Stefan Birner

nextnano GmbH

We compare InAs-based and GaSb-based interband cascade lasers (ICLs) with the same 12-stage active region designed to emit at a wavelength of 4.6 µm. They employ a hybrid cladding architecture with the same geometry and inner claddings consisting of InAs/AlSb superlattices but different outer claddings: The InAs-based ICL employs plasmon enhanced n...

We present a comparison between interband cascade lasers (ICLs) with a six-stage active region emitting at 5 μm with AlSb/InAs superlattice claddings and with bulk Al0.85Ga0.15As0.07Sb0.93 claddings. Utilizing bulk AlGaAsSb claddings with their lower refractive index compared to the more commonly used AlSb/InAs superlattice claddings, the mode-conf...

We present an interband cascade laser (ICL) emitting at 5.2 µm consisting of an 8-stage active region and a hybrid cladding composed of outer plasmon-enhanced InAs0.915Sb0.085 and inner InAs/AlSb superlattice claddings. The hybrid cladding architecture shows an increase in mode confinement in the active region by 11.2% according to the simulation....

In quantum nanoelectronics, numerical simulations have become a ubiquitous tool. Yet the comparison with experiments is often done at a qualitative level or restricted to a single device with a handful of fitting parameters. In this work, we assess the predictive power of these simulations by comparing the results of a single model with a large exp...

The progress of charge manipulation in semiconductor-based nanoscale devices opened up a novel route to realise a flying qubit with a single electron. In the present review, we introduce the concept of these electron flying qubits, discuss their most promising realisations and show how numerical simulations are applicable to accelerate experimental...

A significant enhancement in the output power of a GaAs-based terahertz quantum cascade laser (THz QCL) was achieved by implementing a relatively high impurity doping concentration. The QC structure was precisely designed using the nonequilibrium green function method by considering the band bending effect caused by a higher doping concentration. T...

The progress of charge manipulation in semiconductor-based nanoscale devices opened up a novel route to realise a flying qubit with a single electron. In the present review, we introduce the concept of these electron flying qubits, discuss their most promising realisations and show how numerical simulations are applicable to accelerate experimental...

In quantum nanoelectronics, numerical simulations have become an ubiquitous tool. Yet the comparison with experiments is often done at a qualitative level or restricted to a single device with a handful of fitting parameters. In this work, we assess the predictive power of these simulations by comparing the results of a single model with a large ex...

A GaAs/AlGaAs based terahertz quantum cascade laser with a peak power of 1.31 W and average power of 52 mW is demonstrated. In order to realize a high output power, an effective way is to increase the electron density at active region by using high doping level. This work aims to optimize a design enabling to use a heavy doping level, by re‐alignin...

A 1.31 W peak power and 52 mW average power THz QCL is presented by variable Al composition active structure with high doping concentration based on NEGF method design. Device has thick growth active layers and large mesa size with the consideration of heat dissipation.

We report electroluminescence originating from L-valley transitions in n-type Ge/Si0.15Ge0.85 quantum cascade structures centered at 3.4 and 4.9 THz with a line broadening of Δ f / f ≈ 0.2. Three strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. The design is based on...

We report electroluminescence originating from L-valley transitions in n-type Ge/Si$_{0.15}$Ge$_{0.85}$ quantum cascade structures centered at 3.4 and 4.9 THz with a line broadening of $\Delta f/f \approx 0.2$. Three strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. T...

In this manuscript, we develop a generalized theory for the scattering process produced by interface roughness on charge carriers that is suitable for any semiconductor heterostructure. By exploiting our experimental insights into the three-dimensional atomic landscape of Ge/Ge-Si heterointerfaces obtained by atom probe tomography, we are able to d...

While GeSn alloys with high Sn content constitute direct group-IV semiconductors, their growth on Si remains challenging. The deposition of a few monolayers of pure Sn on Ge and their overgrowth with Ge using molecular beam epitaxy can be a means of obtaining Sn-rich quantum wells with very high Sn content while maintaining high crystal quality. He...

The waveguide losses from a range of surface plasmon and double metal waveguides for Ge/Si1−xGe x THz quantum cascade laser gain media are investigated at 4.79 THz (62.6 μm wavelength). Double metal waveguides demonstrate lower losses than surface plasmonic guiding with minimum losses for a 10 μm thick active gain region with silver metal of 21 cm⁻...

We develop a generalized theory for the scattering process produced by interface roughness on charge carriers and which is suitable for any semiconductor heterostructure. By exploiting our experimental insights into the three-dimensional atomic landscape obtained on Ge/GeSi heterointerfaces obtained by atom probe tomography, we have been able to de...

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)...

Operating at high temperatures in the range of thermoelectric coolers is essential for terahertz quantum cascade lasers to real applications. The use of scattering-assisted injection scheme enables an increase in operating temperature. This concept, however, has not been implemented in a short-period structure consisting of two quantum wells. In th...

n-type Ge/SiGe terahertz quantum cascade lasers are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs quantum cascade laser design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling ope...

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...

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...

n-type Ge/SiGe terahertz quantum cascade laser are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs QCL design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling operation up to room t...

Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared t...

Carrier transport in GaN terahertz (THz) quantum cascade laser (QCL) structures is theoretically investigated using a non-equilibrium Green's function method. Although scattering due to polar optical phonons in GaN is greatly enhanced with respect to GaAs/AlGaAs THz QCLs, the phonon-induced broadening of the laser levels is found to remain much sma...

We used high hydrostatic pressure to perform photoluminescence measurements on polar ZnO/ZnMgO quantum well structures. Our structure oriented along the c-direction (polar direction) was grown by plasma-assisted molecular beam epitaxy on a-plane sapphire. Due to the intrinsic electric field, which exists in polar wurtzite structure at ambient pres...

Dislocations exhibit a number of exceptional electronic properties resulting in a significant increase in the drain current of MOSFETs if defined numbers of these defects are placed in the channel. Measurements on individual dislocations in Si refer to a supermetallic conductivity. A model of the electronic structure of dislocations is proposed bas...

We present a novel and very efficient method for calculating quantum transport in quantum cascade lasers (QCLs). It follows the nonequilibrium Green’s function (NEGF) framework but sidesteps the calculation of lesser self-energies by replacing them by a quasi-equilibrium expression. This method generalizes the phenomenological Büttiker probe model...

In this contribution all the various definitions of the k·p parameters available in the literature are summarized, and equations that relate them to each other are provided. We believe that such a summary for both zinc blende and wurtzite crystals on a few pages is very useful, not only for beginners but also for experienced researchers that quickl...

Photovoltaic nano-devices have largely been relying on charge separation in conventional p-n junctions. Junction formation via doping, however, imposes major challenges in process control. Here, we report on a concept for photovoltaic energy conversion at the nano scale without the need for intentional doping. Our approach relies on charge carrier...

We present theoretical investigations of miniband structures and optical properties of InAs/GaAs one-dimensional quantum dot superlattices (1D-QDSLs). The calculation is based on the multi-band k·p theory, including the conduction and valence band mixing effects, the strain effect, and the piezoelectric effect; all three effects have periodic bound...

We present numerical simulations of the capacitive coupling between graphene nanoribbons of various widths and gate electrodes in different configurations. We compare the influence of lateral metallic or graphene side gate structures on the overall back gate capacitive coupling. Most interest- ingly, we find a complex interplay between quantum capa...

DOI:https://doi.org/10.1103/PhysRevLett.111.249901

Here we report our research on quantum-dot structures with collective barriers surrounding groups of quantum dots (planes, clusters etc) and preventing photoelectron capture. Employing Monte-Carlo simulations, we investigate photoelectron kinetics and calculate the photoelectron lifetime as a function of geometrical parameters of the structures, do...

The charging of quantum dots provides two strong effects which improve Quantum Dot Infrared Photodetector (QDIP) performance. First, electrons placed in the quantum dots enhance IR-induced transitions and increase electron coupling to IR radiation. Second, the built-in-dot charge creates potential barriers around dots and these barriers strongly su...

Whereas thermoelectric performance is normally limited by the figure of merit ZT, transverse thermoelectrics can achieve arbitrarily large temperature differences in a single leg even with inferior ZT by being geometrically tapered. We introduce a band-engineered transverse thermoelectric with p-type Seebeck in one direction and n-type orthogonal,...

Whereas thermoelectric performance is normally limited by the figure of merit ZT, transverse thermoelectrics can achieve arbitrarily large temperature differences in a single leg even with inferior ZT by being geometrically tapered. We introduce a band-engineered transverse thermoelectric with p-type Seebeck in one direction and n-type orthogonal,...

Here we report our research on quantum-dot structures with collective barriers surrounding groups of quantum dots (planes, clusters etc) and preventing photoelectron capture. Employing Monte-Carlo simulations, we investigate photoelectron kinetics and calculate the photoelectron lifetime as a function of geometrical parameters of the structures, do...

Narrow gap materials with anisotropic electron and hole band conductance are shown to function as anisotropic two-band transverse (A2T) thermoelectrics, whereby longitudinal electrical currents generate transverse Peltier heat flow. Unlike the Ettingshausen effect which requires external magnetic field, a large transverse Seebeck coefficient in A2T...

Leo Esaki originally proposed that by increasing the layer thickness, InAs/GaSb superlattices can be tuned from a semiconducting to a semimetallic state, where electron and hole wavefunctions are spatially localized to InAs and GaSb layers, respectively. Because of the tunably small spatially indirect gap of InAs/GaSb and the anisotropy of the supe...

The hydrophobic interaction of surfaces with water is a well-known phenomenon, but experimental evidence of its influence on biosensor devices has been lacking. In this work we investigate diamond field-effect devices, reporting on Hall effect experiments and complementary simulations of the interfacial potential at the hydrogen-terminated diamond/...

Graphene, with its unique combination of physical and electronic properties, holds great promise for biosensor and bioelectronic applications. In this respect, the development of graphene solution-gated field-effect transistor (SGFET) arrays capable of operation in aqueous environments will establish the real potential of graphene in this rapidly e...

This paper details a complete formalism for calculating electron subband energy and degeneracy in strained multi-valley quantum wells grown along any orientation with explicit results for the AlAs quantum well case. A standardized rotation matrix is defined to transform from the conventional- cubic-cell basis to the quantum-well-transport basis whe...

We have studied the excitation-and polarization-dependent optical properties of GaN/AlN self-assembled quantum dots QDs grown on Si111 substrates. Ensembles of QDs were subject to various external stress configurations that resulted from the thermal expansion coefficient mismatch between the GaN/AlN layers and the Si111 substrate and ranged from in...

The contact block reduction (CBR) method is a variant of the nonequilibrium Green’s function formalism and can be used to describe quantum transport in the ballistic limit very efficiently. We present a numerical implementation of a charge self-consistent version of the CBR algorithm. We show in detail how to calculate the electronic properties of...

We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well (MQW) structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3–1.9 μm wavelength range, originating from the photoexcitation of electrons from the first to the se...

We studied a doping series of (110)-oriented AlAs quantum wells (QWs) and observed transport evidence of single anisotropic-mass valley occupancy for the electrons in a 150 \AA wide QW. Our calculations of strain and quantum confinement for these samples predict single anisotropic-mass valley occupancy for well widths $W$ greater than 53 \AA. Below...

For a bio-sensor device based on a silicon-on-insulator structure, we calculate the sensitivity to specific charge distributions in the electrolyte solution that arise from protein binding to the semiconductor surface. This surface is bio-functionalized with a lipid layer so that proteins can specifically bind to the headgroups of the lipids on the...

The dependence of the photoluminescence (PL) emission wavelength of SiGe islands embedded into a Si matrix on their Ge concentration and gradient was investigated. Intense PL signals at wavelengths that can be shifted over most of the telecom wavelength range (1.38-1.77 mu m) by varying the Ge concentration were observed. Using the structural islan...

Quantum dots (QDs) are of special interest for both fundamental physics and optoelectronic applications. Controlled growth of ensembles of self assembled GaN/AIN QDs showing efficient room-temperature luminescence has been demonstrated recently. On the other hand, GaN-based devices have been shown useful to detect chemically induced changes of the...

Modern nanotechnology offers routes to create new artificial materials, widening the functionality of devices in physics, chemistry, and biology. Templated self-organization has been recognized as a possible route to achieve exact positioning of quantum dots to create quantum dot arrays, molecules, and crystals. Here we employ extreme ultraviolet i...

nextnano is a semiconductor nanodevice simulation tool that has been developed for predicting and understanding a wide range of electronic and optical properties of semiconductor nanostructures. The underlying idea is to provide a robust and generic framework for modeling device applications in the field of nanosized semiconductor heterostructures....

By using two‐photon absorption spectroscopy we have performed time‐integrated and time‐resolved photoluminescence measurements on several coupled asymmetric GaN quantum discs, with embedded AlGaN barriers. We observe free‐carrier screening, with an enhancement due to tunneling between the coupled quantum discs.

We present a systematic investigation of free-carrier screening in coupled asymmetric GaN quantum discs with embedded AlGaN barriers using time-integrated and time-resolved micro-photoluminescence measurements, supported by three-dimensional multi-band k·p computational modeling. Free-carrier screening effects decreased with the barrier thickness,...