A. Frova

Hydrogen activated radiative states in GaAs/GaAlAs heterostructures and InGaAs/GaAs multiquantum wells

Direct observation of optical emission from H‐related complexes in molecular beam epitaxy grown bulk GaAs and GaAlAs, as well as InGaAs/GaAs strained multiquantum wells (MQWs), is obtained from liquid He photoluminescence experiments. Hydrogenation is achieved by low‐energy ion irradiation from a Kaufman source. The volume incorporation of hydrogen, for equal treatment, is dependent upon the density of impurities and defects where H can bind. For moderate H treatment, in addition to passivation of shallow acceptors, in GaAs we observe novel emission bands, δ, peaking at 1.360, and γ, peaking at 1.455 eV. After heavy hydrogen treatment in GaAs of low radiative efficiency−even p type originally−there appears a deeper structure α at ∼1.20 eV, of the kind known for ‘‘internal’’ transitions in the Ga vacancy‐donor complex. Equivalent bands are found in bulk GaAlAs and also in InGaAs/GaAs MQWs. The results allow an approximate estimate of the various optically active Ga‐vacancy levels, as affected by the different degrees of hydrogenation of the dangling bonds, and a comparison with theoretical values. Moreover, they provide evidence for the creation of a H‐related donor whose binding energy is of order 25 meV. Finally, the γ band is suggestive of a transition between localized conduction and valence states associated with the local distortion that is introduced in the lattice when H binds to impurities, defects, and lattice atoms.

Role of hydrogen in III–N–V compound semiconductors

The role of hydrogen in altering the electronic properties of the InxGa1?xAs1?yNy/GaAs system has been investigated by photoluminescence (PL) spectroscopy. Several heterostructures whose nitrogen concentration, y, spans from the dilute (0.0001 ? y ? 0.001) to the alloy (0.01 ? y ? 0.052) limit have been studied. The most remarkable effects observed are a quenching of the PL lines related to exciton recombination in N complexes in the dilute limit, and a bandgap blueshift of the N-containing material towards that of the N-free reference samples in the alloy limit. Differences and similarities found between In-free and x = 0.25?0.41 samples are highlighted. In all cases, the system fully recovers by thermal annealing the optical properties it had before hydrogenation. These behaviours can be accounted for by the formation of N?H bonds, which leads to an effective electronic passivation of the N atoms in the lattice. An analysis of the annealing experiments provides some clues on the geometry of the N complexes in the dilute limit as well as an estimate of the N?H bond strength in both dilute and alloy limits. All these results show that the charge distribution around the N atoms maintains in the alloy limit the strongly localized character it has in the impurity limit.

Luminescence efficiency of near‐surface quantum wells before and after ion‐gun hydrogenation

We have studied the effects of the proximity of a bare Al0.3Ga0.7As surface on the luminescence of an underlying GaAs quantum well (QW) before and after hydrogenation. The mechanism which is affected by H is tunneling to surface states through the surface barrier. Its thickness was varied by wet etching from 60 to 350 A. Our experiments reveal that the degradation of luminescence efficiency from the QW is dependent on the surface barrier thickness and the excitation energy used in the photoluminescence measurements. A complete recovery or even further enhancement of luminescence efficiency was observed in the near‐surface QW after low‐energy ion‐beam hydrogenation, even at room temperature.

Scattering effects on resonant tunneling in double‐barrier heterostructures

In this paper we use the transfer matrix approach to investigate the effect of scattering in the well on resonant tunneling in double‐barrier heterostructures. We consider a double‐barrier heterostructure, in which there is a uniform distribution of scattering centers throughout the well, and use the δ‐type function to describe the perturbation potential of the centers. For simplicity, we only consider one dimension. From model calculations of the transmission probability of the system we find, in agreement with experimental observations, that resonant peaks arising from the quantized states in the well are always present, provided the concentration of the scattering centers is not too high. The effect of scattering is to lower the tunneling current and to shift and broaden the peak of the resonant tunneling spectrum.

High temperature photoluminescence efficiency and thermal stability of (InGa)(AsN)/GaAs quantum wells

The temperature dependence of the photoluminescence (PL) efficiency of (InGa)(AsN)/GaAs single quantum wells (QWs) has been studied from 10 to 500 K. The PL intensity of N-containing samples is almost constant from room temperature to 500 K, in contrast to what is observed in (InGa)As QWs grown under the same conditions. This thermal stability increases for an increase in nitrogen content. We discuss these effects in terms of strain compensation at high nitrogen concentrations.

The spectrum of energy levels of the Ga‐vacancy/deuterium complexes in p‐GaAs

Energy levels of the complexes formed by Ga vacancies binding one or more deuterium (or hydrogen) atoms are investigated by low‐temperature photoluminescence. Since the as‐grown, nondeuterated material is unintentionally doped p type, the emission is possible because of ‘‘internal’’ recombination from a D donor in an adjacent bond‐center position and the ground levels of the different possible vacancy configurations, with zero, one, or more D atoms trapped in its dangling bonds. At 2 K, the transitions occur at 1.14, 1.19–1.22, 1.33–1.30, and 1.34 eV, and the last two have never been observed before. The given energy ranges correspond to laser excitation increasing from the lowest to highest level, for a given D treatment of the material. We explain the new transitions and the multiplet character of some of the bands in terms of the possible different ways of accommodating the D atoms inside the vacancy. The level separation in our data is less than that predicted theoretically for the ‘‘bare’’ vacancy wi...

Interaction mechanisms of near‐surface quantum wells with oxidized and H‐passivated AlGaAs surfaces

The tunneling mechanism of electrons and holes to surface states from near‐surface Al0.3Ga0.7As/GaAs quantum wells has been investigated by steady‐state and time‐resolved photoluminescence spectroscopy, near liquid‐helium temperature, of the excitonic e1‐hh1 transition in the well. The ensemble of the data, taken over a wide range of optical excitation levels, for various values of the tunneling‐barrier thickness, and before and after passivation of the surface by hydrogen, allows a description both of the details of the tunneling mechanism and of the character and behavior of relevant surface states. The main results are summarized as follows: (i) steady‐state tunneling is ambipolar, namely, separate for electrons and holes, rather than excitonic; (ii) Spicer’s advanced unified defect model for an oxidized GaAs surface, antisite‐As donors as dominating surface traps, provides an appropriate description of the state distribution at the interface between AlGaAs and its oxide; (iii) hole accumulation in sur...

Hydrogen passivation of interface defects in GaAs/AlAs short‐period superlattices

We have observed an increase of overall luminescence intensity and carrier lifetimes of GaAs/AlAs short‐period superlattices after exposure of the samples to low‐energy hydrogen ion‐gun irradiation. We conclude from our data that interface defects which may become important as nonradiative recombination centers in short‐period superlattices with only a few monolayers period can be passivated by hydrogen.

Giant photoluminescence enhancement in deuterated highly strained InAs/GaAs quantum wells

The photoluminescence of InAs/GaAs pseudomorphic single quantum wells, of width 1, 1.2, and 1.6 monolayers, is studied before and after diffusion of monoatomic deuterium into the samples. The luminescence shows a red shift for increasing nominal well width, suggesting an interface roughness on a scale much smaller than the exciton size. The luminescence efficiency increases by several orders of magnitude after sample deuteration. A discussion about the origin of radiative recombination in these heterostructures, before and after deuteration, is also given.

Atomic equilibrium concentrations in (InGa)As quantum dots

In InxGa1−xAs quantum dots (QDs), the plot of peak emission energies versus the total amount of indium exhibits a well defined pattern, which is independent of the indium concentration. Moreover, photoluminescence spectra of InAs QDs grown by atomic layer molecular beam epitaxy (ALMBE) roughly coincide with those of In0.5Ga0.5As QDs grown by metalorganic vapor phase deposition. We suggest that the total amount of In rather than the nominal In concentration determines the emission energy of these two sets of QDs, and that In interdiffusion is rather strong in ALMBE growth.