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Charge carrier transport anisotropy in ultrananocrystalline diamond films
Diamond & Related Materials 19 (2010) 238–241
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Charge carrier transport anisotropy in ultrananocrystalline diamond films M.C. Rossi a, A. Minutello a, S. Carta a, P. Calvani a,⁎, G. Conte a, V. Ralchenko b a b
University of Roma Tre, Electronic Engineering Department, IFN, and CNISM, Via della Vasca navale 84, 00146 Rome, Italy Russian Academy of Science, Vavilova ul. 38, 119991 Moskow, Russia
a r t i c l e
i n f o
Available online 29 September 2009 Keywords: Ultrananocrystalline diamond Transient photocurrent Characteristic times Defect density
a b s t r a c t The optoelectronic properties of ultrananocrystalline diamond films (UNCD) grown using N2 = 0 and 5% in the deposition gas mixture, are investigated by transient photocurrent measurements under nanosecond light pulses, both in planar and sandwich contact arrangements. Independent of contact configuration and N2% value, very similar characteristic times in the 6-7 ns range are detected in the nanosecond range, reflecting a homogeneous distribution of states responsible for such decay times. On a longer time scale, nitrogen addition appears to slow down carrier transport promoting trapping and detrapping processes responsible for single and two power law photocurrent decays in films deposited using N2 = 5% for sandwich and planar contact arrangements, respectively. Such a result suggests a nitrogen induced transport anisotropy tentatively related to structural modifications occurring at relatively low N2%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Ultra-nanocrystalline diamond (UNCD) is a unique form of carbon with nanometer grain size and extremely high grain boundary concentration. Owing to the N induced changes of resistivity over several orders of magnitude [1], UNCD films has been recently used for a wide variety of electronic applications, such as MESFETs [2], and chemFETs [3] device realization. UNCD films also appear very attractive for electrochemical and biosensors, as well as for surface acoustic wave devices [4]. The development of these applications requires a deep understanding of optical and electronic properties of the material. Previous investigations [5,6] have already shown a clear contribution of defect states in optical absorption as well as in carrier transport, where grain boundary conductivity dominate transport properties of UNCD films. Further informations can be gained from probing carrier transport and dynamics by transient photocurrent measurements, where a time-varying light excitation is used to promote the formation of charge carriers excess, which is collected at the device contacts. The resulting time-dependent photocurrent signal elucidates the kinetics of carrier generation, trapping and recombination, which are largely affected by the material properties. In particular, recombination times depend on the density of defects lying around midgap, whereas trapping processes are mainly related to the energy distribution of defect states within the gap. In this context the characteristic times of photogenerated carriers in UNCD films deposited without and with relatively small nitrogen percentage are here investigated in detail.
3–15 μm thick UNCD films were deposited at 800 °C by MWCVD technique using hydrogen poor Ar/CH4/N2 gas mixture with N2 in the range 0 ÷ 5%. Details on the preparation conditions and structural characteristics are described elsewhere [7]. Before stopping the deposition process, the samples were kept in Ar/H2 plasma in order to prevent the deposition of a surface graphitic phase during the sample cooling. Under this fabrication condition, pinhole free UNCD films were obtained. Typical morphology of films employed in this study is depicted in Fig. 1 for N2 = 0% (a) and N2 = 5% (b) in the deposition gas mixture. At large scale, about 1 μm globules of nanocrystals with average size of about 10 nm are observed, independent of N2 percentage. Such nanocrystals are further arranged in small blocks of 40–50 nm for N2 = 0% (a), whereas appear fully dispersed for N2 = 5% (b). From the morphological point of view, nitrogen addiction then mainly affects the presence of a sort of amorphous connective tissue embedding nanocrystalline domains. Metal-semiconductor-metal (MSM) structures were realized using Ag contacts with 3 mm apart double finger geometry on the front surface and a single finger one on the substrate surface. Similar electric field values were applied in both configurations by properly choosing the bias voltage. Spectrally resolved photocurrent measurements were performed in the 200–800 nm range using a Xe 300 W lamp filtered by a double grating monochromator (SPEX 1680). The photocurrent signal was amplified with an EG&G 5182 transimpedance amplifier and measured with an EG&G 5210 lock-in amplifier. Transient photocurrent has been investigated using the UV pulses from a ArF excimer laser at λ = 193 nm. Photocurrent signal was detected through the 50 Ω input resistance of a 2 GHz oscilloscope
⁎ Corresponding author. E-mail address: [email protected] (P. Calvani). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.09.007
M.C. Rossi et al. / Diamond & Related Materials 19 (2010) 238–241
239
Fig. 2. Spectrally resolved photocurrent yield of UNCD films deposited using N2 = 0% and N2 = 5% in the deposition gas mixture. Continuous line highlights the contribution of band to band transitions in diamond nanocrystals.
The normalized transient photocurrent signals in UNCD samples deposited with N2 = 0% and N2 = 5% in the gas phase are compared in Fig. 3 on a nanosecond time scale.
Fig. 1. SEM images of UNCD films grown using (a) N2 = 0% and (b) N2 = 5% in the deposition gas mixture.
(Lecroy wavepro 960). DC and AC signal paths were separated by using a 10 GHz bias tee (Picoseconds model 5575A). 3. Results and discussion Spectral photocurrent yield of 0% and 5% N2 UNCD films are compared in Fig. 2. In both cases, an increase of the photocurrent yield is observed when photon energies is close to the diamond energy gap (5.5 eV), due to absorption in diamond nanocrystals. In the subgap spectral region a featureless signal is observed for N2 = 0%, monotonously decreasing with photon energy downward to the photocurrent onset at about 1 eV. Such a broad photocurrent band is attributed to the absorption of amorphous carbon phases predominantly located at grain boundaries. Photocurrent spectra then reflect that UNCD films are constituted by diamond nanocrystals embedded in a nondiamond carbon tissue. These spectral details slightly changes with the addition of nitrogen to the deposition gas mixture, only promoting a change of the ratio between below and above the band gap energy and a shift of the photocurrent onset towards 4 eV [8].
Fig. 3. Comparison between normalized photocurrent recorded on a short time scale by planar contact arrangement in UNCD films deposited using N2 = 0% (dashed line) and N2 = 5% (continuous line) in the deposition gas mixture.
240
M.C. Rossi et al. / Diamond & Related Materials 19 (2010) 238–241
Independent of contact configuration, a single photocurrent decay is detected for the sample grown without nitrogen (Fig. 3, dashed curve) whereas two different decay mechanism are distinguished in sample grown with N2 = 5% (Fig. 3, continuous line). As far as the evaluation of the characteristic time is concerned, the pulse decay of photogenerated carriers p(t) can be described by an exponential function, according to: pðtÞ = pð0Þ expð−t = τÞ
ð1Þ
where only a single kind of charge carrier created by pulsed photoexcitation is assumed (hole minority carrier in nitrogen doped UNCD exhibiting n-type dark conductivity, with N defects acting as recombination centers). Here τ = σ N is the recombination time, with σ the capture cross section and p(0) is the initial carrier density which depends on the absorption coefficient and on laser pulse intensity. For the sample grown with N2 = 0% a single time constant τ = 6 ns is found. Similarly, in the sandwich contact configuration a fast exponential decay with time constant τ = 8 ns is detected, although followed by a long-lived tail extending towards the microsecond time scale. The appearance of such a photocurrent tail is indicative of trap mediated transport and suggests the existence of localized states which extend the carrier lifetime. More precisely, following the UV pulse excitation, the photogenerated carriers occupy extended states and promptly contribute to the transport. As the time proceeds, the free carrier density is reduced by band to band carrier recombination and localization at defect sites. Trapped carriers can be released into extended states via thermal emission processes, then contributing once more to the photocurrent signal. In order to analyze the possible occurrence of minority carrier transport anisotropy related to the structural one clearly observed in UNCD films grown with N2 = 25% or after high temperature annealing [9,10], the transient photocurrent has been investigated both perpendicularly and in parallel to the direction of the impinging laser pulses. The comparison between photocurrent pulse shape obtained on a nanosecond time scale in planar and sandwich contact geometry in UNCD sample deposited with N2 = 5% is reported in Fig. 4. In both contact geometries, the photocurrent pulses decay according to an exponential dependence with roughly the same time constant τ = 6– 8 ns, although the pulse shape appears slightly wider (FWHM about 5 ns) for the sandwich arrangement. Moreover, the weight of the slow components is higher for the planar contact geometry. A more detailed investigation of these slow components has been performed at different bias voltages by extending the measurement time scale to the μs range, where the detection of residual photocurrent signals clearly suggests the existence of dispersive transport occurring by hopping among localized states. As shown in Fig. 5, in planar contact arrangement the photocurrent varies according to two power laws with different slopes, changing from (β−1) for times shorter than the transit time Tt to (−β −1) for T >Tt, following the prediction of the model developed by Scherr and Montroll [11]. For increasing bias field β is almost constant, but the signal is enhanced and shifts towards shorter times, owing to the enhanced probability for carrier hopping in the direction of the applied field. This leads to a sort of "universality" of the photocurrent decay shapes, once the curves are properly shifted. Within this frame, dispersion is thought to originate from the existence of a wide distribution of carrier hopping or trapping times [12]. By using the scaling law of the photocurrent decays (not shown), best fitting of the experimental data reported in Fig. 5 gives in the planar case an average dispersion coefficient β = 0.69 and a transit time Tt = 390 ns. The sum of these two slopes is close to 2, in rather good agreement with theoretical prediction.
Fig. 4. Comparison between normalized photosignal obtained on a short time scale for planar and sandwich contact configurations.
As usually reported for disordered semiconductors, the dispersion parameter β is related to the slope E0 of an exponentially energy distributed trapping tail states through the equation [11]: β = kT = E0
ð2Þ
Fig. 5. Double logarithmic plot of transient normalized photocurrent showing a single and double power law decay detected for the film deposited using N2 = 5% in planar and sandwich contact arrangements, respectively.
M.C. Rossi et al. / Diamond & Related Materials 19 (2010) 238–241
giving for UNCD films with N2 = 5% a band tail distribution of defect states with a slope E0 = 37 meV. Conversely, in the sandwich contact arrangements a featureless single decay curve is observed according to a single power low with exponent close to 0.9 (see Fig. 5). Such a power law decay follows the short time exponential dependence with τ = 6 ns, already observed in the planar configuration. The latter result suggests that defects acting as recombination centers are almost homogeneously distributed, possibly close to the diamond nanograin periphery, as previously suggested [13]. As far as the single power law decay is concerned, an algebraic photocurrent decay could occur in a defect free material, according to the relation: pðtÞ = pð0Þ = ½ð1 + γpð0Þt
ð3Þ
where γ is the bimolecular recombination coefficient, which is fairly high in wide bandgap semiconductors. However a similar situation seems to be unreasonable in our case, since the algebraic law decay, although with exponent slightly less than one, install only after the exponential time dependence, and a defect free material parallel to the growth direction is not expected. More reasonably, we tentatively suggest that such a decay shape results from a particular band tail profile and defect distribution. It is worth to be noticed that the observation of a featureless decay curve according to a single power law in the vertical contact arrangement could be also due to the fact that photogenerated carriers may never reach the rear electrode, at least in the measurement time scale. 4. Conclusions The effect of relatively small quantity of nitrogen in the deposition gas mixture (N2 = 0 and 5%) on the electronic properties of ultrananocrystalline diamond films have been investigated by transient photocurrent measurements, both in planar and sandwich contact arrangements. Independent of N2 value, on a nanosecond time scale the films exhibit a very similar decay constant suggesting that amount of
241
defects affecting the short time carrier dynamics is influenced by nitrogen only to a lesser extent. Instead, relatively small nitrogen addition to the deposition gas phase strongly enhance trapping and detrapping processes which slow down the carrier transport and introduce anomalous dispersion of transit time. In particular, when N2 = 5%, a transport anisotropy is detected on a μs time scale, reflecting into single and two power law photocurrent decays in sandwich and planar contact configuration, respectively. Such a result suggests a possible nitrogen induced structural change already at relatively low nitrogen content in the gas phase, although clearly observed only into UNCD grown using N2 = 25%. Acknowledgments The authors wish to thank O. Lebedev and I. Vlasov for morphological characterization by SEM images and S. Salvatori for spectrally resolved photocurrent measurements. References [1] O.A. Shenderova, D.M. Gruen, Ultrananocrystalline diamond: synthesis, properties and application, William Andrew Publishing, NY, USA, 2006. [2] M. Kubovic, K. Janischowsky, E. Kohn, Diamond Relat. Mater. 14 (2005) 514. [3] T. Guzdek, J. Szmidt, M. Dudek, P. Niedzielski, Diamond Relat. Mater. 13 (2004) 1059. [4] O. Elmazria, F. Benedic, M. El Hakiki, H. Moubchir, M.B. Assouar, F. Silva, Diamond Relat. Mater. 15 (2006) 193. [5] P. Achatz, J.A. Garrido, M. Stutzmann, O.A. Williams, D.M. Gruen, A. Kromka, D. Steinmüller, Appl. Phys. Lett. 88 (2006) 101908. [6] P. Achatz, O.A. Williams, D.M. Gruen, J.A. Garrido, M. Stutzmann, Phys. Rev. B 74 (2006) 155429. [7] S.M. Pimenov, V.G. Ralchenko, V.I. Konov, A.V. Khomich, E.V. Zavedeev, V.D. Frolov, Proc. SPIE 5850 (2004) 230. [8] W. Gajewski, P. Achatz, O.A. Williams, K. Henen, E. Bustarret, M. Stutzmann, J.A. Garrido, Phys. Rev. B 79 (2009) 045206. [9] R. Arenal, P. Bruno, D.J. Miller, M. Bleuel, J. Lal, D.M. Gruen, Phys. Rev. B 75 (2007) 195431. [10] I.I. Vlaslov, O.I. Lebedev, V.G. Ralchenko, E. Goovaerts, G. Bertoni, G. Van Tendeloo, V. Konov, Adv. Mater. 19 (2007) 4058. [11] H. Scher, E.W. Montroll, Phys. Rev. B 12 (1975) 2455. [12] T. Tiedje, A. Rose, Solid State Commun. 37 (1980) 49. [13] M.C. Rossi, S. Salvatori, A. Minutello, G. Conte, V. Ralchenko, Proc. SPIE 6591 (2007) 65910B.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Charge carrier transport anisotropy in ultrananocrystalline diamond films M.C. Rossi a, A. Minutello a, S. Carta a, P. Calvani a,⁎, G. Conte a, V. Ralchenko b a b
University of Roma Tre, Electronic Engineering Department, IFN, and CNISM, Via della Vasca navale 84, 00146 Rome, Italy Russian Academy of Science, Vavilova ul. 38, 119991 Moskow, Russia
a r t i c l e
i n f o
Available online 29 September 2009 Keywords: Ultrananocrystalline diamond Transient photocurrent Characteristic times Defect density
a b s t r a c t The optoelectronic properties of ultrananocrystalline diamond films (UNCD) grown using N2 = 0 and 5% in the deposition gas mixture, are investigated by transient photocurrent measurements under nanosecond light pulses, both in planar and sandwich contact arrangements. Independent of contact configuration and N2% value, very similar characteristic times in the 6-7 ns range are detected in the nanosecond range, reflecting a homogeneous distribution of states responsible for such decay times. On a longer time scale, nitrogen addition appears to slow down carrier transport promoting trapping and detrapping processes responsible for single and two power law photocurrent decays in films deposited using N2 = 5% for sandwich and planar contact arrangements, respectively. Such a result suggests a nitrogen induced transport anisotropy tentatively related to structural modifications occurring at relatively low N2%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Ultra-nanocrystalline diamond (UNCD) is a unique form of carbon with nanometer grain size and extremely high grain boundary concentration. Owing to the N induced changes of resistivity over several orders of magnitude [1], UNCD films has been recently used for a wide variety of electronic applications, such as MESFETs [2], and chemFETs [3] device realization. UNCD films also appear very attractive for electrochemical and biosensors, as well as for surface acoustic wave devices [4]. The development of these applications requires a deep understanding of optical and electronic properties of the material. Previous investigations [5,6] have already shown a clear contribution of defect states in optical absorption as well as in carrier transport, where grain boundary conductivity dominate transport properties of UNCD films. Further informations can be gained from probing carrier transport and dynamics by transient photocurrent measurements, where a time-varying light excitation is used to promote the formation of charge carriers excess, which is collected at the device contacts. The resulting time-dependent photocurrent signal elucidates the kinetics of carrier generation, trapping and recombination, which are largely affected by the material properties. In particular, recombination times depend on the density of defects lying around midgap, whereas trapping processes are mainly related to the energy distribution of defect states within the gap. In this context the characteristic times of photogenerated carriers in UNCD films deposited without and with relatively small nitrogen percentage are here investigated in detail.
3–15 μm thick UNCD films were deposited at 800 °C by MWCVD technique using hydrogen poor Ar/CH4/N2 gas mixture with N2 in the range 0 ÷ 5%. Details on the preparation conditions and structural characteristics are described elsewhere [7]. Before stopping the deposition process, the samples were kept in Ar/H2 plasma in order to prevent the deposition of a surface graphitic phase during the sample cooling. Under this fabrication condition, pinhole free UNCD films were obtained. Typical morphology of films employed in this study is depicted in Fig. 1 for N2 = 0% (a) and N2 = 5% (b) in the deposition gas mixture. At large scale, about 1 μm globules of nanocrystals with average size of about 10 nm are observed, independent of N2 percentage. Such nanocrystals are further arranged in small blocks of 40–50 nm for N2 = 0% (a), whereas appear fully dispersed for N2 = 5% (b). From the morphological point of view, nitrogen addiction then mainly affects the presence of a sort of amorphous connective tissue embedding nanocrystalline domains. Metal-semiconductor-metal (MSM) structures were realized using Ag contacts with 3 mm apart double finger geometry on the front surface and a single finger one on the substrate surface. Similar electric field values were applied in both configurations by properly choosing the bias voltage. Spectrally resolved photocurrent measurements were performed in the 200–800 nm range using a Xe 300 W lamp filtered by a double grating monochromator (SPEX 1680). The photocurrent signal was amplified with an EG&G 5182 transimpedance amplifier and measured with an EG&G 5210 lock-in amplifier. Transient photocurrent has been investigated using the UV pulses from a ArF excimer laser at λ = 193 nm. Photocurrent signal was detected through the 50 Ω input resistance of a 2 GHz oscilloscope
⁎ Corresponding author. E-mail address: [email protected] (P. Calvani). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.09.007
M.C. Rossi et al. / Diamond & Related Materials 19 (2010) 238–241
239
Fig. 2. Spectrally resolved photocurrent yield of UNCD films deposited using N2 = 0% and N2 = 5% in the deposition gas mixture. Continuous line highlights the contribution of band to band transitions in diamond nanocrystals.
The normalized transient photocurrent signals in UNCD samples deposited with N2 = 0% and N2 = 5% in the gas phase are compared in Fig. 3 on a nanosecond time scale.
Fig. 1. SEM images of UNCD films grown using (a) N2 = 0% and (b) N2 = 5% in the deposition gas mixture.
(Lecroy wavepro 960). DC and AC signal paths were separated by using a 10 GHz bias tee (Picoseconds model 5575A). 3. Results and discussion Spectral photocurrent yield of 0% and 5% N2 UNCD films are compared in Fig. 2. In both cases, an increase of the photocurrent yield is observed when photon energies is close to the diamond energy gap (5.5 eV), due to absorption in diamond nanocrystals. In the subgap spectral region a featureless signal is observed for N2 = 0%, monotonously decreasing with photon energy downward to the photocurrent onset at about 1 eV. Such a broad photocurrent band is attributed to the absorption of amorphous carbon phases predominantly located at grain boundaries. Photocurrent spectra then reflect that UNCD films are constituted by diamond nanocrystals embedded in a nondiamond carbon tissue. These spectral details slightly changes with the addition of nitrogen to the deposition gas mixture, only promoting a change of the ratio between below and above the band gap energy and a shift of the photocurrent onset towards 4 eV [8].
Fig. 3. Comparison between normalized photocurrent recorded on a short time scale by planar contact arrangement in UNCD films deposited using N2 = 0% (dashed line) and N2 = 5% (continuous line) in the deposition gas mixture.
240
M.C. Rossi et al. / Diamond & Related Materials 19 (2010) 238–241
Independent of contact configuration, a single photocurrent decay is detected for the sample grown without nitrogen (Fig. 3, dashed curve) whereas two different decay mechanism are distinguished in sample grown with N2 = 5% (Fig. 3, continuous line). As far as the evaluation of the characteristic time is concerned, the pulse decay of photogenerated carriers p(t) can be described by an exponential function, according to: pðtÞ = pð0Þ expð−t = τÞ
ð1Þ
where only a single kind of charge carrier created by pulsed photoexcitation is assumed (hole minority carrier in nitrogen doped UNCD exhibiting n-type dark conductivity, with N defects acting as recombination centers). Here τ = σ N is the recombination time, with σ the capture cross section and p(0) is the initial carrier density which depends on the absorption coefficient and on laser pulse intensity. For the sample grown with N2 = 0% a single time constant τ = 6 ns is found. Similarly, in the sandwich contact configuration a fast exponential decay with time constant τ = 8 ns is detected, although followed by a long-lived tail extending towards the microsecond time scale. The appearance of such a photocurrent tail is indicative of trap mediated transport and suggests the existence of localized states which extend the carrier lifetime. More precisely, following the UV pulse excitation, the photogenerated carriers occupy extended states and promptly contribute to the transport. As the time proceeds, the free carrier density is reduced by band to band carrier recombination and localization at defect sites. Trapped carriers can be released into extended states via thermal emission processes, then contributing once more to the photocurrent signal. In order to analyze the possible occurrence of minority carrier transport anisotropy related to the structural one clearly observed in UNCD films grown with N2 = 25% or after high temperature annealing [9,10], the transient photocurrent has been investigated both perpendicularly and in parallel to the direction of the impinging laser pulses. The comparison between photocurrent pulse shape obtained on a nanosecond time scale in planar and sandwich contact geometry in UNCD sample deposited with N2 = 5% is reported in Fig. 4. In both contact geometries, the photocurrent pulses decay according to an exponential dependence with roughly the same time constant τ = 6– 8 ns, although the pulse shape appears slightly wider (FWHM about 5 ns) for the sandwich arrangement. Moreover, the weight of the slow components is higher for the planar contact geometry. A more detailed investigation of these slow components has been performed at different bias voltages by extending the measurement time scale to the μs range, where the detection of residual photocurrent signals clearly suggests the existence of dispersive transport occurring by hopping among localized states. As shown in Fig. 5, in planar contact arrangement the photocurrent varies according to two power laws with different slopes, changing from (β−1) for times shorter than the transit time Tt to (−β −1) for T >Tt, following the prediction of the model developed by Scherr and Montroll [11]. For increasing bias field β is almost constant, but the signal is enhanced and shifts towards shorter times, owing to the enhanced probability for carrier hopping in the direction of the applied field. This leads to a sort of "universality" of the photocurrent decay shapes, once the curves are properly shifted. Within this frame, dispersion is thought to originate from the existence of a wide distribution of carrier hopping or trapping times [12]. By using the scaling law of the photocurrent decays (not shown), best fitting of the experimental data reported in Fig. 5 gives in the planar case an average dispersion coefficient β = 0.69 and a transit time Tt = 390 ns. The sum of these two slopes is close to 2, in rather good agreement with theoretical prediction.
Fig. 4. Comparison between normalized photosignal obtained on a short time scale for planar and sandwich contact configurations.
As usually reported for disordered semiconductors, the dispersion parameter β is related to the slope E0 of an exponentially energy distributed trapping tail states through the equation [11]: β = kT = E0
ð2Þ
Fig. 5. Double logarithmic plot of transient normalized photocurrent showing a single and double power law decay detected for the film deposited using N2 = 5% in planar and sandwich contact arrangements, respectively.
M.C. Rossi et al. / Diamond & Related Materials 19 (2010) 238–241
giving for UNCD films with N2 = 5% a band tail distribution of defect states with a slope E0 = 37 meV. Conversely, in the sandwich contact arrangements a featureless single decay curve is observed according to a single power low with exponent close to 0.9 (see Fig. 5). Such a power law decay follows the short time exponential dependence with τ = 6 ns, already observed in the planar configuration. The latter result suggests that defects acting as recombination centers are almost homogeneously distributed, possibly close to the diamond nanograin periphery, as previously suggested [13]. As far as the single power law decay is concerned, an algebraic photocurrent decay could occur in a defect free material, according to the relation: pðtÞ = pð0Þ = ½ð1 + γpð0Þt
ð3Þ
where γ is the bimolecular recombination coefficient, which is fairly high in wide bandgap semiconductors. However a similar situation seems to be unreasonable in our case, since the algebraic law decay, although with exponent slightly less than one, install only after the exponential time dependence, and a defect free material parallel to the growth direction is not expected. More reasonably, we tentatively suggest that such a decay shape results from a particular band tail profile and defect distribution. It is worth to be noticed that the observation of a featureless decay curve according to a single power law in the vertical contact arrangement could be also due to the fact that photogenerated carriers may never reach the rear electrode, at least in the measurement time scale. 4. Conclusions The effect of relatively small quantity of nitrogen in the deposition gas mixture (N2 = 0 and 5%) on the electronic properties of ultrananocrystalline diamond films have been investigated by transient photocurrent measurements, both in planar and sandwich contact arrangements. Independent of N2 value, on a nanosecond time scale the films exhibit a very similar decay constant suggesting that amount of
241
defects affecting the short time carrier dynamics is influenced by nitrogen only to a lesser extent. Instead, relatively small nitrogen addition to the deposition gas phase strongly enhance trapping and detrapping processes which slow down the carrier transport and introduce anomalous dispersion of transit time. In particular, when N2 = 5%, a transport anisotropy is detected on a μs time scale, reflecting into single and two power law photocurrent decays in sandwich and planar contact configuration, respectively. Such a result suggests a possible nitrogen induced structural change already at relatively low nitrogen content in the gas phase, although clearly observed only into UNCD grown using N2 = 25%. Acknowledgments The authors wish to thank O. Lebedev and I. Vlasov for morphological characterization by SEM images and S. Salvatori for spectrally resolved photocurrent measurements. References [1] O.A. Shenderova, D.M. Gruen, Ultrananocrystalline diamond: synthesis, properties and application, William Andrew Publishing, NY, USA, 2006. [2] M. Kubovic, K. Janischowsky, E. Kohn, Diamond Relat. Mater. 14 (2005) 514. [3] T. Guzdek, J. Szmidt, M. Dudek, P. Niedzielski, Diamond Relat. Mater. 13 (2004) 1059. [4] O. Elmazria, F. Benedic, M. El Hakiki, H. Moubchir, M.B. Assouar, F. Silva, Diamond Relat. Mater. 15 (2006) 193. [5] P. Achatz, J.A. Garrido, M. Stutzmann, O.A. Williams, D.M. Gruen, A. Kromka, D. Steinmüller, Appl. Phys. Lett. 88 (2006) 101908. [6] P. Achatz, O.A. Williams, D.M. Gruen, J.A. Garrido, M. Stutzmann, Phys. Rev. B 74 (2006) 155429. [7] S.M. Pimenov, V.G. Ralchenko, V.I. Konov, A.V. Khomich, E.V. Zavedeev, V.D. Frolov, Proc. SPIE 5850 (2004) 230. [8] W. Gajewski, P. Achatz, O.A. Williams, K. Henen, E. Bustarret, M. Stutzmann, J.A. Garrido, Phys. Rev. B 79 (2009) 045206. [9] R. Arenal, P. Bruno, D.J. Miller, M. Bleuel, J. Lal, D.M. Gruen, Phys. Rev. B 75 (2007) 195431. [10] I.I. Vlaslov, O.I. Lebedev, V.G. Ralchenko, E. Goovaerts, G. Bertoni, G. Van Tendeloo, V. Konov, Adv. Mater. 19 (2007) 4058. [11] H. Scher, E.W. Montroll, Phys. Rev. B 12 (1975) 2455. [12] T. Tiedje, A. Rose, Solid State Commun. 37 (1980) 49. [13] M.C. Rossi, S. Salvatori, A. Minutello, G. Conte, V. Ralchenko, Proc. SPIE 6591 (2007) 65910B.