Impurity and defect incorporation in diamond films deposited at low substrate temperatures

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Diamond and Related Materials 7 (1998) 193-199

Impurity and defect incorporation in diamond films deposited at low substrate temperatures J. Stiegler a'*, A. B e r g m a i e r b, j. M i c h l e r ~, Y. y o n K a e n e l a , G . D o l l i n g e r b, E. B l a n k a " Ecole Polytechnique F6d~rale de Lausanne, D~partement ties Mat6riaux, CH- 1015 Lausanne, S;i'itzerland b Technische Universitiit Miincheel. Physik Department El2, D-85747 Garching, Germany Received 23 June 1997; accepted 2 August 1997

Abstract

The quality of CVD diamond films degrades severely with decreasing substrate temperatures. In this report, the impurity and defect incorporation in diamond films deposited from a carbon-hydrogen-oxygen gas system at substrate temperatures between 560 and 345 C has been investigated using elastic recoil detection (ERD), FTIR and micro-Raman spectroscopy. In approaching the low temperature limit which coincides with the formation of cauliflower morphologies, the hydrogen incorporation rises steeply. Hydrogen contents beyond I at.% have been measured, roughly 20 times higher than in the upper temperature range. By contrast, there was a much smaller rate of rise in the concentration of nitrogen and oxygen, despite a

marked change in the microstructure of the deposited films. At the lowest substrate temperatures, the absolute hydrogen content measured by ERD increases more steeply than those measured by FTIR spectroscopy, which refers to C-H stretch vibrations only. There is evidence that hydrogen is incorporated also in the bulk rather than being concentrated at grain boundaries as at higher temperatures. Tnis conclusion is supported by micro-Raman spectroscopy exhibiting significant peak broadening h~. the low temperature region. :© 1998 Elsevier Science S.A. Keywords: Low substrate temperatures: Impurities: Characterization; Hydrogen

I. introduction Low temperature growth of diamond (LTGD) lilms represents a promising lield Ibr a large number of industrial applications, involving optical materials, mechanical coatings, semiconductors, and heat spreading devices [I ]. While typical substrate temperatures in chemical vapor deposition (CVD) of diamond range from 750 to 1000 C , far less information is available on growth at lower temperatures as required by numerous substrate materials which cannot be exposed to a high temperature growth process. LTGD suffers from several serious drawbacks [2]. The diamond growth rate usually diminishes with decreasing substrate temperatures because growth kinetics at the diamond surface is a thermally activated process [3]. Incorporation of amorphous carbon and other defects leads to deterioration of the crystalline quality, and thereby the physical properties of the thin film [2]. In view of a widespread commercialization of the low temperature diamond CVD process, these problems must be solved. * Corresponding author. Tel: 41 21 693 3920: Fax: 41 21 693 5891):. e-mail: stiegler(a)mxsgl.eptt.ch 0925-9635/98/$19.00 tl 1998 Elsevier Science B.V. All rights reserved. Pil S0925-9635(97)00164-7

There are only scarce reports in the literature addressing impurity condensation and defect formation in CVD diarnond tih'ns, particularly at low substrate temperatures [4]. In thi,~ paper, the incorporation of impurities in films grown in a microwave plasma-assisted CVD process from a carbon--hydrogen roxygen (C H~O) gas system is investigated. The structural modifications associated with low temperature deposition as observed by transmission electron microscopy ! TEM ) were described in a previous paper [5]. There, it was shown that a twodimensional (2D) nucleation mechanism superseding re-entrant cornet" growth is responsible for the transition to nanocrystalline diamond at lower temperatures. Such changes in growth influence the film morphology and the defect content. The present study provides data, mainly chemical, which are complementary to these LTGD films.

2. Experimental The film deposition was carried out in a classical fttsed silica tube microwave reactor onto (lO0) silicon. The substrate temperature was independently controlled

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J. Stiegler ct al. / Diamond and Rehtted Materials 7 (1998) 193-199

between 560 and 275 :C by an active cooling system fitted to the holder stage. All other d,~pu~itioil parameters were kept constant (microwave power: 600 W, pressure: 80mbar, gas flow: 29/31 sccm CH4/CO2). The temperature of the silicon substrate (T~) was measured by means of an infrared pyrometer directed toward the surface of the substrate. The pyrometer operated at a wavelength of approximately 8 lam where emission from the plasma did not influence the measurements. All films were grown to a constant thickness of 2.5 lam. Two sample series were deposited under identical conditions, because two destructive characterization methods were applied. Both sets delivered very similar results with respect to growth rate, morphology and structural transition. Further experimemal details may be found elsewhere [3,6]. High-resolution elastic recoil detection (ERD) using heavy ions was employed to determine the content of it~apurities like nitrogen, oxygen, and hydrogen. The advantage of this method is that it simultaneously measures and quantifies different impurity concentrations with very high sensitivity [7]. Probing by ERD concerned only the near-surface region of the films, down to a depth of approximately 1 lam and did not include the interfacial region. For comparison, carbonhydrogen (C-H) stretch vibrations in the near-infrared range (3050--2750cm -t) were observed by means of Fourier transform infrared (FTIR) spectroscopy. These non-destructive measurements wea'e performed in transmission and involved the entire film section including the interracial region. A thin, solution-cast polymer tilm of known thickness (polypropylene, 5(1 nmt was used to calibrate the C~H stretch absorbance. The hydrogen content of the lilms then was estimated by comparing the areas under the stretch bands in the 3050..... 2750 cm t spectral range. Micro-Raman spectra were recorded using a DILOR XY 800 spectrometer (Ar + laser, 514nm). The laser spot size of approximately 1 ~tm was smaller than the observed morphological grain size. The spectra were taken from thin foils (thickness: several 100 nm) originally prepared for and investigated by TEM [5]. The probed volume thus was very similar to the one probed by ERD, For each sample, the laser power was reduced until peak shift and broadening caused by sampl'e heating had disappeared (down to I mW for the sample with the lowest quality). The position of the diamond peak was repeatedly calibrated using a large type-lb HPHT diamond. The po::ition a~d shape of the characteristic zerocentered phonon peak of diamond, situated at 1332.5 cm - t, is influenced by stress, impurities and grain size (phonon confinement). In order to eliminate stress effects, the Raman spectra were taken from grains almost disconnected from the edge of the electron transparent area of the thin foils. In this way, peak shift

and broadening can be attributed to impurity and grain size effects only.

3. Results

Representative microstructures of two films grown at two different substrate ~emperature~ are shown in Fig. 1. At elevated temperatures, the only observable ci'ystalline defects are isolated twin boundaries running through whole grains. While the morphological grain size in the order of 1 ~tm is retained at much lower temperatures, such crystals contain a very high density of microtwins and stacking faults with spacings down to 10 nm, giving rise to numerous incoherent twin boundaries and amorphous inclusions [5]. This change of microstructure has been attributed to the transition from re-entrant corner growth to 2D-nucleation on {111 } surfaces, as explained elsewhere [5]. Macro-Raman spectra of these films indicated a disappearing diamond peak, increasing intensities of defect bands in the 1400-1500 cm-~ region commonly referred to as disordered or amorphous carbon [8], as well as a strongly enhanced luminescence background [9]. While the growth rate follows an Arrhenius-type relationship with an apparent activation energy of 5 +0.4 kcal mol- ~ in the upper range of substrate temperatures, no such correlation exists any more in the temperature range below 430 '~C (Fig. 2). This unusual behavior of the growth rate coincides with the structural transition reported earlier [5]. The absolute concentrations of the impurities nitrogen and oxygen measured by ERD are shown by Fig. 3. Although the crystalline quality deteriorates at low temperatures, the nitrogen (approximately 100 ppm) and oxygen (approximately 400 ppm) concentrations remain low. There is a trend for increasing oxygen and slightly decreasing nitrogen incorporation with decreasing temperature. Hydrogen incorporation has been investigated by FTIR spectroscopy and ERD. Concerning the former method, all films revealed more or less clear signals originating from C-H stretch vibrations in the nearinfrared range (3050--2750cm-t), see Fig. 4. The hydrogen content associated with C-H stretch bands has been plotted in Fig. 5 where hydrogen is shown to rise to almost 2 at.% when the substrate tetnperature is lowered to 345 C . Fig. 5 additionally compares the hydrogen contents obtained fi'om FTIR and ERD, the latter method measuring absolute concentrations. While the hydrogen content in the upper temperature range remains almost constant, a steep increase is noticed for both methods beginning from 430 '~'C downwards. The differences between both measurements mainly are due to difl'erent probe depths as will be discussed later..,No stretch or bend modes involving nitrogen o~" oxygen

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bonds could be detected by FTIR spectroscopy, contirming the relatively low levels of these irnpurities alreacty found by ERD. Micro-Raman spectra taken from grains located at the edges of the electron transparent areas of the various thin foils are shown by Fig. 6. The diamond component can be clearly discerned for all films, but the peak shape undergoes a major change with decreasing temperature, which mainly is due to broadening. In an earlier article,

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the same group of authors reported on the disappearance of the diamond peak in macro-Raman spectra of tilms grown.in the low substrate temperature range [6]. This statement must be corrected because very faint diamond peaks appear in the macro-Raman mode when extremely long integration times are applied. Origin:,!ly, this

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4. Discussion

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information was hidden in the background because of line broadening. Observation of the diamond peak on thin TEM specimens in micro-mode takes advantage of peak narrowing by stress relaxation during fihn preparation tbr TEM. The phonon confinement model, which predicts asymmetric line broadening and peak shift to lower wavenumbers for decreasing crystal sizes [li}], was used to analyze observed peak shift and broadening in more detail. Based on the mean domain size determined from X R D measurements, see ref. [5 ], peak shift and broadenhag according to phonon confinement were calculated. it turns out from a comparison with micro-Raman measurements of the line widtL :~c~: fable 1, that the measured dmmond peaks are .:...,,,,-',~erflly broader than predicted by the phonon continemertI model•

The following discussion lbcuses on impurity incorporation during low temperature growth of diamond from C-O-rich C-H-O gas mixtures. The experimental aspects of low temperature diamond growth from this particular gas systenl have been reported previously [6]• The interpretation of the present dala is closely related to the structural development of these films which have been discussed in detail elsewhere [5]. Concerning nitrogen, it has been suggested that this impurity if supplied from the gas atmosphere is incorporated honlogeneously in bulk diamond [11]. Incorporation coefficients given in the literature which express the nitrogen concentratit,n in the diamond lilm with respect to the nitrogen to carbon ratio in the gas phase range from 5 x 10- 4 for ( ! ! 1)-homoepitaxial films [12] to 4 x 10 -3 for polycrystalline films [11]. For the experiments described here, the gas phase nitrogen concentration was not known, but it was likely to be constant. Although there might be a trend for a slightly decreasing nitrogen incorporation with decreasing substrate temperature {see Fig. 3) a clear correlation cannot be established because the variation of nilrogen is within the experimental errors, it should be noticed that the nitrogen incorporation can be suppressed if its gas phase concentration is lowered sufficiently [11]. However, it is not necessarily useful to further reduce this type of impurity, especially at low temperatures already low, because the growth rate might be reduced as well, similar to what has been reported for high deposition temperetures [ 13]. Oxygen, existing ie the gas phase in great excess, is incorporated in the films with a clear trend for higher concentrations with decreasing temperature. Surprisingly, films grown under similar conditions fi'om

197

J. Stiegler et ul. , Diamond ami Rehtted Materials 7 (1998) 193 199

Tab[e I Domain size determined by X R D [5] and the corresponding line shift and broadening of the diamond Raman peak according to the phonon confinement model, compared with micro-Raman measurements of the samples of Fig. 6. The bottom line of the table gives domain size and peak broadening as calculated from the experimental peak shift via the confinement model Substrate temp. I C I

HPHT 560 430 345 345

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Peak broadening, FWHM (cm-~)

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0 < 0.09 0.15 0.63 2.41

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9

the "conventional' CH4-H:, gas system in the same reactor exhibited only slightly smaller oxygen impurities. Although leaks and etching of the silica tube might have increased the oxygen contamination in the plasma, gas phase concentrations of oxygen were several orders of magnitude higher in the CH4-CO_, system compared to the CH4-H2 system (by a factor greater than l 0 4 according to estimations from optical emission spectroscopy). From this observation it must be concluded that the incorporation of oxygen either does not scale with its partial pressure in the gas phase or becomes saturated at a rather low gas phase level, as a result of condensation at structural defects, for example. This latter interpretation could be rationalized with the observed dependence on substrate temperature, because the concentration of structural defects also increases with decreasing temperature. Anolher aspect concerns the participation of oxygen in the growth process [14]. If the growth surface were terminated by oxygen containing species like CO to any noticeable extent [15], although this seems unlikely in the concurrence of atomic hydn'ogen [16], one would expect the oxygen content of the film to depend on the oxygen concentration in the gas phase. In such a case, some of tim surface terminating oxygen atoms may be supposed to be captured by the solid, in grain boundaries, for example, similar to what occurs with hydrogen. The present observations do not provide evidence for such a mechanism. FTIR and ERD collect signals of different physical origin. ERD as a nuclear method measures irnpurities at an absolute scale whereas FTIR in the spectral range 3050-2750cm-~ responds to the various C H stretch vibrations, see ref. [17]. Measurement of hydrogen via FTIR thus is more restrictive than ERD as all hydrogen is supposed to be bonded in CH, CI-12 c,r CH3 groups. Another fundamental difference between the two methods refers to the probe depth comprising the entire tiim section in FTIR, but only a layer of the order of 1 lam close to the fihn surface in ERD. Concerning hydrogen in po!ycrystalline diamond

films, McNamara et al. [18] have pointed out that for concentrations below 0.1 at.%, corresponding to substrate temperatures of 475 :C and above in the present case, hydrogen can be found preferentially at grain boundaries terminating the crystal surfaces and giving rise to C - H stretch vibrations in FTIR spectroscopy. It is very likely that hydrogen found in the upper temperature range (475 :C and above) predomi,.;antly is located at morphological grain boundaries, those which are observed by SEM, and that the differences between FTIR and ERD in this range are to be attributed to the different probe depths. This interpretation is justified as ibllows. Beginning with a nucleation density of about 5 x 10 ~° cm ~ -" which was constant for all films, competitive growth leads to rapid grain coarsening and about 2 ~tm large grains are found at the surlace of 2.5 l.tm thick tihns. Making the assumption that {i) the grain sizc increases linearly with film thickness and {ii) the grain boundaries constitute l l l l l surfaces with a hydrogen coverage of 3.14 × l 0 t5 c m • i~ydrogcn contents of 5 x 10 'z at.% for the upper layer of ! lain thickness, seen by ERD, are calculated. On the other hand, values about one order of magnitude greater are obtained if the variation of grain boundary density across the film section is taken into account. This latter situation is representative of FTIR. Both methods thus can be supposed to provide equivalent hydrogen concentrations. With decreasing temperature (430 C or less), the morphological grain size does not change much and the live-fold increase of the FTIR or the more than 20-fold increase of the ERD signals cannot be explained without postulating a several-fold specific grain boundary occupancy with hydrogen. According to high resolution electron microscopy this is not the case and it is more reasonable to link hydrogen incorporation to the defect evolution. At high temperatures where growth occurs with an activation energy of 5 kcal moi ~, the density of grqwth sites is limited by the number of ledges emitted from re-entrant corners [5]. Toward lower temperatures, the deviation of the growth rate from an Arrhenius

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J, Stiegler et aL / Diamond amt Rektted Materials 7 (1998) ! 93-199

behavior coincides with the beginning of nano-crystalliue diamond formation, 2D-nucleation on top of existing terraces enables many more growth species attached to the surface to become incorporated in the lattice, i.e. the number of new growth sites grows substantially with decreasing temperature. If it is imagined that hydrogen does not escape when terraces originating from 2D-nucleation impinge on each other, the impressive increase in hydrogen in the low temperature range, stated independently with both methods, becomes plausible. Even incoherent grain boundaries and amorphous inclusions, where much more hydrogen can be included, have been reported in a previous paper [5]. Both the Raman diamond and the XRD peaks, see ref. [4], experience considerable broadening in the lower temperature range. It should be noticed, however, that already in the upper temperature range, where the domain size is in the order of 1 lam and the diamond lattice according to TEM is virtually free of two-dimensional defects, the diamond line gradually broadens with decreasing temperature. If the XRD domain size for temperatures of 430 °C and above is used as input for the confinement model, only the non-existent experimental peak shift but not peak broadening can be reproduced, see Table 1. At low temperature (345 :'C in Table 1), the peak shift according to phonon confinement is too small. The influence of long-range stresses to explain the observed shift can be ruled out° Therefore, the observed peak shift was taken to calculate an equivalent domain size of 9 nm via the confinement model, which is in reasonable agreement with high resolution TEM [5]. Whichever approach is taken, the experimental peak width always is signilicantly larger than broadening owing to the domain size. The remaining line broadening of the Raman diamond peak thus should be the result of reduced phonon lifetime related to phonon scattering at impurities [19]. The structural units responsible for scattering could be vacancies with hydrogen terminated carbon bonds as described by Mehandru et al. [20] or hydrogen captured between impinging ledges as proposed before.

5. Conclusions The deposition of diamond films by PACVD from CO-rich C-H-O gas mixtures at low subst~ate temperatures (560-.345 'C) is accompa~aied by the incorporation of hydrogen, nitrogen and oxygen impurities. The nitrogen content depends only weakly on t~mperature. The formerly defined low temperature limit for diamond growth coincides with the beginning of n~aocrystalline diamond formation and is characterized by an increase of the oxygen and a steep rise of the hydrogen (up to 2 at.%) concentrations with decreasing temperature. In the temperature range beyond 750 ~C, hydrogen concen-

trations as found here at the lowest deposition temperature only have been observed at the interface of heteroepitaxial layers [21]. Nitrogen is supposed to be homogeneously incorporated in the bulk from the gas phase. Although the sites for oxygen incorporation could not be clearly defined, there seems to be a preference for condensation on defects. Hydrogen incorporatioti is concentrated on grain boundaries in the upper temperature range. From the analysis of broadening of the diamond Raman peak it is concluded that substantial amounts of hydrogen are incorporated in the bulk at low temperatures. Growth of high quality diamond requires very low nitrogen levels in the gas phase and suppression of crystalline defects acting as sites for impurity condensation. On the other hand, high defect densities as a consequence of 2D-nucleation can accelerate growth and counteract the effect of decreasing temperature on thermally activated growth.

Acknowledgement This work is supported by the "Kommission ftir Technologie und Innovation', Bern (Switzerland), by the 'Beschleunigerlaboratorium der LudwigMaximilian-Universit/it Mtinchen und der Technischen Universit/it Mtinchen' (Germany), and by the 'Deutsche Forschungsgemeinschaft' (DFG) carried out under the trinational 'D-A-CH' German, Austrian and Swiss cooperation on the 'Synthesis of Superhard Materials'.

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