Donor-acceptor Pair Emission Research Articles

Chemical structure and optical signatures of nitrogen acceptors in MgZnO

Nitrogen can be incorporated into MgZnO using low-temperature deposition. Donor–acceptor pair emission from N-doped MgZnO is attributed to molecular N2.

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.

Cathodoluminescene study of Mg implanted GaN: the impact of dislocation on Mg diffusion

Magnesium (Mg) ion implanted homoepitaxial GaN layers is investigated by cathodoluminescence (CL) and secondary ion mass spectrometry (SIMS). The impact of dislocations on Mg diffusion is clarified by CL monitoring the Mg-related donor-acceptor pair (DAP) emission on novel angle cutting specimen. CL results suggest that: (1) there exist high concentration of nonradiative defects in a Mg implanted layer; and (2) Mg shows pipe diffusion along threading dislocations throughout epilayer to substrate. To achieve successful Mg doping by ion implantation, it is necessary to suppress the formation of a dead region in the Mg implanted layer and the pipe diffusion along threading dislocations.

Correlation of donor-acceptor pair emission on the performance of GaN-based UV photodetector

High responsivity UV photodetector has been realized by fabricating single crystalline epitaxial GaN-based devices grown on silicon substrate by using molecular beam epitaxy system. The influence of trap states existing within the forbidden gap on GaN-based optoelectronic devices has been investigated. The quality and performance of the fabricated GaN based UV photodetector with distinct AlN buffer layer grown at low and high substrate temperature are substantiated. A detailed analysis reveals that high temperature buffer can yield improved crystalline quality of GaN with similar morphological properties as compared to buffer layer grown at low temperature. However, the distinct buffer layer influence the optical properties as the photoluminescence analysis explains that both the films possess superior band-to-band edge emission where GaN grown with high temperature AlN buffer consists of dominant acceptor defect states in comparison to GaN with low temperature AlN buffer layer. The existence of acceptor states has been accredited by the presence of Ga vacancy related states in the band gap. The fabricated devices yield high photoresponsivity of 2.1 and 1.5 A/W at 1 V from GaN films bearing low and high accepter defect states which explain a clear correlation of device performance with trap states within the band-gap region.

Time-resolved photoluminescence spectral analysis of phonon-assisted DAP and e-A recombination in N+B-doped n-type 4H-SiC epilayers

It is crucial to clarify the roles of phonon-assisted donor-acceptor pairs (DAPs) and free-to-acceptor (e-A) emissions in n-type 4H-SiC doped with nitrogen (N) and boron (B), where N and B induce the shallow donor and the D center (deep B) acceptor levels, respectively, in order to understand the complicated carrier recombination mechanism, as well as developing fluorescent SiC with a high color rendering index by controlling the ratio of the two overlapped emissions. Here, time-resolved photoluminescence (TRPL) spectral analyses were performed, in which phonon-assisted DAPs and e-A components were individually recognized. The D center-related green luminescence (1.6–2.8 eV) shows a non-exponential decay followed by a very slow decay in TRPL measurements at room temperature (RT). It emerges that most of the DAP emission intensities decay much faster than e-A emissions and contribute to what is initially fast and non-exponential decay at low temperatures, while the slow decay at RT is mainly from e-A emissions. At a much higher temperature, such as 473 K, only the e-A emission remains and the decay transforms from non-exponential to exponential behavior. High-temperature thermal quenching of e-A emissions exhibits different behaviors for samples with differing B doping concentrations. An activation energy of 0.6 eV was estimated from the Arrhenius plot of e-A emission intensity from a B-doped sample, which matches the D center level. This indicates that the hole thermal emission rate is greatly enhanced at a high temperature, which accelerates the decay of e-A emission intensity at high temperatures.

Growth of CaAl2Se4: Co Single Crystal Thin Film for Solar Cell Development and Its Solar Cell Application

After the CuInSe₂films grown by hot-wall epitaxy method were annealed in Cu-, Se- and In-atmospheres, the point defects were investigated by using the photoluminescence (PL) experiment. In the as-grown CuInSe₂films, the free excitons corresponding to the light hole and the heavy hole have been observed, and their splitting gap was 8.2 meV. The gap is associated with the strain caused by the lattice mismatch between the substrate and the film in the heterojunction growth. By means of thermal annealing in various atmospheres, the (D˚, X) emission was observed to originate from the Vse or CUint. From this emission, the ED value of the donor impurity level is extracted to 0.1950 eV. Also, the emission between the free electrons and the acceptor holes (FA) became a dominant peak after Se atmosphere treatment, and the EA value of the acceptor level is turned out to be 0.2371 eV. Thus, the origin of the FA emission is related to Vcu or Seint. These PL results led us to confirm that CuInSe₂:Se is converted into the optical n-type. In addition, the origin of the donor-acceptor pair emissions is caused by the recombination between donors such as Vse or CUint and acceptors such as Vcu or Seint. Based on these PL results, we have schemed a new energy-level diagram of the recombination process in CuInSe₂.

Eu–Mg defects and donor–acceptor pairs in GaN: photodissociation and the excitation transfer problem

We have investigated the temperature-dependent photoluminescence (TDPL) profiles of Eu3+ ions implanted in an HVPE-grown bulk GaN sample doped with Mg and of donor–acceptor pairs (DAP) involving the shallow Mg acceptor in GaN(Mg) (unimplanted) and GaN(Mg):Eu samples. Below 125 K, the TDPL of Eu3+ in GaN(Mg):Eu correlates with that of the DAP. Below 75 K, the intensity of Eu3+ emission saturates, indicating a limitation to the numbers of Eu–Mg defects available to receive excitation transferred from the host, while the DAP continues to increase, albeit more slowly in the implanted than the unimplanted sample. Prolonged exposure to UV light at low temperature results in the photodissociation of Eu–Mg defects in their Eu1(Mg) configuration, with a corresponding increase in shallow DAP emission and the emergence of emission from unassociated EuGa (Eu2) defects.

Photoluminescence and X-ray luminescence of Pb0.30Cd0.70I2 solid solutions. Comparative study

The nature and relative contributions (RCs) of various radiative recombination processes to the photoluminescence (PL) spectra for bulk nanostructured Pb0.30Cd0.70I2 solid solutions have been established at different temperatures. The analysis indicates that the PL is caused by free, bound and self-trapped excitons as well as by donor-acceptor pairs emission with the participation of shallow and deep acceptor centers. It was shown that X-ray luminescence (XRL) spectra are also determined by these recombination processes. However, their RC and the temperature evolution are considerably different. Besides, XRL spectra contain an intense long-wavelength band associated with the emission of many LO-phonons. It was shown that the deep luminescence surface states, associated with the self-trapped excitons and deep intrinsic defects, mainly determine the intensity of XRL spectra both at 80K and room temperature. The results obtained open the way to the optimization of the scintillator properties of the investigated materials.

Photoluminescence Characterization of Carrier Recombination Centers in 4H-SiC Substrates by Utilizing below Gap Excitation

Though the crystal growth technology of SiC is improving steadily, it is still crucial to reduce crystalline defects which act as carrier recombination (CR) centers and deteriorate device performance. We detected CR centers in a p-type 4H-SiC substrate by observing the intensity change of photoluminescence due to the addition of a below-gap excitation (BGE) light of 0.93[eV]. We noticed the temperature and the BGE density dependence of band edge (BE) emission in addition to donor acceptor pair (DAP) emission and discriminated the temperature effect from that of BGE. The BGE density dependence of the PL intensity quenching is different among the BE emission, B0- and C0-lines of the DAP, respectively. It gives us an important clue for understanding CR mechanisms inside the bandgap of SiC.

Optical study of the band structure of wurtzite GaP nanowires

We investigated the optical properties of wurtzite (WZ) GaP nanowires by performing photoluminescence (PL) and time-resolved PL measurements in the temperature range from 4 K to 300 K, together with atom probe tomography to identify residual impurities in the nanowires. At low temperature, the WZ GaP luminescence shows donor-acceptor pair emission at 2.115 eV and 2.088 eV, and Burstein-Moss band-filling continuum between 2.180 and 2.253 eV, resulting in a direct band gap above 2.170 eV. Sharp exciton α-β-γ lines are observed at 2.140–2.164–2.252 eV, respectively, showing clear differences in lifetime, presence of phonon replicas, and temperature-dependence. The excitonic nature of those peaks is critically discussed, leading to a direct band gap of ∼2.190 eV and to a resonant state associated with the γ-line ∼80 meV above the Γ8C conduction band edge.

Characteristics of InGaAsBi with various lattice mismatches on InP substrate

To develop bismuth-containing infrared optoelectronic devices, InGaAsBi/InP films with different lattice mismatches have been investigated. The lattice mismatch was tailored by changing the Bi content in conjunction with the In content simultaneously. X-ray diffraction analysis revealed that alloy lattice constants have been extended positively by incorporation of Bi into the crystal lattice. Electrical and optical characteristics were investigated by Hall-effect, optical absorption and photoluminescence measurements. A bandgap shrinking of about 56.4 meV/Bi% was deduced by X-ray diffraction and optical absorption measurements. From the excitation dependent photoluminescence measurement at 10 K, the donor-acceptor pair emissions were inferred for samples containing moderate and high levels of Bi. The temperature dependence of the PL peak energy is as small as 0.06 meV/K in In0.5Ga0.5As0.987Bi0.013, which is fairly low compared with that of In0.5Ga0.5As.

Characterization of radiative recombination in Ag(In,Ga)Se2 thin films by photoluminescence

A detailed analysis of the radiative recombination processes in Ag(InGa)Se2 thin films grown by a three-stage method was carried out by photoluminescence. The temperature and excitation dependence of the photoluminescence spectra was used to identify the recombination types and determine the ionization energy of the defects in the films. Significant differences were observed between the spectra of the Ag-rich and Ag-poor samples. The Ag-rich films were dominated by two emission peaks of donor acceptor pairs (DAPs). The DAP at lower energy level is attributed to recombination of donor level 13.8 meV (Agi) with acceptor level 70.3 meV (AgIn), while the one at high energy level is assigned to recombination of donor level 18.5 meV (Agi) with acceptor level 108.9 (AgSe). When Ag/III atomic ratio was near 2.00, a phonon related-structure began to appear, which is attributed to the phonon replica of the high energy level DAP. In the case of Ag-poor AIGS samples, the dominant broad asymmetric peaks of AIGS films with different Ag/III atomic ratios were related to potential fluctuation at low temperature, and the compensation level decreased with increasing Ag/III atomic ratio. The emission line was assigned to recombination of donor level 12.7 meV (Agi) with acceptor level 175 meV (AgGa2). When the excitation power and temperature were increased, new free-bound and DAP emission lines began to appear. The free-bound was assigned to the transition from the conduction band to an acceptor level of 80 meV (AgIn). The DAP was assigned to recombination of donor level 20 meV (VSe) with acceptor level 145 meV (AgGa).

The ratio Oxygen/Zinc effect on photoluminescence emission line at 3.31 eV in ZnO nanowires

We have studied the photoluminescence emission line at 3.31 eV in ZnO nanowires. In undoped ZnO, this band strongly depends on high oxygen concentration and could originate from recombination of bound-exciton complex related to structural defects. Conversely, in doped ones, the photoluminescence emission appears notably at a low VI/II ratio and with the emergence of donor-acceptor pair emission due to the presence of α-No nitrogen complex, which acts as a shallow acceptor in ZnO. We found that this band corresponds to 3LO, the third phonon replica of resonant Raman scattering. Furthermore, a remarkable variation is detected in a number of resonant Raman scattering multiphonons.

Fluorescent P-Type 4H-SiC Grown by PVT Method

Fluorescent SiC, which contains donor and acceptor impurities with optimum concentrations, can work as a phosphor for visible light emission by donor-acceptor-pair (DAP) recombination. In this work, 3 inch N-B-Al co-doped fluorescent 4H-SiC crystals are prepared by PVT method. The p-type fluorescent 4H-SiC with low aluminum doping concentration can show intensive yellow-green fluorescence at room temperature. N-B DAP peak wavelength shifts from 578nm to 525nm and weak N-Al DAP emission occurred 403/420 nm quenches, when the temperature increases from 4K to 298K. The aluminum doping induces higher defect concentration in the fluorescent crystal and decreases optical transmissivity of the crystal in the visible light range. It triggers more non-radiative recombination and light absorption losses in the crystal.

Effects of Mg/Ga and V/III source ratios on hole concentration of N-polar p-type GaN grown by metalorganic vapor phase epitaxy

The effects of growth conditions such as Mg/Ga and V/III ratios on the properties of N-polar p-type GaN grown by metalorganic vapor phase epitaxy were studied. Photoluminescence spectra from Mg-doped GaN depended on Mg/Ga and V/III ratios. For the lightly doped samples, the band-to-acceptor emission was observed at 3.3 eV and its relative intensity decreased with increasing V/III ratio. For the heavily doped samples, the donor–acceptor pair emission was observed at 2.8 eV and its peak intensity monotonically decreased with V/III ratio. The hole concentration was maximum for the Mg/Ga ratio. This is the same tendency as in group-III polar (0001) growth. The V/III ratio also reduced the hole concentration. The higher V/III ratio reduced the concentration of residual donors such as oxygen by substituting nitrogen atoms. The surface became rougher with increasing V/III ratio and the hillock density increased.

Photoluminescence due to impurity-cluster-bound exciton in highly doped and highly compensated Si

We have investigated photoluminescence (PL) at 4.2 K in highly doped and highly compensated Si with donor and acceptor impurities in the intermediate concentration range from 1 × 1016 to 3 × 1018 cm−3. PL spectra were dominated by the radiative recombination of excitons bound by impurity clusters and the donor–acceptor pair emission. The peak position of the exciton emission shifts to the lower energy side monotonically with an increase in the sum of the donor and acceptor concentrations, where the relationship between the position and the concentration is universal regardless of the species of impurities and is valid also for uncompensated Si. This allowed us to suggest that the cluster consists of multiple species of donor and acceptor impurities and that the difference in the species does not cause a detectable variation in the binding energy of an exciton. A possible method for quantifying the donor and acceptor impurities is proposed.

Molecular nitrogen acceptors in ZnO nanowires induced by nitrogen plasma annealing

X-ray absorption near-edge spectroscopy, photoluminescence, cathodoluminescence, and Raman spectroscopy have been used to investigate the chemical states of nitrogen dopants in ZnO nanowires. It is found that nitrogen exists in multiple states: ${\mathrm{N}}_{\mathrm{O}},\phantom{\rule{0.16em}{0ex}}{\mathrm{N}}_{\mathrm{Zn}}$, and loosely bound ${\mathrm{N}}_{2}$ molecule. The results establish a direct link between a donor-acceptor pair emission at 3.232 eV and the concentration of loosely bound ${\mathrm{N}}_{2}$. This work confirms that ${\mathrm{N}}_{2}$ at Zn site is a potential candidate for producing a shallow acceptor state in N-doped ZnO as theoretically predicted by Lambrecht and Boonchun [Phys. Rev. B 87, 195207 (2013)]. Additionally, shallow acceptor states arising from ${\mathrm{N}}_{\mathrm{O}}$ complexes have been ruled out in this paper.

Donor-acceptor-pair emission in fluorescent 4H-SiC grown by PVT method

Fluorescent SiC, which contains donor and acceptor impurities with optimum concentrations, can work as a phosphor for visible light emission by donor-acceptor-pair (DAP) recombination. In this work, 3 inch N-B-Al co-doped fluorescent 4H-SiC crystals are prepared by PVT method. The p-type fluorescent 4H-SiC with low aluminum doping concentration can show intensive yellow-green fluorescence at room temperature. N-B DAP peak wavelength shifts from 578nm to 525nm and weak N-Al DAP emission occurred 403/420 nm quenches, when the temperature increases from 4K to 298K. The aluminum doping induces higher defect concentration in the fluorescent crystal and decreases optical transmissivity of the crystal in the visible light range. It triggers more non-radiative recombination and light absorption losses in the crystal.

The Zn-vacancy related green luminescence and donor–acceptor pair emission in ZnO grown by pulsed laser deposition

A low temperature (10 K) photoluminescence study shows that green luminescence peaking at 2.47 eV and near band edge emission at 3.23 eV from the Zn-vacancy related defect are introduced in undoped ZnO grown by pulsed laser deposition after annealing at 900 °C.

Ultraviolet-emitting ZnO thick layer grown by thermal oxidation with gallium

The ZnO layer with thickness of 1.6 μm in ZnO/ZnGa2O4 composite structure was grown by the thermal oxidation of ZnS substrate with gallium. The optical property of the ZnO thick layer was investigated by time-resolved photoluminescence. A single UV emission around 375 nm with short lifetime was observed at room temperature while the visible emission was absolutely quenched. The UV emission band was composed of the neutral donor bound exciton (D0X) and donor-acceptor pair (DAP) emission peaks with large full-width at half-maximums (FWHMs) at 3.367 and 3.318 eV, respectively, at 10 K. However, the intensity of the D0X emission was stronger than that of the DAP emission at measuring temperatures of 10–300 K.