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NON-DISPERSIVE AND DISPERSIVE TRANSPORT IN AMORPHOUS GERMANIUM SELENIDE AND
HYDROGENATED SILICON
J. Shirafuji, G. Kim, H. Matsui, M. Inoue, K. Yoshino, Y. Inuishi
To cite this version:
J. Shirafuji, G. Kim, H. Matsui, M. Inoue, K. Yoshino, et al.. NON-DISPERSIVE AND DISPERSIVE TRANSPORT IN AMORPHOUS GERMANIUM SELENIDE AND HYDROGENATED SILICON.
Journal de Physique Colloques, 1981, 42 (C4), pp.C4-583-C4-586. �10.1051/jphyscol:19814127�. �jpa-
00220745�
CoZZoque C4, suppzdment au nOIO, Tome 4 2 , octobre 1981 page C4-583
NON-DISPERSIVE AND DISPERSIVE TRANSPORT IN AMORPHOUS GERMANIUM SELENIDE AND HYDROGENATED SILICON
J. S h i r a f u j i , G . I . K i m , H. Matsui, M. I n o u e , K. Yoshino and Y. I n u i s h i
Department of EZectricaZ Engineering, FacuZty o f Engineering, Osaka Urriversity, Yarnada-W, Suita, Osaka, 565, Japan
Sbstract.- The coexistence of non-dispersive ( f a s t ) and dispersive (slm) elec- tron transport is observed in C=-Se evaporated films i n time-of-flight measure- ment. The saturation of the induced charge with increasing e l e c t r i c f i e l d in the non-dispersive cmpcment allows us t o estiiMte the reccmbination l i f e t h e of the optically injected electrons. In hydrogenated a-Si, on the other hand, the coex- istence of two transport processes is not found s o far. Electron transport i n GD a-Si is non-dispersive, but highly dispersive in SP a-Si. Holes shm dispersive process i n both ia) and SP a-Si.
Introduction.- It is of much importance in applications of amorphous semiconductors to optoelectronic devices such a s t h i n film solar cells, holcgraphic m r i e s and electronic photography t o investigate behaviors of photoexcited carriers. Time-of- f l i g h t technique would be a m e r f u l tool t o study characteristic behaviors of opt- i c a l l y injected carriers in amrphous semiconductors as w e l l a s i n crystals. In amarphous materials it i s frequently rnet t h a t the kink corresponding t o the carrier transit time does not appear due t o dispersive nature of the transport. Even i n the dispersive transport, the-of-flight method is still effective t o estimate the d r i f t m b i l i t y of carriers £ran log I vs. log t (I:current, t:time) plot.
A phenanenolcyical theory of the c?iswsive t r a n s p r t has been f i r s t presented by Scher and Montroll (1) under the assmption of large hopping-time distribution.
(X1 the other hand, Pollak (2) has pointed out t h a t the results of Scher and Montroll can not be explained by hopping conduction and are rather a gad. resresentation of the trap-limited transport under bread conditions. It has been sham more directly by several workers ( 3 , 4 ) that the generalized trap-limited transport can yield sam situations a s treated by Scher a n d - b n t r o l l . Because of the existence of dense and widely spread localized levels, dispersive nature of c a r r i e r transport is m r e or less contained i n amorphous materials: i n s m ~ cases dispersive nature daninates and i n other cases non-dispersive character i s observed, depending on materials o r preparation d t i o n s . In this paper we w i l l s h m the eqxrimental results of the coexistence of non-dispersive and dispersive electron transport i n evaporated G-Se glasses. Cchnparison i s made between sputtered (SP) and glmdischarged (Q) hydro- genated amorphous Si. The ccexistence of dispersive and ncn-dispersive conduction i s not found, but either of them dcarcinates depending an c a r r i e r type and preparation
~ t h d .
Expefinkxital Results and Discussion.
a) @-Se ev.pratted films.- GexSel-x (0.2 X $ 0.4) films were deposited on Sn02 coated glass substrate. A semitransparent Au electrode was evaporated onto the film f o r providing sandwich-type cell. The thickness o:f the f i l m ranged fran 1.4 t o 4.9 )rm. The -imental setup for time-of-flight measuremnt was similar t o t h a t de- scribed in previous papers (5,6). Pulsed l i g h t (10 ns, 337.1 m) £ran a nitrogen laser was focused on the sample through the Au electrode. The t r a n s i t tim t of p t o c a r r i e r s was e s t h t e d fran the induced charge waveform Q(t) for non-di&rsive
a s t electrons o r f r m log I ( t ) vs. log t p l o t of the transient current I ( t ) f o r dispersive slm eiectrons and holes. The applied f i e l d E was varied up t o 5xldl V/m.
The measwement was done a t ram temperature (R.T.) unless m k e d .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814127
C4-584 JOURNAL DE PHYSIQUE The electron transport i n a-Se (2 p thick) was non-dispersive, but when 20 % was added t o a-=, it changed drastically t o be dispersive. On further
increasing of Ge content, there appeared a sharp E = lo4 ~ l c m r i s e ( f a s t conponent) i n the induced charge signal
followed by a slm dispersive region (slm c- nent). This situation i s shown i n Fig. l. The height Q? of the f a s t c q n e n t tends t o increase
with increasing Ge content. The appearance of b m 0 ccmponents was influenced a l s o by the applied field.
{
For example in
Gee.
7 ~ 3 % ~ 66 glass (4.7p
thick), 0 40 80 120 160 200 the induced charge signal had only slm capcnent TlME ( P S ) a t 9.6~103 V/cm, m i l e a t 1 . 9 ~ 1 0 ~ V/cm non-disper-sive f a s t component b e c m clearly caning out. The
tim a t the boundary between f a s t and slow c m U Q25 075 nents correspnded t o the t r a n s i t t k of high- a
mobility electrons. In fact, the electron mobility 0 0 2 04 06 08 10 estirmted had the sm mgnitude and tenp=rature
=)
a TlME ( m s ) dependence a s those of non-dispersive electronsobserved in thick s a p l e s (150-303
p
thick) (6).
The non-dispersive electron n-obility i n Ge-Se was GeO2SeoB (1.5 pm) a s high as 0.4-0.1 cm2fl.s a t R.T.:this value can
be compared w i t h those i n Q) a-Si. The height of 0 f a s t (Qf) and slow (Q,) camponents of the induced
charge increased as the applied f i e l d was raised. 0 02 0 4 06 08 1.0 Both 06 and Oc i n the s a m l e with thickness of TlME (ms) 1.6 @showedda tendency t o s a t m a t e a t high fiel+.Fig. l
by elec- For the f a s t conponent (non-dispersive electron in &-Se with different.
transport) the saturation of the induced charge ccanpositions.
a t high fields i s caused by the a r r i v a l of a l l injected electrons t o the opposite electrode.
Fram the analysis of Qf
-
E characteristics of the 1.6 p Geo.33Se0.66 sanple,the was obtained t o be p ~ = 1 . 5 The slow c m p n e n t did not s h w a kink in the induced charge signal o r the trans- i e n t current, indicating the characteristic feature of dispersive transport. Figure 2 shows vs. E relation f o r samples with different glass conpositions. i s not p r o p r t i o n a l but superlinear t o E. The values of the n-obility estimated fram the slope in the low f i e l d region are l i s t e d i n Table I. The mgnitude of the dispersive electron mobility is comparable t o those observed by P a i (71, a l t h o a he e s t i m t e d the mobility by a conventional e t h d . I n the case of hole transport, only the dis- persive s l m caqmnent was observed in the induced charge signal. The estimatedn-obility is given in Table I.
The mechanism concerning the coexistence of two kinds of electron transport is not conclusive yet, but possibilities are i n the following:
i ) Morphological inhamogeneity--When the sanple contains two regions of high and low mobilities such a s crystalline and mrphous regions o r segre- gation of glass conpasition, the induced charge can
W 2 have two ccmpcnents. However, fram x-ray diffrac-
Table I Drift nobility of dispersive
( 4 5pm) carriers (300 K) in Ge-Se
O o r 2 3 4 5 (Electron)
Gee.
2Seo. 2 . 3 ~ 1 6 ~ cn2/v. s E ( x ~ ~ 4 ~ / c m )Fig. 2 Applied f i e l d dependence *0.35~0.65
of inverse t r a n s i t t* of slm (Ih31e) G e ~ . 3%0.7 7.4x10-~
electrons i n Ge-Se -0. 43e0. 6 1.7x10-~
e x p e r h t ,
GeSe). It has been k n m (7,8) that the addition of &l amount of branching atoms such as A s and Ge causes a remarkable redwtion of the electron mobility i n a-%.
I f there i s a nonuniform distribution of branching atoms, tsm kinds (fast and slow) of electrons can exist. However, the f a s t electrons observed has a much higher m b i l i t y (-10-I an2fl.s) than that of a-Se (6x10-3 m2fl.s). As seen in Fig. 1, a t the sarne applied f i e l d the f a s t capment becoms significant when the content is increased. This tenciency seems t o suggest the essential role of GeSeq mlecules.
meal segregation of structures wnsisting of GeSeq mlecules may responsible for high-mobility region, although the possibility of contribution of Ge-Ge bonds can not be s t i l l excluded. ii) Coexistence of different conduction mchanisns--- Electrcms can possess differentrmbilities depending on which energy levels electron w i l l d r i f t through. The appearance of two conpnents could be caused from conduc- tions a t conduction band tail (fast electron) and through gap states ( s l m electron).
Hawever, the activation energy (0.28 eV for Ge0.~%0.8) of the f a s t electron robil- i t y seem t o be t m large ccanpared w i I 5 the band t a i l extent (0.1 eV in Ge0.2*0.8 glass which was est-ted fram the optical energy gap (-1.9 eV) and the mobility gap (.v2 eV). There i s another plausible possibility of the presence of M kind5 of dominant electron traps a t 0.28 eV and deeper with broad distribution.
b) Hydrogenated a-Si.- GD a-Si f i w (4.5 )rm thick) were deposited on I T 0 glass sub- s t r a t e (250 K) using SiH4/H2 mixture. A conventional sputtering mthod was used t o prepare SP a-Si f i l m (1p thick) onto Sn02 coated glass substrate (220
oC)
using Si target and &/H2 mixture. The t o t a l gas pressure was l x l r 2 Torr and the hydrogen partial pressure w a s 7 A r 4 Torr.GD and SP hydrogenated a-Si films did not show an indication of the coexistence of nm-dispersive and dispersive transport. I n GD a-Si films electrons showed rela- tively nm-dispersive transportwith R.T. imbility of 6x10-~ &/V-s a t 8 . 3 ~ 0 3 V/m.
This value can be ccarpared with those &served by LeCon&r e t al. (9) and Tiedje e t al.(10). bhen the applied field w a s changed, the t r a n s i t time shaved nonlinear change. As seen from Fig. 3, a t low temperatures the electron m b i l i t y increases, although weakly, with increasing applied field, while a t high temperatures the elec- tron mobility decreases with increasing applied field. This feature a t low tempera- ture i s similar t o that &served i n Ge-Se glasses. CXI the other hand, the condudion a t high temperatures is apparently similar t o that in crystalline materials, althou- gh the origin i s not clear a t present. The fact that the conduction miss varies wit!! the teqxrature range w a s also d-trated on the temperature dependence of the electron mobility as shmn i n Fig. 4. In the low tenperatme region the activa- t i m energy is 0.19 eV which i s canparable t o that reported so far (9) and may cor- respond t o the depth of electron traps. A t high tenpratures the slope beccnws reduced and the m b i l i t y tends t o saturate. In the tenperatme dependence measure- mt, it was found that the current waveform tends t o be dispersive as the terrpera- ture was l m r e d .
3---- -
A 3 2 3 K ELECTRON
= 298 K 2 8 8 K
Fig. 3 Applied field dependene of Fig. 4 Temperature dependence inverse transit tirre of electrons in of electron d r i f t mobility in
GD a-Si G9 a-Si
C4-586 JOURNAL DE PHYSIQUE
Holes i n Ca) a-Si shmed apparently dispersive transport. The log I ( t ) vs. log t curve gave the hole nobility of 2.4xl0-4
&/'V.s
a t R.T. and 8 . 4 ~ 0 3 V / m , which is much larger than those &served by Allan (2-
7xl0-~ cm2/V-S) (11). However, the t r a n s i t tim was independent of the applied f i e l d up t o 1 . 5 A d V/m: this may sug- gest t h a t t h e transient current is dominated by non-exponential c a r r i e r recombination p r e s s rather than c a r r i e r transport.In SP a-Si films, on the other hand, electrons and holes shcwed highly dispersive transport: even i n the log I ( t ) vs. log t curve the appearance of t h e kink was very weak. The electron nasbility was about 5 A r 5
d / V . s
a t R.T. and 5 x l d V/m. It was seen a t lm applied f i e l d s t h a t the t r a n s i t tirre is apparently independent of the applied f i e l d in a similar way t o the case of Q a-Si. The temperature dependence of the t r a n s i t tim in the temperature range from 325 t o 405 K gave its activation energy t o be 0.3 eV.The hole m b i l i t y i n SP a-Si f i l m was 2
-
4x10-5 cm2/V-S which can be cohnparedwith the observation i n GD a-Si (11). Hawever, the activation energy of 0.2 eV cletembed f r m the t-rature dependence of the transit t i r r e was much s m l l e r than t h a t observed in Q a-Si (0.45 eV) (11). The similarity of the hole n o b i l i t i e s between in SP and Gh) a-Si films -lies t h a t the distribution of gap states near the valence band i s not s a t i s f a c t o r i l y inproved by hydrogenation.
S-.- We have d m n s t r a t e d the coexistence of non-dispersive ( f a s t ) and disper- sive (slcw) electrons i n evaporated Ge-Se glasses. P o s s i b i l i t i e s responsible f o r the ccexistence are discussed. The non-dispersive f a s t electrons has the roomtenpe- rature m b i l i t y as high a s 0.5
&/v.s
being comparable with t h a t of GD a-Si.In hydrogenated a-Si, only electrons in Q f i l m show the non-dispersive nature.
The coexistence of non-dispersive and dispersive transport is not observed. Holes i n Q) a-Si show apparently dispersive transport, but the position of the kink i n log I ( t ) vs. log t p l o t is rather independent of the applied field. The reconbina- tion process seem t o be dominant over the transport process. I n SP a-Si films, hiqhly dispersive transport i s observed f o r both electrons and holes.
The authors are much grateful t o Prof. H.Harrakawa of @aka University and Prof.
S.Nitta of Gifu University f o r their kind supply of Q a-Si sanples.
References
1) Scher H. and B n t r o l l E.W., Phys. Rev. (1975) 2455.
2) Pollak M - , Phil. Mag.
36
(1977) 1157.3) Marshall J.M., Phil. Mag. 36 (1977) 959, 4) Sdmidlin F.W., Phys. ~ e v . - s (1977) 2362.
5) Kim G.I., Shirafuji J. and Inuishi Y., Tech. Rep. @aka Univ. 28 (1978) 111.
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Garmischl~artenkirchen (1974) p. 355.
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