GaN microelectronics technology
Growing AlInN
At first sight it appears quite challenging to grow high- quality AlInN films. There are huge differences in growth conditions of AlN and InN, which have typical growth temperatures of 1100 °C and 500 °C, respectively. In addition, the lattice mismatch between these materials can be as high as 13 percent. However, it is possible to realize a homogeneous ternary through careful optimization of the growth conditions.
At EPFL in Lausanne, Switzerland, AlInN/GaN epilayers are grown in an Aixtron 200/4 RF-S MOCVD system on 2-inch substrates made from c-plane sapphire, silicon, and SiC. Growth on sapphire is initiated by a low- temperature GaN nucleation layer, and an AlN buffer is employed for silicon and SiC. In all cases, a 0.5-2 µm- thick, undoped GaN layer follows the buffer. This is grown using conditions to minimize possible parasitic conduction
paths, and its net residual doping concentration (ND-NA) is typically below 1014 cm-3.
All HEMT structures are free from cracks. Dislocation densities in the epilayers are governed by the substrate choice, and range from 7 x 108 cm-2 for sapphire to 5 x 109 cm-2 for silicon. Typical X-ray diffraction (XRD) rocking curve linewidths are less than 1000 arcsec for GaN deposited on silicon, below 500 arcsec for GaN grown on sapphire, and under 200 arcsec for GaN epitaxial layers on SiC.
The AlInN/GaN heterostructure features a thin GaN channel, grown under conditions specifically chosen to improve surface morphology so as to form a good interface with the barrier material. An AlN interlayer just a nanometer thick is inserted between the channel and the AlInN barrier to limit the detrimental impact of alloy scattering. When properly done, this delivers a massive improvement in the 2DEG lateral transport properties. Growth of the AlInN layer is typically carried out at 800- 850°C, using deposition rates of 0.2-0.6 µm/h. If the temperature is too low, the crystal quality degrades, according to high-resolution x-ray diffraction (XRD). On the other hand, if the temperature is too high, it impairs indium incorporation and prevents formation of near-lattice matched alloys. Treading the fine line between these two unwanted scenarios is crucial to realizing good 2DEG properties, and ultimately is the key to great HEMT performance. Material uniformity of thick, nearly lattice-matched AlInN epilayers grown on GaN-on- sapphire templates can be assessed by energy dispersive X-ray analysis. The indium composition slowly varies across the wafer indicating good homogeneity (see Figure 2). It is typically 17 ± 1 percent for lattice-matching to GaN. The surface of the AlInN barrier is very smooth and has a root-mean-square roughness of just 0.5 nm (see Figure 3).
Figure 5. Sapphire and SiC still offer superior platforms to silicon for the growth of AlInN/GaN HEMTs, but silicon is not too far behind
Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDXS) analysis reveals a sharp interface, with no evidence of gallium diffusion into the AlInN barrier (see Figure 4) [13]. If this element, gallium, had significantly contaminated the barrier, it would decrease the 2DEG electron density in the channel. By varying the indium content of the AlInN barrier and its thickness, it is possible to realize 2DEG densities ranging from 0.5-3.5 x 1013 cm-2.
Room-temperature mobility in the 2DEG depends on both the sheet carrier density and the type of substrate (see Figure 5). The lowest sheet resistances, typically 200 Ω/ are obtained on sapphire and SiC. On silicon, the sheet resistance is slightly larger, due to the lower crystalline
Figure 4. Gallium does not diffuse into the AlInN barrier,
according to STEM and EDXF analysis. These are reproduced courtesy of Dr. L. Zhou and Prof. D.J. Smith, Arizona State University
August / September 2010
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