Simple plasma assisted atomic layer deposition technique for high substitutional nitrogen doping of TiO2

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Alshehri, A. H., Nelson-Fitzpatrick, N., Ibrahim, K. H., Mistry, K., Yavuz, M., & Musselman, K. P. (2018). Simple plasma assisted atomic layer deposition technique for high substitutional nitrogen doping of TiO2. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 36(3), 031602 and may be found at https://doi.org/10.1116/1.5019170In this work, a plasma assisted atomic layer deposition system was used to deposit nitrogen-doped titanium dioxide. A simple approach was developed that requires only a nitrogen plasma and short plasma exposure times to effectively dope TiO2. A range of nitrogen concentrations were achieved by varying the flow rate and exposure times of nitrogen and oxygen plasmas. A nitrogen content as high as 23 ± 0.5 at. % was observed when only the nitrogen plasma was used. It was also possible to vary the type of nitrogen doping from almost entirely interstitial to purely substitutional, as measured by x-ray photoelectron spectroscopy. Ultraviolet-visible spectroscopy measurements showed a shifting in the absorption edge from 350 to 520 nm with doping, indicating bandgap narrowing from 3.1 to 1.9 eV.Canada Foundation for InnovationOntario Ministry of Research and Innovation, Industry, CanadaMike and Ophelia Lazaradi

Mechanism of Thermal Atomic Layer Etch of W Metal Using Sequential Oxidation and Chlorination:A First-Principles Study

Thermal atomic layer etch (ALE) of W metal can be achieved by sequential self-limiting oxidation and chlorination reactions at elevated temperatures. In this paper, we analyze the reaction mechanisms of W ALE using the first-principles simulation. We show that oxidizing agents such as O2, O3, and N2O can be used to produce a WOx surface layer in the first step of an ALE process with ozone being the most reactive. While the oxidation pulse on clean W is very exergonic, our study suggests that runaway oxidation of W is not thermodynamically favorable. In the second ALE pulse, WCl6 and Cl2 remove the oxidized surface W atoms by the formation of volatile tungsten oxychloride (WxOyClz) species. In this pulse, each adsorbed WCl6 molecule was found to remove one surface W atom with a moderate energy cost. Our calculations further show that the desorption of the additional etch products is endothermic by up to 4.7 eV. Our findings are consistent with the high temperatures needed to produce ALE in experiments. In total, our quantum chemical calculations have identified the lowest energy pathways for ALE of tungsten metal along with the most likely etch products, and these findings may help guide the development of improved etch reagents

Self-Limiting Temperature Window for Thermal Atomic Layer Etching of HfO2 and ZrO2 Based on the Atomic-Scale Mechanism

HfO2 and ZrO2 are two high-k materials that are important in the downscaling of semiconductor devices. Atomic-level control of material processing is required for the fabrication of thin films of these materials at nanoscale device sizes. Thermal atomic layer etching (ALE) of metal oxides, in which up to one monolayer of the material can be removed, can be achieved by sequential self-limiting (SL) fluorination and ligand-exchange reactions at elevated temperatures. However, to date, a detailed atomistic understanding of the mechanism of thermal ALE of these technologically important oxides is lacking. In this paper, we investigate the hydrogen fluoride (HF) pulse in the first step in the thermal ALE process of HfO2 and ZrO2 using first-principles simulations. We introduce Natarajan–Elliott analysis, a thermodynamic methodology, to compare reaction models representing the self-limiting (SL) and continuous spontaneous etching (SE) processes taking place during an ALE pulse. Applying this method to the first HF pulse on HfO2 and ZrO2, we found that thermodynamic barriers impeding continuous etching are present at ALE-relevant temperatures. We performed explicit HF adsorption calculations on the oxide surfaces to understand the mechanistic details of the HF pulse. A HF molecule adsorbs dissociatively on both oxides by forming metal–F and O–H bonds. HF coverages ranging from 1.0 ± 0.3 to 17.0 ± 0.3 HF/nm2 are investigated, and a mixture of molecularly and dissociatively adsorbed HF molecules is present at higher coverages. Theoretical etch rates of −0.61 ± 0.02 Å/cycle for HfO2 and −0.57 ± 0.02 Å/cycle for ZrO2 were calculated using maximum coverages of 7.0 ± 0.3 and 6.5 ± 0.3 M–F bonds/nm2, respectively (M = Hf, Zr

Isotropic atomic layer etching of GaN using SF₆ plasma and Al(CH₃)₃

GaN is an enabling material for light emitting diodes, advanced radio frequency, and power semiconductor devices. However, fabrication of GaN devices often relies on harsh etch processes, which can leave an etch damage layer, limiting final device performance. In this work, an isotropic atomic layer etching (ALE) process involving SF6 plasma and trimethylaluminium [Al(CH3)3] is presented for the controlled etching of GaN, which reduces oxygen and carbon contamination while smoothing the surface. The ALE chemistry was first examined with density functional theory. A comparison between proposed thermal and plasma-driven reactions is made by implementing Natarajan-Elliott analysis, highlighting that the plasma process is a good candidate for GaN ALE. Saturation was experimentally confirmed for both ALE half-cycles at 150 and 300 °C, with etch rates of 0.31 ± 0.01 and 0.40 ± 0.02 nm/cycle, respectively. Analysis of the films post-ALE shows that the RMS roughness of the films decreases from 2.6 ± 0.1 to 1.9 ± 0.1 nm after 25 nm of etching at 300 °C, in agreement with a previously developed curvature-dependent smoothing model. Taken together, this ALE process enables accurate GaN thickness tuning, surface cleaning, and surface smoothing, allowing for further development of GaN devices.</p

Gravure de semiconducteurs à large bande interdite III-N pour les transistors Normally-OFF à base de GaN/Algan

Le nitrure de gallium (GaN) est un semi-conducteur à large bande interdite utilisé en optoélectronique et dans les dispositifs de puissance et/ou haute fréquence. Les matériauxbinaires III-N à large bande interdite sont les plus prometteurs pour fabriquer des dispositifs à haute puissance et/ou à haute fréquence pouvant fonctionner à haute température. Les procédés basés sur les plasmas chlorés sont particulièrement adaptés à la gravure de ces matériaux. Ces derniers présentent une parfaitereproductibilité et permettent une gravure « fine » ou plus ou moins rapide en fonction des exigences sur la qualité de surface recherchée. Plusieurs exigences doivent être satisfaites pour réaliser la gravure sèche de GaN pour les structures HEMTs communes à base de GaN/AlGaN dans le cadre d’un procédé de fabrication existant : après gravure, i) La rugosité de surface sur la couche barrière AlGaN doit être la plus petite possible ; ii) cette même surface doit présenter un faible endommagement et une faible contamination en impuretés. L’objectif de ce projet est de trouver un procédé qui respecte les exigences imposées après gravure et qui permet de donner une sélectivité optimale entre le p-GaN par rapport à AlGaN

Characterization of Plasma-Enhanced Atomic Layer Deposited Ga2O3 using Ga(acac)3 On GaN

abstract: This research has studied remote plasma enhanced atomic layer deposited Ga2O3 thin films with gallium acetylacetonate (Ga(acac)3) as Ga precursor and remote inductively coupled oxygen plasma as oxidizer. The Ga2O3 thin films were mainly considered as passivation layers on GaN. Growth conditions including Ga(acac)3 precursor pulse time, O2 plasma pulse time, N2 purge time and deposition temperature were investigated and optimized on phosphorus doped Si (100) wafer to achieve a saturated self-limiting growth. A temperature growth window was observed between 150 ℃ and 320 ℃. Ga precursor molecules can saturate on the substrate surface in 0.6 s in one cycle and the plasma power saturates at 150 W. A growth rate of 0.31 Å/cycle was observed for PEALD Ga2O3. Since the study is devoted towards Ga2O3 working as passivation layer on GaN, the band alignment of Ga2O3 on GaN were further determined with X-ray Photoemission Spectroscopy and Ultraviolet Photoemission Spectroscopy. Two models are often used to decide the band alignment of a heterojunction: the electron affinity model assumes the heterojunction aligns at the vacuum level, and the charge neutrality level model (CNL) which considers the presence of an interface dipole. The conduction band offset (CBO), valence band offset (VBO) and band bending (BB) of PEALD Ga2O3 thin films on GaN were 0.1 ±0.2 eV, 1.0±0.2 eV and 0.3 eV respectively. Type-I band alignments were determined. Further study including using PEALD Ga2O3 as passivation layer on GaN MOS gate and applying atomic layer etching to GaN was described.Dissertation/ThesisDoctoral Dissertation Physics 201

Atomic Layer Etching of Metal Films, Metal Nitrides, and Metal Oxides with Bcl3 and Xef2

The continued miniaturization and increase in architectural complexity of transistor-based devices require new process methods. One such method is called atomic layer etching (ALE). ALE is based upon sequential self-limiting or pseudo-self-limiting reactions that remove materials with atomic level control. Until recently, ALE was based upon halogen adsorption followed by high energy ion bombardments for isotropic (directional) etching of materials. However, recently, new thermal based techniques have been developed for anisotropic (nondirectional) etching of semiconductor-based devices. New approaches for thermal ALE have been demonstrated for crystalline AlN, W/WO₃, TaN/Ta₂O₅, MOCVD GaN and Ga₂O₃. Crystalline AlN was demonstrated to etch with Sn(acac)₂ and HF as the reactants. WO₃, Ta₂O₅ and Ga₂O₃ were demonstrated to etch with BCl₃ and HF. TaN and W were etched with O₂/O₃ in conjunction with BCl₃ and HF. MOCVD GaN was etched with BCl₃ and XeF₂. All systems were analyzed with spectroscopic ellipsometry as the main technique for thickness analysis. Thermal ALE of all systems was shown to be possible in the general temperature range of 150 to 250&deg;C. Investigations into the reaction pathway were conducted for each system. Ga₂O₃, MOCVD GaN, and AlN proceeded through a fluorination and ligand exchange. In this process, the surface is fluorinated, and the resulting fluoride is transferred for the ligand on Sn(acac)₂ or BCl₃ creating volatile complexes. In the case for WO₃ and Ta₂O₅, BCl₃ converts the surface to a B₂O₃ layer while forming volatile metal chlorides. B₂O₃ is then spontaneously removed by interaction with HF which does not fluorinate WO₃ or Ta₂O₅. This pathway is important when a fluorination step would create volatile fluorides leading to noncontrolled spontaneous etching. TaN and W are etched similarly, except that an oxidation step is required. The thermal ALE of other metal derivatives such as metal phosphides, sulfides, tellurides and elemental metals can be etched through these two pathways.</p

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

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