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Optical and Electronic Structures and Devices |
Device Programs in the Department are mainly focussed on semiconductor optoelectronic / photonic devices such as lasers (edge emitters and VCSELs), light emitting diodes, infrared photodetectors, optical modulators, waveguides, saturable absorbers, photonic integrated circuits. Research in this field covers III-V compound semiconductor as well as silicon optoelectronics. Electronic device structures include high electron mobility transistors, high voltage power device structures based on SiC, GaN, ZnO. Examples of research in this area are given below.
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Active waveguide devices and structures employing silicon nanocrystals
Robert G Elliman, , ,
Silicon nanocrystals exhibit novel optical properties, including strong room temperaturte luminescence, and can be formed within silica substrates by annealing silicon-rich SiO2 formed either by high-enegy ion implantation or plasma-enhanced chemical vapour deposition (PECVD). The higher refractive index of the nanocrystal-rich region can be used to define waveguides. Such structures exhibit novel wave-guiding behaviour and form a basis for studying the optical and electronic properties of nanoscale silicon.
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Bose-Einstein Condensation of Excitons
, H. Hoe Tan, Chennupati Jagadish
Coupled quantum wells under applied electric fields are a popular system for trapping cold excitons to investigate collective quantum properties. Applying an electric field breaks the well symmetry resulting in extended dipole excitons (spatially indirect). Due to the small spatial overlap of the electron and hole wavefunctions the indirect excitons in this system have a long radiative lifetime and the dipole alignment gives an overall repulsive interaction. This system creates the expected conditions necessary for achieving a Bose-Einstein Condensate (BEC) of excitons. These excitons are trapped only in one dimension and diffuse rapidly upon creation by optical excitation. In order to make best use of the quantum properties of cold exciton gases further localization is advantageous. We have created lateral traps using a spatially varying electric field and is shown to achieve good trap depths and profile control. These traps also offer the possibility to observe spatial/angular condensation as a definitive signature of BEC and remove the restriction of working with an infinite 2D system. Exciton condensation is expected in this system at around 2K for densities well inside the dilute limit.
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Optoelectronic Device Integration
Mykhaylo Lysevych, , Lan Fu, H. Hoe Tan, Chennupati Jagadish
The study of quantum well and quantum dot intermixing is important to realise integrated optoelectronic devices. Monolithic integration of optoelectronic devices is desirable for practical applications, as this would lead to low loss, high speed modules and operating at lower currents. To achieve this, the in-plane band-gap tuning is required. Our research in this area involves the investigation of three different techniques to achieve the lateral band-gap control - (i) ion implantation induced intermixing, (ii) impurity free induced disordering (using a dielectric cap) and
(iii) selective area epitaxy.
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Photonic Crystal Devices
Ian McKerracher, Hattori Haroldo, H. Hoe Tan, Chennupati Jagadish
A photonic crystal is a periodic structure of high index contrast. These crystals have the property of preventing light from propagating in certain directions with specified energies, creating a photonic band gap which is analogous to the electronic band gap in semiconductors. Photonic crystals allow the fabrication of new ultra-compact optoelectronic devices because of their ability to confine light and guide and control its propagation. By controlling their sizes and periodicity, they can be used to select or extinct certain modes of the optical field and hence opens up new applications in photonic devices, such as highly efficient light sources. The addition of photonic crystals to quantum dot lasers can considerably enhance the performance of these devices, reducing their size and threshold current and improving their quantum efficiency. This project is targeted at integrating photonic crystals to quantum dot lasers and infrared photdetectors to achieve a new class of high performance lasers. This novel technology will lead to applications in communications, computers, defence and sensing.
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Quantum Dot Infrared Photodetectors
Greg Jolley, Ian McKerracher, Lan Fu, Chennupati Jagadish
Quantum dot infrared photodetectors (QDIPs), based on self-assembled quantum dots (QDs), have attracted extensive interest in the past decade for mid- to long-wavelength (3–20μm) infrared photodetection. Due to three-dimensional carrier confinement, QDIPs outperform their quantum well (QW) counterparts in sensitivity to normal-incident IR radiation, responsivity, detectivity, and ability to operate at higher temperatures. Based on mature III-V compound semiconductor technology that offers high uniformity and low cost, QDIPs are also competitive with conventional HgCdTe detectors, especially for fabrication of two-dimensional focal-plane arrays.
Our research effort is focused on making multi-colour QDIPs for high-performance infrared systems in applications such as remote temperature sensing, chemical analysis, target identification and spectrometers. One techniuque is to fabricate two-colour QDIPs which involves stacking two QD structures made from different materials (such as InGaAs/GaAs and InAs/GaAs). Another method uses the so-called dots-in-a-well (DWELL). QDs embedded in a QW can detect two or more colours based on photocurrent contributions from transitions between QD and QW states, and between QD states and the continuum. Yet another approach is the use of interdiffusion techniques such as impurity free vacancy disordering and ion implantation induced intermixing which avoid complicated growth processes and structure designs.
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Quantum Dots Lasers
, , H. Hoe Tan, Chennupati Jagadish
Self-assembled quantum dots have received much excitement due to their three dimensional carrier confinement. The atom-like density of states in quantum dots leads to a narrow gain spectrum and, hence, improved lasing characteristics. This project involves to growth, fabrication and characterisation of InAs and InGaAs quantum dots lasers (on GaAs and InP substrates).
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Semiconductor THz Emitters and Detectors
Lan Fu, Chennupati Jagadish
The generation and detection of THz radiation using semiconductors depend strongly on the material from which the device is fabricated. Material dependent carrier trapping and recombination times play an essential role in photoconductive receiver device performance. Specifically, long carrier lifetimes will permit the reception of large amounts of noise and short carrier lifetimes will reduce the signal level and accuracy. The use of ion implantation to modify the material properties (such as the carrier trapping times) of the semiconductors are investigated. This work is carried out in close collaboration with Oxford University.
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