Martin Florovič

Influence of the AP MOVPE process parameters on properties of (In, Ga)(As, N)/ GaAs heterostructures for photovoltaic applications

GaAsN and InGaAsN semiconductor alloys with a small amount of nitrogen, so called dilute nitrides, are especially attractive for telecom lasers and very efficient multijunction solar cells applications. The epitaxial growth of these materials using MBE and MOVPE is a big challenge for technologists due to the large miscibility gap between GaAs and GaN. Additionally, elaboration of the growth process of quaternary alloys InGaAsN is more complicated than GaAsN epitaxy because a precise determination of their composition requires applying different examination methods and comparison of the obtained results. This work presents the influence of the growth parameters on the properties and alloy composition of the triple quantum wells 3×InGaAsN/GaAs grown by atmospheric pressure metal organic vapour phase epitaxy AP MOVPE. Dependence of the structural and optical parameters of the investigated heterostructures on the growth temperature and the nitrogen source concentration in the reactor atmosphere was analyzed. Material quality of the obtained InGaAsN quantum wells was studied using high resolution X-Ray diffraction HRXRD, contactless electro-reflectance spectroscopy CER, photoluminescence PL, secondary ion mass spectrometry SIMS, photocurrent PC and Raman RS spectroscopies, deep level transient spectroscopy DLTS and transmission electron microscopy TEM.

Electrical Properties of Recessed Algan/Gan Schottky Diodes Under off–State Stress

Abstract Electrical properties of recessed and non-recessed AlGaN/GaN Schottky diodes under off-state stress were investigated. The samples were consecutively stressed by the stepped negative bias (−60 V). Before and after the stress I−V and C−V characteristics were evaluated to verify the device degradation process. Finally, the degradation mechanism and the influence of AlGaN recessed layer thickness on the electrical properties of the Schottky diodes were analysed. It was found that the short time stress influence on I−V characteristics was most negligible for the non-recessed sample. Shallow and deep recessed samples exhibited initial trap filling and reverse current decrease. Generally it was found that the stress voltage near 60 V caused recoverable device degradation

Influence of the MOVPE growth parameters on the properties of InGaAsN/GaAs MQW structures for solar cells application

This work presents the epitaxial growth of undoped multiple quantum well (MQW) - 3 × In_yGa_1-yAs_1-xN_x/GaAs - structures obtained by atmospheric pressure metalorganic vapour phase epitaxy (APMOVPE). The relations between the hydrogen flow rate through the bubblers with TBHy, TMGa, TMIn and the composition of the resulting In_yGa_1-yAs_1-xN_x films were investigated. The influence of the growth temperature on the quantum well In_yGa_1-yAs_1-xN_x parameters were observed. Based upon this experiment, the technology parameters were estimated. These results were applied to the optimization of the hydrogen flow rate through the bubblers with TBHy, TMGa, TMIn and temperature for the growth of quantum well of In_yGa_1-yAs_1-xN_x.

Preparation and properties of Zn delta-doped GaAs/AlGaAs heterojunction phototransistor

We have prepared, characterized and discussed the performance of AlGaAs/GaAs heterojunction bipolar phototransistor (HPT) including Zn delta-doped base. Due to the existence of Δ-doped sheets located in the middle of undoped GaAs base the Δ-doped HPT devices exhibit low dark current, nearly zero offset voltage, saturation voltage ~0.4V, and rise and fall times in ns range at wavelength of 850nm up to 6V of applied voltage. Due to avalanche multiplication behavior at the collector junction, an increased optical gain G>10 can be reached for applied voltages in the range of 6-12V. For voltages higher than the device breakdown voltage (~12V) switching and negative differential resistance (NDR) effect is measurable in the inverted mode of operation.

Fabrication and characterization of Si/SiO2/TiO2/ZnO heterostructures from sputtered and oxidized Ti-film

Abstract The present work reports the fabrication of p-Si/SiO2 /TiO2 and p-Si/SiO2 /TiO2 /ZnO heterostructures deposited by RF sputtering on p-Si substrate. The structural properties of the heterostructures were characterized by X-ray reflectivity and SIMS depth profiling. The electrical and optical properties of the heterostructures were investigated by I − V, C-V measurements and VIS spectroscopy, respectively. The measurements reveal that I − V characteristics in dark show semiconductor-insulator-semiconductor (SIS) structure properties. The I − V characteristics under illumination exhibit changes with significant increase of photocurrent due to photoassisted tunnelling and injection through SiO2 /TiO2 interlayer.

Trap analysis of GaN-based heterostructures using current transients mesurements

This work deals with the trap analysis of the GaN-based structures using current time response on the voltage pulse applied to Schottky and ohmic contacts containing AlGaN and InAlN layers with different composition and thickness. Monitoring of the current time evolution for the investigated samples allows identifying and comparing particular traps. The analysis of the current transients measured at various temperatures via fitting up to four exponentials is used to resolve traps located in the particular layers and their activation energies were determined.

Spectral analysis of In X Ga 1−x N/GaN quantum well structures for III-nitride based solar cells

This paper reports on the optical and electrical characteristics of III-nitride epi-structures for visible wavelength absorption. Structures, incorporating InxGa1-xN/GaN (0.20<;x<;0.35) quantum well active regions have been analysed by photocurrent spectroscopy, optical transmission and electro/photo-luminescence techniques to identify the key spectral absorption characteristics of the structures. In addition the influence of indium composition and number of quantum wells on the spectral response of such structures was investigated. In order to demonstrate the effect of different indium compositions on the photovoltaic performance of an InGaN solar cell, an active silicon solar cell was stacked under two InGaN structures with differing indium composition. For higher indium composition, a lower spectral response is observed at zero bias and correspondingly lower quantum efficiency by a factor of approximately two. The obtained results provide insight how multi-junction solar cells incorporating III-nitride solar cells might be designed in a multi-junction configuration in order to boost the overall efficiency of conventional cell materials such as silicon.

Degradation processes in AlGaN/GaN HEMTs under high drain-bias off-state stress

This paper reports on degradation processes in the AlGaN/GaN HEMTs under high drain-bias off-state stress. The devices were off-stressed at the drain voltage of 60 V and the gate voltage of -5 V for 30 min, resp.60 min. Static device performance was evaluated before, during and after the stress. Performed measurements show a slight degradation in the drain current and a decrease of the threshold voltage. However, these effects are partially recovered as follows from the measurement performed 30 min after the stress. All these indicates that the electrons that accumulate in the structure as well as on the surface of AlGaN layer create an electrostatic charge induced by high electric field between gate and drain. This causes a reduction in the channel current and plays a major role during the off-state degradation of AlGaN/GaN HEMTs.

Variable temperature photocurrent characterization of quantum dots intermediate band photovoltaic devices

The key issue for enhancing the efficiency of semiconductor photovoltaic material devices is to reduce point defects recombination phenomena and to extend the absorption wavelength range. By inserting InAs Quantum Dots in a host GaAs semiconductor structure, new energy levels can be generated resulting in wavelength absorption enhancement. Thus, the main objective of this work was to design a material based on GaAs host semiconductor with extended absorption wavelength in the infrared region. We extend our previous characterisation of GaAs/InAs material systems by studying variable temperature photocurrent spectroscopy from 300K down to 50K in order to study the effect of different inter-dot doping profiles on cell efficiency.

Low temperature investigation of electrical and optical properties of InGaAsN/Gas QW Schottky barrier photodetectors

This work presents electrical and optical properties of Schottky barrier photodetectors containing the triple quantum well (MQW) InGaAsN/GaAs structures grown by atmospheric pressure metal organic vapour phase epitaxy. The peaks corresponding to optical transitions in MQW were identified by photocurrent spectroscopy and compared with simulations. The measurements revealed that there are two embedded and overlapped QW which can be simulated as InxGa1-xAs1-yNy/GaAs QW and InxGa1-xAs/GaAs QW with appropriate width and composition. This effect can be assigned to the interdiffusion between QW and GaAs during the structure growth. The low temperature spectral measurements revealed the shift and intensity of optical transitions in particular QWs.