Min-Cherl Jung

Ge nitride formation in N-doped amorphous Ge2Sb2Te5

The chemical state of N in N-doped amorphous Ge2Sb2Te5 (a-GST) samples with 0–14.3Nat.% doping concentrations was investigated by high-resolution x-ray photoelectron spectroscopy (HRXPS) and Ge K-edge x-ray absorption spectroscopy (XAS). HRXPS showed negligible change in the Te 4d and Sb 4d core-level spectra. In the Ge 3d core-level spectra, a Ge nitride (GeNx) peak developed at the binding energy of 30.2eV and increased in intensity as the N-doping concentration increased. Generation of GeNx was confirmed by the Ge K-edge absorption spectra. These results indicate that the N atoms bonded with the Ge atoms to form GeNx, rather than bonding with the Te or Sb atoms. It has been suggested that the formation of Ge nitride results in increased resistance and phase-change temperature.

Substantial improvement of perovskite solar cells stability by pinhole-free hole transport layer with doping engineering

We fabricated perovskite solar cells using a triple-layer of n-type doped, intrinsic, and p-type doped 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) (n-i-p) as hole transport layer (HTL) by vacuum evaporation. The doping concentration for n-type doped spiro-OMeTAD was optimized to adjust the highest occupied molecular orbital of spiro-OMeTAD to match the valence band maximum of perovskite for efficient hole extraction while maintaining a high open circuit voltage. Time-dependent solar cell performance measurements revealed significantly improved air stability for perovskite solar cells with the n-i-p structured spiro-OMeTAD HTL showing sustained efficiencies even after 840 h of air exposure.

Observation of molecular nitrogen in N-doped Ge2Sb2Te5

Ge2Sb2Te5 (GST) film in the crystalline state was nitrogen doped using the reactive sputtering method in order to increase sheet resistance. High-resolution x-ray absorption spectroscopy revealed that molecular nitrogen (N2) existed in the N-doped GST film. This finding implies that both molecular nitrogen and atomic-state nitrogen should be taken into account in understanding the structures of N-doped GST film. The molecular nitrogen is believed to exist at interstitial and vacancy sites, and more likely at grain boundaries.

High-resolution x-ray photoelectron spectroscopy on oxygen-free amorphous Ge2Sb2Te5

Amorphous Ge2Sb2Te5 (a-GST) film, 100nm thick, was grown by cosputtering from GeTe and Sb2Te3 targets on a silicon wafer at room temperature. The native oxidized layer, which was formed in air and about 20nm thick measured by secondary ion mass spectroscopy, was removed by Ne+ ion sputtering for 1h with 0.6kV beam energy. Core-level spectra of the Te 3d and 4d, Sb 3d and 4d, and Ge 3d of the oxygen-free a-GST were obtained by high-resolution x-ray photoelectron spectroscopy with synchrotron radiation and compared with those from Ge and GeTe. The analysis implies that the a-GST is composed on the base of chemical states of GeTe.

Self-Assembled Nanowires with Giant Rashba Split Bands

We investigated Pt-induced nanowires on the Si(110) surface using scanning tunneling microscopy (STM) and angle-resolved photoemission. High resolution STM images show a well-ordered nanowire array of 1.6 nm width and 2.7 nm separation. Angle-resolved photoemission reveals fully occupied one-dimensional (1D) bands with a Rashba-type split dispersion. Local dI/dV spectra further indicate well-confined 1D electron channels on the nanowires, whose density of states characteristics are consistent with the Rashba-type band splitting. The observed energy and momentum splitting of the bands are among the largest ever reported for Rashba systems, suggesting the Pt-Si nanowire as a unique 1D giant Rashba system. This self-assembled nanowire can be exploited for silicon-based spintronics devices as well as the quest for Majorana fermions.

Properties and solar cell applications of Pb-free perovskite films formed by vapor deposition

We deposited lead-free CH3NH3SnBr3 – organometal halide perovskite films using two vapor deposition-based methods (i.e., co-evaporation and sequential evaporation) using SnBr2 and CH3NH3Br. We obtained comprehensive information about the structural and electronic properties of these Pb-free perovskite films. X-ray diffraction results confirmed the crystalline structure of MASnBr3. Using UV-Vis measurements, we determined the optical bandgap of these films to be ∼2.2 eV. On the basis of ultraviolet photoemission spectroscopy results, the work function and ionization energy were measured to be 4.3 and 6.1 eV, respectively. Solar cells employing such a perovskite film in a planar structure were fabricated with various hole transport layers (HTLs) (spiro-OMeTAD, C60, and P3HT). For perovskite films prepared by co-deposition, we obtained solar cell efficiencies ranging from 0.03 to 0.35%. On the other hand, when we used sequential deposition, a higher efficiency up to 1.12% was obtained using P3HT as the HTL. We confirmed that the low efficiency of MASnBr3 based perovskite solar cells is due to their relatively high resistance and the fast formation of Sn–Br oxide on the top surface by air exposure during fabrication. The sequential deposition method helped avoid such oxidation resulting in higher efficiencies.

The presence of CH3NH2 neutral species in organometal halide perovskite films

We report the presence of CH3NH2 neutral species not only on the surface but also at grain boundaries in the interior of thin polycrystalline films of methylammonium lead iodide perovskite CH3NH3PbI3 (thickness ∼ 50 nm) that were prepared using a standard solution method. Different chemical states for C K-edge were observed at the surfaces and in the interiors of perovskite films. Salient features of σ*(CH3-NH3+: methylammonium cation) and σ*(CH3-NH2: methylamine neutral species) were observed at 290.3 and 292.8 eV in both partial (surface-sensitive) and total (bulk) electron yield modes by near-edge x-ray absorption fine structure measurements. Consistently, two chemical states originated from CH3NH3+ and CH3NH2 in C 1s and N 1s core-level spectra were observed using high-resolution x-ray photoelectron spectroscopy. Using density functional theory calculations, we show that CH3NH2 cannot reside stably in the MAPbI3 perovskite crystal structure. Therefore, we propose that these CH3NH2 neutral species are ...

Effect of indium on phase-change characteristics and local chemical states of In–Ge–Sb–Te alloys

We introduce single-phase In–Ge–Sb–Te (IGST) quaternary thin film (fcc structure when crystallized) deposited by cosputtering from Ge2Sb2Te5(GST) and In3Sb1Te2 targets. This film, compared with the GST ternary system, provides a significant increase of amorphous-to-crystalline transformation temperature. High-resolution x-ray photoelectron spectroscopy (HRXPS) revealed that, with increasing In amounts, the Sb 4d and Ge 3d core peaks shift toward lower binding energies (BEs), with negligible changes in spectral linewidths, whereas the In 4d and Te 4d core peaks show insignificant changes in BEs. HRXPS interpretation suggests that the Na site in IGST can be occupied by Te, Sb, In, and vacancy, whereas in GST it is occupied only by Te.

Chemical state and atomic structure of Ge2Sb2Te5 system for nonvolatile phase-change random access memory

We present chemical state information on contamination-free Ge2Sb2Te5 thin film using high-resolution x-ray photoelectron spectroscopy (HRXPS) and the corresponding theoretical understanding of the chemical states, on both amorphous and metastable phases, illuminating the phase-change mechanism of the system. HRXPS data revealed that the Sb 4d shallow core level was split into two components having different binding energies and that the spin-orbit splitting feature of the Ge 3d level was enhanced as the system became metastable. Negligible change was observed in the Te 4d shallow core level, and in contrary to the previous report’s prediction less change in valance band spectra was observed. The results imply that Sb movement is also involved in the phase-change mechanism and that acquisition of shallow core-level spectra can be a useful measure for understanding phase-change mechanism. Hydrogenated SbTe6 octahedral-like cluster model was introduced to schematically interpret the generation of the two co...

Degradation mechanism of organic light-emitting device investigated by scanning photoelectron microscopy coupled with peel-off technique

The authors present space-resolved spectroscopic data on organic layers of a degraded organic light-emitting device. The data were obtained using a scanning photoelectron microscope (SPEM) coupled with peel-off technique to directly probe the uncontaminated organic layers, which were covered with cathode layer. The SPEM images of the degraded device show different and small size distributions of tris-8-hydroxy quinoline aluminum (Alq3) and hole-transport layers compared to that of as-prepared device. The analysis indicates that bonding strength between Alq3 and cathode layers and between the Alq3 and hole transport layers becomes weak as the device degrades, presumably due to structural deformation of the organic layers.