CRYSTAL GROWTH & DESIGN
InN Growth by Plasma-Assisted Molecular Beam Epitaxy with Indium Monolayer Insertion
2008 VOL. 8, NO. 3 1073–1077
Yongzhao Yao,*,†,‡ Takashi Sekiguchi,†,‡ Yoshiki Sakuma,† Naoki Ohashi,§ Yutaka Adachi,§ Hanako Okuno,4 and Masaki Takeguchi4 AdVanced Electronic Materials Center, National Institute for Materials Science, Tsukuba 305-0044, Japan, Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba 305-0005, Japan, Optronic Materials Center, National Institute for Materials Science, Tsukuba 305-0044, Japan, and High Voltage Electron Microscopy Station, National Institute for Materials Science, Tsukuba 305-0003, Japan ReceiVed September 28, 2007; ReVised Manuscript ReceiVed NoVember 28, 2007
ABSTRACT: An etching effect of N-plasma on a GaN buffer layer was found in the initial stage of InN growth by plasma-assisted molecular beam epitaxy. We proposed to predeposit 1–2 monolayers (ML) of In (referred to as “In insertion”) on the GaN buffer layer to protect it from etching, thus preserving the flat buffer surface for InN growth. Atomic force microscopy (AFM), highresolution X-ray diffraction (HRXRD), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) were carried out to evaluate the GaN buffers and InN films and compare the effect of the In insertion. It has been shown that the In insertion significantly improves the structural quality and optical property of InN. A flatter InN surface, narrower XRD full-width at half-maximum, sharper InN/GaN interface, and stronger PL were observed in optimal samples with 1.8 ML In insertion. The effect of In insertion is discussed in terms of buffer surface protection and enhancement of surface migration of In adatoms. Introduction InN, an indispensable member of III-nitrides, has attracted increasing attention in the past decade due to its remarkable electrical and optical properties, such as small effective mass of electron, high mobility, high peak and saturation velocity of electron and near-IR light emission.1 However, InN is still one of the most mysterious compounds known, and some of its physical properties remain unclear. As an example, its bandgap is a subject of controversy, previously being reported at 2 eV2 and now being revised down to ∼0.65–0.70 eV.3–6 This is mainly due to the great difficulties in growing high-quality crystals, which can be traced back to the physical properties of InN, such as low dissociation temperature, stoichiometric instability, and extremely high equilibrium vapor pressure of nitrogen over InN. Recently, the quality of InN grown by plasma-assisted molecular beam epitaxy (PA-MBE) on sapphire has been improved very quickly7–9 by carefully controlling the growth condition of InN as well as by optimizing the growth of buffer layers. The interfacial quality of buffer layer, especially the surface flatness, is one of the most important factors for InN growth because it has great influence on the initial stage of InN growth. In our previous study,10 we found that the GaN film could be etched by the N-plasma at ∼500 °C, resulting in surface roughening. Thus, if GaN is used as buffer layer for InN growth, we need to know whether the GaN preserves the surface flatness when N-plasma is supplied to grow InN. Furthermore, if N-plasma etches the GaN surface when InN growth starts, how shall we protect the buffer from roughening. On the basis of this consideration, we propose to predeposit 1–2 monolayers (ML) of In onto the GaN buffer layer as a surface protection layer, on which InN films are grown. The influence of this In * To whom correspondence should be addressed. Tel: +81-29-851-3354 (ext: 8895). Fax: +81-29-860-4794. E-mail:
[email protected]. † Advanced Electronic Materials Center, National Institute for Materials Science. ‡ University of Tsukuba. § Optronic Materials Center, National Institute for Materials Science. 4 High Voltage Electron Microscopy Station, National Institute for Materials Science.
Table 1. Sample Denotations and Preparation Processes Step 1
GaN growth
Step 2
In insertion
thermal clean/880 °C, 1 h nitridation/880 °C, 30 min growth/880 °C, 2 h/300 nm 500 °C 0 ML
0.9 ML
1.8 ML
For Set “Buf” (GaN samples, shown in Figure 1) Step 3
N* exposure
500 °C, 20 s Buf-1
Buf-2
Buf-3
For Set “Epi” (thick InN samples, shown in Figure 2) Step 3
InN growth
500 °C, 1 h, 100 nm Epi-1
Epi-2
Epi-3
For set “Thin” (thin InN samples, shown in Figure 3) Step 3
InN growth
500 °C, 10 min, ∼15 nm Thin-1
Thin-2
Thin-3
insertion on the structural quality and optical properties of InN are investigated by atomic force microscopy (AFM), highresolution transmission electron microscopy (HRTEM), high resolution X-ray diffraction (HRXRD), and photoluminescence (PL). Experimental Section InN films with GaN buffer layers were grown on c-plane sapphire substrates by PA-MBE using a radio frequency (RF) plasma source for nitrogen and conventional Knudsen effusion cells for gallium and indium. The N2 flow rate was 0.5 sccm for all samples, which was translated to a nitrogen partial pressure in the growth chamber at approximately 2.5 × 10-6 Torr. The power of the RF generator was fixed at 500 W. An ion-trapper was used to deflect the high energetic ion species that induce defects into the epitaxial layer.1 The flux rate of the Group-III element was adjusted by changing the temperature of Knudsen cell for gallium or indium. The c-plane sapphire substrate with 50-nm-thick titanium on the backside was cleaned with acetone and ethanol in an ultrasonic bath at room temperature before being loaded into the growth chamber. The PA-MBE processes are shown in Table 1. The substrate was first thermally cleaned for 1 h and nitrided by N-plasma for 30 min both at 880 °C. Immediately after that, a 300-nm-thick GaN buffer layer was grown under a slightly Ga-rich condition. The substrate stayed at 880
10.1021/cg700947g CCC: $40.75 2008 American Chemical Society Published on Web 02/12/2008
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Figure 1. Morphologies of GaN buffers with different treatments (set “Buf”): (a) as-grown GaN buffer, rms ) 0.7 nm, (b) Buf-1 (20-s N*), rms ) 1.7 nm, (c) Buf-2 (0.9 ML In + 20-s N*), rms ) 0.8 nm, and (d) Buf-3 (1.8 ML In+20-s N*), rms ) 0.6 nm. Horizontal dotted lines indicate the traces of profile. °C without a N-plasma supply for 30 s to consume the small amount of excess gallium on the GaN surface. Then it was cooled to the InN growth temperature of 500 °C, and an In layer was deposited by opening the shutter for the In source for a short duration. For comparison, three different nominal coverages of In were deposited: 0 ML (no In insertion), 0.9 ML and 1.8 ML (shutter opened for 0, 10, and 20 s, respectively). The nominal coverage of In was calculated based on the growth rate of InN film (5.65 MLs/min, 1 ML ) 0.285 nm). After the In deposition, three different categories of samples were grown: set “Buf” was completed by exposing to N-plasma for 20 s after the In insertion, to investigate the etching effect of N-plasma on GaN buffer; set “Epi” was completed by growing a 100-nm-thick InN film on the In insertion layer under a slightly N-rich condition at 500 °C; set “Thin” was completed by growing a 15-nm-thick InN film on the In insertion layer under the exact same condition with set “Epi”, to study the initial stage of InN growth. In each set, the denotation of 1, 2, or 3 corresponds to the In nominal coverage of 0, 0.9, and 1.8 ML, respectively. The morphology of the InN film was revealed by an AFM operated in the tapping mode. HRTEM measurements were performed with JEOL-JEM3000F operated at 300 keV. HRXRD scans were performed with a Philips PANalytical X’Pert MRD using Cu KR1 radiation. PL measurements were performed at 77 K using the 514.5 nm line of an
Ar+ laser as the excitation source. The luminescence was detected by Ge detector and reconfirmed by PbS detector.
Results and Discussion Figure 1 shows the surface morphologies of set “Buf” as well as the as-grown GaN buffer for comparison. The as-grown GaN buffer shows a flat surface with atomic steps, while Buf-1, Buf2, and Buf-3 show different morphologies depending on their coverage of In insertion. In the case of Buf-1, GaN buffer shows a roughened surface after exposure to N-plasma for 20 s, whose rms roughness is 1.7 nm. Some 10-nm-deep hexagonal pits appear on it. The pit density is on the order of mid-109 cm-2 measured from a low-magnification AFM image. This result indicates that the N-plasma etches the GaN surface even at an exposure time as short as 20 s. This etching process may involve the decomposition of GaN described by the reaction: GaN + N* f Ga(adsorbed) + N2v and the desorption of metallic gallium: Ga(absorbed) f Gav.11 In contrast, after exposure to N-plasma Buf-2 and Buf-3 preserve flat surfaces with rms roughnesses of less than 1 nm (0.8 nm for Buf-2 and 0.6 nm
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Figure 2. Morphologies of 100-nm-thick InN films with different coverage of In insertion (set “Epi”): (a) Epi-1 (no In insertion), rms ) 7.4 nm, (b) Epi-2 (0.9 ML In), rms ) 0.5 nm, and (c) Epi-3 (1.8 ML In), rms ) 0.2 nm. The line-profile path is shown by an arrowed line in each AFM image.
for Buf-3). Their line-profiles show that the surface fluctuation is on the same order with one GaN monolayer. Etch pits do not appear on Buf-2 and Buf-3 at all. These results suggest that 1–2 MLs In insertion can effectively protect the GaN surface from etching by N-plasma and preserve the atomically flat surface for InN growth. Figure 2 shows the morphologies and line profiles of set “Epi”, that is, the 100-nm-thick InN films grown on different coverage of In insertion. On the surface of Epi-1, some holes about 50 nm in depth can be seen. This is reasonable if the etching effect of N-plasma on the GaN buffer is considered. We believe that in the case of Epi-1 when the shutters for the In source and N-plasma are opened to grow InN, two reactions may simultaneously occur with N-plasma involved: InN growth (In + N* f InN) and GaN etching (GaN + N* f Gav + N2v). The latter causes the formation of pits as shown in Figure 1b and consequently results in the surface roughening of Epi-1. Note that the pits on Buf-1 and the holes on Epi-1 have a similar density and diameter, but they differ in depth (several nanometers for pits and ∼50 nm for holes). This implies that the deep holes on Epi-1 may originate from the pits on Buf-1. The initial surface roughness of buffer is enhanced in depth after the 100-nm InN growth. In contrast, Epi-2 and Epi-3 show the atomically flat surfaces with the rms roughness as low as 0.5 and 0.2 nm, respectively. Along the line-profile path as shown by the arrowed line in the AFM images, two adjacent terraces differ in height by about 1.10 nm, 1.65 or 2.20 nm for Epi-2 and 0.55 nm for Epi-3. These values correspond to 4, 6, 8, and 2 MLs of InN, respectively. These multiatomic steps indicate that the growth mode for Epi-2 and Epi-3 is a two-dimensional (2D) layer-by-layer mode. Comparing the morphology of Epi1, Epi-2, and Epi-3, the influence of In insertion is very obvious. It is clear that the In insertion effectively protects the flat surface of GaN buffer layer from etching by N-plasma, thus allowing the InN growth to start on an atomically flat buffer surface. The flat buffer surface is crucial for InN growth, because it allows the longer migration of In adatoms. During InN growth, if In adatoms are able to migrate long enough to nucleate at a terrace edge, then the layer-by-layer growth mode may be favored. On the other hand, a rough buffer surface will greatly shorten the migration length of In adatoms,12 and threedimensional (3D) growth may occur. To corroborate the above view, we also fabricated another set of InN samples (set “Thin”) using identical growth conditions
with set “Epi”, except for the smaller film thickness of ∼15 nm. This set of samples enables us to investigate the InN growth at the initial growth stage, thus confirming that the etching and surface roughening indeed occur during this period as we expect. Figure 3 shows the morphologies of set “Thin”. Hexagonal etch pits ∼10 nm in depth can be found in Thin-1 (Figure 3a), and an InN layer composed of small 3D islands has covered both the unetched area and the bottom of the etched area. The surface morphology of Thin-1 resembles that of Buf-1, and the pit areas might be inherited from Buf-1 when InN growth proceeds. In contrast, Thin-2 and Thin-3 (Figure 3b,c) show much flatter surface without any etch pits. In our former study, we found InN is chemically more stable than GaN against N-plasma. InN film almost preserves its morphology after a 30 min exposure to N-plasma, whereas the GaN film surface dramatically changes even when the exposure to N-plasma is only several minutes. A possible reason could be related to the different atomic weight of metallic gallium and indium and their difference in desorption coefficient. We believe that when the InN layer is grown thick enough to isolate the GaN from N-plasma, the N-plasma etching becomes negligible afterward. However, at the initial stage of InN growth, protecting the GaN surface from N-plasma etching is the key to preserving the atomically flat surface and leading the growth to 2D mode. In the above discussion at a micron-scale, it was shown that In insertion reduced the etch pits; we next take a closer look at the etch-pit-free interface region at a nanometer-scale. Figure 4a,b shows the HRTEM images of InN/GaN interfacial regions in Epi-1 and Epi-3, respectively, both viewed along the [112j0] zone axis. Figure 4, panels c and d are their Fourier filtered (FFT) images using the in-plane 101j0 spatial frequencies. Etchpits were not seen in the observed regions. First we compare the InN/GaN interfaces. In Figure 4a, some black contrast was observed at the interface. These black contrasts arise from the lattice disorder. This implies that the interface in Epi-1 is not uniform and relatively rough at an atomic scale. In contrast, one can clearly locate the interface in Figure 4b because the InN and GaN layers gave sharp change in the atomic arrangement at the interface. The interface in Epi-3 is very flat and abrupt. Its fluctuation is less than one monolayer, which indicates that the InN growth almost simultaneously started on the same GaN monolayer. From FFT images, InN/GaN interface in Figure 4c is difficult to distinguish because of the irregular appearance of extra half-planes, which has no periodicity. However, in
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Figure 3. Morphologies of 15-nm-thick InN films with different coverage of In insertion (set “Thin”): (a) Thin-1 (no In insertion), rms ) 1.7 nm, (b) Thin-2 (0.9 ML In), rms ) 0.4 nm, and (c) Thin-3 (1.8 ML In), rms ) 0.2 nm. Horizontal dotted lines indicate the traces of profile.
Figure 4. HRTEM images of InN/GaN interfacial area in (a) Epi-1 and (b) Epi-3, both viewed along [112j0] and the corresponding Fourier filtered images (c) and (d) using the in-plane 101j0 spatial frequencies. The horizontal white arrows indicate the InN/GaN interfaces. The black arrows indicate the defective regions in Epi-1.
Figure 4d, we clearly observed the regular appearance of extra half-planes in GaN with the periodicity of 2.76 nm. In one period, 10 GaN {101j0} planes match 9 InN {101j0} planes at the interface. Along the interface, misfit dislocations regularly formed at the positions where the extra half-plane of GaN terminated. Second, we compare the InN side above the interface. In Figure 4c,d, the blurred parts in Epi-1 (denoted by the black arrows) are observable, indicating the presence of defects or large strain in InN. In contrast, Epi-3 has a very clear and smooth stripe pattern of lattice planes. We did not find any disorder regions like those in Epi-1. These experiments indicate that the buffer surface in Epi-3 was more uniform than that in Epi-1 due to the presence of In insertion. The In insertion makes the interface and InN more perfect. A possible reason may be associated with the different migration length of In adatoms on bare GaN buffer and on In-covered GaN buffer.
The results of XRD measurements on the set “Epi” are shown in Figure 5. The full-width at half-maximum (fwhm) in XRD of ω-2θ scan, symmetric (0002) and asymmetric (101j4) ω-rocking curve (XRC) scans of InN films become narrower when the coverage of In insertion increases in the range from 0 (Epi-1) to 1.8 ML (Epi-3). Because the thicknesses of InN films in set “Epi” are almost the same, the narrowing of fwhm should be attributed to the improvement in structural quality of InN by the In insertion. The XRD measurements are consistent with the HRTEM measurements. The improved crystal quality of Epi-3 can be traced back to less lattice disorder from the very beginning of InN growth as shown in Figure 4. Figure 6 shows the 77 K PL spectra of InN films detected by a Ge detector. A Ge detector was used because of its high sensitivity around 0.7 eV. A PbS detector with a cutoff energy at 0.516 eV (2.4 µm) was also used to confirm the PL spectra
InN Growth by Plasma-Assisted Molecular Beam Epitaxy
Figure 5. Plot of fwhm in XRD measurements of ω-2θ scan, symmetric (0002) and asymmetric (101j4) ω-rocking curve scans versus the coverage of In insertion.
Crystal Growth & Design, Vol. 8, No. 3, 2008 1077
demonstrated. It is worth noting that we also tried to grow InN film with thicker In insertion, for example, 3.6 or 7.2 MLs; however, InN film was not successfully grown on them. This phenomenon may be explained as follows: when the thickness of In layer is smaller than a critical value, the topmost In atoms bond to the N atoms of GaN and keep the covalent bond. If the N-plasma is supplied, N atoms can bond to the In and form InN. On the other hand, if the In layer is above the critical thickness, it becomes metallic and loses its covalency. Thus, the N atoms cannot bond to the topmost In, and InN growth therefore is suppressed. This is in agreement with Xu et al.’s report.13 By in situ reflection high-energy electron diffraction (RHEED) and spectroscopic ellipsometry (SE) monitoring, they found that, once the surface was covered with excess indium, the growth would stop. A critical thickness of In insertion should be between 1.8 and 3.6 MLs, above which the InN film cannot be grown. Conclusion Indium monolayer insertion was introduced to the growth of InN films on GaN buffer. The InN films grown with the In insertion showed great improvement in the film flatness, structural quality, and PL intensity. It has been found that In insertion protected GaN from etching by N-plasma when InN growth started, thus preserving the flat buffer surface for InN growth. Another function of In insertion was to facilitate the migration of adatoms and make the InN/GaN interface more abrupt and highly regular. The optimized coverage of In insertion has been determined around 1–2 MLs. The In insertion, in our experiment, is the key to obtaining high-quality InN.
References Figure 6. PL spectra of InN with different coverage of In insertion: (a) Epi-1 (no In insertion), (b) Epi-2 (0.9 ML), and (c) Epi-3 (1.8 ML), measured at 77 K with Ge detector. Inset shows a comparison of PL spectra of Epi-3 measured at 77 K and room temperature.
detected by Ge detector. The results show that the cutoff effect of Ge detector did not significantly affect the peak energy and the shape of PL spectra. The PL peak energies for Epi-1, Epi2, and Epi-3 all locate at 0.743 eV, which is in agreement with the recent reports.3,4 This suggests the InN epifilms grown in this work have good crystal quality and relatively low electron concentration. The great improvement of PL peak intensity by In insertion can be seen in Figure 6. The PL peak intensities of Epi-3 and Epi-2 are 20 times and 8 times stronger than that of Epi-1, respectively. This indicates that the crystal quality is improved and the density of nonradiative recombination centers might be lowered by the In insertion. We would like to point out that the PL in Epi-3 was quite strong, which could be observed even at room temperature (RT) as shown in the inset of Figure 6. The ratio of PL integral intensity between 77 K and RT was about 6.8. The PL peak energy at 77 K and RT did not show any significant shift. By a comprehensive evaluation, the beneficial effect of In insertion on the InN growth with GaN buffer layer has been
(1) Bhuiyan, A. G.; Hashimoto, A.; Yamamoto, A. J. Appl. Phys. 2003, 94, 2779–2808. (2) Tansley, T. L.; Foley, C. P. J. Appl. Phys. 1986, 59, 3241–3244. (3) Davydov, V. Y.; Klochikhin, A. A.; Seisyan, R. P.; Emtsev, V. V.; Ivanov, S. V.; Bechstedt, F.; Furthmüller, J.; Harima, H.; Mudryi, A. V.; Aderhold, J.; Semchinova, O.; Graul, J. Phys. Status Solidi B 2002, 229, R1–R3. (4) Wu, J.; Walukiewicz, W.; Yu, K. M.; Arger, J. W., III; Haller, E. E.; Lu, H.; Schaff, W. J.; Saito, Y.; Nanishi, Y. Appl. Phys. Lett. 2002, 80, 3967–3969. (5) Xu, K.; Yoshikawa, A. Appl. Phys. Lett. 2003, 83, 251–253. (6) Nanishi, Y.; Saito, Y.; Yamaguchi, T. Jpn. J. Appl. Phys. 2003, 42, 2549–2559. (7) Saito, Y.; Teraguchi, N.; Suzuki, A.; Araki, T.; Nanishi, Y. Jpn. J. Appl. Phys. 2001, 40, L91–L93. (8) Walukiewicz, W.; Ager, J. W., III; Yu, K. M.; Liliental-Weber, Z.; Wu, J.; Li, S. X.; Jones, R. E.; Denlinger, J. D. J. Phys. D 2006, 39, R83–R99. (9) Yoshitani, M.; Akasaka, K.; Wang, X. Q.; Che, S. B.; Ishitani, Y.; Yoshikawa, A. J. Appl. Phys. 2006, 99, 044913 1–6. (10) Yao, Y. Z.; Sekiguchi, T.; Sakuma, Y.; Ohashi, N. J. Cryst. Growth 2007, 301–302, 521–524. (11) Myers, T. H.; Millecchia, M. R.; Ptak, A. J.; Ziemer, K. S.; Stinespring, C. D. J. Vac. Sci. Technol. B 1999, 17, 1654–1658. (12) Liu, B.; Leone, S. R.; Kitajima, T.; Zhang, T. H.; Borca, C. J. Appl. Phys. 2005, 97, 023509 1–8. (13) Xu, K.; Yoshikawa, A. Appl. Phys. Lett. 2003, 83, 251–253.
CG700947G
