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Large changes in the electrical conductivity of tetrahedrally-coordinated chalcogenide glasses containing oxygen
Journal of Non-Crystalline Solids 164--166 (1993) 1191-1194 North-Holland
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Large changes in the electrical conductivity of tetrahedrally-coordinated chalcogenide glasses containing oxygen P.C. Taylor a, R.E. Shirey a, S. Girlani a and J. H a u t a l a b aDepartment of Physics, University of Utah, Salt Lake City, U T 84112, U.S.A. bTEL America, Inc., 123 Brimble Avenue, Berverly, MA 01915, U.S.A.
For glassy (Cu2/381/3)x(As2/sS3/5)1.x with x = 9/19 [Cu6As4S ~ the S and Cu atoms are tetrahedrally coordinated, the As atoms are three-fold coordinated, and there exist only Cu-S and As-S bends. With the addition of oxygen to films sputtered at this composition, the activation energy for the conductivity decreases from approximately 0.4 eV to less than 0.1 eV and the room temperature conductivity increases from about 10 -4 fflcml to greater than 1 flicm'Â. Optical absorption measurements are consistent with these conductivity results. These results are compared to similar measurements in the Cu-As-Se system.
1. I N T R O D U C T I O N There is strong evidence that the addition of metal atoms (atoms from groups I, II or HI of the periodic table) to chalcogenide glasses leads to an increase in the average local coordination n u m b e r of the chalcogen atoms from two in most standard chalcogenide glasses to four when the metal concentrations are great enough. This evidence comes from models [1-4] which are based on the covalent nature of the local bonding in these glasses and from experimentalx-ray scattering and xray absorption free structure (EXAFS) measurements [5-7]. The optical and electronic properties of the metal chalcogenide glasses change dramatically as the metal concentration increases. For low metal concentrations where the vast majority of the chalcogen atoms are two-fold coordinated, the glasses behave like the standard chalcogenide glasses such as Se or As2Se 3. In these glasses the valence band is formed from non-bending p states on the chalcogen atoms and the flexibility of the network ensures that the materials cannot be doped (i.e., the F e r m i level remains near the middle of the gap regardless of the addition of "impurity"atoms). On the
other hand, when the metal concentrations are high enough the average local coordination number approaches four for allthe constituent atoms and the valence band is comprised of sp 3 hybridized states. In glasses where the average coordination number approaches four, the F e r m i level can be moved close to at least one band edge [8]. The Formal Valence Shell (FVS) model [3,4] provides the framework within which one can determine the average coordination number for each atomic species in a given glass as well as t h e stoichiometric compositions where the bonding arrangements are the least complex. For example, in the IV-VI ternary glass-forming systems, of which the prototypical compositions based on Cu-AsSe and Cu-As-S are examples, the average coordination number of the chalcogen (S or Se) can be continuously increased from two to four. When ternary compositions are made in the form (I2/3VI1/3)x(V2/5VI3/5)l.x, then only I-VI and V-VI bends are present. In these "stoichiometric"compositions the group I and V atoms act like the analog of the cation sublattice in a crystalline semiconductor and the group VI atoms form the analog of the anion sublattice. The group I metal atoms are
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
1192
P.C. Taylor et al. / Tetrahedrally-coordinated chalcogenide glasses containing oxygen
a l w a y s t e t r a h e d r a l l y coordinated, t h e group V a t o m s are three-fold coordinated for x _< 0.47,
and the group VI chalcogen atoms increase in a v e r a g e c o o r d i n a t i o n n u m b e r f r o m two to four as x goes f r o m 0 to 0.47. T h u s by e m p l o y i n g compositions that satisfythe above expression one c a n control t h e a v e r a g e coordination number of the chalcogen atoms with no c h a n g e i n t h e c o o r d i n a t i o n n u m b e r s of the o t h e r two c o n s t i t u e n t species and w i t h only two t y p e s of bends p r e s e n t i n t h e glass. W e shall concentrate on the composition x = 9/19 -- 0.47 w h e r e b e t h t h e m e t a l a n d the chalcogen a t o m s are t e t r a h e d r a l l y c o o r d i n a t e d , [At this composition t h e a v e r a g e coordination n u m b e r is 3.8, which e q u a l s t h e a v e r a g e c o o r d i n a t i o n n u m b e r i n device-quality a-Si:H w i t h a p p r o x i m a t e l y 7 at. % H.] We h a v e p r e v i o u s l y s h o w n u s i n g t h e Cu-As-Se system, t h a t w h e n t h e Se a t o m s are t e t r a h e d r a l l y bonded, t h e m a g n i t u d e s and a c t i v a t i o n e n e r g i e s of t h e d a r k conductivity can be v a r i e d o v e r a wide r a n g e by t h e i n c o r p o r a t i o n of o x y g e n [8]. W h e n t h e Se is p r e d o m i n a n t l y three-fold c o o r d i n a t e d (x = 0.315), the a d d i t i o n of o x y g e n h a s essentially no effect on t h e conductivity [8]. T h e optical p r o p e r t i e s also e x h i b i t s t r i k i n g differences as x goes f r o m - 0 . 3 to - 0 . 5 [9]. I n t h e p r e s e n t p a p e r we e x t e n d t h e s e studies to t h e Cu-As-S s y s t e m and compare our results to the Cu-As-Se glasses. For glassy (Cu2/3S1/3)x(As2/583/5)1.x with x = 9/19 [Cuo.32Aso.21Seo.47 or Cu6As4S 9] the S and Cu atoms are tetrahedrally coordinated, the As atoms are three-fold coordinated, and there exist only Cu-S and As-S bonds. W e will show that, with the addition of oxygen to films sputtered at this composition, the magnitudes and activation energies of the dark conductivity vary dramatically just as they do in the Cu-As-Se system,
2. E X P E R I M E N T A L
DETAILS
All thin-film samples were sputtered in argon using a Cu-As-S target designed to
,
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i , I , I , I 0.2 0.4 0.s 0.8 • 0 2 In Sputtering Gos
i
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F i g u r e 1. O x y g e n c o n c e n t r a t i o n i n t h e films as a f u n c t i o n of t h e 0 2 p a r t i a l p r e s s u r e in t h e s p u t t e r i n g gas. O p e n t r i a n g l e s a n d solid circles are d a t a for glassy Cuo.32Aso. 218eo.47 and Cuo.32Aso.21So.47,respectively.
produce films w i t h t h e stoichiometric composition Cuo.32As0.2180.47. Details are a v a i l a b l e e l s e w h e r e [10].
3. R E S U L T S The incorporation of oxygen into the CuAs-S glassy films from the sputtering gas is much more efficientthan it is in the Cu-As-Se system. This fact is illustratedin Fig. i which shows the at. % O incorporated in the films as a function of the partialpressure of 0 2 in the sputteringgas. In the S system (filledcircles in Fig. 1) there is approximatelyfour times as m u c h oxygen as there is in the Se films for the same partial pressure in the sputtering gas. Over the ranges shown in Fig. 1, the incorporation of oxygen, at constant pressure in the sputtering gas, is approximatelya linear function of the concentration of 02 in the sputtering gas.
P.C. Taylor et al. / Tetrahedrally-coordinated chalcogenide glasses containing oxygen
I
t
t
o.~IT-
0.
-
-~ 0.2
0.1
; ~.
I 5
I I0
_
15
At.~OInFtlms
Figure 2. Activation energy for electrical conductivity as a function of oxygen concentration. Open triangles and solidcircles are data for glassy Cuo.32Aso.21Seo.47 and Cuo.32Aso.21So.47,respectively.
In the Se system the room-temperature electrical conductivity a can be varied from approximately 10 -3 fflcml to greater than 1 flicml [8]. The optical energy gap Eg of glassy Cuo.32Aso. 21Se0.47 is - 1.3 eV (defihed as the energy at which the absorption coefficient a = 104 cml), and the activation energy A E for the electrical conductivity is 0.32 eV in the nominally-undoped samples, Thus, at this composition, even the nominally undoped samples have the Fermi level about one fourth of the gap from the valence band edge. We believe that this position of the F e r m i level is determined by residual defects, perhaps controlled by the residual oxygen in the glass. At high oxygen concentrations, where a >- 1 fl-1 cm-1, AE < 0.1 eV. In the S system (glassy Cuo.32Aso.2180.47 ) the optical band gap is larger (AE = 1.8 eV), and the effect of oxygen on the electronic properties is greater. In this material the room t e m p e r a t u r e conductivity is 10 "4 f f l cm-1 in the nominally undoped glass while the
1193
glasses with _> 10 at. % 0 have AE < 0.1 eV. The variation of the activation energies with oxygen concentration are shown in Fig. 2 for both the Se (open triangles) and S (flled circles) systems. It is apparent from this figure that the effect of oxygen is greater in the S system. There exists an optical absorption below the optical gap that also scales with the oxygen concentration. In the Se system this absorption exhibits an onset near 0.2 eV and a nearly constant magnitude above 0.2 eV that scales with the conductivity. In the S system the analogous absorption rises at about 0.4 eV as shown in Fig. 3. There also exists a dark electron spin resonance (ESR) signal that scales roughly as the below-gap optical absorption.
4. D I S C U S S I O N Certain structural and electronic properties of glassy Cuo.32Aso.21Seo.47 and CUo.32Aso.21So.47 are similar to those of a-Si:H. First, these two chalcogenide semiconductors are p r e d o m i n a n t l y tetrahedrallycoordinated and the dominant electronicdefects probably exhibit positive effectiveelectron-electron correlation energies (positiveUeff)[8,9]. Second, atleastforp-type electricalconductivity, films of these two glassy chalcogenides can be made with conductivitiesthat vary by several orders of magnitude and with activation energies that can be quite small (< 0.1 eV). At this point, however, the similarities between the group IV and the group VI tetrahedral amorphous semiconductors end and major differences appear. One major difference is that, although the covalent nature of the bonding tends to control the local structural order (the fundamental assumption of the FVS model [2,3,4]), most of the bonds in these two metal chalcogenide glasses have strong ionic character. In fact the ionicity is similar to that which exists in crystalline II-VI semiconductors. In the II-VI
P.C. Taylor et al. / Tetrahedrally-coordinated chalcogenide glasses containing oxygen
1194 105
,
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i
,
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,
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,
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,
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Cuo.32Aso.21Seo.47 and about 0.4 eV above the valence band edge in Cu0.32As0.21S0.47 . The conductivity mechanism is still uncertairL The data on the Se system are consistent with a band-type conductivity over most of the doping range [8], b u t because the optical absorption onset in the S system is near 0.4 eV, one must question this interpretation for beth glasses.
ACKNOWLEDGEMENT I
a0
a~
0.s
1.2
l.e
h,(,v)
Figure
3.
This research was supported by the N S F under grant no. DMR-909-01596.
Optical Absorption in glassy
CUo.32As0.21S0.47 as a function of oxygen
REFERENCES
concentration- A nominally undoped; x, • , D, +, represent 2.9, 2.2, 5.4 and l4 at. %oxygen, respectively.
1.
semiconductors the dominant doping m e c h a n i s m i s u s u a l l y t h e presence of vacancies and/or interstitials. Cation vacancies are generally acceptors and anion vacancies generaUydonors. Because one type of vacancy is preferred in a given composition, it is diWicult to change the carrier type in most II-VI semiconductors. Only recently has substitutional doping become more common in II-VI systems [11]. In glassy Cuo.32Aso.21Seo.47 and Cu0.32As0.21S0.47 it has been suggested [8] that, although most of the oxygen atoms go into the glass substitutionally for S or Se, the p-type doping is controlled by the presence of oxygen atoms which compete for C u b e n d s and generate the amorphous analog of Cu vacancies(Se-Se bends and danglingso bends) in the glass. The dark ESR has been attributed to a "dangling bond" at a threefold-coordinated S or So site, and the below gap absorption to the excitation of electrons into these defects [8]. From the absorption data one can conclude that the defect level is about 0.2 eV above the valence band edge in
J.Z. L i u a n d P . C . Taylor, Phys. Rev. Lett. 59 (1987) 1938. 2. J.Z. Liu andP. C. Taylor, Solid State Commun. 70 (1989) 81. 3. J.Liu and P.C. Taylor, J. Non-Cryst. Solids 114 (1989) 25. 4. P.C. Taylor, Z.M. Salah andJ. Z. Liu, in Advanced in Disordered Semiconductors, H. Fritzsche, ed. (World Scientific, Singapore, 1990), Vol. 3, p. 23. 5. K.S. Liang, A. Bienenstock ad C. W. Bates, Phys. Rev. B10 (1974) 1528. 6. S. Laderman, A. Bienenstock and K.S. Liang, Solar Energy Mat. 8 (1982) 15. 7. S.II. IIunter, A. Bienenstock and T.M. IIayes, in Amorphous and Liquid Semiconductors, W. E. Spear, ed. (University of Edinburgh, Edinburgh, 1977), p. 78. 8. J. IIautala, B. Moosman and P.C. Taylor, J. Non-Cryst. Solids 137 & 138 (1991) 1043. 9. J. Hautala, S. Y a m a s a k i a n d P.C. Taylor, J. Non-Cryst Solids 114 (1989) 85. 10. R.E. Shirey and P.C. Taylor, U. U t a h J. Undergrad. Res. 3 (1992) 40. 11. See, for example, T. Yasuda, I. Miksuishi and H. Kukimoto, Appl. Phys. Lett. 52 (1988) 57.
~ou~N,~ L or
~
U
Large changes in the electrical conductivity of tetrahedrally-coordinated chalcogenide glasses containing oxygen P.C. Taylor a, R.E. Shirey a, S. Girlani a and J. H a u t a l a b aDepartment of Physics, University of Utah, Salt Lake City, U T 84112, U.S.A. bTEL America, Inc., 123 Brimble Avenue, Berverly, MA 01915, U.S.A.
For glassy (Cu2/381/3)x(As2/sS3/5)1.x with x = 9/19 [Cu6As4S ~ the S and Cu atoms are tetrahedrally coordinated, the As atoms are three-fold coordinated, and there exist only Cu-S and As-S bends. With the addition of oxygen to films sputtered at this composition, the activation energy for the conductivity decreases from approximately 0.4 eV to less than 0.1 eV and the room temperature conductivity increases from about 10 -4 fflcml to greater than 1 flicm'Â. Optical absorption measurements are consistent with these conductivity results. These results are compared to similar measurements in the Cu-As-Se system.
1. I N T R O D U C T I O N There is strong evidence that the addition of metal atoms (atoms from groups I, II or HI of the periodic table) to chalcogenide glasses leads to an increase in the average local coordination n u m b e r of the chalcogen atoms from two in most standard chalcogenide glasses to four when the metal concentrations are great enough. This evidence comes from models [1-4] which are based on the covalent nature of the local bonding in these glasses and from experimentalx-ray scattering and xray absorption free structure (EXAFS) measurements [5-7]. The optical and electronic properties of the metal chalcogenide glasses change dramatically as the metal concentration increases. For low metal concentrations where the vast majority of the chalcogen atoms are two-fold coordinated, the glasses behave like the standard chalcogenide glasses such as Se or As2Se 3. In these glasses the valence band is formed from non-bending p states on the chalcogen atoms and the flexibility of the network ensures that the materials cannot be doped (i.e., the F e r m i level remains near the middle of the gap regardless of the addition of "impurity"atoms). On the
other hand, when the metal concentrations are high enough the average local coordination number approaches four for allthe constituent atoms and the valence band is comprised of sp 3 hybridized states. In glasses where the average coordination number approaches four, the F e r m i level can be moved close to at least one band edge [8]. The Formal Valence Shell (FVS) model [3,4] provides the framework within which one can determine the average coordination number for each atomic species in a given glass as well as t h e stoichiometric compositions where the bonding arrangements are the least complex. For example, in the IV-VI ternary glass-forming systems, of which the prototypical compositions based on Cu-AsSe and Cu-As-S are examples, the average coordination number of the chalcogen (S or Se) can be continuously increased from two to four. When ternary compositions are made in the form (I2/3VI1/3)x(V2/5VI3/5)l.x, then only I-VI and V-VI bends are present. In these "stoichiometric"compositions the group I and V atoms act like the analog of the cation sublattice in a crystalline semiconductor and the group VI atoms form the analog of the anion sublattice. The group I metal atoms are
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
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P.C. Taylor et al. / Tetrahedrally-coordinated chalcogenide glasses containing oxygen
a l w a y s t e t r a h e d r a l l y coordinated, t h e group V a t o m s are three-fold coordinated for x _< 0.47,
and the group VI chalcogen atoms increase in a v e r a g e c o o r d i n a t i o n n u m b e r f r o m two to four as x goes f r o m 0 to 0.47. T h u s by e m p l o y i n g compositions that satisfythe above expression one c a n control t h e a v e r a g e coordination number of the chalcogen atoms with no c h a n g e i n t h e c o o r d i n a t i o n n u m b e r s of the o t h e r two c o n s t i t u e n t species and w i t h only two t y p e s of bends p r e s e n t i n t h e glass. W e shall concentrate on the composition x = 9/19 -- 0.47 w h e r e b e t h t h e m e t a l a n d the chalcogen a t o m s are t e t r a h e d r a l l y c o o r d i n a t e d , [At this composition t h e a v e r a g e coordination n u m b e r is 3.8, which e q u a l s t h e a v e r a g e c o o r d i n a t i o n n u m b e r i n device-quality a-Si:H w i t h a p p r o x i m a t e l y 7 at. % H.] We h a v e p r e v i o u s l y s h o w n u s i n g t h e Cu-As-Se system, t h a t w h e n t h e Se a t o m s are t e t r a h e d r a l l y bonded, t h e m a g n i t u d e s and a c t i v a t i o n e n e r g i e s of t h e d a r k conductivity can be v a r i e d o v e r a wide r a n g e by t h e i n c o r p o r a t i o n of o x y g e n [8]. W h e n t h e Se is p r e d o m i n a n t l y three-fold c o o r d i n a t e d (x = 0.315), the a d d i t i o n of o x y g e n h a s essentially no effect on t h e conductivity [8]. T h e optical p r o p e r t i e s also e x h i b i t s t r i k i n g differences as x goes f r o m - 0 . 3 to - 0 . 5 [9]. I n t h e p r e s e n t p a p e r we e x t e n d t h e s e studies to t h e Cu-As-S s y s t e m and compare our results to the Cu-As-Se glasses. For glassy (Cu2/3S1/3)x(As2/583/5)1.x with x = 9/19 [Cuo.32Aso.21Seo.47 or Cu6As4S 9] the S and Cu atoms are tetrahedrally coordinated, the As atoms are three-fold coordinated, and there exist only Cu-S and As-S bonds. W e will show that, with the addition of oxygen to films sputtered at this composition, the magnitudes and activation energies of the dark conductivity vary dramatically just as they do in the Cu-As-Se system,
2. E X P E R I M E N T A L
DETAILS
All thin-film samples were sputtered in argon using a Cu-As-S target designed to
,
i
,
,
,
i
,
,
,
i /
15
T
// / ~ I0 ~ -~ o be~ < 5
/ / /
/
/
/]
/
/ Q
,,
/ 'i l~
// i"
//
/
//
/i/// ,"
I
t/ 0.
i , I , I , I 0.2 0.4 0.s 0.8 • 0 2 In Sputtering Gos
i
I 1.0
F i g u r e 1. O x y g e n c o n c e n t r a t i o n i n t h e films as a f u n c t i o n of t h e 0 2 p a r t i a l p r e s s u r e in t h e s p u t t e r i n g gas. O p e n t r i a n g l e s a n d solid circles are d a t a for glassy Cuo.32Aso. 218eo.47 and Cuo.32Aso.21So.47,respectively.
produce films w i t h t h e stoichiometric composition Cuo.32As0.2180.47. Details are a v a i l a b l e e l s e w h e r e [10].
3. R E S U L T S The incorporation of oxygen into the CuAs-S glassy films from the sputtering gas is much more efficientthan it is in the Cu-As-Se system. This fact is illustratedin Fig. i which shows the at. % O incorporated in the films as a function of the partialpressure of 0 2 in the sputteringgas. In the S system (filledcircles in Fig. 1) there is approximatelyfour times as m u c h oxygen as there is in the Se films for the same partial pressure in the sputtering gas. Over the ranges shown in Fig. 1, the incorporation of oxygen, at constant pressure in the sputtering gas, is approximatelya linear function of the concentration of 02 in the sputtering gas.
P.C. Taylor et al. / Tetrahedrally-coordinated chalcogenide glasses containing oxygen
I
t
t
o.~IT-
0.
-
-~ 0.2
0.1
; ~.
I 5
I I0
_
15
At.~OInFtlms
Figure 2. Activation energy for electrical conductivity as a function of oxygen concentration. Open triangles and solidcircles are data for glassy Cuo.32Aso.21Seo.47 and Cuo.32Aso.21So.47,respectively.
In the Se system the room-temperature electrical conductivity a can be varied from approximately 10 -3 fflcml to greater than 1 flicml [8]. The optical energy gap Eg of glassy Cuo.32Aso. 21Se0.47 is - 1.3 eV (defihed as the energy at which the absorption coefficient a = 104 cml), and the activation energy A E for the electrical conductivity is 0.32 eV in the nominally-undoped samples, Thus, at this composition, even the nominally undoped samples have the Fermi level about one fourth of the gap from the valence band edge. We believe that this position of the F e r m i level is determined by residual defects, perhaps controlled by the residual oxygen in the glass. At high oxygen concentrations, where a >- 1 fl-1 cm-1, AE < 0.1 eV. In the S system (glassy Cuo.32Aso.2180.47 ) the optical band gap is larger (AE = 1.8 eV), and the effect of oxygen on the electronic properties is greater. In this material the room t e m p e r a t u r e conductivity is 10 "4 f f l cm-1 in the nominally undoped glass while the
1193
glasses with _> 10 at. % 0 have AE < 0.1 eV. The variation of the activation energies with oxygen concentration are shown in Fig. 2 for both the Se (open triangles) and S (flled circles) systems. It is apparent from this figure that the effect of oxygen is greater in the S system. There exists an optical absorption below the optical gap that also scales with the oxygen concentration. In the Se system this absorption exhibits an onset near 0.2 eV and a nearly constant magnitude above 0.2 eV that scales with the conductivity. In the S system the analogous absorption rises at about 0.4 eV as shown in Fig. 3. There also exists a dark electron spin resonance (ESR) signal that scales roughly as the below-gap optical absorption.
4. D I S C U S S I O N Certain structural and electronic properties of glassy Cuo.32Aso.21Seo.47 and CUo.32Aso.21So.47 are similar to those of a-Si:H. First, these two chalcogenide semiconductors are p r e d o m i n a n t l y tetrahedrallycoordinated and the dominant electronicdefects probably exhibit positive effectiveelectron-electron correlation energies (positiveUeff)[8,9]. Second, atleastforp-type electricalconductivity, films of these two glassy chalcogenides can be made with conductivitiesthat vary by several orders of magnitude and with activation energies that can be quite small (< 0.1 eV). At this point, however, the similarities between the group IV and the group VI tetrahedral amorphous semiconductors end and major differences appear. One major difference is that, although the covalent nature of the bonding tends to control the local structural order (the fundamental assumption of the FVS model [2,3,4]), most of the bonds in these two metal chalcogenide glasses have strong ionic character. In fact the ionicity is similar to that which exists in crystalline II-VI semiconductors. In the II-VI
P.C. Taylor et al. / Tetrahedrally-coordinated chalcogenide glasses containing oxygen
1194 105
,
10~
i
,
i
,
t
,
t
~_.~_~_,~_~~
,
÷
1~ "
Cuo.32Aso.21Seo.47 and about 0.4 eV above the valence band edge in Cu0.32As0.21S0.47 . The conductivity mechanism is still uncertairL The data on the Se system are consistent with a band-type conductivity over most of the doping range [8], b u t because the optical absorption onset in the S system is near 0.4 eV, one must question this interpretation for beth glasses.
ACKNOWLEDGEMENT I
a0
a~
0.s
1.2
l.e
h,(,v)
Figure
3.
This research was supported by the N S F under grant no. DMR-909-01596.
Optical Absorption in glassy
CUo.32As0.21S0.47 as a function of oxygen
REFERENCES
concentration- A nominally undoped; x, • , D, +, represent 2.9, 2.2, 5.4 and l4 at. %oxygen, respectively.
1.
semiconductors the dominant doping m e c h a n i s m i s u s u a l l y t h e presence of vacancies and/or interstitials. Cation vacancies are generally acceptors and anion vacancies generaUydonors. Because one type of vacancy is preferred in a given composition, it is diWicult to change the carrier type in most II-VI semiconductors. Only recently has substitutional doping become more common in II-VI systems [11]. In glassy Cuo.32Aso.21Seo.47 and Cu0.32As0.21S0.47 it has been suggested [8] that, although most of the oxygen atoms go into the glass substitutionally for S or Se, the p-type doping is controlled by the presence of oxygen atoms which compete for C u b e n d s and generate the amorphous analog of Cu vacancies(Se-Se bends and danglingso bends) in the glass. The dark ESR has been attributed to a "dangling bond" at a threefold-coordinated S or So site, and the below gap absorption to the excitation of electrons into these defects [8]. From the absorption data one can conclude that the defect level is about 0.2 eV above the valence band edge in
J.Z. L i u a n d P . C . Taylor, Phys. Rev. Lett. 59 (1987) 1938. 2. J.Z. Liu andP. C. Taylor, Solid State Commun. 70 (1989) 81. 3. J.Liu and P.C. Taylor, J. Non-Cryst. Solids 114 (1989) 25. 4. P.C. Taylor, Z.M. Salah andJ. Z. Liu, in Advanced in Disordered Semiconductors, H. Fritzsche, ed. (World Scientific, Singapore, 1990), Vol. 3, p. 23. 5. K.S. Liang, A. Bienenstock ad C. W. Bates, Phys. Rev. B10 (1974) 1528. 6. S. Laderman, A. Bienenstock and K.S. Liang, Solar Energy Mat. 8 (1982) 15. 7. S.II. IIunter, A. Bienenstock and T.M. IIayes, in Amorphous and Liquid Semiconductors, W. E. Spear, ed. (University of Edinburgh, Edinburgh, 1977), p. 78. 8. J. IIautala, B. Moosman and P.C. Taylor, J. Non-Cryst. Solids 137 & 138 (1991) 1043. 9. J. Hautala, S. Y a m a s a k i a n d P.C. Taylor, J. Non-Cryst Solids 114 (1989) 85. 10. R.E. Shirey and P.C. Taylor, U. U t a h J. Undergrad. Res. 3 (1992) 40. 11. See, for example, T. Yasuda, I. Miksuishi and H. Kukimoto, Appl. Phys. Lett. 52 (1988) 57.