Preparation And Properties Of Evaporated CdTe Films .
Progress Report No . 6PREPARATION AND PROPERTIES OF EVAPORATED CdTe FILMSCOMPARED WITH SINGLE-CRYSTAL CdTeFebruary 1, 1982 - April 30, 1982SERI/PR--9330-1-TGDE82 017062Subcontract No . XW-1 9330-1Solar Energy Research InstituterDeoa-r ttmen tE,,.o.f.-EnergyGolden, ColoradoDISCLAIMER-----,,.--of the United States Government.Th1s book wa preoored as an account of work soonsored bv an agencyof the1f employees. makes anyNeither the United States Government nor .,ny agencv theteof. r'!Cr anvre5ponsibilitv for m accuracv.l)rtlile; alanyassumesorwrrantv, li'XPres5 or fmolied. rodUet. or orocess diSClOsed rcom )1eteness. ut usefulness of anv information, JPDaratus,.rlgnts. ?.eierei'IG! herein t o anv saec1hcreort Sents that its use 'M)Uid not infrin!Je privately owneodOescum rcial urodiJCt. rocess. or serVIC :: bv trade name. tr emark, manufacturer, or othetwise,lidO i vor favoring bY tl'l e Unitednot necessarily oonstitute or imply its ndOtS8ment. reco,r:mendation.of authors o!XOressed herem do notStates Governmt!nt or arw aqencv tl'lefeoi. Tht! views and ooiniartsany agency thereof.necessarily state or reflect !l'letse .;:n the U11ited States G lvernment orRichard H. Bube , Principal InvestigatorAlan L . Fahrenbruch, Senior Research AssociateWalter Huber , Post-Doctoral FellowCharles Fortmann and Thomas Thorpe, Graduate Research AssistantsDepartment of Materials S cience and EngineeringStanford UniversityS tanford, California 94305OJSTRJBUTJON OE IHIS DOCUMENT IS ONLIMITE[)
CONTENTSINTRODUCTION3HOT-WALL VACUUM EVAPORATIONOF n-TYPE CdTe:In LAYERS5CdTe Films on 705 9 GlassSub s trates58CdTe Films on BaF2Qualitative ElectronicTransport Model for Poly Crystalline n-type CdTe Films 11CdTe Films on Metal/7059Glass Sub s trates12DEVICES: n-CdTe:In LAYERS ONp-CdTe:P SINGLE CRYSTALSUBSTRATES15GRAIN BOUNDARY STUDIES19FUTURE PLANS24REFERENCES26APPENDIX: "Large Grain CdTe Thin Films onSb-Bi Alloy Coated Ta Sub s trates" byN . Romeo and R . H . Bub e
2.ABSTRACTFilms of n-type CdTe:In have b een deposited by hot-wall vacuumevaporation (HWVE) on 7059 glass sub strates, BaF2s ingle crys tal substrates,metal (Pt, Cr, Mo , Al) coated glass substrates, and single crystal p-typeCdTe sub s trates .Films deposited on 7059 glass show typically a darkres istivity of 2 x 1052ohm-em and a light resistivity of 3 x 10 ohm-em .With increasing In source temperature, the resis tivity decreases, butactually increases slightly again if the Tin is raised above 600 C .Photoexcitation increases the electron density but does not affect theelectron mobility .It appears that the grains are depleted in the dark .Films deposited on BaF show dark resistivity of about 5 ohm-em2and light resistivity of about 2 ohm-em , corresponding to electron densitiesof about 3 x 1016-32and electron mobilities of about 30 cm /V-sec .emFor16-3doping levels above 10cmphotoexcitation increases the mobility, butnot the electron density; it appears that the grains are not depleted inthe dark in this case .Cr coated 705 9 glass makes an ohmic contact to n-type CdTe f ilms .A S chottky diode formed with a lOOA thick Au layer showed VJsc 29 mA/cm and a solar efficiency of about 2% .oc 0 . 46 V,An n/p homojunction device was made b y HWVE deposition of a 1 . 5 mthick n-type CdTe layer on a p-type CdTe single crys tal substrate, reproducingthe earlier cells made in this way in Linz .Jsc fValues of Voc 0 . 73 V and20 . 24 mA cm (because o f the thickness of the n-layer) were obtained .Modelling of the spectral response indicated Lp 0 . 28 m .Grain boundary investigations showed the additive quality of twoindependent grain boundaries when measured in series, and tes ted the effectsof" "passivation" by Au, Cu, Li and HIn in n-type CdTe grain boundaries .2in p-type CdTe grain boundaries, andMarked decreases in grain boundary res is tancewere observed after Li diffusion and H2diffusion in p-type CdTe .
3.INTRODUCTIONDuring this contract period the emphasis has been placed on growthand characterization of n-type C dTe:In films on various substrates in orderto explore the effects of growth parameters on the photoelectronic andphysical properties of the films .These films have been grown on substrateso f 7059 glas s , metal-coated 7059 glass, BaFp-type CdTe:P single crystals .2single crystals , andAlthough the principal thrust o f ourresearch is toward growth o f polycrystalline films on amorphous substrates( such as glass or stainless steel) , the analysis of such polycrystallinefilms is difficult .Since films grown on single crystal substrates havemuch larger grains with low angle grain boundaries , these are used asdiagno stic tools to elucidate film properties that would otherwise b eobscured by grain boundary phenomena .The most important of these propertiesare carrier density and diffusion length inside the grains .In many casescomparisons are made between films simultaneously grown on single crystaland amorphous substrates .Low cost polycrystalline film solar cells will ultimately be depos itedon glass , ceramic , or metal substrates .In the case o f insulating substrates ,a high conductivity layer that serves to collect current and to form theohmic contact to the active layers must exist b etween the substrate and theCdTe film .To choose such a conductive layer or metal substrate , the followingmust be known: (1) the effects of the conducting layer on growth and propertieso f the film ( i . e . , growth rate , grain size , doping density , etc . ) ; ( 2 ) chemicalreactions between the conducting layer and the CdTe layer at the growtht emperature ; and (3 ) the ohmic characterof the contact thus formed .Thereforegrowth on metal-coated 705 9 substrates has been initiated to answer thesequestions .Thin metal films (59-llOA) were used so that large-scale chemicalr eactions would be observable and thick films of the same metals were used
4.t o eliminate s eries resistance effects for electrical characterization .A particular research proj ect along this line was carried out withthe participation of Professor Nicola Romeo , a visitor from the University ofP arma , and an authority on grapho-epitaxy .Thin CdTe films were deposited onSb-Bi alloy films deposited on tantalum substrates.o f 20 m were achieved .Grain sizes of the orderA manuscript has been submitted to the Journal ofV acuum S cience and Technology , and a copy of this manuscript is included asan Appendix to this report .Measurements of the electrical properties o f grain boundaries inlarge grain single crys tal material (bicrystals ) are being continued to(1) provide input for modeling of the electrical transport in thinpolycrystalline films , and ( 2 ) give information on possible grain boundaryp assivation metho ds .
5.HOT WALL VACUU}f EVAPORATION OF n-TYPE CdTe:In LAYERSCdTe Films on 705 9 Glass Substrates1.GrowthThe CdTeThe growth conditions are lis ted in Table I . source temperature was maintained at 575 C for all the films grown .The indium source temperature was varied between 540were grown either under excess Te or excess Cd (TSubs trate temperatures TTs in the range of 430s and 6 80 C .Films0Cd 205-245 C; TTe to 510 C were used . 4 80 C film nucleation and growth did not occur .typically of the order of 7 m/h . 0 360 C) .AboveThe growth rate wasThe typical grain s ize is approximately1 m and X-ray diffractometer measurements indicate no preferential orientationo f the grains .Microprobe analyses of these films indicate good stoichiometryand spectrophotometer traces show the expected optical transmiss ion spectra .Electrical Properties2.The electrical characteristics of some representative samplesare listed in Table II .the resistivitywas measured using a four-point method .varied between 1054000and 6 80 C .Contacts of In were evaporated onto these films and8and 105The dark resistivityohm-em for indium source temperature TinbetweenIllumination with s imulated AM1 . 5 sunlight caused theresis tivity to -drop by orders of magnitude with the lowest observed valuebeing 225 ohm-em .This is quite adequate for a solar cell absorber layerwith complete coverage by the ohmic contact .The resistivity variesexponentially with 1/T with an activation energy AE .Dark values of theactivation energy are between 0 . 79 and 0 . 48 eV , while illumination reducesthe activation energy to 0 . 1 eV or less for all these films .The variation of the electrical properties with excess Te or Cdwas also examined.The results (shown in Tab le II) give no clear indication
6.TABLE IGrowth Conditions for CdTe FilmsFilm flSubs trateT0CCdTe'0Ti ' Cn0Te ' cdTCTe' T ' Cst , llm0Growth Rate ,llm/h17059 glass5 755403604302.08.027059 glass5755403604702.08.037059 glass5755403605100047059 glass5755403604 800057059 glass5 755402454 303.067059 glass5 755402"4 54700077059 glass5 756 302454501.35 .287059 glass5 756 802454502.16.39A7059 glass5 756 802504500.96.89Bp-CdTe crystal5756 802504501.511 . 3lOB7059 glass5755902454501.85 .4llB7059 glass55059024545012A7059 glass5755902454501.61.912BPt/ 7059 glass5 755902454501.61.9137059 glass5 755902054 501.85 .4147059 glass5 755902204502.57 .519APt/7059 glass56056125047019Bcr/7059 glass56056125047020AMo/7059 glass5655612504700020BCr/7059 glass56556125047021BCr/ 7059 glass56151024548022Al/7059 glass5705 8125045523Al/7059 glass5755 7525045524Al/7059 glass5255 2525045526Cr/7059 glass5 7059025047012 . 0
,'7.TABLE II Electrical Properties of Representative CdTe Films on 705 9 Glass at 300 KFilm #1Resistivity , ohm-emDarkLight (AM 1 . 5 )3 .1X510144.2X10X10455465Activation Energy of Resistivity , eVLight (AM 1 . 5 )Dark43 .5X102 .3X102.0X105.0X2101.8X106 .3X102 .2X106 .4X2103 .3.5 .8X10X104430 . 790 . 100 . 480 . 080 . 4022220 . 480 . 050 . 560 . 040 . 490 . 040 . 48
8. o f the influence o f the Te or Cd source for an In s ource temperature of 5 40 C .A simple calculation shows that the Cd and Te pressures supplied by the CdTes o urce are of the same order of magnitude as the vapor pressure of the excessCd or Te sources at 245 and 360 C respectively .Therefore a lowering of theexcess element source temperature should not result in a s trong variationo f the vapor species above the substrate .The film resistivity as a function of the indium source temperaturei s plotted in Figure 1 .0o f about 600 C,The resistivity has a plateau for Tinand further increase of Ti does not result in a reduction of the resistivity .n-311inemHall effect measurements indicate carrier densities of 10the dark.Figure 2 shows a plo t of the electron density and mobility asa function of layer resistivity under varying light intensity .The mobilityi s not changed by illumination , whereas the electron density changes byo rders of magnitude .It may be c oncluded , therefore , that the photoconductivityo f these CdTe f ilms on glass results entirely from a change in the dens ity off ree electrons .C dTe Films on BaF21.GrowthThe purpose of the growth of CdTe layers on BaF subs trates2was to obtain large grain CdTe films and to compare the electricalp roperties of thes e films with those grown on 7059 glass substrates .The films were grown on air-cleaved (111) BaF sub strates.2BaF2i s an insulator with high optical transmission in the visible range , a latticecons tant of 6 . 2A compared to 6 . 48A for CdTe, and a thermal expans ion coefficientthree times larger than than of CdTe .The growth conditions are listed in Table III .The substrate temperatures used are ;n the range 450-480 C, while typical CdTe source
109--CdTe ON7059 GLASS IN THE DARKoIN THE LIGHTCdTe ON---- \ BaF2 IN THE DARK\1 IN THE LIGHT- c:105-Q-. -. 104.-(/)Cl)UJ0::1 0 -- ----. -- 110350400INDIUMFigure 1.450500550600SOURCE TEMPERATURE650( C)The dark and light resistivity o f CdTe films grown on7059 glass sub s trates, and on s ingle crystal BaFs ubstrates as a func tion o f the temperature of t e indiumreservoir T .fi
, AMl.S 410 1DARKCdTe ON 7059 GLASS". !,.-(") J.E13 10 - ·ItJ)z·w20 10 1.\i.I C3CdTe #10-310'·-i-f- -!-2-2- 0::10 .f210 Figure 2 .410 '"'(,):t. I Jlil0 -L-----L---- . . , --J.310 .-110210(,)Q.)Cl)E111 o1o4CdTe :1/ g 2LU.0::010510RESISITIVITY p {.O.cm)610o10Variation o f the electron density and mobility underoptical excitation for CdTe films grown on 7059 glasss ub s trates .
TABLE III'Growth Conditions and Characteristic Properties of CdTe Films on BaF2 SubstratesFilmIIooooTC T ' C T ' c Ts, cInCdCdTe'GrowthRate,t, llm lJm/haResistivity, 0245480o255623742454800.84bLight(AM1.5)Hall Measurementsp,ohm-cm n,cm-3 l1 5cc35a Measured by four-point probe method.b Deposition occured, but film peeled off after growth.c These values were obtained from photo-Hall effect measurementsunder illumination.\0.
·'10 .0temperatures were 550-575 C .The Cd reservoir temperature was maintained at 245 C, and the In temperature was varied between 374 and 630 C .For the typical surface morphology of CdTe layer growth atTs 480 C, as shown in Figure 3, one observes a featureless background withshallow pyramids arising from i t .in the crystal .1These pyramid soriginate from defectsThe distribution of the pyramids over the film surfaceis nonuniform and the layers of the pyramids are typically in the rangeX-ray diffractometer measurements indicate a potentialo f 30 ]lm.orientation of the grains in the ( 111) direction .BaF has a cubic2s tructure and the cleaved surface is ( 111) oriented .2.Electrical PropertiesThe electrical properties of these films were obtained usinga four-point measurement for resistivity and the Van der Pauw geometryThe results are s ummar ized in Table III .for Hall measurements .methods give the same result for the resistivity .4from 10 to 1 ohm-em with TFigure 1 .inbetween 320 BothThe resis tivity changes and 630 C, as also shown in greater than 500 CThe variation o f the resistivity for Tinis similar to that for the CdTe layers grown on 7059 glass subs trates .The change of resis tivity for the low-resistivity films under illuminationis much less pronounced than for the CdTe samples on glass substrates .The resistivity of film #lOA gr own on BaF simul taneously2 with a film #lOB on 7059 glass at T 450 C, is more than an order ofsmagnitude lower than for sample #lOB .at Ts On the other hand, a film grown 480 C and at the same indium temperature ( e . g . , film #16 ) hadan even smaller resis tivity than s ample lflOA.The resistivity offilm #lOA varies wi th temperature according to an activation energy of0 . 25 eV in the dark, compar ed wi th an activation ener gy of 0 . 06 eV fora film deposited at Ts 480 C ( e . g . , film # 15 ) .These results show
Figure 3 .Surface morphology o f a CdTe film grown on BaFat I0 480 C and a growth rate o f 1.4 pm/h .sMagni fication: SOOX2
-100 1-- - 0 J-(1) 0 - .J.-NCOEOu::E::L40 0 0 20 I-II17101II CdTe ON BaF21d5 IN THE DARK0IN THE LIGHTL-3504004505005506 00-- ---- -- 650INDIUM SOURCE. TEMPERATURE ( C.)Figure 4.(a) The electron mobility , and ( b ) the electron density ofCdTe layers on BaF as a function of·the indium sourcetemperature The solid dots are dark measurements andthe open circ f s are measurements under 80 mW/ cm2 of white. light . t
11 .that the grain boundary effects are more pronoun ced in this sample than forfilms grown at a higher substrate temperature .are shown inThe electron density and mobility as a function of TinFigure 4 .We were unable to measure the dark value of the electron densityf or the most photosensitive sample (#25 ) grown at T0in 374 C .By varyingthe light intensity one observes a change in the mobility, butno change inthe electron density .We assume, therefore, that the experimental pointf or sample #25 represents also the electron density in the dark .For doping levels above 1016-3the dens ity of electrons doescmnot change with illumination, whereas the mobility increases .Theelectron density in CdTe #18 was also determined from capacitance vs voltagedata for a Au S chottky barrier, and is in good agreement with the Hallmeasurement .Electron densities above 1016-30emfor Tgreater than 500 Cinwere als o found in the earlier resear ch on hot-wall vacuum evaporation ofCdTe films at Linz .Qualitative Electronic Transport Model for Polycrystalline n-type CdTe FilmsAlthough the CdTe layers on glass and on BaF were grown at2similar indium temperatures, the electron density differs by several orderso f magnitude ( 1011-3-316on BaF ) .emcmon glass compared to 102Thisresult can be explained by the grain boundary transport proposed by Setoand Baccarani .32These authors showed that the grains in small grain s izef ilms (such as CdTe on 705 9 glas s ) can be completely depleted, and thed ensity of free carriers is reduced by orders of magnitude .On the otherhand, the carrier density in large grain films ( such as CdTe on BaF )2approaches the bulk doping dens ity .Illumination modulates the density o f trapped charge at thegrain boundaries.For layers with completely depleted grains, the increasein conductivity with illumination can result almost entir ely from an
In CONTACTS -.Fig. 5 .fabricated on metal-coated 7059 glass.Geometry of devlces.
12 .increase in the carrier density, whereas in the case o f partly depleted grainsthe modulation of the grain boundary barrier height changes the mobility .4This is in agreement with our experimental results .A more quantitative electrical transport computer model forp olycrys talline CdTe films is being formulated .CdTe Films on Metal/7059-Glass Sub s tratesFilms of CdTe:In were deposited on substrates of metal-coated705 9 glass in order to observe the effects on f ilm growth and to determinethe feasibility of ohmic back-contact formation .The substrates wereprepared by deposition of f ilms of Pt, Cr, and Mo on 7059 glass substratesby electron beam evaporation .Thin metal films, 50 to lOOA thick, werechosen so that: ( 1 ) large s cale chemical interactions between metal and CdTewould be directly observable, and (2) illumination of the CdTe through themetal film would be possible .Thicker films, greater than SOOOA, were usedf or electrical characterization when series resistance was a problem.The growth parameters of these CdTe films are included in Table I .Samples with metal-coated glass substrates were grown at higher substrate temperatures (470 C ) , allowing comparison of these f ilms with thosegrown under similar conditions on BaF 2The interaction energy betweenCdTe and Cr or Pt appears to be larger than that of either Mo/7059 or07059 alone, since no deposition occurred on the latter substrates at 470 C .The n-CdTe:In layer s were greater than 1 m thick and appeared tobe extremely uniform ; these films had a grain s ize of 1 to 2 m and a grains tructure similar to samples grown on 705 9 glass without metal .There didnot appear to be any appreciable diffusion of the s ubtrate metal into theCdTe layer or chemical reaction, since the thin metal layers remained brightand without defects .Strip contacts of In were applied (Figure 5 ) to serve as ohmic
13.2contacts and thin (lOOA) Au dots with area of 3mmwere applied to the top sur faceof the CdTe grown on the Cr-coated substrates to serve as diagnos tic diodes.The Cr-coated 7059 substrates are the most completely characterized to date.The In strips made ohmic contact to the n-CdTe: In as expected.Measurementsof resistivity through the film using the top In contacts and the back Crcontact (which proved to be a low resis tance contact) yielded an upper bound4for the bulk n-CdTe resistivity of 1.3 x 10 ohm-em in the dark (withoutaccounting for any possible contact resis tance) .The dark log J vs V plot for s ample #19 is shown in Figure 6 ; thevalues o f A and J0-62A/cm respectively.are 2.1 and 5 x 10I vs V plo ts are shown in Figure 7 .The lOOA Au layer forming the j unctionb arrier absorbs 30 to 50% of the incident light.Jscand VocLight and darkThe measured values for2at an illumination of AM 1.5 are 9'mA/cm and 0 . 46 V respectively.The measured value of Vocis in reasonable agreement with the value calculatedfromv.A kToc which yields a value of 0.41 V.qln{ (J/J ) 1}sc 0(1)The fill factor is 0.41 and the solarefficiency was 1.9% for this cell.Samples #19 and #20, grown unders imilar conditions, showed almost the same photovoltaic parameters.Capacitance vs voltage measurements at 10 kHz on sample #19yielded a barrier height of 0.42 V and a carrier density of 10as indicated in Figur e15-3cm8.Two aspects of the electrical characteris tics of this Au/CdTej unction (#10) suggest that it is more complex than a simple S chottkydiode.First, the diode factor is 2.2, rather than the value of 1.0as expected for a simple Schottky diode.Second, the Vthe barrier height measured by C-V techniques.presence ofaocis larger thanThis suggests thethin insulating layer between the Au and the n-CdTe:Inf ilm and/ or the involvement o f considerable depletion region recombination
10 r---- -- HWVE#19-NEHWVE#910-5(.). (-. .Cf)zw0-tzUJ0:::0:::::lu1(}610-71 o- 891 0-10-;10-- -L -- ------ --------L-------L------00.20.41.00.60.81.2BIAS (V)Figure 6 .andDark J-V charac terist ics for an n/p homoj unctio n (fiNE 119 ) /'"'-'"' .: ---- .:--/UT.TIT'C':!!10'1
.- (-fi10-.zUJa a::: (J10 510-J32110-s-1-2Figure 7.Dark and 1i ht (AM 1. 5) I-V characteristics for Au/n-CdTe(HWVE #19),VArea3 mm0.46 V , J9 mA/cm2, ff0.41.
X X XCD X X X X X X(.0 X fJ). !: .·o cL .o :o; r-f XLn . s!I c.c. .;-1- -u l.c. I\13: tIs: .,0. - OJr-1 '1 I0. QJ ·0 ia t.n!.nUJC\lUJ.Nm.co. E-.,wo5en.
1/.,., .in the j unction transpo.r t .Through the film resistivity measurements made on n-CdTe:In on aP t ( 5 0A) -coated 7059 glass substrate indicated.that the Pt alo makes arelatively low resis tance contact, although with some rectifyingcharacteristics .Au/n-CdTe/Pt diodes proved difficult to characterizedue to the large s eries resistance of the thin Pt layer, however .Four samples (HWVE #22, #23, and #24A and B) were grown onAl coated 705 9 glass sub s trates (1 . 3 m Al deposited by r f sputtering) .Samples #24A and #24B were prepared on sub s trates that had been annealedin a partical pressure o f Cd ( 10to the n-CdTe:In deposition .-2mmHg) for 45 minutes in situ priorThe Cd partial pressure anneal was used toprevent the diffusion of Cd from the growing n-CdTe:In film into the5aluminum by saturating the Al with Cd .In all cases ( #23, 24A and B ) eithera very resistive contact or a diode was formed .Diodes between theAl-coated 7059 sub s trate and the n-CdTe:In had A factors of about 3 .The formation of a low resistance contact between Cr andn-CdTe:In is expected s in ce the indicated barrier height (using awork function of 4 . 5 eV for Cr ) is only 0 . 1 to 0 .2 eV .However, thef ormation of a low resistance contact to Pt is quite unexpected s in cea barrier height of 1 . 3 eV is calculated, and other workers havemeasured barrier heights of 0 . 6 to 0 . 7 eV ( usually slightly larger thanthe Au/n-CdTe barrier ) .Similarly the high resistance and/ or diodecontacts formed when n-CdTe is deposited on Al are surprising .Thework function of Al indicates a barrier height o f less than 0 . 1 eV ;in addition Al is an n-type dopant in CdTe .These results, althoughquite preliminary, suggest either than the out-diffusion of Cd formsan insulating region in the CdTe adj acent to the Al or that acompound (e . g . , Al Te , a semiconductor with baridgap o f about 2 . 5 eV)2 3forms between the Al and the n-CdTe at high growth temperatures .
15 .DEVICES:n-CdTe:In LAYERS ON p-CdTe:P SINGLE CRYSTAL SUB STRATESIn previous Progress Repor ts result s were reported on n/p homojunctions formed by HWVE deposition o f n-CdTe:In layers at Linz, Austria,In order toon p-CdTe:P single crystal sub strates supplied by Stanford .reproduce and refine these results, similar devices were made bydeposition with the Stanford HWVE system .The properties of thesejunctions, with thinner n-type layers , show good agreement with spectralresponse modeling and provide estimates o f the minority carrier diffusionlength L in thes e materials .pA large-grain (3 to 5mm )polycrystalline p-CdTe: P substratewas prepared by mechanical polishing and then etching a 1with a 5% Br:MeOh etch .mmthick waferThe growth conditions for the film on thissubstrates (sample #9 ) were chosen to be similar to samples #1 through #8grown on 7059 glass, thus providing a large grain epitaxial thin filmmaterial with which the small grain (0 . 5 to 1 m) films on 7059 glasscould be compared (Figure 9 ) .Ohmic contacts to the n-CdTe film were fabricated by thedep osition o f In s tripes by vacuum evaporation followed by annealing inThen Cu:Au alloy contactsHz for 10 minutes to finish the contacting .were deposited on the surface of the p-CdTe previously etched withH so :K cr o :H o, thus forming ohmic contacts to the p-side of the cell.2 4 2 2 7 2The bulk resistivity of the grown n-CdTe:In layer was 2500 ohm-emin the dark and 2 . 5 ohm-em under simulated AM 1 . 5 solar illumination .For comparison, the resistivities of samples grown on glass under similarconditions were 3x3510 ohm-em in the dark, and 10 ohm-em in the light.Dark log J vs V measurements (Figure 6 ) reveal normal diodebehavior with an A factor of 2 . 3 and JAM 1. 5 illumination, the values o f Voc0 3xand Jsc10-lO2A/cm Under simulated2were 0 . 73 V and 0 . 24 mA/cm
.··' . · . :9-. .:.- ·. .·. . .·) · :. . . . r ·.:. -. -.:: .: ·-.· ; :. .··':.:. .:.:.·-.-· ; - .-- ··-. - . · -. - .·--: ·-·. . .'"':" ::: - ·:.:· --- - .---.-·.-. ,.,---:--------.-.- --.-. . . -.;. -.20Figure 9. m. Optical micrographs o f n- CdTe layer grown ona p-type single crys tal CdTe subs trate(HWVE /!9 ) .
16.respectively (Figure 10).The low fill factor is characteristic of thelarge series resistance of the film (about 5 times larger than that requiredfor negligible R loss for this grid spacing),and the effect of a bias svoltage dependent light-generated current.A laser scan of sample #9 provided by the Institute of EnergyConversion at the University of Delaware (Figure 11) shows the region of thefront contact s trips and a defect (grain boundary).The scan was taken withthe diode in reverse bias (1 V) to amplify the collection features of thedevice.The defects are also apparent with zero applied bias.The laserscan is sensitive only to n-layer properties s ince the film thickness (1.5 pm)i s much larger than the optical absorption length for the laser light (6228A)used (0.2 pm).A grain boundary present in the CdTe sub strate is reproduced inthe HWVE grown n-layer (Figure 11) and is apparent as a resistive drop inthe collection features of the laser scan.The n-layer collection region isseen to be rather small , in agreement wi th the high series resistance observedfor this diode.The spectral response measurement for this device was compared tothose obtained by a theoretical model based on the equations found in Hovel'sb ook.6The parameters chosen for the spectral response model were as follows:surface recombination velocity at the front surface ,S 610em/sec;thickness of the n-layer (j unction depth) , 1.5 pm; mobility of holes .and22electrons , 39 cm /V-sec and 100 cm /V-sec , respectively; depletion layerwidth , 0.1 pm; and diffusion length of the holes in the front layer Ltaken as the maj or variational parameter in the modeling.pwasAs can be seenfrom Figure 12 , the diode described above has a large sensitivity to the valueof Lpwith the quantum efficiency values for a diode with Lps ignificantly larger than those for a similar diode with LThe model of a diode with Lp p 0.5 pm being0.28 pm.0.28 pm yielded an extremely good fit to
- t-.zwa::a::::: 01681 571()60.30.210-410-30.75Figure 10.--------VOLTAGE ( V)Dark and light (AM 1.5) I-V characteristics for an n/p homojunctionCdTe (HWVE #9). Area 0.25 cm2 V 0.73 V,J 0.24 mA/cm2,scocA 2. J.---------------------------------
F gure 11 .Laser scan of n/p homoj unc tion (HWVE #9) in the region of a grainboundary s een in the optical micrograph. This grain boundaryappears to be a s imple res istive barrier to current collection.
AaLLLLw :: Iz :: G25I20A. 15ItI1I x.fxI1(. 10. 05 . · ml\0. 00/K::X x x x:x-x x -:X x ·x ·iKC:. '-'5.6:J,. p.75WAVELENGTH (microns)Figure 12.Calculated quantum effici ncyvswa ele g h . p . xx 2 tm·'
17.the measured efficiency of HWVE sample #9.This value for Lwith EBIC measurements which yielded a value of Lp pis consistent0.35 ]lm.This valuefor L is in agreemen t with those determined by Mimila-Arroyo et a1 .p5The most eviden t d ifference between this film and those described in ProgressReport No. 1 which had a larger L ' is a considerab ly higher T "inpIt ispossible that the same imperfections responsible for the photoconductivity(three orders of magnitude) are responsible for the reduced diffusion length.The characteristics of the HWVE cells grown in Linz and atS tanford are compared in Table IV.evident.The values of Jsc A remarkable agreement in properties is20.24 mA/cm and Voc 0.73 V are in fairlygood agreemen t with the general diode equation given in Eq. (1).
18.TABLE IVComparison of p/n CdTe Homoj unction Cells Prepared at Linz and StanfordLinzOriginalLinzThinnedStanfordS ubstratesBoule-3p , em38b171038b1710Resis tivity , ohm-em1.11.134504504505655655755605606 80Growth Conditions T ' Cs0CTCdTe'0T ' Cin0T ' CCdThickness , ].lmJ0310X3X51.57710- lO3X101.511-g3X10- lOA2.22 .72.3vee' v0 . 620 . 67a0 . 190 . 73(AM 1 . 5)
Films of n-type CdTe:In have been deposited by hot-wall vacuum evaporation (HWVE) on 7059 glass substrates, BaF 2 single crystal substrates, metal (Pt, Cr, Mo, Al) coated glass substrates, and single crystal p-type CdTe substrates. Films deposited on
Thermally Evaporated Mg Nanoblades and Thin Films Deposition conditions: Tilt angle : Thin films :0 Nanorods :83.7 Pressure: 6.9 *10-6 mbar Rotation: 1 RPM Substrate : Si (100) Evaporated thin film 3000 nm (top view SEM image) Evaporated nanorod 300 nm (top view SEM image) Evaporated thin film 3000 nm (
Research Journal of Chemical Sciences _ ISSN 2231-606X Vol. 1(7), 76-80, Oct. (2011) Res.J.Chem.Sci. International Science Congress Association 76 Optical Properties of Vacuum Evaporated WO 3 Thin Films Rao M.C. 1 and Hussain O.M. 2 1Department of P
Primera Analytical Solutions Corp. I Page17of41 solution was evaporated to dryness and re-dissolved in hexane, and then cleaned up with a silica SPE cartridge. The eluate was evaporated to dryness and reconstituted in methanol for LC-MS/MS injection. The extraction/clean-up procedures were summarized in Tables 3A and
Food Image Recognition Could be very challenging Singapore Tea or Teh Teh, tea with milk and sugar Teh-C, tea with evaporated milk Teh-C-kosong, tea with evaporated milk and no sugar Teh-O, tea with sugar only Teh-O-kosong, plain tea without milk or sugar Teh tarik, the Malay tea Teh-halia, tea with ginger water Teh-bing, tea with ice, aka Teh-ice
O is a versatile metal oxide which finds application in number of fields such as solar cells, photocatalysis, electrochromic devices and sensors [1-4]. Cuprous oxide (Cu 2 O) is regarded as a . The electron beam evaporated films
480 Introduction to AutoCAD for Civil Engineering Applications . 11.2. Introduction . Consider a cup of coffee and assume that the coffee has evaporated. Figure 11-1a shows the coffee marks as circular rings in the top view and straight lines in the front view. In the coffee marks plotting the parameter is the time when the coffee was evaporated.
Feb 25, 2020 · metal–organic chemical vapor deposi-tion, resulting in complicated manufac-turing procedures and subsequent high cost.[8–12] Although evaporated Ge xSn 1-x films have been explored as alternative materials for SWIR photodetectors, their practical applications are limited by the
Diffusion in evaporated films of magnesium-aluminium Y. N. TREHAN VACUUM deposition can easily produce metal films of uniform thickness . When two such metal films are deposited, one over the other, and the r
