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aPhys. Status Solidi C 11, No. 3–4, 428–431 (2014) / DOI 10.1002/pssc.201300533

Impact of potassium and water on the electronic properties of InN(0001) surfaces S. Reiß, A. Eisenhardt, S. Krischok*, and M. Himmerlich

Institut für Physik and Institut für Mikro- und Nanotechnologien, TU Ilmenau, PF 100565, 98684 Ilmenau, Germany

Received 20 September 2013, revised 8 November 2013, accepted 13 December 2013 Published online 19 February 2014

Keywords indium nitride, surface electronic properties, adsorption, potassium, water * Corresponding author: e-mail stefan.krischok@tu-ilmenau.de, Phone: +49 3677 69 3405, Fax: +49 3677 69 3365

In this work we investigate the interaction of potassium and water with 2×2 reconstructed InN(0001) surfaces prepared by plasma-assisted molecular beam epitaxy. The influence of adsorbate-substrate-interaction on sur-face properties is characterized in-situ by photoelectron spectroscopy. Potassium exposure leads to a strong re-duction in the work function Φ to 1.6 eV revealing a charge transfer from the adsorbate to the InN surface. In parallel, a reduction of the surface downward band bend-

ing by 0.2 eV and hence a reduced electron accumulation density is observed. While interaction of water with clean InN(0001)-2×2 surfaces induces only minor changes in the surface band bending, water adsorption at potassium covered InN(0001) leads to a reversal of the K-induced reduction in surface band bending and a slight increase of Φ to 2.4 eV. These results show that surrounding water modifies the interaction of potassium with InN(0001) surfaces.

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1 Introduction InN is a low band gap [1, 2] and high electron mobility [3] semiconductor with application po-tential for the use in optoelectronic and high frequency electronic devices. The often observed high surface elec-tron concentration [4, 5] is partially linked to the unique band structure of InN, which results in n-type behavior due to ionization of donor-type defects and impurities [6]. Re-cent studies demonstrate that particular surface configura-tions and surface preparation methods can influence the electron concentration in dependence of the crystal orienta-tion. For instance flat band conditions were found after growth for N-polar, and nonpolar InN [7, 8] or after sur-face preparation using HCl for InN(000-1) [9]. The control of the surface accumulation is a big challenge for the reali-zation of InN based devices. However, its tendency to form a surface electron accumulation layer and its biocompati-bility makes InN also an interesting material for biosensors [10-13]. For the implementation of InN in sensors and oth-er devices, the control and modification of the surface elec-tron density is a crucial point and demand a detailed under-standing of the influence of adsorbates on the surface accumulation layer and electronic properties of InN. Previ-ous experiments performing ozone-induced oxidation of

InN(0001) films [14, 15], anodic oxidation [16] as well as in-situ oxidation [17] have shown a depletion in electron concentration and also adsorbates containing strong elec-tronegative sulfur induce a comparable effect [18]. Earlier studies also reported on modifications of the electronic and optical properties of polycrystalline InN in aqueous eletro-chemical reactions [19, 20] and via water electrolysis [21].

Investigating the influence of adsorbed potassium in dry and aqueous environment is of great importance for bi-osensor applications, since K is involved in many cell me-tabolism processes. For other III-V semiconductors, stud-ies on K adsorption revealed modification of the surface electronic properties [22, 23], but the effect of K for the group III nitride material system is not well established: only one study exists that discusses comparable effects for the interaction of Cs and Ba with InN surfaces [24]. In this work we focus on the interaction of K with InN(0001) sur-faces, which exhibit a strong surface downward band bend-ing directly after growth [7]. Furthermore, the influence of water on the K-covered InN(0001) surface is investigated with a special focus on surface band bending and accumu-lation layer density as well as charge transfer mechanisms and the formation of surface dipoles.

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2 Experimental InN(0001) films with a 2×2 surface re-construction were prepared by plasma-assisted molecular beam epitaxy (PAMBE) on GaN/Al2O3(0001) templates in an UHV system (base pressure < 2×10-10 mbar) consisting of a growth chamber directly connected to an analysis chamber. For more details about the epitaxial growth see Refs. [7] and [25]. After growth, the samples were in-situ characterized by X-ray photoelectron spectroscopy (XPS) using monochromated AlKα (1486.7 eV) radiation and ul-traviolet photoelectron spectroscopy (UPS) using He I (21.2 eV) and He II (40.8 eV) radiation [26]. All experi-ments were performed in normal emission.

Potassium was offered by a commercial alkali dis-penser (SAES Getters) in a self-built evaporator. The po-tassium flux was kept constant during the experiments by controlling the dispenser current (5.2 A). Prior deposition, the device was carefully outgassed. Water was offered by backfilling the chamber (partial pressure of 2×10-9 mbar as monitored by a Bayard-Alpert ionization gauge). To ensure a maximum of purity, bidestilled water was used and cleaned by repeated freeze-pump-thaw-cycles prior the ex-periments. Furthermore, the composition of the inserted gas was examined by quadrupole mass spectrometry. One should note, that partial activation of the molecular water by hot filaments cannot be completely ruled out.

3 Results and discussion Photoemission experi-

ments performed directly after the epitaxial growth show the expected spectral features of InN and no indication for impurities like oxygen and carbon. Moreover, no indica-tions for existence of excess metallic indium or nitrogen defects at the surface were found. The electronic properties of the InN(0001)-2×2 surface are described in detail in Refs. [7] and [25]. Briefly, the work function of the as-grown film is 4.4 eV as extrapolated from the low kinetic energy onset of the UPS spectra. The position of the va-lence band maximum (VBM) is 1.4 eV below the Fermi energy EF. Taking into account the degenerated character of the grown InN films and the typical bulk electron con-centration of these films between 8×1018 and 1×1019 cm-3

[7, 25], the bulk Fermi level is ~0.2 eV [27] above the conduction band minimum (CBM). This parameter is not affected by surface adsorbates and consequently changes in the VBM and core level position are direct indicators for a variation in valence band bending Vbb which equals -0.5 eV for the as-grown InN(0001) surface [7].

Potassium was directly deposited onto the 2×2 recon-structed InN(0001) surface. Again XPS measurements did not indicate the presence of any unexpected ele-ments/impurities within the detection limit (~0.1 at.%), but a nonlinear increase of the potassium core level intensity is detected. The K layer thickness was calculated on the basis

Figure 1 XPS core level spectra of the In3d5/2 and N1s state of as-grown InN(0001), after deposition of ~1 ML K and subsequent water adsorption at the K-covered InN(0001) surface. of the measured intensities of the In3d5/2 and K2p core lev-el spectra using a two layer model. Both, valence band as well as the In4d, N1s and In3d spectra were investigated. The last two are shown in Fig. 1. For the as-grown surface, these states are located at binding energies (BE) of 17.5 eV (In4d), 396.5 eV (N1s) and 444.1 eV (In3d5/2). During po-tassium adsorption, a shift of the measured VBM and core level spectra to 0.2 eV lower BE is observed.

This shift of occupied electron states is associated with a reduction of VBB from the initial value of -0.5 eV to -0.3 eV. In parallel, the measured core level spectra be-come narrower and more symmetric (Fig. 1), indicating a reduction of influences caused by plasmon losses [28]. Both effects, the reduced VBB as well as the reduced plas-mon energy, are linked to a reduction in surface electron concentration. During continuous K deposition, the K2p3/2 peak is initially detected at 294.3 eV and shifts to 294.0 eV for higher coverage (see Fig. 3). Its shape undergoes slight changes due to alterations of the bonding character of the adsorbed potassium, which are indicated by changes in the work function as discussed later on. In addition, a new component is formed in the N1s core level spectra at 398.2 eV (Fig. 1, right). N-O bonds caused by adsorbates from the residual gas can be ruled out because their bind-ing energy would have a lower BE (typically ~397.4 eV) [29, 30] and an uptake of oxygen during the experiment is not detected. A possible explanation for this feature ac-cording to the measured BE and the deposition of K is the formation of potassium nitrogen bonds at the surface.

The coverage dependence of ΔVBB and changes in the work function Φ as determined by UPS are presented in Fig. 2. The work function decreases rapidly in the low coverage regime from initially 4.4 eV to 2.2 eV for 0.25 ML K/InN. Afterwards, Φ slowly decreases further to 1.6 eV. The strong reduction of Φ together with the re-duced surface downward band bending point to the forma-tion of a strong K-induced surface dipole [31] during po-tassium adsorption. The change in surface dipole (ΔΦdip)

430 S. Reiß et al.: Electronic properties of InN(0001) surfaces

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Figure 2 Absolute values and changes in work function ΔΦ and surface band bending ΔVBB in dependence on the K adlayer thickness and subsequent H2O exposure.

is a combined result of ΔVbb and ΔΦ, as visualized by the arrow in Fig. 2. Its positive pole is pointing towards the vacuum and therefore facilitates electron emission from the surface as one might expect for the adsorption of alkali at-oms with their low electron affinity. It is very likely that a complete ionization of the potassium takes place at least in the initial stage of K deposition and K+ is formed at the InN surface. With increasing coverage, the adsorption be-comes more and more neutral due to coulomb interaction between the positive potassium species leading to the gradual change in work function [32].

After the deposition of ~1 ML of potassium onto the InN(0001) surface (1 ML K/InN), the heterostructure was exposed to water. This experiment serves as simple model for an InN sensor surface exposed to K species in aqueous environment, as it is important for sensing biological sys-tems. Prior presenting and discussing the obtained data, previous results on the interaction of water with clean InN(0001) surfaces [17] are briefly summarized. Initially, water molecules interact strongly with the InN surface. The work function decreases from 4.4 eV to 4.0 eV for very low exposures up to ~0.5 L (adsorbate coverage < 0.1 ML). For higher doses the surface becomes less reactive and a slow increase to 4.2 - 4.3 eV is observed until the satura-tion coverage of ~1 ML (1012 L) is reached. At the same time a small effect on the surface band alignment occurs: exposure of 2×2 reconstructed InN(0001) surfaces to more than 10 L water leads to a shift of the VBM and the core level towards 0.1 eV lower BE, correlated with a slight de-crease in ΔVBB and surface electron density.

In contrast, water interaction with potassium-covered InN(0001) films leads to different results. It causes a shift of the VBM and core level states to 0.2 eV higher BE and therefore to the same positions as for the as-grown InN(0001) surface (see Fig. 1). The consequence is a downbending of the electronic bands at the surface of about 0.2 eV. Consequently, VBB is modified to the initial

value of -0.5 eV, associated with a re-increase of the sur-face electron concentration. This finding is in good agree-ment with the observed broadening and increase of asym-metrical shape of the core level spectra (Fig. 1) after water interaction. In parallel, water adsorption at the ~1 ML K/InN(0001) structureleads to changes in the valence band spectra monitored by UPS (not shown). These spectra re-veal the rapid formation of two new states at 9.8 eV and 5.5 eV. We assign them to hydroxide states, which have typically an energy distance of 3.0 - 4.5 eV (see Ref. [33] and references therein) and a peak area ratio (1:3), as ob-served in our case as well. Moreover, the formation of hy-droxide groups is also supported by the measured O1s core level spectrum. During water exposure, the state at 531.6 eV, labelled O1 in Fig. 3, rises quickly in intensity. Its binding energy can be interpreted to be an indication for the existence of hydroxide bonds when compared to earlier studies [34]. However, this is an indirect proof, since non-hydrogen containing adsorbates also resulted in O1s states in this energy range [17]. The fact that the reaction mecha-nisms and surface chemistry during water interaction at the ~1 ML K/InN structure is rather complex is also consoli-dated by the existence of a second feature at 529.8 eV (la-belled O2), which could be induced by the formation of in-dium, nitrogen or potassium oxides [13, 29, 35, 36]. Due to unknown details of surface chemistry in this part of the ex-periment we describe the observed effects in dependence of the water exposure in Langmuir instead of calculating an effective adlayer thickness.

Figure 3 XPS spectra of the K2p state measured during the dep-osition of potassium onto the 2×2 reconstructed InN(0001) sur-face (left) and of the O1s state during subsequent interaction of the ~1 ML K/InN(0001) interface with water.

After initiation of water exposure, the work function

undergoes a rapid increase from 1.6 eV to 2.3 eV (see Fig. 2) demonstrating the high reactivity between the alkali adsorbate and the water molecules. During continuous in-teraction, Φ is slowly further increasing to 2.4 eV. The combined increase of Φ and VBB results in a weakening of the still positive surface dipole. Thereby, negative [OH]-

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groups are predominantly formed and attached to the K-covered InN.

Obviously, water interaction has a strong influence on the electronic properties of the ~1 ML K/InN(0001) sur-face, whereas water adsorption at as-grown samples causes only minor changes [17]. The presence of potassium strongly influences the dissociation process of water at InN surfaces with consequences on the electronic properties. It is anticipated that the interaction of water and potassium and the resulting surface reaction influences the charge transfer at the InN(0001) surface. Understanding the un-derlying mechanisms that lead to changes in band align-ment and surface electron accumulation requires a detailed study on the surface reaction processes. These aspects are beyond the scope of this manuscript and are discussed for K adsorption at InN(0001) in Ref. [37]. The details of wa-ter/potassium interaction at the InN(0001) surface will be subject of a future publication. 4 Conclusion The influence of potassium and water ad-sorption on InN(0001) surfaces was investigated by XPS and UPS focussing on changes in the work function and surface band bending as well as adsorbate formation. Po-tassium forms a positive dipole at the surface and most likely donates its additional electron to the InN surface. At the same time it reduces the surface downward band bend-ing from -0.5 eV to -0.3 eV, synonymous with a reduction of the surface electron density. Subsequent water adsorp-tion leads to a reverse increase of the band bending to the initial value of -0.5 eV and formation of potassium hydrox-ide. Although a microscopic model for explanation of the examined changes is not established yet, this study clearly demonstrates that different adsorbates and combinations thereof have strong impact on the surface electronic prop-erties of InN which need to be understood for clarification of e.g. sensor device function.

Acknowledgements We are grateful for financial support by the DFG under grant SCHA435/25, the state of Thuringia via the graduate school “PhotoGrad” and the Carl-Zeiss-Stiftung.

References [1] V. Y. Davydov et al., Phys. Status Solidi B 299, R1 (2002). [2] R. Goldhahn, P. Schley, A. Winzer, G. Gobsch, V. Cimalla,

O. Ambacher, M. Rakel, C. Cobet, N. Esser, H. Lu, and W. J. Schaff, Phys. Status Solidi A 203, 42 (2006).

[3] V. M. Polyakov and F. Schwierz, Appl. Phys. Lett. 88, 032101 (2006).

[4] H. Lu, W. J. Schaff, L. F. Eastman, and C. E. Stutz, Appl. Phys. Lett. 82, 1736 (2003).

[5] I. Mahboob, T. D. Veal, C. F. McConville, H. Lu, and W. J. Schaff, Phys. Rev. Lett. 92, 036804 (2004).

[6] P. D. C. King et al., Phys. Rev. B 77, 045316 (2008). [7] A. Eisenhardt, S. Krischok, and M. Himmerlich, Appl. Phys.

Lett. 102, 231602 (2013). [8] C.-L. Wu, H.-M. Lee, C.-T. Kuo, C.-H. Chen, and S. Gwo,

Phys. Rev. Lett. 101, 106803 (2008).

[9] C.-T.Kuo, S.-C. Lin, K.-K. Chang, H.-W. Shiu, L.-Y. Chang, C.-H. Chen, S.-J. Tang, and S. Gwo, Appl. Phys. Lett. 98, 052101 (2011).

[10] N. Chaniotakis, N. Sofikiti, Anal. Chim. Acta 615, 1 (2008). [11] Y.-H. Chang, Y.-S. Lu, Y.-L. Hong, S. Gwo, and J. A. Yeh,

IEEE Sens. J. 11, 1157 (2011). [12] Y.-S. Lu, C.-L. Ho, J. A. Yeh, H.-W. Lin, and S. Gwo, Appl.

Phys. Lett. 92, 212102 (2008). [13] K.-W. Kao, M.-C. Hsu, Y.-H. Chang, S. Gwo, and J. A. Yeh,

Sensors 12, 7157 (2012). [14] V. Lebedev et al., J. Appl. Phys. 101, 123705 (2007). [15] V. Cimalla, V. Lebedev, C. Y. Wang, M. Ali, G. Ecke,

V. M. Polyakov, F. Schwierz, O. Ambacher, H. Lu, and W. J. Schaff, Appl. Phys. Lett. 90, 152106 (2007).

[16] A. Denisenko, C. Pietzka, A. Chuvilin, U. Kaiser, H. Lu, W. J. Schaff, and E. Kohn, J. Appl. Phys. 105, 033702 (2009).

[17] A. Eisenhardt, S. Reiß, M. Himmerlich, J. A. Schaefer, and S. Krischok, Phys. Status Solidi A 207, 1037 (2010).

[18] L. R. Bailey, T. D. Veal, C. E. Kendrick, S. M. Durbin, and C. F. McConville, Appl. Phys. Lett. 95, 192111 (2009).

[19] N. Asai, Y. Inoue, H. Sugimura, and O. Takai, J. Electro-chem. Soc. 146, 2365 (1999).

[20] Y. Inoue, A. Yamaguchi, T. Fujihara, J. Yamazaki, and O. Takai, J. Electrochem. Soc. 154, J212 (2007).

[21] K. S. A. Butcher, M. Wintrebert-Fouquet, P.P.-T. Chen. R. Wuhrer, and M. R. Phillips, Phys. Status Solidi C 2, 2293 (2005).

[22] K. O. Magnusson and B. Reihl, Phys. Rev. B 40, 7814, (1989).

[23] O. Pankratov and M. Scheffler, Phys. Rev. Lett. 70, 351 (1993).

[24] G. V. Benemanskaya, M. N. Lapushkin, S. N. Timoshnev, V. Yu. Davydov, and V. N. Zhmerik, Bull. Russ. Acad. Sci. Phys. 76, 309 (2012).

[25] M. Himmerlich, A. Eisenhardt, J. A. Schaefer, and S. Kri-schok, Phys. Status Solidi B 246, 1173 (2009).

[26] M. Himmerlich, S. Krischok, V. Lebedev, O. Ambacher, and J. A. Schaefer, J. Cryst. Growth 306, 6 (2007).

[27] P. Schley, R. Goldhahn, G. Gobsch, M. Feneberg, K. Thon-ke, X. Wang, and A. Yoshikawa, Phys. Status Solidi B 246, 1177 (2009).

[28] P. D. C. King, T. D. Veal, H. Lu, S. A. Hatfield, W. J. Schaff, and C. F. McConville, Surf. Sci. 602, 871 (2008).

[29] I. J. Lee, C.-J. Yu, T.-B. Hur, H.-K. Kim, C.-O. Kim, and J.-Y. Kim, J. Korean Phys. Soc. 49, 2176 (2006).

[30] V. Lebedev et al., J. Appl. Phys. 100, 094902 (2006). [31] V. M. Bermudez, J. Appl. Phys. 80, 1190 (1996). [32] A. Neumann, S. L. M. Schroeder, and K. Christmann, Phys.

Rev. B 51, 17007 (1995). [33] S. Krischok, O. Höfft, J. Günster, J. Stultz, D. W. Good-

mann, and V. Kempter, Surf. Sci. 495, 8 (2001). [34] L. Avalle, E. Santos, E. Lerva, and V. A. Macagno, Thin

Solid Films 219, 7 (1992). [35] G. Brodén, G. Pirug, and H. P. Bonzel, Chem. Phys. Lett.

73, 506 (1980). [36] M. Ayyoob and M. S. Hegde, Surf. Sci. 133, 516 (1983). [37] A. Eisenhardt, S. Reiß, S. Krischok, and M. Himmerlich,

Reduction of electron accumulation at InN(0001) surfaces via saturation of surface states by potassium and oxygen as donor- or acceptor-type adsorbates, submitted.