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Quantum confinement effect and size-dependent photoluminescence in laser ablated ultra-thin GZO films
Accepted Manuscript Quantum confinement effect and size-dependent photoluminescence in Laser ablated ultra-thin GZO films Ali Hassan, Muhammad Irfan, Yijian Jiang PII: DOI: Reference:
S0167-577X(17)31401-5 http://dx.doi.org/10.1016/j.matlet.2017.09.061 MLBLUE 23175
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
26 July 2017 24 August 2017 17 September 2017
Please cite this article as: A. Hassan, M. Irfan, Y. Jiang, Quantum confinement effect and size-dependent photoluminescence in Laser ablated ultra-thin GZO films, Materials Letters (2017), doi: http://dx.doi.org/10.1016/ j.matlet.2017.09.061
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Quantum confinement effect and size-dependent photoluminescence in Laser ablated ultra-thin GZO films Ali Hassana*, Muhammad Irfanb, Yijian Jianga a
Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, PR China.
b
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese
Academy of Sciences, Beijing, 100190, China.
*Corresponding Author ([email protected])
Abstract: The structural, morphological, and optical characteristics of GZO ultra-thin films were investigated using XRD (X-ray diffraction), FE-SEM (field emission electron microscopy), in-situ EDS (Energy Dispersive X-ray) spectroscopy, UV-VIS-IR spectroscopy and photoluminescence spectroscopy (PL) respectively. Morphological analysis reveals the noodle, seed and particle like structure of GZO for GaN, sapphire and Si substrates respectively with average grain size ranging from 5 to 20nm. An effective mass model (EM-Model) for particle in a cylindrical wave function of e-h pair was correlated with experimental results. The reduction in FWHM value (from 31nm to 13nm) of NBE (near-band-edge) emission peak and enhanced NBE intensity have been achieved with small grain size. Blue shift in optical band gap is explained in term of grain radius by EM-model. Improved optical and structural properties were found in relation with quantum confinement effect. The current study states that grain size plays vital role in order to tailor optical properties of GZO thin films. Keywords: Thin films, Luminescence, Laser Processing, Semiconductor, Epitaxial growth.
Introduction: Zinc Oxide (ZnO) having wide and direct band gap (3.37eV) with comparatively large exciton binding energy (60meV) is regarded as one of the best candidate for replacing the rare earth transparent conductive oxides and other functional semiconductors [1]. ZnO based nanowires (NWs) [2] and nanorods (NRs) [3] have attracted much interest due to their unique electrical and optoelectronic properties in the field of modern device fabrication technology. Some recent studies show the obvious milestone in developing precisely controllable size and shape of ZnO nanoparticles (NPs) and NRs having excellent electrical and optical properties [4]. Beside these, the favorable doping elements in ZnO matrix is a hot issue in order to attain desired electronic and optoelectronic properties. Rare earth metals always produce deep alteration of spin dependent phenomena in ZnO NWs and NRs, this helps to establish a system with unique and enhanced functionality, such as sensitive 1
biomedical sensors[5], scintillators materials with low time delay[6], high efficient laser diodes and solid state lightning [7-9]. Recent research shows that the ZnO doped with group III elements (B, In, Al, Ga) exhibits improved optoelectronic properties[10], so these films gain much interest as an alternate for indium tin oxide (ITO). Among all group III elements Ga has some distinct advantages as; its ionic and covalent radius (0.62 and 1.26Å) are comparable with those of Zn (0.74 and 1.34Å) and hence the occurrence of small lattice mismatches even for high Ga doping in ZnO. Moreover, Ga is less reactive and more resistive against oxidization as compared to its competitors during the deposition process. Recently Ga-doped ZnO (GZO) based stable transparent electrodes have been used in organic light emitting diodes (OLEDs) and efficiency has increased to 25% at 20mA[11]. Different methods have been adopted to synthesis ZnO NWs and NRs worldwide. In this study we have used pulsed laser deposition [12] technique to develop NPs, nano-seeds (NSs) and nano-noodles (NNs) on Si, sapphire and GaN substrate, respectively. The morphology, substitutional kinetics and stability dependence of Ga doping in ZnO host matrix is analyzed and discussed using EM-Model to understand quantum confinement effect. Experimental Details: Lambda physik LPX305iF KrF excimer laser having (λ=248nm, pulse duration tp=20ns, laser Energy=450mJ, frequency= 3Hz, laser fluence= 200mJ/cm2) was used to ablate the GZO target. Laser beam was focused on the target which was mounted in the deposition chamber with circular target holder. Target holder and mechanical rotator were rotated at 10rpm to avoid texturing and pitting of target during deposition. Prior to deposition, all the substrates were ultrasonically cleaned for 20min each, via Acetone, Ethanol and Deionized water respectively. The cleaned substrates then dried with 99.999% Nitrogen and were placed at Si heater exactly parallel to the target with 5cm.substrate temperature was kept constant at 400ºC. the chamber was evacuated to the base pressure of 1 × 10 Pa. for 30min. then high purity Oxygen (99.999%) was introduced to the chamber and pressure was kept at 4.5 × 10 Pa. during the ablation. 5400 pulses of laser beam were used for each deposition. WYKO NT1100 3D non-contact Optical profiler was used to measure the films thickness in VSI (vertical Scanning Interferometry) mode. X-ray diffraction of Bruker -D8 with step size 0.02° was performed to check the crystalline quality, FWHM, and shift in d values in θ-2θ scan mood. Ni filtered Cu Kα radiation with 1.542Å was used in this technique. Field emission electron microscopy (FESEM, JSM 6500F) was used to found the surface morphology of deposited films. In-situ EDS analysis attached with FESEM was used to confirm the elemental composition and microstructure examination of GZO thin films. The transmission spectra were calculated using UV-3600 Plus UV-VIS-IR spectrophotometer. Photoluminescence spectra were measured using He-Cd laser with a wavelength of 325nm. All the spectra were recorded at room temperature.
2
Results & Discussion: XRD patterns of ultra-thin GZO films have been shown in Fig. 1. Strong Bragg’s reflection from (002) plane indicates the preferred orientation of GZO along (002) direction with hexagonal wurtzite structure. The highly intense (002) peak points towards epitaxial growth of GZO thin films along c-axis. Samples grown on sapphire substrate have additional peak at angle 42º which belongs to sapphire substrate because of ultra-thin growth of GZO, effect of substrate material was also starts to contribute in the XRD analysis [13]. Slight shift of highly textured (002) peak towards the higher angle (average ∆(2θ=0.320º) indicates the lowest crystallite size ever reported to the best of our knowledge. Well-known Scherer formula has been used to calculate the crystallite size. =
.
(1)
Here “Dc” is the crystallite size, “λ” is the X-ray wavelength “θ” is the Bragg angle and “β” is the diffraction broadening at full width and half maxima. Tab. 1 summarize the average crystallite size for each GZO films, variation in crystallite size with different substrate at particular deposition time have been observed which can be understood by the difference in lattice mismatches between substrate and deposited materials. Decrease in c-axis parameter have also been observed with 0.5wt% Ga doping in ZnO crystal, c-axis slightly shrinks from 5.2260Å to 5.195Å. This behavior strongly suggests that; a) Ga atoms are perfectly substituted by Zn atoms in ZnO crystal lattice due to slight variation in ionic and covalent radii of Ga (0.62 and 1.26Å) and Zn (0.74 and 1.34Å). The similar effect was reported for GZO thin films grown on Quartz substrate via RF magnetron sputtering [14].Thickness of thin films was measured using 3D surface profiler, which varied from 40nm, 90nm and 230nm for 30min, 20min and 10min samples respectively. Fig.2 exhibits more interesting and distinct FE-SEM micrographs and surface morphology of all GZO thin films. Thin film grown on GaN substrate shows noodle like structure whereas on sapphire substrate it shows seed like structure and on Si substrate the nanoparticles were formed, which transform into smaller grains with periodic decrease in deposition time (30min-10min). There are voids in the structure of GZO grown on sapphire and silicon for higher thickness but with decrease in thickness the grain size value also reduces which helps to overcome this structural disorder. The voids and less uniformity with higher grain size may be due to the strain produce at higher thickness level. For ultra-thin size of film, the Quantum confinement effect plays dominant role to overcome these structural mismatches and voids produced by lattice strain. It is also observed that with the decrease in thickness of thin films the grain size reduces and film uniformity enhances which may be the cause to improve the optical properties of GZO thin films have discussed in the next section [15]. The in-situ EDS analysis were performed in order to confirm the elemental composition in all samples to verify the purity level of grown thin films (supplementary data fig. S1). 3
Room temperature PL spectra of as-deposited GZO films have been recorder and shown in Fig.3. Two main features are visible in these spectra: the near-band-edge emission (NBE) and broad deep-level-emission (DLE). All the samples have dominated peak of near-band-edge transition; this is because of radiative recombination of free excitons from near conduction band edge to the valance band. It has been observed that GZO/GaN films with 37nm thickness is free from DLE emission peak (550-650nm) which indicates that the growth of GZO on GaN has no oxygen vacancy (Vo). These deep-level-emission originate from the radiative recombination of electron hole pair[11]. Electrons with singly ionized Oxygen vacancies have deeply trapped by holes in valance band. Strong green emission has been observed in Fig.5(a & b) for GZO/Al2O3 and GZO/Si which weakens in Fig.3(c) indicating that quantum confinement effect plays important role in order to overcome defects(Vo and Zni) related photoluminescence [16]. When the grain size becomes smaller than 100nm quantum confinement effect emerge to overcome lattice mismatches and structural defects which limits the optoelectronic properties of thin films [17, 18]. The more interesting thing is; when the grain size reached 5nm, full width half maxima of NBE emission peak becomes shorter (from 31nm to 13nm) with sharp increase in the peak intensity. The abrupt increase in emission intensity and reduction of FWHM of NBE peak can be well understood in contrast with reduced Huang-Rhys (H-R) factor correlated with EM-Model which explains the size-dependent photoluminescence in semiconductors. The H-R factor describes the reduction of FWHM of NBE peak which associated with the Longitudinal optical (LO) exciton-phonon coupling. The LO exciton -phonon coupling is related with strain relaxation in semiconductors[19]. The reduced FWHM value in NBE peak from 31nm to 13nm indicates the decrease in stress intensity in thin films which helps to control the efficiency droop in ZnO-based LEDs which is one of the major challenge in the field of ZnO-based optoelectronics. The EM-Model developed for cylindrical potential for electrons and holes is [20]. Ѱ, , , = !"
, #
$ %&' (*
, +
)
(2)
2 Here -. [0. 1 3,4657] is the zeroth-order Bessel function, 0. is the first zero of the zeroth-order Bessel function (2.405), L is the length of cylinder, and N is the normalization constant. The calculated energy shifts ∆E, relative to the bulk band gap as a function of the grain radius R, is given by 9: = <=∗ ? ħ<
@" < #
$ +
* <
$ B − DѰ(E )Ѱ(E )F + G|E
<
E|
FѰ(E)Ѱ(E )I
(3)
Here m* is the reduced effective exciton massJ3 J4 ⁄(J3 + J4 ), ħ is Planck’s constant, e is electron charge, and L is dielectric constant for GZO. First part of the Eq. 3 is size-dependent kinetic energy confinement imposed by the nano grains on 4
wall of substrate material. The second term is the attractive Coulomb interaction between electron and hole in first order perturbation theory. Columbic interaction can be numerically evaluated using the Green’s function expansion in terms of Bessel functions: "
|E E |
O=(P P) = ∑T = (Q ) = (Q)Q|RR| SQ =UT N T
(4)
Where -V (W23,4 ) is the mth order Bessel function. According to our experimental findings, (Fig.S3 in supplementary data) inverse relation of band shift (9:) with square of grain radius (R) has been found which is well accordingly with the proposed EM-Model (as the first part of Eq.3 describes the inverse relation of radius of grains with band-gap energy), these results shows that radius of nano noodle and nano granular are effecting parameter for optical band tailoring. Conclusion: Structural, morphological and optical properties of ultra-thin GZO films have been investigated in this article. FE-SEM micrographs reveal the growth of different nanostructures on different type of substrate materials, where noodle-like GZO/GaN structure with 5nm average grain size is found comparatively best choice for growth of high efficient ZnO-based luminescence devices. With grain size about 5nm, lattice mismatches and intrinsic defects vanishes and more uniform crystal structure is achieved which indicates that quantum confinement effect plays vital role to suppress defect related emissions and enhance near band emission (NBE) intensity with sharp band edge emission. The reduction in FWHM (from 31nm to 13nm) with high intense NBE peak and suppression of DLE peak is a great achievement of this work which helps to enhance the luminesce efficiency of ZnO-based LEDs. Acknowledgements: This work was supported by the National Natural Science Foundation of China (No. 11374031), China Scholarship Council. The authors would like to express thanks to Dr. Yinzhou Yan and Dr. Yong Zeng for many valuable suggestions. References: [1]
L. Zhang, Y. Ruan, Y. Liu, Y. Zhai, Effect of growth temperature on the structure and optical properties of ZnO nanorod arrays grown on ITO substrate, Crystal Research and Technology, 48 (2013) 996-1002.
[2]
L. Zhao, Y. Chen, Y. Liu, G. Zhang, J. She, S. Deng, N. Xu, J. Chen, Integration of ZnO nanowires in gated field emitter arrays for large-area vacuum microelectronics applications, Current Applied Physics, 17 (2017) 85-91.
[3]
Z. Ye, T. Wang, S. Wu, X. Ji, Q. Zhang, Na-doped ZnO nanorods fabricated by chemical vapor deposition and their optoelectrical properties, Journal of Alloys and Compounds, 690 (2017) 189-194.
5
[4]
G. del C. Pizarro, O.G. Marambio, M. Jeria-Orell, D.P. Oyarzún, K.E. Geckeler, Size, morphology and optical properties of ZnO nanoparticles prepared under the influence of honeycomb-porous
poly[(2-hydroxyethylmethacrylate)m-block-poly(N-phenyl
maleimide)n] copolymer films, Materials & Design, 111 (2016) 513-521. [5]
K. Karthika, K. Ravichandran, Tuning the Microstructural and Magnetic Properties of ZnO Nanopowders through the Simultaneous Doping of Mn and Ni for Biomedical Applications, Journal of Materials Science & Technology, 31 (2015) 1111-1117.
[6]
M.J.F. Empizo, A.B. Santos-Putungan, K. Yamanoi, H.T. Salazar Jr, E.P. Anguluan, K. Mori, R. Arita, Y. Minami, M.V. Luong, T. Shimizu, E.S. Estacio, A.S. Somintac, A.A. Salvador, R.V. Sarmago, T. Fukuda, N. Sarukura, Structural and optical characterization and scintillator application of hydrothermal-grown ZnO microrods, Optical Materials, 65 (2017) 82-87.
[7]
N. Nripasree, N.K. Deepak, Structural, optical and electrical properties of SnN codoped p type ZnO
thin films prepared by spray pyrolysis technique for diode applications,
Materials Science and Engineering: B, 211 (2016) 121-127. [8]
H.Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Metalorganic vapor phase epitaxial growth of a high
quality GaN film using an AlN buffer layer, Applied Physics Letters, 48 (1986)
353-355. [9]
S. Pimputkar, J.S. Speck, S.P. DenBaars, S. Nakamura, Prospects for LED lighting, Nat
[10]
X. Zhang, L. Li, J. Su, Y. Wang, Y. Shi, X. Ren, N. Liu, A. Zhang, J. Zhou, Y. Gao,
Photon, 3 (2009) 180-182. Bandgap engineering of GaxZn1–xO nanowire arrays for wavelength‐tunable light‐emitting diodes, Laser & Photonics Reviews, 8 (2014) 429-435. [11]
V. Gokulakrishnan, V. Purushothaman, E. Arthi, K. Jeganathan, K. Ramamurthi, Effect of vacuum annealing on the structural, optical, and electrical properties of spray-deposited Ga-doped ZnO thin films, physica status solidi (a), 209 (2012) 1481-1486.
[12]
Y. Zhao, Y. Jiang, Y. Fang, The influence of substrate temperature on ZnO thin films prepared by PLD technique, Journal of Crystal Growth, 307 (2007) 278-282.
[13]
H. Wu, Z. Hu, B. Li, H. Wang, Y. Peng, D. Zhou, X. Zhang, High-quality ZnO thin film grown on sapphire by hydrothermal method, Materials Letters, 161 (2015) 565-567.
[14]
B. Singh, S. Ghosh, Highly Conducting Gallium-Doped ZnO Thin Film as Transparent Schottky Contact for Organic-Semiconductor-Based Schottky Diodes, Journal of Electronic Materials, 44 (2015) 2663-2669.
[15]
X. Xu, M. Funato, Y. Kawakami, K. Okamoto, K. Tamada, Grain size dependence of
[16]
L.-C. Chao, J.-W. Chen, H.-C. Peng, C.-H. Ho, Characterization of nitrogen doped p-type
surface plasmon enhanced photoluminescence, Opt. Express, 21 (2013) 3145-3151. ZnO thin films prepared by reactive ion beam sputter deposition, Surface and Coatings Technology, 231 (2013) 492-495. [17]
M. Zheng, L. Zhang, G. Li, Z. Jiang, Preparation and optical properties of composite thin films with embedded InP nanoparticles, Chinese Science Bulletin, 46 (2001) 461-465.
[18]
G. Cantele, D. Ninno, Size-dependent structural and electronic properties of Bi(111) ultrathin nanofilms from first principles, Physical Review Materials, 1 (2017) 014002.
[19]
P. Renwick, H. Tang, J. Bai, T. Wang, Reduced longitudinal optical phonon-exciton interaction in InGaN/GaN nanorod structures, Applied Physics Letters, 100 (2012)
6
182105. [20]
M.S. Gudiksen, J. Wang, C.M. Lieber, Size-Dependent Photoluminescence from Single Indium Phosphide Nanowires, The Journal of Physical Chemistry B, 106 (2002) 4036-4039.
7
Fig. 1. X-ray diffractograms of as-deposited GZO thin films with (a) 30min (b) 20min(c) 10min, deposition time.
Fig. 2. FE-SEM micrographs for GaN, Sapphire and Si substrate (from left to right) with 30min (a, b, c), 20min (d, e, f) and 10min (g, h, i) deposition time (from top to bottom) respectively.
8
Fig. 3. Room temperature photoluminescence spectra of GZO/GaN, GZO/Al2O3 and GZO/Si with (a) 19nm (b) 11nm (c) 5nm grain size.
9
Table.1: Crystallographic results and deposition parameters for all ultra-thin GZO films Substrate Material
GaN
Al2O3
Si
Dep. Time (min)
c (Å)
Average Grain Size(nm)
Film thickness (nm)
30
5.1948
19
190nm
20
5.1674
11
91nm
10
5.1492
5
37nm
30
5.17
22
195nm
20
5.1914
8
90nm
10
5.1666
7
40nm
30
5.192
18
188nm
20
5.1876
9
95nm
10
5.1842
5
30nm
10
Highlights Ultra-thin Single crystal GZO films have been prepared. Quantum confinement effect plays vital role to enhance NBE Luminesce. Grain size and shape mediated photoluminescence has been discussed.
11
S0167-577X(17)31401-5 http://dx.doi.org/10.1016/j.matlet.2017.09.061 MLBLUE 23175
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
26 July 2017 24 August 2017 17 September 2017
Please cite this article as: A. Hassan, M. Irfan, Y. Jiang, Quantum confinement effect and size-dependent photoluminescence in Laser ablated ultra-thin GZO films, Materials Letters (2017), doi: http://dx.doi.org/10.1016/ j.matlet.2017.09.061
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Quantum confinement effect and size-dependent photoluminescence in Laser ablated ultra-thin GZO films Ali Hassana*, Muhammad Irfanb, Yijian Jianga a
Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, PR China.
b
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese
Academy of Sciences, Beijing, 100190, China.
*Corresponding Author ([email protected])
Abstract: The structural, morphological, and optical characteristics of GZO ultra-thin films were investigated using XRD (X-ray diffraction), FE-SEM (field emission electron microscopy), in-situ EDS (Energy Dispersive X-ray) spectroscopy, UV-VIS-IR spectroscopy and photoluminescence spectroscopy (PL) respectively. Morphological analysis reveals the noodle, seed and particle like structure of GZO for GaN, sapphire and Si substrates respectively with average grain size ranging from 5 to 20nm. An effective mass model (EM-Model) for particle in a cylindrical wave function of e-h pair was correlated with experimental results. The reduction in FWHM value (from 31nm to 13nm) of NBE (near-band-edge) emission peak and enhanced NBE intensity have been achieved with small grain size. Blue shift in optical band gap is explained in term of grain radius by EM-model. Improved optical and structural properties were found in relation with quantum confinement effect. The current study states that grain size plays vital role in order to tailor optical properties of GZO thin films. Keywords: Thin films, Luminescence, Laser Processing, Semiconductor, Epitaxial growth.
Introduction: Zinc Oxide (ZnO) having wide and direct band gap (3.37eV) with comparatively large exciton binding energy (60meV) is regarded as one of the best candidate for replacing the rare earth transparent conductive oxides and other functional semiconductors [1]. ZnO based nanowires (NWs) [2] and nanorods (NRs) [3] have attracted much interest due to their unique electrical and optoelectronic properties in the field of modern device fabrication technology. Some recent studies show the obvious milestone in developing precisely controllable size and shape of ZnO nanoparticles (NPs) and NRs having excellent electrical and optical properties [4]. Beside these, the favorable doping elements in ZnO matrix is a hot issue in order to attain desired electronic and optoelectronic properties. Rare earth metals always produce deep alteration of spin dependent phenomena in ZnO NWs and NRs, this helps to establish a system with unique and enhanced functionality, such as sensitive 1
biomedical sensors[5], scintillators materials with low time delay[6], high efficient laser diodes and solid state lightning [7-9]. Recent research shows that the ZnO doped with group III elements (B, In, Al, Ga) exhibits improved optoelectronic properties[10], so these films gain much interest as an alternate for indium tin oxide (ITO). Among all group III elements Ga has some distinct advantages as; its ionic and covalent radius (0.62 and 1.26Å) are comparable with those of Zn (0.74 and 1.34Å) and hence the occurrence of small lattice mismatches even for high Ga doping in ZnO. Moreover, Ga is less reactive and more resistive against oxidization as compared to its competitors during the deposition process. Recently Ga-doped ZnO (GZO) based stable transparent electrodes have been used in organic light emitting diodes (OLEDs) and efficiency has increased to 25% at 20mA[11]. Different methods have been adopted to synthesis ZnO NWs and NRs worldwide. In this study we have used pulsed laser deposition [12] technique to develop NPs, nano-seeds (NSs) and nano-noodles (NNs) on Si, sapphire and GaN substrate, respectively. The morphology, substitutional kinetics and stability dependence of Ga doping in ZnO host matrix is analyzed and discussed using EM-Model to understand quantum confinement effect. Experimental Details: Lambda physik LPX305iF KrF excimer laser having (λ=248nm, pulse duration tp=20ns, laser Energy=450mJ, frequency= 3Hz, laser fluence= 200mJ/cm2) was used to ablate the GZO target. Laser beam was focused on the target which was mounted in the deposition chamber with circular target holder. Target holder and mechanical rotator were rotated at 10rpm to avoid texturing and pitting of target during deposition. Prior to deposition, all the substrates were ultrasonically cleaned for 20min each, via Acetone, Ethanol and Deionized water respectively. The cleaned substrates then dried with 99.999% Nitrogen and were placed at Si heater exactly parallel to the target with 5cm.substrate temperature was kept constant at 400ºC. the chamber was evacuated to the base pressure of 1 × 10 Pa. for 30min. then high purity Oxygen (99.999%) was introduced to the chamber and pressure was kept at 4.5 × 10 Pa. during the ablation. 5400 pulses of laser beam were used for each deposition. WYKO NT1100 3D non-contact Optical profiler was used to measure the films thickness in VSI (vertical Scanning Interferometry) mode. X-ray diffraction of Bruker -D8 with step size 0.02° was performed to check the crystalline quality, FWHM, and shift in d values in θ-2θ scan mood. Ni filtered Cu Kα radiation with 1.542Å was used in this technique. Field emission electron microscopy (FESEM, JSM 6500F) was used to found the surface morphology of deposited films. In-situ EDS analysis attached with FESEM was used to confirm the elemental composition and microstructure examination of GZO thin films. The transmission spectra were calculated using UV-3600 Plus UV-VIS-IR spectrophotometer. Photoluminescence spectra were measured using He-Cd laser with a wavelength of 325nm. All the spectra were recorded at room temperature.
2
Results & Discussion: XRD patterns of ultra-thin GZO films have been shown in Fig. 1. Strong Bragg’s reflection from (002) plane indicates the preferred orientation of GZO along (002) direction with hexagonal wurtzite structure. The highly intense (002) peak points towards epitaxial growth of GZO thin films along c-axis. Samples grown on sapphire substrate have additional peak at angle 42º which belongs to sapphire substrate because of ultra-thin growth of GZO, effect of substrate material was also starts to contribute in the XRD analysis [13]. Slight shift of highly textured (002) peak towards the higher angle (average ∆(2θ=0.320º) indicates the lowest crystallite size ever reported to the best of our knowledge. Well-known Scherer formula has been used to calculate the crystallite size. =
.
(1)
Here “Dc” is the crystallite size, “λ” is the X-ray wavelength “θ” is the Bragg angle and “β” is the diffraction broadening at full width and half maxima. Tab. 1 summarize the average crystallite size for each GZO films, variation in crystallite size with different substrate at particular deposition time have been observed which can be understood by the difference in lattice mismatches between substrate and deposited materials. Decrease in c-axis parameter have also been observed with 0.5wt% Ga doping in ZnO crystal, c-axis slightly shrinks from 5.2260Å to 5.195Å. This behavior strongly suggests that; a) Ga atoms are perfectly substituted by Zn atoms in ZnO crystal lattice due to slight variation in ionic and covalent radii of Ga (0.62 and 1.26Å) and Zn (0.74 and 1.34Å). The similar effect was reported for GZO thin films grown on Quartz substrate via RF magnetron sputtering [14].Thickness of thin films was measured using 3D surface profiler, which varied from 40nm, 90nm and 230nm for 30min, 20min and 10min samples respectively. Fig.2 exhibits more interesting and distinct FE-SEM micrographs and surface morphology of all GZO thin films. Thin film grown on GaN substrate shows noodle like structure whereas on sapphire substrate it shows seed like structure and on Si substrate the nanoparticles were formed, which transform into smaller grains with periodic decrease in deposition time (30min-10min). There are voids in the structure of GZO grown on sapphire and silicon for higher thickness but with decrease in thickness the grain size value also reduces which helps to overcome this structural disorder. The voids and less uniformity with higher grain size may be due to the strain produce at higher thickness level. For ultra-thin size of film, the Quantum confinement effect plays dominant role to overcome these structural mismatches and voids produced by lattice strain. It is also observed that with the decrease in thickness of thin films the grain size reduces and film uniformity enhances which may be the cause to improve the optical properties of GZO thin films have discussed in the next section [15]. The in-situ EDS analysis were performed in order to confirm the elemental composition in all samples to verify the purity level of grown thin films (supplementary data fig. S1). 3
Room temperature PL spectra of as-deposited GZO films have been recorder and shown in Fig.3. Two main features are visible in these spectra: the near-band-edge emission (NBE) and broad deep-level-emission (DLE). All the samples have dominated peak of near-band-edge transition; this is because of radiative recombination of free excitons from near conduction band edge to the valance band. It has been observed that GZO/GaN films with 37nm thickness is free from DLE emission peak (550-650nm) which indicates that the growth of GZO on GaN has no oxygen vacancy (Vo). These deep-level-emission originate from the radiative recombination of electron hole pair[11]. Electrons with singly ionized Oxygen vacancies have deeply trapped by holes in valance band. Strong green emission has been observed in Fig.5(a & b) for GZO/Al2O3 and GZO/Si which weakens in Fig.3(c) indicating that quantum confinement effect plays important role in order to overcome defects(Vo and Zni) related photoluminescence [16]. When the grain size becomes smaller than 100nm quantum confinement effect emerge to overcome lattice mismatches and structural defects which limits the optoelectronic properties of thin films [17, 18]. The more interesting thing is; when the grain size reached 5nm, full width half maxima of NBE emission peak becomes shorter (from 31nm to 13nm) with sharp increase in the peak intensity. The abrupt increase in emission intensity and reduction of FWHM of NBE peak can be well understood in contrast with reduced Huang-Rhys (H-R) factor correlated with EM-Model which explains the size-dependent photoluminescence in semiconductors. The H-R factor describes the reduction of FWHM of NBE peak which associated with the Longitudinal optical (LO) exciton-phonon coupling. The LO exciton -phonon coupling is related with strain relaxation in semiconductors[19]. The reduced FWHM value in NBE peak from 31nm to 13nm indicates the decrease in stress intensity in thin films which helps to control the efficiency droop in ZnO-based LEDs which is one of the major challenge in the field of ZnO-based optoelectronics. The EM-Model developed for cylindrical potential for electrons and holes is [20]. Ѱ, , , = !"
, #
$ %&' (*
, +
)
(2)
2 Here -. [0. 1 3,4657] is the zeroth-order Bessel function, 0. is the first zero of the zeroth-order Bessel function (2.405), L is the length of cylinder, and N is the normalization constant. The calculated energy shifts ∆E, relative to the bulk band gap as a function of the grain radius R, is given by 9: = <=∗ ? ħ<
@" < #
$ +
* <
$ B − DѰ(E )Ѱ(E )F + G|E
<
E|
FѰ(E)Ѱ(E )I
(3)
Here m* is the reduced effective exciton massJ3 J4 ⁄(J3 + J4 ), ħ is Planck’s constant, e is electron charge, and L is dielectric constant for GZO. First part of the Eq. 3 is size-dependent kinetic energy confinement imposed by the nano grains on 4
wall of substrate material. The second term is the attractive Coulomb interaction between electron and hole in first order perturbation theory. Columbic interaction can be numerically evaluated using the Green’s function expansion in terms of Bessel functions: "
|E E |
O=(P P) = ∑T = (Q ) = (Q)Q|RR| SQ =UT N T
(4)
Where -V (W23,4 ) is the mth order Bessel function. According to our experimental findings, (Fig.S3 in supplementary data) inverse relation of band shift (9:) with square of grain radius (R) has been found which is well accordingly with the proposed EM-Model (as the first part of Eq.3 describes the inverse relation of radius of grains with band-gap energy), these results shows that radius of nano noodle and nano granular are effecting parameter for optical band tailoring. Conclusion: Structural, morphological and optical properties of ultra-thin GZO films have been investigated in this article. FE-SEM micrographs reveal the growth of different nanostructures on different type of substrate materials, where noodle-like GZO/GaN structure with 5nm average grain size is found comparatively best choice for growth of high efficient ZnO-based luminescence devices. With grain size about 5nm, lattice mismatches and intrinsic defects vanishes and more uniform crystal structure is achieved which indicates that quantum confinement effect plays vital role to suppress defect related emissions and enhance near band emission (NBE) intensity with sharp band edge emission. The reduction in FWHM (from 31nm to 13nm) with high intense NBE peak and suppression of DLE peak is a great achievement of this work which helps to enhance the luminesce efficiency of ZnO-based LEDs. Acknowledgements: This work was supported by the National Natural Science Foundation of China (No. 11374031), China Scholarship Council. The authors would like to express thanks to Dr. Yinzhou Yan and Dr. Yong Zeng for many valuable suggestions. References: [1]
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Fig. 1. X-ray diffractograms of as-deposited GZO thin films with (a) 30min (b) 20min(c) 10min, deposition time.
Fig. 2. FE-SEM micrographs for GaN, Sapphire and Si substrate (from left to right) with 30min (a, b, c), 20min (d, e, f) and 10min (g, h, i) deposition time (from top to bottom) respectively.
8
Fig. 3. Room temperature photoluminescence spectra of GZO/GaN, GZO/Al2O3 and GZO/Si with (a) 19nm (b) 11nm (c) 5nm grain size.
9
Table.1: Crystallographic results and deposition parameters for all ultra-thin GZO films Substrate Material
GaN
Al2O3
Si
Dep. Time (min)
c (Å)
Average Grain Size(nm)
Film thickness (nm)
30
5.1948
19
190nm
20
5.1674
11
91nm
10
5.1492
5
37nm
30
5.17
22
195nm
20
5.1914
8
90nm
10
5.1666
7
40nm
30
5.192
18
188nm
20
5.1876
9
95nm
10
5.1842
5
30nm
10
Highlights Ultra-thin Single crystal GZO films have been prepared. Quantum confinement effect plays vital role to enhance NBE Luminesce. Grain size and shape mediated photoluminescence has been discussed.
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