- Email: [email protected]
Si(0 0 1)
Journal of Crystal Growth 192 (1998) 175—184
Study of the epitaxial growth of CeO (0 0 1) 2 on yttria-stabilized zirconia/Si(0 0 1) V. Trtı´ k*,1, R. Aguiar, F. Sa´nchez, C. Ferrater, M. Varela Departament de Fı& sica Aplicada i Electro% nica, Universitat de Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Spain Received 16 March 1998; accepted 10 April 1998
Abstract Thin CeO films have been grown by pulsed laser deposition on Si(0 0 1) with yttria-stabilized zirconia buffer layers. 2 The influence of substrate temperature (20—800°C), oxygen pressure (10~5—1 mbar), and laser repetition rate (1—20 Hz) on the CeO crystal and surface quality has been investigated. For an oxygen pressure lower than 10~2 mbar, the CeO film 2 2 grows epitaxially in full temperature range, high-quality epitaxial films were deposited even at room temperature. The out-of-plane CeO lattice parameter decreased as the substrate temperature increased, but no dependence on oxygen 2 pressure has been observed. Rocking curve value of CeO (0 0 2) showed a weak dependence on the oxygen pressure 2 within the range from 10~5 to 10~2 mbar, decreasing when the substrate temperature increases. The crystal quality of CeO layers deposited at 100°C decreased as the laser repetition rate increased, but increased with higher repetition rate 2 for deposition at 800°C. Smooth, featureless surfaces were observed for all the temperatures studied and for oxygen pressure lower than 10~2 mbar. The study of the deposition on a wide range of technological parameters showed reproducible growth of the CeO thin films of excellent crystal and surface quality on Si(0 0 1). ( 1998 Elsevier Science 2 B.V. All rights reserved. PACS: 81.15.Fg; 68.55.!a Keywords: CeO thin film; Pulsed laser deposition; Epitaxial growth; Crystal structure; Surface morphology; Low2 temperature epitaxy
1. Introduction Pulsed laser deposition (PLD) is known to be a versatile deposition technique for preparing * Corresponding author. Tel.: #34 93 4021134; fax: #34 93 4021138; e-mail: [email protected]. 1 Permanent address: Institute of Physics, Academy of Sciences of Czech Republic, Na Slovance 2, 18040 Prague 8, Czech Republic.
multicomponent thin films owing to the congruent transfer of material from the target to the substrate [1]. A further major advantage of PLD is the generation of highly excited particles with high kinetic energy [2] (up to 150 eV for CeO target) [3] which 2 can provide crystal growth at relatively low temperatures. Low-temperature processes are preferable for semiconductor-based device production. CeO thin films are attractive for various 2 electronic and optical applications, such as
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silicon-on-insulator structures, miniaturized stable capacitors, oxygen sensors, buffer layer for hightemperature superconductor films, and optical coatings [4—11]. CeO is a very stable ionic metal 2 oxide with a high melting point (2600°C) and fluorite structure with a lattice constant of 0.541 nm, which is similar to that of silicon (0.35% lattice mismatch) and to that of other important oxide materials exhibiting superconducting, metallic, insulating, ferroelectric, magnetic or optical properties. CeO has been shown to grow epitaxially on 2 oxide-free Si(1 1 1) and Si(0 0 1) substrates. While epitaxial CeO (1 1 1) layers on Si(1 1 1) were suc2 cessfully deposited even at room-temperature [12], epitaxial growth on Si(0 0 1) requires a substrate temperature above 750°C and leads to CeO (0 1 1) 2 orientation [13]. Preferentially (1 1 1) oriented CeO thin films can be grown on oxidized Si(0 0 1) 2 and Si(1 1 1) substrates but this preferential growth is thought to be based on a chemical effect rather than on a crystallographic one [14,15]. These experimental observations are consistent with theoretical predictions by Tasker [16], based on the calculation of the surface energy of ionic materials. According to this theory, (1 1 1) and (0 1 1) oriented growth of CeO layers is energetically favorable 2 while (0 0 1) orientation is “forbidden” due to electric dipole moment perpendicular to the surface, and can occur only with the concurrent neutralization of the surface charge by a complex defect structure. The lattice parameter of CeO increases 2 with decreasing oxygen content due to the higher effective radius of reduced Ce ions [8]. In terms of application, the heteroepitaxial growth of metal oxides on to Si(0 0 1) is more interesting than that of Si(1 1 1) since (0 0 1) oriented substrates are more widely used in the semiconductor industry [17]. CeO (0 0 1) epitaxial 2 layers interesting for c-axis oriented high-¹ super# conductor growth have been obtained on Si(0 0 1) substrates at temperatures above 700°C using intermediate yttria-stabilized zirconia buffer layers (YSZ) [18]. The deposition parameters reported for CeO (0 0 1) layer deposition usually copy the de2 position parameters of either YSZ buffer or high-¹ # superconductor layer without optimization and with no fundamental understanding of the relation-
ship between deposition parameters and layer quality. This paper reports a complete structural and morphological analysis of CeO layers deposited 2 by PLD on Si(0 0 1) substrates with YSZ(0 0 1) buffer layers. The influence of technological parameters, such as substrate temperature, oxygen pressure and laser repetition rate have been studied. A comparison of CeO growth at 100 and 800°C is 2 made as this is important in order to understand the crystal growth of ionic oxides and the PLD process.
2. Experimental procedure Both YSZ and CeO were prepared by PLD 2 using a KrF excimer laser (248 nm wavelength, 34 ns pulse duration). The beam was focused through a spherical lens on to a rotating ceramic target of the required material. The fluence was about 3 J/cm2. The distance between the substrate and the target was 5.5 cm. For correct stoichiometry, laser deposition was carried out in an oxygen ambient, the pressure of which was controlled via a PC-based automatic apparatus. Epitaxial layers of YSZ were grown in the same manner as reported earlier [19,20]. Si(0 0 1) substrates were ultrasonically cleaned in trichlorethylene, acetone, and methanol, without chemical removal of the native oxide layer prior to YSZ deposition. The typical root-mean-square (rms) roughness of the substrate surface was about 0.2 nm. Unless otherwise mentioned, rms-roughness (roughness) was estimated by scanning over a 5]5 lm2 area. YSZ films around 80 nm thick were deposited at 800°C under 3]10~4 mbar oxygen pressure. Growth rate at a laser repetition rate of 10 Hz was 0.25 A_ per pulse. Immediately after deposition, the sample was quickly cooled without filling the deposition chamber with oxygen. The typical rocking curve full-width at half-maximum (FWHM) of the YSZ(0 0 2) peak was around 0.9° and the surface roughness was about 0.2 nm. After deposition and characterization, YSZ/Si samples were cut into pieces and used in preparing CeO 2 layers. A separate set of YSZ/Si chips was used for each particular experimental series to ensure that
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Table 1 CeO deposition conditions 2 Series
A B C D E F
YSZ/Si annealing pressure (mbar)
CeO deposition conditions 2 Temperature (°C)
Pressure (mbar)
Frequency (Hz)
10 10 10 10 10 1]10~5—10
20—800 100 800 100 800 20 (300)
3]10~4 3]10~5—3]10~1 3]10~5—3]10~1 3]10~4 3]10~4 3]10~4
10 10 10 1—20 1—20 10
the properties of the YSZ layers were kept unaltered. In order to exclude the possible influence of YSZ air-exposure or chemical cleaning after characterization, the YSZ/Si samples were treated before deposition of CeO by annealing at a 2 temperature of 800°C at an oxygen pressure of 10 mbar for 10 min. The effect of YSZ annealing at different oxygen pressures will be discussed later. CeO layers were grown at temperatures from 20 to 2 800°C, oxygen pressure from 10~5 to 1 mbar, and laser repetition rate from 1 to 20 Hz. CeO layers 2 were typically 80 nm thick. At higher oxygen pressures, the number of laser pulses was increased to ensure the same thickness for all layers. The deposition conditions of CeO for different experimental 2 series are summarized in Table 1. Film thickness was measured by stylus profilometry. Film structural quality and the dependence of out-of-plane CeO lattice parameter on 2 deposition conditions were investigated by X-ray diffraction (XRD) measurements in a four circle X-ray diffractometer with Cu K radiation. Posia tion and width of XRD peaks were fitted by the Pearson VII formula. Lattice parameter of CeO 2 was estimated from the 2h position of CeO (0 0 2) 2 peak. Film morphology was studied using scanning electron microscopy (SEM). Atomic force microscopy (AFM) working in tapping mode was used to characterize the surface and to measure the roughness of the films. Depth profiles of chemical composition were obtained by secondary ion mass spectrometry (SIMS) together with Ar` sputter etching.
3. Results and discussion 3.1. Effect of substrate temperature The effect of substrate temperature on CeO 2 crystallinity in the range from 20 to 800°C was studied for the CeO films grown at an oxygen 2 pressure of 3]10~4 mbar and with a laser repetition rate of 10 Hz (A series in Tables 1 and 2). All the layers belonging to this experimental series were (0 0 l) cube-on-cube epitaxial. Epitaxial growth of CeO at room-temperature and 100°C is 2 reported in detail elsewhere [21]. Fig. 1 shows the XRD 2H—u and rocking curves of the films deposited at 20 and 800°C; only the
Table 2 Deposition conditions and resulting properties of CeO films for 2 a temperature series with constant YSZ/Si annealing oxygen pressure of 10 mbar, oxygen pressure during deposition of 3]10~4 mbar, and laser repetition rate of 10 Hz Sample
Temperature (°C)
*u (0 0 2) (°)
d (0 0 1) (A_ )
A1 A2 A3 A4 A5 A6 A7 A8 A9
25 75 100 200 300 400 500 600 800
1.84 1.7 1.55 1.05 0.95 0.8 0.69 0.63 0.42
5.497 5.469 5.448 5.432 5.418 5.416 5.409 5.405 5.403
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Fig. 1. XRD 2H—u scans of the CeO films deposited on 2 YSZ/Si(0 0 1) at an oxygen pressure of 3]10~4 mbar and at a temperature of (a) 20°C and (b) 800°C. The inset shows the rocking curves of the (0 0 2) peak of CeO . 2
Fig. 2. XRD /-scan of (2 2 2) family of reflections for CeO 2 films deposited at an oxygen pressure of 3]10~4 mbar and at a temperature of (a) 20°C (solid line) and (b) 800°C (dashed line). The CeO crystal axes are [1 0 0]-aligned with YSZ layer 2 and Si substrate.
YSZ(0 0 2) and CeO (0 0 2) diffraction peaks are 2 observed in the 2H—u scans with FWHM values of rocking curves of 1.8° and 0.4°, respectively. It should be pointed out, for sake of comparison, that the typically reported FWHM values for layers deposited at temperatures about 800°C are above 1° [22]. A shift in the CeO (0 0 2) peak position indicat2 ing a variation in the out-of-plane lattice parameter is clearly seen. The position of YSZ(0 0 2) reflection was the same for all deposited layers. The /-scan of the CeO (2 2 2) reflections corresponding to films 2 deposited at 20 and 800°C shown in Fig. 2 indicates a cube-on-cube epitaxial growth at both temperatures. FWHM of /-scan peaks was 2.15 and 0.61° for CeO deposited at 20 and 800°C, respectively. 2 The dependence of the rocking curve FWHM of CeO (0 0 2) peak on the deposition temperature is 2 depicted in Fig. 3. Extrapolation of the curve suggests that the low-temperature limit for epitaxial growth (“epitaxial temperature”) [23] of CeO is 2 below 0°C. The influence of particle energy on crystal growth of thin films can also be documented by comparing PLD with a purely thermal particle energy (&0.1 eV) deposition method such as electron-beam evaporation. The epitaxial temperature of CeO prepared by conventional electron-beam 2 evaporation was empirically found to be around 700°C for a well-matched substrate (LaAlO ) [24]. 3
Fig. 3. Effect of substrate temperature on the FWHM values of rocking curves of the CeO (0 0 2) diffraction peak for films 2 deposited at an oxygen pressure of 3]10~4 mbar.
Using substrate bias in an electron-beam evaporation apparatus allowed to decrease epitaxial temperature by more than 55°C for CeO layers 2 deposited on Si(1 0 0) substrates [25]. Fig. 4 shows the dependence of the out-of-plane lattice parameter of CeO layers on the growth 2 temperature. Lattice parameter decreases monotonically with increasing temperature within the studied temperature range. A similar behavior was reported for SrTiO deposited on MgO [26—28]. 3 Expansion of SrTiO and even the CeO lattice 3 2
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Table 3 Deposition conditions and resulting properties of CeO films for 2 deposition oxygen pressure series with constant YSZ/Si annealing pressure of 10 mbar, temperature of 100°C, and laser repetition rate of 10 Hz
Fig. 4. Effect of substrate temperature on the out-of-plane lattice parameter (d ) of CeO for films deposited at an oxygen 001 2 pressure of 3]10~4 mbar.
parameter with decreasing substrate temperature is possibly raised by frozen oxygen vacancies, as well as by other effects, such as anisotropic stress, due to lattice mismatch and differences in thermal expansion coefficients, or complex lattice defects. Although oxygen deficiency induces lattice shrinkage for SrRuO , a similar dependence of the lattice 3 parameter on the deposition temperature was reported for SrRuO thin films deposited on 3 SrTiO (0 0 1) and MgO(0 0 1) single crystals [29]. 3 These results suggest that a more common mechanism could cause a decrease in the lattice parameters with increasing temperature for different oxide materials. In order to investigate the influence of deposition temperature on the interdiffusion of elements between materials, SIMS depth profiles were measured for samples with CeO layers deposited at 20 2 and 800°C. Both samples showed abrupt element profiles without any evidence of interdiffusion. No significant differences between SIMS profiles for layers deposited at both temperatures were observed. 3.2. Effect of oxygen pressure The effects of oxygen pressure in the range from 10~5 to 1 mbar on the deposition of CeO films 2 was studied for temperatures of 100 and 800°C and 10 Hz laser repetition rate (B and C series in Tables 1 and 3).
Sample
Pressure (mbar)
*u (0 0 2) (°)
d (0 0 1) (A_ )
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10
3]10~5 8]10~5 3]10~4 8]10~4 3]10~3 8]10~3 3]10~2 8]10~2 3]10~1 8]10~1
1.80 1.73 1.55 1.42 1.28 1.70 1.84 A A A
5.463 5.463 5.469 5.469 5.464 5.464 5.425 A A A
A — amorphous.
Fig. 5. Effect of oxygen pressure on the FWHM values of rocking curves of the CeO (0 0 2) diffraction peak for films deposited 2 at 100°C. The dashed line serves as a visual guide.
The dependence of the rocking curve FWHM of CeO films prepared at 100°C on the oxygen pres2 sure is depicted in Fig. 5. All layers deposited at pressures above 8]10~2 mbar were amorphous. Oxygen pressure has a large influence on the energy of particles reaching the substrate and subsequently on the migration energy of adatoms. For pressures from 10~5 to 10~2 mbar the average energy of particles is almost constant, but for oxygen pressures above 10~2 mbar it decreases substantially by scattering with ambient gas atoms, and is nearly
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V. Trtı& k et al. / Journal of Crystal Growth 192 (1998) 175–184 Table 4 Deposition conditions and resulting properties of CeO films for 2 laser repetition rate series with constant YSZ/Si annealing oxygen pressure of 10 mbar, oxygen pressure during deposition of 3]10~4 mbar, and temperature of 100°C Sample
Frequency (Hz)
*u (0 0 2) (°)
d (0 0 1) (A_ )
D1 D2 D3 D4
1 5 10 20
1.51 1.57 1.63 1.86
5.475 5.470 5.471 5.469
Fig. 6. Effect of oxygen pressure on the out-of-plane lattice parameter (d ) of CeO for films deposited at 100°C. 001 2
thermallized at pressures above 10~1 mbar [30]. Thus, increasing the deposition pressure over a certain limit causes a deterioration in the crystal quality of the growth layer. Oxygen pressures above 8]10~2 mbar prevent the crystal growth of CeO . 2 No remarkable dependence of the out-of-plane lattice parameter of CeO was observed when 2 changing oxygen pressure, as can be seen from Fig. 6. The only different value in the lattice parameter at 3]10~2 mbar deposition pressure might have been caused by the reduced kinetic energy of particles which strongly affected layer growth. On the other hand, crystal quality and lattice parameter of CeO films deposited at 800°C were 2 not affected by oxygen pressure in the given range. Therefore, it is thought that the surface migration energy at 800°C is predominantly determined by the thermal energy of adatoms, supplied by substrate heating. Differences in growth mode at lower and higher deposition pressures were also documented by AFM. More details are given below. 3.3. Effect of repetition rate The influence of laser repetition rate in the 1—20 Hz range on CeO properties was studied for 2 substrate temperatures of 100 and 800°C and an oxygen pressure of 3]10~4 mbar (D and E series in Tables 1 and 4). The dependence of the rocking curve FWHM on the laser repetition rate of CeO films prepared at 2 100°C is depicted in Fig. 7. The deterioration in the
Fig. 7. Effect of laser repetition rate on the FWHM values of rocking curves of the CeO (0 0 2) diffraction peak for films 2 deposited at 100°C and at an oxygen pressure of 3]10~4 mbar.
crystal quality of CeO with a higher laser repeti2 tion rate is probably caused by limited adatom mobility at 100°C. In contrast, an increase in the FWHM rocking curve value with decreasing laser repetition rate was found for CeO deposited at 800°C. In this 2 case, adatom mobility was not the limiting factor that determined the layer quality but rather the longer processing time at high temperature, which promoted detrimental chemical reactions and interdiffusion of atoms. Similar results have previously been obtained for YSZ layers deposited at 800°C [20] and a significant degradation was observed in YSZ thin film crystal quality for samples kept at the deposition temperature for 50 min, which is the time needed for deposition at 1 Hz. Since it is not possible to determine the influence of
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Table 5 Deposition conditions and resulting properties of CeO films for YSZ/Si annealing oxygen pressure series with constant laser repetition 2 rate of 10 Hz Sample
YSZ/Si annealing pressure (mbar)
Temperature (°C)
Pressure (mbar)
*u (0 0 2) (°)
d (0 0 1) (A_ )
F1 F2 F3 F4 F5 F6 F7 F8
1]10~5 1]10~5 1]10~5 — — — 10 1]10~5
25 25 25 25 25 25 25 300
3]10~4 3]10~3 3]10~1 3]10~4 8]10~5 3]10~5 3]10~4 3]10~4
A A A 2.18 2.46 2.74 1.84 1.1
— — — 5.489 5.470 5.460 5.495 5.420
A — amorphous.
YSZ crystal quality on CeO , the details of this 2 experiment are not reported here. 3.4. Effect of YSZ/Si annealing The effects of the YSZ surface state, changed by YSZ/Si annealing, on CeO growth at 20°C were 2 investigated by various surface heat treatments at different oxygen pressures for 10 min, at 800°C. CeO layers were deposited under the same condi2 tions at 20°C with an oxygen pressure of 3] 10~4 mbar, and 10 Hz repetition rate on YSZ/Si samples, treated by annealing at 10~5 mbar, 10 mbar of oxygen, and without annealing (F series in Tables 1 and 5). Additional experiments with CeO 2 deposition at 300°C were carried out. While no changes in YSZ properties were observed (crystal structure and surface morphology) after annealing, a clear influence on CeO crystal 2 structure was found. The CeO layers deposited on 2 the YSZ surface annealed at 10~5 mbar were amorphous, and all the other layers were epitaxially (0 0 l) oriented. The minimum FWHM of CeO (0 0 2) rocking curve (1.8°) was found for the 2 sample annealed at 10 mbar of oxygen before deposition of CeO . Rocking curve FWHM of 2 CeO (0 0 2) layers deposited on nonannealed 2 YSZ/Si samples were wider than 2.2°. One day after deposition, all the amorphous CeO layers depos2 ited on YSZ buffer annealed at 10~5 mbar started to crack and peel-off, while all crystalline and even
amorphous layers from the other experimental series were brittle with good adhesion and time stability. However, epitaxially grown CeO layers were 2 obtained irrespective of the YSZ surface heat treatment, at a substrate temperature as high as 300°C. This observation is in agreement with the assumption that Ce is preferentially attracted and bonded to the surface oxygen atoms during the early growth stages [5,6]. When the growth temperature exceeds a certain point, limitations to epitaxial growth driven by the chemical state of the surface are overcome by a rise in the adatom mobility supported by the substrate heating. Further research which focuses on the chemical state of annealed YSZ surfaces is required in order to acquire a complete understanding of this phenomenon. 3.5. Effect on surface morphology SEM micrographs of the CeO layers deposited 2 at an oxygen pressure below 10~2 mbar, irrespective of the other deposition conditions, showed an homogenous, featureless, smooth surface, almost free of droplets. The surfaces of CeO layers depo2 sited at 100°C and an oxygen pressure below 10~2 mbar were very smooth, as shown by AFM, with a roughness in the range of 0.3—0.4 nm. A very low surface roughness of 0.4 nm for the sample deposited at oxygen pressure of 3]10~2 mbar (Fig. 8a) strongly contrasts with a roughness of
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Fig. 9. AFM image of CeO layers deposited on YSZ/Si(0 0 1) 2 at 100°C and 3]10~4 mbar oxygen pressure.
Fig. 8. AFM images of CeO layers deposited on YSZ/Si(0 0 1) 2 at 100°C and (a) 3]10~2 mbar and (b) 8]10~2 mbar oxygen pressure.
4.8 nm for an higher oxygen pressure of 8] 10~2 mbar (Fig. 8b). It can be clearly seen that the oxygen pressure determining the energy of the ablated particles changes the epitaxial and smooth 2-D growth at oxygen pressures below 10~2 mbar to amorphous and rough 3-D growth at pressures over 10~1 mbar. An AFM image in Fig. 9 shows flat spherical valleys with diameters from 1-2 lm, surrounded by flat plateaus. The step height is about 0.6 nm, close to the CeO lattice constant. 2 Similar results were published for CeO films de2 posited on to YBa Cu O surfaces [31]. In this 2 3 7~x case, valleys with roughly 50 nm diameter and
a step height of +4CeO lattice constants were 2 observed. The importance of oxygen pressure for CeO 2 deposited at 800°C is shown in Fig. 10. Surface roughness of CeO deposited at an oxygen pressure 2 of 3]10~4 mbar (Fig. 10a) and 3]10~1 mbar (Fig. 10b) was 0.2 and 1.2 nm, respectively. While the film deposited at 3]10~4 mbar was atomically smooth, the film deposited at 3]10~1 mbar showed a needle-shaped morphology with two different perpendicular population of grains. On the other hand, laser repetition rate within the studied range had no marked effect on surface roughness for CeO layers deposited at an oxygen 2 pressure of 3]10~4 mbar and temperatures of 100 and 800°C. Similarly, deposition temperature had no major influence on roughness for deposition pressures below 3]10~2 mbar. At higher deposition temperatures the characteristic low-temperature shallow valleys disappear.
4. Conclusions A detailed study of the growth of CeO thin films 2 deposited on Si(0 0 1) substrates with YSZ buffer layers by PLD has been carried out. Epitaxial CeO thin films were obtained for all the samples 2 prepared at deposition temperatures ranging from
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show a strong dependence on oxygen pressure. While the CeO layers deposited under oxygen 2 pressures in the range between 10~5 and 10~2 mbar were epitaxial with extremely smooth surfaces, the layers deposited at oxygen pressures above 10~1 mbar were amorphous and showed rough surfaces. Flat spherical valleys, with a step height around the CeO lattice constant, observed 2 by AFM for CeO deposited at 100°C suggested 2 smooth 2-D growth even for deposition at low temperatures. No noticeable influence of oxygen pressure on the crystal quality of CeO deposited at 2 800°C was observed. The surface morphology was shown to be independent both on substrate temperature and laser repetition rate for an oxygen pressure below 3]10~2 mbar.
Acknowledgements The authors acknowledge the collaboration of the Scientific and Technical Services of the Universitat de Barcelona. V.T. acknowledges the financial support of a NATO grant. This work is part of a research project financed by CICYT of the Spanish Government (Project MAT96-0911) and DGR of the Catalan Government.
References Fig. 10. AFM images of CeO layers deposited on YSZ/Si(0 0 1) 2 at 800°C and (a) 3]10~4 mbar and (b) 3]10~1 mbar oxygen pressure.
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Study of the epitaxial growth of CeO (0 0 1) 2 on yttria-stabilized zirconia/Si(0 0 1) V. Trtı´ k*,1, R. Aguiar, F. Sa´nchez, C. Ferrater, M. Varela Departament de Fı& sica Aplicada i Electro% nica, Universitat de Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Spain Received 16 March 1998; accepted 10 April 1998
Abstract Thin CeO films have been grown by pulsed laser deposition on Si(0 0 1) with yttria-stabilized zirconia buffer layers. 2 The influence of substrate temperature (20—800°C), oxygen pressure (10~5—1 mbar), and laser repetition rate (1—20 Hz) on the CeO crystal and surface quality has been investigated. For an oxygen pressure lower than 10~2 mbar, the CeO film 2 2 grows epitaxially in full temperature range, high-quality epitaxial films were deposited even at room temperature. The out-of-plane CeO lattice parameter decreased as the substrate temperature increased, but no dependence on oxygen 2 pressure has been observed. Rocking curve value of CeO (0 0 2) showed a weak dependence on the oxygen pressure 2 within the range from 10~5 to 10~2 mbar, decreasing when the substrate temperature increases. The crystal quality of CeO layers deposited at 100°C decreased as the laser repetition rate increased, but increased with higher repetition rate 2 for deposition at 800°C. Smooth, featureless surfaces were observed for all the temperatures studied and for oxygen pressure lower than 10~2 mbar. The study of the deposition on a wide range of technological parameters showed reproducible growth of the CeO thin films of excellent crystal and surface quality on Si(0 0 1). ( 1998 Elsevier Science 2 B.V. All rights reserved. PACS: 81.15.Fg; 68.55.!a Keywords: CeO thin film; Pulsed laser deposition; Epitaxial growth; Crystal structure; Surface morphology; Low2 temperature epitaxy
1. Introduction Pulsed laser deposition (PLD) is known to be a versatile deposition technique for preparing * Corresponding author. Tel.: #34 93 4021134; fax: #34 93 4021138; e-mail: [email protected]. 1 Permanent address: Institute of Physics, Academy of Sciences of Czech Republic, Na Slovance 2, 18040 Prague 8, Czech Republic.
multicomponent thin films owing to the congruent transfer of material from the target to the substrate [1]. A further major advantage of PLD is the generation of highly excited particles with high kinetic energy [2] (up to 150 eV for CeO target) [3] which 2 can provide crystal growth at relatively low temperatures. Low-temperature processes are preferable for semiconductor-based device production. CeO thin films are attractive for various 2 electronic and optical applications, such as
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silicon-on-insulator structures, miniaturized stable capacitors, oxygen sensors, buffer layer for hightemperature superconductor films, and optical coatings [4—11]. CeO is a very stable ionic metal 2 oxide with a high melting point (2600°C) and fluorite structure with a lattice constant of 0.541 nm, which is similar to that of silicon (0.35% lattice mismatch) and to that of other important oxide materials exhibiting superconducting, metallic, insulating, ferroelectric, magnetic or optical properties. CeO has been shown to grow epitaxially on 2 oxide-free Si(1 1 1) and Si(0 0 1) substrates. While epitaxial CeO (1 1 1) layers on Si(1 1 1) were suc2 cessfully deposited even at room-temperature [12], epitaxial growth on Si(0 0 1) requires a substrate temperature above 750°C and leads to CeO (0 1 1) 2 orientation [13]. Preferentially (1 1 1) oriented CeO thin films can be grown on oxidized Si(0 0 1) 2 and Si(1 1 1) substrates but this preferential growth is thought to be based on a chemical effect rather than on a crystallographic one [14,15]. These experimental observations are consistent with theoretical predictions by Tasker [16], based on the calculation of the surface energy of ionic materials. According to this theory, (1 1 1) and (0 1 1) oriented growth of CeO layers is energetically favorable 2 while (0 0 1) orientation is “forbidden” due to electric dipole moment perpendicular to the surface, and can occur only with the concurrent neutralization of the surface charge by a complex defect structure. The lattice parameter of CeO increases 2 with decreasing oxygen content due to the higher effective radius of reduced Ce ions [8]. In terms of application, the heteroepitaxial growth of metal oxides on to Si(0 0 1) is more interesting than that of Si(1 1 1) since (0 0 1) oriented substrates are more widely used in the semiconductor industry [17]. CeO (0 0 1) epitaxial 2 layers interesting for c-axis oriented high-¹ super# conductor growth have been obtained on Si(0 0 1) substrates at temperatures above 700°C using intermediate yttria-stabilized zirconia buffer layers (YSZ) [18]. The deposition parameters reported for CeO (0 0 1) layer deposition usually copy the de2 position parameters of either YSZ buffer or high-¹ # superconductor layer without optimization and with no fundamental understanding of the relation-
ship between deposition parameters and layer quality. This paper reports a complete structural and morphological analysis of CeO layers deposited 2 by PLD on Si(0 0 1) substrates with YSZ(0 0 1) buffer layers. The influence of technological parameters, such as substrate temperature, oxygen pressure and laser repetition rate have been studied. A comparison of CeO growth at 100 and 800°C is 2 made as this is important in order to understand the crystal growth of ionic oxides and the PLD process.
2. Experimental procedure Both YSZ and CeO were prepared by PLD 2 using a KrF excimer laser (248 nm wavelength, 34 ns pulse duration). The beam was focused through a spherical lens on to a rotating ceramic target of the required material. The fluence was about 3 J/cm2. The distance between the substrate and the target was 5.5 cm. For correct stoichiometry, laser deposition was carried out in an oxygen ambient, the pressure of which was controlled via a PC-based automatic apparatus. Epitaxial layers of YSZ were grown in the same manner as reported earlier [19,20]. Si(0 0 1) substrates were ultrasonically cleaned in trichlorethylene, acetone, and methanol, without chemical removal of the native oxide layer prior to YSZ deposition. The typical root-mean-square (rms) roughness of the substrate surface was about 0.2 nm. Unless otherwise mentioned, rms-roughness (roughness) was estimated by scanning over a 5]5 lm2 area. YSZ films around 80 nm thick were deposited at 800°C under 3]10~4 mbar oxygen pressure. Growth rate at a laser repetition rate of 10 Hz was 0.25 A_ per pulse. Immediately after deposition, the sample was quickly cooled without filling the deposition chamber with oxygen. The typical rocking curve full-width at half-maximum (FWHM) of the YSZ(0 0 2) peak was around 0.9° and the surface roughness was about 0.2 nm. After deposition and characterization, YSZ/Si samples were cut into pieces and used in preparing CeO 2 layers. A separate set of YSZ/Si chips was used for each particular experimental series to ensure that
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Table 1 CeO deposition conditions 2 Series
A B C D E F
YSZ/Si annealing pressure (mbar)
CeO deposition conditions 2 Temperature (°C)
Pressure (mbar)
Frequency (Hz)
10 10 10 10 10 1]10~5—10
20—800 100 800 100 800 20 (300)
3]10~4 3]10~5—3]10~1 3]10~5—3]10~1 3]10~4 3]10~4 3]10~4
10 10 10 1—20 1—20 10
the properties of the YSZ layers were kept unaltered. In order to exclude the possible influence of YSZ air-exposure or chemical cleaning after characterization, the YSZ/Si samples were treated before deposition of CeO by annealing at a 2 temperature of 800°C at an oxygen pressure of 10 mbar for 10 min. The effect of YSZ annealing at different oxygen pressures will be discussed later. CeO layers were grown at temperatures from 20 to 2 800°C, oxygen pressure from 10~5 to 1 mbar, and laser repetition rate from 1 to 20 Hz. CeO layers 2 were typically 80 nm thick. At higher oxygen pressures, the number of laser pulses was increased to ensure the same thickness for all layers. The deposition conditions of CeO for different experimental 2 series are summarized in Table 1. Film thickness was measured by stylus profilometry. Film structural quality and the dependence of out-of-plane CeO lattice parameter on 2 deposition conditions were investigated by X-ray diffraction (XRD) measurements in a four circle X-ray diffractometer with Cu K radiation. Posia tion and width of XRD peaks were fitted by the Pearson VII formula. Lattice parameter of CeO 2 was estimated from the 2h position of CeO (0 0 2) 2 peak. Film morphology was studied using scanning electron microscopy (SEM). Atomic force microscopy (AFM) working in tapping mode was used to characterize the surface and to measure the roughness of the films. Depth profiles of chemical composition were obtained by secondary ion mass spectrometry (SIMS) together with Ar` sputter etching.
3. Results and discussion 3.1. Effect of substrate temperature The effect of substrate temperature on CeO 2 crystallinity in the range from 20 to 800°C was studied for the CeO films grown at an oxygen 2 pressure of 3]10~4 mbar and with a laser repetition rate of 10 Hz (A series in Tables 1 and 2). All the layers belonging to this experimental series were (0 0 l) cube-on-cube epitaxial. Epitaxial growth of CeO at room-temperature and 100°C is 2 reported in detail elsewhere [21]. Fig. 1 shows the XRD 2H—u and rocking curves of the films deposited at 20 and 800°C; only the
Table 2 Deposition conditions and resulting properties of CeO films for 2 a temperature series with constant YSZ/Si annealing oxygen pressure of 10 mbar, oxygen pressure during deposition of 3]10~4 mbar, and laser repetition rate of 10 Hz Sample
Temperature (°C)
*u (0 0 2) (°)
d (0 0 1) (A_ )
A1 A2 A3 A4 A5 A6 A7 A8 A9
25 75 100 200 300 400 500 600 800
1.84 1.7 1.55 1.05 0.95 0.8 0.69 0.63 0.42
5.497 5.469 5.448 5.432 5.418 5.416 5.409 5.405 5.403
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Fig. 1. XRD 2H—u scans of the CeO films deposited on 2 YSZ/Si(0 0 1) at an oxygen pressure of 3]10~4 mbar and at a temperature of (a) 20°C and (b) 800°C. The inset shows the rocking curves of the (0 0 2) peak of CeO . 2
Fig. 2. XRD /-scan of (2 2 2) family of reflections for CeO 2 films deposited at an oxygen pressure of 3]10~4 mbar and at a temperature of (a) 20°C (solid line) and (b) 800°C (dashed line). The CeO crystal axes are [1 0 0]-aligned with YSZ layer 2 and Si substrate.
YSZ(0 0 2) and CeO (0 0 2) diffraction peaks are 2 observed in the 2H—u scans with FWHM values of rocking curves of 1.8° and 0.4°, respectively. It should be pointed out, for sake of comparison, that the typically reported FWHM values for layers deposited at temperatures about 800°C are above 1° [22]. A shift in the CeO (0 0 2) peak position indicat2 ing a variation in the out-of-plane lattice parameter is clearly seen. The position of YSZ(0 0 2) reflection was the same for all deposited layers. The /-scan of the CeO (2 2 2) reflections corresponding to films 2 deposited at 20 and 800°C shown in Fig. 2 indicates a cube-on-cube epitaxial growth at both temperatures. FWHM of /-scan peaks was 2.15 and 0.61° for CeO deposited at 20 and 800°C, respectively. 2 The dependence of the rocking curve FWHM of CeO (0 0 2) peak on the deposition temperature is 2 depicted in Fig. 3. Extrapolation of the curve suggests that the low-temperature limit for epitaxial growth (“epitaxial temperature”) [23] of CeO is 2 below 0°C. The influence of particle energy on crystal growth of thin films can also be documented by comparing PLD with a purely thermal particle energy (&0.1 eV) deposition method such as electron-beam evaporation. The epitaxial temperature of CeO prepared by conventional electron-beam 2 evaporation was empirically found to be around 700°C for a well-matched substrate (LaAlO ) [24]. 3
Fig. 3. Effect of substrate temperature on the FWHM values of rocking curves of the CeO (0 0 2) diffraction peak for films 2 deposited at an oxygen pressure of 3]10~4 mbar.
Using substrate bias in an electron-beam evaporation apparatus allowed to decrease epitaxial temperature by more than 55°C for CeO layers 2 deposited on Si(1 0 0) substrates [25]. Fig. 4 shows the dependence of the out-of-plane lattice parameter of CeO layers on the growth 2 temperature. Lattice parameter decreases monotonically with increasing temperature within the studied temperature range. A similar behavior was reported for SrTiO deposited on MgO [26—28]. 3 Expansion of SrTiO and even the CeO lattice 3 2
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Table 3 Deposition conditions and resulting properties of CeO films for 2 deposition oxygen pressure series with constant YSZ/Si annealing pressure of 10 mbar, temperature of 100°C, and laser repetition rate of 10 Hz
Fig. 4. Effect of substrate temperature on the out-of-plane lattice parameter (d ) of CeO for films deposited at an oxygen 001 2 pressure of 3]10~4 mbar.
parameter with decreasing substrate temperature is possibly raised by frozen oxygen vacancies, as well as by other effects, such as anisotropic stress, due to lattice mismatch and differences in thermal expansion coefficients, or complex lattice defects. Although oxygen deficiency induces lattice shrinkage for SrRuO , a similar dependence of the lattice 3 parameter on the deposition temperature was reported for SrRuO thin films deposited on 3 SrTiO (0 0 1) and MgO(0 0 1) single crystals [29]. 3 These results suggest that a more common mechanism could cause a decrease in the lattice parameters with increasing temperature for different oxide materials. In order to investigate the influence of deposition temperature on the interdiffusion of elements between materials, SIMS depth profiles were measured for samples with CeO layers deposited at 20 2 and 800°C. Both samples showed abrupt element profiles without any evidence of interdiffusion. No significant differences between SIMS profiles for layers deposited at both temperatures were observed. 3.2. Effect of oxygen pressure The effects of oxygen pressure in the range from 10~5 to 1 mbar on the deposition of CeO films 2 was studied for temperatures of 100 and 800°C and 10 Hz laser repetition rate (B and C series in Tables 1 and 3).
Sample
Pressure (mbar)
*u (0 0 2) (°)
d (0 0 1) (A_ )
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10
3]10~5 8]10~5 3]10~4 8]10~4 3]10~3 8]10~3 3]10~2 8]10~2 3]10~1 8]10~1
1.80 1.73 1.55 1.42 1.28 1.70 1.84 A A A
5.463 5.463 5.469 5.469 5.464 5.464 5.425 A A A
A — amorphous.
Fig. 5. Effect of oxygen pressure on the FWHM values of rocking curves of the CeO (0 0 2) diffraction peak for films deposited 2 at 100°C. The dashed line serves as a visual guide.
The dependence of the rocking curve FWHM of CeO films prepared at 100°C on the oxygen pres2 sure is depicted in Fig. 5. All layers deposited at pressures above 8]10~2 mbar were amorphous. Oxygen pressure has a large influence on the energy of particles reaching the substrate and subsequently on the migration energy of adatoms. For pressures from 10~5 to 10~2 mbar the average energy of particles is almost constant, but for oxygen pressures above 10~2 mbar it decreases substantially by scattering with ambient gas atoms, and is nearly
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V. Trtı& k et al. / Journal of Crystal Growth 192 (1998) 175–184 Table 4 Deposition conditions and resulting properties of CeO films for 2 laser repetition rate series with constant YSZ/Si annealing oxygen pressure of 10 mbar, oxygen pressure during deposition of 3]10~4 mbar, and temperature of 100°C Sample
Frequency (Hz)
*u (0 0 2) (°)
d (0 0 1) (A_ )
D1 D2 D3 D4
1 5 10 20
1.51 1.57 1.63 1.86
5.475 5.470 5.471 5.469
Fig. 6. Effect of oxygen pressure on the out-of-plane lattice parameter (d ) of CeO for films deposited at 100°C. 001 2
thermallized at pressures above 10~1 mbar [30]. Thus, increasing the deposition pressure over a certain limit causes a deterioration in the crystal quality of the growth layer. Oxygen pressures above 8]10~2 mbar prevent the crystal growth of CeO . 2 No remarkable dependence of the out-of-plane lattice parameter of CeO was observed when 2 changing oxygen pressure, as can be seen from Fig. 6. The only different value in the lattice parameter at 3]10~2 mbar deposition pressure might have been caused by the reduced kinetic energy of particles which strongly affected layer growth. On the other hand, crystal quality and lattice parameter of CeO films deposited at 800°C were 2 not affected by oxygen pressure in the given range. Therefore, it is thought that the surface migration energy at 800°C is predominantly determined by the thermal energy of adatoms, supplied by substrate heating. Differences in growth mode at lower and higher deposition pressures were also documented by AFM. More details are given below. 3.3. Effect of repetition rate The influence of laser repetition rate in the 1—20 Hz range on CeO properties was studied for 2 substrate temperatures of 100 and 800°C and an oxygen pressure of 3]10~4 mbar (D and E series in Tables 1 and 4). The dependence of the rocking curve FWHM on the laser repetition rate of CeO films prepared at 2 100°C is depicted in Fig. 7. The deterioration in the
Fig. 7. Effect of laser repetition rate on the FWHM values of rocking curves of the CeO (0 0 2) diffraction peak for films 2 deposited at 100°C and at an oxygen pressure of 3]10~4 mbar.
crystal quality of CeO with a higher laser repeti2 tion rate is probably caused by limited adatom mobility at 100°C. In contrast, an increase in the FWHM rocking curve value with decreasing laser repetition rate was found for CeO deposited at 800°C. In this 2 case, adatom mobility was not the limiting factor that determined the layer quality but rather the longer processing time at high temperature, which promoted detrimental chemical reactions and interdiffusion of atoms. Similar results have previously been obtained for YSZ layers deposited at 800°C [20] and a significant degradation was observed in YSZ thin film crystal quality for samples kept at the deposition temperature for 50 min, which is the time needed for deposition at 1 Hz. Since it is not possible to determine the influence of
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Table 5 Deposition conditions and resulting properties of CeO films for YSZ/Si annealing oxygen pressure series with constant laser repetition 2 rate of 10 Hz Sample
YSZ/Si annealing pressure (mbar)
Temperature (°C)
Pressure (mbar)
*u (0 0 2) (°)
d (0 0 1) (A_ )
F1 F2 F3 F4 F5 F6 F7 F8
1]10~5 1]10~5 1]10~5 — — — 10 1]10~5
25 25 25 25 25 25 25 300
3]10~4 3]10~3 3]10~1 3]10~4 8]10~5 3]10~5 3]10~4 3]10~4
A A A 2.18 2.46 2.74 1.84 1.1
— — — 5.489 5.470 5.460 5.495 5.420
A — amorphous.
YSZ crystal quality on CeO , the details of this 2 experiment are not reported here. 3.4. Effect of YSZ/Si annealing The effects of the YSZ surface state, changed by YSZ/Si annealing, on CeO growth at 20°C were 2 investigated by various surface heat treatments at different oxygen pressures for 10 min, at 800°C. CeO layers were deposited under the same condi2 tions at 20°C with an oxygen pressure of 3] 10~4 mbar, and 10 Hz repetition rate on YSZ/Si samples, treated by annealing at 10~5 mbar, 10 mbar of oxygen, and without annealing (F series in Tables 1 and 5). Additional experiments with CeO 2 deposition at 300°C were carried out. While no changes in YSZ properties were observed (crystal structure and surface morphology) after annealing, a clear influence on CeO crystal 2 structure was found. The CeO layers deposited on 2 the YSZ surface annealed at 10~5 mbar were amorphous, and all the other layers were epitaxially (0 0 l) oriented. The minimum FWHM of CeO (0 0 2) rocking curve (1.8°) was found for the 2 sample annealed at 10 mbar of oxygen before deposition of CeO . Rocking curve FWHM of 2 CeO (0 0 2) layers deposited on nonannealed 2 YSZ/Si samples were wider than 2.2°. One day after deposition, all the amorphous CeO layers depos2 ited on YSZ buffer annealed at 10~5 mbar started to crack and peel-off, while all crystalline and even
amorphous layers from the other experimental series were brittle with good adhesion and time stability. However, epitaxially grown CeO layers were 2 obtained irrespective of the YSZ surface heat treatment, at a substrate temperature as high as 300°C. This observation is in agreement with the assumption that Ce is preferentially attracted and bonded to the surface oxygen atoms during the early growth stages [5,6]. When the growth temperature exceeds a certain point, limitations to epitaxial growth driven by the chemical state of the surface are overcome by a rise in the adatom mobility supported by the substrate heating. Further research which focuses on the chemical state of annealed YSZ surfaces is required in order to acquire a complete understanding of this phenomenon. 3.5. Effect on surface morphology SEM micrographs of the CeO layers deposited 2 at an oxygen pressure below 10~2 mbar, irrespective of the other deposition conditions, showed an homogenous, featureless, smooth surface, almost free of droplets. The surfaces of CeO layers depo2 sited at 100°C and an oxygen pressure below 10~2 mbar were very smooth, as shown by AFM, with a roughness in the range of 0.3—0.4 nm. A very low surface roughness of 0.4 nm for the sample deposited at oxygen pressure of 3]10~2 mbar (Fig. 8a) strongly contrasts with a roughness of
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Fig. 9. AFM image of CeO layers deposited on YSZ/Si(0 0 1) 2 at 100°C and 3]10~4 mbar oxygen pressure.
Fig. 8. AFM images of CeO layers deposited on YSZ/Si(0 0 1) 2 at 100°C and (a) 3]10~2 mbar and (b) 8]10~2 mbar oxygen pressure.
4.8 nm for an higher oxygen pressure of 8] 10~2 mbar (Fig. 8b). It can be clearly seen that the oxygen pressure determining the energy of the ablated particles changes the epitaxial and smooth 2-D growth at oxygen pressures below 10~2 mbar to amorphous and rough 3-D growth at pressures over 10~1 mbar. An AFM image in Fig. 9 shows flat spherical valleys with diameters from 1-2 lm, surrounded by flat plateaus. The step height is about 0.6 nm, close to the CeO lattice constant. 2 Similar results were published for CeO films de2 posited on to YBa Cu O surfaces [31]. In this 2 3 7~x case, valleys with roughly 50 nm diameter and
a step height of +4CeO lattice constants were 2 observed. The importance of oxygen pressure for CeO 2 deposited at 800°C is shown in Fig. 10. Surface roughness of CeO deposited at an oxygen pressure 2 of 3]10~4 mbar (Fig. 10a) and 3]10~1 mbar (Fig. 10b) was 0.2 and 1.2 nm, respectively. While the film deposited at 3]10~4 mbar was atomically smooth, the film deposited at 3]10~1 mbar showed a needle-shaped morphology with two different perpendicular population of grains. On the other hand, laser repetition rate within the studied range had no marked effect on surface roughness for CeO layers deposited at an oxygen 2 pressure of 3]10~4 mbar and temperatures of 100 and 800°C. Similarly, deposition temperature had no major influence on roughness for deposition pressures below 3]10~2 mbar. At higher deposition temperatures the characteristic low-temperature shallow valleys disappear.
4. Conclusions A detailed study of the growth of CeO thin films 2 deposited on Si(0 0 1) substrates with YSZ buffer layers by PLD has been carried out. Epitaxial CeO thin films were obtained for all the samples 2 prepared at deposition temperatures ranging from
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show a strong dependence on oxygen pressure. While the CeO layers deposited under oxygen 2 pressures in the range between 10~5 and 10~2 mbar were epitaxial with extremely smooth surfaces, the layers deposited at oxygen pressures above 10~1 mbar were amorphous and showed rough surfaces. Flat spherical valleys, with a step height around the CeO lattice constant, observed 2 by AFM for CeO deposited at 100°C suggested 2 smooth 2-D growth even for deposition at low temperatures. No noticeable influence of oxygen pressure on the crystal quality of CeO deposited at 2 800°C was observed. The surface morphology was shown to be independent both on substrate temperature and laser repetition rate for an oxygen pressure below 3]10~2 mbar.
Acknowledgements The authors acknowledge the collaboration of the Scientific and Technical Services of the Universitat de Barcelona. V.T. acknowledges the financial support of a NATO grant. This work is part of a research project financed by CICYT of the Spanish Government (Project MAT96-0911) and DGR of the Catalan Government.
References Fig. 10. AFM images of CeO layers deposited on YSZ/Si(0 0 1) 2 at 800°C and (a) 3]10~4 mbar and (b) 3]10~1 mbar oxygen pressure.
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