00>-oriented LEO GaN stripes grown on silicon substrates are shown to have similar structural properties as LEO GaN grown on GaN/Al1. Introduction
Lateral epitaxial overgrowth (LEO) is an attractive method to produce GaN films with a low density of extended defects, which is beneficial both to studies of the fundamental properties of the GaInAlN materials system and to GaN-based device technology. Recent studies have confirmed that the density of threading dislocations (TDs) is reduced by 3-4 orders of magnitude in the LEO material grown on 6H-SiC [1] and Al2O3 [2] Reference Zheleva, Nam, Bremser and Davis[3] Reference Sakai, Sunakawa and Usui[4] substrates, and the mechanisms of threading dislocations evolution during LEO have been investigated. [1] [2] Reference Sakai, Sunakawa and Usui[4] Reference Marchand, Ibbetson, Fini, Kozodoy, Keller, DenBaars, Marchand, Speck and Mishra[5] Reference Wu, Ibbetson, Fini, Kozodoy, Keller, Speck, DenBaars and Mishra[6] Studies of the optical properties of LEO GaN Reference Nam, Bremser, Zheleva and Davis[7] Reference Sakai, Sunakawa and Usui[8] Reference Chichibu, Marchand, Minski, Keller, Fini, Ibbetson, Deguchi, Sota and Nakamura[9] and InGaN quantum wells Reference Nam, Bremser, Zheleva and Davis[7] Reference Freitas, Nam, Davis, Saparin and Obyden[10] have revealed that TDs act as non-radiative recombination centers. However, the minority carrier diffusion length (<200 nm) is smaller than the average distance between TDs, Reference Freitas, Nam, Davis, Saparin and Obyden[10] such that the emission mechanisms of the carriers that do recombine radiatively appear to be unaffected by moderate TD densities (~106−109 cm−2). On the other hand, reducing the TD density has been shown to reduce the reverse leakage current by ~3 orders of magnitude in GaN p-n junctions, Reference Li, Bishop and Coleman[11] InGaN single Reference Rosner, Girolami, Marchand, Fini, Ibbetson, Zhao, Keller, Mishra, DenBaars and Speck[12] and multiple Reference Kozodoy, Ibbetson, Marchand, Fini, Keller, DenBaars, Speck and Mishra[13] quantum well light emitting diodes, and GaN/AlGaN heterojunction field-effect transistors Reference Mukai, Takekawa and Nakamura[14] fabricated on LEO GaN. More recently, ultraviolet p-i-n photodetectors fabricated on LEO AlGaN have exhibited a similar reduction of the reverse leakage current by up to 6 orders of magnitude. Reference Sasaoka, Sumakawa, Kimura, Nido, Usui and Sakai[15] The use of LEO GaN has also resulted in marked improvements in the lifetime of InGaN/GaN laser diodes. Reference Vetury, Marchand, Ibbetson, Fini, Keller, Speck, Denbaars, Mishra and Hirayama[16]
Such improvements in structural properties and device performance have revived interest in alternative substrates such as Si(111), which has potential advantages for device integration, thermal management, and cost. Reference Parish, Keller, Kozodoy, Ibbetson, Marchand, Fini, Fleisher, DenBaars and Mishra[17] Reference Nakamura, Senoh, Nagahama, Iwasa, Yamada, Matsushita, Kiyoku, Sugimoto, Kozaki, Umemoto, Sano and Chocho[18] Reference Guha and Bojarczuk[19] For GaN on Si(111) in general, the difference in lattice parameters and the strength of the Si-N bond prevent the formation of smooth, single-crystal GaN. Reference Osinsky, Gangopadhyay, Yang, Gaska, Kuksenkov, Temkin, Shmagin, Chang, Muth and Kolbas[20] Reference Stevens, Kinniburgh and Beresford[21] Reference Chu[22] This has been alleviated by using a two-step method involving various buffer layers such as SiC, Reference Manasevit, Erdmann and Simpson[23] Reference Lei and Moustakas[24] GaN, Reference Chu[22] AlN, Reference Parish, Keller, Kozodoy, Ibbetson, Marchand, Fini, Fleisher, DenBaars and Mishra[17] Reference Takeuchi, Amano, Hiramatsu, Sawaki and Akasaki[25] Reference Steckl, Devrajan, Tran and Stall[26] Reference Watanabe, Takeuchi and Hirosawa[27] Reference Kung, Saxler, Zhang, Walker, Wang, Ferguson and Razeghi[28] Reference Meng and Perry[29] GaAs, Reference Ohtani, Stevens and Beresford[30] AlAs, Reference Godlewski, Bergman, Monemar, Rossner and Barski[31] and SiNx, Reference Yang, Sun, Chen, Anwar, Khan, Nikishin, Seryogin, Osinsky, Chernyak, Temkin, hu and Mahajan[32] which typically yields a smooth morphology and a columnar microstructure with a TD density of 1010−1011 cm−2.
We have recently demonstrated a 3-4 orders of magnitude reduction of the TD density in LEO GaN grown on Si(111) using an intermediate AlN buffer layer partially covered by a SiO2 mask. Reference Kobayashi, Kobayashi, Dapkus, Choi, Bond, Zhang and Rich[33] GaN pyramids have also been fabricated on Si(111) by selective-area growth and LEO using an AlGaN buffer layer. Reference Nakada, Aksenov and Okumura[34] More recently GaN stripes were grown on etched GaN/AlN/SiC/Si(111) substrates Reference Marchand, Zhang, Zhao, Golan, Fini, Ibbetson, Keller, DenBaars, Speck, Mishra and Hirayama[35] (‘pendeo-epitaxy’) and on SiO2-patterned GaN/Si(111) layers; Reference Kawaguchi, Honda, Yamaguchi, Hiramatsu, Sawaki and Hirayama[36] however as of yet no analysis of the microstructural properties has been published.
In this paper we report on the structural and optical properties of GaN stripes grown on SiO2-patterned AlN/Si(111) substrates using LEO. The extended defect reduction is characterized by transmission electron microscopy (TEM), x-ray diffraction (XRD), atomic force microscopy (AFM), and cathodoluminescence (CL) imaging. The optical properties are examined using CL and photoluminescence (PL) spectroscopy. It is shown that there is a relationship between the AlN buffer thickness and the stripe morphology which, in turn, affects the microstructure of the LEO GaN. Finally, the issues of chemical compability and thermal expansion mismatch are discussed.
2. Experimental
Two inch-diameter Si(111) wafers were etched in buffered HF for one minute before growth. After heating to the growth temperature of 900°C under hydrogen, the TMAl and NH3 precursors were introduced in the metalorganic chemical vapor deposition (MOCVD) growth chamber and the AlN buffer layer was deposited at a total pressure of 76Torr. The thickness of the AlN layer was ~60nm (‘sample A’) or ~180 nm (‘sample B’). In both cases the AlN layer was crack-free over the entire wafer, and the RMS roughness measured by AFM was on the order of 15nm. [a] The wafers were then coated with 200 nm-thick SiO2 using plasma-enhanced chemical vapor deposition, and 5 µm-wide stripes oriented in the <1
Samples were characterized by scanning electron microscopy (SEM) using a JEOL 6300F field emission microscope operating at 15kV. Specimens for TEM were prepared by wedge polishing followed by standard Ar+ ion milling. Images were recorded on a JEOL 2000FX microscope operated at 200 kV. X-ray rocking curves were measured using CuKα radiation from a Bede double-crystal diffractometer. The surface topography was imaged using a Digital Instruments Dimension 3000 AFM operating in tapping mode. The room-temperature PL was excited using a HeCd laser (325nm, ~20 mW/cm2) and detected using a 1/8m grating monochromator and a photomultiplier tube. The CL measurements were performed in a scanning electron microscope (SEM) at 10 kV (penetration depth of ~0.3-0.4µm determined by Monte Carlo modeling) using an Oxford MonoCL mirror and grating spectrometer system for collecting the generated light and dispersing it to provide wavelength resolution. Reference Walker, Hamilton, Diaz and Razeghi[38]
3. Results
Figure 1 shows cross-section SEM micrographs of typical LEO GaN stripes overgrown for 60 minutes from the SiO2-masked AlN buffer layers. The ‘seed’ region corresponds to GaN grown vertically from the AlN buffer layer, whereas the ‘LEO’ regions correspond to GaN overgrown laterally over the SiO2 mask, as indicated in Figure 1. The AlN buffer layer cannot be readily distinguished in Figure 1. For sample A (60 nm-thick AlN, Figure 1a) the stripe was bound on top by the (0001) facet and on the edges by vertical {11
Figure 1a. Cross section SEM micrograph of a typical GaN LEO stripe on Si(111) after 60 minutes of growth: sample A, 60 nm-thick AlN.
Figure 1b. Cross section SEM micrograph of a typical GaN LEO stripe on Si(111) after 60 minutes of growth: sample B, 180 nm-thick AlN.
Figure 2 shows the surface topography of samples A and B measured by AFM. Figure 2a is a wide-area image (144 µm2) covering the seed and LEO regions of sample A. The topography of the seed region is dominated by c/2-height steps which tend to form partial spirals due to the high density (~2×109 cm−2) of pure screw (Burgers vector b=<0001>) and mixed character (b = 1/3 <11
Figure 2a. Surface topography measured by tapping mode AFM: sample A, large-area scan.
Figure 2b. Surface topography measured by tapping mode AFM: sample A, height mode. The “S” arrow indicates an instance of step termination associated with a screw-character threading dislocation intersection the surface of the film. The “E” arrow indicates an instance of a smaller surface depression typically associated with pure edge threading dislocation.
Figure 2a shows that the atomic steps in the LEO region tend to form a paired structure along the <1
Figure 2c. Surface topography measured by tapping mode AFM: sample B, height mode. “S” and “E” arrows as in Figure 2b.
Pure edge TDs (b = 1/3 <11
Cross-section TEM micrographs of samples A and B are shown in Figure 3a and 3b, respectively. In both cases the seed region has a TD density on the order of ~1011 cm−2, consisting predominantly of pure edge dislocations. In contrast, the LEO regions have a very low density of TDs and an essentially single-crystalline microstructure. Mixed-character dislocations parallel to the basal plane with <11
Figure 3a. Cross section bright-field TEM micrographs of sample A. The left and right panels correspond to = 11
Figure 3b. Cross section bright-field TEM micrographs of sample B. The left and right panels correspond to = 11
As reported earlier in the case of LEO of GaN on GaN/Al2O3 substrates, Reference Sakai, Sunakawa and Usui[4] [2] the c-planes of the LEO regions grown on Si(111) are tilted relative to that of the seed region towards the <11
Figure 4a. .X-ray rocking curves of sample A. The dotted (solid) line corresponds to an ω-scan with the stripe direction parallel (perpendicular) to the diffraction plane.
Figure 4b. X-ray rocking curves of sample B. The dotted (solid) line corresponds to an ω-scan with the stripe direction parallel (perpendicular) to the diffraction plane.
Figure 5 shows room-temperature PL spectra of samples A and B. For both samples the GaN band-edge emission at ~365.8nm and an intense deep level-related band centered at 570nm are observed. It is typical for uncoalesced LEO GaN stripes to exhibit yellow luminescence, which gradually decreases as the growth proceeds towards complete coalescence. Reference Marchand[45] This is most likely related to the large initial growth rate resulting from the enhancement of the Ga precursor supply from the mask regions. Reference Marchand[45] The band-edge emission is associated with free excitons Reference Tsukamoto, Taki, Kuwano, Oki, Shibata, Sawaki, Hiramatsu, Onabe, Hiramatsu, Itaya and Nakano[46] and suggests that the GaN is under tensile stress, as is commonly observed for growth on silicon substrates. Reference Takeuchi, Amano, Hiramatsu, Sawaki and Akasaki[25] Reference Tsukamoto, Taki, Kuwano, Oki, Shibata, Sawaki, Hiramatsu, Onabe, Hiramatsu, Itaya and Nakano[46] In this experiment the emission from both the seed and the LEO regions was measured, therefore the contributions from the different regions could be not separated (see e.g. Ref. Reference Nam, Bremser, Zheleva and Davis[7]).
Figure 5. Room temperature PL spectra of samples A and B. In this experiment the emission from both the LEO and the seed region was collected.
Figure 6a shows a plan-view SEM micrograph of sample A; Figure 6b and 6c show monochromatic CL images of the same region with the monochromator set at the band edge emission of GaN. The dark areas in the seed region are associated with threading dislocations, which have been postulated to act as non-radiative recombination centers. Reference Walker, Hamilton, Diaz and Razeghi[38] The LEO regions show spatially uniform luminescence, which is consistent with a low density of electrically active defects and corroborates previous indications that the threading dislocations have a deleterious effect on the photoluminescence intensity in GaN. Reference Nam, Bremser, Zheleva and Davis[7] Reference Sakai, Sunakawa and Usui[8] Reference Freitas, Nam, Davis, Saparin and Obyden[10] Dark straight lines parallel to the <11
Figure 6a. (a) Plan-view SEM micrograph of sample A; (b) monochromatic (λ=367nm) CL images of the same region; (c) high magnification image of (b).
4. Discussion
Although the LEO of GaN on AlN/Si(111) substrates occurs in a very similar way as on GaN/Al2O3 substrates, significant structural differences can be noted, as discussed below.
4.1 Stripe morphology and microstructure
Figure 1 indicates that the LEO stripes grown on AlN/Si(111) are bound at least in part by inclined sidewalls. For identical pattern and growth conditions, the LEO growth on GaN/Al2O3 substrates would result in vertical sidewalls only (see e.g. Refs Reference Zheleva, Nam, Bremser and Davis[3], Reference Sakai, Sunakawa and Usui[4]). In addition, the sidewall morphology appears to depend on the thickness of the AlN buffer layer (compare Figure 1a and 1b). The two effects are reproducible and are not associated with any change in the controllable growth parameters, such as susceptor temperature, input flow rates, and history of the growth chamber.
Another particularity of the growth on Si(111) is that, unlike the equivalent process on GaN/Al2O3 substrates, which appears to be extremely robust, the LEO stripes grown on AlN/Si(111) undergo a gradual degradation as the growth duration is increased. The affected stripes exhibit regions of rough morphology associated with an erosion of the SiO2 mask and the underlying silicon substrate near the edge of the stripes, as well as a significant loss of growth selectivity. The rough regions are sometimes bounded by cracks in the LEO stripe (cracking is discussed below). The degradation appears to be essentially independent of the thickness of the SiO2 mask, but is strongly dependent on the thickness of the AlN layer. For example, approximately 50% of the surface of the wafer is degraded in structures such as sample A (60 nm-thick AlN buffer) after two hours of growth, whereas wafers with 180 nm-thick buffers can be grown to full coalescence without any signs of degradation.
Although the exact cause of this degradation phenomenon is unknown, its relationship with the AlN buffer thickness suggests that it involves chemical reactions between the precursors and silicon out-diffusing from the Si(111) wafer through the buffer layer. Recent results have shown that silicon doping affects the configuration of surface steps Reference Marchand, Ibbetson, Fini, Wu, Rosner, Keller, Speck, Mishra and DenBaars[47] Reference Chichibu, Azuhata, Sota, Amano and Akasaki[48] during growth of planar GaN films, as well as the morphology of GaN pyramids during selective-area growth and LEO. Reference Keller, Chichibu, Minsky, Hu, Mishra and DenBaars[49] Based on these results, it is reasonable to assume that the differences in facet morphology between samples overgrown using different substrates (GaN/Al2O3 and AlN/Si(111)) and different buffer thicknesses can be explained at least in part by the presence of silicon out-diffusing from the substrate at a rate that decreases as the AlN buffer thickness is increased. The exact mechanisms of this process, for example, whether the silicon actually diffuses through the GaN stripe, the SiO2 mask, or both, are not known at this time but do not affect the conclusion in a fundamental way. Further studies on the effect of intentional Si-doping during LEO will be published elsewhere. We note that Linthicum etal. have recently used a SiC diffusion barrier Reference Marchand, Zhang, Zhao, Golan, Fini, Ibbetson, Keller, DenBaars, Speck, Mishra and Hirayama[35] to prevent degradation of the GaN stripes during growth on silicon. However our results indicate that an AlN layer alone is sufficient to prevent this degradation provided that it is sufficiently thick.
We note that the polarity of GaN on AlN/Si(111) grown by MOCVD has not been determined. Based on the considerable body of literature on the relation between polarity and morphology in GaN, Reference Shen, Tanaka, Iwai and Aoyagi[50] significant morphological differences should be expected between Ga-face LEO GaN and N-face LEO GaN. Our preliminary experiments with the LEO of GaN by MOCVD on thick N-face GaN grown on sapphire by molecular beam epitaxy have shown that the stripe morphology is considerably rougher than for Ga-face LEO GaN, which suggests that LEO GaN on AlN/Si(111) is of Ga-face polarity. Work towards the explicit determination of the polarity is underway.
Finally, other process parameters could be affected by the choice of substrate. For example, the better thermal conductivity of silicon compared to sapphire (124 vs 25 W/mK at 298K) could result in a difference in substrate temperature, which in turn would affect the stripe morphology. Reference Zhou, Perkins, Rehder, Kuech and Babcock[39] Such effects could conceivably explain the difference in facet morphology between GaN/Al2O3 and AlN/Si(111) substrates; however they do not explain the effect of the AlN buffer thickness.
4.2 Thermal mismatch
Although the cracking issue is not well documented in other studies, it is common to observe cracking for bulk GaN on Si grown by MOCVD. Reference Manasevit, Erdmann and Simpson[23] Reference Kapolnek, Wu, Heying, Keller, Keller, Mishra, DenBaars, Nishida and Speck[43] Reference Haffouz, Beaumont and Gibart[51] This is to be expected since the thermal expansion mismatch between GaN (α = 5.6×10−6 K−1 (a axis)) and Si (α = 3.6×10−6 K−1) causes GaN to be under tensile stress after cooldown. Our PL and CL data (Figs. 5 and 6) indeed show that samples A and B are under tensile stress, and similar PL peak energies were also observed for planar GaN grown on Si(111). Reference Takeuchi, Amano, Hiramatsu, Sawaki and Akasaki[25] Reference Kapolnek, Wu, Heying, Keller, Keller, Mishra, DenBaars, Nishida and Speck[43] Reference Tsukamoto, Taki, Kuwano, Oki, Shibata, Sawaki, Hiramatsu, Onabe, Hiramatsu, Itaya and Nakano[46]
Cracking occurs along the three equivalent {1
Although no modeling of the thermal stress in LEO GaN was performed in our studies, our experimental results indicate that cracking depends on most growth and processing parameters, such as the thickness of AlN buffer layer (as discussed above) and of the SiO2 mask, pattern geometry, V/III ratio, and growth temperature. A more systematic study of the effect of processing parameters on the thermal strain in GaN/AlN/Si(111) is underway.
5. Conclusions
We have obtained LEO GaN stripes on Si(111) substrates using a thin AlN buffer layer and characterized their structural and optical properties. A reduction in threading dislocation density of 3-4 orders of magnitude was corroborated by AFM, TEM, and CL measurements. The AlN buffer thickness was shown to affect the stripe morphology and, in turn, the microstructure and the extent of cracking in the LEO stripes. The stripe morphology and growth selectivity gradually degrade as the growth duration is increased; a 180 nm-thick AlN buffer was shown to prevent degradation, such that full coalescence can be achieved on a 40 µm-period pattern. Further studies on the electrical properties of the LEO GaN on Si(111) are underway.
This work was supported by the Office of Naval Research through a contract supervised by Dr. C. Wood and made use of the MRL Central Facilities supported by the NSF under award DMR-9123048. HM acknowledges financial support from NSERC (Canada). PF ackowledges financial support from a National Defense Science and Engineering Graduate Fellowship provided by ONR.
