Amorphization and graphitization of single-crystal diamond — A transmission electron microscopy study

The amorphization and graphitization of single-crystal diamond by ion implantation were explored using transmission electron microscopy (TEM). The effect of ion implantation and annealing on the microstructure was studied in (100) diamond substrates Si⁺ implanted at 1 MeV. At a dose of 1 × 10¹⁵ cm^− 2, implants done at 77 K showed a damage layer that evolves into amorphous pockets upon annealing at 1350 °C for 24 h whereas room temperature implants (303 K) recovered to the original defect free state upon annealing. Increasing the dose to 7 × 10¹⁵ Si⁺/cm² at 303 K created an amorphous-carbon layer 570 ± 20 nm thick. Using a buried marker layer, it was possible to determine that the swelling associated with the amorphization process was 150 nm. From this it was calculated that the layer while obviously less dense than crystalline diamond was still 15% more dense than graphite. Electron diffraction is consistent with the as-implanted structure consisting of amorphous carbon. Upon annealing, further swelling occurs, and full graphitization is achieved between 1 and 24 h at 1350 °C as determined by both the density and electron diffraction analysis. No solid phase epitaxial recrystallization of diamond is observed. The graphite showed a preferred crystal orientation with the (002)g//(022)d. Comparison with Monte Carlo simulations suggests the critical displacement threshold for amorphization of diamond is approximately 6 ± 2 × 10²² vacancies/cm³.     

Thermal analysis for graphitization and ablation depths of diamond films

In the present study, a thick diamond film deposited on a 6″ silicon wafer was planarized by an ArF excimer laser. In order to predict the graphitization thickness formed in the diamond film by a laser beam moving with a sliding velocity as well as a fluence, the general solution of the three-dimensional temperature distribution is successfully developed to be a function of the fluence and pulse duration of the laser beam. A reported model developed for the graphitization probability of diamond film is adopted in the present study to determine the graphitization thickness if the temperature solutions are substituted into this model. The ablation thickness in the graphitization layer can be determined so long as the local temperature in the specimen is higher than the ablation temperature of the graphitization layer. Then, the thickness of graphite residual on the working surface after ablation can be obtained by the subtraction of the ablation thickness from the graphitization thickness. This graphite thickness is proved to be very close to the value obtained from scratching tests. This implies that the temperature solutions predicted by the present model are trustworthy. The effect of the scanning velocity is insignificant to the surface temperature if the pulse duration time is much shorter than the time period between two adjacent laser pulses.     

Homoepitaxial single crystal diamond grown on natural diamond seeds (type IIa) with boron-implanted layer demonstrating the highest mobility of 1150 cm/V s at 300 K for ion-implanted diamond

The homoepitaxial single crystal diamond growth by microwave plasma assisted CVD at high microwave power density 200 W/cm³ in a 2.45 GHz MPACVD reactor using natural diamond seeds (type IIa) was investigated. The semiconductor CVD diamond of p-type was obtained by doping technique of ion implantation. Boron ions were implanted at the acceleration energy of 80 keV with two cases of dose: 5 · 10¹⁴ and 3 · 10¹⁵ cm^− 2. To recover the damage layer and activate dopants in CVD diamond the rapid annealing at nitrogen atmosphere at 1380° C was used. B-implanted diamond layer showing the mobility of 1150 cm²/V s at 300 K which is the highest for ion-implanted diamond was obtained.     

Fabrication of graphitic layers in diamond using FIB implantation and high pressure high temperature annealing

We have used high pressure high temperature annealing (HPHT) for graphitisation of implanted layers in diamond created by 30 keV Ga⁺ focused ion beam. Electron microscopy has been used to investigate the implanted layers. It has been revealed that, unlike annealing at vacuum pressure, the graphitization during HPHT annealing occurred through epitaxial growth of graphite (002) planes parallel to (111) diamond planes. High quality of graphite was confirmed by high resolution electron microscopy and electron energy loss spectroscopy.     

The properties of free-standing diamond films after plasma high temperature treatment of the rapid heating

The plasma treatment of rapid heating was introduced for increasing fracture strength of free-standing diamond films. The effects of plasma high temperature annealing treatment on surface morphology, internal stress, vacancy defects, impurities and fracture strength of free-standing diamond films were investigated by scanning electron microscopy (SEM), Raman, positron annihilation technique (PAT) and mechanical property testing. It showed that the fracture strength of the diamond films increases up to 70% for lower fracture diamond films with treating temperature (1500-1600 °C). The graphitization in surface and interior of diamond films would be produced by high temperature treatment more than 1700 °C. Fracture strengths of diamond films could be enhanced after high temperature treatment and the main factor of that was compressive stress state in diamond films induced by graphitization. The impurity of N was segregated and integrated with vacancy cluster to become [N-V]⁰ and [N-V]⁻.     

Surface engineering of diamond tips for improved field electron emission

Results are reported on the study of surface engineering of diamond microtips for improved field electron emission. Two-dimensional (2D) arrays of diamond pyramids were prepared using a molding technique. As-grown diamond pyramids of 3 and 9 μm in size with sharp apexes were insulating and showed a relatively poor field electron emission. Several ways are examined to provide an electrical conductivity of diamond in order to supply electrons for the emission: diamond boron doping, partial graphitization/amorphization by nitrogen ion implantation and/or high temperature annealing, formation of built-in conductive metal layers. A perceptible improvement of surface electrical conductivity and reduction of field electron emission thresholds down to 10 V/μm was observed. For explanation of the results it was enough to take into account only the geometric field enhancement on pyramids apexes.     

High-pressure and high-temperature annealing effects of boron-implanted diamond

We show that high-pressure and high-temperature (HPHT) annealing of ion-implanted diamond is efficient as a doping technique. The HPHT annealing condition is located in the thermodynamically stable region for diamond. The HPHT annealing is highly effective for the recovery of damage induced by ion implantation. In the entire annealing temperature range, the HPHT annealing is more efficient than conventional thermal annealing methods such as vacuum annealing. At 1400 °C, we obtained the highest boron doping efficiency of 7.1%, which is ten times higher than that by vacuum annealing.     

Effect of vacuum annealing on field emission from heavily phosphorus doped homoepitaxial (111) diamond

Vacuum anneal treatments effects on field emission properties of phosphorus doped diamond are discussed. The C1s core level of diamond is characterized by X-ray photoelectron spectroscopy (XPS). With increasing annealing temperature, firstly oxygen desorption takes place from surface, which induce surface reconstruction followed by graphitization of surface. Field emission properties, therefore, strongly depend on vacuum anneal temperature. The threshold voltage of diamond annealed at 900 °C is the lowest. Fowler–Nordheim plots indicate that diamond annealed at 900 °C has the lowest barrier height, which is in good agreement with electron affinities as measured on carbon reconstructed surface. Further increased annealing temperature induces surface graphitization, which causes a threshold voltage increase of field emission.     

Modification of the structure of diamond with MeV ion implantation

We present experimental results and numerical simulations to investigate the modification of structural–mechanical properties of ion-implanted single-crystal diamond. A phenomenological model is used to derive an analytical expression for the variation of mass density and elastic properties as a function of damage density in the crystal. These relations are applied together with SRIM Monte Carlo simulations to set up finite element simulations for the determination of internal strains and surface deformation of MeV-ion-implanted diamond samples. The results are validated through comparison with high resolution X-ray diffraction and white-light interferometric profilometry experiments. The former are carried out on 180 keV B implanted diamond samples, to determine the induced structural variation, in terms of lattice spacing and disorder, whilst the latter are performed on 1.8 MeV He implanted diamond samples to measure surface swelling. The effect of thermal processing on the evolution of the structural–mechanical properties of damaged diamond is also evaluated by performing the same profilometric measurements after annealing at 1000 °C, and modeling the obtained trends with a suitably modified analytical model. The results allow the development of a coherent model describing the effects of MeV-ion-induced damage on the structural–mechanical properties of single-crystal diamond. In particular, we suggest a more reliable method to determine the so-called diamond “graphitization threshold” for the considered implantation type.     

Molecular dynamics simulation of shear-induced graphitization of amorphous carbon films

The shear-induced graphitization of amorphous carbon (a-C) films in sliding contact with a diamond counterface is investigated by molecular dynamics (MD) simulations. The gradual formation of a graphene-like sp² dominant layer on the a-C film surface is observed after steady-state sliding has been achieved, which provides direct evidence for the experimental observations of friction induced graphitization of a-C film. After the graphitized layer is formed, the relative sliding occurs between the graphitized atomic layers. During the shearing process, the biaxial stress in the graphitized layer experiences a transition from highly compressive (42 GPa) to tensile (−3 GPa). It is the relaxation of the local biaxial stress that leads to the sp³-to-sp² structural transformation.     

Decrystallization of diamond by Nd:YAG and excimer lasers and subsequent lattice relaxation

This paper reports a new phenomenon—decrystallization of a solid by 1–5 eV photons, in a few seconds. A polycrystalline, 20 μm thick layer of CVD diamond on a WC substrate was exposed to two simultaneous pulsed laser beams 308 nm excimer, and 1.06 μm Nd:YAG in air ambient. The rough surface was ablated and smoothed, but the upper exposed half of the diamond layer was shown by Raman spectroscopy and X-ray diffraction to have been transformed to a non-crystalline phase. The 1332 cm^− 1 Raman peak disappeared, demonstrating that lattice periodicity had been destroyed. Interestingly, the atomic disorder was gradually relaxed during five years at room temperature, and the Raman signature reappeared. The effect of decreasing intensity of diamond Raman peak has been demonstrated in single-crystal diamond. The distorted diamond lattice relaxed upon annealing in hydrogen plasma. The combination of pressure waves and the heating and cooling cycles is responsible for the creation and subsequent freezing of the atomic disorder, all in the solid state. The metastable states are introduced when atoms in the perfect lattice become displaced from the equilibrium positions. This is the process we term decrystallization—to distinguish it from glass formation, which requires the intermediary liquid state.     

Laser ablation of diamond particles suspended in ethanol: Effective formation of long polyynes

Polyynes were synthesized by laser ablation of diamond particles with various sizes suspended in ethanol. Chain length distributions and total yields of polyynes produced were compared with those produced from graphitized diamond particles and graphite particles. The relative amounts of long polyynes such as C₁₄H₂ and C₁₆H₂ produced from diamond particles were found to be larger than those from graphitized diamond particles and graphite particles. From the change of the chain length distribution with the laser irradiation time, it is concluded that the long polyynes are produced directly from diamond particles at the initial stage of ablation. Furthermore, the total yield of polyynes was found to increase with the size of diamond particles and decrease as their graphitization proceeds. Possible mechanisms of these results are discussed.     

Three-dimensional laser writing in diamond bulk

Laser fabrication of micrometer-scale graphitic structures in the bulk of monocrystalline CVD diamond was investigated. Ultra-short (1 ps) pulses of Ti:sapphire laser were tightly focused inside the sample. Initiation of graphitization in the focal volume and propagation of graphitization front towards the laser beam are considered. Electrical resistivity of the produced graphitic material was measured (3.6-3.9 Ω cm). Problems and strategies of 3D laser writing are discussed paying special attention to graphitization homogeneity, minimization of the wire diameter, control of the structure shape and reduction of the surrounding diamond damage. Laser fabrication of complex microstructures including pillars, plates, hexagonal chain and periodic arrays of straight wires has been demonstrated.     

Formation of diamond-like carbon by fs laser irradiation of organic liquids

The high intensities generated in femtosecond (fs) laser interactions offers the possibility of novel formation routes for diamond and diamond-like carbon materials. Coulomb explosion, a common phenomenon in intensive fs irradiation, has recently been shown to lead to a direct graphite–diamond phase transition on the surface of graphite. In this paper we report the results of fs irradiation of a variety of liquid organic compounds at intensities in the Coulomb explosion regime. The products of laser-induced chemistry under these conditions have been studied using visual/surface enhanced Raman and transmission electron microscopy (TEM). Surface enhanced Raman spectra/TEM show that an intermediate diamond phase, trans-polyacetylene chains and amorphous carbon are present after fs irradiation of liquid alkanes. We also find that the diamond component can be enhanced by irradiation in the presence of certain transition metals; however the origin of this effect is still uncertain. Diamond films deposited in this way are found to exhibit a nano-assembled structure involving individual nanodiamonds extending over an area of about 1 cm². This process represents a wet chemical method for room temperature formation of diamond-like carbon films     

Chemical composition,thermal stability and hydrogen plasma treatment of laser-cut single-crystal diamond surface studied by X-ray Photoelectron Spectroscopy and Atomic Force Microscopy

X-ray photoelectron spectroscopy examination shows that after laser cutting under ambient condition, the upper surface of diamond consists of a heavy oxidized layer consisting of a variety of carbon–oxygen chemical states comprising –C═O, –C–O–C– and –C–O–H species. The thickness of the oxide layer was estimated to be ～22 ?. Upon vacuum annealing to 700 °C the thickness of the oxide layer decreases to ～10 A and the upper surface layer becomes more diamond-like through desorption of C–O species. Exposure of the laser cut diamond surface to a microwave hydrogen (MW-H) plasma results in removal of the oxide layer and exposure of the diamond phase. This is evidenced by the appearance of characteristic diamond surface and bulk plasmons which accompanied the C (1s) X-ray photoelectron peak. Our studies show that the surface chemical composition and thermal stability of the laser cut and polished surfaces both after MW-H exposure are nearly similar. The morphology of the laser cut surface shows an ill-defined laminar structure without any characteristic features which is not significantly affected by MW-H plasma exposure. This is in contrast to the polished surfaces for which exposure to the MW-H may result in its planarization.     

Phenomenological model of Silicon-On-Diamond laser bonding

A recently proposed method of fabrication of a Silicon-On-Diamond material, based on pulsed laser irradiation of the diamond-silicon interface [1,2], has been modeled by means of a finite element one-dimensional numerical algorithm. A wide set of mechanisms of energy-transport and exchange has been taken into account: reflection, transmission and absorption of light; generation, diffusion, recombination and energy relaxation of the electron-hole plasma; heat transfer; phase changes; generation and propagation of pressure waves. The model allows us to determine the energy thresholds for the adhesion of diamond and silicon at each pulse width and also gives predictions about the behavior of other measurable quantities, like thickness of the amorphized layer and reflectivity of the interface during and after irradiation. This work is meant to be both a basis for the optimization of the process, in view to its application in microelectronics, and a reference to explore wider ranges of process parameters with respect to those reported in the previous literature.     

Adhesion enhancement of diamond coating on minor Al-modified copper substrate

We report on the enhanced interfacial adhesion of diamond coating on copper substrate modified by a small fraction of Al. For pure copper substrate, the diamond coating formed tends to crack and delaminate, primarily caused by a slight accumulation of detrimental graphite intermediate layer and thermal stress induced by mismatch of the coefficients of thermal expansion. Additions of 1 and 3 at.% Al to the copper substrate gradually decrease the intermediate graphitic phase. At the higher Al concentration, an aluminium oxide forms at the coating–substrate interface, and graphitic/amorphous carbon is completely inhibited, leading to significantly enhanced interfacial adhesion of diamond coating. The electron structure of copper is not observed to significantly alter on this Cu–Al dilute alloy. The alumina barrier layer preferentially formed on copper surface is believed to play a key role in preventing graphitization and adhesion enhancement.     

Structural and optical studies on sol–gel derived ZnO thin films by excimer laser annealing

High-quality polycrystalline ZnO thin films were deposited onto alkali-free glasses at a temperature of 300°C in air ambience by combining sol–gel spin coating and KrF excimer laser annealing. The effects of laser irradiation energy density on the crystallization, microstructure, surface morphology, and optical transmittance of as-prepared ZnO thin films were investigated and compared to the results of thermally annealed ZnO thin films. The crystallinity level and average crystallite size of laser annealed ZnO thin films increased as laser energy density increased. The crystallinity levels and average crystallite size of excimer laser annealed (ELA) thin films were greater than those of the thermally annealed (TA) thin films. However, laser annealed thin films had abnormal grain growth when irradiation energy density was 175 mJ/cm². Experimental results indicated that the optimum irradiation energy density for excimer laser annealing of ZnO sol–gel films was 150 mJ/cm². The ELA 150 thin films had a dense microstructure, an RMS roughness value of 5.30 nm, and an optical band gap of 3.38 eV, close to the band gap of a ZnO crystal (3.4 eV).     

Effect of XeF laser treatment on structure of nanocrystalline diamond films

Nanocrystalline diamond (NCD) films were deposited on Si substrates by microwave plasma-enhanced chemical vapor deposition (MPECVD) using methane/hydrogen/oxygen (30/169/0.2 sccm) as process gases. Subsequently a thin (0.33 μm) and a thick (1.01 μm) NCD films were irradiated with XeF excimer laser (λ = 351 nm) with 300 and 600 mJ cm^? 2 of energy densities in air. The NCD films became rougher after laser irradiations. Fraction of graphitic clusters decreased but oxygen content increased in the thin NCD film after laser irradiation. Opposite phenomena were observed for the thick NCD films. Effect of laser irradiation to oxygenation and graphitization of NCD films was correlated with structural properties of free surface and grain boundaries of the thin and thick NCD films.     

Theoretical aspects of diamond films and laser action

The thresholds for laser action at 309 nm wavelength from highly n-doped homogeneous diamond crystals (as opposed to the F-centre diamond laser) at excitation by electron beam irradiation have been evaluated by analogy with calculations which predicted the laser thresholds of electron beam irradiated GaAs lasers. With the condition that the material has to be a direct band semiconductor, the results of Picket and Mehl (SPIE, 877 (1988) 64) are used for calculations at very high stresses, allowing the laser wavelength to be shifted even further into the UV. In the present case, this stress is produced by crystal defects, and is similar to the stress produced in silicon after strong ion implantation. By comparison with other e-beam excited lasers, the UV diamond laser should achieve a high efficiency of up to 35%.