Secondary Ion Mass Spectrometry Depth Research Articles

Depth profile study of LaAl1‐xCrxO3/SrTiO3 (x = 0, 0.2, 0.6, and 1) using time of flight secondary ion mass spectrometry (TOF‐SIMS)

The conducting two‐dimensional electron gas (2DEG) behavior in LaAlO3 (LAO)/SrTiO3 (STO) and their associated mechanisms from various aspects have brought tremendous attention in the concerned area of research. To correlate the 2DEG behavior with their compositions, we have performed time of flight secondary ion mass spectrometry (TOF‐SIMS) depth profile analysis of thin films of Cr‐doped LAO/STO system as LaAl1‐xCrxO3 (x = 0, 0.2, 0.6 and 1) deposited over TiO2 terminated STO substrate, which includes two parent compounds LAO/STO (metallic) and LCO/STO (insulating). The uniform decrease of La and Al concentration at the interface of LAO/STO (metallic) system and in the contrary the nonuniformity of La and Cr concentration in LCO/STO (insulating) system have been highlighted. The uniform variation of ionic concentration at the interface of LAO/STO may increase the career concentrations to make the system metallic. The upward and downward diffusion at the interfaces of intermediate compositions varies differently from their parent ones due to the mixing of Al and Cr. Our results may help to understand the conducting nature of LAO/STO system for future developments and applications in such system.

Oxygen diffusion coefficient in the γ phase of a TiAl GE alloy determined by SIMS

First reliable experimental oxygen diffusion coefficient data have been obtained in a Ti48.3Al47.7Cr1.9Nb2.1 near-γ GE alloy through secondary ion mass spectrometry (SIMS) depth profiling measurements of 18O isotopes between 500 °C and 700 °C. The following expression of diffusion coefficient D has thus been derived: D(m2/s)=10−10.6±1∙exp(−107±10kJ.mol−1/RT). These data have been compared with theoretical calculations from literature, showing reasonable agreement concerning the activation energy, but significant discrepancy regarding the D values.

Oxide dissolution and oxygen diffusion scenarios in niobium and implications on the Bean–Livingston barrier in superconducting cavities

We generalize a native Nb2O5 dissolution model [G. Ciovati, Appl. Phys. Lett. 89, 022507 (2006)] to sequential overlayer dissolutions, multilayer dissolution, and realistic temperature profiles, which may be applicable to other materials. The model is applied to secondary ion mass spectrometry depth profile measurements for varying temperature profiles and two-step oxide dissolution in Nb and found to agree well. In the context of the Meissner screening response due to impurity profiles on the length scale of the London penetration depth, the shallow diffusion of O impurities results in a substantial decrease in the peak supercurrent density near the surface. In this framework, oxide dissolution and oxygen diffusion can account for a rise in peak supportable magnetic field in SRF cavities with baking time and a suppression after the optimal baking time is reached, in good agreement with peak-field baking temperatures and times as well as recent quench field measurements.

High‐Temperature Annealing of Si‐Doped AlGaN

This study explores the impact of Si doping on the material properties of high‐temperature annealed (HTA) Al0.71Ga0.29N layers, which are grown on AlN/sapphire templates. The AlGaN layers are doped with Si by applying different IV/III ratios during epitaxial growth and compared to undoped Al0.71Ga0.29N. Before HTA, the threading dislocation density (TDD) for all samples is about 6.0 × 109 cm−2. After HTA, the Si‐doped AlGaN grown with the highest IV/III ratio of 3.6 × 104 shows the lowest TDD of 1.2 × 109 cm−2. Secondary ion mass spectrometry depth profiles reveal an accelerated Ga diffusion from the doped AlGaN into the AlN buffer layer compared to undoped AlGaN. This suggests that the Ga diffusion process is mediated by Si diffusion. Consequently, the Ga diffusion leads to a decrease in the Ga mole fraction of annealed Si‐doped AlGaN. Furthermore, strain relaxation is higher for the Si‐doped AlGaN than for the undoped AlGaN, before and after HTA. The results from this study suggest that Si doping can be a new promising approach in enhancing the quality of HTA‐AlGaN as a useful template for the growth of UV LED heterostructures.

Investigation of tetrakis(ethylmethylamido)hafnium adsorption mechanism in initial growth of atomic layer deposited-HfO2 thin films on H-/OH-terminated Si (100) surfaces

The continuous scaling down of dynamic random access memory devices has necessitated a comprehensive understanding of the initial growth mechanism in atomic layer deposition. In this study, HfO2 was deposited using tetrakis(ethylmethylamido)hafnium (TEMAHf)-H2O on H-/OH-terminated Si (100) surfaces. By analyzing the Hf-O peak in the FTIR spectra and medium-energy ion scattering measurements, it was determined that a monolayer was formed on the H-Si surface at a rate (10 cycles) lower than that for the OH-Si surface (4 cycles). The ligand variations during each cycle, as determined by FTIR measurements, enabled the suggestion of the initial precursor adsorption mechanism. An analysis of the infrared spectra and secondary ion mass spectrometry depth profiles revealed surface-dependent differences in interfacial bonding. This explained the variation in the rate of formation of 1 Ml. Additionally, theoretical investigations using density functional theory calculations identified the reaction pathway with the lowest energy barrier, thereby validating the experimentally proposed mechanism. This study to elucidate the Si surface and the TEMAHf-H2O reaction mechanism provided insights into the analysis of the initial precursor adsorption mechanism for other types of precursors.

Hydroxyl ion absorption in on-chip high-Q resonators.

Thermal silica is a common dielectric used in all-silicon photonic circuits. Additionally, bound hydroxyl ions (Si-OH) can provide a significant component of optical loss in this material on account of the wet nature of the thermal oxidation process. A convenient way to quantify this loss relative to other mechanisms is through OH absorption at 1380 nm. Here, using ultra-high-quality factor(Q-factor) thermal-silica wedge microresonators, the OH absorption loss peak is measured and distinguished from the scattering loss baseline over a wavelength range from 680 nm to 1550 nm. Record-high on-chip resonator Q-factors are observed for near-visible and visible wavelengths, and the absorption limited Q-factor is as high as 8 billion in the telecom band. Hydroxyl ion content level around 2.4 ppm (weight) is inferred from both Q measurements and by secondary ion mass spectroscopy (SIMS) depth profiling.

Formation of Highly-Activated N-Type Shallow Junction in Germanium Using Nanosecond Laser Annealing and Fluorine Co-Doping

By employing a 355-nm nanosecond (ns) ultraviolet (UV) laser annealing, the impact of fluorine (F) co-doping on the formation of a highly activated N-type shallow junction in germanium (Ge) is investigated. Secondary ion mass spectrometry (SIMS) depth profiling of phosphorus (P) demonstrated that an ultra high P concentration of 9 × 1020 cm−3 at a shallow junction of 55 nm with less dopant diffusion can be obtained using ns laser annealing. F co-doping was confirmed to be an efficient way to improve the activation of the P dopants, but show less influence on the redistribution of P dopants within the NLA melted region. However, the activation level of the shallow junction could be increased to approximately 1 × 1020 cm−3 in the presence of F at an optimized concentration.

High Current Density Trench CAVET on Bulk GaN Substrates with Low-Temperature GaN Suppressing Mg Diffusion

We report that, for the first time, a low-temperature GaN (LT-GaN) layer prepared by metal–organic chemical vapor deposition (MOCVD) regrowth was used as a Mg stopping layer (MSL) for a GaN trench current–aperture vertical electron transistor (CAVET) with p-GaN as a carrier blocking layer (CBL). Inserting LT-GaN on top of the p-GaN effectively suppresses Mg out-diffusion into the regrown AlGaN/GaN channel, contributing to the high current capability of GaN vertical devices with a p-GaN CBL. With different MOCVD growth conditions, MSLs inserted in trench CAVETs were comprehensively investigated for the influence of MSL regrowth temperature and thickness on device performance. With the best on-state current performance obtained in this study, the trench CAVET with a 100 nm thick MSL regrown at 750 °C shows a high drain current of 3.2 kA/cm2 and a low on-state resistance of 1.2 mΩ∙cm2. The secondary ion mass spectrometry (SIMS) depth profiles show that the trench CAVET with the 100 nm thick MSL regrown at 750 °C has a dramatically decreased Mg diffusion decay rate (~39 nm/decade) in AlGaN/GaN channel, compared to that of the CAVET without a MSL (~104 nm/decade). In developing GaN vertical devices embedded with a Mg-doped p-type layer, the LT-GaN as the MSL demonstrates a promising approach to effectively isolate Mg from the subsequently grown layers.

Microstructured silicon substrates impregnated with bis(2,4,4-trimethylpentyl) phosphinic acid for selective scandium recovery

The recovery of trace amounts of scandium from process tailings represents a challenging yet critical task for the growing demand for sustainable and renewable technologies. As an important step toward addressing the environmental and technical concerns related to current scandium separation approaches, we developed novel extractant-impregnated surfaces (EIS) that held a thin layer of extractant within a micropillar texturized substrate. Compared with traditional solvent extraction, the EIS requires significantly less volume of extractant and no agitation. About 95 % of scandium is separated from the complex feedstock prepared from industrial waste bauxite residue, and the selectivity of scandium is more than 200 times higher than that of aluminum. Adsorption isotherm and kinetic studies are performed, and the experimental data are in good agreement with the Langmuir isotherm model and indicate pseudo-second-order kinetics. Cryo time-of-flight secondary ion mass spectrometry depth profiling is conducted on post-extraction EIS to unveil the scandium ions transport behaviour. Moreover, it is confirmed the EIS scheme and its underlying principles can be applied to sandblasted polytetrafluoroethylene substrates, which proves the potential of using EIS design at larger scales.

Insights on the degradation mechanism for large format prismatic graphite/LiFePO4 battery cycled under elevated temperature

Increasingly, batteries are being exposed to elevated temperatures to promote charging capability and satisfy various requirements. A comprehension of the degradation mechanism under elevated temperature is essential for advanced battery management. Here, commercial 100 Ah prismatic graphite/LiFePO4 batteries cycled under 45 °C are investigated. Less than 500 cycles are reached when capacity fades to 90 % of the nominal value, while more than 1300 cycles are reached under room temperature. Comprehensive post-mortem analysis is conducted to explore the mechanism of accelerated aging for 45 °C cycled batteries. The concentration of electrolyte salt dropping from 13.19 wt% to 12.98 wt% indicates the decomposition of electrolyte which brings about the gas release and battery volume expansion. Loss of electrolyte associated with enhanced evolution of solid electrolyte interface (SEI) film also leads to loss of lithium inventory. Larger LiFePO4 particles tend to fracture and Fe dissolves from them. It is proposed that Fe deposition accelerates the SEI film generation and the deposition blocks the graphite layers and limits lithium intercalation. From time-of-flight secondary ion mass spectroscopy depth profiling results, it is found that cathode electrolyte interphase film thickens moderately while SEI film thickens sharply, causing anode porosity to drop from 46.22 % to 40.11 %.

Hydrogen donor formation in a solution-processed ZnO film by UV light irradiation

A transparent ZnO film was deposited by the spin-spray method at a low temperature of 90 °C. Ultraviolet (UV) light irradiation to the film for 60 min decreased the film resistivity drastically by three orders of magnitude from 55.3 to 4.43 × 10−2 Ω cm. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profile and 1H solid-state nuclear magnetic resonance (NMR) spectroscopy analysis revealed that the as-deposited ZnO film before UV treatment contained a large amount of water in the ZnO crystal lattice, and hydrogen donors, including interstitial hydrogen in bond-centered sites (Hi), substitutional hydrogen on the oxygen lattice site (HO), and three O–H bonds in a zinc vacancy (VZn−H3), were generated in the ZnO film after UV treatment. From these results, the mechanism of the hydrogen donor formation was proposed in which UV irradiation to photocatalytic ZnO decomposed the water in the ZnO crystal lattice to form H+ and OH− ions, which associated with the oxygen and zinc vacancies, resulting in the formation of Hi, HO, and VZn−H3.

Low-temperature copper–copper quasi-direct bonding with cobalt passivation layer

In this work, an ultra-thin cobalt film is used as a passivation layer to achieve a Cu–Cu quasi-direct bonding process, and successful bonding was achieved at the bonding temperature of 230 °C with the bonding pressure of 0.5 MPa in non-vacuum conditions. A tensile bonding strength of over 12.8 MPa can be achieved. With the cobalt layer, the total film roughness is greatly reduced to around 1 nm, and the cross-sectional transmission electron microscope images reveal the presence of a void-free bonding interface. Time-of-flight secondary ion mass spectrometry depth analysis is carried out using a PVD deposited multilayer sample, which simulates the bonding process without pressure. It is found that a Cu–Co intermixing layer formed between the Cu and Co interfaces during annealing, and Cu has diffused into the Co layer. Due to the nanocrystalline structure of the Co film and the low roughness of the system, a low bonding temperature can be achieved. These results demonstrate that Co, which can be selectively grown on the Cu surface, can be used as a passivation layer to achieve low-cost and high-quality interconnect for the three-dimensional integrated circuits.

AFM Study of Roughness Development during ToF-SIMS Depth Profiling of Multilayers with a Cs+ Ion Beam in a H2 Atmosphere.

The influence of H2 flooding on the development of surface roughness during time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling was studied to evaluate the different aspects of a H2 atmosphere in comparison to an ultrahigh vacuum (UHV) environment. Multilayer samples, consisting of different combinations of metal, metal oxide, and alloy layers of different elements, were bombarded with 1 and 2 keV Cs+ ion beams in UHV and a H2 atmosphere of 7 × 10–7 mbar. The surface roughness Sa was measured with atomic force microscopy (AFM) on the initial surface and in the craters formed while sputtering, either in the middle of the layers or at the interfaces. We found that the roughness after Cs+ sputtering depends on the chemical composition/structure of the individual layers, and it increases with the sputtering depth. However, the increase in the roughness was, in specific cases, approximately a few tens of percent lower when sputtering in the H2 atmosphere compared to the UHV. In the other cases, the average surface roughness was generally still lower when H2 flooding was applied, but the differences were statistically insignificant. Additionally, we observed that for the initially rough surfaces with an Sa of about 5 nm, sputtering with the 1 keV Cs+ beam might have a smoothing effect, thereby reducing the initial roughness. Our observations also indicate that Cs+ sputtering with ion energies of 1 and 2 keV has a similar effect on roughness development, except for the cases with initially very smooth samples. The results show the beneficial effect of H2 flooding on surface roughness development during the ToF-SIMS depth profiling in addition to a reduction of the matrix effect and an improved identification of thin layers.

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) for Analysis of Surface and Interface Chemistry of Porous Transport Layers

As the United States energy infrastructure moves towards the integration of hydrogen energy, the reliable production of hydrogen through water electrolyzers is imperative. In proton exchange membrane water electrolyzes (PEMWE’s), the porous transport layer (PTL) plays an important role. When choosing PTL material, one should consider corrosion resistance, electrical conductivity, and ability to support the structure of the electrochemical cell. Due to these requirements, titanium is the current state-of-the-art anode PTL material. However, titanium quickly forms a layer of titanium oxide which significantly decreases conductivity of the PTL and respectively decreases the overall efficiency of the PEMWE system. Coatings are commonly applied to the PTL to combat this complication, but the use of noble metals leads to cost concerns. Additionally, the degradation of coated PTLs is not yet well known.Advanced physicochemical characterization of PTLs at various stages of fabrication and testing is vital to understand their properties and the impact on electrochemical performance in order to improve durability and meet cost targets. Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) is currently used to cross section areas of the PTL for subsequent analysis with Scanning Transmission Electron MicroscopyEnergy-dispersive X-ray Spectroscopy (STEM-EDS) analysis to visualize the elemental information of the PTL materials and PTL coatings, specifically looking at detrimental oxide layer formation. Unfortunately, this approach is time-consuming and difficult, motivating the development of alternative approaches that allow the characterization of wide sample sets more efficiently. Additionally, STEM-EDS analysis only provides elemental information, so if several oxide layers preside, it can be difficult to differentiate them. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiling is a very powerful technique that has been commonly used to characterize thin films and buried interfaces. Unlike the FIB-SEM and STEM-EDS combination, ToF-SIMS can be performed relatively quickly in several areas of the PTL, provides chemical information, and is sensitive to trace elements. An added advantage of ToF-SIMS is ability to do 2D and 3D chemical analysis on different scales. This enables visualization of elemental and chemical distribution on both larger scale (70x70 um) areas, and smaller areas (20x10 um) which provides more detailed surface and interface information. This allows for detailed tracking of all major constituents of the PTL and coating. It also allows to follow changes in oxide layers upon fabrication and after electrochemical testing, as well as track homogeneity of the surface and oxide layer interfaces throughout different parts of the PTL. This presentation will demonstrate capabilities of this technique for characterization of PTLs, along with progress and potential challenges. ToF-SIMs results will be compared to TEM analysis of cross-sections obtained with FIB-SEM. This study will highlight similarities and differences between the techniques, technique optimization for these morphologically challenging samples, and paths for future investigation moving forward.

(Invited) ALD HfO2/β-Ga2O3 Reliability for Future Power Devices

The ultra-wide bandgap (UWBG) semiconductor, β-gallium oxide, has shown great potential in power devices due to its excellent electrical properties and availability of high-quality single-crystalline substrates. Significant progress has been made in the development of β-Ga2O3 metal-oxide-semiconductor (MOS) devices, but further progress will require optimization of the gate-dielectric interface. In this talk, I will describe the investigation of HfO2 dielectric reliability on β-Ga2O3 for future power devices. I fabricated β-Ga2O3 (010) MOS capacitors with atomic layer deposited (ALD) HfO2 and used capacitance-voltage (C-V) and current density-voltage (J-V) techniques to analyze the electrical reliability of ALD HfO2/β-Ga2O3 MOSCAPs. Positive bias stress measurements indicated that HfO2/β-Ga2O3 MOSCAPs are more susceptible to charge trapping at lower electric stress fields compared to HfO2/Si MOSCAPs, likely due to the smaller conduction band offset between HfO2 and β-Ga2O3. Initial flatband voltage shifts were observed for C-V sweeps of HfO2/β-Ga2O3 MOSCAPs, showing the significance of border traps within the dielectric. The lack of mobile holes in the β-Ga2O3 impedes the recombination of these trapped charges in the dielectric while the MOSCAP is in depletion. We observed full recovery from stress-induced charge trapping in the HfO2/β-Ga2O3 MOSCAPs upon deep ultra-violet illumination. The negative valence band offset between the dielectric and semiconductor allows the recombination of photo-generated holes with the trapped dielectric charges [1]. Because many fabrication steps require high temperature processing, we also investigated the thermal reliability of the ALD HfO2/β-Ga2O3 (010) interface. Negligible Ga atom diffusion was observed in secondary ion mass spectrometry (SIMS) depth profiles for anneals at temperatures up to 850°C in N2 environment for 10 minutes. Significant Ga diffusion into the HfO2 layer was observed upon annealing at 900°C, resulting in a severely degraded MOS structure. Additional analysis was performed to investigate the impact of high temperature processing on the MOS electrical characteristics [2].H.N.M was supported by a National Science Foundation Graduate Research Fellowship under Grant No. DGE 1256260. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Portions of this work were performed at the Lurie Nanofabrication Facility and J. D. Hanawalt X-Ray MicroAnalysis Laboratory, which are supported by the College of Engineering at the University of Michigan.[1] H. N. Masten, J. D. Phillips, and R. L. Peterson, “Charge trapping and recovery in ALD HfO2/β-Ga2O3 (010) MOS capacitors,” Semicond. Sci. Technol., vol. 36, no. 4, p. 04LT01, Mar. 2021, doi:10.1088/1361-6641/abe880.[2] H. N. Masten, J. D. Phillips, and R. L. Peterson, “Effects of High Temperature Annealing on the Atomic Layer Deposited HfO2 /β-Ga2O3 (010) interface,” Journal of Applied Physics, Accepted.

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Effects of sputtering induced artifacts on the determination of diffusion coefficient: Application to Ni/Cu system

The influence of the deformation of the measured depth profile as compared to the originals, on the determination of diffusion coefficient was evaluated in depth profiling. Based on Fick's second law, the original simulation depth profiles were obtained. By application of the mixing-roughness-information depth model, the influences of sputtering induced artifacts on the original depth profiles could be predicted and, as a result, we could investigate the effects of those artifacts on the determination of the diffusion coefficient. Firstly, a numerical simulation was applied to evaluate the effects of those sputtering induced artifacts on the determination of the diffusion coefficient. Subsequently, an experiment was performed to obtain the diffusion coefficient in a Ni–Cu system and the influences of the sputtering induced artifacts on the measured diffusion coefficient were estimated according to the previous simulation results. The simulation results showed that, for roughness less than 10 nm, and mixing length or information depth less than 4 nm, the effects of sputtering induced artifacts on the determination of the diffusion coefficient could be ignored. In addition, Cu(100 nm)/Ni(100 nm)/SiOx/Si(100) thin films were synthesized using electron-beam evaporation and then annealed at different temperatures under vacuum. The samples were characterized by time-of-flight secondary ion mass spectrometry depth profiling. The effects of sputtering induced artifacts on the depth profiles of the Ni/Cu films were very small since the roughness and mixing length are very small. The measured diffusion coefficient agrees well with the results of others.

Characterization of Cu2ZnSnS4 thin films prepared with and without thin Al2O3 barrier layer

We investigated the effect of thin Al2O3 barrier layer (∼20 nm) deposited by radio frequency magnetron sputtering technique on the structural, morphological, optical, and electrical properties of Cu2ZnSnS4 (CZTS) films. Major differences in the crystalline qualities of the films were not observed with the addition of the Al2O3 layer except for a small decrease in crystallite size. Raman spectra of the film with the Al2O3 layer exhibited the peaks belonging to ZnS and Cu2-xS secondary phases. X-ray photoelectron spectroscopy analysis showed that SO42- and CuSO4 compounds were formed on the surfaces of the films. The formation of the CuSO4 phase was associated with the bonding between CuO and S in an oxidized CZTS surface. The presence of the ZnS, Cu2-xS secondary phases, and CuSO4 on the surfaces of the films were also confirmed by scanning electron microscopy images. The film with the Al2O3 layer differed from the other films with its surface that have randomly distributed porous-structured agglomerations and a decrease in the ZnS secondary phases on its surface. All films exhibited different Na distributions in secondary ion mass spectroscopy depth profiles. The Al2O3 layer caused a slower gradient in Na diffusion with decreasing the accumulation of Na element at a distinct region. The activation energy for the thermally activated band conduction was calculated as 0.030 eV and attributed to the thermal release of the holes from the vacancy of copper (VCu) defect states. The results of temperature-dependent conductivity measurements indicated that there were different conduction mechanisms in the films.

Mutual Diffusion of Model Acceptor/Donor Bilayers under Solvent Vapor Annealing as a Novel Route for Organic Solar Cell Fabrication

The fabrication of bulk heterojunction organic solar cells (OSCs) is primarily based on a phase demixing during solution deposition. This spontaneous process is triggered when, as a result of a decrease in the solvent concentration, interactions between donor and acceptor molecules begin to dominate. Herein, we present that interdiffusion of the same molecules is possible when a bilayers of donors and acceptors are exposed to solvent vapor. Poly(3-hexyl thiophene) (P3HT), and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) were used as donors and two types of fullerene derivatives were used as acceptors: phenyl-C61-butyric acid methyl ester (PC60BM) and phenyl-C71-butyric acid methyl ester (PC70BM), Secondary ion mass spectrometry depth profiling revealed that the interpenetration of donors and acceptors induced by solvent vapor annealing was dependent on solvent vapor and component compatibility. Exposure to chloroform vapor resulted in a complete intermixing of both components. The mutual mixing increased efficiency of inverted solar cells prepared by solvent vapor annealing of model donor/acceptor bilayers. These results provide a new means for mixing incompatible components for the fabrication of organic solar cells.

Effects of high temperature annealing on the atomic layer deposited HfO2/β-Ga2O3(010) interface

Atomic layer deposited HfO2 is a primary candidate for metal–oxide–semiconductor (MOS) power devices based on the ultra-wide bandgap semiconductor β-Ga2O3. Here, we investigated the thermal stability of this stack. Out-diffusion of gallium into HfO2, measured by secondary ion mass spectroscopy depth profile, was observed after annealing at 900 °C. Electrical characterization of MOS capacitors (MOSCAPs) showed that this diffusion caused a dramatic increase in leakage current. For annealing temperatures between 700 and 850 °C, no significant Ga diffusion into the HfO2 layer was observed. Nonetheless, MOSCAPs made with stacks annealed at 700 °C have significantly higher forward bias leakage compared to as-prepared MOSCAPs. Through photo-assisted capacitance–voltage measurements (C–V), we found that this leakage is due to an increase in interface traps (Dit) lying 0.3–0.9 eV below the conduction band. We thus have identified how thermal treatments influence HfO2/Ga2O3 behavior: for anneals at 700–850 °C, we observe an increase in Dit and leakage, while annealing at >900 °C results in notable Ga out-diffusion and a catastrophic degradation in leakage. This understanding is key to improving the performance and reliability of future β-Ga2O3 MOS power devices.