Semiconductor Science and Technology

The most successful example of large lattice-mismatched epitaxial growth of semiconductors is the growth of III-nitrides on sapphire, leading to the award of the Nobel Prize in 2014 and great success in developing InGaN-based blue emitters. However, the majority of achievements in the field of III-nitride optoelectronics are mainly limited to polar GaN grown on c-plane (0001) sapphire. This polar orientation poses a number of fundamental issues, such as reduced quantum efficiency, efficiency droop, green and yellow gap in wavelength coverage, etc. To date, it is still a great challenge to develop longer wavelength devices such as green and yellow emitters. One clear way forward would be to grow III-nitride device structures along a semi-/non-polar direction, in particular, a semi-polar orientation, which potentially leads to both enhanced indium incorporation into GaN and reduced quantum confined Stark effects. This review presents recent progress on developing semi-polar GaN overgrowth technologies on sapphire or Si substrates, the two kinds of major substrates which are cost-effective and thus industry-compatible, and also demonstrates the latest achievements on electrically injected InGaN emitters with long emission wavelengths up to and including amber on overgrown semi-polar GaN. Finally, this review presents a summary and outlook on further developments for semi-polar GaN based optoelectronics.

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.

The effect of the quantum well (QW) number (n_QW) in far ultraviolet-C light emitting diodes (LEDs) on the optical power, external quantum efficiency (EQE) and degradation has been investigated. AlGaN-based multi-QW (MQW) LEDs designed for emission at 233 nm and 226 nm with n_QW between 1 and 30 are compared. A positive correlation between the optical power at 200 mA and L70 lifetime for large n_QW was observed. For the 233 nm LEDs QW numbers 6 ⩽ n_QW ⩽ 15 result in optical powers of 4–5 mW at 200 mA (corresponding to a maximum EQE of 0.47% for n_QW = 15) and L70 lifetimes of 9–13 h. For n_QW = 30 a reduction of output power and L70 lifetime was found indicating an optimum n_QW for 233 nm LEDs. For the 226 nm LEDs a constant optical power of 0.5 mW at 200 mA (corresponding to an EQE of 0.05%) was measured independent of n_QW. However, the L70 lifetime continuously increases from 7 h for 3 QWs to 13 h for 18 QWs. The enhanced optical power accompanied by a reduced degradation is attributed to a reduced hole leakage from the MQW into the n-side and reduced local charge carrier density per QW for large n_QW.

Gallium nitride (GaN) has gained traction in replacing silicon for power electronics applications, due to its high breakdown field, high mobility 2D electron gas, and effective n/p-type doping. This paper reviews three important topics of GaN power devices. One is the voltage-blocking structures needed to operate at high voltage while minimizing conduction loss and switching loss. Another one is the structure used to achieve normally-off operation, which is often required for power electronics. The third topic is the monolithic integration of gate drivers and power switches to achieve the ultimate switching speed at a low cost.

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.

An easy to fabricate ohmic-contact to moderately-doped p-type GaAs has been achieved. The tri-layer Au/Ni/Au contact is deposited by thermal evaporation, followed by rapid thermal annealing in nitrogen atmosphere. A series of annealing times and temperatures are explored to determine the influence of annealing conditions on the low-resistance ohmic contacts. The resulting contacts show more than three orders of magnitude reduction in contact resistance compared to alternative Ti/Au depositions.

This paper presents a comprehensive investigation on the role and manifestation of the FinFET effect in low voltage 4H-SiC MOSFETs as compared to their Si counterparts. For this purpose, a finite element model of a fabricated SiC FinFET with a fin width of 55 nm is constructed and calibrated to experimental data at a range of operating temperatures. The resulting TCAD model is then applied to examine the impact of the FinFET effect on the threshold voltage and the spatial variation of the carrier density and the drift mobility in the channel for a range of doping concentrations N_A of the p-well. It is thereby shown that by reducing the fin width $W_\mathrm{fin}$ from conventional values ${\gtrsim}$500 nm down to an interval of optimal values ∼40 nm (keeping everything else constant), it is possible to enhance the channel mobility ∼2.5 times. This improvement is not only found to be much larger than the one predicted in Si (where it is ∼ 15% according to the TCAD model) but also arises at a larger fin width (for a given N_A). In this respect, it is demonstrated that this optimal range of fin widths can be moved to even larger, more practical values by reducing the doping of the p-well. As an alternative, the on-state performance of the gate-all-around FET is also examined in detail following a similar procedure. It is hence shown that this structure can display the FinFET effect at an even larger, nearly conventional $W_\mathrm{fin}$ of ∼250 nm, whilst attaining a higher channel mobility and inversion layer density even than a stripe FinFET operated at the same overdrive. Thus, in light of all these advantages, the FinFET topology can play a key role in reducing the channel resistance of SiC power MOSFETs.

This is a review article on the current status and future prospects of the research and development on gallium oxide (Ga₂O₃) power devices. Ga₂O₃ possesses excellent material properties, in particular for power device applications. It is also attractive from an industrial viewpoint since large-size, high-quality wafers can be manufactured from a single-crystal bulk synthesized by melt–growth methods. These two features have drawn much attention to Ga₂O₃ as a new wide bandgap semiconductor following SiC and GaN. In this review, we describe the recent progress in the research and development on fundamental technologies of Ga₂O₃ devices, covering single-crystal bulk and wafer production, homoepitaxial thin film growth by molecular beam epitaxy and halide vapor phase epitaxy, as well as device processing and characterization of metal–semiconductor field-effect transistors, metal–oxide–semiconductor field-effect transistors and Schottky barrier diodes.

The Shockley–Queisser limit poses a significant challenge in solar technology research, limiting the theoretical efficiency to around 30%. Thermophotovoltaic (TPV) systems have emerged as a solution by incorporating a thermal absorber in traditional solar cell setups to achieve total efficiency beyond the limits. The efficiency of the overall system heavily depends on the performance and quality of the thermal absorber, which absorbs photons from the heat source and transfers them to the TPV cell. However, complex and expensive fabrication processes have hindered widespread adoption of TPV technology. The well-established metal-assisted chemical etching (MACE) method could be the best choice to mitigate these as it is a cost-effective, scalable, and mass-production-friendly process, which is widely used for surface texturization, creating nanostructures like nanopores, pyramids, and nanowires. MACE technique is also suitable for producing highly efficient silicon-based thermal absorbers with over 90% absorption rate, which can contribute to increased total conversion efficiency. However, it does not come without challenges such as maintaining control over the etch rate in order to achieve uniformity. This paper comprehensively reviews the utilization of MACE for fabricating silicon-based thermal absorbers in TPV systems with the range of effective wavelengths of 600–2000 nm which corresponds to the energy level of 0.55–1.85 eV. The advantages and challenges of MACE, along with characterization techniques, are extensively discussed. By providing valuable insights, this paper aims to support researchers interested in advancing TPV technology.

PbS colloidal quantum dot (CQD) photodiodes are becoming increasingly significant for their utility in short-wave infrared detection and imaging. Despite their promise, challenges persist in synthesizing large PbS quantum dots that extend their response wavelength beyond 1500 nm. The usual hot-injection method has problems because of changes in temperature, solution ratios, and the Ostwald ripening process, all of which make it harder to make large PbS quantum dots that are all the same size as the temperature rises. In this study, we introduced small-sized PbS quantum dots, which absorb light at approximately 600 nm, into a reaction system containing seed quantum dots that absorb light at approximately 1050 nm. This process was repeated on multiple occasions to facilitate the growth of the quantum dots, resulting in larger PbS quantum dots with an absorption peak at 1550 nm. By meticulously regulating parameters such as dosage, reaction time, and the number of injections, we successfully synthesized PbS quantum dots of varying sizes. This approach not only enables the production of PbS quantum dots with specific size requirements but also introduces novel insights into the synthesis mechanism of PbS. Furthermore, PbS CQD photodiodes were fabricated with reduced dark current densities by employing SnO₂ synthesized via the sol–gel method as an electron transport layer, which better matches the energy bands of the larger PbS quantum dots. At ambient temperature, the photodiode demonstrated a responsivity of 290 mA W⁻¹ with a normalized detectivity of 1.16 × 10¹¹ Jones, thereby underscoring its promising potential for practical applications.

The growing demand for efficient solar-blind ultraviolet photodetectors (SBPDs) has fueled interest in the exceptional properties of $\beta$-${\text{G}}{{\text{a}}_2}{{\text{O}}_3}$. Here, SBPDs with a metal–semiconductor–metal structure are fabricated based on $\beta$-${\text{G}}{{\text{a}}_2}{{\text{O}}_3}$ thin films. The ($\bar 2$01) oriented $\beta$-${\text{G}}{{\text{a}}_2}{{\text{O}}_3}$ is grown on $c$-plane sapphire using metal–organic chemical vapor deposition. Hydrogen as a shallow donor is introduced into the $\beta$-${\text{G}}{{\text{a}}_2}{{\text{O}}_3}$ surface layer selectively on the area underneath contacts by plasma treatment to enhance the ohmic contact. As a result, superexcellent performances are demonstrated, including a remarkable responsivity of $3.0 \times {10^4}{\text{ }}$A W⁻¹ at a wavelength of 238 nm, specific detectivity of $1.2 \times {10^{17}}$ Jones and an external quantum efficiency of $1.5 \times {10^7}\%$. Also, faster rise and decay speed are achieved. The hydrogen plasma treatment (HPT) brings fast transit time of carriers and effective gain electron injection, which are used to explain the enhancement of performance. This work highlights that HPT selectively on the area underneath contacts is an efficient strategy for achieving high-performance $\beta$-${\text{G}}{{\text{a}}_2}{{\text{O}}_3}$ solar-blind ultraviolet photodetectors.

Semiconductor nanowires can be utilized to create quantum dot qubits. The formation of quantum dots is typically achieved by means of bottom gates created by a lift-off process. As an alternative, we fabricated flat buried bottom gate structures by filling etched trenches in a Si substrate with sputtered TiN, followed by mechanical polishing. This method achieved gate line pitches as small as 60 nm. The gate fingers have low gate leakage. As a proof of principle, we fabricated quantum dot devices using InAs nanowires placed on the gate fingers. These devices exhibit single electron tunneling and Coulomb blockade.

Interband cascade lasers (ICLs) are becoming desirable mid-infrared semiconductor laser sources particularly in the 3–6 µm wavelength range due to their low power consumption. In this work, we report the demonstration of room temperature (RT) ICLs based on hybrid-cladding layers and a modified type-II quantum well active region with an emission wavelength near 7.7 µm, the longest ever reported for RT ICLs. By investigating two ICL wafers with different structural qualities and strains, we show their correlation and importance to the corresponding device performance in terms of their maximum operating temperature, threshold current density and threshold voltage, as well as their operational robustness at high currents. This comparative study also reveals connection between different cladding configurations in terms of electrical resistance and voltage drop across them. Furthermore, by comparing with previous reported ICLs at similar wavelengths, but with different designs, we show and discuss how significantly the doping levels in ICLs could affect free-carrier absorption related device performance in terms of external quantum efficiency, maximum operating temperature and threshold current density. Our experimental results and analyses provide insights and guidance for further development and optimization of ICLs in the long wavelength region.

Thin film solar cells based on the wide band gap absorber antimony sulfide (Sb₂S₃) have great application potential and prospect. Typical Sb₂S₃ thin-film solar cells usually use a low-cost solution method (such as hydrothermal deposition) to prepare the light-absorbing material, and use a toxic CdS film with a narrow band gap (about 2.4 eV) as the electron transport layer (ETL). TiO₂, a non-toxic ETL alternative with a wide band gap (3.1–3.4 eV), fulfills the photoelectric needs of the ETL. But when the TiO₂ is used as a template for Sb₂S₃ deposition, it shows an island of uneven growth. Herein, an ion layer adsorption and reaction method was used to modify the surface of TiO₂ with an ultra-thin layer of In₂S₃, thereby improving the nucleation and growth characteristics of Sb₂S₃ during hydrothermal deposition. In this way, the Sb₂S₃ film forms a compact and specular state on the TiO₂ surface similar to that on the CdS surface, realizing the quasi-epitaxial growth of the Sb₂S₃ film, which provides a new solution for efficient and environmentally friendly solar cells.

In recent years, Cu–Ag–Bi–I quaternary lead-free perovskites have emerged as promising candidates for optoelectronic applications, offering an environmental alternative to traditional lead-based perovskites. This review presents a comprehensive analysis of the current advancements in the synthesis, structural characterization, and photoelectric properties of Cu–Ag–Bi–I compounds, with a focus on their photoelectric applications, including solar cells, indoor photovoltaics, and photodetectors. The unique combination of metal cations in Cu–Ag–Bi–I materials leads to tunable bandgaps, high absorption coefficients, and favorable charge transport properties, positioning them as versatile materials for various optoelectronic applications. Despite their potential, challenges remain in optimizing their performance and stability. We discuss current strategies, such as additive engineering and doping, to enhance material properties and suggest future directions for the development of these materials. Ultimately, Cu–Ag–Bi–I lead-free perovskites has significant potential for commercialization as a burgeoning green and efficient photoelectric materials.

Traditional semiconductor lasers, despite their versatility, face significant challenges in beam control, power output, and temperature sensitivity. Photonic crystal surface emitting lasers (PCSELs) overcome these limitations by employing a two-dimensional photonic crystal structure, enabling large-area, high-power, coherent laser emission with narrow beam divergence and narrow linewidth. This review offers a concise overview of PCSEL technology, concentrating on its design principles, fabrication processes, and potential applications. We trace the development of PCSELs from their initial demonstration in 1999 to recent breakthroughs achieving 50 W output power with ultra-narrow beam divergence. Furthermore, we explore the fundamental design principles of PCSELs, including mode analysis, threshold current, and injection design. Key steps in PCSEL fabrication are outlined, emphasizing methods such as regrowth epitaxy and transparent conductor deposition. Finally, we compare PCSELs with established semiconductor laser types, highlighting their applications and prospects.

Gallium nitride (GaN) has gained traction in replacing silicon for power electronics applications, due to its high breakdown field, high mobility 2D electron gas, and effective n/p-type doping. This paper reviews three important topics of GaN power devices. One is the voltage-blocking structures needed to operate at high voltage while minimizing conduction loss and switching loss. Another one is the structure used to achieve normally-off operation, which is often required for power electronics. The third topic is the monolithic integration of gate drivers and power switches to achieve the ultimate switching speed at a low cost.

This paper provides a comprehensive review of multi-stage infrared detectors, including interband cascade infrared photodetectors (ICIPs) and quantum cascade detectors (QCDs). These detectors exhibit low dark current, high detectivity, and high 3 dB bandwidth positioning them as promising candidates in infrared (IR) detector technology. The review covers the history of multi-stage IR detectors, the corresponding device physics, materials systems, DC and RF performance, and recent advancements. Additionally, a comparative analysis of ICIPs and QCDs is provided, along with discussions on optimization strategies. This review is intended to be a valuable resource for researchers and engineers in the field of IR detector technology, offering a detailed insight into the most advanced multi-stage IR detector technology and providing guidance for future development.

Perovskite/silicon tandem solar cells (PSTSCs) have garnered global interest owing to their potential for achieving high power conversion efficiencies (PCEs) at reduced costs. Although single-junction solar cells presently dominate the photovoltaic (PV) market, PSTSCs recently achieved a record-breaking PCE of 33.9%, thereby showing considerable promise as a next-generation PV technology. The development of highly efficient PSTSCs at a low production cost could markedly impact the future of the PV industry. Perovskite-based TSCs have demonstrated superior efficiency in converting light compared to their standalone counterparts, owing to their more effective utilization of the photon spectrum and reduced energy loss. However, several challenges must be addressed to surpass the Shockley–Queisser limit of 29.4% for silicon solar cells, which necessitates a deeper understanding of this promising technology. This review offers a distinct perspective on the routes toward commercializing PSTSCs and scrutinizes the current forefront of scientific and engineering hurdles in this domain. By leveraging insights from existing research on perovskites and silicon, we summarize the technical issues anticipated during the large-scale production of PSTSCs and suggest potential avenues for future research.

We present a detailed investigation on the properties of supersaturated GaAs using Cr implantation followed by nanosecond Pulsed Laser Melting (PLM). A comparison between two different lasers was carried out (ArF+ and Nd-YAG). We have analyzed supersaturated samples by means of structural, electrical, optical, and optoelectronic techniques. Raman spectroscopy and transmission electron microscopy (TEM) results show a recovery of crystallinity after the PLM process. ToF-SIMS results reveal a Cr concentration above the solid solubility limit after the PLM process, featuring depth distribution depending on the type of laser used. The sheet resistance results obtained using van der Pauw configuration measurements show an activation of the implanted Cr in semi-insulating GaAs after PLM. Optical transmittance and Reflectance measurements show a sub-bandgap absorption (up to 10% for = 1250 nm) of the supersaturated GaAs:Cr related to the Cr incorporation, besides absorption induced by the PLM process itself. The origin of this sub-bandgap absorption has been analyzed by means of opto-electronic measurements revealing sub-bandgap photoresponsivity related to the absorption analyzed. The photoresponsivity measured below the bandgap originates from both the defects introduced by the PLM process and the states within the bandgap associated to the implanted Cr.

In this work, five dual-channel vertical trench GaN-based metal oxide semiconductor field-effect transistors (MOSFETs) are proposed and studied. The dual-channel structures include dual two-dimensional electron gas (2DEG) structure, 2DEG and three-dimensional electron gas (3DEG) hybrid structures, and dual 3DEG structure. Compared to the MOSFET with 2DEG from the abrupt GaN/Al_0.3Ga_0.7N heterostructure, the MOSFET with 3DEG from the graded Al_xGa_1-xN (0 ≤ x ≤ 0.3) layer can provide more channel electrons, and significantly improve the saturation current and reduce the on-resistance. By introducing three-dimensional hole gas (3DHG) as the current barrier layer (CBL) to optimize the dual-3DEG channel vertical MOSFET, the saturation current of the optimized MOSFET is 2.8 times higher and the on-resistance is reduced by 53% to 1.11 mΩcm² at V_gs=12 V, and the breakdown voltage is increased from 955 V to 1510 V compared with traditional single-channel MOSFET. Therefore, the optimized dual-channel MOSFET has excellent DC characteristics and can be used as a high-power switching device in next-generation energy and communications applications.

This study investigates the focused ion beam (FIB)-induced damage on the transmission electron microscopy (TEM) lamellae of terbium-doped oxygen-rich silicon oxide (ORSO:Tb) thin films by comparing the TEM images prepared using FIB and conventional mechanical methods. While there are significant advantages to the FIB technique, there is a potential that energetic ions used during the FIB process impose damage to the lamellae. The results indicate that TEM images of FIB-prepared lamellae exhibit higher resolution, allowing for a more detailed examination of nanocrystal structures and quantum dots. In contrast, the lack of sufficient clarity of the mechanically prepared TEM images reduced the number of nanocrystals visible in the field of view, resulting in a less effective and detailed study of the thinned films. We found no evidence of Ga implantation or mixing into the thinned film, and there were no observable FIB-induced damages such as recrystallization, or amorphization. Photoluminescence (PL) spectra exhibited red and blue shifts with increasing annealing temperature at blue and green emissions, respectively. X-ray diffraction (XRD) patterns verify that the formation of crystalline nanostructures begins at 1100 ⁰C, and at least at 1200 ⁰C, two phases of Tb4Si3(SiO4)O10 and Tb2O3 in the sample are recognized.

This paper reports an effective way of improving the linearity of AlGaN/GaN High Electron Mobility Transistors (HEMTs) by modulating the AlGaN barrier thickness under the gate electrode along its width. This produces an offset in the local equivalent threshold voltage (VT ), and consequently, the proposed device acts as multiple transistors with a VT offset connected in parallel configuration. Due to the resulting gm3 cancellation, the new device
structure exhibits 10-fold suppressed gm3 peak compared to conventional planar-gated AlGaN/GaN HEMTs. A linearization of Cgs −VGS curve is also demonstrated. Compact modeling and large signal simulation of the proposed device exhibits 7 dB improvement in the linearity figure-of-merit, OIP3/PDC at 5 GHz, compared to the conventional planar gated devices. Additionally, around 10 dB suppression in two-tone third-order intermodulation distortion (IMD3) and improved efficiency across 3 dB, 6 dB and 10 dB output back-offs (OBOs) is observed compared to the conventional planar-gated AlGaN/GaN HEMTs at 5 GHz for deep class AB applications. Hence, the proposed longitudinally-varying gate recess technology is a promising candidate for mm-wave linear and efficient amplifiers, suitable for 5G applications.

This article introduces the line tunneling-based gate-overlapped-source-only nanotube tunnel FET (GoS-NT-TFET) for the implementation of a low-power standard ternary inverter (STI). The proposed device exhibits a higher ON current with a steep subthreshold slope of 30 mV/decade while operating at VDD = 0.5 V. It is optimized for ternary operation, demonstrating gate-bias-independent point tunneling and gate-bias-dependent line tunneling. This device is meticulously optimized by varying the channel length, source doping, and drain doping, a process that is detailed in this article. The optimized device shows ~96% improvement in static power dissipation, ~50% improvement in dynamic power dissipation, and ~49% improvement in the power-delay product (PDP) compared to the conventional tunnelling based ternary CMOS (T-CMOS).

Semiconductor nanowires can be utilized to create quantum dot qubits. The formation of quantum dots is typically achieved by means of bottom gates created by a lift-off process. As an alternative, we fabricated flat buried bottom gate structures by filling etched trenches in a Si substrate with sputtered TiN, followed by mechanical polishing. This method achieved gate line pitches as small as 60 nm. The gate fingers have low gate leakage. As a proof of principle, we fabricated quantum dot devices using InAs nanowires placed on the gate fingers. These devices exhibit single electron tunneling and Coulomb blockade.

This study investigates the focused ion beam (FIB)-induced damage on the transmission electron microscopy (TEM) lamellae of terbium-doped oxygen-rich silicon oxide (ORSO:Tb) thin films by comparing the TEM images prepared using FIB and conventional mechanical methods. While there are significant advantages to the FIB technique, there is a potential that energetic ions used during the FIB process impose damage to the lamellae. The results indicate that TEM images of FIB-prepared lamellae exhibit higher resolution, allowing for a more detailed examination of nanocrystal structures and quantum dots. In contrast, the lack of sufficient clarity of the mechanically prepared TEM images reduced the number of nanocrystals visible in the field of view, resulting in a less effective and detailed study of the thinned films. We found no evidence of Ga implantation or mixing into the thinned film, and there were no observable FIB-induced damages such as recrystallization, or amorphization. Photoluminescence (PL) spectra exhibited red and blue shifts with increasing annealing temperature at blue and green emissions, respectively. X-ray diffraction (XRD) patterns verify that the formation of crystalline nanostructures begins at 1100 ⁰C, and at least at 1200 ⁰C, two phases of Tb4Si3(SiO4)O10 and Tb2O3 in the sample are recognized.

Heavily-doped, amorphous and polycrystalline silicon (poly-Si) layers play important roles in silicon solar cell fabrication and performance. Here we demonstrate applications of time-resolved photoluminescence decay to measure recombination lifetimes in such regions, which are generally below 1 µs, and difficult to measure with other techniques. Firstly, we demonstrate the measurement of Auger lifetimes in uniformly heavily-doped silicon wafers, and show the impact of surface recombination in samples with phosphorus or boron doping concentrations below 1 × 10¹⁹ cm⁻³. We also assess the possible impact of high concentrations of iron contamination on the extraction of such Auger lifetimes. We then report recombination lifetimes measured in thin deposited intrinsic amorphous silicon films, and heavily-doped poly-Si films, as commonly used in passivating contact structures. Interestingly, recombination lifetimes in intrinsic amorphous silicon films can be significantly enhanced by a hydrogenation process. By contrast, recombination lifetimes in heavily-doped poly-Si films vary with different doping profiles for samples fabricated with different deposition techniques, but are not improved by hydrogenation.

Germanium (Ge), characterized by its indirect bandgap energy of 0.66 eV, faces limitations in optoelectronic applications. However, applying strain transforms Ge into a direct bandgap semiconductor, potentially broadening its technological utility. This study investigates the effects of intense femtosecond (fs) laser irradiation on crystalline Ge to induce such strain and examine its consequent structural and electronic alterations. Employing micro-Raman spectroscopy, transmission electron microscopy (TEM), x-ray diffraction (XRD), and spectrophotometric analyses, we aim to elucidate the underlying mechanisms of strain-induced transformations. Our findings reveal a maximum Raman shift of up to 10.5 cm⁻¹, indicative of significant localized tensile strain. TEM analysis shows polycrystalline structures with rich defects, corroborating Raman data and suggesting strained nanostructures. XRD results point to anisotropic type of strain, which could facilitate the transition towards a direct bandgap semiconductor compared to uniaxial or biaxial strain. Optical measurements further indicate bandgap enlargement to 0.78 eV, close to the direct transition energy at 0.8 eV. These comprehensive analyses demonstrate that fs laser irradiation can effectively induce strains to transform Ge, thereby enhancing its application potential in photonic and optoelectronic devices.

The effect of the quantum well (QW) number (n_QW) in far ultraviolet-C light emitting diodes (LEDs) on the optical power, external quantum efficiency (EQE) and degradation has been investigated. AlGaN-based multi-QW (MQW) LEDs designed for emission at 233 nm and 226 nm with n_QW between 1 and 30 are compared. A positive correlation between the optical power at 200 mA and L70 lifetime for large n_QW was observed. For the 233 nm LEDs QW numbers 6 ⩽ n_QW ⩽ 15 result in optical powers of 4–5 mW at 200 mA (corresponding to a maximum EQE of 0.47% for n_QW = 15) and L70 lifetimes of 9–13 h. For n_QW = 30 a reduction of output power and L70 lifetime was found indicating an optimum n_QW for 233 nm LEDs. For the 226 nm LEDs a constant optical power of 0.5 mW at 200 mA (corresponding to an EQE of 0.05%) was measured independent of n_QW. However, the L70 lifetime continuously increases from 7 h for 3 QWs to 13 h for 18 QWs. The enhanced optical power accompanied by a reduced degradation is attributed to a reduced hole leakage from the MQW into the n-side and reduced local charge carrier density per QW for large n_QW.

Gallium nitride (GaN) has gained traction in replacing silicon for power electronics applications, due to its high breakdown field, high mobility 2D electron gas, and effective n/p-type doping. This paper reviews three important topics of GaN power devices. One is the voltage-blocking structures needed to operate at high voltage while minimizing conduction loss and switching loss. Another one is the structure used to achieve normally-off operation, which is often required for power electronics. The third topic is the monolithic integration of gate drivers and power switches to achieve the ultimate switching speed at a low cost.

We report on the fabrication and micro-transfer printing (µ-TP) of InGaAs/InP avalanche photodiodes (APDs) onto silicon substrates. A process flow was developed to suspend the devices using semiconductor tethers. The developed process reduces the number of fabrication steps required compared to methods based on the use of photoresist tethers. Furthermore, our process is compatible with devices that may be susceptible to damage induced by the photoresist removal process. APDs were characterised in linear mode operation both before suspension and after printing. Despite the additional fabrication steps required to suspend the APD membranes and the physical nature of the µ-TP process, the electrical characteristics of the devices were preserved. No degradation in the optical performance of the devices was measured. Our work represents the first demonstration of µ-TP of InGaAs/InP APDs onto silicon substrates. The results highlight the viability of µ-TP for effective heterogeneous integration of InGaAs/InP APDs with silicon photonic integrated circuits for optical and quantum communication and other light detection applications.

Arrays of far-UVC micro light emitting diodes (LEDs) based on AlGaN and emitting at 233–235 nm have been fabricated on different types of AlN-sapphire templates and the optical polarization, output power, and efficiencies have been studied in dependence of the template technology and the mesa diameter of the micro-pixels. While LEDs fabricated on metal organic vapor phase epitaxy (MOVPE) AlN-sapphire templates show dominant TM polarized emission with a degree of polarization (DoP) of −0.2, LEDs on high temperature annealed AlN-sapphire templates show dominant TE polarized emission with a DoP of 0.2–0.3. The output power and external quantum efficiency increases with decreasing diameter of the slanted and reflective micro LED mesa. Peak output powers of 18 mW at 200 mA and peak external quantum efficiencies of up to 2.7% for mesa diameters of 1.5 µm on annealed templates were measured, corresponding to peak wall plug efficiencies of 1.7%, while conventional LEDs with large mesa areas on the same template showed maximum EQEs of 1.1%. The relative increase in output power by using the micro LED approach as compared to a conventional large area emitter is stronger for LEDs on MOVPE AlN templates than on annealed templates (about a factor of 3.7 vs. 2.3, respectively, at 50 mA) which is attributed to the polarization dependence of the light extraction.

The quantum spin Hall effect in non-magnetic and Mn-doped HgTe quantum well (QW) is strongly affected by Kondo scattering of edge electrons by holes localized on acceptors. A generalized eigenvalue method is usually employed for determining impurity binding energies from the multiband Kohn–Luttinger Hamiltonians in bulk samples and semiconductor quantum structures. Such an approach provides accurate values of the level positions but its applicability for determining the impurity localization radius can be questioned. As an alternative method we propose here the Gram–Schmidt orthogonalization procedure allowing to employ the standard eigenvalue algorithms and, thus, to determine both impurity level energies and the set of normalized eigenvectors. We apply this approach to singly-ionized acceptor states in HgTe QWs and obtain impurity level energies and localization radiuses even for states degenerate with the continuum of band states. Such information allows us to assess the energy of bound magnetic polarons in QWs doped with magnetic ions. We determine the polaron energies and discuss consequences of the resonant polaron formation on band transport in the bulk samples and QWs in the regimes of quantum Hall effects.

The Shockley–Queisser limit poses a significant challenge in solar technology research, limiting the theoretical efficiency to around 30%. Thermophotovoltaic (TPV) systems have emerged as a solution by incorporating a thermal absorber in traditional solar cell setups to achieve total efficiency beyond the limits. The efficiency of the overall system heavily depends on the performance and quality of the thermal absorber, which absorbs photons from the heat source and transfers them to the TPV cell. However, complex and expensive fabrication processes have hindered widespread adoption of TPV technology. The well-established metal-assisted chemical etching (MACE) method could be the best choice to mitigate these as it is a cost-effective, scalable, and mass-production-friendly process, which is widely used for surface texturization, creating nanostructures like nanopores, pyramids, and nanowires. MACE technique is also suitable for producing highly efficient silicon-based thermal absorbers with over 90% absorption rate, which can contribute to increased total conversion efficiency. However, it does not come without challenges such as maintaining control over the etch rate in order to achieve uniformity. This paper comprehensively reviews the utilization of MACE for fabricating silicon-based thermal absorbers in TPV systems with the range of effective wavelengths of 600–2000 nm which corresponds to the energy level of 0.55–1.85 eV. The advantages and challenges of MACE, along with characterization techniques, are extensively discussed. By providing valuable insights, this paper aims to support researchers interested in advancing TPV technology.