Jong-In Shim

Direct comparison of structural and optical properties of a nitride-based core-shell microrod LED by means of highly spatially-resolved cathodoluminescence and μ-Raman(Conference Presentation)

We present a nanometer-scale correlation of the structural, optical, and electronic properties of InGaN/GaN core-shell microrod LEDs: The microrods were fabricated by MOVPE on a GaN/sapphire template covered with an SiO2-mask. Through the mask openings, Si-doped n-GaN cores were grown with high SiH4 flow rate at the base. Subsequently, the SiH4 flow rate was reduced towards the microrod tip to maintain a high surface quality. The Si-doped GaN core was finally encased by an InGaN single quantum well (SQW) followed by an intrinsic GaN layer and a thick Mg-doped p-GaN shell. Highly spatially resolved cathodoluminescence (CL) directly performed in a scanning transmission electron microscope (STEM) was applied to analyze the free-carrier concentration within the Si-doped GaN core and the luminescence properties of the individual functional layers. The CL was supported by Raman spectroscopy directly carried out at the same microrod on the thin TEM-lamella. The cross-sectional CL of a single microrod resolves the emission of the single layers. CL and Raman measurements reveal a high free-carrier concentration of 7x1019 cm 3 in the bottom part and a decreasing doping level towards the tip of the microrod. Moreover, structural investigations exhibit that initial Si-doping of the core has a strong influence on the formation of extended defects in the overgrown shells. However, we observe the most intense emission coming from the InGaN QW on the non-polar side walls, which shows a strong red shift along the facet in growth direction due to an increased QW thickness accompanied by an increased indium concentration right at the intersection of generated defects and InGaN QW, a red shifted emission appears, which indicates indium clustering.

Suppressing the incorporation of carbon impurity in AlGaN:Mg for green LDs with low operation voltage (Conference Presentation)

This paper reports the influence of carbon impurities on electrical properties of AlGaN: Mg layer which is used in InGaN-based blue/green laser diodes (LDs) as the cladding layer. AlGaN layer grown by MOCVD usually contains more carbon impurity than GaN, especially when AlGaN is grown at a low temperature to avoid the degradation of InGaN/GaN quantum wells in green LD with high indium content. However, no experimental study on the effect of carbon impurity on the electrical properties of the AlGaN:Mg layer has been reported. All AlGaN:Mg samples were grown on c-plane GaN/sapphire template at various growth pressure, growth rate and V/III ratio to suppress the carbon impurity concentration. These sample were then activated at 950℃ for 90s in nitrogen ambient. The hole concentration and resistivity of Al0.07Ga0.93N:Mg samples depend on carbon concentration. By reducing carbon concentration from 2×1018 to 5×1016 cm-3, the hole concentration increase from 7.5×1016 to 3.5×1017 cm-3, and thus the resistivity of p-Al0.07Ga0.93N decreases from 7.4 to 2.2 Ω·cm . Based on the analysis of the charge neutrality equation, we believe that the compensation effect of CGa(Al) as a shallow donor in AlGaN:Mg explains the dependence of the hole concentration and the resisitivity on the carbon concentration in our samples, which will be discussed in detail in this report. By applying the optimized AlGaN:Mg grown at low temperature as the cladding layer, we have obtained green LD structures without thermal degradation in the InGaN active region. The differential resistance is 2.4 Ω leading to V= 4.9 V at 4 kA/cm2. It lases at 508nm with Jth= 1.8 kA/cm2, and has a output power of 58 mW at a current density of 6 kA/cm2.

Freezing of quantum confinement Stark effect at low temperatures? (Conference Presentation)

There are two physical phenomena governing the light emission in InGaN quantum structures: the internal electric fields and the In composition fluctuations. Both these effects manifest through the blue shift of the wavelength emission with the excitation intensity and both of them have the pronounced influence on the light emitting properties of these structures. In order to discriminate between these two effects, we fabricated two identical structures: one with the quantum barriers doped with silicon (method for internal electric field screening) and the other with an undoped active region. Under the optical excitation the emission peak shifts by almost 35 nm (Si doped) and 50nm (without Si). Additionally, we studied temperature dependence of the emission peak position. In case of low temperatures and at RT and high pumping energy, emission energy position is almost the same for both samples. Our observations lead us to the conclusion that at low temperatures and at high pumping regime the Quantum Confined Stark Effect (QCSE) is totally suppressed. While this is understandable that at high carrier injection QCSE is screened, the origin of the low temperature effect is much less clear. We can speculate that at the lowest temperature the carriers are localized eliminating the spatial separation of holes and electrons wavefunctions. Measured cathodoluminescence (CL) maps show the same level of the indium fluctuations for both samples. At higher excitation the fluctuations starts to be less visible suggesting band filling of states. Finally we compare recombination times by means of time resolved photoluminescence.

Demonstration of nitride-based lasers excited by electron beam (Conference Presentation)

The UV semiconductor-based laser sources are important for a variety of fields, including medical, mechanical processing, chemical processing, biology, and photonics. However, the development of UV-B and UV-C laser diodes is strongly hampered because of the difficulties with current injection technology such as the realization of both a high hole concentration and low resistivity p-type AlGaN with a high AlN molar fraction. Because laser oscillation from AlGaN, with a high AlN molar fraction, can be obtained under optical pumping, UV lasers with controllable wavelengths should be realized if this problem can be solved. One promising technique for avoiding these problems is the use of electron beam excitation. Till date, nitride semiconductor-based lasers have been designed to achieve population inversion of the carrier and to oscillate due to current injection. However, as previously discussed, it is difficult to achieve wavelengths in UV-B and UV-C using this method. The conductivity control of nitride semiconductors is unnecessary using electron beam excitation. Therefore, it would be possible to expand the wavelength region for the laser action of nitride semiconductor-based lasers from deep UV to infrared if a nitride semiconductor-based laser could be oscillated via electron beam excitation. In this study, nitride-based lasers excited by electron beam were investigated, and laser emission was observed for the first time from a GaInN/GaN and GaN/AlGaN-based MQWs excited by an electron beam.

Single-photon emission from a high-purity hexagonal boron nitride crystal (Conference Presentation)

Point-like defects in wide-bandgap materials are at the heart of a broad range of emerging applications including quantum information processing and metrology [1]. A well-known example is the nitrogen-vacancy (NV) defect in diamond, which can be used as a solid-state qubit to perform elaborate quantum information protocols [2] and highly sensitive magnetic field sensing [3]. These results motivate the search of new defects in other wide-bandgap materials, which would offer an expanded range of functionalities compared to NV defects in diamond. In that context, hexagonal boron nitride (hBN) appears as an appealing material. First, it has a 6-eV bandgap, which is ideally suited to host optically active defects with energy levels deeply buried between the valence band and the conduction band. Second, hBN is an electrical insulator with a two-dimensional (2D) honeycomb structure, which is a key element of Van der Waals heterostructures. Such “artificial” materials are currently attracting a great interest owing to their unique mechanical, electrical and optical properties [4]. Combining these properties with individual quantum systems would likely open new perspectives in quantum technologies. In this talk, I will report on the optical detection of individual defects hosted in a high-purity hBN crystal. Stable single photon emission is demonstrated under ambient conditions by means of photon correlation measurements [5]. A detailed analysis of the photophysical properties of the defect reveals a highly efficient radiative transition, leading to one of the brightest single photon source reported to date from a bulk, unpatterned, material. These results make a bridge between the physics of 2D materials and quantum technologies, and pave the way towards applications of van der Waals heterostructures in photonic-based quantum information science, metrology and optoelectronics. References [1] D. D. Awschalom, L. C. Bassett, A. S. Dzurak, E. L. Hu, and J. R. Petta, Science 339, 1174 (2013). [2] B. Hensen et al., Nature 526, 682-686 (2015). [3] J.-P. Tetienne, T. Hingant, J.-F. Roch, P. Maletinsky, and V. Jacques, Rep. Prog. Phys. 77, 056503 (2014). [4] A. K. Geim and I. V. Grigorieva, Nature 499, 419-425 (2013). [5] L. J. Martinez, T. Pelini, V. Waselowski, J. R. Maze, B. Gil, G. Cassabois, and V. Jacques, preprint arXiv:1606.04124.

Impact of InGaN alloy disorder on LED properties (Conference Presentation)

It emerges that LEDs properties are strongly impacted by intrinsic disorder induced by random In compositional fluctuations. They obviously impact the light emission spectrum and carrier mobilities. The quantitative evaluation of their impact in full heterostructures is made difficult by the extreme demand on computing resources when calculating solutions of the Schrodinger equation for a disordered 3D potential map. Calculations are then limited to small volumes and to the first few quantum states, not allowing for simulations of transport properties in full devices. It was recently shown in a simplified model that disorder can account for a turn-on voltage of LEDs smaller by 1V compared to standard simulations. We will present novel theoretical and modeling tools of disorder, namely the Filoche-Mayboroda 3D localization landscape theory, which from the original disordered energy map provides an effective potential which allows to use standard transport equations while accounting for microscopic disorder. We thus gain a deep understanding of various effects of disorder in nitride heterostructures on their electrical and optoelectronic properties. As a first application of the new tool we model our detailed measurements of the absorption edge of InGaN/GaN quantum wells with varying In composition. The tool is then applied to carrier transport in full LED structures. The effective potential increases current at a given bias voltage by accounting for two quantum effects of disorder, tunneling and confinement, which together smooth out potential discontinuities in the heterostructure. Quantum efficiency, Auger and leakage droop, ideality factor of the LED will be discussed.

Facet temperature measurement of GaN-based laser diodes using thermoreflectance spectroscopy (Conference Presentation)

Investigation of temperature distribution on the facet of the device, with high spatial and temperature resolution, is crucial to gain insight into thermally activated degradation modes in GaN-based lasers. This work undertakes the problem of temperature distribution measurement on the facet of the nitride lasers. Thermal investigation of the nitride devices is mainly based on thermal imaging. However, this approach is characterized by inherently low spatial resolution, as well as the fact, that the registered image, is averaged over the volume of the device, limiting the ability to observe the enhanced thermal processes occurring at the vicinity of the surface (front facet). Thermoreflectance spectroscopy, provides the possibility of registering of high spatial and temperature resolution images of the surface of the device operating in quasi-CW or pulsed mode. In this work we present development of the unique experimental setup and procedure, devoted to the thermal characterization of the nitride lasers. Thermal characterization of series of devices was performed, providing a mode for comparing different operating conditions, geometries and device designs. Measurement of the temperature profile and high-resolution temperature distribution maps on the front facet of AlGaN/GaN via thermoreflectance spectroscopy will be demonstrated. The results indicate the direction to take in order to improve the laser reliability and performance. Additionally, the degradation mechanisms induced by temperature increase are discussed.

Electrical and optical properties of flexible nanowire blue light-emitting diodes under mechanical bending (Conference Presentation)

III-Nitride-based nanowire LEDs have shown high internal quantum efficiency and stable light emission over a wide current range to enable white phosphor-free white-light emission. The structures provide a unique advantage for flexible electronics where the out-of-plane three-dimensional nanowires are invariant to applied bending. Testing this concept, InGaN dot-in-wire light-emitting diodes on sapphire substrates were transferred onto flexible polyethylene terephthalate (PET) substrates using a bonding and laser-liftoff process. In0.15Ga0.85N nanowire blue LEDs were grown on sapphire substrates having Ni/Au contacts applied to the top p-doped region and a lateral insulating polyimide layer applied between the nanowires. The nanowire structures were then bonded onto a transfer wafer and separated from its sapphire growth substrate by laser-liftoff (LLO) using a 266 nm KrF laser. A double-transfer technique, where the inverted LED structures were then transferred and bonded onto the PET with a silver-based adhesive that formed the final bottom contact. The LEDs before and after the transfer process did not show measurable degradation in the I-V and optical characteristics. The 425 nm luminescence peak was found to remain constant during applied mechanical strain on the flexible substrate suggesting the nanowire LEDs did not experience any plane-strain during bending. A constant 2.5 V turn-on voltage, and a forward current of 0.4 mA at 4 V was measured under concave and convex bending. Atomic force and scanning electron microscopy characterization will also be shown of the nanowire device before and after double transfer as well as numerical simulation of the mechanical motion of the nanowire structures during bending.

Semi-insulating HVPE-GaN grown on native seeds (Conference Presentation)

Hydride Vapor Phase Epitaxy (HVPE) is the most popular method for fabrication of high structural quality and high-purity GaN substrates. The technology of obtaining a low level of impurities together with high crystallographic quality of HVPE-GaN crystals enables the next step, namely introducing intentional doping to the growth process and obtaining semi-insulating crystals. This work describes developing a method for incorporation of acceptors (carbon or iron) into HVPE-grown GaN while maintaining high structural quality and low level of other impurities in the material. Ammonothermally grown GaN crystals and substrates will be used as seeds. All growth processes will be carried out in a home-built quartz horizontal HVPE reactor. Methane (CH4) will be used as the precursor of carbon. The FeCl2 precursor will be created inside the reactor chamber by an HCl-stream over elemental iron. HVPE crystallization runs with different flows of acceptor precursors will be performed. HVPE-GaN:C and HVPE-GaN:Fe crystals will be characterized with X-ray diffraction, Raman spectroscopy, low-temperature photoluminescence, optical as well as transmission electron microscopies, Hall measurements, and Secondary Ion Mass Spectrometry. The properties of crystallized HVPE-GaN:C and HVPE-GaN:Fe will be compared in detail. It will be shown that concentrations of impurities (carbon or iron) in the new-grown material is always very uniform across the (0001) surface and along the c-direction. This result together with a high crystalline quality of the crystallized material will allow to obtain the semi-insulating HVPE-GaN crystals with resistivity of the order of 109 Ω.cm at room temperature.

Novel device designs enabled by lattice-matched GaN-ZnGeN2 heterostructures(Conference Presentation)

Group III-nitride (Al-, In-, Ga-, N) material system has been well studied and widely applied in optoelectronics such as light emitting diodes (LEDs) for solid state lighting. In contrast, the group II-IV-nitride is rarely studied, yet it can expand the material properties provided by III-nitrides. For example, ZnGeN2 has a similar bandgap and lattice constant as those of GaN. Recently, theoretical studies based on first principle calculation indicate a large band offset between GaN and ZnGeN2 (Delta_Ec=1.4 eV; Delta_Ev=1.5 eV). Utilizing the novel heterostructures of GaN (InGaN)/ZnGeN2, we studied the following two types of device structures: 1) Type-II InGaN-ZnGeN2 quantum wells (QWs) for high efficiency blue and green LEDs; 2) Lattice-matched GaN-ZnGeN2 coupled QWs for near-IR intersubband transitions. The design of type-II InGaN-ZnGeN2 QWs leads to a significant enhancement of the electron-hole wavefunction overlap due to the strong confinement of the holes in the ZnGeN2 layer as well as the engineered band bending. Simulation studies based on a self-consistent 6-band k∙p method indicate an enhancement of 5-7 times of spontaneous emission rate for an appropriately designed type-II InGaN-ZnGeN2 QWs for LED applications. For the coupled QW structure, it is comprised of two GaN QWs separated by a thin ZnGeN2 barrier layer, with thick ZnGeN2 layers as outer barriers surrounding the QWs. Our studies indicate that with optimized ZnGeN2 barrier thickness, the energy separation between E1 and E2 can be tuned to 92 meV for the resonance of the electron and LO-phonon scattering.