J. Phys. Chem. 1994, 98, 13308-13313
13308
Crystal Structure and Growth Mechanism of Lead Monoxide Thin Films Produced by Pulsed Laser Deposition Mitra I. Baleva,* Luchezar N. Bozukov, and Veneta D. Tuncheva Faculty of Physics, University of Sofia, 1126 Sofia, Bulgaria Received: February 24, 1994; In Final Form: July 7, 1994@
PbO films, grown by pulsed laser deposition (PLD) on two types of substrates (monocrystal quartz and amorphous glass) at different substrate temperatures, are investigated. The overall films' crystal structure is investigated by X-ray diffraction. The dependence of the film structure on the substrate temperature is considered in the framework of the classical thermodynamic theory. The considerations make clear why films with the a-PbO phase solely or predominantly cannot be grown.
Introduction PbO exists in two polymorphic forms: the low-temperature tetragonal phase, a-PbO, and the high-temperature orthorhombic phase, p-PbO. At 489 "C and atmospheric pressure a-PbO undergoes a phase transition to P-PbO.' At low temperatures, below 489 "C, the orthorhombic phase free energy is very close to that of the tetragonal phase.2 As a result, the orthorhombic phase, although metastable, exists below the transition temperature. An interest in the deposition of PbO films was first stimulated by the application of this material as a basis of the Vidicon camera tubes and lasts up to now. In particular, nowadays lead monoxide is regarded as a potential solid lubricant for use at elevated temperatures in oxidizing environm e n t ~ .As ~ the two phases of lead monoxide have different properties, a number of investigations have been undertaken to understand how to control the distribution of the two phases in the films. The structural investigations of the films, deposited by different techniques at different technological conditions, show that the films grow mostly with the orthorhombic phase with some admixture of the tetragonal This is supported by the reflectance high energy electron diffraction (RHEED) and Raman spectroscopy investigations of our PbO films produced by pulsed laser evaporation (PLD).8,9 The aim of the present work is the investigation of the overall PbO film structure by X-ray diffraction. The influence of the substrate temperature and substrate type on the film structure is considered in the framework of the general crystallization theory. Samples and Experiment Samples. The PbO films investigated are deposited by PLD on two types of substrates: (101) monocrystal quartz and amorphous glass. The substrates are polished to 0.01 p m and cleaned ultrasonically. The PLD is a highly nonequilibrium process for thin film production. It consists of interaction of the laser beam with the solid target, resulting in ejection of new phases, which condense on the substrate in the form of a film. A free generating Nd:glass laser (2. = 1.06 pm) with a pulse duration of 400 pus and a time interval between the pulses of 8 s was used. The pressure in the vacuum chamber was about 1.3 Pa. The parameters which can be varied in this technique are (1) substrate-target distance, Lst;(2) substrate temperature, T,; (3) pressure in the vacuum chamber, P; (4) energy of the laser pulse (or laser power density), P,;and ( 5 ) number of pulses, @
Abstract published in Advance ACS Abstracts, November 1, 1994.
0022-3654/94/2098-13308$04.50/0
N. The details of the PLD experimental setup are given earlier.1° Tablets of tetragonal PbO powder pressed at 1.1 x 104 kg/cm2 are used as targets. It is found" that PbO films can be grown at T, in the range from 20 to 300 "C and distance L,,= 3 cm. The number of pulses N is limited by the target damage. At Pj = 0.86 x lo5 W/cm*, for example, N has to be less than 150. Thus, the films cannot be thicker than 500 nm. The deposition rate, defined as v d = d/Nz (where d is film thickness and z is the laser pulse duration), varies from 5 pmls to 20 pm/s depending on the remaining technological parameters. The thicknesses of the films are determined from the interference fringes of the transmittance and reflectance spectra measured in the range from 3000 to 30 000 cm-' according to the methods described by Ohlidal et a1.12 and by Manifacier et a1.13 The data on the films investigated are summarized in Table 1. Experiment. The X-ray diffraction patterns of the films are obtained by the standard X-ray diffractometer URD-6 with graphite monochromatized Co K a radiation and with a step size of 0.02". The angular range 28 is varied from 15" to 37" in the case of the films deposited on quartz substrates and from 15" to 60" in the case of the films deposited on glass substrates. The diffraction patterns of the films deposited on quartz substrates are shown in Figure 1 and those of the films deposited on glass substrates in Figure 2. The polycrystal diffraction spectra of a-PbO and p-PbO with intensities and positions taken from PDF-2 Sets 1-43 database14 are shown by solid lines in Figures l a and 2a and in Figures l b and 2b, respectively. The (101) Si02 substrate reflection seen in the range investigated is indicated by a dashed line in Figure la. The fact that the reflection of the substrate is seen in the experimentaldiffraction patterns indicates that the X-ray penetration depth is greater than the thickness of the samples. The diffraction pattern of the glass substrate alone, shown in Figure 2c, indicates that the amorphous halo detected can be attributed to the substrate. The most intense reflections in the films deposited at different substrate temperatures on both types of substrates are those of the B-PbO phase; (Ool),(002), and (1 11) reflections of this phase are readily apparent in Figures 1 and 2. The ratio of the (002) and (1 11) reflections intensities in the polycrystal P-PbO phases is Z(mjZ(111)= 0.31.14 The ratio Z(002j4111) in the films, deposited at different T,, determined from the experimental X-ray diffraction patterns, is given in Table 1. It is seen that at T, = 50 "C the P-PbO phase is polycrystalline (sample Q1) on quartz substrate; the value of the ratio Zt00$(lll) is close to 0.31. On raising the substrate temperature, the value 4 ~ 2 j Z ( 1 1 1 )strongly increases, indicating in this way a preferred growth in the (001) 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98,No. 50, 1994 13309
Crystal Structure of Lead Monoxide Thin Films TABLE 1: PbO Film Characteristics" sample Q1
Ts,"C
50 120 d, nm B-PbO 1(00z)/1(111) 0.26 a-Pbo not detected
Q3
Q4
Q6
Q7
G2
G13
150 120 2.95 not detected
190 90 13.70 not detected
260 110 31.00 [OO 1I
310 120 121.00 roo11
150 180 0.55 not detected
300 300 164.00 polycrystal
a In the sample labeling, Q stands for quartz, G for glass substrate. The remaining technological parameters for all samples are L,, = 3 cm, P = Torr, E, = 0.734 J, and N = 150. The ratios 1(~&111) of the intensities of the (002) and (1 11) reflections of the p-PbO phase, determined from the X-ray diffraction pattems, are given.
0 30
J W c)
Ts=5O0C
w
-
3
f a 1
x al
1
2 0 , degrees Figure 1. X-ray diffraction pattems of PbO films, deposited on quartz
Figure 2. X-ray diffraction pattems of PbO films, deposited on glass
substrate at different substrate temperatures (c-g). The polycrystal diffraction spectral4 of (a) a-PbO and (b) B-PbO are shown by solid lines. The positionsI4 of the (100)* reflection of the PbO1.4 phase and the (111)x reflection of the metallic Pb are shown by dashed lines in part a. The (101) reflection of the quartz (SiOz) substrate is indicated by a dashed line in part a as well.
substrate at two different substrate temperatures (d, e). The X-ray diffraction pattem of (c) the amorphous glass substrate is given also. The polycrystal diffraction spectra of (a) a-PbO and (b) B-PbO are shown by solid lines. The positions of the (loo)* reflection of the Pb01.4 phase and the (1 1l ) x reflection of the Pb are shown14in part a.
direction of crystallization, growth along the c-axis, perpendicular to the substrate plane. Thus, the P-PbO phase in the film at T, = 310 "C grows almost entirely in the (001) direction. The same holds for the film grown on glass substrates. The difference is that the p-PbO phase is polycrystalline up to higher substrate temperatures. In the film deposited on glass substrate at T, = 150 "C the ratio of the intensities 4 ~ 4 4 1 1 1 = ) 0.55, while in the film deposited at the same temperature on quartz substrate Z(m2)/Z(111) = 2.95 (see Table 1). The a-PbO phase is detected only in the films deposited at high substrate temperatures. The (001) reflection of the a-PbO phase is seen only in the diffraction patterns of the films deposited on quartz substrates at T, = 260 and 310 "C; see Figure lf,g. The intensity of the (001) reflection of the polycrystal a-PbO is 20 times lower than its most intense (101) refle~ti0n.l~ The fact that only the less intense (001) reflection is seen in the experimental diffraction patterns of the films deposited on quartz substrates means that the a-PbO phase grows in the preferred [OOl] direction of crystallization. Thus, in the films deposited on quartz substrates the a-PbO phase
grows along the c-axis, perpendicular to the substrate plane. The (101) and (1 10) reflections of the a-PbO phase, denoted as lOlt and 110t in Figure 2e, are seen in the diffraction pattern of the film deposited on glass substrate at T, = 300 "C. These two reflections are the most intense reflections in the spectrum of the polycrystal a-PbO phase,14and the ratio of their intensities in the experimental spectrum is the same as in the spectrum shown in Figure 2a. Thus, the a-PbO phase detected in the films deposited on glass substrates at high substrate temperature is polycrystalline. Its quantity in the film is obviously small, the intensities of the a-PbO phase reflections are 1 order of magnitude lower than those of the p-PbO. Along with the reflections of the a-PbO and p-PbO phases two additional reflections are seen in all the experimental diffraction pattems. The intensities of these reflections are very low in the films produced on quartz substrates. The reflection at 28 = 32.8" is indexed as a (100) reflection of the cubic Pb01.4 phase and that at 28 = 36.4" as a Pb (1 11) reflection. Their positions are marked by dashed lines in Figure l a and Figure 2a.
Baleva et al.
13310 J. Phys. Chem., Vol. 98, No. 50, 1994 The analysis of the X-ray diffraction patterns of PbO films deposited by PLD on quartz and glass substrates leads to the following conclusions: (1) The films grow in the B-PbO phase predominantly. The crystallization of the phase depends strongly on the substrate temperature. On elevating the substrate temperature, an oriented growth in the (001) direction takes place. The oriented growth on the B-PbO phase depends on the substrate type. In the films deposited on quartz substrates it takes place at lower substrate temperatures. (2) The a-PbO phase is present in the films produced at high substrate temperatures, being in the preferred [0011 direction of crystallization in the films deposited on quartz substrates and polycrystalline in those deposited on glass substrates. (3) The presence of the Pb01.4 phase and metallic lead, though in small quantities, is detected in all of the films.
Obviously the validity of these relations is limited by the condition Vk > 1 or
TmM os- 0 assuming a lower value of Tm (Tm = 1120 K), and thus corresponding to polycrystal growth, are shown by dashed lines. In the same figure, the nucleation rate temperature dependences for the monocrystal a-PbO (curve 2a) and p-PbO (curve la) are reproduced for comparison. It is seen that at a high degree of supercooling (low Ts)the nucleation rate for the polycrystal /3-PbO phase is the highest (curve lb). On elevating the substrate temperature at a certain T,, the nucleation rate of the monocrystal j3-PbO (curve la) becomes the highest one. The X-ray investigations show that the growth of j3-PbO in the [Ool] direction prevails to a great extent in the films deposited at Ts > 150 "C on quartz substrates (Figure 1). It is clear from Figure 4 that the growth of the polycrystal a-PbO is thermodynamically unfavorable in the whole temperature region investigated. The a-PbO phase, which is always monocrystal in the films grown on quartz substrates, is present only in the films grown at high substrate temperatures. At these temperatures, as seen from Figure 4, the probability for growth of the monocrystal phase (curve 2a) is higher than that for the growth of the polycrystal phase (curve 2b).
In the case of deposition on glass substrates the curves represented in Figure 4 are obviously shifted to higher temperatures, as shown in Figure 3, where curves 3a and 4a correspond to growth on the hospitable glass substrate. As can be seen from Figures 1 and 2, the p-PbO phase grows on glass with the prevailing (001) orientation at higher Tsthan in the case of growth on quartz substrates. The range of probability for growth of monocrystal a-PbO phase on glass substrates is shifted to too high substrate temperatures. At the highest substrate temperatures used in our experiment the probability for growth of polycrystal a-PbO is obviously still higher. It is worth mentioning that the consideration presented here is semiquantitative. We know the monocrystal PbO melting temperature, but the polycrystal phase melting temperature is taken with an arbitrary value. Nevertheless, the PbO films' structure is understood in the framework of the classical thermodynamic theory. Moreover, on the grounds of these considerations we can conclude that PbO films with the a-PbO phase solely cannot be grown at all. The a-PbO phase can be present as an admixture with the p-PbO phase at high substrate temperatures. The probability for growth of this phase in our point of view increases with the increase of the film thickness as a result of the increase of the crystallization front temperature and the fluctuations in it during the film growth. As a result, the a-PbO phase quantity is not homogeneously distributed over the film thickness. It has to be negligible in the initial stage of growth and reaches the highest values on the top of the films. An excellent confiiation of this supposition is the paper of Tompsett et aL5 Investigating in situ the structure of thermally evaporated lead monoxide films by high-energy electron diffraction, they find that the initial nucleation is always with the orthorhombic P-PbO phase. A nucleation with the tetragonal a-PbO phase takes place only at a certain thickness governed by the conditions of deposition. Our W E D investigations of the already grown films, reported recently: indicate that the a-PbO phase predominates on the film surface. Thus, it is obvious that the relatively small quantity of the a-PbO phase in the films, detected by the X-ray investigations, is situated close to the film surface. The data of other authors3-' about the structure of the films deposited by different methods are consistent with the consideration of the PbO films' growth in the framework of the classical thermodynamic theory as presented here. Conclusion The overall crystal structure of PbO films, grown by PLD at different substrate temperatures, is understood, considering the growth process, in the framework of the classical thermodynamic theory. Our considerations clarify that films with the a-PbO phase solely or even predominantly cannot be grown. The reason for this is the higher value of the kinetic term, governed by the higher linear thermal expansion coefficient of the a-PbO phase. Acknowledgment. This work has been supported financially by the Ministry of Education and Science under Contract a-47. References and Notes (1) Trinquire, G.;Hoffmann, R. J . Phys. Chem. 1984,88,6696-6711. ( 2 ) Fiziko-chimicheskie svojstva okislov, Handbook, Moskva, Metalurgiya, 1978; p 465 (in Russian). (3) Zabinski, J.; Donley, M.; Dyhouse, V.; Moore, R.; McDevitt, N. Phase Formation and Modification by Beam-Solid Interactions. Mat. Res. SOC.Symp. Proc. 1992, 235, 848. (4) Schottmiller, J. J . Appl. Phys. 1966, 37, 3505. (5) Tompset, M.; Noble, J. Thin Solid Films 1970, 5 , 81. (6) Heijne, L.Philips Res. Per?. Suppl. 1%1,4, 2.
Crystal Structure of Lead Monoxide Thin Films (7) Kramarenko, N.; Miloslavskii, V.; Naboikin, Yu. Opt. Spektrosk. 1968, 24, 971. ( 8 ) Baleva, M.; Tuncheva, V. J . Muter. Sci. Lett., in press. (9) Baleva, M.; Tuncheva, V. J. Solid State Chem. 1994, 108. (IO) Baleva, M.; Maksimov, M.; Metev, S.; Sendova, M. J . Muter. Sci. Lett. 1986, 533. (11) Baleva, M.; Tuncheva, V. J. Muter. Sci. Lett., in press. (12) Ohlidal, I.; Navratil, K.; Schmidt, E. Appl. Phys. 1982, 157. (13) Manifacier, J.; Gasiot, J.; Fillard, J. J. Phys. E: Sci. Instrum. 1976, 1003. (14) PDF-2 Sets 1-43 database, JCPDS-ICDD.
J. Phys. Chem., Vol. 98, No. SO, 1994 13313 (15) Frenkel, J. Kinetic Theory ofliquids; Dover Publication, Inc.: New York, 1955; p 415. (16) Walton, D. J. Chem. Phys. 1962, 37, 2182. (17) Kashchiev, D. J . Cryst. Growth 1977, 40, 29. (18) Eskertova, L. Physics of Thin Films;Plenum Press: New York, and London SNTL PUBLISHERS of Technical Literature: Prague, 1986; p 103. (19) Sirota,N. Zh. Tekh. Fiz. 1948, 28, 1136. (20) Sorrel, Ch. J. Am. Cerum. SOC. 1970, 53, 641. (21) Sorrel, Ch. J. Am. Cerum. SOC. 1970, 53, 552.
