Microscopic Identification of Hot Spots in Multibarrier Schottky

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 59, NO. 3, MARCH 2012

Microscopic Identification of Hot Spots in Multibarrier Schottky Contacts on Pulsed Laser Deposition Grown Zinc Oxide Thin Films Stefan Müller, Holger von Wenckstern, Otwin Breitenstein, Jörg Lenzner, and Marius Grundmann

Abstract—Multiple barriers have been observed in the I–V characteristics of various Schottky diodes (SDs). Here, multibarrier PdOx /ZnO Schottky contacts (SCs) exhibiting three different discrete barrier heights are investigated. Using a model considering three parallel connected SDs with individual barrier potential, ideality factor, and active area, the forward current–voltage characteristic of the PdOx /ZnO SC was accurately modeled. Regions of low barrier height are visualized by dark lock-in thermography as a function of forward current. The microscopic origin of local patches with the lowest barrier height has been identified as incorporation of Al2 O3 particles affecting the morphological and structural thin film properties on the micrometer scale. Index Terms—Barrier inhomogeneities, dark lock-in thermography (DLIT), energy-dispersive X-ray spectroscopy, reactive sputtering, Schottky barrier, thin film, zinc oxide (ZnO).

I. I NTRODUCTION

I

N RECENT years, zinc oxide (ZnO)-based Schottky diodes (SDs) have been investigated with renewed interest [1]– [25]. This research utilized Schottky contacts (SCs) for material characterization [2], [5], [18], as gate contacts in field-effect transistors [11], [20], [23], and on the optimization of their rectifying properties. For the optimization, different pretreatments of the ZnO surface, particularly by wet chemical methods [4], [9], [10], [12], [17] or remote plasma treatment [3], [4], [6], [8], have been investigated and led to high-quality SCs. Multibarrier SCs, which exhibit one or more kinks in the forward direction of the semi-logarithmic current–voltage (I–V ) characteristic, are a well known phenomenon for diodes based on silicon carbide (SiC). In 1999, Defives et al. [26] published the I–V characteristics of two-barrier SCs on SiC. The fitting of such I–V characteristics was carried out by a simple model

Manuscript received August 9, 2011; revised October 14, 2011; accepted November 15, 2011. Date of publication January 11, 2012; date of current version February 23, 2012. This work was supported in part by the European Social Fund (ESF) within Nachwuchsforschergruppe “Multiscale functional structures” and by the Deutsche Forschungsgemeinschaft in the framework of Sonderforschungsbereich 762 “Functionality of Oxidic Interfaces.” The review of this paper was arranged by Editor G. Jeong. S. Müller, H. von Wenckstern, J. Lenzner, and M. Grundmann are with the Fakultät für Physik und Geowissenschaften, Institut für Experimentelle Physik II, Universität Leipzig, 04103 Leipzig, Germany (e-mail: stefan.mueller@ physik.uni-leipzig.de; [email protected]; [email protected]; [email protected]). O. Breitenstein is with the Max-Planck-Institut für Mikrostrukturphysik, 06120 Halle, Germany (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2011.2177984

of two SDs connected in parallel. Skromme et al. [27] substantiated the multibarrier behavior to a discrete distribution of crystal defects due to a direct correlation between defects visualized by electron beam induced current (EBIC) measurements and electrical nonidealities. In the next few years, these results were validated by other reports, e.g., by a comprehensive report about the different morphological defects in SiC by Wang et al. [28]. Another possible reason for the multibarrier behavior was discussed by Tumakha et al. [29], relating localized inhomogeneities to localized defect states pinning the Fermi level. This result was revisited in the investigations of Ewing et al. [30]. They suggested a local pinning of the Fermi level by three defect states 0.6, 0.8, and 1.05 eV below the conduction band, respectively, which explains the appearance of three different barrier heights for the low barrier regions. EBIC measurement on many of the SCs indicated that these defects are related to stacking fault clusters in the SiC. For ZnO, several I–V characteristics of SCs on hydrothermally grown bulk ZnO [10], [14] and on pulsed laser deposition (PLD)-grown thin film [4] with a multibarrier behavior were reported; however, possible reasons for this behavior have not been discussed so far. Here, we present investigations of this phenomenon for selected SCs on PLD-grown ZnO thin films. We discuss a representative three-barrier I–V characteristic fitted by a model of three individual SDs connected in parallel. The low barrier regions were visualized by dark lock-in thermography (DLIT). Using scanning electron microscopy (SEM), focused ion beam (FIB) preparation of cross sections, and energy dispersive X-ray microanalyses (EDX), the microscopic origin of the regions with the lowest barrier was identified. II. E XPERIMENTAL D ETAILS For the present investigation, we used ZnO thin films grown by PLD. The thin films contained two different ZnO layers. First, an approximately 200 nm thick highly aluminum doped ZnO:Al layer was deposited on a-plane sapphire substrates. This highly conducting layer with effective donor density and resistivity of 1 × 1020 cm−3 and 1 × 10−3 Ω cm, respectively, is used as an ohmic back-contact and ensures a low series resistance of the Schottky barrier diode [13]. Subsequently, about 1 μm thick of nominally undoped ZnO layer with an effective donor density of about 7 × 1017 cm−3 was grown. Both layers were deposited at a growth temperature of

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MÜLLER et al.: HOT SPOTS IN MULTIBARRIER SCs ON PLD-GROWN ZnO THIN FILMS

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650 ◦ C and an oxygen partial pressure of 0.016 mbar. Before the fabrication of the SCs, the thin film surface was cleaned with acetone. Using a photolithography mask, circular contacts with diameters between 150 and 750 μm were defined. The SCs were fabricated by dc sputtering of palladium. First, a layer of PdOx was fabricated in a 1/1 volume % Ar/O2 atmosphere with a thickness of about 15 nm. At the beginning of the reactive sputter process, surface cleaning occurs due to the negatively charged oxygen ions and molecules impinging on the sample surface and cleaning off residual contaminants like hydrocarbonates or a hydroxide layer. Subsequently, a second Pd layer is sputtered in pure argon atmosphere. This layer ensures an equipotential surface. The thickness of this layer is about 10 nm. The PdOx /ZnO SCs were investigated by I–V measurements at room temperature and for selected SCs additionally in a temperature range between 20 and 320 K. The I–V characteristics were recorded using an Agilent 4155C semiconductor parameter analyzer. Capacitance–voltage (C–V ) measurements were performed with an Agilent 4294 A precision impedance analyzer. The DLIT images were acquired by a TDL 640 SM “Lock-in” by Thermosensorik GmbH Erlangen. The SEM investigations were performed with a Nova NanoLab 200 by FEI Company, which has an integrated FIB and an EDX detector. III. R ESULTS AND D ISCUSSION First, we discuss the I–V characteristic of a typical threebarrier ZnO SC (diameter of 400 μm), which is depicted in Fig. 1(a). The forward characteristic at 200 K clearly indicates a three-barrier behavior; therefore, we considered three parallel connected SDs for modeling. Two kinks in the forward direction at voltage of about 0.5 and 0.9 V, respectively, separate regions with different exponential slopes. These kinks indicate that the low and intermediate barrier regions, respectively, are under flatband conditions. The current transport through ZnO SCs can be described by thermionic emission [15], [22] e(VA − IRs ) VA − IRs (1) −1 + I = Is exp nkB T Rp where e is the elementary charge, VA is the applied voltage, T is the absolute temperature, and kB is Boltzmann’s constant. This model incorporates the voltage dependence of the barrier through the ideality factor n and the series and parallel resistances Rs and Rp of the diode. The saturation current Is is given by −eΦB,eﬀ ∗ 2 Is = A0 A T exp (2) kB T where A∗ is the Richardson constant, with a theoretical value of 32 A cm−2 K−2 (using m∗e = 0.27 me,0 ), A0 is the area, and ΦB,eﬀ is the effective barrier height of the SD. The model used here considers three parallel connected SDs with individual discrete values for barrier height, ideality factor, area, and series resistance [see the equivalent circuit diagram in the inset of Fig. 1(b)]. Additionally, the resistance RP is connected in parallel to the three SDs.

Fig. 1. (a) Experimental I–V characteristic of a multibarrier PdOx /ZnO SC at 200, 250, and 300 K. (b) I–V characteristic of a multi-barrier SC at 200 K. The black solid line visualizes the measured experimental data. The lower dotted (low barrier), dashed (intermediate barrier), and upper dotted (high barrier) line, respectively, depicts the characteristics of the individual SDs. The sum of the three characteristics (model) is shown as dot-dashed line. The inset shows the equivalent circuit diagram of the used model. (c) Residuum and relative error (inset) of the fitting (dot-dashed curve) in (b).

Using this model, a good agreement between experimental and modeled data is achieved. The fitted characteristics of the three individual SDs and the sum of the currents are shown in Fig. 1(b). Fig. 1(c) shows the residuum and relative error of the fit. The calculated contact parameters of this SC are summarized in Table I. The effective barrier height of 810 meV (1000 meV) at 200 K (at room temperature) of the high barrier region is larger than the effective barrier heights reported for Pd/ZnO SCs so far [4], [5], [13] and can be explained by the oxidation of the SC. The area of the three SDs differs by orders of magnitude, similar to

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TABLE I CHARACTERISTIC CONTACT PARAMETERS (A0 = 1.26 × 10−3 cm2 , T = 200 K) CALCULATED FOR THE I–V C HARACTERISTIC S HOWN IN F IG . 1(a)

Fig. 3. Effective barrier height versus ideality factor of the low barrier region of 37 multibarrier PdOx /ZnO SCs fabricated identically on one wafer.

Fig. 2. Effective barrier height versus ideality factor of the high and intermediate barrier regions of several multibarrier PdOx /ZnO SCs of one sample.

the case of SiC. The areas of the low and intermediate barrier SDs are 2.1 × 10−6 A0 and 6.4 × 10−4 A0 , respectively. The diode with the lowest barrier has the lowest ideality factor of 1.4. In contrast, the SD with an area of 6.4 × 10−4 A0 (intermediate barrier diode) shows the highest dependence of the barrier height on applied voltage (n = 3). The ideality factor of the high barrier regions is 2.3; hence, no direct correlation between the area of the SDs and their ideality factor exists. The mean barrier height of this contact determined from C–V measurements [32] is 1310 meV and is in good agreement (within the error of the experiment) with the mean barrier height of 1350 meV of the high barrier region determined by I–V –T measurements. The I–V characteristics of 41 (37 of which showed threebarrier behavior) SCs were fitted. These SCs are on one wafer and were processed simultaneously under nominally identical conditions. Fig. 2 depicts the dependence of the effective barrier heights on the ideality factor for the high (black squares) and intermediate barrier region (white circles). A linear dependence between both parameters is revealed for the high barrier part of the contacts; however, two regions with different slopes are visible. The point of intersection of both regions is at an ideality factor of about 2. The slope of the linear part is for smaller ideality factors larger than for the region with ideality factors above 2. This behavior is explainable by Tung’s theory of inhomogeneous SCs [33], showing a substantial increase of the ideality factor of SCs for a high density of nanoscale barrier inhomogeneities. For small values of the ideality factor (small density of nanoscale barrier inhomogeneities), the effective barrier height is proportional to the ideality factor, whereas for higher ideality factors (high density of nanoscale barrier inhomogeneities), it flattens to be a fixed value independent of the ideality factor [34]. By linear fitting of the low ideality factor region, we determined the barrier height Φnif B,high to be

1.27 eV for this sample. This value is slightly higher than the reported mean barrier height of palladium SCs on ZnO thin film [13], which can also be explained by the oxidation of our SCs. The dependence of the effective barrier height on the ideality factor for the intermediate barrier region is also linear; however, the distribution is large, which is caused by a larger error of the fitting parameters for the intermediate barrier region. For some SCs, the contribution of intermediate barrier region is hardly visible in the room temperature I–V characteristic, as depicted, for example, in Fig. 1(a), causing the aforementioned larger error of the fitting parameters. The extrapolation of the linear dependence between both parameters yields for the intermediate barrier height Φnif B,im = 0.74 eV. Fig. 3 shows the dependence of the effective barrier height on the ideality factor for the low barrier region of the 37 SCs. The graph exhibits no dependence between both parameters. The effective barrier height deduced for the low barrier region varies between 0.45 and 0.65 eV independent of the ideality factor. The histogram of the observed barrier heights in the right side of Fig. 3 depicts a distribution around a maximum of about 0.57 eV. The visualization of the low barrier regions was performed by DLIT [35]. Fig. 4(a) shows an overlay of the temperature modulation amplitude and a topography image of a selected PdOx /ZnO three-barrier SC. The applied voltage was 1 V (current of 1 × 10−4 A). The current density through the low barrier region is at 1 V, about 60 times larger than that through the high barrier region. Measurements at lower voltages do not provide additional information due to noise. This is caused by the very low emissivity of the smooth metal surface of the SC, resulting in weak heat radiation. The image shows several small patches with increased temperature modulation. The locally increased current flow is due to a lower barrier height according to (1) and (2). For a voltage of 1.5 V (current of 2 × 10−3 A), the DLIT measurement [Fig. 4(b)] reveals an increased number of observable patches, particularly in the left part of the contact highlighted by the white square. When the current density through the regions of different barrier heights becomes similar, the temperature signal homogenizes accordingly, like exemplarily shown in Fig. 4(c) for V = 2 V. The low barrier patches revealed by DLIT were investigated with a higher lateral resolution using SEM. This technique revealed several randomly distributed droplets with diameters between 500 nm and 5 μm at the surface of the contact, as shown in Fig. 5. The three droplets in the center of Fig. 5


Fig. 4. Overlay of the temperature modulation amplitude and topography of a selected multibarrier Pd/ZnO SC (contact diameter of 450 μm) measured by DLIT. The measurements were performed at three different biasing voltages (a) 1 V, (b) 1.5 V, and (c) 2 V. (d) Current density of the DLIT measured SC at 290 K.

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Fig. 6. Magnified SEM view of two different types of droplets. (a) Droplet with an increased temperature modulation at low biasing voltages (type A). (b) Droplet without an observable temperature modulation at low biasing voltages (type B). (c) and (d) SEM pictures of cross sections prepared by FIB of the droplets in (a) and (b).

Fig. 7. Elemental analysis performed by EDX of the type A particle incorporated in the thin film.

Fig. 5. SEM picture of a magnified section (white square in Fig. 4) of the DLIT measured SC.

show a different morphology in contrast to the others. A comparison between the DLIT and SEM measurements indicates an increased DLIT signal only at the surface of such droplets. A high-resolution SEM image of the two different types of droplets depicted in Fig. 6 illustrates the different morphologies strikingly. Fig. 6(a) shows a droplet contributing to the DLIT signal (type A), whereas the droplet depicted in Fig. 6(b) (type B) does not contribute. For further investigations of the origin of the morphological differences between the different types of droplet, we prepared cross sections by FIB. In Fig. 6(c) and (d), SEM images of cross sections of selected droplets of types A and B are shown. At the bottom of the ZnO layer, particles are incorporated in the thin film for both types of droplets; however, the shape of the particles is different. The particle in Fig. 6(c) is spherical, whereas in Fig. 6(d), a disc-shaped particle is visible. A second difference exists in the quality of the ZnO thin film on top of the particles. The layer above the spherical particle is not

continuously connected with the rest of the thin film, and many different ZnO grains are visible [see Fig. 6(c)]. In contrast, the ZnO layer above type B particles is connected to the ZnO thin film and appears homogeneous. EDX microanalysis of the cross sections revealed the chemical composition of the particles as aluminum oxide, as depicted in Fig. 7. The particles were incorporated in the thin film during the growth of the ohmic back contact layer. IV. C ONCLUSION In summary, we have investigated lateral variations of barrier height of PdOx SCs on PLD-grown ZnO thin films using macroscopic and microscopic techniques. An I–V characteristic having three distinct exponential slopes was modeled considering three individual Schottky barriers connected in parallel. We imaged the low barrier regions by DLIT at different biasing voltages. The microscopic origin of the low barrier regions was found to be the incorporation of Al2 O3 droplets. Although the diameter of the spherical droplets varies from 500 nm to 5 μm, the barrier heights of the low barrier regions observed are similar. To understand this rather unexpected behavior, the process of barrier formation in the vicinity of the spherical as well as disc-like shaped droplets requires further investigations.

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The variation of crystalline quality above incorporated particles cannot be the reason for the multibarrier behavior of SCs on hydrothermally grown bulk ZnO [10], [14]. The process responsible for the second local decrease of barrier height of our SCs possibly has another origin. One reasonable explanation can be the local pinning of the Fermi level at a local surface defect state, as reported for SiC [29], [30]. To clarify such origins, further investigations with high spatial resolution are needed. ACKNOWLEDGMENT We gratefully thank H. Hochmuth for PLD growth of the investigated samples, G. Ramm for the preparation of ZnO targets, and M. Hahn for the preparation of the SCs. We particularly thank Prof. M. Henry (Dublin City University) for proofreading the manuscript. R EFERENCES [1] S. Liang, H. Sheng, Y. Liu, Z. Huo, Y. Lu, and H. Shen, “ZnO Schottky ultraviolet photodetectors,” J. Cryst. Growth, vol. 225, no. 2, pp. 110–113, May 2001. [2] F. D. Auret, S. A. Goodman, M. J. Legodi, W. E. Meyer, and D. C. Look, “Electrical characterization of vapor-phase-grown single-crystal ZnO,” Appl. Phys. Lett., vol. 80, no. 8, pp. 1340–1342, Feb. 2002. [3] B. J. Coppa, R. F. Davis, and R. J. Nemanich, “Gold Schottky contacts on oxygen plasma-treated, n-type ZnO(0001-bar),” Appl. Phys. Lett., vol. 82, no. 3, pp. 400–402, Jan. 2003. [4] H. von Wenckstern, E. M. Kaidashev, M. Lorenz, H. Hochmuth, G. Biehne, J. Lenzner, V. Gottschalch, R. Pickenhain, and M. Grundmann, “Lateral homogeneity of Schottky contacts on n-type ZnO,” Appl. Phys. Lett., vol. 84, no. 1, pp. 79–81, Jan. 2004. [5] U. Grossner, S. Gabrielsen, T. M. Borseth, J. Grillenberger, A. Y. Kuznetsov, and B. G. Svensson, “Palladium Schottky barrier contacts to hydrothermally grown n-ZnO and shallow electron states,” Appl. Phys. Lett., vol. 85, no. 12, pp. 2259–2261, Sep. 2004. [6] K. Ip, B. P. Gila, A. H. Onstine, E. S. Lambers, Y. W. Heo, K. H. Baik, D. P. Norton, S. J. Pearton, S. Kim, J. R. LaRoche, and F. Ren, “Improved Pt/Au and W/Pt/Au Schottky contacts on n-type ZnO using ozone cleaning,” Appl. Phys. Lett., vol. 84, no. 25, pp. 5133–5135, Jun. 2004. [7] T. Lin, S. Chang, Y. Su, B. Huang, M. Fujita, and Y. Horikoshi, “ZnO MSM photodetectors with Ru contact electrodes,” J. Cryst. Growth, vol. 281, no. 2–4, pp. 513–517, Aug. 2005. [8] H. L. Mosbacker, Y. M. Strzhemechny, B. D. White, P. E. Smith, D. C. Look, D. C. Reynolds, C. W. Litton, and L. J. Brillson, “Role of near-surface states in ohmic-Schottky conversion of Au contacts to ZnO,” Appl. Phys. Lett., vol. 87, no. 1, pp. 012102-1–012102-3, Jul. 2005. [9] S.-H. Kim, H.-K. Kim, and T.-Y. Seong, “Electrical characteristics of Pt Schottky contacts on sulfide-treated n-type ZnO,” Appl. Phys. Lett., vol. 86, no. 2, pp. 022101-1–022101-3, Jan. 2005. [10] S.-H. Kim, H.-K. Kim, and T.-Y. Seong, “Effect of hydrogen peroxide treatment on the characteristics of Pt Schottky contact on n-type ZnO,” Appl. Phys. Lett., vol. 86, no. 11, pp. 112101-1–112101-3, Mar. 2005. [11] J. Nishii, A. Ohtomo, K. Ohtani, H. Ohno, and M. Kawasaki, “Highmobility field-effect transistors based on single-crystalline ZnO channels,” Jpn. J. Appl. Phys., vol. 44, no. 38, pp. L1193–L1195, Sep. 2005. [12] S.-H. Kim, H.-K. Kim, S.-W. Jeong, and T.-Y. Seong, “Characteristics of Pt Schottky contacts on hydrogen peroxide-treated n-type ZnO(0001) layers,” Superlatt. Microstruct., vol. 39, no. 1–4, pp. 211–217, Jan.– Apr. 2006. [13] H. von Wenckstern, G. Biehne, R. A. Rahman, H. Hochmuth, M. Lorenz, and M. Grundmann, “Mean barrier height of Pd Schottky contacts on ZnO thin films,” Appl. Phys. Lett., vol. 88, no. 9, pp. 092102-1–092102-3, Feb. 2006. [14] M. W. Allen, M. M. Alkaisi, and S. M. Durbin, “Metal Schottky diodes on Zn-polar and O-polar bulk ZnO,” Appl. Phys. Lett., vol. 89, no. 10, pp. 103520-1–103520-3, Sep. 2006. [15] M. W. Allen, S. M. Durbin, and J. B. Metson, “Silver oxide Schottky contacts on n-type ZnO,” Appl. Phys. Lett., vol. 91, no. 5, pp. 053512-1– 053512-3, Jul. 2007.

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Stefan Müller received the Diploma in physics in 2010 from the Universität Leipzig, Leipzig, Germany, where he is currently working toward the Ph.D. degree in the Semiconductor Physics Group lead by Prof. Grundmann and the Young Researchers Group “Functional multiscale structures” in the Graduate School of Natural Sciences BuildMona. His research interests include the investigation of metal–semiconductor junctions on oxide thin films grown by pulsed-laser deposition and the investigation of their electrical and optoelectronic properties.

Jörg Lenzner received the Diploma in physics from the Universität Leipzig, Leipzig, Germany, in 1980. Since 1982, he has been a Member of the technical staff with the Institut fär Experimentelle Physik II, Universität Leipzig. In this time, he worked on several scanning electron microscopes and became an expert for measurements and sample preparation with these devices.

Holger von Wenckstern received the M.S. degree and the Ph.D. degree in physics from the Universität Leipzig, Leipzig, Germany. As a member of the Semiconductor Physics group of Prof. Grundmann, he is currently leading the young researchers’ group Functional multiscale structures, being a part of the Graduate School BuildMona. His publication record includes more than 100 refereed articles. His research interests include thin-film growth by pulsed-laser deposition and investigation of their electrical and optoelectronic properties. A special research interest is metal–semiconductor junctions and their application to deep defect level characterization. His research is focused on oxide-based visible-blind devices such as photodetectors, metal–semiconductor FETs, metal–insulator–semiconductor FETs, and junction FETs.

Marius Grundmann received the Diploma in physics and the Ph.D. (Dr. rer. nat.) from the Technische Universität Berlin (TUB), Berlin, Germany, in 1988 and 1991, respectively. In 1992, he was a Postdoc with Bellcore, Red Bank, NJ, working on quantum wires, and with the TUB, working on self-organized semiconductor quantum dots. Since 2000, he has been a Professor for semiconductor physics with the UniversitLeipzig, Leipzig, Germany. He has published three books and over 330 papers with an h-index of 53 [Institute for Scientific Information (ISI)]. His research is focused on thin films, nanostructures, and transparent devices based on oxide materials, heterostructures, and microcavities. Dr. Grundmann was a recipient of the Akademiepreis of the BerlinBrandenburg Academy of Sciences, the Gerhard-Hess-Preis of the German Science Foundation (DFG), the Heinz-Maier-Leibnitz-Preis of the DFG and Bundesministerium für Bildung und Forschung (BMBF), and the Leipziger Wissenschaftspreis of the Saxonian Academy of Sciences. He is a member of the DPG, the American Physical Society (APS), and the Materials Research Society (MRS).

Otwin Breitenstein received the Ph.D. degree in physics from the University of Leipzig, Leipzig, Germany, in 1980. Since 1992, he has been with Max Planck Institute of Microstructure Physics, Halle, Germany, where he investigated defects in semiconductors. Since 1999, he has been using lock-in thermography for detecting internal shunts in silicon solar cells. In 2001, he introduced this technique on a microscopic scale for isolating faults in integrated circuits. He is giving lectures on photovoltaics with Halle University, Halle, Germany. He is the author of a book on lock-in thermography. He has published more than 100 contributions about his research in scientific journals and international conference proceedings.