Kazuma Ikeda

Analysis of the main acceptor defect in p-type GaAsN grown by chemical beam epitaxy

The properties of the main acceptor state in GaAsN crystal grown by chemical beam epitaxy (CBE) are studied based on the changes of carrier concentration and temperature dependence of the junction capacitance due to the annealing. The junction capacitance in p-type films shows a significant increase between 50 and 80 K and the energy level of the acceptor defect in as-grown film was obtained at 0.161 eV above the valence band maximum. This energy level decreases and its density increases with annealing time. Therefore, this acceptor state is not thermally stable and its structure and electrical properties change by the annealing. This acceptor state is mainly responsible for the high background doping in unintentionally doped GaAsN grown by CBE, since the carrier concentration estimated by using the energy level and the density obtained by capacitance–temperature (C–T) measurements explain well the carrier concentration at room temperature provided by capacitance–voltage (C–V) results.

Possibility of static low concentrator PV optimized for vehicle installation

One of the most direct and efficient ways in eliminating greenhouse gas emission from cars is to install a PV panel on the roof. Since the area of the roof of the car is limited, high-efficiency III-V cells will be useful. Because of the space limitation, it should be high-efficiency panel. III-V cells may be a candidate, but it should be CPV for saving cost of the cell. Considering its quick movement and appearance, trackers were thought difficult to implement. Our choice was a static concentrator customized to automobile. Special static concentrator that collects sunlight from shallow incident angle was successfully developed.

III-V-N materials for super high-efficiency multijunction solar cells

We have been studying concentrator multi-junction solar cells under Japanese Innovative Photovoltaic R&D program since FY2008. InGaAsN is one of appropriate materials for 4-or 5-junction solar cell configuration because this material can be lattice-matched to GaAs and Ge substrates. However, present InGaAsN single-junction solar cells have been inefficient because of low minority-carrier lifetime due to N-related recombination centers and low carrier mobility due to alloy scattering and non-homogeneity of N. This paper presents our major results in the understanding of majority and minority carrier traps in GaAsN grown by chemical beam epitaxy and their relationships with the poor electrical properties of the materials.

Real-time observation of rotational twin formation during molecular-beam epitaxial growth of GaAs on Si (111) by x-ray diffraction

The formation and evolution of rotational twin (TW) domains introduced by a stacking fault during molecular-beam epitaxial growth of GaAs on Si (111) substrates were studied by in situ x-ray diffraction. To modify the volume ratio of TW to total GaAs domains, GaAs was deposited under high and low group V/group III (V/III) flux ratios. For low V/III, there was less nucleation of TW than normal growth (NG) domains, although the NG and TW growth rates were similar. For high V/III, the NG and TW growth rates varied until a few GaAs monolayers were deposited; the mean TW domain size was smaller for all film thicknesses.

Wagging and stretching modes of N–H complexes in GaAsN grown by chemical beam epitaxy

The origin of N–H related local vibration modes (LVMs) in GaAsN grown by chemical beam epitaxy is discussed on the basis of the change in peak intensity caused by annealing. The peak areas at 973 and 3124 cm−1 increase as the annealing temperature increases. Their ratio is almost one, independent of the annealing temperature and time. On the other hand, the intensities at 960, 2952, 3014, and 3098 cm−1 decrease owing to annealing. Although the peak area at 960 cm−1 becomes half after annealing at 700 °C, those at 2952, 3014, and 3098 cm−1 to almost zero. These results indicate that the wagging mode at 973 cm−1 and the stretching mode at 3124 cm−1 are originated from the same N–H related defect in GaAsN, and that the wagging mode at 960 cm−1 does not correspond to the stretching modes at 2952, 3014, and 3098 cm−1.

Scattering Anisotropy Effect on Layered Thermoelectric Materials

The effect of scattering anisotropy caused by mass anisotropy on the thermoelectric figure of merit is investigated. The simplest electron transport model is considered assuming a single-band, parabolic energy dispersion relation and ionized impurity scattering, which is assumed to be elastic. The wave-vector dependence of the relaxation time is fully considered within the relaxation-time approximation. Transport coefficients are calculated using the anisotropic relaxation time at finite temperatures. The figure of merit perpendicular to the axis with the largest effective mass is shown to decrease as the effective mass ratio increases. This result indicates that the weak confinement of electrons into layers of bulk materials cannot be the direct cause of the enhancement of the figure of merit along the layers.

Correlations between N–H local vibrational modes in GaAsN grown by chemical beam epitaxy

Correlations between N–H local vibrational modes (LVMs) in GaAsN grown by chemical beam epitaxy are studied by polarized Fourier transform infrared spectroscopy. The N–H bonds of LVMs at 960, 2952, 3011, and 3098 cm−1 are shown to exhibit polarizations along in the GaAsN(001) plane. By comparing the polarization degrees of those LVMs, it is shown that the four LVMs have no correlations with each other. This result indicates that there are four types of N–H complexes whose bond directions are 〈111〉. Those structures are considered to be of the bond-center or antibonding type with interstitial host atoms, antisite host atoms, and vacancies.

GaAsN Grown by Chemical Beam Epitaxy for Solar Cell Application

InGaAsN is a candidate material to realize the ultrahigh efficiency lattice-matched multijunction solar cell. This material has the 1eV band gap energy and same lattice constant as GaAs or Ge substrate by controlling the In and N compositions to be 9% and 3%, respectively [1, 2]. So far, the highest conversion efficiency of the lattice-matched multi-junction solar cell is 43.5% at 418-suns which was achieved by the 3-junction solar cell, GaInP/GaAs/GaInNAs [3]. By realizing the 4-junction device, InGaP/InGaAs/InGaAsN/Ge, the efficiency is expected to be 41% at 1-sun under the AM1.5G spectrum and 51% at 500-suns under the AM1.5D spectrum [4]. In order to achieve the expected super high efficiency, the short circuit current, Jsc, under a GaAs filter has to be 17 mA/cm2 at 1-sun under AM0 conditions. However, the highest Jsc under that condition has been 10.9mA/cm2. This corresponds to the 6.2% conversion efficiency and Jsc=26.0 mA/cm2 without GaAs filters at 1-sun under AM1.5 [5]. The diffusion length of the minority carrier in this case is supposed to be much shorter than 1 μm which is required to achieve the ultrahigh efficiency. Therefore, the mobility and lifetime of the minority carrier should be improved.

Optical DLTS for the study of recombination centers in GaAsN grown by chemical beam epitaxy

New broad DLTS peak signals in GaAsN solar cell, grown by chemical beam epitaxy, were obtained using the combination of optical-irradiation and conventional Deep Level Transient Spectroscopy (DLTS). Those broad peak signals cannot be detected by conventional DLTS method in the dark. The broad peak signals were overlapped with three deep level states at least and showed the increase of DLTS peak intensity. However, the other deep level state (EV+0.60eV) showed no significant change of DLTS peak signals in the dark and optical excitation. The condition of minority carrier injection by optical irradiation indicated that the mechanism of carrier capture and emission at some deep centers had been changed.

Real-time observation of crystallographic tilting InGaAs layers on GaAs offcut substrates

In situ X-ray reciprocal space mapping during InxGa1−xAs/GaAs(001) MBE growth is performed to investigate effects of substrate misorientations on crystallographic tilting. It was found that evolution of the crystallographic tilt for the InGaAs films is strongly dependent on both layer structures and substrate misorientations. We discuss these observations in terms of an asymmetric distribution of dislocations.