Silicon-on-insulator (SOI) is fast becoming a popular technology platform for silicon photonic devices as well as RFand
nanoelectronics, while the field of nanoscale silicon structures remains an active topic in optoelectronic research.
Silicon light sources with an active material only a couple of nanometers in dimension has several distinct advantages for
enhanced light emission due to carrier confinement and improved external light extraction efficiency when compared to
bulk CMOS light sources. This work focuses on these inherent advantages, where the design of nanoscale silicon light
source arrays implemented in a custom SOI process, is presented utilizing hot carrier luminescence generated by
avalanching pn-junctions. SOI light sources have shown an improvement in electroluminescent power emitted through
various techniques inducing carrier confinement effects. These techniques are typically applied by either thinning the
silicon active material in one or two dimensions. Conventional nanoscale SOI LEDs usually depend on recombination in
forward-biased junctions. The wavelengths of interest are consequently in the near-infrared range around the band gap
energy. In contrast, the device structures presented in this paper is based on hot carrier electroluminescence utilizing
reach-through and punch-through techniques. We present the power spectral densities of the structures over a very wide
spectral range also covering the visible wavelengths. The spectral characteristics of the SOI light sources were
investigated and the dominant light generation mechanisms were identified. This work compares various device
structures and light source architectures and the results presented exemplify the possibilities of SOI light sources for
future applications.
KEYWORDS: Night vision, Silicon, Imaging systems, CMOS technology, Near infrared, LCDs, Image processing, Scanning electron microscopy, Visible radiation, Video
The unrivalled integration potential of CMOS has made it the dominant technology for digital integrated circuits. With the advent of visible light emission from silicon through hot carrier electroluminescence, several applications arose, all of which rely upon the advantages of mature CMOS technologies for a competitive edge in a very active and attractive market. In this paper we present a low-cost night vision viewer which employs only standard CMOS technologies. A commercial CMOS imager is utilized for near infrared image capturing with a 128x96 pixel all-CMOS microdisplay implemented to convey the image to the user. The display is implemented in a standard 0.35 μm CMOS process, with no process alterations or post processing. The display features a 25 μm pixel pitch and a 3.2 mm x 2.4 mm active area, which through magnification presents the virtual image to the user equivalent of a 19-inch display viewed from a distance of 3 meters. This work represents the first application of a CMOS microdisplay in a low-cost consumer product.
Silicon-on-insulator (SOI) is becoming an important technology platform in nanometer scale CMOS integrated circuits. The platform offers a number of distinct advantages over bulk CMOS for materializing silicon light sources based on hot carrier luminescence. This work describes the design of nanoscale silicon structures for enhanced light emission with improved power efficiency, which allows the use of SOI light sources in short-haul optical communication links with extended possibilities for other applications. It has been shown experimentally that reducing the dimensions of the active material results in an improvement of electroluminescent power emitted from forward-biased pn-junctions. Previously published results show a similar trend for light sources based on hot carrier luminescence. Building on our previous work in SOI light sources, multiple fingerlike junctions are manufactured in an arrayed fashion for coupling into large diameter core optical fibers for CMOS optical communications up to a few hundred meters. The manufacturing methodology and associated challenges are discussed for the scaling down of device dimensions, and difficulties in realizing the structures are investigated. The optical power characteristics are discussed as well as the spectral nature of emission along with the advantages and disadvantages thereof. This work compares different architectures of light sources that were implemented where a comparison is drawn between previous SOI devices as well as bulk CMOS. We believe the improved SOI light sources are fully compatible with modern CMOS technologies based on SOI and may provide such technologies with a much needed light source as part of the circuit designer’s toolkit.
Microdisplay technology, the miniaturization and integration of small displays for various applications, is predominantly
based on OLED and LCoS technologies. Silicon light emission from hot carrier electroluminescence has been shown to
emit light visibly perceptible without the aid of any additional intensification, although the electrical to optical
conversion efficiency is not as high as the technologies mentioned above. For some applications, this drawback may be
traded off against the major cost advantage and superior integration opportunities offered by CMOS microdisplays using
integrated silicon light sources. This work introduces an improved version of our previously published microdisplay by
making use of new efficiency enhanced CMOS light emitting structures and an increased display resolution.
Silicon hot carrier luminescence is often created when reverse biased pn-junctions enter the breakdown regime where
impact ionization results in carrier transport across the junction. Avalanche breakdown is typically unwanted in modern
CMOS processes. Design rules and process design are generally tailored to prevent breakdown, while the voltages
associated with breakdown are too high to directly interact with the rest of the CMOS standard library. This work shows
that it is possible to lower the operating voltage of CMOS light sources without compromising the optical output power.
This results in more efficient light sources with improved interaction with other standard library components.
This work proves that it is possible to create a reasonably high resolution microdisplay while integrating the active
matrix controller and drivers on the same integrated circuit die without additional modifications, in a standard CMOS
process.
The emission spectra of pn-junction and punch-through (PT) carrier injection silicon (Si) CMOS light sources were
measured at various current densities and temperatures. In contrast to the narrow-band forward-biased junction spectrum,
that peaks around 1.1 μm (1.1 eV), the reverse-bias spectrum was found to extend from about 350 nm (3.4 eV) to about
1.7 μm (0.7 eV) covering the UV, Vis and NIR regions. Since the photon energy decreases with increasing wavelength,
the significant NIR radiation implies that the quantum conversion efficiency of Si avalanche light sources is appreciably
higher than previously reported. The spectrum of PT light source constitutes a scaled combination of both the forwardand
reverse-biased junction spectra. Calculating the photon flux at the emission source within the Si against photon
energy allowed the deduction and quantification of the physical light emission processes with respect to silicon's
electronic band structure. Intra-conduction-band (c-c) electron (e-) transitions seem to be the dominant physical
mechanism responsible for the wide avalanche spectrum. The effect of current densities up to 106 A/cm2 and
temperatures between 22 °C and 122 °C on the emission spectrum and consequently the physical light generation
mechanism are investigated and quantified.
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