Deep Impurity Research Articles

The ab initio study of n-type nitrogen and gallium co-doped diamond

Abstract In order to better understand the influence of different complexes on the diamond co-doping system, N and Ga are chosen as co-dopants in diamond. The properties of several substitutional NmGan (m+n <= 3) complexes with vacancy (Va) in the bulk diamond have been investigated by ab initio density functional technique, including their optimized lattice structures, formation energies, binding energies and thermodynamic transition levels. The calculational results show that NmGan complexes in the donor-acceptor-donor (DAD) configuration can provide ionization energies similar to phosphorus-doped diamond. All other complexes provide deep impurity levels. For the DAD configuration, the adsorption process on the diamond surface has been studied to demonstrate the feasibility of growing diamonds containing N-Ga-N in experiments. The desired complex configuration is not uniquely present in the co-doped system. Investigating these properties of different complexes beyond NGaN provides insight into the N and Ga codoped diamond system, yielding a more comprehensive understanding of its potential and limitations. Our research ideas can also be extended to other co-doped systems and help to evaluate other potential co-dopants for diamond.

First-principle calculations of the electronic structure and optical properties of β-Ga2O3 with various vacancy defects

In this work, the effects of five types of vacancy defects on the electronic, optical and thermodynamic properties of β-Ga2O3 were investigated using first-principle calculations. The findings revealed that oxygen vacancies were more prevalent than gallium vacancies, with tetrahedral gallium vacancies being easier to form than octahedral ones. Vacancy defects induced a shift in the energy bands towards the lower energy end, with gallium vacancies reducing the bandgap from 4.9 eV to 3.1 eV, while the effect of oxygen vacancies on the bandgap was negligible. Oxygen vacancies introduced deep impurity levels near the Fermi level, while gallium vacancies introduced shallow impurity levels, primarily due to contributions from O-2p, Ga-4s, and Ga-4p orbital electrons. The differential charge density diagrams illustrated that gallium vacancies had a more pronounced impact on electronic distribution compared to oxygen vacancies, with VO3 exhibiting the least influence. As the temperature increases, the β-Ga2O3 with VO1 and VO2 exhibit higher thermodynamic stability compared to the intrinsic β-Ga2O3 and the one with VO3, while the intrinsic β-Ga2O3 has a superior heat capacity value compared to the β-Ga2O3 containing the five vacancy defects. The defect energy levels resulting from vacancies enhanced light absorption and conductivity, particularly oxygen vacancies, which improved the absorption capacity of β-Ga2O3 in the visible light range. These findings provide valuable insights for elucidating optical absorption experimental phenomena and photoluminescence.

Combined experimental and first principles look into (Ce, Mo) doped BiVO4

Here we investigated the effects of Ce and Mo doping on hydrothermally synthesized bismuth vanadate BiVO4 nanoparticles (NPs). The existence of monoclinic scheelite and tetragonal zircon phases of NPs was validated from Rietveld refinement of the powdered X-ray diffraction, room temperature Raman, and Fourier-transform infrared spectroscopy. The co-doping of Bi and V sites with respective Ce and Mo dopants in a mixed tetragonal zircon and monoclinic scheelite phases of BiVO4 lattice was corroborated from high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The photoluminescence measurements revealed enhancement of photo-generated carrier recombination in (Ce, Mo) co-doped BiVO4 NPs which may have hampered its photocatalytic efficiency in degrading the methylene blue dye. The simulations based on Hubbard U corrected density functional theory (DFT+U) suggest that Mo and Ce co-doping introduced deep impurity states which may have facilitated the photo-generated carrier recombination detrimental to photocatalytic performance. The UV-vis diffuse reflectance measurements provided evidence for the presence of these defect states. In summary, this work may have presented a comprehensive experimental analysis of (Ce, Mo) doped BiVO4 supported by DFT simulations.

Space charge region recombination in highly efficient silicon solar cells

The recombination rate in the space charge region (SCR) of a silicon-based barrier structure with a long Shockley–Reed–Hall lifetime is calculated theoretically by taking into account the concentration gradient of excess electron-hole pairs in the base region. Effects of the SCR lifetime and applied voltage on the structure ideality factor have been analyzed. The ideality factor is significantly reduced by the concentration gradient of electron-hole pairs. This mechanism provides an increase of the effective lifetime compared to the case when it is insignificant, which is realized at sufficiently low pair concentrations. The theoretical results have been shown to be in agreement with experimental data. A method of finding the experimental recombination rate in SCR in highly efficient silicon solar cells (SCs) has been proposed and implemented. It has been shown that at the high excess carrier concentration exceeding 1015 cm–3 the contribution to the SCR recombination velocity from the initial region of SCR that became neutral is significant. From a comparison of theory with experiment, the SCR lifetime and the ratio of the hole to the electron capture cross sections are determined for a number of silicon SCs. The effect of SCR recombination on the key characteristics of highly efficient silicon SCs, such as photoconversion efficiency and open-circuit voltage, has been evaluated. It has been shown that they depend not only on the charge carrier lifetime in SCR, but also on the ratio of hole to electron capture cross sections σp /σn. When σp /σn < 1, this effect is significantly strengthened, while in the opposite case σp /σn > 1 it is weakened. It has been ascertained that in a number of highly efficient silicon SCs, the distribution of the inverse lifetime in SCR is described by the Gaussian one. The effect described in the paper is also significant for silicon diodes with a thin base, p-i-n structures, and for silicon transistors with p-n junctions. In Appendix 2, the need to take into account the lifetime of non-radiative excitonic Auger recombination with participation of deep impurities in silicon is analyzed in detail. It has been shown, in particular, that considering it enables to reconcile the theoretical and experimental dependences for the effective lifetime in the silicon bulk.

Visible-Light-Induced photocatalytic oxidation of gaseous ammonia on Mo, c-codoped TiO2: Synthesis, performance and mechanism

Ammonia gas (NH3) is a notorious malodorous pollutant with detrimental effects on environment and human health. Removing NH3 through photocatalytic oxidation remains a challenging task due to unclear reaction mechanisms. In this study, DFT simulation results revealed that Cdoped TiO2 could extend the photo-response range to the visible region, but the introduction of deep impurity levels was found to accelerate photogenerated carrier recombination. Fortunately, the rational doping of Mo was found to mitigate this negative effect. Accordingly, Mo, C co-doped TiO2 was synthesized via a facile low-temperature calcination process, resulting in unique band structure that enhanced the photocatalytic oxidation of NH3. Notably, in-situ DRIFTS and DFT results demonstrated that the gradual reaction of NH3 adsorbed on Mo atoms with oxygen species through the N2H4 mechanism, promoting the formation of N2. These findings offer insights into the design of efficient photocatalysts for NH3 removal under visible light, and present a promising technology for indoor air pollution control.

Broadband-Spectral-Responsivity of black silicon photodetector with high gain and sub-bandgap sensitivity by titanium hyperdoping

Silicon, as one of the most essential semiconducting materials, is widely applied in various photodetection applications due to its high portability, convenient preparation, and compatibility with conventional complementary metal–oxide–semiconductor technology. Nowadays, black silicon, prepared by enforcing ultrafast laser micro-structuring and hyperdoping techniques, has received considerable attention for developing next-generation silicon photonics and silicon optoelectronics devices. In this work, high-comprehensive-property black silicon photodetectors with broadband spectral responsivity and high gain were achieved by femtosecond laser irradiation and titanium hyperdoping. The proposed device can respond to the incident light from 400 nm to at least 1550 nm, demonstrating a high responsivity of 40.59 A/W for 950 nm under the application of −5 V bias, and the corresponding detectivity can reach 3.61 × 1012 cm·Hz1/2W−1. Moreover, the fabricated photodetector exhibits a stable sub-bandgap photocurrent at 1550 nm, with an average responsivity of 3.42 mA/W by applying a −5 V bias. These improvements were achieved by the hyperdoping of titanium in silicon, which shows deep impurity levels, low electroactivity, and increased effective carrier lifetimes, as well as the formation of shallow n-i heterojunction and trapping centers induced by the femtosecond laser irradiation. Our work provides significant insights into the applications of black silicon in various optical-related fields, including weak and infrared photon detection, silicon photonic chips, remote sensing, and large-scale optoelectronic integration.

Effect of p-block metal doping on the optical properties of blue-phosphorene phase monolayer GeSe

Expanding the optical absorption of blue-phosphorene phase GeSe and designing it as infrared light driven photocatalysts and photodetectors is of particular significance. In this work, the mechanism of p-block metals (Al, Ga, In, An, Pb, Sb and Bi) doping enhancing the optical performance of monolayer blue-phosphorene phase GeSe was investigated. The effect of all p-block metal doping on the lattice constant of GeSe is very weak. Pb doped system has the smallest formation energy (−4.82 eV). Except for Sn and Pb, other p-block metal doping can introduce deep impurity levels into the band gap of monolayer GeSe, and they can significantly absorb infrared light, especially Ga doped systems. This is due to the emergence of impurity levels that assist the photon transition. The results of this work provide guidance for promoting the application of blue-phosphorene phase GeSe in the field of photocatalysts and photodetectors.

Anion optimization for bifunctional surface passivation in perovskite solar cells.

Pseudo-halide (PH) anion engineering has emerged as a surface passivation strategy of interest for perovskite-based optoelectronics; but until now, PH anions have led to insufficient defect passivation and thus to undesired deep impurity states. The size of the chemical space of PH anions (>106 molecules) has so far limited attempts to explore the full family of candidate molecules. We created a machine learning workflow to speed up the discovery process using full-density functional theory calculations for training the model. The physics-informed machine learning model allowed us to pinpoint promising molecules with a head group that prevents lattice distortion and anti-site defect formation, and a tail group optimized for strong attachment to the surface. We identified 15 potential bifunctional PH anions with the ability to passivate both donors and acceptors, and through experimentation, discovered that sodium thioglycolate was the most effective passivant. This strategy resulted in a power-conversion efficiency of 24.56% with a high open-circuit voltage of 1.19 volts (24.04% National Renewable Energy Lab-certified quasi-steady-state) in inverted perovskite solar cells. Encapsulated devices maintained 96% of their initial power-conversion energy during 900 hours of one-sun operation at the maximum power point.

Effect of ultrasound power on HCl leaching kinetics of impurity removal of aphanitic graphite

This study aimed to investigate the effect of ultrasonic power and temperature on the impurity removal rate during conventional and ultrasonic-assisted leaching of aphanitic graphite. The results showed that the ash removal rate increased gradually (∼50 %) with the increase in ultrasonic power and temperature but deteriorated at high power and temperature. The unreacted shrinkage core model was found to fit the experimental results better than other models. The Arrhenius equation was used to calculate the finger front factor and activation energy under different ultrasonic power conditions. The ultrasonic leaching process was significantly influenced by temperature, and the enhancement of the leaching reaction rate constant by ultrasound was mainly reflected in the increase of the pre-exponential factor A. Ultrasound treatment improved the efficiency of impurity mineral removal by destroying the inert layer formed on the graphite surface, promoting particle fragmentation, and generating oxidation radicals. The poor reactivity of hydrochloric acid with quartz and some silicate minerals is a bottleneck limiting the further improvement of impurity removal efficiency in ultrasound-assisted aphanitic graphite. Finally, the study suggests that introducing fluoride salts may be a promising method for deep impurity removal in the ultrasound-assisted hydrochloric acid leaching process of aphanitic graphite.

Enhanced thermoelectric performance of InSb through deep level impurity donor state induced by La doping

As one member of III-V semiconductors that have a variety of applications, InSb also shows great potential in thermoelectrics owing to its high mobility. In this work, the thermoelectric performance of n-type polycrystalline InSb is improved by La doping, which induces deep level impurity donor state and massive defects, synergistically optimizing both electrical and thermal transport properties. The peak power factor of La-doped InSb is enhanced from 42 to 58 μW cm−1 K−2 at 773 K due to the enlarged carrier concentration provided by the deep level donor state. Meanwhile, lattice thermal conductivity decreases from 3.2 to 1.7 W m−1 K−1 at 773 K due to In precipitates, point defects and local atomic disorder. Consequently, the peak zT of La-doped InSb is promoted to 0.85 at 773 K, which is among the best value in III-V compounds.

Introduction of deep level impurities, S, Se, and Zn, into Si wafers for high-temperature operation of a Si qubit

To realize high-temperature operation of Si qubits, deep impurity levels with large confinement energy, which are hardly thermally excited, were introduced into Si wafers. Group II impurity Zn and group VI impurities S and Se, which are known to form deep levels, were introduced into the Si substrates by ion implantation. These samples were analyzed for concentration-depth profiles, energy level depths, and absence of defects. To introduce deep impurities into thin channels such as 50 nm thick Si, we found impurity introduction conditions such that the concentration depth profiles had maximum values at less than 50 nm from the Si surface. Then, the formation of the deep levels and absence of defects were experimentally examined. Using the conditions to introduce deep impurities into the Si wafer obtained from the experiments, single-electron transport at room temperature, high-temperature operation of qubit, and room-temperature quantum magnetic sensors are promising.

Microplasma Breakdown in GaAs‐Based Avalanche S‐Diodes Doped with Deep Fe Acceptors

The article reports investigations into the microplasma breakdown in GaAs‐based avalanche S‐diodes doped with deep Fe acceptor impurities. The experiment shows the effect of current limitation in a reverse I–V curve with “soft” avalanche breakdown. It proposes 2D single microplasma models and calculates I–V curves of diodes with a deep impurity during microplasma breakdown. By comparing experimental and calculation data, authors propose an explanation for the effect of current limitation during avalanche breakdown. The effect is associated with capture of avalanche holes at negatively charged Fe centers, which enhances the depletion region and minimizes the maximum electric field in a reverse‐biased p–n junction of an S‐diode.

Classification of different post-hyperdoping treatments for enhanced crystallinity of IR-sensitive femtosecond-laser processed silicon

Crystalline silicon becomes photosensitive and absorbing in the sub-bandgap spectral region if hyperdoped, i.e. supersaturated to a level above the solubility limit in thermal equilibrium, by deep impurities, such as sulfur. Here we apply femtosecond laserpulses to crystalline silicon in a SF6 atmosphere as hyperdoping method. The ultrashort laser pulses cause crystal damage and amorphous phases that would decrease quantum efficiency in a potential optoelectronic device application. We investigate five different post-hyperdoping methods: three etching techniques (ion beam etching IBE, reactive ion etching RIE, and wet-chemical etching HNA) as well as ns-annealing and minute-long thermal annealing and study their impact on crystallinity by Raman spectroscopy and absorptance in the visible and near infrared wavelength regime. We use femtosecond laser hyperdoped silicon (fs-hSi) with two different levels of surface roughness to study a potential dependence on the impact of post-treatments. In our investigation, ns-annealing leads to the best results, characterized by a high Raman crystallinity and a high remaining absorptance in the sub-bandgap spectral region of silicon. Within the used etching methods IBE outperforms the other etching methods above a certain level of fs-hSi surface roughness. We relate this to the specific anisotropic material removal behavior of the IBE technique and back this up with simulations of the effect of the various etching processes.

An analytical model for organic bulk heterojunction solar cells based on Saha equation for exciton dissociation.

The power conversion efficiencies of organic solar cells (OSCs) have been greatly improved in recent years. However, latest experimental data of high efficiency OSCs, the sublinear relationship between the short circuit current density (Jsc) and light intensity (Pin), and the effects of energetic disorder in bulk heterojunction organic solar cells have not been understood. An analytical model for high-efficiency OSCs is proposed, which takes most physical factors into account that have been ignored in most previous models, including practical solar spectra and absorption spectra, degeneracy effect, exciton effect, space charge limited current, and unified mobility expression dependent on temperature, electric field and density, etc. Three analytical iterative methods are proposed to solve the strong non-linear Poisson equation and the drift-diffusion equations. The method for the drift-diffusion equations involves introducing two constant coefficients and determining their values self-consistently by demanding the space averages of approximate drift and diffusion currents equal to the averages of accurate ones. The theoretical results for five high-efficiency OSCs are in good agreement with experimental data, including current-voltage curves, light intensity-dependent Jsc and open-circuit voltage (Voc) curves. The effects of energetic disorder in bulk heterojunction organic solar cells, and the sublinear relationship Jsc ∝ Pαin (α < 1) can be well explained. The Saha equation for exciton dissociation and the space-charge-limited-current (SCLC) effect are important for modelling high-efficiency OSCs. The Voc ∼ Pin relationship can be influenced by many factors. But, the Jsc ∼ Pin relationship can be mainly and slightly influenced by the exciton effect and energetic disorder, respectively. When aiming to realize higher performance OSCs, one should decrease six material parameters, including the energetic disorder, exciton mass, deep level impurity concentration, the ratios of electron and hole mobilities, densities of states for electrons and holes, and potential barriers at the anode and cathode. The performance parameters of 15 triad compounds are predicted by using ab initio Eg and absorption spectra from the literature along with other input parameters taken from previous optimized values, and the efficiency of two compounds was found to exceed 35%.

Visualizing localized, radiative defects in GaAs solar cells

We have used a calibrated, wide-field hyperspectral imaging instrument to obtain absolute spectrally and spatially resolved photoluminescence images in high growth-rate, rear-junction GaAs solar cells from 300 to 77 K. At the site of some localized defects scattered throughout the active layer, we report a novel, double-peak luminescence emission with maximum peak energies corresponding to both the main band-to-band transition and a band-to-impurity optical transition below the band gap energy. Temperature-dependent imaging reveals that the evolution of the peak intensity and energy agrees well with a model of free-to-bound recombination with a deep impurity center, likely a gallium antisite defect. We also analyzed the temperature dependence of the band-to-band transition within the context of an analytical model of photoluminescence and discuss the agreement between the modeling results and external device parameters such as the open circuit voltage of the solar cells over this broad temperature range.

Modulating the valence of Ga and the deep level impurity for high thermoelectric performance of n-type Pb0.98Ga0.02Te1-xSex compounds

It has been found that the doped Ga has a distinct role in PbTe and PbSe compounds though they possess the same crystal structure. Therefore, to understand how the thermoelectric performance evolves in the Ga-doped PbTe1-xSex solid solution is crucial. Herein, we report that a maximum ZT value of 1.57 at 721 K and a high average ZT value of 1.13 in the temperature range of 300–843 K are attained for Pb0.98Ga0.02Te0.96Se0.04 compound. The experimental studies show that doping Ga into PbTe introduces a shallow level impurity state Ga3+ and a deep level associated with the Ga + state simultaneously. Interestingly, upon Se-alloying all of the Ga + are oxidized into Ga3+ at low temperatures, which increases the room-temperature carrier concentration from 1.16 × 1019 cm−3 to 2.17 × 1019 cm−3. This leads to an improved power factor of 3.04 mW m−1 K−2 for Pb0.98Ga0.02Te0.96Se0.04 sample at 482 K. Additionally, Se-alloying further enhances the point defect phonon scattering and lowers the lattice thermal conductivity to 0.46 W m−1 K−1 for Pb0.98Ga0.02Te0.96Se0.04 sample at 690 K. As a result, the ZT value is increased by about 112% compared with the maximum ZT of 0.74 for pristine PbTe. This work provides a new avenue for tuning the doping behavior of cation site dopant via alloying on anion site to further improve the ZT value in thermoelectric materials.

Inducing a deep impurity level in Li2SnO3 by ionic Bi–O bonding to enhance light absorption in photocatalysis

Orbital engineering is an important strategy for modulating light absorption in photocatalysis. Here, Bi doping of the oxide photocatalyst Li2SnO3 to enhance light absorption was rationally designed by orbital engineering. Based on density functional theory, owing to the lower Bi 6s energy level compared with that of Sn 5s, a deep impurity energy level induced by ionic Bi–O bonds is generated in the middle band gap of Li2SnO3. The impurity energy level can facilitate the utilization efficiency of light absorption, leading to remarkably enhanced photocatalytic performance. For Li2Sn0.9Bi0.1O3, the photodegradation rate of tetracycline solution reached 76% within 30 min, which was approximately 2.6 times higher than that for Li2SnO3. A radical trapping experiment revealed that the holes (h+) play a dominant role in the elimination reaction. Finally, liquid chromatography–mass spectrometry was performed to monitor the photocatalytic process. This study lays a foundation for rational design of photocatalysts with excellent light absorption for photocatalysis.

Structure, optical absorption and photochromic effect in Rb0.95Nb1.375Mo0.625O5.79

The compound with β-pyrochlore structure with rare for structure type symmetry (orthorhombic, Pnma) has been prepared recently. In polycrystalline sample Rb0.95Nb1.375Mo0.625O5.79 with beta-pyrochlore structure type prepared by solid-state reaction, the diffuse reflection and transmission spectra have been studied. The spectral dependence of the photochromic effect and the temperature dependence of its kinetics have been investigated. It was found that Rb0.95Nb1.375Mo0.625O5.79 is an indirect-gap semiconductor with band gap of 3 ​eV. A study of the transmission spectrum and a detailed investigation of the photochromic effect made it possible to determine some characteristics of the deep impurity defect level and make a conclusion about the energy ranges in which the Fermi level can be localized in a band gap.

Characterization of Schottky-diode X/[formula omitted]-ray detectors based on CdTe crystals with different uncompensated impurity concentrations

The Schottky-diode X/ γ-ray detectors based on semi-insulating p-like CdTe single crystals with four values of uncompensated impurity concentration (N≈ 2 × 1012 cm−3, 2 × 1011 cm−3, 1 × 1011 cm−3, 4 × 1010 cm−3) were developed and characterized by I-V measurements at low reverse bias (0.005–100 V) and detection of 137Cs and 241Am radioisotope emission spectra. All the detectors were fabricated by the same technique using vacuum evaporation of a Cr electrode (rectifying contact) and chemical deposition of an Au electrode (Ohmic contact) onto the B- and A-face of CdTe(111) crystals, respectively, pre-treated with Ar-ion bombardment. The change of the charge transport mechanism from over-barrier (thermionic) to generation current along with the analysis of the temperature dependence of the differential resistivity evidenced a high concentration of deep level impurities near the middle of the semiconductor bandgap. The detection efficiency η of the detectors was calculated using the corrected expressions for the coordinate dependence of the electrical field strength in a diode which took into account the fact that the depletion region width exceeded the CdTe crystal thickness at higher bias voltages. It was shown that the dependence η(N) decreased with increasing N without a maximum corresponding to an optimal value Nopt as reported earlier. The Cr/CdTe/Au Schottky-diode detectors, fabricated using CdTe with the lowest N, demonstrated the highest energy resolution (0.6% @662 keV and 7.5% @59.5 keV) and it degraded when crystals with higher N were used.

Electrical and Optical Doping of Silicon by Pulsed-Laser Melting

Over four decades ago, pulsed-laser melting, or pulsed-laser annealing as it was termed at that time, was the subject of intense study as a potential advance in silicon device processing. In particular, it was found that nanosecond laser melting of the near-surface of silicon and subsequent liquid phase epitaxy could not only very effectively remove lattice disorder following ion implantation, but could achieve dopant electrical activities exceeding equilibrium solubility limits. However, when it was realised that solid phase annealing at longer time scales could achieve similar results, interest in pulsed-laser melting waned for over two decades as a processing method for silicon devices. With the emergence of flat panel displays in the 1990s, pulsed-laser melting was found to offer an attractive solution for large area crystallisation of amorphous silicon and dopant activation. This method gave improved thin film transistors used in the panel backplane to define the pixelation of displays. For this application, ultra-rapid pulsed laser melting remains the crystallisation method of choice since the heating is confined to the silicon thin film and the underlying glass or plastic substrates are protected from thermal degradation. This article will be organised chronologically, but treatment naturally divides into the two main topics: (1) an electrical doping research focus up until around 2000, and (2) optical doping as the research focus after that time. In the first part of this article, the early pulsed-laser annealing studies for electrical doping of silicon are reviewed, followed by the more recent use of pulsed-lasers for flat panel display fabrication. In terms of the second topic of this review, optical doping of silicon for efficient infrared light detection, this process requires deep level impurities to be introduced into the silicon lattice at high concentrations to form an intermediate band within the silicon bandgap. The chalcogen elements and then transition metals were investigated from the early 2000s since they can provide the required deep levels in silicon. However, their low solid solubilities necessitated ultra-rapid pulsed-laser melting to achieve supersaturation in silicon many orders of magnitude beyond the equilibrium solid solubility. Although infrared light absorption has been demonstrated using this approach, significant challenges were encountered in attempting to achieve efficient optical doping in such cases, or hyperdoping as it has been termed. Issues that limit this approach include: lateral and surface impurity segregation during solidification from the melt, leading to defective filaments throughout the doped layer; and poor efficiency of collection of photo-induced carriers necessary for the fabrication of photodetectors. The history and current status of optical hyperdoping of silicon with deep level impurities is reviewed in the second part of this article.