Browsing by Author "Suihkonen, Sami, Dr., Aalto University, Department of Micro and Nanosciences, Finland"

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Diffusion injected light emitting diode
(2015) Riuttanen, Lauri
School of Electrical Engineering | Doctoral dissertation (article-based)
Lighting plays a major role in consumption of electrical energy in the world. Thus, increasing the efficiency of light sources is one key element in reducing the green house gas emissions. Light emitting diodes (LEDs) are gaining a foothold in general lighting. Despite their rapid development in light output and their superior efficiency compared to other light sources, LEDs still need improvements in order to become the ultimate lighting technology. A typical LED is a double heterojunction (DHJ) structure, in which the active region fabricated from a lower band gap material is sandwiched between higher band gap p- and n-doped regions. By biasing such a structure electrons and holes are transferred by current into the active region, where they recombine releasing energy as photons. The carrier injection in a conventional LED structure is typically efficient. However, in more exotic novel structures based on nanowires or near surface nanostructures, fabricating a DHJ becomes difficult. This thesis presents the experimental studies on a novel current injection method for light emitting applications. The method is based on bipolar diffusion of charge carriers. Unlike in the conventional method, the active region does not have to placed between the p- and n-layers of the pn-junction. The diffusion injection method is experimentally demonstrated by two types of prototype structures. The first prototype was fabricated using a multi quantum well (MQW) stack buried under the pn-junction. The second prototype was fabricated using a near surface quantum well (QW) placed on top of the pn-junction. The first prototype showed that the diffusion current components can be used to excite an active region outside of the pn-junction. The second prototype showed a large improvement in injection efficiency as well as the suitability of the method for exciting surface structures. The applications of diffusion injection can be found in blue galliun nitride based LEDs studied in this thesis as well as in green solid-state light sources, light sources integrated into silicon technology and devices based on nanostructures and plasmonics.
Synchrotron radiation x-ray topography of crystallographic defects in GaN
(2014) Sintonen, Sakari
School of Electrical Engineering | Doctoral dissertation (article-based)
In this thesis, the crystal structures of bulk, homoepitaxial and heteroepitaxial GaN were characterized by synchrotron radiation x-ray topography (SR-XRT), x-ray diffraction (XRD) and defect selective etching (DSE). The SR-XRT image contrast of threading screw dislocations and threading mixed dislocations in GaN were determined. The images caused by the strain fields of threading screw dislocations and threading screw dislocation clusters were simulated, and the simulated and experimental topograph images of low defect density ammonothermal GaN were in excellent agreement. Topograph images corresponding to strain fields of large dislocations with Burgers vector magnitudes equal to multiples of the elemental screw dislocation Burgers vector were observed. DSE experiments were performed to determine whether these large defect images originate from single dislocation strain fields, i.e. super screw dislocations or micropipes, or from combined strain fields of several elemental dislocations in close proximity. DSE images revealed that all observations but one were caused by groups of adjacent elemental threading dislocations. The determined topograph image contrasts of threading mixed and threading screw dislocations were utilized in large area studies of bulk GaN grown by the ammonothermal method. In one of the studies, threading mixed dislocations were observed to form arrays consisting of mixed dislocations with identical Burgers vectors. The minute lattice tilt and twist caused by the dislocation arrays were calculated based on dislocation spacing and confirmed with high resolution XRD measurements. SR-XRT analysis of grain boundaries and basal plane dislocations in ammonothermal GaN were also discussed. The structural quality of homoepitaxial GaN and heteroepitaxial GaN on patterned substrates was characterized by SR-XRT and XRD. The GaN layers were grown by metallo-organic vapour phase epitaxy (MOVPE) on ammonothermal GaN substrates, patterned sapphire substrates and substrates consisting of patterned MOVPE grown GaN layers on sapphire. SR-XRT is a technique especially suited for imaging the defect structure of materials with low crystalline defect density, whereas XRD is applicable to characterization of materials with higher defect density as well. SR-XRT measurements enabled imaging of individual dislocations and identification of different dislocation types.