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Chemical modification of the properties of amorphous arsenic
Journal of Non-Crystalline Solids 35 & 36 (1980) 883-888 ~North-Holland Publishing Company
C H E M I C A L M O D I F I C A T I O N OF THE P R O P E R T I E S OF A M O R P H O U S A R S E N I C E. M y t i l i n e o u * and E. A. Davis U n i v e r s i t y of Cambridge, Cavendish L a b o r a t o r y M a d i n g l e y Rd., C a m b r i d g e CB30HE U.K.
A m o r p h o u s films of arsenic c o n t a i n i n g a few p e r c e n t of Ni, Ge, S, Se or Te have been p r e p a r e d on w a t e r - c o o l e d substrates by c o - s p u t t e r i n g in pure Ar or A r / H 2 mixtures. M e a s u r e m e n t s of d.c. conductivity, t h e r m o p o w e r and optical absorption were made to c h a r a c t e r i z e the effects of chemical modification. I n c o r p o r a t i o n of Ni and Ge appear to shift slightly the Fermi level but this does not occur for h y d r o g e n a t e d films. Tentative models for the states in the gap p r o d u c i n g the o b s e r v e d effects will be described.
EXPERIMENTAL Thin s p u t t e r e d films of arsenic and arsenic with d i f f e r e n t amounts of impurities were p r e p a r e d in pure argon or in d i f f e r e n t a r g o n / h y d r o g e n mixtures, from a polyc r y s t a l l i n e arsenic target. Desired amounts of additives were p l a c e d on the face of the arsenic cathode, in a g e o m e t r y (3 p i e c e s in the p e r i p h e r y and one in the center) that was found to give h o m o g e n e o u s films. The substrates were C o m i n g 7059 glass; they were water cooled. A base p r e s s u r e of 2x10 -5 torr was o b t a i n e d before i n t { o d u c i n g 5x10 -3 torr of pure Ar or Ar/H 2 mixtures. The s p u t t e r i n g rate was % 0.5 A/sec with a r.f. p o w e r of about 70 watts. The sample thickness varied from 0.3~ to 0.5~. D e t a i l e d chemical analysis, with an X-ray fluorescence electron microscope, has net given any s a t i s f a c t o r y results due to c a l i b r a t i o n difficulties. It is b e l i e v e d that a linear relation holds b e t w e e n the d i f f e r e n t alloys of the same additives, because of the similar s p u t t e r i n g rates of As and the additives, and that the chemical analysis would give simply a correction factor, not far from unity. A larger d e v i a t i o n is, however, e x p e c t e d for alloys with Se, since this m a t e r i a l has a much higher s p u t t e r i n g rate than As. The p e r c e n t a g e n u m b e r given b e l o w will p r e s e n t the "surface ratio" of d i f f e r e n t elements on the As target.
DILUTE ALLOYS OF As WITH Ni. All the films of As with x% Ni, where x = 0.4, 0.7, 1.0 and 1.5 were p r e p a r e d by co-sputtering, on w a t e r - c o o l e d substrates and in a pure Ar atmosphere. At about room t e m p e r a t u r e (290 K), the absQlute value of the c o n d u c t i v i t y increases 4 orders of magnitude, from 2.2 1 0 - / ~ - i c m -I for pure As to 2.6 x 1 0 - 3 ~ - i c m -I for the 1.5% Ni alloy. [i] A t e m p e r a t u r e range from 330 to 160 K was covered. Two d i f f e r e n t regimes may be distinguished: (i) The region above room temperature, where there is an i n d i c a t i o n of an a c t i v a t i o n energy, EU, v a r y i n g from 0.40 eV for 0.4% Ni to 0.21 eV for 1.5% Ni. The intercepts lie b e t w e e n 80 and 13 ~ - i c m -I , w h i c h may indicate c o n d u c t i o n in l o c a l i z e d states near the band edge EA; (ii) B e l o w room temperature, v a r i a b l e - r a n g e h o p p i n g at the Fermi level is indicated. The t h e r m o p o w e r is p r a c t i c a l l y t e m p e r a t u r e independent with values of the order of 100D V/deg. Its value is c o n s i s t e n t with v a r i a b l e - r a n g e h o p p i n g conduction.
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E~ Mytilineou, EoA. Davis / Properties of Amorphous Arsenic
884
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The optical absorption coefficient is shifted toward lower photon energies as the concentration of Ni is I increased. In fig. 1 the optical gap, Eop t - determined from the plot of (~hv) ~ v.s. photon energy - and the "electrical gap", 2Eu, are plotted as a function of the Ni concentration. The electrical gap shows a dramatic decrease of 0.78 eV between pure As and As with 1.5% Ni. A fraction of a percent (~ 0.4) of Ni changes the electrical gap by about 0.4 eV. The optical gap decreases too, with the addition of Ni; its variation follows qualitatively that of the electrical gap but the overall change is much less. Nickel has been used successfully as a modifier by Ovshinsky (1977) and Flasck et al. (1977). They report that modification of chalcogenides, Si or As, is only possible by co-sputtering at very low substrate temperatures (77 K) where e q u i l i b r i u m between the charged additives and the valence alternation centers is prevented. Our results prove that modification of As with Ni, is possible even at higher substrate temperatures (water-cooled). Ovshinsky and Adler (1978) suggest that Ni is introduced into semiconductors in general, and chalcogenides in particular, w i t h different oxidation states, i.e. Ni l+, Ni 2+, Ni 3+, etc., the charge state d e p e n d i n g on the exact position of the Fermi energy. The effect of Ni is to compensate the C +~ (or D +) states, reducing greatly their concentration: to preserve charge neutrality, when the positively charged Ni sites exceed in density the number of initial C~ sites, the concentration of C 1 (or D ) is increased but remains always somewhat larger than the concentration of the Ni ions
Eo Mytilineou, E.A. Davis / Properties of Amorphous Arsenic
885
If a s u f f i c i e n t c o n c e n t r a t i o n of Ni is present, the Fermi level rises until it enters a band a r i s i n g from states of the modifier. This would o r d i n a r i l y result in e x t r i n s i c n-type conduction. Fig. 2a displays a sketch of the p r o p o s e d d e n s i t y of e l e c t r o n i c states of a m o r p h o u s arsenic. The v a l e n c e a l t e r n a t i o n pairs (VAPs) are P2 and P4 (Kastner and F r i t z s c h e 1978, Greaves, Elliott and Davis 1979). The a c t i v a t i o n energy for c o n d u c t i o n in e x t e n d e d states at E C is equal to Eg = (£i + C2)/2 (Fritzsche and K a s t n e r 1978). Ni is e x p e c t e d to be i n t r Q d u c e d as a p o s i t i v e l y charged impurity in amorphous arsenic. The N i l + a n d N i 3 + a r e s i n g l e - e l e c t r o n states, w h i l e N i 0 a n d N i 2+ a r e d o a b l e - e l e c t r o n states respectively; giving rise to defect states with h i g h e r energy than the s i n g l e - e l e c t r o n defect states. The defects form p o s i t i v e - U states (Adler and Yoffa 1976). Very small c o n c e n t r a t i o n s of Ni in amorphous arsenic decrease the c o n c e n t r a t i o n of P~_ over P2- centers. If P 4 , + P~z are still the p r e d o m i n a n t defects no change is expected in the optical E _ and the ' op5 ' electrical gap, 2Eu, as shown by the i n t e r p o l a t e d dashed curve in fig. l. , + C o n c e n t r a t i o n s of Ni (~ 0.4%) are s u f f l c l e n t to saturate the P4 centers as can be d e d u c e d from the changes in the electrical and optical gaps. A sketch of the density of e l e c t r o n i c states, in this case, is i l l u s t r a t e d in fig. 2b. It is b e l i e v e d that for such small c o n c e n t r a t i o n s of Ni the e n e r g e t i c a l l y p r e f e r r e d defects are the lower energy s i n g l e - e l e c t r o n states of Ni l+ (not shown) and Ni3+; their c o r r e s p o n d i n g d o u b l e - e l e c t r o n states b e i n g higher in energy (by a correlation energy) will be empty. The exact energy and thus the p o s i t i o n of these states in the gap, is not known, so fig. 2b is a q u a l i t a t i v e diagram. The Fermi level enters the band arising from singly o c c u p i e d Ni states. The a c t i v a t i o n energy for conduction is then E~ = C 3. E has been lowered from that in pure As for two reasons (i) the Fermi l e v e l has been shifted upwards and (ii) the p r e d o m i n a n t c o n d u c t i o n p a t h has changed from b e i n g at E to E A as d i s c u s s e d C earlier. At lower t e m p e r a t u r e s v a r i a b l e - r a n g e hopping, in the s l n g l e - e l e c t r o n states of Ni, at E F occurs. On i n c r e a s i n g the c o n c e n t r a t i o n of Ni up to ~ 0.7%, the Fermi level remains almost constant. H o w e v e r as far as the Ni c o n c e n t r a t i o n increased above 0.7% the density of double o c c u p i e d electron states m a y become important resulting in a further decrease of E@. It should be n o t i c e d that the optical gap, Eop t, is a f f e c t e d only slightly by the addition of Ni.
DILUTE ALLOYS OF As WITH Ge. Films of AS with x% Ge, where x = 0.64, 1.9, 5.7 and 12 were c o - s p u t t e r e d in pure Ar or in d i f f e r e n t H2/Ar mixtures. The s u b s t r a t e s were water-cooled. The t e m p e r a t u r e d e p e n d e n c e of the c o n d u c t i v i t y of films c o - s p u t t e r e d in pure Ar was m e a s u r e d b e t w e e n 345-180 K. Two d i f f e r e n t regimes may be distinguished, In the first, just above room temperature, the c o n d u c t i v i t y a c t i v a t i o n energies and the intercepts slowly ~ e c r e a s e with x, from 0.62 eV and 3 x 1 0 3 ~ - i c m -I for 0.64% Ge to 0.44 eV and 5~- cm -I for 12% Ge~ while the absolute value of the c o n d u c t i v i t y is almost u n c h a n g e d (8-10 x 10-7Q - cm -I at 300 K). F r o m the values of the intercepts it is c o n c l u d e d t h a t the t r a n s p o r t m e c h a n i s m changes smoothly from e x t e n d e d states c o n d u c t i o n at E C to h o p p i n g at the localized states at E A for the 12% Ge alloy and p e r h a p s for the 5.7% Ge alloy. B e l o w room t e m p e r a t u r e a v a r i a b l e - r a n g e h o p p i n g region appears with the addition of Ge. The a b s o r p t i o n c o e f f i c i e n t as a function of p h o t o n energy, is shifted toward h i g h e r energies, as the c o n c e n t r a t i o n of Ge is increased. The v a r i a t i o n of the optical gap, E , and the e l e c t r i c a l gap 2E , as a function of the Ge content o t ' q are p l o t t e d in ~lg. 3a. For small c o n c e n t r a t l o n s of Ge (~ 0.64%) the optical gap d e c r e a s e s while the e l e c t r i c a l gap increases. For h i g h e r c o n c e n t r a t i o n s of Ge the optical gap increases (up to x = 5.7) and remains stable, while the electrical
E. Mytilineou, E.Ao Davis / Properties of Amorphous Arsenic
886
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The v a r i a t i o n of the optical gap, E _, and the e l e c t r i c a l gap, • OD 2Eu, as a function (a) of the Ge content and (b) of the H_ c o n c e n t r a t l o n zn the s p u t t e r ± n g gas of the As 12% Ge alloy. Sketch of the p r o p o s e d density of electronic states of amorphous arsenic that has been c h e m i c a l l y m o d i f i e d by a sufficient c o n c e n t r a t i o n of Ge to obtain extrinsic conduction. Occupied states at T = 0 are shaded.
gap d e c r e a s e s about ~ 0.36 eV as the t r a n s p o r t m e c h a n i s m changes for c o n d u c t i o n in e x t e n d e d states at E C, to h o p p i n g at the l o c a l i z e d states at E AThe effects of h y d r o g e n a t i o n d u r i n g c o - s p u t t e r i n g of As with Ge in 20/80 and 40/60 H 2 in Ar m i x t u r e s were studied. Cor~non features are o b s e r v e d in all cases. The value of the conductivity, at room temperature, d e c r e a s e d by about two orders of m a g n i t u d e when 40/60 H 2 in Ar mixture was used. The a c t i v a t i o n energy for c o n d u c t i o n shows an increase of 0.15 eV for the alloy with 0.64% Ge and up to 0.30 eV for the alloy with 12% Ge. The v a r i a b l e range h o p p i n g regime disappears. The a b s o r p t i o n c o e f f i c i e n t in all cases shifts to higher energies. Fig. 3b displays the v a r i a t i o n of the E and 2E O for the 12% Ge alloy as a function of ot the H 2 content in the s p u t t e r i n ~ g a s . Both, optical and electrical gaps increase at d i f f e r e n t rates and at 40% H 2 in the s p u t t e r i n g gas they b e c o m e almost equal at about 1.46 eV, c r e a t i n g a new intrinsic alloy. In t e t r a h e d r a l a m o r p h o u s semiconductors, such as Si and Ge, t h r e e - f o l d c o o r d i n a t e d atoms w i t h a d a n g l i n g bond, are the common defects. They give rise to a high density of states at the Fermi level. If Ge is i n t r o d u c e d into the t h r e e - f o l d c o o r d i n a t e d n e t w o r k of amorphous AS, it may be r e a s o n a b l e to suggest two possibilities; (i) The Ge keeps its tetrahedral coordination, c o n n e c t e d by strained bonds to P~ atoms (a rather e n e r g e t i c a l l y u n f a v o r a b l e situation). This is not an e l e c t r i c a l l y active reaction as the concentrations of p o s i t i v e and negative charged defects remain equal. (ii) The Ge is t h r e e - f o l d c o o r d i n a t e d with a d a n g l i n g bond. The d a n g l i n g bonds may give rise to deep donor and acceptor levels (as in a m o r p h o u s Ge or Si); their relative p o s i t i o n in the gap of amorphou~ arsenic could be above m i d - g a p as can be c o n c l u d e d from the m o l e c u l a r states d i a g r a m of K a s t n e r (1972). The c o n c e n t r a t i o n of the YAPs is unchanged. Fig. 3c displays a sketch of the p r o p o s e d density of e l e c t r o n i c states of amorphous
F. Mytilineou, E.A. Davis / Properties of Amorphous Arsenic
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arsenic that has been chemically modified with Ge. From fig. 3a it could be concluded that small concentrations of Ge (< 0.64%) are enough to shift the Fermi level into the band arising from the lower energy (donor) states of the Ge dangling bonds. This band may lie below the Fermi level of pure amorphous arsenic as the electrical gap increases with the addition of 0.64% Ge. For temperatures above 300 K, the electrical conduction is in the extended states at E C and its value is almost unchanged; at low temperatures variable-range hopping occurs, due to the high density of Ge dangling bonds at the Fermi level. As the concentration of Ge increases, the Fermi level is shifted towards the conduction band, as it becomes gradually unpinned by the positive-U states of the Ge dangling bonds. Hydrogenation of the 12% Ge alloy (fig. 3b) results in a saturation of all the Ge dangling bonds. The new intrinsic alloy, when 40% H 2 in Ar is used, has an optical gap similar to that of pure As prepared in a similar sputtering gas.
DILUTE ALLOYS OF As WITH CHALCOGENIDES. Three different alloys of As with each of the chalcogenides S, Se or Te were made by co-sputtering in pure Ar, on water-cooled substrates. The chalcogen concentration was < 1%. Similar features are observed in all of them. With addition of 0.40% chalcogen the absolute value of the conductivity increases slightly (~ - 2.6x10-7~-icm -I) while the activation energy remains the same as in pure As. Below room temperature variable-range hopping conduction is indicated. Higher concentrations decrease the absolute value of the conductivity, about an order of magnitude for 1% chalcogen alloys and increase the activation energy. The optical absorption coefficient at room temperature shifts towards lower energies for the two first alloys (x < 0.65) and towards higher energies for the 1% chalcogen alloys. The variation of the optical, Eopt, and electrical gap, 2EG, as a function of the S content is plotted in fig. 4a. For low values of x(x ~ 0.4), the electrical gap seems to follow the variation of the optical gap; as x increases the electrical gap increases at a slightly higher rate than the optical gap. For the 1% S alloy the difference between the two gaps is almost double than for the 0.36% S alloy; this may be evidence of a shift of the Fermi level towards the valence band.
E. Mytilineou, E.A. Davis / Properties of Amorphous Arsenic
888
Arsenic with 1% Te have been co-sputtered in different x% H2/Ar mixtures, with x = i0, 20, 30 and 40. After hydrogenation the absolute value of the conductivity decreases about two orders of magnitude for the 40% H 2 in At. Initially an increase of 0.13 eV in the activation energy is observed for 20% H 2 in the sputtering gas; higher concentrations of H 2 decrease slightly the activation energy (fig. 4b). The temperature dependence of the conductivity is a straight line for the temperature range covered. The states that caused the deviation of linearity in the alloy sputtered in pure Ar, seem to disappear with the addition of H 2. The optical absorption coefficient is shifted towards higher energies with the addition of H 2 in the sputtering gas. The optical gap increased ~ 0.2 eV when 40% H 2 is added (fig. 4b). When both chalcogenides and pnictides are present in a glass (for example As2Se3) all the specles • P4' + C3' + P2, C 1 will be present but in different concentrations (Kastner and Fritzsche 1978). The more energetically favorable reactions seem to be the 2C
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The addition of the chalcogenide elements in amorphous arsenic, may introduce a small concentratmon of C 3 and C VAPs in addltlon to the P~ = and P2. The charged 1 defect centers formed by eq. (2) do not change the balance of the charged centers. The relatlve posltion of the C3, C 1 states in the gap depends on the specific element (Kastner 1972), since the lone-pair level of the chalcogenides shifts towards the bonding states of As as their energy gap increases. In the case of S (with the larger gap) it is reasonable to assume that the C~ states lie below midgap (fig. 4c), gradually shifting the Fermi level towards the valence band. •
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REFERENCES *Now at the University of Athens, [i] [2] [3]
[4] [5] [6] [7] [8] [9]
Solonos 104, Athens 144, Greece
Data will be presented in more detail elsewhere. Adler, D. and Yoffa, E. J., Electronic Structure of Amorphous Semiconductors, Phys. Rev. Lett. 36 (1976) 1197. Flasck, R., Izu, M., Sapru, K., Anderson, T., Ovshinsky, S. R. and Fritzsche, H., Optical and Electronic Properties of Modified Amorphous Materials, in: Spear, W. E. (ed), Amorphous and Liquid Semiconductors, (1977) 523 (Center of Industrial Consultance and Liaison, University of Edinburgh). Fritzsche, H. and Kastner, M., The Effect of Charged Additives on the Carrier Concentrations in Lone-pair Semiconductors, Phil. Mag. B. 37 (1978) 285. Greaves, G. N., Elliott, S. R., and Davis, E. A., Amorphous Arsenic, Adv. Phys. 28 (1979) 49. Kastner, M., Bonding bands, and Impurity States in Chalcogenide Semiconductors, Phys. Rev. Lett. 28 (1972) 355. Kastner, M. and Fritzsche, H., Defect Chemistry of Lone-pair Semiconductors, Phil. Mag. B., 37 (1978) 199. Ovshinsky, S. R., Chemical Modification of a-Semiconductors, ibid [3] p. 519. Ovshinsky, S. R. and Adler, D., Local Structure Bonding and Electronic Properties of Covalent Amorphous Semiconductors., Contemp. Phys. 19 (1978) 109.
C H E M I C A L M O D I F I C A T I O N OF THE P R O P E R T I E S OF A M O R P H O U S A R S E N I C E. M y t i l i n e o u * and E. A. Davis U n i v e r s i t y of Cambridge, Cavendish L a b o r a t o r y M a d i n g l e y Rd., C a m b r i d g e CB30HE U.K.
A m o r p h o u s films of arsenic c o n t a i n i n g a few p e r c e n t of Ni, Ge, S, Se or Te have been p r e p a r e d on w a t e r - c o o l e d substrates by c o - s p u t t e r i n g in pure Ar or A r / H 2 mixtures. M e a s u r e m e n t s of d.c. conductivity, t h e r m o p o w e r and optical absorption were made to c h a r a c t e r i z e the effects of chemical modification. I n c o r p o r a t i o n of Ni and Ge appear to shift slightly the Fermi level but this does not occur for h y d r o g e n a t e d films. Tentative models for the states in the gap p r o d u c i n g the o b s e r v e d effects will be described.
EXPERIMENTAL Thin s p u t t e r e d films of arsenic and arsenic with d i f f e r e n t amounts of impurities were p r e p a r e d in pure argon or in d i f f e r e n t a r g o n / h y d r o g e n mixtures, from a polyc r y s t a l l i n e arsenic target. Desired amounts of additives were p l a c e d on the face of the arsenic cathode, in a g e o m e t r y (3 p i e c e s in the p e r i p h e r y and one in the center) that was found to give h o m o g e n e o u s films. The substrates were C o m i n g 7059 glass; they were water cooled. A base p r e s s u r e of 2x10 -5 torr was o b t a i n e d before i n t { o d u c i n g 5x10 -3 torr of pure Ar or Ar/H 2 mixtures. The s p u t t e r i n g rate was % 0.5 A/sec with a r.f. p o w e r of about 70 watts. The sample thickness varied from 0.3~ to 0.5~. D e t a i l e d chemical analysis, with an X-ray fluorescence electron microscope, has net given any s a t i s f a c t o r y results due to c a l i b r a t i o n difficulties. It is b e l i e v e d that a linear relation holds b e t w e e n the d i f f e r e n t alloys of the same additives, because of the similar s p u t t e r i n g rates of As and the additives, and that the chemical analysis would give simply a correction factor, not far from unity. A larger d e v i a t i o n is, however, e x p e c t e d for alloys with Se, since this m a t e r i a l has a much higher s p u t t e r i n g rate than As. The p e r c e n t a g e n u m b e r given b e l o w will p r e s e n t the "surface ratio" of d i f f e r e n t elements on the As target.
DILUTE ALLOYS OF As WITH Ni. All the films of As with x% Ni, where x = 0.4, 0.7, 1.0 and 1.5 were p r e p a r e d by co-sputtering, on w a t e r - c o o l e d substrates and in a pure Ar atmosphere. At about room t e m p e r a t u r e (290 K), the absQlute value of the c o n d u c t i v i t y increases 4 orders of magnitude, from 2.2 1 0 - / ~ - i c m -I for pure As to 2.6 x 1 0 - 3 ~ - i c m -I for the 1.5% Ni alloy. [i] A t e m p e r a t u r e range from 330 to 160 K was covered. Two d i f f e r e n t regimes may be distinguished: (i) The region above room temperature, where there is an i n d i c a t i o n of an a c t i v a t i o n energy, EU, v a r y i n g from 0.40 eV for 0.4% Ni to 0.21 eV for 1.5% Ni. The intercepts lie b e t w e e n 80 and 13 ~ - i c m -I , w h i c h may indicate c o n d u c t i o n in l o c a l i z e d states near the band edge EA; (ii) B e l o w room temperature, v a r i a b l e - r a n g e h o p p i n g at the Fermi level is indicated. The t h e r m o p o w e r is p r a c t i c a l l y t e m p e r a t u r e independent with values of the order of 100D V/deg. Its value is c o n s i s t e n t with v a r i a b l e - r a n g e h o p p i n g conduction.
883
E~ Mytilineou, EoA. Davis / Properties of Amorphous Arsenic
884
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2. Sketch of the p r o p o s e d density of electronic states of (a) amorphous arsenic and (b) amorphous arsenic that has been chemically modified by a sufficient concentration of Ni to obtain extrinsic conduction. The activation energy for conduction in the extended states is (£1 +E2)/2 for case (a) and ~3 in case (b). Occupied states at T = 0 are shaded.
The optical absorption coefficient is shifted toward lower photon energies as the concentration of Ni is I increased. In fig. 1 the optical gap, Eop t - determined from the plot of (~hv) ~ v.s. photon energy - and the "electrical gap", 2Eu, are plotted as a function of the Ni concentration. The electrical gap shows a dramatic decrease of 0.78 eV between pure As and As with 1.5% Ni. A fraction of a percent (~ 0.4) of Ni changes the electrical gap by about 0.4 eV. The optical gap decreases too, with the addition of Ni; its variation follows qualitatively that of the electrical gap but the overall change is much less. Nickel has been used successfully as a modifier by Ovshinsky (1977) and Flasck et al. (1977). They report that modification of chalcogenides, Si or As, is only possible by co-sputtering at very low substrate temperatures (77 K) where e q u i l i b r i u m between the charged additives and the valence alternation centers is prevented. Our results prove that modification of As with Ni, is possible even at higher substrate temperatures (water-cooled). Ovshinsky and Adler (1978) suggest that Ni is introduced into semiconductors in general, and chalcogenides in particular, w i t h different oxidation states, i.e. Ni l+, Ni 2+, Ni 3+, etc., the charge state d e p e n d i n g on the exact position of the Fermi energy. The effect of Ni is to compensate the C +~ (or D +) states, reducing greatly their concentration: to preserve charge neutrality, when the positively charged Ni sites exceed in density the number of initial C~ sites, the concentration of C 1 (or D ) is increased but remains always somewhat larger than the concentration of the Ni ions
Eo Mytilineou, E.A. Davis / Properties of Amorphous Arsenic
885
If a s u f f i c i e n t c o n c e n t r a t i o n of Ni is present, the Fermi level rises until it enters a band a r i s i n g from states of the modifier. This would o r d i n a r i l y result in e x t r i n s i c n-type conduction. Fig. 2a displays a sketch of the p r o p o s e d d e n s i t y of e l e c t r o n i c states of a m o r p h o u s arsenic. The v a l e n c e a l t e r n a t i o n pairs (VAPs) are P2 and P4 (Kastner and F r i t z s c h e 1978, Greaves, Elliott and Davis 1979). The a c t i v a t i o n energy for c o n d u c t i o n in e x t e n d e d states at E C is equal to Eg = (£i + C2)/2 (Fritzsche and K a s t n e r 1978). Ni is e x p e c t e d to be i n t r Q d u c e d as a p o s i t i v e l y charged impurity in amorphous arsenic. The N i l + a n d N i 3 + a r e s i n g l e - e l e c t r o n states, w h i l e N i 0 a n d N i 2+ a r e d o a b l e - e l e c t r o n states respectively; giving rise to defect states with h i g h e r energy than the s i n g l e - e l e c t r o n defect states. The defects form p o s i t i v e - U states (Adler and Yoffa 1976). Very small c o n c e n t r a t i o n s of Ni in amorphous arsenic decrease the c o n c e n t r a t i o n of P~_ over P2- centers. If P 4 , + P~z are still the p r e d o m i n a n t defects no change is expected in the optical E _ and the ' op5 ' electrical gap, 2Eu, as shown by the i n t e r p o l a t e d dashed curve in fig. l. , + C o n c e n t r a t i o n s of Ni (~ 0.4%) are s u f f l c l e n t to saturate the P4 centers as can be d e d u c e d from the changes in the electrical and optical gaps. A sketch of the density of e l e c t r o n i c states, in this case, is i l l u s t r a t e d in fig. 2b. It is b e l i e v e d that for such small c o n c e n t r a t i o n s of Ni the e n e r g e t i c a l l y p r e f e r r e d defects are the lower energy s i n g l e - e l e c t r o n states of Ni l+ (not shown) and Ni3+; their c o r r e s p o n d i n g d o u b l e - e l e c t r o n states b e i n g higher in energy (by a correlation energy) will be empty. The exact energy and thus the p o s i t i o n of these states in the gap, is not known, so fig. 2b is a q u a l i t a t i v e diagram. The Fermi level enters the band arising from singly o c c u p i e d Ni states. The a c t i v a t i o n energy for conduction is then E~ = C 3. E has been lowered from that in pure As for two reasons (i) the Fermi l e v e l has been shifted upwards and (ii) the p r e d o m i n a n t c o n d u c t i o n p a t h has changed from b e i n g at E to E A as d i s c u s s e d C earlier. At lower t e m p e r a t u r e s v a r i a b l e - r a n g e hopping, in the s l n g l e - e l e c t r o n states of Ni, at E F occurs. On i n c r e a s i n g the c o n c e n t r a t i o n of Ni up to ~ 0.7%, the Fermi level remains almost constant. H o w e v e r as far as the Ni c o n c e n t r a t i o n increased above 0.7% the density of double o c c u p i e d electron states m a y become important resulting in a further decrease of E@. It should be n o t i c e d that the optical gap, Eop t, is a f f e c t e d only slightly by the addition of Ni.
DILUTE ALLOYS OF As WITH Ge. Films of AS with x% Ge, where x = 0.64, 1.9, 5.7 and 12 were c o - s p u t t e r e d in pure Ar or in d i f f e r e n t H2/Ar mixtures. The s u b s t r a t e s were water-cooled. The t e m p e r a t u r e d e p e n d e n c e of the c o n d u c t i v i t y of films c o - s p u t t e r e d in pure Ar was m e a s u r e d b e t w e e n 345-180 K. Two d i f f e r e n t regimes may be distinguished, In the first, just above room temperature, the c o n d u c t i v i t y a c t i v a t i o n energies and the intercepts slowly ~ e c r e a s e with x, from 0.62 eV and 3 x 1 0 3 ~ - i c m -I for 0.64% Ge to 0.44 eV and 5~- cm -I for 12% Ge~ while the absolute value of the c o n d u c t i v i t y is almost u n c h a n g e d (8-10 x 10-7Q - cm -I at 300 K). F r o m the values of the intercepts it is c o n c l u d e d t h a t the t r a n s p o r t m e c h a n i s m changes smoothly from e x t e n d e d states c o n d u c t i o n at E C to h o p p i n g at the localized states at E A for the 12% Ge alloy and p e r h a p s for the 5.7% Ge alloy. B e l o w room t e m p e r a t u r e a v a r i a b l e - r a n g e h o p p i n g region appears with the addition of Ge. The a b s o r p t i o n c o e f f i c i e n t as a function of p h o t o n energy, is shifted toward h i g h e r energies, as the c o n c e n t r a t i o n of Ge is increased. The v a r i a t i o n of the optical gap, E , and the e l e c t r i c a l gap 2E , as a function of the Ge content o t ' q are p l o t t e d in ~lg. 3a. For small c o n c e n t r a t l o n s of Ge (~ 0.64%) the optical gap d e c r e a s e s while the e l e c t r i c a l gap increases. For h i g h e r c o n c e n t r a t i o n s of Ge the optical gap increases (up to x = 5.7) and remains stable, while the electrical
E. Mytilineou, E.Ao Davis / Properties of Amorphous Arsenic
886
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The v a r i a t i o n of the optical gap, E _, and the e l e c t r i c a l gap, • OD 2Eu, as a function (a) of the Ge content and (b) of the H_ c o n c e n t r a t l o n zn the s p u t t e r ± n g gas of the As 12% Ge alloy. Sketch of the p r o p o s e d density of electronic states of amorphous arsenic that has been c h e m i c a l l y m o d i f i e d by a sufficient c o n c e n t r a t i o n of Ge to obtain extrinsic conduction. Occupied states at T = 0 are shaded.
gap d e c r e a s e s about ~ 0.36 eV as the t r a n s p o r t m e c h a n i s m changes for c o n d u c t i o n in e x t e n d e d states at E C, to h o p p i n g at the l o c a l i z e d states at E AThe effects of h y d r o g e n a t i o n d u r i n g c o - s p u t t e r i n g of As with Ge in 20/80 and 40/60 H 2 in Ar m i x t u r e s were studied. Cor~non features are o b s e r v e d in all cases. The value of the conductivity, at room temperature, d e c r e a s e d by about two orders of m a g n i t u d e when 40/60 H 2 in Ar mixture was used. The a c t i v a t i o n energy for c o n d u c t i o n shows an increase of 0.15 eV for the alloy with 0.64% Ge and up to 0.30 eV for the alloy with 12% Ge. The v a r i a b l e range h o p p i n g regime disappears. The a b s o r p t i o n c o e f f i c i e n t in all cases shifts to higher energies. Fig. 3b displays the v a r i a t i o n of the E and 2E O for the 12% Ge alloy as a function of ot the H 2 content in the s p u t t e r i n ~ g a s . Both, optical and electrical gaps increase at d i f f e r e n t rates and at 40% H 2 in the s p u t t e r i n g gas they b e c o m e almost equal at about 1.46 eV, c r e a t i n g a new intrinsic alloy. In t e t r a h e d r a l a m o r p h o u s semiconductors, such as Si and Ge, t h r e e - f o l d c o o r d i n a t e d atoms w i t h a d a n g l i n g bond, are the common defects. They give rise to a high density of states at the Fermi level. If Ge is i n t r o d u c e d into the t h r e e - f o l d c o o r d i n a t e d n e t w o r k of amorphous AS, it may be r e a s o n a b l e to suggest two possibilities; (i) The Ge keeps its tetrahedral coordination, c o n n e c t e d by strained bonds to P~ atoms (a rather e n e r g e t i c a l l y u n f a v o r a b l e situation). This is not an e l e c t r i c a l l y active reaction as the concentrations of p o s i t i v e and negative charged defects remain equal. (ii) The Ge is t h r e e - f o l d c o o r d i n a t e d with a d a n g l i n g bond. The d a n g l i n g bonds may give rise to deep donor and acceptor levels (as in a m o r p h o u s Ge or Si); their relative p o s i t i o n in the gap of amorphou~ arsenic could be above m i d - g a p as can be c o n c l u d e d from the m o l e c u l a r states d i a g r a m of K a s t n e r (1972). The c o n c e n t r a t i o n of the YAPs is unchanged. Fig. 3c displays a sketch of the p r o p o s e d density of e l e c t r o n i c states of amorphous
F. Mytilineou, E.A. Davis / Properties of Amorphous Arsenic
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arsenic that has been chemically modified with Ge. From fig. 3a it could be concluded that small concentrations of Ge (< 0.64%) are enough to shift the Fermi level into the band arising from the lower energy (donor) states of the Ge dangling bonds. This band may lie below the Fermi level of pure amorphous arsenic as the electrical gap increases with the addition of 0.64% Ge. For temperatures above 300 K, the electrical conduction is in the extended states at E C and its value is almost unchanged; at low temperatures variable-range hopping occurs, due to the high density of Ge dangling bonds at the Fermi level. As the concentration of Ge increases, the Fermi level is shifted towards the conduction band, as it becomes gradually unpinned by the positive-U states of the Ge dangling bonds. Hydrogenation of the 12% Ge alloy (fig. 3b) results in a saturation of all the Ge dangling bonds. The new intrinsic alloy, when 40% H 2 in Ar is used, has an optical gap similar to that of pure As prepared in a similar sputtering gas.
DILUTE ALLOYS OF As WITH CHALCOGENIDES. Three different alloys of As with each of the chalcogenides S, Se or Te were made by co-sputtering in pure Ar, on water-cooled substrates. The chalcogen concentration was < 1%. Similar features are observed in all of them. With addition of 0.40% chalcogen the absolute value of the conductivity increases slightly (~ - 2.6x10-7~-icm -I) while the activation energy remains the same as in pure As. Below room temperature variable-range hopping conduction is indicated. Higher concentrations decrease the absolute value of the conductivity, about an order of magnitude for 1% chalcogen alloys and increase the activation energy. The optical absorption coefficient at room temperature shifts towards lower energies for the two first alloys (x < 0.65) and towards higher energies for the 1% chalcogen alloys. The variation of the optical, Eopt, and electrical gap, 2EG, as a function of the S content is plotted in fig. 4a. For low values of x(x ~ 0.4), the electrical gap seems to follow the variation of the optical gap; as x increases the electrical gap increases at a slightly higher rate than the optical gap. For the 1% S alloy the difference between the two gaps is almost double than for the 0.36% S alloy; this may be evidence of a shift of the Fermi level towards the valence band.
E. Mytilineou, E.A. Davis / Properties of Amorphous Arsenic
888
Arsenic with 1% Te have been co-sputtered in different x% H2/Ar mixtures, with x = i0, 20, 30 and 40. After hydrogenation the absolute value of the conductivity decreases about two orders of magnitude for the 40% H 2 in At. Initially an increase of 0.13 eV in the activation energy is observed for 20% H 2 in the sputtering gas; higher concentrations of H 2 decrease slightly the activation energy (fig. 4b). The temperature dependence of the conductivity is a straight line for the temperature range covered. The states that caused the deviation of linearity in the alloy sputtered in pure Ar, seem to disappear with the addition of H 2. The optical absorption coefficient is shifted towards higher energies with the addition of H 2 in the sputtering gas. The optical gap increased ~ 0.2 eV when 40% H 2 is added (fig. 4b). When both chalcogenides and pnictides are present in a glass (for example As2Se3) all the specles • P4' + C3' + P2, C 1 will be present but in different concentrations (Kastner and Fritzsche 1978). The more energetically favorable reactions seem to be the 2C
~ C +3 + C
(i)
and
P3 +
2
+
P4 + C
(2).
The addition of the chalcogenide elements in amorphous arsenic, may introduce a small concentratmon of C 3 and C VAPs in addltlon to the P~ = and P2. The charged 1 defect centers formed by eq. (2) do not change the balance of the charged centers. The relatlve posltion of the C3, C 1 states in the gap depends on the specific element (Kastner 1972), since the lone-pair level of the chalcogenides shifts towards the bonding states of As as their energy gap increases. In the case of S (with the larger gap) it is reasonable to assume that the C~ states lie below midgap (fig. 4c), gradually shifting the Fermi level towards the valence band. •
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REFERENCES *Now at the University of Athens, [i] [2] [3]
[4] [5] [6] [7] [8] [9]
Solonos 104, Athens 144, Greece
Data will be presented in more detail elsewhere. Adler, D. and Yoffa, E. J., Electronic Structure of Amorphous Semiconductors, Phys. Rev. Lett. 36 (1976) 1197. Flasck, R., Izu, M., Sapru, K., Anderson, T., Ovshinsky, S. R. and Fritzsche, H., Optical and Electronic Properties of Modified Amorphous Materials, in: Spear, W. E. (ed), Amorphous and Liquid Semiconductors, (1977) 523 (Center of Industrial Consultance and Liaison, University of Edinburgh). Fritzsche, H. and Kastner, M., The Effect of Charged Additives on the Carrier Concentrations in Lone-pair Semiconductors, Phil. Mag. B. 37 (1978) 285. Greaves, G. N., Elliott, S. R., and Davis, E. A., Amorphous Arsenic, Adv. Phys. 28 (1979) 49. Kastner, M., Bonding bands, and Impurity States in Chalcogenide Semiconductors, Phys. Rev. Lett. 28 (1972) 355. Kastner, M. and Fritzsche, H., Defect Chemistry of Lone-pair Semiconductors, Phil. Mag. B., 37 (1978) 199. Ovshinsky, S. R., Chemical Modification of a-Semiconductors, ibid [3] p. 519. Ovshinsky, S. R. and Adler, D., Local Structure Bonding and Electronic Properties of Covalent Amorphous Semiconductors., Contemp. Phys. 19 (1978) 109.