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Epitaxial thin films of tantalum‐doped β‐Ga2O3 (Ta‐Ga2O3) are grown on MgO (001) substrates to study the effect of Ta doping on the electrical properties of β‐Ga2O3 films. X‐Ray diffraction (XRD) measurements show that the films with different Ta doping concentrations are (00l)‐oriented single‐crystalline β‐Ga2O3 without impurity phases. The incorporation of the Ta element modifies the electrical properties of Ta‐Ga2O3 films significantly. At a very low doping ratio of 0.05 mol%, the Ta‐Ga2O3 film showed a minimum resistivity of 2.32 Ω cm and a carrier concentration of 2.48 × 1017 cm−3. The corresponding activation energy of Ta element in the film is 16.8 meV, suggesting that the Ta element is a promising shallow donor dopant. The X‐ray photoelectron spectroscopy (XPS) analysis confirms that the Fermi level of the Ga2O3 films shifts toward the conduction band minimum after the introduction of Ta ions. These results indicate that the transition metal element Ta could be an effective n‐type dopant for modulating the carrier transport behavior of β‐Ga2O3 films. [ABSTRACT FROM AUTHOR]

Herein, the use of a Hf doping strategy in epitaxial β‐Ga2O3 thin films is demonstrated to substantially enhance the conductivity and simultaneously achieve an even wider bandgap. Epitaxial β‐Ga2O3 thin films with various Hf dopant ratios are fabricated by pulsed laser deposition. The doping impact on the structural, optical, and transport properties is systematically investigated. At an optimized doping regime of 1%–2% in molar ratio, the heteroepitaxial Hf‐doped β‐Ga2O3 (Hf‐Ga2O3) films grown on the MgO substrate achieve a high n‐type conductivity of 6.5 S cm−1, a carrier mobility of 1.8 cm2 V−1 s−1, a carrier concentration of 2.69 × 1019 cm−3, and an ultrawide bandgap of 4.9 eV. The electron transport is further enhanced for the homoepitaxial Hf‐Ga2O3 film that exhibits a conductivity of 1236 S cm−1 and an ultrahigh mobility of 113 cm2 V−1 s−1. Density functional theory calculations reveal the preferential atomic site of Hf dopants in the β‐Ga2O3 lattice and its impact on the optical bandgap. This study demonstrates an effective way to achieve degenerate n‐type doping in ultrawide‐bandgap semiconductors for developing optoelectronic and power electronic devices. [ABSTRACT FROM AUTHOR]

Semipolar AlGaN multiple quantum wells (MQWs) have unique advantages in deep ultraviolet light emitters due to the weak Quantum-Confined Stark Effect. However, their applications are hampered by the poor crystalline quality of semipolar AlGaN thin films. Different treatments were developed to improve the crystal quality of semipolar AlGaN, including a multistep in situ thermal annealing technique proposed by our group. In this work, temperature-dependent and time-resolved photoluminescence characterizations were performed to reveal the carrier localization in the MQW region. The degree of carrier localization in semipolar AlGaN MQWs grown on top of the in situ-annealed AlN is similar to that of conventional ex situ face-to-face annealing, both of which are significantly stronger than that of the c-plane counterpart. Moreover, MQWs on in situ-annealed AlN show drastically reduced dislocation densities, demonstrating its great potential for the future development of high-efficiency optoelectronic devices. [ABSTRACT FROM AUTHOR]

Periodic nanocone–nanopillar dual‐structured arrays are wet chemical etched on 20 μm‐thick crystalline silicon substrates, enabling the realization of excellent light absorption properties and enhanced electrical contact with PEDOT:PSS. The finally textured silicon/PEDOT:PSS thin film hybrid solar cell shows a power conversion efficiency of 12.2%. These results provide a viable route toward high‐performance thin film silicon/polymer hybrid cells by shape‐controlled nanotexturing. [ABSTRACT FROM AUTHOR]

Crystalline silicon thin film (c-Si TF) solar cells with an active layer thickness of a few micrometers may provide a viable pathway for further sustainable development of photovoltaic technology, because of its potentials in cost reduction and high efficiency. However, the performance of such cells is largely constrained by the deteriorated light absorption of the ultrathin photoactive material. Here, we report an efficient light-trapping strategy in c-Si TFs (~20 μm in thickness) that utilizes two-dimensional (2D) arrays of inverted nanopyramid (INP) as surface texturing. Three types of INP arrays with typical periodicities of 300, 670, and 1400 nm, either on front, rear, or both surfaces of the c-Si TFs, are fabricated by scalable colloidal lithography and anisotropic wet etch technique. With the extra aid of antireflection coating, the sufficient optical absorption of 20-μm-thick c-Si with a double-sided 1400-nm INP arrays yields a photocurrent density of 39.86 mA/cm, which is about 76 % higher than the flat counterpart (22.63 mA/cm) and is only 3 % lower than the value of Lambertian limit (41.10 mA/cm). The novel surface texturing scheme with 2D INP arrays has the advantages of excellent antireflection and light-trapping capabilities, an inherent low parasitic surface area, a negligible surface damage, and a good compatibility for subsequent process steps, making it a good alternative for high-performance c-Si TF solar cells. [ABSTRACT FROM AUTHOR]

ZnGa2O4 is a promising semiconductor for developing high-performance deep-ultraviolet photodetectors owing to a number of advantageous fundamental characteristics. However, Zn volatilization during the ZnGa2O4 growth is a widely recognized problem that seriously degrades the film quality and the device performance. In this study, we report the synthesis of epitaxial ZnGa2O4 thin films by pulsed laser deposition using a non-stoichiometric Zn1+xGa2O4 target. It is found that supplementing excessive Zn concentration from the target is highly effective to stabilize stochiometric ZnGa2O4 thin films during the PLD growth. The influence of various growth parameters on the phase formation, crystallinity and surface morphology is systematically investigated. The film growth behavior further impacts the resulting optical absorption and thermal conductivity. The optimized epitaxial ZnGa2O4 film exhibits a full width at half maximum value of 0.6 degree for a 120 nm thickness, a surface roughness of 0.223 nm, a band gap of 4.79 eV and a room-temperature thermal conductivity of 40.137 W/(m⋅K). This study provides insights into synthesizing epitaxial ZnGa2O4 films for high performance optoelectronic devices. [ABSTRACT FROM AUTHOR]

Silicon (Si) heterojunction (HJ) solar cells (SCs) with dopant‐free interdigitated back contacts (IBCs) for both polarities have attracted considerable attention because of their potential for high‐efficiency performance combined with a simplified process flow. However, the lack of a thorough understanding of this type of SCs is restraining a plethora of screening opportunities for functional materials and HJ design. Here, experiments and simulations are conducted to explore the mechanisms of charge carrier transport/recombination and formulate design principles for a dopant‐free IBC‐HJ SC. The roles of the IBC area fill ratio, device pitch, and interface passivation are addressed and optimized. With a poly(3,4‐ethylenedioxythiophene):polystyrene (PEDOT:PSS)/magnesium oxide thin film as the hole‐transport layer (HTL)/electron‐transport layer (ETL), the bulk of photoelectric loss is understood to stem from recombination at the ETL/n‐Si interface in our case study. The simulated hole (minority carrier) current flow distributions clearly demonstrate the benefits of interfacial passivation on the photovoltaic performance, and a remarkable efficiency above 24% is anticipated in an optimal design for the conditions studied so far. The results presented in this article have wide applicability, providing valuable guidance for the development of a high‐efficiency IBC‐HJ SC with fully dopant‐free contacts. [ABSTRACT FROM AUTHOR]

Ni/Au nanomesh (NM) is utilized as the p‐type contact in the ultraviolet (UV)‐LEDs for improving the light extraction efficiency. The substitution of conventional planar Ni/Au p‐type electrode with highly ordered, large scalable Ni/Au NMs results in enhanced light transmission in the p‐type contact region of the UV‐LEDs. Transmission over 75% is demonstrated for Ni/Au‐NM thin film coated sapphire substrate, whereas light is totally blocked by conventional planar Ni/Au thin film. Linearized I–V characteristic between p‐GaN and Ni/Au NMs is achieved by appropriate thermal annealing of the contact, and specific contact resistivity of 0.90 Ω cm2 is reported. UV‐LED incorporated with Ni/Au NMs exhibits higher output power than that with planar contact due to enhanced light transmission through the metal network structure. Finally, simulation results suggest good agreement with experimental data, illustrating that Ni/Au NMs can serve as a potential candidate in the development of high efficiency UV optoelectronic devices. In this work, the authors present an alternative transparent metal contact on top of UV‐LEDs by using a stack of Ni and Au thin films perforated with hexagonally periodic holes. UV‐LEDs with Ni/Au nanomesh contact demonstrate high optical transmittance over 75%, small specific contact resistivity of 0.90 Ω cm2, and higher output power than that with planar contact. The results presented herein provide an important strategy in achieving high‐efficiency, flexible optoelectronic devices. [ABSTRACT FROM AUTHOR]

Periodic nanocone‐nanopillar dual‐structured arrays are wet chemical etched on 20 μm‐thick crystalline silicon substrates by Pingqi Gao, Xiaofeng Li, Jichun Ye and co‐workers in article number 1501793. This structure enables the realization of excellent light absorption properties and enhanced electrical contact with PEDOT:PSS. The final textured silicon/PEDOT:PSS thin film hybrid solar cell shows a power conversion efficiency of 12.2%. [ABSTRACT FROM AUTHOR]

High-quality AlGaN thin films with precise control of compositions and surfaces play a determined role in the development of short-wavelength optoelectronic as well as high-voltage/frequency power and RF devices. In this work, the interplay among strain, crystal quality, and surface morphology of AlGaN were comprehensively investigated. Increasing the growth rate of AlGaN results in a reduction of the AlN mole fraction and increased in-plane compressive strain. TEM analysis reveals that strain relaxation primarily occurs through generation and bending of dislocations as well as coarsening of the surface. This work provides useful insights into the growth kinetics and strain relaxation mechanism of high-Al-component AlGaN thin films on AlN template, offering guidance in the development of high-performance optoelectronic and electronic devices. • High Al-content AlGaN thin films were grown on AlN template with different growth rates. • Lower growth rate benefits lower strain level, smoother surface morphology and better crystalline quality. • AlGaN with higher growth rate exhibits larger compressive strain and wider strain distribution. • Strain relaxation primarily occurs through generation and bending of dislocations as well as surface roughening. [ABSTRACT FROM AUTHOR]

Doping scandium in aluminum nitride (AlN) can effectively enhance the piezoelectric properties of the material, herein making ScAlN a research hotspot for the application of high-frequency and wide-bandwidth RF devices. However, there has been limited investigations into the evolution of crystal structure and composition stability under thermal annealing, especially at high temperatures. In this work, the crystal quality, surface morphology, phase and elemental distribution of sputter-deposited Sc x Al 1− x N thin films with various Sc contents were comprehensively investigated before and after high-temperature thermal annealing. It is found that the full-width-half-maximum values of the (002) diffraction peaks of Sc x Al 1−x N thin films increase with higher Sc contents, while appropriate annealing condition can enhance crystal quality. It is also revealed that Sc segregation leads to the formation of cubic-phase ScN grains, and Sc atoms diffuse towards the sapphire substrate and surface, forming a Sc 2 O 3 layer on two sides of the ScAlN core layer. These findings provide important insights into the preparation of wurtzite ScAlN films under thermal annealing and offer valuable references for the development of novel ScAlN-based devices. • Sc x Al 1−x N films with 0-20% Sc component were deposited. • Incorporating Sc into the AlN crystal lattice induces lattice strain and distortion. • The FWHM values of the XRD rocking curve of Sc 0.05 Al 0.95 N thin films is improved from 1.5° to 0.85° after annealing at 1550℃ for 30 min. • Cubic-phase ScN grains were formed inside the Al-rich ScAlN matrix after thermal annealing. • Sc atoms diffuse towards the sapphire substrate and surface, forming Sc 2 O 3 layer due to the stronger oxygen affinity of Sc compared to Al. [ABSTRACT FROM AUTHOR]

Abstract A precise characterization of thin film silicon in devices is a great challenge for device optimization because the material properties depend on the substrate and change with film thickness. A specific situation is thin poly-Si layer in tunnel oxide passivated contact (TOPCon) structure in high-efficiency solar cells. In this paper, we present a systematic study using a wide range UV-Raman spectroscopy to measure the crystalline structure and chemical bonding configurations in the TOPCon structure. Because of the high absorption coefficient of 325-nm laser in silicon films, the Raman probes only the top surface region and minimizes the contribution from c-Si substrate. However, the comparison with the Raman spectra collected with a green laser of 532 nm, the UV-Raman spectrum of a-Si:H shows a very different spectral shape with the disappearance of the LA mode of Si-Si vibration and an appearance of additional signal at 325 cm−1. In addition, the UV-Raman spectrum of the pre-crystallized P doped a-Si:H layer shows most of the vibration modes of Si–Si, Si–H, Si–O–Si and Si–O–H bonds, where the Si–O–H bonds are in the top surface oxide layer. After the crystallization, the Si-Si TO mode shows a high degree of crystallization, the Si-H vibration modes disappear, and an additional Si-O-Si mode is observed which is believed from the ultrathin SiO x passivation layer. In addition, we provide a set of UV-Raman measurements on the samples made with the same layer structure but at different annealing temperatures to show the correlation between the UV-Raman spectra and the passivation quality of the TOPCon structure on the n-type c-Si wafers. These observations prove that UV-Raman spectroscopy is a useful characterization tool for high efficiency c-Si solar cells with TOPCon structure. In addition, the impacts of the material properties on the passivation quality are discussed. Highlights • UV-Raman spectra of thin Si films for TOPCon solar cells are measured in real TOPCon structures. • The TA mode disappears and a sharp peak at 325 cm−1 is observed in the UV-Raman spectra of a-Si: H. • Si-Hx wagging/rocking and stretching modes are found to be correlated to passivation quality. • Si-O and Si-O-H vibration modes are detected in real TOPCon structures with the UV-Raman spectroscopy. • Correlation between UV-Raman spectra and TOPCon passivation is observed. [ABSTRACT FROM AUTHOR]

The carrier transport through the silicon-oxide (SiO x ) layer in tunnel oxide passivated contact (TOPCon) c-Si solar cells has been studied experimentally and by simulation. The current intensity versus voltage (J-V) characteristics of GaIn/n-c-Si/SiO x /n + -poly-Si/Al structures shows a linear Ohmic characteristic, while a non-Ohmic behavior is observed in the samples without the n + -poly-Si contact layer. Conductive Atomic Force Microscopy (c-AFM) images reveal some current spikes on the surface of the samples, which could be related to the transport through pinholes. The simulation results show that 1) a rectification characteristic is obtained when only the tunneling mechanism is included, 2) both the reverse saturation current and the forward current increase when a small amount of transport through pinholes is introduced, and 3) finally a linear Ohmic behavior is observed when the pinhole transport component reaches a certain level. Furthermore, the simulation for whole TOPCon solar cells provides some useful results. For very thin SiO x (< 1.2 nm), the tunneling provides sufficient high tunneling probability and high efficiency TOPCon solar cells can be obtained without transport through pinholes if the passivation is ensured; while for a relatively thick SiO x (> 1.2 nm) without the transport through pinholes, the TOPCon solar cell shows a poor fill factor ( FF ) with a high series resistance ( Rs ) because the tunneling does not provide a sufficient high transport channel for carrier transport, and the introduction of a small number of transports through pinholes improves the FF and reduces the Rs , hence improves the PCE. However, a high possibility for carrier going through pinholes reduces all of the performance parameters and degrades PCE for all the cases simulated. Therefore, an optimized pinhole density and size distribution is critical engineering for solar cell performance optimization. However, the establishment of an optimized method to precisely control the pinhole formation and characterization is still on the way. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Zn x Ga 1-x O (0 ≤ x ≤ 1) film with a composition gradient has been fabricated using a combinatorial PLD growth method. • The underlying relationship between the compositional gradient, structural and defect evolution and the photodetection performance is discussed. • The Zn x Ga 1-x O photodetectors with x = 0.18–0.27 exhibit a maximum responsivity of 12.88 A/W at 20 V. Ternary complex oxides exhibit strong compositional dependence on their structure and functionalities. While the profound impact of the cation stoichiometry on the physical properties of ZnGa 2 O 4 is widely recognized, how it affects the ultraviolet photodetection behavior remains not clear. Here we apply a combinatorial pulsed laser deposition technique to establish graded composition in Zn x Ga 1-x O (0 ≤ x ≤ 1) across a 2-inch sapphire substrate. In combination with structural and optical characterizations, a systematic investigation of the compositional effect on the phase, crystallinity and bandgap for the Zn x Ga 1-x O system is performed, and their further impact on the photodetection performance is compared by fabricating multiple metal–semiconductor-metal (MSM) photodetectors based on the graded Zn x Ga 1-x O films. Decent photodetection behavior is seen for the photodetectors with x ≤ 0.33, above which the performance is largely limited by the conductive phase formation. An optimized compositional range of x = 0.18–0.27 is identified to deliver the best device performance, which includes a responsivity of 12.88 A/W and a photo-to-dark current ratio of 2.8 × 108. This slight zinc deficiency is considered beneficial for the formation of moderate level of oxygen vacancies or antisite defects that boosts carrier transport. This study sheds light on the composition-dependent evolution of structure and properties in ternary ultrawide-bandgap semiconductors for high-performance ultraviolet photodetection. [ABSTRACT FROM AUTHOR]

Gallium oxide (Ga 2 O 3) is featured by various types of polymorphs that exhibit distinct properties for a broad range of applications. Selective stabilization of distinct Ga 2 O 3 polymorphs is highly desired, but is usually limited by impurity phase formation. Here we report the controlled growth of metastable gallium oxide films by pulsed laser deposition (PLD), and explore a comprehensive phase evolution picture tuned by key synthesis parameters, substrate orientations, and extrinsic tin (Sn) dopants. We demonstrate the stabilization of both undoped and Sn-doped α -Ga 2 O 3 films by selecting a-plane sapphire substrates, and find that Sn dopants can largely broaden the growth window of α -Ga 2 O 3. Besides, Sn-doped ε -Ga 2 O 3 is more favored than β -Ga 2 O 3 to be stabilized on c-plane substrates, and the oxygen pressure is the critical factor to dictate the ε -Ga 2 O 3 formation. The structural impact on the thermal transport among these metastable Ga 2 O 3 films is investigated, which shows a maximum value of 5.66 Wm−1K−1 in α -Ga 2 O 3 and a minimum value of 1.04 Wm−1K−1 in amorphous Ga 2 O 3. The native defects of these phases are sensitive to the Sn doping, which exhibits distinct response for deep ultraviolet photodetection. This study establishes a synthesis guideline of metastable Ga 2 O 3 polymorphs by PLD and provides insights into selective phase stabilization of transition metal oxides with various crystal phases. [Display omitted] • A comprehensive picture of the Ga 2 O 3 phase evolution has been established by pulsed laser deposition. • Sn dopants can largely broaden the growth window of both ε- and α-Ga 2 O 3 and suppress intrinsic defects. • The thermal conductivity of β-, ε-, α-Ga 2 O 3 and amorphous films are measured by time domain thermoreflectance. [ABSTRACT FROM AUTHOR]

In this work, we report a significantly boosted defect-insensitive ultraviolet emission from InGaN/GaN multiple quantum wells (MQWs) grown on patterned AlN nucleation layer (NL) compared to samples grown on uniform NL. Carrier localization is clearly illustrated for MQWs grown on patterned AlN NL as evidenced by an S-shape profile from temperature-dependent photoluminescence characterization. The underlying mechanism for the carrier localization is demonstrated to correlate with partial relaxation of compressive strains inside epitaxial thin films. This work illustrates that carrier localization can be achieved in MQWs with very low indium content by the adoption of patterned NL during growth, and provides a promising route towards the realization of high-efficiency ultraviolet emitter. • A technique in boosting the UV emission from InGaN MQW is proposed through MOCVD on patterned AlN nucleation layer. • Carrier localization was demonstrated in low indium content of less than 5% by temperature-dependent PL. • Improved optical properties are observed in MQWs grown on patterned AlN nucleation layer despite higher dislocation density. • Partial relaxation of compressive strains through nanopatterned AlN NL is responsible for the carrier localization effect. [ABSTRACT FROM AUTHOR]