Heteroepitaxial electrodeposition of zinc oxide films on gallium nitride

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Received 6 August 1999; accepted for publication 6 October 1999. Epitaxial zinc oxide films have been prepared on gallium nitride 0002 substrates by cathodic.
APPLIED PHYSICS LETTERS

VOLUME 75, NUMBER 24

13 DECEMBER 1999

Heteroepitaxial electrodeposition of zinc oxide films on gallium nitride Th. Pauporte´a) and D. Lincotb) Laboratoire d’Electrochimie et de Chimie Analytique (UMR CNRS 7575), Ecole Nationale Supe´rieure de Chimie de Paris, 75231 Paris cedex 05, France

共Received 6 August 1999; accepted for publication 6 October 1999兲 Epitaxial zinc oxide films have been prepared on gallium nitride 共0002兲 substrates by cathodic electrodeposition in an aqueous solution containing a zinc salt and dissolved oxygen at 85 °C. The films have the hexagonal structure with the c axis parallel to that of GaN and the 关100兴 direction in ZnO parallel to the 关100兴 direction in GaN in the 共0002兲 basal plane. The structural quality is attested by the values of the full width at half maximum in ␪/2␪ x-ray diffraction 共XRD兲 diagrams ¯ 1兲 planes 关0.07° for the 共0002兲 peak兴 and in five circles XRD diagrams 关0.74° for the ZnO 共101 ¯ compared to 0.47° for the GaN 共1011兲 planes兴. The morphology of the layers has been studied by scanning electron microscopy. Before coalescence, arrays of epitaxial single crystalline hexagonal columns are observed with a low dispersion in size, indicating instantaneous tridimensional nucleation. Preliminary results on luminescence properties of the films before and after annealing are presented. © 1999 American Institute of Physics. 关S0003-6951共99兲03348-3兴

semiconductors, including oxides, thus appears to be a new and very promising field. Electrodeposition of ZnO has been discovered recently by our group17,18 and by Izaki and Omi19,20 using dissolved oxygen as the oxygen precursor and nitrate ions, respectively. The overall deposition reaction in the presence of zinc ions and dissolved oxygen is very simple:

Zinc oxide presents interesting electrical, optical, acoustic and chemical properties which find wide applications in the fields of 共opto兲electronics, sensors and catalysis. As a wide gap n type semiconductor ZnO can be used as transparent conducting windows in solar cells and is a promising material for UV light emitting devices. Epitaxial zinc oxide films have been grown on sapphire by several groups1–6 despite the high mismatch between the two compounds and on SiC.7 Vispute et al. have reported the epitaxial growth of ZnO on GaN.8 This combination is very interesting since the lattice mismatch between these two hexagonal compounds is low 共2.4%兲. In this letter, we report on the possibility of depositing epitaxial ZnO films by electrodeposition, a simple and low temperature method, which may become, for some specific applications, alternative or supplementary to gas phase techniques used up to now for epitaxial growth. Preliminary results also show that the electrodeposited films are able to exhibit UV luminescence attesting to their internal quality. Very few works have been devoted to the epitaxial growth of compound semiconductor films by electrochemistry. Pioneering works have concerned chalcogenides. Hodes and Rubinstein have achieved epitaxial electrodeposition of II–VI quantum dots in the 5 nm range on evaporated Au共111兲 films.10 Lincot et al.11,12 have achieved epitaxial films of CdTe on single crystal InP 共111兲 substrate. Subsequently, a similar approach has been used for the growth of CdSe on GaAs and InP 共111兲.13 Epitaxial CdS films have been obtained on InP from a nonaqueous solvent.14 Concerning oxides, Switzer et al.15 have shown recently the epitaxial growth of metastable ␦ -Bi2O3 on Au 共110兲, 共100兲, and 共111兲 by the electrochemical method. The same group has also successfully electrodeposited cuprous oxide (Cu2O) on gold single crystals.16 Epitaxial electrodeposition of compound

Zn2⫹ ⫹0.5O2⫹2e ⫺ ⇒ZnO.

It takes place by applying a cathodic potential to a conducting substrate. To investigate specifically the possibility of epitaxial growth of ZnO, n-type GaN has been chosen both for structural reasons and because the n-type character gives a nonblocking electrode for cathodic reactions. A previous mechanistic study allowed us to optimize the experimental conditions of deposition.17,18 The solution contained 5 ⫻10⫺3 M ZnCl2 and the supporting electrolyte was a reagent grade 0.1 M KCl. Water was of millipore quality. The temperature of the deposition bath was 85 °C and the oxygen concentration was about 2⫻10⫺4 M obtained by mixing 25/75 in volume of O2 and Ar. GaN epitaxial films 共1.3 ␮m thick兲 were grown on 共0002兲 sapphire substrates by MOCVD. The samples were mounted as a working electrode and deposition performed in a classical three electrode electrochemical cell. The counterelectrode was a platinum wire. An active area of 0.125 cm2, isolated with silicone rubber, was in contact with solution. The deposition was performed at a fixed potential of ⫺1.4 V with respect to the saturated mercurous sulfate electrode (E⫽⫹0.65 V vs NHE兲. After potential application, the cathodic current reached a steady state value very rapidly. The corresponding current densities ranged from 0.26 to 0.32 mA cm⫺2. The structural characterizations of the films and the determination of the epitaxy were carried out by x-ray diffraction techniques using ␪/2␪ and five-circle goniometers. Luminescence experiments were done at 1.6 K. The excitation wavelength was the argon laser line at 351 nm. The incident power was 10 mW focused on a spot of 0.2 mm in diameter.

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Electronic mail: [email protected] Electronic mail: [email protected]

b兲

0003-6951/99/75(24)/3817/3/$15.00

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FIG. 1. XRD diagram of ZnO layers electrodeposited on 共0002兲 GaN: 共a兲 ⌿ ¯ 1其 family of planes obtained around an axis normal to the scan of the 兵101 ¯ 1兲 sample surface. The glancing angle was 0.6°. Enlarged view of the 共101 peaks of ZnO 共b兲 and GaN 共c兲.

XRD patterns have been recorded in the ␪ –2␪ scan with a cobalt anticathode over a large ␪ range 共not shown兲 that only planes of the 兵0002其 family were present. The deposited ZnO is thus oriented preferentially with the c axis normal to the GaN surface. The c parameter of the sample is 5.220 Å for ZnO and c is equal to 5.180 Å for GaN. The peaks have a FWHM of about 0.070° in both cases. Grazing angle XRD measurements have been performed with a copper anticathode at a glancing angle of 0.6°. The ␦ angle was fixed at ¯ 1兲 planes in ZnO. 32.45° for the specific detection of 共101 The corresponding ‘‘⌿’’ scan is shown in Fig. 1共a兲. Well¯ 1兲 peaks are observed which demonstrate that defined 共101 the layer is epitaxial. The presence of six peaks at a distance of 60° instead of the three expected is a consequence of a multipositioning effect. Figure 1共b兲 shows an enlarged view ¯ 1兲 peak. The FWHM is about 0.74°. For of the ZnO 共101 ¯ 1兲 peak of comparison, in Fig. 1共d兲, the corresponding 共101 the GaN substrate has been recorded. The FWHM is 0.47°. As a consequence, the epitaxial quality of the ZnO layer is close to that of the underlying GaN. The very low level of the base line in the ⌿ scan is another indication of the quality of the epitaxy. The morphology of the ZnO layers obtained after 70 min of deposition has been investigated by SEM and is shown in Fig. 2. The deposit is composed by an array of columns with a hexagonal section, some hundreds of nanometers in width 关Fig. 2共a兲兴. The parallelism of the crystallite edges is a consequence of the epitaxial growth of the individual nuclei. This view shows that the growth of ZnO proceeds by a tridimensional mechanism. The fact that the grains have about the same size indicates an instantaneous nucleation on the GaN surface. The density of nucleation centers is about (8 – 9)⫻108 cm⫺2 and the vertical growth rate is 0.6 ␮m/h. In some places coalescence between adjacent grains is clearly visible. Complete coalescence was almost achieved

FIG. 2. SEM images of 0.7 ␮m thick electrodeposited layers 共deposition duration of 70 min兲: 共a兲 overview of the ZnO layer, with a tilt angle; 共b兲 cross sectional view; 共c兲 top view of the ZnO layer. The inset shows a pit defect found in the bare GaN substrate near the area with electrodeposited ZnO.

after 2 h of growth giving compact ZnO layers 共not shown兲. The cross sectional view of Fig. 2共b兲 displays the typical shape of the crystallites with an enlarged lower part. The outer surface of the grains is remarkably smooth. In Fig. 2共c兲 a top view of the hexagonal ZnO crystallites is compared with that of a hexagonal pit defect present in an uncovered part of the supporting GaN layer 共inset兲. The directions indicated by thick lines are identical in the two images. This confirms the epitaxial relationship between the two layers obtained by XRD, in which case the relation ZnO 关100兴//GaN 关100兴 is clearly visualized. Preliminary results on photoluminescence 共PL兲 study are shown in Fig. 3. Figure 3共a兲 shows the PL spectrum recorded on the bare GaN substrate. The peak at 3.463 eV with a linewidth of 10 meV 共FWHM兲 is characteristic of luminescence of GaN.8,9 Figure 3共b兲 shows the spectrum obtained after electrodeposition of ZnO 共curve 1兲. No additional luminescence signal is detected for zinc oxide in the investigated wavelength range. After annealing treatment at 400 °C in air for 1 h a specific and intense luminescence peak appears at 3.363 eV. This value is characteristic of ZnO luminescence and is generally associated with the donor to acceptor transition. The peak is asymmetrical due to an Urbach tail. The linewidth is 20 meV 共FWHM兲 instead of 10 meV for the GaN layer. It can be pointed out that the photoluminescence

Th. Pauporte´ and D. Lincot

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ganization during deposition due to easy ionic exchanges between the surface and the solution through near equilibrium dissolution/precipitation processes. Electrodeposited ZnO looks promising for future 共electro兲luminescence applications. Playing with deposition conditions, in particular the composition of the solution 共nature of precursors, anions, additives兲 and a precise control of the growth by the deposition potential, should allow us to improve/engineer further the film properties. Tuning morphological properties, from dense films to localized growth of single crystalline columns, is also an open challenge. The authors are grateful to Dr. Bernard Beaumont from the Center de Recherche sur l’He´te´roe´pitaxie et ses Applications 共CNRS兲 for supplying GaN substrates. They also thank Dr. Robert Cortes from the Laboratoire de Physique des Liquides et E´lectrochimie 共CNRS兲 for performing GAXRD experiments. Dr Andre´e Kahn from the Laboratoire de Chimie applique´ de l’e´tat Solide is acknowledged for the ␪ –2␪ XRD measurements and Dr. Alain Lusson from the Laboratoire de Physique du Solide for the photoluminescence experiments.

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FIG. 3. Photoluminescence spectra recorded at 1.6 K, with an excitation wavelength of 351 nm: 共a兲 GaN substrate, 共b兲 as-deposited ZnO on GaN 共1兲 and after annealing at 400 °C for 1 h 共2兲 .

intensity of the ZnO film is close to that of the GaN substrate, indicating a rather good quality. Questions arise from the comparison of the PL spectra recorded before and after annealing. After annealing, a large attenuation of the GaN luminescence signal occurs, which suggests an absorbing effect of the ZnO layer both for the excitation signal 共at 351 nm兲 and the GaN luminescence signal, due to a lower band gap energy. On the other hand, in Fig. 3共b兲, the GaN peak is almost not reduced in intensity: there is no absorption effect as expected for a larger band gap overlayer. This hypothesis has been confirmed by optical transmission measurements on ZnO layers electrodeposited on transparent SnO2 /glass substrates. Band gap values of 3.5 and 3.3 eV have been determined before and after annealing, respectively. This confirms that as-deposited ZnO is transparent to exciting photons at 351 nm. This effect may be associated with the presence of chloride ions in the bath.18 Electrodeposition performed in the presence of nitrate ions19,20 or perchlorate ions21 lead to band gap values closer to 3.3 eV. In conclusion, epitaxial ZnO films can be deposited on GaN single crystals by an electrochemical method. The crystallographic quality appears to be very good, especially when considering the low temperature deposition and the relatively high growth rate conditions. Such results are related to the growth in the solution environment which, in comparison to the vapor phase, offers additional possibilities for atomic or-

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