M. H. Zoellner

M. H. Zoellner
IHP | Institute for High Performance Microelectronics · Materials Research

Dr. rer. nat.

About

57
Publications
7,303
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468
Citations
Featured research
Article
Full-text available
Indium oxide (InxOy) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) using trimethylindium and oxygen plasma in a low-temperature range of 80–200 °C. The optical properties, chemical composition, crystallographic structure, and electrical characteristics of these layers were investigated by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), as well as current-voltage and capacitance-voltage measurements. The SE results yielded a nearly constant growth rate of 0.56 Å per cycle and a thickness inhomogeneity of ≤1.2% across 4-in. substrates in the temperature range of 100–150 °C. The refractive index (at 632.8 nm) was found to be 2.07 for the films deposited at 150 °C. The PEALD-InxOy layers exhibit a direct (3.3 ± 0.2 eV) and an indirect (2.8 ± 0.1 eV) bandgap with an uptrend for both with increasing substrate temperature. Based on XPS characterization, all InxOy samples are free of carbon impurities and show a temperature-dependent off stoichiometry indicating oxygen vacancies. XRD diffraction patterns demonstrate an onset of crystallization at 150 °C. Consistent with the optical, XPS, and XRD data, the films deposited at ≥150 °C possess higher electrical conductivity. Our findings prove that a low-temperature PEALD process of InxOy is feasible and promising for a high-quality thin-film deposition without chemical impurities on thermally fragile substrates.
Article
Full-text available
The integration of GaN on Si as large scale substrates still faces many hurdles. Besides the large difference in the lattice constant and the high thermal mismatch existing between GaN and Si, spiral hillock growth phenomena are common problems in the development of thick GaN layers. In this work, hexagonal hillocks were observed on GaN/AlGaN high-electron-mobility transistor heterostructures grown on Si(111) by metal-organic chemical vapor deposition. The presence of these morphological and structural defects is attributed to the presence of localized contamination at the AlN/Si interface. These carbon-based defects cause highly defective regions in the AlN seed layer, which propagate through all the AlGaN buffer layers inducing the formation of V-shaped pits at the AlGaN interfaces. In hillock regions of the wafers, Raman spectroscopy indicates disturbed two-dimensional electron gas characteristics resulting from GaN/AlGaN interface roughness and a decreased amount of free carriers in the potential well. Energy-dispersive x-ray spectroscopy reveals Ga accumulation inside the V-pits and in nanopipes below, which is responsible for defective areas and the increased GaN growth rate resulting in hillock formation. Photoluminescence measurements confirm the presence of Ga-rich material reducing the inherent gallium vacancy concentration. Here, the reduced amount of Ga-vacancies acting as a shallow acceptor suppresses the ultraviolet luminescence band from donor–acceptor pair transition.
Article
Full-text available
n-type Ge/SiGe asymmetric coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge-rich SiGe tunnel barrier, as a function of the geometry parameters of the design and the P dopant concentration. Through a comparison of THz spectroscopic data with numerical calculations of intersubband optical absorption resonances, we demonstrated that it is possible to tune, by design, the energy and the spatial overlap of quantum confined subbands in the conduction band of the heterostructures. The high structural/interface quality of the samples and the control achieved on subband hybridization are promising starting points towards a working electrically pumped light-emitting device.
Article
An Al2O3/ZnO heterojunction was grown on a Si single crystal substrate by subsequent thermal and plasma-assisted atomic layer deposition (ALD) in situ. The band offsets of the heterointerface were then studied by consecutive removal of the layers by argon sputtering, followed by in situ X-ray photoelectron spectroscopy. The valence band maximum and conduction band minimum of Al2O3 are found to be 1.1 eV below and 2.3 eV above those of ZnO, resulting in a type-I staggered heterojunction. An apparent reduction of ZnO to elemental Zn in the interface region was detected in the Zn 2p core level and Zn L3MM Auger spectra. This suggests an interface formation different from previous models. The reduction of ZnO to Zn in the interface region accompanied by the creation of oxygen vacancies in ZnO results in an upward band bending at the interface. Therefore, this study suggests that interfacial properties such as the band bending as well as the valence and conduction band offsets should be in situ controllable to a certain extent by careful selection of the process parameters.
Additional affiliations
October 2009 - present
IHP
Position
  • Researcher

Publications

Publications (57)
Article
An Al2O3/ZnO heterojunction was grown on a Si single crystal substrate by subsequent thermal and plasma-assisted atomic layer deposition (ALD) in situ. The band offsets of the heterointerface were then studied by consecutive removal of the layers by argon sputtering, followed by in situ X-ray photoelectron spectroscopy. The valence band maximum and...
Article
Full-text available
Indium oxide (InxOy) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) using trimethylindium and oxygen plasma in a low-temperature range of 80–200 °C. The optical properties, chemical composition, crystallographic structure, and electrical characteristics of these layers were investigated by spectroscopic ellipsometry (S...
Article
Strain relaxed Si1−xGex buffer layers on Si(001) can be used as virtual substrates for the growth of both strained Si and strained SiGe, which are suitable materials for sub-7 nm CMOS devices due to their enhanced carrier mobility. For industrial applications, the threading dislocation density (TDD) has to be as low as possible. However, a reductio...
Article
The threading dislocation density (TDD) in plastically relaxed Ge/Si(001) heteroepitaxial films is commonly observed to decrease progressively with their thickness due to mutual annihilation. However, there exists a saturation limit, known as the geometrical limit, beyond which a further decrease of the TDD in the Ge film is hindered. Here, we show...
Article
Full-text available
The integration of GaN on Si as large scale substrates still faces many hurdles. Besides the large difference in the lattice constant and the high thermal mismatch existing between GaN and Si, spiral hillock growth phenomena are common problems in the development of thick GaN layers. In this work, hexagonal hillocks were observed on GaN/AlGaN high-...
Article
Full-text available
n-type Ge/SiGe asymmetric coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge-r...
Preprint
: n-type Ge/SiGe asymmetric-coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge...
Article
Exploiting intersubband transitions in Ge/SiGe quantum cascade devices provides a way to integrate terahertz light emitters into silicon-based technology. With the view to realizing a Ge/SiGe Quantum Cascade Laser, we present the optical and structural properties of n-type strain-symmetrized Ge/SiGe asymmetric coupled quantum wells grown on Si(001)...
Article
Regular surface undulations, called cross-hatch patterns, appearing at the free surface of lattice-mismatched heteroepitaxial films are a key signature of plastic relaxation. Here we show that the dynamics of cross-hatch formation is accurately described by a continuum model based on strain-mediated surface diffusion, provided that a realistic dist...
Article
InGaN/GaN double heterostructures and multi-quantum wells have been successfully developed since more than 20 years for LED lightning applications. Recent developments show that state-of-the-art LEDs benefit from artificially generated V-pit defects. However, the control of structural and chemical properties plays a tremendous role. In this paper w...
Poster
Full-text available
Theoretical predictions indicate that the n-type Ge/SiGe multi quantum-well system is the most promising material for the realization of a Si-compatible THz quantum cascade laser (QCL) operating at room temperature. As a key step forward for QCL design within the SiGe material platform, we studied both experimentally and theoretically asymmetric co...
Article
Theoretical predictions indicate that the n-type Ge/Si−Ge multi-quantum-well system is the most promising material for the realization of a Si-compatible THz quantum cascade laser operating at room temperature. To advance in this direction, we study, both experimentally and theoretically, asymmetric coupled multi-quantum-well samples based on this...
Article
We provide an in-depth characterization of the dislocation distribution in partially relaxed Si0.92Ge0.08/Si(001) films. This is achieved by an innovative and general method, combining two state-of-the-art characterization techniques through suitable modeling. After having inferred the dislocation positions from transmission-electron-microscopy ima...
Article
In this paper we investigate the structural and optical properties of few strain-unbalanced multiple Ge/GeSi quantum wells pseudomorphically grown on GeSi reverse-graded substrates. The obtained high epitaxial quality demonstrates that strain symmetrization is not a mandatory requirement for few quantum-well repetitions. Photoluminescence data, sup...
Article
Atomic layer deposited aluminum oxide (ALD-Al2O3) is a dielectric material, which is widely used in organic light emitting diodes in order to prevent their organic layers from humidity related degradation. Unfortunately, there are strong hints that in some cases, ALD-Al2O3 itself is suffering from humidity related degradation. Especially, high temp...
Article
Full-text available
Growth mechanism of ScN on Sc 2 O 3 for integration of Ga-polar GaN on Si(111) is investigated by in-situ X-ray photoemission spectroscopy, ex-situ time-of-flight secondary ion mass spectrometry, atomic force microscopy, and ab-initio density functional theory (DFT) calculations. The ScN films are grown by molecular beam epitaxy from e-beam evapora...
Article
The group-IV semiconductor alloy Ge1�x�ySixSny has recently attracted great interest due to its prospective potential for use in optoelectronics, electronics, and photovoltaics. Here, we investigate molecular beam epitaxy grown Ge1�x�ySixSny alloys lattice-matched to Ge with large Si and Sn concentrations of up to 42% and 10%, respectively. The sam...
Article
Full-text available
Growth mechanism of ScN on Sc 2 O 3 for integration of Ga-polar GaN on Si(111) is investigated by in-situ X-ray photoemission spectroscopy, ex-situ time-of-flight secondary ion mass spectrometry, atomic force microscopy, and ab-initio density functional theory (DFT) calculations. The ScN films are grown by molecular beam epitaxy from e-beam evapora...
Article
Full-text available
We report the growth of thin ScN layers deposited by plasma-assisted molecular beam epitaxy on Sc2O3/Y2O3/Si(111) substrates. Using x-ray diffraction, Raman spectroscopy, and transmission electron microscopy, we find that ScN films grown at 600 °C are single crystalline, twin-free with rock-salt crystal structure, and exhibit a direct optical band...
Article
Full-text available
Praseodymia films have been exposed to oxygen plasma at room temperature after deposition on Si(111) via molecular beam epitaxy. Different parameters as film thickness, exposure time and flux during plasma treatment have been varied to study their influence on the oxygen plasma oxidation process. The surface near regions have been investigated by m...
Article
We investigate the structural properties and strain state of Ge nano-structures selectively grown on Si pillars of about 60 nm diameter with different SiGe buffer layers. A matrix of TEOS SiO 2 surrounding the Si nano-pillars causes a tensile strain in the top part at the growth temperature of the buffer that reduces the misfit and supports defect-...
Article
Full-text available
Rare earth praseodymium and cerium oxides have attracted intense research interest in the last few decades, due to their intriguing chemical and physical characteristics. An understanding of the correlation between structure and properties, in particular the surface chemistry, is urgently required for their application in microelectronics, catalysi...
Article
Advanced semiconductor heterostructures are at the very heart of many modern technologies, including aggressively scaled complementary metal oxide semiconductor transistors for high performance computing and laser diodes for low power solid state lighting applications. The control of structural and compositional homogeneity of these semiconductor h...
Article
Full-text available
In this work the structural and morphological changes of Ce$_{1-x}$Pr$_{x}$O$_{2-\delta}$ ($x$ = 0.20, 0.35 and 0.75) films grown on Si(111) due to post depostion annealing are investigated by low energy electron diffraction combined with a spot profile analysis. The surface of the oxide films exhibit mosaics with large terraces separated by monoat...
Article
Full-text available
This paper presents a study of the spatial distribution of strain and lattice orientation in CMOS fabricated strained Ge microstripes using high resolution x-ray micro-diffraction. The recently developed model-free characterization tool, based on a quick scanning x-ray diffraction microscopy technique can image strain down to levels of 10�5 (Da/a)...
Article
Full-text available
The local electronic and atomic structure of (111)-oriented, single crystalline mixed Ce1−xPrxO2−δ (x = 0, 0.1 and 0.6) epitaxial thin films on silicon substrates have been investigated in view of engineering redox properties of complex oxide films. Non-destructive X-ray absorption near edge structure reveals that Pr shows only +3 valence and Ce sh...
Article
Integration of functional oxides on Si substrates could open a pathway to integrate diverse devices on Si-based technology. Oxygen vacancies (Vo˙˙) can strongly affect solid state properties of oxides, including the room temperature ferromagnetism (RTFM) in diluted magnetic oxides. Here, we report a systematical study on the RTFM of oxygen vacancy...
Thesis
Motivation: Ceria attracted a lot of attention for microelectronics (e.g. high-k gate oxide, buffer layer for heteroepitaxy) and catalysis (e.g. selectiveoxidative and dehydrogenating reactions), due to its inherent structural (similar crystal structures and small lattice mismatch with respect to silicon) and electronic (Ce4+/Ce3+ valence state var...
Article
By the growth of a 180 nm thick film on Si(111), we produce a metastable ceria surface with a morphology dominated by terraced pyramids with an oriented triangular base. Changes in the nanoscale surface morphology and local surface potential due to annealing at temperatures ranging from 300 K to 1150 K in the ultra-high vacuum are studied with non-...
Article
The structural changes of ultrathin PrO2(111) films grown on Si(111) due to thermal reduction in ultra high vacuum are investigated. Therefore, praseodymia films are treated with oxygen plasma to obtain completely oxidized PrO2 films. The reduction of these films is studied with thermal desorption spectroscopy combined with ex situ X-ray diffractio...
Article
The inherent properties of epitaxial oxide thin-film layers have attracted the intense interest of different research fields, such as catalysis and microelectronics. The focus of this work is the temperature-dependent oxygen release, oxygen vacancy formation, and lattice rearrangement of Ce1–xPrxO2−δ thin films with systematic stoichiometry variati...
Article
Full-text available
The structural changes of a (111) oriented CeO2 film grown on a Si(111) substrate covered with a hex-Pr2O3(0001) interface layer due to post deposition annealing are investigated. X-ray photoelectron spectroscopy measurements revealing the near surface stoichiometry show that the film reduces continuously upon extended heat treatment. The film is n...
Article
Full-text available
In this work, the morphology of the surface of hexagonal Pr2O3(0 0 0 1) films grown on Si(1 1 1) is studied by high-resolution low-energy electron diffraction combined with spot profile analysis. For this purpose, praseodymia films prepared by molecular beam epitaxy were capped with protecting amorphous germanium films. After removal of the capping...
Article
Full-text available
In this work a 250 nm CeO2(111) film grown on a hex-Pr2O3(0001)/Si(111) system is annealed at 660�°C for 30 min to form the iota bulk phase of Ce7O12 as controlled by x-ray photoelectron spectroscopy. The (111) surface of the stabilized i phase is characterized via high-resolution lowenergy electron diffraction. The i-phase surface exhibits a (r7xr7...
Article
Full-text available
X-ray diffraction and extended X-ray absorption fine structure study of epitaxial mixed ternary bixbyite Pr x Y 22x O 3 (x 5 0–2) films on Si (111) Ternary single crystalline bixbyite Pr x Y 2Àx O 3 films over the full stoichiometry range (x ¼ 0–2) have been epitaxially grown on Si (111) with tailored electronic and crystallographic structure. In t...
Article
Full-text available
Rare earth bixbyite oxides (Re2O3) crystallize on Si (001) in 110-orientation due to either lattice mismatch or oxide/Si interface conditions. In this letter, a 1/2 monolayer (ML) SrO layer was employed as an interface engineering approach to achieve epitaxial ternary oxide PrxY2�xO3 (x¼0–2) films on Si (001) with a sharp oxide/Si (001) interface....
Article
The surface morphology of CeO(2)(111) single crystals and silicon supported ceria films is investigated by non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) for various annealing conditions. Annealing bulk samples at 1100 K results in small terraces with rounded ledges and steps with predominantly one O-Ce-O trip...
Conference Paper
Full-text available
In this work, SrO buffer layers were employed as an interface engineering approach to realize single crystalline mixed ternary rare earth bixbyite oxide (RE2O3) films on Si (001) substrate. Single crystalline mixed PrxY2-xO3 (x=0-2) has been successfully grown on Si (111) (Ref. 7). However, the formation of such mixed oxide can only be realized at...
Article
Epitaxial oxide thin film layers are of interest for model catalytic studies. We report the growth of Ce1−xPrxO2−δ mixed oxide layers of different stoichiometries (x=0–1) and oxygen deficiency (δ>0) on Si(111) by co-evaporating molecular beam epitaxy. The main objective is to identify the crystal phases and to investigate the correlation between co...
Article
Tailored CeO2/Pr2O3 thin-film oxide heterostructures are of interest for model catalysis studies by surface science techniques. For this purpose, thin CeO2(111) films were grown by molecular beam epitaxy on hex-Pr2O3(0001)/Si(111) as well as on cub-Pr2O3(111)/Si(111) support systems. A comparative, rigorous structure investigation by reflection hig...

Projects

Project (1)
Project
The THz part of the electromagnetic spectrum (0.3-10 THz) is currently exploited in commercial security screening systems (weapon detection beneath clothes), medical diagnostics tools (skin and breast cancer, burns, and in ophthalmology) and production-line monitoring (non-destructive test in the pharmaceutical industry). Existing sources of THz radiation are still toolarge and expensive to be a massively deployed in all of the existing and proposed applications, which include large bandwidth wireless communications and the extension of security screening to far-infrared spectroscopic identification of chemicals and explosives up to 10 THz. A lower production cost, a higher level of integration with control electronics, and a broader range of emitted wavelength are all desirable to expand the application of THz radiation. FLASH, will develop a room-temperature THz laser integrated on Si using CMOS technology-compatible processes and materials. The laser, of quantum cascade type, will be assembled using newly developed conduction-band germanium rich heterostructures. It will leverage on the non-polar nature of silicon and germanium crystal lattices to potentially enabling room temperature operation, and will emit over 1 mW of power in the 1-10 THz range. In perspective, the development of the SiGe heterostructure platform will pave the way towards the new field of nonlinear silicon photonics based on band-structure engineering. The consortium includes EU leaders in silicon chip manufacturing, Si/SiGe/Ge epitaxial material growth, laser and band structure modelling, quantum cascade laser design and terahertz/infrared spectroscopy. The proposed device can provide a step-change in compactness, reduced cost, and functionality of source performance, thus enabling large scale use of terahertz radiation in existing fields of application, and open up new fields of application commercially exploited, such as wireless communication and security imaging. FLASH is funded by EC under the FET-OPEN scheme of Horizon2020. Consortium: - Universita Degli Studi Roma Tre, Rome, Italy (Coordinator) - University of Glasgow, UK - IHP GmbH, Frankfurt (Oder), Germany - ETH Zurich, Switzerland - nextnano GmbH, Munich, Germany