Two-electron Satellite Research Articles

Reduction in residual impurities in semi-polar [formula omitted] and [formula omitted] GaN grown by metalorganic vapor phase epitaxy

We report on residual impurities in semi-polar 303¯1¯ and 202¯1¯ GaN homo-epitaxial layer grown by metal-organic chemical vapor deposition. The 303¯1¯ and 202¯1¯ GaN layer showed atomically smooth surface and clear steps toward 101¯6 and 101¯4, respectively. The residual impurity concentrations of oxygen and carbon were below the detection limit of secondary-ion mass spectroscopy. Low-temperature photoluminescence revealed that 202¯1¯ GaN layer consisted free excitons and Si donor bound excitons, along with two-electron satellite lines and longitudinal optical (LO) phonon coupling transitions. The results indicate semi-polar 303¯1¯ and 202¯1¯ GaN epitaxial layers are promising candidates in obtaining a high quality GaN drift layer for vertical GaN devices.

Hydrogen-related excitons and their excited-state transitions in ZnO

The role of hydrogen in the photoluminescence (PL) of ZnO was investigated using four different types of bulk ZnO single crystal, with varying concentrations of unintentional hydrogen donor and Group I acceptor impurities. Photoluminescence spectra were measured at 3 K, with emission energies determined to \ifmmode\pm\else\textpm\fi{}50 \ensuremath{\mu}eV, before and after separate annealing in ${\mathrm{O}}_{2}$, ${\mathrm{N}}_{2}$, and ${\mathrm{H}}_{2}$ atmospheres. Using this approach, several new hydrogen-related neutral-donor-bound excitons, and their corresponding B exciton, ionized donor, and two electron satellite (TES) excited state transitions were identified and their properties further investigated using hydrogen and deuterium ion implantation. The commonly observed ${I}_{4}$ (3.36272 eV) emission due to excitons bound to multicoordinated hydrogen inside an oxygen vacancy (${\mathrm{H}}_{\mathrm{O}}$), that is present in most ZnO material, was noticeably absent in hydrothermally grown (HT) ZnO and instead was replaced by a doublet of two closely lying recombination lines ${I}_{4b,c}$ (3.36219, 3.36237 eV) due to a hydrogen-related donor with a binding energy (${E}_{D}$) of 47.7 meV. A new and usually dominant recombination line ${I}_{6\text{\ensuremath{-}}H}$ (3.36085 eV) due to a different hydrogen-related defect complex with an ${E}_{D}$ of 49.5 meV was also identified in HT ZnO. Here, ${I}_{4b,c}$ and ${I}_{6\text{\ensuremath{-}}H}$ were stable up to approximately 400 and 600 \ifmmode^\circ\else\textdegree\fi{}C, respectively, indicating that they are likely to contribute to the unintentional $n$-type conductivity of ZnO. Another doublet ${I}_{5}$ (3.36137, 3.36148 eV) was identified in hydrogenated HT ZnO single crystals with low Li concentrations, and this was associated with a defect complex with an ${E}_{D}$ of 49.1 meV. A broad near band edge (NBE) emission centered at 3.366 eV was associated with excitons bound to subsurface hydrogen. We further demonstrate that hydrogen incorporates on different lattice sites for different annealing conditions and show that the new features ${I}_{4b,c}$, ${I}_{6\text{\ensuremath{-}}H}$, and ${I}_{5}$ most likely originate from the lithium-hydrogen defect complexes $\mathrm{L}{\mathrm{i}}_{\mathrm{Zn}}\text{\ensuremath{-}}{\mathrm{H}}_{\mathrm{O}}$, $\mathrm{A}{\mathrm{l}}_{\mathrm{Zn}}\text{\ensuremath{-}}{\mathrm{H}}_{\mathrm{O}}\text{\ensuremath{-}}\mathrm{L}{\mathrm{i}}_{\mathrm{Zn}}$, and ${\mathrm{V}}_{\mathrm{Zn}}\text{\ensuremath{-}}{\mathrm{H}}_{3,4}$, respectively.

High-resolution photoluminescence spectroscopy of Sn-doped ZnO single crystals

Group IV donors in ZnO are poorly understood, despite evidence that they are effective n-type dopants. Here we present high-resolution photoluminescence (PL) spectroscopy studies of unintentionally doped and Sn-doped ZnO single crystals grown by the chemical vapor transport method. Doped samples showed greatly increased emission from the I10 bound exciton transition that was recently proven to be related to the incorporation of Sn impurities based on radio-isotope studies. The PL linewidths are exceptionally sharp for these samples, enabling a clear identification of several donor species. Temperature-dependent PL measurements of the I10 line emission energy and intensity dependence reveal a behavior that is similar to other shallow donors in ZnO. Ionized donor bound-exciton and two-electron satellite transitions of the I10 transition are unambiguously identified and yield a donor binding energy of 71meV. In contrast to recent reports of Ge-related donors in ZnO, the spectroscopic binding energy for the Sn-related donor bound exciton follows a linear relationship with donor binding energy (Haynes rule) similar to recently observed carbon related donors, and confirming the shallow nature of this defect center, which was recently attributed to a SnZn double donor compensated by an unknown single acceptor.

Study of optical properties of bulk GaN crystals grown by HVPE

We investigated the optical properties of a series of GaN samples sliced from the same bulk crystal grown using hydride vapor phase epitaxy. The high crystalline quality of the samples was evaluated using cathodoluminescence measurements, and the dislocation density ranged from 2.4 × 106 to 2.3 × 105 cm−2. The impurity concentration was determined using secondary-ion mass spectroscopy, and photoluminescence (PL) measurements were conducted in the range of 3–300 K. We did not find a correlation between the O or C impurities and the weak yellow luminescence (YL) band. As the dislocation density decreased, the intensity of the band edge emission increased and that of the YL band decreased. A competition between the two-electron satellite lines correlated to Si and the YL band was also observed in the low-temperature PL spectra, which demonstrated that the Si impurity also plays an important role in the weak YL band of these GaN samples. These results indicate that the Si donors around the dislocations, as reasonable sources of shallow donors, will recombine with possible deep acceptors and finally respond with the YL.

Ultrafast Electron and Hole Relaxation Pathways in Few-Layer MoS2

Femtosecond optical pump–probe spectroscopy is employed to elucidate the band-selective ultrafast carrier dynamics of few-layer MoS2. Following narrowband resonant photoexcitation of the exciton A transition, the subpicosecond to picosecond relaxation dynamics of the electron and the hole at the K valley are separately interrogated by a broadband probe pulse. The temporal evolution of the spectral first moment reveals nonexponential intravalley relaxation dynamics in the conduction band. Fluence dependence measurements suggest that this relaxation process is predominantly mediated by acoustic phonon emission. Intervalley scattering of carriers from the K valley to the extrema of the conduction and valence bands is also observed via the decay of the spectral zeroth moment. In addition, second-order Raman scattering leads to the emergence of sidebands in the normalized differential transmission spectra. The observed two-phonon energies and the fluence-dependent time constants suggest that the E1g longitudinal optical (LO) phonon and the LA phonon participate in intervalley scattering in the conduction and valence bands, respectively. Ab initio nonadiabatic molecular dynamics simulations yield time constants of 0.80 and 0.72 ps for intra- and intervalley electronic relaxation, respectively; the latter agrees well with experiment. Finally, the normalized differential transmission spectra reveal a two-electron shake-up satellite that originates from band-edge radiative recombination and the simultaneous excitation of a hole from Kv1 to Kv2. From its spectral position, a Kv1–Kv2 spin–orbit splitting of 1166 ± 1 cm–1 is deduced. The observation of the two-electron transition points to the existence of strong electron correlation in photoexcited few-layer MoS2.

Investigation of structural and photoluminescence properties of gas and metal ions doped zinc oxide single crystals

We report the structural and photoluminescence properties of annealing dependent and gas and metal ion implanted zinc oxide (ZnO) single crystals. Gd, Na and N ions were implanted into ZnO at an average implantation depth of 12, 24, and 38nm, respectively from the surface layer. The samples were annealed under high vacuum or oxygen at 650°C and their effects were studied. Raman spectra of Gd implanted and oxygen annealed ZnO revealed the A1 longitudinal optical, A1(LO) mode, usually assigned to intrinsic defects, which suggested a partial recovery of implantation induced disorders, unlike ion implanted and vacuum annealed ZnO. Low temperature photoluminescence spectra from unimplanted and Gd implanted ZnO revealed transitions from neutral bound exciton, two electron satellites (TES) and LO phonon replicas. Deep level transitions in unimplanted and Gd implanted ZnO were affected by the annealing atmosphere, and oxygen annealing reduced the deep level emission, suggesting oxygen vacancy related emissions. A small red-shift in the near-band edge emission and a blue-shift in deep level emissions were observed in N and Na implanted and annealed ZnO, indicating slight changes in their band gap energies.

Photoluminescence Study on O-plasma Treated ZnO Thin Films

A temperature dependent (10K-290K) photoluminescence (PL) study for two differently prepared ZnO thin films (as-grown and O-plasma treated) is presented. Four characteristic peaks were identified for both samples: (i) neutral donor-bound excitons ( $D^oX$ ), (ii) two electron satellites (TES), (iii) phonon replica of $D^oX$ ( $D^oX$ -1LO), and (iv) donor-acceptor pair transition (DAP). As the sample temperature increased, $D^oX$ -1LO and DAP transitions became indistinct. This was accompanied by newly-rising emission of free electron-acceptor transitions (e, $A^o$ ). The spectral evolution with temperature for as-grown samples also showed the optical emission from free excitons, which became dominant at higher temperatures. Some features related to O-plasma were identified in PL spectra: (i) different positions of TES transitions (28meV lower than $D^oX$ for as-grown samples and 35meV for O-plasma treated samples), (ii) the decrease of spectral intensity in both emissions of $D^oX$ and DAP after O-plasma treatment, and (iii) no noticeable transition from free excitons after the O-plasma treatment.

Fabrication and photoluminescence properties of high‐quality ZnO film grown by solution‐vaporizing MOCVD

AbstractHigh‐quality ZnO film has been successfully grown on 2 inch c‐plane sapphire substrate using Zn(DPM)2 and O2 as zinc and oxygen sources by a solution‐vaporizing MOCVD technique. All films deposited at the substrate temperatures of 680 ∼ 760 °C were preferentially oriented along the (0001) direction and increasing the substrate temperature the full width at half maximum (FWHM) of the (0002) peak was decreased, as the crystallinity is improved. At the optimized substrate temperature of 740 °C the film showed the smallest FWHM was about 240 arcsec with the smoothest surface. In photoluminescence analysis of the film deposited at 740 °C, exciton emissions bound to the neutral donor (D0X) and two‐electron satellites (TES) emission dominated the edge emission region while phonon replicas of D0X and TES dominated the lower energy region with a weakly donor‐acceptor‐pair emission. Further, a comparatively negligible defect luminescence was observed, which indicated high‐quality ZnO film obtained. With increased temperature, the PL intensity of D0X and TES emissions decreased, which peaks shifts to the lower energy side, while the intensity of the free exciton emission increases and finally dominates the spectrum at room temperature. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of annealing temperature on optical properties of ZnO nanorods with Mg0.2Zn0.8O capping layers

ZnO nanorods were grown on spin-coated ZnO seed layers by the hydrothermal method. The Mg0.2Zn0.8O capping layers were deposited on ZnO nanorods by the sol-gel method. The ZnO nanorods with Mg0.2Zn0.8O capping layers were annealed at 600°C. Temperature-dependent photoluminescence (PL) spectroscopy was carried out to investigate the mechanism governing the quenching behavior of the PL spectra. For the 12 K PL spectra, the peaks of the Mg0.2Zn0.8O capping layers, excitons bound to neutral donors (D0X), two-electron satellite transitions, donor-acceptor pairs, and free-to-neutral-acceptors and their longitudinal optical (LO) phonon replicas were observed at annealing temperatures from 600°C. At 12 K, the peak of the Mg0.2Zn0.8O capping layers was blue-shifted as the annealing temperature increased. The peaks of the D0X, free excitons and Mg0.2Zn0.8O capping layers merged and the PL intensity of the peaks decreased while the temperature increased from 12 to 300 K.

Carrier transport and photoluminescence properties of Ga-doped ZnO films grown by ion-plating and by atmospheric-pressure CVD

Ga-doped ZnO (GZO) films with carrier concentrations ranging from 5.2×1017 to 2.9×1020cm−3 were grown on r-plane sapphire substrates by atmospheric-pressure CVD (AP-CVD). The gradients of Hall mobility (μ) — temperature (T) curves (denoted by Δμ/ΔT) for the GZO films grown by AP-CVD (CVD-GZO films) plotted as a function of carrier concentration n obey those for the GZO films deposited by ion plating with dc arc discharge (IP-GZO films). This suggests that the dominant carrier scattering mechanism limiting the carrier transport is common between the CVD-GZO and IP-GZO films at any given n. The CVD-GZO films with low n exhibited the Ga-related neutral donor bound exciton and two-electron satellite (TES) lines at low temperature. With increasing n, the above two characteristic lines shifted higher energies accompanied by broadening. The IP-GZO films exhibited two emission lines with opposite n dependences; with increasing n, one line shifted towards higher energies, whereas another line shifted towards lower energies. With an increase in n caused by the donor doping, the relaxation of the momentum-conservation law brought the remarkable changes to the PL and PLE spectra of both the IP-GZO and CVD-GZO films with n of more than 4.0×1019cm−3 that is close to the Mott density.

Two-electron-satellite transition of donor bound exciton in ZnO: Radiative Auger effect

Two-electron-satellite (TES) transition of donor bound exciton (D0X) is an interesting many-body quantum process. In this letter, precise luminescence spectra and temperature behaviors of the TES transition of aluminum bound exciton in two kinds of zinc oxide single crystals were investigated in detail. It is found that the TES transition can be treated as a radiative Auger process in which the temperature dependence of the emission intensity of the transition can be fitted very well with a model taking into account the temperature dependent Auger term and thermal dissociation of the D0X excitons.

온도 변화에 따른 ZnO 박막에 대한 PL 연구

The optical properties of ZnO thin film have been studied using photoluminescence(PL) spectroscopy with the change of sample temperatures from 10 K to 290 K. The spectrum at 10 K showed the characteristic emission lines of ZnO which were as follows: free exciton(FX) at 3.369 eV, neutral donor-bound exciton() at 3.360 eV, two electron satellite(TES) at 3.332 eV, -1LO at 3.289 eV, and donor-acceptor pair(DAP) transiton at 3.217 eV. From the spectral evolution with temperatures, two features could be identified as temperature went higher: (1) the bound excitons changed gradually into free excitons, (2) DAP turned into free electron-acceptor transition(e,). The PL intensity of free exciton increased with the increase of temperatures, which was accompanied by the decrease of the intensity of bound excitions and bound excition-related transitons such as TES and -1LO. At 80 K DAP transition disappeared, while (e,) transition started to appear at 30 K.

Studies on temperature- and excitation-power-dependent photoluminescence of ZnO thin film grown by plasma-assisted molecular beam epitaxy

The temperature- and excitation-power-dependence of near-edge emission in a ZnO thin film were studied. The near-band-edge emission peaks at 3.370, 3.355, 3.338, 3.305, 3.254, 3.184, 3.112, and 3.042 eV were attributed to the emission of free excitons (FX); neutral-donor-bound excitons (D0X); neutral-acceptor or deep donor excitons; two-electron satellites (TES); donor–acceptor pairs (DAP); and the first-order longitudinal optical (1LO) phonon replica of DAP (DAP-1LO), DAP-2LO, and DAP-3LO, respectively. According to Haynes's rule, the factors a and b are 0.3 and 5 meV, respectively (Eloc = 0.3ED + 5 meV). The temperature dependence of the FX transition energy was analyzed using both Varshni's formula and Cody's formula for the variation in the band gap with temperature. Varshni's formula yielded a good fit for FX emission. The dependence of the photoluminescence intensity I on the excitation power L can be described by a power law, i.e., I ∼ Lα, where α is an exponent. Our results show that α is 0.86 for D0X and 0.63 for DAP.

Fine optical spectroscopy of the 3.45 eV emission line in GaN nanowires

GaN nanowires grown by plasma-assisted molecular beam epitaxy are of excellent optical quality, their optical signature being characteristic of homogeneous strain-free GaN. There are however discrepancies between the low temperature luminescence spectra of GaN thin films and nanowires, in particular, a strong emission line around 3.45 eV in nanowires is not found with such a large intensity in thin film GaN. The origin of this emission line in nanowires is still debated; in this article, we shed new light on this debate notably by polarization-resolved luminescence and magneto-luminescence experiments. Our findings demonstrate, in particular, that this line cannot be attributed to a two-electron satellite of the donor bound exciton transition.

Temperature-dependent Photoluminescence Study on Aluminum-doped Nanocrystalline ZnO Thin Films by Sol-gel Dip-coating Method

The photoluminescence (PT) properties of Al-doped ZnO thin films grown by the sol-gel dip-coating method have been investigated. At 12 K, nine distinct PL peaks were observed at 2.037, 2.592, 2.832, 3.027, 3.177, 3.216, 3.260, 3.303, and 3.354 eV. The deep-level emissions (2.037, 2.592, 2.832, and 3.027 eV) were attributed to native defects. The near-band-edge (NBE) emission peaks at 3.354, 3.303, 3.260, 3.216, and 3.177 eV were attributed to the emission of the neutral-donor-bound excitons (<TEX>$D^0X$</TEX>), two-electron satellite (TES), free-to-neutral-acceptors (e,<TEX>$A^0$</TEX>), donor-acceptor pairs (DAP), and second-order longitudinal optical (2LO) phonon replicas of the TES (TES-2LO), respectively. According to Haynes' empirical rule, we calculated the energy of a free exciton (FX) to be 3.374 eV. The thermal activation energy for <TEX>$D^0X$</TEX> in the nanocrystalline ZnO thin film was found to be ~25 meV, corresponding to the thermal dissociation energy required for <TEX>$D^0X$</TEX> transitions.

Photoluminescence characterization of vertically aligned ZnO microrods

A detailed optical characterization of vertically aligned ZnO microrods (μRs) grown using a Ni-based catalyst was carried out by excitation-power- and temperature-dependent photoluminescence (PL) measurements. Low-temperature PL spectra of ZnO μRs are dominated by near-band-edge (NBE) emission consisted of a series of sharp lines typical for the bulk ZnO. Starting from the higher energy free exciton (FX) emission feature, the majority of them can be explained by radiative recombination of excitons bound to neutral donors (D0X), defect bound exciton (DBX), two-electron satellites emission, free-to-bound, i.e. free electrons to the neutral acceptors (eA0) transition, as well as their longitudinal-optical phonon replicas. An additional excitonic line located in between the FX and D0X lines, denoted as the surface excitons (SX) for ZnO μRs is observed. The intensity of the SX line is found to be smaller than that of the nanosized counterpart and has been attributed to the surface–volume ratio effects. The excitation-power-dependent results of FX line at low and high power regimes show quite close values corresponding to, respectively, p=2 and p=1 limits of the theoretical power law expression I∼Lp and larger deviations for the D0X, SX and DBX lines. The temperature-dependent measurements confirmed the presence of eA0 line showing kT/2 influence to the position of eA0 emission line in comparison with FX. FX emissions persist up to 300K and together with the dominant eA0 emission govern the line shape of the NBE emission range, while D0X and SX lines are quenched completely at 150K.

Investigation of ultraviolet carrier recombination of Volmer-Weber type ZnO nanocrystals and nanorods through Varshni’s formula and Arrhenius plots

The carrier recombination in Volmer-Weber-type ZnO nanocrystals (VW ZnO NCs) and Volmer-Weber-type ZnO nanorods (VW ZnO NRs) synthesized by using a physical sputtering method and a hydrothermal method, respectively, was investigated by using temperature-dependent photoluminescence. The main mechanism of UV emission observed in both materials was attributed to the neutral donor bound to the exciton (D0X) and a two-electron satellite (TES). Using the energies of D0X and its TES, we found that the donor binding energy was higher than that of a ZnO crystal. The change in the main carrier recombination and the activation energy were estimated using Varshni’s formula and an Arrhenius plot, respectively. For the VW ZnO NCs, the change from D0X to free exciton (FX) was examined at 150 K, and an activation energy of 6.8 meV was obtained. For the VW ZnO NRs, no switch of the main carrier recombination was obvious at 150 K, but some deviations with different origins appeared at temperatures above 150 K on Varshni’s curve and the Arrhenius plot. The activation energy was double that of the VW ZnO NCs due to the existence of a number of donor impurities.

산소 플라즈마 처리 후 ZnO 박막에 대한 PL 연구

The optical properties of ZnO thin film, being treated by O-plasma, have been studied using Photoluminescence(PL) spectroscopy with the change of temperature from 10 K to 290 K. Two characteristic peaks were identified at 10 K : 3.357 eV(<TEX>$D^{\circ}X$</TEX>) and 3.324 eV(TES). The peak of <TEX>$D^{\circ}X$</TEX> is believed to be due to neutral donor bound excitons where the donor is in the ground state. However, the TES(Two Electron Satellite) peak indicates the excited state of the donor(excitation energy was ~30 meV). The donor binding energy was estimated to be 44 meV, which indicates the possible presence of the neutral donor bound excitons at RT. The thermal effect including thermal broadening was identified from temperature evolution of the spectrum. Both the peak intensity and the peak energy have decreased as the temperature increases. As the temperature approaches to RT, the two peak merges into one broad peak, which is considered a combination of multiple peaks having different physical origins.

Origins of green band emission in high-temperature annealed N-doped ZnO

Nitrogen-doped ZnO sample has been annealed in O 2 ambient at high temperature (1000 °C) to improve its photoluminescence property. Low-temperature photoluminescence spectra of the sample are dominated by three near-band-edge emissions at 3.377, 3.362, and 3.332 eV, which are ascribed to free exciton emission (FX A), and neutral donor-bound exciton (D 0X), and its two-electron satellite (TES), respectively. With increasing temperature in low temperature region, the intensity of FX A increases and the green band (GB) shows a negative thermal quenching effect resulting from thermal dissociation of D 0X with more free excitons and neutral donors formed. The doublet structure with energy space ∼30 meV and repeated separation of longitudinal-optical phonon energy of 72 meV are observed in GB at low temperatures. The temperature independent energy position of GB indicates a typical recombination characteristic within strongly localized complexes. The doublet structures are considered to originate from the ground and exited states of shallow donors recombining with deep acceptors such as zinc vacancies.

Identification of a donor-related recombination channel in ZnO thin films

. Its appearance was only observed in samples grown on a MgO or heavily aluminum-doped ZnO buffer layer. The properties of the recombination mechanism were probed by photoluminescenceand photoluminescence-excitation spectroscopy. The recombination was classified to be an exciton bound to aneutral donor. Both an ionized state and a two-electron satellite transition were observed and could be corre-lated with the central transition. The excitation channels are similar to those of the well-known I