Epitaxial Growth And Mechanism Analysis Of Large Area AlN Single Crystalline Thin Films By Pulsed Laser Deposition

Compound semiconducting AlN has a wide band gap of6.2eV, high thermalconductivity, high chemical and thermal stabilities, and high surface acoustic wave velocity.These properties make AlN a promising material for applications in ultraviolet （UV） and deepUV solid state light sources, and bulk acoustic wave resonators （FBARs）, etc. As we know, inorder to substantially improve the efficiency of AlN-based devices, single crystalline AlN filmis imperative to be used. Meanwhile, if the manufacturing costs of AlN single-crystalline filmcan be reduced, AlN-based devices will be applied widely in industry. All this requires that weachieve single-crystalline AlN films epitaxially grown on large area and low cost substrates.Firstly, many substrates are used for growing AlN films. Among them, sapphire （-Al₂O₃） andsilicon （Si） have been already realized large scale production. And Si has higher thermalconductivity as compared with-Al₂O₃. Secondly, single-crystalline AlN films are usuallygrown through metal-organic chemical vapor deposition （MOCVD）, molecular beam epitaxy（MBE）, and pulsed laser deposition （PLD）. Among these techniques, MOCVD growthundergoes an irreversible parasitic reaction between TMAl and NH3for AlN, and MBEgrowth needs to configure an ultra-high vacuum system and there is interfacial reactionbetween substrate and film. Nevertheless, PLD growth can successfully overcome the aboveproblems. PLD employs a highly energetic pulsed laser to ablate growth targets, so as toassure high kinetic energy of species when arriving substrates, and therefore realize theepitaxial growth of abrupt-interface single-crystalline films at a relatively low temperatureand low vacuum growth conditions. PLD growth is a flexible process without pre-reaction orcrucible contamination. Therefore, we will use PLD to achieve epitaxially grownsingle-crystalline AlN films on2-inch-Al₂O₃and Si substrates. However, it is difficult torealize homogeneous film epitaxially grown on large area substrates by PLD, which is due tothe intrinsic characteristic of the traditional PLD that the flying route of species away fromtargets is highly directionally distributed, resulting in serious thickness inhomogeneity.Moreover, one can hardly find any report about analyzing epitaxial growth mechanism of AlNfilms by PLD. In this thesis, we report on the homogeneous epitaxial growth ofsingle-crystalline AlN films on2inch-Al₂O₃and Si substrates, and systematical analysis ofgrowth mechanism of AlN films by PLD. The main contents and conclusions are as follows.Firstly, we studied PLD parameters such as pressure, laser frequency, substratetemperature, nitriding process, and radio frequency plasma, which influence the crystallinequality and morphology of AlN films epitaxially grown on-Al₂O₃substrates, and finallyobtained the optimal parameters. At the optimum conditions, AlN films epitaxially grown at arelatively low growth temperature on-Al₂O₃substrate was realized. This is mainly due tothat the ablated species with high kinetic energy can still migrate and diffuse rapidly on therelatively low temperature substrate. Then we analyzed epitaxial relationship, mismatch stress,and interfacial layer between-Al₂O₃and AlN films. AlN films revealed an atomically abruptinterface on-Al₂O₃substrates. Because interfacial reaction was effectively suppressed bypre-nitridation of-Al₂O₃substrates and the unique characteristic of PLD. Furthermore, theforming reason of dislocations and growth mechanism of AlN films on-Al₂O₃substrates by PLD were also investigated. Since there is13.3%lattice mismatch between α-Al₂O₃and AlN,dislocations were formed as a way to relieve mismatch strain in the islands coalescence stageat AlN/α-Al₂O₃interface. AlN films growing on α-Al₂O₃substrates by PLD is atwo-dimensional growth process, which is result from two unique effects of PLD. One is thehigh energy effect which considerably facilitates ablated species to overcome energy barrierand migrate rapidly along scattered islands and among layers. The other is the pulse effect thatenables ablated species to have enough time to migrate to the stable position before nextspecies arrive in the interrupted time of a pulse.Secondly, single-crystalline AlN films were epitaxially grown on Si substrates by PLD,which is based on the optimum process of AlN films on α-Al₂O₃substrates. Then the epitaxialrelationship, mismatch stress and interfacial layer between Si substrates and AlN films werealso analyzed. AlN films also revealed an atomically abrupt interface on Si substratesindicated by high-resolution transmission electron microscopy. There is no amorphous SNxlayer existing between between Si substrates and AlN films reported by other researchers.This is attributed to pre-ablation and utilization of AlN ceramic targets. They can reduce thereaction probability between Si substrate and active N, and eventually inhibit Si-N interfacialreaction. In the contrary, an AlSiN layer was found in the AlN/Si （111） interface innon-optimal growth conditions, which is derived from the interfacial interdiffusion andpenetration between active Si atoms and AlN species, resulting in high density of dislocationsand defects in AlN/Si （111） interface.Finally, homogeneous single-crystalline AlN films were successfully grown on2inch-diameter α-Al₂O₃and Si substrates by PLD respectively. By optimizing the laserrastering and PLD growth conditions, the2inch-diameter AlN films exhibit excellentthickness uniformity with a root-mean-square （RMS） inhomogeneity of less than4.3%. Thisindicates that it is quite useful for improving film uniformity by properly changing laserrastering movement. The application of laser rastering technique engineers incident anglesand the locations on substrate of species, as well as plume spatial distribution, and eventuallyleads to homogeneous AlN films over large scale. Also the change of target-substrate distancecan substantially affect the thickness and uniformity of AlN films. This is due to that theplume size and plasma density distribution correspondingly change as the target-substratedistance, so AlN films exhibit varied thickness with different uniformity.In this thesis, the achievement of homogeneous single-crystalline AlN films with abruptinterface grown on2inch-diameter α-Al₂O₃and Si substrates by PLD is a significant appliedfoundation for improving the efficiency and reducing the manufacture costs of AlN-baseddevices. Meanwhile, the systematical investigation on the forming reasons of defects and thegrowth mechanism of AlN films by PLD is an essential theoretical basis for growinglarge-scale and high-quality AlN single-crystalline films.`...