Elimination of columnar microstructure in N-face InAlN ...

Elimination of columnar microstructure in N-face InAlN, lattice-matched to GaN, grown by plasma-assisted molecular beam epitaxy in the N-rich regime Elaheh Ahmadi, Ravi Shivaraman, Feng Wu, Steven Wienecke, Stephen W. Kaun, Stacia Keller, James S. Speck, and Umesh K. Mishra Citation: Applied Physics Letters 104, 072107 (2014); doi: 10.1063/1.4866435 View online: http://dx.doi.org/10.1063/1.4866435 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Publisher’s Note: “Observation of columnar microstructure in lattice-matched InAlN/GaN grown by plasma assisted molecular beam epitaxy” [Appl. Phys. Lett. 100, 232102 (2012)] Appl. Phys. Lett. 101, 049903 (2012); 10.1063/1.4740223 Observation of columnar microstructure in lattice-matched InAlN/GaN grown by plasma assisted molecular beam epitaxy Appl. Phys. Lett. 100, 232102 (2012); 10.1063/1.4725482 Molecular beam epitaxy of InAlN lattice-matched to GaN with homogeneous composition using ammonia as nitrogen source Appl. Phys. Lett. 100, 072107 (2012); 10.1063/1.3686922 High quality InAlN single layers lattice-matched to GaN grown by molecular beam epitaxy Appl. Phys. Lett. 99, 031103 (2011); 10.1063/1.3614434 High mobility AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy on semi-insulating GaN templates prepared by hydride vapor phase epitaxy J. Appl. Phys. 92, 338 (2002); 10.1063/1.1484227

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APPLIED PHYSICS LETTERS 104, 072107 (2014)

Elimination of columnar microstructure in N-face InAlN, lattice-matched to GaN, grown by plasma-assisted molecular beam epitaxy in the N-rich regime Elaheh Ahmadi,1 Ravi Shivaraman,2 Feng Wu,2 Steven Wienecke,1 Stephen W. Kaun,2 Stacia Keller,1 James S. Speck,2 and Umesh K. Mishra1

1 Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106, USA 2 Materials Department, University of California, Santa Barbara, California 93106, USA

(Received 11 December 2013; accepted 3 February 2014; published online 21 February 2014) The microstructure of N-face InAlN layers, lattice-matched to GaN, was investigated by scanning transmission electron microscopy and atom probe tomography. These layers were grown by plasma-assisted molecular beam epitaxy (PAMBE) in the N-rich regime. Microstructural analysis shows an absence of the lateral composition modulation that was previously observed in InAlN films grown by PAMBE. A room temperature two-dimensional electron gas (2DEG) mobility of 1100 cm2/V s and 2DEG sheet charge density of 1.9  1013 cm2 was measured for N-face GaN/AlN/GaN/InAlN high-electron-mobility transistors with lattice-matched InAlN back barriers. C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4866435] V InAlN has received significant attention due to its great potential for electronic1–6 and optoelectronic applications.7–10 In particular, In0.18Al0.82N presents the advantage of being lattice-matched to GaN and simultaneously exhibiting a high spontaneous polarization charge, making In0.18Al0.82N attractive for use as the barrier layer in high-electron-mobility transistors (HEMTs). In this context, excellent DC and radio frequency (RF) performances have been demonstrated on both N-face and metal-face devices with In0.18Al0.82N barriers grown by metal-organic chemical vapor deposition (MOCVD).1,4,11 Also, Wong et al.12 grew high-quality In0.18Al0.82N layers with homogeneous composition, using ammonia-based molecular beam epitaxy. However, in the case of InAlN growth by plasma-assisted molecular beam epitaxy (PAMBE), a strong non-uniformity in the in-plane In distribution was observed for both N-face and metal-face In0.18Al0.82N.13–16 This compositional inhomogeneity manifests itself as a columnar microstructure with AlN-rich cores (5–10 nm in width) and InN-rich intercolumn boundaries.13,15,16 Because of the large differences between the bandgaps and polarization of InN and AlN, this non-uniformity in InAlN composition could be a source of scattering, leading to mobility degradation in HEMTs. In this work, uniform N-face InAlN layers were obtained by optimizing the growth conditions of PAMBE. Samples were grown in a Varian Gen-II MBE system, equipped with conventional thermal effusion cells for Al, Ga, and In sources and a Veeco Unibulb rf plasma N source. The N source consisted of ultrahigh-purity (99.9995%) N2 gas flowing at 0.3 SCCM through the rf-plasma source with 250 W rf power, which corresponded to a growth rate of 250 nm/h for metal-rich GaN layers. Samples used for structural studies were grown on Lumilog ð0001Þ N-face freestanding (FS) GaN substrates. Structure A, shown in Fig. 1(a), was grown for atom probe tomography (APT) and scanning transmission electron microscopy (STEM) studies. The growth of structure A was initiated with a 100-nm-thick GaN buffer under Ga-rich conditions with a substrate temperature of 700  C to ensure a smooth surface. The substrate 0003-6951/2014/104(7)/072107/3/$30.00

temperature was then decreased to 550  C to grow the InAlN layer. The InAlN layer was grown under N-rich conditions using a III/V ratio ((fIn þ fAl)/fN) of 0.4 and fIn/(fAl þ fIn) of 0.5, where fN is the flux of active nitrogen, and fIn and fAl are the In and Al fluxes, respectively. This was followed by the growth of a thin (2 nm) GaN layer at low temperature (LT) to prevent the InAlN layer from decomposing during the increase of the substrate temperature for the subsequent growth of the AlN interlayer and GaN capping layer. Structure A was capped with 100 nm GaN to be used as calibration for APT. Using the aforementioned growth conditions for GaN/AlN/GaN/InAlN, structure B (Fig. 1(b)) was grown directly on the C-face of on-axis 6H-SiC for HEMT fabrication. The growth of structure B began with an AlN initiation layer grown with an Al/N flux ratio of 0.6 at 740  C, followed by growth of a 600-nm-thick GaN buffer under Ga-rich conditions. Threading dislocation densities of 5  107 cm2 and 3  1010 cm2 were estimated for structure A and structure B, respectively, from the rocking curve full widths at half maximum of high-resolution x-ray diffraction (HRXRD) x scans for the GaN ð2021Þ reflection. Symmetric HRXRD x-2h scans were performed across on-axis (0002) reflections using a triple-axis detector to determine the thickness and composition of the InAlN layers. Assuming Vegard’s law, HRXRD yielded In mole fractions of 0.18 for all InAlN layers grown in this study. In addition to structure A and structure B, a sample that consisted of a 70-nm-thick In0.18Al0.82N layer grown on FS GaN was used to verify the coherency of the InAlN layers to the underlying GaN layers. Coherency was confirmed through an HRXRD reciprocal space map (RSM) around the asymmetric GaN ð1015Þ reflection in co-planar geometry on this sample. STEM specimens were extracted from structure A using an FEI Helios 600 Dual Beam Focused Ion Beam instrument. Cross-sectional STEM was carried out with an FEI Titan FEG Microscope, operated at 300 kV. APT of the InAlN layer in structure A was performed using a Cameca Local Electrode Atom Probe 3000X HR to investigate the 3D

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FIG. 1. Schematics of GaN/AlN/GaN/InAlN HEMT structures (a) grown on FS GaN for structural studies (structure A) and (b) grown on SiC for HEMT fabrication (structure B).

distribution of In. APT is a destructive technique by which a 3D map of atomic positions in the specimen can be reconstructed. For this purpose, tapering tips with radii of approximately 50 nm were created from structure A. The specimens were cooled to a base temperature of 30 K, and a high voltage between 4 and 8 kV was applied. The surfaces of the specimens were then evaporated at a rate of 0.008 atoms/ pulse by simultaneous pulsing of a Nd:YAG laser (532 nm second harmonic and 120 ps pulse width) at 200 kHz with a pulse energy of 0.02 nJ and collected at a position sensitive detector. Atomic characterization was then determined by time-of-flight measurements, which gave an approximation of the mass-to-charge ratio. For the data reconstruction, the specimen was modeled as a hemisphere on a truncated cone. Suitably chosen reconstruction parameters demagnify the detector coordinates of the atoms to give their specimen coordinates.17 To investigate the structural properties of the latticematched In0.18Al0.82N layers, cross-sectional STEM studies (Fig. 2) were performed. No compositional modulation was observed in the In0.18Al0.82N layer, as opposed to previous results on samples grown by PAMBE reported in the literature.13–16 We believe that having an excess In flux, which acted as a surfactant for the Al adatoms, and a higher

FIG. 2. Cross-sectional STEM image of structure A, showing uniform contrast in the InAlN layer.

Appl. Phys. Lett. 104, 072107 (2014)

FIG. 3. (a) APT proximity histogram (1D concentration profile) of structure A. (b) 2D map of the in-plane (perpendicular to growth direction) In distribution in the InAlN layer of structure A. (c) 3D APT reconstruction of structure A.

growth temperature in comparison to previous works prevented the formation of the AlN-rich dynamical platelets described by Sahonta et al.15 The relatively rough upper interface of the AlN interlayer, shown in Fig. 2, was the result of poor Al adatom surface diffusion with a substrate temperature of 700  C. This issue was resolved in structure B by keeping the Ga shutter open during the growth of the AlN interlayer.18 The liquid Ga adlayers act as a surfactant and reduce the energy barrier for Al adatom diffusion.19 APT of structure A (Fig. 3) revealed that the spatial distribution of In in the InAlN layer followed that which would result from a random alloy-binomial distribution. The average In mole fraction in the InAlN layer was measured to be 0.27, which was higher than that determined from HRXRD through Vegard’s law. We believe that this discrepancy arose from the lack of bowing parameters applied during HRXRD analysis. An APT 1D composition profile (Fig. 3(a)) indicated that the AlN interlayer was essentially pure with Al occupying nearly 98% of the III-sites.20 It should be noted that some of the elemental intermixing could be due to reconstruction artifacts arising from the large evaporation field differences between GaN and AlN. Structure B was diced into 7  7 mm2 segments for Hall measurements with Van der Pauw geometry. Annealed In contacts were formed at the corners of the segments.

FIG. 4. Temperature-dependence of the 2DEG mobility and sheet charge density for an N-face GaN/AlN/GaN/InAlN HEMT (structure B).

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regime. The elimination of the columnar microstructure previously observed in N-rich PAMBE-grown InAlN layers was achieved through increases in the growth temperature and fIn/(fAl þ fIn). A RT 2DEG mobility of 1100 cm2/V s and sheet charge density of 1.9  1013 cm2 were measured in an N-face GaN/AlN/GaN/InAlN heterostructure, and N-face HEMTs with lattice-matched InAlN back barriers were fabricated. These N-face HEMTs exhibited a maximum drain current of 1.19 A/mm and a high transconductance of 760 mS/mm. FIG. 5. DC-IV measurements of a GaN/AlN/GaN/InAlN HEMT (fabricated on structure B).

The authors would like to acknowledge funding support from the Solid State Lighting and Energy Center at UCSB and the Office of Naval Research (Dr. P. Maki, Program Manager). This work made use of the NSF National Nanofabrication Infrastructure Network and the central facilities supported by the NSF MRSEC at UCSB. The authors also appreciate fruitful discussions with Karine Hestroffer.

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FIG. 6. Extrinsic transconductance (Gm) for a HEMT fabricated on structure B as a function of gate-source voltage (Vgs) for different drain voltages (Vds).

Figure 4 shows the temperature-dependence of the two-dimensional electron gas (2DEG) mobility and sheet charge density of structure B. A 2DEG mobility and sheet charge density of 1100 cm2/V s and 1.9  1013 cm2, respectively, were measured at room temperature (RT), which corresponded to a sheet resistance of 299 X/ⵧ. The drop in sheet charge density at low temperatures, which has been observed previously by Smorchkova et al.21 for AlGaN/GaN heterostructures grown on SiC, is most likely attributed to the nonlinearity of stress states with temperature. HEMTs were then fabricated on structure B. A Ti/Al/Ni/Au (20/120/30/50 nm) multilayer stack, annealed at 820  C for 30 s in N2, was used for the Ohmic source and drain contacts. Mesas were formed with BCl3/Cl2 reactive ion etching. Ni/Au/Ni (30/250/50 nm) was used as the gate metallization. The HEMTs were 2  75 lm wide with a nominal gate length of 0.7 lm, a gate-source spacing of 0.3 lm, and a gate-drain spacing of 0.9 lm. The HEMTs showed a maximum DC drain current (ID) of 1.19 A/mm at a gate-source voltage (Vgs) of 0 V and a soft pinch-off at a Vgs of 2.5 V (Fig. 5). A maximum transconductance (Gm) of 760 mS/mm (Fig. 6) was obtained, which is relatively high for a HEMT structure with the above-mentioned geometry.22 We are currently investigating the reason for such a high Gm by performing AC and DC measurements on samples with different structural designs. In conclusion, N-face InAlN with homogeneous alloy composition was demonstrated using PAMBE in the N-rich

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