Progression of group-III sesquioxides: epitaxy, solubility and desorption

In recent years, ultra-wide bandgap semiconductors have increasingly moved into scientific focus due to their outstanding material properties, making them promising candidates for future applications within high-power electronics or solar-blind photo detectors. The group-III-sesquioxides can appear in various polymorphs, which influences, for instance, the energy of the optical bandgap. In gallium oxide, the optical bandgap ranges between 4.6 and 5.3 eV depending on the polymorph. For each polymorph it can be increased or decreased by alloying with aluminum oxide (8.8 eV) or indium oxide (2.7–3.75 eV), respectively, enabling bandgap engineering and thus leading to an extended application field. For this purpose, an overview of miscibility limits, the variation of bandgap and lattice constants as a function of the alloy composition are reviewed for the rhombohedral, monoclinic, orthorhombic and cubic polymorph. Further, the effect of formation and desorption of volatile suboxides on growth rates is described with respect to chemical trends of the discussed ternary materials

Electronic properties of E3 electron trap in n-type ZnO

International audienceDeep level transient spectroscopy measurements were per- formed on three non-intentionally doped n-type ZnO samples grown by different techniques in order to investigate the electronic properties of E3 electron trap. The ionization energy and the capture cross-section are found respectively at 0.275 eV from the conduction band and 2.3×10^−16 cm2 with no electric field dependence. This center is present irrespective of the synthesis method. In view of its physical properties and recent works published in the literature, its physical origin is discussed. Based mainly on its insensibility to the macroscopic electric field, the best candidates turn out to be dual defects with opposite charges on adjacent sites, like the dual vacancy VO -VZn

Wavelength-selective ultraviolet (Mg,Zn)O photodiodes: Tuning of parallel composition gradients with oxygen pressure

We report on ultraviolet photodiodes with integrated optical filter based on the wurtzite (Mg,Zn)O thin films. Tuning of the bandgap of filter and active layers was realized by employing a continuous composition spread approach relying on the ablation of a single segmented target in pulsed-laser deposition. Filter and active layers of the device were deposited on opposite sides of a sapphire substrate with nearly parallel compositional gradients. Ensure that for each sample position the bandgap of the filter layer blocking the high energy radiation is higher than that of the active layer. Different oxygen pressures during the two depositions runs. The absorption edge is tuned over 360 meV and the spectral bandwidth of photodiodes is typically 100 meV and as low as 50 meV

Visible-blind and solar-blind ultraviolet photodiodes based on (InxGa1-x)2O3

UV and deep-UV selective photodiodes from visible-blind to solar-blind were realized based on a Si-doped (InxGa1–x)2O3 thin film with a monotonic lateral variation of 0.0035<x<0.83. Such layer was deposited by employing a continuous composition spread approach relying on the ablation of a single segmented target in pulsed-laser deposition. The photo response signal is provided from a metal-semiconductor-metal structure upon backside illumination. The absorption onset was tuned from 4.83 to 3.22 eV for increasing x. Higher responsivities were observed for photodiodes fabricated from indium-rich part of the sample, for which an internal gain mechanism could be identified. VC 2016 AIP Publishing LLC

Nickel oxide-based heterostructures with large band offsets

We present research results on the electronic transport in heterostructures based on p-type nickel oxide (NiO) with the n-type oxide semiconductors zinc oxide (ZnO) and cadmium oxide (CdO). NiO is a desirable candidate for application in (opto-)electronic devices. However, because of its small electron affinity, heterojunctions with most n-type oxide semiconductors exhibit conduction and valence band offsets at the heterointerface in excess of 1 eV. ZnO/NiO junctions exhibit a so called type-II band alignment, making electron-hole recombination the only process by which a current can vertically flow through the structure. These heterojunctions are nevertheless shown to be of practical use in efficient optoelectronic devices, as exemplified here by our UV-converting transparent solar cells. These devices, although exhibiting high conversion efficiencies, suffer from two light-activated recombination channels connected to the type-II interface, one of which we identify and analyse in more detail here. Furthermore, CdO/NiO contacts were studied - a heterostructure with even larger band offsets such that a type-III band alignment is achieved. This situation theoretically enables the development of a 2-dimensional electronic system consisting of topologically protected states. We present experiments demonstrating that the CdO/NiO heterostructure indeed hosts a conductive layer absent in both materials when studied separately

Highly rectifying p-ZnCo2O4/n-ZnO heterojunction diodes

We present oxide bipolar heterojunction diodes consisting of p-type ZnCo2O4 and n-type ZnO fabricated by pulsed-laser deposition. Hole conduction of ZnCo2O4 (ZCO) was evaluated by Hall and Seebeck effect as well as scanning capacitance spectroscopy. Both, ZCO/ZnO and ZnO/ZCO type heterostructures, showed diode characteristics. For amorphous ZCO deposited at room temperature on epitaxial ZnO/Al2O3 thin films, we achieved current rectification ratios up to 2x1010, ideality factors around 2, and long-term stability

Low frequency noise of ZnO based metal-semiconductor field-effect transistors

The low frequency noise of metal-semiconductor field-effect transistors (MESFETs) based on ZnO:Mg thin films grown by pulsed laser deposition on a-plane sapphire was investigated. In order to distinguish between noise generation in the bulk channel material, at the semiconductor surface, and at the gate/channel interface, ohmic ZnO channels without gate were investigated in detail, especially concerning the dependency of the noise on geometrical variations. The experiments suggest that the dominating 1/f noise in the frequency range below 1 kHz is generated within the bulk channel material, both for bare ZnO channels and MESFETs

Method of choice for fabrication of high-quality ZnO-based Schottky diodes

We present a comprehensive comparison of electrical properties of differently fabricated high quality Schottky contacts on ZnO thin films grown by pulsed laser deposition. Thermally evaporated Pd/ZnO Schottky contacts exhibit ideality factors as low as 1.06 due to their high lateral homogeneity. The effective Richardson constant determined using these homogeneous contacts is (7.7±4.8)A cm−2 K−2 close to the theoretical value of 32 A cm−2 K−2. However, their rectification ratio is at most five orders of magnitude due to their comparably small barrier height (≈0.7eV). The largest effective barrier height (1.11 eV) and rectification ratio(7×1010) was obtained for reactively sputtered PdOx/ZnO Schottky contacts. Eclipse pulsed laser deposited IrOx/ZnO Schottky contacts were found to combine very good lateral homogeneity (n≈1.1), with a reasonably large barrier height (0.96 eV) and large rectification ratio (≈9 orders of magnitude). Our results for differently fabricated Schottky contacts suggest that the barrier formation is highly dependent on the presence of oxygen vacancies close to the interface and the different compensation mechanisms involved