Highly oriented, (100) textured diamond films have been grown on single-crystal Si substrates via microwave plasma enhanced chemical vapor deposition. A multistep deposition process including bias-enhanced nucleation and textured growth was used to obtain smooth films consisting of epitaxial grains with only low-angle grain boundaries. Boron-doped layers were selectively deposited onto the surface of these oriented films and temperature-dependent Hall effect measurements indicated a 3 to 5 times improvement in hole mobility over polycrystalline films grown under similar conditions. Room temperature hole mobilities between 135 and 278 cm2/V-s were measured for the highly oriented samples as compared to 2 to 50 cm2/V-s for typical polycrystalline films. Grain size effects and a comparison between the transport properties of polycrystalline, highly oriented and homoepitaxial films will be discussed. Metal-oxide- semiconductor field-effect transistors were then fabricated on the highly oriented films and exhibited saturation and pinch-off of the channel current.
Morphologies of vapor-deposited polycrystalline diamond films range from clean multimicron crystallites to nanocrystalline cauliflower nodules, depending on deposition conditions. While previous efforts to connect diamond film quality to growth conditions focus on competitive growth of nondiamond phases, we propose that twinning is a major controlling factor. We use a geometric construction to define growth conditions under which a given twin can outgrow and possibly bury the parent face on which it originated. We then show how the full spectrum of diamond crystallite shapes and film morphologies are explained in terms of penetration twins without reference to the actual mechanistics of diamond growth. This results in a reactor map that serves as a powerful tool in process development and control.
Experimental techniques were developed for the measurement of CVD doped-diamond film thermal conductivity. Three solutions were derived, one each for 1D, 2D, and radial heat flow. Parameters such as characteristic length, film resistivity and thickness were chosen from the model to reduce convective effects, obtain the desired temperature rise, and minimize the uncertainty in the estimation of the thermal conductivity. A diamond film specimen of doped and nondoped diamond layers deposited on a silicon substrate was designed and fabricated. A circular region was chemically etched from the substrate to expose a free-standing diamond diaphragm, 3 mm in diameter. An infrared imaging temperature acquisition system was implemented to improve spatial, temporal, and mechanical limitations posed by contact sensors. Preliminary results for the thermal conductivity were obtained using the method of least squares to minimize the error between the measured temperatures recorded by the infrared temperature acquisition system and the calculated temperatures determined by the optimal radial heat flow model. The thermal conductivity was determined to be 240 +/- 11 W/m K.
Current transient spectroscopy has been used to determine the deep level structure in natural diamond up to activation energies of 1 eV. The material was activated by high-energy electrons. By varying the energy and consequently the range of the electrons in diamond, it was possible to obtain information on the depth distribution of three deep centers. The current transients were evaluated by applying a curve-fitting technique, which provides a better energy resolution than the commonly used window technique.
Aluminum nitride (AlN) films were grown by chemical vapor deposition (CVD) on boron-doped diamond films deposited by the hot-filament CVD (HFCVD) method. The films were characterized by scanning electron microscopy, x-ray diffraction, and Raman spectroscopy. The electrical characterization of the AlN/diamond interface was performed by current-voltage (I-V) and capacitance- voltage measurements. The resulting films showed one x-ray diffraction peak of (100) oriented AlN and three diamond diffraction peaks of (111), (220) and (331) orientation. The Raman spectra showed two peaks, one at 660 cm-1 due to scattering by the AlN lattice and the other at 1335 cm-1 by the diamond lattice. The I-V measurements on the metal(W)/diamond/Si/Al structure showed ohmic behavior from which the diamond film resistivity of 5 X 105 (Omega) -cm was estimated. The I-V measurements on the W/AlN/diamond/Si/Al structure showed rectifying behavior. The capacitance of the film was independent of the applied voltage and was dominated by the diamond bulk capacitance.
Recent improvements in the CVD diamond deposition process have made possible the fabrication of diamond photoconductive diodes with carrier mobility and lifetime exceeding the values typical of natural gemstones. One of the more surprising recent results is that the best room-temperature carrier properties have been measured on polycrystalline diamond films. The combined electron- hole mobility, as measured by transient photoconductivity at low carrier densities, is 4000 square centimeters per volt per second at electric field of 200 volts per centimeter and is comparable to that of the best single-crystal IIa natural diamonds. Carrier lifetimes measured under the same conditions are 150 picoseconds for the CVD diamond films. The collection distance within the diamond films, at the highest applied fields, is comparable to the average film grain size, indicative of little or no carrier scattering at grain boundaries. A comparison of SIMS measurements with electrical results suggest that impurity incorporation in the near grain boundary regions are responsible for controlling the carrier mobility.
Spectrally and temporally resolved cathodoluminescence (CL), micro-Raman spectroscopy and the investigation with a scanning force microscopy in a contact current mode (CCM-SFM) are used for characterizing the properties of diamond films. The diamond films and particles are grown by microwave plasma-assisted CVD (MPCVD) on top of monocrystalline and porous silicon (PS) surfaces. The PS layers with different thicknesses and porosity are formed on (111) and (100) silicon by anodization in 12 percent HF solution (HF : H2O equals 1 : 3) at constant current density. A 15 keV electron-beam is used for CL excitation. The CL investigations are carried out at 77 K using an optical multichannel analysis system with simultaneous resolutions of (Delta) (lambda) equals0.2 nm and (Delta) tequals1 ns. Complementary Raman analysis has shown that the synthesized films exhibit diamond structure with good crystalline quality. Diamond films on monocrystalline silicon mostly yield a Raman peak shift of 3-5 cm-1 towards higher wave numbers compared to those of natural diamond due to the presence of compressive stress. The presence of PS allows to reduce stress in diamond films up to a peak shift of 1-2 cm-1 under the same deposition conditions. Intensity and FWHM of the cathodoluminescence as well as the FWHM of the Raman spectrum on PS decrease compared to those of silicon. This indicates that PS is superior to monocrystalline silicon concerning the crystalline quality of the diamond films. High- lateral-resolution analysis, in order to correlate the surface topography with the electrical properties of these diamond films, is carried out by a CCM-SFM. From these characterization methods crucial material system parameters are deduced revealing the influence of a thin PS layer on the crystalline and electrical properties of the diamond films grown on top.
Diamond and related materials have rather unique properties rendering them not only with commensurately unique applications, but also with unique synthesis and processing requirements. The unique properties, when properly exploited, can lead to enabling new applications. The special processing requirements and the unique applications are addressed.
Schottky diodes have been fabricated on homeoepitaxial p-doped diamond layers grown on p+-diamond substrates. Two distinctly different configurations were investigated to study the influence of the p+-substrate conductivity and obtain a rectifying characteristic with low ohmic loss. A series resistance of 8(Omega) was obtained at 500 degree(s)C for strong forward bias and a 5 X 10-5 cm2 contact area. Due to the low activation energy of the p+-substrate conductivity the minimum series resistance at R.T was 70(Omega) . To our knowledge, these are the lowest series resistances reported so far for a diamond Schottky diode enabling extremely high current densities of 103 A/cm2 in combination with a current rectification ratio of 105 at +/- 2V.
The electrical characteristics of metal-semiconducting diamond (MS), metal-insulating diamond-semiconducting diamond (MiS), and metal-oxide-semiconducting diamond (MOS) structures on single- crystal semiconducting diamond have been compared. Vertical structures were fabricated with the MS diode, MiS diode, and MOS capacitor on one face of the crystals and the ohmic contacts on the opposite face. Ohmic contacts, formed by ion implantation of B, exhibited reduced contact impedance. Current-voltage and capacitance-voltage characteristics of the three structures were measured. The MS and MiS diode structures exhibited large forward bias currents, whereas the insulating SiO2 limited the forward bias current through the MOSC structure. The MiS structure exhibited the lowest reverse bias leakage currents (<10 pA/mm2), whereas the values for the MOSC and MS structures were 1 and 2 orders of magnitude larger, respectively. For all samples the uncompensated acceptor concentration of the semiconducting diamond was 1-2 X 1016 cm-3. These values were consistent with the secondary-ion mass spectroscopy measurements of the B concentration in similar natural type IIb diamonds.
This paper is a review on would-be donor impurities in diamond lattice: N, P, Li, and Na. Other impurities like oxygen and sulfur are also discussed. As the solubility of donor impurities in the diamond lattice is predicted to be low, new methods of forcing the introduction of impurities into the diamond lattice are discussed. We propose a new method of electric-field-assisted diffusion and a method of increasing the sticking coefficient of the impurities by growth under electric bias. We also discuss the method of ion-assisted doping during growth proposed by a research group from SI Technologies.
Diamond has long been recognized as a promising material to fabricate robust, solar-blind radiation detectors. In this work, we present initial results from our program to fabricate a 2D imager using synthetic diamond. We observed the desired low dark current and spectral response properties in MSM detectors made on type-IIa diamond and integrating photoresponse was observed in MIS capacitors fabricated on type IIb diamond. Subsequent modeling of the MIS photoresponse supports the conclusion that electrons are stored at the diamond-insulator interface which makes it feasible to consider a diamond CCD. We discuss relevant processing steps and a plan to make a thin, back-illuminated CCD for VUV and UV imaging applications.
Crystalline diamond has some unique and extreme properties that make it an attractive semiconductor in certain electronic applications such as in high-power systems and high-temperature environments. However, since a source of low-cost single crystal diamond does not exist, its widespread use is not commercially attractive. A cheaper form of diamond is polycrystalline diamond, which has been recently routinely grown on silicon by the high- pressure microwave-source plasma deposition technique. Large- grain thick polycrystalline films have been obtained with properties approaching those of single-crystal diamond. This report describes results obtained from optical and electrical methods used in evaluating these films for use as ultraviolet radiation sensors and as a capacitor dielectric.
Diamond has attractive properties as an advanced electronic material. Its combination of high mobility, breakdown, and thermal conductivity results in the largest Johnson's and Keyes' figures of merit by far. The realization of that potential is another matter. In this paper we review some of the reported achievements related to diamond as an electronic and sensor material. The state of heteroepitaxial films and polycrystalline diamond active electronics will be covered; negative electron affinity is briefly covered, and the piezoresistive effect in diamond and its relevance to sensing is described.
Due to a unique combination of its mechanical, electrical, thermal, and chemical properties, diamond is an excellent material for temperature and heat flux sensors. Although natural diamond and synthetic diamond thermistors were demonstrated for temperatures below 600 K already in the 1960s, they were never commercialized mainly due to high cost. Recently, there has been a renewed interest in diamond thermistors because of rapid progress in diamond film fabrication using the chemical vapor deposition (CVD) process, which can produce diamond films on nondiamond substrates such as Si at a cost comparable to any other material fabricated routinely in the IC industry. Due to these developments, there is a tremendous potential of diamond temperature sensors for high-speed and high-temperature applications, especially in harsh environments. In fact, diamond sensors may be the first application of diamond in the area of passive diamond electronic devices, because sensor structures do not require single crystal diamond and n-type doping which is difficult to achieve. In the present work, we demonstrate that p- type polycrystalline diamond thermistors show temperature and response-time ranges of 80-1373 K and 290ns-25microsecond(s) , respectively. The fabrication technology of a multisensor diamond microchip is discussed for commercialization of diamond sensors in the near term.
The frequency response of thin film sensors can be enhanced by using a film of semiconducting diamond. Conventional sensors made of metallic films such as Ni can only be used for measuring flow turbulences of the order of 60 KHz. The proposed diamond sensor can measure instabilities as high as 2 X 105 Hz. A computer simulation was carried out to compare the response of the proposed sensor with a Ni film sensor. The rationale for choosing the appropriate film and substrate materials has also been explained.