High-quality Homoepitaxial Diamond Films Research Articles

Nitrogen and Bias Effect on Homoepitaxial Diamond Growth by Hot-Filament Chemical Vapor Deposition

The development of homoepitaxial films for advanced device applications has been studied, but high growth rate and diamond film quality have not yet been explored. In the current study, high quality homoepitaxial diamond films were grown on type Ib (100) HPHT synthetic diamond substrate by hot-filament chemical vapor deposition. The reactant gases were mixed by CH4 and H2 with small amounts of N2 (500 to 3000 ppm). Besides, a bias system was used to assist diamond film deposition. The pyramidal crystals on diamond surface can be suppressed and high quality diamond film of FWHM (Full Width at Half Maximum) = 10.76 cm-1 with high growth rate of 8.78 ± 0.2 μm/ hr was obtained at the condition of adding 1000 ppm nitrogen. At the bias voltage of -150 V, the pyramidal crystals can also be suppressed and high quality diamond film of FWHM = 10.19 cm-1 was obtained. With nitrogen addition above 2000 ppm, diamond film was partly doped and some sp2 structures appeared. These homoepitaxial diamond films were characterized by optical microscopy and micro-Raman spectroscopy.

Cathodoluminescence experiments on isotopic clean diamond with carbon isotopes 12C and 13C

High-quality homoepitaxial diamond films grown by microwave plasma-assisted chemical vapor deposition from 12C, 13C and naturalC containing methane (CH 4) gases have been studied by cathodoluminescence (CL) spectroscopy. Near-band-gap CL spectra at low temperatures were characterized in two isotopically enriched C diamond ( 12C, 13C) films. We observe an energy shift of the free-exciton recombination radiation with a change of the isotope composition from 12C to 13C. This result is in good agreement with both theoretical prediction as well as experimental results reported in the literature. In addition, strong excitonic emiss ion is detected from the isotopic clean 12C diamond films at 79 K.

Effects of vicinal angles from (0 0 1) surface on the Boron-doping features of high-quality homoepitaxial diamond films grown by the high-power microwave plasma chemical-vapor-deposition method

Vicinal surface effects on homoepitaxial growth and boron-doping processes have been studied in case of single-crystalline diamond (0 0 1) surfaces grown using the high-power microwave plasma chemical-vapor-deposition (MWPCVD) method. The off-angles inclined from the on-orientation (0 0 1) surfaces ranged to 5° along the [1 1 0] or [1 0 0] direction, while the concentration of doping B(CH 3) 3 gas was kept constant with a B/C ratio of 50 ppm. Although a number of square-like growth hillocks often appeared, depending substantially on the crystalline quality of the high-pressure/high-temperature-synthesized (HPHT) Ib diamond substrates employed, the number and shape of the hillocks changed significantly with the increasing off-angle. For the vicinal surfaces with off-angles of ≈3° inclined along the [1 1 0] direction, macroscopically flat surfaces were obtained, compared with the other off-angle cases examined. Furthermore, the growth rate and acceptor density of substitutional boron atoms in the homoepitaxial layers were found to substantially increase with the increasing off-angle. These indicate that the step density can play important roles not only in the homoepitaxial growth but also in the boron-incorporation process during the high-power MWPCVD growth.

Highly efficient doping of boron into high-quality homoepitaxial diamond films

High-quality boron-doped homoepitaxial diamond (100) films were grown using high-power microwave plasma chemical vapor deposition (MPCVD). Results showed that the incorporation efficiency of boron from gas phase was increased by two orders of magnitude compared to those of conventional growth conditions using high-density plasma. Boron-doped crystal thus grown with the low B / C ratio of 50 ppm in the gas phase had reasonably low resistivity of 2 Ω cm at 290 K, whereas Hall mobility of 830 cm 2 V s − 1 at this temperature was higher than those reported previous for such resistivity. The highly boron-doped film showed strong free exciton recombination emissions with a bound exciton component in the cathodoluminescence spectra taken at room temperature, reflecting the considerably high density of the substitutional boron in the film despite its low density of electronic defects. The diamond growth rate in the high-power MPCVD was 3.5 μm h − 1 . The highest room-temperature Hall mobility achieved in this study was 1620 cm 2 V s − 1 . These results indicate that the resultant high-rate growth with high-power MPCVD is advantageous for depositing high-quality and conductive diamond films.

Effect of increasing the microwave density in both continuous and pulsed wave mode on the growth of monocrystalline diamond films

AbstractDiamond synthetic single crystals exhibiting a high and reliable quality would find many applications in different areas such as optics, mechanics, or electronic devices. In this study, thick high quality homoepitaxial diamond films were grown on synthetic substrates using Micro‐Wave Plasma Assisted Chemical Vapour Deposition. The effect of increasing the density of the hydrogen methane discharge was investigated since it is believed to allow higher deposition rates by enhancing the production of precursor species. In continuous wave mode, it was actually observed that increasing the microwave density in the plasma from 65 to 125 W/cm3 leaded to an increase of diamond growth rates from 3 to 8.5 µm/h and from 11 to 19 µm/h when 4% and 7% of CH4 was added to the gas phase respectively. However, when such a high microwave power density is injected, overheating of the reactor walls, windows and wave‐guides occurs leading to problematic cooling‐down and higher contamination of the diamond film. In particular higher silicon incorporation was found by photoluminescence spectroscopy that probably originates from an enhanced etching of the quartz windows. Thanks to the use of a pulsed discharge with a peak power of 190 W/cm3 the deposition of diamond at rates close to 22 µm/h became possible while limiting this problem of overheating. The use of such pulsed discharges is thus considered to be a very promising method for the growth of high quality diamond single‐crystals at high deposition rates. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High Quality MPCVD Epitaxial Diamond Film for Power Device Application

ABSTRACTAs a wide bandgap (5.47 eV) semiconductor material, single crystal diamond has high electron mobility (reportedly between 2000 and 4400 cm2V-1s-1), high electron saturation velocity (2×107 cms-1), high breakdown voltage (>107 Vcm-1), and high thermal conductivity (>21 Wcm-1K-1). Diamond-based semiconductor devices offer the potential of operation at high voltages, power levels, temperatures and under extreme radiation conditions. In this work, we present our effort to grow high quality homo-epitaxial diamond films on (100)-single crystal diamond substrates by microwave plasma chemical vapor deposition (MPCVD). The growth rate can vary from 0.01 to 100 micrometers per hour, depending on growth conditions, doping, and quality; and using a “lift-off” process, free-standing homo-epi films with remarkably low p-type doping (1×1014–1×1017 cm-3) and exceptionally low compensation ∼ 1×1013 cm-3 have been made. Vertical and lateral structure high voltage diamond Schottky rectifiers have been built for frequency dependent capacitance-voltage (C-V), and current-voltage (I-V) measurements. A breakdown voltage of 8 kV at 100 μm distance and 12.4 kV at 300 μm distance is recorded for lateral structure devices without Ohmic contact (back to back Schottky contacts), while an un-optimized vertical device with an back-side Ohmic contact has demonstrated a forward voltage drop of 7 V at 18 A/cm2 in a device that can only block 600 V. New test results show 3.7 kV blocking voltage vertical devices on 20 μm freestanding MPCVD diamond film. This data shows that the quality of diamond film extremely affect the electrical properties of the built devices.

Growth of high-quality homoepitaxial diamond films by HF-CVD

Defined growth of high-quality crystalline diamond films is necessary for manufacturing of electronic devices. Several groups have already worked on this topic and the main challenges remaining are the suppression of crystalline defects, the reduction of impurities and higher growth rates. High-quality homoepitaxial diamond films were produced in a hot filament chemical vapor deposition (HF-CVD) system with and without addition of nitrogen to the gas phase. Substrates were (100)-oriented synthetic Ib diamonds (Sumitomo) with a maximum misorientation angle of 0.3° as determined by X-ray diffraction. By adjusting the α-parameter independently to a value close to two using polycrystalline diamond films on silicon growth rates of up to 500 nm/h could be achieved while simultaneously keeping the density of unepitaxial crystallites below 10 cm−2. The surface roughness (RMS) was below l nm in a 1×1-μm2 area. Impurities were studied by photoluminescence and cathodoluminescence spectroscopy. For the films deposited without intentional nitrogen addition, nitrogen-related impurities could only be detected by low temperature photoluminescence indicating a very low concentration. At 1.68 eV we observed the signature of the Si-vacancy complex probably caused by evaporation of Si from the tungsten filament.

High quality homoepitaxial CVD diamond for electronic devices

Abstract Recent achievements in homoepitaxial CVD diamond films for electronic devices have been discussed. We have successfully synthesized high-quality homoepitaxial diamond films with atomically flat surface by the microwave plasma chemical vapor deposition (CVD) using a low CH4 concentration of CH4/H2 gas system less than 0.15% CH4/H2 ratio and Ib (001) substrates with low-misorientation angle less than 1.5°. These films are atomically flat over an area as large as 4×4 mm2 and have shown a strong excitonic emission of 5.27 eV line, even at room temperature, with no essential emission lines in the visible light region in the cathodoluminescence (CL) spectra. Furthermore, high-quality Schottky junctions between Al and P type high-conductivity layers near the surface of these films have been obtained. Based on this growth method, we have also successfully synthesized B-doped diamond films using trimethylboron [B(CH3)3,TMB] gas as a B-doping source, whose Hall mobility is 1840 cm2/Vs at 290 K. Schottky junction fabricated by the B-doped diamond also shows excellent performances, indicating that the homoepitaxial diamond films presented here have a high potentiality for electronic devices.

Growth and characterization of hillock-free high quality homoepitaxial diamond films

A suitable pre-treatment process to significantly suppress growth hillocks on homoepitaxial diamond surfaces has been successfully developed. Nanometer-scale morphologies obtained for the homoepitaxial diamond films by means of an atomic force microscope show that the mean roughness (root mean square) of the homoepitaxial diamond films with thickness of 1 μm was approximately 5 nm in the scanning region of 2×2 μm2. The results from cathodoluminescence (CL) measurements indicate that the developed pre-treatment and deposition by a high power ASTeX microwave plasma chemical vapor deposition (MWP-CVD) apparatus led to growth of homoepitaxial diamond films with high crystalline quality yielding only band-edge emissions as the main peak in CL spectra at low temperatures (80 K).

Activation energy in low compensated homoepitaxial boron-doped diamond films

Abstract High-quality homoepitaxial diamond films have been grown on 100-type Ib synthetic diamond, and boron doped by diborane from 5×1016 to 8×1020 cm−3. The current–temperature characteristics from 300 to 1000 K have been measured and fitted by the general expression of the conductivity in a compensated semiconductor in the ionisation regime. The saturation of the conductivity is observed between 680 and 1000 K for the samples doped below 2×1017 cm−3. In the range 1018–1019 cm−3, an activation energy of 185 meV is observed between 500 and 1000 K and around 368 meV between 300 and 500 K. The activation energy of 185 meV corresponds to half the boron ionisation energy, and is related to the conduction regime in an uncompensated semiconductor. The evolution of these activation energies and of the room-temperature resistivity with the boron doping are presented.

Diamond semiconductor for electronic device application

A study of diamond films for electronic device application is reviewed based on our resent experimental results on homoepitaxil diamond films. High quality homoepitaxial diamond films have been successfully grown on (001) diamond substrate by introducing a concept of “clean epitaxiy”. Atomic force microscopy (AFM) images have indicated that the surface of these films has atomically flat region and the films were grown by a step-flow mode. Current-voltage characteristics of Al/diamond Schottky diodes prepared with these films show excellent rectification properties with the very low level of the reverse current which has never been reported. Cathodoluminescence (CL) experiments at room temperature on these films also show a clear spectrum due to a free exciton for first time. These experimental results suggest that the diamond films grown by the step-flow mode have a potential for electronic device application.

Electrical characterization of homoepitaxial diamond films doped with B, P, Li and Na during crystal growth

Abstract Using the microwave plasma CVD method, we have grown high-quality homoepitaxial diamond films on (100)-cut diamond substrates. The films were selectively grown as Hall bars by using a sputtered SiO2 mask on the diamond substrates. Electrical contacts were electron-beam evaporated Mo/Pt/Au layers which were annealed at high temperatures. Boron was mainly used as a p-type dopant, but lithium, sodium and phosphorus were also investigated as potential n-type dopants. Doping was done during film growth by placing the oxides of the different doping materials into the plasma chamber. The electrical conductivity as well as the current-voltage characteristics were measured over a large temperature range. In addition, Hall effect measurements were performed on boron-doped layers from 100 K to 1050 K. At room temperature the investigated Li-, Na- and P-doped films had very high resistivities over 109 Ωcm, undoped films even more than 1016 Ωcm. The activation energies of electrical resistivity were in the range 0–0.43 eV for boron and 0.16 eV for lithium. The undoped films grown on boron-doped substrates had breakdown fields up to 2 MV cm−1.

High Quality Homoepitaxial Diamond Films Grown in End-Launch Type Reactors

ABSTRACTHigh quality diamond films have been successfully grown, by step-flow, on (001) diamond substrates using an end-launch type chemical vapor deposition reactor. Electrical properties of as-deposited diamond films as well as the surface morphology and the film crystallinity were investigated. Optical and atomic-force microscope images indicated that diamond films consisted of atomically flat terraces and macroscopic steps running parallel to [110×l and 1×2 double-domain structure. The currentvoltage characteristics of Al-Schottky contacts to these step-flow grown diamond films showed excellent rectification properties, indicating the potential of this material for electronic applications.

Accumulation of Implantation Damage in MEV Implanted Diamond Crystals

ABSTRACTSingle crystal, type IIa natural diamond substrates have been implanted with 4 MeV carbon ions to doses ranging from 0.5–130×1016 cm-2. The accumulation of implantation damage is studied by Raman and RBS/channeling. Similar effects are observed for crystals of [100], [110], and [111] orientation but with different rates of damage accumulation. With increasing implantation damage, the triply degenerate Raman mode at 1332 cm-1 broadens and shifts down to around 1300 cm-1. This corresponds to a peak in the one-phonon density of states as predicted for Raman from an amorphous sp3 network. There is no evidence for the existence of sp2 carbon in the implanted area. Additional non-graphite Raman peaks appear at 1451, 1494, and 1635 cm-1. At the higher doses, the 1332 cm-1 Raman mode is no longer observed; however, RBS/channeling still shows the surface region to be crystalline and it is possible to grow high quality homoepitaxial diamond films on these substrates.