Browsing by Author "Suihkonen, Sami"
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- 2D electrons and 2D plasmons in AlGaN/GaN nanostructure under highly non-equilibrium conditions
A4 Artikkeli konferenssijulkaisussa(2020-03-25) Loginov, L. A.; Shalygin, V. A.; Moldavskaya, M. D.; Vinnichenko, M. Ya; Firsov, D. A.; Maremyanin, K. V.; Sakharov, A. V.; Zavarin, E. E.; Arteev, D. S.; Lundin, W. V.; Kauppinen, Christoffer; Suihkonen, SamiWe report on studies of electrically excited non-equilibrium 2D electrons and 2D plasmons in an AlGaN/GaN nanostructure. Optical access to 2D plasmons is provided by means of a metal grating fabricated at the nanostructure surface, while the properties of 2D electrons are examined in the samples without metal grating. The paper focuses on the creation of highly non-equilibrium conditions when the effective temperature of 2D electrons is much higher than the crystal lattice temperature. Such conditions are realized by applying short electrical pulses with a low repetition frequency. A method has been developed for independently determining the temperature of hot electrons and the temperature of the crystal lattice under an applied electric field. It has been shown that under highly non-equilibrium conditions the spectral density of terahertz electroluminescence of 2D plasmons can significantly exceed that of 2D electrons at a certain frequency. - Aikaerotteinen fotoluminesenssin mittauslaitteisto lähi-UV-alueella
Elektroniikan, tietoliikenteen ja automaation tiedekunta | Bachelor's thesis(2008) Turunen, Teemu - AlN metal-semiconductor field-effect transistors using Si-ion implantation
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2018-04-01) Okumura, Hironori; Suihkonen, Sami; Lemettinen, Jori; Uedono, Akira; Zhang, Yuhao; Piedra, Daniel; Palacios, TomásWe report on the electrical characterization of Si-ion implanted AlN layers and the first demonstration of metal-semiconductor field-effect transistors (MESFETs) with an ion-implanted AlN channel. The ion-implanted AlN layers with Si dose of 5 × 1014 cm-2 exhibit n-type characteristics after thermal annealing at 1230°C. The ion-implanted AlN MESFETs provide good drain current saturation and stable pinch-off operation even at 250°C. The off-state breakdown voltage is 2370V for drain-to-gate spacing of 25μm. These results show the great potential of AlN-channel transistors for high-temperature and high-power applications. - Atomic layer deposition growth of epitaxial zinc oxide
School of Electrical Engineering | Licentiate thesis(2014) Särkijärvi, SuviIn this work the basic characteristics of ZnO, atomic layer deposition (ALD) and epitaxial ZnO growth by ALD are presented, followed by the experimental results of the epitaxial growth of ZnO on GaN template by ALD. Diethylzinc (DEZn) and water vapour (H2O) were used as precursors. The structure and the quality of the grown ZnO layers were studied with scanning electron microscope (SEM), atomic force microscope (AFM), X-ray diffraction (XRD), photoluminescence (PL) measurements and positron annihilation spectroscopy. The ZnO fims were confrmed epitaxial, and the film quality was found to improve with increasing deposition temperature in the vicinity of the threshold temperature of two dimensional growth. Contrary to previous reports, high-temperature annealing did not enhance the film properties but rather damaged the film. We conclude that high quality ZnO thin films can be grown by ALD. Interestingly only separate Zn-vacancies were observed in the films, although ZnO thin films typically contain fairly high density of surface pits and vacancy clusters. - Atomic Layer Deposition of PbS Thin Films at Low Temperatures
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-10-13) Popov, Georgi; Bačić, Goran; Mattinen, Miika; Manner, Toni; Lindström, Hannu; Seppänen, Heli; Suihkonen, Sami; Vehkamäki, Marko; Kemell, Marianna; Jalkanen, Pasi; Mizohata, Kenichiro; Räisänen, Jyrki; Leskelä, Markku; Koivula, Hanna Maarit; Barry, Seán T.; Ritala, MikkoAtomic layer deposition (ALD) is a viable method for depositing functional, passivating, and encapsulating layers on top of halide perovskites. Studies in that area have only focused on metal oxides, despite a great number of materials that can be made with ALD. This work demonstrates that, in addition to oxides, other ALD processes can be compatible with the perovskites. We describe two new ALD processes for lead sulfide. These processes operate at low deposition temperatures (45-155 °C) that have been inaccessible to previous ALD PbS processes. Our processes rely on volatile and reactive lead precursors Pb(dbda) (dbda = rac-N2,N3-di-tert-butylbutane-2,3-diamide) and Pb(btsa)2 (btsa = bis(trimethylsilyl)amide) as well as H2S. These precursors produce high quality PbS thin films that are uniform, crystalline, and pure. The films exhibit p-type conductivity and good mobilities of 10-70 cm2 V-1 s-1. Low deposition temperatures enable direct ALD of PbS onto a halide perovskite CH3NH3PbI3 (MAPI) without its decomposition. The stability of MAPI in ambient air is greatly improved by capping with ALD PbS. More generally, these new processes offer valuable alternatives for PbS-based devices, and we hope that this study will inspire more studies on ALD of non-oxides on halide perovskites. - Atomic layer etching of gallium nitride (0001)
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017-11-01) Kauppinen, Christoffer; Khan, Sabbir Ahmed; Sundqvist, Jonas; Suyatin, Dmitry B.; Suihkonen, Sami; Kauppinen, Esko I.; Sopanen, MarkkuIn this work, atomic layer etching (ALE) of thin film Ga-polar GaN(0001) is reported in detail using sequential surface modification by Cl2 adsorption and removal of the modified surface layer by low energy Ar plasma exposure in a standard reactive ion etching system. The feasibility and reproducibility of the process are demonstrated by patterning GaN(0001) films by the ALE process using photoresist as an etch mask. The demonstrated ALE is deemed to be useful for the fabrication of nanoscale structures and high electron mobility transistors and expected to be adoptable for ALE of other materials. - Defect studies with positrons: what could we learn on III-nitride heterostructures?
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2010) Tuomisto, Filip; Mäki, Jussi-Matti; Svensk, Olli; Törmä, Pekka; Ali, Muhammad; Suihkonen, Sami; Sopanen, MarkkuWe have applied positron annihilation spectroscopy to study 400 – 500 nm InGaN-based LED structures, as well as InGaN and AlGaN materials with varying In and Al contents. We find that the effect of adding In to GaN on the annihilation parameters obeys the Vegard's law, while in the case of AlGaN the possible effect of Al is completely screened by efficient formation of cation vacancies. The results obtained in the InGaN LED structures are indistinguishable from defect-free GaN, suggesting that the positrons annihilate preferentially in the barriers of the MQW system. - Diffusion injected multi-quantum well light-emitting diode structure
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2014) Riuttanen, Lauri; Kivisaari, Pyry; Nykänen, Henri; Svensk, Olli; Suihkonen, Sami; Oksanen, Jani; Tulkki, Jukka; Sopanen, Markku - Diffusion-Driven Charge Transport in Light Emitting Devices
A2 Katsausartikkeli tieteellisessä aikakauslehdessä(2017-12-12) Kim, Iurii; Kivisaari, Pyry; Oksanen, Jani; Suihkonen, SamiAlmost all modern inorganic light-emitting diode (LED) designs are based on double heterojunctions (DHJs) whose structure and current injection principle have remained essentially unchanged for decades. Although highly efficient devices based on the DHJ design have been developed and commercialized for energy-efficient general lighting, the conventional DHJ design requires burying the active region (AR) inside a pn-junction. This has hindered the development of emitters utilizing nanostructured ARs located close to device surfaces such as nanowires or surface quantum wells. Modern DHJ III-N LEDs also exhibit resistive losses that arise from the DHJ device geometry. The recently introduced diffusion-driven charge transport (DDCT) emitter design offers a novel way to transport charge carriers to unconventionally placed ARs. In a DDCT device, the AR is located apart from the pn-junction and the charge carriers are injected into the AR by bipolar diffusion. This device design allows the integration of surface ARs to semiconductor LEDs and offers a promising method to reduce resistive losses in high power devices. In this work, we present a review of the recent progress in gallium nitride (GaN) based DDCT devices, and an outlook of potential DDCT has for opto- and microelectronics. - Electrical injection to contactless near-surface InGaN quantum well
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2015-08-03) Riuttanen, Lauri; Kivisaari, Pyry; Svensk, Olli; Oksanen, Jani; Suihkonen, SamiCharge injection to the prevailing and emerging light-emitting devices is almost exclusively based on the double heterojunction (DHJ) structures that have remained essentially unchanged for decades. In this letter, we report the excitation of a near surface indium gallium nitride (InGaN) quantum well (QW) by bipolar carrier diffusion from a nearby electrically excited pn-homojunction. The demonstrated near surface QW emitter is covered only by a 10nm GaN capping leaving the light-emitting mesa perfectly free of metals, other contact, or current spreading structures. The presented proof-of-principle structure, operating approximately with a quantum efficiency of one fifth of a conventional single QW reference structure, provides conclusive evidence of the feasibility of using diffusion injection to excite near surface light-emitting structures needed, e.g., for developing light emitters or photo-voltaic devices based on nanoplasmonics or free-standing nanowires. In contrast to the existing DHJ solutions or optical pumping, our approach allows exciting nanostructures without the need of forming a DHJ, absorbing layers or even electrical contacts on the device surface. - Electrical measurement of internal quantum efficiency and extraction efficiency of III-N light-emitting diodes
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2012) Kivisaari, Pyry; Riuttanen, Lauri; Oksanen, Jani; Suihkonen, Sami; Ali, Muhammad; Lipsanen, Harri; Tulkki, JukkaWe propose a direct electrical measurement method for determining the extraction efficiency (EXE) and internal quantum efficiency(IQE) of III-Nitride light-emitting diodes(LEDs). The method is based on measuring the optical output power as a function of injection current at current densities near the external quantum efficiency (EQE) maximum and extracting IQE and EXE from the measurement data. In contrast to conventional methods, our method requires no low temperaturemeasurements or prior knowledge of the device structure. The method is far more convenient than commonly used methods because it enables measuring the EXE and IQE of different LED structures at room temperature directly in a repeatable and consistent way. This enables convenient comparison of LED structures. We apply the method to determine the IQE and EXE of one commercial LED and selected self-grown planar LED chips to compare the effects of different LED structure designs. Our results are in line with published experimental results and also give more insight to our earlier findings regarding the effects of growth parameters on the quantum efficiency. In addition, our measurement method allows estimating the Shockley-Read-Hall and radiative recombination parameters if the Auger parameter is known. - Electrical measurement of internal quantum efficiency and extraction efficiency of III-N light-emitting diodes
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2012) Kivisaari, Pyry; Riuttanen, Lauri; Oksanen, Jani; Suihkonen, Sami; Ali, Muhammad; Lipsanen, Harri; Tulkki, JukkaWe propose a direct electrical measurement method for determining the extraction efficiency (EXE) and internal quantum efficiency (IQE) of III-Nitride light-emitting diodes (LEDs). The method is based on measuring the optical output power as a function of injection current at current densities near the external quantum efficiency (EQE) maximum and extracting IQE and EXE from the measurement data. In contrast to conventional methods, our method requires no low temperature measurements or prior knowledge of the device structure. The method is far more convenient than commonly used methods because it enables measuring the EXE and IQE of different LED structures at room temperature directly in a repeatable and consistent way. This enables convenient comparison of LED structures. We apply the method to determine the IQE and EXE of one commercial LED and selected self-grown planar LED chips to compare the effects of different LED structure designs. Our results are in line with published experimental results and also give more insight to our earlier findings regarding the effects of growth parameters on the quantum efficiency. In addition, our measurement method allows estimating the Shockley-Read-Hall and radiative recombination parameters if the Auger parameter is known. - Elimination of Lateral Resistance and Current Crowding in Large-Area LEDs by Composition Grading and Diffusion-Driven Charge Transport
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017) Kivisaari, Pyry; Kim, Iurii; Suihkonen, Sami; Oksanen, Jani - Elimination of resistive losses in large-area LEDs by new diffusion-driven devices
School of Science | A4 Artikkeli konferenssijulkaisussa(2017) Kivisaari, Pyry; Kim, Iurii; Suihkonen, Sami; Oksanen, JaniHigh-power operation of conventional GaN-based light-emitting diodes (LEDs) is severely limited by current crowding, which increases the bias voltage of the LED, concentrates light emission close to the p-type contact edge, and aggravates the efficiency droop. Fabricating LEDs on thick n-GaN substrates alleviates current crowding but requires the use of expensive bulk GaN substrates and fairly large n-contacts, which take away a large part of the active region (AR). In this work, we demonstrate through comparative simulations how the recently introduced diffusion-driven charge transport (DDCT) concept can be used to realize lateral heterojunction (LHJ) structures, which eliminate most of the lateral current crowding. Specifically in this work, we analyze how using a single-side graded AR can both facilitate electron and hole diffusion in DDCT and increase the effective AR thickness. Our simulations show that the increased effective AR thickness allows a substantial reduction in the efficiency droop at large currents, and that unlike conventional 2D LEDs, the LHJ structure shows practically no added efficiency loss or differential resistance due to current crowding. Furthermore, as both electrons and holes enter the AR from the same side without any notable potential barriers in the LHJ structure, the LHJ structure shows an additional wall-plug efficiency gain over the conventional structures under comparison. This injection from the same side is expected to be even more interesting in multiple quantum well structures, where carriers typically need to surpass several potential barriers in conventional LEDs before recombining. In addition to simulations, we also demonstrate selective-area growth of a finger structure suitable for operation as an LHJ device with 2µm distance between n- and p-GaN regions. - Enhanced Specific Detectivity and UV-to-Visible Rejection-Ratio of Visible-Blind Metal– Semiconductor–Metal Photodetectors, Based on Epitaxial GaN/Si(111)
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2023-06-05) Pal, Pinki; Kaur, Amadeep; Suihkonen, Sami; Lemettinen, Jori; Laha, A.; Dhar, Subhabrata; Mahapatra, SuddhasattaA very high specific detectivity and UV-to-visible rejection-ratio (UVRR) is reported for visible-blind metal–semiconductor–metal (MSM) photodetectors (PDs), fabricated with gallium nitride (GaN) on Si(111) epitaxial layers. Comprehensive analysis of different figures-of-merit (FOM) reveals that the high specific detectivity results from a large responsivity in the UV-A region of the electromagnetic spectrum, and extremely low dark current, of the PDs. The current transport mechanisms in the absence and presence of illumination suggest that an internal gain, which is attributable to photo-induced barrier lowering, is responsible for the large responsivity of the PDs. The dark current is reduced due to the use of platinum-gold (Pt/Au) Schottky contacts, characterized by a relatively high Schottky barrier height. The vast improvement of nearly all FOMs, compared to those reported earlier for GaN/Si PDs, is highly encouraging for the development of low-cost, large-area arrays of visible-blind PDs. - Epi-Gd2O3/AlGaN/GaN MOS HEMT on 150 mm Si wafer: A fully epitaxial system for high power application
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2019-08-05) Sarkar, Ritam; Bhunia, S.; Nag, D.; Barik, B. C.; Das Gupta, K.; Saha, D.; Ganguly, S.; Laha, Apurba; Lemettinen, Jori; Kauppinen, Christoffer; Kim, Iurii; Suihkonen, Sami; Gribisch, Philipp; Osten, Hans JörgIn this letter, we report the impact of epitaxial Gd2O3 on the electrical properties of an AlGaN/GaN high electron mobility transistor (HEMT) grown on a 150 mm diameter Si (111) substrate. Incorporation of epitaxial Gd2O3 grown by the molecular beam epitaxy technique under a metal gate (metal/Gd2O3/AlGaN/GaN) causes six orders of magnitude reduction in gate leakage current compared to metal/AlGaN/GaN HEMT. We observe that epi-Gd2O3 undergoes complete structural changes from hexagonal to monoclinic as the thickness of the layer is increased from 2.8 nm to 15 nm. Such structural transformation is found to have a strong impact on electrical properties whereby the gate leakage current reaches its minimum value when the oxide thickness is 2.8 nm. We find a similar trend in the density of interface traps (Dit) having a minimum value of 2.98 × 1012 cm-2 eV-1 for the epioxide layer of thickness 2.8 nm. Our measurements also confirm a significant increase in the two dimensional electron gas (2DEG) density (∼40%) at AlGaN/GaN interface with epioxide grown on AlGaN, thus confirming the contribution of epitaxial lattice strain on 2DEG modulation. - Epitaxy of gallium nitride films on silicon substrates
Sähkötekniikan korkeakoulu | Master's thesis(2015-08-24) Lemettinen, JoriMetal organic vapour phase epitaxy (MOVPE) of c-plane gallium nitride (GaN) on 6-inch (111) silicon substrates is studied in this thesis. GaN layers were grown by an Aixtron 6" CSS MOVPE employing an aluminium nitride nucleation layer and a step graded aluminium gallium nitride buffer layer. The epitaxial layer thicknesses were mapped over the whole wafer with a spectrophotometer. The total film thickness was approximately 2.05 micro m. The thickness uniformity was good and the standard deviation was 1.1% (10 mm edge exclusion). The wafer geometry was measured using capacitive profiling. The wafer bow was 80.1 micro m after growth. The bow was non-spherical probably due to thickness non-uniformity. Reciprocal space maps around (002) and (105) reflections were measured with x-ray diffraction. The GaN layers were under tensile strain at room temperature. The full width at half maximum of the (002) and (105) omega-scans were 800 and 770 arcsec, respectively. To further improve the structure a thicker buffer layer possibly with higher aluminium content should be used to reduce wafer bow. Also dislocation reduction techniques such as SiNx interlayers should be considered to increase crystal quality. In addition, aluminium predose instead of nitridation could be employed. - Fabrication of InGaN quantum wells for LED applications
Faculty of Electronics, Communications and Automation | Doctoral dissertation (article-based)(2008-04-11) Suihkonen, SamiIn this thesis fabrication and properties of InGaN quantum wells (QWs) for light emitting diode (LED) applications is studied. Metal-organic vapor phase epitaxy (MOVPE) is used to grow InGaN/(InAl)GaN multiple quantum well (MQW) and LED structures on GaN/sapphire substrates. Also a multistep growth method for the growth of GaN on sapphire is investigated. The method enables a tenfold reduction of threading dislocation (TD) density in the GaN layer compared to conventional growth methods. The objective of this work is to study the physics of InGaN QWs and to improve the performance of InGaN MQW structures used in near-UV, blue and green LEDs. The quality of quantum wells is analyzed by x-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements. The LED structures are characterized also by electroluminescence (EL) measurements. Various MOVPE growth parameters of InGaN/GaN QWs are evaluated for growth of MQW structures emitting blue light. Smooth surface morphology of the MQW stack is achieved by introducing a small amount of H2 during the MOVPE growth of the GaN barrier layers. The effect of TD density on the performance of near-UV, blue, and green LEDs is studied by fabricating LED structures on GaN buffers grown by the multistep method. Improved EL output power at high operating current density is observed in the blue LEDs fabricated on the multistep GaN buffers. MOVPE growth of quaternary InAlGaN layers is investigated and InGaN/InAlGaN MQW structures for near-UV emission are presented. The internal quantum efficiency (IQE) of InGaN/InAlGaN MQW structures is found to be sensitive to the InAlGaN barrier layer composition and the strain state of the structure. A MQW structure emitting at 383 nm with an IQE of 45 % is presented. Finally the origin of the high efficiency of InGaN QWs is discussed. The high efficiency is due to self-screening mechanism of TDs in In containing QWs. The height of the potential barrier formed around the TD depends on the In content of the QWs, and thus the effect of TDs on the performance of blue and green LEDs is different. - Free electron concentration dependent sub-bandgap optical absorption characterization of bulk GaN crystals
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2015-12-15) Pimputkar, Siddha; Suihkonen, Sami; Imade, M.; Mori, Y.; Speck, James S.; Nakamura, ShujiOptical transmission measurements were performed on high quality bulk gallium nitride (GaN) crystals grown by sodium flux, hydride vapor phase epitaxy, and the ammonothermal method with varying free electron concentrations ranging from 4×1016 cm-3 to 9×1018 cm-3. The quality of the crystals was analyzed by x-ray diffraction, threading dislocation density determination, impurity concentrations, and Hall mobility measurements. The sub-bandgap absorption coefficient and index of refraction was determined based on illumination wavelengths ranging from 360 nm to 800 nm. Phonon-assisted free carrier absorption was determined to be the dominant absorption mechanism above approximately 0.1 cm-1. The absorption coefficient at 450 nm varied linearly from 0.1 cm- 1 to 5 cm-1 for free electron concentrations ranging from 1×1017 cm-3 to 9×1018 cm-3. The ammonothermal GaN samples exhibited a strong defect related onset of absorption above 2.9 eV which can be explained by the presence of appreciable hydrogenated gallium vacancies having defect states close to the valance band within the electric bandgap of GaN. The presence of hydrogenated gallium vacancies was experimentally confirmed by Fourier transform infrared absorbance measurements and double hydrogenated gallium vacancy defect are speculated to be prominent in ammonothermal GaN. - Identifying threading dislocation types in ammonothermally grown bulk α-GaN by confocal Raman 3-D imaging of volumetric stress distribution
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2018-07-25) Holmi, Joonas; Bairamov, Bakhysh H.; Suihkonen, Sami; Lipsanen, HarriThis study demonstrates all-optical Raman scattering study of dislocations in ammonothermally grown α-GaN crystal and identifies an edge a-type threading dislocation (TD) using a confocal Raman 3-D imaging technique. These findings make possible the characterization of volumetric stress field and low TD density distributions over a large area on bulk α-GaN single crystal. The dislocation type effects on Raman shift are also discussed in detail (in order to identify the edge a-type and mixed a+c-type TDs, and theorize the invisibility of the screw c-type TDs). Authors are not aware of any previous reports using the confocal Raman 3-D imaging to identify the edge a-type and mixed a+c-type TDs.
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