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Physical review materials 7(5), L052401 (2023). doi:10.1103/PhysRevMaterials.7.L052401, We demonstrate the presence of a hysteresis in tunneling spectra acquired at 4.3 K on cleaved MA-Br terminated (001) surfaces of MAPbBr3 single crystals. Simulations of the tunneling spectra reveal an underlying polarization-voltage (P-V) hysteresis, which is caused by an interplay of subsurface field-induced rotation and alignment of the MA molecules, stabilized by dipole-dipole interactions, and an ion-lattice relaxation. The resulting subsurface polarization is counteracted by a compensating surface polarization, detectable by surface-sensitive tunneling spectroscopy., Published by APS, College Park, MD

The electronic properties of (1010) surfaces of ternary solid solution (Al,Ga)N semiconductors with compositions ranging from pure GaN to AlN as well as n- and p-type doping are investigated using cross-sectional scanning tunneling microscopy (XSTM) and spectroscopy (XSTS).The aim is to identify the different intrinsic and extrinsic surface states present after differentsurface treatments (i.e., atomically clean vs. hydroxylated), to unravel the states’ position in the fundamental band gap and their physical origin. Particular emphasis is put on the Fermi-levelpinning as a central parameter defining the surfaces’ electronic properties and passivation. First, a systematic study of the dependence of the tunnel current on the variation of different physical sample and tip properties is done using tunnel current simulations. This provides adetailed understanding of the properties derivable from XSTS data and forms a basis for aproper interpretation of tunnel spectra of ternary nitride surfaces. Using these findings, the surface states of freshly cleaved and air-exposed non-polar (1010)surfaces of GaN n-p-n junctions are investigated quantitatively by XSTS. On freshly cleavedp-type surfaces, the Fermi level is extrinsically pinned by defect states which arise from cleavage induced steps, but not by intrinsic dangling bond states. In contrast, on n-type surfaces, the Fermi level is intrinsically pinned by the empty Ga-derived dangling bond state, while extrinsicdefect states, although present, do not affect the electronic properties. After air exposure and subsequent heating in UHV, the pinning level of both n- and p-doped (1010) surfaces shifts towards the band edge and the density of pinning states decreases. This is attributed to the adsorption and dissociation of water molecules: H+ and OH− groups bind to the surfaces’anions and cations, shifting the intrinsic (and likely also extrinsic) surface states out of the fundamental band gap into the valence band. XSTS data reveal a partially passivated GaNsurface with a reduced band bending and decreased pinning DOS. These results unravel adelicate interplay between intrinsic and extrinsic surface states and emphasize the importance of dopant-dependent Fermi-level pinning for the interpretation of tunnel spectra at the m-planeGaN surfaces. Furthermore, the intrinsic surface states of ternary solid solution (Al,Ga)N (1010) surfaces are investigated. Tunneling on pure AlN is found to be fundamentally different from that of Ga containing (Al,Ga)N compounds where tunneling in bulk states dominates. For pure AlN, tunneling into the conduction band is not possible due to the large band gap, the low carriercon centration, and large tip-induced band bending. Instead, the tunnel spectra are compatible only with tunneling into the intrinsic surface states. On the basis of these findings, the minimum of the empty group III-derived dangling bond state is extracted from tunnel spectra and found to shift towards midgap with increasing Al concentration. This is corroborated by complementary DFT calculations.The trends and physical effects observed here can be anticipated to be not limited to this particular (Al,Ga)N alloy, but rather provide a view on the general behavior for intrinsic and extrinsic surface states of most binary and ternary non-polar III-nitride compound semiconductor surfaces.

We report a magnetic transition region in La_{0.7}Sr_{0.3}MnO_{3} with gradually changing magnitude of magnetization, but no rotation, stable at all temperatures below T_{C}. Spatially resolved magnetization, composition and Mn valence data reveal that the magnetic transition region is induced by a subtle Mn composition change, leading to charge transfer at the interface due to carrier diffusion and drift. The electrostatic shaping of the magnetic transition region is mediated by the Mn valence, which affects both magnetization by Mn^{3+}-Mn^{4+} double exchange interaction and free carrier concentration.