Electronic properties of H-terminated diamond during NO2 and O3 adsorption and desorption

Diamond & Related Materials 24 (2012) 99–103

Contents lists available at SciVerse ScienceDirect

Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond

Electronic properties of H-terminated diamond during NO2 and O3 adsorption and desorption☆ Hisashi Sato ⁎, Makoto Kasu 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 20 December 2011 Keywords: Diamond Hydrogen-termination Molecular adsorption Hole channel

a b s t r a c t We investigated hole sheet concentration and mobility during NO2 or O3 adsorption/desorption on Hterminated diamond surface with a hole sheet concentration (ps) of ~1 × 10 14 cm− 2. During NO2 adsorption, ps first increased with time and eventually saturated. When the NO2 gas concentration increased in a range of b 300 ppm, the saturated value of ps increased. However, in the range of > 300 ppm, the values were the same, and we therefore determined that the high limit of ps is ~ 9 × 1013 cm − 2 for (001) orientation. Further, we found that during the NO2 adsorption process hole mobility (μ) stays constant, while ps is increasing. We propose a NO2 adsorption/desorption and hole-generation model to explain these experimental results. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen (H)-terminated diamond surface exhibits p-type conductivity [1,2] when it is exposed to air [3,4], because a two-dimensional hole channel forms near the surface. Typical hole sheet concentration (ps) is ~1 × 1013 cm− 2. The mobility (μ) is ~100 cm2 V− 1 s − 1, and the sheet resistance (Rs) is ~5 kΩ. The two-dimensional hole channel on H-terminated diamond has been used for field effect transistors (FETs) [5–7]. We reported high cut-off frequencies fT (transition frequency) and fMAX (the maximum frequency of oscillation) of 45 and 120 GHz, respectively [8], and Pout (RF output power density) of 2.1 W/mm at 1 GHz [9], which demonstrates diamond's potential for use in RF high-power amplifiers in microwave and millimeter-wave ranges. Investigations of the mechanism of the hole emergence in air have been performed [3,10–13] and most of them proposed models that assumed thin water layers on the diamond surfaces. Following the discovery of the hole emergence in air, Gi et al. reported enhanced hole emergence in oxidizing gases including NO2 and O3, which are slightly soluble in water, and explained the emergence by a water-free model [14,15]. Recently we also found that NO2 (~ 5 ppb) in air induces p-type conductivity on H-terminated diamond and identified NO2, NO, O3, and SO2 as gases that form surface holes on it [16] and O2 as a gas that increase surface hole slightly [17]. Using 300-ppm NO2 gas, we increased ps up to ~1014 cm− 2, one order of magnitude higher than that in air. From an analysis of the time evolution of adsorption/desorption, we found that the sorption process consists of two or three fundamental steps [18]. ☆ Presented at NDNC 2011, the 5th International Conference on New Diamond and Nano Carbons, Suzhou, China. ⁎ Corresponding author. E-mail address: [email protected] (H. Sato). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.12.004

From secondary ion mass spectroscopy (SIMS), we confirmed that oxygen accumulates on H-terminated diamond surface [19]. By using the technology of NO2 adsorption on H-terminated diamond, drain current ID in the FETs increased 1.8 fold, transconductance gm increased 1.5 fold and power-gain cut-off frequency fMAX increased 1.6 fold [19]. In this paper, we report simultaneous measurements of ps and μ using much more concentrated NO2 and investigate the adsorption/ desorption processes on H-terminated diamond. We propose a model of adsorption/desorption and hole generation to explain the results. 2. Experimental procedures The samples were chemical-vapor-deposited (CVD) (001) singlecrystal diamond free-standing films (5 × 5 × 0.5 mm 3) with extremely low N concentration of b5 ppb and B concentration of b1 ppb. We confirmed that the sample surface before any treatment was highly insulating (>200 GΩ of sheet resistance). As a H-termination surface treatment, the diamond surface was exposed to a plasma ball in H2 atmosphere (50 Torr) in a microwave plasma CVD chamber with the microwave power of 1.3 kW at 700 °C for 30 min. Afterwards, four gold contacts were evaporated near the corner edges on the H-terminated surface in the van der Pauw configuration. The ps and μ were measured simultaneously by Hall measurement at room temperature (RT). The sample was put in a closed box during the Hall measurement. The box volume was 0.05 l, and it was able to be filled with a specific gas and evacuated with a turbo molecular pump (evacuation rate: 70 l/s). Thus, gas filling and evacuation took less than 1 s. NO2 used in the measurements was diluted at the specific gas concentration with N2 gas (purity: 7 N), and the 1-ppm O3 used in this measurement was generated with an ozonizer using dry air as a source.

H. Sato, M. Kasu / Diamond & Related Materials 24 (2012) 99–103

3. Results Fig. 1 compares the time evolution of ps and μ during (a) 5 (b) 300, (c) 20,000-ppm NO2 and (d) 1-ppm O3 exposure. For all cases, the sample was evacuated first and then the gas was supplied to the sample box. For 5-ppm NO2 [Fig. 1 (a)], after the start of the gas flow (t = 0), ps increased from 7.3 × 1012 to 2.8 × 1013 cm− 2 until t ~ 2 h. Afterwards, ps increased more slowly and then at t ~ 165 h saturated at 5.2 × 1013 cm− 2. For 300-ppm NO2 [Fig. 1(b)], after the supply of NO2 gas (t = 0), ps increased from 5.7 × 1012 to saturate at 9.3 × 10 13 cm− 2 in 50 h. For 20,000-ppm NO2 [Fig. 1(c)] after the supply of NO2 gas (t = 0), ps increased from 4.9 × 1013 rapidly within 3 h to saturate at 8.9 × 1013 cm− 2. We checked the reproducibility for the 20,000-ppm NO2, finding that the saturated values of ps coincided within 1% for two independent experimental sequences and that μ was also reproduced within 5% in the two runs. Another interesting finding is that μ remained almost constant while the ps was increasing. For example, for the 5-ppm case [Fig. 1(a)], after the NO2 supply (t = 0), from 20 to 165 h, ps increased slowly but μ stayed constant at around 60 cm2 V− 1 s − 1. Similarly, in the 300-ppm case [Fig. 1(b)], from 20 to 60 h, ps increased slowly but μ stayed at ~ 50 cm 2 V − 1 s − 1. In semiconductors, μ normally decreases as ps increases because more ionized impurities scatter carriers more often. That μ did not change while ps increased which indicates that 1.2 x 10 14

(b) 1.2 x 10

120

14

120

300-ppm NO2 exposure 100

1 x 1014

100

8 x 10 13

80

8 x 10 13

80

6 x 10 13

60

6 x 10 13

60

4 x 10 13

40

4 x 10 13

40

2 x 10 13

20

2 x 10 13

20

0

0

50

100

150

200

ps(cm-2)

1 x 1014

(cm2V-1s-1)

ps(cm-2)

5-ppm NO2 exposure

0

0

0

50

100

Time (hour)

(c) 1.2 x 10

200

250

300

0 350

Time (hour)

(d)

120

14

120

1.2 x 10 14

20000-ppm NO2 exposure

1-ppm O3 exposure 100

1 x 1014

100

8 x 10 13

80

8 x 10 13

80

6 x 10 13

60

6 x 10 13

60

4 x 10 13

40

4 x 10 13

40

2 x 10 13

20

2 x 10 13

20

0

-5

0

Time (hour)

5

0 10

ps(cm-2)

1 x 1014

(cm2V-1s-1)

ps(cm-2)

150

(cm2V-1s-1)

(a)

ionized impurity scattering does not dominate μ. The hole carriers may be scattered by surface roughness or inhomogeneous random potential. As clearly seen for 5-ppm NO2 process [Fig. 1(a)], there are fast and slow increases of ps during NO2 adsorption. As we reported in Ref. [12], an analysis using the Elovich equation showed that at least two fundamental processes occur during both adsorption and desorption. We will explain these results in Section 4. For 1-ppm O3 [Fig. 1(d)] during adsorption, ps increased from 8.7 × 1011 to 1.81× 1013 cm− 2 for ~4 h. However, after t ~ 4 h, contrary to the NO2 case, ps decreased from 1.81 × 1013 cm− 2 to 9.2 × 1012 cm− 2. The reason for this may be that extra O3 molecules break C\H bonds on the surface. Fig. 2 compares the time evolutions of ps and μ during exposure to NO2 with different concentrations. For all cases, saturation of ps during NO2 adsorption was clearly observed. As we reported in Ref. [12], as the NO2 gas concentration increases, the saturated ps value increases. However, as seen for 300- and 20,000-ppm NO2, the saturated ps values were the same at ~9 × 1013 cm− 2. This indicates that the value of ~9 × 1013 cm− 2 is the high limit ps for H-terminated diamond (001) surface. Further, as shown in Fig. 2(b), as the NO2 concentration increased, lower μ was obtained. Fig. 3 compares changes of ps and μ when the sample was evacuated to a vacuum after exposure to the same concentrations of NO2 and O3 (1 ppm) for ~ 0.5 h. The time of 0.5 h was less than the time it

0

0

20

40

60

(cm2V-1s-1)

100

0 80

Time (hour)

Fig. 1. Timewise changes of the sheet hole sheet concentration ps and mobility μ of H-terminated (001) diamond surface at room temperature during exposure to (a) 5-, (b) 300-, and (c) 20,000-ppm NO2 and (d) 1-ppm O3.

H. Sato, M. Kasu / Diamond & Related Materials 24 (2012) 99–103

(a)

(a) 1.4 x 10 14

in Vacuum after 1-ppm gas exposure

2.5 x 10 13 NO2

ps (cm-2)

ps (cm-2)

3 x 10 13

NO2 exposure

1.2 x 10 14 1 x 10

101

start of evacuation

14

20000 ppm 300 ppm

8 x 10 13

2 x 10 13 1.5 x 10 13 O3

in vacuum

1 x 10 13

6 x 10 13

5 x 10 12

4 x 10 13 5 ppm

0

2 x 10 13

0

200

400

600

800

Time (hour) 0

0

10

20

30

40

50

(b)

120

Time (hour)

(b)

80

NO2

µ (cm2V-1s-1)

NO2 exposure 70

µ (cm2V-1s-1)

in Vacuum after 1-ppm gas exposure

100

5 ppm

60

80 60 O3

40 20

300 ppm

0 50

200

400

600

800

Time (hour) in vacuum

20000 ppm start of evacuation

40

0

0

10

20

30

40

50

Fig. 3. Timewise changes of the (a) sheet hole sheet concentration ps and (b) mobility μ of H-terminated (001) diamond surface at room temperature in vacuum after exposure to NO2 and O3 with the same 1 ppm for 30 min.

Time (hour) Fig. 2. Comparison of the (a) sheet hole sheet concentration ps and (b) mobility μ of H-terminated (001) diamond surface at room temperature during exposure to NO2 with different concentrations. Only for the 20,000-ppm case, the changes during evacuation (in vacuum) after exposure are shown as well.

takes for the H-terminated surface to start being oxidized by O3 (~4 h) as shown in Fig. 1(d). As shown in Fig. 3(a), for the NO2, ps decreased from 2.5 × 1013 to 2.0 × 1013 cm− 2 suddenly until t ~ 1 h and then started to decrease from 2.0 × 1013 cm− 2 gradually. For the O3, ps decreased suddenly from 2.3× 1013 to 1.3 × 1013 cm− 2 until t ~ 2 h, and started to increase to 1.6 × 1013 cm− 2 (t = 96 h). Afterwards, ps decreased slowly. As shown in Fig. 3(b), μ exhibited almost no change with time, while ps changed. This can be explained again by a spatial separation between the hole channel and ionized acceptors, as described before. The μ in the O3 case is slightly lower than that in the NO2 case, which would be because the O3 exposure damaged the H-terminated surface more. The interesting finding is that in Fig. 3(a) the slowly decreasing p s with time after ~ 240 h is completely the same for NO2 and O3 . This means that the same residual adsorbed species remain on the surface after 240-h evacuation following both NO2 and O 3 adsorption for 0.5 h. The species should be composed of oxygen, which is the unique common element between NO2 and O 3 . Thus, they should be oxygen atoms (O) or molecules (O2 ). O atoms will easily break C\H surface bonds and form C\O bonds (oxygen surface-termination), and the surface will be highly insulating. However, from experiments, we have found that the surface exhibits p-type conductivity. Therefore, it is likely that O 2 molecules are the residual adsorbed species. This will be discussed in the next section.

4. Discussion As seen in Fig. 4(a), when the diamond surface is exposed to NO2 gas, NO2 molecules diffuse from the vapor phase to the surface. Then, NO2 molecules adsorb and condense on the surface [Fig. 4(b)]. This is like the nucleation process seen in crystal growth. Near RT, NO2 and N2O4 coexist (NO2 N2O4). Therefore condensation of NO2 molecules would be possible. Next, as shown in Fig. 4(c), NO2 molecules take electrons from the diamond and the NO2 molecules are negatively ionized, and positive holes are left on the diamond surface; thus CdH + NO2(gas) => CD+H\NO2−, where Cd is surface carbon, CdH is H-terminated diamond, and CD+H is H-terminated diamond carbon which is positively ionized. Thus, the positive holes in diamond are spatially separated by CdH bonds from negatively-ionized NO2 molecules. Therefore, μ did not change with time, but ps did. Soon, extra NO2 molecules on the surface start to diffuse (migrate), which is like the surface migration and two-dimensional growth seen in crystal growth. When the surface coverage of NO2 molecules on the H-terminated surface reaches one monolayer, the adsorption of NO2 molecules stop and extra NO2 molecules are desorbed to the vapor phase. In this state, the time evolution of ps shows a saturating behavior. As seen in Fig. 4(d), the monolayer of NO2 molecules forms a grain structure, because NO2 molecules migrate on the surface. A high NO2 concentration leads to a high NO2 growth rate, which results in small grain size. Then holes are scattered more frequently and as a result μ decreases. When the NO2-adsorbed H-terminated diamond surface is evacuated, the desorption process proceeds in two steps [Fig. 4(e) and (f)] as seen in Fig. 3 (a). As mentioned above, the slowly decreasing ps with time after ~ 240 h is completely the same for NO2 and O3 in Fig. 3(a), indicating that the same residual adsorbed species remain

102

H. Sato, M. Kasu / Diamond & Related Materials 24 (2012) 99–103

Fig. 4. Model of NO2 adsorption/desorption and hole generation on H-terminated diamond. (a) NO2 is supplied. (b) NO2 adsorption and condensation (nucleation). (c) Adsorbed NO2 removes an electron from diamond and a hole is left in diamond and extra NO2 migrates on the surface. (d) One monolayer of NO2 covers the entire surface. Between grains, boundaries form. The grain boundaries scatter holes in diamond. (e) During evacuation, NO2 is dissociated into N2 and O2, and only N2 desorbs. Remaining O2 molecules are negatively ionized and still supply some number of holes into the diamond, but hole concentration decreases rapidly compared to the state in (d). (f) Remaining O2 molecules are still negatively ionized and holes remain in diamond. O2 adsorption is relatively stable in vacuum for one month.

on the surface after 240-h evacuation following both NO2 and O3 adsorption. It is likely that O2 molecules are the residual adsorbed species because they are composed of oxygen, the unique common element between NO2 and O3, as discussed above. Even though it has not been experimentally confirmed yet, we speculate that NO2 is decomposed into O2 and N2 [Fig. 4(e)] in the first step. Then, N2 molecules desorb into the vapor phase and negatively ionized O2 molecules are left on the H-terminated surface (CD+H\NO2− => (CD+ H\1/2(NO2−)2)metastable => CD+H\O2− + 1/2 N2(gas)) and still generate holes, and this step progresses in a short time (~30 min). After the first step, 20–40% of the total number of generated holes ps are lost. This is because of the difference in the electron affinity between NO2 and O2 molecules on the H-terminated surface. In the second step of the desorption process, O2 molecules desorb from the Hterminated surface slowly due to the recombination of the O2 ions

on the surface and the holes in the diamond, because the recombination requires tunneling of electrons from the O2 ions to the diamond lattice with a time scale of weeks. Thus the O2-adsorbed Hterminated diamond surface and the hole channel supported by the O2 ions on the surface stay chemically stable for one month. The case of O3 is slightly different. For O3 adsorption, the reaction CDH + O3(gas) =>CD+H\O3− progresses firstly to generate holes. Further adsorption causes the additional reaction CD+H\O3− => 1/2CD\ O\CD + O2 + 1/2H2O(gas) and ps is lost. Therefore, as shown in Fig. 1(d), during adsorption, ps initially increased but then started to decrease. Thus moderate coverage of O3 is important in order to keep ps (CD+H\O3−) high. For O3 desorption, the reaction CD+H\O3− =>(CD+ H\1/2(O3−)2)metastable =>CD+H\O2− + 1/2O2(gas) proceeds. As in the NO2 case, negatively ionized O2 molecules are left on the Hterminated surface (CD+H\O2−) and still generate holes. The

H. Sato, M. Kasu / Diamond & Related Materials 24 (2012) 99–103

lifetime of (CD+H\1/2(O3−)2)metastable may be much longer than (CD+H\1/2(NO2−)2)metastable, which would explain why the time evolution of ps for O3 shows a clear dip structure at t ~ 1 h in Fig. 3(a).

5. Conclusions We investigated hole sheet concentration (ps) and mobility (μ) during NO2 or O3 adsorption/desorption on H-terminated diamond surface. The high limit of ps was determined to be ~9 × 10 13 cm − 2 for (001) orientation during NO2 adsorption. Further, we found that during the NO2 adsorption process, μ stays constant while ps is increasing. We proposed a NO2 adsorption/desorption and holegeneration model to explain these experimental results.

Acknowledgment The authors thank Drs. Yoshiharu Yamauchi (NTT Electronics Techno), Yoko Maruo (NTT Energy and Environment Systems Laboratories) for their experimental support, and Drs. Kazuyuki Hirama, Hideki Yamamoto, and Toshiki Makimoto (NTT Basic Research Laboratories) for their discussions. This work was partially supported by the SCOPE “Diamond RF Power Amplifier” Project from the Ministry of Internal Affairs and Communications, Japan.

103

References [1] M.I. Landstrass, K.V. Ravi, Appl. Phys. Lett. 55 (1989) 975. [2] H. Kawarada, Surf. Sci. Rep. 26 (1996) 205. [3] R.S. Gi, T. Mizumasa, Y. Akiba, Y. Hirose, T. Kurosu, M. Iida, Jpn. J. Appl. Phys. 34 (1995) 5550. [4] M. Kasu, M. Kubovic, A. Aleksov, N. Teofilov, Y. Taniyasu, R. Sauer, E. Kohn, T. Makimoto, N. Kobayashi, Diam. Relat. Mater. 13 (2004) 226. [5] H. Kawarada, M. Aoki, M. Ito, Appl. Phys. Lett. 65 (1994) 1563. [6] P. Gluche, A. Aleksov, A. Vescan, W. Ebert, E. Kohn, IEEE Electron Device Lett. 18 (1997) 547. [7] H.J. Looi, L.Y.S. Pang, J.S. Foord, R.B. Jackman, Solid State Electron. 42 (1998) 2215. [8] K. Ueda, M. Kasu, Y. Yamauchi, T. Makimoto, M. Schwitters, D.J. Twitchen, G.A. Scarsbrook, S.E. Coe, IEEE Electron Device Lett. 27 (2006) 570. [9] M. Kasu, K. Ueda, H. Ye, Y. Yamauchi, S. Sasaki, T. Makimoto, Electron. Lett. 41 (2005) 1249. [10] A. Denisenko, A. Aleksov, A. Pribil, P. Gluche, W. Ebert, E. Kohn, Diam. Relat. Mater. 9 (2000) 1138. [11] F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85 (2000) 3472. [12] J. Ristein, M. Riedel, L. Ley, J. Electrochem. Soc. 151 (10) (2004) E315. [13] V. Chakrapani, J.C. Angus, A.B. Anderson, S.D. Wolter, B.R. Stoner, G.U. Sumanasekera, Science 318 (2007) 1424. [14] R.S. Gi, T. Ishikawa, S. Tanaka, T. Kimura, Y. Akiba, M. Iida, Jpn. J. Appl. Phys. 36 (1997) 2057. [15] R.S. Gi, K. Tashiro, S. Tanaka, T. Fujisawa, H. Kimura, T. Kurosu, M. Iida, Jpn. J. Appl. Phys. 38 (1999) 3492. [16] M. Kubovic, M. Kasu, Jpn. J. Appl. Phys. 49 (2010) 110208; M. Kubovic, M. Kasu, H. Kageshima, F. Maeda, Diamond Relat. Mater. 19 (2010) 889. [17] M. Kubovic, M. Kasu, H. Kageshima, Appl. Phys. Lett. 96 (2010) 052101. [18] M. Kubovic, M. Kasu, Y. Yamauchi, K. Ueda, H. Kageshima, Diamond Relat. Mater. 18 (2009) 796. [19] M. Kubovic, M. Kasu, Appl. Phys. Express 2 (2009) 086502.