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1998 characterisation of multilayers by x ray reflection


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1998 characterisation of multilayers by x ray reflection

  1. 1. Surface Science 409 (1998) 229–240 Characterisation of multilayers by X-ray reflection A.J. Steinfort, P.M.L.O. Scholte *, F. Tuinstra Delft Institute of Microelectronics and Submicron Technology (DIMES), Department of Applied Physics, Delft University of Technology, P.O. Box 5046, NL-2600 GA Delft, The Netherlands Received 15 October 1997; received in revised form 2 February 1998; accepted for publication 16 February 1998 Abstract The inclusion of refraction effects in kinematic scattering theory provides a powerful tool for describing diffuse scattering by X-ray reflectivity. The theory is applied to multilayers with roughened interfaces. Islands and miscut-induced steps as well as randomly oriented roughness are included in the theory. The interfacial roughness leads to a broad, diffuse intensity distribution around the multilayer Bragg reflections. From the line shape, the morphology of the interfaces can be deduced. The calculated profiles are compared with experimental data from a 4×(Si/Ge Si ) multilayer with vicinal interfaces. Clear side peaks are x 1−x observed from which the mean island size and the average step height are deduced, which are consistent with AFM images. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Epitaxy; Single crystal surfaces; Stepped single crystal surfaces; Superlattices; Surface structure, morphology, roughness, and topography; Vicinal single crystal surfaces; X-ray scattering, diffraction, and reflection 1. Introduction contains information about the in-plane and out- of-plane interface characteristics. The features of thin layers on a substrate which Several profile calculations of diffuse scattering can be inspected by X-ray scattering are the crys- due to interfacial roughness have been presented tallinity and orientation distribution in the layers, for various roughness profiles [3–7]. Most of the the layer thicknesses, and the global interface calculations were performed in the kinematic scat- morphology. X-ray reflectometry is a powerful tering approach, in which refraction effects are tool to study the morphological aspects of the neglected. For randomly oriented surface rough- layered structures. As the incidence angle is kept ness, intensity profile calculations have been per- small the penetration depth is reduced, resulting formed by Sinha et al. [8]. The calculations were in an enhanced surface layer sensitivity. Scattering performed in the distorted-wave Born approxima- of X-rays from multilayers at small scattering tion, including the local wave amplitudes and local angles results in a specular component containing wave vectors. The calculation was extended to multilayer Bragg peaks due to the periodic super- multilayers by Holy et al. [9]. Good agreement ´ structure. Interfacial roughness leads to a diffuse was found between experimental X-ray reflectivity background contribution and a loss in the specular data and the calculations. intensity [1,2]. The diffuse scattering distribution In the case of stepped surfaces, the intensity distribution is often described using the kinematic * Corresponding author. scattering approach. The stepped surface is 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 02 0 8 -8
  2. 2. 230 A.J. Steinfort et al. / Surface Science 409 (1998) 229–240 described with a pair correlation function. Good much smaller than unity, the intensity I(q) in the agreement is found with electron diffraction data scattered beam is given by [10–14]. However, X-ray reflectivity data from KP K multilayers are poorly described [15], as refraction 2 I(q)= f (r)p(r) e−iqr dr , (1) effects are ignored. Recent work by Holy et al.´ [16 ] presented an intensity calculation for multi- r layers with vicinal interfaces in the distorted-wave with the scattering vector q taken in the vacuum Born approximation. Comparison with reflectivity (i.e. outside the sample). The interfaces are data obtained from miscut GaAs/Ga In As/ described with p(r) being defined by the interface 1−x x GaPAs multilayers resulted in a good agreement. profiles functions p (r) like This confirms the large impact of refraction effects h G on the line profile of X-ray scattering distributions 1 if r at interface h p(r)=∑ p (r)= (2) at small scattering angles. h 0 if r not at interface h In this paper, we present a general expression h for scattering from multilayers in the Born approx- The scattering factor f =f(r) for r in layer h is h imation, including refraction effects. This approach related to the layer-dependent refractive index n h differs from the kinematic scattering approach in by that including refraction effects, its validity still l2e2 remains at small scattering angles. Compared to n =1−d =1− Nf . (3) the distorted-wave Born approximation, the h h 2pmc2 h expression is simplified as no position-dependent The scattering vector q is expressed in the in-plane electric wave amplitudes are included. However, and the out-of-plane components. The in-plane this also implies that the expression is not valid component q is independent of the height position for small scattering angles with incidence angles x in the multilayer. The out-of-plane component of and exit angles close to the critical angle for total the scattering vector qh has to be evaluated in external reflection. The expression has been formu- z every layer, and is expressed in the layer-dependent lated for two general cases. First, the case of incidence and exit angles and the refractive index interfaces containing two levels will be treated. This description is applicable, for example, to according to interfaces with an islanded morphology. The for- qh =n k (sin vh +sin vh ), (4) mation of ripples or islands at multilayer interfaces z h 0 out in has been observed for the case of SiGe multilayers where k is the wave vector in vacuum. The local 0 [17,18]. It has been shown to play an role in the incident angle vh and the exit angle vh from the in out relaxation of misfit-induced strain [18]. Next, an electric wave in layer h are calculated from Snell’s expression for the scattering from miscut interfaces law; starting at the surface (h=0) with n =1 and 0 will be given. In the calculation, the layer-depen- v0 +v0 =2h, the scattering angle for given q is out in dent scattering vector is included, as well as a finite (q , q0 ). In this two-dimensional description, the x z in-plane and out-of-plane correlation length. The interface positions are given by z and the lateral h results are compared with experimental data direction is denoted as x. The roughness at each obtained from Si/Ge Si multilayers. interface is described by steps with step heights ma x 1−x where m is an integer, as shown in Fig. 1a for the case of m=1. The intensity in Eq. (1) can now be 2. Calculations rewritten as a summation over the interfaces h, k, i.e. The incident and exit angles are assumed to be larger than the critical angle, so the relative ampli- tude of the electromagnetic wave at each interface I(q)=∑ f f e−iwhk h,k h k P ∑ C (u , u ) uz hk x z is approximately equal to unity. Under the assump- ux tion that the amplitude in the reflected beam is ×exp{−i[q u +a(qh m−qk n)]} du , (5) x x z z x
  3. 3. A.J. Steinfort et al. / Surface Science 409 (1998) 229–240 231 Fig. 1. (a) Schematic representation of a multilayer system with two-level interfaces. The step height is a and the island lengths are denoted by L . The expression for the correlation is evaluated in different directions. (b) A grey-scale level representation of the n scattering intensity from a multilayer system as shown in (a) as a function of the in-plane and out-of-plane q and q . y z with u =a(m−n). The phase factor w results ability of finding two scatterers at interfaces h and z hk from the interface distance z −z . After correction k separated by a vector (u , u +z −z ). It can be k h x z k h for the optical path length, the expression becomes expressed as G k w = ∑ qp d p=h+1 h z p if h+1≤k , (6) hk x z m h P C (u , u )= ∑ p (x, ma)p (x+u , ma+u ) dx. k x z hk ∑ −qp d if k+1≤h x z p (7) p=k+1 0 if h=k In the following, an expression for the pair where d =z −z is the thickness of layer p. The correlation function in a multilayer system with p p+1 p pair correlation function C (u , u ) gives the prob- rough interfaces will be derived. We start from the hk x z
  4. 4. 232 A.J. Steinfort et al. / Surface Science 409 (1998) 229–240 expression for the pair correlation function from With the relative occupation of the lower level a single surface derived by Lent et al. [10] which given by c, the dependencies between the in-plane will be extended for multilayers. First, the rough- partial correlation functions c (u ) are given by mn x ness at the interfaces is described by two levels. Next, scattering from a descending stepped inter- c (u )+c (u )=c, 00 x 10 x face is given. The expressions are derived under c (u )+c (u )=1−c. (12) the assumptions that no roughness is present on 01 x 11 x the terraces. The step height is assumed to be With Eq. (12), the summation over the step levels smaller than the thickness of the layer. m,n=0,1 in Eq. (9) is carried out. The resulting The multilayer system is defined by M interfaces, expression for the scattered intensity is split into each having a two-level step distribution, as shown the specular part I (0, q0 ) and the diffuse compo- spec z in Fig. 1a. In this two-level description, the levels nent I (q), i.e. diff at each interface are indicated by m and n equal I (q)+I (0, q0 )32 ∑ f f Y to 0 and 1 for the lower level and the top level, diff spec z h k hk h,k respectively. The partial correlation functions C (u ) are defined by the probability of having h≤k P hk,ll+D x a scatterer at interface h, level l and a scatterer at × du e−iqxux C (u )+I (0, q0 )d(q ), (13) interface k, level l+D separated over u in the x hk,01 x spec z x x lateral direction. It is related to the pair correlation where d(q ) is the Kronecker delta, and function by x C (u , Da)=∑ C (u ). (8) Y =cos(Q )+cos(Q ) hk x hk,ll+D x hk hk,01 hk,10 l −cos(Q )−cos(Q ). (14) The intensity in Eq. (5) is now rewritten as hk,00 hk,11 The expression for the specular part of the intensity P 2 is given by I(q)=2 du e−iqxux ∑ ∑ f f x h k −2 h,kh m,n I (0, q0 )32 ∑ f f [2c cos(Q ) spec z h k hk,00 h≤k h,k ×[2 cos(Q )]C (u ), (9) h≤k hk,mn hk,mn x with +(1−c) cos(Q )]. (15) hk,11 Q =a(qh m−qk n)+w . (10) The in-plane pair correlation function c (u ) is hk,mn z z hk mn x given by the sum over all step configurations and In Eq. (1)a the correlation function C (u ) terrace lengths over a distance u . The number of hk,mn x x in the multilayer system is evaluated in different steps over a distance u is denoted as n . An directions. Laterally for u =0, the correlation x x z example of a step configuration with terrace function C (u )=c (u ) is assumed to be inde- lengths L with n =6 is shown in Fig. 1a. hh,mn x mn x 0…nx x pendent of h (i.e. identical at every interface). This Evaluation of C (u ) results for positive values is not unlikely, as the surface forms a template to hk,01 x of u in [10] the following deposited layer. In the z direction, x PPP the correlation function H with the correlation 2 hk C (u )3H G (u ) ∑ … length j is defined by C (u =0)=c (0)H . hk,01 x hk hk x z hk,mn x mn hk nx=0 For u ≠0 and u ≠0, the correlation function x z nx odd C (u ) is assumed to be decomposable, as in hk,mn x C (u )=c (u )H G (u ), hk,mn x mn x hk hk x where c and H are as defined above. G (u ) (11) P × T (L )T (L )T (L ) … T (L ) 0,o 0 1 1 0 2 1,f nx mn hk hk x A B represents the lateral distance j over which the nx x ×d u − ∑ L dL dL dL … dL . (16) profiles at interface h and interface k are replicated. x i 0 1 2 nx i=0
  5. 5. A.J. Steinfort et al. / Surface Science 409 (1998) 229–240 233 T (L) is the probability of finding a scatterer at exponential decay. The calculations are performed 0,o level 0 at the origin a distance L before the first on a multilayer system with four bilayers with step, and T (L) is the probability of finding a island formation on the interfaces. The islands 1,f scatterer at level 1 a distance L after the last step. have an average length of 700 nm and a step height All T (L)s in between give the probability of ˚ of 4 A, and the correlation length in the surface m finding a scatterer at level m a distance l away normal direction is taken to be equal to the from the preceding step. T (L) and T (L) are multilayer thickness. The in-plane correlation 0,o 1,f written in terms of T (L) and T (L) as length is chosen to be 10 mm. 0 1 Because of the interfacial correlation, the diffuse P c 2 scattering profile forms sheets in reciprocal space T (L)= T (x) dx, 0,o L 0 through the multilayer Bragg positions located at 0 L the specular line through q =0. The bending of P 2 x T (L)= T (x) dx, (17) the intensity sheets results from the position depen- 1,f 1 dence of the scattering vector, i.e. inclusion of the L layer-dependent refractive index. In the intensity with sheets, two maxima at each side of the specular P 2 direction are visible. From their position and the L = xT (x) dx, (18) in-plane width, the average size and deviation of i i 0 the terrace width can be deduced. with i=0,1 being the average terrace size of level In the same way, the scattered intensity from a i. For negative u , the in-plane correlation function misoriented surface can also be described. The step x height is given by m a with the corresponding step is i height distribution Z(m a). The pair correlation C (u <0)=C* (u >0). (19) i hk,mn x hk,mn x function which is valid for multilayers is derived Taking an exponential decay as the expression for from the pair correlation function obtained for a the in-plane correlation function G (u ), the total single interface [10], resulting in hk x diffuse scattering distribution ( Eq. (13)) is analyti- cally solvable. With the modified terrace length distributions C (u , u )3G (u )H hk x z hk x hk T (u )d G 0,f x ux,0 T ∞ (L)=T (L)G (u ), (20) 2 2 P P hk,i i hk x 2 2 2 + ∑ ∑ … ∑ … the expression for diffuse scattering becomes: nx=1 m1=−2 mn =−2 x L0=0 Ln =0 ×T (L )Z(m a)T(L )Z(m a)T(L ) x C 8c … I (q)3∑ f f Y H 0 0 1 1 2 2 diff q2 L h k hk hk x 1 h,k ×Z(m a)T(L )d[u −L (n )] nx nx x lat x h≤k ×Re hk,0 x G [1−T ∞ (q )][1−T ∞ (q )] hk,1 x 1−T ∞ (q )T ∞ (q ) HD , (21) ×d[u −L (n )] dL dL dL … dL , z vert x 0 1 2 nx H (23) hk,0 x hk,1 x with with P 2 L Snx L + ma Snx m a T∞ = T ∞ (L) e−iqxL dL. L (n )= i=0 i i=1 i , (24) hk,i hk,i lat x 0 L 2+ ma 2 In Fig. 1b, an example of diffuse scattering according to Eq. (21) is shown. The interface-to- ma Snx L − L Snx m a L (n )= i=0 i i=1 i . (25) interface correlation function H is taken as an vert x L 2+ ma 2 hk
  6. 6. 234 A.J. Steinfort et al. / Surface Science 409 (1998) 229–240 Inserting this expression into Eq. (5) results in and C 1 2 I (q)3 ∑ f f H 2 cos(w ) Z(q∞ )= ∑ Z(m a) e−iq∞ mja,z (28) diff q∞2L h,k h k hk hk z j x j=−2 h≤k where q∞ and q∞ are the reciprocal vectors in the z x x∞ and z∞ directions, as shown in Fig. 2a. G HD [1−T ∞ (q∞ )][1−Z(q∞ )] ×Re hk x z , (26) In Fig. 2b, an example of scattering is shown in 1−T ∞ (q∞ )Z(q∞ ) the case of a multilayer including correlated vicinal hk x z with interfaces. The terrace width is 900 nm and the ˚ step height is 10 A. In contrast to scattering from P 2 the islanded multilayer structure as shown in T ∞ (q∞ )= T ∞ (L) e−iq∞ L dL, x (27) hk x hk Fig. 1b, the miscut structure gives rise to an asym- 0 Fig. 2. (a) Schematic representation of a descending staircase as present on a vicinal surface. The x, z and x∞, z∞ directions correspond to the directions parallel and perpendicular to the physical and to the crystallographic surface, respectively. (b) A grey-scale level representation of the intensity distribution from a multilayer system with interfaces, as shown in the upper figure, as a function of q and q . x z
  7. 7. A.J. Steinfort et al. / Surface Science 409 (1998) 229–240 235 metric intensity distribution along the q direction, geometry, the surface normal is oriented in the x caused by the asymmetry at the interfaces. scattering plane and the anisotropic shape of the In the description of the surfaces, no restrictions instrumental resolution function does not result in are imposed on the size of the islands. In the limit a change in resolution for different scan directions. of the island or terrace size to infinity, the pair The measurements were performed on a correlation function C (u , u ) is a constant. The 4×(Si/Ge Si ) multilayer system on a vicinal hk x z x 1−x diffuse scattering for q ≠0 is zero and the intensity Si(001) substrate. The multilayers were grown by x is localised at q =0. In the other extreme, where molecular beam epitaxy under ultrahigh vacuum. x the in-plane correlation length reaches zero, the The substrate had a miscut of 0.45±0.02°, as diffuse scattering intensity has a constant value for determined by a combination of optical alignment all q . In practice, only a limited range in q is of the surface and the orientation of the Si(001) x x accessible because of shading effects from the direction by X-ray diffraction. First, a 250 nm sample. This range will set the lower limit to which thick Si buffer layer was deposited on the Si spatial frequencies can be measured. The upper substrate. Alternate Si and Ge Si layers were x 1−x limit of spatial frequencies to be measured is given deposited at a deposition temperature of 550°C. by the instrumental broadening or, in the case of In the case of a miscut surface, the surface and high-resolution measurements, the spatial coher- interface characteristics cannot be regarded as ence length of the X-rays, which is of the order of invariant under azimuthal rotation. The reciprocal micrometers. q and q directions were defined as being perpen- x y dicular and parallel to the steps on the surface. The q direction was defined as being perpendicular z to the optical surface. Scans were performed in 3. Measurements two planes in reciprocal space with q =0 and x q =0. The reciprocal planes contain full informa- y 3.1. Experimental tion about the in-plane characteristics, both paral- lel and perpendicular to the steps. In Section 2 we showed that the in-plane charac- teristics of the surface and interfaces can be 3.2. Presentation of the data deduced from the diffuse scattering profile. The observed intensities are normally orders of magni- The layer thicknesses and Ge concentration, the tude smaller in intensity than the intensity in the refractive indexes as well as the root mean square specular direction. Because of the non-periodic (RMS) interface roughness were estimated from nature of the interface roughness, the intensity is the out-of-plane specular reflectivity profile. The not as localised as the multilayer Bragg peaks in experimental profile was compared with a theoreti- the specular direction. cal description [19] to obtain the out-of-plane layer The measurements were performed with a four- parameters. In Fig. 3, the data are presented which circle diffractometer. The Cu K X-ray beam was were collected in the specular direction (circles). a taken from a standard X-ray source fitted with a The theoretical profile is shown as a solid line. commercial tube running at 40 kV and 20 mA. The average RMS interface roughness is The divergence of the primary beam was defined ˚ 7.4±0.1 A and the RMS surface roughness is by slits and was set to 0.03°. The resolution was ˚ 11.2±0.2 A. The Ge concentration x is 0.40 and ˚ set to dq=4.2×10−3 A−1 using a 0.25 mm slit in the Si and Ge Si ˚ layer thicknesses are 120 A 0.40 0.60 ˚ , respectively. front of the detector. The height of the slit was and 49 A 1 mm. A Ni filter was placed in front of the In Figs. 4 and 5 the experimental data in the detector to eliminate the K contribution in the two q =0 and q =0 planes are shown. The data b x y wavelength spectrum. All scans were performed in were collected in the in-plane direction in 50 steps the parallel scan mode by adjusting the rotational ˚ with a step size of 2.2×10−4 A−1. In the out-of- angles v and w of the diffractometer. In this plane direction, the data were collected on 50
  8. 8. 236 A.J. Steinfort et al. / Surface Science 409 (1998) 229–240 multilevel interfaces. Parallel to the steps, a two- level system is assumed at every interface to describe the islanded profile. The intensity profiles are calculated using Eq. (21). In the direction perpendicular to the steps, the interfaces are described as descending staircases, with the result- ing intensity calculated from Eq. (26). The terrace widths are assumed to have a geo- metric (exponential ) distribution according to C D 1 −(L−L ) T (L)=T (L)= exp islands 0 1 dL dL islands islands for L>L (29) islands for the two level description, and C D 1 −(L−L ) T(L) exp steps for L>L dL dL steps steps steps (30) for the miscut-induced stepped morphology. The step height h is chosen to be constant (h ), leading Fig. 3. The specular reflectivity curve from a 0 to the step-height distribution function 4×(Si/Ge Si ) multilayer. From the position of the 0.40 0.60 maxima and the relative intensity, the bilayer thicknesses and Z(h)=d(h−h ). (31) the Ge concentration as well as the interface roughness are 0 established. The calculated intensities are convoluted with the instrumental reponse function and corrected ˚ points separated by 1.4×10−3 A−1. The experi- for geometric factors arising from different inci- mental data show the clear sheet-like appearance dence angles. A satisfactory fit of the calculated of the intensity distribution. This indicates that intensity to the measured data is obtained by the interface-to-interface correlation length is large varying only the distribution function parameters compared to the bilayer thickness. The sheets are and a scale factor. concentrated around the multilayer Bragg peak In Fig. 6, two line scans are shown parallel positions. In the q direction the intensity is asym- and perpendicular to the steps at a height x ˚ metrical around the specular direction. Only at q =0.163 A, which is through the fourth Bragg z one side next to the specular direction is an addi- peak of the profile in Fig. 3. The data are repre- tional maximum observed. This is in agreement sented as open circles. The central intensity with the model calculations presented in Fig. 2b. maxima correspond to the specular intensity, and In the perpendicular q direction, a symmetric the intensity distribution for q ≠0 and q ≠0 con- y x y intensity distribution is observed around q =0. tains information about the lateral roughness dis- x Next to the specular direction, two minor side tribution on the interfaces and the surface. The peaks are visible. This indicates an islanded profile symmetric profile, shown as a solid line in Fig. 6a, at all interfaces in the direction parallel to the steps. is the calculated intensity distribution according to Eqs. (21) and (29), describing scattering from 3.3. Discussion stepped surfaces and interfaces. The optimal values for the parameters as used in the calculations are For further analysis of the data, line scans at given in Table 1. The island height h cannot be 0 constant q are used. The data are described with estimated from this profile, since variation of this z
  9. 9. A.J. Steinfort et al. / Surface Science 409 (1998) 229–240 237 Fig. 4. Logarithmic intensity distribution from a 4×(Si/Ge Si ) multilayer with a miscut angle of 0.45° as a function of q and 0.40 0.60 x q . The q direction is parallel to the miscut-induced steps. Note the sheet-like intensity distribution at multilayer Bragg positions z x ˚ with a side maximum at q =0.00062 A−1. x Fig. 5. Logarithmic intensity distribution from a 4×(Si/Ge Si ) multilayer with a miscut angle of 0.45° as a function of q and 0.40 0.60 y q . The q direction is perpendicular to the miscut-induced steps. The sheet-like intensity distribution at the multilayer Bragg positions z y ˚ −1. is symmetrical around q =0 with side maxima at q =0.001 A x x parameter only results in a uniform amplification mainly influences the lateral width of the specular of all points at constant q . In Fig. 6b, the scatter- reflection. No additional broadening is found apart z ing intensity distribution perpendicular to the steps from instrumental broadening. This means that is shown: a clear side peak appears at the interfacial correlation length is equal or larger ˚ q =0.00062 A−1. This maximum is caused by scat- than the correlation length of the X-rays, and the x tering from terraces arranged as a descending interfaces are highly conformal. From line scans staircase. The asymmetry is caused by the asym- in the q direction, the value for the out-of-plane z metric step geometry. The average size of the correlation length j has been determined to be z terraces determines the position of the maximum. 65±5 nm, which is equal to the total thickness of The optimal parameters used to calculate the inten- the multilayer system (68 nm). The estimated sity distribution are given in Table 1. The solid island height equals an RMS roughness of 7.5 A. ˚ line in Fig. 6b is the calculated intensity with the This is equal to the RMS interface roughness optimal parameters. The correlation length j estimated from the specular profile. Only at the x
  10. 10. 238 A.J. Steinfort et al. / Surface Science 409 (1998) 229–240 ˚ Fig. 6. Intensity distribution from a 4×(Si/Ge Si ) multilayer taken from Figs. 4 and 5 at q =0.163 A−1. The open circles are 0.40 0.60 z the measured data and the solid lines are the best fit. The directions correspond to (a) parallel to the steps and (b) perpendicular to the steps. Table 1 Optimal values for the Si/Ge Si multilayer interface parameters for the in-plane directions x in the step direction and y perpendicu- 0.40 0.60 lar to the step x-direction perpendicular to the steps L (nm) dL (nm) j (nm) j (nm) h (nm) steps steps x 0 855±36 484±36 >104 65±5 1.3±0.3 y-direction parallel to the steps L (nm) dL (nm) j (nm) j (nm) h (nm) island island x 0 675±28 194±11 >104 65±5 – The characteristic length scales are given by L in the x-direction and L in the y-direction. The correlation length in the steps islands x-direction and the z-direction are denoted by j and j , respectively. The height of the islands is denoted by h . x z 0 ˚ surface is a higher RMS value of 11.2 A found. As the step height of these terraces is probably of This means that apart from island formation at the order of monoatomic distances, scattering from the interfaces, no additional interface roughness is these terraces is not observed since the intensity present. The islands are highly interface-to-inter- decreases rapidly with decreasing step height. face correlated throughout the whole multilayer. The results were also compared with an atomic Only at the surface is additional uncorrelated force microscopy (AFM ) image from the surface, roughness present. which is shown in Fig. 7. It is clear that the AFM In the case of flat terraces the vicinal angle is image only provides information about the surface, calculated from the terrace length and the step as the X-ray data are a result of all interfaces. The height, resulting in a miscut angle of 0.1°. This is x and y directions as defined by the diffraction not in agreement with the observed value of 0.45°. experiments are indicated in Fig. 7. The average This implies the presence of steps on the terraces, step direction is from the lower left to the upper resulting in a vicinal angle of 0.35° on the terraces. right corner. In this image, 2D islands are clearly
  11. 11. A.J. Steinfort et al. / Surface Science 409 (1998) 229–240 239 between the experimental data and the calculated intensity profiles. Experimental data are presented from a Si/Ge Si multilayer system. The sample x 1−x has a miscut of 0.45°. The interfaces of the miscut sample contain islands with typical length scales which are different along and perpendicular to the steps. Typical island sizes are 855 nm×675 nm, with the short side parallel to the steps. The standard deviation of the average island size in the direction perpendicular to the steps is significantly larger (484 nm) than the deviation along the steps (194 nm). This is also seen in the AFM image of the multilayer surface. The morphology of the first interface (substrate–multilayer) is repeated at each following interface, resulting in a large correlation length in the surface normal direction. A large part of the roughness at each interface is corre- Fig. 7. AFM image from the surface of a 4×(Si/Ge Si ) lated. Only at the surface is uncorrelated rough- 0.40 0.60 multilayer. The surface miscut is 0.45°. In the inset, the miscut- ness present. induced average step direction (x) is indicated. The grid in the inset shows the avreage island-to-island distance as determined by X-ray diffraction. Acknowledgements visible. The mean distance between the islands in We wish to acknowledge the expert technical the y direction is about 660 nm, and the average assistance of C.W. Laman and R.F. Staakman. ˚ height difference is 6 A. In the perpendicular direc- This work was supported by the Netherlands tion, the island size is about 900 nm with an Foundation for Fundamental Research ( FOM ). ˚ average height difference of 9 A. With the given miscut angle of 0.45°, this confirms the presence of steps on the islands. The overall RMS roughness References ˚ at the surface is estimated to be about 8 A. In the upper left corner of Fig. 7 the mean island-to- [1] R.A. Cowley, T.W. Ryan, Appl. Phys. 20 (1987) 61. island distance as estimated from the X-ray [2] R. Bloch, L. Brugemann, W. Press, Appl. Phys. 22 diffraction experiments is indicated by a grid. The (1989) 1136. [3] R.L. Headrick, J.-M. Baribeau, Y.E. Strausser, Appl. mean island size and height as estimated from the Phys. Lett. 66 (1995) 96. X-ray diffraction experiments (valid for the inter- [4] Y.H. Phang, D.E. Savage, R. Kariotis, M.G. Lagally, faces and the surface) agrees well with those from J. Appl. Phys. 74 (1993) 3181. the surface imaged using AFM. [5] D.K.G. de Boer, A.J.G. Leenaers, W.W. van den Hoogen- hof, Appl. Phys. A 58 (1994) 169. [6 ] D.E. Savage, N. Schimke, Y.-H. Phang, M.G. Lagally, J. Appl. Phys. 71 (1992) 3283. 4. Conclusions [7] J.B. Kortright, J. Appl. Phys. 70 (1991) 3620. [8] S.K. Sinha, E.B. Sirota, S. Garoff, H.B. Stanley, Phys. We have shown that diffuse X-ray scattering Rev. B 38 (1988) 2297. provides a full overview of the roughness at each [9] V. Holy, T. Baumbach, Phys. Rev. B 49 (1994) 10688. ` interface and the correlation between the inter- [10] C.S. Lent, P.I. Cohen, Surf. Sci. 161 (1985) 39. [11] J.E. Houston, R.L. Park, Surf. Sci. 26 (1971) 169. faces. Calculation of the diffuse scattering profiles [12] J.P. Pimbley, T-.M. Lu, J. Vac. Sci. Technol. A 2 (1984) is performed in kinematic scattering theory, includ- 457. ing refraction effects. Good agreement is found [13] M. Henzler, Surf. Sci. 73 (1978) 240.
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