Deep Defects in Fe-Doped GaN Layers Analysed by Electrical and Photoelectrical Spectroscopic Methods
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DEEP DEFECTS IN Fe-DOPED GaN LAYERS ANALYSED BY ELECTRICAL AND PHOTOELECTRICAL SPECTROSCOPIC METHODS H. Witte, K. Fluegge, A. Dadgar, A. Krtschil, A. Krost and J. Christen Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg, D-39016 Magdeburg, Germany Abstract The electrical activity of iron in Fe- doped, and in Si and Mg co-doped GaN layers grown on sapphire substrates by metal organic vapor phase epitaxy was studied as shown by temperature dependent Hall Effect (TDH) measurements. In all samples iron doping generates an acceptor defect, which compensates donors in n-type GaN. Furthermore, iron doping causes strong potential inhomogeneities, which decrease the Hall mobility in the layers. To verify, if iron creates only hole traps, defects in n-type Si:Fe and Fe doped samples were investigated. The well known dominant electron traps in n-type GaN at 520 - 550 meV and 480 meV were found by deep level transient spectroscopy and thermal admittance spectroscopy, respectively. A high Fe-doped GaN layer shows a low p-type conductivity dominated by the iron acceptor. An activation energy of EV+ 460 meV was determined by TDH indicating, that the iron acceptor correlates with this defect level. Introduction Insulating GaN layers are of high interest for applications in high-frequency and high-voltage devices. To generate these insulating properties, the implantation of GaN layers with transition metals such as iron, chromium or vanadium is a useful approach [1]. However, implantation changes the concentration of deep defect levels and introduces additional impurities which influence the transport properties drastically [2]. Therefore, doping with iron is another way for the realization of semi- insulating GaN layers [3]. Nevertheless, the knowledge of the complex compensation mechanism in iron-doped GaN layers is scarce up to now. Essentially, the electrical properties of the iron defect charge states are not understood yet. We have analyzed defects and compensation effects in Fe-doped GaN layers grown by metal-organic vapor phase epitaxy on sapphire in detail. Experimental Details All samples were grown on (0001) oriented sapphire substrates using metal organic vapor phase epitaxy. Ferrocene was used as Fe-precursor. We have investigated three types of samples. At first, iron doped GaN samples were grown on a GaN seed layer. The reference sample #AR is an undoped GaN layer, 2.4 µm thick and with n =3.2*1017 cm-3 at 295 K. In the samples #A1 and #A2 the undoped layers are doped with a ferrocene flux of 50 nmol/min and 100 nmol/min, respectively.
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A second type sample was additionally co-doped with Si and Fe (GaN:Si:Fe). As a reference sample (#BR) a 3.6 µm Si-doped layer (n = 1.4*1018 cm-3) was grown on the top of an undoped 1.2 µm GaN and a GaN seed layer. In the Si and Fe co-doped sample #B1 the Si-doped top layer is additionally doped with iron with a ferrocene flux of 4 nmol/min. The third type of samples is GaN:Mg co-doped with Fe on an AlN seed layer. As a reference (sample #CR
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