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Preparation of zinc sulfide thin films by ultrasonic spray pyrolsis from bis(diethyldithiocarbamato) zinc(II)
Thin Solid Films, 224 (1993) 221 - 226
221
Preparation of zinc sulfide thin films by ultrasonic spray pyrolysis from bis(diethyldithiocarbamato)zinc(II) R. D. Pike, H. Cui, R. Kershaw, K. Dwight and A. Wold Department of Chemistry, Brown University, Providenee, RI 02912 (USA)
T. N. Blanton Analytical Technology Division, Eastman Kodak Company, Rochester, N Y 14650 (USA)
A. A. Wernberg and H. J. Gysling Corporate Research Laboratories, Eastman Kadak Company, Rochester, N Y 14650 (USA) (Received August 6, 1992; accepted October 7, 1992)
Abstract
Thin films of zinc sulfide were prepared by ultrasonically spraying a toluene solution of bis(diethyldithiocarbamato)zinc(II) onto silicon, sapphire and gallium arsenide substrates at 460-520 °C. The films prepared on silicon or sapphire were found to have a highly oriented hexagonal structure, while those deposited onto cubic (100) gallium arsenide showed a highly oriented cubic structure. The films were characterized by X-ray diffraction analysis, ellipsometry, scanning electron microscopy, and IR spectroscopy.
I. Introduction
Zinc sulfide is a I I - V I semiconductor with a large direct band gap in the near-UV region. A substantial effort has been directed at the preparation of ZnS thin films for use in electroluminescent displays [1, 2], cathodoluminescent displays and multilayer dielectric filters [3, 4]. Zinc sulfide shows transparency from the mid-IR through the visible region; hence, films may also be of use in optical phase modulation, IR antireflection coatings, and light guiding in integrated optics [5-7]. Two crystallographic modifications are commonly found for ZnS [8]. Both contain close-packed sulfur layers with zinc atoms occupying one half of the tetrahedral interstices. The low temperature, cubic polymorph, 13-ZnS, is known as sphalerite or zincblende, while the high temperature, hexagonal polymorph, ~-ZnS, is known as wurtzite. The AH ° of the sphalerite to wurtzite phase transition has recently been reported to be ca. - 2 . 5 + 1.5 kJmo1-1
[9]. Although the older literature, and even some of the current literature, indicate that there is an invariant transition temperature (ca. 1020 °C) between the cubic and hexagonal phases, Scott and Barnes [10] have shown this to be incorrect. These workers have estab-
lished that this transformation temperature is a function of both temperature and sulfur fugacity, high sulfur fugacities leading to zinc vacancies and transition temperatures above 1000 °C. Sulfur vacancies, resulting from low sulfur fugacities, can decrease the transition temperature to below 500 °C. Zinc sulfide thin films have been prepared directly by a variety of zinc and sulfur sources (Table 1), as well as by the conversion of zinc oxide films by treatment with H2S [22]. Recently, a variety of high quality metal oxide thin films have been prepared with metal acetyiacetonate precursors by spray pyrolysis with ultrasonic nebulization [23-26]. These metal complexes function as single-source precursors and deposit the corresponding metal oxide films without oxygen in the argon carrier gas. In contrast, most metal sulfide films (see Table 1) have been prepared using separate metal and sulfur sources. The use of single-source precursors for the fabrication of metal sulfide films has been reported by only a few workers [27-34]. Bismuth [30], copper [31], and zinc [32-34] sulfide films have been prepared from the corresponding dithiocarbamate complexes. In this paper, the preparation of both cubic and hexagonal zinc sulfide films by spray pyrolysis of toluene solutions of bis(diethyldithiocarbamato)zinc(II) is reported.
Elsevier Sequoia
R. D. Pike ctal. ,' Zinc .~u(/idefilm/ahricalion
222
TABLE 1. Literature preparations of zinc sullide films Reference
Precursors
Technique
Substratc
Temperature ( ('}
ZnS phase
(orientation) I1, 12 11 12 12 13 14 15 16 17
ZnCl 2, Na, S
Dipping
ZnCI. - .vMeOH, (H2N)2CS ZnEI 2, CS~
l)ipping MOCVD MOCVD MOCVD MOCVD
18 19 20
ZnMe 2, Et~S ZnMe2, tt2S ZnMe 2 or, ZnEt> H , S Zm S~ Zn, H , S Zn, Ss
21 This work
ZnS Zn(S2CNEt2) 2
H z vapor transport Spray pyrolysis
MBE MBE ALE
Indium tin oxide on glass Mo lnP( 1 l I) GaAs(1(/0) Glass Glass Si(lll} Glass Glass GaP( 100} GaAs(100) Sit 10t)) GaAs(100) Glass AI20~ Ta20 ~ GaP(Ill) St(Ill) Sit I00) A120~ 1001) AI20~ (012) GaAs (100)
25
400 500 300 4511 550 151) 500 350 440 34O 360 500 500 5011 710 851) 460 5211
Cubic" Cubic" Cubic ( 11 I1 (Tubic { 100) Hexagonal" 1 lexagonal (002)" Hexagonal {[)02)" Hexagonal (002)" Cubic {111)' Cubic (1tl0) Cubic {100) Cubic ( 100} Cubic (100) [tcxagonaP l texagonal" tfexagonaF' Cubic ( I I 1) and Hexagonal (002)
Hexagonal (002) Hexagonal (002) ]texagonal (002) Hexagonal ( 01121 Cubic (100)
:'Presence of the other phase not excluded. MOCVD. metalorganic chemical vapor deposition; MBE, molecular beam epitaxy: ,ALE, atomic layer epitax?.
2. Experimental details
2.1. Thermal decomposition study of bis(diethyldithiocarbamato )zinc(II) Bis(diethyldithiocarbamato)zinc(II) was purchased from Aldrich and recrystallized from 9:1 toluene:hexane. The pyrolysis system used was a continuous microfurnace-type pyrolyzer (pyrojector, Scientific Glass Engineering Pty. Ltd.). The pyrojector was modified by removing the narrow bore stainless steel tubing located at the exit of the furnace and replacing it with 0.53 mm fused silica tubing. This modification minimized clogging of the tubing at the exit of the reactor. After evaporation of a solution containing approximately 50 btg of the precursor onto a solids injector syringe (Scientific Glass Engineering), the sample was introduced directly into the furnace via injection. On exiting the pyrojector, the pyrolysis products were first concentrated in a trap cooled by liquid nitrogen, and then allowed to pass into the column for analysis. Analyses of the products were obtained using a Hewlett-Packard model 5987A gas chromatograph and mass spectrometer and a 15 m column containing a 0.25 I;m thick DB-5 stationary phase (J & W Scientific). The temperature was programmed from 30 to 300 ~'C at
20 C rain ~ for all the analyses. Highly volatile gases, such as ethylene, would not be detected efficiently using this procedure.
2.2. Preparation qf zinc s'ulfide fihns' The apparatus used for the preparation of the films (Fig. 1) is similar to that previously reported [21 24], except for modifications made to allow purging of the apparatus with argon to remove oxygen. The substrates were held either perpendicular to the flow of gas using a quartz substrate holder, or placed on a graphite block tilted 7' from the horizontal. A thoroughly degassed solution of the precursor dissolved in toluene (0.01 M) was nebulized and transported to the heated substrate using argon as a carrier gas. Silicon( 111 ) ( 5 - 1 0 fl cm and 0.01 f~ cm), Sit 100) 1613 f~ cm), sapphire(012) and (001), and undoped, semiinsulating GaAs, orientation (100), off 2 ;' toward ( 110}, were used as substrates. Silicon substrates were reduced prior to deposition by heating at 800 C in 15%H~ 85%Ar for lb. Sapphire substrates were etched with bromine, followed by hydrogen peroxide, while gallium arsenide wafers were cleaned in acetone, methanol and etched in 5:1:1 H2SOn:H202:H20. Conditions used for preparing the ZnS films are summarized in Table 2.
223
R. D. Pike et al. / Zinc sulfide film fabrication
TABLE 3. Calculated X-ray tilting angles
1
1
Furnace
Primary lattice plane
Additional lattice plane
~ (deg)
c~ (deg)
Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal
Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal
90 0 62.1 43.4 90
0 90 27.9 46,6 0
0, 70.5 54.7 35.3
90, 19.5 35.3 54.7
(001) (001) (001) (001) (001)
Cubic (111) Cubic ( I 11) Cubic (111)
(100) (002) (101) (102) (110)
Cubic (111) Cubic (200) Cubic (220)
Ultrasonic~ Supply ~ I Relaf']
1. Rotoflow Valves 4. Ultrasonic Transducer 2. Purge Stone 5. Spray Nozzle 3. Solution 6. Substrate Fig. 1. Ultrasonic spray pyrolysis apparatus.
TABLE 2. Conditions for preparation of zinc sulfide films Solution Substrates Substrate temperature ("C) Substrate to nozzle distance (cm) ID of nozzle (ram) ID of reactor (mm) Argon flow rate (1 min ~) Nebulization cycle (min) Deposition rate (A min ~)
Zn( $2CNEt2)2, 10 mM in toluene Si(lll), Si(100), AI203 (012), A1203 (001), GaAs(100) 460-520 8.9 9.5 (tip inclined upward at ca. 15° angle) 36 3.2 0.5 on, 1.0 off (continuous gas flow) 90
ID, internal diameter.
calculated. The pole figure angle ~ is equal to 90 ° - ~b. Some calculated values for ~b and ~ for hexagonal (001) and cubic (111) oriented ZnS are listed in Table 3. The samples were held at the calculated angles ~ and were then scanned through 20 along an orthogonal axis. The surface topography of the films was studied by scanning electron microscopy using a JEOL JSM-840F operating at 2.0 kV. Optical measurements of the films on polished sapphire substrates were recorded using a Cary Model 17 dual-beam ratio recording spectrophotometer in the range 300-800 nm. An attempt was made to determine the optical band gap from the transmittance near the absorption edge. IR spectra at room temperature were obtained on a Perkin-Elmer 580 single-beam scanning IR spectrophotometer at an instrumental resolution of 2.8cm -1. The measurements were recorded in the transmission mode over the range 2 0 0 - 4 0 0 0 c m -1. Transmission through the sample was normalized to the signal obtained in the absence of a sample.
2.3. Characterization of zinc sulfide films
3. Results and discussion
The thickness of step-etched films was measured using a Sloan Dektak apparatus equipped with a microtip surface sensor, and uniformity was determined using a Rudolph Research Auto EI-I1 ellipsometer. Standard X-ray diffraction patterns were obtained using a Philips diffractometer with monochromated high intensity Cu K~ 1 radiation (2 = 1.5405 ~). Diffraction patterns were taken with a scan rate of 1°20 min-1 over the range of 12° < 20 < 78 °. Tilt angle X-ray analyses were carried out on a four-circle pole figure diffractometer attached to a Rigaku RU-300 rotating copper anode diffractometer. The tilt angles 0 required to bring additional cubic or hexagonal planes into diffraction were first
The decomposition of bis(diethyldithiocarbamato)zinc(II) was studied by gas chromatography and mass spectrometry (GC-MS) between 400 and 500 °C. The major volatile products detected by GC-MS had molecular ions of m/z = 177 and 87, which are assigned to the isothiocyanate and the thioamide shown in eqn. (1). The presence of diethylamine and carbon disulfide most probably results from the further thermal decomposition of the initially formed thioamide, although this could not be verified since ethylene, the other expected product of the thioamide thermolysis, could not be detected efficiently using our apparatus:
Zn(S2CN(C2H5)2)2
, ZnS + C2HsNCS + (C2Hs)2NCS2CzH5 (C2Hs)2NH + CS2 + C2H4 '
I
(1)
224
R. D. Pike el al. / Zim' ~ullide /ilm /ahricalion
It is possible that other decomposition routes also exist. Uniform films of zinc sulfide, ranging in thickness from 0.1 to 0.5 p,m, were readily prepared on singlecrystal silicon, sapphire and gallium arsenide substrates. The films were highly reflective and adhered well to all three substrates (adhesive tape test). Noticeable darkening of the film occurred when greater thicknesses were grown. Film thicknesses were determined by ellipsometry. Using this technique, a 0.21 pm film was found to have a uniform thickness within 1%. A scanning electron micrograph of a 0.5 pm thick film on S i ( l l l ) is shown in Fig. 2. From the micrograph it can be seen that the average crystallite diameter is approximately 0.3 p m . A high degree of crystallographic orientation was observed when 0.5pm ZnS films on Si(lll), Si(100) and sapphire(001) were analyzed by X-ray diffraction. Less orientation was obtained with the films grown on sapphire(012). The X-ray pattern for ZnS on S i ( l l l ) is shown in Fig. 3. The intense peak at 20 28.6 (d = 3.12 ,~) corresponds to the hexagonal (002) and/or the cubic ( 111 ) planes of ZnS. Analysis of an uncoated substrate revealed only a very weak S i ( l l l ) line at 20 28.4, which is a result of off-axis cutting by ca. 3 4 . Hence, the primary contribution to this peak is due to the zinc sulfde. The very weak lines at 20 51.9 (d = 1.76/~) and 20 5 9 . 3 (d = 1.56/~) correspond to the hexagonal (103) plane and the hexagonal (004)
C t'-
0
20
30
40
50
60
70
2 Theta
Fig. 3. X-ray diffraction pattern of a 0.5 12111ZllS lihn on Si( II I) and/or cubic (222) planes respectively. ZnS films deposited onto sapphire (012) displayed a second intense line at 20 47.8 ( d - 1.91 /~), which can be assigned to the hexagonal (110) and/or cubic (220) planes. The peak at d - 1 . 7 6 / ~ is unique to the hexagonal phase, which must, therefore, be present. Since standard X-ray difl)'action analysis could not unequivocally exclude the coexistence of the cubic phase, tilt angle diffraction was used to characterize further the phases present. Owing to the polycrystalline nature of the films, the tolerance of ~ was l\)und to be rather large. For example, scanning 20 at :~ = 15, the cubic (111t plane at 20 = 2 8 . 6 was not seen, but the hexagonal (100), (101) and (110) planes were observed at 26.9, 30.5' and 47.5 , 20, respectively (see Fig. 4). Using fot.r different tilt angles, no cubic reflections were located by this technique, but all expected hexagonal lines were observed. These results confirm that the films on the HEX 100~
HEX (101)
HEX {110)
09 C mC
. . . .
25
i
. . . .
I
30
. . . .
i
. . . .
I
. . . .
i
. . . .
35
I
40
. . . .
P
. . . .
I
45
. . . .
i
. . . .
t
50
2 Theta
Fig. 2. Scanning electron micrograph of a 0.5 pm ZnS tilm on Si(I II).
Fig. 4. Tilt anglc diffraction scan of a 0.5 pm ZnS fihn on Si(lll) (tile pole ligure tilt angle is 15 1.
R. O. Pike et al. / Zinc sulfide film fabrication
silicon and sapphire substrates consist of highly c-axisoriented hexagonal ZnS. A complete pole figure of a ZnS film on Si(111) is consistent with the films having a fiber texture. In contrast to these results, the cubic phase of zinc sulfide was formed on GaAs(100) under similar deposition conditions. Standard X-ray diffraction revealed intense lines at 20 33.1 ° ( d = 2 . 7 0 / ~ ) and 20 69.5 ° (d = 1.35 A). These peaks correspond to the (200) and (400) lattice planes and are unique to the cubic phase, indicating that a highly oriented (100) ZnS film was formed on this substrate. Unequivocal characterization of the crystallographic phase is difficult for oriented ZnS films, and is often absent in the literature. Table 1 reveals that the growth of hexagonal ZnS films generally occurs at higher temperatures [13, 20] but can occur at temperatures as low as 300 °C. Cubic thin films have been produced at ambient temperatures by dip-coating [11, 12], as well as by molecular beam or atomic layer epitaxial growth techniques carried out below 400 °C. In one instance, however, films assigned as cubic ZnS films were obtained by MOCVD growth on glass at 500 °C [ 16]. The above results suggest that a combination of factors determines the crystalline phase of ZnS that is formed in these thin film deposition processes. Two important variables clearly identified by Scott and Barnes [ 10], are the temperature and the sulfur fugacity. Low temperature formation of the hexagonal phase has been observed by many investigators under sulfur-deficient conditions. Under these conditions, for example, the cubic-hexagonal phase boundary can occur below 500 °C. It is also evident from the literature that the use of I I I - V substrates can lead to cubic films [17-19]. Cubic zinc sulfide shows a reasonable lattice match to cubic GaAs (95.7%) and an excellent match to cubic
225
GaP (greater than 99%) [35] (ZnS a = 5.406 ,~, GaAs a = 5.654/~, GaP a = 5.451 ~). The use of such substrates promotes the deposition of cubic ZnS films even at temperatures where the hexagonal phase is usually formed [21]. IR transmission properties of the films on silicon were also determined. Figure 5 shows the measured transmittance of a 0.5 gm ZnS film on S i ( l l l ) . The absorption due to the silicon substrate is included for comparison. The film displays high transparency in the IR, characteristic of zinc sulfide. The strong, principal lattice absorption band of ZnS occurs at 278 cm -1, which is in good agreement with literature values [22, 36, 371.
4. Conclusions This work illustrates that there are at least three important parameters that influence the zinc sulfide polymorph formed in film fabrication processes: temperature, sulfur fugacity, and the nature and orientation of the substrate. On amorphous or poorly lattice matched substrates, the factors that dominate are the temperature and the sulfur fugacity, while on closely lattice matched substrates, the cubic phase can be formed even under conditions (i.e. high temperatures) that normally favor the hexagonal phase.
Acknowledgments This work was partial!y supported by NSF Contract No. D M R 901-3602 and by Eastman Kodak Company, Rochester, NY.
References I00
1 G. O. R. Mach, Phys. Status Solidi A, 69 (1982) 11. 2 T. E m m a and M. M c D o n o u g h , J. Vac. Sci. Technol. A, 2 (1984) 362. 3 A. Preisinger and H. K. Pulker, Jpn. J. Appl. Phys,, Suppl. 2, Part 1 (1974) 769. 4 A. M. Ledger, Appl. Opt., 18 (1979) 2979. 5 P. L. Jones, D. Moore and D. C. Smith, J. Phys. E, 9(1976) 312. 6 P. L. Jones, D. R. Cotton and D. Moore, Thin SolM Films, 88 (1982) 163. 7 J. A. Aguilera, J. Aguilera, P. Baumeister, A. Bloom, D. Coursen, J. A. Dobrowolski, F. T. Goldstein, D. E. Gaustafson and R. A. Kemp, Appl. Opt., 27(1988) 2832. 8 E. H. Nickel, Department of Mines Technical Survey, Ottawa, Inf.
8o
~o
6o
40
20
0
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0
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I
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i000
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,
2000
Wovenomber
.
.
.
.
T
3000
.
.
.
Circ., No. 170.
.
4000
( c m -I)
Fig~ 5. Transmission IR spectrum of a 0.5 pm ZnS layer on an S i ( l l l ) wafer.
9 P. J. Gardner and P. Pang, J. Chem. Soc., Faraday Trans. 1, 84 (1988) 1879. 10 S. D. Scott and H. L. Barnes, Geoehim. Cosmochim, Acta, 36(1972) 1275. 11 Y. F. Nicolau and J. C. Menard, J. Cryst. Growth, 92 (1988) 128.
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R. D. Pike el al. / Zinc su([ide film ./+,bricalion
12 Y. F. Nicolau, M. Dupuy and M. Brunel, J. Eleetrochem. Sot., 137 119901. 2915. 13 T. M a r u y a m a and T. Kawaguchi, Thin Solid Films, 188(1990) 323, 14 S. Takata, T. Minami, T. Miyata and H. Nanto. J. Crvst. Growth, 86 ( 19881 257. 15 T. Shibata, K. Hirabayashi and H. Kozawaguchi, Ji m. .I. Appl. Phys., 26 (19871 L1664. 16 K. Hirabayashi and O. Kogurc, Jpn. J. Appl. Phys., 24( 19851 1484. 17 S. Fujita, T. T o m o m u r a and A. Sasaki, Jpn..I. AppL Phys.. 22 ( 19831 L583. 18 M. Yokoyama, K.-I. Kashiro and S.-I. Ohta. Appl. Phys. Lett., 49 (1986) 411. 19 S. Kaneda, S. Satou, T. Setoyama, S.-I. Motoyama, M. Yokoyama and N. Ota, J. Co'st. Growth, 76 ( 19861 440. 20 V.-P. Tanninen. M. Oikkonen and T. O. Tuomi, Phys. Status Solidi ,4, 67(1981) 573. 21 S. Fuke, H. Araki, H. K. Kuwahara and T. lmai, J. Ap/d. Phys., 59 (1986) 1761. 22 Y.-M. Gao, P. Wu, J. Baglio, K. Dwight and A. Wold, Mater. Re.~. Bull., 24 ( 19891 1215, 23 Y.-M. Gao, P. Wu, R, Kershaw, K. Dwight and A. Wold, Mater. Res. Butt., 25(1990) 871. 24 Y,-T. Qian, R. Kershaw, K. Dwight and A. Wold, Mater. Res. Bull., 25 (1990) 1243.
25 W. W. Xu, R. Kershaw. K. Dwight and A. Wold, Mater. Res. Bull., 25 (1990) 1385. 26 Y.-M. Gao, P. Wu, K. Dwight and A. Wold, J. Solid Stale ('hem.. O0 (1992/ 228. 27 Y. Takahashi, R. Yuki, S. Motojima and K. Sugiyama, J. O y s t . Growth, 50 (1980) 491. 28 M. A. H. Evans and J. O. Williams, Thin SolidFihm', 87(1982) 1,1. 29 R. Nomura, K. Kanaya and H. Matsuda, Chem. Letl., ( 19881 1849. 30 R. Nomura, K. Kanaya and H. Matsuda. Bull. Chem. Soc..lira.. 62 (1989) 939. 31 R. Nomura, K. Kanaya and H. Matsuda, Ind. Eng. ('hem. Re.~., 2S (1989) 877. 32 D. M. Frigo, O. F. Z. Khan and P. O'Brien, ,/. ('ry,~t. Growth, 9(~ (1989) 989. 33 B. L. Druz, A. 1. Dyadenko, Y. N. Evtukhov, M. Y. Rakhlin and V. E. Rodionov, lnorg. Mater., 26 (1990) 24. 34 A. Saunders, A. Vecht and G. Tyrrell, in S. K. Deb and A. Zunger (eds.), Proc. 7th Inl. Uo~7[~on Ternary and Multinar.v UompoumL~, 5h,owmass, 4"0, 198& Material Research Society, Pittsburgh, PA. p. 213. 35 H. Kukimoto, .I. ()'yst. Growth, 107(19911 637. 36 (-). Brafman and S. S. Mitra, Phys. Rez., 171 ( 19681 931. 37 A. Memon and S. B. Tanner, Phys. Status Solidi B, 12S (1995) 49.
221
Preparation of zinc sulfide thin films by ultrasonic spray pyrolysis from bis(diethyldithiocarbamato)zinc(II) R. D. Pike, H. Cui, R. Kershaw, K. Dwight and A. Wold Department of Chemistry, Brown University, Providenee, RI 02912 (USA)
T. N. Blanton Analytical Technology Division, Eastman Kodak Company, Rochester, N Y 14650 (USA)
A. A. Wernberg and H. J. Gysling Corporate Research Laboratories, Eastman Kadak Company, Rochester, N Y 14650 (USA) (Received August 6, 1992; accepted October 7, 1992)
Abstract
Thin films of zinc sulfide were prepared by ultrasonically spraying a toluene solution of bis(diethyldithiocarbamato)zinc(II) onto silicon, sapphire and gallium arsenide substrates at 460-520 °C. The films prepared on silicon or sapphire were found to have a highly oriented hexagonal structure, while those deposited onto cubic (100) gallium arsenide showed a highly oriented cubic structure. The films were characterized by X-ray diffraction analysis, ellipsometry, scanning electron microscopy, and IR spectroscopy.
I. Introduction
Zinc sulfide is a I I - V I semiconductor with a large direct band gap in the near-UV region. A substantial effort has been directed at the preparation of ZnS thin films for use in electroluminescent displays [1, 2], cathodoluminescent displays and multilayer dielectric filters [3, 4]. Zinc sulfide shows transparency from the mid-IR through the visible region; hence, films may also be of use in optical phase modulation, IR antireflection coatings, and light guiding in integrated optics [5-7]. Two crystallographic modifications are commonly found for ZnS [8]. Both contain close-packed sulfur layers with zinc atoms occupying one half of the tetrahedral interstices. The low temperature, cubic polymorph, 13-ZnS, is known as sphalerite or zincblende, while the high temperature, hexagonal polymorph, ~-ZnS, is known as wurtzite. The AH ° of the sphalerite to wurtzite phase transition has recently been reported to be ca. - 2 . 5 + 1.5 kJmo1-1
[9]. Although the older literature, and even some of the current literature, indicate that there is an invariant transition temperature (ca. 1020 °C) between the cubic and hexagonal phases, Scott and Barnes [10] have shown this to be incorrect. These workers have estab-
lished that this transformation temperature is a function of both temperature and sulfur fugacity, high sulfur fugacities leading to zinc vacancies and transition temperatures above 1000 °C. Sulfur vacancies, resulting from low sulfur fugacities, can decrease the transition temperature to below 500 °C. Zinc sulfide thin films have been prepared directly by a variety of zinc and sulfur sources (Table 1), as well as by the conversion of zinc oxide films by treatment with H2S [22]. Recently, a variety of high quality metal oxide thin films have been prepared with metal acetyiacetonate precursors by spray pyrolysis with ultrasonic nebulization [23-26]. These metal complexes function as single-source precursors and deposit the corresponding metal oxide films without oxygen in the argon carrier gas. In contrast, most metal sulfide films (see Table 1) have been prepared using separate metal and sulfur sources. The use of single-source precursors for the fabrication of metal sulfide films has been reported by only a few workers [27-34]. Bismuth [30], copper [31], and zinc [32-34] sulfide films have been prepared from the corresponding dithiocarbamate complexes. In this paper, the preparation of both cubic and hexagonal zinc sulfide films by spray pyrolysis of toluene solutions of bis(diethyldithiocarbamato)zinc(II) is reported.
Elsevier Sequoia
R. D. Pike ctal. ,' Zinc .~u(/idefilm/ahricalion
222
TABLE 1. Literature preparations of zinc sullide films Reference
Precursors
Technique
Substratc
Temperature ( ('}
ZnS phase
(orientation) I1, 12 11 12 12 13 14 15 16 17
ZnCl 2, Na, S
Dipping
ZnCI. - .vMeOH, (H2N)2CS ZnEI 2, CS~
l)ipping MOCVD MOCVD MOCVD MOCVD
18 19 20
ZnMe 2, Et~S ZnMe2, tt2S ZnMe 2 or, ZnEt> H , S Zm S~ Zn, H , S Zn, Ss
21 This work
ZnS Zn(S2CNEt2) 2
H z vapor transport Spray pyrolysis
MBE MBE ALE
Indium tin oxide on glass Mo lnP( 1 l I) GaAs(1(/0) Glass Glass Si(lll} Glass Glass GaP( 100} GaAs(100) Sit 10t)) GaAs(100) Glass AI20~ Ta20 ~ GaP(Ill) St(Ill) Sit I00) A120~ 1001) AI20~ (012) GaAs (100)
25
400 500 300 4511 550 151) 500 350 440 34O 360 500 500 5011 710 851) 460 5211
Cubic" Cubic" Cubic ( 11 I1 (Tubic { 100) Hexagonal" 1 lexagonal (002)" Hexagonal {[)02)" Hexagonal (002)" Cubic {111)' Cubic (1tl0) Cubic {100) Cubic ( 100} Cubic (100) [tcxagonaP l texagonal" tfexagonaF' Cubic ( I I 1) and Hexagonal (002)
Hexagonal (002) Hexagonal (002) ]texagonal (002) Hexagonal ( 01121 Cubic (100)
:'Presence of the other phase not excluded. MOCVD. metalorganic chemical vapor deposition; MBE, molecular beam epitaxy: ,ALE, atomic layer epitax?.
2. Experimental details
2.1. Thermal decomposition study of bis(diethyldithiocarbamato )zinc(II) Bis(diethyldithiocarbamato)zinc(II) was purchased from Aldrich and recrystallized from 9:1 toluene:hexane. The pyrolysis system used was a continuous microfurnace-type pyrolyzer (pyrojector, Scientific Glass Engineering Pty. Ltd.). The pyrojector was modified by removing the narrow bore stainless steel tubing located at the exit of the furnace and replacing it with 0.53 mm fused silica tubing. This modification minimized clogging of the tubing at the exit of the reactor. After evaporation of a solution containing approximately 50 btg of the precursor onto a solids injector syringe (Scientific Glass Engineering), the sample was introduced directly into the furnace via injection. On exiting the pyrojector, the pyrolysis products were first concentrated in a trap cooled by liquid nitrogen, and then allowed to pass into the column for analysis. Analyses of the products were obtained using a Hewlett-Packard model 5987A gas chromatograph and mass spectrometer and a 15 m column containing a 0.25 I;m thick DB-5 stationary phase (J & W Scientific). The temperature was programmed from 30 to 300 ~'C at
20 C rain ~ for all the analyses. Highly volatile gases, such as ethylene, would not be detected efficiently using this procedure.
2.2. Preparation qf zinc s'ulfide fihns' The apparatus used for the preparation of the films (Fig. 1) is similar to that previously reported [21 24], except for modifications made to allow purging of the apparatus with argon to remove oxygen. The substrates were held either perpendicular to the flow of gas using a quartz substrate holder, or placed on a graphite block tilted 7' from the horizontal. A thoroughly degassed solution of the precursor dissolved in toluene (0.01 M) was nebulized and transported to the heated substrate using argon as a carrier gas. Silicon( 111 ) ( 5 - 1 0 fl cm and 0.01 f~ cm), Sit 100) 1613 f~ cm), sapphire(012) and (001), and undoped, semiinsulating GaAs, orientation (100), off 2 ;' toward ( 110}, were used as substrates. Silicon substrates were reduced prior to deposition by heating at 800 C in 15%H~ 85%Ar for lb. Sapphire substrates were etched with bromine, followed by hydrogen peroxide, while gallium arsenide wafers were cleaned in acetone, methanol and etched in 5:1:1 H2SOn:H202:H20. Conditions used for preparing the ZnS films are summarized in Table 2.
223
R. D. Pike et al. / Zinc sulfide film fabrication
TABLE 3. Calculated X-ray tilting angles
1
1
Furnace
Primary lattice plane
Additional lattice plane
~ (deg)
c~ (deg)
Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal
Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal
90 0 62.1 43.4 90
0 90 27.9 46,6 0
0, 70.5 54.7 35.3
90, 19.5 35.3 54.7
(001) (001) (001) (001) (001)
Cubic (111) Cubic ( I 11) Cubic (111)
(100) (002) (101) (102) (110)
Cubic (111) Cubic (200) Cubic (220)
Ultrasonic~ Supply ~ I Relaf']
1. Rotoflow Valves 4. Ultrasonic Transducer 2. Purge Stone 5. Spray Nozzle 3. Solution 6. Substrate Fig. 1. Ultrasonic spray pyrolysis apparatus.
TABLE 2. Conditions for preparation of zinc sulfide films Solution Substrates Substrate temperature ("C) Substrate to nozzle distance (cm) ID of nozzle (ram) ID of reactor (mm) Argon flow rate (1 min ~) Nebulization cycle (min) Deposition rate (A min ~)
Zn( $2CNEt2)2, 10 mM in toluene Si(lll), Si(100), AI203 (012), A1203 (001), GaAs(100) 460-520 8.9 9.5 (tip inclined upward at ca. 15° angle) 36 3.2 0.5 on, 1.0 off (continuous gas flow) 90
ID, internal diameter.
calculated. The pole figure angle ~ is equal to 90 ° - ~b. Some calculated values for ~b and ~ for hexagonal (001) and cubic (111) oriented ZnS are listed in Table 3. The samples were held at the calculated angles ~ and were then scanned through 20 along an orthogonal axis. The surface topography of the films was studied by scanning electron microscopy using a JEOL JSM-840F operating at 2.0 kV. Optical measurements of the films on polished sapphire substrates were recorded using a Cary Model 17 dual-beam ratio recording spectrophotometer in the range 300-800 nm. An attempt was made to determine the optical band gap from the transmittance near the absorption edge. IR spectra at room temperature were obtained on a Perkin-Elmer 580 single-beam scanning IR spectrophotometer at an instrumental resolution of 2.8cm -1. The measurements were recorded in the transmission mode over the range 2 0 0 - 4 0 0 0 c m -1. Transmission through the sample was normalized to the signal obtained in the absence of a sample.
2.3. Characterization of zinc sulfide films
3. Results and discussion
The thickness of step-etched films was measured using a Sloan Dektak apparatus equipped with a microtip surface sensor, and uniformity was determined using a Rudolph Research Auto EI-I1 ellipsometer. Standard X-ray diffraction patterns were obtained using a Philips diffractometer with monochromated high intensity Cu K~ 1 radiation (2 = 1.5405 ~). Diffraction patterns were taken with a scan rate of 1°20 min-1 over the range of 12° < 20 < 78 °. Tilt angle X-ray analyses were carried out on a four-circle pole figure diffractometer attached to a Rigaku RU-300 rotating copper anode diffractometer. The tilt angles 0 required to bring additional cubic or hexagonal planes into diffraction were first
The decomposition of bis(diethyldithiocarbamato)zinc(II) was studied by gas chromatography and mass spectrometry (GC-MS) between 400 and 500 °C. The major volatile products detected by GC-MS had molecular ions of m/z = 177 and 87, which are assigned to the isothiocyanate and the thioamide shown in eqn. (1). The presence of diethylamine and carbon disulfide most probably results from the further thermal decomposition of the initially formed thioamide, although this could not be verified since ethylene, the other expected product of the thioamide thermolysis, could not be detected efficiently using our apparatus:
Zn(S2CN(C2H5)2)2
, ZnS + C2HsNCS + (C2Hs)2NCS2CzH5 (C2Hs)2NH + CS2 + C2H4 '
I
(1)
224
R. D. Pike el al. / Zim' ~ullide /ilm /ahricalion
It is possible that other decomposition routes also exist. Uniform films of zinc sulfide, ranging in thickness from 0.1 to 0.5 p,m, were readily prepared on singlecrystal silicon, sapphire and gallium arsenide substrates. The films were highly reflective and adhered well to all three substrates (adhesive tape test). Noticeable darkening of the film occurred when greater thicknesses were grown. Film thicknesses were determined by ellipsometry. Using this technique, a 0.21 pm film was found to have a uniform thickness within 1%. A scanning electron micrograph of a 0.5 pm thick film on S i ( l l l ) is shown in Fig. 2. From the micrograph it can be seen that the average crystallite diameter is approximately 0.3 p m . A high degree of crystallographic orientation was observed when 0.5pm ZnS films on Si(lll), Si(100) and sapphire(001) were analyzed by X-ray diffraction. Less orientation was obtained with the films grown on sapphire(012). The X-ray pattern for ZnS on S i ( l l l ) is shown in Fig. 3. The intense peak at 20 28.6 (d = 3.12 ,~) corresponds to the hexagonal (002) and/or the cubic ( 111 ) planes of ZnS. Analysis of an uncoated substrate revealed only a very weak S i ( l l l ) line at 20 28.4, which is a result of off-axis cutting by ca. 3 4 . Hence, the primary contribution to this peak is due to the zinc sulfde. The very weak lines at 20 51.9 (d = 1.76/~) and 20 5 9 . 3 (d = 1.56/~) correspond to the hexagonal (103) plane and the hexagonal (004)
C t'-
0
20
30
40
50
60
70
2 Theta
Fig. 3. X-ray diffraction pattern of a 0.5 12111ZllS lihn on Si( II I) and/or cubic (222) planes respectively. ZnS films deposited onto sapphire (012) displayed a second intense line at 20 47.8 ( d - 1.91 /~), which can be assigned to the hexagonal (110) and/or cubic (220) planes. The peak at d - 1 . 7 6 / ~ is unique to the hexagonal phase, which must, therefore, be present. Since standard X-ray difl)'action analysis could not unequivocally exclude the coexistence of the cubic phase, tilt angle diffraction was used to characterize further the phases present. Owing to the polycrystalline nature of the films, the tolerance of ~ was l\)und to be rather large. For example, scanning 20 at :~ = 15, the cubic (111t plane at 20 = 2 8 . 6 was not seen, but the hexagonal (100), (101) and (110) planes were observed at 26.9, 30.5' and 47.5 , 20, respectively (see Fig. 4). Using fot.r different tilt angles, no cubic reflections were located by this technique, but all expected hexagonal lines were observed. These results confirm that the films on the HEX 100~
HEX (101)
HEX {110)
09 C mC
. . . .
25
i
. . . .
I
30
. . . .
i
. . . .
I
. . . .
i
. . . .
35
I
40
. . . .
P
. . . .
I
45
. . . .
i
. . . .
t
50
2 Theta
Fig. 2. Scanning electron micrograph of a 0.5 pm ZnS tilm on Si(I II).
Fig. 4. Tilt anglc diffraction scan of a 0.5 pm ZnS fihn on Si(lll) (tile pole ligure tilt angle is 15 1.
R. O. Pike et al. / Zinc sulfide film fabrication
silicon and sapphire substrates consist of highly c-axisoriented hexagonal ZnS. A complete pole figure of a ZnS film on Si(111) is consistent with the films having a fiber texture. In contrast to these results, the cubic phase of zinc sulfide was formed on GaAs(100) under similar deposition conditions. Standard X-ray diffraction revealed intense lines at 20 33.1 ° ( d = 2 . 7 0 / ~ ) and 20 69.5 ° (d = 1.35 A). These peaks correspond to the (200) and (400) lattice planes and are unique to the cubic phase, indicating that a highly oriented (100) ZnS film was formed on this substrate. Unequivocal characterization of the crystallographic phase is difficult for oriented ZnS films, and is often absent in the literature. Table 1 reveals that the growth of hexagonal ZnS films generally occurs at higher temperatures [13, 20] but can occur at temperatures as low as 300 °C. Cubic thin films have been produced at ambient temperatures by dip-coating [11, 12], as well as by molecular beam or atomic layer epitaxial growth techniques carried out below 400 °C. In one instance, however, films assigned as cubic ZnS films were obtained by MOCVD growth on glass at 500 °C [ 16]. The above results suggest that a combination of factors determines the crystalline phase of ZnS that is formed in these thin film deposition processes. Two important variables clearly identified by Scott and Barnes [ 10], are the temperature and the sulfur fugacity. Low temperature formation of the hexagonal phase has been observed by many investigators under sulfur-deficient conditions. Under these conditions, for example, the cubic-hexagonal phase boundary can occur below 500 °C. It is also evident from the literature that the use of I I I - V substrates can lead to cubic films [17-19]. Cubic zinc sulfide shows a reasonable lattice match to cubic GaAs (95.7%) and an excellent match to cubic
225
GaP (greater than 99%) [35] (ZnS a = 5.406 ,~, GaAs a = 5.654/~, GaP a = 5.451 ~). The use of such substrates promotes the deposition of cubic ZnS films even at temperatures where the hexagonal phase is usually formed [21]. IR transmission properties of the films on silicon were also determined. Figure 5 shows the measured transmittance of a 0.5 gm ZnS film on S i ( l l l ) . The absorption due to the silicon substrate is included for comparison. The film displays high transparency in the IR, characteristic of zinc sulfide. The strong, principal lattice absorption band of ZnS occurs at 278 cm -1, which is in good agreement with literature values [22, 36, 371.
4. Conclusions This work illustrates that there are at least three important parameters that influence the zinc sulfide polymorph formed in film fabrication processes: temperature, sulfur fugacity, and the nature and orientation of the substrate. On amorphous or poorly lattice matched substrates, the factors that dominate are the temperature and the sulfur fugacity, while on closely lattice matched substrates, the cubic phase can be formed even under conditions (i.e. high temperatures) that normally favor the hexagonal phase.
Acknowledgments This work was partial!y supported by NSF Contract No. D M R 901-3602 and by Eastman Kodak Company, Rochester, NY.
References I00
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