The impact of ultrathin Al2O3 films on the electrical response of p-Ge/Al2O3/HfO2/Au MOS structures

• Published in: Journal of physics. D, Applied physics Applied physics, 2016-08, Vol.49 (38)
• Main Authors: Botzakaki, M A, Skoulatakis, G, Kennou, S, Ladas, S, Tsamis, C, Georga, S N, Krontiras, C A
• Format: Article
• Language: English
• Subjects: ALD; electrical characterization; Ge-MOS devices; HfO; passivation layer; XPS
• ISSN: 0022-3727; 1361-6463
• DOI: 10.1088/0022-3727/49/38/385104

It is well known that the most critical issue in Ge CMOS technology is the successful growth of high-k gate dielectrics on Ge substrates. The high interface quality of Ge/high-k dielectric is connected with advanced electrical responses of Ge based MOS devices. Following this trend, atomic layer deposition deposited ultrathin Al2O3 and HfO2 films were grown on p-Ge. Al2O3 acts as a passivation layer between p-Ge and high-k HfO2 films. An extensive set of p-Ge/Al2O3/HfO2 structures were fabricated with Al2O3 thickness ranging from 0.5 nm to 1.5 nm and HfO2 thickness varying from 2.0 nm to 3.0 nm. All structures were characterized by x-ray photoelectron spectroscopy (XPS) and AFM. XPS analysis revealed the stoichiometric growth of both films in the absence of Ge sub-oxides between p-Ge and Al2O3 films. AFM analysis revealed the growth of smooth and cohesive films, which exhibited minimal roughness (~0.2 nm) comparable to that of clean bare p-Ge surfaces. The electrical response of all structures was analyzed by C-V, G-V, C-f, G-f and J-V characteristics, from 80 K to 300 K. It is found that the incorporation of ultrathin Al2O3 passivation layers between p-Ge and HfO2 films leads to superior electrical responses of the structures. All structures exhibit well defined C-V curves with parasitic effects, gradually diminishing and becoming absent below 170 K. Dit values were calculated at each temperature, using both Hill-Coleman and Conductance methods. Structures of p-Ge/0.5 nm Al2O3/2.0 nm HfO2/Au, with an equivalent oxide thickness (EOT) equal to 1.3 nm, exhibit Dit values as low as ~7.4  ×  1010 eV−1 cm−2. To our knowledge, these values are among the lowest reported. J-V measurements reveal leakage currents in the order of 10-1 A cm−2, which are comparable to previously published results for structures with the same EOT. A complete mapping of the energy distribution of Dits into the energy bandgap of p-Ge, from the valence band towards midgap, is also reported. These promising results contribute to the challenge of switching to high-k dielectrics as gate materials for future high-performance metal-oxide-semiconductor field-effect transistors based on Ge substrates. Making the switch to such devices would allow us toexploit its superior properties.