MgxZn1-xO multiple quantum

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Cite This: ACS Appl. Nano Mater. 2018, 1, 3829−3836

Exciton Localization of High-Quality ZnO/MgxZn1−xO Multiple Quantum Wells on Si (111) with a Y2O3 Buffer Layer Wei-Rein Liu,*,† Wei-Lun Huang,‡ Yung-Chi Wu,§ Liang-Hsun Lai,† Chia-Hung Hsu,*,†,§ Wen-Feng Hsieh,§ Tsung-Hung Chiang,⊥ H. W. Wan,∥ M. Hong,∥ and J. Kao# †

National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan Program for Science and Technology of Accelerator Light Source and §Department of Photonics, College of Electrical and Computer Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan ⊥ Department of Materials Science & Engineering and #Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan ∥ Graduate Institute of Applied Physics and Department of Physics, National Taiwan University, Taipei 10617, Taiwan

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S Supporting Information *

ABSTRACT: We report the structural and optical properties of ten-period ZnO/MgxZn1−xO multiple quantum wells (MQWs) prepared on the most widely used semiconductor material, Si. The introduction of a nanometer thick high-k Y2O3 transition layer between Si (111) substrate and a ZnO buffer layer significantly improves the structural perfection of the MQWs grown on top of it. The high structural quality of the ZnO/MgxZn1−xO MQWs is evidenced by the appearance of pronounced high order satellite peaks in X-ray crystal truncation rods; high resolution cross-sectional transmission electron microscopy images also confirmed the regularly arranged well and barrier layers. When the well width is less than ∼2.7 nm, the quantum-confined Stark effect in MQWs can be negligible. Not only the increasing exciton-binding energy but also reducing exciton−phonon coupling determined in temperature-dependent photoluminescence spectra indicate quantum-size effect. Our results demonstrate that ZnO/MgxZn1−xO MQWs integrated on Si have great potential in UV optoelectronic device applications. KEYWORDS: ZnO, multiple quantum wells, quantum confinement, interface, Si, exciton localization

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INTRODUCTION Functional devices, such as quantum well (QW) lasers, tunneling devices, random access memories, and high-electron mobility transistors, usually comprise nanoscale multilayer heterostructures that exploit quantum confinement effects. Wurtzite ZnO, a wide direct band gap semiconductor of 3.37 eV, offer the driving force for the photonic applications, e.g., the light-emitting diodes (LEDs),1 polariton lasers,2 and photodetectors3 in the UV/blue spectral region because of the thermal stability of free exciton (FX) with a large binding energy of 60 meV at room temperature (RT), almost three times higher than that of GaN (25 meV), and even beyond 100 meV in quantum wells or quantum dots. Therefore, much effort has been devoted toward the investigation and fabrication of ZnO/ZnMgO MQWs for UV light emitter applications. In these studies, ScAlMgO4 or sapphire are the extensively used substrates for the growth of ZnO/ZnMgO MQWs.4,5 Although many MQWs researches on the structural and optical properties using sapphire, ScAlMgO4, ZnO substrates,4−6 etc., were conducted by many groups, there are few literatures about the structural and optical properties of MQWs grown on Si, especially for high quality ZnO/ © 2018 American Chemical Society

MgxZn1−xO multiple quantum wells. Because of the merits of the low cost, excellent quality, and large-area availability of Si wafer, the unique opportunity of integrating well-established Si electronics with ZnO-based optoelectronic devices, the direct growth of high-quality epitaxial ZnO film on Si is highly desired. Nevertheless, the formation of amphorous SiO2 layer at the ZnO/Si interface, the large diversity in lattice constants (15.4%) and thermal expansion coefficient (56%),7,8 either polycrystalline or highly textured ZnO films were commonly obtained.9,10 Because the formation enthalpy of Y2O3 is larger in magnitude than that of SiO2 and ZnO, ZnO epitaxial films with high crystalline quality and excellent optical properties can indeed be grown on Si by using a Y2O3 nanothickness buffer layer without forming amphorous SiO2 layer at the ZnO/Si interface.11 As a result, the domain matching epitaxy with 7 or 8 ZnO {112̅0} planes matching 6 or 7 {44̅0} planes of Y2O3 leads to a significant reduction of residual strain.11 Furthermore, Y2O3 possesses high dielectric constant, high conduction band offset, and thermodynamic stability with Si Received: April 10, 2018 Accepted: July 25, 2018 Published: July 27, 2018 3829

DOI: 10.1021/acsanm.8b00595 ACS Appl. Nano Mater. 2018, 1, 3829−3836

Article

ACS Applied Nano Materials

Figure 1. (a) Schematic drawing of homemade PLD growth system and the ten-period ZnO/ZnMgO MQW structure on nanothick Y2O3/Si (111) composite substrates. (b) XRD radial scan along the surface normal of ZnO/ZnMgO MQWs with well width of 2.7 nm grown on a Y2O3/Si composite substrate. (c) Enlarged XRD radial scan around the ZnO/ZnMgO MQWs (0002) diffraction peak. (d) XRD RSM at the (101̅5) ZnO/ ZnMgO MQWs reflection.

devices.19 Furthermore, as the well width of MQWs structures is reduced to near the Bohr radius of exciton, the internal electric field can be neglected, and so the quantum-size effect would become dominant. The reducing Bohr radius of exciton would decrease the exciton dipole moment and so less probability to interact with LO-phonons, implying the reduction of the exciton−phonon coupling.20−22 However, the investigation of structural properties, quantum-size effect, and QCSE influence on optical properties with the change of well width in ZnO/ZnMgO MQWs grown on Si has seldom been reported. In this study, the structural and optical properties of tenperiod ZnO/Zn0.8Mg0.2O (ZnO/ZnMgO) MQWs prepared by pulsed-laser deposition (PLD) on the most widely used semiconductor material, Si, were comprehensively investigated. The significant improvement of the structural perfection of the MQWs by the introduction of nm-thick Y2O3 transition layer between Si (111) substrate and a ZnO buffer layer was demonstrated by the appearance of pronounced high order satellite peaks in X-ray crystal truncation rods and high resolution cross-sectional TEM images. By performing the power- and temperature-dependent photoluminescence (PL) spectra, we observe the negligible built-in electric field effect, enhancing exciton-binding energy with reducing exciton−

that provides epitaxial growth of insulator layers on Si for achieving Si on insulator (SOI) structures and the long-range goal of 3D integrated circuits.12,13 Accordingly, the growth of high-quality ZnO/ZnMgO MQWs on Y2O3/Si offers attraction to harmonically incorporate related optoelectronic devices in silicon based integrated circuits. Applying ZnO/ZnMgO MQWs structure can enhance the confinement of carriers and photons in the well regions to increase exciton binding energy and interaction strength because the exciton binding energy is inversely proportional to the Bohr radius. Nevertheless, growing ZnO/ZnMgO MQWs along the c-axis direction leads to a built-in electric field, which is attributed to the spontaneous and piezoelectric polarizations resulting from the lattice mismatches between the well and barrier materials, exerting across the quantum well to cause the quantum-confined Stark effect (QCSE),4,14,15 Resulting from the large internal electric field to tilt band diagram, QCSE existence in MQWs reduces the exciton oscillator strength through separating the wave functions of the electron and hole as well as influences on the lifetime of exciton due to reduction of the recombination rate of electron and hole.4,16−18 Therefore, the existence of QCSE in the cplane ZnO MQWs leads to reduce the optical gain and exhibits a barrier for achieving the larger efficiency of optoelectronic 3830


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Figure 2. (a) Magnified cross-sectional BF-STEM micrograph along [112̅]Si projection of Lw = 2.7 nm sample in rectangle-marked region shown in (b). (c) High-resolution TEM image in square-marked region shown in b. (d) Cross-sectional BF-STEM image and (e) high-resolution TEM image in square-marked region shown in (d) of Lw = 1.6 nm sample. (f) Enlarged XRD radial scan around the ZnO/ZnMgO MQWs (0002) diffraction peak of Lw = 1.6 nm sample. detector yield a typical resolution of better than 3.2 × 10−3 Å−1. The cross-sectional TEM samples with a thickness of approximately 90 ± 10 nm were prepared using a focused ion beam. In addition, the microstructure and interface qualities of the samples were analyzed using scanning transmission electron microscopy (STEM) performed with a JEOL 2100F microscope equipped with an energy-dispersive X-ray detector operated at 200 keV. PL measurements were carried out using a He−Cd laser with a wavelength of 325 nm as the pumping source. The emitted light was dispersed by a iHR 550 spectrometer and detected by an UV-sensitive photomultiplier tube.

phonon coupling in MQWs, implying quantum-size effect, when the well width is made less than ∼2.7 nm. Our results indicate that ZnO/ZnMgO MQWs integrated on Si have great potential in UV optoelectronic device applications.

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EXPERIMENTAL METHODS

A Y2O3 buffer layer with a thickness of ∼9 nm was grown on Si (111) by molecular-beam epitaxy method. The details of the growth conditions and the structure of the Y2O3 layer had been reported elsewhere.13,23 The Y2O3/Si composite substrates were then transferred in air to a homemade pulsed laser deposition (PLD) growth system (see Figure 1a). A KrF excimer laser with wavelength of 248 nm was focused on commercial hot-pressed stoichiometric ZnO (4N), and Zn0.8Mg0.2O (3N) targets with pulse energy density of ∼2−3 J/cm2. To minimize the effect of magnesium diffusion into the well layers by thermal driving force at high growth temperature, we adopted temperature-gradient control, in which the growth temperature of the barriers was set lower than that of the well layers.5 Prior to fabricate the ten-period ZnO/ZnMgO MQWs, a 250 nm thick ZnO buffer layer was grown on the Y2O3/Si composite substrates. Then, alternating Zn0.8Mg0.2O barriers, ZnO wells were deposited; finally, a Zn0.8Mg0.2O cap layer was deposited on ZnO/ZnMgO MQWs, whose structures are depicted in Figure 1a. XRD measurements were performed with a nine-circle diffractometer at the IU22 undulator beamline TPS-09A of Taiwan Photon Source and beamline BL13A at the National Synchrotron Radiation Research Center with an incident wavelength of 1.0331 Å. Two pairs of slits located between the sample and the LaCl3 scintillation

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RESULTS AND DISCUSSION A typical radial scan along surface normal of a ten-period ZnO/ZnMgO MQWs sample with well width (LW) of 2.7 nm is illustrated in Figure 1b. Only ZnO MQWs (0002), (0004), and (0006) reflections together with the Si (111), (222) and (333) reflections were observed, elucidating the ZnO/ZnMgO MQWs is c-plane oriented with its [0001] axis parallel to the Si [111] direction. The broad peaks on the high q side of the Si (111), (222), and (333) reflections are attributed to the Y2O3 (222), (444), and (666) reflections, respectively. The fixed ratio in peak position between each Y2O3 reflection and the nearby Si reflection along surface normal reveals the cube-oncube growth of Y2O3 on Si substrate. From its period of thickness fringes near Si (111) Bragg peak, Δq = 0.699 Å−1, the thickness of the Y2O3 buffer layer is ∼9 nm. Furthermore, it is worth mentioning that the appearance of well-resolved satellite 3831


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Figure 3. (a) CL spectra of the ZnO/MgZnO MQWs collected at RT and 20 K. (b) PL spectra of MQWs with various well widths measured at 10 K. The dashed line marked the ZnO buffer layer emission.

smaller, ∼ 1%, than that of pure ZnO. To better understand the cause of peak broadening and to assess the overall strain state of the ZnO buffer layer and ZnO/ZnMgO MQWs, reciprocal space mapping (RSM) near the asymmetric ZnO (101̅5) reflection was performed. Figure 1d shows the H-L plane RSM of the LW = 2.7 nm MQWs. The reciprocal lattice points of ZnO/ZnMgO MQWs and ZnO were lined up at the same H position (green dash line), indicating the MQWs structure is nearly fully strained. The lattice parameters of the ZnO buffer layer are a = 0.3259 nm and c = 0.5191 nm as determined by fitting the positions of several Bragg reflections. As compared with bulk ZnO (a = 0.3244 and c = 0.5204 nm), we found that ZnO buffer layer was tensily strained (∼0.5%) in the lateral direction and the lattice along the growth direction is correspondingly compressed (∼-0.3%). The small strain attests a relaxed growth of our ZnO buffer layer on Y2O3. Moreover, the reciprocal lattice point of the MQWs structure is located at lager L than that of the ZnO buffer, indicating that the average lattice constant c of MQWs structure smaller than that of pure ZnO. This observation is consistent with specular rod results mentioned above. To verify the well width and interfacial structure between ZnMgO and ZnO in MQWs, Figure 2a, b show the alternating light/dark stripes parallel to the substrate surface in the crosssectional bright field scanning TEM (BF-STEM) image of Lw= 2.7 nm sample and bright-field TEM micrograph along [112̅]Si projection, respectively, suggesting the formation of MQW structure. The layers with bright contrast are MgZnO barriers and the dark layers are ZnO wells, respectively. The BF-STEM image also revealed good periodicity and uniformity across the entire 10-pair MQWs structure. Identified from the highresolution TEM and high-angle annular dark field (HAADF) STEM images combined with energy dispersive X-ray (EDX) microanalysis, as shown in Figure 2c and Figure S2, respectively. The HR-TEM image near the interface region exhibits unequivocal ZnO/ZnMgO interfaces. The estimated period of the MQWs is about 28.7 nm, which agrees well with the XRD results. Furthermore, the thickness of the ZnO quantum wells is estimated to be about 2.7 nm from TEM analysis. As show in the EDX line-scan composition profiles of Zn, O, and Mg elements along the growth direction, both the width of the peaks in Zn concentration and the dips in Mg composition are roughly equal to the thickness of the ZnO wells with 2.7 nm, suggesting that nonsignificant atomic interdiffusion occurs, imply that the temperature-gradient

peaks of the ninth order around the (0002) peak (see Figure 1c) arises from interference between X-ray waves reflected from the sample structure. This is an indication of the high crystalline quality and the flatness of the MQW structure, a good layer periodicity as well as the uniformity of the layer thicknesses over the layers because both interface imperfection and composition inhomogeneity would degrade the phase coherence and suppress the intensity of the satellite peaks.24 The calculated one-pair thicknesses from the satellite peaks period Δq is ∼29.1 nm. The result also demonstrate that ZnO/ ZnMgO MQWs can be grown on the Y2O3/Si (111) composite substrate with excellent structural quality. Subsequently, the maximal order of observed satellites and the intensity of the satellites provide us a scale to compare the samples prepared under different growth conditions. A set of XRD radial scans along surface normal for the samples A, B, and C (see Figure S1), which were grown at different temperatures in chronological order and the other growth parameters are the same, show pronounced satellite peaks up to the ninth, fifth, and sixth order, respectively, indicating the flatness of the QW interfaces and the uniformity of the layer thicknesses over the layers. The one-pair thicknesses d of the samples A, B, and C were determined to be 29.1, 22.6, and 22.0 nm, respectively, calculated from the satellite peaks period Δq. The period of sample A is slightly larger than the nominal value but the thickness of the samples B and C are about 20% less than the design value. The growth temperature and the decrease of growth rate due to the coating of the growth chamber viewport for delivering laser beam are the possible cause of the change of superlattice period. The coherence length derived from the width of the third order satellite peaks are 80.3, 69.7, and 68.4 nm, respectively, and are smaller than the nominal MQWs total thickness revealing the MQWs structure is not coherent throughout the entire structure. The presence of more satellite peaks and larger coherence length in the XRD pattern of sample A manifests its better structural characteristics. We thus adopted the growth conditions of sample A to grow MQWs of different well thicknesses. The lattice constants c of the ZnO buffer layer and MQWs are c = 0.5191 and 0.5141 nm as determined by fitting the position of Bragg’s diffraction peak for ZnO (0002) and MQWS (0002). Because Zn1−xMgxO has a smaller c-axis lattice constant than that of ZnO,25 the average lattice constant of MQWs structure, which is weighted by the layer thickness, is 3832


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PL spectra of the samples with Lw = 1.6, 2.7, and 5.6 nm measured at 10 K. We observed no apparent blue shift of the PL peak for Lw = 1.6 and 2.7 nm samples with increasing pumping power that show no QCSE due to the quantum confinement effect, implying Lw is smaller than the Bohr radius of bulk ZnO (∼3 nm). Nevertheless, for MQWs with Lw = 5.6 nm, a large blue shift of ∼120 meV is observed by increasing the pumping power. The blue shift of the PL emission is attributed to the screening of the internal electric field by high densities of electron−hole dipoles, which are known as QCSE screening.5,29 Different from the power-dependent PL spectra of ZnO/ZnMgO MQWs, the ZnO buffer layer exhibits the monotonic red shift and broadening of the UV peak with increasing excitation power, which is due to a stronger local heating effect.30 For understanding the varying of exciton binding energy and exciton−phonon coupling strength under the quantum confinement effect, Figure S4a−c illustrate the temperaturedependent PL spectra of ZnO/ZnMgO MQWs with Lw = 1.6, 2.7, and 5.6 nm, respectively, taken from 10 to 300 K. With the increasing the temperature from 10 K to RT, the PL peaks of all 3 samples exhibit some degree of S-curve dependence of NBE peak with temperature, implying that the MQWs have localized potential wells induced by the local variation in well width or inhomogeneity of alloy potential in the MgZnO barrier layers.31 As compared to MQWs with Lw = 1.6 and 2.7 nm, not only the larger change in S-shaped dependence but also the larger full width at half-maximum value were apparently observed in the MQWs with Lw = 5.6 nm. These are ascribed to the triangle-sharp potential well from QCSE induced by the existence of larger internal electric field32,33 and influences of alloy fluctuation. Moreover, for 10 K < T < 50 K, because of the relatively longer relaxation time of the exciton with increasing temperature, excitons have more opportunity to relax into the lower energy tail states caused by the inhomogeneous potential fluctuations (Epotential = 4−5 meV) before radiative recombination. This behavior produces a red shift of NBE peak with increasing temperature. When the temperature is further increased between 50 and 170 K, due to the decreasing of exciton lifetimes with increasing temperature, the excitons recombine before reaching the lower-energy-tail states. This behavior enhances a broadening of the higherenergy emission and leads to a blue shift of NBE peak. As T increases above 170 K, a red shift appears again with band gap shrinkage. It is caused by a change in the exciton dynamics with temperature due to donor-bound exciton and free exciton transformation.34 The integrated intensity of the NBE emission of ZnO/ ZnMgO MQWs with Lw = 1.6, 2.7, and 5.6 nm as a function of inverse temperature are given in Figure S5a. The temperaturedependent integrated PL intensity can be described as the Arrhenius expression:33,35,36

control can suppress Mg diffusion from barrier into the well layer. However, as the ZnO well decrease to 1.6 nm, we observed that the equivocal contrast, as shown in Figure 2e, exists near the interface between well and barrier layers from of BF-STEM image, suggesting the minor interdiffuson of Mg content induced by larger stain. This composition inhomogeneity decreases the phase coherence and suppresses the intensity of the satellite peaks, which also agrees with the XRD results in Figure 2f. Precisely determining the Mg composition of the MgZnO barrier layers in MQWs is important for engineering the exciton binding energy by varying MgZnO barrier height along with the well width.26 We employed cathodoluminescence (CL) spectra and lattice constant to determine the Mg concentration. Figure 3a shows the CL spectra of ZnO/ MgZnO MQWs measured at RT and 20 K, respectively. We found CL emissions at photon energy >4.0 eV is originated from radiative recombination of excitons in the MgZnO barrier/capping layers and for that emitting at lower photon energy belongs to the confined excitons in the ZnO wells, respectively. With increasing the temperature, the CL emission intensity from the ZnO wells dominate the CL spectra, indicating that ZnO wells have the larger thermal-quenching resistance than ZnMgO barrier/capping layers. This is the fact due to the energy difference of ∼772 meV between the ZnMgO barrier and ZnO band gap. The energy levels for electrons and holes in QWs are located quite deeply in the well, and as a result the energy barrier to be overcome is higher than 26 meV at RT. The result indicates the luminescence from the ZnO well is more stable with increasing temperature than that from the ZnMgO barrier. Comparing the CL emission peak position of ZnMgO layer with that reported by Koike et al.,25 we derived the Mg contents x = 0.33−0.36 in ZnMgO barrier layer. Furthermore, according to the dependence of lattice parameters along the a-axis27 and c-axis25 with Mg content and the lattice parameters of a = 3.268 Å and c = 5.141 Å for ZnMgO of our Lw = 2.7 nm sample, we estimated the Mg concentration to be x = 0.33−0.36 that is consistent with the CL results. The discrepancy of the Mg concentration in ZnMgO between deposited layer and the used target in PLD is attributed to the fact that the vapor pressure of ZnO and Zn at the substrate temperature is much larger than that of MgO and Mg, as a result, Zn-related species can easily desorb from the growing surface and the higher Mg concentration is achieved on the surface, which is in agreement with the report by Ohtomo et al.28 Figure 3b shows the PL spectra of the ZnO/ZnMgO MQWs with three different well thicknesses measured at 10 K. The dashed line marked the ZnO buffer layer emission at ∼3.354 eV, which is a promising peak in Lw = 5.6 nm sample and is barely seen in Lw = 2.7 and 1.6 nm samples. The LT-PL emission of the MQWs blue-shifts from 3.36 to 3.53 eV as decreasing the well width from Lw = 2.7 to 1.6 nm relative to the ZnO buffer. The observed blue shift of the MQWs nearband edge (NBE) emission is attributed to the quantum confinement effect. On the other hand, the NBE emission of Lw = 5.6 nm sample exhibits a red shift to 3.23 eV with respect to that of the ZnO buffer layer, suggesting QCSE dominant. Because the existence of QCSE in the polar heterostructure could be reduced by the photoinduced carriers to screen the Coulomb potential and so to compensate the build-in electric field that induces the blue shift of NBE peak at the high excitation power. Figure S3a−c shows the power-dependent

I (T ) =

I0 1 + Ae−E b1/ kbT + Be−E b2 / kbT

(1)

where I0 is the integrated intensity at 0 K, kb is Boltzmann’s constant, Eb1 and Eb2 are the active energies, and A and B are the corresponding constants. We found Eb1 was about 5 meV and might correspond to the energy of exciton bound to a neutral donor or localization potential due to alloy fluctuation or well width variation; Eb2 responsible for the hightemperature part of I(T) are 41, 60, and 73 meV for Lw = 5.6, 2.7, and 1.6 nm samples, respectively, which correspond to 3833


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Figure 4. (a) Fitting results of Eb1 and Eb2 by Arrhenius relationship with varying ZnO well width, Lw, as shown by dark-triangle and magentasquare symbols, respectively. (b) Coupling strength of LO phonon including A1(LO) and E1(LO) mode, as well as E2(low) phonon with exciton with varying ZnO well width, Lw, as shown by red-circle and blue- triangle symbols, respectively. (c) Transition and binding energies of exciton as a function of the ZnO well width, Lw, are shown by magenta triangle and red star symbols, respectively. The transition and binding energies of exciton for different Lw from Makino et al.4 and Béaure et al.39 are also depicted as reference. The slope of the magenta line fitted by linear relation is ∼688 kV/cm. The dash dot line indicates the exction binding energy of bulk ZnO and the corresponding well width.

the exciton binding energy in the quantum wells. The fitting results of the three samples with different well widths are plotted in Figure 4a. The exciton binding energy exhibits monotonic increase with decreasing well width and it becomes smaller than that of the bulk ZnO (60 meV) for Lw being larger than 3 nm. This result suggests the QCSE may dominate when the well width of polar-ZnO MQWs is larger than 3 nm and is agreement with the report by Pan et al.,35 leading to the exciton binding energy becoming smaller than the bulk value. The exciton−LO phonon coupling has a great influence on the optical and electrical properties. To quantitatively investigate the effect of the quantum confinement on the exciton−LO phonon interaction resulting from the Fröhlich interaction, we also fit the temperature-dependent excitonic emission peak, as shown in Figure S5b, to the Bose−Einstein expression.37 n

Eg (T ) = E0 −

∑ λi i=0

1 e Ei / kbT − 1

plotted in Figure 4b. As the well width decreases, the strength of exciton−LO phonon interaction decreases. The A1(LO) phonon, the lattice vibration along c-axis, shows weaker coupling with the exciton as decreasing the well width that is consistent with quantum dots results.22 The reduction of LOphonon coupling is attributed to less polar nature caused by the decrease in the well width, i.e., the reduction of the exciton Bohr radius and so increase in the exciton binding energy. In addition, different from the nature of increasing E2(low) coupling strength with shrinking well width observed in the mplane ZnO/MgZnO MQWs caused by the spatial phonon confinement and/or the interface phonon,23 we observed that E2(low) phonon does not stronger couples with exciton as decreasing the well width in c-plane ZnO/MgZnO MQWs. Because the E2(low) mode is a two-dimensional vibration along the m-plane, i.e., vibrates on the plane perpendicular to the c-axis, E2(low) phonon in c-plane ZnO/MgZnO MQWs is not significantly affected by the spatial phonon confinement and/or the interface phonon. For determining the nature of QCSE in the polar ZnO MQWs grown on Si(111) with nanothick Y2O3 buffer layer, we replotted the transition and binding energies of exciton with different ZnO well widths and compared that with the results from Makino et al.4 and Béaure et al.39 respectively, shown in Figure 4c. To estimate the minimum of build-in electric field, we ignore the influence of quantum-confinement effect on the exciton binding energy, because QCSE and the exciton binding energy show the opposite-trend influence on excitonic peak position. The relationship between the transition energies and the well thickness can be assumed as linear. The slope of the magenta line leads to the minimum of an internal-electric field, ∼688 kV/cm, which is more than 560 kV/cm reported by Makino et al.4 Nevertheless, we found our fitted exciton binding energies do not match the calculated curve with premeditated QCSE existence in MQWs from a report by Béaure et al.,39 where the bulk value is 60 meV, indicating that the build-in electric field of our MQWs is less than 820 kV/cm. Our results demonstrate that ZnO/MgZnO MQWs integrated on nanothick Y2O3/Si (111) composite substrates possess high structural quality and excellent optical properties that has great potential in UV optoelectronic device applications.

(2)

where Eg(T) is the exciton transition energy at a given temperature T, E0 is the zero-temperature exciton energy, and λi represents the coupling strength of the ith optical phonon mode with energy Ei. Because only the longitudinal phonon modes can interact with excitons, the A1(TO) and E1(TO) modes are not considered. The two nonpolar phonon modes associated with E2 symmetry, the atomic vibration on the plane perpendicular to the c-axis, are E2(high) and E2(low), respectively. The E2(high) associated with oxygen atoms is isolated at the Γ point (k = 0), which does not propagate; E2(low) is dominated by the vibrations of the heavy Zn sublattice.38 Because we are interested in the exciton−phonon coupling via the Fröhlich interaction, only the propagating of modes were discussed. Furthermore, the phonon energy of the A1(LO) mode is ∼574 cm−1 (∼72 meV) and E1(LO) is ∼584 cm−1, which is too close (∼1 meV difference) to be distinguished. Therefore, we only considered the two LOphonon modes E2(low) with E1 = 12.6 meV and coupling strength λ1 as well as E2 = 72 meV which includes A1(LO) and E1(LO) mode with coupling strength λ2, respectively. The fitting results of the corresponding coupling strength between exciton and LO−phonon for three different well widths are 3834


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CONCLUSIONS A set of ten-period ZnO/MgZnO MQWs with well thickness varying from 1.6 to 5.6 nm have been grown by pulsed laser deposition on Si (111) substrates using a nm-thick Y2O3 buffer layer. Single-phase MgxZn1−xO with Mg content of x = 0.33− 0.36, as determined from lattice constant and emitted peak in CL spectra, was adopted as the barrier material. The high crystalline-quality of the MQWs was greatly improved by using the Y2O3 buffer layer on Si (111), as evidence of the appearance of many pronounced satellite peaks in X-ray crystal truncation rods from XRD. The cross-sectional HR-TEM and STEM images also confirmed the regularly arranged well and barrier layers. When the ZnO well width is larger than 2.7 nm, due to the QCSE existence analyzed from the powerdependent PL spectra, the exciton binding energy were less than the 60 meV of bulk ZnO. Furthermore, not only the increasing exciton binding energy but also reducing exciton− phonon coupling determined from temperature-dependent PL spectra indicate the quantum confinement effect as ZnO well width is narrower than 2.7 nm. Finally, our results manifest both quantum-confinement effect and QCSE induced by the built-in electric field (∼688 kV/cm) play important roles in determining the physical properties of these high-quality ZnO/ MgZnO MQWs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00595. Set of XRD radial scans for the samples A, B, and C of Lw= 2.7 nm (Figure S1); HAADF STEM images (Figure S2); excitation power-dependent PL spectra (Figure S3); temperature dependent PL spectra (Figure S4); integrated intensity of the NBE emission and the temperature-dependent excitonic emission peak as a function of temperature (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wei-Rein Liu: 0000-0002-3330-0865 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the staffs of TPS-09A beamline and Dr. HwoShuenn Sheu, Dr. Kuan-Li Yu, Dr. Yu-Chun Chuang, and Dr. Ying-Yi Chang at NSRRC for assistance in X-ray diffraction measurements. This work was supported by the Ministry of Science and Technology of Taiwan, under grant MOST MOST 105-2112-M-213-008.

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REFERENCES

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