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Electronic and surface properties of H-terminated diamond surface affected by NO2 gas
Diamond & Related Materials 19 (2010) 889–893
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d i a m o n d
Electronic and surface properties of H-terminated diamond surface affected by NO2 gas M. Kubovic, M. Kasu ⁎, H. Kageshima, F. Maeda NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi 243-0198, Japan
a r t i c l e
i n f o
Available online 22 February 2010 Keywords: Diamond film Adsorption Passivation Surface characterization High-frequency device
a b s t r a c t Hydrogen-terminated diamond surface exhibits p-type conductivity during its exposure to air. To investigate this phenomenon, we examined the influence of different gases on the surface conductivity. Exposure to NO2 gas resulted in the biggest increase in conductivity, while H2O vapor decreased the surface conductivity. Moreover, even very low concentrations of NO2 molecules in air increased the hole sheet concentration, and with increasing NO2 concentration, the hole sheet concentration increased up to 2.3 × 1014 cm− 2 (at 300 ppm NO2). This increase of hole sheet concentration was observed during exposure to NO2 gas and simultaneous adsorption of NO2 molecules on the diamond surface, while it decreased when the exposure stopped and NO2 molecules desorbed from the surface. X-ray photoelectron spectroscopy investigation showed upward band bending and partial oxidation of the hydrogen-terminated surface after exposure to air and NO2. FETs exposed to NO2 gas exhibited lower source and drain resistances, which led to a 1.8-fold increase of maximum drain current, transconductance increased 1.5-fold and maximum frequency of oscillation increased 1.6-fold. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Diamond is a promising large-bandgap material for RF (radio frequency) high-performance field effect transistors (FETs) due to its exceptional electrical and thermal properties, such as high mobilities [1], high breakdown voltage, and high thermal conductivity [2]. Undoped diamond is highly insulating, whereas hydrogen-terminated diamond surface exhibits p-type conductivity without impurity doping [3,4]. High-frequency, high-power FETs employing such a p-type conductive channel can operate at millimeter-wave frequencies with cut-off frequency fT of 45 GHz and fmax of 120 GHz [5] and RF output power density of 2.1 W/mm at 1 GHz [6]. However, the origin of the conduction mechanism is still under discussion, though several models have been proposed [7–13]. A significant decrease of the conductivity [14] and hole sheet concentration [15] has been observed in vacuum, while a full recovery has been observed in air, indicating that adsorbates from air are essential for the formation of the hole channel. To clarify the origin of this channel, we need to identify specific adsorbates and investigate their adsorption/desorption on the H-terminated surface. An understanding of the conduction mechanism would enable us to increase the surface conductivity and improve the performance of diamond FETs. To identify which specific gas specie is responsible for the p-type conductivity, H-terminated diamond surface has been exposed to
⁎ Corresponding author. E-mail address: [email protected] (M. Kasu). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.02.021
different atmospheres. Ri et al. have reported that N2, O2, CO2, and H2O vapors do not influence the resistance [16] and that NO2 gas decreases the resistance of the conductive layer [17] and increases the hole sheet concentration (ps) [8]. Our Hall measurements have confirmed that ps significantly increases during exposure to NO2 gas [18] and that even a very low concentration of NO2 molecules in air (∼20 ppb) will noticeably increase ps [19]. The increase of ps during exposure to NO2 is attributed to adsorption of NO2 molecules on the H-terminated diamond surface. When the NO2 exposure is stopped, NO2 molecules desorb from the surface and ps decreases again. As reported in present paper, an investigation of the time evolution of hole sheet concentration during adsorption and desorption of different gas molecules and an X-ray photoelectron spectroscopy (XPS) analysis of diamond surface after different treatments will provide a better understanding of the conduction mechanism. 2. Experimental Undoped homoepitaxial (001)-oriented diamond layers were grown and H-terminated by microwave plasma chemical vapor deposition (MPCVD). The flow rates of H2 and CH4 were 300 and 3 sccm, respectively. The gas pressure was 6.7 kPa, substrate temperature was 700 °C, and microwave power was 1.3 kW. Hall measurements were used to measure the hole sheet concentration during the exposure to different gases. After the deposition of thermally evaporated ohmic Au contacts in the Van der Pauw configuration and before the exposure of the H-terminated diamond surface to the gases, the samples were kept in N2 atmosphere at a constant flow rate
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of 0.4 l/min. Due to the high flow rate and small volume of the measurement chamber of 0.05 l, the gas in the chamber is exchanged in less than 10 s. In order to provide the same starting surface before the actual exposure to a specific gas, the sample was heated for 1 h at 150 °C in N2 atmosphere, which promoted the desorption of previously adsorbed molecules. After the sample had been cooled down to room temperature, the hole concentration started to slowly increase to its original value. The NO2 gas was prepared by using a standard gas generator with NO2 permeation tubes as the gas source. The generated NO2 gas was diluted with N2 gas (purity 5 N) to obtain a specified NO2 concentration. The NO2 concentration of the generated gas was measured by using detector tubes. The NO2 concentration in air was monitored by an environmental monitoring system. Diamond FETs were fabricated as described previously [20]. The FETs use thermally evaporated Au for the ohmic contacts and Al for the gate contact, respectively. Aluminum was evaporated directly onto the hydrogen-terminated diamond surface, and a 5-10 nm thick insulator layer naturally formed underneath the Al contact [21]. Ultraviolet (UV) ozone treatment was used to obtain highly insulating oxygen-terminated surface.
Fig. 2. Change of hole sheet concentration during 1 hour exposure to 5 ppm NO2, and 100% O2 and Ar. The sample was kept in N2 atmosphere before and after exposure to these gases.
4. Influence of NO2 gas on hole sheet concentration 3. Influence of different gases on the hole sheet concentration Surface adsorbates from air are essential for the formation of the conductive layer as shown in Fig. 1, where the current of an ungated FET (a two-terminal device composed of an ohmic source and drain contacts but without a gate contact as shown in the inset) decreases during evacuation of the measurement chamber and fully recovers after subsequent venting of the system with air. To clarify which specific gas specie influences the conductivity the most, we performed Hall measurements during exposure of H-terminated diamond surface to pure N2 (purity 5 N), O2 (99.8%), Ar (5 N), and 5 ppm NO2 (diluted in N2). As shown in Fig. 2, during a 1 hour exposure to argon, the change of ps was minimal (dropped by 3%). After the Ar exposure stopped and N2 atmosphere was restored, ps slowly increased. During a 1 h exposure to oxygen, ps increased 10%. When the O2 exposure stopped and N2 atmosphere was restored, ps initially decreased but then started to slowly increase again. When the sample was exposed to 5 ppm NO2 gas for 1 h, the hole sheet concentration increased 2.5-fold compared to its value before the NO2 exposure. When the NO2 exposure stopped, ps started to decrease due to desorption of NO2 molecules from the surface, as reported previously [18,19]. In these experiments, the three main components of air (N2, O2 and Ar) had little effect on the hole sheet concentration. On the other hand, ps increased very significantly during exposure to NO2 gas.
Fig. 1. Change of current of an ungated FET (the distance between contacts was 5 μm) during evacuation and subsequent ventilation of the measurement system with air.
Hall measurements were performed to observe the change in sheet charge density (ps) and mobility (μ) during an 80 min exposure of the H-terminated diamond surface to NO2 gas. As shown in Fig. 3, ps and μ measured in air just before the exposure to NO2 gas were 4.8×1013 cm− 2 and 19 cm2/Vs, respectively. When the sample was exposed to 3 ppm NO2, the ps rapidly increased from 4.8×1013 to 1.1×1014 cm− 2, while μ dropped slightly from 19 to 17 cm2/Vs. Then, the NO2 exposure was stopped and the measurement chamber was purged with air. The adsorbed NO2 molecules started to desorb from the diamond surface back to air and ps started to drop while μ slightly increased. When the desorption process slowed down, the surface was exposed to a higher NO2 concentration. At 300 ppm NO2, the sheet charge density reached 2.3×1014 cm− 2, which is the highest reported value. At the same time, the conductivity increased almost 4-fold compared to the value measured in air. Although mobility slightly decreased (by around 10%) with increasing NO2 concentration, the maximum ps increased severalfold and conductivity therefore increased significantly as well. A similar increase of conductivity was observed on a variety of samples, such as Sumitomo Electric HTHP-synthesized (high-temperature, high-pressure) type-IIa, Element Six CVD-grown single- and polycrystalline diamond, and our CVD homoepitaxial layers grown on HTHP Ib substrates.
Fig. 3. Hall measurements performed during exposure to different concentrations of NO2 gas. The time evolution of hole sheet concentration reflects the adsorption and desorption of NO2 molecules onto and from the H-terminated diamond surface, respectively.
M. Kubovic et al. / Diamond & Related Materials 19 (2010) 889–893
Fig. 4. Change of hole sheet concentration during 1 h exposure to 300 ppm NO2 (adsorption of NO2 molecules), followed by a three-week long purge of the system in nitrogen atmosphere (desorption of NO2 molecules).
In general, during the exposure to NO2 gas, ps increased drastically within a few minutes, followed by a slower increase until an equilibrium between adsorption and desorption of NO2 molecules was reached. When the NO2 exposure stopped, the initial fast decrease of ps was followed by a steady slower decrease, which slowed down significantly after several days as shown in Fig. 4. After three weeks, the hole sheet concentration was still higher than the ps measured before the NO2 exposure. There are two possible explanations for this: (i) a small amount of physisorbed NO2 molecules remained on the surface or (ii) NO2 molecules partly chemisorbed on the surface. This chemisorption can be linked to a dissociation of NO2 molecules, which involves oxidation of the H-terminated surface and electron transfer (The oxidation was confirmed by XPS as described in section 5). After the NO2 exposure experiments, we investigated the influence of humidity on ps during the desorption process. As shown in Fig. 5, ps (normalized to its maximum value) measured at room temperature in N2 atmosphere decreased after exposure to 5 ppm NO2. In this pure nitrogen atmosphere (circles), ps decreased relatively slowly. On the other hand, in humid nitrogen atmosphere (squares) created with a water bubbling system to obtain a nitrogen gas with assumed 100% relative humidity, ps decreased rapidly, dropping below the value measured before the exposure to NO2 (dashed line) within a few minutes. This fast decrease
Fig. 5. Comparison of hole sheet concentration during the desorption process at 150 °C in N2 atmosphere and during desorption in dry and humid N2 atmospheres after exposure to 5 ppm NO2. The dashed line indicates the hole sheet concentration before exposure to NO2 gas.
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Fig. 6. Change of drain current of a FET with LG = 0.1 μm and WG = 100 μm compared to the change of relative ambient humidity measured at the same time.
can be explained by the decomposition of absorbed NO2 molecules in H2O on the H-terminated surface. When the decrease of ps in humid nitrogen atmosphere is compared to its decrease due to the thermally enhanced desorption process, the presence of water vapor decreased ps even more than desorption of NO2 molecules at elevated temperature did. Furthermore, as shown in Fig. 6, we observed a negative influence of increased ambient humidity on the maximum drain current of a FET. During a long-term measurement, the increase of relative humidity resulted in decreased current, which again increased when the humidity dropped. 5. XPS investigation XPS was used to investigate the diamond surface after different surface treatments. First, the sample was O-terminated by wet chemical oxidation in sulfuric acid solution in addition to UV ozone treatment. Then, the surface was H-terminated in a CVD system, this surface was later exposed to air for three days and finally for 1 hour to 300 ppm NO2. After each of those four treatments, XPS spectra were measured and analyzed. In each case, C 1s core-level peaks were measured with and without the employment of a neutralizing flood gun. An energy shifts was hardly observed (at most less than 0.1 eV), indicating that XPS results were not influenced by the charging effect. As shown in Fig. 7, the XPS spectrum of the O-terminated surface shows a strong O 1s peak. The
Fig. 7. XPS spectra measured after different surface treatments. A strong O 1s peak is present in the case of O-terminated surface. This peak disappeared after hydrogenation of the surface. A weaker O 1s peak reappeared after exposure of H-terminated surface to air and NO2. No N 1s peak was observed during all measurements.
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Fig. 10. Schematic cross-section of a diamond FET during exposure to NO2 gas. The gas influenced only the exposed H-terminated surface area between source (S), gate (G) and drain (D) contacts.
The magnified spectra of O 1s and C 1s peaks of the O-terminated surface and the H-terminated surface after exposure to air are shown in Fig. 8a and b, respectively. The exposure of H-terminated surface to air and NO2 resulted in virtually identical XPS spectra. For both surfaces, peaks with binding energy shifted by 2.7 eV toward higher energy from the main C 1s peak were observed. These peaks are attributed to the chemical state of C =O or carbonyl [22]. In addition, in the air-exposed case, a peak with the binding energy shifted by 4.8 eV from the main peak was observed. The origin of this new peak is not clear. However, because only carbon and oxygen peaks were observed, this peak can be attributed to an oxide of carbon. In this case, from the electronegativity point of view [23], because of the larger binding energy difference from the main peak, the coordination number of carbon atoms with oxygen atoms is larger than that of the C =O bond, which could indicate an O = C = O bond. These results indicate that the O-terminated surface is terminated with a single chemical state of an oxide bond (C=O) but that the air-exposed H-terminated surface is not uniformly terminated with the same bond of carbon and oxygen atoms. As shown in Fig. 9, there was a shift of binding energy of C 1s peaks measured after different surface treatments. The 0.5 eV shift towards lower binding energies indicate upward band bending after hydrogenation of the O-terminated surface. This observation is consistent with a previous XPS study [22]. In our case, an additional shift of 0.15 eV was observed after exposure of the H-terminated surface to air or NO2. Fig. 8. Magnified XPS spectra of a) O-terminated surface and b) H-terminated surface after exposure to air.
6. Influence of NO2 gas on FET characteristics
O 1s peak disappeared after H-termination of the surface, while weaker O 1s peaks reappeared after exposure to air and NO2, indicating partial oxidation of the surface. No N 1s peak was observed, not even after exposure to NO2 gas.
As described before, NO2 gas can be used to increase the conductivity of the surface channel, and therefore to increase the drain current of a FET. As shown in Fig. 10, only areas between metal contacts were affected by the gas, which resulted in increased ps, and decreased source and drain resistances (RS and RD). Fig. 11 shows the
Fig. 9. XPS spectra of the C 1s peak measured after different surface treatments. The shift of this peak towards lower binding energies indicates upward band bending, as shown in the inset.
Fig. 11. Output characteristics of a FET with LG = 0.1 μm and WG = 100 μm measured in air and during exposure to 100 ppm NO2. On-state resistance (RON) is indicated by the dashed lines.
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2.3 × 1014 cm− 2 was measured after exposure to 300 ppm NO2. On the other hand, the presence of water vapors strongly decreased the surface conductivity. XPS measurements showed partial oxidation of H-terminated surface after exposure to air, indicating the formation of carbon oxides other than that on the O-terminated surface. The XPS analysis showed upward band bending after hydrogenation of the surface. FETs exposed to NO2 gas exhibited a 1.8-fold increase in maximum drain current, and power gain cut-off frequency increased 1.6-fold. Acknowledgment
Fig. 12. Frequency dependence of power gain of a FET with LG = 0.1 μm and WG = 50 μm in air and during exposure to 100 ppm NO2.
We thank Yoshiharu Yamauchi (NTT Basic Research Laboratories), Yoko Maruo (NTT Energy and Environment System Laboratories), and Hiroshi Ando (NTT Applied Technology) for their technical expertise and helpful discussions. This work was partly supported by the SCOPE project of the Ministry of Internal Affairs and Communications, Japan. References
output drain current–voltage (IV) characteristic of a FET with LG = 0.1 μm and WG = 100 μm measured in air and during exposure to 100 ppm NO2. The on-state resistance between the source and drain (RON) decreased 1.7-fold and the maximum transconductance increased 1.5-fold. The maximum drain current measured at VDS = −10 V and VGS = −3.5 V increased by 80%, from 235 to 425 mA/mm, after the NO2 exposure. Scattering parameters were measured in order to evaluate the RF capabilities of these FETs. Fig. 12 shows the RF power gain plot of a FET (LG = 0.1 μm and WG = 50 μm) measured at VDS = −20 V and VGS = −1 V in air and during exposure to NO2. The maximum frequency of oscillation (fmax) was extracted from the frequency dependence of unilateral power gain using the slope of − 6 dB/oct. Low source resistance is important for achieving high fmax [24]. After the exposure to 100 ppm NO2, which increased the surface conductivity and decreased RS, fmax increased from 25 to 40 GHz and the fmax/fT ratio increased from 1.7 to 2.7. On the other hand, fT changed only slightly from 15 to 16 GHz. The fT depends mainly on the channel properties underneath the gate contact [24]. RS and RD have only minimal influence on fT [25]. Nevertheless, Fig. 11 suggests a slight change of the threshold voltage, which indicates that the NO2 gas probably slightly influenced the channel properties underneath the edges of the gate contact. This would decrease the effective gate length, which could explain the small increase of fT. 7. Conclusion The exposure to NO2 gas increased hole sheet concentration significantly, while the main components of air (N2, O2, and Ar) had little influenced on the ps. A very high hole sheet concentration of
[1] J. Isberg, J. Hammersberg, E. Johansson, T. Wikström, D.J. Twitchen, A.J. Whitehead, S.E. Coe, G.A. Scarsbrook, Science 297 (2002) 1670. [2] A.T. Collins, in: G. Davies (Ed.), Properties and Growth of Diamond, INSPEC London, 1994, p. 228. [3] M.I. Landstrass, K.V. Ravi, Appl. Phys. Lett. 55 (1989) 975. [4] H. Kawarada, Surf. Sci. Rep. 26 (1996) 205. [5] K. Ueda, M. Kasu, Y. Yamauchi, T. Makimoto, M. Schwitters, D.J. Twitchen, G.A. Scarsbrook, S.E. Coe, Electron Devic. Lett. 27 (2006) 570. [6] M. Kasu, K. Ueda, H. Ye, Y. Yamauchi, S. Sasaki, T. Makimoto, Electron. Lett. 41 (2005) 1249. [7] H. Kawarada, H. Sasaki, A. Sato, Phys. Rev. B 52 (1995) 11351. [8] S.G. Ri, K. Tashiro, S. Tanaka, T. Fujisawa, H. Kimura, T. Kurosu, M. Iida, Jpn. J. Appl. Phys. 38 (1999) 3492. [9] F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys Rev Lett 85 (2000) 3472. [10] L. Ley, J. Ristein, F. Meier, M. Riedel, P. Strobel, Physica B 376 (2006) 262. [11] D. Petrini, K. Larsson, J. Phys. Chem. C 111 (2007) 13804. [12] V. Chakrapani, J.C. Angus, A.B. Anderson, S.D. Wolter, B.R. Stoner, G.U. Sumanasekera, Science 318 (2007) 1424. [13] K. Hirama, H. Takayanagi, S. Yamauchi, J.H. Yang, H. Kawarada, H. Umezawa, Appl. Phys. Lett. 92 (2008) 112107. [14] A. Denisenko, A. Aleksov, A. Pribil, P. Gluche, W. Ebert, E. Kohn, Diamond Relat. Mater. 9 (2000) 1138. [15] M. Kasu, M. Kubovic, A. Aleksov, N. Teofilov, Y. Taniyasu, R. Sauer, E. Kohn, T. Makimoto, N. Kobayashi, Diamond Relat. Mater. 13 (2004) 226. [16] S.G. Ri, T. Mizumasa, Y. Akiba, Y. Hirose, T. Kurosu, M. Iida, Jpn. J. Appl. Phys. 34 (1995) 5550. [17] S.G. Ri, T. Ishikawa, S. Tanaka, T. Kimura, Y. Akiba, M. Iida, Jpn. J. Appl. Phys. 36 (1997) 2057. [18] M. Kubovic, M. Kasu, Appl. Phys. Express 2 (2009) 086502. [19] M. Kubovic, M. Kasu, H. Kageshima, Appl. Phys. Lett. 96 (2010) 052101. [20] M. Kasu, K. Ueda, H. Kageshima, Y. Yamauchi, Diamond Relat. Mater. 17 (2008) 741. [21] M. Kubovic, M. Kasu, Y. Yamauchi, K. Ueda, H. Kageshima, Diamond Relat. Mater. 18 (2009) 796. [22] J.I.B. Wilson, J.S. Walton, G. Beamson, J. Electron Spectrosc. Relat. Phenom. 121 (2001) 183. [23] T. Hattori, T. Igarashi, M. Ohi, H. Yamagishi, Jpn. J. Appl. Phys. 28 (1989) L1436. [24] P. Chavarkar, U.K. Mishra, in: M. Golio (Ed.), RF and Microwave Semiconductor Device Handbook, CRC Press, Boca Raton, 2001, p. 8-7. [25] P.J. Tasker, B. Hughes, El. Dev. Lett. 10 (1989) 291.
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 e v i e r. c o m / l o c a t e / d i a m o n d
Electronic and surface properties of H-terminated diamond surface affected by NO2 gas M. Kubovic, M. Kasu ⁎, H. Kageshima, F. Maeda NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi 243-0198, Japan
a r t i c l e
i n f o
Available online 22 February 2010 Keywords: Diamond film Adsorption Passivation Surface characterization High-frequency device
a b s t r a c t Hydrogen-terminated diamond surface exhibits p-type conductivity during its exposure to air. To investigate this phenomenon, we examined the influence of different gases on the surface conductivity. Exposure to NO2 gas resulted in the biggest increase in conductivity, while H2O vapor decreased the surface conductivity. Moreover, even very low concentrations of NO2 molecules in air increased the hole sheet concentration, and with increasing NO2 concentration, the hole sheet concentration increased up to 2.3 × 1014 cm− 2 (at 300 ppm NO2). This increase of hole sheet concentration was observed during exposure to NO2 gas and simultaneous adsorption of NO2 molecules on the diamond surface, while it decreased when the exposure stopped and NO2 molecules desorbed from the surface. X-ray photoelectron spectroscopy investigation showed upward band bending and partial oxidation of the hydrogen-terminated surface after exposure to air and NO2. FETs exposed to NO2 gas exhibited lower source and drain resistances, which led to a 1.8-fold increase of maximum drain current, transconductance increased 1.5-fold and maximum frequency of oscillation increased 1.6-fold. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Diamond is a promising large-bandgap material for RF (radio frequency) high-performance field effect transistors (FETs) due to its exceptional electrical and thermal properties, such as high mobilities [1], high breakdown voltage, and high thermal conductivity [2]. Undoped diamond is highly insulating, whereas hydrogen-terminated diamond surface exhibits p-type conductivity without impurity doping [3,4]. High-frequency, high-power FETs employing such a p-type conductive channel can operate at millimeter-wave frequencies with cut-off frequency fT of 45 GHz and fmax of 120 GHz [5] and RF output power density of 2.1 W/mm at 1 GHz [6]. However, the origin of the conduction mechanism is still under discussion, though several models have been proposed [7–13]. A significant decrease of the conductivity [14] and hole sheet concentration [15] has been observed in vacuum, while a full recovery has been observed in air, indicating that adsorbates from air are essential for the formation of the hole channel. To clarify the origin of this channel, we need to identify specific adsorbates and investigate their adsorption/desorption on the H-terminated surface. An understanding of the conduction mechanism would enable us to increase the surface conductivity and improve the performance of diamond FETs. To identify which specific gas specie is responsible for the p-type conductivity, H-terminated diamond surface has been exposed to
⁎ Corresponding author. E-mail address: [email protected] (M. Kasu). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.02.021
different atmospheres. Ri et al. have reported that N2, O2, CO2, and H2O vapors do not influence the resistance [16] and that NO2 gas decreases the resistance of the conductive layer [17] and increases the hole sheet concentration (ps) [8]. Our Hall measurements have confirmed that ps significantly increases during exposure to NO2 gas [18] and that even a very low concentration of NO2 molecules in air (∼20 ppb) will noticeably increase ps [19]. The increase of ps during exposure to NO2 is attributed to adsorption of NO2 molecules on the H-terminated diamond surface. When the NO2 exposure is stopped, NO2 molecules desorb from the surface and ps decreases again. As reported in present paper, an investigation of the time evolution of hole sheet concentration during adsorption and desorption of different gas molecules and an X-ray photoelectron spectroscopy (XPS) analysis of diamond surface after different treatments will provide a better understanding of the conduction mechanism. 2. Experimental Undoped homoepitaxial (001)-oriented diamond layers were grown and H-terminated by microwave plasma chemical vapor deposition (MPCVD). The flow rates of H2 and CH4 were 300 and 3 sccm, respectively. The gas pressure was 6.7 kPa, substrate temperature was 700 °C, and microwave power was 1.3 kW. Hall measurements were used to measure the hole sheet concentration during the exposure to different gases. After the deposition of thermally evaporated ohmic Au contacts in the Van der Pauw configuration and before the exposure of the H-terminated diamond surface to the gases, the samples were kept in N2 atmosphere at a constant flow rate
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of 0.4 l/min. Due to the high flow rate and small volume of the measurement chamber of 0.05 l, the gas in the chamber is exchanged in less than 10 s. In order to provide the same starting surface before the actual exposure to a specific gas, the sample was heated for 1 h at 150 °C in N2 atmosphere, which promoted the desorption of previously adsorbed molecules. After the sample had been cooled down to room temperature, the hole concentration started to slowly increase to its original value. The NO2 gas was prepared by using a standard gas generator with NO2 permeation tubes as the gas source. The generated NO2 gas was diluted with N2 gas (purity 5 N) to obtain a specified NO2 concentration. The NO2 concentration of the generated gas was measured by using detector tubes. The NO2 concentration in air was monitored by an environmental monitoring system. Diamond FETs were fabricated as described previously [20]. The FETs use thermally evaporated Au for the ohmic contacts and Al for the gate contact, respectively. Aluminum was evaporated directly onto the hydrogen-terminated diamond surface, and a 5-10 nm thick insulator layer naturally formed underneath the Al contact [21]. Ultraviolet (UV) ozone treatment was used to obtain highly insulating oxygen-terminated surface.
Fig. 2. Change of hole sheet concentration during 1 hour exposure to 5 ppm NO2, and 100% O2 and Ar. The sample was kept in N2 atmosphere before and after exposure to these gases.
4. Influence of NO2 gas on hole sheet concentration 3. Influence of different gases on the hole sheet concentration Surface adsorbates from air are essential for the formation of the conductive layer as shown in Fig. 1, where the current of an ungated FET (a two-terminal device composed of an ohmic source and drain contacts but without a gate contact as shown in the inset) decreases during evacuation of the measurement chamber and fully recovers after subsequent venting of the system with air. To clarify which specific gas specie influences the conductivity the most, we performed Hall measurements during exposure of H-terminated diamond surface to pure N2 (purity 5 N), O2 (99.8%), Ar (5 N), and 5 ppm NO2 (diluted in N2). As shown in Fig. 2, during a 1 hour exposure to argon, the change of ps was minimal (dropped by 3%). After the Ar exposure stopped and N2 atmosphere was restored, ps slowly increased. During a 1 h exposure to oxygen, ps increased 10%. When the O2 exposure stopped and N2 atmosphere was restored, ps initially decreased but then started to slowly increase again. When the sample was exposed to 5 ppm NO2 gas for 1 h, the hole sheet concentration increased 2.5-fold compared to its value before the NO2 exposure. When the NO2 exposure stopped, ps started to decrease due to desorption of NO2 molecules from the surface, as reported previously [18,19]. In these experiments, the three main components of air (N2, O2 and Ar) had little effect on the hole sheet concentration. On the other hand, ps increased very significantly during exposure to NO2 gas.
Fig. 1. Change of current of an ungated FET (the distance between contacts was 5 μm) during evacuation and subsequent ventilation of the measurement system with air.
Hall measurements were performed to observe the change in sheet charge density (ps) and mobility (μ) during an 80 min exposure of the H-terminated diamond surface to NO2 gas. As shown in Fig. 3, ps and μ measured in air just before the exposure to NO2 gas were 4.8×1013 cm− 2 and 19 cm2/Vs, respectively. When the sample was exposed to 3 ppm NO2, the ps rapidly increased from 4.8×1013 to 1.1×1014 cm− 2, while μ dropped slightly from 19 to 17 cm2/Vs. Then, the NO2 exposure was stopped and the measurement chamber was purged with air. The adsorbed NO2 molecules started to desorb from the diamond surface back to air and ps started to drop while μ slightly increased. When the desorption process slowed down, the surface was exposed to a higher NO2 concentration. At 300 ppm NO2, the sheet charge density reached 2.3×1014 cm− 2, which is the highest reported value. At the same time, the conductivity increased almost 4-fold compared to the value measured in air. Although mobility slightly decreased (by around 10%) with increasing NO2 concentration, the maximum ps increased severalfold and conductivity therefore increased significantly as well. A similar increase of conductivity was observed on a variety of samples, such as Sumitomo Electric HTHP-synthesized (high-temperature, high-pressure) type-IIa, Element Six CVD-grown single- and polycrystalline diamond, and our CVD homoepitaxial layers grown on HTHP Ib substrates.
Fig. 3. Hall measurements performed during exposure to different concentrations of NO2 gas. The time evolution of hole sheet concentration reflects the adsorption and desorption of NO2 molecules onto and from the H-terminated diamond surface, respectively.
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Fig. 4. Change of hole sheet concentration during 1 h exposure to 300 ppm NO2 (adsorption of NO2 molecules), followed by a three-week long purge of the system in nitrogen atmosphere (desorption of NO2 molecules).
In general, during the exposure to NO2 gas, ps increased drastically within a few minutes, followed by a slower increase until an equilibrium between adsorption and desorption of NO2 molecules was reached. When the NO2 exposure stopped, the initial fast decrease of ps was followed by a steady slower decrease, which slowed down significantly after several days as shown in Fig. 4. After three weeks, the hole sheet concentration was still higher than the ps measured before the NO2 exposure. There are two possible explanations for this: (i) a small amount of physisorbed NO2 molecules remained on the surface or (ii) NO2 molecules partly chemisorbed on the surface. This chemisorption can be linked to a dissociation of NO2 molecules, which involves oxidation of the H-terminated surface and electron transfer (The oxidation was confirmed by XPS as described in section 5). After the NO2 exposure experiments, we investigated the influence of humidity on ps during the desorption process. As shown in Fig. 5, ps (normalized to its maximum value) measured at room temperature in N2 atmosphere decreased after exposure to 5 ppm NO2. In this pure nitrogen atmosphere (circles), ps decreased relatively slowly. On the other hand, in humid nitrogen atmosphere (squares) created with a water bubbling system to obtain a nitrogen gas with assumed 100% relative humidity, ps decreased rapidly, dropping below the value measured before the exposure to NO2 (dashed line) within a few minutes. This fast decrease
Fig. 5. Comparison of hole sheet concentration during the desorption process at 150 °C in N2 atmosphere and during desorption in dry and humid N2 atmospheres after exposure to 5 ppm NO2. The dashed line indicates the hole sheet concentration before exposure to NO2 gas.
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Fig. 6. Change of drain current of a FET with LG = 0.1 μm and WG = 100 μm compared to the change of relative ambient humidity measured at the same time.
can be explained by the decomposition of absorbed NO2 molecules in H2O on the H-terminated surface. When the decrease of ps in humid nitrogen atmosphere is compared to its decrease due to the thermally enhanced desorption process, the presence of water vapor decreased ps even more than desorption of NO2 molecules at elevated temperature did. Furthermore, as shown in Fig. 6, we observed a negative influence of increased ambient humidity on the maximum drain current of a FET. During a long-term measurement, the increase of relative humidity resulted in decreased current, which again increased when the humidity dropped. 5. XPS investigation XPS was used to investigate the diamond surface after different surface treatments. First, the sample was O-terminated by wet chemical oxidation in sulfuric acid solution in addition to UV ozone treatment. Then, the surface was H-terminated in a CVD system, this surface was later exposed to air for three days and finally for 1 hour to 300 ppm NO2. After each of those four treatments, XPS spectra were measured and analyzed. In each case, C 1s core-level peaks were measured with and without the employment of a neutralizing flood gun. An energy shifts was hardly observed (at most less than 0.1 eV), indicating that XPS results were not influenced by the charging effect. As shown in Fig. 7, the XPS spectrum of the O-terminated surface shows a strong O 1s peak. The
Fig. 7. XPS spectra measured after different surface treatments. A strong O 1s peak is present in the case of O-terminated surface. This peak disappeared after hydrogenation of the surface. A weaker O 1s peak reappeared after exposure of H-terminated surface to air and NO2. No N 1s peak was observed during all measurements.
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Fig. 10. Schematic cross-section of a diamond FET during exposure to NO2 gas. The gas influenced only the exposed H-terminated surface area between source (S), gate (G) and drain (D) contacts.
The magnified spectra of O 1s and C 1s peaks of the O-terminated surface and the H-terminated surface after exposure to air are shown in Fig. 8a and b, respectively. The exposure of H-terminated surface to air and NO2 resulted in virtually identical XPS spectra. For both surfaces, peaks with binding energy shifted by 2.7 eV toward higher energy from the main C 1s peak were observed. These peaks are attributed to the chemical state of C =O or carbonyl [22]. In addition, in the air-exposed case, a peak with the binding energy shifted by 4.8 eV from the main peak was observed. The origin of this new peak is not clear. However, because only carbon and oxygen peaks were observed, this peak can be attributed to an oxide of carbon. In this case, from the electronegativity point of view [23], because of the larger binding energy difference from the main peak, the coordination number of carbon atoms with oxygen atoms is larger than that of the C =O bond, which could indicate an O = C = O bond. These results indicate that the O-terminated surface is terminated with a single chemical state of an oxide bond (C=O) but that the air-exposed H-terminated surface is not uniformly terminated with the same bond of carbon and oxygen atoms. As shown in Fig. 9, there was a shift of binding energy of C 1s peaks measured after different surface treatments. The 0.5 eV shift towards lower binding energies indicate upward band bending after hydrogenation of the O-terminated surface. This observation is consistent with a previous XPS study [22]. In our case, an additional shift of 0.15 eV was observed after exposure of the H-terminated surface to air or NO2. Fig. 8. Magnified XPS spectra of a) O-terminated surface and b) H-terminated surface after exposure to air.
6. Influence of NO2 gas on FET characteristics
O 1s peak disappeared after H-termination of the surface, while weaker O 1s peaks reappeared after exposure to air and NO2, indicating partial oxidation of the surface. No N 1s peak was observed, not even after exposure to NO2 gas.
As described before, NO2 gas can be used to increase the conductivity of the surface channel, and therefore to increase the drain current of a FET. As shown in Fig. 10, only areas between metal contacts were affected by the gas, which resulted in increased ps, and decreased source and drain resistances (RS and RD). Fig. 11 shows the
Fig. 9. XPS spectra of the C 1s peak measured after different surface treatments. The shift of this peak towards lower binding energies indicates upward band bending, as shown in the inset.
Fig. 11. Output characteristics of a FET with LG = 0.1 μm and WG = 100 μm measured in air and during exposure to 100 ppm NO2. On-state resistance (RON) is indicated by the dashed lines.
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2.3 × 1014 cm− 2 was measured after exposure to 300 ppm NO2. On the other hand, the presence of water vapors strongly decreased the surface conductivity. XPS measurements showed partial oxidation of H-terminated surface after exposure to air, indicating the formation of carbon oxides other than that on the O-terminated surface. The XPS analysis showed upward band bending after hydrogenation of the surface. FETs exposed to NO2 gas exhibited a 1.8-fold increase in maximum drain current, and power gain cut-off frequency increased 1.6-fold. Acknowledgment
Fig. 12. Frequency dependence of power gain of a FET with LG = 0.1 μm and WG = 50 μm in air and during exposure to 100 ppm NO2.
We thank Yoshiharu Yamauchi (NTT Basic Research Laboratories), Yoko Maruo (NTT Energy and Environment System Laboratories), and Hiroshi Ando (NTT Applied Technology) for their technical expertise and helpful discussions. This work was partly supported by the SCOPE project of the Ministry of Internal Affairs and Communications, Japan. References
output drain current–voltage (IV) characteristic of a FET with LG = 0.1 μm and WG = 100 μm measured in air and during exposure to 100 ppm NO2. The on-state resistance between the source and drain (RON) decreased 1.7-fold and the maximum transconductance increased 1.5-fold. The maximum drain current measured at VDS = −10 V and VGS = −3.5 V increased by 80%, from 235 to 425 mA/mm, after the NO2 exposure. Scattering parameters were measured in order to evaluate the RF capabilities of these FETs. Fig. 12 shows the RF power gain plot of a FET (LG = 0.1 μm and WG = 50 μm) measured at VDS = −20 V and VGS = −1 V in air and during exposure to NO2. The maximum frequency of oscillation (fmax) was extracted from the frequency dependence of unilateral power gain using the slope of − 6 dB/oct. Low source resistance is important for achieving high fmax [24]. After the exposure to 100 ppm NO2, which increased the surface conductivity and decreased RS, fmax increased from 25 to 40 GHz and the fmax/fT ratio increased from 1.7 to 2.7. On the other hand, fT changed only slightly from 15 to 16 GHz. The fT depends mainly on the channel properties underneath the gate contact [24]. RS and RD have only minimal influence on fT [25]. Nevertheless, Fig. 11 suggests a slight change of the threshold voltage, which indicates that the NO2 gas probably slightly influenced the channel properties underneath the edges of the gate contact. This would decrease the effective gate length, which could explain the small increase of fT. 7. Conclusion The exposure to NO2 gas increased hole sheet concentration significantly, while the main components of air (N2, O2, and Ar) had little influenced on the ps. A very high hole sheet concentration of
[1] J. Isberg, J. Hammersberg, E. Johansson, T. Wikström, D.J. Twitchen, A.J. Whitehead, S.E. Coe, G.A. Scarsbrook, Science 297 (2002) 1670. [2] A.T. Collins, in: G. Davies (Ed.), Properties and Growth of Diamond, INSPEC London, 1994, p. 228. [3] M.I. Landstrass, K.V. Ravi, Appl. Phys. Lett. 55 (1989) 975. [4] H. Kawarada, Surf. Sci. Rep. 26 (1996) 205. [5] K. Ueda, M. Kasu, Y. Yamauchi, T. Makimoto, M. Schwitters, D.J. Twitchen, G.A. Scarsbrook, S.E. Coe, Electron Devic. Lett. 27 (2006) 570. [6] M. Kasu, K. Ueda, H. Ye, Y. Yamauchi, S. Sasaki, T. Makimoto, Electron. Lett. 41 (2005) 1249. [7] H. Kawarada, H. Sasaki, A. Sato, Phys. Rev. B 52 (1995) 11351. [8] S.G. Ri, K. Tashiro, S. Tanaka, T. Fujisawa, H. Kimura, T. Kurosu, M. Iida, Jpn. J. Appl. Phys. 38 (1999) 3492. [9] F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys Rev Lett 85 (2000) 3472. [10] L. Ley, J. Ristein, F. Meier, M. Riedel, P. Strobel, Physica B 376 (2006) 262. [11] D. Petrini, K. Larsson, J. Phys. Chem. C 111 (2007) 13804. [12] V. Chakrapani, J.C. Angus, A.B. Anderson, S.D. Wolter, B.R. Stoner, G.U. Sumanasekera, Science 318 (2007) 1424. [13] K. Hirama, H. Takayanagi, S. Yamauchi, J.H. Yang, H. Kawarada, H. Umezawa, Appl. Phys. Lett. 92 (2008) 112107. [14] A. Denisenko, A. Aleksov, A. Pribil, P. Gluche, W. Ebert, E. Kohn, Diamond Relat. Mater. 9 (2000) 1138. [15] M. Kasu, M. Kubovic, A. Aleksov, N. Teofilov, Y. Taniyasu, R. Sauer, E. Kohn, T. Makimoto, N. Kobayashi, Diamond Relat. Mater. 13 (2004) 226. [16] S.G. Ri, T. Mizumasa, Y. Akiba, Y. Hirose, T. Kurosu, M. Iida, Jpn. J. Appl. Phys. 34 (1995) 5550. [17] S.G. Ri, T. Ishikawa, S. Tanaka, T. Kimura, Y. Akiba, M. Iida, Jpn. J. Appl. Phys. 36 (1997) 2057. [18] M. Kubovic, M. Kasu, Appl. Phys. Express 2 (2009) 086502. [19] M. Kubovic, M. Kasu, H. Kageshima, Appl. Phys. Lett. 96 (2010) 052101. [20] M. Kasu, K. Ueda, H. Kageshima, Y. Yamauchi, Diamond Relat. Mater. 17 (2008) 741. [21] M. Kubovic, M. Kasu, Y. Yamauchi, K. Ueda, H. Kageshima, Diamond Relat. Mater. 18 (2009) 796. [22] J.I.B. Wilson, J.S. Walton, G. Beamson, J. Electron Spectrosc. Relat. Phenom. 121 (2001) 183. [23] T. Hattori, T. Igarashi, M. Ohi, H. Yamagishi, Jpn. J. Appl. Phys. 28 (1989) L1436. [24] P. Chavarkar, U.K. Mishra, in: M. Golio (Ed.), RF and Microwave Semiconductor Device Handbook, CRC Press, Boca Raton, 2001, p. 8-7. [25] P.J. Tasker, B. Hughes, El. Dev. Lett. 10 (1989) 291.