Design of step-graded AlGaN buffers for GaN-on-Si heterostructures grown by MOCVD

Abstract

For the growth of low-defect crack-free GaN heterostructures on large-area silicon substrates, compositional grading of AlGaN is a widely adapted buffer technique to restrict the propagation of lattice-mismatch induced defects and balance the thermal expansion mismatch-induced tensile stress. So far, a consolidation of the design strategy of such step-graded buffers has been impaired by the incomplete understanding of the effect of individual buffer design parameters on the mechanical and microstructural properties of the epilayers. Herein, we have analysed a series of metal-organic chemical vapor deposition (MOCVD) grown GaN/graded-AlGaN/AlN/Si heterostructures through in situ curvature measurements and post-growth X-ray diffraction (XRD). Our results reveal that in such epi structures, the GaN layer itself induces more compressive stress than the AlGaN buffer, but the underlying AlGaN layers dictate the magnitude of this stress. Furthermore, for a fixed AlGaN buffer thickness, the mean-stress accumulated during the GaN growth is found to be correlated with its structural properties. Specifically, one µm-thick GaN layers that acquire 1.50 GPa or higher compressive mean-stress are seen to possess XRD ω-FWHM values less than 650 arc-sec. Also, the evolution of instantaneous stresses during the growth of the AlGaN layers is found to be a valuable indicator for buffer optimization, and composition difference between successive layers is established as a crucial criterion. The results also show that increasing the total buffer thickness (for a fixed number of steps) or increasing the number of steps (for a fixed total buffer thickness) may not always be beneficial. Irrespective of the buffer thickness, optimized high electron mobility transistor (HEMT) structures show similarly low sheet-resistance (~ 350 Ω/□) and high mobility (~ 2000 cm2/V-s) at room temperature.

Additive GaN Solid Immersion Lenses for Enhanced Photon Extraction Efficiency from Diamond Color Centers

“…A schematic of the process flow is indicated in Figure while corresponding experimental results are shown in Figure . Figure a illustrates how the GaN solid immersion lenses are defined and suspended on a strain-optimized heteroepitaxially grown GaN-on-Si chip − based on the wafer-scale compatible microfabrication process reported in earlier study . Initially, polymer resist lenses are defined by grayscale lithography and thermal reflow (1), followed by inductively coupled plasma (ICP) dry etching to transfer the lens shape into the 2 μm thick GaN top layer (2).…”

III-Nitride Magnetron Sputter Epitaxy on Si: Controlling Morphology, Crystal Quality, and Polarity Using Al Seed Layers

“…The heteroepitaxial growth on cost-effective Si(111) substrates is the preferred choice for power electronic devices; however, GaN-on-Si epitaxy is characterized by large mismatches in lattice parameter and thermal expansion coefficient. High defect densities and cracking of films exceeding 1 μm in thickness are related issues that still need to be solved in the epitaxy process. , A process window for high-quality growth of GaN/AlN by MSE has already been demonstrated. − Careful design of buffer layers and strain management is necessary to successfully grow high-quality and crack-free GaN/AlN on Si. ,, The integration of an Al seed layer into the process flow has proven beneficial to improve the crystal quality of AlN-on-Si grown by various epitaxy technologies. Enhanced surface diffusion and avoiding the formation of SiN x layers promote the adhesion and subsequent growth of AlN layers. − However, a thorough investigation of the impact of this seed layer on the film morphology, structural quality, and film polarity of sputtered GaN/AlN/Si(111) stacks has rarely been reported.…”

“… 15 − 18 Careful design of buffer layers and strain management is necessary to successfully grow high-quality and crack-free GaN/AlN on Si. 8 , 14 , 19 The integration of an Al seed layer into the process flow has proven beneficial to improve the crystal quality of AlN-on-Si grown by various epitaxy technologies. Enhanced surface diffusion and avoiding the formation of SiN x layers promote the adhesion and subsequent growth of AlN layers.…”

Effect of High Al Content AlGaN Buffer Layer on the Properties of GaN‐on‐Silicon Materials

“…Shen et al [ 17 ] achieved the growth of 2 μm‐thick, crack‐free GaN thin films by introducing ultra‐thin AlN/GaN superlattice interlayers into the superlattices, they controlled the average Al content by varying the thickness of the superlattice AlN and GaN, and found that cracks could be suppressed more effectively if the optimal average Al content is less than 0.5. Ghosh et al [ 18 ] optimized MOCVD‐grown GaN/graded‐AlGaN/AlN/Si heterostructures by in‐situ curvature measurements, and they found that the GaN layer produces more compressive stress than the AlGaN buffer layer, and the underlying AlGaN layer determines the magnitude of this stress. Furthermore, when the buffer layer thickness is fixed, the average stress accumulated during GaN growth is related to its structural properties.…”