Charge transport in chlorine doped amorphous Se:Te xerographic photoreceptor films

Journal of Non-Crystalline Solids 72 (1985) 23-37 North-Holland, A m s t e r d a m

CHARGE T R A N S P O R T IN C H L O R I N E D O P E D A M O R P H O U S XEROGRAPHIC P H O T O R E C E P T O R FILMS

23

Se: Te

S.O. KASAP and C. J U H A S Z Department of Electrical Engineering, Imperial College of Science and Technology, University of London, London SPV7 2BT, UK Received 9 August 1984 Revised manuscript received 9 October 1984

Time-of-flight drift mobility experiments were carried out on a-Se 1 ~Te:, ( x = 0-0.1) with chlorine as an additive up to 70 atomic parts per million to investigate the charge transport mechanism in these xerographically important photoreceptor films. Hole drift mobility-temperature data indicate that hole transport in a-Se: Te alloys is controlled by a relatively discrete set of Te-introduced shallow traps (probably Te~- centres) at - 0.43 eV above E v whose concentration increases nearly linearly with Te addition. Chlorine doping generates an additional set of traps (probably C1 o centres) around the same energy as Te-induced traps in a similar fashion to the effect of CI addition to a-Se. Electron drift mobility-temperature data for a-Se:Te alloys containing no C1 can also be interpreted by assuming Te-introduced electron traps at - 0.49 eV below E c. There was no electron transport observable in Cl-doped a-Se : Te alloys.

1. Introduction

Amorphous Sel_xTex alloys play an important role in the commercial xerographic photoreceptor industry [1,2] due to a more desirable spectral response in comparison with pure and halogen-doped a-Se and a-Se0.995As0.005 photoreceptor semiconductors [1-4]. We carried out time-of-flight (TOF) drift mobility measurements as a function of temperature on a-Se l_xTex (x < 0.l) films with Cl-doping up to 70 at. ppm in order to examine the charge transport mechanism in these photoreceptor materials. Drift mobility-temperature measurements on pure a-Se films (see for example refs. [5-12]) indicate a shallow trap-controlled transport mechanism operating both for holes and electrons as first proposed by Spear [5,6]. The nature of the basic microscopic conduction process, whether by extended state transport or by hopping, for example in the localized tail states, has not been conclusively established [13-15]. The lack of pressure dependence in the thermally activated drift mobility behaviour of both types of carriers up to - 5 kbar and down to - 230 K [16] has been used by various authors as evidence against the measured drift mobility representing a hopping transport [17,18]. Introduction of chlorine to a-Se has been found [15] to increase the hole 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

24

S.O. Kasap, C. Juhasz / Charge transport in chlorine doped amorphous films

drift mobility activation energy E~h by - 0.16 eV which may be interpreted as the result of an additional set of shallow traps at 0.45 eV above E v. The fact that in the Cl-doped a-Se0.995As0.005, E~h is almost the same as in pure a-Se [19] means that the increase of - 0.16 eV in Cl-doped a-Se cannot be due to an increase in energy between the valence band mobility edge E v and the intrinsic shallow traps at a depth Et, for example as a consequence of enhanced disorder by doping. The most plausible interpretation then is a discrete manifold of chlorine-induced traps (probably charged e.g. Clo) at - 0 . 4 5 eV which are destroyed or compensated for by lightly alloying with arsenic. Previous measurements on a - S e : T e alloy films [20-22] (without CI) have shown a sharp fall in both hole and electron drift mobility with an addition of a few per cent Te concomitant with a rise in the activation energy. The mechanism by which Te affects the charge transport in a-Se however could not be fully identified which is not surprising since the nature of microscopic mobility itself has not been resolved. The recent studies on the structure of a-Sel_xTex alloys using neutron diffraction techniques [23] indicate that the freely rotating random chain model proposed for the structure of a-Se still perseveres up to large Te contents (70 at.%) and that Te enters the structure essentially in a substitutional manner as predicted by the isoelectronic property of the Te atom. We would, therefore, expect the a-Se:Te alloys to have the full set of Sei-, Se~-, Te{ and Te~valence alternation pair (VAP) type of structural charged defects with concentrations which are composition- and temperature-dependent as determined by the law of mass action and charge neutrality conditions. Kastner and Fritzsche [24,26] have supposed that the number of densities of these native charged defects are frozen in at the glass transition temperature Tg so that even at temperatures well below Tg there are large concentrations of such defects. In the case of a-Se and a-As2Se 3, however, thermal cycling and annealing effects on the drift mobility, xerographic residual voltage and SCLC measurements of Abkowitz and co-workers [27-31] indicate that at a temperature TA < Tg, the glass relaxes towards its metastable melt phase and at room temperature over a time scale (t A) of hundreds of hours it "reaches" its melt-like equilibrium state when it is said to have aged. Thus, at least in the case of a-Se, the concentration of the native defects after a sufficiently long time depends on TA, the annealing temperature, rather than Tg since the defects would have been equilibrated. An important conclusion from Abkowitz's work is that the densities of these intrinsic defects in a-Se samples prepared even under widely differing conditions may be comparable when aged. Provided that small amounts of Te alloying a-Se generates sufficient density of Tel- and Te~ VAP defects (which are probably more easily formed than Sei- and Se~- pairs [32]) and that their energy levels in the mobility gap allow them to be probed by the T O F technique then it should, in principle, be possible to study their effect on the charge transport mechanism by carrying out drift mobility-temperature measurements.

S.O. Kasap, C. Juhasz / Charge transport in chlorine doped amorphous films

25

2. Experimental details Samples were prepared by vacuum evaporation of'vitreous pellets from stainless steel boats onto heated pre-oxidized AI substrates. Various compositions of S e : T e + C1 alloys were obtained by mixing the correct amounts by weight of pure selenium with portions of the pre-alloyed master batches. The maximum Te concentration used was 12.5 wt% in the Cl-doped alloy. Material characterization was carried out as described previously [15] by Differential Scanning Calorimetry and X-ray work to confirm that the structures were essentially non-crystalline and by Spark Source Mass Spectrometry to obtain the chlorine content (as well as any other impurity). In addition, Scanning Electron Microprobe (SEM) *t investigations gave variations in the Te composition across the sample thickness. Slow deposition rates were used to avoid large compositional variations from the average bulk concentration. Usually, due to fractionation effects, the top surface was richer in Te and the bottom (substrate) surface region was slightly deficient in Te. The "average" Te concentration in a few of the samples was also obtained t using atomic absorption spectroscopy. The compositional profile of the chlorine was not known due to the obvious difficulties of the analysis. The samples were well annealed at room temperature for several weeks. A semi-transparent Au electrode was deposited as a top electrode and the TOF drift mobility measurements were carried out as described previously [15].

3. Results and discussion

3.1. Hole transport Typical drift mobility temperature data /~h ( T ) for a-Se/5 wt% Te and 20 wt ppm Cl-doped a-Se/5 wt% Te films are shown in figs. 1 and 2 respectively as log /'£h versus T -1. The insets display the field dependence of the low temperature mobility activation energy E/t h which seems to fall linearly with the applied field. Fig. 3 shows log ~h versus T -1 plots at low fields ( F - 3 × 104 V cm -1) for various Te compositions and amounts of chlorine doping. At low temperatures, the drift mobility in Se alloyed with 3.5 wt% and 5 wt% Te exhibits a well-defined thermally-activated behaviour with a larger activation energy E~(F) than that for pure a-Se. At high temperatures ( T > 270 K for 5 wt% Te and T > 255 K for 3.5 wt% Te in fig. 3) the activation energy is not as well defined ( - 0 . 3 eV) and is smaller than its value at low temperatures. Similar behaviour with two temperature regimes is also apparent in the drift mobility work of Pai [22] on a-Se : Te films up to 1% Te. Pai finds that at low temperatures (_< 240 K) both 0.5% Te and 1% Te films have E~ = 0.33 eV at * At Imperial College. t At Nottingham University (Wolfson Institute).

S. O. Kasap, C. Juhasz / Charge transport in chlorinedoped amorphousfilms

26

I

I

l

I

I

I

I

"

I

I

I

I

I

1

I

I

0.46 0.42 ~ Eu:0"428eV 10

-1

0.38

a - Seo.968 meo.o32

E~ (eV) 0-34 10 -2

0.30 _

i

0

400

800 . 1200

~h

( c m 2 V4s -~

L --6 - ~ pm

10-:

~

v" loo v

~ "%~ \ ~ , 2

400V 600V

418eV "- ~

I

3"2

I

I

3.4

I

I

I

I

3"6 3"8 103 T-I(K -1) ~

I

I

4.0

o

,.ev

v~,'q-'~7%

0382ev 10-4 30

O.313 eV

/

I

'~\ I

4"2

I

l

4'4

I

4'6

Fig. 1. Semi-logarithmic plot of hole drift mobility versus reciprocal temperature for a-Se/5 wt% Te. The inset shows the field dependence of the low temperature mobility activation energy as E. versus V.

F = 8 × ]04 V cm -a which is reasonably close to the low temperature value, E ~ - 0.37 eV, around the same field for the a - S e : T e films of this work. It therefore seems that the low temperature activation process saturates even at a fraction of one percent Te. At 0.5 wt% Te, the mobility measurements could not be extended to sufficiently low temperatures to obtain well-defined thermally activated behaviour (discussed later). For the chlorine-doped a-Sel_~Tex films, well-defined thermally-activated behaviour extended over the whole temperature range accessed, and is also typical of chlorine-doped a-Se whose properties were reported previously [15]. The zero field mobility activation energy E ° for a - S e : T e alloys, with or without chlorine, seems to remain around 0.42-0.45 eV for the Te concentra-

S.O. Kasap, C. Juhasz /Charge transport in chlorine doped amorphousfilms

27

0.48 a - Seo.966Teo.o 34 + 36 at ppm C[

o

E u =0.445

0.44

eV

(ev) 0.40

10 -2 0"36

I

I

I

I 800

I

I 4.4

10 -3

t

•382 eV

_

( c m 2 V-is ~)

10 -4

o.4o4

--

v 400 + 500 700

10 .5

I

I 3.2

3'0

I

I 3-4

I

I 3.6

I

I 3.8

10 3 T - I ( K - l )

I

I 4'0

I

I 4'2

I 4-6

.---,,

Fig. 2. Semi-logarithmic plot of hole drift mobility versus reciprocal temperature for 20 wt ppm CI doped a-Se/5 wt% Te. The inset shows E~, versus V.

tions investigated (up to - 12.5 wt% Te). Fig. 4 conveniently summarizes the behaviour of the hole drift mobility, /~h and its low temperature "zero field" activation energy, E ° in the system a-Sel_xTex + y ppm C1. Although E ° = 0.44 eV in Cl-doped alloys is slightly larger than that in samples containing no C1, the difference ( - 0.02 eV) is within experimental errors. The two temperature regimes in a-Se 1_xTex alloys without chlorine may be interpreted as arising from two discrete manifolds of shallow traps, since the drift mobility is then given by

/zo(T)

.

N,,

/~h(T) - 1 -v ~

E,,

N,2

exp ~-~ + ~

Et2

exp k T '

(1)

S.O. Kasap, C. Juhasz / Charge transport in chlorine doped amorphousfilms

28 1

L_

l

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

a - S e l - x Tex + y p p m CI

7

.÷

-

10-:

t

P.

(cm 2 V-~s-

10

.

10-

.

.

.

.

.

.

.

62 x',63 +964 v 074 •

•

10 -5

.

I 3.2

82

I

I 3.4

I

I

I

3.6

I 38

I

I 4.0

I

42

4.4

46

48

IOa/T Fig. 3. Temperature dependence of hole drift mobility at low fields ( - 3 x 1 0 4 V cm -1) in the system a-Se l_xTex + y ppm C1. Data for pure a-Se taken from ref. [15].

where E u is the depth in energy of the trap species i from the transport states at E v, and Nti is their density. We may tentatively associate the traps at Etl with the Se atoms and those at Et2 with the Te atoms, i.e. type 1 traps are

S.O. Kasap, C. Juhasz / Charge transport in chlorine doped amorphousfilms

29

1.0

E~h a-- 0.43-0,45 eV

E~h (eV)

(cm2V-ld)I ~

/-

s~,_.r~.

(¢m2V%-~)

_ ~

I

X

o NoC[

\

,o b ..r"

o .2o

\"E~a

cl

o.,ooo < -

1.0

t

O"

(eV) wt °/o Te ---10-4 I I I I I I I I I I I I I I I I I I ~ i 0.1 0 2 4 6 8 10 12 14 16 18 20 Pure S e + y w t pprn Cl

Fig. 4. Hole and electron drift mohilities (at F = 5 × 10 4 V c m - l ) and their low temperature "zero field" activation energies in the system a-Se 1_xTe~, + y ppm C1.

intrinsic to the a-Se structure and type 2 traps are Te induced. To investigate the appropriateness of eq. (1) in describing the mobility-temperature behaviour, eq. (1) was curve-fitted by a least square deviations method to the # h ( T ) data of a-Se~ _xTex with the assumptions #0 = M T - t = 0.1-0.3 cm 2 v -1 s -1 at room temperature, and N , / N v = BiT -3/2. In the two parameter analysis, E , was set to E~,(F) for a-Se and the best Ntl and Nt2 were found. In a three parameter analysis, the best E , was also searched for. In each case Et2 was set to E~(F) obtained from the low temperature range, since in the latter E, is well defined (i.e. the second exponential term dominates the right-hand side of eq. (1)). The results are summarized in table 1.

6.5×10 -4 4.4)<10 -5

0.30

0.10

0.10

Se/3.5%Te

Se/5%Te

8.4 × 10- 4

0.27

Pure Se

3.6 × 10 - 7 2.3 x 10 - 3

1.9 )< 10- 3 5.7)<10 4 6.7 × 10- 5

0.02

0.30

0.30

0.10

0.10

4.0 × 10 s

it0(300 K) (cm2 V -1 s - l )

Composition

Ntl/N~(300 K)

Table 1 Analysis of drift mobility data for a-Se I _~Tex

1.4 × 10- 6

1.6)<10-6

5.2 × 10- 6

5.2 )< 10 - 6

0

8.5)<10 7

1.2)<10 6

3.4 × 10 6

0

Nt2/N~(300K)

best ~ - , ~ ,

N,I U,2 best - - - -

best ~ - ,

best ~ o , ~ -

N,| N,2 best - - - -

best ~ , ~ - v Nd N,2 best - - - -

best ~-~' N~' Etl

?

0.401 0.401 0.401 ? 0.402 0.402 0.402 0.402

0.254

0.260 0.230 0.306 0.402 0.23 0.236 0.230 0.292

Etl

-

U~'Uv U,~ U,~

N~'N~ U,~ U,2 ,Et,

Nv , Etl

N,~

-

N~'N~ U,, U,~

U,, U,~

eq. (4) used

best /to, N-v at F = 1 . 7 × 1 0 4 V c m -~

U,~

(eV)

(eV)

Analysis/comment (eq. (1))

Et2

Etl

.%

5

S.O. Kasap, C. Juhasz / Charge transport in chlorinedoped amorphousfilms

t

~o~0.1 c m2 V-~$-~

t

Ntl

10-14Nt 2 (cm -3) 2. E

3]

-

_. 1017

•

/Nt2

l Ntl

2.4

,

2.ol-

/

1.6

(cm -3)

/

Jlo ''

/

j3r N t l J

I

..o" 1.2

1015

o'81-

/

~-'---~,, N t 2

0.4 0~

1014 0

1'

2 3 4 5 x at. °/o Te

6

7

Fig. 5. Dependence of trap type 1 density Nt] and trap type 2 density Nt2 on atomic percentage of Te content.

It can be seen from table 1 that the density Nt2 of type 2 traps at Et2 ~ 0.42 eV is much smaller than Nt~ and that, as expected, Nt2 increases with Te content. For example, Nt2 = 1.7 x 1014 and 2.6 × 1014 c m -3 for 3.5 wt% Te (2.2 at.%) and 5 wt% Te (3.2 at.%) alloyed with Se respectively. Clearly Nt2 seems to be nearly proportional to the number of Te atoms; this is verified by plotting Nt2 versus x at.% as in fig. 5. The line drawn through the origin contains both Nt2 values and thus confirms the starting assumption that type 2 traps are introduced by Te. Moreover, the linear dependence of N,2 on the number of Te atoms seems to be independent of the absolute value o f / % (300 K) used in the analysis as shown in fig. 5. There is also a slight increase in Etl with the Te content which cannot be

32

S.O. Kasap, C Juhasz / Charge transport in chlorine doped amorphous films

simply explained as due to the movement of E v away from the gap as a consequence of an increase in the disorder since the optical gap has been found to decrease with addition of Te [33]. It is, however, possible for the native shallow defects at Etl to be shifted deeper into the gap. T h e / t h ( T ) data for the Se/0.5 wt% Te composition could not be collected down to sufficiently low temperatures (fig. 3) to observe well-defined thermally-activated behaviour so the N~2 value for this sample was estimated by using Et2 -- 0.40 eV (at F = 3 × 1 0 4 V cm -1) from the inset of fig. 1 and /t0 (300 K) = 0.3 cm 2 V - 1 s- 1 in eq. (1). The estimated Nt2 value falls close to the Nt2 line in fig. 5 and thus further substantiates the identification of type 2 traps with Te atoms. Note that table 1 also indicates an increase in Nt~ with Te addition as displayed in fig. 5. If the type 1 traps are intrinsic Se defects (e.g. Se~-) frozen in at the glass transition temperature Tg, then an increase in Nt~ is not unexpected in as much as Tg rises nearly linearly with Te addition [34,35]. However, as shown by the works of Abkowitz, the mobility controlling traps are an equilibrium property of the well-annealed amorphous sample and thus are not expected to be influenced by Tg. An enhancement in the concentration of Se~- defects is possible with Te addition if the Se~- defect generating reaction involving the S e - T e interaction, i.e. Se° + Te ° ~ Se~- + T e ; is energetically more favourable than the other defect generating reactions. It seems that intuitively this may be the case since the Te atom is more likely to form a three-fold coordinated defect centre than a singly coordinated one, based on the observation that in the liquid, Te is three-fold coordinated [36,37]. If, on the other hand, Nil is assumed to be relatively unaffected by Te alloying in small amounts, then table 1 suggests a fall in /t0 to - 0.1 cm 2 V-1 s-1 with Te introduction. The present analysis cannot conclusively determine whether a rise in N,~ or a fall i n / t o with Te addition takes place. Since the low temperature mobility activation energy decreases with the applied field we may suspect the type 2 traps to be charged centres, similar to type 1 intrinsic traps proposed for a-Se. Under-coordinated Te{ defect centres are probably the most obvious candidates for these type 2 trap species. Chlorine-doped a-Se and a-Sel_xTex both have well-defined thermallyactivated hole drift mobilities from room temperature down to the lowest temperatures studied. In terms of eq. (1), transport controlled by two types of shallow traps, this means that the density Ntl is too small to allow type 1 traps to influence the transport in comparison with the influence of type 2 traps. Taking a reasonable value of /t o (300 K), say - 0 . 3 cm 2 V -1 s -1, we can estimate the shallow trap density responsible for the thermally activated behaviour o f / t h ( T ) . For example, for a-Se/3.5 wt% Te + 20 wt. ppm CI in fig. 3, /th ( F = 3 . 3 × 1 0 4 V c m - 1 ) ~ 6 . 4 × 1 0 -3 cm 2 V -1 s -1 and E~ (at same F ) = 0.401 eV in /th = /t; exp( -- EJkT),

(2)

S.O. Kasap, C Juhasz /Charge transport in chlorine doped amorphous films

33

where/~) = f f o N c / N t , gives ff~)= 3.2 x 10 4 cm 2 V-1 s 1 and N t / N v --- 9.4 x 10 6 ( N t -- 4.7 x 1014 cm 3.) This trap density is about a factor of 2 - 3 higher than that for Te-induced traps in undoped Se/3.5 wt% Te (table 1) but a factor of 10 lower than that for pure a-Se. It can be seen that CI doping, in the shallow trap controlled transport mechanism, leads either to a reduction in N, a n d / o r increases in if0 or alternatively to an introduction of Cl-induced additional traps at Et2 ~ 0.44 eV. The latter conclusion would explain not only the/~h(T) data on a-Se + y p p m C1 [15] but also on CI doped a-Se 1_Te~. The C1 introduced traps may be C1 o centres which are unbound C1 o ions in the structure. Thus, the results of the drift mobility-temperature measurements indicate that adding Te and C1 to a-Se introduces discrete sets of shallow hole traps; one set is due to Te atoms and is at - 0 . 4 3 eV above E v and the other is Cl-induced and is at - 0 . 4 5 eV above E v. It is important to remark that the possibility of a single manifold of shallow traps of one type at 0.43-0.45 eV induced either by Te or by C1 addition or the microscopic mode of conduction becoming thermally-activated (i.e. changing from drift to hopping), within experimental errors, cannot be ruled out. 3.2. Electron transport

Electron transport was only observable in a-Se 1 ,Te, alloys without chlorine. In Cl-doped a-Sel_xTe ~ alloys there was no detectable T O F electron photocurrent probably due to a drastic reduction in the electron range with CI addition [38,39]. The temperature dependence of the drift mobility/~e(T) in a-Se/3.5 wt% Te is shown in fig. 6. At a given field, /~e(T) below - 290 K exhibits a well defined thermally-activated behaviour. Above - 2 9 0 K, E~ decreases with temperature indicating a possible eventual saturation in pie. A similar behaviour for/~e(T) in a-Se occurs at - 270 K in a less sharp manner. The mobility activation energy has a slight field dependence as shown in the inset of fig. 6 of the type E~ = E ° - a e F with E ° = 0.49-0.50 eV and a = 30 ,~. Fig. 4 summarizes the effect of Te alloying a-Se on the drift mobility and its activation energy. It is not difficult to show from eq. (2) that the expected fall in /.% from an increase in E~, alone is a factor of - 10 more than actually observed in fig. 4; a situation similar to the effects of additions of As on electron transport discussed previously [19]. Thus Te addition not only increases E, but also the pre-exponential factor kt~ in eq. (2). The increase in the latter quantity can be accounted for if electron transport is controlled by the localized tail states of extent A E below E c since then the drift m o b i l i t y / ~ is given by ~ = ~o( AE/kT)"

exp(-AE/kT).

(3)

It does, however, require an unusual value of - 4 for the index n (assuming / % - 0.3 cm 2 V 1 s - l ) characterizing the shape of tail density states function

34

S, O. Kasap, C. Juhasz / Charge transport in chlorine doped amorphous films

10-I

w

m

i

i

i

I

I

I

l

I

I

I

I

a-Se0.97BTe0022

0.5;

E~(eVJ

~ o = 0.49 5 eV .....E~

0.~ -

I0-2

\.\

~

a- Se

".\

10-3,

~

T

"~.~7k

E~=O.3L,5eV,~, --7~I03 crn2V-Is-I

"~\'\._a-seo

~e

F:15

\

(cn~V% "~ )

I0-~.

,

L = 63 ~.m VOLTAGE

10-s"

F=O

/.,._

a-Seo.gTaTeo.o22

o -400 v - 500 ,, - 600 + - BOO A -I000

10-6

'

3.2

I

3.~

'

I

3.6

'

I

3.8

'

I

~.0

'

I

'

~.2 103 T-1 ( K -1 )

I

'

I

~.~,

4.6

'

4.8

=

Fig. 6. Semi-logarithmic plot of electron drift mobility versus reciprocal temperature for a-Se/3.5 wt% Te. The inset shows E~, versus V.

via N ( E ) - ( E - EA)". Alternatively, for n = 1, the a b s o l u t e value o f / t o will have to be - 100 c m 2 V -1 s - I T h e p r e - e x p o n e n t i a l f a c t o r / ~ at F = 0 in eq. (2) describing the well d e f i n e d

S.O. Kasap, C. Juhasz /Charge transport in chlorine doped amorphousfilms

35

thermally-activated behaviour at low temperatures is - 1 . 3 8 × 105 cm 2 V 1 s - 1 which with Poe - 0.3 cm 2 V - 1 s - 1 implies a shallow electron trap density 1.2 × 1014 c m - 3 at - 0.49 eV below E c. This trap concentration is comparable within a factor of 2, to the Te-induced hole trap density. It therefore seems reasonable to suppose that the mobility controlling electron traps are Te introduced. Note that the F e ( T ) data cannot conclusively determine the fate of the native Se defects controlling /~ in a-Se as a result of Te addition. This point can be demonstrated by inputting Et~ = 0.34 eV and N d / N c = 5 × 10 -5 for undoped a-Se and Et2 = 0.49 eV and N t z / N ~ = 2.3 x 10 - 6 for a-Se/3.5 wt% Te into eq. (1), which shows clearly that the second exponential term dominates the right-hand side, i.e. type 2 traps at - 0 . 4 9 eV below E~ effectively control the electron transport. The decrease in E, with temperature above T - 2 8 6 K is either due to structural relaxational processes as a consequence of approaching Tg which may result in changes in N t, E t , etc. [40,41] or due to the two exponential terms in eq. (1) becoming comparable ( so that E~, --* E t l for a-Se).

4. Final comments and conclusions

Fig. 4 provides a summary of hole and electron T O F drift mobility data on a-Se l_~Te x films with various amounts of CI doping. Drift mobility-temperature data indicate that hole transport in a-Sel_xTex is controlled by two sets of discrete shallow traps. The first set is already present in a-Se, i.e. at - 0.29 eV above E v and density - 1015 cm-3, whereas the second set is Te induced and appears at - 0.43 eV above Ev. The concentration of Te-introduced traps was found to increase linearly with Te content. They are probably associated with the Te~- centres. Doping a Se 1_xTex with C1 up to 40 wt p p m was found to introduce additional shallow hole traps at - 0 . 4 5 eV above Ev, around the same energy as the Te~- traps. There was no detectable electron transport signal in Cl-doped a-Se 1 xTe, alloys. The electron drift mobility-temperature data for the C1 free a-Se 1_:,Tee films indicate that electron transport is probably controlled by Te introduced ~hallow traps at - 0.49 eV below E c whose density is comparable to the Te induced shallow hole traps. It is therefore likely that hole and electron transport in a - S e : T e alloys are controlled by T e { , Te~- VAP defect centres a,hich are relatively easily formed. Although the interpretation used in this work may not necessarily be mique, it does provide a self-consistent view as well as reasonable values for ihe various trap densities. For example, for a-Seo.978Te0.022Nt2- 1.7 × 1014 : m - 3 m e a n s that 1 in every - 4 × 10 6 Te atoms forms a trap of this type, ~,hich when compared with 1 in every - 3 × 10 7 Se atoms forming a type 1 rap in pure a-Se suggests that Te associated defects are more readily produced han Se associated. It is unlikely that the decrease in Euh with the temperature in a - S e : T e

36

S.O. Kasap, C. Juhasz / Charge transport in chlorine doped amorphous films

alloys can be a t t r i b u t e d to the s a t u r a t i o n of the t r a p c o n t r o l l e d m o b i l i t y t o w a r d s the m i c r o s c o p i c m o b i l i t y it0, i.e. ~d = ~0 1 "[- Nvv e x p

~

---'/~0,

(4)

as was the case for a-Se [8] since the a p p l i c a t i o n of eq. (4) via a c o m p u t a t i o n a l analysis to a - S e / 5 wt% Te with /~0 = M T - l a n d N t / N v = B T -3/2 gives t~0 --0.02 cm 2 V -1 s - l a n d N t / N v = 3.6 x 10 -7 (i.e. N t ~ 1.8 X 1013 c m - 3 ) . This implies that Te a d d i t i o n destroys the shallow hole traps intrinsic to the a-Se structure a n d d r a s t i c a l l y r e d u c e s the m i c r o s c o p i c m o b i l i t y to a value typical of h o p p i n g transport. It w o u l d o b v i o u s l y be useful to e x a m i n e the p l a u s i b i l i t y of o t h e r i n t e r p r e t a tions of d r i f t - m o b i l i t y t e m p e r a t u r e d a t a besides those e x p l o r e d in this paper. In this c o n n e c t i o n note that if the basic m i c r o s c o p i c c o n d u c t i o n process is of h o p p i n g type, i.e. ,,hop e x p ( - W h o p / k r ) /'tO = t~O

(5)

then the analyses b a s e d on eq. (1) r e m a i n relatively unaffected, save the a b s o l u t e values of the p r e - e x p o n e n t i a l factors N t l / N , , N t 2 / N v which w o u l d be m u c h smaller. W e wish to t h a n k the Science a n d Engineering R e s e a r c h Council U K a n d G e s t e t n e r Byfleet ( U K ) L t d for financial support. H e l p f u l discussions with Drs. R. R o l l a s o n a n d J. T u r n e r of G e s t e t n e r Byfleet L t d were a p p r e c i a t e d .

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