Silicon-Integrated III–V Light Emitters and Absorbers Using Bipolar Diffusion

Peer reviewe

Injektiogeometrian vaikutus sähkövirran jakautumiseen puolijohdeledeissä

Diplomityössä esitellään ledien toiminnan kuvauksessa käytettyjä fysikaalisia malleja sekä mallinnetaan kahden valitun ledirakenteen toimintaa numeerisesti. Mallit ratkaistaan käyttämällä hyväksi elementtimenetelmää. Mallinnettavat ledirakenteet perustuvat TFFC- ja TMJ-ledeihin, jotka on hiljattain esitelty tieteellisessä kirjallisuudessa tehokkaina valaistusledirakenteina. Diplomityössä käytetyt numeeriset mallit perustuvat Maxwellin yhtälöihin, Fermi-Dirac-statistiikkaan, puolijohteiden virrankuljetusyhtälöihin ja yleisiin rekombinaatiomalleihin. Työssä mallinnetaan nitridipuolijohteita, joissa esiintyy voimakkaita spontaaneja sekä pietsosähköisiä polarisaatiokenttiä, ja näiden kenttien vaikutus on huomioitu työssä käytetyissä malleissa. Työssä verrataan yllämainittuja ledirakenteita toisiinsa tutkimalla sähkövirran injektiota, ledien hyötysuhdetta, säteilevän valon tehoa sekä säteilevän valon taajuusjakaumaa. Työssä tutkitaan myös virranlevityskerroksen leveyden vaikutusta rakenteiden tehokkuuteen. Lisäksi tarkastellaan spontaanin ja pietsosähköisen polarisaation vaikutusta ledien toimintaan ja hyötysuhteeseen.In this thesis the physical models needed in simulating the operation of semiconductor LEDs are reviewed and used to numerically model the operation of two different LED structures. The numerical models are solved with the finite element method and the simulated structures are variations of the thin-film flip-chip and transverse multi-quantum well LEDs which are recently proposed structures for efficient white light generation. The numerical models used in this thesis are based on Maxwell's equations, Fermi-Dirac statistics, semiconductor current equations and basic recombination models. Modelled materials are nitride semiconductors which exhibit strong spontaneous and piezoelectric polarization fields, and these fields are also included in the models. The two above mentioned LED structures are compared with each other in terms of the current injection properties, overall efficiency, light output power and the frequency distribution of the generated light. The effect of the current injection layer width on the efficiency is studied as well. In addition, the effect of the spontaneous and piezoelectric polarization fields on the LED operation and efficiency is investigated

Monte Carlo study of non-quasiequilibrium carrier dynamics in III–N LEDs

Hot carrier effects have been observed in recent measurements of III–Nitride (III–N) light-emitting diodes. In this paper we carry out bipolar Monte Carlo simulations for electrons and holes in a typical III–N multi-quantum well (MQW) LED. According to our simulations, significant non-quasiequilibrium carrier distributions exist in the barrier layers of the structure. This is observed as average carrier energies much larger than the 1.5kBT1.5kBT corresponding to quasi-equilibrium. Due to the small potential drop over the MQW being modest, the non-quasiequilibrium carriers can be predominantly ascribed to nnp and npp Auger processes taking place in the QWs. Further investigations are needed to determine the effects of hot carriers on the macroscopic device characteristics of real devices

Bipolar Monte Carlo Simulation of Hot Carriers In III-N LEDs

We carry out bipolar Monte Carlo (MC) simulations of electron and hole transport in a multi-quantum well light-emitting diode with an electron-blocking layer. The MC simulation accounts for the most important interband recombination and intraband scattering processes and solves self-consistently for the non-quasiequilibrium transport. The fully bipolar MC simulator results in better convergence than our previous Monte Carlo-drift-diffusion (MCDD) model and also shows clear signatures of hot holes. Accounting for both hot electron and hot hole effects increases the total current and decreases the efficiency especially at high bias voltages. We also present our in-house full band structure calculations for GaN to be coupled later with the MC simulation in order to enable even more detailed predictions of device operation

Elimination of resistive losses in large-area LEDs by new diffusion-driven devices

High-power operation of conventional GaN-based light-emitting diodes (LEDs) is severely limited by current crowding, which increases the bias voltage of the LED, concentrates light emission close to the p-type contact edge, and aggravates the efficiency droop. Fabricating LEDs on thick n-GaN substrates alleviates current crowding but requires the use of expensive bulk GaN substrates and fairly large n-contacts, which take away a large part of the active region (AR). In this work, we demonstrate through comparative simulations how the recently introduced diffusion-driven charge transport (DDCT) concept can be used to realize lateral heterojunction (LHJ) structures, which eliminate most of the lateral current crowding. Specifically in this work, we analyze how using a single-side graded AR can both facilitate electron and hole diffusion in DDCT and increase the effective AR thickness. Our simulations show that the increased effective AR thickness allows a substantial reduction in the efficiency droop at large currents, and that unlike conventional 2D LEDs, the LHJ structure shows practically no added efficiency loss or differential resistance due to current crowding. Furthermore, as both electrons and holes enter the AR from the same side without any notable potential barriers in the LHJ structure, the LHJ structure shows an additional wall-plug efficiency gain over the conventional structures under comparison. This injection from the same side is expected to be even more interesting in multiple quantum well structures, where carriers typically need to surpass several potential barriers in conventional LEDs before recombining. In addition to simulations, we also demonstrate selective-area growth of a finger structure suitable for operation as an LHJ device with 2µm distance between n- and p-GaN regions.Peer reviewe

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 temperaturemeasurements 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 also give more insight to our earlier findings regarding the effects of growth parameters on the quantum efficiency. In addition, our measurement method allows estimating the Shockley-Read-Hall and radiative recombination parameters if the Auger parameter is known.Peer reviewe

Intracavity double diode structures with GaInP barrier layers for thermophotonic cooling

Optical cooling of semiconductors has recently been demonstrated both for optically pumped CdS nanobelts and for electrically injected GaInAsSb LEDs at very low powers. To enable cooling at larger power and to understand and overcome the main obstacles in optical cooling of conventional semiconductor structures, we study thermophotonic (TPX) heat transport in cavity coupled light emitters. Our structures consist of a double heterojunction (DHJ) LED with a GaAs active layer and a corresponding DHJ or a p-n-homojunction photodiode, enclosed within a single semiconductor cavity to eliminate the light extraction challenges. Our presently studied double diode structures (DDS) use GaInP barriers around the GaAs active layer instead of the AlGaAs barriers used in our previous structures. We characterize our updated double diode structures by four point probe IV- measurements and measure how the material modifications affect the recombination parameters and coupling quantum efficiencies in the structures. The coupling quantum efficiency of the new devices with InGaP barrier layers is found to be approximately 10 % larger than for the structures with AlGaAs barriers at the point of maximum efficiency.Peer reviewe