Electronic structure of functional region of the interband cascade infrared photodetector designed to operate with cut-off wavelength of ~10.7 μm is calculated using second nearest neighbor sp3s* tight binding model with spin-orbit interactions. The effective bandgaps and alignment of the band edges are presented. Lattice mismatch of each region to the GaSb substrate is determined. The influence of InAs incorporation into the InSb interfacial layer is investigated. It is shown that up to 5% InAs addition to InSb interface in InAs/GaSb superlattice absorber is allowed if efficient carrier transport is to be kept. Furthermore, interface of up to x=2% InAsxSb1-x can be used in the proposed InAs/AlSb superlattice intraband relaxation region to keep its proper operation.
Measurements of low-frequency noise of type-II superlattice detectors designed for mid-IR wavelengths are used to determine noise limitations, calculate the real detectivity, and study 1/f noise-current correlations in these devices. No 1/f noise connected to the diffusion current is found as opposed to the generation-recombination, shunt, and tunneling currents. The contribution from the shunt current to 1/f noise can be so large that shunt-originated noise dominates in the high-temperature region, in which current is limited by the generation-recombination and diffusion components. It is also demonstrated that devices made of type-II superlattice contain traps generating random processes with thermally activated kinetics, and the activation energies of these traps are determined.
The essential steps in simulations of modern separate absorption, grading, charge, and multiplication avalanche photodiode and their results are discussed. All simulations were performed using two commercial technology computer-aided design type software packages, namely Silvaco ATLAS and Crosslight APSYS. Comparison between those two frameworks was made and differences between them were pointed out. Several examples of the influence of changes made in individual layers on overall device characteristics have been shown. Proper selection of models and their parameters as well as its significance on results has been illustrated. Additionally, default values of material parameters were revised and adequate values from the literature were entered. Simulated characteristics of optimized structure were compared with ones obtained from measurements of real devices (e.g., current-voltage curves). Finally, properties of crucial layers in the structure were discussed.
For high-bit rate and long-haul receivers in optical telecommunication systems the avalanche photodiodes are preferred since they offer an improvement of the receiver sensitivity by several decibels. Recently critical sensing and imaging applications stimulated development of modified avalanche photodiodes structures operating in 1.55 μm spectral range. For these devices speed is not further critical. Instead, very low current densities and low multiplication noises are the main requirements. The most advanced structure of avalanche photodiodes is known as Separate Absorption, Grading, Charge and Multiplication (SAGCM). In the present work the performance of uncooled InGaAs/InAlAs/InP avalanche photodiodes operating near 1.55 μm has been studied theoretically. Device modeling based on advanced drift - diffusion model with commercial Crosslight APSYS software has been performed. Conventional SAGCM avalanche photodiodes as well as devices with a relatively thick undepleted p-type InGaAs absorption region and thin InAlAs multiplication layer have been considered. This type of avalanche photodiodes enables to increase device quantum efficiency, reduce dark current and eliminate impact ionization processes within absorbing layer. Extensive calculations allowed for detailed analysis of individual regions of the device and determination of their influence on diode characteristics.
High-speed, high-sensitivity, avalanche photodiodes operating at 1.55 μm spectral range have been utilized in modern
long-haul and high-bit rate optical communication systems. Related research was focused on developing detectors with
minimized excess noise and maximized gain-bandwidth product.
Recently imaging and critical sensing applications stimulated development of modified avalanche photodiode structures
operating in 1.55 μm spectral range. For these devices speed is no more critical. Instead, very low current densities and
low multiplication noise are the main requirement.
In the present work the performance of uncooled InGaAs/InAlAs/InP avalanche photodiodes operating near 1.5 μm has
been studied. Device modeling based on advanced drift and diffusion model have been performed with commercial
Crosslight APSYS simulator. Conventional separate absorption, charge and multiplication (SACM) avalanche
photodiode as well as devices with a relatively thick undepleted p-type InGaAs absorption region and thin InAlAs
multiplication layer have been considered. The latter type of APD structure enables to increase device quantum
efficiency, reduce dark current and eliminate impact ionization processes within absorbing layer.
Resonant cavity enhanced photodetectors are promising candidates for applications in high-speed optical communications due to their high quantum efficiency and large bandwidth. This is a consequence of placing the thin absorber of the photodetector inside a Fabry-Perot microcavity so the absorption could be enhanced by recycling the photons with resonance wavelength.
The performance of uncooled resonant cavity enhanced InGaAs/InAlAs photovoltaic devices operating near 1.55 μm has been studied both theoretically and experimentally. The analyses include two different types of structures with cavity end mirrors made of semiconducting and metallic reflectors as well as semiconducting and hybrid (dielectric Si3N4/SiO2 + metal) Bragg reflectors. Optimization of the device design includes: absorption layer thickness, position of absorption layer within the cavity and number of layers in distributed Bragg reflectors.
Dependence of absorption on wavelength and incidence angle are discussed. Various issues related to applications of resonance cavity enhanced photodiodes in optical systems are considered.
Practical devices with metallic and hybrid mirrors were fabricated by molecular beam epitaxy and by microwave-compatible processing. A properly designed device of this type has potential for subpicosecond response time.
Fast refractive microlenses are increasingly important as optical concentrators for uncooled infrared photodetectors. They are used in purpose to improve performance and speed of response. Refractive microlenses formed directly onto semiconductor materials draw much attention because they facilitate monolithic integration with active element of infrared photodetector. Gallium arsenide due to its superior optical and mechanical properties is a material of choice for fabrication of microlenses. We have developed process for fabrication of GaAs refractive microlenses monolithically integrated with InGaAs and HgCdTe photodetectors, both as single element devices and two-dimensional arrays. Specially designed machine tool has been used for preparation of relatively large single spherical GaAs microlenses with 0.5 mm-10 mm diameter. The microlens-detector arrays were prepared using a combination of ion milling and wet chemical etching. The typical process involves one photolithography, one ion milling and one or two chemical etching steps. More advanced procedures have also been proposed to improve quality of the lenses. The lenses can be optimized as optical concentrators for IR photodetectors with circular, square, rectangular and other geometries. This process is especially convenient for fabrication of lenses with size less than 50 μm, but larger lenses with size exceeding 300 μm can be prepared as well with some modifications of the fabrication procedures.
We report here uncooled and thermoelectrically cooled InAs photodetectors designed for fast and sensitive detection of IR radiation. This has been achieved by the use of a complex architecture of the device that ensures reduced thermal generation of charge carriers, fast diffusion and drift transport of photogenerated carriers across the absorber region, a low series resistance, and a low capacitance. In addition, the device are monolithically immersed to GaAs hyperhemispherical microlenses that reduces capacitance by more than two orders of magnitude in comparison to non-immersed devices of the same optical area. As a result, the optimized devices are characterized by picosecond response time.
InAs-based alloys has been the subject of intense studies over the past few years, due to its importance as a material for photovoltaic detectors that can be tuned to optimum performance at any wavelength within 1 - 3.6 μm spectral range. We report here new results concerning electrical and photoelectrical characteristics of epitaxial InAs photovoltaic detectors optically immersed to GaAs microlenses and operating at near room temperatures. The devices are based on epitaxial heterostructures grown directly on thick GaAs substrates by molecular beam epitaxy. The performance of the detectors has been significantly improved by the use of multilayer heterostructures with optimized doping and band gap profiles, improved crystalline perfection of the heterostructure, improved processing resulting in reduction of parasitic series resistance and increase of shunt resistance of the device. Additionally, monolithic optical immersion to GaAs substrates has been applied. The devices have been characterized by current and capacitance measurements performed at steady-state and time dependent conditions at temperatures 77-300 K. The measurements provide an indication that tunneling and defect- assisted rather than band-to-band processes limit performance of the devices. The measurements of capacitance and series resistance of devices show picosecond RC time constant even for devices with relatively large apparent area (1 mm2). This is due to the reduction of junction capacitance by two orders of magnitude in comparison with non-immersed devices of the same optical area. The unique properties make possible more widespread applications of the optically immersed InAs devices in various infrared systems.
The effect of high temperature-high pressure (HT-HP) treatment of semiconducting layers on their structural properties was investigated by X-ray methods. The changes of the strain state of the samples induced by the HT-HP treatment depend on the initial strain state and growth method of thin layers. Only for the layers obtained by MBE methods the change of strain state of layers was found. Decrease of the dislocation density was detected for relaxed InAs/GaAs layers after the treatment at 673K - 1.1 Gpa for 1 h. For strained AlGaAs/GaAs samples the pressure - induces stress is responsible for creation of dislocation.
The performance of uncooled InxGa1-xAs photovoltaic devices operating in the 2-3.6 micrometers spectral range has been studied both theoretically and experimentally. Various multilayer homo- and heterojunction devices have been considered. Band-to-band Auger processes were taken into account as the dominant mechanisms of thermal generation and recombination. Calculations show that the best performance can be obtained in devices with lightly doped n-type absorber region as a result of the significant role of Auger S (spin-off band) process. The optimized heterostructures with near intrinsic n-type narrow gap InGaAs absorber were grown using molecular beam epitaxy. The performance of the present InGaAs devices remains significantly below theoretical limits. Better agreement can be observed for devices with longer cutoff wavelength as the result of increasing role of fundamental limitations and improved quality of materials with x approximately equal to 1. Improvement of performance by almost an order of magnitude has been achieved by the use of monolithic optical immersion. This has been accomplished by growth of the InGaAs heterostructures on thick SI GaAs substrates and by formation of the immersion lenses directly in the substrate. These devices are expected to find important applications in infrared systems which require better performance than existing uncooled detectors.
The structure of InGaAs (InAs) layers grown on InP (001) and GaAs (001) surfaces by Molecular Beam Epitaxy was studied by high-resolution x-ray diffractometry. The reciprocal lattice mapping and the rocking curve technique were used to determine distribution of misfit dislocations in the layers. Directional dependence of dislocation density in InGaAs strained layers grown at 2D growth mode was observed. It was found that anisotropic distribution of dislocations in the InGaAs layers resulted from development via bending in the interface plane of dislocations present in the InP substrate. Simultaneously, homogeneous distribution of dislocations in relaxed InAs layers grown on InP as well as GaAs substrates has been detected. At the initial stage these epitaxial layers were grown to 3D island mode. The reciprocal lattice maps confirm that coalescence of islands during the epitaxy generates dislocations, which in turn homogeneously distribute in the layer. It seems that the growth mode rather than lattice mismatch determines density of dislocations in InAs epitaxial layers grown on InP and GaAs substrates. However, lattice mismatch influences relaxation process in lattice-mismatched layers. Transport properties of relaxed InAs layers strongly depend on growth temperature.