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Copper selenide thin films by chemical bath deposition
Journal of Crystal Growth 203 (1999) 113}124
Copper selenide thin "lms by chemical bath deposition V.M. GarcmH a, P.K. Nair, M.T.S. Nair* Department of Solar Energy Materials, Centro de Investigacio& n en Energn& a, Universidad Nacional Auto& noma de Me& xico, Temixco, Morelos 62580, Mexico Received 1 December 1998; accepted 2 January 1999 Communicated by M. Schieber
Abstract We report the structural, optical, and electrical properties of thin "lms (0.05 to 0.25 lm) of copper selenide obtained from chemical baths using sodium selenosulfate or N,N-dimethylselenourea as a source of selenide ions. X-ray di!raction (XRD) studies on the "lms obtained from baths using sodium selenosulfate suggest a cubic structure as in berzelianite, Cu Se with x"0.15. Annealing the "lms at 4003C in nitrogen leads to a partial conversion of the "lm to Cu Se. In the \V case of "lms obtained from the baths containing dimethylselenourea, the XRD patterns match that of klockmannite, CuSe. Annealing these "lms in nitrogen at 4003C results in loss of selenium, and consequently a composition rich in copper, similar to Cu Se, is reached. Optical absorption in the "lms result from free carrier absorption in the near \V infrared region with absorption coe$cient of &10 cm\. Band-to-band transitions which gives rise to the optical absorption in the visible-ultraviolet region may be interpreted in terms of direct allowed transitions with band gap in the 2.1}2.3 eV range and indirect allowed transitions with band gap 1.2}1.4 eV. All the "lms, as prepared and annealed, show p-type conductivity, in the range of (1}5);10 )\ cm\. This results in high near infrared re#ectance, of 30}80%. 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemical bath deposition; Electronic materials; Copper selenide "lms
1. Introduction Selenides of copper with di!erent compositions, stoichiometric (a-Cu Se, Cu Se , CuSe, and CuSe ) and nonstoichiometric (Cu Se), and structural \V forms are well-documented [1]. The phase diagram of copper}selenium system reported in Ref. [2] shows that the thermal stability of these compounds varies depending on the stoichiometric * Corresponding author. E-mail address: [email protected] (M.T.S. Nair)
form. CuSe is reported as hexagonal at room temperature and undergoes transition to orthorhombic at 483C and back to hexagonal at 1203C [1]. At still higher temperatures, CuSe disproportionates into Cu Se and selenium. A face centered cubic struc\V ture is reported for Cu Se with 0.15)x)0.2 at \V room temperature. These materials are semiconductors with p-type conductivity. Cu Se is reported to possess a di\V rect band gap of 2.2 eV and an indirect band gap of 1.4 eV for x"0.2 [3]. The indirect band gap being near the optimum value for solar cell applications
0022-0248/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 0 4 0 - 8
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[4], the material could o!er a high e$ciency of conversion. For the values of x between 0.1 and 0.3, in vacuum evaporated thin "lms of Cu Se, hole \V mobility of the order of 10 cm V\ s\ and carrier concentrations in the range of 10}10 cm\ are reported in Ref. [3]. The use of these "lms to form a junction with n-type semiconductors either as absorber in heterojunction with CdS or as window material in heterojunction with n-Si has been demonstrated [3,5}7]. The CdS/Cu Se devices are \V reported to show conversion e$ciencies of up to 4.25% with < "457 mV, J "14.4 mA/cm, and FF"0.64 and n-Si/Cu Se showed conversion \V e$ciencies of up to 8.8% with < "450 mV and J "2 mA/cm. Thin "lms of copper selenide can be obtained by a variety of techniques like vaccum evaporation [3,5}7], melting of Cu and Se [8}11], electrodeposition [12,13] and chemical bath deposition [14}19]. The composition of the chemically deposited thin "lms using selenosulfate as the source of selenide ions has been reported as CuSe in Refs. [15,16], and as Cu Se in Ref. [17]. The "lms \V obtained using N,N-dimethylselenourea as the source of selenide ions are reported to be of CuSe composition [18]. There has also been a report that depending on the selenosulfate to copper-ion ratio in the bath either Cu Se or CuSe may be obtained [19]. In this work we used sodium selenosulfate and N,N-dimethylselenourea (DMSU) as sources of Se\ ions, to prepare copper selenide thin "lms. The results of structural, optical and electrical studies on these "lms are discussed. X-ray di!raction studies con"rm that "lms produced from the selenosulfate bath are of Cu Se composition, even \V though the precipitate may have a mixed composition Cu Se /Cu Se. The "lms produced from \V dimethylselenourea bath are CuSe, but upon annealing at 4003C, they become Cu Se. The op\V tical absorption in the "lms is interpreted in terms of free carrier absorption in the near infrared and band-to-band transitions in the visible-ultraviolet region. We consider that clari"cation of the structure, composition, optical and electrical properties and thermal stability of chemically deposited Cu SeV thin "lms is important in the context of its use as
a precursor material for forming CuInSe -thin "lms for thin "lm solar cell application. This application calls for large enough crystallite size, of 2 lm, and a "lm thickness of nearly 2 lm [20]. Current research e!orts are directed towards identifying a low-cost deposition technique for a precursor material from which solar cell grade CuInSe can be produced. In recent results, Cu}In}Ga}Se mixture produced by electrodeposition has been subjected to chemical and thermal processing at 5503C in a four-source physical vapor deposition chamber to produce solar cells with conversion e$ciencies of 13.7% [21]. Here it has been considered that the CuSe phase melts at temperature above 5003C, which along with the elements made available would lead to the formation of ternary and quaternary compounds with improved grain size due to re-crystallization at the elevated temperature. It is therefore apparent that CuSe/Cu Se/CuSe(S) obtained by multiple dip V chemical bath deposition technique along with physical vapor deposition technique and simultaneous or subsequent thermal processing could result in alternative solar cell absorber materials as suggested by us recently [22].
2. Experimental details 2.1. Materials employed The chemicals used in the deposition baths for the present "lms are copper sulfate pentahydrate (CuSO ) 5H O) and 30% NH of Baker analyzed reagent grade, copper chloride dihydrate (CuCl ) 2H O), sodium tartrate (Na C H O ) 2H O), and anhydrous sodium sul "te (Na SO ) of analytical reagent quality from Productos QumH micos Monterrey, sodium selenosulfate and N,N-dimethylselenourea. The latter two compounds were prepared in our laboratory. A solution of sodium selenosulfate was prepared by re#uxing 100 ml of &1 M sodium sul"te with different quantities of selenium powder (Aldrich chemicals) for about 2 h. This solution will contain an excess of sodium sul"te which prevents the oxidation of selenide to selenium. N,N-dimethylselenourea was prepared following the method
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cited in Ref. [18], involving chemical reaction of H Se with dimethylcynamide. Corning glass micro scope glass slides of 76;26;1 mm were used as substrates. The substrates were cleaned well using detergent and water and dried prior to "lm deposition. 2.2. Deposition of the xlms using sodium selenosulfate Chemical baths of two di!erent compositions with respect to the concentration of selenosulfate were used for the deposition of thin "lms of copper selenide. These are labelled as Bath S1 and Bath S2 and are constituted by mixing the following reagents in the given sequence of addition in a 100 ml beaker. Bath S1: 10 ml of 0.5 M CuSO ) 5H O, 1.5 ml of 30% NH (&15 M), 30 ml of a freshly prepared solution of &0.18 M Na SeSO and the rest distil led water to make the volume to 100 ml. Bath S2: 10 ml of 0.5 M CuSO ) 5H O, 1.5 ml of 30% NH (&15 M), 12 ml of freshly prepared &0.4 M Na SeSO solution and the rest distilled water to make up to 100 ml. The substrates were placed vertically immersed in the deposition baths against the wall of the beaker. The deposition was allowed to proceed at room temperature, &253C, for di!erent durations, 2}25 h.
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At the end of each predetermined period of deposition, the coated substrates were taken out of the bath, washed well with distilled water and dried. The "lms reported here are specularly re#ective, deposited on both sides of the substrate. Film on the substrate at the surface that faced the wall of the beaker during the deposition was retained for measurements, and the "lm on the opposite surface was removed with cotton swabs moistened with dilute HCl. The thickness of the "lm was measured using an Alpha step 100 (Tencor instruments, CA). 2.4. Characterization of the xlms X-ray di!raction patterns of the "lms were recorded on a Siemens D500 di!ractometer with CuK radiation. The optical transmittance and near a normal specular re#ectance of the "lms were recorded using a UV-VIS-NIR Shimadzu UV-3101 PC spectrophotometer, for "lm side incidence, with air and a front aluminized mirror as references, respectively. To study the electrical properties, pairs of silver-paint electrodes of 5 mm length at 5 mm separation were printed on the surface of the "lm. A Keithley 230 programmable voltage source and a Keithley 619 electrometer were used for the measurement. The bias applied was 0.01 V.
3. Results and discussion 2.3. Deposition of the xlms using N,N-dimethylselenourea
3.1. Growth of the xlms
The bath (Bath M) was prepared following the procedure given in Ref. [18]. To a volume of 3.5 ml of 0.5 M CuCl ) 2H O in a 100 ml beaker was added with stirring 20 ml of 0.8 M of sodium tartrate, Na C H O ) 2H O. This was followed by the addition of a freshly prepared solution of &0.07 M N,N-dimethylselenourea by dissolving 0.2 g of the compound in 20 ml of 0.01 M Na SO . The volume was made up to 100 ml by adding 57 ml of distilled water. Microscope glass slides mentioned above were used as substrates. The deposition was carried out at 503C for di!erent duration, 2}12 h.
Fig. 1 shows the "lm thickness against duration of deposition of the "lms. In all cases the "lm growth sets in after an initial nucleation period of about 1}2 h. The growth slows down after 4}8 h. Durations of deposition longer than those shown in Fig. 1 result in peeling the "lms from the substrates during rinsing. Thus, it was unable to detect the terminal thickness for each deposition process. The thin "lm formation in chemical bath deposition is described as a process of ion-by-ion condensation at the surface of the substrate [23]. In the present case, copper ions are released into the bath
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The selenide ions are liberated into the baths by the dissociations of sodium selenosulfate and dimethylselenourea according to the following reactions: Baths S1 and S2: SeSO\#2OH\Se\#SO\#H O, and
(4)
Bath M: (CH ) NNH CSe#3H O (CH ) NNH CO#Se\#2H O>. (5) The copper ions and selenide ions, thus formed will condense at the substrate to form thin "lms of CuSe, Cu Se or of intermediate composition. 3.2. Structural characterization of the xlms
Fig. 1. Thickness of copper selenide-thin "lms against durations of deposition in Baths S1 and S2 at room temperature and in Bath M at 503C.
by the dissociation equilibria presented by the complexes of copper: Baths S1 and S2: [Cu(NH ) ]>Cu>#4NH ; and
(1)
Bath M: [Cu(C H O )]Cu>#C H O\. (2) The relative stabilities of Cu(I) and Cu(II) depend strongly on the environment [24]. In presence of excess of sodium sul"te in Baths S1 and S2 which is inherent in the preparation of sodium selenosulfate, there exists the possibility of reduction of Cu(II) to Cu(I) based on the reduction potentials [25]: SO\#H O#2e\SO\#2OH\, EF"!0.93 V, Cu>#e\Cu>, EF"0.153 V.
(3)
3.2.1. Films prepared using sodium selenosulfate (Baths S1 and S2) Figs. 2 and 3 show the XRD patterns of the as prepared and annealed samples of the "lms obtained from Baths S1 and S2. The patterns show well-de"ned peaks suggesting that the "lms are crystalline. A comparison of the observed peaks with the standard JCPDS cards show that the as prepared samples possess a structure matching the mineral berzelianite (JCPDS 6-680), Cu Se with \V x"0.15 which belongs to the cubic system with a"0.5739 nm. The observed peak positions are in agreement with those due to re#ections from (1 1 1), (2 2 0) and (3 1 1) planes of the reported structure. Annealing the samples in air or nitrogen up to 2003C (Fig. 2) does not a!ect the composition or structure. Annealing in nitrogen at 3003C (Fig. 3) does not a!ect the composition, but the orientation of the crystallites is now more randomized than in the as prepared "lm. Annealing in air at 3003C (Fig. 2) results in partial conversion of Cu Se to \V Cu O, cuprite (JCPDS 5-667). The peak at 2h"11.83 in this pattern is not assigned. In the pattern of the "lm annealed in nitrogen at 4003C (Fig. 3), we observe a new peak at 2h"13.0363 (d"6.89 As ) along with broadening of peaks at 2h"26, 45, and 543. With the presence of the additional peak at 2h"13.0363, this pattern matches the standard pattern of stoichiometric
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Fig. 2. XRD patterns of samples of 0.12 lm "lm from Bath S1, as-prepared and after annealing in air for 1 h each at 200 and 3003C.
Fig. 3. XRD patterns of samples of a 0.13 lm thin "lm of copper selenide obtained from Bath S2, before and after annealing for 1 h each at 300 and 4003C in 50 m Torr of nitrogen.
Cu Se called bellidoite (JCPDS 29-575). However, the presence of Cu Se cannot be ruled out since \V the last three peaks are in common with those reported in the standard pattern of berzelianite. This leads to the conclusion that upon annealing in nitrogen at 4003C, a part of Cu Se "lm loses Se \V and becomes stoichiometric Cu Se. The result presented for the as prepared "lms in the present case is in agreement with that reported in Ref. [17]. The chemical bath in the case of Ref. [17] was constituted using copper ion to NH molar ratio of 1 : 20 as compared with 1 : 4.5 in the present work. The quantities of sodium selenosulfate and excess sul"te ions in the bath are comparable in both cases. It is apparent that the reduction of Cu(II) to Cu(I) which leads to the deposition of Cu Se is promoted by the presence of sul"te ion \V in the bath typically in a ratio of Cu(II) to SO\of 1 : 5 (Bath S1), 1 : 1.2 (Bath S2) and 1 : 1.8 [17], following Eq. (3), irrespective of the abundance of
ammonia (aq.). However, we observed that the composition of the precipitate could be di!erent from that of the thin "lm. XRD pattern of the precipitate obtained from Bath S2 is given in Fig. 4. Here all the peaks due to berzelianite and in addition all the peaks corresponding to the standard pattern of the copper-selenide phase known as umangite, Cu Se , JCPDS 19-402 are present with intensity ratios in good agreement with that of the standard pattern. The crystallite grain size in the "lms was calculated using the Scherrer formula [26]: D"0.9j/b cos h. Here, D is the grain diameter, j is the wavelength of the X-ray used (1.5406 As , in the present case), b is the full-width at half-maximum height of the peak, and h is the Bragg angle that corresponds to the peak analyzed. Calculations using the peaks at 2h"26.773 (1 1 1) and 2h"44.73 (2 2 0) for the Cu Se thin "lms of Fig. 2 (as prep., and \V 2003C) and Fig. 3 (3003C, nitrogen annealed)
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Fig. 4. XRD patterns of precipitate of copper selenide obtained from Bath S2.
Fig. 5. XRD patterns of CuSe samples from Bath M, as-prepared and N (50 m Torr) annealed for 1 h at 3003C.
indicate an average grain diameter of 20 nm in the as prepared and 2003C annealed "lms, and 29 nm in the "lm annealed at 3003C. We noted that increase in thickness from 0.06 to 0.20 lm of the "lms does not signi"cantly change the grain size in the as prepared or annealed "lms from the typical values cited above.
klockmannite (JCPDS 34-171) which is hexagonal with composition CuSe. The absence of other peaks with signi"cant relative intensities as in the JCPDS pattern is due to crystallite orientation in the thin "lm. This corroborates the previous result [18] on the XRD analyses of the powder scraped o! the substrate as well as on that precipitated in the bath. Thus, the copper selenide obtained from the bath using dimethylselenourea is hexagonal CuSe. Annealing the "lms in air at 3003C was reported earlier [18] to cause decomposition of CuSe to CuSeO ) CuO. However, we see from Fig. 5 that there is no decomposition up to 3003C when the annealing is done in nitrogen. The grain diameter, calculated from the (1 0 2) peak in the as prepared "lm of 0.12 lm thickness is about 12 nm, and is 19 nm in the sample annealed at 3003C. A "lm of 0.14 lm thickness shows similar results. The calculated grain size for the annealed "lm is 22 nm.
3.2.2. Films prepared using N,N-dimethylselenourea (Bath M) Fig. 5 gives the XRD pattern of the as prepared (0.12 lm) and annealed samples of copper selenide prepared using dimethylselenourea (Bath M). It is seen from the "gure that the "lms, as prepared, are of low crystallinity. Annealing the "lms in nitrogen enhances the crystallinity as evidenced from the increase in peak heights in the patterns of the annealed samples. The observed peaks match those (of intensity 100 and 64%, respectively) due to re#ections from (1 0 2) and (1 1 0) planes of mineral
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3.3. Optical and electrical properties 3.3.1. Free carrier optical absorption The optical transmittance, ¹ (%), and near normal re#ectance, R (%), spectra of the "lms prepared by using Bath S2 are presented in Fig. 7. We note that in the near infrared, the transmittance decreases with increase in the "lm thickness where as the re#ectance increases. This characteristic is typical of "lms with high electrical conductivity, which implies a re#ection coe$cient nearing 100% for "lms of metallic conductivity. The electrical conductivity of the Cu Se "lms reported here is \V about 5;10 () cm)\. Calculation of the re#ection coe$cient, R, may be done using [27]:
R"1!4
Fig. 6. XRD patterns of samples of copper selenide with di!erent thicknesses from Bath M after annealing at 4003C for 1 h in N (50 m Torr).
The XRD patterns of the "lm annealed in nitrogen at 4003C, given in Fig. 6 show that the annealing changes the composition and crystallinity of the "lms. The patterns match the standard pattern reported for Cu Se (JCPDS 26-557), considering an absence of certain peaks due to crystallite orientation. We consider that the match to Cu Se is \V inferior, even though we are not ruling out a mixed composition. The intensity of the observed peaks increases with the increase in thickness of the "lms. This is attributable to the increase in the amount as well as the grain size of the crystallites. The grain diameters in the "lms vary only marginally with thickness: 23 nm for the "lm of 0.094 lm and 25 nm for the "lm of 0.14 lm. The change in composition to copper-rich phase of the CuSe "lms is due to the loss of selenium upon heating. This loss has been detected in XRF studies (not reported here): the Cu : Se ratio reduces from 1 : 1 (assigned to as prepared "lm) to 1.7 : 1 in the annealed "lm.
pce . pj
(6)
Here, c is velocity of light, e is the permittivity of free space, p is the conductivity of the material, and j is the wavelength of the incident radiation. The evaluated values of R (%) are 58% (j"1500 nm) and 67% (j"2500 nm) for the material of these "lms. However, the skin depth, d, of the Cu Se \V "lms for the electromagnetic wave of 2500 nm calculated from the equation [27]:
d"
2j , ckp
(7)
where k is the permeability of the material, taken equal to the permeability of free space, shows that for the reduction of the incident amplitude of the electric "eld to 1/e (e"2.718), a thickness of 0.16 lm of the material is required. For a wavelength of 1500 nm of incident radiation, the value of skin depth is 0.13 lm. In the present case, only the highest "lm thickness (0.15 lm) is comparable with this value of the skin depth. The experimentally observed values of R (%), 54 and 75%, respectively, for wavelengths of 1500 and 2500 nm are close to the theoretically predicted values cited above. Values of R (%) of thinner "lms are considerably less than that of the theoretical value predicted for the bulk material. The above consideration of the interaction of the electromagnetic wave with a conducting medium also suggests that the incident intensity (I ) drops to
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value of a was obtained from the ¹ (%) curve shown in Fig. 7 using the procedure detailed below for the evaluation of absorption coe$cient for band-to-band transitions. Annealing a Cu Se "lm of 0.16 lm thickness \V in nitrogen at 300 and 4003C results in change in the optical transmittance and re#ectance spectra of the "lms. Similarly changes in the optical spectra are also observed in a "lm of thickness 0.12 lm deposited from Bath M. These changes are attributed to structural, compositional and crystallinity changes in the "lm, taking place in the "lms during the annealing as discussed in 3.2. The near infrared re#ectance has been found to decrease with annealing temperature causing a corresponding increase in the near infrared transmittance in both cases. This is associated with a decrease in the electrical conductivity with annealing temperature observed in all the "lms, which according to Eq. (6) would produce such e!ects.
Fig. 7. Optical transmittance ¹ (%), near normal specular re#ectance R (%), and ¹ (%) spectra of Cu Se-thin "lms with \V di!erent thickness obtained from Bath S2.
1/e of the initial value, when the wave traverses a length z"1/a , since I "I e\?X and the absorp X M tion coe$cient (a ) for free carrier absorption is 2/d. The value of a for the incident radiation of wavelength 2500 nm in the Cu Se with electrical \V conductivity of 5;10 () cm)\ is 1.23;10 cm\. The values of this parameter evaluated from the transmittance and re#ectance data of Fig. 7 for "lms of di!erent thickness and wavelength of incident radiation of 2500 nm are 1.3;10 cm\ for the 0.15 lm and 1.9;10 cm\ for the 0.06 lm "lm. The optical properties estimated for the "lms in the near infrared region using the electrical conductivity of the "lms are in good agreement with the experimental values in the case of "lm thickness approaching the calculated value of skin depth. The
3.3.2. Optical absorption due to band-to-band transitions Using the optical spectra, the band gap values for the "lms are estimated. For this, the transmittance spectra have been corrected for the loss due to re#ections: ¹ "100¹ (%)/(100!R%), as shown in Fig. 7. We assume that the predominant component in the re#ected intensity is due to the air-"lm interface. In the ¹ curves, the free carrier absorption discussed above leads to low near infrared transmittance, and hence the typical transmittance spectra of nondegenerate semiconductors are not attained. The absorption coe$cients (a ) for band-to-band transition across the gap at di!erent wavelengths are calculated using the corrected transmittance values: a "(1/t) ln(100/¹ ), where t is the "lm thickness. The direct and indirect band gap values are obtained from plots of a and a , respectively, against the corresponding values of photon energy (hl). Such plots for the "lms deposited from Bath S2, as prepared as well as annealed at 3003C in nitrogen, are given in Fig. 8. Extrapolating the straight line part of the curves in Fig. 8 to the energy-axis, where a"0, gives the values of band gap for direct transitions (E ) and to where a "0, gives the values for indirect transitions (E ). The E varies in the range of 2.38}2.35 eV
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Fig. 8. Plots of a and a against hl for band-to-band transitions of Cu Se-thin "lms deposited from Bath S2, of as-prepared and \V annealed "lms, N at 3003C, with "lm thickness of 0.06, 0.12, 0.15 lm.
(as prepared) and 2.20}2.13 eV (annealed), and the E is in the 1.34}1.22 eV for annealed "lms, de pending on the "lm thickness. In the case of as prepared "lms, the optical absorption may also "t a straight-line region in the a against hl, corresponding to an indirect band gap of nearly 1.9 eV.
This however would not be a correct conclusion because the optical absorption coe$cients in the range of photon energy used are '10 cm\ } typical realm of direct gap absorption. In both cases the optical absorption coe$cient does not drop to zero because of the setting in of free carrier absorption
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before the band-to-band transitions disappear. Hence it is not possible to analyze the low-energy region in the band-to-band absorption corresponding to indirect gaps. The values indicated above for the indirect band gap, are considered to be the upper limit. The values of band gaps in the annealed "lms are close to the value 2.2 and 1.4 eV reported for the direct and indirect band gaps, respectively, of Cu Se, x"0.2, thin "lms obtained by vacuum \V evaporation [3,6]. From Section 3.2 on structural analyses, it is seen that the "lms from Baths S1 and S2 are Cu Se (x"0.15). The larger band gap \V values in the as prepared samples compared with that of the annealed samples are due to the quantum con"nement e!ect [28,29] arising from smaller grain size in the former. The increase in grain size upon annealing the "lms at 3003C in nitrogen is evident from the XRD patterns in Fig. 3. Similarly, the decrease in band gap with increase in "lm thickness is also attributable to the grain size increase with increase in thickness. No single crystal data on band gap of Cu Se is available for com\V parison. The direct and indirect band gaps in a thin "lm (0.13 lm) of Cu Se deposited from Bath S2, \V after annealing at 4003C are 2.17 and 1.16 eV, respectively. The observed indirect band gap is close to the band gap 1}1.1 eV of Cu Se [30]. In the case of "lms prepared from Bath M, the direct energy gap varies from 2.16 to 2.1 eV, as the thickness varies from 0.09 to 0.14 lm, for as-prepared samples. These "lms are CuSe, as concluded from the XRD data. No reliable data on optical band gap of CuSe is available to us in the literature for comparison. Annealing the "lms at 4003C in nitrogen (at 50 mTorr) for 1 h results in a change in composition from CuSe to Cu Se . Analyses of the optical spectral data of such annealed "lms in this work showed the presence of both direct and indirect band gaps, the former in the 2.16}2.1 eV range and the latter in the 1.47}1.24 eV range. Such band gap values are close to those reported for Cu Se "lms [3,6]. \V 3.3.3. Electrical properties Hot probe test has shown that all the "lms reported here, Cu Se, Cu Se, CuSe and Cu Se , \V are of p-type conductivity which is known to arise
from copper de"ciency at any given composition. The electrical conductivity, p, of the as-prepared "lms deposited from Bath S2 is (4.2}4.8); 10 )\ cm\. Annealing does not change this conductivity signi"cantly. Similarly high-electrical conductivity (p"5000 )\ cm\) have been reported in evaporated "lms of Cu Se [5]. Hole \V mobilities k +2.5 cm/V s and carrier concentra tions p+1.2;10 cm\ have been reported for such "lms. By analogy, we assume that the present "lms may possess similar carrier concentration and mobility. The "lms obtained from Bath M (CuSe) are generally less conducting than those obtained from Bath S2 (Cu Se). When these "lms are annealed \V at 3003C in nitrogen, the conductivity decreases in the "lms. This is opposite to what would be expected on the basis of the observed grain size improvement upon annealing at this temperature, indicated by the XRD analyses. In the earlier paper [18] on chemically deposited CuSe, we have reported the e!ect of annealing in air on the electrical properties. The expected increase in conductivity upon annealing was observed in "lms annealed at temperatures up to 2003C, above which the "lms started decomposing. In the present case, where the decomposition of CuSe in atmospheric oxygen was eliminated, the decrease in conductivity is attributed to changes in the composition through loss of selenium upon annealing at 3003C. Such loss has been con"rmed in XRF studies (not reported in this paper). The loss of selenium is more pronounced in the samples annealed at 4003C, as indicated by the change in composition to Cu Se . 4. Conclusions Good-quality thin "lms of copper selenide of composition Cu Se (JCPDS 6-680) and CuSe \V with structures of berzelianite (cubic) and Klockmannite (hexagonal, JCPDS 34-171), respectively, are deposited from chemical baths of di!erent compositions, using sodium selenosulfate and N,Ndimethylselenourea. The grain size in the as-prepared samples of Cu Se are in the range of \V 20}30 nm, which increases by about 30% upon annealing in nitrogen at 3003C. When annealed at
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4003C in 50 mTorr of nitrogen, the "lms tend to become Cu Se. Air annealing results in the de composition of the "lms to Cu O at 3003C. The CuSe "lms are obtained from the dimethylselenourea bath, where the concentration of sodium sul"te is very low. Crystallinity is low in as-prepared samples, but this improves upon annealing in nitrogen at 3003C (50 mTorr). The grain size in the annealed "lms is in the 12}22 nm range. Annealing at 4003C in nitrogen (50 mTorr) results in the conversion of CuSe "lms to Cu Se with grain size in the range of about 25 nm. The "lms possess high-electrical conductivity. We have analyzed the optical absorption due to free carrier absorption and band-to-band transitions, both with absorption coe$cients of &10 cm\. The direct band gap values of about 2.2 eV could be evaluated with out interference from the free carrier absorption. However, the accuracy in the calculation of the indirect band gap is limited. This calculation could be made only in the case of annealed "lms and shows values near 1.2 eV, in agreement with previous reports. We presented in this work that the chemically deposited copper selenide thin "lms, Cu Se are \V stable under nitrogen annealing, except for modi"cation in the crystallinity and slight modi"cation of the composition. In the case of CuSe-thin "lms deposited from the dimethylselenourea, annealing results in signi"cant change in the composition. Temperatures above 4003C could not be used for nitrogen annealing in the present work because of experimental limitation. We consider that at higher temperatures, Cu Se-thin "lms deposited from \V the selenosulfate bath as well as CuSe-thin "lms deposited from the dimethylselenourea bath would be converted to Cu Se. The highly conductive copper selenide-thin "lms reported here may be used as radiation "lters. The application of the "lms as absorber material in solar cells would require further work, "rst to modify the deposition technique to attain higher "lm thickness, and then to develop post deposition processing to reduce the conductivity of the "lms by two to three orders of magnitude. This is required to create the depletion region in the copper selenide absorber in the heterojunction con"guration, so that the band-to-band optical absorption results in
123
charge carrier separation. The grain size also would be required to improve to result in high mobilitylife time product. Further work from our laboratory would address these issues in the thin "lm deposition, post deposition processing, and development of applications of the "lms. The possibility of creating new ternary compounds using the chemically deposited CuSe "lm as a precursor, as mentioned in the introduction, may be by far the most prospective application of this deposition technique.
Acknowledgements The authors are grateful to Leticia Ban os of IIM, UNAM, for recording the XRD patterns and to Oscar Gomezdaza of CIE-UNAM for general assistance in the work. We acknowledge the "nancial support given from CONACyT (MeH xico) and DGAPA (UNAM IN502594 and IN500997, MeH xico). One of us (V.M. GarcmH a) is grateful to the Universidad AutoH noma de Zacatecas for the "nancial support.
References [1] A.L.N. Stevels, F. Jellinek, Recueil 111 (1971) 273. [2] R.D. Heyding, Can. J. Chem. 44 (1966) 1233. [3] A.M. Hermann, L. Fabick, J. Crystal Growth 61 (1983) 658. [4] J.J. Loferski, J. Appl. Phys. 27 (1956) 777. [5] H. Okimura, T. Matsumae, R. Makabe, Thin Solid Films 71 (1980) 53. [6] K.L. Chopra, S.R. Das, Thin Film Solar Cells, Plenum Press, New York, 1983. [7] L.F. Buldhaupt, R.A. Mickelson, J.M. Stewart, W.S. Chen, Emerging materials systems for solar cell applications !Cu Se, Boeing Aerospace Company, Final Report \V DE-AC04-79Et-23005, April 1980, as cited in K.L. Chopra, S.R. Das, Thin Film Solar Cells, Plenum Press, New York, 1983. [8] A. Tonejc, Z. Ogorelec, B. Mestnik, Appl. Crystallogr. 8 (1975) 375. [9] T. Sakuma, K. Shibata, J. Phys. Soc. Japan 58 (9) (1989) 3061. [10] K. Yakamoto, S. Kashida, J. Solid State Chem. 93 (1991) 202. [11] M.A. Korzhuev, V.F. Bankina, I.G. Korolkova, G.B. Sheina, E.A. Obraztsova, Phys. Stat. Sol. (A) 123 (1991) 131.
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[12] S. Massaccesi, S. Sanchez, J. Vedel, J. Electrochem. Soc. 140 (1993) 2540. [13] S.K. Haram, K.S.V. Santhanam, Thin Solid Films 238 (1994) 21. [14] I. Grozdanov, Synth. Met. 63 (1994) 213. [15] A. Mondal, P. Pramanik, J. Solid State Chem. 47 (1983) 81. [16] A. Mondal, P. Pramanik, J. Solid State Chem. 55 (1984) 116. [17] G.K. Padam, Thin Solid Films 150 (1987) L89. [18] C.A. Estrada, P.K. Nair, M.T.S. Nair, R.A. Zingaro, E.A. Meyers, J. Electrochem. Soc. 141 (1994) 802. [19] I. Grozdanov, Semicond. Sci. Technol. 9 (1994) 1234. [20] T.R. Tuttle, M.A. Contreras, J.S. Ward, A.L. Tennant, K.R. Ramanathan, J. Keane, R. Nou", Proc. SPIE 2531 (1995) 194. [21] R.N. Bhattacharya, J.E. Granata, W. Batchelor, F. Hasoon, H. Wiesner, K.R. Ramanathan, J. Keane, R. Nou", Proc. SPIE 2531 (1997) 90. [22] P.K. Nair, M.T.S. Nair, V.M. GarcmH a, O.L. Arenas, Y. Pen a, A. Castillo, I.T. Ayala, O. Gomezdaza, A. SaH nchez, J. Campos, H. Hu, R. SuaH rez, M.E. RincoH n, Sol. Energy Mater. Sol. Cells 52 (1998) 313.
[23] K.L. Chopra, R.C. Kainthla, D.K. Pandya, A.P. Thakoor, in: G. Hass, et al., (Eds.), Physics of Thin Films, Vol. 12, Academic Press, New York, 1982, p. 201. [24] A.F. Trotman-Dickenson (Executive Ed.), Comprehensive Inorganic Chemistry, Vol. 3, Pergamon Press, Oxford, 1973, p. 16. [25] G.M. Barrow, Physical Chemistry, 3rd ed., McGraw-Hill, Tokyo, 1973, p. 668. [26] C.S. Barret, T.B. Massalski, Structure of Metals, McGrawHill, New York, 1966, p. 155. [27] H.J. Pain, The Physics of Vibrations and Waves, 2nd ed., Wiley, Bristol, 1979. [28] G. Hodes, A. Albu-Yayor, F. Decker, P. Motisuke, Phys. Rev. B 36 (8) (1987) 4215. [29] V.M. GarcmH a, M.T.S. Nair, P.K. Nair, R.A. Zingaro, Semicond. Sci. Technol. 12 (1997) 645. [30] O. Madelung, Semiconductors, Other than Group IV Elements and III}V Compounds, Springer, Berlin, 1992, p. 51.
Copper selenide thin "lms by chemical bath deposition V.M. GarcmH a, P.K. Nair, M.T.S. Nair* Department of Solar Energy Materials, Centro de Investigacio& n en Energn& a, Universidad Nacional Auto& noma de Me& xico, Temixco, Morelos 62580, Mexico Received 1 December 1998; accepted 2 January 1999 Communicated by M. Schieber
Abstract We report the structural, optical, and electrical properties of thin "lms (0.05 to 0.25 lm) of copper selenide obtained from chemical baths using sodium selenosulfate or N,N-dimethylselenourea as a source of selenide ions. X-ray di!raction (XRD) studies on the "lms obtained from baths using sodium selenosulfate suggest a cubic structure as in berzelianite, Cu Se with x"0.15. Annealing the "lms at 4003C in nitrogen leads to a partial conversion of the "lm to Cu Se. In the \V case of "lms obtained from the baths containing dimethylselenourea, the XRD patterns match that of klockmannite, CuSe. Annealing these "lms in nitrogen at 4003C results in loss of selenium, and consequently a composition rich in copper, similar to Cu Se, is reached. Optical absorption in the "lms result from free carrier absorption in the near \V infrared region with absorption coe$cient of &10 cm\. Band-to-band transitions which gives rise to the optical absorption in the visible-ultraviolet region may be interpreted in terms of direct allowed transitions with band gap in the 2.1}2.3 eV range and indirect allowed transitions with band gap 1.2}1.4 eV. All the "lms, as prepared and annealed, show p-type conductivity, in the range of (1}5);10 )\ cm\. This results in high near infrared re#ectance, of 30}80%. 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemical bath deposition; Electronic materials; Copper selenide "lms
1. Introduction Selenides of copper with di!erent compositions, stoichiometric (a-Cu Se, Cu Se , CuSe, and CuSe ) and nonstoichiometric (Cu Se), and structural \V forms are well-documented [1]. The phase diagram of copper}selenium system reported in Ref. [2] shows that the thermal stability of these compounds varies depending on the stoichiometric * Corresponding author. E-mail address: [email protected] (M.T.S. Nair)
form. CuSe is reported as hexagonal at room temperature and undergoes transition to orthorhombic at 483C and back to hexagonal at 1203C [1]. At still higher temperatures, CuSe disproportionates into Cu Se and selenium. A face centered cubic struc\V ture is reported for Cu Se with 0.15)x)0.2 at \V room temperature. These materials are semiconductors with p-type conductivity. Cu Se is reported to possess a di\V rect band gap of 2.2 eV and an indirect band gap of 1.4 eV for x"0.2 [3]. The indirect band gap being near the optimum value for solar cell applications
0022-0248/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 0 4 0 - 8
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[4], the material could o!er a high e$ciency of conversion. For the values of x between 0.1 and 0.3, in vacuum evaporated thin "lms of Cu Se, hole \V mobility of the order of 10 cm V\ s\ and carrier concentrations in the range of 10}10 cm\ are reported in Ref. [3]. The use of these "lms to form a junction with n-type semiconductors either as absorber in heterojunction with CdS or as window material in heterojunction with n-Si has been demonstrated [3,5}7]. The CdS/Cu Se devices are \V reported to show conversion e$ciencies of up to 4.25% with < "457 mV, J "14.4 mA/cm, and FF"0.64 and n-Si/Cu Se showed conversion \V e$ciencies of up to 8.8% with < "450 mV and J "2 mA/cm. Thin "lms of copper selenide can be obtained by a variety of techniques like vaccum evaporation [3,5}7], melting of Cu and Se [8}11], electrodeposition [12,13] and chemical bath deposition [14}19]. The composition of the chemically deposited thin "lms using selenosulfate as the source of selenide ions has been reported as CuSe in Refs. [15,16], and as Cu Se in Ref. [17]. The "lms \V obtained using N,N-dimethylselenourea as the source of selenide ions are reported to be of CuSe composition [18]. There has also been a report that depending on the selenosulfate to copper-ion ratio in the bath either Cu Se or CuSe may be obtained [19]. In this work we used sodium selenosulfate and N,N-dimethylselenourea (DMSU) as sources of Se\ ions, to prepare copper selenide thin "lms. The results of structural, optical and electrical studies on these "lms are discussed. X-ray di!raction studies con"rm that "lms produced from the selenosulfate bath are of Cu Se composition, even \V though the precipitate may have a mixed composition Cu Se /Cu Se. The "lms produced from \V dimethylselenourea bath are CuSe, but upon annealing at 4003C, they become Cu Se. The op\V tical absorption in the "lms is interpreted in terms of free carrier absorption in the near infrared and band-to-band transitions in the visible-ultraviolet region. We consider that clari"cation of the structure, composition, optical and electrical properties and thermal stability of chemically deposited Cu SeV thin "lms is important in the context of its use as
a precursor material for forming CuInSe -thin "lms for thin "lm solar cell application. This application calls for large enough crystallite size, of 2 lm, and a "lm thickness of nearly 2 lm [20]. Current research e!orts are directed towards identifying a low-cost deposition technique for a precursor material from which solar cell grade CuInSe can be produced. In recent results, Cu}In}Ga}Se mixture produced by electrodeposition has been subjected to chemical and thermal processing at 5503C in a four-source physical vapor deposition chamber to produce solar cells with conversion e$ciencies of 13.7% [21]. Here it has been considered that the CuSe phase melts at temperature above 5003C, which along with the elements made available would lead to the formation of ternary and quaternary compounds with improved grain size due to re-crystallization at the elevated temperature. It is therefore apparent that CuSe/Cu Se/CuSe(S) obtained by multiple dip V chemical bath deposition technique along with physical vapor deposition technique and simultaneous or subsequent thermal processing could result in alternative solar cell absorber materials as suggested by us recently [22].
2. Experimental details 2.1. Materials employed The chemicals used in the deposition baths for the present "lms are copper sulfate pentahydrate (CuSO ) 5H O) and 30% NH of Baker analyzed reagent grade, copper chloride dihydrate (CuCl ) 2H O), sodium tartrate (Na C H O ) 2H O), and anhydrous sodium sul "te (Na SO ) of analytical reagent quality from Productos QumH micos Monterrey, sodium selenosulfate and N,N-dimethylselenourea. The latter two compounds were prepared in our laboratory. A solution of sodium selenosulfate was prepared by re#uxing 100 ml of &1 M sodium sul"te with different quantities of selenium powder (Aldrich chemicals) for about 2 h. This solution will contain an excess of sodium sul"te which prevents the oxidation of selenide to selenium. N,N-dimethylselenourea was prepared following the method
V.M. Garcn& a et al. / Journal of Crystal Growth 203 (1999) 113}124
cited in Ref. [18], involving chemical reaction of H Se with dimethylcynamide. Corning glass micro scope glass slides of 76;26;1 mm were used as substrates. The substrates were cleaned well using detergent and water and dried prior to "lm deposition. 2.2. Deposition of the xlms using sodium selenosulfate Chemical baths of two di!erent compositions with respect to the concentration of selenosulfate were used for the deposition of thin "lms of copper selenide. These are labelled as Bath S1 and Bath S2 and are constituted by mixing the following reagents in the given sequence of addition in a 100 ml beaker. Bath S1: 10 ml of 0.5 M CuSO ) 5H O, 1.5 ml of 30% NH (&15 M), 30 ml of a freshly prepared solution of &0.18 M Na SeSO and the rest distil led water to make the volume to 100 ml. Bath S2: 10 ml of 0.5 M CuSO ) 5H O, 1.5 ml of 30% NH (&15 M), 12 ml of freshly prepared &0.4 M Na SeSO solution and the rest distilled water to make up to 100 ml. The substrates were placed vertically immersed in the deposition baths against the wall of the beaker. The deposition was allowed to proceed at room temperature, &253C, for di!erent durations, 2}25 h.
115
At the end of each predetermined period of deposition, the coated substrates were taken out of the bath, washed well with distilled water and dried. The "lms reported here are specularly re#ective, deposited on both sides of the substrate. Film on the substrate at the surface that faced the wall of the beaker during the deposition was retained for measurements, and the "lm on the opposite surface was removed with cotton swabs moistened with dilute HCl. The thickness of the "lm was measured using an Alpha step 100 (Tencor instruments, CA). 2.4. Characterization of the xlms X-ray di!raction patterns of the "lms were recorded on a Siemens D500 di!ractometer with CuK radiation. The optical transmittance and near a normal specular re#ectance of the "lms were recorded using a UV-VIS-NIR Shimadzu UV-3101 PC spectrophotometer, for "lm side incidence, with air and a front aluminized mirror as references, respectively. To study the electrical properties, pairs of silver-paint electrodes of 5 mm length at 5 mm separation were printed on the surface of the "lm. A Keithley 230 programmable voltage source and a Keithley 619 electrometer were used for the measurement. The bias applied was 0.01 V.
3. Results and discussion 2.3. Deposition of the xlms using N,N-dimethylselenourea
3.1. Growth of the xlms
The bath (Bath M) was prepared following the procedure given in Ref. [18]. To a volume of 3.5 ml of 0.5 M CuCl ) 2H O in a 100 ml beaker was added with stirring 20 ml of 0.8 M of sodium tartrate, Na C H O ) 2H O. This was followed by the addition of a freshly prepared solution of &0.07 M N,N-dimethylselenourea by dissolving 0.2 g of the compound in 20 ml of 0.01 M Na SO . The volume was made up to 100 ml by adding 57 ml of distilled water. Microscope glass slides mentioned above were used as substrates. The deposition was carried out at 503C for di!erent duration, 2}12 h.
Fig. 1 shows the "lm thickness against duration of deposition of the "lms. In all cases the "lm growth sets in after an initial nucleation period of about 1}2 h. The growth slows down after 4}8 h. Durations of deposition longer than those shown in Fig. 1 result in peeling the "lms from the substrates during rinsing. Thus, it was unable to detect the terminal thickness for each deposition process. The thin "lm formation in chemical bath deposition is described as a process of ion-by-ion condensation at the surface of the substrate [23]. In the present case, copper ions are released into the bath
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The selenide ions are liberated into the baths by the dissociations of sodium selenosulfate and dimethylselenourea according to the following reactions: Baths S1 and S2: SeSO\#2OH\Se\#SO\#H O, and
(4)
Bath M: (CH ) NNH CSe#3H O (CH ) NNH CO#Se\#2H O>. (5) The copper ions and selenide ions, thus formed will condense at the substrate to form thin "lms of CuSe, Cu Se or of intermediate composition. 3.2. Structural characterization of the xlms
Fig. 1. Thickness of copper selenide-thin "lms against durations of deposition in Baths S1 and S2 at room temperature and in Bath M at 503C.
by the dissociation equilibria presented by the complexes of copper: Baths S1 and S2: [Cu(NH ) ]>Cu>#4NH ; and
(1)
Bath M: [Cu(C H O )]Cu>#C H O\. (2) The relative stabilities of Cu(I) and Cu(II) depend strongly on the environment [24]. In presence of excess of sodium sul"te in Baths S1 and S2 which is inherent in the preparation of sodium selenosulfate, there exists the possibility of reduction of Cu(II) to Cu(I) based on the reduction potentials [25]: SO\#H O#2e\SO\#2OH\, EF"!0.93 V, Cu>#e\Cu>, EF"0.153 V.
(3)
3.2.1. Films prepared using sodium selenosulfate (Baths S1 and S2) Figs. 2 and 3 show the XRD patterns of the as prepared and annealed samples of the "lms obtained from Baths S1 and S2. The patterns show well-de"ned peaks suggesting that the "lms are crystalline. A comparison of the observed peaks with the standard JCPDS cards show that the as prepared samples possess a structure matching the mineral berzelianite (JCPDS 6-680), Cu Se with \V x"0.15 which belongs to the cubic system with a"0.5739 nm. The observed peak positions are in agreement with those due to re#ections from (1 1 1), (2 2 0) and (3 1 1) planes of the reported structure. Annealing the samples in air or nitrogen up to 2003C (Fig. 2) does not a!ect the composition or structure. Annealing in nitrogen at 3003C (Fig. 3) does not a!ect the composition, but the orientation of the crystallites is now more randomized than in the as prepared "lm. Annealing in air at 3003C (Fig. 2) results in partial conversion of Cu Se to \V Cu O, cuprite (JCPDS 5-667). The peak at 2h"11.83 in this pattern is not assigned. In the pattern of the "lm annealed in nitrogen at 4003C (Fig. 3), we observe a new peak at 2h"13.0363 (d"6.89 As ) along with broadening of peaks at 2h"26, 45, and 543. With the presence of the additional peak at 2h"13.0363, this pattern matches the standard pattern of stoichiometric
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117
Fig. 2. XRD patterns of samples of 0.12 lm "lm from Bath S1, as-prepared and after annealing in air for 1 h each at 200 and 3003C.
Fig. 3. XRD patterns of samples of a 0.13 lm thin "lm of copper selenide obtained from Bath S2, before and after annealing for 1 h each at 300 and 4003C in 50 m Torr of nitrogen.
Cu Se called bellidoite (JCPDS 29-575). However, the presence of Cu Se cannot be ruled out since \V the last three peaks are in common with those reported in the standard pattern of berzelianite. This leads to the conclusion that upon annealing in nitrogen at 4003C, a part of Cu Se "lm loses Se \V and becomes stoichiometric Cu Se. The result presented for the as prepared "lms in the present case is in agreement with that reported in Ref. [17]. The chemical bath in the case of Ref. [17] was constituted using copper ion to NH molar ratio of 1 : 20 as compared with 1 : 4.5 in the present work. The quantities of sodium selenosulfate and excess sul"te ions in the bath are comparable in both cases. It is apparent that the reduction of Cu(II) to Cu(I) which leads to the deposition of Cu Se is promoted by the presence of sul"te ion \V in the bath typically in a ratio of Cu(II) to SO\of 1 : 5 (Bath S1), 1 : 1.2 (Bath S2) and 1 : 1.8 [17], following Eq. (3), irrespective of the abundance of
ammonia (aq.). However, we observed that the composition of the precipitate could be di!erent from that of the thin "lm. XRD pattern of the precipitate obtained from Bath S2 is given in Fig. 4. Here all the peaks due to berzelianite and in addition all the peaks corresponding to the standard pattern of the copper-selenide phase known as umangite, Cu Se , JCPDS 19-402 are present with intensity ratios in good agreement with that of the standard pattern. The crystallite grain size in the "lms was calculated using the Scherrer formula [26]: D"0.9j/b cos h. Here, D is the grain diameter, j is the wavelength of the X-ray used (1.5406 As , in the present case), b is the full-width at half-maximum height of the peak, and h is the Bragg angle that corresponds to the peak analyzed. Calculations using the peaks at 2h"26.773 (1 1 1) and 2h"44.73 (2 2 0) for the Cu Se thin "lms of Fig. 2 (as prep., and \V 2003C) and Fig. 3 (3003C, nitrogen annealed)
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Fig. 4. XRD patterns of precipitate of copper selenide obtained from Bath S2.
Fig. 5. XRD patterns of CuSe samples from Bath M, as-prepared and N (50 m Torr) annealed for 1 h at 3003C.
indicate an average grain diameter of 20 nm in the as prepared and 2003C annealed "lms, and 29 nm in the "lm annealed at 3003C. We noted that increase in thickness from 0.06 to 0.20 lm of the "lms does not signi"cantly change the grain size in the as prepared or annealed "lms from the typical values cited above.
klockmannite (JCPDS 34-171) which is hexagonal with composition CuSe. The absence of other peaks with signi"cant relative intensities as in the JCPDS pattern is due to crystallite orientation in the thin "lm. This corroborates the previous result [18] on the XRD analyses of the powder scraped o! the substrate as well as on that precipitated in the bath. Thus, the copper selenide obtained from the bath using dimethylselenourea is hexagonal CuSe. Annealing the "lms in air at 3003C was reported earlier [18] to cause decomposition of CuSe to CuSeO ) CuO. However, we see from Fig. 5 that there is no decomposition up to 3003C when the annealing is done in nitrogen. The grain diameter, calculated from the (1 0 2) peak in the as prepared "lm of 0.12 lm thickness is about 12 nm, and is 19 nm in the sample annealed at 3003C. A "lm of 0.14 lm thickness shows similar results. The calculated grain size for the annealed "lm is 22 nm.
3.2.2. Films prepared using N,N-dimethylselenourea (Bath M) Fig. 5 gives the XRD pattern of the as prepared (0.12 lm) and annealed samples of copper selenide prepared using dimethylselenourea (Bath M). It is seen from the "gure that the "lms, as prepared, are of low crystallinity. Annealing the "lms in nitrogen enhances the crystallinity as evidenced from the increase in peak heights in the patterns of the annealed samples. The observed peaks match those (of intensity 100 and 64%, respectively) due to re#ections from (1 0 2) and (1 1 0) planes of mineral
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119
3.3. Optical and electrical properties 3.3.1. Free carrier optical absorption The optical transmittance, ¹ (%), and near normal re#ectance, R (%), spectra of the "lms prepared by using Bath S2 are presented in Fig. 7. We note that in the near infrared, the transmittance decreases with increase in the "lm thickness where as the re#ectance increases. This characteristic is typical of "lms with high electrical conductivity, which implies a re#ection coe$cient nearing 100% for "lms of metallic conductivity. The electrical conductivity of the Cu Se "lms reported here is \V about 5;10 () cm)\. Calculation of the re#ection coe$cient, R, may be done using [27]:
R"1!4
Fig. 6. XRD patterns of samples of copper selenide with di!erent thicknesses from Bath M after annealing at 4003C for 1 h in N (50 m Torr).
The XRD patterns of the "lm annealed in nitrogen at 4003C, given in Fig. 6 show that the annealing changes the composition and crystallinity of the "lms. The patterns match the standard pattern reported for Cu Se (JCPDS 26-557), considering an absence of certain peaks due to crystallite orientation. We consider that the match to Cu Se is \V inferior, even though we are not ruling out a mixed composition. The intensity of the observed peaks increases with the increase in thickness of the "lms. This is attributable to the increase in the amount as well as the grain size of the crystallites. The grain diameters in the "lms vary only marginally with thickness: 23 nm for the "lm of 0.094 lm and 25 nm for the "lm of 0.14 lm. The change in composition to copper-rich phase of the CuSe "lms is due to the loss of selenium upon heating. This loss has been detected in XRF studies (not reported here): the Cu : Se ratio reduces from 1 : 1 (assigned to as prepared "lm) to 1.7 : 1 in the annealed "lm.
pce . pj
(6)
Here, c is velocity of light, e is the permittivity of free space, p is the conductivity of the material, and j is the wavelength of the incident radiation. The evaluated values of R (%) are 58% (j"1500 nm) and 67% (j"2500 nm) for the material of these "lms. However, the skin depth, d, of the Cu Se \V "lms for the electromagnetic wave of 2500 nm calculated from the equation [27]:
d"
2j , ckp
(7)
where k is the permeability of the material, taken equal to the permeability of free space, shows that for the reduction of the incident amplitude of the electric "eld to 1/e (e"2.718), a thickness of 0.16 lm of the material is required. For a wavelength of 1500 nm of incident radiation, the value of skin depth is 0.13 lm. In the present case, only the highest "lm thickness (0.15 lm) is comparable with this value of the skin depth. The experimentally observed values of R (%), 54 and 75%, respectively, for wavelengths of 1500 and 2500 nm are close to the theoretically predicted values cited above. Values of R (%) of thinner "lms are considerably less than that of the theoretical value predicted for the bulk material. The above consideration of the interaction of the electromagnetic wave with a conducting medium also suggests that the incident intensity (I ) drops to
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value of a was obtained from the ¹ (%) curve shown in Fig. 7 using the procedure detailed below for the evaluation of absorption coe$cient for band-to-band transitions. Annealing a Cu Se "lm of 0.16 lm thickness \V in nitrogen at 300 and 4003C results in change in the optical transmittance and re#ectance spectra of the "lms. Similarly changes in the optical spectra are also observed in a "lm of thickness 0.12 lm deposited from Bath M. These changes are attributed to structural, compositional and crystallinity changes in the "lm, taking place in the "lms during the annealing as discussed in 3.2. The near infrared re#ectance has been found to decrease with annealing temperature causing a corresponding increase in the near infrared transmittance in both cases. This is associated with a decrease in the electrical conductivity with annealing temperature observed in all the "lms, which according to Eq. (6) would produce such e!ects.
Fig. 7. Optical transmittance ¹ (%), near normal specular re#ectance R (%), and ¹ (%) spectra of Cu Se-thin "lms with \V di!erent thickness obtained from Bath S2.
1/e of the initial value, when the wave traverses a length z"1/a , since I "I e\?X and the absorp X M tion coe$cient (a ) for free carrier absorption is 2/d. The value of a for the incident radiation of wavelength 2500 nm in the Cu Se with electrical \V conductivity of 5;10 () cm)\ is 1.23;10 cm\. The values of this parameter evaluated from the transmittance and re#ectance data of Fig. 7 for "lms of di!erent thickness and wavelength of incident radiation of 2500 nm are 1.3;10 cm\ for the 0.15 lm and 1.9;10 cm\ for the 0.06 lm "lm. The optical properties estimated for the "lms in the near infrared region using the electrical conductivity of the "lms are in good agreement with the experimental values in the case of "lm thickness approaching the calculated value of skin depth. The
3.3.2. Optical absorption due to band-to-band transitions Using the optical spectra, the band gap values for the "lms are estimated. For this, the transmittance spectra have been corrected for the loss due to re#ections: ¹ "100¹ (%)/(100!R%), as shown in Fig. 7. We assume that the predominant component in the re#ected intensity is due to the air-"lm interface. In the ¹ curves, the free carrier absorption discussed above leads to low near infrared transmittance, and hence the typical transmittance spectra of nondegenerate semiconductors are not attained. The absorption coe$cients (a ) for band-to-band transition across the gap at di!erent wavelengths are calculated using the corrected transmittance values: a "(1/t) ln(100/¹ ), where t is the "lm thickness. The direct and indirect band gap values are obtained from plots of a and a , respectively, against the corresponding values of photon energy (hl). Such plots for the "lms deposited from Bath S2, as prepared as well as annealed at 3003C in nitrogen, are given in Fig. 8. Extrapolating the straight line part of the curves in Fig. 8 to the energy-axis, where a"0, gives the values of band gap for direct transitions (E ) and to where a "0, gives the values for indirect transitions (E ). The E varies in the range of 2.38}2.35 eV
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Fig. 8. Plots of a and a against hl for band-to-band transitions of Cu Se-thin "lms deposited from Bath S2, of as-prepared and \V annealed "lms, N at 3003C, with "lm thickness of 0.06, 0.12, 0.15 lm.
(as prepared) and 2.20}2.13 eV (annealed), and the E is in the 1.34}1.22 eV for annealed "lms, de pending on the "lm thickness. In the case of as prepared "lms, the optical absorption may also "t a straight-line region in the a against hl, corresponding to an indirect band gap of nearly 1.9 eV.
This however would not be a correct conclusion because the optical absorption coe$cients in the range of photon energy used are '10 cm\ } typical realm of direct gap absorption. In both cases the optical absorption coe$cient does not drop to zero because of the setting in of free carrier absorption
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before the band-to-band transitions disappear. Hence it is not possible to analyze the low-energy region in the band-to-band absorption corresponding to indirect gaps. The values indicated above for the indirect band gap, are considered to be the upper limit. The values of band gaps in the annealed "lms are close to the value 2.2 and 1.4 eV reported for the direct and indirect band gaps, respectively, of Cu Se, x"0.2, thin "lms obtained by vacuum \V evaporation [3,6]. From Section 3.2 on structural analyses, it is seen that the "lms from Baths S1 and S2 are Cu Se (x"0.15). The larger band gap \V values in the as prepared samples compared with that of the annealed samples are due to the quantum con"nement e!ect [28,29] arising from smaller grain size in the former. The increase in grain size upon annealing the "lms at 3003C in nitrogen is evident from the XRD patterns in Fig. 3. Similarly, the decrease in band gap with increase in "lm thickness is also attributable to the grain size increase with increase in thickness. No single crystal data on band gap of Cu Se is available for com\V parison. The direct and indirect band gaps in a thin "lm (0.13 lm) of Cu Se deposited from Bath S2, \V after annealing at 4003C are 2.17 and 1.16 eV, respectively. The observed indirect band gap is close to the band gap 1}1.1 eV of Cu Se [30]. In the case of "lms prepared from Bath M, the direct energy gap varies from 2.16 to 2.1 eV, as the thickness varies from 0.09 to 0.14 lm, for as-prepared samples. These "lms are CuSe, as concluded from the XRD data. No reliable data on optical band gap of CuSe is available to us in the literature for comparison. Annealing the "lms at 4003C in nitrogen (at 50 mTorr) for 1 h results in a change in composition from CuSe to Cu Se . Analyses of the optical spectral data of such annealed "lms in this work showed the presence of both direct and indirect band gaps, the former in the 2.16}2.1 eV range and the latter in the 1.47}1.24 eV range. Such band gap values are close to those reported for Cu Se "lms [3,6]. \V 3.3.3. Electrical properties Hot probe test has shown that all the "lms reported here, Cu Se, Cu Se, CuSe and Cu Se , \V are of p-type conductivity which is known to arise
from copper de"ciency at any given composition. The electrical conductivity, p, of the as-prepared "lms deposited from Bath S2 is (4.2}4.8); 10 )\ cm\. Annealing does not change this conductivity signi"cantly. Similarly high-electrical conductivity (p"5000 )\ cm\) have been reported in evaporated "lms of Cu Se [5]. Hole \V mobilities k +2.5 cm/V s and carrier concentra tions p+1.2;10 cm\ have been reported for such "lms. By analogy, we assume that the present "lms may possess similar carrier concentration and mobility. The "lms obtained from Bath M (CuSe) are generally less conducting than those obtained from Bath S2 (Cu Se). When these "lms are annealed \V at 3003C in nitrogen, the conductivity decreases in the "lms. This is opposite to what would be expected on the basis of the observed grain size improvement upon annealing at this temperature, indicated by the XRD analyses. In the earlier paper [18] on chemically deposited CuSe, we have reported the e!ect of annealing in air on the electrical properties. The expected increase in conductivity upon annealing was observed in "lms annealed at temperatures up to 2003C, above which the "lms started decomposing. In the present case, where the decomposition of CuSe in atmospheric oxygen was eliminated, the decrease in conductivity is attributed to changes in the composition through loss of selenium upon annealing at 3003C. Such loss has been con"rmed in XRF studies (not reported in this paper). The loss of selenium is more pronounced in the samples annealed at 4003C, as indicated by the change in composition to Cu Se . 4. Conclusions Good-quality thin "lms of copper selenide of composition Cu Se (JCPDS 6-680) and CuSe \V with structures of berzelianite (cubic) and Klockmannite (hexagonal, JCPDS 34-171), respectively, are deposited from chemical baths of di!erent compositions, using sodium selenosulfate and N,Ndimethylselenourea. The grain size in the as-prepared samples of Cu Se are in the range of \V 20}30 nm, which increases by about 30% upon annealing in nitrogen at 3003C. When annealed at
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4003C in 50 mTorr of nitrogen, the "lms tend to become Cu Se. Air annealing results in the de composition of the "lms to Cu O at 3003C. The CuSe "lms are obtained from the dimethylselenourea bath, where the concentration of sodium sul"te is very low. Crystallinity is low in as-prepared samples, but this improves upon annealing in nitrogen at 3003C (50 mTorr). The grain size in the annealed "lms is in the 12}22 nm range. Annealing at 4003C in nitrogen (50 mTorr) results in the conversion of CuSe "lms to Cu Se with grain size in the range of about 25 nm. The "lms possess high-electrical conductivity. We have analyzed the optical absorption due to free carrier absorption and band-to-band transitions, both with absorption coe$cients of &10 cm\. The direct band gap values of about 2.2 eV could be evaluated with out interference from the free carrier absorption. However, the accuracy in the calculation of the indirect band gap is limited. This calculation could be made only in the case of annealed "lms and shows values near 1.2 eV, in agreement with previous reports. We presented in this work that the chemically deposited copper selenide thin "lms, Cu Se are \V stable under nitrogen annealing, except for modi"cation in the crystallinity and slight modi"cation of the composition. In the case of CuSe-thin "lms deposited from the dimethylselenourea, annealing results in signi"cant change in the composition. Temperatures above 4003C could not be used for nitrogen annealing in the present work because of experimental limitation. We consider that at higher temperatures, Cu Se-thin "lms deposited from \V the selenosulfate bath as well as CuSe-thin "lms deposited from the dimethylselenourea bath would be converted to Cu Se. The highly conductive copper selenide-thin "lms reported here may be used as radiation "lters. The application of the "lms as absorber material in solar cells would require further work, "rst to modify the deposition technique to attain higher "lm thickness, and then to develop post deposition processing to reduce the conductivity of the "lms by two to three orders of magnitude. This is required to create the depletion region in the copper selenide absorber in the heterojunction con"guration, so that the band-to-band optical absorption results in
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charge carrier separation. The grain size also would be required to improve to result in high mobilitylife time product. Further work from our laboratory would address these issues in the thin "lm deposition, post deposition processing, and development of applications of the "lms. The possibility of creating new ternary compounds using the chemically deposited CuSe "lm as a precursor, as mentioned in the introduction, may be by far the most prospective application of this deposition technique.
Acknowledgements The authors are grateful to Leticia Ban os of IIM, UNAM, for recording the XRD patterns and to Oscar Gomezdaza of CIE-UNAM for general assistance in the work. We acknowledge the "nancial support given from CONACyT (MeH xico) and DGAPA (UNAM IN502594 and IN500997, MeH xico). One of us (V.M. GarcmH a) is grateful to the Universidad AutoH noma de Zacatecas for the "nancial support.
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