APPLIED PHYSICS LETTERS
VOLUME 78, NUMBER 17
23 APRIL 2001
Electron affinity of Alx Ga1À x N„0001… surfaces S. P. Grabowski, M. Schneider, H. Nienhaus,a) and W. Mo¨nch Laboratorium fu¨r Festko¨rperphysik, Gerhard-Mercator-Universita¨t Duisburg, D-47048 Duisburg, Germany
R. Dimitrov, O. Ambacher, and M. Stutzmann Walter-Schottky-Institut, Technische Universita¨t Mu¨nchen, D-85748 Garching, Germany
共Received 12 October 2000; accepted for publication 23 February 2001兲 The electronic properties and the electron affinities of Alx Ga1⫺x N共0001兲 surfaces were investigated by ultraviolet photoemission spectroscopy 共UPS兲 over the whole composition range. The samples were prepared by N-ion sputtering and annealing. Surface cleanliness and stoichiometry were monitored with x-ray photoemission spectroscopy. Samples with high aluminum content showed traces of oxygen which could not be removed by further cleaning cycles. However, we have evidence that the oxygen is located in the bulk and not at the surface. From the UP spectra the ionization energies and electron affinities as a function of composition x were determined. A decrease in electron affinity with increasing aluminum content was found, but the electron affinity remains positive for all x. Thus, earlier predictions of negative electron affinity for high aluminum content were not confirmed. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1367275兴
The search for semiconductor materials which exhibit a negative electron affinity 共NEA兲 has put the III nitrides into the spotlight lately. The electron affinity is defined as the difference between the vacuum level E vac and the conduction-band minimum E c 共see Fig. 1兲. If becomes negative, that is, E vac lies below the conduction-band edge, any electron that is excited into the conduction band has enough energy to leave the crystal. This no-barrier electron emission has potential for cold-cathode electron emitter applications such as field emitters and flat-panel displays. A so-called effective NEA has been demonstrated for p-GaN共0001兲 surfaces after Cs adsorption.1 In this case, the vacuum level lies only below the conduction-band minimum in the bulk regime E cb , but not below the value at the surface E cs . Then, electrons can only leave the crystal after a ballistic transport to the surface and transmission through the top most dipole layer. In the case of a true NEA, when E vac lies below the conduction-band minimum E cs right at the surface, every electron that thermalizes to the bottom of the conduction band in the surface regime can still leave the solid, since they encounter no barrier. According to Benjamin et al.,2 Alx Ga1⫺x N crystals with high Al content show such a true negative electron affinity. This prediction was based solely on an interpolation from data of AlN and AlGaN samples with low Al content and the appearance of a peak with high intensity at the low-energy onset of photoemission spectra. However, it has been demonstrated that the observation of such a peak is not sufficient evidence of a NEA.3 Moreover, the interpretation of Benjamin et al. is in contrast to results which clearly demonstrate a positive electron affinity for AlN.4,5 In this study we present a systematic investigation of the electron affinity of AlGaN共0001兲 surfaces over the whole composition range in order to elucidate whether or not a NEA at these surfaces occurs. a兲
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The GaN samples were purchased from Cree Research, where they were grown by metalorganic chemical vapor deposition on sapphire. All the other samples were grown at the Walter-Schottky-Institut by plasma-induced molecular beam epitaxy on sapphire substrates. Details about the growth process and the sample quality can be found elsewhere.6 All samples are III polar. This was tested on GaN by atomic force microscopy after a KOH etch has been applied.7 In case of AlGaN, the polarity was determined by locating the two-dimensional electron gas in a AlGaN/GaN heterostructure through the capacitance–voltage profiling technique.8 The samples were cleaned by a standard two-step procedure, developed for GaN.9,10 First, they were dipped in 50% aqueous HF solution, rinsed with de-ionized water and blown dry with N2. Then, in vacuo, repeated cycles of N⫹-ion sputtering at kinetic energies of 3 keV for 10 min and annealing at 800 °C were applied. The annealing step had to be conducted in a Ga flux in order to compensate a loss of surface Ga. It has been shown that for AlGaN alloys a linear interpolation between the decomposition temperatures of GaN 共850 °C兲11 and AlN 共1100 °C兲11,12 is not permitted. Instead, a Ga loss was observed even for Alx Ga1⫺x N samples at least up to x⫽0.75 when heated to temperatures exceeding 800 °C.13 X-ray photoemission spectroscopy 共XPS兲 with Mg(K ␣ ) radiation (ប ⫽1253.6 eV) was applied to monitor surface cleanliness and stoichiometry. The ionization energy I of each sample, which is defined as the difference between E vac and the top of the valence-band E v , was determined by ultraviolet photoemission spectroscopy 共UPS兲 excited with He–I radiation (ប ⫽21.2 eV) from the width W EDC of the respective energy distribution curve 共EDC兲 according to W EDC⫽ប ⫺I.
共1兲
As can be seen in Fig. 1, the ionization energy and the true electron affinity are correlated by
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Appl. Phys. Lett., Vol. 78, No. 17, 23 April 2001
FIG. 1. Schematic drawing of the conduction-band minimum (E c ) and valence band maximum (E v ) in the bulk 共subscript b兲 and at the surface 共subscript s兲. The ionization energy I, the electron affinity , the vacuum level E vac , and the Fermi energy E F are also shown. If E vac lies below E cs , one speaks of true negative electron affinity 共NEA兲.
⫽I⫺E g ,
共2兲
where E g is the respective energy band gap. If E g is known, one can deduce the electron affinity from the ionization energy. The amount of oxygen after our preparation depends on the Al mole fraction as can be seen in Fig. 2. Shown there is the intensity ratio of the O(1s) and N(1s) XPS lines as a function of aluminum content x. Surfaces with low Al content show no or little contamination with oxygen, but with increasing x the amount of oxygen still present after preparation increases. However, we have evidence that the oxygen is located in the bulk and not at the surface. If one measures the O contamination after sputtering but before annealing, one finds a drastic reduction in the O/N intensity ratio, but after annealing, the value increases again. This is a hint that the oxygen is at first reduced by sputtering but than migrates from the bulk regime to the surface during annealing. This assumption has been stated by other groups as well5 and is supported by the fact that dislocations in the crystal are con-
FIG. 3. Electron affinity of Alx Ga1⫺x N共0001兲 surfaces. A decrease of with increasing Al content x is observed and the values remain positive for all x. The dashed line is meant to guide the eye. Symbol 䊉 represents our own data, 䉮: data from Ref. 1, 䊐: data from Ref. 2, 丣 : data from Ref. 5, 䉭: data from Ref. 9, 䊊: data from Ref. 16, and ⫻: data from Ref. 17.
taminated with oxygen. These serve as diffusion paths for oxygen. By assuming that the oxygen is equally distributed in the bulk layers, one can estimate the maximum amount of oxygen present in the top most layer, i.e., at the surface, by applying a simple layer model14 as 3% of a monolayer. Figure 3 depicts the electron affinity of AlGaN alloys as a function of aluminum content. These values were obtained using Eq. 共2兲 from the ionization energies measured with UPS. For the band gap of Alx Ga1⫺x N a band bowing E g 共 Alx Ga1⫺x N兲 ⫽x•E g 共 AIN兲 ⫹ 共 1⫺x 兲 •E g 共 GaN兲 ⫺b•x• 共 1⫺x 兲
共3兲
was considered. The band gaps of GaN and AlN at room temperature are 3.4 and 6.2 eV, respectively. The bowing parameter b was determined as 1.3 eV at a set of samples that were grown by the same method with the same parameters as the ones employed in this study.8 The literature values for b, with the exception of some groups who report negative values, range from b⫽0.53 to 2.6 eV.15 This difference in band bowing can be explained by different strain in the respective layers and different defect densities due to different growth methods.15 The presence of possible surface or defect states renders it difficult to determine the position of the valence band maximum 共VBM兲 and, thus, the ionization energy and the electron affinity.5 Therefore, extra caution has to be used when analyzing the UP spectra. Usually, one determines the position of the VBM by fitting a tangent to the high kinetic energy end of the photoemission spectra. The point of intersection of the tangent with the intensity base line marks the high energy end of the EDC and, thus, the VBM. Surface states might affect the tangent-fitting procedure. Therefore, the tangent has to be fitted in an area where the influence of surface states is negligible. To find out where this is, we compared UP spectra of clean samples with spectra recorded after oxygen exposure.13 It was found that oxygen removes the surface states and changes the photoemission spectra slightly at the very high kinetic energy end. Ignoring the spectral features due to surface states enables us to find a tangent which is a reasonable fit to the VBM. However, one
FIG. 2. Intensity ratio of the O(1s) and N(1s) XPS lines measured with Mg(K ␣ ) radiation (ប ⫽1253.6 eV) and the related O concentration at the surface as a function of Al content for Alx Ga1⫺x N共0001兲. The dashed line is meant to guide the eye. Downloaded 02 May 2001 to 134.91.160.35. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
Grabowski et al.
Appl. Phys. Lett., Vol. 78, No. 17, 23 April 2001
can still find several tangents which are reasonable approximations of the data, but slightly differ in slope. This results in different VBM positions represented by the errors bars in Fig. 3. Despite the data scatter, a decrease of with increasing Al content is definitely observed and the electron affinity remains positive for all x. Good agreement of our experimental data with literature values is found with the exception of the values from Refs. 4, 5, and 17. For GaN we obtain an average value from different samples of 共GaN兲⫽3.1⫾0.2 eV the literature values range from 3.2 to 3.6 eV.1,2,9,16,17 The values of Benjamin et al.2 共䊐 data in Fig. 3兲 provided for AlGaN with various compositions follow nicely the trend of our experimental data, whereas the electron affinities determined by Kozawa et al.17 共⫻ data in Fig. 3兲 are generally larger than our values. For the electron affinity of AlN, Wu and Kahn5 found a value of 1.9 eV, which is much larger than our average value of 共AlN兲⫽0.25⫾0.3 eV. Benjamin et al.2 determined zero electron affinity. The discrepancy between our data and the values from Refs. 5 and 17 might be caused by oxygen contamination at their samples, since oxygen adsorption was found to increase the ionization energy, and thus the electron affinity, of AlN and AlGaN.13 From these results we can draw the following conclusions. First, the electron affinities of these surfaces do not seem to depend on growth technique or substrate layer, since various growth procedures as well as substrate materials were used, but the values, with the exception of the data points from Refs. 5 and 17, follow the same trend. Second, none of the samples show a true negative electron affinity. The prediction by Benjamin et al. was based on an interpolation which was inspired by their electron affinity of AlN which was zero. This is the smallest value that can be experimentally determined. Even when the electron affinity is negative, photoexcited electrons still have to overcome the gap energy, resulting in an ionization energy which equals E g and thus, delivering an electron affinity that is zero and not negative according to Eq. 共2兲. However, our results show that remains positive. In summary, Alx Ga1⫺x N共0001兲 surfaces were cleaned by N⫹-ion sputtering and annealing. The amount of oxygen still present after cleaning was found to depend on the Al content; the higher the Al mole fraction the higher the amount of residual oxygen. Oxygen is believed to be located
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in the bulk and the surface contamination is estimated to be smaller than 3% of a monolayer. From the ionization energies measured with UPS the electron affinities over the whole composition range were determined. A decrease in with increasing Al content x was found, but the electron affinity remains positive for all x. Thus, earlier predictions of NEA for AlGaN were not confirmed. Financial support by the Deutsche Forschungsgemeinschaft under Contract No. Mo 318/28-2 is gratefully acknowledged. 1
M. Eyckeler, W. Mo¨nch, T. U. Kampen, R. Dimitrov, O. Ambacher, and M. Stutzmann, J. Vac. Sci. Technol. B 16, 2224 共1998兲. 2 M. C. Benjamin, M. D. Bremser, T. W. Weeks, Jr., S. W. King, R. F. Davis, and R. J. Nemanich, Appl. Surf. Sci. 104Õ105, 455 共1996兲; M. C. Benjamin, C. Wang, R. F. Davis, and R. J. Nemanich, Appl. Phys. Lett. 64, 3288 共1994兲; R. J. Nemanich, P. K. Baumann, M. C. Benjamin, S. W. King, J. van der Weide, and R. F. Davis, Diamond Relat. Mater. 5, 790 共1996兲. 3 J. E. Yater and A. Shih, Appl. Surf. Sci. 143, 219 共1999兲. 4 V. M. Bermudez, T. M. Jung, K. Doverspike, and A. E. Wickenden, J. Appl. Phys. 79, 110 共1996兲; V. M. Bermudez, C. I. Wu, and A. Kahn, ibid. 89, 1991 共2001兲. 5 C. I. Wu and A. Kahn, Appl. Phys. Lett. 74, 546 共1999兲; C. I. Wu, A. Kahn, E. S. Hellman, and D. N. E. Buchanan, ibid. 73, 1346 共1998兲. 6 H. Angerer, D. Brunner, F. Freudenberg, O. Ambacher, M. Stutzmann, R. Ho¨pler, T. Metzger, E. Born, G. Dollinger, A. Bergmaier, S. Karsch, and H.-J. Ko¨rner, Appl. Phys. Lett. 71, 1504 共1997兲; D. Brunner, H. Angerer, E. Bustarret, F. Freudenberg, R. Ho¨pler, R. Dimitrov, O. Ambacher, and M. Stutzmann, J. Appl. Phys. 82, 5090 共1997兲. 7 M. Seelmann-Eggebert, J. L. Weyher, H. Obloh, H. Zimmermann, A. Rar, and S. Porowski, Appl. Phys. Lett. 71, 2635 共1997兲. 8 M. J. Murphy, B. E. Foutz, K. Chu, H. Wu, W. Yeo, W. J. Schaff, O. Ambacher, L. F. Eastman, T. J. Eustis, R. Dimitrov, M. Stutzmann, and W. Rieger, MRS Internet J. Nitride Semicond. Res. 4S1, G8.4 共1999兲. 9 V. M. Bermudez, J. Appl. Phys. 80, 1190 共1996兲. 10 H. Nienhaus, C. Schepers, S. P. Grabowski, and W. Mo¨nch, Appl. Phys. Lett. 77, 403 共2000兲. 11 O. Ambacher, M. S. Brandt, R. Dimitrov, T. Metzger, M. Stutzmann, R. A. Fischer, A. Miehr, A. Bergmaier, and G. Dollinger, J. Vac. Sci. Technol. B 14, 3532 共1996兲. 12 C. B. Vartuli, S. J. Pearton, C. R. Abernathy, J. D. MacKenzie, E. S. Lambers, and J. C. Zolper, J. Vac. Sci. Technol. B 14, 3523 共1996兲. 13 M. Schneider, Diploma thesis, University of Duisburg, 1999. 14 R. Memeo, F. Ciccacci, C. Mariani, and S. Ossicini, Thin Solid Films 109, 159 共1983兲. 15 S. R. Lee, A. F. Wright, M. H. Crawford, G. A. Petersen, J. Han, and R. M. Biefeld, Appl. Phys. Lett. 74, 3344 共1999兲, and references therein. 16 C. I. Wu, A. Kahn, N. Taskar, D. Dorman, and D. Gallagher, J. Appl. Phys. 83, 4249 共1998兲. 17 T. Kozawa, T. Mori, T. Ohwaki, Y. Taga, and N. Sawaki, Jpn. J. Appl. Phys., Part 2 39, L772 共2000兲.
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