Low-temperature synthesis of AlN films through electron cyclotron resonance plasma-aided reactive pulsed laser deposition

Combination of pulsed laser ablation with electron cyclotron resonance microwave discharge was demonstrated for a novel method for low-temperature thin film growth. Aluminum nitride thin films were synthesized on silicon substrates at temperatures below 80 °C by means of reactive pulsed laser deposition in nitrogen plasma generated from the electron cyclotron resonance discharge. The synthesized films show a very smooth surface and were found to have a stoichiometric AlN composition. X-ray photoelectron spectroscopy analysis evidenced the formation of aluminum nitride compound. Fourier transform infrared spectroscopy revealed the characteristic phonon modes of AlN. The AlN films were observed to be highly transparent in the visible and near-IR regions and have a sharp absorption edge near 190 nm. The band gap of the synthesized AlN films was determined to be 5.7 eV. The mechanisms responsible for the low-temperature film synthesis are also discussed in the paper. The nitrogen plasma facilitates the nitride formation and enhances the film growth. Received: 17 March 2000 / Accepted: 28 March 2000 / Published online: 23 May 2001     

In situ characterisation of thin-film formation in molecular low-temperature plasmas

Thin films can be formed in low-temperature plasma in two ways: as a result of material modification usually in a discharge of non-organic gas and in the course of a deposition process in a discharge of organic vapour. Thin-film formation requires in situ control of plasma processing in order to get information on the growth mechanisms and control the deposition process. Here are shown some effective in situ techniques of thin-film characterisation: various kinds of infrared spectroscopy (infrared reflection absorption spectroscopy, attenuated total reflection spectroscopy, etc.), ellipsometry and microgravimetry. We discuss the application of these methods to polymer modification as well as plasma polymerisation. The applied techniques give very good spatial resolution, up to a few nm. The limitations of method or equipment on time resolution can be compensated by the appropriate experimental arrangement. Received: 4 October 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

立方氮化硼薄膜生长过程中的界面控制

在立方氮化硼薄膜气相生长过程中生成的无定形初期层和乱层结构氮化硼中间层，一直是阻碍立方氮化硼薄膜外延生长的主要原因.系统地分析了硅衬底预处理对立方氮化硼薄膜中无定形初期层成分的影响，发现在等离子体化学气相生长法制备薄膜时，硅衬底上形成无定形初期层的可能原因有氧的存在、离子轰击以及高温下硅的氮化物的形成.在H₂气氛中1200K热处理硅衬底可以有效地减少真空室中残留氧浓度，除去硅表面的自然氧化层，保持硅衬底表面晶体结构.控制衬底温度不超过900 K，就能防止硅的氮化物的形成，成功地除去无 关键词： 立方氮化硼薄膜 等离子体化学气相生长 界面 电子显微镜     

The effects of dc bias voltage on the crystal size and crystal quality. of cBN films

For the deposition of cubic boron nitride thin films in Ar–N₂–BF₃–H₂ system by dc jet plasma chemical vapor deposition, the role of dc substrate bias ranging from -70 V to -150 V was investigated. A critical bias voltage was observed for the formation of cBN phase. The cBN content in the film increased with bias voltage and reached a maximum at the bias voltage of -85 V. Increasing the bias voltage further caused a decrease in cBN content and peeling of the films from the substrate. By combining the results of infrared spectroscopy, Raman spectroscopy and X-ray diffraction, the bias voltage was also found to strongly affect the crystal size, crystal quality and residual stress of the deposited films. A bias voltage a little higher than the critical value was demonstrated to be favorable for the deposition of a high-quality cBN film with large crystal size and low residual stress. Received: 13 June 2000 / Accepted: 21 June 2000 / Published online: 23 August 2000     

Novel technique for hetero-epitaxial diamond film growth on cubic boron nitride from iron carbide at high temperature and high pressure

Cubic boron nitride (c-BN) crystals about 0.1–0.3 mmin dimension were treated with iron carbide powders (high purity 99%) with size of 80–100 mesh at a high temperature of 1620 K and a high pressure of 5.2 GPa. It was found that hetero-epitaxial diamond films have been grown on the c-BN from iron carbide. The formation of dia-mond films on the cubic boron nitride can be confirmed by laser Raman spectra, face scan of elements and reflective high-energy electron diffraction. It was suggested that diamond films could be epitaxially formed on the c-BN through decomposition of iron carbide. This approach provides a possible and very effective way to realize hetero-epitaxial growth of homogeneous and large-area diamond films on c-BN, which is different from the conventional technique using a chemical vapor deposition method. Received: 20 December 2000 / Accepted: 9 January 2001 / Published online: 28 February 2001     

Deposition of hexagonal boron nitride thin films on silver nanoparticle substrates and surface enhanced infrared absorption

Silver nanoparticle thin films with different average particle diameters are grown on silicon substrates. Boron nitride thin films are then deposited on the silver nanoparticle interlayers by radio frequency （RF） magnetron sputtering. The boron nitride thin films are characterized by Fourier transform infrared spectra. The average particle diameters of silver nanoparticle thin films are 126.6, 78.4, and 178.8 nm. The results show that the sizes of the silver nanoparticles have effects on the intensities of infrared spectra of boron nitride thin films. An enhanced infrared absorption is detected for boron nitride thin film grown on silver nanoparticle thin film. This result is helpful to study the growth mechanism of boron nitride thin film.     

Electronic structure of GaN(0001)‐2 × 2 thin films grown by PAMBE

Gallium nitride thin films were grown on silicon carbide (0001) by plasma‐assisted molecular beam epitaxy (PAMBE). The samples were cooled down in nitrogen plasma and characterized in situ by reflection high energy electron diffraction (RHEED), photoelectron spectroscopy (XPS/UPS), and atomic force microscopy (AFM) revealing stoichiometric and smooth GaN films virtually free of contaminations. We present valence band data obtained by UPS with strong emission from surface states inside the fundamental band gap. These states and the observed 2 × 2 surface reconstruction are highly sensitive towards residual molecules. Once these surface states have disappeared the original state could not be recovered by surface preparation methods underlining the necessity of in situ investigations on as‐grown surfaces. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excimer laser surface treatment of aluminum alloy in nitrogen

The excimer laser nitriding process reported is developed to enhance the mechanical and chemical properties of aluminum alloys. An excimer laser beam is focused onto the alloy surface in a cell containing 1-bar nitrogen gas. A vapor plasma expands from the surface and a shock wave dissociates and ionizes nitrogen. It is assumed that nitrogen from plasma in contact with the surface penetrates to some depth. Thus it is necessary to work with a sufficient laser fluence to create the plasma, but this fluence must be limited to prevent laser-induced surface roughness. The nitrogen-concentration profiles are determined from Rutherford backscattering spectroscopy and scanning electron microscopy coupled to energy-dispersive X-ray analysis. Crystalline quality is evidenced by an X-ray diffraction technique. Transmission electron microscopy gives the in-depth microstructure. Fretting coefficient measurements exhibit a lowering for some experimental conditions. The polycrystalline nitride layer obtained is several micrometers thick and composed of a pure AlN (columnar microstructure) top layer (200–500 nm thick) standing on an AlN (grains) in alloy diffusion layer. From the heat conduction equation calculation it is shown that a 308-nm laser wavelength would be better to increase the nitride thickness, as it corresponds to a weaker reflectance R value for aluminum. Received: 17 October 2000 / Accepted: 19 October 2000 / Published online: 23 May 2001     

Hexagonal boron nitride film substrate for fabrication of nanostructures

The fabrication of material with an atomic scale manipulation requires the suitable advanced substrate for epitaxial growth without the effect by the substrate lattice structure. Hexagonal boron nitride (h-BN) can be the advanced substrate for atomic manipulation due to van der Waals’ gap with little attractive force along to c axis. We have successfully synthesized h-BN layer on the co-deposited Cu/BN film by surface segregation phenomena using helicon wave plasma enhanced radio frequency (rf) magnetron sputtering system. Auger electron spectroscopy (AES) and X-ray photon spectroscopy (XPS) analysis showed that the h-BN composite segregated on the surface of Cu/BN film covered over 95% of the film annealed at 900 K for 30 min. Atomic forces microscopy (AFM) and scanning tunneling microscopy (STM) analysis showed that attractive force on the film surface is uniformly distributed to an extent of 2nN and that the h-BN surface can be a good electric insulator like sintered h-BN plate.     

Formation of CNx thin films by reactive pulsed laser deposition assisted by electron cyclotron resonance microwave discharge

Amorphous carbon nitride thin films were synthesized by pulsed laser deposition combined with electron cyclotron resonance (ECR) microwave discharge in nitrogen gas. The ECR discharge supplies active nitrogen species in the deposition environment and to the growing film surface, enhancing the film growth in complex processes accompanied by chemical reaction. The synthesized films were characterized by Rutherford backscattering spectroscopy (RBS), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy. The films were determined to consist purely of carbon and nitrogen with a nitrogen concentration of 42%, and have a thickness of 550 nm over which carbon and nitrogen are well distributed. Structural characterizations based on XPS, FTIR and Raman analysis showed that these films appear to contain several bonding configurations between carbon and nitrogen with a small amount of C≡N bonds compared with other bonding states. Received: 31 August 2000 / Accepted: 12 December 2000 / Published online: 23 May 2001     

Surface properties of cubic boron nitride thin films

Studying the surface properties of cubic boron nitride (c-BN) thin films is very important to making it clear that its formation mechanism and application. In this paper, c-BN thin films were deposited on Si substrates by radio frequency sputter. The influence of working gas pressure on the formation of cBN thin film was studied. The surface of c-BN films was analyzed by X-ray photoelectron spectroscopy (XPS), and the results showed that the surface of c-BN thin films contained C and O elements besides B and N. Value of N/B of c-BN thin films that contained cubic phase of boron nitride was very close to 1. The calculation based on XPS showed that the thickness of hexagonal boron nitride (h-BN) on the surface of c-BN films is approximately 0.8 nm.     

Influence of oxygen on the growth of cubic boron nitride thin films by plasma-enhanced chemical vapour deposition

Cubic boron nitride thin films were deposited on silicon substrates by low-pressure inductively coupled plasma-enhanced chemical vapour deposition. It was found that the introduction of O₂ into the deposition system suppresses both nucleation and growth of cubic boron nitride. At a B₂H₆ concentration of 2.5\% during film deposition, the critical O₂ concentration allowed for the nucleation of cubic boron nitride was found to be less than 1.4\%, while that for the growth of cubic boron nitride was higher than 2.1\%. Moreover, the infrared absorption peak observed at around 1230--1280~cm^-1, frequently detected for cubic boron nitride films prepared using non-ultrahigh vacuum systems, appears to be due to the absorption of boron oxide, a contaminant formed as a result of the oxygen impurity. Therefore, the existence of trace oxygen contamination in boron nitride films can be evaluated qualitatively by this infrared absorption peak.     

Arc Root Attachment on the Anode Surface of Arc Plasma Torch Observed with a Novel Method

The arc-root attachment on the anode surface of a dc non-transferred arc plasma torch has been successfully observed using a novel approach. A specially designed copper mirror with a boron nitride film coated on its surface central-region is employed to avoid the effect of intensive light emitted from the arc column upon the observation of weakly luminous arc root. It is found that the arc-root attachment is diffusive on the anode surface of the argon plasma torch, while constricted arc roots often occur when hydrogen or nitrogen is added into argon as the plasma-forming gas.     

等离子体增强化学气相沉积法制备立方氮化硼薄膜过程中的表面生长机理

利用感应耦合等离子体增强化学气相沉积法以Ar，He，N₂和B₂H₆为反应气体制备了高纯立方氮化硼薄膜.用四极质谱仪对等离子体状况进行了系统的分析，发现B₂H₆完全被电离而N₂只是部分被电离.H₂和过量的N₂在等离子体中生成大量中性的H原子和活化的N^*₂，它们与表面的相互作用严重地阻碍了立方 关键词： 立方氮化硼薄膜 等离子体 质谱     

Plasma deposition of hydrogenated amorphous silicon: Studies of the growth surface

This review focusses on the plasma-surface interactions and surface processes involved in a-Si: H thin film growth. We restrict our discussion of growth fluxes to a summary, and do not address plasma kinetics. In recent years, powerful in situ experiments have been carried out on the growing film surface, which reveal the adsorption, penetration, reaction, and elimination of precursor species, as well as the atomic-scale morphology and composition of the growth zone. Good data sets are available both for PACVD and reactive magnetron sputter deposition. These form an interesting comparison, since the former process is dominated by the hydrogen-rich radical SiH₃ at low energy, and the latter by energetic atomic Si and H. We review the key experiments and conclusions, underlining those aspects which are well established and those which remain qualitative; and we discuss the transition from amorphous to fine-grained polycrystalline film growth at high hydrogen pressures in terms of the surface mechanisms. This field is now entering a scientific stage where a detailed theory of low-temperature, plasma-assisted growth can be developed.     

Preparation of transparent BN films with superhydrophobic surface

A novel approach was investigated to obtain the superhydrophobicity on surfaces of boron nitride films. In this method boron nitride films were deposited firstly on Si(1 0 0) and quartz substrate using a radio frequency (RF) magnetron sputtering system, and then using CF₄ plasma treatment, the topmost surface area can be modified systematically. The results have shown that the water contact angle on such surfaces can be tuned from 67° to 159°. The films were observed to be uniform. The surfaces of films consist of micro-features, which were confirmed by Atomic Force Micrograph. The chemical bond states of the films were determined by Fourier Transform Infrared (FTIR) Spectroscopy, which indicate the dominance of B-N binding. According to the X-ray Photoelectron Spectroscopy analysis, the surface of film is mainly in BN phase. The micro-feature induced surface roughness is responsible for the observed superhydrophobic nature. The water contact angles measured on these surfaces can be modeled by the Cassie's formulation.     

Amine-functionalized boron nitride nanotubes

The surface of boron nitride nanotubes (BNNTs) has been functionalized with amine groups via ammonia plasma irradiation. The functionalized tubes were characterized by Fourier transform infrared spectroscopy and electron energy loss spectroscopy. Amine-functionalized BNNTs were found to be highly dispersible in chloroform, and are predicted to form the basis of a new class of chemically reactive nanostructures.     

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     

On the determination of energy fluxes at plasma–surface processes

A summary is given of different methods for the determination of the energy influx and its influence on the thermal balance and energetic conditions of substrate surfaces during plasma processing. The discussed mechanisms include heat radiation and kinetic and potential energy of charged particles and sputtered neutrals. For a few examples such as magnetron sputtering of a-C:H films, sputter deposition of aluminum on microparticles, and titanium deposition in a hollow-cathode arc evaporation device the energetic balance of substrates during plasma processing is presented. Received: 6 July 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

First-principles density-functional calculations on the field emission properties of BN nanocones

The first-principles density-functional theory is used to study the geometrical structures and field emission properties of different boron nitride nanocones with 240 disclination. It is found that the nanocones can be stable under applied electric field and the emission current is sensitively dependent on the tips of nanocones. The nanocones with homonuclear bonds at the tip can introduce additional energy states near Fermi level, which can reduce the ionization potential and increase the emission current of these boron nitride nanocones. This investigation indicates that the boron nitride nanocone can be a promising candidate as a field emission electron source.