Garg, J C

Jain, S K

Rastogi, R K

Sharma, B D

IACS Institutional Repository

Indian J  Phys. 52A, 440-461 (1978)Anisotropy of selenium thin film obliquely deposited in vacuumJ. C. Cauo , T> Sjiabma, II. K , R-astogi and 8 K . JainDepartment of Phfjsics, Uinvc?\Rih^  of TtajaslJian, Jai2>ut''I’ho (lol)(indoi)(3(i of ro{i,ular roflocitaiKu', of lluii films ol Koloiiium \vitli, th(‘ i]\JckiioKH and vapour incidoTice an|j;l(*, k,as inoasiired atilifforcjit waVolonglJns A difforoiioci in rcdli c^taiioo is found liotwoiin llu3 positive and vM/alive curvos corrospondin" to t]u> inoidonco of I'gUt from tli(‘ sido of vapour boain and to that from tho opposite sido Tins suggests that tb.o surlane ol a soJoiiium film jirejiai-ofl by obliqu(3 inoidoiujo ovajioration is assymotrieal m its Strueturo Tho anisotrojiy ol oJootrieal resistivity with dojioKition angle has boon studied in air. TJui fbl'lbronco botwoon the values of resistivity nioasurixl in tvso flirordjon.s, parallel and porpondicular to the vapour beam oan bo explained by self sliadmving of growing crystallites and tint pri'Seiuie of stiuotural dcJect The jiresence of geomotric anisotropy in films d(3posite<l at an oblique aiigki of incidemu* is su])poi(i‘d by eliictron microHcojwc study1 . Tn t r o d t j c t t o nAmong smuioonductors selenium finds tlu» widest applications and in some of those, it IK irreplacealilo Selonium crystalisis in a trigonal stmctuie and the unit cell of the lattice contains three atoms arranged in thc^  charactoristie spiral chain TIm" l atio of the distances between next neiglibours in one cliain and betAVi'cn twm eh.ains of selennim is afiout 1-49 (Cooper 19()9) Tin* trigonal' sekiiiium thus possesses strong anisotrojiy of the structure and has a number of peculiar and iiiterostiiig pliysical iwopertios A peculiai- feature of the band Structure of selenium is an energy gap wh,ich exists between two gryups of valence bands Tins gap leads to a miiumuni m the spoedra at liigh energies, the main optical aiiLKofropy ],s also located at high energies The electrical conductivity of sedenium depends on the crystal direction Pai-allel to the o-axis the con­ductivity is gi eater than perpendicular to it. T}i,(> ratio 7?^  /^n s^ found to bo between the values 3 and 10Sekmium films have been prepared from the vajiGUr by a number of workers, the structure of thesis films depends upon deposition variables such as substiate material and substrate teiujioraturo (Tauraitieiio & Tau.raitus 1969, \ i^scakas et at 1969, Herd & Cliaudhari 1973) Howevoi, very little is known about tn.e effect of oblique incideiioo on the structure and the physical properties of selo- iman films It is well kiiovji (Konig & Kelwig 1950) that an obliquely incident film reveals chains of crystallites whoso long axi s aie oriented nemendioular to the duection of vapor beam. The oriented chains give rise to macroscopic effects anisotropy in jihotovoltago and photo oondnetivity (Kamiyama el al 1962),440magnt t^ic ani«otropy (Koiug c't Hdwig 1050, Siiiilh et ul 1000), D})1u'.a) ani>iotiox)V 1903, Tanaka t'i. Takaliaslii 1903) and anisotTo])ii! ivnifitaiu; * (Iliuaki 1003, Takalio,slu ct al 1971, Jam el al 1975). Tlio jhvhimico of miorosi'.ttjiio gooinotric aniiiotropjr in obliquo nunlonco lilin,s may thi'.fi bo siuduHl IVom maaohoopu* lueasuroniontH in tb,o windy tlio (kfX^ oiKloiUio ol ivguliui* ivlloct.anoL* nitbtlio thickiiosH and vapour iucidenuo anglo was moosiFrod at dirtori^ nt waudnigilis Tb.o cbango ni tJio josistivity m soIr>nmm films uitli. ihi(;knc,s.s aiul di'posiljon angles Avas also maasurod in air Tlu‘ c'-xx^ ormiontal rosulls an^  diiSi.u.sst'd on tin l)asis of Kolf .shadowing offoot2 E x p e r im e n t a l  P r o c e d u r eAnisotropy of selenium thin film  obliquely deposited 441Soloninin tilius wovii projiarod on to oonnug-7059 glas.s .sub.strato by ova]iora- tion of jmlyarystallmo inijc So from a Tantalum boat in a vaisuuin oJ about 10  ^ Ton Tho goonudrioal {UTangoment oJ tlio .souiro and tbo ,sub.slrat(‘ is slioAAm in figuri'! J. Tbo film tbiolinoNs was monitor(sl using Eizoan',s intorfoi’o-Q SUBSTRATh"  ^ 1 e  = 3 o " T ^ ^A-i V/ IOVe^45 I \/  ✓  I /e= 60*! \'  /\"  0 = 75* ■'1J L i r rSe EVAPORATORFig.I Schematic illustration of oblique deposition CAngleGis the deposition angle,R|| is the parallel resistivity \ Rj i^s the perpendicular resistivity3Fig. 1442 G a rg , S h a rm a , R a s to g i  a n d  J a i nmoiw mothf'fl. Tli« sub.strato wai3 kept at room tomporaturc (27"C), the deposi­tion arif^ h) vvas varied Trom 0'^  to with an interval of 15".Kefleetanee of tlni films were determined as a fmiction of angle of incidenoo between 2r>'^  and 70" at differeat vvaviilengths using the interference filters and polarizers. Th.c schematic diagram of reflectivity apparatus is sliown in figure 3 wliich essentiallj?- is a research, spectrometer fitted with polaroides. The ligh,t sovret  ^ was a high pre.ssure mercury vapour lamp and detector was a 1P28 tube Th(‘ sampkt mount was in the form of a window, being capable of rotating 111 vertical and horizontal planesTlie g(‘ometr^ of tb.e sample for el(*ctrical moasuremonts is slio^vn in figure 2, Large area oh.mic current-voltage contacts of evaporated aluminium were madeSeleniumSchematic representation of reflectometer S b Source of light; C L.^Condensing lense F “ Fllter j C =Collimator j P = Polorizer ;T  = rotating Table i Analyzer tO > telescope ^P.M. B Photo multiplier Tu b e .Fig. 3at the end of the seleTsium fUniti. TKo >spiicing botwwu ajumiiiium oloctrodes was 6 mm and the latoral dimoiusion of tlie film n as 4 mm RoSistiA ity JiioaKured along the diroetion parallel to t1\o incidonco piano of vapom- li(»am .shall bo oallod ‘parallel reKiativity’ and oxx»r(‘SSed as /?n in jH ixiondioular Hiieetion ‘]K'rpendi- eular resisti\ity’ i,s oxpio.ssod asTo moaauro tho realstanue of th.o film, tlio cm rout thi'ongb, the aj)ucimeii \\a,s measured iiiSing an Electrometo '^ Amxililier (BAEO model EA81(b\) ol range to amperes The d.e. biasing ol' tb,e saiuiilo was dom  ^ n.smg an elec­tronically regulated d.c snxDply (Philips maktO of range 2v50-2500 voltsAnisotropy of selenium thin film obliquely deposited 4433 R esults and  D iscussionA oj S^ dcniuiii Fib nsi, 5 and d sho\y tlio lolation botwcon the rofloctance and tlio iiicidiniue angle of ligUt-j 0 iM tile angle of ineidenee, oJ tUe vapour beam and I la the thickmiSS444 G a rg , S h a rm a , R a s to g i a n d  J a inot the film. In thoae tigiiro.s the notation positive oorrosxiond.s to tl\e inuidoneo of light bom the aide of the vapoui beam, negative to that from the oxjposite ■side. A dilformiee in reflectanco iK found between the posilive and the negative curves for low thioknessoa and large deposition angle This suggests that the surfae.e of the selenium film pro])ared by oblique incidence evajioration i,s asymme­trical in its structure'. The asymmetry decreases with increasing film thickness and increases uith angle of deposition and tJio same can bo oxiilainod by the ‘selishadouing’ that occurs during the oblique doiiosition The growing crys­tallites prevent the impinging atoms from reaching the substrate where tlieerydtallitos oa^t siliadows loavmg uiicloi^osiUxl aroas thoro in forming afilm, all tUo crystalhto.s grow long, urosswiso to tUe direoiion oJ dc^positiou WUcui th<» doi)o,sitioii angle is lai-ge,, tl\c rrystallitos grow more inclined, thereljy de\'olop- mg laigo asyniuiptr '^ m tlieir shape l^^ intiler, no detectable dd'forence in reflect­ance is found botweon the Positive and jVegative curves foi tUielviiosscfci largerAnisotropy of selenium thin film  obliquely deposited 445than 1000 A 8ne,h an efloct i,s expected in view oi the following : As the film becomes thicker, the spac'e between tln^  crystallite chains gets fillcsl and as a consoquonce assume svmmetrical shape making the surface of the film almost optically flat Th.e film dopositod at 0° angle of deposition and of thickness 500 A does not show the above effect. Thus it appears that> the surface of the film prepared by normal evaporation {0 =  0'’) possesses reflectance symmetry.8446 G a rg , S lia rm a , K a s t o g i  a n d  J a inTt is also nl)Sf‘fvrsfl tha( if tl'o illuruination is made* l>y tlin polarisod light, th<i intensity of j(»flec*f(Ml liglit rests witli tlu> firiimtation of tJio olotdrio Vector rolative to the direction of tlu’ vapour beam AvJiich imiilios that tho j-efloctivity of tli(! iiJn) has diUorent  ^aliu\s in two directions, parallel and porjxfndioillar to tho direction of deposition. Figure 7 shows tho variation of reflectivity of tho sample ot IhiLkness about tiOO A with the orientation of the (^ Itfctric vwtor ofReflectivity at a function of Orientation of lha Electric VectorFip 7the, iiieidinit plane polari^al light relative to the din^ction of the vapour beam. From these c,urves, it follows that at 0"’ deposition angle tho curve is sj'^mniotrical and is a ciielo As the de position angle increase,s, the curve develops asjnninotry witli unequal radii along some diameter We measure this anisotropy by the ratios aja,, and bjh^, vi^ hcwo a's and /v’.s stands for the radii along two mutually pcrpondieular diameters and are being plotted as a funotioon of deposition angleni ligiiHi 8 ri lollnws Irinu ti\o graph tliat liu‘ i:iTn) is iilmusl iiulopriulonl ui aiiglo ol (loiinsition w luK' tlio ratio rtj 7/, moieSaoS witU tho angle of deposition Tiu^ may he explained hy Se,lf sluniowing' deSeuht l^ earluaA n is o tr o p y  o f  selen iu m  thin film  obliquely deposited  447Deposition Angle 6PiK SFigure 0 shows the reHeetiviiy of the Selenium him of' tiuehiiess IhhoA, being depoSitiHl at an angle ol 15° as a function ot angle oi' meideiiei  ^ of light at (lifioront wavedeiigtlis It is Se^ ii th.d the magnitude of the relleetivity (haireases with deereasing wavelength The refliictiviiieS [R) aie liemg ealc.ulaled using tlio following expression,/('/) AR^ \^ ^^ |^^/(/) (1 - /f)H  4^ 1’rwn ' (f/2. 4tt(V  ^— / sin I.Awhere J{r) and J{i) am the intensities ol relioeted and tim meideiit beams, A is the wavelongth of tho light iisefl,  ^ is 11m angle of meidenee am) t is the ihieUness of the film The observed elToct follows the above expressions448G a rg , S h a rm a , R a s to g i a n d  J a inF ig . 95  ibdsolro'jiylOiguro 10 «lv>w« Ih o  d o p „ . . l e n c «  «1 th e  oh sorvedr  -4 - .  m  .Lnrl /? ) o f  th o  film w ith  depoH ition angle 0 before, and a t e  loaia iv i 10 . 1 1 T h is  figure sh ow s th a t  th e  parallel resistiv ityV S i T d —  *». “ ~ " 5”  f  ,| ■ 1 fiiiji (( — 1000 k )  has a con sta n t va lu e  with d ep ositionC  “  “  • “I — -  « ” > “  • " " • e - e„ r  d o p o d t lm  ■ W k . I -  « l ” ‘  “ «1 ™ "thiokiiossos All obliquely deposited tlim film has cihajii stmcuuo inth its long axis perpendicular to the beam diroetion Tht* sc'lf sliadov ing effect is rxpeciud to bo minimum along the direction perpendicular to the vapour Ivuiin Tliua variation of with deposition angle 0 slioiild be iiiueli lews than that of i?||. The exiieriuuuital results alst seem to be in accordaneci with the abovt'i pictiire.Anisotropy of selenium thin film  obliquely deposited 449The obHl^ rVfKl variation of R\\ for thin films and /?j. for thiclc films with deposition angle may possibly be attnbmed to the structural ch^ hicts, being developed in the Rims during deposition . the structural defects density is assumed to have a direct relation with the angle of deposition. To explain the above variation, let us consider the formula hir the conductivityrr — vie//,W'hcwe )t IS the carrier density, r is tlu^  electionic charge, and /i, the mobility. Tims, the conductivity for a given thickness increaSc^ S if n and/or // increases. For small thicknesses, one gtmiually expects that an increase in the number of delects will increase the carrier density but will decreases tbe mobility.As the selenium films generally show the /)-type (umductivity, one expects that an incioasi  ^ in acceptor sites due to defects will increase the caiTior density n but \vill decrease the mobility // An increase in // may also arise from an increase in grain size which is expected to be enhanced by self shadowing The observed increase in conductivity with deposition angk  ^ may thus bo accounted for the increased grain size Tlie ofi'ect of annealing on the resistivity of the film is depicted in figure 10 Annealing reduces the number of defects and, tluweTore, n. The reduction in defects together with the increased grain size both tend to increase //- The obser\od increase in conductivity associated with the change to the annealed state thus results primarily from an increase in /<.450 Ga-rg, Shanna, Rastogi and JainIvaliofi ai’o plottc-d against tiie dopoMition angle as sliowii in fignio 11.Jt IS found til at \Mth an increase in doijosition angle, anisotropy in resistivity incroaso lor thin films and docreases for thicker films. The effect is found to reduce after annealing the film. The observwl trend is in accordance with the aimve picture. Further, for a film of thicker 1000 A. deposited at 0^  the ratio is fbuiid 1,0 b(i about 2 vvliich is m good agreement with the value for tlio hulk selenium (Cooper 1909).Fie: 11Figure 12 shows the electron micrograp]i, of the film of tliicknoss 300 A deposifcwl at an angle of mcidonoe of 75” In this the agglomeration of crystallites into chain v'hoso long axes are on the average porjieTulieular to the beam diree- tion can lie infeiTod. It thus jirovidos dmwt evirleiice that goometric anisotiojiy on a niiero«copio seale exists m ofiliipie-ini uheici' Sedeniem films and leads to an interpi-etation to the origin of amsotropyAcKNowUiinn e mentThe work was done under tlie C !S I B, n^ soaieli Scheme One oi the authors (SK J ) is grateful to CS f.R., New Delhi for tJui award of J.R.F The eo-OIako, Shaema, Rastooi&.Tak Indian J PhyN. VoJ. fl2A No S 1078Fig 12ojieratiou oxtondod 1)V T)r 0 K. Snvastava at B H U , \"arciii!isi n\ tho uso of olottl/T'on ?Ti](!ro,scopes is ‘ifratoliilly ackllo^ vle.d^ (^^ dA n is o t r o p y  o f  se len iu m  th in  f i lm  o b liq u e ly  d ep osited  451R tcfkhbncesCoopei W C (od ) 1!)G9 The of Selenitnu autl Tcflun'iiru, IV'r'^ amnn l*uws, iiOtidoti.HavaUi B JOfiS 0,</o. Buturi 32. HH.Hard S. R. & Chauilluiri I’ lIHli ./ Appl. Phi/i.. 44, 1102.Jam I P , Thakuj’ T). K- & Gaip J V 1975 Iinbrni A Pure Appl Phys 13. 4U2 Kamiyarnti M., Huiwlomo M. & IvuUimoi-o H. I!)(i2 Japan J. appl Phy,\ 1, 2(»2 Knuig H & Holwifj; G. 1950 Optik 6, 111.Niikai J I9G;1 Oyo BufiinSZ, 91Smith D. 0 . Cohen M. S, & Woi.ss G \\ lllfiO J . appl Ph>p. 31, 1755 Takahashi M , Oriuahi H & Osamii Tada 1971 J. appl Phys 42, H9I1 TmiaUa S &, Talcahashi T 19(K1 Oyo Batun 32, 172.Tauiail,ieno S A. & Tiiurailiis A S 19«9 Vopr, Pacliocdcktromh 25, 21,ViHeakaa J K , Mnninmsm G A SaliV'A. A, 1969 Lvit. Fiz linili 11, S2I.

Anisotropy of Selenium Thin Film Obliquely Deposited In Vacuum

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Anisotropy of Selenium Thin Film Obliquely Deposited In Vacuum

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