GaN/Al(Ga)N heterostructures: MBE growth, electronic properties and polarization fields

The demand of higher and higher storage density in digital data processing applicationslead in the last decade to an increased interest in the development of injection lasers operatingat short wavelengths. The group III-nitrides has revealed to be the most successfullclass of semiconductors for optoelectronic applications for wavelengths from 200 to650 nm. Beside this, the nitrides are moving first promising steps also in the field of radiofrequencydevices: AlGaN/GaN HEMTs grown on seminsulating SiC Show very highpower density at frequencies up to 50 GHz and an excellent thermal and chemical stability.The aim of this work was to achieve experimentally the knowledge of the energy bandscheme along several nitride heterostructures and the surfaces electronic properties, thusproviding a set of basic parameters needed for the design of heterostructure devices.The first step for the realization of GaN/Al(Ga)N heterostructures and for the determinationof their band scheme was the optimization of the GaN and AIGaN deposition bymeans of MBE on 6H-SiC substrates. To this phase belonged the preparation of the substrate,a systematic change of the growth parameters and a consequent control of structural,optical and electrical properties by means of different experimental techniques (XRD, PL,AFM, Hall).The determination of the band offset at various nitride interfaces, including the ones to theSiC substrate was obtained $\textit{in situ}$ by means of monochromatized XPS, after the heterojunctionwas grown by MBE. The systematic analysis of the overlayer core level positionin dependence on the deposited thickness showed the presence of electric fields of theorder of a MV/cm in the thin (1 - 6 nm) nitride layers. These high internal fields arisefrom the high polarization (spontaneous and piezoelectric) of the nitrides, because of theirhexagonal symmetry. The interplay of surface states, as evidenced by adsorption of activatedhydrogen, was responsible of a partial reduction of the field intensity with respectto the predicted value. Because of the internal fields, the measured values of the valenceband offsets (VBO) strongly depend on the thickness and the heterojunction VBO wasextracted through extrapolation at zero thickness. By means of this systematic study a setof VBOs deterrnined by XPS is provided, which for the first time takes into account thepresence of the high polarization fields.The measured valence band offsets, as well as the observed surface level pinning in thicklayers were used for modeling heterostructures, where the polarization fields were utilizedas design parameters. The first type consists of about 30 nm AlGaN on GaN(0001). Inthis rase the discontinuity of the polarizations of the two materials induces a positive fixedCharge at the interface, which attracts electrons with the formation of a high density 2-dimensional electron gas (n$_{s} \approx$ 10$^{13}$cm$^{-2}$), without modulation doping, also favouredalso by the relatively high band discontinuity. The presence of such a high conductivechannel was confirmed by temperature dependent Hall measurements on some optimizedGaN/Al$_{0.25}$Ga$_{0.75}$N heterostructures and represents the first step for a further controlled developmentof optimized HEMT devices. As second application of the polarization fieldsas design parameters were Al(Ga)N/GaN superlattices prepared by MBE. By an appropriatechoice of the GaN well and of the strained AIN barrier thickness we could controlthe fields in the triangular quantum well and consequently the energy of the dominantemission in the PL spectra