Nanowires from Ni Seed Layers

We report a controllably reproducible and spontaneous growth of single-crystalline NiSix nanowires using NiOx/Ni seed layers during SiH4 chemical vapor deposition (CVD). We provide evidence that upon the reactions of SiH4 (vapor)-Ni seed layers (solid), the presence of the NiOx overlayer on Ni seed layers plays the key role to promote the spontaneous one-dimensional growth of NiSix single crystals without employing catalytic nanocrystals. Specifically, the spontaneous nanowire formation on the NiOx overlayer is understood within the frame of the SiH4 vapor-phase reaction with out-diffused Ni from the Ni underlayers, where the Ni diffusion is controlled by the NiOx overlayers for the limited nucleation. We show that single-crystalline NiSix nanowires by this self-organized fashion in our synthesis display a narrow diameter distribution, and their average length is set by the thickness of the Ni seed layers. We argue that our simple CVD method employing the bilayers of transition metal and their oxides as the seed layers can provide implication as the general synthetic route for the spontaneous growth of metal-silicide nanowires in large scales.

Analysis of Compositionally and Structurally Graded Si:H and Si1−xGex:H Thin Films by Real Time Spectroscopic Ellipsometry

ABSTRACTSilicon-germanium (Si1−xGex:H) thin films have been prepared by plasma enhanced chemical vapor deposition of SiH4 and GeH4 and measured during growth using real time spectroscopic ellipsometry. A two-layer virtual interface analysis has been applied to study the structural evolution of Si:H films prepared in multistep processes utilizing alternating intermediate and low H2-dilution material layers, which have been designed to produce predominately amorphous films with a controlled distribution of microcrystalline particles. The compositional evolution of alloy-graded a-Si1−xGex:H has been studied as well using similar methods. In each case, depth profiles of microcrystalline content, fμc, or Ge content, x, have been extracted. Additionally, real time spectroscopic ellipsometry has been used to monitor post-deposition exposure of a-Si:H, a-Si1−xGex:H, and a-Ge:H films to a hydrogen plasma in situ in order to determine sub-surface amorphous film modification similar to that which would occur when a highly H2-diluted layer is deposited on a layer prepared with lower dilution. These analyses provide guidance for enhanced performance of Si:H based solar cells, through controlled bandgap grading using compositionally graded amorphous binary alloys (a-Si1−xGex:H) or the incorporation of controlled fractions of microcrystallites into bulk amorphous i-layer materials, and by providing a fundamental understanding of the modification of component layers during the deposition of subsequent layers in multilayer stacks.

Spontaneous Chemical Vapor Growth of NiSi Nanowires and Their Metallic Properties

One-dimensional metallic nanostructures offer challenging opportunities to investigate their possible applications as interconnects in electronic circuitry and as field emitters in field emission displays. In particular, metallic silicide nanowires are promising candidates, because their growth can be easily integrated with silicon processing technology. Indeed the synthesis of single-crystalline, transition metal silicide nanowires, such as TaSi2, [6] e-FeSi, and CoSi, has been recently reported using diverse methods. The synthetic methods employed involve relatively high temperature processes, and may not be compatible with the low temperature processes required for their practical applications into electronic and display devices. In contrast, single-crystalline Ni-silicide nanowires, which have been experimentally verified to possess intrinsically low resistivity at low dimensions compared with the others, can be processed at relatively low temperatures, thus can provide practical advantages. For example, Wu et al. reported highly metallic properties of single-crystalline NiSi nanowires fabricated by a solid-state reaction at 550 °C between individual Si nanowires and Ni overshells. Here a simpler and controlled synthesis at low temperatures (∼400 °C) is reported, where single-crystalline NiSi nanowires spontaneously grow by SiH4 chemical vapor deposition (CVD) on Ni thin films predeposited on various substrates, such as SiO2/Si, quartz, and indium tin oxide (ITO) substrates. In fact, the Ni-catalyzed vapor phase thin film deposition has been previously investigated for applications into Si microelectronics and photovoltaics. Specifically, the dimensionality of the Ni-silicides have been reproducibly directed from thin films to single-crystalline nanowires, as well as their phases, by fine-tuning of growth parameters during CVD of a SiH4 gas precursor on predeposited Ni thin films. It is noted that Decker et al. and Kim et al. recently reported similar Ni-silicide nanowire growth by CVD and sputtering, however, the detailed growth mechanism, which is the main focus of this study, is unavailable to date. Here, by providing close observations of the morphological and phase evolution of Ni-silicide nanowires under optimized growth conditions, the spontaneous nanowire growth mechanism is discussed based on one-dimensional nucleation and growth of NiSi at a low supersaturation limit along with one-dimensional Ni diffusion during the chemical vapor reaction. Singlecrystalline NiSi nanowires in this study exhibit typical metallic behaviors, and show promising field-emission properties. It is suggested that our simple method to spontaneously grow NiSi nanowires at low temperatures can provide a practical strategy to fabricate metallic nanostructures based on bottom-up synthetic approaches. Our synthetic approach employs the Ni-catalyzed decomposition of SiH4, [14] which can occur well below the thermal decomposition temperature of SiH4 at above 600 °C. Thermally evaporated Ni thin films of 60–80 nm in thickness are used. During the CVD of SiH4 within the wide range of precursor partial pressures and growth temperatures of 10–100 torr and 250–600 °C, the reaction produces either Ni-silicide thin films or nanowires of various phases. Figure 1 illustrates the evolution of the morphology and the phases of Ni-silicides, grown at 250, 400, and 600 °C at 50 torr of SiH4, with the corresponding scanning electron microscopy (SEM) images, X-ray diffraction (XRD), and element depth profiling by Auger spectrometry. Within the entire growth range, Ni-silicide thin films have been commonly formed on the substrates; however, the surface morphology of the Ni-silicide thin films has been modified from planar sheets to nanowires, as well as phases in Ni-to-Si stoichiometry. The observations from Figure 1 are summarized in the following within the framework of SiH4 vapor–Ni solid reactions, where SiH4 decomposition and solid Ni diffusion are both thermally activated. First, at 250 °C, the catalyzed decomposition of SiH4 and the Ni out-diffusion from the pre-deposited Ni thin films are not sufficient to turn the Ni film into a Ni-silicide thin film during the reaction, and C O M M U N IC A IO N

Excitation mechanism of visible, Tb3+ photoluminescence from Tb-doped silicon oxynitride

The excitation mechanism of visible luminescence from Tb3+-doped silicon oxynitride is investigated. Tb-doped silicon oxynitride films were deposited by inductive-coupled plasma-enhanced chemical vapor deposition of SiH4, O2, and N2 with concurrent sputtering of Tb. Luminescences from both the host matrix and the Tb3+ intra-4f transition are observed, but no correlation is found between them as the composition and the annealing conditions were varied. Photoluminescence excitation spectroscopy shows a strong increase in the Tb3+ luminescence intensity as the pump energy is increased above 3.5eV while the host matrix luminescence decreases. Taken together, the results that there is little energy transfer between band-tail states of silicon oxynitride and Tb3+, and that efficient excitation of Tb3+ by carriers requires excitation of carriers into the extended states of oxynitride.

Nd3+ photoluminescence and its implication on the excitation mechanisms of Nd3+ in Nd doped hydrogenated amorphous silicon alloyed with carbon

Nd3+ photoluminescence from Nd doped, hydrogenated amorphous silicon alloyed with carbon (a-Si:H:C) is investigated. Nd-doped, a-Si:H:C films were deposited using electron cyclotron resonance plasma enhanced chemical vapor deposition of SiH4 and CH4 with concurrent sputtering of Nd. Clear Nd3+ luminescence peaks can be observed when excited with 488nm light, indicating that Nd3+ ions in a-Si:H:C are excited via carriers generated in the a-Si:H:C. The energetics of Nd-doped a-Si:H:C rule out defect-related Auger excitation via mid-gap states. Based on the temperature and pump power dependence of Nd3+ and intrinsic a-Si:H:C photoluminescence intensities, we identify excitation via Nd-related defects with a strong coupling to Nd3+ as a possible excitation mechanism for Nd in a-Si:H:C.

Controlling Er–Tm interaction in Er and Tm codoped silicon-rich silicon oxide using nanometer-scale spatial separation for efficient, broadband infrared luminescence

The effect of nanometer-scale spatial separation between Er3+ and Tm3+ ions in Er and Tm codoped silicon-rich silicon oxide (SRSO) films is investigated. Er and Tm codoped SRSO films, which consist of nanocluster Si(nc-Si) embedded inside SiO2 matrix, were fabricated with electron cyclotron resonance-plasma enhanced chemical vapor deposition of SiH4 and O2 with concurrent sputtering of Er and Tm metal targets. Spatial separation between Er3+ and Tm3+ ions was achieved by depositing alternating layers of Er- and Tm-doped layers of varying thickness while keeping the total film thickness the same. The films display broadband infrared photoluminescence (PL) from 1.5 to 2.0μm under a single source excitation due to simultaneous excitation of Er3+ and Tm3+ ions by nc-Si. Increasing the layer thickness from 0 to 72nm increases the Er3+ PL intensity nearly 50-fold while the Tm3+ PL intensity is unaffected. The data are well-explained by a model assuming a dipole–dipole interaction between excited Er3+ and Tm3+ ions, and suggest that by nanoscale engineering, efficient, ultrabroadband infrared luminescence can be obtained in an optically homogeneous material using a single light source.

The thermo-optic effect of Si nanocrystals in silicon-rich silicon oxidethin films

The thermo-optic effect of Si nanocrystals in silicon-rich silicon oxide (SRSO) thin films at 1530nm is investigated. SRSO thin films, which consist of nanocrystal Si (nc-Si) embedded inside the SiO2 matrix, were prepared by electron-cyclotron-resonance plasma-enhanced chemical vapor deposition of SiH4 and O2 followed by a 30min anneal at 1150°C. The refractive indices of all SRSO films increased with increasing temperature, with the thermo-optic coefficient increasing from 1.0to6.6×10−5K−1 as the Si content is increased from 37to45at.%. The thermo-optic coeffecients of nc-Si, obtained by correcting for the volume fraction of nc-Si, also increased with increasing Si content from 1to2.5×10−4K−1. The results indicate that the thermo-optic effect of nc-Si is size-dependent, and that it must be taken into account when interpreting the luminescence data from SRSO films with high density of nc-Si.

Enhancement of Er3+ Luminescence from Er-doped Hydrogenated Amorphous Silicon by Carbon Co-doping

The effect of C co-doping on the Er3+ luminescence properties of Er-doped hydrogenated amorphous Si (a-Si:H) is investigated. Er-doped a-Si:H thin films co-doped with C were deposited using electron–cyclotron resonance plasma enhanced chemical vapor deposition of SiH4 and CH4 with concurrent sputtering of Er. We find that C co-doping greatly increases the room temperature 1.54 µm Er3+ photoluminescence (PL) intensity, with the maximum enhancement by a factor of 4 being obtained at a C concentration of ∼15 at.%. Part of the reason for such enhancement is the suppression of the temperature quenching of the Er3+ PL intensity and liftime, which we attribute to the increased optical bandgap of a-Si:H due to C co-doping. However, unlike other co-dopants such as O that also can enhance the Er3+ PL, C co-doping does not degrade the a-Si:H film quality, demonstrating its suitability for developing Si-based optoelectronic devices.

Nucleation kinetics of Si quantum dots on SiO2

The formation of Si quantum dots on SiO2 by chemical vapor deposition of SiH4 has been investigated in the range from the submonolayer to the complete coverage with Si. In order to investigate the very early stages of the nucleation process of Si on SiO2, the energy filtered transmission electron microscopy has been chosen as the main characterization technique, because of the high spatial resolution typical of the transmission electron microscopy analysis, coupled to the compositional information obtained by the electron energy loss spectroscopy. The plan view configuration has been used to measure the dot size distributions down to dimensions of about 1 nm, and in cross section to evaluate the dot wetting angle. For all the several experimental conditions, a wetting angle distribution has been obtained and has shown to be centered at about 90°. Data on the dot size distributions are shown and discussed in the framework of a continuous nucleation model, which has been implemented to take into account the dot coalescence process contribution. Through the comparison with experimental data the relevant thermodynamic parameters of the process have been evaluated: the critical radius, the free energy barrier for nucleation, and the concentration of nucleation sites.

The Nd-nanocluster coupling strength and its effect in excitation/de-excitation of Nd3+ luminescence in Nd-doped silicon-rich silicon oxide

The Nd-nanocluster Si (nc-Si) coupling strength and its effect in excitation/de-excitation of Nd3+ luminescence in Nd-doped silicon-rich silicon oxide (SRSO) is investigated. Nd-doped SRSO thin films, which consist of nc-Si embedded inside a SiO2 matrix, were prepared by electron-cyclotron-resonance plasma-enhanced chemical vapor deposition of SiH4 and O2 with cosputtering of Nd and subsequent anneal at 950 °C. Efficient Nd3+ luminescence with moderate temperature quenching is observed. Based on an analysis of the temperature dependence of Nd3+ luminescence lifetime, we find a coupling strength between nc-Si and Nd that is strong enough to result in efficient excitation of Nd3+ via quantum-confined excitons, while weak enough to result in a small back-transfer rate is identified as the key to Nd3+ luminescence.

Identification of Si and SiH in catalytic chemical vapor deposition of SiH4 by laser induced fluorescence spectroscopy

Radical species produced in catalytic chemical vapor deposition (CVD), often called hot-wire CVD, processes were identified by using a laser induced fluorescence technique. Ground state Si atoms could be detected at low pressures where collisional processes in the gas phase could be ignored. The electronic temperature of Si atoms just after the formation on the catalyzer (tungsten) surfaces was 1320±490 K, when the catalyzer temperature was 2300 K. By the addition of 0.5 Pa of Ar, the electronic temperature was lowered down to 450±30 K. The absolute density of Si atoms was 3±1×109 cm−3 at 10 cm below the catalyzer when the flow rate and the pressure of SiH4 were 0.5 sccm and 4 mPa, respectively. This density is just 0.3% of that of the parent SiH4 molecules. However, since the decay rate of Si atoms is fast, it can be concluded that atomic silicon is one of the major products on the heated catalyzer surfaces. SiH radicals could also be detected, but the production rate of this species is two orders of magnitude less than that of Si atoms. It was also discovered that volatile SiH4 molecules are produced by the atomic hydrogen attack on the amorphous silicon deposited on the chamber walls.

Silicon nanowire devices

Transport measurements were carried out on 15–35 nm diameter silicon nanowires grown using SiH4 chemical vapor deposition via Au or Zn particle-nucleated vapor-liquid-solid growth at 440 °C. Both Al and Ti/Au contacts to the wires were investigated. The wires, as produced, were essentially intrinsic, although Au nucleated wires exhibited a slightly higher conductance. Thermal treatment of the fabricated devices resulted in better electrical contacts, as well as diffusion of dopant atoms into the nanowires, and increased the nanowire conductance by as much as 104. Three terminal devices indicate that the doping of the wires is p type.

Memory effects in MOS capacitors with silicon rich oxide insulators

ABSTRACTTo form crystalline Si dots embedded in SiO2, we have deposited thin films of silicon rich oxide (SRO) by plasma-enhanced chemical vapor deposition of SiH4 and O2. Then the materials wereannealed in N2 ambient at temperatures between 950 and 1100 °C. Under such processing, the supersaturation of Si in the amorphous SRO film produces the formation of crystalline Si dots embedded in SiO2. The narrow dot size distributions, analyzed by transmission electron microscopy, are characterized by average grain radii and standard deviations down to about 1 nm. The memory function of such structures has been investigated in metal-oxidesemiconductor (MOS) capacitors with a SRO film sandwiched between two thin SiO2 layers as insulator and with an n+ polycrystalline silicon gate. The operations of write and storage are clearly detected by measurements of hysteresis in capacitance-voltage characteristics and they have been studied as a function of bias.

Deposition and 1.54 μm Er3+ luminescent properties of erbium-doped hydrogenated amorphous silicon thin films by electron cyclotron resonance plasma enhanced chemical vapor deposition of SiH4 with concurrent sputtering of erbium

Erbium-doped hydrogenated amorphous silicon (a-Si:H) thin films were deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition of SiH4 with concurrent sputtering of erbium. The oxygen and carbon contamination levels in all films were 1×1019 and 3×1019 cm−3, respectively, and the erbium concentrations could be controlled from 0.13 to 1.1 at. % by adjusting the erbium bias voltage. The half width Γ/2 of the Raman transverse-optic peak of the deposited films ranged from 32±1 to 36±1 cm−1, increasing with the increasing Er concentration. Strong 1.54 μm Er3+ luminescence with little temperature quenching was observed from all samples. The most intense Er3+ luminescence was observed from the as-deposited film with a deposition temperature of 380 °C and an erbium concentration of 0.13 at. %, showing that using the present method, erbium-doped a-Si:H films with good erbium optical activity and low structural disorder can be deposited directly while avoiding excessive contamination.