Neutral Donor-bound Exciton Research Articles

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.

ZnO films grown on glass substrates using high-energy precursors generated by a catalytic reaction

A chemical vapor deposition method using a Pt-catalyzed H2-O2 reaction was applied to the growth of ZnO films on glass substrates. High-energy ZnO precursors formed by the reaction between H2O molecules generated by the exothermic reaction of H2 and O2 and DMZn gas molecules were supplied to the substrate surface. ZnO films were grown on glass substrates at substrate temperatures of 573-873 K. X-ray diffraction patterns from ZnO films grown at temperatures above 673 K exhibited intense (0002) peaks. Photoluminescence (PL) spectra at 17.8 K showed strong emission peaks between 3.355 eV and 3.3604 eV, which were attributed to the neutral donor-bound exciton Dox. The lowest FWHM of the Dox emission was 6.3 meV, less than that previously reported for ZnO (10 meV) grown on a sapphire substrate at 1073 K using conventional MOCVD.

Effects of Growth Temperature on Properties of Nonpolar a-Plane ZnO Films on GaN Templates by Pulsed Laser Deposition

In this work, a-plane GaN/r-sapphire templates have been used to grow nonpolar a-plane ZnO films by pulsed laser deposition. The ZnO film growth temperature was varied in the range of 400°C to 600°C, and the effect of growth temperature on the properties of the ZnO thin films was investigated using x-ray diffraction, atomic force microscopy, photoluminescence (PL) spectroscopy, and Raman measurements. The results show that the crystal quality, surface morphology, strain states, and optical properties of a-plane ZnO films are strongly correlated with the deposition temperature. It is found that the crystallinity of the ZnO films gets better and the surface roughness decreases with increasing growth temperature. At a growth temperature of 600°C, the a-ZnO films display the best crystal quality with x-ray $$ (11\bar{2}0) $$ omega scan full-width at half-maximum values of 0.28° and 0.41° on axis $$ (11\bar{2}0) $$ at azimuth 0° and 90°, respectively. Furthermore, the PL spectrum measured at 83 K is dominated by neutral donor-bound excitons and free-electron-to-bound (e-A 0) emission, and relatively intense LO-phonon replicas of (e-A 0) have also been observed in the a-plane ZnO. The dominance of the free exciton and the appearance of its replicas strongly indicate the high quality of the film.

Deposition of Zinc Oxide Thin Films Using a Surface Reaction on Platinum Nanoparticles

ABSTRACTA new chemical vapor deposition method for the growth of ZnO films using the reaction between dimethylzinc (DMZn) and thermally excited H2O produced by a Pt-catalyzed H2–O2 reaction was investigated. The thermally excited H2O molecules formed by the exothermic reaction of H2 and O2 on the catalyst were ejected from a fine nozzle into the reaction zone and allowed to collide with DMZn ejected from another fine nozzle. The ZnO films were grown directly on a-plane (11-20) sapphire substrates at substrate temperatures of 773-873 K with no buffer layer. X-ray diffraction patterns exhibited intense (0002) and (0004) peaks from the ZnO(0001) index plane. The smallest full width at half maximum (FWHM) value of the ω- rocking curve of ZnO(0002) was less than 0.1º. The largest Hall mobility and the smallest residual carrier concentration of the ZnO films were 169 cm2V−1s−1 and 1.7×1017 cm−3, respectively. Photoluminescence (PL) spectra at room temperature exhibited a band edge emission at 3.29 eV, with a FWHM of 104 meV. Green luminescence from deeper levels was generally about 1.5% of the band edge emission intensity. PL spectra at 5 K showed a strong emission peak at 3.3603 eV, attributed to the neutral donor-bound exciton Dºx. The FWHM was as low as 1.0 meV. Free exciton emissions also appeared at 3.3757 eV (FXA, n=1) and 3.4221 eV (FXA, n=2).

Nonpolar a-plane ZnO films grown on GaN/sapphire templates with SiNx interlayer by pulsed laser deposition

Nonpolar a-plane ZnO films were deposited for the first time on nearly lattice-matched a-plane GaN templates using an in situ SiNx interlayer by the pulsed laser deposition method. The symmetric and asymmetric reciprocal space mappings reveal the broadening effects of the reciprocal lattice points and the residual biaxial in-plane tensile strain of a-plane ZnO to be 0.335% and 0.055% along the [0 0 0 1] c-axis and p-axis, respectively. The photoluminescence spectrum at 85 K is dominated by neutral donor-bound excitons and free-electron-to-bound (e-A0) emissions; relatively intense LO-phonon replicas of (e-A0) have also been observed in a-plane ZnO. Temperature-dependent PL spectra have also been discussed to identify the origin of the emission peaks. Up to fourth A1(LO) phonon mode can be observed to be enhanced significantly from the resonant Raman spectrum.

Characterization of a-plane orientation ZnO film grown on GaN/Sapphire template by pulsed laser deposition

In this study, the authors have investigated the structural and optical properties of ZnO layer grown by pulsed laser deposition on GaN/ r-plane sapphire. X-ray diffraction results demonstrate the ZnO film to be highly preferentially deposited at a-axis orientation; the different rocking curve values along the two orthogonal directions indicate the low C 2 v symmetry in the growth a-plane ZnO. From free stress to large tensile stress (about 1.34 × 10 9 Pa) distribution along the growth direction of ZnO is revealed by visible Raman mapping spectra. The enhanced significantly high-order longitudinal-optical (LO) phonon modes up to 4th and no TO phonons have been observed in Raman spectrum under UV 325 nm by resonance conditions; an intense and broad disorder activated surface phonon mode is also observed, resulting from the increased disorder on the film surface with stripe-like growth features. Low-temperature photoluminescence measurements reveal that the band-edge emission of ZnO is dominated by neutral donor-bound exciton and free electrons to neutral acceptor emissions. Interfacial microstructure of ZnO/GaN has been examined by transmission electron microscopy, with the epitaxial relationship ( 1 0 1 ¯ 0 ) ZnO//( 0 0 0 2 ¯ ) GaN. All these results indicated that GaN template played an important role in the growth of ZnO film, with full advantage of small lattice mismatch.

A study on structural formation and optical property of wide band-gap Be 0.2Zn 0.8O layers grown by RF magnetron co-sputtering system

Wide band-gap BeZnO layers were grown on Al 2O 3 (0 0 0 1) substrate using radio-frequency magnetron co-sputtering. The rate of Be x Zn 1− x O crystallized as a hexagonal structure was x=0.2. From the X-ray photoelectron spectroscopy measurement, the O–Zn bonds relating the crystal structure and the Be–O bonds related to the deviation of the stoichiometry in the BeZnO layer were caught at 530.4 and 531.7 eV in the O 1s spectrum, respectively. Thus, the observance on the Be 1s peak of 113.2 eV associated with the bonding Be–O indicates that the sputtered Be atoms are substituted for the host-lattice site in ZnO. This Be–O bonding shows a relatively low intense and broadening spectrum caused by large fluctuation of Be content in the BeZnO layer. From the photoluminescence and transmittance measurement, the free exciton and the neutral donor-bound exciton (D 0, X) emissions were observed at 3.7692 and 3.7313 eV, respectively, and an average transmittance rate over 95% was achieved in a wide ultraviolet (UV)–visible region. Also, the binding energy for the (D 0, X) emission was extracted to be 37.9 meV. Through the wide band-gap material BeZnO, we may open some possibilities for fabricating a ZnO-based UV light-emitting diode to be utilized as a barrier layer comprised of the ZnO/BeZnO quantum well structure and/or an UV light emitting material itself.

Temperature behavior of electron-acceptor transitions and oxygen vacancy recombinations in ZnO thin films

The present study focuses on the photoluminescence (PL) properties of ZnO films prepared by the reaction of water and zinc. Temperature-dependent PL characteristics of the ZnO films have been investigated in the range from 15 to 260 K. The PL spectrum at 15 K is dominated by neutral donor-bound exciton (D0X) emissions. The emission line at about 3.304 eV can be due to the transition of free electrons to neutral acceptor states (eA0). It is suggested that the ultraviolet emission at room temperature can be attributed to the incorporation of eA0 and free exciton (FX) transitions, rather than sole FX. According to the temperature behavior of green band, it is further confirmed that the green emission is mainly related to the singly ionized oxygen vacancies.

Temperature-Dependent Studies of Si-Implanted Al0.33Ga0.67N with Different Annealing Temperatures and Times

Electrical activation studies were carried out on Si-implanted Al0.33Ga0.67N as a function of ion dose, annealing temperature, and annealing time. The samples were implanted at room temperature with Si ions at 200 keV in doses ranging from 1 × 1014 cm−2 to 1 × 1015 cm−2, and subsequently proximity-cap annealed from 1150°C to 1350°C for 20 min to 60 min in a nitrogen environment. One hundred percent electrical activation efficiency was obtained for Al0.33Ga0.67N samples implanted with a dose of 1 × 1015 cm−2 after annealing at either 1200°C for 40 min or at 1300°C for 20 min. The samples implanted with doses of 1 × 1014 cm−2 and 5 × 1014 cm−2 exhibited significant activations of 74% and 90% after annealing for 20 min at 1300°C and 1350°C, respectively. The mobility increased as the annealing temperature increased from 1150°C to 1350°C, showing peak mobilities of 80 cm2/V s, 64 cm2/V s, and 61 cm2/V s for doses of 1 × 1014 cm−2, 5 × 1014 cm−2, and 1 × 1015 cm−2, respectively. Temperature-dependent Hall-effect measurements showed that most of the implanted layers were degenerately doped. Cathodoluminescence measurements for all samples exhibited a sharp neutral donor-bound exciton peak at 4.08 eV, indicating excellent recovery of damage caused by ion implantation.

Photoluminescence properties of Co-doped ZnO nanorods synthesized by hydrothermal method

Cobalt doped zinc oxide nanorods Zn1−xCoxO (x = 0.01, 0.10) have been synthesized by a hydrothermal process with Zn(NO3)2, Co(NO3)2, NH4OH, CO(NH2)2 and C2H5OH at 150 °C for 1 h. X-ray diffraction and scanning electron microscopy were used to characterize the crystalline structure, size and morphology of the samples. The photoluminescence (PL) and the PL excitation spectra of the nanorods were measured in the range of temperature from 15 K to room temperature. The PL spectra at low temperatures exhibit a group of ultraviolet narrow lines in the near-band-edge region of 3.0–3.4 eV and a very broad band peaked at 3.20 eV. The origin of the near-band-edge PL is interpreted as an emission from free excitons, neutral donor-bound excitons, radiative transitions from a donor to the valence band and donor–acceptor pairs. In particular, a group of emission lines in the red region of 1.8–1.9 eV have been revealed. These emission lines were assigned to the radiative transitions within the tetrahedral Co2+ ions in the ZnO host crystal.

Comparative study of photoluminescences for Zn-polar and O-polar faces of single-crystalline ZnO bulks

The authors have an extensive study of photoluminescences for Zn-polar and O-polar faces of single-crystalline ZnO bulks. In the photoluminescence (PL) spectra at 10 K, Zn-polar and O-polar faces show a common emission feature: neutral donor-bound excitons and their longitudinal-optical (LO) phonon replicas are strong and free excitons are very weak. However, in the PL spectra at room temperature (RT), Zn-polar and O-polar faces show extremely different emission characteristics: the emission intensity of Zn-polar face is 30 times larger than that of O-polar face and the band edge of Zn-polar face is 33 meV redshifted from that of O-polar face. The temperature dependence of photoluminescence indicates that the PL spectra at RT are closely associated with free excitons and their phonon-assisted annihilation processes. As a result, it is found that the RT PL spectra of Zn-polar face are dominated by the first-order LO phonon replica of A free excitons, while that of O-polar face is determined by A free excitons. This is ascribed to the difference in exciton-phonon coupling strengths that Zn-polar phase has larger coupling strength than O-polar face.

Ionized and neutral donor-bound excitons in ZnO

We show for ZnO a correlation between the ionized donor bound exciton ${D}^{+}X$ recombinations ${I}_{0}$, ${I}_{1}$, and ${I}_{2}$ and the neutral donor bound exciton ${D}^{0}X$ recombinations ${I}_{6a}$, ${I}_{8}$, and ${I}_{9}$, respectively. The experimental data on the localization energies of ionized and neutral donor bound excitons are compared to theoretical predictions and a good agreement is found. We give an estimate of the localization energies of neutral acceptor bound excitons. One can conclude that the localization energies of neutral donor and acceptor bound excitons in ZnO will be very close in energy and an assignment of transition lines to donors and/or acceptors (in the absence of decisive data from magneto-optics) will not be straightforward.

Photoluminescence and acceptor level state of p-type nitrogen-doped MgZnO films

A wurtzite nitrogen-doped MgZnO (MgZnO:N) film was grown by plasma-assisted molecular-beam epitaxy (PAMBE) on c-plane sapphire using radical NO as oxygen source and nitrogen dopant. The as-grown film shows n-type conduction at room temperature, but transforms into p-type conduction after annealed. Photoluminescence (PL) spectrum measured at 80 K is dominated by neutral donor-bound exciton emission (D0X) located at 3.522 eV for the n-type MgZnO:N film, but by neutral acceptor-bound exciton emission (A0X) located at 3.515 eV for the p-type MgZnO:N film. By fitting exciton emission intensity of temperature-dependent PL spectra, the binding energies of the D0X and A0X were estimated to be 32 and 43 meV, respectively. Based on the energy shift of exciton emission, the band gap of the MgZnO:N film is estimated to be 3.613 eV, which is 179 meV larger than that of ZnO. Using the Haynes rule, the acceptor energy level of the MgZnO:N film was evaluated to be about 176 meV above the valence band.

Magneto-spectroscopy of donor-bound excitons in GaN

Different emission channels, due to recombination of neutral donor-bound excitons (D 0X) in GaN, are discussed from photoluminescence experiments performed in magnetic fields up to 28 T for a freestanding sample and a heteropepitaxial layer doped with silicon. For these samples, in addition to the principal recombination channel of D 0X, in which donors are left in their ground states, the well-resolved two-electron satellites (TES) involving different donor excitations were observed. A detailed analysis of the magneto-optical data provides important knowledge about the energy structure of the oxygen and silicon donors in wurtzite GaN. The application of high magnetic fields allowed us to tune the intra-donor excitations across the LO-phonon energy. A strong enhancement of TES intensity and the appearance of several anti-crossing processes in the vicinity of the LO-phonon replica of the principal D 0X transition have been observed. These effects are discussed in terms of resonant interaction between LO phonons and donor-bound electrons (magneto-polaron effect).

Optical properties of BeZnO layers studied by photoluminescence spectroscopy

Epitaxial BeZnO layers, the candidates for active barrier layers composed of the ZnO/BexZn1−xO quantum well structure, were successfully grown using the hybrid beam deposition method. The Be rate of x in the grown BexZn1−xO layer was estimated to be 0.11. The optical properties caused by the thermal quenching phenomenon of BeZnO, an ultraviolet light emitting material, were studied using the temperature dependence of photoluminescence spectra. Emission of the neutral donor-bound exciton shows a tendency to quench the intensity and extend the spectral width with increasing temperatures. This tendency is related to the phonons generated by the lattice vibration of the host material in BeZnO. These findings led us to conclude that the phonons may be participating in the quenching process.

Epitaxial growth and optical properties of semipolar (112¯2) GaN and InGaN∕GaN quantum wells on GaN bulk substrates

GaN and InGaN∕GaN multiple quantum well (MQW) were grown on semipolar (112¯2) GaN bulk substrates by metal organic vapor phase epitaxy. The GaN homoepitaxial layer has an atomically flat surface. Optical reflection measurements reveal polarization anisotropy for the A, B, and C excitons. Free A excitons dominate the photoluminescence (PL) spectrum at 10K and are accompanied by a weaker, sharp doublet emission due to neutral donor-bound excitons. The InGaN∕GaN MQW grown on a GaN homoepitaxial layer involves fast radiative recombination processes. The PL decay monitored at 428nm can be fitted with a double exponential curve, which has lifetimes of 46 and 142ps at 10K. These values are two orders of magnitude shorter than those in conventional c-oriented QWs and are attributed to the weakened internal electric field. The emissions from GaN and MQW polarize along the [11¯00] direction with polarization degrees of 0.46 and 0.69, respectively, due to the low crystal symmetry.

Temperature dependence of structural and optical properties of ZnO films grown on Si substrates by MOCVD

Growth of ZnO films on Si (1 1 1) substrates has been carried out at temperatures ranging from 300 to 475 °C by metal organic chemical vapor deposition (MOCVD) technique. A multilayer-tower structure was observed for the film deposited at 300 °C. For the films deposited at higher temperatures, nanorods accompanied with nanotubes were observed. The X-ray diffraction measurement shows that the film deposited at 425 °C has better crystalline quality than the others. Ultraviolet (UV) band edge emissions peaked at 3.27 eV with a full-width at half-maximum (FWHM) of 113–136 meV were observed for all of the films at room temperature. Low-temperature photoluminescence (PL) spectra at 3.5 K are dominated by neutral donor-bound excitons (D 0X) and donor–acceptor pair (DAP) emission. For the films deposited at lower temperature, a deep level emission peaked at 2.80 eV were observed. Temperature dependence of PL shows that the free exciton emission increases with increasing temperature and eventually dominates the PL spectra for the temperature higher than 80 K. Time-resolved PL were carried out at room temperature and 77 K. For the film deposited at 425 °C, the decay times of DAP and D 0X at 77 K were 293 and 389 ps, respectively, while that of the band edge emission was 190 ps at room temperature.

Photoluminescence studies of high-quality ZnO single crystals by hydrothermal method

Photoluminescence and reflectivity spectral measurement have been carried out between liquid helium (4.2 K) and room temperature to evaluate the high-quality ZnO wafers grown by hydrothermal method. The + c, − c and m surface of the ZnO wafer, illuminated by 325 nm ultraviolet light at 4.2 K, emitted the visible luminescence of pale green, pale yellow and pale green, respectively. The broad and weak emissions appeared at 2.43 eV in the + c and m surface, and at 2.29 eV in the − c surface. These peaks were caused by the cation atoms incorporated into the ZnO crystal during the crystal growth. The excitation density dependence of the PL spectra was also investigated, and it was found that no donor–acceptor pair emissions included in the PL spectra of the + c, − c and m surface. The thermal quenching process of the neutral donor-bound and free exciton emissions were observed in the temperature range of 13–300 K, and then the thermal activation energy of A-free exciton was estimated to be 63 and 66 meV in the + c and m surface, respectively.

Time-resolved spectroscopy of excitons bound at shallow neutral donors in HVPE GaN

Time-resolved photoluminescence (TRPL) data for temperatures 2–150 K are presented for two thick HVPE samples grown in two different laboratories. The samples both have residual O and Si shallow donor concentrations in the 10 16 cm −3 range. The radiative decay time for neutral donor-bound excitons (DBEs) related to these donors is found to be about 300 ps. The decay of the DBEs at longer decay times is found to be related to feeding from the free exciton–polariton states. At elevated temperatures the decay of the DBE is very similar to the free exciton decay.

Ultraviolet Luminescence in Mg0.12Zn0.88O AlloyFilms

We investigate the origin of ultraviolet (UV) emission fromMg0.12Zn0.88O alloy thin films with a wurtzite structurefabricated on c-plane Al2O3 substrates by plasmaassisted molecular beam epitaxy. At room temperature, the absorptionedge and UV emission band of the Mg0.12Zn0.88O film shiftto high-energy side compared with ZnO films. Temperature dependence ofthe photoluminescence spectra shows that the UV emission is composed offree exciton and neutral donor bound exciton emissions. Two-stepdissociation processes of the UV emission are observed with theincreasing temperature. The thermal quenching mechanism is attributedto the dissociation of the free exciton from the neutral donor boundexciton in the low temperature region and the dissociation of freeelectron and hole from the free exciton in the high temperature region.