Dr. Holger von Wenckstern

Universität Leipzig
Felix-Bloch-Institut für Festkörperphysik
Abteilung Halbleiterphysik
Linnéstraße 5
D-04103 Leipzig, Germany

Tel.: +49-(0)-341-97 32604
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Research Topics
  • Combinatorial Pulsed-Laser Deposition of Oxide Thin Films
  • Electrical Characterization of Defects in Semiconductors
  • Device Characterization and Optimization
    e.g.: Solar Cells, UV-Photodetectors, Rectifiers, FETs, High-Power Devices...
  • Transport Phenomena of Oxide Heterostructures (2DEGs)
  • Amorphous Oxide Semiconductors for Flexible and Low-Cost Electronics

Selected Recent Publications

Properties of orthorhombic (In,Ga)2O3 thin films and Schottky barrier diodes thereon
The orthorhombic Ga2O3 polymorph has a large predicted spontaneous polarization making the material very interesting for growth of heterostructures with exceptionally high two-dimensional electron density. However, this polymorph is metastable and melt-growth of bulk crystals for homoepitaxy is not possible. We have determined epitaxial relationships between orthorhombic (001)Ga2O3 and numerous substrate materials. Further, ternary thin films with systematic lateral variation of the cation composition were realized by pulsed-laser ablation of segmented targets, a chemical analysis is depicted in the figure. The maximum indium content in orthorhombic gallium oxide was extended to 35%. This allowed describing important material parameters, e.g. the band gap, latticing constant, etc., for a wide range of alloy compositions.Another highlight is the fabrication of the first Schottky barrier diode to orthorhombic Ga2O3 which was enabled by increasing the free electron concentration by Zr doping demonstrating that growth of semiconducting %kappa;-Ga2O3 thin films is feasible. The diodes are stable under high temperatures, high rectification was maintained even after heat treatment at 700K (see figure).
Further information is available at:

A. Hassa, H. von Wenckstern, D.Splith, C. Sturm, M. Kneiß, V. Prozheeva and M. Grundmann
Structural, optical and electrical properties of orthorhombic κ-(InxGa1-x)2O3 thin films
APL Materials (accepted for publication)

and
M. Kneiß, A. Hassa, D.Splith, C. Sturm, H. von Wenckstern, T. Schulz, N. Koch, M. Lorenz and M. Grundmann
Tin-Assisted PLD-Growth of κ-Ga2O3 Thin Films with High Crystalline Quality
APL Materials 7, 022516 (2019)



Properties of (In,Ga)2O3 alloys
For Elsevier's Metal Oxides series the book "Gallium Oxide Technology, Devices and Applications" was edited by Stephen Pearton, Fan Ren and Michael Mastro. We have contributed a chapter covering the physical properties of (InxGa1-x)2O3 thin films. We discuss growth methods, epitaxial relationships, structural, electrical as well as optical properties for the entire phase diagram. Further, properties of demontrator devices such as photo detectors are covered. Samples, realized by a combinatorial material science approach, are used to identify growth conditions that are favorable for stabilization of a certain polymorph.
Further information is available at:
Holger von Wenckstern
Properties of (In,Ga)2O3 alloys
in: Gallium Oxide Technology, Devices and Applications, eds. Stephen Pearton Fan Ren Michael Mastro, Elsevier (2019), ISBN: 9780128145210

Combinatorial material science enabled by pulsed laser deposition using radially segmented targets
We introduce an approach allowing i) realization of discrete composition spread libraries and ii) deposition of samples with continuous composition spread in growth direction. For that, radially PLD targets are manufactured in house and the shape of the inner segment resembles an ellipse. By controlling the position at which the laser radiation is incident on the radially segmented target a direct control of the time averaged particle flux is possible. The larger the radial position of the incident radiation (cf. red dashed circles in the figure), the higher is the share of the material within the outer segment on the composition of the PLD plasma. Discrete sample libraries can be created by deposition of homogeneous thin films for various fixed radial positions of the laser spot. Samples with composition gradient in growth direction are obtained by variation of the radial laser spot position position during growth.
Further information is available at:
M. Kneiß, P. Storm, G. Benndorf, M. Grundmann and H. von Wenckstern
Combinatorial material science and strain engineering enabled by pulsed laser deposition using radially segmented targets
ACS Comb. Sci. 20, 643 (2018)

Influence of Oxygen Deficiency on the Rectifying Behavior of Transparent-Semiconducting-Oxide-Metal Interfaces
The formation of Schottky barrier diodes (SBD) is prerequisite for several device applications including rectifiers, photo detectors and metal-semiconductor field-effect transistors. Concerning transparent semiconducting oxides the realization of high-quality SBDs is commonly not straight forward. Typically, oxygen plasma surface treatment and/or reactive sputtering of the contact metal is required for high rectification. On the basis of X-ray photoelectron spectroscopy and electrical transport measurements we demonstrate the mechanism leading to high-quality SBDs in case of reactively sputtered contacts to zinc-tin oxide. Results are compared to reference diodes realized by non-reactive sputter deposition.
Further information is available at:
Thorsten Schulz, Sofie Bitter, Peter Schlupp, Holger von Wenckstern, Nobert Koch, Marius Grundmann
The influence of oxygen deficiency on the rectifying behavior of transparent semiconducting oxide-metal interfaces
Phys. Rev. Appl. 9, 064001 (2018)

Group-III Sesquioxides: Growth, Physical Properties and Devices
The group-III sesquioxides find potential application as high-power devices or solar-blind UV detectors. For each binary technology for growing single-crystalline bulk material exists, enabling homoepitaxial growth of thin films with high crystalline quality. The bandgap can be tuned in an energy range from about 4 to 8 eV for the ternary alloys and allows growth of heterostructures with large band offset. In this current review past results and recent investigations on the growth, the material properties, contact fabrication and the alloying of group-III sesquioxides are reviewed, and an overview on demonstrator devices is provided. The variation of the lattice constants within monoclinic (InxGa1-x)2O3 and (AlxGa1-x)2O3 is shown examplarily in the figure.
Further information is available at:
Holger von Wenckstern
Group-III Sesquioxides: Growth, Physical Properties and Devices
Adv. Electron. Mater. 3, 1600350 (2017)

The Vital Role of Oxygen for the Formation of Highly Rectifying Schottky Barrier Diodes on Amorphous Zinc-Tin-Oxide with Various Cation Composition
Amorphous, low-temperature deposited semiconducting oxides drive pixel elements of active matrix displays. Up to now, layers containing indium and gallium are used for these application. In order to reduce material costs a substitution of these elements by abundant metals is desired. Zinc-tin-oxide is a promising substitute with promising properties even if deposited at room temperature. We describe properties of reactively sputtered platinum Schottky barrier diodes on zinc-tin-oxide thin films with laterally varying cation composition, which were grown by a continuous composition spread technique for pulsed laser deposition (see below). We found that the rectification of the diodes increases with increasing zinc content and saturates for a zinc content of about 0.4 and higher. The effective barrier height increases monotonously as visible in the figure. Further we could show, that the amount of oxygen offered during contact deposition is essential for the rectification and long-term stability of the diodes.
Further information is available at:
Sofie Bitter, P. Schlupp, H. von Wenckstern, M. Grundmann
The Vital Role of Oxygen for the Formation of Highly Rectifying Schottky Barrier Diodes on Amorphous Zinc-Tin-Oxide with Various Cation Composition
ACS Materials and Interfaces 9, 26574-26581 (2017)



Method of choice for fabrication of high-quality β-gallium oxide-based Schottky diode
Gallium oxide is has for a semiconductor a very large bandgap of about 4.9 eV in its most stable monoclinic modification. This renders the material eminently suited for high-power applications. Rectifying Schottky barrier diodes represent the basic high-power device. We report on properties of differently deposited platinum Schottky diodes on Ga2O3 thin films grown heteroepitaxially by pulsed laser deposition on (00.1) Al2O3 substrates. High Schottky barrier height and low ideally factor is obtained for thermally evaporated metal layers. For sputtered contacts, reactive sputtering leads to higher barriers than for non-reactively or evaporated diodes. Further, long-distance sputtering results in higher Schottky barriers and lower ideality factor compared to sputter processes with much lower target-to-substrate distance. This effect is impressively illustrated by a peripheral sputter process resulting in a variation of the target-to-substrate distance on one thin film. As the figure demonstrates lower distances result in inferior diode properties due to stronger defect formation by the impinging of higher energetic particles.
Further information is available at:
Stefan Müller, Holger von Wenckstern, Florian Schmidt, Daniel Splith, Heiko Frenzel, Marius Grundmann
Method of choice for fabrication of high-quality β-gallium oxide-based Schottky diodes
Semic. Sci. Technol. 32, 065013 (2017)



Ring Oscillators based on ZnO Channel JFETs and MESFETs
Electronic circuits based on oxide semiconductors find application in active matrix displays and present an essential ingredient to future transparent and possibly flexible circuitry. Most approaches are based on metal-insulator-semiconductor gate structures that typically have extremely low leakage current densities. However, Schottky diodes to wide bandgap oxide semiconductors have barrier height of 1eV and higher and therefore very low leakage current density as well. The same holds true for all-oxide pn-heterodiodes that can be used to control the source-drain current of field-effect transistors. In the current manuscript we implemented Schottky barrier diodes and pn-heterodiodes as gate contacts to ZnO-based inverters used as building blocks of ring oscillators; the design is depicted in the image. We demonstrated single stage delay times down to 110 ns and very low power consumption.
Further information is available at:
F.J. Klüpfel, H. von Wenckstern, M. Grundmann
Ring Oscillators based on ZnO Channel JFETs and MESFETs
Adv. Electr. Mater. 2, 1500431 (2016)



Semi-transparent n-ZnO/p-NiO UV solar cells
The design of a transparent photovoltaic cell seems at first glance contradictory since a huge share of the solar irradiation is not used for energy harvesting. However, facades of modern buildings often contain large areas covered by architectural glass and these areas can be exploited by transparent photovoltaic cells. In this publication, we describe a solar cell being highly transparent to the human eye that can harvest photons with energy in the ultraviolet part of the AM1.5 spectrum. As shown in the figure, ZnO:Al is used as transparent backside electrode. ZnO is used as absorber and a thin NiO-layer comprises the all-oxide pn-heterodiode. The devices are characterized by an open-circuit voltage of 520 meV, a short-circuit current of 0.5 mAcm2, a maximum external quantum efficiency of 55% and a UV power conversion efficiency of 3.1%. We discuss loss mechanisms identified by calculations of voltage-dependent efficiency of carrier collection.
Further information is available at:
Robert Karsthof, Paul Räcke, Zhipeng Zhang, Holger von Wenckstern, Marius Grundmann
Semi-transparent n-ZnO/p-NiO UV solar cells
phys. stat. sol. (a) 213, 30 (2015)



All Amorphous Oxide Bipolar Heterojunction Diodes from Abundant Metals
The field of amorphous semiconductor electronics has emerged in the last two decades and was boosted by commercial application first and foremost within the back plane of state-of-the art flat panel displays and tablet computers. This is primarily due to the high electron mobility obtainable already for low deposition temperatures. Besides this enormous success the major drawback compared to Si-based electronics is the struggle in realizing bipolar conduction enabling complementary circuitry. A first step is the fabrication of highly rectifying all amorphous, oxide pn-junctions that we demonstrate in this letter combining p-type zinc cobalt oxide and n-type zinc tin oxide. The diodes discussed were entirely processed below 100ºC and exhibit record-breaking current rectification of six orders of magnitude. This marks an important step from unipolar to bipolar amorphous oxide devices that can also be realized on bendable substrates. The figure depicts histograms of the rectification, ideality facor and series resistivity of sample series of pn- (top) and pin-diodes (bottom) .
Further information is available at:
P. Schlupp, F.-L. Schein, H. von Wenckstern, M. Grundmann
All Amorphous Oxide Bipolar Heterojunction Diodes from Abundant Metals
Adv. Electron. Mater. 1, 1400023 (2015)



Properties of (InxGa1-x)2O3 thin films with lateral composition spread
The group-III sesquioxides are interesting for high-power electronics, UV- and deep UV-photo detectors or quantum well infrared photo detectors. The semiconducting properties of the binaries In2O3 and Ga2O3 were investigated and published recently, however, not much information is available for the (In,Ga) 2 O3 alloy system. We have investigated lattice parameters, Raman-active phonon modes, dielectric function (from the near infrared to the vacuum UV spectral range), transmission and properties of Schottky barrier diodes essentially for the complete phase diagram. For the study we used a novel pulsed-laser deposition approach to combinatorial material science allowing in principle to deposit the entire phase diagram on a single wafer. The figure depicts a false-color representation of room temperaure Raman measurements measured at 18 sample posiitons with different In content. The results are published in these three manuscripts:
Further information is available at:
H. von Wenckstern, D. Splith, M. Purfürst, Z. Zhang, Ch. Kranert, S. Müller, M. Lorenz, M. Grundmann
Structural and optical properties of (In,Ga)2O3 thin films and characteristics of Schottky contacts thereon
Semic. Sci. Technol. 30, 024005 (2015)

R. Schmidt-Grund, Ch. Kranert, T. Böntgen, H. von Wenckstern, H. Krauß, M. Grundmann
Dielectric function in the NIR-VUV spectral range of (InxGa1-x)2O3 thin films
J. Appl. Phys. 116, 053510 (2014)

Ch. Kranert, J. Lenzner, M. Jenderka, M. Lorenz, H. von Wenckstern, R. Schmidt-Grund, M. Grundmann
Lattice parameters and Raman-active phonon modes of (InxGa1-x)2O3 for x < 0.4
J. Appl. Phys. 116, 013505 (2014)



Schottky contacts to In2O3
The binary semiconductors CdO, InN and In2O3 have a very low-lying conduction band minimum (CBM) in common. For these the CBM even lies below the charge neutrality level leading to easy n-type doping and peculiar electronic surface properties such as formation of a two-dimensional electron gas localized within a surface quantum well. Electrical contacts are due to the resulting high electron concentration at the surface ohmic and exploitation of these materials within active devices such as rectifiers, transistors or detectors was out of reach. In collaboration with the Paul-Drude Institut Berlin and the University of California, Santa Barbara, we demonstrate Schottky barrier contacts to In2O3 with rectifying properties being sufficient for space-charge layer spectroscopy. In contrast to thermal evaporation or non-reactive dc-sputtering a reactive dc-sputtering process resulted in rectifying contacts on In2O3 for various metals. Since oxygen vacancies are a likely microscopic origin of the electron accumulating at the surface the sputtering in an oxygen bearing ambient seems to be the key for the removal of surface electrons since it is capable to reduce the density of surface-near oxygen vacancies. Thereby Schottky diodes on nominally undoped, Mg-doped and Sn-doped In2O3 thin films grown by molecular beam epitaxy were realized. Arrays of Au, Pd, or Pt-Schottky barrier contacts were investigated by current-voltage measurements at room temperature (see figure for data of best diodes) showing that the ensemble of Pt-diodes on Mg-doped In2O3 has in average highest rectification. Temperature-dependent current-voltage measurements revealed that thermionic emission is the dominant transport mechanism. Further, this data allowed determining the homogeneous barrier height of Pt/ In2O3 to be about 1 eV. The comparatively high rectification of the diodes allowed thermal admittance spectroscopy. We observed two donor-like defects states with thermal activation energy of about 20 to 25 meV and 290 meV, respectively. The result demonstrate that a reactive sputtering of Schottky contact metals on In2O3 opens new possibilities of material characterization and fabrication of devices such as metal-semiconductor field-effect transistors. The figure depicts room temperature current-voltage characteristics of Au, Pd, Pt Schottky contacts on undoped, Mg-doped and Sn-doped In2O3 thin films. A characteristic of a non-reactively sputtered contact is shown as well.
Further information is available at:
H. von Wenckstern, D. Splith, F. Schmidt, M. Grundmann, O. Bierwagen, J.S. Speck
Schottky contacts to In2O3
APL Materials 2, 046104 (2014))



All Schottky barrier diodes are ideal -- at least at certain temperatures
We have studied in cooperation with the University of Pretoria (RSA) electrical properties of Cu/Ga2O3 Schottky barrier diodes. Group-III sesquioxides such as Ga2O3 are potential materials for solar-blind UV-photo-detectors (PD) and first and foremost high-power electronics. Schottky diodes can be used to create UV-PDs as well as metal-semiconductor field-effect transistors. We present an advanced front-to-back diode layout comprising a highly conducting ZnO:Ga layer used as ohmic back contact, an undoped ZnO-layer for conduction band alignment and Si-doped Ga2O3. Structural and electrical properties at room temperature are detailed. The high thermal stability of the device allowed current-voltage measurements up to temperatures for which the device behaves ideal. Lateral fluctuations of the Schottky barrier potential are common for real Schottky diodes and cause the voltage dependence of the barrier height (ideality factors larger than unity). Further, the temperature dependence of the barrier height and the ideality factor are determined by these inhomogeneities. For Cu/Ga2O3 it is possible to investigate the current-voltage characteristics at temperatures for which these fluctuations are small compared to the thermal energy resulting in an ideal diode behavior. The figure depicts such a characteristic that was acquired at 600K. Both, forward and backward current are perfectly fitted only considering thermionic emission theory and image force lowering.
Further information is available at:
D. Splith, S. Müller, F. Schmidt, H. von Wenckstern, J. J. van Rensburg, W. E. Meyer, M. Grundmann
Determination of the mean and the homogeneous barrier height of Cu Schottky contacts on heteroepitaxial β-Ga2O3 thin films grown by pulsed laser deposition
phys. stat. sol. (a) 211, 40 (2014))



Long-term stability of highly rectifying all-oxide pn-heterojunction diodes
The emerging field of wide bandgap semiconducting oxides is driven by various functionalities such as piezoelectricity, magnetism or, if combined, multiferroic behavior, or simply transparency that cannot be realized using conventional semiconductors. However, one fundamental difference between Si, GaAs, or GaN and wide bandgap semiconducting oxides is the fact that the latter are in principle unipolar. In consequence, pn-homojunction diodes are not realizable and only heterojunction diodes are feasible. We have fabricated such heterodiodes consisting of p-conducting, amorphous ZnCo2O4 and n-conducting ZnO. We demonstrate that for an optimized diode layout rectification of 10 orders of magnitude is obtained. Further we show, that the rectification of these heterojunction is stable over a time period exceeding one year as depicted in the figure (a: comparison of j(V)-characterisitcs, b (from left to right): change of ideality factor, series resistance and rectification). A slight decrease of the rectification is due to an increased series resistance of the diode and not to changes of the p-ZnCo2O4/n-ZnO interface. Further information is available at:
F.-L. Schein, M. Winter, T. Böntgen, H. von Wenckstern, M. Grundmann
Highly rectifying p-ZnCo2O4/n-ZnO heterojunction diodes
Appl. Phys. Lett. 104, 022104 (2014)



Continuous composition spread by using a single, segmented PLD target
Combinatorial strategies for high-throughput analysis of arrays of different substances have been successfully used in biomedicine and by the pharmaceutical industry for many decades. Combinatorial approaches have been introduced to solid-state science in the sixties of the last century. Nowadays, thin films with a continuous composition spread (CCS) are routinely prepared by co-sputtering or by the ablation of different targets by pulsed-laser deposition (PLD). Here, the creation of a CCS by PLD is somewhat involved since parts of the substrate have to be shadowed by a movable mask and the PLD targets have to be exchanged in order to create the CCS. This results in low growth rates and the implementation of this method is for most PLD chambers not possible. In this recent publication we introduce a new method for creating continuous composition spreads within thin films grown by pulsed laser deposition. Instead of ablating several targets of different chemical composition we use a single but segmented target. Here, the composition of the individual segments differs. Within the article we discuss the influence of the PLD chamber geometry and process parameters on the composition of the resulting thin films with CCS. Further, we report on a sample having a one-dimensional CCS that was realized by using a two-fold segmented target and a sample having two independent composition gradients realized by ablating a three-fold segmented target.
The figure depicts possible PLD target segmentations and the calculated elemental distribution on the substrate. Further information is available at:
H. von Wenckstern, Z. Zhang, F. Schmidt, J. Lenzner, H. Hochmuth and M. Grundmann
Continuous composition spread using pulsed-laser deposition with a single, segmented target
CrystEngComm 15, 10020-10027 (2013)



Wavelength-selective UV-photodetectors
The detection of ultraviolet irradiation is important in environmental research and monitoring, flame detection and monitoring of industrial processes like UV-curing of glues, adhesives or disinfection of drinking water by UV irradiation. Wide bandgap semiconducting oxides are highly suited for detectors operating in the UV-A, UV-B and even in the UV-C spectral range. For many applications the spectrally integrated detection of UV irradiation is not sufficient. In this letter we present means towards monolithic, wavelength-selective UV-A photo-detectors by using a continuous composition spread approach. For that, a laterally homogeneous (Mg,Zn)O optical filter layer is deposited on one side of a 2 inch sapphire wafer, while a (Mg,Zn)O layer with lateral variation of the Mg-content is deposited on the other side of the wafer and acts as active layer. The bandgap of the active layer is smaller than the bandgap of the optical filter layer. The bandwidth of the detector is now given by the difference of the absorption edge of the filter and the active layer.
The figure depicts this bandwidth (a) along with the laterally varying absorption edge of the active layer (b) and the in principle constant absorption edge of the optical filter layer (c). Further information is available at:
Zhipeng Zhang, Holger von Wenckstern, Marius Grundmann
Energy-selective multichannel ultraviolet photodiodes based on (Mg,Zn)O
Appl. Phys. Lett. 103, 171111 (2013)