S. Van Elshocht

Deposition of HfO2 on germanium and the impact of surface pretreatments

The deposition behavior of HfO2 by metalorganic chemical vapor deposition on germanium has been investigated. HfO2 films can be deposited on Ge with equally good quality as compared to high-k growth on silicon. Surface preparation is very important: compared to an HF-last, NH3 pretreatments result in smoother films with strongly reduced diffusion of germanium in the HfO2 film, resulting in a much better electrical performance. We clearly show that much thinner interfacial layers can be obtained, approximately half the thickness of what is typically found for depositions on silicon, suggesting the possibility of more aggressive equivalent oxide thickness∕leakage scaling.

Dielectric properties of dysprosium- and scandium-doped hafnium dioxide thin films

Dysprosium- and scandium-doped HfO2 films have been deposited by atomic-vapor deposition on SiO2∕Si substrates. Glancing-incidence x-ray diffraction demonstrates that Dy0.10Hf0.90Ox and Sc0.10Hf0.90Ox films show a cubic crystal structure, whereas HfO2 films are monoclinic. The dielectric permittivity increases strongly from 16 for HfO2 to 32 for Dy0.10Hf0.90Ox and Sc0.10Hf0.90Ox. This leads to a reduction of the leakage current in the tunneling regime by up to three orders of magnitude for constant effective oxide thickness. For thick films (≳6nm), it is shown that leakage occurs via the Poole-Frenkel mechanism and that doping HfO2 increases leakage for constant physical oxide thickness.

Estimation of fixed charge densities in hafnium-silicate gate dielectrics

In this paper, an effective technique and methodology for the estimation of fixed charge components in high-k stacks was demonstrated by varying both the SiO2 and high-k dielectric thicknesses. The SiO2 thickness was scaled on a single wafer by uniformly changing the etch time of a thermally grown SiO2 layer across the wafer. This minimized wafer-to-wafer variations and enables acquisition of statistically significant datasets. Layers with different thickness of both the nitrided and non-nitrided hafnium-silicate layers were then grown on these wafers to estimate all the interfacial and bulk charge components. The reproducibility and validity of this technique were demonstrated, and this method was used to compare the fixed charge levels in Hf-silicates (HfSiO) and nitrided-Hf-silicate (HfSiON) layers

Effect of hafnium germanate formation on the interface of HfO2/germanium metal oxide semiconductor devices

We have studied the thermal stability of HfO2 thin layers on germanium and the substrate interface development. HfO2 was deposited on Ge substrates and annealed in O2 or N2 at 500°C (substrate temperature). After O2 anneal, we observed the formation of hafnium germanate, which is stable at 500°C in N2 as opposed to GeO2 that desorbs as GeO. We believe that this hafnium germanate is an oxygen barrier and as such is at the origin of the much thinner interface between HfO2 and germanium as compared to silicon. In addition, results suggest that the HfGeOx is related to the high interface state density frequently reported for germanium metal oxide semiconductor devices.

Atomic Layer Deposition of Strontium Titanate Films Using Sr ( #2#1Cp ) 2 and Ti ( OMe ) 4

Strontium titanate (STO) is a promising candidate as a high-k dielectric for dynamic random access memory application. STO thin films are deposited by atomic layer deposition using Sr( t Bu 3 Cp) 2 , Ti(OMe) 4 , and H 2 O as precursors. Growth and saturation behavior of STO and binary oxides are evaluated by ellipsometry thickness measurements. The precursor pulse ratio controls the amount of Sr and Ti incorporated in STO films. Stoichiometric SrTiO 3 is characterized by the lowest crystallization temperature and largest refractive index, density, and dielectric constant. An excess of Ti or Sr results in an increase in the crystallization onset temperature and contraction or expansion of the cubic cell constant of perovskite SrTiO 3 . Incorporation of more Sr in STO reduces the leakage current density but also increases the capacitance-equivalent thickness.

Composition influence on the physical and electrical properties of SrxTi1−xOy-based metal-insulator-metal capacitors prepared by atomic layer deposition using TiN bottom electrodes

In this work, the physical and electrical properties of SrxTi1−xOy (STO)-based metal-insulator-metal capacitors (MIMcaps) with various compositions are studied in detail. While most recent studies on STO were done on noblelike metal electrodes (Ru, Pt), this work focuses on a low temperature (250 °C) atomic layer deposition (ALD) process, using an alternative precursor set and carefully optimized processing conditions, enabling the use of low-cost, manufacturable-friendly TiN electrodes. Physical analyses show that the film crystallization temperature, its texture and morphology strongly depends on the Sr/Ti ratio. Such physical variations have a direct impact on the electric properties of SrxTi1−xOy based capacitors. It is found that Sr-enrichment result in a monotonous decrease in the dielectric constant and leakage current as predicted by ab initio calculations. The intercept of the EOT vs physical thickness plot further indicates that increasing the Sr-content at the film interface with the bottom TiN...

Silicate formation and thermal stability of ternary rare earth oxides as high-k dielectrics

Hf-based dielectrics are currently being introduced into complementary metal oxide semiconductor transistors as replacement for SiON to limit gate leakage current densities. Alternative materials such as rare earth based dielectrics are of interest to obtain proper threshold voltages as well as to engineer a material with a high thermal stability. The authors have studied rare earth based dielectrics such as Dy2O3, DyHfOx, DyScOx, La2O3, HfLaOx, and LaAlOx by means of ellipsometry, time of flight secondary ion mass spectroscopy x-ray diffraction, and x-ray photoelectron spectroscopy. The authors show that ellipsometry is an easy and powerful tool to study silicate formation. For ternary rare earth oxides, this behavior is heavily dependent on the composition of the deposited layer and demonstrates a nonlinear dependence. The system evolves to a stable composition that is controlled by the thermal budget and the rare earth content of the layer. It is shown that silicate formation can lead to a severe overe...

Strontium doped hafnium oxide thin films: Wide process window for ferroelectric memories

Ferroelectricity in hafnium oxide has been reported for the incorporation of Al, Si, Y and Gd or in a solid solution with the chemically similar ZrO2. Here, we report strontium as the first bivalent and — so far — largest dopant in terms of atomic radius also inducing ferroelectric behavior. Besides the solid solution of HfO2/ZrO2 for Sr:HfO2, ferroelectricity is observed in the widest concentration range of all dopants used up to now. First results of ab initio simulations also suggest such a comparatively wide window for ferroelectricity. With a coercive field of about 2 MV/cm another figure exceeds the characteristics reported before. A maximum remanent polarization of 23 μC/cm2 also ranks among the highest values reported until now. The fabricated TiN-Sr:HfO2-TiN capacitors exhibit switching times in the nanosecond range and still retain 80 % of their initial remanent polarization after 106 endurance cycles. The 10 nm ferroelectric thin films prepared by atomic layer deposition are capable of integration into 3D capacitors or FinFETs.

Band alignment and electron traps in Y2O3 layers on (100)Si

Y2O3 films deposited by atomic vapor deposition on (100)Si with a 2 or 5 nm thick pregrown thermal SiO2 are investigated as possible charge trapping layers. Analysis of these structures using spectroscopic ellipsometry, photoconductivity, and internal photoemission reveals that Y2O3 has a 5.6 eV wide optical bandgap and a 2.0 eV conduction band offset with silicon. Photo(dis)charging experiments show that the optical energy depth of most of the traps exceeds 1.5 eV with respect to the Y2O3 conduction band, explaining the observed charge retention time of ∼108 s at room temperature, even in the absence of a blocking insulator.

Thermal stability of dysprosium scandate thin films

The thermal stability of DyScO3 thin films in contact with SiO2 or HfO2 during annealing up to 1000°C has been studied. It is found that DyScO3∕SiO2 stacks react during annealing and a phase separation into polycrystalline Sc-rich (and relatively Si-poor) DySc silicate on top of an amorphous Dy-rich DySc silicate is observed. In contrast, DyScO3 is found to be thermodynamically stable in contact with HfO2 and to recrystallize upon annealing. These results demonstrate that the previously reported high crystallization temperature of >1000°C for DyScO3 is not an intrinsic material property but caused by silicate formation.