Oxygen Annealing Research Articles

Effects of high pressure oxygen annealing on Hf0.5Zr0.5O2 ferroelectric device

We report a high-pressure oxygen annealing (HPOA) process to improve the performance of TiN/Hf0.5Zr0.5O2 (HZO)/TiN devices by controlling the number of oxygen vacancies and carbon contaminants. The ferroelectric properties of HZO film after HPOA at 250 °C for 30 min under different oxygen pressures from 0 to 80 bar were evaluated by electrical and structural characterizations. We found that a sample treated with an oxygen pressure at 40 bar exhibited large switchable polarization (2Pr) of approximately 38 and 47 μC cm−2 in its pristine and wake-up states, respectively. Compared to a control sample, an approximately 40% reduction in the wake-up effect was achieved after HPOA at 40 bar. Improved ferroelectric properties of HZO film can be explained by the appropriate amount of oxygen vacancies and reduced carbon contaminants after HPOA.

Increase of YBa2Cu3O7 critical currents by Mo substitution and high-pressure oxygen annealing

We have previously shown for powders that Mo substitution into the CuO chains of YBa2Cu3O7 can create effective pinning centres which significantly increase the critical current density (jc) in 7 T field by a factor of 4 and 10 at 50 and 60 K, respectively (Rogacki et al., 2006) [1]. The present work reports on the influence of the Mo substitution and high-pressure oxygen annealing on the pinning properties and critical currents of YBa2Cu3-xMoxO7-d by comparing pure (x = 0, d > 0) and substituted (x = 0.03, d < 0) single crystals. Pinning properties have been investigated by measurements of magnetization loops and calculations of jc in the ab-plane, in the temperature range from 2 to 90 K and in fields up to 14 T. Depending on the Mo substitution and the oxygen treatment, several types of pinning centres increasing jc have been revealed and analysed in the frame of Dew-Hughes’ and Kramer's models.

Single Atom Catalysts: Surface Reduction State Determines Stabilization and Incorporation of Rh on α‐Fe2O3(11¯02) (Adv. Mater. Interfaces 8/2021)

Rhodium atoms adsorbed on a model iron oxide surface sinter to nanoparticles already at room temperature. Redispersion into isolated single atoms is induced by oxygen annealing. The resulting atoms are incorporated in the first subsurface layer of the oxide, rather than the surface. More details can be found in article number 2001908 by Gareth S. Parkinson and co-workers.

The influence of thermal annealing on the photoconducting properties of BaSnO3 films

Starting from high-quality oxygen-deficient BaSnO3 films, we have monitored the evolution of their electrical conducting and photoconducting properties after subsequent post-thermal annealing in oxygen. In this way, we have been able to modify the electrical conductivity of the film by at least three orders of magnitude (from 18.2 to 0.013 Ω−1 m−1) by simply reducing the oxygen vacancies concentration after each thermal annealing. Even though the film holds its semiconducting-like behavior, we have observed a modification of the hopping parameters concomitant with a decrease in the Fermi energy level as the electrical conductivity is reduced. Similarly, the effective energy gap extracted from photoconductance spectroscopy measurements decreases as the Fermi energy level decreases suggesting the presence of in-gap states generated by oxygen vacancies. A direct energy bulk gap value of (3.8±0.1) eV was obtained. While the photoconductivity increases from ≃4.6 to 73%, its slow time constants become less dominant as the electrical conductivity is decreased in accordance with a reduction of the oxygen vacancies density, which play a key role as electron-traps.

Oxygen annealing induced changes in defects within β-Ga2O3 epitaxial films measured using photoluminescence

In this work, we use photoluminescence spectroscopy (PL) to monitor changes in the UV, blue, and green emission bands from n-type (010) Ga2O3 films grown by metalorganic vapor phase epitaxy induced by annealing at different temperatures under O2 ambient. Annealing at successively higher temperatures decreases the overall PL yield and UV intensity at nearly the same rates, indicating the increase in the formation of at least one non-radiative defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV increase, suggesting that defects associated with these emissions increase in concentration with O2 annealing. Utilizing the different absorption coefficients of 240 and 266 nm polarization-dependent excitation, we find activation energy for the generation of non-radiative defects of 1.34 eV in the bulk but 2.53 eV near the surface. We also deduce activation energies for the green emission-related defects of 1.20 eV near the surface and 2.21 eV at low temperatures and 0.74 eV at high temperatures through the films, whereas the blue-related defects have activation energy in the range 0.72–0.77 eV for all depths. Lastly, we observe hillock surface morphologies and Cr diffusion from the substrate into the film for temperatures above 1050 °C. These observations are consistent with the formation and diffusion of V Ga and its complexes as a dominant process during O2 annealing, but further work will be necessary to determine which defects and complexes provide radiative and non-radiative recombination channels and the detailed kinetic processes occurring at surfaces and in bulk amongst defect populations.

Radiation Tolerance and Charge Trapping Enhancement of ALD HfO2/Al2O3 Nanolaminated Dielectrics.

High-k dielectric stacks are regarded as a promising information storage media in the Charge Trapping Non-Volatile Memories, which are the most viable alternative to the standard floating gate memory technology. The implementation of high-k materials in real devices requires (among the other investigations) estimation of their radiation hardness. Here we report the effect of gamma radiation (60Co source, doses of 10 and 10 kGy) on dielectric properties, memory windows, leakage currents and retention characteristics of nanolaminated HfO2/Al2O3 stacks obtained by atomic layer deposition and its relationship with post-deposition annealing in oxygen and nitrogen ambient. The results reveal that depending on the dose, either increase or reduction of all kinds of electrically active defects (i.e., initial oxide charge, fast and slow interface states) can be observed. Radiation generates oxide charges with a different sign in O2 and N2 annealed stacks. The results clearly demonstrate a substantial increase in memory windows of the as-grown and oxygen treated stacks resulting from enhancement of the electron trapping. The leakage currents and the retention times of O2 annealed stacks are not deteriorated by irradiation, hence these stacks have high radiation tolerance.

Deterministic Influence of Substrate-Induced Oxygen Vacancy Diffusion on Bi2WO6 Thin Film Growth

In oxide epitaxy, the growth temperature and background oxygen partial pressure are considered as the most critical factors that control the phase stability of an oxide thin film. Here, we report an unusual case wherein diffusion of oxygen vacancies from the substrate overpowers the growth temperature and oxygen partial pressure to deterministically influence the phase stability of $Bi_{2}WO_{6}$ thin film grown by the pulsed laser deposition technique. We show that when grown on an oxygen-deficient $SrTiO_{3}$ substrate, the $Bi_{2}WO_{6}$ film exhibits a mixture of (001) and (100)/(010)-oriented domains alongside (001)-oriented impurity $WO_{3}$ phases. The (100)/(010)-oriented $Bi_{2}WO_{6}$ phases form a self-organized 3D nanopillar-structure, yielding a very rough film surface morphology. Oxygen annealing of the substrate or using a few monolayer-thick $SrRuO_{3}$ as the blocking layer for oxygen vacancy diffusion enables growing high-quality single-crystalline $Bi_{2}WO_{6}$ (001) thin film exhibiting an atomically smooth film surface with step-terrace structure. We propose that the large oxide-ion conductivity of $Bi_{2}WO_{6}$ facilitates diffusion of oxygen vacancies from the substrate during the film growth, accelerating the evaporation of volatile Bismuth (Bi), which hinders the epitaxial growth. Our work provides a general guideline for high-quality thin film growth of Aurivillius compounds and other oxide-ion conductors containing volatile elements.

Effects of Tantalum Doping on Electrical Characteristics of Indium-Gallium-Zinc Oxide Thin-Film Transistors

Thin-film transistors (TFTs) that utilize oxide semiconductors as channel materials have been widely used in various electronic device applications, such as displays, sensors, and memory devices. Among the oxide semiconductors, transparent amorphous indium-gallium-zinc oxides (a-IGZO) have superior characteristics, namely, high field-effect mobility (~10cm2/Vs), good uniformity, low processing temperatures, optical transparency, and low-cost fabrication processability compared with conventional amorphous/poly-silicon TFTs. However, oxide TFTs including a-IGZO TFTs have bias stress-induced instabilities, which are still critical issues in launching commercial oxide TFT products. These issues can be attributed to the oxygen vacancy-related defects, which are known to act as a major source of electrons and as carrier traps. To suppress such oxygen vacancy-related defects, many researchers reported various techniques, such as ozone treatment, oxygen annealing on oxide TFTs, passivation, and metal-doped oxide semiconductors.In this study, we investigated the device performance of tantalum (Ta)-doped IGZO TFTs as a function of Ta content. Ta was used as a dopant because of its bond-dissociation-energy (BDE, 805kJ/mol) with oxygen higher than those of In, Ga, and Zn. The Ta-doped IGZO, which was adopted as a channel layer of bottom-gate TFTs, was fabricated using a magnetron co-sputtering method. Heavily doped p-type Si wafer with thermally grown SiO2 (300nm) was used as substrates. The RF power for the a-IGZO (In2O3 :Ga2O3 : ZnO = 1:1:1)target was fixed at 100 W while the power for the Ta target increased from 0 to 50 W. Aluminum (Al) was deposited as source/drain electrodes using an electron beam evaporator. With increasing Ta content in the IGZO films, the field-effect mobility (μFE) of the Ta-doped IGZO TFTs decreased, the threshold voltage (Vth) increased from negative to positive values, and the subthreshold swing (SS) became smaller with higher on/off ratio. Especially, Ta (10W)-doped IGZO TFTs exhibited μFE of 7.11 cm2/Vs, a SS of 0.195V/decade, and an on/off ratio ~1x108. These results show that the Ta-doping improves the switching characteristics of the Ta-IGZO TFTs. Furthermore, to evaluate the bias-stress stability, positive bias stress (PBS), negative bias stress (NBS) tests were conducted.

Amorphization of Vanadium Oxides during the Reversible Insertion of Lithium

The results of studying the structure and phase composition of thin-film positive electrodes of a lithium-ion battery based on vanadium oxides are presented. It is shown that initially polycrystalline films contain higher V2O5 and V3O7 oxides, the proportion of which increases with the time of oxygen annealing. As a result of charge–discharge cycling, the higher vanadium oxides are amorphized, while the content of the VO2 and V2O3 phases decreases slightly, and the proportion of VO oxide remains unchanged. These data are in good agreement with the change in the capacity of the films during their cycling, which suggests that the main reason for the decrease in the capacity of the films is the amorphization of the higher vanadium oxides with the highest specific capacity.

Enhanced High-Temperature (600 °C) NO2 Response of ZnFe2O4 Nanoparticle-Based Exhaust Gas Sensors.

Fabrication of gas sensors to monitor toxic exhaust gases at high working temperatures is a challenging task due to the low sensitivity and narrow long-term stability of the devices under harsh conditions. Herein, the fabrication of a chemiresistor-type gas sensor is reported for the detection of NO2 gas at 600 °C. The sensing element consists of ZnFe2O4 nanoparticles prepared via a high-energy ball milling and annealed at different temperatures (600–1000 °C). The effects of annealing temperature on the crystal structure, morphology, and gas sensing properties of ZnFe2O4 nanoparticles are studied. A mixed spinel structure of ZnFe2O4 nanoparticles with a lattice parameter of 8.445 Å is revealed by X-ray diffraction analysis. The crystallite size and X-ray density of ZnFe2O4 nanoparticles increase with the annealing temperature, whereas the lattice parameter and volume are considerably reduced indicating lattice distortion and defects such as oxygen vacancies. ZnFe2O4 nanoparticles annealed at 1000 °C exhibit the highest sensitivity (0.13% ppm–1), sharp response (τres = 195 s), recovery (τrec = 17 s), and linear response to 100–400 ppm NO2 gas. The annealing temperature and oxygen vacancies play a major role in determining the sensitivity of devices. The plausible sensing mechanism is discussed. ZnFe2O4 nanoparticles show great potential for high-temperature exhaust gas sensing applications.

The fabrication of LiNbO3 memristors for electronic synapses using oxygen annealing

Based on the LiNbO3 (LN) single crystal thin film prepared using Ar+ etching, an LN thin film memristor was fabricated by oxygen annealing. Atomic force microscope, scanning electron microscope and electron paramagnetic resonance test results show that the method uniformly reduces the amount of oxygen vacancies on the surface of the material. The current-voltage scanning (I–V scanning), retention and endurance test results show that this method effectively reduces the possibility of breakdown and increases the retention and endurance performance of the device. By adjusting the parameters of the electric pulse, the annealed sample successfully emulated spike-rate dependent plasticity, pulse-paired facilitation, post-tetanic potentiation, Ebbinghaus forgetting curve and the spike-time dependent plasticity. These results indicate that the device prepared herein could be used as an electronic synapse in the field of brain-like neuromorphic computing systems.

(Invited) Sub-40mV Sigma VTH Igzo nFETs in 300mm Fab

In this work, IGZO device integration is reported leveraging our 300mm-fab facilities. Our objective is mainly to gain insights into the process and material elements which drive the control of the performance parameters of IGZO nFETs.To control the final doping of the IGZO channel, it is well reported in literature that a final oxygen anneal can be applied to passivate the oxygen vacancies which are formed during the device fabrication. This technique is also used in our 300mm flow. As it might be expected, in front gate IGZO nFETs, the passivation efficiency is limited by the presence of the top metal gate stack. Therefore, it seems important to limit the formation of the oxygen vacancies during the deposition of the gate dielectrics. Three oxides were studied: SiO2, Al2O3 and HfO2 on top of the IGZO channel. After oxygen annealing, only PECVD-SiO2 shows a large recovery while for Al2O3 and HfO2, it remains low. This result challenges the implementation of conventional water-based high-K materials in gate first IGZO integration.Contrary to the use of undoped IGZO in the channel for IOFF control, contacts can rely on maximizing the oxygen vacancies to increase the dopant concentration in the S/D regions. When not done locally, it could be an extra source of variability. This doping is made through oxygen scavenging from IGZO by a thin metal contact barrier. Low specific contact resistivity down to 1x107 Ohm.cm2 is demonstrated when Ti is thinner than 5nm. With thicker barrier, the formation of TiO2 and specific alloys is taking place at the IGZO/Contact barrier interface. This is both confirmed by ab initio simulations and by advanced physical characterization techniques.Regarding the channel, X-ray diffraction techniques are extensively here since this is a nondestructive in-fab technique that provides detailed information about the crystallographic structure of the IGZO material. We show a typical spectrum where clear peaks/humps are seen and attributed to: 1. amorphous IGZO, 2. CAAC-IGZO and 3. a previously not reported, to the best of our knowledge, phase called here s-IGZO. The s-phase is only formed under certain conditions of power, temperature and oxygen flow during material deposition.Thick IGZO (>12nm) back-gated nFETs with active layers submitted to final O2 anneal are used to study the different phases of IGZO and their electrical impact on device parameters. In this configuration, the carrier transport preferentially occurs in the bottom half of the IGZO channel while the top half (SiO2/IGZO interface) mostly drives the electrostatic control of BG transistors. The amorphous IGZO has much reduced spread in VTH-ON and higher ID,LIN (~mobility) than that of CAAC-IGZO. Reliability tests have also been carried out to compare the two phases and the results show the existence of two competing PBTI degradation mechanisms for CAAC-IGZO. It is important to report the degradation over time and different VG stress because a cut line at a specific value could have shown no BTI degradation of CAAC-IGZO and then attributing an unfair benefit of this phase over the a-IGZO phase. Combining these findings to some other results where the sheet resistance of IGZO under hydrogen exposure depends on the phases, we can reasonably conclude that CAAC owns different doping levels in comparison to a-IGZO.Based on the previous learning about n-type dopant location in IGZO, we show that the performance of transistors keeps increasing when the channel length is reduced and scales with the channel width. A measure of the VTH-ON variation has been performed at the 300mm wafer scale from long to ~120nm LCH and down to 200nm WCH dimensions. The figure in attachement shows the Id-Vg curves of >100 Back Gated IGZO-nFETs with no failed devices detected. The standard variation of the VTH-ON across LCH and WCH is often less than 40mV with a minimum of 20mV.In conclusion, we have demonstrated in this study scaled IGZO nFETs with excellent VTH_ON control using an industry compatible 300mm process flow. This has been achieved thanks to a careful mapping of n-type doping in the three dimensions of the IGZO channel. While semi-crystalline IGZO seems to be more robust against hydrogen than that of amorphous-IGZO, a new IGZO phase is found to help boost ION at short channel. The demonstration of back-gate IGZO nFETs with low variability and relatively high drive will benefit to the top-gate architecture development. This will provide new opportunities for the IGZO-based devices like an “on-the-fly” VTH setting for the performance control of advanced applications. Figure 1

Perovskite Oxides Tunable by Electromechanical and Electrothermal Couplings

Objectives: Oxides hold the great promise for emerging electronic applications (other than conventional CMOS components such as gate oxides) because they can significantly increase the operational range of temperature and bias voltage due to the much higher bandgap energy than semiconductors. However, using the oxides for such novel purposes requires their electronic properties to be properly tuned and tailored to specific device applications. Therefore, this work seeks to develop the way to address this challenge by focusing on the tunable characteristics of perovskite oxides driven by their unique electromechanical and electrothermal couplings. Among various types of oxides, crystalline perovskite titanate structures such as STO (SrTiO3) have been chosen as the representative material platform in this study, due to their multifunctional properties and well-established growth techniques. New Results: Previously, researchers have demonstrated the heat-induced tunable electrical characteristics of STO doped with various impurities [1-2]. Due to the complex chemistry where both the thermal and doping effects contribute to the observed change in electrical properties, accurate physical origin of the STO’s tunable behavior has not been clearly identified. In this work, we focused on an undoped, 1 mm-thick STO crystal to quantitatively measure its response when solely exposed to an external stimulus such as mechanical pressure.Fig. 1 depicts the C-AFM (conductive atomic force microscope) experimental setup to study the tunable behavior of STO driven by an electromechanical coupling. The commercially available conductive tip (Asyelec.01-R2) was used to apply a bias voltage (Vbias = 2V) while in contact with the top surface of the STO sample. This enables simultaneous measurement of electrical response during the experiment. The force was applied and varied by changing the set-point voltage (Vsetpoint, kept below 3V to prevent any possible damage to the measurement system) that is used to make the cantilever tip bent with the varying height. Conversion from the control parameter (Vsetpoint) to the mechanical pressure (N/m2) was conducted through precise calibration of the C-AFM measurement setup, including spring constant (0.92 N/m) and deflection sensitivity (0.0576 mm/V) (Table 1).Fig. 2 shows the measured electrical conductivity vs. applied pressure plot for the undoped STO sample used in the experiment. To ensure statistically meaningful data, each experiment was performed repetitively (at least 10 times) and both the mean (data points) and standard deviation (error bars) values were presented in the figure. A clear trend of increase in electrical conductivity with increasing pressure was observed, which is best attributed to creation of oxygen vacancies by mechanical stress in STO [3]. This will surely help researchers study the fundamental electromechanical coupling-driven behavior of perovskite oxide thin films. Fig. 3 further presents the C-AFM image when pressure was applied to the STO surface (the white particles are believed to be due to pile-up of electronic charges upon the application of localized electrical bias).We next studied the electrothermal coupling-driven conductance change in STO by performing a systematic study of thermal annealing (at temperatures ranging from 200°C to 800°C and in dry air). For this study, a sufficient amount of oxygen vacancies was purposely introduced when the STO thin film was prepared (see Fig. 4 for the device schematic). Because subsequent annealing annihilated oxygen vacancies, decrease in electrical conductivity was observed in STO samples annealed at high temperatures (Fig. 5). The reasoning of relating the observed trend in Fig. 5 to the change in the number of oxygen vacancies upon thermal annealing is well supported by measuring the hysteretic behavior of the STO thin film. Fig. 6 shows a measured butterfly curve that is typical for memristor devices where removal and restoration of oxygen vacancies play a key role as the switching mechanism. Significance: This work is of significant importance for the development of future electronic devices with significantly enhanced reliability features. By demonstrating that electrical characteristics of multifunctional oxide thin films can be largely tuned by external stimuli such as mechanical pressure and thermal annealing, it will spark various research initiatives in using the oxide for novel, more versatile applications.

Performance Enhancement of Electrospun IGZO-Nanofiber-Based Field-Effect Transistors with High-k Gate Dielectrics through Microwave Annealing and Postcalcination Oxygen Plasma Treatment.

We investigated the effects of various high-k gate dielectrics as well as microwave annealing (MWA) calcination and a postcalcination oxygen plasma treatment on the electrical properties and stability of electrospun indium gallium zinc oxide (IGZO)-nanofiber (NF)-based field-effect transistors (FETs). We found that the higher the dielectric constant of the gate dielectric, the better the electric field is transferred, resulting in the better performance of the IGZO NF FET. In addition, the MWA-calcined IGZO NF FET was superior to the conventional furnace annealing-calcined device in terms of the electrical properties of the device and the operation of resistor-loaded inverter, and it was proved that the oxygen plasma treatment further improved the performance. The results of the gate bias temperature stress test confirmed that the MWA calcination process and postcalcination oxygen plasma treatment greatly improved the stability of the IGZO NF FET by reducing the number of defects and charge traps. This verified that the MWA calcination process and oxygen plasma treatment effectively remove the organic solvent and impurities that act as charge traps in the chemical analysis of NF using X-ray photoelectron spectroscopy. Furthermore, it was demonstrated through scanning electron microscopy and ultraviolet-visible spectrophotometer that the MWA calcination process and postcalcination oxygen plasma treatment also improve the morphological and optical properties of IGZO NF.

Microstructure of YBa2Cu3Oy coated conductor using {100} ⟨001⟩ textured Cu tape with dual functions of metal substrate and electric stabilizing layer in order to develop low-cost high-TC superconducting wires

A new configuration of coated conductors (CCs), REBa2Cu3Oy (RE: rare earth elements such as Sm, Eu, Gd, and Y)/(Sr, La)TiO3 conductive buffer/Ni-electroplated {100}⟨001⟩ textured Cu and SUS316 lamination tape, has been proposed to reduce material costs. This conductive buffer layer is slightly modified from the previously reported Sr (Ti, Nb)O3 conductive buffer layers by changing the substitution element and site. However, a drastic change is found in the electric resistivity of the conductive buffer layers after oxygen annealing of YBa2Cu3Oy. The resistivity of Sr(Ti, Nb)O3 conductive buffer layers increases by more than three orders of magnitude, while the resistivity of (Sr, La)TiO3 is almost maintained and only minimally increases. Microscopic structural analysis of the CCs using the newly proposed conductive buffer layer is conducted using transmission electron microscopy (TEM). A very clean interface between the metal tape and the conductive buffer layer in the 600 nm thick (Sr, La)TiO3 buffer layers is confirmed by the TEM analysis, which has no reactive layers as well as no inter-diffusion of oxygen and metal elements. This result will soon lead to the development of low-cost and high-performance CCs.

P‐48: Study of ITO Electrode on the White Color Uniformity of TFT‐LCD

This paper makes a research on the relationship between TFT transmittance and white color uniformity, which based on 1st ITO process optimization. The results show that the better TFT transmittance uniformity leads to better white color uniformity, and the ITO oxidation is an important factor for TFT transmittance uniformity improvement. Annealing and increase oxygen flow during ITO sputtering when annealing is skipped are both the effective methods to obtain an excellent white color uniformity.

High speed and ultra-low dark current Ge vertical p-i-n photodetectors on an oxygen-annealed Ge-on-insulator platform with GeOx surface passivation.

Germanium (Ge) vertical p-i-n photodetectors were demonstrated with an ultra-low dark current of 0.57 mA/cm2 at -1 V. A germanium-on-insulator (GOI) platform with a 200-mm wafer scale was realized for photodetector fabrication via direct wafer bonding and layer transfer techniques, followed by oxygen annealing in finance. A thin germanium-oxide (GeOx) layer was formed on the sidewall of photodetectors by ozone oxidation to suppress surface leakage current. The responsivity of the vertical p-i-n annealed GOI photodetectors was revealed to be 0.42 and 0.28 A/W at 1,500 and 1,550 nm at -1 V, respectively. The photodetector characteristics are investigated in comparison with photodetectors with SiO2 surface passivation. The surface leakage current is reduced by a factor of 10 for photodetectors via ozone oxidation. The 3dB bandwidth of 1.72 GHz at -1 V for GeOx surface-passivated photodetectors is enhanced by approximately 2 times compared to the one for SiO2 surface-passivated photodetectors. The 3dB bandwidth is theoretically expected to further enhance to ∼70 GHz with a 5 µm mesa diameter.

Ambient aging effects on the effective energy gap of ZnO thin films

Using photoconductance spectroscopy, we have studied the influence of different types of thermal annealing on epitaxial ZnO thin films where band bending effects play a major role. Once the film is exposed to ambient air conditions after a simple thermal annealing in oxygen at 600°C, the effective energy gap is stable with a value of ≃3.15 eV, while after a corresponding annealing in vacuum and subsequent air exposure, it starts at ≃3.24 eV, and then it evolves along the days until it reaches the bulk energy gap value of ZnO. By means of valence band x-ray photoemission spectroscopy (XPS), we have confirmed that these phenomena are related via the Franz–Keldysh effect to a downward band bending in the former case and a time dependent upward band bending in the latter one that slowly tends to a flat band condition, tracking the behavior observed in the effective energy gap. Core level XPS measurements suggest that for each type of thermal annealing, a different adsorption kinetics of water and hydrogen take place.

Crystal growth, finite element analysis and oxygen annealing of ZnO bulk crystal grown by low-temperature-gradient chemical vapor transport

The growth of high-quality ZnO bulk crystals is an outstanding challenge, and the nature of grown-in defects in crystals is far from well understood. In this work, a specially designed low-temperature-gradient CVT growth system was developed based on finite element analysis. High-quality ZnO single crystals were successfully grown. The supersaturation front in the growth system was controlled, and internal stress in the crystal was reduced by introducing a radiation baffle and a special seed pedestal. The structural, electrical, and optical properties of ZnO crystals were studied before and after oxygen annealing. The scanning electron microscope (SEM) and optical microscope (OM) results indicate the stable growth is achieved under the Zn-rich growth condition. The X-ray rocking curve measurements reveal the high crystalline quality of the grown crystal with the full-width at half-maximum (FWHM) value of 17 arcsec for the (004) reflection. The time-of-flight secondary ion mass spectroscopy (ToF-SIMS) results prove the high purity of grown crystals and reveal the variations of trace impurity concentrations in crystals. When annealed in oxygen at 1000 °C, the optical band gap (Eg) changes from 2.50 eV to 3.15 eV. The Raman analysis indicates that oxygen vacancies (Vo) in the as-grown crystal disappear after annealing. The Hall measurements show that the room temperature (RT) mobilities before and after annealing are 197 cm2V−1c−1 and 207 cm2V−1c−1, respectively. The RT carrier concentration decreases by about two orders of magnitude to 5.45 × 1015 cm−3, while the RT resistivity increases by about one order of magnitude to 5.57 Ω cm after annealing. The positron annihilation lifetimes (PALS) results prove the presence of Zn vacancies (Vzn) after annealing. The origins of as-grown ZnO green luminescence (GL) and the temperature-dependent blue shifts of the GL peak positions are explained based on Zni and Vo defects. The fine structures of annealed ZnO GL are found to be related to Vzn native defects rather than Cu impurity.

Performance improvement of amorphous Ga2O3 ultraviolet photodetector by annealing under oxygen atmosphere

We have demonstrated the fast-speed and high-rejection-ratio solar-blind ultraviolet (UV) photodetectors based on amorphous Ga2O3 (a-Ga2O3) films grown by atomic layer deposition. The effect of the annealing under oxygen atmosphere on the performance of a-Ga2O3 photodetectors is investigated. By the oxygen annealing at 500 °C, the 90-10% decay time of a-Ga2O3 photodetector can be decreased to ∼150 ns, and the UV/Visible rejection ratio of the photodetector can reach as high as 2.74×105, due to the significant suppression of the visible light response. Moreover, the dark current of the 500 °C-annealed photodetector is only 9.43 pA at 10 V bias. These phenomena can be explained by the decrease in oxygen vacancy concentration of the a-Ga2O3 films after the oxygen annealing. The combination of high rejection ratios and fast operating speeds offers a viable way for facile and scalable fabrication of the oxide semiconductor solar-blind UV detectors.