PHYSICAL REVIEW B 68, 125322 ~2003!

Band-gap dependence of field emission from one-dimensional nanostructures grownon n-type and p-type silicon substrates

C. S. Chang,* S. Chattopadhyay, and L. C. Chen†

Centre for Condensed Matter Sciences, National Taiwan University, Taipei-106, Taiwan

K. H. Chen‡

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei-106, Taiwan

C. W. ChenDepartment of Materials Science and Engineering, National Taiwan University, Taipei-106, Taiwan

Y. F. ChenDepartment of Physics, National Taiwan University, Taipei-106, Taiwan

R. Collazo and Z. SitarDepartment of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27613, USA

~Received 26 June 2003; published 24 September 2003!

Field emission of electrons from narrow-band-gap and wide-band-gap one-dimensional nanostructures werestudied.N-type silicon substrates enhanced the emission from the low-band-gap silicon nanowires and carbonnanotubes, whereasp-type substrates were a better choice for field emission from wide-band-gap silicon carbonnitride nanocrystalline thin films and nanorods. The role of the substrate-nanostructure interface was modeledbased on different junction mechanisms to explain, qualitatively, the fundamentally different emission behaviorof these nanostructures whenn- andp-type silicon substrates were used. The results could be explained on thebasis of simple carrier transport across the silicon-silicon nanowire interface and subsequent tunneling ofelectrons for the silicon nanowires. Schottky barrier theory can explain the better field emission of electronsfrom the n-type silicon supported carbon nanotubes. The decreased barrier height at the interface of thesilicon-silicon carbon nitride heterojunction, whenp-type silicon substrate was used, could explain the superiorfield emission in comparison to whenn-type silicon substrates were used.

DOI: 10.1103/PhysRevB.68.125322 PACS number~s!: 73.63.Bd, 73.40.Lq, 73.63.Fg

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The development of vacuum microelectronics was inmately linked with the realization of reliable high intensielectron sources. The thermionic emission from conventiocathode materials was challenged by the field emission fgated molybdenum and silicon microtip1 cold cathode emit-ters, for low power applications, that could be scaled doto micrometer sizes to utilize the high electric field to extraelectrons. The advent of nanotechnology then provided ecient electron emitters through a wide gamut of nanostrtures, nanorods, nanotubes, and nanowires, of differentments and even compounds. The microgeometry of thnanostructures demonstrated how some of the parametethe Fowler-Nordheim equation,2 especially the field enhancement factor, used to describe field emission in metals, cobe tailored to achieve lower threshold voltages for electemission. The material,3 its microstructure,4 the microgeom-etry ~that is, whether the emitter is a tip, tube or rod!,5,6 anyadsorbed species and surface modifications,7,8 and electricaland other physical properties of the emitting material wthoroughly studied to give us an insight into this phenoenon of field emission. However, these nanostructuresoften supported on attractive substrates such as siliconfuture integration to the device technology, whose roledetermining the emission efficiency has been unclear.substrate-nanostructure interface is of paramount importasince the availability of electrons undergoing tunneling fro

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one-dimensional nanostructured emitters depends on theficiency of the electron injection mechanism from the sustrate across this interface. This paper, instead of scanthe much visited nanostructure-vacuum interface, proceto examine the substrate-nanostructure interface that hremarkable effect on the field emission patterns of low bagap nanostructures namely, silicon nanowires~SiNWs! andcarbon nanotube~CNTs!, and wide-band-gap nanostructurenamely, silicon carbon nitride~SiCN! nanocrystallites, andnanorods, supported onn- andp-type crystalline silicon sub-strates.

SiNW, multiwall CNTs ~MWCNT!, and SiCN nanocrys-talline thin films and nanorods were deposited on dopSi substrates namely,n1 (resistivity;2 – 5 mV cm), n(resistivity;1 – 10V cm), and p type (resistivity;1– 10V cm). The field emission properties were measureda parallel-plate diode type device structure,8 where the nano-structures grown on silicon served as the cathode andindium tin oxide coated glass plate served as the anode.distance between these two electrodes could be controand was kept at;50 mm. This distance was used to calculathe apparent field at the tip. A vacuum level of 1028 Torrwas maintained during the measurements.

Randomly oriented Si nanowires@inset, Fig. 1~a!# grownby catalyst-assisted chemical vapor deposition onn-type sili-con substrates showed superior field emission properties

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those grown onp-type silicon substrates@Fig. 1~a!#. TheSiNWs grown on n-type substrates showed a two-ordhigher emission current density than those grown onp-typesubstrates at an applied electric field of 17.5 V/mm. Assum-ing a slightly higher band gap for SiNWs~1.4 eV! than Si~1.1 eV! ~neglecting any significant quantum confinemeeffects!, and a small band bending at the interface, the pdominant term for the field emission of electrons is the crier transport across the substrate-SiNW interface followby tunneling from the SiNW to vacuum. In contrast to tenergy barrier faced by electrons in the acceptor levep-type Si substrates to inject into the SiNWs, the electrofrom the donor level of then-type Si substrate can be easiinjected into the conduction band of the SiNW againsmuch smaller energy barrier and then tunnel into vacuuThe smaller barrier at the substrate-nanostructure interexplains the higher field emission for then-type substrates.

FIG. 1. Emitted current density as a function of applied field~a! silicon nanowires grown on~h! n-type Si and~m! p-type Sisubstrates~the inset shows scanning electron micrograph of rdomly oriented silicon nanowires grown on silicon substrates!; ~b!well aligned multiwall carbon nanotubes grown on~,! n-type Siand~.! p-type Si substrates at a deposition temperature of 1000and on~h! n-type Si and~j! p-type Si substrates, for CNTs depoited at 800 °C,~inset shows scanning electron micrograph of waligned multi wall carbon nanotubes grown on silicon substrate!.

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Well aligned MWCNTs@inset, Fig. 1~b!# were grown bythe microwave plasma enhanced chemical vapor deposroute, onn- and p-type silicon substrates, using iron ascatalyst and hydrocarbon feedstock gases.6,9 Here, theI -Vcharacteristics@Fig. 1~b!# shows that, for the CNTs grown othen-type substrates, the field emission is superior to thathe CNTs grown onp-type substrates. MWCNTs grown ahigher substrate temperatures~1000 °C! were found to beless defective than the ones grown at lower substrate tperatures~800 °C!. Raman spectroscopy showed a highertensity ratio of the 1580-cm21 mode ~G band! to the1345-cm21 mode ~D band! for the CNT grown at 1000 °C.Nevertheless, both sets of MWCNT samples showed befield emission whenn-type substrates were used. If a turn-ofield (Von) for field emission is defined as the field requirefor 10 mA/cm2 of emitted current then theVon for then- andp-type substrates were 2.8 and 3.8 V/mm, respectively, forthe MWCNT grown at 1000 °C. For the MWCNT grown a800 °C, theVon for then- andp-type substrates were 7.0 an9.4 V/mm, respectively.

To explain this result, we utilized the theory of Schottkbarriers. Fullerene tubules have been shown to have a cadensity similar to that of metals and a zero band gap at rotemperature.10 CNTs were shown to have a band gap offew hundred meV at room temperature.11 We can then treatthe CNT-Si interface as a metal-semiconductor interfaceapply the theory for Schottky barriers on this system. Sinfield emission is measured by applying a negative bias tosubstrate, the situation of a Si~n-type!-MWCNT combina-tion is somewhat similar to a forward biased Schottky juntion, whereas a Si~p-type!-MWCNT combination is similarto that of a reverse-biased Schottky junction. The builtpotential (Vbi) in each of these cases can be written as

Vbin5~fBn2Vn!2VF ~1!

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Vbip5~fBp2Vp!1VR ~2!

for the Si~n-type!-MWCNT and Si~p-type!-MWCNT cases,respectively. HereqVn andqVp stand for the energy differ-ence between the Fermi level and the conduction or valeband forn- andp-type Si;VF andVR stand for the forwardand reverse bias voltages; andqfBn and qfBp denote thebarrier heights for then- andp-type substrates, respectivelHere, the difference betweenqVn ~;0.22 eV! and qVp~;0.16 eV! is negligible. In other words, the effectivbuilt-in potential is reduced in case of then-type substratesand increased for thep-type substrates under the naturebiasing used in the field emission measurement. This difence in the built-in field, that opposes the transfer of eltrons from the substrate to the MWCNT, determines the dference in the observed field emission current levelsshould be mentioned that the barrier heightsqfBn andqfBpincrease as the work function of the CNTs increases. Itbe shown that in thermal equilibrium, the built-in potention n-type Si is always less than that ofp-type Si for a CNTwork function smaller than 4.57 eV. The presence ofVF and

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BAND-GAP DEPENDENCE OF FIELD EMISSION FROM . . . PHYSICAL REVIEW B68, 125322 ~2003!

VR will shift this limiting work function to higher valuesencompassing the commonly observed MWCNT work futions in the range of 4.6–4.8 eV.12 As a consequence,n-typesubstrates will always show better field emission properby virtue of a lower built-in potential.

Nanocrystalline SiCN films were deposited via electrcyclotron resonance plasma enhanced chemical vapor dsition ~ECR-PECVD!.13 Controlling the ECR-PECVD conditions both high and low density of SiCN crystallites@inset,Fig. 2~a!# were prepared onn1-, n-, andp-type Si substratesIt must be noted that the density of these nanostructuresexemplified in the case of CNTs, has a profound effect onfield emission behavior.14 The I -V emission characteristic@Fig. 2~a!# clearly reveals that the SiCN nanocrystallisamples produced onp-type silicon substrates emit a highcurrent density as compared to those grown onn-type orn1-type silicon; this is true for any given field and irrespe

FIG. 2. Emitted current density as a function of applied field~a! silicon carbon nitride nanocrystals grown on~j! p-type Si and~s! n-type Si and~n! n1-type Si substrates,~inset shows scanningelectron micrograph of silicon carbon nitride nanocrystals grownsilicon substrates!; ~b! silicon carbon nitride nanorods grown o~j! p-type Si and~h! n-type Si substrates@the inset shows thescanning electron micrograph~SEM! of silicon carbon nitride na-norods grown on silicon substrates with silicon carbon nitride nacrystalline buffer layers#.

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tive of the crystallite density. In this case,Von for the nanoc-rystallites grown onp-, n-type, andn1-type silicon sub-strates are 14.5, 16.9, and.20 V/mm, respectively.

SiCN nanocrystalline films were used as buffer layersthe growth of SiCN nanorods via microwave chemical vapdeposition@inset, Fig. 2~b!#.13 High density SiCN nanocrystalline films produced high density SiCN nanorods. Intereingly, the SiCN nanorods grown on these nanocrystallSiCN buffer layers also show a higher field emission whgrown onp-type Si substrates than those grown onn-typesilicon @Fig. 2~b!#, irrespective of the nanorod density. Themission pattern is reproducible over several observatand independent of morphological differences such as dsity and length of the nanorods.Von for the SiCN nanorodsgrown on p- and n-type silicon substrates is 9.7 and 11V/mm, respectively.

The simple transport as that occurring in case of SiNWthe Schottky barrier effect as in a CNT, is inadequateexplain the observed results. To explain the results we reto semiconductor heterojunctions. The analogy of the pressystem, SiCN on Si, with that of a semiconductor heterojution is valid owing to the large difference in their band gaand resistivities. The SiCN material has a high electricalsistivity and a wide band gap~4.2 eV!15 in comparison to thelow electrical resistivity and low band gap~1.1 eV! of thedoped silicon substrates. From an independent ab-initioculation, the work function for SiCN was found to be 5eV,16 whereas that forn-type Si is ;4.15 eV.17 Our x-rayphotoemission spectroscopy measurements on SiCN dmined a minimum work function of 4.5 eV above the Fermlevel of the back contact, as seen in Fig. 3. It is evident tthe electron affinity~x! of wide-band-gap SiCN is less thathat of the low band gap silicon (x54.01 eV).17

The model being introduced is still valid for a range ofxfor SiCN, unless it is too small to make SiCN stronglyptype. Assuming an electron affinityx;2.5 eV for SiCN~Fig.4!, we obtain a ‘‘straddled’’ heterojunction wherein the bagap of the SiCN completely overlaps the band gap of silic

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FIG. 3. Band-edge photoemission spectrum from a nanocrysline SiCN film. Minimum work function is 4.5 eV above the Fermlevel of the back contact.

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FIG. 4. Representative band diagrams for~a!Si ~n-type!-SiCN and ~b! Si ~p-type!-SiCN het-erostructures, in thermal equilibrium.EC , EV ,and EF represent the conduction band, valenband, and Fermi level of SiCN material, respetively; ECN , EVN , andEFN represent the conduction band, valence band, and Fermi leveln-type Si; andECP, EVP , and EFP represent theconduction band, valence band, and Fermi leof p-type Si.

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After constructing the band diagram based on the availadata on silicon and SiCN, it was found that SiCN is slighp type. Indeed, amorphous SiCN was found to be slightlptype.18 When Si and SiCN come into intimate contact andthermal equilibrium is established~Fig. 4!, band bending oc-curs at the interface.17 We expect a difference in the barrieheights for electrons and holes in such an abrupt heterojtion between Si and SiCN. For a Si~n-type!-SiCN junction,the large difference in the Fermi levels ofn-type Si (EFN)and SiCN (EF) before equilibration, would necessitatelarger flow of electrons to SiCN to obtain Fermi level aligment in equilibrium. This will in turn give rise to a severband bending and a higher barrier for further electron moment from silicon to SiCN. In the case of a Si~p-type!-SiCNheterojunction, however, the Fermi level in silicon (EFP) islower than the Fermi level of SiCN (EF) before equilibra-tion, and will necessitate some flow of electrons from SiCduring the Fermi level alignment. This will result in banbending at the Si-SiCN interface, but this time, instead obarrier, a well is formed. The flow of electrons across tjunction and subsequent tunneling during field emisswould be efficient for thep-type silicon substrate due to thabsence of the energy barrier. If there is any voltage dwithin the thickness of the material the work function mbe more than 4.5 eV. Even if we assume a higher work fution ~up to 5.5 eV! for SiCN as found from theoretical estmates, a barrier at the Si~p-type!-SiCN interface will existbut will be smaller than that existing in the Si~n-type!-SiCNinterface, becauseEFP2EF will be always less thanEFN

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2EF .17,19 For p1 Si substratesEFP and EF will be evencloser and a lower barrier height will be seen by the electrtunneling into SiCN. Hence, larger field emission currewere observed for the Si~p-type!-SiCN case as compared tthe Si~n-type!-SiCN case. This model is oversimplified sincwe have neglected the effects of interface states, diffuseffects, and possibly some contribution from the externalasing used during the measurements, however, it giveplausible qualitative explanation of experimentally observeffects.

In general, the different nanostructures studied here hdifferent morphologies, electrical conductivities, and surfastates that may influence the order of emission current dsities in different cases. The change in the emission patfrom these nanostructures when the substrates are chafrom n-type silicon top-type silicon, however, is related tthe substrate-nanostructure interface, and has been explaby different junction mechanisms determined by their bagaps. The change in emission pattern effected by a changsubstrate conduction may be enhanced by proper dopinthese nanostructures, especially in the case of SiCN whecan even be reversed depending on the Fermi level positing in the band diagram~Fig. 4!.

In short, the field emission from silicon nanowires, wealigned carbon nanotubes, silicon carbon nitride nanocrysline thin films, and nanorods were studied. The effect ofp- andn-type silicon substrates in limiting or enhancing thfield emission currents from the nanostructures has bdemonstrated. Then-type silicon substrates proved to be be

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BAND-GAP DEPENDENCE OF FIELD EMISSION FROM . . . PHYSICAL REVIEW B68, 125322 ~2003!

ter for field emission from low-band-gap silicon nanowiand carbon nanotubes systems. The energetically hiFermi level in n-type silicon was conducive for electrotransport across the Si-SiNW interface assisting in fiemission. Field emission from carbon nanotubes wasplained with the help of the Schottky barrier theory, wheren-type silicon substrate offered a lower built-in potentialcarrier transport. The results of improved field emissionthe case of wide-band-gap nanocrystalline SiCN thin filand SiCN nanorods onp-type silicon substrates were explained based on band diagram considerations in semi

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ductor heterojunctions.P-type silicon substrates offeredsignificantly lower injection barrier height, which resultedlarger field emission currents.

This work was carried out under projects funded by tNational Science Council and Ministry of Education, Tawan. The authors are grateful to Professor L. H. Peng ofElectrical Engineering Department, National Taiwan Univsity, for his valuable comments. One of the authors~S.C!acknowledges a post-doctoral fellowship awarded by thetional Science Council, Taiwan, R.O.C.

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