X-ray Photoemission Spectroscopy Results Research Articles

Giant dielectric response and relaxation behavior of Bi3+/W6+ co-doped TiO2 ceramics.

With the rapid development of electronic information technology, dielectric ceramics are widely used in the field of passive devices such as multi-layer ceramic capacitors. In this paper, (Bi2/3W1/3)xTi1-xO2 (BWTOx) ceramics with superior dielectric properties have been prepared by using a traditional solid-state method. Remarkably, at a (Bi2/3W1/3)4+ doping level of 0.01, a (Bi2/3W1/3)0.01Ti0.99O2 ceramic achieved a giant dielectric permittivity of ∼1.5 × 104 and a low loss tangent of ∼0.07 at 1 kHz, as well as a good temperature independence, which could satisfy the operating temperature standards for X9R capacitors. The abnormal dielectric relaxation in the low temperature region can be explained by the interface polarization. Data based on the complex impedance spectroscopy and X-ray photoemission spectroscopy results indicate that the colossal permittivity of BWTOx ceramics is mainly ascribed to the internal barrier layer capacitance effect. The findings of this work could provide valuable insights for achieving large dielectric constants and good temperature stability simultaneously in BWTOx and other related electronic ceramic materials.

Improved performance of deep ultraviolet AlGaN-based light-emitting diode by reducing contact resistance of Al-based reflector

In this study, chlorine (Cl) treatment was carried out on p-AlGaN to enhance the performance of ultraviolet-C light emitting diodes (UVC LEDs) by modifying ITO work function and hence reducing the contact resistance of ITO/Al reflector. The Cl-treated UVC LEDs exhibit the forward voltage of 6.88 V at 20 mA, whereas the reference samples show 7.50 V. The light output power and relative wall plug efficiency (WPE) of the Cl-treated UVC LEDs are enhanced by 17.1% at 500 mW and 19.5% at 100 mA, respectively, as compared to the reference. Additionally, the Cl-treated LEDs also display reduction in both the leakage current and ideality factor. Further, the photoluminescence (PL) intensity of AlGaN micro-disks is also enhanced by the Cl-treatment. X-ray photoemission spectroscopy (XPS) results indicate the formation of Cl-ITO at the ITO/p-AlGaN interface and the passivation of the surface states of AlGaN by Cl radicals. Based on the XPS results, a possible mechanism for the improved performance of Cl-treated UVC AlGaN-based LEDs is described and discussed.

Intriguing triboelectrification behavior of identical P(VDF-TrFE) polymers

We report an intriguing triboelectrification behavior of thermally treated P(VDF-TrFE) polymers. Cooled (C-) and annealed (A-) P(VDF-TrFE) films were prepared by the rapid cooling of the melt solution and the subsequent annealing, respectively. The C–P(VDF-TrFE) had sparse grains and small elastic modulus and surface charge density, while, the A-P(VDF-TrFE) had dense grains and large elastic modulus and surface charge density. When the polymers contacted ITO electrodes, the triboelectric outputs of C-(PVDF-TrFE) were larger than those of A-P(VDF-TrFE). When the polymers contacted each other, the triboelectric outputs of different P(VDF-TrFE) films were significantly larger than those of similar P(VDF-TrFE) films. Based on detailed X-ray photoemission spectroscopy results, we suggested that roughness-induced frictional heat and elastic modulus differences may play an important role in minute material and/or ion transfer during the triboelectrification of identical P(VDF-TrFE) polymers.

Surface Reactivity and Surface Characterization of the Layered β(III)-CoOOH Material: an Experimental and Computational Study

In this article, we focused on the comprehension of the surface reactivity of layered β(III)-cobalt oxyhydroxide, β(III)-CoOOH, by implementing a multiscale study associating both experimental, surface characterization by X-ray photoemission spectroscopy (XPS) and scanning electron microscopy and first-principles calculations. The surface reactivity and the chemical properties of the surface are key factors in the charge-storage mechanism, and β(III)-CoOOH presents interesting characteristics to be used as pseudo-capacitive electrode materials in supercapacitors thanks to its large surface specific area (∼100 m2/gs) and its high electronic conductivity (10–3 to 1 S cm–1). The surface reactivity (basic and redox character) of the synthesized compounds, which consists in aggregates of particles with 60–100 nm length, has been explored from the adsorption of SO2 molecules followed by XPS analyses. A kinetic study of the reactivity allowed us to identify three steps in the adsorption mechanism of our β(III)-CoOOH samples. The coupling of XPS and computational results allows us to establish a link between the surface reactivity in the identified domains, the formation of sulfate and sulfite species, and the cobalt Co3+ and Co4+ species of the active sites along with the underlying electronic processes.

Optimization of Ni/Ag-Based Reflectors to Improve the Performance of 273 nm Deep Ultraviolet AlGaN-Based Light Emitting Diodes

We optimized Ni/Ag-based p-type reflectors for the improvement of efficiency of 273 nm deep ultraviolet (DUV) AlGaN-based flip-chip light emitting diodes (FCLEDs). The Ni(3 nm)/Ag(5–15 nm)/Al/Ni and Ni(25–50 nm)/Ag/Ni contacts exhibited higher reflectance (36.4–39.5%) at 273 nm than reference Ni(5 nm)/Au(5 nm)/Al/Ni contact (26.1%). The Ni(3 nm)/Ag/Al(200 nm)/Ni(20 nm) and Ni/Ag(200 nm)/Ni(20 nm)-based FCLEDs gave forward voltages in the rage of 6.93–7.11 V and 5.5–6.28 V at 20 mA, respectively, whereas the Ni/Au-based sample showed 6.35 V. Further, the Ni(3 nm)/Ag(10 nm)/Al/Ni-based and Ni(50 nm)/Ag(200 nm)/Ni-based samples exhibited 4.85% and 13.4% larger output power at 1.2 W than the reference sample. The Ni(3 nm)/Ag(10 nm)-based and Ni(50 nm)/Ag(200 nm)/Ni-based samples produced 5.6% and 8.5% higher peak external quantum efficiency than the reference sample. It was further shown that the Ni(3 nm)/Ag(10 nm)/Al/Ni-based and Ni(50 nm)/Ag(200 nm)/Ni-based samples experienced less efficiency droop (namely, 27.9 and 26.4%, respectively) than the reference sample (31.4%). Based on the scanning transmission electron microscopy and X-ray photoemission spectroscopy results, the ohmic formation mechanism is described and discussed.

Structural, electronic, and electrical behaviour of MWCNTs: TiO2 (:SiO2) nanocomposites

We have functionalized multiwall carbon nanotubes (MWCNTs) with the composition of SiO2 and TiO2 (MWCNTs:TiO2:SiO2) at different Ti:Si stoichiometric ratios (Ti:Si ≈ 6:6 at% and ≈10:10 at%) using the hydrothermal process. The micro-structural, electronic and electrical properties of the unfunctionalized and functionalized MWCNTs were studied. Changes in surface morphology, degree of hybridization, crystallite structure and bonding structure due to functionalization were studied using field-emission scanning electron microscopy, Raman spectroscopy, x-ray diffraction, x-ray photoemission spectroscopy (XPS) and x-ray absorption near edge structure (XANES) spectroscopy techniques. Memristive and charge storage properties for MWCNTs:TiO2:SiO2 nanocomposites (NCs) are more pronounced on NCs functionalised with high stoichiometric ratio (Ti:Si ≈ 10:10) due to the contribution of Ti3+ and Si 2p core states as indicated by XPS and XANES results. The tunability of electrical conductivity is shown by an increase in the measured current and semiconducting I–V behaviour of the material as a result of high content of Ti-charge transfer. These observed changes in the electrical behaviour and electronic/bonding structure of the NCs indicate that the material could be useful for electrical/electronic applications and photocatalytic activity.

Ultrafast wavelength-dependent carrier dynamics related to metastable defects in Cu(In,Ga)Se2 solar cells with chemically deposited Zn(O,S) buffer layer

The Cu(In,Ga)Se2 (CIGS) solar cells were fabricated with a Zn(O,S) buffer layer via chemical bath deposition method (CBD) using different thiourea (TU) mole concentration. The solar cells showed a substantial increment in the performance after light soaking treatment for 1 h. The performance increment was found to be in proportion to the TU mole concentration used in Z(O,S) layer deposition. To understand the cause of increased efficiency, we examined the metastable defect states that were reversible and reproducible in the CIGS solar cell. Firstly, to understand the film quality and the interface between CBD-Zn(O,S) and the CIGS layer, chemical states and band alignment along the depth direction was studied from the results of X-ray photoemission spectroscopy (XPS). It was observed that as the TU mole concentration increased, the distribution of S atoms in the CBD-Zn(O,S) layer changed from Gaussian to almost straight line due to the existence of numerous Zn–S bonds at the interface. With the actively diffused S atoms toward CIGS, a hollow band region was formed at the interface of the CIGS/CBD-Zn(O,S) layers, suppressing the electron-hole recombination, and thus enhancing the cell efficiency. Further, to measure the metastable defect states, optical pump-THz probe (OPTP) spectroscopy was utilized with two pumps having beam energies of 400 nm and 800 nm, which were sensitive enough to detect the defect states at an ultrafast time scale. We observed a unique and unusual decay curve of a re-excitation process after the initial ultrafast decay. The decay was contributed by “(VSe–VCu) divacancy complex,” which is considered a metastable defect state predominantly found near the surface of CIGS layer. The (VSe–VCu) metastable defect states were filled by photocarriers and then the re-excited photocarriers contributed to the expeditious flow of the photocurrent, giving rise to the increment of the cell efficiency.

Experimental and theoretical study of adsorptive interactions in diesel fuel desulfurization over Ag/MCM-41 adsorbent

Molecular dynamics simulation performed by the DFT/ONIOM method and X-ray photoemission spectroscopy (XPS) data were employed for studying the adsorbate-adsorbent interaction system in diesel desulfurization over Ag/MCM-41 produced from beach sand silica (MPI). The morphology and structure of the materials were characterized via powder X-ray diffraction (XRD), scanning transmission electron microscopy (STEM) and field emission scanning electron microscopy (FESEM) together with energy dispersive spectrometry (EDS) analysis. The results proved a high dispersion of different Ag nanodomains onto MCM-41 and their chemical interaction with support and sulfur compounds by π-complexation. The best fit of kinetic and equilibrium data to pseudo-second order (R2 > 0.99) and Langmuir models (R2 > 0.98), respectively, demonstrate the occurrence of chemisorptive/catalytic interactions with organosulfur compounds, as seen in the XPS results. Its adsorption capacity (qm = 31.25 mgS/g) was 10 times higher than that obtained for pure MCM-41 and double the qm for Ag/MCM-41(C) adsorbent from commercial silica. The computational modeling approach provided valuable insight towards molecular level understanding of the mechanism in aromatic S-compounds adsorption over functionalized MCM-41 and the role of Ag species in this process.

Surface Layer Fluorination of TiO2 Electrodes for Electrode Protection LiBs: Fading the Reactivity of the Negative Electrode/Electrolyte Interface

We demonstrate the possibility to master the negative electrode/electrolyte interface reactivity of a TiO2 based electrode with a protecting fluorinated layer, synthesized by a safe process applied for a first time on electrode materials. Pure XeF2 has been used as fluorinating agent with two different fluorination rates (LowF and HighF). This prospective study provides new insights of the surface fluorination benefits thanks to extreme surface spectroscopy analysis. Indeed, enhanced electrochemical performances have been correlated with the surface reactivity of fluorinated electrodes by the means of the Scanning Auger Mapping analysis (SAM) and to the X-ray Photoemission Spectroscopy (XPS). As shown by the XPS results, fluorinated electrodes exhibit thinner and stabilized solid electrolyte interphase (SEI) layer, providing more effective passivating properties. The SAM chemical mappings confirm this trend. The fluorination of the active material leads to fade the reactivity toward the electrolyte, consequently the products of the electrolyte degradation are deposited in lower quantities on the fluorinated electrode surface, especially the species originated from the salt degradation. Both of the fluorinated electrodes exhibit improved specific capacity by 10% after ten cycles. The polarization is reduced by 12.4% and 17% for LowF and HighF electrodes, respectively.

Plasma and Annealing Treatments to Form Height-Barrier Ni-Based Schottky Contact to n-GaN

We investigated the combined effects of plasma and annealing treatments on the electrical and interfacial reactions of Ni/Au Schottky contacts on n-GaN for high-efficiency electronic and optoelectronic devices. Thermionic emission (TE)-based ideal current-voltage model showed that the Schottky barrier heights (SBHs) ranged from 0.68 to 0.92 eV and the ideality factors were in the range of 1.16–2.15. On the other hand, inhomogeneity model and capacitance-voltage characteristics gave similar SBHs in the range of 1.11–1.45 eV. X-ray photoemission spectroscopy (XPS) results displayed that annealing caused the Ga 2p core level to shift toward lower energies by 0.17–0.18 eV, while plasma treatment induced the Ga 2p core level to shift toward higher energies by 0.12 eV. The XPS Ni 2p core level results exhibited that annealing samples at 600°C gave rise to the formation of NiO and Ni2O3 in addition to Ni3Ga4 and Ni3Ga interfacial phases, which were confirmed by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) results. Based on the XPS, XRD, and STEM results, the annealing and plasma-treatment-induced increase in the SBHs is described and discussed.

Inhomogeneity of barrier heights of transparent Ag/ITO Schottky contacts on n-type GaN annealed at different temperatures

We report on the formation of high barrier-height and transparent Ag/ITO Schottky contacts on n-GaN (nd = 5 × 1018 cm−3) for optoelectronic and transparent electronic devices. Calculations using the thermionic emission model-based current-voltage characteristics of the samples annealed at various temperatures showed small Schottky barrier heights (SBHs) of 0.31–0.37 eV and ideality factors of 1.84–2.19. Conventional activation energy plot showed greatly smaller Richardson constant than the theoretical value. To understand such abnormality, the modified Richardson plot, a model of lateral SBH variation with Gaussian distribution, and capacitance-voltage method were used, where their SBHs were estimated to be in the range 0.74–0.93 eV. Together with the temperature-dependent SBHs and ideality factors, these results imply that SBH behavior could be explained in terms of barrier inhomogeneity at the interfaces. The Ag/ITO samples annealed at 500 °C transmitted 80.9% at 560 nm. The X-ray photoemission spectroscopy (XPS) Ga 2p core level spectra from the interfaces of the samples shifted toward either higher or lower energies. Scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy mapping results revealed the outdiffusion of Ga atoms from n-GaN when annealed at 500 °C. Based on the electrical, XPS and STEM results, the annealing temperature dependence of the SBHs is described and discussed.

Interfacial reactions to form high-barrier-height ITO-based Schottky contacts on p-type GaN using a Ti interlayer

We report the development of high-barrier-height and transparent Ti/ITO Schottky contacts on p-GaN for optoelectronic and transparent electronic devices. The Schottky barrier heights (SBHs) and ideality factors estimated using the current–voltage characteristics were in the ranges of 0.36–0.39 eV and 1.74–2.07, respectively, depending on the annealing temperature. However, the barrier inhomogeneity and modified Richardson plot methods yielded much larger SBHs in the range of 0.82–1.18 eV. At 560 nm, the transmittance of the Ti/ITO samples was in the range of 47.4–89.9%. The Ga 2p core levels obtained from the X-ray photoemission spectroscopy (XPS) of the interface regions of the ITO/Ti/GaN samples shifted toward higher or lower energies, depending on the annealing temperature. The normalized N/Ga atomic ratio showed that N and Ga vacancies were formed at the p-GaN surface region at 300 and 500 °C, respectively. The XPS Ti 2p, N 1s, and O 1s core level results showed the formation of interfacial TiN and TiO2 phases at 300 and 500 °C, respectively. The elemental mapping results obtained using scanning transmission electron microscopy (STEM) demonstrated the outdiffusion of Ga atoms in the sample annealed at 500 °C. On the basis of XPS and STEM results, the dependence of the SBHs on the annealing temperature is described and discussed.

Operational stability of solution-processed indium-oxide thin-film transistors: Environmental condition and electrical stress

We investigate the operational stability of bottom-gate/top-contact-structured indium-oxide (In2O3) thin-film transistors (TFTs) in atmospheric air and under vacuum. Based on the thermogravimetric analysis of the In2O3 precursor solution, we utilize a thermal annealing process at 400 °C for 40 min to prepare the In2O3 films. The results of X-ray photoemission spectroscopy and field-emission scanning electron microscopy show that the electron is the majority carrier in the In2O3 semiconductor film prepared by a spin-coating method and that the film has a polycrystalline morphology with grain boundaries. The fabricated In2O3 TFTs operate in an n-type enhancement mode. When constant drain and gate voltages are applied, these TFTs in atmospheric air exhibit a more acute decay in the drain currents with time compared to that observed under vacuum. In the positive gate-bias stress experiments, a decrease in the field-effect mobility and a positive shift in the threshold voltage are invariably observed both in atmospheric air and under vacuum, but such characteristic variations are also found to be more pronounced for the atmospheric-air case. These results are explained in terms of the electron-trapping phenomenon at the grain boundaries in the In2O3 semiconductor, as well as the electrostatic interactions between electrons and polar water molecules.

Effect of substrate temperature on the morphological, structural, and optical properties of RF sputtered Ge1−xSnx films on Si substrate**Project supported by the Universiti Sains Malaysia (Grant No. 1001/PFIZIK/846072).

In this study, Ge1−xSnx alloy films are co-sputtered on Si(100) substrates using RF magnetron sputtering at different substrate temperatures. Scanning electron micrographs, atomic force microscopy (AFM), Raman spectroscopy, and x-ray photoemission spectroscopy (XPS) are conducted to investigate the effect of substrate temperature on the structural and optical properties of grown GeSn alloy films. AFM results show that RMS surface roughness of the films increases from 1.02 to 2.30 nm when raising the substrate temperature. This increase could be due to Sn surface segregation that occurs when raising the substrate temperature. Raman spectra exhibits the lowest FWHM value and highest phonon intensity for a film sputtered at 140 °C. The spectra show that decreasing the deposition temperature to 140 °C improves the crystalline quality of the alloy films and increases nanocrystalline phase formation. The results of Raman spectra and XPS confirm Ge–Sn bond formation. The optoelectronic characteristics of fabricated metal-semiconductor-metal photodetectors on sputtered samples at room temperature (RT) and 140 °C are studied in the dark and under illumination. The sample sputtered at 140 °C performs better than the RT sputtered sample.

Nitrogen-concentration modulated interface quality, band alignment and electrical properties of HfTiON/Ge gate stack pretreated by trimethylaluminum precursor

Effects of nitrogen incorporation on the interface chemical bonding states, band alignment and electrical properties of sputtering-derived HfTiO high-k gate stacks on Ge substrates pretreated by trimethylaluminum (TMA) precursor have been investigated by X-ray photoemission spectroscopy (XPS), UV–vis transmission spectroscopy, and electrical measurements. XPS results indicate that incorporation of a moderate amount of nitrogen (3sccm) incorporation can effectively suppress the formation of unstable GeO2 and low-k germinate at the interfacial region. Meanwhile, reduction in band gap and valence band offset and increase in conduction band offset have been observed after nitrogen incorporation. Electrical properties were evaluated by capacitance-voltage (C–V) and leakage current density-voltage (J–V) measurements based on MOS capacitors. Small gate leakage current (4.13×10−7A/cm2 at Vg=1V), almost disappeared hysteresis, and large dielectric constant (19.6) that have been observed for MOS capacitor with HfTiON/Ge stacked gate dielectric with N2 flow of 3sccm.

A Thermally Stable NiZn/Ta/Ni Scheme to Replace AuBe/Au Contacts in High-Efficiency AlGaInP-Based Light-Emitting Diodes

We developed NiZn/(Ta/)Ni ohmic contacts to replace expensive AuBe/Au contacts commonly used in high-efficiency AlGaInP-based light-emitting diodes (LEDs), and compared the electrical properties of the two contact types. Unlike the AuBe/Au (130 nm/100 nm) contact, the NiZn/Ta/Ni (130 nm/20 nm/100 nm) contact shows improved electrical properties after being annealed at 500°C, with a contact resistivity of 5.2 × 10−6 Ω cm2. LEDs with the NiZn/Ta/Ni contact exhibited a 4.4% higher output power (at 250 mW) than LEDs with the AuBe/Au contact. In contrast to the trend for the AuBe/Au contact, the Ga 2p core level for the NiZn/Ta/Ni contact shifted toward lower binding energies after being annealed at 500°C. Auger electron spectroscopy (AES) depth profiles showed that annealing the AuBe/Au samples caused the outdiffusion of both Be and P atoms into the metal contact, whereas in the NiZn/Ta/Ni samples, Zn atoms indiffused into the GaP layer. The annealing-induced electrical degradation and ohmic contact formation mechanisms are described and discussed on the basis of the results of x-ray photoemission spectroscopy and AES.

Correlation between active layer thickness and ambient gas stability in IGZO thin-film transistors

Decreasing the active layer thickness has been recently reported as an alternative way to achieve fully depleted oxide thin-film transistors for the realization of low-voltage operations. However, the correlation between the active layer thickness and device resistivity to environmental changes is still unclear, which is important for the optimized design of oxide thin-film transistors. In this work, the ambient gas stability of IGZO thin-film transistors is found to be strongly correlated to the IGZO thickness. The TFT with the thinnest IGZO layer shows the highest intrinsic electron mobility in a vacuum, which is greatly reduced after exposure to O2/air. The device with a thick IGZO layer shows similar electron mobility in O2/air, whereas the mobility variation measured in the vacuum is absent. The thickness dependent ambient gas stability is attributed to a high-mobility region in the IGZO surface vicinity with less sputtering-induced damage, which will become electron depleted in O2/air due to the electron transfer to adsorbed gas molecules. The O2 adsorption and deduced IGZO surface band bending is demonstrated by the ambient-pressure x-ray photoemission spectroscopy results.

Interface quality modulation, band alignment modification and optimization of electrical properties of HfGdO/Ge gate stacks by nitrogen incorporation

Effects of nitrogen incorporation on the interfacial chemical bonding states, band alignment, electrical properties and leakage current conduction mechanisms of HfGdO-based metal-oxide-semiconductor (MOS) capacitors has been investigated by UV–Vis transmission spectroscopy, X-ray photoemission spectroscopy (XPS), and electrical measurements. XPS results indicate that incorporation of a moderate amount of nitrogen incorporation into HfGdO gate dielectrics can effectively suppress the formation of low-k GeO2 and Hf(Gd)-Ge-O interfacial layer at the interfacial region. Meanwhile, reduction in band gap and valence band offset and increase in conduction band offset have been observed after nitrogen incorporation. Electrical measurements based on MOS capacitors have shown that MOS capacitor with HfGdON/Ge stacked gate dielectric with N2 flow rate of 3 sccm exhibits small gate leakage current (1.08 × 10−3 A/cm2 at Vg = 1 V), almost disappeared hysteresis, and large dielectric constant (29.2). The involved leakage current conduction mechanisms for MOS capacitor devices with and without nitrogen incorporation also have been discussed in detail.

A low-cost portable electrical sensor for hydroxyl ions based on amorphous InGaZnO4 thin film at room temperature

The measurement and control of hydroxide ion (OH−) concentration in solution are essential in industrial processes. However, no portable sensing method directly targeting OH− ion with low-cost has been reported till date. Herein, we demonstrate an electrical detection method for OH− concentration in solution based on impedance spectroscopy of hydroxyl ions (OH−) attached to amorphous InGaZnO4 (aIGZO) film surfaces. The systematic examination of impedance response reveals that the resistance component of impedance is sensitive to the OH− ions interaction with the film surface. Results of X-ray photoemission spectroscopy confirm that the change of the impedance property is directly attributed to the amount of hydroxyl radical on the film surface originated from OH− ions in the solution. The impedance behavior of the film upon interaction with OH− was reasonably described by the theoretical analysis of optical measurements based on a vacancy-dependent model. Developed by applying this mechanism as a reference application, an easy-to-use aIGZO thin film based resistance OH− sensor at room temperature shows superior sensitivity, reproducibility, and linearity in the alkali range. This study extends the understanding and usage of aIGZO thin film regarding surface-sensing for the detection of surface interaction and process involving chemical ions and species.

Investigation of Nucleation and Intermixing at Hetero-Interface in III-Nitride-4H-SiC Structures

High sensitivity ultraviolet (UV) avalanche photodiodes (APDs) are useful for various applications, including chemical and biological identification, optical wireless communications, and UV sensing systems. State-of-the-art SiC APDs exhibit a high peak quantum efficiency (QE) of 60% at 268 nm, but the response of these devices diminishes at longer wavelengths due to weak absorption and at shorter wavelengths due to increasing photo-generation of carriers in the top doped layer of this device, the short diffusion length of minority carriers in this doped region and the presence of a high density of surface states [1]. Recently, we have demonstrated that heterostructure III-Nitride/SiC diodes have promise for improving the efficiency of SiC based detectors throughout the UV spectrum. However, these devices require growth techniques for initiating two-dimensional growth on lattice mismatch substrates as well as suppressing impurity migration at the hetero-interface. p- SiC In this paper we explore the impact of in situ substrate preparation and migration enhanced epitaxy (MEE) on the nucleation and impurity concentration of thin AlN films grown by plasma-assisted molecular beam epitaxy on 4H-SiC. A series of 30nm AlN films were deposited on i-p epitaxial SiC structures grown on 4 degree miscut 4H-SiC. The in-situ preparation techniques explored include annealing at 740 °C while depositing and desorbing Ga with and without N plasma active and the shutter closed. The AlN films were grown at temperatures of 600 °C and 740 °C using a standard approach without growth interruption under N limited conditions or employ an MEE, or interrupted growth approach. The surface morphology of the samples were examined by atomic force microscopy and the composition of the films were studies by depth profiling x-ray photoemission spectroscopy (XPS). Figure 1 compares the morphology and the composition profile of the AlN film grown by MEE (sample A) and the standard process (sample B) with the N plasma off during the Ga preparation. Sample A exhibits an atomically smooth, 2D surface, and a nearly stoichiometric Al to N composition in the AlN layer that drops off at the heterointerface. In contrast, sample B has a higher roughness on a 2x2 mm scale with an absence of discernable steps on the substrate. XPS results indicate a slightly larger Al to N composition in the film (no visible droplets) and a detectable presence of Al in the SiC layer indicating migration. These results suggest that MEE approach can both promote the nucleation of AlN growth on SiC in a 2D mode as well as suppress the migration of Al into SiC.Figure 2 demonstrates the impact of an active N plasma source during the Ga preparation process despite closing the source shutter on the morphology and interface composition of the AlN films. Sample C was prepared with the N plasma on and was grown at 600 °C. The film exhibits a rough surface morphology despite the use of MEE, consistent with the observations of Okumura et al. [4]. Sample D, grown under similar conditions at a higher substrate temperature of 740 °C exhibits a 2D morphology characterized by micro-steps and possible step-bunching. Sample E was grown at 600 °C similar to sample D, but with the N plasma off during Ga deposition, exhibits an atomically smooth surface morphology consistent with that of the substrate. XPS studies of sample D indicate the presence of GaN as well as metallic Ga at the hetero-interface in contrast to that of sample E. This suggests that active N that leaks around the closed shutter leads to GaN island formation that prevents the nucleation of AlN in a 2D growth mode at lower substrate temperatures. The potential partial nitridation of the SiC surface during this process does not impede the nucleation of AlN in a 2D growth mode based upon studies of films grown on substrates annealed at 740 °C with the N plasma active but the shutter closed (data not shown). Figure 1