Neutral Donor-bound Exciton Research Articles

All-Oxide Transparent Photodetector Array for Ultrafast Response through Self-Powered Excitonic Photovoltage Operation.

Can photodetectors be transparent and operate in self-powered mode? Is it possible to achieve invisible electronics, independent of the external power supply system, for on-site applications? Here, a ZnO/NiO heterojunction-based high-functional transparent ultraviolet (UV) photodetector operating in the self-powered photovoltaic mode with outstanding responsivity and detectivity values of 6.9 A W-1 and 8.0 × 1012 Jones, respectively, is reported. The highest IUV /Idark value of 8.9 × 104 is attained at a wavelength of 385nm, together with a very small dark current value of 9.15 × 10-12 A. A large-scale sputtering method is adopted to deposit the heterostructure of n-ZnO and p-NiO sequentially. This deposition instinctively forms an abrupt junction, resulting in a high-quality heterojunction device. Moreover, developing a ZnO/NiO-heterojunction-based 4 × 5 matrix array with an output photovoltage of 4.5V is preferred for integrating photodetectors into sensing and imaging systems. This transparent UV photodetector exhibits the fastest photo-response time (83ns) reported for array configurations, which is achieved using an exciton-induced photovoltage based on a neutral donor-bound exciton. Overall, this study provides a simple method for achieving a high-performance large-scale transparent UV photodetector with a self-powered array configuration.

Ammonothermal growth of 2 inch long GaN single crystals using an acidic NH4F mineralizer in a Ag-lined autoclave

Seeded ammonothermal growths of a few-mm-thick GaN crystals on a 2 inch diameter c-plane and a 45 mm long m-plane GaN wafers were carried out by using an NH4F mineralizer in a 60 mm diameter Ag-lined autoclave. As a result of dynamic control of the temperature profile, low dislocation density and nearly bowing-free m-plane GaN was grown: i.e. the full-width at half-maximum values for the X-ray rocking curves of the 100 and 10 2 reflections were smaller than 28 arcsec and the radius of curvature was estimated to be 1460 m. In addition, its low temperature photoluminescence spectrum exhibited free and neutral donor-bound exciton emission peaks.

Temperature dependence of photoluminescence from AlN/GaN-thin nanowires

Well vertically aligned AlN/GaN Nanowires (NWs) with average diameter and length of 25 and 200 nm, respectively, were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si (111) substrates. The photoluminescence (PL) spectra of AlN/GaN NWs show strong emission bands around 3.41 and 3.1 eV at low temperatures in addition to the neutral donor-bound excitons (D0XA). The temperature dependence of the PL peaks is presented and discussed considering the surface effects and structural defects created in GaN due to the mismatch between the different lattice constants of GaN and AlN. The strong emission band, with an energy lower than the widely reported band edge of GaN, is dominant at temperatures higher than 50 K which enhance the application of AlN/GaN in nanophotonic devices working at room temperature.

Time-resolved photon echoes from donor-bound excitons in ZnO epitaxial layers

The coherent optical response from 140~nm and 65~nm thick ZnO epitaxial layers is studied using transient four-wave-mixing spectroscopy with picosecond temporal resolution. Resonant excitation of neutral donor-bound excitons results in two-pulse and three-pulse photon echoes. For the donor-bound A exciton (D$^0$X$_\text{A}$) at temperature of 1.8~K we evaluate optical coherence times $T_2=33-50$~ps corresponding to homogeneous linewidths of $13-19~\mu$eV, about two orders of magnitude smaller as compared with the inhomogeneous broadening of the optical transitions. The coherent dynamics is determined mainly by the population decay with time $T_1=30-40$~ps, while pure dephasing is negligible in the studied high quality samples even for strong optical excitation. Temperature increase leads to a significant shortening of $T_2$ due to interaction with acoustic phonons. In contrast, the loss of coherence of the donor-bound B exciton (D$^0$X$_\text{B}$) is significantly faster ($T_2=3.6$~ps) and governed by pure dephasing processes.

Thermodynamic behaviors of excitonic emission in ZnO nanorods grown by pulsed laser deposition

We investigated the thermodynamic behaviors of the exciton emission in ZnO nanorods that had been grown by laser ablation. The ZnO nanorods exhibited a clear luminescence peak from the neutral donor-bound exciton (D0X), which persisted near room temperature. Through analyzing temperature-dependences of photoluminescence properties, we found out insignificant thermal-quenching of D0X, arising from the large donor binding energy (i.e., EbD(NR) ~ 51.1 ± 7.3meV). A small discrepancy of EbD(NR) from ZnO bulks’ values (i.e., EbD(Bulk) = 53 – 72meV) is associated with inhomogeneous thermal-broadening factors such as defect-scattering at the surface of the nanorod. Despite of inhomogeneous thermal-broadening, the ZnO nanorods still have a high luminescence efficiency because of the weak homogeneous thermal-broadening effect (i.e., low exciton-phonon coupling).

Selective area growth of high-density GaN nanowire arrays on Si(111) using thin AlN seeding layers

Selective area growth (SAG) of high-density (2.5×109cm−2) GaN nanowires (NWs) on Si(111) substrate by plasma-assisted molecular beam epitaxy is presented. The effects of morphology and thickness of the AlN seeding layer on the quality of SAG GaN NWs are investigated. A thin AlN seeding layer of 30nm thick with a surface roughness of less than 0.5nm is suitable for high quality SAG GaN NWs growth. High-density AlN nanopedestal arrays used as seeds for SAG GaN NWs are fabricated from thin AlN seeding layers using soft nanoimprint lithography. By adjusting the growth temperature and Ga/N flux ratio, hexagonal shaped SAG GaN NWs are realized. The quality of SAG GaN NWs is evaluated by low temperature photoluminescence (PL) measurements. Three major groups of PL peaks at 3.47, 3.45, and 3.41eV are identified. The peak at 3.471eV is related to the neutral donor-bound exciton emission, and the 3.41eV broadband emission is attributed to stacking faults or structural defects. The 3.45eV peak is identified as the emission due to exciton recombination at polar inversion domain boundaries of NWs.

Bright green emission and temperature dependent localized bound exciton transitions from undoped ZnO films

Bright green luminescence is achieved from undoped ZnO films prepared by the reaction of oxygen and zinc powder. The green emission band is considered to be mainly due to the singly ionized oxygen vacancies. Temperature dependence of exciton transitions in the undoped ZnO films has been investigated in the range from 10 to 270K. The PL spectrum at 10K is dominated by neutral donor-bound exciton (D°X) emissions. The dominant emission centered at about 3.303eV at above 45K can be attributed to the localized bound exciton (LBX) transitions related to basal plane stacking faults. LBX transitions can survive up to near room temperature due to the LBX binding energy of 68meV.

Annealing effects on Cd0.96Zn0.04Te crystals with Te inclusions probed by photoluminescence spectroscopy

With effectively improved spectral resolution and signal-to-noise ratio, PL features are well resolved in the Te- and Cd-rich CdZnTe crystals and ascribed to the neutral donor-bound exciton (DX), acceptor-bound exciton (AX), A-center, and deep donor–acceptor pair. A detailed analysis indicates that (i) Cd pressure annealing drives the Te inclusions to the surface region of the crystal, destroys the inclusions by excess Cd atoms, and releases abundant donor-like and acceptor-like impurities; (ii) the PL features of the deep energy levels at 1.30–1.55 eV originate in the A-center consisting Cd vacancy and one type shallow donor, and the PL feature at 1.474 eV is the so-called SA luminescence of a donor–acceptor transition between an A-center and another shallow donor. A quantitative description of the recombination scheme is established, and by which the effects of the annealing are clarified.

PREPARATION AND PHOTOLUMINESCENCE PROPERTIES OF RF-SPUTTERED ZnO FILMS

<p class="TRANSAffiliation"><span>ZnO/Si films were prepared by radio frequency (RF) magnetron sputtering at room temperature. By optimizing the heat treatment conditions, we obtained a good quality film annealed at 700 ºC for longer 60 minutes. This process was monitored carefully by Raman scattering spectroscopy, and X-ray diffraction. The photoluminescence study on this film revealed that only ultraviolet emissions due to donor-acceptor pair (DAP), neutral acceptor-bound exciton (AºX) and donor-bound exciton (DºX) were observed. The intensity and peak position of these emissions depend on the measurement temperature and excitation power density.</span></p>

Influence of Exciton Localization on the Emission and Ultraviolet Photoresponse of ZnO/ZnS Core-Shell Nanowires.

The structural and optical properties of ZnO and ZnO/ZnS core-shell nanowires grown by a wet chemical method are investigated. The near-bandgap ultraviolet (UV) emission of the ZnO nanowires was enhanced by four times after coating with ZnS. The enhanced emission was attributed to surface passivation of the ZnO nanowires and localized states introduced during ZnS growth. The emission of the ZnO and ZnO/ZnS core-shell nanowires was attributed to neutral donor-bound excitons and localized excitons, respectively. Localized states prevented excitons from diffusing to nonradiative recombination centers, so therefore contributed to the enhanced emission. Emission from the localized exciton was not sensitive to temperature, so emission from the ZnO/ZnS core-shell nanowires was more stable at higher temperature. UV photodetectors based on the ZnO and ZnO/ZnS core-shell nanowires were fabricated. Under UV excitation, the device based on the ZnO/ZnS core-shell nanowires exhibited a photocurrent approximately 40 times higher than that of the device based on the ZnO nanowires. The differing photoresponse of the detectors was consistent with the existence of surface passivation and localized states. This study provides a means for modifying the optical properties of ZnO materials, and demonstrates the potential of ZnO/ZnS core-shell nanowires in UV excitonic emission and detection.

수소 플라즈마 처리를 거친 ZnO 박막에 대한 PL 연구

The physical effects of H-plasma treatment on ZnO thin film have been studied using photoluminescence(PL) spectroscopy. Four characteristic peaks have been identified: (i) <TEX>$D^0X$</TEX> peak (neutral donor-bound exciton), showing relatively small integrated intensity after H-plasma treatment, indicates that H-plasma passivates the neutral donors in ZnO at low temperatures. The rapid decrease in the integrated intensity of the peak as the temperature goes up is considered to be due to the ionization of neutral donors. (ii) H-related complex-bound exciton peak appears at the low temperatures (10 K~80 K) after H-plasma treatment, showing the same thermal evolution as <TEX>$D^0X$</TEX> peak. (iii) FX (free exciton) peak starts to show up at 60 K and grows more and more as the temperature goes up, which is considered to be related to the increase in free electron concentration in the film. (iv) violet band is intensified after H-plasma, which means more defects and impurities are generated by H-plasma process.

Temperature-related exciton features on the Ga-/N-Faces of a free-standing HVPE GaN

An apparently different effect of excitonic luminescence has been observed on the Ga-/N-faces of a free-standing HVPE GaN. The neutral donor-bound exciton (D20X) emission of the N-face is only dominant at lower temperature as compared with that of the Ga-face. Moreover, the temperature-related ratio of the peak intensity of D20X to XAn=1 (a free exciton) is found to be about 2.8, which is in coincidence with the ratio of the average dislocation density of the N-face to the Ga-face confirmed by Cathodoluminiscence images. These details could provide useful information for the design of GaN-based and related devices.

Optical and structural properties of Zn1−x Mg x O ceramic materials

This paper reports structural, optical and cathodoluminescence characterizations of sintered Zn1−x Mg x O composite materials. The effects of MgO composition on these film properties have been analyzed. X-ray diffraction (XRD) confirms that all composites are polycrystalline with prominent hexagonal wurtzite structure along two preferred orientations (002) and (101) for the crystallite growth. Above doping content x = 10 %, the formation of the hexagonal ZnMgO alloy phase and the segregation of the cubic MgO phase start. From reflectance and absorption measurements, we determined the band gap energy which tends to increase from 3.287 to 3.827 eV as the doping content increases. This widening of the optical band gap is explained by the Burstein–Moss effect which causes a significant increase of electron concentration (2.89 × 1018−5.19 × 1020 cm−3). The luminescent properties of the Zn1−x Mg x O pellets are studied by cathodoluminescence (CL) at room and liquid nitrogen temperatures under different electron beam excitations. At room temperature, the CL spectra of the Zn1−x Mg x O composites exhibit a dominant broad yellow-green light band at 2.38 eV and two ultraviolet emission peaks at 3.24 and 3.45 eV corresponding to the luminescence of the hexagonal ZnO and ZnMgO structures, respectively. For the doped ZnO samples, it reveals also new red peaks at 1.72 and 1.77 eV assigned to impurities’ emissions. However, the CL spectra recorded at 77 K show the presence of excitonic emission peaks related to recombination of free exciton (X A), neutral donor-bound excitons (D0X) and their phonon replicas. The CL intensity and energy position of the green, red and ultraviolet emission peaks are found to depend strongly on the MgO doping content. The CL intensity of the UV and red emissions is more enhanced than the green light when the MgO content increases. CL imaging analysis shows that the repartition of the emitting centers in Zn1−x Mg x O composites is intimately connected to the film composition and surface morphology.

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.

Excitonic emission dynamics in homoepitaxial AlN films studied using polarized and spatio-time-resolved cathodoluminescence measurements

Excitonic emission dynamics in homoepitaxial AlN films grown on a freestanding substrate prepared by the physical-vapor-transport method were examined. Reflecting the low threading dislocation density (&amp;lt;104 cm−2), room-temperature cathodoluminescence intensity images mapped at the free A-exciton energy exhibited homogeneous contrasts. Low-temperature cathodoluminescence peaks at 6.0415 and 6.0287 eV, which were polarized parallel and perpendicular, respectively, to the c-axis, exhibited identical risetimes and short lifetimes; the latter coincided with the temporal delay of neutral donor-bound exciton emissions. These results support the assumption that the two peaks originate from the recombination of free A-excitons of irreducible representations Γ1 and Γ5, respectively.

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.

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.

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.

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.