Claudia Richter

A comprehensive study on the structural evolution of HfO2 thin films doped with various dopants

The origin of the unexpected ferroelectricity in doped HfO2 thin films is now considered to be the formation of a non-centrosymmetric Pca21 orthorhombic phase. Due to the polycrystalline nature of the films as well as their extremely small thickness (∼10 nm) and mixed orientation and phase composition, structural analysis of doped HfO2 thin films remains a challenging task. As a further complication, the structural similarities of the orthorhombic and tetragonal phase are difficult to distinguish by typical structural analysis techniques such as X-ray diffraction. To resolve this issue, the changes in the grazing incidence X-ray diffraction (GIXRD) patterns of HfO2 films doped with Si, Al, and Gd are systematically examined. For all dopants, the shift of o111/t101 diffraction peak is observed with increasing atomic layer deposition (ALD) cycle ratio, and this shift is thought to originate from the orthorhombic to P42/nmc tetragonal phase transition with decreasing aspect ratio (2a/(b + c) for orthorhombic and c/a for the tetragonal phase). For quantitative phase analysis, Rietveld refinement is applied to the GIXRD patterns. A progressive phase transition from P21/c monoclinic to orthorhombic to tetragonal is confirmed for all dopants, and a strong relationship between orthorhombic phase fraction and remanent polarization value is uniquely demonstrated. The concentration range for the ferroelectric properties was the narrowest for the Si-doped HfO2 films. The dopant size is believed to strongly affect the concentration range for the ferroelectric phase stabilization, since small dopants can strongly decrease the free energy of the tetragonal phase due to their shorter metal–oxygen bonds.

Breakdown and Protection of ALD Moisture Barrier Thin Films.

The water vapor barrier properties of low-temperature atomic layer deposited (ALD) AlOx thin-films are observed to be unstable if exposed directly to high or even ambient relative humidities. Upon exposure to humid atmospheres, their apparent barrier breaks down and their water vapor transmission rates (WVTR), measured by electrical calcium tests, deteriorate by several orders of magnitude. These changes are accompanied by surface roughening beyond the original thickness, observed by atomic force microscopy. X-ray reflectivity investigations show a strong decrease in density caused by only 5 min storage in a 38 °C, 90% relative humidity climate. We show that barrier stabilities required for device applications can be achieved by protection layers which prevent the direct contact of water condensing on the surface, i.e., the sensitive ALD barrier. Nine different protection layers of either ALD materials or polymers are tested on the barriers. Although ALD materials prove to be ineffective, applied polymers seem to provide good protection independent of thickness, surface free energy, and deposition technique. A glued-on PET foil stands out as a low-cost, easily processed, and especially stable solution. This way, 20 nm single layer ALD barriers for organic electronics are measured. They yield reliable WVTRs down to 2×10(-5) g(H2O) m(-2) day(-1) at 38 °C and 90% relative humidity, highlighting the great potential of ALD encapsulation.

Atomic layer deposited high-κ nanolaminates for silicon surface passivation

Nanolaminates comprising of TiO2 or HfO2 sublayers within an Al2O3 matrix are grown with atomic layer deposition. These nanolaminates provide an improved silicon surface passivation compared to conventional Al2O3 films. The physical properties of the nanolaminates can be described with a dynamic growth model that considers initial and steady-state growth rates for the involved metal oxides. This model links the cycle ratios of the different atomic layer deposition precursors to the thickness and the material concentrations of the nanolaminate, which are determined by means of spectroscopic ellipsometry. Effective carrier lifetime measurements show that Al2O3-TiO2 nanolaminates achieve values of up to 6.0 ms at a TiO2 concentration of 0.2%. In Al2O3-HfO2 nanolaminates, a maximum effective carrier lifetime of 5.5 ms is reached at 7% HfO2. Electrical measurements show that the TiO2 incorporation causes strong hysteresis effects, which are linked to the trapping of negative charges and result in an enhanced f...

Integration of molecular-layer-deposited aluminum alkoxide interlayers into inorganic nanolaminate barriers for encapsulation of organic electronics with improved stress resistance

Diffusion barrier stacks for the encapsulation of organic electronics made from inorganic nanolaminates of Al2O3 and TiO2 with aluminum alkoxide interlayers have been deposited by atomic layer deposition (ALD) and molecular layer deposition (MLD). As a part of the MLD process development, the deposition of aluminum alkoxide with low a density of about 1.7 g/cm3 was verified. The ALD/MLD diffusion barrier stack is meant to be deposited either on a polymer film, creating a flexible barrier substrate, or on top of a device on glass, creating a thin-film encapsulation. In order to measure the water vapor transmission rate (WVTR) through the barrier, the device is replaced by a calcium layer acting as a water sensor in an electrical calcium test. For the barrier stack applied as thin-film encapsulation on glass substrates, high resolution scanning electron microscopy investigations indicate that the inorganic nanolaminates without MLD interlayers are brittle as they crack easily upon the stress induced by the ...

Lanthanum-Doped Hafnium Oxide: A Robust Ferroelectric Material

Recently simulation groups have reported the lanthanide series elements as the dopants that have the strongest effect on the stabilization of the ferroelectric non-centrosymmetric orthorhombic phase in hafnium oxide. This finding confirms experimental results for lanthanum and gadolinium showing the highest remanent polarization values of all hafnia-based ferroelectric films until now. However, no comprehensive overview that links structural properties to the electrical performance of the films in detail is available for lanthanide-doped hafnia. La:HfO2 appears to be a material with a broad window of process parameters, and accordingly, by optimization of the La content in the layer, it is possible to improve the performance of the material significantly. Variations of the La concentration leads to changes in the crystallographic structure in the bulk of the films and at the interfaces to the electrode materials, which impacts the spontaneous polarization, internal bias fields, and with this the field cycling behavior of the capacitor structure. Characterization results are compared to other dopants like Si, Al, and Gd to validate the advantages of the material in applications such as semiconductor memory devices.

How to make DRAM non-volatile? Anti-ferroelectrics: A new paradigm for universal memories

The major disadvantages of DRAM memory cells are the very short retention time and high power consumption needed to refresh the stored information. Here, we present a new concept using a modified DRAM capacitor stack to enable non-volatile data storage. Recent reports verified anti-ferroelectric properties for pure ZrO2 dielectrics used in DRAM stacks. Anti-ferroelectric materials are well known for high endurance strength but at the same time volatile memory behavior. Based on Landau theory, we propose a simple way how non-volatility can be achieved in state-of-the-art ZrO2 based DRAM stacks. By employing electrodes with different workfunction values, a built-in bias is introduced within the AFE stack, thus creating two stable non-volatile states. Moreover, we report the fabrication of the worlds first non-volatile AFE-RAM. Detailed characterization proved high endurance and reliable operation of this non-volatile DRAM stack equivalent. In addition to the 1T-1C cell, we show a proof of concept for a MIS capacitor device which can be integrated in future AFE-FET based 1T memory architectures.

Film properties of low temperature HfO2 grown with H2O, O3, or remote O2-plasma

A reduction of the deposition temperature is necessary for atomic layer deposition (ALD) on organic devices. HfO2 films were deposited by ALD on silicon substrates in a wide temperature range from 80 to 300 °C with tetrakis[ethylmethylamino]hafnium as metal precursor and H2O, O3, or an remote O2-plasma as oxygen source. Growth rate and density were correlated to electrical properties like dielectric constant and leakage current of simple capacitor structures to evaluate the impact of different process conditions. Process optimizations were performed to reduce film imperfections visible at lower deposition temperatures. Additionally, the influence of postdeposition annealing on the structural and electrical properties was studied.

Anti-ferroelectric ZrO 2 , an enabler for low power non-volatile 1T-1C and 1T random access memories

Recently it was demonstrated that an asymmetric DRAM capacitor stack can introduce non-volatility and at the same time outperform ferroelectric HfO2 based FeRAM in terms of cycle endurance. With the present work, we provide an in-depth study of the underlying mechanisms and perform a comprehensive retention study that characterizes ferroelectric memories. Piezoelectric force microscopy is applied to prove the ultimate scalability of the proposed concept beyond capacitor based application. Finally, switching density plots reveal a much lower device to device variability and high switching uniformity with respect to ferroelectric HfO2 based films.

Nanoscopic studies of domain structure dynamics in ferroelectric La:HfO2 capacitors

Visualization of domain structure evolution under an electrical bias has been carried out in ferroelectric La:HfO2 capacitors by a combination of Piezoresponse Force Microscopy (PFM) and pulse switching techniques to study the nanoscopic mechanism of polarization reversal and the wake-up process. It has been directly shown that the main mechanism behind the transformation of the polarization hysteretic behavior and an increase in the remanent polarization value upon the alternating current cycling is electrically induced domain de-pinning. PFM imaging and local spectroscopy revealed asymmetric switching in the La:HfO2 capacitors due to a significant imprint likely caused by the different boundary conditions at the top and bottom interfaces. Domain switching kinetics can be well-described by the nucleation limited switching model characterized by a broad distribution of the local switching times. It has been found that the domain velocity varies significantly throughout the switching process indicating str...

Impact of field cycling on HfO 2 based non-volatile memory devices

The discovery of ferroelectricity in HfO2 and ZrO2 based dielectrics enabled the introduction of these materials in highly scalable non-volatile memory devices. Typical memory cells are using a capacitor or a transistor as the storage device. These scaled devices are sensitive to the local structure of the storage material, here the granularity of the dielectric doped HfO2 layer, varying the local ferroelectric properties. Detailed studies are conducted to correlate these structural properties to the electrical performance to further optimize the devices for future applications.