Novel AlGdN Alloys for Integrated Detection in Radiation Environments

Metastable ternary nitrides are a growing class of materials that offer broad potential for both new functionality and heterogenous integration. In this work, we are synthesizing Al_xGd_1-xN as a radiation hard ultrawide bandgap semiconductor, with potential for integrated detection high energy particles including neutrons and gamma rays. Metastable Al_1-xGd_xN alloys have been synthesized via non-equilibrium radio frequency co-sputtering in a combinatorial system allowing exploration of a range of Gd compositions. First-principles calculations show that the limiting critical composition for a wurtzite to rocksalt phase transition is x_c = 0.82. Theory suggests that at temperatures below 1000 K there is a large miscibility gap limiting Gd incorporation in AlN to only a few percent. However, by accessing higher effective temperature through non-equilibrium growth we have achieved Gd concentrations up to 24%, the highest Gd³⁺ incorporation into the wurtzite phase reported to date. Single-phase compositions up to x ≈ 0.24 are confirmed by high resolution synchrotron grazing incidence wide angle X-ray scattering and transmission electron microscopy (TEM). <br/>These new materials have been characterized using a range of analytical tools, including X-ray diffraction, Rutherford Backscattering Spectroscopy, cathodoluminescence, temperature dependent X-ray, ellipsometry, and transmission electron microscopy with high resolution energy dispersive X-ray. Energy dispersive X-ray (EDX) imaging in the scanning electron microscope and cathodoluminescence, monitoring an internal UV transition within the Gd atoms, indicate uniform distribution of Gd in thin films with columnar grains. High resolution EDX in the TEM shows homogeneous incorporation of Al, Gd and N in films on the order of ~ 200 nm in thickness, and TEM imaging indicates a wurtzite structure based on fast Fourier transform (FFT) analysis, consistent with the synchrotron grazing incidence wide angle X-ray scattering.<br/>Because Al_1-xGd_xN is a heterostructural alloy (AlN is a wurtzite structure, GdN is a rocksalt structure), a key part of the project is to explore the fundamental science to enable stable integration of new tunable metastable nitride materials into thin film structures for high radiation and high temperature environments. Interfacial understanding and control are critical and could potentially provide both enhanced structural stability for these metastable alloys (e.g., via designed lattice matching) and tunable electronic functionality (e.g., ohmic to rectifying behavior). We will review the status of contact materials for high resistivity high energy particle detectors and discuss paths to device integration.