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 technologies for near-to-eye applications mostly use a complementary metal-oxide semiconductor (CMOS) processing chip as backplane for pixel addressing, with extensive post-processing on top of the CMOS chip to deposit organic LED or liquid crystal layers. Here, we examine the possibility of integrating emissive microdisplays within the CMOS chip, with absolutely no post processing needed. This will dramatically reduce the manufacturing cost of microdisplays and may lead to new microdisplay applications. Visible electroluminescence is achieved by biasing pn junctions into avalanche breakdown mode. The most appropriate CMOS pn junction is selected and innovative techniques are applied to increase the light extraction efficiency from the CMOS chip using the metal layers of the CMOS process. An 8×64 dot matrix microdisplay was designed and manufactured in a 0.35-μm CMOS technology. The experimental results show that a luminance level of 20 cd/m2 can be reached, which is an adequate luminance value in order to comfortably read data being displayed in relatively dark environments. The electrical power dissipation per pixel being activated is 0.9 mW/pixel. It is also shown that the pixels can be switched at a rate faster than 350 MHz.
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.
The idea of integrating a light emitter and detector in the cost effective and mature technology which is CMOS remains
an attractive one. Silicon light emitters, used in avalanche breakdown, are demonstrated to switch at frequencies above
1 GHz whilst still being electrically detected, a three-fold increase on previous reported results. Utilizing novel BEOLstack
reflectors and increased array sizes have resulted in an increased power efficiency allowing multi-Mb/s data rates.
In this paper we present an all-silicon optical communication link with data rates exceeding 10 Mb/s at a bit error rate of
less than 10-12, representing a ten-fold increase over the previous fastest demonstrated silicon data link. Data rates
exceeding 40 Mb/s are also presented and evaluated. The quality of the optical link is established using both eye diagram
measurements as well as a digital communication system setup. The digital communication system setup comprises the
generation of 232-1 random data, 8B/10B encoding and decoding, data recovery and the subsequent bit error counting.
Display technologies always seem to find a wide range of interesting applications. As devices develop towards
miniaturization, niche applications for small displays may emerge. While OLEDs and LCDs dominate the market for
small displays, they have some shortcomings as relatively expensive technologies. Although CMOS is certainly not the
dominating semiconductor for photonics, its widespread use, favourable cost and robustness present an attractive
potential if it could find application in the microdisplay environment. Advances in improving the quantum efficiency of
avalanche electroluminescence and the favourable spectral characteristics of light generated through the said mechanism
may afford CMOS the possibility to be used as a display technology. This work shows that it is possible to integrate a
fully functional display in a completely standard CMOS technology mainly geared towards digital design while using
light sources completely compatible with the process and without any post processing required.
The idea of moving CMOS into the mainstream optical domain remains an attractive one. In this paper we discuss our
recent advances towards a complete silicon optical communication solution. We prove that transmission of baseband
data at multiples of megabits per second rates are possible using improved silicon light sources in a completely native
standard CMOS process with no post processing. The CMOS die is aligned to a fiber end and the light sources are
directly modulated. An optical signal is generated and transmitted to a silicon Avalanche Photodiode (APD) module,
received and recovered. Signal detectability is proven through eye diagram measurements.
The results show an improvement of more than tenfold over our previous results, also demonstrating the fastest optical
communication from standard CMOS light sources. This paper presents an all silicon optical data link capable of 2 Mb/s
at a bit error rate of 10-10, or alternatively 1 Mb/s at a bit error rate of 10-14. As the devices are not operating at their
intrinsic switching speed limit, we believe that even higher transmission rates are possible with complete integration of
all components in CMOS.
For CMOS silicon-based light emitting devices to become practical the external power efficiency must be increased. In
this paper a reach-through technique is described whereby the external power efficiency can be increased as a result of
three phenomena: i) increase in internal quantum efficiency, ii) increase in light extraction efficiency, and iii) lower
operating voltage. The three techniques are discussed and the factor 7 improvement in external power efficiency will be
described in terms of the electrical characteristics as well as the external radiation patterns.
The low cost and mass integration potential of CMOS integrated circuits create an attractive opportunity for investigating
CMOS as an optical platform. Although silicon, as an indirect band gap material, is known for inefficient
electroluminescence, silicon-based optical transmission is still a much sought after capability. This paper shows the
potential of an all silicon transmission system for both clock and data transmission.
By utilizing silicon light emitting diodes operating in avalanche, it is shown that a switching speed of above hundred
megahertz is possible. The transmitter consists of an array of light sources, with metal light directors for improved
external quantum efficiency. The array is pulsed across an optical fibre and received by an avalanche photodiode and
amplifier module. Spectral results of the received signal confirm an optical component in excess of 100 MHz, were the
off-chip driver circuitry and the photodiode receiver currently limit the bandwidth of the system.
As the requirements for wideband data transmission are more stringent than for a narrow band clock signal, the
transmission system was tested as a baseband digital communication system, with transmission speeds of up to 176 kbps.
We also present eye diagrams of the received signal to prove the success of the transmission system, where transmission
speed is limited to detectable optical levels versus allowable in-band noise.
A refinement on these principles might lead to CMOS as a contender in high speed clock transmission as well as an
alternative to III-V devices for low cost optical transmission systems.
To investigate quantum-confinement (QC) effects on silicon (Si) light source electroluminescence (EL) properties like
external power efficiency (EPE) and spectral emission, nanometer-scale Si finger junctions were manufactured in a fully
customized silicon-on-insulator (SOI) production technology. All spectrometer-measured thickness-confined SOI light
sources displayed pronounced optical power for 600 nm < λ < 1 μm. The best thickness-confined SOI light source
emitted about 24 times more optical power around λ = 844 nm and exhibited an EPE improvement factor of about 21
compared to a 350 nm bulk-CMOS avalanche reference light-source operating at the same current. Internal quantum
efficiency (IQE) enhancements factors of about 3.5 were attributed to carrier-confinement. The punch-through (PT)
technique, which introduced breakdown voltages as low as 6 V, increased the SOI light source EPE by about a factor 2.5.
It was estimated that geometric-optical improvement techniques that include Si finger surface profiling, raised the SOI
light source external quantum efficiency (EQE) by about a factor 1.7. It was further shown that the SOI Si handle could
be used to reflect up to about 40 % of light that would otherwise be lost due to downward radiation back up, thereby
increasing the EPE of SOI light sources.
A prototype Silicon CMOS Optical Integrated Circuit (Si CMOS OEIC) was designed and simulated using standard 0.8 micron Bi-CMOS silicon integrated circuit technology. The circuit consisted of an integrated silicon light emitting source, an optical wave-guiding structure, two integrated optical detectors and two high-gain CMOS transimpedance analogue amplifiers. Simulations with MicroSim PSpice software predict a utilizable bandwidth capability of up to 220 MHz for the trans-impedance amplifier for detected photo-currents at the input of the amplifier in the range of 1 nA to 100 nA and driving a 10mV to 1 V signal into a 100 kΩ load. First iteration OEIC structures were realised in 1.2 micron CMOS technology for various source-waveguide-detector arrangements. Current signal ranging from 1nA to 1 micro-amp was detected at detectors. The technology seems favorable for first-iteration implementation for digital communications on chip up to 200Mbps.
A series of light emitting devices were designed and realized with a standard 2 micron CMOS technology, 1.2 micron CMOS technology and 0.8 micron Bi-CMOS integrated circuit fabrication technology. The devices operated in the reverse breakdown avalanche mode, at voltage levels of 8 - 20 V and in the current range 80 (mu) A - 10 mA. The devices emit visible light in the 450 - 750 nm wavelength region at intensity levels of up to 1 nWmicrometers -2 (10 mW.cm-2). A series of optimized optical detectors were developed using the same technologies in order to detect lateral and glancing incidence visible and infrared radiation optimally. A series of waveguiding structures of up to 100 micron in length were designed and realized with CMOS technologies by utilizing the field oxide, the inter- metallic oxides and the aluminum metal layers as construction elements. Signal levels ranging from 60 nA to 1 micro-amperes could be detected at the detectors of waveguiding structures of up to 100 micron in length. Finally, a complete optoelectronic integrated circuit was designed and simulated with 0.8 micron Bi-CMOS technology with some of the developed light sources, detectors, waveguiding structures and added driving and amplification circuitry. In particular a very powerful high gain wide- bandwidth MOSFET signal amplifiers was developed that could be successfully integrated in the optoelectronic integrated circuit. The developed technologies show potential for application of optoelectronic circuits in next generation silicon CMOS integrated circuits.
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