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Role of photoinduced defects in amorphous GexSe1−x photoconductivity
I O U R N A L OF
Journal of Non-Crystalline Solids 155 (1993) 171-179 North-Holland
NO
SO
Role of photoinduced defects in amorphous GexSel_x photoconductivity Z. El G h a r r a s , A. B o u r a h l a ~ and C. V a u t i e r Laboratoire d'Etude et de CaractErisation des Compos~s Amorphes et des Polymdres, UFR Sciences, BP 118, 76134 Mont-Saint-Aignan c~dex, France
Received 18 February 1992 Revised manuscript received 13 October 1992
The spectral dependence of photoconductivityof amorphous pure Se and Ge-Se alloys(4 and 8 at.% Ge) evaporated thin films was measured for a spectral range between 2511and 8(10nm. A comparison of the Se and Ge-Se spectra shows that the magnitude of photocurrent decreases as the Ge content increases. This result indicates a correlation between the magnitude of the photocurrent and the density of photoinduced defects. The contribution of these defects to the enhancement of photoconductivity in a specific region of the spectrum as well as the influence of temperature on their density are also discussed.
1. Introduction Among the various effects arising in chalcogenide glasses under irradiation, the spectral dependence of photoconductivity remains the least well investigated. Most of the numerous studies reported in the literature on photoconductivity of these materials concern the temperature or light intensity dependence (for a brief review, see refs.
[1-4]). The first model of photoconductivity for amorphous chalcogenides was proposed by Street and Mott [5] and is based upon models of charged defects [6,7]. These models are now well established and describe correctly the experimental data on these materials. More recently, Caries and co-workers [8,9] have observed two major discrepancies between their experimental results, obtained on amorphous selenium films, and the
i Permanent address: ENS, BP 227, Mostaganem, Algeria. Correspondence to: Dr Z. El Gharras, Laboratoire d'Etude et
de Caract6risation des Compos6s Amorphes et des Polym~res, UFR Sciences, BP 118, 76134 Mont-Saint-Aignan c~dex, France. Tel: + 33 35 14 68 81. Telefax: + 33 35 14 68 82.
S t r e e t - M o t t model. According to this model, the photoconductivity must be proportional to the product of the quantum efficiency, r/, and the light intensity, F, when the photocurrent, lph, is less than the dark current, !,,, whereas for larger values of F, which yield a photocurrent larger than ! d, the photoconductivity must be proportional to ( ' o F ) ~/2. The Caries et al. experimental data on a-Se films show a sublinear variation of Iph (/ph "~ FX, where x is equal to 0.75) but also a spectral dependence of the photoconductivity which does not follow the variation of quantum efficiency. Concerning this last result, a model for steady-state photoconductivity which explains the spectral dependence of photocurrent in a-Se films has been proposed [9]. These authors have shown that there are different processes, involving specific defects, which are a function of incident photon energy. It is now well established that these photoinduced defects play an important role in the electrical properties of chalcogenide semiconductors. In the photoconductivity process, they contribute, for an optical excitation of suitable wavelength, to the generation and the recombination of free
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
! 72
Z. El Gharras et al. / Photoinduced dt~'cts m a-(h,, Se t _ ~ phot~'onductit'iD,
phous selenium. Moseley and Chamberlain [11] have shown that the photoinduced absorption (PA) decreases in the S e - G e glasses when the concentration of Ge increases. This last result shows that a correlation exists between the density of defects, which are the photoinduced absorption centers, and the flexibility of network. In this paper, we present new experimental results of steady state photocon~.activity obtained on amorphous Ge,Set_, films. We also present
carriers by means of two localized bands in the gap, located on each side of the Fermi level. A close correlation between the density of photoinduced defects, their distribution in the film and some of the structural, electrical or optical properties of chalcogenide glasses have been reported in the literature. Among these, Grenet et al. [10] have shown, by photorelaxation measurements, that the diffusion of photoinduced defects increases the structural relaxation rate of amor-
(Iph/F.~)
~o =.
/
1o'-
l O 0.
10'.. I
1()~ III
II
I
16~
161
I
3
'~
s
hr(ev
Fig. 1. Spectral dependence of Se photoresponse ( . . . . . ) and quanlunl efficiency ( ) (from ref. [12]). Tile saturated high energy values of both curves have normalized to unity.
Z. El Ghan-as et al. / Photoinduced defects in a-Ge,Se,
an analysis of the experimental data which corroborates the Carles photoconductivity model and the Moseley PA measurements. In order to estimate the influence of Ge, we study, in the first part of this paper, the processes involved in the spectral dependence of a-Se photoconductivity. For the concentrations of interest to us, the Ge content influences the processes which control the magnitude of photocurrent. We also discuss the role played by the photoinduced defects in photoconductivity as well as the influence of the temperature and the concentration of Ge on their density.
_ + photoconductility
2. Experimental The GeSe bulk alloys were prepared in a sealed quartz tube where the mixture was heated. for 48 h, at 1000°C and then quenched. In a vacuum af 10B7 mbar, the GeSe and Se thin fiihns were obtained from bulk by thermal evaporation (evaporation rate m 1 rim/s)) on glass substrates at room temperature. Gold electrodes were deposited for bias contacts. The films have a planar structure and the incident light tlux was at right angle to the sample plane. The film thickness was m 1.5 pm and the interelectrode distance 250
Is
I
lo1
,
I
1
.Se rGe
a04
S %96
*~%0.3%9*
3
4
Fig. 2. Spectral dependence of photoconductivity
t-3
in Se. Ge,,,,MSe,,,uh and Ge,,,,,$e,,,,
films at room temperature.
174
Z, El Gharras et aL / Photoinduced defects in a-Ge~St'z _ ~ photoconducticity
p.m. With an applied voltage of 100 V, the electric field (4 × 105 V m -I) is small enough to prevent space charge effects and the dark current is ohmic whatever the temperature. Since the incidence flux depends upon the spectral response of both light source and grating system, the results are presented in the following normalized form:
( lph/FA)/( Ioh/FA)~ = f(h~,), where Iph = li-ld" and lph is the
photocurrent, I i
is the current measured under illumination, I d is the dark current, F is the light intensity for a wavelength, A, and he is the incident photon e n e r g y . (lph/FA) s is the normalized value of photocurrent corresponding to the saturation of the film photoresponse and measured for the photon energies > 4 eV. Finally, it is important for further discussions to point oat that the photocurrent values have been measured step by step with photon energy decreasing from 5 to 1.5 eV.
10 z
4%
1
.lO
,
.10 °
• 300k
=310 k
.1~~
". 3 2 0 k -+" 3 3 0 k
I , i
III
i I
J r
I I
I I
II
I
. . . .
_l
¢
J
2
/
|,
6
3
h i " (ev) Fig. 3. Spectral dependence of Gca0~Se ~
photoresponse for different values of the temperature: 300, 310. 320 and 330 K.
Z. El G h a r r a s et al. / Photoinduced defects in a - G e x S e t _ ~. photoconductil,ity
3. Results
The variation of photocurrent in a-Se thin films versus incident photon energy is reported in fig. 1. In the same figure, the variation of quantum efficiency, r/, as observed by Knights and Davis [12], is also shown. At low energies, the photocurrent increases exponentially as does the quantum efficiency. It exhibits a maximum for 3.6 eV and tends towards a constant value for energies > 4 eV. In order to determine the influence of germanium atoms added in the Se matrix, we used the same experimental protocol for the study of the
175
spectral dependence of photoconductivity of Geo.o4Seo.96 and Geo.0sSe0.92 alloys (fig. 2). We report in the same figure the photoconductivity spectra of both GeSe alloys and pure Se. We note that the variation of photocurrent with photon energy has the same shape for GeSe alloys as for Se. However the magnitude of the peaks decreases as the Ge content increases and the 'shoulder' which exists at hv = 2.1 eV in the selenium spectrum vanishes in the GeSe alloys curves.
The photoconductivity spectra of Ge0.04Se0.96 and Ge0.osSeo.92 alloys, were recorded 'in situ' in the temperature range from 300 to 330 K (figs. 3 _10 z
8%
.10 t
_10 0
N= • 300 K - 310K 320 K "~ 3 3 0 K
462 /'1 III .
.
I I .
.
li
._]_
t_.
_163 -I
i I
I
I
I
2
3
4
5
1~ ~" 6
hr" (ev) Fig. 4. Spectral dependence of Geo0sSe~}~2 photoresponse for different values of the temperature: 300, 310, 320 and 330 K.
176
Z. El Gitarras et al. / Pitotoinduced defects h~ a-GexSe t _ .~ photoconducti~'ity
and 4, respectively). In the investigated temperature range, the temperature increase alters principally the magnitude of photocurrent in part II of the curves. The maximum of the photocurrent decreases as the temperature increases.
~
I
citon
f
4. Discussion
self- trapl~ed/ exciton ]
'°',°"/
4.1. Spectral photoresponse of Se According to Caries et al. [9] we may consider that three energy ranges, with a preponderant photogeneration process in each one, exist in Se spectrum (fig. 1). (a) In part I, the photon energies lie above 4 eV and the photocurrent as well as the quantum efficiency have a constant value. In this region, the photon energy is larger than the optical gap of selenium. Therefore, it can be assumed that each absorbed photon is able to create, after thermalization, a free electron-hole pair as predicted by the Knights-Davis model [12]. At the same time, following Street [13], we may assume that the photogenerated electronhole pairs geminate giving rise to a photoinduced defect. A bond is transferred from one chalcogen atom to another, resulting in two defects: one of them is negatively charged and onefold coordinated and the other one is positively charged and threefold coordinated. These defects are labelled, respectively, D- and D +. The requirement of neutrality leads to an equal density of D + and D - which are associated in pairs (D +, D - ) called valence alternation pairs (VAP). Some of them can be close enough to be localized on neighbouring atoms. In such a case, they are called intimate valence alternation pairs (IVAP). When free carriers are photogenerated, the3 are rapidly trapped by the charged defects which become neutral (D°). In the Street model [13] the photoinduced defect is a (D÷,D - ) pair but the concerned atoms are close together so that the structure is IVAPlike. Because of the strong electron-phonon coupling in chalcogenides, the transition from the ground state to a closed (D+,D -) pair can occur either by self trapping of an exciton (path I (fig.
rr
J
round-state CONFIGURATION
Fig. 5. Configurational coordinate diagram for self-trapped excitoa in chalcogenides (see ref. [13]).
5) [13]), followed by a local relaxation, or by direct excitation (path II, (fig. 5) [14]). However, in this energy range the photon penetration depth is small (about 0.1 ~m) compared with the sample thickness (1.5 p.m). The photoinduced defects are created near the surface, in a small part of the sample and a gradient of IVAP concentration appears between the 'dark part' of the sample and the illuminated one. Grenet et al. [10] have shown that this gradient of IVAPs concentration involves the diffusion of photoinduced defects towards the entire sample. According to them, we may consider that the exposure time (1 h), used at each wavelength in our experimental procedure, is sufficient for the concentration of IVAP to become homogeneous in the entire sample. (b) The part II of photoresponse curve exhibits two distinct regions. In the first region (2.2 eV < hu < 2.6 eV), the photoconductivity exhibits an exponential variation as does the quantum efficiency. In the second region (2.6 eV < hu < 4 eV), the photoconductivity exhibits a maximum (for hu = 3.6 eV)
Z. E l Gharras et al. / Photoinduced detects in a-GexSe t _ x phowconductivity
while the quantum efficiency saturates. The difference between photoconductivity and quantum efficiency data suggest that the direct photogeneration of free carriers is not the main process giving rise to a photoconductive effect. Indeed, the photoinduced defects created during the illumination with higher energies can be photoexcited to a metastable state and converted into (D °, D °) pairs [15]. Following the Street-Mott model [5], the carriers trapped in the D ° centers are thermally excited in the bands, giving rise to photoconductivity. Simultaneously, a charged pair is created in one of the following configurations: (D °, D+), (D °, D-). These reactions can be summarized in the following manner: 2C
,
(D+ D _)
h,,,
(DO, Do )
exciton th
7
(D°' D +) + electron,
>
(D °, D - ) + hole, where C is the chalcogen atom in its normal configuration, "hv' is the photo-activation process and 'th' is the thermal process. Thus, the photoinduced defects would be in one of the following configurations: (D +, D-), (D °, D°), (D °, D+), (D °, D-). Two of them are neutral and the two others are charged. The charged pairs, because of the Coulombic interaction, would be trapping centers for electrons and holes, respectively. The recombination, in the three last configurations, occurs by restoring the VAP which self annihilates and returns to the ground state. Biegelsen and Street [16] have also suggested that a direct generation of a free hole and a correlated (D °, D - ) pair (or an electron and a (D °, D +) pair) may occur through an optical excitation of a (D +, D - ) pair. Thus, it is clear that the photoinduced defects, which are created during the illumination with higher energies, contribute progressively to increase the photoconductivity by means of the intermediate (D °, D °) state or by direct optical excitation of free carriers (3.6 eV < hv < 4 eV). At the same time, the density of trapping
177
centers increases as well as their contribution to the recombination of free carriers. In fact, the variation of the trapping center density is inversely proportional to the density variation of defect centers which participate in photogeneration. As the incident photon energy decreases, the penetration depth increases, and the density of trapping centers increases until the contribution of photoinduced defects to the recombination becomes greater than their contribution to the generation; then the photoconductivity decreases (2.6 eV < hv < 3.6 eV). So, the extrinsic defects created during illumination with higher energies recombine progressively after an important contribution to photoconductivity. Their contribution vanishes effectively when the photoconductivity curve rejoins the quantum efficiency curve (hv = 2.6 eV), then both curves show the same variations (2.2 eV < hv < 2.6 eV). (c) In part 111, we observe that the shape of the curve follows the quantum efficiency curve with the same 'shoulder' at hv = 2.1 eV. In this region, the incident photon energy is smaller than the optical gap and no direct photogeneration of free carriers can be expected. However, the 'shoulder' of the curve, which starts effectively at 2.1 eV, is generally attributed to the formation of excitons near the VAPs which exist in amorphous chalcogenides [12]. This assumption is confirmed by the excitation spectrum of photoluminescence [17] which exhibits a maximum at 2.1 eV. 4.2. The GeSe photoconductivity The addition of germanium, at low concentrations, does not alter basically the processes which control the Se photoconductivity. Moreover, the influence of germanium appears particularly in part 1I of the spectra (fig. 2), where the magnitude of the maximum of photocurrent decreases as the Ge content increases. This experimental result is not surprising if we take into account the structural transformations due to germanium in the G e - S e glasses, as well as their influence on the photoinduced defects density. As a matter of fact, it is now well established that Ge atoms (because of their tetrahedral coor-
178
Z. El Gharras et al. / Photoinduced defects in a-Ge xSe I _ ~. photoconductivity
dination) are branched points for Se chains leading, in the studied concentration range, to a cross-linked quasi planar structure. When the Ge content increases, the number of cross points increases as well as the number of constraints. The polymeric chains of Sc are shortened and the flexibility of network reduced. Moreover, in the Street model [13], the creation of an extrinsic defect by self-trapping of an exciton must be followed by an important local distortion of the site. It is then reasonable to think that the germanium, by increasing the network rigidity, w!!l easily prevent this distortion. As the concentration of Ge increases, the creation of the photoinduced defects becomes more and more difficult and then their density must be reduced. In the case of Se, we have shown that the magnitude of the photocurrent peak is strongly dependent on photoinduced defect density. In the GeSe alloys, the above arguments indicate that this density decreases as Ge content increases, involving a concomitant decreasing in the magnitude of photocurrent in the region II, as observed (fig. 2). The decrease of photocurrent with increasing temperatures (figs. 3 and 4) may be also related to a reduction of the photoinduced defect density which controls the magnitude of the peak in this energy range. Indeed, the number of self-trapped excitons which can cross over the potential energy barrier in the configurational coordinate diagram (see fig 5) and return to the ground state increases as the temperature increases. Therefore, all the photoinduced defects which thermally self annihilate do not contribute to the free carrier generation and thus the photocurrent decreases. It seems then that the germanium introduced, at low rate, in the Se matrix does not interfere directly in the processes which control the photoconductivity. Nevertheless, a strong correlation exists between the network rigidity, which increases with Ge content, and the decrease of the extrinsic defect density. This correlation is in agreement with the photoinduced absorption measurements reported by Moseley and Chamberlain [11]. These authors have shown that, for a given value of the absorp-
tion coefficient, the photoinduced absorption in GeySer_ x (x _<0.33) alloys decreases as the Ge content increases. The photoinduced absorption t,,,, photogeneration centers being centers and "~'" the same, such a result tends to confirm that the addition of germanium atoms leads effectively to a decreasing of their density. The variation of photoconductivity with sample composition may be, also, understood in terms of the argument of Mollot et al. [18]. Following these workers, owing to the presence of Ge, 'hard' and 'soft' regions exist in the glasses. The 'hard' region are small volumes around Ge atoms, exhibiting a medium range order, and may extend up to - 1 . 5 nm. Their concentration increases with Ge content and they would have the largest value for Ge0.33Se0.67. Mollot et al. [18] suggest that the photoluminescence 'originates' in the centers created in the 'hard' regions, whereas the photoinduced absorption and photoinduced ESR occurs predominantly at centers in 'soft' regions. Therefore, an increase of the 'hard' region density will involve a proportional increasing of photoluminescence efficiency (as oL~erved by Ball et al. [19]) because the photoluminescence is associated only with defects in these regions. Inversely, a concomitant decrease in the concentration of 'soft' regions will result in a reduction of photoinduced absorption center density (as observed by Moseley et al. Ill]). Extending the above arguments to the photoconductivity, we may notice that the concept of 'soft' and 'hard' regions is in good agreement with the present experimental results. In the photon energy range below 2 eV (part Ill), we observe that the 'shoulder' which appears in the case of pure Se does not exist for alloys (fig. 2). In the case of Se, this 'shoulder' has been attributed to a contribution of intrinsic defects (VAPs) to photogeneration process. The difference between Se and GeSe alloys in this part of spectra could be attributed to a decrease of the VAP density due to the presence of Ge atoms. However, it is admitted that the germanium, at these concentrations (4 and 8 at.% GeL does not alter the VAP density in the GeSe alloys. Moreover, when the photon energy lies between 1.8 and 2 eV (the range of energy including
Z El Gharras et M. / Pho~oin&~¢~'d de/¢:~s m a - G e ~S e l - • photoconductit,ity
the 'shoulder" in the Se spectrum), we observe that the magnitude of photocurrent, in the G e S e spectra, is about one decade higher than the magnitude of photocurrent in the Se spectrum. ~.t seems that the generation process including the intrinsic defects, in the Se case. is hidden by a more efficient process in the GeSe case. Let us remember that the limit between the energy range corresponding to the r e ~ o n s il and III in the Se spectrum takes place near the optical gap of this material (-~ 2.1 eV), while the optical gap of GeSe alloys, with 4 and 8 at.% Ge. is at a lower value ( ~ 1.9 eV) [20]. The illumination with an energy higher or lower than 2.1 eV involves automatically a change in the generation mode in selenium while in the GeSe films the generation mode may remain unchanged until the optical gap of these alloys is reached. Therefore. taking into account that the decrease of photocurrent is monotonic in the GeSe spectra, it is reasonable to suppose that the beginning of region 11I (1.8 eV
5. C o n c l u s i o n
The exl3,:ziaicntai ,,.~u,,o'" a~ a ,,,r,,,e.~.,,,~,, ,.._.....~;",~;""'-" that the spectral responsivity of pure Se films, as well as that of the G e S e alloys films, is stron~y dependent on the photoinduced defect d e n s i b . These extrinsic defects contribute effectively to increase the sample photoconductivity for a photon energy range lying between 2.6 and 4 eV. Based on our data and deductions therefrom. the concept of "hard' and "soft" regions is a valid description of the behaviour of GeSe allo~. The generation and recombination processes which control the magnitude of photocurrent in region II occur predominantly in the defect centers cre-
179
ated in the "soft' regions. This result is also in good agreement with the configurational coordinate diagram for VAPs [21], which describes the radiative and non-radiative recombination.
References 11] R.H. Bube, Photoconductivity in Solids (Wiley, New York, 1960). 12] A. Rose, Photoconductivity and Related Processes (lntermience, New York, 19.63). 13] S.M. Ryvkin, Photoelectric Effects in Semiconductors (Consultant Bureau, New York, 1964). [4] J. Mort and D.M PaL Photoconductivily and Related Phenomena (El~vier. New York, 1976). 15] R.A. Street and N.F. Moll, Phys. Rev. Lett. 35 (1975) 1293. ]6] N.F. Molt. E.A. Davis and R.A. Street, Philos. Mag. 32 ( 1075} 9.61. IV] M. Kasmer. D. Adler and H. Fritzsche, Phys. Rev. Left 37 (1976) t504. IS] D. Cartes. C. Vaulier and C. Viger, Thin Solid Films 17 (1973) 67. [9] D. Caries. G. Lefran~ois and J.P. Larmagnac, J. Phys. l_ell. (Paris) 45 (1984) L901. ll0] J. Grenet. D. Caries, G. Lefran~ois and J.P. Larmagnac, J. Non-CD'sl. Solids 56 (1983) 285. ]1|] A_t. Moseley and J.M. Chamberlain, Philos. Mag. B43 (19~I l !0.65. [12] J.C. Knights and E.A. Davis, J. Phys. Chem. Solids 35 ( 1974 ~543. [13] R A . Slreet. Solid Stale Commun. 24 (1977) 3.63. [14] R.A. Street. in: Proc. 7th Int. Conf. on Amorphous and Liquid Semiconductors, ed. W.E. Spear (Edinburgh Univer~ily. 1977) p. 509. t15] R.A. Slreel. Phys. Rev. BI7 (1978) 3984. 410] D.K. Biegelsen and R.A. Street, Phys. Rev. Len. 44 ( t9801 803. I17] R.A. Slreel, T.M. Searle and J.G. Austin, Philos. Mag. 29 (19741 1157. IIS] F. Mollol. J. Cernogora and C. Benoil '~ la Guillaumc, J. Non-Cryst. Solids 35&3b (1980) 939. 119] G_I. Ball, J.M. Chamberlain and T. lnstone, Solid State Commun. 27 (1978) 71. ]20] R. Azoulay, H. Thibierge and A. Brenac, J. Non-Cryst. Solids 11,;(1975) 33. ]21] M. Kaslner, J. Phys. ('13 (1980) 3319.
Journal of Non-Crystalline Solids 155 (1993) 171-179 North-Holland
NO
SO
Role of photoinduced defects in amorphous GexSel_x photoconductivity Z. El G h a r r a s , A. B o u r a h l a ~ and C. V a u t i e r Laboratoire d'Etude et de CaractErisation des Compos~s Amorphes et des Polymdres, UFR Sciences, BP 118, 76134 Mont-Saint-Aignan c~dex, France
Received 18 February 1992 Revised manuscript received 13 October 1992
The spectral dependence of photoconductivityof amorphous pure Se and Ge-Se alloys(4 and 8 at.% Ge) evaporated thin films was measured for a spectral range between 2511and 8(10nm. A comparison of the Se and Ge-Se spectra shows that the magnitude of photocurrent decreases as the Ge content increases. This result indicates a correlation between the magnitude of the photocurrent and the density of photoinduced defects. The contribution of these defects to the enhancement of photoconductivity in a specific region of the spectrum as well as the influence of temperature on their density are also discussed.
1. Introduction Among the various effects arising in chalcogenide glasses under irradiation, the spectral dependence of photoconductivity remains the least well investigated. Most of the numerous studies reported in the literature on photoconductivity of these materials concern the temperature or light intensity dependence (for a brief review, see refs.
[1-4]). The first model of photoconductivity for amorphous chalcogenides was proposed by Street and Mott [5] and is based upon models of charged defects [6,7]. These models are now well established and describe correctly the experimental data on these materials. More recently, Caries and co-workers [8,9] have observed two major discrepancies between their experimental results, obtained on amorphous selenium films, and the
i Permanent address: ENS, BP 227, Mostaganem, Algeria. Correspondence to: Dr Z. El Gharras, Laboratoire d'Etude et
de Caract6risation des Compos6s Amorphes et des Polym~res, UFR Sciences, BP 118, 76134 Mont-Saint-Aignan c~dex, France. Tel: + 33 35 14 68 81. Telefax: + 33 35 14 68 82.
S t r e e t - M o t t model. According to this model, the photoconductivity must be proportional to the product of the quantum efficiency, r/, and the light intensity, F, when the photocurrent, lph, is less than the dark current, !,,, whereas for larger values of F, which yield a photocurrent larger than ! d, the photoconductivity must be proportional to ( ' o F ) ~/2. The Caries et al. experimental data on a-Se films show a sublinear variation of Iph (/ph "~ FX, where x is equal to 0.75) but also a spectral dependence of the photoconductivity which does not follow the variation of quantum efficiency. Concerning this last result, a model for steady-state photoconductivity which explains the spectral dependence of photocurrent in a-Se films has been proposed [9]. These authors have shown that there are different processes, involving specific defects, which are a function of incident photon energy. It is now well established that these photoinduced defects play an important role in the electrical properties of chalcogenide semiconductors. In the photoconductivity process, they contribute, for an optical excitation of suitable wavelength, to the generation and the recombination of free
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
! 72
Z. El Gharras et al. / Photoinduced dt~'cts m a-(h,, Se t _ ~ phot~'onductit'iD,
phous selenium. Moseley and Chamberlain [11] have shown that the photoinduced absorption (PA) decreases in the S e - G e glasses when the concentration of Ge increases. This last result shows that a correlation exists between the density of defects, which are the photoinduced absorption centers, and the flexibility of network. In this paper, we present new experimental results of steady state photocon~.activity obtained on amorphous Ge,Set_, films. We also present
carriers by means of two localized bands in the gap, located on each side of the Fermi level. A close correlation between the density of photoinduced defects, their distribution in the film and some of the structural, electrical or optical properties of chalcogenide glasses have been reported in the literature. Among these, Grenet et al. [10] have shown, by photorelaxation measurements, that the diffusion of photoinduced defects increases the structural relaxation rate of amor-
(Iph/F.~)
~o =.
/
1o'-
l O 0.
10'.. I
1()~ III
II
I
16~
161
I
3
'~
s
hr(ev
Fig. 1. Spectral dependence of Se photoresponse ( . . . . . ) and quanlunl efficiency ( ) (from ref. [12]). Tile saturated high energy values of both curves have normalized to unity.
Z. El Ghan-as et al. / Photoinduced defects in a-Ge,Se,
an analysis of the experimental data which corroborates the Carles photoconductivity model and the Moseley PA measurements. In order to estimate the influence of Ge, we study, in the first part of this paper, the processes involved in the spectral dependence of a-Se photoconductivity. For the concentrations of interest to us, the Ge content influences the processes which control the magnitude of photocurrent. We also discuss the role played by the photoinduced defects in photoconductivity as well as the influence of the temperature and the concentration of Ge on their density.
_ + photoconductility
2. Experimental The GeSe bulk alloys were prepared in a sealed quartz tube where the mixture was heated. for 48 h, at 1000°C and then quenched. In a vacuum af 10B7 mbar, the GeSe and Se thin fiihns were obtained from bulk by thermal evaporation (evaporation rate m 1 rim/s)) on glass substrates at room temperature. Gold electrodes were deposited for bias contacts. The films have a planar structure and the incident light tlux was at right angle to the sample plane. The film thickness was m 1.5 pm and the interelectrode distance 250
Is
I
lo1
,
I
1
.Se rGe
a04
S %96
*~%0.3%9*
3
4
Fig. 2. Spectral dependence of photoconductivity
t-3
in Se. Ge,,,,MSe,,,uh and Ge,,,,,$e,,,,
films at room temperature.
174
Z, El Gharras et aL / Photoinduced defects in a-Ge~St'z _ ~ photoconducticity
p.m. With an applied voltage of 100 V, the electric field (4 × 105 V m -I) is small enough to prevent space charge effects and the dark current is ohmic whatever the temperature. Since the incidence flux depends upon the spectral response of both light source and grating system, the results are presented in the following normalized form:
( lph/FA)/( Ioh/FA)~ = f(h~,), where Iph = li-ld" and lph is the
photocurrent, I i
is the current measured under illumination, I d is the dark current, F is the light intensity for a wavelength, A, and he is the incident photon e n e r g y . (lph/FA) s is the normalized value of photocurrent corresponding to the saturation of the film photoresponse and measured for the photon energies > 4 eV. Finally, it is important for further discussions to point oat that the photocurrent values have been measured step by step with photon energy decreasing from 5 to 1.5 eV.
10 z
4%
1
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,
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• 300k
=310 k
.1~~
". 3 2 0 k -+" 3 3 0 k
I , i
III
i I
J r
I I
I I
II
I
. . . .
_l
¢
J
2
/
|,
6
3
h i " (ev) Fig. 3. Spectral dependence of Gca0~Se ~
photoresponse for different values of the temperature: 300, 310. 320 and 330 K.
Z. El G h a r r a s et al. / Photoinduced defects in a - G e x S e t _ ~. photoconductil,ity
3. Results
The variation of photocurrent in a-Se thin films versus incident photon energy is reported in fig. 1. In the same figure, the variation of quantum efficiency, r/, as observed by Knights and Davis [12], is also shown. At low energies, the photocurrent increases exponentially as does the quantum efficiency. It exhibits a maximum for 3.6 eV and tends towards a constant value for energies > 4 eV. In order to determine the influence of germanium atoms added in the Se matrix, we used the same experimental protocol for the study of the
175
spectral dependence of photoconductivity of Geo.o4Seo.96 and Geo.0sSe0.92 alloys (fig. 2). We report in the same figure the photoconductivity spectra of both GeSe alloys and pure Se. We note that the variation of photocurrent with photon energy has the same shape for GeSe alloys as for Se. However the magnitude of the peaks decreases as the Ge content increases and the 'shoulder' which exists at hv = 2.1 eV in the selenium spectrum vanishes in the GeSe alloys curves.
The photoconductivity spectra of Ge0.04Se0.96 and Ge0.osSeo.92 alloys, were recorded 'in situ' in the temperature range from 300 to 330 K (figs. 3 _10 z
8%
.10 t
_10 0
N= • 300 K - 310K 320 K "~ 3 3 0 K
462 /'1 III .
.
I I .
.
li
._]_
t_.
_163 -I
i I
I
I
I
2
3
4
5
1~ ~" 6
hr" (ev) Fig. 4. Spectral dependence of Geo0sSe~}~2 photoresponse for different values of the temperature: 300, 310, 320 and 330 K.
176
Z. El Gitarras et al. / Pitotoinduced defects h~ a-GexSe t _ .~ photoconducti~'ity
and 4, respectively). In the investigated temperature range, the temperature increase alters principally the magnitude of photocurrent in part II of the curves. The maximum of the photocurrent decreases as the temperature increases.
~
I
citon
f
4. Discussion
self- trapl~ed/ exciton ]
'°',°"/
4.1. Spectral photoresponse of Se According to Caries et al. [9] we may consider that three energy ranges, with a preponderant photogeneration process in each one, exist in Se spectrum (fig. 1). (a) In part I, the photon energies lie above 4 eV and the photocurrent as well as the quantum efficiency have a constant value. In this region, the photon energy is larger than the optical gap of selenium. Therefore, it can be assumed that each absorbed photon is able to create, after thermalization, a free electron-hole pair as predicted by the Knights-Davis model [12]. At the same time, following Street [13], we may assume that the photogenerated electronhole pairs geminate giving rise to a photoinduced defect. A bond is transferred from one chalcogen atom to another, resulting in two defects: one of them is negatively charged and onefold coordinated and the other one is positively charged and threefold coordinated. These defects are labelled, respectively, D- and D +. The requirement of neutrality leads to an equal density of D + and D - which are associated in pairs (D +, D - ) called valence alternation pairs (VAP). Some of them can be close enough to be localized on neighbouring atoms. In such a case, they are called intimate valence alternation pairs (IVAP). When free carriers are photogenerated, the3 are rapidly trapped by the charged defects which become neutral (D°). In the Street model [13] the photoinduced defect is a (D÷,D - ) pair but the concerned atoms are close together so that the structure is IVAPlike. Because of the strong electron-phonon coupling in chalcogenides, the transition from the ground state to a closed (D+,D -) pair can occur either by self trapping of an exciton (path I (fig.
rr
J
round-state CONFIGURATION
Fig. 5. Configurational coordinate diagram for self-trapped excitoa in chalcogenides (see ref. [13]).
5) [13]), followed by a local relaxation, or by direct excitation (path II, (fig. 5) [14]). However, in this energy range the photon penetration depth is small (about 0.1 ~m) compared with the sample thickness (1.5 p.m). The photoinduced defects are created near the surface, in a small part of the sample and a gradient of IVAP concentration appears between the 'dark part' of the sample and the illuminated one. Grenet et al. [10] have shown that this gradient of IVAPs concentration involves the diffusion of photoinduced defects towards the entire sample. According to them, we may consider that the exposure time (1 h), used at each wavelength in our experimental procedure, is sufficient for the concentration of IVAP to become homogeneous in the entire sample. (b) The part II of photoresponse curve exhibits two distinct regions. In the first region (2.2 eV < hu < 2.6 eV), the photoconductivity exhibits an exponential variation as does the quantum efficiency. In the second region (2.6 eV < hu < 4 eV), the photoconductivity exhibits a maximum (for hu = 3.6 eV)
Z. E l Gharras et al. / Photoinduced detects in a-GexSe t _ x phowconductivity
while the quantum efficiency saturates. The difference between photoconductivity and quantum efficiency data suggest that the direct photogeneration of free carriers is not the main process giving rise to a photoconductive effect. Indeed, the photoinduced defects created during the illumination with higher energies can be photoexcited to a metastable state and converted into (D °, D °) pairs [15]. Following the Street-Mott model [5], the carriers trapped in the D ° centers are thermally excited in the bands, giving rise to photoconductivity. Simultaneously, a charged pair is created in one of the following configurations: (D °, D+), (D °, D-). These reactions can be summarized in the following manner: 2C
,
(D+ D _)
h,,,
(DO, Do )
exciton th
7
(D°' D +) + electron,
>
(D °, D - ) + hole, where C is the chalcogen atom in its normal configuration, "hv' is the photo-activation process and 'th' is the thermal process. Thus, the photoinduced defects would be in one of the following configurations: (D +, D-), (D °, D°), (D °, D+), (D °, D-). Two of them are neutral and the two others are charged. The charged pairs, because of the Coulombic interaction, would be trapping centers for electrons and holes, respectively. The recombination, in the three last configurations, occurs by restoring the VAP which self annihilates and returns to the ground state. Biegelsen and Street [16] have also suggested that a direct generation of a free hole and a correlated (D °, D - ) pair (or an electron and a (D °, D +) pair) may occur through an optical excitation of a (D +, D - ) pair. Thus, it is clear that the photoinduced defects, which are created during the illumination with higher energies, contribute progressively to increase the photoconductivity by means of the intermediate (D °, D °) state or by direct optical excitation of free carriers (3.6 eV < hv < 4 eV). At the same time, the density of trapping
177
centers increases as well as their contribution to the recombination of free carriers. In fact, the variation of the trapping center density is inversely proportional to the density variation of defect centers which participate in photogeneration. As the incident photon energy decreases, the penetration depth increases, and the density of trapping centers increases until the contribution of photoinduced defects to the recombination becomes greater than their contribution to the generation; then the photoconductivity decreases (2.6 eV < hv < 3.6 eV). So, the extrinsic defects created during illumination with higher energies recombine progressively after an important contribution to photoconductivity. Their contribution vanishes effectively when the photoconductivity curve rejoins the quantum efficiency curve (hv = 2.6 eV), then both curves show the same variations (2.2 eV < hv < 2.6 eV). (c) In part 111, we observe that the shape of the curve follows the quantum efficiency curve with the same 'shoulder' at hv = 2.1 eV. In this region, the incident photon energy is smaller than the optical gap and no direct photogeneration of free carriers can be expected. However, the 'shoulder' of the curve, which starts effectively at 2.1 eV, is generally attributed to the formation of excitons near the VAPs which exist in amorphous chalcogenides [12]. This assumption is confirmed by the excitation spectrum of photoluminescence [17] which exhibits a maximum at 2.1 eV. 4.2. The GeSe photoconductivity The addition of germanium, at low concentrations, does not alter basically the processes which control the Se photoconductivity. Moreover, the influence of germanium appears particularly in part 1I of the spectra (fig. 2), where the magnitude of the maximum of photocurrent decreases as the Ge content increases. This experimental result is not surprising if we take into account the structural transformations due to germanium in the G e - S e glasses, as well as their influence on the photoinduced defects density. As a matter of fact, it is now well established that Ge atoms (because of their tetrahedral coor-
178
Z. El Gharras et al. / Photoinduced defects in a-Ge xSe I _ ~. photoconductivity
dination) are branched points for Se chains leading, in the studied concentration range, to a cross-linked quasi planar structure. When the Ge content increases, the number of cross points increases as well as the number of constraints. The polymeric chains of Sc are shortened and the flexibility of network reduced. Moreover, in the Street model [13], the creation of an extrinsic defect by self-trapping of an exciton must be followed by an important local distortion of the site. It is then reasonable to think that the germanium, by increasing the network rigidity, w!!l easily prevent this distortion. As the concentration of Ge increases, the creation of the photoinduced defects becomes more and more difficult and then their density must be reduced. In the case of Se, we have shown that the magnitude of the photocurrent peak is strongly dependent on photoinduced defect density. In the GeSe alloys, the above arguments indicate that this density decreases as Ge content increases, involving a concomitant decreasing in the magnitude of photocurrent in the region II, as observed (fig. 2). The decrease of photocurrent with increasing temperatures (figs. 3 and 4) may be also related to a reduction of the photoinduced defect density which controls the magnitude of the peak in this energy range. Indeed, the number of self-trapped excitons which can cross over the potential energy barrier in the configurational coordinate diagram (see fig 5) and return to the ground state increases as the temperature increases. Therefore, all the photoinduced defects which thermally self annihilate do not contribute to the free carrier generation and thus the photocurrent decreases. It seems then that the germanium introduced, at low rate, in the Se matrix does not interfere directly in the processes which control the photoconductivity. Nevertheless, a strong correlation exists between the network rigidity, which increases with Ge content, and the decrease of the extrinsic defect density. This correlation is in agreement with the photoinduced absorption measurements reported by Moseley and Chamberlain [11]. These authors have shown that, for a given value of the absorp-
tion coefficient, the photoinduced absorption in GeySer_ x (x _<0.33) alloys decreases as the Ge content increases. The photoinduced absorption t,,,, photogeneration centers being centers and "~'" the same, such a result tends to confirm that the addition of germanium atoms leads effectively to a decreasing of their density. The variation of photoconductivity with sample composition may be, also, understood in terms of the argument of Mollot et al. [18]. Following these workers, owing to the presence of Ge, 'hard' and 'soft' regions exist in the glasses. The 'hard' region are small volumes around Ge atoms, exhibiting a medium range order, and may extend up to - 1 . 5 nm. Their concentration increases with Ge content and they would have the largest value for Ge0.33Se0.67. Mollot et al. [18] suggest that the photoluminescence 'originates' in the centers created in the 'hard' regions, whereas the photoinduced absorption and photoinduced ESR occurs predominantly at centers in 'soft' regions. Therefore, an increase of the 'hard' region density will involve a proportional increasing of photoluminescence efficiency (as oL~erved by Ball et al. [19]) because the photoluminescence is associated only with defects in these regions. Inversely, a concomitant decrease in the concentration of 'soft' regions will result in a reduction of photoinduced absorption center density (as observed by Moseley et al. Ill]). Extending the above arguments to the photoconductivity, we may notice that the concept of 'soft' and 'hard' regions is in good agreement with the present experimental results. In the photon energy range below 2 eV (part Ill), we observe that the 'shoulder' which appears in the case of pure Se does not exist for alloys (fig. 2). In the case of Se, this 'shoulder' has been attributed to a contribution of intrinsic defects (VAPs) to photogeneration process. The difference between Se and GeSe alloys in this part of spectra could be attributed to a decrease of the VAP density due to the presence of Ge atoms. However, it is admitted that the germanium, at these concentrations (4 and 8 at.% GeL does not alter the VAP density in the GeSe alloys. Moreover, when the photon energy lies between 1.8 and 2 eV (the range of energy including
Z El Gharras et M. / Pho~oin&~¢~'d de/¢:~s m a - G e ~S e l - • photoconductit,ity
the 'shoulder" in the Se spectrum), we observe that the magnitude of photocurrent, in the G e S e spectra, is about one decade higher than the magnitude of photocurrent in the Se spectrum. ~.t seems that the generation process including the intrinsic defects, in the Se case. is hidden by a more efficient process in the GeSe case. Let us remember that the limit between the energy range corresponding to the r e ~ o n s il and III in the Se spectrum takes place near the optical gap of this material (-~ 2.1 eV), while the optical gap of GeSe alloys, with 4 and 8 at.% Ge. is at a lower value ( ~ 1.9 eV) [20]. The illumination with an energy higher or lower than 2.1 eV involves automatically a change in the generation mode in selenium while in the GeSe films the generation mode may remain unchanged until the optical gap of these alloys is reached. Therefore. taking into account that the decrease of photocurrent is monotonic in the GeSe spectra, it is reasonable to suppose that the beginning of region 11I (1.8 eV
5. C o n c l u s i o n
The exl3,:ziaicntai ,,.~u,,o'" a~ a ,,,r,,,e.~.,,,~,, ,.._.....~;",~;""'-" that the spectral responsivity of pure Se films, as well as that of the G e S e alloys films, is stron~y dependent on the photoinduced defect d e n s i b . These extrinsic defects contribute effectively to increase the sample photoconductivity for a photon energy range lying between 2.6 and 4 eV. Based on our data and deductions therefrom. the concept of "hard' and "soft" regions is a valid description of the behaviour of GeSe allo~. The generation and recombination processes which control the magnitude of photocurrent in region II occur predominantly in the defect centers cre-
179
ated in the "soft' regions. This result is also in good agreement with the configurational coordinate diagram for VAPs [21], which describes the radiative and non-radiative recombination.
References 11] R.H. Bube, Photoconductivity in Solids (Wiley, New York, 1960). 12] A. Rose, Photoconductivity and Related Processes (lntermience, New York, 19.63). 13] S.M. Ryvkin, Photoelectric Effects in Semiconductors (Consultant Bureau, New York, 1964). [4] J. Mort and D.M PaL Photoconductivily and Related Phenomena (El~vier. New York, 1976). 15] R.A. Street and N.F. Moll, Phys. Rev. Lett. 35 (1975) 1293. ]6] N.F. Molt. E.A. Davis and R.A. Street, Philos. Mag. 32 ( 1075} 9.61. IV] M. Kasmer. D. Adler and H. Fritzsche, Phys. Rev. Left 37 (1976) t504. IS] D. Cartes. C. Vaulier and C. Viger, Thin Solid Films 17 (1973) 67. [9] D. Caries. G. Lefran~ois and J.P. Larmagnac, J. Phys. l_ell. (Paris) 45 (1984) L901. ll0] J. Grenet. D. Caries, G. Lefran~ois and J.P. Larmagnac, J. Non-CD'sl. Solids 56 (1983) 285. ]1|] A_t. Moseley and J.M. Chamberlain, Philos. Mag. B43 (19~I l !0.65. [12] J.C. Knights and E.A. Davis, J. Phys. Chem. Solids 35 ( 1974 ~543. [13] R A . Slreet. Solid Stale Commun. 24 (1977) 3.63. [14] R.A. Street. in: Proc. 7th Int. Conf. on Amorphous and Liquid Semiconductors, ed. W.E. Spear (Edinburgh Univer~ily. 1977) p. 509. t15] R.A. Slreel. Phys. Rev. BI7 (1978) 3984. 410] D.K. Biegelsen and R.A. Street, Phys. Rev. Len. 44 ( t9801 803. I17] R.A. Slreel, T.M. Searle and J.G. Austin, Philos. Mag. 29 (19741 1157. IIS] F. Mollol. J. Cernogora and C. Benoil '~ la Guillaumc, J. Non-Cryst. Solids 35&3b (1980) 939. 119] G_I. Ball, J.M. Chamberlain and T. lnstone, Solid State Commun. 27 (1978) 71. ]20] R. Azoulay, H. Thibierge and A. Brenac, J. Non-Cryst. Solids 11,;(1975) 33. ]21] M. Kaslner, J. Phys. ('13 (1980) 3319.