Al interfaces

Solar Energy Materials 23 (1991) 83-9l North-Holland

Solar Energy Materials

Grazing-incidence X-ray diffraction study of all-sputtered AI/CdS and CdS/AI interfaces A. Bennouna, A. Haouni, N. Ghermani, E.L. A r n e z i a n e L.P.S.C.M., Facult# des Sciences, B.P S-15, Marrakech, Morocco

and

M. B r u n e l Laboratotre de Cnstallographtc !66X, 38042 Grenoble Cede~, France Recewed 17 December 1900 t3,~,,,-,,~ ,.m~..r.,.c of a!um'.num ~.,4 cad.~,ium ~.lfidP are investmated at the C u K a wavelength in order to compute the penetrahon devth versus the angle of incidence. Grazing~i.ctdence X-ray diffraction (GIXD) measurements are used to analyze the structural properties c~f all-sputtere.d AI/CdS and CdS/AI mterface~, The results showy tE,~ exi~ience of a 50 to 80 ?~, thick ,nicrocrystalhne layer when CdS is dep,oslted ,on AI. When del~s~ea on CdS. Ai is lmpianted at a depth wb:ch Is estimated aiound 150/~.

~. Introduction Aluminum and indium are good ohmic contacts to n-type cadmium ~alfide [1] but also to other n-type I I - V I compounds becaus..; their work function is smaller than the electron affimties of most of them [2-4] and the substitution of a metal atom of the semiconductor by an atom f r o m / h e contact gives a f, ee electron that increases the majority charge carriers concentration near the interface. As the a ll-sputtering approach has become a promising technique for producing thin-film solar ce!~,s [5,6], we present i- this paper the study of ;.he structural properties oi" the interface between as-sputtered AI and CdS fflms. The study is made using grazing-lncidence X-ray diffraction (GIXD) which is a v e ~ powerful ~on-destruc'dve method to analyze the structural properties at a few nanometers near a surface [7,8].

2. Experimental details All the films are deposited in an Alcatel SCM 451 diode RF spattering system with u n d o l ~ d 99.999% pui-¢ targets (7.5 cm in diameter for CdS and 10 cm for At). 0165 1633/91/$03.50 © 1991 - Elsevier Science l~ubhshers B.V. All rights reserved

84

A. Bennouna et al / All-sputtered AI / CdS and CdS / A! interfaces

Table 1 Deposltmn parameters t}f the films Target Substrate bias RF power lmttai pressure Argon l'res.~u~e Target voltage Depositmn rate External heating

Aiummlum

CdS Grounded

1.3 W/cm:

3 4 W/cm 2

_
880 V 33 ,~/mm

128 A,/mm No

The target-to-substrate distance is 6.5 cm. Argon is used as sputtering gas (99.995% pur~). To the previous commercial system containing three target holders we added a rotating masks holder concentric wtth the substrate holder. Both are electrically insulated from ground and can be externa!!y biased. This allows multiple growing and multiple masking withouz breaking the vacuum between the deposits, t h e substrate holder can be ih sir~ water-cooled or heated by an external source. The masks are made from_ AI sheets. The target is always presputtered for 5 min and the deposition time is adjusted to the required thickness according to the deposition rate which i¢ aetermiped by different methods. In-situ, rate and thickness are controlled by an XTM quartz thickness mon;tor. After extracting films, visible interferential microscopy, interference fringes in the transmission spectra (for semiconductors thicker than 5000 A) or weighing the deposit (for deposits heavier than 1 mg) allow to check the in-situ monitoring. The deposition parameters we used are fluted in table 1. The samplc~ prepared for this study are all couples of C d S / A I and A I / C d S deposited on to the same amorphous glass substrate without breaking the vacuum between the depositions of the thick back-layer and the top-layer. Optical microscope ebservations reveal that the samples are h~mogeacous h~ transmission and reflection and free of porosi~. Other samples have been made to check ~he ele-'tricai properties: in to[" coplanar structures, the maximum appSed field of about 100 V / c m did not a!!o,us to reach the non-ohmic regime ancl in sandwich structures A ! / C d S / A I , we found that the linearity of the current-voltage characteristic is kept up to fields near 8 × 103 V / c m .

3. Results and discussion 3.1. Preliminary study

In order to get the values of the penetration depth of X-rays tn the films at a given incidence angle, a preimth~arj study of the optical ploperties has been

A Bennouna et aL / Allosputtered A i / C d S and CdS/AI interfaces

85

conducted. The experimental procedure we choose consists in fitting the reflectance versus angle of incidence curves.

3.1.1. Critical an~,;e and pcnctrati6~ depth The complex refraction index fi can be written as:

=n -jk.

(1)

In the X-ray wavelength region, if we take into account the real part dispersion correction A f ' , n can be calculated using [7,8]:

n= l-

---mc: - - ~ N ( Z + A I ' ) = I + n ' ,

(2)

where N Z is the total number of electrons per volume unit. A f ' is equal to +0.204 for AI, +0.319 for S and - 0 . 6 for Cd [9]. The Imaginary part of (1) is a function of the linear absorption coefficient by the law: k .

. . 4~"

.

4~"

,

(3)

where p is the density and tr T is the total absorption cross section per mass unit. For toe elements, values of o"t are tabulated 19,10,11] and for compom~.ds such as C6S, we can compute ,,.t, using the formula: p ( C d S ) = N ( C d / C d S ) trt(Cd ) m ( C d ) + N ( S / C d S ) aT(S ) re(S),

(4)

where N ( X / Y ) is related to the number of X atoms per volume unit in a Y . . . . . pvui,d and ,.'n.(X) to the mass of the atom X. W h e n an unpolarized incident w'we with an angle of incidence i = rr/2 - a falls on a st.rt ~ce separating air and a medium of refractive index fi, the reflectance R(a, n, k) is given by the Fresnel laws modified by an exponential correction term taking into account the rms heigh~ ,r of the surface irregularities [12.13]. If the real part n is lower than unity, below a critical angle a¢, =¢72,,',

(5)

the reflectance rise.~ strongly to near to uniVy valuea and the penetration depth decreases to some nanometers. If the angle a between me mclclent wave end the. mrface is ema~.i enough, the penetration depth of the X-rays no:,mally to the surface can be upproximated by: r(a) = ~

sin arctg! ~t2 + 2n'

(6)

Using formt,;as (2), (3) and (4) we find the theoretical v.~hies oi n' and k. To check these values we compared the shape of calculatea re~ectance curve on a rough ~,utface with the e~perimental one taking also into account the dive'-gence of thc beam. The best fit to the experimental data is presented in fig. 1. Table 2

A Bennouna et at / All-sputtered A I / C d S and CdS/AI interfaces

86

,o

Fig. 1 Experimental (symbols) and calculated (hnes) reflectance rot grazing nncidence angles at the Cu K a wavelength.

Table 2 Optical properties of the films at the C u K a wavelength A! u [cm- II NZ [10 24 cm -3] 10c' n' t~c [ ° ]

Lflb

131 0.798 -8.36 0 236

[~,j ~'(0.15 ° ) [,~] :-(0.20 ° ) [,~,1 7(0.40 o ) [,~.l

959 i.298 - 13.59 0.299

~o

60

39

27 32 485

56 4302

shows the results of our computal..'ens, it also includes the values tr we found far the roughness of the films. Fig. 2 shows the shape of the variation of the penetration depth of the Cu Kor X-ra--'; versus the angle a. Some of its typical values are also listed in table 2.

/

4oo4

1 J

At_

Fig 2. Pen~ tratlou depth of the Cu K a X-rays at grazing incidence angles.

A. Bennouna et aL ./AILsp~atered A I / CdS and CdS / 4! interfaces

B

87

~3~1

5

I

~ 2'5 30 ¢(°) Fig. 3. v.-.-,.~ ....diffraction spectra of CdS(100 ,~)/A! at three angles of incidence (for clarity, spectra are 15

arbltrardy shifted along the ordinate and normalized to 39, 61 and 86 for a = 0.15 °, 0.2 ° and 0.4", respectively).

3.1.2. Str,~cture of the a,~'-depositedfilrrt~ A complete study o f the structure and texture of as-0eposited films is achieved. T h e pJ-i,i,.::pal rcnults a t e the followings: (i) S p u t t e r e d CdS thin-films hav~ a wu, tziie silucture [i4,15] and ha~,e a prefe,'red orientation with the c-axis of erystallites perpendicuiax to the substrate, the m e a n disorientation being around 18 o. T h e size of such crystallites lies i~.ear a few h u n d r e d s fingstriSrhs in the plane as well as in the cross-plane direction. T h e classical X D spectra are essentially c o m p o s e d o f (002), (004) lines and contain small contributions o f (10;) reflections, the o t h e r lines being always very small or non-measurable. (ii) Sputtered AI thin-films have a cubic structure and grow completely disorie n t e d so that the line intensitie~ -ue very ::oar to the powder ones. The m e a n size of the crysta!lites generally lies near a h u n d r e d ~ngstrbms.

3.2. Grazing-mctdence spectra of the samples Figs. 3 and 4 show the G I X D spectra excited by the Cu Kce line at three angles for 100 and 200 ,,~ thickness of CdS deposited on AI. ~351

•

n ~'\.

'

15

9s i ,

I

ZO

?

I

"

Z5

~

0('~

F{~. ~ . X - r a y .,,.,lat "r, -t iL ' n spectra of CdS/200 ,-~, °" "AI at thre," angles of incidence (same c o m m e n t s as fig.

3 but ,h~ normahTation c,-,nstantsare here 57, 85 and i06,

respectwely).

88

A. Bennouna et al / AIl-sput~eredAI / CdS and CdS / A I mterfi,~es

I [2

#362

113

9

6

I

t

r

A

7 ~

I

.

i

F:g,. 5. X-ray d~ffractton soectra of AI( 100 ,~), ~CdS at three angles of incidence (same comments as fig. 3 bat the eormahzat~on constants are here 283, 446 and 535, respectwely)

F=gs. 5 and 6 show the G I X D spectra obtained with the same angles of incidence than above but with 100 and 200 A thickness of A1 grown on CdS. The peaks labelled 1 to 11 are listed in table 3 with the probabl~ indexing of each one. Taking into account the oxidation of AI we added a coluran for A1203 [16] in the table. It has been clearly shown by electron energy-loss spectroscopy (EELS) ,hat ~n,~tals react with CdS when the heats of reaction per metal atom is smaller t h m +0.5 eV and for AI2S 3 this one is near to n 1.43 eV [3]. In spite of that we did nc~t add any column for AIzS 3 because we have no clear structures of a well-crystallized phase corresponding to the stable hexagonal aluminum sulfide [16].

3.3. Comments and discussion 3.3.1. CdS / A I The lines 5, 6 and 8 containing A I ( l l 1), (200) and (220) contributions appear at 100 .A, of thickness of CdS in fig. 3 at a = 0.15 o v, hile r is near 30 A. The same

#352

12

1o 9 {

6

r

]

n

7

111

C~=0.4 at=02.1 C¢:G151

lg

2'0

3b ff[*)

F~g 6. X-ray diffraction spectra of AI(200 A)/CdS at three angles of mc:dence (same comments as fig. 3 but the normalization constants are here 146, 235 and 292, respectwe~y)

,4 Bennouna et ai. / All-sputtered At / CdS and CdS / AI interfaces

89

Table 3 Structures wslble m the figurc "*ah pr ~babh. indexi,~ ~ir~t. ,tand~ for the powder ASTM cards intensity.) Structure

0 (deg)

1 2 3 4 5 6 7 8 9 10 I1 B

12.4 13 2 14.2 17.7 19.3 22.2 26.0 32 5 18.5 24.1 27 3 13 4

AI

(hkl)

CdS Int.

(111) (200)

100 47

(220)

22

A1203

(hkl)

lnt

(hkl;

Int.

(106) (002~ (101)

75 60 100

(012J

75

(104) (110)

90 40

(113) (024j

100 45

(110) (112)

55 45

(102) (103) (004)

25 40 4

(111) reflection is absent near the surface when the CdS thickness is incre,_sed to 200 /k (see fig. 4, a = 0.15 ~) and appears lightly when the analysis depth is increased. These two remarks indicale that in this cas,.'. ~mminum atoms penetrate at least 100 to 170 ,~ in the semiconductc~ overl.~ye .. The exodiffusion of Al enhanced by the cadmium a~ad sulfl~r implantation it. ,,-- back-layer improve the superficial concentration of Al which oxidizes giving/M20 3 contributions to the structures 1, 4, 5, 6 and 7 (see table 3). The wide bump B a~;,pearmg in the two upper curves of fig. 3 splits in three reflections correspondin~ ',~ the CdS(100), (002) and (101) near the surface ( a = 0.15). We can also aote that the split of these three structures labelled 1, 2 and 3 is better when the nenetration depth is smaller (compare ~ -- 0.15, 0.2 and 0.4 ° in fig. 4) or near the surface when the overlayer thickness is increased (compare c~ --0.15 ° from fi~gs. 3 and 4). As the well-defined first three structures change when r reaches 32 A (fig. 3, a = 0.2 o ), we can estimate at 50 to 80/k the thickness of a microct~Jstalline overlayer grown on the aluminum substrate. For a = 0.4 °, it is possible that the structure B contains some contribution of tile amorphous glass substrate but the related line is wider that the width of B reported hel e. From the width of B, the mean size of the microcrystallites is estimat.:,d to 22 approximately. The existence of such microcrystalline overlayer can be due to different reasons: (i) Lattice mismatch between aluminum and both cubic [16] and hexagonal cadmium sulfide can cause the growth of a micro,,rystalline CdS overlayer. Cameron et al. [17] showed that CdS deposited by MIiE on A3 crystallizes in the cubic phase with a preferred orientation (111). The (I10) aluminum faces ( a V ~ = 5 . 7 6 ,~,) represent a good seed of crystallization for the cubic phase of CdS (a = 5.82 ,g,). (ii) Stochiometry change in the first nanometers can stimulate the formation of mlcrocrystalline CdS layer,