N. A. Bert

Low-threshold injection lasers based on vertically coupled quantum dots

We have fabricated and studied injection lasers based on vertically coupled quantum dots (VECODs). VECODs are self-organized during successive deposition of several sheets of (In,Ga)As quantum dots separated by thin GaAs spacers. VECODs are introduced in the active region of a GaAs-A1GaAs GRIN SCH lasers. Increasing the number of periods (N) in the VECOD leads to a remarkable decrease in threshold current density ( ~ 100 A/cm 2 at 300 K for N = 10). Lasing proceeds via the ground state of the quantum dots (QD) up to room temperature. Placing the QD array into an external AIGaAs--GaAs quantum well allows us to extend the range of thermal stability of threshold current density (To = 350 K) up to room temperature. Using (In,Ga)As-(A1,Ga)As VECODs in combination with high temperature growth of emitter and waveguide layers results in further reduction of threshold current density (60-80 A/cm 2, 300 K) and increase in internal quantum efficiency (70%). Room temperature continuous wave operation (light output 160 mW per mirror) and lasing via the states of QDs up to I = (6-7) Ith have been demonstrated.

Optical and structural properties of InAs quantum dots in a GaAs matrix for a spectral range up to 1.7 μm

We demonstrate the possibility of extending the spectral range of luminescence due to InAs quantum dots (QDs) in a GaAs matrix up to 1.7 μm. Realization of such a long wavelength emission is related to formation of lateral associations of QDs during InAs deposition at low substrate temperatures (∼320–400u200a°C).

Volmer–Weber and Stranski–Krastanov InAs-(Al,Ga)As quantum dots emitting at 1.3 μm

Quantum dots (QDs) formed on GaAs(100) substrates by InAs deposition followed by (Al,Ga)As or (In,Ga,Al)As overgrowth demonstrate a photoluminescence (PL) peak that is redshifted (up to 1.3 μm) compared to PL emission of GaAs-covered QDs. The result is attributed to redistribution of InAs molecules in the system in favor of the QDs, stimulated by Al atoms in the cap layer. The deposition of a 1 nm thick AlAs cover layer on top of the InAs–GaAs QDs results in replacement of InAs molecules of the wetting layer by AlAs molecules, leading to a significant increase in the heights of the InAs QDs, as follows from transmission electron microscopy. This effect is directly confirmed by transmission electron microscopy indicating a transition to a Volmer–Weber-like QD arrangement. We demonstrate an injection laser based on this kind of QDs.

Control of the emission wavelength of self-organized InGaAs quantum dots: main achievements and present status

Recent achievements in controlling the electronic spectrum of InAs-based quantum dots (QDs) formed by self-organization phenomena during the initial stages of strained layer epitaxy are reviewed. Three different ways to exercise this control are discussed, based on variation of QD size with the amount of QD material deposited, tuning of the electronic levels in QDs by changing the matrix bandgap, and electronic coupling of neighbouring QDs vertically stacked in the growth direction. Possibilities to prevent thermal evaporation of carriers out of QD states and to tune the emission wavelength in the range 0.85-1.3 µm on GaAs substrates and up to 2 µm on InP substrates are demonstrated.

Local stresses induced by nanoscale As–Sb clusters in GaAs matrix

Microstructure of GaAs films grown by molecular-beam epitaxy at low temperature and delta doped with Sb was studied by transmission electron microscopy. The material contained 0.5 at.u200a% excess of arsenic that precipitated during post growth anneals. The Sb δ doping was found to strongly affect the microstructure of precipitates (clusters) and their ripening rate upon annealing. Segregation of Sb impurity in the clusters was revealed. In contrast to the well known pure As clusters, the As–Sb clusters induced strong local deformations in the surrounding GaAs matrix. Until a threshold diameter of 7–8 nm the clusters and surrounding matrix were coherently strained. Larger clusters were associated with dislocation loops of interstitial type. The cluster-loop orientation relationships were determined. Relaxation of local strains by formation of the dislocation loops was studied both experimentally and theoretically.

Enhanced As–Sb intermixing of GaSb monolayer superlattices in low-temperature grown GaAs

As–Sb compositional intermixing was studied by transmission electron microscopy (TEM) in GaAs films grown by molecular-beam epitaxy at low temperature (LT) and δ doped with antimony. The TEM technique was calibrated by imaging the as-grown films with δ layers containing various amounts of Sb. The calibration allowed us to deduce the effective As–Sb interdiffusion coefficient from apparent thickness of the Sb δ layers in the films subjected to isochronal anneals at 400–600u200a°C. The As–Sb intermixing in LT GaAs was found to be much enhanced when compared to conventional material. Its temperature dependence yields a diffusion coefficient of DAs–Sb=2×10−14u200aexp(−0.62±0.15u200aeV/kt)u200acm2u200as−1. Since the kick-out mechanism operating under equilibrium conditions is valid for As–Sb interdiffusion in GaAs, the enhanced intermixing was attributed to an oversaturation of arsenic self-interstitials in the LT GaAs films. The effective activation energy for As–Sb interdiffusion in LT GaAs seems to be reasonably close to the m...

Investigation of dislocation loops associated with As–Sb nanoclusters in GaAs

A model is developed for the elastic stress relaxation via formation of prismatic dislocation loops in a vicinity of the As–Sb nanoclusters built in GaAs matrix. The model is based on the experimental investigation of the microstructure of the As–Sb nanoclusters, which can be produced in Sb-doped GaAs films by the molecular-beam epitaxy at low temperature and subsequent anneal. A strong anisotropic mismatch between the As–Sb nanoclusters and GaAs matrix has been revealed by transmission electron microscopy. This mismatch was proven to be a reason for the formation of nanoscale dislocation loops near the nanoclusters. Our theoretical model explores the elastic properties of an inclusion with uniaxial dilatation. For such inclusions, the elastic stresses and stored energy are determined in a closed analytical form. The theoretical analysis predicts a specific nonlinear dependence of the dislocation loop diameter on the cluster diameter, which fits well the experimentally observed one. It is demonstrated tha...

Fabrication of submicron topology with ion- and neutral-beam etching

Recently the microelectronics devices fabricated on the basis of the topological structures with conic or point elements are applied widely. The traditional method of formation of the structures using an electron beam lithography, a chemical or ion beam (plasma) etching has some disadvantages. For example, the chemical etching of the submicron elements has rather low reproducibility of the geometry profile. It is clearly displayed in the formation of relief in the mesa of the submicron size. The electronic lithography and the polishing ion etching method of the cone formation is complicated due to the problem with the stability of the masking material and the resolution. What is more important the problem of the compatibility with other technological operations of the microelectronics fabrication. The authors offer the simple and reproducible method of fabrication of the submicron structure of a cone type. It bases on the formation of the surface relief by the neutral projectiles sputtering. The present paper reports the possibility of the cone relief fabrication on the wide surface and in the narrow submicron mesa by the argon neutral bombardment.

Majority carrier accumulation in low-temperature-grown GaAs layer inserted into n-and p-type matrices

Accumulation of electrons and holes has been revealed by capacitance-voltage technique in As-cluster containing GaAs layers sandwiched between n-type or p-type GaAs buffers. As a result of the majority carrier accumulation, a large depletion region forms in adjacent buffers. Simulation of the capacitance-voltage characteristics based on numerical solution of the Poisson equation showed the concentration of accumulated charge to be as high as 1/spl times/10/sup 12/ cm/sup -2/ which is comparable with the concentration of As clusters determined from transmission electron microscopy study.

Influence of Antimony Doping on Nanoscale Arsenic Clusters and Dislocation Loops in Low-Temperature Grown Gallium Arsenide Films

Transmission electron microscopy was employed to study the microstructure of GaAs films grown by molecular-beam epitaxy at low temperature and delta-doped with Sb. Thus obtained material contained 0.5 at.% of excess arsenic that precipitates upon post-growth anneals. The Sb doping was found to strongly affect the microstructure of arsenic clusters and their ripening rate upon annealing. Segregation of Sb impurities in the As clusters was revealed. In contrast to the well known pure As clusters, the As-Sb clusters induced strong local deformations in the surrounding GaAs matrix. Relaxation of these deformations resulted in formation of dislocation loops, which was studied both experimentally and theoretically.