Wlodzimierz Nakwaski

Oxidation kinetics of AlAs and (AlGa)As layers in GaAs-based diode laser structures: comparative analysis of available experimental data

An understanding of the kinetics of the steam oxidation of AlAs and (AlGa)As layers is crucial to maintain good control of the process of manufacturing modern GaAs-based diode microresonator vertical-cavity surface-emitting lasers (VCSELs). Mathematically, the process has been described in our previous publications. Our theoretical equations contain, however, a few adjustable parameters, which should be found experimentally. Therefore, in this paper, an extensive analysis of existing experimental results (including ours) is performed. It enables us to extract the physical parameters describing oxidation kinetics for the AlAs and (AlGa)As oxidation process. While some scattering in the parameters is found from measurements reported by different groups, the same general trends are observed. These results support our physical understanding of the oxidation process. Nevertheless, the oxidation process still needs some experimental investigations to establish all conditions influencing its kinetics and to enable reproducible oxidation results obtained under assumed identical technological conditions. It has been confirmed that currently theoretical modelling cannot offer a reasonable a priori control of the oxidation process without a careful calibration of a given oxidation set-up and procedure. However, the simulation may enable determination of safe limits of the process, especially important in the fabrication of small-size microresonator VCSELs.

Self-consistent thermal-electrical modeling of proton-implanted top-surface emitting semiconductor lasers

A new comprehensive thermal-electrical self-consistent model of proton-implanted top-surface-emitting lasers is described. The model is applied to study thermal characteristics of GaAs/AlGa As/AlAs devices with the active-region diameter of 35 micrometers . Our results show that intense heating occurs at pumping currents exceeding 4X threshold. Long tails of radial temperature distribution will result in severe thermal crosstalk if integration of these devices into densely packed 2D arrays were to be attempted. Minimization of electrical series resistance is shown to be very important for improving the device performance.

Design of InGaN/GaN/AlGaN VCSELs using the effective frequency method

The effective frequency method is applied to analyze complex optical structures of nitride-based intracavity-contacted VCSELs. Numerical results indicate that higher-order lateral modes would be favored in nitride-based intracavity-contacted VCSELs due to strong current-crowding effect. A semi-transparent contact design is proposed to provide more uniform gain distribution within the active region and maintain lasing in the fundamental lateral mode.

Thermal and molecular stresses in multi-layered structures of nitride devices

Mechanical stresses considerably influence the operation of nitride devices because of stress-related effective masses, band gaps and piezoelectric phenomena, to name the most important effects. However, even the most advanced models of those devices rarely contain mechanical parts, mostly because of their complexity and time-consuming numerical calculation procedures. In the present paper, a new analytical mechanical model of nitride devices is presented which enables simple determination of stress fields in their multi-layered volume. The validity of the model has been confirmed by comparing its results with those obtained with the aid of rigorous numerical finite-element calculations.

A novel diagonal-current injection VCSEL design proposed for nitride lasers

An advanced three-dimensional optical-electrical model has been used for nitride lasers to design a novel vertical-cavity surface-emitting laser (VCSEL) configuration with a diagonal-current injection (DCI) mechanism. The design has been optimized for the lowest room-temperature (RT) lasing threshold which has been found to be similar to RT thresholds of advanced arsenide and phosphide VCSELs. The DCI nitride VCSEL demonstrates very promising anticipated RT threshold characteristics. Its optical structure is very selective: the fundamental mode exhibits distinctly the lowest threshold. In addition, the lasing threshold has been found to be exponentially proportional to the barrier width.

Thermal properties of etched-well surface-emitting diode lasers and two-dimensional arrays

A self-consistent thermal-electrical model of etched-well GaAs/A1GaAs doubleheterostructure VCSELs is used to optimize an individual device design with the goal of reducing the relative power loss due to heating and maximizing the optical output power. An optimal active-region diameter is determined, such that the excess of supplied power over the cw lasing threshold power at the corresponding active-region temperature is maximum. The role of other structural parameters, such as thicknesses and doping levels of both cladding layers, is also discussed. The single-emitter analysis is then modified to study very-large-size two-dimensional (2-D) VCSEL arrays. Severe crosstalk is predicted for closely packed arrays and conditions are identified for an array configuration that would not suffer from the excessive cross-talk penalty.

Structure optimisation of modern GaAs-based InGaAs/GaAs quantum-dot VCSELs for optical fibre communication

Room-temperature (RT) continuous-wave (CW) performance of modern 1300-nm oxide-confined In(Ga)As/GaAs quantum-dot (QD) vertical-cavity surface-emitting diode lasers (VCSELs) taking advantage of many QD sheets is investigated using our comprehensive self-consistent simulation model to suggest their optimal design. Obviously, quantum dots should be as uniform as possible and as dense as possible to ensure high enough optical gain. Besides, our simulation reveals that efficient and uniform current injection into VCSEL active regions necessary to enhance excitation of the desired fundamental LP01 mode is accomplished in the VCSEL configuration with the broad-area bottom contact and the ring upper one as well as with the oxide aperture localized within the first period of the upper p-type DBR. The doping of the DBR mirrors is chosen as a compromise between their high enough electrical conductivity and low enough free-carrier absorption. The oxide aperture is additionally introducing the radial optical waveguiding. Moreover, our analysis has been concluded that VCSEL active regions should be composed of at least 9 QD sheets to acquire efficient RT CW operation. Furthermore, rather longer optical cavities are recommended in this case because localization of QD sheets should be adjusted to the anti-node positions of the optical cavity standing wave.

Thermal crosstalk in arrays of proton-implanted top-surface-emitting lasers

An integrated device and package 3D model is developed to computationally investigate the thermal crosstalk in arrays of proton-implanted top-surface emitting lasers. A self- consistent electro-thermo-opto model is employed for the device. The anisotropic thermal property is considered for the package model. Temperature dependency of critical device and material properties is included, as well as multiple heat generation mechanisms. Effects of spacing on lasing performance and non-uniformity of VCSEL arrays are found significant. Thermal crosstalk becomes worse for increased sizes and packaging densities of laser arrays. Degraded laser performance is found due to the thermal crosstalk, especially for the lasers closest to the center of the array package.

Effects of carrier diffusion on thermal properties of proton-implanted top-surface-emitting lasers

A comprehensive, 3D, thermal-electrical self-consistent finite-element model is described and used to investigate thermal properties of GaAs-AlGaAs proton-implanted top-surface-emitting lasers. Special attention is paid to carrier diffusion within the layer containing the active region and to its influence on temperature profiles. In the model, an analytical approximation is used to describe the 3D current spreading between the annular top contact and the broad- area bottom contact. Temperature dependence of many device and material parameters is included. Multiple heat sources are taken into consideration. The carrier diffusion equation, including injection-current generation, ambipolar diffusion as well as bimolecular and spontaneous recombination terms, is solved numerically using the finite-element method for the layer containing the active region. The results indicate that carrier diffusion strongly influences the distribution of main heat sources. As a result, both current-spreading and heat- spreading phenomena are modified.

Effective thermal conductivity analysis of vertical-cavity top-surface-emitting lasers with semiconducting Bragg mirrors

A new approximate analytical approach is developed and applied to investigate thermal properties of top-surface-emitting vertical- cavity diode lasers (VCSELs) mounted substrate-down. Multilayer structure of distributed-Bragg reflectors is taken into account by considering anisotropic thermal conductivity. Design conditions for minimal thermal resistance in short- and long- wavelength systems are specified for devices with various active- region diameters. Our results indicate that difficulties with obtaining the cw operation of long-wavelength VCSELs are primarily associated with intrinsic properties rather than with their thermal resistance.