Deposition and characterization of low temperature silicon nitride films deposited by inductively coupled plasma CVD

Silicon nitride films have been deposited at a low temperature (70 °C) by inductively coupled plasma chemical vapor deposition (ICP-CVD) technique and their physical and chemical properties were studied. For a deposited SiN sample, β-phase was observed and refractive index of 2.1 at 13.18 nm/min deposition rate was obtained. The attained stress of 0.08 GPa is lower as compared to the reported value of 1.1 GPa for SiN thin films. To study the deposited film, characterization was performed using X-ray photoelectron spectra (XPS), X-ray diffraction (XRD), micro Raman spectroscopy, Fourier transfer infrared spectroscopy (FTIR), cross-section scanning electron microscopy (SEM) and atomic force microscopy (AFM).     

DLTS characterization of silicon nitride passivated AlGaN/GaN heterostructures

Passivating the ungated surface of AlGaN/GaN HEMTs with silicon nitride (SiN) is effective in improving the microwave output power performances of these devices. However, very little information is available about surface states in GaN-based HEMTs after SiN passivation. In this work we investigate AlGaN/GaN HEMTs structures having either metal–semiconductor or metal–SiN–semiconductor gate contacts. In short gate devices conductance DLTS measurements point out a hole-like peak that shows an anomalous behaviour and can be ascribed to surface states in the access regions of the device. In insulated gate HEMTs a band of levels is detected and ascribed to surface states, whose energy ranges from 0.14 to 0.43 eV. Capacitance–voltage measurements allow us to point out the existence of a second band of interface states deeper in energy than the former one. This band is responsible for slow transients observed in the characteristics of the insulated gate FAT-HEMT.     

Thermal oxidation of n-type ZnN films made by -sputtering from a zinc nitride target, and their conversion into p-type films

Transparent p-type thin films, containing zinc oxide phases, have been fabricated from the oxidation of n-type zinc nitride films. The zinc nitride thin films were deposited by rf-magnetron sputtering from a zinc nitride target in pure N₂ and pure Ar plasma. Films deposited in Ar plasma were conductive (resistivity 4.7×10⁻² Ω cm and carrier concentrations around 10²⁰ cm⁻³) Zn-rich Zn_xN_y films of low transmittance, whereas Zn_xN_y films deposited in N₂ plasma showed high transmittance (>80%), but five orders of magnitude lower conductivity. Thermal oxidation up to 550 C converted all films into p-type materials, exhibiting high resistivity, 10²–10³ Ω cm, and carrier concentration around 10¹³ cm⁻³. However, upon oxidation, the Zn_xN_y films did not show the zinc oxide phase, whereas Zn-rich Zn_xN_y films were converted into films containing ZnO and ZnO₂ phases. All films exhibited transmittance >85% with a characteristic excitonic dip in the transmittance curve at 365 nm. Low temperature photoluminescence revealed the existence of exciton emissions at 3.36 and 3.305 eV for the p-type zinc oxide film.     

Photoluminescence properties of SiNx/Si amorphous multilayer structures grown by plasma-enhanced chemical vapor deposition

Very thin (nanometric) silicon layers were grown in between silicon nitride barriers by SiH₂Cl₂/H₂/NH₃ plasma-enhanced chemical vapor deposition (PECVD). The multilayer structures were deposited onto fused silica and silicon substrates. Deposition conditions were selected to favor Si cluster formation of different sizes in between the barriers of silicon nitride. The samples were thermally treated in an inert atmosphere for 1 h at 500 °C for dehydrogenation. Room-temperature photoluminescence (RT-PL) and optical transmission in different ranges were used to evaluate the optical properties of the structures. UV-VIS absorption spectra present two band edges. These band edges are well fitted by the Tauc model typically used for amorphous materials. RT-PL spectra are characterized by strong broad bands, which have a blue shift as a function of the deposition time of the silicon layer, even for as-grown samples. The broad luminescence could be associated with the confinement effect in the silicon clusters. After annealing of the samples, the PL bands red shift. This is probably due to the thermal decomposition of N-H bonds with further effusion of hydrogen and better nitrogen passivation of the nc-Si/SiN_x interfaces.     

Nanostructured Au/Si substrate for organic molecule SERS detection

Nanoparticles of noble metals, such as gold and silver, exhibit unique and tunable optical properties on account of their surface plasmon resonance. In particular, gold nanoparticles on silicon substrates are attractive for future nanoscale sensors and optical devices due to their resistance to oxidation and due to their electrical and optical properties. In this study, we developed a nanostructured gold/macroporous silicon (Au/PS) substrate capped with 11-mercaptoundecanoic acid (11-MUA) with ultra-sensitive detection properties achieved in characterization, an approach based on surface-enhanced Raman scattering (SERS). Surface-enhanced Raman scattering allows us to detect substances at a low concentration level and to observe structural details of a thiol molecule bonded to small film thicknesses. Raman measurements were carried out at 514 nm and 785 nm. In order to emphasize the effect of the Si microstructuration on the efficiency of this new substrate (Au/PS) proposed for SERS experiments, the same molecule (11-MUA) was adsorbed on it as well as on gold/atomically flat silicon (Au/Si) and on commercial Klarite (Mesophotonics) substrates. Systematic studies realized by Raman spectroscopy, electron microscopy, and X-ray spectroscopy show the influence of silicon substrate texturing and metallic deposition conditions, including time and temperature on the optical phenomena.     

Radio frequency source power-induced ion energy impact on SiN films deposited by using a pulsed-PECVD in SiH4–N2 plasma at room temperature

Using a radio frequency (rf) pulsed-plasma enhanced chemical vapor deposition system, silicon nitride (SiN) films were deposited in a SiH₄–N₂ inductively coupled plasma. Effect of duty ratio and rf source powers on deposition rate at room temperature were investigated in the ranges 50–90% and 600–900 W, respectively. Plasma diagnostics on ion energy was conducted and rf source power-induced ion energy impact on SiN films were studied as well as some correlations between deposition rate and ion energy. High and low energies ranged from 17.8 to 22.6 eV, and from 23.6 to 33.8 eV, respectively. Higher ion energies observed at lower duty ratios or lower rf powers was attributed to a lower plasma density. Ion energy flux variation was opposite to that for ion energy. Meanwhile, the deposition rate increased with decreasing the duty ratio at all powers but 900 W. This was not clear as a function of rf source power. The deposition rate ranged from 17.0 to 26.5 nm/min.     

Influence of interfaces density and thermal processes on mechanical stress of PECVD silicon nitride

The paper focuses on a particular silicon nitride thin film (SiN_x) produced by plasma enahanced chemical vapor deposition (PECVD) technique with high deposition rate (26 nm/min) and low values of mechanical stress (<100 MPa). This was perfomed with mixed frequency procedure varying the modulation of high frequency at 13.56 MHz and low frequency at 308 kHz of RF power supply during the deposition, without changing the ratio of reaction gases. Low stress silicon nitride is commonly obtained by tailoring the thickness ratio of high frequency vs. low frequency silicon nitride layers.The attention of this work was directed to the influence of the number of interfaces per thickness unit on the stress characteristics of the deposited material. Two sets of wafer samples were deposited with low stress silicon nitride, with a thickness of 260 nm and 2 μm, respectively. Thermal annealing processes at 380 and 520 °C in a inert enviroment were also performed on the wafers.The Stoney–Hoffman model was used to estimate the stress values by wafer curvature measurement with a mechanical surface profilometer: the stress was calculated for the as-deposited layer, and after each annealing process.The thickness and the refractive index of the SiN_x were also measured and charaterized by variable angle spectra elliposometry (VASE) techinique.The experimental measurements were performed at the MT-LAB, IRST (Istituto per la Ricerca Scientifica e Tecnologica) of Bruno Kessler Foundation for Research in Trento.     

Optically pumped InGaN/GaN MQW lift-off lasers grown on silicon substrates

Chemical etching and removal of the silicon substrate was used for the creation of optically pumped lift-off InGaN/GaN multiple quantum well (MQW) lasers from heterostructures grown on silicon substrate by MOVPE. Luminescence and laser properties of these heterostructures on silicon substrates as well as those of MQWs lifted-off from their substrate by chemical etching were investigated. The lowest value of the lasing threshold of the lift-off lasers at room temperature was about 205 kW/cm² for a laser wavelength of 463 nm and about 360 kW/cm² for a wavelength of 475 nm. It was shown theoretically that the reduction of internal losses, caused by the absence of absorption in the substrate (resulting from its removal) is most significant for the high order modes having lower values of mirror losses and can lead to a 50% reduction of the threshold (or material gain in InGaN necessary to achieve the threshold).     

GaN micromachined FBAR structures for microwave applications

This paper presents the manufacturing of GaN membrane supported F-BAR structures. The 2.2 μm thick GaN layer was grown using MOCVD techniques on a high-resistivity 111-oriented silicon substrate. Conventional contact lithography, electron-gun Ti/Au evaporation and lift-off techniques were used to define top-side metallization of the the FBAR structures. Bulk micromachining techniques were used for the release of the GaN membrane. The bottom-side metallization of the micromachined structure was obtained by means of sputtered gold. S-parameter measurements have shown a pronounced resonance around 1.2 GHz. The extracted value of acoustic velocity is in good agreement with those reported by other authors on materials fabricated by other methods. The demonstrated FBAR function in epitaxially grown GaN layers opens new avenues for a low-cost monolithic integration with GaN-based electronics and sensing devices.     

Corrosion resistant surface for vanadium nitride and hafnium nitride layers as function of grain size

In this research it was studied vanadium nitride (VN) and hafnium nitride (HfN) film, which were deposited onto silicon (Si (100)) and AISI 4140 steel substrates via r.f. magnetron sputtering technique in Ar/N₂ atmosphere with purity at 99.99% for both V and Hf metallic targets. Both films were approximately 1.2±0.1 µm thick. The crystallography structures that were evaluated via X-ray diffraction analysis (XRD) showed preferential orientations in the Bragg planes VN (200) and HfN (111). The chemical compositions for both films were characterized by EDX. Atomic Force Microscopy (AFM) was used to study the morphology; the results reveal grain sizes of 78±2 nm for VN and 58±2 nm for HfN and roughness values of 4.2±0.1 nm for VN and 1.5±0.1 nm for HfN films. The electrochemical performance in VN and HfN films deposited onto steel 4140 were studied by Tafel polarization curves and impedance spectroscopy methods (EIS) under contact with sodium chloride at 3.5 wt% solution, therefore, it was found that the corrosion rate decreased about 95% in VN and 99% for HfN films in relation to uncoated 4140 steel, thus demonstrating, the protecting effect of VN and HfN films under a corrosive environment as function of morphological characteristics (grain size).     

氧化随机织构硅表面对单晶硅太阳电池性能的影响研究

热氧化生长的SiO\-2 薄膜经常在高效单晶硅太阳电池中被用作扩散掩膜,化学镀掩膜,钝化层或者基本的减反射层.在这些高效太阳电池中,经常使用碱性溶液对单晶硅表面进行处理,得到随机分布的正金字塔结构的织绒表面,减少表面的光反射.表面氧化后的正金字塔太阳电池暗反向电流-电压呈现"软击穿"现象,并联电阻明显下降.研究结果表明引起这些现象的原因在于氧化正金字塔表面会导致在体内形成位错型缺陷,这些缺陷能够贯穿整个pn 结,导致太阳电池的并联电阻下降,同时载流子在位错型缺陷在能隙中引入的能级处发生复合,导致空间电荷区 关键词： 热氧化 随机织构 位错 太阳电池     

Synthesis of thin silicon nanowires using gold-catalyzed chemical vapor deposition

Silicon nanowires were synthesized using chemical vapor deposition catalyzed by gold nanoparticles deposited on silicon substrates. Silicon nanowires grew epitaxially in 111 directions on (100)-oriented silicon substrates. For a particular set of process parameters, we observed a critical thickness of the nucleating gold film, below which nanowires could not be grown. We studied the dependence of the Au-Si alloy droplet size and size distribution on the starting gold film thickness and the annealing conditions. Increasing the Cl:Si ratio in the gas phase allowed nanowires to grow on smaller Au-Si alloy droplets. We used a modified heating sequence that deconvoluted the effect of silicon substrate consumption and gas-phase silicon supply on the Au-Si alloy formation and allowed growth of nanowires with diameters less than 20 nm. The modified heating sequence was also used to demonstrate the growth of bridging silicon nanowires with diameters less than 20 nm, which is a significant step in producing electronic devices. PACS 81.07.b; 81.15.Gh     

Electrons diffusion study on the nitrogen-doped nanocrystalline diamond film grown by MPECVD method

Nitrogen-doped nanocrystalline diamond (NNCD) films were deposited onto p-type silicon substrates with three different layer structures: (i) directly onto the silicon substrate (NNCD/Si), (ii) silicon with undoped nanocrystalline diamond layer which was deposited in the same way as the above mentioned NNCD by the recipe Ar/CH₄/H₂ with a ratio of 98%/1%/1% (NNCD/NCD/Si), and (iii) silicon wafer with 100 nm thickness SiO₂ layer (NNCD/SiO₂/Si). Atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy were employed to characterize the morphology and microstructure of the as-grown nitrogen-doped diamond films. Silver colloid/silver contacts were made at to measure the current-voltage (I-V) characteristics for the three different structures. Electrons from a CVD reactor hydrogen plasma diffuse toward the p-type silicon substrate during a deposition process under the high temperature (∼800 °C). The study concluded that the SiO₂ layer could effectively prevents the diffusion of electrons.     

Peculiarities of electron field emission from ZnO nanocrystals and nanostructured films

ZnO microcrystals and nanocrystals were grown on silicon substrates by condensation from vapour phase. Nanostructured ZnO films were deposited by plasma enhanced metal organic chemical vapour deposition (PEMOCVD). The parameters of field emission, namely form-factor β and work function , were calculated for ZnO structures by the help of the Fowler–Nordheim equation. The work functions from ZnO nanostructured films were evaluated by a comparison method. The density of emission current from ZnO nanostructures reaches 0.6 mA/cm² at electric force F=2.110⁵ V/cm. During repeatable measurements β changes from 5.810⁴ to 2.310⁶ cm⁻¹, indicating improvement of field emission. Obtained values of work functions were 3.7±0.37 eV and 2.9–3.2 eV for ZnO nanostructures and ZnO films respectively.     

氮化铌纳米线光学特性

氮化铌(NbN)纳米线是超导纳米线单光子探测器(SNSPD)常用的光敏材料,其光学性质是影响SNSPD性能的关键因素.本文结合实验数据和仿真结果,系统研究了多种NbN超导纳米线探测器器件结构的光学特性,表征了以下四种器件结构下的反射光谱以及透射光谱:1)双面热氧化硅衬底背面对光结构;2)双面SiN硅衬底背面对光结构;3)硅衬底上以金层+SiN缓冲层为反射镜的正面对光结构;4)以分布式布拉格反射镜(DBR)为衬底的正面对光结构.并在上述四种器件结构基础上,生长了不同厚度的NbN薄膜,观察不同厚度NbN薄膜的吸收效率.经分析,发现在不同器件结构下的最佳NbN厚度与光吸收率的关系如下:双面热氧化硅衬底上的NbN层在1606 nm处最大吸收率为91.7%,其余结构在最佳NbN厚度条件下吸收率都能达到99%以上.其中双面SiN的硅衬底结构中最大吸收率为99.3%, Au+SiN为99.8%, DBR为99.9%.最后,将DBR器件实测结果与仿真结果进行了差异性分析.这些结果对高效率SNSPD设计与研制具有指导意义.     

Structural characterization of ZnO/ Er2O3 core/shell nanowires

Zinc oxide/erbium oxide core/shell nanowires are of great potential value to optoelectronics because of the possible demonstration of laser emission in the 1.5 μm range. In this paper we present a convenient technique to obtain structures of this composition. ZnO core nanowires were first obtained by a vapor–liquid–solid (VLS) method using gold as a catalyst. ZnO nanowires ranging from 50 to 100 nm in width were grown on the substrates. Erbium was incorporated into these nanowires by their exposure to Er(tmhd)₃ at elevated temperatures. After annealing at 700 C in air, the nanowires presented 1.54 μm emission when excited by any of the lines of an Ar⁺ laser. An investigation of nanowire structure by HRTEM indicates that indeed the cores consist of hexagonal ZnO grown in the 001 direction while the surface contains randomly oriented Er₂O₃ nanoparticles. EXAFS analysis reveals that the Er atoms possess a sixfold oxygen coordination environment, almost identical to that of Er₂O₃. Taken collectively, these data suggest that the overall architectures of these nanowires are discrete layered ZnO/ Er₂O₃ core/shell structures whereby erbium atoms are not incorporated into the ZnO core geometry.     

Epitaxial growth of ZnSiN2 single-crystalline films on sapphire substrates

A new family of wide band gap nitride semiconductors expressed as II–IV-N₂ have recently attracted attention due to their expected properties such as optical non-linearity. In addition, among these compounds, ZnGeN₂ and ZnSiN₂ have lattice parameters close to GaN and SiC respectively. Up to now, there is very little work reported on this class of materials and no systematic thin film growth study has been reported to date. In this paper we present the first study on the growth of ZnSiN₂ on c-sapphire and (100) silicon substrates using low pressure MOVPE technique. Triethylamine:dimethylzinc adduct, silane diluted in H₂ and ammonia were used as source materials. Single crystalline epitaxial ZnSiN₂ layers were obtained on nitridated c-sapphire substrates in the temperature range 873–973 K by using an adapted II/IV molar ratio ranging from 1.2 to 12. Assuming an orthorhombic unit cell, the lattice parameters calculated from the X-ray diffraction data are a=0.534 nm, b=0.617 nm and c=0.504 nm.     

Ni/Au Schottky gate oxidation and BCB passivation for high-breakdown-voltage AlGaN/GaN HEMT

A new AlGaN/GaN high electron mobility transistor (HEMT) employing Ni/Au Schottky gate oxidation and benzocyclobutene (BCB) passivation is fabricated in order to increase a breakdown voltage and forward drain current. The Ni/Au Schottky gate metal with a thickness of 50/300 nm is oxidized under oxygen ambient at 500 C and the highly resistive NiO is formed at the gate edge. The leakage current of AlGaN/GaN HEMTs is decreased from 4.94 μA to 3.34 nA due to the formation of NiO. The BCB, which has a low dielectric constant, successfully passivates AlGaN/GaN HEMTs by suppressing electron injection into surface states. The BCB passivation layer has a low capacitance, so BCB passivation increases the switching speed of AlGaN/GaN HEMTs compared with silicon nitride passivation, which has a high dielectric constant. The forward drain current of a BCB-passivated device is 199 mA /mm, while that of an unpassivated device is 172 mA /mm due to the increase in two-dimensional electron gas (2DEG) charge.