Muhammad Irfan Kazim Integrated Circuits & System … Circuits & System (EKS) Linköping University...

Muhammad Irfan Kazim

Integrated Circuits & System (EKS) Linköping University (LiU)

Optoelectronic Devices

Optoelectronic Devices. Ver A.

Muhammad Irfan Kazim, 2015-05-28,

Optoelectronic Devices - Outline

● Light emitting devices (LED/LASERs)

● Photo-detectors

● Photonic Integrated circuits

● Optical interconnects

Light emitting devices (LED/LASERs)



Direct & Indirect band-gap semiconductors

a) Direct band-gap semiconductors

such as GaAs, InP and GaN

b) An indirect band-gap

semiconductor, such as silicon.

Optical transitions must obey conservation laws of both energy and momentum

Courtesy: doi:10.1038/nphoton.2013.65



Double hetero-junction structures

● Electrons attracted across the left-hand junction from the n-InP to the n-InGaAsP● Holes attracted across the right-hand junction from the p-InP into the n-InGaAsP● Recombination takes place in the n-InGaAsP and spontaneous emission (or lasing) occurs● The hetero-junction allows small active region where the light is produced. The material in the active region has a higher refractive index than the material surrounding it. This means a mirror surface effect is created at the junction which helps to confine and direct the light emitted.



LED vs LASER

LED LASER

Very low in cost compared to communication lasers Expensive due to temperature & power control

Produces band of wavelengths, non-coherent Ideally single wavelength & coherent

Digital modulation not possible Pulse modulation upto 0.5 femto seconds

Device response linear with current flow; analog modulation possible

Output power nonlinear with input signal power; analog modulation not possible

Typical output power ~ 100 µW kW of power; 20 mW; Optical amplifier ~ 250 mW

● LASER Requirements

● The photon that stimulated the emission itself is not absorbed and continues along its original path accompanied by the newly emitted photon.

● When the number of electrons in the excited state multiplied by the probability of stimulation by an incoming photon exceeds the number of electrons in the ground state multiplied by the probability of absorption of an incoming photon.



Other LASERs

Quantum Well Structures

When light is confined into a cavity smaller than its wavelength it behaves as a particle (quantum) rather than as a wave. In the case of semiconductor lasers if we restrict the size of the cavity, quantum behavior changes the operation of the laser in a dramatic and fundamental way.

Distributed Feed-backLASER with bragg gratingallows one mode that conforms towave-lengthof grating



QW & QD

● 1D confinement: quantum wells; structures consisting of a thin well materials sandwiched between two layers of a barrier materials.

● 2D confinement: quantum wires; structures consisting of a thin and narrow well materials surrounded by barrier materials.

● 3D confinement: quantum dots; nano-size particles in a barrier materials.

● The quantum confinement => allowed electron and hole states are quantized in the well region => energy required to generate e-h pair or radiation emitted from the process of e-h pair recombination is modified

● => wavelength tuning of the radiation (used in LED or laser applications)



Quantum Cascade Laser

● In semiconductor laser diodes, electrons and holes are annihilated after recombining across the band gap and can play no further part in photon generation.

● However in a unipolar QCL, once an electron has undergone an intersubband transition and emitted a photon in one period of the superlattice, it can tunnel into the next period of the structure where another photon can be emitted. This process of a single electron causing the emission of multiple photons as it traverses through the QCL structure gives rise to the name cascade and makes a quantum efficiency of greater than unity possible which leads to higher output powers than semiconductor laser diodes.



Crystal Growth & Materials

● AlGaAs-GaAs (direct band-gap heterostructure) – Liquid phase epitaxy

● MOCVD & MBE provide sharp heterointerfaces with better accuracy in composition and thickness control

● MOCVD for first RT pulsed operation AlGaAs DHs laser and first QW

● MOCVD & MBE also used for quantum dots (InAs/GaAs)

● Some of the LED materials

● GaAsP, replaced by AlGaAs because of direct bandgap in visible region & lattice matched to GaAs substrate

● Quaternary III-V AlGaAsP-GaAsP heterojunction to offer bandgap tuning in lattice matched heterojunctions, InAlGaP next quaternary material



The birth of blue LED1

● Three Japanese scientists, Isamu Akasaki, Hiroshi Amano and Shuji Nakamura were being awarded the 2014 Nobel Prize in Physics for the development of efficient blue GaN LED

● In order to form multilayer alloy structures like in DH, QW, MQW layers should be grown under lattice matched conditions to prevent generation of defects at growth interfaces

● SiC has a drawback of indirect band-gap

● In 1991 blue ZnSe LASER was demonstrated first time but there were reliability issues although lattice matched growth on GaAs substrates could be achieved

● Lattice constants between GaN and sapphire differ by about 16% and this difference combined with the dissimilar thermal expansion coefficients made it insurmountably difficult to grow a high-quality GaN film with a flat surface free from pits and cracks.

● To accommodate large lattice mismatch GaN was grown on Sapphire substrate with diverse distribution of growth orientations but they resulted in defects at interfaces which made p-type conduction also impossible thus restricted fabrication of preferred p-n junction LED



The birth of blue LED1 ● Akasaki and Amano discovered the growth of a thin, low-temperature AlN buffer layer prior to high-temperature growth of GaN was highly beneficial, allowing the fabrication of extremely high-quality GaN single-crystal films. Mirror-quality surfaces without cracks and pits, with much lower density of dislocations became possible. Electrical and optical properties of GaN films remarkably improved, with much lower residual carriers, high electron mobility & intense luminescence which led to pn-junction blue LED.

● To realize p-type conductivity, a technique called low-energy electron beam irradiation (LEEBI) was employed to activate the Mg-doped GaN for the first time .

● AlN buffer layer and LEEBI lead to fabrication of p–n junction blue LED in 1989

● Nakamura introduced a two-gas flow system for MOCVD, with a main flow that carries the reactant gas parallel to the substrates and another sub-flow that transports inorganic gas perpendicular to the substrates for the purpose of changing the direction of the main flow to bring the reactant gas into contact with the substrates for growing high quality GaN. Nakamura’s GaN low-temperature buffer growth method is much simpler and suitable for mass production. He used thermal annealing of Mg-doped GaN in a nitrogen gas flow and obtained a uniform and thicker p-type layer than possible with LEEBI.



The birth of blue LED1

● The combination of a simple p-type doping method and the GaN low-temperature buffer layer technology led Nakamura to fabricate bright DH blue LEDs for the first time in 1993.

● Further improvement of GaN and InGaN material-quality combined with improvements to device structure, such as leakage current suppression by AlGaN layer insertion, led to blue LEDs with enhanced performance, reaching an astonishing blue light luminous intensity greater than 1 cd in 1994.These LEDs were almost 250 times brighter than SiC-based predecessors.

● A new form of high-efficiency white light is possible by combining blue LEDs with a yellow phosphor. Such white LEDs now commonly provide backlight in mobile phones, PCs and TVs, saving a huge amount of energy worldwide.

Photo-detectors



Photodetectors

Properties

● Detector Responsivity: ratio of output current to input optical power● Spectral Response Range: range of wavelengths over which the device will operate ● Response Time: how quickly the detector can respond to variations in the input light intensity● Noise Characteristics: critical for low input of light



Photodetectors

● Depletion zone extremely thin, ● Most light passes without absorption in depletion,

● Wide intrinsic layer acts like wide depletion layer because of less dopants,

● Capacitance reduces, ● Faster response time due to drift



Avalanche Photo-detectors

Problems:● High voltage,● Narrow linear region,● Background light amplification

● Avalanche multiplication,

● Impact ionization



Photo-transistors2

● A full plane emitter with an area of 97 х 97 μm2 ● This layout type leads to a low current density resulting in reduced cut-off frequency

● 1.4 μm wide and 8.4 μm wide gap, 1.4 х 1.4 μm2, ● the current density is higher & base-emitter capacitance lower because of the smaller emitter area,

● -3 dB BW of PT increases, ● Electric field is not homogenous, ● Reduced gain



Photo-transistors2

● 0.6 µm OPTO ASIC CMOS process using low doped epitaxial starting wafers,● Do not need high voltages for their internal amplification,● A large photo-diode is formed by base-collector diode● The higher responsivity is achieved due to implementing a deep intrinsic layer for a thick base-collector SCR

● The fact that PTs need only low voltages compared to APDs and show a high responsivity due to the current amplification makes them well suited for different SoC applications like active pixels, light barriers or optocouplers

● Special wafer has a thick (15 µm), low-doped (2 х 10-13 cm3) p-epitaxial layer and on top a shallow (1 μm), low-doped (10-14 cm3) n-epitaxial layer

● Fig. a The increase of SCRs thickness leads to a decrease of the capacitances C

BC and C

BE and to a reduced effective base width W

B.

● Fig. b The higher doped base leads to thinner SCRs (compared to Fig. 2a) which leads to larger capacitance C

BC and C

BE and a thicker effective base width.

● Therefore the device speed is slower and the current gain β decreases. Fig. c A method to vary the base doping concentration in standard CMOS; NW

33: 1 μm stripe with 2 μm space, NW

50: 1 μm stripe with 1 μm space,

NW66

: 2 μm stripe with 1 μm space



Results (β, Responsivity)2



Multi-spectral/hyper-spectral FPA imaging

● Humans see visible light (380 nm to 700 nm)● Goldfish see infrared (700 nm to 1mm)● Bumble bees see ultraviolet (10 nm to 380 nm)

● Multispectral and hyperspectral imagery gives the power to see as humans (red, green and blue), goldfish (infrared), bumble bees (ultraviolet).

http://aviris.jpl.nasa.gov/

http://www.militaryaerospace.com/articles/2014/07/near-ir-fpa.html

http://www.militaryaerospace.com/articles/2014

Photonic integrated circuits



Photonic Integrated Circuits3

● Monolithic integration of optical waveguides, optical couplers, light sources and detectors for signal integration & distribution. They are mostly fabricated using essentially planar fabrication approaches like electron-beam lithography or UV optical lithography.

● Three-dimensional dip-in direct-laser-writing optical lithography to fabricate three- dimensional polymeric functional devices on pre-fabricated planar optical chips containing Si

3N

4 waveguides as well as grating couplers made by standard electron-beam lithography.

● Light is coupled into the circuit through a central focusing grating coupler (close-up in Figure 1b) on port 2. Shortly afterward, the propagating mode is split 50 : 50 by a Y-splitter. The light is then guided by nano-photonic waveguides that are 450 nm thick and 1 mm wide. This choice of waveguide geometry supports two guided modes at a wavelength of 1550 nm, one being transverse electric (TE)-like, the other one transverse magnetic (TM)-like. One half of the light is guided to a reference port (1), the other half to a tapered waveguide (close-up in Figure 1c). The light in the tapered waveguide is coupled into a 3D optical component, which previously was fabricated by DLW such that it closes the gap between the facing tapered waveguides. Eventually, the light is coupled back into the nanophotonic circuit and leaves the chip through another grating coupler (port 3).



Photonics



Photonics IC3



Photonics IC

Optical interconnects



Optical interconnects

● Speed of CMOS based circuits would be limited by interconnection delays instead of gate delays with decreasing feature sizes

● Global metal interconnects within chip for clock routing (Research at LiU)

● High speed metal based memory interconnects or chip-to-chip optical interconnects

● In 2012, IBM announced 1 Tb/s data parallel transfer capacity using an integrated GaAs based VCSEL (vertical-cavity surface emitting laser) chip with 24 transmitters and 90 nm CMOS technology receivers in 150 m short link.



References

[1] Yasushi Nanishi, “The birth of the blue LED”, Nature Photonics, VOL 8, December 2014

[2] P. Kostov, W. Gaberl, H. Zimmermann, “Visible and NIR integrated Phototransistors in CMOS technology”, Elsevier Solid-State Electronics, 65–66 (2011), 211–218

[3] Martin Schumann, Tiemo Buckmann, Nico Gruhler, Martin Wegener, and Wolfram Pernice, “Hybrid 2D–3D optical devices for integrated optics by direct laser writing”, Light Sci Appl, Nature Publishing Group, 2014, 3

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