S. Shimomura

Extremely high uniformity of interfaces in GaAs/AlGaAs quantum wells grown on (411)A GaAs substrates by molecular beam epitaxy

GaAs/AlGaAs quantum wells (QWs) were grown on (411)A‐oriented GaAs substrates by molecular beam epitaxy (MBE). Photoluminescence linewidths at 4.2 K are almost the same as the narrowest linewidths reported so far for GaAs/AlGaAs QWs grown on (100)‐oriented GaAs substrates with the growth interruption at the heterointerfaces. Furthermore, only one sharp peak was observed for each QW on the (411) substrate over the whole area of the wafer (10 mm×10 mm), in contrast with three splitted luminescence peaks for one kind of GaAs/AlGaAs QW grown on the (100) substrates by MBE with growth interruption. This result implies that effectively atomically flat interfaces over a macroscopic area (about 10 mm×10 mm) has been realized for the first time in GaAs/Al0.3Ga0.7As QWs grown on (411)A GaAs substrates by MBE. This is possibly due to the large migration of Ga and Al atoms on the (411)A plane during MBE growth and the step‐flow growth mode on the atomically corrugated (411)A plane.

LATERAL VARIATION OF INDIUM CONTENT IN INGAAS GROWN ON GAAS CHANNELED SUBSTRATES BY MOLECULAR BEAM EPITAXY

Lateral variation of In content in a 1.5 μm thick InxGa1−xAs (x≂0.2) epilayer grown on channeled GaAs (100) substrates having (755)A or (411)A slope regions by molecular beam epitaxy (MBE) was studied by the energy dispersive x‐ray spectroscopy, with high spatial resolution (0.6 μm). Significantly peculiar migration of In atoms on the (411)A was observed. Experimental data of this work strongly suggest that In atoms migrate preferentially in one way, that is to say, they migrate much more in the [122] direction than in the opposite [122] direction on the (411)A plane during MBE growth. The migration length of In atoms on the (100) plane was 5 μm when InGaAs was grown at Ts=570u2009°C and V/III=30.

Metamorphic GaAs/GaAsBi Heterostructured Nanowires.

GaAs/GaAsBi coaxial multishell nanowires were grown by molecular beam epitaxy. Introducing Bi results in a characteristic nanowire surface morphology with strong roughening. Elemental mappings clearly show the formation of the GaAsBi shell with inhomogeneous Bi distributions within the layer surrounded by the outermost GaAs, having a strong structural disorder at the wire surface. The nanowire exhibits a predominantly ZB structure from the bottom to the middle part. The polytipic WZ structure creates denser twin defects in the upper part than in the bottom and middle parts of the nanowire. We observe room temperature cathodoluminescence from the GaAsBi nanowires with a broad spectral line shape between 1.1 and 1.5 eV, accompanied by multiple peaks. A distinct energy peak at 1.24 eV agrees well with the energy of the reduced GaAsBi alloy band gap by the introduction of 2% Bi. The existence of localized states energetically and spatially dispersed throughout the NW are indicated from the low temperature cathodoluminescence spectra and images, resulting in the observed luminescence spectra characterized by large line widths at low temperatures as well as by the appearance of multiple peaks at high temperatures and for high excitation powers.

Highly uniform and high optical quality In0.22Ga0.78As/GaAs quantum wires grown on (221)A GaAs substrate by molecular beam epitaxy

We have grown self-organized InGaAs/GaAs quantum wire (QWR) structures on several kinds of (nnl)A and (nnl)B GaAs substrates by molecular beam epitaxy to optimize the substrate orientation. We observed the most uniform and highest-density corrugation with straight step edges running in the [110] direction on the surface of a 3.0-nm-thick InGaAs layer on the (221)A GaAs substrate among the (nnl)A GaAs substrate. The lateral period of the corrugation was 36 nm and the height was 1.8 nm. On the other hand, GaAs surfaces were almost flat. Hence, a nominally 3.0-nm-thick (221)A InGaAs quantum well sandwiched by GaAs layers is a high density and uniform QWR structure due to the lateral thickness modulation. Photoluminescence (PL) from the (221)A InGaAs/GaAs QWRs structure as 12 K was strongly polarized along the wire direction and its polarization degree P[≡(I∥−I⊥)/(I∥+I⊥)] was 0.20. The PL linewidth was as small as 5.8 meV.

Mobility enhancement by reduced remote impurity scattering in a pseudomorphic In0.7Ga0.3As/In0.52Al0.48As quantum well high electron mobility transistor structure with (411)A super-flat interfaces grown by molecular-beam epitaxy

We have carried out a Shubnikov–de Haas (SdH) measurement at 4 K and investigated the electronic properties and scattering mechanisms in a pseudomorphic In0.7Ga0.3As/In0.52Al0.48As quantum well high electron mobility transistor (QW-HEMT) structure with a thin spacer thickness of 3 nm grown on a (411)A-oriented InP substrate by molecular-beam epitaxy (MBE). Electrons occupied the zeroth and first subbands in the 12-nm-thick In0.7Ga0.3As channel layer at two-dimensional electron gas (2DEG) densities of 3.10×1012 and 0.99×1012 cm−2, respectively. 2DEG mobilities of the (411)A sample for the zeroth and first subbands were μ0=52u200a000 and μ1=66u200a000 cm2/Vu200as, which were much higher than those of the (100) QW-HEMT structure (μ0=22u200a000 and μ1=26u200a000 cm2/Vu200as). The result indicates that the electron mobility of the (411)A sample is enhanced by reduction of remote impurity scattering because the spacer thickness (Lsp=3 nm) and distribution of sheet doped impurities are laterally uniform in the (411)A In0.7Ga0.3As/In0.5...

Photoacoustic spectroscopy of absorption edge for GaAsBi/GaAs nanowires grown on Si substrate

GaAsBi/GaAs nanowires (NWs) grown on Si substrate and proper reference samples have been studied by photoacoustic (PA) spectroscopy. It has been shown that PA signal originating from NWs is quite strong and can be easily identified in the PA spectra, as well as distinguished from the signal originating from the Si substrate. The absorption edge of GaAsBi/GaAs and GaAs NWs has been determined from the analysis of amplitude PA spectra to be 1.26u2009eV and 1.42u2009eV, respectively. These values are consistent with the band gap reduction resulting from the introduction of ∼2% Bi in bulk GaAsBi alloy. The presented results prove that, despite light scattering, which is typical for NWs, PA spectroscopy is an excellent tool to study the absorption edge in semiconductor NWs.

Highly ordered self-assembled nanoscale periodic faceting in GaAs(631) homoepitaxial growth

We report on the self-assembly of large-order-correlated nanoscale faceting on GaAs(631)A substrates grown by molecular beam epitaxy. The surface morphology of the grown samples as a function of the growth temperature and the As-beam equivalent pressure was studied using atomic force microscopy. A two-dimensional autocorrelation function analysis was performed in order to quantitatively determine the uniformity of the surface corrugation. By optimizing the growth conditions, correlated faceted areas as large as 1.7u2009×u20091.7u2009μm2 are obtained. The highly ordered surface corrugation discussed here provides useful insights to prepare highly ordered facet planes for the self organized growth of quantum wires.

Surface migration of group V atoms in GaAsP grown on GaAs channeled substrates by molecular beam epitaxy

Surface migration of group V atoms (As and P) during molecular beam epitaxy was investigated for the first time by measuring lateral profiles of phosphorus content in GaAsP layers grown on (100)u200aGaAs channeled substrates (CSs) with 12–16 μm wide (n11)A side-slope facet region (n=3, 4, and 5) using As4 and P2 beams. The P content (x) of the GaAs1−xPx layer on the (411)A side-slope region was 43% smaller (x=0.11) than that on the (411)A flat substrates, while P contents of GaAsP layers on (311)A and (511)A side-slope regions were larger than those of GaAsP layers on a corresponding flat substrate. The observed lateral P content profile was well reproduced by simulation based on a conventional surface migration model. The migration lengths of As atoms, LAs, were determined as LAs(411)A=18u200aμm, LAs(100)=28u200aμm, LAs(511)A=33u200aμm, and LAs(311)A=35u200aμm, by comparing the observed and simulated profiles.

GaAsBi/GaAs multi-quantum well LED grown by molecular beam epitaxy using a two-substrate-temperature technique

We report a GaAs0.96Bi0.04/GaAs multiple quantum well (MQW) light emitting diode (LED) grown by molecular beam epitaxy using a two-substrate-temperature (TST) technique. In particular, the QWs and the barriers in the intrinsic region were grown at the different temperatures of [Formula: see text] = 350 °C and [Formula: see text] respectively. Investigations of the microstructure using transmission electron microscopy (TEM) reveal homogeneous MQWs free of extended defects. Furthermore, the local determination of the Bi distribution profile across the MQWs region using TEM techniques confirm the uniform Bi distribution, while revealing a slightly chemically graded GaAs-on-GaAsBi interface due to Bi surface segregation. Despite this small broadening, we found that Bi segregation is significantly reduced (up to 18% reduction) compared to previous reports on Bi segregation in GaAsBi/GaAs MQWs. Hence, the TST procedure proves as a very efficient method to reduce Bi segregation and thus increase the quality of the layers and interfaces. These improvements positively reflect in the optical properties. Room temperature photoluminescence and electroluminescence (EL) at 1.23 μm emission wavelength are successfully demonstrated using TST MQWs containing less Bi content than in previous reports. Finally, LED fabricated using the present TST technique show current-voltage (I-V) curves with a forward voltage of 3.3 V at an injection current of 130 mA under 1.0 kA cm-2 current excitation. These results not only demonstrate that TST technique provides optical device quality GaAsBi/GaAs MQWs but highlight the relevance of TST-based growth techniques on the fabrication of future heterostructure devices based on dilute bismides.

Interface roughness characterization by electron mobility of pseudomorphic In0.74Ga0.26As∕In0.52Al0.48As modulation-doped quantum wells grown on (411)A InP substrates by molecular beam epitaxy

Interface roughness of pseudomorphic In0.74Ga0.26As∕In0.52Al0.48As modulation-doped quantum wells (MD-QWs) grown on the (411)A and (100) InP substrates by molecular beam epitaxy was characterized by sheet electron concentration (Ns) dependence of two-dimensional electron gas (2DEG) mobility by applying gate bias at 15K. The (411)A MD-QW with well width of 4nm (8nm) showed value of 1.8–2.1 (1.5–1.6) times higher 2DEG mobility at 15K comparing to the corresponding (100) sample in the range of Ns (1.2–2.3×1012cm−2), which results from reduced interface roughness scattering due to the super-flat (411)A InGaAs∕InAlAs interfaces. By analyzing Ns dependence of the 2DEG mobility, we deduced wave-number dependence of the Fourier components of the lateral well-width fluctuation (Δq0) arising from the interface roughness in the range of q0=0.55–0.7nm−1. Values of ∣Δq0∣2 of the (411)A InGaAs∕InAlAs interface were about half of those of the (100) interface in the whole range of q0=0.55–0.7nm−1.