Laser-Induced Crystallization of Ge-rich GST Thin Films—Insights from In Situ Synchrotron X-Ray Diffraction and Electron Microscopy

Phase Change Materials (PCM) are very promising candidates for future non-volatile memory applications [1]. The contrast between two phases (amorphous and crystalline) and the differences in their physical properties (resistivity, optical reflectivity …) allows modifying and reading the state of the memory. The most studied PCM is Ge₂Sb₂Te₅ (GST), with a crystallization temperature within the 150-170°C range. This temperature is too low for data retention in automotive applications but Ge-rich GST (GGST) with a crystallization temperature of 350°C has been developed [2] and is fully suitable for this purpose. While <i>in situ</i> furnace-annealing of GGST have been recently performed at low heating rates (few °C/min) evidencing a sequential crystallization of the Ge and Ge₂Sb₂Te₅ phases [3], real memories are switched at the ns timescale. In the aim of performing time-resolved investigations of the phase transformations in GGST and reproducing the crystallization dynamics of operating devices, laser irradiation experiments were performed <i>in situ</i>. The X-ray Diffraction (XRD) experiments have been performed on ID09 beamline at ESRF (France). Samples were irradiated <i>in situ</i> thanks to a laser with an 800nm wavelength and a pulse duration of 100ps which matches the minimal pulse length of the X-ray beam. Laser pulses had fluences going from 14 to 219 mJ/cm² and a focused pink (14.5 keV, ΔE/E ~ 1.5 %, size ~ 25 µm (V) x 49 µm (H)) X-ray beam was used for probing the sample response at an incident angle of 1° with high enough flux (~10⁸ photons/100ps pulse). The laser is connected to the synchrotron bunch clock which allows laser and x-ray pulses to be emitted synchronously. The laser had an angle of incidence of 15°. A charge coupled device (CCD) detector was used with a total active area of 170x170mm². A piezoelectric stage allowed positioning the X-ray beam at the maximum of the laser intensity. The investigated samples were 50nm thick GGST thin films with the following structure: Si(001) substrate// 30nm SiO₂ // 15nm SiN// 50nm GGST// 15nm SiN. GGST layers are deposited by physical vapor deposition SiN capping is deposited by chemical vapor deposition). From the 2D diffraction patterns Ge and GST crystallization have been evidenced. The diffraction peaks are fitted with a Gaussian function after background subtraction. The evolution of integrated intensity, integral breadth and peak position for Ge 111 and GST 200 as a function of laser fluence and number of laser pulses will be presented and discussed. These results demonstrate the ability to control the crystallization of GGST with laser irradiation and open the way to future time-resolved experiments of the reversible phase change mechanisms. Time resolved experiments have been also performed on partially crystallized samples at a fluence of 34mJ/cm². The time evolution of the Ge<b> </b>111 peak position has been monitored and shows a 300 ps rise time followed by a slow 20ns relaxation<b>. </b><i>Post mortem </i>scanning electron microscopy observations of the samples allows different regimes of the laser-sample interaction to be distinguished depending on the laser fluence and number of pulses.<br/>1. H.S.P. Wong <i>et</i> <i>al., Proc. IEEE 98</i> (2010) 2201.<br/>2. P. Zuliani <i>et al., Solid State Electron. 111 </i>(2015) 27.<br/>3. O. Thomas <i>et al., Microelectronic Engineering 244-246</i> (2021) 111573