A. C. Maciel

Nonlinear excitonic optical absorption in GaInAs/InP quantum wells

We have studied the saturation of optical absorption in GaInAs/InP quantum wells at room temperature. Using optical excitation from a tunable cw Co:MgF2 laser, we find a saturation intensity of 70 Wu2009cm−2 when exciting resonantly at the n=1 heavy‐hole exciton, and we deduce values of the nonlinear absorption and refraction coefficients −60 cmu2009W−1 and −0.3 cm2u2009kW−1, respectively. The saturation intensity in the quantum well is significantly lower than in bulk GaInAs, and also in GaAs quantum wells.

Formation of self-assembled InAs quantum dots on (110) GaAs substrates

The formation of indium arsenide self-assembled quantum dots (QDs) has been observed on (110) GaAs substrates. The quantum dots were grown using solid source molecular-beam epitaxy. Both (110) and (100) GaAs wafers were placed in the growth chamber and grown on simultaneously. QDs were found to form on the (110) GaAs substrate when grown upon a thin AlAs layer. Buried layers of InAs were studied using photoluminescence spectroscopy, while surface layers were studied by atomic force microscopy. While the luminescence spectra for simultaneously grown (110) and (100) QDs are similar in structure, the changes in each sample’s spectrum as a function of both temperature and excitation intensity suggest quantum dot density is over an order of magnitude lower on (110) samples than it is on the (100) samples. This difference is conditionally confirmed by atomic force microscopy studies.

Optical studies of excitons in Ga0.47In0.53As/InP multiple quantum wells

We report optical absorption and photoluminescence measurements of excitons in Ga0.47In0.53As/InP multiple quantum wells grown by metalorganic chemical vapor deposition. At 4 K the luminescence linewidth for n=1 heavy‐hole excitons is measured to be 7 meV for a 30‐period structure with wells of width 154 A. The absorption spectrum at low temperature shows four peaks which we assign to confined heavy‐hole excitons. A theoretical calculation of the energies of these states indicates that the ratio of the conduction‐band to valence‐band energy discontinuities is 45:55.

Dominant role of surface states in photoexcited carrier dynamics in CdSe nanocrystalline films prepared by chemical deposition

We report on ultrafast carrier dynamics in nanocrystalline CdSe thin films prepared by chemical solution deposition. The photoluminescence and transient absorption dynamics have spectrally dependent decay faster at shorter wavelengths. The spectral behavior of the decay rates in samples with different nanocrystal sizes and the sensitivity of the dynamics on the surface modification indicate the dominant role of the surface states in the photoluminescence.

Electric field dependent exciton energy and photoluminescence quenching in GaInAs/InP quantum wells

We report the observation of electric field induced exciton energy shifts and photoluminescence quenching in GaInAs/InP multiple quantum wells. We have measured both the photocurrent and photoluminescence spectra from 100 A wells contained with p+‐ and n+‐InP layers in a conventional p‐i‐n structure; reverse bias voltages of up to 12 V were applied. The exciton peaks in the photocurrent spectrum are seen to broaden and shift to lower energy; the photoluminescence peak, which is due to n=1 excitonic and free‐carrier recombination, also shifts to lower energy and is completely quenched at high voltages. These results are similar to those reported previously for GaAs quantum wells and ascribed to the quantum‐confined Stark effect.

Nonlinear optical absorption in bulk GaInAs/InP at room temperature

We report cw measurements of band‐gap resonant nonlinear absorption in bulk Ga0.47In0.53As/InP at room temperature. A tunable Co:MgF2 laser was used for these experiments. The nonlinear absorption coefficient α2 is determined by measuring saturation of the optical absorption, and from this the nonlinear refractive index n2 is calculated in the band‐tail region. We find α2≊−6 cmu2009W−1 at 1.67 μm and n2≊−4×10−5 cm2u2009W−1 at 1.69 μm. Within the band‐tail region the nonlinear optical quality factor ‖n2/α‖ is found to lie between 0.9 and 1.8×10−8 cm3u2009W−1.

Electron transport in modulation-doped GaAs v-groove quantum wires

Abstract We report the growth of modulation-doped GaAs/Al x Ga 1− x As v-groove quantum wires and structural, electrical and optical investigations of their electronic states and transport properties. By using alternative group III precursors on partially SiO 2 masked pre-patterned GaAs substrates, samples have been fabricated which permit electrical measurements of single isolated wire structures without the need for additional electron-beam lithography. Magneto-transport was measured as a function of tilt angle of the incident magnetic field to identify the formation of low-dimensional electron gases in different parts of the structure. Photoluminescence investigations reveal 1D and 2D confined states which show different carrier heating when electric fields are applied along the wire structure.

On the choice of precursors for the MOVPE-growth of high-quality Al0.30Ga0.70As/GaAs v-groove quantum wires with large subband spacing

Abstract MOVPE growth is used to prepare high-quality v-groove quantum wires in the AlGaAs/GaAs system. The particular aim of this work is to find suitable precursor combinations and growth conditions with which quantum wire structures can be achieved, where only the lowest subband is occupied and on which truly one-dimensional transport can be studied. Different precursor combinations of triethylgallium (TEGa), trimethylgallium (TMGa), trimethylaluminium (TMAl) and dimethylethylaminenalane (DMEAAl) were employed for growth of the Al0.30Ga0.70As barrier layer. Calculations of the 2D-Schrodinger-equation show, that suitable quantum wire geometries are obtained when using combinations with TMGa. High-quality material is achieved at 575°C for TMGa/DMEAAl and 595°C for TMGa/TMAl. In general, lower growth temperatures lead to the preferential lower radii of curvature for a given precursor combination. A comparison of the best two combinations shows that TMGa/TMAl is best suited for growth of the Al0.30Ga0.70As barrier layer with respect to the intended application.

Nonlinear excitonic optical absorption in GaSb

We have measured the saturation of optical absorption in GaSb at wavelengths close to the band gap, and have determined the nonlinear absorption (α2) and refraction (n2) coefficients. At liquid‐helium temperature a sharp free‐exciton absorption line is observed which saturates with increasing incident laser intensity; we obtain values ‖α2‖=70 cmu2009W−1 and ‖n2‖≊0.2 cm2u2009kW−1. At room temperature we do not observe a well‐defined exciton; saturation of the residual interband absorption occurs at much higher intensity, and it is found to be obscured by strong thermal effects.

Carrier relaxation in GaAs v-groove quantum wires and the effects of localization

Carrier relaxation processes have been investigated in