Improved Structural and Chemical Properties of Nearly Lattice

ARTICLE pubs.acs.org/crystal

Improved Structural and Chemical Properties of Nearly Lattice-Matched Ternary and Quaternary Barriers for GaN-Based HEMTs Jos
e M. M
anuel,† Francisco M. Morales,*,† Rafael García,† Taek Lim,‡ Lutz Kirste,‡ Rolf Aidam,‡ and Oliver Ambacher‡ † ‡

Dep. Ciencia de los Materiales e IM y QI, Universidad de C
adiz, 11510-Puerto Real, C
adiz, Spain Fraunhofer Institute for Applied Solid State Physics, Tullastrasse 72, 79108 Freiburg, Germany ABSTRACT: A complete structural and compositional study was carried out for a series of GaN-based lattice-matched HEMT structures. As barrier materials pseudomorphic to GaN, both ternary InAlN and quaternary InAlGaN were investigated. Growths were performed using molecular beam epitaxy on GaN/ sapphire or GaN/SiC templates. An abrupt triple-layer AlN/GaN/AlN nanothin spacer at the interface is crucial to improve the structural and electrical properties of the heterostructures. In all cases, this resulted in single-crystalline and single-phase barrier layers.

1. INTRODUCTION High electron mobility transistors (HEMTs) are currently one of the most important types of analog electronic devices, due to their outstanding performance in both low noise applications and high power circuits.1 These devices are based on a two-dimensional electron gas (2DEG) that forms in an energy well at the interface between two semiconductor layers with different band gap values (barrier and channel). A high mobility can be achieved in the undoped channel of a HEMT, provided that the involved materials are of a high crystalline quality and homogeneous in composition. Since 1980, materials used to fabricate HEMTs have evolved. Arsenides (such as GaAs) and InP were complemented by GaN,2
4 since it presents a wider band gap and thermal stability, and higher peak electron and saturation velocities.5 The AlGaN/GaN system has much improved in the last two decades and is currently in an early stage of commercialization,6 allowing higher power densities than other material systems for HEMTs. Regardless of this, strain constitutes an intrinsic limitation for AlGaN/GaN junctions, making unfeasible a nonstressed barrier layer. Most recent research points to heterostructures such as InAlN/GaN or InAlGaN/GaN,7,8 which could avoid this problem for the proper barrier composition, obtaining thus a commensurate lattice-matched growth. Nevertheless, the fabrication of these InN-containing alloys presents great difficulties, such as the low miscibility of the ternary or quaternary compound and the narrow window of feasible growth temperatures.9 On the other hand, while for InAlN/GaN a perfect lattice-matching is obtained for a single composition,10 the quaternary alloy permits a wide range of chemical combinations for its unstrained pseudomorphic growth. Several reports on InAlN/GaN heterostructures grown r 2011 American Chemical Society

by molecular beam epitaxy (MBE) or other techniques were published during the past decade but exhibited difficulties in growth.11
14 However, structures with good quality have recently been reported.10,15 This is still not the case for quaternary films nearly lattice-matched to GaN, for which chemical vapor deposition first attempts are from 1996,16 while MBE was first used in 2000.17 Most of these layers suffered from phase separation effects both in compositions by In segregation and atomic ordering18,19 or in the structure presenting mixed cubic and hexagonal phases.20 Recently some of the present authors have demonstrated excellent electronic properties for InAlGaN/GaN HEMTs grown by radiofrequency plasma-assisted MBE (RF-PAMBE).8,21 The present work focuses on the demonstration of the impressive structural and compositional characteristics of analogous specimens by a complete characterization using X-ray and electron beam based techniques. The atomically controlled placement of a sequence of stacked III
N binary nanolayers (AlN/GaN/AlN) between the barrier and the GaN buffer in combination with an optimized growth are the key for yielding the best morphology and electric properties.

2. EXPERIMENTAL SECTION Five specimens, having InxAlyN or InxAlyGazN (x þ y þ z = 1) barriers, and an additional AlyGazN/GaN sample for strain comparisons, are revised. The pseudotemplates consist of c-plane Al2O3 or 4H-SiC wafers on which a polar 1.3
2.0 μm-thick GaN layer was grown by metal
organic chemical vapor deposition (MOCVD). Subsequently, a Veeco GEN20A system was used to deposit by RF-PAMBE, starting Received: March 18, 2011 Revised: April 8, 2011 Published: April 12, 2011 2588

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Table 1. MBE Growth Parameters In-BEP

Al-BEP

Ga-BEP

growth rate

temp

(10
8Torr)

(10
8Torr)

(10
8Torr)

(nm/h)

(°C/K)

S1

0

0.6

7

∼140

610/883.15

S2

1.2

0.79

0

100

420/693.15

S3

1.1

0.73

2.0

∼120

420/693.15

S4

2.0

0.73

1.62

150

580/853.15

2.3

0.73

1.58

150

580/853.15

0.6

7

150

610/883.15

S5 S6

13

with a ∼100 nm polar GaN buffer layer, continued by the triple AlN/ GaN/AlN nanospacer, and terminated by the InxAlyGazN barrier layer. The AlN-based triple spacers have not been widely used before, but their utilization already demonstrated improvements in InAlN-barrier heterostructures.10 The epilayers have distinct compositions of x = 0 for specimen S1 (AlGaN), z = 0 for S2 (InAlN), and varying x/y/z ratios for S3 to S6 (InAlGaN). The MBE growth conditions for the different specimens, including MBE fluxes (beam equivalent pressure values 10
8 Torr), growth temperatures, and rates are listed in Table 1. Please note that flux values are from different calibration series and generally not directly comparable to each other. Consequently, the thicknesses of the barrier layers vary between ∼20 nm (samples S1 and S6) and ∼50 nm (S2 to S5). The X-ray diffraction (XRD) was carried out in a system with a two bounce Ge 220 monochromator and a triple axis Ge 220 with Cu KR1-radiation. Highresolution analyses, consisting in both XRD θ
2θ scans and reciprocal space maps, were made to determine lattice parameters. For the transmission electron microscopy (TEM) characterization, the samples were prepared in cross sections (XTEM) and studied in JEOL-1200EX and -2010FEG microscopes. Selected area electron diffraction (SAED) patterns, consistent with XRD, gave information on materials crystal orientations and lattice attributes. Diffraction contrast (DC-TEM) and high-resolution (HRTEM) images gave an overview of layers sequence, interfaces, and dislocations even at atomic resolution. To analyze the chemical homogeneity in layers, nanoprobe energy dispersive X-ray (EDX) spectra and high angle annular dark field (HAADF) images, both in scanning-TEM mode, were collected.

3. RESULTS AND DISCUSSION Imaging and diffraction analyses confirmed the formation of single-crystalline layers oriented in the directions [0001]Al2O3/ [0001]GaN/[0001]SB, where SB stands for wurtzite structures of spacer plus barrier (stacked AlN/GaN/AlN/InxAlyGazN). For S5, instead of sapphire, a [0001]SiC/[0001]AlN substrate is observed. Pairs of Figures (1a
b and 1c
d) present bright field DC-XTEM panoramic images (g = 0002 near the [11þþ20] zone axis), representative of InAlGaN/GaN systems on sapphire and on SiC, and their respective SAED patterns with reflections indexed into the expanded insets. These overviews prove that every layer is flat and sharp interfaced for extended regions, as well as the lack of significant structural defects, such as stacking faults, pyramidal inverted pits, or grain boundaries, common in these novel In-compounds. Nevertheless, typical threading dislocations (TD) are visible across the GaN template. These line defects are expected to cross and reach the top surface of the barriers due to the lattice coherence with GaN, but they are mostly blocked by the In containing barrier epilayers. The SAED patterns of all samples indicate the pseudomorphic growth of the barrier epilayers with respect to the GaN substrate.

Figure 1. (a) XTEM micrograph of sample S6; (b) SAED pattern of S4; (c) XTEM micrograph of S5; and (d) SAED pattern of S5.

This is clear by the vertical alignment of GaN and InxAlyGazN associated reflections, not only for symmetric but also for asymmetric ones (Figure 1b is an example). Moreover, these patterns, when taken for regions containing many materials, allow the determination of the orientations of the demonstrated single crystals. Note that the selection for the pattern of Figure 1d does not include the barrier. It can be concluded that the layers, in the direction parallel to the growth plane, are heteroepitaxial and are oriented as [0110]Al2O3/[1120]GaN/[1120]SB on sapphire and [1120]SiC/[1120]AlN/[1120]GaN/[1120]SB on silicon carbide. The thick GaN buffer layer in the heterostructures is strain-relaxed, as expected. In HAADF micrographs the intensity is almost proportional to the square of the atomic number, Z, and therefore, any difference in composition is detected with a high sensitivity through contrast changes. For all samples, these types of images and EDX have been used to search for possible inhomogeneities and related defects, such as chemically distinguishable columnar domains. As observed in Figure 2a and b, the barrier layers are, as the spacers, highly homogeneous in composition. This is deduced from invariant smooth contrasts in any layer with sharp interfaces and, thus, no hint of intermixing. This is also deduced from HRTEM, as revealed in the atomically abrupt transitions between single crystals (Figure 2c). The high crystalline quality 2589

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Table 2. Composition, Lattice Parameters, Strain, and Misfit of the Barriers x/y/z (%, EDX)

Figure 2. (a) HAADF images of S4 (on Al2O3); and (b) of S5 (on SiC); (c) HRTEM image of the HEMT active region of S3.

of layers with the characteristic ABAB wurtzite stacking sequence is shown, and no misfit dislocations are visible in the HEMT active interfaces, which constitutes a further evidence of the pseudomorphic growth of the barrier and spacers. EDX has been utilized to determine the composition of each barrier. The chemical homogeneity was confirmed through the low standard deviations of the average values for the composition obtained by measurements at many local positions along and across each epilayer. Averaged values are listed in Table 2 together with their lattice parameters measured by XRD and derived lateral strains (εa and εc). Moreover, the misfit with respect to the underlying GaN (f), i.e. the deviation from ideal pseudomorphicity, is included in that table. These values are calculated using the expressions in eqs 1 and 2, respectively, where d stands for either measured a and c, the lattice parameters, d0 is the associated lattice constant for an unstrained alloy of the same composition according to the Vegard’s law, and aL and aS are the actual basal lattice parameters of the epilayer and the GaN substrate, respectively. d
d0 d0

ð1Þ

aL
aS aS

ð2Þ

εd ¼

f ¼

The barriers of samples S2, S3, S5, and S6 can be considered pseudomorphic, consistent with the observed SAED patterns. For samples S1 (AlGaN) and S4 (InAlGaN with low InN content) with relatively higher in-plane tensile strains, the barrier layer shows a small relaxation due to slightly bigger misfits. The compositions calculated considering the XRD parameters and the Vegard’s law for binary elastic and lattice constants10,21 also present a good consistency with the values determined by the EDX measurements. Although the increase of the Ga percentage from InAlN to the InAlGaN barrier layers implies a higher miscibility based on thermodynamics, an equivalent homogeneity can be assigned to

a (A)

c (A)

εc (%)

εa (%)

%f

S1

0.00/41.88/58.12

3.1820

5.0777


0.43

0.81


0.13

S2 S3

18.02/81.98/0.00 10.52/66.78/22.70

3.1889 3.1854

5.1104 5.1026


0.01
0.01

0.01 0.36

0.02 0.00

S4

1.11/42.14/56.75

3.1811

5.0865


0.36

0.67


0.09

S5

4.46/38.95/56.59

3.1924

5.1176


0.22

0.57

0.00

S6

7.18/37.40/55.42

3.1863

5.1488

0.06

0.04

0.03

any barrier according to comparable HAADF contrasts and EDX standard deviations of the averages. However, considering the electrical properties, better results were obtained as the amount of GaN increased in the InAlGaN alloy.22 Above 50% GaN content in the InAlGaN barrier, the mobility is comparable to well-established AlGaN/GaN HEMTs grown by MBE.23 For example, a mobility of over 1400 cm2/(V s) has been obtained using Hall Effect measurements24 for S6. Many factors may affect the electrical properties, such as the interface quality influenced by the spacer layers, alloy scattering by the barrier, or defects related to a nonideal material quality. The quality of the interface between the GaN channel and spacer, as well as the large-scale crystalline quality of the barrier layers as assessed by the presented TEM investigations and the widths of XRD rocking curves,25 are roughly comparable independent of the barrier composition. Further, the effect of alloy scattering would not be expected to decrease with increasing GaN content. Therefore, the increase in mobility with GaN concentration in the InAlGaN barrier (as authors of the manuscript have measured for similar samples in other works22) is probably caused by a possible improvement in the material quality on a subnanometric scale not revealed by HAADF images. These improvements can likely be attributed to the better miscibility of InAlGaN alloys with sufficient GaN content.

4. CONCLUSIONS RF-PAMBE constitutes an advantageous growth technique for lattice-matched and therefore unstrained InAl(Ga)N/GaN alloys. Both ternary InAlN and quaternary InAlGaN layers with high crystalline quality and homogeneous composition have been achieved. The use of a triple-layer AlN/GaN/AlN ultrathin spacer has contributed to improving the structural quality of the barrier layers, compared to the cases studied in previous works, in which this spacer was not included,10 although the major improvement this spacer layer provides is avoiding leaks of the electron current to the barrier. By combining the advanced spacer with an InAlGaN barrier of suitable composition, heterostructures with exceptionally high mobilities have been realized, enabling the fabrication of HEMT devices.8 ’ AUTHOR INFORMATION Corresponding Author

*Telephone: þ34-956016452. Fax: þ34956016288. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Junta de Andalucía (PAI group TEP-120 and projects P09-TEP-5403 and P08-TEP03516) and by the CICYT project MAT 2010-15206. EU COST MP0805 Action is also to be addressed in these acknowledgments. TEM analyses were carried out at DME, SCCYT-UCA. 2590

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