Strong Confined Optical Emission and Enhanced Thermoelectric

Nov 21, 2013 - Strong Confined Optical Emission and Enhanced Thermoelectric Power Factor by Coupling Homologous In2O3(ZnO)m with In-Doped ZnO ...
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Strong Confined Optical Emission and Enhanced Thermoelectric Power Factor by Coupling Homologous In2O3(ZnO)m with In-Doped ZnO Channels into Heterojunction Belts Yi-Feng Lai,† Hui-Wen Shen,† Yi-Chang Li,† Chao-Hung Wang,† Yen-Chih Chen,† Po-Chien Huang,† Chuan-Pu Liu,*,†,‡ and Ruey-Chi Wang*,§ †

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan § Department of Chemical and Materials Engineering National University of Kaohsiung, Kaohsiung, 81148 Taiwan ‡

ABSTRACT: Multifunctional nanodevices integrated with excellent optoelectric and thermoelectric properties are highly demanded for developing next generation selfpowered optoelectronics. In the work, we demonstrate the viability of a coupled structure of homologous In2O3(ZnO)m with In-doped ZnO into heterojunction belts synthesized by alloy-evaporation deposition, offering multiple functions with strong confined optical emissions and high power factors. Energy-filter secondary electron images reveal a fivelayer contrast in the width direction of the belts due to different ionization energies corresponding to In2O3(ZnO)m/In:ZnO/ZnO/In:ZnO/In2O3(ZnO)m, as confirmed by transmission electron microscopy analysis. The indium-doped ZnO channels confined in two sides behave like quantum wells through band alignment, and become predominant in the main UV emission at 385 nm, as revealed by cathodoluminescence spectroscopic imaging. More intriguingly, this novel heterostructure provides an ideal pathway to enhance electron conduction through the indium doped ZnO layer, and the homologous In2O3(ZnO)m layer contains numerous interfaces to impede phonon transportation. In this way, the power factor of the heterostructure is greatly enhanced to be 2.07 × 10−4 W m−1 K−2 at room temperature, as compared to about 1.03 × 10−5 W m−1 K−2 for the undoped ZnO nanowires. This unique heterostructure achieved by a facile one-step growth process offers a new concept for designing devices by manipulating photons via coupling with electrons and phonons.

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INTRODUCTION Size shrinkage used to be the most common trend over the past few decades guiding the movement of current device fabrication such as by the drive of Moore’s law. However, with decreasing size down to a few tens of nanometers, more physical limitations have emerged as formidable threats for the continued development of nanodevices. To overcome the enormous hurdles, alternatively, new technologies enabling more functions into an integrated nanodevice, as a self-powered smart system become ever more attractive. However, the drastically different design rules for different functions making integration of various functions into a monolithic nanodevice seems challenging. Here, we demonstrate how to devise an epitaxial heterojunction device, by joining two popular structures for uniquely integrating optoelectric and thermoelectric properties based on ZnO. Due to their superior characteristics, such as a wide bandgap of 3.37 eV, large exciton binding energy of 60 meV, noncentrosymmetry, mechanical robustness, and chemical stability, one-dimensional ZnO nanowires and nanobelts have long attracted research interest for various applications, such as those related to piezoelectricity,1 optoelectronics,2 and field emissions.3 To go beyond what can be provided by a single © 2013 American Chemical Society

intrinsic nanomaterial, currently two popular approaches are taken independently to further render them with more functions, namely, through impurity doping and superlattice structure incorporation. In the first approach, ZnO nanomaterials can be selectively doped into n- or p-type with various concentrations, where even the metal−insulator transition is demonstrated4 as the ultimate case. In addition, the bandgap of ZnO nanomaterials can be tuned with impurity doping,5−7 along with the properties of electrical conductivity, photoemission,8 and even ferromagnetism, and thus these materials can be better suited for various heterostructure nanoscale devices, and especially optoelectronic and memory devices. With the other approach, a layered structure is intentionally grown on ZnO by introducing an excessive amount of certain metal elements to create more epitaxial interfaces for the regulation of phonon transportation without any limit with regard to carrier conduction in thermoelectric devices.9 Homologous M2O3(ZnO)m (M = Al, Ga, In) is an example of such a structure, containing atomic metal oxide insertion Received: August 11, 2013 Revised: November 19, 2013 Published: November 21, 2013 25778

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RESULTS AND DISCUSSION The as-synthesized products were first examined by SEM. Figure 1a is a low-magnification SEM image, showing belts of

layers to form a superlattice structure, and relatively low thermal conductivity has been demonstrated with this material. For example, Andrews et al.9 found that the power factor and thermal resistivity of homologous IGZO (M2O3(ZnO)n (M = In and Ga)) nanowires converted from pure ZnO using a facile diffusion scheme were both enhanced. Therefore, although the impurity doping approach is favorable for optoelectronics, superlattice incorporation is preferred for thermal management applications. However, to date it has not been possible to design a structure combining these two approaches to couple electrons, photons and phonons, allowing new properties and devices to be explored. In this study we carry out indium doping and successfully demonstrate a novel epitaxial heterojunction belt made of five layers (In2O3(ZnO)m/In-doped ZnO/ZnO/In-doped ZnO/ In2O3(ZnO)m) along the width by one-step alloy evaporation deposition without the use of catalysts. The heterostructure exhibits quantum-well like optical emission confinement on the intermediate In-doped ZnO layers. In addition, this structure also has a high power factor, which means it can be applied in thermoelectric devices. This work thus presents the concept of a novel structure combining doped layers with a superlattice structure to manipulate electrons, photons and phonons, thus making the design of new devices possible. This concept can be easily generalized for use with similar material systems.

Figure 1. (a) Low-magnification SEM top-view image of assynthesized products on Si substructure. (b), (c) High-magnification SEM images of nanodisks and microbelts.

lower density formed accompanied by higher density nanodisks (NDs) on a silicon substrate, where the red dotted circles mark the nucleation sites of the belts. The belts exhibit a tapered morphology, shrinking from bottom to top, with typical lengths and widths falling in the range 10−20 and 1−2 μm, respectively. Parts b and c Figure 1 show higher magnification SEM images of the NDs and belts, respectively. The NDs are partially hexagons and partially half-hexagons, with a diagonal size of around 0.8−1 μm randomly oriented on the substrate. Careful examination of some vertically oriented NDs reveals that their thicknesses are in the range 60−100 nm, with most having two parallel side edges with a cone on top. Interestingly, the belts with thicknesses in the range 250−350 nm also show a sword-like shape, with two parallel side edges and a tip of around 60−70° on top. The similar features of these structures, except for the aspect ratio and size, indicate that the belts are grown longer, extending from nanodisks as embryos. The SE images acquired from the lower and upper detectors at an accelerating voltage of 1 kV and a working distance of around 3 mm are shown in Figure 2a,b, respectively. The image from the lower detector is obtained by collecting both backscattered and secondary electrons and is an unfiltered image for greater topographic contrast. In contrast, the upper detector can only collect SEs up to a certain energy, which is determined by the bias applied to the side electrodes, forming a low-pass energy filtered image for voltage contrast, which is sensitive to the electronic structure, as dictated by parameters such as doping concentration.11 In Figure 2a, no clear change in contrast can be seen throughout the belt, as indicated in the inset for the intensity line profile projected from the marked white rectangular area. The near constant contrast implies that the belt is characterized by a flat surface morphology. In contrast, Figure 2b exhibits a distinct sandwich-like contrast for the belt, indicative of being composed of a heterojunction layered structure. As revealed in the projected intensity line profile in the inset, two dark bands are sandwiched by the bright central area and two bright bands at either side, forming a five-layer structure, where the two brightest lines at the edges due to edge effect are excluded. The symmetric five-layer structure can also be seen in a higher magnification SEM image in Figure 2c, which clearly shows a heterojunction structure rendered by nonuniform In doping. To confirm this conjecture, panels d−f of Figure 2 show EDS elemental maps of indium,

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EXPERIMENTAL METHODS The ZnO/In:ZnO/In2O3(ZnO)m heterojunction belts (HBs) were synthesized via the alloy-evaporation deposition (AED) method10 on a Si(100) substrate. Zn (purity: 99.8%, 100 mesh) and In (purity: 99.9%, 100 mesh) mixed powders (weigh ratio = 9:1) were placed in a quartz boat located inside an 1 in. diameter (2.54 cm) horizontal quartz tube reactor and heated at a rate of 20 °C/min from room temperature. Argon was introduced as the carrier gas at the beginning of growth with a flow rate of 8 sccm, while the working pressure was kept at 50 Torr. The alloying treatment was first carried out at 500 °C for 30 min. Subsequently, the pressure was decreased to 5 Torr and the system was heated again to 650 °C at a rate of 20 °C/min. Once the temperature of 650 °C was reached, oxygen was introduced into the chamber with a flow rate of 1 sccm to facilitate the growth of the HBs for 1 h. Finally, the substrate was slowly cooled down to room temperature in the furnace. The morphology of the as-synthesized HBs was investigated by acquiring secondary electron (SE) images with a conventional Everhard−Thornley (ET) and a through-the-lens (TTL) E × B detector, also known as lower and upper detectors, respectively, performed at various voltages from 1 to 20 kV in a cold field emission Hitiachi-SU8000 scanning electron microscope (SEM). The macroscopic element distribution and luminescence properties were examined by an X-ray energy dispersive spectrometer (EDS) (XFlash 5010, Bruker) and cathodoluminescence (CL) detector (Mono CL4, Gatan) in the same SEM operated at 15 kV. The microstructure, orientation, and composition were characterized by a highresolution transmission electron microscope (HRTEM) (JEOL-2100F) operated at 200 keV, and the TEM crosssectional sample was prepared by focused ion beam (FIB) milling (SMI 3050). The electronic and thermoelectric devices of the HBs were fabricated by e-beam lithography, by which the electrical conductivity, carrier concentration, and Seebeck coefficient were determined. 25779

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Figure 2. (a) and (b) SE images of a In2O3(ZnO)m/In:ZnO/ZnO HB recorded using ET and TTL E × B detectors, respectively. The insets show the corresponding projected intensity line profiles. (c)−(f) FESEM images and the corresponding EDS element mapping results of In, Zn, and O, respectively.

Figure 3. (a) TEM bright-filed image of an In2O3(ZnO)m/In:ZnO/ZnO HB with indium concentrations in distinct regions (I, II, and III) obtained from EDS. The inset is the corresponding SAED pattern recorded from the entire belt structure. (b) TEM cross-sectional bright-field (up) and HAADF (down) images of an HB. (c) Medium-magnification HAADF image recorded from the rectangular area in (b) with SAED patterns and In EDS data obtained from regions i, ii, and iii. High-magnification HAADF images obtained from region iii in (c) showing (d) alternate stacking of In− O layers and In/Zn−O slabs along the c-axis, and (e) zigzag modulated structures in the In/Zn−O slabs. (f) Atomic-resolution HAADF image showing columns of Zn and In cations. (g) Intensity line profile and d-spacing across an IDB.

zinc, and oxygen, respectively, with reference to the SEM image in Figure 2c. Intriguingly, these three elements demonstrate drastically different spatial distributions. Whereas oxygen is uniformly distributed over the entire structure, In is richer at the bottom and two sides covering from the area corresponding to the side bright bands, and perhaps to the two dark bands in

Figure 2c. Meanwhile, zinc seems to decay slightly and symmetrically toward the two sides, and probably also in the two bright bands. These results reveal the complex dependence of indium doping on the five heterojunction layers through the influence of the electronic band structure, responsible for the strong SE contrast, which demands further study by TEM. 25780

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Figure 4. Schematic diagram of the proposed growth mechanisms of the HBs.

complex structure composed of many indium-enriched layers, as evidenced by bright lines oriented parallel to the ⟨112̅0⟩ direction, with a few lines extended slightly into region ii, causing weak superlattice spots and prominent streaks in the SEAD pattern. The average In concentration in this region is even higher than in region ii of around 2.4 at. %, as measured by EDS, which in combination with the superlattice results suggests that this region has a modulated homologous In2O3(ZnO)m structure. Notably, a slight increase in indium incorporation switches the homogeneous In doping in region ii to the superlattice structure in region iii. According to previous reports,9,12 the In2O3(ZnO)m superlattice structure generally consists of atomic layers of InO2− octahedra separated by slabs of wurtzite InZnnOn+1+ of varying thicknesses, exactly as observed in the high-magnification HADDF image of region iii in Figure 3d. The formation of a superlattice structure will induce a large strain between the ZnO slabs and InO2 insertion layers. The ends of the partial inclusions through incomplete indium lattice diffusion are thus usually associated with edge dislocations, as shown in the white dashed circle in Figure 3d. The accumulative strain is relaxed via two routes. First, indium atoms distribute along the hexagonal c axis in zigzag modulated structures with the inclined angle of 53°, thus reducing total strain, as shown in Figure 3e, preferably for thicker ZnO slabs (>5 ZnO units), consistent with theoretical predictions.13 In addition, the octahedral indium inclusion layers act as inversion domain boundaries (IDB) to relax the strain, as shown in Figure 3f in a high-resolution HAADF image, where the stacking sequence of the close-packed metal layers switches from ABAB to CACA passing the IDB. Figure 3g shows a projected line profile across an IDB, where the d-spacing in the InZnnOn+1+ layer remains almost the same as in pure ZnO (0002) planes (0.26 nm), but with a 19% increase in the d-spacing (0.31 nm) on the either side of the In−O insertion layer. From the above analyses, we demonstrate that a novel fivelayer heterojunction ZnO belt composed of In2O3(ZnO)m/ In:ZnO/ZnO/In:ZnO/In2O3(ZnO)m is synthesized by a facile growth method. The possible growth mechanisms could be rationalized by four steps, as shown in Figure 4. (1) Selfcatalyzed vapor−liquid−solid (VLS) combined with vapor-solid (VS) mechanism: In the beginning, alloy droplets are formed on the substrate from the co-condensation of zinc and indium vapors, followed by the nucleation of ZnO belts with supplied oxygen via a self-catalyzed VLS process. At the same time, indium is incorporated as a dopant. According to the calculations carried out by Gu et al.,14 the surface energy of ZnO (0001) will become smaller than that of the other major planes once indium doping exceeds 1/4. The fact that nucleation region I at the bottom is rich in indium makes the 1D ZnO grow in the form of a belt along the thermodynami-

Figure 3a shows a low-magnification TEM bright-field image of a single HB with the width and length of about 800 nm and 1 μm, respectively. By comparing this with the SEM results, we suggest that the “region I” feature with a larger width may correspond to the bottom part of the heterojunction belt, whereas the sharp tip feature of “region III” may correspond to the top of the belt. The corresponding selected area electron diffraction (SAED) pattern on the [0001] projection in Figure 3a shows that the entire HB is a single-crystalline wurtzite structure growing along the [101̅0] direction without any extra diffraction spots, indicative of epitaxial interfaces across the heterojunctions. However, the average indium concentrations with respect to ZnO acquired from EDS reveal large variations in spatial distribution, at about 10.3, 2.3, and 2.5 at. % for the bottom (I), middle (II), and top (III) regions of the HB, respectively. Notably, the nucleation region corresponds to the highest indium concentration. To gain more insight into the microstructure and composition of the HB, the region marked with a white rectangle in Figure 3a is cut by FIB for TEM cross-sectional characterization. Figure 3b shows a bright field image in the upper part and scanning TEM (STEM) high-angle annular dark-field (HAADF) image in the lower part, which are analogous to the two SE images in Figure 2 with regard to contrast variation. Whereas the TEM bright field image indicates the smooth top and bottom surfaces as revealed in Figure 2a of the SE image from the lower detector, the HAADF image demonstrates layer contrast similar to that for the SE image from the upper detector in Figure 2b, in that the two side regions are brighter than the central part. Due to the effects of incoherent scattering on the Z contrast, the intensity in a HAADF image is proportional to the composition of a material provided the thickness the same. Therefore, the brighter regions might be ascribed to a higher indium concentration, consistent with the EDS In map in Figure 2d. Half of the FIBcut sample is further studied by TEM, EDS, and SAED in Figure 3c for image contrast, indium concentration, and diffraction pattern, respectively, by which three distinct regions (i, ii, and iii) are assigned. Region i (central part) is characterized by the single-crystalline wurtzite structure of ZnO, as the SAED pattern in the inset shows a negligible indium concentration, and this is also evidenced by the darkest contrast in the HAADF image and the EDS characterization. In region ii, the average indium concentration is around 1.7 at. %, which is associated with the brighter contrast in the HAADH image. The DP in the inset also corresponds to the single crystalline wurtzite structure, with the diffraction spots shifted to a slightly lower angle compared with that of region i, suggesting that this region is single crystalline ZnO homogenously doped with In. Upon careful examination of the HAADF image, region iii is found to exhibit an amazingly 25781

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Figure 5. (a) Room-temperature CL spectrum, (b) SEM image, and (c) monochromatic CL image at 385 nm of a typical HB.

Figure 6. (a) Line-scan profiles of indium concentration, energy filter SE image, and CL emission at 385 nm acquired from the rectangular area in the inset. (b) Schematic diagram of the proposed band structure and radiative recombination routes of the HB.

Figure 7. SEM micrographs of an HB-based (a) field effect transistor and (b) TE device prepared by E-beam lithography. (c) Schematic diagram of the proposed mechanisms for electron and phonon transportation in an HB.

cally favored [101̅0] direction rather than the more common [0002] one. Moreover, it is worth noting that the width is nonuniform throughout the belt, indicating that the VS process is also operating,15 and that the decrease in width may be as a result of the varied growth rate in the [112̅0] direction due to the gradual decrease in indium concentration. (2) Surface diffusion of indium: The indium concentration decreases from the bottom to top, implying that indium diffuses upward. Moreover, the large amount of indium available at the bottom establishes a large concentration gradient as the driving force for diffusion. Under this condition, surface diffusion is much more favored than slow lattice diffusion. (3) and (4) Redistribution of indium to form a heterojunction structure: Subsequently, indium diffuses inward to result in a three-zone sandwich-like belt structure, where each layer structure is determined by the average indium concentration and strain involved. Because the average indium concentration may exceed the solid solubility limit in ZnO in the outermost region (region iii), an In2O3(ZnO)m homologous structure is generated to compromise between the indium concentration and the strain. Region ii certainly results from residual diffusion of indium into ZnO lattices, a process that is limited by indium supply and temperature. The equilibrium structure is finally

established through competition between belt growth and indium diffusion inward from the surfaces, causing region i to shrink more than regions ii and iii with regard to the width toward the top of the belt. The room-temperature CL spectrum of HBs in Figure 5a shows a strong ultraviolet (UV) emission centered at 385 nm, as well as a weak and broad green emission centered at 501 nm. In general, the CL emission of undoped ZnO is located at around 372 nm.16 Therefore, the red shift of the UV emission at 385 nm of the HB may be ascribed to the narrowing of the band gap due to doping,7,15,17 as well as a heterojunction band alignment effect. In addition, the peak at 501 nm is attributed to the transition between the photoexcited holes and singly ionized oxygen vacancies of the ZnO structure.18 Figure 5b shows a low-magnification SEM image of an HB, whereas Figure 5c is the corresponding monochromatic CL images for the emission at 385 nm. In spite of all the ZnO-based material, the results reveal an unusual phenomenon in nonuniform UV emissions over the entire structure, where the peak at 385 nm is mainly contributed by the two sided zones (In doped ZnO and In2O3(ZnO)m), and this provides further support for the doping-induced red shift. 25782

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introduced in region iii can impede phonon transportation with an electron conduction channel decoupled in region ii. For this purpose, we aim at exploring the TE power factor of the HB here. Therefore, in addition to the field-effect device shown in Figure 7a, another device incorporating nanoheaters was also fabricated on a SiO2/Si chip by e-beam lithography, as shown in Figure 7b. Both the electrical conductivity (σ) and the Seebeck coefficient (α) were measured at room temperature. Although the four-point probe method was employed for measuring the electrical conductivity as shown in Figure 7a, the Seebeck coefficient was derived by S = −ΔV/ΔT, where ΔV is the thermoelectric voltage and ΔT is the temperature difference across two HB−electrode contacts defined in Figure 7b. The temperature at the near- and far-side electrode to the heater with different currents applied for Joule heating was determined by temperature dependence of resistance of the electrode material, where the temperature coefficient of resistance of the material was measured separately.23,24 In this way, ΔT could then be calculated. Finally, the power factor (P) was calculated from the measured electrical conductivity and the Seebeck coefficient as P = σα2. The electrical conductivity and carrier concentration of the HB determined from Figure 7a are 4.48 × 104 Ω−1 m−1 and 8.8 × 1019 cm−3, respectively, whereas the Seebeck coefficient is determined to be −63.32 μV/K from Figure 7b, and the power factor can thus be calculated as 2.07 × 10−4 W m−1 K−2 at room temperature. Comparing these results with those of previous studies,9 the power factor of a single ZnO/In:ZnO/In2O3(ZnO)m heterojunction belt is larger by a factor of 20 than that of undoped ZnO nanowires (about 1.03 × 10−5 W m−1 K−2), and comparable to the best reported results for ZnO-based nanomaterials, including Sb-doped ZnO microbelts (3.2 × 10−4 W m−1 K−2)25 and IGZO homologous nanowires [(5−6) × 10−4 W m−1 K−2].9 The high power factor of the HB is attributed to the higher carrier concentration due to indium doping, as compared with the level of about 3 × 1017 cm−3 for undoped ZnO nanowires. The unique sandwich-like HB with a high power factor produced in this work may make possible more efficient TE device applications, because the good separation of the high-conductivity In:ZnO channels enables better carrier conduction and the layer-by-layer In2O3(ZnO)m structures can impede the propagation of phonons due to multiple scattering, as schematically shown in Figure 7c for a ZnO/In:ZnO/In2 O3 (ZnO) m HB. The homologous HB also acts as a double core−shell phonon transfer system to reduce thermal conductivity via the depression and localization of long wavelength phonons at the interfaces of the insertion layers in In2O3(ZnO)m regions, and the interfaces between ZnO, In:ZnO, and In2O3(ZnO)m regions.26 Although the size of the HBs is not yet optimized for use with TE devices, the ideal structure to separate electron conduction from phonon transportation can be easily achieved by one-step synthesis. The TE power factor is expected to be further enhanced by reducing the widths of the In:ZnO and In2O3(ZnO)m regions in future work. Overall, this work demonstrates a concept to fabricate a novel TE structure by generating indium doped ZnO channels for efficient carrier transportation and numerous interfaces to resist phonon propagation.

To better understand the reasons behind this abnormal luminescence phenomenon, the line-scan profiles of the indium concentration from EDS, energy-filtered SE imaging, and CL emissions at 385 nm across the same heterojunction belt, as shown in the inset, are compared in Figure 6a. By referring to the STEM results shown in Figure 3c, we can divide the profile into a five-layer sandwich-like symmetric structure with regions i, ii, and iii corresponding to ZnO, In:ZnO, and In2O3(ZnO)m structures, respectively. The EDS line profile confirms the maximum indium concentration occurs in region iii, which decreases gradually inward up to region ii. On the other hand, the SE intensity profile exhibits dark contrast only in region ii, suggesting this region is doped with donors more heavily than the adjacent regions i and iii, leading to a greater ionization energy for less SE signals.19,20 The luminescence at 385 nm is mainly emitted from region ii, and only partially from region iii. To gain a deeper understanding of this structure, a transistor device fabricated by e-beam lithography, as shown in Figure 7a, was used to measure the carrier concentration based on the field effect of IV characteristics. Accordingly, the carrier concentration is determined to be about 8.8 × 1019 cm−3, presumably the signature of the highly doped region ii, which is far above the semiconductor-metal transition of a Mott critical density of 4 × 1018 cm−3 for In doped ZnO.7 Consequently, the energy bandgap would be narrowed21,22 in region ii to form a degenerate semiconductor, verifying the red shift in UV emissions. With these results, the band structure in contact across the five-layer heterojunction in a belt can be tentatively proposed in Figure 6b. Considering band alignment with a constant Fermi energy level, region ii of highly doped In:ZnO closely resembles a quantum-well-like layer, having energy barriers with adjacent lightly doped ZnO and In2O3(ZnO)m layers. Here, we realize ZnO quantum-well-like structures in an all ZnO heterojunction belt, and these produce confined UV light emissions resembling those of a light-emitting-diode, as opposed to dark bands in the SE image, due to the higher ionization potential that occurs in a self-consistent manner. The possible radiative recombination routes are also indicated in Figure 6b. Upon excitation, the electron-excited carriers will be driven to the quantum-well-like region ii for better confinement, and these subsequently recombine as route ① for the major emission. However, there is some uncertainty with regard to the band alignment for the valence band, which depends on the bandgap reduction in both regions ii and iii, resulting in emissions via recombination across interfaces into regions i and iii, like in route ② for a type II semiconductor, and this is also partially due to the band-tail states. However, this type of emission can only occur over a short distance across the interface, as seen for the interface between regions i and ii, and this process cannot account for the weaker emission in region iii. Although the band offset should also exist for the interface between regions ii and region iii, the presence of both a large amount of defects and interfaces in region iii of the homologous structure might stop electrons from diving away and promote recombination through defect levels, as in route ③ depicted in Figure 6b. Nevertheless, CL emissions are successfully confined in the two symmetric In:ZnO regions as quantum wells in this interesting heterojunction belt structure, which may have potential applications in optoelectronic devices. Moreover, this unique HB is not only useful for optoelectronic applications but is also ideal for exploring thermoelectric (TE) peoperties, because the many interfaces

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CONCLUSIONS ZnO/In:ZnO/In2O3(ZnO)m HBs were synthesized via a proposed alloy-evaporation deposition without using catalysts at a relatively low temperature of 650 °C. TEM characterization 25783

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shows the belt is a In2O3(ZnO)m/In:ZnO/ZnO/In:ZnO/ In2O3(ZnO)m five-layer sandwich structure growing along the [101̅0] direction. The homologous In2O3(ZnO)m regions are composed of In−O insertion layers parallel to the (0002) plane with modulated zigzag indium distributed in adjacent ZnO crystals. The growth mechanism is proposed on the basis of a combined self-catalyzed VLS/VS process along the nonpolar direction of [101̅0], because of a change in the favored surface energy due to indium doping. A large amount of indium migrates upward through surface diffusion along two sides of the belt, and this leads to the generation of homologous In2O3(ZnO)m regions surrounding the heterojunction belts. Subsequently, indium diffuses inward due to the concentration gradient, and this helps establish a highly indium doped ZnO region between the homologous structure and lightly doped ZnO core region, which manifests itself as a dark contrast in the energy-filtered SE image. This unprecedented HB structure offers opportunities to couple electrons and phonons with photons for various strategic energy applications. For example, we have demonstrated quantum-well like UV luminescence confined in two symmetric In:ZnO zones of the HBs even without p-type ZnO, which is explained by the proposed energy band alignment model, where the confinement is achieved by the degenerate band and narrowing bandgap in the In:ZnO regions. The heterojunction belts also demonstrate an excellent TE power factor of 2.07 × 10−4 W m−1 K−2 at room temperature, which sheds light on efficient TE applications by providing high carrier conductivity channels and numerous interfaces to limit the propagation of phonons via multiple scattering. This work demonstrates a new epitaxial layered structure by combining doping in one layer and homologous structures in another. With this new concept, this new nanostructure not only can produce strong confined luminescence but also can offer an efficient way of separating the transportation of electrons and phonons for high thermoelectric power factors in semiconductor devices. It is anticipated that this work will have a positive impact on the development of new devices for manipulating photons via coupling with electrons and phonons.

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AUTHOR INFORMATION

Corresponding Authors

*C.-P. Liu: e-mail, [email protected]; ph, +886-62757575 ext. 62943; fax +886-62346290. *R.-C. Wang: e-mail, [email protected]; ph, +886-75919466. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Council of Taiwan under grant NSC 101-2221-E-006-134MY3. The authors thank the Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan, and the National Science Council (NSC, Taiwan) Core Facilities Laboratory for Nano-Science and NanoTechnology in the Kaohsiung-Pintung Area for providing equipment and technical support. This research was supported in part by (received funding from) the Headquarters of University Advancement at the National Cheng Kung University, which is sponsored by the Ministry of Education, Taiwan, ROC. 25784

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