Size quantization effects in InAs self-assembled islands on InP(001) at the onset of 2D-to-3D transition

Photoluminescence spectroscopy has been used to investigate self-assembled InAs islands in InAlAs grown on InP(0 0 1) by molecular beam epitaxy, in correlation with transmission electron microscopy. The nominal deposition of 3.6 monolayers of InAs at 470 degrees C achieves the onset stage of coherent island formation. In addition to one strong emission around 0.74 eV, the sample displaces several emission peaks at 0.87, 0.92. 0.98, and 1.04 eV. Fully developed islands that coexist with semi-finished disk islands account for the multipeak emission. These results provide strong evidence of size quantization effects in InAs islands. (C) 1999 Elsevier Science B.V. All rights reserved.

Uniformity enhancement of the self-organized InAs quantum dots

Growth interruption was introduced after the deposition of GaAs cap layer, which is thinner than the height of quantum dots. Uniformity of quantum dots has been enhanced because the full-width of half-maximum of photoluminescence decrease from 80 to 27 meV in these samples as the interruption time is increased. Meanwhile, we have observed that the peak position of photoluminescence is a function of interruption time, which can be used to modulate energy level of quantum dots. All of the phenomenon mentioned above can be attributed to the diffusion of In atoms from the tops of InAs islands to the top of GaAs cap layer caused by the difference between the surface energies of InAs and GaAs. (C) 1999 Elsevier Science B.V. All rights reserved.

Normally incident infrared absorption in vertically aligned InGaAs/GaAs quantum dot superlattice

30-period InGaAs/GaAs quantum dot superlattice was fabricated by MBE. Using cross sectional transmission electron microscopy, the InGaAs quantum dots were found to be perfectly vertically aligned in the growth direction (100). Under normally incident radiation, a distinct absorption in the 8.5 similar to 10.4 mu m range peaked at 9.9 mu m was observed. The normally incident infrared absorption in vertically aligned quantum dot superlattice in the 8 similar to 12 mu m range was realized for the first time. This result indicates the potential application of the quantum dot superlattice structure without grating as normally incident infrared detector focal plane arrays.

Metamorphosis of self-organized quantum dots into quantum wires

Spontaneous formation of InAs quantum wires in InAlAs on InP(001) via sequential chain-like coalescence of quantum dots along [1 (1) over bar 0] is realized. Theoretical calculations based on the energetics of interacting steps provide a qualitative explanation for the experimental results. Sequential coalescence of initially isolated dots reduces the total free energy strikingly. Thus the wire-like structure is energetically favorable. (C) 1998 Elsevier Science B.V.

Structure and photoluminescence of InGaAs self-assembled quantum dots grown on InP(001)

Molecular beam epitaxy has been used for growing InGaAs self-assembled quantum dots (QDs) in InAlAs on an InP(001) substrate. Nominal deposition of 9.6 monolayers of In0.9Ga0.1As results in QDs of similar to 6.5 nm high with an areal density of 3.3 X 10(11) cm(-2). Conspicuous bimodal size distribution is identified, and is responsible for the observed QDs photoluminescence (PL) emission with two peaks at 0.627 and 0.657 eV. Good agreement is achieved between the observed PL peak energies and calculated results. (C) 1999 American Institute of Physics. [S00218979(99)00101-2].

New method for the growth of highly uniform quantum dots

A new method is realized for the growth of self-formed quantum dots. We identify that dislocation-free islands can be formed by the strain from the strained superlattice taken as a whole. Unlike the Stranski-Krastanow (S-K) growth mode, the islands do not form during the growth of the corresponding strained single layers. Highly uniform quantum dots can be self-formed via this mechanism. The low temperature spectra of self-formed InGaAs/GaAs quantum dot superlattices grown on a (001) GaAs substrate have a full width at half maximum of 26-34 meV, indicating a better uniformity of quantum dot size than those grown in the S-K mode. This method can provide great degrees of freedom in designing possible quantum dot devices. 1998 Published by Elsevier Science B.V. All rights reserved.

Analysis of atomic force microscopic results of InAs islands formed by molecular beam epitaxy

Atomic force microscopy (AFM) measurements of nanometer-sized islands formed by 2 monolayers of InAs by molecular beam epitaxy have been carried out and the scan line of individual islands was extracted from raw AFM data for investigation. It is found that the base widths of nanometer-sized islands obtained by AFM are not reliable due to the finite size and shape of the contacting probe. A simple model is proposed to analyze the deviation of the measured value From the real value of the base width of InAs islands. (C) 1998 Elsevier Science B.V. All rights reserved.

Electronic structures of InAs self-assembled quantum dots in an axial magnetic field

The electronic structure of an InAs self-assembled quantum dot in the presence of a perpendicular magnetic field is investigated theoretically. The effect of finite offset, valence-band mixing, and strain are taken into account. The hole levels show strong anticrossings. The large strain and strong magnetic field decrease the effect of mixing between heavy hole and light hole. The hole energy levels have in general a weaker field dependence compared with the corresponding uncoupled levels.

Effects of growth interruption on self-assembled InAs/GaAs islands

The effect of growth interruption on the InAs deposition and its subsequent growth as self-assembled island structures, in particular the material transport process of the InAs layers has been investigated by photoluminescence and transmission electron microscopy measurements. InAs material in structures with only coherent islands transfers from the wetting layer to the formed islands and the growth interruption causes a red shift of PL peak energy. On the other hand, the PL peak shifts to higher energy in structures containing simultaneously coherent and noncoherent islands with dislocations. In this case, the noncoherent islands capture InAs material from the surrounding wetting layer as well as coherent islands, which casues a reduction in the size of these islands. The variations in the PL intensity and line width are also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.

Asymmetry in the vertically aligned growth induced InAs islands in GaAs

A 10-InAs-island-layer vertically coupled quantum dot structure on (001) GaAs was grown and investigated by molecular beam epitaxy and transmission electron microscopy. The result shows that the vertically aligned InAs islands are asymmetrical along the two < 110 > directions on the (001) growth plane. Such an asymmetry in the vertically coupled quantum dot structure can be explained with the chemical polarity in the III-V compound semiconductors.

InAs quantum dots in InAlAs matrix on (001)InP substrates grown by molecular beam epitaxy

InAs quantum dots grown on InAlAs lattice-matched to (0 0 1) InP substrates by molecular beam epitaxy are investigated by double-crystal X-ray diffraction, photoluminescence and transmission electron microscopy. The growth process is found to follow the Stranski-Krastanow growth mode. The islands formation is confirmed by the TEM measurements. Strong radiative recombination from the quantum dots and the wetting layer is observed, with room temperature PL emission in the 1.2-1.7 mu m region, demonstrating the potential of the InAs/InAlAs QDs for optoelectronic device applications. (C) 1998 Elsevier Science B.V. All rights reserved.

Improvement of Ge self-organized quantum dots by use of Sb surfactant

A Sb-mediated growth technique is developed to deposit Ge quantum dots (QDs) of small size, high density, and foe of dislocations. These QDs were grown at low growth temperature by molecular beam epitaxy. The photoluminescence and absorption properties of these Ge QDs suggest an indirect-to-direct conversion, which is in good agreement with a theoretical calculation. (C) 1998 American Institute of Physics. [S0003-6951(98)00420-3].

Ordered InAs quantum dots in InAlAs matrix on (001) InP substrates grown by molecular beam epitaxy

InAs self-organized quantum dots in InAlAs matrix lattice-matched to exactly oriented (001) InP substrates were grown by solid source molecular beam epitaxy (MBE) using the Stranski-Krastanow mode. Preliminary characterizations have been performed using photoluminescence and transmission electron microscopy. The geometrical arrangement of the quantum dots is found to be strongly dependent on the amount of coverage. At low deposition thickness. InAs QDs are arranged in chains along [1(1) over bar0$] directions. Luminescence from the quantum dots and the wetting layer consisting of quantum wells with well widths of 1, 2, and 3 monolayers is observed. (C) 1998 American Institute of Physics.

Optical properties of Ge self-organized quantum dots in Si

Small-size, high-density, and vertical-ordering Ge quantum dots are observed in strained Si/Ge short-period superlattices grown on Si(001) at low growth temperature by molecular-beam epitaxy. The photoluminescence (PL) peak position, the strong PL at room temperature, and the high exciton binding energy suggest an indirect-to-direct conversion of the Ge quantum dots. This conversion is in good agreement with the theoretical prediction. The characteristic of absorption directly indicates this conversion. The tunneling of carriers between these quantum dots is also observed. [S0163-1829(98)03515-2].

Material transport in self-assembled InAs/GaAs quantum dot ensemble

Introducing the growth interruption between the InAs deposition and subsequent GaAs growth in self-assembled quantum dot (QD) structures, the material transport process in the InAs layers has been investigated by photoluminescence and transmission electron microscopy measurement. InAs material in structures without misfit dislocations transfers from the wetting layer to QDs corresponding to the red-shift of PL peak energy due to interruption. On the other hand, the PL peak shifts to higher energy in the structures with dislocations. In this case, the misfit dislocations would capture the InAs material from the surrounding wetting layer and coherent islands leading to the reduction of the size of these QDs. The variations in the PL intensity and Linewidth are also discussed.