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Conductivity and defects in amorphous silicon doping modulated multilayers
Thin Solid Films, 198 (1991) 43-51 ELECTRONICS AND OPTICS
43
CONDUCTIVITY AND DEFECTS IN A M O R P H O U S SILICON DOPING M O D U L A T E D MULTILAYERS D. H. ZHANG AND D. HANEMAN
School of Physics, Universi(v of New South Wales, P.O. Box 1, Kensington, New South Wales 2033 (Australia) (Received May 15, 1990; revised September 14, 1990; accepted October 2, 1990)
The variations in dark conductivity and photoconductivity with thickness of the intrinsic (i) layer in n-i-p-i.., multilayers show opposite trends. These are explained in terms of dopant diffusion and phosphorus-boron dJeep trap centers. Effects of light soaking and deposition at non-optimal substrate temperature or discharge power are detailed. In all cases they reduce the persistent photoconductivity which is maximized when film defects are held to a minimum.
1. INTRODUCTION We have previously reported that amorphous hydrogenated silicon (ASIL) doping modulated n - i - p - i . . , multilayers made with fixed n and p layer thicknesses d, and dp, but variable i layer thickness di, showed interesting variations in properties as a function of dl. In particular, the persistent photoconductivity (PPC)I, optical band gap E °, and conductivity activation energy 2 E A show a pronounced maximum at a particular value of di. However, the photoluminescence (PL) peak energy shows only a monotonic reduction a s d i increases 3. All these phenomena have been explained 3'4 by considering the diffusion of hydrogen, phosphorus and boron which occurs between the layers during ASIL deposition, and indeed underlines the importance of such processes. In this communication we report various new data that add to the understanding of the above effects. 2. SAMPLE PREPARATION
The samples consisted of six n layers and five (i-p-i) layers with d n = 7.7 n m , dp = 3.3 nm and di ranging from 2.7 to 160 nm. They were prepared by the glow discharge decomposition of silane at a pressure of 300 mT on borosilicate glass or quartz substrates held at 220 °C. Doping was achieved by admitting 0.1 ~o phosphine (n layer) or nominal 0.01~o diborane (p layer) to the silane. R.f. power was approximately 1 W over 20 mm 2 except where noted otherwise. For the component films, the dark and light conductivities are given in Table I. The deposition rate was between 1.5 and 2.5 A s- 1. Multilayers were produced by depositing n,i,p and i layers in sequence. There 0040-6090/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
44
D.H.
TABLE
ZHANG,
D. H A N E M A N
I
PARAMETERS OF SINGLE FILMS AS USED FOR P R E P A R I N G M U L T I L A Y E R S
Layer
Thickness ( n m )
n p i
7.7 3.3 various
cro(f~
Dark ('onductirilv i c m i)
np(f~
Ptu)loconductivil)'
5×10 3 5 x 10 s 1 x 10 s
9.0x10 3 2.3 x 10 s 5.5 × 10 4
icm
i)
4clirati(m energyEaleV) 0.24 0.62 0.82
was a 60 s time interval between d e p o s i t ] o n of each layer, in which the r.f. p o w e r was switched off a n d the gas m i x t u r e a d j u s t e d to be uniform and steady. Electrical c o n t a c t to each layer of the 1.7 cm x 1 cm m u l t i l a y e r s was m a d e by scratching a l o n g the long sides a n d e v a p o r a t i n g a l u m i n u m . Effects of light were o b t a i n e d from a t u n g s t e n - h a l o g e n E L H l a m p t h r o u g h a 5 cm thick w a t e r cell acting as a heat filter. F o r P P C m e a s u r e m e n t s , specimens were first a n n e a l e d for 30 min at 180 C to restore equilibrium, a n d then i l l u m i n a t e d at r o o m t e m p e r a t u r e . All m e a s u r e m e n t s were p e r f o r m e d in a v a c u u m oven e q u i p p e d with t e m p e r a t u r e control. 3.
CONDUCTIVITY
AND PHOTOCONDUCTIVITY
C o n d u c t i v i t i e s in the d a r k a ° a n d u n d e r light of 50 m W cm 2 ae were m e a s u r e d for the series of layers and are shown in Fig. 1. T h e y show a r e m a r k a b l e variation, p e a k i n g at the s a m e value of di as does the P P C p a r a m e t e r G a n d the a c t i v a t i o n energy. G is the r a t i o to the d a r k c o n d u c t i v i t y of that m e a s u r e d 10rain after cessation of a 20 s light flash as described above. T h e values of a c t i v a t i o n energy vs. d~ are s h o w n in Fig. 2. It s h o u l d be n o t e d that whereas ¢rP shows a m a x i m u m , in c o m m o n with the three p a r a m e t e r s G, E A a n d E °, the d a r k c o n d u c t i v i t y cr° shows a m i n i m u m . F u r t h e r m o r e , there is a similar d r o p in b o t h ¢r° a n d a P as di increases from a b o u t 90 nm to a b o u t 170 nm. W e n o w i n t e r p r e t these initially puzzling p h e n o m e n a . -1
I
i
oP(°I
-4
-3
o
~
"
-5
-4 Oe
-6
0
I
I
I
l
I
30
60
90
120
150
180
di(nm) Fig. 1. L o g a r i t h m s o f c o n d u c t i v i t y in t h e d a r k crD a n d u n d e r i l l u m i n a t i o n o f 50 m W c m o f i l a y e r t h i c k n e s s d~ f o r v a r i o u s m u l t i l a y e r s (d. = 7.7 n m . d o = 3.3 n m ) .
2 o_P as a f u n c t i o n
CONDUCTIVITY AND DEFECTS IN a-Si
I
I
45 I
I
I
<0.55~
?/ ,,t 0.50 z
o
I-~ 0.45
0.41
I
30
I
I
60
90
I
I
120
150
di(nm) Fig. 2. ActivationenergyE A a s a functionofdi. The conductivity of the multilayer is in principle determined by the conductivities of the component layers, including interface layers if appropriate. If these are not included one may show that, ifa is the multilayer conductivity, cr(2di + dn + dp) = 2aidi +
a,dn + opdp
where one ignores the single extra n layer for simplicity. From Table I, the value o f a n is so high, (105 times and more) compared with % and cq, and with any interface layers, that it dominates the right-hand side. Hence one might ask why ~rD should be affected by any change in di. The explanation is that there are several processes to consider involving diffusion of hydrogen, phosphorus and boron. As di begins to increase above the minimum value used of 2.7 nm, hydrogen diffusion from the i into the n layers increases their activation energies, causing the observed drop in a in Fig. 1, and increase in EA in Fig. 2. However, phosphorus diffusion from the n into the i layer edges is also occurring, causing their conductivity to increase greatly. This counterbalances the above effect after di of about 20 nm and a increases. However, for larger values of d~, the loss of phosphorus from the n layers reduces their conductivity and gradually outweighs the effect of the increased conductivity in the edges of the i layers, so that a shows an overall reducing tendency after d~ of about 70 nm. Contrary to the behaviour of trD, aP shows a peak as seen in Fig. 1. We believe this is caused by the same mechanism that causes PPC, namely the formation of P-B centers acting as carrier traps. Thus the carriers excited by the (steady) illumination do not recombine as rapidly when the P-B centers are present, thus enhancing ~a. It should be noted that though ~D is reduced owing to increased EA, the latter is not relevant to the first order for a P as the light energy exceeds EA and excites carriers into the conduction band. It should also be noted that the explanation for a D is based on the overriding conductivity of the n layer. However, in the case of PPC and photoconductivity, all layers conduct and there is a significant contribution from the nominal i layers, which actually are converted to partly compensated layers around
46
D. H. ZHANG, D. HANEMAN
d~ = 14nm, and contain the P - B centers 3. F o r multilayers with "i" layers thinner than about 15 nm, o P drops again because the P - B centers have accreted h y d r o g e n and are mostly shallow trap P - B - H centers as explained previously 4. It should be noted that the m a x i m u m o P is greater than 2 x 10 2~') l c m l " which is larger than that o f any individual film (Table I). This is because o f the presence in the multilayer o f the trap centers that reduce recombination. F r o m the above reasoning, one would expect changes in a u p o n annealing. D a t a are shown in Fig. 3. For a multilayer with small di (10nm), the dark conductivity steadily reduces as expected from Fig. 1, owing to enhanced hydrogen diffusion into the n layers. The photoconductivity rises slightly as also expected, owing to formation of P - B centers, and then falls as these are accreted by hydrogen. The multilayer with intermediate di (31.5 nm) shows an initial d r o p in o D due to h y d r o g e n diffusion but is then approximately constant owing to the competing process of p h o s p h o r u s diffusion into the i layer. However, o P drops owing to h y d r o g e n accretion onto the P - B centers. F o r the thicker i layer (52.5nm) multi)ayer, there is a quick initial drop in o D due to hydrogen diffusion but no change thereafter due to p h o s p h o r u s diffusion--data were not obtained thereafter. The p h o t o c o n d u c t i v i t y showed little change, as expected from Fig. 1. Hence one can
-2
-3 "7 O
L
b
djInto )
O
-4
-5-5.30 ANNEALING
10
+
315
•
52.5
o
~ I 50
100 TIME (HOURS)
Fig. 3. Effectof annealing on light and dark conductivity of multilayers, annealing temperature is 215 'C for d~ = 10 and 31.5nm, and 230 °C for d~ - 52.5nm.
CONDUCTIVITY
AND
DEFECTS
a-Si
IN
47
qualitatively account for these data in terms of the explanations for the phenomena in Figs. 1 and 2. 4.
OPTICALLY
INDUCED
CONDUCTIVITY
CHANGES
If the band gap incident light intensity is sufficiently strong, around 200 m W c m - 2 , it is known that defect centers, thought to be dangling bonds, are formed which reduce conductivity 5-1°. As a check on the effects of this on the multilayers, they were exposed to the light source for 10h at an intensity of 2 0 0 m W c m 2, achieved by bringing the light source closer (to keep the spectral content the same). It was found that a P reduced, generally as for single layers. Results for one sample are shown in Fig. 4. This n - i - p - i multilayer consisted of 6 x 6 nm n, 5 x 2.3 n m p and 10 × 10.4nm i layers. The value of G was 170. It should be noted that aP decreases slowly during illumination, as found in unlayered films, while the value of G measured at the end of the 10h exposure is reduced by a factor of 7 to approximately 26. After annealing at 180 °C for 30min, the sample returns to its ground state and the process can be repeated (black dots). The values of trD and aP are reliable to within a factor of 1.5. These data show that the light and dark conductivities of the multilayer are reduced by the illumination, as is not unexpected if the component films develop defect centers. In addition, the reduction in the P P C factor G shows that the action of the light breaks bonds and releases hydrogen in the vicinity of the traps, so that hydrogen can accrete onto them. Thus the number of deep traps is reduced. The broken bonds act as scattering centers reducing the conductivities. After annealing
S'-19
ofter anneal (e}
-2
~ -31 U
.2o_ 4
m o
-5
1
°
m J
-0
_
o
I
I
I
0
2
4
I
I
6 8 t (hours)
I
10
L.._ 0
Fig. 4. Change in a Pfor a multilayerof initial PPC factor G = 170 during illumination of 200mW cm- 2 from a tungsten-halogen lamp. After annealing the initial behavior is restored,(right-hand curve).
48
D. H. Z H A N G , D. HANEMAN
at 180 ~C, the hydrogen atoms are released from the B-P centers and on average reduce the number of broken bonds to the pre-annealing value. 5. EFFECT OF L I G H T INTENSITY
One may ask whether the photoconductivity and the persistent photoconductivity have the same dependence on light intensity, since PPC depends on excess carriers rather than the equilibrium number of excited carriers. In Fig. 5 we show such a dependence for both a P and a PPc where the latter is the conductivity 10 min after cessation of a 5 s flash of light. As a precaution against remanent effects, the sample (d~ --- 13.7 nm) was annealed after each illumination. It is seen that both a P and ~rPPc show effectively the same dependence on light intensity F. This is in agreement with data 11'12 obtained for n - p - n - p ... multilayers, where a relation a ~ F r was found, with r = 0.4. In our data this relationship is fitted quite well with r = 0.33. The 15')/0difference in the value of r is probably not significant. The similar dependences of a P and crPPc, show that increases in the number of photogenerated carriers do not affect the relative action of the deep traps compared with other recombination centers, at least up to the intensities used here. 6.
EFFECTS OF P R E P A R A T I O N CONDITIONS
The results in Section 4 showing that light-induced defects decrease the PPC raise the question whether other defects have a similar effect. It is known that the temperature ~ of the substrate durin8 deposition affects the dangling bond spin density t3'14 and luminescence intensity 15, particularly for samples made at low
aP
(o)
o'PPC(m)
u
"g-2 0
-3 10
[ 100 F (mWcm -2)
Fig. 5. P h o t o c o n d u c t i v i t y a P and persistent p h o t o c o n d u c t i v i t y a PPc, m e a s u r e d l O m i n after cessation of a 20 s light flash of 50 m W cm - z against light intensity F on l o g - l o g scales.
CONDUCTIVITY AND DEFECTS IN
a-Si
49
power like ours. Hence the P P C was measured for samples (di = 13.7 nm) made at different T~. The results in Fig. 6 show a general m a x i m u m in the P P C factor G for multilayers prepared at around 200 °C, and a fall-off at higher or lower temperatures. For room temperature films, G is unity (no PPC). This result is readily explained by the absence of diffusion at such temperatures, so that the P - B centers cannot form. At temperatures between room temperature and 200°C, diffusion occurs but to a lesser extent than at around 200 °C, hence there are fewer deep traps. In addition, more dangling bond defects are present. Both of these factors will cause a lower G. For T~ > 200 °C (250 °C), there is adequate diffusion, and fall-off in G is presumably due to the incidence of a higher density of defects which occurs when is greater than the optimum value. It may be noted that in the case of thick n - p multilayers (d n + d v = 41 nm), Su e t a l . ~6 reported a maximum G for ts = 200°C and much lower values for T~ = 160°C. However, the conditions of deposition in their experiments differed greatly from the more standard conditions used here. Defects can also be induced by using higher r.f. power during deposition 15. ~7 This was found from the decrease in the IR absorption coefficient at 200 c m - 1 and the increase in the wagging mode absorption at 845cm 1 at higher power. Furthermore, the photoluminescence intensity and refractive index decreased. Figure 7 shows the dependence of G for multilayers made at different r.f. powers. The samples made at a nominal r.f. power of ! W have a structure of d, = 5nm, dp = 2 n m and di = 10nm (maximum G). The thicknesses of the other
10 s
0
o
o
o
A
~ 10 2 r, a.
o
2
101
0 100 200 300 Ts (o C ) 5UBSTRATE TEMPERATURE Fig. 6. The PPC parameter G (a PPC/a D ) obtained for multilayers with d i = 13.7 nm, for various substrate temperatures.
50
D. H. ZHANG.
I
1 2.0
D. HANEMAN
I
I I 2.5 3.0 POWER tarb. units)
Fig. 7. The PPC parameter G obtained the discharge during deposition.
3.5 for multilayers
with di = 10 nm, for different r.f. power applied to
specimens differed slightly owing to the difference of deposition rate at different powers. The observed fall-off in G for powers less than or greater than the optimum could conceivably be related to the differences in thickness. However, the ratios of d,$r$, are the same for all the structures and the thickness differences in the power range 2.222.5 in the graph are quite small, about 10%. Therefore, we believe the major cause of the effect is the much higher defect concentration for layers made at greater than or less than the optimum power. For example, pi-n solar cehs fabricated at non-optimum power exhibit a markedly reduced efficiency owing to higher defect concentrations. Thus these data confirm the deleterious effect of film defects on the PPC performance of n--i-p-i . . multilayers. To obtain high values of over 1000 as achieved for our films (Fig. 6). despite their large area, it appears necessary to minimize the formation of defects. 7. CONCLUSION The experiments described above show that the various changes in multilayer properties for various differences in structure, preparation conditions and treatment, can all be consistently explained by the deep trap P-B center hypothesis and by considering the effects of diffusion of dopants and hydrogen in the multilayers. In all cases, processes known to cause defects reduce the persistent conductivity. ACKNOWLEDGMENT
This work was supported
by the Australian
Research
Council.
CONDUCTIVITY AND DEFECTS IN
a-Si
51
REFERENCES
I 2 3 4 5 6 7 8 9 10 I1 12 13 14 15 16 17
D.H. Zhang and D. Haneman, J. Appl. Phys., 63 (I 988) 1591. D.H. Zhang and D. Haneman, Appl. Phys. Lett., 52 (1988) 1392. D.H. Zhang, D. Haneman and Z. R. Shi, J. Appl. Phys., 66 (1989) 4958. D. Haneman and D. H. Zhang, Appl. Surf Sci., 33/34 (1988) 692. D.L. Staebler and C. R. Wronski, Appl. Phys. Lett., 31 (1977) 292. M. Taniglian, N. B. Goodman and H. Fritzsche, J. Phys. (Orsay), Colloq., 42 (1981) Suppl. 10, C4375. R.S. Crandall, Phys. Rev. B, 42 (1981) 7457. H. Dersch, J. Stuke and J. Beichler, Appl. Phys. Lett., 38 (198 I) 456. H. Okushi, M. Miyagawa, Y. Tokumaru, S. Yamasaki, H. Oheda and K. Tanaka, Appl. Phys. Letl., 42 (1983) 895. J.I. Pankove and J. E. Berkeyheiser, Appl. Phys. Lett., 37 (1980) 705. J. Kakalios and H. Fritzsche, J. Non-Cryst. Solids, 77-78 (1985) 1101. J. Kakalios, Philos. Mag. B, 54(1986) 199.
B.A. ScottandJ. A. Reimer, J. Appl. Phys.,54(1983)6853. H.J. Queisser and D. E. Theodorou, Phys. Rev. Lett., 43 (1979) 401. R.A. Street, J. C. Knights and D. K. Biegelsen, Phys. Rev. B, 18 (1978) 1880. F.C. Su, S. Levine, P.G. VanierandF. J. Kampas, Appl. Phys. Lett.,47(1985)612. J.C. Knights, Jpn. J. Appl. Phys., 18(1979) Suppl. 18-1, 10.
43
CONDUCTIVITY AND DEFECTS IN A M O R P H O U S SILICON DOPING M O D U L A T E D MULTILAYERS D. H. ZHANG AND D. HANEMAN
School of Physics, Universi(v of New South Wales, P.O. Box 1, Kensington, New South Wales 2033 (Australia) (Received May 15, 1990; revised September 14, 1990; accepted October 2, 1990)
The variations in dark conductivity and photoconductivity with thickness of the intrinsic (i) layer in n-i-p-i.., multilayers show opposite trends. These are explained in terms of dopant diffusion and phosphorus-boron dJeep trap centers. Effects of light soaking and deposition at non-optimal substrate temperature or discharge power are detailed. In all cases they reduce the persistent photoconductivity which is maximized when film defects are held to a minimum.
1. INTRODUCTION We have previously reported that amorphous hydrogenated silicon (ASIL) doping modulated n - i - p - i . . , multilayers made with fixed n and p layer thicknesses d, and dp, but variable i layer thickness di, showed interesting variations in properties as a function of dl. In particular, the persistent photoconductivity (PPC)I, optical band gap E °, and conductivity activation energy 2 E A show a pronounced maximum at a particular value of di. However, the photoluminescence (PL) peak energy shows only a monotonic reduction a s d i increases 3. All these phenomena have been explained 3'4 by considering the diffusion of hydrogen, phosphorus and boron which occurs between the layers during ASIL deposition, and indeed underlines the importance of such processes. In this communication we report various new data that add to the understanding of the above effects. 2. SAMPLE PREPARATION
The samples consisted of six n layers and five (i-p-i) layers with d n = 7.7 n m , dp = 3.3 nm and di ranging from 2.7 to 160 nm. They were prepared by the glow discharge decomposition of silane at a pressure of 300 mT on borosilicate glass or quartz substrates held at 220 °C. Doping was achieved by admitting 0.1 ~o phosphine (n layer) or nominal 0.01~o diborane (p layer) to the silane. R.f. power was approximately 1 W over 20 mm 2 except where noted otherwise. For the component films, the dark and light conductivities are given in Table I. The deposition rate was between 1.5 and 2.5 A s- 1. Multilayers were produced by depositing n,i,p and i layers in sequence. There 0040-6090/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
44
D.H.
TABLE
ZHANG,
D. H A N E M A N
I
PARAMETERS OF SINGLE FILMS AS USED FOR P R E P A R I N G M U L T I L A Y E R S
Layer
Thickness ( n m )
n p i
7.7 3.3 various
cro(f~
Dark ('onductirilv i c m i)
np(f~
Ptu)loconductivil)'
5×10 3 5 x 10 s 1 x 10 s
9.0x10 3 2.3 x 10 s 5.5 × 10 4
icm
i)
4clirati(m energyEaleV) 0.24 0.62 0.82
was a 60 s time interval between d e p o s i t ] o n of each layer, in which the r.f. p o w e r was switched off a n d the gas m i x t u r e a d j u s t e d to be uniform and steady. Electrical c o n t a c t to each layer of the 1.7 cm x 1 cm m u l t i l a y e r s was m a d e by scratching a l o n g the long sides a n d e v a p o r a t i n g a l u m i n u m . Effects of light were o b t a i n e d from a t u n g s t e n - h a l o g e n E L H l a m p t h r o u g h a 5 cm thick w a t e r cell acting as a heat filter. F o r P P C m e a s u r e m e n t s , specimens were first a n n e a l e d for 30 min at 180 C to restore equilibrium, a n d then i l l u m i n a t e d at r o o m t e m p e r a t u r e . All m e a s u r e m e n t s were p e r f o r m e d in a v a c u u m oven e q u i p p e d with t e m p e r a t u r e control. 3.
CONDUCTIVITY
AND PHOTOCONDUCTIVITY
C o n d u c t i v i t i e s in the d a r k a ° a n d u n d e r light of 50 m W cm 2 ae were m e a s u r e d for the series of layers and are shown in Fig. 1. T h e y show a r e m a r k a b l e variation, p e a k i n g at the s a m e value of di as does the P P C p a r a m e t e r G a n d the a c t i v a t i o n energy. G is the r a t i o to the d a r k c o n d u c t i v i t y of that m e a s u r e d 10rain after cessation of a 20 s light flash as described above. T h e values of a c t i v a t i o n energy vs. d~ are s h o w n in Fig. 2. It s h o u l d be n o t e d that whereas ¢rP shows a m a x i m u m , in c o m m o n with the three p a r a m e t e r s G, E A a n d E °, the d a r k c o n d u c t i v i t y cr° shows a m i n i m u m . F u r t h e r m o r e , there is a similar d r o p in b o t h ¢r° a n d a P as di increases from a b o u t 90 nm to a b o u t 170 nm. W e n o w i n t e r p r e t these initially puzzling p h e n o m e n a . -1
I
i
oP(°I
-4
-3
o
~
"
-5
-4 Oe
-6
0
I
I
I
l
I
30
60
90
120
150
180
di(nm) Fig. 1. L o g a r i t h m s o f c o n d u c t i v i t y in t h e d a r k crD a n d u n d e r i l l u m i n a t i o n o f 50 m W c m o f i l a y e r t h i c k n e s s d~ f o r v a r i o u s m u l t i l a y e r s (d. = 7.7 n m . d o = 3.3 n m ) .
2 o_P as a f u n c t i o n
CONDUCTIVITY AND DEFECTS IN a-Si
I
I
45 I
I
I
<0.55~
?/ ,,t 0.50 z
o
I-~ 0.45
0.41
I
30
I
I
60
90
I
I
120
150
di(nm) Fig. 2. ActivationenergyE A a s a functionofdi. The conductivity of the multilayer is in principle determined by the conductivities of the component layers, including interface layers if appropriate. If these are not included one may show that, ifa is the multilayer conductivity, cr(2di + dn + dp) = 2aidi +
a,dn + opdp
where one ignores the single extra n layer for simplicity. From Table I, the value o f a n is so high, (105 times and more) compared with % and cq, and with any interface layers, that it dominates the right-hand side. Hence one might ask why ~rD should be affected by any change in di. The explanation is that there are several processes to consider involving diffusion of hydrogen, phosphorus and boron. As di begins to increase above the minimum value used of 2.7 nm, hydrogen diffusion from the i into the n layers increases their activation energies, causing the observed drop in a in Fig. 1, and increase in EA in Fig. 2. However, phosphorus diffusion from the n into the i layer edges is also occurring, causing their conductivity to increase greatly. This counterbalances the above effect after di of about 20 nm and a increases. However, for larger values of d~, the loss of phosphorus from the n layers reduces their conductivity and gradually outweighs the effect of the increased conductivity in the edges of the i layers, so that a shows an overall reducing tendency after d~ of about 70 nm. Contrary to the behaviour of trD, aP shows a peak as seen in Fig. 1. We believe this is caused by the same mechanism that causes PPC, namely the formation of P-B centers acting as carrier traps. Thus the carriers excited by the (steady) illumination do not recombine as rapidly when the P-B centers are present, thus enhancing ~a. It should be noted that though ~D is reduced owing to increased EA, the latter is not relevant to the first order for a P as the light energy exceeds EA and excites carriers into the conduction band. It should also be noted that the explanation for a D is based on the overriding conductivity of the n layer. However, in the case of PPC and photoconductivity, all layers conduct and there is a significant contribution from the nominal i layers, which actually are converted to partly compensated layers around
46
D. H. ZHANG, D. HANEMAN
d~ = 14nm, and contain the P - B centers 3. F o r multilayers with "i" layers thinner than about 15 nm, o P drops again because the P - B centers have accreted h y d r o g e n and are mostly shallow trap P - B - H centers as explained previously 4. It should be noted that the m a x i m u m o P is greater than 2 x 10 2~') l c m l " which is larger than that o f any individual film (Table I). This is because o f the presence in the multilayer o f the trap centers that reduce recombination. F r o m the above reasoning, one would expect changes in a u p o n annealing. D a t a are shown in Fig. 3. For a multilayer with small di (10nm), the dark conductivity steadily reduces as expected from Fig. 1, owing to enhanced hydrogen diffusion into the n layers. The photoconductivity rises slightly as also expected, owing to formation of P - B centers, and then falls as these are accreted by hydrogen. The multilayer with intermediate di (31.5 nm) shows an initial d r o p in o D due to h y d r o g e n diffusion but is then approximately constant owing to the competing process of p h o s p h o r u s diffusion into the i layer. However, o P drops owing to h y d r o g e n accretion onto the P - B centers. F o r the thicker i layer (52.5nm) multi)ayer, there is a quick initial drop in o D due to hydrogen diffusion but no change thereafter due to p h o s p h o r u s diffusion--data were not obtained thereafter. The p h o t o c o n d u c t i v i t y showed little change, as expected from Fig. 1. Hence one can
-2
-3 "7 O
L
b
djInto )
O
-4
-5-5.30 ANNEALING
10
+
315
•
52.5
o
~ I 50
100 TIME (HOURS)
Fig. 3. Effectof annealing on light and dark conductivity of multilayers, annealing temperature is 215 'C for d~ = 10 and 31.5nm, and 230 °C for d~ - 52.5nm.
CONDUCTIVITY
AND
DEFECTS
a-Si
IN
47
qualitatively account for these data in terms of the explanations for the phenomena in Figs. 1 and 2. 4.
OPTICALLY
INDUCED
CONDUCTIVITY
CHANGES
If the band gap incident light intensity is sufficiently strong, around 200 m W c m - 2 , it is known that defect centers, thought to be dangling bonds, are formed which reduce conductivity 5-1°. As a check on the effects of this on the multilayers, they were exposed to the light source for 10h at an intensity of 2 0 0 m W c m 2, achieved by bringing the light source closer (to keep the spectral content the same). It was found that a P reduced, generally as for single layers. Results for one sample are shown in Fig. 4. This n - i - p - i multilayer consisted of 6 x 6 nm n, 5 x 2.3 n m p and 10 × 10.4nm i layers. The value of G was 170. It should be noted that aP decreases slowly during illumination, as found in unlayered films, while the value of G measured at the end of the 10h exposure is reduced by a factor of 7 to approximately 26. After annealing at 180 °C for 30min, the sample returns to its ground state and the process can be repeated (black dots). The values of trD and aP are reliable to within a factor of 1.5. These data show that the light and dark conductivities of the multilayer are reduced by the illumination, as is not unexpected if the component films develop defect centers. In addition, the reduction in the P P C factor G shows that the action of the light breaks bonds and releases hydrogen in the vicinity of the traps, so that hydrogen can accrete onto them. Thus the number of deep traps is reduced. The broken bonds act as scattering centers reducing the conductivities. After annealing
S'-19
ofter anneal (e}
-2
~ -31 U
.2o_ 4
m o
-5
1
°
m J
-0
_
o
I
I
I
0
2
4
I
I
6 8 t (hours)
I
10
L.._ 0
Fig. 4. Change in a Pfor a multilayerof initial PPC factor G = 170 during illumination of 200mW cm- 2 from a tungsten-halogen lamp. After annealing the initial behavior is restored,(right-hand curve).
48
D. H. Z H A N G , D. HANEMAN
at 180 ~C, the hydrogen atoms are released from the B-P centers and on average reduce the number of broken bonds to the pre-annealing value. 5. EFFECT OF L I G H T INTENSITY
One may ask whether the photoconductivity and the persistent photoconductivity have the same dependence on light intensity, since PPC depends on excess carriers rather than the equilibrium number of excited carriers. In Fig. 5 we show such a dependence for both a P and a PPc where the latter is the conductivity 10 min after cessation of a 5 s flash of light. As a precaution against remanent effects, the sample (d~ --- 13.7 nm) was annealed after each illumination. It is seen that both a P and ~rPPc show effectively the same dependence on light intensity F. This is in agreement with data 11'12 obtained for n - p - n - p ... multilayers, where a relation a ~ F r was found, with r = 0.4. In our data this relationship is fitted quite well with r = 0.33. The 15')/0difference in the value of r is probably not significant. The similar dependences of a P and crPPc, show that increases in the number of photogenerated carriers do not affect the relative action of the deep traps compared with other recombination centers, at least up to the intensities used here. 6.
EFFECTS OF P R E P A R A T I O N CONDITIONS
The results in Section 4 showing that light-induced defects decrease the PPC raise the question whether other defects have a similar effect. It is known that the temperature ~ of the substrate durin8 deposition affects the dangling bond spin density t3'14 and luminescence intensity 15, particularly for samples made at low
aP
(o)
o'PPC(m)
u
"g-2 0
-3 10
[ 100 F (mWcm -2)
Fig. 5. P h o t o c o n d u c t i v i t y a P and persistent p h o t o c o n d u c t i v i t y a PPc, m e a s u r e d l O m i n after cessation of a 20 s light flash of 50 m W cm - z against light intensity F on l o g - l o g scales.
CONDUCTIVITY AND DEFECTS IN
a-Si
49
power like ours. Hence the P P C was measured for samples (di = 13.7 nm) made at different T~. The results in Fig. 6 show a general m a x i m u m in the P P C factor G for multilayers prepared at around 200 °C, and a fall-off at higher or lower temperatures. For room temperature films, G is unity (no PPC). This result is readily explained by the absence of diffusion at such temperatures, so that the P - B centers cannot form. At temperatures between room temperature and 200°C, diffusion occurs but to a lesser extent than at around 200 °C, hence there are fewer deep traps. In addition, more dangling bond defects are present. Both of these factors will cause a lower G. For T~ > 200 °C (250 °C), there is adequate diffusion, and fall-off in G is presumably due to the incidence of a higher density of defects which occurs when is greater than the optimum value. It may be noted that in the case of thick n - p multilayers (d n + d v = 41 nm), Su e t a l . ~6 reported a maximum G for ts = 200°C and much lower values for T~ = 160°C. However, the conditions of deposition in their experiments differed greatly from the more standard conditions used here. Defects can also be induced by using higher r.f. power during deposition 15. ~7 This was found from the decrease in the IR absorption coefficient at 200 c m - 1 and the increase in the wagging mode absorption at 845cm 1 at higher power. Furthermore, the photoluminescence intensity and refractive index decreased. Figure 7 shows the dependence of G for multilayers made at different r.f. powers. The samples made at a nominal r.f. power of ! W have a structure of d, = 5nm, dp = 2 n m and di = 10nm (maximum G). The thicknesses of the other
10 s
0
o
o
o
A
~ 10 2 r, a.
o
2
101
0 100 200 300 Ts (o C ) 5UBSTRATE TEMPERATURE Fig. 6. The PPC parameter G (a PPC/a D ) obtained for multilayers with d i = 13.7 nm, for various substrate temperatures.
50
D. H. ZHANG.
I
1 2.0
D. HANEMAN
I
I I 2.5 3.0 POWER tarb. units)
Fig. 7. The PPC parameter G obtained the discharge during deposition.
3.5 for multilayers
with di = 10 nm, for different r.f. power applied to
specimens differed slightly owing to the difference of deposition rate at different powers. The observed fall-off in G for powers less than or greater than the optimum could conceivably be related to the differences in thickness. However, the ratios of d,$r$, are the same for all the structures and the thickness differences in the power range 2.222.5 in the graph are quite small, about 10%. Therefore, we believe the major cause of the effect is the much higher defect concentration for layers made at greater than or less than the optimum power. For example, pi-n solar cehs fabricated at non-optimum power exhibit a markedly reduced efficiency owing to higher defect concentrations. Thus these data confirm the deleterious effect of film defects on the PPC performance of n--i-p-i . . multilayers. To obtain high values of over 1000 as achieved for our films (Fig. 6). despite their large area, it appears necessary to minimize the formation of defects. 7. CONCLUSION The experiments described above show that the various changes in multilayer properties for various differences in structure, preparation conditions and treatment, can all be consistently explained by the deep trap P-B center hypothesis and by considering the effects of diffusion of dopants and hydrogen in the multilayers. In all cases, processes known to cause defects reduce the persistent conductivity. ACKNOWLEDGMENT
This work was supported
by the Australian
Research
Council.
CONDUCTIVITY AND DEFECTS IN
a-Si
51
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I 2 3 4 5 6 7 8 9 10 I1 12 13 14 15 16 17
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