Software and hardware for intelligent robots

Various architectures and their respective software for Hierarchically Intelligent Robots are discussed. They conform to the Principle of Increasing Precision with Decreasing Intelligence by following a three-level structure. The architecture of the organization and coordination levels is presented here and their algorithms are outlined

Density matrix modelling of Ge/GeSi quantum cascade terahertz lasers

The prospect of making silicon-based quantum cascade lasers (QCLs) has attracted considerable research interest in recent years, due to their significant potential advantages including a mature Si processing technology, the prospect of integration with Si microelectronics, and superior thermal performance to that of III–V devices. Amongst various proposed designs, with different material compositions and substrate orientations, (001)-oriented n-type Ge/GeSi structures utilising L-valley intersubband transitions appear to be the most promising due to a small quantisation effective mass, and hence large optical matrix elements, and practically realisable layer widths. While all the previous simulations for group IV-based QCLs used the rate equation model, this neglects the coherence effects and is of limited usefulness for predicting QCL performance, particularly in the terahertz range. In this work, a quantum-mechanics transport model for Ge/SiGe QCL simulation has been developed, using the density matrix (DM) approach. In contrast to the existing DM formulations which have been used to simulate III-V based QCLs, the present model accounts for the role of all the QCL states in coherent transport, or in optical transitions, or both. The simulator includes all the principal scattering mechanisms in Ge/SiGe heterostructures: intravalley scattering due to interface roughness, alloy disorder, ionized impurities, electron-acoustic phonon and optical phonon interactions, and intervalley phonon scattering. It was used in conjunction with a semi-automated optimization algorithm to identify heterostructure designs for bound-to-continuum Ge/GeSi QCLs, and to compensate for the gain-reduction associated with diffuse Ge/GeSi interfaces

Intersubband carrier scattering in n- and p-Si/SiGe quantum wells with diffuse interfaces

Scattering rate calculations in two-dimensional Si/Si1−xGex systems have typically been restricted to rectangular Ge profiles at interfaces between layers. Real interfaces however, may exhibit diffuse Ge profiles either by design or as a limitation of the growth process. It is shown here that alloy disorder scattering dramatically increases with Ge interdiffusion in (100) and (111) n-type quantum wells, but remains almost constant in (100) p-type heterostructures. It is also shown that smoothing of the confining potential leads to large changes in subband energies and scattering rates and a method is presented for calculating growth process tolerances

Extended density matrix model applied to tall barrier quantum cascade lasers

Quantum cascade lasers (QCLs) are promising sources of terahertz (THz) radiation that have applications such as security and medical screening. While optical output power has recently exceeded 1 W, their highest operating temperature is currently limited to ~200 K due to mechanisms such as thermal back filling and non-radiative phonon emission between lasing states. Another possible cause of performance degradation is parasitic leakage currents over barriers into continuum states as subband electron temperatures increase with lattice temperature. Novel designs with new injection schemes remain an intensive research area and new efforts are being made assuming that barrier heights no longer need to be constant. A possible advantage of this is using tall barriers to reduce the leakage current, and in this work we present a theoretical study of recent experimental evidence supporting this. Interface roughness (IFR) scattering scales with the conduction band discontinuity squared and the calculations also assume a typical correlation length Λ and root mean roughness value Δ which are related to growth quality of the individual sample. We take typical values of Λ=60 Å and Δ=3 Å for these parameters. The QCL gain and current output characteristics are calculated using an extended density matrix solver which models transport through the injection barrier coherently. We obtain similar current and gain values at resonance for both structures, indicating that the experimentally observed reduction in current density could be accredited to the reduction of parasitic current leakage. Additionally, this work attempted a similar design with all AlAs barriers which did not lase and it was conjectured that this was due to excessive IFR scattering as well as increased susceptibility to monolayer fluctuations with thinner layers. Our model, which accounts for the lifetime broadening in the gain calculation, confirms that modifying the IFR parameters to Λ=100 Å and Δ=1 Å (i.e. unrealistically sharp interfaces) leads to a significant improvement in performance as shown in Figure 1. We extend this work by proposing designs which aim to balance leakage current reduction and excessive scattering to achieve higher operating temperatures

Frequency tunability and spectral control in terahertz quantum cascade lasers with phase-adjusted finite-defect-site photonic lattices

We report on the effect of finite-defect-site photonic lattices (PLs) on the spectral emission of terahertz frequency quantum cascade lasers, both theoretically and experimentally. A central π-phase adjusted defect incorporated in the PL is shown to favor emission selectively within the photonic bandgap. The effect of the duty cycle and the longitudinal position of such PLs is investigated, and used to demonstrate three distinct spectral behaviors: single mode emission from devices in the range 2.2−5 THz, with a side-mode suppression ratio of 40 dB and exhibiting continuous frequency tuning over >8 GHz; discrete tuning between two engineered emission modes separated by ~40 GHz; and, multiple-mode emission with an engineered frequency spacing between emission lines

Theory and design of quantum cascade lasers in (111) n-type Si/SiGe

Although most work towards the realization of group IV quantum cascade lasers (QCLs) has focused on valence band transitions, there are many desirable properties associated with the conduction band. We show that the commonly cited shortcomings of n-type Si/SiGe heterostructures can be overcome by moving to the (111) growth direction. Specifically, a large band offset and low effective mass are achievable and subband degeneracy is preserved. We predict net gain up to lattice temperatures of 90 K in a bound-to-continuum QCL with a double-metal waveguide, and show that a Ge interdiffusion length of at least 8 Å across interfaces is tolerable

Influence of barrier height on interface roughness scattering and coherent transport in AlGaAs quantum cascade lasers

Quantum cascade lasers (QCLs) are promising sources of terahertz (THz) radiation that have applications such as security and medical screening. While optical output power has recently exceeded 1 W, their highest operating temperature is currently limited to ~200 K due to mechanisms such as thermal back filling and non-radiative phonon emission between lasing states. Another possible cause of performance degradation is parasitic leakage currents over barriers into continuum states as subband electron temperatures increase with lattice temperature. Novel designs with new injection schemes remain an intensive research area and new efforts are being made assuming that barrier heights no longer need to be constant. A possible advantage of this is using tall barriers to reduce the leakage current, and in this work we present a theoretical study of the effects of increased barrier heights on transport between states in the structure. Similar to previous efforts, we initially restrict the modification of barrier height to the injection barrier; these are typically the thickest in THz QCLs and allow the reduced barrier widths necessary for AlAs barriers to remain above 1 ML