Ambacher, Oliver

Baumann, Tim

Ecke, Gernot

Digitale Bibliothek Thüringen

  PROCEEDINGS 11-15 September 2006      FACULTY OF ELECTRICAL ENGINEERING AND INFORMATION SCIENCE    INFORMATION TECHNOLOGY AND ELECTRICAL ENGINEERING - DEVICES AND SYSTEMS, MATERIALS AND TECHNOLOGIES FOR THE FUTURE      Startseite / Index: http://www.db-thueringen.de/servlets/DocumentServlet?id=12391 51. IWK Internationales Wissenschaftliches Kolloquium International Scientific Colloquium Impressum  Herausgeber: Der Rektor der Technischen Universität llmenau  Univ.-Prof. Dr. rer. nat. habil. Peter Scharff  Redaktion: Referat Marketing und Studentische Angelegenheiten  Andrea Schneider   Fakultät für Elektrotechnik und Informationstechnik  Susanne Jakob  Dipl.-Ing. Helge Drumm  Redaktionsschluss: 07. Juli 2006  Technische Realisierung (CD-Rom-Ausgabe):  Institut für Medientechnik an der TU Ilmenau  Dipl.-Ing. Christian Weigel  Dipl.-Ing. Marco Albrecht  Dipl.-Ing. Helge Drumm  Technische Realisierung (Online-Ausgabe):  Universitätsbibliothek Ilmenau    Postfach 10 05 65  98684 Ilmenau  Verlag:   Verlag ISLE, Betriebsstätte des ISLE e.V.  Werner-von-Siemens-Str. 16  98693 llrnenau   © Technische Universität llmenau (Thür.) 2006  Diese Publikationen und alle in ihr enthaltenen Beiträge und Abbildungen sind urheberrechtlich geschützt. Mit Ausnahme der gesetzlich zugelassenen Fälle ist eine Verwertung ohne Einwilligung der Redaktion strafbar.   ISBN (Druckausgabe): 3-938843-15-2 ISBN (CD-Rom-Ausgabe): 3-938843-16-0  Startseite / Index: http://www.db-thueringen.de/servlets/DocumentServlet?id=12391  51st Internationales Wissenschaftliches Kolloquium Technische Universität Ilmenau  September 11 – 15, 2006  G. Ecke, T. Baumann, O. Ambacher   Composition Measurements of Group-III Nitride Ternary and Quaternary Compound Nanostructures by AES  Auger Electron Spectroscopy (AES) is a widely applied method for the determination of the composition of surfaces and thin films of compounds and alloys. Nevertheless, the accuracy of the measurements depend heavily on the quality of the standards. Paying attention to this it is necessary to measure own standard samples of the interesting material system under comparable measurement conditions. For these measurements one has to define verry carefully the measurement parameters, for excample primary electron energy, the retard ratio of the analyser and the sample tilt. All parameters must be choosen in order to get the best performance of the Auger spectrometer, that means good spatial resolution, energy resolution and a low detection limit. AES is a surface sensitive technique, because of this it is essential to eliminate surface contaminations by sputtering of the samples with noble gas ions. However, some additional sputtering effects cause alterations of the surface. The original composition can not be detected directly, because of preferential sputtering[1], sputtering induced atomic mixing, surface roughness, diffusion and segregation. These effects can be excluded as far as possible, if the standard sample of a comparable composition is sputtered and measured under the same conditions. Fig. 1 shows the position of the used standard samples in the system GaN-InN-AlN.  There are many publication in which the peak-to-peak-hight (pph) in the diffentiated Auger spectrum is taken as the measure of the Auger intensities in order to estimate the composition of the sample. But it is known, that changes in the Auger peak shape annihilate the proportionality of pph and concentration. We prefer the use of the direct spectrum. Here the measured signal is the Auger peak Fig. 1.: Position of the standard samples in the group-III nitride systemarea, which can be acquired by subtraction of the background and noise. This critical procedure can be performed by the Shirley approximation [2], but attention should be paid on the position of the borders of the peak area. Also the superposition of two peaks need to be analysed. For example, in InN the In MNN and the N KLL peaks are overlapping even with a good analyser energy resolution, so that both peaks can only be separated by a special peak overlapping approximation.  We carried out the quantification procedures for the binary semiconductors GaN, AlN and InN and than for the  AlGaN, the InGaN and the AlInN material systems for two primary electron energies (5 kV and 25 kV) and for two sample tilt angles (0° and 60° with respect to the surface normal). These experimental set-ups lead to the following experimental parameters: sample tilt (°) primary electron energy (kV) primary electron incidence angle (°) angle between sample and analyser (°) angle of ion incidence (°) 0 5 0 60 67.5 0 25 0 60 67.5 60 5 60 0 43.4 60 25 60 0 43.4  Table 1: experimental conditions of the quantification measurements The first step of this work was the estimation of the sensitivity factors for the binary semiconductors GaN, AlN and InN. Table 2 gives the results for the sensitivity factors for the dominant Auger transitions of the three binary semiconductors. Auger transition  Al (KLL) in AlN N (KLL) in AlN Ga (LMM) in GaN N (KLL) in GaN In (MNN) in GaN N (KLL) in InN   5 KV, 0° 25 KV, 0°   5 KV, 60° 25 KV, 60° 0,160 0,181 0,227 0,300 0,220 0,233 0,203 0,281 0,212 0,225 0,258 0,308 0,140 0,155 0,111 0,142 0,497 0,647 0,337 0,405 0,0218 0,0462 0,0262 0,0305 Table 2: Sensitivity factors for the Auger transitions of AlN, GaN and InN The next steps of the work were the measurements and data evaluation of the ternary  nitrides AlGaN, InAlN und InGaN with variable compositions. In the following paragraph the results are presented for the AlxGa1-xN system. The procedure has been the same for the other ternary group III-nitrides. Fig. 2 shows the composition of the AlGaN layers, measured by an additional measurement technique such as x-ray diffraction or ellipsometry and the corresponding surface composition measured by AES after sputter cleaning of the sample surface.       Fig. 2: Surface concentrations measured by AES in dependence on the real film composition of  AlxGa1-xN  films for 0°, 5 kV electron bombardment  Fig. 3 demonstrates the measured Al content x at the surface of the sputtering induced altered layer in dependence on the real film composition of AlxGa1-xN  films.       Fig. 3: Surface Al content x measured by AES in dependence on the real film composition of  AlxGa1-xN  films  The dashed line demonstrates the theoretical behaviour in the case, that no preferential sputtering would occur. The strength of the bending, of the discrepancy from this dashed line depends on the preferential sputtering YAl/YGa with Y being the sputtering yield. From these dependencies the preferential sputtering behaviour has been calculated for all ternary group III-nitride semiconductor systems for ion bombardment under 67.5° and 43.4°, respectively, with respect to the surface normal for 1 kV Argon. The preferential sputtering for AlGaN, InGaN and AlInN is summarized in table 3.  YGa / YAl for AlGaN YIn / YGa for InGaN YIn / YAl for InAlN 67.5° , 1 kV, Ar 1.55 2.8 2.4 43.4° , 1 kV, Ar 1.5 3.5 3.2 Table 3: Preferential sputtering behaviour for AlGaN, InGaN and AlInN, measured by AES   0,0 0,2 0,4 0,6 0,8 1,0020406080100   Csurface [%] N   (KLL) Ga (LMM)  Al  (KLL) x in AlxGa1-xN0,0 0,2 0,4 0,6 0,8 1,0    without prefential           sputteringx in AlxGa1-xN 5 KV,   0° 25 KV,   0° 5 KV, 60° 25 KV, 60°x gem = cAl/(cAl+cGa)With the help of the sensitivity factors and the preferential sputtering behaviour it is possible, to quantify AES measurements of unknown group III-nitride semiconductor layers, even in nanostructures. But the matrix effects have to be corrected during the quantification procedure. These matrix corrections include the attenuation length of the Auger electrons, the backscattering factor and the atomic density correction. Because of the dependence of these three values on the target composition itself, the quantification process becomes an iteration. It could be found, that after 3 to 4 iterations the calculated concentrations converge and the iterations can be aborted. This procedure of the quantification of ternary and quaternary group III-semiconductor layers has been programmed by a simple excel program.  CONCLUSIONS  Finaly, the composition of group III-nitride ternary and quaternary semiconductor materials can be successfully determined by AES with good accuracy. These investigations can be applied even to nanostructures characterising the composition in resonator bars, optical emitters and sensor structures, where the composition can derive from those of unpatterned structures for the same growing conditions. Here the knowledge of the composition is essential because it defines the electrical properties of the device structure.  References:    [1] A.A. Promokhov et al., Vacuum 56 (2000), 247 [2] D.A. Shirley, Phys. Rev. B5 (1972), 4709     Authors:    Dr. Gernot Ecke Dipl.-Ing. Tim Baumann Univ. Prof. Dr. rer. nat. habil. Oliver Ambacher  TU Ilmenau, ZMN, P.O.B. 100560 Institute for Micro- and Nanotechnologies D-98684 Ilmenau Phone: +49 (0) 3677 69 3407 Fax: +49 (0) 3677 69 3355 E-mail: gernot.ecke@tu-ilmenau.de 

Composition measurements of group-III nitride ternary and quaternary compound nanostructures by AES

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Composition measurements of group-III nitride ternary and quaternary compound nanostructures by AES

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