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Excitonic recombinations and energy levels of highly boron doped homoepitaxial diamond films before and after hydrogenation
Diamond & Related Materials 14 (2005) 350 – 354 www.elsevier.com/locate/diamond
Excitonic recombinations and energy levels of highly boron doped homoepitaxial diamond films before and after hydrogenation C. Barona, M. Wadea, A. Deneuvillea,T, E. Bustarreta, T. Kocinievskib, J. Chevalierb, C. Uzan-Saguyb, R. Kalishc, J. Butlerd a
LEPES-CNRS and University of Grenoble, BP 166, 38042 Grenoble Ce´dex 9, France b LPSC-CNRS, 1 place A. Briand, 92195 Meudon Ce´dex, France c Physics Department and Solid State Institute, Technion, Haifa 32000, Israel d Naval Research Laboratory, Code 6174, Washington DC 20375, USA Available online 19 February 2005
Abstract The incorporation of c1019 B-cm 3 has several effects in the cathodoluminescence spectra of homoepitaxial MPCVD film: (1) the TO free exciton and all the boron-bound excitons’ peaks broaden and shift downward in energy (belectronicQ effect). Plasma post-hydrogenation (P-H) destroys all of them. (2) It induces a low concentration of several species of defects (bstructuralQ effect): (i) a broad band around 4.99 eV with a shoulder around 4.86 eV, small bands on a broad 4.68 eV band, peaks and bands between 2.9 and 4.5 eV, the b2.8 eVQ A band. They originate from B-induced species of defects only existing when [B]zc1019 cm 3. Most of them disappear after P-H. (ii) broad bands around 3.65, 4.1 and 4.68 eV. They originate from B-induced species of defects already existing when [B]b1017 cm 3. P-H prevents the formation of free exciton, doesn’t induce any new defect giving radiative recombination above 2.6 eV, heals some structural defects of diamond. D 2005 Elsevier B.V. All rights reserved. Keywords: CVD diamond; Cathodoluminescence; Post-hydrogenation; Excitons and defects
1. Introduction As the interest for diamond devices is increasing, [1,2] a full knowledge of the effect of the incorporation of boron on the electronic properties of the films is more and more necessary. Isolated substitutional boron induces a shallow localized level about 370 meV [3,4] above the diamond valence band edge. For high boron concentrations, delocalized levels appear leading to metallic conductivity on the bboron impurityQ band when [B]z31020 B-cm 3 [4,5]. These belectronicQ effects from high boron concentrations need to be further documented in homoepitaxial films of diamond. Boron incorporation also induces and/or modifies the natures and concentrations of defects in actual diamond T Corresponding author. Tel.: +33 4 76 88 10 09; fax: +33 4 76 88 79 88. E-mail address: [email protected] (A. Deneuville). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.01.009
films (bstructuralQ effects hereafter). Contradictory results on homoepitaxial and polycrystalline films have been published about this point: In polycrystalline films, several independent experimental works report a decrease of the defects concentrations as [B] increases at least up to the 1019 B-cm 3 range [6,4]. This might be ascribed [4] to the Bernholc effect [7]. According to this model, in wide band gap semiconductors, when they have trapped a charge (i.e. in doped films) the vacancies have much higher mobility than the neutral vacancies. As during the film deposition the relaxation of their defects is assisted by the vacancies, they would have a lower concentration of defects. From its origin, such bBernholcQ effect required boron atoms in doping sites and is also expected to hold in homoepitaxial films. However, incorporation of [B]N1016 cm 3 in the best homoepitaxial films was reported to introduce an increasing concentration of undocumented defects [8]. On the other hand in homoepitaxial films, the same species of boron-
C. Baron et al. / Diamond & Related Materials 14 (2005) 350–354
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guishing between the contributions of the various defects, of the excitons, and of the transitions involving the electronic levels of boron. We will show the first cathodoluminescence spectrum of homoepitaxial films at high boron concentration (c1019 cm 3) as-grown and after post-hydrogenation. Taking into account the previous results [9–16], their comparison will help to distinguish between the various species of defects and the belectronicQ and bstructuralQ effects introduced by 1019 B-cm 3 in homoepitaxial films, and will show the healing effect of H incorporation in diamond.
2. Experimental techniques
Fig. 1. As-grown film. Cathodoluminescence spectra in the 4.4–5.4 energy range. Count number 1 and its 7 scale expansion.
related defects inducing broad bands around 2.6, 3.6 and 4.6 eV have been reported with increasing intensities from [B]b1017 cm 3 [9] to [B]c1018 cm 3 [10]. Cathodoluminescence was used to study defects [11,12], excitons [12,13] and signals ascribed to transitions involving the boron electronic levels [11–16] in diamond. However, spectra for [B]z1019 cm 3 remain unreported for homoepitaxial films and unclear for polycrystalline films. Polycrystalline films spectra exhibit: (i) According to Ref. [14], the boron-bound exciton at 5.21 eV with a shoulder at 5.15 eV (ascribed to direct transition from the conduction band to the boron electronic levels) and a broad band at 4.99 eV (ascribed to the O phonon assisted side band of the 5.15 eV band). The intensities of the 5.15 and 4.99 eV structures increase from 61018 to 31019 B-cm 3. (ii) According to Ref. [15], a 5 eV band (ascribed to a new band induced by the heavy boron concentration) with a shoulder around 4.86 eV (ascribed to its TO phonon side band) of increasing intensities from 41019 to 31020 B-cm 3 while the 5.21 eV boron-bound exciton peak disappears, without any structure around 5.15 eV. (iii) According to Refs. [16,12], a boron-bound TO exciton peak whose energy is shifting progressively downward, very slowly from 1018 to 1020 Bcm 3, then very rapidly from 5.21 down to 5.00 eV up to 11021 B-cm 3 while its width increases continuously from 30 to 100 meV from 1018 to 1021 B-cm 3. The plasma post-hydrogenation (P-H) considerably decreases the concentration of electrically active boron [17,18]. This passivation is the result of boron–hydrogen interaction in the (B,H) complexes. However, the boron concentration remains unchanged. Incorporation of hydrogen in diamond might [17] (i) induce an additional passivation of some structural defects and (ii) introduce defects in a lattice already expanded by the boron incorporation [19,20]. The cathodoluminescence spectra of boron sums up the signals from all radiative recombinations without distin-
A 0.32 Am thick film was deposited by MPCVD on Ib synthetic crystal at 900–950 8C in a conventional 1.5 kW Astex apparatus from a gas mixture at 15 Torr with a total flow rate of 906 sccm. This gas mixture contains 0.44% CH4/H2 with ppm of diborane giving a boron concentration (SIMS) varying from 1 to 21019 cm 3. Hall effect at 300 K gives a free hole concentration of 71015 cm 3 with a mobility of 120 cm2 V 1 s 1. Post-hydrogenation (P-H) during 50 min at 550–520 8C with a MPCVD plasma of pure hydrogen in a NIRIM type deposition chamber reduces the film thickness to 0.30 Am. After P-H, this sample was highly insulating and Hall effect experiments were not possible. The cathodoluminescence spectra were recorded at 5 K each 0.09 nm (2 meV around 5.25 eV) [9] under excitation by 15 and 10 kV electron beams before and after P-H.
3. Results Figs. 1 and 2 show the cathodoluminescence spectra of the as-grown film from 4.4 to 5.4 eV and 2.6 to 4.5 eV, respectively. Fig. 1 shows (i) a main nearly symmetrical
Fig. 2. As-grown film. Cathodoluminescence spectra in the 2.6–4.5 energy range.
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4.68 and 4.75 eV and shoulders around 4.50, 4.59, 4.72 and 4.81 eV. Fig. 4 shows a shoulder around 4.42 eV and broad bands around 4.1, 3.65 and 3.1 eV (with a shoulder around 3.05 eV).
4. Discussion
Fig. 3. Post-hydrogenated film. Cathodoluminescence spectra in the 4.4–5.4 energy range.
peak around 5.189 eV with a Full Width at Half Maximum (FWHM) of 50 meV with other nearly symmetrical peaks around 5.028 eV (FWHMc100 meV) and 5.331 eV (FWHMc51 meV) and (ii) on the 7 scale expansion, shoulders around 5.240, 4.99, 4.86, 4.81 and 4.59 eV (see also Fig. 4) and a small broad band around 4.68 eV with a peak around 4.72 eV. Fig. 2 shows (i) small bands around 2.75, 3.1 (with a shoulder at 3.05 eV), 3.5, 3.65, and 3.72 eV, (ii) small peaks around 3.841, 3.909, 4.049, 4.176, 4.255 and 4.447 eV. Figs. 3 and 4 show the CL spectra of the post hydrogenated film from 4.4 to 5.4 eV and 2.6 to 5.4 eV, respectively. Fig. 3 shows, no signal from 5 to 5.4 eV, a wide band around c4.68 eV with small bands around 4.62,
From the penetration depths of the electron beams higher than the film thicknesses they also probe the substrate. From previous works [8,9] there is no signal from the Ib substrate above 2.6 eV. We look first to the spectrum of the as-grown film between 4.75 and 5.4 eV. As their energy differences fit with those between the boron-bound and free excitons, the structures above 5 eV on Fig. 1 are ascribed to excitons. However, in comparison to their characteristics in homoepitaxial films containing low [B] [13,9,12], this [B]c1019 cm 3 induces a (5215 [9] 5189c) 25 meV downward energy shift and a (50 6 [9]c) 44 meV widening of the BETO peak. They are respectively higher and similar to those reported by Refs. [12,16] for polycrystalline films with the same [B]. We additionally obtain similar shifts and widening for the BENP (5.331 eV, 50 meV), BETO+O (5.028 eV, 100 meV), and FETO (5.240 eV shoulder) peaks. The Fig. 1 also shows on the low energy tail of the BETO+O exciton, small additional signals from wide band and shoulder around 4.99 and 4.86 eV. Although some doubts remain about their origin, from their characteristics similar to those of the band and shoulder appearing with increasing intensities from 41019 to 31020 B-cm 3 for polycrystalline films, they are similarly ascribed to a specie of defects induced by [B]zc1019 cm 3 [15] rather than to
Fig. 4. Comparison of the cathodoluminescence spectra between 2.6 and 5.5 eV. A/As-grown film. A30/Count number of curve A with a 30 scale expansion. P-H/Post-hydrogenated film.
C. Baron et al. / Diamond & Related Materials 14 (2005) 350–354
transitions involving the boron electronic levels [14]. Moreover, the energy of any transition involving the boron electronic levels is expected to decrease as the gap shrinks by c0.13 eV with [B] increasing from 1019 to 31020 cm 3 [12,16], while the energies of the band and shoulder remain constant around 4.99 and 4.86 eV. Post-hydrogenation induces drastic modifications in the 4.75–5.4 eV energy range of CL spectrum (Fig. 4). (i) There is no signal from excitons (Fig. 3 and curve P-H of Fig. 4). The removal of the BE peaks was expected because the boron atoms have lost their trapped hole [17–19]. That of the FETO was not expected. Even without introducing defects (see hereafter), H incorporation sufficiently disturbs the diamond lattice [19] to prevent the formation of free excitons. (ii) The main part of the 4.75–5 eV signal (curves A and P-H of Fig. 4), i.e. the 4.99 and 4.86 band and shoulder disappear. As H incorporation suppresses the trapped hole, it also heals diamond from most of the structural defects species induced by [B]zc1019 cm 3 (see hereafter), this does not give additional support to favour one of their possible origins. A continuum from defects remains on the CL spectrum of the P-H film between 2.6 and 4.9 eV. (i) Between 3.3 and 4.9 eV the shape of this spectrum is similar to that obtained for homoepitaxial films with lower [B] (from [B]b1017 [9] to c1018 cm 3 [10]) with wide bands around 3.6, 4.1 and 4.6 eV. Similarly, these bands are ascribed to defect species introduced by boron incorporation. However, while their intensities were reported to increase with increasing [B] as expected [10], they are ten times lower than that of the BETO peak which was missing for [B]c1018 cm 3 in Ref. [10]). Therefore, there is a low concentration of defect when [B]c1019 cm 3. This might be ascribed to the bBernholcQ effect and/or to more favourable deposition conditions. (ii) There are small bands on our 4.68 eV wide band which don’t appear on the 4.6 eV band obtained at lower [B] [9,10]. Therefore, they have to be ascribed to other species of defects induced by [B]zc1019 cm 3. Careful comparison of curves A and P-H of Fig. 4 shows some common structures around 4.50, 4.59, 4.72 and 4.81 eV from these small bands and also around 3.05 and 3.1 eV. Therefore, the wide 4.68 eV band with its small structures are added on the low energy tail of the 4.86 eV shoulder, as well as some other defects, of the as-grown film are not sensitive to hydrogen incorporation. Suitable scale expansion by a factor of 30 of the A spectrum clarifies these points. The A30 and P-H spectra appear nearly identical between 2.9 and 3.05 eV and 4.3 and 4.6 eV. On the other energy ranges, the A30 signal always has a higher intensity than the P-H one. This suggests that the CL signal of the as-grown film between 2.6 and 5 eV sums up contributions from different species of defects, from some species of defects which are not sensitive to the posthydrogenation, and from other ones which are partially or completely healed by the P-H. For the P-H film, we sooner identify the 3.65, 4.1 and 4.68 and structures on it as
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originating from defects species induced by any [B] and [B]z1019 cm 3, respectively. Because the 3.05 eV band is not sensitive to P-H which (on the contrary) partially removes the 3.1 band, these two bands have to originate from different species of defects. As no structure is previously documented at these energies, these defects have to originate from [B]zc1019 cm 3. The 2.75, 3.5, 3.65 and 3.72 eV bands and the 3.841 3.909, 4.049, 4.176, 4.255 and 4.447 eV peaks of the as-grown spectrum appear as additional contributions on the P-H spectrum from other species of defects. Therefore hydrogen incorporation does not induce radiative defects. The 2.75 eV band is similar to the b2.8 eVQ A band [12,9,10]. However, the intensity of this band is reported to first decrease to disappearance as [B] increases [12]. Therefore, its rebirth has to be ascribed to [B]zc1019 cm 3. Because the energies of the peaks are outside the energy ranges from nitrogen–boron or phosphorus–boron pairs [12], these peaks cannot be ascribed to Donor–Acceptor pairs. Because the energies of the 3.841 3.909, 4.049, 4.176, 4.255 and 4.447 eV peaks and of the 3.5, 3.65 and 3.72 eV bands are not documented, they are ascribed to defects induced by [B]zc1019 cm 3 in the asgrown films. Incorporation of c1019 B-cm 3 induces several species of defects. Some of these species already exist for [B]b1017 cm 3 while others appear only when [B]zc1019 cm 3. Most of these other species of defects are removed by the plasma post-hydrogenation which therefore has a healing effect on some structural defects of the diamond as it is doing in the other semiconductors [21].
5. Conclusions The cathodoluminescence spectra of as-grown homoepitaxial diamond films containing c1019 B-cm 3 appears as the sum of the CL signals of the same film after posthydrogenation (mainly from species of defects already existing at lower [B]) plus peaks and bands (originating mainly from species of defects appearing only when [B]z1019 cm 3) and excitons. The incorporation of c1019 B-cm 3 in the as-grown homoepitaxial film has electronic and structural effects (i) It shifts the energy downward (25 meV) and widens (50 meV) the FWHM of the BENP, BETO, BETO+O and FETO peaks (belectronicQ effect). Hydrogen incorporation removes the boron-bound exciton peaks (passivation of the boron acceptors), but also the FETO exciton (disturbance of the diamond lattice). (ii) It induces two broad structures around 4.99 and 4.86 eV healed by H incorporation. Although some doubts remain, we favour their assignment to structural defects induced by [B]zc1019 cm 3 (bstructuralQ effect). (iii) It induces peaks and band (bstructuralQ effect) on broad bands around 3.65, 4.1 and 4.68 eV. P-H
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removes those appearing between 3.2 and 4.6 eV. The species of defects (ascribed to the B incorporation) giving similar broad bands around 4.6, 4.05 and 3.6 eV already appear when [B]V1017 cm 3 [9]. Their decreasing concentration from [B]c1018 to 1019 cm 3 might be ascribed to the bBernholcQ effect [7] and/or to more favourable deposition conditions. (iv) It induces bands around 3.05 and 3.1 eV (bstructuralQ effect). P-H partially heals the defects inducing the 3.1 eV band while those inducing the 3.05 eV band remain unchanged. The as-grown film has a low concentration of defects. PH heals the film from the defects inducing the 2.8 eV A band. Above 2.6 eV, there is no evidence of P-H induced radiative defect.
Acknowledgements The authors acknowledge D. Le Si Dang and F. Donatini for the access at their cathodoluminescence apparatus, and Professor R. Sauer for fruitful discussions.
References [1] A. Aleksov, M. Kubovic, N. Kaeb, U. Spitzberg, A. Bergmaier, G. Dollinger, Th. Bauer, M. Schreck, B. Strizker, E. Kohn, Diamond Relat. Mater. 12 (2003) 391. [2] S. Koizumi, K. Watanabe, M. Hasegawa, H. Kanda, Science 292 (2001) 1899. [3] A.T. Collins, A.W.S. Williams, J. Phys. C.: Solid State Phys. 4 (1971) 1789.
[4] A. Deneuville, in: C. E. Nebel, J. Ristein (Eds.), bThin films diamond IQ, Semiconductors and semimetals, vol. 76, Academic Press, 2003, pp. 142. [5] A.S. Williams, E.C. Lightowlers, A.T. Collins, J. Phys. C.: Solid State Phys. 3 (1970) 1727. [6] S. Dannefaer, W. Zhu, T. Bretagnon, D. Kerr, Phys. Rev., B 53 (1996) 1979. [7] J. Bernholc, A. Antonelli, T.M. Del Sole, Y. Bar-Yam, S. Pantalides, Phys. Rev. Lett. 61 (1988) 2689. [8] H. Okushi, Diamond Relat. Mater. 10 (2001) 281. [9] M. Wade, M. Kadri, E. Bustarret, A. Deneuville, P. Muret, D. Araujo, Phys. Status Solidi, A Appl. Res. 11 (2004) 2457. [10] S.C. Lawson, H. Kanda, H. Kiyota, T. Tsutsumani, H. Kawarada, J. Appl. Phys. 77 (1995) 1729. [11] A.T. Collins, M. Kamo, Y. Sato, J. Phys. Condens. Matter 1 (1989) L4029; A.T. Collins, M. Kamo, Y. Sato, J. Phys. D.: Appl. Phys. 22 (1989) 1402. [12] R. Sauer, in: C. E. Nebel, J. Ristein (Eds.), bThin films diamond IQ, Semiconductors and semimetals, vol. 76, Academic Press, 2003, pp. 79. [13] P.J. Dean, E.C. Lightowlers, D.R. Wight, Phys. Rev. 140 (1965) A352. [14] H. Kawarada, H. Matsuyama, Y. Yokota, T. Sogi, A. Yamaguchi, A. Hiraki, Phys. Rev., B 47 (1993) 3633. [15] Y. Yokata, T. Tachibana, K. Miyata, K. Hayashi, K. Kobashi, A. Hatta, T. Ito, A. Hiraki, Y. Shintani, Diamond Relat. Mater. 8 (1999) 1587. [16] H. Sternschulte, T. Albrecht, K. Thonke, R. Sauer, in: R. Zimmerman, M. Scheffler (Eds.), The physics of semiconductors, World Scientific, Singapore, 1996, pp. 169. [17] J. Chevallier, B. Theys, C. Grattepain, A. Deneuville, E. Gheeraert, Phys. Rev., B 58 (1998) 7966. [18] C. Uzan-Saguy, A. Reznik, C. Cyterman, R. Brener, R. Kalish, E. Bustarret, M. Bernard, A. Deneuville, J. Chevallier, Diamond Relat. Mater. 10 (2001) 453. [19] J.P. Goss, P.R. Briddon, R. Jones, Z. Teukam, D. Ballutaud, F. Jomard, J. Chevallier, M. Bernard, A. Deneuville, Phys. Rev. B 68 (2003) 235209. [20] F. Brunet, P. Germi, M. Pernet, A. Deneuville, E. Gheeraert, F. Laugier, M. Burdin, G. Rolland, J. Appl. Phys. 83 (1998) 181. [21] J. Chevallier, B. Pajot, Solid State Phenomena 85-86 (2002) 203.
Excitonic recombinations and energy levels of highly boron doped homoepitaxial diamond films before and after hydrogenation C. Barona, M. Wadea, A. Deneuvillea,T, E. Bustarreta, T. Kocinievskib, J. Chevalierb, C. Uzan-Saguyb, R. Kalishc, J. Butlerd a
LEPES-CNRS and University of Grenoble, BP 166, 38042 Grenoble Ce´dex 9, France b LPSC-CNRS, 1 place A. Briand, 92195 Meudon Ce´dex, France c Physics Department and Solid State Institute, Technion, Haifa 32000, Israel d Naval Research Laboratory, Code 6174, Washington DC 20375, USA Available online 19 February 2005
Abstract The incorporation of c1019 B-cm 3 has several effects in the cathodoluminescence spectra of homoepitaxial MPCVD film: (1) the TO free exciton and all the boron-bound excitons’ peaks broaden and shift downward in energy (belectronicQ effect). Plasma post-hydrogenation (P-H) destroys all of them. (2) It induces a low concentration of several species of defects (bstructuralQ effect): (i) a broad band around 4.99 eV with a shoulder around 4.86 eV, small bands on a broad 4.68 eV band, peaks and bands between 2.9 and 4.5 eV, the b2.8 eVQ A band. They originate from B-induced species of defects only existing when [B]zc1019 cm 3. Most of them disappear after P-H. (ii) broad bands around 3.65, 4.1 and 4.68 eV. They originate from B-induced species of defects already existing when [B]b1017 cm 3. P-H prevents the formation of free exciton, doesn’t induce any new defect giving radiative recombination above 2.6 eV, heals some structural defects of diamond. D 2005 Elsevier B.V. All rights reserved. Keywords: CVD diamond; Cathodoluminescence; Post-hydrogenation; Excitons and defects
1. Introduction As the interest for diamond devices is increasing, [1,2] a full knowledge of the effect of the incorporation of boron on the electronic properties of the films is more and more necessary. Isolated substitutional boron induces a shallow localized level about 370 meV [3,4] above the diamond valence band edge. For high boron concentrations, delocalized levels appear leading to metallic conductivity on the bboron impurityQ band when [B]z31020 B-cm 3 [4,5]. These belectronicQ effects from high boron concentrations need to be further documented in homoepitaxial films of diamond. Boron incorporation also induces and/or modifies the natures and concentrations of defects in actual diamond T Corresponding author. Tel.: +33 4 76 88 10 09; fax: +33 4 76 88 79 88. E-mail address: [email protected] (A. Deneuville). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.01.009
films (bstructuralQ effects hereafter). Contradictory results on homoepitaxial and polycrystalline films have been published about this point: In polycrystalline films, several independent experimental works report a decrease of the defects concentrations as [B] increases at least up to the 1019 B-cm 3 range [6,4]. This might be ascribed [4] to the Bernholc effect [7]. According to this model, in wide band gap semiconductors, when they have trapped a charge (i.e. in doped films) the vacancies have much higher mobility than the neutral vacancies. As during the film deposition the relaxation of their defects is assisted by the vacancies, they would have a lower concentration of defects. From its origin, such bBernholcQ effect required boron atoms in doping sites and is also expected to hold in homoepitaxial films. However, incorporation of [B]N1016 cm 3 in the best homoepitaxial films was reported to introduce an increasing concentration of undocumented defects [8]. On the other hand in homoepitaxial films, the same species of boron-
C. Baron et al. / Diamond & Related Materials 14 (2005) 350–354
351
guishing between the contributions of the various defects, of the excitons, and of the transitions involving the electronic levels of boron. We will show the first cathodoluminescence spectrum of homoepitaxial films at high boron concentration (c1019 cm 3) as-grown and after post-hydrogenation. Taking into account the previous results [9–16], their comparison will help to distinguish between the various species of defects and the belectronicQ and bstructuralQ effects introduced by 1019 B-cm 3 in homoepitaxial films, and will show the healing effect of H incorporation in diamond.
2. Experimental techniques
Fig. 1. As-grown film. Cathodoluminescence spectra in the 4.4–5.4 energy range. Count number 1 and its 7 scale expansion.
related defects inducing broad bands around 2.6, 3.6 and 4.6 eV have been reported with increasing intensities from [B]b1017 cm 3 [9] to [B]c1018 cm 3 [10]. Cathodoluminescence was used to study defects [11,12], excitons [12,13] and signals ascribed to transitions involving the boron electronic levels [11–16] in diamond. However, spectra for [B]z1019 cm 3 remain unreported for homoepitaxial films and unclear for polycrystalline films. Polycrystalline films spectra exhibit: (i) According to Ref. [14], the boron-bound exciton at 5.21 eV with a shoulder at 5.15 eV (ascribed to direct transition from the conduction band to the boron electronic levels) and a broad band at 4.99 eV (ascribed to the O phonon assisted side band of the 5.15 eV band). The intensities of the 5.15 and 4.99 eV structures increase from 61018 to 31019 B-cm 3. (ii) According to Ref. [15], a 5 eV band (ascribed to a new band induced by the heavy boron concentration) with a shoulder around 4.86 eV (ascribed to its TO phonon side band) of increasing intensities from 41019 to 31020 B-cm 3 while the 5.21 eV boron-bound exciton peak disappears, without any structure around 5.15 eV. (iii) According to Refs. [16,12], a boron-bound TO exciton peak whose energy is shifting progressively downward, very slowly from 1018 to 1020 Bcm 3, then very rapidly from 5.21 down to 5.00 eV up to 11021 B-cm 3 while its width increases continuously from 30 to 100 meV from 1018 to 1021 B-cm 3. The plasma post-hydrogenation (P-H) considerably decreases the concentration of electrically active boron [17,18]. This passivation is the result of boron–hydrogen interaction in the (B,H) complexes. However, the boron concentration remains unchanged. Incorporation of hydrogen in diamond might [17] (i) induce an additional passivation of some structural defects and (ii) introduce defects in a lattice already expanded by the boron incorporation [19,20]. The cathodoluminescence spectra of boron sums up the signals from all radiative recombinations without distin-
A 0.32 Am thick film was deposited by MPCVD on Ib synthetic crystal at 900–950 8C in a conventional 1.5 kW Astex apparatus from a gas mixture at 15 Torr with a total flow rate of 906 sccm. This gas mixture contains 0.44% CH4/H2 with ppm of diborane giving a boron concentration (SIMS) varying from 1 to 21019 cm 3. Hall effect at 300 K gives a free hole concentration of 71015 cm 3 with a mobility of 120 cm2 V 1 s 1. Post-hydrogenation (P-H) during 50 min at 550–520 8C with a MPCVD plasma of pure hydrogen in a NIRIM type deposition chamber reduces the film thickness to 0.30 Am. After P-H, this sample was highly insulating and Hall effect experiments were not possible. The cathodoluminescence spectra were recorded at 5 K each 0.09 nm (2 meV around 5.25 eV) [9] under excitation by 15 and 10 kV electron beams before and after P-H.
3. Results Figs. 1 and 2 show the cathodoluminescence spectra of the as-grown film from 4.4 to 5.4 eV and 2.6 to 4.5 eV, respectively. Fig. 1 shows (i) a main nearly symmetrical
Fig. 2. As-grown film. Cathodoluminescence spectra in the 2.6–4.5 energy range.
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4.68 and 4.75 eV and shoulders around 4.50, 4.59, 4.72 and 4.81 eV. Fig. 4 shows a shoulder around 4.42 eV and broad bands around 4.1, 3.65 and 3.1 eV (with a shoulder around 3.05 eV).
4. Discussion
Fig. 3. Post-hydrogenated film. Cathodoluminescence spectra in the 4.4–5.4 energy range.
peak around 5.189 eV with a Full Width at Half Maximum (FWHM) of 50 meV with other nearly symmetrical peaks around 5.028 eV (FWHMc100 meV) and 5.331 eV (FWHMc51 meV) and (ii) on the 7 scale expansion, shoulders around 5.240, 4.99, 4.86, 4.81 and 4.59 eV (see also Fig. 4) and a small broad band around 4.68 eV with a peak around 4.72 eV. Fig. 2 shows (i) small bands around 2.75, 3.1 (with a shoulder at 3.05 eV), 3.5, 3.65, and 3.72 eV, (ii) small peaks around 3.841, 3.909, 4.049, 4.176, 4.255 and 4.447 eV. Figs. 3 and 4 show the CL spectra of the post hydrogenated film from 4.4 to 5.4 eV and 2.6 to 5.4 eV, respectively. Fig. 3 shows, no signal from 5 to 5.4 eV, a wide band around c4.68 eV with small bands around 4.62,
From the penetration depths of the electron beams higher than the film thicknesses they also probe the substrate. From previous works [8,9] there is no signal from the Ib substrate above 2.6 eV. We look first to the spectrum of the as-grown film between 4.75 and 5.4 eV. As their energy differences fit with those between the boron-bound and free excitons, the structures above 5 eV on Fig. 1 are ascribed to excitons. However, in comparison to their characteristics in homoepitaxial films containing low [B] [13,9,12], this [B]c1019 cm 3 induces a (5215 [9] 5189c) 25 meV downward energy shift and a (50 6 [9]c) 44 meV widening of the BETO peak. They are respectively higher and similar to those reported by Refs. [12,16] for polycrystalline films with the same [B]. We additionally obtain similar shifts and widening for the BENP (5.331 eV, 50 meV), BETO+O (5.028 eV, 100 meV), and FETO (5.240 eV shoulder) peaks. The Fig. 1 also shows on the low energy tail of the BETO+O exciton, small additional signals from wide band and shoulder around 4.99 and 4.86 eV. Although some doubts remain about their origin, from their characteristics similar to those of the band and shoulder appearing with increasing intensities from 41019 to 31020 B-cm 3 for polycrystalline films, they are similarly ascribed to a specie of defects induced by [B]zc1019 cm 3 [15] rather than to
Fig. 4. Comparison of the cathodoluminescence spectra between 2.6 and 5.5 eV. A/As-grown film. A30/Count number of curve A with a 30 scale expansion. P-H/Post-hydrogenated film.
C. Baron et al. / Diamond & Related Materials 14 (2005) 350–354
transitions involving the boron electronic levels [14]. Moreover, the energy of any transition involving the boron electronic levels is expected to decrease as the gap shrinks by c0.13 eV with [B] increasing from 1019 to 31020 cm 3 [12,16], while the energies of the band and shoulder remain constant around 4.99 and 4.86 eV. Post-hydrogenation induces drastic modifications in the 4.75–5.4 eV energy range of CL spectrum (Fig. 4). (i) There is no signal from excitons (Fig. 3 and curve P-H of Fig. 4). The removal of the BE peaks was expected because the boron atoms have lost their trapped hole [17–19]. That of the FETO was not expected. Even without introducing defects (see hereafter), H incorporation sufficiently disturbs the diamond lattice [19] to prevent the formation of free excitons. (ii) The main part of the 4.75–5 eV signal (curves A and P-H of Fig. 4), i.e. the 4.99 and 4.86 band and shoulder disappear. As H incorporation suppresses the trapped hole, it also heals diamond from most of the structural defects species induced by [B]zc1019 cm 3 (see hereafter), this does not give additional support to favour one of their possible origins. A continuum from defects remains on the CL spectrum of the P-H film between 2.6 and 4.9 eV. (i) Between 3.3 and 4.9 eV the shape of this spectrum is similar to that obtained for homoepitaxial films with lower [B] (from [B]b1017 [9] to c1018 cm 3 [10]) with wide bands around 3.6, 4.1 and 4.6 eV. Similarly, these bands are ascribed to defect species introduced by boron incorporation. However, while their intensities were reported to increase with increasing [B] as expected [10], they are ten times lower than that of the BETO peak which was missing for [B]c1018 cm 3 in Ref. [10]). Therefore, there is a low concentration of defect when [B]c1019 cm 3. This might be ascribed to the bBernholcQ effect and/or to more favourable deposition conditions. (ii) There are small bands on our 4.68 eV wide band which don’t appear on the 4.6 eV band obtained at lower [B] [9,10]. Therefore, they have to be ascribed to other species of defects induced by [B]zc1019 cm 3. Careful comparison of curves A and P-H of Fig. 4 shows some common structures around 4.50, 4.59, 4.72 and 4.81 eV from these small bands and also around 3.05 and 3.1 eV. Therefore, the wide 4.68 eV band with its small structures are added on the low energy tail of the 4.86 eV shoulder, as well as some other defects, of the as-grown film are not sensitive to hydrogen incorporation. Suitable scale expansion by a factor of 30 of the A spectrum clarifies these points. The A30 and P-H spectra appear nearly identical between 2.9 and 3.05 eV and 4.3 and 4.6 eV. On the other energy ranges, the A30 signal always has a higher intensity than the P-H one. This suggests that the CL signal of the as-grown film between 2.6 and 5 eV sums up contributions from different species of defects, from some species of defects which are not sensitive to the posthydrogenation, and from other ones which are partially or completely healed by the P-H. For the P-H film, we sooner identify the 3.65, 4.1 and 4.68 and structures on it as
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originating from defects species induced by any [B] and [B]z1019 cm 3, respectively. Because the 3.05 eV band is not sensitive to P-H which (on the contrary) partially removes the 3.1 band, these two bands have to originate from different species of defects. As no structure is previously documented at these energies, these defects have to originate from [B]zc1019 cm 3. The 2.75, 3.5, 3.65 and 3.72 eV bands and the 3.841 3.909, 4.049, 4.176, 4.255 and 4.447 eV peaks of the as-grown spectrum appear as additional contributions on the P-H spectrum from other species of defects. Therefore hydrogen incorporation does not induce radiative defects. The 2.75 eV band is similar to the b2.8 eVQ A band [12,9,10]. However, the intensity of this band is reported to first decrease to disappearance as [B] increases [12]. Therefore, its rebirth has to be ascribed to [B]zc1019 cm 3. Because the energies of the peaks are outside the energy ranges from nitrogen–boron or phosphorus–boron pairs [12], these peaks cannot be ascribed to Donor–Acceptor pairs. Because the energies of the 3.841 3.909, 4.049, 4.176, 4.255 and 4.447 eV peaks and of the 3.5, 3.65 and 3.72 eV bands are not documented, they are ascribed to defects induced by [B]zc1019 cm 3 in the asgrown films. Incorporation of c1019 B-cm 3 induces several species of defects. Some of these species already exist for [B]b1017 cm 3 while others appear only when [B]zc1019 cm 3. Most of these other species of defects are removed by the plasma post-hydrogenation which therefore has a healing effect on some structural defects of the diamond as it is doing in the other semiconductors [21].
5. Conclusions The cathodoluminescence spectra of as-grown homoepitaxial diamond films containing c1019 B-cm 3 appears as the sum of the CL signals of the same film after posthydrogenation (mainly from species of defects already existing at lower [B]) plus peaks and bands (originating mainly from species of defects appearing only when [B]z1019 cm 3) and excitons. The incorporation of c1019 B-cm 3 in the as-grown homoepitaxial film has electronic and structural effects (i) It shifts the energy downward (25 meV) and widens (50 meV) the FWHM of the BENP, BETO, BETO+O and FETO peaks (belectronicQ effect). Hydrogen incorporation removes the boron-bound exciton peaks (passivation of the boron acceptors), but also the FETO exciton (disturbance of the diamond lattice). (ii) It induces two broad structures around 4.99 and 4.86 eV healed by H incorporation. Although some doubts remain, we favour their assignment to structural defects induced by [B]zc1019 cm 3 (bstructuralQ effect). (iii) It induces peaks and band (bstructuralQ effect) on broad bands around 3.65, 4.1 and 4.68 eV. P-H
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removes those appearing between 3.2 and 4.6 eV. The species of defects (ascribed to the B incorporation) giving similar broad bands around 4.6, 4.05 and 3.6 eV already appear when [B]V1017 cm 3 [9]. Their decreasing concentration from [B]c1018 to 1019 cm 3 might be ascribed to the bBernholcQ effect [7] and/or to more favourable deposition conditions. (iv) It induces bands around 3.05 and 3.1 eV (bstructuralQ effect). P-H partially heals the defects inducing the 3.1 eV band while those inducing the 3.05 eV band remain unchanged. The as-grown film has a low concentration of defects. PH heals the film from the defects inducing the 2.8 eV A band. Above 2.6 eV, there is no evidence of P-H induced radiative defect.
Acknowledgements The authors acknowledge D. Le Si Dang and F. Donatini for the access at their cathodoluminescence apparatus, and Professor R. Sauer for fruitful discussions.
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