Raman characterization of boron-doped {111} homoepitaxial diamond layers

Diamond & Related Materials 15 (2006) 572 – 576 www.elsevier.com/locate/diamond

Raman characterization of boron-doped {111} homoepitaxial diamond layers M. Mermoux a,*, F. Jomard b, C. Tavare`s c, F. Omne`s c, E. Bustarret c a

Laboratoire d’Electrochimie et de Physico-chimie des Mate´riaux et des Interfaces, UMR5631 INPG-CNRS, associated to Universite´ Joseph Fourier BP75, 38402 Saint Martin d’He`res cedex, France b Laboratoire de Physique des Solides et de Cristalloge´ne`se, UMR8635 CNRS-UVSQ, 1 pl. A. Briand, 92195 Meudon cedex, France c Laboratoire d’Etudes des Proprie´te´s Electroniques des Solides, UPR11 CNRS, associated to Universite´ Joseph Fourier. Av. des Martyrs, BP166, 38042 Grenoble 9 cedex, France Available online 23 January 2006

Abstract p-type {111} homoepitaxial diamond layers were grown by Microwave Plasma-Enhanced Chemical Vapor Deposition. The variation of the gas phase boron concentration led to solid-state incorporation of boron in the 6 I 1016 – 3 I 1021 cm 3 range. Confocal Raman spectroscopy and Raman imaging have been used to investigate this series of homoepitaxial films. As already observed for undoped or phosphorous-doped {111} epilayers, a first noticeable feature was the presence of many sharp and weak lines peaking at random in the 500 – 2000 cm 1 range. These peaks were all the most observed that the doping level was low. A number of boron-related Raman lines centered at about 610, 925, 1045 cm 1 were observed for solid state boron concentrations in the 1.5 I 1018 – 9 I 1019 cm 3 range. Above a boron concentration of 3 I 1020 cm 3, the usual Raman signal of heavily boron-doped diamond was recorded. The thickness of the epitaxial layers, in the 0.2 – 2 Am range, was too low to allow a more detailed analysis of the zone-center diamond optical phonon. D 2006 Elsevier B.V. All rights reserved. Keywords: Diamond film; Homoepitaxy; Vibrational properties characterization; p-type doping

1. Introduction Diamond films synthesized by chemical vapor deposition (CVD) are attracting increasing interest in many technological fields because of unique properties. In particular, for electronic applications there is great interest in growing high-quality single-crystal diamond films, and diamond homoepitaxy is currently studied by many research groups. The use of diamond as a semiconducting material for electronics is strictly related to the possibility of doping it in order to control its conductivity. In particular, the growth of high quality phosphorous-doped, {100} homoepitaxial diamond was recently reported [1], which is an important step towards the construction of high quality diamond-based devices. Contrary to the success in realizing {100} epitaxial films, the production of high quality {111} epitaxial diamond is still a difficult task. Growth on {111}-oriented diamond is known to be difficult to control and it may be anticipated that the

* Corresponding author. Tel.: +33 4 76 82 65 48; fax: +33 4 76 82 66 77. E-mail address: [email protected] (M. Mermoux). 0925-9635/$ - see front matter D 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.11.035

performances and the long term stability of {111}-oriented diamond-based devices, for example UV-emitting p– n junctions [2,3], will strongly depend on the structural perfection of the epitaxial layers. As a result, despite its technological relevance, boron doping is still poorly documented for homoepitaxial {111}-oriented films. Two characteristics of {111} homoepitaxial diamond films are noteworthy. The first is the occurrence of cracking in the epitaxial layer. Spontaneous fracture of {111}-oriented diamond films, caused by tensile stress in the layers, has been reported in a number of studies ([4], and references herein). The second is the presence of structural defects (graphite incorporation, stacking faults, . . .) that can be observed by transmission electron microscopy or Raman spectroscopy [5 – 7]. For imaging surface morphology and assessing the integrity of CVD diamond films, optical methods, and Raman spectroscopy in particular are well-established tools. The potential of the method for the study of polycrystalline or homoepitaxial diamond layers has been described recently [4]. In this paper, boron-doped {111} homoepitaxial diamond films were examined by confocal micro-Raman techniques, and the presence of residual stress and structural defects,

M. Mermoux et al. / Diamond & Related Materials 15 (2006) 572 – 576

induced in the epilayers as a consequence of the growth process, was investigated. Beside such a technical motivation, another feature of boron-doped diamond is a complex Raman response with a downshifted zone-center Raman peak and wide bands around 1230 and 500 cm 1, a Fano-like deformation and a lowfrequency electronic scattering, as reported few years ago by various groups. Also observed for doping levels in the 1019 cm 3 range are weak features around 610, 925, 1045, 1375 and 1470 cm 1 [8 –10]. In spite of many efforts, the Raman spectrum of boron-doped diamond remains not completely understood, and a second motivation of this work was to examine in further details the dependence of the Raman spectra as a function of the boron content. More generally, we aim here at opening the way for a more detailed qualitative and quantitative understanding of the specific Raman scattering signature of boron-doped diamond. Preliminary results, obtained for a series of thin – a few Am in thickness – {111} homoepitaxial films covering the 6 I 1016 – 3 I 1021 B/cm3 range are presented. 2. Experimental Different boron-doped samples were grown on 2  2 mm2 polished substrates purchased from Sumitomo Electric Ltd. A miscut angle of 3.0 T 1- with respect to the {111} nominal orientation was measured for some of the substrates. For comparison purposes, nominally undoped or phosphorousdoped films, 1 –4 Am in thickness, and boron-doped {111} single crystals (boron content estimated in the 1019 cm 3 range) were available. Before deposition, substrates were cleaned in a solution of 1 part of percloric acid (HClO4), 3 parts of sulphuric acid (H2SO4) and 4 parts of nitric acid (HNO3) for 1/2 h at 100 -C, and then rinsed in deionized water in an ultrasonic bath. All analysed samples were grown at a temperature of about 1160 K in a fused silica tube deposition chamber at a pressure of 50 Torr. Absolute solid-state boron concentrations and film thickness were deduced from secondary ion mass spectroscopy (SIMS) analysis in a CAMECA IMS4f instrument by comparison with the yield of implanted diamond references measured in the same run. Raman spectroscopy measurements were performed using a Jobin-Yvon/Horiba T64000 triple monochromator equipped with a liquid N2 cooled CCD detector. Ar –Kr+ laser lines at 647.1, 487.9, 514.5, and 363.7 nm were used as excitation source. The first measurements were conducted to record spectra in the 400 –2000 cm 1 wavenumber range. In this case, to increase the signal to noise ratio, a Notch rejection filter was placed before the entrance slit of a single spectrograph equipped with a 1800 or 600 grooves/mm grating. These detection conditions were chosen as an acceptable compromise between resolution and luminosity. Other measurements were conducted to study in detail the diamond line profile. In this case, the triple additive mode of the spectrometer was used. Samples were mounted on a computer-controlled XY table, movable in 100 nm steps. In-depth positioning has been achieved by controlling with a piezoelectrical device (accuracy

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0.05 Am) the vertical position of the microscope objective. For microscopic measurements, an Olympus microscope equipped with a  100 (numerical aperture = 0.95) objective was used. A confocal diaphragm adjustable in size from 10 to 1000 Am provided the lateral and axial resolution of the measurements. To create 2D spectral images, the spectra were processed by using a band fitting procedure. Each individual spectrum has been fitted using a Lorentzian line shape. The parameters of the line can thus be plotted as a function of the spatial coordinates. 3. Results and discussion 3.1. Diamond epilayers Different {111} homoepitaxial films were grown with gas phase boron concentrations ([B]/[C]gas) in the 1 –10000 ppm range. The growth conditions, as well as the film thickness and the growth rates are summarized in Table 1. For the highest [B]/[C]gas ratio used here, the boron concentration reaches 3 I 1021 cm 3. From Table 1, it is seen that the solid state boron concentration increases almost linearly with the [B]/[C]gas ratio up to 100 ppm, and quicker above. The growth rate of the films is first observed to slowly decrease up to a boron concentration in the gas phase of 6000 ppm. Above this concentration, the growth rates of the films drastically decreased down to 0.1 Am/ h. Fig. 1 shows the surface morphology of some of the films. Neither non-epitaxial crystallites nor growth hillocks, which have often been seen in {100} homoepitaxial diamond surface, are observed. All the films are free of cracks. As seen in Fig. 1, most of the samples show two separate domains with different surface morphology, in spite of the initially homogeneous surface preparation. These regions are separated by a clear borderline which is parallel to the sides of the crystals. In the center of the samples, growth steps aligned along a preferred orientation are more or less observed for most of the films examined, within the exception of 6000 and 10.000 ppm samples. In the vicinity of some of the edges of the crystals, an area, 20– 200 Am in width, is also observed which seems to display a lower roughness, as observed with optical microscopy. It may be suspected that this specific morphology results from the growth on off-axis {111} substrates [11]. Table 1 Growth conditions of the {111} epitaxial films examined in this work Sample

B/C (ppm)

B content (cm 3)

Thickness (Am)

Deposition rate (Am/h)

94 95 96 97 107 108 109 113 112 110

1 4 20 100 300 1000 3000 4500 6000 10,000

6.0 I 1016 1.0 I 1017 4.0 I 1017 1.5 I 1018 3.0 I 1019 9.0 I 1019 3.0 I 1020 – 1.4 I 1021 3.0 I 1021

2.05 1.9 1.8 1.6 1.1 1.0 0.9 – 0.36 0.2

0.68 0.63 0.6 0.53 0.55 0.5 0.45 – 0.18 0.1

The solid state boron concentration as well as the growth rate was deduced from SIMS measurements.

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aries. A reason for their observation could be the resonance enhancement by two orders of magnitude for Raman scattering for sp2 bonded structures as compared to sp3 carbon. These features have to be associated with defective CVD diamond growth. Line scans and images were recorded within the two different regions described above, at the vicinity of the edges of the crystals. We did not detect any strong signal difference

(a) 5000 Fig. 1. Typical optical image of the {111} homoepitaxial diamond layers examined in this work. Note the two different regions.

30

3.2. Defects related signals To get representative information for each sample, line scans of Raman spectra were obtained from all the samples. To reduce the probing depth, the confocal hole diameter was kept constant at 50 Am. The green or blue lines of the Ar – Kr+ laser were first used, but the photoluminescence background arising from the substrate, that is the excitation of the H3 and nitrogenvacancy centers, partially obscured the signal of the epitaxial layer. Thus, these measurements have been conducted using the 647.1 nm excitation line. Spectra were obtained over a 40 Am linear distance at 1 Am intervals along the profile. Examples of line scans are presented in Fig. 2. They concern the 20, 1000 and 10,000 ppm samples. A first noticeable feature is the presence of many sharp and weak lines peaking at random in the 500 –2000 cm 1 range. These peaks are all the most observed that the doping level is low. As the doping level is increased, they progressively decrease in number and intensity to become undetectable as the boron content approaches 1020 cm 3. These peaks are similar to those observed during the analysis of undoped or phosphorous-doped {111} epitaxial layers [4]. The width of these peaks is too low to be assigned to any specific amorphous sp2 phase. They were observed using the 487.9, 514.5 and 647.1 nm excitation wavelengths, but not using an excitation in the near UV (363.7 nm). Similar peaks have been observed within grain boundaries in high quality CVD polycrystalline films [9] and during bias-assisted nucleation of diamond for specific bias voltages [12]. In polycrystalline films, a strong correlation has been shown between the presence of these lines and a broad PL signal peaking at about 1.9 eV. In agreement with results presented in Ref. [12], a continuous decrease of the non-diamond spectral features was sometimes observed by extending the exposure time, or by increasing the laser power. This behavior may indicate the involvement of metastable defects. The presence and behavior of these peaks suggest that particular defects, most probably sp2-bonded carbon atoms, or distorted sp3 carbon sites, with or without hydrogen, are incorporated in the diamond lattice during growth. Alternatively, this can be an indication that defects are present within some grain bound-

35 40 500

1000

1500

Wavenumber (cm-1) 1000

(b)

30 35 40 45 500

1000

1500

Wavenumber (cm-1)

(c) 500

30

40

500

1000

1500

Wavenumber (cm-1) Fig. 2. Raman spectra obtained at 1 Am intervals along a 40 Am line. The three line scans correspond to (a) the 20 ppm sample, (b) the 1000 ppm sample and (c) the 10,000 ppm sample. These line scans were used to obtain the average spectra presented in Fig. 3.

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between the two different regions: the ‘‘defect’’ lines were observed with a more or less constant number and intensity. Finally, luminescence spectra were also recorded. For some of the samples, the silicon-related emission at 738 nm was observed. The presence or absence of this signal could not be correlated to the solid state boron concentration. 3.3. Boron-related signals Average spectra were obtained from each line scan by adding all the individual spectra. This is a convenient method to get reproducible spectra with a very high signal to noise ration, that are characteristic of a given sample. This procedure allows very weak signals to be extracted from the background. The evolution of the spectra as a function of the boron content is given in Fig. 3. Spectra have been roughly normalized to the diamond line intensity. For the lowest boron contents, 4 and 100 ppm samples, boron concentration lower than 1018 cm 3, a first feature at about 600 cm 1 is identified. This feature is present on most of the individual spectra, with a more or less constant intensity. At present, this feature has not been observed for other excitation wavelengths, 785 nm in particular [13]. Thus we cannot confirm that this signal arise from Raman scattering. When the boron content [B] is increased (1018 < [B] < 91019 cm 3), other lines peaking at about 610, 925, and 1045 cm 1 progressively appear in the spectra. These lines perfectly correspond to those previously observed in polycrystalline boron-doped CVD diamond films or boron-doped HTHP reference crystals [9] with a similar solid-state boron concentration. Again, these lines are present on all the individual spectra with a more or less constant intensity. During the analysis of reference boron-doped HPHT crystals, other low intensity lines were also observed on the high frequency side of

575

the diamond line, at about 1375 and 1470 cm 1. They are not observed for the present samples because of their weak thickness. Finally, when the boron content is higher than 3 I 1020 cm 3, the Raman signal is further modified and now corresponds to the well described Raman signal of heavily boron-doped diamond crystals or films. In particular, we observe a strong increase of the low frequency electronic scattering and the presence of the signals peaking at 500 and 1230 cm 1. Again, all the individual spectra are similar in shape and intensity. It is worth mentioning that the transition between the two kinds of signals appears between 9 I 1019 and 3 I 1020 B/cm3, that is approximately where the insulator-tometal transition is expected to be [14]. Another interesting feature is the wavelength dependence of the Raman signal. In agreement with other observations [15], the intensity of the boron-related signals, especially the weak signals at 610, 925, and 1045 cm 1, increases with decreasing excitation energy. Obviously, the scattering intensity of these bands does not follow x 4 dependence, where x is the frequency of the excitation laser line. There are different possible explanations of the lines observed in the spectra: the first involves the local vibrational modes of the boron, the second is the phonon line allowed due to the defects or the finite crystal size and the third is the electronic scattering of the boron acceptor. The first hypothesis cannot be accepted because the boron atom, which is lighter than carbon, does not form the local vibrational mode in a range lower than the allowed phonon line of diamond. Taking into account the correlation between these spectral features and boron incorporation as described in this paper, this result suggests that electronic Raman scattering caused by impurity induced electronic states should be taken into account for a complete interpretation of these bands.

Intensity (a.u.)

3.4. Frequency of the diamond line

400

600

800

1000

1200

1400

1600

-1

Wavenumber (cm ) Fig. 3. Raman spectra characteristic of the 4 (lower trace), 100, 300, 1000, 3000, 4500, 6000, 10,000 ppm (higher trace) samples. Spectra were displaced vertically, for clarity.

A second set of measurements has been conducted to study in more details the diamond line profile. Here, spectra have been recorded by carefully focusing the laser at the sample’s surface, and the confocal hole was close to the lowest useful value. In all cases, the zone-center Raman diamond signal from the substrate was observed and prevented the examination of the Raman signal of the epilayer. In particular, for boron concentrations below 3 I 1019 cm 3, the signal remained symmetrical and was near-perfectly fitted using a Lorentzian line shape with a band width in the 1.7– 2 cm 1 range. Thus, for such thin epitaxial films, we were unable to study the expected downshift and broadening of the zone center phonon line of diamond, which can be as high as 10 or 15 cm 1 above the insulator-to-metal transition. From a more technical viewpoint, another conclusion is that, within the exception of one particular sample, there was no evidence for tensile stress in the films. Different Raman images were constructed within different regions of the films. At present, as in previous studies [4], only linear defects were identified on the Raman images that exhibited a weak increase of the width of the diamond line

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and of the photoluminescence background. These linear defects are most probably polishing grooves. 4. Summary p-type {111} homoepitaxial diamond layers were grown by Microwave Plasma-Enhanced Chemical Vapor Deposition. The variation of the gas phase boron concentration led to solid-state incorporation of boron in the 6 I 1016 – 3 I 1021 cm 3 range. Confocal Raman spectroscopy and Raman imaging has been used to investigate this series of homoepitaxial films. As already observed for undoped or phosphorous-doped {111} epilayers, a first noticeable feature was the presence of many sharp and weak lines peaking at random in the 500– 2000 cm 1 range. These peaks were all the most observed that the doping level was low. A number of boron-related Raman lines centered at about 610, 925, 1045 cm 1 was observed for solid state boron concentrations in the 1.5 I 1018 –9 I 1019 cm 3 range. Above a boron concentration of 3 I 1020 cm 3, the usual Raman signal of heavily boron-doped diamond was recorded. The thickness of the epitaxial layers, in the 0.2– 2 Am range, was too low to allow a more detailed analysis of the zone-center diamond optical phonon. In this way, it will be interesting to examine thicker films. Future work will also be conducted to correlate optical and electrical properties of the films. Acknowledgments M.M. would like to thank B. Marcus (LEPMI) and A. Crisci (CMTC/INPG) for technical support and fruitful discussions.

References [1] H. Kato, S. Yamasaki, H. Okushi, Appl. Phys. Lett. 86 (2005) 222111. [2] S. Koizumi, K. Watanabe, M. Hasegawa, H. Kanda, Diamond Relat. Mater. 11 (2002) 307. [3] C. Tavares, A. Tajani, C. Baron, F. Jomard, S. Koizumi, E. Gheeraert, E. Bustarret, Diamond Relat. Mater. 14 (2005) 522. [4] M. Mermoux, B. Marcus, A. Crisci, A. Tajani, E. Gheeraert, E. Bustarret, J. Appl. Phys. 97 (2005) 43530. [5] L.F. Sutcu, C.J. Chu, M.S. Thompson, R.H. Hauge, J.L. Margrave, M.P. D’Evelyn, J. Appl. Phys. 71 (1992) 5930. [6] I. Sakagushi, M. Nishitani-Gamo, K.P. Loh, S. Hishita, H. Haneda, T. Ando, Appl. Phys. Lett. 73 (1998) 2675. [7] M. Kasu, T. Makimoto, W. Ebert, E. Kohn, Appl. Phys. Lett. 83 (2003) 3465. [8] Y. Wang, H. Li, Z. Zhangda, K. Feng, Jpn. J. Appl. Phys. 39 (2000) 2795. [9] M. Mermoux, B. Marcus, G.M. Swain, J.E. Butler, J. Phys. Chem., B 106 (2002) 10816. [10] K. Honda, T. Noda, M. Yoshimura, K. Nakagawa, A. Fujishima, J. Phys. Chem., B 108 (2004) 19117. [11] T. Bauer, M. Screck, H. Sternschulte, B. Stritzker, Diamond Relat. Mater. 14 (2005) 266. [12] M. Schreck, C. Grunick, C. Haug, R. Brenn, B. Stritzker, Diamond Relat. Mater. 11 (2002) 487. [13] These spectra were obtained using an InVia Renishaw spectrometer, in its confocal mode. [14] E. Bustarret, E. Gheeraert, K. Watanabe, Phys. Status Solidi, A 199 (2003) 9. [15] M. Mermoux, B. Marcus, A. Crisci, unpublished work.