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Recently, high- k dielectric oxide-based MIS type ultraviolet (UV) photodetectors (PDs) have engrossed the researchers in the area of optoelectronics owing to their superior properties. The current work focuses on the preparation of symmetric interdigitated Au electrodes on Ta 2 O 5 /GaN heterojunction and to check its functioning as a photosensor in UV region. The impact of post-annealing procedure on the bandgap, crystalline quality, surface morphology and chemical composition has been investigated using UV-VIS, XRD, AFM and XPS techniques, respectively. The evaluated optical bandgaps from the Tauc's plots for the Ta 2 O 5 films ranging from 4.52 eV to 4.92 eV. XRD analysis showed the formation of an orthorhombic phase after post-annealing temperature of 700 °C. The post-annealed Ta 2 O 5 film surface morphology comprises prearranged atomic clusters with spherical shapes. The XPS exploration revealed the occurrence of the Ta5+ state and confirmed the formation of Ta 2 O 5 phase. Photodetection parameters of the designed Au/Ta 2 O 5 /GaN MIS BB PDs are studied using I–V, responsivity and temporal responses as a function of the post-annealing process. At 0 V bias, with the illumination of 310 nm UV light, the 800 °C post-annealed MIS PD device exhibited peak responsivity of 212 mA/W, external quantum efficiency (EQE) of 86.8 % and detectivity of 1.5 × 1013 Jones. Further, under self-driven mode with 350 nm light illumination, the 800 °C post-annealed UV PD device generated faster rise and fall times of 90 ms and 790 ms, respectively. The post-annealed Au/Ta 2 O 5 /GaN heterostructure PD device demonstrates improved photoresponsivity, EQE and rise/fall times compared with the as-deposited PD device. This remarkable improvement in the photodetection performance is attributed to the best band configuration of the Ta 2 O 5 /GaN heterostructure because of substantial post-annealing process and facilitating broadband absorption from UV-C to A region with quicker generation, separation and transportation of photogenerated charge carriers. Consequently, the current research work emphasizes the significance of the combination of UV sensing Ta 2 O 5 high- k oxide and n-GaN materials to serve the future necessities of the photodetection technology under self-driven mode i.e., without any external bias. Recently, high- k dielectric oxide-based MIS type ultraviolet (UV) photodetectors (PDs) have engrossed the researchers in the area of optoelectronics owing to their superior properties. The current work focuses on the preparation of symmetric interdigitated Au electrodes on Ta 2 O 5 /GaN heterojunction and to check its functioning as a photosensor in UV region. XRD analysis showed the formation of an orthorhombic phase after post-annealing temperature of 700 °C. The post-annealed Ta 2 O 5 film surface morphology comprises prearranged atomic clusters with spherical shapes. The XPS exploration revealed the occurrence of the Ta5+ state which confirmed the formation of Ta 2 O 5 phase. Photodetection parameters of the designed Au/Ta 2 O 5 /GaN MIS BB PDs are studied using I–V, responsivity and temporal responses as a function of the post-annealing process. At 0 V bias, with the illumination of 310 nm UV light, the 800 °C post-annealed MIS PD device exhibited peak responsivity of 212 mA/W, external quantum efficiency (EQE) of 86.8 % and detectivity of 1.5 × 1013 Jones. Further, under self-driven mode with 350 nm light illumination, the 800 °C post-annealed UV PD device generated faster rise and fall times of 90 ms and 790 ms, respectively. The post-annealed Au/Ta 2 O 5 /GaN heterostructure PD device demonstrates enhanced photoresponsivity, EQE and rise/fall times compared with the as-deposited PD device. Consequently, the current research work emphasizes the significance of the combination of UV sensing Ta 2 O 5 high- k oxide and n-GaN materials to serve the future necessities of the photodetection technology under self-driven mode i.e., without any external bias. [Display omitted] [ABSTRACT FROM AUTHOR]

We report the band gap tuning and facilitated charge transport at perylenediimide (PDI)/GaN interface in organic–inorganic hybrid nanostructure system over flexible titanium (Ti) foil. Energy levels of the materials perfectly align and facilitate high efficiency charge transfer from electron rich n-GaN to electron deficient PDI molecules. Proper interface formation resulted in band gap tuning as well as facilitated electron transport as evident in I–V characteristics. Growth of PDI/GaN hybrid system with band gap tuning from ultra-violet to visible region and excellent electrical properties open up new paradigm for fabrication of efficient optoelectronics devices on flexible substrates. [ABSTRACT FROM AUTHOR]

Recently, heterojunctions-based photodetectors (PDs) with outstanding photosensitivity and fast switching time received substantial attention due to their broadband (BB) photoresponse in various regimes of the electromagnetic spectrum. Still, there is an increasing demand to accomplish BB PDs operating at photovoltaic (self-driven) mode for serving the today's societal energy efficient needs. In this work, Au and NiFe 2 O 4 thin films were deposited onto p-Si substrates to accomplish interdigitated Au/NiFe 2 O 4 /p-Si heterojunction type BB PDs operated in self-powered mode. The structural, optical, morphological and magnetic parameters of the prepared NiFe 2 O 4 thin films as a function of annealing process were studied using XRD, UV–Vis, AFM and XAS/XMCD techniques. Tauc's plot fitted from the UV–Vis absorption data for the as-deposited, 700 °C and 800 °C NiFe 2 O 4 thin films were 4.85, 3.42 and 2.85 eV, respectively. The photodetection parameters of the fabricated interdigitated Au/NiFe 2 O 4 /p-Si BB PDs were analyzed with I–V, photoresponsivity and response (rise/fall) times as a function of annealing process. All the fabricated Au/NiFe 2 O 4 /p-Si BB PD devices with/without annealing process operated in photovoltaic mode (at 0 V). The PD device annealed at 800 °C unveiled better photoresponsivity (12.35 A/W @1060 nm), detectivity (5.9 × 1014 Jones @1060 nm) and rise/fall times (89 ms/185 ms @900 nm) compared to the as-prepared/700 °C annealed PD devices. The reason for this was attributable to the accomplished improvement in crystallinity (XRD), surface morphology (AFM), reduction of bandgap (UV–Vis) and variation of spin/orbital magnetic moments (XAS/XMCD). In this work, we suggest one of the alternative and cost-effective electron beam evaporation method to grow the desired epitaxial NiFe 2 O 4 thin films compared to the other physical vapor deposition approaches such as PLD and ALD to fit the future necessities of optoelectronic field. [ABSTRACT FROM AUTHOR]

We report on the effect of in situ focused ion beam (FIB) trimming on the spin configurations of the hexagonal-shaped elements, characterized by scanning electron microscopy with polarization analysis (SEMPA) imaging. The use of SEMPA and successive in situ FIB trimming makes it possible to study the effect of thickness on the spin configuration of a same hexagonal dot array, eliminating the effect of irregularity in edges caused by the fabrication processes. The results obtained indicate that there exists a transition of a single vortex state to a stable double vortex state when the thickness is reduced to below 8 nm. Micromagnetic modeling has been performed to investigate the effect of reductions in both the thickness and saturation magnetization due to FIB trimming. [ABSTRACT FROM AUTHOR]

Germanium was deposited onto highly oriented pyrolytic graphite (HOPG) with and without antimony in ultra-high vacuum. The surface morphology was analyzed using in situ scanning tunneling microscopy (STM) at room temperature (RT). The film grows exclusively in 3D island mode and was affected significantly by substrate defects. At initial stage, nucleation of cluster occurred at step edges and defect sites. Later, we found various types of Ge nanostructures on HOPG in different deposition conditions and stages, including cluster chains, cluster islands, nanowires, and double layer ramified islands at RT. Compact Ge islands were observed when depositing at a substrate temperature of 450 K or after an annealing at 600 K following RT deposition. In addition, the pre-deposited Sb on graphite enhances the sticking probability and suppresses the surface diffusion of Ge atoms, resulting in a significant increase in Ge cluster island density on HOPG terraces. [ABSTRACT FROM AUTHOR]

In the realm of optoelectronic applications, the incorporation of a metal interlayer between layers of a topological material holds significant potential. The interlayer introduces benefits such as improved charge transfer efficiency, tunable band alignment, plasmonics effect, and enhanced light‐matter interactions, paving the way for more efficient and versatile optoelectronic devices. This work explores the incorporation of gold between Bi2Se3, resulting in an enhancement in the response time of photoresponse of interlayered systems (Bi2Se3‐Au‐Bi2Se3). The introduction of a gold interlayer between Bi2Se3 significantly influences the overlayer structure, crystallinity, crystallite size, and even phonon behavior. Moreover, the responsivity of the interlayered device at a bias (2 V) is found to have high responsivity with fast response and decay time (τr = 0.1 ms, and τd = 24 ms of interlayered systems) as compared to intrinsic Bi2Se3(τr = 0.6 ms, and τd = 28 ms). This change is attributed to the bandgap alteration of the Bi2Se3/Au interface, which is monitored through transient spectroscopy performed at different electric biases. This study suggests that incorporating a gold interlayer holds potential benefits for various optoelectronic applications. [ABSTRACT FROM AUTHOR]

Recently, heterojunctions formed between dissimilar semiconducting materials have been received considerable attention as an elementary building block of photodetectors (PDs) due to their enormous potential of tuning the wavelength selectivity from bandgap limited photoresponsivity in a single band to multiband or broadband. In this work, titania (TiO2) nanorods (NRs) with anatase crystalline structures were synthesized using hydrothermal method and spin coated on p-Si to realize Au/TiO2 NRs/p-Si (lateral) and Au/TiO2 NRs/p-Si/Ag (vertical) heterojunction broadband (BB) PDs. The structural and optical parameters of the prepared TiO2 NRs/p-Si heterojunction were studied using XRD, SEM, EDS, XPS and UV–VIS methods. A direct optical bandgap of 3.22 eV was evaluated using Tauc's method for the TiO2 nanorod (NR) films deposited on quartz substrate. XPS data revealed two strong binding energy (BE) peaks at 464.4 eV and 458.7 eV which correspond to core level spectral lines of Ti2p1/2 and Ti2p3/2, respectively, may arise owing to the interaction of Ti–O–Si bonds. The photoresponse parameters of the prepared two types of BB PDs were evaluated and compared using I–V, responsivity and I–T methods. The vertical structure exhibited betterment in transient responses (@-7 V) due to its reverse bias operation compared to the lateral structure which operated in forward bias (@ 10 V). The Au/TiO2 NRs/p-Si/Ag: vertical BB PD device displayed a peak photoresponsivity of 41 μA/W, detectivity of 4.6 × 108 Jones and rise/fall times of 0.36 s/0.45 s, when compared to Au/TiO2 NRs/p-Si lateral device architecture. This work suggests a simple approach for the development of tunable functionality of inorganic n-TiO2 NRs/p-Si heterojunction-based BB PDs (300–1100 nm) to fit the future necessities of the optoelectronic field. [ABSTRACT FROM AUTHOR]

We have investigated the influence of growth temperature on the in-plane strain, structural, optical and mechanical properties of heteroepitaxially grown GaN layers on sapphire (0001) substrate by laser molecular beam epitaxy (LMBE) technique in the temperature range 500–700 °C. The GaN epitaxial layers are found to have a large in-plane compressive stress of about 1 GPa for low growth temperatures but the strain drastically reduced in the layer grown at 700 °C. The nature of the in-plane strain has been analyzed using high resolution x-ray diffraction, atomic force microscopy (AFM), Raman spectroscopy and photoluminescence (PL) measurements. From AFM, a change in GaN growth mode from grain to island is observed at the high growth temperature above 600 °C. A blue shift of 20–30 meV in near band edge PL emission line has been noticed for the GaN layers containing the large in-plane strain. These observations indicate that the in-plane strain in the GaN layers is dominated by a biaxial strain. Using nanoindentation, it is found that the indentation hardness and Young's modulus of the GaN layers increases with increasing growth temperature. The results disclose the critical role of growth mode in determining the in-plane strain and mechanical properties of the GaN layers grown by LMBE technique. [ABSTRACT FROM AUTHOR]

Heterojunction-based gas sensors are very attractive as they substantially improve the sensing characteristics due to the effective potential barrier present at the interface. Taking the advantages of two excellent semiconducting gas sensing materials i.e., SnO 2 and ZnO, herein, we have constructed ZnO/SnO 2 heterojunction by the combination of vacuum evaporation and r.f. sputtering or atomic layer deposition techniques. The ZnO/SnO 2 heterostructure with optimized thickness of ZnO (∼10 nm) shows a 6-fold enhancement in sensing response compared to bare SnO 2 films against CO gas. The sensing responses of 81 and 85 % have been obtained for ZnO/SnO 2 heterostructures with ZnO deposited by sputtering and atomic layer deposition (ALD) methods, respectively, against 91 ppm of CO gas with an estimated limit of detection of 1.67 and 0.37 ppm. The ALD ZnO/SnO 2 sample displays an extremely fast response time of 5 s. The heterostructure sensors are also highly selective towards CO gas in the presence of other interfering toxic agents. The enhanced sensing characteristics of ZnO/SnO 2 are assigned to the formation of n-n heterojunction as depicted by X-ray photoelectron spectroscopic band alignment study and the strong CO adsorption on ZnO surface as derived from density functional theory calculations. [ABSTRACT FROM AUTHOR]

Mostly, the GaN has been grown on conventional substrates such as sapphire (0001), Si (111), etc. which promote the growth of polar GaN along the c-axis. However, the c-plane GaN exhibits large spontaneous and piezoelectric fields, which influence the radiative recombination rates along with the carrier transport of the heterojunction barriers and hence reduce the efficiency of the device. To address this, we have grown epitaxial nonpolar GaN nanostructures on m-plane sapphire (10−10) by changing the growth sequence in the laser molecular beam epitaxy process. A high-resolution x-ray diffraction (2θ-ω) scan confirmed GaN orientation along non-polar directions for GaN growth on bare m-sapphire whereas mixed-polar GaN was grown on AlN buffered m-sapphire substrate. Field emission scanning electron microscopy study revealed the formation of tilted GaN nanocolumns and island film-like structures that facilitate a high surface-to-volume ratio. The electronic properties of GaN nanostructures were also studied using X-ray photoemission spectroscopy. Further, metal-semiconductor-metal type ultra-violet (UV) photodetectors were fabricated on the various GaN nanostructures. The photo-response behaviour has been studied at different bias voltages using an illumination source of wavelength 325 nm and the photo-responsivity was found to be ∼ 15 A/W at 3 V for non-polar GaN. Finally, we discussed the crystalline and optical properties of various polar GaN nanostructures and their impact on the UV photodetector characteristics for future applications. [Display omitted] • Epitaxial non-polar GaN grown on Sapphire (10-10) using a LMBE technique. • Photodetectors on GaN/sapphire (10-10) revealed a robust response in UV region. • Maximum photoresponsivity in UV-A region of the electromagnetic spectrum. [ABSTRACT FROM AUTHOR]

• Decoration of Bi 2 Se 3 on laser molecular beam epitaxy-grown GaN nanostructures. • Bi 2 Se 3 /GaN heterojunction showed high ultra-violet photodetector responsivity. • Heterojunction Bi 2 Se 3 /GaN for dual-wavelength photodetector applications. Recently, highly responsive photodetectors which are capable of photodetection in a wide range of wavelength spectra are in great demand. To fulfil this, photodetectors formed using heterojunctions of multiple semiconducting materials gain a lot of interest as they cover a broad range of photodetection, and the built-in electric field developed at their interface helps to enhance photoresponse. Here, we have demonstrated a fascinating broadband metal-semiconductor-metal type photodetector (PD) by integrating sputtered Bi 2 Se 3 with the laser molecular beam epitaxy (LMBE) grown GaN film and porous nanowall network on sapphire (0001) substrates. The fabricated photodetectors using a hybrid Bi 2 Se 3 /LMBE-GaN nanowall structure exhibit broadband characteristics with a high responsivity of 51.4 and 1.46 A/W when illuminating with the laser of wavelength 355 nm (UV) and 1064 nm (NIR), respectively. The enhanced UV responsivity and good response in the NIR region using Bi 2 Se 3 /GaN heterojunction nanostructures suggest futuristic application in dual-wavelength detector devices. [Display omitted] [ABSTRACT FROM AUTHOR]

This report discusses the effect of post-oxidation processes and thickness of SnO 2 thin films on their physical properties and CO gas sensing characteristics. SnO 2 films have been grown by simple vacuum evaporation of Sn metal and post-oxidation process. The post-oxidation of Sn films was performed by two different methods, i.e., step-oxidation and direct-oxidation, at a maximum temperature of 850 °C. The thickness of Sn films was varied as 100, 200 and 500 nm. As characterized by x-ray diffraction technique, the SnO 2 thin films have a tetragonal rutile structure with crystallite size of 21 and 27 nm for direct-oxidation and step-oxidation methods, respectively. The SnO 2 films produced by both oxidation methods possess a porous, granular surface morphology as revealed by scanning electron microscopy. The band gap of SnO 2 films calculated using ultraviolet–visible spectroscopy in absorption mode is found to be in the range of 3.36–3.59 eV. The CO sensing performance has been examined at various measurement temperatures up to 400 °C using calibrated CO gas. It is found that the CO sensitivity is significantly higher for the direct-oxidized SnO 2 films as compared to the step-oxidized films. Also, thinner SnO 2 film, i.e. 100 nm of thickness, obtained by direct-oxidation method is found to have the highest sensitivity of 70.7% against CO gas of 915 ppm concentration with a fast response time of 21 s. The effective CO sensing of thinner SnO 2 film is understood based on the formation of smaller grains with the uniform surface distribution. The film is more selective towards CO gas as compared to NH 3 and NO 2 gases and shows a CO limit-of-detection of 10 ppm. [ABSTRACT FROM AUTHOR]

We report the direct growth of self-aligned single crystalline GaN nanorod array on flexible Ta metal foil using laser molecular beam epitaxy. Scanning electron microscopy reveals the vertically aligned nanorods on Ta surface having diameters in the range of 60–80 nm. The nanorods show well-defined hexagonal facets and are quite uniformly distributed across the metal foil. Transmission electron microscopy shows single crystalline nature of the individual rods having c-axis oriented growth with wurtzite structure. Room temperature photoluminescence study exhibits a sharp, intense band-to-band emission without any deep-level bands indicating the excellent optical quality of the GaN nanorod array. X-ray photoemission spectroscopy to elucidate the electronic structure of the nanorods confirms Ga–N bonding and the calculated chemical composition turns out to be slightly Ga rich. Location of valence band maxima also suggests the n -type character of GaN nanorods. The photoelectrochemical water-splitting behaviour of the self-aligned GaN nanorod arrays on Ta foil has been investigated using 1 M oxalic acid as the electrolyte with AM 1.5 G simulated solar radiation under 1 Sun (100 mW/cm2) conditions. The results demonstrate an effective way of fabricating well-aligned GaN nanorods on flexible metal foils for developing simple, relatively inexpensive, and flexible photo-electrodes for photocatalytic solar water-splitting applications. • Direct growth of vertically self-aligned GaN nanorod array on Ta foil is achieved. • The nanorods are single crystalline and have a high density of 1.4–2 × 1010 cm−2. • They exhibit a narrow and intense band-edge emission with negligible defect bands. • Solar water-splitting property of GaN nanorod array on Ta foil is demonstrated. [ABSTRACT FROM AUTHOR]

Highlights: [•] The real-time observation of faster degradation at GB sites of HfO2 dielectric. [•] C-AFM results show enhanced defect generation rate at the GB sites. [•] Preferential breakdown of SiO x IL occurs below the degraded GB region. [•] A breakdown sequence in polycrystalline HfO2/SiO x dielectric stacks was proposed. [Copyright &y& Elsevier]