Photoinduced changes of structure and properties of amorphous chalcogenides

341

Reactivity of Solids, 5 (1988) Ml-349 Elsevier Science Publishers B.V., Amsterdam

- Printed

in The Netherlands

PHOTOINDUCED OF AMORPHOUS

CHANGES OF STRUCTURE CHALCOGENIDES

MILOSLAV

MIROSLAV

FRUMAR,

VLCEK

and JIkI

AND PROPERTIES

KLIKORKA

University of Chemical Technology, Pardubice (Czechoslovakia) (Received

March 3rd, 1987; accepted

January

14th, 1988)

ABSTRACT The photoinduced changes of structure and properties of amorphous chalcogenide layers and powders are discussed. The chemical models of photoinduced effects (bond-redistribution, polymerization, photolytic reactions) are emphasized. The significance of the morphology and of the free volume of amorphous solids for the course of the photoinduced phenomena is illustrated on the amorphous systems Ge-Sb-S and As-Se. The present and potential application of photoinduced phenomena for high resolution optical memories and photoresist production is also mentioned.

INTRODUCTION

Amorphous chalcogenide semiconductors have been studied extensively in the last 20 years because of their large technological and scientific importance [l]. In spite of this, their structure, physical and chemical properties still remain not fully understood. In amorphous chalcogenide glasses the long range order of structure is broken and the physical models of electronic and optical properties based on periodicity of structure cannot be used. But as the short range order is mostly present, the chemical concepts that deal basically with short range interactions (chemical bonds) are very helpful. The S-N rule of covalent bonding is still valid and deviations from optimal bonding can be classified as defects. The positively or negatively charged defects, the density of which is relatively high in any amorphous solid, provide effective traps for photogenerated electrons and holes. The free path of electrons and holes which plays a crucial role in electrophotography is very small and it is also important for the explanation of photoinduced dissolution and diffusion of metals in amorphous semiconductors [2]. Some defects are thermodynamically induced (they are frozen in from the temperature T’) some are introduced during the processing. The former are inevitable, the density of 0168-7336/88/$03.50

0 1988 Elsevier

Science Publishers

B.V.

342

the latter can be at least partly lowered by annealing of the glass, by alloying, or change of processing. The possibility of alloying amorphous semiconductors in large concentration limits and the lack of periodicity, which are the common features of all amorphous solids, allow the properties of these solids to be changed more broadly than is common in crystals. The amorphous materials are thermodynamically metastable and depending on their composition and prehistory (e.g. rate of quenching, irradiation) they can exist in several metastable states. Transition among these states can be induced e.g. by light, heat, pressure and is very often accompanied by a change in physical and chemical properties of the material. Using these changes, the amorphous solids can be used as sensors and memory materials. In this paper we will confine our discussion to the photoinduced effects in amorphous chalcogenides, materials which are not only interesting from the chemical and physical point of view, but also technologically perspective; some of them have already been used as photoresists and optical memory materials with very high resolution.

PHOTOINDUCED

EFFECTS

Irradiation of amorphous chalcogenides by actinic (band gap) light induces changes in physicochemical properties of amorphous chalcogenides such as optical transmittance (Fig. l), optical reflectance, microhardness, index of refraction, solubility of materials in alkaline solutions etc. The intensive irradiation can induce crystallization or melting of material as well. All these changes reflect the change of the inner structure of the material. The crystallization and melting are relatively well understood and both are already applied for production of optical memories. More complex and less understood are the so-called photostructural effects in which the irradiation

500

550

600

A(nml

Fig. 1. Spectral dependence of optical transmittance changes of Ge,,Se,, thin film (d 1 pm). a, layer as evaporated; b, exposed layer; c, annealed layer (180 o C, 1 h); d, layer exposed after annealing. Exposition: 20 min by W lamp (0.4 W/cm’, IR cut filter).

343

4000 z ci B :- 2000

0

200 400 FREQUENCY tcmi’ 1

Fig. 2. Raman spectra of As,& thin films [9]. a, layer as evaporated; b. annealed layer (180 o C, 12 h). The main band ( = 340 cm-‘) corresponds to the stretching vibrations of ASS, pyramids; the narrow sharp bands in the region 110-250 cm-’ are due to unpolymerized “molecular” species of freshly evaporated layers. Intensity in arbitrary units.

induces the changes of physical and chemical properties without any crystallization or melting of the sample. Such effects have been found in many chalcogenides and even chalcogens [l-8]. For the explanation of photostructural effects several ideas were proposed [3-81. Very often, the optically induced redistribution of chemical bonds is supposed where both the strong covalent bonds and weaker “intermolecular” or Van der Waals bonds are considered. In amorphous chalcogenides, due to the presence of non-bonding orbitals of chalcogens and pnictides, electrons of which can be transferred into bonding orbitals, the photoinduced changes of bond-redistribution are relatively easy. The flexibility of the amorphous structure due to larger free volume and also due to the presence of twice-bonded chalcogen atoms also makes the photoinduced changes in the inner structure of amorphous semiconductors easier than in crystalline compounds. In freshly evaporated layers of chalcogenides, which are mostly chemically inhomogeneous due to their dissociation during evaporation, the actinic light induces some polymerization (homogenization) and the thermodynamic state of the system is shifted by light towards equilibrium. Similar homogenization can be induced by heating the layers as evaporated to a temperature near the glass transition temperature Tg (Fig. 2) [9]. The irradiation also changes the density (distribution) of chemical bonds in well-annealed homogeneous amorphous materials and e.g. in As-S system glasses or layers it increases the density of 1As-As ]and IS-S ]bonds as was confirmed by Raman spectroscopy (Fig. 3) [9,11] or by chemical analysis of exposed glass [12]. Such a change can be described by a photoinduced (or photolytic) reaction hv, I,

]As-As 2 ]As-S ( 1 huz, I,; A(T)

( + (S-S ]

(I)

0

200

400

FREQUENCY

icti’)

Fig. 3. Raman spectra of As,S, thin films [9]. a, annealed film; b,c,d, are the spectra excited using the wavelengths 568.2, 530.9 and 488.0 nm, respectively. The band at 231 cm-’ corresponds to the As-As vibrations, the narrow band at 187 cm-’ can be connected with the presence of As,& molecules. Intensity in arbitrary units.

or more generally, hv,.

in MX, chalcogenides

I,

21M-XI,-

IM-M)+IX-XI

(2)

hvz, I,, A(r)

where X is chalcogen, M is metal or semimetal, hv,, 1, are the energy and intensity of actinic light and A(T) stands for annealing at temperature T. The photoinduced bond redistribution which corresponds to the photochemical reactions (eqs. 1, 2) does not involve all atoms (bonds) but a relatively small part of them (e.g. - 5% in As,& [9]) and thus the amorphous structure of exposed material is preserved. The densities of individual chemical bonds in amorphous chalcogenides can be described by the so-called random covalent network model (RCNM) or chemically ordered network model (CONM) or by their combination. In RCNM, the atoms are statistically distributed and no chemical bond formation is preferred, contrary to CONM in which the formation of bonds between chalcogen and metal or metalloid (e.g. As-S) is strongly preferred to M-M or X-X bond formation. The state of well-annealed chalcogenides is close to the CONM. By irradiation the equilibrium of eqs. 1, 2 is shifted slightly towards the state described by RCNM (Fig. 4, dotted lines). The new bonds between like atoms (= identical atoms) remain obviously embedded in the glassy matrix. The irradiation of the amorphous material can also change the coordination of individual atoms and some microscopic parts with overcoordination and undercoordination or with increased and decreased mean valency of the central atom are formed, e.g. according to eq. 3: IlUl. I,

2 MX,,.MX,+,,

+ MX,_,,

(3)

hu?. I,. A(T)

which may correspond

to the disproportion

of the compound.

345

0.2

0.4

0.6

Fig. 4. Compositional

08 x dependence of bond densities in the amorphous As, _,S, system.

It was found from EXAFS experiments [13] that the mean coordination number of germanium in amorphous GeS, and in GeSe, is increased after intensive illumination while the chalcogen decreases its mean coordination number. This behaviour can be described by eq. 3. In GeS, glasses some microscopic parts of glass will be disproportionated towards the GeS structure, the other part possesses the excess of sulphur. The GeS crystallizes in a distorted rock salt lattice [14] and hence the coordination of Ge in crystalline GeS is close to 6. Coordination of Ge in GeS,,, glasses remains at 4 as the excess sulphur forms -S,chains. The slight increase in coordination number of Ge upon illumination means that probably the structure of some microscopic parts of GeS, becomes closer to the GeS structure and some parts have higher sulphur content. The amorphous structure of irradiated glasses is preserved and only a small part of the Me-X bonds is influenced (e.g. in As-S glasses - 5% [9]). The right-hand side of eqs. l-3 represents the state which is further from the thermodynamic equilibrium and the annealing of the material at temperatures (T> T,), when the atoms become mobile, causes the shift of the reaction back, to the left-hand side. As the activation energy of reaction 1 is only AE = 0.8 eV [15], the thermal annealing proceeds slowly even at room temperature as was already found in ref. 9. The equilibrium of the reactions l-3 can be shifted by the change in wavelength and intensity of actinic light as well.

FREE VOLUME INFLUENCE

In the above-mentioned processes the redistribution of chemical bonds requires the change of some atomic positions. Such changes could proceed more easily in materials with larger free volume which is the case of amorphous solids being generally of lower density than the corresponding crystals. The “compactness” of the structure is proportional to the parameter S 6 = P CAixi/Pj [ I

-

CAixi/P i

1

/CAixi i

= (C

K i

KXp)/V,,p

(4)

346

20

40

60

80 Asiat%l

Fig. 5. Compositional dependence of density (a), volume per atom (b), parameter S (c), photoinduced change of refraction index (d), and photoinduced change of absorption edge (e) in thin layers of the As-Se system (161. 1, is for layers as evaporated and 2, for annealed layers.

where Ai, xi, p, stand for atomic weight, atomic fraction and density of the i-th element of the glass, respectively. V, corresponds to the volume of the i-th component in the elemental form and I&, stands for experimentally found volume of the whole sample. The value of S is different in glasses of different composition and even in amorphous solids of one particular system it has a minimum near a particular composition. In such an area the “compactness” of the glass is the lowest and due to the large free volume, the photoinduced redistribution of chemical bonds and atom positions occurs relatively easily. This suggestion was confirmed in papers [16-181 where the compositional dependence of the intensity of photoinduced optical changes followed the curve of compositional dependence of 6 (Fig. 5). We assume that both factors (redistribution of chemical bonds and compactness of the structure) control the photosensitivity of the samples studied. In the first approximation their common effect can be expressed by the product ‘r) = (M-S 16, where ) M-S ) are the densities of bonds which might be changed by incident light. These densities were calculated as given in [18,19]. A good agreement was found between the compositional dependences of the parameter w and the photoinduced optical transmittance change AT (Figs. 6 and 7). In contrast to thin layers, the bulk glasses of the same composition have a much lower sensitivity to actinic radiation, but after being powdered or evaporated they can produce relatively large photoinduced changes. The evaporated layers generally have a lower density, in powdered glasses many

347

0

10

20

30

x

4o

Fig. 6. Compositional dependences of the parameter w and of optically induced AT change of thin layers as well as diffuse reflectance AR of powdered glassy samples in the Ge,Sb,,_,&, system.

internal defects and voids are created by crushing and thus the free volume is effectively increased. The compositional dependence of the change in diffuse reflectance of powdered glasses follows again very well the dependence of the parameter w on the composition (Figs. 6 and 7) of the glass. In some papers [9,11,20] not only photoinduced redistribution of chemical bonds but also formation of amorphous As clusters in As chalcogenides was suggested. In such cases the rupture of As-X bonds is followed by migration of As atoms which is again enhanced by a free volume increase. The kinetics of photodarkening is determined by diffusion of As defects [20]. The clustering of As can also explain the composition dependence of photodarkening in As-S glasses which is enhanced in As-rich glasses where the probability of finding several neighbouring As atoms is higher. The mechanism of breaking some chemical bonds in amorphous chalcogenides can happen in one single step or via an intermediate less stable local defect which can be a self-trapped exciton [3-71 (Fig. 8). The above-discussed mechanism dealt with photoinduced changes of short-range order in amorphous materials. The amorphous solids are in-

or

01

Fig. 7. Compositional dependences of the parameter w and of optically induced AT change of thin layers as well as diffuse reflectance AR of powdered glassy samples in the GexSbIOSs,,,_, system.

348

0

pnctogen

l

chalcogen

Fig. 8. Model of random covalent network (RCNM) structural rearrangement from chemically-ordered network (CONM) configuration which proceeds directly or via a self-trapped exciton in (STE) state.

organic polymers formed by chains, layers or 3D structures and obviously the extent of polymerization can be changed by illumination not only in freshly evaporated but also in annealed glasses (Fig. 3). The actinic light also changes the microhardness, the viscosity flow, glass transition temperature and the solubility of the glasses in alkaline solutions [3-81 and therefore it can be supposed that the intermediate range order or degree of polymerization also play an important role. Griffiths [21] discovered reversible changes in the structure of GeSe, glasses under illumination in which several stages of ordering including microcrystallites were observed. According to ref. 21 small ordered units are formed and they can be reversibly polymerized into 3D units. The photoinduced phenomena also include the photoenhanced dissolution and diffusion of metals (mainly Ag, Cu) into amorphous layers. The diffusion of Ag+ ions proceeds only in the direction of incident light and very sharp patterns with submicron details can be obtained. It was suggested [2] that the extremely small free path of electrons and holes and the larger mobility of Ag+ ions in amorphous chalcogenides are crucial for the explanation of the effect. The above-mentioned photostructural effects are already applied in optidissolution and cal memories production. The effect of photoinduced diffusion of metals into amorphous chalcogenides has been applied in the preparation of inorganic photoresists for very-large-scale integrated circuits production [5,6,22].

CONCLUSION

The band gap irradiation induces several effects and changes the structure as well as physical and chemical properties of the material. We have tried to describe and explain those of them which are connected with photoinduced chemical changes and reactions.

349 ACKNOWLEDGEMENT

The authors thank the editors of the journal Philosophical Magazine (Taylors and Francis Ltd.) for permission to reprint some figures published in the Philosophical Magazine.

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