Starter Kit for Modeling a GaAs-Si interface with Quantum Espresso

1 Starter Kit for Modeling a GaAs-Si interface with Quantum Espresso A.Martí and I. Artacho Instituto de Energía Solar – Universidad Politécnica de Madrid, Spain The motivation behind this upload to Zenodo is to provide the interested visitor with an initial set of files (“starter kit”) so they can model, visualize and perhaps, even carry out further research on the physical and electronic properties of GaAs/Si interfaces. Behind this bundle is the idea of gaining progressive insight into the potential growth of III-Vs on silicon by techniques such as molecular beam epitaxy (MBE): how atoms rearrange, how defects originate, etc. To this end we provide: • The necessary input files to be executed in Quantum Espresso (see Notes below). These are listed in Table I, showing also how they can be executed together with some comments and illustrative results. For the calculations we have used 12 cores (indicated by the use of “mpirun -np 12”). This can be omitted for single core calculations. In our case, using a Lenovo ThinkPad 21KVS0DM00 Laptop, calculation times have been around 2 days for the examples provided. • The Blender file (GaAs_on_Si.blend within the folder blender) that allows visualizing in 3D the atomic positions used in the calculations (Fig. 3) as well as automatically creating a video to visualize the relaxation of the atoms from their initial positions to their final positions. By using this file, the reader can visualize the relaxation from different visual perspectives on their own. Several examples of the videos that can be created are included in this bundle as examples: bottom.mkv, lateral.mkv and front.mkv. The Blender file includes a script in Python where the atomic positions should be inserted in order to automatically generate new videos. For inserting the initial atomic positions, please search into the script for the block commented as : # Parameters: Initial positions of the atoms are inserted here; These positions are inserted in frame 1 of the video. For inserting the final positions, please search for the lines commented as # Insert here the final atomic positions obtained after relaxation. These positions will be inserted in frame 250 and Blender will interpolate in-between. Once the Blender composition is created after running the script, the reader is assumed to have some knowledge about Blender to create their own videos with the perspective they wish. It is also possible to change the color, size of the atoms, etc. We recommend this is done in the python script provided within Blender. If you create your own video, note you might need to change the location where the created video file is stored (in our case, in a tmp folder). Use EEVEE (instead of Cycles) for faster rendering. 2 Table I. Proposed file templates in the starter kit. It is recommended that files are executed in the order the files are provided since some files require the output generated by the previous ones. Name of the file Execution Comments relax.in mpirun -np 12 pw.x < relax.in > relax.out Once executed, by the end of the file “relax.out”, look for the relaxed positions of the Ga, As and Si atoms. Note: “relax.in” only relaxes the atomic positions due to the instruction “relax” within the file. A more accurate calculation can be done using “vc-relax” instead at the cost of a higher computational time. The use of “relax” nevertheless is sufficient to grasp the general idea about how atoms try to find more favorable energetic positions. This visualization is favored by the use of Blender (see later). scf.in mpirun -np 12 pw.x < scf.in > scf.out “Self-consistent-field” calculation. For the first run, we recommend you insert yourself, in the “scf.in” file, the relaxed atomic positions obtained in the “relax.out” file and start the calculations “from_scratch”. This might not be necessary if you use the “restart” option instead, but we have not tested this option. In any case, please, recall that if you need to stop the calculations in a controlled way, you need to create the corresponding empty “.EXIT” file (“GaAs_on_Si.EXIT” in our case if you have not changed our names of the files) at any time in the “tmp” folder and use the option “restart”. nscf.in mpirun -np 12 pw.x < nscf.in > nscf.out Increases the number of k points calculated using the results calculated in the previous scf step. bands.in mpirun -np 12 pw.x < bands.in > bands.out Calculates the band diagram. bands_post.in mpirun -np 12 bands.x < bands_post.in > bands_post.out Creates the files for plotting the band diagram. No gap is expected for this structure. bands.gnuplot gnuplot bands.gnuplot A template for plotting the band diagram of the structure using the output file created by “bands_post.in”. In one of its lines, “bands.gnuplot” includes the position of the Fermi level that can be found in the file “nscf.out” by searching for “Fermi”. This position will vary depending on the research you carry out. Result is shown in Fig. 1. dos.in mpirun -np 12 dos.x < dos.in > dos.out Calculates the density of states. Gives an idea of the energies of the defect formation at the interface even when atoms accommodate to new positions elastically. dos.gnuplot gnuplot dos.gnuplot Template to plot the density of states obtained by running “dos.in”. Results are shown in Fig. 02. As_ONCV_PBE-1.2.upf Ga_ONCV_PBE-1.2.upf Ga_ONCV_PBE-1.2.upf Pseudopotentials for As, Ga and Si. To be included in the folder “pseudo”. 3 Fig. 1. Energy band calculated with Quantum Espresso for the GaAs/Si interface ([001] direction) with the files provided in this “starter kit”. Fig. 2. Density of states calculated with Quantum Espresso for the GaAs/InAs interface with the files provided in this “starter kit”. 4 Fig. 3. Rendering by Blender of the atomic positions used in the simulation: yellow, silicon atoms; dark grey, As atoms; light gray, Ga atoms. NOTES: • Quantum Espresso [1], [2] is an open-source suite of codes for electronic-structure calculations and materials modeling at the nanoscale, based on density functional theory, plane waves, and pseudopotentials. It can be downloaded from www.quantum-espresso.org together with the instructions for installing it. Quantum Espresso can be installed both in Windows as in Linux. The example we provide has been tested on Quantum Espresso 7.3.1 installed under Ubuntu 22.04.5 LTS and tested in a Lenovo ThinkPad 21KVS0DM00 Laptop. • The input files provided contain several instructions, but we make no aim in this document to explain what each instruction does since the reader can easily find this information in the Quantum Espresso manual or, nowadays, they can simply ask AI tools such as ChatGPT (https://openai.com/es-ES/chatgpt/overview/) to obtain a very good summary of what these instructions do adapted to their needs. ChatGPT has assisted us in creating the script for the Blender file and the input files for Quantum Espresso. However, we have not used ChatGPT to review this text. • The output files have not been included to save space. They will be created again once you run the input files. • The unit supercell we use, extends 50 A in the z direction to simulate only one mixed layer of GaAs and Si atoms: CELL_PARAMETERS angstrom 5.43 0.00 0.00 0.00 5.43 0.00 0.00 0.00 50.00 The value of 5.43 A corresponds to the lattice constant of silicon since in our simulation we wanted to put the Ga and As atoms as if initially they were also located preserving this lattice constant. GaAs has a larger lattice constant (5.65 A) and, in fact, the simulation (best appreciated in the videos) reveals an expansion of the position of the atoms as expected from the difference in the value between the two lattice constants. • The pseudopotentials for As, Ga and Si have been downloaded from http://www.quantum-simulation.org/potentials/sg15_oncv/upf/ . They are included within the folder “pseudo”. • The crystal structure of Si and GaAs have been obtained from Wikipedia. • For plotting the actual band diagrams, we shall assume “gnuplot” (www.gnuplot.info/) is installed in your system. 5 • Blender can be downloaded from https://www.blender.org/download/ • Files provided must be understood, as mentioned, as starter kits to initiate research on the electronic properties of the GaAs/Si interface using Quantum Espresso. No rigorous physical significance of the results here presented is claimed at this stage. Results are only illustrative. We think further work should continue by, for example: a) enlarging the supercell; b) carrying out cell relaxation (using vc-relax); c) exploring other crystal orientations. • Some times, the following error is generated: “The following floating-point exceptions are signaling: IEEE_DENORMAL”. This error is considered non-critical and has been ignored. References: [1] P. Giannozzi et al., “QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials,” J. Phys. Condens. Matter, vol. 21, no. 39, p. 395502, 2009, doi: 10.1088/0953-8984/21/39/395502. [2] P. Giannozzi et al., “Advanced capabilities for materials modelling with Quantum ESPRESSO,” J. Phys. Condens. Matter, vol. 29, no. 46, p. 465901, 2017, doi: 10.1088/1361-648X/aa8f79. Acknowledgments: This action has been funded from grant PID2021-124193OBC21 (PVBooster Project) funded by Spanish Ministerio de Ciencia e Innovación with European Funds. This action has been financed through the R+D activities program with reference TEC2024/ECO-72 and acronym 4EVERPV-CM granted by the Community of Madrid through the General Directorate of Research and Technological Innovation through Order 3177/2024.