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Electron Field Emission of Silicon-Doped Diamond-Like Carbon Thin Films

Sekhar Chandra Ray, Sarit Kumar Ghosh, Zivayi Chiguvare, Umesh Palnitkar, Way-Faung Pong,

I-Nan Lin, Pagona Papakonstantinou, and Andre Michael Strydom

Jpn. J. Appl. Phys. 49 (2010) 111301

# 2010 The Japan Society of Applied Physics

Person-to-person distribution (up to 10 persons) by the author only. Not permitted for publication for institutional repositories or on personal Web sites.

Electron Field Emission of Silicon-Doped Diamond-Like Carbon Thin Films

Sekhar Chandra Ray1;2�, Sarit Kumar Ghosh3, Zivayi Chiguvare1, Umesh Palnitkar2, Way-Faung Pong2,

I-Nan Lin2, Pagona Papakonstantinou4, and Andre Michael Strydom3

1School of Physics, DST/NRF Centre of Excellence in Strong Materials and Materials Physics Research Institute,

University of the Witwatersrand, Private Bag 3, WITS 2050, Johannesburg, South Africa2Department of Physics, Tamkang University, Tamsui 251, Taiwan3Physics Department, University of Johannesburg, Auckland Park 2006, South Africa4Nanotechnology and Advanced Materials Research Instiute, School of Engineering, University of Ulster at Jordanstown,

Newtownabbey, County Antrim BT37OQB, Northern Ireland, U.K.

Received June 17, 2010; accepted August 30, 2010; published online November 22, 2010

In this work we demonstrate that the field emission characteristics of disordered Si-doped diamond-like carbon (DLC) thin films depend not only

on properties of the conductive clustered sp2 phase and the insulating sp3 matrix (or sp2/sp3 ratio) but also on the presence of Si–Hn and C–Hn

species in the film. The presence of such species reduces the hardness of the film and simultaneously enhances the field emission performance.

A turn on electric field (ETOF) of 6.76 V/mm produced a field emission current density of �0:2mA/cm2, when an electric field of �20V/mm was

applied. The Fowler–Nordheim (FN) tunneling model is appropriate to explain the field emission mechanism only within limited range of the

current density. However, it is found that there is an apparent crossover between space charge limited current (SCLC) and the Frenkel effect due

to impurities incorporated during the fabrication of Si-DLC films. This combined effect (SCLC + Frenkel) allows for the emission of electrons from

the top of the reduced barriers due to the formation of comparatively soft DLC:Si films. The emission also occurs through tunneling from one

conductive cluster (sp2 C=C) to another separated by an insulating matrix (sp3 C–C) after reducing the effective depth of a trap on application of

high electric field. # 2010 The Japan Society of Applied Physics

DOI: 10.1143/JJAP.49.111301

1. Introduction

Since the 1990s electron field emission (EFE) of carbon-based materials has been widely investigated with a view ofproducing stable and cheap cold flat cathodes. Practical field-emitter applications from diamond-like carbon (DLC) andrelated films have experienced a great impetus due to theirunique properties such as: high hardness, excellent chemicalinertness, atomic-level smoothness, low friction coefficient,excellent abrasion resistance, high thermal conductivity andoptical transparency. The field emission occurs at high fields;one forms a cathode in a needle shape so that the electronscan be emitted at low voltage. The negative electron affinity(NEA) of certain hydrogenated surfaces plays an importantrole1) and the effect of different surface terminating speciescan greatly affect the emission characteristics.2) However,since most of the results from low field emission experimentsare from poorly characterized surfaces it is evident thatNEA is not solely responsible for the emission process.There are many different mechanisms involved as theelectrons travel from the negative end of the power supplythrough the various interfacial contacts through the bulkof the film itself to the film surface, then tunnel throughthe potential barrier, propagate through the vacuum gap,before finally reaching the anode. The exact nature of themechanisms occurring at each step in this process and theway in which they interact is still not well understood. Thetwo basic models for carrier injection at a metal–insulatorinterface or a metal surface are Schottky emission andFowler–Nordheim (FN) injection.3) In both models, apotential barrier is created at the interface corresponding tothe energy difference between the conduction band of theinsulator and the Fermi level of the metal. In Schottkyemission, carriers are injected over this barrier, whereas inthe FN case, carriers tunnel through the barrier. In the mostcurrent literature on field induced emission from DLC films,

the current–voltage relation is simply fitted to a FN modeland a good fit has been taken as evidence that the processoccurring is cold field emission. A question mark, therefore,hangs over the validity of using the FN model for DLC filmsand also, therefore, over whether the dominant emissionmechanism is really cold field emission at all. The Poole–Frenkel (PF)4,5) conduction is a different bulk process basedon space charge limited current (SCLC)6) that relies on thesignificant number of defect or impurity sites. The chargecarriers reside on these defect sites and when the sites areclose together then the wave functions of the charge carriersare overlapping and allow the carriers to ‘‘hop’’ from one siteto another. If the defect sites are too distant then the hoppingmechanism cannot keep up with the applied field and the PFcurrent can become SCLC.7) Apart from this model, differentother models have also been proposed on electron fieldemission based on the mechanism of chemically modulatedwork function of carbon films including the negativeaffinity,1,8–10) antenna effect of conducting channels,11)

impurity gap states,12,13) band bending at depletion layers14)

and surface dipole formation15) but all these modelsaccomplished by field emission and/or reduced thresholdfield remain largely unexplained.8–15)

In this work we have studied the electron field emissioncharacteristics of Si-doped DLC films (including harder andsofter-films), containing H and Si with sp3 content up to�60%. No information has been reported on the fieldemission study of Si-doped DLC films. The results presentedin this work are discussed with the help of the most well-known FN and hence we elucidated the mechanism thatcomprises of a combined model of SCLC + PF for Si-dopedDLC films.

2. Experimental Methods

The Si-doped DLC films were deposited on Si-substratesby the plasma enhanced chemical vapor deposition(PECVD) method using tetramethylsilane [Si(CH3)4, TMS]flow (0– 40 sccm) as a Si-precursor along with Ar (10 sccm)�E-mail address: Sekhar.Ray@wits.ac.za

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and C2H2 (20 sccm) at constant bias voltage (Vb) 400 V.Details of the film preparation and their different micro-structural and optoelectronic properties are discussed in ourprevious reports.16–19) Electron field emission characteristicswere performed using a Keithley 237 power supply. Thecathode voltage was applied by an analog programmable1.0 kV power supply under computer control and themeasured emission current was logged at each voltage.The measurements were carried out under a low 10�6 Torrambient pressure. The movement of the anode tip (1 mmdiameter) was measured digitally and the gap betweenemitter and collector was confirmed by optical microscope.During the measurements the anode and cathode distancein the field emission system was fixed at 200 mm, which isthe thickness of the micro-glass spacer used to isolatethe cathode from the anode.

The electrical resistivity, �, was measured on selectedsamples by means of the four-probe dc-current technique.Current reversal was used to minimize the effect of spuriouscontact voltage. The temperature dependence of � wastracked below room temperature by allowing proper temper-ature equilibrium at each datum point prior to measurementof the resistive voltage. Electrical contact to the samplesurface was achieved using high-purity gold wire affixedwith conducting epoxy.

3. Results and Discussion

The electron field emission current density as a function ofapplied electric field (J–EA curves) for Si-doped DLC filmsgrown using TMS flow rates within the range 0 – 40 sccm(i.e., having different Si at. %) are shown in Fig. 1. Noemission is observed at a low TMS flow rate of 5 sccm, thenthe current density (J) shows an enhancement over the rangeof 10 – 20 sccm, reaching a maximum at �20 sccm and thenupon further increase of TMS flow it decreases, with noemission been observed at 40 sccm. The emission currentdensity (J) of all films estimated at the applied electric field(EA) of 20 V/mm are presented in Table I for comparativepurposes. It is observed that the best emission occurs at aTMS flow of 20 sccm and being �0:2 mA/cm2 at EA ¼20 V/mm. The turn on field (ETOF) of the electron fieldemission was obtained from a linear extrapolation of the FNcurve in the high EA region. The curve was plotted in theform of lnðJ=EA

2Þ vs (1=EA) following the FN field emissionequation20) as follows:

J ¼ Að�EAÞ2 exp �B’3=2

�EA

� �ð1Þ

where J is the current density, EA is the applied electric field,� is the effective emission barrier, � is the field enhancementfactor, A, B are constants and their values are 1:4� 10�2 and6:8� 109, respectively, and ETOE is defined as the field atwhich J exceeds 0.1 mA/cm2. The ETOF of all these films areobtained from the Fig. 1(b) and are given in Table I. Itis found that the ETOF decreases with increase of TMS flowup to 20 sccm (ETOF � 6:76 V/mm for this film). Furtherincrease of TMS flow up to 30 sccm causes an increaes onETOF (decrease of J) and above 30 sccm no emission wasobserved. Decrease of ETOF indicates an enhancement ofelectron field emission. In addition, according to the FN plot,the slope m [given in eq. (1)] would represent the combined

effect of work function and enhancement of local electricfield (�) and is given by m ¼ �ðB�3=2Þ=�, using � ¼ 4:7 eVfor the work function of DLC,21,22) the field enhancementfactor was calculated from the slope of the F–N plot. Theenhancement factors are 3:5� 104 and 1:9� 104, respec-tively for the films prepared at TMS flow 20 and 15 sccm,respectively. In general, the sp2-content acts as a source ofemission sites resulting in low ETOF, but in the present casehighest emission occurs for the films deposited using TMSflow of 20 sccm; which have the lowest sp2-content (seeTable I). This result indicates that the mechanism of thefield emission is different in the present case. In our previ-ous reports16–18) we have observed that the sp2-contentobtained from X-ray absorption near edge structure(XANES) decreases (increase of sp3-content) with increaseof TMS flow during deposition and in the present study withthe same manner, ETOF decreases, i.e., J is increases. Therelationships between sp2-content and turn on field (ETOF)with TMS flow are plotted in Fig. 2(a); whereas the decreaseof J (increase of ETOF) with increase of hardness is shown inFig. 2(b). This figure indicates that the softer-films are goodfield emitters of electrons. The ID=IG ratio obtained from

0 10 20 30 400

2

4

6

J (μ

A/c

m2 )

EA (V /μm)

0 10 20 30 400.0

0.2

0.4

0.6

0.8 (a)

Cur

rent

den

sity

J (

mA

/cm

2 )

TMS (sccm) No FE 0, 05

10 15 20 30

No FE 40

Applied electric field EA (V/μm)

-18

-15

-12

-9

-6

0.0 0.1 0.2 0.3 0.4 0.5

F-N plots(b)

1/EA (μm/V)

ln (

J/E

A

2 ) ((

mA

/cm

2 )/(V

/μm

)2 )

TMS (sccm)No FE 0, 05

10 15 20 30

No FE 40

Fig. 1. (Color online) (a) Electron field emission J–EA plots and (b) FN

plots of Si-doped DLC films deposited at different TMS flow ranging from

10–30 sccm using plasma enhanced chemical vapour deposition method

at the bias voltage Vb ¼ 400V. No field emission is observed for the films

when TMS flow was 0, 5, and 40 sccm.

Jpn. J. Appl. Phys. 49 (2010) 111301 S. C. Ray et al.

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Raman spectra changes as a function of the flow rate inthe same manner as the ETOF (see Table I). These resultsconfirm that the structure and hence the electron fieldemission is not only due to the presence of sp2- or sp3-clusters (and/or ID=IG ratio) but also due to other speciespresent in the film structure like Si–Hn, C–Hn, and so on (seeTable I). Comparatively higher film hardness hinders themovement of sp2- or sp3-cluster and reduces the electronconduction path and hence electron field emission. It issuggested that the presence of higher amounts of Si–Hn

and C–Hn species in DLC:Si films, enhances the electronfield emission and at the same time reduces the hardnessof the film structure.16) As a consequence on applicationof an electric field, the wave function overlap of the

clusters will result in electron delocalization23) and/orenhanced hopping/tunneling between the clusters thatenhance the connectivity. Again, our data suggest that theemission barrier of Si-DLC decreases with both increasingof the sp3-bond and hydrogen content in the film structure asdescribed by Robertson24) in the EFE mechanism of DLCfilms. This is also evidenced by the magnitude of the slopeof the straight line fits in FN curves [Fig. 1(b)]. The slopeof the F–N curves decreases when TMS flow increases from10 to 20 sccm and then increases. This hints that the bestemission properties will be achieved for Si-DLC films withhigher sp3-bond content (lower sp2-content) implying goodconduction paths leading to enhanced field emission. Again,different species (Si–Hn, C–Hn) present in the film structuremay also act as the surface terminating species which greatlyaffect the emission25) as well as film properties.

We have studied the variation of resistivity (�) withtemperature (T) and found that the conductivity trend of theSi-doped DLC films as a function of the TMS flow rateis similar to their EFE characteristics. Figure 3 shows the�–T curve for the films having TMS flow in the range of10 – 30 sccm. It is very clear that the electrical resistivity

Table I. The turn on field (ETOF), electron emission current density (J) at EA ¼ 20V/mm, sp2-content from XANES, Young’s modulus, ID=IG ratio from

Raman spectra and H2 Content from FTIR spectra of Si-DLC films deposited at Vb ¼ 400V with different TMS flow rate in an Ar (10 sccm) and C2H2(20 sccm) atmosphere along with the sp2-content and stress of the films.

TMSFE-Turn on EF-current sp2-content Young’s

ID=IGH2 content

flowelectric field, ETOF density, J (arb. unit) modulus

ratioatoms/cm3 (�1021)

(sccm)(V/mm) (A/cm2) from XANES (GPa)

[Ref. 16][Ref. 17]

(from F-N plots) at EA ¼ 20 V/mm [Refs. 16, 18] [Refs. 16, 18]Si–Hn C–Hn

0 Field emission is not observed 0.95 187 0.36 — 24

5 Field emission is not observed 0.94 157 0.30 2.2 42

10 26.42 0:7614� 10�8 0.90 149 0.23 2.8 62

15 15.50 0:2046� 10�6 0.82 143 0.18 3 78

20 6.76 0:1891� 10�3 0.75 142 0.16 3.8 86

30 24.39 0:1836� 10�7 0.76 146 0.20 2.9 80

40 Field emission is not observed 0.79 160 0.28 2.5 45

11.5 12.0 12.5

0.0000

0.0001

0.0002

0.1891

0.6

0.7

0.8

0.9

Cur

rent

Den

sity

(m

A/c

m2 )

Hardness (GPa)

0 10 20 30 40

10

15

20

25

ETOE

ET

OF (

V/μ

m)

TMS flow (sccm)

sp2 -c

onte

nt (

arb.

uni

t)

sp2-content

Fig. 2. (a) Turn on electric field ETOF and sp2-content with TMS flow

(sccm) and (b) emission current density J (mA/cm2) with Young’s

modulus (GPa) of the Si-DLC films.

180 210 240 270 3000

10

20

30

40R

esis

tivi

ty, ρ

(Ω.c

m)

Temperature (K)

TMS(sccm) 10 15 20 30

Fig. 3. Resistivity versus temperature curves for the Si-DLC films

having TMS flow between 10 and 30 sccm.

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decreases, i.e., that the electrical conductivity improves withTMS flow up to 20 sccm and then decreases again. Hence,20 sccm TMS flow films are yielding the most electricallyconductive material of all the samples investigated in thiswork. At higher TMS flow (>20 sccm) the films becomeharder and field emission is reduced. We speculate that atthese higher TMS flow rates, there may be the formation ofhigher bonding energy species like Si–O bond (8.3 eV)/Si–Cbond (4.6 eV) along with C–C (3.7 eV) besides the Si–H(3.1 eV) and C–H bonds (3.3 eV),16) that enhance thehardness of the film structure and hence reduce the fieldemission as confirmed by the �–T behavior.

As a matter of fact, whenever discussing the field emission,the most commonly used model for the ejection of electronsfrom the surface is the well-known FN equation [eq. (1)]. TheFN model only deals with surface effects (or at the interfacebetween the electrical contact and the film) and the resultingfits are usually taken as the evidence of the process of coldfield emission. Xu et al.26) confirmed that the non-linearity inthe FN plot originated from a transition from thermionicemission (TE) to field emission (FE) as the applied fieldincreases, based on a unified electron-emission equation. TheSchottky emission and SCLC models appear to provide muchbetter descriptions of the overall conduction mechanism thanthe FN model. All these findings suggest that there is anapparent crossover in Schottky emission; SCLC models andFN behavior as the microstructure of the films changes dueto the impurity incorporation in the Si-DLC film structure.

4. Theory

We propose that the enhancement of the field emission maybe due to the different intrinsic conductive properties of sp2

clusters embedded in the surrounding sp3 insulating matrix.It is well known that the electric field near a single con-ductive dielectric sphere in an insulating matrix is increasedby a factor of up to two due to the dielectric mismatchbetween the sphere and the matrix. It has been calculated27)

for closely spaced spheres with the conductivity of gold, anenhancement of the electric field by a factor of 56 is possibleif the spheres have a separation of 5 nm. This enhancementincreases as the separation between the spheres is reducedreaching a factor of 400 for a sphere to sphere separation of1 nm. Although the conductivity of the sp2 clusters will notbe as high as that of Au, these calculations demonstrate thathigh enhancement factors could be obtained by consideringthe effects of just two conductive spheres near the surface. Inthis manner, the emission process of Si-DLC films could beexplained as follows: the high density of defects will act tolocalize and attract the field lines from the anode to a thinregion near the surface of the film. High field enhancementfactors are present if two (or more) sp2 conductive clustersare close to each other along with Si–Hn, C–Hn and/or Siand H. Once the electrons are emitted from the clusters nearthe surface of the film, they can be replaced by the electronsfrom clusters deeper within the film. Such a description aidsus in explaining the non-uniform (on the nano- to micro-scale) nature of the emission across the surface of the film inwhich the local arrangement of the clusters below thesurface is important. The significant mechanism is probablyonly the tunneling of the electrons through the potentialbarrier, since conduction through the film would be

relatively facile. For more insulating films, however, con-duction through the bulk of the film might become importantand potentially rate limiting. Thus, bulk conduction mech-anisms (such as SCLC), as well as mechanisms occurring atthe various interfaces (such as Schottky emission) may beginto play a significant role in the electron transport. If this istrue, the observed current density–electric field dependenceis not based on a single model and may be the combina-tion of two or more mechanisms along with the Fowler–Nordheim surface ejection model. Here we combine theSCLC and Poole–Frenkel models and explain the emissionmechanism of Si-DLC films as follows:

According to the theory of SCLC between plane parallelelectrodes and well known Mott–Gurney law, the currentdensity (J) is as follows:28)

J ¼9

8�""0

V2

L3; ð2Þ

where, � is the free carrier mobility, " the dielectric constantof the material, V the applied voltage, and L the distancebetween the electrodes. The current is assumed to be due tocarriers of one sign only, the effect of diffusion is neglectedand the mobility is assumed to be independent of the field.Equation (2) can be modified for the case of single set oftraps, situated at energy A below the conduction band asshown in Fig. 4. If �f and �t are the free and trap chargecarrier densities in the films respectively then the proportionof total free charge is given by

�f

�f þ �t

¼ �0 ¼Nc

Nt

exp �A

kT

� �; ð3Þ

where Nc is the effective density of states in the conductionband, Nt is the density of traps and the current density:

Ebelow

ETop

Ele

ctro

n en

ergy

AδA

x1

Trap Ground state

Electron trap energy

Conduction band

Fig. 4. Frenkel effect, showing the reduction of the trap depth A by �A.

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J ¼ ��fE. Frenkel29) pointed out that the presence of astrong electric field may cause the reduction of effectivedepth of a trap. If an electron trap is regarded as a positivelycharged centre at a fixed position in a structureless dielectric,then the potential energy of the electron is

VðrÞ ¼�e2

4�""0r� eEx;

where the electric field Ex is applied in the x-direction. In theabsence of the externally applied electric field, the VðrÞ iszero and occurs at infinite distance from the trap. When theelectric field is applied, VðrÞ is reduced on one side of thetrap, as shown in Fig. 4. The VðrÞ now has a maximum onthe x-axis through the trap centre at a distance

x1 ¼e

4�""0E

� �1=2

from the trap and the value of VðrÞ at this point is,

VðrÞ ¼�e3E�""0

� �1=2

:

The effective depth of the trap is considered to be reducedand consequently the proportions of free charge carriers areincreased. Therefore the proportion total free charge carrierswhen an electric field E is applied can be written followingeq. (3) as

�f

�f þ �t

¼Nc

Nt

exp �A

kTþ

1

kT

e3E

�""0

� �1=2" #

¼ �0 expð�E1=2Þ; ð4Þ

where

�0 ¼Nc

Nt

; � ¼1

kT

e3

�""0

� �1=2

; andV

L¼ E

Therefore the SCLC in Frenkel effect can be written aftersimplifying eqs. (2)–(4) as

J ¼9

8�""0

V2

L3�0 expð�E1=2Þ; ð5Þ

and further this equation is modified and could be written as

lnJ

EA2

� �/ EA

1=2: ð6Þ

We have considered that the electron emission takes placeover the top of the reduced barrier (ETop) and by tunneling(EBelow) from the conductive sp2 cluster with the impuritiesincorporated during deposition to another conductive cluster,separated by an insulating sp3 matrix, after reducing theeffective depth of a trap on the application of high EA. Thisgives the increase in probability of emission in the forwarddirection only. We also assumed that it has been increasedby the same factor as the barrier has been decreased. Thiscrossover from SCLC and Frenkel behavior is also evidenceof percolation, whereby the conductivity of insulatingmaterials (sp3-rich matrix) is increased by the modifyingthe material and enable them as electrically heterogeneous(conductor-insulator) composite system by the incorporatedimpurities. We have plotted the current density and electricfield using relation (6) as shown Fig. 5 and a fit (better thanFN plot) has been taken as the evidence that the process

occurring is SCLC in Frenkel effect. These fittings wereobserved only for those films, which are comparativelysofter films and may be due to more effective depth ofreduced trap on applied the same EA. However, the ETOF isnearly two times lower than the results obtained from FNplots. There is no physical basis for this assumption apartfrom the fact that it gives zero current density at zero fields,which is realistic. For further verification, we checked thecurrent density from J–EA plots at the low ETOF and foundthat the J is of the order of mA/cm2, which is also realisticand quite reasonable.

5. Conclusions

The general conclusion from this work is that the fieldemission behavior of the DLC:Si based film cannot beexplained by a single model. The combined effect of spacecharge limited current (SCLC) and the Frenkel effect(SCLC + Frenkel) is used to explain the mechanism ofthe field emission current for the comparatively soft DLC:Sifilms. Indeed, findings are pointing to the multiple roles ofsp2-bonded carbon networks with different species (Si–Hn,C–Hn, H, and Si), on the softness-hardness and electricalemission activity. Incorporation of ‘‘Si’’ from the TMSsource in DLC film increases the emission significantly, buthigh level of ‘‘Si’’ reduces emission due to formation ofharder films. The presence of ‘‘H’’ in the film acts as mobilespecies that could move around the bulk film and disappearfrom the surface by electron-stimulated desorption. Ourstudies provide an understanding for the field emissionmechanisms that occurs in Si-DLC films as well as otherDLC based materials. Hence, the linear plots in Fig. 5,clearly show that the field emission mechanism of thesefilms (DLC:Si) is strongly based on the combination effectof SCLC and the Frenkel models/mechanism.

Acknowledgement

The author (WFP) would like to thank the National ScienceCouncil of the Republic of China for financially supportingthis research under Contract No. NSC 99-2112-M032-004-MY3.

10 20 30 40 50 60 70 80 90

-7

-6

-5

-4

-3

-2

-1

0

ln(J

/EA

2 )

EA

1/2 (V/μm)1/2

TMS 15 TMS 20 linear fitting for TMS 15 linear fitting for TMS 20

Fig. 5. (Color online) Space charge current limited with Frenkel effect

shows the Si-DLC films have the best fittings for the films deposited by

TMS flow of 15 and 20 sccm, respectively.

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