B. J. Robinson

Improved InAsP metamorphic layers grown on an InP substrate using underlying InP grown at low temperatures

The use of an InP epitaxial layer grown at low temperatures before the growth of a step-graded InAsP metamorphic buffer has been shown to provide a large improvement in the crystal quality of the final metamorphic layer. The improvement is evidenced by over an order of magnitude increase in photoluminescence intensity as well as a large reduction of the mosaic spread and the overall tilt of the relaxed layers.

Photoreflectance investigations of quantum well intermixing processes in compressively strained InGaAsP∕InGaAsP quantum well laser structures emitting at 1.55μm

We have investigated the effects of interdiffusion and its technological parameters on the subband structure in compressively strained InGaAsP quantum wells (QWs) using photoreflectance and photoluminescence techniques. p-i-n laser structures with three QWs were grown by gas source molecular beam epitaxy and capped with dielectric films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition and annealed using a rapid thermal annealing process. A numerical real-time wave-packet propagation method including static electric field, strain in the wells and barriers, and error function interface diffusion modeling is used to calculate the transition energies for the diffused QWs. It has been shown that the shift of the energy levels due to the interdiffusion related changes of the well confinement potential profile is a consequence of two competing processes: a change of the well width and an effective increase of the band gap energy resulting in a net blueshift of all optical trans...

Enhanced bandgap blue-shift in InGaAsP multiple-quantum-well laser structures by low-temperature-grown InP

Quantum well intermixingxa0(QWI) in an InGaAsP multiple-quantum-wellxa0(MQW) laser structure is demonstrated using an InP epitaxial layer grown at 300xa0°C, by gas source molecular beam epitaxy, followed by rapid thermal annealing. Photoluminescence is used to compare the magnitude of the QWI process between low-temperature (LT)- and normal-temperature (NT, 470xa0°C)-grown InP layers as a function of both anneal temperature and time. For example, after an anneal at 780xa0°C, a large bandgap blue-shift of ~197xa0nm is observed in MQW structures capped with LT-InP as compared to an ~35xa0nm shift in identical structures capped with NT-InP. Also, the effect of the LT-InP capping is compared to NT-InP, capped with a dielectric (~100xa0nm of SiO2), following anneal at 800xa0°C for 60xa0s. This shows blue-shifts of ~243 and ~142xa0nm, respectively.

Intermixing of InGaAsP/InGaAsP quantum-well structures using dielectric films

The enhancement of quantum-well intermixing by SiOxNy and commercial spin-on glass dielectric films deposited on InP-based practical laser structures with all-quaternary active regions has been studied. Migration of semiconductor atoms into the dielectric films was observed using secondary ion mass spectrometry, and this migration behaviour has been correlated with the amount of band-gap change. Based on the results presented, it is suggested that the enhanced intermixing effect is due to the injection of group V interstitials into the underlying laser structure. These are formed at the dielectric/semiconductor interface as Ga and In atoms migrate into the dielectric layer. However, some interfacial process appears to influence the release of the interstitials from the interface region depending on the composition of the SiOxNy film.

Quantum well intermixing in InGaAsP laser structures using a low temperature grown InP cap layer

Quantum well intermixing (QWI) in a 1.55 µm InGaAsP laser-like structure has been enhanced using the defects incorporated in an InP capping layer grown at low temperature (below the congruent sublimation temperature) by molecular beam epitaxy and subsequently subjected to rapid thermal annealing. The structures used had quantum wells (QWs) and barrier layers with identical group III compositions so the inter-diffusion occurs only on the group V sub-lattice. This inter-diffusion is induced by the diffusion of P-interstitials that result from the dissociation of PIn anti-site defects that are present in large concentrations in the low temperature InP (LT-InP) layer. The magnitude of the QWI is determined by measuring the blueshift in the wavelength of room temperature photoluminescence emission from the QWs. It was found that the magnitude of the blueshift is dependent on the growth conditions of the LT-InP such that larger blueshifts are observed for LT-InP layers either grown at lower temperatures or with increasing P2 overpressures. These features correlate with the expected changes in the concentration of PIn defects with these changes in growth conditions. Also, there is a change in the rate of change in blueshift with the thickness of the LT-InP layer. For thin layers the rate of change of blueshift with thickness is rapid, but at a certain thickness a transition occurs to a lower rate of change with thickness. This transition thickness is temperature dependent such that the transition to the reduced rate occurs at larger thicknesses at higher anneal temperatures. This transition is interpreted as re-trapping of the P-interstitials in the LT-InP by the In-vacancies resulting from the PIn dissociation which leads to a reduced rate of supply of P-interstitials into the underlying laser structure.

Probing the indium clustering in InGaAs∕GaAs quantum wells by room temperature contactless electroreflectance and photoluminescence spectroscopy

Room temperature contactless electroreflectance (CER) supported by photoluminescence (PL) has been proposed as a fast and nondestructive ex situ technique for testing the effect of atom clustering in quantum wells (QWs). The indium clustering in InGaAs∕GaAs QWs was achieved by increasing the growth temperature. It has been shown that this effect causes significant changes in the spectral response. While the line shape of the GaAs-related CER feature remains unaffected there appear broad resonances similar to those for naturally inhomogeneous ensemble of self-assembled quantum dots instead of sharp and intensive lines characteristic for QWs. Additionally, the PL signal exhibits a quantum-dot-like behavior as well, i.e., strongly broadened Gaussian-like peaks with linear excitation power dependence on their intensity and the occurrence of the state filling effect for high excitation.

InAs quantum wire induced composition modulation in an In0.53Ga0.37Al0.10As barrier layer grown on an InP substrate

Composition modulations are observed by transmission electron microscopy in In0.53Ga0.37Al0.10As barrier layers that overgrow both single- and multilayer InAs quantum wire structures grown on an InP substrate. Indium-rich (gallium-deficient) regions were observed in the region of the barrier layer lying directly above individual quantum wires, while indium-deficient (gallium-rich) regions were detected in the barrier above the gaps between adjacent underlying quantum wires. The magnitude of such modulation was typically 7% (atomic percent) for both indium and gallium as estimated from the energy dispersive x-ray analysis. The origin of such composition modulations was determined by modeling the chemical potential distribution for indium and gallium on the growth front of the barrier layer at the initial capping stage of the quantum wires with finite element simulations. It is found that the number and positions of the indium-rich regions are determined by the combined effects of strain and surface energy ...

Comparison of quantum well intermixing in GaAs structures using a low temperature grown epitaxial layer or a SiO2 cap

Studies of quantum well intermixing (QWI) have been performed on Al-free GaAs based structures in which InGaAs quantum wells (QWs) have either GaAs barriers or InGaAsP quaternary barriers such that the barrier-QW compositional change consists solely of a group III change (GaAs barrier) or a group V change (quaternary barrier). These structures permit identification of the sublattice upon which intermixing occurs when the point defects responsible for the QWI are created by annealing in the presence of a (conventional) dielectric (SiO2) cap layer versus an InGaP cap layer grown at low temperature (LT-InGaP). QWI occurs on the group III sublattice via vacancy diffusion in both the LT-InGaP and SiO2 capped samples with identical group V compositions in the QW and barrier layers. For the samples with identical group III compositions for the QW and barriers, QWI is only observed with the LT-InGaP capping and occurs via group V interstitial diffusion and P–As exchange in the QW.

Photoreflectance study of the interdiffusion effects in the InGaAsP-based quantum well laser structures

Abstract Photoreflectance spectroscopy has been used to study interdiffusion effects of dielectric-capped, rapid-thermal-annealed InGaAsP-based quantum well laser structures grown by gas source molecular beam epitaxy. Post-growth modification-induced changes of the quantum well shape influence its energy levels. For the processed laser structures a blue shift of ground and excited state transitions has been observed. It has been found that the energy difference between the two lowest heavy hole levels decreases approximately linearly with the blue shift of the ground state transition.

Investigation of dielectric cap induced intermixing of InxGa1−xAsyP1−y/InP quantum well laser structures by photoreflectance and photoluminescence

Abstract The quantum well intermixing (QWI) of 1.55 μm laser structure through thermal treatment, utilising various cap layers, has been investigated by both photoluminescence (PL, emission-like) and photoreflectance (PR, absorption-like) experiments. A blue shift of the QW ground state transition has been observed for post-grown-modified laser structures. The influence of the cap stoichiometry on QWI has been analysed. It has been found that the magnitude of the blue shift strongly depends on the stoichiometry of the dielectric film. In PR, besides the blue shift of the fundamental transition, a blue shift of the excited state transitions has been observed. The blue shift is evidently stronger for the ground state transition than for the higher energy ones. The character of the recombination process at room temperature has been found as free carrier recombination for both as-grown- and post-grown-modified laser structures.