A New Method for Lift-off of III-Nitride Semiconductors for Heterogeneous Integration
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The release and transfer of GaN epilayers to other substrates is of interest for a variety of applications, including heterogeneous integration of silicon logic devices, III–V power devices and optical devices. We have developed a simple wet chemical etching method to release high-quality epitaxial III-nitride films from their substrates. This method builds on a nanoepitaxial lateral overgrowth (NELO) process that provides III-Nitride films with low dislocation densities. NELO is accomplished using a nanoporous mask layer patterned on GaN substrates. Chemical removal of the SiO2 layer after growth of III-Nitride overlayers causes fracture at the interface between the GaN film and the original GaN substrate, resulting in free-standing GaN films with nanostructured surfaces on one side. These layers can be transferred to other substrates, and the nano-structured surface can be used in photonic devices, or planarized for power devices.
KeywordsNanorod Lift off III-nitride semiconductor
Besides its applications in high efficiency and high power optoelectronics, e.g. Light Emitting Diodes (LEDs) and Laser Diodes (LDs), RF transistors, power electronics or photodetectors , III-nitride compound semiconductors nowadays attract high interests in utilizing their unique optoelectronic properties, and integrating them heterogeneously with other material system to form more functional devices for future computational and telecommunication applications. A robust integration technique would allow the adding of high performance functionalities provided by GaN devices on other substrates for various advanced applications [2–6].
One approach to integrate III-nitride onto other material is direct heteroepitaxial growth on other substrates. For example, the growth of GaN structures on Si (100) or Si (110) substrates by molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD) have been reported [7, 8] for the integration of GaN devices with Si circuits. However, the material quality achieved through direct deposition is far from ideal. High density of deleterious threading dislocations in the active device layer due to large lattice mismatch and thermal mismatch inhibits the device performance. A more attractive approach is heterogeneous integration, where GaN structure that is independently grown and optimized, is released from its original substrate and transferred to favorable substrates. On-wafer integration of Si (100) MOSFETs and GaN HEMTs [9, 10], GaN optical interconnects  and thin film transistors on flexible substrate  have been demonstrated by using release and transfer techniques, such as laser lift off or chemical removal of substrates, layer transfer and wafer bonding technique.
To transfer the GaN devices, one of the key components involves releasing the GaN layers from its original substrate. The most cost effective approach to release GaN layers from its original substrate is to selectively etch a sacrificial layer. For example, Si released from Silicon on Insulator (SOI) and GaAs with AlAs as a sacrificial layers [12, 13]. The epitaxial lift off (ELO) method was well established since 1990s’ due to the availability of the etching selectivity between GaAs and AlAs. Many devices such as LEDs, lasers, MESFETs, and Photodiode were demonstrated based on the ELO method, [12–14] and heterogeneous integration was achieved in these system . However, in the case of GaN material, the use of sacrificial layer etching is still far less optimized in view of the material quality achieved and the device performance [16–18]. Laser lift off (LLO) [19–23], Photoelectrochemical (PEC) lateral etching [24, 25], were alternative approaches to release GaN layers, however, these methods were costly and limits the scope of application. Therefore, there is a need for a low cost and effective chemical process to release GaN from its substrates without compromising the material quality.
In this paper, we report a simple yet effective method to release high quality gallium nitride film from its substrate. The method utilizes nanoepitaxial lateral overgrowth (NELO) process, where GaN film is grown over nanoporous SiO2 mask layer. After chemical removal of the SiO2 nano-networks, the GaN with nanostrutures releases spontaneously from the bottom GaN template layer, leading to a high quality GaN film with a nanostructured surface on one side. The key factors in this method are (1) nanostructured layer materials that can be selectively removed by wet chemicals. (2) the interaction between the nanostructured SiO2 layer and GaN nanostructures at the interface enabling the self release after chemical etching. This method offers the advantages of a low cost simple process with high etching selectivity and it produces GaN of good crystal quality. It could potentially lead to highly integrated and efficient III–V nanoelectronics, nanophotonics and power devices on silicon and mechanically flexible circuits.
Nano-Epitaxial Lateral Overgrown (NELO) GaN is grown over the nanostructured SiO2 mask on the GaN template. A GaN template layer ~1 μm was first grown on sapphire (0001) substrate using metalorganic chemical vapor deposition (MOCVD). A ~100 nm SiO2 film was then deposited on the GaN template layer using plasma enhanced chemical vapor deposition (PECVD), followed by fabricating anodized alumina (AAO) to create nanostructures. The detailed AAO anodization process was reported previously [26–28]. CF4-based inductively coupled plasma (ICP) etching was employed to transfer the nanopore structures of AAO into the SiO2 layer. The AAO template was then removed by chemical etchant, resulting in a closed packed nanopore arrays in the SiO2 layer on the surface of GaN. The mean pore diameter and interpore distance were of 60 and 110 nm, respectively. NELO GaN layer was then regrown on the GaN template covered with nanoporous SiO2 by using MOCVD. The detailed growth condition is reported previously elsewhere [26–28].
Coulombic interaction is expressed in the first term of interatomic potential; the second term is the Gilbert-type short-range repulsion, while covalent bonding and repulsion of the modified Morse types is represented by the third and fourth terms respectively. The last term represents Van der Waals interactions. rij is the distance between the i th and j th atoms, Zi is the effective charge for each atom, εo is the dielectric constant of the vacuum, fo is the constant for unit conversion [41.86 kJ/(nm mol)], ai is the repulsion radius, bj is the softness parameter, D1, D2, β1 and β2 are covalent coefficients, and ci and cj are the Van der Waals coefficient. Detailed descriptions of the parameters are obtained in Ref. 24. The time evolution trajectory of the atoms is solved using Verlet algorithm. The time step of 2 fs is used in the present study. All atomic pairs between Ga and N are taken into account and calculated for all the potentials in the equation.
In conclusion, a simple and reliable method to form high quality freestanding GaN film is demonstrated. The GaN thin film is grown through a SiO2 template with nano-openings on top of a GaN layer and released later by HF based wet chemical etching. The self-release of the GaN thin film is enabled by the built-in stress originating from the interaction of GaN and the SiO2 mask. The GaN film produced by this method offers better crystal quality, beneficial to high efficiency and high power applications. In addition, the nanostructures in GaN produced by this method can lead to improved light extraction for LED applications and can be used to create hierarchical structures for nanophotonic applications. The method is potentially useful for applications that involve heterogeneous integration of III-nitride based optical and power devices on Si or flexible substrates, as well as various other applications, e.g. MEMs system, microcavities and microoptics systems and nanophotonics.
This project was funded by Agency for Science, Technology and Research (A*STAR) of Singapore under grant No. IMRE/09-1P0605.
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