Mayuko Fudeta

Vertical Quantum Confinement Effect on Optical Properties of Strain-Induced Quantum Dots Self-Formed in GaP/InP Short-Period Superlattices

Optical properties of multilayer quantum dots (MQDs) self-formed in the GaP/InP short period superlattice (SL)/InGaP multilayer structures are investigated by changing the SL period (P) and InGaP barrier thickness (B). By decreasing P, photoluminescence (PL) peak energy is shifted toward higher energy due to the quantum size effect along the growth direction (vertical direction). PL linewidth broadening with temperature is reduced by decreasing P and B. This improvement is attributed to the reduction of potential distribution among quantum dots (QDs) and the enhancement of quantum confinement along the vertical direction, and to the enhancement of quantum confinement due to the vertical coupling effect between QDs. Stokes shift for the MQDs structure is observed to be small. Very small temperature variation of PL peak energy is observed in these MQDs, which originates from the existence of the multiaxial strains among the MQDs. Electroluminescence (EL) spectra show split peaks corresponding to the subbands of QDs on increasing injection current.

Improvement of Optical Properties of Multilayer Quantum Dots Self-Formed in GaP/InP Short-Period Superlattices on GaAs(311)A

Multilayer quantum dots (MQDs) structures are fabricated on a GaAs(311)A substrate by sandwiching the quantum dots (QDs) self-formed in (GaP)1.5(InP)1.88 short-period superlattices (SLs) with InGaP/InAlP SL layers instead of InGaP layers, as barrier and cladding layers. Narrower photoluminescence (PL) and electroluminescence (EL) linewidths and weaker temperature variations are observed for the modified MQDs compared with the previously reported best values for MQDs with InGaP barrier and cladding layers. PL and EL peak energies for the modified MQDs are higher than those for the previous MQDs. These results suggest the enhancement of carrier confinement by the use of InGaP/InAlP SL layers as barrier and cladding layers. The temperature dependence of EL intensity is also improved.

Improved Optical Properties of Strain-Induced Quantum Dots Self-Formed in GaP/InP Short-Period Superlattices.

Optical properties of multilayer quantum dots (MQDs) self-formed in the GaP/InP short-period superlattice (SL)/InGaP multilayer structures are investigated as a function of InGaP barrier thickness (B). Photoluminescence (PL) linewidth broadening with temperature is improved and tends to reduce by decreasing B. This is attributed to the vertical coupling effect between QDs and their vertical alignment. Temperature variation of PL properties shows the exciton behavior. At low temperatures, emissions from both bound exciton and free exciton appear under the weak excitation density condition. Integrated PL intensity is quite stable up to 120 K.

Growth Temperature Dependence of Self-Formation Process of Quantum Dot Structures in GaP/InP Short-Period Superlattices Grown on GaAs (311)A Substrate

Growth temperature dependence of the self-formation process of quantum dot (QD) structures in (GaP)1.5 (InP)1.88 short-period superlattices (SLs) grown on GaAs (311)A substrates is studied by scanning tunneling microscopy (STM). SLs are grown by gas-source molecular beam epitaxy (MBE) at 420–500°C. The STM image of the sample grown at 460°C reveals completely self-formed QD structures aligned along both [233] and [011] directions due to the strain-induced lateral composition modulation. On the other hand, both below (420°C) and above (480°C, 500°C) this temperature the self-formation process of QD structures is suppressed and only incomplete structures elongated along the [011] direction are formed, probably due to the suppressed or over-enhanced migration of group III atoms on the surface, respectively. Scanning tunneling spectroscopy (STS) measurements reveal that the amplitude of the lateral periodic variation of the band-gap energy in the self-formed structures also decreases both below and above the optimum growth temperature.

Optical properties of quantum dots self-formed in GaP/InP short period superlattices grown on GaAs (N11) substrates

Optical properties of multilayer quantum dots (MQDs) self-formed in the GaP/InP short period superlattice (SL)/InGaP multilayer structures are investigated by changing SL-period (P) and InGaP barrier thickness (B). By decreasing P, photoluminescence (PL) peak energy shifts toward higher energy due to quantum size effect along the growth direction (vertical direction). PL linewidth broadening with temperature is reduced by decreasing P and B. This improvement is attributed to the reduction of potential distribution among QDs and the enhancement of quantum confinement along the vertical direction, and to the enhancement of quantum confinement due to the vertical coupling effect between QDs. Very small temperature variation of PL peak energy is observed in these MQDs, which is attributed to the existence of the multiaxial strains in the MQDs.

Characterization of self-organized GaP/InP quantum dots with scanning tunneling spectroscopy and time-resolved PL spectroscopy

High lateral density and well-aligned quantum dot (QD) structures are self-formed by growing the (GaP) 1.5 (InP) 1.88 short period superlattices (SLs) on GaAs (311) A substrates. Growth sequence dependence of the QD structures is studied with scanning tunneling spectroscopy and time-resolved photoluminescence spectroscopy. The self-formed dot size for the GaP/InP SLs (GaP is the first growth) is smaller than that for the InP/GaP SLs (InP is the first growth). This difference is considered to be caused by the difference in the strain between the first layer and the substrate. Photoluminescence decay time for the QDs self-formed in the GaP/InP SLs is a little bit shorter than that in the InP/ GaP SLs, which is explained by the enhancement of carrier localization due to the increase in overlapping of wave function.

Scanning tunneling microscopy and time-resolved photoluminiscence spectroscopy study of self-organized GaP/InP quantum dot structures

Abstract GaP/InP short period superlattices (SLs) are grown on GaAs (311)A substrates by gas source molecular beam epitaxy (MBE). Scanning tunneling microscopy/spectroscopy (STM/STS) measurements show that the quantum dot (QD) structures are self-formed with a lateral density of ∼10 11 cm −2 . Growth temperature dependence of self-formed structures is studied with STM/STS and clear temperature dependence is observed. Optimum growth temperature is about 460°C. Time-resolved photoluminescence (PL) spectroscopy measurement on the multilayer QD (MQD) structures shows that the PL decay time strongly depends on emission energy and temperature, and ranges from 0.1 to 2.5 ns, which can be explained by considering the tunneling effect of carriers between QDs.

STM/STS study on high lateral density, well-aligned In-Ga-P quantum dots self-formed in GaP/InP short-period superlattices grown on GaAs(N11)A substrate

High lateral density and well-aligned In-Ga-P quantum dots on GaAs(N11)A substrate were observed by STM/STS. The superlattice cycle and gas source MBE growth temperature dependence of the self-formed structures were studied by STM. The self-formation process and mechanism were discussed based on these results. The optimum growth temperature exists at around 460/spl deg/C under the present growth conditions.

Time-Resolved Photoluminescence Study of Strain-Induced Quantum Dots Self-Formed in GaP/InP Short-Period Superlattice

The nature of emission in quantum dot (QD) structures self-formed in GaP/InP short-period superlattice is investigated in detail by means of time-resolved photoluminescence (TRPL) spectroscopy. PL decay time strongly depends on emission energy and temperature, and ranges from 0.1 ns to 2.5 ns, which can be explained by considering the tunneling effect of carriers and the nonideal dimensionality of dots. The multilayer QD structures with thicker QD layers show a longer PL decay time of up to 1 µs at low excitation densities, which is considered to be caused by an internal piezoelectric field.

Straight Quantum Wires Self-Formed by Growing GaP/InP Short-Period Superlattices on GaAs(011) Substrate

Quantum wires (QWRs) are self-formed by strain-induced composition modulation while growing (GaP)n(InP)m short-period superlattices (SLs) on GaAs(011) substrates by gas source MBE. Improvements in straightness, uniformity and length of these QWRs are observed, compared with those on GaAs(100) substrates. The QWR direction is parallel to the [011] direction and is mainly determined by the easy migration direction of group III atoms on the surface. Multilayer quantum wires (MQWRs), self-formed while growing (GaP)n(InP)m SL/InGaP multilayers exhibit strong polarization anisotropy of as large as 71% in their photoluminescence (PL) emission. Temperature-insensitive variation of the PL peak energy is observed in these MQWR structures.