Single Crystal Diamond Deposited by Dual Radio-Frequency Plasma Jet CVD with High Growth Rate
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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 Crystals 2019, 9, 32; doi:10.3390/cryst9010032 www.mdpi.com/journal/crystals Crystals 2019, 9, 32 2 of 9 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\HnH22\CHnCH4 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. Crystals 2019, 9, 32 3 of 9 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.