TEM Sample Preparation of Irradiated GaN on a Sapphire Substrate

By Prashanth Prasad, Materials Applications Engineer, Gatan, Pleasanton, CA

For much of the past decade, GaN has been a subject of extensive research due to very important technological applications of this material. Current applications of GaN include light-emitting diodes (LED's), laser diodes, UV detectors, and microwave power and ultra-high power switches. In the fabrication of such GaN-based devices, ion bombardment represents a very attractive tool for several technological steps, electrical and optical selective-area doping, dry etching, electrical isolation, quantum well intermixing, and ion-cut. Successful applications of ion implantation depend on understanding the production and annealing of radiation damage. In this regard, transmission electron microscopy (TEM) plays a pivotal role. Consequently, preparation of good TEM samples is paramount in the detailed studies of ion implantation damage in GaN. Thus, knowledge of appropriate TEM sample preparation techniques is not only important for investigating fundamental defect processes in solids under ion bombardment but also essential for the fast developing GaN industry.

Cross-section TEM sample preparation from bulk sample is accomplished using the Gatan Model 601.07000 TEM cross-section kit, the Gatan Model 601 Ultrasonic Cutting Tool, the Gatan Model 656 Dimple Grinder and the Gatan Model 691 PIPS.

Mechanical Preparation:

A 4x5mm rectangular wafer is cut using the Ultrasonic Cutting Tool. The sample wafer and some dummy Si wafers are coated with a thin layer of G-1 epoxy and placed in a Teflon mold such that the interface of interest is at the center of the stack. The glued stack is cured on a hot plate at 130^o C for 10 minutes under pressure, using a spring clamp, to form a strong bond between the wafers. A 2.3mm cylinder is cut from the stack using the Ultrasonic Cutting Tool. The inside of a 3mm diameter brass tube is coated with G-1 epoxy and the 2.3mm stack inserted into the brass tube and cured on a hot plate at 130^o C for 10 minutes. A diamond saw is then used to slice 3mm specimen discs ready for grinding and dimpling. The sample is thinned and parallel polished to about 60 microns, then dimpled and polished from one side, using the Gatan Model 656 Dimple Grinder, to a thickness of about 10-15 microns. Since Sapphire is a brittle material, samples must be handled with special care especially at low thicknesses.

Ion beam Milling:

The sample is milled in the PIPS in two steps, using beam modulation, with the parameters shown below:

1. 3.5 keV, Gun angles of 4^o Top and 3^o Bottom, till perforation reaches interface of interest.
2. 2.5 keV, Gun angles of 3^o Top and 2^o Bottom, 5 min, to improve final polish.

Results:

A bulk “GaN on sapphire (Alumina)” sample, ion implanted with Gold, received from the Australian National University, was prepared using the technique described above. The TEM sample preparation successfully produced a large amount of thin area. The bright field TEM micrograph obtained at 200kV, as seen in Fig. 1, clearly shows the presence of N2 gas “bubbles” that are characteristic of ion implanted GaN. The image also shows the presence of three sub-layers in Alumina. The Gold ions pass through GaN and come to rest in Alumina at a depth of 0.7 um below the GaN surface. This is responsible for the “three sub-layers” appearance. The first sub-layer is significantly damaged Alumina; the second sub-layer is irradiation induced partial structural disorder and the third sub-layer is undamaged Sapphire.

Fig 1: Bright Field TEM image obtained at 200kV showing various layers and sub-layers in the sample.

High-resolution TEM images of the GaN layer showed the presence of pockets of nano-crystalline GaN in an otherwise amorphous matrix of GaN (Fig. 2). It can be seen the orientation of these nano-crystals is quite different from each other, indicating that they are a result of ion induced re-ordering within the material.

The sample had vast amounts of thin area where meaningful EELS analysis could be conducted. Fig. 3 shows an Energy Filtered TEM image that confirms the identity of the various cross-sectional layers. The maps were made from two Energy Filtered TEM Spectrum Images (EFTEM SI) taken on a Model 863 GIF Tridiem: the first series from 200eV to 600eV with a 15eV step and slit size and a 3 second exposure per plane (40 planes), and the second series from an EFTEM SI series over 950-1960eV with a 30 eV step and slit size and a 15 second exposure per plane (33 Planes).

Conclusions:

Cross-section TEM sample preparation of the irradiated “GaN on Sapphire” sample was successfully achieved using the Gatan TEM cross-section kit, Ultrasonic Cutting Tool, Dimple Grinder and PIPS. Large amounts of clean, damage-free electron-transparent areas were observed on the prepared sample. The damage induced by the Gold irradiation on the GaN layer and the Sapphire/Alumina layer was successfully characterized. The TEM sample preparation technique that was followed created a high quality specimen, which allowed extensive, meaningful EELS analysis and HRTEM studies to be performed easily.

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