Chemical Characterization of ZnO Films Pulsed Laser Deposited on InP

To obtain oxygen-stoichiometric films, ZnO films were prepared on InP(100) by means of pulsed laser deposition using a two-stage oxygen-pressure proce...
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J. Phys. Chem. C 2007, 111, 3505-3511

3505

Chemical Characterization of ZnO Films Pulsed Laser Deposited on InP E. Vasco,* O. Bo1 hme, and E. Roma´ n Instituto de Ciencia de Materiales de Madrid, Consejo Superior de InVestigaciones Cientı´ficas, Cantoblanco, 28049 Madrid, Spain ReceiVed: September 22, 2006; In Final Form: December 1, 2006

The composition profile and the chemical states of ZnO films deposited on InP(100) were investigated by complementary chemical characterization techniques (namely, Rutherford backscattering spectroscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy). To obtain oxygen-stoichiometric films, ZnO films were prepared on InP(100) by means of pulsed laser deposition using a two-stage oxygen-pressure procedure. Such a procedure, which consists in an initial growth stage where the growth of the first monolayers takes place at oxygen pressures as low as 10-5 mbar followed by a stationary growth stage at higher oxygen pressure (10-1 mbar), is revealed as suitable to keep the oxygen composition without deteriorating the crystalline quality of the ZnO films nor degrading the substrate surface. The influence of the oxygen pressure used during the stationary growth stage on the optical properties in the visible wavelength range of the deposited films is examined by specular reflectance spectroscopy. Controlling the oxygen composition makes the system ZnO/InP(100) a promising candidate for the implementation of integrated short-wavelength photonic devices.

Introduction A great deal of effort developed by the scientific community during the last years has gone into improving ZnO properties as an UV light emitter.1 ZnO is a II-VI semiconductor with wurtzite crystalline structure, wide band gap (3.37 eV), and stable UV lasing action due to the large binding energy (≈60 meV) of its exciton. It is a promising candidate for a new generation of short-wavelength optoelectronic devices (e.g., light-emitting diodes and laser diodes)2 reliable at room temperature (RT). The latter represents a meaningful progress with regard to other wide band gap semiconductors with green-blue excitonic laser emission such as ZnSe and GaN with substantially lower exciton binding energies (22 and 25 meV, respectively). Moreover, ZnO exhibits additional attractive functional properties as follows:3 (i) doped with Al4 and/or In,5 it is used as transparent electrode in solar cells, (ii) doped with 3dtransition metals (as diluted magnetic semiconductor), it exhibits interesting ferromagnetic properties for Spintronics,6 whereas (iii) doped with Li, it shows ferroelectric activity,7 (iv) ZnO is also a well-known piezoelectric material used in surface acoustic wave devices,8 and (v) it exhibits remarkable optical features as a second harmonic generator9 and waveguide.10 In this context, the ZnO integration with bulk direct band gap semiconductors such as InP results in a particularly attractive way to implement optical integrated circuits (OICs).11 Despite the preliminary progress of externally pumped RT UV lasing from epitaxial ZnO thin films on single-crystalline sapphire substrates, the accurate control of the ZnO film stoichiometry is still one of the prime pending challenges:12-14 The use of ZnO in light-emitting and laser diodes requires a reduced intrinsic concentration of free electrons within the ZnO film, counting on the possibility of inverting the carrier concentrations so that a p-type conductivity is achieved.15 * To whom correspondence [email protected].

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Furthermore, unlike the lasing action based in electron-hole plasma processes, the excitonic lasing does not require a high carrier concentration. Usually, as-grown ZnO exhibits n-type conductivity at RT due to a large concentration of free electrons experimentally ascribed16,17 to oxygen non-stoichiometry (i.e., oxygen vacancies) in reducing atmospheres.14 These vacancies, which are native point defects in ZnO, would originate shallow donor levels partially ionized at RT.14 As previously demonstrated, this results in a compensation effect18 on doping with p-type impurities.15,19 Therefore, among stoichiometric defects within ZnO, the oxygen deficiencies are revealed as the key factor for tuning the reliability and versatility of forthcoming ZnO-based devices.13 Consequently, several studies focused on how to tailor the oxygen stoichiometry in vapor-deposited ZnO films by means of different growth conditions,13 designed insitu deposition procedures,12,20 and/or postdeposition processing have been executed.21 In particular, those procedures12,20 that propose the use of a two-stage oxygen-pressure growth seem to be suitable to control the oxygen stoichiometry without degrading the crystalline quality of the ZnO films nor damaging the substrate surface. The InP surface is unstable at moderate temperatures in oxygenenriched atmospheres: above 365 °C, P evaporates preferentially22 and the remaining surface In oxidizes into In2O3-x.22,23 The two-stage oxygen-pressure growth consists of the following: (i) an initial growth stage for the formation of early nuclei and the growth of the first monolayers at oxygen pressures as low as 10-5 mbar and subsequently, (ii) a stationary growth stage (second stage hereafter) taking place at higher oxygen pressures (not higher than 10-1 mbar). In contrast, films deposited directly at pressures as high as those used in the stationary stage show poor crystalline qualities,20 rough surface morphologies,20 low electron mobility, and poor luminescence properties.12 Besides, the in-situ deposition procedures12,20 exhibit remarkable advantages with respect to postdeposition annealing to control the oxygen stoichiometry of films thicker

10.1021/jp0662294 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/03/2007

3506 J. Phys. Chem. C, Vol. 111, No. 8, 2007 than 100 nm due to the reduced oxygen diffusivity in crystalline ZnO at temperatures lower than 1000 °C.24 Notwithstanding the relative success of the proposed twostage oxygen-pressure procedure, there is so far no thorough chemical characterization by high sensitivity techniques (such as X-ray photoelectron spectroscopy, XPS, or Auger electron spectroscopy, AES) that confirms its healing effect. The present work intends to fill this knowledge gap. Here, we investigate with complementary characterization techniques (such as Rutherford backscattering spectroscopy (RBS), AES, and XPS) the composition and chemical state profiles of ZnO thin films prepared via pulsed laser deposition at different oxygen pressures on thermally annealed InP substrates. Special attention is paid to the chemical states at the ZnO/InP interface taking into consideration the nature of the involved InP native oxides. In addition, from comparing RBS with AES/XPS results, the yield for a potential phenomenon of preferential oxygen sputtering as induced by film eroding for depth profiling studies (with a 3 keV Ar+ ion beam) is estimated. The thus-estimated magnitude of such a preferential sputtering phenomenon allows us to differentiate between extrinsic and intrinsic oxygen deficiencies. Experimental Section The ZnO thin films were grown on semi-insulating Fe-doped InP(100) substrates by pulsed laser deposition. The following parameters correspond to the used experimental setup: KrF excimer laser emitting 16-ns UV (λ ) 248 nm) pulses at a frequency of 10 Hz was focalized onto a 20-rpm rotating ZnO ceramic target at a power density of 5 J/cm2. Prior to sample preparation, the substrate surface was degreased and washed using trichloroethylene, acetone, and isopropanol and then dried on a flux of highly pure nitrogen. Subsequently, the substrate was promptly introduced into the deposition chamber, which is at nitrogen overpressure to avoid potential contaminations. The InP(100):Fe substrates were placed 60 mm opposite the target and held at 350 °C during the entire growth. The deposits were carried out within a high-vacuum chamber (base pressure ) 1 × 10-7 mbar) in which a flux of high-purity molecular oxygen (99.999%) was introduced through a leak valve until reaching dynamic pressures ranging between 10-5 to 1 mbar.25 Independently of the nominal deposition pressuresas reported for each samplesthe first regime of all the investigated growths (which corresponds to the initial growth stage as described above) was performed at the lowest working pressure (1 × 10-5 mbar), and just after 300 pulses (≈3-4 nm deposited), the pressure was quickly increased (with a stabilization time e30 s) up to reach its deposition value. The thus-grown films exhibit (as reported in ref 20) the followingsindependently of nominal deposition pressure: a wurtzite structure with (0001)ZnO//(100)InP outof-plane preferential orientation, no in-plane order, and deposition rates ranging between 0.10 and 0.13 Å/pulse (higher as the oxygen pressure increases). RBS spectra were measured with a Van der Graaf accelerator, using a 1.6 MeV He+ ion beam. Back-scattered ions were detected by means of two surface barrier detectors of 12 and 18 keV energy resolution tilted 180° and 140° from the beam direction, respectively. Simulations of the RBS spectra were performed with RUMP software.26 The sample grain structure was investigated by scanning electron microscopy (SEM) in cleaved samples. Cross-sectional images were taken using an ISI DS-130C high-resolution microscope operating at 19 kV. AES and XPS measurements were carried out in an ultrahighvacuum chamber (base pressure ) 1 × 10-10 mbar), equipped

Vasco et al. with a Perkin-Elmer ESCA/Auger spectrometer based on a double pass cylindrical mirror analyzer. The AES and XPS emissions were excited using a 3 keV primary electron beam and Mg KR X-ray radiation, respectively. The narrow scan XPS spectra (detailed spectra, hereafter) were acquired at an analyzer pass energy of 50 eV, providing an energy resolution of 1 eV. The so-collected signals are processed as follows: AES Spectra. The relative stoichiometry of each of the involved elements is estimated from the peak-to-peak amplitude of its specific Auger transition in the derivative signal. Amplitudes were normalized with their standard elemental sensitivity factor.27 XPS Spectra. The spectra were subjected to a background subtraction formalism.28 Before analyzing the XPS data, the contribution of the Mg KR3,4 satellite lines were removed. Only once, in the narrow scan of the P 2p spectrum, the Zn 3s KR3 satellite lines were not removed, to have a clear demonstration that no misinterpretation due to signal overlap might have occurred. The peak fit was performed using Gaussian functions, which is justified because of the limited resolution (1.0 eV) of the used energy analyzer. The energy range was calibrated using a pure gold standard as reference.29 No significant charging effects were detected due to the moderate electrical conductivity of the analyzed samples whose resistivities range between 7 × 10-3 and 9 Ω‚cm for oxygen pressures rising between 10-5 and 10-1 mbar. Depth profiling was carried out with a 3 keV Ar ion beam over an area of 6 × 10 mm2 and a total current of 0.5 µA, which was chosen to minimize any phenomenon of preferential sputtering of light elements (e.g., oxygen). The erosion rate under these conditions and the depth uncertainty due to the nonflatness of the erosion front were estimated30 to 1 Å/µA-min and 10% of the penetration depth, respectively. The normal spectral reflectance measurements were carried out in a double-beam UV-vis-NIR spectrophotometer Varian Cary 5E equipped with a tungsten halogen lamp emitting in the visible wavelength range. The reflectance spectra were recorded at RT with a photomultiplier tube detector R928 PMT by using an absolute specular reflectance accessory and subsequently baseline corrected. Results and Discussion Figure 1 shows the RBS spectra of two ZnO films deposited at 350 °C in oxygen dynamic pressures of 10-5 and 10-1 mbar (top and bottom curves, respectively). The data simulation, whose results are plotted as solid traces overlapping the corresponding data (symbols), reveals that the film grown at the higher oxygen pressure is stoichiometric whereas that deposited at the lower pressure is largely oxygen-depleted (up to 30%) as estimated from the atomic O/Zn ratio which is lower than that expected for stoichiometric ZnO. X-ray diffraction results (not shown here) allow us to confirm that the structure of the crystalline bulk of the oxygen-depleted film corresponds to that of wurtzite ZnO. The grain structure of each one of these films (as imaged by cross-sectional SEM) is shown in the closer inset. The higher-pressure stoichiometric film exhibits a compact columnar structure, with grain boundaries running up along the direction of growth. In contrast, the lower-pressure nonstoichiometric films show a noncolumnar structure characterized as a less compact background of assembled equiaxed grains whose boundaries spreading out, in average, along the in-plane and out-of-plane directions. Thus, the shift of the band at ≈1.0 MeV, which is ascribed to buried In, toward higher kinetic energies of the back-scattered He+ ions for oxygen-depleted

ZnO Films Pulsed Laser Deposited on InP

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Figure 1. RBS spectra of two ZnO films deposited at 350 °C in oxygen pressures of 10-5 and 10-1 mbar (top and bottom curves, respectively). The symbols represent the collected experimental data whereas the solid traces correspond to their best simulations, which were achieved from the sketched configurations (i.e., average stoichiometry and film thickness). The surface (Zn and O) and buried (In) contributions of the involved elements to the RBS spectra are indicated. Inset: crosssectional SEM images of the analyzed samples.

ZnO is the result of two factors: (i) a smaller film thickness due to the oxygen-pressure dependence of the deposition rate and (ii) the resulting less compact structure. The worsening of the crystalline quality of the ZnO films, likely a result of the decrease in average grain size, as the oxygen pressure drops has been previously reported.20 The increase in the density of grain boundaries for the non-stoichiometric films would suggest that such boundaries correspond mostly to the oxygen-depleted regions, such that the depletion phenomenon would be spatially nonuniform. AES survey spectra measured at different depths within a lower-pressure 36-nm thick film are shown in Figure 2a. The spectra are sorted from the top to the bottom in function of the analysis depth referred to the ZnO/InP interface. The spectrum labeled as “surface” (approximately -36 nm) corresponds to the pre-erosion surface of the ZnO film. It shows evidence of environmental contamination, for example, the contribution of surface carbon. The film stoichiometry was estimated from peakto-peak amplitudes of those labeled Auger transitions.27 Figure 2b shows the composition profiles obtained by AES of two ZnO/ InP systems with ZnO films deposited at different oxygen pressures, namely, 10-5 (open symbols) and 10-1 mbar (solid ones). In good agreement with the results achieved by RBS (Figure 1), the bulk of the lower-pressure film is ≈25% oxygendepleted, whereas the film deposited at 10-1 mbar is fairly ((3%) stoichiometric. The stoichiometry of the lower-pressure film is partially recovered a few nanometers above the interface within that bulk region grown during the initial growth stage (first 300 pulses). The width of the ZnO/InP interface is estimated from the narrowest transient of the analyzed profiles as the difference between the corresponding depths to 10 and 90% of the stationary composition.30 Thus, a minimum interface width of ≈4 nm was calculated from the Zn profile. This value agrees with the depth uncertainty (expected as 10% of the penetration depth). Larger widths could be associated with interdiffusion across the interface, except in the case of oxygen where a 6-nm width transient suggests (as demonstrated below) that the growth takes place on an oxidized InP surface rather than on a bare one. In any case, the composition profiles in

Figure 2. (a) Series of AES spectra collected at increasing depths (referred to the ZnO/InP interface) within a 36-nm thick ZnO film deposited at 350 °C in 10-5 mbar. The characteristic AES emissions of the involved elements are denoted, except that at 215 eV, which corresponds to implanted Ar from the Ar+ ion beam used for depth profiling. (b) AES-derived composition profiles of two ZnO films deposited at 350 °C in oxygen pressures of 10-5 and 10-1 mbar (open and solid labeled symbols, respectively). The interface and oxygen stoichiometry transient regions (both with widths of 4 and 6 nm, respectively) are indicated by vertical dotted lines. The solid and dashed curves serve as visual guides.

Figure 2b clearly demonstrate the absence of massive interdiffusion in the growth/system studied here. XPS survey spectra collected at different depths (i.e., film surface, film bulk, interface, and substrate bulk denoted as a, b, c, and d, respectively) are shown in Figure 3. The labels identify the characteristic peaks, whereas the dotted frames enclose Auger emissions in the XPS spectra. The fact that the C 1s peak does neither appear beyond the film surface along the thickness nor appear at the ZnO/InP interface indicates that the deposition atmosphere is contaminant-free and the surface adsorption of carbon occurs when the sample is exposed to room conditions. Detailed XPS spectra (within the ranges: 1016-1028 eV for Zn 2p3/2; 527-535 eV, O 1s; 441-449, In 3d5/2; and 124-137 eV, P 2p) of a higher-pressure (10-1 mbar) ZnO film are shown in Figures 4 (Zn 2p and O 1s) and 5 (In 3d and P 2p). The spectra were taken at different depths, as labeled, and they are compared (excluding Zn 2p) with those recorded from the surface of uncovered InP(100), which was prepared as a standard substrate (i.e., heated up to 350 °C at 1 × 10-5 mbar) except for the fact that the growth was not carried out. Consequently, the latter is denoted as the “pre-deposition InP surface”. It should

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Figure 3. Series of XPS spectra collected at different labeled depths within a 36-nm thick ZnO film deposited at 350 °C in 10-1 mbar. The characteristic XPS emissions of the involved elements are labeled, whereas some AES emissions (also labeled) are enclosed within dotted frames.

be stressed that the spectra of the respective surfaces shown in Figures 4 and 5 (i.e., all those labeled as “surface”) correspond to spectra acquired once the carbon contamination was removed. The contributions obtained from fitting the identified peaks were compared (in terms of binding energies and full widths at half-maximum, fwhm) with those summarized in Table 1. The comparison is performed in a qualitative way, because of the limited resolution, which does not allow an accurate separation of contributions with binding energy differences lower than 0.5 eV. This limitation is reinforced by unspecified offsets (see functions φ in Table 1) of the binding energies as a result of stoichiometric gradients and/or oxygen-deficient compounds. The Zn 2p3/2 peak is shown in Figure 4a. This peak shows a single contribution with binding energy (fwhm) of 1021.9 eV (2.3 eV), which can be attributed to slightly oxygen-deficient ZnO (i.e., ZnO1-x).31,32 On the other hand, the O 1s peaks (Figure 4b) exhibit a large number of contributions: Three chemical states with binding energies (fwhm) of 530.3 eV (1.95 eV), 531.9 eV (1.7 eV), and 532.6 eV (2.4 eV) are resolved at the carbon-free ZnO surface. The contribution at lower binding energy is ascribed to O2- ions within the hexagonal ZnO matrix.32 The contribution at intermediate binding energy can be connected with O2- ions in oxygen-deficient regions.32 The fact that this contribution is detected within the compact bulks of ZnO films grown at temperatures as high as 700 °C (as those reported in ref 20) discards that such a contribution is connected with adsorbed -OH species; not that way, the higher binding energy contribution, which is attributed to chemisorbed and/or dissociated -OH and OH‚‚‚O type of surface species.32,33 This third contribution disappears in the spectra collected within the ZnO film bulk and at the ZnO/InP interface. The presence of the two contributions (at 530.3 and 531.9 eV) implies that the oxygen deficiencies are spatially nonuniform such that stoichiometric and non-stoichiometric regions alternate. The ratio of the areas of both contributions (i.e., Area531.9/Area530.3) is used

Figure 4. 0.1 eV/step XPS spectra collected at different labeled depths within a 36-nm thick ZnO film (350 °C, 10-1 mbar) showing details of the peaks: (a) Zn 2p3/2 and (b) O 1s. The vertical, dashed lines serve to compare the binding energies of the contributions identified from the peak fit with those predicted (Table 1).

to estimate the oxygen deficiencies to be e6%. This reduced concentration of oxygen deficiencies is not intrinsic but originates during depth profiling with Ar+ ions. This statement is supported by further analysis (not shown here), which showed an increase in the deficiency ratio as the Ar+ ion beam current rises (9% at 1 µA). Such extrinsic deficiencies would originate from preferential oxygen sputtering with an enhanced yield in grain boundary regions and thus are spatially nonuniform. Preferential oxygen sputtering during XPS depth profiling also explains why such deficiencies have not been detected in the analysis performed by RBS, even when taking into account the reduced sensitivity of this latter technique for the detection of non-stoichiometries of light elements. On the contrary, the AES spectra shown in Figure 2 are insensitive to extrinsic oxygen deficiencies as low as that detected since the atomic fractions are computed with regard to normalized Zn/O atomic ratio. Hence, for the stoichiometric film with extrinsic oxygen deficiencies of R ) 6% (Zn1O1-R), the Zn atomic fraction [1/(2 - R) ≈ 51.5%] is included within the estimated error bar ((3%). Besides, it is feasible to suppose that the preferential sputtering effect on the oxygen concentration is minor in largely oxygen-depleted ZnO films, as those deposited at 10-5 mbar. The O 1s peak collected at the ZnO/InP interface can be decomposed into three main contributions at 530.3 ev (2.0 eV), 531.5 eV (1.9 eV), and 531.9 eV (1.5 eV). These contributions cannot be connected unambiguously with particular chemical states due to the complexity of the pre-deposition surface covered with InP native oxides (see the peak fit at the bottom of Figure 4b). Thus, the lower binding energy contribution is interpreted as the result of combining two minor

ZnO Films Pulsed Laser Deposited on InP

J. Phys. Chem. C, Vol. 111, No. 8, 2007 3509 Figure 5a shows the In 3d5/2 peak. Two major contributions can be seen at 444.8-444.4 eV (1.8 eV) and 445.4 eV (2.0 eV), each comprising several non-isolable subcontributions. The contribution at lower binding energy results from the combination of at least two subcontributions: (i) In3+ ascribed to InP (nominally23 at 444.4 eV (1.17 eV)) and (ii) In3+ bonded with O2- forming oxygen-deficient In2O3-x,23,31 whose nominal binding energy shifts from 444.8 eV to 444.4 as the amount of oxygen drops. Thus, at the pre-deposition surface, the latter (In2O3-x) is the major subcontribution, whereas the peak shifting toward lower binding energies and its shrink (fwhm decreases from 1.8 to 1.4 eV) in the ZnO-covered substrate indicate that the former (InP) prevails. This implies that oxygen-deficient In2O3-x reduces in the presence of Zn bonded with oxygen vacancies, likely during the initial growth stage (first 300 pulses at 10-5 mbar). The contribution at 445.4 eV is attributed to the group of polyphosphates.23 This contribution, which is detected in the pre-deposition surface, remains at the ZnO/InP interface exhibiting just slight changes in the ratio of the contributing oxides, as revealed by the Gaussian function broadening. In the pre-deposition surface, the P 2p peak (Figure 5b) can be decomposed into two main fitting contributions at 128.8 eV (1.8 eV) and 133.9 eV (300 nm) where the refractive index exhibits a thickness-independent value. The solid and dashed curves serve as visual guides. Inset: specular reflectance spectrum of a 1.4-µm thick ZnO film (350 °C, 10-1 mbar). The symbols, the solid curve and the dotted one correspond to the experimental data points, their best fit using the modified envelope method36 and the simulated reflectance spectrum for an infinite stoichiometric ZnO film, respectively.

using the following parameters and assumptions: (i) The InP optical constants reported elsewhere37 were fitted to Sellmeiner’s equations. (ii) The film thicknesses estimated from the corresponding deposition rates and tested by cross-sectional SEM in cleaved samples are used as input data. (iii) Possible compositional and/or crystalline inhomogeneities within the ZnO films were ignored. (iv) Due to the moderate absorption of ZnO within the investigated wavelength range, the wavelength dependence of its extinction coefficient was neglected. One of the fitted spectra (with symbols and solid trace being the experimental data points and their best fit, respectively) is depicted in the inset of Figure 6. The fitting deviation around 400 nm reveals that the fourth assumption (see above) ceases to be valid for wavelengths within near to the band gap region (nominally ≈390 nm). The oxygen-pressure dependence of the averaged ordinary refractive index (Figure 6) shows an increase in the refractive index (and consequently, of the reflectivity) as the oxygen content in the film drops. Such an increase takes place in a moderate way until oxygen pressures reach as low as 10-3 mbar and in an extreme manner for lower pressures. An explanation of this effect based on the evolution of the film grain structure with the decreasing oxygen pressure is not plausible since lower refractive indexes are expected for the less compact structures. So, the observed behavior (i.e., the increase in refractive index as the oxygen pressure drops) is due to a probable decrease of the material effective band gap as a result of an increase in density of the shallow donor levels ascribed to increasing concentration of oxygen vacancies. Conclusions The ZnO films deposited according to the proposed procedure (comprising the two oxygen-pressure stages) are stoichiometric,

Vasco et al. highly crystalline-as previously reported in ref 20sfree of massive interdiffusions, and exhibit a compact columnar structure. The concentration of oxygen vacancies in these films is strongly reduced. A detected rest level of oxygen vacancies (max 6%) can be clearly ascribed to an Ar+ ion beam-induced phenomenon of preferential oxygen sputtering and thus correspond to nonintrinsic deficiencies. Oxygen non-stoichiometries related to the low oxygen pressure (10-5 mbar) used during the initial growth stage were not detected. This stage rather promotes In2O3-x reduction by excess Zn and enhances the chemical reactivity of phosphorus, whereas the rest of the native oxides in the pre-deposition InP surface (e.g., the polyphosphates) are not affected. This initial growth stage has to be followed by a second stationary stage of higher oxygen pressure. Using only one oxygen-pressure stage for the entire film growth resulted in poor film quality. On the one hand, this is because extending the low oxygen-pressure condition (initial stage) for the entire growth results in large oxygen depletion and poor crystalline quality. On the other hand, a poor crystalline quality is also the consequence of skipping the low oxygen-pressure stage and starting the growth directly at high oxygen-pressure stage (second one). In particular, the largely oxygen-depleted films show a noncolumnar structure and an ordinary refractive index within the visible wavelength range higher than that of stoichiometric columnar ZnO films. Acknowledgment. E.V. acknowledges and appreciates the support from the Spanish Education and Science ministry under the Ramo´n y Cajal program. References and Notes (1) Cao, H.; et al. Phys. ReV. Lett. 2000, 84, 5584. Huang, M. H.; et al. Science 2001, 292, 1897. Scharrer, M.; et al. Appl. Phys. Lett. 2006, 88, 201103. Zhang, Z.; et al. J. Phys. Chem. B 2006, 110, 8566. (2) Aoki, T.; Hatanaka, Y.; Look, D. C. Appl. Phys. Lett. 2000, 76, 3257. Ohta, H.; et al. Appl. Phys. Lett. 2003, 83, 1029. Tsukazaki, A.; et al. Nat. Mater. 2005, 4, 42. (3) Ozgur, U.; et al. J. Appl. Phys. 2005, 98, 041301. (4) Gupta, A. A.; Compaan, A. D. Appl. Phys. Lett. 2004, 85, 684. (5) Xu, L. A.; et al. J. Phys. Chem. B 2006, 110, 6637. (6) Kittilstved, K. R.; Liu, W. K.; Gamelin, D. R. Nat. Mater. 2006, 5, 291. (7) Ni, H. Q.; et al. Appl. Phys. Lett. 2001, 79, 812. Dhananjay; Nagaraju, J.; Krupanidhi, S. B. J. Appl. Phys. 2006, 99, 034105. (8) Wu, T. T.; Wang, W. S. J. Appl. Phys. 2004, 96, 5249. Liu D. S.; et al. Jpn. J. Appl. Phys. 2006, 45, 3531. (9) Johnson, J. C.; et al. Nano Lett. 2004, 4, 197. Prasanth, R. R.; Van Vugt, L. K.; Vanmaekelbergh, D.; Gerritsen, H. C. Appl. Phys. Lett. 2006, 88, 181501. (10) Huang, H. B.; et al. J. Phys. Chem. B 2005, 109, 20746. Leong, E. S. P.; Yu, S. F.; Abiyasa A. P.; Lau, S. P. Appl. Phys. Lett. 2006, 88, 091116. (11) Shim, E. S.; et al. Mater. Sci. Eng. B 2003, 102, 366. Bang, K. H.; et al. Appl. Surf. Sci. 2003, 210, 177. (12) Choopun, S.; et al. Appl. Phys. Lett. 1999, 75, 3947. (13) Jeong, S. H.; Kim, B. S.; Lee, B. T. Appl. Phys. Lett. 2003, 82, 2625. Chichibu, S. F.; et al. J. Appl. Phys. 2006, 99, 093505. (14) Yu, Z. G.; Gong, H.; Wu, P. Chem. Mater. 2005, 17, 852. (15) Routbort, G. W.; Mason, T. O. J. Appl. Phys. 2000, 87, 117. Ma, Y.; et al. J. Appl. Phys. 2004, 95, 6268. (16) On the basis of experimental results reported in ref 15 (oxygenpressure dependence of the n-type conductivity) without being theoretically confirmed (see ref 17). (17) Janotti, A.; Van de Walle, C. G. J. Cryst. Growth 2006, 287, 58. (18) Independently of the shallow or deep naturescurrently under discussionsof the donor levels originated by the oxygen vacancies, their compensation effect is considered proven (see refs 15 and 17). (19) Joseph, M.; et al. Jpn. J. Appl. Phys. 1999, 38, L.1205. Ryu, Y. R.; et al. J. Cryst. Growth 2000, 216, 330. Look, D. C.; Claftin, B. Phys. Status Solidi B 2004, 241, 624. (20) Vasco, E.; et al. J. Vac. Sci. Technol., B. 2001, 19, 224. (21) Chu, S. Y.; Water, W.; Liaw, J. T. J. Eur. Ceram. Soc. 2003, 23, 1593. Yadav, H. K.; Sreenivas, K.; Gupta, V. J. Appl. Phys. 2006, 99, 083507.

ZnO Films Pulsed Laser Deposited on InP (22) Farrow, R. F. C. J. Phys. D 1974, 7, L121. Wagner, J. F.; Wilmsen, C. W. J. Appl. Phys. 1982, 58, 5798. (23) Hollinger, G.; Bergignat, E.; Joseph, J.; Robach, Y. J. Vac. Sci. Technol., A 1985, 6, 2082. (24) Sabioni, A. C. S.; Ramos, M. J. F.; Ferraz, W. B. Mater. Res. 2003, 6, 173. (25) Oxygen pressures on the order of 10-4 mbar are avoided because the system of differential vacuum is unstable for dynamic pressures within this range. (26) Doolite, L. R. Methods Phys. Res. B. 1985, 9, 334. (27) The following relative elemental Auger sensitivities for a 3 keV primary electron beam were taken from L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach, and R. W. Weber: Handbook of Auger Electron Spectroscopy Physical Electronics Industries Inc.: Minnesota, 1976; p 13, 0.50 for O KLL emission at 503 eV; 0.17 for Zn LMM at 994 eV; and 0.52 for P LMM at 120 eV. The relative sensitivity for In MNN at 404 eV was computed as 0.23 from assuming a stoichiometric substrate bulk.

J. Phys. Chem. C, Vol. 111, No. 8, 2007 3511 (28) Shirley, D. A. Phys. ReV. B 1972, 5, 4709. (29) Fuggle, J. C.; Kallne, E. L.; Watson, M.; Fabian, D. J. Phys. ReV. B 1977, 8, 759. (30) Vasco, E.; et al. Chem. Mater. 2001, 13, 1061. Vasco, E.; Bo¨hme, O.; Roma´n, E. Zaldo, C. Appl. Phys. Lett. 2001, 78, 2037. (31) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder J. F.; Muilemberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corporation: Minnesota, 1979. (32) Cao, H. T.; et al. J. Solid State Chem. 2004, 177, 1480. (33) Chen, M.; et al. Appl. Surf. Sci. 2000, 158, 134. (34) Wolan, J. T.; Hoflund, G. B. J. Vac. Sci. Technol., A 1998, 16, 2546. (35) Hollinger, G.; et al. Surf. Sci. 1986, 168, 617. (36) Rusli; Amaratunga, G. A. J. Appl. Opt. 1995, 34, 7914. Mu¨llerova´, J.; Mundron, J. Acta Phys. SloVaca 2000, 50, 477. (37) Aspnes, D. E.; Studna, A. A. Phys. ReV. B 1983, 27, 985.