Presence Of Nonradiative Recombination Centers Research Articles

Detection of Nonradiative Recombination Centers in GaPN by Combining Two‐Wavelength Excited Photoluminescence and Time‐Resolved Photoluminescence

Presence and influence of nonradiative recombination (NRR) centers in an intermediate band (IB)‐type material, GaP1–xNx (), are studied by two‐wavelength excited photoluminescence (TWEPL) method and time‐resolved photoluminescence (TRPL) measurement at 77 K. With the use of below‐gap excitation (BGE) light in addition to an above‐gap excitation (AGE), the PL peak intensity is found to increase which indicates the presence of NRR centers and a secondary excitation from the IB to conduction band (CB). Depending on the effect of different BGE energies, an energy diagram on the distribution of NRR centers and NRR process is interpreted. The saturation of PL increase is attributed to the trap‐filling effect in NRR centers, which allows us to modify the rate equation. The NRR parameters are evaluated by a qualitative simulation of the modified rate equations of one‐level model together with the lifetime determined by TRPL. In continuation of evaluating NRR parameters by rate equation analysis, the addition of TRPL measurement improves accuracy and approaches the determination of NRR parameters. A successful characterization of NRR centers leads to a proper optimization of IB‐type solar cells (IBSCs).

Оптические свойства трехмерных островков InGaP(As), сформированных методом замещения элементов пятой группы

Photoluminescence spectroscopy (PL) has been used to study the optical properties of three-dimensional quantum-sized InGaPAs islands formed by substituting phosphorus by arsenic in an InGaP layer deposited on GaAs directly during epitaxial growth. PL line of the formed array of islands is in the range of 950–1000 nm at room temperature. Studies of PL in the temperature range 78–300 K indicate a significant inhomogeneity of the island array, the presence of nonradiative recombination centers, and carrier transport between islands. We observe in the photoluminescence excitation spectra a line associated with absorption in the residual two-dimensional InGaPAs layer. Annealing of the structures results in 300% increase of the PL intensity at room temperature with an insignificant short-wavelength shift of the island PL line, and also in improvement of the homogeneity within the island array.

Influence of ZnO seed layer precursor molar ratio on the density of interface defects in low temperature aqueous chemically synthesized ZnO nanorods/GaN light-emitting diodes

Low temperature aqueous chemical synthesis (LT-ACS) of zinc oxide (ZnO) nanorods (NRs) has been attracting considerable research interest due to its great potential in the development of light-emitting diodes (LEDs). The influence of the molar ratio of the zinc acetate (ZnAc): KOH as a ZnO seed layer precursor on the density of interface defects and hence the presence of non-radiative recombination centers in LT-ACS of ZnO NRs/GaN LEDs has been systematically investigated. The material quality of the as-prepared seed layer as quantitatively deduced by the X-ray photoelectron spectroscopy is found to be influenced by the molar ratio. It is revealed by spatially resolved cathodoluminescence that the seed layer molar ratio plays a significant role in the formation and the density of defects at the n-ZnO NRs/p-GaN heterostructure interface. Consequently, LED devices processed using ZnO NRs synthesized with molar ratio of 1:5 M exhibit stronger yellow emission (∼575 nm) compared to those based on 1:1 and 1:3 M ratios as measured by the electroluminescence. Furthermore, seed layer molar ratio shows a quantitative dependence of the non-radiative defect densities as deduced from light-output current characteristics analysis. These results have implications on the development of high-efficiency ZnO-based LEDs and may also be helpful in understanding the effects of the ZnO seed layer on defect-related non-radiative recombination.

Visible light emission and energy transfer processes in Sm-doped nitride films

Even though the great interest in studying the near-infrared light emission due to Er3+ ions for telecommunication purposes, efficient visible radiation can be achieved from many different rare-earth (RE) ions. In fact, visible and/or near-infrared light emission takes place in RE-doped wide bandgap semiconductors following either photon or electron excitation, suggesting their technological potential in devices such as light-emitting diodes (LED's) and flat-panel displays, for example. Taking into consideration these aspects, the present contribution reports on the investigation of AlN, BeN, GeN, and SiN thin films doped with samarium. The samples were prepared by sputtering and as a result of the deposition method and conditions they present an amorphous structure and Sm concentrations in the low 0.5 at. %. After deposition, the samples were submitted to thermal annealing treatments and investigated by different spectroscopic techniques. A detailed examination of the experimental data allowed to identify optical transitions due to Sm3+ and Sm2+ ions as well as differences in their mechanisms of photon excitation and recombination. Moreover, it is shown that the Sm-related spectral features and emission intensity are susceptible, respectively, to the atomic environment the Sm3+/Sm2+ ions experience and to the presence of non-radiative recombination centers.

Effect of high energy proton irradiation on InAs/GaAs quantum dots: Enhancement of photoluminescence efficiency (up to ∼7 times) with minimum spectral signature shift

We demonstrate 7-fold increase of photoluminescence efficiency in GaAs/(InAs/GaAs) quantum dot hetero-structure, employing high energy proton irradiation, without any post-annealing treatment. Protons of energy 3–5 MeV with fluence in the range (1.2–7.04) × 10 12 ions/cm 2 were used for irradiation. X-ray diffraction analysis revealed crystalline quality of the GaAs cap layer improves on proton irradiation. Photoluminescence study conducted at low temperature and low laser excitation density proved the presence of non-radiative recombination centers in the system which gets eliminated on proton irradiation. Shift in photoluminescence emission towards higher wavelength upon irradiation substantiated the reduction in strain field existed between GaAs cap layer and InAs/GaAs quantum dots. The enhancement in PL efficiency is thus attributed to the annihilation of defects/non-radiative recombination centers present in GaAs cap layer as well as in InAs/GaAs quantum dots induced by proton irradiation.

Suppression of nonradiative processes in long-lived polar GaN/AlN quantum dots

We present a temperature-dependent time-resolved photoluminescence study of the nonradiative processes in polar GaN/AlN quantum dots and quantum wells. The photoluminescence decay times of quantum wells drop above 50 K due to the presence of nonradiative recombination centers. In contrast, the three-dimensional carrier confinement in quantum dots efficiently suppresses nonradiative processes up to room temperature, even for radiative decay times reaching the microsecond range.

Cathodoluminescence study of visible luminescence in hydrothermal ZnO crystals

We present a study of the visible luminescence in ZnO hydrothermal crystals. The study is carried out on oxygen polarity basal plane (000-1) plates that are sliced through several distinct growth sectors of the crystal. The plates reveal three distinct regions that display differences in the luminescence emission, especially in the visible range. These differences should be related to the variation in the distribution of impurities and native defects in the various growth sectors of hydrothermal ZnO. CL images also expose the presence of nonradiative recombination centers that are associated with point defects or complexes of point defects. CL measurements varying the acceleration voltage of the e-beam reveal that the deep levels responsible for the green luminescence, and in a minor extension those responsible for the yellow-orange luminescence are depleted near the surface. A discussion about the main hypothesis accounting for such behavior is presented.

Physical Mechanisms of Photoluminescence of InGaAs(N) Alloy Films Grown by MOVPE

Reported here are some results of temperature and excitation power dependences of photoluminescence (PL) from the InGaAs(N) (In = 10.5%, 17.0% and N = 0-2.3%) alloy films grown at 530 °C and 600 °C by metalorganic vapour phase epitaxy (MOVPE). The InGaAs(N) alloy films emitting at room temperature in the wavelength (λ) range of 0.97-1.23 μm have been investigated. The low-temperature PL spectra in this set of samples are dominated by multiple peak emissions associated with both near-band-edge emission of InGaAs(N) (high energy peak, E PH ) and strongly localized states (lower energy peaks, E PL ) much lower than the InGaAs(N) band-gap. The temperature dependence of integrated PL intensity indicates the presence of non-radiative recombination centers with the localization energy (E loc = 9.4-25.8 meV), which increased with increasing N concentration. Here we noted that E loc are in agreement with the energy difference of E PH and E PL peaks. The α values extracted from the relation I PL I α ex are used to examine the recombination process. In the N-containing layers it is demonstrated that free-excitons, not free-carriers, mainly govern the radiative recombination.

Imaging charge trap distributions in GaN using environmental scanning electron microscopy

We present direct experimental evidence for a field assisted component in images acquired using the gaseous secondary electron detector (GSED) employed in environmental scanning electron microscopes. Enhanced secondary electron (SE) emission was observed in GSED images of epitaxial GaN bombarded with MeV He ions. The increase in SE emission is attributed to an electric field generated by electrons trapped at defects produced by ion implantation. The presence of nonradiative recombination centers and of trapped charge in implanted GaN was established by cathodoluminescence spectroscopy and energy dispersive x-ray spectrometry. The field assisted SE component is distinguishable from the “normal” GSED signal by characteristic pressure and temperature dependencies. The presented results demonstrate the utility of the GSED for imaging charge trap distributions in semiconductors.

Electronic coupling and thermal relaxation in self-assembled InAs quantum dot superlattices

We report optical and structural characterizations of InAs quantum dot superlattices grown by molecular beam epitaxy on GaAs (001). Cross-sectional electron microscopy imaging reveals ~ 20 nm diameter InAs islands well aligned along the growth direction. Low temperature photoluminescence spectra exhibit a clear redshift with decreasing island vertical separation, as a result of electronic coupling between stacked dots, given that the larger shift is obtained for 70 A spacing. At high excitation power, the photoluminescence spectra show a doublet structure corresponding to coupled and uncoupled quantum dot states. The temperature dependence of the photoluminescence indicates that thermionic emission activation energy is much lower than the estimated barrier height. Such a difference is explained by the presence of non-radiative recombination centers due to strain relaxation and In segregation.

Excitation mechanism of Er in Si studied with a free-electron laser

We investigate the influence of infrared illumination using a free-electron laser on the photoluminescence of erbium-implanted silicon material. In addition to the earlier reported quenching of the Er-related photoluminescence due to dissociation of the intermediate excitation stage, two more features of the energy transfer mechanism are revealed. In the wavelength dependence of the quenching effect a local extreme is detected for a beam energy of approximately 100 meV. A possible origin of this effect is discussed. Further, the current experiment revealed the presence of non-radiative recombination centers which could transfer their energy to Er ions under the influence of the infrared beam. The centers were found to be characterized by extremely slow generation and decay kinetics.

Crystal Growth of ZnSe in Closed Tube by Chemical Transport with Iodine

The crystal growth of ZnSe was performed by chemical transport using iodine as the transport agent. The growth parameters were : source zone temperature ; 1098-1123 K, temperature difference between source and growth zones ; 25 - 50 K, and amount of iodine (M) ; 0.10∼30 kg/m 3 . The transport rate was measured experimentally and calculated by using thermodynamical data and gas-phase transport equation. With an increase of M, the observed transport rate was divided into three regions ; a gradual increase at M 5.0 kg/m 3 where fine granular, block-form and needle-like crystals resulted, respectively. The observed transport rate was almost along the calculated one which reproduced the steep increase in the transport rate at M> 5.0 kg/m 3 . The results indicate that the transport was limited by the diffusion and laminar flow in the gas phase at M≤5.0 kg/m 3 and then modified by the thermal convection at M>5.0 kg/m 3 . All the grown crystals only showed the self-activated emission peak in photoluminescence spectra indicating the presence of non-radiative recombination centers. Those centers were attributed to the defects caused from the Se 2 overpressure and the iodine incorporation in the crystals.

Cathodoluminescence and transmission electron microscopy study of dark line defects in thick In0.2Ga0.8As/GaAs multiple quantum wells

The spatial distribution of the long-wavelength luminescence in thick In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) has been investigated using cathodoluminescence (CL) imaging and spectroscopy. The CL spectra show defect-induced broad bands between 1000≲λ≲1600 nm. These bands exhibit spatial variations which correlate with the dark line defects (DLDs) observed in the λ=950 nm exciton luminescence imaging. Transmission electron microscopy showed that [110]-oriented misfit dislocations occur primarily at the substrate-to-MQW and GaAs capping layer-to-MQW interfaces. The large spatial variation of the luminescence intensities indicates that the DLDs observed in CL images are caused by the presence of nonradiative recombination centers occurring in the MQW region located between the interface misfit dislocations. This study provides new information describing the origin and nature of DLDs and differs from previous models which have regarded the electronic nature of dislocation cores as the primary mechanism for inducing DLD radiative contrast in luminescence imaging of strained InGaAs/GaAs.

Defects in strained In0.2Ga0.8As/GaAs multiple quantum wells on patterned and unpatterned substrates: A near-infrared cathodoluminescence study

The spatial distribution of the long-wavelength luminescence in thick In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) grown on patterned and unpatterned substrates has been investigated using cathodoluminescence (CL) imaging and spectroscopy. By spatially correlating the luminescence arising from the MQW exciton recombination (λ≊950 nm) with the longer wavelength (1000≲λ≲1200 nm) luminescence arising from the defect-induced recombination, we demonstrate that it is possible to determine the regions of highest film quality in both the mesa and valley regions for growth on patterned GaAs substrates. The present approach enables a judicious determination of the optimal regions to be used for active pixels in InGaAs/GaAs devices. For growth on unpatterned substrates, the CL spectra show defect-induced broad bands between 1000≲λ≲1600 nm. These bands exhibit spatial variations which correlate with the dark line defects (DLDs) observed in the λ=950 nm exciton luminescence imaging. Transmission electron microscopy showed that [110]-oriented misfit dislocations occur primarily at the substrate-to-MQW interface. The large spatial variation of the luminescence intensities indicates that the DLDs observed in CL images are caused by the presence of nonradiative recombination centers occurring in the MQW region located above the interface dislocations. This study provides new information describing the origin and nature of DLDs and differs from previous models, which have regarded the electronic nature of dislocation cores as the primary mechanism for inducing DLD radiative contrast in luminescence imaging of strained InGaAs/GaAs.

Influences of trace metal impurities on the thermal quenching of photoluminescence in hydrogenated amorphous silicon by homogeneous chemical vapor deposition

The influence of trace metal impurities of low-temperature undoped amorphous silicon by homogeneous chemical vapor deposited a-Si:H has been explored for the first time. The metal impurities Ni, Cr, and Fe cause a shift of the transition temperature for the double-activated regime to a relatively low value. However, Ga and B impurities quench the photoluminescence intensity at a low temperature. Both of them cause weak photoemission of the films at room temperature. The shift of the transition temperature can be explained by the presence of non-radiative deep recombination centers. The quenching of the photoluminescence intensity is caused by the presence of nonradiative recombination centers.

Influence of growth temperature on Be incorporation in molecular beam epitaxy GaAs epilayers

Photoluminescence investigation of several Be-doped molecular beam epitaxy GaAs epilayers (including post-epi-growth-annealed layers) revealed the presence of nonradiative recombination centers having the following properties. In low-temperature (550 °C)-grown layers their concentration is found to increase with Be acceptor concentration, while high-temperature (640 °C)-grown layers exhibited only a low concentration of these centers. Transmission electron microscope measurements on these layers showed the presence of extrinsic stacking faults involving interstitial atoms. Since this stacking faults density is also found to increase with doping level, we tentatively conclude that Be interstitial atoms are most likely participating in the formation of nonradiative centers.

Microstructure of plasma-deposited a-Si : H films

Using transmission and scanning electron microscopy, it is shown that plasma deposition of amorphous silicon hydrogen films from silane or silane/argon mixtures proceeds via nucleation and growth of islands of average lateral dimensions ∼100 Å. If these islands do not coalesce into a homogeneous film, subsequent growth produces columnar morphology with low-density interstitial regions. There is a strong correlation between the columnar structure and the presence of nonradiative recombination centers.

Luminescent Time Decay of Excitons Bound to Zn–O Complexes in Gap

Time-decay measurements of the photoexcited red luminescence near room temperature in a series of GaP (Zn, O) samples with varying zinc concentrations and varying thermal histories (annealing conditions) are presented. A phenomenological analysis of the electron recombination processes in GaP (Zn, O) based on semiconductor statistics and a simple kinetic model is then discussed which takes the (accurately measurable) time decays as independent variables and predicts the values of less accurately known quantities. It is demonstrated that such elusive but important parameters as the internal quantum efficiency, and the relative strengths of radiative and nonradiative centers, can be quantitatively predicted using time-decay and free-hole-concentration measurements. Curves of internal quantum efficiency are presented as functions of the experimentally measurable time decays and free-hole concentrations; as is the ratio of the capture time of the nonradiative centers to the capture time of the Zn–O complexes. These curves lead to an explanation of the dependence of the luminescent time decay of a sample on its previous thermal history. The explanation depends on (1) thermalization of electrons captured by Zn–O complexes back into the conduction band, (2) the presence of nonradiative recombination centers, and (3) a change in the relative concentrations of radiative and nonradiative recombination centers with annealing. From an analysis of the luminescent time decays it is predicted that the energy level of an electron on a Zn–O complex is 0.24±0.02 eV below the conduction-band edge at room temperature. The temperature dependences of the luminescent decay and the external quantum efficiency (measured in the range 300°–450°K using both above and below bandgap photoexcitation) can also be understood on the basis of the model. When the intrinsic radiative excitonic lifetime is allowed to vary as a function of exciton binding-energy agreement between predicted and measured photoluminescent quantum efficiencies results.