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The study of lead sulphide films. VI. Influence of oxidants on the chemically deposited PbS thin films
MATERIALS SCIEWCE & EWClNEERlNG B
ELSEVIER
Materials
Science
and Engineering
B41 (1996)
235-240
The study of lead sulphide films. VI. Influence of oxidants on the chemically deposited PbS thin films Cristina
Nagcu, Valentina Institute
of Chemistry
Vomir,
” Rahm
Ripan”
Ileana Pop, Violeta Ionescu, Rodica Grew
Cluj-Napocn, Received
30 F&trinele 3 April
Street,
3400 C&f-Napoca,
Romania
1996
Abstract By treating Ihe PbS films in H,Oz solution an increase of electrical resistance and photosensitivity (the last being about ten-fold greater) was observed. In the case of K,S,O,, a similar phenomenon was ascertained, but with a very high electrical resistance and a lower photosensitivity. The presence of PbSO, in thin films was proved by IR spectroscopy. The SOi- content varies within the range 15-26 wt.%. The PbS films obtained in the presence of oxidants are characterized by electrical resistances which increase with the quantity of oxidant added. The photosensitivity has a maximum of 0.035 mol 1-l H,O,. The IR spectrum shows the appearance of lead cyanamide PbCN, (13.1 wt.% in the sample with highest photosensitivity). The oxidation products and other detected impurities substantially influence the electrical and photoelectrical properties of PbS layers. Krywo~tis:
Lead
sulphide;
Thin
films;
Electrical
resistance;
Photosensitivity;
1. Introduction The use of PbS films as infrared detectors, photoresistances, temperature and humidity sensors requires a high photosensitivity. In order to increase the photosensitivity of these films, many investigations have been undertaken. Though the literature data in the field of the sensitization of PbS films in air or oxygen are numerous [I], there have not been many studies concerning the sensitization in solutions containing oxidants. Few data that exist regarding the oxidation of PbS in oxidant agents solutions do not treat the topic of increasing PbS films photosensitivity, but only the PbS powder oxidation with H,O, [2,3], or the PbS film oxidation with Fe(CN)zor S20i- for other purposes [4,51. On the contrary, some of the researchers in this field were interested in the utilization of oxidant agents directly in the PbS deposition medium to obtain films with a better photosensitivity [6-121. The influence of H,Oz on the crystalline orientation of chemically deposited PbS films was investigated by Torriani et al. [131. In this paper we report an experimental study on the 0921.5107/96/$15.00
0 1996 -
PII SO921-5107(96)01611-X
Elsevier
Science
S.A. All rights
reserved
Oxidants
influence of some oxidants (HzO, and K,S,O,) on PbS films and the preparation of PbS films in the presence of oxidants in the deposition baths.
2. Experimental
details
On optical glass slides K 515, 6 mm x 3.5 mm x 0.8 mm in size, two PbS layers from solutions with identical molar concentration Pb(NO,),, 1.4 x lo-’ mol 1-l; SC(NH&, 5.7 x lo-’ mall-‘; NaOH, 1.58 x 10-l mol 1-l; NH,OH.HCl, 4.1 x 10V2 mol 1-l were deposited. The plates were pretreated for the chemical deposition in the following stages: degreasing with trichlorethy lene, washing with aqueous solution containing cleaning agents, treating with HNO, 6.5 mol 1-l for 24 h at ambient temperature, washing with distilled water and finally with tridistilled water. The quality of PbS film obtained on these plates depends on the cleanliness of the support. Before the preparation of solution, lead nitrate, thiourea and hydroxylamine hydrochloride p.a. were recrystallized twice from tridistilled water. The deposition baths were prepared in 50 ml beakers using
236
C. Nap
er al. / Marerink
Science
and Engineering
B41 (1996)
235-240
Table 1 Influence of the treatment of PbS thin films with Hz02 solutions on their electrical and photoelectrical properties Sample
Treatment in HzO,
Untreated
cone. (mol 1-l)
time (min)
11/l 11/z 11/3
3.32 x 10-x 3.32 x lo-” 3.32 x 10-s
30 60 90
67 71 60
14/l 14/2 14/3
1.66 x 10-3 1.66 x 1O-3 1.66 x 10-A
30 60 90
68 56
27/l 2712 2113
0.83 x lo-” 0.83 x 1o-3 0.83 x 10-s
30 60 90
Treated
0.13 0.17
0.49
0.11
217 227 305
58
0.12 0.23 0.12
242 236 375
0.87 0.37 1.37
62 63 65
0.16 0.15 0.14
241 382 450
0.45 0.52 1.06
* Error limits = 6.2%. sheet electrical resistance in the dark; S photosensitivity; S = (R 0d - R,,)/R,,;
R Od,
tridistilled water. The glass plates were immersed in the bath by being held with polyethylene clamps. The temperature was maintained with an ultrathermostat U lo VEB MLW (Germany). The deposition time for each layer was 30 min and deposition temperature 25 “C. After the deposition of the first layer, the plates were washed with tridistilled water and were then introduced in a freshly prepared bath, for the second layer deposition. Then, the plates with the PbS layers on both sides were washed and the film from one side of the substrate was removed with cotton swabs moistened with diluted HCI. The plates were again washed and dried using nitrogen gas flow [14,15]. Then, the films obtained were treated in H,O, solutions (with or without NaOH) or in K2S208 solutions. After treatment these films were washed and dried as above. To obtain PbS thin films in solutions containing oxidants two layers were deposited (30 min for each layer) at 25 “C from solutions with the following concentrations: Pb(NO,),, 1.4 x lo-’ mol 1-I; SC(NH,),, 5.7 x 10e2 mol 1-l; NaOH, 1.58 x 10-i mol I-‘, to which different quantities of H,O, or K,S208 were added. The oxidants were mixed in the reaction medium before the alkalization of solution. Sheet electrical resistance in the dark (R,,) and under illumination with tungsten lamp of 10 mW cmm2 (Rmi) have been measured at 25 “C using an E 0302 multimeter (IAEMI, Bucharest, Romania) after application of silver paste contacts and terminals from silvered copper thin wires. For discussion, we consider the photosensitivity S of the films as S = (R,,R,,)/R,, [16,17]. The IR spectra of PbS powder from scraped-off film (prepared in varied conditions) were recorded with a Specord ‘75 IR spectrophotometer.
0.37 1.04
R,,, sheet electrical resistance under illumination.
The method for microchemical determination of SOa- ion in PbS films is based on sample digestion by an alkaline (ammonia) EDTA solution, followed by spectrophotometric titration of the sulfate in 80% acetone with a barium chloride solution [l&19]. For the determination of lead cyanamide content in PbS films, these were treated with acetic acid, followed by addition of silver nitrate solution and the excess of silver nitrate was titrated with ammonium thiocyanate
PO1*
3. Results and discussion 3.1. PbS thin films treatment in solutions containing oxidm.ts
3.1.1. H202 solutions H,O, solutions in the range 0.83 x 10°3-3.33 x 10e3 mol l-*, at 25 “C were used. At higher concentrations than mentioned above, the PbS films dissolve in a short time. Otherwise, in more diluted solutions the photosensitivity modifications were insignificant. From Table 1 an increase of electrical resistances may be seen for all samples treated with H,O, solutions, as compared with the untreated samples. With a few exceptions, an increase of electrical resistances with the time of treatment has been observed. Approximately the same effect may be ascertained and for the photosensitivity as well. Photosensitivity for a 90 min period of treatment is 8 - 10 times greater as compared with the initial values. No regularity in increasing the photosensitivity with the time of treatment or concentration of H,O, solutions may be observed.
C. Nap
et al. / Marerials
Science
In our work [14] we have reported that the initial thickness of PbS films was in the range 0.17-0.20 urn. The time of treatment with the oxidant solution was chosen so that the film thickness would not change in order to avoid influencing the electrical resistance mea-‘ surements (because R, depends on film thickness). When the time of treatment with the highest concentration of H,O, exceeds 90 min, a thinning of the film followed by its solving was observed. To estimate the photosensitivity, S, both the electrical resistances in the dark and under illumination of untreated and treated films have been determined. Treatment with oxidant does not only change the electrical resistances in the dark, but also the electrical resistances under illumination (the photocurrents increase implicitly) e.g., sample 1 l/l, Table 1, for the untreated sample, ROri = 67 kQ/O, R,, = 59 kQ2/0; for the treated sample, Rod= 217 kQ/U, R,, = 158 ksZ/O. Further information on the uniformity and surface structure of PbS films can be obtained by electron microscopy. Our electron microscopy determinations [14] show for untreated films a uniform distribution of particles having the size between 0.17-0.20 pm, the same with the film thickness. After treatment with oxidant solutions, the film surface is uniform too, but the particles size is lower (0.1-0.12 urn). The homogeneity of both untreated and treated films has been checked by electrical measurements, by determining the potential variation with distance from one of the electrodes. This variation was linear [6]. Lunekok-Burmakina and Gerasenkova [3] supposed that in the S*- step-by-step oxidation process, S(OH), is forming whose hydroxyls are replaced with perhydroxyls, S(OOH),, the end product being SO;-. S’+ 2H,O, + S(OH), + 20H-
and Engixeerixg
B41 j1996)
237
235-240
The ratios of final/initial photosensitivity change between l-2.5, a smaller increase as compared with the previous case. 3.1.3. K,S,O, solutions As can be seen from Table 3, after treatment of PbS films in K,S,O, oxidant solutions, the electrical resistances increase very much. The photosensitivity was higher too, but insignificantly. Fig. l(b) illustrates the presence of PbS04 in this case. Above the specified K2S208 concentrations (see Table 3) the PbS films are dissolved in a short time. The treatment of PbS films in K,S,O, solutions with higher concentrations than H,O, solutions (see Section 3.1.1) explain the shorter treatment time and the greater values of electrical resistances (owing ‘to the higher SOi- content, 26 wt.%). 3.2. Obtaining of PbS thin jlrns in the presence of oxidants 3.2.1. In the presence of H202 The experiments have been made at concentrations of H,O, between 3.1 x 10-3-9.5 x lo-* mol 1-l. The data obtained are presented in Fig. 2. A progressive increase of electrical resistances with the added quantity
L
S(OH), + 2H,02 + SO;- + 2H+ + 2H,O The presence of PbSO, in the treatment of PbS thin films with H,O, solutions was detected by IR spectroscopy (Fig. l(a)). The absorption maxima from 610630 cm-’ and 1050-1150 cm-’ are specific for PbSO, (Fig. l(c)). The content of SO:- in PbS thin films treated by HZO, solutions varies in the range of 1521.5 wt.%. In the presence of PbSO, the PbS thin films show, according to our experiments, a higher photosensitivity. This agrees well with Wolten’s results [lo], but not with Kunze’s data [8]. 3.1.2. Alkaline H20, solutions In Table 2 data concerning the electrical and photoelectrical properties of PbS thin films, before and after treatment in H,O, solutions at various molar ratios H20,:NaOH are shown. An increase of electrical resistance and photosensitivity of the films can be seen.
q
3000
2000 wavenumber
1000 (cm” 1
Fig. 1. PbS thin films IR spectra: (a) treated in H,O,; (b) treated in K&O,; (c) PbSO, prepared; (d) PbS film prepared in the pfesence oi H&b
238
C. Napzi
et al. / Materials
Science
and Engineering
341 (1996)
235-240
Table 2 Treatment of PbS thin films in H,O, solutions in alkaline medium Sample
NaOH (mol I-‘)
Untreated
Treated s
S
go) 681/l
0.41 x IO-' 0.61 x 10-J 0.82x IO-" 10-3 10-3
68112 68113 684/4 68415 68416 684/l 68418 68419
2.5 x 1O-2 2.5 x 10-2 2.5 x 10-2
2.5 x lo-*
10-X 1.3 x 10-3 2.28x10-3 2.90 x lo-”
684/10
3.11 x 10-3 3.21 x lo-’ 3.32 x 10W3
684/l 1 684112
* Error limits = 4.5%. Treatment time, 30 min; R,,
91 58
0.17 0.11
61 55
0.13
162 131 109
3.7 x lo-* 1.85 x lo-* o.92x10w2
52 60
68
0.31
160
0.17 0.30 0.34
63
3.5 x lo-’
58
169 115
0.49 0.31
2.27 x IO-* 1.56 x 10-Z 8.05 x 10-S
66 52 56
0.42 0.24 0.15 0.37 0.33
215 145 263
0.17 0.37 0.34
119
solutions Treated
R (k&)
s
81 68 74
mined by the thermoelectric probe method. Concordantly with observations of other authors, the PbS layers obtained without added oxidant are less photosensitive than the layers prepared in the presence of oxidants [9,11,12]. It is accepted that the PbS photoconductive films are composed of a system of crystallites separated by intercrystalline barriers [9,24-291. According to Petritz the crystallites are about 0.1 to 1 urn and the width of the intercrystalline barriers is about 5 to 20 A [24]. Recently, Indrea and Barbu show that the sensibilised PbS film is a heterogeneous system of 320 A crystallites separated by 160 A intercrystalline barriers [30]. The
1 N
Rod 80.
I800
-u cr"
Untreated
* Error limits = 6.5%. Treatment time, 5 min; R,, 1.
0.31
3.0 x 10-Z 3.5 x 10-z 3.0 x 10-2 2.7 x 10-z
(mol I-‘)
685 686 687
0.26 0.22 0.33
and S have the significations from Table 1.
Table 3 Treatment of PbS thin film in K&OS fG20,
259 116
0.42 0.17 0.28
of H,O, can be seen. The photosensitivity increase too, showing a maximum around the value of 3.5 x lo-* mol 1-l H,02 and then decrease (a fact which is observed by other authors as well [ll]). At above 4.7 x 1W2 mol 1-l H,O,, the electrical resistance exceeds 2 Ma/C!. The IR absorption spectrum of PbS films deposited in the presence of H,O, (Fig. l(d)) shows the appearance of PbCN, (lead cyanamide) having a maximum at 19.50 cm-’ [Zl]. The presence of PbCN, in some PbS films was found by Biller et al. [22] and Kitaev et al. [23]. They assert that the formation of PbCN, is a consequence of thiourea decomposition and depends on the alkalinity of chemical deposition bath. At higher alkalinities no PbCN, results. The amount of PbCN, found in the highest photosensitivity sample was 13.1 wt.%. In the same sample a 3.4 wt.% SO:- content has also been detected, though it could not be identified in the IR spectrum. We have established that all PbS films prepared with or without oxidant show p-type conductivity as deter-
Sample
&I)
fbd (k-Q/~)
a’$
0.37
1400
0.41
0.11
1520
0.18
0.21
1200
0.28
60-
_ 600
40.
-400
20-
-200
1
3
Hz02 and S have the significations from Table
5
[mole /liter
x10-2
1
Fig. 2. Variation of photosensitivity (S) and sheet electrical resistance in the dark (R,,) as a function of H,O, concentration.
C. Nap
et al. / Materials Science and Etzgiizeering 841 (1996) 235-240
of the electrical resistances with the increase of K,S,O, concentration has been observed. In these conditions the. photosensitivities values are somewhat smaller than in the case of H,O, (at maximum) and vary insignificantly. The effect of K,S,O,, much lower as compared with H,O,, may be explained by stereo impediments. The voluminous S,Oiion cannot enter between PbS crystallites, acting only at the surface of the layer.
u
?60.X I/)
Ioo 4
8 K&08
12 (mole
16 /titer
x10-4 )
Fig. 3. Variation of photosensitivity (S) and sheet electrical resistance in the dark CR,,) as a function of K&O, concentration.
intercrystalline barriers are the most affected zones concerning the chemical composition. The segregation of impurities, the film oxidation and the trapping centers will produce at intercrystalline zones level [24,31,32]. The resistance of the PbS films may be considered to be owing to a large number of barriers between crystallites (the intercrystalline barriers are of high resistance compared with the crystallites) [33,34]. Concerning the effect of oxidants it is mentioned in the literature that the PbS grains are surrounded by PbSO, and PbO phases, which substantiaily determine electrical and photoelectrical properties of the PbS layer [34-371. The primary photoeffect consisting of an increase of the majority carriers density, results from absorption of light in the PbS crystallites, creating hole-electron pairs. In our case, the majority of carriers are holes, because the films show P-type conductivity. Secondary amplification effects can result from lowering intercrystalline barriers by trapping minority carriers (electrons). Acharya and Bose [34] have shown experimentally that the barrier height is reduced by illumination. According to Espevik et al. [9] photoexcitation produces trapped electrons in the vicinity of intergrain barriers, thus reducing the depletion layer width in the grain and increasing the tunnelling transmission coefficient. The same phenomenon is reported by Aleshin et al. [37]. The increase of electrical resistances as a result of oxidant action may be ascribed to an enlargement of the number of intercrystalline barriers because the size of the crystallites becomes smaller and their number greater. 3.2.2.
239
In the presence of K,S,08
PbS thin films have been obtained in a similar way with those deposited in the presence of HzO, except for the fact that in this case, the concentration of K&O8 was lower than that of H,O, (Fig. 3). The electrical resistances were greater than those measured in the absence of an oxidant. Practically, a small modification
4. Conclusions
By treating chemically deposited PbS thin films in H,O, and K,S,O, solutions the increase of electrical resistances and photosensitivities is observed. The presence of PbSO, in the treated films has been found. The PbS thin films obtained in the presence of oxidants in the deposition bath are characterized by electrical resistances which increase with the added quantity of oxidant. The photosensitivity has a maximum at 3.5 x 10e2 mol 1-l H,O, concentration. In PbS films, PbCN, and PbSO, were detected.
Acknowledgements
This work was financially supported by the Ministry of Research and Technology, Romania. The authors are grateful to Dr. E. Indrea for helpful discussions.
References
Ul I. Pop, V. Ionescu,C. NaFcu,V. Vomir, R. Grecu and E. Indrea,
Thin Solid Films, in press. Lunekok-Burmakina, A.P. Potemskaya and A.N. Gerasenkova, Khim. Perekisnykh. Soedin., Akad. Nauk SSSR, Inst. Obshch. i Neorg. Khim., (1963) 105. and A.N. Gerasenkova,Zhur. ??eorg. [31 V.A. Lunekok-Burmakina Khim., 9 (1964) 270. [41 D.N. Gritsan, V.D. Kalugin and N.S. Opaleva, USSR Patent 985147, 1982. [51 V.D. Kalugin, N.S. Opaleva, D.N. Gritsan, A.A. Borodkina and N.V. Ilceeva, Hark. IIUL, Harkov, (1984) 49. [61 C.W. Mahlman, Plzys. Rev., 103 (1956) 1619. 171 O.A. Kunze, O.G. Malan, P.A. Bi.iger and W. Pink, Z. )Vatwfomh., 26b (1971) 8. [8] O.A. Kunze, J. Electrochem. Sue., I.23 (1976) 946. [9] S. Espevik, Ch. Wu and R.H. Bube, J. Appl. Phys., 42 (1971) 3513. [IO] G.M. Wolten, J. Electrochem. Sot., 122 (1975) 1149. [Ill G.P. Kothiyal, B. Ghosh and J. Desphande, J. Phys. D. Appl. Phys., 13 (1980) 869. [12] A.V. Burlak, V.V. Zotov, A.V. Ignatov, A.V. Tyurin and V.G. Tsukerman, .Fiz. Tekn. Poluprovod., 26 (1992) 548. [13] LC. Torriani, M. Tomiyama, S. Bilac, G. Rego, J.I. Cisneros and Z.P. Arguello, Tfzin Solid Films, 77 (1981) 347. [14] C. Naqcu, V. Vomir, V. Ionescu and I. Pop, Rev. Roum. Chin?., 40 (1995) 127.
PI V.A.
240
C. Nqcu
et al. / Materials
Scieme
P51 C. Nascu, I. Pop, V. Ionescu and V. Vomir, Stuciiu Universitatis Babe@olyai,
Cilenlia,
37 (1992)
Phys., 1171
75 (1994)
Appl.
1.557.
M.T.S. Nair, P.K. Nair, R.A. Zingaro and E.A. Meyers, J. Appl. Phys.,
74 (1993)
841 (1996)
235-240
[25] J. Volger, Ph.r~s.Rev., 79 (1950)
1023. Appl. Phys~ Lelt., JJ (1967) 237. ~271 D.P. Snowden and A.M. Portis, Pliys. Rev., 120 (1960) 1983. WI R.H. Bube, Appl. Phys. Left., 13 (1968) 136. WI G.H. Blount, R.H. Bube and A.L. Robinson, J. Appl. Pllys.> 41 (1970) 2190. Processes DO1 E. Indrea and A. Barbu, 2ud ht. Conj: on Photoexcited and Applications, 17-21 September, J995, Jerusalem ICPEPA,
WI J.L. Davis and R.F. Greene,
55.
1161M.T.S. Nair, P.K. Nair, R.A. Zingaro and E.A. Meyers, J.
and Engheering
1879.
USI V.V. Kuznetsov, I.V. Shimanogova, O.L. Samorukova and E.K.
Zakharov, Z/IL~~.Analit. K/h., 44 (1989) 225. V.V. Kuznetsov, Yu.V. Ermolenko, O.L. Samorukova and E.K. Zakharov, Ziw. halit. H&z., 42 (1957) 1909. amnmtici WI H. Sanielevici and Fl. Urseanu, Analize de intermediari si colnx@ orgaizici, Ed. Technic%. Bucuresti, 1985, p. 165. Pll R.A. Nyquist and R.C. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, New York, 1971, p. 73. P21 L.N. Biller, V.N. Vertsner, MS. Davydov, V.S. Kosnyrev and G.P. Tihomirov, Izv. Akad. Nauk. SSSR, Ser. Fizic., 30 (1966) 799. ~231 G.A. Kitaev, L.N. Mashaeva, V.F. IMarkov, A.Yu. Kurkin and L.E. Vasiunina, Iza. Akad, Hauk SSSR, Neorg. Mater. 25 (1989) 1262. v41 R.L. Petritz, Phys. Rev., JO4 (1956) 1508. t191
[311 ~321
[331 [341 [351 [361 [371
1995, Abstracts, p. 37. J. Humphrey and W. Scanlon, Phys. Rev., 10.5 (1957) 469. R. Candea, D. DBdarlat, P. Fitori, R. Turcu, E. Indrea, A. Darabont, L. Biro, I. Bratu and N. Aldea, Stdii ,~i Cemtdvi Fiz., 38 (1986) 410. J.F. Woods, Phys. Rev., 106 (1957) 235. H.N. Acharya and H.N. Bose, Indim J. Phys., 5% (1979) 6. K. Duh and J.N. Zemel, T/if/? Solid Films, 26 (1975) 165. MS. Said and J.N. Zemel, J. Appl. Phys., 47 (1976) S71. A.N. Aleshin, A.V. Burlak, A.V. Ignatov, V.A. Pasternak and A.V. Tyurin, horg. Mater. (Transl. of Neorg. Mater.), 31 (1995) 394.
ELSEVIER
Materials
Science
and Engineering
B41 (1996)
235-240
The study of lead sulphide films. VI. Influence of oxidants on the chemically deposited PbS thin films Cristina
Nagcu, Valentina Institute
of Chemistry
Vomir,
” Rahm
Ripan”
Ileana Pop, Violeta Ionescu, Rodica Grew
Cluj-Napocn, Received
30 F&trinele 3 April
Street,
3400 C&f-Napoca,
Romania
1996
Abstract By treating Ihe PbS films in H,Oz solution an increase of electrical resistance and photosensitivity (the last being about ten-fold greater) was observed. In the case of K,S,O,, a similar phenomenon was ascertained, but with a very high electrical resistance and a lower photosensitivity. The presence of PbSO, in thin films was proved by IR spectroscopy. The SOi- content varies within the range 15-26 wt.%. The PbS films obtained in the presence of oxidants are characterized by electrical resistances which increase with the quantity of oxidant added. The photosensitivity has a maximum of 0.035 mol 1-l H,O,. The IR spectrum shows the appearance of lead cyanamide PbCN, (13.1 wt.% in the sample with highest photosensitivity). The oxidation products and other detected impurities substantially influence the electrical and photoelectrical properties of PbS layers. Krywo~tis:
Lead
sulphide;
Thin
films;
Electrical
resistance;
Photosensitivity;
1. Introduction The use of PbS films as infrared detectors, photoresistances, temperature and humidity sensors requires a high photosensitivity. In order to increase the photosensitivity of these films, many investigations have been undertaken. Though the literature data in the field of the sensitization of PbS films in air or oxygen are numerous [I], there have not been many studies concerning the sensitization in solutions containing oxidants. Few data that exist regarding the oxidation of PbS in oxidant agents solutions do not treat the topic of increasing PbS films photosensitivity, but only the PbS powder oxidation with H,O, [2,3], or the PbS film oxidation with Fe(CN)zor S20i- for other purposes [4,51. On the contrary, some of the researchers in this field were interested in the utilization of oxidant agents directly in the PbS deposition medium to obtain films with a better photosensitivity [6-121. The influence of H,Oz on the crystalline orientation of chemically deposited PbS films was investigated by Torriani et al. [131. In this paper we report an experimental study on the 0921.5107/96/$15.00
0 1996 -
PII SO921-5107(96)01611-X
Elsevier
Science
S.A. All rights
reserved
Oxidants
influence of some oxidants (HzO, and K,S,O,) on PbS films and the preparation of PbS films in the presence of oxidants in the deposition baths.
2. Experimental
details
On optical glass slides K 515, 6 mm x 3.5 mm x 0.8 mm in size, two PbS layers from solutions with identical molar concentration Pb(NO,),, 1.4 x lo-’ mol 1-l; SC(NH&, 5.7 x lo-’ mall-‘; NaOH, 1.58 x 10-l mol 1-l; NH,OH.HCl, 4.1 x 10V2 mol 1-l were deposited. The plates were pretreated for the chemical deposition in the following stages: degreasing with trichlorethy lene, washing with aqueous solution containing cleaning agents, treating with HNO, 6.5 mol 1-l for 24 h at ambient temperature, washing with distilled water and finally with tridistilled water. The quality of PbS film obtained on these plates depends on the cleanliness of the support. Before the preparation of solution, lead nitrate, thiourea and hydroxylamine hydrochloride p.a. were recrystallized twice from tridistilled water. The deposition baths were prepared in 50 ml beakers using
236
C. Nap
er al. / Marerink
Science
and Engineering
B41 (1996)
235-240
Table 1 Influence of the treatment of PbS thin films with Hz02 solutions on their electrical and photoelectrical properties Sample
Treatment in HzO,
Untreated
cone. (mol 1-l)
time (min)
11/l 11/z 11/3
3.32 x 10-x 3.32 x lo-” 3.32 x 10-s
30 60 90
67 71 60
14/l 14/2 14/3
1.66 x 10-3 1.66 x 1O-3 1.66 x 10-A
30 60 90
68 56
27/l 2712 2113
0.83 x lo-” 0.83 x 1o-3 0.83 x 10-s
30 60 90
Treated
0.13 0.17
0.49
0.11
217 227 305
58
0.12 0.23 0.12
242 236 375
0.87 0.37 1.37
62 63 65
0.16 0.15 0.14
241 382 450
0.45 0.52 1.06
* Error limits = 6.2%. sheet electrical resistance in the dark; S photosensitivity; S = (R 0d - R,,)/R,,;
R Od,
tridistilled water. The glass plates were immersed in the bath by being held with polyethylene clamps. The temperature was maintained with an ultrathermostat U lo VEB MLW (Germany). The deposition time for each layer was 30 min and deposition temperature 25 “C. After the deposition of the first layer, the plates were washed with tridistilled water and were then introduced in a freshly prepared bath, for the second layer deposition. Then, the plates with the PbS layers on both sides were washed and the film from one side of the substrate was removed with cotton swabs moistened with diluted HCI. The plates were again washed and dried using nitrogen gas flow [14,15]. Then, the films obtained were treated in H,O, solutions (with or without NaOH) or in K2S208 solutions. After treatment these films were washed and dried as above. To obtain PbS thin films in solutions containing oxidants two layers were deposited (30 min for each layer) at 25 “C from solutions with the following concentrations: Pb(NO,),, 1.4 x lo-’ mol 1-I; SC(NH,),, 5.7 x 10e2 mol 1-l; NaOH, 1.58 x 10-i mol I-‘, to which different quantities of H,O, or K,S208 were added. The oxidants were mixed in the reaction medium before the alkalization of solution. Sheet electrical resistance in the dark (R,,) and under illumination with tungsten lamp of 10 mW cmm2 (Rmi) have been measured at 25 “C using an E 0302 multimeter (IAEMI, Bucharest, Romania) after application of silver paste contacts and terminals from silvered copper thin wires. For discussion, we consider the photosensitivity S of the films as S = (R,,R,,)/R,, [16,17]. The IR spectra of PbS powder from scraped-off film (prepared in varied conditions) were recorded with a Specord ‘75 IR spectrophotometer.
0.37 1.04
R,,, sheet electrical resistance under illumination.
The method for microchemical determination of SOa- ion in PbS films is based on sample digestion by an alkaline (ammonia) EDTA solution, followed by spectrophotometric titration of the sulfate in 80% acetone with a barium chloride solution [l&19]. For the determination of lead cyanamide content in PbS films, these were treated with acetic acid, followed by addition of silver nitrate solution and the excess of silver nitrate was titrated with ammonium thiocyanate
PO1*
3. Results and discussion 3.1. PbS thin films treatment in solutions containing oxidm.ts
3.1.1. H202 solutions H,O, solutions in the range 0.83 x 10°3-3.33 x 10e3 mol l-*, at 25 “C were used. At higher concentrations than mentioned above, the PbS films dissolve in a short time. Otherwise, in more diluted solutions the photosensitivity modifications were insignificant. From Table 1 an increase of electrical resistances may be seen for all samples treated with H,O, solutions, as compared with the untreated samples. With a few exceptions, an increase of electrical resistances with the time of treatment has been observed. Approximately the same effect may be ascertained and for the photosensitivity as well. Photosensitivity for a 90 min period of treatment is 8 - 10 times greater as compared with the initial values. No regularity in increasing the photosensitivity with the time of treatment or concentration of H,O, solutions may be observed.
C. Nap
et al. / Marerials
Science
In our work [14] we have reported that the initial thickness of PbS films was in the range 0.17-0.20 urn. The time of treatment with the oxidant solution was chosen so that the film thickness would not change in order to avoid influencing the electrical resistance mea-‘ surements (because R, depends on film thickness). When the time of treatment with the highest concentration of H,O, exceeds 90 min, a thinning of the film followed by its solving was observed. To estimate the photosensitivity, S, both the electrical resistances in the dark and under illumination of untreated and treated films have been determined. Treatment with oxidant does not only change the electrical resistances in the dark, but also the electrical resistances under illumination (the photocurrents increase implicitly) e.g., sample 1 l/l, Table 1, for the untreated sample, ROri = 67 kQ/O, R,, = 59 kQ2/0; for the treated sample, Rod= 217 kQ/U, R,, = 158 ksZ/O. Further information on the uniformity and surface structure of PbS films can be obtained by electron microscopy. Our electron microscopy determinations [14] show for untreated films a uniform distribution of particles having the size between 0.17-0.20 pm, the same with the film thickness. After treatment with oxidant solutions, the film surface is uniform too, but the particles size is lower (0.1-0.12 urn). The homogeneity of both untreated and treated films has been checked by electrical measurements, by determining the potential variation with distance from one of the electrodes. This variation was linear [6]. Lunekok-Burmakina and Gerasenkova [3] supposed that in the S*- step-by-step oxidation process, S(OH), is forming whose hydroxyls are replaced with perhydroxyls, S(OOH),, the end product being SO;-. S’+ 2H,O, + S(OH), + 20H-
and Engixeerixg
B41 j1996)
237
235-240
The ratios of final/initial photosensitivity change between l-2.5, a smaller increase as compared with the previous case. 3.1.3. K,S,O, solutions As can be seen from Table 3, after treatment of PbS films in K,S,O, oxidant solutions, the electrical resistances increase very much. The photosensitivity was higher too, but insignificantly. Fig. l(b) illustrates the presence of PbS04 in this case. Above the specified K2S208 concentrations (see Table 3) the PbS films are dissolved in a short time. The treatment of PbS films in K,S,O, solutions with higher concentrations than H,O, solutions (see Section 3.1.1) explain the shorter treatment time and the greater values of electrical resistances (owing ‘to the higher SOi- content, 26 wt.%). 3.2. Obtaining of PbS thin jlrns in the presence of oxidants 3.2.1. In the presence of H202 The experiments have been made at concentrations of H,O, between 3.1 x 10-3-9.5 x lo-* mol 1-l. The data obtained are presented in Fig. 2. A progressive increase of electrical resistances with the added quantity
L
S(OH), + 2H,02 + SO;- + 2H+ + 2H,O The presence of PbSO, in the treatment of PbS thin films with H,O, solutions was detected by IR spectroscopy (Fig. l(a)). The absorption maxima from 610630 cm-’ and 1050-1150 cm-’ are specific for PbSO, (Fig. l(c)). The content of SO:- in PbS thin films treated by HZO, solutions varies in the range of 1521.5 wt.%. In the presence of PbSO, the PbS thin films show, according to our experiments, a higher photosensitivity. This agrees well with Wolten’s results [lo], but not with Kunze’s data [8]. 3.1.2. Alkaline H20, solutions In Table 2 data concerning the electrical and photoelectrical properties of PbS thin films, before and after treatment in H,O, solutions at various molar ratios H20,:NaOH are shown. An increase of electrical resistance and photosensitivity of the films can be seen.
q
3000
2000 wavenumber
1000 (cm” 1
Fig. 1. PbS thin films IR spectra: (a) treated in H,O,; (b) treated in K&O,; (c) PbSO, prepared; (d) PbS film prepared in the pfesence oi H&b
238
C. Napzi
et al. / Materials
Science
and Engineering
341 (1996)
235-240
Table 2 Treatment of PbS thin films in H,O, solutions in alkaline medium Sample
NaOH (mol I-‘)
Untreated
Treated s
S
go) 681/l
0.41 x IO-' 0.61 x 10-J 0.82x IO-" 10-3 10-3
68112 68113 684/4 68415 68416 684/l 68418 68419
2.5 x 1O-2 2.5 x 10-2 2.5 x 10-2
2.5 x lo-*
10-X 1.3 x 10-3 2.28x10-3 2.90 x lo-”
684/10
3.11 x 10-3 3.21 x lo-’ 3.32 x 10W3
684/l 1 684112
* Error limits = 4.5%. Treatment time, 30 min; R,,
91 58
0.17 0.11
61 55
0.13
162 131 109
3.7 x lo-* 1.85 x lo-* o.92x10w2
52 60
68
0.31
160
0.17 0.30 0.34
63
3.5 x lo-’
58
169 115
0.49 0.31
2.27 x IO-* 1.56 x 10-Z 8.05 x 10-S
66 52 56
0.42 0.24 0.15 0.37 0.33
215 145 263
0.17 0.37 0.34
119
solutions Treated
R (k&)
s
81 68 74
mined by the thermoelectric probe method. Concordantly with observations of other authors, the PbS layers obtained without added oxidant are less photosensitive than the layers prepared in the presence of oxidants [9,11,12]. It is accepted that the PbS photoconductive films are composed of a system of crystallites separated by intercrystalline barriers [9,24-291. According to Petritz the crystallites are about 0.1 to 1 urn and the width of the intercrystalline barriers is about 5 to 20 A [24]. Recently, Indrea and Barbu show that the sensibilised PbS film is a heterogeneous system of 320 A crystallites separated by 160 A intercrystalline barriers [30]. The
1 N
Rod 80.
I800
-u cr"
Untreated
* Error limits = 6.5%. Treatment time, 5 min; R,, 1.
0.31
3.0 x 10-Z 3.5 x 10-z 3.0 x 10-2 2.7 x 10-z
(mol I-‘)
685 686 687
0.26 0.22 0.33
and S have the significations from Table 1.
Table 3 Treatment of PbS thin film in K&OS fG20,
259 116
0.42 0.17 0.28
of H,O, can be seen. The photosensitivity increase too, showing a maximum around the value of 3.5 x lo-* mol 1-l H,02 and then decrease (a fact which is observed by other authors as well [ll]). At above 4.7 x 1W2 mol 1-l H,O,, the electrical resistance exceeds 2 Ma/C!. The IR absorption spectrum of PbS films deposited in the presence of H,O, (Fig. l(d)) shows the appearance of PbCN, (lead cyanamide) having a maximum at 19.50 cm-’ [Zl]. The presence of PbCN, in some PbS films was found by Biller et al. [22] and Kitaev et al. [23]. They assert that the formation of PbCN, is a consequence of thiourea decomposition and depends on the alkalinity of chemical deposition bath. At higher alkalinities no PbCN, results. The amount of PbCN, found in the highest photosensitivity sample was 13.1 wt.%. In the same sample a 3.4 wt.% SO:- content has also been detected, though it could not be identified in the IR spectrum. We have established that all PbS films prepared with or without oxidant show p-type conductivity as deter-
Sample
&I)
fbd (k-Q/~)
a’$
0.37
1400
0.41
0.11
1520
0.18
0.21
1200
0.28
60-
_ 600
40.
-400
20-
-200
1
3
Hz02 and S have the significations from Table
5
[mole /liter
x10-2
1
Fig. 2. Variation of photosensitivity (S) and sheet electrical resistance in the dark (R,,) as a function of H,O, concentration.
C. Nap
et al. / Materials Science and Etzgiizeering 841 (1996) 235-240
of the electrical resistances with the increase of K,S,O, concentration has been observed. In these conditions the. photosensitivities values are somewhat smaller than in the case of H,O, (at maximum) and vary insignificantly. The effect of K,S,O,, much lower as compared with H,O,, may be explained by stereo impediments. The voluminous S,Oiion cannot enter between PbS crystallites, acting only at the surface of the layer.
u
?60.X I/)
Ioo 4
8 K&08
12 (mole
16 /titer
x10-4 )
Fig. 3. Variation of photosensitivity (S) and sheet electrical resistance in the dark CR,,) as a function of K&O, concentration.
intercrystalline barriers are the most affected zones concerning the chemical composition. The segregation of impurities, the film oxidation and the trapping centers will produce at intercrystalline zones level [24,31,32]. The resistance of the PbS films may be considered to be owing to a large number of barriers between crystallites (the intercrystalline barriers are of high resistance compared with the crystallites) [33,34]. Concerning the effect of oxidants it is mentioned in the literature that the PbS grains are surrounded by PbSO, and PbO phases, which substantiaily determine electrical and photoelectrical properties of the PbS layer [34-371. The primary photoeffect consisting of an increase of the majority carriers density, results from absorption of light in the PbS crystallites, creating hole-electron pairs. In our case, the majority of carriers are holes, because the films show P-type conductivity. Secondary amplification effects can result from lowering intercrystalline barriers by trapping minority carriers (electrons). Acharya and Bose [34] have shown experimentally that the barrier height is reduced by illumination. According to Espevik et al. [9] photoexcitation produces trapped electrons in the vicinity of intergrain barriers, thus reducing the depletion layer width in the grain and increasing the tunnelling transmission coefficient. The same phenomenon is reported by Aleshin et al. [37]. The increase of electrical resistances as a result of oxidant action may be ascribed to an enlargement of the number of intercrystalline barriers because the size of the crystallites becomes smaller and their number greater. 3.2.2.
239
In the presence of K,S,08
PbS thin films have been obtained in a similar way with those deposited in the presence of HzO, except for the fact that in this case, the concentration of K&O8 was lower than that of H,O, (Fig. 3). The electrical resistances were greater than those measured in the absence of an oxidant. Practically, a small modification
4. Conclusions
By treating chemically deposited PbS thin films in H,O, and K,S,O, solutions the increase of electrical resistances and photosensitivities is observed. The presence of PbSO, in the treated films has been found. The PbS thin films obtained in the presence of oxidants in the deposition bath are characterized by electrical resistances which increase with the added quantity of oxidant. The photosensitivity has a maximum at 3.5 x 10e2 mol 1-l H,O, concentration. In PbS films, PbCN, and PbSO, were detected.
Acknowledgements
This work was financially supported by the Ministry of Research and Technology, Romania. The authors are grateful to Dr. E. Indrea for helpful discussions.
References
Ul I. Pop, V. Ionescu,C. NaFcu,V. Vomir, R. Grecu and E. Indrea,
Thin Solid Films, in press. Lunekok-Burmakina, A.P. Potemskaya and A.N. Gerasenkova, Khim. Perekisnykh. Soedin., Akad. Nauk SSSR, Inst. Obshch. i Neorg. Khim., (1963) 105. and A.N. Gerasenkova,Zhur. ??eorg. [31 V.A. Lunekok-Burmakina Khim., 9 (1964) 270. [41 D.N. Gritsan, V.D. Kalugin and N.S. Opaleva, USSR Patent 985147, 1982. [51 V.D. Kalugin, N.S. Opaleva, D.N. Gritsan, A.A. Borodkina and N.V. Ilceeva, Hark. IIUL, Harkov, (1984) 49. [61 C.W. Mahlman, Plzys. Rev., 103 (1956) 1619. 171 O.A. Kunze, O.G. Malan, P.A. Bi.iger and W. Pink, Z. )Vatwfomh., 26b (1971) 8. [8] O.A. Kunze, J. Electrochem. Sue., I.23 (1976) 946. [9] S. Espevik, Ch. Wu and R.H. Bube, J. Appl. Phys., 42 (1971) 3513. [IO] G.M. Wolten, J. Electrochem. Sot., 122 (1975) 1149. [Ill G.P. Kothiyal, B. Ghosh and J. Desphande, J. Phys. D. Appl. Phys., 13 (1980) 869. [12] A.V. Burlak, V.V. Zotov, A.V. Ignatov, A.V. Tyurin and V.G. Tsukerman, .Fiz. Tekn. Poluprovod., 26 (1992) 548. [13] LC. Torriani, M. Tomiyama, S. Bilac, G. Rego, J.I. Cisneros and Z.P. Arguello, Tfzin Solid Films, 77 (1981) 347. [14] C. Naqcu, V. Vomir, V. Ionescu and I. Pop, Rev. Roum. Chin?., 40 (1995) 127.
PI V.A.
240
C. Nqcu
et al. / Materials
Scieme
P51 C. Nascu, I. Pop, V. Ionescu and V. Vomir, Stuciiu Universitatis Babe@olyai,
Cilenlia,
37 (1992)
Phys., 1171
75 (1994)
Appl.
1.557.
M.T.S. Nair, P.K. Nair, R.A. Zingaro and E.A. Meyers, J. Appl. Phys.,
74 (1993)
841 (1996)
235-240
[25] J. Volger, Ph.r~s.Rev., 79 (1950)
1023. Appl. Phys~ Lelt., JJ (1967) 237. ~271 D.P. Snowden and A.M. Portis, Pliys. Rev., 120 (1960) 1983. WI R.H. Bube, Appl. Phys. Left., 13 (1968) 136. WI G.H. Blount, R.H. Bube and A.L. Robinson, J. Appl. Pllys.> 41 (1970) 2190. Processes DO1 E. Indrea and A. Barbu, 2ud ht. Conj: on Photoexcited and Applications, 17-21 September, J995, Jerusalem ICPEPA,
WI J.L. Davis and R.F. Greene,
55.
1161M.T.S. Nair, P.K. Nair, R.A. Zingaro and E.A. Meyers, J.
and Engheering
1879.
USI V.V. Kuznetsov, I.V. Shimanogova, O.L. Samorukova and E.K.
Zakharov, Z/IL~~.Analit. K/h., 44 (1989) 225. V.V. Kuznetsov, Yu.V. Ermolenko, O.L. Samorukova and E.K. Zakharov, Ziw. halit. H&z., 42 (1957) 1909. amnmtici WI H. Sanielevici and Fl. Urseanu, Analize de intermediari si colnx@ orgaizici, Ed. Technic%. Bucuresti, 1985, p. 165. Pll R.A. Nyquist and R.C. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, New York, 1971, p. 73. P21 L.N. Biller, V.N. Vertsner, MS. Davydov, V.S. Kosnyrev and G.P. Tihomirov, Izv. Akad. Nauk. SSSR, Ser. Fizic., 30 (1966) 799. ~231 G.A. Kitaev, L.N. Mashaeva, V.F. IMarkov, A.Yu. Kurkin and L.E. Vasiunina, Iza. Akad, Hauk SSSR, Neorg. Mater. 25 (1989) 1262. v41 R.L. Petritz, Phys. Rev., JO4 (1956) 1508. t191
[311 ~321
[331 [341 [351 [361 [371
1995, Abstracts, p. 37. J. Humphrey and W. Scanlon, Phys. Rev., 10.5 (1957) 469. R. Candea, D. DBdarlat, P. Fitori, R. Turcu, E. Indrea, A. Darabont, L. Biro, I. Bratu and N. Aldea, Stdii ,~i Cemtdvi Fiz., 38 (1986) 410. J.F. Woods, Phys. Rev., 106 (1957) 235. H.N. Acharya and H.N. Bose, Indim J. Phys., 5% (1979) 6. K. Duh and J.N. Zemel, T/if/? Solid Films, 26 (1975) 165. MS. Said and J.N. Zemel, J. Appl. Phys., 47 (1976) S71. A.N. Aleshin, A.V. Burlak, A.V. Ignatov, V.A. Pasternak and A.V. Tyurin, horg. Mater. (Transl. of Neorg. Mater.), 31 (1995) 394.