Laser Operating At Terahertz Frequencies Biology Essay
Terahertz spread is defined slackly as the spread between the electronic ( ?300um ) and optical ( ?30um ) electromagnetic spectrum, see Fig1.
( 0.3THz-10THz ) . ( Borak 2005 ) It is the most developing portion of the spectrum due to the deficiency of a convenient radiation beginning. Therefore terahertz coevals is chiefly produced from upward transition of electronic beginnings or downward converting from optical beginnings.
( Ferguson and Zhang 2002 ) Both methods are non ideal as they each have their ain restrictions ; pulsed operation, cryogenically cooled, limited by size, complexness and excessively expensive. ( Williams 2007 )As there exists an extended scope of applications in the THz part changing from biomedical to industrial and security imagination, production monitoring, uranology and chemical analysis. To run into the demands of these applications requires a compact, coherent, continuous-wave ( cw ) solid province beginning runing near room temperature. The first quantum cascade optical maser ( QCL ) operating in the terahertz part at 4.
4 THz was demonstrated in 2002 and fabricated on GaAs/AlGaAs. ( Kohler, Tredicucci et Al. 2002 ) .
The QCL represents terrific promise in run intoing the key standards outlined supra. This undertaking is on the development and fiction of a Si/SiGe QCL.One of the chief grounds why this subject is of import is for the realization of electrically pumped Si optical maser which so far to day of the month has non been achieved.
If this could be achieved so it could let massive integrating with CMOS/BICMOS devices and the realization of a individual incorporate bit. The benefits from this would be cheaper industry costs as there would be less constituents required to bring forth a to the full working system and besides a public presentation betterment from taking the constriction of interconnects between devices. ( Paul 2005 )Another motive for theOne of the chief grounds why this subject is of import is that a SiGe QCL will hold built-in advantages over a ( group 35 ) QCL ( Sorel 1996 ) .
Some of these advantages are the lower rates of electron-phonon or hole-phonon sprinkling between subbands. Due to the covalent bonding of Si and Ge, the group IV nonionic phonon sprinkling is weaker than the polar phonon dispersing found in ionic III-V semiconducting materials. Since optical maser addition is relative to the life-time between subband provinces this means that group 4 should hold higher addition. Besides the deficiency of polar optical phonon dispersing below the optical phonon energy of around 62meV in group 4 compared to that of a group 5 which has the phonon dispersing which consequences in a non radiative life-time between optical maser subband provinces holding a strong temperature dependance ( Paul 2005 ) . By working on this undertaking it is hoped to play a function in the fiction of the first of all time electrically pumped Si based optical maser.
Intro needs serious redrafting! !Fig 1: Electromagnetic spectrum demoing THz spread between electronic and optical spectrumFig2: Massive integrating of CMOS/ BICMOS with Si devicesSemiconductor Laser Diode BasicsA homojunction optical maser rectifying tube ( LD ) is really similar in footings of design with a standard visible radiation breathing rectifying tube ( LED ) . For a criterion LED, radiation occurs under forward prejudice of a pn junction ( see fig3 ) . For a direct bandgap stuff where negatrons are injected from the N to the P and holes are injected from the P to the N there is an surplus of minority carries in the depletion part besides known as the active part.
When the bearers recombine they emit a photon in random waies and hence this procedure is known as self-generated emanation.For self-generated emanation to happen a short bearer life-time and diffusion length are required. This is the instance for direct bandgap stuffs like GaAs where no alteration in crystal impulse is required for the recombination procedure, so there is a high chance of self-generated emanation. An indirect set spread stuff would necessitate at least one phonon involved in the recombination procedure and so there is a low chance of self-generated emanation to happen. ( Add equations from talk notes i.e.
internal quantum efficiency ) ( Wood 1994 )Fig3: PN junction under forward prejudice demoing electron hole recombination. E-K diagrams of direct/indirect bandgap stuffsStimulated emanation ( Equations from optoelectronics book )A optical maser rectifying tube differs from an LED in that it relies upon a procedure known as stirred emanation ( light elaboration by stirred emanation ) . This procedure occurs when a photon interacts with a conductivity set negatron which recombines with a hole in the valency set that produces another photon. Therefore one photon has stimulated the emanation of another photon and these two can excite farther photons ( see fig4 ) .
( Zeghbroeck 2007 ) The photons produced all have the same stage, way, polarisation and frequence of the incoming photon. To guarantee that a photon interacts with a conductivity set negatron and non with an negatron in the valency set which consequences in soaking up, have to engineer a population inversion. In a semiconducting material optical maser population inversion is achieved by holding a larger population of negatrons in the lower portion of the conductivity set than the negatrons in the higher portion of the valency set ( see fig5 ) .Fig4: Stimulated emanation procedureFig5: Difference in electron population between the conductivity and valency setThis can be done by utilizing a pn junction like the one used for the LED except that that each side is to a great extent doped.
By using a big prejudice electromotive force ( eV & A ; gt ; Eg ) the Fermi degree of the n side is moved degree or greater than the p conductivity set ( frailty versa ) . Then the built in potency is removed and so the active part can make full up rapidly with negatrons and holes and will give rise to self-generated emanation where the photons produced will excite farther emanation.Optical FeedbackFurther considerations of doing a successful optical maser apart from optical addition ( population inversion ) is the demand of optical parturiency. This is particularly true in the instance of the homojunction LD where the active part is highly little compared to the overall size of the device. Any photon that escapes the active part into the majority stuff is likely to be absorbed and wont farther any stirred emanation. Optical parturiency can be achieved by holding a lower refractive index than the nucleus ( Needs serious repair! ) With a big optical parturiency the addition can so be greatly increased by optical feedback by manner of an optical pit. An optical feedback can be made by puting brooding mirrors at the terminals of the pit which ensures photons will go multiple times along the pit, hence increasing the optical addition.
Such a pit is called a Fabroy-Perot optical resonating chamber and can be produced by spliting the terminals of the semiconducting material. Traveling moving ridges inside the pit must organize standing moving ridges puting limitations on the wavelengths that can vibrate.( Equations sing pit manners )2 QCL FundamentalssThe Quantum cascade optical maser is a unipolar device in that there is merely one bearer ( negatrons or holes ) as opposed to the LD and LED which produce photons through the interband recombination of negatrons with holes. ( Harrison 2005 ) As already seen for the LD the emitted photon has wavelength governed by the bandgap of the majority stuff and so there is no suited semiconducting material stuff to breathe terahertz radiation utilizing standard LD design as a bandgap of less than 60meV would be required. This is even true of the quantum good LD shown in fig6. The QCL on the other manus emits photons based on the energy difference between electronic passages between subband provinces in quantum Wellss. ( fig6b ) .
( Equation ) Therefore the QCL is alone in that it is non bound by the bandgap of the host stuff but can be specifically engineered ( Capasso, Tredicucci et Al. 1999 ) to run over a big scope of frequences. Thus it can be tailored for the THz spread.
Fig6: Interband recombination and intersubband passageQuantum WellEngineering subband provinces for intersubband ( intrasubband ) passage requires some cognition about quantum Wellss. A quantum well is formed when a thin bed of a semiconducting material stuff with a narrow bandgap is sandwiched between two beds of a wider bandgap. This creates a heterostructure and if the bed of the narrow bandgap is thin plenty ( ? 10nm ) so it is susceptible to quantisation effects which produce distinguishable subband provinces. An illustration of these subband provinces can be seen from the text edition illustration of the infinitely deep square good ( fig7 ) .
This is a strictly idealized illustration where the barriers have possible space in tallness and are infinite in length. Outside the well the moving ridge map must be zero and inside the good the clip independent Schr & A ; ouml ; dinger equation is equal toEquationwithA solution to equation must organize edge provinces and a non pervert energy spectrum hence chooseEquationUsing uninterrupted boundary conditions the moving ridge map vanishes at the walls of the well henceTherefore the energy of each province is found from equation 2, where N is the quantum figure and labels the provinces. The normalized moving ridge map depicting each province is equation 3.
From fig7b it is clear that by restricting a atom to a part of infinite it produces distinct energy degrees which are known as the subbands.Equation 2Fig7a: Infinite square good b: Subband provinces ( right )A major benefit from intersubband passage is that since there is no negatron hole recombination, the negatron is non annihilated and can be utilized farther by cascading the negatron through a staircase potency of conjugate Wellss. The negatron will breathe a photon each clip it makes a passage from the active subband provinces in the quantum good and by quantum mechanical tunnelling, the negatron can burrow from the land province of a well into an aroused province in the neighbouring good and get down the whole procedure over once more see fig8. Therefore by cascading coupled quantum wells the optical addition in the QCL can be amplified, which is particularly of import for terahertz radiation as the power of the optical maser is straight relative to the frequence of the emitted radiation. As terahertz radiation is on an order of 103 magnitudes less than seeable light by cascading can convey the power back up to approximately the same degree. ( Kohler, Tredicucci et Al. 2002 ; Paul 2005 )EquationFig8: Staircase potency with negatron cascading breathing multiple photonsQuantum mechanical tunnellingSingle BarrierA atom meeting a barrier that has a possible energy greater than the energy of the atom would anticipate classically that the atom would be reflected back of the barrier.
Quantum automatically nevertheless there is a finite chance that the atom will go through directly through ( fig8 ) . This is a strictly quantum mechanical consequence which is due to the wave nature of atoms and is known as quantum mechanical tunnelling. ( Zettili 2009 )Fig8: Quantum mechanical tunnelling through a individual barrier of possible greater than atom energyFor the individual barrier instance the chance of transmittal can be found asEquationWhereIf E & A ; lt ; & A ; lt ; V0 Then equation 3 can be approximated asEquationTherefore for a high chance of burrowing requires a short tunnelling distance L, little possible barrier V0 and light effectual mass. Therefore for the QCL to be efficient demand to utilize a stuff that has a light effectual mass.Double barrier-Resonant tunnellingAn illustration that is more closely related to the conveyance of negatrons in a QCL by burrowing is the dual barrier instance. If a quantum good like the one in fig7 has barriers either side that are thin plenty so an negatron is no longer genuinely jump to a subband province but can burrow out giving rise to a resonating province.
This is besides the edifice block of the resonant burrowing rectifying tube.By pick of bed thickness and applied electric field, life-times and burrowing chances of each degree are engineered in order to obtain population inversion between two sub-bandsFig9: resonating burrowing rectifying tube demoing NDR
History of QCL First QCL and design, progresss in IR to FIR, SiGe designs why p type
History of QCL
The first suggestion of the quantum cascade optical maser was in 1971 when Kazarinov and Suris proposed the usage of intersubband passages in a superlattice construction, electrically pumped by burrowing for light elaboration. ( Kazarino.
Rf and Suris 1971 ) A superlattice construction can be seen as an extension beyond that of the dual barrier presented in fig9. By increasing the figure of barriers/wells within a semiconducting material this creates a periodic potency and the negatron ( hole ) is no longer confined to a distinct subband province in a quantum good but is every bit likely to be found in any of the quantum wells ensuing in the formation of a miniband. Much like the manner the conductivity and valency set are formed by the periodic potency of the crystalline construction there can be multiple minibands with minigaps in between. Therefore a superlattice can be used to filtrate the energy of negatrons, leting merely those within the minibands to go through or reflecting those in the minigaps. ( Davies 1998 ) )Due to the innovation of molecular beam epitaxy ( MBE ) in 1969, superlattice constructions could be eventually realized by the possible to turn monolayers with precise control. ( Beere, Fowler et Al. 2005 ) This engineering paved the manner for bring forthing devices that could work quantum mechanical effects.
It was n’t until 1988 nevertheless that the first intersubband luminescence was witnessed and this was in the FIR part ( Helm, Colas et Al. 1988 ) .MIR QCLThe first intersubband optical maser followed later, some 23 old ages after the first proposal by Kazarinov and Suris in 1994 when Jerome Faist at Bell labs demonstrated the quantum cascade optical maser. The design is shown in fig10 and is based on a three quantum good active part where the radiative passage is diagonal ( intersubband ) between subband provinces 3 and 2. Population inversion is obtained by a fast depopulation of negatrons from degree 2 to 1 by puting the subband separation between them to the longitudinal optical ( LO ) phonon resonance energy.
By making this it allows to take advantage of an highly fast and dominate sprinkling procedure, where the non radiative LO phonon dispersing clip is much quicker than the radiative passage clip. Each active part is separated by a doped superlattice injector/collector that collects negatrons from the lower provinces and injects them into the aroused radiative province in the following phase and so the procedure is cascaded.The first device was grown on InGaAs/AlInAs/InP and operated at 4.2um. Lasing was achieved in pulsed manner bring forthing up to 10mW at cryogenic temperatures. ( Faist, Capasso et Al.
1994 )Fig10: First working QCL based on a 3QW active partIt was merely a twelvemonth after their innovation that QCLs achieved uninterrupted moving ridge ( cw ) operation ( Faist, Capasso et Al. 1995 ) at cryogenic temperatures, and in pulsed manner up to room temperature a twelvemonth after that ( Faist, Capasso et Al. 1996 ) . In 2002 came the first cw QCL operating at room temperature.
( Beck, Hofstetter et Al. 2002 ) The device lased at 9.1um with up to 17mW of end product power. Ever since its creative activity the QCL has become the dominate beginning over the MIR part.
FIR QCL challengesThe production of a QCL operating in the FIR part though, has been far more ambitious compared to its opposite number in the MIR. This is chiefly due to three issues ; the energy spacing between the radiative subband provinces for THz radiation is highly little ( 10 THz – 1 THz ? 40meV- 4meV ) . Therefore it is harder to selectively inject negatrons into the upper aroused radiation province and non the lower 1. Besides to accomplish population inversion requires fast depopulation of the lower radiative province but since the energy degrees are lower than the LO phonon resonance energy this can non be used as the chief sprinkling mechanism. Therefore there is trouble in commanding dominate sprinkling rates at low energies ; electron-electron, electron dross and interface raggedness dispersing. Another challenge is the development of a suited wave guide to restrict photons of such long wavelength.
In standard LD design as already seen, optical parturiency comes from holding a dielectric facing that has a lower refractive index than the nucleus. If this was implemented in a FIR QCL it would necessitate an unpractical and expensive sum of cladding. Free bearer soaking up which scales with ?2, to a great extent impacting the wave guide losingss. ( Need mentions )First FIR QCLIt was n’t until October 2001 that the first QCL with photon energy less than the semiconducting material optical phonon was demonstrated at 4.4 THz.
The device was grown on GaAs/AlGaAs and had extremum powers up to 2mW at 50K. ( Kohler, Tredicucci et Al. 2002 ) .From so on, FIR QCLs have been demonstrated over a broad part of the THz spread from 5 THz down to 0.85THz.
( Needs coating )SiGe QCL
A treatment of the intended attack to the subject. What methods are to be used?
SiGe QCL FabricationIn order to make a on the job QCL that can be so tested and characterised requires utilizing device fiction techniques to do contacts to the device and besides pattern the device into the right characteristic. This portion of the study will depict the standard photolithography techniques ( shown below ) that will be used in the processing of the SiGe QCL.
Photolithography is an optical agencies for reassigning forms onto a substrate. Forms are foremost transferred to a photoresist bed. Photoresist is a liquid movie that can be spread out onto a substrate, exposed with a coveted form, and developed into a selectively placed bed for subsequent processing via etching or metal deposition.• Surface Preparation• Coating ( Spin Resist )• Pre-Bake ( Soft Bake )• Alignment• Exposure• Development•MetallizationSurface PreparationThe first phase of any photolithography procedure is the sample must be cleaned exhaustively. This is required to take any dust atoms from spliting of the wafer to guarantee that there are no contaminations in the photoresist. The standard clean used is a 5 min soak in propanone with supersonic agitation followed by a 5 min soak in isopropyl intoxicant ( IPA ) with extremist sonic agitation and so in conclusion a N ( N2 ) blow prohibitionist.
Coating ( Spin Resist )A demand of a good photoresist is that it is has to be contaminant free, has good uniformity and adheres good to the surface of the sample. The Shipley Microposit S1800 series meets these demands and so will be used. For good adhesion ideally want no H2O on the surface of the sample. Therefore after the surface readying phase the sample is given a desiccation bake in a convection oven for 10 min at 120C.
Then the sample is given an adhesion booster besides known as a surface primer merely before the photoresist is applied. The surface primer is applied in the same manner as the photoresist as can be seen in fig11. The sample is held on a spinster chow by vacuity and resist is applied manually by a syringe. For the Shipley Microposit S1818 photoresist a unvarying coating is achieved by whirling at 4000rpm for 30 seconds and this corresponds to a resist thickness of about 1.8um.
Fig11: Set up for using the resist.Pre-Bake ( Soft Bake )After the resist has been spun a soft bake is required to vaporize extra dissolver and increase the denseness of the resist. The Softbake for the S1818 resist is performed on a hot home base set at 85C for 120 seconds.Alignment and ExposureAfter the softbake the sample is ready for photolithography. The coveted beds that are to be transferred to the sample are done by utilizing a exposure mask and a mask aligner. By utilizing a CAD tool such as L-edit all beds required for processing are drawn together so separated by package to guarantee that they accurately match up. Once the beds have been separated a maestro transcript of the mask is written utilizing negatron beam lithography.
The mask is made of a quartz substrate coated with Cr where the form is etched into the Cr. The mask aligner is so used to aline the first bed to the sample. Subsequent beds are aligned to by utilizing 3 grades of control between mask and the sample ( x, y, ?Y ) . To guarantee an accurate alliance between beds, alignment Markss with a vernier graduated table are used as can be seen in fig12. Once alliance has been carried out the sample is ready for UV exposure.
A figure of contact methods between the mask and sample are available ; soft contact, difficult contact and vacuity contact. For S1818 an exposure clip of 5 seconds is required.Fig12: Alignment Markss with vernier graduated tableDevelopmentThis is the phase that removes the open resist so that subsequent processing can take topographic point. S1800 series is a positive exposure resist hence after exposure to extremist violet light the photoresist remotion rate in the developer greatly increases. The development of S1818 is a 1:1 ratio of Microposit MF developer with RO H2O agitated for 75 seconds followed by a rinse in RO H2O.Post-Bake ( Hard Bake )A Post bake is merely needed if the subsequent processing measure is dry or wet etching, where a 30 minute bake at 120C in a convection oven will stabilise and indurate the resist. A typical measure that will necessitate the difficult bake in the SiGe QCL processing will be etching square or ridge tables utilizing reactive ion etching ( RIE ) to organize the optical maser construction.
This is a subtractive procedure where photoresist is used as a mask and any countries without photoresist are etched off.MetallizationIf nevertheless the following phase in processing is doing contacts to the QCL by metallization so a soft resist is required and no difficult bake is applied. Metallization will be accomplished utilizing a procedure known as lift off as seen in fig13. The first measure is to expose the countries where contacts will be made at the photolithography phase. After the development an O plasma is applied utilizing a barrel Asher to take photoresist residue which can do hapless metal adhesion. A Hydrofluoric ( Hf ) acid dip is besides used to take any native oxide to guarantee the best possible adhesion.
Metal is so deposited either by sputtering or vaporization. Raise off is so achieved by utilizing acetone heated to 50C by a H2O bath and agitation either by manus or extremist sonic. The last phase of the lift off procedure is to utilize the rapid thermic annealer ( RTA ) to guarantee best possible ohmic contacts.
Fig13: Raise off procedure for metallizationTrial constructionsIn order to guarantee the best possible fiction procedure for the SiGe QCLs, trial constructions will be used to let characterizing of each processing phase. Therefore some trial constructions have been designed and a mask home base has been fabricated for standard photolithography processing. The following subdivision will speak about the theory behind the trial structures that have so far been designed.Transportation length methodOne such trial construction is the Transfer length method ( TLM ) . TLM is a contact measuring technique that gives a complete word picture of the contact by supplying the sheet opposition, the contact opposition and the specific contact electric resistance of the metal-semiconductor interface.
By utilizing this trial construction it will let an accurate estimation of what metals to utilize for doing the best ohmic contacts to the QCL. The design of the TLM consists of four contacts with unequal spacing runing from 5um to 20um. To forestall current spreading and better truth a mesa etch is performed by RIE.
The contacts are made reasonably big ( 150um by 150um ) to enable probing by the parametric analyzer. A four point investigation method ( shown in fig11b ) is employed to extinguish the parasitic opposition introduced by the investigations.Fig11: TLM with spacing from 5um to 20um Bs: Four point Probe methodEntire opposition RT measured between two contacts is equal to equation 5, where vitamin D is spacing between contacts, is sheet electric resistance, W is contact breadth and is metal contact opposition.
Rc can be approximated to equation 6, where is the specific contact electric resistance and LT is the transportation length found from the intercept at RT = 0.Equation 5Equation 6By mensurating the entire opposition for each spread from 5 to 20um and plotting RT as a map of vitamin D ( see Fig 12 ) can pull out the sheet opposition from the gradient of the line, the contact opposition from the intercept at vitamin D = 0 and the specific contact electric resistance by happening LT and utilizing equation 6. ( Schroder 1990 )Fig 12: Plot of RT vs. vitamin DVan der Pauw and Hall saloon