Browsing by Author "Suihkonen, Sami, Dr., Aalto University, Finland"

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Approaches for optimizing III-N based devices
(2022) Kim, Iurii
School of Electrical Engineering | Doctoral dissertation (article-based)
Three-nitride (III-N) materials have been widely introduced into our everyday lives. Indium gallium nitride (InGaN) light emitting diodes (LEDs) are the backbone of modern lighting sources, while high-mobility electron transistors (HEMTs) based on aluminium gallium nitride (AlGaN) are widely used for high-power high-frequency applications, and GaN power-amplifier devices in 5G technology. However, despite technological and manufacturing advances, devices based on III-N suffer from numerous problems at all stages of their production; beginning with the choice of substrate, continuing with the device design stage and ending with the metallization and characterization stages. There is clearly much room for making improvements at all stages of the fabrication process. This dissertation presents several approaches for fabrication and design optimization that could improve III-N based devices. GaN epitaxy on patterned 6-inch silicon (Si) substrates was studied. It was shown that thicker layers can be obtained compared to a planar Si substrate. The spatial distribution of the strain in the grown GaN patterns was mapped out using confocal Raman spectroscopy. The studies helped to highlight the shape and size of the cracks in the films. It was also observed that the shape of the corners of the patterned unit affected the uniformity of the strain distribution. With various growth parameters, a 500 × 500 µm2 crack-free area for a 1.5 µm thick GaN film was achieved. In addition to that, the AlN transition layer grown by atomic layer deposition (ALD) was studied as an alternative approach to overcome direct growth issues between GaN and Si substrates. It was shown that AlN ALD layers could be used as a template for further overgrowth. The drawbacks of the conventional current injection principle and the recent progress in novel diffusion-driven current transport (DDCT) design were reviewed. The next generation of such DDCT based devices require the use of selective area growth (SAG), which was implemented and optimized to fabricate lateral heterojunction LED structures. Thus, a finger structure with 2 µm distance between the n- and p-GaN regions was achieved. Besides that, the effect of the geometric dimensions of the fingers on injection efficiency were studied on fabricated back-contacted LED structures with finger widths between 1-20 µm. Finally, the SAG method was also implemented to fabricate heavily doped n+-GaN layers to use them as non-alloyed ohmic contacts.
Deposition and characterization of aluminum nitride thin films for piezoelectric MEMS
(2020) Österlund, Elmeri
School of Electrical Engineering | Doctoral dissertation (article-based)
Piezoelectric actuation and sensing would improve many sensors based on microelectromechanical systems (MEMS), such as gyroscopes used in inertial measurement units. Inertial MEMS sensors, especially gyroscopes, require in-plane actuation and sensing, in addition to out-of-plane actuation and sensing in order to measure movement in all three directions. Effective piezoelectric implementation requires the deposition of high crystal quality material on the vertical sidewalls of MEMS structures, which can then move in the in-plane direction. Currently used methods for thin film deposition and processing do not have adequate conformal coverage on vertical sidewalls. Previous research on the deposition of piezoelectric aluminum nitride (AlN) has focused on using AlN in applications, such as resonators, where conformal coverage is not needed, using line-of-sight physical vapor deposition (PVD), which generally has poor conformal coverage and results in tilted crystallites when deposited on vertical surfaces. Chemical vapor deposition (CVD) on the other hand, should result in better conformal coverage on the vertical sidewalls of the three-dimensional structures needed for optimum in-plane actuation and sensing, and metalorganic CVD (MOCVD) and atomic layer deposition (ALD) were used to deposit AlN thin films on vertical sidewalls. The conformal coverage of the MOCVD and ALD processes were studied by growing AlN thin films on patterned silicon substrates, and the crystal quality and microstructure of the films were studied using a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). Furthermore, the mechanical properties, reliability, and microstructural stability of AlN and Sc-alloyed AlN (AlScN) thin films were studied. The purpose of this dissertation was to the find a suitable method and process parameters for the deposition of high crystal-quality piezoelectric AlN on vertical sidewalls that can then be used in the fabrication of piezoelectric inertial MEMS sensors and enable the use of AlN and AlScN in new applications by studying their mechanical reliability and microstructural stability.
Experimental investigations on growth of GaN-based materials for light emitting applications
(2012) Ali, Muhammad
School of Engineering | Doctoral dissertation (article-based)
This work investigates the fabrication, characterization and application of GaN-based layers in light emitting structures. All the thin films and light emitting diodes (LEDs) discussed in this thesis were grown by metal organic vapor phase epitaxy (MOVPE). The objective of this thesis was to improve the material quality and light extraction of III-N optoelectronic structures. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), photoluminescence (PL) and electroluminescence (EL) were used to characterize the samples. GaN layers were grown on GaN templates having hexagon shaped openings. Embedded voids were created at the GaN-sapphire interface with a novel process described in this work. A control over the shape of the voids was demonstrated. It was observed that changing the diameter of the hexagonal openings has an impact on the inclination angle of the sidewalls of embedded voids. The GaN layers having the embedded voids with low inclination angle sidewalls showed an improved material quality. Significant threading dislocation (TD) bending was observed near such voids. InGaN/GaN LEDs were grown on GaN layers with embedded voids of different shapes. Improved material quality and more efficient light extraction due to the introduced void geometry enhanced the light output of 60 degrees inclined sidewall embedded void LEDs. Light extraction enhancement was also studied by mask-less chemical roughening of the back side of the sapphire substrate. An optimized roughening process improved the light extraction from the LED structure by more than 20 %. The compositional dependence of indium and aluminum on MOVPE growth conditions in quaternary InAlGaN layers was investigated. InGaN multi-quantum well structures (MQWs) having quaternary barriers with near-UV emission were also studied. It was observed that the internal quantum efficiency (IQE) of InGaN/InAlGaN MQW structure was sensitive to the barrier layer composition. A proof-of-concept LED based augmented reality application was demonstrated. A working InGaN/GaN single micro-pixel light source was integrated into a contact lens. The LED micropixel display was controlled via a radio frequency transmitter in free space and tested on a rabbit.
Improvements to epitaxial III-N field-effect transistor technology
(2020) Lemettinen, Jori
School of Electrical Engineering | Doctoral dissertation (article-based)
This dissertation aims to improve three-nitride (III-N) technology on two fronts, namely, to shift it to a higher level of integration, and to pave the way for future ultra-wide band gap (UWBG) aluminum nitride (AlN) based transistors. Gallium nitride (GaN) epitaxy on silicon-on-insulator (SOI) substrates was studied. These SOI substrates could offer better electrical isolation for power electronics and improved radio-frequency (RF) characteristics by reducing substrate conduction losses. Higher crystalline quality, lower strain and improved electrical characteristics were demonstrated compared to bulk silicon (Si). The buried oxide layer increased the vertical breakdown voltage by approximately 400 V. Synchrotron radiation X-ray topography analysis confirmed that the stress relief mechanism in GaN-on-SOI epitaxy is the formation of a dislocation network in the SOI device Si layer. P-channel GaN high electron mobility transistors (HEMTs) were monolithically fabricated on silicon together with the more common n-channel HEMTs. The fabricated devices show state-of-the-art performance when compared with other GaN/AlGaN p-FETs on sapphire substrates. The development of GaN-based complementary metal oxide semiconductor (CMOS) technology would tremendously increase the possible level of integration by enabling a monolithic gate driver logic. A front-end-of-line fabrication process was developed for an nitrogen-polar (N-polar) AlN-based transistor and an ion-implanted metal-polar AlN metal-semiconductor field-effect transistor (MESFET). UWBG AlN holds tremendous potential for high power applications due to its critical electric field being four times that of GaN and forty times that of Si. We demonstrated high-quality N-polar AlN by using a reduced substrate miscut combined with high-growth temperature, reduced growth rate, and high V/III-ratio. A resistive N-polar AlN buffer was developed. It was discovered that high temperature growth leads to unintentional Si incorporation. Therefore, a layer grown at reduced temperature was utilized to act as electrical insulation. The AlN buffer was used as a foundation for an N-polar AlGaN/AlN polarization-doped field-effect transistor (PolFET) on silicon carbide (SiC). This is the first demonstration of an N-polar AlN-based polarization doped (PolFET, HEMT) field-effect transistor (FET). The combination of the N-polar structure, an improved edge-based contact technology, and the large charge density of the polarization-generated three-dimensional (3D) electron gas allows the fabrication of devices with more than 120 mA/mm current density. This is the highest value reported for an AlGaN/AlN heterostructure thus far. In addition, ion-implanted metal-polar AlN MESFET was demonstrated. The off-state breakdown voltage was 2370 V showing the great potential of AlN high-power applications.