Lauri Riuttanen

Electrical measurement of internal quantum efficiency and extraction efficiency of III-N light-emitting diodes

We propose a direct electrical measurement method for determining the extraction efficiency (EXE) and internal quantum efficiency (IQE) of III-Nitride light-emitting diodes (LEDs). The method is based on measuring the optical output power as a function of injection current at current densities near the external quantum efficiency (EQE) maximum and extracting IQE and EXE from the measurement data. In contrast to conventional methods, our method requires no low temperature measurements or prior knowledge of the device structure. The method is far more convenient than commonly used methods because it enables measuring the EXE and IQE of different LED structures at room temperature directly in a repeatable and consistent way. This enables convenient comparison of LED structures. We apply the method to determine the IQE and EXE of one commercial LED and selected self-grown planar LED chips to compare the effects of different LED structure designs. Our results are in line with published experimental results and...

Diffusion injected multi-quantum well light-emitting diode structure

The attention towards light-emitting diode (LED) structures based on nanowires, surface plasmon coupled LEDs, and large-area high-power LEDs has been increasing for their potential in increasing the optical output power and efficiency of LEDs. In this work we demonstrate an alternative way to inject charge carriers into the active region of an LED, which is based on completely different current transport mechanism compared to conventional current injection approaches. The demonstrated structure is expected to help overcoming some of the challenges related to current injection with conventional structures. A functioning III-nitride diffusion injected light-emitting diode structure, in which the light-emitting active region is located outside the pn-junction, is realized and characterized. In this device design, the charge carriers are injected into the active region by bipolar diffusion, which could also be utilized to excite otherwise challenging to realize light-emitting structures.

Diffusion Injection in a Buried Multiquantum Well Light-Emitting Diode Structure

Devices based on nanostructures hold great potential to improve the performance of present light emitter technologies. In this paper, we examine a buried multiquantum well III-nitride diffusion injected light-emitting diode (DILED), where the active region is located outside the p-n junction and current injection to the active region takes place through bipolar diffusion. We study the current-voltage behavior and light emission characteristics of the DILED as a function of temperature experimentally and theoretically. We show that in contrast to conventional LEDs, the light output efficiency of the DILED increases when temperature is increased from 0 °C to 100 °C. This anomalous temperature behavior is shown to be linked to the strong temperature dependency of ionized acceptor density in the p-doped region, which increases the hole diffusion current into the active region. This highlights the fundamental difference in the operating principle of the DILED compared with conventional LEDs. In addition to optical and electrical characterization of a DILED, we also study the relation of the observed yellow-band luminescence to Shockley-Read-Hall recombination, compare the measurements to charge carrier transport simulations, and present an equivalent circuit model of the DILED structure for additional insight into the new current injection scheme.

Electrical injection to contactless near-surface InGaN quantum well

Charge injection to the prevailing and emerging light-emitting devices is almost exclusively based on the double heterojunction (DHJ) structures that have remained essentially unchanged for decades. In this letter, we report the excitation of a near surface indium gallium nitride (InGaN) quantum well (QW) by bipolar carrier diffusion from a nearby electrically excited pn-homojunction. The demonstrated near surface QW emitter is covered only by a 10 nm GaN capping leaving the light-emitting mesa perfectly free of metals, other contact, or current spreading structures. The presented proof-of-principle structure, operating approximately with a quantum efficiency of one fifth of a conventional single QW reference structure, provides conclusive evidence of the feasibility of using diffusion injection to excite near surface light-emitting structures needed, e.g., for developing light emitters or photo-voltaic devices based on nanoplasmonics or free-standing nanowires. In contrast to the existing DHJ solutions or optical pumping, our approach allows exciting nanostructures without the need of forming a DHJ, absorbing layers or even electrical contacts on the device surface.

Optical and electrical properties of GaN: Si-based microstructures with a wide range of doping levels

The optical and electrical properties of silicon-doped epitaxial gallium nitride layers grown on sapphire have been studied. The studies have been performed over a wide range of silicon concentrations on each side of the Mott transition. The critical concentrations of Si atoms corresponding to the formation of an impurity band in gallium nitride (∼2.5 × 1018 cm−3) and to the overlap of the impurity band with the conduction band (∼2 × 1019 cm−3) have been refined. The maximum of the photoluminescence spectrum shifts nonmonotonically with increasing doping level. This shift is determined by two factors: (1) an increase in the exchange interaction leading to a decrease in the energy gap width and (2) a change in the radiation mechanism as the donor concentration increases. The temperature dependence of the exciton luminescence with participating optical phonons has been studied. The energies of phonon-plasmon modes in GaN: Si layers with different silicon concentrations have been measured using Raman spectroscopy.

Vertical excitation profile in diffusion injected multi-quantum well light emitting diode structure

Due to their potential to improve the performance of light-emitting diodes (LEDs), novel device structures based on nanowires, surface plasmons, and large-area high-power devices have received increasing amount of interest. These structures are almost exclusively based on the double hetero junction (DHJ) structure, that has remained essentially unchanged for decades. In this work we study a III-nitride diffusion injected light-emitting diode (DILED), in which the active region is located outside the pn-junction and the excitation of the active region is based on bipolar diffusion of charge carriers. This unorthodox approach removes the need of placing the active region in the conventional current path and thus enabling carrier injection in device structures, which would be challenging to realize with the conventional DHJ design. The structure studied in this work is has 3 indium gallium nitride / gallium nitride (InGaN/GaN) quantum wells (QWs) under a GaN pn-junction. The QWs are grown at diferent growth temperatures for obtaining distinctive luminescence peaks. This allows to obtain knowledge on the carrier diffusion in the structure. When the device is biased, all QWs emit light indicating a significant diffusion current into the QW stack.

Interaction of surface plasmon polaritons in heavily doped GaN microstructures with terahertz radiation

We present the results of experimental and theoretical studies of the surface plasmon polariton excitations in heavily doped GaN epitaxial layers. Reflection and emission of radiation in the frequency range of 2–20 THz including the Reststrahlen band were investigated for samples with grating etched on the sample surface, as well as for samples with flat surface. The reflectivity spectrum for p-polarized radiation measured for the sample with the surface-relief grating demonstrates a set of resonances associated with excitations of different surface plasmon polariton modes. Spectral peculiarities due to the diffraction effect have been also revealed. The characteristic features of the reflectivity spectrum, namely, frequencies, amplitudes, and widths of the resonance dips, are well described theoretically by a modified technique of rigorous coupled-wave analysis of Maxwell equations. The emissivity spectra of the samples were measured under epilayer temperature modulation by pulsed electric field. The emi...

Terahertz reflection and emission associated with nonequilibrium surface plasmon polaritons in n-GaN

Surface plasmon polaritons are investigated in heavily doped n-GaN epitaxial layers. The grating etched on the surface of the epitaxial layer is used to convert photons into the surface plasmon polaritons and vice versa. The spectral study of reflection demonstrates the possibility of nonequilibrium surface plasmon polaritons excitation due to terahertz radiation scattering on the grating. Terahertz electroluminescence is investigated under lateral electric field. The luminescence spectrum demonstrates a significant contribution of nonequilibrium surface plasmon polariton scattering to terahertz radiation emission.

Diffusion-driven current transport to near-surface nanostructures

Diffusion-driven current transport (DDCT) has recently been proposed as a new way to organize the current injection in nanoscale optoelectronic devices. The very recent first proof-of-principle experiments have also shown that DDCT works as predicted theoretically. In this work we perform simulations on DDCT-based III-Nitride devices and demonstrate how the optimization of DDCT differs significantly from the optimization of conventional double heterostructure based devices.