Selective area epitaxy of ultra-high density InGaN quantum dots by diblock copolymer lithography

Highly uniform InGaN-based quantum dots (QDs) grown on a nanopatterned dielectric layer defined by self-assembled diblock copolymer were performed by metal-organic chemical vapor deposition. The cylindrical-shaped nanopatterns were created on SiNx layers deposited on a GaN template, which provided the nanopatterning for the epitaxy of ultra-high density QD with uniform size and distribution. Scanning electron microscopy and atomic force microscopy measurements were conducted to investigate the QDs morphology. The InGaN/GaN QDs with density up to 8 × 1010 cm-2 are realized, which represents ultra-high dot density for highly uniform and well-controlled, nitride-based QDs, with QD diameter of approximately 22-25 nm. The photoluminescence (PL) studies indicated the importance of NH3 annealing and GaN spacer layer growth for improving the PL intensity of the SiNx-treated GaN surface, to achieve high optical-quality QDs applicable for photonics devices

Fabrication of Nanodot Decorated Sapphire Substrates for Abbreviated Growth Mode Deposition of Gallium Nitride

The overarching theme of this body of work is the development and demonstration of sapphire substrates with sub-micron scale surface features laid out in arrays with controlled shape, size, and distribution. The key contributions of the work are: (1) the collaborative demonstration that such substrates enable novel GaN fabrication options like the Abbreviated Growth Mode (AGM) approach that can lead to lower cost, higher quality LED devices, (2) the proof-of-concept demonstration that large scale surface patterning with the use of anodic aluminum oxide (AAO) templates is a feasible approach for creating low-cost patterns that should be compatible with AGM, and (3) that the Aluminum-to-sapphire conversion process used to fabricate the surface structures has distinct zones of behavior with regard to feature size and temperature that can be used to suggest an optimized set of process conditions

Development of telecom wavelength InAs Quantum Dot lasers by MOCVD

The subject of this thesis is to develop quantum dot lasers around the telecom wavelengths of 1300nm and 1550nm for optical fibre communications. Quantum dots (QD) were grown by Metal-Organic Vapour Deposition (MOCVD) utilising both the conventional Stranski-Krastanov (S.K) method and a novel droplet epitaxy (DE) approach. The first section compares 1.1 µm InAs/GaAs QD lasers grown on the on-axis GaAs(100) substrates and substrates offcut 3° towards (110). QD lasers on the off-axis substrates had a lower threshold current density (Jth) and higher gain. An ~20% increase in the QD density for the 3-degree off-axis sample was found compared to the on-axis samples. The higher QD density is related to the change of morphology of the GaAs spacer layer, with more steps formed on the surface of the off-axis GaAs, providing a favourable nucleation site for QDs. These 1.1µm QDs are the first step towards the future realisation of 1.3µm QD lasers by MOCVD in comparison to MBE literature of 1310nm QDs. The longer term aim of the research is to incorporate these QD lasers on Silicon substrate for photonic integration. The second section presents the design, growth and characterisation of 1550nm InAs QD lasers grown by Droplet Epitaxy. The DE approach has several potential advantages, including removing the influence of the well-known wetting layer seen in the SK growth of QDs. The QD structure with the InP waveguide layer blue-shifts the QDs' wavelength to 1530nm, close to the target wavelength of ~1550nm. An approximately five times increase in emission intensity from QD samples was achieved when the Zn doping was reduced. The waveguide material was changed to InP, which significantly improved the carrier injection into the QDs. A high tail of diffused arsenic and a high oxygen concentration observed on the QD & QW samples may be preventing lasing. A new Quantum Well structure incorporating AlInGaAs as waveguide layer material was also introduced and formed the basis of further optimisation in future work to achieve room temperature lasing of these DE QD lasers

Semiconductor Laser Based on Thermoelectrophotonics

This dissertation presents to our knowledge the first demonstration of a quantum well (QW) laser monolithically integrated with internal optical pump based on a light emitting diode (LED). The LED with high efficiency is operated in a thermoelectrophotonic (TEP) regime for which it can absorb both its own emitted light and heat. The LED optical pump can reduce internal optical loss in the QW laser, and enables monolithically integrated TEP heat pumps to the semiconductor laser. The design, growth and fabrication processes of the laser chip are discussed, and its experimental data is presented. In order to further increase the TEP laser efficiency the development of QDs as the active region for TEP edge emitting laser (EEL) is studied. The usage of QD as TEP laser\u27s active region is significant in terms of its low threshold current density, low internal optical loss and high reliability, which are mainly due to low transparency in QD laser. The crystal growth of self-organized QDs in molecular beam epitaxial (MBE) system and characterization of QDs are mentioned. The design, growth, processing and fabrication of a QD laser structure are detailed. The characteristics of laser devices with different cavity length are reported. QD active regions with different amount of material are grown to improve the active region performance. Theoretical calculations based on material parameters and semiconductor physics indicate that with proper design, the combination of high efficiency LED in TEP regime with a QD laser can result in the integrated laser chip power conversion efficiency exceeding unity

Red-Emitting III-Nitride Self-Assembled Quantum Dot Lasers.

Visible and ultra-violet light sources have numerous applications in the fields of solid state lighting, optical data storage, plastic fiber communications, heads-up displays in automobiles, and in quantum cryptography and communications. Most research and development into such sources is being done using III-nitride materials where the emission can be tuned from the deep UV in AlN to the near infrared in InN. However due to material limitations including large strain, piezoelectric polarization, and the unavailability of cheap native substrates, most visible devices are restricted to emission near GaN at 365nm up to around 530nm. These dots are formed by the relaxation of strain, and it has been shown both theoretically and experimentally that the piezoelectric field and the resultant quantum confined stark effect are significantly lower than those values reported in comparable QWs. As a result, the radiative carrier lifetimes in such dots are typically around 10-100 times smaller than those in equivalent QWs. Furthermore, the quasi-three dimensional confinement of carriers in the InGaN islands that form the dots can reduce carrier migration to (and therefore recombination at) dislocations and other defects. In the present study, molecular beam epitaxial growth and the properties of InGaN/GaN self-assembled quantum dots have been investigated in detail. The quantum dots, emitting at 630nm, have been studied optically through temperature-dependent, excitation-dependent, and time-resolved photoluminescence. A radiative lifetime of ~2ns has been measured in these samples. Samples with varying number of dot layers were grown and characterized structurally by atomic force microscopy. The growth conditions of the dots have been optimized including the InGaN and GaN thickness and the nitrogen interruption time. The optimized dots have been incorporated into edge-emitting laser heterostructures. Other optimizations including the novel use of an all In0.18Al0.82N cladding are incorporated into the laser heterostructure to optimize the output power and reduce loss.The first red emitting quantum dot lasers, emitting at up to 630nm have been realized in the present study. These lasers show good performance compared with other material systems, including InGaAlP/GaAs and AlGaAs based red lasers.The maximum measured output power is 30mW, making them suitable for the applications discussed above.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120878/1/tfrost_1.pd

From Challenges to Solutions, Heteroepitaxy of GaAs-Based Materials on Si for Si Photonics

Monolithic growth of III-V materials onto Si substrates is appealing for realizing practical on-chip light sources for Si-based photonic integrated circuits (PICs). Nevertheless, the material dissimilarities between III-V materials and Si substrates inevitably lead to the formation of crystalline defects, including antiphase domains (APBs), threading dislocations (TDs), and micro-cracks. These nontrivial defects lead to impaired device performance and must be suppressed to a sufficiently low value before propagating into the active region. In this chapter, we review current approaches to control the formation of defects and achieve high-quality GaAs monolithically grown on Si substrates. An APB-free GaAs on complementary-metal-oxide semiconductor (CMOS)-compatible Si (001) substrates grown by molecular beam epitaxy (MBE) only and a low TD density GaAs buffer layer with strained-layer superlattice (SLS) and asymmetric step-graded (ASG) InGaAs layers are demonstrated. Furthermore, recent advances in InAs/GaAs quantum dot (QD) lasers as efficient on-chip light sources grown on the patterned Si substrates for PICs are outlined

CMOS Integration of High Performance Quantum Dot Lasers For Silicon Photonics

Integration of III-V components on Si substrates is required for realizing the promise of Silicon Photonic systems. Specifically, the direct bandgap of many III-V materials is required for light sources, efficient modulators and photodetectors. Several different approaches have been taken to integrate III-V lasers into the silicon photonic platform, such as wafer bonding, direct growth, butt coupling, etc. Here, we have devised a novel laser design that overcomes the above limitations. In our approach, we use InAs quantum dot (QD) lasers monolithically integrated with silicon waveguides and other Si photonic passive components. Due to their unique structures, the QD lasers have been proven by several groups to have the combination of high temperature stability, large modulation bandwidth and low power consumption compared with their quantum well counterparts, which makes it an ideal candidate for Si photonic applications. The first section of this dissertation introduces the theory and novelty of QD lasers, the DC and RF characterization methods of QD lasers are also discussed. The second section is focused on the growth of QD gain chip which a broadband gain chip based on QD inhomogeneous broadening properties was demonstrated. In third section, the lasers devices are built on Si substrate by Pd wafer bonding technology. Firstly, a ridge waveguide QD laser is demonstrated in this section which have better heat dissipation and lower threshold current compared to the unbonded lasers. In section four, a on Si comb laser is also developed. Due to inhomogeneous broadening and ultrafast carrier dynamics, InAs quantum dots have key advantages that make them well suited for Mode-locked lasers (MLLs). In section five, a passively mode-locked InAs quantum dots laser on Si is achieved at a repetition rate of ~7.3 GHz under appropriate bias conditions. In section six, a butt-joint integration configuration based on QD lasers and silicon photonics ring resonator is tested by using to translation stage. In order to achieve the on chip butt-joint integration, an on chip laser facet was created in section seven. A novel facet etching method is developed by using Br-ion beam assist etching (Br-IBAE). In section eight, a Pd-GaAs butt-joint integration platform was proposed, a hybrid tunable QD laser which consist of a QD SOA gain chip butt joint coupled with a passive Si3N4 photonic integrated circuit is proof of concept by using an external booster SOA coupled with a Si3N4 ring reflector feedback circuit. The final section summarized the work discussed in this thesis and also discussed some future approaches by using QD lasers integrated with silicon photonics integrated circuits based on the Pd-GaAs wafer bonding butt-joint coupled platform

Distributed feedback lasers and integrated laser arrays for wavelength-division multiplexing systems

Distributed Feedback (DFB) lasers and integrated laser arrays are of great importance in Wavelength-Division Multiplexing (WDM) systems in fiber optic communication systems. High-performance, low-cost DFB lasers and laser arrays are highly desirable for applications in intra-datacenter transport and in local access networks. This dissertation is focused on the design, fabrication and achievement of high-performance, low-cost DFB Lasers and Integrated DFB Arrays for WDM Systems. It investigates the use of a novel sampled grating approach, called the equivalent phase shift method, to achieve integrated DFB laser arrays with single-mode lasing at uniformly-spaced and precisely-positioned wavelengths. First, laterally-Coupled DFB (LC-DFB) lasers with first-order sidewall gratings are realized, with gratings fabricated by optical interference lithography instead of e-beam. Then, LC-DFB lasers and LC-DFB laser arrays with sampled gratings and equivalent phase shifts are proposed, numerically analyzed and experimentally demonstrated. Each LC-DFB laser with an equivalent quarter-wave phase shift is shown to lase at the pre-specified wavelength in a single longitudinal mode, with good side-mode suppression ratio (SMSR) over a very wide range of injection currents. Integrated LC-DFB laser arrays with five uniformly-spaced wavelength channels are demonstrated, in close agreement with the design. For better performance, buried heterostructure (BH)-DFB laser and laser arrays are also demonstrated using the same sampled-grating technology. A 6-wavelenth laser array with a 300 μm cavity length and a 8-wavelength laser array with 250 μm cavity length are successively demonstrated, each showing precisely positioned lasing wavelengths, good SMSR, and uniformly good lasing characteristics under a wide range of operating currents and temperatures. Finally, it is demonstrated that the wavelength of a monolithic WDM laser array can be continuously tuned over a very wide wavelength range of nearly 40 nm. The proposed method offers a practical and cost-effective solution for the manufacture of high-performance, monolithic multi-wavelength DFB laser arrays as well as widely wavelength-tunable DFB lasers for integrated WDM systems.Electrical and Computer Engineerin

OPTICAL CHARACTERIZATION OF INHOMOGENEITIES IN BLUE-EMITTING INGAN/GAN MQWS

The growth of blue-emitting InGaN/GaN MQWs, the system setup of a low temperature PL/EL/IV system for temperature dependent PL/EL/IV spectroscopy, and the system setup of a CLSM with nanometer-scale spectrum measurement and TRPL measurement abilities are described. A range of temperature-dependent PL experimental work, CLSM imaging experimental work and TRPL experimental work on blue-emitting InGaN/GaN MQWs are presented. In temperature-dependent PL measurements, the decreasing of spectrum- integrated PL intensity with increasing temperature is explained with a two-nonradiative- channel model, in which the two nonradiative channels correspond to the thermal activation of carriers out of the strongly localized states and the weakly localized states, respectively. The ‘S-shaped’ red-blue-red shift of PL peak energy and the ‘inverse S- shaped’ change of PL FWHM when temperature increases from 10 K to 300 K are explained with carrier localization and carrier dynamics. CLSM imaging and nanometer-scale PL spectral measurements show that the PL intensity fluctuates in micrometer scale, and that the bandgap energy in bright region is tens of meV smaller than that in dark region. The small-bandgap-energy regions are localization centers which limit the diffusion of the carriers and prevent carriers from diffusing to the NRRCs. Nanometer-scale TRPL measurements are conducted on blue-emitting InGaN/GaN MQWs for the first time, as far as the author knows. The measurements show that both bright region and dark region are characterized by two lifetimes: fast decay lifetime t1 is smaller than 3 ns and slow decay lifetime t2 is longer than 10 ns. The fast decay with shorter lifetime t1 corresponds to the carrier localization in weakly localized states, where the radiative recombination is more quenched by NRRCs and also competes with carrier transfer intro strongly localized states. And the slow decay with longer lifetime t2 corresponds carrier localization in strongly localized states. The fact that both fast decay and slow decay exist in both bright region and dark region indicates that both bright region and dark region has small bandgap energy fluctuation in themselves. Measurements show that the slow decay lifetime t2 in bright region is longer than that in dark region, indicating a higher probability of nonradiative recombination in dark region or carrier transporting from dark region to bright region. Measurements show that larger bandgap energy difference between small- bandgap-energy regions and large-bandgap-energy regions provides stronger carrier localization effect, via the presence of higher CLSM image average intensity, larger PL intensity ratio and longer smaller-bandgap-energy slow decay lifetime t2 when larger bandgap energy difference occurs. The effect of MOCVD growth parameters on MQW bandgap energy fluctuations and average intensity was analyzed. It was found out that by increasing growth pressure, decreasing growth rate, increasing growth temperature, increasing effective V/III ratio, and increasing gas speed, the bandgap energy difference between bright region and dark region increases, leading to higher average PL intensity