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Hole emission defect states in amorphous As2Se3
Journal of Non-Crystalline Solids 35 & 36 (1980) 837-842 © North-Holland Publishing Company
ttole Emission Defect States in Amorphous As2Se 3 Andrew R. Melnyk Webster Research Center Xerox Corporation Rochester, New York
The influence of composition (30 to 40% As and 0 to 0.5% 1), deposition parameters, and subsequent annealing on charge emission centers in vacuum deposited As2Se3 films was studied. The filermal emission of holes flora deep gap states was measured by a time resolved charge depletion technique. The observed states appear to be a tail of a distribution that peaks above 1.1 eV. The emission rate decreased with reduction of arsenic and increased with the addition of iodine. The rate was sensitive to sample fabrication; decreasing with substrate temperature in the vicinity of the glass temperature Tg. Freshly made fihns had a high initial rate which relaxed with a thne constant of ahout 100 days at room temperature. Annealing by heating the samples to T g and cooling slowly relaxed the emission rate further. The results are • dmcussed in terms of the recent charged defect models.
1. Introduction Localized gap states play a fundamental role in determining the electronic and optical properties of amorphous semiconductors such as the chalcogenide glasses. Some ten years ago Cohen, Fritzsche and Ovshinsky 1 proposed that with all bonds satisfied the disorder in an amorphous solid leads to featureless tails of localized states extending from the valence and conduction bands into the gap. In the last four years models have been developed on the idea of gap states due to charged palm defects. Street and Matt 2 proposed that topological constraints caused by disorder will result in a finite density of dangling bonds (D). Pairs of these D's exchange an electron to fbrm charge defects D + and D', the reaction being exothermic. Kasmcr, Adler and Vritzsche 3 using a chemical approach proposed a similar model, but replaced the D by a three-fold-coordinated chalcogen atom C3°. The dangling bond, they argued, is energetically too expensive and so reacts with the kme pair electrons of a neighboring normal two-fuld-coordinated chalcogen forming half a bond wilh an unpaired electron in the antibonding state. Removing that electron results in the charged defect C3-~, while adding an electron results in a singly coordinated chalcogen C1". One technique to probe gap states in chalcogenide glasses is the dark discharge measurement, which observes states that thermally emit charge carriers. The technique was first used by both lng and Neyhart, 4 and Montrimas, et. al. 5 on amorphous As2Se 3 and later by Schein (' on Se. Recently Melnyk, Neyhart and Sperry 7 (MNS) refined the measurement and analysis, employing the phcnom~.'non of charge depletion to directly measure the thermally generated charge. This study examines the gap states observed by the dark charge depletion (DCD) measurement and compares the results with the defect theories. Section 2 briefly reviews the DCD measurement and analysis, and summarizes the experimental details. The energy distribution of the gap states is examined in Section 3, and from the data of MNS it is shown that the measurement observes the tail of a distribution of states deeper than 1.0 eV. Section 4 prcscnts rcsuhs on the effects of small additions of iodine and reduction of arsenic, and effects of annealing, deposition rate and substrate temperature on the gap states. These experiments suggest a link between the observed states and defects frozen-in by cooling from the glass temperature. This is discussed in the last section and an attempt is made to interpret the results with the charged defect theories, namely, that the observed thermal generation is C + --~ C" + 2 holes.
837
838
AoRo Melnyk / Hole Emission Defect States in Amorphous As2Se 3
2, I)cpletion Measurement and Analysis The dark discharge measurement involves capacitivcly charging a layer of the material and measuring the surface potential in an open circuit configuration, Figure I. The charging is usually done with a corona device, but could also be done with a momentary contact closure between an electrndcd sample and a ~oltage source, '['he essential requirement is that the two surfaces or electrodes do not inject charge into the sample, so the discharge is due only to the generation and transport of the charge in the bulk. If the transport rate is much larger t'~an the generation rate, the voltage discharge rate dV/dt is proportional to the thermal generation rate. Unfi}rtunately a fraction of the surface charge is injected, even with thin blocking layers, and may initially dominate the discharge. The DCD measurement solves this problem by relying on the fact that only one charge carrier, the hole, is mobile. The discharge proceeds by the depletion of holes leaving behind a space charge of localized electrons. So the voltage is Or
I-
+ ÷ 4 + + 4 + 4 + + + + + +
BLOCKING
-
SURFACES~ / As-Se GUaSS "1. . ALUMINUM~ SUBSTRATE
-
-
i-
. . . . . -
-
"
(L ) ~i ~
where a is thesurfacccharge, n i s t h e spacccharge density of removed holes, 1, is the sample thickness and e the dielectric. When the generated charge equals the surface charge; e n L = a , equation (1) is replaced by
i~l
' -L
Figure I. Schematic ,~fthe dark charge depiction experiment.
V = o2/2en~.
(2)
It is readily evident that the discharge rate, dV/dt, will have an inflcctior at the time e n L = a , even if d a / d t is finite. This depletion time t b marks the time required to generate n carriers which are experimentally determined by V(t D) = enL2/2e
(3)
Hereafter t D will denote the time to thermally generate n holes. If not specified, n will be 1014 cm "3 and the temperature will be 293K. The measurement*; used in this study were as described by MNS. The samples were dark rested at least 20 minutes between measurements and at least 24 hours if exposed to room lights. Unless otherwise noted, the samples were prepared by vacuum depositing As-Se alloys at the rate of about 2 # m / m i n (crucible temperature 660-680K) on Aluminum substrates heated to 455K. After deposition, the samples were cooled by exposing to air at room temperature and their surfaces were treated to reduce surface dark discharge.
3. Energy Distribution of Hole Emission States
The time and temperature dependence of n is governed by the thermal hole emission rate from electron trap states which by definition are at or above E F. For a discrete density of these states N o at energy Eo above the valence band, the generation rate is by first order kinetics n=N0[1 - exp(-Rt)], with
(4)
R = v exp(-E0/kT)
(5)
where v is the attempt to escape frequency. For n<
839
AoR. Melnyk / Hole Emission Defect States in Amorphous As2Se 3
electrons, the distribution of states can be mapped out by measuring the activation energy of tl) for different n. Figure 2 is stuch a plot using the data of MNS. The measurements were taken on samples of varying thickness charged to the same surface charge density, ~6XI0 I1 cln 2. The data indicates an approximately exponential distribution instead of a discrete density of states at 1.0 cV as originally proposed by MNS. In fact, one can calculate the generation for an exponential tail distribution of width W and show that ~ n = n0(Rt)kT/W
(6)
Here n0/W is the density at El:, where the distribution is truncated, and E 0 in expression (5) for R is replaced by E F. The calculation assumcs the emission is from the quasi-Fermi energy E = E F + W In(n/n0)
(7)
and n0
[ (3
I
A%rSes3Io.zs
Figure 2. I)cplcted charge density n versus its activation energy. 1)ata is from Reference 7.
10'5
/
=
,o'"I IO t s | 0.8
01.9
II .0 E (eV)
PP
I
-
~" + x: 0.25 ~" o x,05 f [~T~ ~r----~
/
/I
04;-
<+'
0.7
i I. I
1.2
+ ÷
++
'
Figure 3. Temperature dependcnceofthepowerfactor in the timcdcpendenceofn.
~/~l AS37 r - ' ~Se63~IX[
/4'40
] ! 06~
/
:'~
]
n~t
os
oz / 11
OL~ 0
I 200
0.6~'~...'_ ~~ 28o i
I 400 T(°K)
*
~_L^-_. 300 n
n 600
5~
reported random variation in p with temperature for a given sample. Only by averaging over many samples ( + ) or measurements (o) is the predicted dependence found with W=38 mcV. It is interesting to note that W=kTg, where Ts=440K is the glass temperature of the material. If an exponential tail is the appropriate distribution, the measurement directly yields the attempt to e~apc frequency u. Combining equations (6) and (7) the activation energy of t D is ~,tD = exp(E/kT)
(8)
with only v as an adjustable parameter. For tD~100 seconds at 293K and E~I.0 eV, equation (8) requires v = 1015see"1. Although the activation energy could not be measured accurately (0.9+0.08 eV), depletion below 1013 cm "3 showed a break in the time dependence that could bc interpreted as a discontinuity in the density of states. If the Fermi energy is placed in the middle of the gap. about 0.91 eV for As2Se 3, an exponential extrapolation gives 1014 cm 3 eV-I states at the Fermi energy. States at energies above 1.05 eV were not measured. A lower limit estimate is 10 ]5 - 1016 cm "3 located below 1.2 eV, and more if the the peak of the distribution is at higher energy,
4. Composition and Structure Effects To determine the influence of non-stoichiometric compositk)n as well as small iodine additions a range of AsSe-I glass compositions was measured with the DCD technique. The results, in terms of file depletion time for
840
A.R. Melnyk / Hole Emission Defect States in Amorphous As2Se 3
1014 charge carriers cm 3 at 293K, are shown in Figure 4. The generation rate decreases approximately exponentially by a factor of ten for an 8% As reduction from As2Sey The effect of iodine addition is as reported earlier 7, doubling the rate with the first 0.05% 1 and thereafter increasing the rate more gradually, roughly a factor of two for every 0.25% 1. The influcnce of iodine and arsenic appear additive, that is, the effects of each are independent of the relative concentration of the uther. Activation energy measurements were performed on some of the compositions and revealed no significant diffincnces except a slight trend toward higher activation energies with longer depletion timcs. On close examination, the power of the time dcpcndence p, showed an increase with As reduction. IO00
I000
1014era-3
Figure 4. The dependence o'f tl) on the sample arsenic and iodine.
293 K
0.05 % 1"
vo3
10'14cmJ3 - - ~
z9~ K
i
1
i
It
/od
co v 3 IOO
I00
5
i01 a 50
n n J
t i I a 35 %As
I
Figure 5. Effect ofsubstrate temperature during vacuum deposition on t D.
I I 40
3130
I I ~oo .~I .~o ~o 40 5~
SUBSTRATE
TEMPERATURE (°K)
The preceding data was all obtained on "aged" samples made by "normal" vacuum deposition conditions. The effect of substrate temperature on the depletion time on agcd samples of two iodine compositions is illustrated in Figure 5. Varying substrate temperature caused a slight (1.5%) temperature-dependent decrease in As above about 430K on the 0.3% 1 sample, so the data was corrected to 37% As. Unfortunately the film composition of the 0% I samples was not measured, hence the increasing depletion time above 450K may be an artifact of decreasing As. Varying the dcposition rate by crucible temperature (from 633K :o 703K) caused no effect on tD, after the effect on As was factored out. "Aged" refers to the fact the samples had been stored (in the dark) for several months, to years in some cases, prior to the DCD measurements. Figure 6 shows the effect 150
o
~
o
Oo o o
100 •
r-
o
1014cm-~ 293 K As37Se63io.o 5
i5o %
510
i IOO
- i 150
i 200
AGE ( DAYS FROM DEPOSITION )
'fable I. Effect of annealing on t D . time Temp Initial t D Final t D (h~) (°K) (sec) (sec) 0 455 16 455 32 433 124 48 433 56 393 113 71 72 393 80 353 119 169 142 353 120 189 150 313 115 245
Figure 6. Room temperature relaxation of t D. of sample age on the depletion time, revealing that the generation rate is relaxing with a time scale of months at room temperature. The data in Figure 6 was taken on many samples fabricated at different times with a fixed "normal" process. Compositional effects were eliminated by chosing samples with a narrow composition range of 37.2+0.2%As and 54+4ppma I. The effect was not due to some unknown drift in sample fabrication
A.R. Melnyk / Hole Emission Defect States in Amorphous As2Se 3
8~I
as illustrated by one sample monitored over a peried of time (solid circles in Figure 6). Table I shows the effects of annealing. Four well-relaxed samples were heated above 455K overnight in an ovcn and removed at different temperatures while the oven was slowly cooled o~er a period of a week. The samples were cooled by exposing them to room temperature air. This rcsultcd in rapid cooling (minutes) to about 315K and progrcsively slower cooling to ruom temperature (about an hour). The sample removed at 393K resembles a freshly made sample, while the remaining slowly-cooled samples relaxed beyond the initial equilibrium.
5. Discussion The I)C1) experiment is explained by the thermal generation and depletion of holes from gap states above El:. A total density of states increasing fi'om about 1013 cm "3 to 1015 cm "3 is observed in an energy range between 0.9 and 1.1 cV above the valence band, The activation energy and time-dependence of generation indicate the states are an exponential-tail distribution with a 38 meV width. The peak of this distribution is located deeper than 1.1 eV but the upper limit cannot be determined. About 1015 - 1016 states cm "3 are estimated if the peak is below 1,2 eV. In the earlier studies, Ing and Ncyhart 4 interpreted their results in terms of about 1014 hole emission centers cm 3 located between 0.6 and 0.8 eV above the valence band and 1016-10 t7 cm -3 between 0.8 and 0.86 eV, while Montrimas ct. al.5 found 1.9XI015 cm -3 at 0.91 eV and claimed four additional shallower levels. The difference with the present results appears to bc duc to the mode of analysis, particularly the use of the initial discharge rate. In an elegant series of experiments Abkowitz and Pai 9 have shown that the electronic properties of glassy Se are influenced by thermal cycling through Tg, and have interpreted this to be due to defects introduced as a result of bond breaking and bond rearrangement in the neighborhood of Tg. Here, I~h,eaging, annealing and sample preparatiun experiments similarly indicate that the gap states are associated with defects arising from the structural disorder of the glass. Indirect confimmtion that the relaxation in Figure 6 is due to bulk structure relaxation is provided by accoustic velocity measurements on samples that were cooled from Tg on similar time scales, l~,obinettc 1° found the longitudinal velocity showed a similar increase as the samples relaxed with age, and from correlation with density measurements concluded the effect was due to the densification of glass. Since Tg decreases rapidly with As in the 30-40% composition region, the observed decrease in gap states with As reduction may be a manifestation of structural relaxation rather than a direct chemical effect. Iodine however in low concentrations (<1017 cm 3} definitely increases the states in the tail but at significantly lower rates than its atomic concentration. Since the gap sl:ates observed by the DCD measurement indicate a structural origin they may be interpreted in terms of the charged defect models. 2,3 Although the valence-alternating pair model terminology 3 will be employed here, the discussion can equally bc made in the terminology of Street and Mort. 2 "lhe C + acts as an electron trap or hole emitter while the C is the hole trap. According to the energy diagram proposed by Street and Mott, the C + is moved up toward the conduction band while the C" is lowered toward the valence band. Adding one electron (emitting a hole) requires a large amount of energy to convert C + to C t, but adding the next elcctrun for C O to C" requires less energy thus the average thermalization energy is half way between the two energy levels. So thermal generation may be the two step process C + ~ C" + 2 holes.
(9)
Initially when their densities are equal, [ C + ] = [ C ] , the activation energy is E F, half the energy to remove the two holes from one defect. This process would favor non-intimate pairs, that is isolated C+'s, and the low (<1013 cm 3) density of states near E F may reflect a low concentration of non-intimate VAP's. As more intimate pairs are thermally converted their coulomb repulsion increases the required thermal energy. An alternate explanation is that the thermal generation initially is due to
842
AoRo Melnyk / Hole Emission Defect States in Amorphous As2Se 3
CO ~ C" + hole,
(10)
requiring that there be a small but finite density of the neutral defects. As tlle C°'s are depleted, larger energies are required for the direct conversion. Two arguments can bc made in fi~vor of the latter process. The first is based on the light enhanced dark discharge effect: Pre-exposing the sample to light and charging after some dark relaxation period enhan~zes the dark discharge. Analysis indicates the same thermal generation/depletion process takes place as in dark rested samples except the light pre-exposed region generates charge from a higher density of shallower states (near El,)8 Since photopunqping produces states that are detected in spin resonance experimentsn, this suggests that pre-expost~re to light may be increasing the density of the neutral defects C 0. The second argument is based on the large attempt to escape frequency factor, v = 1015see-1. Since typical phonon frequencies for As2Se3 are of the order of 1012see"1 it appears too large by about a factor of 103. Such a factor could be explained by the temperature dependence of the band gap, 12 exp(y/k)~102, and field enhancement The lower thermal energy process (10) would also enhance the rate, but the situation is made worse if the two step process (9) is involved since the rate should be limited by the slower, higher energy step C +--~ C o + hole.
References 1. Cohen, M.H., Fritzsche, H. and Ovshinsky, S.R., Phys. Rev. Iktt. 22, (1969) 1065. 2. Street, R.A. and Mott, N.F., Phys. Rev. Lctt. 35, (1975) 1293. 3. Kastncr, M., Adler, D. and Fri~sehc~ H., Phys. Rev. Lett. 37, (1976) 1504. 4. Ing, S.W. Jr. and Neyhart~ J.H., J. Appl. Phys. 43, (1972) 2670. 5. Montrimas, E., Tauzaitiene, S. and Tauraitis A., in: W.F. Berg and K. Hauffe (eds)., Current Problems in Eleetrophotography (de Gruyter, Berlin, 1972). 6. Schein, L.B., Phys. Rev. B 10, (1974) 3451. 7. Melnyk, A.R, Ncyhart, J.H., and Sperry, Eleetrophotography Conference (in print).
R.H. in: M. Tubbs (ed)., Proceedings of the 1976
8. Melnyk, A.R. and Chung L. (unpublished). 9. Abowitz, M. and Pai, D.M., Phys. Rev. Lett. 38, (1977) 1190. 10. Robinette S., to be published in J. Non-Cryst. Solids. 11. Bishop, S.G., Strom, U. and Taylor, P.C, Phys. Rev. Lett. 36, (1976) 543. 12. Mott, N.F. and Davis, E.A., Electronic Processes in Non-crystalline Materials (Oxford, Clarcdon Press, 1971).
ttole Emission Defect States in Amorphous As2Se 3 Andrew R. Melnyk Webster Research Center Xerox Corporation Rochester, New York
The influence of composition (30 to 40% As and 0 to 0.5% 1), deposition parameters, and subsequent annealing on charge emission centers in vacuum deposited As2Se3 films was studied. The filermal emission of holes flora deep gap states was measured by a time resolved charge depletion technique. The observed states appear to be a tail of a distribution that peaks above 1.1 eV. The emission rate decreased with reduction of arsenic and increased with the addition of iodine. The rate was sensitive to sample fabrication; decreasing with substrate temperature in the vicinity of the glass temperature Tg. Freshly made fihns had a high initial rate which relaxed with a thne constant of ahout 100 days at room temperature. Annealing by heating the samples to T g and cooling slowly relaxed the emission rate further. The results are • dmcussed in terms of the recent charged defect models.
1. Introduction Localized gap states play a fundamental role in determining the electronic and optical properties of amorphous semiconductors such as the chalcogenide glasses. Some ten years ago Cohen, Fritzsche and Ovshinsky 1 proposed that with all bonds satisfied the disorder in an amorphous solid leads to featureless tails of localized states extending from the valence and conduction bands into the gap. In the last four years models have been developed on the idea of gap states due to charged palm defects. Street and Matt 2 proposed that topological constraints caused by disorder will result in a finite density of dangling bonds (D). Pairs of these D's exchange an electron to fbrm charge defects D + and D', the reaction being exothermic. Kasmcr, Adler and Vritzsche 3 using a chemical approach proposed a similar model, but replaced the D by a three-fold-coordinated chalcogen atom C3°. The dangling bond, they argued, is energetically too expensive and so reacts with the kme pair electrons of a neighboring normal two-fuld-coordinated chalcogen forming half a bond wilh an unpaired electron in the antibonding state. Removing that electron results in the charged defect C3-~, while adding an electron results in a singly coordinated chalcogen C1". One technique to probe gap states in chalcogenide glasses is the dark discharge measurement, which observes states that thermally emit charge carriers. The technique was first used by both lng and Neyhart, 4 and Montrimas, et. al. 5 on amorphous As2Se 3 and later by Schein (' on Se. Recently Melnyk, Neyhart and Sperry 7 (MNS) refined the measurement and analysis, employing the phcnom~.'non of charge depletion to directly measure the thermally generated charge. This study examines the gap states observed by the dark charge depletion (DCD) measurement and compares the results with the defect theories. Section 2 briefly reviews the DCD measurement and analysis, and summarizes the experimental details. The energy distribution of the gap states is examined in Section 3, and from the data of MNS it is shown that the measurement observes the tail of a distribution of states deeper than 1.0 eV. Section 4 prcscnts rcsuhs on the effects of small additions of iodine and reduction of arsenic, and effects of annealing, deposition rate and substrate temperature on the gap states. These experiments suggest a link between the observed states and defects frozen-in by cooling from the glass temperature. This is discussed in the last section and an attempt is made to interpret the results with the charged defect theories, namely, that the observed thermal generation is C + --~ C" + 2 holes.
837
838
AoRo Melnyk / Hole Emission Defect States in Amorphous As2Se 3
2, I)cpletion Measurement and Analysis The dark discharge measurement involves capacitivcly charging a layer of the material and measuring the surface potential in an open circuit configuration, Figure I. The charging is usually done with a corona device, but could also be done with a momentary contact closure between an electrndcd sample and a ~oltage source, '['he essential requirement is that the two surfaces or electrodes do not inject charge into the sample, so the discharge is due only to the generation and transport of the charge in the bulk. If the transport rate is much larger t'~an the generation rate, the voltage discharge rate dV/dt is proportional to the thermal generation rate. Unfi}rtunately a fraction of the surface charge is injected, even with thin blocking layers, and may initially dominate the discharge. The DCD measurement solves this problem by relying on the fact that only one charge carrier, the hole, is mobile. The discharge proceeds by the depletion of holes leaving behind a space charge of localized electrons. So the voltage is Or
I-
+ ÷ 4 + + 4 + 4 + + + + + +
BLOCKING
-
SURFACES~ / As-Se GUaSS "1. . ALUMINUM~ SUBSTRATE
-
-
i-
. . . . . -
-
"
(L ) ~i ~
where a is thesurfacccharge, n i s t h e spacccharge density of removed holes, 1, is the sample thickness and e the dielectric. When the generated charge equals the surface charge; e n L = a , equation (1) is replaced by
i~l
' -L
Figure I. Schematic ,~fthe dark charge depiction experiment.
V = o2/2en~.
(2)
It is readily evident that the discharge rate, dV/dt, will have an inflcctior at the time e n L = a , even if d a / d t is finite. This depletion time t b marks the time required to generate n carriers which are experimentally determined by V(t D) = enL2/2e
(3)
Hereafter t D will denote the time to thermally generate n holes. If not specified, n will be 1014 cm "3 and the temperature will be 293K. The measurement*; used in this study were as described by MNS. The samples were dark rested at least 20 minutes between measurements and at least 24 hours if exposed to room lights. Unless otherwise noted, the samples were prepared by vacuum depositing As-Se alloys at the rate of about 2 # m / m i n (crucible temperature 660-680K) on Aluminum substrates heated to 455K. After deposition, the samples were cooled by exposing to air at room temperature and their surfaces were treated to reduce surface dark discharge.
3. Energy Distribution of Hole Emission States
The time and temperature dependence of n is governed by the thermal hole emission rate from electron trap states which by definition are at or above E F. For a discrete density of these states N o at energy Eo above the valence band, the generation rate is by first order kinetics n=N0[1 - exp(-Rt)], with
(4)
R = v exp(-E0/kT)
(5)
where v is the attempt to escape frequency. For n<
839
AoR. Melnyk / Hole Emission Defect States in Amorphous As2Se 3
electrons, the distribution of states can be mapped out by measuring the activation energy of tl) for different n. Figure 2 is stuch a plot using the data of MNS. The measurements were taken on samples of varying thickness charged to the same surface charge density, ~6XI0 I1 cln 2. The data indicates an approximately exponential distribution instead of a discrete density of states at 1.0 cV as originally proposed by MNS. In fact, one can calculate the generation for an exponential tail distribution of width W and show that ~ n = n0(Rt)kT/W
(6)
Here n0/W is the density at El:, where the distribution is truncated, and E 0 in expression (5) for R is replaced by E F. The calculation assumcs the emission is from the quasi-Fermi energy E = E F + W In(n/n0)
(7)
and n0
[ (3
I
A%rSes3Io.zs
Figure 2. I)cplcted charge density n versus its activation energy. 1)ata is from Reference 7.
10'5
/
=
,o'"I IO t s | 0.8
01.9
II .0 E (eV)
PP
I
-
~" + x: 0.25 ~" o x,05 f [~T~ ~r----~
/
/I
04;-
<+'
0.7
i I. I
1.2
+ ÷
++
'
Figure 3. Temperature dependcnceofthepowerfactor in the timcdcpendenceofn.
~/~l AS37 r - ' ~Se63~IX[
/4'40
] ! 06~
/
:'~
]
n~t
os
oz / 11
OL~ 0
I 200
0.6~'~...'_ ~~ 28o i
I 400 T(°K)
*
~_L^-_. 300 n
n 600
5~
reported random variation in p with temperature for a given sample. Only by averaging over many samples ( + ) or measurements (o) is the predicted dependence found with W=38 mcV. It is interesting to note that W=kTg, where Ts=440K is the glass temperature of the material. If an exponential tail is the appropriate distribution, the measurement directly yields the attempt to e~apc frequency u. Combining equations (6) and (7) the activation energy of t D is ~,tD = exp(E/kT)
(8)
with only v as an adjustable parameter. For tD~100 seconds at 293K and E~I.0 eV, equation (8) requires v = 1015see"1. Although the activation energy could not be measured accurately (0.9+0.08 eV), depletion below 1013 cm "3 showed a break in the time dependence that could bc interpreted as a discontinuity in the density of states. If the Fermi energy is placed in the middle of the gap. about 0.91 eV for As2Se 3, an exponential extrapolation gives 1014 cm 3 eV-I states at the Fermi energy. States at energies above 1.05 eV were not measured. A lower limit estimate is 10 ]5 - 1016 cm "3 located below 1.2 eV, and more if the the peak of the distribution is at higher energy,
4. Composition and Structure Effects To determine the influence of non-stoichiometric compositk)n as well as small iodine additions a range of AsSe-I glass compositions was measured with the DCD technique. The results, in terms of file depletion time for
840
A.R. Melnyk / Hole Emission Defect States in Amorphous As2Se 3
1014 charge carriers cm 3 at 293K, are shown in Figure 4. The generation rate decreases approximately exponentially by a factor of ten for an 8% As reduction from As2Sey The effect of iodine addition is as reported earlier 7, doubling the rate with the first 0.05% 1 and thereafter increasing the rate more gradually, roughly a factor of two for every 0.25% 1. The influcnce of iodine and arsenic appear additive, that is, the effects of each are independent of the relative concentration of the uther. Activation energy measurements were performed on some of the compositions and revealed no significant diffincnces except a slight trend toward higher activation energies with longer depletion timcs. On close examination, the power of the time dcpcndence p, showed an increase with As reduction. IO00
I000
1014era-3
Figure 4. The dependence o'f tl) on the sample arsenic and iodine.
293 K
0.05 % 1"
vo3
10'14cmJ3 - - ~
z9~ K
i
1
i
It
/od
co v 3 IOO
I00
5
i01 a 50
n n J
t i I a 35 %As
I
Figure 5. Effect ofsubstrate temperature during vacuum deposition on t D.
I I 40
3130
I I ~oo .~I .~o ~o 40 5~
SUBSTRATE
TEMPERATURE (°K)
The preceding data was all obtained on "aged" samples made by "normal" vacuum deposition conditions. The effect of substrate temperature on the depletion time on agcd samples of two iodine compositions is illustrated in Figure 5. Varying substrate temperature caused a slight (1.5%) temperature-dependent decrease in As above about 430K on the 0.3% 1 sample, so the data was corrected to 37% As. Unfortunately the film composition of the 0% I samples was not measured, hence the increasing depletion time above 450K may be an artifact of decreasing As. Varying the dcposition rate by crucible temperature (from 633K :o 703K) caused no effect on tD, after the effect on As was factored out. "Aged" refers to the fact the samples had been stored (in the dark) for several months, to years in some cases, prior to the DCD measurements. Figure 6 shows the effect 150
o
~
o
Oo o o
100 •
r-
o
1014cm-~ 293 K As37Se63io.o 5
i5o %
510
i IOO
- i 150
i 200
AGE ( DAYS FROM DEPOSITION )
'fable I. Effect of annealing on t D . time Temp Initial t D Final t D (h~) (°K) (sec) (sec) 0 455 16 455 32 433 124 48 433 56 393 113 71 72 393 80 353 119 169 142 353 120 189 150 313 115 245
Figure 6. Room temperature relaxation of t D. of sample age on the depletion time, revealing that the generation rate is relaxing with a time scale of months at room temperature. The data in Figure 6 was taken on many samples fabricated at different times with a fixed "normal" process. Compositional effects were eliminated by chosing samples with a narrow composition range of 37.2+0.2%As and 54+4ppma I. The effect was not due to some unknown drift in sample fabrication
A.R. Melnyk / Hole Emission Defect States in Amorphous As2Se 3
8~I
as illustrated by one sample monitored over a peried of time (solid circles in Figure 6). Table I shows the effects of annealing. Four well-relaxed samples were heated above 455K overnight in an ovcn and removed at different temperatures while the oven was slowly cooled o~er a period of a week. The samples were cooled by exposing them to room temperature air. This rcsultcd in rapid cooling (minutes) to about 315K and progrcsively slower cooling to ruom temperature (about an hour). The sample removed at 393K resembles a freshly made sample, while the remaining slowly-cooled samples relaxed beyond the initial equilibrium.
5. Discussion The I)C1) experiment is explained by the thermal generation and depletion of holes from gap states above El:. A total density of states increasing fi'om about 1013 cm "3 to 1015 cm "3 is observed in an energy range between 0.9 and 1.1 cV above the valence band, The activation energy and time-dependence of generation indicate the states are an exponential-tail distribution with a 38 meV width. The peak of this distribution is located deeper than 1.1 eV but the upper limit cannot be determined. About 1015 - 1016 states cm "3 are estimated if the peak is below 1,2 eV. In the earlier studies, Ing and Ncyhart 4 interpreted their results in terms of about 1014 hole emission centers cm 3 located between 0.6 and 0.8 eV above the valence band and 1016-10 t7 cm -3 between 0.8 and 0.86 eV, while Montrimas ct. al.5 found 1.9XI015 cm -3 at 0.91 eV and claimed four additional shallower levels. The difference with the present results appears to bc duc to the mode of analysis, particularly the use of the initial discharge rate. In an elegant series of experiments Abkowitz and Pai 9 have shown that the electronic properties of glassy Se are influenced by thermal cycling through Tg, and have interpreted this to be due to defects introduced as a result of bond breaking and bond rearrangement in the neighborhood of Tg. Here, I~h,eaging, annealing and sample preparatiun experiments similarly indicate that the gap states are associated with defects arising from the structural disorder of the glass. Indirect confimmtion that the relaxation in Figure 6 is due to bulk structure relaxation is provided by accoustic velocity measurements on samples that were cooled from Tg on similar time scales, l~,obinettc 1° found the longitudinal velocity showed a similar increase as the samples relaxed with age, and from correlation with density measurements concluded the effect was due to the densification of glass. Since Tg decreases rapidly with As in the 30-40% composition region, the observed decrease in gap states with As reduction may be a manifestation of structural relaxation rather than a direct chemical effect. Iodine however in low concentrations (<1017 cm 3} definitely increases the states in the tail but at significantly lower rates than its atomic concentration. Since the gap sl:ates observed by the DCD measurement indicate a structural origin they may be interpreted in terms of the charged defect models. 2,3 Although the valence-alternating pair model terminology 3 will be employed here, the discussion can equally bc made in the terminology of Street and Mort. 2 "lhe C + acts as an electron trap or hole emitter while the C is the hole trap. According to the energy diagram proposed by Street and Mott, the C + is moved up toward the conduction band while the C" is lowered toward the valence band. Adding one electron (emitting a hole) requires a large amount of energy to convert C + to C t, but adding the next elcctrun for C O to C" requires less energy thus the average thermalization energy is half way between the two energy levels. So thermal generation may be the two step process C + ~ C" + 2 holes.
(9)
Initially when their densities are equal, [ C + ] = [ C ] , the activation energy is E F, half the energy to remove the two holes from one defect. This process would favor non-intimate pairs, that is isolated C+'s, and the low (<1013 cm 3) density of states near E F may reflect a low concentration of non-intimate VAP's. As more intimate pairs are thermally converted their coulomb repulsion increases the required thermal energy. An alternate explanation is that the thermal generation initially is due to
842
AoRo Melnyk / Hole Emission Defect States in Amorphous As2Se 3
CO ~ C" + hole,
(10)
requiring that there be a small but finite density of the neutral defects. As tlle C°'s are depleted, larger energies are required for the direct conversion. Two arguments can bc made in fi~vor of the latter process. The first is based on the light enhanced dark discharge effect: Pre-exposing the sample to light and charging after some dark relaxation period enhan~zes the dark discharge. Analysis indicates the same thermal generation/depletion process takes place as in dark rested samples except the light pre-exposed region generates charge from a higher density of shallower states (near El,)8 Since photopunqping produces states that are detected in spin resonance experimentsn, this suggests that pre-expost~re to light may be increasing the density of the neutral defects C 0. The second argument is based on the large attempt to escape frequency factor, v = 1015see-1. Since typical phonon frequencies for As2Se3 are of the order of 1012see"1 it appears too large by about a factor of 103. Such a factor could be explained by the temperature dependence of the band gap, 12 exp(y/k)~102, and field enhancement The lower thermal energy process (10) would also enhance the rate, but the situation is made worse if the two step process (9) is involved since the rate should be limited by the slower, higher energy step C +--~ C o + hole.
References 1. Cohen, M.H., Fritzsche, H. and Ovshinsky, S.R., Phys. Rev. Iktt. 22, (1969) 1065. 2. Street, R.A. and Mott, N.F., Phys. Rev. Lctt. 35, (1975) 1293. 3. Kastncr, M., Adler, D. and Fri~sehc~ H., Phys. Rev. Lett. 37, (1976) 1504. 4. Ing, S.W. Jr. and Neyhart~ J.H., J. Appl. Phys. 43, (1972) 2670. 5. Montrimas, E., Tauzaitiene, S. and Tauraitis A., in: W.F. Berg and K. Hauffe (eds)., Current Problems in Eleetrophotography (de Gruyter, Berlin, 1972). 6. Schein, L.B., Phys. Rev. B 10, (1974) 3451. 7. Melnyk, A.R, Ncyhart, J.H., and Sperry, Eleetrophotography Conference (in print).
R.H. in: M. Tubbs (ed)., Proceedings of the 1976
8. Melnyk, A.R. and Chung L. (unpublished). 9. Abowitz, M. and Pai, D.M., Phys. Rev. Lett. 38, (1977) 1190. 10. Robinette S., to be published in J. Non-Cryst. Solids. 11. Bishop, S.G., Strom, U. and Taylor, P.C, Phys. Rev. Lett. 36, (1976) 543. 12. Mott, N.F. and Davis, E.A., Electronic Processes in Non-crystalline Materials (Oxford, Clarcdon Press, 1971).