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Flexible electronics and mechanically bendable devices based on Group III-N semiconductor materials are emerging; however, there are several challenges in manufacturing, such as cost reduction, device stability and flexibility, and device-performance improvement. To overcome these limitations, it is necessary to replace the brittle and expensive semiconductor wafers with single-crystalline flexible templates for a new-bandgap semiconductor platform. The substrates in the new concept of semiconductor materials have a hybrid structure consisting of a single-crystalline III-N thin film on a flexible metal tape substrate which provides a convenient and scalable roll-to-roll deposition process. We present a detailed study of a unique and simple direct epitaxial growth technique for crystallinity transformation to deliver single-crystalline GaN thin film with highly oriented grains along both a-axis and c-axis directions on a flexible and polycrystalline copper tape. A 2-dimensional (2D) graphene having the same atomic configuration as the (0001) basal plane of wurtzite structure is employed as a seed layer which plays a key role in following the III-N epitaxy growth. The DC reactive magnetron sputtering method is then applied to deposit an AlN layer under optimized conditions to achieve preferred-orientation growth. Finally, single-crystalline GaN layers (∼1 μm) are epitaxially grown using metal organic chemical vapor deposition (MOCVD) on the biaxially-textured buffer layer. The flexible single-crystalline GaN film obtained using this method provides a new way for a wide-bandgap semiconductor platform pursuing flexible, high-performance, and versatile device technology. [ABSTRACT FROM AUTHOR]

We investigate the effect of strain on the two-dimensional electron gas (2DEG) channel in a flexible Al0.25Ga0.75N/GaN high-electron-mobility transistor (HEMT) by mechanical bending to prove the concept of active polarization engineering to create multifunctional electronic and photonic devices made of flexible group III-nitride thin films. The flexible HEMTs are fabricated by a layer-transfer process and integrated with a 150-μm-thick Cu film. The strain values are estimated from high-resolution x-ray diffraction and Raman spectroscopy in 4-cm bend-down and −4-cm bend-up test conditions. The strain-induced piezoelectric polarization can alter the charge density of the 2DEG in the channel at the AlGaN/GaN interface and thus modify the output characteristics of the flexible HEMTs. Accordingly, output characteristics show an increase in output current by 3.4% in the bend-down condition and a decrease by 4.3% in the bend-up condition. Transfer characteristics show a shift of threshold voltage, which also supports the 2DEG channel modulation during bending. Computational simulation based on the same structure confirms the same current modulation effect and threshold voltage shift. Furthermore, the electrical characteristics of the flexible HEMTs show a repeatable dependence on the strain effect, which offers potential for electro-mechanical device applications. [ABSTRACT FROM AUTHOR]

We investigate the polarization modulation effect of a single-crystalline BeO layer on AlGaN/GaN high-electron-mobility transistors (HEMTs). The BeO layer with macroscopic polarization on top of the AlGaN barrier layer increases the 2-dimensional electron gas density in the triangular quantum well (QW) at the interface of the AlGaN/GaN heterostructure. Electronic band bending of BeO and a deeper triangular QW observed from the simulated conduction band profile indicate that the BeO layer can modify the polarization field at the AlGaN/GaN interface. A ∼20-nm-thick single-crystalline BeO thin film is grown on AlGaN/GaN HEMTs by atomic-layer deposition. Room-temperature and variable-temperature Hall-effect measurements confirm that the HEMT with BeO forms a channel with a 14% increase of the sheet carrier concentration as compared with a conventional HEMT. An improved output performance is also observed in the I-V characteristics which confirms the polarization modulation effect of the BeO layer. [ABSTRACT FROM AUTHOR]

We demonstrated the growth of wurtzite‐crystalline beryllium oxide (BeO) thin films on GaN and ZnO substrates using atomic layer deposition (ALD). Single‐crystalline BeO were epitaxially grown on GaN. Despite the inherently large lattice mismatch of BeO and GaN atoms, the 6/5 and 7/6 domain‐matched structures dramatically reduced the residual strain in BeO thin films. On the other hand, the lattice mismatch of BeO and ZnO was not effectively accommodated in the mixed domains. X‐ray diffraction (XRD) confirmed the in‐plane crystallization of BeO‐on‐substrates in the (002){102}BeO||(002){102}Sub orientation and relaxation degrees of 20.8% (GaN), 100% (ZnO). The theoretical critical thicknesses of BeO for strain relaxation were 2.2 μm (GaN) and 1.6 nm (ZnO), calculated using a total film energy model. Transmission electron microscopy (TEM) and Fourier‐filtered imaging supported the bonding configuration and crystallinity of wurtzite BeO thin films on GaN and ZnO substrates. [ABSTRACT FROM AUTHOR]

This study demonstrates the first flexible single‐junction III‐V photovoltaic solar cells (SCs) based on single‐crystal‐like gallium arsenide (GaAs) thin films on a low‐cost metal substrate by direct and continuous deposition, which can bypass expensive single crystal wafer fabrication. The two‐dimensional modeling of the GaAs SC is developed and used to study feasibility of single‐crystal‐like GaAs thin films for high performance SC devices. A promising SC device performance characteristic with an open‐circuit voltage of 560 mV and short circuit current of 19.4 mA/cm2, resulting in a conversion efficiency of ~7.6%, is demonstrated. We developed flexible thin film III‐V solar cells on a new photovoltaics (PV) platform where nearly single‐crystalline III‐V thin films are directly grown on inexpensive polycrystalline metal substrates using crystallinity‐transformational buffer layers. This PV technology combines the efficiency advantages of high‐quality III‐V materials with continuous roll‐to‐roll deposition process of thin film on a low‐cost tape substrate to produce flexible, cost‐competitive, and high‐efficiency SCs with application versatility. [ABSTRACT FROM AUTHOR]

Thermal management in flexible electronic has proven to be challenging thereby limiting the development of flexible devices with high power densities. To truly enable the technological implementation of such devices, it is imperative to develop highly thermally conducting flexible substrates that are fully compatible with large-scale fabrication. Here, we present the thermal conductivity of state-of-the-art flexible yttria-stabilized zirconia (YSZ) substrates measured using the 3ω technique, which is already commercially manufactured via roll-to-roll technique. We observe that increasing the grain size increases the thermal conductivity of the flexible 3 mol.% YSZ, while the flexibility and transparency of the sample are hardly affected by the grain size enlargement. We exhibit thermal conductivity values of up to 4.16 Wm−1 K−1 that is at least 4 times higher than state-of-the-art polymeric flexible substrates. Phonon-hopping model (PHM) for granular material was used to fit the measured thermal conductivity and accurately define the thermal transport mechanism. Our results show that through grain size optimization, YSZ flexible substrates can be realized as flexible substrates, that pave new avenues for future novel application in flexible electronics through the utilization of both their ceramic structural flexibility and high heat dissipating capability. [ABSTRACT FROM AUTHOR]

We investigate thermo-electronic behaviors of flexible AlGaN/GaN heterostructure field-effect transistors (HFETs) for high-power operation of the devices using Raman thermometry, infrared imaging, and current-voltage characteristics. A large negative differential conductance observed in HFETs on polymeric flexible substrates is confirmed to originate from the decreasing mobility of the two-dimensional electron gas channel caused by the self-heating effect. We develop high-power transistors by suppressing the negative differential conductance in the flexible HFETs using chemical lift-off and modified Ti/Au/In metal bonding processes with copper (Cu) tapes for high thermal conductivity and low thermal interfacial resistance in the flexible hybrid structures. Among different flexible HFETs, the ID of the HFETs on Cu with Ni/Au/In structures decreases only by 11.3% with increasing drain bias from the peak current to the current at VDS=20 V, which is close to that of the HFETs on Si (9.6%), solving the problem of previous flexible AlGaN/GaN transistors. [ABSTRACT FROM AUTHOR]

We study the potential and feasibility of high hole mobility transistors (HHMTs) based on flexible AlGaInN/GaN heterostructures using numerical simulation. We develop a map for the sheet density of two-dimensional hole gas (2DHG) at different mole fractions of AlN, GaN, and InN of AlGaInN with mechanical bending conditions. External compressive strain via bending can induce relatively high density of 2DHG (e.g., > 8\times 10^12 cm ^-2 ) while keeping InN fraction low (<0.3) so that 2DHG channel can be formed in a GaN layer. We show the electronic energy band diagrams, family curves of I-V characteristics, and transfer characteristics of an In0.25 Al0.75N/GaN heterostructure in different bending conditions. By bending up, 2DHG is formed to perform the device function as HHMTs. Without bending, 2-D electron gas is induced and the devices function as high electron mobility transistors. This unique property can be used in complementary integrated circuits for high-power and high-temperature applications. [ABSTRACT FROM AUTHOR]

We theoretically study the change of the performance characteristics with various mechanical bending conditions for flexible thin-film transistors (TFTs) by two-dimensional device simulation. The characteristics of newly developed flexible TFTs with high crystalline quality and high carrier mobility are more sensitive to the degree of bending. We developed a model to estimate the change in the characteristics as a function of curvature radius of the channel with a focus on scattering of carriers in a bent TFT. Field-effect mobility decreases by bending, e.g., ~11% with a radius at R\,\,=30 mm and a knee voltage increases, while a threshold voltage remains the same. This model can be extended to other flexible TFTs with bending. [ABSTRACT FROM PUBLISHER]

Patterned gallium oxide (Ga2O3) using one-step patterning followed by oxygen plasma treatment on a p-GaN layer can function as both current-blocking and optical-scattering regions for uniform current spreading and improved light-extraction efficiencies. The results showed that the optical output power of the LED with patterned Ga2O3 increased by 16.8% at 60 mA compared to that of conventional LEDs. Numerical studies on light tracing and emission pattern, and external quantum efficiency evaluation support the improvement of both uniform current spreading and light-extraction efficiencies by patterned Ga2O3. [ABSTRACT FROM AUTHOR]

Demonstrated are visible GaN-based light-emitting diodes (LEDs) on economical large-area Si substrates using an advanced device and packaging architecture to improve optical output power, while reducing manufacturing costs. The process employs thin-film-flip-chip devices and wafer-level chip-scale packages and uses through-Si-via substrate and anisotropic conductive film for bonding. The improved curvature control region is applied in the epitaxial growth of the LED structure on a Si substrate to achieve flat wafers for epitaxial structures at room temperature, which is critical for wafer-level bonding. External quantum efficiency and light-output power at 350 mA increase by $\sim 12$ % compared with those of conventional flip-chip LEDs grown on a sapphire substrate. The devices also show a reverse-bias leakage current failure rate of <10%. [ABSTRACT FROM PUBLISHER]

We report on the influence of lattice-matched InAlGaN quaternary electron-blocking layer (Q-EBL) on hole transport and distribution in InGaN/GaN multiple quantum wells (MQWs) of visible light-emitting diodes (LEDs). Triple-wavelength (TW)-emitting active region was introduced to deduce carrier transport and distribution from emission intensities of different QWs in TW-LEDs. The electro-optical characteristics of TW-LEDs were compared with respect to the Q-EBL and silicon doping in a selected QW barrier. In addition, the efficiency droop characteristics of TW-LEDs according to existence of the Q-EBL were also investigated. The results show that holes were preferentially injected into a QW adjacent to a p-type layer in the TW-LED without the Q-EBL, while enhanced hole transport to lower QWs closed to an n-type layer and following uniform distribution were observed in the TW-LED with the Q-EBL. The modified kinetic energy of holes overcoming the Q-EBL is responsible for the improved hole transport and changes in the carrier capturing efficiency of each different QW, resulting in the improved peak efficiency and the efficiency droop of TW-LEDs. [ABSTRACT FROM AUTHOR]

This paper presents strain-effect transistors (SETs) based on flexible III-nitride high-electron-mobility transistors (HEMTs) through theoretical calculations. We show that the electronic band structures of InAlGaN/GaN thin-film heterostructures on flexible substrates can be modified by external bending with a high degree of freedom using polarization properties of the polar semiconductor materials. Transfer characteristics of the HEMT devices, including threshold voltage and transconductance, are controlled by varied external strain. Equilibrium 2-dimensional electron gas (2DEG) is enhanced with applied tensile strain by bending the flexible structure with the concave-side down (bend-down condition). 2DEG density is reduced and eventually depleted with increasing compressive strain in bend-up conditions. The operation mode of different HEMT structures changes from depletion- to enchantment-mode or vice versa depending on the type and magnitude of external strain. The results suggest that the operation modes and transfer characteristics of HEMTs can be engineered with an optimum external bending strain applied in the device structure, which is expected to be beneficial for both radio frequency and switching applications. In addition, we show that drain currents of transistors based on flexible InAlGaN/GaN can be modulated only by external strain without applying electric field in the gate. The channel conductivity modulation that is obtained by only external strain proposes an extended functional device, gate-free SETs, which can be used in electro-mechanical applications. [ABSTRACT FROM AUTHOR]

We demonstrate flip-chip light-emitting diodes (FC-LEDs) on a flexible yttria-stabilized zirconia (YSZ) substrate and compare them with FC-LEDs on a polymeric substrate. Degradation of luminescence intensity and red-shift of peak wavelength are not observed for the LED on the flexible YSZ, unlike one on the polyimide substrate, due to improved capability to remove the generated heat from the chip to the substrate. Thermal distribution measurements and finite-element simulations show improved thermal management by the flexible ceramic as compared with previously developed flexible LEDs on polymeric substrates. The results present an improved solution to high power operation of flexible LEDs. [ABSTRACT FROM PUBLISHER]

The cover image is based on the Research Article High‐efficiency flexible III‐V photovoltaic solar cells based on single‐crystal‐like thin films directly grown on metallic tapes by Sara Pouladi et al., https://doi.org/10.1002/pip.3070. [ABSTRACT FROM AUTHOR]

The effect of controlling the c-axis alignment (mosaicity) to the cross-plane thermal transport in textured polycrystalline aluminum nitride (AlN) thin films is experimentally and theoretically investigated. We show that by controlling the sputtering conditions we are able to deposit AlN thin films with varying c-axis grain tilt (mosaicity) from 10° to 0°. Microstructural characterization shows that the films are nearly identical in thickness and grain size, and the difference in mosaicity alters the grain interface quality. This has a significant effect to thermal transport where a thermal conductivity of 4.22 vs 8.09 W/mK are measured for samples with tilt angles of 10° versus 0° respectively. The modified Callaway model was used to fit the theoretical curves to the experimental results using various phonon scattering mechanisms at the grain interface. It was found that using a non-gray model gives an overview of the phonon scattering at the grain boundaries, whereas treating the grain boundary as an array of dislocation lines with varying angle relative to the heat flow, best describes the mechanism of the thermal transport. Lastly, our results show that controlling the quality of the grain interface provides a tuning knob to control thermal transport in polycrystalline materials.

Piezoelectric generators (PEGs) are a promising power source for future self-powered electronics by converting ubiquitous ambient mechanical energy into electricity. However, most of the high-output PEGs are made from lead zirconate titanate, in which the hazardous lead could be a potential risk to both humans and environment, limiting their real applications. III-Nitride (III-N) can be a potential candidate to make stable, safe, and efficient PEGs due to its high chemical stability and piezoelectricity. Also, PEGs are preferred to be flexible rather than rigid, to better harvest the low-magnitude mechanical energy. Herein, a high-output, lead-free, and flexible PEG (F-PEG) is made from GaN thin film by transferring a single-crystalline epitaxial layer from silicon substrate to a flexible substrate. The output voltage, current density, and power density can reach 28 V, 1 μA·cm -2 , and 6 μW·cm -2 , respectively, by bending the F-PEG. The generated electric power by human finger bending is high enough to light commercial visible light-emitting diodes and charge commercial capacitors. The output performance is maintained higher than 95% of its original value after 10 000-cycle test. This highly stable, high-output, and lead-free GaN thin-film F-PEG has the great potential for future self-powered electronic devices and systems.

We have grown a single-crystal beryllium oxide (BeO) thin film on a gallium nitride (GaN) substrate by atomic-layer deposition (ALD) for the first time. BeO has a higher thermal conductivity, bandgap energy, and dielectric constant than SiO 2 . As an electrical insulator, diamond is the only material on earth whose thermal conductivity exceeds that of BeO. Despite these advantages, there is no chemical-vapor-deposition technique for BeO-thin-film deposition, and thus, it is not used in nanoscale-semiconductor-device processing. In this study, the BeO thin films grown on a GaN substrate with a single crystal showed excellent interface and thermal stability. Transmission electron microscopy showed clear diffraction patterns, and the Raman shifts associated with soft phonon modes verified the high thermal conductivity. The X-ray scan confirmed the out-of-plane single-crystal growth direction and the in-plane, 6-fold, symmetrical wurtzite structure. Single-crystalline BeO was grown on GaN despite the large lattice mismatch, which suggested a model that accommodated the strain of hexagonal-on-hexagonal epitaxy with 5/6 and 6/7 domain matching. BeO has a good dielectric constant and good thermal conductivity, bandgap energy, and single-crystal characteristics, so it is suitable for the gate dielectric of power semiconductor devices. The capacitance-voltage (C-V) results of BeO on a GaN-metal-oxide semiconductor exhibited low frequency dispersion, hysteresis, and interface-defect density.

Single-crystal-like silicon (Si) thin films on bendable and scalable substrates via direct deposition are a promising material platform for high-performance and cost-effective devices of flexible electronics. However, due to the thick and unintentionally highly doped semiconductor layer, the operation of transistors has been hampered. We report the first demonstration of high-performance flexible thin-film transistors (TFTs) using single-crystal-like Si thin films with a field-effect mobility of ∼200 cm 2 /V·s and saturation current, I/l W > 50 μA/μm, which are orders-of-magnitude higher than the device characteristics of conventional flexible TFTs. The Si thin films with a (001) plane grown on a metal tape by a "seed and epitaxy" technique show nearly single-crystalline properties characterized by X-ray diffraction, Raman spectroscopy, reflection high-energy electron diffraction, and transmission electron microscopy. The realization of flexible and high-performance Si TFTs can establish a new pathway for extended applications of flexible electronics such as amplification and digital circuits, more than currently dominant display switches.