Structural and electrical characteristics of high quality ...

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G. Z. Xing,1,2,a) D. D. Wang,2 B. Yao,1,b) L. F. N. Ah Qune,2 T. Yang,1 Q. He,1 J. H. Yang,3 and L. L. Yang3. 1State Key Laboratory of Superhard Materials and ...
JOURNAL OF APPLIED PHYSICS 108, 083710 共2010兲

Structural and electrical characteristics of high quality „100… orientated-Zn3N2 thin films grown by radio-frequency magnetron sputtering G. Z. Xing,1,2,a兲 D. D. Wang,2 B. Yao,1,b兲 L. F. N. Ah Qune,2 T. Yang,1 Q. He,1 J. H. Yang,3 and L. L. Yang3 1

State Key Laboratory of Superhard Materials and Department of Physics, Jilin University, Changchun 130023, People’s Republic of China 2 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 3 Institute of Condensed Matter Physics, Jilin Normal University, Siping 136000, People’s Republic of China

共Received 22 May 2010; accepted 26 August 2010; published online 21 October 2010兲 We report on highly crystalline zinc nitride 共Zn3N2兲 thin films which were grown by rf magnetron sputtering on quartz substrates. The substrate temperature during growth is found to strongly affect the crystal quality of the thin films. The chemical bonding states were determined by x-ray photoelectron spectroscopy. Large chemical shifts in core-level N 1s peaks with binding energy of 396.4 eV were observed as compared to N 1s of free amine 共398.8 eV兲, indicating Zn–N bond formation. Two N 1s states were found: one is N1 formed by Zn–N bonds and another is 共N2兲 produced by substitution of N molecules at N ion sites, which leads to larger lattice constants, consistent with x-ray diffraction results. Temperature-dependent Hall effect measurements of our Zn3N2 films exhibited distinct conduction mechanisms at specific different temperature ranges. © 2010 American Institute of Physics. 关doi:10.1063/1.3493208兴 I. INTRODUCTION

The vast variety of interesting properties and potential applications of nitride semiconductor materials have lead to their tremendous growth over the last decade. Group III nitride semiconductors such as AlN, GaN, and InN are now common ingredients in optical and electronic devices.1–4 Zinc oxide 共ZnO兲 materials are also receiving ample attention because of their attractive properties, in particular, their wide band gap 共3.37 eV兲 and large exciton binding energy 共BE兲 共60 meV兲 at room temperature.5–8 Though Zn3N2 powder was first synthesized in 1940 by Juza and Hahn9 and was found to have an antiscandium oxide 共Sc2O3兲 structure, zinc nitride materials have received much attention only recently upon the demonstration of Zn3N2 films to produce p-type ZnO:N,5 which has obviously great potential in optoelectronics, electronics, and spintronics applications.10–12 These potentials, however, are yet to be capitalized, and few advances have been achieved using this Zn3N2 material. Zn3N2 films have since been grown by many techniques such as, to name but a few, magnetron sputtering,13–15 metal organic chemical vapor deposition,16 and molecular beam epitaxy.17 In previous studies, Zn3N2 films have shown electron carrier concentrations up to ⬃1020 cm−3, and relatively high electron mobilities of approximately 100 cm2 V−1 s−1.13 However, optical band gap measurements of Zn3N2 thin films, which give a measure of their optical absorption spectra, have yielded conflicting values such as 3.2 eV reported by Kuriyama et al.18 and Zong et al.,19 2.4 eV by Zhang et al.,20 1.23 eV by Futsuara,13 and 1.01 eV by Kazuaki Toyoura et al.21 Though being within the same order of magnitude, an a兲

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

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accurate and precise value of the band gap of Zn3N2 thin films is essential for their implementation in devices. An accurate measurement of the band gap of Zn3N2 requires the reproducible preparation of very high quality Zn3N2 samples as standard; a feat that has been rather elusive. The difficulty in obtaining high quality films resides in the difficulty to precisely control the growth conditions of Zn3N2. This in turn limits the elucidation of the still many undetermined properties and phenomena exhibited by Zn3N2 materials.18–21 Our aim is to develop a recipe to reproducibly prepare very high quality Zn3N2 films and study their significant structural and electrical properties. In the present work, we report the preparation of 共100兲 orientated-Zn3N2 thin films by using rf magnetron sputtering under various substrate temperatures. A comprehensive investigation of the morphology, structure, and chemical bonding states of the Zn3N2 films were carried out using atomic force microscopy 共AFM兲, x-ray diffraction 共XRD兲, Raman spectroscopy, and x-ray photoelectron spectroscopy 共XPS兲. Moreover, the electrical properties of optimally prepared Zn3N2 thin films were characterized using temperature-dependent Hall effect measurements. II. EXPERIMENTAL

Zn3N2 films were deposited on quartz substrates by reactive rf magnetron sputtering at a base pressure better than 1.5⫻ 10−4 Pa. The experimental setup was described in previous reports.22–24 Typically, a zinc target 共99.999%, sigma aldrich兲 was cleaned by chemical etching, mechanically polished, thoroughly washed in deionized water and blown dry in pure N2 gas, prior to being inserted in the vacuum chamber. N2 gas with purity of 99.999% was used as sputtering gas at a flow rate of 20 SCCM 共standard cubic centimeter per

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FIG. 1. 共Color online兲 XRD patterns of the Zn3N2 films grown at different substrate temperatures: 共a兲 423 K, 共b兲 498 K, 共c兲 573 K, 共d兲 648 K and 共e兲 723 K. Inset: AFM image 共2.6 ␮m ⫻ 2.2 ␮m兲 showing the morphology of Zn3N2 epilayer ⬃600 nm thick, grown at 498 K.

minute兲. The working pressure and sputtering power were kept at 1.0 Pa and 100 W respectively. In order to obtain high crystal quality, the effects of substrate temperature on crystal growth were systematically studied by depositing Zn3N2 films at various substrate temperatures, while other growth parameters such as sputtering power and working pressure were kept constant. Zn3N2 films were grown with the quartz substrates maintained at 423, 498, 573, 648, and 723 K by a heating element with direct contact behind the substrate holder. The structure of the films was characterized by XRD using Cu K␣1 radiation 共␭ = 0.154 18 nm兲. Further post annealing treatments 共PATs兲 were performed under the pressure of 1 ⫻ 10−4 Pa. Raman spectra were carried out with a WITEC CRM200 Raman system. The excitation source was a 532 nm laser, while the samples were mounted on a x-y piezostage. AFM images were obtained using a scanning probe microscope 共Veeco Digital Instruments兲 in tapping mode with Si probe tips. The chemical composition and bonding state of the films were analyzed by XPS using Al K␣ at a base pressure better than 10−8 Pa. Measured binding energies were referenced to aliphatic carbon peak at 284.6 eV. Samples were sputtered using Ar+ ions prior to XPS measurements to remove any surface contamination.25–28 Electrical properties were measured using a Lakeshore’s 7707 Hall measurement system by a Van der Pauw four-point configuration at a temperature range of 85–400 K. To compensate for various electromagnetic effects, compiled data were averaged over both positive and negative regions of currents and magnetic fields. Ohmic contacts were made using indium electrodes annealed under a pressure of 10−3 Pa for 20 min at 573 K. III. RESULTS AND DISCUSSION

Figure 1 shows XRD patterns of produced thin films deposited at different substrate temperatures. All observed diffraction peaks correspond to Zn3N2 peaks;29 no Zn and ZnO peaks were observed within detection limit. XRD analysis determined the Zn3N2 films to be polycrystalline films. Figures 1共a兲–1共e兲 indicate the formation of single

J. Appl. Phys. 108, 083710 共2010兲

FIG. 2. 共Color online兲 共a兲 Raman spectrum of as-grown Zn3N2 film vs annealed sample. Growth temperature was 498 K. Raman spectrum obtained by a 532 nm laser, annealing pressure: 1 ⫻ 10−4 Pa. Typical XPS narrow scan of 共b兲 N 1s and 共c兲 Zn 2p of the Zn3N2 film deposited at 498 K.

phase Zn3N2 films with crystal orientation strongly dependent on the substrate temperature. A strong and sharp 400 peak is observed in all patterns, indicating the Zn3N2 films have a preferred 100 orientation. All 400 peaks shift toward lower diffraction angles relative to standard powder values.29 The substrate temperature has a remarkable influence on the structural characteristics as observed from the relative XRD peak ratios at various temperatures 共Fig. 1兲. We presume that, at low substrate temperatures, particles nucleating at the substrates would have insufficient kinetic energies to undergo diffusion.30 Furthermore, with an activation energy corresponding to 423 K, nitrogen would have insufficient energy at 423 K to react with zinc, leading to a low yield of Zn-N bond formation. It is reasonable to assume some amount of free N2 molecules would be formed on the surface and be deposited along with Zn3N2. Higher growth temperatures would increase the rate of Zn-N bond formation and there would exist an optimum temperature at which the conditions, along with vacuum base pressure, fluence of sputtering gas 共N2兲 and sputtering power, would yield the highest Zn3N2 bond formation at the ideal stoichiometry. Raising the growth temperature beyond that optimum would upset the unstable balance of Zn3N2 formation and decomposition: at growth temperatures higher than their activation energies, the already formed Zn3N2 could further react with incoming species of Zn and N, as well as undergo Zn–N bond breaking, in turn introducing a large number of localized point defects in the film’s crystallinity.30 The inset of Fig. 1 shows an AFM image of the Zn3N2 film grown at 498 K as compact and uniform grains with a surface roughness of ⬃7 ⫾ 0.5 nm. As illustrated in Fig. 2共a兲, compared to our as-grown Zn3N2 film grown at 498 K, no shifts in the characteristic 257 and 565 cm−1 Raman-active vibration modes20 were observed during PATs processes. This indicates our thin films do not undergo strains resulting from lattice mismatch between films and substrates, and any difference among their lattice constants during XRD measurements is inherent to their growth at their respective temperatures. Figures 2共b兲 and 2共c兲 show XPS results obtained for a sample grown at 498 K. Figure 2共b兲 shows Zn 2P3/2 with a BE of 1020.7 eV. This is close to 1022.0 eV measured by Futsuhara et al.13 No interstitial zinc 共Zni兲 in ZnO state was observed.22 Figure 2共c兲 depicts the N 1s peak consisting of two main compo-

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TABLE I. Room temperature electrical properties of Zn3N2 films grown at 498 K in different measuring magnetic fields.

FIG. 3. FWHM and d spacing values estimated by Gaussian fits of 共800兲 XRD Zn3N2 peaks plotted against substrate temperature.

nents: a higher intensity N1s peak found at a BE of 396.4 eV and a lower intensity peak at BE= 404.9 eV. The higher intensity component is found to undergo a large chemical shift in 2.4 eV toward lower BE relative to free amine referenced at 398.8 eV. This indicates the formation of very strong ionic bonds within Zn3N2, as compared to the chemical shifts in other nitride compounds such as BN 共0.7 eV兲, AlN 共1.4 eV兲, and GaN 共1.7 eV兲.13 The lower intensity N 1s peak found at a higher BE of 404.9 eV indicating the presence of nitrogen 共N2兲 molecules.12 The d spacing values and full width at half maximum 共FWHM兲 were determined from the high diffraction angle of 共800兲 peaks. Figure 3 shows d spacing values and FWHM undergoing similar changes as a function of substrate temperature, with both quantities decreasing to a minimum as the substrate temperature 共during growth兲 is increased from 423 to 498 K, followed by a monotonous increase as the substrate temperature is increased beyond 498 K. We hereby combine our XRD, Raman spectroscopy and XPS results to explain this phenomenon. Since our Raman spectroscopy results indicate there is no increase or relief in stress during PATs, and our XPS results suggest only Zn from the clean zinc target and N from the sputtering gas are present on a clean Ar+ sputtered thin film 共therefore, the change in lattice constant is not due to impurities within the thin film兲, this leads us to believe free N2 molecules exist within the lattice of our grown Zn3N2 thin films, leading to the larger lattice constant as measured by XRD, owing to their large radius.12 This is supported by our XPS results which indicate the presence of molecular nitrogen 共N2兲. Our XPS results further indicate all zinc existing in Zn2+ state, hence the increase in d and broadening in FWHM at temperatures other than 498 K does not result from a different stoichiometry of the zinc nitride film. This implies Zn3N2 films grown at 498 K contain the least amount of free N2 molecules within the lattice, in turn suggesting that 498 K offers the optimum growth

Measuring field 共G兲

Resistivity 共⍀ cm兲

Carrier type

3 ⫻ 103 6 ⫻ 103 9 ⫻ 103

2.60⫻ 10−2 2.60⫻ 10−2 2.60⫻ 10−2

n n n

Mobility Concentration 共cm2 V−1 s−1兲 共cm−3兲 7.74⫻ 1018 1.21⫻ 1019 1.78⫻ 1019

31.0 19.8 13.5

condition with Zn3N2 films grown with near-ideal stoichiometry and hence having the best crystal quality. Temperature-dependent Hall effect and electrical transport measurements were conducted to characterize the electrical properties of the Zn3N2 films grown at 498 K. Ohmic contacts were made by vacuum-sintering small indium dots at four corners of square-shaped samples. Excellent linearity was found from the I-V curves, indicating good Ohmic contact between the Zn3N2 and indium electrodes. The magnitude of the magnetic field was 3, 6, and 9 ⫻ 103 Gauss, and the optimal current was determined automatically by the Hall system. The results of room temperature Hall effect measurements under different magnetic fields are listed in Table I. The Zn3N2 films showed n-type conduction because of their negative Hall coefficients. The resistivity was relatively low with typical value at about 2.60⫻ 10−2 ⍀ cm and the electron charge carrier concentration ne did not vary significantly with the magnetic field. It might be needful to note that there might be an increase in the electron charge carrier concentration from additional electron charge carriers originating from defects such as unintentional nitride vacancy 共VN兲 in the Zn3N2 film.17 Figure 4 shows the relationship of electrical conductivity ␴ as a function of temperature. The Zn3N2 film behaves like a nonintrinsic semiconductor with the conductivity ␴ decreasing as a function of the temperature. Generally, there exist two factors controlling the conductivity of nonintrinsic semiconductors, the first being ionized impurities and intrinsic excitations, the second being two scattering mechanisms involving ionic impurities and lattice scatterings.31 As shown in the inset of Fig. 4, the nonlinear relationship of ␴ with temperature is evident between 85 and 400 K, pointing to two distinct conductive mechanisms in different temperature ranges. In region I, referring to low

FIG. 4. 共Color online兲 Arrhenius plot of conductivity for Zn3N2 film grown at 498 K. The inset depicts the temperature dependence of conductivity for Zn3N2 film grown at 498 K, measured between 85 and 400 K. Solid lines show the theoretical simulation and solid circles show experimental data points.

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ionization temperature 共T ⬍ ⬃ 275 K兲, the scattering events largely depend on ionized impurities, as their numbers increase with rising temperature, hence increasing the mobility. The concentration of such impurities being limited, the increase in conductivity with respect to an increase in temperature is rather moderate and results in a linear relationship between conductivity and temperature as observed. At high temperature however, i.e., in region II 共T ⬎ ⬃ 275 K兲, all impurities are deemed to be ionized. Hence excitations of intrinsic valence electrons to the conduction band leads to an abundant generation of charge carriers, easily exceeding the effect of reduced mobility due to lattice vibrations. The conductivity thus increases exponentially as a function of temperature as observed. For intrinsic semiconductors, the relationship between conductivity ␴ and temperature can be expressed by

␴ = 2共kBT/2␲ប2兲2/3共memh兲3/4e共␮e + ␮h兲exp共− Eg/2kBT兲, 共1兲 where me and mh are the electron and hole effective mass, while ␮e and ␮h are the electron and hole mobility respectively, e being the electronic charge, T the absolute temperature, and kB the Boltzmann’s constant. For a nonintrinsic semiconductor, the relationship between ␴ and temperature is more complicated. However, at higher temperatures 共region II兲, the relationship between conductivity ␴ and temperature can be approximated to an exponential term since the linear term 共region I兲 would then be negligible. Therefore, whether being an intrinsic or a nonintrinsic semiconductor, the relationship between conductivity ␴ and temperature can be expressed in the form ␴ = A exp共−Eg / 2kBT兲, where A is a proportionality constant. The band gap Eg can then be estimated by measuring the relationship between ␴ and T using the above equation. To adopt the nonlinear curve fitting, ␴ versus T can be expressed as ␴ = ␣T3/2 in region I and ␴ = A exp共a / T兲 in region II respectively, where ␣ , A and a are all constants. From our results and assumptions, we therefore estimate the band gap Eg of our Zn3N2 film to be 1.01 eV. This is closest to the value previously reported by Toyoura et al.,21 and indicates the Zn3N2 film behaves as a typical narrow band gap semiconductor.

IV. CONCLUSIONS

In summary, Zn3N2 共100兲 films of high crystal quality were deposited on quartz substrates by rf magnetron sputtering. The effects of substrate temperature on the structure of Zn3N2 film were studied. It was found that Zn3N2 films obtained at 498 K have the best crystal quality. XPS N 1s peak 共396.4 eV兲 for Zn3N2 indicates the formation of N–Zn bonds. Zn3N2 films show high carrier concentrations and demonstrated different conductive mechanisms in the temperature range of 85–400 K. The band gap Eg was estimated to be 1.01 eV by temperature-dependent Hall effect measurements. Our high quality Zn3N2 films have been shown to be a promising candidate for nitride materials based optical and electronic devices.

ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China under Grant Nos. 50532050, 60776011, 60506014, 10674133, 60806002, and 10874178, the “97” program under Grant No. 2006CB604906 and National Found for Fostering Talents of Basic Science under Grant No. J0730311. 1

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