Microstructural Analysis of Void Formation Due to a NH4Cl Layer for Self-Separation of GaN Thick Films Hyun-Jae Lee,† Jun-Seok Ha,† Takafumi Yao,† Chinkyo Kim,*,‡ Soon-Ku Hong,# Jiho Chang,§ Jae Wook Lee,| and Jeong Yong Lee|
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2877–2880
Center for Interdisciplinary Research, Tohoku UniVersity, Aramaki, Aoba-ku, Sendai, 980-8578, Japan, Department of Physics and Research Institute for Basic Sciences, Kyunghee UniVersity, 1 Hoegi-dong Dongdaemun-gu, Seoul 130-701, Korea, School of Nano Science and Technology, Chungnam National UniVersity, Daejeon, Republic of Korea, Major of Semiconductor Physics, Korea Maritime UniVersity, Pusan, Republic of Korea, and Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea ReceiVed February 16, 2009; ReVised Manuscript ReceiVed March 12, 2009
ABSTRACT: The role of a NH4Cl layer grown between GaN and a sapphire substrate for self-separation of GaN films was investigated in detail. Microstructural analysis of the NH4Cl layer reveals that self-separation of a GaN film was effectively assisted by the sub-micrometer-sized voids formed due to etching of GaN by evaporated NH4Cl. An areal fraction of nonetched GaN in contact with sapphire substrate was found to be linearly dependent on the thickness of NH4Cl layer and to have two different proportionality constants below and above an NH4Cl thickness at which GaN begins to be self-separated. This intriguing behavior was well explained by a simple model that takes account of void merging. Introduction Heteroepitaxial growth of GaN films on foreign substrates due to the lack of a GaN substrate inevitably generates unwanted extended defects. To develop a method to fabricate GaN substrates, many attempts such as laser lift-off (LLO), voidassisted separation (VAS), and facet-controlled epitaxial lateral overgrowth (FACLEO) have been made to effectively separate a sapphire substrate from a thick GaN film.1-3 The LLO technique, however, has a fracturing problem during laser irradiation,4 while the other approaches require complex processes such as deposition of mask materials and growth of GaN template by metal organic chemical vapor deposition, prior to growth of thick GaN by hydride vapor phase epitaxy (HVPE).1,2 A more simple and easy approach is necessary for wide use of freestanding GaN substrates. Recently, self-separation of a GaN film from substrate by an evaporable NH4Cl layer was reported, and application of this technique for fabrication of freestanding substrates was quite promising.5 In this work, detailed microstructural analysis of the role of the NH4Cl layer was carried out to fully elucidate the mechanism of this interesting and straightforward self-separation.
Figure 1. Gibbs free energy as a function of temperature with different NH3/HCl ratios. as low-temperature-grown (LTG) GaN. Subsequently, a thick GaN film was grown at a substrate temperature of 1040 °C. Voided interface structure was formed by NH4Cl during heating for the growth of GaN at 1040 °C (labeled as high-temperature-grown (HTG) GaN). GaN was self-separated from sapphire while cooling after completion of growth.
Results and Discussion Experimental Procedures Growth was carried out in a horizontal HVPE system at atmospheric pressure. An evaporable buffer layer (EBL) consisting of NH4Cl and GaN crystallites was grown at 450 °C for 5 min after nitridation of c-plane (0002) at 1080 °C. NH3, HCl, and total flow were 1.5 slm, 37.5 sccm, and 3 slm, respectively. A carrier gas was N2. An NH4Cl layer in an EBL was formed by in situ reaction of HCl and NH3 gases on a sapphire substrate. Simultaneously GaN crystallites in an EBL were grown by the reaction of GaCl and NH3. After the growth of an EBL, the substrate temperature was increased to 550 °C. At this temperature, GaN film was grown on this EBL, and this layer is labeled * Corresponding author. Phone: +82-2-961-0379. Fax: +82-2-957-8408. E-mail:
[email protected]. † Tohoku University. ‡ Kyunghee University. # Chungnam National University. § Korean Maritime University. | KAIST.
The optimum deposition condition of NH4Cl as a sacrificial layer for self-separation of GaN was theoretically estimated by considering the thermodynamic properties of NH4Cl. A chemical reaction equation of NH4Cl from NH3 and HCl can be written as shown in eq 1, and it should be noted that NH4Cl is in liquid state when the temperature is between 338 and 520 °C.6
HCl(g) + NH3(g) ) NH4CL(l)
(1)
The difference of Gibbs free energies, (∆G), between reactants and product in the chemical reaction equation can be obtained from eq 2, and it is plotted as a function of growth temperature with different NH3/HCl ratios in Figure 1.7 GaCl conversion ratio from HCl flowed over metal Ga at temperature of 850 °C (typical setting for HTG GaN) is almost unity, so the NH3/HCl ratio is virtually identical to the V/III ratio. Since the GaCl or GaCl3 conversion ratio is, however, very small at
10.1021/cg900193k CCC: $40.75 2009 American Chemical Society Published on Web 04/08/2009
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Figure 3. EDS spectra shows clear signal of Cl from NH4Cl for asgrown sample, but after annealing no Cl signal is observed.
Figure 2. Surface morphologies of (a) as-grown and (b) annealed 2.5µm-thick LTG GaN grown on EBL consisting of NH4Cl and GaN crystallites, respectively.
this range of low temperature, it should be noted that the NH3/ HCl ratio and V/III ratio are not equal.8
∆G ) ∆G° + RT ln
[NH4Cl] [NH3][HCl]
(2)
As shown in Figure 1, decomposition temperature of NH4Cl decreases with increasing V/III ratio. Hence, the deposition of NH4Cl can be easily controlled by changing the growth temperature. NH4Cl was easily deposited below 350 °C, but GaN crystallites were not grown at such a low temperature. So, the growth temperature of NH4Cl was set to 450 °C with a V/III ratio of 40 for reliable deposition of both NH4Cl and GaN crystallites. As a preliminary experiment to investigate the evaporation property of EBL, EBL was grown on c-sapphire at 450 °C and subsequently 2.5-µm-thick GaN was grown at 550 °C on top of the EBL. Figure 2 shows the surface morphologies of asgrown and annealed (at 1040 °C) samples with LTG GaN/EBL structure. Many cracks with liquid-like substance were found in the as-grown sample, and liquid-like material, which was originally in the liquid phase at growth temperature, is solidified NH4Cl. After the sample was annealed at 1040 °C for 10 min, as shown in Figure 2b the grown LTG GaN films burst up due to stress caused by vaporized NH4Cl. Thus, simply inserting a NH4Cl layer between GaN and sapphire does not help selfseparation at all, but there should be escape paths for vaporizing NH4Cl within EBL and LTG GaN. In our design of the experiment, the thickness of LTG GaN was set to be below 200 nm in such a way that LTG GaN did not make a continuous film on top of EBL, but many voids were intentionally formed, which served as escape paths for vaporized NH4Cl. These escape paths are of great importance not only in preventing GaN films from bursting up, but also in acting as voids assisting selfseparation of GaN films.
Figure 4. Cross-sectional high-resolution transmission electron microscopy (HRTEM) image showing an EBL. A magnified image shows the area near domain boundary between the embedded domain and the surrounding matrix. SADP of this region reveals a mixture of 6-fold symmetric pattern with additional two peaks in circles. The calculated value of d-spacing for these additional peaks corresponds to 0.248 nm. This agrees with the d-spacing for the surrounding matrix, which implies that the embedded domain is GaN and surrounding matrix is NH4Cl.
Figure 3 shows cross-sectional SEM images, energy dispersive spectroscopy (EDS) spectra, and EDS elemental maps for the as-grown and annealed sample with LTG GaN/ EBL/sapphire structure. Cl signal is observed only for the as-grown sample, and EDS elemental maps shown in insets clearly reveal that NH4Cl layers exist between GaN and sapphire substrate. After annealing at 1040 °C, the Cl peak from NH4Cl disappeared due to evaporation of NH4Cl. From these results, it was confirmed that LTG GaN layers were successfully grown on EBL. Figure 4 shows a high resolution TEM image and a selective area diffraction pattern (SADP) of a 25-nm-thick EBL grown on a nitrided sapphire (0002) substrate. In an EBL, a domain marked by a white dotted line is embedded in the surrounding matrix. SADP from the area marked by a red box, which contains both embedded domain and surrounding matrix, shows a diffraction pattern with a 6-fold symmetry and additional peaks
NH4Cl Layer for Self-Separation of GaN Films
Figure 5. Cross-sectional SEM images of HTG GaN/LTG GaN/EBL structures with different EBL thicknesses of (a) 10, (b) 22, and (c) 50 nm. White-dotted lines represent the interface between LTG GaN and HTG GaN for each sample. Note that NH4Cl etched in both GaN in an EBL and LTG GaN. Panel (d) shows a TEM image of an interface between an EBL and sapphire, where sapphire is partly etched out. Panels (e) and (f) are plan-view SEM images of the interface between an EBL and sapphire after self-separation with an EBL thickness of 30 and 50 nm, respectively. Remaining GaN grains in an EBL and bare sapphire surface on which NH4Cl nucleated are distinctively seen. The areal fraction of remaining GaN domains is smaller for the sample with a thicker EBL.
circled in white. A d-spacing calculated from the additional peaks is 0.248 nm, which corresponds to a d-spacing in surrounding matrix as shown in the figure and also well agrees with that of NH4Cl {100} planes.9 Thus, 6-fold symmetric diffraction pattern is due to GaN, which is an embedded domain, and this confirms the coexistence of GaN and NH4Cl in EBL. This TEM image clearly shows that NH4Cl partly covers sapphire substrate, which can effectively form voids at the interface in the subsequent annealing process. Figure 5 shows cross-sectional SEM images of HTG GaN/LTG GaN/EBL structures with different EBL thicknesses of (a) 10, (b) 22, and (c) 50 nm. White-dotted lines show the interface between LTG and HTG GaN. There are a few notable features in these images. First, the thickness of EBL is less than 50 nm for all these three samples, but the volume of voids extends through an LTG GaN layer. This is because NH4Cl in an EBL not only evaporates at high temperature, but also etches out surrounding GaN because decomposed NH4Cl reproduces HCl. Even some part of a sapphire substrate was found to be etched out due to evaporable NH4Cl as shown in Figure 5d. Second, the lateral size of void increased with EBL thickness since GaN domains were partially etched laterally. This implies that the areal fraction of GaN domains in contact with a sapphire substrate decreases with increasing thickness of an EBL. As shown in Figure 5e,f, plan-view SEM images of sapphire substrates after self-separation of GaN films with EBL thickness of 30 and 50 nm actually reveal that the areal fraction decreased with EBL thickness. Since the areal fraction of GaN in contact with sapphire determines how easily the GaN film can be selfseparated from the sapphire substrate, the EBL thickness dependence of areal fraction of GaN is of great importance in EBLassisted self-separation mechanism. Third, voids began to merge above a certain thickness. Merging of voids needs to be carefully
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Figure 6. Panels (a-c) and (d-f) show schematic diagrams for the cross-sectional view of LTG/EBL structure with increasing thickness of an EBL before and after annealing at 1040 °C, respectively. Orange-, green-, blue-colored region represent NH4 in an EBL, GaN in an EBL, LTG GaN, respectively. Red segments in (d-f) are cross-sectional lateral lengths of an area of GaN domains in contact with a sapphire substrate, seen from the perpendicular direction to the page. Panels (g-i) show schematic diagrams for the plan-view of corresponding structures on the left.
Figure 7. Fraction of contact area of GaN on a sapphire substrate, f, as a function of EBL thickness. Note that f has different slopes in two regimes. D ) 10 µm, d ) 10 nm, No ) 1.27 × 106 (regime I), 9.55 × 105 (regime II) were used for fitting.
analyzed because the contact area under merged voids do not contribute to uphold the GaN layer above. In our case, GaN was self-separated when the EBL thickness was above 30 nm. The interesting features mentioned above are schematically drawn in Figure 6. Figure 6a-c and d-f shows schematic diagrams for LTG GaN/EBL before and after annealing at 1040 °C with increasing EBL thicknesses, respectively. Red segments in d-f are cross-sectional lateral lengths of an area of GaN domains in contact with a sapphire substrate seen from the perpendicular direction to the page. Corresponding planviews of interfacial region between an EBL and sapphire are shown in g-i. Note that the contact area under merged voids do not contribute to uphold GaN layer as explained above (see Figure 6i). Figure 7 shows an areal fraction of GaN domains in contact N dj/D with a sapphire substrate, which is measured by f ) ∑j)1 as shown in Figure 6d. f linearly depends on the thickness of EBL, but slopes are observed to change at an EBL thickness of 30 nm at which GaN begins to be self-separated. To explain the different slopes in two regimes, we have developed a simple model as follows. We assume there are N cylindrical GaN domains in an EBL with a diameter, d, on a square-shape surface
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with a side, D (see Figure 6g). The areal fraction, farea, of GaN on the substrate is given by farea ) Nπd2/4D2. Since the contact area decreases due to etching with increasing EBL thickness, N is considered to be linearly dependent on EBL thickness and is given by N ) No - Rt where No and R are an initial number of cylinders covering contact areas and a proportionality factor of etching with a thickness, respectively. Thus, the thickness dependence of an areal fraction can be written by farea ) (No - tR)πd2/4D2, and fitting results are shown in Figure 7. A reduced slope implies that the reduction rate of an areal fraction decreased, but there is no reason for GaN etching rate at the interfacial region to decrease at thicker EBL. It is more likely due to the fact that an actual number of available domains to be etched decreases when voids merge because the domains under merged voids are not considered to uphold upper layers and they are not counted in the summation of dj (see Figure 6i). Even though the etching speed is the same, this void merging effect makes the reduction rate of an areal fraction appear to decrease above 30 nm. The thickness of 30 nm, at which a slope changes, corresponds to the point when voids began to merge with one another. Note that we interchangeably used f and farea in the discussion above. In our model, the estimation of f by measuring crosssectional contact lengths can be shown to be equivalent to farea, which is the literal definition of an areal fraction. One of the great advantages to estimate an areal fraction by f is that the areal fraction of GaN in contact with sapphire can be estimated even if GaN is not separated when the EBL thickness is below 30 nm. Conclusions In conclusion, microstructural analysis of void formation due to evaporable NH4Cl layer was carried out. NH4Cl in an EBL
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was found to form many sub-micrometer voids at the interfacial region of GaN and sapphire by etching out surrounding GaN matrix. With increasing thickness of an EBL, the areal fraction of GaN decreased. The reduction rate of areal fraction was linearly dependent, but it decreased due to merged voids above an EBL thickness of 30 nm, which also corresponds to the thickness above which GaN is self-separated from sapphire. Acknowledgment. H.J.L. thanks the Japan Society for the Promotion of Science (JSPS) for providing financial support in the form of a JSPS Research Fellowship for Young Scientists. C.K. was supported in part by the Seoul Research and Business Development Program-Grant No. 10583.
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