48 Laün/vmerican Jnumal ofMctallurgy 1O1l1 Materials, Vol. J() Nos. 1 & 2 (1990)

Microstructure and opto- electrical studies of cadmium selenide thin film.

S. MAHMOUD and A.H. EIDElectron Microscope and Thin Films Department , National Research Centre, Dokki, Cairo, Egypt.Nucleation and growth of cadmiumselenide thin film' areof considerable interest because of their directeffect on theoptical andclectrical properties of the material.Vacuum depositad layers on amorphous and crystalline suhstrates showed a polycrystalline structure. 111elayers were deposited at a pressure of lO Pa, withtwo deposition rates of 12 mn/sec and !5 nm/sec. 111eelectrical resistí vity and the optica! absorption were studied on quartz substrate, The morphology and themicrostructurc were.alsoinvestigated by X-ray ami electron ruicroscope. Some activationenergies wereestirnated fromthe optical absorbanceand thermo-resistancerelationship, Their values are consistent with the published data.Micoestructura y propiedades opt ••-~Iktrica.. de capa.<ddgada.. de CdSe,la nucleación y crecimiento de capa, delgadas de CdSe ha cobrado reciente interés debido a sus propiedades ópticas y eléctrica. s. La. capa. depositadas en vacíosobre sustratos arnorfns y cristalinos tienen una estructura poli cristalina. Se depositaron las capa.. a una presión de 1O"Pa, con velocidades de deposición de 12nmls. y 15 nm/s. Se estudiaron laresistividad eléctrica y la absorción óptica paracapasdepositadas encuarzo. Seinvestigótarnbiénla morfología y la rnicroestructuramando DRX yTEM. Se.estimaron las energías de activación a partir de relaciones de absorbancia óptica y termo-resistencia. Los valores obtenidos coinciden condatos publicados en la literatura.

1. INTRODUCTION

Evaporated layers of CdSe are widely used in the faoricationoí' thin film field eífect transistors ano solar ccii:.;.[1]. Thecadmíum selenide films contain the hexagonai :;h::~~~:~eandgrain growth occurs during anneal . The electriral propertíesoí' these films depend largeJy on their strucrure and cornposi-tion, and consequently on {he deposition environrnents. Theresistivity and carrier concentration, as well as the energy gaphad been followed by many authors [2-5]. Some cornplexchanges oí' the resistivity due lo duration and temperature ofannealing in air or under vacuum is then due to recrystalli-zation and re-ordering 01' the CdSe network [6,7], 111eelectronic properties of thin CdSe filrns prepared throughphysical vapour transport were investigated usingphotolurninescence and electronic measurements [8]. 111e

Iilms were studied at each oí' the main preparation steps, i. e.evaporation, annealing, etching, and tinally photo-etching. Al3 K two distínct donor-acceptor transitions al 1.75 andI.? eVwere found in the photoluminescence spectra in addition todeep states at about 1.55 eV at 20 K.Many differences still exist among the published data dealingwith the electro-optical behaviour of the thermaííy evaporatedCdSe films especially ofthose prepared under normal vacuwnoí' 10-4 - lO'5 Pa. Since this range of vacuum is mainly of theorder employed in any mass-production technology, morework is now needed lo furnish a c1ear background about theelectro-optical characteristics of thermodeposíted CdSe layers.

-~-

Fig.l. X-ray diffractogram of CdSe powder.

Revista Latinoamericana de Metalurgia y Materiales, Vol. ]() Nos. 1 & 2 (1990) 49

d: 500 nm d ~ MO nm

'"

~.2ª~I _

I

)0zo

Fig.2. X-ray diffractogram of a freshly deposited Cd<;efilms 011 quartz. slides of different thicknesses and rates.

(2.a)15 nm/sec.

d : J()O nm

d. 200 nm

32

ti = 500nm

-~-30 20

(2.b) 12 nm/sec.

2. EXPERIMENTAL PROCEDURE

Groups of CdSe layers of different thicknesses were ther-mally deposited by vacuum sublimation, on carefully cleanedoptically fIat quartz slide, very thin layers of evaporatedamorphous carbon and freshly cleaved surfaces of mica androcksalt. The substrates were maintained at room tempera-ture during the deposition. Two different deposition rates of12 mnlsec 15 mnlsec were selected. Electron microscopecopper grids which were previously covered with a thin filmor carbon were used for finishing the layers for transmíssíonelectrón microscope investigations. The microstructure wasstudied by X-ray and electron díffraction. The optical densitywas recorded in visible and UV regions using a Beckmann5260double beam spectrophotorneter. Theelectrical resistiv-ity was mea.sured as a functíon of temperature. The specimenholder in contact with a calibrated thermocouple were placed

inside a temperature regulated oyen. Thick vacuum depositedgold layers served as ·electrodes of ohrnic contacts. Filmthicknesses were measured interfero- metrically [9].

3. RESULTS AND DISCUSSION.

Since CdSe can have both wurtzite and zinc blend structure[10], X-ray diffraction was carried on the powder employed inthe present work. The corresponding ditfraction is shown in fig.1 with the analysis listed in table 1compared with the AS1MCardo The calculated data is in good agreement with a matrixof wurtzite plus traces of zinc blend. Figs. 2 and 3 show X-raydiffractograms of CdSe films of dífferent thicknesses deposi tedwith different rates on quartz slides and freshly cleaved planeof mica respectively. At a rate 01' 12nm1sec, the preferredorientation of (002) plane normal to tne substrate is clear

50 Latin/vmericanIournal of Metallurgy and Material s, Vol. to No s. 1 & 2 (l990)

rJ ; ~OOnm

-..,~~ ~~ -<:>- :::....• ,,~

<:>- <:>

'" '" -~ C) Ul~'"~

--'•.... - •....

d = 800 nm.., - C)!::' ::: :::•... •...

•....'"<:><:>

::: •...•....

~ '"<:>

'"

U - .- .•....... ~

especially for layers oí" 200 nm and 300 JUn. Besídes, theintensity of the planes increased with thickness implying thatsorne coalescence took place leading to the tormation oí"bigger crystals, As therate of dcpositíon increases (Ió nm/sec)more planes started lo appear indicating secondary nucleationof polycrystalline CdSe nuclei. For film thickness 800 nm, the(101) plane 01' metallic cadmium appears. This rnay be duo tothe more prolonged time of heating due to the evaporationfilament and the high t1ux 01' deposition,

50 50

nucleation is then influenced by the presence of theseimpurities, besides some other pararneters; deposition rate,substrate temperature, contamination, type of substrates andstate oí' vacuum,Figures 4, 5 and 6 are typical transmission elcctron micro-graphs of thin CdSe film deposited on carbon and freshlycleaved Surfaces oí'rocksaltand micarespecti vely, nopreferredorientation could be noticed. The ring electron diffractionpattem with the corresponding micrographs indicate that, in

"'o JO

Fig. 3. X-ray diffraction patiern of CdSe film.l' depasited (In a mica cleavage plane al different thicklesses with.a rafe ()f l Snm/sec.

Keeping in mind that the nucleation and growth as wellas thepossihility oí' epitaxy on crystalline suhstrates are criticallydepend upon the physic-chemical state of the substraresurtace whose cleanless up to the molecular level is essential.At a vacuum close to 10-4 Pa enough molecular of residualsimpinge on the surtace of the substrates every second lofurnish a complete layer of condensed molecules.Most oí' them are re-evaporated, but the available lime torworking un a clean surtace rernains very short. Accordingly,using high rates of deposition is now essential to increase theflux oí' the deposited molecules such that the conraminatingmolecules are diluted in the deposits. In the light oí' this resulr,using high rates of deposition during the preparation of thinCdSe layers at normal high vacuum, is needed now forreleasing the discrepancies between the experimental results.This conclusión is confirmcd by thc thcorctical approach ofHirth and Pound [11] for the nucleation and overgrowth.They assumed that the substrate surtace contains a number oí'mohile adsorbed atoms and/or molecules plus a number oí'clusters in the form oí' sphcrical caps. The first stage of

the early stages of nucleation and growth, the film is mainlycomposed oí' ultra smallnuclei which at some places quicklycoalesce and form aggregates of srnall crystallites randomlyoriented respect of the substrate type. These results consistentwith previously published work [12]. The electron diffractionpattems were indexed according to the wurtzite structure,Accordingly, one may conclude that the deposited thick layerswere polycrystalline at high rates of deposition. As the ratedecreases, some preferred orientation with (002) plane normallo the substrate started to take place for films of thicknessgreater UlaIl200 nrn which were investigated by X-rayo On theother hand, for films 01' thickness less than 100 nrn, thetransmission electrón micrographs showed small nuclei be-tween large aggregates oí' polycrystalline structnre randomlyoriented on the substrates.Figure 7 shows the optical density oí' various CdSe layers overthe spectral range from 200 nm to 900 nm, The absorption edgedcpcnds on the thickness oí' the sarnple and shifts towardshigher photon energies as the ñlm thickness decreases, Theestimated values for the optical energy gaps are 1.65 eV, 1.58

Revista Latinoamericana de Metalurgia y Materiales, Vol. lO Nos. 1 & 2 (1990) 51

Fig.4. Transmission electron micrographs ami electron diffraction p/ntems (ir CdSe deposiis 011 carbon substrateat a rafe of 15 nm/sec.

Fig.5. Transmission electro" micrographs ami elcctron diffraction pattems ofCdSe deposits 011 a cleaved o/rock salt al arate of 15nm/sec.

52 LatinAmerican Iournal al Metallurgy and Material s, Vol. 10 Nos. 1 & 2 (1990)

Fig. 6. Transmission electron micrographs and electron diffraction patterns o/ CdSe film deposited on a freshlycleaved surfuce o/ mica at a rate 0/15 nm/sec.

Revista Latinoamericana de Metalurgia y Materiales, VoL 10 Nos. 1 & 2 (1990)

eVand 1.55 eVfor layers of 200 nm, 300 nm and 500 nmrespectively . These values are consistent with the value of1.55 eV obtained by Jaeger-Waldau et al. [8]. This indicatesthat besides the intrinsic activation energy, there is an energyactivated mechanism which may exist in thin films. Similarphenomena were observed in thermo-resistance relationship~d was attributed to the effect offilm surface and scatteringpf the charge carriers at the grain surface [13].For an electrically active impurity [14] the absorptionspectrum may be observed only in the very low temperaturerange when the free charge carriers are still bound to theirimpurityatoms. While, as theimpurity atoms get ionized withthe rise in temperature their absorption spectrum vanishes.

O. '" - •....•.." "" ,

\ \\ \

\ \\ ," '.\ \(bl\ \

\rtl \\ \" \" '\ \

\ \

\ \\ \\ \\ \\ \

\ \\ \\ \

\ \\ \

... \\ \i \\. \\ ~\

\

"\\\\ -,

él' d ~ 500 nm

bl d~ 300"",

e J d ir 700 "'"

700 <00 600

Fzg.7. Dependence of optical density o/ vacuum-depositedCdSe fi lms on wavelegth.

Accordingly, one might expect that the activation energiesestimated from the resistance-temperature relationship canhave values with no correspondence in the absorption spec-trum which is recorded at room temperature and above. Thetemperature may indirectly affect the value ofthe intrinsic gapwidth. If the density of states in the conduction band is small,they will be rapidly filled with electrons of donors and unableto accommodate new electrons with the result that the valueof intrinsic activation energy estimated from resistance-temperature relationship could be than the real value calcu-lated for impurity free sample.Since the X-ray showed excess cadmium in the depositedCdSe film (fig. 2 a), accordingly one might expect theoccurrence of cadmium donor levels into- the energy gap.

53

These \evels have a fundamental rule in the activation proccss.fig. 8 is a representative example for log R versus '/T for a layerof about 800 nm thick. This relation is divided into three linearsections with different slopes. The first one covers the rangefrom room temperature up to 353 K with an activation energyof 0.4 eV. It might be due to the ionization of excess cadmiumlevels into the conduction bando The second section covers therange from 353 K to 493 K with activation energy of 1.12 eVmay be due to raising electrons from the valence band into theempty states ofthe cadmium level until complete filling oncemore. The last section of activationenergy of 1.73 eV is anintrinsic activated process. 11represents a transition ofelectrons

lO

·E:I.7JrV

e .8 ".~t;;;0·.(0 rV ••". D.[~ ".•.. ~,~ E¡ 1.I]fV ~'" '"

jv B'f; ~ 1. .52rV

I~~--------~--------.7--------~,---~r.s 2.0 2.S 3.0

iooo!» drg-I

Fig. 8. Log R "s. l/T for a layer o/about 800 nm thickwith a model o[ expected transitions.

from the valencc band to the partially empty states into theconduction bando These states, then are expected to be aboutfew hundredsof meV abovethe bottom oftheconduction band:depcnds on the concentration of donors ,"Cadmiul11" in thematrix layer. One might then expect that the measuredintrinsic gap width of 1.73 eV could be greater than thc sumof the activation energies of the first and second section.Since the depth ofthe donor leve! "0.4 eV" is n01certain to bemeasured frorn the bottom of the conduction band, then thesum of the transitions donor ~ conduction band plus valenceband donor level can be greaterthan the intrinsic gap width "E¡"of the impurity-free layer (fig. 8 ) . The effect of impurityconcentration was noticed in indium antimonidc: as theclectron concentration changes frOI1110'7 to 5 x 1018cm-3,the available allowed energy states into the conduction bandshift up by about 0.3 eV [14].

54 Latin/imerican Journal of'Metallurgy and Materials, Vol. 10Nos. 1 & 2 (1990)

- (lila) % (lIlo) %d calco dx-ray hklcalc. x-ray

3.7198 100 3,72 100 100

3.517 56 3.51 002 70

3.2873 72 3.29 101 75

3.0074 8.6 3.01200

100(cubic)

2.5543 34.4 2.554 102 35

2.1493 90 2.151 110 85

1.9794 82.8 1.98 103 70

1.861 20 1.863 200 12

1.8294 59 1.834 112 50

1.7957 15.6 1.8 201 12

1.646 9.7 1.645 202 8

1.4555 23.6 1.456 203 20

1.4104 12.4 1.407 210 8

1.3792 11.8 1.38 211 8

1.3152 17.2 1.312 105 14

1.312 14.5 13059 212 6

1.2442 12.4 1.2411 300 10

1.2428 8.6 1.2218 310 1

1.2084 25.8 1.2056 213 18-

1.1729 9.7 1.17 302 8

1.1231 8.6 1.1201 205 8

1.119 5.4 1.144 106 2

TABLE 1 X-r~ dijJractionofCdSe Powder

REFERENCES

1. M.l Lee, S.W Wright and C. P.Judge, Solid State Electron 23 (1980) 671.2. S. R. Jawalekar and M. K-Rail, Indian 1. Phys.,54 (1980) 223.3. A-VanCalster, A-Vervaet, Lde Rycke andJ. de Baets,J. Cryst.Growth86(1988) 924.4. P.Singh and B.Baishya, Indian J. Phys,61 (1987) 230.5. N. S. A1vi and L Kaufman, Solid State Electron Jl (1988)45.6. V. V. Serdyuk and V, A. Smyntyna, Poluprovodn. Tekh, and Mikroelektron,(USSR) 22 (1976) 5. . "7. L Spanulescu, L Secareanu, N. Baltateanu, 1.Z. Abdi, and T. Khalass, ThinSolid Films, 143 (1986) 1.8. R. Jaeger-Waldau, N. Stuecheli; M. Braun, M. Lux.Steiner, E.Bucher, R.Teune, H. Flaisher, W. Kerfin, R.Braun and W Koschel, 1.Appl. Phys; 64(1988)2601.9. S. Tolansky, "Introduction to Interferometry'', longans Green and Co. ,london, 2nd Edition (1973) 157.10. R. W. C. Wyckoff, "Crystal Structures" Interscience, New York (1963).11.. 1. P. Hirth and G. M. Poud, "Progress in Materials Science" VoI.II.Macmillan New York (1963).12. B. D. Chas, G. C. S, Collins, FAHuntley and 1. W. Streeds, Thin SolidFilms 67 (1980) 207.13. V, K Miloslavskii, E. N. Naboikina, V. A- Lehedev and v'LKramlsova,UKV, Fiz. Zh, 14(1969) 819.14. P. S. Kireev,"Semiconductor Physics",Mir Publishers Mosko,( 1975) 568.