High-pressure and high-temperature annealing effects on CVD homoepitaxial diamond films

High-pressure and high-temperature (HPHT) annealing effects on the chemical vapor-deposited (CVD) homoepitaxial diamond films were investigated. By the HPHT annealing, the intensity of free-exciton (FE)-related emission was increased by  2 times and the luminescence bands from 270 to 320 nm, which originate from 5RL and 2BD bands, were almost completely eliminated in the cathodoluminescence (CL) spectrum. The CL intensity of band-A emission, which is related to crystal defects in diamond, was also decreased. The hole mobility at room temperature was increased from 826 to 1030 cm²/Vs by HPHT annealing. These results suggest that HPHT annealing decreases the crystalline defects and improves the optical and electronic properties of homoepitaxial diamond films.     

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.     

High quality homoepitaxial CVD diamond for electronic devices

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.     

Formation of a conducting layer by high energy metal ion implantation into diamond

A conducting layer of Al or Cu was formed under the surface of synthetic Ib diamond (100) by high-energy metal ion implantation of 1×10¹⁵–2.8×10¹⁷ ions cm⁻² with ion energies of 3 MeV (Al) and 8 MeV (Cu). The distribution peaks of heavily implanted Al and Cu, investigated by secondary ion mass spectroscopy (SIMS), were approximately 1.3 and 2.2-μm deep from the diamond surfaces, respectively. The results agreed with simulation results of TRIM. The Raman line of implanted diamond surfaces was observed at approximately 1333 cm⁻¹, though they were broad and slightly shifted from the diamond Raman peak. The diamond structure was clearly observed at the implanted surfaces by reflection high energy electron diffraction (RHEED) measurements. From these results, it was concluded that the surfaces of the implanted diamond were not totally damaged. The sheet resistance of the metal implanted layers measured by a four-probe method decreased with increasing the ion dose and reached a minimum value of approximately 170 Ω/□. By comparing this resistance value with that of a similarly implanted layer in a SiO₂ glass substrate, it was concluded that electrons were transported mainly through the implanted metal layer for high dose specimens, while electron transport via defects was dominant for low dose specimens.     

EPR study of hydrogen-related defects in boron-doped p-type CVD homoepitaxial diamond films

Boron-doped p-type single crystalline chemical vapor deposition (CVD) homoepitaxial diamond films were investigated by electron paramagnetic resonance (EPR). Carbon dangling bond defects, which were accompanied by a nearby hydrogen atom, were observed in boron-doped p-type CVD diamond films on a IIa substrate similar to those observed in undoped diamond. This result suggested that the energy level position of the defects is located below the Fermi energy of boron-doped diamond, at around 0.3 eV above the valence-band top. The reason why the Fermi energy could be changed by the incorporation of boron atoms at low density (10¹⁶–10¹⁷/cm³) in the film in spite of the existence of the large defect density of EPR centers (10¹⁸/cm³) is thought to be that the singly occupied electron states of defects are located near the band edge. As for the thermal annealing effect of the defects, it was revealed that the concentration of the defects and the mobility of the p-type film did not change after annealing up to 1200 °C which is much higher than the temperature of boron–hydrogen pair dissociation.     

Recovery treatments for ion-induced defects in high-quality homoepitaxial CVD diamond

Effects of vacuum annealing and hydrogen plasma exposure on ion-implantation-induced defects have been investigated in case of high-quality chemical-vapor deposited (CVD) diamond mainly using cathodoluminescence (CL) measurements. The well-focused 30-keV Ga ions were implanted into regions with different ion doses from 1×10¹² to 1×10¹⁵ ions/cm². The free-exciton emission and the N–V center were observed at 235 and 575 nm, respectively, in room temperature CL spectra for as-grown homoepitaxial CVD diamond. The former vanished completely after all the implantation processes examined while the latter was destroyed more strongly with increasing ion doses. On one hand, the band edge emissions at 235 nm were hardly recovered even after any treatments examined. On the other hand, the CL peak at 575 nm reappeared either after a 30-min vacuum annealing at 900°C for the Ga dose of 1×10¹² ions/cm² or after a suitable hydrogen plasma treatment for all the Ga ion dosages examined. Thus, it is found that the band edge emission signal is required to investigate the beam damages to ‘high’ crystalline quality diamond. A removal of the damaged surface layer by the plasma etching is also discussed in relation to the recovery process mainly for the 575-nm peak at heavier ion doses.     

Electrical Properties of B-doped homoepitaxial diamond films grown from UHP gas sources

It is confirmed that a small amount of nitrogen incorporated into chemical vapor deposited diamond films dramatically affects their electrical properties. Nitrogen can be incorporated into diamond films through the leak of vacuum system and/or from the impurity in source gases. Because a nitrogen atom can be a deep donor in diamond crystal, the p-type semiconducting properties of boron doped diamond films can be degraded even by the small amount of nitrogen. The small amount of nitrogen in chemical vapor deposited diamond films was measured by cathodoluminescence spectroscopy. For the detection of nitrogen, the N–V center was intentionally induced by defect formation through ion beam irradiation and subsequent annealing. The luminescence intensity of the N–V center was decreased by reducing the leak of the vacuum system and by upgrading the purity of the source gases. Both the carrier density and the Hall mobility of the boron doped diamond films were successfully improved by the control of nitrogen contamination. Using extremely high pure CH₄, H₂ and B₂H₆ in a tightly sealed vacuum system, the total amount of nitrogen impurity in the source gas was controlled to <80 ppm in the N/C atomic ratio resulting in a Hall mobility of 1600 cm²/Vs with a hole concentration of >10¹⁴ cm⁻³ at the room temperature in a 10-ppm-boron doped homoepitaxial diamond film.     

Reduced influences of the HPHT substrates on the electronic quality of homoepitaxial CVD diamond layers and on their ultraviolet detector performance

We have carried out a detailed estimation of the influences of the high-pressure/high-temperature-synthesized (HPHT) Ib substrate on the crystalline quality of the homoepitaxial diamond and on the performance of the ultraviolet (UV) detector. The H3 center related luminescence peaks were observed even from the homoepitaxial diamond film having a thickness of 250 μm on a HPHT Ib substrate, suggesting that carriers excited in the epitaxial diamond layer can diffuse over a rather long distance to the HPHT substrate when the quality of the epitaxial layer is sufficiently high. Furthermore, we have attempted to efficiently reduce the long-distance carrier diffusion phenomenon by inserting a boron-doped layer between the epitaxial layer for the detection and the HPHT Ib substrate. The electrically-floating B-doped layer inserted between the homoepitaxial layer and the HPHT substrate efficiently reduced the long-distance carrier diffusion phenomenon, and substantially improved the performance of the UV detector fabricated on a low-quality HPHT Ib substrate.     

Characterization of substrate off-angle effects for high-quality homoepitaxial CVD diamond films

We have studied the substrate off-angle effects for the crystalline quality of the homoepitaxial diamond films mainly by using steady-state cathodoluminescence (CL) and time-resolved photoluminescence (PL) measurements. By means of the microwave plasma chemical vapor deposition method under high-power microwave power with high methane concentrations, the homoepitaxial diamond films were grown on the high-pressure/high-temperature-synthesized (HPHT) Ib (001) substrates inclined along either <110> or <100> direction by different off-angles ranging from 2° to 5°. In spite of high growth rates, we have succeeded in improving crystalline quality by employing the HPHT substrates with considerably large off-angles. Both steady-state CL and time-resolved PL measurements clearly indicate that larger off-angles lead to better crystalline quality of the homoepitaxial film, suggesting that further improvements in crystalline quality can be expected when using substrates having even larger off-angles.     

Characterization of nanocrystalline diamond films implanted with nitrogen ions

Nanocrystalline diamond films, prepared by a microwave plasma-enhanced CVD, were implanted using 110-keV nitrogen ions under fluence ranging from 10¹⁶–10¹⁷ ions cm⁻². AFM, XRD, XPS and Raman spectroscopy were used to analyze the changes in surface structure and chemical state of the films before and after implantation. Results show that high-fluence nitrogen ions implanted in the nanocrystalline diamond film cause a decline in diamond crystallinity and a swelling of the crystal lattice; the cubic-shaped diamond grains in the film transform into similar roundish-shaped grains due to the sputtering effect of implanted nitrogen ions. Nitrogen-ion implantation changes the surface chemical state of the nanocrystalline diamond film. After high-fluence implantation, the surface of the film is completely covered by a layer of oxygen-containing groups. This phenomenon plays an importance role in the reduction of the adhesive friction between an Al₂O₃ ball and the nanocrystalline diamond film.     

Ion implantation in diamond using 30 keV Ga focused ion beam

Focused ion beam (FIB) technique is a well established technique for processing and modifying materials at micro- and nanoscale. FIB implantation with 30 keV Ga⁺ ions into a single crystal diamond has been studied via a combination of transmission electron microscopy (TEM) imaging and spectroscopy in the attempt to understand the damage formation in diamond. The damage formation has been studied as a function of implantation dose with eight different doses ranging from 6 × 10¹⁴ to 1 × 10¹⁶ ions/cm². The TEM studies have revealed different structure of low-dose and high-dose implanted regions. 3.5 nm diamond cap layer was observed in the low-dose implanted layer. TEM analysis has shown volume extension of around 50% in the amorphous region and up to 7% in diamond at the crystal-amorphous interface. The density of amorphous damage layer was measured to be 2.51 g/cm³ and 2.24 g/cm³ in the low-dose and high-dose implanted regions, respectively. The amorphisation threshold for ion implantation in diamond at room temperature was determined to be 5.2 × 10²² vacancies/cm³.     

The influences of oxygen ion implantation on the structural and electrical properties of B-doped diamond films

The oxygen ion with a dose of 10¹⁴ (called CVDBO14) and 10¹⁵ cm^− 2 (called CVDBO15) was implanted into boron doped diamond films synthesized in chemical vapor deposition. The structural and electrical properties of different samples were characterized by XPS, Raman spectroscopy and 4-probe resistivity measurements. The results show that oxygen ion exists both in the diamond surface and the subsurface of the films. The FWHM values of CVDBO15 samples are higher than those of CVDBO14 samples, indicating that more damages existed in CVDBO15 samples. The resistivity of CVDBO15 sample series is smaller than those of CVDBO14 sample series, and the film with a larger FWHM value exhibits low resistivity. In the 1150 °C annealed sample, the activation energy decreases from 0.50 eV to 0.39 eV with the oxygen ion dose increasing from 10¹⁴ to 10¹⁵ cm^− 2. It is indicated that oxygen ion and the defects produced by ion implantation give contributions to the conductivity in diamond films. Some surface hydrogen is removed and pi-bonded carbon as well as C-H vibration is formed after annealing, which is also relative to the lower resistivity in the samples.     

Charge transport in high mobility single crystal diamond

Time of Flight (TOF) measurements using conventional laser TOF and α-particle TOF setups have been carried out on high quality CVD diamond samples to study the electron and drift mobility and to compare them with the mobility data for IIA diamond. The measured mobilities for all samples investigated are in the range 2000–2250 cm²/Vs for holes and 2200–2750 cm²/Vs for electrons, thus close to the theoretical prediction as well as to IIa diamond mobility values. The charge transient profile measured in the laser TOF measurements is influenced by the electric field profile in the sample, which might be changed based on the charge trapping at low electric fields applied, depending on the surface atomic termination. The temperature dependence of the drift mobility indicates that at room temperature the scattering on acoustic phonons is the main dominant scattering mechanism and the contribution of other types of carrier scattering mechanism is negligible.     

On the two-phonon absorption of CVD diamond films

It is well known that the absorption coefficient of diamond in the two-phonon region is constant, for example at 2000 cm^{− 1}, the absorption coefficient is 12.3 cm^{− 1}. This means that the infrared absorbance in the two-phonon region is proportional to the thickness of the samples, which is generally used as standard to normalize the infrared absorption spectra of diamond samples according to their thickness. This is true for natural and HPHT synthetic single crystal diamond. However for polycrystalline or nanocrystalline CVD diamond films, we found that the situation may be different. For high quality thick CVD diamond films of thickness > 150 μm, the infrared absorbance in the two-phonon region is proportional to its thickness. While CVD diamond films of equal thickness but of different quality show variable absorbance in the two-phonon absorption region in terms of thickness. Our investigation on this observation primarily indicates that the grain size of CVD diamond films has influence on the two-phonon absorption. In this work, we present this new result and discuss the mechanism of this phenomenon in the light of the growth mechanism of CVD diamond.     

Metal–insulator transition in boron-ion implanted type IIa diamond

Electrical conductivity measurements for selected boron-ion dopant concentrations have been made on type IIa diamond specimens in the temperature range 1.5–30 K. Samples have been implanted using the CIRA (cold implantation–rapid annealing) process, in which small implantation increments were used followed by high-temperature annealing to achieve a significant reduction in the levels of implantation-induced radiation damage and to obtain maximum boron activation. Further post anneals at temperatures up to 1700°C were carried out. Using this procedure, we have recorded, for the first time, metallic conductivity behaviour in implanted surface layers in single-crystal diamond specimens with boron concentrations measured by secondary-ion mass spectrometry to be of the order of n=10²¹ cm⁻³. The occurrence of a metal–insulator transition in this system is discussed.     

Electroanalytical application of modified diamond electrodes

Metal-modified diamond electrodes were fabricated by using ion implantation method for electroanalytical applications. Nickel and copper ion were implanted at a different film of boron-doped diamond (BDD) with a dose of 5×10¹⁴ cm⁻² for each type of ion. The electrochemical behavior has been studied for glucose oxidation in alkaline media by using cyclic voltammetry and flow injection analysis. Those electrodes exhibited high catalytic activity and excellent electrochemical stability with low background current even after strong ultrasonication in the cleaning process. The results indicate that metal-implanted method could be a promising method for controlling the electrochemical properties of diamond electrodes.     

Epitaxial lateral overgrowth (ELO) of homoepitaxial diamond through an iridium mesh

In the present work iridium layers forming a mesh on diamond have been studied as potential candidates for buried electrodes or stopping layers in an ELO process for heteroepitaxial diamond. Thin iridium layers (∼ 15 nm) were deposited by e-beam evaporation at ∼ 700 °C on the facets of individual (001)-oriented CVD diamond crystallites and macroscopic Ib HPHT substrates with off-axis angles of several degrees. The heteroepitaxial iridium films formed a mesh with 10–200 nm large holes. These were penetrated by homoepitaxial diamond in a microwave plasma chemical vapour deposition process (MWPCVD) burying the iridium layer completely after 15 min of diamond growth. High resolution X-ray diffraction including reciprocal space mapping and Raman spectroscopy was used to characterize the structural properties of the diamond overlayer on the Ib HPHT substrate. It was monocrystalline with an FWHM of 0.03–0.05° of the X-ray rocking curve. Its lattice planes were tilted by ∼ 0.01° with respect to the substrate and showed a macroscopic strain of − 10^− 4 perpendicular to the surface. Besides the smaller lattice constant due to the lack of nitrogen the strain is mostly attributed to a tensile in-plane stress state. Strain and tilt can be attributed to the lateral overgrowth and the off-axis angle of the substrate.     

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.     

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).     

High dose N ion implantation effects on surface treated UNCD films

Ion implantation is commonly used to modify the surface or near-surface properties of materials. In this work, plasma treated ultrananocrystalline diamond (UNCD) films were implanted using 100 and 200 keV high dose (10¹⁶ ions/cm²) nitrogen ions and annealed. Detailed studies have been carried out to reveal the structural and chemical states of the surface treated UNCD films before implantation, as-implanted, and after annealing by using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron field emission (EFE) measurements. The high dose N ion implantation induced the formation of amorphous phase, which are converted into graphitic phase after annealing, and improved the field emission properties of UNCD films. The improved field emission is attributed to the surface charge transfer doping mechanism.