The Role of the Multi Buffer Layer Technique on the Structural Quality of GaN
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INTRODUCTION GaN and related alloys are of particular interest since their ability to cover a wide spectral range [1] that is not possible with any combination of any other semiconductor materials. Since no cheap crystalline substrate with a lattice parameter close enough to that of GaN can be available, growth techniques have been improved in order to limit the defect density in the GaN layer. The most commonly used technique consists in the deposition of an AlN buffer layer to form the junction between the substrate and the GaN layer and leads to
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the formation of more than 109 dislocations/cm2. Elaborated techniques have since emerged. The so-called "lateral epitaxial overgrowth" LEO technique appears today to give the best results. The lowest dislocation density is observed at the top of the GaN layers despite the presence of grain boundaries with mixed dislocations to compensate a misorientation [2]. Another technique is to first deposit a thick low-temperature (LT) GaN on the substrate. The first stage of the growth gives 3-D islands of GaN on the substrate. Afterwards, deposition of GaN at high-temperature (HT) makes the growth follow the same principles as those for LEO. These two latter techniques are based upon the faster growth along the < 2 110> directions. In this paper, we report on a Transmission Electron Microscopy study of GaN layers obtained by using the deposition of successive LT-GaN layers between HT-GaN layers.
EXPERIMENTAL GaN layers were grown on (0001) sapphire substrates in an horizontal reactor as described in ref [3]. TMGa, TMAl and amonia were used as Ga, Al and N source respectively, at around 140 Torr growth pressure.
fig 1: Schematic drawing of the structure of the two studied samples The structure of the samples used in this study is represented in fig. 1. A 1 µm thick GaN layer is deposited at high-temperature (HT) over a low-temperature LT-GaN or LT-AlN buffer layer. Afterwards, a second buffer layer is deposited at low-temperature (LT). This second buffer layer is called interlayer and its goal is to improve the quality of the subsequent 1 µm thick HT-GaN layer. It is a few nanometer thick AlN or GaN layer. For the investigations, the TEM samples were prepared by standard procedure using ion-milling at liquid
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nitrogen temperature and examined using a Jeol 200 CX operating at 200 kV. Before the observations, the samples were mounted on a gold covered grid and then dipped in a KOH (50%) solution for 5 min. This etch step was aimed at removing damage created during the ion milling.
RESULTS A- GaN/GaN interlayer/GaN Fig 2 is a dark field image of the sample the structure of which is represented in fig 1.a. The diffraction conditions were chosen so that all dislocations are visible in the image. It is noticed that the first HT-GaN layer is highly defective with a huge density of threading dislocations. Conventional diffraction contrast method was used to determine their Burgers vectors b using the invisibility criterion, g.b=0. Diffraction analysis showed that
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