This article presents a comprehensive investigation into the suitability of diverse p‐type materials for the fabrication of pn‐heterojunctions and junction field‐effect transistors (JFETs) with α‐Ga2O3. The electrical properties of these materials are examined in detail and compared with one another in order to ascertain their relative merits. Therefore copper iodide (CuI), zinc cobalt oxide (ZCO), and nickel oxide (NiO) are used as p‐type contacts. The smallest ideality factors are obtained for pn‐heterojunctions utilizing CuI, whereas the highest current rectification is obtained for diodes with NiO and ZCO contacts, if an oxygen plasma treatment of the α‐Ga2O3 is performed before the deposition of the p‐type electrode. This way current rectification ratios of up to are achieved. In a next step, these findings are used to fabricate highly functional JFETs with a α‐Ga2O3 channel. These devices exhibit current on/off ratios of up to and small mean sub‐threshold swings of at a low drain voltage of 3 V. High breakdown voltages of up to 418 V are achieved, regardless of the gate material.

The feasibility of zirconium doping of alpha-Ga 2 O 3 grown by pulsed laser deposition is demonstrated. Targets with different zirconium contents are used to adjust the zirconium content in the thin films. Therefore, a two-step growth process is utilized, where first an undoped alpha-Ga 2 O 3 thin film is grown as a high -temperature buffer layer and the zirconium -doped alpha-Ga 2 O 3 layer is subsequently deposited at a lower growth temperature. Highly conductive thin films are obtained with resistivities as low as 3.3 x 10 -3 O cm. An electron mobility as high as 38 cm 2 V -1 s -1 is measured for a high free carrier density of 6.5 x 10 18 cm -3 . The crystallization in the alpha-phase was confirmed by x-ray diffraction measurements. Further, a strong influence of the growth temperature on the zirconium incorporation is observed, which can be explained by the increased desorption of volatile Ga 2 O 3 suboxides at high growth temperatures. Depth -resolved x-ray photoelectron spectroscopy measurements were employed to investigate the doping profile in the thin films. They reveal a strongly depth -dependent incorporation of the zirconium, with a decreased incorporation towards the surface. First Schottky barrier diodes on zirconium -doped alpha-Ga 2 O 3 thin films with rectification ratios as high as 8.5 orders of magnitude at +/- 3 V are presented.

The feasibility of zirconium doping of $\ensuremath{\alpha}$-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ grown by pulsed laser deposition is demonstrated. Targets with different zirconium contents are used to adjust the zirconium content in the thin films. Therefore, a two-step growth process is utilized, where first an undoped $\ensuremath{\alpha}$-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin film is grown as a high-temperature buffer layer and the zirconium-doped $\ensuremath{\alpha}$-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ layer is subsequently deposited at a lower growth temperature. Highly conductive thin films are obtained with resistivities as low as $3.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.2em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.2em}{0ex}}\mathrm{cm}$. An electron mobility as high as $38\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ is measured for a high free carrier density of $6.5\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The crystallization in the $\ensuremath{\alpha}$-phase was confirmed by x-ray diffraction measurements. Further, a strong influence of the growth temperature on the zirconium incorporation is observed, which can be explained by the increased desorption of volatile ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ suboxides at high growth temperatures. Depth-resolved x-ray photoelectron spectroscopy measurements were employed to investigate the doping profile in the thin films. They reveal a strongly depth-dependent incorporation of the zirconium, with a decreased incorporation towards the surface. First Schottky barrier diodes on zirconium-doped $\ensuremath{\alpha}$-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films with rectification ratios as high as 8.5 orders of magnitude at $\ifmmode\pm\else\textpm\fi{}3\phantom{\rule{0.2em}{0ex}}\mathrm{V}$ are presented.

To tailor electrical properties of often degenerate pristine CuI, Ni is introduced as alloy constituent. Cosputtering in a reactive, but also in an inert atmosphere as well as pulsed laser deposition (PLD), is used to grow thin films. The Ni content within the alloy thin films is systematically varied for different growth techniques and growth conditions. A solubility limit is evidenced by an additional phase for Ni contents , observed in X‐Ray diffraction and atomic force microscopy by a change in surface morphology. Furthermore, metallic, nanoscaled nickel clusters, revealed by X‐Ray photoelectron spectroscopy and high‐resolution transmission electron microscopy (HRTEM), underpin a solubility limit of Ni in CuI. Although no reduction of charge carrier density is observed with increasing Ni content, a dilute magnetic behavior of the thin films is observed in vibrating sample magnetometry. Further, independent of the deposition technique, unique multilayer features are observed in HRTEM measurements for thin films of a cation composition of . Opposite to previous claims, no transition to n‐type behavior was observed, which was also confirmed by density functional theory calculations of the alloy system.

We present alpha -Ga2O3:Zr based metal-semiconductor field-effect transistors (MESFETs) with PtOx/Pt gate contacts. Pulsed laser deposition is used to grow the alpha -Ga2O3:Zr thin films in a two-step process on m-plane alpha -Al2O3. A nominally undoped alpha -Ga2O3 layer is grown at high growth temperature as growth template. Subsequently, a alpha -Ga2O3:Zr layer is grown at a lower growth temperature. We compare the performance of Ring-FET devices on a planar 30 nm thick zirconium doped layer deposited at 465 degrees C and mesa-FETs on a 35 nm thick thin film deposited at 500 degrees C. The Ring-FETs have current on/off ratios as high as 1.7 x 10 9 and a threshold voltage of - 0.28 V, and they exhibit very low mean sub-threshold swing of ( 110 +/- 20 ) mV/dec. For the mesa-FETs, smaller current on/off ratios of 4 x 10 7 are measured and a threshold voltage of - 1.5 V was obtained due to the larger thin film thickness. The on/off ratio is limited by a higher tunneling current in the off-regime. We present high voltage measurements, which show a breakdown of the mesa-FET device at - 340 V, corresponding to a high breakdown field of 1.36 MV/cm and significantly exceeding the previously achieved breakdown voltage for alpha -Ga2O3 based MESFETs.

AbstractCombinatorial pulsed laser deposition (C‐PLD) based on segmented targets has led to new possibilities in the pace of discovery of novel advanced materials with significantly reduced deposition time and material consumption. However, fabrication of established segmented targets may be complex or not even possible for certain material combinations. In this article, two alternative C‐PLD techniques based on easy‐to‐fabricate segmented targets are presented. One approach uses two semi‐circular segments A and B with a systematic adjustable lateral shift of the target rotation axis from the target center. The other approach is based on a new target design defined by an ABA‐segmentation where B is a horizontal bar between two semi‐circular segments of A. Both approaches enable growth of discrete composition libraries. The concepts and important parameters are introduced and computer simulations as function of the geometric parameters are made to yield the expected thin film compositions. As proof‐of‐concept, the techniques are employed on the transparent, semiconducting ternary alloy zinc‐tin‐oxide. The simulations are in very good agreement with the experimental data. Physical properties of films grown by the demonstrated approaches are compared with those obtained by established PLD processes.

Phase-pure α-Ga2O3 thin films with high surface quality and crystallinity have been grown on m-plane sapphire using pulsed laser deposition (PLD). Therefore, the influence of growth temperature, oxygen background pressure, and film thickness on the structural properties is investigated to determine the growth window for phase-pure corundum structured α-Ga2O3. Samples were analyzed using x-ray diffraction (XRD), atomic force microscopy (AFM), and spectroscopic ellipsometry measurements. A distinct growth window in favor of phase-pure (10.0)-oriented α-Ga2O3 for growth temperatures above 480 °C and low oxygen partial pressures p(O2) of 3 × 10−4 mbar is identified. The growth rate increases significantly with increasing oxygen pressure. Furthermore, it shows an Arrhenius-like decrease for lower temperatures, caused by the increasing desorption of volatile Ga2O suboxides. It was found that for thicker layers, the growth of monoclinic β-Ga2O3 is promoted likely facilitated by the c-facets of the α-Ga2O3 grains. This leads to a (010)-oriented island growth corroborated by stripe-like features in AFM scans and a corresponding in-plane orientation confirmed by XRD ϕ-scans. For oxygen partial pressures above 3 × 10−4 mbar and Tg < 540 °C, the formation of mixed (10.0) α-Ga2O3 and spinel-defective (110)-γ-Ga2O3 manifesting as defective inclusions was observed independent of the layer thickness d. A corresponding p(O2) − d − T phase diagram for the growth of Ga2O3 on m-plane sapphire by PLD is provided.

Structural and electrical properties of undoped and doped α‐Ga2O3 thin films grown by pulsed laser deposition on m‐plane sapphire in a two‐step process are presented. A buffer layer of undoped α‐Ga2O3 is introduced below the electrically active thin film to improve the crystal quality and enable the stabilization of the α‐phase at lower substrate temperatures for sufficient dopant incorporation. Donor doping of the active layers with tin, germanium, and silicon, respectively, is realized below a critical substrate temperature of 600 °C. Depth‐resolved X‐ray photoelectron spectroscopy measurements on tin‐doped samples reveal a lower amount of tin in the bulk thin film compared to the surface and a lower tin incorporation for higher substrate temperatures, indicating desorption or float‐up processes that determine the dopant incorporation. Electron mobilities as high as 17 cm2 V−1 s−1 (at ) and 37 cm2 V−1 s−1 (at ) are achieved for tin‐ and germanium doping, respectively. Further, a narrow window of suitable annealing temperature from 680 to 700 K for obtaining ohmic Ti/Al/Au layer stacks is identified. For higher annealing temperatures, a deterioration of the electrical properties of the thin films is observed suggesting the need for developing low temperature contacting procedures for α‐Ga2O3‐based devices.

Deposition of semiconductors on bendable substrates is a crucial step toward flexible circuitry and deposition by a roll-to-roll process. Since most bendable substrates have limited temperature stability (normally degradation starts between 150 °C and 300 °C), processing temperatures are typically below that of rigid substrates. Amorphous oxide semiconductors (AOSs) such as indium gallium zinc oxide (IGZO) can be fabricated at room temperature (RT) and exhibit electron mobilities >10 cm2 V−1 s−1 being a pre-requisite for application in backplanes of displays. While IGZO is already commercially exploited, the search for alternative materials is highly relevant because indium and gallium are rare and expensive. Zinc tin oxide (ZTO) is a promising AOS since zinc and tin are highly abundant and cheap. In this letter, we discuss RT-fabricated n-type ZTO thin films used as the channel material in flexible inverter circuits based on junction field-effect transistors. RT-fabricated nickel oxide is used as a semitransparent p-type gate material. The devices are fabricated on flexible polyimide and exhibit an excellent peak gain magnitude of 464 and uncertainty level as low as 130 mV at a supply voltage of only 3 V. They are characterized before and after bending at various radii. Even after bending at 2 mm radius, the inverters behave still very well.

Metal-semiconductor and junction n-channel field-effect transistors (MESFETs and JFETs) have been fabricated on glass substrates using room temperature deposited amorphous zinc-tin oxide (ZTO) channel layers. Characteristics of transistors and inverter circuits are compared. Best FET devices exhibit ON-to-OFF current ratios over eight orders of magnitude, subthreshold swings as low as 250 mV/dec and field-effect mobilities of 5 cm(2)/Vs. Furthermore, all devices show long-term stability over a period of more than 200 days. Inverters fabricated using either MESFETs or JFETs exhibit remarkable peak gain magnitude values of 350 and voltage uncertainty levels as low as 260 mV for an operating voltage of 5 V. A Schottky diode FET logic (SDFL) approach is applied to shift the switching voltage which is a requirement for cascading of inverters for realization of ring oscillators.

Schottky diode FET logic (SDFL) ring oscillator circuits comprising metal-semiconductor field-effect transistors (MESFETs) based on amorphous zinc-tin-oxide (ZTO) n-channels are presented. The ZTO channel layers are deposited entirely at room temperature by long-throw magnetron sputtering. Best MESFETs exhibit on/off current ratios as high as 8.6 orders of magnitude, a sub-threshold swing as low as 250 mV dec(-1), and a maximum transconductance of 205 mu S. Corresponding inverters show peakge gain magnitude (pgm) values of 83 with uncertainty levels as low as 0.5 V at an operating voltage of 5 V. Single stage delay times down to 277 ns are measured for three-stage ring oscillators, corresponding to oscillation frequencies as high as 451 kHz. Oscillations are observed at operating voltages as low as 3 V. These results prove the feasibility of room-temperature-deposited, amorphous semiconducting oxide based integrated circuits with SDFL layout. The presented approach provides more efficient as well as fail-safe device fabrication and similar oscillation frequencies at significantly lower operating voltages compared to conventional, high-temperature processed logic circuits based on insulating gates.

Room temperature fabrication of amorphous oxide semiconductors enables a cost-efficient production of devices on flexible and large-area substrates. Metal-semiconductor field-effect transistors using amorphous zinc-tin-oxide (ZTO) thin films with a cation composition of 1:1 Zn:Sn are presented. The n-type ZTO channel is deposited by long-throw magnetron sputtering from a ceramic target at room temperature on glass substrates. Reactively sputtered platinum is used as a gate contact material. We report on/off current ratios as high as 1.8 × 106, a threshold voltage of 0.47 V, and a sub-threshold swing of 124 mV dec−1 for as-fabricated devices. Using negative bias treatment, an improvement in device properties is observed, namely, a decrease in the off-current by two orders of magnitude and a reduction of the sub-threshold swing. An inverter based on as-deposited ZTO metal-semiconductor field-effect transistors exhibits a high peak gain magnitude of 119 and a small uncertainty level of 160 mV for a supply voltage of 3 V.

Transparent semiconducting oxides (TSO) are promising candidates for the fabrication of flexible and low-cost electronic devices, as they contain only abundant materials, are nontoxic, and exhibit high carrier mobilities. The formation of rectifying Schottky-barrier contacts is a prerequisite for devices, such as rectifiers, photodetectors, and metal-semiconductor field-effect transistors, and it was found that the presence of oxygen plays an essential role during the formation of the Schottky contacts. With electrical measurements on Pt/zinc-tin-oxide (ZTO) and PtOx/ZTO Schottky-barrier contacts and depth-resolved x-ray photoelectron spectroscopy measurements we demonstrate the important role of oxygen at the interface between TSOs and the metal contact for the rectifying behavior of diodes. In the vicinity of the interface, PtOx is reduced to Pt in a two-step process. Pt(OH)(4) is reduced within one day, whereas the reduction of PtO takes place over a time period of several weeks. The reduction results in improved rectification compared to Pt/ZTO, due to a filling of oxygen vacancies, which leads to a reduction of the free-carrier concentration in the vicinity of the PtOx/ZTO interface. This increases the depletion layer width and subsequently reduces the tunneling current, resulting in a higher rectification ratio. The time scale of the permanent performance improvement can be shortened significantly by applying a reverse bias to the diode. The described mechanism is most likely also present at other transparent-semiconducting-oxide-metal interfaces.