Semiconductor Science and Technology

III-nitride semiconductors are promising optoelectronic and electronic materials and have been extensively investigated in the past decades. New functionalities, such as ferroelectricity, ferromagnetism, and superconductivity, have been implanted into III-nitrides to expand their capability in next-generation semiconductor and quantum technologies. The recent experimental demonstration of ferroelectricity in nitride materials, including ScAl(Ga)N, boron-substituted AlN, and hexagonal BN, has inspired tremendous research interest. Due to the large remnant polarization, high breakdown field, high Curie temperature, and significantly enhanced piezoelectric, linear and nonlinear optical properties, nitride ferroelectric semiconductors have enabled a wealth of applications in electronic, ferroelectronic, acoustoelectronic, optoelectronic, and quantum devices and systems. In this review, the development of nitride ferroelectric semiconductors from materials to devices is discussed. While expounding on the unique advantages and outstanding achievements of nitride ferroelectrics, the existing challenges and promising prospects have been also discussed.

With the explosive growth of digital data in the era of the Internet of Things (IoT), fast and scalable memory technologies are being researched for data storage and data-driven computation. Among the emerging memories, resistive switching memory (RRAM) raises strong interest due to its high speed, high density as a result of its simple two-terminal structure, and low cost of fabrication. The scaling projection of RRAM, however, requires a detailed understanding of switching mechanisms and there are potential reliability concerns regarding small device sizes. This work provides an overview of the current understanding of bipolar-switching RRAM operation, reliability and scaling. After reviewing the phenomenological and microscopic descriptions of the switching processes, the stability of the low- and high-resistance states will be discussed in terms of conductance fluctuations and evolution in 1D filaments containing only a few atoms. The scaling potential of RRAM will finally be addressed by reviewing the recent breakthroughs in multilevel operation and 3D architecture, making RRAM a strong competitor among future high-density memory solutions.

Photonic integrated circuits (PICs) are considered as the way to make photonic systems or subsystems cheap and ubiquitous. PICs still are several orders of magnitude more expensive than their microelectronic counterparts, which has restricted their application to a few niche markets. Recently, a novel approach in photonic integration is emerging which will reduce the R&D and prototyping costs and the throughput time of PICs by more than an order of magnitude. It will bring the application of PICs that integrate complex and advanced photonic functionality on a single chip within reach for a large number of small and larger companies and initiate a breakthrough in the application of Photonic ICs. The paper explains the concept of generic photonic integration technology using the technology developed by the COBRA research institute of TU Eindhoven as an example, and it describes the current status and prospects of generic InP-based integration technology.

Gallium nitride high-electron-mobility transistors (GaN HEMTs) are at a point of rapid growth in defense (radar, SATCOM) and commercial (5G and beyond) industries. This growth also comes at a point at which the standard GaN heterostructures remain unoptimized for maximum performance. For this reason, we propose the shift to the aluminum nitride (AlN) platform. AlN allows for smarter, highly-scaled heterostructure design that will improve the output power and thermal management of III-nitride amplifiers. Beyond improvements over the incumbent amplifier technology, AlN will allow for a level of integration previously unachievable with GaN electronics. State-of-the-art high-current p-channel FETs, mature filter technology, and advanced waveguides, all monolithically integrated with an AlN/GaN/AlN HEMT, is made possible with AlN. It is on this new AlN platform that nitride electronics may maximize their full high-power, high-speed potential for mm-wave communication and high-power logic applications.

Ultraviolet light-emitting diodes (UV-LEDs) have started replacing UV lamps. The power per LED of high-power LED products has reached 12 W (14 A), which is 100 times the values observed ten years ago. In addition, the cost of these high-power LEDs has been decreasing. In this study, we attempt to understand the technologies and potential of UV-LEDs.

GaN-based light-emitting devices have the potential to realize all visible emissions with the same material system. These emitters are expected to be next-generation red, green, and blue displays and illumination tools. These emitting devices have been realized with highly efficient blue and green light-emitting diodes (LEDs) and laser diodes. Extending them to longer wavelength emissions remains challenging from an efficiency perspective. In the emerging research field of micro-LED displays, III-nitride red LEDs are in high demand to establish highly efficient devices like conventional blue and green systems. In this review, we describe fundamental issues in the development of red LEDs by III-nitrides. We also focus on the key role of growth techniques such as higher temperature growth, strain engineering, nanostructures, and Eu doping. The recent progress and prospect of developing III-nitride-based red light-emitting devices will be presented.

Accurately measuring and controlling the electrical properties of semiconductor nanowires is of paramount importance in the development of novel nanowire-based devices. In light of this, terahertz (THz) conductivity spectroscopy has emerged as an ideal non-contact technique for probing nanowire electrical conductivity and is showing tremendous value in the targeted development of nanowire devices. THz spectroscopic measurements of nanowires enable charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities to be measured with high accuracy and high throughput in a contact-free fashion. This review spans seminal and recent studies of the electronic properties of nanowires using THz spectroscopy. A didactic description of THz time-domain spectroscopy, optical pump–THz probe spectroscopy, and their application to nanowires is included. We review a variety of technologically important nanowire materials, including GaAs, InAs, InP, GaN and InN nanowires, Si and Ge nanowires, ZnO nanowires, nanowire heterostructures, doped nanowires and modulation-doped nanowires. Finally, we discuss how THz measurements are guiding the development of nanowire-based devices, with the example of single-nanowire photoconductive THz receivers.

A novel GaN trench gate vertical MOSFET (PSGT-MOSFET) with a double-shield structure composed of a separated gate (SG) and a p-type shielding layer (P_shield) is proposed and investigated. The P_shield is positioned within the drift region, which can suppress the electric field peak at the bottom of the trench during the off state. This helps to prevent premature breakdown of the gate oxide layer. Additionally, the presence of P_shield enables the device to have adaptive voltage withstand characteristics. The SG can convert a portion of gate-to-drain capacitance (C_gd) into drain-to-source capacitance (C_ds), significantly reducing the gate-to-drain charge of the device. This improvement in charge distribution helps enhance the switching characteristics of the device. Later, the impact of the position and length of the P_shield on the breakdown voltage (BV) and specific on-resistance (R_{on_sp}) was studied. The influence of the position and length of the SG on gate charge (Q_gd) and BV was also investigated. Through TCAD simulations, the parameters of P_shield and SG were optimized. Compared to conventional GaN TG-MOSFET with the same structural parameters, the gate charge was reduced by 88%. In addition, this paper also discusses the principle of adaptive voltage withstand in PSGT-MOSFET.

Over the last decade, there has been increasing interest in transferring the research advances in radiofrequency (RF) rectifiers, the quintessential element of the chip in the RF identification (RFID) tags, obtained on rigid substrates onto plastic (flexible) substrates. The growing demand for flexible RFID tags, wireless communications applications and wireless energy harvesting systems that can be produced at a low-cost is a key driver for this technology push. In this topical review, we summarise recent progress and status of flexible RF diodes and rectifying circuits, with specific focus on materials and device processing aspects. To this end, different families of materials (e.g. flexible silicon, metal oxides, organic and carbon nanomaterials), manufacturing processes (e.g. vacuum and solution processing) and device architectures (diodes and transistors) are compared. Although emphasis is placed on performance, functionality, mechanical flexibility and operating stability, the various bottlenecks associated with each technology are also addressed. Finally, we present our outlook on the commercialisation potential and on the positioning of each material class in the RF electronics landscape based on the findings summarised herein. It is beyond doubt that the field of flexible high and ultra-high frequency rectifiers and electronics as a whole will continue to be an active area of research over the coming years.

Gallium nitride (GaN) is becoming a mainstream semiconductor for power and radio-frequency (RF) applications. While commercial GaN devices are increasingly being adopted in data centers, electric vehicles, consumer electronics, telecom and defense applications, their performance is still far from the intrinsic GaN limit. In the last few years, the fin field-effect transistor (FinFET) and trigate architectures have been leveraged to develop a new generation of GaN power and RF devices, which have continuously advanced the state-of-the-art in the area of microwave and power electronics. Very different from Si digital FinFET devices, GaN FinFETs have allowed for numerous structural innovations based on engineering the two-dimensional-electron gas or p–n junctions, in both lateral and vertical architectures. The superior gate controllability in these fin-based GaN devices has not only allowed higher current on/off ratio, steeper threshold swing, and suppression of short-channel effects, but also enhancement-mode operation, on-resistance reduction, current collapse alleviation, linearity improvement, higher operating frequency, and enhanced thermal management. Several GaN FinFET and trigate device technologies are close to commercialization. This review paper presents a global overview of the reported GaN FinFET and trigate device technologies for RF and power applications, as well as provides in-depth analyses correlating device design parameters to device performance space. The paper concludes with a summary of current challenges and exciting research opportunities in this very dynamic research field.

We theoretically study biexcitons and quadrons in quantum dots with parabolic confinement and give a complete comparison between the two excitations. The calculation of quadron and biexciton binding energies as functions of electron-to-hole confinement potentials and mass ratios, using the unrestricted Hartree–Fock method, shows the essential differences between biexcitons and quadrons. The crossover between the negative and positive binding energies is indicated. The effect of an external magnetic field on the quadron and biexciton binding energies has also been investigated. In addition, the crossover between anti-binding and binding of both excited quadron and biexciton states in a certain range of the electron-to-hole oscillator length ratios has been found.

We investigated the optimization effects of KOH solution treatment and SiN_x sacrificial layer treatment on the contact characteristics of V/Al/Ni/Au on plasma-etched n-AlGaN. The results showed that the contacts on n-Al_0.5Ga_0.5N with both surface treatments are truly Ohmic in nature, while the contacts on untreated plasma-etched samples are still rectifying. Surface atomic concentration analysis revealed that both surface treatment methods effectively reduced the nitrogen vacancies on n-AlGaN induced by plasma etching, which mostly act as acceptor-like states, leading to severe compensation and hindering of the formation of Ohmic contact. Moreover, the operating voltage was reduced by 0.9 V at 20 mA for 285 nm ultraviolet light-emitting diodes, demonstrating that the surface treatment could work well for plasma-etched n-AlGaN Ohmic contacts.

Doping engineering has been widely utilized to increase the efficiency of White organic light-emitting diodes (WOLEDs). In this study, a blue phosphor material named DMAC-DPS and an orange phosphor material named PO-01 are integrated into the host materials Bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) and carbazole-based 4,4'-biscarbazole-p-biphenyl (CBP) by incorporating the principle of complementary color luminescence, resulting in a doped double-luminescent layer hybrid WOLED. The developed device structure consists of ITO/MoO₃/TCTA/DPEPO:DMAC-DPS/CBP:PO-01 (or CBP:PO-01/DPEPO:DMAC-DPS)/TAZ/Alq₃/LiF/Al. The transfer of energy between the host and guest materials is achieved by controlling the thickness and position of the emitting layer, leading to a more balanced emission of blue and yellow light and an overall increase in device efficiency. The developed WOLED exhibits a maximum current efficiency of 26.8 cd A⁻¹, a power efficiency of 16.8 lm W⁻¹, and an external quantum efficiency of 10.95%. The stable color coordinates of the device remains consistent, varying from (0.34, 0.40) to (0.33, 0.39) at brightness levels ranging from 100 to 1000 cd m⁻². Technically, the incorporation of blue and orange phosphor materials into the host materials DPEPO and CBP, respectively, resulting in a doped double-luminescent layer hybrid WOLED, has shown a more balanced emission of blue and yellow light and resulted in increased efficiency. The reliable color coordinates corroborate the good color stability, making it a promising candidate for various applications. Furthermore, the controlled transfer of energy between the host and guest materials has led to a more balanced emission of blue and yellow light. Our developed doping engineering methods have shown potential for increased efficiency and good color stability, making the developed WOLED a promising candidate for various applications.

The present research work based on the newly prepared organic-inorganic hybrid heterostructure will be exploited to develop a multifunctional device including non-volatile resistance switching memory devices, and ultraviolet (UV) light detection behavior for the first time based on p-PEDOT:PSS/i-BFO/n-ZnO junctions. Using a spray pyrolysis technique, n-type zinc oxide (ZnO) and i-type bismuth ferrite (BiFeO₃) thin film layers were prepared on the clean glass substrates at temperature 673 K. Using a spin coater method, the p-PEDOT:PSS were grown upon a bismuth ferrite (BiFeO₃) thin film with a constant spin velocity of 2000 rpm and heated at 363 K. The current (I)–voltage (V), photoresponse characteristics and resistive switching (RS) behavior of the fabricated p-PEDOT:PSS/i-BFO/n-ZnO hybrid devices were carried out. The device shows high photoresponsivity (R) of 0.001 285 A W⁻¹ and fast photoresponse switching speed with the measured rise and fall time of 493 and 970 ms respectively. Based on the electrical properties, a conductive filament formation/rupture mechanism is proposed to explain the observed RS characteristics.

This paper primarily focusses on developing an analytical model with a non-zero bandgap of boron (B)/nitrogen (N) substitution doped graphene field-effect transistors (GFETs) to mimic the synaptic behaviour. The trap charges at the channel and gate-insulator interface are utilized to induce the hysteresis conduction mechanism, which is further exploited to accomplish synaptic plasticity. The proposed recurrence, that is the time-dependent trap drain current model, accurately captures the physical insights of trap charges using an equivalent metal–insulator–graphene model. An interesting feature of the proposed model is that it is compatible with both the doped (B/N) and the undoped GFETs. The model is also investigated to generate the hysteresis characteristics of the GFET that are further utilized to simulate the synaptic behaviour. Another fact that must be noticed is the existence of complete OFF regions for doped B/N GFETs, unlike the undoped case, which manifest undesirable ambipolar behaviour. The synapse made up of B/N-doped GFETs predicts an optimistic learning and memory mechanism, termed as spike time-dependent plasticity (STDP). The STDP characteristics of B/N doped synaptic GFETs have been enhanced by more than 18 × compared to artificial synapses made of undoped GFETs. Hence, the hysteresis behaviour along with the non-zero bandgap of B/N substitution doped GFETs makes them highly favourable for the dynamic mimicking of synaptic plasticity to be efficiently biologically plausible.

Wide bandgap semiconductor gallium oxide (β-Ga₂O₃) has emerged as a prominent material in the field of high-power microelectronics and optoelectronics, due to its excellent and stable performance. However, the lack of high-quality p-type β-Ga₂O₃ hinders the realization of its full potential. Here, we initially summarize the origins of p-type doping limitation in β-Ga₂O₃, followed by proposing four potential design strategies to enhance the p-type conductivity of β-Ga₂O₃. (i) Lowering the formation energy of acceptors to enhance its effective doping concentration. (ii) Reducing the ionization energy of acceptors to increase the concentration of free holes in the valence band maximum (VBM). (iii) Increasing the VBM of β-Ga₂O₃ to decrease the ionization energy of acceptors. (iv) Intrinsic defect engineering and nanotechnology of β-Ga₂O₃. For each strategy, we illustrate the design principles based on fundamental physical theories along with specific examples. From this review, one could learn the p-type doping strategies for β-Ga₂O₃.

Magnetic tunnel junctions (MTJs), as the core storage unit of magneto resistive random-access memory, plays important role in the cutting-edge spintronics. In the MTJ devices, there are multiple internal magnetic/nonmagnetic heterojunction structures. The heterojunction always consists of magnetic metals and magnetic insulators or nonmagnetic metals. The interface of the heterojunction has certain physical effects that can affect the performance of MTJ devices. In the review, combined with the existing research results, the physical mechanism of magnetic/non-magnetic heterojunction interface coupling is discussed. The influence of the interface effect of the heterojunction on the performance of MTJ devices is studied. The optimization method is proposed specifically. This work systematically summarizes the interface effect of magnetic/non-magnetic heterojunction, which could be the critical aspect for the device's yield and reliability.

This paper reviews the development of three-dimensional (3D) structure-controlled InGaN quantum wells (QWs) for highly efficient multiwavelength emitters without using phosphors. Specifically, two representative structures are reviewed: 3D structures composed of stable planes with low surface energies and 3D structures composed of unstable planes. In the early stage of the research, 3D structures were grown on the (0001) polar plane through the selective area growth (SAG) technique based on metalorganic vapor phase epitaxy. Because GaN cannot grow on dielectric masks, different mask patterns were used to create various 3D facetted structures composed of stable facet planes. The InGaN QW parameters depend on the facet planes, which led to polychromatic emission, including white-light emission. After polychromatic light-emitting diodes (LEDs) on the (0001) polar plane were demonstrated, 3D QWs and LEDs were also demonstrated on the ($\bar{1}\bar{1}$2$\bar{2}$) semipolar plane through SAG. There, the (0001) facet plane was excluded; consequently, all the facet QWs showed short radiative recombination lifetimes, which are beneficial for future applications in visible-light communication. To further enhance the controllability of the emission spectra from 3D QWs or LEDs, convex-lens-shaped 3D structures have been proposed. The smooth surface of such structures is composed of unstable planes and has continuously varying crystal tilts. Because QW parameters are dependent on the crystal tilt, polychromatic emission is achieved. This method demonstrates greater flexibility of the structure design, which is expected to result in greater controllability of emission spectra.

Photocatalytic H₂ evolution and CO₂ reduction are promising technologies for addressing environmental and energy issues. g-C₃N₄ is one of most promising materials to form improved catalysts because of its exceptional electrical structure, physical and chemical characteristics, and distinctive metal-free feature. This article provides a summary of current advancements in g-C₃N₄-based catalysts from innovative design approaches and their applications. Hydrogen evolution has reached 6305.18 µmol g⁻¹ h⁻¹ and >9 h of stability using the SnS₂/g-C₃N₄ heterojunction. Additionally, the ZnO/Au/g-C₃N₄ maintains a constant CO generation rate of 689.7 mol m⁻² during the 8 h reaction. To fully understand the interior relationship of theory–structure performance on g-C₃N₄-based materials, modifications are studied simultaneously. Furthermore, the synthesis of g-C₃N₄ and g-C₃N₄-based materials, as well as their respective instances, have been reported. The reduction of CO₂ and H₂ generation is summarized. Lastly, a short overview of the present issues and potential alternatives for g-C₃N₄-based materials is provided.

Silicon carbide (SiC) is a typical wide band-gap semiconductor material that exhibits excellent physical properties such as high electron saturated drift velocity, high breakdown field, etc. The SiC material contains many polytypes, among which 4H-SiC is almost the most popular polytype as it possesses a suitable band-gap and high electron saturated drift velocity. In order to produce 4H-SiC power devices with a high barrier voltage of over several thousand volts, the minority carrier lifetime of 4H-SiC single crystals must be carefully managed. In general, both bulk defects and surface defects in 4H-SiC can reduce the minority carrier lifetime. Nevertheless, as surface defects have received less attention in publications, this study reviews surface defects in 4H-SiC. These defects can be classified into a number of categories, such as triangle defect, pit, carrot, etc. This paper discusses each one individually followed by the introduction of industrially feasible methods to characterize them. Following this, the impact of surface defects on the minority carrier lifetime is analyzed and discussed. Finally, a particular emphasis is put on discussing various passivation schemes and their effects on the minority carrier lifetime of 4H-SiC single crystals. Overall, this review paper aims to help young researchers comprehend surface defects in 4H-SiC single crystal material.

In this work, the terahertz (THz) time-domain spectroscopy was employed in studying the carrier dynamics in low-temperature grown (LT-) and semi-insulating (SI-) gallium arsenide (GaAs) photoconductive antenna (PCA) at above- (λ = 780 nm, Eg = 1.59 eV) and below- (λ = 1.55 μm, Eg 0.80 eV) bandgap excitation. We measured the excitation power dependence of the LT-GaAs (SI-GaAs) THz emission. Then,
the equivalent circuit model (ECM) which considers the (i) photogeneration, (ii) screening effects, and (iii) transport of carriers was utilized in analyzing the THz radiation mechanisms in the above- and below-bandgap excitation of the two substrates. In simulating the above-bandgap THz emission of both PCAs, we employed the direct bandgap excitation model which takes into account the
band-to-band transitions of photoexcited carriers. Meanwhile, to simulate the LT-GaAs (SI-GaAs) THz emission at below-bandgap excitation we utilized the two-step photoabsporption facilitated by the mid-gap states. In this model the
photoexcited carriers jump from the valence band to the mid-gap states and then to the conduction band. Results suggest that the THz emission from LT-GaAs and SI-GaAs at above- and below-bandgap excitation occur due to band-to-band
transitions, and two-step photoabsorption process via midgap states, respectively.

This paper demonstrates low-resistance and high-transparency p-type contact materials for UV micro-LEDs at 365 nm. As a commonly used p-type LED contact, Indium tin oxide (ITO) and nickel/indium tin oxide (Ni/ITO) contacts were studied before and after rapid thermal annealing (RTA) treatments. The transmittance at 365 nm wavelength of 200 nm thick ITO films increased from approximately 57% to 90% after RTA at a temperature exceeding 400°C, while the Ni/ITO film had a transmittance of about 73% after annealing. Micron-sized UV-LEDs (light-emitting diodes) with Ni/ITO p-contact were fabricated. Electrical characterization shows that Ni/ITO films annealed at 600°C demonstrated good ohmic contact behavior and the highest on-wafer external quantum efficiency (EQE), despite slightly lower transmittance. This paper shows the potential of annealed Ni/ITO films as promising p-contact materials for high-performance 365 nm UV-LEDs.

A novel GaN trench gate vertical MOSFET (PSGT-MOSFET) with a double-shield structure composed of a separated gate (SG) and a p-type shielding layer (P_shield) is proposed and investigated. The P_shield is positioned within the drift region, which can suppress the electric field peak at the bottom of the trench during the off state. This helps to prevent premature breakdown of the gate oxide layer. Additionally, the presence of P_shield enables the device to have adaptive voltage withstand characteristics. The SG can convert a portion of gate-to-drain capacitance (C_gd) into drain-to-source capacitance (C_ds), significantly reducing the gate-to-drain charge of the device. This improvement in charge distribution helps enhance the switching characteristics of the device. Later, the impact of the position and length of the P_shield on the breakdown voltage (BV) and specific on-resistance (R_{on_sp}) was studied. The influence of the position and length of the SG on gate charge (Q_gd) and BV was also investigated. Through TCAD simulations, the parameters of P_shield and SG were optimized. Compared to conventional GaN TG-MOSFET with the same structural parameters, the gate charge was reduced by 88%. In addition, this paper also discusses the principle of adaptive voltage withstand in PSGT-MOSFET.

The overall performance of the multilayer resulting in a sol-gel bismuth ferrite (BiFeO₃) film will be primarily determined by the properties of the first layer, but this has yet to receive much attention, even though chemical and morphological defects of this layer can accumulate as the number of layers increases. Here, we perform an optical, conductive, and ferroelectric study of first layer (L₁) dip-coating sol-gel BiFeO₃ films using two routes that vary only in the dissolvent; the first one is based on 2-methoxyethanol (MOE), and the second one on acetic acid (AA) with some MOE (AA-MOE). Tauc plots reveal a band gap of 2.43 eV and 2.75 eV for MOE (30 ± 5 nm thick) and AA-MOE (35 ± 5 nm thick) films, respectively. MOE films showed a dielectric function with features at ∼2.5 eV, ∼3.1 eV, and ∼3.9 eV, which were associated with charge-transfer transitions, but such features are absent in AA-MOE films. Advanced atomic force microscopy techniques were used to identify the fine features or defects of the BiFeO₃ films: The conductive maps show that the charge transport pathways in both film routes are controlled by nanometer defects rather than grain or grain boundary defects. Current-voltage curves reveal high conductive pathway at a lower voltage for the MOE films than for AA-MOE films. The piezoelectric coefficient for MOE films was ∼20% higher than AA-MOE films. Both deposition methods yield ferroelectric films with an electromechanical strain controlled by the piezoelectric effect and minimal contribution from electrostriction. An optimization for the AA-MOE-based route in the withdrawal speed results in a significant reduction of morphological defects and a more than twofold increase in the piezoelectric coefficient. Our results broaden the understanding of optical and ferroelectric BiFeO₃ films based on a chemical solution by dip-coating.

An experimental study of straight and bent distributed Bragg reflector (DBR) ridge waveguide (RW) lasers and Fabry–Pérot (FP) RW lasers emitting at 785 nm is presented. To determine the losses introduced by the bent waveguides within DBR-RW lasers, different laser designs were manufactured and characterized. The bent waveguides investigated here within DBR-RW laser diodes are sine-shaped S-bends. S-bends with three different lateral offsets are manufactured. The experimental characterization of FP lasers and the straight DBR-RW lasers with different coatings at the rear facet enables a rough estimation of the losses caused by the DBR grating and the determination of the DBR reflectivity. Furthermore, additional losses in the bent DBR-RW lasers caused by the S-bend (i.e. radiation and scattering losses) are quantified by comparing them to the straight DBR-RW lasers. Within the active resonator, the S-bend losses amount to α_Bend = 0.6 cm⁻¹ (α_Bend = 0.5 dB) for the smallest manufactured lateral S-bend offset H = 40 μm. For both straight and bent DBR-RW lasers spectrally narrow single-mode emission is obtained. A lateral beam width of 3.8 μm (using second moments) and a lateral far-field angle of about 18° and 19.5° (using second moments) for the straight and S-bend DBR-RW are measured, respectively. This gives a lateral beam propagation ratio of 1.2 and 1.3 (using second moments) for straight and S-bend DBR-RW, respectively. The radiation loss in dependency of the lateral S-bend offset is simulated and compared to experimentally estimated S-bend losses for bent DBR-RW lasers (H = 40 μm, H = 60 μm and H = 70 μm).

The Franz–Keldysh effect in the optical absorption edge of a bulk TlGaSe₂ layered semiconductor poled under an external electric field was investigated in the present work. The Franz–Keldysh shift below the optical bandgap absorption region, as well as the quasi-periodic oscillations above the fundamental bandgap of TlGaSe₂, were observed. The measured changes in optical light absorption of the TlGaSe₂ sample were revealed after poling processing. The poling technique is used to produce the built-in internal electric field within the TlGaSe₂ semiconductor. The frozen-in internal electric field in TlGaSe₂ was experimentally monitored through changes in the lineshape of the absorption spectra at the fundamental band edge. The observed results are accurately fitted with the theoretical lineshape function of the Franz–Keldysh absorption tail below the bandgap of TlGaSe₂ and quasi-periodic oscillations above the bandgap. A good agreement between the theoretical and experimental results was observed. The present study demonstrated that the Franz–Keldysh effect can be used to identify and characterize the localized internal electric fields originating from electrically active native imperfections in the TlGaSe₂ crystals.

Suspended epitaxial gallium nitride (GaN) on silicon (Si) photonic crystal devices suffer from large residual tensile strain, especially for long waveguides, because fine structures tend to crack due to large stress. By introducing spring-like tethers, designed by the combination of a spring network model and finite element method simulations, the stress at critical locations was mitigated and the cracking issue was solved. Meanwhile, the tethered-beam structure was found to be potentially a powerful method for high-precision strain measurement in tensile thin films, and in this case, a strain of $2.27(\pm0.01)\times10^{-3}$ was measured in 350 nm epitaxial GaN-on-Si.

In this study, the effect of depositing CdSeTe and CdTe layers at different substrate temperatures (STs) by evaporation in vacuum on the properties of the CdSeTe/CdTe stacks was investigated. First, CdSeTe layers in stack structure were grown at STs of 150 °C, 200 °C and 250 °C and then CdTe layers on the CdSeTe produced with the optimum temperature were coated at STs of 150 °C, 200 °C and 250 °C. The employing of STs up to 150 °C on both CdSeTe and CdTe films in CdSeTe/CdTe stacks demonstrated the presence of Te and/or oxide phases as well as the alloying, while more stable phase structures at higher temperatures. In the CdSeTe/CdTe stack, the increase in ST of CdSeTe promoted the alloying, while it weakened the alloy in which was applied in CdTe. It was concluded that under the applied experimental conditions, STs of 250 °C and 200 °C with the graded alloying structure, suitable absorption sites, more homogeneous surface morphology for potential solar cell applications would be more suitable for CdSeTe and CdTe, respectively. As a result, the application of ST to CdSeTe or CdTe in the stacks can be used as a tool to control the properties of the stack structure.

This paper presents a novel approach for reducing the gate resistance (R_g) of K and Ka-band GaN HFETs with 150 nm gate length through a new gate metallization technique. The method involves increasing the gate cross-section via galvanic metallization using FBH's Ir-sputter gate technology, which allows an increase in gate metal thickness from the current 0.4 μm to approximately 1.0 μm for the transistors under investigation. This optimization leads to a substantial 50% reduction in gate series resistance, resulting in significant improvements in the RF performance. Specifically, the devices achieve 20% higher output power density and 10% better power-added efficiency at 20 GHz and V_ds = 20 V. The decreased gate resistance enables new degrees of freedom in design, such as longer gate fingers and/or shorter gate lengths, for more efficient power cells operating in this frequency range.

Defect mitigation of electronic devices is conventionally achieved using thermal annealing. To mobilize the defects, very high temperatures are necessary. Since thermal diffusion is random in nature, the process may take a prolonged period of time. In contrast, we demonstrate a room temperature annealing technique that takes only a few seconds. The fundamental mechanism is defect mobilization by atomic scale mechanical force originating from very high current density but low duty cycle electrical pulses. The high-energy electrons lose their momentum upon collision with the defects, yet the low duty cycle suppresses any heat accumulation to keep the temperature ambient. For a 7 × 10⁵ A cm⁻² pulsed current, we report an approximately 26% reduction in specific on-resistance, a 50% increase of the rectification ratio with a lower ideality factor, and reverse leakage current for as-fabricated vertical geometry GaN p–n diodes. We characterize the microscopic defect density of the devices before and after the room temperature processing to explain the improvement in the electrical characteristics. Raman analysis reveals an improvement in the crystallinity of the GaN layer and an approximately 40% relaxation of any post-fabrication residual strain compared to the as-received sample. Cross-sectional transmission electron microscopy (TEM) images and geometric phase analysis results of high-resolution TEM images further confirm the effectiveness of the proposed room temperature annealing technique to mitigate defects in the device. No detrimental effect, such as diffusion and/or segregation of elements, is observed as a result of applying a high-density pulsed current, as confirmed by energy dispersive x-ray spectroscopy mapping.