Donor-acceptor Pair Emission Research Articles

Enhancement of Donor–Acceptor Pair Emissions in Colloidal AgInS2 Quantum Dots with High Concentrations of Defects

We report on mechanisms of highly fluorescent donor–acceptor pair emissions in chalcopyrite AgInS2 quantum dots (QDs). We observed that photoluminescence quantum yields (PL-QYs) and radiative recombination rates strongly depend on the QD size; smaller QDs exhibit higher PL-QYs and higher radiative recombination rates of up to 60% and 106 s –1, respectively. Such characteristics were examined in terms of concentrations of donors and acceptors estimated from PL decay behaviors analyzed by the theory of Thomas and Hopfield. These analyses revealed that concentrations of donors and acceptors are higher for smaller QDs; this significantly enhances radiative recombination rates and PL-QYs in smaller QDs. In turn, higher concentrations of donors and acceptors in smaller QDs can be attributed to the use of lower synthesis temperatures, which increase the population of donor–acceptor-type lattice defects.

Inhomogeneous distribution of defect-related emission in Si-doped AlGaN epitaxial layers with different Al content and Si concentration

The spatial distribution of luminescence in Si-doped AlGaN epitaxial layers that differ in Al content and Si concentration has been studied by cathodoluminescence (CL) mapping in combination with scanning electron microscopy. The density of surface hillocks increased with decreasing Al content and with increasing Si concentration. The mechanisms giving rise to those hillocks are likely different. The hillocks induced surface roughening, and the compositional fluctuation and local donor-acceptor-pair (DAP) emission at hillock edges in AlGaN epitaxial layers were enhanced irrespective of the origin of the hillocks. The intensity of local DAP emission was related to Si concentration, as well as to hillock density. CL observation revealed that DAP emission areas were present inside the samples and were likely related to dislocations concentrated at hillock edges. Possible candidates for acceptors in the observed DAP emission that are closely related in terms of both Si concentration and hillock edges with large deformations are a VIII-SiIII complex and SiN, which are unfavorable in ordinary III-nitrides.

CuIn<sub>1-x</sub>Ga<sub>x</sub>Se<sub>2</sub> Thin Films Prepared by Co-Evaporation Technique

The optical properties of Cu-poor CuIn1-xGaxSe2 thin films with different gallium contents grown by co-evaporated technique were studied. Measurements of photoluminescence and photoreflectance were performed on the samples. The photoluminescence and photoreflectance emission peaks observed around 1.1 eV are attributed to donor-acceptor pair luminescence. These donor-acceptor pair emissions are considered to originate from relatively shallow acceptor and donor energy levels. With increasing gallium content, the emission peaks shift towards higher levels of photon energy, and the linewidths of the luminescence spectra for the samples become wider, which we attributes to the greater statistical disorder between indium and gallium. Moreover, the conversion efficiency of the CuIn1-xGaxSe2-based solar cells is obtained. The measured results coincide with the inference given by the photoluminescence spectra.

Photoluminescence study of Si doped and undoped Chalcopyrite CuGaSe2 thin films

Photoluminescence (PL) characteristics have been studied on undoped and Si-doped CuGaSe2 single crystal thin films grown on GaAs (001) substrate by migration-enhanced epitaxy. Room temperature PL spectrum of an undoped layer clearly shows free excitonic emission bands related to the minimum band-edge and to the split-off valence band, but no discernible emission has been observed in the low energy area. At 4.2 K, the excitonic emission due to the split-off valence band disappears. Instead, two additional emissions appear at 1.68 and 1.715 eV which are attributed to the bound exciton and band-to-acceptor transition. The Si doping to CuGaSe2 produces two additional PL bands around 1.61 and 1.64 eV. These PL bands are attributed to the donor acceptor pair emissions due to the doped Si impurity which probably occupies Cu or Ga sites and intrinsic Cu vacancy.

Electrical and optical inhomogeneity in N-face GaN grown by hydride vapor phase epitaxy

A detailed study on electrical and optical inhomogeneity in N-face GaN grown by hydride vapor phase epitaxy is presented. There are two distinctive regions with non-homogeneous electrical and optical properties in N-face of GaN revealed by wet etching, Raman and cathodoluminescence. One region exclusively exhibited stronger donor–acceptor pair emission and higher carrier concentration due to impurity incorporation. A strong red-shift of near band edge emission of ∼24meV was also observed owing to additional carrier concentration. Furthermore, the activation energy difference of the etching process between the two regions is ∼0.12eV. The distinctive etching topography was probably due to the difference of surface potentials related to the level of carrier concentration in GaN.

Defects in N, O and N, Zn implanted ZnO bulk crystals

Comprehensive characterization of defects formed in bulk ZnO single crystals co-implanted with N and Zn as well as N and O atoms is performed by means of optically detected magnetic resonance (ODMR) complemented by Raman and photoluminescence (PL) spectroscopies. It is shown that in addition to intrinsic defects such as Zn vacancies and Zn interstitials, several N-related defects are formed in the implanted ZnO. The prevailed configuration of the defects is found to depend on the choices of the co-implants and also the chosen annealing ambient. Specifically, co-implantation with O leads to the formation of (i) defects responsible for local vibrational modes at 277, 511, and 581 cm−1; (ii) a N-related acceptor with the binding energy of 160 ± 40 meV that is involved in the donor-acceptor pair emission at 3.23 eV; and (iii) a deep donor and a deep NO acceptor revealed from ODMR. Activation of the latter defects is found to require post-implantation annealing in nitrogen ambient. None of these defects are detected when N is co-implanted with Zn. Under these conditions, the dominant N-induced defects include a deep center responsible for the 3.3128 eV PL line, as well as an acceptor center of unknown origin revealed by ODMR. Formation mechanisms of the studied defects and their role in carrier recombination are discussed.

Microstructure and point defects in CdTe nanowires for photovoltaic applications

Defects in Au-catalysed CdTe nanowires vapour–liquid–solid-grown on polycrystalline underlayers have been critically evaluated. Their low-temperature photoluminescence spectra were dominated by excitonic emission with rarely observed above-gap emission also being recorded. While acceptor bound exciton lines due to monovalent metallic impurities (Ag, Cu or Na) were seen, only deeper, donor–acceptor-pair emission could be attributed to the Au contamination that is expected from the catalyst. Annealing under nitrogen acted to enhance the single crystal-like PL emission, whilst oxidizing and reducing anneals of the type that is used in solar cell device processing caused it to degrade. The incidence of stacking faults, polytypes and twins was related only to the growth axes of the wires (〈111〉 50%, 〈112〉 30% and 〈110〉 20%), and was not influenced by annealing. The potential electrical activity of the point and extended defects, and the suitability of these nanowire materials (including processing steps) for solar cell applications, is discussed. Overall they have a quality that is superior to that of thin polycrystalline films, although questions remain about recombination due to Au.

The Luminescence of a CuI Film Scintillator Controlled by a Distributed Bragg Reflector

We investigate the emission properties of CuI film scintillators and the effect of distributed Bragg reflectors. The free-exciton emission and the donor-acceptor pair emission from the CuI thin film with the peak wavelengths of 410 nm and 420 nm are observed. However, for the two emission bands, the distributed Bragg reflector reflection results in different enhancements, which is interpreted by the varying transmittance with wavelength. Angle-dependence of emission profile is decided by the transmittance and DBR reflection, which may be useful in scintillation detection applications.

Study on proton irradiation induced defects in GaN thick film

Proton-irradiation-induced defects threaten seriously the stable performance of GaN-based devices in harsh environments, such as outer space. It is therefore urgent to understand the behaviors of proton-irradiation-induced defects for improving the radiation tolerance of GaN-based devices. Positron annihilation spectroscopy (PAS) has been used to study proton-induced defects in GaN grown by HVPE. The result shows that VGa is the main defects and no (VGaVN) or (VGaVN)2 is formed in 5 MeV proton-irradiated GaN. Photoluminescence (PL) spectrum is carried out at 10K. After irradiation, the band edge shows a blue-shift, but the donor-acceptor pair (DAP) emission band and its LO-phonon replicas is kept at the original position. The intensity of yellow luminescence (YL) band is decreased, which means that the origin of YL band has no relation with VGa. The increased FWHM of GaN (0002) peak in proton-irradiated GaN indicates a degradation of crystal quality.

Photoluminescence Study of Deep Levels in CuInS2 Thin Films Grown by Sulfurization Using Ditertiarybutylsulfide

Photoluminescence (PL) spectroscopy measurements are carried out to determine the deep defect levels of Cu-rich CuInS2 thin films prepared by sulfurization using ditertiarybutylsulfide [(t-C4H9)2S: DTBS]. Several PL emission peaks were detected at 1.07, 1.01, and 0.93 eV in Cu-rich CuInS2 thin films. These peaks are considered to be due to both donor–acceptor pair emission as well as transitions related to trap. On the basis of excitation power dependent and temperature dependent PL measurements, the defect levels are calculated. Copper interstitial (Cui) was determined to be the deep donor level, which is easier to create in samples prepared under Cu-rich conditions. A new trap level at 625 meV below the conduction band was found. Using these data and our previous study data, the intrinsic defects are easier to exist in what kind of CuInS2 thin films is analyzed, a complete defect levels diagram of CuInS2 thin films is also proposed.

Improvement of doping efficiency in Mg-Al0.14Ga0.86N/GaN superlattices with AlN interlayer by suppressing donor-like defects

We investigate the mechanism for the improvement of p-type doping efficiency in Mg-Al0.14Ga0.86N/GaN superlattices (SLs). It is shown that the hole concentration of SLs increases by nearly an order of magnitude, from 1.1×1017 to 9.3×1017 cm−3, when an AlN interlayer is inserted to modulate the strains. Schrödinger—Poisson self-consistent calculations suggest that such an increase could be attributed to the reduction of donor-like defects caused by the strain modulation induced by the AlN interlayer. Additionally, the donor-acceptor pair emission exhibits a remarkable decrease in intensity of the cathodoluminescence spectrum for SLs with an AlN interlayer. This supports the theoretical calculations and indicates that the strain modulation of SLs could be beneficial to the donor-like defect suppression as well as the p-type doping efficiency improvement.

Optical Properties of Undopeda-Plane GaN Grown with Different Initial Growth Pressures

The optical properties of undoped a-plane GaN films grown by metal organic vapor phase epitaxy (MOVPE) with different initial growth pressures were investigated using photoluminescence (PL) measurements. Compared to GaN sample grown with higher initial grown pressure, which exhibited the dominant emission band at 3.423 eV, the dominant PL spectra for GaN sample grown with lower initial growth pressure was the donor-acceptor pair (DAP) band at 3.268 eV. Interestingly, the PL intensity of DAP longitudinal optical (LO) phonon replica was stronger than DAP emission above 50 K, indicating strong phonon coupling. The emission band at 3.359 eV observed for the sample grown with higher initial growth pressure was not observed for the sample grown with lower initial growth pressure. Based on the results obtained from Si doping, it was suggested that this band might be related with the improved crystalline quality through Si doping.

Optical Properties of Undoped a-Plane GaN Grown with Different Initial Growth Pressures

The optical properties of undoped a-plane GaN films grown by metal organic vapor phase epitaxy (MOVPE) with different initial growth pressures were investigated using photoluminescence (PL) measurements. Compared to GaN sample grown with higher initial grown pressure, which exhibited the dominant emission band at 3.423 eV, the dominant PL spectra for GaN sample grown with lower initial growth pressure was the donor-acceptor pair (DAP) band at 3.268 eV. Interestingly, the PL intensity of DAP longitudinal optical (LO) phonon replica was stronger than DAP emission above 50 K, indicating strong phonon coupling. The emission band at 3.359 eV observed for the sample grown with higher initial growth pressure was not observed for the sample grown with lower initial growth pressure. Based on the results obtained from Si doping, it was suggested that this band might be related with the improved crystalline quality through Si doping.

Photoluminescence investigation of the indirect band gap and shallow impurities in icosahedral B12As2

The indirect band gap of icosahedral B12As2 (IBA) has been determined by variable temperature photoluminescence measurements (8 K-294 K) on solution-grown bulk samples. In addition, evidence of three shallow acceptor levels and one shallow donor level is reported. The low-temperature spectra were characterized by broad and intense deep defect emission, donor-acceptor pair (DAP) bands, and exciton recombination. The appearance of DAP emission verifies the incorporation of a donor in IBA, which has not been reported previously. The temperature dependence of the free exciton (FE) intensity reflected a FE binding energy of 45 meV. The variation of the FE peak position with temperature was fitted with both Varshni and Pässler models to determine an expression for the temperature dependence of the indirect band gap. The resulting low and room temperature band gaps are Eg(0) = 3.470 eV and Eg(294 K) = 3.373 eV, respectively. The latter is not consistent with previous reports of the room temperature band gap, 3.20 eV and 3.47 eV, derived from band structure calculations and optical absorption, respectively. The origin of these discrepancies is discussed. The DAP spectra reveal three relatively shallow acceptors with binding energies of ≈175, 255, and 291 meV, and a shallow donor with binding energy ≈25 meV. Although the identity of the individual acceptors is not known, they appear to be associated with the light-hole band. The small donor binding energy is suggestive of an interstitial donor impurity, which is suspected to be Ni.

Optical properties of edge dislocations on (11¯00) prismatic planes in wurtzite ZnO introduced at elevated temperatures

An arbitrary number (3×108−1×1010 cm−2) of edge dislocations on (11¯00) prismatic planes, with the Burgers vector of (a/3)[112¯0] and with the dislocation lines nearly parallel to [0001], were introduced intentionally in wurtzite ZnO bulk single crystals at elevated temperatures of 923–1073 K, and the optical properties were examined. After the introduction of the dislocations, the intensity of the intrinsic emissions existing in pre-dislocated crystals, i.e., near-band edge emissions and deep level emissions, was almost unchanged, and donor-acceptor pair (DAP) emissions with photon energies of 2.20 and 2.50 eV at temperature of 12 K appeared. The intensity of the DAP emissions increased with increasing the dislocation density. The origin of the DAP emissions was determined as acceptor levels of 0.9 and 1.2 eV depth introduced with the dislocations.

Influence of dopant transport rate upon photoluminescence and electrical properties of phosphorus‐doped ZnMgTe layers grown by MOVPE

Abstractp‐type doping of Zn1‐xMgx Te has been investigated by metalorganic vapour phase epitaxy using tris‐dimethylaminophosphorus as a dopant source. The photoluminescence (PL) and electrical properties of the as‐grown and post‐annealed layers have been clarified as a function of the dopant transport rate. In PL spectra at 4.2 K, the strong donor acceptor pair emission vanished and instead enhancement of P acceptor‐related excitonic emission and/or appearance of free‐to‐bound emission were observed for all the layers by the post‐annealing treatment. In accordance with the PL behavior, the hole concentration at room temperature of the annealed layers was significantly enhanced from 2×1015∼2×1016 cm‐3 to 5×1016∼2×1018 cm‐3 by a maximum factor of ∼600. The enhanced hole concentration shows saturation tendency with increasing dopant transport rate. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of donor–acceptor-pair emission in fluorescent 6H-SiC

We investigated donor–acceptor-pair emission in N–B-doped 6H-SiC samples by using photoluminescence (PL) and angle-resolved PL. It is shown that n-type doping with concentrations larger than 1018 cm−3 is favorable for observing luminescence, and increasing nitrogen results in stronger luminescence. A dopant concentration difference greater than 4×1018 cm−3 is proposed to help achieve intense PL. Angular-dependent PL was observed that was attributed to the Fabry–Pérot microcavity interference effect, and a strong luminescence intensity in a large emission angle range was also achieved. The results indicate that N–B-doped fluorescent SiC is a good wavelength converter in white LED applications.

Different temperature dependence of excitonic and defect-related photoluminescence spectra in ZnS nanobelts and nanowires

In this paper, both excitonic and defect-related information of ZnS nanobelts and nanowires have been investigated by a temperature-dependent photoluminescence (PL) spectrum. PL spectra of ZnS nanobelts and nanowires differ significantly in the ultraviolet (UV) and visible emission regions. In UV emission regions, due to high-quality crystals, free exciton B (FXB), free exciton A (FXA), FXA-one longitudinal optical (LO) phonon replica are observed in ZnS nanobelts, as well as free-to-bound (e, A) with its one LO phonon replica, while neutral-donor bound exciton (Do, X) and free-to-bound (e, A) are observed in ZnS nanowires at 10 K. The peak and relative intensity of the FX and (Do, X) versus temperature follow well with conventional empirical relations. In the visible emission regions, weak donor–acceptor pair (DAP) and self-activated (SA) emission from ZnS nanowires are commonly observed, but the Y band emission is only observed at 10 K in ZnS nanobelts. The Y band emission disappears at some temperature lower than 50 K. The peak position and full width at half maximum of DAP and SA emission bands display different temperature dependences. Detailed study on temperature-dependent PL spectra of ZnS nanobelts and nanowires provides crucial information on the nature of the electronic states and recombination mechanisms in these nanostructures.

Room temperature luminescence properties of fluorescent SiC as white light emitting diode medium

The high quantum efficiency of donor–acceptor-pair emission in N and B co-doped 6H–SiC opens the way for SiC to constitute as an efficient light-emitting medium for white light-emitting diodes. In this work, we evidence room temperature luminescence in N and B co-doped 6H–SiC fluorescent material grown by the Fast Sublimation Growth Process. Three series of samples, with eight different N and B doping levels, were investigated. In most samples, from photoluminescence measurements a strong N–B donor–acceptor-pair emission band was observed at room temperature, with intensity dependent on the nitrogen pressure in the growth chamber and boron doping level in the source. Low temperature photoluminescence spectra showed that N bound exciton peaks exhibited a continuous broadening with increasing N2 pressure during the growth, unambiguously indicating an opportunity to control the N doping in the epilayer by conveniently changing the N2 pressure. Finally, the crystal quality of the N and B doped 6H–SiC was evaluated by X-ray diffraction measurements. The ω rocking curves of (0006) Bragg diffractions from the samples grown with lower and higher N2 pressure show almost the same value of the full width at half maximum as that collected from the substrate. This suggests that the N and B doping, which is expected to give rise to an efficient donor–acceptor-pair emission at room temperature, does not degrade the crystal quality.

Vertically arrayed Ga-doped ZnO nanorods grown by magnetron sputtering: The effect of Ga contents and microstructural evaluation

Vertically aligned Ga-doped ZnO (ZnO:Ga) nanorods were grown on sapphire substrates without buffer layers under conditions designed to ensure rapid growth rates with a magnetron sputtering system. Unlike the undoped ZnO layer with microsize protrusions, the 1 and 2 wt% Ga doping induced the formation of uniform nanorod arrays with epitaxial growth behavior. In the initial growth stage, the ZnO:Ga layers exhibited island growth with pyramidal shapes due to enhanced misfit strain. This promoted preferential growth along the c-axis, resulting in the formation of nanorod arrays ,without buffer layers. However, the increased stress stored in the ZnO:Ga layers generated high density of stacking faults, which led to an intense donor–acceptor pair emission at 3.31 eV. In addition, the sample doped with a high Ga amount (2 wt%) had rotated crystal phases, resulting in rough sidewalls with nano-branch or sawtooth shapes.