Jaewook Kim

High‐efficiency Cr‐MIS solar cells on single and polycrystalline silicon

Cr‐MIS solar cells having a 2‐cm2 area have been fabricated to produce 12.2% efficiency on single crystal and 8.8% efficiency on polycrystalline Si. Surface‐state data were used to predict open‐circuit voltages of 0.60 and 0.50 V, respectively, for the single‐crystal and polycrystalline Si. Spectral response measurements and Cr metal thickness confirm the differences in short‐circuit current density using these two types of Si.

Factors which maximize the efficiency of Cr–p‐Si Schottky (MIS) solar cells

Processing steps used in fabricating silicon Schottky solar cells have been evaluated to optimize energy conversion efficiency. Factors such as wafer resistivity and orientation, etching, oxide growth, heat treatment, depostiion technique, and grid design are included in the study. The proper heat treatment and metal deposition rate are most crucial in achieving a high open‐circuit voltage. The process has produced a solar cell with Voc=0.55 V, fill factor of 0.71, and 8.6% AM1 efficiency with a potential improvement to 12.5%.

Auger, ellipsometry, and environmental studies of thin films applied to schottky (MIS) solar cells

Schottky (MIS) solar cells typically consist of the structure 5Å Cr/50Å Cu/40Å Cr/20Å oxide/silicon. A study of metal diffusion and precise oxide thickness is important to predict the stability of such a device. Devices fabricated using the same process are shown to have almost identical electronic performance. Auger profiles show that the metal films do not penetrate through adjacent regions except when Ag is used in place of Cu. Ag does penetrate through the Cr and into the Si which alters the electronic properties. Ellipsometer studies show the insulator to range in thickness from 20Å to 28Å which may be controlled by variation of the heat-treatment cycle. Environmental studies of encapsulated solar cells show that polystyrene offers more protection than Sylgard l84. Failures due to dirt coatings, discoloration, thermal stresses, poor wire bonds, voltage degradation, and fill factor degradation have been observed. Properly fabricated and encapsulated cells have performed well for more than a year. A hermetic encapsulation may be necessary for future long term stability.

Interrelation between domain structures and polarization switching in hybrid improper ferroelectric Ca3(Mn,Ti)2O7

Ca3Mn2O7 and Ca3Ti2O7 have been proposed as the prototypical hybrid improper ferroelectrics (HIFs), and a significant magnetoelectric (ME) coupling in magnetic Ca3Mn2O7 is, in fact, reported theoretically and experimentally. Although the switchability of polarization is confirmed in Ca3Ti2O7 and other non-magnetic HIFs, there is no report of switchable polarization in the isostructural Ca3Mn2O7. We constructed the phase diagram of Ca3Mn2-xTixO7 through our systematic study of a series of single crystalline Ca3Mn2-xTixO7 (x = 0, 0.1, 1, 1.5, and 2). Using transmission electron microscopy, we have unveiled the unique domain structure of Ca3Mn2O7: the high-density 90° stacking of a- and b-domains along the c-axis due to the phase transition through an intermediate Acca phase and the in-plane irregular wavy ferroelastic twin domains. The interrelation between domain structures and physical properties is unprecedented: the stacking along the c-axis prevents the switching of polarization and causes the irregula...

Magnetic-field-induced phases in anisotropic triangular antiferromagnets: Application toCuCrO2

We introduce a minimal spin model for describing the magnetic properties of

Bending and breaking of stripes in a charge ordered manganite

\mathrm{CuCrO_2}

Interface state measurements and computer simulation studies of MIS solar cells

. Our Monte Carlo simulations of this model reveal a rich magnetic field induced phase diagram, which explains the measured field dependence of the electric polarization. The sequence of phase transitions between different mutiferroic states arises from a subtle interplay between spatial and spin anisotropy, magnetic frustration and thermal fluctuations. Our calculations are compared to new measurements up to 92 T.

Vortex ferroelectric domains, large-loop weak ferromagnetic domains, and their decoupling in hexagonal (Lu, Sc)FeO 3

In charge-ordered phases, broken translational symmetry emerges from couplings between charge, spin, lattice, or orbital degrees of freedom, giving rise to remarkable phenomena such as colossal magnetoresistance and metal–insulator transitions. The role of the lattice in charge-ordered states remains particularly enigmatic, soliciting characterization of the microscopic lattice behavior. Here we directly map picometer scale periodic lattice displacements at individual atomic columns in the room temperature charge-ordered manganite Bi0.35Sr0.18Ca0.47MnO3 using aberration-corrected scanning transmission electron microscopy. We measure transverse, displacive lattice modulations of the cations, distinct from existing manganite charge-order models. We reveal locally unidirectional striped domains as small as ~5 nm, despite apparent bidirectionality over larger length scales. Further, we observe a direct link between disorder in one lattice modulation, in the form of dislocations and shear deformations, and nascent order in the perpendicular modulation. By examining the defects and symmetries of periodic lattice displacements near the charge ordering phase transition, we directly visualize the local competition underpinning spatial heterogeneity in a complex oxide.Charge-lattice coupling plays a central role in the exotic behaviors of multiferroic complex oxides, such as manganites, however, obtaining a microscopic picture is challenging. Here, Savitzky et al. map periodic lattice displacement fields at the picometer scale to study local order-disorder competition.

Nature and evolution of incommensurate charge order in manganites visualized with cryogenic scanning transmission electron microscopy

Experimental surface state studies on MIS diodes show average surface state density values of 2.5 × 10¹¹, 5.5 × 10¹¹, 3 × 10¹², and 4 × 10¹²ev^-1cm^-2for

Partially disordered antiferromagnetism and multiferroic behavior in a frustrated Ising system CoCl2–2SC(NH2)2

The direct domain coupling of spontaneous ferroelectric polarization and net magnetic moment can result in giant magnetoelectric (ME) coupling, which is essential to achieve mutual control and practical applications of multiferroics. Recently, the possible bulk domain coupling, the mutual control of ferroelectricity (FE) and weak ferromagnetism (WFM) have been theoretically predicted in hexagonal LuFeO3. Here, we report the first successful growth of highly-cleavable Sc-stabilized hexagonal Lu0.6Sc0.4FeO3 (h-LSFO) single crystals, as well as the first visualization of their intrinsic cloverleaf pattern of vortex FE domains and large-loop WFM domains. The vortex FE domains are on the order of 0.1–1 μm in size. On the other hand, the loop WFM domains are ~100 μm in size, and there exists no interlocking of FE and WFM domain walls. These strongly manifest the decoupling between FE and WFM in h-LSFO. The domain decoupling can be explained as the consequence of the structure-mediated coupling between polarization and dominant in-plane antiferromagnetic spins according to the theoretical prediction, which reveals intriguing interplays between FE, WFM, and antiferromagnetic orders in h-LSFO. Our results also indicate that the magnetic topological charge tends to be identical to the structural topological charge. This could provide new insights into the induction of direct coupling between magnetism and ferroelectricity mediated by structural distortions, which will be useful for the future applications of multiferroics.Multiferroics: decoupled domainsExperiments reveal the decoupling between vortex ferroelectric domains and loop weak ferromagnetic domains in hexagonal Lu0.6Sc0.4FeO3. A team led by Sang-Wook Cheong from the Rutgers University in the USA grows cleavable Sc-stabilized hexagonal Lu0.6Sc0.4FeO3 (h-LSFO) single crystals and visualize cloverleaf patterns of vortex ferroelectric (FE) domains and large-loop weak ferromagnetic (WFM) domains. The WFM domains are much larger in size than FE domains. The distinct sizes and shapes demonstrate the decoupling between ferroelectricity and out-of-plane weak ferromagnetism, which is consistent with a former prediction on the absence of coupled magnetoelectric effects in h-LSFO. Furthermore, they indicate that the magnetic topological charge tends to be identical with the structural topological charge. The results provide insights into coupling between magnetism and ferroelectricity mediated through structural distortions, which might be useful for future applications.