Photodarkening in amorphous GeSnSe4Cu0.5 films

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Pergamon

Vacuum/volume 51/number 3/pages 453 to 455/1998 © 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042–207X/98 $19.00+.00

PII: S0042-207X(98)00195-X

Photodarkening in amorphous GeSnSe4Cu0.5 films K. Sedeek, Physics Department, Faculty of Science (Girls), Al-Azhar University, Cairo, Egypt received 18 February 1998

Amorphous GeSnSe4Cu0.5 films were studied to follow the photodarkening property. Illuminating the films with a He/Ne laser, shifts the absorption edge towards the red, creates an absorption tail at lower energies and decreases the refractive index. Subsequent annealing suppresses this photodarkening property. Exposure to laser radiation seems to introduce some structural disorder at the band edges. This disorder relaxes away on annealing. Characterization of the bulk material used to deposit the films is reported. © 1998 Elsevier Science Ltd. All rights reserved.

0[ Introduction Some Ge based chalcogenide glasses such as GeSe1\ Ge0!xSnxSe1 display an interesting property known as photodarkening[ When exposed to light of photon energy near or above the optical gap\ the absorption edge shifts towards lower energies\ thereby appearing darker in the visible range[ This shift is suppressed by thermal annealing below the glass transition temperature ð0\ 1Ł[ Recent study of the e}ect of laser exposure on the electrical properties of Ge0!xSnxSe5 glasses shows that it depends on the Sn fraction ð2Ł[ A study of the e}ect of Cu on the photodarkening property of glassy "Ge9[84Sn9[94Se1#0!xCux _lms gave evidence that this property was not observed for x × 9[0 ð3Ł[ The present work was undertaken to gain more understanding of the mechanism of photodarkening through the study of the changes occurred in the absorption coe.cient\ the refractive index and the optical gap width as the amorphous GeSnSe3Cu9[4 _lms undergo illumination and subsequent annealing[ Charac! terization of the as quenched bulk material used for _lm depo! sition is also presented[

6948 Corning glass substrates held at room temperature at a rate of 29 A:sec[ The _lm thickness was calculated from the re~ectivity minima knowing the order of the interference fringes ð5Ł[ A thick! ness of 0[82 mm was calculated for the studied _lms[ The d[c[ conductivity of the _lms was measured in a specially designed cryostat ð4Ł under a vacuum of 09−2 torr using a 505 Keithley electrometer[ For optical experiments\ the re~ectivity "R# and the transmittance "T# were measured three times] _rstly for the as deposited _lm\ secondly after illuminating the _lm with a He:Ne laser "9[84 mW#\ and thirdly after annealing the _lm at 049>c for 1 h[ 2[ Results and discussion 2[0[ The as!quenched material[ "Figure 0Ðb# reproduces the x!ray powder di}raction pattern of the bulk material[ It is abundantly

1[ Experimental techniques The bulk sample was prepared by the well!known melt quen! ching!technique ð4Ł[ Pure elemental components "88[888)# in a cleaned quartz tube evacuated to 09−4 torr were used[ The tube was heated to 0949>c in a vertical position for 03 h and was rocked intermittently to insure homogenization[ The tube was then dropped vertically into iced water[ The chemical com! position was determined using the EDX "energy dispersive x! ray spectroscopy# technique on a Joel 5399 scanning electron microscope[ An x!ray di}ractometer equipped by a nickel!_ltered K copper radiation was used to determine the structure[ The transition temperatures were obtained using a di}erential scan! ning calorimeter "DSC#model Shimadzu 29[ The _lms studied were prepared by electron beam deposition from the synthesized material[ The _lms were deposited onto

Figure 0[ The x!ray di}raction patterns of "a# the deposited _lm and "b# the as quenched material[ 453

K. Sedeek: Photodarkening in amorphous GeSnSe4Cu0.5 films Table 0[ The X!ray di}raction data of the most important di}raction lines of the as quenched GeSnSe3Cu9[4 material Peak

1u>

dexp "A ý#

dcal "A ý#

I:I9

0 1 2 3

17[02 36[74 03[77 06[66

2[06 0[89 4[84 3[88

2[06 0[89 4[84 3[88

099 25 21 20

clear that the prepared ingot has polycrystalline structure[ Data of dexp "the experimental lattice spacing#\ dcal "the calculated lattice spacing# and I:I9 "the relative intensity# are given in Table 0 for the most important di}raction lines[ The DSC results given in Figure 1 shows no T` "the glass transition temperature# endothermic peak as the investigated material was already polycrystalline[ Two exothermic crys! tallization peaks with Tc0  403>C and Tc1  417>C are also dis! played in Figure 1[ The presence of these two values re~ects the existence of two phases in the synthesized material[ The measured relative hydrostatic density of the bulk sample equals 2[4[ 2[1[ The deposited _lms[ 2[1[0[ Electrical properties[ The XRD pattern given in "Figure 0Ða# re~ects the amorphous structure of the studied _lms as no de_nite di}raction lines appear[ The variation of the d[c[ conductivity "s# with reciprocal tem! perature "0:T# for the GeSnSe3Cu9[4 _lm is displayed in Figure 2[ The plot of log s "0:T# gives only one straight portion obeying the exponential relation ð6Ł] s  C exp −DE:kT

"0#

where C is the pre!exponential factor\ DE is the thermal activation energy and k is the Boltzmann constant[ The calculated values of sRT "the conductivity measured at room temperature#\ DE and C are respectively 2[2×09−6"Vcm#−0\ 9[5 eV and 4[6×092 "Vcm#−0[ The values of DE and C suggest thermally activated

Figure 2[ The variation of the d[c[ conductivity with reciprocal tem! perature for the deposited _lm[

conduction at extended states beyond the mobility edge over the studied temperature range "24Ð024>c#[ 2[1[1[ Optical properties and photodarkening[ The absorption coe.cient a"v# of the amorphous GeSnSe3Cu9[4 _lm was cal! culated according to the relation] a"v#  0:d ln ð"0−R#1:TŁ

where d is the _lm thickness[ Figure 3 shows the variation of a"v# with photon energies :v for the _lm] as deposited\ illuminated with a He:Ne laser and when subsequently annealed at 049>c for 1 h[ It is clear from the plots that illumination shifts the absorp! tion edge as a whole to lower energies and increases the optical absorption[ The e}ect of laser exposure on the refractive index "n# was also investigated[ Figure 4 displays the variation of "n# with :v for the studied _lm[ Apart from the general trend of "n# as nearly stable with :v\ the most remarkable feature here is the decrease of "n# with illumination and its subsequent increase on annealing[ The optical gap E` was calculated following the relation ð7Ł]

a:v  b":v−E`#s !0

Figure 1[ The di}erential scanning calorimeter pattern of the as quenched material[ 454

1

"2#

where b is the edge width parameter and s is an integer usually equal to 0\ 1 or 2 in non!direct amorphous sem! iconductors[ The plots of a:v vs[ photon energy of the studied composition are given in Figure 5[ The most inter! esting result of this study is the shift of the absorption edge to the red "E` decreases from 0[20 to 0[04 eV# when exposed to the laser radiation and its subsequent shift to the blue on annealing "E` increases from 0[04 to 0[20 eV#[ It is to be also noted that the absorption tail induced by illumi! nation is suppressed by annealing[ Recent studies on the e}ect of Cu on the photodarkening

K. Sedeek: Photodarkening in amorphous GeSnSe4Cu0.5 films

Figure 3[ The variation of the absorption coe.cient vs[ photon energies for the studied _lm "a# as deposited\ "b# illuminated and "c# annealed at 049>C for 1 h[

Figure 5[ The variation of "a:v#0:1 vs[ photon energies for the studied _lm "a# as deposited\ "b# illuminated and "c# annealed at 049>C for 1 h[

of the structural evolution during the annealing of Ge0!xSnxSe1 ð8Ł showed that the optical gap width increased on annealing[ Moreover\ the fraction of homopolar bonded Sn which broaden the valence and conduction band edges decreases on annealing ð09Ł[ This would be consistent with an increase of the gap width as predicted in our experiments[ The photodarkening property has many possible applications in optical memory devices so more experiments are required to follow the structure disorder induced by laser[ Conclusions

Figure 4[ The variation of the refractive index vs[ photon energies for the studied _lm "a# as deposited\ "b# illuminated and "c# annealed at 049>C for 1 h[

We have performed a study of photodarkening on electron beam deposited GeSnSe3Cu9[4 amorphous _lms[ Illumi! nation with a He:Ne laser creates an absorption tail at lower energies and decreases the refractive index[ The value of the optical gap also decreases from 0[20 to 0[04 eV[ Subsequent annealing of the illuminated _lm at 049>c for 1 h suppresses this photodarkening e}ect\ eliminates the excess tail and increases the refractive index[ References

properties of glassy "Ge9[84Sn9[94Se1#0!xCux ð3Ł showed that the photodarkening was not a consequence of a masking e}ect due to a reduction in the optical gap\ rather it is related to a change in the structure of the glass network[ In our case we see that laser may cause some energy dis! turbance resulting in chemical and mechanical disorder that a}ects the short range order of the studied GeSnSe3Cu9[4 amorphous _lm[ The disorder level is located at the band edges and relaxes away on annealing[ Studies

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