Cobet, Christoph

Esser, Norbert

Goldhahn, Rüdiger

Rakel, Munise

Digitale Bibliothek Thüringen

     Cobet, Christoph; Rakel, Munise; Goldhahn, Rüdiger; Esser, Norbert:  Conduction band properties of III-nitrides characterized by synchrotron ellipsometry on core level excitations  Zuerst erschienen in: Highlights / Annual report 2005 // Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H. - Berlin : BESSY, 2006, S. 212-214  Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H.BESSY - Annual Report2005Published by:Berliner Elektronenspeicherring-Gesellschaftfür Synchrotronstrahlung m.b.H. (BESSY)Albert-Einstein-Str.1512489 BerlinTel. +49 30 6392 2999Fax +49 30 6392 2990 info@bessy.dewww.bessy.deEdited by:Dr. Kai GodehusenConduction band properties of III-nitrides characterized bysynchrotron ellipsometry on core level excitationsC. Cobet1, M. Rakel1,2, R. Goldhahn3, and N. Esser11ISAS- Institute for Analytical Sciences, Department Berlin, Albert-Einstein-Str. 9, D-12489 Berlin2Institut fu¨r Festko¨rperphysik, TU Berlin, Hardenbergstr. 36, D-10623 Berlin3Inst. f. Physik, Zentrum f. Mikro- und Nanotechnologien, TU Ilmenau, D-98684 IlmenauGroup III nitrides such as Ga-, Al- and In-nitride and compounds are a class of semicon-ductors with one of the highest technological potential today. But in contrast to the recentadvances in epitaxial growth and technology, the understanding of fundamental material prop-erties is surprisingly poor. Many questions concerning the electronic band structure are stillopen. A prominent example is the ongoing discussion about the exact value of the fundamen-tal band gap of InN. A mayor problem for theoretical calculations is the correct treatment ofIn4d/Ga3d-core electrons, which slightly interact with the nitrogen 2s valence electrons. In GaNthis interaction is much weaker than in InN. The s-d-coupling is difficult to handle, because itresults also in an additional p-d repulsion effect [2]. This coupling is may be responsible forthe very low band gap of InN. The determination of the relative energetic positions betweenthe valence and conduction bands could provide already helpful information [7]. But a com-plete experimental analysis of electronic transitions (excitations) is mandatory for testing andvalidating theoretical results. Common methods are either photoemission experiments, whichdetermine e.g. the valence band DOS with respect to the Fermi level, or optical reflection andabsorption measurements, which reveal interband electronic transitions in the visible and VUVspectral range.We use the synchrotron light at BESSYFigure 1: Imaginary part of the measured DF of hexagonaland cubic GaN (solid) in comparison to the calculated PDOSof the p-like empty states around the Ga-atoms (patterned)[10]. The Ga3d transition structures are denoted with 1DIII−5DIII according to the notation of Cardona et.al. [3].II in order to access the optical propertiesof GaN and InN in the far VUV spec-tral range by spectroscopic ellipsometry[4, 11]. This measurement technique isbased on the determination of the polar-ization of light before and after the reflec-tion on the sample surface under a cer-tain angle of incidence. The measuredamplitude and phase differences can betranslated subsequently into the dielec-tric function (DF) or the commonly usedrefractive and absorption coefficient.Above 18 eV the DF of GaN and InNis dominated by optical transitions be-tween the Ga3d/In4d core states and thep-like unoccupied (conduction) electronbands. Transitions to conduction stateswith s-character are not allowed due tothe dipole selection rules. However, thelocalized d-states have a sharp bindingenergy without significant dispersion in the entire Brillouin zone. Thus, these d-levels canbe used to explore the site specific density of empty states in a similar manner as known fromX-ray absorption on the N1s core level edge [8]. The stable crystal structure of III-nitrides isthe hexagonal wurtzite lattice where the hexagonal closed packed III-nitride layers are stackedin an ABAB... sequence. Thus, the electronic and optical properties reveal an extraordinaryaxis, the c-axis, parallel to the stacking sequence. All other perpendicular directions remain de-generated. The metastable cubic zincblende crystal structure of GaN was successfully stabilized212in thin films on appropriate cubic substrates [1]. The electronic as well as optical properties ofthis cubic material are assumed to be isotropic. However, the integrated density of conductionband states should be almost identical in both crystal structures. This is, in fact, observed inour ellipsometric measurements on a wurtzite and zincblende GaN sample. Figure 1 shows theimaginary part of the DF in the spectral range of Ga3d transitions. The DF is proportionalto the number of excited electrons and should directly relate to the partial density of states(PDOS) with dominant p-character in the conduction bands. Both line shapes are almost thesame. The lower overall amplitude and the broadening of transition features are related to thelower crystal quality of the cubic sample.We compare these measurements inFigure 2: Imaginary part of the ordinary and extraordinaryDF of GaN between 18 and 27eV measured on M-plane GaN[1-100].figure 1 also to the calculated PDOS ofp-like states on the Ga site (patterned)of cubic GaN. As assumed, the DF is ap-proximately reproduced by the calculatedPDOS although excitonic effects as wellas k and energy dependent variations ofmatrix elements could influence the DF.The DF of wurtzite GaN was determinedon a c-plane sample, where the c-axis isperpendicularly oriented to the surface.In this orientation we have determinedthe ordinary DF, which relates to exci-tations with the electric field vector per-pendicular to the c-axis. In order to ac-cess also the extraordinary DF we use M-plane GaN [0100] sample, where the c-axis lies in the surface plane.By measuring in the two high sym-Figure 3: Imaginary part of the measured DF of hexagonalInN (solid) in comparison to the calculated PDOS of the p-like empty states around the In-atoms (patterned) [5]. TheIn4d transition structures are denoted with 1DIII −6DIII .All these valence band characteristic features split up due toa spin orbit splitting of the In4d-states, which is notable e.g.in the 2DIII and2DIII + ∆d structure.metry orientations we can determine bothdielectric tensor components which are pre-sented in figure 2. According to these me-asurements we observe a reasonable shiftto lower energies and a slightly differentline shape of the extraordinary DF whilethe ordinary component is almost identi-cal to the measurements on the c-planesample. This shift could be explainedby a distortion of the chemical bondingsalong the c-axis. Polarization effects aswell as a charge accumulation at inter-faces along the extraordinary crystal axisare already known in III nitrides.Figure 3 shows a comparison betweenthe imaginary part of the ordinary DF ofwurtzite InN and the calculated PDOS ofp-like InN conduction band states. Be-tween 16 and 28 eV the DF is again dom-inated by excitations of In4d core-states.The line shape of the DF and the assumed PDOS are nearly identical to the respective GaNmeasurements but shifted by about 2 eV to lower energies. This shift correlates to the lowerband gap of InN in comparison to GaN. Recent photoemission experiments report a binding213energy of the In4d5/2 electrons of 17.4±0.1 eV with respect to the valence band maximum [9].On the other hand, we can determine the onset of In4d transitions to conduction bands at about18 eV (fig. 3). The comparison of both measurements further support the new results about aInN band gap at 0.68 eV [6].We gratefully acknowledge the BESSY assistance and the financial support of the BMBF underthe contract number 05 KS4KTB/3.[1] D. J. As, D. Schikora, and K. Lischka. Phys. Stat. Sol. (c), 0(6):1607–1626, 2003.[2] B. Bouhafs, F. Litimein, Z. Dridi, and P. Ruterana. Phys. Stat. Sol. (b), 236(1):61–81, 2003.[3] M. Cardona, W. Gudat, E. E. Koch, M. Skibowski, B. Sonntag, and P. Y. Yu. Phys. Rev. Lett., 25(10):659–661, 1970.[4] N. Esser, M. Rakel, C. Cobet, W. G. Schmidt, W. Braun, and M. Cardona. phys. stat. sol. (b), 242(13):2601–2609, 2005.[5] F. Fuchs, 2004. Institut fu¨r Festko¨rpertheorie und -optik, Friedrich-Schiller-Universita¨t Jena, Fro¨belstieg 1,07743 Jena, Germany, priv. com.[6] R. Goldhahn, P. Schley, A. T. Winzer, G. Gobsch, V. Cimalla, O. Ambacher, M. Rakel, C. Cobet, N. Esser,H. Lu, and W. J. Schaff. phys. stat. sol. (a), 203(1):42–49, 2006.[7] W. R. L. Lambrecht, B. Segall, M. Yoganathan, W. Suttrop, P. Davaty, W. J. Choyke, J. A. Edmond, J. A.Powell, and M. Alouani. Phys. Rev. B, 50(15):10722–10726, 1994.[8] K. Lawniczak-Jablonska, T. Suski, I. Gorczyca, N. E. Christensen, K. E. Attenkofer, R. C. C. Perera, E. M.Gullikson, J. H. Underwood, D. L. Ederer, and Z. Liliental Weber. Phys. Rev. B, 61(24):16623– 16632, 2000.[9] L. F. J. Piper, T. D. Veal, P. H. Jefferson, C. F. McConville, F. Fuchs, J. Furthmu¨ller, F. Bechstedt, HaiLu, and W. J. Schaff. Phys. Rev. B, 72:245319, 2005.[10] W. G. Schmidt, 2004. Theoretische Physik, Univ. Paderborn, Warburger Str. 100, 33098 Paderborn, Ger-many, priv. com.[11] H. G. Tompkins and E. A. Irene. Handbook of Ellipsometry. William Andrew Publishing, Norwich, 2005.214

Conduction band properties of III-nitrides characterized by synchrotron ellipsometry on core level excitations

https://www.db-thueringen.de/servlets/MCRFileNodeServlet/dbt_derivate_00009266/BESSY2005_CRGE.pdf

Conduction band properties of III-nitrides characterized by synchrotron ellipsometry on core level excitations

Abstract

Similar works

Full text

Available Versions

Digitale Bibliothek Thüringen