714 Nuclear Instruments and Methods in Physics Research B55 (1991) 714-717

North-Holland

Raman scattering and photoluminescence analysis of SOI/SIMOX structures obtained by sequential implantation and annealing correlated with cross sectional TEM

A. PCrez a, J. Portillo a, A. Cornet a, J. JimCnez b, J.R. Morante a, P.L.F. Hemment ’ and K.P. Homewood ’ a Ccitedra d’Electrlnica, Facultat de Fisica, Universitat de Barcelona, Avda. Diagonal 645-647, 08028-Barcelona, Spain

b Departamento de Fisica de la Materia Condensada, Universidad de Valladolid, PO de la Magdalena SN, 4701I- Valladolid, Spain

’ Department of Electronic and Electrical Engineering, University of Surrey, Guildford, Surrey, GU2 5XH, UK

In this paper we describe the analysis by Raman scattering and photolnminescence spectroscopy of silicon-on-insulator (SOI) structures obtained by sequential implantation and annealing (%A). TEM observations have allowed correlation of the tensile strain observed from the Raman spectra obtained with excitation wavelengths higher than 488 nm and the D bands from the photoluminescence spectra to the presence of a region with precipitates and dislocations close to the buried oxide. Likewise, the comparison between the Raman spectra obtained with lower wavelength (457.9 nm) from SIA samples and from equivalent ones obtained by a single implantation and anneal, has corroborated the higher quality of the surface silicon layer from the SIA structure.

1. Introduction

Ramau scattering and photoluminescence spec- troscopy are techniques which have special interest when characterizing semiconductor materials for microelec- tronic applications. This is due to their nondestructive character, as well as their suitability for the study of thin layered structures, such as silicon-on-insulator (SOI) substrates. Between the different techniques for forma- tion of these substrates, SIMOX (separation by im- planted oxygen) appears to be the leading one [1,2]. Analysis carried out on these structures using the above techniques have been already reported by different authors [2-51. They have observed from their measure- ments the existence of a tensile strain in the top silicon layer, as well as its possible recovery by high tempera- ture anneal (HTA) stages.

In this work we have performed an analysis by Raman scattering and photoluminescence techniques of SOI/SIMOX structures which, unlike those previously reported, have been obtained by sequential implanta- tion and annealing (SIA). SIA constitutes one of the more interesting methods of production of high quality SOI material [1,2]. The substrates obtained by this method have defect densities several orders of magni- tude lower than those obtained by a single implant and anneal. However, the mechanisms related to the defects and their dependence on the technological parameters are not well known, which gives a strong interest in the characterization of these structures and comparison with

the substrates formed by conventional processing (single implant and anneal).

Raman scattering measurements have been obtained using different excitation wavelengths. This has allowed information about the structure of the different regions of the material to be obtained, and the presence of strain for different penetration depths to be detected. By correlating these data with the microscopic structure of the material (observed by cross sectional TEM) it is possible to determine the contribution of each region to the measured spectra, which constitutes relevant infor- mation for the analysis of the transport properties of the devices built in these materials.

2. Results and discussion

SIMOX wafers were produced by Ibis Technology Corp. [6]. Two different sets of samples were measured which, according to their preparation, were labelled SS and SIA. The SS samples were made by a single step oxygen implantation at an energy of 200 keV and a dose of 1.7 X 10” cmP2. During implantation the sub- strate was held at a temperature of 640” C. Subse- quently, samples were annealed at a temperature of 1300” C for 6 h. The SIA samples were obtained by implanting one third of the total dose at 640°C after which the wafer was annealed at 1300 O C for 2 h. This cycle was carried out three times to give a total dose of

0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

A. Ptrez et al. / Analysis of SOI/SDfOX structures 715

Fig. 1. Cross section image of SS sample. The arrow indicates the upper surface.

1.5 x fP cm-2 and a total annealim~ time of 6 h at 1300°C.

The observation of the saqles by cross sectional TEM has revealed the existence of significant dif- ferences between the SS and SIA structures. The cross section structure of the SS samples is shown in .fig. 1. It is formed by a top silicon layer free of Si& preci@tates and a buried oxide layer with silicon islands. Wqhave not been able to detect the presence of dislocatio,ns in the top layer. The thickness nf these layers (observed

along 2 pm of th& structure) are about 295 and 310‘ mn, respectively. In the case of the SIA sample (shown in fig. 2), the structure has three different regions: the first one is a 235 mn thick silicon layer free of precipitates and dislocations. Below this layer there is a region of silicon with oxide precipitates and disloc&ions, with an average thi&ness about 200 mn, md the buried oxide layer. The upper interface of this layer is very wavy, due to the coalescence of some of the precipitates. The total structure has a thickness of about 650 nm.

Fig. 2. Cross section image of SIA sample. The arrow indicates the upper surface.

VI. B&ATIZRIALS SCIEiWE

716 A. P&z et al. / Analysis of SOI/SIMOX structures

Raman scattering measurements have been per- formed with a DILOR XY spectrometer. The samples were excited with the 457.9, 488 and 514 run lines from an Ar+ laser. For these lines, the penetration depths for the scattered light have been estimated to be approxi- mately 300, 600 and 800 nm, respectively [7]. Detection has been made using the multichannel mode with pho- ton counting electronics. Moreover, all the spectra have been compared to these obtained under the same condi- tions in bulk silicon.

Fig. 3 shows the spectra obtained from the different samples with an excitation wavelength of 457.9 nm. In this figure the intensity is normalized to the maximum value. These spectra show a shift towards the low fre- quencies and a symmetric broadening in relation to that obtained in bulk silicon. This indicates that these layers are under a tensile strain. The symmetrical shape of the lines confirms the crystalline nature of the layers, and their broadening points to the existence of a distribu- tion of strains. No amorphous-like bands are observed. Moreover, the spectra from SS samples present higher shifts and broadenings of the Raman lines than those from SIA samples. This points to the existence of a higher tensile strain in the surface region of the SS structure in relation to the SIA one.

The results obtained using excitation wavelengths of 488 and 514 nm show a different behaviour. In fig. 4 the spectra measured for a wavelength of 514 nm are plotted. In this case, the higher shifts and broadenings occur in the SIA structure. Moreover, the spectra obtained for this structure show a small contribution in the higher frequencies side, which indicates the presence in the SIA sample of a compressive stress.

The increase in the shift and the broadening of the Raman lines from SIA samples as the penetration depth increases is in agreement with the observations of Olego et al. [4] and Takahashi et al. [7] from SS samples. These authors correlated the strain in the top silicon layer with

_ bulk

. . . . . . . . . SS ___ SIA

530 520 510 50( ;

shift (cm-‘)

Fig. 3. First-order Raman spectra obtained from the different

samples with an excitation wavelength of 457.9 nm. The Ra-

man intensities are normalized (I,, = 1 for every sample).

‘\ _ bulk

‘; ; . . 5 s

: --_ SIA i ‘,

3 ’ i i, ’

5 ; i‘ idL ,q i ‘\

,I; i ‘, ,f;. \I,

,‘.:’ : i, ,, ;: : \\

_, ,:’ . . _ _--,./

\\ /_I--.. ,.... . ..I’ ‘Y . . . .._ ‘.\ . . . . ..y.--___

550 530 510 490

shift (cm-‘)

Fig. 4. First-order Raman spectra obtained with an excitation

wavelength of 514 nm. The Raman intensities are normalized.

the presence of oxide precipitates. As the penetration depth increases, the contribution to the spectra of the region with precipitates and dislocations increases. However, from the measurements carried out with the lower penetration depth we observe a lower strain in the SIA structure than in the SS one (free of oxygen pre- cipitates). This gives a strong indication about the higher quality of the surface region of the SIA structure (free of precipitates and dislocations). According to this, higher mobility values (closer to those of bulk silicon) are to be expected in the surface region of the SIA sample. The small compressive strain component found for higher penetration depths could be due to the ex- istence of a higher compressive strain below the buried layer (similar to that observed in as-implanted struc- tures [2]) or maybe in the silicon regions below the precipitates.

The higher quality of the SIA structure has been corroborated by photoluminescence (PL) measure- ments. These measurements have been made over the energy range 0.750 to 1.160 eV. The sample has been excited with the 514 nm line from the Ar+ laser. The temperature of the sample has been estimated to be 25 K. The spectral response was determined with a grating monochromator (Jobin-Yvon, model HR2) with a reso- lution of 13 A/mm, using a cooled InAs detector and a conventional lock-in amplifier. The spectra were nor- malized to compensate for the detector response.

Fig. 5 shows PL spectra from SS (curve a) and SIA samples (curve b). Both curves show a peak located in the near band-gap region which is due to the recombi- nation of intrinsic excitons. The intensity of this peak in the SIA sample is a factor of 2 greater than in the SS sample, indicating a larger number of non-radiative defects in the single implant material, which is in agree- ment with the lower strain observed in the top Si layer of the SIA material.

Moreover, the spectrum from the SIA sample shows at lower energies the presence of broad defect related

A. Perez et al. /Analysis of SOI/SIMOXstructures 731

0.04 8 I I I I I 0.76 0.84 0.92 1.00 1.06 1.16

ENERGY (eV)

Fig. 5. PL spectra obtained from (a) SS and @) SIA samples.

peaks labelled D,, Da, D, and D4. Their positions fall within the range of energies previously reported by Duncan et al. [S], namely, 0.802 to 0.814 eV, 0.862 to

0.875 eV, 0.930 to 0.948 eV and 0.992 to 1.000 eV. The broadest peak, which appears at the lowest energy, includes the D, and Da bands. These peaks were first observed in plastically deformed silicon, and are be- lieved to be related to dislocations [S]. They have been observed also in bulk silicon in which oxygen is known to have precipitated and in SS SIMOX material [5,8,9]. In this latter case, the width of the D lines was found to be much broader than in the plastic~y deformed sam- ples. Drozdov et al. [lo] have reported that a high oxygen concentration can increase considerably the width of the D lines, specially if the samples are cooled down slowly from high temperatures to produce a high concentration of oxygen precipitates. According to this, Weber et al. [S] have attributed this broadening of the D lines to inhomogeneous strains in the layers, determined by the presence of oxygen precipitates. So, the broad bands from the SIA samples (curve b) would be due to excess oxygen in these samples, maybe as oxygen pre- cipitates. The appearance of the D bands in the PL spectrum is consistent with the observation by TEM of dislocations in the second region of the SIA structure pinned to the precipitates.

3. Conclusions

From cross sectional TEM analysis of both sets of samples we have deduced the high morphological qual- ity of the studied structures. In both cases we have

observed the existence of a Si surface layer free of dislocations within the resolution of our observations (about 10’ cm-‘). However, the ch~acte~ation of the samples by Raman scattering and photoluminescence spectroscopy has allowed demonstration of the better quality of the top silicon layer free of precipitates from the structure obtained by sequential implant and an- nealing and showing a lower strain and a lower con- ~entration of defects such as non-radiative recombina- tion centers, in relation to the SS substrate.

The presence of a layer of SiO, precipitates close to the buried oxide with dislocations pinned to them de- termines a higher tensile strain in this region, which is observed by the shifts and broadenings of the Raman peaks, as well as the appearance of the D bands in the PL spectra. A similar structure has been observed by Hill et al. [ll] from SIA samples implanted at 150 keV. The cause of these precipitates is not clear. It seems to be related to initial wafer conditions affecting the oxygen precipitation. In any case, they indicate the need for an opti~~tion of the technological parameters related to the SIA process.

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VI. MATERIALS SCIENCE