GIPO newsletter

Regularly-patterned Multi-section GaN Nanorod Arrays Grown with a Pulsed Growth Technique

Professor C. C. (Chih-Chung) Yang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The growth of regularly-patterned multi-section GaN nanorod (NR) arrays based on a pulsed growth technique with metalorganic chemical vapor deposition is demonstrated. Such an NR with multiple sections of different cross-sectional sizes is formed by tapering a uniform cross section to another through the decrease of Ga supply duration stepwise for reducing the size of the catalytic Ga droplet. Contrast line structures are observed in either a scanning electron microscopy or a transmission electron microscopy image of an NR. Such a contrast line-marker corresponds to a thin Ga-rich layer formed at the beginning of GaN precipitation of a pulsed-growth cycle and illustrates the boundary between two successive growth cycles in pulsed growth. By analyzing the geometry variation of the contrast line-markers, the morphology evolution in the growth of a multi-section NR, including a tapering process, can be traced. Such a morphology variation is controlled by the size of the catalytic Ga droplet and its coverage range on the slant facets at the top of an NR. The comparison of emission spectrum between single-, two-, and three-section GaN NRs with sidewall InGaN/GaN quantum wells (QWs) indicates that a multi-section NR can lead to a significantly broader sidewall emission spectrum.

Fig. 1 (a)-(d): Tilted SEM images of a single-section (sample S), two 2-section (samples T1 and T2), and one 3-section (sample T3) n-GaN NR arrays. (e)-(h): Tilted SEM images after three periods of InGaN/GaN QWs are grown on the sidewalls of the n-GaN NRs in samples S and T1-T3, respectively. (i)-(l): Cross-sectional SEM images of the QW NR arrays in samples S and T1-T3, respectively. Numbers 1-3 are marked for indicating the locations of local cathodoluminescence measurements.

Fig. 2 (a)-(d): Local cathodoluminescence spectra at the three locations for samples S and T1-T3, respectively.

Fig. 3 Normalized photoluminescence spectra of the QW NR arrays in samples S and T1-T3.

Coupling Behaviors of a Radiating Dipole with the Surface Plasmon Induced on a Metal Protrusion

Professor Y. W. Kiang’s Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The coupling behaviors of a radiating dipole with localized surface plasmon (LSP) resonance modes induced on an Ag protrusion in a GaN layer are numerically studied with an algorithm including the feedback effect of LSP resonance to modify the emission behavior of the radiating dipole. Two spectral ranges (400 – 570 and 570 – 800 nm) of radiated power enhancement can be observed. The radiated power enhancement in the long-wavelength (short-wavelength) range is generated through the coupling of the radiating dipole with the dipole and quadrupole (higher-order) LSP resonance modes induced on the protrusion. Because the LSP electromagnetic fields of those higher-order modes are mainly distributed around the tip of the protrusion, which is close to the radiating dipole, the radiated power enhancement is quite effective in the blue-green range even though the corresponding metal absorption can be higher than that of a lower-order LSP mode. Compared with the configurations of surface metal nanoparticle (NP) and embedded metal NP, the protrusion structure has the advantages of relatively higher radiated power enhancement in the blue-green spectral range and relatively lower fabrication cost from a practical application viewpoint.

Fig. 1. Problem geometry of numerical simulation. A truncated-cone-shaped Ag protrusion with the base radius r, height h, and inclination angle θ in GaN is connected to the upper half-space of Ag. The vertical distance between the protrusion tip and the designated QW, which consists of a radiating dipole array in the x-y plane, is d. The lateral position of a radiating dipole is defined as s.

Fig. 2. Normalized radiated powers as functions of wavelength by fixing the distance between the protrusion tip and the radiating dipole, d, at 30 nm and varying the protrusion length, h. The lateral distance of the radiating dipole s is set at s=0. The radiating dipole is oriented along the x-axis. The dashed curve is plotted for the normalized total radiated power of a radiating dipole located in GaN 100 nm away from an air/GaN interface.

MBE growth of the highly mismatched alloy InAsPSb

Professor Hao-Hsiung Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Because of the strong internal bond distortion resulting from highly mismatched bond lengths, the InPSb and InAsPSb alloys are with a lot of peculiar properties which have not been fully understood yet. We used VG V80H GSMBE to grow the two alloys on InAs and GaAs. Also, with the surprising property that the lattice constant of InAsPSb remains approximately equal to that of InAs, we can treat the quaternary alloy as a buffer layer between GaAs and InAs. Results show that the buffer layer is feasible, although there are some stacking faults in the interface.

Fig. 1. XRD of InPSb grown on InAs and GaAs substrates. C3723 shows a sharp and narrow-line width epilayer signal without phase separation.

Fig. 2. XRD of InAsPSb grown on InAs and GaAs substrates. No matter how the composition changes, the diffraction angles of InAsPSb keep approximately equal.

Fig. 3. (004) and (115) RSM of InAs/InAsPSb/GaAs structure.

Fig. 4. HRTEM cross-section view of InAs/InAsPSb/GaAs structure.

High Efficient Planar Heterojunction Perovskite Solar Cells via Sandwich Evaporation Technique

Professor Ching-Fuh Lin’s laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Due to the gradual depletion of fossil fuels, the energy issue has received wide attention in this century. The world is making many efforts to develop alternative energy. Solar energy is an inexhaustible and readily available resources on earth. On the top of that, it is eco-friendly to our environment. For these advantages, the development of solar cells has become an excellent strategy to solve the shortage of energy. Among many types of solar cells, perovskite solar cells suddenly appear on the horizon and the conversion efficiency has been made significant progress from 3% to above 20% in just six years.

Although high-efficiency perovskite solar cells have been achieved, perovskite formed by solution process, in general, is very susceptible to the atmosphere and moisture. The perovskite grains usually become small and messy by the rapid formation of perovskite. In this research, we developed a new method using a homemade chamber, called Sandwich Evaporation Technique (SET), to form the perovskite layer, as shown in Fig.1. The process can be mostly conducted in the atmosphere and not be affected by the environment easily. The process includes the following steps. First, we raised a sandwich-like structure, MAI/PbI₂/MAI, to fabricate the active layer of perovskite to enhance the current density, as shown in Fig. 2. Second, the perovskite solar cells are fabricated with the traditional structure of ITO/PEDOT-4083/Perovskite/PCBM/PEI/Ag, which can successfully achieve the power conversion efficiency of 14.03%.

Fig. 1. The picture of the homemade chamber of Sandwich Evaporation Technique (SET) process.

Fig. 2. The picture of Sandwich Evaporation Technique (SET) process.

Furthermore, we modify those techniques that can make the better film of the active layer. IPA, CB have been applied on the substrates to slow down the reaction. Here this mechanism is refined for the SET process. Vials with solvent are placed inside the chamber directly. With the refined process, we are able to achieve 14.62% of perovskite solar cells with a higher current density and large crystals of perovskite, as shown in Fig. 3. This brand new method to fabricate the perovskie under atmosphere environment could potentially enable the mass production in the future using the Sandwich Evaporation Technique (SET), as shown in Fig.4.

Fig. 3. The SEM image of the perovskite.

Fig. 4. The schematic diagram of the Sandwich Evaporation Technique (SET).

Improved Internal-Quantum Efficiency of GaN-Based LEDs

Professor Chieh-Hsiung Kuan

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Instead of the limited exposure feature sizes of conventional photolithography, the ELIONIX ELS-7000 E-beam lithography system and the wet-etching technology were conducted to carry out the investigation. All patterned-sapphire substrates (PSSs) were all arranged in squares with the period of 2000nm and with various post-duty cycles (PDCs) of 1, 4 and 8%, which is defined in Fig. 1(a), respectively. Each square had a hexagonal post. SEM images of accomplished PSSs are shown in Fig. 1(b)-(d). The PSSs and a conventional sapphire substrate (CSS) with polished surface were grown using low pressure metal organic chemical vapor deposition (LP-MOCVD). The light-emitting diodes (LEDs) were encapsulated with TO-46 metal can package with a chip size of 17×34 mil².

Figure 2 shows the residual compressive strain and the associated Raman width of the GaN-based PSS LEDs. With the increase of PDC, the residual compressive strain of the GaN-based PSS LEDs reduces first and then slightly increases as compared to the sample with the CSS. Fig. 3 shows the relative IQE versus PDC of the GaN-based PSS LEDs biased at injection currents of 120mA and 360mA, respectively. With the increase of PDC in contrast to the CSS, the reduced residual compressive strain leads to the enhanced relative IQE under the same crystalline quality.

Fig. 1. The SEM images of different PSSs and the definition of post-duty cycle.

Fig. 2. The residual compressive strain with the associated Raman width. The inserted figure is the Raman spectra.

Fig. 3. The relative IQE of the LEDs, calculated from the inserted figure regarding the EQE versus PDC.