Characterization of homoepitaxial CVD diamond grown at moderate microwave power

Diamond & Related Materials 15 (2006) 517 – 521 www.elsevier.com/locate/diamond

Characterization of homoepitaxial CVD diamond grown at moderate microwave power M.G. Donato a , G. Faggio a , G. Messina a,⁎, R. Potenza b , S. Santangelo a , M. Scoccia c , C. Tuvé b , G. Verona Rinati c a

INFM, Dipartimento di Meccanica e Materiali, Facoltà di Ingegneria dell'Università, Località Feo di Vito, 89060 Reggio Calabria, Italy b INFN, Dipartimento di Fisica, Università di Catania, Italy c INFM, Dipartimento di Ingegneria Meccanica, Università di Roma Tor Vergata, 00133 Roma, Italy Available online 26 January 2006

Abstract In this paper, we report on the characterization of homoepitaxial CVD diamond grown onto HPHT Ib diamond substrates by varying systematically the methane to hydrogen ratio in the deposition gas mixture (1–7%) and the microwave power (520–720 W). Growth rates up to approximately 2.2 μm/h have been achieved. X-ray diffraction, Raman spectroscopy and photoluminescence (PL) have been used to characterize the diamond samples. Raman measurements point out an excellent crystalline quality and phase purity of the homoepitaxial specimens even at the highest CH4 concentration used. Completely flat PL spectra registered in a wide energy range (1.7–2.7 eV) exclude impurity contamination of the diamond samples. Such results show that homoepitaxial CVD diamond can be grown, at moderate microwave power and with moderate growth rate, preserving a good crystalline quality. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond film; Plasma CVD; Homoepitaxy; Defect characterization

1. Introduction Diamond possesses many outstanding physical properties which make it an ideal candidate for a number of technological applications where extreme operating conditions are required. In fact, its transparency to visible light, high breakdown field, high radiation hardness and chemical inertness would allow the realization of solar-blind UV detectors or high-energy particle detectors, which might substitute Si-based devices in harsh environments. However, in spite of the promising characteristics of the material, diamond-based electronics has to face up to some issues regarding mainly the material quality, its largescale availability and costs. Natural diamond is not proposable as an engineering material, because it lacks in standardization, is expensive and sometimes may present the inclusion of defects and impurities that drastically compromise its electronic ⁎ Corresponding author. Dip. Meccanica e Materiali, Facoltà di Ingegneria, Università di Reggio Calabria, Via Graziella, Località Feo di Vito, 89100 Reggio Calabria Italy. Tel.: +39 0965 875485, +39 0965 875317, +39 349 3605802 (mobile); fax: +39 0965 875201. E-mail address: [email protected] (G. Messina). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.11.040

properties. Thus, a great effort has been devoted to establish highly reproducible deposition processes allowing the largescale synthesis of diamond crystals. Specimens grown by High Pressure–High Temperature (HPHT) methods have reduced sizes and generally contain many impurities as nitrogen or metal catalysts. In addition, the deposition equipment is quite expensive. The most widely used technique to produce synthetic diamond is chemical vapor deposition (CVD), due to the high reliability of the deposition process. However, the polycrystalline nature of the heteroepitaxial CVD samples constitutes a serious limitation to its technological exploitation. In fact, grain boundaries act as preferential sites for the incorporation of defects and impurities that may reduce drastically the electronic properties of polycrystalline films. Thus, recently, the scientific community has devoted much interest to the deposition of diamond onto diamond substrates, resulting in single-crystal specimens. The very high values of the electron and hole mobility measured on homoepitaxial CVD diamond samples [1] suggest that such material might be successfully used for the realization of diamond-based highly performing electronic devices. However, the optimization of the deposition process leading to very high-quality diamond samples is not

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complete yet. Japanese researchers (ETL, Tsukuba) [2–4] succeeded in depositing atomically flat homoepitaxial diamond using a very low methane concentration (approx. 0.05%) in the deposition gas mixture; however, the growth rates were too low (tens of nanometers per hour) for the technological exploitation of the material. To enhance the growth rate, different routes have been proposed. Teraji et al. [5,6] using a CVD process at high microwave power (approx. 3.8–4.2 kW), succeeded in depositing flat homoepitaxial samples at higher methane concentrations (approx. 4%) and relatively higher growth rate (approx. 2.5 μm/h). Great advances in the deposition of high-quality homoepitaxial diamond have been made by the French group at LIMHP/CNRS [7–9]. In particular, these authors used both high microwave power (3.2 kW) and etching of the diamond substrate with a O2–H2 plasma before the deposition [7,8]. In these conditions, excellent quality samples have been grown at a growth rate of approximately 6 μm/h. To further improve the growth rate, they attempted to increase both the methane concentration in the gas mixture and the microwave power [9]. However, in these conditions the diamond samples exhibited the inclusion of impurities, such as Si and N, probably due to overheating and etching of the reactor windows [9]. To avoid this problem, the same authors proposed a pulsed wave process, which allowed the deposition of highquality, impurity-free samples at a high growth rate of approximately 22 μm/h. Bogdan et al. [10] succeeded in depositing thick samples (∼120 μm) with near-atomically flat surface making a pre-etching of the diamond substrate, but using a considerably lower microwave power (1000 W). Finally, to suppress the formation of unepitaxial crystallites, Bauer et al. [11] grew homoepitaxial diamond onto slightly off-axis substrates at 1400 W microwave power and 10% CH4 in H2. In a previous work [12], we have obtained preliminary results about the deposition of homoepitaxial CVD diamond onto unselected diamond substrates. Accordingly to the literature results [7–11], we have found that the quality of the initial diamond substrates has a driving effect on the final morphology of the deposited single-crystal diamond samples, even if their crystalline quality and phase purity may be very high. Thus, in this work, we report on the growth and characterization of homoepitaxial CVD diamond samples grown onto nominally identical HPHT Ib diamond substrates (Sumitomo), by varying systematically the methane concentration in the CH4 / H2 gas mixture (from 1% to 7%) and the microwave power (from 520 to 720 W). Aim of this study is to establish if at moderate microwave powers, which are by far the most commonly available in research laboratories, it is possible to find deposition conditions which guarantee the growth of detector-grade homoepitaxial CVD diamond. We will show that diamond samples of excellent crystalline quality (linewidths lower than 1.7 cm− 1) and purity can be obtained even at the highest CH4 concentration used. This study suggests the possibility to find a different working condition of the growth reactor (microwave power, methane concentration, pressure and flow of the gas mixture), enabling to obtain samples of acceptable quality for electronic applications at growth rates sensibly higher than those we have previously obtained with polycrystalline samples [13].

2. Experimental Homoepitaxial diamond samples were deposited in a properly modified microwave plasma chemical vapor deposition tubular reactor [14] onto HPHT type Ib single-crystal diamond substrates (Sumitomo) at the University of Rome Tor Vergata. The samples have been deposited, at fixed CH4 / H2 concentration, by varying systematically the microwave power or, at fixed microwave power, by varying systematically the methane concentration in the growth mixture. Details of the deposition are reported in Table 1. Before the homoepitaxial growth, the standard chemical cleaning procedure of the diamond substrates was carried out. The Raman scattering measurements were carried out at room temperature with an Instrument S.A. Ramanor U1000 double monochromator, equipped with a microscope Olympus BX40 for micro-Raman sampling and with an electrically cooled Hamamatsu R943-02 photomultiplier for photon-counting detection. The 514.5 (2.41 eV) and 457.9 nm (2.71 eV) lines of an Ar+ ion laser (Coherent Innova 70) were used to excite Raman scattering. Using a ×100 objective, the laser beam was focused to a diameter of about 1 μm. A depth resolution of about 4 μm was obtained with a confocal aperture of 200 μm. To minimise the slit-induced line broadening of the diamond Raman peak, a very narrow slit aperture (50 μm) was used. The resolution of the double monochromator is approximately 0.15 cm− 1. Micro-photoluminescence (PL) measurements were carried out at room temperature by using the same experimental set-up used for micro-Raman spectroscopy. The PL spectra have been taken in the spectral region 1.7–2.7 eV. 3. Results and discussion In Fig. 1, the X-ray diffraction spectrum of the sample grown at the highest CH4 concentration (SCD27, 7% CH4 / H2) is shown. Only the (400) reflections for the Cu Kα1 and Cu Kα2 are observed, thus pointing out the single crystalline structure of the sample. The lattice parameter, calculated by Bragg's law, is d = 3.567 Å. In the inset, the rocking curve of the (400) reflection is shown. A FWHM = 0.06° is obtained, that represents the instrumental limit of the X-ray diffractometer. In Fig. 2, one of the diamond Raman peaks (circles) registered on the samples and its Lorentzian fit (solid curve) are shown. The peak position (1331.8 cm− 1) and the FWHM (1.6 cm− 1) witness the excellent crystalline quality of the samples, even higher than the diamond substrate Table 1 Details on the deposition characteristics of the homoepitaxial CVD diamond samples studied in this work Sample

CH4 / H2 (%)

PMW (W)

Growth rate (μm/h)

SCD28 SCD25 SCD23 SCD24 SCD26 SCD27

1 1 1 2 4 7

520 620 720 720 720 720

0.47 0.84 0.92 1.10 1.67 2.15

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12000

x10 (400)

10000 8000

FWHM=0.06° d=3.567 Å

6000 4000 2000 0 59.5

59.6

59.7

59.8

59.9

60.0

θ (deg)

0 30

60

90

120

2nd order Raman band

Normalised PL intensity (a. u.)

Intensity (a.u.)

Intensity (a.u.)

12000

6000

519

500

1500

Raman peak

2500

Wavenumber shift (cm-1)

2θ (deg) Fig. 1. X-ray diffraction spectrum (Cu anode) of the sample SCD27. In the inset, the rocking curve of the (400) reflection is shown. The lattice parameter d and the full width at half maximum (FWHM) of the peak are also indicated.

(linewidth approx. 2 cm− 1). The unshifted value of the peak position indicates that almost no stress distribution is present in the film. Linewidth values of the diamond Raman peak comparable to those observed in the present work have been obtained on thicker samples grown at different deposition conditions, namely performing a pre-treatment of the substrate with an O2/H2 plasma [10], and/or utilising higher microwave power [7–9] for the diamond deposition, or by carefully controlling the misorientation angle of the (100) substrate [11]. In this respect, we remind that, besides the standard cleaning procedure, no special pre-treatment of the substrates has been performed. Several diamond Raman peaks have been registered on each sample, to calculate statistically significant average values of position and width, which, in turn, may give information on the overall quality of the sample and on its spatial homogeneity. It has been observed that, within the experimental errors, the crystalline quality of the different samples is quite comparable.

1.8

2.0

2.2

2.4

2.6

PL energy (eV) Fig. 3. Photoluminescence spectrum registered on the same sample of Fig. 2. Inset: enlarged view of the same spectrum in wavenumber shift units, showing the second order Raman band of the diamond structure. Excitation wavelength λ = 457.9 nm.

Moreover, it is worth noting that a good crystalline quality is observed even in the samples grown at the highest CH4 concentration, which suggests that in the present deposition conditions, the growth rate can be increased from 0.92 to 2.15 μm/h, by increasing the methane concentration in the growth mixture, without worsening the sample crystalline quality. In the same way, the increase of microwave power from 520 to 720 W at 1% CH4 / H2 does not cause appreciably variations in the crystalline quality of the samples. In Fig. 3, a typical photoluminescence spectrum obtained from the samples is shown. Besides the sharp diamond Raman peak, no spectral features originating from non-diamond carbon phases and/or N-impurity inclusions are observed in the 1.7– 2.7 eV energy range, pointing out an excellent purity of the deposited samples. Moreover, in the inset an enlarged view of the same PL spectrum is shown in wavenumber shift units,

1320

Normalised PL intensity (a.u)

Raman signal (a. u.)

X pos= 1331.8 cm-1

1.6 cm-1

1326

1332

1338

st

1 order Raman peaks

CH4/H2=7% CH4/H2=4% CH4/H2=2% CH4/H2=1%

1344

Raman shift (cm-1)

0

2000

4000

6000

8000

Wavenumber shift (cm-1) Fig. 2. Representative diamond Raman peak (circles) registered on one of the samples studied of this work (SCD25) and its Lorentzian fit (solid curve). The values of the position and width of the peak are also indicated. Excitation wavelength λ = 514.5 nm.

Fig. 4. Photoluminescence spectra registered on the homoepitaxial samples studied in this work. Data have been displaced vertically for clarity. Excitation wavelength λ = 457.9 nm.

M.G. Donato et al. / Diamond & Related Materials 15 (2006) 517–521

Normalised intensity (a. u.)

520

CH4=2.2 % CH4=1.8 % CH4=1.4 % CH4=1 % CH4=0.6 %

0

2000

4000

6000

8000

Wavenumber shift (cm-1) Fig. 5. Photoluminescence spectra registered on polycrystalline samples grown at progressively higher CH4 concentration [16]. Data have been displaced vertically for clarity. The solid lines have been drawn as a guide for eyes. Excitation wavelength λ = 514.5 nm.

clearly exhibiting the second order Raman band of the diamond structure [7–9,15]. This band is very weak if compared with diamond first order Raman peak and, as it happens in polycrystalline samples, may be easily obscured by spectral features originating from defects and impurities embedded in the diamond matrix. Thus, the observation of the second order Raman band may be regarded as a qualitative indicator of the purity of homoepitaxial diamond [7–9]. The PL spectra obtained by increasing systematically the CH4 concentration are shown in Fig. 4. All the spectra are dominated by the sharp diamond Raman peak, thus indicating the great purity of the samples even at the highest methane concentration used (7% CH4 / H2). This fact represents an important progress in CVD diamond synthesis, because in polycrystalline samples the quality of the diamond grains, although large in size, is extremely sensitive to the increase of both CH4 concentration and in the microwave power [13,16]. In fact, the local crystalline quality of polycrystalline samples decreases, as witnessed by the higher PL background in Fig. 5, when the methane concentration in the CH4 / H2 gas mixture increases from 0.6% to 2.2%. In addition, in case of polycrystalline samples grown on Si substrates, the utilization of high microwave powers would result in detrimental incorporation of Si atoms in the growing diamond film, due to the etching of the Si substrate by the highly reactive plasma [17]. Thus, in homoepitaxial CVD diamond deposition, the growth rate can be increased using both higher CH4 concentrations in the growth mixture and higher microwave power without compromising the crystalline quality and phase purity of the obtained diamond samples. However, an extremely weak feature at about 8289 cm− 1 (approx. 1.68 eV) is observed only in the spectrum of the SCD27 sample (7% CH4 / H2) (not shown in Fig. 4), which indicates the presence of the Si–V defect centre already observed by other authors [8,9]. The contamination has been attributed [9] to the etching of the reactor quartz walls during the deposition. Further analy-

sis must be done to properly evaluate this possible source of contamination of the samples and to determine to what extent it is possible to increase the microwave power without lowering significantly the homoepitaxial diamond quality. Moreover, as a further indirect confirmation of the very high quality of the homoepitaxial samples grown at moderate microwave power, these samples have been utilized to fabricate UV and particle detectors [18,19]. In particular, the UV detectors show a very good signal to noise ratio and no persistence of photocurrent nor pumping effects [18], which are the common limiting factors in the polycrystalline-diamond based UV photodetection. Also particle detectors based on homoepitaxial diamond have shown interesting detection properties, such as very high energy resolution of about 1.1% and collection efficiency close to 100% [19]. 4. Conclusions In this work, a characterization study of homoepitaxial CVD diamond by means of X-ray diffraction, Raman spectroscopy and photoluminescence has been presented. The samples have been deposited onto identical HPHT Ib diamond substrates by varying systematically the CH4 / H2 ratio in the deposition gas mixture and the microwave power. XRD measurements have shown that the deposited samples have a good crystalline structure. High resolution Raman measurements have shown unshifted diamond peaks having linewidths lower than 1.7 cm− 1, which are even better than the values measured on the diamond substrates. Moreover, such narrow lines have been measured also on the samples grown at the highest methane concentrations, confirming the excellent quality of the deposited samples. As a further confirmation of their very good crystalline quality, the diamond second order Raman band has also been observed in all the deposited samples. Photoluminescence measurements at room temperature have shown completely flat spectra, thus pointing out that no impurity contamination is present in these samples, except for the very weak PL feature related to the Si–V optical centre observed only at the highest CH4 / H2 concentration (7%). Thus, we can deduce that high-quality homoepitaxial CVD samples can be grown at moderate microwave powers, which are still the most widely used in the research laboratories. The growth rates can be increased by increasing the methane concentration in the deposition gas mixture without compromising the crystalline quality and the phase purity of the samples. References [1] J. Isberg, J. Hammersberg, E. Johansson, T. Wikstrom, D.J. Twitchen, A.J. Whitehead, S.E. Coe, G.A. Scarsbrook, Science 297 (2002) 1670. [2] H. Watanabe, D. Takeuchi, S. Yamanaka, H. Okushi, K. Kajimura, T. Sekiguchi, Diamond Relat. Mater. 8 (1999) 1272. [3] D. Takeuchi, H. Watanabe, S. Yamanaka, H. Okushi, K. Kajimura, Diamond Relat. Mater. 9 (2000) 231. [4] H. Okushi, Diamond Relat. Mater. 10 (2001) 281. [5] T. Teraji, S. Mitani, C. Wang, T. Ito, J. Cryst. Growth 235 (2002) 287. [6] T. Teraji, M. Hamada, H. Wada, M. Yamamoto, K. Arima, T. Ito, Diamond Relat. Mater. 14 (2005) 255.

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