Electron accumulation and doping in InN and InGaN alloys

• Main Author: Linhart, W. M.
• Published: University of Warwick 2012
• Subjects: 530.412; QC Physics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.560340

InN and group III nitride materials have attracted great interest due to their potential applications for optoelectronic devices, as the range of band gaps cover the ultra-violet to the near infrared. InN and all In-rich InxGa1−xN alloys exhibit a surface electron accumulation layer. This is due to the unusually low conduction band minimum (CBM) at the Brouillon zone centre (Γ-point) with respect to the charge- neutrality level. Electron accumulation has been observed at the surface of almost all n-type and p-type InN, making proof of p-type doping of this material very difficult. Routine characterization of p-type conductivity of Mg-doped samples using single-field Hall effect is prevented by the presence of a surface inversion space-charge layer, and hence the surface electron-rich region dominates the measurements. In this thesis, the results of investigations on non-polar InN surfaces, Mg-doped InN surfaces and a range of InxGa1−xN alloys across the composition entire range are presented. Considerable improvement of the quality of a- and m-plane InN thin films has been achieved using free standing GaN substrates in conjunction with a GaN buffer layer and grown by PAMBE. Using a combination of infrared reflectivity (FTIR), x-ray photoemission spectroscopy (XPS) and electrochemical capacitance voltage (ECV) measurements, the surface space charge properties of these samples have been investigated. The surface Fermi level has been determined to be lower than previously observed on non- cleaved InN samples. Additionally a high carrier concentration has been found on the non-polar InN, close to the interface with the GaN buffer layer, associated with unintentionally incorporated oxygen impurities. The increased concentration of oxygen impurities near the InN/GaN interface, confirmed by secondary ion mass spectrometry (SIMS), is due to the relatively low growth temperature (380 - 450 ◦C) used to produce the non-polar InN films. XPS has been also used in the investigations of Mg-doped InN. A significant lowering of the surface Fermi level has been observed with increasing Mg-doping for the highest Mg concentration (> 1 × 1019 cm−3) indicating a highly desirable reduction in the degree of surface electron accumulation. While for moderate Mg concentrations the surface Fermi level is at the previously determined ‘universal’ value of ~ 1.4 eV above the valence band maximum, for [Mg]=1.2×1020 cm−3, a value of 0.83 eV is found. As a consequence, for [Mg]> 1 × 1019 cm−3 the donor surface state density increases while the surface electron density decreases enormously, resulting in a transition from electron accumulation to almost just hole depletion layer. This reduction of electron accumulation in high Mg-doped InN can be improved by additional surface treatment, therefore results of a series of sulfur treated Mg-doped InN sample are also reported in this thesis. Finally, the electronic properties of InxGa1−xN alloys with a composition range of 0.20 >= x >= 1.00 have been investigated, using XPS and FTIR. The transition from electron accumulation to electron depletion has been observed at a composition of x = 0.20, while for x >= 0.20 an increasing electron accumulation with decreasing Ga fraction has been observed.