SINGLE-MODE ENGINEERING IN SEMICONDUCTOR LASERS USING PARITY-TIME-SYMMETRY AND
COUPLED-CAVITY STRUCTURES
A THESIS SUBMITTED TO
THE GRADUATE SCHOOL OF ENGINEERING AND SCIENCE OF BILKENT UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE IN
MATERIAL SCIENCE AND NANOTECHNOLOGY
By Enes Şeker
July 2021
- ---�
Single-Mode Engineering in Semiconductor Lasers Using Parity-Time-Symmetıy and Coupled-Cavity Structures
By Enes Şeker July 2021
We certify that we have read this thesis and that in our opinion it is fully adequate, in scope and in quality, as a thesis for the degree of Master of Science.
Abdullah Demir (Advisor)
Hasan Yılmaz
Emre Yüce
Approved for the Graduate Schoolf of Engineering and Science
Ezhan Karaşan V Director of the Graduate School
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ABSTRACT
SINGLE-MODE ENGINEERING IN SEMICONDUCTOR LASERS USING PARITY-TIME-SYMMETRY AND COUPLED-CAVITY
STRUCTURES
Enes Şeker
M.S. in Material Science and Nanotechnology Advisor: Abdullah Demir
July 2021
High power single spatial mode semiconductor lasers are of interest for various applications, including optical communication, material processing, and pumping single-mode optical fibers. The output power of a typical index guided ridge waveguide single-mode laser is limited by its narrow waveguide width required to cut off higher-order optical modes. To overcome the output power limitation, conventional techniques rely on structures increasing the mode size without introducing new modes. These methods are based on lateral mode discrimination in a single waveguide to enforce single-mode operation. In contrast to the conventional methods, our work utilizes the concept of parity-time-symmetry (PTS) and coupled-cavity (CC) structures. By exploiting these two approaches, we employ multi-mode waveguides to achieve single-mode lasing in edge-emitting laser diodes. The PTS laser is based on coupling two identical waveguides. By electrically tuning the gain and loss in each waveguide, the optical modes are manipulated to realize a single-mode operation. On the other hand, the CC approach is based on the resonant coupling of waveguides with different widths to realize single-mode operation. In contrast to the PTS method, CC lasers have an unpumped
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waveguide to introduce loss instead of tuning the loss with an electrical pump.
Towards this goal, the design parameters are numerically explored by detailed optical simulations, and their sensitivities are investigated for PTS and CC methods. I fabricated and experimentally demonstrated the control of the optical mode profiles promising single-mode operation for PTS and CC structures. The results are encouraging for future research and industrial applications.
Keywords: Parity-time-symmetry, coupled cavity, single-mode operation, mode filtering, high power semiconductor lasers.
V
ÖZET
PARİTE-ZAMAN-SİMETRİSİ VE BAĞLAŞIK-KOVUK YAPILARI KULLANARAK YARI İLETKENLER LAZERLERDE TEK-KİP
MÜHENDİSLİĞİ
Enes Şeker
M.S. in Material Science and Nanotechnology Advisor: Abdullah Demir
July 2021
Yüksek güçlü tek uzaysal kipli yarı iletken lazerler, optik iletişim, malzeme işleme ve tek-kipli optik fiberlerin pompalanması dahil olmak üzere çeşitli uygulamalar için ilgi çekicidir. Tipik indis-kılavuzlu sırt dalga kılavuzlu lazerlerde tek-kipli lazerin çıkış gücü, yüksek dereceli optik kipleri kesmek için gereken dalga kılavuzu genişliği ile sınırlıdır. Çıkış gücü sınırlamasını aşabilmek için, geleneksel teknikler, yeni kipler eklemeden temel kip boyutunu artıran yapılara dayanır. Bu yöntemler, rakip kiplere kayıplar vererek ve tek kipli çalışmayı zorlayarak yanal mod ayrımcılığına dayanır. Geleneksel yöntemlerin aksine, çalışmamız parite-zaman- simetrisi (PTS) ve bağlaşık-kovuk (CC) kavramlarının kullanılmasına dayanmaktadır. Bu iki yaklaşımdan yararlanarak, geniş ve çok-kipli dalga kılavuzu yapılarında tek-kipli lazer çıkışı elde etmeyi amaçladık. PTS lazer, bağlaşımlı iki dalga kılavuzuna dayanmaktadır. Her dalga kılavuzundaki kazancı ve kaybı elektriksel olarak ayarlayarak, tek-kipli çıkış elde etmek için optik kipler manipüle edilir. Öte yandan, CC yaklaşımı, kenar ışımalı lazer diyotlarda tek-kipli çalışmayı
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gerçekleştirmek için rezonansla eşleşmiş dalga kılavuzlarına dayanmaktadır. PTS yönteminin aksine, CC lazerler, kaybı bir elektrikli pompa ile ayarlamak yerine kaybı sağlamak için pompalanmamış bir dalga kılavuzuna sahiptir. Bu amaç doğrultusunda, bu tezde cihaz tasarım parametreleri nümerik simülasyonlarla detaylı bir şekilde analiz edilmiş, PTS ve CC yöntemleri için duyarlılıkları araştırılmıştır. Üretim ve test çalışmalarını tamamlayarak, hem PTS hem de CC yapıları için tek-kipli operasyonla sonuçlanan kenar ışımalı lazerler elde ettik.
Sonuçlar gelecekteki araştırmalar ve endüstriyel uygulamalar için umut vericidir.
Anahtar Kelimeler: Parite-zaman-simetrisi, bağlaşık kovuklar, tek kipli ışıma, kip filtreleme, yüksek güçlü yarı iletken lazerler.
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Acknowledgement
To begin with, I would like to thank my advisor Asst. Prof. Abdullah Demir for his support and amazing guidance, especially in the optimization and device design discussions. I wish to thank Asst. Prof. Hasan Yılmaz and Assoc. Prof. Emre Yüce for reviewing this thesis work and their valuable feedback.
I would like to thank all my companions, colleagues, and individuals of the Nanophotonic Devices Research Laboratory (nanoPhD Lab), Abdulmalik Abdulkadir Madigawa, Dr. Khalil Dadashi, Serdar Şengül, Turgay Bebek, Doğukan Apaydın, Kaveh Ebadi, Ali Kaan Sünnetçioğlu, Dr. Babak Olyaeefar, Ebru Buhara, Ataollah Kalantari Osgouei, Melike Batgıray Abboud, Gamze Altınyaprak, Muhammed Abboud, and Pouria Hasani for making my time here an energizing one. It has been a pleasure to share numerous great recollections with you all. Special thanks to Dr. Seval Arslan and Dr. Sinan Gündoğdu for their coaching in the cleanroom and scientific discussions, and Serdar Şengül for the characterization of devices. Also, I would like to thank to UNAM cleanroom staff Murat Güre, Ergün Karaman, Mustafa Özer, İsa Murat Çalık, Abdullah Kafadenk for their support in the cleanroom.
Last but not least, I wish to extend my gratitude to my mother Gülsel Şeker, my father Ahmet Şeker and my sister Azize Şeker for their endless love. Thank you for being there.
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In Memory of MD. İlker Tosun
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Table of Contents
List Of Figures... XII List of Tables ... XVIII
1. INTRODUCTION... 1
1.1. Lasers ... 1
1.2. Single Spatial Mode Operation ... 2
1.3. Outline of The Thesis ... 4
2. PTS IN SEMICONDUCTOR LASERS ... 6
2.1. Background ... 6
2.2. PTS Laser Design Simulations ... 8
2.2.1. Effect of Ridge Depth ... 10
2.2.2. Effect of Spacing Width on Coupling ... 12
2.2.3. Coupling Calculation and Waveguide Width ... 13
2.3. PTS Laser Fabrication ... 17
2.3.1. Mask Layout ... 17
2.3.2. Lithographic Processes ... 20
2.3.3. Device Packaging... 37
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2.4. PTS Laser Characterization ... 37
2.4.1. Set-up ... 37
2.4.2. LI Characteristics ... 38
2.4.3. Near Field Profiles ... 40
3. COUPLED-CAVITY (CC) LASER ... 43
3.1. Background ... 43
3.2. CC Laser Design Simulations ... 45
3.2.1. Simulation and Mode Overlap Calculation Overview ... 45
3.2.2. Ridge Depth Optimization ... 46
3.2.3. Dispersion Curve ... 46
3.2.4. Sensitivity Analysis and Design Optimization ... 47
3.2.5. Optimization of Main Waveguide Width ... 50
3.2.6. Effect of Waveguide Spacing ... 51
3.3. CC Laser Fabrication ... 53
3.3.1. Mask Layout ... 53
3.3.2. Lithographic Processes ... 53
3.3.3. Device Packaging... 56
3.4. CC Laser Characterization ... 56
3.4.1. LI Characteristics ... 56
3.4.2. Near-Field Profiles ... 57
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4. CONCLUSION AND FUTURE OUTLOOK ... 60 References ... 62
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ist Of FiguresFigure 1.1 : a) Absorption b) Spontaneous Emission c) Stimulated Emission. ... 2 Figure 1. 2: a) Sechematics of a semiconductor laser b) Near-field simulation of single mode laser c) Near-field simulation of multi mode laser ... 3 Figure 2.1: Illustration of PTS laser device with high order modes and coupling. ... 9 Figure 2.2: X represents the waveguide width, and R is the ridge depth both in µm. As expected, larger waveguide widths can support more modes. The first high-order comes earlier in the deeper ridges due to its higher confinement. ... 11 Figure 2. 3:Simulation results show the effect of different spacing for 7 µm waveguide and 1.145 µm ridge depth, "S" here represents the spacing between the waveguides. ... 12 Figure 2. 4: Illustration of modes to calculate Ƙ0 and Ƙ1. ... 14 Figure 2.5: Ƙ0 and Ƙ1 result for the 8 µm waveguide with different spacing widths.
Blackline represents the loss level. Between Ƙ0 and Ƙ1 is the target working range. ... 16 Figure 2. 6: General view of the mask design. ... 18 Figure 2.7: Green fields are the first mask, red fields are the second mask. ... 19 Figure 2.8 a) Cap layer etch b) Ridge etch c) Metal window opening d)P-Metal deposition.
e) Electroplating, f) SEM picture after full fabrication. ... 20
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Figure 2. 9:a) Not to scale drawing of facet after cap layer etch. b) Top view of the device after cap layer etch taken by optical microscopy with 10x lens. ... 21 Figure 2. 10. a) Not to scale drawing of facet after cap layer etch. b) Not to scale drawing of facet after mesa etches c) Top view of the device after cap layer etch taken by optical microscopy d) Top view of the device after mesa etch taken by optical microscopy. ... 22 Figure 2.11: Three different etch solutions, parallel and perpendicular, represent the crystal orientation. ... 24 Figure 2. 12: a) Facet view of not to scale schematics of structure b) BCl3:Cl2 (5:15) c) BCl3:Cl2 (10:15) d) BCl3:Cl2 (15:15) e) BCl3:Cl2:Ar:N2 (10:10:10:4.5). ... 25 Figure 2.13: a) SEM picture of the facet b) Optical microscope image of the surface after etching the nitride film. ... 25 Figure 2.14: The spacing between the waveguides after etching the substrate. ... 26 Figure 2. 15: Spacing between the waveguides after silicon nitride etching a) CHF3:O2
(100:5 sccm) 20 µbar 71 watt b) CHF3:O2 (100:5 sccm) 20 µbar 71 watt + 10-sec BOE etching c) SF6:O2 (20:5 sccm) 20 µbar 71 watt d) SF6:O2 (20:5 sccm) 20 µbar 71 watt + 10-sec BOE. ... 27 Figure 2.16: a) Etching recipe I used with RIE CHF3:O2 (100:5 sccm) 20 µbar 71 watt b) Etching recipe I used with ICP CHF3:O2 (100:5 sccm), 15 mTorr, ICP Power: 0 W, Bias Power: 71 W. The pressure might seem different but when we convert µbar to torr they are the same. ... 29 Figure 2.17: a) Etching results of the recipe with low-frequency ICP power b) Etching results of the recipe with high-frequency ICP power. ... 30
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Figure 2.18: SEM pictures from the facet view after following recipes a) Bias power: 300 watt, ICP power: 30 watt, CF4:O2 (100:20 sccm), Pressure: 40 mTorr b) Bias power: 100 watt, ICP power: 30 watt, CF4:O2 (100:20 sccm), Pressure: 40 mTorr c) Bias power: 70 watt, ICP power: 30 watt, CF4:O2 (20:15 sccm), Pressure: 5 mTorr d) Bias power: 70 watt, ICP power: 30 watt, CF4:O2 (20:15 sccm), Pressure: 40 mTorr. ... 30 Figure 2.19: SEM pictures from the facet view after following recipes a) Bias power: 70 watt, ICP power: 30 watt, CF4:O2 (100:20 sccm), Pressure: 40 mTorr b) Bias power: 70 watt, ICP power: 30 watt, CF4:O2 (20:15 sccm), Pressure: 40 mTorr c) Bias power: 100 watt, ICP power: 30 watt, CF4:O2 (20:20 sccm), Pressure: 40 mTorr ... 31 Figure 2.20: SEM pictures from the facet view after best recipe (Bias power: 400 watt, ICP power: 100 watt, CF4:CHF3 (20:3 sccm), Pressure: 5 mTorr) with different etch duration a) 1-minute b) 2-minute c) 2.5-minute d) 3-minute e) 3.5-minute. ... 32 Figure 2 21: The facet view of the implantation process. ... 32 Figure 2.22: a) Facet view of the structure after window opening. b) optical microscopy image from the top after window opening, purple areas are silicon nitride, and white areas are the window opening. ... 33 Figure 2.23: a) Not to scale schematics after metal window opening etch b) SEM picture of facet view after metal window opening etch with RIE, nitride layer is comparable to the schematic... 34 Figure 2.24: a) The facet view of the PR profile after image reversal lithography b) Side view of the PR profile that will sit on the spacing between the waveguides, after image reversal lithography. ... 35
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Figure 2.25:a) Facet view of of final structure schematic b) Top view optical microscopy picture after p-metal deposition (145 nm gold) c) Top view optical microscopy image after electroplating (~1µm gold). ... 36 Figure 2.26: Fabricated laser bar soldered on a PCB card and attached to the microstage.
... 37 Figure 2.27: Characterization setupconsisting of an objective, camera, and a power meter.
... 38 Figure 2.28: The photograph of the characterization setup. ... 38 Figure 2.29: a) Schematics for pumping b) LI curve for single waveguide laser c) LI curves for the PTs concept lasers for different pump levels. ... 39 Figure 2.30: Demonstration of optical mode and the leakage emission from the loss waveguide ... 41 Figure 2.31: a) Schematics of a SWG laser b) 2D near-field results of SWG laser c) Illustration of PTS breaking d) 2D near-field image of the PTS-unbroken regime e) 2D near-field image of the PTS-breaking regime ... 42
Figure 3.1. Schematic of the CC laser design. ... 44 Figure 3 2: a)3D schematic of the CC laser structure. b) Simulation results show three supported modes in the device. c) Simulation results for single waveguide (SWG) laser.
Colors are representing the intensity distribution of the E-field; red is the highest blue is the lowest. ... 45 Figure 3.3: Calculation of mode coupling percentage. For every mode, I calculate data individually. ... 46
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Figure 3.4: a) Dispersion curve for the structure, plotted by extracting effective refractive index data from the simulations for every waveguide value. b) Wp versus Wm is derived from the dispersion curve.. ... 47 Figure 3.5: a) Modal overlap in Wp for the 8 µm Wm and 3.2 µm spacing. Resonant is very close to 2.35 µm, which we find from the dispersion curve. b) Simulation results for the 8 µm Wm and 1 µm Wp case. c) Simulation results for the 8 µm Wm and 2.35 µm Wp case. d) Simulation results for the 8 µm Wm and 3.4 µm Wp case. ... 49 Figure 3.6: a) Modal overlap in Wp for the 6 µm Wm and 3.2 µm spacing. b) Simulation results for the 7 µm Wm and 3.2 µm spacing c) Simulation results for the 9 µm Wm and 3.2 µm spacing. ... 50 Figure 3. 7: Resonant tolerance of different Wm values. Larger Wm has slightly better coupling than smaller waveguides. ... 51 Figure 3.8: Effect of different spacing values. As expected, the coupling percentage is high around the resonant waveguide. ... 52 Figure 3.9: Resonant cases with different spacing widths a) 1 μm b) 2 μm c) 3 μm d) 4 μm ... 53 Figure 3.10: a) Cap layer etch b) Ridge etch c) Metal window opening d) P-metal deposition e) N-metal deposition ... 53 Figure 3.11: a) Schematics of pumping the CC laser b) SEM picture after ridge etch .... 56 Figure 3.12: a) LI curve comparison for the SWG and the CC with a) Wp=1.5 µm, a) Wp=1.7 µm, and a) Wp=1.9 µm. ... 57
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Figure 3.13 a) Schematics of a SWG laser b) 2D near-field results of SWG laser c) Illustration of On-Resonance case d) 2D near-field image of the Off-Resonance case e) 2D near-field image of the On-Resonance case... 59
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List of Tables
Table 2. 1: Kappa values for different waveguide widths for a fixed ridge depth (1.145 µm) ... 15 Table 2. 2: Kappa calculations with achievable spacing widths for different waveguide widths. ... 16 Table 2. 3: We named the recipes for the people who will use the same machine within our facility; UNAM and ARL are the names of two different clean rooms within our facility.
For example, the difference between UNAM He and UNAM N2 recipes is the carrier in the Silane gas, UNAM He has %2 Silane and %98 He, UNAM N2 %2 Silane %98 N2, these carriers shouldn't change the quality of silicon nitride because the only difference is the carrier gas. I already give the details of the etching recipes in the previous discussion. .. 28 Table 2. 4: Near-field profiles of fabricated PTs lasers. ... 40 Table 3. 1: Near-field profiles for CC concept devices ... 58
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Chapter 1
1. Introduction 1.1. Lasers
LASER (Light Amplification by Stimulated Emission of Radiation) word is used by Gordon Gould and developed by Theodore Maiman in 1960 [1]. It is always desired to have a higher power in a smaller area, and lasers fulfill this expectation. With the results of the improvements in technology, lasers became the main component of many different applications. Different types of lasers are developed within the last decades; one of the smallest is the semiconductor lasers. In contrast with the first ruby lasers, semiconductor lasers are electrically pumped, and compact devices. Therefore, semiconductor lasers are the most demanded laser types amongst other types.
Lasing is the result of series of events with suitable components. Laser components are; an active medium with a higher refractive index than the environment, a pump source for exciting the electrons, and a feedback mechanism suitable for the emission wavelength.
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With the existence of these components, the lasing process starts with absorption. Next, atoms in the gain medium are excited by the pump source, and electrons move to the excited states. Population inversion is achieved when the number of electrons in the excited states is higher than that of the ground state. The excited state is filled with electrons after population inversion, and electrons go back to the ground state by emitting a photon. This process is called spontaneous emission. The emitted photon interacts with the other excited electrons; while lowering them to the ground state identical photon is created with the same frequency and phase. The number of photons is doubled with every interaction, and this process is called stimulated emission. Figure 1.1 shows the lasing steps.
Figure 1.1 : a) Absorption b) Spontaneous Emission c) Stimulated Emission.
The light generated in the laser has unique properties such as narrow spectral wavelength (monochromatic), in phase emission (coherent), smaller divergence angle (directional).
These properties make lasers an appropriate light source for many applications ranging from medical imaging to information technology.
1.2. Single Spatial Mode Operation
Lasers can support a certain number of modes depending on the cavity length and waveguide width relative to the emission wavelength. In a Fabry Perot cavity, the gain medium is in the middle of two different mirrors. Modes are standing waves fitting the length of the cavity and calculated by the below Eq.1.1. L here is the cavity length, λ is
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emission wavelength, and q is an integer giving the number of supported modes in a cavity.
This formula can be applied to longitudinal, lateral and vertical directions.
𝐿 = 𝑞λ
2 (1.1)
Semiconductor lasers have many optical modes in the longitudinal direction and much less or even one mode in the lateral or vertical direction because of their long structure in the longitudinal and small size in the lateral and vertical direction, as shown in Figure 1.1 Within the scope of this thesis, single spatial mode operation refers to the single optical mode in the lateral direction. Due to its smaller width, the most crucial feature of the single-mode semiconductor laser is its high brightness. It has higher power in a relatively smaller area compared to the broad-area waveguide lasers. This high brightness output beam makes single-mode lasers ideal for fiber coupling, laser cutting/welding, pump source, and optical fiber communication.
Figure 1. 2: a) Sechematics of a semiconductor laser b) Near-field simulation of single mode laser c) Near- field simulation of multi mode laser
Single-mode operation can be obtained by employing a narrow lateral waveguide relative to the emission wavelength. Using Eq 1.1, one can describe a range for the suitable"L", in this case waveguide width, for a single-mode operation High power single-mode semiconductor lasers have been demonstrated by controlling the lateral waveguide width
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and utilizing narrow waveguides [2], [3]. Another approach for single-mode operation is to filter out high-order optical modes of multi-mode operating broad-area waveguide lasers. Alternative methods have been demonstrated based on waveguide engineering in the lateral or lateral direction such as tapered designs [4], and groove structures [5].
Using the PTS concept, micro-ring resonators, toroids, optical pumped edge-emitting lasers, and various designs are reported in the literature. Optically pumped lasers other designs have limited output power depending on the pump source, and they are not suitable for industrial applications. We report an electrically pumped edge-emitting laser with its systematic characterization, which is ideal for industrial applications.
CC concept is a novel term for the literature, and no scientific research has been published in the journals so far. There are conference proceedings using electrically pumped edge-emitting lasers, but the systematic investigation about the device parameter is missing. We report a detailed examination of the concept using simulations and experimentally demonstrate the single-mode operation.
According to the results we get, PTS and CC concepts provide single-mode operation, but the CC concept is simpler to fabricate and characterize since there is one pump.
Our work employs a Gallium Arsenide (GaAs) based laser device; the emission wavelength for these devices is at the 9xx nm level. The emission wavelength strongly depends on the bandgap of the active medium material.
1.3. Outline of The Thesis
Chapter 1 introduces the background on lasers and their single spatial mode operation.
Also, the applications and literature on single-mode lasers are discussed. The two novel concepts and motivation of this work is briefly introduced.
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In Chapter 2, the concept of parity-time symmetry in photonics and lasers is explained. The device simulations, fabrication process and experimental results are demonstrated.
Chapter 3 focuses on the concept of couple-cavity lasers. The device simulations, fabrication process and experimental results are presented.
Chapter 4 concludes by comparing the results of the two approaches and possible future directions are presented.
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Chapter 2
2. PTS in Semiconductor Lasers 2.1. Background
Single-mode semiconductor lasers are essential in many aspects as discussed in the introduction chapter. Although it is straightforward to achieve single-mode lasing by narrowing the waveguide width relative to the wavelength, there are several concepts that we can implement in semiconductor lasers to achieve single-mode operation with higher output power. Parity Time Symmetry concept is one of those methods [6], and we implement it in the semiconductor lasers to achieve single-mode lasing.
In an optoelectronic device design, the expectation is to decrease the losses or propose ideas to overcome losses. The losses are diminishing the device performance in terms of output power and stability. It seems impossible to eliminate the losses and this question showed up; "Can we use losses instead of getting away from them"? [7] The answer to the question is coming from quantum field theory. In a study from 1998, the
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classical and quantum properties of Parity-Time (PT) symmetry theory was investigated [8]. This study paves the way to the realization of PT-symmetric systems in various fields.
To utilize PTS in an optical system, gain and loss in the structure are employed in conjunction with the coupling (Kappa, Ƙ) between the components. The theory was introduced for coupled optical PTS structures [9]. The exceptional point is defined as the PT symmetry breaking condition [10] at which the system's behavior changes drastically.
There are various applications of PTS in the literature. Optically coupled PTS system on a lithium niobate substrate was reported [11]. In another study, the output beam was tilted by PTS breaking with complex PTSarrays [12]. Single-mode lasing from a micro-ring laser with optical pumping was presented for the first time in 2014 using synthetic lasers [13]. PTS breaking was utilizied to eliminate the high-order modes.
Various research groups reported on the coupled cavities from micro-ring lasers to toroid’s [14]–[17].
Another study achieved a transparency in a wide range of the spectrum by using PTS breaking [18]. In another work, invisibility with meta-surfaces was achieved by breaking PTS [19][20]. PTS breaking photonic lattices was reported [21].
There are few studies employing edge-emitting lasers in the literature. One study reports optical pumping of the laser waveguide [22], and another work reports on the electrically pumped edge-emitting laser device to achieve nearly single-mode operation [6]. Our work employs an electrically pumped edge-emitting laserto achieve high power single-mode operation from a large area waveguide. Such a waveguide intrinsically
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supports multi-mode operation. By judiciously utilizing PTS in our structure, we study the optical mode profile and output performance of our high power single-mode lasers..
A laser waveguide can support several optical modes (see Eq 1.1) depending on the emission wavelength and the waveguide width. A laser with a narrow waveguide width that can support only fundamental mode has a single lateral spatial mode operation. In the cavity, the threshold for high-order mode is too high, and it cannot overcome losses to lase.
This device suffers from limited output power compared to a wider waveguide laser.
Altough, broad-area semiconductor lasers can achieve power levels exceeding 20W [23]–
[25], and their output [26], [27] and reliability [28]–[30] are limited by various mechanisms. On the other hand, single-mode lasers are more robust against facet degradation, which allows higher output power density but its lateral waveguide width limits the single-mode output power to around 2 W [31].
In the PTS concept, we are using losses to gain. There are two symmetric waveguides separated with a small distance to control the coupling of its supported optical modes. Each waveguide supports two modes, the fundamental mode and the first high- order mode. After electrically isolating these two waveguides from each other, we pump them separately with different current levels. While the gain waveguide is pumped above the threshold current to achieve lasing, we pump the loss waveguide with a current lower than the lasing threshold to control its loss.
2.2. PTS Laser Design Simulations
Figure 2.1 shows the illustration of the concept. The width of the high-order mode is larger than the fundamental mode. Therefore, it has less confinement in the waveguide than the
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fundamental mode. It has a tail outside of the waveguide. When we pump the lasers, these two high-order modes of gain and loss waveguides interact, and the gain waveguide's high- order mode will couple to the loss waveguide. The coupling coefficient (Kappa, Ƙ) defines the coupling between these two waveguides calculated separately for each mode. Kappa is strongly related to the spacing between the waveguides; the smaller the spacing higher the Kappa. The detailed information about Kappa is provided in the next section.
Figure 2.1: Illustration of PTS laser device with high order modes and coupling.
In the PTS laser, there are two waveguides with the same and constant width along the cavity, and separated by a small spacing. To understand the behavior of kappa (к), we implemented a systematic simulation study and analyzed the key parameters, which are thewaveguide width, spacing between the waveguides and the ridge depth.. The ridge depth is important to ensure that both waveguides can support two optical modes.
For the PTS laser design simulations, we employed two different epi-design to compare the effect of epi-design. The difference between them is their stack structure, thus, the effective refractive indices. According to the simulation results from Lumerical Mode
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Solution, slightly changing the system affects the intensity distribution and loss. Thus, calculated kappa, which defines the coupling, is changing from design to design.
We explain the simulations on PTS lasers in this part of the thesis. The simulations' main point is to understand the effect of spacing and calculating the kappa to decide on the design parameters of the device that we will fabricate. There are several parameters to set within the simulations; optimum ridge depth, optimum spacing width, and waveguide width. Ridge depth is essential to obtain a stable multi-mode lasing defined by the intrinsic waveguide structure. The waveguide width is critical for mode control and coupling.
Waveguide spacing affects the optical mode coupling and current leakage to the neighbor waveguide. We present the detailed simulation results and analysis of these key parameters in the following subsections. We employed Lumerical MODE software for all the simulations presented.
2.2.1. Effect of Ridge Depth
The number of optical modes and their behavior are modified by the ridge depth and waveguide width, which depend on the epitaxial design because the effective refractive index changes by the epitaxy. Thus, it affects the mode confinement inside the waveguide and the ridge depth needs to be optimized. We first investigated the sensitivity of our structure on the ridge depth and waveguide width to set their values for the fabrication.
Deciding on the optimum sizes was the first step of the laser device design in our study.
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Figure 2.2: X represents the waveguide width, and R is the ridge depth both in µm. As expected, larger waveguide widths can support more modes. The first high-order comes earlier in the deeper ridges due to its higher confinement.
Figure 2.2 demonstrates the supported optical mode profile results of the waveguide simulations for various ridge depths and waveguide widths. In each row, the ridge depth is fixed and the waveguide width varies from 3.0 to 9.0 µm. Ridge depths are approximately 1.1 µm, 1.15 µm, and 1.2 µm from top to bottom. One can see that the number of supported modes in the waveguide increases with the waveguide width. For example, 3 µm waveguide in the first row supports only the fundamental mode while larger than 6 µm waveguide supports both the fundamental and high-order modes. When we check the columns in Figure 2.2, we see that a 5 µm waveguide supports a different number of modes with varying ridge depths. This variation is the sensitivity of our design to the ridge depth.
50 nm etch depth difference results in several modes supported. The deeper ridge depth provides stronger confinement, and the first high-order mode shows up earlier compared to the shallow ridges. According to these results, the optimum ridge depth is 1.15 µm. Even if the structure is off by 50 nm in the fabrication, it will still support two optical modes for
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6, 7, and 8 µm waveguide widths. Hence the optimum value for a two-mode single waveguide is to have 1.15 µm ridge depth and 7.0 µm wide waveguide for minimum sensitivity to their variations. However, the waveguide width needs to be optimized and set by considering coupling effects as discussed in the following sections.
2.2.2. Effect of Spacing Width on Coupling
In the previous section, we discussed the optimum ridge depth. The optimum ridge depth ensures that the device will support two modes and the next step is to optimize the spacing width of the two-waveguide design. We implemented simulations with the optimum ridge depth to see the impact of spacing between the waveguides. Using Lumerical MODE software, we plot the intensity distributions of the electric fields and demonstrate them in Figure 2. 3 . In Figure 2. 3, you can see the results for different spacing widths with the fixed ridge depth of 1.145 µm and waveguide width of 7 µm.
Figure 2. 3:Simulation results show the effect of different spacing for 7 µm waveguide and 1.145 µm ridge depth, "S" here represents the spacing between the waveguides.
As you can see in Figure 2. 3, the fundamental mode's confinement is strong, and it has very weak coupling to the neighbor waveguide for increased spacing. But the high order mode shows coupling to the neighbor waveguide for all cases.
The two waveguides are named gain and loss due to their assignment in the concept.
Since the fundamental mode's confinement is very high, the fundamental mode cannot couple to the loss waveguide under these circumstances. But the high order mode can
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couple to the loss waveguide under specific conditions. Spacing should be small enough to maintain interaction between the high-order modes. If the spacing is too much that the tails of high-order modes cannot interact and the coupling doesn't occur.
To set an optimum spacing width between the waveguides, we should also consider the fabrication limits. The minimum resolution of the mask manufacturing needs to be considered as well. For example, we cannot produce masks with spacings smaller than 2.5 µm in UNAM. However, it is possible for professional lithographic mask companies to manufacture openings as small as 1.3 µm. Another problem with the smaller opening is that it is challenging to electrically isolate the waveguides if they are too close. The higher the spacing the higher the resistance between the waveguides. Due to these limitations, we decided to set the spacing as 3.2 µm. With 3.2 µm spacing, it will be easier to fabricate the device with repeatable lithography. Also, the current leakage between the waveguides will be limited and the coupling is still within the acceptable range.
2.2.3.
Coupling Calculation and Waveguide Width
The waveguide width is less critical in the concept compared to the spacing width. To confirm PTS conditions, we carefully choose the width based on the simulation results from the ridge depth part. We also consider the fabrication tolerance in the lateral axis while deciding the optimum width. We calculate the kappa (Ƙ) to ensure that the fabricated device will satisfy the PTS conditions. The loss value should be in between the two kappa values.
For a coupler with identical waveguides, the coupling can be described by the following equations:
∆𝑛 = 𝑛1− 𝑛2 (2. 1)
14 𝐿𝑐 = 𝜆0
2∆𝑛 (2. 2) Ƙ = 𝜋
2𝐿𝑐 (2. 3) Each optical mode has a coupling coefficient, where Ƙ0 is for the fundamental mode and Ƙ1 is for the high order mode, 𝑛1,2 is the refractive index of each mode, 𝜆0 is the wavelength of light and 𝐿𝑐 is the coupling length (i.e. the length required to couple 100%
of the light from one to the other waveguide). There are four modes for the two waveguide structure since each waveguide individually supports two modes. I start with calculating
∆𝑛 which defines the effective refractive index difference between the two supermodes.
As shown in Figure 2. 4, I use mode #1 & mode#2 to calculate Ƙ0, which defines the coupling coefficient of the fundamental mode. With the same method, we use mode#5 and mode#6 to calculate Ƙ1, which establishes the coupling coefficient of the high order mode.
We use this data to calculate coupling length (Lc) by using Eq. (2.2). With the calculated coupling length, one obtains the Kappa values with Eq. (2.3).
Figure 2. 4: Illustration of modes to calculate Ƙ0 and Ƙ1.
After calculating Ƙ0 and Ƙ1, we compare them with the cavity loss (includes internal absorption loss and outcoupling losses), which is 3.5 cm-1. Cavity loss should be in-between Ƙ0 and Ƙ1 values to satisfy the PTS conditions [REF: 2014 Science paper,
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“Parity-time–symmetric microring lasers”]. We calculate the coupling values for different waveguide widths of 6.0 to 9.0 µm with a fixed ridge depth (1.145 µm) to choose the optimum waveguide width. Table 2.1 presents the results of these simulations for TE modes since our laser structure supports only the TE mode because of its gain region.
Table 2.1: Kappa values for different waveguide widths for a fixed ridge depth (1.145 µm)
You can see from the results that the cooupling decreases with the larger spacing.
To make it easy to understand, I plot the Ƙ0 and Ƙ1 versus spacing width for a fixed waveguide of 8 µm. In Figure 2.5 , there is a line at 3.5 cm-1 demonstrating the cavity loss level. This level indicates that 1.5 µm to 4.5 µm spacing width will be suitable for the PTS condition.
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Figure 2.5: Ƙ0 and Ƙ1 result for the 8 µm waveguide with different spacing widths. Blackline represents the loss level. Between Ƙ0 and Ƙ1 is the target working range.
Targeting the spacing width between 3 and-3.5 µm, Table 2.2 presents a simplified version of the previous table.
Table 2.2: Kappa calculations with achievable spacing widths for different waveguide widths.
One can see in Figure 2.5 that kappa changes almost linearly so that the waveguide width can be selected based on the current simulations. It means that kappa values will be in-between 6 µm and 7 µm waveguide widths values for a 6.5 µm waveguide width. Also, considering the fabrication limits, we set the waveguide widths to be 6.5 µm, 7.25 µm, and 8 µm with a spacing of 3.2 µm.
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0
1 2 3 4 5
Kappa cm-1
Spacing µm
8 µm
Kappa 0 Kappa 1
Loss = 3.5 cm-1 PTS-breaking range
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2.3. PTS Laser Fabrication 2.3.1. Mask Layout
Mask manufacturing is an expensive and complicated process. The researcher should carefully design the optical lithography mask layout and then check it a few times by putting all mask layers on top of each other to see if the patterns fit correctly on each other as expected. Mask production employs a laser light source instead of a UV light source. In this method, the laser source patterns the design by exposing the photoresist to laser light.
With the proper developer solution, the exposed photoresist is etched of and the rest remains solid. The Cr layer under the opening photoresist is etched away by appropriate chemicals. Since the Cr layer is as thin as 5nm, undercut is not an issue in this step. Mask is ready to use after removing all the photoresists left.
Device fabrication starts with the design of an optical lithography mask. We use the layout editor program L-edit from Tanner, EDA (Electronic Design and Automation), for this purpose. In the first mask design, we designed to have H+ Implantation electrical isolation of the waveguides. However, this device did not lase due to damage in the active region. Then we realized another solution to isolate the waveguides according to the results of the simulations and previous characterizations. By changing the epi-design, we significantly reduced the current leakage issue. Hence, a new mask layout was prepared for the new epi design.
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Figure 2.6: General view of the mask design.
There are two optical lithography masks; a "dark field mask" and the "clear field mask" Suppose it has more open areas than closed areas. In that case, it is called a "clear field mask," If the mask has more Cr parts than the openings, it is called a "dark field mask." If you have a dark field mask, it won't be easy to find your sample using a camera while aligning the sample to the mask during optical lithography because the only thing you can see is the mask itself. To solve this problem, we put a frame in every step that makes it easy to find and do the rough alignment. I use this frame to do rough alignment, and for the fine alignment, I have markers on every step. For example, in Figure 2.6, you can see the frame and the markers on a clear field mask. This frame might seem not crucial in the first look, but it makes alignment very easy, especially if you have a dark field mask.
The most challenging part of doing optical lithography is the alignment of the sample. If your mask aligner is a manual machine with a camera, you need to spend more time designing markers for the fine alignment. We have six lithography steps in this project, which means we do five alignments for one device fabrication. In the first step, we
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put three initial markers on each side of the chip for the second mask alignment, the top, middle, and the bottom of the chip, as shown in Figure 2.6. In the second step, we put a marker to align with the initial marker. In Figure 2.7 , you can see the alignment process.
There are two options to design six steps lithography masks; 1) You can put all the initial markers in the first mask, 2) You can put two markers in every step for aligning with the initial one and the next one. We choose both to increase the tolerance of fabrication to the misalignment. In the first option, every misalignment is adding up together. At the end of the device fabrication, we might be too far away from our tolerance. In this project, the second step is the most crucial one. Then we put the initial markers on the second mask.
Thus, the first misalignment will not affect the fabricated device.
Figure 2.7: Green fields are the first mask, red fields are the second mask.
In Figure 2.7 , green-colored areas are Cr parts of the first mask and the red-colored areas are for the second mask. The exposure light will not transfer from those areas, and the remaining areas expose to the light. After development, the same pattern will be on the sample.
In Figure 2.7 , you see two masks and markers in different colors. The green part is the initial marker from the first mask. The other four of them are the later markers for
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the next steps. Here you see the alignment of the second mask. The initial markers for the next steps will be defined here. The letters in every initial marker represent the name of the step; for example, "I" stands for isolation mask and "M" stands for metallization, which is P-metal deposition.
2.3.2. Lithographic Processes
Standart fabrication techniques are used for the device fabrication. In Figure 2.8 one can see the outline of the steps.
Figure 2.8 a) Cap layer etch b) Ridge etch c) Metal window opening d)P-Metal deposition. e) Electroplating, f) SEM picture after full fabrication.
Cap Layer Etch
We have a 100 nm highly dopped GaAs (Gallium Arsenide) cap layer in the epitaxial design. In the first step, I etch all the places but the top of the waveguide. As shown in Figure 2.9., we also define the laser cavity length etching near the facet regions. We remove the cap layer by chemical etching due to its repeatable etch rate and ease. We diluted Phosphoric acid solution with hydrogen peroxide for the etchant (add the chemical formulas and rations). Hydrogen peroxide oxidizes the surface in this solution, and phosphoric acid is etching the highly doped 100 nm GaAs layer in approximately 20
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seconds. Therefore etch rate is 300 nm/min. We will use the same solution also for the mesa etch.
Figure 2.9. a) Schematic of facet after cap layer etch. b) Top view of the device after cap layer etch taken by optical microscopy with 10x lens.
Chemically etching everywhere results in a rough surface which increases the adhesion of the silicon nitride layer. We use silicon nitride as the electrical isolation layer not to inject current outside the waveguide.We will discuss the details about the insulator layer in the p-metal window opening part. Cap layer etching is also decreasing the catastrophic optical mirror damage by protecting the facet from the current leakage. We will also explain it in the next chapter.
Mesa Etch
Mesa step is where we define the ridges. It creates the refractive index difference between the ridge and outside. The waveguide should have a higher refractive index than the surrounding. As we discussed earlier, we have two waveguides close to each other to make mode coupling possible between them.
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As we mentioned in the cap layer section, an essential point after mesa etching decreases the catastrophic optical mirror damage effect. We have a gap between the end of the cap layer etch, and the mesa etch. When we cleave the laser bar to define facets, we still have an etched part where the current does not flow. Thus, it limits possible leakage towards the laser facet. This leaking current would result in damaging the facet and decrease the lifetime of the laser device. Since the current does not flow to the facet, it will not damage the facet, as shown in Figure 2.10 . I label this gap as un-pumped region.
Therefore, the current will directly flow through the waveguide as shown in Figure 2.10 .
Figure 2.10. a) Sechematic of facet after cap layer etch. b) Sechematic of facet after mesa etches c) Top view of the device after cap layer etch taken by optical microscopy d) Top view of the device after mesa etch taken by optical microscopy.
The challenging part starts with the lithography. We set the spacing between the waveguides to 2 µm in the initial design in this step because we planned to do H+
Implantation. It adds extra steps to the device fabrication. We use a hard mask to protect waveguides from implantation. While implanting, they accelerate the atoms and hit the
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substrate; then atom sits in a suitable place in the crystal structure. The hard mask protects the waveguide so that particles cannot pass through the mask because it is thick enough to hold them. As you can imagine, this extra step will result in an undercut that increases the spacing between the waveguides. To keep the final spacing around 2 µm, we design the mask to 1.3 µm. It becomes 1.7 µm after hard mask etching, and reaches 2 µm at the end of substrate etching.
In a standard semiconductor laser device fabrication, the etching method is chemical etching. The main difference between the chemical and physical etching is the anisotropy. For anisotropic etching, we use physical etching, and for isotropic etching, we use chemical etching. Etching type affects the wall shapes of the ridge. In the P-metal deposition step, we need to have a continuous metal layer on the walls. Chemical etching provides this positive angled wall shape for a continuous metal layer. Negative angled mesa walls will result in discontinued top metal contact, which makes the plating step impossible.
Chemical etching is essential for laser fabrication. Based on a systematic study published on GaAs etching [32], [33], we aimed to obtain the same wall shape for my epitaxial design. We successfully repeated the results for GaAs, but it didn't give good results for the laser structure we employed for the device fabrication. The results are summarized in Figure 2.11 . As seen in Figure 2.11., chemical etching strongly depends on the crystal orientation and substrate. We have two different crystal orientations in this experiment, perpendicular and parallel orientation, defined according to major flat. The results for the 0-deg-off GaAs wafer with HCl: H2O2:DI (1:4:40) satisfy our expectations, but the same solution does not give a similar wall shape with the 0-deg-off laser sample.
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According to these results, the undercut is more than expectations as it expands the spacing width to 3 µm. Then, we focused on anisotropic etch optimization with ICP to have less undercut. To overcome discontinues in the top metal contact layer, we will make the top contact thicker by electrochemical gold deposition. If the metal layer is thicker than my ridge height, the contact metal will be continuous as desired.
Figure 2.11. Three different etch solutions, parallel and perpendicular, represent the crystal orientation.
We start optimizing the anisotropic etching for silicon nitride with RIE and laser sample etching with ICP. We will discuss the results of anisotropic silicon nitride etching in the next chapter. Here we will show the final results of anisotropic substrate (mostly GaAs and AlGaAs) etching with ICP. We used the recipes from the literature to optimize for our structure [34]–[38]. After obtaining a working recipe, we modify the gas ratios to make them suitable for our case. Figure 2.12 presents a schematic and the etching results of four different recipes and the red circle in the schematics shows SEM picture area. The first three are with the same gases, and the difference between them is the gas ratios.
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Different ratios change the undercut but not in a linear way. The spacing width for the recipes is 1.463 µm, 1.571 µm, 1.473 µm, and 1.968 µm, respectively. Changing the gasses is directly affecting the etch rate. Etch rates for the recipes are 1.4 µm/min, 1.6 µm/min, 1.4 µm/min and 1.9 µm/min respectively. According to the results, recipe b and d look suitable but recipe b has some roughness on the walls. Then, we decided to continue with recipe d.
Figure 2.12. a) Schematic of structure b) BCl3:Cl2 (5:15) c) BCl3:Cl2 (10:15) d) BCl3:Cl2 (15:15) e) BCl3:Cl2:Ar:N2
(10:10:10:4.5).
After physical etching with ICP, we continue with chemical etching for 20 seconds to obtain smooth wall morphology. We represent the SEM pictures of defined mesa steps after etching in Figure 2.13. The SEM pictures from the facet and optical images of top surface are presented in Figure 2.13.
Figure 2.13. a) SEM picture of the facet b) Optical microscope image of the surface after etching the nitride film.
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Etch Process Development for H+ Implantation
In the project, the first idea was to do hydrogen implantation for electrical isolation of the waveguides. This section discusses the details of the hard mask required for the implantation process. We need a hard mask to protect some regions from implanted hydrogen ions during the implantation process.
We employed the mesa etch step mask for patterning the hard mask, so it does not add a step to the lithography process. We deposit silicon nitride to the surface with PECVD, then pattern it with PR. We continue with etching silicon nitride with RIE, etching the substrate with ICP, and then chemical etching. To deposit 800 nm silicon nitride, we used a PECVD recipe with Silane (SiH4,%2): NH3 (180:45 sccm) at 250 C° and 1 Torr. Then, it is patterned by ~500 nm thick PR. We then use an RIE recipe with CHF3:O2 (100:5 sccm), 20 µbar, 71-W power to etch the silicon nitride. This recipe gives excellent results as presented in Figure 2.14. The SEM picture shows the facet profile after mesa etch with the hard mask on top, where the final spacing is around 2 µm.
Figure 2.14. SEM picture after etching the nitride and substrate.
Then the silane gas we are using for depositing silicon nitride was depleted.
Replacing gas took around six months and we focused on optimizing new recipes using
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another PECVD machine to deposit silicon nitride. We first deposit with the same recipe I used before in the new machine. Then we tried to etch it with the same RIE recipe, but it didn't give the same results. In Figure 2.15. you see four pictures from two different recipes; a-b) are the same recipe that I used before, c-d) are the SF6:O2 (20:5 sccm) 20µbar 71 W. Figure 2.15 a-c) shows the silicon nitride residues in the opening part after the RIE process. I added 10-sec BOE etching to remove the leftover silicon nitride residues. In Figure 2.15 b-d) you can see the clean version of the opening after BOE etching.
Figure 2.15. Spacing between the waveguides after silicon nitride etching a) CHF3:O2 (100:5 sccm) 20 µbar 71 watt b) CHF3:O2 (100:5 sccm) 20 µbar 71 watt + 10-sec BOE etching c) SF6:O2 (20:5 sccm) 20 µbar 71 watt d) SF6:O2 (20:5 sccm) 20 µbar 71 watt + 10-sec BOE.
The spacing width is ~2 µm in Figure 2.15.a), and ~3 µm in b). Since there are still residues, we need to do BOE etch for ten or twenty more seconds, increasing the spacing.
In
Table 2.3, all the results from our RIE experiments are summarized. In the meantime, RIE broke down, and we had to move to ICP for etching silicon nitride. It has various gas options, and etch rate is higher than RIE.
We did optimization with ICP to etch silicon nitride anisotropically. In this part of the thesis, we will summarize the results of these experiments. There are few things to optimize with ICP etching: ICP Power, Bias Power, frequency of ICP power, and gas ratios. First, we need to find a recipe that etches the silicon nitride we deposit. We tried some recipes from literature and colleagues. The recipes from the literature generally do
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not lead to the results presented due to the difference of machine architectures. However, the recipes from the colleagues provide better results. ICP machine has two power supplies;
ICP Power and Bias Power. ICP Power provides higher density plasma, and it increases the etch rate. If you apply zero ICP Power, ICP becomes RIE because RIE has only Bias Power. Previously we tried the same etching recipe we used in RIE, with zero ICP power.
We didn't get the same results that we used to get on the same silicon nitride layer in Figure 2.16. Comparing the results from this etching experiment, a) represents the results from RIE etching b) ICP etching. Although we set the same values, the results are not similar.
In Figure 2.16, RIE etched all the silicon nitride but ICP caused undercut with a ratio of almost one.
Table 2.3. We named the recipes for the people who will use the same machine within our facility; UNAM and ARL are the names of two different clean rooms within our facility. For example, the difference between UNAM He and UNAM N2 recipes is the carrier in the Silane gas, UNAM He has %2 Silane and %98 He, UNAM N2 %2 Silane %98 N2, these carriers shouldn't change the quality of silicon nitride because the only difference is the carrier gas. I already give the details of the etching recipes in the previous discussion.
Machine Name of the recipe
Deposition Rate (nm per minute)
ETCH RATE (nm per minute)
BOE CHF3:O2 100:5
CH4F3:O2 60:40
SF6:O2 20:5
PECVD 2
UNAM
He - - 8.625 - -
UNAM
N2 21.34
~1400 14 (40 min etch with PR)
7.75
20 (10 min etching)
~300 8.75 (40 min etch WO PR) 16.67 (30 min etching)
~180 9.34 (30 min etch WO PR) 13.4 (50 min etching)
OLD PECVD
ARL He 7.27
- 11.25 - -
6.82
ARL N2 12.7 ~180 21 - -
ARL
SiO2 34
Hot: 450
- - -
R.T.:
150
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The goal of these experiments is to have a repeatable working recipe with maximum selectivity. The PR thickness is 460 nm, and the silicon nitride thickness is 700 nm as required for the implantation process. Therefore, selectivity should be higher than 1.52. In Figure 2.17, you can see the effect of ICP power frequency. There are two frequency options for the ICP Power; 380 kHz and 13.56 mHz. Higher frequency provides higher plasma density. As you can see in Figure 2.17, surprisingly, higher density plasma etches less PR than lower density plasma, and undercut is also less compared to the lower density plasma.
Figure 2.16: a) Etching recipe I used with RIE CHF3:O2 (100:5 sccm) 20 µbar 71 watt b) Etching recipe I used with ICP CHF3:O2 (100:5 sccm), 15 mTorr, ICP Power: 0 W, Bias Power: 71 W. The pressure might seem different but when we convert µbar to torr they are the same.
Since plasma is etching the substrate physically by hitting the surface atoms with ions and ejecting them, ICP is categorized among the physical etching methods. However, due to the interaction between the gasses and the substrate, chemical etching is occurring.
These minor interactions may increase due to the ratio of etching gasses. The results of this chemical etching are the undercut. Therefore, we can decrease the undercut by changing the gas ratio. In Figure 2.19. you see the three different gas ratios and subsequent undercut
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values. We have two other gases, CHF4 and O2 ratio, in the recipes, representing the flow rate of CHF4/O2. In Figure 2.19 ratio is 5, 1.34, and 1, respectively, where undercut values are on the bottom of the pictures. They are not similar to our expectation. They do not change linearly due to the huge difference between the ratios. For example, by setting the ratio to 2 instead of 5, we would see the linear change in the undercut values. Because when we increase the volume of gas inside the chamber, other unknown interactions might happen. It is better the put values close to each other. It seems that from Figure 2.19 b-c), a higher gas ratio lowers the undercut due to the lower interaction between the gasses and the substrate.
Figure 2.17. a) Etching results of the recipe with low-frequency ICP power b) Etching results of the recipe with high-frequency ICP power.
Figure 2.18. SEM pictures from the facet view after following recipes a) Bias power: 300 watt, ICP power:
30 watt, CF4:O2 (100:20 sccm), Pressure: 40 mTorr b) Bias power: 100 watt, ICP power: 30 watt, CF4:O2
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(100:20 sccm), Pressure: 40 mTorr c) Bias power: 70 watt, ICP power: 30 watt, CF4:O2 (20:15 sccm), Pressure: 5 mTorr d) Bias power: 70 watt, ICP power: 30 watt, CF4:O2 (20:15 sccm), Pressure: 40 mTorr.
Figure 2.19. SEM pictures from the facet view after following recipes a) Bias power: 70 watt, ICP power:
30 watt, CF4:O2 (100:20 sccm), Pressure: 40 mTorr b) Bias power: 70 watt, ICP power: 30 watt, CF4:O2 (20:15 sccm), Pressure: 40 mTorr c) Bias power: 100 watt, ICP power: 30 watt, CF4:O2 (20:20 sccm), Pressure: 40 mTorr
We showed some of the results from different experiments; we end up with this recipe which might work for our case. Bias power: 400 watt, ICP power: 100 watt, CF4:CHF3 (100:20 sccm), Pressure: 40 mTorr. After deciding on the recipe, we should check if it is repeatable. For this, we tried the same recipe for a different amount of time.
In Figure 2.20. you can see the results from different etch times from 1 to 3.5 min.
According to the results, we don't have a repeatable recipe; even the etch rates change from one process to another. We realized that inside the chamber etching process is not homogenous. If we don't place the sample in the same location every time, we have different results.
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Figure 2.20: SEM pictures from the facet view after best recipe (Bias power: 400 watt, ICP power: 100 watt, CF4:CHF3 (20:3 sccm), Pressure: 5 mTorr) with different etch duration a) 1-minute b) 2-minute c) 2.5- minute d) 3-minute e) 3.5-minute.
We implemented a two-step implantation process. First step is 3E15/cm2 at 50keV then second step is 2E15/cm2 at 20keV. The implant energy determines the depth for waveguide isolation as illustrated in Figure 2.21. The H+ implanted areas protect the lasing part so that we can isolate waveguides from each other.
Figure 2.21. The facet view of the implantation process.
P-Metal Window Opening
Now we deposit the isolation layer make a path for the current to follow through the waveguide. On top of the ridges, we etch some part as a window to ensure that the current will flow through there. We represent the structure after metal window opening, both schematic, and optical microscope image in Figure 2.22. From those openings, current will flow through the waveguide.
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Figure 2.22.: a) Facet view of the structure after window opening. b) optical microscopy image from the top after window opening, purple areas are silicon nitride, and white areas are the window opening.
We have the most critical alignment in this step. If it is shifted more than 1.25 µm due to the misalignment, window opening will be on the spacing to pump the area we want to isolate. To protect from this, in the new mask design, we increased the separation between the windows as much as possible. We shift them in the opposite direction through the edges of the waveguides. Another thing here is, we were using BOE for etching the silicon nitride for opening windows. Silicon nitride thickness is 150 nm, and BOE etch rate is too high, see
Table 2.3. due to this high etch rate, controlling the undercut was difficult. To protect from opening more expansive windows than ridges, we did the window opening with RIE to have anisotropic walls. In Figure 2.23, you can see the results from the anisotropic window opening.
