Dislocation density dependent electroabsorption in epitaxial lateral overgrown InGaN/GaN quantum structures

Cataloged from PDF version of article.We study electroabsorption (EA) behavior of InGaN/GaN quantum structures grown using epitaxial lateral overgrowth (ELOG) in correlation with their dislocation density levels and in comparison to steady state and time-resolved photoluminescence measurements. The results reveal that ELOG structures with decreasing mask stripe widths exhibit stronger EA performance, with a maximum EA enhancement factor of 4.8 compared to the reference without ELOG. The analyses show that the EA performance follows similar trends with decreasing dislocation density as the essential parameters of the photoluminescence spectra (peak position, width and intensity) together with the photoluminescence lifetimes. While keeping the growth window widths constant, compared to photoluminescence behavior, however, EA surprisingly exhibits the largest performance variation, making EA the most sensitive to the mask stripe widths. (C) 2013 Optical Society of Americ

Electric field dependent optoelectronic nature of InGaN/GaN quantum structures and devices

Ankara : The Department of Electrical and Electronics Engineering and the Graduate School of Engineering and Science of Bilkent University, 2012.Thesis (Ph. D.) -- Bilkent University, 2012.Includes bibliographical references leaves 91-101.In the past two decades we have been witnessing the emergence and rapid development of III-Nitride based optoelectronic devices including InGaN/GaN light-emitting diodes (LEDs) and laser diodes with operation wavelengths ranging from green-blue to near-UV. These InGaN/GaN devices are now being widely used in applications important for lighting, displays, and data storage, collectively exceeding a total market size of 10 billion USD. Although InGaN/GaN has been studied and exploited very extensively to date, its field dependent nature is mostly unknown and is surprisingly prone to quite unexpected behavior due to its intrinsic polarization property. In this thesis, we report our systematic study on the electric field dependent characteristics of InGaN/GaN quantum structures and devices including modulators and LEDs. Here we present our comparative study of electroabsorption in polar c-plane InGaN/GaN multiple quantum wells (MQWs) with different built-in polarization induced electrostatic fields. Analyzing modulator structures with varying structural MQW parameters, we find that electroabsorption grows stronger with decreasing built-in electrostatic field strength inside the well layer, as predicted by our theoretical model and verified by our experimental results. To further explore the field dependent optoelectronic nature of c-plane grown InGaN/GaN quantum structures, we investigate radiative carrier dynamics, which is of critical importance for LEDs. Our time and spectrum resolved photoluminescence measurements and numerical analyses indicate that the carrier lifetimes, the radiative recombination lifetimes, and the quantum efficiencies all decrease with increasing field. We also study the physics of electroabsorption and carrier dynamics in InGaN/GaN quantum heterostructures grown intentionally on nonpolar a-plane of the wurtzite crystal structure, which are free of the polarization-induced electrostatic fields. We compare these results with the conventional c-plane grown polar structures. In the polar case, we observe blue-shifting absorption profile and decreasing carrier lifetimes with increasing electric field. In the nonpolar case, however, we observe completely the opposite: a red-shifting absorption profile and increasing carrier lifetimes. We explain these observations in the context of basic physical principles including Fermi‟s golden rule and quantum-confined Stark effect. Also, we present electroabsorption behavior of InGaN/GaN quantum structures grown using epitaxial lateral overgrowth (ELOG) in correlation with their dislocation density levels and in comparison to steady state and time-resolved photoluminescence measurements. The results reveal that ELOG structures with decreasing mask stripe widths exhibit stronger electroabsorption performance. While keeping the ELOG window widths constant, compared to photoluminescence behavior, however, electroabsorption surprisingly exhibits the largest performance variation, making the electroabsorption the most sensitive to the mask stripe widths. This thesis work provides significant insight and important information for the optoelectronics of InGaN/GaN quantum structures and devices to better understand their field dependent nature.Sarı, EmrePh.D

Evaluation of growth methods for the heteroepitaxy of non-polar (1120) GAN on sapphire by MOVPE

Non-polar a-plane gallium nitride (GaN) lms have been grown on r-plane (1102) sapphire by metal organic vapour phase epitaxy (MOVPE). A total of ve in-situ defect reduction techniques for a-plane GaN are compared, including two variants with a low temperature GaN nucleation layer (LTNL) and three variants without LTNL, in which the high- temperature growth of GaN is performed directly on the sapphire using various crystallite sizes. The material quality is investigated by photoluminescence (PL), x-ray di raction, cathodoluminescence, atomic force and optical microscopy. It is found that all layers are anisotropically strained with threading dislocation densities over 109 cm2. The PL spectrum is typically dominated by emission from basal plane stacking faults. Overall, growth techniques without LTNL do not yield any particular improvement and even result in the creation of new defects, ie. inversion domains, which are seldom observed if a low temperature GaN nucleation layer is used. The best growth method uses a LTNL combined with a single silicon nitride interlayer.This work is supported by the Engineering and Physical Sciences Research Council (United Kingdom) under EP/J003603/1 and EP/H0495331. The European Research Council has also provided nancial support under the European Community's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no 279361 (MACONS).This is the final published version, also available from Elsevier at: http://dx.doi.org/10.1016/j.jcrysgro.2014.09.00

Role of substrate quality on the performance of semipolar (11 2 - 2) InGaN light-emitting diodes

We compare the optical properties and device performance of unpackaged InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) emitting at ∼430 nm grown simultaneously on a high-cost small-size bulk semipolar (11 2 - 2) GaN substrate (Bulk-GaN) and a low-cost large-size (11 2 - 2) GaN template created on patterned (10 1 - 2) r-plane sapphire substrate (PSS-GaN). The Bulk-GaN substrate has the threading dislocation density (TDD) of ∼ and basal-plane stacking fault (BSF) density of 0 cm-1, while the PSS-GaN substrate has the TDD of ∼2 × 108cm-2 and BSF density of ∼1 × 103cm-1. Despite an enhanced light extraction efficiency, the LED grown on PSS-GaN has two-times lower internal quantum efficiency than the LED grown on Bulk-GaN as determined by photoluminescence measurements. The LED grown on PSS-GaN substrate also has about two-times lower output power compared to the LED grown on Bulk-GaN substrate. This lower output power was attributed to the higher TDD and BSF density

Characterization of Growth and Real Structure of Nitride Based Semiconductor Devices by Use of Synchrotron Radiation

Thin GaN films with different dislocation densities were characterized by X-ray scattering methods to study the influence of in-situ deposited SiN. The reciprocal space maps in both coplanar and grazing incidence geometry were measured to support the development of new method for determination of dislocation densities. The structure and morphology of InGaN quantum dots before and after capping were studied by X-ray scattering and by AFM. The results are discussed with predicted phase separation

Growth, processing and chracterization of gallium nitride based coaxial LEDs grown by MOVPE

Gallium nitride (GaN) based coaxial (core-shell type) light emitting diodes (LEDs) offer a wide range of advantages. The active region of these LEDs is located on non-polar, {1-100} m-plane GaN sidewalls, which helps eliminate the quantum confined Stark effect (QCSE) and improve the radiative recombination efficiency of LEDs. The recent evolution of a catalyst free, scalable, repeatable and industrially viable device quality GaN nanowire and nanowall metal organic vapor phase epitaxy (MOVPE) growth process has enhanced the possibility of these LEDs going into production from laboratory. Previous work has shown that these nanowires exhibited an intense photoluminescence (PL), in spite of their large surface-area to volume ratio, and lasing was observed when these nanowires were optically pumped at high intensity. In this dissertation, it is shown that as long as the GaN three dimensional (3D) structures have their critical dimension below a micron, the threading defect (TD) density along the c- direction approaches zero. A TD that enters into this structure bends towards the surface vii ({1-100} m-plane side wall) in its vicinity, thereby reducing its dislocation line energy. The possibility of growing zero defect GaN templates is extremely important in the breakdown voltage improvement, the reverse bias leakage current reduction and efficiency droop reduction. This growth method has also been extended to device quality micron sized features, thereby presenting us with opportunity to study and explore LEDs of different sizes and shapes. In addition to the microstructure growth, two different repeatable approaches have been identified and demonstrated for the microelectronic processing of these micron-sized LEDs. Despite being far from perfect, the characterization results obtained from these LEDs have been encouraging. The technological challenges associated with the fabrication of the coaxial LEDs are also discussed in this dissertation

Beyond conventional c-plane GaN-based light emitting diodes: A systematic exploration of LEDs on semi-polar orientations

Despite enormous efforts and investments, the efficiency of InGaN-based green and yellow-green light emitters remains relatively low, and that limits progress in developing full color display, laser diodes, and bright light sources for general lighting. The low efficiency of light emitting devices in the green-to-yellow spectral range, also known as the “Green Gap”, is considered a global concern in the LED industry. The polar c-plane orientation of GaN, which is the mainstay in the LED industry, suffers from polarization-induced separation of electrons and hole wavefunctions (also known as the “quantum confined Stark effect”) and low indium incorporation efficiency that are the two main factors that contribute to the Green Gap phenomenon. One possible approach that holds promise for a new generation of green and yellow light emitting devices with higher efficiency is the deployment of nonpolar and semi-polar crystallographic orientations of GaN to eliminate or mitigate polarization fields. In theory, the use of other GaN planes for light emitters could also enhance the efficiency of indium incorporation compared to c-plane. In this thesis, I present a systematic exploration of the suitable GaN orientation for future lighting technologies. First, in order to lay the groundwork for further studies, it is important to discuss the analysis of processes limiting LED efficiency and some novel designs of active regions to overcome these limitations. Afterwards, the choice of nonpolar orientations as an alternative is discussed. For nonpolar orientation, the (1-100)-oriented (m-plane) structures on patterned Si (112) and freestanding m-GaN are studied. The semi-polar orientations having substantially reduced polarization field are found to be more promising for light-emitting diodes (LEDs) owing to high indium incorporation efficiency predicted by theoretical studies. Thus, the semi-polar orientations are given close attention as alternatives for future LED technology. One of the obstacles impeding the development of this technology is the lack of suitable substrates for high quality materials having semi-polar and nonpolar orientations. Even though the growth of free-standing GaN substrates (homoepitaxy) could produce material of reasonable quality, the native nonpolar and semi-polar substrates are very expensive and small in size. On the other hand, GaN growth of semi-polar and nonpolar orientations on inexpensive, large-size foreign substrates (heteroepitaxy), including silicon (Si) and sapphire (Al2O3), usually leads to high density of extended defects (dislocations and stacking faults). Therefore, it is imperative to explore approaches that allow the reduction of defect density in the semi-polar GaN layers grown on foreign substrates. In the presented work, I develop a cost-effective preparation technique of high performance light emitting structures (GaN-on-Si, and GaN-on-Sapphire technologies). Based on theoretical calculations predicting the maximum indium incorporation efficiency at θ ~ 62º (θ being the tilt angle of the orientation with respect to c-plane), I investigate (11-22) and (1-101) semi-polar orientations featured by θ = 58º and θ = 62º, respectively, as promising candidates for green emitters. The (11-22)-oriented GaN layers are grown on planar m-plane sapphire, while the semi-polar (1-101) GaN are grown on patterned Si (001). The in-situ epitaxial lateral overgrowth techniques using SiNx nanoporous interlayers are utilized to improve the crystal quality of the layers. The data indicates the improvement of photoluminescence intensity by a factor of 5, as well as the improvement carrier lifetime by up to 85% by employing the in-situ ELO technique. The electronic and optoelectronic properties of these nonpolar and semi-polar planes include excitonic recombination dynamics, optical anisotropy, exciton localization, indium incorporation efficiency, defect-related optical activities, and some challenges associated with these new technologies are discussed. A polarized emission from GaN quantum wells (with a degree of polarization close to 58%) with low non-radiative components is demonstrated for semi-polar (1-101) structure grown on patterned Si (001). We also demonstrated that indium incorporation efficiency is around 20% higher for the semi-polar (11-22) InGaN quantum wells compared to its c-plane counterpart. The spatially resolved cathodoluminescence spectroscopy demonstrates the uniform distribution of indium in the growth plane. The uniformity of indium is also supported by the relatively low exciton localization energy of Eloc = 7meV at 15 K for these semi-polar (11-22) InGaN quantum wells compared to several other literature reports on c-plane. The excitons are observed to undergo radiative recombination in the quantum wells in basal-plane stacking faults at room temperature. The wurtzite/zincblende electronic band-alignment of BSFs is proven to be of type II using the time-resolved differential transmission (TRDT) method. The knowledge of band alignment and degree of carrier localization in BSFs are extremely important for evaluating their effects on device properties. Future research for better understanding and potential developments of the semi-polar LEDs is pointed out at the end

Characterisation of polar (0001) and non-polar (11-20) ultraviolet nitride semiconductors

UV and deep-UV emitters based on AlGaN/AlN heterostructures are very inefficient due to the high lattice mismatch of these films with sapphire substrates, leading to high dislocation densities. This thesis describes the characterisation of the nanostructures of a range of UV structures, including c-plane (polar) AlGaN epilayers grown on AlN template, and nonpolar GaN/AlGaN MQWs grown on a-plane GaN template. The results are based primarily on transmission electron microscopy (TEM), cathodoluminescence in the scanning electron microscope (SEM-CL), high-resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) measurements. The structural and optical properties of various types of defect were examined in the c-plane AlGaN epilayers. Strain analysis based on in-situ wafer curvature measurements was employed to describe the strain relief mechanisms for different AlGaN compositions and to correlate the strain to each type of defect observed in the epilayers. This is followed by the investigation of AlN template growth optimisation, based on the TMA pre-dose on sapphire method to enhance the quality and the surface morphology of the template further. The initial growth conditions were shown to be critical for the final AlN film morphology. A higher TMA pre-dose has been shown to enable a better Al coverage leading to a fully coalesced AlN film at 1 μm thickness. An atomically smooth surface of the template was achieved over a large 10 x 10 μm AFM scale. Finally, the investigation of UV emitters based on nonpolar crystal orientations is presented. The SiNx interlayer was able to reduce the threading dislocation density but was also found to generate voids with longer SiNx growth time. The relationship between voids, threading dislocations, inversion domain boundaries and their associated V-defects and the variation in MQW growth rate has been discussed in detail

III-Nitride Nanocrystal Based Green and Ultraviolet Optoelectronics

Extensive research efforts have been devoted to III-nitride based solid-state lighting since the first demonstration of high-brightness GaN-based blue light emitting diodes (LEDs). Over the past decade, the performance of GaN-based LEDs including external quantum efficiency (EQE), wall-plug efficiency, output power and lifetime has been improved significantly while the cost of GaN substrate has been reduced drastically. Although the development of blue and near ultraviolet (UV) LED is mature, achieving equally excellent performance in other wavelengths based on III-nitrides is still challenging. Especially, the significant efficiency droop in the green wavelength, known as “green gap” and the extremely low EQE in the UV regime, known as “UV threshold”, have become two most urgent issues. Green LEDs emit light that is most sensitive to human eye, implicating its importance in a variety of applications such as screen- and projection-based displays. UV light sources have a variety of applications including water and air purification, sterilization/disinfection of medical tools, medical diagnostics, phototherapy, sensing, which make solid-state deep UV (DUV) light sources with compactness, low operating power and long lifetime highly desirable. The deterioration of performance with green LEDs originates from increased indium content of the active region, which could degrade material quality and increase quantum confined Stark effect due to the high polarization fields in c-plane InGaN/GaN quantum wells (QWs). Meanwhile, limiting factors in III-nitride UV LEDs include low internal quantum efficiency due to large densities of dislocations, poor carrier injection efficiency and low light extraction efficiency. In this dissertation, we have investigated the molecular beam epitaxial growth, structural characterization, and electrical and optical properties of low-dimensional III-nitride nanocrystals as potential solutions to above-mentioned issues. Through a combination of theoretical calculation and experimental investigation, we show that defects formation in AlN could be precisely controlled under N-rich epitaxy condition. With further optimized p-type doping, AlN nanowire-based LEDs emitting at 210 nm were fabricated. We report DUV excitonic LEDs with the incorporation of monolayer GaN with emission wavelengths of ~238 nm, and exhibit suppressed Auger recombination, negligible efficiency droop and a small turn on voltage ~5 V. To enhance the light extraction efficiency of AlGaN nanowires grown on Si substrate, we demonstrated epitaxy of AlGaN nanowires on Al coated Si(001) substrate wherein Al film functions as a UV light reflective layer to enhance the light extraction efficiency. AlGaN nanowire-based DUV LEDs on Al film were successfully grown and fabricated and measured with a turn-on voltage of 7 V and an electroluminescence emission at 288 nm. Green-emitting InGaN/GaN nanowire LEDs on Si(001) substrate were demonstrated, wherein the active region and p-contact layer consist of InGaN/GaN disks-in-nanowires and Mg-doped GaN epilayers. The incorporation of planar p-GaN layer significantly reduces the fabrication complexity of nanowire-based devices and improves the robustness of electrical connection, leading to a more stable device operation. We also demonstrated micrometer scale InGaN photonic nanocrystal green LEDs with ultra-stable operation. The emission features a wavelength of ~548 nm and a spectral linewidth of ~4 nm, which is nearly five to ten times narrower than that of conventional InGaN QW LEDs in this wavelength range. Significantly, the device performance, in terms of the emission peak and spectral linewidth, is nearly invariant with injection current. Work presented in this thesis provides a new approach for achieving high-performance green and DUV LEDs by using III-nitride nanostructures.PHDElectrical and Computer EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163122/1/ypwu_1.pd