Applied Surface Science 256 (2009) 1006–1009

Field emission properties of nano-structured phosphorus-doped diamond

Takatoshi Yamada a,*, Christoph E. Nebel b, Shin-ichi Shikata a

a Diamond Research Center, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305-8568, Japanb Fraunhofer Institute of Applied Solid State Physics, Tullastrasse 72, Freiburg 79108, Germany

A R T I C L E I N F O

Article history:

Available online 6 June 2009

Keywords:

Diamond

Field emission

Nano-fabrication

Electron affinity

Field enhancement

A B S T R A C T

Nano-structured phosphorus-doped diamonds were fabricated for field emitters and their field emission

properties were characterized. Two kinds of nano-structures were prepared; tip array structures and

whiskers on tip structures. The tips, which have 100 nm radius and 10 mm height, are used in tip array

structures; whiskers have tip radii of 5 nm and height of 500 nm. Following nano-structure formation, a

reduction of threshold fields is observed compared to non-patterned flat surfaces. This is ascribed to field

concentration at the tips. However, at higher electric fields, a saturation of the emission current is

observed due to non-negligible bulk resistances in tips and whiskers.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

In field emission applications and vacuum nano-electronicdevices, fabrication of field emitter arrays with low operatingvoltages and stable emission currents is a key requirement [1].Generally, field emission properties are dominated by surfacepotential barriers, which limit emission currents as well as currentstability. Since the surface potential barrier is related to theelectron affinities, the study for low electron affinity materialshave gained much attention. In addition, high chemical stability ofthe emitting surface is also required to achieve reproducible andconstant emission currents.

Diamond is one of the most appropriate field emitter materials[1–3], because the electron affinity of diamond can be tuned from�1.3 to +1.7 eV by variation of surface terminations [4,5]. In addition,diamond shows physical and chemical stabilities. Although manyreports on the field emission characteristics of diamond exist, mostare focused on undoped insulating or boron (B)-doped p-typesemiconducting diamonds [1–3]. These results showed that theemission of electrons from conduction band was most effective incase of negative or small positive electron affinity and that n-typedoping of diamond was required to achieve high emission currents.N-type doping of diamond with phosphorus (P) was discovered in1997 [6]. Since then activities aimed at the realization of emitterdevices based on diamond grown continuously. Recently, wereported that carbon-reconstructed heavily P-doped (1 1 1)-oriented diamond flat surfaces showed a low threshold voltageand a more stable field emission current compared to hydrogen (H)or oxygen (O)-terminated P-doped diamonds [7–9]. For H-termi-

* 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 � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.05.119

nated diamond, the electron affinity is negative. This gives rise to aninternal depletion layer by ionized donors, which prevents electronsfrom approaching the emitting surface and therefore higher electricfields are necessary for field emission. Positive electron affinitiesdominate field emission from O-terminated and carbon-recon-structed surfaces. As a positive electron affinity prevents theformation of a significant depletion layer at the surface, a carbon-reconstructed surface in combination with phosphorus dopingseems to be the most promising solution for this problem.

In order to realize highly efficient emitter devices fromdiamond, a combination of optimized electron affinity withnano-structuring will be a promising approach. Realization ofnano-structures such as tips or wires, gives rise to fieldenhancement effects, which lead to a reduction of requiredthreshold fields. Nano-structuring techniques for diamond fieldemitter devices have already been reported in the literature[10,11]. However, such techniques are not suitable for thefabrication of P-doped diamond tip arrays with high aspect ratio,because cracks were formed to relieve tensile stress in thick P-doped homoepitaxial diamond films [12]. In this paper, weintroduce two new fabrication techniques to manufacture nano-structures; namely (i) formation of a tip array by top-downreactive ion etching (RIE) technique and (ii) nano-whiskers on suchtip structures. By formation of such nano-structures, a significantreduction of threshold fields is obtained compared to non-patterned flat diamond surfaces.

2. Fabrication of nano-structured phosphorus-dopeddiamonds

The procedure for fabrication of nano-structured P-dopeddiamond is schematically shown in Fig. 1. In order to form a tiparray structure, (1 1 1)-oriented insulating Ib diamond substrates

Fig. 1. Procedure for fabrication of nano-structured phosphorus-doped diamonds.

T. Yamada et al. / Applied Surface Science 256 (2009) 1006–1009 1007

(3 mm � 3 mm) were mounted on silicon plates in an etchingchamber using an oxygen plasma etching process (Fig. 1(a)).Oxygen plasma physically etched silicon and oxidized silicon, andthen oxide particles deposited on the fresh etched diamondsurface. These silicon oxide particles formed effectively micro-/nano-etching masks [13], leading to the formation of tip structures(Fig. 1(b)). This is the key process for tip array formation. Then, thenano-structured diamond was exposed to a diamond growthprocess in micro-wave plasma CVD (Fig. 1(c)). Such a surface(sample A) was examined to detect the effective geometricalshaping on field emission. Additionally, nano-whiskers werefabricated on such tip structures (sample B) by oxygen/argon(O2/Ar) plasma etching (Fig. 1(d)). For Ohmic contacts, titanium/platinum (Ti/Pt) films with a thickness of 30/50 nm were formedon a wet-chemical oxidized surface [7–9], followed by annealing at400 8C in vacuum.

Inductively coupled plasma (ICP) etching was performed on flatdiamond surfaces to produce tip structures. Etching conditionswere as follows: antenna power: 1000 W, bias power: 100 W,oxygen gas pressure: 267 Torr (2 Pa), time: 60 min. As etchingmask we used a self-aligned silicon oxide particles. This mask issputtered Si from the base plate where Si-nano-particles arereleased and re-deposited on the diamond surface [13]. In order toenhance the formation, a supply of silicon oxide particles duringthe etching process by a silicon base plate as holders isimplemented. This leads to the formation of nano-diamond tiparrays and structured tips with high aspect ratio [14].

Growth of P-doped (n-type) diamond on these tips was carriedout using micro-wave plasma assisted CVD. We used CH4 as acarbon source and PH3 as the doping gas. Under our reportedgrowth condition [7–9], diamond tips were etched and broken byhydrogen plasma. In this paper, we changed the growth conditionsin order to coat diamond films on tip array structures. The growth

conditions were as follows: pressure: 25 Torr, substrate tempera-ture: 900 8C, microwave power: 750 W, CH4/H2: 0.1%. The P/C ratioin the CVD gas phase during growth was 5% to obtain a high dopingdensity. These conditions are different compared to our standardheavy P-doping of homoepitaxial (1 1 1) diamond films [7–9]. Toavoid hydrogen etching of diamond tips during diamond growth[15], we increased the CH4/H2 ratio compared to the growthconditions for non-patterned flat surface (CH4/H2: 0.05%) [7–9]. Toachieve uniform growth on tip structures, a low growth rate wasapplied by lowering the gas pressure. After 3 h deposition, the filmthickness was about 100 nm. The film grown under new conditionsin this study was evaluated. The electric resistivity at roomtemperature obtained by van der Pauw method was 5 � 102 V cmand the phosphorus concentration by secondary ion massspectroscopy (SIMS) in the films was estimated to be7 � 1019 cm�3. The obtained data are almost same as results offilms obtained by previous reported conditions [7–9].

In order to form whiskers on tip structures (sample B), weapplied ICP etching of a diamond tip array (sample A). Etchingconditions were as follows: antenna power: 500 W, bias power:50 W, oxygen gas pressure: 267 Torr (2 Pa), O2/Ar flow rate: 70/30and time: 10 min.

Fabricated nano-structured P-doped diamonds were observedby scanning electron microscopy (SEM). These results show thatthe coating of intrinsic diamond with phosphorus-doped diamondwas successful (sample A) as shown in Fig. 2(a). To evaluate tipheight, we prepared samples by braking tips as shown in the insetof Fig. 2(a). The height (hA) is about 10 mm. Observation withhigher magnification (Fig. 2(b)) revealed the typical tip radius (rA)is to be about 100 nm. The SEM results also reveal whiskers on thetip structure generated by ICP etching as shown in Fig. 2(c) and (d).Whiskers have tip radii (rB) of less than 5 nm and heights (hB) ofabout 500 nm. Non-patterned flat surface as a comparison is alsoshown in Fig. 2(e). Although steps originated from substrate bymechanically polishing are observed, the flat surface is obtained.Atomic force microscopy (AFM) observation indicated that averageroughness was about 40 nm. From SEM images, field enhancementfactors can also be calculated from the geometrical properties of atip using Eq. (1) where r is the tip radius and h is the tip height [16]:

b ¼ h

r(1)

The field enhancement factors b of sample A (bA) and B (bB) wereestimated to be about 100. It is expected that the reduction ofthreshold fields of samples A and B compared to sample C by nano-structuring.

3. Field emission characteristics

Field emission properties were measured in a high vacuumsystem with a base pressure of �1 � 10�9 Torr. The diamondsamples were set as cathodes and the field emission current as afunction of anode voltage was measured. Here, a tungsten carbide(WC) needle with 50 mm diameter was used as anode to evaluatethe field emission locally. The distances between anode anddiamond were 25 mm for samples A and B and 10 mm for sample C.Details can be found in Refs. [7–9]. For formation of carbon-reconstructed surfaces on diamond, samples were annealed in highvacuum at 900 8C for 10 min prior to field emission measurements,which were the optimized parameter typically used on flatdiamond surfaces [17].

The field emission properties of samples A and B are shown inFig. 3. The electric field was calculated by taking into account theapplied voltage and the anode–diamond distance. The non-patterned flat homoepitaxial diamond (sample C) used asreference, is also shown in Fig. 3. Significant reductions of

Fig. 2. SEM images of a nano-structured P-doped diamond tip array (a) and (b), of whiskers on tip structures (c) and (d).

T. Yamada et al. / Applied Surface Science 256 (2009) 1006–10091008

threshold fields, where 10�10 A emission current was measured,are obtained by nano-structure formation. The lowest thresholdfield is detected from sample B. The field emission threshold ofsample B is about half of that detected on sample A. This is inagreement with the values obtained on tip structures withprotrusions [18].

Note that the field emission properties of samples A and B showfour and two different regions, respectively. For sample A, the fieldemission current increases exponentially from 165 V/mm (A1),then it deviates from the exponential increasing form (A2), and at270 V/mm it increases again exponentially (A3), followed by asaturation (A4), which means that emission currents deviates fromthe exponential increasing. The field emission of sample B startsfrom 66 V/mm and increases exponentially (B1), then the currentsaturates at fields >90 V/mm (B2).

Fowler–Nordheim (F–N) plots of all samples are shown inFig. 4. The F–N plot of sample A shows clearly the four differentregions while only two are revealed on sample B. Using F–N

Fig. 3. Emission current vs electric field characteristics of nano-structured P-doped

diamonds and a non-patterned phosphorus-doped flat surface.

plots, we can discuss field emission properties in the regions of(A1), (A3) and (B1). The slopes of the F–N plot in regions (A1)and (A3) of sample A are �2.7 � 103 and �3.1 � 103 V/(cm eV3/

2), respectively. The slope of the F–N plot in the region (B1) ofsample B is �1.5 � 103 V/(cm eV3/2). These data indicate thattunneling through the barrier dominates the emission process inregions of (A1), (A3) and (B1). On sample C, a straight line with aslope of �1.34 � 104 is detected. The relationship betweenslope of F–N plot (D) and emission barrier height (f) is givenby Eq. (2), taking into account the field enhancement factor (b)[1]:

D ¼ �6:8� 103 f3=2=b (2)

Assuming that emission barrier heights for all samples are samesince the field emission properties were dominated by the electron

Fig. 4. Fowler–Nordheim plots of nano-structured P-doped diamonds and non-

patterned phosphorus-doped flat surface.

T. Yamada et al. / Applied Surface Science 256 (2009) 1006–1009 1009

affinity of annealed n-type diamond surface [7–9], the fieldenhancement factor ratio (R) can be calculated by followingequation:

R ¼b0tip

b0C¼ DC

Dtip(3)

where btip is the field enhancement factor of sample A (b0A) or B(b0B), b0C is the field enhancement factor of sample C, Dtip is theslope of the F–N plot of sample A (DA) or sample B (DB), DC is theslope of the F–N plot of sample C. In regime A1, the fieldenhancement factor (b0A) is 5.6 times higher than that of sample C(b0C). For sample B (region B1), the field enhancement factor (b0B) is10.1 times higher than sample C (b0C). Obviously, the fieldenhancement caused by nano-structuring gives rise to an effectivereduction of threshold field. However, the obtained field enhance-ment factor of sample A (b0A) and B (b0B), using F–N plots, aresmaller than the field enhancement factors obtained from tipgeometries (bA and bB). When we consider the field enhancementfactor, it is necessary to take into account the field screening effecton the nano-structured tip array due to the formation of dense tips[19]. Our fabricated samples are consisted of dense tips, which isenough to take place field screening effect on tip arrays. Thedistance between tips as well as detailed geometrical properties,have to be taken into account. The distances between tips ofsample A are less than 1 mm, which prevents the increase of fieldenhancement. b0B is twice as large as b0A, however field enhance-ment factors samples A and B (bA and bB) obtained from Eq. (1) arealmost same. By formation of whiskers on tips, field concentrationat whiskers increases [18].

The field emission property of sample A shows two currentincreasing regions (A1) and (A3), respectively; these imply thatfield emission from sample A is dominated by two different tipstructures. By comparison of the slopes of the F–N plot, the fieldenhancement factor of region A3 is slightly higher than that of A1.Variations of tip radii are observed in Fig. 2(b). We assumetherefore, that the electron emission initially starts from sharp tipsfollowed by emission from slightly larger ones.

The field emission current becomes saturated due to the limitedfilm conductivity of P-doped diamond in region (A4) and (B2). Inthe regions of (A4) and (B4), the series resistances during fieldemission were estimated to be 3.7 � 108 V for sample A and2.5 � 108 V for sample B in Fig. 3. Assuming active tip ratio was30% [1] and resistivity was 5 � 102 V cm, resistances of bothsamples were in the order of 108 V. Such higher series resistancesare enough for field emission saturation [20]. For region (A2), theactive tip ratio was less than region (A4). In addition, the seriesresistance of (A2) was expected to be much higher than that of (A4)due to difference in tip size. Thus, sample A shows two saturationregions. Usually, the doping efficiency of phosphorus depends oncrystal orientation [21]. For (1 1 1) oriented diamond, theincorporation efficiency is significantly higher than on (1 0 0).We used therefore (1 1 1)-oriented diamond substrates. However,the deposition of n-type diamond on the side-walls of tips will begoverned by growth on deviations of (1 1 1) orientations. The

phosphorus concentration in film is lower than anticipated on(1 1 1) diamond, leading to a higher than expected resistance.

4. Summary

Two kinds of nano-structured P-doped diamonds were preparedby plasma etching and by CVD overgrowth to realize field emitterarrays. By SEM observations, the tip array structure (sample A)shows tip radii of about 100 nm and tip length of about 10 mm. Theformation of whiskers on tips is also confirmed. The whiskers showtip radii of less than 5 nm and a typical length of 500 nm. In fieldemission experiments, a reduction of threshold field is obtained,both on tip arrays as well as on tips with nano-whiskers. The lattershow the lowest threshold field. Non-linear F–N plots were obtainedfor both samples. Comparing the slopes of F–N plots, the largest fieldenhancement factor is calculated for tips with whiskers, whichobviously lead to a reduction of the threshold field. At higher electricfields, the emission current saturates due to limiting bulkconductivity. We assume if we combine both, nano-structuresand reconstruction, that phosphorus-doped diamond tip arrayemitters could become potential candidates for field emissiondevices with low threshold fields and constant emission properties.

Acknowledgement

This work was partially supported financially by KAKENHI(#19205026).

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