Intermixing of InGaAs/GaAs quantum wells and quantum dots using sputter-deposited silicon oxynitride capping layers

Various approaches can be used to selectively control the amount of intermixing in III-Vquantum well and quantum dotstructures. Impurity-free vacancy disordering is one technique that is favored for its simplicity, however this mechanism is sensitive to many experimental parameters. In this study, a series of silicon oxynitride capping layers have been used in the intermixing of InGaAs/GaAs quantum well and quantum dotstructures. These thin films were deposited by sputter deposition in order to minimize the incorporation of hydrogen, which has been reported to influence impurity-free vacancy disordering. The degree of intermixing was probed by photoluminescence spectroscopy and this is discussed with respect to the properties of the SiOxNyfilms. This work was also designed to monitor any additional intermixing that might be attributed to the sputtering process. In addition, the high-temperature stress is known to affect the group-III vacancy concentration, which is central to the intermixing process. This stress was directly measured and the experimental values are compared with an elastic-deformation model.This work has been made possible with access to the ACT Node of the Australian National Fabrication Facility and through the financial support of the Australian Research Council

Post-growth spectral tuning of InGaAs/GaAs quantum dot infrared photodetectors

Infrared photodetectors are essential in many industries and modern applications require devices with enhanced capabilities. High-performance detectors can be used for spectroscopy in medicine and environmental monitoring. Imaging scenarios include the identification of military targets and predicting equipment failure. These thermal imaging systems benefit from multicolour photodetectors. For example, some heat-seeking missiles incorporate two-colour HgCdTe arrays to discern target aircraft from decoy flares. Hyperspectral imaging describes the fusion of imaging and spectroscopy. These systems exhibit high spatial and spectral resolution, generally by dispersing different wavelengths onto a focal-plane array. Agricultural surveys, extraterrestrial exploration and medical procedures can all benefit from this powerful technique. High-end detectors in the mid-wavelength and long-wavelength infrared are usually made from HgCdTe alloys. These narrow-bandgap semiconductors exhibit favourable optoelectronic properties, however fabrication challenges lead to extravagant costs. In comparison, mature fabrication processes are available for III-V materials. Interband photodetectors made from these compounds are only sensitive at shorter infrared wavelengths. In recent years, intersubband devices have been developed for longer wavelengths and quantum well infrared photodetectors are now commercially available. Focal-plane arrays made from these structures are cheaper and the yield is better than with the HgCdTe technology. Quantum dot infrared photodetectors can also be fabricated from III-V materials. These architectures are inherently sensitive to normal-incidence radiation and have long carrier lifetimes, so they are expected to out-perform their quantum well counterparts. The devices studied here may be applicable to meteorology, atmospheric monitoring, molecular biology and medicine. High-quality quantum dots are normally grown by self-assembly and this restricts their size and composition. Hence, directly fabricating devices to operate at different wavelengths is an ongoing challenge. Post-growth techniques can instead be used to tailor the spectral response and two such approaches are considered in this thesis. Firstly, guided-mode resonances have been exploited in narrowband transmission filters. This design is agnostic to the detector technology and suitable for rugged environments. Germanium and calcium fluoride were selected for the dielectric layers and deposited films were thoroughly characterised. Guided-mode resonance filters based on photonic crystal slabs were integrated with quantum dot infrared photodetectors. The photoresponse of these devices was linearly tunable with the radius of the photonic crystal holes. These detectors are shown to be suitable for hyperspectral imaging with further optimisation of the device architectures. Intermixing shifts the response of InGaAs/GaAs quantum dot infrared photodetectors, so it is an effective approach to spectral tuning. Dielectric capping layers can be used to control the amount of intermixing and this allows multicolour detectors to be monolithically fabricated. In these studies, the compositional and thermomechanical properties of different dielectrics were measured. Preliminary intermixing experiments were performed on different heterostructures to extract the dominant physical processes. Ultimately, multicolour quantum dot infrared photodetectors were fabricated on a single sample. Silica was used to enhance intermixing through impurity-free vacancy disordering, whereas titania suppressed intermixing. Finally, the performance of each device was correlated with the properties of each dielectric. These detectors are found to be ideal for multispectral applications in the long-wavelength infrared band

Liquid crystal based nonlinear fishnet metamaterials

We study experimentally the nonlinear properties of fishnet metamaterials infiltrated with nematic liquid crystals and find that moderate laser powers result in significant changes of the optical transmission of the composite structures. We also show that the nonlinear response of our structure can be further tuned with a bias electric field, enabling the realization of electrically tunable nonlinear metamaterials.We acknowledge the support by the Australian Research Council, the Australian National Computational Infrastructure, and the ACT Node of Australian National Fabrication Facility

Tilted response of fishnet metamaterials at near-infrared optical wavelengths

We study experimentally the transmission properties of Au-TiO2 -Au fishnet metamaterials in the near-infrared spectral range and analyze the change in the transmission resonances at varying angles of incidence and different input polarizations. The results show that the main transmission peak through the fishnet is due to the excitation of hole modes. This high-transmission region is significantly influenced by surface plasmon coupling when the incident electric field has a component normal to the metal plates, while little change with respect to tilt is observed when the electric field is parallel to the two metal films of the fishnet

Integration of bandpass guided-mode resonance filters with mid-wavelength infrared photodetectors

Guided-mode resonances have been exploited to filter the normal-incidence transmission of mid-infrared wavelengths through a photonic crystal slab. A two-dimensionally periodic structure has been integrated with a quantum dot infrared photodetector to narrow its mid-wavelength infrared photoresponse spectrum. Finite-difference time-domain simulations were employed to extract the filter transmittance, which is dominated by a peak near 6 m. The simulated resonance is linearly tunable with the air-hole radius but it is insensitive to small changes in the incidence angle. To realize this filter, a patterned Ge slab was fabricated on a CaF2 cladding layer, on the InGaAs/GaAs photodetector. Filters fabricated on a plain GaAs substrate were also characterized by Fourier-transform infrared spectroscopy. This transmittance was consistent with the corresponding simulation, however the resonance peak was degraded in comparison to the filtered photodetector and its associated simulations

Spectral tuning of InGaAs/GaAs quantum dot infrared photodetectors with bandpass guided-mode resonance filters

Quantum dot infrared photodetectors can be coupled with micro-structured filters to create narrowband sensors. Guided-mode resonance filters based on a high-index dielectric slab can exhibit bandpass characteristics that are suitable for monolithic integration with focal-plane arrays. Here, patterned Ge filters were integrated with InGaAs/GaAs quantum dot detectors to linearly tune their 77 K photoresponse peaks from 5.6 μm to 6.2 μm. The dark current was not influenced by these filters but the ability to narrow the photoresponse linewidth was limited by substrate scattering, which is often encountered with front-side illumination architectures

Thermal Annealing Study on InGaAs/GaAs Quantum Dot Infrared Photodetectors

In this work, rapid thermal annealing was performed on InGaAs/GaAs quantum dot infrared photodetectors (QDIPs) at different temperatures. The photoluminescence showed a blue-shifted wavelength and the spectral response exhibited red-shift from the annealed QDIPs in comparison with the as-grown sample. The overall device performance was not affected by low annealing temperature however for high annealing temperature, some degradation in device detectivity (but not responsivity) was observed. This is a consequence of increased dark current due to defect formation and increased ground state energy

Impurity-free vacancy disordering of quantum heterostructures with SiOxNy encapsulants deposited by magnetron sputtering

Post-growth techniques such as impurity-free vacancy disordering (IFVD) are simple and effective avenues to monolithic integration of optoelectonic components. Sputter deposition of encapsulant films can enhance quantum well intermixing through IFVD and an additional mechanism involving surface damage during the sputtering process. In this study, these two mechanisms were compared in a multi-quantum well structure. The compositions of different silicon oxy-nitride films were controlled by sputter deposition in different ambient gases. These different encapsulants were used to initiate IFVD in the same heterostructure and the observed intermixing is compared to the film properties

Thermal expansion coefficients and composition of sputter-deposited silicon oxynitride thin films

Modern technology is heavily reliant on silicon dioxide and silicon nitride thin films. These films have many electronic and optical applications, and in some cases silicon oxynitride films of intermediate composition are desirable. We have systematically deposited several SiOxNy films by magnetron sputter deposition and thoroughly investigated their composition with Rutherford backscattering spectrometry and optical measurements. The as-deposited stress in these thin films was also measured and all were found to be compressive. Temperature-dependent stress measurements up to 450 °C were then used to extract the biaxial modulus and coefficient of thermal expansion for each SiOxNy . The SiO2-like films exhibit negative thermal expansion, which is consistent with a strong but porous structure. Increasing the nitrogen content results in the thermal expansion coefficient increasing towards values reported elsewhere for Si3N4