P. R. Stone

Correlation between structure and electrical transport in ion-irradiated graphene grown on Cu foils

Graphene grown by chemical vapor deposition and supported on SiO2 and sapphire substrates was studied following controlled introduction of defects induced by 35 keV carbon ion irradiation. Changes in Raman spectra following fluences ranging from 1012 cm-2 to 1015 cm-2 indicate that the structure of graphene evolves from a highly-ordered layer, to a patchwork of disordered domains, to an essentially amorphous film. These structural changes result in a dramatic decrease in the Hall mobility by orders of magnitude while, remarkably, the Hall concentration remains almost unchanged, suggesting that the Fermi level is pinned at a hole concentration near 1x1013 cm-2. A model for scattering by resonant scatterers is in good agreement with mobility measurements up to an ion fluence of 1x1014 cm-2.

Demonstration of homojunction ZnTe solar cells

We report on the proof of photovoltaic activity of homojunction ZnTe solar cells in which n-ZnTe layers are fabricated by thermal diffusion of Al into p-ZnTe at several diffusion times to control the junction depth. An open circuit voltage of approximately 0.9 V was obtained under 1× sun AM1.5G condition in all solar cells, independent of diffusion times, while a short circuit current dropped down with increasing the diffusion time due to an increased light absorption in heavily defective Al-diffused layer. These fundamental results provide a basis for future development of intermediate band solar cells based on ZnTe materials.

Electrical transport and ferromagnetism in Ga1−xMnxAs synthesized by ion implantation and pulsed-laser melting

We present a detailed investigation of the magnetic and magnetotransport properties of thin films of ferromagnetic Ga1−xMnxAs synthesized using ion implantation and pulsed-laser melting (II-PLM). The field and temperature-dependent magnetization, magnetic anisotropy, temperature-dependent resistivity, magnetoresistance, and Hall effect of II-PLM Ga1−xMnxAs films have all of the characteristic signatures of the strong p‐d interaction of holes and Mn ions observed in the dilute hole-mediated ferromagnetic phase. The ferromagnetic and electrical transport properties of II-PLM films correspond to the peak substitutional Mn concentration meaning that the nonuniform Mn depth distribution is unimportant in determining the film properties. Good quantitative agreement is found with films grown by low temperature molecular beam epitaxy and having the similar substitutional MnGa composition. Additionally, we demonstrate that II-PLM Ga1−xMnxAs films are free from interstitial MnI because of the high-temperature proce...

Embedded Binary Eutectic Alloy Nanostructures: A New Class of Phase Change Materials

Phase change materials are essential to a number of technologies ranging from optical data storage to energy storage and transport applications. This widespread interest has given rise to a substantial effort to develop bulk phase change materials well suited for desired applications. Here, we suggest a novel and complementary approach, the use of binary eutectic alloy nanoparticles embedded within a matrix. Using GeSn nanoparticles embedded in silica as an example, we establish that the presence of a nanoparticle/matrix interface enables one to stabilize both nanobicrystal and homogeneous alloy morphologies. Further, the kinetics of switching between the two morphologies can be tuned simply by altering the composition.

Tuning of ferromagnetism through anion substitution in Ga–Mn–pnictide ferromagnetic semiconductors

Abstract We have synthesized Ga 1− x Mn x As 1− y P y and Ga 1− x Mn x P 1− y N y by the combination of ion implantation and pulsed-laser melting. We find that the incorporation of isovalent impurities with smaller atomic radii leads to a realignment of the magnetic easy axis in Ga 1− x Mn x P 1− y N y /GaP and Ga 1− x Mn x As 1− y P y /GaAs thin films from in-plane to out-of-plane. This tensile-strain-induced magnetic anisotropy is reminiscent of that observed in Ga 1− x Mn x As grown on larger lattice constant (In, Ga)As buffer layers indicating that the role of strain in determining magnetic anisotropy is fundamental to III–Mn–V materials. In addition, we observe a decrease in the ferromagnetic Curie temperature in Ga 1− x Mn x As 1− y P y with increasing y from 0 to 0.028. Such a decrease may result from localization of holes as the P/As ratio on the Group V sublattice increases.

Device characteristics of a 17.1% efficient solar cell deposited by a non-vacuum printing method on flexible foil

Thin film CuInSe2-based (hereafter broadly referred to as CIGS) solar cells are promising due to their high efficiencies. Here, we present a 17.1% efficient CIGS solar cell produced using a low-cost non-vacuum printing method on flexible foil. This power conversion efficiency is the highest demonstrated efficiency of any thin film solar cell with the absorber deposited by non-vacuum techniques. In this paper, we compare both the champion device and median device results from our baseline process to champion devices from other laboratories. It is found that the offset between absorber bandgap and open circuit voltage (Eg/q-Voc) for the champion device is ~0.45V, comparable to champion co-evaporated CIGS. The low (Eg/q - Voc) values for this deposition process is believed to be due to a high minority carrier lifetime in the CIGS as there is a correlation between steady-state photoluminescence (PL) intensity measured after absorber formation and (Eg/q - Voc). External quantum efficiency and spectral PL measurements were used to determine the average minimum bandgap for the champion device and the bandgap uniformity on the sub-micron length scale. The carrier collection length in the absorber layer for this deposition process was determined to be ~1.3μm based on electron beam induced current measurements. These results demonstrate that low-cost, non-vacuum printing of CIGS can achieve comparable efficiencies and materials quality to more traditional vacuum co-evaporation and sputtering techniques.

MnL3,2 x-ray absorption and magnetic circular dichroism in ferromagnetic Ga1−xMnxP

We have measured the X-ray absorption and X-ray magnetic circular dichroism (XMCD) at the Mn L{sub 3,2} edges in ferromagnetic Ga{sub 1-x}Mn{sub x}P for 0.018 {le} x {le} 0.042. Large XMCD asymmetries at the L{sub 3} edge indicate significant spin-polarization of the density of states at the Fermi energy. The temperature dependence of the XMCD and moment per Mn of 2.67 {+-} 0.45 {mu}{sub B} calculated using sum rules are consistent with magnetometry values. The spectral shapes of the X-ray absorption and XMCD are nearly identical with those for Ga{sub 1-x}Mn{sub x}As indicating that the hybridization of Mn d and anion p states is similar in the two materials.

Comparison of the magnetic properties of GeMn thin films through Mn L-edge x-ray absorption

Comparison of the magnetic properties of GeMn thin films through Mn L-edge x-ray absorption S. Ahlers, 1 P. R. Stone, 2, 3 N. Sircar, 1 E. Arenholz, 4 O. D. Dubon, 2, 3 and D. Bougeard 1 Walter Schottky Institut, Technische Universit¨ t M¨ nchen, a u Am Coulombwall 3, D-85748 Garching, Germany Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Abstract X-ray absorption spectroscopy of epitaxial GeMn thin films reveals an experimentally indistin- guishable electronic configuration of Mn atoms incorporated in Ge 1−𝑥 Mn 𝑥 nanoclusters and in precipitates of the intermetallic compound Mn 5 Ge 3 , respectively. However, the average magnetic response of thin films containing Ge 1−𝑥 Mn 𝑥 nanoclusters is lower than the response of films contain- ing Mn 5 Ge 3 precipitates. This reduced magnetic response of Ge 1−𝑥 Mn 𝑥 nanoclusters is explained in terms of a fraction of Mn atoms being magnetically inactive due to antiferromagnetic coupling or the presence of structural disorder. A determination of the role of magnetically inactive Mn atoms in the self-assembly of the thermodynamically metastable Ge 1−𝑥 Mn 𝑥 nanoclusters seems to be an essential ingredient for an enhanced control of this promising high Curie temperature magnetic semiconductor.

Magnetocrystalline anisotropy and magnetization reversal in Ga1- xMnxP synthesized by ion implantation and pulsed-laser melting

We report the observation of ferromagnetic resonance (FMR) and the determination of the magnetocrystalline anisotropy in (100)-oriented single-crystalline thin film samples of GaMnP with x=0.042. The contributions to the magnetic anisotropy were determined by measuring the angular- and the temperature-dependencies of the FMR resonance fields and by superconducting quantum interference device magnetometry. The largest contribution to the anisotropy is a uniaxial component perpendicular to the film plane; however, a negative contribution from cubic anisotropy is also found. Additional in-plane uniaxial components are observed at low temperatures, which lift the degeneracy between the in-plane [011] and [01-1] directions as well as between the in-plane [010] and [001] directions. Near T=5K, the easy magnetization axis is close to the in-plane [01-1] direction. All anisotropy parameters decrease with increasing temperature and disappear above the Curie temperature T_C. A consistent picture of the magnetic anisotropy of ferromagnetic GaMnP emerges from the FMR and magnetometry data. The latter can be successfully modeled when both coherent magnetization rotation and magnetic domain nucleation are considered.

Reversible phase changes in Ge–Au nanoparticles

We demonstrate a reversible phase transition in nanoparticles composed of a binary eutectic alloy, Ge–Au. The structure, 9 nm diameter nanoparticles embedded in silica, can be switched from bilobe to mixed using a 30 ns ultraviolet laser pulse. The structure can be switched back to bilobe by heating at 80 °C. The bilobe/mixed switching can be performed on the same sample at least ten times. Synchrotron X-ray diffraction studies reveal that the bilobe structure contains crystalline Ge and Au while the mixed structure consists of crystalline Ge and β Ge–Au.