Chunlei Wang

Growth of high-quality homoepitaxial CVD diamond films at high growth rate

Abstract High growth-rate deposition of ∼2xa0μm/h has been achieved for high-quality diamond (1xa00xa00) film growth by means of high-power microwave-plasma chemical-vapor deposition (MPCVD). The growth rate is at least 40 times higher than the values reported from conventional low-power MPCVD in the case of high-quality diamond film growth. Cathodoluminescence spectra from the grown diamond films reveal strong band-edge emissions even at room temperature, indicating high crystalline quality of the diamond films. With increasing growth temperature, the film surface became flat and the intensity of the band-edge emission strong.

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

Abstract 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 μm 2 . 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).

Fabrication and properties of lateral p–i–p structures using single-crystalline CVD diamond layers for high electric field applications

Abstract Chemical-vapor-deposited diamond p–i–p structures have been fabricated on homoepitaxially grown single-crystalline layers using focused ion beam etching in order to investigate carrier transport properties of diamond in high electric fields more than 10 6 V/cm. The examined structures included a 200-nm-thick intrinsic (undoped) diamond region laterally sandwiched between two p-type (B-doped) diamond regions. At high fields above ≈3×10 7 V/cm, I – V characteristics of the lateral p–i–p structure revealed abnormal increases in current. The observed performances can be explained more reasonably in terms of impact ionization events from the valence band to the conduction band in the intrinsic diamond than in terms of direct Fowler–Nordheim tunneling events of electrons from the valence band of the negatively biased p diamond to the conduction band of the intrinsic diamond.

Highly efficient electron emitting diode fabricated with single-crystalline diamond

Abstract Highly efficient electron emitting diodes have successfully been fabricated using single-crystalline diamond films epitaxially grown on high-pressure synthesized (100) diamond. It turns out that the crystalline quality of the diamond used is essential to the present-type electron emitter operated under extremely high electric fields of ∼107 V/cm. For the electron emitting diode, the current efficiency defined as the ratio of the emission current to the driving current can reach unity in the best case. The mechanism of the highly efficient electron emission observed is discussed in relation to possible carrier excitations in the undoped diamond layer under the high fields. Using a high-power microwave-plasma chemical vapor deposition apparatus, high-quality diamond thin films were homoepitaxially grown. It is found from measuring the threshold energy of total photoelectron yields that a moderate surface oxidation process can change the surface electric dipole layer and, therefore, the electron affinity for hydrogen-terminated diamond surfaces as the Topping model shows.

Boron-Doped Diamond Film Homoepitaxially Grown on High-Quality Chemical-Vapor-Deposited Diamond (100)

In this study, high-quality homoepitaxial diamond (100) without any growth hillocks or abnormal particles has been investigated as a buffer layer for boron-doped diamond overgrowth to improve the electrical properties. For the undoped buffer layer used, very strong band-edge emissions were observed at room temperature (RT) in the cathodoluminescence (CL) spectra. After the overgrowth of B-doped homoepitaxial layer, CL characteristics and electrical properties were compared with cases when diamond buffer layers with inferior quality from the viewpoint of CL spectra were employed. The results showed that employing such a high-quality buffer layer led to an efficient acceptor doping of B atoms of 1.6×1019 cm-3 to the overgrown layer while the RT hole mobility was kept at 910 cm2/Vs, although bound-exciton CL peaks were well resolved only at lower temperatures. The usefulness of the high-quality buffer layer is discussed.

Interface characterization of chemical-vapour-deposited diamond on Cu and Pt substrates studied by transmission electron microscopy

A novel method has been employed to study the interfacial structures of the chemical-vapour-deposited (CVD) diamond on Cu and Pt substrates using high resolution electron microscopy (HREM) without thinning the specimens. The CVD diamond used for the HREM study was directly deposited on the Cu and Pt transmission electron microscopy (TEM) grids by means of microwave plasma-assisted chemical vapour deposition. The HREM images clearly reveal that there exists a graphite intermediate layer between diamond and copper, in which a small amount of amorphous carbon is embedded. A 4 nm graphite transitional layer has also been observed on the platinum substrate. The (0002) planes of the intermediate graphite phase closely parallel the Cu and Pt surfaces. The registry between {111} planes of some diamond particles and the beneath graphites (0002) planes is evidenced by HREM, and the appearance of the (111) texturing diamond grains on Cu and Pt is supposed to be closely related to the graphite intermediate layers.

Investigation of distribution of defects and impurities in boron-doped CVD diamond film by cathodoluminescence spectroscopy

Abstract Distribution of defects and impurities was investigated by cathodoluminescence spectroscopy on as-grown boron-doped diamond film synthesized by microwave plasma chemical vapor deposition method. Luminescence profiles in a boron-doped diamond film were observed by cross-section CL images. Intense emission of 535 nm (~2.32 eV) band was found in a thin layer of the growth surface and in a rather thick layer of interface to the silicon substrate. Complementary to the 535-nm emission, the edge emission appeared in the bulk region excluding the surface layer. The origination of the 535-nm band can be explained by a boron-vacancy center.

High-Quality Homoepitaxial Diamond Films Grown at Normal Deposition Rates

High-quality homoepitaxial diamond films without any growth hillocks and abnormal particles have been successfully deposited at a reasonably high deposition speed of ~0.5 µm/h by a 5 kW microwave plasma chemical vapor deposition system. These films show cathodoluminescence (CL) spectra predominated by strong free-exciton recombination radiation assisted by a transverse optical phonon and its replicas even at room temperature. The temperature dependence of the CL feature has been analyzed quantitatively. The results suggest that a considerable reduction of non-radiative relaxation paths is accomplished in the present case of high-quality diamond films.

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

Abstract 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 1×10 12 to 1×10 15 ions/cm 2 . 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 1×10 12 ions/cm 2 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.

Structural analysis of ion-implanted chemical-vapor-deposited diamond by transmission electron microscope

Abstract A transmission electron microscope (TEM) study of ion-implanted chemical-vapor-deposited (CVD) diamond is presented. CVD diamond used for transmission electron microscope observation was directly deposited onto Mo TEM grids. As-deposited specimens were irradiated by C (100 keV) ions at room temperature with a wide range of implantation doses (1012–1017/cm2). Transmission electron diffraction (TED) patterns indicate that there exists a critical dose (Dc) for the onset of amorphization of CVD diamond as a result of ion induced damage and the value of critical dose is confirmed to be about 3 × 1015/cm2. The ion-induced transformation process is clearly revealed by high resolution electron microscope (HREM) images. For a higher dose implantation (7 × 1015/cm2) a large amount of diamond phase is transformed into amorphous carbon and many tiny misoriented diamond blocks are found to be left in the amorphous solid. The average size of these misoriented diamond blocks is only about 1–2 nm. Further bombardment (1017/cm2) almost kills all of the diamond phase within the irradiated volume and moreover leads to local formation of micropolycrystalline graphite.