Low-temperature Chemical Vapor Deposition Research Articles

Building Robust and Reliable Molecular Constructs: Patterning, Metallic Contacts, and Layer-by-Layer Assembly

We describe recent progress in our laboratories to build stable complex two- and three-dimensional molecular constructs. We have introduced a simple and robust method for constructing complex molecular devices using top-down and bottom-up techniques based on self-assembled monolayers (SAMs), lithography, and site-selective reactions. It has significant advantages over other methods; it is easily scaled up, affords precise nanoscale placement, and is extensible to many different materials. Several recent developments are discussed including the UV photopatterning and electron beam lithography of SAMs adsorbed on semiconductors, the site-selective deposition of metals using electroless deposition and low-temperature chemical vapor deposition, and layer-by-layer assembly using covalent coupling. Optimization and further development of these techniques requires a detailed understanding of the reaction pathways involved in the lithography of SAMs and of the interaction of SAMs with metals, organometallic compounds, ions, and other compounds.

Si 1− xGe x growth using Si 3H 8 by low temperature chemical vapor deposition

Low temperature epitaxial growth of group-IV alloys is a key process step to realize the advanced Si-based devices. In order to keep high growth rate below 600 °C, trisilane (Si 3H 8) was used for their growth as an alternative Si precursor gas. Then, we compared the use of Si 3H 8 versus SiH 4 for Si 1− x Ge x growth in H 2 and N 2 as carrier gas by low temperature chemical vapor deposition. By using Si 3H 8 and controlling GeH 4 flow rate, Si 1− x Ge x growth with high growth rate and wide range of Ge concentration has been achieved compared to SiH 4-based process. The growth rate and Ge concentration in Si 1− x Ge x with Si 3H 8 grown at 600 °C ranged from 11 to 74 nm/min and from 0 to 40%, respectively. The obtained growth rates with Si 3H 8 are between 1.5 and 6 times higher than for SiH 4 at a given growth condition. Si 3H 8-based in-situ B- and C-doped Si 1− x Ge x growth with high growth rate was also demonstrated.

Remote plasma treatment of Si surfaces: Enhanced nucleation in low-temperature chemical vapor deposition

The nucleation density on Si(100):H is increased by two orders of magnitude after exposing the surface to a remote argon plasma. We study HfB2 growth from Hf(BH4)4 and MgO growth from Mg(DMDBA)2 plus H2O. In the latter case, pretreatment allows the growth of MgO films with an rms roughness below 0.5 nm, whereas in absence of plasma treatment no nucleation is observed. The plasma does not damage the substrate and is compatible with microelectronics technology. We propose that H desorption is the key mechanism leading to nucleation enhancement, and that remote plasma activation is likely to be generally applicable.

Tungsten Coatings as a Protective Layer on Surfaces in Contact with Molten Metals

Hot wall low temperature (500{degree sign}C) Chemical Vapour Deposition (CVD) is used to coat steel surfaces with a W or WC coating to protect the steel substrate against the attack of molten metals. Wettability measurements show that the W coating is almost not wetted by Zn, indicating excellent barrier properties against corrosion and soldering. It was also found that surface roughness influences the thermal barrier properties of the W(C) coating which can be beneficial to minimize heat checking.

Low Temperature Synthesis of Vertically Aligned Carbon Nanotubes with Electrical Contact to Metallic Substrates Enabled by Thermal Decomposition of the Carbon Feedstock

Growth of vertically aligned carbon nanotube (CNT) carpets on metallic substrates at low temperatures was achieved by controlled thermal treatment of ethylene and hydrogen at a temperature higher than the substrate temperature. High-resolution transmission electron microscopy showed that nanotubes were crystalline for a preheating temperature of 770 degrees C and a substrate temperature of 500 degrees C. Conductive atomic force microscopy measurements indicated electrical contact through the CNT carpet to the metallic substrate with an approximate resistance of 35 kOmega for multiwall carpets taller than two micrometers. An analysis of the activation energies indicated that thermal decomposition of the hydrocarbon/hydrogen gas mixture was the rate-limiting step for low-temperature chemical vapor deposition growth of CNTs. These results represent a significant advance toward the goal of replacing copper interconnects with nanotubes using CMOS-compatible processes.

Experimental and theoretical study of the evolution of surface roughness in amorphous silicon films grown by low-temperature plasma-enhanced chemical vapor deposition

Using real-time spectroscopic ellipsometry the evolution of the surface roughness of amorphous silicon thin films grown by low-temperature $(200\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C})$ plasma-enhanced chemical vapor deposition (PECVD) at high process gas pressure (3 mTorr) has been studied as a function of the hydrogen dilution gas-flow ratio ${R}_{H}=[{\text{H}}_{2}]/[{\text{SiH}}_{4}]$ with $15\ensuremath{\le}{R}_{H}\ensuremath{\le}60$. To describe the roughness evolution, we have used a $3\mathrm{D}$ linearized continuum equation which includes a negative surface-tension term to take into account the destabilizing effects of short-range attraction and/or shadowing, as well as a smoothing term corresponding to surface diffusion. Using this model we have obtained very good agreement with experimental results for the evolution of the surface roughness in the case of large dilution ratio. However, our results indicate that for small dilution ratio the surface slopes are significantly larger and as a result additional nonlinear terms need to be included at large thicknesses. Our results also indicate that surface diffusion plays an important role during PECVD film growth while the diffusion rate increases with increasing hydrogen dilution ratio. We also find that the early stages of island nucleation play an important role in determining the subsequent roughness evolution. In particular, the assumption of a large wetting angle $({\ensuremath{\theta}}_{W}\ensuremath{\simeq}90\ifmmode^\circ\else\textdegree\fi{})$ for the $3\mathrm{D}$ islands formed in the initial stages leads to significantly better agreement with experiments than a smaller wetting angle $({\ensuremath{\theta}}_{W}\ensuremath{\simeq}45\ifmmode^\circ\else\textdegree\fi{})$. This is consistent with recent experiments on liquid Si droplets on ${\text{SiO}}_{2}$ [H. Kanai et al., J. Mater. Sci. 42, 9529 (2007)] substrates in which a wetting angle of $90\ifmmode^\circ\else\textdegree\fi{}$ was observed.

Core-shell InGaAs/GaAs quantum well nanoneedles grown on silicon with silicon-transparent emission

In(x)Ga(1-x)As wurtzite nanoneedles are grown without catalysts on silicon substrates with x ranging from zero to 0.15 using low-temperature metalorganic chemical vapor deposition. The nanoneedles assume a 6 degrees - 9 degrees tapered shape, have sharp 2-5 nm tips, are 4 microm in length and 600 nm wide at the base. The micro-photoluminescence peaks exhibit redshifts corresponding to their increased indium incorporation. Core-shell InGaAs/GaAs layered quantum well structures are grown which exhibit quantum confinement of carriers, and emission below the silicon bandgap.

Thin film solar cells based on microcrystalline silicon–germanium narrow-gap absorbers

We report on the improved performance of p–i–n junction solar cells incorporating hydrogenated microcrystalline silicon–germanium (μc-Si 1− x Ge x :H) absorber i-layers prepared by low-temperature (∼200 °C) plasma-enhanced chemical vapor deposition. While the optical absorption increases with Ge content, the photocarrier transport in the solar cells for x>0.2 is dominated by the carrier recombination due to the increased dangling bond defects and the illumination-induced field distortion in the i-layer. In contrast, the solar cells with smaller Ge contents ( x∼0.2) exhibit better carrier collection characteristics with extended infrared sensitivities even higher than those of double-thickness μc-Si:H solar cells. As a result, we have achieved a 6.3% efficiency using a 1-μm-thick μc-Si 0.8Ge 0.2:H i-layer.

Growth Inhibition to Enhance Conformal Coverage in Thin Film Chemical Vapor Deposition

We introduce the use of a growth inhibitor to enhance thin film conformality in low temperature chemical vapor deposition. Films of TiB(2) grown from the single source precursor Ti(BH(4))(3)(dme) are much more highly conformal when grown in the presence of one of the film growth byproducts, 1,2-dimethoxyethane (dme). This effect can be explained in terms of two alternative inhibitory mechanisms: one involving blocking of surface reactive sites, which is equivalent to reducing the rate of the forward reaction leading to film growth, the other analogous to Le Chatelier's principle, in which the addition of a reaction product increases the rate of the back reaction. The reduction in growth rate corresponds to a reduction in the sticking probability of the precursor, which enhances conformality by enabling the precursor to diffuse deeper into a recessed feature before it reacts.

Comparative luminescence properties of ZnO nanorods grown on various substrates by low-temperature metalorganic chemical vapour deposition

In this study, we report on a comparative optical investigation of ZnO nanorods grown by a low-temperature metalorganic chemical vapour deposition technique on various substrates such as sapphire, silicon, GaN/sapphire, and glass. The near-band-edge excitonic features from these nanorods depend on the structural quality of ZnO. The band-edge photoluminescence (PL) decay times in these ZnO nanorods are studied by the low-temperature time-resolved PL technique. The nanorods on p-Si(1 0 0) template show comparatively longer PL decay time of 365 ps at 4.5 K, which could be due to an improved crystal quality and reduced interfacial strain. Due to the inferior crystalline quality, a slight reduction in the PL decay time has been observed in nanorods grown on glass substrate.

Atomically sharp catalyst-free wurtzite GaAs∕AlGaAs nanoneedles grown on silicon

We report a catalyst-free, self-assembled growth mode generating single-crystal wurtzite phase ultrasharp GaAs∕AlGaAs nanoneedles on both GaAs and Si substrates via low-temperature metal-organic chemical vapor deposition. The needles exhibit record-narrow tip diameters of 2–4nm wide and sharp 6°–9° taper angles. The length is dependent on growth time and up to 3–4μm nanoneedles are attained. The structures do not exhibit twinning defects, contrary to typical GaAs nanowires grown by vapor-liquid-solid catalyzed growth. AlGaAs layered nanoneedle structures are also demonstrated.

Synthesis and characterization of silica nanosprings by a low temperature chemical vapor deposition

Silica nanosprings were synthesized using a simple, low temperature, chemical vapor deposition method via a vapor–liquid–solid mechanism. Nanosprings with excellent uniformity and helicity in high and repeatable yields have been observed. The morphology and crystal structure of the nanosprings were characterized by scanning electron microscopy and transmission electron microscopy. The chemical composition of the nanosprings was determined using the energy-filtered transmission electron microscopic method. The as-grown nanomaterials were confirmed to be amorphous silica with irregularly shaped Au catalytic particles located at the tips. In addition, we propose a spontaneous spinning growth model to explain the formation of such helical nanostructures.

Highly Symmetrical CdS Tetrahedral Nanocrystals Prepared by Low-Temperature Chemical Vapor Deposition Using Polysulfide as the Sulfur Source

In this paper, we report a new scheme for chemical vapor deposition (CVD) of CdS nanocrystals, in which polysulfide and cadmium chloride were used as the sulfur and cadmium source, respectively. During the reaction, hydrogen sulfide was generated by controlled hydrolysis of polysulfide and reacted with cadmium chloride to give CdS. Tetrahedral CdS nanocrystals with homogeneous size and nearly perfect Td point group symmetry were obtained in high yield. The source and deposition temperature were only 540 and 400 °C, respectively, which were significantly lower than those in conventional CVD methods. The low deposition temperature was found critical for the formation of the tetrahedral nanostructures. The photoluminescence spectra of the tetrahedral nanocrystals consisted of a sharp band edge emission peak and a broad band related to trapped states.

Low-temperature chemical vapor deposition of highly doped n-type epitaxial Si at high growth rate

We investigated the growth of in-situ n-type doped epitaxial Si layers with arsenic and phosphorus by means of low-temperature chemical vapor deposition using trisilane as Si-precursor. Indeed, in order to prevent the alteration of the characteristics of the devices which are already present on the wafer, an epitaxy process at low temperature is highly desired for applications such as BiCMOS. In this work, the varying parameters are the deposition temperature, the Si-precursor mass flow and the dopant gas flow. As a result, a process for the deposition of heavily doped epilayers was demonstrated at 600 °C with high deposition rate, which is important for maintaining high throughput and low process cost. We showed that using trisilane as a Si-precursor resulted in a much more linear n-type doping behavior than using dichlorosilane. Therefore it allowed an easier process control and a wider dynamic doping range. Our process is an interesting route for the epitaxy of a low-resistance emitter layer for bipolar transistor application.

Performance of a-Si:H n–i–p photodiodes on plastic substrate

This paper reports on a-Si:H n–i–p photodiodes on PEN substrates with performance characteristics suitable for imaging applications. Segmented n–i–p photodiodes were fabricated using a process sequence and design rules that are compatible with industrial technology. Low-temperature (150°C) plasma-enhanced chemical vapor deposition (PECVD) was employed for the a-Si:H and passivation dielectric layers. Device measurements included current–voltage characteristics, dark current decay, and spectral response. To identify the sources of the reverse dark current, the measurements were performed on variable area test structures with device sizes ranging from 126μm to 2mm. The n–i–p photodiodes on PEN substrates demonstrate quantum efficiencies as high as 83% and reverse current density lower than 500pA/cm2 at −3V, as measured on 126μm photodiodes. Thus, the performance characteristics of the n–i–p diodes on PEN substrates meet the requirements for bio-medical X-ray imaging. We also discuss the mechanisms underlying the reverse dark current and the effect of the substrate on device characteristics.

Carrier collection characteristics of microcrystalline silicon–germanium p–i–n junction solar cells

We report on the carrier collection characteristics of hydrogenated microcrystalline silicon–germanium (μc-Si1−xGex:H) p–i–n junction solar cells fabricated by low-temperature (∼200°C) plasma-enhanced chemical vapor deposition. Spectral response measurements reveal that the Ge incorporation into absorber i-layer reduces the quantum efficiencies at short wavelengths. Furthermore, the illumination of the solar cells for x⩾0.35, particularly in the wavelength range of <650nm, induces a strong injection-level-dependent p–i interface recombination and a weak collection enhancement in the bulk. These results indicate that space charges near the p–i interface are largely negative, which gives rise to an electric field distortion in the i-layer. We attribute the negative space charges to the presence of the acceptor-like states that are responsible for the strong p-type conduction observed in undoped μc-Si1−xGex:H films for large Ge contents.

Effort to produce textured CeO2 and MgO films by the spray pyrolysis technique as buffer layers for coated conductors

The possibility of using spray pyrolysis as a simple, chemical, low cost method for the production of CeO2 and MgO thin films widely used as buffer layers for second generation coated conductors was examined. CeO2 films were produced on borosilicate glass substrates and four different surface morphologies were observed at different deposition temperatures. The smoothest films were obtained when the process was described as low temperature chemical vapour deposition. In addition, c-axis textured CeO2 films have been deposited on Si (100) single crystal, but the surface morphology was quite rough, consisting of distinct particles indicating that further optimisation is needed. On the other hand c-axis textured MgO films with smooth morphologies were deposited on Si (100) single crystal. Rocking curves revealed an excellent out of plane texture with a FWHM between 0.950 and 1.010

Surface Selective Growth of Ruthenium Films under Low-Temperature Pulsed CVD Conditions

Ruthenium film growth has been studied in the temperature range 110-275°C using tricarbonyl{η 4 -cyclohexa-1,3--diene}ruthenium, ammonia, nitrous oxide, hydrogen, and pulsed chemical vapor deposition (CVD) conditions. Spectroscopic ellipsometry was used for thickness measurements. Below ∼150°C no ruthenium growth occurred on Si, SiO 2 , Al 2 O 3 surfaces; at >150°C, the film growth was limited by the surface nucleation that caused formation of granular films. Implementation of sputtered subnanometer thick Pt-Pd alloy seed layers allowed us to obtain smooth ruthenium films with very small grains, providing a region of surface selective ruthenium film growth at 110-150°C with deposition rates in the range of 0.03-0.2 nm/cycle.

Complete composition tunability of InGaN nanowires using a combinatorial approach

The III nitrides have been intensely studied in recent years because of their huge potential for everything from high-efficiency solid-state lighting and photovoltaics to high-power and temperature electronics. In particular, the InGaN ternary alloy is of interest for solid-state lighting and photovoltaics because of the ability to tune the direct bandgap of this material from the near-ultraviolet to the near-infrared region. In an effort to synthesize InGaN nitride, researchers have tried many growth techniques. Nonetheless, there remains considerable difficulty in making high-quality InGaN films and/or freestanding nanowires with tunability across the entire range of compositions. Here we report for the first time the growth of single-crystalline In(x)Ga(1-x)N nanowires across the entire compositional range from x=0 to 1; the nanowires were synthesized by low-temperature halide chemical vapour deposition and were shown to have tunable emission from the near-ultraviolet to the near-infrared region. We propose that the exceptional composition tunability is due to the low process temperature and the ability of the nanowire morphology to accommodate strain-relaxed growth, which suppresses the tendency toward phase separation that plagues the thin-film community.

NOx sensing properties of In2O3 nanoparticles prepared by metal organic chemical vapor deposition

In 2 O 3 nanoparticles were deposited by low-temperature metal organic chemical vapor deposition. The response of 10-nm thick In 2 O 3 particle containing layers to NO x and O 2 gases is investigated. The lowest detectable NO x concentration is ∼200 ppb and the sensor performance is strongly dependent on the gas partial pressure as well as on the operating temperature. The sensor response towards 200 ppm of NO x is found to be above 10 4 . Furthermore, the cross-sensitivity against O 2 is very low, demonstrating that the In 2 O 3 nanoparticles are very suitable for the selective NO x detection.