Comparison of InGaN-Based LEDs Grown on Conventional Sapphire ...

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 1, JANUARY 2010

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Comparison of InGaN-Based LEDs Grown on Conventional Sapphire and Cone-Shape-Patterned Sapphire Substrate Jae-Hoon Lee, Senior Member, IEEE, Dong-Yul Lee, Bang-Won Oh, and Jung-Hee Lee, Senior Member, IEEE

Abstract—To improve the external quantum efficiency, a highquality InGaN/GaN film was grown on a cone-shape-patterned sapphire substrate (CSPSS) by using metal–organic chemical vapor deposition. The surface pattern of the CSPSS seems to be more helpful for the accommodative relaxation of compressive strain related to the lattice mismatch between GaN and a sapphire substrate because the growth mode of GaN on the CSPSS was similar to that of the epitaxial lateral overgrowth. The output power of a light-emitting diode (LED) grown on the CSPSS was estimated to be 16.5 mW at a forward current of 20 mA, which is improved by 35% compared to that of a LED grown on a conventional sapphire substrate. The significant enhancement in output power is attributed to both the increase of the extraction efficiency, resulted from the increase in photon escaping probability due to enhanced light scattering at the CSPSS, and the improvement of the crystal quality due to the reduction of dislocation. Index Terms—Compressive stress, dislocation, external efficiency, GaN, light-emitting diode (LED), patterned sapphire.

I. I NTRODUCTION

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ROUP-III-NITRIDE compound semiconductors have been attracting great interest for optoelectronic device applications, such as light-emitting diodes (LEDs), laser diodes, and high-power and high-temperature electronics [1]–[4]. InGaN/GaN LEDs have already been extensively used in fullcolor displays, traffic displays, and other various applications. In particular, white LEDs based on InGaN/GaN are regarded as the most promising solid-state lighting devices to replace conventional incandescent and fluorescent lamps, and they become now commercially available in the backlight of a liquid crystal display. In spite of their recent success, however, the output power of InGaN/GaN LEDs needs to be improved for

Manuscript received June 2, 2009; revised September 28, 2009. First published November 13, 2009; current version published December 23, 2009. This work was supported in part by the Korea Science and Engineering Foundation through the National Research Laboratory Program funded by the Ministry of Science and Technology (M10600000273-0650000-27310), Brain Korea 21, and Korea Electronics Technology Institute. The review of this paper was arranged by Editor E. F. Schubert. J.-H. Lee, D.-Y. Lee, and B.-W. Oh are with the Manufacturing Technology Group, LS Division, Samsung LED Company, Ltd., Suwon 443-743, Korea (e-mail: [email protected]). J.-H. Lee is with the School of Electrical Engineering and Computer Science, Kyungpook National University, Daegu 702-701, Korea (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2009.2034495

commercial use. The limitation on light output power is mainly attributed to the low internal quantum efficiency and light extraction efficiency. The low internal quantum efficiency results from the high threading dislocation density of GaN films. It is well known that high-density threading dislocations are inherent in epitaxial GaN films on sapphire substrates due to the large difference in lattice constant between the epitaxial layer and the sapphire substrate. Therefore, how to reduce the dislocation density is an important issue for fabricating high-performance LEDs [5], [6]. Moreover, the refractive index of nitride films (n = 2.5) is higher than that of air (n = 1) and sapphire substrate (n = 1.78). The critical angle of the escape cone is about 23◦ , which indicates that about only 4% of the total light can be extracted from the surface [7]. Most of the light generated in the active layer is absorbed by the electrode at each reflection and gradually disappears, due to the total internal reflection, and is then converted to heat. Recently, it has been reported that one can not only reduce the threading dislocation density in GaN films but also enhance the light extraction efficiency by using a patterned sapphire substrate (PSS) [8]–[12]. The PSS technique has attracted much attention for its high production yield due to the single growth process without any interruption. However, it requires a relatively long growth time because the growth in this mode starts on both etched and nonetched c-plane sapphire and inevitably needs long time in merging the layers grown on both regions at the same growth level and, hence, to obtain a smooth growth surface. In our previous study [13], a high-quality InGaN/GaN film was grown on a cone-shapepatterned sapphire substrate (CSPSS) by using metal–organic chemical vapor deposition (MOCVD). The growth mode of GaN on the CSPSS was similar to that of the epitaxial lateral overgrowth (ELOG) because the growth, in the initial stage, proceeds only on a flat basal sapphire substrate, and there is no preferential growth plane on the cone region. This enables reducing the growth time required in obtaining a smooth growth surface over the patterned region, compared to the cases grown on a conventional PSS. To apply for the manufacturing whiteLED application, it is also important to know the influence of the optical scattering from the GaN and patterned sapphire interface and the color conversion to combine blue LEDs and phosphors. In this paper, we report a remarkable improvement in the electrical and optical properties of InGaN/GaN LEDs grown on a CSPSS by finding the difference between the growth mode on a CSPSS and a conventional sapphire substrate (CSS).

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Fig. 1. SEM images of the fabricated CSPSS process.

II. E XPERIMENTS Fig. 1 shows a scanning electron microscope (SEM) image of the fabricated CSPSS process. The preparation of the CSPSS is as follows [13]. After a photoresist (PR) with 3.5-μm thickness had been coated on a c-plane (0001) sapphire substrate, the PR was patterned first to be a rectangular shape [Fig. 1(a)] with different interval and reflowed during a hard-baking process at 140 ◦ C to make a cone shape as shown in Fig. 1(b). The sapphire substrate was then etched by using inductively coupled plasma reactive ion etching employing reactive Cl2 gas. The diameter and interval of each cone-shaped pattern were 3 and 1 μm, respectively. The height of the cone shape was about 1.5 μm. Four InGaN/GaN multiquantum well (MQW) LEDs were grown on the CSPSS and also on the CSS by MOCVD at the same growth condition. The sapphire substrate was first cleaned in H2 at 1020 ◦ C, followed by the growth of a 25-nmthick GaN buffer layer at 550 ◦ C. After high-temperature annealing of the buffer layer, 5-μm-thick undoped GaN and Si-doped n-GaN layers were sequentially grown at a temperature of 1100 ◦ C. An InGaN/GaN MQW active region, which consists of five pairs of 3-nm-thick undoped InGaN wells and 12-nm-thick Si-doped GaN barriers, was then grown on the n-GaN layer. A p-type cladding layer, which consists a Mgdoped strained layer AlGaN/GaN superlattice, was grown directly on top of the active region, followed by the growth of a 150-nm p-type GaN contact layer. Devices of 260 μm × 690 μm dimensions were fabricated by a normal side-view LED chip process using an ITO transmittance p-contact and Cr/Au metals for the n-contact. To examine the edge sidewall emission, the ITO transmittance metal was replaced with Cr/Au (nontransmittance) in the p-GaN region. The sample was equipped on 0.6t side-view PKG with a horizontal thickness of 6 mm. The LED chips were mounted on lead frames and connected by Au wires. Then, the LED chips were encapsulated with or without phosphor. For the characterization of the surface morphology of the grown layers, we used a SEM, a transmission electron microscope (TEM), and a cathodoluminescence (CL) microscope. The crystal quality of the grown films was investigated by X-ray diffraction and time-integrated

Fig. 2. Time evolution of interference micrographs for a GaN surface grown on a CSPSS. (a) Buffer. (b) Annealing. (c) 10 min. (d) 30 min.

photoluminescence (PL). The stress in the films was measured using FLX2320S stress measurement. The output power of the fabricated LEDs was measured by using an integrating sphere to collect the light emitted in all directions from the LEDs at room temperature. III. R ESULTS AND D ISCUSSION The time evolution of interference micrographs for a GaN surface grown on a CSPSS is shown in Fig. 2, i.e., after the growth of the initial buffer layer at low temperature [Fig. 2(a)], after temperature ramping (annealing the buffer layer) [Fig. 2(b)], and after 10- and 30-min growth of the epitaxial layer [Fig. 2(c) and (d)]. In general, the III-nitride layer preferentially grows on the (0001) crystallographic c-plane of a sapphire substrate. Although there is no growth plane in the cone-shape-patterned region, the low-temperature GaN amorphous buffer layer is well formed on the plane and patterned sapphire region as shown in Fig. 2(a). When the temperature increases from low temperature (550 ◦ C) to high temperature (1020 ◦ C) in the ramping stage of the growth, the random recrystallized GaN islands are formed on the etched flat basal sapphire surface, which is similar to a typical growth mode on the CSS [14]. In the cone-shape-patterned region, however, the larger regular GaN islands are formed on the six corners of a hexagon-shaped cone as shown in Fig. 2(b), which explains that Ga species tend to migrate from the top region of the cone surface to the bottom region of the cone as increasing temperature [15]–[17]. In the initial stage of the high-temperature growth on the CSPSS, therefore, the coalescence starts on the etched flat basal sapphire surface, indicating that the growth mode on the CSPSS is considerably different from that on the conventional PSS such as stripe, hexagonal, or rectangular geometry. As expected, the growth of GaN on the CSPSS was only initiated from the etched basal surface with the (0001) crystallographic plane because there was no growth plane in the cone-shapepatterned region as shown in Fig. 2(c). As the growth proceeds, the growth also laterally propagates toward the peak of the cone as shown in Fig. 2(d). This lateral growth greatly decreases the

LEE et al.: COMPARISON OF InGaN-BASED LEDs GROWN ON CONVENTIONAL SAPPHIRE AND CONE-SHAPE-PATTERNED SAPPHIRE SUBSTRATE

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Fig. 3. Schematic view of the planarization of GaN grown on the conventional PSS and on the proposed CSPSS.

Fig. 6.

Fig. 4.

TEM image of a film grown on a CSPSS.

Fig. 5.

CL image of (a) film grown on a CSS and (b) film grown on a CSPSS.

dislocation density in the grown film, similarly to the ELOG mode [12]. This also enables reducing the time required in obtaining a smooth growth surface over the patterned region, compared to that for those grown on a conventional PSS, where the growth starts both on the etched and nonetched regions at the same time as shown in Fig. 3 [18]. Fig. 4 shows the cross-sectional TEM images under g = 0002 two-beam condition of the interface region between the CSPSS and a GaN layer grown on it, demonstrating that the ELOG-like mode on the CSPSS effectively suppresses the propagation of dislocation into the cone region, even though many dislocations were observed in the film grown on the basal plane of the sapphire. This reduction of dislocation was also confirmed by performing a CL measurement at room temperature as shown in Fig. 5. In bright regions, a radiative process dominates over a nonradiative process because of the lower density of structural defects. The dark spot density in the film grown on a CSS is roughly estimated to about 7 × 108 cm−2 . Apparently, the dark spot density in the film grown on a CSPSS was decreased to about 2 × 108 cm−2 in number.

PL measurements of InGaN/GaN films grown on a CSS and a CSPSS.

The room-temperature PL spectra for both InGaN/GaN samples grown on a CSS and a CSPSS are shown in Fig. 6. The band-edge emission intensity of the sample grown the CSPSS was about four times higher in magnitude than that of the sample grown on the CSS. The full-width half-maximum (FWHM) values of the samples grown on the CSS and the CSPSS are 19 and 17 nm, respectively. In conjunction with a considerable enhancement in emission intensity and FWHM in PL, it is clear that the crystal quality of the film grown on a CSPSS is improved. The typical corresponding spectral PL peak wavelength from the InGaN/GaN MQWs was shifted from 452 nm for a film grown on a CSS to 454 nm for a film grown on a CSPSS. In our previous work [13], we reported that the surface geometry of the CSPSS may be adequate for the easy relaxation of compressive strain during the growth on sapphire. The lattice constant c of films grown on a CSPSS is very close to the lattice constant of the bulk GaN of 5.185 Å, while the lattice constant of the GaN film grown on a CSS exhibits a relatively large lattice constant. The slight red shift in PL for the film grown on a CSPSS is due to the reduced band-gap energy which is caused by the decreased lattice constant c [19]. The reduction of compressive stress in the films grown on a CSPSS was also confirmed by a thin-film stress measurement using FLX-2320S as shown in Fig. 7. The bowing and stress of the 5.12-μm-thickness film grown on a CSS were 34 μm and −437 MPa, respectively, with sapphire Young’s modulus of 4.08 MPa. On the other hand, the values were reduced to 28 μm and −364 MPa for the film with a thickness of 5.48 μm grown on a CSPSS. Consequently, the ELOG-like mode for the GaN layer grown on a CSPSS results in less lattice mismatch and incoherency between the GaN layer and the sapphire substrate. Fig. 8 shows the I–V characteristics for both CSS-LEDs (which are called the LEDs grown on a CSS) and CSPSSLEDs (which are called the LEDs grown on a CSPSS). The forward voltage measured at an injection current of 20 mA showed negligible difference, 3.15 and 3.18 V for the CSSLED and CSPSS-LED, respectively. The leakage currents of the CSS-LED and CSPSS-LED at a reverse voltage of −10 V were around −0.23 μA and −0.46 nA, respectively, as shown in

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Fig. 7. Wafer bowing of films grown on a CSS and a CSPSS. Fig. 9. Light output power for the fabricated LEDs as a function of the injection current. TABLE I S IMULATED R ESULTS FOR THE R AY E XTRACTION R ATIO ( IN P ERCENTS ) OF THE CSS-LED AND CSPSS-LED

Fig. 8. Typical I–V characteristics for the CSS-LED and CSPSS-LED.

the inset in Fig. 8. The decreased reverse leakage current in the CSPSS-LED is also attributed to the decrease in the threading dislocation density in the structure grown on a CSPSS. Fig. 9 shows the light output power for the CSS-LED and CSPSS-LED as a function of the injection current. The total output power of the devices was measured by using sideview PKG without phosphor to collect the light emitted in all directions from the LEDs. With increasing injection current, the light output power of the CSPSS-LED exhibited a rapid increase over the CSS-LED. The output power at 20 mA was estimated to be 12.2 and 16.5 mW for the CSS-LED and CSPSS-LED, respectively. To investigate the sidewalldirection-emitted power only from the devices, ohmic contact metallization with Cr/Au was replaced with ITO. The output power at 20 mA was estimated to be 6.1 and 6.95 mW for the sidewall CSS-LED and sidewall CSPSS-LED, respectively. Notice that the 20-mA output power of the CSPSS-LED shows 35% enhancement compared to that of the CSS-LED. On the other hand, the sidewall-directed output power from the CSPSS-LED exhibited only 14% enhancement compared to

Fig. 10. Schematic ray tracing of light for the (a) CSS-LED and (b) CSPSS-LED.

that of the CSS-LED, which clearly indicates that the light extraction from the CSPSS-LED is higher than that from the CSS-LED due to the enhanced light scattering on the coneshaped sapphire substrate toward a vertical direction rather than the sidewall direction. To clarify the experimental observance, a simulation was performed by using a point-and-shoot ray tracing method. The material parameters such as the reflective index of GaN (n = 2.5), sapphire (n = 1.78), and air (n = 1) and the reflectivity of a bottom mirror (90%) were obtained from the literature and our measured data [20]. Table I shows the expected extraction ratio of the CSS-LED and CSPSS-LED, calculated along two different directions. Although the values of the simulated results are a little different from those of the experiment data, the improvement of the reflection ratio in the top direction of the CSPSS-LED (227%) is much higher than that in the lateral direction of the CSPSS-LED (72%), which reasonably agrees with the increased vertical-direction


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Fig. 11. Far-field emission patterns of the CSS-LED and CSPSS-LED at an injection current of 20 mA.

Fig. 13. CIE chromaticity coordinates obtained from the measured spectra of the PSS-LED and CSPSS-LED under various current.

Fig. 12. Emission spectra of the (a) CSS-LED and (b) CSPSS-LED under various forward-bias current.

scattering convinced by the experimental results. Fig. 10 shows a schematic ray tracing of light for CSS-LEDs [Fig. 10(a)] and CSPSS-LEDs [Fig. 10(b)]. Due to the large refractive index difference between GaN of 2.5 and air of 1, the total internal reflection is mainly responsible for the photon trapping. According to Snell’s law, light confined within a critical angle of 23◦ can cross the air when traveling from a GaN surface to air [7]. In the case of the CSS-LED, a small fraction of light generated in the active region of the LED can escape to the

surrounding air. A significant portion of the photons is reflected within the LED until they are absorbed and converted to heat. In the case of the CSPSS-LEDs, on the other hand, the inclined planes of the cone-shaped patterns can easily redirect the photons back into the escape cone on the top of the device surface, which significantly enhances the light escaping probability, and, hence, the light extraction toward the surface direction, which is different from that of the CSS-LED where most lights are laterally guided by the air/GaN/planar sapphire waveguide and emitted in the sidewall direction. This argument is clear if we analyze the far-field emission patterns under 20-mA current injection as shown in Fig. 11. The output beam pattern for the CSPSS-LED explains that both the light extraction toward the vertical direction and the total light extraction are greatly increased. Fig. 12 shows the EL spectra of the white LEDs fabricated on the CSS and the CSPSS, combined with a yellow phosphor, under various forward-bias current [21]. With increasing current, the spectral position of the blue peak was changed, first the blue shift and then the red shift. The variation of the spectra peak position and FWHM of the peak for the CSPSS-LED is smaller than those of the CSS-LED. The Commission Internationale de 1’Eclairage (CIE) chromaticity

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coordinates obtained from the measured spectra of the CSSLED and CSPSS-LED under various current are shown in Fig. 13. Even though the EL peak wavelength of the CSS-LED (456.4 nm) at 20 mA is very similar to that of the CSPSS-LED (457.1 nm), the color coordinate (x, y) of the CSS-LED (0.329, 0.327) is different from that of the CSPSS-LED (0.326, 0.321) even at the same ratio of phosphor mixture. The change of color coordinates is mainly attributed to the different white conversion efficiency of the CSPSS-LED, resulting from the enhanced light extraction toward a vertical direction compared with that of the CSS-LED, as mentioned previously. It is noticeable that the chromaticity point moves toward lower color temperatures with the increasing current in the range from 20 to 100 mA [22], [23]. The phosphor used in this paper has more effective luminescence efficiency in shorter wavelengths, since the ratio of blue light absorbed by phosphors becomes effectively higher, caused by the blue shift of the InGaN blue LED [24], [25]. For further increasing current to 200 mA, on the other hand, the chromaticity of white LEDs is shifted to the higher color temperatures because changes in the junction temperature of the LED lead to changes in the light output, wavelength, and spectral width [26], [27]. IV. C ONCLUSION The ELOG-like mode on a CSPSS effectively suppresses the probability of dislocation propagation into the cone region and, hence, greatly improves the crystal quality of the GaN films, and also the light extraction efficiency can greatly be increased with the increase in photon escaping probability due to enhanced light scattering near the cone region. In the LED grown on the CSPSS, the improvement of the output power of the top directional emitting at a forward current of 20 mA is increased to 35% compared to that of the sidewall directional emitting because the cone-shaped pattern on sapphire scatters the photon generated in the MQW toward the top direction. Although the wavelength of the CSS-LED and CSPSS-LED at 20 mA is similar, the coordinate of the CSS-LED and CSPSSLED is different when the mixture of phosphor is the same. ACKNOWLEDGMENT The authors would like to thank Dr. J.-W. Lee and Dr. J.-B. Lee of Samsung LED Company, Ltd., for the useful measurements and discussions. R EFERENCES [1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Mat-sushita, Y. Sugimoto, and H. Kiyoku, “High-power, long-lifetime InGaN multi-quantum-well-structure laser diodes,” Jpn. J. Appl. Phys., vol. 36, no. 8B, pp. L1 059–L1 061, Aug. 1997. [2] S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama, “High brightness InGaN blue, green and yellow light-emitting-diodes with quantum well structure,” Jpn. J. Appl. Phys., vol. 34, no. 7A, pp. L797–L799, Jul. 1995. [3] J. H. Lee and J. H. Lee, “Enhanced performance of GaN-based light emitting diode with isoelectronic Al doping layer,” J. Appl. Phys., vol. 105, no. 6, pp. 064 508-1–064 508-6, Mar. 2009. [4] D. H. Youn, J. H. Lee, V. Kumar, K. S. Lee, J. H. Lee, and I. Adesida, “The effects of isoelectronic Al doping and process optimization for the fabrication of high-power AlGaN–GaN HEMTs,” IEEE Trans. Electron Devices, vol. 51, no. 5, pp. 785–789, May 2004.

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Jae-Hoon Lee (M’08–SM’09) was born in Seoul, Korea, in 1971. He received the B.S. degree in electronic engineering from Kwandong University, Gangneung, Korea, in 1995 and the M.S. and Ph.D. degrees in electronic engineering from Kyungpook National University, Daegu, Korea, in 2000 and 2003, respectively. His doctoral research concerned the enhanced performance of an AlGaN/GaN HFET using isoelectronic Al doping effect. From 2003 to 2009, he was with the LM Research and Development 1 Group, Samsung ElectroMechanics Company, Suwon, Korea. Since April 2009, he has been with Samsung LED Company, Ltd., Suwon, where he is currently a Senior Engineer with the Manufacturing Technology Group. He has authored and coauthored 41 technical papers in international peer-reviewed journals and is the holder of 16 U.S.- and 34 Korean-granted patents in his fields of expertise. His main interests are III-nitride growth using metal–organic chemical vapor deposition, gallium-nitride-based electronic and optoelectronic devices, and physics of III-nitride compounds. Dr. Lee is a member of the MRS. He is profiled in Marquis Who’s Who in the World 2010.

Dong-Yul Lee was born in Daegu, Korea, in 1974. He received the B.S. degree in physics from Daegu Hanny University, Gyeongsan, Korea, in 1997 and the M.S. and Ph.D. degrees in physics from Yeungnam University, Daegu, in 1999 and 2004, respectively. From 2004 to 2009, he was with the Lighting Module Research Group, Samsung ElectroMechanics Company, Suwon, Korea, where he developed the III-nitride epistructures with high quantum efficiency (QE). He also investigated the optical and electrical properties on the evaluation of QE and leakage source in III-nitride LEDs. Since April 2009, he has been with Samsung LED Company, Ltd., Suwon, where he is currently a Senior Engineer with the Manufacturing Technology Group. His research interests are the growth of III-nitride compounds using MOCVD and the characterization of LEDs using modulation spectroscopy, time-resolved photoluminescence, cathodoluminescence, and capacitance–voltage.

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Bang-Won Oh received the B.S. degree from Seoul National University, Seoul, Korea, and the M.S. and Ph.D. degrees in material science and engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Korea. From 1995 to 2009, he was with Samsung ElectroMechanics Company, Suwon, Korea. Since April 2009, he has been with Samsung LED Company, Ltd., Suwon, where he is currently the Vice President with the Manufacturing Team. He is the holder of over 100 patents in his fields of expertise.

Jung-Hee Lee (SM’01) was born in Daegu, Korea, in 1957. He received the B.S. and M.S. degrees in electronic engineering from Kyungpook National University, Daegu, in 1979 and 1983, respectively, the M.S. degree in electrical and computer engineering from the Florida Institute of Technology, Melbourne, in 1986, and the Ph.D. degree in electrical and computer engineering from North Carolina State University, Raleigh, in 1990. His doctoral research concerned carrier collection and laser properties in monolayer-thick quantum well heterostructures. From 1990 to 1993, he was with the Compound Semiconductor Research Group, Electronics and Telecommunication Research Institute, Daejeon, Korea. Since 1993, he has been a Professor with the School of Electrical Engineering and Computer Science, Kyungpook National University, Daegu. His current research work is focused on gallium-nitride-based electronic and optoelectronic devices, atomic layer epitaxy for advanced CMOS applications, and characterizations and analyses for 3-D Si devices such as FinFETs or MuGFETs.