Artificial superlattices exhibit exceptional electronic, magnetic, optical, and mechanical properties which make them unique candidates for applications in a broad range of technologies. A common key feature of superlattices is the need for atomically abrupt interfaces. However, superlattices comprised of materials with different properties, such as melting points and diffusivities, pose large challenges for achieving high crystal quality of both constituents with abrupt interfaces. By employing ion-assisted magnetron sputter epitaxy, we present an innovative solution to this problem with utilizing a unique combination of thermal radiation and kinetic energy that enable sufficient adatom mobility for epitaxial growth of both materials. The research was implemented for the case of CrB2/TiB2 heteroepitaxial superlattices, as neutron interference mirrors, wherein the constituents’ melting points differ by 1100 K. Ion-induced intermixing was avoided by commencing growth of each TiB2 and CrB2 layer by up to 3 unit cells (uc) without ion assistance, forming a buffer to protect the interface during the ion-assisted growth of the remainder of each layer. Heteroepitaxial superlattice growth with interface widths σCrB2 ∼1 uc and σTiB2 ∼2 uc was confirmed for different modulation periods. More than 3000 uc (∼1 µm) thick superlattices with abrupt interfaces were demonstrated for neutron mirror applications.

This study investigates the effects of incorporating 11B4C interlayers into Fe/Si multilayers, with a focus on interface quality, reflectivity, polarization, and magnetic properties for polarizing neutron optics. It is found that the introduction of 1-2 & Aring; 11B4C interlayers significantly improves the interface sharpness, reducing interface width and preventing excessive Si diffusion into the Fe layers. X-ray reflectivity and polarized neutron reflectivity measurements show enhanced reflectivity and polarization, with a notable increase in polarization for 30 & Aring; period multilayers. The inclusion of interlayers also helps prevent the formation of iron-silicides, improving both the magnetic properties and neutron optical performance. However, the impact of interlayers is less pronounced in thicker-period multilayers (100 & Aring;), primarily due to the ratio between layer and interface widths. These results suggest that 11B4C interlayers offer a promising route for optimizing Fe/Si multilayer performance in polarizing neutron mirrors.

Semiconductor devices are constructed from stacks of materials with different electrical properties, making deposition of thin layers central in producing semiconductor chips. The shrinking of electronics has resulted in complex device architectures which require deposition into holes and recessed features. A key parameter for such deposition is the step coverage (SC), which is the ratio of the thickness of material at the bottom and at the top. Here, we show that adding a co-flow of a heavy inert gas affords a higher SC for deposition by chemical vapor deposition (CVD). By adding a co-flow of Xe to a CVD process for boron carbide using a single source precursor with a lower molecular mass than the atomic mass of Xe, the SC increased from 0.71 to 0.97 in a 10:1 aspect ratio feature. The concept was further validated by a longer deposition depth in lateral high aspect ratio structures. We suggest that competitive co-diffusion is a general route to conformal CVD.

Single-crystal CrB₂/TiB₂ diboride superlattices with well-defined layers are promising candidates for neutron optics. However, excess B in sputter-deposited TiB_y using a single TiB₂ target deteriorates the structural quality of CrB_x/TiB_y (0001) superlattices. We study the influence of co-sputtering of TiB₂ + Ti on the stoichiometry and crystalline quality of 300-nm-thick TiB_y single layers and CrB_x/TiB_y (0001) superlattices on Al₂O₃(0001) substrates grown by DC magnetron sputter epitaxy at growth-temperatures T_S ranging from 600 to 900 °C. By controlling the relative applied powers to the TiB₂ and Ti magnetrons, y could be reduced from 3.3 to 0.9. TiB_2.3 grown at 750 °C exhibited epitaxial domains about 10x larger than non-co-sputtered films. Close-to-stoichiometry CrB_1.7/TiB_2.3 superlattices with modulation periods Λ = 6 nm grown at 750 °C showed the highest single crystal quality and best layer definition. TiB_2.3 layers display rough top interfaces indicating kinetically limited growth while CrB_1.7 forms flat and abrupt top interfaces indicating epitaxial growth with high adatom mobility.

Artificial superlattices with their exceptional properties have been popular in a broad range of applications such as electronic, magnetic, optical, and hard coating. Another potential application for single crystal artificial superlattices is highly efficient interference neutron optics, owing to an ultimate interface width of just ±½ atomic layer. Moreover, studies of superlattices have been instrumental in understanding the hardening mechanisms in transition metal nitrides and carbides while such studies on transition metal diborides is lacking, despite extensive studies on monolithic transition metal diboride thin films.

This work is an initiative to grow CrB₂/TiB₂ (0001) diboride superlattices epitaxially onto Al₂O₃ (0001) substrates by direct current magnetron sputter epitaxy implementing two different approaches; compound diboride targets, and co-sputtering of a metal target with a compound target. Effects of substrate temperature, B stoichiometry (B/TM ratio), modulation period Λ = D_CrB2 + D_TiB2, layer thickness ratio , and relative applied power to magnetrons on the structural and interface quality of superlattices are studied and discussed.

Using compound targets, superlattices with thickness ratio Γ = 0.3 and modulation periods Λ between 1 and 10 nm, and with Λ = 6 nm and thickness ratios between 0.2 to 0.8 were synthesized at the optimum sputter gas pressure of p_Ar = 4 mTorr and a substrate temperature of 600 °C. It is found that superlattices with Λ = 6 nm and Γ in the range of 0.2-0.4 exhibit the highest structural quality. However, B segregation in the over-stoichiometric TiB_y layers (y > 2), grown from TiB₂ compound target, results in narrow epitaxial superlattice columnar growth with structurally distorted B-rich boundaries. By co-sputtering from Ti and TiB₂ targets, y could be reduced from 3.3 to 0.9 in TiB_y layers through controlling the relative applied target power. Co-sputtered TiB_y single layers and superlattices were grown at substrate temperatures between 600 and 900 °C. 300-nm-thick TiB_2.3 single layers grown at 750 °C exhibited epitaxial domains about 10x larger than non-co-sputtered films.A significant enhancement for close-tostoichiometry CrB_1.7/TiB_2.3 superlattices with modulation periods Λ = 6 nm was achieved at 750 °C. X-ray diffraction, time of flight elastic recoil detection analysis, scanning transmission electron microscopy, electron energy loss spectroscopy, selected area electron diffraction, and nano-indentation are used for characterization.

Artificial superlattices with their exceptional properties have been popular in a broad range of applications such as electronic, magnetic, optical, and hard coating. Another potential application for single crystal artificial superlattices is highly efficient interference neutron optics, owing to an ultimate interface width of just ±½ atomic layer. Moreover, studies of superlattices have been instrumental in understanding the hardening mechanisms in transition metal nitrides and carbides while such studies on transition metal diborides are lacking, despite extensive studies on monolithic transition metal diboride thin films.

This thesis describes a logical series of studies solving a range of fundamental issues which enabled epitaxial growth of high quality CrB₂/TiB₂ (0001) diboride superlattices onto Al₂O₃ (0001) and 4H-SiC (0001) substrates by direct current magnetron sputter epitaxy. This involved implementation of several different strategies; sputtering from compound diboride targets, co-sputtering from separate metal and compound targets, sputtering from titanium boride (Ti:B = 1:1) and chromium diboride targets, monolayer-control of growth kinetics by Ar-ion assistance, as well as choice of low lattice mismatch substrate. Effects on the structural and interface quality of the superlattices are studied with respect to substrate temperature, B stoichiometry (B/TM ratio), modulation period Λ = D_CrB2 + D_TiB2, layer thickness ratio Γ = D_TiB2/(D_CrB2+ D_TiB2), ion-assistance energies, use of interfacial protective buffer layers, as well as choice of Al₂O₃ or 4H-SiC substrates. X-ray diffraction, X-ray reflectivity, Neutron reflectivity, time of flight elastic recoil detection analysis, reciprocal space map, scanning transmission electron microscopy, electron energy loss spectroscopy, selected area electron diffraction, and nano-indentation are used for characterization.

For all experimental conditions, it is found that the highest structural quality of the superlattices are obtained for the layer thickness ratio Γ ≈ 0.3. Using Al₂O₃ substrates and compound targets, an optimum sputter gas pressure of p_Ar = 4 mTorr and a substrate temperature of 600 °C, it is found that superlattices with modulation period Λ = 6 nm and Γ in the range of 0.2-0.4 exhibit the highest structural quality. However, B segregation in over-stoichiometric TiB_y layers (y > 2), grown from TiB₂ compound target, results in narrow epitaxial superlattice columnar growth with structurally distorted B-rich boundaries.

By co-sputtering from Ti and TiB₂ targets, y can be tailored in the range 0.9 to 3.3 in TiB_y layers through controlling the relative applied target powers. Co-sputtered TiB_2.3 single layers exhibit 10x larger epitaxial domains than non-co-sputtered films and high quality close-to-stoichiometric CrB_1.7/TiB_2.3 superlattices are obtained at a higher growth temperature of 750 °C. The individual CrB_1.7 layers grow in a 2D-fashion leading to smooth surfaces and atomically abrupt interfaces when TiB_2.3 is grown on top. In contrast, TiB_2.3 layers exhibit rough interfaces where CrB_1.7 is grown on top which indicate kinetically limited 3D growth of the TiB_2.3. The difference in growth mode between CrB_1.7 and TiB_2.3 is attributed to a 1100 K difference in melting temperatures. Introducing the use of a titanium boride target, close-to-stoichiometric continuous TiB_1.8 single layers of high crystal quality are achieved directly, which relaxes any process constraints imposed by the need of balanced fluxes from Ti and TiB₂ targets. Moreover, ion-assisted epitaxial growth is implemented by concurrent low energy Ar-ion extracted from the sputtering plasma, to stimulate the mobility of the adatoms, in particular for TiB_1.8 which has the highest melting temperature. CrB_1.9/TiB_1.8 superlattices with single crystal quality and superlattice layer definition exceeding any of the previously obtained superlattices is obtained using TiB target combined with Ar-ion energies of 31 eV and 60 eV for CrB_1.9 and TiB_1.8, respectively, at a growth temperature of 1173 K. Further enhancement of interface abruptness is obtained by commencing the growth of each individual layer without ion-assistance for one unit cell, forming a buffer layer protecting the just formed interfaces from ion-induced intermixing during the ion-assisted growth of the remaining parts of the layers.

The benefits of using a lattice matched substrate are demonstrated by using 4H-SiC which exhibit only -1.29 % lattice mismatch as compared to -26 % for Al₂O₃ which is used in the earlier experiments in this work. The SiC allows for growing high quality single crystal superlattices also with thickness ratios Γ = 0.5 and Γ = 0.7. This is attributed to few misfit dislocations generated due to substrate misfit which lead to a strained layer superlattice growth. The superlattices exhibit less structural crystal defects, smoother initial growth, and smaller interface roughness when using SiC substrates.

Konstgjorda supergitter med sina exceptionella egenskaper har varit populära i ett brett spektrum av användningsområden såsom elektroniska, magnetiska, optiska och hårda beläggningar. En annan potentiell tillämpning för enkristallsupergitter är högeffektiv interferensneutronoptik, tack vare att gränssnitten mellan lagren kan vara så tunna som endast ±½ atomlager. Studier av supergitter har även varit avgörande för att förstå härdnings-mekanismerna i övergångsmetall-nitrider och -karbider medan sådana studier på övergångsmetalldiborider saknas, trots omfattande studier av monolitiska tunna filmer.

Denna avhandling beskriver en logisk serie av studier som löser en rad grundläggande problem som möjliggjort epitaxiell tillväxt av högkvalitativa CrB₂/TiB₂-diboridsupergitter på substrat av enkristallin safir och kiselkarbid genom magnetronsputtring. Detta innebar implementering av flera olika strategier; sputtring från diboridkällor, samsputtring från separata metall- och diboridkällor, sputtring från titanmonoborid- och kromdiboridkällor, kontroll av tillväxt-kinetiken på atomlagernivå genom argonjon-assisterad epitaxi samt val av substrat med liten gittermissanpassning. Effekter på supergittrens kristallstruktur och gränssnittskvalitet studeras med avseende på substrattemperatur, borstökiometri (B/metall-förhållande), modulerings-period Λ = D_CrB2 + D_TiB2, skikttjockleksförhållande Γ = D_TiB2/(D_CrB2 + D_TiB2), jonassistans-energi, användning av gränssnittsskyddande buffertlager, samt val av safir- eller kiselkarbid-substrat. Röntgendiffraktion, röntgenreflektivitet, neutronreflektivitet, jonstråleanalys, transmissionselektronmikroskopi, elektronenergiförlustspektroskopi, elektron-diffraktion och nanoindentation används för karakterisering.

För alla experimentella förhållanden har det visat sig att den högsta strukturella kvaliteten hos supergittren erhålls för skikttjockleksförhållandet Γ ≈ 0,3. Vid användning av safirsubstrat och sputtring från Cr- och Ti-diboridkällor med ett optimalt sputtergastryck p_Ar = 4 mTorr och en substrattemperatur på 600 °C, har vi funnit att supergitter med moduleringsperiod Λ = 6 nm och Γ i intervallet 0,2-0,4 uppvisar den högsta strukturella kvaliteten. Emellertid resulterar överstökiometriska TiB_y-skikt (y > 2) i segregering av bor som i sin tur leder till tillväxt av supergitter i smala kolumnära epitaxiella domäner med strukturellt förvrängda borrika gränser.

Genom att samsputtra från en Ti- och en TiB₂-källa, och välja det rätta effektförhållandet till sputterkällorna, kan y skräddarsys i intervallet 0,9 till 3,3 i TiB_y-lagren. Samsputtrade TiB_2,3 enkellager uppvisar 10x större epitaxiella domäner än icke-samsamsputtrade filmer, samt ger högkvalitativa, nära stökiometriska, CrB_1,7/TiB_2,3 supergitter vid en högre tillväxttemperatur på 750 °C. De individuella CrB_1.7-skikten växer på ett tvådimensionellt sätt vilket leder till släta ytor och atomärt abrupta gränssnitt när TiB_2,3 odlas ovanpå. Däremot uppvisar TiB_2,3-skikt ojämna gränssnitt då CrB_1,7 odlas ovanpå vilket tyder på en kinetiskt begränsad tredimensionell tillväxtmod av TiB_2,3. Skillnaden i tillväxtsätt mellan CrB_1,7 och TiB_2,3 tillskrivs en 1100 K skillnad i smälttemperaturer. Genom att introducera användningen av en sputterkälla av titanmonoborid uppnås kontinuerliga och nästan stökiometriska TiB_1,8-epilager av hög kristallkvalitet, vilket lättar på alla processbegränsningar tack vare behovet av balanserade flöden från Ti- och TiB₂-källorna. Samtidigt implementeras jonassisterad epitaxiell tillväxt, genom lågenergetiska Ar-joner som extraheras från sputterplasmat, för att stimulera rörligheten hos adatomerna, särskilt för TiB_1,8 som har den högsta smälttemperaturen. De bästa supergittren erhålls genom användningen av en TiB-källa kombinerat med Ar-jonenergier på 31 eV och 60 eV för CrB_1,9 respektive TiB_1,8, vid en tillväxttemperatur på 1173 K. CrB_1,9/TiB_1,8 supergitter med enkristallkvalitet och supergitterskiktsdefinition som överstiger något av de tidigare erhållna supergittren. Ytterligare förbättring av gränssnittems abrupthet erhålls genom att påbörja tillväxten av varje enskilt skikt med ett atomlager utan jonassistans, som bildar skyddande buffertskikt på de nyss bildade lagergränsytorna, vilka hindrar joninducerad intermixning av av atomer mellan skikten under den följande jonassisterade tillväxten av återstoden av varje lager.

Fördelarna med att använda ett gitteranpassat substrat påvisas genom att använda kiselkarbid (4H-SiC) som endast uppvisar -1,29 % gittermissanpassning mot supergittren jämfört med -26 % för safir (Al₂O₃) som använts i de tidigare experimenten i detta arbete. SiC möjliggör odling av högkvalitativa enkristallsupergitter även med tjockleksförhållanden Γ = 0,5 och Γ = 0,7. Detta tillskrivs supergittertillväxt med töjda lager och mindre relaxering av gitterspänningar under kylningen efter tillväxt.

Sputter-deposited titanium diborides are promising candidates for protective coatings in harsh and extreme conditions. However, growing these layers from TiB₂ diboride targets by DC magnetron sputtering usually leads to over-stoichiometric layers with low crystal qualities. Moreover, superlattices with TiB₂ as one of the constituents have been becoming popular, owing to their superior mechanical properties compared to single layer constituents in addition to their use in other applications such as neutron optics. Here, we propose the use of a TiB (Ti:B = 1:1) sputtering target in an on-axis deposition geometry and demonstrate the growth of epitaxial sub-stoichiometric TiB_1.8 thin films. Furthermore, we present the growth of CrB_1.7/TiB_1.8 superlattices, from TiB (Ti:B = 1:1) and stoichiometric CrB₂ targets, with abrupt interfaces as promising materials system for neutron interference mirrors. The high crystal quality structure with well-defined interfaces is the common feature of superlattices which, regardless of application, should be addressed during the growth process.

Utilizing TiB target, all films crystallize in the hexagonal AlB₂ structure. The sub-stoichiometry of the TiB_1.8 films was accompanied by the presence of planar defects embedded in the films. CrB_1.7/TiB_1.8 superlattices exhibited a homogeneous boron distribution within the layers with no sign of B-rich tissue phases through the layers. This study demonstrates the feasibility for TiB as sputter target material, that offers a solution for deposition of TiB₂-based superlattices without the need to adjust the deposition parameters. Such adjustments would otherwise be unavoidable for tuning the TiB2 composition and could affect the growth of the other constituent materials.

We systematically study the oxidation properties of sputter-deposited TiB2.5 coatings up to 700 °C. Oxide-scale thickness dox increases linearly with time ta for 300, 400, 500, and 700 °C, while an oxidation-protective behavior occurs with dox=250∙ta0.2 at 600 °C. Oxide-layer’s structure changes from amorphous to rutile/anatase-TiO2 at temperatures ≥ 500 °C. Abnormally low oxidation rate at 600 °C is attributed to a highly dense columnar TiO2-sublayer growing near oxide/film interface with a top-amorphous thin layer, suppressing oxygen diffusion. A model is proposed to explain the oxide-scale evolution at 600 °C. Decreasing heating rate to 1.0 °C/min plays a noticeable role in the TiB2.5 oxidation.

Refractory transition-metal (TM) diborides have high melting points, excellent hardness, and good  chemical  stability.  However, these properties are not sufficient for applications involving extreme  environments that require high mechanical strength as well as oxidation and corrosion resistance. Here, we study the effect of Cr addition on the properties of ZrB₂-rich Zr₁-_xCr_xB_y thin films grown by hybrid high-power impulse and dc magnetron co-sputtering (Cr-HiPIMS/ZrB₂-DCMS) with a 100-V Cr-metal-ion synchronized potential. Cr metal fraction, x = Cr/(Zr+Cr), is increased from 0.23 to 0.44 by decreasing the power P_zrb2 applied to the DCMS ZrB₂ target from 4000 to 2000 W, while the average power, pulse width, and frequency applied to the HiPIMS Cr target are maintained constant. In addition, y decreases from 2.18 to 1.11 as a function of P_zrb2, as a result of supplying Cr to the growing film and preferential B resputtering caused by the pulsed Cr-ion flux. ZrB_2.18, Zr_0.77Cr_0.23B_1.52, Zr_0.71Cr_0.29B_1.42, and Zr_0.68Cr_0.32B_1.38 2 films have hexagonal AlB₂ crystal structure with a columnar nanostructure, while Zr_0.64Cr_0.36B_1.30 and Zr_0.56Cr_0.44B_1.11 are  amorphous. All films show hardness above 30 GPa. Zr_0.56Cr_0.44B_1.11 alloys exhibit much better toughness, wear, oxidation, and corrosion resistance than ZrB_2.18. This combination of properties   makes Zr_0.56Cr_0.44B_1.11 ideal candidates for numerous strategic applications.