Low temperature RPCVD epitaxial growth of Si1−xGex using Si2H6 and Ge2H6

Solid-State Electronics 83 (2013) 2–9

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Low temperature RPCVD epitaxial growth of Si1xGex using Si2H6 and Ge2H6 S. Wirths a,⇑, D. Buca a, A.T. Tiedemann a, P. Bernardy a, B. Holländer a, T. Stoica a, G. Mussler a, U. Breuer b, S. Mantl a a b

Peter Grünberg Institute (PGI 9-IT) and JARA – Fundamentals of Future Information Technologies, Forschungszentrum Juelich, Juelich 52425, Germany Central Division of Analytical Chemistry (ZCH), Forschungszentrum Juelich, Juelich 52425, Germany

a r t i c l e

i n f o

Article history: Available online 20 February 2013 Keywords: SiGe Chemical vapor deposition technique Semiconductor doping Channeling

a b s t r a c t The growth of intrinsic SiGe and, n- and p-type doping of Si and SiGe layers was studied using a Reduced Pressure Chemical Vapor Deposition AIXTRON TRICENTÒ cluster tool. Most emphasis was placed on the growth kinetics in the low temperature regime of 450–600 °C which is characterized by surface limited reactions. A low growth activation energy of 0.667 eV was achieved by using Si2H6 and Ge2H6 precursors. Fully strained SiGe layers with Ge contents up to 53% at a record thickness of 29 nm were grown at a very low growth temperature of 450 °C. The dopant incorporation in Si strongly increases with the B2H6 flux but saturates rapidly with increasing PH3 flow. High dopant concentrations of 1.1  1020 cm3 and 1  1021 cm3 were obtained for Si:P and Si:B doping, respectively, at a growth temperature of 600 °C. For Si0.56Ge0.44 layers the maximum dopant concentrations achieved were 5  1020 cm3 for P at 500 °C and 4  1020 cm3 for B doping at 600 °C. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Heteroepitaxial growth of silicon–germanium structures are of increasing interest to expand and combine the capabilities of silicon-based opto- and nano-electronics. SiGe alloys and multi-quantum wells are suitable for high-speed photodetectors and telecomband applications [1,2] but also for optical interconnects [3]. Although strain engineering [4–6] pushed impressively the limits of Si-devices for further progress of the CMOS performance new materials with higher intrinsic carrier mobility, like Si1xGex [7– 9] and novel architectures are investigated. Recently, a new type of transistor based on quantum mechanical tunnelling, the Tunnel field-effect transistor (TFET), gained a lot of attention due to its potential for sub-60 mV/decade inverse subthreshold slopes. The latter is a prerequisite for scaling the supply voltage well below 1 V, a necessity for ultralow power applications [10]. Strained SiGe alloys are very promising for TFET fabrication [11] and high performance MOSFETs due to their enhanced carrier mobilities [7,8] and the possibilities to adjust the band-gaps and band-offsets via the Ge content and the elastic strain [12,13]. In this work, we investigated the epitaxial growth of Si1xGex layers using a Reduced Pressure Chemical Vapor Deposition (RPCVD) technique with a non-standard combination of gas precursors Si2H6–Ge2H6. Although these gases have been used to grow Si and SiGe layers before [14–16], we emphasize here the growth of fully strained high Ge content layers at a growth temperature ⇑ Corresponding author. E-mail address: [email protected] (S. Wirths). 0038-1101/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2013.01.032

of 450 °C. This was enabled by the use of a new CVD showerhead reactor. Furthermore, low temperature p- and n-doping of Si and SiGe are addressed.

2. Experimental For the growth studies, we used an industry compatible metal cold-wall Reduced Pressure AIXTRON TRICENTÒ reactor (RPCVD) with a showerhead technology for 200 mm wafers. The cold-walls suppress the parasitic deposition, and thus memory-effects, and reduce the maintenance efforts of the reactor. The wafer temperature is measured by six thermocouples inside the graphite susceptor heated by IR-lamps. The showerhead assures a uniform gas precursor distribution over the wafer and, economically and environmental more important, reduces the total gas consumption. The strained/relaxed SiGe layers were grown on 200 mm Si(001) wafers using Si2H6 and Ge2H6 (10% diluted in H2) gases. Other than SiH4 and SiH2Cl2, Si2H6 promises reasonable growth rates at very low growth temperatures because of the weaker Si– Si bond energy compared to the Si–H bond energy [15,17]. Li et al. [18] have shown that pyrolytic Si homoepitaxy and Ge heteroepitaxy using Si2H6 and Ge2H6 can be achieved at temperatures as low as 400 °C and 350 °C, respectively. For the growth of p- and n-type doped Si(Ge) layers B2H6 (100 ppm in H2) and PH3 (100 ppm in H2) were employed. As a carrier gas H2 and/or N2 was employed. Multiple characterization methods were applied for a comprehensive material characterization. The crystalline quality of the

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SiGe layers was investigated using Rutherford Backscattering Spectrometry in the ion channeling mode (RBS/C). The degree of singlecrystallinity is evaluated by the minimum yield of backscattered He+ ions defined as the ratio of the intensity of the aligned and random spectra taken below the surface peak signal [19]. The Ge atomic fraction was determined by simulating the RBS spectra using the RUMP code. RBS/C measurements were performed using a Tandetron accelerator with 1.4 MeV He+ ions using a backscattered angle of 170°. Complementary information regarding composition and thickness of selected SiGe samples was obtained by X-ray Diffraction (XRD) measurements using a Bruker high resolution diffractometer. From Reciprocal Space Mapping (RSM), the degree of strain relaxation of SiGe layers was evaluated to determine the critical thickness for plastic relaxation. The elastic strain in the Si1xGex epilayers was additionally measured by Raman spectroscopy using an excitation wavelength of 532 nm and restricted to low laserpower density in order to avoid local thermal heating. Several experimental techniques in particular Atomic Force Microscopy (AFM) and for detailed information on the layer morphology cross-sectional and plane-view transmission electron microscopy (TEM) with a JEOL 4000 FX microscope were used. Doping profiles and concentrations of Boron and Phosphorus were analysed by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS).

3. Results 3.1. Epitaxial SiGe growth studies The aim of our study was to determine the optimal experimental conditions for epitaxial growth of pseudomorphic SiGe layers with Ge contents exceeding 40 at.%. For this growth study the total pressure and the Si2H6 partial pressure were kept constant at 6000 Pa (60 mbar) and 15 Pa, respectively. Moreover, a constant total flow of a few standard litres per minute has been applied. The layers were grown at temperatures between 450 °C and 600 °C by varying the Ge2H6 partial pressure in the range of 1.8– 9 Pa. The Ge percentage in the epitaxial SiGe layers, as determined by RBS and XRD is presented in Fig. 1a as a function of the Ge2H6 partial pressure for different temperatures. The Ge concentration can be varied between 18 at.% and 62 at.% and increases linearly with

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the Ge2H6 partial pressure for all investigated growth temperatures. At constant Ge2H6 partial pressure and the maximum used Ge2H6/Si2H6 flux ratio, the Ge content in the SiGe layers increases with decreasing growth temperature: 45% at 600 °C, 50% at 550 °C, 58% at 500 °C and 62% at a growth temperature of 450 °C. The maximum Ge content given above represents the value at which highly strained layers were obtained. The temperature effect on the SiGe deposition rate is illustrated by Fig. 1b. As expected, with decreasing temperature, the growth rate decreases, while the Ge content increases for a constant Ge2H6/Si2H6 ratio. At lower temperatures the decomposition rate of both Si and Ge precursors are reduced, but stronger for Si than Ge sources resulting in an increase of the Ge content in the epitaxial layers. Despite of reduced decomposition rate, even for the lowest growth temperature of 450 °C, a growth rate of 3 nm/min was achieved for a Ge concentration of 62%, while at higher temperatures of 600 °C a maximum growth rate of 20 nm/min can be reached. In Fig. 2a the mole fraction ratio between Ge and Si in the SiGe layers as a function of the partial pressure ratio of Ge2H6 and Si2H6 precursors is shown. The Ge mole fraction x can be obtained as [14]:

x pðGe2 H6 Þ ¼m x1 pðSi2 H6 Þ The value of the reactive sticking probability m, extracted by linear fitting [20], is temperature dependent and increases with decreasing growth temperature. For our CVD conditions, the growth takes place in a surface limited regime, controlled by hydrogen desorption. The rate of hydrogen desorption from (1 0 0) surfaces of both silicon and germanium was intensively studied in the literature and it is known to strongly depend on temperature [21,22]. Experiments demonstrate that hydrogen desorption for SiGe alloy growth differs from the pure Si and Ge growth. The growth rate enhancement is attributed to accelerated hydrogen desorption from the growing surface in the presence of Germane atoms [22]. To determine the activation energies we plotted on logarithm scale the growth rate as a function of the inverse temperature, for two Ge2H6 partial pressure values of 1.8 Pa and 3 Pa, at a constant Si2H6 partial pressure of 15 Pa (see Fig. 2b). The extracted activation energies associated to the SiGe growth rates are Eact = 0.831 eV for pGe2H6 = 1.8 Pa and Eact = 0.667 eV for pGe2H6 = 3.0 Pa. Kolahdouz et al. [16] also reported a decrease of the activation energy with increasing Ge2H6 partial pressure, but for Ge2H6 partial pressure values one order of magnitude lower

Fig. 1. (a) Ge composition as a function of the Ge2H6 partial pressure and (b) SiGe growth rate as a function of Ge content for four different deposition temperatures. The reactor total pressure, the Si2H6 partial pressure and the total flow were kept constant during growth.

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Fig. 2. (a) Ratio of mole fraction between Ge and Si in the SiGe layers as a function of the partial pressure ratio of Ge2H6 and Si2H6. The Ge sticking coefficient m is given for every temperature. (b) SiGe growth rate as a function of the inverse temperature (Arrhenius plot) for the reaction rate limited growth regime for two different partial pressure ratios.

than in our case. This has been attributed to a lower activation energy for Ge deposition and to a reduced desorption energy of H atoms from the Ge atoms [23]. All layers shown here are single crystalline as demonstrated by RBS and X-ray diffraction. Typical RBS random (dotted blue line1) and channeling (solid black line) spectra of a SiGe layer grown at 600 °C are presented in Fig. 3a. To assess the crystal quality, the minimum yield value, vmin, is compared to the value of structurally perfect SiGe layers of vmin = 5% [19]. The vmin values are presented in Fig. 3b as a function of the Ge concentration. For all layers grown at 600 °C vmin values of 5–6% are obtained indicating a nearly perfect single crystal structure. Additionally, the spectra show no defects at the SiGe/Si substrate interface, as checked by ion channeling and by TEM analysis (Fig 3). This is an indication for pseudomorphic growth. High single crystallinity was also observed for layers grown at 550 °C with Ge concentrations below 45%. For the layers grown at 500 °C and 450 °C, the vmin values are between 4% and 7%. Slightly increased channeling values may arise for thin surface layers, e.g. 30 nm SiGe layers grown at 450 °C, due to the influence of the surface peak on the determination of the minimum channeling yield. For these samples, complementary, XRD and

Raman spectroscopy were employed to determine the strain state in the layers, as discussed in the next chapter. In Fig. 4 an exemplary TEM image of a Si/Si0.56Ge0.44 heterostructure is shown indicating besides high crystal quality smooth layer surface morphology and sharp Si/SiGe interfaces.

3.2. SiGe critical thickness for strain relaxation Due to the large lattice mismatch of 4.2% between Si and Ge, SiGe heteroepitaxy faces certain restrictions. Up to a particular layer thickness which is dependent on the Ge concentration, SiGe layers can be grown pseudomorphically on Si substrates [24,25]. Above the so-called ‘‘critical thickness for strain relaxation’’, dislocations are generated at the Si/SiGe interface which lead to strain relaxation [24]. Another limitation in growing uniform, strained SiGe films is related to the change of the growth mode to undulated layers or/and island formation, usually observed for a heteroepitaxy with high lattice mismatch. Here, we show, that using low temperature deposition and keeping a relative high growth rate yield to smooth SiGe films.

Fig. 3. (a) RBS/C spectra of a 100 nm thick SiGe layer with a Ge concentration of 29%. (b) Channeling minimum yield as function of Ge content for SiGe layers grown at different temperatures.

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Fig. 4. TEM image of a 3 nm/9.5 nm Si0.56Ge0.44 heterostructure as used for quantum well SiGe MOSFET devices [7,8].

To determine the degree of strain relaxation and hence the critical thickness for pseudomorphic growth of high Ge concentration layers, XRD  2h scans around the (0 0 4) Bragg peak and RSM measurements around the (2 2 4) reflection are the methods of choice. In Fig. 5 three x  2h scans on samples grown at 450 °C and 500 °C are shown. The extracted Ge concentrations and layer thicknesses are in good agreement with the RBS results. For 450 °C the Ge concentrations vary between 45% and 64% (RBS: 45–62%) and for 500 °C between 31% and 57% (RBS: 32–58%). The fringes for higher Ge concentrations systematically shift to smaller 2h values, as a result of the increasing out-of-plane lattice constant for higher Ge percentage. In addition, the well-shaped fringes on each side of the XRD layer peaks indicate atomically smooth surfaces and interfaces. The results obtained from the RSM measurements on samples grown at 450 °C and 500 °C are summarized in Fig. 6. Here the degree of strain relaxation and the layer thicknesses are plotted as a function of the Ge concentration. Considering the resolution of the diffractometer of 0.01 Å a 3% error has to be taken in consideration. For the 450 °C growth temperature, the 31 nm and 29 nm thick SiGe layers with Ge content of 45% and 53%, respectively are fully strained while the 21 nm layer with Ge content of 62% is slightly relaxed (about 7%). All the SiGe layers grown at 500 °C are fully

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strained even for high Ge concentrations of 55% and 49% with thicknesses of 24 nm and 28 nm, respectively. These values significantly exceed the theoretical critical thicknesses for high Ge content SiGe layers [24]. For the Ge concentrations of 35% and 57% achieved at 500 °C the measured layer thicknesses of 101 nm and 30 nm, respectively, exceed the critical layer thickness of People and Bean [24] by a factor larger than 2. Moreover, these results are slightly above the state-of-the-art critical thicknesses explored recently by Hartmann et al. [25]. All the results are summarized in Fig. 7. In this plot the determined critical layer thicknesses using XRD for 450 °C and 500 °C are shown as a function of the Ge concentration and compared to recent literature data. Raman spectroscopy and AFM were also employed for strain and structural characterization. The Raman results are summarized in Fig. 8 where the theoretical lines for fully strained and fully relaxed SiGe layers, taken from the recent work of Perova et al. [26], are extrapolated, assuming linear behaviour, to high Ge contents and added for comparison. The Raman peak shifts are in good agreement with the XRD data and confirm that the combination of Si2H6 and Ge2H6 allows the growth of highly strained SiGe layers on Si, with Ge concentrations up to 62 at.%. AFM is used to study the topography of strained SiGe films. Strain relaxation processes are found to determine the surface morphology, with distinct morphological features arising from both misfit dislocation formation and three-dimensional growth of coherent islands and pits on the surface. In the diamond cubic lattice structures of SiGe, and Ge the dislocations propagate primarily by glide on (1 1 1) planes inclined to the (0 0 1) interface. The geometry of the interfacial dislocation array is, therefore defined by the intersection of the inclined glide planes with the interface. The misfit dislocation lines run along orthogonal [1 1 0] directions. The AFM image in Fig. 9a indicate the existence of atomic steps on the surface of the 20 nm Si0.38Ge0.62 layer grown at 450 °C. The existence of surface atomic steps corresponds to a misfit network at the SiGe/Si substrate interface, while their absence indicates pseudomorphic layer growth as presented in Fig. 9b. All SiGe layers present Root Mean Square (RMS) roughness values below 0.25 nm, similar to homoepitaxial Si growth. 3.3. In situ Si doping using PH3 and B2H6 For CMOS device processing, in situ doped epitaxially grown layers offer several advantages compared to doping by ion implantation: high dopant substitutionality, single crystalline quality

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Fig. 5. XRD  2h scans around the (0 0 4) Bragg peak for fully strained SiGe layers with Ge concentrations between 45% and 57% grown at: (a) 450 °C and (b) 500 °C.

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Fig. 6. Degree of strain relaxation (left scale) and layer thickness (right scale) determined by X-ray diffraction as a function of the Ge concentration for: (a) 450 °C and (b) 500 °C growth temperatures.

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without ion induced end-of-range defects and, last but not the least, abrupt p–n junctions. Moreover, CVD growth of doped layer avoids the necessity of high temperature post ion implantation treatments for dopant activation and defect healing which may induce strain relaxation of elastically strained layers [27,28]. The thermal budget for processing may be further reduced if Si and Ge precursors like Si2H6 and Ge2H6 are employed which enables low temperature CVD growth even with high growth rates, as discussed above. For our in situ Si-doping studies we employed PH3 and B2H6 precursors (both 100 ppm in H2) as n-type and p-type dopants, respectively. In order to investigate the concentration and homogeneity of the dopant atoms incorporation the Si2H6 partial pressure was maintained constant at 15 Pa while the dopant precursor partial pressure was systematically increased. Epitaxial Si doped layers with thicknesses between 40 nm and 60 nm were grown at 600 °C and the PH3 partial pressure was varied from 0.003 Pa to 0.028 Pa. In Fig. 10a ToF-SIMS-spectra for four P-doped Si layers (Si:P) are shown. An increase of the PH3 partial pressure results in an increase of P volume concentration of about one order of

magnitude. For all studied samples, even for the highest P concentration of 1.1  1020 cm3, a homogeneous P concentration within the layer was obtained. For Boron doped Si (Si:B), layers of about 580 nm were grown in one run at 600 °C while the B2H6 partial pressure was increased during the growth in four steps from 0.001 Pa to 0.019 Pa. For each growth step a constant growth time was maintained. The B2H6 partial pressure values have been chosen lower than that of the PH3 precursor, due to the fact that there are twice as many B atoms in one precursor molecule. The SIMS spectrum (Fig. 10b) shows very smooth B doping plateaus, hence very homogeneous distribution within each layer. To compare the two different types of doping the extracted B and P atomic volume concentration, as well as the growth rates are summarized in Fig. 11. For low dopant partial pressures a five times higher B than P concentration is measured and at higher partial pressures the B concentration becomes ten times higher than the P concentration. The highest impurity concentrations as determined from SIMS analyses are 1.1  1020 cm3 for P and 1  1021 cm3 for B. For a B2H6 partial pressure of 0.019 Pa the growth rate increases nearly linearly up to 11 nm/min. In the case of P, the dopant concentration saturates at a partial pressure of about 0.012 Pa and the growth rate seems to be independent of the PH3 partial pressure. This might be attributed to limited free active sites density at the growing surface by P atoms segregation [29,30]. The growth rate value of about 1.2 nm/min for Si:P is similar to the growth rate for intrinsic Si at the same growth temperature, but it is one order of magnitude lower than the growth rate for Si:B. The RBS-Channeling spectra of both, P and B doped Si layers (not show here), provide very low minimum channeling yields of 4% (3–3.5% is a typical value for high quality Si bulk) indicating that most of the dopants occupy substitutional lattice sites [28,31].

3.4. In situ SiGe doping using PH3 and B2H6 n- and p-type doped SiGe layers are essential for state-of-theart MOSFETs [8] devices. Moreover, steep junction formation is one of the key requirements for good TFET device functionality [10]. Low temperature is a necessity for the growth and processing of highly strained doped layer in order to avoid strain relaxation and atom interdiffusion. Using Si2H6, Ge2H6 and B2H6 or PH3 doped Si0.56Ge0.44 layers with thicknesses of 85 nm to 100 nm were grown at 500 °C and 600 °C, respectively. During the SiGe layer growth the dopant

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partial pressure was systematically increased in four steps as for the doped Si:B layer in the previous section. The SIMS profiles for p-type doped Si0.56Ge0.44 grown at 600 °C and for n-type doped SiGe grown at 500 °C and 600 °C are shown in Fig. 12. Similar to

the Si doping case, homogeneous distribution of dopants within the layer sequences was found. Maximum dopant concentrations of 3.5  1020 cm3 and 3  1020 cm3 are obtained for B and P doping of Si0.56Ge0.44 layer grown at 600 °C, respectively (Fig. 12a). The

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maximum P volume concentration increases to 4.7  1020 cm3 by decreasing the growth temperature to 500 °C at constant gas precursor fluxes. It should be mentioned that in this case the SiGe alloying concentration also changes. These values are comparable (SiGe:B) to or even higher (SiGe:P) than those reported in literature using different (SiH2Cl2 and GeH4) precursors at similar temperatures [32]. The doping efficiency behaviour for B and P in SiGe significantly differ from the case for B and P doping of Si. For instance, at a PH3 partial pressure of 0.028 Pa using only Si2H6 and PH3 a volume concentration of P atoms of 1.1  1020 cm3 within the Si:P layer is obtained. Adding Ge2H6 results in a three times higher P atom concentration within the SiGe:P layer. For B doping, the behaviour changes: at the dopant partial pressure of 0.019 Pa the measured volume concentration of B atoms within the Si:B layers is three times higher than in SiGe:B layers. In all doping studies the Si2H6 and Ge2H6 partial pressures were kept constant at 15 Pa and 7.5 Pa, respectively. According to the intrinsic SiGe growth section, at a growth temperature of 600 °C a Ge concentration of 41% and a growth rate of 18 nm/min are expected. The growth rates for doped SiGe layers are presented in Fig. 13b. Compared to the undoped case (18 nm/min) slightly smaller values of 10 nm/min (SiGe:B) and 14 nm/min (SiGe:P) were observed at low dopant partial pressures, 0.0014 Pa for B2H6 and 0.0029 Pa for PH3. By increasing the amount of dopant precursors to 0.0284 Pa for PH3 and 0.0193 Pa for B2H6 the intrinsic SiGe growth rate was achieved. In conclusion we can state that for B doped SiGe an increase of dopant partial pressure results in an increase of the growth rate, for SiGe:P, however, nearly no growth rate change could be observed. Further experiments especially concerning the growth temperature are essential for a more detailed analysis of the growth kinetics for doped SiGe. 4. Conclusions We have studied the low temperature epitaxial growth of highly strained sige and p- and n-type doped Si and SiGe layers using Si2H6 and Ge2H6 precursors. Fully strained epitaxial single crystalline SiGe layers have been obtained at temperatures as low as 450 °C, with high Ge concentrations up to 53%. A comprehensive analysis using RBS, XRD, RSM and Raman spectroscopy confirms the pseudomorphic growth of the SiGe layer and the high critical layer thickness for plastic relaxation for all growth temperatures down to 450 °C. Furthermore, high doping concentrations of 1.1  1020 cm3 for Si:P and 1  1021 cm3 for Si:B, were obtained at 600 °C using PH3 and B2H6, respectively. Even higher doping concentrations of 3.0  1020 cm3 and 3.5  1020 cm3 have been obtained for Si0.56Ge0.44 layers at 600 °C, using PH3 and B2H6 precursors, respectively. The growth at 500 °C enhances also the Ge content to 57% and the P concentration to 4.7  1020 cm3. All the growth studies were performed using a new designed 200 mm AIXTRON CVD reactor with showerhead technology, being the first report of epitaxial growth in such a reactor design. Acknowledgments The authors thank to M. Heuken and P.K. Baumann from AIXTRON SE for continuous support on the cluster tool. References [1] Rauter P, Mussler G, Grützmacher D, Fromherz T. Tensile strained SiGe quantum well infrared photodetectors based on a light-hole ground state. Appl Phys Lett 2011;98(21):211106. [2] Buca D, Winnerl S, Lenk S, Buchal C, Xu D-X. Fast time response from Si–SiGe undulating layer superlattices. Appl Phys Lett 2002;80(22):4172.

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