Donor-bound Exciton Research Articles

Localized excitons mediate defect emission in ZnO powders

A series of continuous-wave spectroscopic measurements elucidates the mechanism responsible for the technologically important green emission from deep-level traps in ZnO:Zn powders. Analysis of low-temperature photoluminescence (PL) and PL excitation spectra for bound excitons compared to the temperature-dependent behavior of the green emission reveals a deep correlation between green PL and specific donor-bound excitons. Direct excitation of these bound excitons produces highly efficient green emission from near-surface defects. When normalized by the measured external quantum efficiency, the integrated PL for both excitonic and green emission features grows identically with excitation intensity, confirming the strong connection between green emission and excitons. The implications of these findings are used to circumscribe operational characteristics of doped ZnO-based white light phosphors whose quantum efficiency is almost twice as large when the bound excitons are directly excited.

Optical and electrical properties of hydrothermally grown Al-doped ZnO nanorods on graphene/Ni/Si substrate

We present a simple way to prepare low-resistance ZnO nanorods by hydrothermal self-assembled growth at 95°C and in situ doped with Al. The NRs were grown on graphene/Ni/Si and annealed at 400°C. Few layer graphene was used to assist aligned growth of the NRs and acted as an electrode during electric measurement. The measurement showed resistance of the Al-doped ZnO NRs 100 times lower than that of undoped ZnO NRs. Photoluminescence measurement showed enhanced deep level emission for the Al-doped NRs and low temperature photoluminescence study showed coexistence of acceptor bound-exciton (3.353eV) and donor bound-exciton (3.362eV).

Local stress distribution in GaN vertical light-emitting diodes fabricated using CLO and LLO methods

We report on the local stress distribution in a GaN-based vertical light-emitting diode (V-LED) fabricated using two types of separation methods: a chemical lift-off (CLO) procedure and a laser lift-off (LLO) technique. The CLO LED exhibits a stronger donor-bound exciton (D0X) emission than the LLO LED, owing to its textured surface morphology and lower amount of damage to the structure. On the basis of the photoluminescence and Raman spectroscopy results, we determine that the CLO GaN LED has an 82 MPa lower residual stress than the LLO GaN LED. Therefore, the CLO technique can be considered as a more effective method to fabricate stress-relieved high-brightness LEDs.

Optical properties of ZnO powder prepared by using a proteic sol-gel process

We have studied the optical properties of ZnO powder synthesized by using a proteic sol-gel process with coconut water as the precursor. The energy dispersive X-ray spectrometer and X-ray diffraction results show high purity of the synthesized ZnO powder. From the low-temperature (12 K) and power-dependent PL spectra, the donor-bound exciton, the acceptor-bound exciton, the donor-to-acceptor pair (DAP), and the phonon-replica of the DAP transition have been observed at 3.38, 3.34, 3.26, and 3.19 eV, respectively. The free exciton emission (∼3.3 eV) is also observed at 300 K in the temperature-dependent PL spectra.

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.

Surface plasmon enhanced photoluminescence and Raman scattering of ultra thin ZnO-Au hybrid nanoparticles

We report substantially enhanced photoluminescence (PL) and Raman signal from ZnO nanoparticles (NPs) deposited on Si with Au NPs. The resonant excitation of surface plasmon of Au NPs is responsible for the enhanced PL intensity and Raman signal from the ZnO-Au hybrid NPs. Low temperature PL measurements exhibit 14 fold enhancements in the PL intensity of ZnO-Au sample in comparison with ZnO/Si sample. It is found that, at low temperature, the donor bound exciton (DX) has dominant contribution in the PL emission of ZnO-Au hybrid NPs. At high temperature, the free exciton (FX) has dominant contribution in PL emission of these NPs. The thermal activation energies of DX and FX are found to be 5 meV and 50 meV, respectively. The defect related visible PL emission of ZnO is completely suppressed in the ZnO-Au hybrid NPs.

Zeeman splitting and dynamics of an isoelectronic bound exciton near the band edge of ZnO

Comprehensive time-resolved photoluminescence and magneto-optical measurements are performed on a bound exciton (BX) line peaking at 3.3621 eV (labeled as ${I}^{*}$). Though the energy position of ${I}^{*}$ lies within the same energy range as that for donor bound exciton (DX) transitions, its behavior in an applied magnetic field is found to be distinctly different from that observed for DXs bound to either ionized or neutral donors. An exciton bound to an isoelectronic center with a hole-attractive local potential is shown to provide a satisfactory model that can account for all experimental results of the ${I}^{*}$ transition.

Photoluminescence Dynamics of GaN/Si Nanowires

In this work we present analysis of carriers dynamics in samples of GaN nanowires grown on silicon. The samples exhibit bright luminescence of bulk donor-bound excitons at 3.472 eV, surface defect-bound excitons at 3.450 eV (SDX) and a broad (0.05 eV) band centered at 3.47 eV caused probably by single free exciton and bi-exciton recombination. The SDX emission has long lifetime τ = 0.6 ns at 4 K and can be observed up to 50 K. At higher temperatures luminescence is dominated by free excitons. The broad excitonic band is best visible under high excitation, and reveals fast, non-exponential dynamics. We present mathematical model assuming exciton exciton interaction leading to the Auger processes. The model includes n (Langevin) term and describes well the non-exponential dynamics of the excitonic band.

Thickness effect on luminescent properties of sol–gel derived ZnO thin films

We studied the photoluminescence (PL) characteristics of ZnO thin films grown by sol–gel spin-coating technique. The ZnO thin films are preferentially oriented along the c-axis perpendicular to the substrate. The radiative recombination of donor-bound excitons dominates the low-temperature PL spectra. Defect-related transitions enhance as thickness decreases. The lineshape of room-temperature PL spectra from sol–gel deposited ZnO thin film is greatly influenced by exciton–phonon interaction, which is governed by defects in the thin films.

Optical properties of ZnO thin films grown on glass substrates using catalytically generated high-temperature H2O

The optical properties of zinc oxide (ZnO) films grown by chemical vapor deposition using high-temperature H2O generated by H2–O2 reactions on Pt nanoparticles were measured. High-energy ZnO precursors formed by the reaction between dimethylzinc gas molecules and H2O molecules were supplied to the substrate surface. ZnO films were grown on glass substrates at substrate temperatures in the range 573–873K. X-ray diffraction patterns from films grown at temperatures above 673K exhibited intense (0002) peaks. Most films had visible and infrared transparencies of higher than 90%. Photoluminescence spectra obtained at 17.8K exhibited strong emission peaks between 3.355 and 3.3604eV, which were attributed to the neutral donor-bound exciton Dox. The lowest full width at half maximum of Dox emission was 6.3meV.

Comparative study of photoluminescences for the Ga-/N-faces of a free-standing GaN bulk fabricated by using hydride vapor-phase epitaxy and self-separation technique

The authors report on a comparative study of photoluminescences for the Ga-/N-faces of a freestanding GaN bulk, fabricated by using hydride vapor-phase epitaxy and self-separation technique. It is found that the Ga-face and the N-face of the same bulk have apparently different optical properties: i) the Ga-face has well-defined excitionic emission lines while the N-face has just broad emission peaks at similar energy levels and ii) the Ga-face is characterized by a transition crossover between DoX (neutral donor-bound exciton) and FXA (A free exciton) in changing temperature while the N-face is characterized by changes in the dominant emission mechanism in the order of DoX, 3.43-eV emission, DoAo (neutral donor-acceptor) pair and eAo (neutral electron-acceptor), and FXA with increasing temperature from 10 K to room temperature. On the other hand, the N-face is shown to have a rougher surface morphology and a larger density of edge dislocations, which is due to structural imperfections generated during the lift-off process. Also, the N-face is shown to have a higher conductivity and a larger electron concentration, which indicates that the N face contains a larger density of donor-type defects. It is suggested that the different emission mechanism of the Ga-/N-faces in the free-standing GaN bulk is ascribed to the fact that the N-face contains a larger density of electrically active radiative and nonradiative recombination centers, which are generated during the self-separation process.

Structural and optical properties of annealed ZnO single crystals in a helium ambient

We have used X-ray diffraction and photoluminescence measurements to investigate the effects of helium annealing of ZnO single crystals grown by using the hydrothermal method. The crystal structure of the grown ZnO single crystal was hexagonal with cell parameters a = 0.3249 and c = 0.5206 nm. The crystal structure did not change after helium annealing. The full width at half-maximum (FWHM) of the (002) diffraction peak following the helium annealing treatment decreases to 0.048° from 0.093° for the as-grown sample, indicating improved crystalline quality. The PL spectra of the crystals showed near-band-edge and visible (green and yellow) emission bands. The PL intensity of the near-band edge emissions of helium-annealed sample was increased while that of the yellow emission band was decreased. From the temperature dependence of the PL spectra, the peak positions of the free exciton (FE) and the donor-bound exciton (BE) shifted to lower energy whereas the visible emission bands show an anomalous temperature-dependent behavior.

Study of Ultraviolet Emission Spectra in ZnO Thin Films

Photoluminescence (PL) of ZnO thin films prepared on c-Al2O3substrates by pulsed laser deposition (PLD) are investigated. For all samples, roomtemperature (RT) spectra show a strong band-edge ultraviolet (UV) emission with a pronounced low-energy band tail. The origin of this UV emission is analyzed by the temperature dependence of PL spectra. The result shows that the UV emission at RT contains different recombination processes. At low temperature donor-bound exciton (D0X) emission plays a major role in PL spectra, while the free exciton transition (FX) gradually dominates the spectrum with increasing temperatures. It notes that at low temperature an emission band (FA) appears in low energy side of D0X and FX and can survive up to RT. Further confirmation shows that the origin of the band FA can be attributed to the transitions of conduction band electrons to acceptors (e, A0), in which the acceptor binding energy is estimated to be approximately 121 meV. It is concluded that at room temperature UV emission originates from the corporate contributions of the free exciton and free electrons-to-acceptor transitions.

Fabrication and low-temperature photoluminescence spectra of Mn-doped ZnO nanowires

Mn-doped ZnO nanowires have been fabricated through a high temperature vapor-solid deposition process. The low-temperature photoluminescence spectra of the samples show that there are multipeak emissions at the ultraviolet (UV) region (about 3.4–3.0 eV). The excitonic and phonon-assisted transitions in Mn-doped ZnO nanowires were investigated. The results show that there is an obvious oscillatory structure emission at the UV region under low temperature from 12–125 K. The oscillatory structure has an energy periodicity about 70 meV and the oscillatory structure is mainly attributed to longitudinal optical (LO) phonon replicas of free excitons (FX). The multipeak emissions at 12 K are attributed to a donor-bound exciton (DBX, 3.3617 eV), 1LO-phonon replicas of a free exciton (FX-1LO, 3.3105 eV), 2LO-phonon replicas of a free exciton (FX-2LO, 3.2396 eV), and 3LO-phonon replicas of a free exciton (FX-3LO, 3.1692 eV), respectively. The intensity of UV emission and the efficiency of emission from the Mn-doped ZnO nanowires are improved.

Efficient upconversion of photoluminescence via two-photon absorption in bulk and nanorod ZnO

Efficient upconversion of photoluminescence from donor-bound excitons is revealed in bulk and nanorod ZnO. Based on excitation power-dependent PL measurements performed with different energies of excitation photons, two-photon absorption (TPA) and two-step TPA (TS-TPA) processes are concluded to be responsible for the upconversion. The TS-TPA process is found to occur via a defect/impurity (or defects/impurities) with an energy level (or levels) lying within 1.14–1.56 eV from one of the band edges, without involving photon recycling. One of the possible defect candidates could be VZn. A sharp energy threshold, different from that for the corresponding one-photon absorption, is observed for the TPA process and is explained in terms of selection rules for the involved optical transitions.

ZnO/(ZnMg)O single quantum wells with high Mg content graded barriers

ZnO/Zn1−xMgxO single quantum wells (SQWs) were grown by plasma-assisted molecular beam epitaxy on c-plane sapphire substrates. Compositional grading allows the application of optimized growth conditions for the fabrication of Zn1−xMgxO barriers with high crystalline quality and a maximum Mg content of x = 0.23. High resolution x-ray diffraction reveals partial relaxation of the graded barriers. Due to exciton localization, the SQW emission is found to consist of contributions from donor-bound and free excitons. While for narrow SQWs with well width dW≤2.5nm, the observed increase of the exciton binding energy is caused by quantum confinement, the drop of the photoluminescence emission below the ZnO bulk value found for wide SQWs is attributed to the quantum-confined Stark effect. For a Mg content of x = 0.23, a built-in electric field of 630 kV/cm is extracted, giving rise to a decrease of the exciton binding energy and rapid thermal quenching of the SQW emission characterized by an activation energy of (24 ± 4) meV for dW = 8.3 nm.

The exciton luminescence in Zn(Cd)Se/ZnMgSSe quantum wells

We study exciton states in Zn(Cd)Se/ZnMgSSe quantum wells (QWs) with various degrees of diffusion blurring in the interfaces by the methods of optical spectroscopy. We show that at low temperatures the QW emission spectra are determined by free and neutral donor-bound excitons. Blurring of the heterointerfaces leads to the increase in the energy shift between the emission line maxima of free and bound excitons. We explain the nonlinear dependence of the steady-state photoluminescence intensity on the excitation-power density in terms of the neutralization of charged donors at the photoexcitation of heterostructures. We observed a complex long-time dynamics of the reflection coefficient, evoked by the charge-redistribution processes in the heterostructure, near the QW exciton resonances under the irradiation.

Photoluminescence study of ZnO nanostructures grown on silicon by MOCVD

Abstract ZnO nanostructures with a size ranging from 20 to 100 nm were successfully deposited on (1 0 0)-Si substrates at different temperatures (500–800 °C) using MOCVD. It could be confirmed that the size of ZnO nanostructures decreased with increasing growth temperature. From photoluminescence (PL) studies it was found, that intensive band-edge PL of ZnO nanostructures consists of emission lines with maxima at 368.6 nm, 370.1 nm, 373.7 nm, 383.9 nm, 391.7 nm, 400.7 nm and 412 nm. These lines can be dedicated to free excitons and impurity donor-bound excitons, where hydrogen acts as donor impurity with an activation energy of about 65 meV. A UV shift of the band-edge PL line with increasing growth temperature of ZnO nanostructures was observed as a result of the quantum confinement effect. The results suggest that an increase of growth temperature leads to increased band-edge PL intensity. Moreover, the ratio of band-edge PL intensity to green- (red-) band intensity also increases, indicating better crystalline quality of ZnO nanostructures with increasing growth temperature.

The photoluminescence characterization of the N-doped ZnO films produced by wet chemical deposition

The ZnO:N films are prepared by a wet chemical method. The temperature-dependent photoluminescence (PL) is used to investigate those ZnO: N films. Due to the introduction of nitrogen atoms into ZnO film, another phase appears in the ZnO film, which can release the stress and improve the film quality. As a result, a neutral donor-bound exciton (D0X) emission peak is shown in low temperature PL spectrum. With the increasing temperature, the D0X line gradually loses its intensity and shifts to 3.30 eV, which is consistent with the well-known conversion from bound exciton to free exciton at elevated temperatures. Then, due to the thermal quenching effect, the D0X line vanishes in room temperature. In addition, no shift is shown in the location of visible band emission and only the intensity decreases with the increasing temperature.

Dominant free exciton emission in ZnO nanorods

Nanostructured ZnO is considered to be a promising building block in the design of nanoscale optoelectronic devices. It usually shows dominant donor-bound exciton (DX) emission at low temperatures. In this study, ZnO nanorods with high crystallinity and optical quality were grown by metal-organic chemical vapor deposition on a-plane sapphire (1120) substrates. Dominant free exciton (FX) emission at a low temperature (14 K) was observed by photoluminescence spectroscopy. It was attributed to both the enhancement of the FX emission induced by the high crystalline quality of the nanorods and the suppression of the DX emission induced by hydrogen out-diffusion. The latter reason is believed to be more important from the analysis of the hydrogen distribution in the nanorods through photoluminescence spectroscopy and secondary ion mass spectrometry. A slow cooling process during the deposition is suggested to result in a better optical quality. These results can promote our understanding of the optical properties of ZnO nanostructures.