Diamond & Related Materials 31 (2013) 47–49

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Diamond & Related Materials

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Maximum hole concentration for Hydrogen-terminated diamond surfaces withvarious surface orientations obtained by exposure to highly concentrated NO2

Hisashi Sato a,⁎, Makoto Kasu b,c

a NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, 243-0198, Japanb Green Electronics Laboratories, Saga University, Saga, Japanc Graduate School of Electrical and Electronic Engineering, Saga University, Saga, 840-8502, Japan

⁎ Corresponding author.E-mail addresses: satoh.hisashi@lab.ntt.co.jp (H. Sato

(M. Kasu).

0925-9635/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.diamond.2012.10.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 4 April 2012Received in revised form 18 October 2012Accepted 22 October 2012Available online 8 November 2012

Keywords:DiamondHydrogen terminationMolecular adsorptionSurface orientation

NO2 exposure drastically increases the hole concentration on the surface of hydrogen (H)-terminateddiamond. When the NO2 gas concentration is higher than 300 ppm, the saturated hole sheet concentrationps stays the same. Therefore, the ps value is regarded as the high limit of the concentration of holes onH-terminated diamond surface, ps,max. In this work, we compared ps,max, mobility μ, and sheet resistance Rsfor (100), (110), and (111) H-terminated surfaces of chemical-vapor-deposited single-crystal diamond. On(110), (111), (100) surfaces, the ps,max values are 1.717×1014 and 1.512×1014 cm−2, and 0.981×1014,respectively. This result supports the first-principle calculations: the hole concentration depends on theenergy difference between the valence band maximum and the unoccupied orbitals of adsorbent NO2

molecules. We have achieved Rs of 719.3 Ω/sq (ps=1.456×1014 cm−2 and μ=59.6 cm2 V−1 s−1), the lowestreported so far, on (111) surfaces under 20,000-ppm NO2 atmosphere.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Diamond is a wide-gap semiconductor with the bandgap of5.47 eV, and is expected to show the highest performance in powerelectronic devices [1] among all semiconductors. The p-type conduc-tion on hydrogen (H)-terminated diamond surfaceswas first observedby Landstrass and Revi [2], and the surface hole channels have beenused in field-effect transistors (FETs) [3–5], which show high RF-power performance in the microwave and millimeter-wave ranges[6,7].

However, the p-type conduction mechanism is still controversial.From the early stage, many reports have pointed out that the p-typeconduction appears when the surfaces are exposed to air [8–11]. Onthe H-terminated surface in air, typical values of the hole sheet con-centration ps, mobility μ, and sheet resistance Rs for single-crystal(100) surfaces are ~1.5×1013 cm−2, ~32 cm2 V−1 s−1, and~13 kΩ/sq., respectively [12]. Maier et al. explained the mechanismof hole emergence with exposure to air in terms of charge-transferdoping [13]. Thus, they postulated that the adsorbed water layer pro-vides the electron acceptors necessary for establishing the surfaceconductivity. Accordingly, Garrido et al. attributed the increase inconductivity to a change in the pH of this water layer [14]. Zhang etal. claimed that such a pH change affects the electrochemical potentialof the electron acceptors and, consequently, the surface conductivity

), kasu@cc.saga-u.ac.jp

rights reserved.

[15]. However, we have recently confirmed that neither H2O nor CO2

has any influence on the hole concentration [16], or rather that therelative humidity in air decreases the surface conductivity [17]. Giet al. demonstrated enhanced hole emergence in oxidizing gasescontaining NO2 and O3 [18,19]. Moreover, we have found that onlyexposure to NO2, NO, O3, or SO2 can substantially increase the holeconcentration [16].

Ueda et al. used (110)-oriented polycrystalline diamond, on whichFETs showed higher drain current and cut-off frequencies than thoseon (100) diamond [6]. Kasu et al. used (111) orientation and obtainedhigher ps (1.7×1013 cm−2) than on (100) (~1×1013 cm−2) [20].Motivated by these results, Hirama et al. investigated hole channelsformed in air on H-terminated (110) and (111) surfaces of single-crystal diamond [21] and reported higher ps values on (110) and(111) surfaces than on (100) surfaces. They suggested that ps isproportional to the C―H bond density on the surface and explainedthe mechanism in terms of the spontaneous formation of electricdipoles in a C―H bond [22].

Kubovic and Kasu reported that during the NO2 and O3 adsorptionprocess, ps increases suddenly at first and finally saturates [23]. Withincreasing NO2 concentration, the saturated ps increases. However,for NO2 concentration>300 ppm, the saturated ps does not increasefurther [24]. On the basis of these results, we concluded that themaximum ps in the saturation region is the upper limit of the concen-tration of holes on H-terminated diamond surface.

Here, we report the measured maximum ps values in the satura-tion region obtained under >300-ppm NO2 atmosphere for variousorientations of H-terminated diamond single crystals and discuss

48 H. Sato, M. Kasu / Diamond & Related Materials 31 (2013) 47–49

the hole-doping mechanism. For the device application, we explorethe highest ps and the lowest sheet resistance for specific surfaceorientations.

2. Experimental

The diamond crystals used were chemical-vapor-deposited (100),(110), and (111) single-crystal diamond free-standing films (5×5×0.5 mm3) with extremely low N concentration of b5 ppb and Bconcentration of b1 ppb, commercially available from Element Six.The (100) films were electronics-grade type IIa. The (110) and(111) films were type IIIa. The roughness (root-mean square) wasb5 nm in an area of 1×1 mm2. The sample surface before any treat-ment was highly insulating (>200-GΩ sheet resistance). As anH-termination surface treatment, the diamond surface was exposedto a plasma ball in H2 atmosphere (50 torr) in a microwave plasmachemical vapor deposition chamber with the microwave power of1.3 kW at 700 °C for 30 min. Afterwards, four gold contacts wereevaporated thermally through a shadow mask very near the corneredges of the H-terminated surface in the van der Pauw configuration.The values of ps and μ were measured simultaneously by Hallmeasurements at room temperature (RT). During the measurement,the sample was set in a closed box (volume: 0.05 l) of the Hallmeasurement system, which can be filled with a specific gas andevacuated with a turbo molecular pump (evacuation rate: 70 l/s).Thus, the time necessary for gas filling and evacuation is estimatedto be less than 1 s. NO2 gas used in the measurements was dilutedto obtain specific gas concentrations with N2 gas (purity: 7 N). Inthis work, we used 20,000-ppm NO2 gas, because in a NO2 gas con-centration range >300 ppm, the saturated ps is almost unchanged[24]. Thus, the saturated ps value for >300 ppm can be regarded asthe upper limit of the concentration of holes on H-terminated dia-mond surface [24].

3. Results

Fig. 1 compares the time evolution of ps for (100), (110), and (111)surfaces during 20000-ppmNO2 exposure. After the start of the gasflow(t=0), ps increased rapidly for 10 min and then saturated. The ps,max

values on (110), (111), and (100) surfaces are 1.717×1014, 1.512×1014 cm−2, and 0.981×1014, respectively. The ps,max is the highest on

-10 0 10 20 30 40 50

Time (min)

0

2x1014

1x1014

20000-ppm NO2 exposure

(100)

(111)

(110)

p s (cm

-2)

Fig. 1. Time-wise changes of the sheet hole sheet concentration ps of H-terminated(100), (110), and (111) diamond surfaces at RT during exposure to 20000-ppm NO2.The data for (100), (110), and (111) are shown by green dashed lines, blue brokenlines, and red solid lines, respectively.

(110) surfaces and the lowest on (100) surfaces; that is, ps,max on(110)>(111)>(100).

Fig. 2 compares the time evolution of μ for (100), (110), and (111)surfaces during 20000-ppm NO2 exposure in the same measurement.After the start of the gas flow (t=0), μ increased rapidly for a fewminutes and then saturated on the (111) surface. On (100) and(110) surfaces, μ decreased just after the start of the gas flow andthen saturated. The μ value when ps saturated, μsat, is lower on (110)surfaces than on (100) and (111) surfaces. On (100) and (111)surfaces, the μsat values are almost the same.

Fig. 3 compares the time evolution of Rs for (100), (110), and(111) surfaces during 20000-ppm NO2 exposure. The Rs value iscalculated as Rs=1/(qpsμ), where q is electron charge. After thestart of the gas flow (t=0), Rs decreased rapidly within a fewminutesand then saturated. The Rs value when ps almost completely saturated,Rs,sat, is lower on (111) surfaces than on (100) and (110) surfaces. On(100) and (110) surfaces, the Rs,sat values are almost the same.

Fig. 4 plots sets of ps,max and μsat. Table 1 summarizes ps,max andμsat for adsorption of 20000-ppm NO2 diluted with N2 on (100),(110), and (111) H-terminated diamond surfaces where ps,max almostcompletely saturated. Here, on (111) surfaces, we obtained the Rs,satvalue of 719.3 Ω/sq., which is the lowest ever reported. Here, ps,max=1.456×1014 cm−2 and μsat=59.6 cm2 V−1 s−1. We have reportedthat μsat in the hole channels of H-terminated diamond surfaces isdominated by phonon scattering of degenerated hole carriers [25],which may have a surface-orientation dependence. As a result, thevalue of μ in the hole channels of H-terminated diamond surfaces isaround 100 cm2 V−1 s−1, which is much lower than that in bulk dia-mond (1840 cm2 V−1 s−1 [26]).

4. Discussion

The ps,max values on (110), (111), and (100) surfaces are 1.717×1014

and 1.512×1014 cm−2, and 0.981×1014, respectively. On the otherhand, C―H bond density; 2.22×1015, 1.82×1015, 1.58×1015 cm−2,for (110), (111), (100) surfaces, respectively. Thus, the sequence of theorientations for ps,max is the same as that for C―H bond density. Hiramaet al. claimed that the ps,max may be proportional to the C―H bond den-sity and that holes are generated by the spontaneous formation of elec-tric dipoles in a C―H bond [22]. However, our model is totally different.Experimentally, we have found that exposure to NO2, NO, SO2, orO3

-10 0 10 20 30 40 500

20

40

60

80

100

120

(cm

2 V-1

s-1)

Time (min)

20000-ppm NO2 exposure

(100)

(111)

(110)

Fig. 2. Time-wise changes of the mobility μ of H-terminated (100), (110), and (111)diamond surfaces at RT during exposure to 20000-ppm NO2. The data for (100),(110), and (111) are shown by green dashed lines, blue broken lines, and red solidlines, respectively.

-10 0 10 20 30 40 500

500

1000

1500

2000

2500

3000R

s (/s

q)

Time (min)

20000-ppm NO2 exposure

(100)

(110)

(111)

Fig. 3. Time-wise changes of the sheet resistance Rs of H-terminated (100), (110), and(111) diamond surfaces at RT during exposure to 20000-ppm NO2. The data for (100),(110), and (111) are shown by green dashed lines, blue broken lines, and red solidlines, respectively.

Table 1The sheet carrier density (ps), mobility (μ), and sheet resistance (Rs) for NO2 adsorptionwith 20000-ppm NO2/N2 gases on (100), (110), and (111) H-terminated diamondsurfaces. Two different crystals, labeled #1 and #2, were investigated.

Sample no. Surface orientation (100) (110) (111)

#1 ps (×1014 cm−2) 0.889 1.541 1.456μ (cm2 V−1 s−1) 50.5 30.1 59.6Rs (Ω/sq) 1388 1347 719.3

#2 ps (×1014 cm−2) 0.981 1.717 1.512μ (cm2 V−1 s−1) 57.9 31.6 55.0Rs (Ω/sq) 1099 1151 750.7

49H. Sato, M. Kasu / Diamond & Related Materials 31 (2013) 47–49

gases increases hole concentration remarkably [16]. From first-principlecalculations, we confirmed that when NO2, NO, SO2, and O3 moleculesadsorb on the H-terminated surface, unoccupied molecular orbitals,such as LUMO (the lowest unoccupied orbitals) and SOMO (single occu-pied orbitals), form near the valence band maximum (VBM) top ofH-terminated diamond surface [27]. Then, the valence electrons transferto the unoccupied states until the unoccupied states in the moleculesbalance with the VBM of H-terminated diamond and, consequently,the band bends upward. Therefore, we concluded that the hole concen-tration depends on the energy difference between the LUMO/SOMO andtheVBM. TheVBM level depends on the negative electron-affinity value,which is affected by the C―H dipole. From first-principle calculations,we showed that the VBM level is the highest for (110), second highestfor (111), and the lowest for (100) [26]. Therefore, the hole concentra-tion depends on the C―H bond density but is not exactly proportionalto it. [22].

0

20

40

60

80

100

120

(cm

2 V-1

s-1)

ps (cm-2)1x1014 1.5x1014 2x10145x1013

(100) (111)

(110)

Rs = 1 k /sq

Rs = 2 k /sq

Rs = 5 k /sq

H-terminated diamond surfaces with 20000-ppm NO

2 adsorption

Fig. 4. Sets of mobility μ and the sheet hole sheet concentration ps obtained forH-terminated (100), (110), and (111) diamond surfaces at RT during exposure to20000-ppm NO2. The data for (100), (110), and (111) are shown in green, blue, andred, respectively. Calculated sets of μ and ps which fulfill Rs=1, 2, and 5 kΩ/sq areindicated by solid lines.

5. Conclusion

Wemeasured and compared ps, μ, and Rs during NO2 adsorption on(100), (110), and (111) H-terminated diamond surfaces. The ps,max

values on (110), (111), and (100) surfaces are 1.717×1014, 1.512×1014 cm−2, and 0.981×1014, respectively. The sequence of surfaceorientations for ps,max is the same as that for C―H bond density. Thisresult can be explained in terms of the energy balance between theVBM of H-terminated diamond and the unoccupied molecular orbitalsof NO2 adsorbent. We have achieved Rs of 719.3 Ω/sq (ps=1.456×1014 cm−2 and μ=59.6 cm2 V−1 s−1), the lowest reported so far, on(111) surfaces under 20,000-ppm NO2 atmosphere.

Acknowledgements

The authors thank Dr. Yoshiharu Yamauchi (NTT Electronics Tech-nology) for his experimental support, and Drs. Kazuyuki Hirama, HidekiYamamoto, and Toshiki Makimoto (NTT Basic Research Laboratories)for their discussions. This work is supported by JSPS grant-in aidno. 24360124.

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