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N-polar AlN nucleation layers grown by hot-wall MOCVD on SiC: Effects of substrate orientation on the polarity, surface morphology and crystal quality
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N-polar AlN nucleation layers grown by hot-wall MOCVD on SiC: Effects of substrate orientation on the polarity, surface morphology and crystal quality Hengfang Zhang a , Plamen P. Paskov a , Olof Kordina a,b , Jr-Tai Chen a,b , Vanya Darakchieva a,c ,∗ a b c
Center for III-Nitride Technology, C3NiT - Janzén, Linköping University, 581 83 Linköping, Sweden SweGaN AB, Teknikringen 8D, 583 30 Linköping, Sweden Terahertz Materials Analysis Center, THeMAC, Linköping University, 581 83 Linköping, Sweden
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Keywords: Hot-wall AlN nucleation layer Nitrogen-polar Substrate orientation effect MOCVD
ABSTRACT Hot-wall metalorganic vapor phase epitaxy enables a superior quality of group-III nitride epitaxial layers and high electron mobility transistor structures, but has not yet been explored for N-polar growth. In this work, we aim at achieving N-polar AlN nucleation layers (NLs) with optimized properties for subsequent growth of GaN device heterostructures. The effects of substrate orientation on the polarity, surface morphology and ̄ C-face SiC (0001) ̄ off-cut towards the [1120] ̄ by crystalline quality of AlN NLs on on-axis C-face SiC (0001), 4◦ , and Si-face SiC (0001) are investigated. The results are discussed in view of growth mode evolution with growth temperature and substrate orientation. It is demonstrated that N-polar AlN NLs with step-flow growth mode and 0002 rocking curve widths below 20 arcsec can be achieved on off-axis C-face SiC substrates.
1. Introduction Group III-nitride semiconductors have continuously attracted strong research interest due to their application in optoelectronic [1] and electronic devices [2,3]. In particular, GaN-based high electron mobility transistor (HEMT) heterostructures have been intensively investigated for radio frequency electronic and power switching applications [4,5]. Group III-nitrides have a wurtzite crystal structure and epitaxial layers of these materials are typically grown along the polar 𝑐-axis. Most works so far have been focused on metal (Ga, Al, In)-polar epitaxy due to easier growth, and the majority of devices utilize metal-polar structures grown along the (0001) direction. Recently, much efforts have ̄ been focused on N-polar epitaxy because structures grown along (0001) direction are found to be advantageous for some device applications [6]. For example, compared with conventional metal-polar HEMTs, the N-polar counterparts exhibit improved characteristics such as feasibility to fabricate low-resistance ohmic contacts, enhanced carrier confinement with a natural back barrier, as well as better device scalability [7]. A record breakdown voltage over 2 kV and low on-resistance have been reported for N-polar HEMT grown on miscut sapphire substrates [8]. Epitaxial N-polar GaN layers have been demonstrated on different substrates such as GaN [9], sapphire [10] and SiC [11]. The presence of hexagonal hillocks on the surface was identified as a common problem for the growth on on-axis substrates including the case of homoepitaxy. It was found that the formation of hexagonal hillocks
can be suppressed or eliminated by employing vicinal substrates with different misorientation angles [9–12]. In general, when GaN is grown on SiC a thin AlN nucleation layer (NL) is employed in order to improve crystal quality and morphology. The main role of the AlN NL is to reduce the density of the misfit dislocations as a result of the smaller lattice mismatch of 1% between AlN and the SiC compared with the lattice mismatch of 3.5% between GaN and SiC. High-quality thin AlN NL was also demonstrated to reduce the thermal boundary resistance at the interface between the SiC substrate and the GaN buffer layer in HEMT structures [13]. The nucleation conditions have a strong impact on the polarity of AlN NL and of subsequently grown GaN layers [14]. Specifically for N-polar III-nitrides epitaxy, AlN NL significantly affects the hexagonal hillock formation and crystallization of N-polar GaN layers [12,15]. Won et al. [12] investigated the effect of different growth conditions (e.g. V/III ratio) and the thickness of AlN NL on the surface morphology and structural properties of N-polar GaN films grown on vicinal C-face ̄ by 3.6◦ and ⟨1010⟩ ̄ by SiC substrates misoriented towards the ⟨1120⟩ ◦ 4 . Lemettinen et al. [16] have performed a comprehensive study of N-polar and Al-polar AlN on 4H-SiC substrates and reported on the effect of the substrate miscut angle on the quality of N-polar AlN. The substrate pre-treatment have been found to have a strong impact on the polarity of AlN and GaN layers grown on C-face SiC [17]. All these investigations proved that the SiC substrate orientation and the
∗ Correspondence to: Center for III-Nitride Technology, C3NiT - Janzén, Linköping University E-mail address: [email protected] (V. Darakchieva).
https://doi.org/10.1016/j.physb.2019.411819 Received 6 June 2019; Received in revised form 22 October 2019; Accepted 24 October 2019 0921-4526/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Hengfang Zhang, Physica B, https://doi.org/10.1016/j.physb.2019.411819
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mode with increasing growth temperature is observed. With increasing the temperature to 950 ◦ C [Fig. 1(e)] the island size increases and formation of steps start to manifest in the surface morphology. The island size further increases with increasing the growth temperature to 1050 ◦ C, and well defined steps can be clearly resolved [Fig. 1(h)]. Similarly to the layers grown on on-axis C-face SiC substrates the island density decreases and the RMS roughness increases. Previously Won et al. [12] showed that for N-polar AlN NLs grown at 1100 ◦ C on C̄ face SiC with a 3.6◦ miscut towards the [1120], the growth mode of AlN NLs changed from island-growth mode to step-flow growth mode as the V/III ratio increased. On the other hand, for 200 nm — thick N̄ polar AlN grown on 4H-SiC substrates with 4◦ miscut towards the [1100] exhibited all step-flow growth mode regardless of the V/III ratio [16]. For the AlN NLs grown on Si-face SiC substrates a similar trend of increasing island size with increasing growth temperature is observed [Figs. 1 (c), (f) and (i)]. However, in this case the islands start to coalesce at temperature of 950 ◦ C resulting in trench-like morphology and a larger RMS roughness of 5.4 nm [Fig. 1(f)]. When the temperature is further increased to 1050 ◦ C, the AlN NLs growth mode changes from island-like to layer-by-layer growth mode [Fig. 1(i)]. Pits with depth of 5 nm are observed on the surface, and which result from incomplete island coalescence. This result is in agreement with previous reports that AlN NLs grown on Si-face 4H-SiC on-axis exhibit many pits [23]. Heavily pitted surfaces were also observed for 50-nm-thick AlN layers on Si-face 6H-SiC substrates with similar surface morphology [14]. The observed morphology of the layers grown on different substrates can be understood in view of different adatom kinetics on Al-polar and N-polar surfaces [7,24,25]. In MOCVD the growth rate is generally limited by the transport of metal atoms. For growth on an Alpolar surface the bonds between surface atom (Al𝑠𝑢𝑟𝑓 ) and Al adatom are fairy week which facilitate the diffusion and layer-by-layer growth is achieved at relatively low temperatures. When the Al impinges on a N-polar surface strong N𝑠𝑢𝑟𝑓 -Al bonds form and the adatom diffusion is suppressed. Note that the Al adatom diffusion barrier were calculated to be 1.17 eV and 1.87 eV on Al-polar and N-polar surfaces, respectively [25]. Thus, for the case of AlN NLs epitaxy on Si-face SiC substrates, the diffusion barrier is lower resulting in a more mobile Al adatoms. Consequently, island coalescence and layer-by layer growth mode occur for lower temperatures as compared to AlN epitaxy on C-face SiC. Indeed, even at the highest growth temperature of 1050 ◦ C the AlN NLs on C-face on-axis SiC exhibit island-like growth mode [Fig. 1(g). For off-cut C-face SiC substrates the density of surface steps is higher and the terrace widths are smaller compared to on-axis SiC, which enables easier adatom incorporation at the step edges during growth. As a result, the adatoms’ energies become large enough to overcome higher energy barriers (e.g the potential of a step) and the adatoms diffusion lengths exceed the step lengths leading to step-flow growth mode at lower temperatures. This explains why for growth on off-cut axis C-face SiC, the transition from island to step-flow growth mode occurs at a temperature of 950 ◦ C. In order to get further insight into the structural properties of the AlN NLs, HR-XRD rocking curve (RC) measurements were performed. The full width at half maximum (FWHM) of the AlN symmetric (0002) RC is affected by screw or mixed type of dislocations with [0001] line component. The results on the AlN (0002) RC FWHMs depending on growth temperature and substrate used are summarized in Fig. 2. The (0002) RC FWHMs of the AlN NLs grown on both off-axis and on-axis C-face SiC first increase with increasing growth temperature to 950 ◦ C, and then decrease with further increase of temperature to 1050 ◦ C. In contrast, for the case of AlN NLs grown on the Siface SiC, the (0002) RC decreases linearly with increasing growth temperature. The observed variation of the (0002) RC FWHM can be associated with a change of screw and mixed type of dislocations, and mosaicity. In principle, the broadening of the (0002) RC could also be due to a presence of domains with small lateral sizes [26]. However, this is unlikely in our case since a clear trend of increasing
growth conditions play an important role for the polarity control and crystalline quality of AlN NLs, which in turn affects the polarity and the properties of GaN grown on top. In this work, we report on the effect of the substrate orientation on the polarity, surface morphology and crystal quality of AlN NLs grown by hot-wall metalorganic chemical vapor deposition (MOCVD) on 4HSiC. Hot-wall MOCVD has demonstrated a superior quality of group-III nitride epitaxial layers and HEMT structures [13,18,19]. Compared to the conventional cold-wall MOCVD in which only the substrate is heated from the back, hot-wall MOCVD employs a heated susceptor providing highly uniform temperature distribution [13,20]. In addition, it enables better cracking efficiency of the precursors which prevents growth-limited species consumption by gas-phase adduct formation [21]. The hot-wall MOCVD allows a larger growth temperature window up to 1600 ◦ C, while it can also achieve high quality AlN layers at reduced growth temperatures. Despite the number of advantages, the hot-wall MOCVD has not yet been explored for N-polar growth. Here, we aim at achieving N-polar AlN NLs and optimizing their quality, which is of a significant importance for subsequent growth of GaN layers in HEMT heterostructures. 2. Experimental details Epitaxial AlN NLs with a thickness of 50 nm were grown by hot-wall MOCVD simultaneously on on-axis semi-insulating (SI) 4H-SiC (0001) ̄ as well as on n-type off-cut 4H-SiC (0001) ̄ with [0001] and (0001), ̄ by 4◦ . The substrates were pre-cleaned misoriented towards the [1120] first in acetone, and then in methanol in a liquid ultrasound bath for 3 min and second in 80 ◦ C heated ammonia solution for 5 min, followed by 5 min etching in 80 ◦ C heated hydrochloric acid solution and rinsing by deionized water. Trimethylaluminum (TMAl) and ammonia (NH3 ) were used as the source precursors for Al and N respectively. A mixture of N2 and H2 was used as a carrier gas, and the pressure was set at 50 mbar. Prior to growth, the substrates were annealed at 1360 ◦ C in H2 followed by NH3 pre-flow. All AlN NLs were grown for 10 min using the same V/III ratio at of 811, with a TMAl volume flow of 0.9 ml/min and a NH3 volume flow of 2 l/min. The set of flow rates of carrier gas and precursors generally replicate the condition developed for efficient and uniform deposition of heteroepitaxial and homoepitaxial Al-polar AlN thin layers by hot-wall MOCVD [21]. The growth was performed at three different temperatures: 850 ◦ C, 950 ◦ C and 1050 ◦ C. The surface morphology of the AlN layers were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). High-resolution X-ray diffraction measurement (HR-XRD) was used to evaluate the crystalline quality of the AlN NLs. The polarity of the epitaxial layers was determined by examining the etching rate in potassium hydroxide solution (KOH) [22] combined with HR-XRD radial scans in the vicinity of the AlN 0002 diffraction peak. 3. Results and discussion Figs. 1 (a)–(i) show AFM images of the as-grown AlN NLs for each growth temperature and the tree types of substrates used. It is seen that the AlN NLs exhibit island-like morphology with a variation of grain size and density that is dependent of the growth temperature and the substrate orientation. The surfaces of the AlN NLs grown at low temperature of 850 ◦ C [Figs. 1 (a), (b) and (c)] exhibit high densities of small islands. With increasing the growth temperature the island size and the root-mean-square (RMS) roughness of the AlN NLs grown on C-SiC on-axis substrates increase [Figs. 1 (a), (d) and (g)]. This is accompanied with a decrease of island density as expected due to the coalescence of islands at higher temperature. Similar results have been previously reported for 75 nm-thick AlN layers grown at a high V/III ratio on C-face 6H-SiC substrates [14]. For the AlN NLs grown on C-SiC substrates with a 4◦ off-cut [Figs. 1 (b), (e) and (h)], a clear tendency of developing step-flow growth 2
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Fig. 1. AFM images of the AlN NLs grown at different temperatures: (a), (b), (c) — 850 ◦ C; (d), (e), (f) — 950 ◦ C and (g), (h), (i) — 1050 ◦ C. For each temperature the growth is performed simultaneously on three types of SiC substrates: (a), (d), (g) C-face SiC on-axis; (b), (e), (h) C-face SiC 4◦ off-axis; (c), (f), (i) Si-face SiC on-axis. The root-mean-square (RMS) roughness of the AlN NLs are indicated on each image. The scale bar of 1 𝜇m indicated in (h) refers to all images.
grain size is observed with increasing growth temperature. The (0002) RCs of the AlN NLs on off-axis C-face SiC substrates are narrower than the respective RCs for the NLs on on-axis C-face SiC substrates independent of growth temperature. This indicates a better crystalline quality and lower density of screw and mixed type of dislocations for the films grown on the off-cut substrates. Note that very narrow (0002) RC below 20 arcsec is measured for the AlN NL on off-axis SiC at the highest growth temperature (Fig. 2). Minimization of (0002) RC FWHM of the NL is very important as possible deviation of the AlN NL 𝑐-axis from the perfect alignment perpendicular to the surface (i.e. the mosaicity) would be replicated into a subsequent GaN growth [27]. Then, in the initial stage of GaN growth slightly misoriented grains may be formed disturbing the subsequent growth by dislocation generation when grain coalescence occurs. The threading dislocations in the NL layers might also be reproduced in the GaN layer. For the optimized growth conditions of N-polar AlN NL on off-axis substrates ̄ and (1015) ̄ RCs with FWHMs we measured narrow asymmetric (1012) below 200 arcsec. The broadening of these asymmetric peaks is related to the presence of edge type dislocations and mosaic twist [26]. The KOH wet chemical etching is a simple and direct way to determine the polarity of III-nitride film on the macroscopic scale [28]. It has been proved that the etch rate of the N-polar surface of AlN is very fast, while the etch rate for the Al-polar AlN surface is extremely low [22]. KOH etched N-polar AlN epitaxial layers usually exhibit rough surfaces with hexagonal hillocks in different sizes while Al-polar surfaces are much flatter [29]. Hence, monitoring the surface morphology after KOH etching can be used to evaluate epitaxial layers polarity. We also performed XRD measurements of the AlN (0002) radial scans before and after KOH etching in order to enable quantitative and conclusive assignment of film polarity. Fig. 3 shows SEM images of the AlN NLs grown at 1050 ◦ C simultaneously on the three types of SiC substrates before: Fig. 3(a), (b) and (c)
Fig. 2. Full width at half maximum (FWHM) of the AlN (0002) rocking curves as a function of the growth temperature of AlN NLs. Different symbols indicate the type of substrate used.
and after KOH etching: Fig. 3(d), (e) and (f). Fig. 3: (g), (h) and (i) show the respective AlN (0002) 2𝜃 −𝜔 scans before and after KOH etching for the three types of substrates, respectively. The SEM topographies of the as-grown AlN NLs are consistent with the AFM surface morphologies (see Fig. 1: (g), (h) and (i) ). The topographies of the AlN NLs on both on-axis and off-axis C-face SiC after etching [Fig. 3: (d) and (e)] are very similar to the topography of a bare substrate (not shown here). It is also seen that the AlN (0002) diffraction peak can no longer be detected in the 2𝜃 − 𝜔 scans of the AlN NLs after etching [Fig. 3: (g), (h)]. These results clearly indicate that the AlN NLs grown on C-face SiC, both on — and off-axis, have N-polarity. In contrast, in the case of the AlN NL grown on Si-face SiC, the KOH etching does not cause any change in the 2𝜃 − 𝜔 scans [Fig. 3: (i)] where the AlN (0002) diffraction peaks before and after etching are practically identical. Apart from the pits formation 3
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Fig. 3. SEM images of the AlN NLs grown at 1050 ◦ C simultaneously on the three types of SiC substrates: (a), (d) — C-face SiC on-axis; (b), (e) — C-face SiC 4◦ off-axis; and (c), (f) — Si-face SiC on-axis. (a), (b) and (c) show the topographies of the as grown AlN NLs while (d), (e) and (f) show the corresponding topographies after KOH chemical etching. The scale bar of 1 μm indicated in (f) refers to all images. (g), (h) and (i) display the AlN (0002) 2𝜃 − 𝜔 scans of the respective AlN NLs before and after KOH etching.
Acknowledgments
as a result of the etching, no change of film surface morphology is observed [Fig. 3: (c) and (f)]. These observations imply an Al-polarity for the AlN NLs grown on Si-face SiC. We note that similar results are obtained for all growth temperatures.
This work is performed within the framework of the competence center for III-Nitride technology, C3Nit — Janzén supported by the Swedish Governmental Agency for Innovation Systems (VINNOVA) under the Competence Center Program Grant No. 2016-05190, Linköping University, Chalmers University of Technology, ABB, Ericsson, Epiluvac, FMV, Gotmic, On Semiconductor, Saab, SweGaN, and UMS. We further acknowledge support from the Swedish Research Council VR under Award No. 2016-00889, Swedish Foundation for Strategic Research under Grants No. FL12-0181, No. RIF14-055, and No. EM160024, and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No. 2009-00971. We acknowledge fruitful discussions with Dr. Pitsiri Sukkaew.
4. Conclusion The effects of substrate orientation on the polarity, surface morphology and crystal quality of AlN NLs grown by hot-wall MOCVD on SiC were investigated. AlN NLs with N-polarity are achieved on both on̄ while the layers grown on Si-face axis and off-cut C-face SiC (0001), SiC (0001) posses Al-polarity. It is shown that with increasing growth temperature the growth mode changes from island-like to step-flow on ̄ and to layer-by-layer on SiC (0001), respectively. off-axis SiC (0001) In contrast, island-like mode is preserved when AlN NLs are grown on on-axis C-face SiC independent of growth temperature. These results are explained in view of different adatom mobility on Al — and Npolar surfaces and the effects of off-cut on substrate step density and terrace width. All layers exhibit good crystalline quality as revealed by narrow symmetric RCs. In particular, 0002 RC FWHM below 20 arcsec are achieved for the films grown at 1050 ◦ C on Si-face and offaxis C-face SiC. For the N polar AlN NLs grown at optimized growth ̄ RC FWHM conditions on off-axis C-face SiC narrow assymetric 1012 below 200 arcsec is also achieved. N-polar AlN nucleation layers with smooth surfaces and good crystalline quality that can be employed for subsequent growth of N-polar device heterostructures are demonstrated for the first time by hot-wall MOCVD.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 4
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N-polar AlN nucleation layers grown by hot-wall MOCVD on SiC: Effects of substrate orientation on the polarity, surface morphology and crystal quality Hengfang Zhang a , Plamen P. Paskov a , Olof Kordina a,b , Jr-Tai Chen a,b , Vanya Darakchieva a,c ,∗ a b c
Center for III-Nitride Technology, C3NiT - Janzén, Linköping University, 581 83 Linköping, Sweden SweGaN AB, Teknikringen 8D, 583 30 Linköping, Sweden Terahertz Materials Analysis Center, THeMAC, Linköping University, 581 83 Linköping, Sweden
ARTICLE
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Keywords: Hot-wall AlN nucleation layer Nitrogen-polar Substrate orientation effect MOCVD
ABSTRACT Hot-wall metalorganic vapor phase epitaxy enables a superior quality of group-III nitride epitaxial layers and high electron mobility transistor structures, but has not yet been explored for N-polar growth. In this work, we aim at achieving N-polar AlN nucleation layers (NLs) with optimized properties for subsequent growth of GaN device heterostructures. The effects of substrate orientation on the polarity, surface morphology and ̄ C-face SiC (0001) ̄ off-cut towards the [1120] ̄ by crystalline quality of AlN NLs on on-axis C-face SiC (0001), 4◦ , and Si-face SiC (0001) are investigated. The results are discussed in view of growth mode evolution with growth temperature and substrate orientation. It is demonstrated that N-polar AlN NLs with step-flow growth mode and 0002 rocking curve widths below 20 arcsec can be achieved on off-axis C-face SiC substrates.
1. Introduction Group III-nitride semiconductors have continuously attracted strong research interest due to their application in optoelectronic [1] and electronic devices [2,3]. In particular, GaN-based high electron mobility transistor (HEMT) heterostructures have been intensively investigated for radio frequency electronic and power switching applications [4,5]. Group III-nitrides have a wurtzite crystal structure and epitaxial layers of these materials are typically grown along the polar 𝑐-axis. Most works so far have been focused on metal (Ga, Al, In)-polar epitaxy due to easier growth, and the majority of devices utilize metal-polar structures grown along the (0001) direction. Recently, much efforts have ̄ been focused on N-polar epitaxy because structures grown along (0001) direction are found to be advantageous for some device applications [6]. For example, compared with conventional metal-polar HEMTs, the N-polar counterparts exhibit improved characteristics such as feasibility to fabricate low-resistance ohmic contacts, enhanced carrier confinement with a natural back barrier, as well as better device scalability [7]. A record breakdown voltage over 2 kV and low on-resistance have been reported for N-polar HEMT grown on miscut sapphire substrates [8]. Epitaxial N-polar GaN layers have been demonstrated on different substrates such as GaN [9], sapphire [10] and SiC [11]. The presence of hexagonal hillocks on the surface was identified as a common problem for the growth on on-axis substrates including the case of homoepitaxy. It was found that the formation of hexagonal hillocks
can be suppressed or eliminated by employing vicinal substrates with different misorientation angles [9–12]. In general, when GaN is grown on SiC a thin AlN nucleation layer (NL) is employed in order to improve crystal quality and morphology. The main role of the AlN NL is to reduce the density of the misfit dislocations as a result of the smaller lattice mismatch of 1% between AlN and the SiC compared with the lattice mismatch of 3.5% between GaN and SiC. High-quality thin AlN NL was also demonstrated to reduce the thermal boundary resistance at the interface between the SiC substrate and the GaN buffer layer in HEMT structures [13]. The nucleation conditions have a strong impact on the polarity of AlN NL and of subsequently grown GaN layers [14]. Specifically for N-polar III-nitrides epitaxy, AlN NL significantly affects the hexagonal hillock formation and crystallization of N-polar GaN layers [12,15]. Won et al. [12] investigated the effect of different growth conditions (e.g. V/III ratio) and the thickness of AlN NL on the surface morphology and structural properties of N-polar GaN films grown on vicinal C-face ̄ by 3.6◦ and ⟨1010⟩ ̄ by SiC substrates misoriented towards the ⟨1120⟩ ◦ 4 . Lemettinen et al. [16] have performed a comprehensive study of N-polar and Al-polar AlN on 4H-SiC substrates and reported on the effect of the substrate miscut angle on the quality of N-polar AlN. The substrate pre-treatment have been found to have a strong impact on the polarity of AlN and GaN layers grown on C-face SiC [17]. All these investigations proved that the SiC substrate orientation and the
∗ Correspondence to: Center for III-Nitride Technology, C3NiT - Janzén, Linköping University E-mail address: [email protected] (V. Darakchieva).
https://doi.org/10.1016/j.physb.2019.411819 Received 6 June 2019; Received in revised form 22 October 2019; Accepted 24 October 2019 0921-4526/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Hengfang Zhang, Physica B, https://doi.org/10.1016/j.physb.2019.411819
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mode with increasing growth temperature is observed. With increasing the temperature to 950 ◦ C [Fig. 1(e)] the island size increases and formation of steps start to manifest in the surface morphology. The island size further increases with increasing the growth temperature to 1050 ◦ C, and well defined steps can be clearly resolved [Fig. 1(h)]. Similarly to the layers grown on on-axis C-face SiC substrates the island density decreases and the RMS roughness increases. Previously Won et al. [12] showed that for N-polar AlN NLs grown at 1100 ◦ C on C̄ face SiC with a 3.6◦ miscut towards the [1120], the growth mode of AlN NLs changed from island-growth mode to step-flow growth mode as the V/III ratio increased. On the other hand, for 200 nm — thick N̄ polar AlN grown on 4H-SiC substrates with 4◦ miscut towards the [1100] exhibited all step-flow growth mode regardless of the V/III ratio [16]. For the AlN NLs grown on Si-face SiC substrates a similar trend of increasing island size with increasing growth temperature is observed [Figs. 1 (c), (f) and (i)]. However, in this case the islands start to coalesce at temperature of 950 ◦ C resulting in trench-like morphology and a larger RMS roughness of 5.4 nm [Fig. 1(f)]. When the temperature is further increased to 1050 ◦ C, the AlN NLs growth mode changes from island-like to layer-by-layer growth mode [Fig. 1(i)]. Pits with depth of 5 nm are observed on the surface, and which result from incomplete island coalescence. This result is in agreement with previous reports that AlN NLs grown on Si-face 4H-SiC on-axis exhibit many pits [23]. Heavily pitted surfaces were also observed for 50-nm-thick AlN layers on Si-face 6H-SiC substrates with similar surface morphology [14]. The observed morphology of the layers grown on different substrates can be understood in view of different adatom kinetics on Al-polar and N-polar surfaces [7,24,25]. In MOCVD the growth rate is generally limited by the transport of metal atoms. For growth on an Alpolar surface the bonds between surface atom (Al𝑠𝑢𝑟𝑓 ) and Al adatom are fairy week which facilitate the diffusion and layer-by-layer growth is achieved at relatively low temperatures. When the Al impinges on a N-polar surface strong N𝑠𝑢𝑟𝑓 -Al bonds form and the adatom diffusion is suppressed. Note that the Al adatom diffusion barrier were calculated to be 1.17 eV and 1.87 eV on Al-polar and N-polar surfaces, respectively [25]. Thus, for the case of AlN NLs epitaxy on Si-face SiC substrates, the diffusion barrier is lower resulting in a more mobile Al adatoms. Consequently, island coalescence and layer-by layer growth mode occur for lower temperatures as compared to AlN epitaxy on C-face SiC. Indeed, even at the highest growth temperature of 1050 ◦ C the AlN NLs on C-face on-axis SiC exhibit island-like growth mode [Fig. 1(g). For off-cut C-face SiC substrates the density of surface steps is higher and the terrace widths are smaller compared to on-axis SiC, which enables easier adatom incorporation at the step edges during growth. As a result, the adatoms’ energies become large enough to overcome higher energy barriers (e.g the potential of a step) and the adatoms diffusion lengths exceed the step lengths leading to step-flow growth mode at lower temperatures. This explains why for growth on off-cut axis C-face SiC, the transition from island to step-flow growth mode occurs at a temperature of 950 ◦ C. In order to get further insight into the structural properties of the AlN NLs, HR-XRD rocking curve (RC) measurements were performed. The full width at half maximum (FWHM) of the AlN symmetric (0002) RC is affected by screw or mixed type of dislocations with [0001] line component. The results on the AlN (0002) RC FWHMs depending on growth temperature and substrate used are summarized in Fig. 2. The (0002) RC FWHMs of the AlN NLs grown on both off-axis and on-axis C-face SiC first increase with increasing growth temperature to 950 ◦ C, and then decrease with further increase of temperature to 1050 ◦ C. In contrast, for the case of AlN NLs grown on the Siface SiC, the (0002) RC decreases linearly with increasing growth temperature. The observed variation of the (0002) RC FWHM can be associated with a change of screw and mixed type of dislocations, and mosaicity. In principle, the broadening of the (0002) RC could also be due to a presence of domains with small lateral sizes [26]. However, this is unlikely in our case since a clear trend of increasing
growth conditions play an important role for the polarity control and crystalline quality of AlN NLs, which in turn affects the polarity and the properties of GaN grown on top. In this work, we report on the effect of the substrate orientation on the polarity, surface morphology and crystal quality of AlN NLs grown by hot-wall metalorganic chemical vapor deposition (MOCVD) on 4HSiC. Hot-wall MOCVD has demonstrated a superior quality of group-III nitride epitaxial layers and HEMT structures [13,18,19]. Compared to the conventional cold-wall MOCVD in which only the substrate is heated from the back, hot-wall MOCVD employs a heated susceptor providing highly uniform temperature distribution [13,20]. In addition, it enables better cracking efficiency of the precursors which prevents growth-limited species consumption by gas-phase adduct formation [21]. The hot-wall MOCVD allows a larger growth temperature window up to 1600 ◦ C, while it can also achieve high quality AlN layers at reduced growth temperatures. Despite the number of advantages, the hot-wall MOCVD has not yet been explored for N-polar growth. Here, we aim at achieving N-polar AlN NLs and optimizing their quality, which is of a significant importance for subsequent growth of GaN layers in HEMT heterostructures. 2. Experimental details Epitaxial AlN NLs with a thickness of 50 nm were grown by hot-wall MOCVD simultaneously on on-axis semi-insulating (SI) 4H-SiC (0001) ̄ as well as on n-type off-cut 4H-SiC (0001) ̄ with [0001] and (0001), ̄ by 4◦ . The substrates were pre-cleaned misoriented towards the [1120] first in acetone, and then in methanol in a liquid ultrasound bath for 3 min and second in 80 ◦ C heated ammonia solution for 5 min, followed by 5 min etching in 80 ◦ C heated hydrochloric acid solution and rinsing by deionized water. Trimethylaluminum (TMAl) and ammonia (NH3 ) were used as the source precursors for Al and N respectively. A mixture of N2 and H2 was used as a carrier gas, and the pressure was set at 50 mbar. Prior to growth, the substrates were annealed at 1360 ◦ C in H2 followed by NH3 pre-flow. All AlN NLs were grown for 10 min using the same V/III ratio at of 811, with a TMAl volume flow of 0.9 ml/min and a NH3 volume flow of 2 l/min. The set of flow rates of carrier gas and precursors generally replicate the condition developed for efficient and uniform deposition of heteroepitaxial and homoepitaxial Al-polar AlN thin layers by hot-wall MOCVD [21]. The growth was performed at three different temperatures: 850 ◦ C, 950 ◦ C and 1050 ◦ C. The surface morphology of the AlN layers were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). High-resolution X-ray diffraction measurement (HR-XRD) was used to evaluate the crystalline quality of the AlN NLs. The polarity of the epitaxial layers was determined by examining the etching rate in potassium hydroxide solution (KOH) [22] combined with HR-XRD radial scans in the vicinity of the AlN 0002 diffraction peak. 3. Results and discussion Figs. 1 (a)–(i) show AFM images of the as-grown AlN NLs for each growth temperature and the tree types of substrates used. It is seen that the AlN NLs exhibit island-like morphology with a variation of grain size and density that is dependent of the growth temperature and the substrate orientation. The surfaces of the AlN NLs grown at low temperature of 850 ◦ C [Figs. 1 (a), (b) and (c)] exhibit high densities of small islands. With increasing the growth temperature the island size and the root-mean-square (RMS) roughness of the AlN NLs grown on C-SiC on-axis substrates increase [Figs. 1 (a), (d) and (g)]. This is accompanied with a decrease of island density as expected due to the coalescence of islands at higher temperature. Similar results have been previously reported for 75 nm-thick AlN layers grown at a high V/III ratio on C-face 6H-SiC substrates [14]. For the AlN NLs grown on C-SiC substrates with a 4◦ off-cut [Figs. 1 (b), (e) and (h)], a clear tendency of developing step-flow growth 2
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Fig. 1. AFM images of the AlN NLs grown at different temperatures: (a), (b), (c) — 850 ◦ C; (d), (e), (f) — 950 ◦ C and (g), (h), (i) — 1050 ◦ C. For each temperature the growth is performed simultaneously on three types of SiC substrates: (a), (d), (g) C-face SiC on-axis; (b), (e), (h) C-face SiC 4◦ off-axis; (c), (f), (i) Si-face SiC on-axis. The root-mean-square (RMS) roughness of the AlN NLs are indicated on each image. The scale bar of 1 𝜇m indicated in (h) refers to all images.
grain size is observed with increasing growth temperature. The (0002) RCs of the AlN NLs on off-axis C-face SiC substrates are narrower than the respective RCs for the NLs on on-axis C-face SiC substrates independent of growth temperature. This indicates a better crystalline quality and lower density of screw and mixed type of dislocations for the films grown on the off-cut substrates. Note that very narrow (0002) RC below 20 arcsec is measured for the AlN NL on off-axis SiC at the highest growth temperature (Fig. 2). Minimization of (0002) RC FWHM of the NL is very important as possible deviation of the AlN NL 𝑐-axis from the perfect alignment perpendicular to the surface (i.e. the mosaicity) would be replicated into a subsequent GaN growth [27]. Then, in the initial stage of GaN growth slightly misoriented grains may be formed disturbing the subsequent growth by dislocation generation when grain coalescence occurs. The threading dislocations in the NL layers might also be reproduced in the GaN layer. For the optimized growth conditions of N-polar AlN NL on off-axis substrates ̄ and (1015) ̄ RCs with FWHMs we measured narrow asymmetric (1012) below 200 arcsec. The broadening of these asymmetric peaks is related to the presence of edge type dislocations and mosaic twist [26]. The KOH wet chemical etching is a simple and direct way to determine the polarity of III-nitride film on the macroscopic scale [28]. It has been proved that the etch rate of the N-polar surface of AlN is very fast, while the etch rate for the Al-polar AlN surface is extremely low [22]. KOH etched N-polar AlN epitaxial layers usually exhibit rough surfaces with hexagonal hillocks in different sizes while Al-polar surfaces are much flatter [29]. Hence, monitoring the surface morphology after KOH etching can be used to evaluate epitaxial layers polarity. We also performed XRD measurements of the AlN (0002) radial scans before and after KOH etching in order to enable quantitative and conclusive assignment of film polarity. Fig. 3 shows SEM images of the AlN NLs grown at 1050 ◦ C simultaneously on the three types of SiC substrates before: Fig. 3(a), (b) and (c)
Fig. 2. Full width at half maximum (FWHM) of the AlN (0002) rocking curves as a function of the growth temperature of AlN NLs. Different symbols indicate the type of substrate used.
and after KOH etching: Fig. 3(d), (e) and (f). Fig. 3: (g), (h) and (i) show the respective AlN (0002) 2𝜃 −𝜔 scans before and after KOH etching for the three types of substrates, respectively. The SEM topographies of the as-grown AlN NLs are consistent with the AFM surface morphologies (see Fig. 1: (g), (h) and (i) ). The topographies of the AlN NLs on both on-axis and off-axis C-face SiC after etching [Fig. 3: (d) and (e)] are very similar to the topography of a bare substrate (not shown here). It is also seen that the AlN (0002) diffraction peak can no longer be detected in the 2𝜃 − 𝜔 scans of the AlN NLs after etching [Fig. 3: (g), (h)]. These results clearly indicate that the AlN NLs grown on C-face SiC, both on — and off-axis, have N-polarity. In contrast, in the case of the AlN NL grown on Si-face SiC, the KOH etching does not cause any change in the 2𝜃 − 𝜔 scans [Fig. 3: (i)] where the AlN (0002) diffraction peaks before and after etching are practically identical. Apart from the pits formation 3
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Fig. 3. SEM images of the AlN NLs grown at 1050 ◦ C simultaneously on the three types of SiC substrates: (a), (d) — C-face SiC on-axis; (b), (e) — C-face SiC 4◦ off-axis; and (c), (f) — Si-face SiC on-axis. (a), (b) and (c) show the topographies of the as grown AlN NLs while (d), (e) and (f) show the corresponding topographies after KOH chemical etching. The scale bar of 1 μm indicated in (f) refers to all images. (g), (h) and (i) display the AlN (0002) 2𝜃 − 𝜔 scans of the respective AlN NLs before and after KOH etching.
Acknowledgments
as a result of the etching, no change of film surface morphology is observed [Fig. 3: (c) and (f)]. These observations imply an Al-polarity for the AlN NLs grown on Si-face SiC. We note that similar results are obtained for all growth temperatures.
This work is performed within the framework of the competence center for III-Nitride technology, C3Nit — Janzén supported by the Swedish Governmental Agency for Innovation Systems (VINNOVA) under the Competence Center Program Grant No. 2016-05190, Linköping University, Chalmers University of Technology, ABB, Ericsson, Epiluvac, FMV, Gotmic, On Semiconductor, Saab, SweGaN, and UMS. We further acknowledge support from the Swedish Research Council VR under Award No. 2016-00889, Swedish Foundation for Strategic Research under Grants No. FL12-0181, No. RIF14-055, and No. EM160024, and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No. 2009-00971. We acknowledge fruitful discussions with Dr. Pitsiri Sukkaew.
4. Conclusion The effects of substrate orientation on the polarity, surface morphology and crystal quality of AlN NLs grown by hot-wall MOCVD on SiC were investigated. AlN NLs with N-polarity are achieved on both on̄ while the layers grown on Si-face axis and off-cut C-face SiC (0001), SiC (0001) posses Al-polarity. It is shown that with increasing growth temperature the growth mode changes from island-like to step-flow on ̄ and to layer-by-layer on SiC (0001), respectively. off-axis SiC (0001) In contrast, island-like mode is preserved when AlN NLs are grown on on-axis C-face SiC independent of growth temperature. These results are explained in view of different adatom mobility on Al — and Npolar surfaces and the effects of off-cut on substrate step density and terrace width. All layers exhibit good crystalline quality as revealed by narrow symmetric RCs. In particular, 0002 RC FWHM below 20 arcsec are achieved for the films grown at 1050 ◦ C on Si-face and offaxis C-face SiC. For the N polar AlN NLs grown at optimized growth ̄ RC FWHM conditions on off-axis C-face SiC narrow assymetric 1012 below 200 arcsec is also achieved. N-polar AlN nucleation layers with smooth surfaces and good crystalline quality that can be employed for subsequent growth of N-polar device heterostructures are demonstrated for the first time by hot-wall MOCVD.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 4
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