Examensarbeten för masterexamen // Master Theses
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- PostCharacterization of Active Regions for InP-Based MEMS-VCSELs(2010) Haglund, Erik; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceWidely tunable MEMS-VCSELs with a tuning range exceeding 50 nm are currently being developed for applications in reconfigurable optical networks and sensor systems. The tuning range of MEMS-VCSELs is today mainly limited by the width and magnitude of the gain spectra. In order to design new devices with improved performance, especially improved tuning range, detailed knowledge of the gain properties is required. Due to the difficulties associated with performing reliable gain spectra imulations, this thesis is concerned with implementing an experimental procedure to determine the gain haracteristics of an active region. An MBE-grown active region with two AlGaInAs quantum wells was characterized. The gain spectra of the active region was determined by measuring the ASE-spectra and using the Hakki-Paoli method to compute the gain [1]. Care was taken to reduce the influence of the OSA limited linewidth. The Hakki-Paoli method can not be applied directly on a VCSEL, due to its very long free spectral range. The active region was therefore characterized by measurements on edge-emitting Metal-Clad Ridge-Waveguide (MCRW) laser diodes. The lasers had ridge widths of 2-30 μm and lengths of 300-1500 μm. The laser wavelength was 1.5 μm. Due to the lack of lateral current confinement nderneath the ridge, the lasers showed a lateral carrier diffusion with almost 20% of the injected current flowing outside of the ridge at threshold for 5 μm wide ridges. To be able to predict the VCSEL gain from the gain measured in the MCRW lasers, the empirical material gain parameter g0 was calculated from the measurements. Only the relevant fundamental TE mode was measured by coupling the light into the OSA using a smallaperture polarization maintaining fiber with an in-fiber polarizer. The influence of the current density and heatsink temperature on the gain peak wavelength, FWHM and gain magnitude was investigated. A g0=1441 cm¡1 was measured at 15oC. The peak gain was red-shifted with 0.77 nm/K, and blue shifted with 0.91 nm/mA. The peak net gain was found to decrease with 0.49 cm¡1/K.
- PostCharacterization of Bismuth Telluride Thin Films Grown by MBE(2013) Fülöp, Attila; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceBismuth telluride, Bi2Te3, has been predicted to be a topological insulator; it has however been difficult to manufacture good quality thin films of it. This project focuses on the characterization and analysis of bismuth telluride thin films that have been manufactured using molecular beam epitaxy in the Chalmers cleanroom. Substrate material and several growth condition parameters have been varied and are here analyzed using X-ray diffraction and atomic force microscopy to draw conclusions about the optimal growth conditions. It turns out that the growth mode of the bismuth telluride thin films changes when grown on vicinal vs. flat substrates. Other effects include a phase change from Bi2Te3 to Bi4Te3 when the tellurium flux is decreased below 20 times the bismuth flux. This phase change leads to the destruction of the topological insulator properties.
- PostElectrical Conductivity of n-doped GaN-based Distributed Bragg Reflectors(2016) Hjort, Filip; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceVertical-cavity surface-emitting lasers (VCSELs) could find new applications in areas ranging from displays to bio-medicine if their wavelengths were extended into the blue spectrum. However, the realization of GaN-based VCSELs have proven to be challenging. One of the major hurdles have been to achieve epitaxially grown distributed Bragg reflectors (DBRs) with high reflectivity and vertical electrical conductivity. A vertically conductive DBR would allow the n-contact to be moved outside the cavity, reducing the cavity length and leading to significantly improved laser performance for blue-emitting GaN-based VCSELs. To electrically characterize a DBR, low-temperature annealed ohmic contacts to n-doped GaN are essential, since the DBRs often have built-in-strain and thus could crack when exposed to temperature rampings. Ohmic contact formation of Al, Ta/Al/Ta, and Ti/Al/Ti/Au contacts on n-GaN with different concentrations doping was studied for annealing temperatures up to 700°C by transmission line measurements. On highly doped n-GaN (n = 2.5 × 10^19 cm−3) the Al contacts were ohmic as-deposited and had a minimum specific contact resistivity of 3 × 10−7 cm^2. Al contacts had the lowest contact resistivity reported for ohmic contacts on n-GaN annealed under 500°C. Ta/Al/Ta contacts had lowest resistivity for annealing over 500°C and were thermally stable up to at least 700°C. No ohmic contact to low-doped n-GaN (n < 3 × 10^17 cm−3) was found. The effect of interlayers on the vertical electrical conductivity of AlN/GaN DBRs was investigated by IV-measurements on four different 10.5 DBR pair samples (Sample A-D). Sample A had no interlayers, samples B and C had one pair of AlN/GaN interlayers with thicknesses of 2 nm/2 nm and 0.5 nm/0.5 nm at each interface, respectively, and sample D had graded interlayers from GaN to AlN over 1.5 nm thickness. The electrical conductivity of 8 and 9 DBR pairs was investigated by etching mesas to different etch depths and depositing Al-contacts on top and on the etched surface next to the mesa. The series resistance scaled with mesa area and the lateral and contact resistance contribution was less than 10% of the total resistance. Sample A had a mean specific series resistance of 0.043 cm^2 for 8 pairs, which is comparable to state-of-the-art for n-doped AlN/GaN DBRs. The mean specific series resistances for Sample B and D were 4 times higher than that of sample A, and 10 times higher for Sample C, indicating that interlayers used for strain compensation can increase the electrical resistivity of n-doped AlN/GaN DBRs. An MBE-grown n-doped ZnO/GaN DBR was characterized in a similar manner and showed a specific series resistance of less than 10−3 cm2 for 3 pairs. The lateral and contact resistance were dominating over the vertical resistance, which made an accurate determination of the DBR conductivity difficult. Ga-droplets, which covered the DBR surface, were ruled out as the reason for the low vertical resistance by investigating small droplet-free mesas. This is the first report on electrically conducting ZnO/GaN DBRs.
- PostEnergy Efficient High Data Rate RF-DAC based PAM Modulator(2018) Strömbeck, Frida; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceTo achieve a high-data rate wireless communication system with PAM-4 modulationis tested in this project. A MMIC solution is developed and fabricated with TeledyneScientific Instruments, US, in their 250nm InP DHBT-process with f_max=650GHz.The simulated system consists of a RF-DAC, a PA, transmitter and receiver antennawith 40dBi gain, an LNA, a VGA and a PD. ADS simulation shows that the systemlooks promising. The critical part in the system is the RF-DAC based modulator,which is why two different solutions are tested and evaluated.The modulators aredesigned for an LO frequency of 70GHz, though the plan is to double the frequencywhen the function of the chosen RF-DAC is confirmed. The reason to not designfor an LO frequency of 140GHz straight away is to simplify the testing process.Symbol rate for one of the fabricated modulators is tested and confirmed to supporta 40Gbps transmission with a BER of3.7×10−6, and the energy efficiency is betterthan 1.2pJ/bit.
- PostInfluence of damping on high-speed VCSEL data transmission(2013) Haglund, Emanuel P.; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe vertical-cavity surface-emitting laser (VCSEL) is a low-cost light source with properties well suited for short-reach high-speed data transmission. Due to the increasing demand for high-speed data communication capacity, both in data centers and home connections, there is need for low-cost solutions operating at high bit rates, which makes the VCSEL an attractive option in such optical systems. Commercially available VCSELs are, however, limited to bit rates of around 10 Gbit/s, which is far lower than the data rates specified in upcoming standards. This means that the development of higher speed VCSELs is required. This work has studied the influence of damping on high-speed VCSEL data transmission performance, through an investigation of the large signal properties. VCSELs with different damping (obtained by shallow surface etches of different depths) were characterized through static and dynamic measurements, with emphasis on characterizations during large signal data transmission. Through investigations of the timing jitter and the bit error rate (BER), it was found that using a VCSEL with a K-factor of 0.16 ns and a D-factor of 9 GHz/mA^(1/2), provided the best high-speed properties among the tested devices. This particular VCSEL was able to achieve record high data rates, with 57 Gbit/s back-to-back (BTB), and 55 Gbit/s and 43 Gbit/s over 50 m and 100 m of OM4-fiber, respectively.
- PostQuantum state tomography of 1D resonance fluorescence(2017) Strandberg, Ingrid; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceTomography is the name under which all state reconstruction techniques are denoted, one of the most recognized examples being medical tomography. Quantum state tomography is a procedure to determine the quantum state of a physical system. By performing homodyne measurements on resonance fluorescence from an artificial atom coupled to a one-dimensional transmission line, its quantum state can be reconstructed. Resonance fluorescence is one of the simplest setups that results in non-classical states of light. If these states are non-classical in the sense that they have a negative Wigner function, they can be used as a computational resource for quantum computing. There are many different approaches to quantum computing. Some, like gate based quantum computing using discrete variables like qubits, have been extensively researched, both theoretically and experimentally. There exists and alternative approach: continuous variable quantum computing. The continuous variables we will be concerned with are the components of the electromagnetic field that constitute the resonance fluorescence. There are different parameters that affect the nature of the resonance fluorescence, for example, the number of transmission lines the atom is coupled to, or the strength of the driving field. In this work, we develop the tools necessary to numerically simulate homodyne detection of resonance fluorescence for different sets of parameters, and reconstruct the quantum state as well as calculating the Wigner negativity.