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Rare earth dopants as probes of localized states in chalcogenide glasses
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Journal of Non-Crystalline Solids 223 (1998) 105-113
Rare earth dopants as probes of localized states in chalcogenide glasses D.A. Turnbull a,b,c,,, S.G. Bishop a,b,c a Center for Compound Semiconductor Microelectronics, Microelectronics Laboratory, University oflllinois, 208 N. Wright Street, Urbana, IL 61801, USA b Center for Optoelectronic Science and Technology, University of Illinois, Urbana, IL 61801, USA c Department of Electrical and Computer Engineering, University oflllinois, Urbana, IL 61801, USA Received 9 April 1997; revised 4 August 1997
Abstract The recently observed phenomenon of broad band excitation of rare earth dopants in chalcogenide glasses indicates an interaction between the rare earth ions and localized background defect states in the glass. Low temperature (5 K) photoluminescence and photoluminescence excitation spectroscopy of Er2S3-doped As12Ge33Se55 samples show similarities in excitation lineshape, Stokes shift, and spectral position to the intrinsic luminescence of an undoped As 12Ge33Se55 sample. Fatiguing of the broad band excitation has been observed in Er-doped As12Ge33Se55 and As2S 3 samples. These results indicate that the broad excitation band involves interaction with the same native defects in the host glass which give rise to important optical, electrical, and magnetic resonance properties of the chalcogenides. A model for the energy transfer process from the host glass to the rare earths, mediated by intrinsic defect states in the glass is presented. Some features of existing models for the behavior of chalcogenide glasses will be evaluated in light of our results. © 1998 Elsevier Science B.V.
1. Introduction Recently, chalcogenide glasses have become the subject of study as potential host materials for rare earth-doped optical fiber amplifiers in the 1.3 ~ m spectral range [1-4]. These glasses, based on sulfur, selenium and tellurium, offer the prospect of smaller non-radiative decay rates and larger emission crosssections for rare earth dopants. In contrast to the wide band gap oxide glasses, which are the usual host materials for Er-doped fiber amplifiers operating at 1.55 Ixm, the chalcogenide glasses have band
* Corresponding author. Tel.: + 1-217 244 6378; fax: + 1-217 244 6375; e-mail: [email protected]. 0022-3093//$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 7 ) 0 0 3 4 6 - 3
gaps in the visible or near infrared and are not transparent in the visible and near ultraviolet spectral ranges. This narrow bandgap means that the intrinsic optical transitions of the glass, including the interband absorption, the band edge Urbach tail, and the below-gap weak absorption tail (WAT) [5,6], will overlap some of the inter F-band transitions of the rare earth dopants. This overlap raises the possibility of interactions between the optical absorption and emission processes of the rare earth dopants and the chalcogenide host glass. We initiated a study of photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy in rare earth doped chalcogenide glasses to investigate the interactions between the host glasses
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D.A. Turnbull, S. G. Bishop~Journal of Non-Crystalline Solids 223 (1998) 105-113
and the rare earth dopants and the possible influence of such interactions upon the suitability of these materials for optical amplifiers. Our initial studies [7-10] detected a, broad, near-band edge PLE band in the excitation spectra of rare earth emission bands in chalcogenide glasses. This excitation mechanism has proven to be a general feature of rare earth dopants in a variety of chalcogenide glass hosts with potentially important technological implications. Fig. 1 shows both the PL and PLE spectra taken from a 0.1 wt% Er-doped ASlzGe33Se55 sample [8]. The PL spectrum shows the usual 1540 nm Er emission line [11]. The PLE spectrum, detecting at 1540 nm, also shows the expected inter-F-band absorptions of the Er atoms at 980 and 810 nm [11]. However, these relatively sharp Er bands are superimposed on an excitation band with a larger amplitude and width, stretching from ~ 500 to 1000 nm. Similar broad PLE bands were observed in Pr- and Dy-doped [9,10] As]2Ge33Se55 glasses, as well as in Er-doped As2S 3 [7]. This broad PLE band is due to the absorption of light by the host glass and the subsequent transfer of this energy to the rare earth dopants. In this paper we present new low temperature measurements of the PL and PLE in rare earth-doped
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Wavelength (nm) Fig. 1. PL and PLE spectra taken at 300 K from a 0.1 wt% Er-As 12Ge33Se55 glass. The PL spectrum,on the right, shows the expected 1540 nm emission line. The PLE spectra on the left, detecting at 1540 nm, shows the normal Er 810 nm and 980 nm absorption bands. These bands, however, are superimposedon a strong, broad excitation band, due to absorptionby the host glass and subsequent energy transfer to the Er dopants.
chalcogenide glasses which demonstrate similarities between the PL excitation mechanisms of the rare earth-doped glasses and those of the undoped-host glass. These results indicate that study of the interaction between the host glasses and the rare earth dopants can provide information about the intrinsic properties of the host glass and new insights conceming the phenomenological models which have been proposed [12,13] to explain the fundamental optical, electrical, and magnetic resonance properties of localized electronic states in chalcogenide glasses.
2. Background The experimental results and their interpretation presented here must be discussed within the context of a broad range of previous experimental and theoretical work on chalcogenide glasses. Of particular importance are the unique optical absorption and emission properties of the chalcogenide glasses which are intimately involved in the broad band excitation mechanism of the rare earth dopant emissions. For a more complete overview of the properties of the chalcogenides than is possible to present here, see the excellent review article by Street [14]. The absorption edge in chalcogenide glasses can be divided into three distinct regions [15]. At the largest energies, the absorption coefficient shows the standard, quadratic dependence on energy of bandto-band absorption. At slightly smaller energies, the band edge absorption is broadened into an exponential absorption tail, usually referred to as the Urbach edge or Urbach tall. A similar Urbach edge has been observed in crystalline semiconductors [16], in which it is attributed to electric field broadening of the band-to-band or exciton absorption. However, the Urbach absorption edge is broadened much more substantially in the chalcogenides than it is in crystalline semiconductors. In order to account for the Urbach edge in the chalcogenides with the same explanations as are used for crystalline materials, extremely high electric field strengths are required [17]. At still smaller energies, extending well into the bandgap, there is a shallower exponential absorption tail, commonly called the weak absorption tail (WAT). The strength of the WAT is linked to the presence of impurities in the sample [6,18], but the exact origin of this absorption is not known.
D.A. TurnbulL S.G. Bishop~Journal of Non-Crystalline Solids 223 (1998) 105-113
At low temperature (below ~ 100 K), chalcogenide glasses exhibit a broad luminescence band centered approximately in the middle of the bandgap [14]. The PLE spectrum of this mid-gap PL follows the absorption spectrum of the glass until it reaches a maximum about halfway up the Urbach edge, and then decreases at larger energy. Any model of excitation and recombination in these glasses must account for the large Stokes shift between the peak energies of the PLE and PL bands. The host glass PL band also fatigues under exciting light [14], with the most rapid fatiguing occurring for exciting light at the peak wavelength of the PLE spectrum. The fatiguing is accompanied by the induction of an electron paramagnetic resonance (EPR) signal and an absorption band stretching from the band edge to approximately half the bandgap energy [19]. Most pure chalcogenide glasses show no EPR signals when unexposed to light [15]. The lack of any EPR signal in this case, combined with the pinning of the Fermi level, was one of the major questions confronting theorists in the 1970s [15]. Anderson [20] solved this problem with the suggestion that strong lattice coupling at impurity sites could produce an effective negative correlation energy, U, at defect sites. This energy results in all defect sites being either unoccupied or doubly occupied, which explains the absence of any EPR signal in the glasses in the dark, while also allowing a pinning of the Fermi level. This concept was developed in the work of Mott, Davis and Street [12,21] (MDS model) and further explored in later work of Street [22,23]. They proposed that charged dangling bonds would act as lattice coupled impurity sites in the glass, and were able to explain most of the observed features of chalcogenides glasses. A similar model was independently developed by Kastner, Adler and Fritzsche (KAF model) [13], and later developed further by Kastner and co-workers [24,25]. Kastner proposed that the important sites were actually pairs of oppositely charged defects, which he termed intimate valence alternation pairs (IVAPs), rather than single dangling bonds. These phenomenological models successfully explained or were consistent with our experimental understanding of the optical and electronic properties of the chalcogenide glasses at the time of the models' conceptualization. However, some important
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questions still remain unanswered. Most notable among these are the lack of an understanding of the different absorption processes and the properties of the radiative recombination centers in the glass. Our discovery of interactions between the absorption and emission processes of the chalcogenide glasses and the rare earth dopants provides a valuable opportunity to gain new insights concerning the intrinsic optical and electronic properties of these amorphous semiconductors, and to subject the established defect models to additional experimental tests.
3. Experimental techniques A variety of excitation sources were used for different aspects of this paper. PL spectra were excited with an Argon ion laser, a Ti-doped sapphire laser, or a tungsten lamp dispersed by a 0.5 m double monochromator. PLE spectra were taken using the tungsten lamp as the excitation source. The luminescence was collected and focused onto the aperture of a 1 m single grating monochromator, which dispersed the PL. The resultant signal was detected with a 77 K cooled Ge pin detector and analyzed using a lock-in detector (Stanford Research Systems SR850). The samples were cooled in a liquid Helium cryostat (Janis Supervaritemp). Glass samples were prepared at the Crystal Growth Laboratory at the University of Utah from a mixture of crystalline Er2S 3 and commercially available, high purity As12Ge33Se55 or As2S 3 glass. The mixtures were heated (over 25 h) in a quartz ampoule to 1000°C, rocked for 100 h, and then cooled to room temperature. Fig. 2a shows the PL spectrum taken at 5 K from a 0.1 wt% Er-doped Asi2Ge33Se55 sample, excited at 600 nm by the tungsten lamp. In this low temperature spectrum, the 1540 nm Er emission is superimposed on the broad, mid gap PL of the host glass. The apparent structure in the host glass PL band is caused by atmospheric absorption and the spectral dependence of the spectrometer grating efficiency. The host glass PL band is actually smooth and featureless [14]. Fig. 2b shows PLE spectra of the Er and host glass PL bands, normalized for comparison. Because the peak position of the host glass PLE is known to shift with the detected PL wavelength [26], in order to obtain a valid comparison between these
D.A. Turnbull, S.G. Bishop~Journal of Non-CrystaUine Solids 223 (1998) 105-113
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two PLE spectra it was necessary to obtain the host glass PLE spectrum while detecting at the same wavelength as was used for the Er PLE spectrum. Consequently, the host glass PLE spectrum detecting at 1540 nm was obtained from an undoped As 12Ge33Se55 sample which contained no conflicting Er emission. The low energy sides of the two PLE spectra are nearly identical. There is a slight deviation in the Er PLE spectra, with a small peak at ~ 850 nm which we think is due to a small background concentration of Cr impurities (see Ref. [27] for a description of the effect of Cr co-dopants on the PLE lineshape). The similarity of the PLE lineshapes suggests that in both cases, the initial absorption mechanism is the same, with different recombination paths resulting in the excitation of the Er or the host glass PL. The high energy roll-offs of the two PLE curves differ, however. The roll-off of the Er PLE is faster, both occurring at a slightly smaller energy and decreasing more quickly with increasing energy, resulting in a
quicker fall off in the PL intensity as excitation energies are increased. The implications of this difference will be discussed below. The data presented in Fig. 3 demonstrate for the first time that the low temperature Er PL, when pumped in the broad band range of the PLE spectrum, exhibits a fatiguing phenomenon which is similar to the well-known fatiguing of the mid-gap host glass PL [19]. We believe this fatiguing is caused by a decrease in the efficiency of the energy transfer process to the rare earth dopants rather than by a decrease in the radiative quantum efficiency of the Er dopants. Fig. 3a shows the fatiguing of both the host glass and the rare earth luminescence intensity from a 0.1 wt% Er-As12Ge33Se55 sample under optical excitation at 5 K. The luminescence is excited by a focused Ar laser at 514.5 nm, on the high energy side of the broad excitation band. The two spectra were taken from different spots on the same sample, and are normalized to the same initial intensity to show differences in the fatiguing rates. It can
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Fig. 2. (a) shows a PL spectrum taken at 5 K from a 0.1 wt% Er-doped As12Ge33Se55 glass, excited at 600 nm. This spectrum shows the sharp Er emission band at 1540 nm, superimposed on the broad host glass PL band, which extends over the whole range of the spectrum. The apparent structure in the host glass PL band is due to artifacts from atmospheric absorption and the spectral response of our spectrometer grating. (b) shows two PLE spectra, taken at 5 K PL, both detecting at 1540 nm. The dashed line is from a 0.1 wt% Er-As12Ge33Se55 sample, while the solid line is taken from a pure As12Ge33Ses5 sample. Note the remarkable similarity in lineshape and spectral position of the two spectra. The spectrum from the Er-doped glass falls off slightly more quickly at high energies, though. The implications of this difference are discussed in the text.
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be seen that the host glass luminescence fatigues somewhat more rapidly than the Er PL. A similar fatiguing effect was observed in 0.1% Er-doped As2S 3. Fig. 3b shows fatiguing curves taken from this sample, again detecting both the Er and the host glass PL. In the As2S 3 sample, when exciting with the 514.5 nm line of the Ar laser, the two fatiguing rates are similar, with the host glass PL fatiguing slightly faster. This difference in the relative fatiguing rates of the host glass and the rare-earth emissions can be explained by the faster large energy roll-off of the Er PLE mentioned above. As mentioned previously, the fatiguing rate of the host glass PL varies with the excitation wavelength, with the fastest fatiguing occurring at the peak of the PLE spectrum. Since the Er PLE falls off more rapidly at large energies, then the Er fatiguing rate should also decrease more than the host glass fatiguing as the excitation light is moved to larger energies. Because the As]2Ge33Se55 glass has a smaller bandgap than the As2S 3, the difference in energy of the Ar light (514.5 nm) used as the excitation source and the peak energy of the PLE spectrum in As12Ge33Se55 is larger than in the case of As2S 3. Thus, when excited at 514.5 nm the . . . .
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Er PL fatiguing will be slower in comparison to the host glass PL fatiguing in the As]2Ge33Se55 than in the As2S 3 glass. There are two other parallels which we had previously observed between the host glass PLE and the broad band excitation of the rare earth dopants, which strengthen the link between these two processes. First, a shift in the peak wavelength of the PLE spectra with the detecting wavelength has been observed in the case of the host glass [26]. That is, as the detecting wavelength is increased, the peak wavelength of the PLE shifts to longer wavelength. Similarly, in the rare earth doped glasses, the peak position of the broad PLE band was found to shift when different rare earth emission bands were excited. That is, the peak of the broad PLE band for the 1700 nm Dy emission occurs at a longer wavelength than that for the 1340 nm Dy emission. The peaks of the broad PLE bands for the 1540 nm Er and 1640 nm Pr also fit this pattern. The second parallel concerns the range of rare earth emission energies that are efficiently excited by the broad band PLE mechanism. We observed that there was a high energy cutoff to the broad band excitation mechanism, that rare earth transitions which required more '
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Fig. 3. Fatiguing curves for the host glass (solid lines) and Er (dashed lines) PL, excited with the 514.5 nm line from an Ar laser, taken at 5 K. The Er PL curves are corrected by subtracting out the underlying host glass PL signal. (a) shows the curves from a 0.1 wt% Er-As12Ge33Se55 glass. It can be seen that the host glass PL fatigues considerablymore quickly than the Er PL. (b) shows the curves from a 1.0 wt% Er-As2S 3 glass. In this glass, the host glass PL also fatigues more quickly than the Er PL, but the rates are closer than those from the As12Ge33Se55 glass. The Er PL from the As2S 3 is much weaker than that from the As12Ge33Se55, which is why the Er PL fatiguing curve is noisy in (b).
D.A. Turnbull, S.G. Bishop/ Journal of Non-Crystalline Solids 223 (1998) 105-113
110
than N 1 eV to excite did not show any broad PLE Preliminary results, to be band in As,,Ge,,Se,,. presented in a future paper, also indicate that there is a low energy cutoff to the transfer mechanism as well, and the range of energies of transitions which are efficiently excited by the broad band corresponds approximately to the range of the host glass PL band. Simply stated, the Stokes shift between the excitation and emission energies for the host glass PL is similar to that between the host glass absorption and the excitation energy transferred to the rare earth dopants.
4. Discussion These similarities led us previously [lo] to propose an excitation mechanism based on the MDS model for the host glass PL. Fig. 4 shows a schematic of the proposed energy transfer process. The absorption of light in the Urbach edge spectral range excites an electron from the valence band into the conduction band. Subsequently, the resultant hole is captured by a nearby defect-related site in the glass. (The complementary process involving a capture of the electron is also possible.) This capture changes the charge state of the defect-related site, which
results in a structural relaxation that moves the state deeper into the gap. This relaxation is responsible for the observed Stokes shift. The electron then recombines with the hole, transferring the energy nonradiatively to a nearby Er atom. In order to evaluate the implications of the broad band excitation of the rare earths on existing models for the properties of chalcogenide glasses, the nature of the defect site involved in the transfer process described above must be determined. In the MDS model, the broad PLE band, the radiative recombination and its fatigue, and the optically induced EPR and below gap absorption were all explained in terms of charged defects due to dangling bonds which are present in the glass. In contrast, we originally proposed that the local sites involved in the broad band excitation of the rare earth dopants we have observed are induced by impurities which enter the glass during the doping procedure. There were several reasons for proposing that these sites, although sharing many of the properties of the intrinsic defect states in the glass, were impurity related. First, no special precautions were taken to maintain the purity of the glasses prepared at the University of Utah, and SIMS measurements indicated the presence of numerous transition metal and other impuri-
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Fig. 4. A schematic of the proposed energy transfer process is shown. The excitation light is absorbed near an impurity site in the glass, creating an electron-hole pair. The hole is subsequently captured by the impurity. Because of the strong electron-phonon coupling in the system, after the hole is captured, the impurity level will move deeper into the gap. An electron from the conduction band can then recombine with the hole trapped at the impurity site, transferring its energy non-radiatively to a nearby rare-earth atom. After recombination, the lattice around the impurity will distort, moving the impurity site back to its original energy state.
D.A. Turnbull, S. G. Bishop/Journal of Non-Crystalline Solids 223 (1998) 105-113
ties in these glasses. Second, samples prepared by implanting Er into high purity As2S 3 showed the 1540 nm Er PL and sharp Er 3+ F-band PLE bands, but no broad PLE band. Also, the broad rare earth PLE bands exhibit a very different temperature dependence than the host glass PL in As2S 3 and As12Ge33Se55. The host glass PL is quenched at room temperature, while the broad PLE band is still strong [8,14]. Lastly, samples of GeAsS prepared with care taken to maintain high purity in the melt (and using elemental Er as the dopant rather than Er2S 3) showed only a weak or non-existent broad band PLE, indicating that the broad band PLE of the rare earth emissions is dependent on sample preparation, while the intrinsic host glass properties are not [15]. However, this last conclusion was in error. While the high purity GeAsS samples (made at Coming) showed no broad excitation of the rare earth dopants at room temperature, when cooled to liquid helium temperatures, they too exhibited a broad PLE band for the rare earth emissions. The difference in the thermal quenching of the broad band in these sampies, compared to the Asl2Ge33Se55 samples, could be explained by their different glass composition. In addition, a Pr-doped As12Ge33Se55 sample was prepared at the Naval Research Laboratory under similar conditions designed to maintain a high purity and reduce the level of background impurities in the melt. This sample also exhibits a broad PLE band for the 1640 nm Pr PL band at room temperature, just as the samples prepared at Utah do. The existence of broad PLE bands in high purity samples prepared at different laboratories, with different purification and distillation techniques, together with the existence and nearly identical rate of fatiguing of the rare earth and host glass PL, has led us to conclude that the same native defects responsible for the midgap PL and other properties of the host glass are also the sites which mediate the transfer of energy from the host glass to the rare earth dopants. Although the presence of the broad excitation band in high purity glasses indicates that impurities may not be crucial to the excitation of the rare earths, the other pieces of evidence mentioned above supporting the role of impurities must still be explained before accepting that native defects play the key role in the energy transfer process. While no
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broad band was observed in the excitation spectrum of an Er-implanted sample, this absence could be due to differences in the incorporation of the Er in the implanted samples as compared to samples doped in the melt. Alternatively, the implanting process could have damaged the host glass in such a way that the energy transfer process was quenched. The difference between the temperature dependence of the PLE of the rare earth emission and that of the host glass PL can also be explained without invoking impurities. After an electron-hole pair is created in the glass, there are three main recombination pathways. The pair can recombine non-radiatively through multi-phonon relaxation, radiatively by emitting a photon, or by transferring their energy to a rare-earth dopant atom. Of these, only the non-radiative recombination will be strongly temperature dependent, and this pathway dominates in the pure glass at room temperature. However, if the probability of energy transfer is large enough, the non-radiative recombination will not dominate at room temperature, and hence the broad band excitation of the rare earths will not be completely thermally quenched at room temperature. Work is currently underway to obtain a more quantitative understanding of the temperature dependence of the broad band excitation process. If it is true that the defects involved in the broad band excitation of rare earth dopants are the same as those giving rise to the host glass PL and other properties of the chalcogenides, then the rare earths can act as probes to determine some of the properties of the intrinsic defects. Thus, the broad band PLE can serve as a useful tool for evaluating the models of the intrinsic processes in chalcogenide glasses. In particular, it may be fruitful to examine features of the broad band rare earth PLE which differ from the corresponding features of the host glass PLE. It is these aspects which supply new tests for the existing theories, which must explain the observed variations. One feature about which some conclusions can immediately be drawn is the high energy roll-over of the PLE spectra for the host glass and rare earth PL. A successful explanation of this phenomenon must be able to account for the faster roll-off for the broad band PLE of the rare earth emissions. There are two plausible existing explanations for the high energy decrease in the PLE, neither of which is wholly
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D.A. Turnbull, S.G. Bishop~Journal of Non-Crystalline Solids 223 (1998) 105-113
satisfactory. The first model for the high energy behavior of the PLE was proposed by Davis [28], who used it to explain the photoconductivity of a-Se. He suggested that at low energies the electron and hole excited by the light were likely to be captured in localized bandtail states without diffusing apart. However, at higher energies, where more energy is imparted to the excited electron-hole pair, they are more likely to diffuse apart reducing the likelihood of subsequent radiative recombination. This separation would then produce a decrease in the efficiency of the PL at higher excitation energies and a corresponding roll-off in the PLE. An objection to this model is provided by the spectral dependence of the fatiguing process and optically induced EPR. The induction of an EPR signal is attributed to the hopping or diffusion of electrons (or holes) away from holes (or electrons) bound at the defect sites in the glass [19]. If Davis's model were correct, the fatiguing and induced EPR should occur more quickly at larger excitation energies, which is the opposite of what is observed. This model also has no clear explanation for the different high energy behaviors of the rare earth and host glass PLE spectra. The second existing explanation is due to Street. He proposed [14] that the localized defects where recombination occurs are charged. These charged defects will then produce electric fields in their vicinity, which cause the broadening of the absorption edge into its characteristic exponential Urbach form. The absorption at lower energies will occur in regions of high field, which will be close to the charged defects, making subsequent capture and recombination at these defects more likely. At larger energies, it becomes less and less likely that the photon will be absorbed in the vicinity of a charged defect and, since the diffusion length of carriers in the glass is small compared to the density of defect states [14], this will result in a roll over in the PLE at higher energies. It is possible that the local electric fields of defects near rare earth atoms could be different than the local fields of isolated defects, which could account for differences in the high energy behavior of the PLE spectra. However, Street used the same electric fields to explain the broadening of the Urbach absorption edge, which is identical in both the rare earth and host glass PLE. Thus, this model is unable to explain our results. It requires the
local electric fields around defects to be the same in the doped and undoped glasses in order to explain the identical Urbach edge absorptions. In contradiction, it also requires that the local electric fields be different in order to explain the change in the high energy roll-off in the PLE spectra. Neither model for the PLE can satisfactorily explain all the experimental results. However, our previous work presents some additional information about the large energy roll off which may be helpful in developing a more complete explanation of the process. While the large energy roll off of the broad band PLE for the 1540 nm Er absorption occurred more rapidly with increasing energy than that of the host glass PL, the addition of transition metals such as Cr and Fe decreased the slope of the large energy roll-off of the broad band PLE [27]. Another possible clue to the origin of the high energy roll-over is provided by an earlier observation of ours [9]. While the exponential slope of both the large and small energy tails of the broad band rare earth PLE changed slightly from sample to sample, it was found that the difference between the two slopes was constant in all samples. Further work is necessary to determine the mechanism responsible for the high energy behavior of the PLE spectra.
5. Summary and conclusions In conclusion, the broad excitation bands of rare earth dopants in chalcogenide glasses show a number of similarities with the PLE of the host glass PL, including a fatiguing effect. These similarities lead to the conclusion that the same defect states which give rise to the properties of the host glass are involved in the energy transfer between the host and the rare earth dopants. We found that proposed mechanisms for the high energy behavior of the PLE do not explain the observed phenomena.
Acknowledgements The authors would like to thank M.C. Delong and the Crystal Growth Laboratory of the Department of Physics, University of Utah, for preparing most of the glasses; Bruce Aitken and Coming, Inc., and Jas
D.A. Turnbull, S. G. Bishop/Journal of Non-Crystalline Solids 223 (1998) 105-113
Sanghera and the Naval Research Laboratory for providing samples; and P.C. Taylor for many helpful discussions. This work has been supported by the NSF under the Engineering Research Centers (ECD89-43166) and the Materials Research Science and Engineering Centers (DMR-89-20538) Programs, and by ARPA under the Center for Optoelectronic Science and Technology (Grant #MDA972-94-1-0004) Program.
References [1] K. Wei, D.P. Machewirth, J. Wenzel, E. Snitzer, G.H. Sigel Jr., J. Non-Cryst. Solids 182 (1995) 257. [2] K. Wei, D.P. Machewirth, J. Wenzel, E. Snitzer, G.H. Sigel Jr., Opt. Lett. 19 (1994) 904. [3] D.W. Hewak, R.S. Deol, J. Wang, G. Wylangowski, J.A. Medeiros Neto, B. Samson, R.I. Laming, W.S. Brocklesby, D.N. Payne, A. Jha, M. Poulain, M. Otero, S. Surinach, M.D. Baro, Electron. Lett. 29 (1993) 237. [4] D.W. Hewak, J.A. Medeiros Neto, B. Samson, R.S. Brown, K.P. Jedrzejewski, J. Wang, E. Taylor, R.I. Laming, G. Wylangowski, D.N. Payne, IEEE Photon. Tech. Lett. 6 (1994) 609. [5] J. Tauc, A. Menth, D.L. Wood, Phys. Rev. Lett. 25 (1970) 759. [6] J. Tauc, F.J. DiSalvo, G.E. Peterson, D.L. Wood, in: H.O. Hooper, A.M. de Graaf (Eds.), Amorphous Magnetism, Plenum, New York, 1973, p. 119.
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[7] S.Q. Gu, S. Ramachandran, E.E. Reuter, D.A. Turnbull, J.T. Verdeyen, S.G. Bishop, J. Appl. Phys. 77 (1995) 3365. [8] S.Q. Gu, S. Ramachandran, E.E. Reuter, D.A. Turnbull, J.T. Verdeyen, S.G. Bishop, Appl. Phys. Lett. 66 (1995) 670. [9] S.Q. Gu, D.A. Turnbull, S.G. Bishop, IEEE Photon. Tech. Lett. 8 (1996) 1041. [10] D.A. Turnbull, S.Q. Gu, S.G. Bishop, J. Appl. Phys. 80 (1996) 2436. [11] W.J. Miniscalco, J. Lightwave Tech. 9 (1991) 234. [12] R.A. Street, N.F. Mott, Phys. Rev. Lett. 35 (1975) 1293. [13] M. Kastner, D. Adler, H. Fritzsche, Phys. Rev. Lett, 37 (1976) 1504. [14] R.A. Street, Adv. Phys. 25 (1976) 397. [15] N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon, Oxford, 1979, ch. 9. [16] J.D. Dow, D. Redfield, Phys. Rev. Lett. 26 (1971) 762. [17] P.C. Taylor, in: W.M. Yen (Ed.), Laser Spectroscopy of Solids II, Springer, Berlin, 1989, ch. 7. [18] A.R. Hilton, D.J. Hayes, M.D. Rechtin, J. Non-Cryst. Solids 17 (1975) 319. [19] S.G. Bishop, U. Strom, P.C. Taylor, Phys. Rev. Lett. 34 (1975) 1346. [20] P.W. Anderson, Phys. Rev. Lett. 34 (1975) 953. [21] N.F. Mott, E.A. Davis, R.A. Street, Philos. Mag. 32 (1975) 961. [22] N.F. Mott, R.A. Street, Philos. Mag. 36 (1977) 33. [23] R.A. Street, Phys. Rev. B 17 (1978) 3984. [24] M. Kastner, J. Non-Cryst. Solids 31 (1978) 223. [25] M. Kastner, H. Fritzsche, Philos. Mag. B 37 (1978) 199. [26] G.S. Higashi, M. Kastner, Phys. Rev. Lett. 47 (1981) 124. [27] D.A. Tumbull, S.G. Bishop, J. Non-Cryst. Solids 213&214 (1997) 288. [28] E.A. Davis, J. Non-Cryst. Solids 4 (1970) 107.
ELSEVIER
Journal of Non-Crystalline Solids 223 (1998) 105-113
Rare earth dopants as probes of localized states in chalcogenide glasses D.A. Turnbull a,b,c,,, S.G. Bishop a,b,c a Center for Compound Semiconductor Microelectronics, Microelectronics Laboratory, University oflllinois, 208 N. Wright Street, Urbana, IL 61801, USA b Center for Optoelectronic Science and Technology, University of Illinois, Urbana, IL 61801, USA c Department of Electrical and Computer Engineering, University oflllinois, Urbana, IL 61801, USA Received 9 April 1997; revised 4 August 1997
Abstract The recently observed phenomenon of broad band excitation of rare earth dopants in chalcogenide glasses indicates an interaction between the rare earth ions and localized background defect states in the glass. Low temperature (5 K) photoluminescence and photoluminescence excitation spectroscopy of Er2S3-doped As12Ge33Se55 samples show similarities in excitation lineshape, Stokes shift, and spectral position to the intrinsic luminescence of an undoped As 12Ge33Se55 sample. Fatiguing of the broad band excitation has been observed in Er-doped As12Ge33Se55 and As2S 3 samples. These results indicate that the broad excitation band involves interaction with the same native defects in the host glass which give rise to important optical, electrical, and magnetic resonance properties of the chalcogenides. A model for the energy transfer process from the host glass to the rare earths, mediated by intrinsic defect states in the glass is presented. Some features of existing models for the behavior of chalcogenide glasses will be evaluated in light of our results. © 1998 Elsevier Science B.V.
1. Introduction Recently, chalcogenide glasses have become the subject of study as potential host materials for rare earth-doped optical fiber amplifiers in the 1.3 ~ m spectral range [1-4]. These glasses, based on sulfur, selenium and tellurium, offer the prospect of smaller non-radiative decay rates and larger emission crosssections for rare earth dopants. In contrast to the wide band gap oxide glasses, which are the usual host materials for Er-doped fiber amplifiers operating at 1.55 Ixm, the chalcogenide glasses have band
* Corresponding author. Tel.: + 1-217 244 6378; fax: + 1-217 244 6375; e-mail: [email protected]. 0022-3093//$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 7 ) 0 0 3 4 6 - 3
gaps in the visible or near infrared and are not transparent in the visible and near ultraviolet spectral ranges. This narrow bandgap means that the intrinsic optical transitions of the glass, including the interband absorption, the band edge Urbach tail, and the below-gap weak absorption tail (WAT) [5,6], will overlap some of the inter F-band transitions of the rare earth dopants. This overlap raises the possibility of interactions between the optical absorption and emission processes of the rare earth dopants and the chalcogenide host glass. We initiated a study of photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy in rare earth doped chalcogenide glasses to investigate the interactions between the host glasses
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D.A. Turnbull, S. G. Bishop~Journal of Non-Crystalline Solids 223 (1998) 105-113
and the rare earth dopants and the possible influence of such interactions upon the suitability of these materials for optical amplifiers. Our initial studies [7-10] detected a, broad, near-band edge PLE band in the excitation spectra of rare earth emission bands in chalcogenide glasses. This excitation mechanism has proven to be a general feature of rare earth dopants in a variety of chalcogenide glass hosts with potentially important technological implications. Fig. 1 shows both the PL and PLE spectra taken from a 0.1 wt% Er-doped ASlzGe33Se55 sample [8]. The PL spectrum shows the usual 1540 nm Er emission line [11]. The PLE spectrum, detecting at 1540 nm, also shows the expected inter-F-band absorptions of the Er atoms at 980 and 810 nm [11]. However, these relatively sharp Er bands are superimposed on an excitation band with a larger amplitude and width, stretching from ~ 500 to 1000 nm. Similar broad PLE bands were observed in Pr- and Dy-doped [9,10] As]2Ge33Se55 glasses, as well as in Er-doped As2S 3 [7]. This broad PLE band is due to the absorption of light by the host glass and the subsequent transfer of this energy to the rare earth dopants. In this paper we present new low temperature measurements of the PL and PLE in rare earth-doped
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Wavelength (nm) Fig. 1. PL and PLE spectra taken at 300 K from a 0.1 wt% Er-As 12Ge33Se55 glass. The PL spectrum,on the right, shows the expected 1540 nm emission line. The PLE spectra on the left, detecting at 1540 nm, shows the normal Er 810 nm and 980 nm absorption bands. These bands, however, are superimposedon a strong, broad excitation band, due to absorptionby the host glass and subsequent energy transfer to the Er dopants.
chalcogenide glasses which demonstrate similarities between the PL excitation mechanisms of the rare earth-doped glasses and those of the undoped-host glass. These results indicate that study of the interaction between the host glasses and the rare earth dopants can provide information about the intrinsic properties of the host glass and new insights conceming the phenomenological models which have been proposed [12,13] to explain the fundamental optical, electrical, and magnetic resonance properties of localized electronic states in chalcogenide glasses.
2. Background The experimental results and their interpretation presented here must be discussed within the context of a broad range of previous experimental and theoretical work on chalcogenide glasses. Of particular importance are the unique optical absorption and emission properties of the chalcogenide glasses which are intimately involved in the broad band excitation mechanism of the rare earth dopant emissions. For a more complete overview of the properties of the chalcogenides than is possible to present here, see the excellent review article by Street [14]. The absorption edge in chalcogenide glasses can be divided into three distinct regions [15]. At the largest energies, the absorption coefficient shows the standard, quadratic dependence on energy of bandto-band absorption. At slightly smaller energies, the band edge absorption is broadened into an exponential absorption tail, usually referred to as the Urbach edge or Urbach tall. A similar Urbach edge has been observed in crystalline semiconductors [16], in which it is attributed to electric field broadening of the band-to-band or exciton absorption. However, the Urbach absorption edge is broadened much more substantially in the chalcogenides than it is in crystalline semiconductors. In order to account for the Urbach edge in the chalcogenides with the same explanations as are used for crystalline materials, extremely high electric field strengths are required [17]. At still smaller energies, extending well into the bandgap, there is a shallower exponential absorption tail, commonly called the weak absorption tail (WAT). The strength of the WAT is linked to the presence of impurities in the sample [6,18], but the exact origin of this absorption is not known.
D.A. TurnbulL S.G. Bishop~Journal of Non-Crystalline Solids 223 (1998) 105-113
At low temperature (below ~ 100 K), chalcogenide glasses exhibit a broad luminescence band centered approximately in the middle of the bandgap [14]. The PLE spectrum of this mid-gap PL follows the absorption spectrum of the glass until it reaches a maximum about halfway up the Urbach edge, and then decreases at larger energy. Any model of excitation and recombination in these glasses must account for the large Stokes shift between the peak energies of the PLE and PL bands. The host glass PL band also fatigues under exciting light [14], with the most rapid fatiguing occurring for exciting light at the peak wavelength of the PLE spectrum. The fatiguing is accompanied by the induction of an electron paramagnetic resonance (EPR) signal and an absorption band stretching from the band edge to approximately half the bandgap energy [19]. Most pure chalcogenide glasses show no EPR signals when unexposed to light [15]. The lack of any EPR signal in this case, combined with the pinning of the Fermi level, was one of the major questions confronting theorists in the 1970s [15]. Anderson [20] solved this problem with the suggestion that strong lattice coupling at impurity sites could produce an effective negative correlation energy, U, at defect sites. This energy results in all defect sites being either unoccupied or doubly occupied, which explains the absence of any EPR signal in the glasses in the dark, while also allowing a pinning of the Fermi level. This concept was developed in the work of Mott, Davis and Street [12,21] (MDS model) and further explored in later work of Street [22,23]. They proposed that charged dangling bonds would act as lattice coupled impurity sites in the glass, and were able to explain most of the observed features of chalcogenides glasses. A similar model was independently developed by Kastner, Adler and Fritzsche (KAF model) [13], and later developed further by Kastner and co-workers [24,25]. Kastner proposed that the important sites were actually pairs of oppositely charged defects, which he termed intimate valence alternation pairs (IVAPs), rather than single dangling bonds. These phenomenological models successfully explained or were consistent with our experimental understanding of the optical and electronic properties of the chalcogenide glasses at the time of the models' conceptualization. However, some important
107
questions still remain unanswered. Most notable among these are the lack of an understanding of the different absorption processes and the properties of the radiative recombination centers in the glass. Our discovery of interactions between the absorption and emission processes of the chalcogenide glasses and the rare earth dopants provides a valuable opportunity to gain new insights concerning the intrinsic optical and electronic properties of these amorphous semiconductors, and to subject the established defect models to additional experimental tests.
3. Experimental techniques A variety of excitation sources were used for different aspects of this paper. PL spectra were excited with an Argon ion laser, a Ti-doped sapphire laser, or a tungsten lamp dispersed by a 0.5 m double monochromator. PLE spectra were taken using the tungsten lamp as the excitation source. The luminescence was collected and focused onto the aperture of a 1 m single grating monochromator, which dispersed the PL. The resultant signal was detected with a 77 K cooled Ge pin detector and analyzed using a lock-in detector (Stanford Research Systems SR850). The samples were cooled in a liquid Helium cryostat (Janis Supervaritemp). Glass samples were prepared at the Crystal Growth Laboratory at the University of Utah from a mixture of crystalline Er2S 3 and commercially available, high purity As12Ge33Se55 or As2S 3 glass. The mixtures were heated (over 25 h) in a quartz ampoule to 1000°C, rocked for 100 h, and then cooled to room temperature. Fig. 2a shows the PL spectrum taken at 5 K from a 0.1 wt% Er-doped Asi2Ge33Se55 sample, excited at 600 nm by the tungsten lamp. In this low temperature spectrum, the 1540 nm Er emission is superimposed on the broad, mid gap PL of the host glass. The apparent structure in the host glass PL band is caused by atmospheric absorption and the spectral dependence of the spectrometer grating efficiency. The host glass PL band is actually smooth and featureless [14]. Fig. 2b shows PLE spectra of the Er and host glass PL bands, normalized for comparison. Because the peak position of the host glass PLE is known to shift with the detected PL wavelength [26], in order to obtain a valid comparison between these
D.A. Turnbull, S.G. Bishop~Journal of Non-CrystaUine Solids 223 (1998) 105-113
108
two PLE spectra it was necessary to obtain the host glass PLE spectrum while detecting at the same wavelength as was used for the Er PLE spectrum. Consequently, the host glass PLE spectrum detecting at 1540 nm was obtained from an undoped As 12Ge33Se55 sample which contained no conflicting Er emission. The low energy sides of the two PLE spectra are nearly identical. There is a slight deviation in the Er PLE spectra, with a small peak at ~ 850 nm which we think is due to a small background concentration of Cr impurities (see Ref. [27] for a description of the effect of Cr co-dopants on the PLE lineshape). The similarity of the PLE lineshapes suggests that in both cases, the initial absorption mechanism is the same, with different recombination paths resulting in the excitation of the Er or the host glass PL. The high energy roll-offs of the two PLE curves differ, however. The roll-off of the Er PLE is faster, both occurring at a slightly smaller energy and decreasing more quickly with increasing energy, resulting in a
quicker fall off in the PL intensity as excitation energies are increased. The implications of this difference will be discussed below. The data presented in Fig. 3 demonstrate for the first time that the low temperature Er PL, when pumped in the broad band range of the PLE spectrum, exhibits a fatiguing phenomenon which is similar to the well-known fatiguing of the mid-gap host glass PL [19]. We believe this fatiguing is caused by a decrease in the efficiency of the energy transfer process to the rare earth dopants rather than by a decrease in the radiative quantum efficiency of the Er dopants. Fig. 3a shows the fatiguing of both the host glass and the rare earth luminescence intensity from a 0.1 wt% Er-As12Ge33Se55 sample under optical excitation at 5 K. The luminescence is excited by a focused Ar laser at 514.5 nm, on the high energy side of the broad excitation band. The two spectra were taken from different spots on the same sample, and are normalized to the same initial intensity to show differences in the fatiguing rates. It can
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Fig. 2. (a) shows a PL spectrum taken at 5 K from a 0.1 wt% Er-doped As12Ge33Se55 glass, excited at 600 nm. This spectrum shows the sharp Er emission band at 1540 nm, superimposed on the broad host glass PL band, which extends over the whole range of the spectrum. The apparent structure in the host glass PL band is due to artifacts from atmospheric absorption and the spectral response of our spectrometer grating. (b) shows two PLE spectra, taken at 5 K PL, both detecting at 1540 nm. The dashed line is from a 0.1 wt% Er-As12Ge33Se55 sample, while the solid line is taken from a pure As12Ge33Ses5 sample. Note the remarkable similarity in lineshape and spectral position of the two spectra. The spectrum from the Er-doped glass falls off slightly more quickly at high energies, though. The implications of this difference are discussed in the text.
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D.A. Turnbull, S.G. Bishop/Journal of Non-Crystalline Solids 223 (1998) 105-113
be seen that the host glass luminescence fatigues somewhat more rapidly than the Er PL. A similar fatiguing effect was observed in 0.1% Er-doped As2S 3. Fig. 3b shows fatiguing curves taken from this sample, again detecting both the Er and the host glass PL. In the As2S 3 sample, when exciting with the 514.5 nm line of the Ar laser, the two fatiguing rates are similar, with the host glass PL fatiguing slightly faster. This difference in the relative fatiguing rates of the host glass and the rare-earth emissions can be explained by the faster large energy roll-off of the Er PLE mentioned above. As mentioned previously, the fatiguing rate of the host glass PL varies with the excitation wavelength, with the fastest fatiguing occurring at the peak of the PLE spectrum. Since the Er PLE falls off more rapidly at large energies, then the Er fatiguing rate should also decrease more than the host glass fatiguing as the excitation light is moved to larger energies. Because the As]2Ge33Se55 glass has a smaller bandgap than the As2S 3, the difference in energy of the Ar light (514.5 nm) used as the excitation source and the peak energy of the PLE spectrum in As12Ge33Se55 is larger than in the case of As2S 3. Thus, when excited at 514.5 nm the . . . .
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Er PL fatiguing will be slower in comparison to the host glass PL fatiguing in the As]2Ge33Se55 than in the As2S 3 glass. There are two other parallels which we had previously observed between the host glass PLE and the broad band excitation of the rare earth dopants, which strengthen the link between these two processes. First, a shift in the peak wavelength of the PLE spectra with the detecting wavelength has been observed in the case of the host glass [26]. That is, as the detecting wavelength is increased, the peak wavelength of the PLE shifts to longer wavelength. Similarly, in the rare earth doped glasses, the peak position of the broad PLE band was found to shift when different rare earth emission bands were excited. That is, the peak of the broad PLE band for the 1700 nm Dy emission occurs at a longer wavelength than that for the 1340 nm Dy emission. The peaks of the broad PLE bands for the 1540 nm Er and 1640 nm Pr also fit this pattern. The second parallel concerns the range of rare earth emission energies that are efficiently excited by the broad band PLE mechanism. We observed that there was a high energy cutoff to the broad band excitation mechanism, that rare earth transitions which required more '
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Fig. 3. Fatiguing curves for the host glass (solid lines) and Er (dashed lines) PL, excited with the 514.5 nm line from an Ar laser, taken at 5 K. The Er PL curves are corrected by subtracting out the underlying host glass PL signal. (a) shows the curves from a 0.1 wt% Er-As12Ge33Se55 glass. It can be seen that the host glass PL fatigues considerablymore quickly than the Er PL. (b) shows the curves from a 1.0 wt% Er-As2S 3 glass. In this glass, the host glass PL also fatigues more quickly than the Er PL, but the rates are closer than those from the As12Ge33Se55 glass. The Er PL from the As2S 3 is much weaker than that from the As12Ge33Se55, which is why the Er PL fatiguing curve is noisy in (b).
D.A. Turnbull, S.G. Bishop/ Journal of Non-Crystalline Solids 223 (1998) 105-113
110
than N 1 eV to excite did not show any broad PLE Preliminary results, to be band in As,,Ge,,Se,,. presented in a future paper, also indicate that there is a low energy cutoff to the transfer mechanism as well, and the range of energies of transitions which are efficiently excited by the broad band corresponds approximately to the range of the host glass PL band. Simply stated, the Stokes shift between the excitation and emission energies for the host glass PL is similar to that between the host glass absorption and the excitation energy transferred to the rare earth dopants.
4. Discussion These similarities led us previously [lo] to propose an excitation mechanism based on the MDS model for the host glass PL. Fig. 4 shows a schematic of the proposed energy transfer process. The absorption of light in the Urbach edge spectral range excites an electron from the valence band into the conduction band. Subsequently, the resultant hole is captured by a nearby defect-related site in the glass. (The complementary process involving a capture of the electron is also possible.) This capture changes the charge state of the defect-related site, which
results in a structural relaxation that moves the state deeper into the gap. This relaxation is responsible for the observed Stokes shift. The electron then recombines with the hole, transferring the energy nonradiatively to a nearby Er atom. In order to evaluate the implications of the broad band excitation of the rare earths on existing models for the properties of chalcogenide glasses, the nature of the defect site involved in the transfer process described above must be determined. In the MDS model, the broad PLE band, the radiative recombination and its fatigue, and the optically induced EPR and below gap absorption were all explained in terms of charged defects due to dangling bonds which are present in the glass. In contrast, we originally proposed that the local sites involved in the broad band excitation of the rare earth dopants we have observed are induced by impurities which enter the glass during the doping procedure. There were several reasons for proposing that these sites, although sharing many of the properties of the intrinsic defect states in the glass, were impurity related. First, no special precautions were taken to maintain the purity of the glasses prepared at the University of Utah, and SIMS measurements indicated the presence of numerous transition metal and other impuri-
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Fig. 4. A schematic of the proposed energy transfer process is shown. The excitation light is absorbed near an impurity site in the glass, creating an electron-hole pair. The hole is subsequently captured by the impurity. Because of the strong electron-phonon coupling in the system, after the hole is captured, the impurity level will move deeper into the gap. An electron from the conduction band can then recombine with the hole trapped at the impurity site, transferring its energy non-radiatively to a nearby rare-earth atom. After recombination, the lattice around the impurity will distort, moving the impurity site back to its original energy state.
D.A. Turnbull, S. G. Bishop/Journal of Non-Crystalline Solids 223 (1998) 105-113
ties in these glasses. Second, samples prepared by implanting Er into high purity As2S 3 showed the 1540 nm Er PL and sharp Er 3+ F-band PLE bands, but no broad PLE band. Also, the broad rare earth PLE bands exhibit a very different temperature dependence than the host glass PL in As2S 3 and As12Ge33Se55. The host glass PL is quenched at room temperature, while the broad PLE band is still strong [8,14]. Lastly, samples of GeAsS prepared with care taken to maintain high purity in the melt (and using elemental Er as the dopant rather than Er2S 3) showed only a weak or non-existent broad band PLE, indicating that the broad band PLE of the rare earth emissions is dependent on sample preparation, while the intrinsic host glass properties are not [15]. However, this last conclusion was in error. While the high purity GeAsS samples (made at Coming) showed no broad excitation of the rare earth dopants at room temperature, when cooled to liquid helium temperatures, they too exhibited a broad PLE band for the rare earth emissions. The difference in the thermal quenching of the broad band in these sampies, compared to the Asl2Ge33Se55 samples, could be explained by their different glass composition. In addition, a Pr-doped As12Ge33Se55 sample was prepared at the Naval Research Laboratory under similar conditions designed to maintain a high purity and reduce the level of background impurities in the melt. This sample also exhibits a broad PLE band for the 1640 nm Pr PL band at room temperature, just as the samples prepared at Utah do. The existence of broad PLE bands in high purity samples prepared at different laboratories, with different purification and distillation techniques, together with the existence and nearly identical rate of fatiguing of the rare earth and host glass PL, has led us to conclude that the same native defects responsible for the midgap PL and other properties of the host glass are also the sites which mediate the transfer of energy from the host glass to the rare earth dopants. Although the presence of the broad excitation band in high purity glasses indicates that impurities may not be crucial to the excitation of the rare earths, the other pieces of evidence mentioned above supporting the role of impurities must still be explained before accepting that native defects play the key role in the energy transfer process. While no
111
broad band was observed in the excitation spectrum of an Er-implanted sample, this absence could be due to differences in the incorporation of the Er in the implanted samples as compared to samples doped in the melt. Alternatively, the implanting process could have damaged the host glass in such a way that the energy transfer process was quenched. The difference between the temperature dependence of the PLE of the rare earth emission and that of the host glass PL can also be explained without invoking impurities. After an electron-hole pair is created in the glass, there are three main recombination pathways. The pair can recombine non-radiatively through multi-phonon relaxation, radiatively by emitting a photon, or by transferring their energy to a rare-earth dopant atom. Of these, only the non-radiative recombination will be strongly temperature dependent, and this pathway dominates in the pure glass at room temperature. However, if the probability of energy transfer is large enough, the non-radiative recombination will not dominate at room temperature, and hence the broad band excitation of the rare earths will not be completely thermally quenched at room temperature. Work is currently underway to obtain a more quantitative understanding of the temperature dependence of the broad band excitation process. If it is true that the defects involved in the broad band excitation of rare earth dopants are the same as those giving rise to the host glass PL and other properties of the chalcogenides, then the rare earths can act as probes to determine some of the properties of the intrinsic defects. Thus, the broad band PLE can serve as a useful tool for evaluating the models of the intrinsic processes in chalcogenide glasses. In particular, it may be fruitful to examine features of the broad band rare earth PLE which differ from the corresponding features of the host glass PLE. It is these aspects which supply new tests for the existing theories, which must explain the observed variations. One feature about which some conclusions can immediately be drawn is the high energy roll-over of the PLE spectra for the host glass and rare earth PL. A successful explanation of this phenomenon must be able to account for the faster roll-off for the broad band PLE of the rare earth emissions. There are two plausible existing explanations for the high energy decrease in the PLE, neither of which is wholly
112
D.A. Turnbull, S.G. Bishop~Journal of Non-Crystalline Solids 223 (1998) 105-113
satisfactory. The first model for the high energy behavior of the PLE was proposed by Davis [28], who used it to explain the photoconductivity of a-Se. He suggested that at low energies the electron and hole excited by the light were likely to be captured in localized bandtail states without diffusing apart. However, at higher energies, where more energy is imparted to the excited electron-hole pair, they are more likely to diffuse apart reducing the likelihood of subsequent radiative recombination. This separation would then produce a decrease in the efficiency of the PL at higher excitation energies and a corresponding roll-off in the PLE. An objection to this model is provided by the spectral dependence of the fatiguing process and optically induced EPR. The induction of an EPR signal is attributed to the hopping or diffusion of electrons (or holes) away from holes (or electrons) bound at the defect sites in the glass [19]. If Davis's model were correct, the fatiguing and induced EPR should occur more quickly at larger excitation energies, which is the opposite of what is observed. This model also has no clear explanation for the different high energy behaviors of the rare earth and host glass PLE spectra. The second existing explanation is due to Street. He proposed [14] that the localized defects where recombination occurs are charged. These charged defects will then produce electric fields in their vicinity, which cause the broadening of the absorption edge into its characteristic exponential Urbach form. The absorption at lower energies will occur in regions of high field, which will be close to the charged defects, making subsequent capture and recombination at these defects more likely. At larger energies, it becomes less and less likely that the photon will be absorbed in the vicinity of a charged defect and, since the diffusion length of carriers in the glass is small compared to the density of defect states [14], this will result in a roll over in the PLE at higher energies. It is possible that the local electric fields of defects near rare earth atoms could be different than the local fields of isolated defects, which could account for differences in the high energy behavior of the PLE spectra. However, Street used the same electric fields to explain the broadening of the Urbach absorption edge, which is identical in both the rare earth and host glass PLE. Thus, this model is unable to explain our results. It requires the
local electric fields around defects to be the same in the doped and undoped glasses in order to explain the identical Urbach edge absorptions. In contradiction, it also requires that the local electric fields be different in order to explain the change in the high energy roll-off in the PLE spectra. Neither model for the PLE can satisfactorily explain all the experimental results. However, our previous work presents some additional information about the large energy roll off which may be helpful in developing a more complete explanation of the process. While the large energy roll off of the broad band PLE for the 1540 nm Er absorption occurred more rapidly with increasing energy than that of the host glass PL, the addition of transition metals such as Cr and Fe decreased the slope of the large energy roll-off of the broad band PLE [27]. Another possible clue to the origin of the high energy roll-over is provided by an earlier observation of ours [9]. While the exponential slope of both the large and small energy tails of the broad band rare earth PLE changed slightly from sample to sample, it was found that the difference between the two slopes was constant in all samples. Further work is necessary to determine the mechanism responsible for the high energy behavior of the PLE spectra.
5. Summary and conclusions In conclusion, the broad excitation bands of rare earth dopants in chalcogenide glasses show a number of similarities with the PLE of the host glass PL, including a fatiguing effect. These similarities lead to the conclusion that the same defect states which give rise to the properties of the host glass are involved in the energy transfer between the host and the rare earth dopants. We found that proposed mechanisms for the high energy behavior of the PLE do not explain the observed phenomena.
Acknowledgements The authors would like to thank M.C. Delong and the Crystal Growth Laboratory of the Department of Physics, University of Utah, for preparing most of the glasses; Bruce Aitken and Coming, Inc., and Jas
D.A. Turnbull, S. G. Bishop/Journal of Non-Crystalline Solids 223 (1998) 105-113
Sanghera and the Naval Research Laboratory for providing samples; and P.C. Taylor for many helpful discussions. This work has been supported by the NSF under the Engineering Research Centers (ECD89-43166) and the Materials Research Science and Engineering Centers (DMR-89-20538) Programs, and by ARPA under the Center for Optoelectronic Science and Technology (Grant #MDA972-94-1-0004) Program.
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