Satoshi Shimomura

Extremely Flat Interfaces in GaAs/AlGaAs Quantum Wells Grown on GaAs (411)A Substrates by Molecular Beam Epitaxy

GaAs/Al0.3Ga0.7As quantum wells (QWs) grown on (411)A-oriented GaAs substrates by molecular beam epitaxy (MBE) showed extremely flat interfaces over a macroscopic area (about 200 µm ) even for the case of no growth interruption, which is mainly due to the intrinsically large migration of Ga atoms and layer growth in the step-flow mode on the (411)A plane. Photoluminescence linewidths at 4.2K were almost the same as or better than the narrowest linewidths reported for GaAs/AlGaAs and GaAs/AlAs QWs grown on GaAs (100) substrates with growth interruption at each GaAs/AlGaAs(AlAs) interface. Only one sharp luminescence peak was observed for each QW on the (411)A substrates, in contrast with three luminescence peaks for the QWs on the (100) substrates, indicating that extremely flat and uniform interfaces over a macroscopic area of laser excitation (200 µm diameter) are realized in the GaAs/AlGaAs QWs grown on (411)A GaAs substrates.

High-density GaAs/AlAs quantum wires grown on (775)B-oriented GaAs substrates by molecular beam epitaxy

GaAs/AlAs quantum wires (QWRs) were found to be naturally formed by a regularly corrugated AlAs/GaAs interface and a flat GaAs/AlAs interface in an AlAs/GaAs/AlAs quantum well with a well width of 3.3 nm grown on a (775)B GaAs substrate by molecular beam epitaxy. The lateral period and vertical amplitude of the AlAs/GaAs interface corrugation were 12 nm and 1.2 nm, respectively. The QWRs were formed side by side with an extremely high density of 8×105 QWRs/cm. A photoluminescence peak at λ=715 nm from the QWRs with a cross section of about 12×3 nm2 showed a polarization degree of (I∥ - I⊥) / (I∥ + I⊥) = 0.11 and a very small full width at half maximum of 15 meV at 4.2 K.

Self-organized GaAs quantum-wire lasers grown on (775)B-oriented GaAs substrates by molecular beam epitaxy

Self-organized GaAs/(GaAs)4(AlAs)2 quantum-wire (QWR) lasers were grown on (775)B-oriented GaAs substrates by molecular-beam epitaxy. The QWRs were naturally formed at thick parts in the GaAs/(GaAs)4(AlAs)2 quantum well with a corrugated AlAs-on-GaAs upper interface and a flat GaAs-on-AlAs lower interface. The density of the QWRs was as high as 8×106 cm−1. Stripe-geometry lasers with the self-organized (775)B GaAs/(GaAs)4(AlAs)2 QWRs as an active region oscillated at 20 °C with threshold current densities of about 3 kA/cm2 for uncoated mirrors.

Highly uniform and high-density GaAs/(GaAs)4(AlAs)2 quantum wires grown on (775)B-oriented GaAs substrates by molecular beam epitaxy

Extremely uniform and high-density GaAs/(GaAs)4(AlAs)2 quantum wires (QWRs) were self-organized in a thin GaAs/(GaAs)4(AlAs)2 quantum well grown on a (775)B GaAs substrate with a regularly corrugated AlAs–on–GaAs interface and a flat GaAs–on–AlAs interface by molecular beam epitaxy. A strong photoluminescence (PL) peak at λ=692 nm from the GaAs/ (GaAs)4(AlAs)2 QWRs showed large polarization anisotropy [P=(I∥−I⊥)/(I∥+I⊥)=0.19]. A full width at half-maximum of the PL peak from the QWRs was as small as 15 meV at 14 K, which is smaller than those of any self-organized GaAs/AlGaAs QWRs reported so far. Density of the QWRs ( 8×105 QWRs/cm) is the same as the highest ever reported. These results indicate that the QWRs grown on the (775)B GaAs substrate meet requirements for applications to QWR lasers (high uniformity, high density, high optical quality, and simple fabrication process).

Surface Corrugation of GaAs Layers Grown on (775)B-Oriented GaAs Substrates by Molecular Beam Epitaxy

Surface morphologies of GaAs layers grown on (775)B-oriented GaAs substrates by molecular beam epitaxy were studied using atomic force microscopy. The surface of a GaAs layer grown on a (775)B GaAs substrate is flat when it is grown at a substrate temperature (T s) below 580°C, but corrugates regularly for T s ≥640°C. Similar periodic corrugation of the (775)B GaAs surface formed during thermal annealing at T s=640°C under As4 atmosphere (10-6 Torr), which is similar in shape to that of a GaAs layer in the early stage of growth (a GaAs layer thinner than 5 nm). As the thickness of a GaAs layer increases from 5 nm, the lateral period and step height of the corrugation increase and tend to saturate for thick GaAs layers (200 nm thick or more). The precise structure of the surface corrugation of a 3-nm-thick GaAs layer (the lateral period of 12 nm and step height of 1.2 nm) was observed as a corrugated AlAs-on-GaAs interface of a GaAs/(GaAs)5(AlAs)5 quantum well structure with an average well width of 3 nm by cross-sectional transmission electron microscopy observation.

Low temperature etching of GaAs substrates and improved morphology of GaAs grown by metalorganic molecular beam epitaxy using trisdimethylaminoarsenic and triethylgallium

Metalorganic molecular beam etching of a GaAs surface is observed at temperatures as low as 370 o C, for the first time, when the GaAs surface is exposed to TDMAAs (trisdimethylaminoarsenic) only. At this low temperature, not only the oxide layer is removed, but a smoothening of the surface also occurs. At higher temperatures (≥480 o C), however, this reaction of GaAs with TDMAAs is responsible for a roughening of the (100) GaAs surface, forming (411)A micro-facets. At low V/III beam pressure ratios (≤0.5), smooth surfaces as well as good optical and electrical properties are obtained for GaAs layers grown even at high temperatures (550≤T sub ≤ 650 o C). TDMAAs proves to be an extremely efficient arsenic source which can supply enough As even for a small V/III ratio of 0.25, and provides high GaAs growth-rates up to 2 μm/h for a TDMAAs flux of 0.15 SCCM and TEG of 0.4 SCCM

Polarization control of vertical cavity surface emitting laser structure by using self-organized quantum wires grown on (775)B-oriented GaAs substrate by molecular beam epitaxy

Vertical cavity surface emitting laser (VCSEL) structures with self-organized InGaAs quantum wires (QWRs) in their active region were grown on (775)B-oriented GaAs substrate by molecular beam epitaxy. The (775)B self-organized InGaAs QWRs have a regularly corrugated AlAs-on-InGaAs upper interface with average lateral period of 40 nm and vertical amplitude of 1.5 nm and a flat InGaAs-on-AlAs lower interface. The (775)B VCSEL structure was optically pumped and showed room temperature lasing action for wavelength of around 830 nm. The light output was linearly polarized and the polarization direction was fixed to the QWR direction. This is the first demonstration of polarization control of VCSELs by self-organized QWRs in their active region.

Super-flat interfaces in pseudomorphic In0.72Ga0.28As/In0.52Al0.48As quantum wells grown on (411)A InP substrates by molecular beam epitaxy

We investigated flatness of heterointerfaces in pseudomorphic In0.72Ga0.28As/In0.52Al0.48As quantum wells (QWs) with well widths Lw of 1.3, 1.7, and 3.4 nm grown on (411)A InP substrates by molecular beam epitaxy (MBE). Full width at half maximum of a photoluminescence (12 K) peak from the (411)A QW with Lw=1.3 nm was as narrow as 20.7 meV, which is 28% smaller than that (28.9 meV) of the corresponding QW grown on a conventional (100) InP substrate. This result indicates that effectively atomically flat interfaces [(411)A super-flat interfaces] were successfully formed, for the first time, in pseudomorphic In0.72Ga0.28As/In0.52Al0.48As QWs grown on the (411)A InP substrate by MBE.

Laser operation at room temperature of self-organized In0.1Ga0.9As/(GaAs)6(AlAs)1 quantum wires grown on (775)B-oriented GaAs substrates by molecular beam epitaxy

In0.1Ga0.9As/(GaAs)6(AlAs)1 quantum wires (QWRs) were naturally formed in a 3.0-nm- or 4.8-nm-thick In0.1Ga0.9As/(GaAs)6(AlAs)1 quantum well grown on a (775)B-oriented GaAs substrate by molecular beam epitaxy which has a corrugated AlAs-on-InGaAs upper interface (a period of about 40 nm and a vertical amplitude of about 2 nm) and a flat InGaAs-on-AlAs lower interface. Strong polarization dependence [P≡(I∥−I⊥)/(I∥+I⊥)=0.15] of photoluminescence spectrum from the (775)B InGaAs QWR structures (Lw=3.0 nm) was observed at 11 K, indicating their good one dimensionality. Graded index separate confinement heterostructure-type self-organized (775)B In0.1Ga0.9As/(GaAs)6(AlAs)1QWR lasers were fabricated, and they showed laser oscillation with threshold current densities of 1.7–3.1 kA/cm2 and lasing wavelengths of 833–868 nm at room temperature (27 °C) under pulsed current condition.

Extremely flat interfaces in InxGa1-xAs/Al0.3Ga0.7As quantum wells grown on (411)A GaAs substrates by molecular beam epitaxy

Effectively atomically flat interfaces over a macroscopic area (super-flat interfaces) have been achieved in pseudomorphic In x Ga 1-x As/Al 0.3 Ga 0.7 As (x = 0.0, 0.04, 0.07) quantum wells (QWs) with well widths (L w ) of 1.2-11.8 nm grown on (411)A GaAs substrates at 520°C by molecular beam epitaxy (MBE). A single, sharp photoluminescence (PL) peak was observed for each QW over the large area of the wafer (8 mm x 5 mm). The linewidths for narrow QWs (L w = 2.4nm) were 8.9meV (x = 0.04) and 9.9 meV (x = 0.07) at 4.2 K, which were about 30% smaller than those of QWs simultaneously grown on conventional (100) GaAs substrates.