Plasma-enhanced atomic layer deposition of vanadium phosphate as a lithium-ion battery electrode material

Vanadium phosphate films were deposited by a new process consisting of sequential exposures to trimethyl phosphate (TMP) plasma, O2 plasma, and either vanadium oxytriisopropoxide [VTIP, OV(O-i-Pr)3] or tetrakisethylmethylamido vanadium [TEMAV, V(NEtMe)4] as the vanadium precursor. At a substrate temperature of 300 °C, the decomposition behavior of these precursors could not be neglected; while VTIP decomposed and thus yielded a plasma-enhanced chemical vapor deposition process, the author found that the decomposition of the TEMAV precursor was inhibited by the preceding TMP plasma/O2 plasma exposures. The TEMAV process showed linear growth, saturating behavior, and yielded uniform and smooth films; as such, it was regarded as a plasma-enhanced atomic layer deposition process. The resulting films had an elastic recoil detection-measured stoichiometry of V1.1PO4.3 with 3% hydrogen and no detectable carbon contamination. They could be electrochemically lithiated and showed desirable properties as lithium-ion battery electrodes in the potential region between 1.4 and 3.6 V versus Li+/Li, including low capacity fading and an excellent rate capability. In a wider potential region, they showed a emarkably high capacity (equivalent to three lithium ions per vanadium atom), at the expense of reduced cyclability.status: publishe

Atomic layer deposition of aluminum phosphate based on the plasma polymerization of trimethyl phosphate

Aluminum phosphate thin films were deposited by plasma-assisted atomic layer deposition (ALD) using a sequence of trimethyl phosphate (TMP, Me3PO4) plasma, O-2 plasma, and trimethylaluminum (TMA, Me3Al) exposures. In situ characterization was performed, including spectroscopic ellipsometry, optical emission spectroscopy, mass spectrometry and FTIR. In the investigated temperature region between 50 and 320 degrees C, nucleation delays were absent and linear growth was observed, with the growth per cycle (GPC) being strongly dependent on temperature. The plasma polymerization of TMP was found to play an important role in this process, resulting in CVD-like behavior at low temperatures and ALD-like behavior at high temperatures. Films grown at 320 degrees C had a GPC value of 0.37 nm/cycle and consisted of amorphous aluminum pyrophosphate (Al4P6O21). They could be crystallized to triclinic AlPO4 (tridymite) by annealing to 900 degrees C, as evidenced by high-temperature XRD measurements. The use of a TMP plasma might open up the possibility of depositing many other metal phosphates by combining it with appropriate organometallic precursors

Modeling the conformality of atomic layer deposition: the effect of sticking probability

The key advantage of atomic layer deposition (ALD) is undoubtedly the excellent step coverage, which allows for conformal deposition of thin films in high-aspect-ratio structures. In this paper, a model is proposed to predict the deposited film thickness as a function of depth inside a hole. The main model parameters are the gas pressure, the deposition temperature, and the initial sticking probability of the precursor molecules. Earlier work by Gordon et al. assumed a sticking probability of 0/100% for molecules hitting a covered/uncovered section of the wall of the hole, thus resulting in a stepwise film-thickness profile. In this work, the sticking probability is related to the surface coverage theta by Langmuir’s equation s(theta) = s0(1−theta), whereby the initial sticking probability s0 is now an adjustable model parameter. For s0~=100%, the model predicts a steplike profile, in agreement with Gordon et al., while for smaller values of s0, a gradual decreasing coverage profile is predicted. Furthermore, experiments were performed to quantify the conformality for the trimethylaluminum (TMA)/H2O ALD process using macroscopic test structures. It is shown that the experimental data and the simulation results follow the same trends