Acceptor Transition Research Articles

Exploring hybrid dihydrogen phosphate systems: Experimental and theoretical investigation

The investigation into the crystal structure of 1-(carboxymethyl)cyclohexyl methanaminium dihydrogenphosphate endeavors to unravel the intricate molecular interactions at play. Utilizing Hirshfeld surface analysis, we delineate the specific types of interactions within the molecules. Fingerprint plots and molecular surface contours (dnorm, di, and de) vividly illustrate the proportional contributions of H…H (53.8%), O…H (42.9%), and O…O (2.2%). Further characterization through FTIR and UV–Vis spectroscopy enhances our comprehension.In contrast, the molecular arrangement’s reinforcement stems from the NH… O interaction, leading to a linear chain. Through FTIR and FT-Raman experiments, we discern functional groups, contrasting experimental results with theoretical values. Mulliken atomic charges are meticulously examined, indicating reduced reactivity and heightened stability across diverse environments.Exploration of energy gaps in HOMO-LUMO and Density of States (DOS) spectra reveals coherence, supported by Natural Bond Orbital (NBO) Analysis, accentuating the equilibrium in energy during donor and acceptor transitions. This holistic approach enriches our understanding of the structural intricacies and stability factors inherent in the examined compound.

Structural, Optical, and Electronic Properties of Epitaxial β‐(AlxGa1‐x)2O3 Films for Optoelectronic Devices

AbstractBandgap engineering in monoclinic gallium oxide (β‐Ga2O3) is a powerful strategy for designing semiconductor optoelectronic devices with specific functionalities. In this work, aluminum doping is utilized to modulate the bandgap of Ga2O3. By growing epitaxial β‐(AlxGa1‐x)2O3 (0≤ x≤ 0.84) films on c‐plane sapphire substrates using RF magnetron sputtering, it allowed to tune the energy bandgap, achieving values as high as 6.10 eV. The increased luminescence intensity is attributed to the recombination between donor and acceptor transitions induced by Al doping, resulting in more defects. Additionally, the luminescent band experienced blueshifts due to the enhanced bandgaps. Moreover, density of functional theory (DFT) simulations confirmed the sensitivity of the bandgap to Al content, distinguishing between Ga‐dominated (x < 0.5) and Al‐dominated (x > 0.5) β‐(AlxGa1‐x)2O3. Notably, the bandgap increased more rapidly in Ga‐dominated structures compared to Al‐dominated ones. The electronic structure analysis revealed a redistribution of Ga d states from valence to conduction bands, attributed to the introduction of numerous Al p states. These combined experimental and detailed electronic structure investigations proved crucial insights for designing the structure and exploring potential applications of β‐(AlxGa1‐x)2O3 in photonic devices.

Investigation of carbon-related complexes in highly C-doped GaN grown by metalorganic vapor phase epitaxy

We investigated the C-related complexes in highly C-doped GaN by electron spin resonance (ESR) spectroscopy, Fourier transform IR spectroscopy (FTIR), and minority carrier transient spectroscopy (MCTS) measurements. In the ESR spectra, two resonances with g values of 2.02 and 2.04 were found to be assigned by (0/−) deep acceptor and (+/0) charge transition levels of carbon substituting for nitrogen site (CN). In the FTIR spectra, two local vibrational modes positioned at 1679 and 1718 cm−1 were confirmed to be associated with tri-carbon complexes of CN–CGa–CN (basal) and CN–CGa–CN (axial), respectively. In the MCTS spectra, we observed the hole trap level of E v + 0.25 ± 0.1 eV associated with the tri-carbon complexes, which are the dominant C-related defects, suggesting that these complexes affect the electronic properties in the highly C-doped GaN.

Influence of the Fluorophore Mobility on Distance Measurements by Gas-Phase FRET.

Gas-phase Förster resonance energy transfer (FRET) combines mass spectrometry and fluorescence spectroscopy for the conformational analysis of mass-selected biomolecular ions. In FRET, fluorophore pairs are typically covalently attached to a biomolecule using short linkers, which affect the mobility of the dye and the relative orientation of the transition dipole moments of the donor and acceptor. Intramolecular interactions may further influence the range of motion. Yet, little is known about this factor, despite the importance of intramolecular interactions in the absence of a solvent. In this study, we applied transition metal ion FRET (tmFRET) to probe the mobility of a single chromophore pair (Rhodamine 110 and Cu2+) as a function of linker lengths to assess the relevance of intramolecular interactions. Increasing FRET efficiencies were observed with increasing linker length, ranging from 5% (2 atoms) to 28% (13 atoms). To rationalize this trend, we profiled the conformational landscape of each model system using molecular dynamics (MD) simulations. We captured intramolecular interactions that promote a population shift toward smaller donor-acceptor separation for longer linker lengths and induce a significant increase in the acceptor's transition dipole moment. The presented methodology is a first step toward the explicit consideration of a fluorophore's range of motion in the interpretation of gas-phase FRET experiments.

A Combination of Ion Implantation and High‐Temperature Annealing: Donor–Acceptor Pairs in Carbon‐Implanted AlN

Herein, carbon‐implanted high‐temperature annealed (HTA) AlN layers are analyzed and donor–acceptor pair (DAP) transitions probably between the two most abundant impurities, carbon and oxygen, are identified. Both are regarded as the main, hard‐to‐avoid impurities in crystal growth. Oxygen is believed to lead to absorption in the deep UV below a wavelength of 250 nm. In contrast, carbon is the most likely candidate to be responsible for a distinct absorption band around 265 nm. This interpretation has recently been challenged. In this study, carbon‐implanted and HTA AlN layers with ion fluences above 8.1 × 1015 cm−2 are analyzed using low‐temperature and time‐resolved cathodoluminescence spectroscopy. Due to the high concentration of oxygen inside the AlN, as a result of the HTA process, a DAP transition between a most likely carbon‐related acceptor and ON is observed. The measured temperature‐ and power‐dependent blueshift of the peak emission energy as well as the luminescence transients can be clearly explained by a continuous change from a DAP transition at low temperature to a free electron to acceptor transition with increasing temperature. The findings are supported by a configurational coordinate model that describes the measured behavior qualitatively.

Coherent coupling of metamaterial resonators with dipole transitions of boron acceptors in Si.

We investigate the coherent coupling of metamaterial resonators with hydrogen-like boron acceptors in Si at cryogenic temperatures. When the resonance frequency of the metamaterial, chosen to be in the range 7-9 THz, superimposes the transition frequency from the ground state of the acceptor to an excited state, Rabi splitting as large as 0.4 THz is observed. The coherent coupling shows a feature of cooperative interaction, where the Rabi splitting is proportional to the square root of the density of the acceptors. Our experiments may help to open a possible route for the investigation of quantum information processes employing strong coupling of dopants in cavities.

Si−doped In0.145Ga0.855As0.123Sb0.877: A novel p−type quaternary alloy with high crystalline quality

Antimonide−based p−n junctions are particularly attractive for a wide variety of optoelectronic applications in the near and mid-infrared wavelength range. In this work, novel p−type Si−doped In0.145Ga0.855As0.123Sb0.877 epitaxial layers were grown on GaSb(100) substrates by Liquid Phase Epitaxy (LPE) technique. The XPS spectra measured on Si−doped In0.145Ga0.855As0.123Sb0.877 layers indicate that Si atoms behave as acceptors. The analysis of the surface depletion region and its relationship with phonon−plasmon L_ coupling were investigated using Raman spectroscopy, giving an acceptor concentration of NA ∼ 5.8 × 1017 cm−3 for a depletion region thickness of d ∼ 9.4 nm. Low temperature photoluminiscence (PL) spectrum for the Si−doped In0.145Ga0.855As0.123Sb0.877 layers showed a bound excitonic emission peak associated to neutral Sb acceptors, with an activation energy of ∼10meV and a donor−acceptor transition. It was observed that the Si doping reduces the excitonic emission intensity and increases the donor−acceptor pair recombination intensity.

Intracenter dipole transitions of a hydrogen-like boron acceptor in diamond: Oscillator strengths and line broadening

Substitutional boron in diamond acts as an acceptor center with the ionization energy of about 372 meV. Unlike its analogues in elemental semiconductors (silicon, germanium), boron bound states are poorly described by the electronic mass approximation due to the large ionization energy and the small spin-orbit splitting of the valence band in diamond. Thereby experimental investigations appear to be a main way of study of discrete electronic states of boron in diamond.In this paper, we report infrared (IR) absorption spectroscopy results obtained for HPHT-grown single crystal diamonds doped with natural boron (80% 11B – 20% 10B), and with isotopically enriched boron (99% 11B – 1% 10B). A moderate dopant concentration (1016–1017 cm−3), low concentration of lattice defects and vanishing thermal impact at cryogenic temperatures provided significantly reduced line broadening so that more than 60 boron transitions could be spectrally resolved. Mathematical approximation of the spectral lines revealed the main empiric features of zero-phonon dipole intracenter transitions: their energy, linewidth and integrated absorption. This allowed us to calculate oscillator strength of the boron acceptor transitions in diamond. The Lorentzian shape of the spectral lines indicates dominating uniform broadening of the transitions. Together with the relatively broad linewidths (0.1–0.5 meV) it reveals strong electron-phonon interaction in boron-doped diamond.

First-Principles Study of Intrinsic Point Defects of Monolayer GeSSupported by the National Natural Science Foundation of China (Grant Nos. 61922077, 11804333, 11704114, 11874347, 61121491, 61427901, 11634003, and U1930402), the National Key Research and Development Program of China (Grant Nos. 2016YFB0700700 and 2018YFB2200100), the Science Challenge Project (Grant No. TZ2016003), and the Youth Innovation Promotion Association of Chinese Academy of Sciences

The properties of six kinds of intrinsic point defects in monolayer GeS are systematically investigated using the “transfer to real state” model, based on density functional theory. We find that Ge vacancy is the dominant intrinsic acceptor defect, due to its shallow acceptor transition energy level and lowest formation energy, which is primarily responsible for the intrinsic p-type conductivity of monolayer GeS, and effectively explains the native p-type conductivity of GeS observed in experiment. The shallow acceptor transition level derives from the local structural distortion induced by Coulomb repulsion between the charged vacancy center and its surrounding anions. Furthermore, with respect to growth conditions, Ge vacancies will be compensated by fewer n-type intrinsic defects under Ge-poor growth conditions. Our results have established the physical origin of the intrinsic p-type conductivity in monolayer GeS, as well as expanding the understanding of defect properties in low-dimensional semiconductor materials.

Study on shallow donor-type impurities of GaN epilayer regrown by epitaxial lateral overgrowth technique

p-doped gallium nitride (GaN) regrowth by epitaxial lateral overgrowth using a SiO2 mask is studied. A comparison between SiO2 and Al2O3 masked p-GaN by cathodoluminescence spectroscopy and scanning electron microscopy indicates that donor-type impurities are related to the SiO2 mask. A domain peak of 3.25 eV induced by shallow-donor and acceptor transitions and the dark contrast of obtuse triangles have been detected in SiO2 masked p-type GaN. Secondary ion mass spectroscopy is simultaneously employed for the analysis of SiO2 and Al2O3 masked p-GaN and identifies that the source of donor-type impurities is from Si atoms. Furthermore, the experimental results of cross-sectional microstructures at different regrowth times have been investigated. It is found that the donor-type impurities tend to cluster in semi-polar 112¯2 facets before the coalescence at the bottom of adjacent triangular stripes starts. The explanation for the non-uniform distribution of impurities is that semi-polar 112¯2 facets exhibit more dangling bond densities than the (0001) plane, and the SiO2 mask exposed to the vapor phase would likely introduce more impurities before the coalescence of GaN stripes.

Property manipulation through pulsed laser annealing in high dose Mg-implanted GaN

The generation of p-type GaN through ion implantation is an attractive proposition in the massive production of GaN-based bipolar devices, whereas the removal of implantation induced lattice disturbances and defects is a difficult exercise and hampers the conversion of conductivity in GaN. Pulsed laser annealing is an effective annealing technique to recover lattice crystallinity and activate dopants with the preserved implanted profile. In this work, the effect of pulsed laser annealing on structural and optical recovery in high-dose magnesium (Mg) ion-implanted GaN has been investigated. The structural evolution and vibrational dynamics indicate an obvious structural recovery and partial strain release of Mg-implanted GaN during the pulsed laser annealing process, with a threshold laser fluence of 400 mJ/cm2, while rough surface structures are a result of the regrowth mechanism similar to liquid phase epitaxy. The enhanced donor–acceptor transition at 3.35 eV after pulsed laser irradiation is a sign of the effective activation of Mg from interstitial sites into the substitution of Ga ions. These results suggest that further optimization of the laser annealing technique has promising potential to manipulate the p-type conductivity of Mg-implanted GaN and to be implemented in GaN bipolar devices for practical applications.

Luminescence line shapes of band to deep centre and donor–acceptor transitions in AlN

Energy structure and electron coupling with local lattice vibrations have been investigated for deep centres in AlN using hybrid functional density functional theory. Local phonon energies and Huang–Rhys parameters have been calculated for defects and defect complexes containing most common unintentional impurities of carbon, oxygen and silicon, and for intrinsic vacancies, nitrogen split-interstitial defect, and complexes of Al and N vacancies in AlN. Luminescence line shapes of band to deep centre transitions in AlN have been calculated in dependence on temperature for most abundant defects in AlN. Donor–acceptor luminescence line shapes for shallow donor to deep acceptor and deep donor to deep acceptor transitions have been considered theoretically. Configuration diagrams of oxygen and silicon DX centres have been calculated by density functional theory with hybrid functional, and peak energies of optical transitions of an electron from the DX-centres to deep acceptors have been estimated. Possible assignments of the experimental luminescence bands in AlN based on the calculations have been discussed.

Doubled Thermoelectric Figure of Merit in p-Type β-FeSi2 via Synergistically Optimizing Electrical and Thermal Transports.

β-FeSi2 has long been investigated as a promising thermoelectric (TE) material working at high temperatures due to its combining features of environmental friendliness, good thermal stability, and strong oxidation resistance. However, the real application of β-FeSi2 is still limited by its low TE figure of merit (zT). In this study, nearly doubled zT in p-type β-FeSi2 has been achieved via synergistically optimizing electrical and thermal transports. Based on the first-principles calculations, Al with shallow acceptor transition level and high carrier donation efficiency is chosen to dope β-FeSi2. Significantly improved electrical transport, particularly in the low temperature range, has been obtained in the Al-doped β-FeSi2 system. The power factor for FeSi1.96Al0.04 at 300 K is even higher than that of p-type β-FeSi2-based compounds reported previously at high temperatures. By alloying β-FeSi2 with Os at the Fe sites, we further lower the lattice thermal conductivity. Fe0.80Os0.20Si1.96Al0.04 possesses the lowest lattice thermal conductivity among the β-FeSi2 compounds prepared by the equilibrium method. Finally, a record-high zT value of 0.35 is obtained for p-type Fe0.80Os0.20Si1.96Al0.04. This study is expected to accelerate the application of β-FeSi2.

Modulating donor-acceptor transition energies in phosphorus-boron co-doped silicon nanocrystals via X- and L-type ligands.

In this work, we explore the effect of ligand binding groups on the visible and NIR photoluminescent properties within phosphorus-boron co-doped silicon nanocrystals (PB:Si NCs) by exploiting both the X-type (covalent) and L-type (Lewis donor molecule) bonding interactions. We find that the cooperative nature of both X- and L-type bonding from alkoxide/alcohol, alkylamide/alkylamine, and alkylthiolate/alkylthiol on PB:Si NCs results in photoluminescence (PL) energy blue shifts from the as-synthesized, hydride-terminated NCs (PB:Si-H) in excess of 0.4 eV, depending on the surface termination. These PL blue shifts appear greatest in the most strongly confined samples with diameters <4 nm where the surface-to-volume ratio is high and, therefore, the ligand effects are most pronounced. A correlation between the donor group strength (either X-type or L-type) and the degree of D-A state modulation is found, and the proportion of the PL blue shift from the X- and L-type interactions is quantified. Raman spectroscopy is used to provide additional evidence of the strength of the L-type donor groups. Additionally, we probe how the nature of the ligand chemistry affects the radiative lifetime and PL efficiency and find that the ligands do not significantly change the D-A emission dynamics, and all samples retain the long 50-130 μs lifetimes characteristic of these transitions. Finally, we describe three mechanisms that operate to affect the D-A recombination energies: (1) X-type ligands that modulate the PB:Si-X NC wavefunction; (2) L-type ligands that perturb the donor and acceptor states via a molecular orbital theory picture; and (3) X- and L-type ligands that cause a dielectric increase around the PB:Si NC core, which provides Coulomb screening and modulates the donor and acceptor states even further.

Photoluminescence Line‐Shape Analysis of Highly n‐Type Doped Zincblende GaN

An investigation of different n‐type doped zincblende gallium nitride thin films measured by photoluminescence from 7 K to room temperature is presented. The spectra change with increasing free‐carrier concentration due to many‐body effects such as the Burstein–Moss shift and band‐gap renormalization. The samples are grown by molecular beam epitaxy on a 3C‐SiC/Si (001) substrate, and a free‐carrier concentration above 1020 cm−3 is achieved by introducing germanium as a donor. The analysis of the measured spectra by a line‐shape fit yields different transition processes for different doping concentrations and temperatures, such as a band–band transition and a band–acceptor transition. The conduction band dispersion of Kane's model is perfectly suited to explain the experimental data quantitatively.

Observation of photoluminescence edge emission in CuSbSe2 absorber material for photovoltaic applications

CuSbSe2 has great potential for being an earth-abundant absorber material in efficient thin film solar cells. In this study, the radiative recombination in CuSbSe2 polycrystalline material was studied by temperature and laser power dependent photoluminescence spectroscopy which revealed the band-to-band and band-to-deep acceptor transitions, with the last one involving a deep acceptor defect with an activation energy of 165 ± 6 meV. The observation of the edge emission in CuSbSe2 refers to the high quality of the synthesized material.

Donor–acceptor pair emission via defects with strong electron–phonon coupling in heavily doped AlxGa1−xN:Si layers with Al content x > 0.5

We report the results of time-resolved and temperature-dependent stationary photoluminescence investigations of the defects responsible for emission in the visible spectral range in heavily silicon-doped AlxGa1–xN layers grown by molecular beam epitaxy on sapphire substrates. The emission band was attributed to donor–acceptor transitions. The transitions were described using the one-dimensional configuration coordinate model taking into account the high-doping regime. An increase in Al content from 0.56 to 1 leads to an increase in the acceptor ionization energy from 1.4 to 1.87 eV. The value of the Franck–Condon shift is about 1 eV at x = 0.56–0.74 and decreases to 0.8 eV at x > 0.74. The changes in the donor–acceptor transition energy parameters with increasing silicon concentration are discussed.

Nonthermal Plasma-Synthesized Phosphorus–Boron co-Doped Si Nanocrystals: A New Approach to Nontoxic NIR-Emitters

We report on the successful creation of nonthermal plasma-synthesized phorphorus and boron co-doped Si nanocrystals (PB:Si NCs) with diameters (DNC) ranging from 2.9 to 7.3 nm. Peak photoluminescence (PL) emission energies for all PB:Si NC diameters are ca. 400–500 meV lower than the excitonic emission values in intrinsic Si NCs, which can be attributed to prevalent donor–acceptor (D–A) transitions within the co-doped system. This D–A transition model is further evidenced by PL lifetimes within the range of 40–80 μs, faster than what is observed for intrinsic Si NCs. By reducing the level of confinement within PB:Si NCs (i.e., DNC > 4 nm), we are able to red-shift the near-infrared (NIR)-emitting D–A transitions to below the band gap of bulk Si (1.12 eV). We quantify the PL quantum yield (PLQY) for a range of DNC and show that the plasma method can achieve reasonably high PLQY values (12% for DNC = 2.9 nm), even without any optimization of the synthesis or surface chemistry. We posit that perfect charge c...

Deep level acceptors of Zn-Mg divalent ions dopants in β-Ga2O3 for the difficulty to p-type conductivity

β-Ga2O3 exhibits huge potential applications in next-generation power electronics with its wide band gap, large breakdown field and high Baligas's figure of merit. The achievement of p-type conductive β-Ga2O3 faces hugely challenge due to the self-compensating process, while that is critically important to further extend its utility and applications. Although the acceptor deep levels of divalent ions dopants can explain the difficulty of p-type Ga2O3 theoretically, while the experimental evidence is lacked. Herein, Zn and Zn-Mg co-doped β-Ga2O3 thin films were prepared to investigate the acceptor levels of ZnGa and MgGa. The XPS results indicate that both Zn and Mg exhibit divalent ions when they replaced trivalent Ga ion. The doping concentration and band gap can modulate by solely changing the numbers of Zn and Mg pieces during the deposition process. The levels of ZnGa and MgGa are located at 0.79 eV and 1.00 eV respectively speculated by photoluminescence, such deep acceptor transition levels proved that it is difficult to obtain highly conductive p-type Ga2O3 by doping with divalent ions of Zn and Mg.

Different natures of sub-gap states in 135-CIGS/112-CIGS and 112-CIGS/135-CIGS heterostructures investigated by photoluminescence technique

CuIn1-xGaxSe2 (112-CIGS) has been proposed as a material for high efficiency thin film solar cells with reliably applicable photovoltaic properties. Higher efficiency devices can be achieved by various surface treatments and are believed to have Cu deficiency at the surface. One of the possible techniques is to have a very thin Cu-depleted surface, e.g. Cu(In1-xGax)3Se5 (135-CIGS) layer on top of 112-CIGS layer, thus the heterostructure of 135-CIGS/112-CIGS is formed. Our previous work show that the photoluminescence (PL) spectra of 135-CIGS with various thicknesses on 112-CIGS (135-CIGS/112-CIGS) heterostructure are identified as donor-to-acceptor pairs (DAPs) and free (conduction band) -to-bound (acceptor) transitions and show the temperature and excitation power dependence on the PL spectra. On the contrary, when the thin 112-CIGS layer is deposited on top of 135-CIGS (112-CIGS/135-CIGS), the PL spectra show more pronounced and resolved peaks which are surprisingly independent of temperature and excitation power and significantly distinguishable in its natures observed in those broad emissions of 135-CIGS/112-CIGS heterostructures. When the thickness of 112-CIGS on 135-CIGS is increased, the red shift in the order of ten meV is noticed in the emission lines from both 112-CIGS and 135-CIGS layers. These results suggest that the strains play more important roles in thinner capping layer and freeze out the temperature and excitation power dependence in the 112-CIGS/135-CIGS rather than the 135-CIGS/112-CIGS heterostructures.