Post-deposition Annealing Research Articles

Plasma-enhanced atomic layer deposition of nickel and nickel oxide on silicon for photoelectrochemical applications

The production of hydrogen fuel through sunlight-driven water splitting has the potential to harness and store large quantities of solar energy in a clean and scalable chemical state, suitable for later use in a range of energy applications. Silicon (Si) possesses many of the required properties to be used effectively as a photoelectrochemical (PEC) water-splitting photoanode. However, its sensitivity to corrosion during the oxygen evolution reaction limits its performance in photoanode applications, thus requiring additional overlayer materials to protect the underlying Si substrate. Nickel oxide (NiO) is one material that acts as an effective protective layer, being transparent, suitably conductive and stable. In this work, we present NiO deposition via state-of-the-art atomic layer deposition and photoemission studies to grow and characterize NiO and Ni–metal protective films. Early-stage nucleation of deposited thin films is illustrated along with the effects of post-deposition annealing and argon milling on depth profile information. Previous reports on the effects of slow argon milling are explored and counter arguments are proposed. Protective films are subjected to PEC testing, which shows enhancement of stability and photocurrent output as a result of the deposited films and plasma annealing on these thin films.

Exploring hafnium oxide's potential for passivating contacts for silicon solar cells

We investigate the potential of ultra-thin HfO2 films grown by atomic layer deposition for passivating contacts to silicon focusing on variations in film thickness and post-deposition annealing temperature. A peak in passivation quality – as assessed by carrier lifetime measurements – is reported for 2.2 nm thick films annealed at 475 °C, for which a surface recombination velocity <1 cm/s is determined. For films <2.2 nm thick, there is a marked decrease in passivation quality. X-ray diffraction highlights a change from crystallised monoclinic to amorphous HfO2 as film thickness decreases from 12 nm to 2.2 nm. Kelvin probe results indicate that as-deposited 2.2–12 nm films have similar effective work functions, although the work function of 1 nm films is considerably lower. Upon post-deposition annealing in vacuum, all films exhibit a reduction in effective work function at temperatures coincident with the onset of passivation in air-annealed samples. An initial investigation into the contact resistivity in a passivating contact structure utilizing HfO2 reveals a strong post-deposition annealing temperature dependence, with the lowest resistance achieved below 375 °C, followed by a decrease in performance as temperature increases towards the optimal temperature for passivation (475 °C). Limitations of the contact structure used are discussed.

Suppression of Visible Emission in Low-Temperature Synthesized Cobalt-Doped ZnO Nanoparticles and Their Photosensing Applications.

Cobalt (Co)-doped ZnO nanoparticles have been synthesized at 100 °C using a simple chemical technique, without post-deposition annealing. These nanoparticles are of excellent crystallinity and show a significant reduction in defect density upon Co-doping. By varying the Co solution concentration, it is observed that oxygen-vacancy-related defects are suppressed at lower Co-doping, while the defect density shows an increasing trend at higher doping densities. This suggests that mild doping can significantly suppress the defects in ZnO for electronic and optoelectronic applications. The effect of Co-doping is studied using X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), electrical conductivity, and Mott-Schottky plots. Photodetectors fabricated using pure and Co-doped ZnO nanoparticles show a noticeable reduction in the response time upon Co-doping, which again affirms the reduction in the defect density after Co-doping.

Post-deposition annealing effects on microstructure, mechanical and tribological properties of AlCrN films

The post-deposition annealing of AlCrN films was conducted in an atmospheric environment for 2 h, with annealing temperatures ranging from 25 to 1000 °C. The microstructure, mechanical, and tribological properties were systematically investigated as a function of the annealing temperature. The results show that the AlN phase in the film began to oxidize at 600 °C, resulting in deterioration of the film's toughness and resistance to plastic deformation. Subsequently, the wear resistance also deteriorated, with the wear rate being nearly 16 times that of the as-deposited film. At 800 °C, the AlN obviously underwent oxidization to form Al2O3. The high temperature annealing promoted the coalescence of Cr on the film surface and caused a reaction with the Al2O3 to form a replacement solid solution of Cr–O–Al, thereby improving the mechanical properties of the annealed films. At the same time, the tribological properties of the films were not significantly deteriorated by annealing at temperatures other than 600 °C, similar to those of the films without any annealing treatment.

X-ray diffraction study of epitaxial CuO nanostructures obtained through post-deposition annealing of Cu on SrTiO3(001)

Orientation, structure and morphology in the early growth stages of CuO films on strontium titanate were studied by synchrotron radiation X-ray diffraction. Nanostructured CuO films were obtained by ex situ heat treatment at 970 K in oxygen flow at ambient pressure after an initial deposition of Cu on SrTiO3(001) at 10−4 Pa oxygen pressure using the molecular beam epitaxy technique. These films at low coverages grow forming islands a few tens of nanometers wide, with the [010] direction perpendicular to the substrate surface. Two types of epitaxies are observed, CuO[001]//SrTiO3[100] and CuO[100]//SrTiO3[100], the former being the preferred one. The combination of these epitaxies with the P4mm surface symmetry of the substrate generates 16 different orientations of nanostructures. The lattice constant values obtained from X-ray diffraction are very close to those of bulk tenorite, with the exception of the monoclinic angle β, which is reduced from 99.54° to 96.76(8)° by the epitaxy constraints. This angle is a key parameter in determining the magnetic properties of CuO.

Improved ferroelectric properties of CMOS back-end-of-line compatible Hf0.5Zr0.5O2 thin films by introducing dielectric layers

Hf0.5Zr0.5O2 (HZO) ferroelectric thin films have gained significant attention for the development of next-generation ferroelectric memories by complementary-metal-oxide semiconductor (CMOS) back-end-of-line (BEOL) processing, due to their relatively low crystallization temperature. However, it remains challenging to achieve excellent ferroelectric properties with post deposition annealing (PDA) process at a BEOL compatible temperature. Along these lines, in this work, it is demonstrated that the ferroelectricity of 15 nm thick HZO thin film prepared by PDA process at 400 °C can be improved to varying degrees, via depositing 2 nm thick dielectric layers of Al2O3, HfO2, or ZrO2 at either the bottom or the top of the film. Notably, the HZO thin film with the top-Al2O3 layer exhibits remarkable ferroelectric properties, which are independent of the thickness of HZO. The 6 nm thick HZO thin film shows a total remanent polarization (2Pr) of 31 μC/cm2 under an operating voltage of 2.5 V. These results represent a significant advancement in the fabrication of high-performance, BEOL compatible ferroelectric memories, as compared to previously reported state-of-the-art works.

Synthesis of Micron-Sized WS2 Crystallites Using Atomic Layer Deposition and Sulfur Annealing

The synthesis of micron-sized WS2 crystallites via atomic layer deposition (ALD) is reported for the first time using bis(t-butylimido)bis(trimethylsilylmethyl)tungsten and H2S as reactants, followed by post-deposition annealing. Self-limiting growth on silicon is demonstrated between 315 and 350 °C. As-deposited films are nanocrystalline, comprising a mix of WS2 and WO3 phases along with residual amounts of WOxNy and SiOx impurities. Post-deposition annealing in elemental sulfur at 600 °C induces the emergence of the 2LA(M), E2g1(Γ), and A1g(Γ) Raman vibration modes for WS2 and increases crystallite size up to a few microns, the largest reported to date for ALD WS2. The growth substrate was found to impact WS2 morphology. Films grown and annealed on silicon produced two distinct morphologies of WS2 crystallites with (i) in-plane multilayered flake pyramids predominantly seen at 650 °C and (ii) out-of-plane “flowers” growing at 700 and 800 °C. Films grown and annealed on ZnS showed only out-of-plane “crumpled” flowers with a high density of WS2 edge sites suitable for catalysis, while films grown and annealed on Al2O3 showed predominately few-layered in-plane WS2 flakes, suitable for electronic devices.

Defect generation and recovery in high-k HfO2/SiO2/Si stack fabrication

The defect generation and recovery are studied in a high-k HfO2/SiO2/Si stack for MOSFETs, at each fabrication step. The stack is fabricated in a well-established manner, via chemical oxidation for a SiO2 interfacial layer and atomic layer deposition for a HfO2 layer, followed by post-deposition annealing (PDA), O2 plasma treatment, and forming gas annealing (FGA). Throughout the fabrication, the carrier lifetime is measured for monitoring the defects in the stack. The measurements indicate that the defects are generated by the HfO2/SiO2 stack formation as well as PDA and O2 plasma treatment, whereas those defects are mostly recovered by FGA.

Exploring thermally stable metal-oxide/SiO2 stack for metal oxide semiconductor memory and demonstration of pulse controlled linear response

We fabricated Al2O3/SiO2 stack structures with atomically thin Ti oxide layers at the interfaces using atomic layer deposition and investigated the capacitance–voltage (C–V) hysteresis of the metal-oxide-semiconductor (MOS) capacitors. We studied the effect of post-deposition annealing in the temperature range of 150 °C−500 °C on the C–V hysteresis and found that the Al2O3/SiO2-based stacks are thermally stable compared to ZrO2/SiO2- and HfO2/SiO2-based stacks. Using Al2O3/SiO2-based stacks, we investigated the impact of oxide layer thickness and gate electrode materials and studied pulse-induced current changes in MOS field-effect transistors.

Hafnium oxide: A thin film dielectric with controllable etch resistance for semiconductor device fabrication

Thin film dielectrics are ubiquitous in the manufacture of electronic devices and are frequently deposited and etched away at various stages of device fabrication. We demonstrate that hafnium oxide (HfO2) thin films grown via atomic layer deposition on silicon and silicon pre-coated with aluminum oxide (Al2O3) have etch resistance properties, which can be tuned simply by changing the post-deposition annealing temperature. The etching rates of films in hydrofluoric acid (HF) solutions were found to be dependent on annealing temperature, with the etch rate decreasing with increasing temperature. A transition region in the etch rate was identified between 300 and 350 °C, corresponding to the crystallization of the HfO2 films, as identified via x-ray diffraction. HfO2 films deposited directly onto silicon annealed above 350 °C were resistant to 10% HF solutions over the course of several hours. In the case of Si/Al2O3/HfO2 stacks, closer inspection reveals the existence of channels, which reduces the etch resistance of HF acid, as evidenced by tetramethylammonium hydroxide etching of the silicon substrate. Crystallized HfO2 can be used to protect other dielectrics in device processing, and we demonstrate its use in single-sided fabrication of patterned structures of Al2O3, which can control the effective charge-carrier lifetime in silicon wafers for use in modulating THz and mm-wave radiation.

High Mobility Silicon Dioxide Layers on 4H-SiC Deposited by Means of Atomic Layer Deposition

A study on the impact of different growth and deposition techniques on the reliability of silicon dioxide (SiO2) layers on silicon carbide (SiC) metal-oxide-semiconductor capacitors (MOSCAPs) is presented and compared to channel mobilities that were extracted from lateral metal-oxide-semiconductor field-effect transistors (LMOSFETs). Oxide layers were formed using atomic layer deposition (ALD), low pressure chemical vapour deposition (LPCVD) and direct thermal growth, including post-deposition anneals (PDAs) in nitrious oxide and forming gas (FG) for the ALD-and LPCVD-deposited oxides. Electrical characterisation results at elevated temperatures show that a PDA in FG leads to the highest average breakdown electric field of 10.08 MV/cm, outperforming all other device splits. Time-dependent dielectric breakdown (TDDB) results showed that the time to failure of 63% of the investigated samples at 9MV/cm in the FG-annealed samples was about 50% higher than in LPCVD-deposited oxides that had undergone an N2O PDA. Channel mobilities of the FG-treated samples averaged about three to four times higher than in other datasets, showing excellent peak field-effect mobilities of 60 cm2/V.s and 108 cm2/V.s at room temperature and 175°C, respectively.

Evolution of Interface State Density and Near Interface Oxide Traps under Controlled Nitric Oxide Annealing in SiO&lt;sub&gt;2&lt;/sub&gt;/SiC Lateral MOSFETs

In this paper, the effect of different post oxide deposition nitridation processes in NO on n-channel lateral MOSFETs fabricated on implanted 4H-SiC were investigated. In particular, the electrical behavior of the MOSFETs was deeply investigated not only in terms of SiO2/SiC interface state density and field effect mobility, but also considering the threshold voltage stability effect. The aim of this work was to explore to which extent post oxide deposition annealing in NO is beneficial for the MOS interface behavior and when their detrimental effects start to become predominant on the device performances. Here, the separation of the trapping states at the interface – either close to the conduction and valence band edges – and the near interface oxide traps are reported for the different duration of the post oxide deposition annealing. In fact, cyclic gate bias stress was employed in order to analyze the behavior of the trapping states and to correlate them with the variation of the benefits in terms of the channel mobility (that saturates at about 80 cm2V-1s-1), and on the threshold voltage instability effect. In particular, prolonged PDAs may induce an increase of the amount of trapping states close to the valence band edge and inside the insulator of about 20% and 50 %, respectively.

Effects of electrode materials on the performance of Zr0.75Hf0.25O2-based ferroelectric thin films via post deposition annealing

In this work, the effects of top electrode (TE) and bottom electrode (BE) on the ferroelectric properties of zirconia-based Zr0.75Hf0.25O2 (ZHO) thin films annealed by post-deposition annealing (PDA) are investigated in detail. Among W/ZHO/BE capacitors (BE = W, Cr or TiN), W/ZHO/W delivered the highest ferroelectric remanent polarization and the best endurance performance, revealing that the BE with a smaller coefficient of thermal expansion (CTE) plays a vital role in enhancing the ferroelectricity of fluorite-structure ZHO. For TE/ZHO/W structures (TE = W, Pt, Ni, TaN or TiN), the stability of TE metals seems to have a larger impact on the performance over their CTE values. This work provides a guideline to modulate and optimize the ferroelectric performance of PDA-treated ZHO-based thin films.

Effect of Deposition Temperature on Long-Term Residual Stress Evolution of Au Films.

To increase the residual stress stability of Au films while reducing the residual stress level, the effect of deposition temperature on long-term residual stress evolution of Au films under different conditions were studied. Au films with a thickness of 360 nm were deposited using e-beam evaporation on fused silica under different temperatures. Observations and comparisons were made of the microstructures of Au films deposited under different temperatures. Results showed that by increasing the deposition temperature, a more compact microstructure of Au film was obtained, which was manifested in increased grain size and reduced grain-boundary voids. After deposition, a combined process consisting of natural placement and 80 °C thermal holding was conducted on the Au films, and the residual stresses were monitored using the curvature-based technique. Results showed that the initial tensile residual stress of the as-deposited film decreased with the deposition temperature. The Au films with higher deposition temperatures showed better residual stress stability, maintaining low stress levels in the subsequent long-term combination of natural placement and thermal holding. The mechanism was discussed based on the differences in microstructure. Comparisons were made between post-deposition annealing and increased deposition temperature.

Evolution of Frenkel defect pairs in β-Ga2O3 thin film with the introduction of oxygen and its application in GaN-based ultraviolet light-emitting diode

In this work, β-Ga2O3 thin films were deposited by radio frequency (RF) magnetron sputtering technique. The effect of oxygen-argon flow ratio (OAFR) on the formation of Frenkel defect pairs was systematically analyzed. The Frenkel defect pairs would become abundant when the OAFR was 25:25 sccm, resulting in a severe damage in crystalline quality and an increase in the density of disorder. With the further increase of OAFR, Frenkel defect pairs would dramatically decrease or even vanish due to the significant reduction of oxygen vacancies (V0). Immediately after that, on the basis of avoiding the influence of Frenkel defect pairs inside the β-Ga2O3 films to the devices, the p-NiO/i-Ga2O3/n-GaN diode was fabricated from the as-prepared high-quality β-Ga2O3 film after post-deposition annealing as the electron-blocking layer. Subsequently, its electrical characteristics were investigated in detail which exhibited excellent rectification characteristics under all test temperatures. Correspondingly, the device achieved electroluminescence (EL) with a dominant sharp emission peak in the ultraviolet range (∼375 nm). In the end, the energy band diagram was used for in-depth analysis of the EL mechanism.

Thermal Evolution, Hydrocarbon Generation, and Heat Accumulation of a High Geothermal Coalfield: A Case Study of Pingdingshan Coalfield, China.

As an important energy base in central China, the Pingdingshan coalfield has abundant coal and geothermal resources. The cooperative exploration of coal and geothermal resources is significant for the comprehensive utilization of energy resources. This work collected coal-bearing samples from the Pingdingshan coalfield to investigate the tectono-thermal evolution of a high geothermal coalfield, especially the present geothermal field and hydrocarbon generation model. The geochemical results show that the Shanxi and Taiyuan source rocks have average R o values of 0.88 and 0.97%, respectively, with an average Rock-Eval T max value of 442 °C. Hydrocarbon generation of source rocks started at ∼205 Ma, with the highest rates at ∼170 Ma, reaching the maximum transformation ratio of 40-50% in the middle of the Early Cretaceous. The age and length of apatite fission tracks (AFTs) indicate that coal-bearing strata underwent significant post-depositional annealing after the Late Permian and suggest an abnormal thermal event that occurred in the Late Mesozoic. Meso-Cenozoic thermal event was mainly caused by the plutonic metamorphism of the Early Jurassic and magmatic thermal metamorphism of the Early Cretaceous, achieving a maximum paleotemperature of ∼140 °C. The magmatic thermal event resulted from the intensive post-orogenic extension of the Qinling-Dabie Orogenic Belt caused by the tectonic transition of the North and South China Plates. The present-day high geotemperature of Pingdingshan Coalfield is dominated by the horst structure caused by the regional extension of the basin-mountain system. The Cambrian limestone with a high thermal conductivity underlying coal measure collects deep heat, forming a heat accumulation center of this horst structure with a heat flow of 74 mW/m2 and a maximum temperature of ∼50 °C nowadays.

Forming-Free HfOx-Based Resistive Memory With Improved Uniformity Achieved by the Thermal Annealing-Induced Self-Doping of Ge

In this work, a forming-free HfOx-based resistive random access memory (RRAM) with improved uniformity is successfully demonstrated without an additional doping process, ionic injection, or special layer deposition. The significantly enhanced performances can be attributed to the self-doping of Ge atoms from the Ge bottom electrode (BE) into the HfO2 layer during the postdeposition annealing (PDA) process. Besides, the RRAM provides a lower reset current and a higher ON/OFF ratio than the reported forming-free RRAM. These excellent properties are beneficial to the application of power-saving RRAM and its circuit operation.

Transformation kinetics for low temperature post-deposition crystallization of TiO 2 thin films prepared via atomic layer deposition (ALD) from tetrakis(dimethylamino)titanium(IV) (TDMAT) and water

Background: We report on the fundamental crystallization kinetics of atomic layer deposited (ALD) TiO2 thin films undergoing a post-deposition anneal (PDA) at low temperatures to probe differences in the as-deposited film microstructure. Methods: The system of study is ALD TiO2 thin films prepared from tetrakis(dimethylamino)titanium(IV) (TDMAT) and water at 120 °C, 140 °C and 160 °C followed by ex situ low temperature annealing at temperatures ranging from 140 °C to 220 °C. All as-deposited TiO2 thin films are amorphous by X-ray diffraction (XRD). Post-deposition annealing (PDA) produces large grain anatase crystals, confirmed by XRD and top-view scanning electron microscopy (SEM). A detailed SEM study is performed to quantify the nucleation and growth kinetics by fitting microstructural data to the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. Finally, a time-temperature-transformation (TTT) diagram is constructed to summarize the differences in crystallization behavior at different ALD deposition temperatures. Results and conclusions: Fitting microstructural data to the JMAK equation reveals an Avrami exponent close to 3 with continuous nucleation, suggesting two-dimensional, plate-like crystal growth. Applying an Arrhenius relationship to the phase transformation data, the combined activation energy for nucleation and growth is found to be 1.40–1.58 eV atom-1. Nucleation rates are determined, and an Arrhenius relationship is used to calculate the critical Gibbs free energy for nucleation (~1.3–1.4 eV atom-1). As such, nucleation is the rate-limiting step for the amorphous to anatase phase transformation. ALD growth temperature is found to dictate film microstructure with lower deposition temperatures reducing the nucleation rate and leading to larger grain sizes irrespective of PDA conditions. The nucleation rate pre-exponential frequency factor increases with increasing deposition temperature, thereby increasing the likelihood for nucleation. Interestingly, it is this difference in the vibrational modes of the amorphous structure, as indicated by the variation in the nucleation rate pre-exponential frequency factor, that alters the phase transformation rates and not a change in the activation energies for the transformation.

Phase separation paths in metastable Zr1-xAlxN monolithic layers compared to multilayers with TiN: Growth versus annealing temperatures

Metastable super-saturated Zr1−xAlxN alloys tend to phase separate into the equilibrium cubic (c) ZrN and wurtzite (w) AlN due to a deep miscibility gap. Transformation is shown here to follow distinctly different paths depending on if Zr1−xAlxN (x = 0.3 and 0.6) is sputter deposited as a single layer or multi-layered with TiN, and further varied by post-deposition annealing. Using in situ high-energy synchrotron wide-angle X-ray scattering and analytical transmission electron microscopy, surface segregation effects are compared to secondary phase transformations occurring in as-deposited layers during thermal annealing up to 1000°C. For the primary phase transformation from the vapor phase, w-AlN nucleates and an AlN-ZrN labyrinthine structure evolves at elevated deposition temperature with semi-coherent interfaces over several nanometers, where the higher Al content narrows the structure in both single and multilayers. Transformation in thinner alloy layers is governed by epitaxial forces which stabilize single-phase c-Zr0.4Al0.6N, which enables c-Zr0.4Al0.6N/TiN superlattice growth at temperatures as low as 350°C. Regardless of the decomposition route, the formation of c-AlN is impeded and w-AlN instantaneously forms during transformation. In contrast, isostructural decomposition into w-AlN and w-Zr(Al)N occurs in w-Zr0.4Al0.6N alloys during annealing.

Low-temperature synthesis of crystalline vanadium oxide films using oxygen plasmas

Vanadium oxide (VOx) compounds feature various polymorphs, including V2O5 and VO2, with attractive temperature-tunable optical and electrical properties. However, to achieve the desired material property, high-temperature post-deposition annealing of as-grown VOx films is mostly needed, limiting its use for low-temperature compatible substrates and processes. Herein, we report on the low-temperature hollow-cathode plasma-enhanced atomic layer deposition (ALD) of crystalline vanadium oxide thin films using tetrakis(ethylmethylamido)vanadium and oxygen plasma as a precursor and coreactant, respectively. To extract the impact of the type of plasma source, VOx samples were also synthesized in an inductively coupled plasma-enhanced ALD reactor. Moreover, we have incorporated in situ Ar-plasma and ex situ thermal annealing to investigate the tunability of VOx structural properties. Our findings confirm that both plasma-ALD techniques were able to synthesize as-grown polycrystalline V2O5 films at 150 °C. Postdeposition thermal annealing converted the as-grown V2O5 films into different crystalline VOx states: V2O3, V4O9, and VO2. The last one, VO2 is particularly interesting as a phase-change material, and the metal-insulator transition around 70 °C has been confirmed using temperature-dependent x-ray diffraction and resistivity measurements.