J Mater Sci: Mater Electron DOI 10.1007/s10854-017-7241-7
Yttrium oxide nanostructured thin films deposited by radio frequency sputtering: the annealing optimizations and correlations between structural, morphological, optical and electrical properties Saleh Abubakar1,2 · Senol Kaya1,2 · Aliekber Aktag1,2 · Ercan Yilmaz1,2
Received: 13 March 2017 / Accepted: 29 May 2017 © Springer Science+Business Media New York 2017
Abstract In this study, structural, morphological, optical, electrical properties and their correlations of the Yttrium Oxide (Y2O3) thin films were studied in details. The variations in these parameters by annealing of the samples at 500, 700, 900 °C were examined and optimum annealing conditions for the Y2O3 thin films were also determined. The structural parameters were studied by X-ray diffractometer analysis while scanning electron microscopy (SEM) was used for investigating the morphological properties of the devices. The reflection measurements were performed and band gap (Eg) calculations have been done by using the spectroscopic reflectometer measurements. The electrical parameters were examined by specifying surface state density and alternating-current (a.c.) conductivity. The results have revealed that the crystallizations, grain sizes of the thin films were improved with annealing due to agglomeration of the small particles around the bigger cluster thanks to high thermal energy which can also be seen in SEM measurements. On the other hand, both the reflection and the E g were enhanced with annealing. The films having disorder structure, and higher defects density localised in the energy gap of dielectrics layer caused additionally allowed states. These additionally allowed states may affect the optical characteristics. Hence, it may deflect the optical performance of the films. The surface state densities almost decrease and the a.c. the conductivity of the thin films increases with increasing in annealing temperature due to * Ercan Yilmaz
[email protected] 1
Center for Nuclear Radiation Detectors Research and Applications, AIBU, 14280 Bolu, Turkey
2
Physics Department, Abant Izzet Baysal University, 14280 Bolu, Turkey
rise in the grain sizes of the films. The number of the defect centres localised in the intra-crystallites boundary of the grains cause lattice and impurity scattering hence increase the bulk resistivity of layers. Therefore, the films having higher grain sizes decrease the number of the grain boundary; hence, increase the a.c. the conductivity of devices. Considering these results, strong relations were observed among structural, morphological, optical and electrical characteristics of the thin films and the devices which were annealed at 900 °C exhibited demanding characteristics for microelectronic applications.
1 Introduction A considerable attentions have been paid towards the growth and research of the novel dielectric materials such as HfO2 [1], Al2O3 [2], Sm2O3 [3], ZnO2 [4], TiO2 [5] etc. for state-of-the-art microelectronic technology. Among these materials, Yttrium Oxide has some demanding characteristics including the high dielectric constant, thermodynamically stability with Si, and especially low lattice mismatch betweenY2O3 and Si [6–9]. The demanding basic characteristics of the Y2O3 numbers of parameters should be considered for the Y 2O3 for microelectronic applications. It is well-known that the fabrication parameters highly affect the structural, optical and electrical characteristics of the devices [10–12] and it is also important to understand the correlation among structural, optical and electrical characteristics in detail. Several methods can be used to deposit Y2O3 films. Some of them are molecular beam epitaxy [13], electron beam evaporation [14], pulsed laser deposition [15, 16], DC/RF sputtering [17], and chemical vapour deposition [18]. Among all methods, the process or the act of
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crystallising temperatures, electro-optics, electrical and microstructures features of the Y2O3 thin films are strongly related to how the interface surface structures, morphologies behave. Also, the fabrication techniques, deposition conditions, and post-growth process also have the huge impact on these features. However, in both physical and chemical deposition methods, the ultra-microstructure regarding the chemical valence state of Y and the surface/ interface chemistry have a high effect on electronic and optical properties of the grown thin films and, hence, their device applications. Factors like crystal qualities, lattice parameters, and the defect structures are the causes of sensitivity toward the optical constant and electrical parameters for the changes in thin films features. Additionally, the quality of the developing film surface and interfacial compounds buried at the film-substrate interface are quite important for oxide dielectrics, such as Y 2O3 in this case, for their effective utilisation in optical, electronic, and optoelectronic device applications. In previous works, researchers have investigated annealing effects on devices parameters effects separately [8, 17, 19, 20]. In the current study, an effort was performed to investigate tohether with the effects of post-annealing temperatures for determining the microstructural, morphological, optical properties and electrical characteristics of Y2O3 thin films and correlations among these parameters. The structural parameters including grain size and lattice parameters were investigated by X-ray diffractometer (XRD) while morphological changes were studied by Scanning Electron Microscopy (SEM). On the other hand, the variations microstructural effects on optical characteristics were analysed by Spectroscopic Reflectometers. Capacitance–voltage and conductance measurements were carried out to investigate surface state density and alternating current conductivity on the electrical characteristics of the thin films.
As soon as surface cleaning is done, a wafer was loaded into the chamber of the sputtering system, for deposition of Y2O3 onto the silicon layer. The Y2O3 target was used for the deposition of the oxide layer, with the dimension 4-in. and purity 99.99%. The base pressure of the sputter chamber was adjusted below 4.0 × 10−4 Pa. After the argon gas flow rate was adjusted to 16 sccm, the sputtering pressure was adjusted to 1.0 Pa. Pre-sputtering was performed for about 3 h at 300 W so that any impurities on the surface of the target would be removed, then commercial sputtering was performed under the same parameters for 30 min. The thickness of the Y 2O3 thin film was measured is about 150 nm by the spectroscopic reflectometer. The deposited films were divided into four groups. The first group samples were kept as-deposited, and the others were annealed at 500, 700, and 900 °C for 1 h under a Nitrogen environment, separately. The XRD, SEM and optical reflection measurements were carried out on the annealed samples and as-deposited samples for determining the structural, morphological and optical characteristics of the films. To calculate the surface state density and alternating current conductivity of the thin films, metallization processes were performed for all samples. For back ohmic contact, Aluminium (Al) was deposited on the back side of the film by using RF sputtering technique. Also, the same process was made for front Ohmic contact, but for the front side round dots mask with the diameter of 1.50 mm was used to cover the surface of the films, only the dots open areas were deposited with aluminium (Al) on the front side of the film. The capacitance–voltage and the conductance–voltage measurements were performed at the high frequency of (1 MHz) to calculate surface state density and a.c. the conductivity of the fabricated samples.
2 Experimental details
X-ray diffraction (XRD) measurements were taken for fabricated Y2O3 thin films and the spectra were analysed, and different results were obtained about the formation and properties of the crystal including crystallisation, grain sizes, inter-planer distance (d-spacing) and lattice parameter. The obtained XRD measurement results of the fabricated Y 2O3 thin films annealed at various temperatures are shown in Fig. 1. We have observed that the intensities of the peaks increase with the increasing annealing temperatures. This indicates that atoms on the structure change from disordered states to ordered state with annealing temperatures. Hence, the films annealed at 500, 700, and 900 °C show a continuous crystallisation development. The obtained peaks in the XRD measurement results were analysed by ICDD programme and
In order to fabricate Y2O3 thin films, the silicon wafer which is 500 μm thick, p-type (100) Si substrate with a resistivity of 1–4 Ω was cleaned by the standard Radio Corporation of America (RCA) cleaning process initially. In RCA cleaning process, a silicon wafer was cleaned in 6:1:1 deionized (DI) water:ammonia:H2O2 mix solution for 5 min to remove organic residue and then to remove ionic residue, silicon wafer was cleaned in a mixture of 5:1:1 deionized water–H2O2–HCl solution about 5 min. For eliminating natural oxides on the silicon surface, the wafer was cleaned in 100:1 deionized water–HF solution for 30 s and the cleaned wafer was dried by 6N pure N2.
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3 Results and discussions 3.1 X‑ray diffraction studies
(622)
A s -D e p . 0 500 C 0 700 C 0 900 C A n n e a le d in N 2
(611)
(440)
(134)
400
(400)
500
(211)
Intensity (a.u.)
600
(420)
700
(411)
(222)
J Mater Sci: Mater Electron
300 200 100 0 20
30
40 2theta (deg.)
50
60
Fig. 1 XRD patterns of Y2O3 thin films annealed at various temperatures
the peaks on the XRD measurements are well-matched with the 71-0099 card number. It is seen that annealed films are in cubic phase structure with (222) preferred orientation. The inter-planer distance (d) and lattice parameter (a) were calculated using the characterised peaks by well-known Bragg’s law,
2dhkl sin 𝜃 = n𝜆
(1a)
a d= √ 2 h + k2 +l2
(1b)
where θ is the Bragg angle, λ is diffracted wavelength and n is a positive integer. The calculated d and a values are given in Table 1. As shown in Table 1, the d and a values decrease after 900 °C annealing temperature. The expected ideal d-value is 3.054 Å for bulk Y2O3 obtained from ICDD data base [21, 22]. It is seen that d value after 900 °C annealing is close to the expected value. The higher d values for lower annealing temperature may be attributed to the attractive stress and strains on the film structure [19, 23]. On the other hand, average grain sizes, P, were calculated by Debye–Scherrer equation given [12],
P=
0.9𝜆 𝛽cos𝜃
(2)
where β is FWHM (full width at half max), 𝜆 is X-ray wavelength (1.54 Å), and θ is the Bragg angle. The (222)
Table 1 Some structural parameters of Y2O3 films annealed at different temperatures
characteristic peak was used to evaluate the grain sizes of the films. Using Eq. 2, the P values were obtained, and the results are given in Table 1. It is seen that P values are 22.18, 25.63, and 30.39 nm for 500, 700, and 900 °C, respectively. During annealing at high temperature, individual nanostructures join in a bigger cluster, and the activation energy becomes much larger [23–25], and crystallinity increases. Hence, grain size of the films increase with temperature. Also, the similar studies results show that annealed Y 2O3 thin films parameters improve with the increasing temperature [21, 24]. The XRD analyses reveal that the Y2O3 films annealed at 900 °C exhibit demanded crystalline structure with the possible presence of the attractive stress. 3.2 Scanning electron microscopy (SEM) analysis On account of finding information regarding the surface morphologies and microstructures of the Y 2O3 thin films, scanning electron microscopy (SEM) measurements have been accomplished. Figure 2a–d show the effects of annealing temperature behaviour on the scanning electron microscopy images of the Y2O3 thin films. It’s seen that the Y 2O3 thin films are exhibited different nanostructure. Figure 2a shows that the film has amorphous surface morphology. With the annealing, the presences of Nanograins become evident. It is seen from SEM images that grain size of the sample becomes larger meaning that sample crystallinity is increasing with annealing temperature. As Fig. 2b–d show, the grainy structure became more prominent with a rise in the annealing temperature. The X-ray diffraction outcomes also support the obtained SEM measurements of Y2O3 thin films. All these results indicate that the annealed films crystallinity enhance with higher annealing temperatures. 3.3 Optical characterizations The optical analysis of Y 2O3 samples was carried out and the spectra were analysed to obtain different results. The annealing temperature dependency of reflectance (R) of Y 2O3 thin films as functions of photon energies is shown in Fig. 3. The spectral characteristics as a function of the incident photon energy reveal that the reflectance of the fabricated Y2O3 films slowly enhances with annealing temperature. The variations
Annealing temperature (°C)
Peak (2θ) (°)
Lattice parameter (a) (Å)
d-spacing (d) (Å)
FWHM (β) (°)
Grain size (P) (nm)
500 700 900
29.09 29.09 29.16
10.584 10.584 10.422
3.066 3.066 3.058
0.37 0.32 0.27
22.18 25.63 30.39
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Fig. 2 SEM pictures of the Y 2O3 films a as-deposited, and annealed b 500, c 700 and d 900 °C
Reflection (%)
Y2O3 films increases with increasing annealing temperature. This indicates that the packing density for Y 2O3 films increases upon higher temperature annealing [7, 8, 20, 26]. Correlation between film quality and optics of the band gap of the fabricated Y 2O3 films was investigated by calculating absorption coefficient from Eq. 3. The optical absorption coefficient of the Y 2O3 film, α, is given by the following equation [27, 28]:
A s-D ep 0 500 C 0 700 C 0 900 C
80 60 40 20 0 1
2
3
hv (eV)
4
5
6
Fig. 3 The spectral reflectance curves of Y2O3 films
of the measured reflectance may be attributed to the changes in film crystallinity. The reflectance increases, which may be due to the decrease of the defects and the change of crystalline structures in Y 2O3 films. The films having higher defect densities and low grain sizes may enhance lattice scattering in the oxide structure which enhance the interaction and absorption possibility of the indecent light. Hence, at lower temperatures due to disordered structure, the reflectance decreases. Therefore, films having higher reflectance are demonstrating higher crystallinity [7, 20]. It is clear that the reflectance of
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] [ ] [ 1 T α= ln d (1 − R)2
(3)
where T, R, t are the transmittance, the reflectance and t the thickness of the films, respectively. Reflectance, R, is the amount of flux reflected by a surface. Transmittance T is the amount of flux transmitted through a surface. Any flux which is not reflected or transmitted is absorbed (α) [29, 30]. The conservation of energy requires that:
R+T +𝛼 =1 (4) In the high absorption regions, E g is greater than the fundamental absorptions edges, so a power law comes behind with the absorption of the forms [27, 30–32]: (𝛼hv) = B(hv − Eg )n
(5)
J Mater Sci: Mater Electron
where hv is the energy of the photon, B the absorption edge breadth parameter, Eg is the band-gap energy, and n is the exponent. The exponent, n, determines the types of electronics transmissions/reflections bringing about the optical absorptions and with the values 1/2 direct-allowed, 3/2 direct-forbidden, 2 indirect-allowed and 3 indirect-forbidden transitions orderly [7–9]. The absorptions data and the plots acquired for Y2O3 thin films were shown in Fig. 4. It is obvious that (𝛼hv)2 vs. hv results in the high absorptions regions shows linearity, 𝛼 > 104 cm−1, signifying directallowed transition across E g of Y 2O3 films. The regressions analyses are performed to obtain linear fits. In Fig. 4, the linear fit of the absorption shows the direct band-gap of Y2O3 films. Eg values are indicated with lines. Suffix illustrates the difference of band-gap with annealing temperatures for Y2O3 thin films. The band-gap values extracted by extrapolating the straight lines close to the band-edge to zero are in the order of 5.41, 5.46 and 5.47 eV for the films annealed at 500, 700 and 900 °C. It clearly shows that the optical band-gap increases with increasing annealing temperatures. It is believed that the presence of defects and disorder in the film would produce localized states in the band structure which is responsible for the low value of the energy gap. In other words, the films having higher defect densities may trap the mobile charges and produce like-defect-caused-allowed (DCAS) states within the conventional band gap that enables the optoelectronic conversion of sub-band-gap photons. Hence, these states cause the impurity to the band transition resulting in decreasing the band gap energy. After high-temperature annealing, the extinction of the oxygen defects or vacancies and disorder in the film results in decreasing density of the localised states in the band structural [33, 34], and the impurity- to band transition effects will be decreased. So, only the expected band- toband transitions occurs, which in turn, would increase the band gap of the films [6, 35]. The mentioned mechanism can be seen in Fig. 5 (a) and (b). The XRD results have
shown that crystallisations start at the annealing temperature of 500 °C, and also the crystalline degree increases with increasing the annealing temperature. So, the increase of band-gap could be as a result of modification of the composed crystal structures. 3.4 Electrical characterizations Considering the structural progression of the films, capacitance–voltage, surface charge density and alternating current a.c. conductivity were calculated to investigate the electrical features of the fabricated films. The capacitance–voltage curves of Y 2O3 MOS capacitors annealed at temperatures: As-deposited, 500, 700 and 900 °C are illustrated in Fig. 6, it can be seen that the electrical property of the device increases with the increase in the annealing temperatures. And the samples annealed at 900 °C have shown a better electrical feature, in Fig. 7, the capacitance–voltage curves of 900 °C annealed Y2O3 MOS capacitors at different frequencies are illustrated. In general, the capacitance of the film decreases with the increases in frequency. The characteristics of normal C-V curves are observed, which means the samples annealed at the high-temperature exhibit the demanded results. The capacitance values of the MOS capacitors at low and high frequencies are strongly depended on the signal resulting from the carrier abilities of the charges. Fermi level of the films is adjusted in accordance with the interface-trap
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As-Dep 0 500 C 0 700 C 0 900 C
11
1,6x10
2
-2
-2
(αhν) (cm eV )
2,4x10
10
8,0x10
5,4
5,6
5,8
6,0
hν (eV)
Fig. 4 (𝛼hv)2 vs. hv plots for Y2O3 films
Fig. 5 Diagram of the different optical transitions a contribution of DCAS, and b expected ideal defects free band to band transitions
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Capacitance (F)
J Mater Sci: Mater Electron 4.50x10
-10
3.00x10
-10
1.50x10
using total oxide surface charge ( Qeff = qNss) obtained by using the equation below [12, 38]: As-Dep 0 500 C 0 700 C 0 900 C
VFB = 𝜙ms −
-10
0.00
-9
-6
-3
0
Voltage (V)
Capacitance (F)
Fig. 6 Capacitance–voltage curves of Y 2O3 MOS capacitors at different annealed temperatures: as-deposited, 500, 700 and 900 °C
6.00x10
-10
4.50x10
-10
3.00x10
-10
1.50x10
-10
1 MHz 500 kHz 100 kHz 50 kHz
-8
-4
0
Voltage (V)
Qeff
where q is the electrical charge, 𝜙ms is the difference in the work function between the metal and the semiconductor, and Cox (= ε0εox/d) is the oxide capacitance per unit volume in cm−2. The calculated Nss values were given in Table 2 for various annealing temperature and it is seen that the impurity Nss values almost decrease with annealing temperature. In addition, the Nss are order of 1011 cm−2 which is higher than conventional SiO2 based devices. However, it is still in in the same order as other promising dielectrics reported in [12, 39–41]. The frequency dispersion of Nss values also calculated for devices annealed at 900 °C which exhibits demanding characteristics among other devices annealed different temperatures. It is seen that the Nss 1 MHz; 2.77 × 1011 cm−2, 500 kHz; 2.71 × 1011 cm−2, 100 kHz; 2.64 × 1011 cm−2, and 50 kHz; 2.57 × 1011 cm−2 respectively. The observed small variation on the Nss under different frequency is also reason dispersions on the capacitance characteristics in Fig. 7. On the otherhand, the structural and morphological variations effects the a.c. conductivity of devices. The dielectric constants have been calculated by investigating the dielectric constant of devices by the following equation [8, 36]:
𝜀∗ = 𝜀� + 𝜀�� = Fig. 7 Capacitance–voltage curves of 900 °C annealed Y2O3 MOS capacitors at different frequencies
level due to applied A.C. signal. The charge in the interface trap changes when the MOS capacitor is in the depletion region. The random distribution of insulator charges and the charged interface region cause a nonuniform distribution of surface band bending at the interface. It is also observed from both Figs. 6 and 7 that flatband voltages were changed due to the total surface state ss values can be calculated density (Nss) [36, 37]. The N
(6)
Cox
Cd Gd −j A𝜀0 𝜔A𝜀0
(7)
(8) where ε ׳is the real and ε ׳׳is the imaginary value of the dielectric constant. A is the front contact area of the capacitor, C is the measured capacitance, G is the conductance of the film at high frequency (1 MHz) in the accumulation of −7.0 V gate voltage, and the permittivity of the vacuum is ε0, which is (= 8.85 × 10−14 F cm−1), the thickness is d, which is 150 nm, and ω is the angular frequency of the applied voltage. Calculated parameters are given in
tan 𝛿 = 𝜀� ∕𝜀��
Table 2 The conductivity and related electrical parameters of the Y2O3 MOS capacitors annealed at different temperatures ID
Capacitance (×10−10 F)
Thickness (nm) Conductance Angular (×10−4 S) frequency (Rad s−1)
Area (cm− 2)
Nss (×1011 cm−2)
A.C. conductivity Resistivity (×10−7 ohm−1 cm−1) (×105ohm cm)
As 500 700 900
4.04 5.47 8.11 8.92
6.94 16.7 23.5 27.0
0.01766 0.01766 0.01766 0.01766
7.79 8.71 7.92 2.77
5.89 14.2 20.0 22.9
13
6,280,000 6,280,000 6,280,000 6,280,000
150 150 150 150
17.0 7.05 5.01 4.36
J Mater Sci: Mater Electron
Table 2. Alternating current a.c. conductivity was calculated through [8, 35], ( ) d 𝜎a.c = 𝜔 C tan (𝛿) = 𝜀�� 𝜔𝜀0 (9) A The values of variation of a.c. conductivity with annealing temperature are tabulated in Table 2. It is seen that conductivity is directly proportional to the annealing temperature. When annealing temperature increases, the conductivity increases. Since the resistivity (ρ) is inversely proportional to the conductivity (𝜎a.c = 1∕𝜌), ρ decreases when the annealing temperature increases. Resistivity and conductivity are related to these factors; impurity in the crystalline, scattering cause by dislocations, and the potential barriers which are due to the crystallite boundaries effects [20, 26]. In the SEM and XRD analysis, we have experienced the enhancement of annealing temperature with grain size and the crystallite of the fabricated Y2O3 thin films. The total area of the boundary of two different grains where numbers of defects are localized, decrease for the films having higher grain size. Therefore, the developments on the film structures show that the impurity density of the film decreases. Consequently, due to the increase in the density of the films, ionized impurity scattering, the removal photons scattering, and the resistivity of the film decrease [8, 27, 42]. Similar effects were also observed and discussed in optical analysis of the films.
4 Conclusion The results of this work have shown that the microstructures and the morphology of the fabricated Y 2O3 thin films are remarkably affected by annealing temperature. The XRD analyses reveal that the Y2O3 films annealed at 900 °C exhibit demanded crystalline structure. The SEM measurements show that the Y2O3 thin films have exhibited different nanostructure. The X-ray diffraction (XRD) results also support the obtained SEM measurements of the Y2O3 thin films. On the other hand, increasing in annealing temperature causes increases in the observed optical band gap. The ordered structure, lower number of grain boundaries and the treatment of the defects in the structure are basic reasons for the variations in the optical characteristics with high-temperature annealing. The Nss were also decrease with annealing temperatures and approves the ideal excepted values. Moreover, the a.c conductivity increases with annealing. The conductivity variations are related to lattice scattering, especially, impurity scattering. The developments on the film structures show that the decrements of the impurity density of the films are basically related to the boundaries between grains; hence, it improves
the conductivity of the layers up to some points. All these results indicate that the strong relations exist among structural, morphological, optical and electrical characteristics on thin films. Moreover, the annealed films uniformity and crystallinity enhance with higher annealing temperatures and the devices annealed at 900 °C exhibits demanding characteristics in MOS based devices. Acknowledgements This work is supported by Abant Izzet Baysal University under Contract Number: AIBU, BAP. 2015.03.02.870, and BAP. 2014.03.02.722 and the Ministry of Development of Turkey under Contract Numbers: 2016K121110.
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