Electrical properties of diamond films grown at low temperature

The unique electronic properties of diamond, associated with the emergence of chemical vapour deposition (CVD) methods for the growth of thin films on non-diamond substrates, have led to considerable interest in electronic devices fabricated from this material. In our previous work, we found that polycrystalline diamond films can be deposited at 250 °C using CH₄---CO₂ gas mixtures. Studying the electrical properties and the upcoming problems of applications of low-temperature diamond films are relevant concerns.

In this work, the electrical properties of diamond films grown at low temperatures were studied and compared with those of conventional diamond films. Platinum was used as the upper electrode. The resistivity of low-temperature diamond was around three orders of magnititude lower than that of conventional diamond. However, both the low temperature and conventional growth diamond exihibited rectifying behavior when platinum was used as the upper electrode.     

Electron emission characterization of diamond thin films grown from a solid carbon source

Diamond films of various morphologies and compositions have been deposited on silicon substrates by a plasma-enhanced chemical transport (PECT) process from a solid carbon source. Electron emission efficiency of these diamond films is related to their morphology and composition. The electric field required to excite emission in a boron-doped polycrystalline diamond film ranged between 20 to 50 MV m⁻¹. In an undoped conducting nanocrystalline diamond composite film, the field was as low as 5–11 MV m⁻¹. The cold field electron emission of these films is confirmed from the Fowler-Nordhelm plots of the data. Enhancement of electron emission by band-bending and by the nanocrystalline microstructure are discussed. New diamond emitters made of nanocrystalline boron-doped diamond composite are proposed.     

Pulsed ion beam characterization of CVD diamond surfaces under thin film deposition conditions

Diamond and diamond-like carbon have properties which in principle make them ideally suited to a wide variety of thin-film applications. The widespread use of diamond thin films, however, has been limited for a number of reasons related largely to the lack of understanding and control of the nucleation and growth processes. Real-time, in-situ studies of the surface of the growing diamond film are experimentally difficult because these films are normally grown under a relatively high pressure of hydrogen, and conventional surface analytical methods require an ultrahigh vacuum environment. Pulsed ion beam based analytical methods with differentially pumped ion sources and particle detectors are able to characterize the uppermost atomic layer of a film during growth at ambient pressures in the range 0.7–27 Pa (4–6 orders of magnitude higher than other surface-specific analytical methods). We describe here a system which has been developed for the purpose of determining the hydrogen concentration and bonding sites on diamond surfaces as a function of sample temperature and ambient hydrogen pressure under hot-filament chemical vapor deposition (CVD) growth conditions. It is demonstrated that as the hydrogen partial pressure increases the saturation hydrogen coverage of the surface of a CVD diamond film increases, but that the saturation level depends on the atomic hydrogen concentration and substrate temperature. At the highest temperatures studied (700 °C), it was found that the surface hydrogen concentration did not exceed 1/4 monolayer.     

Physical and tribological properties of diamond films grown in argoncarbon plasmas

Nanocrystalline diamond films have been deposited using a microwave plasma consisting of argon, 2–10% hydrogen and a carbon precursor such as C₆₀ or CH₄. It was found that it is possible to grow the diamond phase with both carbon precursors, although the hydrogen concentration in the plasma was 1–2 orders of magnitude lower than normally required in the absence of the argon. Auger electron spectroscopy, X-ray diffraction measurements and transmission electron microscopy indicate the films are predominantly composed of diamond. Surface roughness, as determined by atomic force microscopy and scanning electron microscopy indicate the nanocrystalline films grown in low hydrogen content plasmas are exceptionally smooth (30–50 nm rms) to thicknesses of 10 m. The smooth nanocrystalline films result in low friction coefficients (μ = 0.04–0.06) and low average wear rates as determined by ball-on-disk measurements.     

High-temperature Raman study in CVD diamond

Measurements of the Raman spectra in chemical vapour deposition (CVD) diamond films at temperatures up to 1200 K are presented. Specifically, the evolution of Raman line position, line width, and intensity were monitored as a function of heating time. The red shifting and the line width broadening of CVD diamond's Raman line with temperature are very similar to that of natural diamond's. However, the detailed temperature dependence of Raman line width depends on the orientation of the CVD diamond crystal and the ambient gas used during thermal treatment. Since the CVD diamond usually exhibited a broader Raman spectra than natural diamond, the evolution of the line width upon heating is thus expected to depend on the origins and the annealing effects of the residual stress. For (111) CVD diamond subjected to annealing in air at 973 K, the line width decreased by more than one wavenumber while the line intensity increased by more than an order of magnitude before it decreased subsequently. In contrast, there is hardly any observable changes of the line width for (100) CVD diamond heated in air at 1173 K. Measurements conducted in He versus in air suggested that the reduction of the non-diamond carbon phase (therefore, the reduction of stress) is likely due to oxidation, which occurs more readily in (111) than in (100).     

Structure of diamond and diamond like carbon thin films grown by hot-filament chemical vapour deposition technique

Micro-crystalline diamond (MCD) and diamond like carbon (DLC) thin films were deposited on silicon (100) substrates by hot-filament CVD process using a mixture of CH₄ and H₂ gases at substrate temperature between 400–800°C. The microstructure of the films were studied by X-ray diffraction and scanning electron microscopy. The low temperature deposited films were found to have a mixture of amorphous and crystalline phases. At high temperatures (> 750°C) only crystalline diamond phase was obtained. Scanning electron micrographs showed faceted microcrystals of sizes up to 2μm with fairly uniform size distribution. The structure of DLC films was studied by spectroscopic ellipsometry technique. An estimate of the amount of carbon bonds existing insp ² andsp ³ form was obtained by a specially developed modelling technique. The typical values ofsp ³/sp ² ratio in our films are between 1·88–8·02. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Electronic conduction in SiO2:N thin films grown by thermal oxidation of silicon in N2O

Silicon oxynitride [SiO₂:N] thin films have been grown by oxidizing silicon in N₂O at 900, 1000 and 1100 °C and at 760 and 1520 torr. It is shown that the dominant electrical conduction mechanism, for high electric fields, is the field assisted thermionic emission from the traps (Poole-Frenkel effect), and is not direct or Fowler-Nordheim tunneling, as typically occurs in thermal silicon oxide with similar thickness. Electrical conduction in these films occurs by field assisted electron emission from donor traps with energy levels varying in the range from 0.5 to 1 eV from the conduction band. The results shown here indicate that the best quality films are those grown at low temperature and pressure, since they give films with a higher critical electric field, a higher energy barrier depth at the traps and less donors compensated by acceptors than those grown at high temperatures and pressures.     

Nanomorph silicon grown on template alumina substrate by plasma-enhanced CVD

Novel morphology of amorphous/nanocrystalline (nanomorph) silicon has been obtained by plasma enhanced CVD using template porous alumina substrate. The growing heterogeneous Si layer is composed of nanocrystalline and amorphous distinct areas, conformal to the tipped/ribbed alumina template. Raman spectroscopy and XRD data evidence the plasma-assisted preferential growth of nanocrystalline Si bunches forming the honeycomb net and presumably amorphous Si:H areas between them.     

Photoelectron spectroscopy of ion-irradiated B-doped CVD diamond surfaces

Chemical vapour deposition (CVD) diamond films were irradiated by 1 keV argon ions at room temperature with doses ranging from 3.6 × 10¹² to 1.1 × 10¹⁶ Ar⁺ cm². The influence of sputtering on the valence band density of states of a boron-doped CVD diamond film was investigated by ultraviolet photoelectron spectroscopy and the changes in the plasmon features were observed by X-ray photoelectron spectroscopy of the carbon Is core level and its loss region. A gradual change from typical diamond features to amorphous carbon was observed after prolonged bombardment times. Above a critical dose D_crit of 5.8 × 10¹⁴ Ar⁺ cm² the damaged surface layer is characterized by a splitting of the C Is bulk peak into two components: a bulk-like diamond peak at 285.3 eV binding energy and a defect peak with 1 eV lower binding energy, which is attributed to the production of an amorphous sp²-rich carbon matrix. Moreover additional occupied states in the range of 0–4 eV binding energy, completely different to those observed on reconstructed diamond surfaces, were observed in the valence band spectra of the ion-irradiated diamond surface. These filled states can also be attributed to the amorphous carbon matrix which is formed at high doses. At very low doses (< 3 × 10¹⁴ ions cm²) only a band bending of the C Is diamond core level peak, along with the formation of some occupied states in the band structure at around 3.8 eV binding energy was observed. A comparison with annealed hydrogen-free CVD diamond surfaces shows some similarities concerning these filled states. The obtained spectra are compared with other crystalline and amorphous forms of carbon and the results are discussed in terms of an irradiation-induced change in the atomic structure of the surface. A comparison of ion bombarded and annealed diamond samples clearly shows that no graphitization takes place in the latter case.     

Synthesis and optical characterization of amorphous carbon nitride thin films by hot filament assisted RF plasma CVD

Carbon nitride thin films were synthesized by hot filament assisted radio frequency plasma chemical vapour deposition using methane and nitrogen gas mixture on silicon and glass substrates. The films were deposited at different substrate bias and at different substrate temperatures. At higher substrate bias (>−120 V) there was no deposition on the substrate, but complete etching of the deposited layer was observed. X-ray diffraction studies indicated the films were amorphous. The Fourier transform infrared spectra showed that the films produced exhibited high transmittance with the presence of the C-N stretching band at 1260 cm⁻¹. For the films deposited at a lower substrate temperature C=N peaks were also present. Raman spectroscopic study indicated the presence of D and G peaks whose relative height varied with substrate temperature. The transmittance versus wavelength measurement in the UV-VIS-NIR region showed the high transmittance in the NIR region. The optical band gap of the films was calculated to be 2.0 eV and the refractive index varied within 1.6-1.7 for the wavelength range 800-1800 nm.     

Optical and mechanical properties of diamond like carbon films deposited by microwave ECR plasma CVD

Diamond like carbon (DLC) films were deposited on Si (111) substrates by microwave electron cyclotron resonance (ECR) plasma chemical vapour deposition (CVD) process using plasma of argon and methane gases. During deposition, a d.c. self-bias was applied to the substrates by application of 13·56 MHz rf power. DLC films deposited at three different bias voltages (−60 V, −100 V and −150 V) were characterized by FTIR, Raman spectroscopy and spectroscopic ellipsometry to study the variation in the bonding and optical properties of the deposited coatings with process parameters. The mechanical properties such as hardness and elastic modulus were measured by load depth sensing indentation technique. The DLC film deposited at −100 V bias exhibit high hardness (∼ 19 GPa), high elastic modulus (∼ 160 GPa) and high refractive index (∼ 2·16–2·26) as compared to films deposited at −60 V and −150 V substrate bias. This study clearly shows the significance of substrate bias in controlling the optical and mechanical properties of DLC films.     

Conductivity and dielectric properties of silicon nitride thin films prepared by RF magnetron sputtering using nitrogen gas

Silicon nitride is an important material in very-large-scale integration fabrication and processing. Recent work on films prepared by radio frequency magnetron sputtering using nitrogen gas have shown that the relative permittivity is typically 6.3 and that aluminium forms an ohmic contact to this material. Under direct current (DC) bias the films exhibited space-charge-limited conductivity with a bulk trap density of the order of 2×10²⁴ m⁻³. In the present work alternating current electrical measurements were made on identical samples as a function of frequency and temperature. Conductivity appeared to be by hopping at lower temperatures, giving way to a free-band conduction process with activation energy of typically 0.44 eV at higher temperatures. Over a limited range of frequency and temperature the model of Elliott was applicable, and yielded a value of 2.87×10²³ m⁻³ for the density of localised states, in reasonable agreement with our estimate of the trap density from DC measurements. As in the DC measurements capacitance followed a geometric relationship with relative permittivity 6.3, and showed a moderate decrease with increasing frequency and an increase with increasing temperature, tending towards a constant value at high frequencies and low temperatures. The loss tangent showed a minimum in its frequency dependence, which appeared to shift to higher frequencies with increasing temperature. The measurements are consistent with the model of Goswami and Goswami for samples having ohmic contacts, and are typical of results obtained on other insulating thin film structures.     

Effect of high temperature annealing on the electrical performance of titanium/platinum thin films

In this study, the influence of post deposition annealing steps (PDA) on the electrical resistivity of evaporated titanium/platinum thin films on thermally oxidised silicon is investigated. Varying parameters are the impact of thermal loading with maximum temperatures up to T_PDA = 700 °C and the platinum top layer thickness ranging from 24 nm to 105 nm. The titanium based adhesive film thickness is fixed to 10 nm. Up to post deposition annealing temperatures of T_PDA = 450 °C, the film resistivity is linearly correlated with the reciprocal value of the platinum film thickness according to the size effect. Modifications in the intrinsic film stress strongly influence the electrical material parameter in this temperature regime. At T_PDA > 600 °C, diffusion of titanium into the platinum top layer and its plastic deformation dominate the electrical behaviour, both causing an increase in film resistivity above average.     

Growth and morphological studies of (100) textured diamond thin films by microwave plasma-enhanced chemical vapor deposition

Textured (100) diamond films have been successfully grown using the plasma-enhanced chemical vapor deposition technique and characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction. The thickness of such a (100)-oriented diamond film can be as thin as 4 μm, and the just-emerged transitional layer is found to be only 1.5 μm, which is very thin compared with the computer simulation value of 700d₀, where d₀ is the average distance between the nuclei. A systematic study of various parameters in the carburization and bias steps on the growth of textured (100) diamond films and the subsequent change of surface morphology has been investigated. Experimental results show that these two pre-growth steps seem to ease the growth of textured (100) diamond films and they should be optimized for a set of growth conditions. It is suggested that varying these parameters in the pre-growth steps may cause a change of microstructure, alignment of nuclei, and defect states in the diamond-like layer, resulting in the morphological change of textured (100) diamond films.     

Deposition and characterization of magnetic Ti-Fe-C nanocomposite thin films

Nanocomposite (Ti_1−xFe_x)C_y films with different compositions have been deposited by dc magnetron sputtering at 450 °C. The sputtered films could dissolve as much as 20-30 at.% of Fe on the Ti sites which is far above the maximum solid solubility at equilibrium. The solubility was dependent on the carbon content and more carbon-rich films could dissolve more Fe without the formation of Fe-precipitates. The addition of Fe also reduced the grain size of the carbide particles. Upon annealing, α-Fe starts to precipitate and the amount and size of these precipitates can be controlled by the annealing procedure and from the total composition of the as-deposited films. Mechanical and tribological studies show that some compositions of the (Ti_1−xFe_x)C_y films have very good wear-resistant properties. These results together with magnetization measurements suggest that Ti-Fe-C films can be used as a wear-resistant magnetic thin film material.     

Carbon nanotubes growth on silicon nitride substrates

Carbon nanotubes were synthesized on silicon nitride substrates by thermal chemical vapour deposition using an iron precursor catalyst. The nanotubes were characterized by AFM, FESEM, TEM and micro-Raman spectroscopy. The surface topography of the substrate, dense and flat or porous and rough, controlled the catalyst distribution and carbon nanotubes growth. Flat surfaces led to the synthesis of single-walled carbon nanotubes, whereas the porous ones promoted the growth of multi-walled carbon nanotubes of 60 nm diameter. These nanotubes preferentially grew on the porous sites, exhibiting a good substrate-nanotube interface.     

Electrical sensing properties of silica aerogel thin films to humidity

Mesoporous silica aerogel thin films have been fabricated by dip coating of sol-gel derived silica colloid on gold electrode-patterned alumina substrates followed by supercritical drying. They were evaluated as the sensor elements at relative humidity 20-90% and temperature 15-35 °C under an electrical field of frequency 1-100 kHz. Film thickness and pore structure were two main parameters that determined the sensor performance. The film with a greater thickness showed a stronger dielectric characteristic when moisture abounded, and presented a smaller hysteresis loop and a higher recovery rate, due to the large size of pore throats. As the film thickness decreased, at low humidity the surface conductivity enhanced and the response rate increased. The silica aerogel based humidity sensor can be modeled as an equivalent electrical circuit composed of a resistor and a capacitor in parallel, and is driven by ionic conduction with charged proton carriers.     

Synthetic routes to colossal magnetoresistance manganites thin films containing unstable or highly volatile metal oxides

We demonstrate that the colossal magnetoresistance (CMR) manganites doped with the monovalent cations of the large ionic radius A (A = Na⁺, K⁺, Ag⁺) reveal enhanced sensitivity of the electrical resistance to the magnetic field near the room temperature. The main obstacle to their practical application is severe difficulty of the synthesis, in particular, that of the thin films because of the high volatility or low thermal stability of the oxides of these doping elements. But the preparation of the films in two steps allows for the materials with the superior CMR characteristics. At the first step, epitaxial single phase precursor La_1−xA_yMnO₃ (0.3 ≥ x > y) is formed on the single crystal substrate due to the epitaxial stabilization by the single source MOCVD (metalorganic chemical vapor deposition) using thd-complexes. The second step is the isopiestic annealing with the powder mixture determining the high concentration of the volatile oxide in the gas phase resulting in the saturation of La_1−xA_xMnO₃ (x = y) films by the gas phase transport. The film transformations are studied with XRD and HREM. The high magnetoresistance MR and magnetotransmission MT with the linear magnetic field (H) dependence near the room temperature (dMR/dH up to 45%/T, dMT/dH up to 15%/T) are reported.     

Preparation and characterization of diamond films

Diamond films were deposited by magnetron sputtering of vitreous carbon disc and also by plasma CVD technique using C₂H₂ + H₂ or CO₂ + H₂ gas mixtures. The films were characterized by measuring the electrical, optical and microstructural properties. FTIR and Raman studies were carried out to study the effect ofsp ² andsp ³ bonds present in the films. The films had a high mechanical stress which was determined from the broadening of the optical absorption tail in the films.     

Growth of diamond thin films by chemical vapour deposition

Deposition of diamond thin films on non-diamond substrates at low pressures (<760 torr) and low temperatures (<2000°C) by chemical vapour deposition (CVD) has been the subject of intense research in the last few years. The structural and the electrical properties of CVD diamond films grown on p-type 〈111〉 and high-resistivity (>100 kΩ-cm) 〈100〉 oriented silicon substrates by hot filament chemical vapour deposition technique are described in this review paper.