Control of Crystal Morphologies and Interface Structures of AlN Grown

Appropriately designing the growth procedure can realize self-separation of AlN films from sapphire and spontaneous, nanometer-scale lateral overgrowt...
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Control of Crystal Morphologies and Interface Structures of AlN Grown on Sapphire by Elementary Source Vapor Phase Epitaxy PeiTsen Wu,† Katsuhiro Kishimoto, Mitsuru Funato, and Yoichi Kawakami* Department of Electronic Science and Engineering, Kyoto University, Kyoto 615-8510, Japan ABSTRACT: Elementary source vapor phase epitaxy (EVPE) is an environmentally friendly method for producing aluminum nitride (AlN) crystals from Al metal and N2 gas. Here, we demonstrate that the morphology of AlN grown by EVPE is predominantly affected by the molar flow ratio of the source materials (V/III ratio): high V/III ratios result in AlN film growth, whereas low ratios produce whiskers. These growth characteristics lead to self-separation of AlN films from sapphire, because the local V/III ratio at the AlN/ sapphire interface is generally lower than the input V/III ratio due to sapphire decomposition, which generates mechanically weak whiskers at the interface. To avoid excessive separation, which causes rolling of the separated AlN film, we demonstrate that the pretreatment of sapphire with Al and N2, as well as addition of Al2O3 to the Al source, has beneficial effects on the film growth. This controlled interface formation improves the crystalline quality through spontaneous, nanometer-scale lateral overgrowth.

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restricted to growth of AlN films, but may also be used to produce AlN powder and whisker crystals. However, except for the AlN powder synthesis, the demarcation of growth conditions between AlN films and whisker crystals has yet to be clarified. In this paper, we demonstrate that Al-rich conditions lead to whisker growth, whereas Al poor regimes promote film formation. Related to this, AlN whiskers are often formed at the interface between the AlN film and sapphire substrate: this arises due to the decomposition of sapphire, which proceeds via reduction by the Al source vapor. Consequently, the interfacial region becomes Al-rich, which, in turn, promotes growth of the AlN whiskers. In this study, by controlling the interfacial AlN whisker formation, we achieve both self-separation of AlN from sapphire and high-quality AlN films on sapphire.

INTRODUCTION AlN has the largest direct bandgap among the representative III−nitride compound semiconductors (about 6.0 eV at room temperature (RT)) and is considered a promising material for ultraviolet (UV) light-emitters.1−9 To date, however, the film growth techniques used for the fabrication of AlN-based UV emitters are still in development. One of the greatest challenges to overcome is the dearth of suitable bulk substrates, which consequently restricts AlN film synthesis to foreign substrates such as sapphire and silicon carbide (SiC), resulting in heteroepitaxial growth. Unfortunately, large mismatches between the lattice parameters of AlN and these substrates lead to the formation of high densities of dislocations in the films, which ultimately limit the device performance: for example, the external quantum efficiency of AlGaN-based UV light-emitting diodes (LEDs) emitting below 285 nm is at most 15%.1−3,5,9 Fabrication of devices on AlN bulk crystals would circumvent the problems related to the lattice mismatch, thereby greatly improving the performance of the resulting UVLEDs. The most commonly reported methods used for growth of AlN bulk crystals are hydride vapor phase epitaxy (HVPE)10−13 and sublimation growth.14−17 However, these technologies have some critical restrictions. For instance, HVPE usually uses hazardous hydrochloric acid in order to activate the AlN synthesis reactions, while the sublimation growth method needs extremely high temperatures of around 2000 °C and is therefore a costly route. As such, finding a new, economical, and safe route for AlN bulk growth is a pressing issue. Recently, we have demonstrated a new approach for AlN growth, elementary source vapor phase epitaxy (EVPE), which uses just elemental Al and N2 and can achieve growth rates of ∼18 μm/h at 1550 °C.18 In addition, this method is not © 2016 American Chemical Society

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EXPERIMENTAL SECTION

AlN crystals were prepared by EVPE on sapphire (0001) substrates, using Al metal and N2 gas as source precursors.18 The EVPE apparatus consists of a horizontal reactor with two independently controllable temperature zones, one for the Al source and the other for AlN growth. The source zone is divided into upper and lower channels. Argon (Ar) was introduced into the lower channel to act as a carrier gas, transferring the heat-generated Al vapor to the growth zone. N2, the main reaction gas, was supplied separately into the upper channel. These were subsequently mixed in the growth zone. In this study, the molar flow ratio between N2 gas and Al vapor is referred to as the V/ III ratio, which is typically much greater than unity due to less reactivity of N2 than Al. Received: June 30, 2016 Revised: October 7, 2016 Published: October 24, 2016 6337

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The source zone temperature was set to 1400 °C to provide sufficient quantities of Al vapor, as described in ref 18. The growth pressure was 10 kPa. Unless stated otherwise, N2 and Ar gases were simultaneously introduced into the growth reactor when the temperatures of the source and growth zones reached the target temperatures. The obtained AlN crystals were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and cathodoluminescence (CL) spectroscopy.

different V/III ratios, all for 1 h, and the results were summarized in Figure 2. The plot shows a clear U-shaped

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RESULTS AND DISCUSSION Typical Crystal Morphologies. Two types of AlN crystal morphologies, whisker and film, were obtained under different growth conditions. Figure 1a,b shows SEM images of

Figure 2. Plot of AlN crystal morphologies as functions of growth temperatures and V/III ratios.

boundary between the two AlN crystal morphologies. On the whole, AlN films are obtained under a higher V/III ratio, whereas whisker formation is associated with lower ratios. This trend is consistent with GaN metalorganic vapor phase epitaxy:19 a low V/III ratio promotes nanowire formation. The mechanism has yet to be fully understood, but it is presumed that a low V/III ratio enhances the surface migration of group III adatoms, creating (group III) metal islands which may act as a catalyst for wire formation.20 The U-shaped boundary can be explained as follows: as the growth temperature increases, the decomposition of N2 is promoted, which increases the effective N concentration that can contribute to AlN growth. This accounts for why the input V/III ratio for film growth at 1550 °C can be lower than that at 1500 °C. However, for deposition of AlN films at 1600 °C, a higher V/III ratio is required which is contrary to what one would expect. We consider that the sapphire substrates undergo a reduction reaction in the presence of Al vapor as follows: 2Al(g) + Al2O3(s) = 2Al2O(g) + 1/2O2(g). The rate of reduction is accelerated as higher temperatures, and, consequently, the effective V/III ratio is reduced at the higher temperatures due to the increase of Al and Al-based species gases in the vapor. In short, the growth morphology is determined by the competition between the decomposition reactions of nitrogen gas and sapphire. In addition, we also found that AlN whisker and film crystals can coexist when grown around the U-shaped boundary. This result yields an interesting self-separation phenomenon, as discussed in the next section. Self-Separation of AlN from Sapphire. Figure 3 displays SEM images of (a) ∼200 nm thick and (b) 6 μm thick AlN films grown on sapphire substrates under conditions around the U-shape boundary. In both SEM images, the formation of whiskers can clearly be seen. It is noteworthy that the 6 μm AlN film spontaneously separates from the sapphire substrate after growth and is crack-free, as shown in Figure 3c. Generally, sapphire is an extremely hard material and is difficult to separate from epilayers. However, Figure 2 suggests the sapphire decomposition at around 1550 °C. Because Al vapor is sufficiently reducing to react with metal oxides such as sapphire (Al2O3), we consider that the reducing reaction as well as thermal decomposition of sapphire plays key roles in the observed self-separation phenomenon. To confirm the former

Figure 1. SEM micrograph and the corresponding XRD 2θ−ω profile for (a) and (c) AlN whisker morphology, and (b) and (d) an AlN film.

representative samples of AlN whiskers and films, respectively, and Figure 1c,d presents their respective XRD 2θ−ω profiles. The AlN whisker in Figure 1 was grown at 1300 °C with a V/ III ratio of 450, while the AlN film was grown at 1550 °C with a V/III ratio of 2700. The effects of the growth temperature and V/III ratio on the growth morphologies are discussed below. The dimensions of the whisker crystals were estimated from the SEM images, and it was determined that the diameters were on the order of nanometers, whereas the lengths were typically on the order of several 100 μm. From the XRD profile in Figure 1c, AlN (0002) and (101̅1) reflection peaks are observed at 36.02° and 37.90°, respectively, indicating preferred c-oriented growth (the peak located at 41.69° is attributed to the sapphire (0006) orientation). Although the result of XRD seemingly conflicts with the SEM image (Figure 1a) that suggests the random orientation of the whiskers, it is understandable by considering that a highly oriented region is formed in the vicinity of the interface SEM cannot visualize. For the AlN film (Figure 1b,d), the thickness was determined from SEM to be 3.4 μm, which corresponds to a growth rate of 3.4 μm/h. Only the AlN (0002) reflection peak is observed in the XRD data and, additionally, an XRD ϕ scan of an asymmetric plane shows 6-fold symmetry, indicating that the grown film is c-oriented, single phase AlN. Key Factors Determining Crystal Morphologies. Here, we systematically investigate how the growth temperature and V/III ratio affect the growth morphologies. Many AlN crystals were prepared at different growth temperatures and using 6338

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Figure 4. Optical microscope images of thermally treated sapphire substrates (a) without and (b) with Al vapor at 1400 °C. (c) Schematic diagram of the mechanism of AlN self-separation from the sapphire substrate.

formation of cracks and removes the requirement for ex-situ processing to separate the crystal from the substrate. In fact, an AlN film with a large area of ∼5 × 5 mm2 was separated from sapphire with this phenomenon, as demonstrated in Figure 5.

Figure 3. SEM micrographs of AlN films grown by EVPE. (a) Bird’seye view of a thin AlN film (∼200 nm) showing the self-separation of the film from the substrate which occurred during the initial stage of crystal growth, (b) cross-sectional and (c) bird’s-eye views of a 6 μm thick AlN film self-separated from the sapphire substrate. Figure 5. Peeled-off AlN film from sapphire substrate. Note the rolling of the AlN foil.

hypothesis, two sapphire substrates were heated at 1400 °C for 1 h under an Ar atmosphere, one in the presence of Al vapor and one without. Figure 4 shows photographs of the two sapphire samples after heating (a) in Ar only and (b) with Al vapor. Comparison between Figure 4a,b indicates that Al vapor promotes the sapphire decomposition. As such, the reducing reaction (and thermal decomposition as well) can produce additional Al-containing gases from the sapphire surface. The excess Al vapor decreases the effective V/III ratio near the interface, coinciding with where whisker formation is observed. As the growth proceeds, the contribution to the deposition vapor from sapphire decomposition gradually declines. Consequently, the V/III ratio returns to the value determined by the flow rates of the input gas, and as a result, the AlN film morphology is obtained. Because the AlN whiskers are mechanically weak, the AlN film can be easily separated from the sapphire around the interfacial region. In addition, since the interfacial whiskers effectively mitigate the influence of in-plane tensile strain, there are no cracks on the surface of the 6 μm AlN film. The proposed mechanism of this self-separation is illustrated in Figure 4c. This interesting phenomenon is a beneficial property for the growth of bulk crystals as it can suppress the

However, Figure 5 also suggests a negative impact that excessive self-separation particularly in the early stage of the growth degrades the crystalline quality due to rolling of AlN foils. Therefore, a way for controlling the degree of selfseparation has to be established. Control of Interface Structures. In order to suppress excessive self-separation, we propose two methods. One is a pretreatment of sapphire substrates with Al and N2, and the other is adding alumina (Al2O3) to the source. The growth temperature and pressure were 1550 °C and 10 kPa, respectively, and the V/III ratio was 2200. In the previous experiments, N2 and Ar gases were introduced into the growth reactor when the temperatures of the source and growth zones reached the target temperatures. In the experiments outlined in this section, this procedure was modified such that the gases were loaded into the reactor at the onset of the temperature increase: we refer to this process as AlN pretreatment. The AlN pretreatment has two possible effects: the first is the deposition of a thin AlN layer at lower temperatures, which may act as a low-temperature buffer layer 6339

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Figure 7 compares (0002) XRD 2θ−ω profiles of separated AlN and those with the AlN pretreatment and additional Al2O3.

similar to those identified in metalorganic vapor phase epitaxy of GaN on sapphire; the second is sapphire nitridation, which may occur as the Al vapor pressure is drastically reduced at low temperatures, and under these conditions, only N2 reacts with the substrateto illustrate, the Al vapor pressure at 1100 °C is as low as 1.9 × 10−6 atm, whereas it rises to 2.2 × 10−4 atm at 1400 °C. Figure 6a shows a cross-sectional SEM image of the interface between the AlN epilayer and sapphire substrate for a film

Figure 7. AlN (0002) XRD 2θ−ω profiles of a separated AlN layer and interface-controlled AlN layers. The separated AlN layer is nearly unstrained, while the pretreated AlN layers are compressively strained.

The film thickness is in a range of 10−20 μm. The separated AlN due to whisker formation is nearly unstrained, while the AlN-pretreated AlN is compressively strained. The degree of strain seems unaffected by the presence of Al2O3 in the source. Their peak positions correspond to c lattice parameters of 0.4989−0.4990 nm, which are well accounted for by considering that AlN films are completely relaxed at growth temperatures and accommodate the thermal stress at RT due to the difference in the thermal expansion coefficients between AlN and sapphire. The crystalline qualities are initially evaluated using XRD ω scans for the (0002) symmetric and (11̅02) asymmetric reflections. Figure 8 shows the results for the same AlN films

Figure 6. SEM cross-sectional images of the AlN/sapphire interfaces grown (a) with sapphire pretreatment and (b) with pretreatment and with Al2O3 added to the source.

grown with the AlN pretreatment step. There are small voids present at the interface, but AlN is not completely separated from the sapphire. That is, the decomposition of sapphire was effectively suppressed by the substrate pretreatment. One possible reason is that the low-temperature AlN layer protects the sapphire surface. Another explanation is the formation of an interfacial layer of AlON21−24 during the nitridation process: once full coverage of sapphire with AlON is achieved, the direct redox reactions between sapphire and Al vapor do not occur. Although redox reactions between AlON and Al vapor may happen and extract O from AlON, such reactions do not affect the local effective V/III ratio to a significant degree: in this case, only a small quantity of Al species byproducts would be generated due to the lower oxygen content of AlON as compared to Al2O3. Thus, AlN films tend to grow from the very beginning of the growth phase. Next, another technique to prevent excessive self-separation is introduced. We added Al2O3 (∼0.2 g) to the Al source (∼1.0 g) with the aim of protecting the sapphire substrate. Al2O3 was heated together with Al in the source zone to supply the Albased vapors such as Al2O via the reducing reaction of Al2O3 with Al. As the reduction of sapphire with Al proceeds as follows, 2Al(g) + Al2O3(s) = 2Al2O(g) + 1/2O2(g), the presence of additional Al2O supplied from the source zone to growth zone is expected to reduce the rate of sapphire decomposition. Figure 6b shows a cross-sectional SEM image of an AlN film grown after AlN pretreatment of the substrate and addition of Al2O3 to the source. Comparing Figure 6, panels a and b suggest that the addition of Al2O3 to the source reduces the size of the voids, although it does not totally eliminate them; therefore, Al2O3 addition provides some minor benefits to AlN growth. Structural Properties. Unlike self-separated AlN, which shows large hillocks on the surface (Figure 3c), the interfacecontrolled AlN layers on sapphire have V-pits on the surface. Additional Al2O3 in the source does not affect that feature. The reason for this difference is unclear at present, and further investigation is necessary.

Figure 8. XRD ω scans of the (a) (0002) and (b) (11̅02) planes. Selfseparated and AlN-pretreated AlN layers are compared. The numbers in italics indicate FWHMs (arcsec).

as in Figure 7. The full widths at half-maximum (FWHMs) are much wider for the self-separated AlN. There are two possible reasons for this observation. One is larger dislocation densities most likely due to misaligned whisker (as suggested in Figure 1(c)), and the other is the rolling of the separated AlN at the initial growth stage (Figure 5). In any case, the interface control 6340

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is a key to obtain high quality AlN. For the AlN-pretreated AlN, the FWHMs of the (0002) symmetric and (11̅02) asymmetric reflections are 298 and 293 arcsec, respectively. Adding Al2O3 into the source slightly reduces them to 216 and 270 arcsec. From these values, the screw and edge dislocation densities (N) can be estimated using the equation N = fwhm2/4.35|b|2, where b is the Burgers vector of the corresponding dislocation.18,25 Those are 1.9 × 108 and 4.6 × 108 /cm2, respectively, for the AlN-pretreated AlN, and 1.0 × 108 and 5.8 × 108 /cm2, respectively, for the AlN-pretreated AlN with additional Al2O3. [Note that the FWHMs for the twist components (i.e., for edge dislocations) are determined with (fwhm(11̅02))2 = (fwhm(0002) cos χ)2 + (fwhmtwist sin χ)2, where χ is the angle between the (0001) and (11̅02) planes. Those are 287 and 322 arcsec for AlN without and with additional Al2O3, respectively.] The total numbers of dislocations are hardly affected by the additional Al2O3 source. To further provide some insights into the structural properties, transmission electron microscopy (TEM) was performed. The TEM specimen was prepared by Ar+ milling or focused ion beam. The acceleration voltage for TEM was 200 keV. The sample was a 5-μm-thick, AlN-pretreated AlN with additional Al2O3. Initially, a (112̅0) cross section was observed. The invisibility of some dislocations in dark field TEM images (g·b = 0 criterion) using g = [110̅ 0] and [0002] indicated that the ratio of dislocation densities is pure screw/ pure edge/mixed = 1:4:3. Here, g is the reciprocal vector of the plane of interest, and b is the Burgers vector. This ratio agrees reasonably well with that estimated by XRD, which is screw/ edge = 1:5, as mentioned above. Figure 9 is a plan view bright

Figure 10. Cross-sectional STEM-ADF image of an AlN/sapphire interface viewed along the [112̅0] zone axis. Threading dislocations form preferentially above voids.

dislocations are preferentially generated on top of voids, which suggests that selective area growth occurs. As discussed in the previous section, the AlN pretreatment forms low temperature AlN or Al(O)N on the sapphire surface, but the presence of the interfacial voids suggests that the coverage is not 100%. Therefore, AlN should initially nucleate on partially deposited AlN or Al(O)N (but not on voids) and then grow laterally. The coalescence of the laterally grown AlN nuclei leaves voids at the interface and creates threading dislocations above them. This situation resembles air-bridged lateral epitaxial growth (ABLEG) of GaN, where coalescence of GaN occurs above the voids.26 Different from ABLEG, our growth process spontaneously forms voids on the nanometer scale, but similar to ABLED, it can contribute to the improvement of the crystalline quality. In fact, the best sample in this study has screw and edge dislocation densities of ∼1 × 108 and ∼5 × 108 /cm2, respectively, which are one of the lowest among the reported values for AlN on sapphire, as discussed above.18 Growth Chracteristics. Let us first discuss the consumption efficiency of the source materials. The maximum growth rate of EVPE is 18 μm/h,18 when the source zone temperature is 1400 °C, and the corresponding Al vapor pressure is 2.2 × 10−4 atm. In HVPE, a typical growth rate is 25 μm/h with an AlCl3 partial pressure of 4.0 × 10−4 atm.13 As for the sublimation method, a growth rate of ∼500 μm/h has been achieved at above 2000 °C.15 Assuming that source AlN is decomposed into liquid Al and gaseous N2 at 2000 °C, and the Al vapor dominates the growth, the estimated Al partial pressure for the growth is 6.4 × 10−2 atm. Although direct comparison of the source consumption efficiencies among the growth methods is difficult without knowing the flow rates of the carrier gases, the ratio of (growth rate)/(source pressure) implies comparably better efficiencies in EVPE and HVPE than in the sublimation growth. In metalorganic vapor phase epitaxy, a premature reaction between Al and N sources reduces the AlN growth rate particularly when the growth pressure is high. In EVPE, on the other hand, the growth rate tends to be faster along the gas stream, suggesting the negligible contribution of premature reactions. One of the reasons may be a low growth pressure of 10 kPa. In a future study, the Al flow rate has to be increased to increase the growth rate, when such a reaction would play a role in determining the EVPE growth characteristics.

Figure 9. Plan view STEM bright field image of a 5-μm-thick, AlNpretreated AlN with additional Al2O3. Dark contrasts are due to dislocations.

field image acquired with the scanning TEM (STEM) mode. Dislocations are observable as dark spots or lines. The dark contrast clarifies a total dislocation density of 4 × 108 /cm2, which supports the estimate with XRD. The ratio of dislocation densities revealed by the cross sectional observation suggests the densities of pure screw, pure edge, and mixed dislocations are 5 × 107, 2 × 108, and 1.5 × 108 /cm2, respectively. Those are one of the lowest among the reported values for AlN grown on sapphire.18 Spontaneous, Nano Selective Area Growth. Figure 10 shows a cross-sectional STEM annular dark-field (ADF) image of the same sample as in Figure 9. It is interesting to note that

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CONCLUSIONS We investigated the morphologies of AlN grown by EVPE at various temperatures and V/III ratios. The V/III ratio 6341

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(16) Epelbaum, B. M.; Bickermann, M.; Winnacker, A. J. Cryst. Growth 2005, 275, e479−e484. (17) Epelbaum, B. M.; Bickermann, M.; Nagata, S.; Heimann, P.; Filip, O.; Winnacker, A. J. Cryst. Growth 2007, 305, 317−325. (18) Wu, P. T.; Funato, M.; Kawakami, Y. Sci. Rep. 2015, 5, 17405. (19) Choi, K.; Arita, M.; Arakawa, Y. J. Cryst. Growth 2012, 357, 58− 61. (20) Colombo, C.; Spirkoska, D.; Frimmer, M.; Abstreiter, G.; Fontcuberta i Morral, A. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 155326. (21) Moustakas, T. D.; Molnar, R. J.; Lei, T.; Menon, G.; Eddy, C. R. MRS Online Proc. Libr. 1992, 242, 427−432. (22) Fukuyama, H.; Nakao, W.; Susa, M.; Nagata, K. J. Am. Ceram. Soc. 1999, 82, 1381−1387. (23) Losurdo, M.; Capezzuto, P.; Bruno, G. J. Appl. Phys. 2000, 88, 2138−2145. (24) Nakao, W.; Fukuyama, H. J. Cryst. Growth 2003, 259, 302−308. (25) Chierchia, R.; Böttcher, T.; Heinke, H.; Einfeldt, S.; Figge, S.; Hommel, D. J. Appl. Phys. 2003, 93, 8918−8925. (26) Kidoguchi, I.; Ishibashi, A.; Sugahara, G.; Tsujimura, A.; Ban, Y. Jpn. J. Appl. Phys. 2000, 39, L453−L456.

dominantly affects the morphologies: high V/III ratios produce AlN films, whereas low ratios are found to promote whisker formation. By increasing the growth temperature from 1500 to 1550 °C, decomposition of the N2 gas increases the effective V/ III ratio, such that the input V/III ratio necessary for the film growth can be lowered. However, further increases in the temperature cause sapphire decomposition to yield Al2O, which lowers the effective V/III ratio and requires higher input V/III for the film growth. The whisker growth under a low V/III ratio causes self-separation of AlN from sapphire, because the local V/III ratio at the AlN/sapphire interface is typically lower than the input V/III ratio due to sapphire decomposition. This is a quite attractive characteristic for AlN bulk growth; however, excess separation during the initial growth stages causes rolling of AlN foils. We demonstrate that two methods, AlNpretreatment of sapphire and additional Al2O3, are effective to control the degree of self-separation. The controlled interface formation contributes to film quality improvement through spontaneous selective area growth.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

(P.T.W.) Electronic Devices Laboratory, Toshiba Research and Development Center, Kawasaki, Kanagawa 212-8582, Japan. Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.cgd.6b00979 Cryst. Growth Des. 2016, 16, 6337−6342