Reproducing DFT Calculations In

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

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