Lithuanian Journal of Physics, Vol. 46, No. 2, pp. 211–215 (2006)

SURFACE POTENTIAL OF DIAMOND AND DIAMOND-LIKECARBON THIN FILMS ON Si SUBSTRATE ∗

M. Dłużniewski a, T. Lozovski b, R. Pūras b, and S. Sakalauskas ba Institute of Physics, Technical University of Lodz, Wolczanska 219, 93-005 Lodz, Poland

b Vilnius University, Saulėtekio 9, LT-10222 Vilnius, LithuaniaE-mail: tadeusas.lozovskis@ff.vu.lt

Received 13 December 2005

Semiconductor films are increasingly applied for cathodes of autoelectronic emission. In the case of autoelectronic emissionthe cathode heats itself, therefore the wide band gap semiconductors, such as diamond films, are more suitable for cathodes.Moreover, considering high thermal conduction of diamond films, it is easy to optimize the process of autoelectronic emission.

Primary experimental results regarding the investigation of the surface electric potential of diamond thin films and diamond-like carbon (DLC) thin films are presented in the paper. The films, intended for autoelectronic emission cathodes, weredeposited on differently processed silicon substrate. Values of electron work function, as well as unambiguously related to itsurface electric potential distribution, were examined by contactless Kelvin–Zisman method.

Keywords: thin films, diamond, diamond-like carbon, surface electric potential

PACS: 81.05.Uw

1. Introduction

Field emission from clean metal surfaces takes placeat surface field strengths of the order of ∼109 V·m−1.As a result, for decades field emission was believed tobe a property of sharp tips where the macroscopic fieldcould be magnified up to a desired level. This philos-ophy has dominated the development of field emissionelectron sources until the last ten years. Until relativelyrecently, work to improve the performance of tips hasbeen directed towards tips shape and work function.Sharper tips are more sensitive to damage by ions orohmic heating and nature abhors a low work functionin the same way as it does a vacuum.

In the late 80’s field emission from planar diamond-like and diamond film surfaces was demonstrated atreasonable macroscopic fields stimulating research intoa new class of broad-area electron sources. It was ini-tially believed that broad-area field emission was a spe-cial property of diamond films, because some crystalplanes when terminated with hydrogen have a negativeelectron affinity. However, looking back it is clear thatin fact it is a general property of thin wide band gap(i. e. insulating) layers on conducting substrates. More-∗ The report presented at the 36th Lithuanian National Physics Con-

ference, 16–18 June 2005, Vilnius, Lithuania.

over, although negative electron affinity helps, it is byno means an essential condition for broad-area emis-sion.

Over the past 15 years it has become obvious thatthe earlier concern about surface properties (i. e. elec-tron affinity) was misplaced and that the critical step, ingeneral, is the injection of electrons into the conductionband of the insulator. Defects or insulator doping en-able the formation of a Schottky barrier, which makesthis injection possible by tunnelling, despite band off-sets. The importance of this cannot be overstressedsince it moves the critical area away from the solid–vacuum interface, where it can be affected by residualgases and ions, to the protected conductor–insulator in-terface. As a result, such emitters are more tolerant ofpoor vacuum conditions than tips. If a negative elec-tron affinity surface exists, electrons injected into theconduction band will be able to leave the surface.

2. Samples and their growth technology

Diamond films were grown on silicon substrates bythe hot filament chemical vapour deposition (HF CVD)method [1]. The method is simple and inexpensive.Hot filament usually operates near to substrate, there-fore it is simple to control the substrate temperature, but

c© Lithuanian Physical Society, 2006c© Lithuanian Academy of Sciences, 2006 ISSN 1648-8504

212 M. Dłużniewski et al. / Lithuanian J. Phys. 46, 211–215 (2006)

different diamond structures with many defects growdue to irregular temperature distribution. In order toenhance diamond nucleation density, the standard pro-cedure was performed, i. e. the substrate surface waspolished mechanically and then cleaned in acetone,methanol, and distilled water. Mixture of either acety-lene or methanol with hydrogen was used for deposi-tion process. Content of acetylene was 1% (specimenA1) or 3% (specimen A2) at growing one group of spec-imens. The flow rate was 2.5 cm3/s and the gas pres-sure was 5.5·103 Pa. Second group of specimens wasgrown from mixture of methanol and hydrogen: speci-men B1 – 1% of methanol, gas pressure of 5.5·103 Pa,film thickness of 0.5 µm on n-type Si〈111〉, doped withP, specific resistance

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defect. These levels can be associated with acceptor,donor, electron, hole, recombination, or trapping cen-tre and thus they can significantly change the proper-ties of semiconductor structure. Obviously it was no-ticed that the surface energy levels influence surfaceproperties. Generally it was seeked to create a posi-tive electric charge on the surface in order to reduce theelectron affinity (data about positive electron affinity ispublished, e. g., in [3]) and to improve conditions ofthe autoelectronic emission. Another way to improveautoelectronic emission was the creation of rough sur-face, for example by making pits and cones. Consid-erably stronger electric field is at cone tips (it lowersthe height and the width of the barrier), and as a con-sequence the probability of electron tunnelling over thebarrier is increased.

Generally electrical resistance of the investigatedcarbon films is high and usage of contact measurementmethods is limited. Therefore we used a contactless

Kelvin–Zisman method [4]. The method’s principle isthat a probing microelectrode vibrates beyond inves-tigated surface and the induced electric charge (pro-portional to electron work function) is converted intoalternating voltage. Subsequent signal processing byelectronic equipment and an introduced 100% nega-tive feedback enables one to optimize the metrologicalcharacteristics of measuring device and allows to de-termine the absolute value of specimen surface electricpotential and its in-plane distribution (minimal sam-pling area diameter has been 10 µm). Microscopic in-vestigation revealed a rough surface relief (“spotted”)of examined specimens. We had measured the effec-tive surface electric potential ϕef :

ϕef =1

S

∫

S

ϕdS = ϕ1S1S

+ ϕ2S2S

, (1)

where S1 and S2 are areas of pits and hillocks, ϕ1 andϕ2 are corresponding surface potentials, S = S1 + S2.

(a) (b)

(c) (d)Fig. 2. Images of the investigated film surfaces: (a) specimen A1 (magnification 50), (b) specimen A2 (magnification 50), (c) specimen B2,

(d) specimen B3.

214 M. Dłużniewski et al. / Lithuanian J. Phys. 46, 211–215 (2006)

The surface potential is negative and its value corre-sponds to thermodynamic electron work function. Val-ues of surface electric potential Up are measured withrespect to the calibrated probe and are associated to ef-fective surface potential as follows:

−ϕef =W0q

− Up = −ϕ0 − Up , (2)

where W0 is electron work function of probe, q is elec-tron charge.

According to the experimental results presented inFig. 1(a–c), practically all specimens (except C1) areinhomogeneous. Additionally, surface electric poten-tial of A and B groups of specimens was found to be in-dependent of illumination with white light. Specimensof group A were irradiated with α-particles (239Pu).A positive electric charge adsorbed on the surface ofthe specimens was detected. Therefore surface energybands were bended downwards hereby improving auto-electron emission yield. According to the surface elec-tric potential value and its distribution we state that thespecimen A1 is better than specimen A2. The value ofwork function of specimen A1 is nearest to that of dia-mond. Figures 2(a–d) show the images of the film sur-faces. The surface of specimen A1 is the most uniformaccording to the presented specimen images.

The surface electric potential fluctuation observedfor the C group of specimens due to the white light ir-radiation was less than 50 mV, but the sign of the signalidentified a negligible upward band bending. The val-ues of electron work function of the mentioned groupare higher for thicker films. It means that the propertiesof thicker DLC films are similar to those of diamondfilms. This result was partially confirmed by autoelec-tron emission investigation of the specimen C2. Theelectron field emission behaviour of the film was char-acterized using a sphere-to-plane electrode configura-tion under UHV conditions at the pressure of 10−5 Pain a special chamber. The distance between the elec-trodes was 7, 14, or 25 µm. Current–voltage charac-teristics were recorded using a 5 mm diameter stain-less steel ball anode. The voltage in the range of50–2500 V was applied between the electrodes dur-ing the emission measurements and was changed upand down several times in order to stabilize the I–Vcurve. Current–voltage characteristics indicated that

the autoelectron emission properties corresponded tothe Fowler–Nordheim model [5].

4. Conclusions

Early studies of diamond and diamond-like carbonthin films (appointed to autoelectronic emission cath-odes) deposited onto differently processed Si substratesby optical, electrostatic induction, and autoelectronicemission methods revealed:

• the films are non-uniform, exhibiting a distributionof in-plane values of surface electric potential;

• thicker diamond-like carbon films in group C arecharacterized by better autoelectronic emissionproperties;

• broader data on the surface electric potential valuesof the same type films would give comprehensiveinformation about availability of the films for auto-electronic emission cathodes.

Acknowledgement

The work was cofinanced by the Polish State Com-mittee for Scientific Research (KBN), projectNo. 3 T11B 050 26.

References

[1] K. Fabisiak, A. Banaszak, and M. Kaczamarski, Nucle-ation and growth of diamond films by using HF CVDtechnique, Functional Mater. 10, 117–120 (2003).

[2] Z. Haś and S. Mitura, Nucleation of allotropic carbon inan external electric field, Thin Solid Films 128, 353–360(1985).

[3] J.E. Field, The Properties of Natural and Synthetic Dia-mond (Academic Press, London, 1992).

[4] S. Sakalauskas and A. Sodeika, Automatized measur-ing instrument of the surface electric potential and po-tential’s distribution, Rev. Sci. Instrum. 69, 466–468(1998).

[5] R.H. Fowler and L.W. Nordheim, Electron emission inintense electric fields, Proc. R. Soc. London, Ser. A 119,173–181 (1928).

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DEIMANTO IR Į JĮ PANAŠIŲ ANGLIES SLUOKSNIŲ ANT Si PADĖKLO PAVIRŠINISPOTENCIALAS

M. Dłużniewski a, T. Lozovski b, R. Pūras b, S. Sakalauskas b

a Lodzės technikos universitetas, Lodzė, Lenkijab Vilniaus universitetas, Vilnius, Lietuva

SantraukaPuslaidininkiniai sluoksniai vis plačiau taikomi autoelektroni-

nės emisijos katodams gaminti. Tos emisijos metu katodai įkaista,todėl jiems gaminti geriau tinka plačiatarpės puslaidininkinės me-džiagos, tokios kaip deimantiniai sluoksniai, pasižymintys dar ir la-bai dideliu šiluminiu laidumu, tuo padėdami lengviau optimizuotipačią autoelektroninę emisiją.

Pateikti deimanto ir į deimantą panašių anglies sluoksnių, išau-gintų ant įvairiai apdorotų silicio padėklų ir numatomų taikyti au-toelektroninės emisijos katodams, pirminiai eksperimentiniai ty-rimai, susiję su nesąlytiniu Kelvino ir Zismano (Kelvin–Zisman)būdu išmatuotomis išlaisvinimo darbų vertėmis arba vienareikš-miškai jas atitinkančiomis paviršinio elektrinio potencialo ver-tėmis, kurias lemia tiriamųjų sluoksnių paviršinės savybės.