Doping Profile Research Articles

Impact of post-ion implantation annealing on Se-hyperdoped Ge

Hyperdoped germanium (Ge) has demonstrated increased sub-bandgap absorption, offering potential applications in the short-wavelength-infrared spectrum (1.0–3.0 μm). This study employs ion implantation to introduce a high concentration of selenium (Se) into Ge and investigates the effects of post-implantation annealing techniques on the recovery of implantation damage and alterations in optical properties. We identify optimal conditions for two distinct annealing techniques: rapid thermal annealing (RTA) at a temperature of 650 °C and ultrafast laser heating (ULH) at a fluence of 6 mJ/cm2. The optimized ULH process outperforms the RTA method in preserving high doping profiles and achieving a fourfold increase in sub-bandgap absorption. However, RTA leads to regrowth of single crystalline Ge, while ULH most likely leads to polycrystalline Ge. The study offers valuable insights into the hyperdoping processes in Ge for the development of advanced optoelectronic devices.

Gallium arsenide octave-bandwidth microwave voltage-controlled oscillator having discrete varicap

Modern reconfigurable communication systems demand microwave voltage-controlled oscillators (VCOs) possessing an octave-wide bandwidth. Many advanced applications require also a low phase noise and a low current consumption for such VCOs. To meet an octave-bandwidth specification in combination with a low phase noise and a low current consumption it is desirable to implement a microwave monolithic integrated circuit (MMIC) technology. In-house gallium arsenide (GaAs) MMIC technology is implemented to build VCOs in this work. At the same time, in order to achieve a high-Q wide frequency tuning range, we implemented also a discrete variable capacitance diode (varicap). The discrete varicap is stacked above MMIC oscillator chip. A vertical design structure of the discrete varicap guarantees a high overlap capacitance ratio (K>20) along with reduced affect of the parasitic reactance. A high overlap capacitance ratio is needed for an octave-wide frequency tuning, while reduced affect of the parasitic reactance increases Q-factor of the VCO resonator. Hence, a sequential integration of the discrete varicap over monolithic oscillator chip resolves referred tough challenges in the design of VCO. Integrated VCO is finally packaged as a surface-mount device (SMD). At that SMD ceramic package is sealed with the metal lid. The oscillator circuitry is based on the conventional negative resistance approach with a positive feedback realized by common-source capacitance. An active element of the oscillator is a pseudomorphic high electron-mobility transistor (pHEMT). The gate length of a transistor is 0.15 µm. The width of the gate varies from 240 µm to 360 µm depending on working frequencies of the VCO. The varicap is built on the GaAs epitaxial heterostructure having customized gradient doping profile with abrupt junction. Nonlinear behavioral model of the varicap is extracted from the diode measurements and is further used for VCO simulation. Two types of VCO have been designed, fabricated and tested. First (second) type of VCO provides a tuning range from 5 to 10 GHz (from 10 GHz to 20 GHz), respectively. The measurement results demonstrate an output power of 3.9 dBm; a current consumption of 30 mA at a supply voltage of 5 V; a phase noise of minus 90 dBc/Hz at 100 kHz offset (minus 85 dBc/Hz at 100 kHz offset) for the first (second) type of VCO, respectively.

Improving the performance of industrial TOPCon solar cells through the insertion of intrinsic a-Si layer

Passivating contact solar cells have gradually become the mainstream cell technology due to their excellent performance, and further improving the conversion efficiency has become a focus of subsequent research. Typically, achieving excellent field-effect passivation and low contact resistivity in doped polycrystalline silicon (poly-Si) requires heavy phosphorus doping. However, this approach can lead to a predicament wherein excessive phosphorus diffuses into the silicon (Si) substrate during annealing, consequently causing recombination losses. In response to this challenge, a structure incorporating an intrinsic amorphous silicon (a-Si (i)) layer within the passivation layers has been introduced. The primary objective of this structure is to retard the diffusion of phosphorus into the Si substrate. This study entails comprehensive characterizations to delve into the underlying mechanisms of films with the integrated a-Si (i) layer, including surface microscopy, active dopants profile, crystallographic structure, elemental distribution, and electrical properties. Finally, we have fabricated the industrial-sized TOPCon solar cells with an average efficiency of 23.83 %, which is 0.25 % higher than that of Baseline counterparts (23.58 %) on the production line. The above results have demonstrated the introduction of a-Si (i) film can be a buffer layer, retarding the diffusion of phosphorus into the Si substrate and obtaining a better passivation effect, enabling us to further tailor the doping profile for high-efficiency solar cells. Our work highlights a promising strategy to improve the performance of TOPCon solar cells, showcasing the substantial potential for implementation in industrial manufacturing.

First results on monolithic CMOS detector with internal gain

In this paper we report on a set of characterisations carried out on the first monolithic LGAD prototype integrated in a customised 110 nm CMOS process having a depleted active volume thickness of 48 μm. This prototype is formed by a pixel array where each pixel has a total size of 100 μm× 250 μm and includes a high-speed front-end amplifier. After describing the sensor and the electronics architecture, both laboratory and in-beam measurements are reported and described. Optical characterisations performed with an IR pulsed laser setup have shown a sensor internal gain of about 2.5. With the same experimental setup, the electronic jitter was found to be between 50 ps and 150 ps, depending on the signal amplitude. Moreover, the analysis of a test beam performed at the Proton Synchrotron (PS) T10 facility of CERN with 10 GeV/c protons and pions indicated that the overall detector time resolution is in the range of 234 ps to 244 ps. Further TCAD investigations, based on the doping profile extracted from C(V) measurements, confirmed the multiplication gain measured on the test devices. Finally, TCAD simulations were used to tune the future doping concentration of the gain layer implant, targeting sensors with a higher avalanche gain. This adjustment is expected to enhance the timing performance of the sensors of the future productions, in order to cope with the high event rate expected in most of the near future high-energy and high-luminosity physics experiments, where the time resolution will be essential to disentangle overlapping events and it will also be crucial for Particle IDentification (PID).

Comparative Cryogenic Investigation of FD-SOI Devices with Doped Epitaxial and Metallic Source/Drain

Defects induced by the source/drain process have a significant impact on the scattering mechanism of PMOS at cryogenic temperatures. Here, the cryogenic characteristics of FD-SOI devices with heavily doped epitaxial source/drain (Epi FD-SOI devices) and metallic Schottky barrier source/drain (SB FD-SOI devices) were investigated from 300 K down to 6 K. The doping profile along the channel was analyzed by TCAD simulation analysis. Experimental comparison of transistor performance at cryogenic temperatures was carried out for these devices with gate lengths (L G ) of 100 nm and 40 nm. The I-V characteristics of the FD-SOI devices were measured with a liquid helium cooling environment. The cryogenic effect of the two types of devices on Key parameters including transconductance (G m ), field effect mobility (μ FE ), threshold voltage (V th ) and subthreshold slope (SS) were systematically analyzed. The doping distribution of the heavily doped epitaxial SiGe source/drain structure were subjected to more Coulomb scattering at cryogenic temperatures, whereas the doping distribution of the Schottky-barrier source/drain structure dictates that the device is mainly subjected to phonon scattering at cryogenic temperatures.

GaN-based low-power JLDG-MOSFETs: Effects of doping and gate work function

The purpose of this study is to investigate the possibilities of the junction-less double-gate (JLDG) MOSFET structure with gallium nitride (GaN) channel material to overcome the limitations of conventional MOSFET structures in improving device performance at scaled gate lengths and voltages. The design targets of this study are the doping profile (ND), and gate work function (Ф). The device has been modeled using the Silvaco Atlas 2D device simulator. The proposed model has been validated by calibrating it against the parabolic potential-based analytical model for short-channel JLDG MOSFETs in the subthreshold regime of Jazaeri et al. Device figure of merits (FOMs) such as ON-current (ION), ON-OFF current ratio (ION/IOFF), subthreshold swing (SS), and drain-induced barrier lowering (DIBL) have been evaluated. The maximum on-current, (ION) = 0.9 mA/μm, has been achieved by tuning the channel doping concentration (ND) to 1 × 1019 cm−3. Tuning the gate work function, (Ф) also has a substantial effect on the behavior of the device. The lowest OFF-state current (IOFF) of 1.24 × 10−16 A/μm and power dissipation of 9.69 × 10−17 W/μm have been found for gate work-function (Ф) = 5.1 eV (Au). In addition, the on-current to off-current ratio (ION/IOFF) of 7.56 × 1012 revolutionizes the applicability of the device in the semiconductor industry. Thus, the remarkable improvements in device FOMs prove that GaN-based JLDG MOSFETs are a strong contender for applications requiring low-power logic switching in the next generation.

Evaluation of a Simplified Modeling Approach for SEE Cross-Section Prediction: A Case Study of SEU on 6T SRAM Cells

Electrical models play a crucial role in assessing the radiation sensitivity of devices. However, since they are usually not provided for end users, it is essential to have alternative modeling approaches to optimize circuit design before irradiation tests, and to support the understanding of post-irradiation data. This work proposes a novel simplified methodology to evaluate the single-event effects (SEEs) cross-section. To validate the proposed approach, we consider the 6T SRAM cell a case study in four technological nodes. The modeling considers layout features and the doping profile, presenting ways to estimate unknown parameters. The accuracy and limitations are determined by comparing our simulations with actual experimental data. The results demonstrated a strong correlation with irradiation data, without requiring any fitting of the simulation results or access to process design kit (PDK) data. This proves that our approach is a reliable method for calculating the single-event upset (SEU) cross-section for heavy-ion irradiation.

Influence of boron doping profile on emitter and metal contact recombination for n-PERT silicon solar cells

Increasing interest in n-type silicon (Si) solar cells due to the high-quality material properties of the n-Si wafers, their relatively low sensitivity to donor-like impurities like iron and their resistance to light-induced degradation brings more recent attention to the studies on the p+ emitter and its metal contact. In this work, we investigate the effect of the Boron doping profile on the quality of p+ emitter and their contacts with screen-printed Ag/Al metal fingers. The emitters within the scope of this study are created with BBr3 diffusion in an atmospheric diffusion furnace on n-type Si wafers and also applied to n-PERT Si cells to analyze their effects on device performance. Our results highlight the significance of the doping profile for solar cell performance, as it considerably affects the contact resistivity at the emitter-metal interface and the saturation current densities for emitter (J0, emitter) and Ag/Al metal contacts (J0, AgAl). Reducing the Boron concentration within the emitter leads to a decrease in carrier recombination in the non-contacted region of the emitter, which in turn decreases J0, emitter. However, this reduction increases the recombination in the contacted areas of the emitter, consequently elevating the J0, AgAl, and increasing the contact resistance values. Besides, the power loss analysis and simulation results based on experimental values obtained for our best solar cells group show how much J0, emitter and J0, AgAl values contribute to and would improve solar cell efficiency. The manuscript presents a case study of experimental emitter doping profile optimization applicable to similar n-type PERT or TOPCon cells with varying design parameters.

Global well‐posedness of strong solutions to the compressible magnetohydrodynamic equations with Coulomb force and non‐flat doping profile

This paper establishes the global well‐posedness of strong solutions for the 3D compressible magnetohydrodynamics (MHD) with non‐flat doping profile and Coulomb force, which is of hyperbolic–parabolic–elliptic mixed type. It is essentially shown that for the initial value problem with initial density allowed to contain vacuum states, the strong solution exists globally under small energy but possible large oscillations.

Dielectrically-Modulated GANFET Biosensor for Label-Free Detection of DNA and Avian Influenza Virus: Proposal and Modeling

This paper introduces a novel device called the Gate All Around Engineered Gallium Nitride Field Effect Transistor (GAAE-GANFET), designed specifically for label-free biosensing applications. This innovative gate-all-around engineering in GANFET integrates various device engineering techniques, such as channel engineering, gate engineering, and oxide engineering, to enhance biosensing performance. The channel engineering techniques refer to the use of a gallium nitride channel with a step-graded doping profile, divided into three distinct regions. In contrast, the gate engineering technique refers to the cylindrical split-gate-underlap architecture. The oxide engineering technique involves stacking Al2O3 and HfO2. Moreover, this biosensor incorporates two-sided gate underlap cavities that facilitate the immobilization of biomolecules. These open cavities not only provide structural stability but also simplify the fabrication process to a significant extent. The viability of this biosensor as a label-free biosensor has been evaluated using an antigen and an antibody from the Avian Influenza virus and DNA as the target biomolecules. The proposed analytical model and TCAD simulation results are in excellent agreement, demonstrating the reliability of the proposed device. Additionally, the biosensor’s sensitivity, which depends on cavity length, doping concentration, gate metal work function, and temperature variation, has been thoroughly explored. The gate-all-around structure, along with the integration of tri-step graded doping, GaN as the channel material, gate oxide stacking, and dual open cavity structure in the proposed biosensor, leads to significantly improved biosensing capabilities.

Effect of Fe Doping Profile on Current Collapse in GaN-based RF HEMTs.

Using computer-aided design (TCAD) simulation, the impact of the Fe doping profile, including concentration, decay rate, and depth of the doping region on current-collapse magnitude (▵CC) in 0.5-μm gated GaN-based high electron mobility transistors (HEMTs) is systematically investigated. Accurate simulation models are established and developed to facilitate the fabrication of electronics. It is elucidated that the intricate interplay between trapping and de-trapping of Fe-related traps at the gate-drain edge is responsible for current collapse. The concentration and decay rate of the doping region have a more significant impact on current collapse than the depth. Increased trap state density near two-dimensional electron gas (2DEG) channel caused by deep-level acceptors would boost ▵CC. However, a minor dynamic reduction in 2DEG density (nT) induces a relatively small ▵CC. By adjusting the concentration, decay rate, and depth of the doping region, ▵CC of GaN-based Radio Frequency (RF) HEMTs can be reduced by approximately 50.3 %. The optimized distribution of Fe doping discussed in this work helps to prepare GaN-based RF HEMTs with a limited current collapse effect.

Data-driven solutions of ill-posed inverse problems arising from doping reconstruction in semiconductors

ABSTRACT The non-destructive estimation of doping concentrations in semiconductor devices is of paramount importance for many applications ranging from crystal growth to defect and inhomogeneity detection. A number of technologies (such as LBIC, EBIC and LPS) have been developed which allow the detection of doping variations via photovoltaic effects. The idea is to illuminate the sample at several positions and detect the resulting voltage drop or current at the contacts. We model a general class of such photovoltaic technologies by ill-posed global and local inverse problems based on a drift-diffusion system which describes charge transport in a self-consistent electrical field. The doping profile is included as a parametric field. To numerically solve a physically relevant local inverse problem, we present three approaches, based on least squares, multilayer perceptrons, and residual neural networks. Our data-driven methods reconstruct the doping profile for a given spatially varying voltage signal induced by a laser scan along the sample's surface. The methods are trained on synthetic data sets which are generated by finite volume solutions of the forward problem. While the linear least square method yields an average absolute error around 10%, the nonlinear networks roughly halve this error to 5%.

Analyzing the effect of doping concentration in split-well resonant-phonon terahertz quantum cascade lasers.

The effect of doping concentration on the temperature performance of the novel split-well resonant-phonon (SWRP) terahertz quantum-cascade laser (THz QCL) scheme supporting a clean 4-level system design was analyzed using non-equilibrium Green's functions (NEGF) calculations. Experimental research showed that increasing the doping concentration in these designs led to better results compared to the split-well direct-phonon (SWDP) design, which has a larger overlap between its active laser states and the doping profile. However, further improvement in the temperature performance was expected, which led us to assume there was an increased gain and line broadening when increasing the doping concentration despite the reduced overlap between the doped region and the active laser states. Through simulations based on NEGF calculations we were able to study the contribution of the different scattering mechanisms on the performance of these devices. We concluded that the main mechanism affecting the lasers' temperature performance is electron-electron (e-e) scattering, which largely contributes to gain and line broadening. Interestingly, this scattering mechanism is independent of the doping location, making efforts to reduce overlap between the doped region and the active laser states less effective. Optimization of the e-e scattering thus could be reached only by fine tuning of the doping density in the devices. By uncovering the subtle relationship between doping density and e-e scattering strength, our study not only provides a comprehensive understanding of the underlying physics but also offers a strategic pathway for overcoming current limitations. This work is significant not only for its implications on specific devices but also for its potential to drive advancements in the entire THz QCL field, demonstrating the crucial role of e-e scattering in limiting temperature performance and providing essential knowledge for pushing THz QCLs to new temperature heights.

Impact of Pulse Duration on the Properties of Laser Hyperdoped Black Silicon

The impact of three different pulse durations (100 fs, 1, and 10 ps) on the formation of laser hyperdoped black silicon with respect to surface morphology, sub‐bandgap absorptance, the sulfur concentration profile, and the effective minority carrier lifetime after Al2O3 surface passivation is investigated. The current flow behavior is compared through the hyperdoped layer by I–V measurements after hyperdoping with different pulse durations. For conditions that give the same absolute sub‐bandgap absorptance, an increase in pulse duration from 100 fs to 10 ps results in a shallower sulfur concentration profile. Findings are explained by an increasing ablation threshold from 0.19 J cm−2 for a pulse duration of 100 fs to 0.21 J cm−2 for 1 ps and 0.34 J cm−2 for 10 ps. The formation of an equally absorbing layer with a shallower doping profile results in a reduction in contact and/or sheet resistance. Despite the higher local sulfur concentration, the samples show no decrease in carrier lifetime measured by quasi‐steady‐state photoconductance decay on Al2O3 surface‐passivated samples. The investigation shows that longer pulses of up to 10 ps during laser hyperdoping of silicon result in advanced layer properties that promise to be beneficial in a potential device application.

Plasma-Assisted N2O Oxidation (PANO) in an Industrial Direct Plasma Reactor for TOPCon Production

Plasma-Enhanced Chemical Vapor Deposition (PECVD) is an attractive tool for TOPCon production, as it enables uniformly in situ doped amorphous silicon (a-Si) and dielectric layer depositions with high throughput. However, a lean process requires in situ interfacial oxide growth in the same tool. In this work, we use Plasma-Assisted N2O Oxidation (PANO) in an industrial kHz direct plasma reactor (centrotherm c.PLASMA) to grow the oxide and deposit in situ phosphorus doped a-Si(n) as well as SiNx on asymmetric lifetime samples. Before optimization, the oxide thickness is non uniform on the wafer, and we show that it correlates with the passivation, the contact resistivity, and the doping profile in n-type TOPCon test structures. The passivation seems to benefit more from moderate in-diffusion in the case with PANO than in the case with thermal oxidation. This is probably due to enhanced field-effect passivation compensating for lower chemical passivation, which likely results from plasma-induced damage. After studying the influence of PANO process parameters on the oxide thickness and uniformity, we optimize them to obtain a non-uniformity as low as ±2% and a recombination current density down to 2.3 fA/cm² on planar wafers.

Bi-layer in-situ phosphorus doped poly-Si films by PECVD for blistering-free high-efficiency industrial TOPCon solar cells

The passivating contact concept stands out as one of the most promising and industrially viable photovoltaic (PV) technologies. Further improving the quality of physical contact has become a focus of ongoing research. The film blistering issue has been identified as one of the major bottlenecks for the polysilicon (poly-Si) films deposited by the PECVD approach. In this study, we investigated how the in-situ phosphorus (P) doping level within the poly-Si films contributes to the occurrence of blistering. Our investigations into the film blistering mechanisms reveal that a high in-situ P-doping suppresses hydrogen release levels and reduces the accumulation of residual stress during annealing, which leads to the blistering-free appearance, especially observed in heavily P-doped poly-Si films. However, as excessive P-doping could weaken the interfacial passivation quality, we propose a bi-layer structure of P-doped poly-Si films which allows the doping profile to be tailored and maintain good quality passivating contacts. Based on the bi-layer structure, we fabricated industrial-sized tunnel oxide passivated contact (TOPCon) solar cells, which attained an average efficiency of 23.84%. Our work not only presents a promising strategy for improving the performance of passivating contacts via the PECVD approach but also underscores the significant potential for its widespread implementation in industrial TOPCon solar cell manufacturing.

Surface heterojunction based on n-type low-dimensional perovskite film for highly efficient perovskite tandem solar cells.

Enhancing the quality of junctions is crucial for optimizing carrier extraction and suppressing recombination in semiconductor devices. In recent years, metal halide perovskite has emerged as the most promising next-generation material for optoelectronic devices. However, the construction of high-quality perovskite junctions, as well as characterization and understanding of their carrier polarity and density, remains a challenge. In this study, using combined electrical and spectroscopic characterization techniques, we investigate the doping characteristics of perovskite films by remote molecules, which is corroborated by our theoretical simulations indicating Schottky defects consisting of double ions as effective charge dopants. Through a post-treatment process involving a combination of biammonium and monoammonium molecules, we create a surface layer of n-type low-dimensional perovskite. This surface layer forms a heterojunction with the underlying 3D perovskite film, resulting in a favorable doping profile that enhances carrier extraction. The fabricated device exhibits an outstanding open-circuit voltage (VOC) up to 1.34V and achieves a certified efficiency of 19.31% for single-junction wide-bandgap (1.77eV) perovskite solar cells, together with significantly enhanced operational stability, thanks to the improved separation of carriers. Furthermore, we demonstrate the potential of this wide-bandgap device by achieving a certified efficiency of 27.04% and a VOC of 2.12V in a perovskite/perovskite tandem solar cell configuration.

Measurement of mix at the fuel–ablator interface in indirectly driven capsule implosions on the National Ignition Facility

The interface between the capsule ablator and fuel ice layer is susceptible to hydrodynamic instabilities. The subsequent mixing of hot ablator material into the ice reduces fuel compression at stagnation and is a candidate for reduced capsule performance. The ability to diagnose ice–ablator mix is critical to understanding and improving stability at this interface. Combining the crystal backlighter imager with the single line of sight camera on the National Ignition Facility (NIF) allows direct measurement of ice–ablator mix by providing multiple quasi-monochromatic radiographs of layered capsule implosions per experiment with high spatial (∼12 μm) and temporal (∼35 ps) resolution. The narrow bandwidth of this diagnostic platform allows radiography of the inner edge of the capsule limb close to stagnation without capsule self-emission contaminating the data and removes opacity uncertainties typically associated with the spectral content of the radiograph. Analysis of radiographic data via a parameterized forward-fitting Abel inversion technique provides measurements of the distribution of mix mass inwards from the ice–ablator interface. The sensitivity of this mix measurement technique was demonstrated by applying it to layered experiments in which the stability of the ice–ablator interface was expected to vary significantly. Additional experiments suggest that high-density carbon capsules that employ a buried-layer dopant profile suffer from mixing at the innermost doped–undoped interface. Data from these experiments suggest that opacity models used in hydrodynamic simulations of NIF experiments can potentially over-predict the opacity of doped capsules. LLNL-JRNL-850535-DRAFT.

Impact of doping profiles on the formation of laser-active centers in bismuth-doped GeO2–SiO2 glass fibers

Multi-wavelength-band transmission technology based on the exploitation of the extended spectral region is considered as a potential approach to increase the transmission capacity in the deployed fiber-optic communication infrastructure. The development of optical amplifiers operating in the O-, E-, S-, and U-telecom bands is an extremely important challenge for the successful implementation of this technology. Bismuth-doped fibers are of increasing interest as gain materials, which can be used to provide broadband amplification in the mentioned telecom bands. This is due to the ability of Bi ions incorporated into glass network to form bismuth active centers (BACs) with specific optical properties, which are primarily determined by the glass modifiers. In this work, the impact of the doping profiles of both Ge atoms as glass modifiers and Bi ions on the BACs formation is studied using a series of bismuth-doped fibers fabricated by the modified chemical vapor deposition (MCVD) technique. The Bi-to-BACs conversion efficiency in various spatial regions of the studied samples is presented. It is turned out that for high-Bi concentration regions, the conversion efficiency is very low (less than 10%). In addition, the relationship of the conversion efficiency to the distribution of Bi ions and/or Ge atoms is discussed. Finally, a continuous-wave laser at 1.46 μm with a record slope efficiency of 80% is demonstrated using a Bi-doped fiber with confined doping profile, where the Bi-to-BACs conversion efficiency is 35%. This paper provides new information which might help to facilitate understanding of the features of Bi-doped fibers and their potentially achievable characteristics.

Quantum Efficiency Measurement and Modeling of Silicon Sensors Optimized for Soft X-ray Detection

Hybrid pixel detectors have become indispensable at synchrotron and X-ray free-electron laser facilities thanks to their large dynamic range, high frame rate, low noise, and large area. However, at energies below 3 keV, the detector performance is often limited because of the poor quantum efficiency of the sensor and the difficulty in achieving single-photon resolution due to the low signal-to-noise ratio. In this paper, we address the quantum efficiency of silicon sensors by refining the design of the entrance window, mainly by passivating the silicon surface and optimizing the dopant profile of the n+ region. We present the measurement of the quantum efficiency in the soft X-ray energy range for silicon sensors with several process variations in the fabrication of planar sensors with thin entrance windows. The quantum efficiency for 250 eV photons is increased from almost 0.5% for a standard sensor to up to 62% as a consequence of these developments, comparable to the quantum efficiency of backside-illuminated scientific CMOS sensors. Finally, we discuss the influence of the various process parameters on quantum efficiency and present a strategy for further improvement.