Epitaxial growth and characterization of GaAs-based type-II (GaIn)As/Ga(AsSb)/(GaIn)As “W”-quantum well heterostructures and lasers

Our current telecommunication is based on the optical transmission of data using semiconductor lasers, which are typically fabricated based on indium phosphide substrates. The layers of material in which the emitted light is generated are typically a few nanometers thick and are therefore referred to as quantum wells. Although this approach enables the fabrication of semiconductor lasers emitting in the technologically important wavelength ranges around 1.3 μm and 1.55 μm, the investigation of alternative concepts is of great interest, because the efficiency of existing semiconductor lasers is limited by non-radiative loss processes. A possible alternative are gallium arsenide-based type-II heterostructures, in which electrons and holes are spatially separated. Therefore, the electronic properties of the two charge carrier species are dominated by different materials and can be adjusted independently of each other, which could enable a systematic reduction of non-radiative loss processes. In order to ensure that the spatial separation of electrons and holes in these systems does not lead to inefficient radiative recombination of the charge carrier species, they are often arranged as a so-called "W"-quantum well heterostructure. In this case, a hole quantum well is embedded in between two electron quantum wells resulting in an increased spatial overlap. The present thesis describes the fabrication of type-II "W"-quantum well heterostructures using metalorganic vapor phase epitaxy and their application as active medium in near-infrared lasers. The gallium arsenide-based (GaIn)As/Ga(AsSb)/(GaIn)As material system serves as a model system. Since any follow-up investigations of type-II heterostructures and lasers are based on the fabrication in a sufficiently high quality, the metalorganic vapor phase epitaxy-based growth of such heterostructures is studied first. The metalorganic compounds triethylgallium (TEGa), trimethylindium (TMIn), tertiarybutylarsine (TBAs), and triethylantimone (TESb) serve as precursors in this study. Due to the large number of possible composition and layer thickness combinations, the indium content, the (GaIn)As layer thicknesses, and the Ga(AsSb) layer thickness were held constant at 20 %, 6 nm, and 4 nm, respectively. Consequently, the antimony concentration remains as the last free parameter allowing for an investigation of the growth conditions of Ga(AsSb). The basis for this approach was a theoretical study in which the material gain of the present structures was optimized. The sample growth was carried out at a growth temperature of 550 °C with V/III gas phase ratios between 2.0 and 7.5. Furthermore, the TESb/V gas phase ratio was varied to determine the maximum achievable antimony concentration and thus, to determine the maximum achievable wavelength range. It was possible to demonstrate high-quality type-II heterostructures with antimony concentrations between 19.3 % and 30.2 % in this study. These concentrations corresponded to photoluminescence peak wavelengths between 1.22 μm and 1.47 μm implying a high spectral flexibility of these heterostructures. The results of the growth study are used to fabricate electrical injection lasers emitting at 1.2 µm. Electroluminescence measurements below laser threshold reveal a blue shift as a function of the injection current density of (93 ± 14) meV/(kA/cm2). This blue shift ends as soon as stimulated emission, which is indicated by a narrowing of the line width as well as a distinct threshold behavior of the laser characteristic, is observed. The evaluation of the laser characteristic yields a threshold current density of 0.4 kA/cm2, an optical efficiency of 0.35 W/A per facet corresponding to a differential efficiency of 66 %, and a maximum pulsed optical output power of 1.4 W, which is limited by the measurement setup. The temperature-dependent characterization of a single and a double “W”-quantum well laser shows that higher order type-II transitions can dominate the emission spectra. Higher order transitions are only observed in case of the single “W”-quantum well laser. This finding highlights that it is important to operate these devices at sufficiently low charge carrier densities. Furthermore, the temperature stability of the threshold current density as well as the differential efficiency are described within the framework of an exponential model. The so-called characteristic temperatures T0 and T1 are used as parameters which allow for an assessment of the temperature stability. The investigation yields characteristic temperatures of T0 = (56 ± 2) K and T1 = (105 ± 6) K in case of the single “W”-quantum well laser and T0 = (60 ± 2) K and T1 = (107 ± 12) K in case of the double “W”-quantum well laser. These relatively low T0 values in combination with the previously described blue shift result in a modification of the temperature-induced shift rate of the emission wavelength. Therefore, it is even possible to demonstrate negative shift rates. This modification can be considered as a fundamental difference with respect to the behavior of type-I lasers. As such, it enables the investigation of novel device concepts and may result in the optimization of existing device concepts. In addition to electrical injection lasers, it was also possible to demonstrate vertical-external-cavity surface-emitting lasers (VECSELs) based on “W”-quantum wells as active medium. These optically pumped devices exhibited a maximum optical output power of 4 W under continuous wave operation conditions. The characteristic blue shift also plays an important role in these devices resulting in the requirement of a positive detuning of the resonator with respect to the emission wavelength at low output powers. While the previously mentioned results are promising and highlight the application potential of type-II heterostructures, an important feature of semiconductor lasers was neglected so far. Their spectral flexibility is in important argument in favor of their application since it is possible to tune the emission wavelength for a certain application. An important wavelength range is the window around 1.3 µm, which is used for telecommunication applications. A theoretical optimization of the “W”-quantum well heterostructure for this emission wavelength range results in the in the successful demonstration of a double “W”-quantum well laser emitting at 1.3 µm. This device can be operated up to temperatures of at least 100 °C and electroluminescence measurements show that laser operation is based on the fundamental type-II transition up to 100 °C. The temperature-dependent characterization yields characteristic temperatures of T0 = (132 ± 3) K und T1 = (109 ± 12) K. Furthermore, a threshold current density of 1.0 kA/cm2, a differential efficiency of 41 %, and a maximum pulsed optical output power of 0.68 W per facet, which is once again limited by the measurement setup, are observed at a temperature of 20 °C

Defect investigations in InAs/GaSb type-II strained layer superlattice

InAs/GaSb type-II strained layer superlattices are a material used for infrared detection. By adjusting the thickness of the InAs and GaSb layers, the material bandgap can be tuned to absorb photons from 3-30 um. Compared to competing materials such as HgCdTe and InSb, InAs/GaSb superlattices are more mechanically robust, have reduced tunneling currents, and can use strain to suppress Auger recombination. In spite of these advantages, this material still faces several challenges, including low minority carrier lifetime, resulting from trap levels that cause Schockley-Read-Hall recombination. These low lifetimes lead to reduced signal-to-noise ratio and higher dark current. Therefore, increasing the lifetime is important for improving this material\\u27s performance. However, to increase the carrier lifetimes, the origin of the traps must first be understood. In this work, several key suspect causes of the killer defect were evaluated. A commonly explored suspect in literature, the interfaces, was studied using time-resolved photoluminescence for three different samples. This characterization method was also used to determine if the doping atom and its layer placement significantly impacted the minority carrier lifetime. There is a substantial amount of evidence that the presence of gallium, or the GaSb layer itself harbors the defect. Thus, the rest of the study focused on aspects of GaSb. Layer intermixing of the In and As atoms into the GaSb layer was studied by intentionally incorporating In and As in bulk GaSb and using photocapacitance characterization to observe any possible defect level formation. In addition, trap level formation for different GaSb growth temperatures was also explored with this characterization technique. Finally, in an attempt to reduce trap densities, GaSb was grown with an increased level of Sb monomers rather than dimers. This material was characterized using dark current density measurements and photoluminescence.\u2

Investigation of Optical and Structural Properties of GeSn Heterostructures

Silicon (Si)-based optoelectronics have gained traction due to its primed versatility at developing light-based technologies. Si, however, features indirect bandgap characteristics and suffers relegated optical properties compared to its III-V counterparts. III-Vs have also been hybridized to Si platforms but the resulting technologies are expensive and incompatible with standard complementary-metal-oxide-semiconductor processes. Germanium (Ge), on the other hand, have been engineered to behave like direct bandgap material through tensile strain interventions but are well short of attaining extensive wavelength coverage. To create a competitive material that evades these challenges, transitional amounts of Sn can be incorporated into Ge matrix to form direct bandgap GeSn alloys that have led to the increasing possibility of engineering a suite of low-cost, light emission sources that applies to a wide range of infrared photonics and optoelectronics systems. Hence, the importance of studying the structural and optical properties of these GeSn heterostructures cannot be overemphasized. The first part of this dissertation investigates the structural and optical properties of SiGeSn/GeSn/SiGeSn quantum wells (QWs) where the photoluminescence (PL) behaviors of thick (22 nm in well) and thin (9 nm in well) GeSn QW samples are compared. Using PL results from two excitation lasers (532 nm and 1550 nm lasers) as well as studying their respective optical transitions, the result reveals that the thicker well sample shows i) a more direct bandgap outcome in addition to a much lower ground energy Г valley; ii) a higher carrier density within the well, and iii) an increased barrier height coupled with improved carrier confinement. All of these resulted in a significantly enhanced emission that allows for the first-ever estimation of GeSn QWs quantum efficiency (QE) while also suggesting a path towards efficient mid-infrared devices. To further improve the carrier confinement while also reducing the carrier leakage in the thicker well design, a SiGeSn/GeSn/GeSn/SiGeSn separate confinement heterostructure (SCH) is introduced. The sample is characterized and the optical properties are compared with the previously reported 9 nm and 22 nm well non-SCH samples. Based on the optical transition analysis, the SCH QW also shows significantly higher carrier confinement compared to reference samples. In addition to these studies, an attempt is made to investigate advanced quantum well structures through an all-inclusive structural and optical study of SiGeSn/GeSn/SiGeSn multi-quantum wells (MQWs). The resulting analysis shows evidence of intermixing diffusion during growth. The second part of this work provides insights into the behavior of annealed GeSn bulk samples near the indirect-to-direct transition point. The study attempts to provide connections between the strain, composition, and defect densities before and after annealing. The result reveals the impact of annealing on a sample may either i) lower the strain giving rise to an increased PL while reducing the energy separation or ii) introduce misfit dislocation/ surface roughness leading to an affected or decreased PL. Finally, this work also explores the low-temperature capability of our in-house plasma-enhanced ultra-high vacuum chemical vapor deposition system through the growth of Si-on-Ge epitaxy and pressure-dependent growth of GeSn bulk heterostructures

On the characterisation of solar cells using light beam induced current measurements

The presence of inhomogeneities in semiconductor materials used to fabricate solar cell devices may result in spatial non uniformities in the device properties which may affect current generation in these devices. Besides, current reducing defects such as inclusions, local shunts and optical blockages may be introduced during the various device manufacturing processes which may adversely affect the performance and overall efficiency of solar cells. Diagnostic techniques are therefore needed to identify these defects so as to improve the production technology. This thesis presents the Light Beam Induced Current (LBIC) technique for mapping spatial non uniformities in solar cell devices. The LBIC is a non destructive characterisation technique that uses a focused light beam to raster scan a solar cell surface as the photo-generated current is recorded as a function of position to generate a photo-response map. The technique was used to obtain photoresponse maps for a mc-Si, Back contact Back junction (BC-BJ) silicon solar cell and the InGaP/InGaAs/Ge concentrating triple junction (CTJ) solar cell from which various local current reducing defects were mapped. A reflection signal detector was incorporated into the LBIC measurement system to enable us distinguish between optical blockages on the cell surface and current reducing defects within the solar cell devices. By dynamically biasing the solar cell devices, the electrical activity of the identified defects was investigated and also point-by-point current-voltage (I-V) characteristics were obtained. An interval division algorithm was applied to the measured point-by-point I-V characteristics to extract device and performance parameters from which device and performance parameter uniformity of the devices were mapped. Dark and full cell solar illumination I-V characteristics were also measured to extract device parameters. Analysis of extracted parameters revealed differences between extracted dark and illuminated device parameters which was attributed to departure from the superposition principle due to non-linearity of the semiconductor device equations with respect to carrier concentration. An investigation into the effect of illumination intensity on the I-V parameters of a spot illuminated BC-BJ Si solar cell showed a linear increase and a logarithmic increase of the short circuit current and open circuit voltage respectively with intensity while the series resistance decreased with intensity, which was attributed to increase in conductivity of the active layer. The ideality factor and saturation current were observed to increase while the shunt resistance initially increased before decreasing at higher intensity levels. Under monochromatic illumination, the photo-response of the BC-BJ Si cell was higher at 785nm than at 445nm due to low absorption coefficient of Si for longer wavelength radiations, resulting in carrier generation within the bulk, where there is a higher probability of carriers being collected at the p-n junction before they recombine. Under solar illumination, as the spectral content was altered using long pass colour filters with cut off wavelengths of 610nm and 1000nm, the performance parameters were observed to decrease and this was mainly due to decrease in intensity. For the CTJ solar cell, however, blocking of radiations below 610nm resulted in current mismatch that severely degraded the short circuit current (Isc). The current mismatch affected the extracted device and performance parameters. With a 1000nm long pass filter, a dark I-V was obtained since only the bottom Ge subcell was activated

Advanced Photonic Sciences

The new emerging field of photonics has significantly attracted the interest of many societies, professionals and researchers around the world. The great importance of this field is due to its applicability and possible utilization in almost all scientific and industrial areas. This book presents some advanced research topics in photonics. It consists of 16 chapters organized into three sections: Integrated Photonics, Photonic Materials and Photonic Applications. It can be said that this book is a good contribution for paving the way for further innovations in photonic technology. The chapters have been written and reviewed by well-experienced researchers in their fields. In their contributions they demonstrated the most profound knowledge and expertise for interested individuals in this expanding field. The book will be a good reference for experienced professionals, academics and researchers as well as young researchers only starting their carrier in this field

Spectroelectrochemical Investigations of Anisotropic Semiconductor Nanoparticles

Nanomaterialen beginnen sich aus der akademischen Welt heraus langsam in kommerzielle Produkte zu entwickeln. Dabei helfen ihnen einzigartige optoelektronische Eigenschaften. Damit ein solcher Übergang gelingt, ist es notwendig, die Nanoteilchen durch geeignete Analyseverfahren zu verstehen und Wege zur gezielten Manipulation bestimmter Eigenschaften analytisch zu begleiten. Die vorliegende Arbeit hat sich zum Ziel gemacht eine spektroelektrochemische Methode, die Potential-modulierte Absorptionsspektroskopie (EMAS), weiterzuentwickeln. Mit EMAS können die optoelektronischen Eigenschaften von Halbleiternanomaterialen untersucht werden, wobei die Methode durch ihre besondere Empfindlichkeit im Vergleich zu anderen spektroelektrochemischen Methoden beeindruckt. Im Rahmen der Arbeit wurde das Spektrum von EMAS zunächst von sphärischen Nanopartikeln auf anisotrope Nanoplatelets erweitert, wobei sowohl ein als auch mehrphasige Systeme betrachtet wurden. Anschließend konnte der Messbereich vom sichtbaren Bereich bis ins nahe Infrarot erweitert werden, was auch die Untersuchung von Partikeln möglich macht, die eine potentielle Anwendung in der Konversion solarer Energie haben. Weiterhin wurden neue Werkzeuge entwickelt und EMAS Varianten betrachtet. Zusammenfassend präsentiert sich EMAS als leistungsstarke spektroelektrochemische Analysemethode deren Weiterentwicklung positiv auf die voranschreitende Nanopartikelforschung wirken wird

Trapping and Reliability Investigations in GaN-based HEMTs

GaN-based high electron mobility transistors (HEMTs) are promising candidates for future microwave equipment, such as new solid state power amplifiers (SSPAs), thanks to their excellent performance. A first demonstration of GaN-MMIC transmitter has been developed and put on board the PROBA-V mission. But this technology still suffers from the trapping phenomena, principally due to lattice defects. Thus, the aim of this research is to investigate the trapping effects and the reliability aspects of the GH50 power transistors for C-band applications. A new trap investigation protocol to obtain a complete overview of trap behavior from DC to radio-frequency operation modes, based on combined pulsed I/V measurements, DC and RF drain current measurements, and low-frequency dispersion measurements, is proposed. Furthermore, a nonlinear electro-thermal AlGaN/GaN model with a new additive thermal-trap model including the dynamic behavior of these trap states and their associated temperature variations is presented, in order to correctly predict the RF performance during real RF operating conditions. Finally, an advanced time-domain methodology is presented in order to investigate the device’s reliability and to determine its safe operating area. This methodology is based on the continual monitoring of the RF waveforms and DC parameters under overdrive conditions in order to assess the degradation of the transistor characteristics in the RF power amplifier