Olli Svensk

Analysis of Dislocations Generated during Metal–Organic Vapor Phase Epitaxy of GaN on Patterned Templates

We report on patterning and subsequent metal–organic vapor phase epitaxy overgrowth of GaN films on patterned GaN/sapphire templates. Templates with a hexagonal hole pattern were prepared by photolithography and dry etching. After GaN overgrowth voids were formed at the GaN/sapphire interface. Threading dislocations were found to bend and terminate at void sidewalls during the overgrowth resulting in improved material quality. The dislocations were analyzed by transmission electron microscopy combined with energy dispersive X-ray spectroscopy. Areas with increased Ga concentration were found at the tips of coalesced voids that introduced additional dislocations to the overgrown films.

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.

Stress distribution of GaN layer grown on micro-pillar patterned GaN templates

High-resolution Raman mapping of the stress distribution in an etched GaN micro-pillar template and a 5 μm thick GaN layer grown on a micro-pillar patterned GaN template is investigated. Raman mapping of the E2 (high) phonon shows differences in stress between the coalescing boundary, the top surface of the pillar region and around the GaN micro-pillar. Increased compressive stress is observed at the coalescing boundary of two adjacent GaN micro-pillars, when compared to the laterally grown GaN regions. The electron channeling contrast image reveals the reduction of threading dislocation density in the GaN layer grown on the micro-pillar patterned GaN template.

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.

InGaN-based 405 nm near-ultraviolet light emitting diodes on pillar patterned sapphire substrates

A study of GaN films and nitride based light emitting diodes (LEDs) grown on low density pillar structure (LDPS) and high density pillar structure (honeycomb like) sapphires patterned by chemical wet etching is described. Both types of patterned sapphire substrate (PSS) offered reduced defect density and improved performance of near-ultraviolet LED. In the case of LDPS patterned sapphire the correct choice of the pillar depth was found to be crucial for high quality crystal growth. A reduction of threading dislocation (TD) density from the level of 108 cm−2 down to the level of 2 × 109 cm−2 was observed. It was found that mostly enhanced light extraction rather than improved material quality caused the improvement of the LED performance.

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.

Band-edge luminescence degradation by low energy electron beam irradiation in GaN grown by metal-organic vapor phase epitaxy in H2 and N 2 ambients

The processing and characterization of optical components often requires the use of low energy electron beam (e-beam) techniques, such as scanning electron microscopy or electron beam lithography. The e-beam irradiation has been shown to produce band-edge luminescence degradation in GaN films grown by metal–organic vapor phase epitaxy (MOVPE), down to 20% of the original intensity in both photoluminescnece and cathodoluminescence measurements. The degradation is shown to be strongly related to activation of gallium vacancies in the GaN lattice. In this paper, this effect has been studied with GaN samples grown in two different carrier gases, N2 and H2. The degradation behavior appears almost identical in both cases, implying the vacancy formation to be independent of the carrier gas. Hence, MOVPE GaN electron beam irradiation resistance cannot be improved with the change of the carrier gas.

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.

Smoothing of microfabricated silicon features by thermal annealing in reducing or inert atmospheres

In this work, high-temperature annealing of reactive ion etched silicon microstructures in H2 and Ar gases is studied. Three types of structural features were etched with four different masks into 100-oriented silicon wafers. Scanning electron microscope and atomic force microscope (AFM) results show that when smoothing due to surface diffusion and desorption of silicon is taking place in both the argon and hydrogen gas environments, the three types of features respond differently to the treatments. The surface diffusion was observed to be strongly dependent on temperature in argon, whereas the transport was more linear and controllable in the hydrogen gas environment. For hydrogen, AFM studies were performed to observe the details of the smoothing process. Finally, some potential applications of these transport phenomena are discussed.

Migration kinetics of ion-implanted beryllium in ZnO and GaN

Migration kinetics of ion-implanted beryllium in ZnO and GaN has been studied using the modified radiotracer technique utilizing 7Be tracers. For ZnO the studies were carried out in the temperature range 650–750 °C. Clear Be migration following Arrhenius type behaviour was noted. The process is suggested to be limited due to formation of the BeZnO compound. An activation enthalpy of EA = (2.9 ± 0.3) eV and pre-exponential factor of D0 = 4 × 10−3 m2 s−1 were deduced for Be diffusion in ZnO. In the case of GaN two annealing temperatures were employed, 850 and 950 °C. Distinct trapping of Be in defects induced via implantation was noted. The activation enthalpy for Be diffusion in GaN free of implantation induced defects is estimated to be ~4 eV in agreement with theoretical predictions presented in the literature.