Simple Processing of ZnO from Solution: Homoepitaxial Film and Bulk

Jul 1, 2008 - E-mail: [email protected]., †. Institute of ... The concentration of ZnO in the solution determines the film growth mode. The ro...
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Simple Processing of ZnO from Solution: Homoepitaxial Film and Bulk Single Crystal Dirk Ehrentraut,*,† Miyuki Miyamoto,‡ Hideto Sato,§ Ju¨rgen Riegler,† K. Byrappa,| Katsushi Fujii,⊥ Katsuhiko Inaba,# Tsuguo Fukuda,† and Tadafumi Adschiri†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2814–2820

Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan, Mitsubishi Gas Chemical Co., Inc., 6-1-1 Niijuku, Katsushika-ku, Tokyo 125-0051, Japan, Murata Mfg. Co., Ltd., 2-26-10 Tenjin, Nagaokakyo, Kyoto 617-8555, Japan, Department of Geology, UniVersity of Mysore, Manasagangothri, Mysore 570 006, India, Center for Interdisciplinary Research, Tohoku UniVersity, Aramaki aza Aoba 6-3, Aoba-ku, Sendai 980-8578, Japan, and Rigaku Corp., 3-9-12 Matsubara-cho, Akishima, Tokyo 196-8666, Japan ReceiVed NoVember 5, 2007

ABSTRACT: The system LiCl-ZnCl2-K2CO3 is employed to fabricate homoepitaxial (0001) ZnO films by liquid phase epitaxy (LPE). The effect of ZnO concentration in the solution on the microstructure of the film is analyzed. Transition of growth modes was observed, evolving from island growth at 300 K), as a superfast scintillator (time of response ≈ 0.65 ns), in surface acoustic devices due to excellent electromechanical coupling (k2 ) 8), as photocatalyst, etc.3–8 Most applications are based on thin-film devices, and more recently also microand nanocrystalline ZnO seems interesting. In any case, control of the crystal quality is indispensable to guarantee proper device performance. An increasingly important aspect is the fabrication of materials by technologies which not only guarantee reproducibly high quality but also facilitate one-step fabrication of the functional materials with a minimum effort in producing them by so-called green technologies.8 The green technology of materials processing is vastly being employed at the moment for a wide range of materials owing to several advantages such as cost of processing, environmentally benign, one-step fabrication of the functional materials with desired shape and size, reduction in the processing time, etc. In the case of supercritical fluid (SCF) * Corresponding author. E-mail: [email protected]. † Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. ‡ Mitsubishi Gas Chemical Co., Inc. § Murata Mfg. Co., Ltd. | University of Mysore. ⊥ Center for Interdisciplinary Research, Tohoku University. # Rigaku Corp.

Figure 1. Overview of the technologies employed to fabricate ZnO homoepitaxial film and single crystal.

technology, the use of flow reactors can generate micron-sized to nanosized particles (either pure or hybrid materials) in a few seconds with high control over size and morphology. In this context, we employ the hydrothermal synthesis of ZnO under subcritical conditions (Figure 1). Recently, we have developed the fabrication of singlecrystalline, low surface roughness homoepitaxial ZnO films by liquid phase epitaxy (LPE) under ambient air conditions (Figure 1).9 Alkaline metal chlorides were found suitable with lithium chloride (LiCl) being superior. The continuous reaction of zinc chloride (ZnCl2) with polycrystalline potassium carbonate (K2CO3) and thereby permanent feeding of the solution is applied to overcome the comparably low solubility of ZnO. This way, ZnO films exceeding 10 µm thickness have been demonstrated. Other advantages includes the sharp interface between substrate and film.10 Low-defect ZnO homostructures and Mg1-xZnxO/ZnO heterostructures were fabricated.9 The X-ray rocking curve (XRC) full-width half-maximum (FWHM) from the (0002) reflection peak was as low as 30 arcsec. The potential

10.1021/cg7010919 CCC: $40.75  2008 American Chemical Society Published on Web 07/01/2008

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for production of high-quality films on square centimeter size ZnO substrates was demonstrated. The entire process does not require big efforts due to the simplicity of the equipment and process conditions, nor is high temperature or vacuum needed. Moreover, the flux is completely recyclable after the process has been terminated. It is interesting to note that ZnO films were recently produced from ZnCl2 as source on sapphire substrate by atomic layer epitaxy (ALE) under atmospheric pressure.11 However, the XRC FWHM from the (0002) reflection was high at 558 arcsec indicating unsatisfactory crystallinity. Homoepitaxy of ZnO offers the tool to control the polarity of ZnO film and to fabricate unstrained films due to zero lattice and thermal misfit. A great advantage of growth processes from the liquid phase like LPE is that the activation energy required to trigger nucleation and growth, expressed as ∆G, is extremely low in contrast to vapor phase growth technologies (VPT) such as molecular beam epitaxy (MBE) or metal-organic vapor phase deposition (MOCVD). As example, for the epitaxy of GaAs at 1000 K, ∆G ) 75 and 77 kcal mol-1 for MBE and MOCVD, respectively, but ∆G < 0.03 kcal mol-1 for LPE was reported.12 Due to the vicinity to the thermodynamic equilibrium in LPE, where ∆G for nucleation and dissolution is counterbalanced, high crystallinity of films exhibiting a surface morphology characterized by small step height, ideally monatomic, and large interstep distance of say micrometer range or even faceting over millimeter range13 is attainable by careful control of the supersaturation. In this case, the growth follows a twodimensional (2D) growth mode. In contrast, coalescence of islands is often observed from VPT, but not desired because of atomically rough surface and impurity-trapping grain boundaries, which strongly alters physical properties, e.g. carrier mobility, resistivity, optical transmittance, etc. What is more, in chemical vapor deposition techniques like ALE the application of ZnO substrate may be limited since the ability of ZnO to easily react under corroding atmospheres is certainly a limiting factor in ZnO homoepitaxy. Here we present our study on the impact of LPE growth conditions on the microstructure of such films for better understanding of the complex growth process, particularly the evolution of the film morphology with changing ZnO concentration. Surface roughness and structural perfection of the ZnO films are jointly described by a structural quality factor (SQF), which is introduced for ZnO films in this paper. The luminescence of LPE grown ZnO films is discussed in comparison with hydrothermal ZnO processed under mild and supercritical conditions as shown in Figure 1.

of a substrate was directly exposed to a constant flow of the solution generated through substrate rotation of 5-20 rpm. Undesired shading effects on the (0001) face could thus be reduced. Film growth on the (0001j) face was not a subject of study in this paper due to lower quality.9 Constant temperature, air atmosphere, and pressure was applied during all experiments. The substrate was rinsed with distilled water and subsequently with 2-propanol after withdrawing from the solution and cooling to room temperature. Substrates. (0001) substrates of sizes 0.7 × 1 and 1 × 1 cm2, both with a disorientation angle R e 0.5° toward 〈1010〉, were cut from hydrothermally grown ZnO crystals.15 They were polished to obtain optical grade surfaces, root-mean-square (rms) roughness typically around 1 nm. They were annealed at 1100 °C under flowing oxygen atmosphere to obtain monolayer steps.9 Final rms roughness was around 0.12-0.3 nm. Microcrystal Synthesis. Mild hydrothermal conditions at temperature 150 °C and maximum 10 bar pressure have been generated in Teflon-lined autoclaves of 30 to 50 mL capacity (Berghof, Germany). The starting materials were 99.998% Zn(NO3)2 · 6H2O, 99.999% NH4NO3, HPLC H2O, 99.9% MoO3, 99.99% In2O3, 99.9% Bi2O3, and 99.99% Sb and were taken in an appropriate stoichiometry in the Teflonlined autoclaves, and held at 150 °C for 24 h. After the experimental run, the autoclaves were quenched using ice blocks. The products were recovered through repeated washing in distilled water and centrifuged. Film Characterization. The film thickness was calculated from weight gain, scale accuracy (0.1 mg, and by secondary ion mass. The surface morphology was examined by NDIM (Nikon Eclipse ME600); AFM (Seiko Instruments Inc., SPA-300) working in contact mode under air atmosphere and using Si3N4 tips, aspect ratio 1:1; and FESEM (JEOL JSM-7000F), accelerating voltage of 3 kV. The crystallinity of films was determined by high-resolution XRC analysis (Rigaku, ATX-E) using Cu KR radiation (50 kV, 300 mA) and employing a four Ge (440) channel monochromator (optical resolution about 0.0015°) in combination with an incident slit (0.2 width × 2 height mm2). The scan speed was 0.01° min-1 at the step width of 10-4°. We used the (0002) reflection from the film and the standards by the Joint Committee on Powder Diffraction and Standards (JCPDS, card number JCPDS 89-1397). Concentration of impurities has been analyzed by secondary-ion mass spectroscopy (SIMS). The primary beam species was Cs+ (5 kV, 350 nA); sputter speed around 100 nm min-1. Prior to SIMS, hydrothermally grown undoped and the In-doped ZnO bulk sample were measured by inductively coupled plasma mass spectrometry (ICP-MS) to calibrate the SIMS measurements. Unpolarized PL was measured at 4 K. The PL signal from excitation with a He-Cd laser (λ ) 325 nm, Pout ) 1-1.6 mW) after dispersion on a 30 cm triple grating monochromator was detected by a CCD camera (Princeton Instruments Inc.). Microcrystal Characterization. Powder XRD and FESEM as above and TEM were employed to investigate phase purity and crystallinity. PL (λ ) 325 nm, Pout ) 3 mW) in the above geometry was measured at 10 K. The powder samples were attached to a glass holder using silver paste and aluminum foil.

2. Experimental Section

3. Results and Discussion

Film Synthesis. LPE was carried out by the dipping technique. We used commercial LPE machines each comprising 3 computer-controlled heating zones (accuracy (1 K) and a mechanical unit for substrate lift and rotation (0-300 rpm). A detailed description on the process and formation of solid solution has been given elsewhere.9 Polycrystalline K2CO3 (99.997%) as oxygen source was used in the correct molar ratio to ZnCl2 (99.999%), which is the zinc source. Doping additives were Mg2+, Cd2+, Cu2+, Ga3+, In3+, Ge4+, and Sb5+ through the following compounds: 99.9% MgCl2, 99.998% CdCl2, 99.99% CuCl2, >99.999% GaCl3, 99.999% InCl3, 99.999% GeI4, and 99.999% ZnSb, respectively. ZnCl2 was thoroughly mixed with LiCl (99.999%), was then filled into the crucible along with K2CO3 and was heated immediately. After liquefaction of the solution, the ZnO substrate was slowly immersed into the solution to produce ZnO films at growth temperatures between 630 and 640 °C. A detailed thermal analysis of the whole process has been reported recently.14 Throughout all experiments, the (0001) face

3.1. Film Growth by LPE. Film Microstructure. A high crystallinity with low defect concentration and very flat surface at the same time is required for thin crystalline films in device application. For example, columnar growth may yield excellent quality with respect to X-ray crystallinity, however, the surface possesses a certain roughness, and, moreover, a high number of grain boundaries may notably limit the application. By contrast, an amorphous film of low crystallinity may form a surface of negligible roughness. On the other hand, the highly disordered structure is limiting its device application. The concept of SQF is introduced here to jointly describe both surface roughness and crystallinity: SQF ) (RC)-1, where R, the root-mean-square roughness, is derived from AFM scans of 2 × 2 µm2 areas and C, the X-ray crystallinity, is derived

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Figure 2. Evolution of film structure and transition to 2D growth as relation of structural quality factor SQF over cZnO. H + CMP denotes the value for the best polished ZnO substrate. Highest film quality is obtained at 13 mmol of ZnO. Indexing (a)-(f) refers to Figure 3. Note that transitions between distinct film structures are not as sharp as might be assumed from the graphic.

Figure 3. SEM micrographs illustrating the dependence of surface morphology on concentration of ZnO: (a) 10 mmol of ZnO. Figure 3e shows a film with steps of a few nanometer heights and spacing of 2-5 µm, grown according to step-flow mechanism. The appearance of well-developed {0001} facets clearly demonstrates the growth under conditions near the thermodynamic equilibrium. Highest crystallinity and a smooth surface are found for 12.5 mmol e cZnO < 14 mmol, which is indicated by highest SQF. The capability to grow films with SQF better than substrate H + CMP is demonstrated by the marker at 4.1 × 1011 grd-1 m-1. Any notable increase of cZnO lowers the morphological quality as demonstrated by the micrograph Figure 3f. The latter film was grown at cZnO ≈ 17 mmol according to step-bunching mechanism. Now, pronounced macrosteps are on display developing facets (indexed in Figure 3f), which successively will disappear with further rising cZnO. The SQF exactly follows the mentioned trend: SQF ) 0.3 × 1011 grd-1 m-1 for cZnO ) 15.7 mmol and ≈ 0.1 × 1011 grd-1 m-1 for cZnO ) 17 mmol (Figure 3f). Figure 4 represents the image of an AFM scan of the area 10 × 10 µm2. The (0001) film thickness is about 3.8 µm. Monatomic steps about 0.5 nm in height and spacing of 1-3 µm are indicated by the line scan (raw data). On the atomic scale, a certain roughness might be due to the fact that some crystallization process from attached liquid solution is still under way while cooling down the film, see above. Nevertheless, the rms roughness of only 0.29 nm demonstrates the extreme flatness of the sample in Figure 4. The high crystallinity of the film is further signified by the small XRC FWHM from the (0002) reflection of 31 arcsec, the same as the substrate. The accompanying SQF calculates 4.1 × 1011 grd-1 m-1. Doping. In the next step we tried to grow doped ZnO films by LPE with several dopants of interest like Mg2+, Cd2+, Cu2+, Ga3+, In3+, Ge4+, and Sb5+. A highly uniform Sb, Li codoped ZnO film is shown in the micrograph of Figure 5. The Li content stems from the LiCl solution. The concentration of Sb and Li is about 1018 and 1019 cm-3, respectively, as monitored by secondary-ion mass spectroscopy (SIMS). Parallel aligned faceted steps with spacing of 2-3 µm are formed across the entire film, which was grown from cZnO ) 13 mmol. XRC curves from the (0002) reflection reveal a low-angle shift for

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Figure 6. FWHM mapping over full substrate size of typical In, Li (blue 2) and Ge, Li (black 9) codoped ZnO film. Figure 4. AFM scan from the intentionally undoped (0001) ZnO film grown at cZnO) 13 mmol, SQF ) 4.1 × 1011 grd-1 m-1, showing monolayers with the interstep distance up to 3 µm. Double line scan was recorded for noise reduction.

Figure 7. TEM image from Sb, Li codoped well-faceted ZnO crystals fabricated from the LiCl solution.

Figure 5. Optical interference micrograph from a Sb, Li codoped (0001) ZnO film shows highly uniform steps. The insets show XRC curves in log scale taken from the (0001) and (0001j) face and a contrast-enhanced magnified view of a fraction of the film.

the film grown on the O face, which is attributed to a higher Sb concentration in comparison to the Zn face. This example is another demonstration of the potential of the LPE technique to fabricate homoepitaxial ZnO films with and without doping. Similar to Sb, doping with In3+ and Ge4+ is of particular interest in light of scintillator application with emission in the blue wavelength region around 400-460 nm where selfabsorption is not effective.17 Figure 6 shows the mapping of the XRC FWHM from an In and a Ge doped film. The concentration of In and Ge in the film was 1020 and e1017 cm-3, respectively, and Li accounting with around 5 × 1018 cm-3. Whereas In easily enters the ZnO lattice, Ge concentration was only slightly increased. This was explained in terms of different mechanisms for incorporating In3+ and Ge4+ into ZnO structure.17 In Figure 6 we see the high uniformity of the XRC FWHM using the (0002) reflection across the substrate. Particularly the large central region about 1-2 mm apart from the rim of the substrate is of high crystallinity with FWHM e 35 arcsec. The region near the rim experiences different flow conditions, and consequently fluid transport and deposition of ZnO are different as well.18 However, we may conclude here that the LPE technique as presented is a useful and simple tool for the fabrication of structurally very homogeneous ZnO films with

and without doping. XRC FWHM as low as 22 and 24 arcsec for Ge, Li, and In, Li codoped homoepitaxial ZnO films have been reported recently.19 The fabrication of ZnO films by the LPE technique goes along with coprecipitation of microcrystal whose size and morphology strongly differs as a function of the nature of dopant. Coprecipitation in fact has been employed for homoepitaxial LPE growth of LiNbO3 and KY(WO4)2 in order to achieve a low supersaturation to engineer high-optical-quality films.18,20 Figure 7 presents a TEM image of well-faceted coprecipitated Sb, Li codoped ZnO crystals sizing around 50-200 nm. Their luminescence characteristics will be also investigated later in this paper. 3.2. Low-Temperature Hydrothermal Synthesis. Morphology Control and Doping. Study on morphology control is an important aspect of investigation in the case of crystals with device potential. The change in crystal morphology is the result of relative growth rates of its different faces, the general rule being that the faces which grow slowest are expressed in the crystal habit which is more governed by the chemical kinetics rather than the chemical equilibrium. However, the crystal morphology depends upon various parameters like pH of the growth medium, solvent type, degree of supersaturation, temperature, nature of the dopants and their concentration. Especially the dopants play a crucial role in controlling the crystal morphology. In the processing of nanoparticles with a desired shape and size, surfactants or organic capping agents are used. But in crystal growth the use of various metal ions as dopants is still a popular trend. The use of such selected metal

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Figure 8. TEM image from undoped well-faceted ZnO crystals fabricated under mild hydrothermal conditions.

Figure 10. Low-temperature NBE photoluminescence from (a) the undoped hydrothermal bulk ZnO crystal, (b) Bi and Sb doped ZnO hydrothermal microcrystal, (c) Sb, Li codoped ZnO microcrystal prepared from LiCl solution, and (d) the Sb, Li codoped ZnO film prepared by LPE.

Figure 9. TEM image from Mo doped ZnO crystals.

ions stunts the growth of crystals along certain crystallographic planes and promotes the growth along certain other planes. In the present work, metal ion dopants like Sb, Bi, In, and Mo have been used in order to obtain different morphologies like plates, prisms, rods and needles with different aspect ratios for ZnO crystals. Here, the metal ions have been used in very small quantities (initial concentrations from 8 to 30 mmol with respect to Zn in ZnO), and a significant effect on the morphology of ZnO crystals was noticed. Since the experiments were carried out at mild conditions (temperature at 150 °C and pressure less than 10 bar), it is predicted that the molecular dynamic forces are weak and the growth rate is also not very high, and hence the diffusion is highly controlled, such that the crystal quality is generally high with minimum possible lattice defects. Also because of the moderate crystal growth rates the crystals are usually faceted even if they are very small in size (Figure 8). The powder XRD analysis turned out that phase-pure ZnO according to the standards by the JCPDS has been synthesized in all experiments. However, some modification in intensity was obtained with In doped ZnO being the worst. Dopants do modify the morphology of the ZnO crystal such that the growth along the 〈0001〉 axis becomes prevalent when Sb or Mo (Figure 9) is used. On the other hand, doping In into ZnO bulk crystals at high pressure and temperature around 100 MPa and 400 °C strongly reduces the growth along the 〈0001〉 axis.21 As pointed out, the high quality and sufficiently large size of the doped ZnO crystals allow for photoluminescence (PL) investigation without interference by quantum defined effects such as is typical for nanocrystals. 3.3. Photoluminescence. Luminescence is strongly correlated to crystallinity and impurity of the crystal, which depends on

the fabrication technology. Since we employ three different routes, two of which include water as solvent at different pressure and temperature and the other is water-free with LiCl the solvent, we shall find striking differences in the lowtemperature unpolarized PL spectra. Moreover, the use of a phase-stabilizing substrate also shows great effect as we will demonstrate soon. PL of ZnO is generally characterized by a deep emission band centered in the green spectral range around 2.2-2.5 eV (not shown here) and the sharply structured near band edge (NBE) emission at 3-3.4 eV. The green emission does not strikingly differ for the samples in discussion. It is generally accepted that the origin of this luminescence is an electron-hole radiative recombination at oxygen vacancy, although the detailed transition process is still not clear. We will focus on the NBE emission, which in turn shows great changes upon dopants and sample processing. In Figure 10 are shown the PL spectra from the NBE region of the hydrothermal ZnO crystal serving as standard, the hydrothermal Bi and Sb doped ZnO microcrystal, Sb, Li codoped microcrystal, and the Sb, Li codoped LPE-grown film. The high quality of the standard ZnO sample has been proven by the temperature-dependent Hall-effect technique with Van der Pauw geometry (carrier concentration, mobility, and resistivity around 4 × 1013 cm-3, 530 cm2 V-1 s-1, and 90 Ω cm at 100 K, respectively) and SIMS measurement (Li, K, Al, and Fe around 2 × 1016, 2 × 1015, 4 × 1015, and 5 × 1015, all cm-3, respectively) very recently.19 In the PL spectrum we see the typical emissions from neutral donor bound excitation (D0X), their two-electron satellite (TES), and longitudinal phonon replicas (D0XnLO with n ) 1 to 4 at 73 meV spacing each) peaking around 3.36, 3.32, and 3.29 eV (1LO), respectively. There are weaker peaks starting from 3.251 eV, which are phonon replicas related to the TES around 3.32 eV. The ionized donor bound exciton (D+X) contributes at 3.366 eV from both the (0001) and (0001j) faces. Not shown here is the deep level

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Figure 11. Low-temperature NBE photoluminescence from the Sb, Li codoped (0001) ZnO film showing a variety of emission lines.

emission peaking around 2.4 eV with the intensity comparable to about 10-2 arbitrary units in Figure 10a. As a whole, this sample is of high optical quality quite in agreement with recent reports.22,23 By contrast, the spectrum from the purely Bi and Sb doped microcrystals grown at 150 °C in Figure 10b only displays the broad D0X peak. Note that the plasma line at 3.187 eV is due to the laser power used for excitation. Surprisingly, in spite of the larger ionic radius, the Bi doped samples exhibit better optical characteristics than the Sb doped ones. The TES accounts for the weak peak in the Bi doped ZnO. However, since the deep emission (not shown here) is much stronger in intensity than the NBE, we conclude that doping ZnO with Bi and Sb under applied conditions is not effective enough to synthesize high-quality material with modified optical characteristics. Now, the picture of doping ZnO with Sb completely changes when applying the LiCl solvent (Figure 10c). This sample with its peaks strongly resembles the undoped standard sample in Figure 10a, except for the lower intensity from FXA and FXB. Taking into account that the crystals were nucleated upon selfnucleation and that PL from a powder sample generally contains the fraction from all crystallographic orientations, the spectrum clearly indicates that crystals with high structural and optical quality were obtained. This result is further supported by the TEM observation in Figure 7. If we now apply a lattice-matched ZnO substrate to control nucleation and successive growth of the Sb, Li codoped LPE film in Figure 5, then a spectrum can be measured as shown in Figure 10d. Compared to all above portrayed samples, a strong DAP emission peaking at 3.269 eV and longitudinal phonon replicas each 73 meV gets dominant. The intensity of the DAP zero-phonon line (ZPL) from the (0001j) face is about half compared to the D0X at around 3.36 eV. The D0XnLO are simply overlapped by the intense DAP and their phonon replica. This gets more evident from the following figure. Figure 11 details the spectrum from the Sb, Li codoped (0001) ZnO film of Figure 10d for the energy range 3.25-3.4 eV. Here the splitting of the D0X gets clearer and we spot some characteristic lines in accordance with the literature at 3.372 eV (I0), 3.3607 eV (I6), 3.357 (I9), and 3.353 eV (I10).23 Further peaks at 3.375, 3.378, and 3.39 eV are ascribed to the appearance of A and B-free exciton and generally indicate a high optical quality. In contrast, they are not resolved in the spectrum obtained from the self-nucleated Sb, Li codoped ZnO crystals produced during the same growth run. Obviously, using a lattice-matched substrate has great phase stabilizing effect. Likewise to the doping with Sb, we now discuss in Figure 12 the effect of doping ZnO with In under hydrothermal

Figure 12. Low-temperature NBE photoluminescence from (a) the undoped hydrothermal bulk ZnO crystal, (b) In doped hydrothermal ZnO microcrystal and ZnO bulk crystal, (c) In, Li codoped ZnO microcrystal prepared from LiCl solution, and (d) the In, Li codoped ZnO film prepared by LPE.

conditions and from the LiCl solution in comparison to our standard sample in Figure 12a. It turned out that the fabrication of In doped crystals without lattice-matched substrate is worse compared to the case of Sb and Bi doping (Figure 10b). The low-temperature hydrothermal growth yields extremely poor optical quality with the strongest line due to the plasma as a result of high excitation energy (Figure 12b). In the same image, the In doped ZnO bulk crystal grown on a ZnO substrate under supercritical conditions near 400 °C gains a sharp I9 line due to In-related D0X and a broad emission centered around 3.375 eV.21,24 Now the In, Li codoped ZnO crystal in Figure 12c again shows the prominent I9 line but at higher relative intensity compared to the pure In doped ZnO of Figure 12b. Additional weak peaks are due to D0XnLO. Compared to the case of Sb doping (Figure 10c) we find that doping ZnO with In generally deteriorates the crystal and consequently optical quality. Using a ZnO crystal as substrate again yields higher PL intensities and the same shift in the order of highest PL intensities to the favor of DAP emission. However, the intensity of the DAP from the (0001j) face is now greater by a factor of 1.8 compared to the I9. This is a much stronger gain than for the Sb, Li codoped ZnO film and may be exploited in thin film devices since considerable self-absorption is excluded at this wavelength.17 Recent investigation of the PL decay from In, Li codoped ZnO films at room temperature has revealed superfast decay in the subnanosecond range.17 A tunnel-assisted donor-acceptor recombination process was ascribed the possible mechanism behind. Although this field is not well exploited yet, potential scintillator application with high two-dimensional resolution seems very promising.

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This clearly shows the effect of lattice-matched substrates in the stabilization of crystal quality. Acknowledgment. This work was supported partially by the Special Coordination Funds of the Ministry of Education, Culture, Sports, Science, and Technology of the Japanese Government. Some of the substrates were provided by Tokyo Denpa. We thank S. Ohara (Tohoku University) for support with TEM measurements.

References

Figure 13. Low-temperature NBE photoluminescence from Mo doped ZnO microcrystal showing a variety of emission lines.

What is more, a new strong line at 3.371 eV appears on the O-terminated face of the sample in Figure 12d which is possibly the I1 line.23 Again, emission intensities from the (0001j) face in general are stronger than those from the (0001) face quite in agreement to the Sb, Li codoped film in Figure 10d. Doping ZnO with Mo is another fine example of the versatility of ZnO as host crystal. Here we aimed at improving the photocatalytic effect of ZnO. The photocatalytic efficiency for sunlight in the treatment of industrial effluents and disintegration of toxic organics enhanced from 65% to more than 80% when Mo was doped into ZnO and used as a photocatalyst for the same effluent under similar conditions.25 Figure 13 shows the low-temperature NBE PL spectrum from the Mo doped ZnO microcrystal rich in pronounced emission lines. Besides the typical lines related to D0X (3.364 eV) and TES (3.312 eV) and the plasma line from the laser at 3.185 eV, we find a number of lines ranging from 2.92 to 3.24 eV the nature of which is not quite clear yet. The D0X at 3.364 eV coincides with the I4 line described due to hydrogen in the crystal.23 Interestingly, the TES is very sharp and appears with a strong intensity of 30-1 compared to D0X which compares to about 300-1 for the undoped hydrothermal standard sample in Figure 12a.

4. Conclusions We have clearly demonstrated that the choice of the growth method leads to significant changes in the quality of ZnO crystals. The system LiCl-ZnCl2-K2CO3 can be employed for the growth of (In, Sb) Li codoped ZnO film by LPE. We have shown that the concentration of ZnO determines the growth mechanism with consequences for the morphology of the film as revealed by the transition to 2D growth. Highest-quality single-crystalline film requires 13 mmolof ZnO in the solution. This is confirmed by small X-ray FWHM and highest value for SQF (4.1 × 1011 grd-1 m-1), which jointly describes surface roughness and crystallinity of a film. A film with SQF g 1011 grd-1 m-1 is of high structural quality. Doping of ZnO bulk crystals with Mo, In, Sb, and Bi under the condition of thermodynamic equilibrium can be achieved by subcritical hydrothermal synthesis. Homogeneous ZnO films and bulk crystals with new luminescent properties have been prepared. Luminescence of high intensity in the blue spectral region has been demonstrated for In, Li and Sb, Li codoped ZnO films which were both fabricated on ZnO substrate. The luminescence characteristics dramatically decline for the self-nucleated In, Li and Sb, Li codoped or purely In and Sb doped ZnO crystal.

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