No abstract

Colloidal quantum dots (QDs) are a promising material for optoelectronic applications. Typically, device integration requires QDs to be embedded in a host material. Atomic layer deposition (ALD) is often considered as a deposition technique for such purposes. However, it is known that ALD and vacuum processes often influence the optical properties of QDs in a negative way. Here, we describe an in situ photoluminescence (PL) measurement setup and use it to monitor the PL of QDs under vacuum and during ALD. For CdSe-based core/shell QDs, a reduction in the QD PL was observed upon exposure to vacuum. Water was identified as crucial for maintaining a high PL as evidenced by re-exposure to different gases. Furthermore, we addressed the influence of vacuum, different plasmas (O 2 , H 2 O, H 2 , H 2 S/Ar, and Ar), precursors (trimethylaluminum, diethylzinc, tetrakis(dimethylamido)titanium, and tetrakis(ethylmethylamido)hafnium), reactants (H 2 O, H 2 S, and O 3 ), and ALD processes (Al 2 O 3 , TiO 2 , HfO 2 , and ZnS) on QDs. We observed a PL reduction by up to 90% upon plasma treatments. Furthermore, we found that trimethylaluminum and diethylzinc reduced the PL efficiency by more than 70% while exposure to tetrakis(dimethylamido)titanium and tetrakis(ethylmethylamido)hafnium lowered the PL by only 10−20%. Surprisingly, tetrakis(dimethylamido)titanium and H 2 O, which by themselves had only a minor influence on the QD PL, still caused an 80% drop of the PL efficiency when combined as an ALD process. On the other hand, ALD growth of HfO 2 by combining tetrakis(ethylmethylamido)hafnium and O 3 preserved 80% of the initial PL quantum yield, making it a promising process for QD embedding. These results put forward in situ PL measurements as a versatile technique to identify suitable precursors, reactants and ALD processes for QD embedding and investigate the interaction between QDs and reactive gaseous species in general.

A method to obtain full mass-over-charge (m/z), time-resolved quadruple mass spectrometry (QMS) spectra of an atomic layer deposition (ALD) cycle is proposed. This method allows one to circumvent the limitations of traditional approaches for obtaining QMS information in ALD, as all m/z values can be simultaneously screened for the formation of reaction products in an efficient way. As a proof of concept this method was applied to the trimethylaluminum (TMA)-water process. This process has been studied extensively over the past decades. Besides the expected formation of CH 4 , the formation of gaseous HOAl(CH 3 ) 2 during the water pulse is observed, revealing a secondary reaction pathway for the water. The reaction energy and Gibbs free energy for different reactions are investigated computationally using density functional theory calculations, and confirm that the secondary reaction pathway is thermodynamically allowed for certain surface conditions.

Atomic layer deposition (ALD) of lithium-containing films is of interest for the development of next-generation energy storage devices. Lithium hexamethyl disilazide (LiHMDS) is an established precursor to grow this type of films. The LiHMDS molecule can either be used as a single-source precursor molecule for lithium, or as a dual-source precursor molecule for lithium and silicon. Single-source behaviour of LiHMDS is observed in the deposition process with trimethylphosphate (TMP) resulting in the deposition of crystalline lithium phosphate (Li 3 PO 4 ). In contrast, LiHMDS exhibits dual-source behavior when combined with O 2 plasma, resulting in a lithium silicate. Both processes were characterized with in situ ellipsometry, in situ time-resolved full-range mass spectrometry, x-ray photoelectron spectroscopy (XPS) and elastic recoil detection analysis (ERDA). When we combined both reactants into a three-step LiHMDS-TMP-O 2 * or LiHMDS-O 2 *-TMP process, the dual-source nature of LiH-MDS emerged again. By carefully combining our measurements, it is shown that film growth with LiHMDS (in combination with TMP and O 2 plasma) is driven by dipole-driven selfsaturated surface interactions combined with dissociative chemisorption. We show that when hydroxyl groups are present on the surface, silicon will be incorporated in the films. These insights benefit the general understanding of the behaviour of the LiHMDS and TMP precursors, and may facilitate their effective use in ternary or quaternary processes.

Atomic layer deposition (ALD) of ruthenium dioxide (RuO2) thin films using metalorganic precursors and O2 can be challenging because the O2 dose needs to be precisely tuned and significant nucleation delays are often observed. Here, we present a low-temperature ALD process for RuO2 combining the inorganic precursor ruthenium tetroxide (RuO4) with alcohols. The process exhibits immediate linear growth at 1 Å/cycle when methanol is used as a reactant at deposition temperatures in the range of 60–120 °C. When other alcohols are used, the growth per cycle increases with an increasing number of carbon atoms in the alcohol chain. Based on X-ray photoelectron spectroscopy (XPS) and conventional X-ray diffraction, the deposited material is thought to be amorphous RuO2. Interestingly, pair distribution function (PDF) analysis shows that a structural order exists up to 2–3 nm. Modeling of the PDF suggests the presence of Ru nanocrystallites within a predominantly amorphous RuO2 matrix. Thermal annealing to 420 °C in an inert atmosphere crystallizes the films into rutile RuO2. The films are conductive, as is evident from a resistivity value of 230 μΩ·cm for a 20 nm film grown with methanol, and the resistivity decreased to 120 μΩ·cm after crystallization. Finally, based on in situ mass spectrometry, in situ infrared spectroscopy, and in vacuo XPS studies, an ALD reaction mechanism is proposed, involving partial reduction of the RuO2 surface by the alcohol followed by reoxidation of the surface by RuO4 and concomitant deposition of RuO2.