Photoluminescence Intensity Mapping Research Articles

Nanoscopic spectroscopy of potential barriers formed around V-pits in InGaN/GaN multiple quantum wells on moderate temperature GaN pit expansion layers

We used photoluminescence (PL) spectroscopy combined with scanning near-field optical microscopy to directly observe the potential barriers in InGaN/GaN multiple quantum wells (MQWs) on a GaN layer grown under moderate temperatures (MT-GaN) as the V-pit expansion layer. Area-averaged PL spectra revealed shoulders on the higher-energy side of the InGaN/GaN MQW emission (HE emission) even at room temperature (RT), as well as at low temperature. These HE emission regions, which possessed island-like shapes in the PL intensity maps at RT, corresponded to the dark regions in the PL intensity maps of GaN emission. Low-temperature local PL spectra acquired at the measurement points where HE emissions were observed revealed multiple HE emission peaks at two distinct energies. Most of the regions where the HE emission occurred corresponded to the dark regions of GaN emission and their outer circumference. These experimental observations demonstrate the formation of potential barriers around threading dislocations in the InGaN/GaN MQWs on an MT-GaN layer as the pit expansion layer. The energy difference between the HE emissions and the MQW emissions on the MT-GaN layer tended to increase with increasing V-pit diameter and was much larger than the energy difference for an InGaN/GaN superlattice structure. This was rationalized by the greater change in transition energy for thinner MQWs on V-pits than for planar MQWs, when the thickness in the MQWs on MT-GaN decreases owing to strain effects.

Anomalous Corrosion of Bulk Transition Metal Diselenides Leading to Stable Monolayers.

In this paper we provide insight into an anomalous corrosion process, referred to as electroablation (EA), which converts multilayer flakes of transition metal diselenides like MoSe2 into their corresponding monolayers when micromechanically exfoliated on a conductive electrode and subsequently subjected to a high anodic potential inside a conventional electrochemical cell. Photoluminescence intensity maps and scanning transmission electron microscopy (STEM) images confirmed the single crystalline nature and 2H-hexagonal lattice structure of the remnant monolayer MoSe2 flakes, indicating the superior corrosion stability of the monolayers compared to that of the bulk counterpart. It is noted that the EA technique is a low-cost alternative for high-yield synthesis of single crystalline monolayer MoSe2 at room temperature. We also found that the dynamics of such an electro-oxidation-mediated and self-limiting corrosion process differs significantly for MoSe2 and WSe2. While we were able to engineer the corrosion conditions for the EA process to obtain monolayers of MoSe2, our attempts to obtain monolayers of WSe2 were largely unsuccessful. Finally, we constructed a phenomenological physical chemistry framework to explain such anomalous corrosion processes in transition metal diselenides.

Near-infrared nano-imaging spectroscopy using a phase change mask method.

We propose a technique that employs an optical mask layer of a phase-change material, e.g. GeSbTe, which is widely used for rewritable optical recording media, for realizing highly sensitive near-field imaging spectroscopy of single semiconductor quantum constituents at optical telecommunication wavelengths. Semiconductor quantum dots (QDs) have shown great promise as efficient single photon emitters and entangled photon sources, making them attractive for quantum communication and quantum information processing applications. Self-assembled InAs QDs on InP substrate are promising as near-infrared (NIR) single photon and entangled photon emitters. In order to clarify and control the optical properties of QDs for telecommunication devices, photoluminescence (PL) spectroscopy studies of single QDs with high spatial resolution at NIR wavelength is necessary. The most useful technique to attain this is by using near-field scanning optical miscroscopy (NSOM). However, NSOM has a lower PL collection efficiency at NIR wavelength than at visible wavelength [1]. This problem inhibits NIR-PL spectroscopy based on NSOM to be practically realized. Therefore, we deveopled a method to overcome the low NIR-PL spectroscopy by using a nanoaperture on an optical mask layer of phase-change material (PCM) [2]. Due to the large optical contrast between the crystalline and amorphous phases of the phase-change material at visible wavelengths and its high transparency at NIR wavelengths, an amorphous nanoaperture can be used to realize imaging spectroscopy with a high spatial resolution and a high collection efficiency (Fig.1). We demonstrate the effectiveness of the proposed method by performing numerical simulations and PL measurements of InAs/InP QDs.jmicro;63/suppl_1/i10-a/DFU089F1F1DFU089F1Fig. 1.Schematic illustration of phase change mask method PCM mask effect has also the potential to be applied in emission energy control of QDs. One of the main problems for realization of quantum communication applications is precise control of energy in QDs. We proposed a new approach to control the emission energy of QDs by applying a local strain using volume expansion of phase-change material [3-5]. We calculated the stress and energy shift distribution induced by volume expansion using finite element method. Simulation result reveals that redshift is obtained beneath the flat part of amorphous mark, while blueshift is obtained beneath the edge region of amorphous mark. Simulation result is accompanied by two experimental studies; two-dimensional PL intensity mapping of InAs/InP QD sample deposited by a layer of PCM, and an analysis on the relationship between PL intensity ratio and energy shift were performed.

Redshifted and blueshifted photoluminescence emission of InAs/InP quantum dots upon amorphization of phase change material.

We present the mechanisms underlying the redshifted and blueshifted photoluminescence (PL) of quantum dots (QDs) upon amorphization of phase change material (PCM). We calculated the stress and energy shift distribution induced by volume expansion using finite element method. Simulation result reveals that redshift is obtained beneath the flat part of amorphous mark, while blueshift is obtained beneath the edge region of amorphous mark. Simulation result is accompanied by two experimental studies; two-dimensional PL intensity mapping of InAs/InP QD sample deposited by a layer of PCM, and an analysis on the relationship between PL intensity ratio and energy shift were performed.

Photoluminescence characterization of photovoltaic effect in ZnO/CdS/Cu(In,Ga)Se2 heterostructure

A series of studies on photoluminescence (PL) and time-resolved PL in Cu(In,Ga)Se2 (CIGS) solar cells is presented with emphasis on the photovoltaic effect in the ZnO/CdS/CIGS heterostructure. The photovoltage created by an excitation light increases PL yield and PL decay time of the near-band-edge PL of the CIGS absorber layer in the CIGS solar cell. Under the open circuit condition, the photovoltage reduces the carrier separation field in the space charge region. This reduces the collection of photoexcited carriers, which results in the high PL yield and long PL decay time. The two-dimensional PL intensity mapping of the CIGS solar cell reflects the photovoltage distribution, and the results are compared with the light-beam-induced current image. PL characterization of the CIGS solar cell process has been studied. A chemical-bath deposition of the CdS buffer layer and the following sputter-deposition of the high-resistivity ZnO buffer layer do not change PL intensity and PL decay time of the CIGS layer. On the other hand, a metalorganic chemical vapor deposition (MOCVD) of the high-resistivity ZnO layer leads to high PL intensity and the long PL decay time. The result is discussed in terms of the thermal diffusion of Cd from CdS layer into the CIGS absorber layer during the MOCVD process at an elevated temperature. The formation of the buried pn junction in the CIGS layer due to the Cd-diffusion is discussed in terms of an electron-beam induced current measurement. The utilization of PL as a measure of the junction formation is discussed.

Characterization of Cu(In,Ga)Se2Solar Cell Fabrication Process by Photoluminescence

Photoluminescence (PL), PL intensity mapping and time-resolved PL (TR-PL) studies have been applied to the Cu(In,Ga)Se2 (CIGS) solar cell fabrication process. Measurements have been done just after the respective cell process for the preparation of the Al/ZnO:Al/ZnO/CdS/CIGS structure, in which CdS has been formed by the chemical-bath deposition (CBD) while undoped and Al-doped ZnO layers were deposited by RF magnetron sputtering. PL intensity does not change by depositions of CdS and undoped ZnO buffer layers. PL intensity decreases by the deposition of the ZnO:Al film due to the cell shunt at the edge. The electrical cell isolation by the mechanical scribing leads to the increase in PL intensity because of the formation of the hetero-junction under the open circuit condition. The decay curves of the as deposited CIGS film, CdS/CIGS and ZnO/CdS/CIGS are non-exponential and composed of dominant fast decay and weak slow decay components. After the ZnO:Al deposition, PL decay is represented by the single exponential curve with long decay time. They are discussed in terms of the junction formation. PL intensity mapping after cell processes has been correlated with the solar cell performance.

Characterization of Cu(In,Ga)Se2 Solar Cell Fabrication Process by Photoluminescence

Photoluminescence (PL), PL intensity mapping and time-resolved PL (TR-PL) studies have been applied to the Cu(In,Ga)Se2 (CIGS) solar cell fabrication process. Measurements have been done just after the respective cell process for the preparation of the Al/ZnO:Al/ZnO/CdS/CIGS structure, in which CdS has been formed by the chemical-bath deposition (CBD) while undoped and Al-doped ZnO layers were deposited by RF magnetron sputtering. PL intensity does not change by depositions of CdS and undoped ZnO buffer layers. PL intensity decreases by the deposition of the ZnO:Al film due to the cell shunt at the edge. The electrical cell isolation by the mechanical scribing leads to the increase in PL intensity because of the formation of the hetero-junction under the open circuit condition. The decay curves of the as deposited CIGS film, CdS/CIGS and ZnO/CdS/CIGS are non-exponential and composed of dominant fast decay and weak slow decay components. After the ZnO:Al deposition, PL decay is represented by the single exponential curve with long decay time. They are discussed in terms of the junction formation. PL intensity mapping after cell processes has been correlated with the solar cell performance.

Deep-level photoluminescence due to dislocations and oxygen precipitates in multicrystalline Si

We have demonstrated the presence of a dislocation-related component and a component due to oxygen precipitates in a broad deep-level photoluminescence (PL) band in multicrystalline Si at room temperature. In PL intensity mapping, the lower-energy side of the deep-level PL band at about 0.79 eV appeared as a dark line along a small-angle grain boundary (SA-GB) surrounded by a bright band on either side, while the higher-energy side at about 0.87 eV as a bright line along the SA-GB. These intensity variations agree with the low-temperature PL intensity patterns for well-established dislocation-related lines of D1/D2 and those for oxygen precipitates, respectively. These patterns were observed around SA-GBs with a misorientation angle of 1–2°, and were assumed to be due to the distribution of secondary defects or impurities trapped by the strain field around dislocation clusters forming SA-GBs and that of preferential oxygen precipitation on the dislocations. A spectral component associated with the D3/D4 lines was also extractable from the deep-level PL at about 0.94 eV. The intensity increased on SA-GBs with the angle of <1°, where oxygen precipitation did not occur. This corresponds to the generally accepted idea that the D3/D4 lines are related to the intrinsic nature of dislocations.

Three-dimensional mapping of stresses in plastically deformed diamond using micro-Raman and photoluminescence spectroscopy

The extreme mechanical properties of diamond have made it the material of choice for many industrial applications, ranging from cutting and grinding to wire-drawing dies. A detailed knowledge of its mechanical properties, also at high temperature, is thus of importance. Micro-Raman and photoluminescence (PL) spectroscopy were used to map the three-dimensional (3D) stress distribution surrounding a plastic impression made in a synthetic, type Ib single crystal diamond. The impression was created on a (100) face of the crystal with an Si3N4 impressor at 1400 °C using the so-called soft impressor technique. The diamond Raman peak was mapped at room temperature at the surface and at fixed intervals of 10 μm below the surface using a motorised X-Y stage. The depth (Z)-resolution was limited to 10 μm by means of a confocal pinhole. Using data from the Raman peak position, a 3D map of the stress contours surrounding the impression was generated, while the Raman width data yielded a map of the plastic deformation volume. The surface stress map shows a cross-shaped rosette pattern that corresponds very closely with micrographs imaging the pile-up on the surface due to dislocation movement. The “arms” of the pattern are in compression (∼1.5 GPa), while the center of the impression is in tension (∼1 GPa). The deformation map shows a radially symmetric area of deformation centered on the impression, with the maximum degree of deformation at the center. The stress contours compare favorably with the resolved shear stress contours calculated for diamond. PL intensity maps of the zero phonon line (ZPL) associated with the [N–V]− defect center at 1.945 eV provide images of the extent of vacancy formation and movement during the impression process. Data concerning the position and width of the ZPL correspond well with the Raman results.

Room-Temperature Photoluminescence Observation of Stacking Faults in 3C-SiC

We investigated the optical properties of stacking faults (SFs) in cubic silicon carbide by photoluminescence (PL) spectroscopy and mapping. The room-temperature PL spectra consisted of a 2.3 eV peak due to nitrogen and two undefined broad peaks at 1.7 eV and 0.95 eV. On the PL intensity mapping for the 2.3 eV peak, SFs appeared as dark lines. SFs which expose carbon atoms (SFC) and silicon atoms (SFSi) on the surface appeared as bright lines and dark lines, respectively, in PL mapping for the 1.7 eV and 0.95 eV peaks. We believe the two undefined peaks are associated with SFC. This technique allows us to detect SFs nondestructively and to distinguish between SFC and SFSi. We further suggest the presence of inhomogeneous stress around SFCs based on the broadening of the 2.3 eV peak.

Structural properties of GaN grown by MOVPE turbodisc mass-production reactor

The structural properties of GaN films grown on sapphire in a turbodisc multiwafer reactor have been investigated using high-resolution X-ray diffraction and transmission electron microscopy. We found that the line shape of the (0 0 2) rocking curve can be fitted with two peaks. Therefore, we suggest that the GaN films consist of two layers: one related to extended structural defects such as dislocations originating from the film-buffer layer interface and one that is of high quality. The crystalline quality of the second layer continues to improve with increasing layer thickness. We also show that the relatively large width of the (0 0 2) peaks (270 arcsec for a 3 μm film) has no causal link with the GaN film quality. In contrast, we demonstrate that there is a correlation between the in-plane structural features and optical and electrical properties. Using the (1 0 2) asymmetric reflection, 2″ wafer mapping of the (1 0 2) peak width closely mimics the resistivity mapping and photoluminescence intensity mapping. The (1 0 2) peak width is also sensitive to growth conditions of the buffer layer. By optimization of the buffer layer, the (1 0 2) peak width can be reduced to less than 600 s for a 3 μm GaN film. A uniformity better than 2% can be achieved in both the (1 0 2) width and the resistivity mapping over 2″ wafers for all six wafers in the same growth run.

Growth and characterization of 2 in InP crystals by the vertical gradient freeze technique

Using the example of a {100};, 2 in InP crystal grown with the vertical gradient freeze (VGF) technique, the advantages of this method are demonstrated. It shows the possibility of growing more uniform {;100}; InP, with lower etch pit density (EPD) compared with material which is grown using the liquid encapsulated Czochralski (LEC) technique. The VGF growth was performed in a cylindrical pyrolytic boron nitride (pBN) crucible, without a cone region. A 2 in diameter, LEC-grown crystal was prepared as a seed. The interface between the seed and the grown VGF crystal exhibited the main property changes between LEC- and VGF-grown material. At the seeding interphase, a reduction of half an order of magnitude in EPD was seen, along with the disappearance of the striations which are characteristic of LEC-grown material. Photoluminescence intensity mapping over a {;100}; VGF wafer showed a variation of about 1.8% with no detectable striations.

Space and Time Resolved Photoluminescence of Defects at Dislocations in In-Alloyed GaAs Substrate Material

ABSTRACTWe report a study of dislocations in In-alloyed GaAs substrate material using space and time resolved photoluminescence (PL). PL intensity maps show that an isolated dislocation cluster is in the center of a dark region with a 50μm radius surrounded by a bright region with an outer radius of 150μm. Lifetime measurements were made in the bright and dark regions. Values as long as 3.5 ns and as short as 250 psec were observed in adjacent bright and dark regions. These measurements indicate that the PL intensity contrast is explained by lifetime variations in these features. This supports the view that the dislocation cluster acts as a source and sink for defects which govern the lifetime in the surrounding material. Temperature dependence of the lifetime indicates two different defects may be involved. Both of these produce deep levels, neither one of which is EL2. A surface passivation technique is used to show that surface recombination is not important to the PL intensity contrast.