Improving HfO 2 Thick Films for SiC Power Devices by Si, Y and La Doping

. We investigated the electrical and structural effects of silicon (Si), yttrium (Y) and lanthanum (La) doping in 10-45 nm thick hafnium dioxide (HfO 2 ) films on silicon carbide (SiC) and Si substrates. We show that the introduction of Si dopants leads to a significant enhancement of the electric breakdown field and a reduction of the leakage current density by elevating the crystallization temperature. This effect becomes stronger with higher Si content. In contrast, Y and La doping does not raise T C but increases the tetragonal and orthorhombic phase portion within the crystalline films and therefore enhances the dielectric constant k. Furthermore, we show that larger grains in crystalline films are associated with a higher leakage current density.


Introduction
Silicon carbide (SiC) offers distinct advantages over silicon (Si) due to its wider bandgap, enabling higher electric fields and higher temperatures in SiC devices.However, the conventional silicon dioxide (SiO2) gate oxide limits the full exploitation of these benefits as it cannot withstand the high electric fields present in the dielectric of such a device [1].Replacing SiO2 (dielectric constant k = 3.9) by dielectrics with a higher k alters the electric field distribution in the device.The reduced electric field strength within the gate dielectric allows for higher applied voltages or further device downscaling [1].Hafnium dioxide (HfO2) is a promising alternative with a dielectric constant of 20-40 [2], depending on the crystalline phase content.Yet, it has low band offsets to the conduction and valence band of SiC (ΔEC = 0.7 eV and ΔEV = 1.7 eV) [3], leading to high leakage currents [3,4].In recent years HfO2 thin films have shown significant advantages over SiO2 in low power devices [1].Thus, it is reasonable to adapt the properties of HfO2-based films for high power applications [5,6].Current research has shown that introducing dopants into HfO2 films can significantly enhance their electrical properties [2, 7,8].
This study focuses on the electrical and structural properties of stacks of Si / Y / La-doped HfO2 deposited on a thin thermally grown SiO2 layer on SiC and Si substrates.The objective is to find a stack with a high dielectric constant k, high electric breakdown field (EBD) and low leakage current density (Jleak).

Methods
The HfO2-based films with a thickness ranging from 15 to 45 nm were deposited by plasmaenhanced atomic layer deposition (PE-ALD) at 300°C.Si doping was incorporated using specific ALD cycle ratios, resulting in a percentage of Si cations of 4% and 6% in the HfO2 film.For Y doping, the cation percentage was 5% and 7%, and for La doping it was 9%.A sketch of the samples along with a TEM image of the pure HfO2 film is presented in Fig. 1.The samples were annealed at ~500°C or ~1000°C for one minute in Argon ambient, and the layer thickness was determined by Xray reflectometry (XRR) or transmission electron microscopy (TEM).The crystallographic phase was determined from the peak positions in Grazing-incidence X-ray diffraction measurements (GIXRD), and the size of the crystal grains was estimated from the peak width using the Scherrer relation [9].The dielectric constant k was calculated from the MOS capacitance in the accumulation regime.The obtained k value represents an "effective" k that accounts for the entire dielectric stack including SiO2 and the HfO2-based layer.The k value of the HfO2-based layer was calculated using an equivalent circuit model consisting of two capacitors in series, the measured layer thicknesses, and the k value of SiO2.The leakage current density Jleak at an applied electric field of 2 MV/cm was determined using current-voltage (IV) measurements.The electric breakdown field EBD was defined at a current density of 10 mA/cm 2 .

Results and Discussion
Fig. 2 shows the relationship between Jleak at 2 MV/cm and the k value for the different capacitor structures.For the samples annealed at 1000°C, Jleak was enhanced by four to eight orders of magnitude, rendering them unsuitable as gate dielectrics.Moreover, the high leakage inhibits the saturation of the accumulation capacitance during the CV measurement, which makes the k determination unreliable for these samples [10].Consequently, these samples are excluded from further evaluation.
In contrast, the samples annealed at ~500°C show promising results.For SiC samples, Jleak is typically one order of magnitude higher than for Si samples, which can be attributed to the lower conduction band offset between SiC and HfO2.This is supported by the dependence of EBD on the leakage behavior (Fig. 3): samples with higher leakage demonstrate a lower breakdown voltage.The higher Jleak for the SiC samples draws a distinction between the two datasets and shifts the graph for SiC to the right.For a SiC power device in operation, typically, a leakage current of less than 100 nA/cm 2 is desirable [11], which is fulfilled for all samples.Additionally, the ranking of high-k materials remains consistent for both substrates.The Si-doping enhances EBD, while Y and La-doping result in a lower EBD and higher Jleak as compared to the pure HfO2 films.This observation is expected given that the high-k material was deposited on an interfacial SiO2 layer for both substrates which effectively eliminates the effects of possibly different ALD growth behavior on the different substrates.
The difference in the atomic radii of the dopant and the Hf atoms induces internal tensile stress in the HfO2 lattice, which affects the TC and the resulting crystalline phase [12,13].GIXRD measurements confirm that pure HfO2 crystallizes in the monoclinic phase [2] (Fig. 4).Consequent annealing experiments revealed TC of about 375°C for the 30 nm undoped HfO2 film, which ranks at the lower end of values reported in literature [3][4][5].Fig. 1c shows a TEM image of the crystalline HfO2.For the Si-doped films, GIXRD indicates the presence of randomly oriented nanocrystallites

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Formation of Solid-State Structures within a predominantly amorphous matrix.This can be concluded from the broad peak observed in the measurements performed on both as deposited and annealed films.TC is increased to approximately 650°C and 750°C for the 4 % and 6 % Si-doped films, respectively.In contrast, La or Y doping does not increase TC above 500°C and leads to a mixture of crystallographic phases present in the crystalline films.This goes along with an increase in the k value, as tetragonal or orthorhombic HfO2 have k values of 30-40 compared to approximately 20 for the monoclinic HfO2 [2].Fig. 2. Dependence of Jleak at 2 MV/cm on the dielectric constant of the (doped-)HfO2/SiO2 stack on Si or SiC substrates after annealing.Jleak is higher for SiC samples and increases with higher annealing temperature and higher k value.Fig. 3. Dependence of EBD on Jleak at 2MV/cm.Samples with lower Jleak exhibit a higher EBD.Empty datapoints stand for SiC, filled for Si samples, the shape of the datapoints indicates the material.Dotted lines are a guide to the eye.Fig. 5 shows that stacks with HfO2 layers with higher k have a lower EBD, which is consistent with [1,3,4].Furthermore, these results reveal that the amorphous films have a significantly higher EBD than the crystalline films.At the same time, they exhibit a lower k value since they do not profit from the higher k of the tetragonal or orthorhombic phase portion.Generally, amorphous oxides are advantageous as insulators because they lack grain boundaries, which could provide leakage paths for charge carriers [6,13].Solid State Phenomena Vol.359 may seem negligible at an electric field of 2 MV/cm, the difference becomes significant at higher fields.We assume that the amorphous films or films with a high number of small grains provide much more complex leakage paths compared to films with fewer large grains, subsequently reducing the probability of carrier conduction [14].Notably at 5 MV/cm, a few nanometers reduction in crystal grain size results in a Jleak almost four orders of magnitude lower.This observation highlights the critical impact of crystal grain size on charge carrier conduction mechanisms in our HfO2-based films.
For a better understanding of the doping impact, Fig. 7 illustrates the EBD dependence on the doping concentration for the three doping materials.The HfO2-based film with the highest Si doping exhibits the highest EBD.For the Y and La-doped HfO2 films EBD decreased compared to the pure HfO2.The advantage of Si doping can be attributed to two factors.Firstly, the Si-doped layers are amorphous when annealed at 500°C in contrast to the other films.Secondly, the Si atom is fourvalent, which matches Hf, in contrast to Y and La, which are tri-valent.The mismatch of valency results in dangling bonds within the film, leading to an increased defect density in the bulk of the material [8,13].Such defects can accumulate charge, resulting in higher Jleak and lower EBD [12,13].
To explore the trade-off between EBD and k, Fig. 8 displays their product as a figure of merit (FOM).The plot demonstrates that EBD*k is higher for the stack with pure HfO2 compared to a stack with pure SiO2, and it can be further enhanced by introducing the dopants investigated in this study.Notably, all the presented stacks surpass the value of 39 for SiO2 (with literature values of k = 3.9 and EBD = 10 MV/cm) [3].This FOM provides a powerful tool to rank the films, but to decide whether a specific film is suitable for a specific application, one must consider which of the two factors, EBD or k, is more critical.
Si-doped films demonstrate the most promising characteristics regarding high EBD, low Jleak and increased TC.To further investigate the breakdown behavior, Si-doped films of various thicknesses were deposited on Si substrates.Fig. 9 illustrates that thicker Si-doped layers have a lower EBD, which is consistent with [15,16].This observation could provide another explanation for the higher EBD exhibited by the Si-doped films.Assuming that the diffusion of dopants is negligible after deposition, then the doping is not uniformly distributed within the stack.Each Si ALD cycle creates a Si-rich layer, dividing the bulk pure HfO2 into thinner layers.For instance, in case of the highly Si-doped film, this would lead to HfO2 layers with a thickness of about 1.2 nm.This phenomenon also explains the increase in TC, as several researchers have reported an exponential rise of TC by reducing layer thickness down to a few nanometers [17][18][19].In their work [17] Zacharias et al presented a calculation for TC of SiO2 films depending on their thickness.By adapting this equation to HfO2, we derived equation (1), where TCbulk represents the TC of HfO2 thick films (375°C) and d is the layer thickness of the film under investigation (1.2 nm for 6 % and 1.8 nm for 4 %).Our calculated TC values of 774°C and 688°C for the 6% and 4% Si-doped films, respectively, align well with our

Summary
This study demonstrated that amorphous doped HfO2-based films exhibit higher electric breakdown fields and lower leakage current density than corresponding crystalline films.The small atomic radius of Si enables increasing the crystallization temperature and the Si-doped HfO2 films remain amorphous after annealing at ~500°C.As a result, Si-doped films demonstrate lower leakage current densities and higher electric breakdown fields.In contrast, La-and Y-doped HfO2 films do not show a significant increase in the crystallization temperature but exhibit a higher dielectric constant, which can be attributed to the presence of the tetragonal and orthorhombic phases within the crystalline films.In summary, Si doped amorphous HfO2 films demonstrate superior electrical performance, making them more suitable as gate dielectrics than their crystalline counterparts.This can be attributed to the size and valency of the Si-atoms.Moreover, it is noteworthy that the investigated films exhibit consistent structural and electrical behavior on both Si and SiC substrate.This finding enhances their potential for application versatility across various semiconductor platforms.
Our study demonstrates that while pure HfO2 may not be a viable replacement for SiO2 as gate dielectric in SiC power devices, Si-doping significantly enhances the structural and electrical properties of a 30 nm HfO2 film.To further enhance the suitability of Si-doped HfO2 films for the integration into state-of-the-art SiC power devices, future research should focus on increasing the thermal budget that the stack can withstand during the fabrication processes.Fig. 8.All investigated HfO2-based films have a higher EBD*k than SiO2, with a tendency for an increased EBD*k with higher doping levels.The sample with the highest Y-doping did not exhibit the highest EBD (Fig. 4) but it has the highest EBD*k due to its high k value.Solid State Phenomena Vol.359

Fig. 1 .
Fig. 1.Schematics of the sample with a) pure HfO2 and b) doped HfO2.The blue layer represents the ALD-deposited oxide, and the orange dots resemble the dopants.The TEM image c) shows the sample with pure HfO2 after annealing.The features in the HfO2 film are the lattice planes allowing for differentiation of individual crystal grains, while the amorphous SiO2 interlayer lacks this texture.

Fig. 6 Fig. 4 .
Fig.6further illustrates the relationship between crystallographic structure and charge carrier conduction.Samples with larger crystal grains exhibit a higher leakage current.Although this effect

Fig. 5 .
Fig. 5. Dependence of EBD on k.EBD is generally higher for the amorphous films.The highest EBD was measured for the HfO2 film with the highest Si doping.The k value of the HfO2-based layer was calculated from the k value of the entire stack and the physical thicknesses of the layers as measured by XRR or TEM.

Fig. 9 .
Fig. 9. Dependence of EBD on the physical thickness of the stack for Si-doped HfO2 films on Si substrate.Thicker layers have a higher Jleak and a lower EBD value.
It is important to note that TC for the Si-doped films was determined through annealing in steps of 100°C, leading to an uncertainty of about 50°C in the measured values.