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n-type diamond with high room temperature electrical conductivity by deuteration of boron doped diamond layers
Diamond and Related Materials 13 (2004) 700–704
n-type diamond with high room temperature electrical conductivity by deuteration of boron doped diamond layers C. Saguya,*, R. Kalisha, C. Cytermanna, Z. Teukamb, J. Chevallierb, F. Jomardb, A. Tromson-Carlib, J.E. Butlerc, C. Barond, A. Deneuvilled a
b
Physics Department and Solid State Institute, Technion City, Haifa 32000, Israel ´ ` UMR CNRS 8635, 1 place A.Briand, 92195 Meudon Cedex, France Laboratoire de Physique des Solides et de Cristallogenese, c Naval Research Laboratory, Code 6174, Washington, DC 20375, USA d ´ ´ Electroniques des Solides, CNRS B.P.166, 38042 Grenoble Cedex 09, France Laboratoire d’Etudes des Proprietes
Abstract The conversion of the conductivity of B-doped homoepitaxially grown diamond layers from p- to n-type upon deuteration and its reconversion to p-type following annealing is extensively studied. Several B doped samples have been converted to n-type when exposed to a deuterium plasma at approximately 500 8C. The n-type features are related to D uptake of the samples. The donors, thus formed, have an ionization energy of approximately 0.34 eV below the conduction band edge and a high RT (room temperature) mobility (up to 430 cm2 yV s), as determined by Hall effect measurements as a function of temperature. In the Bdoped layers that do convert to n-type upon deuteration, the D to B concentration ratio is found to be in the range of one to two. Higher D uptake (D to B ratio 42) must be connected to B-doped layers containing a large amount of growth defects, which trap the D. The exact nature of the donor is yet unknown. However, the reversibility of the effect and its relationship with the D concentrations suggest that deuterium is involved in the formation of some complex with B or with some other defects. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Doped diamond; Electrical properties; Defect-complexes
1. Introduction Many potential applications of diamond, as an electronic material, are hampered by the lack of n-type semiconducting diamond with suitable properties at room temperature. Hence, many attempts to achieve electron conduction in diamond have been made over the last few years, however, with no fully satisfactory results. Phosphorus, a group V donor, has been shown to yield n-type doping of diamond by several groups w1,2x during CVD growth on HPHT N111M oriented type Ib substrates using PH3 as the source gas, however, with rather high ionization energy (0.6 eV) and limited conductivity w2–5x. Some reports exist in the recent literature which describe n-type features found in diamond co-doped with a group VI (S), and a group III *Corresponding author. Tel.: q972-4-8293908; fax: q972-48235107. E-mail address: [email protected] (C. Saguy).
elements (B) w6x and for diamond implanted with O or S ions w7–9x (hence perhaps include implantationrelated-defectydopant complexes). Hydrogen, a group I element, forms a mid-gap state in diamond; hence it does not act as a useful donor w10,11x. Most recently, a very encouraging discovery was made w12x. It has been shown that high conductivity ntype diamond can be achieved by deuteration of homoepitaxial (100) boron-doped diamond films. It was already known w13,14x that when a B-doped layer is exposed to a deuterium plasma, deuterium diffusion occurs and (B, D) pairs are formed. As a result, the acceptors are passivated by the deuterium resulting in an almost electrically insulating diamond. However, it was found in Ref. w12x, that when some particular ptype diamond layers are treated under special D plasma condition, deuterium-related complexes are most probably formed and they act as shallow n-type dopants which seem to be responsible for the p-to-n conversion. It has been assumed that these complexes are due to some excess of deuterium above the amount of D needed
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for boron passivation. These complexes were found to break-up under annealing at temperatures below those required for breaking (B, H) neutral complexes (550 8C). Following deuteration, the samples were found to be converted to n-type with an activation energy of the order of 0.22 eV and a room temperature mobility up to 430 cm2 yV s w12x. These electrical properties are remarkable, as the ionization energy of the new donorcomplex is much lower than that of P-doped n-type diamond. Whereas the reported results w12x have clearly demonstrated the n-type features of the deuterated B doped layers, several questions are still open, as listed below: 1. What does characterize the B-doped layers which do show p-to-n-type conversion upon deuteration? In particular, can this conversion be obtained in Bdoped samples grown in other laboratories and in lightly B-doped samples? 2. What are the deuterium plasma conditions, which lead to the correct uptake of D by the B-doped layer to turn it n-type? 3. What is the exact ionization energy of the D-related donor centers? 4. What is the nature of the donor centers? The first results were obtained for heavily borondoped samples originating from NRL w12x. Here, we present new results on the p-type to n-type conversion showing that this conversion also occurs in a slightly boron-doped sample. This sample has been grown at the AIST laboratory in Japan w15x. The results of electrical measurements indicate that the activation energy associated with the donor state related to the formation of some D-related complex is approximately 0.34 eV. Moreover, additional SIMS (secondary ion mass spectroscopy) analysis, searching for possible impurities, which may give rise to donor states are presented. Some results on samples, which did not show the pyn conversion are also discussed, to illuminate the particular material properties required to lead to the observed phenomena. 2. Experimental The growth conditions of the highly B doped homoepitaxial samples produced at NRL are described in detail in Ref. w12x. A new, lightly B doped (wBxs2–3=1017 cmy3, p(RT)s2.5=1014 cmy3 and mp(RT)s900 cm2 Vy1 sy1) homoepitaxial sample, grown at the AIST laboratories in Japan w15x was exposed in the current work, to a more gentle deuteration (475 8C for 2 h) followed by thermal treatments and evaluations, similar to those described in Ref. w12x. The electrical properties of the
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samples were determined by Hall effect measurements in the van der Pauw configuration over a wide temperature range (y100 to 600 8C), as described in Ref. w12x. No thermopower effect was performed to confirm the type of conductivity of the samples. Detailed SIMS measurements (using a CAMECA 4f at Meudon and at the Technion) were performed on selected samples, searching not only for the D and B profiles but also for possible impurities such as N, O, P, S, As, Te, Sb. 3. Results 3.1. Structural The surface quality and the crystallinity of all of the samples were investigated by ECP (electron channeling pattern) and by SEM (scanning electron microscope). These have shown that the samples in their as-grownstate were of high quality (see Ref. w12x for details). 3.2. Electrical measurements Fig. 1 presents the temperature dependence of the free carrier concentration (a) and the carrier mobility (b) after deuteration for a heavily B doped (1–4=1019 Bycm3, sample 1 in Ref. w12x). The evolution of the electrical properties of the sample following deuteration and subsequent annealings at increasing temperatures are marked with increasing numbers. The carrier concentrations in state 1 (as-grown) and 2 (as-deuterated) were measured at CNRS Meudon at room temperature only. Measurements over a large temperature range were made at Technion, at states 3, 4, 5, 6 and 7, as defined in Fig. 1. As seen in Fig. 1b, the room temperature mobility of the n-type converted layer has reached 430 cm2 Vy1 sy1 following annealing at 520 8C for 1 h. The electrical carrier type conversions (from p to n and back to p) and the deuteration and thermal treatments, which have led to them are shown in Fig. 2 displaying the carrier concentration measured at room temperature, following the different stages. The results of Hall measurements for the AIST, lightly B doped sample are shown in Fig. 3. As can be seen this sample also underwent the p to n-type conversion. The analysis of the dependence of the free electron concentration on inverse temperature was performed by fitting the results to the equation describing the electron concentration in a partially compensated n-type semiconductor w16x. For the highly B-doped sample, the very high carrier concentrations as well as the very low mobility (2–5 cm2 Vy1 sy1) indicate that in both states (1 and 2), the conduction mechanism is through hopping in an impurity band (B acceptor band for state 1 and deuterationrelated donor band for state 2). In state 3 (after an annealing at 520 8C for 0.5 h), the electron concentration
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Fig. 2. Histogram showing the conductivity type and room temperature carrier concentration as a function of the various treatments undergone by sample 1.
Fig. 1. (a) Carrier concentration as a function of the reciprocal temperature in as-grown (1), as-deuterated (2), deuterated and annealed at 520 8C for 0.5 h (3) and for 1 h (4), annealed at 650 8C for 1 h (5) and annealed at 750 8C for 1 h (6) states. (b) Mobility as a function of temperature in deuterated and annealed at 520 8C for 0.5 h (d) and for 1 h (m), annealed at 650 8C for 1 h (h) and annealed at 750 8C for 1 h (q) states. The full lines are obtained by fitting the data for states 3 and 4 to the equation describing n(T) in a partially compensated n-type semiconductor w16x. The broken lines for state 5 and 6 are to guide the eyes.
has decreased to 7=1016 cmy3 with a mobility of 180 cm2 Vy1 sy1 at room temperature. With such a high mobility, the conduction mechanism in state 3 cannot be attributed anymore to hopping conduction in the impurity band but to thermally activated conductivity in the conduction band. The fit of the data of curve 3 yields to an effective activation energy of 0.23 eV (full line in curve 3, Fig. 1a). In state 4 (after an annealing at 520 8C for 1 h), the electron concentration further decreases to 2=1016 cmy3 and the mobility increases to 430 cm2 Vy1 sy1, ruling out again impurity band conduction. The parameters extracted from the fit of
curve 4 are: Eds0.31 eV, NdyNas5=1019 cmy3 and a low compensation ratio Na yNd (-0.2%). Finally, for the n-type converted AIST lightly B-doped layer, the activation energy of the donors extracted from the temperature dependence of the electron concentration is 0.34 eV (see full line in Fig. 3). From this analysis, it appears that the ionization energy of the isolated deuterium-related donors is 0.34 eV. The effective activation energy calculated from the fit of curve 3 (Fig. 1a) is lower than the ionization energy of isolated donors centers. Indeed, since the concentration of donors in state 3 is above 5=1019 cmy3, an impurity band forms and the measured effective activation energy corresponds to the position of the Fermi level at the top of this impurity band.
Fig. 3. Carrier concentration as a function of the reciprocal temperature for as-deuterated lightly B-doped sample (j) and its fit. The (q) symbol represents the carrier concentration in the as grown state.
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Fig. 4. SIMS depth profiles of D and B for the lightly B-doped sample from AIST upon deuteration at 475 8C for 2 h.
3.3. SIMS SIMS was used to look for impurity atoms (O, N, S, P, As, Sb, Te), which might give rise to the observed donor states. The SIMS profiles of these potential impurities show (for sample 1) that the concentrations of all the elements searched for are all below the SIMS detection limits before and after deuterium plasma exposure (P: 1=1017, S: 1=1017, O: 5=1018, N: 3=1017, As: 5=1015, Sb: 5=1015, Te: 5=1015 cmy3). Not all samples studied converted from p to n type on deuteration. Fig. 4 shows the D and B SIMS profiles following deuteration for the AIST lightly B-doped sample which converted from p- to n-type. The D to B ratio somewhat exceeds 1 in the first 0.8 mm. The two samples described in Ref. w12x which did show p-to-n conversion exhibit also a D to B ratio above 1. The SIMS deuterium profiles (not shown here) measured in two samples which did not exhibit the n-type conversion clearly show that the whole B layer has taken up much more D than the B concentration (D to B ratio above 5). 4. Discussion Several questions that could have risen from the data presented in Ref. w12x have found an answer in the present extension of that work. The first question relates to the origin of the n-type found for deuterated samples. One could argue that residual impurities or defects, which could give rise to donor states were obscured by the dominating p-type features of the heavy B doping of the original layer. Upon deuteration, this p-type activity is strongly quenched, possibly revealing n-type features that were previously hidden. The SIMS data presented here, which does not show excessive amounts of any possible donor impurity par-
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tially answers this question. The fact that an additional sample originating from another source (hence most probably does not include the same unintentional impurities) has also showed the n-type conversion further strengthens this point. As for other sources for possible donor impurities, we find it most improbable that donor impurities were introduced into the sample during the deuteration. The only candidates for such could be N or O, which were not found in sufficient amount in the SIMS analysis nor can they enter into the sample in any conceivable way during the deuterium plasma exposure (diffusion, low energy implantation). Furthermore, had these ‘hidden’ donors been in the sample they should have compensated the boron acceptors, which are present in the sample in large amounts. Hence no electron would be available on the donor level following deuteration to be thermally activated in the conduction band. The main question, to which no complete answer exists at the moment, is what the nature of the D related complex, which gives rise to the donor activity. The reversibility of the effect and its relationship with the D concentrations suggest that deuterium is involved in the complex formation. Taking also into account the large free electron concentrations achieved at high temperatures in the highly B-doped sample, it is reasonable to attribute the donors to some complexes containing boron and deuterium atoms w12x. One possible configuration could be that at the first deuteration stage all boron acceptors have been passivated by deuterium cations. Further deuteration forms a new donor center located 0.34 eV below the conduction band edge. It could be a B–D related complex, most likely a B atom bonded to two deuterium atoms. However, computations on the possible B–Hn complexes, their structure, stability, and energy levels in the diamond gap have shown that these complexes should induce very deep levels in diamond w17x and, for the moment, this hypothesis seems to be open to discussion. Another possibility would be the formation of D-defect complexes. Computations of such complexes are under way w18x. It seems as if the temperature at which the break-up of D-related complexes which are responsible for the n-type conductivity starts, depends on initial B concentration (and hence also of the n-type concentration). The higher the hole concentration of the B-doped layer before deuteration, the higher the temperature at which the n-type conductivity gets lost; It is Ts550 8C for ps2=1019 cmy3, Ts350 8C for ps6=1015 cmy3 and Ts250 8C and for ps3=1014 cmy3. Complete dissociation of these donor complexes results in a highly compensated material for which no clear conduction type can be determined: this is the ‘transition state’ between the n-type conductivity of Drelated complexes and the pure p-type boron related conductivity. The break-up of (B, D) pairs related to B
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acceptor passivation sets in above 550 8C and the conversion from n- to p-type is clear above 600 8C. Out-diffusion of deuterium starts at 700 8C. Samples that have taken up much more D than the B concentration in the layer seem to contain large amounts of growth-related defects (probably dislocations and penetration twins), which trap the D and may also serve as deep electronic traps. Electrical measurements performed on the samples, which did not convert to n-type upon deuteration have shown that in the defect-containing-samples, the passivation of the B is much less efficient than in good quality B-doped diamond layers. 5. Summary We have elucidated several points related with the recent discovery of a way of obtaining n-type diamond by the exposure of a B doped layer to a deuterium plasma. We have checked all possible impurities that could be directly responsible for the n-type features. Their concentrations are too low to supply a sufficient concentration of donor centers. The deuteration procedure yields, when done under certain conditions, the best n-type electrical carrier features reported to date in diamond, namely an acceptably low activation energy of the responsible donor state (Eds0.34 eV) and a rather high room temperature electron mobility. These properties, and further improvements on them, should open up many applications in diamond based electronic devices. Methods of consistently and reproducibly forming this unique donor state need to be experimentally devised, and the exact properties of the state have to be thoroughly studied. Nevertheless, we believe that this is the first time that an acceptor-hydrogen related complex with donor features has been found, not only in diamond, but in any elemental semiconductor. Acknowledgments We are very grateful to C. Grattepain, Thales Research and Technology, Corbeville for complementary SIMS
analysis. Dr H. Okushi is kindly acknowledged for furnishing p-type homoepitaxial B-doped diamond layers. References w1x S. Koizumi, M. Kamo, Y. Sato, H. Ozaky, T. Inuzuka, Appl. Phys. Lett. 71 (1997) 1065. w2x A. Tajani, E. Gheeraert, N. Casanova, E. Bustarret, J.A. Garrido, G. Rumen, et al., Phys. Status Solidi (A) 3 (2002) 541. w3x S. Koizumi, M. Kamo, Y. Sato, S. Mita, A. Sawabe, A. Reznik, et al., Diamond Relat. Mater. 7 (1997) 540. w4x A. Reznik, R. Kalish, S. Koizumi, Proceedings of the 6th NIRIM International Symposium on Advanced Materials (Tsukuba-Japan 1999) p. 27. w5x E. Gheeraert, S. Koizumi, T. Teraji, H. Kanda, M. Nesladek, Electronic states of phosphorus in diamond, Diamond Relat. Mater. 9 (2000) 948. w6x S.C. Eaton, A.B. Anderson, J.C. Angus, Y.E. Evstefeeva, Y. Pleskov, Electrochem. Solid-State Lett. G65–G68 (2002) 5. w7x M. Hasegawa, D. Takeuchi, S. Yamanaka, M. Ogura, K. Watanabe, N. Kobayashi, et al., Jpn. J. Appl. Phys. 38 (1999) L1519–L1522. w8x J. Prins, Diamond Relat. Mater. 10 (2001) 1756. w9x R. Kalish, C. Saguy, R. Walker, S. Prawer, J. Appl. Phys. 94 (2003) 3923. w10x J.P. Goss, R. Jones, M.I. Heggie, C.P. Ewels, P.R. Briddon, S. Oberg, Phys. Rev. B 65 (2002) 115207. w11x D. Saada, J. Adler, R. Kalish, Phys. Rev. B 61 (2000) 10 711. w12x Z. Teukam, J. Chevallier, C. Saguy, R. Kalish, D. Ballutaud, M. Barbe, et al., Nature Mater. 2 (2003) 482. w13x J. Chevallier, B. Theys, A. Lusson, C. Grattepain, A. Deneuville, E. Gheeraert, Phys. Rev. B 58 (1998) 7966. w14x C. Uzan-Saguy, A. Reznik, C. Cytermann, R. Brener, R. Kalish, E. Bustarret, et al., Diamond Relat. Mater. 10 (2001) 453. w15x S. Yamanaka, H. Watanabe, S. Masai, D. Takeuchi, H. Okushi, K. Kajimura, Jpn. J. Appl. Phys. 37 (1998) L1129. w16x J.S. Blakemore, Solid State Physics, 2nd ed, Cambridge University Press, 1985, p. 320. w17x J.P. Goss, P.R. Briddon, R. Jones, S. Sque, to be published in Diamond Relat. Mater. w18x D. Segev, S.-H. Wei, Phys. Rev. Lett. 91 (2003) 126406.
n-type diamond with high room temperature electrical conductivity by deuteration of boron doped diamond layers C. Saguya,*, R. Kalisha, C. Cytermanna, Z. Teukamb, J. Chevallierb, F. Jomardb, A. Tromson-Carlib, J.E. Butlerc, C. Barond, A. Deneuvilled a
b
Physics Department and Solid State Institute, Technion City, Haifa 32000, Israel ´ ` UMR CNRS 8635, 1 place A.Briand, 92195 Meudon Cedex, France Laboratoire de Physique des Solides et de Cristallogenese, c Naval Research Laboratory, Code 6174, Washington, DC 20375, USA d ´ ´ Electroniques des Solides, CNRS B.P.166, 38042 Grenoble Cedex 09, France Laboratoire d’Etudes des Proprietes
Abstract The conversion of the conductivity of B-doped homoepitaxially grown diamond layers from p- to n-type upon deuteration and its reconversion to p-type following annealing is extensively studied. Several B doped samples have been converted to n-type when exposed to a deuterium plasma at approximately 500 8C. The n-type features are related to D uptake of the samples. The donors, thus formed, have an ionization energy of approximately 0.34 eV below the conduction band edge and a high RT (room temperature) mobility (up to 430 cm2 yV s), as determined by Hall effect measurements as a function of temperature. In the Bdoped layers that do convert to n-type upon deuteration, the D to B concentration ratio is found to be in the range of one to two. Higher D uptake (D to B ratio 42) must be connected to B-doped layers containing a large amount of growth defects, which trap the D. The exact nature of the donor is yet unknown. However, the reversibility of the effect and its relationship with the D concentrations suggest that deuterium is involved in the formation of some complex with B or with some other defects. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Doped diamond; Electrical properties; Defect-complexes
1. Introduction Many potential applications of diamond, as an electronic material, are hampered by the lack of n-type semiconducting diamond with suitable properties at room temperature. Hence, many attempts to achieve electron conduction in diamond have been made over the last few years, however, with no fully satisfactory results. Phosphorus, a group V donor, has been shown to yield n-type doping of diamond by several groups w1,2x during CVD growth on HPHT N111M oriented type Ib substrates using PH3 as the source gas, however, with rather high ionization energy (0.6 eV) and limited conductivity w2–5x. Some reports exist in the recent literature which describe n-type features found in diamond co-doped with a group VI (S), and a group III *Corresponding author. Tel.: q972-4-8293908; fax: q972-48235107. E-mail address: [email protected] (C. Saguy).
elements (B) w6x and for diamond implanted with O or S ions w7–9x (hence perhaps include implantationrelated-defectydopant complexes). Hydrogen, a group I element, forms a mid-gap state in diamond; hence it does not act as a useful donor w10,11x. Most recently, a very encouraging discovery was made w12x. It has been shown that high conductivity ntype diamond can be achieved by deuteration of homoepitaxial (100) boron-doped diamond films. It was already known w13,14x that when a B-doped layer is exposed to a deuterium plasma, deuterium diffusion occurs and (B, D) pairs are formed. As a result, the acceptors are passivated by the deuterium resulting in an almost electrically insulating diamond. However, it was found in Ref. w12x, that when some particular ptype diamond layers are treated under special D plasma condition, deuterium-related complexes are most probably formed and they act as shallow n-type dopants which seem to be responsible for the p-to-n conversion. It has been assumed that these complexes are due to some excess of deuterium above the amount of D needed
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.066
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for boron passivation. These complexes were found to break-up under annealing at temperatures below those required for breaking (B, H) neutral complexes (550 8C). Following deuteration, the samples were found to be converted to n-type with an activation energy of the order of 0.22 eV and a room temperature mobility up to 430 cm2 yV s w12x. These electrical properties are remarkable, as the ionization energy of the new donorcomplex is much lower than that of P-doped n-type diamond. Whereas the reported results w12x have clearly demonstrated the n-type features of the deuterated B doped layers, several questions are still open, as listed below: 1. What does characterize the B-doped layers which do show p-to-n-type conversion upon deuteration? In particular, can this conversion be obtained in Bdoped samples grown in other laboratories and in lightly B-doped samples? 2. What are the deuterium plasma conditions, which lead to the correct uptake of D by the B-doped layer to turn it n-type? 3. What is the exact ionization energy of the D-related donor centers? 4. What is the nature of the donor centers? The first results were obtained for heavily borondoped samples originating from NRL w12x. Here, we present new results on the p-type to n-type conversion showing that this conversion also occurs in a slightly boron-doped sample. This sample has been grown at the AIST laboratory in Japan w15x. The results of electrical measurements indicate that the activation energy associated with the donor state related to the formation of some D-related complex is approximately 0.34 eV. Moreover, additional SIMS (secondary ion mass spectroscopy) analysis, searching for possible impurities, which may give rise to donor states are presented. Some results on samples, which did not show the pyn conversion are also discussed, to illuminate the particular material properties required to lead to the observed phenomena. 2. Experimental The growth conditions of the highly B doped homoepitaxial samples produced at NRL are described in detail in Ref. w12x. A new, lightly B doped (wBxs2–3=1017 cmy3, p(RT)s2.5=1014 cmy3 and mp(RT)s900 cm2 Vy1 sy1) homoepitaxial sample, grown at the AIST laboratories in Japan w15x was exposed in the current work, to a more gentle deuteration (475 8C for 2 h) followed by thermal treatments and evaluations, similar to those described in Ref. w12x. The electrical properties of the
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samples were determined by Hall effect measurements in the van der Pauw configuration over a wide temperature range (y100 to 600 8C), as described in Ref. w12x. No thermopower effect was performed to confirm the type of conductivity of the samples. Detailed SIMS measurements (using a CAMECA 4f at Meudon and at the Technion) were performed on selected samples, searching not only for the D and B profiles but also for possible impurities such as N, O, P, S, As, Te, Sb. 3. Results 3.1. Structural The surface quality and the crystallinity of all of the samples were investigated by ECP (electron channeling pattern) and by SEM (scanning electron microscope). These have shown that the samples in their as-grownstate were of high quality (see Ref. w12x for details). 3.2. Electrical measurements Fig. 1 presents the temperature dependence of the free carrier concentration (a) and the carrier mobility (b) after deuteration for a heavily B doped (1–4=1019 Bycm3, sample 1 in Ref. w12x). The evolution of the electrical properties of the sample following deuteration and subsequent annealings at increasing temperatures are marked with increasing numbers. The carrier concentrations in state 1 (as-grown) and 2 (as-deuterated) were measured at CNRS Meudon at room temperature only. Measurements over a large temperature range were made at Technion, at states 3, 4, 5, 6 and 7, as defined in Fig. 1. As seen in Fig. 1b, the room temperature mobility of the n-type converted layer has reached 430 cm2 Vy1 sy1 following annealing at 520 8C for 1 h. The electrical carrier type conversions (from p to n and back to p) and the deuteration and thermal treatments, which have led to them are shown in Fig. 2 displaying the carrier concentration measured at room temperature, following the different stages. The results of Hall measurements for the AIST, lightly B doped sample are shown in Fig. 3. As can be seen this sample also underwent the p to n-type conversion. The analysis of the dependence of the free electron concentration on inverse temperature was performed by fitting the results to the equation describing the electron concentration in a partially compensated n-type semiconductor w16x. For the highly B-doped sample, the very high carrier concentrations as well as the very low mobility (2–5 cm2 Vy1 sy1) indicate that in both states (1 and 2), the conduction mechanism is through hopping in an impurity band (B acceptor band for state 1 and deuterationrelated donor band for state 2). In state 3 (after an annealing at 520 8C for 0.5 h), the electron concentration
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Fig. 2. Histogram showing the conductivity type and room temperature carrier concentration as a function of the various treatments undergone by sample 1.
Fig. 1. (a) Carrier concentration as a function of the reciprocal temperature in as-grown (1), as-deuterated (2), deuterated and annealed at 520 8C for 0.5 h (3) and for 1 h (4), annealed at 650 8C for 1 h (5) and annealed at 750 8C for 1 h (6) states. (b) Mobility as a function of temperature in deuterated and annealed at 520 8C for 0.5 h (d) and for 1 h (m), annealed at 650 8C for 1 h (h) and annealed at 750 8C for 1 h (q) states. The full lines are obtained by fitting the data for states 3 and 4 to the equation describing n(T) in a partially compensated n-type semiconductor w16x. The broken lines for state 5 and 6 are to guide the eyes.
has decreased to 7=1016 cmy3 with a mobility of 180 cm2 Vy1 sy1 at room temperature. With such a high mobility, the conduction mechanism in state 3 cannot be attributed anymore to hopping conduction in the impurity band but to thermally activated conductivity in the conduction band. The fit of the data of curve 3 yields to an effective activation energy of 0.23 eV (full line in curve 3, Fig. 1a). In state 4 (after an annealing at 520 8C for 1 h), the electron concentration further decreases to 2=1016 cmy3 and the mobility increases to 430 cm2 Vy1 sy1, ruling out again impurity band conduction. The parameters extracted from the fit of
curve 4 are: Eds0.31 eV, NdyNas5=1019 cmy3 and a low compensation ratio Na yNd (-0.2%). Finally, for the n-type converted AIST lightly B-doped layer, the activation energy of the donors extracted from the temperature dependence of the electron concentration is 0.34 eV (see full line in Fig. 3). From this analysis, it appears that the ionization energy of the isolated deuterium-related donors is 0.34 eV. The effective activation energy calculated from the fit of curve 3 (Fig. 1a) is lower than the ionization energy of isolated donors centers. Indeed, since the concentration of donors in state 3 is above 5=1019 cmy3, an impurity band forms and the measured effective activation energy corresponds to the position of the Fermi level at the top of this impurity band.
Fig. 3. Carrier concentration as a function of the reciprocal temperature for as-deuterated lightly B-doped sample (j) and its fit. The (q) symbol represents the carrier concentration in the as grown state.
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Fig. 4. SIMS depth profiles of D and B for the lightly B-doped sample from AIST upon deuteration at 475 8C for 2 h.
3.3. SIMS SIMS was used to look for impurity atoms (O, N, S, P, As, Sb, Te), which might give rise to the observed donor states. The SIMS profiles of these potential impurities show (for sample 1) that the concentrations of all the elements searched for are all below the SIMS detection limits before and after deuterium plasma exposure (P: 1=1017, S: 1=1017, O: 5=1018, N: 3=1017, As: 5=1015, Sb: 5=1015, Te: 5=1015 cmy3). Not all samples studied converted from p to n type on deuteration. Fig. 4 shows the D and B SIMS profiles following deuteration for the AIST lightly B-doped sample which converted from p- to n-type. The D to B ratio somewhat exceeds 1 in the first 0.8 mm. The two samples described in Ref. w12x which did show p-to-n conversion exhibit also a D to B ratio above 1. The SIMS deuterium profiles (not shown here) measured in two samples which did not exhibit the n-type conversion clearly show that the whole B layer has taken up much more D than the B concentration (D to B ratio above 5). 4. Discussion Several questions that could have risen from the data presented in Ref. w12x have found an answer in the present extension of that work. The first question relates to the origin of the n-type found for deuterated samples. One could argue that residual impurities or defects, which could give rise to donor states were obscured by the dominating p-type features of the heavy B doping of the original layer. Upon deuteration, this p-type activity is strongly quenched, possibly revealing n-type features that were previously hidden. The SIMS data presented here, which does not show excessive amounts of any possible donor impurity par-
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tially answers this question. The fact that an additional sample originating from another source (hence most probably does not include the same unintentional impurities) has also showed the n-type conversion further strengthens this point. As for other sources for possible donor impurities, we find it most improbable that donor impurities were introduced into the sample during the deuteration. The only candidates for such could be N or O, which were not found in sufficient amount in the SIMS analysis nor can they enter into the sample in any conceivable way during the deuterium plasma exposure (diffusion, low energy implantation). Furthermore, had these ‘hidden’ donors been in the sample they should have compensated the boron acceptors, which are present in the sample in large amounts. Hence no electron would be available on the donor level following deuteration to be thermally activated in the conduction band. The main question, to which no complete answer exists at the moment, is what the nature of the D related complex, which gives rise to the donor activity. The reversibility of the effect and its relationship with the D concentrations suggest that deuterium is involved in the complex formation. Taking also into account the large free electron concentrations achieved at high temperatures in the highly B-doped sample, it is reasonable to attribute the donors to some complexes containing boron and deuterium atoms w12x. One possible configuration could be that at the first deuteration stage all boron acceptors have been passivated by deuterium cations. Further deuteration forms a new donor center located 0.34 eV below the conduction band edge. It could be a B–D related complex, most likely a B atom bonded to two deuterium atoms. However, computations on the possible B–Hn complexes, their structure, stability, and energy levels in the diamond gap have shown that these complexes should induce very deep levels in diamond w17x and, for the moment, this hypothesis seems to be open to discussion. Another possibility would be the formation of D-defect complexes. Computations of such complexes are under way w18x. It seems as if the temperature at which the break-up of D-related complexes which are responsible for the n-type conductivity starts, depends on initial B concentration (and hence also of the n-type concentration). The higher the hole concentration of the B-doped layer before deuteration, the higher the temperature at which the n-type conductivity gets lost; It is Ts550 8C for ps2=1019 cmy3, Ts350 8C for ps6=1015 cmy3 and Ts250 8C and for ps3=1014 cmy3. Complete dissociation of these donor complexes results in a highly compensated material for which no clear conduction type can be determined: this is the ‘transition state’ between the n-type conductivity of Drelated complexes and the pure p-type boron related conductivity. The break-up of (B, D) pairs related to B
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acceptor passivation sets in above 550 8C and the conversion from n- to p-type is clear above 600 8C. Out-diffusion of deuterium starts at 700 8C. Samples that have taken up much more D than the B concentration in the layer seem to contain large amounts of growth-related defects (probably dislocations and penetration twins), which trap the D and may also serve as deep electronic traps. Electrical measurements performed on the samples, which did not convert to n-type upon deuteration have shown that in the defect-containing-samples, the passivation of the B is much less efficient than in good quality B-doped diamond layers. 5. Summary We have elucidated several points related with the recent discovery of a way of obtaining n-type diamond by the exposure of a B doped layer to a deuterium plasma. We have checked all possible impurities that could be directly responsible for the n-type features. Their concentrations are too low to supply a sufficient concentration of donor centers. The deuteration procedure yields, when done under certain conditions, the best n-type electrical carrier features reported to date in diamond, namely an acceptably low activation energy of the responsible donor state (Eds0.34 eV) and a rather high room temperature electron mobility. These properties, and further improvements on them, should open up many applications in diamond based electronic devices. Methods of consistently and reproducibly forming this unique donor state need to be experimentally devised, and the exact properties of the state have to be thoroughly studied. Nevertheless, we believe that this is the first time that an acceptor-hydrogen related complex with donor features has been found, not only in diamond, but in any elemental semiconductor. Acknowledgments We are very grateful to C. Grattepain, Thales Research and Technology, Corbeville for complementary SIMS
analysis. Dr H. Okushi is kindly acknowledged for furnishing p-type homoepitaxial B-doped diamond layers. References w1x S. Koizumi, M. Kamo, Y. Sato, H. Ozaky, T. Inuzuka, Appl. Phys. Lett. 71 (1997) 1065. w2x A. Tajani, E. Gheeraert, N. Casanova, E. Bustarret, J.A. Garrido, G. Rumen, et al., Phys. Status Solidi (A) 3 (2002) 541. w3x S. Koizumi, M. Kamo, Y. Sato, S. Mita, A. Sawabe, A. Reznik, et al., Diamond Relat. Mater. 7 (1997) 540. w4x A. Reznik, R. Kalish, S. Koizumi, Proceedings of the 6th NIRIM International Symposium on Advanced Materials (Tsukuba-Japan 1999) p. 27. w5x E. Gheeraert, S. Koizumi, T. Teraji, H. Kanda, M. Nesladek, Electronic states of phosphorus in diamond, Diamond Relat. Mater. 9 (2000) 948. w6x S.C. Eaton, A.B. Anderson, J.C. Angus, Y.E. Evstefeeva, Y. Pleskov, Electrochem. Solid-State Lett. G65–G68 (2002) 5. w7x M. Hasegawa, D. Takeuchi, S. Yamanaka, M. Ogura, K. Watanabe, N. Kobayashi, et al., Jpn. J. Appl. Phys. 38 (1999) L1519–L1522. w8x J. Prins, Diamond Relat. Mater. 10 (2001) 1756. w9x R. Kalish, C. Saguy, R. Walker, S. Prawer, J. Appl. Phys. 94 (2003) 3923. w10x J.P. Goss, R. Jones, M.I. Heggie, C.P. Ewels, P.R. Briddon, S. Oberg, Phys. Rev. B 65 (2002) 115207. w11x D. Saada, J. Adler, R. Kalish, Phys. Rev. B 61 (2000) 10 711. w12x Z. Teukam, J. Chevallier, C. Saguy, R. Kalish, D. Ballutaud, M. Barbe, et al., Nature Mater. 2 (2003) 482. w13x J. Chevallier, B. Theys, A. Lusson, C. Grattepain, A. Deneuville, E. Gheeraert, Phys. Rev. B 58 (1998) 7966. w14x C. Uzan-Saguy, A. Reznik, C. Cytermann, R. Brener, R. Kalish, E. Bustarret, et al., Diamond Relat. Mater. 10 (2001) 453. w15x S. Yamanaka, H. Watanabe, S. Masai, D. Takeuchi, H. Okushi, K. Kajimura, Jpn. J. Appl. Phys. 37 (1998) L1129. w16x J.S. Blakemore, Solid State Physics, 2nd ed, Cambridge University Press, 1985, p. 320. w17x J.P. Goss, P.R. Briddon, R. Jones, S. Sque, to be published in Diamond Relat. Mater. w18x D. Segev, S.-H. Wei, Phys. Rev. Lett. 91 (2003) 126406.