Deep Level Transient Spectroscopy

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