Electrical Profiles in GaN/Al 2 O 3 Layers with Conductive Interface Regions
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Internet Journal Nitride Semiconductor Research
Electrical Profiles in GaN/Al2O3 Layers with Conductive Interface Regions D.C. Look12, J. E. Hoelscher12, J. L. Brown2 and G. D. Via3 1Semiconductor
Research Center, Wright State University, Dayton, OH 45435, Force Research Laboratory, Materials and Manufacturing Directorate, Dayton, OH 45433, 3Air Force Research Laboratory, Sensors Directorate, Dayton, OH 45433, 2Air
(Received Wednesday, April 4, 2001; accepted Wednesday, May 9, 2001)
Differential Hall-effect measurements are used to obtain profiles of the mobility, µ, and carrier concentration, n, in a 6-µm-thick GaN layer grown on Al2O3 by hydride vapor phase epitaxy (HVPE). In the top 1-µm region (surface), µ ≈ 1000 cm2/V-s and n ≈ 3 x 1016 cm-3, whereas in the bottom 0.75-µm region (interface), µ ≈ 50 cm2/V-s and n ≈ 2 x 1019 cm-3. Throughout the layer, the carrier concentration correlates well with the O and Si concentrations, with [Si] dominant near the surface, and [O] dominant near the interface, proving the shallow-donor nature of O. The average mobility and carrier concentration in the top 5 µm, i.e., the “bulk” region, are close to the values deduced by a much simpler analysis, introduced previously.
1 Introduction GaN is typically grown epitaxially on lattice-mismatched substrates, since large-area GaN wafers are not available [1]. The most commonly used substrate is Al2O3, but its 13.8% lattice mismatch with GaN can lead to very poor interface characteristics. A partial solution to this problem involves low-temperature growth of a thin GaN or AlN buffer layer between the Al2O3 and the main GaN layer, and this technique works well for layers grown by metal-organic chemical vapor deposition (MOCVD) or by molecular-beam epitaxy (MBE). On the other hand, buffer layers are typically not employed in hydride vapor-phase epitaxy (HVPE), although sometimes the Al2O3 is pretreated with ZnO before the GaN growth [2]. Whatever the case, reports indicate that the HVPE growth process always leads to a thin, highly conductive interface region, which is not only detrimental to certain devices, but which also obscures the bulk electrical properties [3]. This problem is of practical importance, because it appears that thick HVPE GaN layers, separated from their Al2O3 substrates, are becoming a material of choice for GaN homoepitaxy [4] [5]. Thus, it is essential to develop techniques to isolate the bulk electrical properties from those near the interface. Here we show how differential Hall-effect (D-H) measurements can be used for that purpose, and we compare D-H results with
those achieved by a much simpler method, presented previously. 2
Two-layer Model
Roughly speaking, we can model HVPE GaN samples as being composed of a bulk region with thickness db, mobility µb, and carrier concentration nb, and a thin interface region, with similar parameters di, µi and ni. (In this study, we will deal with Hall mobilities and Hall concentrations. The connections with conductivity mobilities and the true concentrations, res
