Subject index to volume 26

The reflectivity of picosecond pulses from a Nd: glass laser on Ge has been measured. A marked increase (∽ 20%) has been found at excitation intensities just below the damage threshold. The effect is discussed in terms of the contribution to the dielectric constant of the crystal, arising from direct intraband transitions within the branches of the valence band. The relevance of these transitions to explain the “plateau” observed in the transmission experiment, is also briefly pointed out.     

Analysis of electron mobility versus temperature after photoexcitation in Si-doped AlxGa1−xAs

The low-temperature Hall mobility of photoexcited electrons has been measured in Si-doped MBE AlGaAs samples and compared with calculated data using the background acceptor density and the alloy scattering potential as free parameters. The possibility of discriminating between negative- or positive-U electron correlation energy for the DX centre has been investigated through a careful analysis of mobility versus temperature curves relating to different photoexcited electron densities. A crucial role of the acceptor density to explain the experimental data within the positive-U model has been evidenced.     

A photoluminescence study of the effects of hydrogen on deep levels in MBE grown GaAlAs:Si

We present a study on low-temperature photoluminescence (PL) of Si-doped

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grown by MBE in the presence and in the absence of a hydrogen backpressure, and post-growth hydrogenated or not, by exposure to a hydrogen plasma. The PL spectra of GaAlAs grown without hydrogen are dominated by transitions involving relatively deep donors and/or acceptors independently on whether the material is post-growth hydrogenated. On the contrary, the spectra of GaAlAs grown in the presence of hydrogen are characterized by recombinations related to excitons and/or to shallow donors and acceptors. Both the in-situ and the ex-situ processes result in PL efficiency enhancements, which are definitely larger (by a factor of up to 20) when the former treatment is used. All of the above results suggest that the ex-situ and the in-situ treatments may affect deep levels of different origin, such as DX centers (related to the band structure of the semiconductor) and levels associated to Al-O complexes, respectively.     

On the intersubband interactions in an electron-hole plasma confined in a quntum well

We calculate the shrinkage of the gaps between the different subbands of a quantum well induced by the presence of a photo-excited electron-hole plasma. The reduction of the higher gaps is found to be almost as strong as that of the fundamental gap. We discuss the importance of the interactions between the different subbands. Our theoretical results compare well with optical transmission experiments.     

Optical properties of a ‘quasi-disordered’ semiconductor: ZnIn2S4

Optical absorption and thermoluminescence measurements have been performed on single crystals of ZnIn₂S₄. An absorption edge and a distribution of traps which depend exponentially on the radiation energy with the same ‘slope’, have been found. These results provide evidence for the existence of a considerable amount of intrinsic disorder in this compound, and allow one to relate the discussion to the wider problem of the optical properties of amorphous semiconductors.     

Carrier recombination dynamics in self-ordered InAs quantum dots

Il Nuovo Cimento D - A photoluminescence study of self-ordered InAs quantum dots grown by molecular beam epitaxy on a GaAs substrate is reported. Short pulses and high excitations have been used in...     

Shallow donors and deep levels in GaAs grown by atomic layer molecular beam epitaxy

We correlate the Si concentration measured by secondary ion mass spectrometry (SIMS) and the net donor concentration in GaAs:Si grown by atomic layer molecular beam epitaxy (ALMBE); Si was supplied during: (a) both the As and the Ga subcycles, (b) the As subcycle, and (c) the Ga subcycle; the layers were grown at temperatures in the 300-530°C range. The results show that Si incorporation and its compensation depend on the Si-supply scheme and that the extent of compensation decreases with the growth temperature. We also study the deep levels in the ALMBE GaAs grown under the above conditions. Our results show the occurrence of M1, M3 and M4 levels with concentrations that are: (i) essentially independent of both the Si supply scheme and the ALMBE growth temperature, (ii) close to those of MBE GaAs grown at 600°C, and (iii) up to 2 orders of magnitude lower than that of GaAs prepared by molecular beam epitaxy (MBE) at similar temperatures.     

Indium surface segregation in InGaAs-based structures prepared by molecular beam epitaxy and atomic layer molecular beam epitaxy

We present a comparative study on In surface segregation in InGaAs/GaAs structures prepared by molecular beam epitaxy (MBE) and atomic layer MBE (ALMBE) at different growth temperatures. The effect of segregation is evaluated by the energy position of exciton transitions in pseudomorphic 10 ML thick In_xGa_1−xAs/GaAs (0.15≤x≤0.30) and in 1 ML thick InAs/GaAs quantum wells. We show that: (i) In segregation decreases with the growth temperatures and is minimized at ALMBE and MBE growth temperatures lower than 260 and 340°C, respectively, and (ii) the segregation is more effective in ALMBE structures than in the MBE counterparts. The growth conditions that have been singled out allow the preparation of structures with high photoluminescence efficiencies even at the low growth temperatures required to minimize In segregation.