Deposition and characterization of Si-rich silicon oxide films using HMDS for integrated photonics

Silicon-rich silicon oxide (SRSO) films were deposited using thermal-CVD system with organic precursor hexamethyldisilazane (HMDS) and oxygen (O₂) at the temperature range of 760–820 °C. The deposited SRSO films were characterized by using ellipsometry, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDAX). The effect of deposition temperature on the physical and optical properties of deposited SRSO films has been studied and analyzed. It has been observed that the refractive index (RI) of the deposited films increases while the stress of compressive nature decreases with the corresponding increase in deposition temperature. The peak positions of Si–O–Si stretching bond and the full-width at half maxima (FWHM) of these peaks have been investigated by using FTIR spectroscopy. It was found that the peak position of Si–O–Si stretching bond move toward lower wave number while the corresponding FWHM increases with increase in the deposition temperature. The peak intensities of Si–H, O–H and Si–OH bonds decrease with corresponding increase in deposition temperature, which signifies an improvement in the quality of the deposited SRSO films. The SEM and EDAX analysis clearly reveals the successful deposition of SRSO films with some dense clusters of silicon having size about 500 nm on the surface of the deposited films.     

New insights into acidic wet chemical silicon etching by HF/H2O–NOHSO4–H2SO4 solutions

Acidic wet chemical etching of crystalline silicon has been examined by utilization of HF–NOHSO₄–H₂SO₄ mixtures. In light of our previous studies the effects of nitrosyl ion concentrations on etching rates were studied time- and temperature resolved. The reactivity of crystalline silicon surfaces in HF/H₂SO₄ solutions is determined by NO⁺-ion concentrations at the silicon/electrolyte interface, measured by ion chromatography. Quantitative solution analysis proofed accumulation of ammonium ions and indicated the conversion of NO⁺ as limiting for the overall etching process. Direct participation in the rate-limiting step was confirmed by calculation of activation energies. Increasing NO⁺-ion contents cause transition from reaction (E_A=55 kJ mol^?1) to diffusion controlled (E_A=10 kJ mol^?1) etching procedures. In combination with time and concentration dependent studies of produced structures a convenient regime for selective texturing or polishing polycrystalline silicon surfaces is reported. Qualitative analysis by ¹⁹F-NMR and Raman spectroscopy identified SiF₅^?/HF₂^? complexes as well as elementary hydrogen (H₂) as hitherto unknown products of silicon dissolution reactions in HF–NOHSO₄–H₂SO₄ mixtures. Based on our findings a strategy for fundamental investigations of relevant reaction pathways is presented and discussed with regard to reported mechanistic concepts.     

Purification of metallurgical-grade silicon powder via chemical attack by hydrofluoric and nitric acids followed by thermal treatment

We investigated a novel process for purifying metallurgical-grade silicon (MG-Si). MG-Si powder was first treated to form a thin porous silicon layer. This was heated at 900 °C under oxygen to weaken impurity–Si bonds. Samples were then chemically etched with dilute aqueous hydrofluoric acid. To understand the mechanisms in this purification process, structural, chemical composition and optical properties of MG-Si powder before and after treatment were characterized using Fourier-transform infrared (FTIR), inductively coupled plasma-atomic emission (ICP-AES), and photoluminescence (PL) spectroscopy techniques. FTIR studies of treated MG-Si powder revealed the formation of a thin porous silicon layer on the top surface, as evidenced by SiH_x vibration peaks. PL spectra show that 30-min HF etching of MG-Si led to an increase in red emission, indicating the formation of porous silicon and suggesting a decrease in impurities. ICP-AES revealed that the process led to significant decreases in the concentrations of 15 different elemental impurities.     

Composition,structure and electrical properties of DC reactive magnetron sputtered Al2O3 thin films

Thin films of alumina (Al₂O₃) were deposited over Si 〈1 0 0〉 substrates at room temperature at an oxygen gas pressure of 0.03 Pa and sputtering power of 60 W using DC reactive magnetron sputtering. The composition of the as-deposited film was analyzed by X-ray photoelectron spectroscopy and the O/Al atomic ratio was found to be 1.72. The films were then annealed in vacuum to 350, 550 and 750 °C and X-ray diffraction results revealed that both as-deposited and post deposition annealed films were amorphous. The surface morphology and topography of the films was studied using scanning electron microscopy and atomic force microscopy, respectively. A progressive decrease in the root mean square (RMS) roughness of the films from 1.53 nm to 0.7 nm was observed with increase in the annealing temperature. Al–Al₂O₃–Al thin film capacitors were then fabricated on p-type Si 〈1 0 0〉 substrate to study the effect of temperature and frequency on the dielectric property of the films and the results are discussed.     

Electrical properties of low-dielectric-constant films prepared by PECVD in O2/CH4/HMDSO

Low-dielectric constant (low-k) films have been prepared by plasma-enhanced chemical vapor deposition (PECVD) from hexamethyldisiloxane (HMDSO) mixed with oxygen or methane. The films are analyzed by ellipsometry, infrared absorption spectroscopy while their electrical properties are deduced from C–V, I–V and R–f measurements performed on Al/insulator/Si structures. For an oxygen and methane fraction equal to 50% and 22%, respectively, the dielectric constant and losses are decreased compared with those of the film prepared in a pure HMDSO plasma. The effect of adding 22% of CH₄ in HMDSO plasma increases the Si–CH₃ bonds containing in the polymer film and as the constant of methyl groups in the film increased the dielectric constant of the film decreases. For this film, the dielectric constant is 2.8, the dielectric losses at 1 kHz are equal to 2×10⁻³, the leakage current density measured for an electric field of 1 MV/cm is 3×10⁻⁹ A/cm² and the breakdown field is close to 5 MV/cm.     

High responsivity MSM black silicon photodetector

The effect of annealing temperature on photoelectric properties of metal–semiconductor–metal (MSM) black silicon photodetector has been studied. The black silicon was fabricated by alkaline etching and metal assisted etching. The nanopores and micro-columns formed by the etching process enhance spectral absorptance significantly at wavelength from 250 nm to 1100 nm. The MSM black silicon photodetectors were annealed at different temperatures in N₂ ambient with a rapid thermal annealing (RTA) process. The fast ramp-up and cool-down rate of RTA is a key factor that eliminates the tensile stress and point defects in Si nanoparticle made from metal assisted wet etching, leading to significant increase of mobility, conductivity and carrier concentration. In addition, the photocurrent and spectral responsivities of detectors increase with annealing temperature. At the wavelength of 600 nm, the responsivity (76.8 A/W) at 673 K is almost three orders of magnitude greater than that of the unannealed sample.     

Elimination of impurities from the surface of silicon using hydrochloric and nitric acid

Acid leaching of silicon is insufficient in order to achieve solar grade silicon. Leaching of silicon previously purified by the copper gathering method can significantly reduce amount of impurities congregated in the Cu–Si intermetallic phase during solidification process. Two samples of 50 wt% Cu–50 wt% Si alloy were solidified at 0.5 and 1.0 °C/min cooling rate. They were treated with 10 vol% HNO₃ and 5 vol% HCl and 7 vol% HNO₃. The inductively Coupled Plasma Mass Spectrometry technique was employed to measure traces of impurities before and after the treatment. It was determined that the overall impurity level in purified silicon was reduced from 5277 ppm_wt to 225.5 ppm_wt. The samples cooled at 0.5 °C/min achieved lower impurities levels in all instances while the sample leached with 10% HNO₃ produced the greatest reduction in impurity level. Scanning electron microscopy and Energy Dispersive X-Ray Spectroscopy analysis showed that the traces of Cu–Si intermetallic together with gathered impurities can be found only in the large silicon particles after the acid leaching treatment. In all instances, the surface of the silicon particles was free of impurities while Si yield was preserved at above 97%.     

Structural,optical and dielectric properties of HfSiO films prepared by co-evaporation method

HfSiO dielectric films were prepared on Si substrate by the co-evaporation method. The chemical composition, crystalline temperature, optical and electrical properties of the compound film were investigated. X-ray photoelectron spectroscopy analysis illustrated that the atom ratio of Hf to Si was about 4:1 and Hf–Si–O bonds appeared in the film. The X-ray diffraction analysis revealed that the crystalline temperature of the film was higher than 850 °C. Optical measurements showed that the refractive index was 1.82 at 550 nm wavelengths and the optical band gap was about 5.88 eV. Electrical measurements demonstrated that the dielectric constant and a fixed charge density were 18.1 and 1.95×10¹² cm⁻² respectively. In addition, an improved leakage current of 7.81 μA/cm² at the gate bias of −3 V was achieved for the annealed HfSiO film.     

Mixed phase silicon thin films grown at high rate using 60 MHz assisted VHF-PECVD technique

Mixed phase amorphous and nanocrystalline silicon (a-Si:H and nc-Si:H) thin films were deposited by VHF-PECVD (60 MHz) using Argon (Ar) as the diluent of silane. These amorphous and crystalline silicon thin films were deposited by varying the argon dilution (f_Ar) from 10–97.5% while keeping other process parameters constant. The effects of argon dilution on deposition rate, structural and optical properties of micro/nanocrystalline silicon thin films are studied. It has been observed that the films deposited from f_Ar 10–70% showed the deposition rate >20 Å/s with the highest deposition rate achieved of ~25 Å/s. Structural characterization has been performed by micro-Raman analysis and Atomic force microscopy. Raman shift towards higher wave number (515 cm⁻¹) with increase of f_Ar indicates variation in crystallinity of silicon films. HRTEM studies revealed the distribution of grain size and the degree of crystallinity. Optical absorption spectroscopy confirmed the increase in band gap of the materials from 1.5 to 2.1 eV.     

BCB-to-oxide bonding technology for 3D integration

Process optimization of BCB polymer to silicon oxide bonding was investigated. The suitable bonding temperature is about 300 °C, while bond failure of BCB-to-oxide bonding is observed starting from 400 °C. Bonding interface morphologies and bond strengths of BCB-to-oxide bonding were investigated as well. PECVD oxide to BCB bonding has better bonding quality than that of thermal oxide to BCB bonding. Si–O–Si bonds may be the reason of a strong BCB to oxide bonding. Water molecules link BCB and oxide surfaces during the initial contact, while Si–O–Si bonds are formed during bonding. This proposed mechanism of BCB-to-oxide bonding provides a guideline for polymer to oxide hybrid bonding technology in 3D integration.     

Investigation of optical and chemical bond properties of hydrogenated amorphous silicon nitride for optoelectronics applications

The aim of this work is to determine optimal deposition parameters of silicon nitride for optical applications. The authors present the investigation of hydrogenated amorphous silicon nitride SiN_x:H deposited by the low temperature PECVD method in high frequency reactors. The study of hydrogen bonds in the SiN_x:H thin films were detailed. The impact of NH₃, SiH₄ and N₂ flow ratio and radio frequency power on optical coefficients in relation to chemical composition and roughness of the film is studied. The correlation between chemical bonds (N–H, Si–H) and refractive index and extinction coefficients is systematically verified. The experimental results show that the films with high refractive indexes superior to 2.05 and low roughness of about 0.35 nm can be achieved for optoelectronics applications by tuning the flow ratio or decreasing the RF power. A variety of processes have been suggested as compatible with low thermal budget (under 350 °C) in order to integrate optical waveguides with lower loss. In particular, the incorporation of N₂ as dilution gas is suited to the fabrication of SiN_x:H films optical waveguide requiring low N–H bonds, low concentration of hydrogen [H] and high refractive index.     

Experimental optimization of solar cells edge junction passivation

We have developed in this study a simple procedure to determine the optimal etching time to passivate the parasitic edge junction of solar cells. The principle of the technique is based on the control of cells electrical characteristics evolution during the gradual elimination of this edge junction. Using plasma technique, the experiments were conducted on monocrystalline and multicrystalline 4 in silicon solar cells round and square in shape respectively. For monocrystalline silicon, the edge junction etch rates of 55.5 nm/min and 90.0–96.5 nm/min has been found for a batch of 20 cells with chemically phosphorus silica glass (PSG) etched and non-etched respectively. The deduced selectivity S=Si/PSG is about 10. For a batch of 100 multicrystalline silicon solar cells, 34 min were sufficient to remove 0.4 μm parasitic junction depth. For the three batches, the difference between the etch rates is explained by the phosphorus concentration and silicon loading effect. As well as for etching uniformities, they are considered good to acceptable.     

Effects of RTA temperatures on conductivity and micro-structures of boron-doped silicon nanocrystals in Si-rich oxide thin films

In this work, the B-doped Si rich oxide (SRO) thin films were deposited and then annealed using rapid thermal annealing (RTA) to form SiO₂-matrix silicon nanocrystals (Si NCs). The effects of the RTA temperatures on the structural properties, conduction mechanisms and electrical properties of B-doped SRO thin films (BSF) were investigated systematically using Hall measurements, Fourier transform infrared spectroscopy and Raman spectroscopy. Results showed that the crystalline fraction of annealed BSF increased from 41.3% to 62.8%, the conductivity was increased from 4.48×10⁻³ S/cm to 0.16 s/cm, the carrier concentration was increased from 8.74×10¹⁷ cm⁻³ to 4.9×10¹⁸ cm⁻³ and the carrier mobility was increased from 0.032 cm² V⁻¹ s⁻¹ to 0.2 cm² V⁻¹ s⁻¹ when the RTA temperatures increased from 1050 °C to 1150 °C. In addition, the fluctuation induced tunneling (FIT) theory was applicable to the conduction mechanisms of SiO₂-matrix boron-doped Si-NC thin films.     

Low temperature SnO2 films deposited by APCVD

SnO₂ films were deposited by atmospheric pressure chemical vapor deposition (APCVD) on glass substrates using tin tetrachloride as the tin precursor, H₂O vapor and O₃–O₂ as oxidizing agents and O₃–O₂ with HF as the fluorine dopant source. The deposition temperatures varied from 200 to 350 °C. It is shown that the deposition temperature and the oxidizing agent used are related with the structure, optical transmission percent, and resistivities of the films. Finally, films with good transmission percent between 85% and 90% in the visible spectrum and lower resistivities ranged from 0.1 to 0.02 Ω cm are obtained.     

Investigations on phosphorus doped amorphous/nanocrystalline silicon films deposited by a filtered cathodic vacuum arc technique in the presence of hydrogen gas

Phosphorus doped amorphous/nanocrystalline silicon (a-Si:H/nc-Si:H) thin films have been deposited by a filtered cathodic vacuum arc (FCVA) technique in the presence of hydrogen gas at different substrate temperatures (T_s) ranging from room temperature (RT) to 350 °C. The films have been characterized by using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, dark conductivity (σ_D), activation energy (ΔE), optical band gap (E_g) and secondary ion mass spectroscopy. The XRD patterns show that RT grown film is amorphous in nature but high temperature (225 and 350 °C) deposited films exhibit nanocrystalline structure with (111) and (220) crystal orientations. The crystallite size of higher temperature grown silicon film evaluated was between 13 and 25 nm. Raman spectra reveal the amorphous nature of the film deposited at RT, whereas higher temperature deposited films show crystalline nature. The crystalline volume fraction of the silicon film deposited at higher temperatures (225 and 350 °C) was estimated to be 58 and 72%. With the increase of T_s, the bonding configuration changes from mono-hydride to di-hydride as revealed by the FTIR spectra. The values of σ_D, ΔE and E_g of silicon films deposited at different T_s were found to be in the range of 5.37×10⁻⁴–1.04 Ω⁻¹ cm⁻¹, 0.05–0.45 eV and 1.42–1.83 eV, respectively. Photoconduction of 3.5% has also been observed in n-type nc-Si:H films with the response and recovery times of 9 and 12 s, respectively. A n-type nc-Si:H/p-type c-Si heterojunction diode was fabricated which showed the diode quality factor between 1.6 and 1.8.     

Thin films of silica imbedded silicon and germanium quantum dots by solution processing

Hydrophilic silicon (0.9 nm) and germanium (2.7 nm) quantum dots (QDs), synthesized utilizing micelles to control particle size, were coated with silica using liquid phase deposition. The use of dodecyltrimethylammonium bromide as a surfactant yielded uniform spheres (Si@SiO₂=57 nm; Ge@SiO₂=32 nm), which could then be arrayed in three dimensions using a vertical deposition method on quartz plates. The silica coated QDs were characterized by UV–visible spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron microscopy. The thin films were characterized by UV–visible spectroscopy, scanning electron microscopy, and the measurement of a photocurrent.     

Chemical bonding states and local structures of the oriented hexagonal BCN films synthesized by microwave plasma CVD

In order to synthesize oriented hexagonal boron carbonitride (h-BCN) films, borane-triethylamine complex (C₆H₁₈BN) was used as a single-source precursor. The films were deposited on Si (1 0 0) substrate by microwave plasma-enhanced chemical-vapor deposition using CH₄+H₂ as the carrier gas. The deposition was performed at different microwave powers of 200–500 W at working pressure of 5.0 Torr. The microhardness, estimated by nano-indentation test, of the films was found to be around 4 GPa. Fourier transform infrared spectroscopy (FT-IR) confirmed the formation of hexagonal BCN phase in a short-range order. The chemical composition and the local structures of films were studied by X-ray photoelectron spectroscopy (XPS) and the near-edge X-ray absorption fine structure (NEXAFS) spectroscopic measurements. XPS revealed that B, C and N atoms in the deposited films are in various chemical environments such as B–N, B–C, C–N and B–C–N atomic hybrid configuration. The NEXAFS measurement suggested that the B atoms are bonded not only to the N atoms but also to the C atoms to form various local structures of sp² B–C–N hybrid configurations. The polarization dependence of NEXAFS suggested that the local structures of the sp² BCN layers have different atomic orientations to the substrate.     

Microbridge preparation through spincoating and photolithography without etching

It is possible to produce HTSC thin films of polymer metal precursors by the simple spincoating technique. This method can be used to manufacture of Y–Ba–Cu–O- and Bi–Sr–Ca–Cu–O–HTSC thin films. The microbridges are generated into the precursor film by photolithography. The etching process step is cancelled. After that the superconducting phases are formed at 950°C respectively 865°C during the tempering process. The HTSC structures serve as a previous stage for SNS contact. The critical temperatures (T_c) measured on the 20 and 200 μm wide microbridges are 82 K for Y–Ba–Cu–O and 108 K for Bi–Sr–Ca–Cu–O. The critical current density (j_c) obtained is 10⁵ A/cm² for 65 K.     

Structural and optical properties of ZrB2 and HfxZr1−xB2 films grown by vicinal surface epitaxy on Si(1 1 1) substrates

ZrB₂ and Hf_xZr_1?xB₂ films were grown on 4° miscut Si(1 1 1) substrates by chemical vapor deposition of gaseous Hf(BH₄)₄ and Zr(BH₄)₄. The films display superior structural and optical properties when compared with ZrB₂ films grown on on-axis Si(1 1 1). The observed improvements include an optically featureless surface with rms roughness of ～2.5–3.5 nm, a ～50% reduction in the amount of residual strain, and a ～50% lower resistivity. These properties should promote the use of diboride films as buffer layers for nitride semiconductor epitaxy on large-area Si substrates.     

Enhancement in electrical performance of indium gallium zinc oxide-based thin film transistors by low temperature thermal annealing

We have studied the characteristics of transparent bottom-gate thin film transistors (TFTs) using In–Ga–Zn–O (IGZO) as an active channel material. IGZO films were deposited on SiO₂/Si substrates by DC sputtering techniques. Thereafter, the bottom-gate TFT devices were fabricated by depositing Ti/Au metal pads on IGZO films, where the channel length and width were defined to be 200 and 1000 μm, respectively. Post-metallization thermal annealing of the devices was carried out at 260, 280 and 300 °C in nitrogen ambient for 1 h. The devices annealed at 280 °C have shown better characteristics with enhanced field-effect mobility and high on–off current ratio. The compositional variation of IGZO films was also observed with different annealing temperatures.