Applied Surface Science 254 (2008) 7921–7924

Surface modification by vacuum annealing for field emission from heavilyphosphorus-doped homoepitaxial (1 1 1) diamond

Takatoshi Yamada *, Christoph E. Nebel, Kumaragurubaran Somu, Shin-ichi Shikata

Diamond Research Center, National Institute for Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8568, Japan

A R T I C L E I N F O

Article history:

Available online 8 April 2008

Keywords:

Field emission

Surface modification

Vacuum annealing

Phosphorus-doped diamond

Electron affinity

A B S T R A C T

The relationship between field emission properties and C 1s core level shifts of heavily phosphorus-

doped homoepitaxial (1 1 1) diamond is investigated as a function of annealing temperature in order to

optimize surface carbon bonding configurations for device applications. A low field emission threshold

voltage is observed from surfaces annealed at 800 8C for hydrogen-plasma treated surface, while a low

field emission threshold voltage of wet-chemical oxidized surface is observed after annealing at 900 8C.

The C 1s core level by X-ray photoelectron spectroscopy (XPS) showed a shoulder peak at 1 eV below the

main peak over 800 and 900 8C annealing temperature for hydrogen-plasma treated and wet-chemical

oxidized surfaces, respectively. When the shoulder peak intensity is less than 10% of the main peak

intensity, lower threshold voltages are observed. This is due to the carbon-reconstruction which gives

rise to a small positive electron affinity. By increasing annealing temperature, the shoulder peak ratios

also increase, which indicates that a surface graphitization takes place. This leads to higher threshold

voltages.

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1. Introduction

Fabrication of promising field emitters is one of the mostimportant issues for field emission displays and vacuum nano-electronic devices. For practical applications, both a reduction ofoperating voltage and obtaining a stable emission currents areremaining problems [1]. Generally, field emission properties aredominated by surface potential barriers, which limits the emissioncurrents as well as stability. Since the surface potential barrier isstrongly related to the electron affinity, low electron affinitymaterials gained much attention. Diamond is a promising fieldemitter material, because the electron affinity of hydrogen (H)-terminated diamond is negative (�1.1 eV) or reconstruction andoxygen (O)-terminated surfaces have small positive electronaffinities (<+1.7 eV) [2,3]. After the achievement of n-typeconduction by phosphorus (P)-doping [4], diamond field emittersare considered as important candidates for applications [5–10]. Wehave reported low threshold field emission and stable emissioncurrent from carbon-reconstructed P-doped diamond in ourprevious studies [8–10]. The reconstructed surface can be formedby vacuum annealing, which is a reasonable technique for practicaldevices. From the literature, electron affinities depend on the

* Corresponding author. Tel.: +81 29 861 3851; fax: +81 29 861 2771.

E-mail address: takatoshi-yamada@aist.go.jp (T. Yamada).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.03.156

annealing conditions [3]. The relationship between surfaceconditions and field emission properties is, however, not clear.

This paper describes annealing temperature effects on fieldemission properties of heavily P-doped diamond. X-ray photoelec-tron spectroscopy (XPS) is used to characterize surface conditions.We report the relationship between field emission properties andsurface bonding conditions after application of different annealingtemperatures for practical device performance. We also discusspossible field emission mechanism of P-doped diamond based onfield emission data and surface electric properties.

2. Experimental

Heavily P-doped homoepitaxial diamond films were grown on(1 1 1) Ib synthetic diamond substrates using micro-wave plasmachemical vapor deposition (CVD). Growth conditions were asfollows: the carbon source: CH4, doping gas: PH3, pressure:25 Torr. P/C ratio in the CVD apparatus during growth: 1%. Typicalresistivities of these films are in the range of (5–8) � 102 V cm atroom temperature. A weak temperature dependence on resistivitywas observed, which indicates a hopping or an impurity bandtransport in the film [11].

Vacuum annealing was carried out in the field emissionmeasurement set up (base pressure of 1 � 10�9 Torr). Annealtemperatures were 700, 800, 900 and 1000 8C. Annealing wascarried out for 10 min, which is enough to change field emission

T. Yamada et al. / Applied Surface Science 254 (2008) 7921–79247922

properties in our experiments. In this study, H-plasma treated andwet-chemical oxidized surfaces were used as initial surfaces. ForH-termination in this study, H-plasma treatment was carried outusing CVD apparatus for diamond growth. Treatment temperaturewas 850 8C and time was 10 min. Wet-chemical oxidization wasperformed by boiling in the mixed acid of H2SO4 and HNO3 (=3:1)at 220 8C for 60 min [12].

Field emission was measured at room temperature in a vacuumsystem. The diamond samples were mounted as cathodes, using amolybdenum (Mo) top plate with a circular hole of 1.5 mm indiameter, to avoid edge effects. A tungsten carbide (WC) needle(20 mm in tip radius) is used as an anode electrode. The anodeelectrode can be moved in x, y, z directions by micrometer screwswithout breaking vacuum. The distance between the anode and thediamond surface was kept constant at 5 mm. The details can beseen in Ref [8]. It must be noticed that titanium/platinum (Ti/Pt)were formed as contact for electron injection. Ti forms carbide afterannealing of 400 8C, which gives rise to Ohmic properties. In orderto avoid oxidization of Ti, Pt was covered with the Ti layer.

Fig. 1. Field emission properties of annealed P-doped diamond for (a) H-plasma

treated and (b) wet-chemical oxidized surfaces.

After annealing or field emission measurements, the sampleswere immediately transferred in air to XPS chamber. The XPSmeasurements were carried out at room temperature in a ThermoVG Scientific ESCALAB Theta Probe spectrometer at a base pressureof better than 2 � 10�8 Pa. A monochromatized Al Ka radiationfrom a twin anode X-ray source with photon energy of 1486.6 eVwas used. Energy resolution was 100 meV. The area was 400 mm indiameter. The analyzer pass energy was set to 50 eV. The energystep size was 50 meV.

3. Results and discussion

Field emission properties of P-doped diamond after applicationof various vacuum annealing temperatures are shown in Fig. 1(a)and (b) for initially H-plasma treated and initially wet-chemicaloxidized surfaces. It is clear from this figure that field emissionproperties strongly depend on annealing temperatures. The lowestthreshold voltages are obtained after annealing at 800 and 900 8Con initially H-plasma treated and initially wet-chemical oxidizedsurface, respectively. By further increasing of the annealingtemperature, threshold voltages increase again and a higherthreshold voltage is observed for surfaces annealed at 1000 8C. It isinteresting to note that we observed no major difference in surfacemorphology by AFM characterizations [10]. We, therefore, con-clude that the change in field emission properties of P-dopeddiamond is due to surface electric property variations at thesurface induced by atomic re-arrangements [10].

C 1s core level shifts of H-plasma treated surface characterizedby XPS are shown in Fig. 2. We also detect a small amount ofcontamination due to water, which comes from air during transfer.The energy position was calibrated by O 1s peak position. Ashoulder peak (C2) of 1 eV below main peak (C1) appears atannealing temperature of 700 8C. The peak intensity increases withincreasing annealing temperature. This peak (C2) is reported to bedue to carbon-reconstruction or graphitization [13]. This is similarto boron-doped p-type diamond [13]. The results clearly show thatthe vacuum annealing modulates surface bonding conditions. Bymeans of reflection high energy electron diffraction (RHEED)(1 � 1) surfaces were observed on H-plasma treated and on wet-chemical oxidized surfaces, C(1 1 1)-2 � 1 reconstructed surfacewas observed for diamond annealed at 800 8C, and hallo patterns

Fig. 2. Carbon 1s core level shifts of H-plasma treated P-doped diamond.

Fig. 3. (a) Peak intensity ratio of carbon shoulder peak at 1 eV below main peak of

annealed P-doped diamond. (b) Threshold voltage of annealed P-doped diamond.

T. Yamada et al. / Applied Surface Science 254 (2008) 7921–7924 7923

was observed on diamond annealed over 1000 8C. We alsoconfirmed surface graphitization and surface reconstruction bysurface plasmon spectra [14]. We observed similar results oninitially wet-chemical oxidized surface. The C2 peak appears atannealing temperature of 800 8C and the peak intensity increaseswith increasing annealing temperature [12]. Fig. 3(a) shows peakintensity ratio of C2/(C1 + C2) for initially H-plasma treated andinitially wet-chemical oxidized surfaces. The difference in tem-peratures where C2 peak appeared indicates that desorptiontemperatures are different between hydrogen and oxygen [2]. Fieldemission threshold voltages for various annealing temperatures

Fig. 4. Field emission mechanism of (a) H-terminated, (b) wet-chemi

are shown in Fig. 3(b). In this study, the threshold voltage wasdefined as anode voltage where 10�11 A of emission current wasmeasured. Although different temperatures are necessary toobtain the lowest threshold voltages for initial H-plasma treated(800 8C) and initial wet-chemical oxidized surfaces (900 8C), bothsurface conditions are almost the same C2/(C1 + C2) ratio. For lowthreshold voltages, it is considered that almost same surfacebonding conditions were formed regardless of different initialsurface properties. By vacuum annealing, hydrogen desorption atthe diamond surface takes place and carbon-reconstruction startsat about 1100 K [3]. This temperature is in good agreement withannealing temperature to obtain the lowest threshold voltage(800 8C). Hydrogen desorption changes the electron affinity fromnegative to positive, and decreases the H-induced surface densityof state. For an ideal carbon-reconstructed surface, no surfacedefects are expected. The internal barrier height reduces to thelowest positive value as shown in Fig. 4. This small positiveelectron affinity dominates field emission properties. For initialoxidized surface, oxygen desorbs from diamond until annealingtemperature of 900 8C and a carbon-reconstructed surface is, then,formed (Fig. 4) [2]. Further increasing the vacuum annealingtemperature gives rise to the threshold voltage due to surfacegraphitization [13]. On graphitic surfaces, the Fermi level may bepinned in the middle of the band gap to dangling bond states [2].This induces an up-ward band bending as shown in Fig. 4. Thehigher internal barrier (Vint) and positive electron affinitydominates field emission at such surface. During field emission,surface oxidization might take place. Carbon-reconstructedsurfaces can be re-produced by vacuum annealing from oxidizedsurfaces. This indicates that carbon-reconstructed surfaces aremost appropriate surfaces for practical diamond field emitter.

4. Summary

We investigated the relationship between field emissionproperties and C 1s core level shift on P-doped (1 1 1) diamondsafter various annealing temperatures. Field emission thresholdvoltages depend on annealing temperatures and the lowestthreshold voltages are observed after annealing of 800 and 900 8Cfor initially H-plasma treated and initially wet-chemical surfaces,respectively. A small peak (C2) at 1 eV below main peak (C2) isobserved on such surfaces. For the peak intensity ratios of C2/

cal oxidized, (c) carbon-reconstructed and (d) graphitic surfaces.

T. Yamada et al. / Applied Surface Science 254 (2008) 7921–79247924

(C1 + C2) less than 10%, low threshold voltages are observed.Under such annealing conditions, hydrogen or oxygen deso-rptions takes place from diamond surface, which leads to asmallest positive electron affinity. Therefore, the thresholdvoltages decrease. We believe that carbon-reconstructed surfacesare most appropriate surfaces for practical diamond field emitterapplications, since such surfaces can easily be formed by vacuumannealing.

Acknowledgements

The authors would like to thank Dr. S. Koizumi, NationalInstitute for Material Science (NIMS), Japan, Dr. H. Yamaguchi andProf. K. Okano, International Christian University (ICU), Japan forexperimental supports and fruitful discussion. This work wasfinancially supported by Nano Tech Challenge Project by the NewEnergy and Industrial Technology Development Organization(NEDO).

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