ZnO

Sample Solution

    Zinc Oxide, or ZnO, is a unique material that has been used in various applications for centuries. While it has been studied for many years, the recent surge of interest in its properties and potential applications makes it an important topic to review. In this literature review we will be exploring the physical and chemical properties of ZnO; structural characteristics, electronic band structure, electrical properties, optical properties and thermal properties.

Structural Characteristics of ZnO The crystal structure of ZnO can vary depending on synthesis methods – thus forming different polymorphs such as hexagonal wurtzite (WZ) and orthorhombic scheelite (OS). The most common polymorph found in nature is WZ-type due to its stability at room temperature and pressure. The lattice parameters are typically a= 3.249 Ångstroms and c = 5.205 Ångstroms with a space group P63mc symmetry ~ 4mm2/V unit cell volume while the OS type has a= 5.411Ångstroms b= 7.883 Ångströms c = 5.187 Ångströms with a space group Pbca symmetry ~ 40 mm2/V unit cell volume [1]. Its cubic unit cell consists of four zinc atoms surrounded by oxygen atom octahedra [2]. For WZ-type materials grown from vapor phase techniques such as metal organic chemical vapor deposition (MOCVD), growth direction is along the <001> crystallographic axes whereas growth direction for OS-type materials grown by solid state reaction usually proceeds along <110> direction indicating different establishment layers arrangement [3]. Electronic Band Structure The electronic band structure determines the electronic transport characteristics which affects optoelectronic devices performance [4]. In general, ZnO belongs to direct wide gap semiconductor family with an energy gap between 2 eV - 3 eV corresponding to their UV emission spectra peak wavelengths around 365 nm - 400 nm [5]. Depending on doping levels or lattice strain conditions small variations occur on their energy bands domain leading to visible range emissions between 500 nm - 700nm[6] . For instance under n-type doping electrons are added into conduction band hence lowering its energy level relative to valence band edge which consequently shifts UV emission spectrum towards lower wavelength values resulting in blue light emission peaks up to 475 nm[7] .Under p-typedoping holes are added into valence band resulting inthe reverse effect i.e shifting ultraviolet spectra towards higher wavelength values corresponding red lightemissions up 500nm[8][9] . Electrical Properties When considering electrical conductivity two main factors must be taken into account: carrier concentrationand mobility factor[10], where both these factors depend heavilyon dopants employed during synthesis processes.[11][12] Thus pure undoped polycrystalline samples show low electrical conductivities due to natural minor impurities present in raw material sources whereas doped samples show improved conductivities mainly attributedto Schottky barrier height reduction allowing easiercharge carriers’ tunneldiffusion.[13][14] These effects have been observed bothin n-typesamplesdopedwith aluminum showing improved hole concentrations per cm3as well asin p-typesamples doped withexcess zinc withconsequentlyimproved electron concentrationsper cm3.[15][16] Althoughconductivity enhancementhas alsobeenobserved when using other transition metals such as manganese(Mg)[17],beryllium(Be)[18], gallium(Ga)[19], silicon (Si) [20], cobalt(Co) [21],[22],molybdenum(Mo)etc.,aluminum still remains oneof themost popular choicefor p-typeddopingdueits betterstabilityat high temperaturesandnonvolatilityeffects when comparedwithothertransitionmetalsallowingbetter control over dopinglevelsduringprocessvariations.[23][24 ][25 ]Moreover these materials show good semi insulatingcharacteristicsatlowtemperaturesless than150Kmakingthem idealcandidatesforthermalmanagementapplicationssuchasLEDproduction processes.[26 ] Optical Properties As mentioned beforecertainamountsofdonorsandacceptorscan leadtovisiblelightemissiondependingontypedopantsaddedto rawmaterialcompositionproviding goodcosteffective solutionsforphotonicdevicesfabricationapplicationseitherby reducingenergygapvaluesornarrowingbandwidthdistributionsaccordingly.[27 ][28] Forthispurposebandgapengineeringtechniqueshavebeenimplementedindifferentways varyingfromsimplesubstitutionmethodswithchalcogenides elements likeBr ,Sulphur ,SeleniumortransitionmetaloxidessuchasTiOx ,SnxorAl x[29–31 ],to novelheterojunctionstructurescombiningdifferentgroupIII–VIcompoundslikealloyTernaryInGax AIx Asy providingmoreflexible tuningparametersforspecificwavelengthbasedoptoelectronicdevices designs[32 ][33 ]. AdditionallythesematerialshavebeenreallyusefulforUV detectionpurposesduetotheirhighabsorptionpropertiesshowing larger absorptioncoefficientsreachingvaluesaround 10^4 cmat300nmregionwhichismorethanoneorderof magnitudelargerthanGaAsspectrumsshowingonly102cm absorbtioncoefficientsat same region makingthem moresuitablecandidatesforUVsensorapplications.[34 35 36 37 ] Thermal Properties Thermalproperties playessentialrolefordeviceslifetimepredictionespecially formobilenanoelectronicswheregoodcontrolandquickresponsearerequiredwhen triggeringvariousoperationalstatesundergivenconditionssuchastemperatureregimeschangingfromnormalambientroomtemperaturesup tobothlowextremeconditionsdownbelow100K reachingablankstateforthesystemshutdownwheneverdevice trip currentsarereachedorpulsedcurrent drivingisneededtotriggerchosenoperationalstatesinthedevicesignalprocessingcircuitriesdesiredperformancevaliditychecksespeciallywhen employingnanoscaledeviceshavingverysmallmarginsoferrorcalculationsduringoperationprocessesduetoshrinkingdimensionsizesatthelowerboundariesofthemicroscopicworld.*38*39*40 All abovementionedcharacteristicsoftheZnovariableparametersallowstimenoustrappingtheirenhancedpropertiesleadingoptimal performancesneededforendusersatisfactiontobeachievedinawiderangeof real time scenarioscoveringwidevarietyoffieldsandindustriesranging frombiomedicalapplicationsthroughdataprocessingschemestomaterialscienceresearchfieldsbeingutilizedeconomicallyfeasiblesolutionsforyourdaytoday problemsolutionneedsinthepresenttimesituationwefindourselveslivingin.*41 *42 *43* To summarize all aforementioned data regarding physical/chemicalpropertiesofZnoitcanbeclearlyseenthatthesisamaterialpossesses widevarietyoffeaturesbenefitingenduserinnumerousrealtimeapplicationsschemes provingconstantlythatitstillholdsalotpotentialundiscoveredyetawaitingfurtherexplorationandresearchesinvestigationstopushtherightboundaries neededformakingdreamscometruefindingrespectiveanswersfortoday’schallengingeverydaysituationalissuesinsolvingournowadaysmostdemanded needsandsolutionsforthosequestionsraisedbyourenvyingworldmarketrequirements demandsintroducedintooursocietiesdailybasistodaysstandards.*44*45