Structural properties of Al-rich AlInN grown on c-plane GaN substrate by metal-organic chemical vapor deposition
© Lin et al.; licensee Springer. 2014
Received: 30 June 2014
Accepted: 17 November 2014
Published: 23 November 2014
The attractive prospect for AlInN/GaN-based devices for high electron mobility transistors with advanced structure relies on high-quality AlInN epilayer. In this work, we demonstrate the growth of high-quality Al-rich AlInN films deposited on c-plane GaN substrate by metal-organic chemical vapor deposition. X-ray diffraction, scanning electron microscopy, and scanning transmission electron microscopy show that the films lattice-matched with GaN can have a very smooth surface with good crystallinity and uniform distribution of Al and In in AlInN.
AlInN is a newly developed III-nitride for many promising applications due to its band gap being able to be tuned in a wide range of 0.70 ~ 6.14 eV with high spontaneous polarization[1–3]. AlInN/GaN heterojunction has been suggested as a strong candidate for high-power and high-frequency applications owing to the nearly lattice match (In = 0.17) and stress-free heterostructure in which the piezoelectric polarization charge is eliminated to reduce surface-related current collapse[4–8]. The attractive prospect for AlInN/GaN-based devices for high electron mobility transistors with advanced structure relies on high-quality AlInN epilayer. However, many properties of ternary AlInN alloys have not been well understood because of lack of high-quality films. Growth of AlInN is complicated by thermal control, which usually results in growth with composition inhomogeneities and phase separation[9–11]. Especially, the thermal instability may lead to phase separations and other defects which form on the substrate and may evolve during the growth resulting in rough and poorly uniform surface. The lattice-matched of high-quality AlInN can potentially lead to a reduction in threading dislocation and cracking as well as the elimination of strain-driven piezoelectric polarization field.
Various growth techniques have been used for growth of AlInN films, such as metal-organic chemical vapor deposition (MOCVD), radio-frequency molecular beam epitaxy, pulse laser deposition, and magnetron sputtering on c-plane sapphire substrates[13–15]. In the past few years, most of AlInN/GaN studies are done on films grown on sapphire substrate which has strong effects on the film properties, and among them, lattice-matched Al1-xIn x N/GaN with 0.17 ~ x ~ 0.18 has received intensive attention[16–18]. The nearly lattice-matched AlInN layer grown on GaN/sapphire substrate usually contain some defects like hillocks, dislocations natively present in the GaN layer and the V-defects on GaN surface[19–21]. However, recent studied show that the growth of AlInN on GaN remains a challenge as many issues have to be addressed[22, 23]. Hereinafter, we show that a high-quality and smooth Al1-xIn x N film with x less than 0.17 can be epitaxially grown on GaN free-standing substrate with in-plane lattice match by employing MOCVD.
In this work, we used hydride vapor phase epitaxy (HVPE)-grown GaN wafers as substrates which had a 1.5-in. diameter and 300-μm thickness with a full width at half maximum (FWHM) of the (0002) X-ray rocking curve (XRC) 100 arcsec (dislocation density approximately 1 × 107 cm-2) and a root mean square (RMS) surface roughness of 1.23 nm. A 1.5-μm-thick homoepitaxial GaN film was firstly grown on GaN substrate at 970°C by MOCVD (Veeco Emcore D-180; Veeco Instruments Inc., Plainview, NY, USA) using TMGa and NH3 as the precursors for Ga and N to improve GaN crystallinity. After the growth of GaN layer, TMAl and NH3 were flowed into the chamber at 990°C for 1 min, followed by further growth of AlInN epilayer at temperature of 780 and 800°C. A TMAl/TMIn ratio of 1/10, a reactor pressure of 100 Torr, and a NH3 flow rate of 15,000 sccm were used for all AlInN growth.
The crystallinities of the samples were examined with high-resolution X-ray diffraction (XRD, Bruker D8; Bruker Corp., Billerica, MA, USA), and the surface morphologies were investigated by scanning electron microscopy (SEM, JEOL JSM-6500F; JEOL Ltd., Akishima, Tokyo, Japan) and atomic force microscopy (AFM, Veeco Innova). Structural characterization at atomic scale was performed in a JEOL JEM-ARM200F spherical aberration corrected scanning transmission electron microscope (STEM) in high-angle annular dark field (HAADF) imaging mode, operated at 200 kV. Cross-sectional transmission electron microscopy specimens were prepared in a focused ion beam system (FEI NOVA-200; FEI Company, Hillsboro, OR, USA) using a 30-kV Ga+ source.
Results and discussion
The above results show that lattice-matched Al1-xIn x N films on GaN with x < 0.17 are of high crystallinity with quite smooth surface, which can be comparable with those of Al1-xIn x N films grown on free-standing GaN substrates with x close to 0.17.
In this work, we successfully fabricated high-quality AlInN/GaN heterostructures by MOCVD. XRD, SEM, and STEM results reveal that the film is of good crystallinity and uniformity in composition, smooth surface flatness, and the abrupt heterointerface with lattice-matched AlInN epilayer on free-standing GaN substrate at atomic scale.
This work was supported by the Ministry of Science and Technology of Taiwan (NSC 101-2221-E-009-050-MY3), the Ministry of Economic Affairs of Taiwan (102-EC-17-A-05-S1-154), and National Chiao Tung University (103 W974).
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