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Epitaxial growth of (111)-oriented ZrxTi1−xN thin films on c-plane Al2O3 substrates
Journal of Crystal Growth 404 (2014) 1–8
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Epitaxial growth of (111)-oriented ZrxTi1 xN thin films on c-plane Al2O3 substrates Ruiteng Li, Jateen S. Gandhi, Rajeev Pillai, Rebecca Forrest, David Starikov, Abdelhak Bensaoula n Physics Department, University of Houston, Houston, TX 77004, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 16 April 2014 Received in revised form 22 June 2014 Accepted 23 June 2014 Communicated by D.W. Shaw Available online 1 July 2014
A systematic study is presented on the effects of process parameters of S-gun configured DC magnetron sputtered ZrN thin films on c-plane Al2O3 substrates. Using a quartz crystal microbalance the deposition rate of ZrN is investigated as a function of Ar and N2 flow rates, target power, chamber pressure and gas injection position in the chamber. Selected growth conditions for ZrN show the interrelation of growth parameters on film orientation and crystallinity. (111) oriented ZrN thin films exhibit X-ray diffraction rocking curve FWHM as low as 0.361. Additionally, (111) oriented ternary ZrxTi1 xN thin films (0 rx r 1) are also deposited on c-plane Al2O3 substrates. High resolution X-ray diffraction characterization shows that ZrxTi1 xN (x ¼0, 0.64, 0.80, 0.93, 1) layers exhibit rocking curve FWHM values of 0.0045–0.0061 for the (111) reflection, indicating highly crystalline thin films. Atomic force microscopy characterizations show ZrxTi1 xN thin films with a surface roughness between 1.2 nm and 2.9 nm. & 2014 Elsevier B.V. All rights reserved.
Keywords: A1. DC reactive magnetron sputtering A1. Thin films A1. XRD A1. AFM B1. ZrTiN
1. Introduction Group-III nitrides and their alloys are excellent materials for LED devices [1] and photodetectors [2] as well as high frequency and high power transistors [3]. InxGa1 xN and AlxGa1 xN band gap energies cover a wide range of the solar spectrum from the ultraviolet to near infrared [4], and high temperature and radiation tolerance opens the door for advanced detectors and photovoltaic applications [5]. However current high end substrates for III-nitrides, such as SiC and GaN wafers, are costly or introduce large strain for the desired AlGaN and InGaN compositions. Sapphire substrates have a 16% lattice mismatch and a 27% thermal expansion coefficient mismatch with GaN. Pretreatments of the sapphire surface [6] or buffer layers [7,8] are usually used to reduce the high defects density generated from the misfit during the III-nitrides growth process. Transition metal nitrides such as ScN interlayers [9], TiNx networks [10], and CrN nanoislands [11] have demonstrated reduction of dislocations in GaN grown on sapphire. In addition the refractory metallic nature of transition nitrides is very attractive for device fabrication. Among transition nitrides families, ternary alloys such as ZrxTi1 xN (0.49 rx r1) with effective lattice constant in the (111) plane covering lattice constants of AlxGa1 xN (0 rxr 1) and InxGa1 xN (0 rx r0.14)
n
Corresponding author. E-mail address: [email protected] (A. Bensaoula).
http://dx.doi.org/10.1016/j.jcrysgro.2014.06.043 0022-0248/& 2014 Elsevier B.V. All rights reserved.
are promising buffer layer candidates for III-nitrides growth on sapphire or silicon substrates. High quality TiN growths on Al2O3 [12,13], MgO [14,15], and Si [16] substrates via various techniques have been reported. Using reactive magnetron sputtering, TiN epitaxial films were grown at cryogenic temperatures [17]. On the other hand synthesis of ZrN thin films has been challenging due to the high substrate temperature growth required [18,19], and the existence of other metastable Zr3N4 [20] and ZrN2 [21] phases. Epitaxial ZrN thin films have been deposited on r-plane Al2O3 [19], (100)-silicon [18], (111)-silicon [22] and (100)-MgO [23] substrates by reactive magnetron sputtering. ZrN thin films have been deposited on cplane Al2O3 at low temperature [24] and on unknown surface orientation Al2O3 at growth temperature of 900 1C [25], but were reported as polycrystalline. Experimental [26,27] and theoretical [28,29] studies of ternary ZrxTi1 xN films have been conducted due to their outstanding physical and chemical properties. In particular, ZrxTi1 xN has been employed as a seed layer for the growth of AlxIn1 xN films on (111)-MgO and nanorod arrays of cplane sapphire substrates [30,31]. However, the crystalline quality of the ZrxTi1 xN thin films was not fully characterized in those reports. In this work we report synthesis of high quality ZrxTi1 xN (0 rx r1) thin films deposited on c-plane sapphire substrates using an S-gun configured DC magnetron sputtering system. The deposition rate and structural quality of ZrN films as a function of (1) target power, (2) Ar/N2 flow ratio, (3) chamber pressure and (4) substrate temperature are investigated. A quartz crystal
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microbalance (QCM) is used for the deposition rate study. The crystalline quality is analyzed via both powder (PXRD) and high resolution X-ray diffraction (HRXRD). From PXRD and HRXRD data the crystalline quality of binary and ternary films is characterized by the following two criteria: (a) film orientation, expressed as the intensity ratio of (111) peak to total intensity of (111) and (200) peaks, and (b) film crystallinity, expressed by (111) rocking curve FHWM. Surface morphology, roughness and surface structure of both binary and ternary films are characterized using atomic force microscopy (AFM) and reflection high energy electron diffraction (RHEED).
2. Experimental procedure 2.1. System description Fig. 1 (left) is a schematic diagram of the growth reactor used in this study. It consists of a DC magnetron sputtering system equipped with three MDX 500 DC supplies powering three 1.5 in. size target sources. A detailed schematic of the target as a self-contained sputter source in an unbalanced magnetron mode is shown in Fig. 1 (right). The target sits on a cathode and a grounded shield acts as an anode with a 5 cm tall chimney placed on top to prevent cross contamination between targets. Gas tubing for Ar and N2 injection passes through the grounded shield for each target. A radio frequency (RF) generator is connected to the substrate holder. The substrate is radiatively heated using two halogen lamps with a K-type thermocouple, placed behind the sample holder, measuring the reflected heat. The distance between the target and the substrate is approximately 12 cm. The base pressure of the chamber is 1.4 10 8 Torr. During deposition, gas pressure is monitored by a Baratron type gauge. Ar is used as the main work gas while N2 acts as the reactive gas. As shown in the diagram, the flow rates for Ar and N2 are separately controlled by two flow meters. Ar and N2 can be injected in three combinations: (1) mixed and injected from the work target area, (2) mixed and injected from a different target area other than the work target or substrate surface area and (3) separately as Ar from the target and N2 from the substrate area. A quartz crystal microbalance system, Inficon XTC/2, is available for monitoring the deposition rate.
2.2. Experimental procedure The ZrN deposition rate study is conducted by employing a QCM under various target powers, chamber pressures, Ar and N2 flow rates, and different gas injection combinations. Due to the small target size and cooling efficiency, DC target power is operated below 100 W. In this paper we present results for only Zr target powers of 80 and 50 W, chamber pressures of 6 and 3 mTorr, and Ar flow rates of 21 and 10 sccm in different combinations, as typical growth conditions. One-fourth of 2 in. size c-plane Al2O3 substrates, back-coated with 1200 Å thick Ti for thermal absorption, are cleaned by dipping for 5 min each in boiling trichloroethylene, acetone and methanol, followed by DI water rinsing for two cycles, and then blow-dried using nitrogen. Prior to deposition, Zr (purity 99.7%) and Ti (purity 99.997%) targets are deoxidized using a 3 min sputtering sequence in pure Ar plasma followed by 10 min sequence in Ar and N2 mixtures for a nitridation and plasma stabilization step. Table 1 shows parameters selected for a coarse ZrN structural optimization. A fine optimization at N2 flow rates of 1.00 sccm, 1.25 sccm, and 1.50 sccm is conducted at a target power of 65 W, substrate temperature of 800 1C, and chamber pressure of 6 mTorr and Ar flow of 21 sccm. Based on the crystalline quality data of ZrN films, the deposition conditions for ZrxTi1 xN are selected as follows: Zr power is adjusted to keep the same deposition rate as previously used for ZrN growth at a target power of 65 W and a substrate temperature of 800 1C, chamber pressure of 6 mTorr, Ar flow of 21 sccm, and N2 flow of 1.25 sccm while Ti target power is changed to 30, 50 and 70 W to achieve the desired alloy compositions. Both gases are mixed before injection
Table 1 Deposition conditions for ZrN and PXRD characterization analysis results. Sample No.
Zr power (W)
Temp. (1C)
Ar/N2 (sccm)
Chamber pressure (mTorr)
I(1 1 1) /(I(1 1 1) þ I(2 0 0)) by PXRD (%)
(111) RC FWHM by PXRD ( 7 0.011)
1 2 3 4 5 6 7
80 80 50 80 80 50 80
720 760 760 760 720 760 720
21/1 21/1 21/1 21/2 21/1 21/0.5 21/0.75
6 6 6 6 3 6 6
499.9 100 476 478 497 499.98 499.8
0.63 0.50 – – – 0.48 –
Fig. 1. (Left) Schematic diagram of the sputtering system and (right) target configuration.
R. Li et al. / Journal of Crystal Growth 404 (2014) 1–8
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from the Zr target area. TiN is deposited at a Ti power of 70 W, substrate temperature of 800 1C, chamber pressure of 6 mTorr, Ar flow of 21 sccm and N2 flow of 1.25 sccm mixed before injection from the Zr target area. No substrate RF bias is applied during the deposition process. Film crystallinity, orientation and lattice constants are determined using both a Siemens D5000 powder X-ray diffractometer (PXRD) using Cu Kα radiation and two Four-circle high resolution X-ray diffractometers (HRXRD) using Cu Kα1 radiation, one with a Ge 220 analyzer before the detector for ZrxTi1 xN lattice constant determination and the other for off axis phi scans. For post-growth surface roughness and surface crystallinity evaluation, AFM and RHEED are utilized. ZrN and TiN film thicknesses are measured by HRXRD X-ray reflectivity. All samples are deposited for approximately 2 h with thicknesses ranging from 70 nm to 200 nm.
inversely relates to the mean free path of Zr atoms. With increasing chamber pressure a higher number of collisions occur between the sputtered flux and the background gas thereby reducing their mean free path and their density at the substrate. With decreasing Ar flow from 21 sccm (black square) to 10 sccm (cyan triangles), we find that at the same Ar/N2 ratio (21/1 sccm, 10/0.47 sccm), Ar flow of 10 sccm yields a slightly higher deposition rate. Based on the trend observed in deposition rate relationship with pressure, this is probably due to the pressure differential between local target area and global main chamber pressure. Though the global pressure is kept constant at 6 mTorr by manually adjusting the main chamber gate valve position, the local pressure around target area is certainly affected by the flow rate. To verify this hypothesis, a pressure differential study was conducted and the results are discussed in the next section.
3. Results and discussion
3.1.2. Local vs. global pressure To investigate the pressure differential between the local target area and global chamber area, a series of deposition rate studies are conducted under various conditions. Fig. 3(a), (b) and (d) shows plots of ZrNx deposition rate dependence on N2 flow rate. Fig. 3(c) is a plot of Zr deposition rate dependence on chamber pressure. In Fig. 3(a), ZrNx deposition rate is monitored when a mixture of Ar and N2 is injected from Zr target (black square) and Ti target (red circle). It shows that at the same N2 flow rate and chamber pressure at 6 mTorr, ZrNx deposition rate increases when injection port is changed from Zr target to Ti target. It is to be noted that injecting the Ar þN2 mixture from the Ti target or sample surface area yields the same deposition rate for ZrNx (data not presented here). In Fig. 3(b) ZrNx deposition rate is monitored when Ar is injected from Zr target but N2 is injected either from Zr target area (black square) or substrate area (red circle). Since N2 flow rate is relatively low compared to Ar flow rate (21 sccm), the difference in deposition rate caused by N2 injection position is due to N2 concentration difference between target area and main chamber. At 6 mTorr, the N2 mean free path is around 1 cm. Considering that the distance between target and substrate surface is 12 cm there would be a N2 concentration gradient along the line from target area to substrate area. In an attempt to quantify the pressure differential, Zr is sputtered under Ar atmosphere with a flow rate of 21 sccm injected from Zr target (black square) and Ti target (red circle) at different chamber pressures as seen in Fig. 3 (c). The two curves with different slopes show that the pressure difference varies with main chamber pressure. Using a chamber pressure of 6 mTorr with Ar injected from Zr target area yields the same deposition rate as that at a chamber pressure at 8.7 mTorr with Ar injected from Ti target area. Based on this observation, ZrNx deposition rate is investigated for various Ti target powers with Ti target gun shutter closed and Ar and N2 mixed prior to injection from Ti target at a chamber pressure of 8.7 mTorr. At low N2 flow rates, which we call N2 deficient region later, the deposition rate of ZrNx increases with increasing Ti target power. At high N2 flow rates, when N2 is sufficient, ZrNx deposition rate tends to be independent of Ti target power conditions. As a summary of the above study, ZrNx deposition rate is influenced by target power, chamber pressure, Ar and N2 flow rates by affecting Zr and N2 density in the deposition process. In our system configuration, we find a strong influence of the gas mixture injection position and how N2 is injected into the chamber on the deposition rate and film quality. The latter is attributed to the difference between the local target pressure and the global chamber pressure. As a high local target pressure is preferable for obtaining a stable plasma, Ar flow rate is kept at 21 sccm instead of 10 sccm and the gas injection position is selected from the Zr target area for all ZrxTi1 xN compound growth conditions. Another important factor
3.1. Study of ZrN deposition rate by QCM 3.1.1. ZrN deposition rate Fig. 2 shows a plot of ZrNx (x indicates the unknown Zr/N ratio with increasing N2 flow rate) deposition rate (DR), measured via QCM, as a function of N2 flow rate with ArþN2 gas mixture injected from the Zr target area. The following parameters are selected for comparison: (a) target powers of 50 W (blue triangle) and 80 W (black square), (b) chamber pressures of 3 mTorr (red circle) and 6 mTorr (black square) and (c) Ar flows of 10 sccm (cyan triangle) and 21 sccm (black square). The deposition rates are calibrated with ZrN thickness measured by X-ray reflectivity. Fig. 2 shows that for all of the target powers the deposition rate decreases with increasing N2 flow rate, which indicates a transition from pure Zr metal to ZrNx with higher N2 flow rates. From the same figure, increasing target power and decreasing chamber pressure increase the ZrNx deposition rate. The explanation here is that target power is directly related to the density of Zr atoms sputtered from the target surface while the chamber pressure
Fig. 2. ZrNx deposition rate dependence on (1) N2 flow rate (all of the curves), (2) Zr target power (50 W: blue triangle, 80 W: black square) at 6 mTorr and Ar of 21 sccm, (3) chamber pressure (3 mTorr: red circle, 6 mTorr: black square) at target power of 80 W and Ar of 21 sccm and (4) Ar flow rate (10 sccm: cyan triangle, 21 sccm: black square) at target power of 80 W and Ar of 21 sccm, with Ar and N2 mixed prior to injection from Zr target area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. (a) ZrNx deposition rate dependence as a function of N2 flow rate when the Ar and N2 mixture is injected from either Zr (black square) or Ti (red circle) target areas; (b) ZrNx deposition rate when Ar is injected from Zr target area and N2 injected from either Zr target (black square) or the substrate area (red circle); (c) Zr deposition rate dependence on pressure when Ar is injected from Zr (black square) or Ti (red circle) target areas and (d) ZrNx deposition rate under different Ti target power conditions (black square: 0 W, red circle: red circle 50 W and blue triangle: 100 W) when the mixture of Ar and N2 is injected from Ti target area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
which needs to be considered for reproducible and systematic thin film deposition studies is the coupling between targets under codeposition process conditions at low (deficient) N2 flow regimes [32]. These studies are beyond the scope of this paper and will not be presented here. 3.2. ZrN thin film orientation and crystallinity Table 1 lists the growth conditions for seven selected ZrN samples grown under various conditions for coarse process optimization and study purposes. The results from PXRD characterization, peak (111) intensity ratio to the total intensity of peaks (111) and (200) for all samples, along with (111) peak rocking curve FWHMs for three samples are also listed. The preferred (111) crystallographic film orientation is the primary property being optimized. For ZrN growth, four parameters, Zr target power, substrate temperature, N2 flow rate and chamber pressure, are varied. By comparing the results, we find that the target power, chamber pressure and N2 flow strongly affect the crystallographic film orientation. Sample No. 2 exhibits the highest
I(111)/(I(111) þI(200)) ratio with a rocking curve (RC) FWHM of 0.501. The (111) intensity ratio is reduced by (a) lowering the target power from 80 W for No. 2 to 50 W for sample No. 3 (b) lowering the chamber pressure from 6 mTorr for sample No. 1 to 3 mTorr for sample No. 5 and (c) lowering N2 flow rate from 1 sccm for sample No. 1 to 0.75 sccm for sample No. 7. However, doubling the N2 flow rate from sample No. 2 to sample No. 4 results in a large drop of the (111) peak intensity ratio. In addition, a 40 1C increase in the substrate temperature from No. 1 to No. 2 results in the elimination of the small (200) peak at 39.31 present in sample No. 1. This also decreases the (111) peak rocking curve FWHM from 0.631 to 0.501. In spite of different target powers and N2 flow conditions both sample No. 2 and sample No. 6 exhibit near identical RC FWHMs with full (111) and almost full (111) crystallographic orientations, having (200) peak intensity less than 10 counts per second, compared to 74k counts per second at (111) peak. This indicates that to get pure (111) oriented ZrN film at different deposition rates, at least two parameters should be varied. From the above results we find that the ZrN crystallographic film orientation depends on target power, chamber pressure and
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Fig. 4. (Left) PXRD scans of ZrN deposited at N2 flow rates at: 1.00 sccm, 1.25 sccm, and 1.50 sccm and (right) Rocking curves of ZrN (111) reflection and Al2O3 (0006) reflection by PXRD. Intensity is plotted in arbitrary units (a.u.).
N2 flow rate. Changing only one parameter from those that optimize (111) oriented deposition in either direction will result in reduction of the (111) intensity ratio. Increasing substrate temperature helps improve the film crystallinity. Based on the results that sample No. 2 has single (111) orientation and that temperature improves film crystallinity we raised the growth temperature to 800 1C, close to the maximum possible in our system, with all other parameters unchanged. We note that the (200) peak now shows up in XRD scans, and its intensity increases on decreasing N2 flow further (data not shown). This is due to the fact that as the study progresses, the target erodes and deposition rate drops for unchanged deposition parameters. Conversely when a new target is installed a lower power yields the same DR. As such the power used for the samples shown in Fig. 4 is 65 W, equivalent to the 80 W for sample No. 2. For a more detailed study the N2 flow rate is varied from 1.00 sccm (4.5%) to 1.25 sccm (5.6%) and 1.50 sccm (6.7%), with target power at 65 W, growth temperature at 800 1C, chamber pressure at 6 mTorr and Ar flow of 21 sccm mixed with N2 before injection from the Zr target. Fig. 4 shows PXRD ω–2θ scans and rocking curves of (111) reflections. Increasing the N2 flow rate from 1.00 sccm to 1.25 sccm reduces the FWHM from 0.491 to 0.361; however a further increase in the flow rate to 1.50 sccm increases the FWHM to 0.571 and increases the presence of (100) oriented material. This is in good agreement with trends observed in published studies for HfN deposition [33] where stoichiometric samples exhibit lowest FWHM values. It is to be noted that single crystalline Al2O3 substrates exhibit a rocking curve FWHM of 0.171 at the (0006) reflection due to the low instrument resolution of the powder X-ray diffractometer. From the analysis above it is clear that parameters such as target power, chamber pressure, N2 flow rate and substrate temperature have strong effects on the film orientation and crystallinity, and they are interrelated. 3.3. ZrxTi1 xN thin film crystallinity and epitaxial relationship to cplane Al2O3 ZrxTi1 xN thin films are deposited using the optimum ZrN conditions ((111) rocking curve FWHM value of 0.361) and adding Ti with Zr power slightly adjusted to keep the ZrN deposition rate unchanged. The latter is monitored by the QCM with the Ti shutter closed prior to thin film deposition. Our main interest is ternary
compounds with a ZrN fraction above 0.5, to cover the important lattice constant range from AlN to In0.14Ga0.86N. Fig. 5 (left) shows the ω–2θ scans of the ZrxTi1 xN (111) reflections for different ZrN fractions in ternary ZrxTi1 xN (x ¼0, 0.64, 0.80, 0.93, 1), with composition determined by HRXRD applying Vegard's law [34]. HRXRD characterization gives a ZrN lattice constant of 4.571 Å, nearly matching the reference values of bulk ZrN lattice constant of 4.574 Å [35], and TiN of 4.237 Å, also very close to the bulk value of 4.238 Å [36]. As such the Zr concentration is calculated using our experimental ZrN and TiN lattice constants, which differs less than 1% from that using the accepted bulk values for the two binaries. Though a deviation from Vegard's law has been reported [27,29] for ZrxTi1 xN, our XPS results on samples not part of this study seem to point out that for our growth conditions such a correction is not necessary. As an example the XPS results on one ZrxTi1 xN sample give a Zr/(Zr þTi) ratio of 0.75, the same as determined from HRXRD (data not shown). Considering the suggested positive deviation b ¼0.011 nm in a(ZrxTi1 xN) ¼ a(ZrN) xþ a(TiN)(1 x)þ bx(1 x), from reference [27], would yield a Zr/ (Zr þTi) ratio of 0.68. The authors in reference [27] mentioned that strain in their thin films might be the source of the deviation from Vegard's law, and that sample annealing at 850 1C for 3 h relaxed the lattice parameters to those predicted by Vegard's law. Our films deposited at 800 1C should match the annealed samples, explaining the match between XRD and XPS data without the need for a correction to Vegard's law. Fig. 5 (right) presents the HRXRD rocking curves for ZrxTi1 xN (111) reflections using a step size of 0.0021. The extremely low rocking curve (RC) FHWMs vary between 0.00451 and 0.0061, very close to instrument resolution of 0.0041 (as shown in Fig. 5 (right)). RC FWHM values, for the (222) reflections, range from 0.241 to 0.301 (except for TiN which is 0.061). This is another proof of the high crystallinity of our samples. Broad diffuse scattering can also be seen in the figure indicating the presence of localized defects, but these defects do not degrade the average crystallinity of the films. We employed X-ray coherence length relationship from reference [37] to our samples (lateral coherence length ε J ¼ 2π=jΔg J j ¼ λ=ð2Γ ω sin θÞ and perpendicular coherence length ε ? ¼ 2π=jΔg ? j ¼ λ=ð2Γ 2θ cos θÞ, with Δg ? and Δg J standing for the perpendicular and parallel width of diffraction vector, respectively, from diffraction intensity distributions, Γ ω and Γ 2θ for the FWHM value in ω-scan and ω–2θ scans, θ for Bragg angle, and λ for
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Fig. 5. (Left) ZrxTi1 xN (x ¼0, 0.64, 0.80, 0.93, 1) ω–2θ scans by PXRD and normalized (111) rocking curves by HRXRD (right).
7301 rotation about the [111] direction relative to c-plane Al2O3 substrates also typically occurs when III-nitrides are grown on sapphire substrates [40] and TiN on sapphire, as previously reported [12]. TiN and Zr0.80Ti0.20N phi scans of the (200) peaks reveal 6-fold symmetry and 7301 rotation as well. This rotation is due to the large lattice mismatch between ZrxTi1 xN and c-plane Al2O3. The epitaxial relationship of ZrxTi1 xN films on c-plane Al2O3 substrates is ZrxTi1 xN [110](111) ||Al2O3[1100](0001). 3.4. Surface morphology
Fig. 6. Phi scans of the (200) peaks for ZrN and (102) peaks for Al2O3.
X-source wavelength). Our samples result in a lateral coherence length of up to 3 mm from RC FWHM values at (111) reflections. As to the perpendicular coherence length, which is calculated from FWHM in ω–2θ scans, our results show values above 100 nm except for TiN thin films which are thinner than that value (TiN films have a thickness of 7072 nm as calculated from X-ray reflectivity). The TiN calculated perpendicular coherence length is 76 nm and is slightly larger than the actual film thickness. Further studies are underway to analyze the variation of the diffuse scattering HRXRD profiles with composition. Phi scans of the ZrN (200) off-axis Bragg peaks, inclined 54.741 with respect to the [111] direction, and the Al2O3 (102) off-axis peaks, inclined 57.611 with respect to the sapphire [0001] direction, reveal a 6-fold symmetry for ZrN films and are shown in Fig. 6. Since the (111) plane in a perfect cubic crystal has a 3-fold symmetry the 6-fold symmetry of this ZrN indicates that there are twin (111) planes in mirror reflection to each other, as reported earlier for TiN on sapphire [38] and ZrN on AlN buffered silicon [39], due to underlying layers' hexagonal symmetry. A single crystal sapphire substrate has 6-fold symmetry, but only 3-fold symmetry of the (102) peaks about the [0001] axis. The observed
Surface roughness and structure of ZrN, Zr0.80Ti0.20N and TiN thin films are characterized post-growth using tapping mode AFM technique and RHEED imaging. The results are shown in Fig. 7. The best ZrN sample exhibited a root mean square (RMS) roughness of 1.9 nm with a mixed streaky and spotty RHEED pattern, which indicates a smooth surface with high crystallinity. The spotty pattern could be a result of kinetic surface roughening, which is exacerbated by the non-normal flux during the sputtering process [41]. This effect is also a possible reason behind higher surface roughness (2.9 nm) and a spotty RHEED pattern with complete absence of any streaky features exhibited by the Zr0.80Ti0.20N sample. During ZrxTi1 xN growth the Zr and Ti fluxes impinge on the sample surface from two different directions with possible atomic flux shadowing resulting in a rougher surface for the ternary compounds. As for the TiN sample, the RHEED exhibits the streakiest pattern of all and the lowest RMS roughness of 1.2 nm.
4. Conclusions Using a QCM technique we are able to systematically study the reactive magnetron sputtering process of ZrN as a function of different parameters. Deposition rate is found to be dependent on target power, chamber pressure, Ar and N2 flow rates, and gas injection positions as allowed in our system configuration. The sputtering parameters are found to be interconnected and strongly influence the orientation and crystallinity of ZrN films. Increasing the substrate temperature improves the film crystallinity. The variation of N2 flow rate alone for ZrN thin films gives a minimum rocking curve FWHM of 0.361 at (111) reflections by PXRD. HRXRD characterization on ZrxTi1 xN (x ¼0, 0.64, 0.80, 0.93, 1) thin films reveals extremely low rocking curve FWHM values at (111)
R. Li et al. / Journal of Crystal Growth 404 (2014) 1–8
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Fig. 7. AFM images under 1 mm2 for (a) ZrN with RMS ¼1.9 nm, (b) Zr0.80Ti0.20N with RMS¼ 2.9 nm, and (c) TiN RMS¼ 1.2 nm; RHEED images for (d) ZrN, (e) Zr0.80Ti0.20N and (f) TiN along (110) axis.
reflections ranging from 0.00451 to 0.0061. Despite the high crystallinity of our thin films, as indicated by XRD results, further optimization of ZrxTi1 xN thin films using target power and N2 flow rate is believed to be still possible.
[8]
[9]
Acknowledgements
[10]
The authors would like to thank Mr. Alan Price for his technical support during this work and Dr. Pavel Dutta and Dr. Jae-Hyun Ryou for their help and giving us access to their HRXRD instrument.
[11]
[12]
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Epitaxial growth of (111)-oriented ZrxTi1 xN thin films on c-plane Al2O3 substrates Ruiteng Li, Jateen S. Gandhi, Rajeev Pillai, Rebecca Forrest, David Starikov, Abdelhak Bensaoula n Physics Department, University of Houston, Houston, TX 77004, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 16 April 2014 Received in revised form 22 June 2014 Accepted 23 June 2014 Communicated by D.W. Shaw Available online 1 July 2014
A systematic study is presented on the effects of process parameters of S-gun configured DC magnetron sputtered ZrN thin films on c-plane Al2O3 substrates. Using a quartz crystal microbalance the deposition rate of ZrN is investigated as a function of Ar and N2 flow rates, target power, chamber pressure and gas injection position in the chamber. Selected growth conditions for ZrN show the interrelation of growth parameters on film orientation and crystallinity. (111) oriented ZrN thin films exhibit X-ray diffraction rocking curve FWHM as low as 0.361. Additionally, (111) oriented ternary ZrxTi1 xN thin films (0 rx r 1) are also deposited on c-plane Al2O3 substrates. High resolution X-ray diffraction characterization shows that ZrxTi1 xN (x ¼0, 0.64, 0.80, 0.93, 1) layers exhibit rocking curve FWHM values of 0.0045–0.0061 for the (111) reflection, indicating highly crystalline thin films. Atomic force microscopy characterizations show ZrxTi1 xN thin films with a surface roughness between 1.2 nm and 2.9 nm. & 2014 Elsevier B.V. All rights reserved.
Keywords: A1. DC reactive magnetron sputtering A1. Thin films A1. XRD A1. AFM B1. ZrTiN
1. Introduction Group-III nitrides and their alloys are excellent materials for LED devices [1] and photodetectors [2] as well as high frequency and high power transistors [3]. InxGa1 xN and AlxGa1 xN band gap energies cover a wide range of the solar spectrum from the ultraviolet to near infrared [4], and high temperature and radiation tolerance opens the door for advanced detectors and photovoltaic applications [5]. However current high end substrates for III-nitrides, such as SiC and GaN wafers, are costly or introduce large strain for the desired AlGaN and InGaN compositions. Sapphire substrates have a 16% lattice mismatch and a 27% thermal expansion coefficient mismatch with GaN. Pretreatments of the sapphire surface [6] or buffer layers [7,8] are usually used to reduce the high defects density generated from the misfit during the III-nitrides growth process. Transition metal nitrides such as ScN interlayers [9], TiNx networks [10], and CrN nanoislands [11] have demonstrated reduction of dislocations in GaN grown on sapphire. In addition the refractory metallic nature of transition nitrides is very attractive for device fabrication. Among transition nitrides families, ternary alloys such as ZrxTi1 xN (0.49 rx r1) with effective lattice constant in the (111) plane covering lattice constants of AlxGa1 xN (0 rxr 1) and InxGa1 xN (0 rx r0.14)
n
Corresponding author. E-mail address: [email protected] (A. Bensaoula).
http://dx.doi.org/10.1016/j.jcrysgro.2014.06.043 0022-0248/& 2014 Elsevier B.V. All rights reserved.
are promising buffer layer candidates for III-nitrides growth on sapphire or silicon substrates. High quality TiN growths on Al2O3 [12,13], MgO [14,15], and Si [16] substrates via various techniques have been reported. Using reactive magnetron sputtering, TiN epitaxial films were grown at cryogenic temperatures [17]. On the other hand synthesis of ZrN thin films has been challenging due to the high substrate temperature growth required [18,19], and the existence of other metastable Zr3N4 [20] and ZrN2 [21] phases. Epitaxial ZrN thin films have been deposited on r-plane Al2O3 [19], (100)-silicon [18], (111)-silicon [22] and (100)-MgO [23] substrates by reactive magnetron sputtering. ZrN thin films have been deposited on cplane Al2O3 at low temperature [24] and on unknown surface orientation Al2O3 at growth temperature of 900 1C [25], but were reported as polycrystalline. Experimental [26,27] and theoretical [28,29] studies of ternary ZrxTi1 xN films have been conducted due to their outstanding physical and chemical properties. In particular, ZrxTi1 xN has been employed as a seed layer for the growth of AlxIn1 xN films on (111)-MgO and nanorod arrays of cplane sapphire substrates [30,31]. However, the crystalline quality of the ZrxTi1 xN thin films was not fully characterized in those reports. In this work we report synthesis of high quality ZrxTi1 xN (0 rx r1) thin films deposited on c-plane sapphire substrates using an S-gun configured DC magnetron sputtering system. The deposition rate and structural quality of ZrN films as a function of (1) target power, (2) Ar/N2 flow ratio, (3) chamber pressure and (4) substrate temperature are investigated. A quartz crystal
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microbalance (QCM) is used for the deposition rate study. The crystalline quality is analyzed via both powder (PXRD) and high resolution X-ray diffraction (HRXRD). From PXRD and HRXRD data the crystalline quality of binary and ternary films is characterized by the following two criteria: (a) film orientation, expressed as the intensity ratio of (111) peak to total intensity of (111) and (200) peaks, and (b) film crystallinity, expressed by (111) rocking curve FHWM. Surface morphology, roughness and surface structure of both binary and ternary films are characterized using atomic force microscopy (AFM) and reflection high energy electron diffraction (RHEED).
2. Experimental procedure 2.1. System description Fig. 1 (left) is a schematic diagram of the growth reactor used in this study. It consists of a DC magnetron sputtering system equipped with three MDX 500 DC supplies powering three 1.5 in. size target sources. A detailed schematic of the target as a self-contained sputter source in an unbalanced magnetron mode is shown in Fig. 1 (right). The target sits on a cathode and a grounded shield acts as an anode with a 5 cm tall chimney placed on top to prevent cross contamination between targets. Gas tubing for Ar and N2 injection passes through the grounded shield for each target. A radio frequency (RF) generator is connected to the substrate holder. The substrate is radiatively heated using two halogen lamps with a K-type thermocouple, placed behind the sample holder, measuring the reflected heat. The distance between the target and the substrate is approximately 12 cm. The base pressure of the chamber is 1.4 10 8 Torr. During deposition, gas pressure is monitored by a Baratron type gauge. Ar is used as the main work gas while N2 acts as the reactive gas. As shown in the diagram, the flow rates for Ar and N2 are separately controlled by two flow meters. Ar and N2 can be injected in three combinations: (1) mixed and injected from the work target area, (2) mixed and injected from a different target area other than the work target or substrate surface area and (3) separately as Ar from the target and N2 from the substrate area. A quartz crystal microbalance system, Inficon XTC/2, is available for monitoring the deposition rate.
2.2. Experimental procedure The ZrN deposition rate study is conducted by employing a QCM under various target powers, chamber pressures, Ar and N2 flow rates, and different gas injection combinations. Due to the small target size and cooling efficiency, DC target power is operated below 100 W. In this paper we present results for only Zr target powers of 80 and 50 W, chamber pressures of 6 and 3 mTorr, and Ar flow rates of 21 and 10 sccm in different combinations, as typical growth conditions. One-fourth of 2 in. size c-plane Al2O3 substrates, back-coated with 1200 Å thick Ti for thermal absorption, are cleaned by dipping for 5 min each in boiling trichloroethylene, acetone and methanol, followed by DI water rinsing for two cycles, and then blow-dried using nitrogen. Prior to deposition, Zr (purity 99.7%) and Ti (purity 99.997%) targets are deoxidized using a 3 min sputtering sequence in pure Ar plasma followed by 10 min sequence in Ar and N2 mixtures for a nitridation and plasma stabilization step. Table 1 shows parameters selected for a coarse ZrN structural optimization. A fine optimization at N2 flow rates of 1.00 sccm, 1.25 sccm, and 1.50 sccm is conducted at a target power of 65 W, substrate temperature of 800 1C, and chamber pressure of 6 mTorr and Ar flow of 21 sccm. Based on the crystalline quality data of ZrN films, the deposition conditions for ZrxTi1 xN are selected as follows: Zr power is adjusted to keep the same deposition rate as previously used for ZrN growth at a target power of 65 W and a substrate temperature of 800 1C, chamber pressure of 6 mTorr, Ar flow of 21 sccm, and N2 flow of 1.25 sccm while Ti target power is changed to 30, 50 and 70 W to achieve the desired alloy compositions. Both gases are mixed before injection
Table 1 Deposition conditions for ZrN and PXRD characterization analysis results. Sample No.
Zr power (W)
Temp. (1C)
Ar/N2 (sccm)
Chamber pressure (mTorr)
I(1 1 1) /(I(1 1 1) þ I(2 0 0)) by PXRD (%)
(111) RC FWHM by PXRD ( 7 0.011)
1 2 3 4 5 6 7
80 80 50 80 80 50 80
720 760 760 760 720 760 720
21/1 21/1 21/1 21/2 21/1 21/0.5 21/0.75
6 6 6 6 3 6 6
499.9 100 476 478 497 499.98 499.8
0.63 0.50 – – – 0.48 –
Fig. 1. (Left) Schematic diagram of the sputtering system and (right) target configuration.
R. Li et al. / Journal of Crystal Growth 404 (2014) 1–8
3
from the Zr target area. TiN is deposited at a Ti power of 70 W, substrate temperature of 800 1C, chamber pressure of 6 mTorr, Ar flow of 21 sccm and N2 flow of 1.25 sccm mixed before injection from the Zr target area. No substrate RF bias is applied during the deposition process. Film crystallinity, orientation and lattice constants are determined using both a Siemens D5000 powder X-ray diffractometer (PXRD) using Cu Kα radiation and two Four-circle high resolution X-ray diffractometers (HRXRD) using Cu Kα1 radiation, one with a Ge 220 analyzer before the detector for ZrxTi1 xN lattice constant determination and the other for off axis phi scans. For post-growth surface roughness and surface crystallinity evaluation, AFM and RHEED are utilized. ZrN and TiN film thicknesses are measured by HRXRD X-ray reflectivity. All samples are deposited for approximately 2 h with thicknesses ranging from 70 nm to 200 nm.
inversely relates to the mean free path of Zr atoms. With increasing chamber pressure a higher number of collisions occur between the sputtered flux and the background gas thereby reducing their mean free path and their density at the substrate. With decreasing Ar flow from 21 sccm (black square) to 10 sccm (cyan triangles), we find that at the same Ar/N2 ratio (21/1 sccm, 10/0.47 sccm), Ar flow of 10 sccm yields a slightly higher deposition rate. Based on the trend observed in deposition rate relationship with pressure, this is probably due to the pressure differential between local target area and global main chamber pressure. Though the global pressure is kept constant at 6 mTorr by manually adjusting the main chamber gate valve position, the local pressure around target area is certainly affected by the flow rate. To verify this hypothesis, a pressure differential study was conducted and the results are discussed in the next section.
3. Results and discussion
3.1.2. Local vs. global pressure To investigate the pressure differential between the local target area and global chamber area, a series of deposition rate studies are conducted under various conditions. Fig. 3(a), (b) and (d) shows plots of ZrNx deposition rate dependence on N2 flow rate. Fig. 3(c) is a plot of Zr deposition rate dependence on chamber pressure. In Fig. 3(a), ZrNx deposition rate is monitored when a mixture of Ar and N2 is injected from Zr target (black square) and Ti target (red circle). It shows that at the same N2 flow rate and chamber pressure at 6 mTorr, ZrNx deposition rate increases when injection port is changed from Zr target to Ti target. It is to be noted that injecting the Ar þN2 mixture from the Ti target or sample surface area yields the same deposition rate for ZrNx (data not presented here). In Fig. 3(b) ZrNx deposition rate is monitored when Ar is injected from Zr target but N2 is injected either from Zr target area (black square) or substrate area (red circle). Since N2 flow rate is relatively low compared to Ar flow rate (21 sccm), the difference in deposition rate caused by N2 injection position is due to N2 concentration difference between target area and main chamber. At 6 mTorr, the N2 mean free path is around 1 cm. Considering that the distance between target and substrate surface is 12 cm there would be a N2 concentration gradient along the line from target area to substrate area. In an attempt to quantify the pressure differential, Zr is sputtered under Ar atmosphere with a flow rate of 21 sccm injected from Zr target (black square) and Ti target (red circle) at different chamber pressures as seen in Fig. 3 (c). The two curves with different slopes show that the pressure difference varies with main chamber pressure. Using a chamber pressure of 6 mTorr with Ar injected from Zr target area yields the same deposition rate as that at a chamber pressure at 8.7 mTorr with Ar injected from Ti target area. Based on this observation, ZrNx deposition rate is investigated for various Ti target powers with Ti target gun shutter closed and Ar and N2 mixed prior to injection from Ti target at a chamber pressure of 8.7 mTorr. At low N2 flow rates, which we call N2 deficient region later, the deposition rate of ZrNx increases with increasing Ti target power. At high N2 flow rates, when N2 is sufficient, ZrNx deposition rate tends to be independent of Ti target power conditions. As a summary of the above study, ZrNx deposition rate is influenced by target power, chamber pressure, Ar and N2 flow rates by affecting Zr and N2 density in the deposition process. In our system configuration, we find a strong influence of the gas mixture injection position and how N2 is injected into the chamber on the deposition rate and film quality. The latter is attributed to the difference between the local target pressure and the global chamber pressure. As a high local target pressure is preferable for obtaining a stable plasma, Ar flow rate is kept at 21 sccm instead of 10 sccm and the gas injection position is selected from the Zr target area for all ZrxTi1 xN compound growth conditions. Another important factor
3.1. Study of ZrN deposition rate by QCM 3.1.1. ZrN deposition rate Fig. 2 shows a plot of ZrNx (x indicates the unknown Zr/N ratio with increasing N2 flow rate) deposition rate (DR), measured via QCM, as a function of N2 flow rate with ArþN2 gas mixture injected from the Zr target area. The following parameters are selected for comparison: (a) target powers of 50 W (blue triangle) and 80 W (black square), (b) chamber pressures of 3 mTorr (red circle) and 6 mTorr (black square) and (c) Ar flows of 10 sccm (cyan triangle) and 21 sccm (black square). The deposition rates are calibrated with ZrN thickness measured by X-ray reflectivity. Fig. 2 shows that for all of the target powers the deposition rate decreases with increasing N2 flow rate, which indicates a transition from pure Zr metal to ZrNx with higher N2 flow rates. From the same figure, increasing target power and decreasing chamber pressure increase the ZrNx deposition rate. The explanation here is that target power is directly related to the density of Zr atoms sputtered from the target surface while the chamber pressure
Fig. 2. ZrNx deposition rate dependence on (1) N2 flow rate (all of the curves), (2) Zr target power (50 W: blue triangle, 80 W: black square) at 6 mTorr and Ar of 21 sccm, (3) chamber pressure (3 mTorr: red circle, 6 mTorr: black square) at target power of 80 W and Ar of 21 sccm and (4) Ar flow rate (10 sccm: cyan triangle, 21 sccm: black square) at target power of 80 W and Ar of 21 sccm, with Ar and N2 mixed prior to injection from Zr target area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. (a) ZrNx deposition rate dependence as a function of N2 flow rate when the Ar and N2 mixture is injected from either Zr (black square) or Ti (red circle) target areas; (b) ZrNx deposition rate when Ar is injected from Zr target area and N2 injected from either Zr target (black square) or the substrate area (red circle); (c) Zr deposition rate dependence on pressure when Ar is injected from Zr (black square) or Ti (red circle) target areas and (d) ZrNx deposition rate under different Ti target power conditions (black square: 0 W, red circle: red circle 50 W and blue triangle: 100 W) when the mixture of Ar and N2 is injected from Ti target area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
which needs to be considered for reproducible and systematic thin film deposition studies is the coupling between targets under codeposition process conditions at low (deficient) N2 flow regimes [32]. These studies are beyond the scope of this paper and will not be presented here. 3.2. ZrN thin film orientation and crystallinity Table 1 lists the growth conditions for seven selected ZrN samples grown under various conditions for coarse process optimization and study purposes. The results from PXRD characterization, peak (111) intensity ratio to the total intensity of peaks (111) and (200) for all samples, along with (111) peak rocking curve FWHMs for three samples are also listed. The preferred (111) crystallographic film orientation is the primary property being optimized. For ZrN growth, four parameters, Zr target power, substrate temperature, N2 flow rate and chamber pressure, are varied. By comparing the results, we find that the target power, chamber pressure and N2 flow strongly affect the crystallographic film orientation. Sample No. 2 exhibits the highest
I(111)/(I(111) þI(200)) ratio with a rocking curve (RC) FWHM of 0.501. The (111) intensity ratio is reduced by (a) lowering the target power from 80 W for No. 2 to 50 W for sample No. 3 (b) lowering the chamber pressure from 6 mTorr for sample No. 1 to 3 mTorr for sample No. 5 and (c) lowering N2 flow rate from 1 sccm for sample No. 1 to 0.75 sccm for sample No. 7. However, doubling the N2 flow rate from sample No. 2 to sample No. 4 results in a large drop of the (111) peak intensity ratio. In addition, a 40 1C increase in the substrate temperature from No. 1 to No. 2 results in the elimination of the small (200) peak at 39.31 present in sample No. 1. This also decreases the (111) peak rocking curve FWHM from 0.631 to 0.501. In spite of different target powers and N2 flow conditions both sample No. 2 and sample No. 6 exhibit near identical RC FWHMs with full (111) and almost full (111) crystallographic orientations, having (200) peak intensity less than 10 counts per second, compared to 74k counts per second at (111) peak. This indicates that to get pure (111) oriented ZrN film at different deposition rates, at least two parameters should be varied. From the above results we find that the ZrN crystallographic film orientation depends on target power, chamber pressure and
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Fig. 4. (Left) PXRD scans of ZrN deposited at N2 flow rates at: 1.00 sccm, 1.25 sccm, and 1.50 sccm and (right) Rocking curves of ZrN (111) reflection and Al2O3 (0006) reflection by PXRD. Intensity is plotted in arbitrary units (a.u.).
N2 flow rate. Changing only one parameter from those that optimize (111) oriented deposition in either direction will result in reduction of the (111) intensity ratio. Increasing substrate temperature helps improve the film crystallinity. Based on the results that sample No. 2 has single (111) orientation and that temperature improves film crystallinity we raised the growth temperature to 800 1C, close to the maximum possible in our system, with all other parameters unchanged. We note that the (200) peak now shows up in XRD scans, and its intensity increases on decreasing N2 flow further (data not shown). This is due to the fact that as the study progresses, the target erodes and deposition rate drops for unchanged deposition parameters. Conversely when a new target is installed a lower power yields the same DR. As such the power used for the samples shown in Fig. 4 is 65 W, equivalent to the 80 W for sample No. 2. For a more detailed study the N2 flow rate is varied from 1.00 sccm (4.5%) to 1.25 sccm (5.6%) and 1.50 sccm (6.7%), with target power at 65 W, growth temperature at 800 1C, chamber pressure at 6 mTorr and Ar flow of 21 sccm mixed with N2 before injection from the Zr target. Fig. 4 shows PXRD ω–2θ scans and rocking curves of (111) reflections. Increasing the N2 flow rate from 1.00 sccm to 1.25 sccm reduces the FWHM from 0.491 to 0.361; however a further increase in the flow rate to 1.50 sccm increases the FWHM to 0.571 and increases the presence of (100) oriented material. This is in good agreement with trends observed in published studies for HfN deposition [33] where stoichiometric samples exhibit lowest FWHM values. It is to be noted that single crystalline Al2O3 substrates exhibit a rocking curve FWHM of 0.171 at the (0006) reflection due to the low instrument resolution of the powder X-ray diffractometer. From the analysis above it is clear that parameters such as target power, chamber pressure, N2 flow rate and substrate temperature have strong effects on the film orientation and crystallinity, and they are interrelated. 3.3. ZrxTi1 xN thin film crystallinity and epitaxial relationship to cplane Al2O3 ZrxTi1 xN thin films are deposited using the optimum ZrN conditions ((111) rocking curve FWHM value of 0.361) and adding Ti with Zr power slightly adjusted to keep the ZrN deposition rate unchanged. The latter is monitored by the QCM with the Ti shutter closed prior to thin film deposition. Our main interest is ternary
compounds with a ZrN fraction above 0.5, to cover the important lattice constant range from AlN to In0.14Ga0.86N. Fig. 5 (left) shows the ω–2θ scans of the ZrxTi1 xN (111) reflections for different ZrN fractions in ternary ZrxTi1 xN (x ¼0, 0.64, 0.80, 0.93, 1), with composition determined by HRXRD applying Vegard's law [34]. HRXRD characterization gives a ZrN lattice constant of 4.571 Å, nearly matching the reference values of bulk ZrN lattice constant of 4.574 Å [35], and TiN of 4.237 Å, also very close to the bulk value of 4.238 Å [36]. As such the Zr concentration is calculated using our experimental ZrN and TiN lattice constants, which differs less than 1% from that using the accepted bulk values for the two binaries. Though a deviation from Vegard's law has been reported [27,29] for ZrxTi1 xN, our XPS results on samples not part of this study seem to point out that for our growth conditions such a correction is not necessary. As an example the XPS results on one ZrxTi1 xN sample give a Zr/(Zr þTi) ratio of 0.75, the same as determined from HRXRD (data not shown). Considering the suggested positive deviation b ¼0.011 nm in a(ZrxTi1 xN) ¼ a(ZrN) xþ a(TiN)(1 x)þ bx(1 x), from reference [27], would yield a Zr/ (Zr þTi) ratio of 0.68. The authors in reference [27] mentioned that strain in their thin films might be the source of the deviation from Vegard's law, and that sample annealing at 850 1C for 3 h relaxed the lattice parameters to those predicted by Vegard's law. Our films deposited at 800 1C should match the annealed samples, explaining the match between XRD and XPS data without the need for a correction to Vegard's law. Fig. 5 (right) presents the HRXRD rocking curves for ZrxTi1 xN (111) reflections using a step size of 0.0021. The extremely low rocking curve (RC) FHWMs vary between 0.00451 and 0.0061, very close to instrument resolution of 0.0041 (as shown in Fig. 5 (right)). RC FWHM values, for the (222) reflections, range from 0.241 to 0.301 (except for TiN which is 0.061). This is another proof of the high crystallinity of our samples. Broad diffuse scattering can also be seen in the figure indicating the presence of localized defects, but these defects do not degrade the average crystallinity of the films. We employed X-ray coherence length relationship from reference [37] to our samples (lateral coherence length ε J ¼ 2π=jΔg J j ¼ λ=ð2Γ ω sin θÞ and perpendicular coherence length ε ? ¼ 2π=jΔg ? j ¼ λ=ð2Γ 2θ cos θÞ, with Δg ? and Δg J standing for the perpendicular and parallel width of diffraction vector, respectively, from diffraction intensity distributions, Γ ω and Γ 2θ for the FWHM value in ω-scan and ω–2θ scans, θ for Bragg angle, and λ for
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Fig. 5. (Left) ZrxTi1 xN (x ¼0, 0.64, 0.80, 0.93, 1) ω–2θ scans by PXRD and normalized (111) rocking curves by HRXRD (right).
7301 rotation about the [111] direction relative to c-plane Al2O3 substrates also typically occurs when III-nitrides are grown on sapphire substrates [40] and TiN on sapphire, as previously reported [12]. TiN and Zr0.80Ti0.20N phi scans of the (200) peaks reveal 6-fold symmetry and 7301 rotation as well. This rotation is due to the large lattice mismatch between ZrxTi1 xN and c-plane Al2O3. The epitaxial relationship of ZrxTi1 xN films on c-plane Al2O3 substrates is ZrxTi1 xN [110](111) ||Al2O3[1100](0001). 3.4. Surface morphology
Fig. 6. Phi scans of the (200) peaks for ZrN and (102) peaks for Al2O3.
X-source wavelength). Our samples result in a lateral coherence length of up to 3 mm from RC FWHM values at (111) reflections. As to the perpendicular coherence length, which is calculated from FWHM in ω–2θ scans, our results show values above 100 nm except for TiN thin films which are thinner than that value (TiN films have a thickness of 7072 nm as calculated from X-ray reflectivity). The TiN calculated perpendicular coherence length is 76 nm and is slightly larger than the actual film thickness. Further studies are underway to analyze the variation of the diffuse scattering HRXRD profiles with composition. Phi scans of the ZrN (200) off-axis Bragg peaks, inclined 54.741 with respect to the [111] direction, and the Al2O3 (102) off-axis peaks, inclined 57.611 with respect to the sapphire [0001] direction, reveal a 6-fold symmetry for ZrN films and are shown in Fig. 6. Since the (111) plane in a perfect cubic crystal has a 3-fold symmetry the 6-fold symmetry of this ZrN indicates that there are twin (111) planes in mirror reflection to each other, as reported earlier for TiN on sapphire [38] and ZrN on AlN buffered silicon [39], due to underlying layers' hexagonal symmetry. A single crystal sapphire substrate has 6-fold symmetry, but only 3-fold symmetry of the (102) peaks about the [0001] axis. The observed
Surface roughness and structure of ZrN, Zr0.80Ti0.20N and TiN thin films are characterized post-growth using tapping mode AFM technique and RHEED imaging. The results are shown in Fig. 7. The best ZrN sample exhibited a root mean square (RMS) roughness of 1.9 nm with a mixed streaky and spotty RHEED pattern, which indicates a smooth surface with high crystallinity. The spotty pattern could be a result of kinetic surface roughening, which is exacerbated by the non-normal flux during the sputtering process [41]. This effect is also a possible reason behind higher surface roughness (2.9 nm) and a spotty RHEED pattern with complete absence of any streaky features exhibited by the Zr0.80Ti0.20N sample. During ZrxTi1 xN growth the Zr and Ti fluxes impinge on the sample surface from two different directions with possible atomic flux shadowing resulting in a rougher surface for the ternary compounds. As for the TiN sample, the RHEED exhibits the streakiest pattern of all and the lowest RMS roughness of 1.2 nm.
4. Conclusions Using a QCM technique we are able to systematically study the reactive magnetron sputtering process of ZrN as a function of different parameters. Deposition rate is found to be dependent on target power, chamber pressure, Ar and N2 flow rates, and gas injection positions as allowed in our system configuration. The sputtering parameters are found to be interconnected and strongly influence the orientation and crystallinity of ZrN films. Increasing the substrate temperature improves the film crystallinity. The variation of N2 flow rate alone for ZrN thin films gives a minimum rocking curve FWHM of 0.361 at (111) reflections by PXRD. HRXRD characterization on ZrxTi1 xN (x ¼0, 0.64, 0.80, 0.93, 1) thin films reveals extremely low rocking curve FWHM values at (111)
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Fig. 7. AFM images under 1 mm2 for (a) ZrN with RMS ¼1.9 nm, (b) Zr0.80Ti0.20N with RMS¼ 2.9 nm, and (c) TiN RMS¼ 1.2 nm; RHEED images for (d) ZrN, (e) Zr0.80Ti0.20N and (f) TiN along (110) axis.
reflections ranging from 0.00451 to 0.0061. Despite the high crystallinity of our thin films, as indicated by XRD results, further optimization of ZrxTi1 xN thin films using target power and N2 flow rate is believed to be still possible.
[8]
[9]
Acknowledgements
[10]
The authors would like to thank Mr. Alan Price for his technical support during this work and Dr. Pavel Dutta and Dr. Jae-Hyun Ryou for their help and giving us access to their HRXRD instrument.
[11]
[12]
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