In-rich InGaN/GaN quantum wells grown by metal-organic chemical vapor deposition

Growth mechanism of In-rich InGaN/GaN quantum wells (QWs) was investigated. First, we examined the initial stage of InN growth on GaN template considering strain-relieving mechanisms such as defect generation, islanding, and alloy formation at 730 degrees C. It was found that, instead of formation of InN layer, defective In-rich InGaN layer with thickness fluctuations was formed to relieve large lattice mismatch over 10% between InN and GaN. By introducing growth interruption (GI) before GaN capping at the same temperature, however, atomically flat InGaN/GaN interfaces were observed, and the quality of In-rich InGaN layer was greatly improved. We found that decomposition and mass transport processes during GI in InGaN layer are responsible for this phenomenon. There exists severe decomposition in InGaN layer during GI, and a 1-nm-thick InGaN layer remained after GI due to stronger bond strength near the InGaN/GaN interface. It was observed that the mass transport processes actively occurred during GI in InGaN layer above 730 degrees C so that defect annihilation in InGaN layer was greatly enhanced. Finally, based on these experimental results, we propose the growth mechanism of In-rich InGaN/GaN QWs using GI.open9

InGaN/GaN light-emitting diode with a polarization tunnel junction

Cataloged from PDF version of article.We report InGaN/GaN light-emitting diodes (LED) comprising in situ integrated p(+)-GaN/InGaN/n(+)-GaN polarization tunnel junctions. Improved current spreading and carrier tunneling probability were obtained in the proposed device architecture, leading to the enhanced optical output power and external quantum efficiency. Compared to the reference InGaN/GaN LEDs using the conventional p(+)/n(+) tunnel junction, these devices having the polarization tunnel junction show a reduced forward bias, which is attributed to the polarization induced electric fields resulting from the in-plane biaxial compressive strain in the thin InGaN layer sandwiched between the p(+)-GaN and n(+)-GaN layers. (C) 2013 AIP Publishing LLC

Quantum dot emission from site-controlled ngan/gan micropyramid arrays

InxGa1−xN quantum dots have been fabricated by the selective growth of GaN micropyramid arrays topped with InGaN/GaN quantum wells. The spatially, spectrally, and time-resolved emission properties of these structures were measured using cathodoluminescence hyperspectral imaging and low-temperature microphotoluminescence spectroscopy. The presence of InGaN quantum dots was confirmed directly by the observation of sharp peaks in the emission spectrum at the pyramid apices. These luminescence peaks exhibit decay lifetimes of approximately 0.5 ns, with linewidths down to 650 me

Graded InGaN Buffers for Strain Relaxation in GaN/InGaN Epilayers Grown on sapphire

Graded InGaN buffers were employed to relax the strain arising from the lattice and thermal mismatch in GaN/InGaN epilayers grown on sapphire. An enhanced strain relaxation was observed in GaN grown on a stack of five InGaN layers, each 200 nm thick with the In content increased in each layer, and with an intermediate thin GaN layer, 10 nm thick inserted between the InGaN layers, as compared to the conventional two-step growth of GaN epilayer on sapphire. The function of the intermediate layer is to progressively relax the strain and to annihilate the dislocations that build up in the InGaN layer. If the InGaN layers were graded too rapidly, more dislocations will be generated. This increases the probability of the dislocations getting entangled and thereby impeding the motion of the dislocations to relax the strain in the InGaN layer. The optimum growth conditions of the intermediate layer play a major role in promoting the suppression and filling of the V-pits in the GaN cap layer, and were empirically found to be a thin 10 nm GaN grown at 750 0°C and annealed at 1000 0°C.Singapore-MIT Alliance (SMA

Graded InGaN Buffers for Strain Relaxation in GaN/InGaN Epliayers Grown on Sapphire

Graded InGaN buffers are employed to relax the strain arising from the lattice and thermal mismatches between GaN/InGaN epilayers grown on sapphire. The formation of V-pits in linearly graded InGaN/GaN bulk epilayers is illustrated. The V-pits were sampled using Atomic Force Microscopy and Scanning Electron Microscopy to examine their variation from the theoretical geometry shape. We discovered that the size of the V-pit opening in linearly graded InGaN, with and without GaN cap layer, has a Gaussian distribution. As such, we deduce that the V-pits are produced at different rates, as the growth of the InGaN layer progresses. In Stage I, the V-pits form at a slow rate at the beginning and then accelerate in Stage II when a critical thickness is reached before decelerating in Stage III after arriving at a mean size. It is possible to fill the V-pits by growing a GaN cap layer. It turns out that the filling of the V-pits is more effective at lower growth temperature of the GaN cap layer and the size of the V-pits opening, which is continued in to GaN cap layer, is not dependent on the GaN cap layer thickness. Furthermore, graded InGaN/GaN layers display better strain relaxation as compared to conventionally grown bulk GaN. By employing a specially design configuration, the V-pits can be eliminated from the InGaN epilayer.Singapore-MIT Alliance (SMA

Fabrication of InGaN quantum wells for LED applications

In this thesis fabrication and properties of InGaN quantum wells (QWs) for light emitting diode (LED) applications is studied. Metal-organic vapor phase epitaxy (MOVPE) is used to grow InGaN/(InAl)GaN multiple quantum well (MQW) and LED structures on GaN/sapphire substrates. Also a multistep growth method for the growth of GaN on sapphire is investigated. The method enables a tenfold reduction of threading dislocation (TD) density in the GaN layer compared to conventional growth methods. The objective of this work is to study the physics of InGaN QWs and to improve the performance of InGaN MQW structures used in near-UV, blue and green LEDs. The quality of quantum wells is analyzed by x-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements. The LED structures are characterized also by electroluminescence (EL) measurements. Various MOVPE growth parameters of InGaN/GaN QWs are evaluated for growth of MQW structures emitting blue light. Smooth surface morphology of the MQW stack is achieved by introducing a small amount of H2 during the MOVPE growth of the GaN barrier layers. The effect of TD density on the performance of near-UV, blue, and green LEDs is studied by fabricating LED structures on GaN buffers grown by the multistep method. Improved EL output power at high operating current density is observed in the blue LEDs fabricated on the multistep GaN buffers. MOVPE growth of quaternary InAlGaN layers is investigated and InGaN/InAlGaN MQW structures for near-UV emission are presented. The internal quantum efficiency (IQE) of InGaN/InAlGaN MQW structures is found to be sensitive to the InAlGaN barrier layer composition and the strain state of the structure. A MQW structure emitting at 383 nm with an IQE of 45 % is presented. Finally the origin of the high efficiency of InGaN QWs is discussed. The high efficiency is due to self-screening mechanism of TDs in In containing QWs. The height of the potential barrier formed around the TD depends on the In content of the QWs, and thus the effect of TDs on the performance of blue and green LEDs is different.Väitöskirjassa tutkittiin InGaN-kvanttikaivojen valmistusta ja ominaisuuksia. Erityisesti keskityttiin kvanttikaivojen käyttöön valoa emittoivissa diodeissa (LED). Kvanttikaivo- ja LED-rakenteet valmistettiin metallo-orgaanisella kaasufaasiepitaksialla (MOVPE) GaN/safiiri-alustakiteiden päälle. Työssä esitettiin myös monivaiheinen menetelmä GaN-ohutkalvojen valmistamiseksi. Monivaiheisella menetelmällä GaN kerroksessa etenevien dislokaatioiden määrää pystyttiin pienentämään kymmenesosaan verrattuna tavanomaisiin valmistusmenetelmiin. Väitöskirjatyön tavoitteena oli tutkia InGaN-kvanttikaivojen fysiikkaa ja parantaa InGaN-kvanttikaivojen ominaisuuksia UV-, sinisissä ja vihreissä LED-rakenteissa. Kvanttikaivojen laatua tutkittiin röntgendiffraktiolla, atomivoimamikroskopialla ja fotoluminesenssimittauksilla. LED-rakenteita tutkittiin myös elektroluminesenssimittauksilla. Tässä työssä tutkittiin useiden eri MOVPE-valmistusprosessien vaikutusta sinisissä LED-rakenteissa käytettävien InGaN/GaN-kvanttikaivorakenteiden laatuun. Tasainen pinnan morfologia saavutettiin käyttämällä vetyä GaN-vallien valmistuksen aikana. Etenevien dislokaatioden vaikutusta UV-, sinisten ja vihreiden LED-rakenteiden ominaisuuksiin tutkittiin valmistamalla LED-rakenteet monivaiheisella menetelmällä valmistettujen GaN-kerrosten päälle. Tämä lisäsi sinisten LED-rakenteiden elektroluminesenssia, kun käytettiin suurta virrantiheyttä. Myös InAlGaN-neliyhdisteiden MOVPE valmistusta tutkittiin ja UV-alueella emittoivia InGaN/InAlGaN-kvanttikaivorakenteita valmistettiin. InGaN/InAlGaN-kvanttikaivorakenteiden sisäisen kvanttihyötysuhteen havaittiin riippuvan InAlGaN-vallien kompositiosta ja kvanttikaivorakenteen jännityksestä. Tässä työssä esiteltiin myös 383 nm:n aallonpituudella ja 45 % kvanttihyötysuhteella emittoiva InGaN/InAlGaN kvanttikaivorakenne. Lopuksi käsiteltiin InGaN kvanttikaivojen korkean hyötysuhteen syitä. Korkean hyötysuhteen havaittiin aiheutuvan dislokaatioiden itsesuojaavasta luonteesta InGaN kvanttikaivoissa. Dislokaatioden ympärille muodostuvan potentiaalivallin korkeus havaittiin riippuvan kaivojen In-pitoisuudesta, joten dislokaatioiden vaikutus sinisten ja vihreiden LED-rakenteiden ominaisuuksiin oli erilainen.reviewe

Facet recovery and light emission from GaN/InGaN/GaN core-shell structures grown by metal organic vapour phase epitaxy on etched GaN nanorod arrays

The use of etched nanorods from a planar template as a growth scaffold for a highly regular GaN/InGaN/GaN core-shell structure is demonstrated. The recovery of m-plane non-polar facets from etched high-aspect-ratio GaN nanorods is studied with and without the introduction of a hydrogen silsesquioxane passivation layer at the bottom of the etched nanorod arrays. This layer successfully prevented c-plane growth between the nanorods, resulting in vertical nanorod sidewalls (∼89.8°) and a more regular height distribution than re-growth on unpassivated nanorods. The height variation on passivated nanorods is solely determined by the uniformity of nanorod diameter, which degrades with increased growth duration. Facet-dependent indium incorporation of GaN/InGaN/GaN core-shell layers regrown onto the etched nanorods is observed by high-resolution cathodoluminescence imaging. Sharp features corresponding to diffracted wave-guide modes in angle-resolved photoluminescence measurements are evidence of the uniformity of the full core-shell structure grown on ordered etched nanorods

Enhanced Luminescence in InGaN Multiple Quantum Wells with Quaternary AlInGaN Barriers

We report on the comparative photoluminescence studies of AlGaN/GaN, GaN/InGaN, and AlInGaN/InGaN multiple quantum well(MQW) structures. The study clearly shows the improvement in materials quality with the introduction of indium. Our results point out the localized state emission mechanism for GaN/InGaN structures and the quantum well emission mechanism for AlInGaN/InGaN structures. The introduction of indium is the dominant factor responsible for the observed differences in the photoluminescence spectra of these MQW structures

Strain induced variations in band offsets and built-in electric fields in InGaN/GaN multiple quantum wells

The band structure, quantum confinement of charge carriers, and their localization affect the optoelectronic properties of compound semiconductor heterostructures and multiple quantum wells (MQWs). We present here the results of a systematic first-principles based density functional theory (DFT) investigation of the dependence of the valence band offsets and band bending in polar and non-polar strain-free and in-plane strained heteroepitaxial In x Ga1- xN(InGaN)/GaN multilayers on the In composition and misfit strain. The results indicate that for non-polar m-plane configurations with [12¯10]InGaN // [12¯10]GaN and [0001]InGaN // [0001]GaN epitaxial alignments, the valence band offset changes linearly from 0 to 0.57 eV as the In composition is varied from 0 (GaN) to 1 (InN). These offsets are relatively insensitive to the misfit strain between InGaN and GaN. On the other hand, for polar c-plane strain-free heterostructures with [101¯0]InGaN // [101¯0]GaN and [12¯10]InGaN // [12¯10]GaN epitaxial alignments, the valence band offset increases nonlinearly from 0 eV (GaN) to 0.90 eV (InN). This is significantly reduced beyond x ≥ 0.5 by the effect of the equi-biaxial misfit strain. Thus, our results affirm that a combination of mechanical boundary conditions, epitaxial orientation, and variation in In concentration can be used as design parameters to rapidly tailor the band offsets in InGaN/GaN MQWs. Typically, calculations of the built-in electric field in complex semiconductor structures often must rely upon sequential optimization via repeated ab initio simulations. Here, we develop a formalism that augments such first-principles computations by including an electrostatic analysis (ESA) using Maxwell and Poisson\u27s relations, thereby converting laborious DFT calculations into finite difference equations that can be rapidly solved. We use these tools to determine the bound sheet charges and built-in electric fields in polar epitaxial InGaN/GaN MQWs on c-plane GaN substrates for In compositions x = 0.125, 0.25,…, and 0.875. The results of the continuum level ESA are in excellent agreement with those from the atomistic level DFT computations, and are, therefore, extendable to such InGaN/GaN MQWs with an arbitrary In composition