Characterization of thin films of amorphous GaP using optical and electron spectroscopy

Thin Solid Films, 82 (1981) 377-391 PREPARATION AND CHARACTERIZATION

377

CHARACTERIZATION OF THIN FILMS OF AMORPHOUS GaP USING OPTICAL AND ELECTRON SPECTROSCOPY J. PERNAS, M. ERMAN, J. B. THEETEN AND F. SIMONDET

Laboratoires d'Electronique et de Physique AppliquOe, 3 avenue Descartes, 94450 Limeil Brbvannes (France) A. GHEORGHIU AND M. L. THEYE

Laboratoire d'Optique des Solides, Equipe de Recherche associbe au CNRS 462, Universitk Pierre et Marie Curie, 4 place Jussieu, 75230 Paris Ckdex 05 (France) L. NEVOT Institut d'Optique, Centre Universitaire d'Orsay, Bgttiment 503, B.P. 43, 91406 Orsay C~dex (France) (Received March 31, 1981 ; accepted M a y 5, 1981)

Thin films of flash-evaporated GaP were examined by reflectancetransmittance techniques, spectroscopic ellipsometry in the visible range, grazingincidence X-ray reflectivity and Auger electron spectroscopy. The interfaces of the film with both the substrate and the atmosphere are oxidized. A multilayer structure is proposed from the ellipsometry data and compared with the results of the X-ray and Auger analyses.

1. INTRODUCTION

This work is part of a general programme of investigation of amorphous III-V compounds deposited as thin films by flash evaporation. In the case of amorphous Gap (a-GaP) previous investigations ~ have suggested the existence of a thin superficial layer of oxide at the film surface. Therefore a multilayer description of the deposited material appeared more appropriate, and more sensitive techniques were required to characterize these films completely. In this work a combination of Auger electron spectroscopy (AES) and ion milling is used to obtain a qualitative estimate of the adequacy of the multilayer description. The use of this technique allows the existence of different regions of uniform composition to be detected and provides models for comparison with the results of more sophisticated techniques such as spectroscopic ellipsometry (SE), which can be used to carry out non-destructive depth profiling of thin films 2, and grazing-incidence X-ray reflectometry (XR), which can be used to characterize both the surface geometry (in particular the roughness) and the surface composition of various samples 3. We have used these techniques to show that an evaporated a-GaP film a few hundred gmgstr6ms thick is oxidized at its interfaces with both the substrate and the atmosphere. This is indicated by the AES data and quantitatively determined by SE and XR. In particular we show that the surface region can best be described as a layer of GaP oxide on top of a layer of a-GaP which is much denser than the bulk GaP. This is confirmed by analysing an a-GaP film of thickness 1 ~tm. 0040-6090/81/0000-0000/$02.50

© Elsevier Sequoia/Printed in The Netherlands

378 2.

J. PERNASet al.

EXPERIMENTAL DETAILS

2.1. Sample preparation Thin films of a-GaP were deposited onto substrates at room temperature under high vacuum conditions (10- 8 Torr rising to the order of 10- v Torr at the end of the deposition process) by the flash evaporation of crystalline G a P powder of a selected grain size which fell onto a heated tungsten crucible at a constant rate. The films were deposited onto several substrates simultaneously. (1) A polished monocrystalline G a P substl;ate was used for AES investigations in order to have a stoichiometric reference. (2) A polished silicon wafer was chosen for the SE analysis because it is a good reflecting substrate with a well-known dielectric function. (3) Optically polished glass (borosilicate Crown B 1664) substrates were used for the reflectance and transmittance measurements, from which the dielectric function g of the deposited film was calculated, and for the X-ray interference method. (4) A vitrified carbon-chip substrate was used for the Rutherford backscattering (RBS) analysis. Two types of films were investigated: (i) two thin films, of thicknesses 296 A (film I) and 585 A (film II), which were prepared from a small-grain (150 tam) powder and were slightly non-stoichiometric (the atomic ratios of phosphorus to gallium were 0.94 and 1.05 for films I and II respectively according to RBS determinations); (ii) one thick film (film II1), of thickness 9750 A, which was prepared from a stoichiometric powder with larger grains (200 ~tm). 2.2. Reflectance and transmittance measurements The complex dielectric function g = el + iE2 of a-GaP films prepared under the same conditions as the films investigated here and with thicknesses between 300 and 2000 A has been measured between 0.5 and 6 eV. This result was obtained by substituting data from measurements of the reflectance R and the transmittance T, performed at near-normal incidence in air with a Cary 14 spectrophotometer, into exact thin film formulae 4. The thicknesses of the films used for this determination were measured by the X-ray interference technique 5 which is discussed in Section 3.2. A Kramers-Kronig analysis of the reflectance data alone was used to obtain ~ for films which were not semitransparent over the whole spectral range. At energies hcoN greater than 6.2 eV the reflectance curves were extrapolated according to the formula R(og) = RION The value ofs was determined by fitting the result to a value ofgwhich was obtained by using the reflectance-transmittance method either on the same film in a spectral region in which it was transparent or on a thinner film prepared simultaneously. Since the reflectance measurements are only available over a limited spectral range and the spectra are rather structureless, the results of the Kramers-Kronig analysis depend sensitively on the g value chosen for the fit. This is particularly true for the position and height of the maximum in e 2.

CHARACTERIZATION OF THIN FILMS OF AMORPHOUS GaP

379

Figure 1 shows the el and/~2 spectra between 2 and 5 eV which are characteristic of the approximately stoichiometric a-GaP films prepared under the conditions described in Section 2.1 and which are taken as a reference for the dielectric function of a-GaP. The two thin films investigated (I and II) had complex dielectric functions very close to these reference values. The absorption edge of the thick film (III) was located at the same energy as in the reference film, but the ~2 maximum, although not shifted, had a lower value; the el values in the near-IR were also smaller. These effects can be attributed to the presence of voids in the thick film, and this was confirmed by SE. 20

\

\ -5

III

I

2.

3.

4.

I

I

I

~.' E(~v)

Fig. 1. Dielectricfunctionof a-GaP whichis used as a reference. It should be emphasized that these reflectance and transmittance measurements are almost insensitive to the presence of a superficial oxide layer. Model computations show that T is barely changed in the presence of a film 30 A thick which has the same dielectric function as the anodic oxide on crystalline G a P 6, while R is appreciably lowered only above 4 eV and AR/R reaches at most 6~o at 5 eV.

2.3. Spectroscopic ellipsometry In this technique the complex reflectance ratio p = Rp/R s = tan ~Uexp(iA) where Rp and R s are respectively the parallel and perpendicular coefficients of light reflected from the sample, is evaluated as a function of the energy E of the light and its angle of incidence 0. The fundamental improvement in SE compared with conventional fixed-energy ellipsometry is that the energy dispersion p(E) can be related to the dielectric functions g(E) which are characteristic of the material being illuminated. Since the energy dependence of the dielectric functions varies markedly from one material to another (an example will be given later when the oxide and aG a P are compared), it is possible to distinguish their various contributions to p(E). A xenon lamp is used as the source. It has a "flat" spectrum over the whole energy range investigated here which is limited at high energy (above 5.4 eV) by the polarizers (calcite prisms) and at low energy (below 1.6 eV) by the detector (UV-

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380

extended S-20 photocathode). The optical sequence is as follows: source; rotating polarizer; sample; fixed analyzer; double monochromator; photomultiplier (PM). The energy selection is thus operated at the end of the optical path. If the analyser is fixed and the polarizer rotates, the intensity Ipr~ is a sinusoidal function of the angle 2P: IpM = a cos(2P) + b sin(2P) + c

(c+at1"2

(1)

tan ~u = tan A ~ !

(2)

cos A = b(c 2

(3)

-

a 2 ) 1/2

By sampling the photomultiplier signal as a function of the polarizer angular position and undertaking a Fourier analysis of the data tan 5u and cos A can be obtained from eqns. (2) and (3). The sampling, data reduction, control of the angular position of the analyser and the energy selection of the double monochromator are carried out with the aid of a minicomputer. The SE data can be used to evaluate the dielectric function of a thin film on a substrate of known dielectric function. In practice the film composition varies with depth and the dielectric function obtained from this evaluation is an effective value. It is more appropriate to assume that the film is composed of a succession of sublayers with thicknesses d i. These sublayers are composed of a mixture of the type xiA (1 -x~)B between materials A and B which have known dielectric functions ~:A[E) and eBIE). The dielectric function e(E) of such a mixture is approximated using the effective medium approximation (EMA) v '~A--C

~ /]B--E

x,~+2~e +(1-xi,e-~+~e = 0

(4)

where x i is the volume fraction of the constituent A. This approximation is valid when the film thickness is small compared with the wavelength of the light. This allows us to account for different effects such as the presence of voids in the material, mixing of the material and its oxide etc. These types of mixtures can be defined as physical, i.e. the compounds of the mixture keep their chemical entity, as opposed to a chemical mixture where new bonds are formed between the various compounds. The description of such a chemical mixture is clearly beyond the capability of the EMA. We shall return to this point later. However, the EMA can compensate to some extent for an incorrect evaluation of the dielectric function taken as a reference. The experimental data are then compared with the predictions of such a multilayer model using a limited number of parameters, i.e. the thicknesses d~and the compositions xi which are such that F = ~ [tan { ~/(En)calc} - - tan { ~(E,)exp} 12 + n

Icos{ le°l , }-cos{AIE.loxpl 12 n

is a minimum.

CHARACTERIZATION OF THIN FILMS OF AMORPHOUS

GaP

381

The mean square deviation 6 is then defined by 62 -

1

N-1

F

(5)

where N is the number of experimental points.

2.4. Grazing-incidence X-ray reflectometry This technique consists in analysing the specularly reflected X-ray intensity I r as a function of the angle of incidence 0 with respect to the average surface plane. Since the values of the refractive index in the X-ray spectral range are very close to unity (n = 1 - ct- ifl (ref. 8) where c~is of the order of 10- 5 and fl is of the order of 10 -6 for ,~ ~ 1 ,~), the reflected intensity is measurable for very small 0 values only; typically Ir decreases from the incident beam intensity I o to 1 0 - 6 x I 0 or even 1 0 - 7 × I o when 0 increases from 0 to about 2 °. These measurements therefore require an extremely accurate goniometer and a highly stabilized and collimated Xray source. The experimental arrangement essentially consists of a goniometer with an angular accuracy of 1" controlled by a PR 8000 computer which allows the specular reflection curve Ir(O ) to be visualized directly. The source is usually an X-ray tube with a copper anticathode (2 = 1.54 ,~ for K~ 1 radiation). The rate and the temperature of the cooling circuit of the tube are regulated. The detector is a proportional counter with a noise level below 0.1 counts s- 1. A simulation of the 1~(0) data was attempted using a model proposed by Nevot and Croce 3 in which it is assumed that the sample consists of a stack of elemental sublayers with well-defined thicknesses and refractive indices and that the surface or interface roughnesses obey a gaussian statistical distribution with a mean displacement ~r. Since high spatial frequencies are predominant in the roughness spectra, the reflection coefficient amplitude at a rough interface can be written as r(O) = ro(O) exp( - 8~2KlnK2n o-2)

(6)

where %(0) is the reflection coefficient for a plane interface and is given by the Fresnel equations 9 and K1. and K2n are the normal components of the wavevectors in the two homogeneous media on each side of the interface. The reflection coefficient of the stratified sample is then deduced by recurrence using an iterative impedance method. The use of XR analysis allows the sample to be characterized geometrically and structurally, i.e. the thicknesses and refractive indices of the sublayers and the roughness of the interfaces can be determined. It should be emphasized that the real p a r t , of the refractive index in the X-ray spectral range is related to the electronic density of the material which, particularly for light elements, varies with its density, a can be expressed in c.g.s, units by = 2.7019 × 101° x 22D ~ Ni(Z i + Af,') where D is the density, A is the atomic mass, N i is the number of atoms with atomic number Z i and Afi' is the real part of the anomalous dispersion correction1° which is

J. PERNASet al.

382

a few per cent of Zi at most. ~ is thus essentially proportional to the electronic density q. Table I lists the numerical data for the compounds likely to be present in the aG a P samples. The values of ( Z + Af')/A are the same to within 10~. Therefore the modifications of ~ will essentially reflect modifications of the density of the medium due to the presence of a different compound, variations in its compactness etc. TABLE I REFRACTIVE INDICES OF G a P AND OXIDES FOR C u K ~ 1 RADIATION

Compound

Density

(Z + Af')A

~z× 106

fl × 106

a-GaP Pz 0 5 Ga203

4.0 a 2.7 5.9

0.446 0.498 0.445

11.44 8.63 16.84

0.33 0.132 0.35

F r o m ref. 11.

The imaginary part fl of the refractive index is computed from the values of the mass absorption coefficient It/D of the elements in the compound using the relation = --OZg 4~

, i

D i

where gi is the mass fraction of element i.

2.5. Auger electron spectroscopy and ion milling Destructive depth profiling of the G a P film is performed by examining the Auger lines of oxygen (503 eV), gallium (1070 eV), phosphorus (120 eV) and carbon (272 eV) while bombarding the sample with a 2 kV Ar + beam several millimetres in diameter at an incident angle of 70 °. The Auger gun bombards a spot 100 gm in diameter with 2.5 keV electrons at a current of 10 gA. After the analysis the total depth eroded is measured using a Talysurf system with a precision of about 100 A. If a constant erosion rate is assumed, the development of the Auger lines with time can be related to a depth in the film. 3.

RESULTS A N D DISCUSSION

3.1. Auger electron spectroscopy Figure 2 shows the variation in the peak-to-peak amplitudes of the phosphorus, oxygen, carbon, gallittm and silicon lines with erosion time for a-GaP films 300 A thick (film I) deposited onto G a P and silicon monocrystalline substrates. In both cases oxygen lines are present at the vacuum-film and film-substrate interfaces. Examination of the silicon line shows that oxygen is not combined with silicon but rather with gallium and phosphorus. The presence of oxygen must thus be due to P205 and/or G a 2 0 3. While the presence of oxides at the air-film interface is expected, it is rather surprising to find a significant amount of oxide at the filmsubstrate interface. We believe that this is due to the oxidation of the fine-grained G a P powder used in the evaporation of this type of film. At the beginning of the evaporation the oxide from the grains is transferred to the substrate. After some time unoxidized phosphorus liberated by the evaporation process is present in sufficient

CHARACTERIZATION OF THIN FILMS OF AMORPHOUS

GaP

383

quantity to getter the evaporated oxide. In addition to the oxygen line, Fig. 2 indicates that the stoichiometry of the film is not constant with depth. C o m p a r i s o n of the ratios of p h o s p h o r u s to gallium for the monocrystalline substrate and the film shows that the film has a global p h o s p h o r u s deficiency which is in agreement with the RBS data. Peak to Peak Amplitude t

/

/"~ . •

\

/

(\.

IO0

il

10(

i/

Sputtering Time 0

20

(a)

4 0 min

0

20

4 0 rain

(b)

Fig. 2. AES depth profiles of a-GaP films about 3130A thick (film I) deposited onto (a) crystalline silicon and (b) crystalline GaP: --. --, phosphorus ;.., carbon; - - , oxygen; - - + --, gallium; - - -, silicon. The presence of a GaP oxide layer at the interface between the film and the substrate should be noted. These AES results show that this layer c a n n o t properly be described as a h o m o g e n e o u s a - G a P film. At least three regions can be distinguished. We shall now attempt to obtain more quantitative information from the SE and XR data.

3.2. Grazing-incidence X-ray reflectometry The specular reflection curve It(O) obtained for the thin a - G a P film I deposited o n t o polished glass is shown in Fig. 3 on a semilogarithmic scale. A simple qualitative analysis of this curve gives precise information a b o u t the film composition• The oscillations observed b e y o n d the total reflection limit 0 c .~ (2~)1/1, which is indicated by an arrow in Fig. 3, are due to interference between the X-ray beams reflected by the two film interfaces. If a n o n - a b s o r b i n g h o m o g e n e o u s layer of uniform thickness e is assumed, the angular positions 0 m of the maxima of the interference fringes 12 must satisfy the equation 2e(0,, 2 - 0~2)1/2 - ~ = m~ where m is an integer.

(7)

384

J. PERNASdl al.

.q

,v~] ;~

t

~ x l 0

}

2

//i//''1/ // 77

iA,,

//;/

//7/

% i

0

t

,

i 5

,

i

I

,

i 10

,

,

i

Fig. 3. X-ray specular reflection curve It(O) for the a-GaP film I (about 300 A thick) on a glass substrate: 0 , experimental data; - - , theoretical curve obtained with the three-layer model (see text). Fig. 4. (0m2 --0c2)1/2 t'S. the interference order m deduced from the data of Fig. 3: O, maxima;@, minima.

A plot of(0m 2 --2~) 1/2 v e r s u s m (Fig. 4) gives a straight line with slope 2/2e from which the thickness e can be deduced. If the first few experimental points deviate from this line, it is necessary to modify the ~ value. O t h e r graphical methods, which are also derived from eqn. (7), can be used to determine e and c~ independently, particularly when the m values are uncertain 13. Figure 4 shows that the angular positions of the first interference fringes (m < 7) verify eqn. (7) and we obtain = 11.9 × 10- 6 and e = 296.5 A. A n o t h e r straight line with a different slope is found for m ~> 10 from which we obtain e = 311.2 A. The shift occurs for fringes of order 8 and 9. In this angular range a strong attenuation of the amplitude of oscillation is observed on the experimental Ir(O ) curve. Such an effect suggests that a layer behaving like a quarter-wave plate for 01 ~ 24.24 m r a d (1.39 °) is present at the a i r film interface. The thickness of the superficial layer can be estimated from the relation es

2/4 (02 - 2 ~ ) 1/2

2 401

This yields e s ~ 15.9 A. Accurate values of the refractive index cq of the superficial layer and of the roughness of the interfaces can only be obtained by using the model described in Section 2.4 and fitting the theoretical curve to the experimental results. The theoretical curve corresponding to a two-layer model, the characteristics of which (thickness, refractive index and roughness) are given in Table II, gives satisfactory agreement with the experimental curve. However, there is a very slight shift in the

CHARACTERIZATION

OF THIN FILMS OF AMORPHOUS

GaP

385

angular position of some fringes and a discrepancy in the average intensity of the higher order fringes (m > 7). TABLE

H

X-RAY REFLECTOMETRY OF a - G a P

Layer

FILM I

Thickness (A)

Refractive index

R.m.s. roughness (h)

1 - 10

8.5

Two layers on glass 1

16

2

290

6(5.9+i0.09)

1 - 10-6(11.9 + i0.34)

8.5

1 - l0 - 6(5.2 q- i0.08) 1 - 1 0 6(11.9 +i0.34) 1 - 10-6(12.2 +i0.35)

7.5 9 9

Three layers on glass 1 2 3

15.75 280 10

Previous computations have indicated that the substrate roughness has an increasing effect on the values of the reflected intensity at large angles of incidence. It is therefore necessary to confirm for this sample the cr value of 5.4 A r.m.s, which has already been found for B1664 glass polished with cerium oxide 3. A value of 5.0 A r.m.s, is deduced for a bare substrate identical with that used for the a-GaP samples, which rules out a possible substrate effect. We have attempted to introduce a transition layer at the film-substrate interface. The theoretical curve corresponding to this three-layer model, the characteristics of which are also reported in Table II, is shown in Fig. 3 (broken curve). A shift in the angular positions of the higher order fringes (m > 12) is observed which suggests that the transition layer probably has a more complex composition and cannot be reduced to a single homogeneous layer. The corresponding ~(z) profile is shown in Fig. 5 (full curve). The profile for planar interfaces is also indicated (broken curve); it allows a direct localization of the various interfaces and a visualization of the indices associated with each layer.

a-6aP10

b

,

vacuum

i'k~Ner

2nd~r (~k)

0

5. Refractiveindex profile planar interfaces. Fig.

i

z

i

,- .....................

i

!

!3~lk ~ sub~h-a~e(~s~)

i I i

~(z)

of the a-GaP film I deducedfrom the data of Fig. 3: - - , profilefor

Sample I can therefore be represented from the X-ray viewpoint by three layers: (i) a superficial layer about 16 A thick with a very low density (1.6-1.8 g c m - 3) which

386

J. PERNASet al.

must be very porous regardless of whether it is composed of G a P or of Ga203 and P205 or of a mixture of these compounds; (ii) a layer about 280 A thick, the density of which (4.16 g cm-3) is very close to the density of crystalline G a P (4.13 g c m - 3) but is higher than the density (4.0 g c m - 3) of flash-evaporated thick a-GaP films which have been studied previouslyl'~; (iii) a transition layer about 10/~ thick which could be a mixture of oxides (P205 and GazOa) together with some voids and contamination with light elements. The average refractive index (c~) corresponding to a mixture of P205 and Ga203 in equal proportions should be equal to 12.73 × l0 -6 which is slightly higher than the experimental value of 12.2 × 10 -6. However, it must be emphasized that the difference between the refractive indices of a-GaP and a mixture of oxides may be quite small which will make the two materials difficult to distinguish by X-ray analysis. The transition layer between the substrate and the homogeneous a-GaP may be much thicker than the value deduced here (approximately 10 A) as the refractive index gradient in this layer is too small to have an effect on the specular reflection curve. 3.3. Spectroscopic ellipsometry SE was performed at several angles of incidence (60 °, 63 °, 66 ° and 70°). A typical spectrum for tan ~Uand cos A taken at 63 ° is shown in Fig. 6 for an a-GaP film 300 thick (film I) on crystalline silicon. Several models of increasing complexity are compared with the data. The results are given in Tables III and IV. The models and the experimental data are compared in Fig. 7 where d(tan qJ) = (tan

~/)¢alc- (tan

g-J)~xp

and d(cos A) = (cos A)c,lc - ( c o s A)exv are plotted against the energy. The models are for mixtures of a-GaP with its oxide. The dielectric function of the anodic oxide grown on monocrystalline G a P is taken as the reference for the oxide 6 while a-GaP is described by the average dielectric function given in Fig. 1. The dielectric function of the crystalline silicon substrate is taken from ref. 11. The first model is restricted to a single homogeneous layer of a-GaP and oxide on crystalline silicon. Table llI gives the thicknesses and the volume fractions of aG a P in this hypothetical layer. The average values predicted by this model give a film thickness of 290 A and a composition of 89~o a-GaP and l 1~ oxide. The thickness is in fair agreement with the results of the XR technique. However, the mean square deviation fi is large. As already indicated by AES, it appears more appropriate to split the single layer into three layers. Three distinct mixtures of a-GaP and oxide are found as shown in Table III. The mean square deviation for the three-layer model is decreased by a factor of 5. The main reason for the improvement of the model is the presence of an oxide-rich layer with a thickness of 120 A and a composition of 89~o oxide and 11~o a-GaP at the film-substrate interface. The value offi is comparable with the noise level of the data, indicating that increasing the number of parameters by adding another layer is unlikely to improve the fit. The two top layers are not satisfactory in this model: the intermediate layer is

TABLE III

290-+25

312.9 302.5 288.6 255.6

d (A)

89.1+1.3

87.5 89.3 88.9 90.7

x (%)

One-layer model

15.9 17.1 18.0 22.3

18.3+2.8

60 63 66 70

Average values

10 -3 10 -3 10 -3 10 -3

157+20

184.5 157.6 150.1 137.4

227-+36

227+4.2

228.5 229.1 229.1 220.8

Layer 2

97.3+-1.3

95.7 98.3 97.0 98.2

90.2-+4.5

87.0 87.7 89.4 96.9

Layer 3

43.3+4.5

48.3 45.8 40.8 38.5

OF

82.3+0.8

82.5 83.4 81.9 81.5

10- 3 10 -3 10 -3 10 -3

FILM I ON

6.7 x 7.4 × 10.6 x 13.8 ×

6

a-GaP

132+20

104.2 133.6 138.5 151.5

d2 (A)

x~ (%)

dl (A)

110.5-+4.2

116.5 110.1 108.0 107.3

xa (Yo)

118+5

113.0 116.9 117.8 123.9

d3 (A)

Layer 3

FILM 1 ON CRYSTALLINE SILICON

Layer 2

a-GaP

Layer 1

MODEL a FOR THE SPECTROSCOPIC ELLIPSOMETRY

27.8 x 37.2 x 47.8 x 60.8 x

6

Three-layer model

"Oxide plus voids (layer 1) on a-GaP plus voids (layer 2) on a-GaP + oxide (layer 3).

Layer 1

0 (deg)

259 252 218 179

THREE-LAYER

CRYSTALLINE SILICON

AN A L T E R N A T I V E

TABLE IV

"All layers consist of a-GaP plus oxide.

values

Average

60 63 66 70

0 (deg)

ONE-LAYER AND THREE-LAYER MODELS a FOR THE SPECTROSCOPIC ELLIPSOMETRY OF

11.0±5.1

7.1 14.9 15.8 6.2

x3 (%) 5.75 × 7.92 x 8.33 x 9.42 x

10 3 10 -3 10 -3 10-3

OO

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8F

COS A

<2

d/'an )

tan¥

J

i

L

....

____~,/

tanb)

/

//

/

j/

/ ,/

/~ /

/" .O~=~f/ L I 2

[ LLI 3

L LL

l k L I I~.L/_A4 4

5. E(eV)

-.1

I

2.

l

3.

I

l

4.

L

5. E(eV)

Fig. 6. Typical SE result for the a - G a P film I (about 300 A thick) on crystalline silicon at an angle of incidence of 63 t Fig. 7. Comparison of the data of Fig. 6 with various models: - - , one layer of a - G a P plus voids on crystalline silicon: , three layers of a - G a P plus voids on crystalline silicon; - - . - - , three layers of aG a P plus oxide on crystalline silicon; - - + - - , one layer of oxide plus voids on two layers of a - G a P plus voids on crystalline silicon.

too dense and the oxide content in the top layer is rather high (17.7~o oxide in a film 57 A thick corresponds to an oxide thickness of 28 A). Furthermore, since the oxide at the surface corresponds to the natural oxidation of the G a P film in air, a thin layer of pure oxide is more probable than a mixture of a - G a P and oxide. This is assumed in the third model where the film is assumed to consist of a superficial layer of oxide and voids on top of the "bulk" film layer of a - G a P and oxide and an oxide-rich interface. The results for this model are given in Table IV. Although ~ is rather higher than in the previous model, this model is considered to be a better physical description. Table IV indicates that the density of the "bulk" of the film is close to that of the reference a - G a P (x 2 = 97~o) and that its thickness is 227 A. The filmsubstrate interface is still present (90 A thick with 57~o oxide content). The top layer is 18 A thick, which is a reasonable value for a native oxide. However, its density is very high (227~, of that of anodic oxide). This means that a description of the natural oxide grown on a - G a P in terms of the reference anodic oxide is not totally satisfactory and that this top layer must be characterized by larger values of the dielectric function. This suggests that some a - G a P is incorporated in the film as a result of the surface roughness. Since the value of 6 is again close to the noise level it is not possible to improve on a three-layer model for this sample and a better description of the top layer is not feasible. Similar results and conclusions were obtained with the other thin a - G a P film (film II), i.e. an oxide layer at the film-substrate interface and a very dense oxide top layer. We now consider a rather simpler case, i.e. a much thicker a - G a P film. In this

C H A R A C T E R I Z A T I O N OF T H I N FILMS OF A M O R P H O U S

GaP

389

case the substrate-film interface will not be detected by light since a-GaP absorbs strongly in the SE spectral range. This corresponds to one layer fewer in the model since the "bulk" a-GaP film is considered as the substrate. The results for an a-GaP film about 1 Ixm thick (film III) are gives in Tables V and VI. The first model (Table V) comprises a pure oxide film on top of a pure a-GaP substrate. The average value obtained for the thickness of this oxide layer is about 23 /k but the mean square deviation is large compared with the noise level. This model can be improved by assuming that the densities of both the oxide film and the a-GaP substrate can change. Table V shows that fi is decreased by a factor of 3 when the substrate contains only 98.7~o of the reference a-GaP. The top film has a thickness of about 13 A and a very high density (442~o). This is a similar result to that obtained for the thin films. Since 6 is still high compared with the noise level, we can introduce an extra layer into the model to obtain better information about the surface region. The results obtained using this three-layer model, in which the top layer is split into a void-containing oxide film on a void-containing a-GaP film, are given in Table VI. TABLE V ONE-LAYER AND TWO-LAYER MODELS FOR THE SPECTROSCOPIC ELLIPSOMETRY OF a - G a P FILM III 0

(deg)

Pure oxide on pure a-GaP

Oxide plus voids on u-GaP plus voids

Oxide thickness

Oxide layer

b

(A)

Thickness

Density

(A)

(%)

Amount of a-GaP in substrate (%)

3

73 71 70

24.4 24.0 21.4

41.0 × 10 -3 36.6 x 10 - 3 37.4 × 10 - 3

13 14 13

457 423 482

98.4 98.7 99.4

13.9 × 10 - 3 13.9 × 10 3 16.3 × 10-3

65

20.7

26.9 x 10 -3

13

405

98.3

11.5 x 10 - 3

Average values

22.6-+1.9

13.3-+0.5

442-+34.9

98.7-+0.5

T A B L E VI THREE-LAYER MODEL a FOR THE SPECTROSCOPIC ELLIPSOMETRY OF a - G a P FILM 111

Layer 1

0 (deg)

Layer 2

Amount of a-GaP in substrate (%)

Thickness

Density

Thickness

Density

(A)

(%)

(A)

(%)

73 71 70 65

31.9 32.3 32.7 34.6

182 173 178 183

59.4 59.3 59.1 44.4

116 116 118 128

83.3 89.2 88.5 91.7

Average values

32.8-+7.2

179-+4.5

55.5+_7.4

119+_5.7

88.2-+3.5

a Oxide

plus voids (layer 1) on a-GaP plus voids (layer 2) on a-GaP plus voids (substrate).

4.47 x 1 0 - 3 3.41 x 10 - 3 3.33 × 10 - 3 2.7 x 10 3

390

J. PERNASet al.

The best fit is obtained with a substrate containing about 88% of the reference aGaP. This decreased density can be accounted for by the large thickness of the deposited film; in this case strains are present which introduce voids. This is consistent with the conclusion deduced from the determination of g from the reflectance-transmittance measurements, i.e. a decrease in the maximum ez intensity and an increase in the e 1 values in the IR with respect to the reference a-GaP. The superficial region is composed of an oxide layer 33 A thick with a density of 180~o of that of the reference anodic oxide, and is situated on top of an a-GaP film 55 A thick with a.etensity of 120~o. This intermediate dense film has the effect of decreasing the density of the oxide film to values which have more physical significance. The thickness of 30 A obtained for this film appears to be reasonable. The extra a-GaP layer can thus be said to improve the description of the superficial region of the aG a P film. It may account for a very rough and diffuse interface between the a-GaP and its oxide, as is observed in the case of crystalline Gap 6. This is about the best result that we can obtain with the EMA on the basis of the physical mixture of materials. An improvement on this description would require some "chemical insight" in terms of a new dielectric function of some intermediate compound between a-GaP and its anodic oxide. 4. CONCLUSIONS The different methods that we used to characterize flash-evaporated thin aG a P films, i.e. AES, XR and SE, give results in fair agreement. These methods are in fact complementary since they are sensitive to different physical properties and emphasize different aspects of the sample structure. All the methods clearly show the existence of a superficial oxide layer which can be characterized very accurately using X-ray reflectometry. It has a thickness of the order of 16 A and a very low average electronic density which indicates a very porous structure. The SE data show that it must be characterized by a dielectric constant which is significantly larger than that of anodic oxide on crystalline GaP. This indicates a different composition and/or structure (probably roughness). The results of studies performed on a thicker opaque film suggest that the interface between the homogeneous a-GaP and the superficial oxide layer has a complex structure. The SE data also indicate very clearly that a transition layer of appreciable thickness, which is probably due to oxidation at the beginning of the deposition process, exists at the interface between homogeneous a-GaP and the substrate. A transition layer can also be deduced from the XR analysis, but a much smaller thickness is indicated. This apparent disagreement is due to the lack of sensitivity of the XR method in this particular case because the refractive indices of a-GaP and the mixture of oxides are too close to be differentiated in the X-ray spectral range. ACKNOWLEDGMENT

We wish to thank J. P. Dupin (University Lyon I) for performing the RBS measurements.

CHARACTERIZATION OF THIN FILMS OF AMORPHOUS GaP

391

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