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Uniform GaN thin films grown on (100) silicon by remote plasma atomic layer deposition
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 014002 (http://iopscience.iop.org/0957-4484/26/1/014002) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.108.9.184 This content was downloaded on 29/12/2014 at 22:30
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Nanotechnology Nanotechnology 26 (2015) 014002 (7pp)
doi:10.1088/0957-4484/26/1/014002
Uniform GaN thin films grown on (100) silicon by remote plasma atomic layer deposition Huan-Yu Shih1, Ming-Chih Lin2, Liang-Yih Chen4 and Miin-Jang Chen1,3 1
Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan Taiwan Textile Research Institute, Taipei 23674, Taiwan 3 National Nano Device Laboratories, Hsinchu 30078, Taiwan 4 Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan 2
E-mail: [email protected] Received 1 July 2014, revised 5 September 2014 Accepted for publication 29 September 2014 Published 10 December 2014 Abstract
The growth of uniform gallium nitride (GaN) thin films was reported on (100) Si substrate by remote plasma atomic layer deposition (RP-ALD) using triethylgallium (TEG) and NH3 as the precursors. The self-limiting growth of GaN was manifested by the saturation of the deposition rate with the doses of TEG and NH3. The increase in the growth temperature leads to the rise of nitrogen content and improved crystallinity of GaN thin films, from amorphous at a low deposition temperature of 200 °C to polycrystalline hexagonal structures at a high growth temperature of 500 °C. No melting-back etching was observed at the GaN/Si interface. The excellent uniformity and almost atomic flat surface of the GaN thin films also infer the surface control mode of the GaN thin films grown by the RP-ALD technique. The GaN thin films grown by RP-ALD will be further applied in the light-emitting diodes and high electron mobility transistors on (100) Si substrate. Keywords: atomic layer deposition, gallium nitride, thin films, grazing incidence x-ray diffraction, crystal structure (Some figures may appear in colour only in the online journal) Introduction
step coverage, which restrains the deposition of GaN on the substrates with complicated surface morphologies. Nowadays, sapphire is the most widely used substrate for GaN heteroepitaxy. However, Si has been recognized as the most promising substrate for GaN due to its merits: large wafer size, low material cost, and feasible integration with the Si electronic circuits. However, it is very difficult to grow high-quality GaN films directly on Si substrates because of the great difference in their lattice constants and thermal expansion coefficients. Another critical issue in the growth of GaN on Si is the melt-back etching: the inter-diffusion occurs at the GaN/Si interface during the high-temperature process, resulting in a very rough surface with deep hollows in the substrate [7, 8]. Atomic layer deposition (ALD) is a low-temperature chemical vapor deposition technique, by which the films were
Gallium nitride (GaN) has attracted great attention because of its large direct bandgap (3.43 eV) together with excellent mechanical, chemical, and thermal stability. GaN has found a variety of applications, such as light-emitting diodes (LEDs) [1], laser diodes [2], and high electron mobility transistors (HEMTs) [3]. High-quality GaN has been conventionally grown by a number of techniques, such as metal-organic chemical vapor deposition (MOCVD) [4], molecular beam epitaxy [5], and pulse laser deposition [6]. However, the high deposition temperatures of GaN (usually greater than 600 °C) cause a limit in the applications on thermally fragile substrates and low-temperature processes. In addition, those techniques are not capable of preparing the GaN thin films with sufficient 0957-4484/15/014002+07$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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Nanotechnology 26 (2015) 014002
grown in a layer-by-layer manner based on surface-limited reactions. As compared with other thin-film deposition techniques, ALD has unique characteristics including accurate thickness control, high uniformity over a large area, excellent conformality on very high-aspect-ratio structures, good reproducibility, and low deposition temperatures [9]. Those advantages make ALD a strong candidate for manufacturing nano-photonic devices and future-generation integrated nanoelectronic circuits. Actually, ALD has been exploited to prepare a variety of materials including oxides [10–12], nitride [13–15], and pure metals [16]. The ALD growth of GaN has been reported by several groups. In early study, GaN was prepared by the so-called ‘atomic layer epitaxy’ [17–19]. However, the ALD window was not carefully investigated. Kim et al reported thermal ALD of GaN using GaCl3 and NH3 as the precursors [20]. The ALD window of this process was between 500 and 750 °C with the deposition rate of ∼0.2 nm per ALD cycle. Recently, Ozgit-Akgun et al reported the GaN thin filmsgrown by plasma-enhanced ALD in the temperature range from 100 to 500 °C using trimethylgallium (TMG, Ga (CH3)3) and NH3 [15]. The self-limiting ALD mode was observed from 185 to 385 °C with the growth rate of ∼0.05 nm/cycle. In 2014, they also reported the growth of GaN, AlN and AlxGa1−xN by hollow cathode ALD and the application in GaN thin film transistors [21, 22]. Recently, the researches of ALD growth of GaN were reviewed by Miikkulainen et al [23]. Nevertheless, the crystallinity of the thin films plays an essential role in GaN epitaxy and the related electrical and photonic devices, but few studies have reported on the improved crystallinity of the GaN thin films prepared by the ALD technique. On the other hand, the use of triethylgallium (TEG, Ga (C2H5)3) rather than TMG as the precursor for Ga is of particular interest because the improved film quality can be obtained with TEG, as reported by the studies in which the MOCVD was used to grow the GaN or InGaN epilayers [24– 26]. The GaN and InGaN thin films exhibited improved crystal quality, stronger near-band-edge photoluminescence, higher mobility, and lower carbon and oxygen concentrations, as TEG was used as the precursor [24, 25]. In this study, we report the characteristics of GaN thin films on (100) Si substrates grown by remote plasma ALD (RP-ALD) using TEG as the precursor, in the temperature ranges from 200 to 500 °C. The growth rate, crystallinity, chemical composition, surface roughness, and uniformity of the films have been investigated. Uniform GaN thin films with almost atomic flat surface were obtained and meltingback etching was not observed at the GaN/Si interface. The crystallinity and the nitrogen content of the ALD GaN films were improved with the increase of the deposition temperature.
Figure 1. The scheme setup of the RP-ALD system.
quartz tube. The direction of plasma gases and precursors is down flow, and the distance between the plasma source and substrate is approximately 50 cm. The GaN thin films were deposited on (100) Si substrates. TEG and remote NH3/H2 plasma were utilized as the precursors for gallium and nitrogen, respectively. The flow rate of H2 is fixed at 5 sccm and the carrier gas is Ar. The remote N radicals were produced by a radio-frequency coil at a power of 300 W under the injection of NH3/H2 pulses, and the hydrogen plasma was used to facilitate NH3 dissociation [27]. Each ALD cycle consisted of four steps: (1) TEG pulse, (2) Ar purge, (3) remote NH3/H2 plasma for 30 s, and finally (4) Ar purge. The films were deposited in the temperature range of 200 ∼ 500 °C with the working pressure of approximately 0.4 Torr. The thickness of the GaN thin films was measured by spectroscopic ellipsometer (SE, Elli-SE, Ellipso Technology) in the wavelength range of 300–1000 nm at an incident angle of 70°. The film thickness was further confirmed by highresolution transmission electron microscopy (HRTEM). Crystalline phases of the GaN films were identified by highpower grazing incidence x-ray diffractometer (GIXRD; Rigaku TTRAX 3, 18 kW) in θ−2θ mode with Cu Kα radiation, where the scan was performed with a low grazing angle of incidence in order to avoid intensive signal from the substrate. The chemical compositions and bonding states in the films were characterized with x-ray photoelectron spectrometer (XPS) using Al Kα (1486.6 eV) radiation. A presputtering step with 3 kV (1.2 μA) Ar+ ion was carried out for 10 s to remove the contamination on the surface. Atomic force microscopy (AFM; NT-MDT) was used to evaluate the topography of the GaN thin films.
Experimental section A scheme of the remote plasma setup is shown in figure 1. The system is inductively coupled plasma (ICP) with a 2
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Figure 3. (a) The GaN growth rate at different deposition
Figure 2. Dependence of GaN growth rate on the (a) TEG pulse time
temperatures. (b) GaN film thickness as a function of the applied ALD cycles. The TEG pulse time is 0.12 s and flow rate of NH3/H2 is 30/5 sccm.
and (b) NH3 flow rate.
Result and discussion shows the film thickness as a function of the applied ALD cycles, revealing a linear dependence between the film thickness and the ALD cycle at the deposition temperatures at 200 and 500 °C. This result also implies that the deposition follows the ALD mode. The GaN thin films grown at 200 and 500 °C also exhibit excellent thickness uniformity, as revealed by the nine-point measurement of the film thickness on a 2 inch (100) Si substrate. The distribution of the film thickness is presented graphically in figure 4. The variation coefficients of film thickness are only 1.31% and 1.54% for the GaN films grown at 200 and 500 °C, respectively, revealing the high uniformity of GaN films as a result of the beneficial characteristics of ALD. The excellent uniformity over a 2 inch substrate can be attributed to the growth of GaN films with the self-limiting mode. Figure 5 shows the crystalline phases of the ∼30 nm thick GaN films grown at different temperatures from 200 to 500 °C. As shown in figure 5, no reflections in the XRD patterns were observed from the GaN thin films grown at a low temperature of 200 °C, indicating that the film is completely amorphous. As the deposition temperature was greater
Figure 2 shows the growth rate of the GaN thin films as a function of the TEG pulse time and NH3 flow rate, at a low deposition temperature of 200 °C. The NH3 flow rate was fixed at 30 sccm in figure 2(a) and the TEG pulse time was fixed at 0.2 s in figure 2(b), respectively. The growth rate was defined in terms of the film thickness divided by the total applied ALD cycles. It is seen in figure 2(a) that the growth rate increases with the TEG dose and then remains constant at 0.025 nm/cycle when the TEG pulse time is greater than 0.1 s. Figure 2(b) reveals that the growth rate also saturates with the NH3 flow rate above 25 sccm. The results suggest that the GaN thin films were grown in a self-limiting manner by the RP-ALD technique. Figure 3(a) shows the dependence of GaN growth rate on the deposition temperature. It is seen that the growth rate increases from 0.025 to 0.062 nm per ALD cycle with the increase of deposition temperature from 200 to 500 °C. Similar behaviors have been observed in the investigations on the growth of AlN using TMA and NH3 by plasma-enhanced ALD, in which the increase of deposition rate of AlN with the deposition temperature was reported [14, 28]. Figure 3(b) 3
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Figure 4. Thickness distributions of the GaN thin films grown at 200 and 500 °C on 2 inch (100) Si substrate. The dash lines represent the average film thickness.
Figure 5. XRD patterns of GaN thin films grown at the temperatures from 200 to 500 °C.
than 200 °C, the films exhibited the diffraction characteristics of hexagonal GaN (h-GaN) structure of the peak intensity correlated positively with the growth temperature. The reflection observed at 32.7°, 34.8°, 36.6° and 57.9° are referring to the h-GaN (101¯0), h-GaN (0002), h-GaN (101¯1) and h-GaN (112¯0 ), respectively,revealing that the films are polycrystalline with hexagonal structure. It can be seen that the peak intensities increases with increasing growth temperature, indicating that the improved crystalline quality of hGaN can be obtained at a higher deposition temperature due to the increase in the reaction and mass transport kinetics [23]. The difference in the crystalline phases of the GaN films prepared at distinct temperatures may be one of the possible reasons to explain the temperature dependence of growth rate as shown in figure 3(a). Because the GaN film deposited at a low temperature is amorphous in structure as shown in figure 5, the random arrangement of atoms results in serious
steric hindrance, thus reducing the growth rate. Such a steric hindrance effect may be suppressed at high deposition temperatures due to the improved crystalline quality, thereby resulting in the increase of growth rate with the growth temperature. It can be also observed that the h-GaN (101¯0) peak becomes dominant at the deposition temperature at 500 °C. According to crystallographic symmetry, the fourfold symmetry of the Si (100) is hardly matched to the threefold symmetry of h-GaN (0001). On the other hand, h-GaN (101¯0) exhibits two fold symmetry, and accordingly h-GaN (101¯0) is better matched to Si (100). Figure 6 shows the XPS spectra of the GaN thin film grown at 500 °C. The binding energy at 396.9 eV as seen in figure 6(a) can be ascribed to the N1s state of the nitrogen compound, as a result of the shift from the binding energy of the N1s core level in elemental nitrogen at 399 eV. The asymmetric XPS line shape may originate from the overlap of 4
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Figure 6. (a) N1s, (b) Ga2p3/2, (c) Ga3d, and (d) O1s XPS spectra of the GaN thin film grown at 500 °C.
the N1s peak with the Ga LMM Auger peak [29]. Figure 6(b) shows the Ga2p3/2 peaks with binding energy of 1118.3 eV, shifted from elemental Gaat 1117 eV. This result is consistent with the Ga2p3/2 peak at 1118 eV in the GaN films as reported by Dinescu et al [30]. Therefore, the shifts in the binding energies of the N1s and Ga2p3/2 levels confirm the bonding between Ga and N [31]. Figure 6(c) shows the Ga3d spectrum peaked at 19.5 eV, which is close to the binding energy of GaN [32]. No carbon signals were observed by the XPS measurement, indicating that the ethyl groups in TEG were almost completely removed during the exposure of remote NH3/H2 plasma. The compositions in the GaN thin film grown at the temperatures from 200 °C to 500 °C were calculated from the relative area under the curve of XPS spectra, giving that the average atomic compositions of Ga, N, and O are 42.1 ± 0.9%, 44.9 ± 0.9%, and 13.0 ± 0.4%, respectively, as shown in figure 7. The most likely origin of oxygen may be the quartz tube of the ICP source [21]. It can be found that the nitrogen composition slightly increases with the growth temperature, which might result from the improved reactivity of NH3 plasma at high growth temperature. This may be another reason to explain the temperature dependence of the growth rate. The carbon composition is low and exhibits no significant change with growth temperature, suggesting that
Figure 7. Composition of GaN film grown on (100) Si at different temperature.
no CVD mode happens in this growth temperature range. In fact, it has been reported that the residual carbon in the GaN films grown by MOCVD [33] and PECVD [34] is much higher than that in ALD GaN. This result suggests that the carbon content in GaN would increase and change if the growth follows the CVD mode. 5
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Figure 9. The refractive index and extinction coefficient as a function of wavelength of the GaN thin film grown at 500 °C.
and remote NH3/H2 plasma. The increase of the deposition temperature from 200 to 500 °C improves the crystalline quality of the GaN films from the amorphous to polycrystalline phase with hexagonal structure. The growth rate and composition of nitrogen also increases with the deposition temperature, which may be explained by the improvement of NH3 plasma reactivity and suppressed steric hindrance due to the improved crystalline quality. No melting-back etching was observed at the GaN/Si interface. TheGaN thin films grown by RP-ALD also exhibit the unique characteristics of ALD, including the saturation of growth rate with the increasing precursor doses, linear dependence of the film thickness on the applied ALD cycle, excellent uniformity, and almost atomic flat surfacewith negligible roughness. The distinguished benefits of ALD also indicate that these GaN thin films can be further applied in the LEDs and HEMTs on (100) Si substrate in the near future.
Figure 8. Cross-sectional HRTEM images of the GaN thin film grown at 500 °C.
Figure 8 displays the cross-sectional HRTEM image of the GaN thin film prepared with 300 ALD cycles at 500 °Con (100) Si substrate. The image shows the substrate of single crystal Si, a thin interfacial layer, and a layer of GaN. The GaN film thickness is about 18.5 nm, revealing the growth rate is approximately 0.06 nm/cycle. This growth rate is in very good agreement witht hat given by the SE measurement as shown in figure 3. It should be noted that the interface between the ALD GaN and Si is smooth and no melt-back etching was observed at a high growth temperature of 500 °C. The AFM measurement indicates that the root-mean-square roughness of the films grown with 600 ALD cycles at 200 and 500 °C are 3.2 and 5.5 Å, respectively. The surface roughness approaches the resolution limit of AFM, accordingly suggesting the almost atomic flat surface with negligible roughness of the GaN thin films grown by RP-ALD, owing to the self-limiting and layer-by-layer growth. The refractive index and extinction coefficient of the ALD GaN thin film grown at 500 °C was investigated by the SE measurement. Figure 9 shows the dispersion of the refractive index and extinction coefficient in the wavelength range between 260 and 1050 nm. The SE data was fitted by the Tauc–Lorentz model, which is widely used for the amorphous and polycrystalline semiconductors. It can be seen that the refractive index of ALD GaN at λ = 633 nm is 2.26 and its band gap is 3.21 eV, pretty close to the reported data of polycrystalline GaN films [35].
Acknowledgment This work was partially supported by Sino-American Silicon Products Inc., Hsinchu Science Park, Taiwan.
References [1] Nakamura S, Mukai T and Senoh M 1991 High-power GaN PN junction blue-light-emitting diodes Japan. J. Appl. Phys. 30 L1998 [2] Nakamura S, Senoh M, Nagahama S, Iwasa N, Yamada T, Matsushita T, Kiyoku H and Sugimoto Y 1996 InGaN multi-quantum-well structure laser diodes grown on MgAl2O4 substrates Appl. Phys. Lett. 68 2105 [3] Khan M A, Bhattarai A, Kuznia J N and Olson D T 1993 Highelectron-mobility transistor based on a GaN-AlxGa1−xN heterojunction Appl. Phys. Lett. 63 1214 [4] Khan M A, Kuznia J N, Vanhove J M, Olson D T, Krishnankutty S and Kolbas R M 1991 Growth of high optical and electrical quality GaN layers using low-pressure metalorganic chemical vapor deposition Appl. Phys. Lett. 58 526
Conclusion In summary, GaN thin films on (100) Si substrate were grown by the RP-ALD technique with the alternating supply of TEG 6
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[5] Daudin B, Widmann F, Feuillet G, Samson Y, Arlery M and Rouviere J L 1997 Stranski–Krastanov growth mode during the molecular beam epitaxy of highly strained GaN Phys. Rev. B 56 R7069 [6] Vispute R D, Talyansky V, Sharma R P, Choopun S, Downes M, Venkatesan T, Jones K A, Iliadis A A, Khan M A and Yang J W 1997 Growth of epitaxial GaN films by pulsed laser deposition Appl. Phys. Lett. 71 102 [7] Ishikawa H, Yamamoto K, Egawa T, Soga T, Jimbo T and Umeno M 1998 Thermal stability of GaN on (111) Si substrate J. Cryst. Growth 189 178 [8] Dadgar A et al 2003 MOVPE growth of GaN on Si(111) substrates J. Cryst. Growth 248 556 [9] George S M 2010 Atomic layer deposition: an overview Chem. Rev. 110 111 [10] Alnes M E, Monakhov E, Fjellvag H and Nilsen O 2012 Atomic layer deposition of copper oxide using copper(II) acetylacetonate and ozone Chem. Vapor Depos. 18 173 [11] Li X F et al 2012 Tin oxide with controlled morphology and crystallinity by atomic layer deposition onto graphene nanosheets for enhanced lithium storage Adv. Funct. Mater. 22 1647 [12] Hwang Y J, Hahn C, Liu B and Yang P D 2012 Photoelectrochemical properties of TiO2 nanowire arrays: a study of the dependence on length and atomic layer deposition coating ACS Nano. 6 5060 [13] Song L et al 2010 Large scale growth and characterization of atomic hexagonal boron nitride layers Nano. Lett. 10 3209 [14] Nepal N, Qadri S B, Hite J K, Mahadik N A, Mastro M A and Eddy C R 2013 Epitaxial growth of AlN films via plasmaassisted atomic layer epitaxy Appl. Phys. Lett. 103 082110 [15] Ozgit C, Donmez I, Alevli M and Biyikli N 2013 Atomic layer deposition of GaN at low temperatures J. Vac. Sci. Technol. A 30 01A124 [16] Kariniemi M, Niinisto J, Hatanpaa T, Kemell M, Sajavaara T, Ritala M and Leskela M 2011 Plasma-enhanced atomic layer deposition of silver thin films Chem. Mater. 23 2901 [17] Khan M A, Skogman R A, Vanhove J M, Olson D T and Kuznia J N 1992 Atomic layer epitaxy of GaN over sapphire using switched metalorganic chemical vapor deposition Appl. Phys. Lett. 60 1366 [18] Karam N H, Parodos T, Colter P, Mcnulty D, Rowland W, Schetzina J, Elmasry N and Bedair S M 1995 Growth of device-quality GaN at 550 °C by atomic layer epitaxy Appl. Phys. Lett. 67 94 [19] Tsuchiya H, Akamatsu M, Ishida M and Hasegawa F 1996 Layer-by-layer growth of GaN on GaAs substrates by alternate supply of GaCl3 and NH3 Japan. J. Appl. Phys. 2 L748 [20] Kim O H, Kim D and Anderson T 2009 Atomic layer deposition of GaN using GaCl3 and NH3 J. Vac. Sci. Technol. A 27 923 [21] Ozgit-Akgun C, Goldenberg E, Okyay A K and Biyikli N 2014 Hollow cathode plasma-assisted atomic layer deposition of
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crystalline AlN, GaN and AlxGa1−xN thin films at low temperatures J. Mater. Chem. C 2 2123 Bolat C O-A S, Tekcan B, Biyikli N and Okyay A K 2014 Low temperature thin film transistors with hollow cathode plasma-assisted atomic layer deposition based GaN channels Appl. Phys. Lett. 104 243505 Miikkulainen V, Leskela M, Ritala M and Puurunen R L 2013 Crystallinity of inorganic films grown by atomic layer deposition: overview and general trends J. Appl. Phys. 113 021301 Saxler A, Walker D, Kung P, Zhang X, Razeghi M, Solomon J, Mitchel W C and Vydyanath H R 1997 Comparison of trimethylgallium and triethylgallium for the growth of GaN Appl. Phys. Lett. 71 3272 Lee K H, Chang P C, Chang S J, Su Y K, Wu S L and Pilkuhn M 2012 Comparison studies of InGaN epitaxy with trimethylgallium and triethylgallium for photosensors application Mater. Chem. Phys. 134 899 Nishizawa J, Abe H, Kurabayashi T and Sakurai N 1986 Photostimulated molecular layer epitaxy J. Vac. Sci. Technol. A 4 706 Siow K S, Britcher L, Kumar S and Griesser H J 2006 Plasma methods for the generation of chemically reactive surfaces for biomolecule immobilization and cell colonization—a review Plasma Process. Polym. 3 392 Bosund M, Sajavaara T, Laitinen M, Huhtio T, Putkonen M, Airaksinen V M and Lipsanen H 2011 Properties of AlN grown by plasma enhanced atomic layer deposition Appl. Surf. Sci. 257 7827 Wagner C D, Riggs W M, Davis L E, Moulder J F and Muilenberg G E 1979 Handbook of X-ray Photoelectron Spectroscopy (Eden Prairie, MN: Perkin-Elmer Physical Electronics Division) Dinescu M, Verardi P, Boulmer-Leborgne C, Gerardi C, Mirenghi L and Sandu V 1998 GaN thin films deposition by laser ablation of liquid Ga target in nitrogen reactive atmosphere Appl. Surf. Sci. 127 559 Shi F, Li H and Xue C S 2010 Fabrication of GaN nanowires and nanorods catalyzed with tantalum J. Mater. Sci. Mater Electron. 21 1249 Hedman J and Martensson N 1980 Gallium nitride studied by electron-spectroscopy Phys. Scr. 22 176 Ambacher O, Angerer H, Dimitrov R, Rieger W, Stutzmann M, Dollinger G and Bergmaier A 1997 Hydrogen in gallium nitride grown by MOCVD Phys. Status Solidi A 159 105–19 Choi S W, Bachmann K J and Lucovsky G 1993 Growthkinetics and characterizations of gallium nitride thin-films by remote pecvd J. Mater. Res. 8 847–54 El-Naggar A M, Ei-Zaiat S Y and Hassan S M 2009 Optical parameters of epitaxial GaN thin film on Si substrate from the reflection spectrum Opt. Laser Technol. 41 334–8
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Uniform GaN thin films grown on (100) silicon by remote plasma atomic layer deposition
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 014002 (http://iopscience.iop.org/0957-4484/26/1/014002) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.108.9.184 This content was downloaded on 29/12/2014 at 22:30
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 014002 (7pp)
doi:10.1088/0957-4484/26/1/014002
Uniform GaN thin films grown on (100) silicon by remote plasma atomic layer deposition Huan-Yu Shih1, Ming-Chih Lin2, Liang-Yih Chen4 and Miin-Jang Chen1,3 1
Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan Taiwan Textile Research Institute, Taipei 23674, Taiwan 3 National Nano Device Laboratories, Hsinchu 30078, Taiwan 4 Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan 2
E-mail: [email protected] Received 1 July 2014, revised 5 September 2014 Accepted for publication 29 September 2014 Published 10 December 2014 Abstract
The growth of uniform gallium nitride (GaN) thin films was reported on (100) Si substrate by remote plasma atomic layer deposition (RP-ALD) using triethylgallium (TEG) and NH3 as the precursors. The self-limiting growth of GaN was manifested by the saturation of the deposition rate with the doses of TEG and NH3. The increase in the growth temperature leads to the rise of nitrogen content and improved crystallinity of GaN thin films, from amorphous at a low deposition temperature of 200 °C to polycrystalline hexagonal structures at a high growth temperature of 500 °C. No melting-back etching was observed at the GaN/Si interface. The excellent uniformity and almost atomic flat surface of the GaN thin films also infer the surface control mode of the GaN thin films grown by the RP-ALD technique. The GaN thin films grown by RP-ALD will be further applied in the light-emitting diodes and high electron mobility transistors on (100) Si substrate. Keywords: atomic layer deposition, gallium nitride, thin films, grazing incidence x-ray diffraction, crystal structure (Some figures may appear in colour only in the online journal) Introduction
step coverage, which restrains the deposition of GaN on the substrates with complicated surface morphologies. Nowadays, sapphire is the most widely used substrate for GaN heteroepitaxy. However, Si has been recognized as the most promising substrate for GaN due to its merits: large wafer size, low material cost, and feasible integration with the Si electronic circuits. However, it is very difficult to grow high-quality GaN films directly on Si substrates because of the great difference in their lattice constants and thermal expansion coefficients. Another critical issue in the growth of GaN on Si is the melt-back etching: the inter-diffusion occurs at the GaN/Si interface during the high-temperature process, resulting in a very rough surface with deep hollows in the substrate [7, 8]. Atomic layer deposition (ALD) is a low-temperature chemical vapor deposition technique, by which the films were
Gallium nitride (GaN) has attracted great attention because of its large direct bandgap (3.43 eV) together with excellent mechanical, chemical, and thermal stability. GaN has found a variety of applications, such as light-emitting diodes (LEDs) [1], laser diodes [2], and high electron mobility transistors (HEMTs) [3]. High-quality GaN has been conventionally grown by a number of techniques, such as metal-organic chemical vapor deposition (MOCVD) [4], molecular beam epitaxy [5], and pulse laser deposition [6]. However, the high deposition temperatures of GaN (usually greater than 600 °C) cause a limit in the applications on thermally fragile substrates and low-temperature processes. In addition, those techniques are not capable of preparing the GaN thin films with sufficient 0957-4484/15/014002+07$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
H-Y Shih et al
Nanotechnology 26 (2015) 014002
grown in a layer-by-layer manner based on surface-limited reactions. As compared with other thin-film deposition techniques, ALD has unique characteristics including accurate thickness control, high uniformity over a large area, excellent conformality on very high-aspect-ratio structures, good reproducibility, and low deposition temperatures [9]. Those advantages make ALD a strong candidate for manufacturing nano-photonic devices and future-generation integrated nanoelectronic circuits. Actually, ALD has been exploited to prepare a variety of materials including oxides [10–12], nitride [13–15], and pure metals [16]. The ALD growth of GaN has been reported by several groups. In early study, GaN was prepared by the so-called ‘atomic layer epitaxy’ [17–19]. However, the ALD window was not carefully investigated. Kim et al reported thermal ALD of GaN using GaCl3 and NH3 as the precursors [20]. The ALD window of this process was between 500 and 750 °C with the deposition rate of ∼0.2 nm per ALD cycle. Recently, Ozgit-Akgun et al reported the GaN thin filmsgrown by plasma-enhanced ALD in the temperature range from 100 to 500 °C using trimethylgallium (TMG, Ga (CH3)3) and NH3 [15]. The self-limiting ALD mode was observed from 185 to 385 °C with the growth rate of ∼0.05 nm/cycle. In 2014, they also reported the growth of GaN, AlN and AlxGa1−xN by hollow cathode ALD and the application in GaN thin film transistors [21, 22]. Recently, the researches of ALD growth of GaN were reviewed by Miikkulainen et al [23]. Nevertheless, the crystallinity of the thin films plays an essential role in GaN epitaxy and the related electrical and photonic devices, but few studies have reported on the improved crystallinity of the GaN thin films prepared by the ALD technique. On the other hand, the use of triethylgallium (TEG, Ga (C2H5)3) rather than TMG as the precursor for Ga is of particular interest because the improved film quality can be obtained with TEG, as reported by the studies in which the MOCVD was used to grow the GaN or InGaN epilayers [24– 26]. The GaN and InGaN thin films exhibited improved crystal quality, stronger near-band-edge photoluminescence, higher mobility, and lower carbon and oxygen concentrations, as TEG was used as the precursor [24, 25]. In this study, we report the characteristics of GaN thin films on (100) Si substrates grown by remote plasma ALD (RP-ALD) using TEG as the precursor, in the temperature ranges from 200 to 500 °C. The growth rate, crystallinity, chemical composition, surface roughness, and uniformity of the films have been investigated. Uniform GaN thin films with almost atomic flat surface were obtained and meltingback etching was not observed at the GaN/Si interface. The crystallinity and the nitrogen content of the ALD GaN films were improved with the increase of the deposition temperature.
Figure 1. The scheme setup of the RP-ALD system.
quartz tube. The direction of plasma gases and precursors is down flow, and the distance between the plasma source and substrate is approximately 50 cm. The GaN thin films were deposited on (100) Si substrates. TEG and remote NH3/H2 plasma were utilized as the precursors for gallium and nitrogen, respectively. The flow rate of H2 is fixed at 5 sccm and the carrier gas is Ar. The remote N radicals were produced by a radio-frequency coil at a power of 300 W under the injection of NH3/H2 pulses, and the hydrogen plasma was used to facilitate NH3 dissociation [27]. Each ALD cycle consisted of four steps: (1) TEG pulse, (2) Ar purge, (3) remote NH3/H2 plasma for 30 s, and finally (4) Ar purge. The films were deposited in the temperature range of 200 ∼ 500 °C with the working pressure of approximately 0.4 Torr. The thickness of the GaN thin films was measured by spectroscopic ellipsometer (SE, Elli-SE, Ellipso Technology) in the wavelength range of 300–1000 nm at an incident angle of 70°. The film thickness was further confirmed by highresolution transmission electron microscopy (HRTEM). Crystalline phases of the GaN films were identified by highpower grazing incidence x-ray diffractometer (GIXRD; Rigaku TTRAX 3, 18 kW) in θ−2θ mode with Cu Kα radiation, where the scan was performed with a low grazing angle of incidence in order to avoid intensive signal from the substrate. The chemical compositions and bonding states in the films were characterized with x-ray photoelectron spectrometer (XPS) using Al Kα (1486.6 eV) radiation. A presputtering step with 3 kV (1.2 μA) Ar+ ion was carried out for 10 s to remove the contamination on the surface. Atomic force microscopy (AFM; NT-MDT) was used to evaluate the topography of the GaN thin films.
Experimental section A scheme of the remote plasma setup is shown in figure 1. The system is inductively coupled plasma (ICP) with a 2
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Figure 3. (a) The GaN growth rate at different deposition
Figure 2. Dependence of GaN growth rate on the (a) TEG pulse time
temperatures. (b) GaN film thickness as a function of the applied ALD cycles. The TEG pulse time is 0.12 s and flow rate of NH3/H2 is 30/5 sccm.
and (b) NH3 flow rate.
Result and discussion shows the film thickness as a function of the applied ALD cycles, revealing a linear dependence between the film thickness and the ALD cycle at the deposition temperatures at 200 and 500 °C. This result also implies that the deposition follows the ALD mode. The GaN thin films grown at 200 and 500 °C also exhibit excellent thickness uniformity, as revealed by the nine-point measurement of the film thickness on a 2 inch (100) Si substrate. The distribution of the film thickness is presented graphically in figure 4. The variation coefficients of film thickness are only 1.31% and 1.54% for the GaN films grown at 200 and 500 °C, respectively, revealing the high uniformity of GaN films as a result of the beneficial characteristics of ALD. The excellent uniformity over a 2 inch substrate can be attributed to the growth of GaN films with the self-limiting mode. Figure 5 shows the crystalline phases of the ∼30 nm thick GaN films grown at different temperatures from 200 to 500 °C. As shown in figure 5, no reflections in the XRD patterns were observed from the GaN thin films grown at a low temperature of 200 °C, indicating that the film is completely amorphous. As the deposition temperature was greater
Figure 2 shows the growth rate of the GaN thin films as a function of the TEG pulse time and NH3 flow rate, at a low deposition temperature of 200 °C. The NH3 flow rate was fixed at 30 sccm in figure 2(a) and the TEG pulse time was fixed at 0.2 s in figure 2(b), respectively. The growth rate was defined in terms of the film thickness divided by the total applied ALD cycles. It is seen in figure 2(a) that the growth rate increases with the TEG dose and then remains constant at 0.025 nm/cycle when the TEG pulse time is greater than 0.1 s. Figure 2(b) reveals that the growth rate also saturates with the NH3 flow rate above 25 sccm. The results suggest that the GaN thin films were grown in a self-limiting manner by the RP-ALD technique. Figure 3(a) shows the dependence of GaN growth rate on the deposition temperature. It is seen that the growth rate increases from 0.025 to 0.062 nm per ALD cycle with the increase of deposition temperature from 200 to 500 °C. Similar behaviors have been observed in the investigations on the growth of AlN using TMA and NH3 by plasma-enhanced ALD, in which the increase of deposition rate of AlN with the deposition temperature was reported [14, 28]. Figure 3(b) 3
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Figure 4. Thickness distributions of the GaN thin films grown at 200 and 500 °C on 2 inch (100) Si substrate. The dash lines represent the average film thickness.
Figure 5. XRD patterns of GaN thin films grown at the temperatures from 200 to 500 °C.
than 200 °C, the films exhibited the diffraction characteristics of hexagonal GaN (h-GaN) structure of the peak intensity correlated positively with the growth temperature. The reflection observed at 32.7°, 34.8°, 36.6° and 57.9° are referring to the h-GaN (101¯0), h-GaN (0002), h-GaN (101¯1) and h-GaN (112¯0 ), respectively,revealing that the films are polycrystalline with hexagonal structure. It can be seen that the peak intensities increases with increasing growth temperature, indicating that the improved crystalline quality of hGaN can be obtained at a higher deposition temperature due to the increase in the reaction and mass transport kinetics [23]. The difference in the crystalline phases of the GaN films prepared at distinct temperatures may be one of the possible reasons to explain the temperature dependence of growth rate as shown in figure 3(a). Because the GaN film deposited at a low temperature is amorphous in structure as shown in figure 5, the random arrangement of atoms results in serious
steric hindrance, thus reducing the growth rate. Such a steric hindrance effect may be suppressed at high deposition temperatures due to the improved crystalline quality, thereby resulting in the increase of growth rate with the growth temperature. It can be also observed that the h-GaN (101¯0) peak becomes dominant at the deposition temperature at 500 °C. According to crystallographic symmetry, the fourfold symmetry of the Si (100) is hardly matched to the threefold symmetry of h-GaN (0001). On the other hand, h-GaN (101¯0) exhibits two fold symmetry, and accordingly h-GaN (101¯0) is better matched to Si (100). Figure 6 shows the XPS spectra of the GaN thin film grown at 500 °C. The binding energy at 396.9 eV as seen in figure 6(a) can be ascribed to the N1s state of the nitrogen compound, as a result of the shift from the binding energy of the N1s core level in elemental nitrogen at 399 eV. The asymmetric XPS line shape may originate from the overlap of 4
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Figure 6. (a) N1s, (b) Ga2p3/2, (c) Ga3d, and (d) O1s XPS spectra of the GaN thin film grown at 500 °C.
the N1s peak with the Ga LMM Auger peak [29]. Figure 6(b) shows the Ga2p3/2 peaks with binding energy of 1118.3 eV, shifted from elemental Gaat 1117 eV. This result is consistent with the Ga2p3/2 peak at 1118 eV in the GaN films as reported by Dinescu et al [30]. Therefore, the shifts in the binding energies of the N1s and Ga2p3/2 levels confirm the bonding between Ga and N [31]. Figure 6(c) shows the Ga3d spectrum peaked at 19.5 eV, which is close to the binding energy of GaN [32]. No carbon signals were observed by the XPS measurement, indicating that the ethyl groups in TEG were almost completely removed during the exposure of remote NH3/H2 plasma. The compositions in the GaN thin film grown at the temperatures from 200 °C to 500 °C were calculated from the relative area under the curve of XPS spectra, giving that the average atomic compositions of Ga, N, and O are 42.1 ± 0.9%, 44.9 ± 0.9%, and 13.0 ± 0.4%, respectively, as shown in figure 7. The most likely origin of oxygen may be the quartz tube of the ICP source [21]. It can be found that the nitrogen composition slightly increases with the growth temperature, which might result from the improved reactivity of NH3 plasma at high growth temperature. This may be another reason to explain the temperature dependence of the growth rate. The carbon composition is low and exhibits no significant change with growth temperature, suggesting that
Figure 7. Composition of GaN film grown on (100) Si at different temperature.
no CVD mode happens in this growth temperature range. In fact, it has been reported that the residual carbon in the GaN films grown by MOCVD [33] and PECVD [34] is much higher than that in ALD GaN. This result suggests that the carbon content in GaN would increase and change if the growth follows the CVD mode. 5
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Figure 9. The refractive index and extinction coefficient as a function of wavelength of the GaN thin film grown at 500 °C.
and remote NH3/H2 plasma. The increase of the deposition temperature from 200 to 500 °C improves the crystalline quality of the GaN films from the amorphous to polycrystalline phase with hexagonal structure. The growth rate and composition of nitrogen also increases with the deposition temperature, which may be explained by the improvement of NH3 plasma reactivity and suppressed steric hindrance due to the improved crystalline quality. No melting-back etching was observed at the GaN/Si interface. TheGaN thin films grown by RP-ALD also exhibit the unique characteristics of ALD, including the saturation of growth rate with the increasing precursor doses, linear dependence of the film thickness on the applied ALD cycle, excellent uniformity, and almost atomic flat surfacewith negligible roughness. The distinguished benefits of ALD also indicate that these GaN thin films can be further applied in the LEDs and HEMTs on (100) Si substrate in the near future.
Figure 8. Cross-sectional HRTEM images of the GaN thin film grown at 500 °C.
Figure 8 displays the cross-sectional HRTEM image of the GaN thin film prepared with 300 ALD cycles at 500 °Con (100) Si substrate. The image shows the substrate of single crystal Si, a thin interfacial layer, and a layer of GaN. The GaN film thickness is about 18.5 nm, revealing the growth rate is approximately 0.06 nm/cycle. This growth rate is in very good agreement witht hat given by the SE measurement as shown in figure 3. It should be noted that the interface between the ALD GaN and Si is smooth and no melt-back etching was observed at a high growth temperature of 500 °C. The AFM measurement indicates that the root-mean-square roughness of the films grown with 600 ALD cycles at 200 and 500 °C are 3.2 and 5.5 Å, respectively. The surface roughness approaches the resolution limit of AFM, accordingly suggesting the almost atomic flat surface with negligible roughness of the GaN thin films grown by RP-ALD, owing to the self-limiting and layer-by-layer growth. The refractive index and extinction coefficient of the ALD GaN thin film grown at 500 °C was investigated by the SE measurement. Figure 9 shows the dispersion of the refractive index and extinction coefficient in the wavelength range between 260 and 1050 nm. The SE data was fitted by the Tauc–Lorentz model, which is widely used for the amorphous and polycrystalline semiconductors. It can be seen that the refractive index of ALD GaN at λ = 633 nm is 2.26 and its band gap is 3.21 eV, pretty close to the reported data of polycrystalline GaN films [35].
Acknowledgment This work was partially supported by Sino-American Silicon Products Inc., Hsinchu Science Park, Taiwan.
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Conclusion In summary, GaN thin films on (100) Si substrate were grown by the RP-ALD technique with the alternating supply of TEG 6
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