Skaalattavia nanovalmistustekniikoita III-V-yhdistepuolijohteille ja dielektreille

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.Fotoniikan ja elektroniikan uusimpien nanorakenteiden ja laitteiden toteuttaminen vaati nykyisten nanovalmistustekniikoiden rajoitteiden ylittämistä. Tämä väitöskirja käsittelee uusia skaalattavia menetelmiä nanorakenteiden valmistukseen III-V-yhdistepuolijohteista ja eristemateriaaleista.  Työssä kehitetyistä menetelmistä tärkeimmät ovat: (1) menetelmä suuren pinta-alan GaAs-nanolankahilojen valmistamiseen käyttäen laserinterferenssilitografiaa, kuivaetsausta ja metallo-orgaanista kaasufaasiepitaksiaa, (2) menetelmä uuden matalataitekertoimisen nanohuokoisen konformaalisen alumiinioksidinanoruohoheijastuksenestokalvon valmistamiseen lasille ja (3) GaN (0001) -kidetason atomikerrosetsausprosessi.  Suuren pinta-alan nanolankahilojen valmistaminen on haastavaa, koska monet nanovalmistusmenetelmät perustuvat peräkkäiseen valmistukseen, esimerkiksi elektronisuihkulla piirtämiseen. Tässä työssä kehitetty menetelmä perustuu valon inteferenssiin ja suuri pinta-ala voidaan kirjoittaa samalla kertaa. GaAs-nanolankahilat ovat mielenkiintoisia, koska niitä voidaan käyttää esimerkiksi seuraavan sukupolven halpojen ja tehokkaiden aurinkokennojen valmistamiseen.  Alumiinioksidinanoruoho valmistetaan pinnoittamalla haluttu kappale alumiinioksidilla käyttäen atomikerroskasvatusta ja käsittelemällä kyseinen kalvo lämpimällä vedellä. Alumiinioksidinanoruoho on täysin uudenlainen tapa valmistaa optisia pinnoitteita, sillä sitä voidaan käyttää satojen optisten komponenttien pinnoittamiseen samaan aikaan. Alumiinioksidinanoruohoa voidaan käyttää jopa pinnoilla, joita ei voida pinnoittaa muilla menetelmillä johtuen äärimmäisen hankalista pinnanmuodoista. Alumiinioksidinanoruoho toimii lasin päällä heijastuksenestokalvona mahdollistaen laajakaistaisen ja kulmariippumattoman läpäisevyyden valon näkyvällä alueella, johtuen nanoruohon taitekerroingradientista. Tämän ilmiön löytäminen on yllättävää, sillä atomikerroskasvatettu alumiinioksidi on hyvin tunnettu ja laajalti käytetty materiaali.  Atomikerrosetsausprosessi kehitettiin GaN (0001) -kidetasoa varten. Menetelmällä voidaan poistaa yksi molekulaarinen kerros (Ga + N) kerrallaan galliumnitridiä. Menetelmää voidaan käyttää atomintarkkojen nanorakenteiden ja korkealiikkuvuustransistorien valmistamiseen käyttäen tavanomaisia fotoresistejä etsausmaskeina. Menetelmän kehittämisestä saatua kokemusta käytettiin lisäksi piin atomikerrosetsauksella tehdyn nanomittakaavan kuvioiden siirron ja korkean tarkkuuden nanoleimaisimien valmistuksen analysointiin. Myös muuta III-N-teknologiaa kehitettiin. Tutkittiin GaN:n kasvua SOI-kiekkojen (eng., silicon on insulator) päälle ja typpi-polaarisen AlN:n kasvua 4H-SiC:in päälle

Fabrication and characterization of plasmonic nanogratings on indium gallium nitride quantum wells

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.In this work a process for fabricating subwavelength silver gratings on GaN was developed, using azo-polymer surface relief gratings as etch masks. The azo-polymer gratings were fabricated using laser interference lithography. The period of the gratings was 255 nm. The patterned areas were approximately 4 cm2. The purpose of the gratings was to couple energy from an InGaN quantum well inside the GaN to surface plasmons, and to couple these surface plasmons to radiative modes. Two-dimensional arrays of gold spheres were fabricated with the same technique. The diameter of the spheres was approximately 40 nm. These were used to grow GaAs nanopillars on GaAs using metalorganic vapour phase epitaxy. Surface plasmon coupling experiments were also performed on the processed GaN samples. An optical setup was built for this purpose. The results from the coupling experiments were inconclusive, although the experimental data agrees qualitatively with theory

Air stable plasma passivation of GaAs at room temperature

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.</p

Grass-like Alumina with Low Refractive Index for Scalable, Broadband, Omnidirectional Antireflection Coatings on Glass Using Atomic Layer Deposition

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.Peer reviewe

Superhydrophobic Antireflection Coating on Glass Using Grass-like Alumina and Fluoropolymer

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.Peer reviewe

Metalorganic vapor phase epitaxy of wurtzite InP nanowires on GaN

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.Peer reviewe

Grass-like Alumina with Low Refractive Index for Scalable, Broadband, Omnidirectional Antireflection Coatings on Glass Using Atomic Layer Deposition

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.Peer reviewe

Back-Contacted Carrier Injection for Scalable GaN Light Emitters

Lisää OA-julkaisu, kun saatavilla.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.Peer reviewe

Atomic layer etching of gallium nitride (0001)

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.Peer reviewe