Ph. Ebert

Nano-scale properties of defects in compound semiconductor surfaces

Abstract The present work reviews atomic-scale properties of point defects and dopant atoms exposed on and in cleavage surfaces of III–V and II–VI semiconductors. In particular, we concentrate on the identification of the types of defects and dopant atoms, the determination of the localized defect states, the electrical charge, and lattice relaxation, as well as the measurement of the interactions between different defects and/or dopant atoms. The physical mechanisms governing the formation of defect complexes, the compensation of dopant atoms, the pinning of the Fermi level, and the stability of defects are discussed in the light of the available theoretical information and experimental results obtained mostly by scanning tunneling microscopy.

Surface states and origin of the Fermi level pinning on nonpolar GaN(11¯00) surfaces

GaN(11¯00) cleavage surfaces were investigated by cross-sectional scanning tunneling microscopy and spectroscopy. It is found that both the N and Ga derived intrinsic dangling bond surface states are outside of the fundamental band gap. Their band edges are both located at the Γ¯ point of the surface Brillouin zone. The observed Fermi level pinning at 1.0 eV below the conduction band edge is attributed to the high step and defect density at the surface but not to intrinsic surface states.

Atomic structure of point defects in compound semiconductor surfaces

Abstract The present work reviews the progress in the determination and understanding of the atomic structure of point defects and dopant atoms exposed in and below cleavage surfaces of III–V semiconductors during recent years. By critically evaluating the present experimental data and theoretical concepts, we discuss the methods of identification of the types of defects and dopant atoms, the determination of defect energy levels, electrical charge states, as well as lattice relaxation, and the deduction of the physical mechanisms governing the interactions between different defects and/or dopant atoms, the formation of defect complexes, the compensation of dopant atoms, the pinning of the Fermi-level, and the stability of defects. Finally, the methodology to extract the concentrations and types of bulk defects and the physics governing bulk defects is examined.

Direct measurement of the band gap and Fermi level position at InN(112¯0)

A nonpolar stoichiometric InN(112¯0) surface freshly cleaved inside UHV was investigated by scanning tunneling microscopy and spectroscopy. Due to the absence of intrinsic surface states in the band gap, scanning tunneling spectroscopy yields directly the fundamental bulk band gap of 0.7±0.1 eV. The Fermi energy is pinned 0.3 eV below the conduction band minimum due to cleavage induced defect states. Thus, intrinsic electron accumulation can be excluded for this surface. Electron accumulation is rather an extrinsic effect due to surface contamination or material decomposition, but not an intrinsic material property of InN.

Phosphorus vacancies and adatoms on GaP(110) surfaces studied by scanning tunneling microscopy

Abstract Atomic defects were investigated on p-doped GaP(110) surfaces by scanning tunneling microscopy. The defects were identified as positively charged phosphorus vacancies and uncharged phosphorus adatoms. They are found to migrate on the phosphorus sublattice. Their motion was observed with atomic resolution and the dependence of the jump probability on the tunneling voltage was measured. The position changes of the defects were found to be induced by tip-specimen interactions.

Direct observation of electrical charges at dislocations in GaAs by cross-sectional scanning tunneling microscopy

We demonstrate the possibility of simultaneous determination of the type and electrical charge state of dislocations in GaAs by cross-sectional scanning tunneling microscopy (STM). The methodology is demonstrated for a dissociated perfect dislocation in highly Si-doped GaAs(110) surfaces. The STM images of the dislocation penetrating GaAs cleavage surface show that both partial dislocation cores as well as the stacking fault between the two partial dislocation cores are negatively charged.

Nanoscale dopant-induced dots and potential fluctuations in GaAs

We identified p-type nanoscale dopant-induced dots that are formed by fluctuations of the dopant atom distribution in sufficiently thin GaAs p–n multilayers. Their electronic structure and the resulting potential variations were investigated by cross-sectional scanning tunneling microscopy and spectroscopy as a function of the number of dopant atoms within the dot. We find significant changes in the current–voltage characteristics of the dots compared to spatially nonconfined material, due to a reduced ability to screen the tip’s electric field. This indicates a limited ability to deplete the dots of free holes arising from the presence of confining potentials surrounding the dopant-induced dots.

Electronic properties of dislocations in GaN investigated by scanning tunneling microscopy

We investigated the type, spatial distribution, line direction, and electronic properties of dislocations in n-type GaN by scanning tunneling microscopy. We found uncharged perfect dislocations with a/3{112¯0} Burgers vectors and negatively charged Shockley partial dislocations with a/3{11¯00} Burgers vectors interconnected by a negatively charged stacking fault. The charges are traced to different charge transfer levels associated with the particular core structure.

Determination of the charge carrier compensation mechanism in Te-doped GaAs by scanning tunneling microscopy

We identified the charge carrier compensation mechanism in Te-doped GaAs with atomically resolved scanning tunneling microscopy. Three types of defects were found: tellurium donors (TeAs), Ga vacancies (VGa), and Ga vacancy–donor complexes (VGa–TeAs). We show quantitatively that the compensation in Te-doped bulk GaAs is exclusively caused by vacancy–donor complexes in contrast to Si-doped GaAs. This is explained with the Fermi-level effect as the universal mechanism leading to Ga vacancy formation in n-doped GaAs, and a Coulomb interaction leading to the formation of the complexes. The quantification of the carrier compensation yields a −3e charge state of VGa in bulk GaAs.

Band offsets at zincblende-wurtzite GaAs nanowire sidewall surfaces

The band structure and the Fermi level pinning at clean and well-ordered sidewall surfaces of zincblende (ZB)-wurtzite (WZ) GaAs nanowires are investigated by scanning tunneling spectroscopy and density functional theory calculations. The WZ-ZB phase transition in GaAs nanowires introduces p-i junctions at the sidewall surfaces. This is caused by the presence of numerous steps, which induce a Fermi level pinning at different energies on the non-polar WZ and ZB sidewall facets.