Hybrid-gate deep depletion mosfethigh-k zro2/diamond-based power devices

Resumen

The improvement of power electronic devices, making them durable and reliable in high power environments is the key to the efficient low-carbon electrical energy production and distribution for our future energy system, eliminating the auxiliary systems and reducing the losses. Despite silicon is a well-established material it presents narrow and inadequate electrical characteristics to be used in power electronics. More efficient green electronic systems will be reached by wide band gap or ultra wide band gap semiconductors since they can provide larger blocking capabilities, higher performance-cost ratios and they can reduce the thermal requirements. Among the candidates, diamond is found to be the ultimate material to meet the power electronic trade-off between the on-resistance and the blocking capabilities. The ultra wide bandgap (UWBG) of 5.5 eV leads to a non linear increase of the performance. The higher critical electric field (> 10 MV.cm-1) allows the use of higher doping concentrations comparing with Si-technology. Which in combination with a superior bulk carrier mobility at room temperature for both electrons and holes (1060 cm2.V-1s-1 and 2100 cm2.V-1s-1, respectively) favours the reduction of the devices¿ resistance. Moreover, the resistivity will be reduced with increasing temperature due to the deep dopants in diamond (ionization energies of B, P or N of 0.38 eV, 0.58 eV and 1.7 eV, respectively [2, 3, 4, 5]), which result in incomplete ionization at room temperature. Likewise, the outstanding thermal conductivity (2200 W.m-1K-1) and the low concentration of intrinsic carriers (it is required a larger thermal energy to promote electrons from the valence band to the conduction band than in semiconductors with smaller bandgap) make diamond the most suitable material for high temperature operation. The most widely used device whether in analog or digital circuits in the Electronic Industry is the metal-oxide-semiconductor field effect transistor (MOSFET). The focus will therefore be on this type of device, particularly based on diamond, which is also the subject of this thesis. However, the upgrade to the new generation of diamond-based power MOSFETs is limited. The main issues are caused by the lack of a native oxide that meets the demanding requirements of the diamond interface. This latter degrades the performance of the devices and leads to a premature breakdown. And also by the difficulty of growing quality n-type diamond layers and the lack of efficient p-channel devices comparable to their n-channel counterparts reached by other WBGSs. It is still challenging provide devices that exploit the diamond¿s properties to their full potential. This thesis proposes an alternative high-k ZrO2/diamond-based hybrid-gate p-channel MOSFET taking advantage of the outstanding properties of these materials. In addition to the superior properties of diamond, mentioned above, zirconium dioxide contributes a barrier for holes of 2 eV, a critical electric field of 2 MV.cm-1, a high enthalpy of formation and an excellent thermodynamic and thermal stability. An attempt has been made to correlate the electrical behaviour of the MOS structure with the microstructural analysis. This has required an in-depth characterization that can be divided in two main blocks: - Materials characterization by the use of microscopy techniques such as high resolution transmission electron microscopy (HREM), energy dispersive X-ray spectroscopy (EDX) and valence electron energy loss spectroscopy (VEELS). - Fabrication and electrical characterization of the electronic devices through different growth techniques (for both diamond and oxide thin films) such as microwave plasma assisted chemical vapour deposition (MPCVD), physical vapour deposition (PVD) and atomic layer deposition (ALD); laser lithography, metals deposition using electron beam (ebeam) evaporation and various surface and thermal treatments to finally perform the electrical measurements. In summary, this manuscript consists of the following chapters: - Chapter 1 will introduce the fabrication process methodology and state of the art of the diamond-based MOSFETs. - Chapter 2 is dedicated to the full analysis of the MOS capacitors, specially focus on understanding the behaviour of ZrO2 thin-films. - Chapter 3 will deal with the implementation and characterization of this MOS structure in a novel hybrid-gate p channel deep depletion MOSFET (D3MOSFET). - The Overall conclusion and further outlook chapter will summarize the current status and will provide future prospective for such electronic devices.