Deep Level Transient Spectroscopy

摘要: INTRODUCTIONDefectsareresponsibleformanydifferentcharacteristicpropertiesofasemiconductor.Theyplayacriticalroleindetermining the viability of a given material for deviceapplications. The identification and control defectshavealwaysbeenamongthemostimportantandcrucialtasks in materials electronic device development.The performance reliability devices can be signif-icantly affected by only minute concentrations unde-sirable defects. Since ultimate quality materialdepends on sensitivity characterization tech-nique, challenge hasbeen to develop detection methods with improvedsensitivity.Whereas electrical are moresensitive than physical techniques, theymay arguably less sensitive some optical techni-ques. However, since operation depends largely onthe properties, istherefore more relevant. Also, activation defects dueto processes requires scrutiny as it has directimpact device.Thedeepleveltransientspectroscopy(DLTS)techniqueprobesthe temperature dependenceofthe charge carriersescaping from trapping centers formed point inthematerial.Thistechniqueisabletocharacterizeeachtypeoftrappingcenterbyprovidingtheactivationenergyofthedefectlevelrelativetooneoftheenergybandedgesandthe capture cross-section traps. This technique canalso used compute concentration depthprofiling centers.Although there exist several characteriza-tion techniques such Hall effect, current–voltage,capacitance–voltage, carrier lifetime measure-ments, (see example Effect Semiconductorsand Capacitance-Voltage (C-V) Characterization ofSemiconductors) none them is spectroscopicnature. spectroscopic nature DLTS key fea-ture that provides both convenience sensitivity.Deep level transient spectroscopy been widelyusedformanydifferentsemiconductors.Thistechniquehasdistinguished itselfincontributing totheresolutionof many defect-related problems technologi-callyimportantsemiconductorssuchassilicon,theIII–Vand II–VI compounds, alloys. Many different varia-tions basic have also devel-oped improved specializedapplications structures nor-mal p–n or Schottky barrier diodes.DLTS not able identify chemistry theoriginofadefect.DLTSdatashouldthereforebeusedinconjunctionwithothertechniques.Asuccessfulstudyofdefectsrequiresaconcertedeffortofvariousresearchersusing various order toderive accurate consistent picture thedefect structure material.Defects SemiconductorsInareal crystal,the periodicsymmetryofthe latticecanbe broken Lattice produce localizedenergy states may energy levels occurringwithin band gap. A (electron hole) boundto defect ina lattice decaying wavefunctionas opposed allowed bands(conduction valence bands) free move.Crystal imperfections known bevacancies orimpuritiesthatareintroducedeitherdelib-erately unintentionally during growth process.Processing fabrication introduce Some unavoid-able they play role determining prop-ertiesofasemiconductor.Chemicalimpuritiesthatformpoint interstitially substitutionallyin lattice. An interstitial atom samespeciesastheatomsinthelattice(intrinsicdefects)orofadifferentspecies(extrinsicdefects).Defectscanalsobeformed vacant sites. There defectcomplexes conglomerations pointdefects.Besidespointdefects,therearealsoone-dimensionaldefects dislocations, two-dimensional defectssuch surfaces grain boundaries, three-dimensional micropipes cavities.Defects occur accordance laws thermo-dynamics mass action. Hence, theremovalorsuppressionofonetypeofdefectwillenhancethe effects another type. For instance, removal ofdefects boundaries dislocationsincreases significance defects.The presence semiconductors beeitherbeneficialordetrimental,dependingonthenatureof actual application materialin devices.Gold impurities silicon junctions pro-vide fast recombination resulting faster switchingtime. Impurities gold, zinc, mercury, etc., insiliconandgermaniumproducehighquantumefficiencyphotodetectors. emission wavelength light-emit-tingdiodes(LEDs)isdeterminedbythepresenceofdeeplevels. In undoped semi-insulating GaAs, family ofdeep donor levels, commonly EL2, compen-sates acceptor due carbon impurity giverise high-resistivity property.Chromium dope GaAs GaAs:Cr, although this no longer wide-spread use.Ontheotherhand,deviceperformanceandreliabilityare greatly suc-cess fiber optics–based telecommunication systemsemployinglaserdiodesdependscriticallyonthelifetimeof laser diodes LEDs. degradation laserdiodesandLEDshasbeenwidelyattributedtoformationof local regions where nonradiative