Diamond and Related Materials 11(2002) 1329–1331

0925-9635/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0925-9635Ž01.00638-0

Optical, anti-reflective and protective properties of diamond and diamond-like carbon films

T.V. Semikina*, A.N. Shmyryeva

National Technical University of Ukraine ‘KPI’, Pr. Poheda 37, Faculty of Electronics, Microelectronics Dept., Corpus 12, 252056 Kiev,Ukraine

Abstract

For the first time the degradation results of solar cells parameters with diamond-like carbon films after high-energy proton andelectron flux exposure like space condition are represented. It is obtained that DLC films prepared by the CDD method at biasvoltagey300 V and nitrogen concentration 10–15% in working gas provide better blooming and protective cover for solar cellsunder space condition.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Diamond-like carbon; Nitrogen; Optoelectronic properties; Application

1. Introduction

The aim of presenting this work was that to choosefrom two deposition methods, those films which depositon solar cells which will allow to decrease characteristicdegradation considerably under high energy electron andproton flux action. For execution of this problem weinvestigated the optical properties of diamond films(DF) and diamond-like carbon films(DLC), and deter-mine technology parameters for directional regulation ofthese films’ optical properties under a change of tech-nology regimes.

2. Experimental

The diamond films prepared by laser sputteringw1xof a carbon target have a polycrystalline structure andare 200 nm thick. This gave rise to high- and low-resistivity films. The spectrum of absorption and trans-mission of these films were investigated. These filmshave low absorption coefficients(Fig. 1). The reflectioncoefficient of high-resistivity films is higher than forlow-resistivity films. From the coefficient absorptiondependence on energy(Taut plot, Fig. 2), we calculatedthe optical band gap(E ) of DF that is 1.2–1.5 eV foropt

low-resistivity and 2.2–2.5 eV for high-resistivity films.The result is thatE s2.5 eV serves as a reference foropt

*Corresponding author.E-mail address: semikina@ee.ntu-kpi.kiev.ua(T.V. Semikina).

diamond films with an sp stationary concentration of3

approximately 85%. The refraction coefficient was 2.2.We investigated the diamond-like carbon films(DLC)

deposited by the chemical vapour deposition method atbias voltage ofy100 V (DLC ) w2,3x and y300 Vsoft

(DLC ) w4–7x. The prepared films have an amorphoushard

structure and are 300–500 nm thick. The second tech-nology factor of change DLC properties was the altera-tion of nitrogen concentration in the working gas. It wasobtained that for DLC theE and refraction coeffi-soft opt

cient n change from 3.2 to 1.9 eV and from 1.6 to 2.2accordingly under nitrogen content alteration from 2–5to 15–20% in working gas.

3. Results

Under a subsequent nitrogen concentration rise to45%, E changes from 1.9 to 3.8–4.0 eV andn fromopt

1.6 to 2.1. For DLC filmsE does not exceed 2.0hard opt

eV, ns2.1. At a nitrogen input of approximately 10–15% relative to the working gas(CH , H and N) it4 2 2

was possible to achieve films with a demanding refrac-tion coefficient nsl.9. Therefore, we can regulariseoptical film properties via alteration of nitrogen concen-tration in working gas.

The DF and DLC films were deposited on industrialsilicon solar cells for investigation of reflection andprotective properties of these films under Earth(AM1.5,power radiation 1000 Wym , Ts20 8C — condition for2

1330 T.V. Semikina, A.N. Shmyryeva / Diamond and Related Materials 11 (2002) 1329–1331

Fig. 1. Absorption coefficient of DF with different resistivity depend-ence on wave number. Fig. 3. Short-circuit current dependence on wavelength of solar cell

without cover(a), with DF low-resistivity(b) and with DF high resis-tivity (c).

Fig. 2. Taut plot of DF with different resistivities.Fig. 4. Current density dependence on wavelength of solar cell with-out cover(2) and with DLC (1).hard

calculated efficiency) and space(AM0, power radia-tions1360 Wym , Ts40 8C) condition. The reflection2

coefficient of solar cells with DF was decreased morethan 3.5 times in the range 460–600 nm(Fig. 3) incomparison with solar cells without cover. Due to theblooming effect the efficiency of the solar cell wasincreased by 0.8%. Solar cells with DLC under Earthsoft

condition of operation demonstrated such good results:U s627 mV; I s39.4 mAycm ; FFs0.79; and effi-2

oc sc

ciency hs19.51%. The DLC deposition gave ansoft

increase in short-circuit current density of 42% andopen-circuit voltage of 12%. For solar cells withDLC the reflection was decreased to 0.1%, andhard

efficiency increased by 1.3–1.4 times due toI increas-sc

ing by 1.4–1.5 times(Fig. 4). Solar cells with aDLC cover had such parameters under Earth condi-hard

tion operation:U s625 mV; I s40 mAycm ; FFs2oc sc

0.792; andhs19.8%(space condition:hs15.8%).For the first time, the experiments were carried out at

high energy electron flux with 10-MeV energy exposure(equipment: microtron M30); the combined action oflow energy electrons and protons,Es170 keV withdifferent equivalent state of orbit space(670, 8000 and

36 000 km); proton action atEs20 MeV. These exper-iments were conducted in the nuclear equipment. Weobtained a short-circuit current decrease under electronaction(Fig. 5). The results demonstrated that solar cellswith DLC cover had the least degradation in com-hard

parison with ZnS, DF and DLC covers, and DLCsoft soft

cover had the worst results. It can be explained thatstructure of DLC film is more like a polymeric-likesoft

compound than diamond-like. DLC film does notsoft

have hardness and adhesion that is proper for DLC .hard

The short-circuit current had the biggest relative altera-tion: with DLC on 31–32%; with ZnS on 37–40%;hard

DF on 38–47%; and with DLC on 52–54%. Thesoft

open-circuit voltage relative change was in the range 8–11%, and full factor change was 14–16% for all typecovers. At high-energy electron flux exposure, the maindegradation factors were lifetime minority carriersdecreasing in base and cover transmission deterioration.

1331T.V. Semikina, A.N. Shmyryeva / Diamond and Related Materials 11 (2002) 1329–1331

Fig. 5. Load current–voltage characteristics(a) solar cell and depend-ence power of solar cells on voltage(b) under electron action withenergy of 1 MeV at different electron fluxes: 1s10 ; 2s10 ; 3s15 14

10 ; and 4s10 cm .13 12 y2

At exposure proton flux increasing more than 1013

cm , the efficiency of solar cells dropped sharply byy2

80–85% on average. The high energy electron andproton action led to large structural change and higherdefect formation and consequently, the density of recom-bination centres increased followed by a life time dropin the solar cell with DLC in comparison withsoft

amorphous DLC films that have lower structuralhard

disordering.

4. Conclusion

So, from our results we propose to use the DLChard

films prepared by the chemical vapour deposition meth-od at bias voltagey300 V and nitrogen concentration15% in working gas, for blooming and as a protectivecover for the solar cell, which allows to decrease thedegradation change by 10–15 times in comparison withthe solar cell without cover(only with quartz glass)under space condition operation.

References

w1x E.I. Tochitsky, O.G. Sviridovich, N.M. Beliavsky, Proceedingsof the Fourth International Symposium on Diamond Films andRelated Materials ISDF4, Kharkov, Ukraine, September 20–22, 1999, pp. 150–153.

w2x V.A. Semenovich, N.I. Klyui, J. Chem. Vapor Deposition 4(1995) 29–37.

w3x T.V. Semikina, A.N. Shmyrjeva, V.A. Semenovich, J. Chem.Vapor Deposition 6(2) (1997) 98–103.

w4x V.A. Semenovich, N.I. Klyui, J. Chem. Vapor Deposition 4(1995) 29–37.

w5x A.G. Gontar, A.A. Doroshenko, A.M. Kutsay, S.I. Khandozhko,J. Chem. Vapor Deposition 4(1) (1995) 15–21.

w6x L.S. Aivazova, N.V. Novikov, S.I. Khandozhko, A.G. Gontar,A.M. Kutsay, J. Chem. Vapor Deposition 6(1) (1997) 52–56.

w7x N.V. Novikov, S.I. Khandozhko, V.M. Perevertailo, L.Yu.Ostrovskaya, A.G. Gontar, O.B. Loginova, J. Diamond Rel.Mater. 7(9) (1998) 1263.