Growth of κ-([Al,In]xGa1-x)2O3 Quantum Wells and Their Potential for Quantum-Well Infrared Photodetectors.

The wide band gap semiconductor κ-Ga2O3 and its aluminum and indium alloys have been proposed as promising materials for many applications. One of them is the use of inter-sub-band transitions in quantum-well (QW) systems for infrared detectors. Our simulations show that the detection wavelength range of nowadays state of the art GaAs/AlxGa1-xAs quantum-well infrared photodetectors (QWIPs) could be substantially excelled with about 1-100 µm using κ-([Al,In]xGa1-x)2O3, while at the same time being transparent to visible light and therefore insensitive to photon noise due to its wide band gap, demonstrating the application potential of this material system. Our simulations further show that the QWIPs efficiency critically depends on the QW thickness, making a precise control over the thickness during growth and a reliable thickness determination essential. We demonstrate that pulsed laser deposition yields the needed accuracy, by analyzing a series of (InxGa1-x)2O3 QWs with (AlyGa1-y)2O3 barriers with high-resolution X-ray diffraction, X-ray photoelectron spectroscopy (XPS) depth profiling, and transmission electron microscopy (TEM). While the superlattice fringes of high-resolution X-ray diffraction only yield an average combined thickness of the QWs and the barrier and X-ray spectroscopy depth profiling requires elaborated modeling of the XPS signal to accurately determine the thickness of such QWs, TEM is the method of choice when it comes to the determination of QW thicknesses.

Band Offsets at κ-([Al,In]xGa1-x)2O3/MgO Interfaces.

Conduction and valence band offsets are among the most crucial material parameters for semiconductor heterostructure device design, such as for high-electron mobility transistors or quantum well infrared photodetectors (QWIP). Because of its expected high spontaneous electrical polarization and the possibility of polarization doping at heterointerfaces similar to the AlGaN/InGaN/GaN system, the metastable orthorhombic κ-phase of Ga2O3 and its indium and aluminum alloy systems are a promising alternative for such device applications. However, respective band offsets to any dielectric are unknown, as well as the evolution of the bands within the alloy systems. We report on the valence and conduction band offsets of orthorhombic κ-(AlxGa1-x)2O3 and κ-(InxGa1-x)2O3 thin films to MgO as reference dielectric by X-ray photoelectron spectroscopy. The thin films with compositions xIn ≤ 0.27 and xAl ≤ 0.55 were grown by pulsed laser deposition utilizing tin-doped and radially segmented targets. The determined band alignments reveal the formation of a type I heterojunction to MgO for all compositions with conduction band offsets of at least 1.4 eV, providing excellent electron confinement. Only low valence band offsets with a maximum of ∼300 meV were observed. Nevertheless, this renders MgO as a promising gate dielectric for metal-oxide-semiconductor transistors in the orthorhombic modification. We further found that the conduction band offsets in the alloy systems are mainly determined by the evolution of the band gaps, which can be tuned by the composition in a wide range between 4.1 and 6.2 eV, because the energy position of the valence band maximum remains almost constant over the complete composition range investigated. Therefore, tunable conduction band offsets of up to 1.1 eV within the alloy systems allow for subniveau transition energies in (AlxGa1-x)2O3/(InxGa1-x)2O3/(AlxGa1-x)2O3 quantum wells from the infrared to the visible regime, which are promising for application in QWIPs.

Combinatorial Material Science and Strain Engineering Enabled by Pulsed Laser Deposition Using Radially Segmented Targets.

Vertical composition gradients of ternary alloy thin films find applications in numerous device structures. Up to now such gradients along the growth direction have not been realized by standard pulsed laser deposition (PLD) systems. In this study, we propose an approach based on a single elliptically segmented PLD target suited for the epitaxial growth of vertically graded layers. The composition of the thin films can be varied by a simple adjustment of the position of the PLD laser spot on the target surface. We demonstrate this principle for the Mg xZn1- xO alloy system. Such vertically composition-graded Mg xZn1- xO thin films exhibit high optical quality and a well-defined Mg-content for each layer. No signs of interdiffusion of Mg-atoms between the layers have been found. Further, this method is capable to deposit homogeneous thin films with any desired, well-defined cation composition having the same high optical and structural quality as films grown by conventional PLD.

Room-temperature synthesized copper iodide thin film as degenerate p-type transparent conductor with a boosted figure of merit.

A degenerate p-type conduction of cuprous iodide (CuI) thin films is achieved at the iodine-rich growth condition, allowing for the record high room-temperature conductivity of ∼156 S/cm for as-deposited CuI and ∼283 S/cm for I-doped CuI. At the same time, the films appear clear and exhibit a high transmission of 60-85% in the visible spectral range. The realization of such simultaneously high conductivity and transparency boosts the figure of merit of a p-type TC: its value jumps from ∼200 to ∼17,000 MΩ-1 Polycrystalline CuI thin films were deposited at room temperature by reactive sputtering. Their electrical and optical properties are examined relative to other p-type transparent conductors. The transport properties of CuI thin films were investigated by temperature-dependent conductivity measurements, which reveal a semiconductor-metal transition depending on the iodine/argon ratio in the sputtering gas.

Room-temperature Domain-epitaxy of Copper Iodide Thin Films for Transparent CuI/ZnO Heterojunctions with High Rectification Ratios Larger than 10(9).

CuI is a p-type transparent conductive semiconductor with unique optoelectronic properties, including wide band gap (3.1 eV), high hole mobility (>40 cm(2)V(-1)s(-1) in bulk), and large room-temperature exciton binding energy (62 meV). The difficulty in epitaxy of CuI is the main obstacle for its application in advanced solid-state electronic devices. Herein, room-temperature heteroepitaxial growth of CuI on various substrates with well-defined in-plane epitaxial relations is realized by reactive sputtering technique. In such heteroepitaxial growth the formation of rotation domains is observed and hereby systematically investigated in accordance with existing theoretical study of domain-epitaxy. The controllable epitaxy of CuI thin films allows for the combination of p-type CuI with suitable n-type semiconductors with the purpose to fabricate epitaxial thin film heterojunctions. Such heterostructures have superior properties to structures without or with weakly ordered in-plane orientation. The obtained epitaxial thin film heterojunction of p-CuI(111)/n-ZnO(00.1) exhibits a high rectification up to 2 × 10(9) (± 2 V), a 100-fold improvement compared to diodes with disordered interfaces. Also a low saturation current density down to 5 × 10(-9)Acm(-2) is formed. These results prove the great potential of epitaxial CuI as a promising p-type optoelectronic material.