Reproducible Science @ nanoHUB.org Reproducing DFT calculations in Role of surface orientation on atomic layer deposited Al2O3/GaAs interface structure and Fermi level pinning Ganesh Hegde, Gerhard Klimeck, and Alejandro Strachan Applied Physics Letters, 99, 093508 (2011) Ale Strachan Network for Computational Nanotechnology (NCN) School of Materials Engineering [email protected] The paper Ale Strachan Key results STEP 1 Used density functional theory (DFT) to predict atomic structure after 1st monolayer of Al2O3 is deposited on GaAs (111)A and (111)B surfaces Ga terminated As terminated STEP 2 Computed their electronic density of states of resulting structures. Kohn-Sham eigenvalues (underestimate band-gap but trends should be accurate) Main results • (111) B interface (blue line) exhibits electronic states within the band-gap • (111) A interface (black line) leads to no electronic states within the band-gap and some near the valence band maximum • Good agreement with MOSFET experiments that exhibit large currents when built on (111) A with little Fermi level pinning and no current in (111) B devices Ale Strachan M. Xu, et al. Electron Devices Meeting (IEDM), 2009 IEEE pp. 1–4. 3 The simulation tool From the tools menu launch “nanoMATERIALS SeqQest DFT” About the tool: SeqQuest, a density functional theory (DFT) code from Sandia National Laboratories: http://dft.sandia.gov/Quest/ Learn more: • Designing meaningful density functional theory calculations in materials science— a primer, Mattsson, et al. Modelling Simul. Mater. Sci. Eng. 13, R1-31 (2004). • nanoHUB-U course “Atoms to Materials” https://nanohub.org/courses/FATM Ale Strachan 4 What we will do Objective We will compute the electronic density of states of the (111)A and (111)B interfaces from the paper Approach • We will select the appropriate structures • Perform a density functional theory electronic calculation using the generalized gradient approximation (GGA) • Analyze the electronic density of states for both configurations using Kohn-Sham eigenvalues Ale Strachan 5 Specify the input structure From the dropdown menu select GaAs-111A You can check the atomic positions and cell parameters to be used in the calculation The cell is periodic along the a and b directions and open along the normal direction Ale Strachan 6 Specify simulation details Select GGA as the exchange and correlation functional Specify 0.1 eV as the width of the Gaussian smearing to be used to plot the electronic density of states. Each Kohn-Sham eigenvalue will be replaced by a Gaussian with this standard deviation. Ale Strachan 7 Finish setup and run No need to compute forces on atoms This simulations takes a while, about 20 minutes, go read about DFT of SeqQuest and come back… Submit the run Ale Strachan 8 Visualizing the DoS From the dropdown menu in the output select “Density of States” Change the plot ranges: Click on the x and y axes select Manual • For x (electron energy) we will plot from -6 to -3 eV (the Fermi energy falls in this range – see next slide) • For y (DoS) we will plot from 0 to 5 Ale Strachan 9 Visualizing the DoS To find out where the Fermi energy lies, plot the electron occupation statistics Ale Strachan 10 Simulating the (111)B case 1. Without deleting the run, go back to the input to run the other interface 2. Select the (111) B interface 3. Re-select GGA and 0.05 eV for the DoS broadening 4. Click Simulate Ale Strachan 11 Make a publication quality plot You can visualize the results of both simulations at the same time An make a publication quality plot directly from nanoHUB You can customize your plot with line colors, symbols, fonts, sizes, etc. Ale Strachan 12 Compare with predictions nanoHUB result • Similar trends in both DoS calculations, with (111)B interface showing significant states in the gap and the (111) A states near the conduction band edge • Possible reasons for discrepancy: different numerical values in k-points and real- space grid, differences in basis sets and pseudopotentials Ale Strachan 13 Further work • Explore the role of in-plane strain on the atomic and electronic structure of the two interfaces in the paper • Study other semiconductor/oxide interfaces Ale Strachan 14.