Matthias Brandt - Academia.edu (original) (raw)

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Papers by Matthias Brandt

Thin Solid Films, 2011

We report electro-optical measurements of BaTiO 3 /ZnO heterostructures grown by pulsed laser dep... more We report electro-optical measurements of BaTiO 3 /ZnO heterostructures grown by pulsed laser deposition. The optical properties of the heterostructures were examined with and without an applied bias. A change in the heterostructure optical properties is found and attributed to a linear electro-optical effect causing a change in the band gap. Moreover the formation of an electric polarization in the BaTiO 3 layer causes a remanent change in the dielectric function if the bias is removed. The change could be estimated to be around 5 meV.

Heterojunctions composed of wurtzite-structure (piezoelectric) ZnO and perovskite-structure (ferr... more Heterojunctions composed of wurtzite-structure (piezoelectric) ZnO and perovskite-structure (ferroelectric) BaTiO 3 are very interesting because of the previously observed ionic lattice polarization coupling at their interfaces. We report electric Sawyer-Tower polarization hysteresis measurements and analysis of a ZnO-BaTiO 3 heterostructure with Pt front and back contacts deposited by pulsed laser deposition onto a (001) silicon substrate. The ZnO layer is n-type (N c = 5.5 • 10 16 cm-3), and the BaTiO 3 (BTO) layer is highly resistive. We observe a strong asymmetric ferroelectric hysteresis, which we attribute to a rectifying depletion layer formation between the ZnO and BaTiO 3 layers. The coupling between the wurtzite-structure and perovskitestructure interface polarization influences the depletion layer formation. We develop a physical model for the electric Sawyer-Tower measurements. Our model includes the effects of the depletion layer formation inside the ZnO layer, the interface charge coupling between the ZnO and BaTiO 3 layers, and the field-dependent ferroelectric polarization inside the BTO. We obtain a very good agreement between our model-generated data and our experiment. We identify voltages in forward and reverse direction at which the depletion layer opens or closes. These voltages are asymmetric, and reveal the effect of the spontaneous piezoelectric (nonswitchable) interface charge of ZnO, which we determine from our analysis here as P sz =-4.1 lC/cm 2 .

Thin Solid Films, 2010

The properties of ZnO/MgZnO heterostructures grown by pulsed-laser deposition on sapphire (112̄0)... more The properties of ZnO/MgZnO heterostructures grown by pulsed-laser deposition on sapphire (112̄0) and ZnO (0001̄) have been compared. Electron accumulation layers have been observed for ZnO/MgZnO heterostructures grown on sapphire by capacitance–voltage (C–V) spectroscopy. The formation of a two-dimensional electron gas (2DEG) in these structures has been confirmed by temperature dependent Hall effect measurements. From C–V measurements the sheet carrier

Semiconductor Science and Technology, 2011

The properties of semiconducting oxide heterostructures are demonstrated using ZnO and its ternar... more The properties of semiconducting oxide heterostructures are demonstrated using ZnO and its ternary alloy MgxZn1 - xO as a model system. This system is of particular importance, as it shows by far the most detailed research activities among oxide semiconductors. MgxZn1 - xO can be grown pseudomorphically on single crystalline ZnO in the step flow growth mode yielding atomically flat

Physical Review Letters, 2015

A pair of conjugate observables, such as the quadrature amplitudes of harmonic motion, have funda... more A pair of conjugate observables, such as the quadrature amplitudes of harmonic motion, have fundamental fluctuations which are bound by the Heisenberg uncertainty relation. However, in a squeezed quantum state, fluctuations of a quantity can be reduced below the standard quantum limit, at the cost of increased fluctuations of the conjugate variable. Here we prepare a nearly macroscopic moving body, realized as a micromechanical resonator, in a squeezed quantum state. We obtain squeezing of one quadrature amplitude 1.1 ± 0.4 dB below the standard quantum limit, thus achieving a long-standing goal of obtaining motional squeezing in a macroscopic object.

Influence of the electric polarization on carrier transport and recombination dynamics in ZnO-bas... more Influence of the electric polarization on carrier transport and recombination dynamics in ZnO-based heterostructures' Universität Leipzig, Dissertation 201 S. * , 269 Lit. * , 78 Abb., 3 Anlagen Referat: Die vorliegende Arbeit befasst sich mit dem Einfluss der elektrischen Polarisation auf Eigenschaften freier Träger in ZnO basierten Halbleiterheterostrukturen. Dabei werden insbesondere Transporteigenschaften freier Träger sowie deren Rekombinationsdynamik untersucht. Die Arbeit behandelt vier inhaltliche Schwerpunkte. Der erste Schwerpunkt liegt auf den physikalischen Eigenschaften der verwendeten Materialen, hier wird der Zusammenhang der Bandlücke und der Gitterkonstanten von Mg x Zn 1−x O Dünnfilmen und deren Magnesiumgehalt beschrieben. Weiterhin wird die Morphologie solcher Filme diskutiert. Auf unterschiedliche Substrate und Abscheidebedingungen wird dabei detailliert eingegangen. Der zweite Schwerpunkt behandelt die Eigenschaften undotierter und phosphordotierter ZnO und Mg x Zn 1−x O Dünnfilme. Die strukturellen, Transport-und Lumineszenzeigenschaften werden hier verglichen und Rückschlüsse auf die Züchtungsbedingungen gezogen. Im dritten Schwerpunkt werden Quanteneffekte an ZnO/Mg x Zn 1−x O Grenzflaechen behandelt. Hierbei wird insbesondere auf den Einfluss der elektrischen Polarisation eingegangen. Die Präsenz eines zweidimensionalen Elektronengases wird nachgewiesen, und die notwendigen Bedingungen zur Entstehung des sogenannten qunatum confined Stark-effects werden dargelegt. Insbesondere wird hier auf züchtungsrelevante Parameter eingegangen. Den vierten Schwerpunkt stellen Kopplungsphänomene in ZnO/BaTiO 3 Heterostrukturen dar. Dabei werden zuerst die experimentell beobachten Eigenschaften verschiedener Heterostrukturen die auf unterschiedlichen Substraten gezüchtet wurden aufgezeigt. Hier stehen strukturelle und Transporteigenschaften im Vordergrund. Ein Modell zur Beschreibung der Ausbildung von Raumladungszonen in derartigen Heterostrukturen wird eingeführt und zur Beschreibung der experimentellen Ergebnisse angewandt. Die Nutzbarkeit der ferroelektrischen Eigenschaften des Materials BaTiO 3 in Kombination mit halbleitendem ZnO wurden untersucht. Hierzu wurden ferroelektrische Feldeffekttransistoren unter Verwendung beider Materialien hergestellt. Die prinzipielle Eignung der Bauelemente als nichtflüchtige Speicherelemente wurde nachgewiesen.

Zinc Oxide Materials and Devices III, 2008

We have investigated the morphology, crystalline quality, the transport and electronic properties... more We have investigated the morphology, crystalline quality, the transport and electronic properties of homoepitaxial ZnO and ZnO:P thin films grown by pulsed-laser deposition. Atomic surface steps are visible for growth temperatures of 650°C and higher. The unit cell volume of undoped thin films is smaller than that of the hydrothermal substrates. Phosphorous doping increases the unit cell volume such that a perfect lattice match is achieved for a nominal phosphorous content of 0.01 wt.%. Undoped thin films have a net doping concentration below 10 15 cm -3 , whereas the phosphorous doping increases the free electron concentration at room temperature to 10 17 cm -3 and above. Temperature dependent Hall effect measurements show that interstitial zinc with a thermal activation energy of 34 meV is a dominant donor in homoepitaxial ZnO:P thin films. The Hall mobility of such samples is similar to ZnO single crystals grown by seeded chemical vapor transport. Low temperature photoluminescence measurements reveal recombination of free excitons and excitons bound to interstitial zinc and excitons bound to neutral and ionized aluminum donors. Defect related deep luminescence is not observed for undoped homoepitaxial thin films. In contrast phosphorous doping introduces two broad recombination bands centered at 2.9 eV and 1.9 eV.

Zinc Oxide Materials and Devices III, 2008

Phosphorous-doped ZnO (ZnO:P) nanowires were prepared by a high-pressure pulsed laser deposition ... more Phosphorous-doped ZnO (ZnO:P) nanowires were prepared by a high-pressure pulsed laser deposition process. To extend the size range of available wires, µm-thick ZnO:P microwires were grown additionally by a direct carbothermal deposition process. Low-temperature cathodoluminescence of single ZnO:P nanowires grown by both processes exhibit characteristic phosphorus acceptor-related peaks: neutral acceptor-bound exciton emission ((A 0 , X), 3.356 eV), freeelectron to neutral-acceptor emission ((e, A 0 ), 3.314 eV), and donor-to-acceptor pair emission (DAP, ~3.24 and ~3.04 eV). This proves that stable phosphorus acceptor levels have been induced into the ZnO:P nano-and microwires. From the quantitative evaluation of the spectroscopic features we deduct an acceptor binding energy of 122 meV. The ZnO:P microwires were used as channels in bottom-gate field effect transistors (FET) built on Si substrates with SiO 2 gate oxide. The electrical FET-characteristics of several wires show reproducibly clear qualitative indication for p-type conductivity for variation of gate voltage. This behavior is opposite to that of nominally undoped, n-type conducting wires investigated for comparison. The p-type conductivity was found to be stable over more than six months.

Zinc Oxide Materials and Devices IV, 2009

ABSTRACT Homoepitaxial ZnO thin films doped with phosphorus (0.01% to 1% P) and/or alloyed with m... more ABSTRACT Homoepitaxial ZnO thin films doped with phosphorus (0.01% to 1% P) and/or alloyed with magnesium (1% to 4% Mg) show pseudomorphic growth with compressive or tensile strain in dependence on the dopant concentration. The structural quality of the used O-face ZnO(001) substrates was inspected by the rocking curves of the symmetric (002) and the skew-symmetric (101) peaks. Preselection of the substrate batches by the supplier decreased the twist dislocation density and increased the structural homogeneity within the batches considerably. TEM cross sections show increasing density of c-plane defects with increasing phosphorus concentration in the films. ZnO(002) rocking curves of MgZnO:P films on ZnO were as narrow as 27 arcsec with a FWHM of the substrate peak of 23 arcsec. The in-plane lattice match was confirmed for all dopant concentrations by HR-XRD triple axis scans of the (002) and (101) peaks. The results show the balance between tensile strain induced by Mg and compressive strain by P in ZnO. Two-dimensional growth with terrace-like surface structure is most prominent for the Mg-alloyed films without P. High electron mobilities up to 190 cm2/Vs at 300K and up to 800 cm2/Vs at 70 K were found in the homoepitaxial MgZnO:P thin films.

MRS Proceedings, 2006

ABSTRACTThe electrical properties of epitaxial ZnO thin films grown on sapphire substrates by pul... more ABSTRACTThe electrical properties of epitaxial ZnO thin films grown on sapphire substrates by pulsed laser deposition were investigated by temperature dependent Hall measurents. The thin films investigated were grown at different oxygen partial pressures ranging from 10-2 mbar to 1 mbar. The formation of a degenerate layer, determining the low temperature Hall data, depends on the oxygen partial pressure applied during growth. Further, the formation of such a layer can be correlated to the grain size of the samples. The thermal activation energy of dominant donors decreases in tendency with increasing oxygen partial pressure p(O2); it is about 100 meV for p(O2) ≤ 3 × 10-2 mbar and about 30 meV for p(O2) ≥ 0.1 mar. The concentration of donors and compensating acceptors increases with increasing p(O2).

Zinc Oxide Materials and Devices II, 2007

Thin Solid Films, 2009

Thin Solid Films, 2011

Thin Solid Films, 2010

Semiconductor Science and Technology, 2011

Physical Review Letters, 2015

Zinc Oxide Materials and Devices III, 2008

Zinc Oxide Materials and Devices IV, 2009

MRS Proceedings, 2006

Zinc Oxide Materials and Devices II, 2007

Thin Solid Films, 2009