Takuo Sasaki

In situ Real-Time X-ray Reciprocal Space Mapping during InGaAs/GaAs Growth for Understanding Strain Relaxation Mechanisms

In situ real-time X-ray diffraction measurements during In0.12Ga0.88As/GaAs(001) epitaxial growth are performed for the first time to understand the strain relaxation mechanisms in a lattice-mismatched system. The high resolution reciprocal space maps of 004 diffraction obtained at interval of 6.2 nm thickness enable transient behavior of residual strain and crystal quality to be observed simultaneously as a function of InGaAs film thickness. From the evolution of these data, five thickness ranges with different relaxation processes and these transition points are determined quantitatively, and the dominant dislocation behavior in each phase is deduced.

Real-time observation of anisotropic strain relaxation by three-dimensional reciprocal space mapping during InGaAs/GaAs (001) growth

Real-time three-dimensional reciprocal space mapping (3D-RSM) measurement during In0.12Ga0.88As/GaAs(001) molecular beam epitaxial growth has been performed to investigate anisotropy in relaxation processes along [110] and [1¯10] directions caused by α and β misfit dislocations (MDs). Anisotropies, strain relaxation, and crystal quality in both directions were simultaneously evaluated via the position and broadness of 022 diffraction in 3D-RSM. In the small-thickness region, strain relaxation caused by α-MDs is higher than that caused by β-MDs, and therefore crystal quality along [110] is worse than that along [1¯10]. Rapid relaxation along both [110] and [1¯10] directions occurs at almost the same thickness. After rapid relaxation, anisotropy in strain relaxation gradually decreases, whereas crystal quality along [1¯10] direction, presumably due to β-MDs, becomes better that along [110] direction and the ratio does not decay with thickness.

Radiation response analysis of wide-gap p-AlInGaP for superhigh-efficiency space photovoltaics

We present here the direct observation of the majority and minority carrier defects generation from wide-band-gap (2.04eV) and thick (2μm) p-AlInGaP diodes and solar cells structures before and after 1MeV electron irradiation by deep level transient spectroscopy (DLTS). One dominant hole-emitting trap H1 (EV+0.37±0.05eV) and two electron-emitting traps, E1 (EC−0.22±0.04eV) and E3 (EC−0.78±0.05eV) have been observed in the temperature range, which we could scan by DLTS. Detailed analysis of the minority carrier injection annealing experiment reveals that the H1 center has shown the same annealing characteristics, which has been previously observed in all phosphide-based materials such as InP, InGaP, and InGaAsP. The annealing property of the radiation-induced defects in p-AlInGaP reveals that multijunction solar cells and other optoelectronic devices such as light-emitting diodes based on this material could be considerably better to Si and GaAs in a radiation environment.

Role of Liquid Indium in the Structural Purity of Wurtzite InAs Nanowires That Grow on Si(111)

InAs nanowires that grow catalyst-free along the [111] crystallographic orientation are prone to wurtzite-zincblende polytypism, making the control of the crystal phase highly challenging. In this work, we explore the dynamic relation between the growth conditions and the structural composition of the nanowires using time-resolved X-ray scattering and diffraction measurements during the growth by molecular beam epitaxy. A spontaneous buildup of liquid indium is directly observed in the beginning of the growth process and associated with the simultaneous nucleation of InAs nanowires predominantly in the wurtzite phase. The highly arsenic-rich growth conditions that we used limited the existence of the liquid indium to a short time interval, which is defined as the nucleation phase. After their nucleation, the nanowires grow in the absence of liquid indium, and with a highly defective wurtzite structure. Complementary ex-situ diffuse X-ray scattering measurements and modeling revealed that this structural degradation is due to the formation of densely spaced stacking faults. Thus, high wurtzite phase purity is associated with the presence of liquid indium. This finding implies that pure wurtzite nanowires may be obtained only if the growth is performed under the continuous presence of liquid indium at the growth interface, that is, in the vapor-liquid-solid mode.

Carrier removal in lattice-mismatched InGaP solar cells under 1-MeV-electron irradiation

Radiation-induced majority carrier removal is investigated from n+∕p− lattice-mismatched In0.56Ga0.44P solar cells under 1-MeV-electron irradiation. The change in carrier concentration in the 1×1017cm−3p− base layer is determined using standard capacitance–voltage techniques and found to proceed at a rate Rc=1.3cm−1, in agreement with that observed in lattice-matched InGaP. However, the observation of an increased short-circuit current and short-wavelength quantum efficiency over the unirradiated values at electron fluence levels in excess of 3×1015cm−2, allows the carrier concentration from the n+ emitter layer to be measured. By modeling the quantum efficiency of these solar cells, it is shown that the main photoresponse from these lattice-mismatched solar cells is due to drift transport, making the spectral response highly sensitive to changes in the width of the depletion region. Using this technique, the carrier concentration in the 2×1018cm−3 n+ emitter layer is found to be reduced to 1×1018cm−3 aft...

X-ray reciprocal space mapping of dislocation-mediated strain relaxation during InGaAs/GaAs(001) epitaxial growth

Dislocation-mediated strain relaxation during lattice-mismatched InGaAs/GaAs(001) heteroepitaxy was studied through in situ x-ray reciprocal space mapping (in situ RSM). At the synchrotron radiation facility SPring-8, a hybrid system of molecular beam epitaxy and x-ray diffractometry with a two-dimensional detector enabled us to perform in situ RSM at high-speed and high-resolution. Using this experimental setup, four results in terms of film properties were simultaneously extracted as functions of film thickness. These were the lattice constants, the diffraction broadenings along in-plane and out-of-plane directions, and the diffuse scattering. Based on correlations among these results, the strain relaxation processes were classified into four thickness ranges with different dislocation behavior. In addition, the existence of transition regimes between the thickness ranges was identified. Finally, the dominant dislocation behavior corresponding to each of the four thickness ranges and transition regimes ...

High-speed three-dimensional reciprocal-space mapping during molecular beam epitaxy growth of InGaAs

This paper describes the development of a high-speed three-dimensional reciprocal-space mapping method designed for the real-time monitoring of the strain relaxation process during the growth of heterostructure semiconductors. Each three-dimensional map is obtained by combining a set of consecutive images, which are captured during the continuous rotation of the sample, and calculating the reciprocal-space coordinates from the detector coordinate system. To demonstrate the feasibility of this rapid mapping technique, the 022 asymmetric diffraction of an InGaAs/GaAs(001) thin film grown by molecular beam epitaxy was measured and the procedure for data calibration was examined. Subsequently, the proposed method was applied to real-time monitoring of the strain relaxation process during the growth of a thin-film heterostructure consisting of In0.07Ga0.93As and In0.18Ga0.82As layers consecutively deposited on GaAs(001). The time resolution of the measurement was 10 s. It was revealed that additional relaxation of the first In0.07Ga0.93As layer was induced by the growth of the second In0.18Ga0.82As layer within a short period of time corresponding to the deposition of only two monolayers of InGaAs.

Effect of Base Doping Concentration on Radiation-Resistance for GaAs Sub-Cells in InGaP/GaAs/Ge

National Institute for Astronomy and Geophysics Research, Helwan, Cairo 11421, EgyptReceived July 30, 2010; revised September 13, 2010; accepted October 1, 2010; published online December 20, 2010GaAs solar cells with the lower base carrier concentration under low energy proton irradiations had shown experimentally the better radiation-resistance. Analytical model based on fundamental approach for radiative and non-radiative recombination has been proposed for radiationdamage in GaAs sub-cells. The radiation resistance of GaAs sub-cells as a function of base carrier concentration has been analyzed by usingradiative recombination lifetime and damage coefficient for minority carrier lifetime. Numerical analysis shows good agreement with experimentalresults. The effect of carrier concentration upon the change of damage constant and carrier removal rate have been studied.# 2010 The Japan Society of Applied PhysicsDOI: 10.1143/JJAP.49.121202

Direct observation of strain in InAs quantum dots and cap layer during molecular beam epitaxial growth using in situ X-ray diffraction

Direct measurements on the growth of InAs quantum dots (QDs) and various cap layers during molecular beam epitaxy are performed by in situ X-ray diffraction (XRD). The evolution of strain induced both in the QDs and cap layers during capping is discussed based on the XRD intensity transients obtained at various lattice constants. Transients with different features are observed from those obtained during InGaAs and GaAs capping. The difference observed is attributed to In-Ga intermixing between the QDs and the cap layer under limited supply of In. Photoluminescence (PL) wavelength can be tuned by controlling the intermixing, which affects both the strain induced in the QDs and the barrier heights. The PL wavelength also varies with the cap layer thickness. A large redshift occurs by reducing the cap thickness. The in situ XRD observation reveals that this is a result of reduced strain. We demonstrate how such information about strain can be applied for designing and preparing novel device structures.

Effects of growth temperature and growth rate on polytypes in gold-catalyzed GaAs nanowires studied by in situ X-ray diffraction

The polytypism of GaAs nanowires was investigated by in situ X-ray diffraction under different growth conditions. The growth of nanowires was found to start with the formation of the zincblende structure, followed by the growth of the wurtzite structure. The observed growth process of nanowires was well reproduced by simulations based on a layer-by-layer nucleation model. At a low growth temperature and a high growth rate, stacking faults were found to be frequently introduced owing to the reduction in energy barrier. As a result, the zincblend and wurtzite segments in nanowires were highly fragmented and the wurtzite structure was formed in the early stage of growth.