Prosiding Pertemuan Ilmiah Sains Materi IIISerpong, 20 -21 Oktober 1998 ISSN 1410-2897

SURFACE DEFECT STATES ON SPUTTERED HYDROGENATEDS3b AMORPHOUS SILICON THIN FILMS

A. N uruddinDepartment of Engineering Physics, Bandung Institute of Technology, Jalan Ganesha 10, Bandung 401.32

J. R. AbelsonCoordinated Science Laboratory and the Department of Materials Science and Engineering, University of Dlinois at

Urbana-Champaign, illinois USA.

ABSTRACTSURFACE DEFECf STATES ON SPUTTERED HYDROGENATED AMORPHOUS SILICON THIN FILMS.

We have determined the density of surface defect state on undoped a-Si:H and B-doped a-Si,C:H thin film. The films weredeposited by a dc reactive magnetron sputtering of Si (B) target in (Ar+UJ plasma. An in-situ Kelvin probe was employed toreveal the contact potential profile during the deposition of the film. The surface density of states on undoped a-Si:Hand borondoped a-Si,C:H was determined by simulated the contact potential profile to be I ' 1011 cm-2 and 2 .10.. cm-\ respectively. The

low surface state density means that the Fermi energy is not pinned, and the surface potential profile is closely related to theinternal potential of the complete device.

ABSTRAK

DEFECT STATE PADA SPUTTER LAPISAN TIPIS SILIKON AMORF TERHmROGENASI. Densitas de-fect state pad a pennukaan laplsan tipis a-Si:H clan a-Si,C:H(B) telah ditentukan. Lapisan tipis dideposisi menggunakanmagnetron sputtering dengan target Si(B) dalam plasma (Ar+HJ. Profil potensial kontak diukur menggunakan metoda Kelvinprobe secara in-situ Densitas defect state pada permukaan lapisan tipis ditentukan dengan simulasi potensial kontak, clandiperoleh sebesar I .1011 cm-2 untuk a-Si:H clan 2 ' 1011 cm-2 untuk a-Si,C:H(B)- Densitas state yang rendah ini menunjukkan

bahwa energi Fermi pada permukaan tidak pin, clan profil potensial permukaan sangat mirip dengan profil potensial dalamdivais yang komplit

INTRODUCTION

the work function difference between the sample sur-face and a vibrating probe, and therefore electrostaticpotential profile. The link between the surface poten-tial profile and the charge states occupancy can bequantitatively established through solution of Poisson'sequationas as demonstrated by Abelson and deRosny[1]. Their approach is to vary the surface charge den-sity of device quality a-Si:H through controlled chemi-so~tion of a donor molecule, and to simultaneouslymeasure work function and conductivity changes. Thebulk defect state density is calculated by analysing thecorrelation of the band bending and conductivity.Siefert et al. employed in-situ Kelvin probe to periodi-cally measure the work function changes vs. thicknessof a-Si:H deposited by plasma assisted chemical vapordeposition (PACVD) [2]. The bulk and surface den-sity of states are quantitatively calculated through solv-ing Poisson's equation of the potential profile. The abil-ity of Kelvin probe to evaluate the density of defectstates during layer growth is due to low surface statedensity on a-Si:H. In this paper, we present the studyof the surface state on undoped and boron doped hy-drogenated amo~hous silicon deposited by reactive

magnetron sputtering (RMS).

For a semiconductor, like a-Si:R with a con-stant electron affinity, the observed potential variationswith thickness are identical to changes in the surfaceFermi level position with respect to the conductionband. In other words, the semiconductor can have adifferent surface band bending depending on the in-ternal charge distribution, where these charges residein surface and bulk defect states. These defect statescan be introduced intentionally by doping, or uninten-tionally through impurities or treatment, such as ionbombardment implantation or thermal populations.

The localized defect states in hydrogenatedamorphous silicon dominate the electronic propertiesof this material. They govern the free carrier lifetimeand thus place a fundamental limit on the performanceof minority carrier devices such as solar cells. Becausethe density of trapped charges far exceeds the densityof free carriers, the defect state occupancy determinesthe band bending through solutions of the Poissonequation. Thus, the shape of the band bending is cru-cial to device design and performance because highelectric fields must be established for efficient chargecollection.

Kelvin probe method has the ability to measure

Prosiding Pertemuan Ilmiah Sains Materi IIISerpong, 20 -21 Oktoher 1998 ISSN 1410-2897

EXPERIMENTAL riodically intemJpting the deposition process and insert-ing the probe close to the sample surface.

During film deposition, the probe is secluded ina separate chamber to prevent modification of its workfunction by adsorbed species. The probe work func-tion is routinely checked by measuring the contact po-tential of a reference gold sheet in the separate cham-ber. This procedure ~rmits the determination the con-tact potential of the sample with an accuracy betterthan:!: 5 m V. The interruption of the deposition pro-cess for contact potential measurement takes less than2 minutes; the optical and electrical properties of thefilm are identical with those of films grown continu-

ously.

Film ~rowth and data collection

Schematic diagram of the deposition systemis shown in Fig.I. The deposition system used inthis experiment is available in the Engineering ScienceBuilding. University of lllinois at Urbana-Champaign.

Simulation of the contact potential profile

The density of states of bulk, interface and,sur-face can be determined from data fitting. The mea-suredcontact potential is related to the density of states bysolving Poisson's equation,

d2V(x) -P2 -dx

Figure I. Schematic diagram of the deposition system anKelvin probe.

(1)The system consists of three interconnected ultrahighvacuum chambers with base pressure on the order of10-0 Torr. It is equipped with a loadlock, which can beused to transfer the sample or to evaporate metal con-tacts. This chamber configuration allows us to sequen-tially deposit p+, i , n+ layer, and metal back contact tomake a complete solar cell without breaking the vacuum

system.

-SSG

where V(x) is the electrostatic potential in the film, r is thecharge density and eeo is the dielectric function of a-Si:H. Equation (I) can be solved numerically. We usethe computer program AMPS (Analysis of Micro-electronic and Photonic Structure) that uses finitedifferences and the Newton Raphson technique [4].AMPS uses finite temperature statistics to computeequilibrium free and trapped carrier populations over ageneral distribution of donor-like and acceptor-likelocalized states, including both exponential band tailstates and the midgap states in a-Si:H.

Equation (I) is solved subject to boundary con-ditions. Figure 3 shows the band diagram of a-Si:Hfilm

(2)

The undoped a-Si:H film is deposited with hy-drogen partial pressure of 0.8 mTorr at a substrate tem-perature (T ,) = 230 °C. These are standard conditionsfor depositing device quality film by reactive magne-tron sputtering [3). The phosphorous-doped a-Si:H isdeposited with P{HJ = 4 mTorr at T. = 200 °C, andthe boron-doped a-Si.C:H is deposited with P(H2) = 4mTorr and P(CH4) = 0.1 mTorr at T, = 200 °C. Theargon partial pressure is kept constant at 1.5 mTorr forboth undoped and B-doped film deposition.

Figure 2 shows in more detail the Kelvin probeapparatus which is connected to the deposition cham-ber. The probe can be retracted back and forth, andthe contact potential measurement is performed by pe-

deposited on a conductive substrate used in thesimulation. The boundary condition at interface iswhere c is the electron affinity of a-Si:H and F sub is thework function of the substrate metal or conducting oxide.The other boundary condition for the free surface of thefilm of finite thickness x is.

q.. (V.. (x))=- (3)EE-

dVdx, , x=x u

0

where q, is the surface charge determined by thesurface potential V.(x) as a result of the deposition; andthe densities of bulk and surface states are input asadjustable parameters. It is worth to mention that thesurface neutrality level can be different form the bulk.

Figure 2. Detail schematic i)f the in-situ Kelvin

prohe apparatus

Prosiding Pertemuan Ilmiah Sains Materi IIISerpong, 20 -21 Oktober 1998 ISSN 1410-2897

RESULTS AND DISCUSSION

Figure 5 shows the ability of the in-situ Kelvinprobe to directly monitor the movement of the surfaceFermi level through the band bending and effect ofdoping during a-Si:H deposition. The undoped a-Si:His first deposited on Pd substrate. As the film thick-ness increases, the work function shifts to a plateaucharacteristic of undoped film in the flat band condi-tion. When the P-doped a-Si:H film is deposited ontop of un doped a-Si:H, the decrease in the contact po-tential difference, i.e., lower the work function, is im-mediately observed, providing direct evident of Fermilevel shift towards the conduction band edge due tophosphorous doping. When the film growth isswitched to B-doped a-Si,C:H the work functionchanges oppositely. The doping efficiency can be foundusing this measure of the work function change and in-dependent know.ledge of the dopant concentration. Fi-nally, the deposition of undoped a-Si:H on B-doped a-

Figure 3 The band diagram of a-Si:H film depo-~ited on a conductive substrate used inthe simulation

The program iteratively solves equation (I) andgenerates a potential profile V,(x) for com- pari son withthe experimentally measured Kelvin probe profile. In asemiconductor, likea-Si:H. the surface is never free fromsurface states. To simulate the surface states, a thin ~ 10

A layer with high density of states is introduced. Thesensitivity of the calculated contact potential profiles tothe assumed values is shown in Fig. 4.. Under the samebulk defect density. a factor of two difference in surfacestate density can result up to 30 m V difference in themeasurements On the other hand. For the same surfacestate density, a high bulk defect density narrows the

Figure S. Contact potential profile measurement ofundoped, P-doped a-Si:H, and B-dopeda-Si,C:H films deposited on Pd coated glasssubstrate.

Figure 4. The sensitivity of the contact potentialprofiles. (a) constant surface statedensity and vary the bulk state density,and (b) constant bulk state density andvary surface state density.

Si,C:H shows a potential profile similar to a Schottkyjunction, e.g. Pd/a-Si:Hjunction. We observe here thatthe depletion width of Pd/a-Si:H < 0.2 ~, which islow for typical device quality a-Si:H. Siefert et al.found the depletion width ~ 0.2 ~m for a-Si:H depos-

ited by PACVD on PtSi substrate [2). The short deple-tion width in both cases may be due to silicide induceddefect states.

We solved the Poisson equation as function ofthe film thickness, and input the bulk and surface den-sity of states as adjustable parameters to fit the experi-mental potential profile. As shown in Fig. 6a, thesedata are best fit with a bulk density of states (Nd) =4.5 .1016 cm-3 eY-I, and surface state density (N.) = 1

.1011 cm-2 (dashed line). The dot curves correspondto the internal potential for particular film thicknesses,at which the contact potential measurement is con-ducted. These show that the field does not go to zero

depletion width. A factor of four difference in bulk statedensit). can produce about 40 m V difference in thecontact potential measurements. However. it must bestressed that these analyses do not apply to asemiconductor with high surface state density. whichpins the surface Fermi level position.

Prosiding Pertemuan Ilmiah Sains Materi IIISerpong, 20 -21 Oktober 1998 ISSN 1410-2897

RESULTS AND DISCUSSION

Figure 5 shows the ability of the in-situ Kelvinprobe to directly monitor the movement of the surfaceFermi level through the band bending and effect ofdoping during a-Si:H deposition. The undoped a-Si:His first deposited on Pd substrate. As the film thick-ness increases. the work function shifts to a plateaucharacteristic of undoped film in the flat band condi-tion. When the P-doped a-Si:H film is deposited ontop of undoped a-Si:H, the decrease in the contact po-tential difference, i.e., lower the work function, is im-mediately observed, providing direct evident of Fermilevel shift towards the conduction band edge due tophosphorous doping. When the film growth isswitched to B-doped a-Si,C:H the work functionchanges oppositely. The doping efficiency can be foundusing this measure of the work function change and in-dependent know.ledge of the dopant concentration. Fi-nally, the deposition of undoped a-Si:H on B-doped a-

model

Figure 3 The band diagram of a-Si:H film depo-sited on a conductive substrate used inthe simulation

The program iteratively solves equation (I) andgenerates a potential profile V.(x) for com- pari son withthe experimentally measured Kelvin probe profile. In asemiconductor, likea-Si:R the surface is never free fromsurface states. To simulate the surface states, a thin ~ 10A layer with high density of states is introduced. Thesensitivity of the calculated contact potential profiles tothe assumed values is shown in Fig. 4.. Under the samebulk defect density, a factor of two difference in surfacestate density can result up to 30 m V difference in themeasurements On the other hand For the same surfacestate density, a high bulk defect density narrows the

Figure 5 Contact potential profile measurement ofun doped, P-doped a-Si:H, and B-dopeda-Si,C:H films deposited on Pd coated glasssubstrate.

Si,C:H shows a potential profile similar to a Schottkyjunction, e.g. Pd/a-Si:Hjunction. We observe here thatthe depletion width of Pd/a-Si:H < 0.2 ~m, which islow for typical device quality a-Si:H. Siefert et al.found the depletion width -0.2 ~m for a-Si:H depos-ited by PACVD on PtSi substrate [2J. The short deple-tion width in both cases may be due to silicide induceddefect states.

We solved the Poisson equation as function ofthe film thickness, and input the bulk and surface den-sity of states as adjustable parameters to fit the experi-mental potential profile. As shown in Fig. 6a, thesedata are best fit with a bulk density of states (Nd) =4.5 .1016 cm-3 eY-1, and surface state density (N.) = I

.1011 cm-2 (dashed line). The dot curves correspondto the internal potential for particular film thicknesses,at which the contact potential measurement is con-ducted. These show that the field does not go to zero

depletion width, A factor of four difference in bulk statedensii.)' can produce about 40 m V difference in thecontact potential measurements, However. it must bestressed that these analyses do not apply to asemiconductor with high surface state density, whichpins the surface Fermi level position,

A. Nuruddin dan .f.R. Abelson364

Pro.\"iding Pertemuan Ilmiah ,\'ains Materi III,\'erpong, 20 -21 Oktober 199R ISSN 1410-2897

CONCLUSION

~""

J

We have detennined the density of surface de-fect state on sputtered thin film of un doped a-Si:H andB-doped a-Si,C:H. The Kelvin probe was shown to bea useful method for measuring the potential profile.Through numerical simulation of the measured poten-tial profile, the bulk and surface defect density of stateswere extracted. The density of surface states onundoped a-Si:H and on boron-doped a-Si,C:H was I1011 cm-2 and 2 ' 1011 cm-2, respectively. The low state

density is due to hydrogen tennination of surface dan-gling bonds which occurs during the film deposition.The measured contact potential profile is closely relatedto the internal potential of the complete device becauseof the low surface state density. Critical device prop-erties, such as p+ layer thickness and built-in poten-tial, and the field strength across the absorber undopeda-Si:H laye~, can be optimized during the growth.

tl l...' , ,. ,..'

0 DXI 4IXXI ~

Thick.-. (A)

'0)

Figure 6 Cont.'lct potential profile of (a) un-doped a-Si: H film deposited on Pd coatedglass substrate. and (b) the boron-doped a-Si.C:H film deposited on Cr coated gla.o;s sub-strate. The solid line represents the internalpotential for the complete film thickness.

inside tIle sample beneath the surface. Figure 6b showsthe potential profile for the boron-doped a-Si,C:H filmdeposited on Cr coated glass substrate. These dataare well fit \vith Nd = I ' 1018 cm-3 eV-I, and 2 ' 1011

cm-2. The density of surface states on undoped a-Si:Hand boron-doped a-Si,C:H surfaces is small, comparedto that on cleaved c-Si surface (- 1014 cm-2) [5). Forcomparison. Siefert et aI. found N, = 1.7 ' 1011 cm-2 for

a-Si:H deposited from silane glow discharge [2).Winer et al.. using in-.\"itu total-yield spectroscopy,measured the density of surface states on 0.5 ~m thickundoped a-Si:H and boron-doped a-Si:H (no carbon)surface to be 2 ' 1011 cm-2 and < 1011 cm-2, respectively

[6J. Jackson et aL using combination of ex-situ electronspin resonance (ESR) and photodeflection spectroscopictechniques. deduced the density of bulk and surfacestates of undoped a-Si:H to be 5 ' 1016 cm-J. and 3 ' 1012cm-2. and boron doped a-Si:H (no carbon) to be 2 ' 1017cm-3. and 5 ' 1010 cm-2. respectively [7]. Note that these

films were exposed to air. The important results are: first,the low N, is due to hydrogen termination of surfacedangling bonds. which occurs during the deposition ofa-Si:H. At this density. the surface states do not pin theFermi level [8]. Second. the measured contact potentialprofile is fit exactly by the calculated contact potentialprofile. because of the low density of surface states.This implies tI\at the space charge density and the built-in electric field distribution can be determined from contactpotential profiles. And tIle total contact potential changedetermines the built-in potential of any junction in a-Si:H.

REFERENCE

[1]. J. ABELSON, D G. DEROSNY, J. de Physique,

44(1983).993.[2]. J. M. SIEFERT, G. DE ROSNY, Phil. Mag.

Letter, BS4 (1986) L57.[3]. M. PINARBASl, N. MALEY, A. MYERS, J. R.

ABELS"ON, Thin ,Solid Films, 171 (1989) 217.[4]. J. K. ARCH, S. J. FONASH, J. Appl. Phys., 68

(1990) 591.[5]. D. E. EASTMAN, W. D. GROBMAN, Phys. Rev.

Lett., 21(1972) 572..K. WINER, L. LEY, in Amorphous Silicon and

Related Materials, edited by H: Fritzsche (WorldScientific Pub., Singapore, 1989), p. 365.

[7]. W. B. JACKSON, D. K. BIEGELSEN, R. J.NEMANICH, J. C. KNIGHT, Appl. Phys. Lett.42 (1983) 105.

(8]. L. LEY, in .Semiconductor and ,Semimetals, ed-ited by R. K. Willardson and A. C. Beer (Aca-demic Press, Orlando, 1984), Vol. 21 B, p. 385.

[6)

A. Nuruddin dun .T.R. Ahel.\'on 365