Optical recording characteristics of implanted tellurium films

Thin Solid Films, 182 (1989) 47 52

47

ELECTRONICS AND OPTICS

OPTICAL R E C O R D I N G C H A R A C T E R I S T I C S OF I M P L A N T E D T E L L U R I U M FILMS JACQUES BEAUVAIS, PIERRE GALARNEAU* AND ROGER A. LESSARD Centre d'Optique Photonique et Laser ( COPL ), Universitb Laval, Qubbec G I K 7P4 (Canada) AMARJIT SINGH* AND EMILE J. KNYSTAUTAS Centre de Recherches sur les Atomes et les Molbcules ( C R A M ) , Dbpartement de Physique, Unit,ersit~ Laval, Qukbec G I K 7P4 (Canada) (Received August 8, 1988: revised December 6, 1988; accepted July 10, 1989)

Tellurium films were vapour deposited on glass substrates held at room temperature and subsequently implanted with different ions. Optical recording parameters such as threshold writing energy and contrast ratio were evaluated. The experimental results indicate that the threshold writing energy is dependent on the type of implanted ion.

1. INTRODUCTION

Optical data storage technology has developed rapidly in the last few years and has encouraged new research efforts into different materials that can be used as recording media. Ablative techniques in semiconductor thin films are one of the major focuses of this research ~. Recently it has been reported that hydrogenated tellurium films can be obtained by sputtering tellurium in a hydrogen-argon atmosphere 2. These films were found to have a higher chemical resistance and a lower writing threshold energy. A similar effect has also been observed in tellurium films implanted with varying concentrations of hydrogen 3. In this paper, a short summary of these results will be presented followed by a comparative study on the effect of implanting different species (He +, Ar ÷ and N + ) on the optical recording parameters of tellurium films. 2. HYDROGEN IMPLANTATION Ion implantation in semiconductors can have large effects on many properties of these materials, and in particular modifications of optical properties seem very interesting'*. Previous results of a study of the effect of hydrogen implantation in tellurium thin films on the optical recording properties of these films (presented in ref. 3 and summarized in Fig. 1) have shown that a reduction in the threshold writing energy WT follows an increase in the implanted ion dose. The threshold was seen to decrease by a factor of 2.4 when the implanted dose was increased to 101 ~ ions c m - 2 and then to remain unchanged for higher doses. The maximum value of * Present address: lnstitut National d'Optique, 369 rue Franquet, Ste-Foy, Qu+bec, Canada. 0(00-6090/89/$3.50

© Elsevier Sequoia/Printed in The Netherlands

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J. BEAUVAIS el a/.

the contrast ratio CR {defined below) and its slope measured with respect to the incident beam writing energy are seen to decrease as the number of implants is increased. These results indicate a strong relationship between the number of implants present in the tellurium thin films and the optical recording properties of these films. +

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I

I

I

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0.2

0.4

0.6

0.8

1.0

INCIDENT ENERGY DENSITY (Te THRESHOLD)

Fig. I. Plot ofconlrast ratio vs. incident energydensity for H ~-implanted tellurium thin films with doses of6xl01`*(x),8.6xl01'*l+t, lxl01S(O)and6xl0]Si[])H ~ cm 2. These results suggest that the dominant mechanism affecting the recording properties of the films is the presence of the impurities in the films, since the implantation energy of the hydrogen ions was low (10kV) and the mass of these impurities is very small. In order to confirm this hypothesis, three other species of ions, He +, N + and Ar +, were implanted in tellurium films and the measurements of the optical recording parameters of these films are presented here. Two of these ions (He + and Ar + ) are rare gases and should interact with the tellurium lattice very differently from the third ion tN~t because of the differences in their electronic configurations. However, these results cannot be directly compared with those of the H+implanted films since these were obtained in a different region of the phase transformation kinetics diagram of the tellurium filmsS; the writing beam used for the hydrogen experiments was provided by a Q-switched Nd +:YAG laser with a 15 ns wide pulse. 3. EXPERIMENT Tellurium films 75nm in thickness were prepared by evaporating pure tellurium from a tantalum heater onto room temperature glass substrates and were subsequently implanted with different ions. All the films were evaporated simultaneously since it has been shown that the conditions under which the films are

OPTICAL RECORDING CHARACTERISTICS OF IMPLANTED

Te

49

deposited, such as substrate temperature and pressure, can have a strong infuence on the film properties 6. The details of ion implantation are described elsewhere7. The implanted profile C(x) in the thin film is given by 8

C(x) = (N o / x / ~ fi) exp { - ( x - Rp)2/262 }

(1)

where N O is the implanted dose, Rp is the projected range and 6 is the projected standard deviation (also called the straggle). The number N s of implants per unit of area in the thin film is then given by

Ns = .[' C(x) dx

(2)

where Tis the thickness of the film. N o was chosen in order to obtain an equal value of N s in all the thin films (N s ~ 1015 i o n s c m - 2 ) . The accelerating voltage was selected such that the projected range was equal to one-half of the thickness of the film 9. In the case of helium, the voltage required to achieve this condition was too low to produce a stable ion beam. Therefore a more energetic beam was used, requiring a greater value of No to keep N~ constant. A schematic diagram of the experimental set-up for optical recording is shown in Fig. 2. The recording was performed with a 40 ns pulse train (2 = 532 nm) using a mode-locked N d : Y A G laser equipped with a frequency doubler. The reading was performed with a continuous wave HeNe laser beam ()~ = 632.8 nm) coilinear with the writing beam. The energy of the writing beam was measured with a S C I E N C E T E K joulemeter (model MJA-10) and the reflectivity changes were monitored using an RCA 30807 photodiode. ~.

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Fig. 2. Digital optical data storage experimental set-up.

4. RESULTS In the following discussion, the contrast ratio CR is given by R b- R a

CR -- - R b + R~

(3)

J. BEAUVAIS e l

50

al.

where R b and Ra represent reflectivity signals before and after the exposure to the writing beam, The threshold writing energy Wv is defined as the energy for which a contrast ratio of 10% is obtained. To stress the effect of implantation, the CR dependence on incident energy density is presented in units normalized with respect to the pure tellurium writing threshold, since the writing threshold depends strongly on laser characteristics. The optical recording characteristic behavior of an as-deposited tellurium film is shown in Fig. 3. Figure 4 illustrates the same curves near WT for tellurium thin films implanted with the ion species helium, nitrogen and argon, in addition to the pure tellurium film data presented in this figure as a reference. The lines drawn through the data are intended merely to guide the eye. Three observations should be noted concerning Fig. 4. 45 40 35-

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INCIDENT ENERGY DENSITY ('re THRESHOLD) Fig. 3. Plot of c o n t r a s t r a t i o v,s. incident e n e r g y d e n s i t y for a p u r e t e l l u r i u m film.

(1) The observed thresholds WT of all implanted films are lower than for the pure tellurium film, with values of 0.16 for helium, 0.20 for nitrogen and 0.53 for argon implants. The ion species with smallest mass thus has the lowest measured value of

wT. (2) For all the implanted films, the slopes ACR/AE~n c (where E~,c is the incident writing beam energy) are higher than for the pure tellurium film. The film with helium impurities has the sharpest rise in CR and the most massive implant (Ar + ) has the smallest observed slope of the implanted films. (3) Although the writing properties of the implanted films were studied only near the threshold writing energy, it is still clear that the maximum values of CR for both the He +- and the Ar +-implanted films are much higher than that observed for the pure tellurium film. The N +-implanted film exhibits a decrease in the maximum value of CR (Table I).

O P T I C A L R E C O R D I N G C H A R A C T E R I S T I C S OF I M P L A N T E D

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I I I I I I 0.2 0.4 0.6 0.8 1.0 1.2 INCIDENT ENERGY DENSITY ('re THRESHOLD) Fig. 4. Plot of c o n t r a s t r a t i o vs. i n c i d e n t e n e r g y d e n s i t y for a p u r e t e l l u r i u m film a n d t h r e e different ion i m p l a n t a t i o n s in t e l l u r i u m films, for d o s e s of 1015 ions c m - 2: + , H e + ;/x, N + ; O , A r + ; IS], p u r e tellurium. TABLE i SUMMARY OF RESULTS FOR IMPLANTED DOSES OF 1015 IONS CM - 2 OF H +, H e +, N + AND A r + IMPLANTED IN TELLURIUM THIN FILMS

1replant

Energy

Atomic' number

Threshold

CR(max)

-H+ He + N + Ar +

-10 30 30 85

-1 2 7 18

I 0.42 0.16 0.20 0.53

40% 22% 52% 33~,,, 70~

kV kV kV kV

All m e a s u r e m e n t s w e r e p e r f o r m e d with a m o d e - l o c k e d N d ÷ : Y A G laser except for the H ÷ - i m p l a n t e d films, w h e r e a 15 ns Q - s w i t c h e d N d + : Y A G laser w a s used.

5. D I S C U S S I O N

The experimental results indicate that, by implanting He +, Ar + and N + ions, one can greatly reduce the value of WT. Since the writing beam used in these experiments was mode locked, all measurements were performed in the ablative region of the phase transformation kinetics diagram. Thus the recording mechanism was based on pit formation. A comparison of the results obtained for the He +- and N+-implanted films shows that an interpretation of the measurements based on the single mechanism of the presence of the impurities in the films is insufficient to explain the behavior of

52

J. BEAUVAIS el a[.

their recording properties. The nitrogen ions have chemical properties which are very different from those of the helium ions, yet their values of I/Vv and ACR/'AEm~are quite similar. This contrasts sharply with a comparison of the He ~- and Ar +implanted film results which show that although their interaction with the tellurium lattice would be expected to be similar, the optical writing parameters arc very different with an increase in WT by a factor of 3 for the Ar + case. An explanation of these results based only on implantation damage is also incomplete. More massive ions are expected to induce larger modifications of the recording parameters as a result of greater damage production, but this is in contradiction with a comparison of the results for N +- and Ar +-implanted films. Furthermore, for the maximum observed values of CR, the rare gas ion implanted films both exhibit a similar increase in this parameter while for the case of the nitrogen-implanted film, in a behavior similar to the hydrogenated films :~, its value is seen to decrease. A more complete description of the effects of ionic implantation on the optical recording properties of thin tellurium films should thus include both a defect formation mechanism induced by the implantation process in addition to a mechanism based on the interaction of the impurities with the tellurium lattice in the films. 6. CONCLUSI()N

In conclusion, we have shown that ion implantation in tellurium films greatly reduces the writing threshold energy at )~ = 532 nm. Both defect formation due to the implantation process and the interaction of the impurities with the tellurium ions in the films probably contribute to the changes in optical recording characteristics of these films and further work is planned to establish their relative contributions. REFERENCES I 2

M.A. Bosch, ,Ippl. Phy.~. l,elr., 40 ( 19821 ,~, and references cited therein. L.M. Schiavonc. M. A. Bosch, H. R. Hallt.'r, K. E. Hubbard, E. Good and J. L. Shay, A Technical

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L. Song, P. GalarneauandR. A. Lessard, Opt. Eng.,28(1989)290.

L. Song, P. Galarneau, M. Cote and R. A. Lessard, Appl. Opt., in the press. 7 A. Singh, in A. Niki-kari (ed.). Adram'es m Sur/~,'e Treatmentx, Vol. 3, Pergamon. Oxford, 1986, p. 155. 8 H. Ryssel, in H. Ryssel and H. Glawischnig (eds.), Ion Implantation Techniques, Springer Series in Electrophysics, Vol. 10, Springer, Berlin, 1982, p. 177. 9 A . F . Burenkov, F. F. Komarov, M. A. Kumakhov and M. M. Temkin, Table q/Ion Implantation Spatial Distributions, Gordon and Breach. London, 1986.