The effect of Ar flow rate in the growth of SiGe:H thin films by PECVD

In this article, by investigating the influence of Ar flow rate on deposition rate and structural properties of hydrogenated silicon germanium (SiGe:H) films, we showed that the addition of Ar in the diluted gas efficiently improve the deposition rate and crystallinity due to an enhanced dissociation of source gases and bombardment on growth surface. The hydrogen content and SE results suggest that the defect density and void volume fraction increases with increasing Ar flow rate, which is attributed to the injection of higher energy Ar⁺ ions into the film led to a displacement of the atoms and an increased possibility of argon being trapped with the films. The optoelectronic properties are investigated by absorption coefficient and dark conductivity measurements and a reasonable explanation is presented.     

Electrochemical corrosion behaviors of a-C:H and a-C:NX:H films

The a-C:H and a-C:N_X:H films were deposited onto silicon wafers using radio frequency (rf) plasma enhanced chemical vapor deposition (PECVD) and pulsed-dc glow discharge plasma CVD, respectively. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize chemical nature and bond types of the films. The results demonstrated that the a-C:H film prepared by rf-CVD (rf C:H) has lower I_D/I_G ratio, indicating smaller sp² cluster size in an amorphous carbon matrix. The nitrogen concentrations of 2.9 at.% and 7.9 at.% correspond to carbon nitride films prepared with rf and pulse power, respectively.Electrochemical corrosion performances of the carbon films were investigated by potentiodynamic polarization test. The electrolyte used in this work was a 0.89% NaCl solution. The corrosion test showed that the rf C:H film exhibited excellent anti-corrosion performance with a corrosion rate of 2 nA cm⁻², while the carbon nitride films prepared by rf technique and pulse technique showed a corrosion rate of 6 nA cm⁻² and 235 nA cm⁻², respectively. It is reasonable to conclude that the smaller sp² cluster size of rf C:H film restrained the electron transfer velocity and then avoids detriment from the exchange of electrons.     

金刚石表面无定形碳氢薄膜生长的分子动力学模拟

利用分子动力学模拟方法研究了CH₂基团轰击金刚石(111)面所形成的无定形碳氢薄膜(a-C:H)的生长过程. 结构分析表明, 得到的无定形碳氢薄膜中碳原子的局域结构(如C–C第一近邻数)与其中氢原子的含量密切相关. CH₂ 基团入射能量的增加会导致得到的薄膜的氢含量降低, 从而改变薄膜中类sp³成键碳原子的比例.     

超薄类金刚石膜生长和结构特性的分子动力学模拟

利用分子动力学模拟方法研究了2—3nm厚的类金刚石(DLC)薄膜在金刚石基体(100)表面上的生长过程. 分析了用来表征沉积后无定型碳膜质量的重要结构特性，如sp³杂化比例、薄膜密度、径向分布函数等，计算结果和现有的实验结果基本一致. 不同入射原子能量对结构特性进而对薄膜性能有重要影响，入射原子能量在20—60eV时，薄膜可以获得最优性能. 载能原子入射是生长均匀、连续、致密固体薄膜的前提，稳定的中间区域对于保证薄膜优良的力学性质是必需的. 关键词： DLC膜 分子动力学模拟 3杂化比例')" href="#">sp³杂化比例     

CH基团与金刚石(111)面的碰撞反应及其对碳膜生长的影响

等离子体增强化学气相沉积技术中的碳膜选择性自组装机理是高性能碳膜制备过程中的挑战性基础课题.采用经典分子动力学方法,模拟了不同能量(1.625-65 eV)的CH基团在清洁金刚石和吸氢金刚石(111)面上的轰击行为,获得了吸附、反弹、反应等各类事件的发生概率,并据此探讨了含氢碳膜制备过程中CH基团的贡献.结果表明,随着入射能量的增加,CH基团对薄膜生长的贡献由单纯的吸附、反弹机理向反应、吸附混合机理转变,其中最主要的反应过程是释放一个或两个氢原子的反应,而释放氢分子的反应则很少发生.这些反应不仅使薄膜生长过程更均匀、薄膜表面更平整,还降低了薄膜的氢含量.生长机理的转变导致低能量条件下所成薄膜中的多数碳原子都包含一个氢原子作为配位原子,而高能量条件下的薄膜中的碳原子则很少有氢原子作为配位原子.另外,通过分析sp~3-C和sp~2-C数目的变化,研究了CH基团对金刚石基底的破坏作用.     

Structure and hardness of a-C:H films prepared by middle frequency plasma chemical vapor deposition

a-C:H films were prepared by middle frequency plasma chemical vapor deposition (MF-PCVD) on silicon substrates from two hydrocarbon source gases, CH₄ and a mixture of C₂H₂ + H₂, at varying bias voltage amplitudes. Raman spectroscopy shows that the structure of the a-C:H films deposited from these two precursors is different. For the films deposited from CH₄, the G peak position around 1520 cm⁻¹ and the small intensity ratio of D peak to G peak (I(D)/I(G)) indicate that the C-C sp³ fraction in this film is about 20 at.%. These films are diamond-like a-C:H films. For the films deposited from C₂H₂ + H₂, the Raman results indicate that their structure is close to graphite-like amorphous carbon. The hardness and elastic modulus of the films deposited from CH₄ increase with increasing bias voltage, while a decrease of hardness and elastic modulus of the films deposited from a mixture of C₂H₂ + H₂ with increasing bias voltage is observed.     

Diamond-like C2H nanolayer, diamane: Simulation of the structure and properties

We consider a new C₂H nanostructure based on bilayer graphene transformed under the covalent bond of hydrogen atoms adsorbed on its external surface, as well as compounds of carbon atoms located opposite each other in neighboring layers. They constitute a “film” of the 〈111〉 diamond with a thickness of less than 1 nm, which is called diamane. The energy characteristics and electron spectra of diamane, graphene, and diamond are calculated using the density functional theory and are compared with each other. The effective Young’s moduli and destruction thresholds of diamane and graphene membranes are determined by the molecular dynamics method. It is shown that C₂H diamane is more stable than CH graphane, its dielectric “gap” is narrower than the band gap of bulk diamond (by 0.8 eV) and graphane (by 0.3 eV), and is harder and more brittle than the latter.     

Simultaneous interaction of methyl radicals and atomic hydrogen with amorphous hydrogenated carbon films, as investigated with optical in situ diagnostics

Methyl radicals (CH₃) and atomic hydrogen (H) are dominant radicals in low-temperature plasmas from methane. The surface reactions of these radicals are believed to be key steps leading to deposition of amorphous hydrogenated carbon (a-C:H) films or polycrystalline diamond in these discharges. The underlying growth mechanism is studied, by exposing an a-C:H film to quantified radical beams of H and CH₃. The deposition or etching rate is monitored via ellipsometry and the variation of the stoichiometry is monitored via isotope labeling and infrared spectroscopy. It was shown recently that, at 320 K, methyl radicals have a sticking coefficient of 10^-4 on a-C:H films, which rises to 10^-2 if an additional flux of atomic hydrogen is present. This represents a synergistic growth mechanism between H and CH₃. From the interpretation of the infrared data, a reaction scheme for this type of film growth is developed: atomic hydrogen creates dangling bonds by abstraction of bonded hydrogen within a surface layer corresponding to the range of H in a-C:H films. These dangling bonds serve at the physical surface as adsorption sites for incoming methyl radicals and beneath the surface as radicalic centers for polymerization reactions leading to carbon–carbon bonds and to the formation of a dense a-C:H film. Received: 18 July 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

类金刚石薄膜力学特性的分子动力学模拟

基于Brenner的REBO势函数,利用分子动力学方法模拟了含氢量不同的类金刚石薄膜的纳米压痕过程,依据得到的加载卸载曲线,计算了薄膜的刚度、硬度以及弹性模量.结果表明:类金刚石薄膜的硬度由氢含量和sp³键含量两个因素共同决定;当薄膜中氢含量小于39% 时,薄膜硬度主要取决于sp³键含量,sp³键越多,硬度越高;当薄膜中氢含量达到52%,薄膜硬度则显著下降,此时氢的作用占据主导地位. 关键词： 类金刚石薄膜 分子动力学模拟 纳米压痕 硬度     

Study of structural and electronic environments of hydrogenated amorphous silicon carbonitride (a-SiCN:H) films deposited by hot wire chemical vapor deposition

Hydrogenated amorphous silicon carbon nitride (a-SiCN:H) thin films were deposited by hot wire chemical vapor deposition (HWCVD) using SiH₄, CH₄, NH₃ and H₂ as precursors. The effects of the H₂ dilution on structural and chemical bonding of a-SiCN:H has been investigated by Raman and X-ray photoelectron spectroscopy (XPS). Increasing the H₂ flow rate in the precursor gas more carbon is introduced into the a-SiCN:H network resulting in decrease of silicon content in the film from 41 at.% to 28.8 at.% and sp² carbon cluster increases when H₂ flow rate is increased from 0 to 20 sccm.     

Molecular dynamics simulation of CH3 interaction with Si(1 0 0) surface

Molecular dynamics simulations of the CH₃ interaction with Si(1 0 0) were performed using the Tersoff-Brenner potential. The H/C ratio obtained from the simulations is in agreement with available experimental data. The results show that H atoms preferentially react with Si. SiH is the dominant form of SiH_x generated. The amount of hydrogen that reacts with silicon is essentially energy-independent. H atoms do not react with adsorbed carbon atoms. The presence of C-H bonds on the surface is due to molecular adsorption.     

The effect of applied negative bias voltage on the structure of Ti-doped a-C:H films deposited by FCVA

Ti-doped hydrogenated diamond-like carbon (DLC) films were deposited on Si(1 0 0) substrates by a filtered cathodic vacuum arc (FCVA) method using Ar and CH₄ as the feedstock. The composition and microstructure of the films were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy and IR spectroscopy. The internal stress was determined by the radius of curvature technique. The influence of the bias voltage on the microstructure of the as-deposited films was investigated. It was found that the graphite-like bonds was dominated in the Ti-doped DLC film deposited at 0 V bias voltage. When bias voltage was increased to −150 V, more diamond-like bond were produced and the sp³ content in film reached the maximum value, after which it decreased and more graphite-like bonds feature produced with further increase of the negative bias voltage. The compressive internal in the Ti-doped DLC films also exhibited a maximum value at −150 V bias voltage. IR results indicated that CH bonded intensity reduced, and H atoms bonded with C atoms were substituted for the Ti atoms as the negative bias voltage increasing. All the composition and microstructure change can be explained by considering the plasma conditions and the effect of negative bias voltage applied to the substrate.     

Structure and phase composition of thin a-C:H films modified by Ag and Ti

The structure and phase composition of thin a-C:H and a-C:H〈M〉 films (M = Ag, Ti, or Ag + Ti) have been studied by Raman and X-ray photoelectron spectroscopy. The a-C:H〈M〉 films were prepared by ion-plasma magnetron sputtering of a combined target of graphite and metal in an Ar–CH₄ gas mixture. The Raman spectra of these films indicate that their structure is amorphous. The a-C:H〈Ag + Ti〉 films have a more graphitized structure in comparison with pure a-C:H films and films containing only one metal. It is established that carbon in the a-C:H〈Ag + Ti〉 films is in the sp ², sp ³, and C=O states, which are characteristic of the a-C:H, a-C:H〈Ag〉, and a-C:H〈Ti〉 films. In addition, there are also ether (–C–O–C–) or epoxy (?C?O–) carbon groups in the a-C:H〈Ag + Ti〉 films. It has been revealed that silver atoms in the a-C:H〈Ag〉 and a-C:H〈Ag + Ti〉 films form no chemical bonds with carbon, oxygen, and titanium. Titanium in the a-C:H〈Ti〉 and a-C:H〈Ag + Ti〉 films exists in the form of titanium IV oxide (TiO₂).     

A 13C isotopic tracing study of a-C:H thin-film growth

The growth mechanisms of amorphous carbon (a-C:H) thin films obtained by the decomposition of methane gas in a multipolar plasma set up were studied using ¹³C as a tracer. Hydrogenated carbon films first grown in a methane plasma with natural isotopic composition (¹²C) were further grown in highly enriched ¹³C methane discharge (experiments were also carried out with the reverse order). ¹³C profiles were obtained by resonant nuclear reaction depth profiling using the narrow (FWHM<100 eV) resonance at 1747.6 keV, in the ¹³C(p, )¹⁴N nuclear reaction. Plasma conditions were varied so as to vary the energy of particles arriving at the sample surface from 30 eV to 500 eV. In all cases, a system of two layers was observed: a pure ¹³C film overlaying a pure ¹²C film (or the reverse order) with a rather sharp interface between the two layers. These results suggest layer by layer amorphous hydrogenated carbon film growth with minimal mixing or interdiffusion of the isotopic layers induced by the ionic bombardment during the growth.     

Simulations of C60 bombardment of Si, SiC, diamond and graphite

Molecular dynamics simulations of the 20-keV C₆₀ bombardment at normal incidence of Si, SiC, diamond and graphite targets were performed. The unique feature of these targets is that strong covalent bonds can be formed between carbon atoms from the C₆₀ projectile and atoms in the solid material. The mesoscale energy deposition footprint (MEDF) model is used to gain physical insight into how the sputtering yields depend on the substrate characteristics. A large proportion of the carbon atoms from the C₆₀ projectile are implanted into the lattice structure of the target. The sputtering yield from SiC is ∼twice that from either diamond or Si and this can be explained by both the region of the energized cylindrical tract created by the impact and the number density. On graphite, the yield of sputtered atoms is negligible because the open lattice allows the cluster to deposit its energy deep within the solid. The simulations suggest that build up of carbon with a graphite-like structure would reduce any sputtering from a solid with C₆₀⁺ bombardment.     

XPS study of the chemical bonding in hydrogenated amorphous germanium–carbon alloys

A systematic study of the chemical bonding in hydrogenated amorphous germanium–carbon (a-Ge_1-xC_x:H)alloys using X-ray photoelectron spectroscopy (XPS) is presented. The films, with carbon content ranging from 0 at. % to 100 at. %, were prepared by the rf co-sputtering technique. Raman spectroscopy was used to investigate the carbon hybridization. Rutherford backscattering spectroscopy (RBS) and XPS were used to determine the film stoichiometry. The Ge 3d and C 1s core levels were used for investigating the bonding properties of germanium and carbon atoms, respectively. The relative concentrations of C–Ge, C–C, and C–H bonds were calculated using the intensities of the chemically shifted C 1s components. It was observed that the carbon atoms enter the germanium network with different hybridization, which depends on the carbon concentration. For concentrations lower than 20 at. %, the carbon atoms are preferentially sp³ hybridized, and approximately randomly distributed. As the carbon content increases the concentration of sp² sites also increases and the films are more graphitic-like. Received: 4 May 1999 / Accepted: 24 November 1999 / Published online: 24 March 2000     

a-C∶H(N)薄膜结构的X射线光电子能谱分析

采用直流-射频等离子增强化学汽相沉积技术制备a-C∶H(N)薄膜，用X射线光电子能谱研究了混合气体中N₂含量对薄膜成分与结构的影响.a-C∶H(N)薄膜中含氮量可达9.09%.对a-C∶H(N)薄膜的C1s和N1s结合能谱的分析表明a-C∶H(N)薄膜的结构是由C₃N₄相镶嵌在sp²键结合的CN_x基体中组成.其中C₃N₄相中N和C原子比接近4∶3，不随薄     

Structure and elastic recovery of Cr-C:H films deposited by a reactive magnetron sputtering technique

Cr-containing hydrogenated amorphous carbon (Cr-C:H) films were deposited on silicon substrates using a DC reactive magnetron sputtering with Cr target in an Ar and C₂H₂ gas mixture. The composition, bond structure, mechanical hardness and elastic recovery of the films were characterized using energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and nano-indentation. The film tribological behavior was also studied by a ball-on-disc tribo-tester. The results showed that the films deposited at low C₂H₂ flow rate (<10 sccm) presented a feature of composite Cr-C:H structure, which consisted of hard brittle chromium carbide phases and amorphous hydrocarbon phase, and thus led to the observed low elastic recovery and poor wear resistance of the films. However, the film deposited at high C₂H₂ flow rate (40 sccm) was found to present a typical feature of polymer-like a-C:H structure containing a large amount of sp³ C-H bonds. As a result, the film revealed a high elastic recovery, and thus exhibited an excellent wear resistance.     

Thermal conductive performance of deposited amorphous carbon materials by molecular dynamics simulation

A series of diamond-like carbon (DLC) films with different microstructure were prepared by depositing carbon atoms on diamond surface with incident energy ranging from 1 to 100 eV. The thermal conductivity of the deposited films and the Kapitza resistance between the film and the diamond substrate were investigated. Results show that the average density, the average fraction of sp³ bonding and the thermal conductivity of the DLC films increase first, reaching a maximum around 20–40 eV before decreasing, while the Kapitza resistance decreases gradually with increased deposition energy. The analysis suggests that the thermal resistance of the interface layer is in the order of 10^?10 m²K/W, which is not ignorable when measuring the thermal conductivity of the deposited film especially when the thickness of the DLC film is not large enough. The fraction of sp³ bonding in the DLC film decreases gradually normal to the diamond surface. However, the thermal conductivity of the film in normal direction is not affected obviously by this kind of structural variation but depends linearly on the average fraction of sp³ bonding in the entire film. The dependence of the thermal conductivity on the fraction of sp³ bonding was analysed by the phonon theory.     

LiNH2储氢材料中间隙H与掺杂原子交互作用对其释氢性能影响机理研究

采用基于密度泛函理论的赝势平面波第一性原理方法,研究了LiNH₂缺陷及其掺杂原子交互作用对其释氢影响.通过对其进行优化求得它们的局域最稳定结构并计算了含间隙H原子缺陷的LiNH₂及其掺杂合金的结合能、间隙缺陷形成能、态密度和电荷布居.结果表明: 系统结合能不能反映LiNH₂及其掺杂合金的释氢性质;平衡时,LiNH₂中有一定的间隙氢原子存在,Mg,Ti掺杂使形成能大大降低,大大增大了间隙氢的浓度. 间隙H原子在带隙引入了缺陷能级使带隙大大减小,提高释氢能力.间隙H原子导致[NH₂]^-中N-H原子间相互作用减弱,容易释氢.间隙H与[NH₂]^-中N存在共价作用,可以解释LiNH₂释氢反应中NH₃的放出.当存在掺杂时,N-H键的键强不均衡,部分较弱,部分较强,较弱的N-H键中H容易放出. 关键词： 储氢材料 第一性原理 缺陷 释氢机理