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Infrared absorption of Ag- and Cu-photodoped chalcogenide films
JOURNAL OF
!1$411 I,I M ELSEVIER
Journal of Non-Crystalline Solids 202 (1996) 113-121
Infrared absorption of Ag- and Cu-photodoped chalcogenide films A.I. Stetsun ,,., I.Z. Indutnyi a, V.G. Kravets b Institute of Semiconductor Physics, prospekt Nauki 45, 252028 Kiet'-28, Ukraine b Institute oflnJbrmation Storage Problems, ul. Shpaka 2, 252113 KieL'-113, Ukraine
Received 17 July 1995;revised 28 November 1995
Abstract Infrared reflectance and transmittance spectra of chalcogenide glass layers (As2S 3, GeS 2, GeSe2), non-doped and photodoped by Ag and Cu, were measured. The spectra were analyzed in terms of a molecular model for optic-mode vibrational frequencies. With Ag photodoping of As2S 3 the main band (312 cm-~) shifts to higher frequencies. The area under the ~oe2(w) contour is unchanged (ez-imaginary part of the dielectric permeability). The ratio of areas under such contours for photodoped layers to undoped ones is 0.87 ___0.3 for GeS 2 and 0.80 _+ 0.03 for GeSe2. In the photodoped As2S 3 and GeS 2 layer spectra a broad weak band at low frequencies (200 cm -~) is observed. This band is affected by the presence of metal-chalcogen bonds. Variations in the IR-spectra with photodoping by Ag or Cu are consistent with the supposition about normal covalent and coordinative bond formation between metal additives and chalcogen atoms. It results in the creation of the two types of impurity centers.
1. Introduction The chalcogenide glasses (ChG) have attracted attention of many investigators as a media of which properties (structure, optical, electrical, chemical and so on) are changed when irradiated by electromagnetic waves, electron or ion beams. One of the most interesting radiation-induced phenomenon is the photostimulated diffusion of Ag or Cu into ChG in the thin-films ChG-metal structures (photodoping effect) [1]. The essence of the phenomenon is that, under illumination of the C h G - A g (Cu) structure by light with energy greater than the chalcogenide band gap, the metal dissolves into the ChG film. The intermediate photodoped layer, thus formed, has physical
Corresponding author. Tel.: + 380-44265 6342; fax: + 380-44 265 8342; e-maih [email protected].
and chemical properties essentially different from non-doped ChG properties. On the basis of the photodoping effect, high-resolution registering media have been created and have found applications in optical memories, lithography, holography, and others. A large number of scientific papers have been devoted to investigation of the photodoping mechanism, the basic parameters of which define the photostimulated metal flow [2]. However, peculiarities of the doping metal atoms interaction with the glass network and local structure of the impurity centers are less well investigated. The structure of photodoped ChG layers were investigated by using grazing-incidence X-ray scattering, extended X-ray-absorption fine-structure, and others [3-5]. There are some difficulties in studying these systems by direct structural X-ray techniques. The photodoped film thickness does not exceed 1-3 ~ m so that the measured signal from the film is both
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A.I. Stetsun et al. / Journal of Non-Co,stalline Solids 202 (1996) 113-121
weak and overwhelmed by the scattering from the rest of the sample (including the substrate). Besides this effect the stimulated structural changes that occur in the photodoped layers under exposure to X-ray irradiation hinders the use of powerful synchrotron beams. Very useful information about the amorphous layers' structure and peculiarities of the impurity atoms chemical bonding can be obtained from Raman and infrared (IR) spectroscopy. The optical vibrational frequencies are determined by vibrations of the amorphous matrix molecular units [6], and integral intensities of absorption bands depend on oscillator concentration and strengths [7]. Variation of these parameters with chalcogenide photodoping can indicate a change in the sample structure and formation of new bonds when metal atoms are embedded into ChG. The Raman spectra of photodoped chalcogenide layers were studied earlier [8,9]. But detailed quantitative analysis of these spectra are hindered due to the long-wavelength shift of the interband absorption edge with photodoping. This shift results in a substantial increase of excitation light absorption in comparison with non-doped layers and a decrease of Raman spectra intensities. The IR-spectra of photodoped ChG layers have been studied on an As33S67 film only [10,11]. It was determined [10] that the main IR-absorption band (310 cm ~ in undoped films) shifts to 340 cm -~ with photodoping by Ag, but more detailed investigations and data analysis have not been performed. In this paper the investigation of As- and Gechalcogenides (AszS3, GeS 2, GeSe 2) IR-spectra, and their modifications with photodoping by Ag and Cu are presented and treated. The peculiarities of the metal-chalcogenide bonds formation are discussed.
2. Experimental techniques The samples for the measurements were prepared by thermal vacuum evaporation at a pressure 10 -3 Pa and sequential metal (Ag or Cu) and ChG deposition onto silica substrates (the total two-layer thickness was 100-400 nm). After deposition of two layers the sample was exposed to light in order to photodissolve and uniformly spread the metal in the ChG film; then the next two layers (metal and ChG)
were deposited on the sample and the process was repeated. This method made it possible to produce rather homogeneous samples of 1-3 i,zm thicknesses. The sample thicknesses were measured by an optical multiple interference fringe method and by ellipsometry. Besides the photodoped sample, an undoped control sample with the same ChG content was prepared simultaneously. The thickness of the photodoped layer, dp, relates to the thickness of the undoped layer, d u, by the expression dp= du + d m, where d m is the total thickness of the photodissolved metal layers. This means that the ChG matrix 'swelling' is approximately equal to the photodissolved metal volume, and the concentration of ChG molecular units, Nu, decreases under photodoping (to Np) proportionally with layer thickness increasing:
d. -
Nu
(1)
dr
For exposure of the metal-As: S 3 and -GeSe: structures an incandescent lamp was used, but for metalGeS 2 a mercury high-pressure lamp was utilized. During deposition the sample thickness was controlled by a quartz crystal oscillator monitoring system (KIT-I). The concentrations of the metal in the photodoped samples were determined by the deposited metal and ChG mass proportion. The interval of metal concentrations in the investigated samples were chosen with account of the photodoping peculiarities in such structures. The metal concentration profile that was observed in the ChG-metal structures under photodoping is step-like with a well defined interface between doped and undoped material. The metal concentration in the doped layer remains constant under exposure until complete dissolution of the metal layer occurs, and a constant concentration value is characteristic for each structure [2]. For As 2S3-Ag this value (frequently called the 'optimal' concentration) is about 32 at.%, for GeS 2 16 at.%; and for GeSe 2 16.7 at.% [12]. The composition of investigated layers was determined by a microprobe analyzer (JCXA-733). The content of each element in the samples corresponds to their chemical formula with accuracy up to 0.5%. The transmittance and reflection spectra in the region 50 to 500 cm-I were measured by a Fourier spectrophotometer (LAFS-1000) with a spectral reso-
A.I. Stetsun et al. / Journal of Non-Cry'stalline Solids 202 (1996) 113-121
lution ~ 3 cm ~. The optical constants (indexes of refraction, n, and absorption, k) of the investigated layers were calculated by using the measured values of the normal incidence transmittance T, the air-incident R and substrate-incident R' reflectances, the sample thickness, and the optical constants of the silica substrate [13]. The theoretical dependencies of R and T on n and k are complex which complicates the calculations and yields multi-valued solutions. For simplicity we used the functions S = (1 + R)/T and Q = ( 1 - R)/T which more smoothly depend on n and k [14] and are more convenient for calculation of the optical constants. The results obtained are expressed in the form of the imaginary part of the complex dielectric permeability e 2 = 2nk.
115
2
0.4
02
0.5
05." 0.t
I 100
, [ I 1 - 200 300 400 500 WAVE N'UMILER Corn-I)
Fig. 2. Spectra of infrared transmittance, T. of GeS 2 (1) and GeS2 Ago 6 (2) layers.
3. Experimental results Fig. l shows the IR-transmission spectra of the undoped and photodoped As2S 3 layers. The reflec0.60.4-
0.20.5
0.3-~
3
02-
~r"
-
-
03ffI-
0"5I 0.3
0.1
I
tO0
I
1
[
200 300 400 WAVE NU~I~E'R (cm-'~)
1
500
Fig. I. Spectra of infrared transmittance, T, of As2S 3 (l), As2S3Ago.9~ (2), As2S3Ag2. 4 (3), As2S 2 (4) and As2SsCuI.25 (5) layers. The initial thickness of ChG in all samples was 2.25 /a,rn.
tion spectra are not presented, but they were used in the optical constant calculations. For the undoped (control) As2S 3 layer (curve 1) a distinct IR-transmission minimum occurs at 312 cm-~; on the highfrequency side of this band an inflection at 372 c m - ] is observed. With increasing Ag concentration the main transmission band shifts to 325 cm-~ for an As2S3Ag0.95 layer (16 at.%, curve 2), and to 345 cm -~ for an As2S3Ag2. 4 layer (32 at.% Ag, curve 3). The inflection at 372 cm ] is shown for all samples and its location is basically unchanged. In the spectrum of the most heavily doped layer (As2S3Ag2. 4) a weak broad band at 200 cm -j is detected. This band was not observed at lower Ag concentration. There is a rather high content of As4S 4 molecular units in thermally deposited As2S 3 films [15]. The As4S 4 molecule vibrations are displayed in the IRspectra of these films also. For comparison, the spectrum of the As2S 2 film, deposited under the same conditions as samples 1-3, is shown in Fig. 1 (curve 4). This spectrum consists of two distinct transmission bands at 343 cm-~ and 372 cm -t , and a weaker band at 225 c m - ] . In the spectrum of the evaporated As2S 3 layer the A s 4 S 4 molecule frequency band at 343 cm ~ is overlapped by the main
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A.I. Stetsun et al. / Journal of Non-Cm,staUine Solids 202 (1996) 113-121
As2S 3 band and is not resolved. A more intense band at 372 c m - ] appears as an inflection on the high frequency side of the main As2S 3 band. The spectral position and the intensity of the inflection at 372 cm-~ remain unchanged under photodoping. It corroborates the slight interaction of the As4S 4 molecules with Ag. Fig. 1 (curve 5) contains plots of IR transmission spectrum of As 2S 3 sample photodoped by Cu (the metal concentration is 20 at.%). The position of the main transmission band is not changed with Cu photodissolution, but on the high frequency side of this band a slight additional band has appeared. For GeS 2 layer the structure of the IR-transmission spectrum consists of a distinct minimum, located at 370 cm -~, and two inflections, at 327 and 425 cm -~ (Fig. 2, curve 1). The weaker band is located at lower frequencies near 150 cm ~. For the photodoped layer GeS2Ag0. 6 the position of the main band is the same as for the undoped layer, but the high frequency side of the band is more convex (Fig. 2, curve 2). A broad and weak band are observed at 200 cm ~, similar to that in As2S3Ag24. The IR transmission spectrum of the GeSe 2 layer (Fig. 3, curve 1) consists of the main band at 252 cm -~ and weaker bands at 310 cm L, 100 cm -I 200 cm -~. The spectra of the layers, photodoped by Ag to 20 at.% metal concentration, Fig. 3, curve 2
0,6
3
I
,~aooo 3 z
tooo
t00
2OO WAYS
'~0 NU~iSE~ (cm -~)
400
Fig. 4. Infrared response for As2S 3 ( | ) and As2S3Ag2. 4 (2). The spectra are decomposed for a comparison of the main band intensity.
and Cu curve 3 have the same band frequencies and intensities. The shift of the main band with photodoping is not observed, but the location of the weak high frequency band changes from 310 cm -~ to 285 cm -~. The low frequency band at 100 c m disappears as well as the band at 150 cm ~ for GeS 2. In order to examine quantitatively the spectra obtained we calculated the optical constants by using measured spectra of T and R. The results are expressed as a frequency function we2(to), because this quantity is proportional to the product of vibration state density, g(to), and oscillator strength, f(to), which takes the frequency dependence of the matrix element into account [16]: 2,rr 2Ne 2 toe2(to) = ~ . f ( t o )
g(w).
(2)
0.4 0.2
2
0.4
Here, to is the frequency, M is the oscillator mass and N is the number of oscillators per unit volume. If f(~o) is a slowly varying function of the frequency, then Eq. (2) gives a good approximation of the vibration state density spectrum. The integral of Eq. (2) will be proportional to the number of oscillators per unit volume (in our case to the density of glass molecular units) [7]: 2 "rr2Ne * 2
0.2 I
I
L
WAVE NUME,tR
[
f,
1"O8'2(O)) 809
m
'
(3)
(cm -1 )
Fig. 3. Spectra of infrared transmittance, T, of GeSe 2 (l), GeS% Ag0.7.s (2) and GeSe~Cu0.75 (3).
where e * is the effective charge, which can change under photodoping. If Eq. (2) is integrated over the
A.L Stetsun et al. / Journal of Non-Cr3,stalline Solids 202 (1996) 113-121
photodoping of As2S 3 and GeS 2 layers. In As2S3, spectrum redistribution displays as maximum shift. In GeS 2, photodoping results in a more convex high frequency wing of the main band. (iii) In the spectra of the As2S3Ag2. 4 and GeS2Ago. 6 layers, broad weak bands at low frequencies (200 cm ~) are observed. It may be that these bands are caused by the metal-chalcogen chemical bonds.
3 '.~ 3000
i
3
117
I
2000 A' t',
~IO00 ..=
z_
100
2O0
1100 400 WAVE NU~41IER(cm-I)
500
Fig. 5. Infrared response for GeS 2 (l) and GeS 2Ago.6 (2).
main IR-band spectral interval, then e* is the mode effective charge. Figs. 4 - 6 contain plots of we2(w) undoped (curve 1) and Ag pbotodoped (curve 2) As2S 3, GeS 2 and GeSe 2 layers. Analysis of these dependencies allows us to summarize the main features of spectrum modification with photodoping. (i) The area under we2(w) contour in the main 1R frequency band spectral region is practically unchanged when As2S 3 is photodoped by Ag. For Ge-chalcogenides the photodoped layer to undoped layer area ratio is 0.87 +_ 0.3 for GeS 2 and 0.80 +_ 0.3 for GeSe~. (ii) In the spectral region of the main IR frequency band, a vibrational state density redistribution toward higher frequencies is observed with
4. D i s c u s s i o n
According to the molecular model developed by Lucovsky and Martin [6,17] the vibrational modes of ChG are associated with the vibrations of the glass molecular units. The applicability of such a model is based on a weak coupling between the molecular units in comparison with the intramolecular bond forces. The main structural unit for arsenic sulfide is the pyramidal ASS3/2 molecule. Neighboring units are bound through the bridging twofold coordinated sulphur atoms. It has been shown, by valence field model calculations and by other estimations [ 17], that the main band in As2S 3 infrared absorption spectrum (312 cm -~, Fig. 1) is associated with ~'3(E) asymmetrical bond-stretching vibrations of these pyramids. The symmetrical bond-stretching vibration u~(A) displays lesser activity in an infrared spectrum and provides asymmetrical spreading of the main band into the high frequency region. The value of As-S bond-stretching force constants, k r, used in valence field model calculations is estimated in accordance with Gordy's rule [18]:
kr = aN( --U-f KAKB ]3/4 +b,
3
(4)
3 ~2000 Q Z
~0o
400
WAVE NUMI~:R ('¢rn-l)
I
~0
Fig. 6. Infrared response for GeSe 2 (l) and GeSe 2Ag0.75 (2).
where a and b are empirical constants, N is an order of the bond, KA and KB are electronegativities, in this case of arsenic and sulphur atoms, and d is the bond length. The main structural units of germanium chalcogenides are GeX~/2 (X = S, Se) tetrahedrons. Table 1 contains values of the optic-mode frequencies of these molecules, obtained in previous investigations and this work. Almost all modes of GeX~/2 tetrahedrons are active in infrared spectra due to inter-
A.I. Stetsun et al./Journal of Non-Cry'stalline Solids 202 (1996) 113-121
118
Table 1 The frequencies of the fundamental modes of germanium chalcogenide display activity in an infrared spectrum Material
Frequency ( c m - t ) v ~,(F2)
v4(F2 )
GeS 2
328 327
-
367,409 370, 425
149 150
[19] this work
GeSe
200 200
80 -
255 258,310 252, 310
100 115 100
[20] [21] this work
v I(A 1)
v2(E)
References
molecular interaction. The existence of such an interaction also leads to the splitting of the v3(F2) mode band. The bond-stretching mode v3(F2) and vl(A) frequencies are situated in the high frequency region of the spectrum. The low frequency bond-bending v4(F2) mode is damped with photodoping of germanium chalcogenides by the metals (Figs. 5 and 6). As shown below, this experimental fact may be explained by the concrete space configuration of chemical bonds between metal additive and structural elements of the glass. The special features of the chemical bond formation by metal atoms introduced as an additive into the ChG matrix were predicted by Kastner [22]. According to this theory an atom of monovalent metal being introduced into the melted ChG forms one normal covalent bond and a few (usually three) coordinative bonds. This configuration of chemical bonds corresponds to that type of center in which the metal atom is bound as a neutral fourfold coordinated additive. The normal covalent bond is created by a union of a metal valence electron and a chalcogen dangling bond electron. An electrons lone pair of a chalcogen atom and the empty orbital on the metal atom are utilized for the formation of a coordinative bond. The center in which metal additive is bound as ions by the four coordinative bonds is created by dissociation of the normal covalent bond and subsequent replacing by a coordinative bond. The concentration ratio of the two types of centers is determined by the law of mass action and an entropy factor [22]. The low-temperature introduction of metal into ChG by means of photodoping differs from doping in the melt, where the structure of new compounds depends on the chemical equilibrium of the glass component. In the case of photodoping the matrix of
ChG has been formed and fixed already. Therefore the possibility for creation of chemical bonds by metal atoms is determined beforehand and depends on the structure of this matrix. That is why the chemical bonds between metal impurities and the ChG matrix are formed preferentially as coordinative bonds. The presence of coordinative metal-chalcogen bonds in photodoped ChG layers was proved experimentally by photoemission spectroscopy [5] and grazing-incidence scattering [3]. The results of investigations of interband transition spectra for A s 2 S 3 and GeS 2 photodoped by Ag corroborate the chemical bond formation between chalcogen and metal atoms by participation of chalcogen lone pair electrons, too [23,24]. The peculiarities of photodoped ChG layer infrared spectra which we observed in this work also agree well with the assumption about the coordinative and normal covalent metal-chalcogen bonds formation. The lone pair electrons of chalcogen do not take part in the creation of the chemical bonds of glass structural units (As-S f o r A S S 3 / 2 pyramids or G e - X for GeX4/2 tetrahedrons). Therefore the formation of metal-chalcogen coordinative bonds does not lead to destruction of the ChG matrix units. Some fraction of the metal atoms form normal covalent metal-chalcogen bonds in the sites of dangling bonds. However, the number of such defects in CbG layers is significantly smaller than the quantity of metal atoms at the optimal photodoping concentration. Consequently, the formation of coordinative bonds play a dominant role in the photodoping phenomena. The creation of these two types of bonds is consistent with conservation of the main band integrated intensity in infrared spectra (Fig. 4). The wide and weak bands near 200 cm-1 in the As~ $3 Ag24 and GeS2 Ag0.6 layers spectra are assigned to the bond-stretching motion of the coordinative and normal covalent metal-chalcogen bonds. Some difference in the spectral position of this band from the Ag2S absorption band (180 cm -~) [25] is caused by the above mentioned specific features of these bonds. The creation of coordinative bonds causes the redistribution of lone-pair electron charge density from chalcogen atoms to metal ones. This brings about an increase of chalcogen effective electronega-
A.I. Stetsun et al./ Journal of Non-Co,stalline Solids 202 (1996) 113-121
tivity and, as a result, an increase of the stretching A s - S and G e - S force constants in accordance with Gordy and Samayajulu's [19] rules. From such an approach, it follows that the increase of the bond-stretching force constants is a consequence of the increasing ionicity of these bonds, since an electronegativity difference determines them. Such force constant growth under photodoping manifests itself in the arsenic sulphide IR (Fig. 1) and Raman spectra, as a high frequency shift of the main band. According to our measurements [26] the main band of the Raman spectra shifts from 344 c m - ~ for As2S 3 to 372 cm -1 for As2S3Ag2.4 . For germanium sulphide, as mentioned above, the position of the main band does not change, but the redistribution of the phonon state density results in a more convex high frequency wing of this band for the photodoped layer (Fig. 2). The increasing of the v I and v 3 mode frequencies with heavy Ag photodoping allows us to suggest that coordinative bonds are weak and some types of structural unit vibrations may be considered as independent from these bonds. Proof of this vibration isolation may also be found in certain space bond configurations of photodoped ChG. For the ChG considered in this paper, the angle between bonds of neighboring structural units at the bridging chalcogen atom is 90-136 ° [27]. In accordance with the theory of valence shell electron pairs repulsion [28] the angle between the coordinative bond and GhG matrix bond at this bridging atom will be close to 90 ° . Therefore the creation of coordinative bonds does not affect the bond-stretching vibrations, but suppresses bond-bending vibrations. These features are observed experimentally. For all investigated materials the frequency of v I and v 3 vibrations either remain unchanged or increase. At the same time the bands of v4 vibrations in the spectra of GeS 2 and GeSe~ photodoped layers are damped (Figs. 5 and 6). Some quantitative estimations of the photodoped layer structure may be made from comparison of spectra integrated intensities (the areas under we,,(~o) contours in the region of main bands) for the photodoped and the corresponding undoped layer. In accordance with Eq. (1) the concentration of ASS3/2 structural units decreases to 82% of their original value in As2S 3 film with arsenic sulfide photodop-
119
ing by Ag to the optimal Ag concentration (32 at.%). The smaller number of oscillators can provide the same area band contour by means of increasing their effective charge value in accordance with Eq. (3). Such estimation demonstrates, that for As~S3Ag2. 4 the effective charge increases 10.5 _+ 3.5% in comparison with As2S 3. Since v I and u 3 modes of ASS3/2 pyramidal units are relatively independent on Ag additives, the dynamical contribution to the effective charge of these modes for photodoped and undoped As2S 3 layers is the same. Consequently the increase of the effective charge has a mostly static nature, i.e. it is conditioned by an increase of glass matrix bond ionicities. Such an increase of ChG matrix bond ionicity with coordinative bond creation is connected with an ion-dipole mechanism of that bond formation. This mechanism includes the interaction between the ion and chalcogen dipole moment conditioned by the space distribution of the lone pair density, as well as the interaction ion and induced dipole moment on the semiconductor matrix bond. Given the peculiarities of the spectra of silver photodoped As 2S 3 layers, an interpretation was made using the force constant formalism. The splitting of twice degenerated ~'3(E) vibration is observed in the spectrum of the As2S 3 layer photodoped by copper (Fig. 1, curve 5). This splitting shows the lowering of ASS3/2 pyramidal units symmetry from C3,. to C, due to formation of S - C u coordinative bonds. The difference between spectra of A s ; S 3 layers photodoped by Ag and Cu is probably connected with a greater strength of the S - C u coordinative bond and the smaller mass of the Cu atom. This leads to a more significant influence of metal additives on the vibrational activity of the structural units in the glass matrix. The suggestion regarding the strength difference of these bonds agrees with observations of higher ionic conductivity in ChG with a structure similar to As2S 3 doped by silver rather than copper [29]. The formation of M e - C h G bonds in photodoped layers depends on the structure of the initial undoped ChG layers. An as-deposited As2S 3 layer contains a significant number of defects, including dangling bonds of S atoms. The concentration of these broken bonds exceeds 7% [15]. When a relatively small concentration of metal, for example Ag, photodis-
120
A.l. Stetsun et al. / Journal of Non-Crystalline Solids 202 (1996) 113-121
\/
\/
2\
/\
a
b
Fig. 7. Schematic representation of the chemical bond formation by silver additive in a photodoped ChG layer. Normal covalent bonds are denoted by straight lines, dots represent the coordinative bonds. The asterisk show an electron localized near the Ag+-ion. (a) The bonding configuration for the Ag additive at the site of the amorphous network defect. This configuration is the same as for doping in the melt [22]. (b) The impurity center at the undefective site of the amorphous network where the metal is bound by coordinative bonds only.
solves, silver binds itself with such onefold coordinated S atoms, forming the center where metal appears as neutral additive (Fig. 7(a)). The concentration of these centers in photodoped As2S 3 layers is determined by the concentration of defects in the ChG matrix. After filling the defect sites the photodiffused metal is bound by coordinative bonds only (Fig. 7(b)). This type of center is created in a natural way by diffusion of the metal ion to the sites of lone pair electrons. This type of center differs from those in a melt, where similar center is formed as a result of normal covalent bond dissociation in the first type of center (Fig. 7(a)), resulting in the creation of a C i defect and subsequent substitution of a normal covalent bond by a coordinative one [22]. Such structure of the second type center is supported by a higher conductivity of Ag photodoped than undoped As2 $3 layers. When the concentration of metal become greater than the number of defects and chalcogen lone pair electrons divided by four, the coordinative number of additive atoms decreases significantly. This is consistent with experimental data about a sharply increasing ionic conductivity in photodoped As2S 3 layers with an Ag concentration of more than 20 at.% [30]. In the spectra of photodoped germanium chalcogenide the main maximum of absorption does not shift. Only a small redistribution of phonon state density in the form of a more convex high frequency
wing of the absorption band at 370 cm -~ in the GeS 2 spectra is observed. Therefore we can ignore the change in the e * value in the photodoped layers and estimate the number of structural units destroyed as a the result of metal additive introduction according to Eq. (3) and (1). The photodoping of GeS 2 with silver at concentrations of 16 at.% results in the destruction of 6_+ 3.5% of molecular units. In the GeSe 2 layer photodoped up to 20 at.% by silver 12 _+ 3.5% of the initial number of GeSe tetrahedrons are destroyed. The demolishing of structural units in germanium chalcogenide with photodoping and their conservation in arsenic sulfide are obviously conditioned by the more rigid structure of the GeSe 2 and GeS 2 amorphous network. The introduction of a significant number of metal ions in the ChG matrix causes the expansion of the semiconductor matrix volume and the local stretching. In As2S 3 layers which have a lower coordination and more flexible structure, the stress relaxation does not lead to the breakage of a perceptible amount of A s - S bonds even under higher concentration of doping metal. The structural network of germanium chalcogenide is more durable. It is formed by G e X 4 / 2 tetrahedron chains linked to one another via common angle-shared chalcogen atoms. These chains are joined by transversal bonds which form tetrahedrons coordinated through the common edge [31]. In the A s 2 S 3 s t r u c t u r e these bonds are absent, and the chains o f ASS3/2 pyramids interact weakly through Van der Waal's like forces. Therefore, unlike A s 2 S 3 , s o m e part of GeS% and GeS 2 bonds are destroyed during the photodoping. Being created in such a way, new dangling bonds take part in the formation of the first type centers also. The number of new dangling bonds depend on volume expansion and tension of the glass matrix under introduction of a large quantity of the metal additive. Consequently the concentration ratio of two types of centers in a photodoped chalcogenide germanium layer will be determined not only by the initial number of defects but also by the dynamical equilibrium between the stretching of the amorphous network and the electrical forces stimulating the metal diffusion. In accordance with the molecular model [19,21], splitting of the v3(F2) mode in the infrared spectra of germanium chalcogenide depends on the strength of
A.L Stetsun et al./Journal of Non-Cr3'stalline Solids 202 (1996) 113-121
c o u p l i n g b e t w e e n glass structural units. T h e r e f o r e the shift o f the w e a k band f r o m 310 c m - J to 285 c m - ~ (Fig. 3) with p h o t o d o p i n g o f a G e S e 2 layer is a c o n s e q u e n c e o f decreasing i n t e r m o l e c u l a r interaction strength. The c o u p l i n g b e t w e e n structural units is changed due to the two factors: the destruction o f s o m e part o f the C h G matrix bonds and the formation o f new c h e m i c a l bonds by metal additives. The f o r m a t i o n o f w e a k c o o r d i n a t i v e S e - A g bonds under an angle to S e - G e bonds close to 90 ° does not lead to a perceptible increase o f i n t e r m o l e c u l a r c o u p l i n g strength, while the destruction o f a p p r o x i m a t e l y 12% of G e - S e bonds causes w e a k e n i n g o f such coupling. A similar situation takes place for a G e S e layer p h o t o d o p e d by copper.
[3] [4] [5] [6] [7] [8] [9] [10] [1 l]
5. Conclusion P h o t o d o p i n g o f the C h G layers by A g or Cu results in the formation o f two types o f impurity centers, T h e creation of c o o r d i n a t i v e bonds play a d o m i n a n t role in these centers. The introduction o f high concentration ( ~ 32 at.%) of doping metal in A s 2 S 3 does not lead to destruction of a large fraction o f structural unit bonds. In g e r m a n i u m c h a l c o g e n i d e unlike A s 2 S 3 the amorphous n e t w o r k is rigid. T h e r e f o r e s o m e part of bonds are destroyed during the photodoping. The n e w dangling bonds take part in f o r m a t i o n o f the first type centers.
Acknowledgements The authors wish to express their gratitude to A.A. K u d r y a v t s e v and A.V. Stronski for valuable discussion and critical reading o f the manuscript.
[12]
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
References [1] M.T. Kostyshin. E.V. Michailovskaya and P.F. Romanenko, Sov. Phys. Solid. State 8 (1966) 571. [2] I.Z. Indutnyi, M.T. Kostyshin, O.P. Kasyarum, E.V, Michailovskaya, V.I. Minko and P.F. Romanenko, Photo-
[29] [30] [31]
121
stimulated Interactions in Metal-Semiconductor Structures (Naukova Dumka, Kiev, 1992). A. Fischer-Colbrie. A. Bienenstock, P.H. Fuoss and Matthew A. Marcus, Phys. Rev. B38 (1988) 12388. J.M. Oldale, J. Rennie and S.R. Elliot, Thin Solid Films 164 (1988) 467. K. Inoue, T. Katayama, M. Kolbayashi, K. Kawamoto and K. Murase, Phys. Rev. B42 (1990) 5154. G. Lucovsky and R.M. Martin, J. Non-Cryst, Solids 8-10 (1971) 185. R.W. Stimets, J. Waldman, J. Lin, T.S. Chang, R.J. Temkin and G.A.N. Connell, Solid State Commun. 13 (1973) 1485. O.E. Owen, A.P. Firth and P.J,S. Ewen, Philos. Mag. B52 (1985) 347. P.J.S. Ewen, A. Zakery, A.P. Firth and A.E. Owen. Philos. Mag. B57 (1988) I. E. Hajto, P.J.S. Ewen, P.G, Hill and A.E. Owen, Phys. Status Solidi (a)114 (1989) 587. Y. Wagner, M. Vlcek, V. Smrcka, P.J.S. Ewen and A.E. Owen, J. Non-Cryst. Solids 164-166 (1903) 1255. M.T. Kostishin, O.V. Stronski and Yu.V. Ushenin, in: Proc. of All-Union Conf. -Vitreous semiconductors" (Nauka, Leningrad, 1985) p. 316. I.Z. Indutnyi and A.I. Stetsun, Proc. SPIE 2113 (1993) 55. R.E. Denton, R.D. Campbell and S,G. Tomlin. J. Phys. D5 (1972) 852. F. Kosek, Z. Gimple. J. Tulka and J. Chlebny, J. Non-Cryst. Solids 90 (1987) 401. G.A.N. Connell and G. Lucovsky, J. Non-Cryst. Solids 31 (1978) 123. G. Lucovsky, Phys. Rev. B6 (1972) 1480. W. Gordy, J. Chem. Phys. 14 (1946) 303. G. Lucovsky, l.P. De Neufville and F.L. Gallener. Phys. Rev. B9 (1971) 1591. N. Kumagai, J. Shirafuji and Y, Inuishi, J. Phys. Soc. Jpn. 42 (1977) 1262. T. Fukunaga, Y. Tanaka and K. Murase, Solid State Commun. 42 (1982) 513. M. Kastner, Philos. Mag. B37 (1978) 127. I.Z. Indutnyi and A.I. Stetsun, Opt. Spectrosc. 71 (1991) 83. I.Z. Indutnyi, A.I. Stetsun, V.I, Zimenko and V.G. Kravets. Opt. Spectrosc. 73 (1993) 1262. L. Jun, 1,I. Videau, B. Janguy, J. Pottier, J.M. Reau and P. Hagenmuller, Mater. Res. Bull. 23 (1988) 1315. I.Z. Indutnyi, A.I. Stetsun, V.G. Kravets and B.D. Nechiporuk, Ukr. J. Phys. 38 (1993) 377. P.N. Sen and M.F. Thorpe, Phys. Rev. Bl5 (1977) 4030. R.J. Gillespie, Molecular Geometry (Van Nostrand Reinhold. London, 1972). Z.U. Borisova, Chalcogenide Semiconductor Glasses (Leningrad University, Leningrad, 1983). V.A. Dan'ko. I.Z. Indutnyi and V.1. Min'ko, Phys. Chem. Glass 18 (1992) 128. J.E. Griffits, G.P. Espinoza, J.P. Remeika and J.C. Phillips, Phys. Rev. B25 (1982) 1272.
!1$411 I,I M ELSEVIER
Journal of Non-Crystalline Solids 202 (1996) 113-121
Infrared absorption of Ag- and Cu-photodoped chalcogenide films A.I. Stetsun ,,., I.Z. Indutnyi a, V.G. Kravets b Institute of Semiconductor Physics, prospekt Nauki 45, 252028 Kiet'-28, Ukraine b Institute oflnJbrmation Storage Problems, ul. Shpaka 2, 252113 KieL'-113, Ukraine
Received 17 July 1995;revised 28 November 1995
Abstract Infrared reflectance and transmittance spectra of chalcogenide glass layers (As2S 3, GeS 2, GeSe2), non-doped and photodoped by Ag and Cu, were measured. The spectra were analyzed in terms of a molecular model for optic-mode vibrational frequencies. With Ag photodoping of As2S 3 the main band (312 cm-~) shifts to higher frequencies. The area under the ~oe2(w) contour is unchanged (ez-imaginary part of the dielectric permeability). The ratio of areas under such contours for photodoped layers to undoped ones is 0.87 ___0.3 for GeS 2 and 0.80 _+ 0.03 for GeSe2. In the photodoped As2S 3 and GeS 2 layer spectra a broad weak band at low frequencies (200 cm -~) is observed. This band is affected by the presence of metal-chalcogen bonds. Variations in the IR-spectra with photodoping by Ag or Cu are consistent with the supposition about normal covalent and coordinative bond formation between metal additives and chalcogen atoms. It results in the creation of the two types of impurity centers.
1. Introduction The chalcogenide glasses (ChG) have attracted attention of many investigators as a media of which properties (structure, optical, electrical, chemical and so on) are changed when irradiated by electromagnetic waves, electron or ion beams. One of the most interesting radiation-induced phenomenon is the photostimulated diffusion of Ag or Cu into ChG in the thin-films ChG-metal structures (photodoping effect) [1]. The essence of the phenomenon is that, under illumination of the C h G - A g (Cu) structure by light with energy greater than the chalcogenide band gap, the metal dissolves into the ChG film. The intermediate photodoped layer, thus formed, has physical
Corresponding author. Tel.: + 380-44265 6342; fax: + 380-44 265 8342; e-maih [email protected].
and chemical properties essentially different from non-doped ChG properties. On the basis of the photodoping effect, high-resolution registering media have been created and have found applications in optical memories, lithography, holography, and others. A large number of scientific papers have been devoted to investigation of the photodoping mechanism, the basic parameters of which define the photostimulated metal flow [2]. However, peculiarities of the doping metal atoms interaction with the glass network and local structure of the impurity centers are less well investigated. The structure of photodoped ChG layers were investigated by using grazing-incidence X-ray scattering, extended X-ray-absorption fine-structure, and others [3-5]. There are some difficulties in studying these systems by direct structural X-ray techniques. The photodoped film thickness does not exceed 1-3 ~ m so that the measured signal from the film is both
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093(96)001 48-2
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A.I. Stetsun et al. / Journal of Non-Co,stalline Solids 202 (1996) 113-121
weak and overwhelmed by the scattering from the rest of the sample (including the substrate). Besides this effect the stimulated structural changes that occur in the photodoped layers under exposure to X-ray irradiation hinders the use of powerful synchrotron beams. Very useful information about the amorphous layers' structure and peculiarities of the impurity atoms chemical bonding can be obtained from Raman and infrared (IR) spectroscopy. The optical vibrational frequencies are determined by vibrations of the amorphous matrix molecular units [6], and integral intensities of absorption bands depend on oscillator concentration and strengths [7]. Variation of these parameters with chalcogenide photodoping can indicate a change in the sample structure and formation of new bonds when metal atoms are embedded into ChG. The Raman spectra of photodoped chalcogenide layers were studied earlier [8,9]. But detailed quantitative analysis of these spectra are hindered due to the long-wavelength shift of the interband absorption edge with photodoping. This shift results in a substantial increase of excitation light absorption in comparison with non-doped layers and a decrease of Raman spectra intensities. The IR-spectra of photodoped ChG layers have been studied on an As33S67 film only [10,11]. It was determined [10] that the main IR-absorption band (310 cm ~ in undoped films) shifts to 340 cm -~ with photodoping by Ag, but more detailed investigations and data analysis have not been performed. In this paper the investigation of As- and Gechalcogenides (AszS3, GeS 2, GeSe 2) IR-spectra, and their modifications with photodoping by Ag and Cu are presented and treated. The peculiarities of the metal-chalcogenide bonds formation are discussed.
2. Experimental techniques The samples for the measurements were prepared by thermal vacuum evaporation at a pressure 10 -3 Pa and sequential metal (Ag or Cu) and ChG deposition onto silica substrates (the total two-layer thickness was 100-400 nm). After deposition of two layers the sample was exposed to light in order to photodissolve and uniformly spread the metal in the ChG film; then the next two layers (metal and ChG)
were deposited on the sample and the process was repeated. This method made it possible to produce rather homogeneous samples of 1-3 i,zm thicknesses. The sample thicknesses were measured by an optical multiple interference fringe method and by ellipsometry. Besides the photodoped sample, an undoped control sample with the same ChG content was prepared simultaneously. The thickness of the photodoped layer, dp, relates to the thickness of the undoped layer, d u, by the expression dp= du + d m, where d m is the total thickness of the photodissolved metal layers. This means that the ChG matrix 'swelling' is approximately equal to the photodissolved metal volume, and the concentration of ChG molecular units, Nu, decreases under photodoping (to Np) proportionally with layer thickness increasing:
d. -
Nu
(1)
dr
For exposure of the metal-As: S 3 and -GeSe: structures an incandescent lamp was used, but for metalGeS 2 a mercury high-pressure lamp was utilized. During deposition the sample thickness was controlled by a quartz crystal oscillator monitoring system (KIT-I). The concentrations of the metal in the photodoped samples were determined by the deposited metal and ChG mass proportion. The interval of metal concentrations in the investigated samples were chosen with account of the photodoping peculiarities in such structures. The metal concentration profile that was observed in the ChG-metal structures under photodoping is step-like with a well defined interface between doped and undoped material. The metal concentration in the doped layer remains constant under exposure until complete dissolution of the metal layer occurs, and a constant concentration value is characteristic for each structure [2]. For As 2S3-Ag this value (frequently called the 'optimal' concentration) is about 32 at.%, for GeS 2 16 at.%; and for GeSe 2 16.7 at.% [12]. The composition of investigated layers was determined by a microprobe analyzer (JCXA-733). The content of each element in the samples corresponds to their chemical formula with accuracy up to 0.5%. The transmittance and reflection spectra in the region 50 to 500 cm-I were measured by a Fourier spectrophotometer (LAFS-1000) with a spectral reso-
A.I. Stetsun et al. / Journal of Non-Cry'stalline Solids 202 (1996) 113-121
lution ~ 3 cm ~. The optical constants (indexes of refraction, n, and absorption, k) of the investigated layers were calculated by using the measured values of the normal incidence transmittance T, the air-incident R and substrate-incident R' reflectances, the sample thickness, and the optical constants of the silica substrate [13]. The theoretical dependencies of R and T on n and k are complex which complicates the calculations and yields multi-valued solutions. For simplicity we used the functions S = (1 + R)/T and Q = ( 1 - R)/T which more smoothly depend on n and k [14] and are more convenient for calculation of the optical constants. The results obtained are expressed in the form of the imaginary part of the complex dielectric permeability e 2 = 2nk.
115
2
0.4
02
0.5
05." 0.t
I 100
, [ I 1 - 200 300 400 500 WAVE N'UMILER Corn-I)
Fig. 2. Spectra of infrared transmittance, T. of GeS 2 (1) and GeS2 Ago 6 (2) layers.
3. Experimental results Fig. l shows the IR-transmission spectra of the undoped and photodoped As2S 3 layers. The reflec0.60.4-
0.20.5
0.3-~
3
02-
~r"
-
-
03ffI-
0"5I 0.3
0.1
I
tO0
I
1
[
200 300 400 WAVE NU~I~E'R (cm-'~)
1
500
Fig. I. Spectra of infrared transmittance, T, of As2S 3 (l), As2S3Ago.9~ (2), As2S3Ag2. 4 (3), As2S 2 (4) and As2SsCuI.25 (5) layers. The initial thickness of ChG in all samples was 2.25 /a,rn.
tion spectra are not presented, but they were used in the optical constant calculations. For the undoped (control) As2S 3 layer (curve 1) a distinct IR-transmission minimum occurs at 312 cm-~; on the highfrequency side of this band an inflection at 372 c m - ] is observed. With increasing Ag concentration the main transmission band shifts to 325 cm-~ for an As2S3Ag0.95 layer (16 at.%, curve 2), and to 345 cm -~ for an As2S3Ag2. 4 layer (32 at.% Ag, curve 3). The inflection at 372 cm ] is shown for all samples and its location is basically unchanged. In the spectrum of the most heavily doped layer (As2S3Ag2. 4) a weak broad band at 200 cm -j is detected. This band was not observed at lower Ag concentration. There is a rather high content of As4S 4 molecular units in thermally deposited As2S 3 films [15]. The As4S 4 molecule vibrations are displayed in the IRspectra of these films also. For comparison, the spectrum of the As2S 2 film, deposited under the same conditions as samples 1-3, is shown in Fig. 1 (curve 4). This spectrum consists of two distinct transmission bands at 343 cm-~ and 372 cm -t , and a weaker band at 225 c m - ] . In the spectrum of the evaporated As2S 3 layer the A s 4 S 4 molecule frequency band at 343 cm ~ is overlapped by the main
116
A.I. Stetsun et al. / Journal of Non-Cm,staUine Solids 202 (1996) 113-121
As2S 3 band and is not resolved. A more intense band at 372 c m - ] appears as an inflection on the high frequency side of the main As2S 3 band. The spectral position and the intensity of the inflection at 372 cm-~ remain unchanged under photodoping. It corroborates the slight interaction of the As4S 4 molecules with Ag. Fig. 1 (curve 5) contains plots of IR transmission spectrum of As 2S 3 sample photodoped by Cu (the metal concentration is 20 at.%). The position of the main transmission band is not changed with Cu photodissolution, but on the high frequency side of this band a slight additional band has appeared. For GeS 2 layer the structure of the IR-transmission spectrum consists of a distinct minimum, located at 370 cm -~, and two inflections, at 327 and 425 cm -~ (Fig. 2, curve 1). The weaker band is located at lower frequencies near 150 cm ~. For the photodoped layer GeS2Ag0. 6 the position of the main band is the same as for the undoped layer, but the high frequency side of the band is more convex (Fig. 2, curve 2). A broad and weak band are observed at 200 cm ~, similar to that in As2S3Ag24. The IR transmission spectrum of the GeSe 2 layer (Fig. 3, curve 1) consists of the main band at 252 cm -~ and weaker bands at 310 cm L, 100 cm -I 200 cm -~. The spectra of the layers, photodoped by Ag to 20 at.% metal concentration, Fig. 3, curve 2
0,6
3
I
,~aooo 3 z
tooo
t00
2OO WAYS
'~0 NU~iSE~ (cm -~)
400
Fig. 4. Infrared response for As2S 3 ( | ) and As2S3Ag2. 4 (2). The spectra are decomposed for a comparison of the main band intensity.
and Cu curve 3 have the same band frequencies and intensities. The shift of the main band with photodoping is not observed, but the location of the weak high frequency band changes from 310 cm -~ to 285 cm -~. The low frequency band at 100 c m disappears as well as the band at 150 cm ~ for GeS 2. In order to examine quantitatively the spectra obtained we calculated the optical constants by using measured spectra of T and R. The results are expressed as a frequency function we2(to), because this quantity is proportional to the product of vibration state density, g(to), and oscillator strength, f(to), which takes the frequency dependence of the matrix element into account [16]: 2,rr 2Ne 2 toe2(to) = ~ . f ( t o )
g(w).
(2)
0.4 0.2
2
0.4
Here, to is the frequency, M is the oscillator mass and N is the number of oscillators per unit volume. If f(~o) is a slowly varying function of the frequency, then Eq. (2) gives a good approximation of the vibration state density spectrum. The integral of Eq. (2) will be proportional to the number of oscillators per unit volume (in our case to the density of glass molecular units) [7]: 2 "rr2Ne * 2
0.2 I
I
L
WAVE NUME,tR
[
f,
1"O8'2(O)) 809
m
'
(3)
(cm -1 )
Fig. 3. Spectra of infrared transmittance, T, of GeSe 2 (l), GeS% Ag0.7.s (2) and GeSe~Cu0.75 (3).
where e * is the effective charge, which can change under photodoping. If Eq. (2) is integrated over the
A.L Stetsun et al. / Journal of Non-Cr3,stalline Solids 202 (1996) 113-121
photodoping of As2S 3 and GeS 2 layers. In As2S3, spectrum redistribution displays as maximum shift. In GeS 2, photodoping results in a more convex high frequency wing of the main band. (iii) In the spectra of the As2S3Ag2. 4 and GeS2Ago. 6 layers, broad weak bands at low frequencies (200 cm ~) are observed. It may be that these bands are caused by the metal-chalcogen chemical bonds.
3 '.~ 3000
i
3
117
I
2000 A' t',
~IO00 ..=
z_
100
2O0
1100 400 WAVE NU~41IER(cm-I)
500
Fig. 5. Infrared response for GeS 2 (l) and GeS 2Ago.6 (2).
main IR-band spectral interval, then e* is the mode effective charge. Figs. 4 - 6 contain plots of we2(w) undoped (curve 1) and Ag pbotodoped (curve 2) As2S 3, GeS 2 and GeSe 2 layers. Analysis of these dependencies allows us to summarize the main features of spectrum modification with photodoping. (i) The area under we2(w) contour in the main 1R frequency band spectral region is practically unchanged when As2S 3 is photodoped by Ag. For Ge-chalcogenides the photodoped layer to undoped layer area ratio is 0.87 +_ 0.3 for GeS 2 and 0.80 +_ 0.3 for GeSe~. (ii) In the spectral region of the main IR frequency band, a vibrational state density redistribution toward higher frequencies is observed with
4. D i s c u s s i o n
According to the molecular model developed by Lucovsky and Martin [6,17] the vibrational modes of ChG are associated with the vibrations of the glass molecular units. The applicability of such a model is based on a weak coupling between the molecular units in comparison with the intramolecular bond forces. The main structural unit for arsenic sulfide is the pyramidal ASS3/2 molecule. Neighboring units are bound through the bridging twofold coordinated sulphur atoms. It has been shown, by valence field model calculations and by other estimations [ 17], that the main band in As2S 3 infrared absorption spectrum (312 cm -~, Fig. 1) is associated with ~'3(E) asymmetrical bond-stretching vibrations of these pyramids. The symmetrical bond-stretching vibration u~(A) displays lesser activity in an infrared spectrum and provides asymmetrical spreading of the main band into the high frequency region. The value of As-S bond-stretching force constants, k r, used in valence field model calculations is estimated in accordance with Gordy's rule [18]:
kr = aN( --U-f KAKB ]3/4 +b,
3
(4)
3 ~2000 Q Z
~0o
400
WAVE NUMI~:R ('¢rn-l)
I
~0
Fig. 6. Infrared response for GeSe 2 (l) and GeSe 2Ag0.75 (2).
where a and b are empirical constants, N is an order of the bond, KA and KB are electronegativities, in this case of arsenic and sulphur atoms, and d is the bond length. The main structural units of germanium chalcogenides are GeX~/2 (X = S, Se) tetrahedrons. Table 1 contains values of the optic-mode frequencies of these molecules, obtained in previous investigations and this work. Almost all modes of GeX~/2 tetrahedrons are active in infrared spectra due to inter-
A.I. Stetsun et al./Journal of Non-Cry'stalline Solids 202 (1996) 113-121
118
Table 1 The frequencies of the fundamental modes of germanium chalcogenide display activity in an infrared spectrum Material
Frequency ( c m - t ) v ~,(F2)
v4(F2 )
GeS 2
328 327
-
367,409 370, 425
149 150
[19] this work
GeSe
200 200
80 -
255 258,310 252, 310
100 115 100
[20] [21] this work
v I(A 1)
v2(E)
References
molecular interaction. The existence of such an interaction also leads to the splitting of the v3(F2) mode band. The bond-stretching mode v3(F2) and vl(A) frequencies are situated in the high frequency region of the spectrum. The low frequency bond-bending v4(F2) mode is damped with photodoping of germanium chalcogenides by the metals (Figs. 5 and 6). As shown below, this experimental fact may be explained by the concrete space configuration of chemical bonds between metal additive and structural elements of the glass. The special features of the chemical bond formation by metal atoms introduced as an additive into the ChG matrix were predicted by Kastner [22]. According to this theory an atom of monovalent metal being introduced into the melted ChG forms one normal covalent bond and a few (usually three) coordinative bonds. This configuration of chemical bonds corresponds to that type of center in which the metal atom is bound as a neutral fourfold coordinated additive. The normal covalent bond is created by a union of a metal valence electron and a chalcogen dangling bond electron. An electrons lone pair of a chalcogen atom and the empty orbital on the metal atom are utilized for the formation of a coordinative bond. The center in which metal additive is bound as ions by the four coordinative bonds is created by dissociation of the normal covalent bond and subsequent replacing by a coordinative bond. The concentration ratio of the two types of centers is determined by the law of mass action and an entropy factor [22]. The low-temperature introduction of metal into ChG by means of photodoping differs from doping in the melt, where the structure of new compounds depends on the chemical equilibrium of the glass component. In the case of photodoping the matrix of
ChG has been formed and fixed already. Therefore the possibility for creation of chemical bonds by metal atoms is determined beforehand and depends on the structure of this matrix. That is why the chemical bonds between metal impurities and the ChG matrix are formed preferentially as coordinative bonds. The presence of coordinative metal-chalcogen bonds in photodoped ChG layers was proved experimentally by photoemission spectroscopy [5] and grazing-incidence scattering [3]. The results of investigations of interband transition spectra for A s 2 S 3 and GeS 2 photodoped by Ag corroborate the chemical bond formation between chalcogen and metal atoms by participation of chalcogen lone pair electrons, too [23,24]. The peculiarities of photodoped ChG layer infrared spectra which we observed in this work also agree well with the assumption about the coordinative and normal covalent metal-chalcogen bonds formation. The lone pair electrons of chalcogen do not take part in the creation of the chemical bonds of glass structural units (As-S f o r A S S 3 / 2 pyramids or G e - X for GeX4/2 tetrahedrons). Therefore the formation of metal-chalcogen coordinative bonds does not lead to destruction of the ChG matrix units. Some fraction of the metal atoms form normal covalent metal-chalcogen bonds in the sites of dangling bonds. However, the number of such defects in CbG layers is significantly smaller than the quantity of metal atoms at the optimal photodoping concentration. Consequently, the formation of coordinative bonds play a dominant role in the photodoping phenomena. The creation of these two types of bonds is consistent with conservation of the main band integrated intensity in infrared spectra (Fig. 4). The wide and weak bands near 200 cm-1 in the As~ $3 Ag24 and GeS2 Ag0.6 layers spectra are assigned to the bond-stretching motion of the coordinative and normal covalent metal-chalcogen bonds. Some difference in the spectral position of this band from the Ag2S absorption band (180 cm -~) [25] is caused by the above mentioned specific features of these bonds. The creation of coordinative bonds causes the redistribution of lone-pair electron charge density from chalcogen atoms to metal ones. This brings about an increase of chalcogen effective electronega-
A.I. Stetsun et al./ Journal of Non-Co,stalline Solids 202 (1996) 113-121
tivity and, as a result, an increase of the stretching A s - S and G e - S force constants in accordance with Gordy and Samayajulu's [19] rules. From such an approach, it follows that the increase of the bond-stretching force constants is a consequence of the increasing ionicity of these bonds, since an electronegativity difference determines them. Such force constant growth under photodoping manifests itself in the arsenic sulphide IR (Fig. 1) and Raman spectra, as a high frequency shift of the main band. According to our measurements [26] the main band of the Raman spectra shifts from 344 c m - ~ for As2S 3 to 372 cm -1 for As2S3Ag2.4 . For germanium sulphide, as mentioned above, the position of the main band does not change, but the redistribution of the phonon state density results in a more convex high frequency wing of this band for the photodoped layer (Fig. 2). The increasing of the v I and v 3 mode frequencies with heavy Ag photodoping allows us to suggest that coordinative bonds are weak and some types of structural unit vibrations may be considered as independent from these bonds. Proof of this vibration isolation may also be found in certain space bond configurations of photodoped ChG. For the ChG considered in this paper, the angle between bonds of neighboring structural units at the bridging chalcogen atom is 90-136 ° [27]. In accordance with the theory of valence shell electron pairs repulsion [28] the angle between the coordinative bond and GhG matrix bond at this bridging atom will be close to 90 ° . Therefore the creation of coordinative bonds does not affect the bond-stretching vibrations, but suppresses bond-bending vibrations. These features are observed experimentally. For all investigated materials the frequency of v I and v 3 vibrations either remain unchanged or increase. At the same time the bands of v4 vibrations in the spectra of GeS 2 and GeSe~ photodoped layers are damped (Figs. 5 and 6). Some quantitative estimations of the photodoped layer structure may be made from comparison of spectra integrated intensities (the areas under we,,(~o) contours in the region of main bands) for the photodoped and the corresponding undoped layer. In accordance with Eq. (1) the concentration of ASS3/2 structural units decreases to 82% of their original value in As2S 3 film with arsenic sulfide photodop-
119
ing by Ag to the optimal Ag concentration (32 at.%). The smaller number of oscillators can provide the same area band contour by means of increasing their effective charge value in accordance with Eq. (3). Such estimation demonstrates, that for As~S3Ag2. 4 the effective charge increases 10.5 _+ 3.5% in comparison with As2S 3. Since v I and u 3 modes of ASS3/2 pyramidal units are relatively independent on Ag additives, the dynamical contribution to the effective charge of these modes for photodoped and undoped As2S 3 layers is the same. Consequently the increase of the effective charge has a mostly static nature, i.e. it is conditioned by an increase of glass matrix bond ionicities. Such an increase of ChG matrix bond ionicity with coordinative bond creation is connected with an ion-dipole mechanism of that bond formation. This mechanism includes the interaction between the ion and chalcogen dipole moment conditioned by the space distribution of the lone pair density, as well as the interaction ion and induced dipole moment on the semiconductor matrix bond. Given the peculiarities of the spectra of silver photodoped As 2S 3 layers, an interpretation was made using the force constant formalism. The splitting of twice degenerated ~'3(E) vibration is observed in the spectrum of the As2S 3 layer photodoped by copper (Fig. 1, curve 5). This splitting shows the lowering of ASS3/2 pyramidal units symmetry from C3,. to C, due to formation of S - C u coordinative bonds. The difference between spectra of A s ; S 3 layers photodoped by Ag and Cu is probably connected with a greater strength of the S - C u coordinative bond and the smaller mass of the Cu atom. This leads to a more significant influence of metal additives on the vibrational activity of the structural units in the glass matrix. The suggestion regarding the strength difference of these bonds agrees with observations of higher ionic conductivity in ChG with a structure similar to As2S 3 doped by silver rather than copper [29]. The formation of M e - C h G bonds in photodoped layers depends on the structure of the initial undoped ChG layers. An as-deposited As2S 3 layer contains a significant number of defects, including dangling bonds of S atoms. The concentration of these broken bonds exceeds 7% [15]. When a relatively small concentration of metal, for example Ag, photodis-
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A.l. Stetsun et al. / Journal of Non-Crystalline Solids 202 (1996) 113-121
\/
\/
2\
/\
a
b
Fig. 7. Schematic representation of the chemical bond formation by silver additive in a photodoped ChG layer. Normal covalent bonds are denoted by straight lines, dots represent the coordinative bonds. The asterisk show an electron localized near the Ag+-ion. (a) The bonding configuration for the Ag additive at the site of the amorphous network defect. This configuration is the same as for doping in the melt [22]. (b) The impurity center at the undefective site of the amorphous network where the metal is bound by coordinative bonds only.
solves, silver binds itself with such onefold coordinated S atoms, forming the center where metal appears as neutral additive (Fig. 7(a)). The concentration of these centers in photodoped As2S 3 layers is determined by the concentration of defects in the ChG matrix. After filling the defect sites the photodiffused metal is bound by coordinative bonds only (Fig. 7(b)). This type of center is created in a natural way by diffusion of the metal ion to the sites of lone pair electrons. This type of center differs from those in a melt, where similar center is formed as a result of normal covalent bond dissociation in the first type of center (Fig. 7(a)), resulting in the creation of a C i defect and subsequent substitution of a normal covalent bond by a coordinative one [22]. Such structure of the second type center is supported by a higher conductivity of Ag photodoped than undoped As2 $3 layers. When the concentration of metal become greater than the number of defects and chalcogen lone pair electrons divided by four, the coordinative number of additive atoms decreases significantly. This is consistent with experimental data about a sharply increasing ionic conductivity in photodoped As2S 3 layers with an Ag concentration of more than 20 at.% [30]. In the spectra of photodoped germanium chalcogenide the main maximum of absorption does not shift. Only a small redistribution of phonon state density in the form of a more convex high frequency
wing of the absorption band at 370 cm -~ in the GeS 2 spectra is observed. Therefore we can ignore the change in the e * value in the photodoped layers and estimate the number of structural units destroyed as a the result of metal additive introduction according to Eq. (3) and (1). The photodoping of GeS 2 with silver at concentrations of 16 at.% results in the destruction of 6_+ 3.5% of molecular units. In the GeSe 2 layer photodoped up to 20 at.% by silver 12 _+ 3.5% of the initial number of GeSe tetrahedrons are destroyed. The demolishing of structural units in germanium chalcogenide with photodoping and their conservation in arsenic sulfide are obviously conditioned by the more rigid structure of the GeSe 2 and GeS 2 amorphous network. The introduction of a significant number of metal ions in the ChG matrix causes the expansion of the semiconductor matrix volume and the local stretching. In As2S 3 layers which have a lower coordination and more flexible structure, the stress relaxation does not lead to the breakage of a perceptible amount of A s - S bonds even under higher concentration of doping metal. The structural network of germanium chalcogenide is more durable. It is formed by G e X 4 / 2 tetrahedron chains linked to one another via common angle-shared chalcogen atoms. These chains are joined by transversal bonds which form tetrahedrons coordinated through the common edge [31]. In the A s 2 S 3 s t r u c t u r e these bonds are absent, and the chains o f ASS3/2 pyramids interact weakly through Van der Waal's like forces. Therefore, unlike A s 2 S 3 , s o m e part of GeS% and GeS 2 bonds are destroyed during the photodoping. Being created in such a way, new dangling bonds take part in the formation of the first type centers also. The number of new dangling bonds depend on volume expansion and tension of the glass matrix under introduction of a large quantity of the metal additive. Consequently the concentration ratio of two types of centers in a photodoped chalcogenide germanium layer will be determined not only by the initial number of defects but also by the dynamical equilibrium between the stretching of the amorphous network and the electrical forces stimulating the metal diffusion. In accordance with the molecular model [19,21], splitting of the v3(F2) mode in the infrared spectra of germanium chalcogenide depends on the strength of
A.L Stetsun et al./Journal of Non-Cr3'stalline Solids 202 (1996) 113-121
c o u p l i n g b e t w e e n glass structural units. T h e r e f o r e the shift o f the w e a k band f r o m 310 c m - J to 285 c m - ~ (Fig. 3) with p h o t o d o p i n g o f a G e S e 2 layer is a c o n s e q u e n c e o f decreasing i n t e r m o l e c u l a r interaction strength. The c o u p l i n g b e t w e e n structural units is changed due to the two factors: the destruction o f s o m e part o f the C h G matrix bonds and the formation o f new c h e m i c a l bonds by metal additives. The f o r m a t i o n o f w e a k c o o r d i n a t i v e S e - A g bonds under an angle to S e - G e bonds close to 90 ° does not lead to a perceptible increase o f i n t e r m o l e c u l a r c o u p l i n g strength, while the destruction o f a p p r o x i m a t e l y 12% of G e - S e bonds causes w e a k e n i n g o f such coupling. A similar situation takes place for a G e S e layer p h o t o d o p e d by copper.
[3] [4] [5] [6] [7] [8] [9] [10] [1 l]
5. Conclusion P h o t o d o p i n g o f the C h G layers by A g or Cu results in the formation o f two types o f impurity centers, T h e creation of c o o r d i n a t i v e bonds play a d o m i n a n t role in these centers. The introduction o f high concentration ( ~ 32 at.%) of doping metal in A s 2 S 3 does not lead to destruction of a large fraction o f structural unit bonds. In g e r m a n i u m c h a l c o g e n i d e unlike A s 2 S 3 the amorphous n e t w o r k is rigid. T h e r e f o r e s o m e part of bonds are destroyed during the photodoping. The n e w dangling bonds take part in f o r m a t i o n o f the first type centers.
Acknowledgements The authors wish to express their gratitude to A.A. K u d r y a v t s e v and A.V. Stronski for valuable discussion and critical reading o f the manuscript.
[12]
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