Andre Wachowiak

NaMLab GmbH

Inconsistencies in the concentrations of unintentional donor impurities and free charge carriers in GaN/AlGaN layer stacks hosting a two-dimensional electron gas (2DEG) can be attributed to the measurement procedure and solely depend on the way in which the free charge carrier concentration is extracted. Particularly, when the 2DEG acts as the bott...

The trench gate MOSFET represents a prominent device architecture among the GaN based vertical devices currently investigated for the next generation of power electronics. A low leakage current level in off-state under high drain bias is of great importance for vertical transistors since it is a crucial feature for high breakdown voltage and device...

Insulated-gate GaN-based transistors can fulfill the emerging demands for the future generation of highly efficient electronics for high-frequency, high-power and high-temperature applications. However, in contrast to Si-based devices, the introduction of an insulator on (Al)GaN is complicated by the absence of a high-quality native oxide for GaN....

Trap states at the dielectric/GaN interface of AlGaN/GaN‐based metal–insulator–semiconductor high electron mobility transistors (MIS‐HEMTs) can cause threshold voltage (Vth) instability especially under positive gate bias stress. Herein, the influence of O2 plasma surface preconditioning (SPC) before the atomic layer deposition of the Al2O3 gate di...

The presence of a two‐dimensional electron gas (2DEG) in GaN/AlxGa1‐xN heterostructures with low aluminum mole fraction is found to depend on the residual background impurity concentration in the GaN/AlGaN layer stack. At a residual donor level of 2×1016cm−3 a 2DEG is absent at 300 K in dark environment. Such a 2DEG can be generated at the GaN/AlGa...

We report and discuss the performance of an enhancement mode n-channel pseudo-vertical GaN metal oxide semiconductor field effect transistor (MOSFET). The trench gate structure of the MOSFET is uniformly covered with an Al2O3 dielectric and TiN electrode material, both deposited by atomic layer deposition (ALD). Normally-off device operation is dem...

Gallium nitride (GaN) has emerged as an essential semiconductor material for energy-efficient lighting and electronic applications owing to its large direct bandgap of 3.4 eV. Present GaN/AlGaN heterostructures seemingly feature an inherently existing, highly-mobile 2-dimensional electron gas (2DEG), which results in normally-on transistor characte...

In this report the influence of the growth conditions on the surface morphology of AlGaN/GaN heterostructures grown on sapphire-based and bulk GaN substrates is nondestructively investigated with focus on the decoration of defects and the surface roughness. Under Ga-rich conditions specific types of dislocations are unintentionally decorated with s...

Scanning spreading resistance microscopy (SSRM) with its high spatial resolution and high dynamic signal range is a powerful tool for two-dimensional characterization of semiconductor dopant areas. However, the application of the method is limited to devices in equilibrium condition, as the investigation of actively operated devices would imply pot...

In this report, the operation of a normally-off vertical gallium nitride (GaN) metal-oxide field effect transistor with a threshold voltage of 5 V is demonstrated. A crucial step during device fabrication is the formation of the highly n-doped source layer. The authors infer that the use of molecular beam epitaxy (MBE) is highly beneficial for supp...

Metal–insulator–semiconductor (MIS) capacitor structures were fabricated on AlGaN/GaN two-dimensional electron gas heterostructure material in order to investigate important aspects of the gate module of a corresponding MIS-high electron mobility transistor device. The process sequence started with an initial wet chemical surface treatment of the a...

We report on the fabrication and characterisation of an enhancement mode n-channel pseudo-vertical GaN metal oxide semiconductor field effect transistor (MOSFET), which utilizes a high-k dielectric covered trench gate and a top side drain contact. The processing technology has been developed to be easily transferrable to a truly vertical MOSFET on...

The p-doping of GaN is typically done with Mg. Because of its high ionization energy Mg is not completely ionized at room temperature. This has to be considered in simulations for defining characteristic values and designing devices, if p-GaN with Mg incorporation is involved. In this work the influence of simulation with a constant ionization rate...

We report on the investigation of the Vth drift behaviour of AlGaN/GaN MISHEMTs upon forward gate voltage stress in dependence of stress bias and stress time. The pulsed measurements allow for the evaluation of the operational regime for optimum device efficiency. We compared the effect of two different high-κ gate dielectric materials with similar...

This paper investigates the origin of the threshold switching effect in NbO2. It is found that the effect is independent of the metal-insulator-transition but can be explained by a trap-assisted Frenkel-Poole like conduction mechanism in combination with a moderate temperature increase by only 150 K due to Joule heating. These findings lead to the...

In this work, we present the terrace etching technique to obtain excessive thickness series of atomic layer deposition (ALD) grown Al2O3 and HfO2 on GaN-cap/AlGaN/GaN heterostructures allowing for the detailed study of oxide charge distribution and its impact of the metal-insulator-semiconductor high electron mobility transistor (MISHEMT) threshold...

We propose and demonstrate an efficient approach to extract key parameters of GaN-based 2DEG heterostructures grown on conducting Silicon substrates. The methodology enables an electrical feedback on different epitaxial design or MOCVD growth conditions in a very short-time frame by means of vertical capacitance–voltage (C–V) measurement on simple...

Abstract Scanning Spreading Resistance Microscopy (SSRM) is applied to investigate single failing CMOS transistors within an integrated circuit. The failing devices are affected by increased gate depletion, and therefore, lower on-current (ION) compared to reference devices. Two different scenarios exhibiting enhanced gate depletion as root cause f...

The breakdown mechanisms of a vertical GaN-MOSFET under variation of several structure parameters were investigated. The device simulation results show different breakdown locations and breakdown voltages according to structure parameters. The maximum breakdown voltage reaches 1.6 kV without a field plate. Furthermore the influence of a field plate...

In this paper, we present mobility investigations of strained nMOS and pMOS short-channel transistors with dimensions down to 30-nm gate length. Using the geometrical magnetoresistance (MR) effect, carrier mobility of electrons and holes in the inversion channel of a recent state-of-the-art CMOS technology is presented from linear to saturation ope...

Atomic layer deposition-grown Al2O3 thin films are grown on n-type GaN and annealed at 300 or 500 °C in various atmospheres. Metal–insulator–semiconductor capacitors (MISCAPs) are used as simplified test structures for AlGaN/GaN heterostructure field effect transistors with an Al2O3 gate dielectric. Electrical characterization of the unannealed MIS...

In this work, we investigate the temperature dependence of electrical switching properties of back-gated, undoped Si-nanowire field-effect transistors with Ni-silicided source/drain contacts. A simple, phenomenological model illustrates the leading order temperature dependence of the source-drain current, which originates predominantly from charge...

Scanning Spreading Resistance Microscopy (SSRM) is successfully applied to investigate failing nLDMOS test devices that exhibit a lowered break down voltage (BVDSS) in electrical test. Cross-sectional, two-dimensional maps of the local sample resistivity from fail and reference (pass) devices reveal significant differences of the dopant concentrati...

We describe a well-controlled experiment enabling the generation of multiple locally blocked implant sites within an integrated circuit in order to evaluate Scanning Spreading Resistance Microscopy (SSRM) as analysis method. Transistor gate structures with polycrystalline (poly-)Si material and source/drain regions were locally affected by lower im...

Hot-phonon effects on hot-electron noise and transport are investigated in nominally undoped two-dimensional Al0.23Ga0.77N/GaN channel with electron density of 7.4 × 1012 cm-2. The electrons are subjected to electric field applied in the channel plane. The dependence of noise temperature on supplied electric power yields hot-phonon lifetime of (370...

In this work we present a new operational principle of an adjustable, high resolution color sensor. The concept is based on a single photodiode, which was specifically designed for a variable spectral response, corresponding to the extension of its space charge region. A wide range of spectral sensitivity functions (i.e. the receptor function of th...

This work presents a HEMT test structure technology which is developed for fast on-wafer characterization of 150mm epitaxial GaN-on-Si material. The test structures allow for extraction of key device and GaN-based material parameter. Information on wafer homogeneity can be obtained via wafer maps. This technology is suitable and essential to evalua...

Molecular beam deposited zirconium dioxide (ZrO2) was assessed as high-kappa gate dielectric for future GaN based devices. To compare and study electrical and structural properties, thin ZrO2 films were deposited on three different substrates, n(++)-c-plane GaN, p-(100) Si, and TiN. The films were fabricated by electron beam evaporation from a sing...

The gate leakage (IGate, table 1) is reduced compared to the conventional 65nm process with SiON dielectric (Fig. 2). The leakage current due to direct tunneling is simulated using the CET as fitting parameter. High-k PFETs with an oxide extension spacer show a decrease in leakage density with reducing channel length, due to an average CET increase...

In this article, the authors analyze the impact of germanium amorphization on the interface defect concentration of state of the art high-k metal gate metal-oxide-semiconductor field-effect transistors. The gate etch is a crucial process step for the high-k gate first integration approach. Germanium implantation is used to amorphize the annealed an...

We have examined the local electronic behavior of titanocene chloride dimer molecules, [Cp2TiCl]2 (Cp = C5H5), on Au(111) using both scanning tunneling microscopy (STM) and density functional theory (DFT). Isolated dimeric molecules are seen to decorate gold step edges at low surface coverages and two coexisting monolayer phases are observed at hig...

C60 fullerides are uniquely flexible molecular materials that exhibit a rich variety of behaviour, including superconductivity and magnetism in bulk compounds, novel electronic and orientational phases in thin films and quantum transport in a single-C60 transistor. The complexity of fulleride properties stems from the existence of many competing in...

We have performed a scanning tunneling microscopy and spectroscopy study of potassium-doped C60 monolayers (KxC60) on Ag(001) in the regime of x ~ 1. Low temperature annealing (640 K) leads to the formation of two well-ordered KxC60 phases that exhibit differing levels of electron charge transfer. Further annealing (710 K) distills out the higher e...

STM studies on K(x)C(60) monolayers reveal new behavior over a wide range of the phase diagram. As x increases from 3 to 5 K(x)C(60) monolayers undergo metal-insulator-metal reentrant phase transitions and exhibit a variety of novel orientational orderings, including a complex 7-molecule, pinwheel-like structure. The proposed driving mechanism for...

STM studies on KxC60 monolayers reveal new behavior over a wide range of the phase diagram. As x increases from 3 to 5 KxC60 monolayers undergo metal-insulator-metal reentrant phase transitions and exhibit a variety of novel orientational orderings, including a complex 7-molecule, pinwheel-like structure. The proposed driving mechanism for the orie...

We investigate the effect of dimensionality on the properties of potassium doped C60 (KxC60) by studying thin films with precisely controlled doping levels and layer structures using scanning tunneling microscopy and spectroscopy. We observe systematic variation in spatial and electronic structure as the films change from the 2D to the quasi-3D reg...

Orientational ordering, which originates from broken rotational symmetry, is a central feature of a broad range of materials including liquid crystals, quantum magnets, and some biological systems. By doping C60 monolayers with alkali impurities, the symmetry of C60 molecules can be broken, opening up the possibility for unique two-dimensional mole...

Diamondoids are hydrocarbon molecules with diamond-like cage structures. Their structural complexity and chemical bond tunability make them ideal building blocks for creating novel nanostructures. We have used cryogenic scanning tunneling spectroscopy to examine the electronic structure of individual diamondoid molecules on the Au(111) surface. We...

The fullerene molecule C60 is known to undergo a strong Jahn-Teller distortion when electrons are added. Recent STM/STS experiments indicate that a single C60 molecule on a Ag(100) surface can be controllably doped with charge-donating potassium atoms; [1] moreover, the experiments suggest that the molecular electronic structure can be tuned so tha...

The titanocene dimer ([Cp2TiCl]2, where Cp = C5H5 ) is an interesting magnetic molecule because it incorporates two spin-1/2 Ti atoms in an antiferromagnetic configuration. We have used cryogenic scanning tunneling microscopy to study the local electronic properties of titanocene dimer molecules adsorbed onto metal surfaces. Ordered patterns of tit...

We present a low-temperature scanning tunneling microscopy (STM) study of KxC60 monolayers on Au(111) for 3 ≤ x ≤ 4. The STM spectrum evolves from one that is characteristic of a metal at x = 3 to one that is characteristic of an insulator at x = 4. This electronic transition is accompanied by a dramatic structural rearrangement of the C60 molecule...

We have observed variable negative differential resistance (NDR) in scanning tunneling spectroscopy measurements of a double layer of C60 molecules on a metallic surface. Minimum to maximum current ratios in the NDR region are tuned by changing the tunneling barrier width. The multilayer geometry is critical, as NDR is not observed when tunneling i...

Future molecular technologies are expected to rely on the ability to locally control the electronic properties of single molecules and molecular complexes. It is critical therefore to understand the influence on molecular properties of different molecular spatial and electronic configurations in the presence of metal electrodes. Here we present a n...

We describe the design and development of a scanning tunneling micoscope (STM) working at very low temperatures in ultra-high vacuum (UHV) and at high magnetic fields. The STM is mounted to the 3He pot of an entirely UHV compatible 3He refrigerator inside a tube which can be baked out to achieve UHV conditions even at room temperature. A base tempe...

The growth morphology and electronic structure of Co(0001) grown on W(110) are studied using scanning tunneling microscopy and scanning tunneling spectroscopy (STS) at T=6 K. Depending on growth conditions, continuous Co films or Co islands on top of a wetting layer are formed. Within the continuous films, dislocation lines appear and increase in d...

Recent developments in spin-polarized scanning tunneling microscopy and spectroscopy have led to an unprecedented insight into magnetism at the nanometer length scale and, in some cases, even down to the atomic level. The correlation between structural, local electronic and local magnetic structure can now be studied beyond the exchange length. Mos...

We report a method for controllably attaching an arbitrary number of charge dopant atoms directly to a single, isolated molecule. Charge-donating K atoms adsorbed on a silver surface were reversibly attached to a C60 molecule by moving it over K atoms with a scanning tunneling microscope tip. Spectroscopic measurements reveal that each attached K a...

We have studied the thickness dependent domain configuration of single-crystal nanoscale Fe islands on W(110) by spin-polarized scanning tunneling microscopy. The experimental results are compared with micromagnetic calculations. For very thin islands, the uniaxial surface anisotropy of Fe/W(110) leads to a single domain state. With increasing isla...

We present a novel method for controllably changing the charge state of a single C_60 molecule by reversibly attaching individual dopant K atoms to its outer surface. Molecular manipulation with a cryogenic scanning tunneling microscope was used to place individual C_60 molecules in contact with charge donating K adsorbates on a Ag(001) surface. By...

The electronic properties of C_60 compounds can be tuned by charge-doping them with alkali impurities. This results in an interplay between molecular charge transfer, Coulomb repulsion, phonon coupling, and nearest neighbor interactions. Here we present a scanning tunneling microscopy/spectroscopy study of K doped C_60 monolayers on Ag(001). We fin...

A key to the success of future molecular technologies is the ability to locally control the electronic properties of single molecules and molecular complexes. Using the molecular manipulation capabilities of the scanning tunneling microscope (STM), we have demonstrated the ability to fabricate K_xC_60 (0

Thin film nanoscale elements with a curling magnetic structure (vortex) are a promising candidate for future nonvolatile data storage devices. Their properties are strongly influenced by the spin structure in the vortex core. We have used spin-polarized scanning tunneling microscopy on nanoscale iron islands to probe for the first time the internal...

The coverage dependence of the Co-induced band bending on p-type InAs(110) (NA=4.6×1017 cm-3) is investigated by angle-resolved photoelectron spectroscopy and interpreted in terms of the morphology of the adsorbate layer deduced from scanning-tunneling-microscopy (STM) images. After room temperature deposition Co forms small islands with a density...