Browsing by Author "Kauppinen, Christoffer"

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  • 2D electrons and 2D plasmons in AlGaN/GaN nanostructure under highly non-equilibrium conditions
    (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, Sami
     A4 Artikkeli konferenssijulkaisussa
    We 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.
  • Air stable plasma passivation of GaAs at room temperature
    (2023-12-21) Kauppinen, Christoffer
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    GaAs surfaces require electrical and chemical passivation for semiconductor devices, but in order to have air stable passivation, high temperatures have been previously required in the passivation step. Here, we demonstrate air-stable, ex situ plasma passivation of GaAs using consecutive hydrogen and nitrogen plasmas at room temperature. No pre-clean using deoxidizing wet chemistry or other means is required. The hydrogen plasma step removes surface oxides and As, which leaves a Ga-rich layer that the nitrogen plasma then turns to GaN. The formed GaN layer efficiently passivates the surface. The plasma-passivated GaAs shows upto 5 × room-temperature photoluminescence after 1 year, and room-temperature time-resolved photoluminescence demonstrates robust passivation even after 3 years, both comparisons to similarly aged unpassivated GaAs. Atomic force microscopy was used to confirm that the passivated surfaces can be made smooth enough for microelectronic applications. Grazing incidence x-ray diffraction indicated that the nitride films are amorphous, and energy-dispersive x-ray spectroscopy was used to estimate the nitrogen content. We used a common inductively coupled plasma reactive ion etching system for plasma passivation, thus enabling the rapid adoption of this technique.
  • Atomic layer etching of gallium nitride (0001)
    (2017-11-01) Kauppinen, Christoffer; Khan, Sabbir Ahmed; Sundqvist, Jonas; Suyatin, Dmitry B.; Suihkonen, Sami; Kauppinen, Esko I.; Sopanen, Markku
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In 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.
  • Back-Contacted Carrier Injection for Scalable GaN Light Emitters
    (2022-01) Kim, Iurii; Kauppinen, Christoffer; Radevici, Ivan; Kivisaari, Pyry; Oksanen, Jani
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    It has recently been proposed that back-contacted III-V light-emitting diodes (LEDs) could offer improved current spreading as compared to conventional mesa or double side contacted structures. This has inspired also experimental efforts to realize such structures, but fabrication methods for them have not yet been fully established. Herein, the use of unintentionally doped and partially carrier-selective contacts (SC) is studied to realize back-contacted indium gallium nitride (InGaN) LEDs. The sharp electroluminescence peak at 439 nm from the multiquantum well stack demonstrates that the approach allows fabricating back-contacted InGaN LEDs without intentionally doped n-GaN layers and without inflicting damage in the active region, often observed in alternative approaches relying on lateral doping and the use of high energy particles during fabrication. The samples are fabricated on a finger configuration with several finger widths between 1 and 20 mu m. It is observed that the emission spreads most uniformly throughout the structure for fingers with the width of 5 mu m. As shown by the simulations, with improved contact resistances, the structures reported herein could enable fabricating back-contacted LEDs with unity injection efficiency and improved current spreading, offering a path toward large-area LEDs without contact shading even in materials where n-doping is elusive.
  • Chemical compatibility of fused filament fabrication-based 3-D printed components with solutions commonly used in semiconductor wet processing
    (2018-10-01) Heikkinen, Ismo T.S.; Kauppinen, Christoffer; Liu, Zhengjun; Asikainen, Sanja M.; Spoljaric, Steven; Seppälä, Jukka V.; Savin, Hele; Pearce, Joshua M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    3-D printing shows great potential in laboratories for making customized labware and reaction vessels. In addition, affordable fused filament fabrication (FFF)-based 3-D printing has successfully produced high-quality and affordable scientific equipment, focusing on tools without strict chemical compatibility limitations. As the additives and colorants used in 3-D printing filaments are proprietary, their compatibility with common chemicals is unknown, which has prevented their widespread use in laboratory chemical processing. In this study, the compatibility of ten widely available FFF plastics with solvents, acids, bases and solutions used in the wet processing of semiconductor materials is explored. The results provide data on materials unavailable in the literature and the chemical properties of 3-D printable plastics that were, are in line with literature. Overall, many 3-D printable plastics are compatible with concentrated solutions. Polypropylene emerged as a promising 3-D printable material for semiconductor processing due to its tolerance of strongly oxidizing acids, such as nitric and sulfuric acids. In addition, 3-D printed custom tools were demonstrated for a range of wet processing applications. The results show that 3-D printed plastics are potential materials for bespoke chemically resistant labware at less than 10% of the cost of such purchased tools. However, further studies are required to ascertain if such materials are fully compatible with clean room processing.
  • Epi-Gd2O3/AlGaN/GaN MOS HEMT on 150 mm Si wafer: A fully epitaxial system for high power application
    (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 Jörg
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In 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.
  • Grass-like alumina coated window harnesses the full omnidirectional potential of black silicon photodiodes
    (2021-11-20) Kauppinen, Christoffer; Pasanen, Toni; Isakov, Kirill; Serue, Michael; Heinonen, Juha; Vähänissi, Ville; Lipsanen, Harri; Savin, Hele
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Packaged photodiodes suffer from Fresnel reflection from the package window glass, especially at high angles of incidence. This has a notable impact particularly on black silicon (b-Si) photodiodes, which have extreme sensitivity. In this work, we show that by adding a simple grass-like alumina antireflection (AR) coating on the window glass, excellent omnidirectional sensitivity and high external quantum efficiency (EQE) of b-Si photodiodes can be retained. We demonstrate that EQE increases at all angles, and up to 15% absolute increases in EQE at a 70° angle of incidence compared to conventional uncoated glass. Furthermore, even at the incidence angle of 50°, the double-sided coating provides higher EQE than bare glass at normal incidence. Our results demonstrate that grass-like alumina coatings are efficient and omnidirectional AR coatings for photodiode package windows in a wide wavelength range across the visible spectrumto near-infrared radiation.
  • Grass-like alumina nanoelectrodes for hierarchical porous silicon supercapacitors
    (2022-12-01) Isakov, Kirill; Sorsa, Olli; Rauhala, Taina; Saxelin, Santeri; Kallio, Tanja; Lipsanen, Harri; Kauppinen, Christoffer
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    With the development of microscale and standalone electronic devices the demand for microscale energy storage is increasing. Micro-supercapacitors are emerging as the candidate for microscale energy storage, especially when combined with energy harvesters. In this work, we enhance the capacitance of porous silicon (PS) supercapacitor electrodes up to 4× by adding a new high-surfacearea nanoelectrode on the existing topography, thus forming a hierarchical 3D supercapacitor electrode that can be used in micro-supercapacitor applications. The nanoelectrode is based on grass-like alumina (GLA) - a recently reported conformal nanoporous coating - with two surrounding TiN films, all materials deposited by atomic layer deposition. The GLA nanoelectrode can be deposited conformally on complex topographies like here on PS, as the total thickness of the electrode structure does not exceed 200 nm allowing it to be fitted in existing electrodes. The GLA nanoelectrode increased the capacitance of the PS supercapacitors alone by up to fourfold or 4× and reduced the self-discharge to a mere 25% loss after 20 h compared to a TiN coated PS reference and state-of-the-art PS both with significantly higher losses. The GLA nanoelectrode showed remarkable stability for 10 000 galvanostatic cycles with a decrease in the capacitance only by 5% and no structural changes were identified from SEM images. Microelectronics compatible processing, the conformal deposition process and the nanoscale thickness of the GLA nanoelectrode allow it to enhance 3D electrodes commonly used in micro-supercapacitors.
  • Grass-like Alumina with Low Refractive Index for Scalable, Broadband, Omnidirectional Antireflection Coatings on Glass Using Atomic Layer Deposition
    (2017-04-11) Kauppinen, Christoffer; Isakov, Kirill; Sopanen, Markku
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We present a new type of nanoporous antireflection (AR) coating based on grass-like alumina with a graded refractive index profile. The grass-like alumina AR coating is fabricated using atomic layer deposition (ALD) of alumina and immersion in heated deionized water. Optical transmittance of 99.5% at 500 nm was achieved with average transmittance of 99.0% in the range of 350–800 nm at normal incidence for double-sided coated glass. Angular spectral transmittance (0–80°) of the double-sided AR coated glass was also measured in the range of 350–800 nm and found to have mean spectral transmittance of 94.0% at 60°, 85.0% at 70°, and 53.1% at 80° angles of incidence, respectively. The grass-like alumina AR coating is suitable for mass production with the presented technique: even hundreds of optical components can be coated in parallel. Furthermore, as an ALD-based technique, the coating can be deposited conformally on surfaces with extreme topography, unlike many spin-coating, physical vapor deposition or glancing angle deposition-based coatings used today.
  • Induktiivisen energiansiirron teoriaa ja simulointia
    (2011) Kauppinen, Christoffer
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Metalorganic vapor phase epitaxy of wurtzite InP nanowires on GaN
    (2020-03-02) Kauppinen, Christoffer; Haggren, Tuomas; Lipsanen, Harri; Sopanen, Markku
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The metalorganic vapor phase epitaxy of wurtzite InP nanowires on GaN (0001) is demonstrated. The InP nanowires exhibit the same wurtzite structure as the underlying wurtzite GaN. The photoluminescence studies indicate that the InP nanowires are single-phase wurtzite with high crystalline quality which is supported by transmission and scanning electron microscopy images. The position of the second valence band or valence band splitting energy is also deduced from the photoluminescence data to be ΔAB = 30 meV at room temperature. The InP/GaN heterojunction can enable exotic optoelectronic and spintronic experiments and applications. In addition, these results can enable traditional III–V growth on III-N materials for heterojunction devices.
  • New nanoporous anti-reflective coating by water treatment of ALD deposited Al2O3
    (2016-06-13) Isakov, Kirill
     Sähkötekniikan korkeakoulu | Master's thesis
    In this work a process for fabricating nanoporous alumina anti-reflective coatings was developed. Anti-reflective coatings were designed for air-glass interface and soda-lime glass was used in the experiments. Alumina was deposited on glass by atomic layer deposition, and then transformed into nanoporous state by immersion in heated de-ionized water. The conditions for nanoporous alumina creation by de-ionized water treatment were investigated and the resulted structure was analysed with ellipsometry, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The transmittance of the coated glass was measured in a Cary 7000 spectrometer, showing excellent broadband and omnidirectional performance. The average transmittance for a visible range of spectrum of a double-side coated glass is up to 99.3 % versus 91.9 % for a non-coated glass. The transmittance for 70 ° incident light is 85.6 % which is 13.9 % higher than for pure glass.
  • Fabrication and characterization of plasmonic nanogratings on indium gallium nitride quantum wells
    (2014-03-31) Kauppinen, Christoffer
     Sähkötekniikan korkeakoulu | Master's thesis
    Tässä työssä kehitettiin prosessi hopeahilojen valmistamiseksi galliumnitridin päälle, käyttäen atsopolymeerihiloja etsausmaskeina. Atsopolymeerihilat valmistettiin laserinterferenssilitografialla. Hopeahilojen periodi oli 255 nm. Kuvioidut näytteet olivat pinta-alaltaan noin 4 cm2. Hopeahilojen tarkoitus oli kytkeä energiaa galliumnitridin sisällä olevasta indiumgalliumnitridikvanttikaivosta pintaplasmoneihin, ja kytkeä energiaa pintaplasmoneista säteileviin moodeihin. Kaksiulotteisia kultapallomatriiseja valmistettiin samalla tekniikalla. Kultapallojen halkaisija oli noin 40 nm. Näitä käytettiin galliumarsenidinanopilareiden kasvattamiseksi galliumarsenidin päälle käyttäen metallo-orgaanista kaasufaasiepitaksiaa. Pintaplasmonien kytkeytymiskokeita suoritettiin prosessoiduille galliumnitridinäytteille. Optinen mittausjärjestely rakennettiin tätä varten. Plasmonista kytkeytymistä ei varmuudella voitu osoittaa, vaikka kytkeytymiskokeiden tulokset sopivat kvalitatiivisesti teorian kanssa yhteen.
  • Scalable nanofabrication techniques for III-V compound semiconductors and dielectrics
    (2018) Kauppinen, Christoffer
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Realization of the newest photonic and electronic nanostructures and devices requires overcoming the limits of present nanofabrication techniques. This thesis presents scalable techniques to fabricate III-V compound semiconductor and dielectric nanostructures.  The central techniques developed in this work are: (1) a method for fabricating large-area position-controlled GaAs nanowire arrays using azopolymers with laser interference lithography (LIL) followed by dry etching and metalorganic vapour phase epitaxy (MOVPE), (2) a new type of low refractive index nanoporous conformal antireflection (AR) coating for glass called grass-like alumina with broadband omnidirectional transmittanceand is made from de-ionized water treated atomic layer deposited alumina, and (3) the atomic layer etching process for the GaN (0001) crystal plane.  The significance of the large-area position-controlled GaAs nanowire arrays is that such high-surface-area, low-volume GaAs nanowire arrays can be used for example in next generation inexpensive and efficient solar cells.  The grass-like alumina presents a paradigm shift on optical coatings as it is suitable for production of hundreds of optical components coated in parallel conformally even on surfaces where no other technique is available due to extreme topography. The grass-like alumina on glass has a graded refractive index profile and acts as an AR coating enabling broadband and omnidirectional transmittance in the visible spectrum of light. What is remarkable is that a completely new type of behaviour was found from such a well known and widely used material as ALD alumina.  GaN (0001) atomic layer etching (ALE) process was developed, which can remove one molecular layer of GaN at a time and is suitable for fabrication of atomic fidelity nanostructures and normally-off high electron mobility transistors, using conventional photoresists as etch masks. This expertize was further used in analyzing ALE of silicon for nanoscale pattern transfer and high-resolution nanoimprint stamp preparation.  In addition to developing the GaN ALE process for the (0001) crystal plane other III-N technologies were developed. GaN growth on silicon on insulator wafers was demonstrated and the films characterized, and N-polar AlN growth on 4H-SiC was characterized.
  • Site-specific growth of oriented ZnO nanocrystal arrays
    (2019-01-01) Bai, Rekha; Pandya, Dinesh; Chaudhary, Sujeet; Dhaka, Veer; Khayrudinov, Vladislav; Lemettinen, Jori; Kauppinen, Christoffer; Lipsanen, Harri
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report on the growth of ZnO nanocrystals having a hexagonal, prismatic shape, sized 700 nm × 600 nm, on bare indium tin oxide (ITO) substrates. The growth is induced by a low ion flux and involves a low-temperature electrodeposition technique. Further, vertically aligned periodic nanocrystal (NC) growth is engineered at predefined positions on polymer-coated ITO substrates patterned with ordered pores. The vertical alignment of ZnO NCs along the c-axis is achieved via ion-by-ion nucleation-controlled growth for patterned pores of size ≈600 nm; however, many-coupled branched NCs with hexagonal shape are formed when a patterned pore size of ≈200 nm is used. X-ray diffraction data is in agreement with the observed morphology. A mechanism is proposed to interpret the observed site-specific oriented/branched growth that is correlated to the pore size. As ordered NC arrays have the potential to generate new collective properties different from single NCs, our first demonstration of a cost effective and facile fabrication process opens up new possibilities for devices with versatile functionalities.
  • Superhydrophobic Antireflection Coating on Glass Using Grass-like Alumina and Fluoropolymer
    (2020-11-04) Isakov, Kirill; Kauppinen, Christoffer; Franssila, Sami; Lipsanen, Harri
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This work presents a superhydrophobic antireflective (AR) coating on glass. The coating consists of a grass-like alumina layer capped with plasma-deposited fluoropolymer. The grass-like alumina is formed by hot water treatment of atomic layer-deposited alumina on glass, and the fluoropolymer is plasma-deposited from CHF3. Excellent broadband AR performance is observed in the visible spectrum with an average transmission of 94.9% for single-sided coated glass, which is close to the maximum 95.3% possible for this glass. Extremely desirable contact angles are obtained with 5-7 min-long fluoropolymer treatments on grass-like alumina with 173° advancing and 160° receding contact angles. This type of multifunctional coating can be beneficial in a multitude of applications like self-cleaning AR coating for solar panels, windows in high-rise buildings, sensors, and aerospace applications as well as just utilizing the excellent water repellent behavior in applications where only superhydrophobicity is required.
  • Two-dimensional plasmons in a GaN/AlGaN heterojunction
    (2019-04-17) Vinnichenko, M. Ya; Shalygin, V.A.; Moldavskaya, M.D.; Artemyev, A.A.; Melentev, Grigory A; Vorobjev, Leonid E; Firsov, D.A.; Korotyeyev, V. V.; Sakharov, A.V.; Zavarin, E.E.; Arteev, D.S.; Lundin, W.V.; Kauppinen, Christoffer; Suihkonen, Sami
     A4 Artikkeli konferenssijulkaisussa
    We report the studies on optical properties of a GaN/AlGaN heterostructure with a surface metal grating. The fabricated structures were optimized for the observation of 2D plasmon resonances in the spectral range of 2–5 THz. The spectra of the equilibrium optical transmission were experimentally investigated and the 2D plasmon resonance was found. The current-voltage characteristics of the grating sample and a reference sample without grating were measured and the dependence of the hot 2D electron temperature on electric field was established. Terahertz electroluminescence was studied in both samples in the sensitivity band of the Ge:Ga detector in electric fields of up to 400 V/cm. It has been shown that, due to the contribution of nonequilibrium 2D plasmons, the integral photoresponse signal for the sample with a surface metal grating increases 2–4 times as compared with the sample without grating, where the terahertz emission is due only to hot 2D electrons.