Ex-situ incorporation of Al in Ge by sputter deposition and pulsed laser melting: a new approach to fabricate hyper-doped Ge:Al alloys.

Ge-based alloys have attracted considerable interest over the last two decades due to their promising properties for high-performance nanoelectronics, mid-infrared photonics, gamma- and X-ray radiation detectors, and quantum computing. A key challenge is the fabrication of alloys incorporating high concentrations of electrically active dopants. In recent years, nanosecond Pulsed Laser Melting (PLM) has emerged as a powerful technique to overcome the limitations of conventional out-of-equilibrium growth processes, such as epitaxy, enabling improved dopant incorporation and activation [1,2]. In this work, hyper-doped Ge:Al films were fabricated through a novel approach combining two-step RF sputter deposition and subsequent PLM [3]. The method consists of sequential deposition of a thin Al layer followed by an amorphous Ge capping layer to prevent Al oxidation before and during laser processing (fig. 1). Laser treatments were performed under a broad range of experimental conditions, using single or multiple pulses and finely tuning the Energy Density (ED). To achieve an in-depth understanding of the melting and recrystallization mechanisms, including Al diffusion and electrical activation, a comprehensive set of electrical, structural, and chemical characterizations was carried out. SIMS analyses were employed to determine Al concentration profiles and detect possible contaminants. Van der Pauw-Hall measurements allowed evaluation of carrier concentration and mobility, revealing a hyper-doping regime with hole concentrations exceeding 1 × 10²¹ cm⁻³ (fig. 2). Surface morphology and microstructure were investigated by AFM and SEM/TEM techniques, enabling direct observation of lattice reconstruction and the formation of Al-rich regions after laser irradiation. Raman spectroscopy was correlated with the other analyses to provide insight into amorphous-to-crystalline phase transformations and the contribution of active dopants to Raman peak shifts. Overall, this work demonstrates that PLM is an effective strategy for ex-situ p-type doping of Ge and offers an interpretative framework for understanding laser-induced recrystallization under different processing conditions. [1] R. Duffy, E. Napolitani, F. Cristiano, Materials science issues related to the fabrication of highly doped junctions by laser annealing of Group IV semiconductors, in Laser Annealing Processes in Semiconductor Technology, Elsevier (2021). [2] E. Di Russo et al., Fully relaxed Ge0.9Sn0.1/Ge by Sn/Ge deposition and nanosecond pulsed laser melting, Applied Surface Science 612, 155817 (2023). [3] E. Di Russo et al., Ex-situ incorporation of Al in Ge by sputter deposition and pulsed laser melting: a new approach to fabricate hyper-doped Ge: Al alloys. Applied Surface Science, 710, 163946 (2025).