Abstract
1. Introduction
An AlN single crystal has been considered as an optimum substrate for a III-nitride epitaxial layer due to its superior physical and chemical properties. And the application of AlN covers from deep ultraviolet detectors, deep ultraviolet LEDs to ultraviolet lasers as well as microwave high power devices[
2. Experimental process
An AlN bulk crystal growth experiment was carried out by PVT using a home-made resistance furnace. The resistance furnace, with sufficiently optimized thermal fields, was composed of tungsten heaters and multilayer tungsten shields. The AlN powder source used for vapor phase deposition was pre-sintered to eliminate any excess Al and oxygen components. Before the PVT experiment, GDMS measurement was conducted to ensure that the content of oxygen was below 10 ppm. Then the PVT process was carried out as follows: an AlN boule grown on the SiC seed, obtained according to our previous reported work[
An optical microscope and differential interference microscope (DICM) as well as a scanning electron microscope (SEM) were used to evaluate the morphology of the as-grown AlN boules. The crystalline quality was characterized by a high-resolution X-ray diffractometer (HRXRD, JV-DX) for both AlN boules and AlxGa1–xN epilayer. Raman spectroscopy (LabRAM HR800) was performed at room temperature using a 488 nm laser as excitation source, and the scanning wavelength was 200–1000 nm. High-resolution rocking curves were obtained by using a triple axis Bruker X-ray diffractometer equipped with a light source of Cu Kα1 X-ray. An AlN wafer was placed in a KOH/NaOH melt for 3 min and the dislocation density was counted by SEM. The quality of the homoepitaxially grown AlxGa1–xN epilayer was estimated by HRXRD, X-ray reciprocal space mapping (RSM) and an atomic force microscope (AFM).
3. Results and discussion
3.1. Surface morphology
With the purpose of revealing the relationship between growth mode and growth condition, the morphology of the as-prepared AlN crystal, grown under a relative low seed surface temperature of 2200 °C, was depicted in Fig. 1(a). The epitaxial crystals consist of a 100–500 μm pyramidal structure along the c direction on the main region and 1–5 mm irregular grains (mainly m plane) at the seed edge. According to reported references and our prior studies[
Figure 1.(Color online) Surface topography of the AlN crystal grown at a seed surface temperature of 2200 °C. (a) Photograph. (b) Differential interference microscope. (c) SEM image.
where
For crystals grown on the AlN seed, as the amplified DICM image presented in Fig. 1(b), the morphology of the as-grown AlN crystal within the marked region is composed of a “primary” hexagonal pyramid with numerous surrounding “secondary” pyramids. We can also find that the primary pyramid grows perpendicularly on the initial Al-polar plane, while the secondary pyramids show a tilted angle of ~30° to the primary one, indicating a dominated growth towards c-axis with 3D islands. It is known that AlN gaseous segments are firstly driven to the seed interface by the axial temperature gradient ΔT, and then they nucleate at the seed surface. During this process, the quantities and orientations of nucleation are concerned with the nucleated energy, specifically the supersaturation in the seed interface. If the gas partial pressure keeps constant and axial temperature gradient ΔT is larger, the supersaturation required for crystal growth is more easily realized. Thereby, numerous AlN hexagonal pyramids are formed. In the sequent growth, the AlN gaseous are more easily nucleated at the top of pyramids formed as hexagon islands, as the SEM image shown in Fig. 1(c). However, the transverse expanded rate of pyramid islands are slow due to the low seed temperature and radial temperature gradient compared with an axial gradient, causing the tendency of axial growth. Some islands may merge into a bigger island. Some islands may develop lonely. So the growth surface still leaves lots of alone AlN islands after long time growth. So it can be inferred that the growth mode under seed surface temperature of 2200 °C[
After increasing the surface temperature of AlN seed from 2200 to 2240 °C, as shown in Fig. 2(a), a smooth topography with intrinsic plane of the {
Figure 2.(Color online) Surface topography of the AlN crystal grown at an increasing seed surface temperature of 2240 °C. (a) Photograph of the AlN crystal. (b) Differential interference microscope image in the center of the crystal. (c) Differential interference microscope image of terraces at the rim. (d) The AlN ingot after 20 h growth.
Then the as-prepared AlN was used as the seed to go another run (20 h) with furnace parameters unchanged. The lateral grown AlN crystal is presented in Fig. 2(d). A smooth surface with a {
A further optimized surface temperature from 2240 to 2260 °C was then used to grow the AlN crystal. The morphology of the AlN crystal is given in Fig. 3(a). The as-grown AlN surface with well-developed intrinsic {
Figure 3.(Color online) The AlN crystal grown at optimized growth temperature of 2260 °C. Photograph of (a) the AlN crystal, (b) the AlN crystal grown after 24 h, and (c) the AlN ingot grown after 72 h.
3.2. Crystallization quality
The crystallization quality of the AlN wafer was characterized by means of measurement methods, as shown in Fig. 4. Before carrying out the characterizations, the AlN boule in Fig. 3(c) was sliced and polished into wafers. In Fig. 4(a), nine individual points were measured using Raman spectrum to evaluate the crystalline quality and uniformity of the obtained AlN wafer. Three peaks with the same wavenumbers of 247, 657 and 889 cm–1 can be seen in Raman results, corresponding to the phonon modes of E2 (low), E2 (high) and A1 (LO), respectively. These values are inconsistent with the modes of c-axis orientation AlN crystal[
Figure 4.(Color online) Crystallization properties of the AlN wafer. (a) 9 point Raman spectrums of the AlN crystal. (b) Mapping of 657 cm–1 Raman shifts. (c) HRXRD pattern of the AlN crystal with
An AlN polished wafer with the size of 10 × 10 mm2 and a thickness of 500 μm shown in Fig. 5(a) was used as the substrate for further epitaxial growth. The surface of the polished AlN wafer is smooth and the surface roughness (Ra) is 0.097 nm, as shown in Fig. 5(b). An AlxGa1–xN epitaxial layer was grown on the AlN substrate by the MOCVD method and the structure of epitaxial layers is shown in Fig. 5(c). Before depositing the AlxGa1–xN layer, a 500 nm AlN epitaxial layer was deposited on the AlN substrate as a buffer layer. The AFM image with a scan range of 10 × 10 μm2 of the as-fabricated AlxGa1–xN epitaxial layer is shown in Fig. 5(d). The smooth surface with atomic steps is clearly seen, which demonstrates the superiority of homogeneous epitaxy on the AlN substrate. Rocking curve FWHMs of (002) symmetric and (102) asymmetric reflection peaks of AlxGa1–xN epitaxial are shown in Table 1. It should be noted here that the FWHM of (102) reflection peak of AlxGa1–xN epitaxial layer is less than 500 arcsec, indicating a lower dislocation density compared with that on the sapphire substrate.
Figure 5.(Color online) (a) AlN polished wafer. (b) Surface roughness of the AlN polished wafer @ 10 × 10
The HRXRD results given in Fig. 5(e) was obtained using ω–2θ scan mode. The left peak at 17.6° relates to the AlxGa1–xN epitaxial layer, while the right peak at 18.0°, stands for the AlN epitaxial layer or the AlN substrate. Eventually, the Al content x is derived to be 0.54 from a cubic equation according to a reported work[
where
4. Conclusion
We demonstrate in this work a high-quality AlN boule grown using the PVT method. The relationship between surface morphology and growth mode was investigated by comparing the as-grown AlN crystals under different growth conditions. Briefly, the AlN crystal grown under a lower seed temperature has a higher temperature gradient along c-axial, resulting in a rough surface and hexagonal pyramids grown in a dislocation-modulated 3D islands growth mode. After increasing the seed temperature, the growth mode changes to single spiral growth mode. And the as-grown AlN crystal has a smooth surface, hexagonal nucleation center and intrinsic {
Acknowledgements
This work was supported by the National Key Research and Development Plan of China (2017YFB0404103) and the National Natural Science Foundation of China (No.51702297).
References
[1] C Hartmann, A Dittmar, J Wollweber et al. Bulk AlN growth by physical vapour transport. Semicond Sci Technol, 29, 084002(2014).
[2] O Filip, B M Epelbaum, M Bickermann et al. Effects of growth direction and polarity on bulk aluminum nitride crystal properties. J Cryst Growth, 318, 427(2011).
[3] G A Slack, T F McNelly. Growth of high purity AlN crystals. J Cryst Growth, 34, 263(1976).
[4] E Mokhov, I Izmaylova, O Kazarova et al. Specific features of sublimation growth of bulk AlN crystals on SiC wafers. Phys Status Solidi C, 10, 445(2013).
[5] M Bickermann, O Filip, B M Epelbaum et al. Growth of AlN bulk crystals on SiC seeds: Chemical analysis and crystal properties. J Cryst Growth, 339, 13(2012).
[6] I Nagai, T Kato, T Miura et al. AlN bulk single crystal growth on 6H-SiC substrates by sublimation method. J Cryst Growth, 312, 2699(2010).
[7] C Hartmann, M Albrecht, J Wollweber et al. SiC seed polarity-dependent bulk AlN growth under the influence of residual oxygen. J Cryst Growth, 344, 19(2012).
[8] T Y Chemekova, O V Avdeev, I S Barash et al. Sublimation growth of 2 inch diameter bulk AlN crystals. Phys Status Solidi C, 5, 1612(2008).
[9] M Bickermann, B M Epelbaum, O Filip et al. Faceting in AlN bulk crystal growth and its impact on optical properties of the crystals. Phys Status Solidi C, 9, 449(2012).
[10] R R Sumathi, R U Barz, T Straubinger et al. Structural and surface topography analysis of AlN single crystals grown on 6H-SiC substrates. J Cryst Growth, 360, 193(2012).
[11] R Dalmau, H S Craft, J Britt et al. High quality AlN single crystal substrates for AlGaN-based devices. Mater Sci Forum, 924, 923(2018).
[12] C Hartmann, L Matiwe, J Wollweber et al. Favourable growth conditions for the preparation of bulk AlN single crystals by PVT. CrystEngComm, 22, 1762(2020).
[13] R R Sumathi. Bulk AlN single crystal growth on foreign substrate and preparation of free-standing native seeds. CrystEngComm, 15, 2232(2013).
[14] L Zhang, H T Qi, H J Cheng et al. Preparation and characterization of AlN seeds for homogeneous growth. J Semicond, 40, 102801(2019).
[15] M Bickermann, B M Epelbaum, O Filip et al. Deep-UV transparent bulk single-crystalline AlN substrates. Phys Status Solidi C, 7, 1743(2010).
[16] M Bickermann, B M Epelbaum, O Filip et al. UV transparent single-crystalline bulk AlN substrates. Phys Status Solidi C, 7, 21(2010).
[17] W X Yang, Y K Zhao, Y Y Wu et al. Deep-UV emission at 260 nm from MBE-grown AlGaN/AlN quantum-well structures. J Cryst Growth, 512, 213(2019).
[18] M Bobea Graziano, I Bryan, Z Bryan et al. Structural characteristics of m-plane AlN substrates and homoepitaxial films. J Cryst Growth, 507, 389(2019).
[19] W Zheng, R S Zheng, F Huang et al. Raman tensor of AlN bulk single crystal. Photon Res, 3, 38(2015).
[20] P Lu, R Collazo, R F Dalmau et al. Seeded growth of AlN bulk crystals in m- and c-orientation. J Cryst Growth, 312, 58(2009).
[21] K Jiang, X J Sun, J W Ben et al. Suppressing the compositional non-uniformity of AlGaN grown on a HVPE-AlN template with large macro-steps. CrystEngComm, 21, 4864(2019).
Set citation alerts for the article
Please enter your email address