Single Crystal Diamond Deposited by Dual Radio-Frequency Plasma Jet CVD with High Growth Rate

crystals

Article Single Crystal Diamond Deposited by Dual Radio-Frequency Plasma Jet CVD with High Growth Rate

Hao Liu , Jia-jun Li, Zhen-rui Li, Kai Xu, Zheng-jia Chen and Guang-chao Chen * College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China; [email protected] (H.L.); [email protected] (J.-j.L.); [email protected] (Z.-r.L.); [email protected] (K.X.); [email protected] (Z.-j.C.) * Correspondence: [email protected]

Received: 3 December 2018; Accepted: 6 January 2019; Published: 10 January 2019

Abstract: Single crystal diamonds were deposited on high pressure high temperature (HPHT) substrate with high growth rate, up to 18.5 µm/h, by using dual radio-frequency inductive coupled plasma jet. The methane flux was found to influence the growth rate of single crystal diamond. The reason for this might be ascribed to the electron temperature increase, raising the flux of methane, based on the plasma diagnosis results by optical emission spectra (OES). The results of Raman spectroscopy and the X-ray rocking-curve indicated that as-deposited diamonds are of good quality.

Keywords: dual radio frequency; inductive coupled plasma jet; single crystal diamond; chemical vapor deposition (CVD); methane ﬂux; growth rate

1. Introduction Diamonds have a large range of potential applications [1–5]. Due to their high atomic density, strong bonding, and crystal structure of high symmetry, diamonds show high Raman gain coefficient, which makes them suitable for high power diamond Raman lasers [6]. With a wide bandgap of 5.5 eV, a high breakdown field, high carrier mobility, and high radiation hardness, diamonds are an ideal material for high energy particle and ionizing radiation detection [7–10]. Furthermore, diamonds exhibit exceptional electronic properties, such as a high breakdown field (>10 MV/cm) and high carrier mobility (4500 cm2/Vs for electrons and 3800 cm2/V for holes), which makes diamond an excellent candidate for next generation semiconductor materials [11,12]. Due to the fact that high energy plasma can dissociate reaction gas very effectively and can provide high atomic hydrogen concentration, which is necessary for fabricating high-quality, large area single crystal diamond (SCD), plasma enhanced chemical vapor deposition (PECVD) has been widely used for SCD growth [13]. According to plasma sources, PECVDs can be classified as microwave-, direct current (DC)-, and radio frequency (RF) plasma CVD. Among all these methods, microwave plasma CVD is the most widely used method, the reported largest area reached 2 inches in diameter, and the fastest growth rate reached 105 µm/h [14–17]. Like microwave plasma, RF plasma is free from metal electrode contamination. However, RF plasma possess longer wavelengths, so that electrons and positive charged particles in the plasma can get energy from the whole electromagnetic wave vibration period, which leads to more uniform spatial distribution [18]. According to previous reports, plasma density higher than 1018 m−3 can be achieved by radio frequency inductively coupled plasma (RF-ICP). Since the 1980s, researchers have been growing diamond films by RF CVD, and the reported diamond films can be deposited uniformly on a substrate as large as 100 mm in diameter, and the growth rate can reach 30 µm/h [19,20]. At the

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Crystals 2018, 8, x FOR PEER REVIEW 2 of 9 beginning of the 21st century, the growth rate reached 70 µm/h [21]. However, some drawbacks exist in the theannular RF plasma discharge CVD zone growth caused method, by skin such effect as “holloweasily, which shaped” results plasma in a non-diamond found in the annular composition, discharge andzone “ion caused bombardment” by skin effect caused easily, by which plasma results self-bias in a non-diamond voltage, which composition, damages and diamond “ion bombardment” growth [22,23].caused According by plasma to our self-bias past research, voltage, whichdual RF-ICP damages can diamond be considered growth as [ 22an,23 effective]. According way to to avoid our past theseresearch, problems. dual Moreover, RF-ICP can poly-crystal be considered and assingle an effective-crystal waydiamond to avoid films these have problems. been fabricated Moreover, successfullypoly-crystal by andvery single-crystal low growth diamondrates [24–27]. ﬁlms In have this been work, fabricated by adjusting successfully growth by condition, very low growththe growthrates rate [24– of27 the]. In SCD this films work, have by adjusting been greatly growth enhanced, condition, and the the growth maximum rate ofgrowth the SCD rate ﬁlms can exceed have been 18 μgreatlym/h. enhanced, and the maximum growth rate can exceed 18 µm/h.

2. Materials2. Materials and and Methods Methods SingleSingle crystal crystal HPHT HPHT Ιb (100) Ib (100) diamonds diamonds (5 mm (5 mm × 5× mm5 mm and and 6 mm 6 mm × 6 ×mm)6 mm) were were used used as seed as seed substrates.substrates. A diagram A diagram of the of dual the dualRF-ICP RF-ICP jet CVD jet CVDsystem system used usedin this in work this workis shown is shown in Figure in Figure 1, in 1, whichin which commercial commercial 13.56 13.56MHz MHzand 4 and MHz 4 MHzRF power RF power coils coilswere weretandemly tandemly set on set the on plasma the plasma generator. generator. TheThe high high frequency frequency coils coils were were used used to exciteexcite plasma,plasma, and and the the low low frequency frequency coils coils were were used used to increase to increaseplasma plasma energy. energy. Based Based on this on this arrangement, arrangement, the the plasma plasma generator, generator, as wellas well as as the the reactive reactive gas gas inlet, inlet,were were further further optimized optimized to to increase increase the the SCD SCD deposition deposition rate. rate. AA mixturemixture ofof ArAr\H\H22\CH\CH4 waswas used used as as workingworking feed feed gas gas with with flow ﬂow rate rate controlled controlled by by the the mass-flow mass-ﬂow controllers. controllers. Before Before deposition deposition process, process, ◦ the thesubstrate substrate was was heated heated to 300 to 300 °C inC an in anacid acid mixture mixture (H2 (HSO24SO:HNO4:HNO3 = 5:1)3 = 5:1)for 30 for min, 30 min, followed followed by by ultrasonicultrasonic clean clean up under up under acetone, acetone, ethanol, ethanol, and and deionized deionized water water to remove to remove excess excess solvent solvent residues. residues. DetailedDetailed deposition deposition parameters parameters are arelisted listed in Table in Table 1. 1.

FigureFigure 1. A 1. diagramA diagram of the of thedual dual radio radio frequency frequency induct inductivelyively coupled coupled plasma plasma (RF-ICP) (RF-ICP) jet chemical jet chemical vaporvapor deposition deposition (CVD) (CVD) system system used used in this in thiswork. work. Table 1. Deposition parameters. Table 1. Deposition parameters. Parameters Value Parameters Value The gas ﬂow of Ar/H2/CH4 3.6 slm/1.2 slm/0.04–0.096 slm The gas flow of Ar/H2/CH4 3.6 slm/1.2 slm/0.04–0.096 slm Temperature 800 ◦C ± 25 ◦C TemperatureChamber pressure 800 °C (8 ±± 250.2) °C kPa ChamberPlasma pressure power 1 (8 kW(HF) ± 0.2) kPa + 9 kW(LF) Plasma power 1 kW(HF) + 9 kW(LF) The gas phase species in dual-RF plasma WERE diagnosed by optical emission spectra (OES) (sofnThe instruments,gas phase species Beijing, in dual-RF China). plasma The deposited WERE diagnosed SCDs were by characterizedoptical emission by X-rayspectra diffraction (OES) (sofn(XRD) instruments, (Rigaku Beijing, Ultima IV,China). Tokyo, The Japan), deposited ﬁeld SCDs emission were scanning characterized electron by X-ray microscopy diffraction (SEM) (XRD) (KYKY (RigakuTechnology, Ultima Beijing, IV, Tokyo, China) Japan), and Raman field spectroscopyemission scanning (Renishaw, electron London, microscopy England). (SEM) (KYKY Technology, Beijing, China) and Raman spectroscopy (Renishaw, London, England).

3. Results and Discussion

3.1. Plasma Diagnosed by Optical Emission Spectra (OES)

OES was applied to diagnose the dual-RF plasma, with the working feed gas of Ar/H2/CH4. OES of plasma was collected through a quartz window under the same instrument parameter setting. The scan range was from 400 nm to 700 nm with a 0.2 nm scanning step.

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3. Results and Discussion

3.1. Plasma Diagnosed by Optical Emission Spectra (OES) Crystals 2018, 8, x FOR PEER REVIEW 3 of 9 OES was applied to diagnose the dual-RF plasma, with the working feed gas of Ar/H2/CH4. OES of plasma was collected through a quartz window under the same instrument parameter setting. Two introduction methods of feed gas were compared. One was the regular feed gas The scan range was from 400 nm to 700 nm with a 0.2 nm scanning step. introduction method, which was used to introduce the mixture of CH4/H2/Ar to pass through the RF Two introduction methods of feed gas were compared. One was the regular feed gas introduction electromagnetic field. The other method was the modified method which was used to introduce the method, which was used to introduce the mixture of CH4/H2/Ar to pass through the RF electromagnetic mixture of H2/Ar to pass through the RF electromagnetic field, form high density H2/Ar plasma beam, field. The other method was the modified method which was used to introduce the mixture of H2/Ar to and introduce the CH4 downstream of the H2/Ar. This means that the methane was dissociated by pass through the RF electromagnetic field, form high density H2/Ar plasma beam, and introduce the CH4 H2/Ar plasma rather than the RF electromagnetic. Figure 2 is the OES result under the different feed downstream of the H /Ar. This means that the methane was dissociated by H /Ar plasma rather than gas introduction methods.2 The insets are the images of the corresponding 2plasma. The observed the RF electromagnetic. Figure2 is the OES result under the different feed gas introduction methods. emission peaks correspond to C2 (471 nm, 516.1 nm, 561 nm swan bands), Hα (656.1 nm), Hβ (486.1 The insets are the images of the corresponding plasma. The observed emission peaks correspond to nm) and CH (388.9 nm and 431.5 nm) for both gas introduction methods, which is a typical spectrum C (471 nm, 516.1 nm, 561 nm swan bands), Hα (656.1 nm), Hβ (486.1 nm) and CH (388.9 nm and for 2diamond deposition, just like that of microwave plasma and DC-arc plasma [28,29]. However, the 431.5 nm) for both gas introduction methods, which is a typical spectrum for diamond deposition, just intensity of C2 and Hα were higher in the modified method than those in the regular method, which like that of microwave plasma and DC-arc plasma [28,29]. However, the intensity of C and Hα were means that the concentration of the H-related radicals and the C-related radicals were higher2 in case higher in the modified method than those in the regular method, which means that the concentration of of modified feed gas introduction method. From the insets, it can be seen that the plasma volume in the H-related radicals and the C-related radicals were higher in case of modified feed gas introduction the modified method was larger than that in the regular method, which is suitable for large area method. From the insets, it can be seen that the plasma volume in the modified method was larger than diamond deposition. As the quality and growth rate of the deposited diamond films depend on active that in the regular method, which is suitable for large area diamond deposition. As the quality and radical densities on the substrate surface, the modified feed gas introduction method was adopted in growth rate of the deposited diamond films depend on active radical densities on the substrate surface, the following deposition procedures. the modified feed gas introduction method was adopted in the following deposition procedures.

Figure 2. Optical emission spectra and plasma torches under two process conditions: (a) old process Figureconditions, 2. Optical (b) improvedemission spectra process and conditions. plasma torches under two process conditions: (a) old process conditions, (b) improved process conditions. Figure3 shows the OES results of the dual-RF plasma. With the methane concentration varying fromFigure 3.3% 3 to shows 8%, thethe intensityOES results of theof the C2 dual-RF(516.1 nm), plasma. Hα (656.1 With the nm), methane Hβ (486.1 concentration nm), CH (431.5 varying nm) fromunder 3.3% different to 8%, the CH intensity4 concentration of the C is2 (516.1 plotted nm), in H Figureα (656.13. Asnm), shown Hβ (486.1 in Figure nm), CH3, a (431.5 slight nm) increase under of differentCH4 concentration CH4 concentration led to ais strong plotted increase in Figure of both3. As C shown and H in related Figure radicals. 3, a slight This increase phenomenon of CH4 is concentrationdue to the CH led4 dissociation to a strong increase reaction. of The both change C and of H H relatedα,Hβ, andradicals. CH intensityThis phenomenon was negligible is due when to theCH CH4 concentration4 dissociation increasedreaction. The from change 4% to 6%,of H however,α, Hβ, and there CH intensity was a noticeable was negligible increase when when CH CH4 4 concentrationconcentration increased was higher from than 4% 6%. to C6%,2 intensity howeve showedr, there awas rapid a growthnoticeable when increase CH4 concentration when CH4 concentrationreached 5%. was The higher condition than of6%. the C plasma2 intensity can showed be deduced a rapid from growth the ratiowhen between CH4 concentration Hβ and H α. reached 5%. The condition of the plasma can be deduced from the ratio between Hβ and Hα. Compared to our previous work [27], the electron temperature (Te) of the plasma was increased from 2.48 eV to 3.92 eV, indicating a strong enhancement of the plasma energy.

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Compared to our previous work [27], the electron temperature (Te) of the plasma was increased Crystalsfrom 2018 2.48, 8 eV, x FOR to 3.92 PEER eV, REVIEW indicating a strong enhancement of the plasma energy. 4 of 9

Figure 3. The emission intensity of the C2 (516.1 nm), Hα (656.1 nm), Hβ (486.1 nm), CH (431.5 nm) with methane concentration varying from 3.3% to 8%.

3.2. Characterization of Diamond Films FigureFigure 3. 3. TheThe emission emission intensity intensity of of the the C C2 2(516.1(516.1 nm), nm), H Hα α(656.1(656.1 nm), nm), H Hβ β(486.1(486.1 nm), nm), CH CH (431.5 (431.5 nm) nm) Figurewithwith methane methane4 shows concentration concentration the effect of varying varyingmethane from from concentrat 3.3% 3.3% to to 8%. 8%.ion on the surface morphology of the deposited diamond3.2. Characterization films. With ofthe Diamond increase Films of methane concentration, the morphology of the as-grown diamond3.2. Characterization film changed of Diamond from stepped Films growth mechanism to overlapping “hills”, which means that Figure4 shows the effect of methane concentration on the surface morphology of the deposited the growthFigure mode 4 shows changed the effect from of 2D methane layer growth concentrat to 3Dion island on the growth surface [30]. morphology From Figure of the4a–c, deposited we can diamond films. With the increase of methane concentration, the morphology of the as-grown diamond seediamond that the films. diamond With filmthe surfaceincrease was of methanecovered byconcentration, step flows, andthe withmorphology the increase of the of as-grownmethane film changed from stepped growth mechanism to overlapping “hills”, which means that the growth concentrationdiamond film thechanged steps frombecome stepped denser growth and moremechanism disordered. to overlapping When the “hills”, methane which concentration means that mode changed from 2D layer growth to 3D island growth [30]. From Figure4a–c, we can see that the reachedthe growth 6%, modesmooth changed growth from surface 2D layerwas achieved. growth to With 3D island further growth increases [30]. of From methane Figure concentration, 4a–c, we can diamond film surface was covered by step flows, and with the increase of methane concentration the overlappingsee that the “hills”diamond can filmbe observed, surface wasthe growth covered mechanism by step flows, changed and to with island the growth, increase which of methane means steps become denser and more disordered. When the methane concentration reached 6%, smooth thatconcentration the crystal qualitythe steps was become decreased denser [31]. and more disordered. When the methane concentration growth surface was achieved. With further increases of methane concentration, overlapping “hills” can reachedThe 6%,film smooth deposition growth rate surface with wasdifferent achieved. methane With concentrationfurther increases is plottedof methane in Figure concentration, 5. The be observed, the growth mechanism changed to island growth, which means that the crystal quality depositionoverlapping rate “hills” increased can be rapidly observed, with the the growth increase mechanism of methane changed concentration. to island After growth, 10 h which deposition, means was decreased [31]. thethat thickness the crystal of qualitythe samples was decreasedincreased by[31]. 73 μm, 80 μm, 125 μm, 161 μm, and 185 μm, respectively. The film deposition rate with different methane concentration is plotted in Figure 5. The deposition ratea) increased rapidly with theb) increase of methane concentration. c) After 10 h deposition, the thickness of the samples increased by 73 μm, 80 μm, 125 μm, 161 μm, and 185 μm, respectively. a) b) c) 30 μm 30 μm 30 μm

30d) μ m e)30 μm 30 μm

d) e) 30 μm 30 μm

FigureFigure 4. 4.SurfaceSurface morphology morphology of ofthe the deposited deposited diam diamondond films. ﬁlms. the the methane methane concentration: concentration: (a) (3.3%;a) 3.3%; (b)( b4%;) 4%; (c) ( c5%;) 5%; (d) (d 6%) 6% and and (e) ( e8%.) 8%. 30 μm 30 μm

Figure 4. Surface morphology of the deposited diamond films. the methane concentration: (a) 3.3%; (b) 4%; (c) 5%; (d) 6% and (e) 8%.

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The film deposition rate with different methane concentration is plotted in Figure5. The deposition rate increased rapidly with the increase of methane concentration. After 10 h deposition, the thickness Crystalsof the 2018 samples, 8, x FOR increased PEER REVIEW by 73 µm, 80 µm, 125 µm, 161 µm, and 185 µm, respectively. 5 of 9

Figure 5. Figure 5. GrowthGrowth rates rates of of the the deposited deposited diamond diamond films, films, with with the the methane methane concentration concentration varying varying from from 3.3% to 8%. 3.3% to 8%. Figure6 shows the Raman spectra of the diamond films. When methane concentration increased Figure 6 shows the Raman spectra of the diamond films. When methane concentration increased from 3.3% to 8%, the diamond peaks were 1330.8 cm−1, 1330.9 cm−1, 1330.9 cm−1, 1330.9 cm−1 and from 3.3% to 8%, the diamond peaks were 1330.8 cm−1, 1330.9 cm−1, 1330.9 cm−1, 1330.9 cm−1 and 1332.1 1332.1 cm−1, respectively. The position of the diamond peak shows obvious negative shift, indicating cm−1, respectively. The position of the diamond peak shows obvious negative shift, indicating a a certain amount of tensile stress existed inside the films. According to Boppart’s study [32], we can certain amount of tensile stress existed inside the films. According to Boppart’s study [32], we can figure out that the specific value of tensile stress was 0.592 GPa, 0.557 GPa, 0.557 GPa, 0.557 GPa and figure out that the specific value of tensile stress was 0.592 GPa, 0.557 GPa, 0.557 GPa, 0.557 GPa and 0.139 GPa, respectively. The tensile stress in the diamond films decreased quickly when methane 0.139 GPa, respectively. The tensile stress in the diamond films decreased quickly when methane concentration was higher than 6%. Compared with the surface morphology, when the methane concentration was higher than 6%. Compared with the surface morphology, when the methane concentration reached 6%, the growth mechanism changed from 2D layer growth to 3D island growth. concentration reached 6%, the growth mechanism changed from 2D layer growth to 3D island We guess that the change of growth mechanism led to the reduction of stress in the crystals. From the growth. We guess that the change of growth mechanism led to the reduction of stress in the crystals. Raman spectra, we can see that, when the methane concentration was higher than 6%, the scattering From the Raman spectra, we can see that, when the methane concentration was higher than 6%, the broadband among 1350.0–1600.0 cm−1 was observed, which came from the sp2 C–C bond. Figure7 scattering broadband among 1350.0–1600.0 cm−1 was observed, which came from the sp2 C–C bond. shows the full wave at half maximum (FWHMs) of X-ray rocking curves and Raman spectrums Figure 7 shows the full wave at half maximum (FWHMs) of X-ray rocking curves and Raman of the deposited diamond films with methane concentration varying from 3.3% to 8%. When the spectrums of the deposited diamond films with methane concentration varying from 3.3% to 8%. concentration of methane increased, the quality of the SCD films displayed a decreased trend. When the concentration of methane increased, the quality of the SCD films displayed a decreased When the methane concentration reached 6%, the deposited films showed a relatively high trend. growth rate and good crystal quality. We tried a 50 h deposition at two 6 × 6 mm2 HPHT seeds at the same time. The fourier transform infrared spectroscopy (FTIR) of the detached grown layer and HPHT diamond substrate are shown in Figure8. The inset is the image of the grown diamond. The transmission coefficient of the deposited film was better than the HPHT diamond substrate. At 1130 cm−1, 1282 cm−1, and 1344 cm−1, deposited diamond film showed lower absorption coefficient, indicating lower nitrogen impurity. The strong bands at 1900 to 2300 cm−1 were the inherent two phonon lines of diamond associated with C–C bonds and were present in both deposited diamond films and HPHT substrates [33]. Figure9 is the high resolution transmission electron microscopy (HRTEM) micrograph of the deposited single crystal diamond and d-spacing of the deposited diamond was measured to be 0.206 nm, coinciding with that of the diamond (111) plane. There were no defects observed, like dislocations and stacking faults, in the detected zone.

Figure 6. Raman spectra of as-grown diamond films when methane concentration varying from 3.3% to 8%.

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Figure 5. Growth rates of the deposited diamond films, with the methane concentration varying from 3.3% to 8%.

Figure 6 shows the Raman spectra of the diamond films. When methane concentration increased from 3.3% to 8%, the diamond peaks were 1330.8 cm−1, 1330.9 cm−1, 1330.9 cm−1, 1330.9 cm−1 and 1332.1 cm−1, respectively. The position of the diamond peak shows obvious negative shift, indicating a certain amount of tensile stress existed inside the films. According to Boppart’s study [32], we can figure out that the specific value of tensile stress was 0.592 GPa, 0.557 GPa, 0.557 GPa, 0.557 GPa and 0.139 GPa, respectively. The tensile stress in the diamond films decreased quickly when methane concentration was higher than 6%. Compared with the surface morphology, when the methane concentration reached 6%, the growth mechanism changed from 2D layer growth to 3D island growth. We guess that the change of growth mechanism led to the reduction of stress in the crystals. From the Raman spectra, we can see that, when the methane concentration was higher than 6%, the scattering broadband among 1350.0–1600.0 cm−1 was observed, which came from the sp2 C–C bond. Figure 7 shows the full wave at half maximum (FWHMs) of X-ray rocking curves and Raman spectrums of the deposited diamond films with methane concentration varying from 3.3% to 8%. When the concentration of methane increased, the quality of the SCD films displayed a decreased trendCrystals. 2019, 9, 32 6 of 9 Crystals 2018, 8, x FOR PEER REVIEW 6 of 9

Crystals 2018, 8, x FOR PEER REVIEW 6 of 9 Figure 6. Raman spectra of as-grown diamond ﬁlms when methane concentration varying from 3.3% Figureto 8%.6. Raman spectra of as-grown diamond films when methane concentration varying from 3.3% to Figure8%. 7. The FWHMs of X-ray rocking curves and Raman spectra of as-grown diamond films with methane concentration varying from 3.3% to 8%.

When the methane concentration reached 6%, the deposited films showed a relatively high growth rate and good crystal quality. We tried a 50 h deposition at two 6 × 6 mm2 HPHT seeds at the same time. The fourier transform infrared spectroscopy (FTIR) of the detached grown layer and HPHT diamond substrate are shown in Figure 8. The inset is the image of the grown diamond. The transmission coefficient of the deposited film was better than the HPHT diamond substrate. At 1130 cm−1, 1282 cm−1, and 1344 cm−1, deposited diamond film showed lower absorption coefficient, indicating lower nitrogen impurity. The strong bands at 1900 to 2300 cm−1 were the inherent two phonon lines of diamond associated with C–C bonds and were present in both deposited diamond films and HPHT substrates [33]. Figure 9 is the high resolution transmission electron microscopy (HRTEM) micrograph of the deposited single crystal diamond and d-spacing of the deposited diamond was measured to be 0.206 nm, coinciding with that of the diamond (111) plane. There were no defectsFigure observed, 7. The FWHMs like dislocations of X-ray rocking and stacking curves and faults, Raman in the spectra detected of as-grown zone. diamond ﬁlms with Figuremethane 7. The concentration FWHMs of X-ray varying rocking from curves 3.3% to and 8%. Raman spectra of as-grown diamond films with methane concentration varying from 3.3% to 8%.

When the methane concentration reached 6%, the deposited films showed a relatively high growth rate and good crystal quality. We tried a 50 h deposition at two 6 × 6 mm2 HPHT seeds at the same time. The fourier transform infrared spectroscopy (FTIR) of the detached grown layer and HPHT diamond substrate are shown in Figure 8. The inset is the image of the grown diamond. The transmission coefficient of the deposited film was better than the HPHT diamond substrate. At 1130 cm−1, 1282 cm−1, and 1344 cm−1, deposited diamond film showed lower absorption coefficient, indicating lower nitrogen impurity. The strong bands at 1900 to 2300 cm−1 were the inherent two phonon lines of diamond associated with C–C bonds and were present in both deposited diamond films and HPHT substrates [33]. Figure 9 is the high resolution transmission electron microscopy (HRTEM) micrograph of the deposited single crystal diamond and d-spacing of the deposited diamond was measured to be 0.206 nm, coinciding with that of the diamond (111) plane. There were no defects observed, like dislocations and stacking faults, in the detected zone.

Figure 8. FTIR of the deposited CVD ﬁlm and HPHT diamond substrate. Figure 8. FTIR of the deposited CVD film and HPHT diamond substrate.

Figure 8. FTIR of the deposited CVD film and HPHT diamond substrate.

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FigureFigure 9. HRTEM 9. HRTEM micrograph micrograph of ofthe the deposited deposited single single crystal crystal diamond. diamond.

4. 4.Conclusion Conclusions InIn this this article, article, we we investigated investigated single-crystal single-crystal diamond diamond growth growth by by dual dual RF-plasma RF-plasma jet jetCVD. CVD. MethaneMethane concentration concentration stronglystrongly influenced influenced the the concentrations concentrations of CH, of C 2CH,,Hβ ,HC2,α ,H andβ, H theirα, and proportions their proportionsin the gas phase,in the gas especially phase, especially C2. The morphology C2. The morphology of the as-grown of the as-grown diamond diamond film changed film changed from step fromflows step to flows islands, to islands, which meanswhich thatmeans the that growth the growth mode mode changed changed from 2Dfrom layer 2D layer growth growth to 3D to island 3D islandgrowth. growth. The SCDThe SCD films films were were rapidly rapidly deposited deposited by dual by dual RF CVD,RF CVD, and and the the growth growth rate rate increased increased with withmethane methane concentration. concentration. To ourTo our knowledge, knowledge, this this was was the firstthe first time time the fast the depositionfast deposition of single of single crystal crystaldiamond diamond by RF by plasma RF plasma CVD CVD has been has realized.been realized.

Author Contributions: Conceptualization, G.-c.C. and H.L.; methodology, H.L. and J.-j.L.; software, Z.-r.L. and Author Contributions: Conceptualization, G.C. and H.L.; methodology, H.L. and J.L.; software, Z.L. and K.X.; K.X.; validation, H.L., Z.-r.L. and K.X.; formal analysis, H.L.; investigation, Z.-j.C.; resources, Z.-j.C.; data curation, validation,H.L.; writing—original H.L., Z.L. and draft K.X.; preparation, formal analysis, H.L.; writing—reviewH.L.; investigation, and editing,Z.C.; resources, G.-c.C. and Z.C.; H.L.; data supervision, curation, H.L.; G.-c.C.; writing—originalproject administration, draft preparation, G.-c.C.; funding H.L.; writing—review acquisition, G.-c.C. and editing, G.C. and H.L.; supervision, G.C.; project administration, G.C.; funding acquisition, G.C. Funding: This work is supported by National Natural Science Foundation of China (50128790), Hundred program Funding:of Chinese This Academy work is ofsupported Sciences, by Hundred National Talents Natura Programl Science of Foundation the Chinese of Academy China (50128790), of Sciences, Hundred Scientific Research Equipment Project of the Chinese Academy of Sciences (yz201356), Hebei Science and Technology Plan program(14291110D), of Chinese Beijing Academy Science and of Sciences, Technology Hundred Project (Z151100003315024).Talents Program of the Chinese Academy of Sciences, Scientific Research Equipment Project of the Chinese Academy of Sciences (yz201356), Hebei Science and Conflicts of Interest: The authors declare no conflict of interest. Technology Plan (14291110D), Beijing Science and Technology Project (Z151100003315024).

ConflictsReferences of Interest: The authors declare no conflict of interest.

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