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Photo-Induced Metastable Effects in Hydrogenated Amorphous Silicon (a-Si H).

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Annalcn der Physik. 7. Folge, Band 42, Heft 2, 1985, S. 187-197
J. A. Barth, Leipzig
Photo- Induced Metastable Effects in Hydrogenated
Amorphous Silicon ( a 4 i :H)
By W. FUHS,
H. MELL, J. STUEE,P. THOMAS,
and G. WEISER
Fachbereich Physik, Universitiit Marburg, F.R.G.
Herrn Prof. Dr. W . Walcher 2um Y5. Geburfstag gewidmel
A b s t r a c t . Light induced changes of electronic properties are studied in glow discharge deposited
a-Si:H-films by ESR, field effect, DLTS and elect,ronir transport. These effects are caused by silicon
da,ngling bonds generated by recombination processes, which raise the density of states near midga,p.
The concomitant, increase of potential fluctuations indicates that these defects are inhomogeneously
distribnted.
Licht-induzierte metastabile Effekte in hydrogenisiertem amorphem Silizium (a-Si :H)
I n l i a l t s u b e r s i c h t . Es wird uber Licht-induzierteAnderungender elektronischenEigenscha.ft.en
von a&: H-Filmen berichtet. Dazu wurde der EinfluD liingerer Belichtung auf ESR, Feldeffekt, DLTS
und Transporteigenschaftsn untersucht. Die metastabilen Effekte entstehen durch freie Si-Bindungen, die bei der Rekombination gebildet werden und zu einer Erhohung der Zustandsdiclite nahe der
Mitte der verbotenen Zone fuhren. Die dabei auftretende Verstarkung von Potentialfluktuntionen
dentet darauf hin, daB diese Defekte inhomogen verteilt sind.
1. Irttroduction
Light. induced changes of the properties of a-Si :H-films were first reported by STAEBLER and WRONSKI
[l].These authors found that prolonged exposure to light strongly
decreases both the dark- and photoconductivity of a-Si: H-films. The initial state could
be restored by annealing above 150°C. From a study of the time dependence of the
relaxation in the temperature range 140- 190°C t,heseauthors concluded that the relaxation process involves an activation energy of 1.5 eV [l].The light induced changes have
been attributed t o changes in the density and/or occupation of deep gap states resulting
in a shift of the Fermi level towards midgap. These states act as recombination centers
and decrease the carrier lifetimes. Electron-spin resonance studies identified these states
a s Si-dangling bonds [a]. The Staebler-Wronski effect (SW) has been observed for a large
variety of deposition conditions and compositions of the gas in the glow discharge [3].
The light induced changes of the conductivity are most pronounced when the Fermi level
lies about half way between the gap center and either mobility edge [4]. The SW-effect
is of utniost iniport,ance for the stability of solar cells made on the basis of hydrogenated
amorphous silicon. These solar cells degrade by extended light exposure, the degradation
being the more pronounced the higher the initial efficiency is [ 5 ] . It may depend on thc
solution of this stability problem whether amorphous silicon deposit,ed in a glow discharge can successfully be used for solar cells. This paper suniniarizes investigations of
light induced effects by electron spin resonance (LESR),field effect, deep level transient
spectroscopy (DLTS) and transport properties which have hecn performed in our lahoratory and discusses the mechanism of defect creat#ion.
188
Ann. Physik Leipzig 42 (1985) 2
2. Electron-Spin Resonance
Measurements of the electron spin resonance give insight into the nature of the light
induced defects [2, 61. Extended exposure to light leads t o a n increase of the ESRresonance a t g = 2.0065 which is corninonly assigned t o singly occupied Si-dangling
bonds. Fig. 1 displays ESR spectra of an undoped sample in the annealed state A and
in state B after exposure to light (6 h exposure, white light, 300 mW/cni2 a t T = 350 K).
The spin density in the light soaked state B amounts to about lo1' cm3 and is higher
by a factor of 10 than in state A. It is important to emphasize that thedark conductivity
in these undoped films is not altered upon light exposure, which indicates that there is
no shift of the Fermi level. Hence the increase of the spin density does not arise from a
change in the occupation of the singly occupied dangling bonds but froin a real increase
in the defect density. The linewidth (7.5 G ) and the g-value (2.0055)remain essentially
unchanged. The lineshape of the signaI (Fig. 1) indicates that the spins are isolated.
A
I
A I
/ \i
10 G
Fig. 1. ESR-spectra of undoped a-Si:H before (A) and afkr (B) light exposure (white light, 300 mW/
em2, G h, T = 80°C) [6]
It has often been discussed that the defects are created by breaking of weak Si-Si
bonds in the amorphous network which leads t o two neighbouring dangling bonds with
a distance of less than 5 8. I n this case one would expect a change in lineshape due t o
exchange narrowing which, however, is not observed. It has therefore been suggested
that hydrogen atonis from nearlying 5i:H sites saturate one of the two created free
bonds. This process is supposed t o enlarge the separation of the defects and to stabilize
them.
The number of light induced spins, AN,, does not simply depend on the absorbed
dose, i.e. on the product of light intensity I and the exposure time t. As is shown in
Figs. 2 and 3
-
A N , W I O ~for
~ t = 1 h,
AN,
for I = 320 niW/cm2.
The data inFig. 2 were obtained by starting a t each value of theilight intensity from thc
annealed state. Fig. 3 presents measurements on two samples of different thickness
which coincide fairly well. From this result it is concluded that the defects are not
W. FUESet al., Metastable Effects in Silicon
I
I
10
30
300
100
Intensity (rnw/cd)
Fig. 2. Light generated spin density A N , as a function of tho light intensity (white light, T = 8OoC,
exposure time 1h) [6]
undoped a-Si:H
T= 8OoC
- d = 1.38pm
x - d=0.78pm
A
I = 320 mW/cm2
I
I
1
10
I
100
Time (mid
I
loo0
Fig. 3. Light generated spin density A N , after an exposure with white light of 320 mW/cm* a t 80°C
as a function of the exposure time and light induced change of the photoconductivity uph[6].Sample
thickness: ( A ) 1.38 pm, ( x ) 0.78 pm
Ann. Physik Leipzig 42 (1985) 2
190
created at the surface but in the bulk of the films. I n addition, the behaviour in Fig. 3
indicates that the number of light induced defects saturates a t long exposure times.
Similar dependences on I and t have recently been reported by STUTZMANN
et al. [7]The light induced spin density depends on the temperature during illmninat ion
(Fig. 4). In this measurement the sample was illuminated for 1 h with white light
(320 mW/cin2)starting at each temperature from the annealed state. I n the temperature
range T < 400 K an activated behaviour is found with an activation energy near
0.04 eV. Above 400 K the competing annealing process leads t o a decrease of AN,.
According to these results there is no simple relationship between the increase of t h e
defect concentration and the photoconductivity as suggested in older reports [ 11. The
temperature dependence of AN, in Fig. 4 is quite similar to that of the light induced
increase of the photoluminescence AL/L in the defect band [8] which is shown for coinparison. Light exposure causes a decrease of the intrinsic luminescence band at 1.4eV
and an increase of the intensity of the defect luminescence near 0.8 eV. The intensity of
the defect band is correlated with the density of dangling bonds.
undoped a-SH
loo
0
3x10-'
<
a
lo-'
I
2
3
1
1
1
4
5
6
lo3 KIT
Fig. 4. Light generated spin density A N , of undoped a-Si:H versus the temperature of light exposure
(white light, 320 mW/cm2, 1 h) [GI. For comparison the relative change of the luminescence intensity
AL/L of the defect band is shown [8]
Annealing of the a-Si :H-films at temperatures above 150°C completely restores the
original state. Isothermal and isochronal annealing studies lead to the conclusion that
the annealing process cannot be described by a single activation energy but only by a
h a d distribution of energies. The upper limit of this distribution is near 1.5eV [el.
W. FUHSet a]., Metastable Effects in Silicon
191
3. Density of Localized Gap States
Light induced changes in the density of gap states have been observed by various
techniques such as field effect [9] and deep level transient spectroscopy (DLTS) [ll,121.
The field effect data (Fig. 5) clearly reveal an increase in the density of gap states upon
light exposure. After strong illumination the current a t V, = 0 has decreased by an
order of magnitude. By annealing a t 170°C the original curve is quite well repoduced.
From the form of the I - V,-curves we conclude that the Staebler-Wronski effect is
a bulk effect and does not arise from a decrease of an accumulation layer a t the surface
or film-substrate interface. After illumination we find that the density of states is raised
near midgap to as much as lo1*
eV-l (curve 2). No significant changes are observed
in the region of the tail states in accordance with measurement)s of the drift mobility
on annealed and light exposed samples [lo].
lo-*
3
A
10'~
lo-"
0.7
0.6
0.5
0.4
Ec- E/eV
0.3
Fig. 5. Current as a function of the gate potential (left) and N ( E ) (right) of undoped a-Si:H [9].
(1)annealed a t 17OoC, (2) after strong illumination, (3) after 0.5 h annealing a t 170°C,(4) after 4 11
annealing a t 170°C. Arrows indicate the position of the Fermi level
From capacitance-DLTS LANGet a]. [Ill deduced that light exposure creates states
with a concentration on the order of lo1*om3 eV-', 0.5 eV above the mobility edge of
the valence band. These states are supposed t o be donorlike, because acceptors in this
concentration would cause a n unreasonably large shift of the Fermi energy. I n order
to account for the weak Fermi level shift these authors postulate the additional creation
of acceptor levels which due t o their low concentration of about 10'6
eV-l could
not be detected in the DLTS-experiment. The DLTS-data obtained in our laboratory
[la], on the other hand, indicate a pronounced increase of the density of states near
midgap (Fig. 6). I n this case N(E) was deduced from current-DLTS-measurements on a
Pt/a-Si :H Schottky barrier, the same trend has also been obtained from capacitanceDLTS data. Starting from the annealed state (curve A) the film was illuminated under
various conditions. The largest changes are observed by illuminating for 60 hours with
white light (500 mW/cm2) and applying forward bias, V > V,,, t o the diode during
Ann. Physik Leipzig 42 (1985) 2
192
illuminating (curve B3). The increase of N(E) is less pronounced but still amounts to
a n order of magnitude if hotnogeneously absorbed light is used of lower intensity
(hv < 1.7 eV, 15 h, 50 mW/cm2) and the diode kept forward biased (curve Bl). Most
remarkably, there is only a small increase of N(E) when during illumination a reverse
bias is applied (curve B2). Since with strong reverse bias the photogenerated carriersare
effectively extracted from the space charge region, recombination is inhibited. This behaviour therefore strongly suggests that the defect creation is connected with the recombination processes.
1
0.0
0.6
E,-
0.4
0.2
E/eV
Fig. 6. Density of states distribution from current-DLTS data on a Pt/a-Si:H-diode. Doping level:
100 ppm PH,; A: before light exposure; B,- B3: after light exposure under different conditions.
Details see text [12]
4. Transport Properties
The important question whether the light induced defects also affect the transport
path has been addressed by concomitant investigations of the conductivity a and the
thermoelectric power S [13]. The results shown in Fig. 7 were obtained for a film deposited from SiH, doped with 100 ppm PH, [ 131. I n order to remove adsorbates and
unintentionally created defects the sample was first annealed a t 280°C (upper curves).
After light exposure u and S have changed appreciably (lowest curves, B), the increase
of both activation energies, E, and E,, confirms that the Fermi level is shifted towards
midgap. The remarkable result is that the difference of the activation energies, E9 =
E, - E,, has increased from 0.09 eV to 0.21 eV. This behaviour indicates that light
exposure does not merely shift the Fermi level but in addition affects the current path
in the a-Si:H-film. The light soaked state is stable against annealing up to 80°C. For
higher annealing temperatures T,, u and S gradually approach their original values
(Fig. 7).
Different activation energies of u and S are observed quite commonly in amorphous
films. In fact, model calculations [ 14,151 have shown that EQin the range 0.05-0.08 eV
necessarily results in situations where transport is controlled by disorder and electronphonon coupling even in a homogeneous system. On the other hand, BEYER
and OVERHOF [16] have suggested that larger values of E, can arise from spatial potential fluctua-
W. FUHSet al., Metastable Effects in Silicon
193
I
-08
-09
-1 0
--
-I I
Y
> -12
-cnE
-1 3
-1 4
a-Si:H
100 ppm PH3
-1.5
2
5
4
3
-1.6
3
2
4
5
lo3 KIT
103KTI
Fig. 7. Conductivity c and thermoelectric power S of a-Si:H (100 ppm PH,) as a function of 1/T.
A annealed, B light exposed (100 rnW/cm2,1.2-2 eV, 60 h). The other curves have been obtained by
stepwise annealing at the given temperatures, the slopes E , and Es are indicated [13]
,
0
I
I
I
I
3
Fig. 8. Eg = E , - E s versus E,. (1)a-Si:H (100 ppm PH,) light exposure and stepwise annealing
(samedata as in Fig. 7). (2) 8,varied by phosphorous doping [13]
Ann. Pliysik Leipzig 42 (1985) 2
194
tions which may originate from density fluctuations, internal strain and/or potential
fluctuations due to charged centers. This model predicts that the value of EQis directly
related to the height of the potential fluctuations. The interesting result shown in Fig. 7
is that the quantity EQdepends on illumination and on the annealing state. Fig. 8 shows
the dependence of E, on E, for the various annealing states (curve 1). It is interesting
to note that a conipletely different dependence is found when E, is changed without
illuniination by phosphorus doping (curve 2). Thus the increase of E, - E, by optical
exposure does not result from the shift of the Ferini level but is due to a change of the
properties of the current path. We conclude that the increase of EQto values above 0.1 eV
is due to an enhancement of potential fluctuations caused by charged light induced defects. However, quantitative difficulties arise if one assuines that the light induced
centers are randomly distributed. I n case of an inhoniogeneous distribution much larger
potential fluctuations are to be expected. It seems thus likely that the light induced
defects are inhomogeneously distributed in the sample.
A completely different behaviour is found in compensated a-Si:H films which are
doped both with phosphorous and boron [17]. Fig. 9 shows the time dependence of the
dark conductivity, od, and photoconductivity, opk,when the samples are illuminated
by a tungsten-halide lamp (100 mW/cm2). For the undopedfilms (U) the typical decrease
U
I
1
10
I -
I.
lo2
t (s)
10’
1000 ppm PH3
1000 pprn B ~
UNDOPED
H ~
lo4
Fig. 9. Dependence of the dark conductivit,y ad and of t,he photocondnctivity up),on the exposure
time t for a compensated film C and an undoped film U [17]
W. FUHSot al., Metastable Effects in Silicon
195
of opkand od with exposure time is found. Jn the compensated n-type film (C), however,
both ,a
,, and od increase with t , the latter by more than two orders of magnitudeafter an
exposure of 15 h. The annealing behaviour, too, is different in both types of samples.
I n the compensated film the initial state is retained a t a much lower temperature and
the relaxation process is determined by a much lower activation energy (-0.6 eV).
An interesting additional feature of this metastable increase of the conductivity in
compensated films is that the difference of the activation energies of conductivity and
thermoelectric power, E, - E,, decreases upon light exposure (Fig. 10) namely from
0.19 t o 0.11 eV. This behaviour contrasts with the results shown in Fig. 7 and 8 of the
singly doped films. A possible reason for this reduction of the potential fluctuations is
separation of the photogenerated electrons and holes and trapping in deep states. It
appears thiisprobable that the excess conductivity in compensated films arises from photo generated carriers trapped a t isolated centers. AKER et a]. [18] have reported on a
similarly large increase of od in p-type, singly boron doped a-Si :H films and have shown
that this behaviour was related to the presence of a negatively charged surface oxyde.
Since the above effect has been observed in both p-type and n-type compensated saniples, it appears unlikely that it is caused by light induced changes of the charge on the
surface oxide.
a-Si:H
10-2.
TA(OC)
- 0.8
1
2
3
-1.0 -
-
LO
120
180
E,(eVI
ES(eV)
060
029
0.50
06L
036
045
Y
> E
Y
v)
-1.4 -
-1.6-
108
2
3
I/T (10'~ K-')
4
-1.8
2
3
- I/T
4
~-9
Fig. 10. Conductivity u and thermopower S a s , a function of 1/T measured after annealing a lightsoaked compensated film at three temperatures T A [17]
5. Mechanism of Defect Creation
The light induced changes of the electronic properties of a-Si:H films can be explained by the creation of silicon dangling bonds and the resulting changes in thedensity
of localized gap states. This is in accordance with the observation that quite similar
changes of the dark conductivity, photoconductiyity, ESR, photoluminescence and
absorption spectra can be achieved when the defects are created by bombardment
196
Ann. Physik Leipzig 42 (1986) 2
with electrons of high energy (-1 MeV) [6]. The defects are produced by electron-hole
recombination processes. The rate of defect creation has been estimated t o be near
10-5 defects/recombination act a t the beginning of light exposure 1191 and decreases
defects one needs roughly 10%
considerably with time. For the creation of lo1'
recoinbination processes [19]. This low efficiency of the defect production indicates that
the defect creation is not linked to the dominant non-radiative recombination channel
via dangling bonds but to a less effective channel.
STUTZMANN
e t al. [ 71 proposed that non-radiative recombination between the tail
states of the conduction and valence bands leads to the breaking of weak Si-Si bonds.
These authors described the kinetics of the defect creation and showed that this model
can indeed explain the dependences of the light induced defect density AN, on intensity
I and exposure time t: AN, -12/3t1/3. Furthermore this model readily explains the
decrease of the creation rate with exposure time. When the density of dangling bonds
increases, the dominant recombination path via dangling bonds is enhanced and the
tail-tail recombination which leads to defect production is more and more suppressed.
This model for defect creation is an intrinsic model which means that the defect creation
is an inherent property of the disordered structure of the hydrogenated amorphous
films. Earlier studies had indicated that the Staebler-Wronski-effect might be related
to inipurities and indeed the films prepared in conventional glow discharge systems contain high concentrations of impurities like oxygen and nitrogen. Recently, TSAI et al.
[SO] were able to reduce the concentration of 0, N and C by 2-3 orders of magnitude
by preparing the films in a bakeable UHV-glow discharge system. It was found that
below impurity concentrations of 1020
the number of defects created by light did
not depend on the oxygen and nitrogen content. The original suggestion, that the first
step of defect creation is the breaking of weak Si-Si bonds [2], is equivalent to the defect
creation by nonradiative tail-tail recombination since weak bonds lead to localized tail
states due to the smaller separation of bonding and antibonding states. This model needs
weak bonds with hydrogen atoms bonded nearby which can switch and stabilize the
defects. However, to date there is no proof for a direct partizipation of hydrogen. I n
particular, there is no proof for a correlation of the SW-effect with the hydrogen content of the films.
.
An important question is whether the created defects are homogeneously distributed
in the film. The changes in the properties of the current path on light exposure suggest
that the defects are charged (at least in part) and inhomogeneously distributed. This is
in accordance with proton magnetic resonance studies which lead to the conclusion
that the hydrogen is inhomogeneously distributed [21]. It is argued that there are
regions where hydrogen atoms are randomly distributed preferably bonded in Si-H
configuration and that most of the hydrogen is clustered in H-rich regions. The result
that the potential fluctuations are enlarged in singly doped films shows that the light
induced defects enhance the inhomogeneity of the films. It is, therefore, likely that the
defects are created predominantly either in the regions with clustered or in those with
diliited hydrogen.
There are a number of results which indicate that the situation is even more complex
and different types of defects might be generated. I n a study of single and dual-beam
photoconductivity HANand BRITZSCHE
[22] found evidence for the creation of two types
of metastable centers. One kind is supposed to decrease the put-product, the other one
to enhance suh-handgap absorption. Moreover, KRUHLER
et al. [23] recently reported
that also hole trapping can produce metastable defects which are different from those
generated by reconhination.
Acknowledgenient. This work has been supported by the Brindesminister fiir
Forschung und Technologie (BMFT).
U’.PUHSet a]., Metastable Effects in Silicon
197
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H.: J. Non-Cryst. Solids 59/60 (1983) 509.
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Bei der Redaktion eingegangen am 10. Dezember 1984.
Anschr. d. Verf.: Prof. Dr. W. FUHS,H. MELL,J. STUKE.P. THOMAS,
G. WEISER
Fachbereich Physik der Universitkt Marburg
Renthof 5 , D-3550 Marburg
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