Vertical-external-cavity surface-emitting lasers and quantum dot lasers

The use of cavity to manipulate photon emission of quantum dots (QDs) has been opening unprecedented opportunities for realizing quantum functional nanophotonic devices and also quantum information devices. In particular, in the field of semiconductor lasers, QDs were introduced as a superior alternative to quantum wells to suppress the temperature dependence of the threshold current in vertical-external-cavity surface-emitting lasers (VECSELs). In this work, a review of properties and development of semiconductor VECSEL devices and QD laser devices is given. Based on the features of VECSEL devices, the main emphasis is put on the recent development of technological approach on semiconductor QD VECSELs. Then, from the viewpoint of both single QD nanolaser and cavity quantum electrodynamics (QED), a single-QD-cavity system resulting from the strong coupling of QD cavity is presented. A difference of this review from the other existing works on semiconductor VECSEL devices is that we will cover both the fundamental aspects and technological approaches of QD VECSEL devices. And lastly, the presented review here has provided a deep insight into useful guideline for the development of QD VECSEL technology and future quantum functional nanophotonic devices and monolithic photonic integrated circuits (MPhICs).Comment: 21 pages, 4 figures. arXiv admin note: text overlap with arXiv:0904.369

Optical Properties of Ultrathin In(Ga)As/GaAs and In(Ga)N/GaN Quantum Wells

Recently, structures based on ultrathin quantum wells (QWs) began to play a critical role in modern devices, such as lasers, solar cells, infrared photodetectors, and light-emitting diodes. However, due to the lack of understanding of the formation mechanism of ultrathin QWs during the capping process, scientists and engineers cannot fully explore the potential of such structures. This study aims to investigate how structural parameters of ultrathin QWs affect their emission properties by conducting a systematic analysis of the optical properties of In(Ga)As/GaAs and In(Ga)N/GaN ultrathin QWs. Specifically, the analysis involved photoluminescence measurements combined with effective bandgap simulation, x-ray diffraction, and transmission electron microscopy characterization. By controlling the growth temperature, indium content depth profile modifications were achieved for the In(Ga)As/GaAs QWs, leading to substantial changes in the emission properties. The analysis was supported by the effective bandgap simulation, which allowed not only to probe the exact shape of the indium depth profile but also to predict and design the structures with the desired optical characteristics. In the case of In(Ga)N/GaN ultrathin QWs, the growth temperature change affected the total indium incorporation within the QW. Further analysis suggested that the total amount of indium is the dominant factor when dealing with optical emission from ultrathin QWs. The ultimate goal of this research was to characterize, understand, and control ultrathin QW structures in various applications

Semiconductor quantum dots for mid-IR light emission

Mid-infrared wavelength range (3 μm to 30 μm) is important in many applications such as environmental monitoring, industrial process control, bio- medical imaging, security and defense. The mid-IR is the spectral home of the distinct vibrational and rotational absorption resonance signatures of a wide range of molecular species, giving mid-IR sensing systems the potential to enable the monitoring and identification of molecules for gas sensing or chemical and biological imaging applications. To enable many of the above applications, compact, high efficiency, and low-cost mid-IR emitters and detectors are required. The goal of this project is to develop highly efficient and low-cost mid-IR emitters. The first part of this thesis gives an introduction to why mid-IR light is important and the state-of-the-art mid-IR sources. We discuss the working theory, structure, advantages and disadvantages of quantum cascade laser (QCL), interband cascade laser and type-I quantum well lasers, which together cover the wavelength range from 2 μm to 11 μm. Then, in the last part of chapter 1, we provide an introduction to quantum dots and a discussion as to why we might want to use quantum dots to improve the performance of QCLs. Here we will discuss primarily two types of quantum dots: those that are generally referred to as patterned (either etched top down, or site-selectively grown), and self-assembled QDs (SAQD). In this project, we studied studied mechanisms for control of energy states in both top-down nanolithographically defined QD and bottom-up InAs submono-layer QD grown by MBE. We demonstrate strong carrier confinement in, and electroluminescence (EL) from, quantum nanostructures fabricated from epitaxially grown quan- tum wells (QWs) using a top-down nanosphere lithography (NSL), dry-etch, mass-transport, and overgrowth fabrication process. Optically active nano- pillars with diameters as small as 90 nm are fabricated, and narrow linewidth(18 meV) electroluminescence from a fabricated diode structure is observed, with an emission blue-shift of over 37 meV from the original quantum well sample luminescence. The results presented offer the potential for low-cost, large-area patterning of quantum nanostructures for optoelectronic applications. However, the NSL defined QD density is limited by the size of the NS used. For 200 nm diameter size NS, the QD density can only reach to 2.5 ∗ 109/cm2 which is likely not nearly high enough for most optoelectronic applications. For this reason, we started to study InAs submonolayer quantum dots (in the next chapter), aiming to use InAs SML QD in the QCL active region. The thesis then goes on to discuss work demonstrating control of energy states in epitaxially-grown quantum dot structures formed by stacked sub- monolayer InAs depositions via engineering of the internal bandstructure of the dots. Transmission electron microscopy of the stacked submonolayer re- gions shows compositional inhomogeneity, indicative of the presence of quantum dots. The quantum dot ground state is manipulated not only by the number of deposited InAs layers, but also by control of the thickness and material composition of the spacing layers between submonolayer InAs de- positions. In this manner, we demonstrate the ability to shift the quantum dot ground state energy at 77K from 1.38 eV to 1.88 eV. The results pre- sented offer a potential avenue towards enhanced control of dot energies for a variety of optoelectronic applications. The SML QD structures were then integrated into QCL-like structures, with our SML deposition in the active region. We also demonstrate infrared light emission from thin epitaxially grown In(Ga)Sb layers in InAs(Sb) matrices across a wide range (3-8 μm) of the mid- infrared spectral range. Our structures are characterized by x-ray diffraction, photo-electron spectroscopy, atomic force microscopy and transmission electron microscopy. Emission is characterized by temperature- and power- dependent infrared step-scan photoluminescence spectroscopy. The epitaxial In(Ga)Sb layers are observed to form either quantum wells, quantum dots, or disordered quantum wells, depending on the insertion layer and substrate material composition. The observed optical properties of the monolayer scale insertions are correlated to their structural properties, as determined by transmission electron and atomic force microscopy. In this work, we employ time resolved PL to study the carrier recombination mechanism in a thin type II material system. With the experimental system we set up and the analysis process developed, we are able to resolve the Shockley-Read-Hall and radiative rates from our materials. This provides a powerful way to study the emitter quality. According to our TRPL study as well as the optical study, we find that the sample with obvious nano- structure formation has the best optical performance and material quality, which makes the QD structure the best candidate for mid-IR emitter and laser applications. Finally, the thesis ends with a study of the growth of InGaSb QDs using MBE by systematically changing growth parameters such as substrate temperature, Ga/In ratio and layer thickness. A high density and high uniformity QD sample is grown and studied at the end of chapter 6. This sample shows a better temperature performance and a better material quality than any other samples without QD formation. From this work we are able to draw the conclusion that the type-II QD structure has the best potential in the future to be made into a low-cost, simple structure and high-performance room-temperature mid-IR emitters

High operating temperature mid-infrared InGaAs/GaAs submonolayer quantum dot quantum cascade detectors on silicon

Monolithic integration of infrared photodetectors on a silicon platform is a promising solution for the development of scalable and affordable photodetectors and infrared focal plane arrays. We report on integration of submonolayer quantum dot quantum cascade detectors (SML QD QCDs) on Si substrates via direct growth. Threading dislocation density has been reduced to the level of ~10 7 cm -2 with the high-quality GaAs-on-Si virtual substrate. We also conducted a morphology analysis for the SML QD QCDs through a transmission electron microscope strain contrast image and to the best of our knowledge, high quality InGaAs/GaAs SML QDs were clearly observed on silicon for the first time. Photoluminescence decay time of the as-grown SML QD QCDs on Si was measured to be around 300 ps, which is comparable to the reference QCDs on lattice-matched GaAs substrates. With the high-quality III-V epitaxial layers and SML QDs, the quantum cascade detectors on Si achieved a normal incident photoresponse temperature up to 160 K under zero bias

Time-resolved spectra of a self-pulsing quantum dot laser

Self-sustained pulsations in the output of an InAs quantum dot laser diode in the MHz range are reported for the first time. The characteristics (shape, range and frequency) are presented for the free running laser and when optical feedback in the Littrow configuration is applied. The frequency resolved optical spectra reveal different envelope shifts between the two cases. This might be related to a change of phase-amplitude coupling across the gain maximum in agreement with the expectation for a two level system. The time scale and bifurcation scenario suggest that these are opto-thermal pulsation like those reported in quantum well amplifiers.(1

Engineered quantum dots for infrared photodetectors

Quantum Dot Infrared Photodetector (QDIP) Focal Plane Arrays (FPAs) have been proposed as an alternative technology for the 3rd generation FPAs. QDIPs are emerging as a competitive technology for infrared detection and imaging especially in the midwave infrared (MWIR) and longwave infrared (LWIR) regime. These detectors are based on intersubband transitions in self-assembled InAs quantum dots (QDs) and offer several advantages such as normal incidence detection, low dark currents and high operating temperatures, while enjoying all the benefits of a mature GaAs fabrication technology. However, due to Stranski-Krastanov (SK) growth mode and the subsequent capping growth, the conventional SK QDs are pancake shaped\u27 with small height to base ratio due to interface diffusion. Thus they cannot fully exploit the 3D \u27artificial atom\u27 properties. This dissertation work investigates two approaches for shape engineered QDs: (1) Selective capping techniques of Stranski-Krastanov QDs, and (2) Growth of Sub-Monolayer (SML) QDs. Using Molecular Beam Epitaxy (MBE) growth, engineered QDs have been demonstrated with improved dot geometry and 3D quantum confinement to more closely resemble the 3D \u27artificial atom\u27. In SK-QDs, the results have demonstrated an increased dot height to base aspect ratio of 0.67 compared with 0.23 for conventional SK-QD using Transmission Electron Microscope (TEM) images, enhanced s-to-p polarized spectral response ratio of 37% compared with 10% for conventional SK-QD, and improved SK-QDIP characterization such as: high operating temperature of 150K under background-limited infrared photodetection (BLIP) condition, photodetectivity of 1x109 cmHz1/2/W at 77K for a peak wavelength of 4.8 μm, and photoconductive gain of 100 (Vb=12V) at 77 K. In SML-QDs, we have demonstrated dots with a small base width of 4~6 nm, height of 8 nm, absence of wetting layer and advantage optical property than the SK-QDs. SML-QD shows adjustable dot height to base aspect ratio of 8nm/6nm, increased s-to-p polarized spectral response ratio of 33%, and a narrower full width at half maximum (FWHM), long wavelength 10.5 μm bound-to-bound intersubband transition, and higher responsivity of 1.2 A/W at -2.2 V at 77K and detectivity of 4x109 cmHz1/2/W at 0.4 V 77K.\u2

Luminescence from Semiconductor Quantum Wires, Quantum Dots, and Monolayer Quantum Wells: Bottleneck and Localization Issues

Semiconductors nanostructures are fabricated using a range of techniques which inevitably have an impact in the resulting optical properties. Multilayers are grown by epitaxial techniques with a varying degree of uniformity in thickness, composition, etc., all leading to localisation effects in two-dimension. These multilayers are patterned to fabricate wires and dots using, in this case, electron beam lithography and dry etching. The fabrication steps contribute to modifications of the optical properties, beyond the expected purely confinement-related effects. An overview of linear and modulation spectroscopy is presented to demonstrate the impact of fabrication steps as well as of lateral confinement upon the emission from wires and dots. We focus on photoreflectance of GaAs-GaA1As dots and Si-SiGe wires as a probe of strain relaxation. Near-field scanning optical microscopy of single dots of GaAs-GaA1As at helium temperatures illustrates the potentials of using scanning probe techniques to study the underlying quantum mechanics of nanostructures. Finally, we suggest that a combination of lateral exciton confinement and exciton localization is a possible way forward to realise high emission efficiency nanostructures