Aluminium nitride (AlN) crystals are grown by physical vapor phase transport (PVT) in crucibles of TaC or W and must have low dislocation densities and relevant diameters (1") regardless of the intended application. Prerequisites for the growth of AlN crystals of high crystalline quality are the availability of low-defect seeds, the avoidance of impurity precipitation during heating up and an optimal T-field design (e.g. high T).

Diameter increase is achieved by gradual seed increase in several successive growth runs with suppression of parasitic growth and slow lateral growth using suitable lateral T-gradients. For the application as a substrate for UV emitters (<= 280 nm) a sufficient transparency in the targeted wavelength range is required, which can be adjusted by specifically influencing the concentrations of the main impurities O, C and Si.

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At the IKZ the installation of a technology-ready processing line for the preparation and characterization of semiconductor substrates with diameters up to 2" is planned. Currently, the IKZ is already able to produce demonstrator substrates (e.g. aluminium nitride) with small diameters and high quality. For the production of prototypes and small series, the machining effort must be reduced and the process reproducibility must be improved.

Central technical or scientific challenges in the production of semiconductor substrates are:

• Low-loss sawing of crystals with different mechanical properties, especially of brittle materials.
• Precise orientation of crystals or substrates.
• Realization of a high structural surface quality through optimal polishing processes.
• Realization of a high surface purity by avoiding contamination during the machining processes and suitable final cleaning processes.

Semiconductor crystals of group III-V (e.g. InP, AlN) are substrate materials of the future for microelectronic, photonic, and opto-mechanical components that find applications in areas such as mobile communication, sensing, medical technology, and automotive. Integrating them into mature Si or sapphire-based technology platforms has the potential to realize cost-effective semiconductor devices with superior properties.

To enable this fundamentally new and future-oriented device generation, we want to develop preparation and transfer processes for integration on Si and other target substrates for III-V bulk crystals grown at the IKZ. Our activities are conducted in close collaboration with the bulk crystal growth (VGF-III-V, Aluminium Nitride Crystal Growth) and Wafering research groups, the IKZ-IRIS joint lab „Layer Transfer" as well as partners specialized in device development.

The R&D activities include:

• Preparation of InP and AlN single crystals with high crystalline perfection and tailored bulk properties (especially doping) into thin layers or micro-platelets with atomically smooth surfaces and low defect densities in preparation for the transfer.
• Optimization of the necessary chemical-mechanical polishing processes (surface) and lithography techniques (micro-structuring).
• Development of methods for characterizing the structural, optical, and electrical properties of the thin layers and micro-platelets.
• Development of transfer processes for the heterointegration of thin layers and micro-platelets onto Si substrates and other material platforms.
• Development of device demonstrators in collaboration with external project partners.
• Investigation of the influence of doping, micro-preparation, and bonding on crystal properties and device parameters.
• Adaptation and optimization of the developed methods for other application-relevant materials.

An important prerequisite for the reliable evaluation of experimental results in crystal growing is the regular measurement of the chemical compositions and impurity concentrations of starting materials and crystals, as well as the measurement of surface impurities on substrate surfaces. Therefore, existing methods are continuously optimized and new methods will be established.

The current focus of the work is on the development of ICP-OES digestion recipes for oxide- and fluoride crystals produced at the institute and by project partners. Technical and scientific challenges include the measurement of Si impurities and the avoidance of HF-based digestion methods. In the medium term, methods for the measurement of light elements (e.g. O and C in AlN source material) and metallic surface impurities (e.g. Fe) should also be established at the institute.