Electronic properties of adsorbates on Ga As ( 001 ) - c ( 2 × 8 ) ∕ ( 2 × 4 )

A systematic experimental and theoretical study was performed to determine the causes of oxide-induced Fermi level pinning and unpinning on Ga As ( 001 ) - c ( 2 × 8 ) ∕ ( 2 × 4 ) . Scanning tunneling spectroscopy (STS) and density functional theory (DFT) were used to study four different adsorbates' ( O 2 , In 2 O , Ga 2 O , and SiO) bonding to the Ga As ( 001 ) - c ( 2 × 8 ) ∕ ( 2 × 4 ) surface. The STS results revealed that out of the four adsorbates studied, only one left the Fermi level unpinned, Ga 2 O . DFT calculations were used to elucidate the causes of the Fermi level pinning. Two distinct pinning mechanisms were identified: direct (adsorbate induced states in the band gap region) and indirect pinnings (generation of undimerized As atoms). For O 2 dissociative chemisorption onto Ga As ( 001 ) - c ( 2 × 8 ) ∕ ( 2 × 4 ) , the Fermi level pinning was only indirect, while direct Fermi level pinning was observed when In 2 O was deposited on Ga As ( 001 ) - c ( 2 × 8 ) ∕ ( 2 × 4 ) . In the case of SiO on Ga As ( 001 ) - c ( 2 × 8 ) ∕ ( 2 × 4 ) , the Fermi level pinning was a combination of the two mechanisms.