Donor-acceptor Pair Spectra Research Articles

Discrete Donor–Acceptor Pair Transitions in CH3NH3PbI3 Perovskite Single Crystals

Achieving a better understanding of the physics of defects in halide perovskites (HPs) is a key challenge for improving the efficiency of the devices. Herein, a comprehensive study of the defect emission of CH3NH3PbI3 perovskite single crystals is presented. The emission of the pristine surface of cleaved crystals is systematically investigated based on steady‐state and time‐resolved micro‐photoluminescence (micro‐PL) spectroscopy. Donor–acceptor pair (DAP) recombination is observed due to the presence of native shallow defects. The DAP spectra present an important variability depending on the location on the surface of the crystals due to inhomogeneous defect distribution. A strong blueshift of the emission is measured as a function of excitation power and is explained by fluctuating potential caused by compensated defects. With increasing photocarrier density, a transition from a structureless to a structured DAP emission with several longitudinal optical phonon replicas is observed. The DAP transition is characterized by the redshift of the emission with time and a slow, non‐exponential PL decay. Sharp discrete lines with sub‐meV widths, the most recognizable spectral signature of DAP transition, are revealed. Herein, the apparently contradictory previous observations on defect emission of CH3NH3PbI3 are reconciled by the results and new insights are provided into the properties of defects in HPs.

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.

Photoluminescence of Mg‐doped m‐plane GaN grown by MOCVD on bulk GaN substrates

AbstractPhotoluminescence (PL) properties are reported for a set of m‐plane GaN films with Mg doping varied from mid 1018 cm−3 to above 1020 cm−3. The samples were grown with MOCVD at reduced pressure on low defect density bulk GaN templates. The sharp line near bandgap bound exciton (BE) spectra observed below 50 K, as well as the broader donor–acceptor pair (DAP) PL bands at 2.9–3.3 eV give evidence of several Mg related acceptors, similar to the case of c‐plane GaN. The dependence of the BE spectra on excitation intensity as well as the transient decay behaviour demonstrate acoustic phonon assisted transfer between the acceptor BE states. The lower energy donor–acceptor pair spectra suggest the presence of deep acceptors, in addition to the two main shallower ones at about 0.23 eV. Similar spectra from Mg‐doped GaN nanowires (NWs) grown by MOCVD are also briefly discussed.

Splitting of type-I (N-B, P-Al) and type-II (N-Al, N-Ga) donor-acceptor pair spectra in 3C-SiC

Discrete series of lines have been observed for many years in donor-acceptor pair (DAP) spectra in 3C-SiC. In this work, the splitting of both type-I (N-B, P-Al) and type-II (N-Al, N-Ga) DAP spectra in 3C-SiC has been systematically investigated by considering the multipole terms. For type-I spectra, in which either N or B substitutes on C sites or P and Al replace Si, the splitting energy of the substructure for a given shell is almost the same for both pairs. For type-II spectra, in which N is on the C site while Al and Ga acceptors replace Si, we find that, when compared with literature data, the splitting energy for a given shell is almost independent of the identity of the acceptor. For both type-I and type-II spectra, this splitting energy can be successfully explained by the octupole term V-3 alone with k(3)=-2x10(5) angstrom(4) meV. Comparing the experimental donor and acceptor binding energies with the values calculated by the effective-mass model, this suggests that the shallow donor (N, P) ions can be treated as point charges while the charge distribution of the acceptor ions (Al, Ga, B) is distorted in accord with the T-d point group symmetry, resulting in a considerable value for k(3). This gives a reasonable explanation for the observed splitting energies for both type-I and type-II DAP spectra.

Mg‐related acceptors in GaN

AbstractPhotoluminescence spectra of c ‐plane Mg doped GaN samples grown by MOVPE on bulk GaN templates reveal previously unknown properties, like the presence of several Mg‐related acceptors. The use of unstrained samples allows a study of both bound exciton (BE) and donor‐acceptor pair (DAP) spectra. Two main acceptors A1 and A2 are observed strongly in BE spectra as well as in DAP spectra, they have similar binding energies, i.e. about 220 meV. The common assignment of the deeper blue PL emission at 2.8–3.0 eV to a deep donor‐shallow acceptor transition is questioned, and discussed in connection with the compensation problem in p‐GaN. It seems like the Fermi level in p‐GaN is controlled by a set of Mg‐related acceptors at energies 0.2–0.6 eV from the valence band top. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

LTPL Investigation of N-Ga and N-Al Donor-Acceptor Pair Spectra in 3C-SiC Layers Grown by VLS on 6H-SiC Substrates

Ga-doped 3C-SiC layers have been grown on on-axis 6H-SiC (0001) substrates by the VLS technique and investigated by low temperature photoluminescence (LTPL) measurements. On these Ga-doped samples, all experimental spectra collected at 5K were found dominated by strong N-Ga donor-acceptor pair (DAP) transitions and phonon replicas. As expected, the N-Ga DAP zero-phonon line (ZPL) was located at lower energy (~ 86 meV) below the N-Al one. Fitting the transition energies for the N-Al close DAP lines gave 251 meV for the Al acceptor binding energy in 3C-SiC and, by comparison, 337 meV for the Ga acceptor one.

Observation of donor-acceptor pair spectra in the photoluminescence of H- and Zn-implanted ZnO single crystals

Donor-acceptor (D-A) pair spectra have been observed in the photoluminescence radiative recombination of selected donor bound exciton complexes in zinc oxide (ZnO) single crystals that have been ion implantation doped with H and Zn atoms and subsequently annealed in a nitrogen (N2) atmosphere at temperatures in the range of 700–800°C. Observations of the D-A pair spectra are explained as the bound states of Hopfield’s bound exciton complex model, in terms of the effective mass approximation of the wurtzitic ZnO energy band structure.

Optical signatures of dopants in GaN

The characteristic optical spectra for shallow donors and acceptors in GaN are discussed. The most accurate photoluminescence (PL) data are obtained from samples grown on freestanding GaN substrates, where strain shifts are absent and a low spectroscopic line width is obtained. Recent PL data for excitons bound to the O and Si donors are discussed in some detail, giving accurate values for the binding energies and excited bound donor states. The Mg-acceptor is the most important one for p-doping, but the related optical spectra are controversial. We show that there are two acceptors present in Mg-doped GaN, with two different acceptor bound exciton peaks, and also two corresponding lower energy donor–acceptor pair spectra. We give tentative evidence for their interpretation.

Impurity-Related Luminescence of Homoepitaxial GaN Studied with High Magnetic Fields

Recent magnetoluminescence results obtained for homoepitaxial GaN layers are presented. The neutral impurity-bound excitons and donor–acceptor pair emission lines have been studied in magnetic fields up to 27 T. Low-temperature luminescence spectra have been measured with the magnetic field parallel and perpendicular to the hexagonal c-axis of the GaN layers. Experimental results allowed us to evaluate diamagnetic shifts, effective g-factors of electrons and holes involved in neutral donor and neutral acceptor complexes as well as the electron–hole exchange constant for close donor–acceptor pairs. Both the fine structure of the neutral acceptor-bound exciton emission and the specific properties of donor–acceptor pair spectra observed in magnetoluminescence experiments are tentatively attributed to the internal structure of the acceptor state.

Characterization of impurities in II–VI semiconductors by time-resolved lineshape analysis of donor-acceptor pair spectra

Strength and energy position of donor-acceptor pair (DAP) luminescence bands are often used for a crude estimate of impurity incorporation and nature in II–VI epilayer structures. We demonstrate that analysis of DAP emission has to be performed more carefully to get reliable information of that kind. A thorough lineshape analysis allows one to determine the acceptor binding energy quite exactly if the donor energy is known, and beyond this the phonon-coupling parameters. A missing or reduced line narrowing of the DAP bands in thin epilayers for decreasing excitation densities is due to inhomogeneous impurity distribution within the strain-profiled layers. In ns time-resolved analysis, DAP decay is shown to depend sensitively on detection energy within the band but is, at all energies, perfectly described by biexponential fits. Fit parameters involve impurity binding energies and, thus, are an independent tool of verifying data obtained from lineshape analysis. Spectrally unresolved double bands can be identified and distinguished by this method.

Novel technique for p-type nitrogen doped ZnSe epitaxial layers

We report a novel technique to obtain p-type ZnSe layers doped with nitrogen. The layers were grown in a low-pressure metalorganic vapor phase epitaxy system using ammonia as the dopant source. A rapid thermal anneal was used to enhance the activation of the nitrogen acceptors. Net acceptor concentration values as high as 3×1016/cm3 were obtained from capacitance-voltage measurements and the profile was uniform over the thickness of the epitaxial layers. The 7 K photoluminescence spectrum was dominated by the acceptor bound exciton peak; the donor-acceptor pair spectra were also observed.

Acoustic phonon-broadened line shape of donor-acceptor pair spectra — a first-principles study

It is well known that donor-acceptor pair spectra are broadened by the distribution of pair separations, but observed spectra are appreciably broader than predicted by this effect. We have therefore included, in addition, the effect of acoustic phonons via a first-principles approach. Our results show that the parameters of primary importance for the acoustic phonon broadening are those associated with the member of the donor-acceptor pair having the deeper energy level. We have compared the present theory to literature data on ZnSe. In this material, the acceptor level is the deeper one, and thus the comparison requires the proper valence band parameters. With use of a recent value of the valence-band deformation potential we have obtained good agreement with experimental data, with no adjustable parameters.

Sharp line donor-acceptor recombination in Mn-implanted GaAs

More than 40 sharp low temperature photoluminescent (PL) lines were observed in the energy range 1.44 – 1.48 eV in Mn implanted VPE GaAs. These are due to a donor-acceptor pair (DAP) recombination where the individual lines arise from a transition at pairs with different lattice separation and a corresponding Coulomb interaction. This is the first observation of a discrete DAP luminescence in GaAs that exists in spite of the large Bohr radii of donors. S As or Si Ga are most likely involved in DAP recombination being usual residual donors in VPE GaAs. The line energy positions are in a good agreement with the Coulomb interaction on the discrete lattice site distances. It is necessary to take into account that two distinct types of DAP spectra are seen in this material: with pairs in one sublattice and in the different sublattices. The observation of the sharp line decay after the laser excitation pulse shows that the lines with higher energy exhibit the lower lifetime. Over 90% sharp line PL intensity decays during the first 100 ns confirming DAP nature of the recombination process observed.

Observation of discrete donor-acceptor pair spectra in MBE grown GaAs

We report the first observation of discrete donor-acceptor pair photo- luminescence spectra in GaAs. Sixty sharp lines are observed in the region between 1.5110-1.5040 eV. These are interpreted as discrete donor- acceptor pair lines resulting from preferential pairing. The pairs are believed to result from silicon on the gallium sublattice and carbon on the arsenic sublattice.

Optical properties of ZnSe doped with Ag and Au

We present bound-exciton (BE) and donor-acceptor-pair (DAP) spectra of ZnSe grown by liquid-phase epitaxy and doped with the transition metals (TM) Ag and Au. Luminescence, luminescence excitation, and time decay spectra establish the assignments of the spectral features and show that Ag forms a medium deep acceptor, ${({E}_{A})}_{\mathrm{Ag}}=431\ifmmode\pm\else\textpm\fi{}2$ meV, consistent with the activation energy for thermal quenching of the DAP spectra. This thermal technique, together with the less precise spectral measurements available for the more-strongly-phonon-coupled Au acceptor indicate that ${({E}_{A})}_{\mathrm{Au}}\ensuremath{\sim}550$ meV, appreciably less than the probable value for Cu, \ensuremath{\sim}650 meV. Peculiarities in the BE properties within this TM sequence are discussed with reference to the influence of their $d$-state characteristics. Strong BE luminescence with no-phonon energy near 2.747 eV is attributed to a neutral ${\mathrm{Ag}}_{\mathrm{Zn}}$-${\mathrm{Ag}}_{\mathrm{I}}$ associate, possibly a split interstitial. Reasons for its absence in ZnSe: Au are discussed. Isotope effects in this spectrum and that of the Li neutral acceptor BE are contrasted. The latter provides proof that ${\mathrm{Li}}_{\mathrm{Zn}}$ is the persistent shallow ${T}_{d}$ site acceptor in ZnSe. Further associate BE luminescence is tentatively identified for ZnSe: Ag and ZnSe: Au.

Central cell effects in donor-acceptor pair spectra of polar semiconductors

The central cell effect in donor-acceptor (DA)-pair spectra is calculated by means of a semi-empirical short range potential and within the effective mass approximation (EMA). The electron-hole recombination energy is determined variationally as a function of the (DA)-pair separation R. The results are compared with previous theoretical work and with experiment in the case of ZnSe with In-donor and Li-acceptor dopants.

Determination of the dielectric constant of GaP from S-C donor-acceptor pair spectra

Determination of the dielectric constant of GaP from S-C donor-acceptor pair spectra

Some effects of electron-phonon interaction on donor-acceptor pair spectra

Intensities and shapes of zero-phonon pair spectra and their phonon replicas were measured for GaP as a function of temperature. Lines are Gaussian, with intensities dependent on two activated, nonradiative processes; bands are quenched at higher temperatures, with redistribution of intensities among pairs and replicas. Effects of hydrostatic pressure are predicted.

Phonon Sidebands in GaP: Bi

The photoluminescence of GaP crystals doped with bismuth, the isoelectronic impurity, has been measured at temperatures between 2K and 77K. The spectra contain many sharp lines at lower temperatures. These lines arise from the donor-Bi pair recombinations and from the recombinations of excitons bound to Bi-traps. The local-mode phonon (26.0±0.4 meV) and the zone-center phonon (49±1 meV) associate with these transitions. The spectral shapes of two series of zone-center phonon sidebands, which dominate the whole spectra, have been analyzed by the configuration coordinate model. The phonon-coupling parameter S for the spectra due to Bi-traps is fairly large in comparison with that for the shallow donor-acceptor pair spectra. This corresponds to the fact that the impurity-lattice interaction is strong for the Bi-isoelectronic trap, since holes or excitons are tightly bound to this center by the short range potential.

On the Spectral Intensity Distribution of Donor–Acceptor Pair Recombination in GaP

AbstractThe spectral intensity distribution of donor–acceptor pair spectra in GaP (donors at P sites) is theoretically investigated by use of a model recently developed for the evaluation of photon energies, and is compared with experimental data. For high excitation levels the intensity distribution is governed by the overlap integral of electron and hole wave functions and is found to depend on the impurity concentration, but hardly on the chemical nature of the impurities involved, i.e. not on the central cell correction. The implications of these observations are discussed.