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Si islands
Microelectronic Engineering 70 (2003) 240–245 www.elsevier.com / locate / mee
Electrical properties of W/ Si interfaces with embedded Ge / Si islands a, a a a b b c A. Hattab *, F. Meyer , Vy Yam , D. Bouchier , R. Meyer , O. Schneegans , C. Clerc a
ˆ 220, Universite´ Paris Sud, 91405 Orsay Cedex, France Institut d’ Electronique Fondamentale, CNRS UMR 8622, Bat b ´ Paris VI et XI, Supelec ´ , 91192 Gif-sur-Yvette Cedex, France LGEP, CNRS URA 0127, Universites c ˆ 108, Universite´ Paris Sud, 91405 Orsay Cedex, France C.S.N.S.M. /CNRS IN2 P3, Bat
Abstract In this work, we investigated electrical and morphological properties of W/ p-type Si Schottky diodes with intentional inhomogeneities introduced by macroscopic Ge-islands embedded beneath the interface. The Si-cap layer thickness (or the island-distance to the interface) was progressively reduced by successive chemical etching cycles. Electrical characterizations were achieved through reverse current–voltage (I–V ) at room temperature and forward I–V measurements as a function of the temperature. In parallel, Rutherford backscattering spectroscopy analyses were performed to follow the Si-cap / Ge islands chemical thinning down with increasing the number of etching cycles. In addition, the comparison of topographical and electrical properties of the etched silicon-cap layer was carried out by conductive atomic force microscopy analyses with a nanometer-scale resolution. Our results indicate that the areas on the top of islands exhibit lower resistance than those which covered the wetting layer. This lateral variation of resistance at the surface of the semiconductor may correspond to Schottky barrier height inhomogeneities observed on broad area I–V characteristics of Schottky contacts. 2003 Elsevier B.V. All rights reserved. Keywords: Schottky barrier height inhomogeneities; Ge-islands; Conductive AFM
1. Introduction Current–voltage (I–V ) characteristics of Schottky diodes often exhibit significant device-to-device variations and non-ideal behavior which cannot be explained adequately by assuming an abrupt junction with a constant barrier height. One way to account for the abnormal behavior is to consider the existence of fluctuations in barrier heights due to the presence of inhomogeneities over the metal–semiconductor contact area [1–3]. Recently, different authors modeled the influence of spatial interface *Corresponding author. Tel.: 133-1-6915-4037; fax: 133-16915-4020. E-mail address: [email protected] (A. Hattab).
inhomogeneities by assuming a Gaussian distribution either of the Schottky barrier heights (SBHs) [1] or of the nanometer-scale patches with reduced SBHs [2,3] and explained the strong correlation that is observed between the SBH and the ideality factor. Various attempts have been made to test the limits of these models through nanometer-scale measurements. Lateral variation in the SBH has been measured on length scales ranging from a few to several hundred nanometers by using ballistic electron emission microscopy (BEEM) and related to macroscopic I–V characteristics [4,5]. This comparison shows that most of the macroscopic SBHs obtained from I–V measurements can be successively interpreted by using the model proposed by Tung [3] with the nm-scale parameters determined by BEEM.
0167-9317 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00433-7
A. Hattab et al. / Microelectronic Engineering 70 (2003) 240–245
Inhomogeneities can exist even in carefully prepared samples and are mostly uncontrolled. In a recent work [6], we investigated electrical properties of Schottky diodes with intentional inhomogeneities introduced by precursor Ge-dots (height of about 2 nm and density of about 6310 7 cm 22 ) located just below the W/ Si interface. Different phenomena may contribute to the SBH fluctuations: Ge segregation, interdiffusion, tensile strain in the Si-cap layer on the island top, strain relaxation in the Ge-island, and more generally the whole energetic band diagram which depends on the Ge-island presence. In this paper, to complete the previous study, we achieved other analyses on islands with larger size and density (macroscopic islands) [7]. The existence of the inhomogeneities is related to the presence of Ge-islands located below the metal / Si contact. Their effect on inhomogeneity is controlled by the Si-cap thickness which is reduced by successive chemical oxidation / etching cycles. We report a simultaneous microscopic and macroscopic study of the contacts. The electrical properties of Schottky barrier were first investigated by conventional current–voltage characteristics. In addition, studies based on an alternative approach to BEEM, conductive atomic force microscopy (AFM) [8], in which simultaneous cartographies of surface roughness and local resistance, have been performed. The evolution of the Ge content as a function of the number of the etching cycles is followed by Rutherford backscattering spectroscopy (RBS) on similar samples.
2. Experimental Samples with Ge-islands were grown on p-Si(100) substrates by ultrahigh-vacuum chemical vapor deposition (UHV-CVD) at temperature of 700 8C. Pure SiH 4 and hydrogen diluted (10%) GeH 4 were used as gas sources. The films can be doped p-type by using B 2 H 4 . The system has a base pressure better 210 than 1310 Torr, and the pressure during growth was about 5310 24 Torr (1 Torr5133.322 Pa). Details of growth and sample preparation have been reported elsewhere [7]. The Ge / Si growth follows a Stranski Krastanow
241
growth mode in which a layer-by-layer grown flat wetting layer (of about 2.5 ML) is followed by the formation of Ge-islands. The two-dimensional– three-dimensional transition is determined by in-situ RHEED (reflection high energy electron diffraction). On a p-type Si (100) substrate (5 V cm), a Si buffer layer of 250 nm thickness with a doping concentration of p55310 16 cm 23 was deposited. Afterwards, 4.5 ML of Ge was grown to form the islands. The size and the density of islands were measured by AFM on as-deposited samples. The islands exhibit two different sizes and shapes: pyramids and domes. The latter are of similar base size but are two times higher. The domes have an average diameter of 115 nm and a height of 22 nm, while the pyramids have an average height of 12 nm [7]. The areal density of these macroscopic islands is about 3310 8 cm 22 and 1310 8 cm 22 for pyramids and domes, respectively. The Ge / Si single layer was subsequently covered with a 12-nm-thick non-intentionally p-doped Si layer. All the samples were then cleaned using a standard degreasing procedure, followed by a dip in diluted HF (1:10) for 45 s and a final rinse in deionized water. For some samples, the Si-cap layer is thinned by chemical etching cycles of repeated (1–2 nm in thick) oxidation in boiling HNO 3 and HF reduction. Each cycle must result in the removal of about 1 nm of Si [9]. This etching is performed to change the distance between the islands and the interface with the metal, in order to vary the island effect on the SBH inhomogeneities. After a rapid loading, a W-film with a thickness of about 100 nm was then deposited by magnetron sputtering system at room temperature. Low cathode voltages were used (#300 V) to prevent any radiation damage. This procedure leads to high quality Schottky diodes on silicon substrate [10]. Several diodes (80 for each sample) with different areas are obtained by lithography. Electrical characterizations of Schottky contacts were achieved through reverse current–voltage measurements at room temperature and forward I–V characterizations as a function of the temperature. The current through the metal–semiconductor interface was assumed to be governed by thermionic emission. An effective SBH FBR was deduced from the reverse current measured at a given reverse voltage VR 53 V while the zero-voltage-SBH FB0 is determined from the forward I–V characteristic. This
242
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procedure leads to underestimated SBH values which differ from the zero-field-SBH FFB . This last SBH can be determined by taking into account the image force lowering corresponding to the electric field and the doping profile in the samples [11]. RBS was used to follow the reduction of the Ge content as a function of the etching number. In addition to the macroscopic I–V characterizations, simultaneous topography and resistance measurements have been carried out with a nanoscale resolution by using a modified commercial AFM with a conductive probe (Nanoscope III, Digital Instruments) [8]. The probe is placed in contact with the sample and a bias of a few 10 22 to a few 10 21 V is applied. The resulting current gives an instantaneous image of the tip to sample resistance.
3. Results and discussion In order to complete a previous study relative to precursor islands [6] and to get a better understanding of inhomogeneous interfaces with buried islands, we achieved analyses on samples with larger and denser islands (macroscopic islands). Firstly, we checked that there is no effect of the different pretreatment on the electrical properties of W-contacts formed by W magnetron sputtering on p-type pure-Si wafers (boron-doped 4310 15 cm 23 ). The SBH FBR remains constant and close to 0.43 eV, regardless the number of chemical oxidation / etching cycles performed prior to the metallization (Fig. 1). This value corresponds to a zero-field-SBH FFB of about 0.46 eV. We have also plotted the average values of the
Fig. 1. Schottky barrier height extracted from reverse I–V characteristics as a function of the number of (HNO 3 –HF) cycles for a sample with macroscopic islands beneath the metal / Si interface. For comparison, some results are given for W/ pure-Si. The SBH was calculated by using the current measured at VR 53 V. 0 cycle corresponds to a dip in an aqueous solution of 10% HF.
A. Hattab et al. / Microelectronic Engineering 70 (2003) 240–245
SBH FBR for contacts to samples with macroscopic islands as a function of the number of (HNO 3 –HF) cycles. We can observe that the SBH for these samples depends on the Si-cap thickness. When the number of the etching cycles is limited (less than three), the SBH is rather low and the diodes exhibit a nearly ohmic behavior. Beyond three etching cycles, the SBH increases slowly and reaches, after nine cycles, the expected value for pure-Si. It is noteworthy that the SBH corrections due to the image force lowering are similar for the Si substrate and the samples with Ge-islands, and are both close to 30 meV. In addition to analyses of reverse I–V characteristics at room temperature, we studied forward I–V characteristics as a function of the temperature and determined the zero-voltage-SBH as well as the ideality factor. Forward I–V curves lead to SBHs which well agree with those obtained from reverse I–V characteristics. This result indicates that the barrier is the same for the forward and the reverse regimes and suggests that the SBH reduction cannot be neither explained by the presence of the shallow Si / Ge / Si heterojunction nor by localized holes in the Ge-islands. When increasing the number of etching cycles beyond 3, the ideality factor, n, decreases and the SBH increases and reaches the value expected for pure-Si. For all the samples, a SBH decrease and an increase of n with decreasing temperature is observed with a linear dependence between these two parameters. Meanwhile, the non-idealities are more pronounced for samples with thicker Si-cap layers and suggest that the inhomogeneities are stronger in these cases. This result strongly differs from that obtained on samples with precursor buried islands [6]. In this last case, the first HNO 3 –HF dipping, which must remove the Ge excess at the surface due to segregation, lead to a large increase of the SBH towards that to pure-Si. This SBH increase is not observed for samples with macroscopic islands although Ge segregation has also occurred during the Si-cap layer growth. This trend indicates that another contribution has to be taken into account to explain the results with macroscopic islands. In addition, for samples with precursor islands, when the Si-cap layer was still thick enough (4 to 5 nm), the effect of the islands was limited and the diode exhibited roughly an ideal behavior: the ideality factor was
243
close to unity, and the variations of the SBH and the ideality factor with temperature were very limited. The Si-cap layer was able to screen the effects due to these small islands. This screening effect cannot occur when the islands are as large as the macroscopic islands. To observe this screening effect on macroscopic islands, the Si-cap layer thickness should have to be increased. Samples with thicker Si-cap layers are presently under investigations to verify this assumption. In order to get a better understanding of these results, we have first performed RBS analyses to follow the Ge content after the different etching cycles on similar samples. The Ge content in samples with macroscopic islands is only slightly reduced after three cycles, divided by about ten after seven cycles and is below the detection limit after nine cycles. This last result indicates that in this case, the Ge-layer must have been strongly reduced. Therefore, each cycle must result in the removal of about 1 to 1.5 nm of Si. As noticed by other authors, during the capping process, the shape, the composition and the density of the Ge-islands change and lead to a flattening of the surface [12]. These modifications are related to interdiffusion, Ge-segregation and strain relaxation of the islands. To study the effect of the capping process, we have investigated the surface roughness of the as-deposited sample by AFM. These analyses evidence a flattening of the surface even if protusions (up to 9–10 nm) still remain on the surface. Fig. 2 shows the height distribution of the surface just after the Si-cap layer deposition and demonstrates that the surface is still characterized by two kinds of islands with different heights. These two populations must correspond to the Si-growth over pyramids (smaller protusions) and domes (larger protusions). We can also observe that the density is still higher for pyramids than for domes, and that the total density is about 5310 8 cm 22 . This result indicates that no significant density change is observed after the capping process. On the contrary, an important change in the width is observed. The average width of about 300 nm is about three times higher than before capping. These results confirm that the Siovergrowth results in a pronounced flattering of the islands, especially of the domes. The first etch of the surface leads to a reduction of
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Fig. 2. Surface roughness distribution for the as-deposited sample with macroscopic buried islands determined by AFM.
the number of domes although the topography image is not significantly modified. This trend is enhanced after three etch cycles. This large reduction of the domes must correspond to the decrease of the Ge content observed by RBS analyses and can be explained by the AFM images. These images indicated that the reduction in the Ge content after three
cycles (Fig. 3) can be related to the larger etch rate for Ge than for Si (two or three time higher). When the Ge is no longer protected by Si at the top of the islands, it is rapidly removed by etching and the Ge-islands are progressively changes into volcanoes with crater depths increasing with the number of etching cycles. The earlier etch of domes must
Fig. 3. Topography image of a sample with macroscopic buried islands after three (HNO 3 –HF) etching cycles.
A. Hattab et al. / Microelectronic Engineering 70 (2003) 240–245
evidence a thinner Si-cap layer on their tops and consequently a higher tensile strain of the silicon. This faster removal of domes could be also enhanced by a preferential etch of tensile strained Si and (or) relaxed Ge. A resistance map with nanometer-scale resolution was carried out simultaneously to topography cartography by using conductive AFM. The distribution of conductivity is not uniform and is correlated to the topography of the surfaces: the resistance is lower on the top (and on the edges) of the islands than over the wetting layer, 10 10 against 10 12 V. The reduction of the resistance is much more important on the top of the domes. This non-uniformity of conductivity over the surface may be related to a local lowering of the SBH due to a reduction of the tensile-strained-Si band gap over the islands, correspond to SBH inhomogeneities and explain the non-ideal macroscopic I–V characteristics. After three cycles, the Ge islands are progressively removed, the areas with low resistance (or low SBH) are removed, the sample becomes more and more homogenous, and as a result the SBH increases. These results differ from those we have obtained on samples with smaller islands which exhibit less inhomogeneities (larger SBHs, lower ideality factors) and an increase of the inhomogeneity effect with increasing the number of etching cycles. In this last case, the first etching cycles lead only to a decrease of the distance between the island tops and the interface, without any destruction of the small islands. Therefore, the inhomogeneities increase with the number of cycles in this case.
4. Conclusion Our investigations on W-contacts to samples with
245
Ge-islands buried below the interface show that we are able to control the island effect on SBH inhomogeneities, either by the change of the island shape and density (precursor and macroscopic islands), or by etching the Si-cap layer. The inhomogeneities are due either directly to the presence of Ge islands in Si and (or) to the lowering of the surface band-gap in the strained parts of the Si-cap above buried islands. The screening effect of the Si-cap layer depends on its thickness and on the island size, and is more effective for small islands. Finally, we have investigated the nanoscale properties of buried Ge-islands and related these measurements to non-ideal macroscopic I–V characteristics.
References ¨ [1] J. Werner, H. Guttler, J. Appl. Phys. 69 (1991) 1522. ¨ [2] W. Monch, J. Vac. Sci. Technol. B 17 (1999) 1867. [3] R. Tung, Appl. Phys. Lett. 58 (1991) 2821; R. Tung, Phys. Rev. B 45 (1992) 13509. [4] C. Detavernier, R.L. Van Meirhaeghe, R. Donaton, K. Maex, F. Cardon, J. Appl. Phys. 84 (1998) 3226. [5] H.J. Im, Y. Ding, J.P. Pelz, W.J. Choyke, Phys. Rev. B 64 (2001) 75310. [6] A. Hattab, V. Aubry Fortuna, F. Meyer, Vy. Yam, V. Le Than, D. Bouchier, C. Clerc, Microelectron. Eng. 64 (2002) 435– 441. [7] V. Le Thanh, Surf. Sci. 492 (2001) 255. ´ R. Meyer, O. Schneegans, L. Boyet, Appl. Phys. [8] F. Houze, Lett. 69 (1996) 1975. [9] Y. Ishimaru, M. Yoshiki, T. Hatanaka, Mater. Res. Soc. Symp. Soc. Proc. 259 (1992) 405. [10] V. Aubry-Fortuna, M. Barthula, G. Tremblay, F. Meyer, P. Warren, K. Lyutovitch, J. Appl. Phys. 89 (2001) 5533. [11] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1969. ¨ [12] T. Meyer, M. Klemenc, H. von Kanel, Phys. Rev. B 60 (1999) 8473.
Electrical properties of W/ Si interfaces with embedded Ge / Si islands a, a a a b b c A. Hattab *, F. Meyer , Vy Yam , D. Bouchier , R. Meyer , O. Schneegans , C. Clerc a
ˆ 220, Universite´ Paris Sud, 91405 Orsay Cedex, France Institut d’ Electronique Fondamentale, CNRS UMR 8622, Bat b ´ Paris VI et XI, Supelec ´ , 91192 Gif-sur-Yvette Cedex, France LGEP, CNRS URA 0127, Universites c ˆ 108, Universite´ Paris Sud, 91405 Orsay Cedex, France C.S.N.S.M. /CNRS IN2 P3, Bat
Abstract In this work, we investigated electrical and morphological properties of W/ p-type Si Schottky diodes with intentional inhomogeneities introduced by macroscopic Ge-islands embedded beneath the interface. The Si-cap layer thickness (or the island-distance to the interface) was progressively reduced by successive chemical etching cycles. Electrical characterizations were achieved through reverse current–voltage (I–V ) at room temperature and forward I–V measurements as a function of the temperature. In parallel, Rutherford backscattering spectroscopy analyses were performed to follow the Si-cap / Ge islands chemical thinning down with increasing the number of etching cycles. In addition, the comparison of topographical and electrical properties of the etched silicon-cap layer was carried out by conductive atomic force microscopy analyses with a nanometer-scale resolution. Our results indicate that the areas on the top of islands exhibit lower resistance than those which covered the wetting layer. This lateral variation of resistance at the surface of the semiconductor may correspond to Schottky barrier height inhomogeneities observed on broad area I–V characteristics of Schottky contacts. 2003 Elsevier B.V. All rights reserved. Keywords: Schottky barrier height inhomogeneities; Ge-islands; Conductive AFM
1. Introduction Current–voltage (I–V ) characteristics of Schottky diodes often exhibit significant device-to-device variations and non-ideal behavior which cannot be explained adequately by assuming an abrupt junction with a constant barrier height. One way to account for the abnormal behavior is to consider the existence of fluctuations in barrier heights due to the presence of inhomogeneities over the metal–semiconductor contact area [1–3]. Recently, different authors modeled the influence of spatial interface *Corresponding author. Tel.: 133-1-6915-4037; fax: 133-16915-4020. E-mail address: [email protected] (A. Hattab).
inhomogeneities by assuming a Gaussian distribution either of the Schottky barrier heights (SBHs) [1] or of the nanometer-scale patches with reduced SBHs [2,3] and explained the strong correlation that is observed between the SBH and the ideality factor. Various attempts have been made to test the limits of these models through nanometer-scale measurements. Lateral variation in the SBH has been measured on length scales ranging from a few to several hundred nanometers by using ballistic electron emission microscopy (BEEM) and related to macroscopic I–V characteristics [4,5]. This comparison shows that most of the macroscopic SBHs obtained from I–V measurements can be successively interpreted by using the model proposed by Tung [3] with the nm-scale parameters determined by BEEM.
0167-9317 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00433-7
A. Hattab et al. / Microelectronic Engineering 70 (2003) 240–245
Inhomogeneities can exist even in carefully prepared samples and are mostly uncontrolled. In a recent work [6], we investigated electrical properties of Schottky diodes with intentional inhomogeneities introduced by precursor Ge-dots (height of about 2 nm and density of about 6310 7 cm 22 ) located just below the W/ Si interface. Different phenomena may contribute to the SBH fluctuations: Ge segregation, interdiffusion, tensile strain in the Si-cap layer on the island top, strain relaxation in the Ge-island, and more generally the whole energetic band diagram which depends on the Ge-island presence. In this paper, to complete the previous study, we achieved other analyses on islands with larger size and density (macroscopic islands) [7]. The existence of the inhomogeneities is related to the presence of Ge-islands located below the metal / Si contact. Their effect on inhomogeneity is controlled by the Si-cap thickness which is reduced by successive chemical oxidation / etching cycles. We report a simultaneous microscopic and macroscopic study of the contacts. The electrical properties of Schottky barrier were first investigated by conventional current–voltage characteristics. In addition, studies based on an alternative approach to BEEM, conductive atomic force microscopy (AFM) [8], in which simultaneous cartographies of surface roughness and local resistance, have been performed. The evolution of the Ge content as a function of the number of the etching cycles is followed by Rutherford backscattering spectroscopy (RBS) on similar samples.
2. Experimental Samples with Ge-islands were grown on p-Si(100) substrates by ultrahigh-vacuum chemical vapor deposition (UHV-CVD) at temperature of 700 8C. Pure SiH 4 and hydrogen diluted (10%) GeH 4 were used as gas sources. The films can be doped p-type by using B 2 H 4 . The system has a base pressure better 210 than 1310 Torr, and the pressure during growth was about 5310 24 Torr (1 Torr5133.322 Pa). Details of growth and sample preparation have been reported elsewhere [7]. The Ge / Si growth follows a Stranski Krastanow
241
growth mode in which a layer-by-layer grown flat wetting layer (of about 2.5 ML) is followed by the formation of Ge-islands. The two-dimensional– three-dimensional transition is determined by in-situ RHEED (reflection high energy electron diffraction). On a p-type Si (100) substrate (5 V cm), a Si buffer layer of 250 nm thickness with a doping concentration of p55310 16 cm 23 was deposited. Afterwards, 4.5 ML of Ge was grown to form the islands. The size and the density of islands were measured by AFM on as-deposited samples. The islands exhibit two different sizes and shapes: pyramids and domes. The latter are of similar base size but are two times higher. The domes have an average diameter of 115 nm and a height of 22 nm, while the pyramids have an average height of 12 nm [7]. The areal density of these macroscopic islands is about 3310 8 cm 22 and 1310 8 cm 22 for pyramids and domes, respectively. The Ge / Si single layer was subsequently covered with a 12-nm-thick non-intentionally p-doped Si layer. All the samples were then cleaned using a standard degreasing procedure, followed by a dip in diluted HF (1:10) for 45 s and a final rinse in deionized water. For some samples, the Si-cap layer is thinned by chemical etching cycles of repeated (1–2 nm in thick) oxidation in boiling HNO 3 and HF reduction. Each cycle must result in the removal of about 1 nm of Si [9]. This etching is performed to change the distance between the islands and the interface with the metal, in order to vary the island effect on the SBH inhomogeneities. After a rapid loading, a W-film with a thickness of about 100 nm was then deposited by magnetron sputtering system at room temperature. Low cathode voltages were used (#300 V) to prevent any radiation damage. This procedure leads to high quality Schottky diodes on silicon substrate [10]. Several diodes (80 for each sample) with different areas are obtained by lithography. Electrical characterizations of Schottky contacts were achieved through reverse current–voltage measurements at room temperature and forward I–V characterizations as a function of the temperature. The current through the metal–semiconductor interface was assumed to be governed by thermionic emission. An effective SBH FBR was deduced from the reverse current measured at a given reverse voltage VR 53 V while the zero-voltage-SBH FB0 is determined from the forward I–V characteristic. This
242
A. Hattab et al. / Microelectronic Engineering 70 (2003) 240–245
procedure leads to underestimated SBH values which differ from the zero-field-SBH FFB . This last SBH can be determined by taking into account the image force lowering corresponding to the electric field and the doping profile in the samples [11]. RBS was used to follow the reduction of the Ge content as a function of the etching number. In addition to the macroscopic I–V characterizations, simultaneous topography and resistance measurements have been carried out with a nanoscale resolution by using a modified commercial AFM with a conductive probe (Nanoscope III, Digital Instruments) [8]. The probe is placed in contact with the sample and a bias of a few 10 22 to a few 10 21 V is applied. The resulting current gives an instantaneous image of the tip to sample resistance.
3. Results and discussion In order to complete a previous study relative to precursor islands [6] and to get a better understanding of inhomogeneous interfaces with buried islands, we achieved analyses on samples with larger and denser islands (macroscopic islands). Firstly, we checked that there is no effect of the different pretreatment on the electrical properties of W-contacts formed by W magnetron sputtering on p-type pure-Si wafers (boron-doped 4310 15 cm 23 ). The SBH FBR remains constant and close to 0.43 eV, regardless the number of chemical oxidation / etching cycles performed prior to the metallization (Fig. 1). This value corresponds to a zero-field-SBH FFB of about 0.46 eV. We have also plotted the average values of the
Fig. 1. Schottky barrier height extracted from reverse I–V characteristics as a function of the number of (HNO 3 –HF) cycles for a sample with macroscopic islands beneath the metal / Si interface. For comparison, some results are given for W/ pure-Si. The SBH was calculated by using the current measured at VR 53 V. 0 cycle corresponds to a dip in an aqueous solution of 10% HF.
A. Hattab et al. / Microelectronic Engineering 70 (2003) 240–245
SBH FBR for contacts to samples with macroscopic islands as a function of the number of (HNO 3 –HF) cycles. We can observe that the SBH for these samples depends on the Si-cap thickness. When the number of the etching cycles is limited (less than three), the SBH is rather low and the diodes exhibit a nearly ohmic behavior. Beyond three etching cycles, the SBH increases slowly and reaches, after nine cycles, the expected value for pure-Si. It is noteworthy that the SBH corrections due to the image force lowering are similar for the Si substrate and the samples with Ge-islands, and are both close to 30 meV. In addition to analyses of reverse I–V characteristics at room temperature, we studied forward I–V characteristics as a function of the temperature and determined the zero-voltage-SBH as well as the ideality factor. Forward I–V curves lead to SBHs which well agree with those obtained from reverse I–V characteristics. This result indicates that the barrier is the same for the forward and the reverse regimes and suggests that the SBH reduction cannot be neither explained by the presence of the shallow Si / Ge / Si heterojunction nor by localized holes in the Ge-islands. When increasing the number of etching cycles beyond 3, the ideality factor, n, decreases and the SBH increases and reaches the value expected for pure-Si. For all the samples, a SBH decrease and an increase of n with decreasing temperature is observed with a linear dependence between these two parameters. Meanwhile, the non-idealities are more pronounced for samples with thicker Si-cap layers and suggest that the inhomogeneities are stronger in these cases. This result strongly differs from that obtained on samples with precursor buried islands [6]. In this last case, the first HNO 3 –HF dipping, which must remove the Ge excess at the surface due to segregation, lead to a large increase of the SBH towards that to pure-Si. This SBH increase is not observed for samples with macroscopic islands although Ge segregation has also occurred during the Si-cap layer growth. This trend indicates that another contribution has to be taken into account to explain the results with macroscopic islands. In addition, for samples with precursor islands, when the Si-cap layer was still thick enough (4 to 5 nm), the effect of the islands was limited and the diode exhibited roughly an ideal behavior: the ideality factor was
243
close to unity, and the variations of the SBH and the ideality factor with temperature were very limited. The Si-cap layer was able to screen the effects due to these small islands. This screening effect cannot occur when the islands are as large as the macroscopic islands. To observe this screening effect on macroscopic islands, the Si-cap layer thickness should have to be increased. Samples with thicker Si-cap layers are presently under investigations to verify this assumption. In order to get a better understanding of these results, we have first performed RBS analyses to follow the Ge content after the different etching cycles on similar samples. The Ge content in samples with macroscopic islands is only slightly reduced after three cycles, divided by about ten after seven cycles and is below the detection limit after nine cycles. This last result indicates that in this case, the Ge-layer must have been strongly reduced. Therefore, each cycle must result in the removal of about 1 to 1.5 nm of Si. As noticed by other authors, during the capping process, the shape, the composition and the density of the Ge-islands change and lead to a flattening of the surface [12]. These modifications are related to interdiffusion, Ge-segregation and strain relaxation of the islands. To study the effect of the capping process, we have investigated the surface roughness of the as-deposited sample by AFM. These analyses evidence a flattening of the surface even if protusions (up to 9–10 nm) still remain on the surface. Fig. 2 shows the height distribution of the surface just after the Si-cap layer deposition and demonstrates that the surface is still characterized by two kinds of islands with different heights. These two populations must correspond to the Si-growth over pyramids (smaller protusions) and domes (larger protusions). We can also observe that the density is still higher for pyramids than for domes, and that the total density is about 5310 8 cm 22 . This result indicates that no significant density change is observed after the capping process. On the contrary, an important change in the width is observed. The average width of about 300 nm is about three times higher than before capping. These results confirm that the Siovergrowth results in a pronounced flattering of the islands, especially of the domes. The first etch of the surface leads to a reduction of
244
A. Hattab et al. / Microelectronic Engineering 70 (2003) 240–245
Fig. 2. Surface roughness distribution for the as-deposited sample with macroscopic buried islands determined by AFM.
the number of domes although the topography image is not significantly modified. This trend is enhanced after three etch cycles. This large reduction of the domes must correspond to the decrease of the Ge content observed by RBS analyses and can be explained by the AFM images. These images indicated that the reduction in the Ge content after three
cycles (Fig. 3) can be related to the larger etch rate for Ge than for Si (two or three time higher). When the Ge is no longer protected by Si at the top of the islands, it is rapidly removed by etching and the Ge-islands are progressively changes into volcanoes with crater depths increasing with the number of etching cycles. The earlier etch of domes must
Fig. 3. Topography image of a sample with macroscopic buried islands after three (HNO 3 –HF) etching cycles.
A. Hattab et al. / Microelectronic Engineering 70 (2003) 240–245
evidence a thinner Si-cap layer on their tops and consequently a higher tensile strain of the silicon. This faster removal of domes could be also enhanced by a preferential etch of tensile strained Si and (or) relaxed Ge. A resistance map with nanometer-scale resolution was carried out simultaneously to topography cartography by using conductive AFM. The distribution of conductivity is not uniform and is correlated to the topography of the surfaces: the resistance is lower on the top (and on the edges) of the islands than over the wetting layer, 10 10 against 10 12 V. The reduction of the resistance is much more important on the top of the domes. This non-uniformity of conductivity over the surface may be related to a local lowering of the SBH due to a reduction of the tensile-strained-Si band gap over the islands, correspond to SBH inhomogeneities and explain the non-ideal macroscopic I–V characteristics. After three cycles, the Ge islands are progressively removed, the areas with low resistance (or low SBH) are removed, the sample becomes more and more homogenous, and as a result the SBH increases. These results differ from those we have obtained on samples with smaller islands which exhibit less inhomogeneities (larger SBHs, lower ideality factors) and an increase of the inhomogeneity effect with increasing the number of etching cycles. In this last case, the first etching cycles lead only to a decrease of the distance between the island tops and the interface, without any destruction of the small islands. Therefore, the inhomogeneities increase with the number of cycles in this case.
4. Conclusion Our investigations on W-contacts to samples with
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Ge-islands buried below the interface show that we are able to control the island effect on SBH inhomogeneities, either by the change of the island shape and density (precursor and macroscopic islands), or by etching the Si-cap layer. The inhomogeneities are due either directly to the presence of Ge islands in Si and (or) to the lowering of the surface band-gap in the strained parts of the Si-cap above buried islands. The screening effect of the Si-cap layer depends on its thickness and on the island size, and is more effective for small islands. Finally, we have investigated the nanoscale properties of buried Ge-islands and related these measurements to non-ideal macroscopic I–V characteristics.
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