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Cu stacked elemental layers
Vacuum 62 (2001) 61}73
CuIn(S Se ) "lms prepared by graphite box annealing V \V of In/Cu stacked elemental layers S. Bandyopadhyaya, S. Roy, S. Chaudhuri, A.K. Pal* Department of Materials Science, Indian Association for the Cultivation of Science, Calcutta-700 032, India
Abstract CuIn(S Se ) "lms were synthesized by sulphurization of In/Cu stacked elemental layers deposited onto glass and V \V Mo-coated glass substrates followed by selenization by graphite box annealing. The "lms, thus synthesized, were characterized by measuring electrical, optical and microstructural properties. The microstructure and hence the physical properties of the "lms depended critically on the amount of sulphur incorporation. Nature of charge carriers depended on Cu/In, (S#Se)/(Cu#In) and S/(S#Se) ratios while their concentrations varied between 10 and 10 cm\. Grain boundary scattering e!ects were critically studied by measuring the electrical conductivity () and Hall mobility () simultaneously on the same sample. Optical transmittance studies indicated the band gap to vary within 0.98}1.40 eV with x values. The photoluminescence spectra, recorded at 80 K were dominated by the excitonic peak located within 1.40}1.16 eV followed by a small peak within &0.96}0.98 eV arising due to transition from conduction band to neutral acceptor (< ) or exciton bound to ionized acceptor (Cu ) states. 2001 Elsevier Science Ltd. All rights reserved. ! ' Keywords: Ternary semiconductors; Electrical conductivity; Photo-luminescence
1. Introduction Thin "lm solar cells based on polycrystalline chalcopyrite semiconductors CuInSe and Cu(In Ga )Se as the absorber layers attracted V \V much attention during the last decade due to high absorption coe$cient and near zero degradation of the cell performance made out of these materials. Solar cells based on Cu(In Ga )Se have alV \V ready attained a conversion e$ciency &18.8% [1,2] while for solar cells based on CuInS in dicated an e$ciency &12% [3]. Intrinsic defects in the absorber material often controlled the per-
* Corresponding author.
formance of the photovoltaic devices. Band o!set in between the absorber layer and the window layer (CdS) was one of the major limiting factors towards increasing the e$ciency value. It was observed that the band o!set might be improved if sulphur is incorporated in the selenide lattice [4}7]. In fact, the semiconductor CuInS has an edge over CuInSe and Cu(In Ga )Se since it does not V \V contain any toxic constituent like selenium. Formation of alloys of CuInSe and CuInS are also advantageous for solar cell application due to the fact that in CuIn(S Se ) the band gap along V \V with other material properties can easily be tuned to the most desirable value by adjusting the sulphur content in the "lm. In#uence of sulphur on the electrical and optical properties of CuIn(S Se ) V \V
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 1 5 6 - 7
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single crystals was studied by Eisener et al. which indicated that the activation energies of the acceptor levels shift to higher values when selenium is replaced by sulphur [8]. The density of states in CuIn(S Se ) thin "lms were estimated by HerV \V berholz et al. from modulated photocurrent measurements which indicated a continuous distribution of traps inside the band gap [9]. Cheng et al. reported their work on the conversion of CuInSe "lm to CuIn(S Se ) by immersing CuInSe "lm V \V in (NH ) S [10]. Four source evaporation tech V nique was utilized by Walter et al. for synthesizing CuIn(S Se ) thin "lm for the fabrication of soV \V lar cells [11]. Incorporation of sulphur and selenium was found to depend on copper content in the "lms. Preparation of CuIn(S Se ) "lms by V \V sulphurization of Cu/In stack by H S followed by selenization was reported by Ohashi et al. [12]. Resistivity of the "lms, thus produced, was seen to increase by one order of magnitude by KCN treatment. The substitution of selenium by sulphur in CuInSe is still a challenging task. E!orts are underway to "nd a viable technological route to produce CuIn(S Se ) "lms as an e!ective abV \V sorber layer for solar cells in a simple and cost e!ective way. Graphite box annealing of stacked elemental layer precursor seems to be a viable alternative to satisfy the above need. Synthesis of CuInS "lms by sulphurization of Cu/In stacked elemental layers was reported earlier [13]. In this paper, synthesis of CuIn(S Se ) "lms by sulV \V phurization of In/Cu stacked elemental layers (SEL) followed by selenization is presented. The "lms thus synthesized are characterized by measuring electrical, optical, microstructural and photoluminescence (PL) properties.
2. Experimental Cu and In (all with purity of &99.995%) were evaporated from alumina crucibles heated indirectly by tungsten heaters in a conventional 12 in. diameter vacuum coating unit at a system pressure of &10\ Pa. The #uxes of the individual sources (Cu&0.1 nm/s and In&0.1 nm/s) could be monitored by two quartz crystal thickness moni-
tors for obtaining the stacked layers with the requisite thicknesses. The substrate could be rotated for uniformity of the thickness of the deposit. For SEL precursor, the layers were deposited in sequence of In/Cu with predetermined thicknesses of the individual layers in order to attain the required composition of the synthesized "lm. The precursors were deposited onto sodalime glass substrates (with and without Mo coating) at room temperature (300 K). These precursors were placed in an appropriate rectangular graphite box for annealing inside a cylindrical (6.5 cm diameter) quartz chamber in partial argon (purity&99.99%) atm. Requisite amount of sulphur (99.99%) was placed inside the quartz chamber for sulphurization. The quartz chamber was heated in an electronically controlled furnace. Selenization was carried out in another quartz chamber similar to the one used for sulphurization. Electrical and galvanomagnetic properties were measured simultaneously on the same specimen by Van der Pauw technique at temperatures ranging from 190 to 400 K [14]. Microstructural studies were carried out by a SEM-EDX (Hitachi S-2300 with Kevex Delta Class-I) while the optical transmittance was measured by a UV-VIS-NIR spectrophotometer (Hitachi-U3410). PL measurements were carried out at 80 K and by using 300 W Xe arc lamp as the emission source, Hamamatsu photomultiplier as the detector along with a 1/4 meter monochromator. The spectra were recorded with excitation at 500, 600 and 700 nm radiations
3. Results and discussion Four possible schemes (Fig. 1) for synthesizing CuIn(S Se ) "lms from stacked elemental V \V layers were tried here. In Scheme a, selenium was incorporated in the stack which was placed in the quartz chamber for annealing in the presence of sulphur. There was no signi"cant change in the band gap beyond 1.07 eV and the XRD indicated separate peaks for CuInS (lower intensity) and CuInSe (higher intensity) along with peaks for a host of binaries (Fig. 2a). The shift in the peak position for (3 1 2/1 1 6) planes indicated the presence of CuIn(S Se ) phase also. The CuInSe V \V
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
Fig. 1. Four di!erent schemes for stacked elemental layer precursor for the preparation of CuIn(S Se ) "lms. Thickness of V \V each layer &0.1 m.
was the most predominant phase and incomplete sulphurizaion was observed from SEM micrograph (inset of Fig. 2a). EDAX measurement (area scan) of this representative "lm showed x"0.55 [Cu"28.07 : In"30.57 : S"22.79 : Se"18.57]. From these results it is apparent that once CuInSe is formed, the substitution of Se by S is not favoured and as such the upper surface only gets sulphurized and the rest of the "lm shows the existence of CuInSe phase only. Increase of sul phur content in the reaction chamber made the "lm brittle, which easily peeled o! the substrate. The stack was then modi"ed by eliminating selenium layers from the bottom or top layers of the SEL as shown in Schemes b and c, respectively, to allow homogeneous sulphurization-cum-selenization of the SEL by annealing in the presence of sulphur. The end product for Scheme-c was always a layered structure as revealed by SEM micrograph (inset of Fig. 2b) and XRD (Fig. 2b). EDX analysis (inset of Fig. 2b) indicated that the layer adjacent to the glass substrate was predominantly CuInSe while the top layer was CuInS . The intermediate layers consisted of several binaries as revealed by the XRD traces (Fig. 2b). Here again, the shift in the peak position for (2 2 0/2 0 4) and (3 1 2/1 1 6) planes indicated the presence of CuIn(S Se ) phase. V \V But, after sulphurization of SEL in Scheme-b, the "lm was found to contain predominant CuInSe phase with binaries of sulphur and selenium. Re-
63
duction in sulphur content in the annealing chamber resulted in the formation of selenide binaries only. The possibility of synthesizing CuIn(S Se ) V \V "lms by simultaneous selenization and sulphurization of the In/Cu layers (Scheme-d) was also examined by putting di!erent proportions of S/Se material in the annealing chamber. It was observed that selenization was a predominant process and the resultant "lms did not show band gaps more than &1.01 eV. XRD spectra did not indicate any signi"cant presence of sub-phases (Fig. 2c) although the peaks could be identi"ed as arising due to the presence of CuIn(S Se ) phase only. V \V However, the peaks were not sharp and intense as compared to the well-synthesized CuIn(S Se ) V \V "lms. The crystallites did not indicate any wellfaceted structure (inset of Fig. 2c). It was found that this process is not conducive for obtaining "lms with predetermined composition. This is due to the higher sulphur vapour pressure, which suppressed the relative selenium vapour pressure in the reaction chamber, and as such primary selenization was hindered. The above observations are in conformity with the fact that the formation energy for CuInSe is lower than that of CuInS and as such the thermo dynamical formation of CuInSe will always be favoured and Se atoms cannot be replaced by S atoms despite their small atomic radii [15}18]. Interestingly, as we go down the group VIb in the periodic table, atomic radii increases from S to Te (R "1.13 As , R "1.22 As and R "1.40 As ) [19]. 1 1 2 Not only the atom of smaller atomic size will be strongly bound, electro-negativity of S is greater than that of Te. So, the S bond which is more ionic than the other members in the chalcogenide series would share the electron cloud more. Thus, the bond strength of CuInS is higher than that of CuInSe which is also evident from the ionization energies of sulphur (< "90 meV) and selenium 1 (< "70 meV) [20]. Keeping the above informa1 tion in mind, the well-synthesized CuIn(S Se ) V \V "lms were successfully obtained here by sulphurization followed by selenization by graphite box (GB) annealing of In/Cu layers (Scheme-d). Sulphurization of the stacked elemental layer precursor as shown in Scheme-d was performed by
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Fig. 2. SEM micrographs (inset) and corresponding XRD patterns of representative "lms synthesized with: (a) Scheme-a; (b) Scheme-b and (c) Scheme-c.
annealing them in a closed graphite box in partial argon atmosphere (&0.5 atm) with sulphur inside the quartz chamber. Quantity of sulphur placed in the annealing chamber was varied to "nd
the optimum value of it. The details of sulphurization has been discussed elsewhere [13]. For selenization of the sulphurized layer, the annealing temperature (¹ ) was kept &770 K while the
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
Fig. 3. Interdependence of the selenium content in and graphite box (GB) and annealing time for obtaining CuIn(S Se ) V \V "lms with di!erent band gaps (E ).
annealing time (t) and the amount of selenium were varied to obtain "lms with di!erent selenium content. Fig. 3 shows the interdependence of the amount of selenium in GB and the corresponding time of annealing for obtaining "lms with di!erent selenium content as indicated by the band gap of the "lms thus synthesized. In this paper, the properties of CuIn(S Se ) "lms synthesized by sulphuV \V rization followed by selenization using GB annealing of In/Cu SEL as the precursor are reported. 3.1. Microstructural studies Figs. 4(a}f ) indicate the SEM micrographs of some representative CuIn(S Se ) "lms synV \V thesized by sulphurization followed by selenization. One can observe that well-crystallized grains for all the "lms could be obtained by this technique and the sizes of the grains decreased from 2.01 to 0.91 m with the increase of sulphur content x"S/(S#Se) in the "lms (Table 1). The XRD of
65
three representative "lms corresponding to the micrographs 4a, d and f are shown in Fig. 5. There are mainly three peaks for re#ections from (1 1 2), (2 2 0/2 0 4) and (3 1 2/1 1 6) planes. The XRD traces in Fig. 5c also indicate an additional peak for the plane (3 1 6/3 3 2). No additional peaks corresponding to possible subphases were noticed which indicated complete synthesization of CuIn(S Se ) V \V "lms by this technique. The compositions of the above "lms were determined by EDX and EPMA analysis which are shown in Table 1. The compositional deviations from the ideal chemical formula would lead to the formation of donor}acceptor pairs and the type of the majority charge carriers. It may be seen from Table 1 that all the "lms are Cu-poor which would promote the presence of copper vacancies (V ) ! acting like acceptor states. Also, "lms synthesized by selenization of CuInS would mean that seleniz ation would proceed through the replacement of sulphur by selenium. Thus, the presence of sulphur interstitials (S ) arising from the parent CuInS "lm and the replaced sulphur from the CuInS lattice by selenium is more likely to describe the defect chemistry of these "lms. As the "lms were selenized in the presence of overpressure of selenium, the presence of selenium interstitials (Se ) could not be ruled out. The presence of copper vacancies (V ), sulphur interstetials (S ) and sel! enium interstitials (Se ) would re#ect predominant p-type conductivity in these CuIn(S Se ) "lms V \V which was con"rmed from electrical and PL measurements. The lattice parameters (`aa and `ca) evaluated from the XRD studies are shown in Fig. 6. The value of `aa varied from 0.557 to 0.579 nm while `ca varied from 1.112 to 1.159 nm with the decrease of sulphur content (x) in the "lms. The corresponding c/a ratio varied within 1.996}2.004. 3.2. Optical properties The transmittance (T ) spectra for four represen tative CuIn(S Se ) "lms synthesized with di!erV \V ent sulphur contents are shown in Fig. 7. The increase in the transmittance in the NIR region with the reduction of the sharpness of fall of the T } plot (Fig. 7) near the band edge for "lms with
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Fig. 4. SEM micrographs of six representative CuIn(S Se ) "lms synthesized at 770 K with Scheme-d x&0.98 and t&250 min; (b) V \V x&0.84 and t&120 min; (c) x&0.72 and t&110 min; (d) x&0.5 and t&80 min; (e) x&0.34 and t&70 min; (f ) x&0.13 and t&20 min.
increasing sulphur content could be observed. The "lms were highly absorbing. The reduction of sharpness of fall can be attributed to the decrease in grain size of the CuIn(S Se ) "lms with increase V \V in the sulphur content in the "lms. The absorption coe$cients () of the CuIn(S Se ) "lms were determined by measurV \V ing transmittance and re#ectance in these "lms
[21]. In general, the absorption coe$cient () may be written as a function of the incident photon energy (h) so that [21]: "(A/h)h!E K, (1) where A is a constant which is di!erent for di!erent transitions indicated by di!erent values of m and E is the corresponding band gap.
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
is found, the value of m can easily be calculated (Eq. (2)) from the slope of the plot of ln(h) versus ln(h!E ). Fig. 8(a) shows the plot of d[ln(h)]/d[h] versus h for a representative "lm from which one can identify the band gap &1.24 eV. The value of m (&0.45), obtained from the plot of ln(h) versus ln(h!E ), as shown in
Now, ln (h)"ln A#m ln (h!E ) and
67
(2)
m d[ln (h)] " . (3) h!E d[h] Eq. (3) suggests that a plot of d[ln(h)]/d[h] versus h will indicate a divergence at h"E from which the value of E may be obtained. Once E
Fig. 6. Variation of lattice constants with x for CuIn(S Se ) V \V "lms: (a) variation of c; (b) variation of a; (c) variation of c/a; x"S/(S#Se).
Fig. 5. XRD spectra of three representative CuIn(S Se ) V \V "lms (a) x&0.98; (b) x&0.5; (c) x&0.13.
Table 1 Composition (x) and optical band gap (E ) of CuIn(S Se ) "lms V \V Sample
E (eV)
x"S/(S#Se)
Cu/In
(S#Se)/(Cu#In)
Grain size (SEM) (m)
IC(8)6 IC(9)14 IC(9)6 IC(11)3 IC(11)8 IC(11)7 IC(8)4 IC(6)6 IC(6)2
1.40 1.30 1.24 1.23 1.18 1.14 1.07 0.99 0.98
0.98 0.94 0.84 0.72 0.61 0.52 0.34 0.21 0.13
0.72 0.70 0.92 0.91 0.93 0.92 0.83 0.86 0.90
1.04 1.00 0.91 1.04 0.93 0.96 0.95 0.98 1.00
0.91 1.28 1.39 1.42 1.53 1.58 1.62 1.81 2.01
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Fig. 7. Transmittance versus wavelength () traces for four representative CuIn(S Se ) "lms: * x&0.98; } } }x&0.84; V \V 2.. x&0.52; -*-*-* x&0.13.
the inset of Fig. 8(b), indicated direct transition. The band gap was determined by extrapolating the linear portion of the plot of (h) versus h (Fig. 8(b)) which indicated E &1.236. E values of other "lms are shown in Table 1. It may be noted that the band gap varied linearly with x x"S/(S#Se). The refractive index (r.i.) of the CuIn(S Se ) "lms V \V was evaluated [22] from the transmittance and re#ectance measurements. It was found that the value of refractive index increased with decreasing sulphur content varying within 1.6}1.9 with variation of x within 0.98}0.13. 3.3. Photoluminescence studies PL spectra (Fig. 9) of CuIn(S Se ) "lms, reV \V corded at 80 K, were dominated by emission peaks which shifted from 1.16 to 1.4 eV with increasing sulphur content. These peaks may be identi"ed to be due to bound excitonic transitions. These excitonic peaks are followed by a smaller peak
Fig. 8. Plot of (a) d[ln(h)]/ d(h) versus h; (b) (h) versus h for a representative CuIn(S Se ) "lm. Inset shows plot of V \V ln(h) versus ln (h!E ).
located at &0.98 eV for "lms with x value ranging from 0.85 to 0.99. The peaks located at &0.98 may be identi"ed to be due to the transitions from conduction band to neutral acceptors V or Cu . ! ' As our "lms are generally copper de"cient, we think that transitions to V are more probable here for ! transitions at &0.98 eV. The "lms with x value less than 0.85, the excitonic peak is accompanied by a peak at &0.96 eV. The intensity of this peak was smaller than that for V and may be identi"ed to !
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
69
Fig. 9. Photoluminescence spectra for two representative "lms with di!erent x values [x"S/(S#Se)]: (a) x&0.98; (b) x&0.13.
be due to the exciton bound to ionized acceptor (Cu ) states. ' 3.4. Electrical and galvanomagnetic studies The measurement of electrical conductivity and Hall mobility were carried out simultaneously on the same specimen by Van der Pauw technique [14] using cross-shaped sample geometry [23] to eliminate "nite contact e!ects. The "lms were predominantly p-type as indicated by the Hall measurements. The variation of electrical conductivity () for seven representative CuIn(S Se ) V \V "lms with temperature (¹) is shown in Fig. 10. Exponential variation of with temperature could be observed for all the "lms studied here and there are two di!erent activation energy regimes. The transition temperature between the two regimes varied from 248 to 343 K as x increases from 0.13 to 0.98. The corresponding temperature dependence of mobility () and carrier concentration (N ) are shown in Figs. 11 and 12, respectively. These exponential variations are indicative of grain boundary scattering as proposed by Petritz [24]. We have evaluated the activation energies E , E and N E from the inverse temperature variations of ln , , ln and ln N , respectively. The values of the di!er-
Fig. 10. Plot of ln versus 1/T ( in mho-cm\) for six representative CuIn(S Se ) "lms with di!erent x values V \V [x"S/(S#Se)]: (a) 䢇 x&0.13; (b) 䊐 x&0.21; (c) 䉱 x&0.34; (d) x&0.52; (e) 䉬 x&0.61; (f ) * x&0.72, (g) 䊏 x&0.84.
ent activation energies obtained as above are shown in Table 2 which implies the following relation to be satis"ed [25]: E "E #E . (4) N , Now, the presence of interface states along with the thermionic emission across the grain boundaries would directly a!ect the charge transport in polycrystalline "lms and the Hall mobility may be expressed as [26]
"¸e
E 1 exp ! , 2mHk¹ k¹
(5)
where ¸ is the grain size, E is the barrier height across the grain boundary and mH is the e!ective mass of the carriers. It is apparent from Eq. (5) that a plot of ln(¹ ) versus 1/T should be a straight line, the slope of which would give the value of the
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Fig. 11. Plot of ln versus 1/T ( in cm/V-sec) for six representative CuIn(S Se ) "lms with di!erent x values V \V [x"S/(S#Se)]: (a) 䢇 x&0.13 ; (b) 䊐 x&0.21; (c) 䉱 x &0.34; (d) x &0.52; (e) 䉬 x&0.61; (f ) * x&0.72, (g) 䊏 x&0.84.
barrier height (E ). Fig. 13 shows the above plots for seven representative "lms. The values of E ob tained from the slopes of these plots are shown in Table 3. The density of trap states (Q ) has been computed from the relation [26]: eQ , E " 8 N
(6)
where N is the carrier concentration (for p-type material) and is the dielectric constant. It is quite apparent from Table 3 that E decreases with the increase of N indicating partially depleted grains in the CuIn(S Se ) "lms. The values of Q , comV \V puted as above, are also shown in Table 3. It may be observed that the "lms with higher sulphur content have less grain boundary trap states. Further, LN
Fig. 12. Plot of ln N versus 1/T (N in cm\) for six representative CuIn(S Se ) "lms with di!erent x values [x"S/(S#Se) V \V x&0.52; ]: (a) 䢇 x&0.13; (b) 䊐 x&0.21; (c) 䉱 x&0.34; (d). (e) 䉬 x&0.61; (f ) * x&0.72, (g) 䊏 x&0.84.
Table 2 Di!erent activation energies of CuIn(S Se ) "lms V \V Sample
E (meV) N
E (meV)
E (meV) ,
IC(9)6 IC(11)3 IC(11)8 IC(11)7 IC(8)4 IC(6)6 IC(6)2 IC(8)6 IC(9)14
53.6 42.6 35.7 29.3 21.2 14.1 11.1 16.4 11.8
25.0 21.1 17.6 15.0 10.9 7.3 5.7 8.7 5.9
27.3 20.7 17.2 14.9 9.8 7.1 5.2 6.8 5.7
will not be adequate to explain the electron transport processes in these "lms since it does not take into account the possibility of partially "lled traps when the depletion regions do not extend throughout the entire crystallite. Baccarani et al. [27] modi"ed the grain boundary trapping model by
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considering monovalent trapping states and the presence of continuous energy distribution of the interface states within the band gap. The modi"ed expressions of conductivities for two limiting cases are
71
(i) When the trap states are below the Fermi level, i.e., E !(E #E )
"
e¸n < E exp ! , k¹ k¹
(7)
with the activation energy E "E . Here, n is the electron concentration in the neutral region and < is the collection velocity, given by [27]
k¹ < " 2mH
(8)
(ii) when the trap states lie above the Fermi level, i.e., E !(E #E );kT,
"
e¸N< k¹Q
2 E E exp ! , N k¹
(9)
with activation energy E "E /2!E , E being the band gap and N , the e!ective density of states given by N "2
2mHk¹ . h
(10)
Using the above expressions of < and N in Eqs. (7) and (9), the expression for gets modi"ed to Eqs. (11) and (12), respectively, Fig. 13. Plot ln (T ) versus 1/T ( in cm/V-sec) for six representative CuIn(S Se ) "lms with di!erent x values V \V [x"S/(S#Se) ]: (a) 䢇 x&0.13; (b) 䊐 x&0.21; (c) 䉱 x&0.34; (d) x&0.52; (e) 䉬 x&0.61; (f ) * x&0.72, (g) 䊏 x&0.84.
"
e¸n E exp ! (2mHk¹) k¹
(11)
Table 3 Grain size (¸), barrier height (E ), carrier concentration, trap state density (Q ), depletion width (=) and position of trap energy level (E ) with respect to Fermi level (E ) Sample
Cu/In
(S#Se)/ (Cu#In)
x"S/ (S#Se)
L (m)
E (meV)
Carrier Conc. (cm\)
Q (10 cm\)
W (nm)
E !E (meV)
IC(8)6 IC(9)14 IC(9)6 IC(11)3 IC(11)8 IC(11)7 IC(8)4 IC(6)6 IC(6)2
0.72 0.70 0.92 0.91 0.93 0.92 0.83 0.86 0.90
1.04 1.00 0.91 1.04 0.93 0.96 0.95 0.98 1.01
0.98 0.94 0.84 0.72 0.61 0.52 0.34 0.21 0.13
0.91 1.28 1.39 1.42 1.53 1.58 1.62 1.81 2.01
18.8 16.3 35.5 31.5 27.8 25.0 20.4 16.9 15.5
1.00;10 1.82;10 3.21;10 8.82;10 2.34;10 6.05;10 1.67;10 5.58;10 1.84;10
9.2 11.6 2.3 3.7 5.8 8.9 13.8 23.1 40.7
4.6 3.2 36.3 21.0 12.3 7.4 4.1 2.1 1.1
192 188 208 204 200 198 195 189 187
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4. Conclusions
Fig. 14. Plot ln (¹ ) versus 1/T ( in mho-cm\) for six representative CuIn(S Se ) "lms with di!erent x values V \V [x"S/(S#Se) ]: (a) 䢇 x&0.13; (b) 䊐 x&0.21; (c) 䉱 x&0.34; x&0.52; (e) 䉬 x&0.61; (f ) * x&0.72, (g) 䊏 x&0.84. (d) Inset: Plot of ln (¹\ ) versus 1/T.
for E !(E #E )
(12)
for E !(E #E );k¹. It was observed that the experimental data "tted well with expression (11) as is evident from the plot of ln(¹ ) versus 1/T (Fig. 14). The "t of the data with Eq. (13) was quite poor (inset of Fig. 14). This indicates that the trap states (E ) lie below the Fermi level (E ) and the relative position of E and E may be obtained from the relation: eQ E !E "E !k¹ ln 2 !1 . (13) (8 N E ) The values of E !E obtained from expression (13) are shown in Table 3. The width of the depletion region (=) was evaluated [27] from Eq. (14) and is shown in Table 3.
="
2 E . eN
(14)
CuIn(S Se ) "lms were synthesized by sulV \V phurization followed by selenization of In/Cu stacked elemental layers using GB annealing with elemental sulphur and selenium as sulphurization and selenization agents. The "lm, thus synthesized, showed chalcopyrite structure with preferred orientation along (1 1 2) planes perpendicular to the substrate plane. Linear variation of `ca, `aa and c/a values with increasing sulphur content was observed. The values of the lattice constants and their ratio c/a decreases with increase of sulphur (x). The transmittance increases and sharpness of fall decreases as x value increases from zero to one. All the "lms showed p-type conductivity. The "lms were mainly Cu-poor. Copper vacancy (V ), sul! phur and selenium interstitial (S ,Se ) act as prob able acceptor states in our "lms. PL studies showed an excitonic peak which varied from 1.40 to 1.16 eV as x value decreased. The peak at 0.98 eV could be assigned due to the transition from conduction band to neutral donor. The peak at &0.96 eV arising for lower x value may be identi"ed with exciton bound to ionized acceptor (Cu ) states. ' Grain boundary scattering e!ects dominated the electron transport mechanism in these "lms at lower temperature of measurements. The density of trap states at the grain boundary decreased with the increasing sulphur content in the "lms. Acknowledgements The authors wish to acknowledge with thanks the CSIR, Govt. of India, for supporting this programme. Authors are grateful to Mr. Shekhar Chandra Ghosh for recording the SEM pictures. One of the authors (S.B.) wishes to thank the Department of Atomic Energy, Government of India, for sanctioning him the Senior Research Fellowship. References [1] Contreras MA, Egaas B, Ramanathan K, Hiltner J, Swartzlander A, Hasoon F, Nou" R. Prog Photovolt 1999;7:311.
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73 [2] Green MA, Emery K, Bucher K, King DI, Igari S. Prog Photovolt Res Appl 1999;7:31. [3] Klaer J, Bruns J, Henninger R, Siemer K, Klenk R, Ellmer K, Braunig D. Semicond Sci Technol 1998;13:1456. [4] Hashimoto Y, Takeuchi K, Ito S. Appl Phys Lett 1995;67:980. [5] Walter T, Menner R, Koble Ch, Schock HW, Proceedings of the 12th EC Photovoltaic Solar Energy Conference, Amesterdam, Dordrecht: Kluwer, 1994. p. 1775. [6] Schock HW, Twelth European Photovoltaic Solar Energy Conference, Amsterdam, 1994. p. 87. [7] Ohashi T, Inakoshi K, Hashimoto Y, Ito K. Sol Energy Mater Solar Cells 1998;50:37. [8] Eisener B, Wolf D, Muller G. Thin Solid Films 2000;361}362:126. [9] Herberholz R, Walter T, Schock HW. J Appl Phys 1994;76:2904. [10] Cheng YH, Tseng BH, Loferski JJ, Hwang HL. Ninth International Photovoltaic Science and Engineering Conference, Miyazaki, Japan, 1996, p. 591. [11] Walter T, Ruckh M, Velthaus KO, Schock HW, Eleventh E.C. Photovoltaic Solar Energy Conference, Montreaux, Switzerland, 1993, p. 124. [12] Ohashi T, Inakoshi K, Hashimoto Y, Ito K. Ninth International Photovoltaic Science and Engineering Conference, Miyazaki, Japan, 1996. p. 393.
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[13] Bandyopadhyay S, Chaudhuri S, Pal AK. Sol. Energy Sol Cells 2000;60:323. [14] Van der Pauw LJ. Philips Res Rep 1958;13:1. [15] Gombia E, Leccabue F, Pelosi C. Mater Lett 1984; 2:429. [16] Glasov VM, Lebedov VV, Molkyn AD, Pashiskin AS. Izv Akad Nauk SSSR Neorg Mater 1979;15: 1865. [17] Cahen D, Nou" R. J Phys Chem Solids 1991;52:947. [18] Klenk R, Walter T, Schmid D, Schock HW. Jpn J Appl Phys 1993;32(Suppl. 32}3):57. [19] Masse G. J Appl Phys 1990;68:2206. [20] Masse G, Lahlou N, Butti C. J Phys Chem Solids 1981;42:449. [21] Bhattacharyya D, Chaudhuri S, Pal AK. Vacuum 1992;43(4):313. [22] Bhattacharyya D, Chaudhuri S, Bhattacharyya SK, Pal AK. Vacuum 1993;44(10):979. [23] de Mey G. Electron Commun 1973;28:309. [24] Petritz RL. Phys Rev 1956;104:1508. [25] Berger H. Phys Stat Solidi 1961;1:739. [26] Seto JYW. J Appl Phys 1975;46:5247. [27] Baccarani G, Ricco B, Spadini G. J Appl Phys 1978;49:5565.
CuIn(S Se ) "lms prepared by graphite box annealing V \V of In/Cu stacked elemental layers S. Bandyopadhyaya, S. Roy, S. Chaudhuri, A.K. Pal* Department of Materials Science, Indian Association for the Cultivation of Science, Calcutta-700 032, India
Abstract CuIn(S Se ) "lms were synthesized by sulphurization of In/Cu stacked elemental layers deposited onto glass and V \V Mo-coated glass substrates followed by selenization by graphite box annealing. The "lms, thus synthesized, were characterized by measuring electrical, optical and microstructural properties. The microstructure and hence the physical properties of the "lms depended critically on the amount of sulphur incorporation. Nature of charge carriers depended on Cu/In, (S#Se)/(Cu#In) and S/(S#Se) ratios while their concentrations varied between 10 and 10 cm\. Grain boundary scattering e!ects were critically studied by measuring the electrical conductivity () and Hall mobility () simultaneously on the same sample. Optical transmittance studies indicated the band gap to vary within 0.98}1.40 eV with x values. The photoluminescence spectra, recorded at 80 K were dominated by the excitonic peak located within 1.40}1.16 eV followed by a small peak within &0.96}0.98 eV arising due to transition from conduction band to neutral acceptor (< ) or exciton bound to ionized acceptor (Cu ) states. 2001 Elsevier Science Ltd. All rights reserved. ! ' Keywords: Ternary semiconductors; Electrical conductivity; Photo-luminescence
1. Introduction Thin "lm solar cells based on polycrystalline chalcopyrite semiconductors CuInSe and Cu(In Ga )Se as the absorber layers attracted V \V much attention during the last decade due to high absorption coe$cient and near zero degradation of the cell performance made out of these materials. Solar cells based on Cu(In Ga )Se have alV \V ready attained a conversion e$ciency &18.8% [1,2] while for solar cells based on CuInS in dicated an e$ciency &12% [3]. Intrinsic defects in the absorber material often controlled the per-
* Corresponding author.
formance of the photovoltaic devices. Band o!set in between the absorber layer and the window layer (CdS) was one of the major limiting factors towards increasing the e$ciency value. It was observed that the band o!set might be improved if sulphur is incorporated in the selenide lattice [4}7]. In fact, the semiconductor CuInS has an edge over CuInSe and Cu(In Ga )Se since it does not V \V contain any toxic constituent like selenium. Formation of alloys of CuInSe and CuInS are also advantageous for solar cell application due to the fact that in CuIn(S Se ) the band gap along V \V with other material properties can easily be tuned to the most desirable value by adjusting the sulphur content in the "lm. In#uence of sulphur on the electrical and optical properties of CuIn(S Se ) V \V
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 1 5 6 - 7
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single crystals was studied by Eisener et al. which indicated that the activation energies of the acceptor levels shift to higher values when selenium is replaced by sulphur [8]. The density of states in CuIn(S Se ) thin "lms were estimated by HerV \V berholz et al. from modulated photocurrent measurements which indicated a continuous distribution of traps inside the band gap [9]. Cheng et al. reported their work on the conversion of CuInSe "lm to CuIn(S Se ) by immersing CuInSe "lm V \V in (NH ) S [10]. Four source evaporation tech V nique was utilized by Walter et al. for synthesizing CuIn(S Se ) thin "lm for the fabrication of soV \V lar cells [11]. Incorporation of sulphur and selenium was found to depend on copper content in the "lms. Preparation of CuIn(S Se ) "lms by V \V sulphurization of Cu/In stack by H S followed by selenization was reported by Ohashi et al. [12]. Resistivity of the "lms, thus produced, was seen to increase by one order of magnitude by KCN treatment. The substitution of selenium by sulphur in CuInSe is still a challenging task. E!orts are underway to "nd a viable technological route to produce CuIn(S Se ) "lms as an e!ective abV \V sorber layer for solar cells in a simple and cost e!ective way. Graphite box annealing of stacked elemental layer precursor seems to be a viable alternative to satisfy the above need. Synthesis of CuInS "lms by sulphurization of Cu/In stacked elemental layers was reported earlier [13]. In this paper, synthesis of CuIn(S Se ) "lms by sulV \V phurization of In/Cu stacked elemental layers (SEL) followed by selenization is presented. The "lms thus synthesized are characterized by measuring electrical, optical, microstructural and photoluminescence (PL) properties.
2. Experimental Cu and In (all with purity of &99.995%) were evaporated from alumina crucibles heated indirectly by tungsten heaters in a conventional 12 in. diameter vacuum coating unit at a system pressure of &10\ Pa. The #uxes of the individual sources (Cu&0.1 nm/s and In&0.1 nm/s) could be monitored by two quartz crystal thickness moni-
tors for obtaining the stacked layers with the requisite thicknesses. The substrate could be rotated for uniformity of the thickness of the deposit. For SEL precursor, the layers were deposited in sequence of In/Cu with predetermined thicknesses of the individual layers in order to attain the required composition of the synthesized "lm. The precursors were deposited onto sodalime glass substrates (with and without Mo coating) at room temperature (300 K). These precursors were placed in an appropriate rectangular graphite box for annealing inside a cylindrical (6.5 cm diameter) quartz chamber in partial argon (purity&99.99%) atm. Requisite amount of sulphur (99.99%) was placed inside the quartz chamber for sulphurization. The quartz chamber was heated in an electronically controlled furnace. Selenization was carried out in another quartz chamber similar to the one used for sulphurization. Electrical and galvanomagnetic properties were measured simultaneously on the same specimen by Van der Pauw technique at temperatures ranging from 190 to 400 K [14]. Microstructural studies were carried out by a SEM-EDX (Hitachi S-2300 with Kevex Delta Class-I) while the optical transmittance was measured by a UV-VIS-NIR spectrophotometer (Hitachi-U3410). PL measurements were carried out at 80 K and by using 300 W Xe arc lamp as the emission source, Hamamatsu photomultiplier as the detector along with a 1/4 meter monochromator. The spectra were recorded with excitation at 500, 600 and 700 nm radiations
3. Results and discussion Four possible schemes (Fig. 1) for synthesizing CuIn(S Se ) "lms from stacked elemental V \V layers were tried here. In Scheme a, selenium was incorporated in the stack which was placed in the quartz chamber for annealing in the presence of sulphur. There was no signi"cant change in the band gap beyond 1.07 eV and the XRD indicated separate peaks for CuInS (lower intensity) and CuInSe (higher intensity) along with peaks for a host of binaries (Fig. 2a). The shift in the peak position for (3 1 2/1 1 6) planes indicated the presence of CuIn(S Se ) phase also. The CuInSe V \V
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
Fig. 1. Four di!erent schemes for stacked elemental layer precursor for the preparation of CuIn(S Se ) "lms. Thickness of V \V each layer &0.1 m.
was the most predominant phase and incomplete sulphurizaion was observed from SEM micrograph (inset of Fig. 2a). EDAX measurement (area scan) of this representative "lm showed x"0.55 [Cu"28.07 : In"30.57 : S"22.79 : Se"18.57]. From these results it is apparent that once CuInSe is formed, the substitution of Se by S is not favoured and as such the upper surface only gets sulphurized and the rest of the "lm shows the existence of CuInSe phase only. Increase of sul phur content in the reaction chamber made the "lm brittle, which easily peeled o! the substrate. The stack was then modi"ed by eliminating selenium layers from the bottom or top layers of the SEL as shown in Schemes b and c, respectively, to allow homogeneous sulphurization-cum-selenization of the SEL by annealing in the presence of sulphur. The end product for Scheme-c was always a layered structure as revealed by SEM micrograph (inset of Fig. 2b) and XRD (Fig. 2b). EDX analysis (inset of Fig. 2b) indicated that the layer adjacent to the glass substrate was predominantly CuInSe while the top layer was CuInS . The intermediate layers consisted of several binaries as revealed by the XRD traces (Fig. 2b). Here again, the shift in the peak position for (2 2 0/2 0 4) and (3 1 2/1 1 6) planes indicated the presence of CuIn(S Se ) phase. V \V But, after sulphurization of SEL in Scheme-b, the "lm was found to contain predominant CuInSe phase with binaries of sulphur and selenium. Re-
63
duction in sulphur content in the annealing chamber resulted in the formation of selenide binaries only. The possibility of synthesizing CuIn(S Se ) V \V "lms by simultaneous selenization and sulphurization of the In/Cu layers (Scheme-d) was also examined by putting di!erent proportions of S/Se material in the annealing chamber. It was observed that selenization was a predominant process and the resultant "lms did not show band gaps more than &1.01 eV. XRD spectra did not indicate any signi"cant presence of sub-phases (Fig. 2c) although the peaks could be identi"ed as arising due to the presence of CuIn(S Se ) phase only. V \V However, the peaks were not sharp and intense as compared to the well-synthesized CuIn(S Se ) V \V "lms. The crystallites did not indicate any wellfaceted structure (inset of Fig. 2c). It was found that this process is not conducive for obtaining "lms with predetermined composition. This is due to the higher sulphur vapour pressure, which suppressed the relative selenium vapour pressure in the reaction chamber, and as such primary selenization was hindered. The above observations are in conformity with the fact that the formation energy for CuInSe is lower than that of CuInS and as such the thermo dynamical formation of CuInSe will always be favoured and Se atoms cannot be replaced by S atoms despite their small atomic radii [15}18]. Interestingly, as we go down the group VIb in the periodic table, atomic radii increases from S to Te (R "1.13 As , R "1.22 As and R "1.40 As ) [19]. 1 1 2 Not only the atom of smaller atomic size will be strongly bound, electro-negativity of S is greater than that of Te. So, the S bond which is more ionic than the other members in the chalcogenide series would share the electron cloud more. Thus, the bond strength of CuInS is higher than that of CuInSe which is also evident from the ionization energies of sulphur (< "90 meV) and selenium 1 (< "70 meV) [20]. Keeping the above informa1 tion in mind, the well-synthesized CuIn(S Se ) V \V "lms were successfully obtained here by sulphurization followed by selenization by graphite box (GB) annealing of In/Cu layers (Scheme-d). Sulphurization of the stacked elemental layer precursor as shown in Scheme-d was performed by
64
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
Fig. 2. SEM micrographs (inset) and corresponding XRD patterns of representative "lms synthesized with: (a) Scheme-a; (b) Scheme-b and (c) Scheme-c.
annealing them in a closed graphite box in partial argon atmosphere (&0.5 atm) with sulphur inside the quartz chamber. Quantity of sulphur placed in the annealing chamber was varied to "nd
the optimum value of it. The details of sulphurization has been discussed elsewhere [13]. For selenization of the sulphurized layer, the annealing temperature (¹ ) was kept &770 K while the
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
Fig. 3. Interdependence of the selenium content in and graphite box (GB) and annealing time for obtaining CuIn(S Se ) V \V "lms with di!erent band gaps (E ).
annealing time (t) and the amount of selenium were varied to obtain "lms with di!erent selenium content. Fig. 3 shows the interdependence of the amount of selenium in GB and the corresponding time of annealing for obtaining "lms with di!erent selenium content as indicated by the band gap of the "lms thus synthesized. In this paper, the properties of CuIn(S Se ) "lms synthesized by sulphuV \V rization followed by selenization using GB annealing of In/Cu SEL as the precursor are reported. 3.1. Microstructural studies Figs. 4(a}f ) indicate the SEM micrographs of some representative CuIn(S Se ) "lms synV \V thesized by sulphurization followed by selenization. One can observe that well-crystallized grains for all the "lms could be obtained by this technique and the sizes of the grains decreased from 2.01 to 0.91 m with the increase of sulphur content x"S/(S#Se) in the "lms (Table 1). The XRD of
65
three representative "lms corresponding to the micrographs 4a, d and f are shown in Fig. 5. There are mainly three peaks for re#ections from (1 1 2), (2 2 0/2 0 4) and (3 1 2/1 1 6) planes. The XRD traces in Fig. 5c also indicate an additional peak for the plane (3 1 6/3 3 2). No additional peaks corresponding to possible subphases were noticed which indicated complete synthesization of CuIn(S Se ) V \V "lms by this technique. The compositions of the above "lms were determined by EDX and EPMA analysis which are shown in Table 1. The compositional deviations from the ideal chemical formula would lead to the formation of donor}acceptor pairs and the type of the majority charge carriers. It may be seen from Table 1 that all the "lms are Cu-poor which would promote the presence of copper vacancies (V ) ! acting like acceptor states. Also, "lms synthesized by selenization of CuInS would mean that seleniz ation would proceed through the replacement of sulphur by selenium. Thus, the presence of sulphur interstitials (S ) arising from the parent CuInS "lm and the replaced sulphur from the CuInS lattice by selenium is more likely to describe the defect chemistry of these "lms. As the "lms were selenized in the presence of overpressure of selenium, the presence of selenium interstitials (Se ) could not be ruled out. The presence of copper vacancies (V ), sulphur interstetials (S ) and sel! enium interstitials (Se ) would re#ect predominant p-type conductivity in these CuIn(S Se ) "lms V \V which was con"rmed from electrical and PL measurements. The lattice parameters (`aa and `ca) evaluated from the XRD studies are shown in Fig. 6. The value of `aa varied from 0.557 to 0.579 nm while `ca varied from 1.112 to 1.159 nm with the decrease of sulphur content (x) in the "lms. The corresponding c/a ratio varied within 1.996}2.004. 3.2. Optical properties The transmittance (T ) spectra for four represen tative CuIn(S Se ) "lms synthesized with di!erV \V ent sulphur contents are shown in Fig. 7. The increase in the transmittance in the NIR region with the reduction of the sharpness of fall of the T } plot (Fig. 7) near the band edge for "lms with
66
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
Fig. 4. SEM micrographs of six representative CuIn(S Se ) "lms synthesized at 770 K with Scheme-d x&0.98 and t&250 min; (b) V \V x&0.84 and t&120 min; (c) x&0.72 and t&110 min; (d) x&0.5 and t&80 min; (e) x&0.34 and t&70 min; (f ) x&0.13 and t&20 min.
increasing sulphur content could be observed. The "lms were highly absorbing. The reduction of sharpness of fall can be attributed to the decrease in grain size of the CuIn(S Se ) "lms with increase V \V in the sulphur content in the "lms. The absorption coe$cients () of the CuIn(S Se ) "lms were determined by measurV \V ing transmittance and re#ectance in these "lms
[21]. In general, the absorption coe$cient () may be written as a function of the incident photon energy (h) so that [21]: "(A/h)h!E K, (1) where A is a constant which is di!erent for di!erent transitions indicated by di!erent values of m and E is the corresponding band gap.
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
is found, the value of m can easily be calculated (Eq. (2)) from the slope of the plot of ln(h) versus ln(h!E ). Fig. 8(a) shows the plot of d[ln(h)]/d[h] versus h for a representative "lm from which one can identify the band gap &1.24 eV. The value of m (&0.45), obtained from the plot of ln(h) versus ln(h!E ), as shown in
Now, ln (h)"ln A#m ln (h!E ) and
67
(2)
m d[ln (h)] " . (3) h!E d[h] Eq. (3) suggests that a plot of d[ln(h)]/d[h] versus h will indicate a divergence at h"E from which the value of E may be obtained. Once E
Fig. 6. Variation of lattice constants with x for CuIn(S Se ) V \V "lms: (a) variation of c; (b) variation of a; (c) variation of c/a; x"S/(S#Se).
Fig. 5. XRD spectra of three representative CuIn(S Se ) V \V "lms (a) x&0.98; (b) x&0.5; (c) x&0.13.
Table 1 Composition (x) and optical band gap (E ) of CuIn(S Se ) "lms V \V Sample
E (eV)
x"S/(S#Se)
Cu/In
(S#Se)/(Cu#In)
Grain size (SEM) (m)
IC(8)6 IC(9)14 IC(9)6 IC(11)3 IC(11)8 IC(11)7 IC(8)4 IC(6)6 IC(6)2
1.40 1.30 1.24 1.23 1.18 1.14 1.07 0.99 0.98
0.98 0.94 0.84 0.72 0.61 0.52 0.34 0.21 0.13
0.72 0.70 0.92 0.91 0.93 0.92 0.83 0.86 0.90
1.04 1.00 0.91 1.04 0.93 0.96 0.95 0.98 1.00
0.91 1.28 1.39 1.42 1.53 1.58 1.62 1.81 2.01
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S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
Fig. 7. Transmittance versus wavelength () traces for four representative CuIn(S Se ) "lms: * x&0.98; } } }x&0.84; V \V 2.. x&0.52; -*-*-* x&0.13.
the inset of Fig. 8(b), indicated direct transition. The band gap was determined by extrapolating the linear portion of the plot of (h) versus h (Fig. 8(b)) which indicated E &1.236. E values of other "lms are shown in Table 1. It may be noted that the band gap varied linearly with x x"S/(S#Se). The refractive index (r.i.) of the CuIn(S Se ) "lms V \V was evaluated [22] from the transmittance and re#ectance measurements. It was found that the value of refractive index increased with decreasing sulphur content varying within 1.6}1.9 with variation of x within 0.98}0.13. 3.3. Photoluminescence studies PL spectra (Fig. 9) of CuIn(S Se ) "lms, reV \V corded at 80 K, were dominated by emission peaks which shifted from 1.16 to 1.4 eV with increasing sulphur content. These peaks may be identi"ed to be due to bound excitonic transitions. These excitonic peaks are followed by a smaller peak
Fig. 8. Plot of (a) d[ln(h)]/ d(h) versus h; (b) (h) versus h for a representative CuIn(S Se ) "lm. Inset shows plot of V \V ln(h) versus ln (h!E ).
located at &0.98 eV for "lms with x value ranging from 0.85 to 0.99. The peaks located at &0.98 may be identi"ed to be due to the transitions from conduction band to neutral acceptors V or Cu . ! ' As our "lms are generally copper de"cient, we think that transitions to V are more probable here for ! transitions at &0.98 eV. The "lms with x value less than 0.85, the excitonic peak is accompanied by a peak at &0.96 eV. The intensity of this peak was smaller than that for V and may be identi"ed to !
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
69
Fig. 9. Photoluminescence spectra for two representative "lms with di!erent x values [x"S/(S#Se)]: (a) x&0.98; (b) x&0.13.
be due to the exciton bound to ionized acceptor (Cu ) states. ' 3.4. Electrical and galvanomagnetic studies The measurement of electrical conductivity and Hall mobility were carried out simultaneously on the same specimen by Van der Pauw technique [14] using cross-shaped sample geometry [23] to eliminate "nite contact e!ects. The "lms were predominantly p-type as indicated by the Hall measurements. The variation of electrical conductivity () for seven representative CuIn(S Se ) V \V "lms with temperature (¹) is shown in Fig. 10. Exponential variation of with temperature could be observed for all the "lms studied here and there are two di!erent activation energy regimes. The transition temperature between the two regimes varied from 248 to 343 K as x increases from 0.13 to 0.98. The corresponding temperature dependence of mobility () and carrier concentration (N ) are shown in Figs. 11 and 12, respectively. These exponential variations are indicative of grain boundary scattering as proposed by Petritz [24]. We have evaluated the activation energies E , E and N E from the inverse temperature variations of ln , , ln and ln N , respectively. The values of the di!er-
Fig. 10. Plot of ln versus 1/T ( in mho-cm\) for six representative CuIn(S Se ) "lms with di!erent x values V \V [x"S/(S#Se)]: (a) 䢇 x&0.13; (b) 䊐 x&0.21; (c) 䉱 x&0.34; (d) x&0.52; (e) 䉬 x&0.61; (f ) * x&0.72, (g) 䊏 x&0.84.
ent activation energies obtained as above are shown in Table 2 which implies the following relation to be satis"ed [25]: E "E #E . (4) N , Now, the presence of interface states along with the thermionic emission across the grain boundaries would directly a!ect the charge transport in polycrystalline "lms and the Hall mobility may be expressed as [26]
"¸e
E 1 exp ! , 2mHk¹ k¹
(5)
where ¸ is the grain size, E is the barrier height across the grain boundary and mH is the e!ective mass of the carriers. It is apparent from Eq. (5) that a plot of ln(¹ ) versus 1/T should be a straight line, the slope of which would give the value of the
70
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
Fig. 11. Plot of ln versus 1/T ( in cm/V-sec) for six representative CuIn(S Se ) "lms with di!erent x values V \V [x"S/(S#Se)]: (a) 䢇 x&0.13 ; (b) 䊐 x&0.21; (c) 䉱 x &0.34; (d) x &0.52; (e) 䉬 x&0.61; (f ) * x&0.72, (g) 䊏 x&0.84.
barrier height (E ). Fig. 13 shows the above plots for seven representative "lms. The values of E ob tained from the slopes of these plots are shown in Table 3. The density of trap states (Q ) has been computed from the relation [26]: eQ , E " 8 N
(6)
where N is the carrier concentration (for p-type material) and is the dielectric constant. It is quite apparent from Table 3 that E decreases with the increase of N indicating partially depleted grains in the CuIn(S Se ) "lms. The values of Q , comV \V puted as above, are also shown in Table 3. It may be observed that the "lms with higher sulphur content have less grain boundary trap states. Further, LN
Fig. 12. Plot of ln N versus 1/T (N in cm\) for six representative CuIn(S Se ) "lms with di!erent x values [x"S/(S#Se) V \V x&0.52; ]: (a) 䢇 x&0.13; (b) 䊐 x&0.21; (c) 䉱 x&0.34; (d). (e) 䉬 x&0.61; (f ) * x&0.72, (g) 䊏 x&0.84.
Table 2 Di!erent activation energies of CuIn(S Se ) "lms V \V Sample
E (meV) N
E (meV)
E (meV) ,
IC(9)6 IC(11)3 IC(11)8 IC(11)7 IC(8)4 IC(6)6 IC(6)2 IC(8)6 IC(9)14
53.6 42.6 35.7 29.3 21.2 14.1 11.1 16.4 11.8
25.0 21.1 17.6 15.0 10.9 7.3 5.7 8.7 5.9
27.3 20.7 17.2 14.9 9.8 7.1 5.2 6.8 5.7
will not be adequate to explain the electron transport processes in these "lms since it does not take into account the possibility of partially "lled traps when the depletion regions do not extend throughout the entire crystallite. Baccarani et al. [27] modi"ed the grain boundary trapping model by
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
considering monovalent trapping states and the presence of continuous energy distribution of the interface states within the band gap. The modi"ed expressions of conductivities for two limiting cases are
71
(i) When the trap states are below the Fermi level, i.e., E !(E #E )
"
e¸n < E exp ! , k¹ k¹
(7)
with the activation energy E "E . Here, n is the electron concentration in the neutral region and < is the collection velocity, given by [27]
k¹ < " 2mH
(8)
(ii) when the trap states lie above the Fermi level, i.e., E !(E #E );kT,
"
e¸N< k¹Q
2 E E exp ! , N k¹
(9)
with activation energy E "E /2!E , E being the band gap and N , the e!ective density of states given by N "2
2mHk¹ . h
(10)
Using the above expressions of < and N in Eqs. (7) and (9), the expression for gets modi"ed to Eqs. (11) and (12), respectively, Fig. 13. Plot ln (T ) versus 1/T ( in cm/V-sec) for six representative CuIn(S Se ) "lms with di!erent x values V \V [x"S/(S#Se) ]: (a) 䢇 x&0.13; (b) 䊐 x&0.21; (c) 䉱 x&0.34; (d) x&0.52; (e) 䉬 x&0.61; (f ) * x&0.72, (g) 䊏 x&0.84.
"
e¸n E exp ! (2mHk¹) k¹
(11)
Table 3 Grain size (¸), barrier height (E ), carrier concentration, trap state density (Q ), depletion width (=) and position of trap energy level (E ) with respect to Fermi level (E ) Sample
Cu/In
(S#Se)/ (Cu#In)
x"S/ (S#Se)
L (m)
E (meV)
Carrier Conc. (cm\)
Q (10 cm\)
W (nm)
E !E (meV)
IC(8)6 IC(9)14 IC(9)6 IC(11)3 IC(11)8 IC(11)7 IC(8)4 IC(6)6 IC(6)2
0.72 0.70 0.92 0.91 0.93 0.92 0.83 0.86 0.90
1.04 1.00 0.91 1.04 0.93 0.96 0.95 0.98 1.01
0.98 0.94 0.84 0.72 0.61 0.52 0.34 0.21 0.13
0.91 1.28 1.39 1.42 1.53 1.58 1.62 1.81 2.01
18.8 16.3 35.5 31.5 27.8 25.0 20.4 16.9 15.5
1.00;10 1.82;10 3.21;10 8.82;10 2.34;10 6.05;10 1.67;10 5.58;10 1.84;10
9.2 11.6 2.3 3.7 5.8 8.9 13.8 23.1 40.7
4.6 3.2 36.3 21.0 12.3 7.4 4.1 2.1 1.1
192 188 208 204 200 198 195 189 187
72
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73
4. Conclusions
Fig. 14. Plot ln (¹ ) versus 1/T ( in mho-cm\) for six representative CuIn(S Se ) "lms with di!erent x values V \V [x"S/(S#Se) ]: (a) 䢇 x&0.13; (b) 䊐 x&0.21; (c) 䉱 x&0.34; x&0.52; (e) 䉬 x&0.61; (f ) * x&0.72, (g) 䊏 x&0.84. (d) Inset: Plot of ln (¹\ ) versus 1/T.
for E !(E #E )
(12)
for E !(E #E );k¹. It was observed that the experimental data "tted well with expression (11) as is evident from the plot of ln(¹ ) versus 1/T (Fig. 14). The "t of the data with Eq. (13) was quite poor (inset of Fig. 14). This indicates that the trap states (E ) lie below the Fermi level (E ) and the relative position of E and E may be obtained from the relation: eQ E !E "E !k¹ ln 2 !1 . (13) (8 N E ) The values of E !E obtained from expression (13) are shown in Table 3. The width of the depletion region (=) was evaluated [27] from Eq. (14) and is shown in Table 3.
="
2 E . eN
(14)
CuIn(S Se ) "lms were synthesized by sulV \V phurization followed by selenization of In/Cu stacked elemental layers using GB annealing with elemental sulphur and selenium as sulphurization and selenization agents. The "lm, thus synthesized, showed chalcopyrite structure with preferred orientation along (1 1 2) planes perpendicular to the substrate plane. Linear variation of `ca, `aa and c/a values with increasing sulphur content was observed. The values of the lattice constants and their ratio c/a decreases with increase of sulphur (x). The transmittance increases and sharpness of fall decreases as x value increases from zero to one. All the "lms showed p-type conductivity. The "lms were mainly Cu-poor. Copper vacancy (V ), sul! phur and selenium interstitial (S ,Se ) act as prob able acceptor states in our "lms. PL studies showed an excitonic peak which varied from 1.40 to 1.16 eV as x value decreased. The peak at 0.98 eV could be assigned due to the transition from conduction band to neutral donor. The peak at &0.96 eV arising for lower x value may be identi"ed with exciton bound to ionized acceptor (Cu ) states. ' Grain boundary scattering e!ects dominated the electron transport mechanism in these "lms at lower temperature of measurements. The density of trap states at the grain boundary decreased with the increasing sulphur content in the "lms. Acknowledgements The authors wish to acknowledge with thanks the CSIR, Govt. of India, for supporting this programme. Authors are grateful to Mr. Shekhar Chandra Ghosh for recording the SEM pictures. One of the authors (S.B.) wishes to thank the Department of Atomic Energy, Government of India, for sanctioning him the Senior Research Fellowship. References [1] Contreras MA, Egaas B, Ramanathan K, Hiltner J, Swartzlander A, Hasoon F, Nou" R. Prog Photovolt 1999;7:311.
S. Bandyopadhyaya et al. / Vacuum 62 (2001) 61}73 [2] Green MA, Emery K, Bucher K, King DI, Igari S. Prog Photovolt Res Appl 1999;7:31. [3] Klaer J, Bruns J, Henninger R, Siemer K, Klenk R, Ellmer K, Braunig D. Semicond Sci Technol 1998;13:1456. [4] Hashimoto Y, Takeuchi K, Ito S. Appl Phys Lett 1995;67:980. [5] Walter T, Menner R, Koble Ch, Schock HW, Proceedings of the 12th EC Photovoltaic Solar Energy Conference, Amesterdam, Dordrecht: Kluwer, 1994. p. 1775. [6] Schock HW, Twelth European Photovoltaic Solar Energy Conference, Amsterdam, 1994. p. 87. [7] Ohashi T, Inakoshi K, Hashimoto Y, Ito K. Sol Energy Mater Solar Cells 1998;50:37. [8] Eisener B, Wolf D, Muller G. Thin Solid Films 2000;361}362:126. [9] Herberholz R, Walter T, Schock HW. J Appl Phys 1994;76:2904. [10] Cheng YH, Tseng BH, Loferski JJ, Hwang HL. Ninth International Photovoltaic Science and Engineering Conference, Miyazaki, Japan, 1996, p. 591. [11] Walter T, Ruckh M, Velthaus KO, Schock HW, Eleventh E.C. Photovoltaic Solar Energy Conference, Montreaux, Switzerland, 1993, p. 124. [12] Ohashi T, Inakoshi K, Hashimoto Y, Ito K. Ninth International Photovoltaic Science and Engineering Conference, Miyazaki, Japan, 1996. p. 393.
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