J. Brault

Role of buffer surface morphology and alloying effects on the properties of InAs nanostructures grown on InP(001)

We show the role played by the buffer surface morphology and by alloying effects on the size, shape and lateral distribution of InAs nanostructures grown on InP(001) substrates by molecular beam epitaxy. Three buffers, viz., In0.53Ga0.47As, In0.52Al0.48As, and InP lattice matched on InP have been studied. Differences in nanostructure morphology and in carrier confinement have been evaluated by atomic force microscopy and by low-temperature photoluminescence measurements, respectively. Alongside the classical relaxation mode through two-dimensional/three-dimensional surface morphology change, a chemical relaxation mode has to be introduced as a competitive mode of relaxation of strained layers. This chemical relaxation mode, due to alloying between the InAs deposit and the buffer, is thought to be responsible for most of the observed differences in the InAs nanostructure properties.

Surface effects on shape, self-organization and photoluminescence of InAs islands grown on InAlAs/InP(001)

InAs nanostructures were grown on In0.52Al0.48As alloy lattice matched on InP(001) substrates by molecular beam epitaxy using specific growth parameters in order to improve island self-organization. We show how the change in InAs surface reconstruction via growth temperature from (2×4) to (2×1) and/or the use of InAlAs initial buffer surface treatments improve the island shape homogeneity (either as quantum wires or as quantum dots). Differences in island shape and in carrier confinement are shown by atomic force microscopy and by photoluminescence measurements, respectively. We point out that such shape amendments induce drastic improvements to island size distribution and discernible changes in photoluminescence properties, in particular concerning polarization.

Strong normal-incidence infrared absorption in self-organized InAs/InAlAs quantum dots grown on InP(001)

InAs self-assembled quantum dots in InAlAs matrix grown on InP(001) substrates have been fabricated using Stranski–Krastanov growth mode. A strong in-plane polarized intraband absorption in the 10.6–20 μm wavelength region has been observed and ascribed to a transition from the ground electron state to an excited state confined in the layer plane along the [110] direction. The absorption at normal-incidence reaches 7.8% for ten layers of n-doped quantum dots. The oscillator strength of the intraband transition is comparable to that achieved in quantum wells for a conduction band intersubband transition. The dependence of the intraband absorption on carrier concentration and temperature suggests a quantum-wire type confinement potential.

From large to low height dispersion for self-organized InAs quantum sticks emitting at 1.55 μm on InP (001)

We show how the height dispersion of self-organized InAs/InP(001) quantum islands emitting at 1.55 μm was reduced by optimizing the epitaxial growth parameters. Low height dispersion was obtained when the InAs deposit thickness was much greater than the critical thickness for two-dimensional/three-dimensional growth mode transition, and when adatom surface diffusion was favored by increasing the growth temperature or reducing the arsenic pressure during the InAs growth. When these growth conditions are not respected, the multicomponent photoluminescence spectrum obtained is explained through the common interpretation of island height varying with monolayer fluctuation. In optimized growth conditions, the multicomponent spectrum obtained is interpreted as emission from fundamental and excited levels of InAs islands with low height dispersion. Transmission electron microscopy (TEM) imaging shows that these InAs islands are stick-like, 50–100 nm in length and 22±1.2 nm in width. Cross-sectional TEM reveals f...

Staggered vertical self-organization of stacked InAs/InAlAs quantum wires on InP(001)

Abstract Using atomic force microscopy (AFM) imaging, transmission electron microscopy (TEM) and photoluminescence (PL), we have studied InAs stacked islands on InP(001) versus the InAlAs spacer layer thickness (SLT). We have found that first wire-like island shape is strongly favored by such a stacking process and second in the 10–25 nm SLT range, the wire size and height are dependent on the SLT. TEM images show off a new surprising staggered vertical island organization that can be explained by the phase separation appearing in the InAlAs spacer layers.

Optical properties of self-assembled InAs quantum islands grown on InP(001) vicinal substrates

We have investigated the effect of misorientated InP(001) substrates on the optical properties of InAs quantum islands (QIs) grown by molecular-beam epitaxy in the Stranski–Krastanow regime. Detailed temperature-dependent photoluminescence (PL), excitation density PL, and polarization of photoluminescence (PPL) are studied. PPL shows a high degree of linear polarization (near 40%) for the nominally oriented substrate n and for the substrate with 2° off miscut angle toward the [110] direction (2°u200aF), while it is near 15% for the substrate with 2° off miscut angle towards [010] direction (2°u200aB), indicating the growth of InAs quantum wires on nominal and 2°u200aF substrates and of InAs quantum dots on 2°u200aB substrate. These island shapes are confirmed by morphological investigations performed by atomic force microscopy. The integrated PL intensity remains very strong at room temperature, as much as 36% of that at 8 K, indicating a strong spatial localization of the carriers in the InAs QIs grown on InP(001).

Infrared spectroscopy of self-organized InAs nanostructures grown on InAlAs/InP(001) for infrared photodetection applications

Abstract InAs nanostructures on InAlAs/InP(0xa00xa01) have been fabricated using Stranski–Krastanov growth mode. Depending on growth conditions, a full coverage of the InAlAs surface by either InAs quantum wires or quantum dots can be achieved. Giant intraband absorptions are observed at mid-infrared wavelengths. The intraband resonances are strongly polarized in the layer plane as a consequence of the quantum confinement along the [1xa01xa00] direction. The absorption at normal incidence reaches 26% for 10 layers of n-doped (1×10 12 cm −2 ) InAs elongated dots. We also report on femtosecond pump–probe experiments aimed at measuring the electron capture time. Typical times range from 3 ps for broad wires to 6 ps for narrow wires.

Quantum dot infrared photodetectors in new material systems

Abstract Infrared detectors were implemented on InAs self-assembled quantum dots fabricated using Stranski–Krastanov growth mode on InAlAs matrix, lattice matched to InP (0xa00xa01) substrates. These dots grow with a shape of small elongated boxes, with their long axis along the [ 1 1 0] direction, and with a high concentration of 7×10 10 cm −2 . Photoconductive measurements were performed in all three polarizations. Rich spectra in the range of 50–500 meV, with different polarization selection rules were observed. The bias dependence of peak intensity of the intraband transitions serves as an additional tool to identify their origin. Some of the peaks, which increase linearly with bias, are attributed to bound-to-continuum transitions. Others, which appear only at larger biases, and increase superlinearly, are due to bound-to-bound transitions. The magnitude of detector responsivity at normal-incidence is similar to that obtained for polarization normal to the layers, and is comparable to that achieved in QWIPs. BLIP conditions prevail at 77 K for integral photocurrent response at F#1. The effect of unintentional doping is discussed. It is shown that this doping can be destructive for detector operation unless the density of dots is large.

Alloying effects in self-assembled InAs/InP dots

Abstract We have studied the influence of alloying induced chemical exchange reactions on the formation of self-assembled InAs dots prepared on InP and In 0.52 Al 0.48 As buffers lattice-matched to InP(001). Atomic force microscopy and low-temperature photoluminescence measurements have been used to characterize the dot properties. Strong differences in the islanding process are observed depending on the growth conditions and on the nature of the buffer layer. They are associated to variation in intermixing between the InAs deposit and the buffer layer.

Optical properties of self-organized InAs nanostructures grown on InAlAs/InP(0 0 1)

Abstract We report on photoluminescence (PL) and polarized PL studies of self-organized InAs quantum dots (QDs) and quantum wires (QWRs) grown on InAlAs lattice matched to InP(0xa00xa01). A strong linear polarization degree is observed for the single-layer QWR structures. However, the QDs are found merely isotropic ones and their luminescence is a mixture of QDs ground and excited states.