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Nanocrystalline silicon superlattices: building blocks for quantum devices
Materials Science and Engineering B69 – 70 (2000) 303 – 308 www.elsevier.com/locate/mseb
Nanocrystalline silicon superlattices: building blocks for quantum devices L. Tsybeskov a,*, G.F. Grom a, M. Jungo a, L. Montes a, P.M. Fauchet a, J.P. McCaffrey b, J.-M. Baribeau b, G.I. Sproule b, D.J. Lockwood b a
Department of Electrical Engineering, Uni6ersity of Rochester, RC Box 270231, Computer Studies 511, Rochester, NY14627, USA b Institute for Microstructural Sciences, National Research Council, Ottawa, Ont. Canada
Abstract A nanocrystalline silicon superlattice (nc-Si SLs) is a structure consisting of Si nanocrystal layers separated by nanometer-thick SiO2. A long range order in the nc-Si SL is obtained along the direction of growth by periodically alternating layers of Si nanocrystals and SiO2. A number of characterization techniques such as transmission electron microscopy (TEM) and atomic force microscopy (AFM), Auger elemental microanalysis. X-ray diffraction and X-ray small angle reflection have proved that the nc-Si SL exhibits a very narrow nanocrystal size distribution (less than 5% in average) and very abrupt and flat nc-Si/SiO2 interfaces with a roughness of B4 A, . Conductance tunnel spectroscopy and capacitance-voltage (C – V) measurements showed that the nc-Si SL is a nearly defect free structure. The results hold promise for nc-Si SL quantum device applications. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Nanofabrication; Silicon nanocrystals
1. Introduction Semiconductor nanocrystals represent a new class of materials where, in general, the electronic structure is determined by the size of the nanocrystals. Semiconductor nanocrystals, also called quantum dots (QDs), can be produced by a wide variety of techniques such as aerosol reactions (gas phase), precipitation in organic liquids (liquid phase) or Stranski-Krastanow growth (solid phase). Existing chemical synthesis methods work well in partially ionic II – VI materials and are also applicable to III– V materials. In the case of covalent group IV semiconductors such as Si and Ge, however, a narrow size distribution of nanocrystals has not yet been achieved [1]. Nevertheless, silicon is the dominant material in microelectronics and there is an increasing interest in silicon nanocrystals due to their potential application in nanoelectronics. The use of self-organization, and possible manufacturing cost reduction makes this interest even stronger. Among the many approaches to fabricate Si QDs, the thermal crystallization of amorphous Si (a-Si)/SiO2 * Corresponding author. Tel.: +1-716-2755342. E-mail address: [email protected] (L. Tsybeskov)
layered structures [2] shows great potential. The crystallized Si nanocrystals are packed in the form of ordered layers, and their size and packing density can be controlled precisely. This type of structure has been called a nanocrystalline Si (nc-Si) SL because it consists of periodically alternating layers of Si nanocrystals and SiO2. Preliminary results clearly demonstrated strong evidence of self-organization in the crystallization of nanoscale Si. The role of self-organization and manufacturing details will be discussed elsewhere. In this work we show that the nc-Si SL prepared by controlled crystallization exhibits nearly perfect nc-Si/ SiO2 interfaces and a narrow nanocrystal size distribution. We show that several techniques which have been developed and used for conventional SL characterization are applicable for nc-Si SL metrology. In addition, we discuss the electrical properties of nc-Si SLs and their application in quantum devices.
2. Sample preparation and structural characterization The magnetron sputtering of the a-Si/SiO2 multilayers was performed at Rochester in a Perkin-Elmer 2400 sputtering system by radio-frequency (RF) sputtering
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Fig. 1. TEM images showing (a) an example of a nc-Si SL and (b) a magnified picture detailing two individual Si nanocrystals.
and plasma oxidation. In the sputtered samples, the a-Si thickness ranged from 20 to 250 A, and that of the SiO2 from 25 to 60 A, . The number of periods varied from 1 to 60. The crystallization was performed by rapid (40–60 s) thermal annealing (RTA) at 800– 900°C followed by furnace annealing. In the furnace annealing step, the temperature was increased by 10°C min − 1 from 600 to 1050°C. This annealing is defined as quasi-equilibrium annealing (QEA).
The thermal crystallization of a-Si/SiO2 multilayers is controlled by several factors. Silicon has a very low diffusivity in SiO2 and the initial RTA process forms nanocrystals without distorting the SL periodic structure. The shape of the Si nanoclusters is expected to be close to spherical due to the competition between surface and volume tension. The strain in nc-Si/SiO2 multilayers can be released by QEA. High-temperature annealing also results in a decrease of the Si/SiO2 interface defect density (see [3,4] for details). The role of the double-step annealing is to create crystalline nuclei by RTA (nucleation stage) and to complete the crystallization of the Si nanocrystals and improve their surface passivation by QEA (growth stage). After nucleation, the nanocrystalline nuclei are surrounded by an amorphous tissue and occupy not more than 20–30% by volume of the Si layer (estimated from Raman scattering). After furnace annealing and completed crystallization, nearly 90–95% of the Si layer volume was found to be crystallized in samples with Si nanocrystals greater than 30 A, in diameter. A wide variety of characterization techniques have been applied to study the nc-Si SLs. X-ray specular reflectivity measurements were carried out using a Philips PW1820 vertical goniometer and simulated using the Philips HRS calculation package. The Raman experiments were carried out at room temperature in a quasi backscattering geometry using a Jobin-Yvon triple spectrometer or a Spex 14018 double spectrometer. Low-frequency Raman scattering was also measured using a SOPRA DMDP 2000 double monochromator with a cooled Hamamatsu R928P photomultiplier. The excitation source was 300 mW of 457.9 nm Ar+ laser light with an angle of incidence of 77.7°. The incident light was polarized in the scattering plane, while the scattered light was collected without polarization analysis. Auger elemental microanalysis was performed with a Physical Electronics Industries PHI 650 instrument with a 5 keV electron beam 30° off normal. An argon ion
Fig. 2. (a) AFM plan-view of the surface of a nc-Si SL showing the spherical shape of the Si nanocrystals, which are nearly the same size. (b) Selected area electron diffraction pattern proves that after crystallization the material is crystalline, not amorphous.
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Fig. 3. (a) The calculated phonon dispersion curve (solid line) fitted to the experimental points (empty circles) with 61 and n1 as the free parameters. (b) The longitudinally polarized Raman spectrum of as-crystallized (8.6 nm nc-Si/3.7 nm SiO2)20 SL taken in a quasibackscattering geometry with l= 457.9 nm from an argon laser.
Fig. 4. (a) Auger elemental microanalysis showing no diffusion or layered structure disruption after 1050°C annealing. (b) Small-angle X-ray reflection data confirming no damage in the layered structure after crystallization and annealing.
gun operating at 1 keV and 51° off normal was used for depth profiling. The Si LMM and O KLL Auger lines at 96 eV and 510 eV, respectively, were used to obtain the SL composition profile. The TEM analysis was performed with a Philips EM 430 microscope operated at 300 kV. Samples were prepared by a cleavage method [5]. Electrical measurements were performed at temperatures ranging from 20 to 300 K in a closed-cycle CTI-Cryogenic system using a Keithly 220 current source and a Keithly 595 quasi-static CV meter. High frequency C–V measurements were obtained using a Bonton 72BD 1 MHz capacitance meter. Fig. 1 shows a TEM picture of a crystallized a-Si/ SiO2 sample. The nc-Si/SiO2 layers are very flat with an average roughness less than 3 – 4 A, . The size of the Si nanocrystals within the layers is close to the thickness of the a-Si layer while the volume packing density is controlled by the thickness of the SiO2 layer. Fig. 1(b) shows the same sample at higher magnification and
depicts individual nanocrystals of nearly perfect spherical shape. The nanocrystal size is around 40 A, with size distribution of 2–3%. The lateral size distribution has been studied by a Digital Instruments AFM using commercial software for crystallite size recognition: in all cases, the grain size distribution was found to be narrow, 5% on average (Fig. 2). Selected area electron diffraction measurements proved that the material is nanocrystalline rather than amorphous (see Fig. 2).
3. Low frequency Raman scattering Raman scattering in low-dimensional systems is a well known method to study the phonon dispersion relationship and crystal translational symmetry. In addition, periodicity and interface quality in the layered structure can be studied using low-energy acoustic phonon scattering.
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A quantization effect of the phonon vibrations could be observed in the case when the phonon coherence length is larger than the nc-Si layer thickness. Low frequency acoustic modes fulfill this condition because they are an in-phase motion of a large number of atoms and are not strongly influenced by the disruption of the translational symmetry. Low fre-
Fig. 5. Second derivative of the I–V curve in a structure consisting of Al/3000 A, n + polycrystalline Si contact/15 A, a-SiO2/130 A, nc-Si/15 A, a-SiO2/p+ c−Si substrate. Different phonon energies are shown. The tunnel current threshold of the 46 mV indicates a small, built-in potential, possibly due to a localized charge.
Fig. 6. High-frequency (1 MHz) room temperature C– V characteristic measured in an 8-period nc-Si/a-SiO2 SL with 20 A, thick nc-Si layers separated by 20 A, of a-SiO2. The arrows show the calculated voltages for the Cmax and Cmin in the ideal MIS structure. The value of the flat-band voltage VFB 500 mV indicates a small amount of localized charge.
quency acoustic modes reflect the average bulk elastic properties of the materials and can be very useful to help estimate the periodicity and the interface roughness in a multilayer structure. In a SL made of alternating layers of Si of thickness d1 and SiO2 of thickness d2, the new period d = d1 + d2 leads to a reduction of the bulk crystal Brillouin zone in the SL growth direction to a minizone with a maximum wavevector qmz = p/d. Since the nc-Si SL contain more than 10 atomic layers per period, it can be treated as an elastic medium. According to theory, the acoustic phonon dispersion in a SL is obtained by folding the acoustic phonon branches of an average bulk compound into the SL minizone (Fig. 3). For phonons with frequencies less than 100 cm − 1, the dispersion is linear in any bulk material, and in a SL it can be described by v= 2m6SLp/d9 q6SL where m= 0,9 1,92,... is the folding index and q is the phonon wavevector. The SL sound velocity is given by 1/6SL = j/61 + (1− j)/62, where j=d1/d is the thickness fraction of nc-Si layers in the SL. In the case of Raman scattering, the phonon wavevector is qp = 4pnSL(l)/l, and in the backscatteing geometry the effective SL refractive index is nSL(l)= jn 21 +(1− j)n 22 (at the laser wavelength l). The case of m=0 corresponds to the Brillouin scattering (vBr =qp6SL) which is present in any bulk material. Each folding index m" 0 gives rise to a doublet (v 9 m =2m6SLp/ d9 vBr), which is the signature of a SL in Raman scattering. In the exact theory, small energy gaps appear in the phonon dispersion at the minizone center and at the edge (when qp/qmz is 0 or 1), due to the mismatch between acoustic impedance (r6, where r is the density) of the nc-Si and a-SiO2 ( 40% for r1 = 2.3 g cm − 3, r2 = 2.2 g cm − 3, 61 = 8.8·105 cm s − 1, 62 = 6.0·105 cm s − 1). The presence of gaps can be ignored because the qp/qmz is not close to 0 or 1 (Fig. 3). For details see reference [5,6]. More importantly, the post treatment steps such as steam oxidation at 750°C following annealing at 1050°C do not change the folded mode spectra [7]. A simple calculation shows that folded acoustic phonon modes will disappear from Raman spectrum in the case of an average interface roughness larger than 3 A, . We would like to emphasize that nc-Si SLs represent a new class of layered structures with a unique thermal stability. Our result from the folded acoustic mode spectroscopy study is supported by TEM, Auger elemental microanalyses, and small-angle X-ray reflection (Fig. 4). In all measurements, damage of the multilayer structure from annealing has not been indicated. Indeed, the simulation of the X-ray reflection data (Fig. 4b) shows that the nc-Si/SiO2 interface in nc-Si SLs even improves after annealing at 1000°C.
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Fig. 7. (a) C – V characteristics showing a hysteresis at room temperature in a structure with nc-Si layers imbedded in the isolated gate. No hysteresis is observed in a test structure with a uniform SiO2 layer. (b) Room temperature charge relaxation measured by capacitance recovery. The C(t) kinetics indicate a non-dispersive carrier transport and a significant retention time dependence as a function of the Si nanocrystal size.
4. Electrical characterization We have used current tunnel spectroscopy (CTS), one of the most sensitive techniques available to characterize the nc-Si/SiO2 interfaces. In order to apply the CTS technique, we have fabricated a double-barrier structure with a single 130 A, -thick nc-Si layer sandwiched between two tunnel-transparent a-SiO2 layers of thickness 15 A, . The structure was deposited on a heavily doped p+Si substrate (resistivity of 0.01 V cm) with an Al/n +doped polycrystalline Si top contact (Fig. 5). The structure is similar to the Esaki diode, because the electron injected from the top contact should tunnel across the 130 A, layer of Si to the heavily B-doped substrate. The experiment was performed at 15 K to minimize thermal broadening and the electronphonon interaction. In order to clarify the details, we have used the second derivative of the I – V data. Several features corresponding to different phonon and phonon-combination energies are observed (Fig. 5). The TA- and TO- phonon assisted transitions are clearly observed (also in [8]). The observation of phonon-assisted transitions in the interband tunneling indicates clean nc-Si/a-SiO2 interfaces. A silicon nanocrystal \ 100 A, in size has an indirect bandgap structure, and the interband tunneling requires phonon assistance to conserve momentum. The interband tunneling is always accompanied by an excess current. In the majority of cases the origin of the excess current has been associated with tunneling via interface defect states. The inelastic scattering due to interaction with structural defects changes the electron momentum. Thus, the phonon-assisted tunneling is no longer the
dominant mechanism, and the features in the I–V curve should be washed out. The observation of phonon-assisted tunneling indicates a very low density of interface defects in the nc-Si/a-SiO2 structure. One way to estimate the interface defect density in a nc-Si/a-SiO2 SL is to use conventional C –V measurements [9]. The sample used for this work consisted of 200 A, thermally grown a-SiO2 on an n-type ( 1–5 V cm) c-Si substrate followed by an 8-period SL. The nc-Si layers with a thickness of 20 A, are separated by a-SiO2 layers with a thickness of 20 A, . The 200 A, of thermally grown a-SiO2 blocks direct current, and the 1 MHz frequency C–V characteristic is very similar to the C–V curve in a standard metal-insulator-semiconductor (MIS) structure (Fig. 6). Fig. 6 shows the C–V data with a saturation at Cmax, which corresponds to the capacitance of the insulating layer (including the thermally grown a-SiO2 and the nc-Si/a-SiO2 SL) with an average dielectric constant of 1.8–2.0 (depending on the volume ratio between Si and SiO2). In addition, the C–V plateau at Cmin is found to be in excellent agreement with the calculated value of the maximum depletion layer thickness ( 0.5 mm) for the 1016 cm − 3 concentration of free carriers that corresponds to the resistivity of 5 V cm of the n-type c-Si substrate. This result justifies the use of a standard high-frequency C–V analysis [9] to estimate the value of the interface defect density which is found to be 1011 cm − 2. What does this defect density value mean in the case of the 8 nc-Si layers (or 16 interfaces) separated by a-SiO2? Considering a Si nanocrystal area density of 1012 –1013 cm − 2 and assuming that all the defects are on the nc-S/a-SiO2 interfaces, we estimate that the only
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one nanocrystal per 100 – 1000 nanocrystals is defective. This estimate is supported by our observation of a relatively high photoluminescence quantum efficiency in nc-Si/a-SiO2 SLs. Our latest analysis indicates that the defect density can be significantly reduced using an optimized preparation condition and a cleaner manufacturing environment. In other words, the density of nc-Si/a-SiO2 interface defects is approaching the le6el of defect tolerance in standard CMOS processing. The low defect density and narrow nanocrystal size distribution possibly responsible for another interesting effect which is recently has been observed in nc-Si SLs. In a MOS structure with 200 A, thick thermally grown SiO2 and 8–20 period SL we studied a charging effect. The structure exhibits a hysteresis in C(V) measurements with a hysteresis loop of 2 – 3 volts. The hysteresis loop direction shows on negative sign of stored charges, e.g. electrons. (Note, that no C – V hysteresis has been found in a similar MOS structure with sputtered SiO2 but without Si nanocrystals (Fig. 7)). Since the hysteresis loop goes around V = 0, the structure can hold the charge without bias, or shows a non-volatile memory effect. Experimental results are shown in Fig. 7. First, the storage time strongly depends on the nanocrystal size. Assuming that Coulomb blockade effect is responsible for charging, the storage time dependence of nanocrystals size is easy to understand. Second, and the most important, the structure shows no leak, or in other words, as long as the capacitor holds the charge, the value of capacitance remains constant. The discharging occurs drastically with a sharp transition between ‘charged’ and ‘not charged’ states, and possibly indicates ‘non-dispersive’ [10] carrier relaxation in nc Si SL [9]. For example in porous silicon, where nanocrystal size varies up to 100%, our measurements showed a dispersive character of carrier relaxation [11]. Our conclusion looks extremely promising for Si nanocrystal application in memory devices. For example, it can be used in the DRAM memory cell with a long refresh time or, may be, even in a nonvolatile memory.
5. Conclusion In conclusion, we list several important results from this study: 1. Nanocrystalline silicon superlattices can be reproducibly fabricated using nearly standard microelectronic fabrication steps. 2. The nc-Si/SiO2 SL exhibits an excellent interface
.
roughness (B 4 A, ), which has been established by a number of characterization techniques including small angle X-ray reflection and low-frequency Raman scattering. 3. The perfect structure of the nc-Si/SiO2 interface is accompanied by a very low interface defect density, or in other words, Si nanocrystal surface defect density. The number of defected nanocrystals is estimated to be from one per 100 to one per 1000. This low density is verified by a number of electrical measurements. 4. A simple test structure with the nc-Si SL exhibits a memory effect currently attributed to Coulomb blockade. The properties of the structure are very promising for practical application in semiconductor memory device. Acknowledgements This work was supported by the US Army Research Office, National Science Foundation, Semiconductor Research Corporation and Motorola. References [1] L. Tsybeskov, MRS Bulletin, April 1998, pp. 33 – 39. [2] L. Tsybeskov, K.D. Hirschman, S.P. Duttagupta, P.M. Fauchet, M. Zacharias, P. Kohlert, J.P. McCaffrey, D.J. Lockwood, in: M. Cahay, J.-P. Leburton, D.J. Lockwood, S. Bandyopadhyay (Eds.), Quantum Confinement IV: Nanoscale Materials, Devices, and Systems, The Electrochemical Society, Pennington, NJ, 1997, pp. 134 – 145. [3] G.F. Grom, P.M. Fauchet, J.P. McCoffrey, J.-M. Baribeau, H.J. Labbe, G.I. Sproule, D.J. Lockwood, in: M. Cahay, J.-P. Leburton, D.J. Lockwood, S. Bandyopadhyay (Eds.), Quantum Confinement: Nanostructures, The Electrochemical Society, Pennington, NJ, 1999, pp. 76 – 94. [4] L. Tsybeskov, K.D. Hirschman, S.P. Duttagupta, M. Zacharias, P.M. Fauchet, J.P. McCaffrey, D.J. Lockwood, Appl. Phys. Lett. 72 (1998) 43. [5] J.P. McCaffrey, Microsc. Res. Tech. 24 (1993) 180. [6] D.J. Lockwood, M.W.C. Dharma-wardana, J.-M. Baribeau, D.C. Houghton, Phys. Rev. B 35 (1987) 2243. [7] G.F. Grom, L. Tsybeskov, P.M. Fauchet, J.P. McCoffrey, J.-M. Baribeau, H.J. Labbe, G.I. Sproule, D.J. Lockwood (in press). [8] L. Tsybeskov, G.F. Grom, P.M. Fauchet, J.P. McCoffrey, J.-M. Baribeau, G.I. Sproule, D.J. Lockwood, Appl. Phys. Lett., (in press). [9] S.M. Zee, Physics of Semiconductor Devices, Wiley Inter-Science, New York, 1969, p. 812. [10] H. Scher, M.F. Shlesinger, J.T. Bendler, Physics Today, January 1991, pp. 26 – 34. [11] P. Rao, E.A. Schiff, L. Tsybeskov, P. M. Fauchet, in: Advances in Microcrystalline and Nanocrystalline Semiconductors-1996, Mat. Res. Soc. Symp. Proc. 454 (1997) 613.
Nanocrystalline silicon superlattices: building blocks for quantum devices L. Tsybeskov a,*, G.F. Grom a, M. Jungo a, L. Montes a, P.M. Fauchet a, J.P. McCaffrey b, J.-M. Baribeau b, G.I. Sproule b, D.J. Lockwood b a
Department of Electrical Engineering, Uni6ersity of Rochester, RC Box 270231, Computer Studies 511, Rochester, NY14627, USA b Institute for Microstructural Sciences, National Research Council, Ottawa, Ont. Canada
Abstract A nanocrystalline silicon superlattice (nc-Si SLs) is a structure consisting of Si nanocrystal layers separated by nanometer-thick SiO2. A long range order in the nc-Si SL is obtained along the direction of growth by periodically alternating layers of Si nanocrystals and SiO2. A number of characterization techniques such as transmission electron microscopy (TEM) and atomic force microscopy (AFM), Auger elemental microanalysis. X-ray diffraction and X-ray small angle reflection have proved that the nc-Si SL exhibits a very narrow nanocrystal size distribution (less than 5% in average) and very abrupt and flat nc-Si/SiO2 interfaces with a roughness of B4 A, . Conductance tunnel spectroscopy and capacitance-voltage (C – V) measurements showed that the nc-Si SL is a nearly defect free structure. The results hold promise for nc-Si SL quantum device applications. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Nanofabrication; Silicon nanocrystals
1. Introduction Semiconductor nanocrystals represent a new class of materials where, in general, the electronic structure is determined by the size of the nanocrystals. Semiconductor nanocrystals, also called quantum dots (QDs), can be produced by a wide variety of techniques such as aerosol reactions (gas phase), precipitation in organic liquids (liquid phase) or Stranski-Krastanow growth (solid phase). Existing chemical synthesis methods work well in partially ionic II – VI materials and are also applicable to III– V materials. In the case of covalent group IV semiconductors such as Si and Ge, however, a narrow size distribution of nanocrystals has not yet been achieved [1]. Nevertheless, silicon is the dominant material in microelectronics and there is an increasing interest in silicon nanocrystals due to their potential application in nanoelectronics. The use of self-organization, and possible manufacturing cost reduction makes this interest even stronger. Among the many approaches to fabricate Si QDs, the thermal crystallization of amorphous Si (a-Si)/SiO2 * Corresponding author. Tel.: +1-716-2755342. E-mail address: [email protected] (L. Tsybeskov)
layered structures [2] shows great potential. The crystallized Si nanocrystals are packed in the form of ordered layers, and their size and packing density can be controlled precisely. This type of structure has been called a nanocrystalline Si (nc-Si) SL because it consists of periodically alternating layers of Si nanocrystals and SiO2. Preliminary results clearly demonstrated strong evidence of self-organization in the crystallization of nanoscale Si. The role of self-organization and manufacturing details will be discussed elsewhere. In this work we show that the nc-Si SL prepared by controlled crystallization exhibits nearly perfect nc-Si/ SiO2 interfaces and a narrow nanocrystal size distribution. We show that several techniques which have been developed and used for conventional SL characterization are applicable for nc-Si SL metrology. In addition, we discuss the electrical properties of nc-Si SLs and their application in quantum devices.
2. Sample preparation and structural characterization The magnetron sputtering of the a-Si/SiO2 multilayers was performed at Rochester in a Perkin-Elmer 2400 sputtering system by radio-frequency (RF) sputtering
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Fig. 1. TEM images showing (a) an example of a nc-Si SL and (b) a magnified picture detailing two individual Si nanocrystals.
and plasma oxidation. In the sputtered samples, the a-Si thickness ranged from 20 to 250 A, and that of the SiO2 from 25 to 60 A, . The number of periods varied from 1 to 60. The crystallization was performed by rapid (40–60 s) thermal annealing (RTA) at 800– 900°C followed by furnace annealing. In the furnace annealing step, the temperature was increased by 10°C min − 1 from 600 to 1050°C. This annealing is defined as quasi-equilibrium annealing (QEA).
The thermal crystallization of a-Si/SiO2 multilayers is controlled by several factors. Silicon has a very low diffusivity in SiO2 and the initial RTA process forms nanocrystals without distorting the SL periodic structure. The shape of the Si nanoclusters is expected to be close to spherical due to the competition between surface and volume tension. The strain in nc-Si/SiO2 multilayers can be released by QEA. High-temperature annealing also results in a decrease of the Si/SiO2 interface defect density (see [3,4] for details). The role of the double-step annealing is to create crystalline nuclei by RTA (nucleation stage) and to complete the crystallization of the Si nanocrystals and improve their surface passivation by QEA (growth stage). After nucleation, the nanocrystalline nuclei are surrounded by an amorphous tissue and occupy not more than 20–30% by volume of the Si layer (estimated from Raman scattering). After furnace annealing and completed crystallization, nearly 90–95% of the Si layer volume was found to be crystallized in samples with Si nanocrystals greater than 30 A, in diameter. A wide variety of characterization techniques have been applied to study the nc-Si SLs. X-ray specular reflectivity measurements were carried out using a Philips PW1820 vertical goniometer and simulated using the Philips HRS calculation package. The Raman experiments were carried out at room temperature in a quasi backscattering geometry using a Jobin-Yvon triple spectrometer or a Spex 14018 double spectrometer. Low-frequency Raman scattering was also measured using a SOPRA DMDP 2000 double monochromator with a cooled Hamamatsu R928P photomultiplier. The excitation source was 300 mW of 457.9 nm Ar+ laser light with an angle of incidence of 77.7°. The incident light was polarized in the scattering plane, while the scattered light was collected without polarization analysis. Auger elemental microanalysis was performed with a Physical Electronics Industries PHI 650 instrument with a 5 keV electron beam 30° off normal. An argon ion
Fig. 2. (a) AFM plan-view of the surface of a nc-Si SL showing the spherical shape of the Si nanocrystals, which are nearly the same size. (b) Selected area electron diffraction pattern proves that after crystallization the material is crystalline, not amorphous.
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Fig. 3. (a) The calculated phonon dispersion curve (solid line) fitted to the experimental points (empty circles) with 61 and n1 as the free parameters. (b) The longitudinally polarized Raman spectrum of as-crystallized (8.6 nm nc-Si/3.7 nm SiO2)20 SL taken in a quasibackscattering geometry with l= 457.9 nm from an argon laser.
Fig. 4. (a) Auger elemental microanalysis showing no diffusion or layered structure disruption after 1050°C annealing. (b) Small-angle X-ray reflection data confirming no damage in the layered structure after crystallization and annealing.
gun operating at 1 keV and 51° off normal was used for depth profiling. The Si LMM and O KLL Auger lines at 96 eV and 510 eV, respectively, were used to obtain the SL composition profile. The TEM analysis was performed with a Philips EM 430 microscope operated at 300 kV. Samples were prepared by a cleavage method [5]. Electrical measurements were performed at temperatures ranging from 20 to 300 K in a closed-cycle CTI-Cryogenic system using a Keithly 220 current source and a Keithly 595 quasi-static CV meter. High frequency C–V measurements were obtained using a Bonton 72BD 1 MHz capacitance meter. Fig. 1 shows a TEM picture of a crystallized a-Si/ SiO2 sample. The nc-Si/SiO2 layers are very flat with an average roughness less than 3 – 4 A, . The size of the Si nanocrystals within the layers is close to the thickness of the a-Si layer while the volume packing density is controlled by the thickness of the SiO2 layer. Fig. 1(b) shows the same sample at higher magnification and
depicts individual nanocrystals of nearly perfect spherical shape. The nanocrystal size is around 40 A, with size distribution of 2–3%. The lateral size distribution has been studied by a Digital Instruments AFM using commercial software for crystallite size recognition: in all cases, the grain size distribution was found to be narrow, 5% on average (Fig. 2). Selected area electron diffraction measurements proved that the material is nanocrystalline rather than amorphous (see Fig. 2).
3. Low frequency Raman scattering Raman scattering in low-dimensional systems is a well known method to study the phonon dispersion relationship and crystal translational symmetry. In addition, periodicity and interface quality in the layered structure can be studied using low-energy acoustic phonon scattering.
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A quantization effect of the phonon vibrations could be observed in the case when the phonon coherence length is larger than the nc-Si layer thickness. Low frequency acoustic modes fulfill this condition because they are an in-phase motion of a large number of atoms and are not strongly influenced by the disruption of the translational symmetry. Low fre-
Fig. 5. Second derivative of the I–V curve in a structure consisting of Al/3000 A, n + polycrystalline Si contact/15 A, a-SiO2/130 A, nc-Si/15 A, a-SiO2/p+ c−Si substrate. Different phonon energies are shown. The tunnel current threshold of the 46 mV indicates a small, built-in potential, possibly due to a localized charge.
Fig. 6. High-frequency (1 MHz) room temperature C– V characteristic measured in an 8-period nc-Si/a-SiO2 SL with 20 A, thick nc-Si layers separated by 20 A, of a-SiO2. The arrows show the calculated voltages for the Cmax and Cmin in the ideal MIS structure. The value of the flat-band voltage VFB 500 mV indicates a small amount of localized charge.
quency acoustic modes reflect the average bulk elastic properties of the materials and can be very useful to help estimate the periodicity and the interface roughness in a multilayer structure. In a SL made of alternating layers of Si of thickness d1 and SiO2 of thickness d2, the new period d = d1 + d2 leads to a reduction of the bulk crystal Brillouin zone in the SL growth direction to a minizone with a maximum wavevector qmz = p/d. Since the nc-Si SL contain more than 10 atomic layers per period, it can be treated as an elastic medium. According to theory, the acoustic phonon dispersion in a SL is obtained by folding the acoustic phonon branches of an average bulk compound into the SL minizone (Fig. 3). For phonons with frequencies less than 100 cm − 1, the dispersion is linear in any bulk material, and in a SL it can be described by v= 2m6SLp/d9 q6SL where m= 0,9 1,92,... is the folding index and q is the phonon wavevector. The SL sound velocity is given by 1/6SL = j/61 + (1− j)/62, where j=d1/d is the thickness fraction of nc-Si layers in the SL. In the case of Raman scattering, the phonon wavevector is qp = 4pnSL(l)/l, and in the backscatteing geometry the effective SL refractive index is nSL(l)= jn 21 +(1− j)n 22 (at the laser wavelength l). The case of m=0 corresponds to the Brillouin scattering (vBr =qp6SL) which is present in any bulk material. Each folding index m" 0 gives rise to a doublet (v 9 m =2m6SLp/ d9 vBr), which is the signature of a SL in Raman scattering. In the exact theory, small energy gaps appear in the phonon dispersion at the minizone center and at the edge (when qp/qmz is 0 or 1), due to the mismatch between acoustic impedance (r6, where r is the density) of the nc-Si and a-SiO2 ( 40% for r1 = 2.3 g cm − 3, r2 = 2.2 g cm − 3, 61 = 8.8·105 cm s − 1, 62 = 6.0·105 cm s − 1). The presence of gaps can be ignored because the qp/qmz is not close to 0 or 1 (Fig. 3). For details see reference [5,6]. More importantly, the post treatment steps such as steam oxidation at 750°C following annealing at 1050°C do not change the folded mode spectra [7]. A simple calculation shows that folded acoustic phonon modes will disappear from Raman spectrum in the case of an average interface roughness larger than 3 A, . We would like to emphasize that nc-Si SLs represent a new class of layered structures with a unique thermal stability. Our result from the folded acoustic mode spectroscopy study is supported by TEM, Auger elemental microanalyses, and small-angle X-ray reflection (Fig. 4). In all measurements, damage of the multilayer structure from annealing has not been indicated. Indeed, the simulation of the X-ray reflection data (Fig. 4b) shows that the nc-Si/SiO2 interface in nc-Si SLs even improves after annealing at 1000°C.
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Fig. 7. (a) C – V characteristics showing a hysteresis at room temperature in a structure with nc-Si layers imbedded in the isolated gate. No hysteresis is observed in a test structure with a uniform SiO2 layer. (b) Room temperature charge relaxation measured by capacitance recovery. The C(t) kinetics indicate a non-dispersive carrier transport and a significant retention time dependence as a function of the Si nanocrystal size.
4. Electrical characterization We have used current tunnel spectroscopy (CTS), one of the most sensitive techniques available to characterize the nc-Si/SiO2 interfaces. In order to apply the CTS technique, we have fabricated a double-barrier structure with a single 130 A, -thick nc-Si layer sandwiched between two tunnel-transparent a-SiO2 layers of thickness 15 A, . The structure was deposited on a heavily doped p+Si substrate (resistivity of 0.01 V cm) with an Al/n +doped polycrystalline Si top contact (Fig. 5). The structure is similar to the Esaki diode, because the electron injected from the top contact should tunnel across the 130 A, layer of Si to the heavily B-doped substrate. The experiment was performed at 15 K to minimize thermal broadening and the electronphonon interaction. In order to clarify the details, we have used the second derivative of the I – V data. Several features corresponding to different phonon and phonon-combination energies are observed (Fig. 5). The TA- and TO- phonon assisted transitions are clearly observed (also in [8]). The observation of phonon-assisted transitions in the interband tunneling indicates clean nc-Si/a-SiO2 interfaces. A silicon nanocrystal \ 100 A, in size has an indirect bandgap structure, and the interband tunneling requires phonon assistance to conserve momentum. The interband tunneling is always accompanied by an excess current. In the majority of cases the origin of the excess current has been associated with tunneling via interface defect states. The inelastic scattering due to interaction with structural defects changes the electron momentum. Thus, the phonon-assisted tunneling is no longer the
dominant mechanism, and the features in the I–V curve should be washed out. The observation of phonon-assisted tunneling indicates a very low density of interface defects in the nc-Si/a-SiO2 structure. One way to estimate the interface defect density in a nc-Si/a-SiO2 SL is to use conventional C –V measurements [9]. The sample used for this work consisted of 200 A, thermally grown a-SiO2 on an n-type ( 1–5 V cm) c-Si substrate followed by an 8-period SL. The nc-Si layers with a thickness of 20 A, are separated by a-SiO2 layers with a thickness of 20 A, . The 200 A, of thermally grown a-SiO2 blocks direct current, and the 1 MHz frequency C–V characteristic is very similar to the C–V curve in a standard metal-insulator-semiconductor (MIS) structure (Fig. 6). Fig. 6 shows the C–V data with a saturation at Cmax, which corresponds to the capacitance of the insulating layer (including the thermally grown a-SiO2 and the nc-Si/a-SiO2 SL) with an average dielectric constant of 1.8–2.0 (depending on the volume ratio between Si and SiO2). In addition, the C–V plateau at Cmin is found to be in excellent agreement with the calculated value of the maximum depletion layer thickness ( 0.5 mm) for the 1016 cm − 3 concentration of free carriers that corresponds to the resistivity of 5 V cm of the n-type c-Si substrate. This result justifies the use of a standard high-frequency C–V analysis [9] to estimate the value of the interface defect density which is found to be 1011 cm − 2. What does this defect density value mean in the case of the 8 nc-Si layers (or 16 interfaces) separated by a-SiO2? Considering a Si nanocrystal area density of 1012 –1013 cm − 2 and assuming that all the defects are on the nc-S/a-SiO2 interfaces, we estimate that the only
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one nanocrystal per 100 – 1000 nanocrystals is defective. This estimate is supported by our observation of a relatively high photoluminescence quantum efficiency in nc-Si/a-SiO2 SLs. Our latest analysis indicates that the defect density can be significantly reduced using an optimized preparation condition and a cleaner manufacturing environment. In other words, the density of nc-Si/a-SiO2 interface defects is approaching the le6el of defect tolerance in standard CMOS processing. The low defect density and narrow nanocrystal size distribution possibly responsible for another interesting effect which is recently has been observed in nc-Si SLs. In a MOS structure with 200 A, thick thermally grown SiO2 and 8–20 period SL we studied a charging effect. The structure exhibits a hysteresis in C(V) measurements with a hysteresis loop of 2 – 3 volts. The hysteresis loop direction shows on negative sign of stored charges, e.g. electrons. (Note, that no C – V hysteresis has been found in a similar MOS structure with sputtered SiO2 but without Si nanocrystals (Fig. 7)). Since the hysteresis loop goes around V = 0, the structure can hold the charge without bias, or shows a non-volatile memory effect. Experimental results are shown in Fig. 7. First, the storage time strongly depends on the nanocrystal size. Assuming that Coulomb blockade effect is responsible for charging, the storage time dependence of nanocrystals size is easy to understand. Second, and the most important, the structure shows no leak, or in other words, as long as the capacitor holds the charge, the value of capacitance remains constant. The discharging occurs drastically with a sharp transition between ‘charged’ and ‘not charged’ states, and possibly indicates ‘non-dispersive’ [10] carrier relaxation in nc Si SL [9]. For example in porous silicon, where nanocrystal size varies up to 100%, our measurements showed a dispersive character of carrier relaxation [11]. Our conclusion looks extremely promising for Si nanocrystal application in memory devices. For example, it can be used in the DRAM memory cell with a long refresh time or, may be, even in a nonvolatile memory.
5. Conclusion In conclusion, we list several important results from this study: 1. Nanocrystalline silicon superlattices can be reproducibly fabricated using nearly standard microelectronic fabrication steps. 2. The nc-Si/SiO2 SL exhibits an excellent interface
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roughness (B 4 A, ), which has been established by a number of characterization techniques including small angle X-ray reflection and low-frequency Raman scattering. 3. The perfect structure of the nc-Si/SiO2 interface is accompanied by a very low interface defect density, or in other words, Si nanocrystal surface defect density. The number of defected nanocrystals is estimated to be from one per 100 to one per 1000. This low density is verified by a number of electrical measurements. 4. A simple test structure with the nc-Si SL exhibits a memory effect currently attributed to Coulomb blockade. The properties of the structure are very promising for practical application in semiconductor memory device. Acknowledgements This work was supported by the US Army Research Office, National Science Foundation, Semiconductor Research Corporation and Motorola. References [1] L. Tsybeskov, MRS Bulletin, April 1998, pp. 33 – 39. [2] L. Tsybeskov, K.D. Hirschman, S.P. Duttagupta, P.M. Fauchet, M. Zacharias, P. Kohlert, J.P. McCaffrey, D.J. Lockwood, in: M. Cahay, J.-P. Leburton, D.J. Lockwood, S. Bandyopadhyay (Eds.), Quantum Confinement IV: Nanoscale Materials, Devices, and Systems, The Electrochemical Society, Pennington, NJ, 1997, pp. 134 – 145. [3] G.F. Grom, P.M. Fauchet, J.P. McCoffrey, J.-M. Baribeau, H.J. Labbe, G.I. Sproule, D.J. Lockwood, in: M. Cahay, J.-P. Leburton, D.J. Lockwood, S. Bandyopadhyay (Eds.), Quantum Confinement: Nanostructures, The Electrochemical Society, Pennington, NJ, 1999, pp. 76 – 94. [4] L. Tsybeskov, K.D. Hirschman, S.P. Duttagupta, M. Zacharias, P.M. Fauchet, J.P. McCaffrey, D.J. Lockwood, Appl. Phys. Lett. 72 (1998) 43. [5] J.P. McCaffrey, Microsc. Res. Tech. 24 (1993) 180. [6] D.J. Lockwood, M.W.C. Dharma-wardana, J.-M. Baribeau, D.C. Houghton, Phys. Rev. B 35 (1987) 2243. [7] G.F. Grom, L. Tsybeskov, P.M. Fauchet, J.P. McCoffrey, J.-M. Baribeau, H.J. Labbe, G.I. Sproule, D.J. Lockwood (in press). [8] L. Tsybeskov, G.F. Grom, P.M. Fauchet, J.P. McCoffrey, J.-M. Baribeau, G.I. Sproule, D.J. Lockwood, Appl. Phys. Lett., (in press). [9] S.M. Zee, Physics of Semiconductor Devices, Wiley Inter-Science, New York, 1969, p. 812. [10] H. Scher, M.F. Shlesinger, J.T. Bendler, Physics Today, January 1991, pp. 26 – 34. [11] P. Rao, E.A. Schiff, L. Tsybeskov, P. M. Fauchet, in: Advances in Microcrystalline and Nanocrystalline Semiconductors-1996, Mat. Res. Soc. Symp. Proc. 454 (1997) 613.