Jan Voves

Optical characterisation of MOVPE grown vertically correlated InAs/GaAs quantum dots

Structures with vertically correlated self-organised InAs quantum dots (QDs) in a GaAs matrix were grown by the low-pressure metal-organic vapour phase epitaxy (MOVPE) and characterised by different microscopic techniques. Photoluminescence in combination with photomodulated reflectance spectroscopy were applied for characterisation of QDs structures. We show that combination of both methods allows detecting optical transitions originating both from QDs and wetting (separation) layers, which can be than compared with those obtained from numerical simulations. On the basis of obtained results, we demonstrate that photoreflectance spectroscopy is an excellent tool for characterisation of QDs structures wetting layers and for identification of spacer thicknesses in vertically stacked QDs structures.

InAs δ-layer structures in GaAs grown by MOVPE and characterised by luminescence and photocurrent spectroscopy

Abstract Photocurrent, electroluminescence, and photoluminescence spectroscopy were used for the characterisation of laser structures containing ultrathin InAs δ -layers in GaAs matrix surrounded by AlGaAs waveguide and grown by low-pressure metal organic vapour phase epitaxy. Three types of δ -layer structures for laser active layers were investigated: single layers with different thickness ( W L ), different numbers of identical layers ( N ), and seven identical δ -layers separated by GaAs spacers of variable thickness ( S L ). Measurement revealed two fundamental optical transition between electron and heavy and light hole states in the δ -layers. Both transitions are shifted to lower energies by hundreds of meV when W L and N increases or S L decreases. While the effect of W L can be explained by a quantum model accounting for the influence of stress and quantum state coupling, data obtained from multilayer structures exhibit significant deviation from theory.

Lasers with δ InAs layers in GaAs

Abstract Electroluminescence of lasers with different numbers (1, 3, 5, 7) of δ InAs layers in GaAs prepared by Low Pressure Metal–Organic Vapor Phase Epitaxy was investigated in a broad temperature range from 10 to 400 K. The dependence of the electroluminescence spectra on the number of δ InAs layers and on the separation of these δ InAs layers was studied under pulse excitation in a wide range of current densities. Results show that by increasing the number of δ InAs layers and decreasing the distance between these layers it is possible to decrease the lasing emission energy below 1.15 eV. Our δ InAs lasers operate even at temperatures above 100 °C, they exhibit weak temperature dependence of threshold current density with values lower than 0.2 kA cm−2 and their differential quantum efficiency lies between 12 and 18%.

Gas-sensing behaviour of ZnO/diamond nanostructures

Microstructured single- and double-layered sensor devices based on p-type hydrogen-terminated nanocrystalline diamond (NCD) films and/or n-type ZnO nanorods (NRs) have been obtained via a facile microwave-plasma-enhanced chemical vapour deposition process or a hydrothermal growth procedure. The morphology and crystal structure of the synthesized materials was analysed with scanning electron microscopy, X-ray diffraction measurements and Raman spectroscopy. The gas sensing properties of the sensors based on i) NCD films, ii) ZnO nanorods, and iii) hybrid ZnO NRs/NCD structures were evaluated with respect to oxidizing (i.e., NO2, CO2) and reducing (i.e., NH3) gases at 150 °C. The hybrid ZnO NRs/NCD sensor showed a remarkably enhanced NO2 response compared to the ZnO NRs sensor. Further, inspired by this special hybrid structure, the simulation of interaction between the gas molecules (NO2 and CO2) and hybrid ZnO NRs/NCD sensor was studied using DFT calculations.

SiC graphene FET with polydimethylglutharimide as a gate dielectric layer

Graphene is perspective material for future carbon based electronics, flexible electronics and other applications. The necessary condition for the commercial use is the high quality graphene growth and semiconductor technology compatible process of whole field effect transistor (FET). One of suitable method for large scale graphene monolayer preparation is the thermal annealing of semi-insulating SiC substrate. One important task of graphene FET process is reliable, cheap and simple gate structure preparation. In this work we present our results of using MicroChem Lift-Off Resist (LOR) layer as a dielectric layer for SiC graphene FETs. LOR resist is based on polydimethylglutharimide. Its unique properties enable to perform exceptionally well resolution imaging, easy process tuning, high yields and superior deposition line width control. In the case of polymer based dielectric layers the breakdown voltage is important parameter. We prepared two sets of different capacitor structures with LOR dielectric layer and Au/Cr electrodes. The first set exhibits very low breakdown voltages (about 3 V). The optimisation of the LOR layer deposition process in the second set increased the breakdown voltage over 40 V keeping the leakage current lower than 2 nA. The second process with LOR layer was used for the preparation of graphene FETs on SiC substrates. The first measurements show resistivity dependence on gate voltage.

Hydrogen silsesquioxane as a gate dielectric layer for SiC graphene FET

Graphene can be prepared by annealing of SiC wafer. That allows large scale patterning by standard UV photolithography. Unfortunately SiC substrate does not allow backgating in contrast to graphene on silicon substrate (with thin silicon dioxide layer). The major challenge is to find suitable dielectric layer that can be used for electrostatic gating without significant influence on carrier mobility or another properties of graphene. We examined electrical behavior of electron exposed hydrogen silsesquioxane (HSQ) layer used as dielectric layer of topgated SiC graphene. We prepared seven sets of capacitor structures for test of HSQ layer electrical properties. The capacitors have different dimensions (100x5 - 100x100 μm) and each set had different exposure energy (12 - 480μC/cm2) and annealing process. Electrodes and contacts were prepared by evaporation of 3/30 nm thick Cr/Au layer. The influence of exposure energy to electrical properties of HSQ layer was observed. The dimension of capacitor structures had lower effect than exposure energy. The gated Hall-bar structure of SiC graphene will be prepared subsequently. Hall-bar will be defined by e-beam lithography, etched by oxygen plasma and contacted by evaporation. HSQ will be used as a gate dielectric.

The characterization of the hole transport in Sb based strained quantum wells

Compressively strained InSb structures aimed for p-channel heterostructure field effect transistors (HFETs) are analysed in order to maximise their hole mobility. We optimise the heterostructure by the change of the material composition, thickness of the layers and position of δ-doping. The splitting of light and heavy hole bands in compressively strained channels are calculated by nextnano3 software as a function of material mole fraction and quantum well thickness. The 8 × 8 k.p method is used for the determination of corresponding hole eigenvalues. The strained layer bandstructure calculation is carried out with Quantumwise ATK for the Kohn-Sham DFT using double-zeta polarised basis set orbitals as well.

Quantum size InAs/GaAs lasers-preparation and properties

Semiconductor lasers based on quantum dots or very thin InAs strained quantum wells have been intensively studied during the last few years. The advantage of a zero-dimensional structure lies in its /spl square/-function density of states but this is devalued by the large fluctuation of the size and shape of quantum dots. However, the higher electroluminescence efficiency and higher working temperature remain. That is why the use of very thin strained quantum wells can be a reasonable compromise for the preparation of highly efficient lasers emitting near 1 /spl mu/m. In this paper, we report the preparation and parameters of laser structures based on very thin strained single and multiple quantum wells working at room and elevated temperatures. The WA characteristics, threshold current density and efficiency of these lasers were studied in the temperature range from 10 K to 370 K.

The Study of Graphene Gas Sensor

The graphene is suitable for gas sensing applications for its two dimensional char-acter which gives the best possible ratio between sensor surface and volume. The interactionbetween graphene surface and gas molecules can signicantly change the graphene layer trans-port properties. Therefore graphene can serve as a sensitive layer in a gas sensor. This work isconcentrated on the analysis of the conductivity of graphene layer exposed to dierent gases(NH3, CO2 ...). Together with the electrical measurement on the interdigital graphene sensora simulation based on quantum atomistic approach has been performed. We used ATK toolkitby Quantuwise based on density functional theory (DFT) models. The exchange-correlationpotential is approximated within the generalized gradient approximation (GGA). The trans-port properties of the electrode-device- electrode geometry were calculated by means of non-equilibrium Greens function formalism as implemented in ATK. Experimental conductivitychanges are compared with the simulation results.

Hole transport in the p-type RTD

Results of a RTD structure simulation by means of the nextnano3 [4] are shown. IV curve simulation by means wingreen program [3] and comparison with experimental results are presented in this paper. Our approach is based on the non-equilibrium greens function (NEGF). We used the program Wingreen [3] only with the single band model apart for the heavy and light holes. We summarized the HH and LH transmissivity. The transmissivities and the local density of states are calculated by means of nextnano3 [4] using Contact Block Reduction methodfor the comparison. Our simulations can serve as the relatively fast estimate of the p-type RTD behavior. Our results are verified by the comparison with the experimental data measured on the MBE grown p-type RTD structures. Measured and simulated I-V characteristics for the p-RTD are shown in the Figure.