Strain Effects on the Optical Polarization Properties of R-Plane Wurtzite GaN

Using the effective-mass Hamiltonian for an arbitrary direction wurtzite semiconductor on the basis of k.p theory, we investigate the strain effects on the transition energies and optical properties in the R-plane ([1012]-oriented plane) GaN. The results show that (1) the transition energies decrease with the biaxial strains changing from -0.5 to 0.5%; and (2) giant optical anisotropy appears in the R-plane which is significantly affected by the biaxial strains. We clarify the relation between the strains and the polarization properties. Finally, we discuss the application of these properties to the R-plane GaN based devices. (c) 2009 The Japan Society of Applied Physics

Growth of nonpolar a-plane GaN on nano-patterned r-plane sapphire substrates

Sapphire substrates were nano-patterned by inductive coupled plasma etching process. Nonpolar a-plane GaN films were grown on planar and nano-patterned r-plane sapphire substrates by metal organic chemical vapor deposition. The anisotropic characteristic and the crystalline quality of the a-plane GaN films were studied through XRD rocking curves. The cross section and surface morphologies of the a-plane GaN films were studied using SEM and AFM measurements, respectively. The crystal quality and surface flatness of the nonpolar a-plane GaN were greatly improved through the usage of the nano-patterned r-plane sapphire substrates. (C) 2008 Elsevier B.V. All rights reserved.

Low-threshold-current and high-out-power 660 nm laser diodes with a p-GaAs current blocking layer for DVD-RAM/R

We investigate a new structure of high-power 660-nm AlGaInP laser diodes. In the structure, a p-GaAs layer is grown on the ridge waveguide serving as the current-blocking layer, and nonabsorbing windows are only fabricated near the cavity facets to increase the catastrophic-optical-damage level. Stable fundamental mode operation was achieved at up to 80 mW without kinks, and the maximum output power was 184 mW at 22 degrees C. The threshold current was 40 mA.

Luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates

In this study, we report comparative luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). High-density InGaN quantum dots (QDs) are formed on GaN templates by decreasing the growth temperature and increasing the adatom hopping-barrier through surface passivation. Atomic force microscopy (AFM) has been employed to estimate the size and height of these dots. Photoluminescence (PL) spectra recorded from (1120) InGaN QDs/(1102) sapphire show much stronger emission intensity compared to spectra recorded from (0001) InGaN QDs/(0001) sapphire. Due to the absence of strong spontaneous polarization and piezoelectric field, such (1150) InGaN QDs in the active layers would lead to high efficiency light emitting devices. (c) 2005 Elsevier B.V. All rights reserved.

Structure and magnetic properties of ScFe6Ga6-type RCo5Ga7 (R = Y, Tb, Dy, Ho and Er)

The structure and magnetic properties of the RCo5Ga7 (R = Y, Tb, Dy, Ho and Er) compounds with the ScFe6Ga6-type structure have been studied. The stability of RCo5Ga7 is closely related with the ratio of the metal radii R-RE/R-(Co,R-Ga). With R-RE/R-(Co,R-Ga) less than or equal to 1.36, the compounds can be stabilized in the ScFe6Ga6-type structure. The lattice of RCo5Ga7 shrinks as the atomic order of R increases, and it is consistent with the lanthanide contraction. The structure analysis based on X-ray diffraction patterns reveals that in the orthorhombic RCo5Ga7 (Immm), R occupies the 2a site, and Co enters into the 8k and the 4h sites, and Ga is at the 4e, 4f, 4g, 4h and 8k sites. The interatomic distances and the coordination numbers of RCo5Ga7 are provided from the refinement results. The short interatomic distance (less than 2.480 Angstrom) between the Co ions results in the negative magnetic interaction, which does not favor ferromagnetic ordering. The magnetic moment of YCo5Ga7 is absent, and RCo5Ga7 (R = Tb, Dy, Ho and Er) may have long-range magnetic ordering with the paramagnetic Curie temperature lower than 5 K. (C) 2004 Elsevier Inc. All rights reserved.

Polarized Raman scattering studies of nonpolar a-plane GaN films grown on r-plane sapphire substrates by MOCVD

Nonpolar (1120) a-plane GaN thin films were grown on r-plane (1102) sapphire substrates by low-pressure metal organic chemical vapor deposition (MOCVD). The stress characteristics of the a-plane GaN films were investigated by means of polarized Raman scattering spectra in backscattering configurations. The experimental results show that there are strong anisotropic in-plane stresses within the epitaxial a-plane GaN films by calculating the corresponding stress tensors. The temperature dependence of Raman scattering spectra was studied in the range from 100 K to 550 K. The measurements reveal that the Raman phonon frequencies decrease with increasing temperature. The temperature at which nonpolar a-plane GaN films are strain free is discussed. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Heterojunction solar cells with n-type nanocrystalline silicon emitters on p-type c-Si wafers

Hydrogenated nanocrystalline silicon (nc-Si:H) n-layers have been used to prepare heterojunction solar cells on flat p-type crystalline silicon (c-Si) wafers. The nc-Si:H n-layers were deposited by radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD), and characterized using Raman spectroscopy, optical transmittance and activation energy of dark-conductivity. The nc-Si:H n-layers obtained comprise fine grained nanocrystallites embedded in amorphous matrix, which have a wider bandgap and a smaller activation energy. Heterojunction solar cells incorporated with the nc-Si n-layer were fabricated using configuration of Ag (100 nm)/1T0 (80 nm)/n-nc-Si:H (15 nm)/buffer a-Si:H/p-c-Si (300 mu m)/Al (200 nm), where a very thin intrinsic a-Si:H buffer layer was used to passivate the p-c-Si surface, followed by a hydrogen plasma treatment prior to the deposition of the thin nanocrystalline layer. The results show that heterojunction solar cells subjected to these surface treatments exhibit a remarkable increase in the efficiency, up to 14.1% on an area of 2.43 cm(2). (c) 2006 Elsevier B.V. All rights reserved.

Mossbauer spectroscopic study of R3Fe29-xCrx and R3Fe29-xCrxH,(y)(R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

Fe-57 Mossbauer spectra for the series of R3Fe29-xCrx (R = Y,Ce, Nd, Sm, Gd, Tb, and Dy) compounds and their hydrides have been measured at 4.2 K. The weighted average hyperfine field at the Fe sites was separated into a 3d-electron contribution, proportional to the average Fe moment, and a transferred contribution due to rare earth moments. The latter was found to increase with the rare earth effective spin (g(J) - 1) J. Hyperfine fields in the hydrides were only slightly larger than in the corresponding alloys.

Fe-57 Mossbauer spectroscopic and magnetic studies of R3Fe29-xVx (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

Mossbauer spectra for Fe atoms in the series of R3Fe29-xVx (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) compounds were collected at 4.2 K. The ratio of 14.5 T/mu(B) between the average hyperfine field B-hf and the average Fe magnetic moment mu(Fe)(MS), obtained from our data, in Y3Fe29-xVx is in agreement with that deduced from the RxTy alloys by Gubbens et al. The average Fe magnetic moments mu(Fe)(MS) in these compounds at 4.2 K, deduced from our Mossbauer spectroscopic studies, are in accord with the results of magnetization measurement. The average hyperfine field of the Fe sites for R3Fe29-xVx at 4.2 K increases with increasing values of the rare earth effective spin (g(J) - 1) J, which indicates that there exists a transferred spin polarization induced by the neighboring rare earth atom.

Mossbauer spectroscopy study of Fe-57 in R3Fe29-xTx (R=Y, Ce, Nd, Sm, Gd, Tb and Dy; T=V and Cr)

Fe-57 Mossbauer spectra for the Fe atoms in the R3Fe29-xTx (R=Y, Ce, Nd, Sm, Gd, Tb, Dy; T=V, Cr) compounds were collected at 4.2 K. The analysis of Mossbauer spectra was based on the results of magnetization and neutron powder diffraction measurements. The average Fe magnetic moments at 4.2 K, deduced from our data, are in accord with magnetization measurements. The average hyperfine field of Tb3Fe29-xCrx (x=1.0, 1.5, 2.0, and 3.0) decreases with increasing Cr concentration, which is also in accordance with the variation of the average Fe magnetic moment in the Tb3Fe29-xCrx compounds.

Crystallographic and magnetic properties of hydride R3Fe29-xTxHy (R=Y, Ce, Nd, Sm, Gd, Tb and Dy; T=V and Cr)

The crystallographic and intrinsic magnetic properties of hydride R3Fe29-xTxHy (R=Y, Ce, Nd, Sm, Gd, Tb, and Dy; T=V and Cr) have been investigated. The lattice constants and the unit cell volume of R3Fe29-xTxHy decrease with increasing R atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Regular anisotropic expansions, mainly along the a- and b-axis rather than along the c-axis, are observed for all the compounds upon hydrogenation. Hydrogenation leads to an increase in Curie temperature. First-order magnetization processes (FOMP) occur in magnetic fields of around 1.5 T and 4.0 T at 4.2 K for Nd3Fe24.5Cr4.5H5.0 and Tb(3)Fc(27.0)Cr(2.0)H(2.8), and around 1.4 T at room temperature for Gd3Fe28.0Cr1.0H4.2 Abnormal crystallographic and magnetic properties of Ce3Fe29-xTxHy suggest that the Ce ion is non-triply ionized.

Hydrogenation and anisotropic expansions in R3Fe29-xTx (R = Y, Ce, Nd, Sm, Gd, Tb and Dy; T = V and Cr)

A systematic study of the phase formation, structure and magnetic properties of the R3Fe29-xTx compounds (R=Y, Ce, Nd, Sm, Gd, Tb, and Dy; T=V and Cr) has been performed upon hydrogenation. The lattice constants and the unit cell volume of R3Fe29-xTxHy decrease with increasing R atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Regular anisotropic expansions mainly along the a- and b-axis rather than along the c-axis are observed for all of the compounds upon hydrogenation. Hydrogenation leads to an increase in the Curie temperature and a corresponding increase in the saturation magnetization at room temperature for each compound. First order magnetization processes (FOMP) occur in the external magnetic fields for Nd3Fe24.5Cr4.5H5.0, Tb3Fe27.0Cr2.0H2.8, and Gd3Fe28.0Cr1.0H4.2 compounds.

Structural and magnetic properties of hydrides R3Fe29-xVxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

A systematic investigation of crystallographic and intrinsic magnetic properties of the hydrides R3Fe29 - xVxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed in this work. The lattice constants a, b, and c and the unit cell volume of R3Fe29 - xVxHy decrease with increasing rare-earth atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Hydrogenation results in regular anisotropic expansions along the a-, b-, and c-axes in this series of hydrides. Abnormal crystallographic and magnetic properties of Ce3Fe27.5V1.5H6.5, like Ce3Fe27.5V1.5, suggest that the Ce ion is non-triply ionized. Hydrogenation leads to the increase in both Curie temperature for all the compounds and in the saturation magnetization at 4.2 K and RT for R3Fe29 - xVx with R = Y, Ce, Nd, Sm, Gd, and Dy, except for Tb. Hydrogenation also leads to a decrease in the anisotropy field at 4.2 K and RT for R3Fe29 - xVx with R = Y, Ce, Nd, Gd, Tb, and Dy, except for Sm. The Ce3Fe27.5V1.5 and Gd3Fe28.4V0.6 show the larger storage of hydrogen with y = 6.5 and 6.9 in these hydrides. (C) 1998 Elsevier Science B.V. All rights reserved.

Crystallographic and magnetic properties of the hydrides R3Fe29-xCrxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

A systematic study of the structural and intrinsic magnetic properties of the hydrides R3Fe29-xCrxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed. Hydrogenation lends to a relative volume expansion of the unit cell and a decrease in x-ray density for each compound. Anisotropic expansions mainly along the n- and b-axes rather than along the c-axis for all of the compounds upon hydrogenation are observed. The lattice constants and the unit-cell volume of R3Fe29-xCrx and R3Fe29-xCrxHy decrease with increasing R atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Hydrogenation results in an increase in the Curie temperature and a corresponding increase in the saturation magnetization at room temperature for each compound. After hydrogenation a decrease of 0.34 mu(B)/Fe in the average Fe atomic magnetic moment and a slight increase in the anisotropy field for Y3Fe27.2Cr1.8 are achieved at 4.2 K. First-order magnetization processes (FOMP) occur in magnetic fields of around 1.5 T and 4.0 T at 4.2 K for Nd3Fe24.5Cr4.5H5.0 and TD3Fe27.0Cr2.0H2.8, and around 1.4 T at room temperature for Gd3Fe28.0Cr1.0H4.2. The abnormal crystallographic and magnetic properties of Ce3Fe25.0Cr4.0 and Ce3Fe25.0Cr4.0H5.4 suggest that the Ce ion non-triply ionized.