Epitaxial growth and characterization of InGaN layers for photovoltaics applications

Resumen

In this work, we have grown the constituent functional blocks of a photovoltaic device based on III-Nitrides compound semiconductors, focusing on the non-intentionally doped (NID) InGaN layer and on the Mg-doped GaN layer. We have introduced two growth methods (MME and DERI) with the purpose of improving the quality of those constituent blocks of a photovoltaic device. All samples have been grown in a molecular beam epitaxy (MBE) reactor. The Metal Modulated Epitaxy (MME) makes use of the modulation of the shutters of the metallic and dopant sources (In, Ga, Al, Mg, Si) alternating open and close conditions. When the shutters are opened, the growth is performed under metal-rich conditions, while when the shutters are closed it is performed under N-rich conditions. The main goal of the MME method is enhancing the proper incorporation of Mg (p-type dopant) atoms into the crystalline structure (Mg atoms on Ga substitutional site MgGa), while keeping a good morphology of the surface. Under N-rich conditions, the Mg atoms are preferentially incorporated in MgGa sites, but the surface is degraded. The surface is flattened back when growing under metal-rich conditions. The final result is an enhancement of the hole concentration with a good surface morphology. The overall growth conditions are N-rich. Non-intentionally doped and magnesium doped GaN layers have been grown on GaN(0001) pseudo-substrates by Metal Modulated Epitaxy (MME). In order to characterize the hole concentration obtained, the Mg-doped GaN layers were also grown on highly resistive (GaN:Fe) pseudo-substrates MME. The Hall effect results have been compared with GaN layers grown in the intermediate regime (the usual growth method by MBE for high quality layers). The layers grown by MME showed 5 times higher hole concentration than those grown in the intermediate regime. The most important parameters to control the InGaN grown on GaN(0001) pseudo-substrates by MME have been studied. The ratio In/Ga has been identified as the most critical parameter that controls the indium incorporation into the alloy. Diode p-i-n structures (p-GaN / i-InGaN / n-GaN) have been grown on GaN(0001) pseudo-substrates, processed and characterized, and crucial mechanisms have been identified in order to improve their performance. The other growth method used in this thesis is Droplet Elimination by Radical-beam Irradiation (DERI). Unlike the MME case, DERI growth of InGaN only modulates opening/closing of the In shutter. DERI consists of two process, the Metal Rich Growth Process (MRGP) and the Droplet Elimination Process (DEP). During the MRGP the In shutter is opened and the layer is grown accumulating In droplets on the surface. When In shutter is closed (DEP) the In droplets are reincorporated epitaxially into the InGaN layer. The overall growth is stoichiometric. In addition, we have studied the effect of four different buffer layers on the structural and optical properties of InGaN layers grown on Si(111) substrates and their correlation with electrical characteristics. The vertical electrical conduction on the InGaN / buffer / Si, with In composition near 46%, which theoretically produces a band alignment, is analysed. InGaN layers were grown for the first time, up to the best of our knowledge, on Si using droplet elimination by radical-beam irradiation (DERI). The results obtained lead to the possibility of fabricating double junctions InGaN/Si solar cells without the need of tunnel junctions between the two sub-cells, therefore simplifying the design of the final device.