GROWTH AND LUMINESCENCE CHARACTERISTICS OF INDIUM NITRIDE FOR OPTOELECTRONICS APPLICATIONS IN THE 1.55-MICRON REGION SEETOH PEIYUAN, IAN (M.Eng. Massachusetts Institute of Technology) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN ADVANCED MATERIALS FOR MICRO- AND NANOSYSTEMS (AMM&NS) SINGAPORE-MIT ALLIANCE NATIONAL UNIVERSITY OF SINGAPORE 2014 DECLARATION I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. ______________________ Seetoh Peiyuan, Ian 30 November 2014 i Acknowledgements I would like to thank Prof. Chua Soo Jin and Prof. Eugene Fitzgerald for their guidance and support over the years, enabling me to approach my PhD research with confidence, helping me overcome difficulties, as well as challenging me towards greater excellence along the way. I would also like to express my gratitude to Dr. Soh Chew Beng for his patient mentoring and also for imparting to me essential experimental skills. Also, I would like to thank staff and students of the Centre of Optoelectronics, namely: Dr. Huang Xiaohu, Dr. Tay Chuan Beng, and Mr. Patrick Tung for their help with the photoluminescence equipment, Dr. Wee Qixun, Mr. Ho Jian Wei, Mr. Zhang Li, Mr. Kwadwo, and Mr. Rayson Tan for their help with the metal organic chemical vapor deposition system, Mr. Tan Beng Hwee and Ms. Musni bte Hussain for their technical and administrative support, as well as many others who have contributed to a very pleasant working environment. In addition, I would like to thank the Institute of Materials Research and Engineering for providing the necessary fabrication and characterization tools, where Mr. Glen Goh, Ms. Doreen Lai, Ms. Vivian Lin, Mr. Lim Poh Chong, and Ms. Tan Hui Ru were especially helpful in providing assistance to the use of several essential tools. Lastly, I would like to thank SMA for providing the funding and facilitating my PhD program, especially to Prof. Choi Wee Kiong and Ms. Hong Yanling for their support. Also, I would like to thank my fellow SMA students, with whom I had memorable experiences taking lessons at the MIT. ii Table of contents Summary vi List of Tables viii List of Figures . viii List of Symbols . xv Chapter : Introduction . 1.1. Background . 1.1.1. InN’s band gap controversy 1.1.2. Recent developments in the epitaxial growth of InN 1.2. Motivation . 10 1.2.1. InN for optical communications . 10 1.2.2. Challenges in the MOCVD growth of InN . 14 1.3. Objectives . 23 1.4. Major Contributions 24 1.5. Outline of thesis 25 Chapter : Experimental tools and procedures . 27 2.1. Metal organic chemical vapor deposition . 27 2.2. Photoluminescence measurements . 31 2.3. UV-assisted electrochemical etching of GaN . 33 2.4. Imaging tools 36 2.5. High resolution X-ray diffraction . 38 Chapter : Growth of InN by MOCVD 41 3.1. Introduction . 41 3.2. Important considerations . 41 iii 3.2.1. Controlling morphology of InN grown on GaN . 41 3.2.2. Reducing dislocation densities 43 3.2.3. Reducing background electron concentration . 45 3.3. Growth experiments 45 3.3.1. Substrates and growth conditions . 45 3.3.2. General observations . 48 3.4. Summary . 52 Chapter : Characteristics of carrier recombination and optical emission from InN . 54 4.1. Introduction . 54 4.2. Non-radiative Auger and SRH processes . 56 4.2.1. Theory: Double-Arrhenius law and power law . 56 4.2.2. Influence of Auger and SRH processes on PL intensities 64 4.3. Radiative recombination in InN 72 4.3.1. Theory: ‘Band-to-tail’ PL lineshape model 72 4.3.2. Lineshape analysis of PL spectra 79 4.4. Summary . 83 Chapter : Nucleation characteristics of InN on GaN 85 5.1. Introduction . 85 5.2. Effects of surface treatment with InGaN 87 5.3. Effects of adjusting V/III ratio on nucleation . 91 5.4. Summary . 95 Chapter : Morphology and optical emission properties of InN grown on nanoporous GaN templates . 97 6.1. Introduction . 97 6.2. Fabrication of nanoporous GaN substrates . 97 iv 6.3. Microstructure of InN grown on nanoporous GaN . 100 6.3.1. Structural analysis . 100 6.3.2. Growth mechanism . 107 6.4. Optical emission from InN grown on nanoporous GaN . 111 6.5. Summary . 116 Chapter : Conclusion 118 7.1. Summary . 118 7.2. Future work . 121 7.2.1. Reducing free electron concentrations 121 7.2.2. Device applications . 123 7.3. Concluding remarks 124 Appendices 126 A.1. List of MOCVD-grown InN samples 126 A.2. Calculation of dislocation densities by X-ray diffraction 128 A.3. Angle and distance between hexagonal planes 132 A.4. Biography . 133 A.5. Publication list . 133 A.5.1. Academic theses . 133 A.5.2. Research journals . 133 A.5.3. Participation in international conferences 134 Bibliography . 135 v Summary Since the revision of its optical band gap from 1.9 eV to around 0.75 eV, InN has received considerable attention to its ability to emit near-infrared radiation at around the 1.55 μm region that is essential for fibre-optics communication. Nevertheless, InN-based light-emitting or laser diodes are currently unavailable commercially due to poor material quality linked to difficulties in epitaxial growth by metal organic chemical vapor deposition, resulting in rough films with high dislocation densities of above 1010 cm-2. Also, the optical processes leading to InN’s optical emission are poorly understood, especially with regards to its relationship to crystalline quality. To address these issues, we have studied the carrier recombination behavior in InN through a series of photoluminescence measurements and developed improved techniques of InN epitaxy on GaN surfaces. The radiative recombination of InN was found to obey a ‘band-to-tail’ model, whereby the near-infrared optical emission is primarily due to recombination between degenerate electrons in the conduction band and photoexcited holes in the tail of the valence band. Non-radiative recombination, comprising of Auger and Shockley-Read-Hall processes, lowered the internal quantum efficiency of the material to as low as 3%, resulting in poor emission intensities at room temperature. Both Auger and Shockley-Read-Hall recombination were found to be thermally-activated, with Auger recombination dominating at low temperatures. Shockley-Read-Hall recombination is more significant in samples of poor crystalline quality, resulting in its dominance over Auger recombination at room temperature. vi We have developed several useful growth techniques that led to growth of smoother InN material on GaN, along with higher internal quantum efficiencies. As InN typically grows in an N-rich condition that is limited by the availability of In precursors, decreasing the V/III ratio during growth increases the thermodynamic driving force that leads to much higher nucleation rates and resulting in better surface coverage. In addition, the high surface energies that act as a barrier to nucleation of InN on GaN can be reduced by performing an InGaN surface treatment procedure or by using nanoporous GaN substrates. Nanoporous GaN was fabricated by the electrochemical etching of GaN by aqueous potassium hydroxide in the presence of ultraviolet light. The process creates many hexagonal sub-100 nm pores in the otherwise smooth GaN surface, which presents numerous surfaces for InN rapid nucleation. The internal quantum efficiency of InN rises to 20% in samples grown on nanoporous GaN. This is attributed to the relief of biaxial stress arising from the lattice mismatch between GaN and InN through the lateral overgrowth of InN over GaN nanopores. This results in the resultant InN having fewer dislocations, which act as non-radiative Shockley-Read-Hall recombination centers. Overall, a combination of InGaN preflow, high V/III ratios, and nanoporous GaN substrates leads to improved morphology, crystalline quality, and optical emission efficiencies, which is vital for future applications in optoelectronic devices. vii List of Tables Table 1-1: Selected physical parameters of materials in InN/GaN and InGaAsP material systems related to optical emission at 1.55 μm [38-40, 42-43]. 13 Table 1-2: Lattice mismatch of relevant substrate materials with c-InN (Inplane lattice parameter aInN = 3.538 Å) 18 Table 4-1: k values under various conditions in the power law: I  Pk. 64 Table 4-2: Calculated values of parameters related to the PL lineshape analysis of InN’s temperature-induced blue-red spectral shifts . 81 Table 6-1: List of InN samples used for structural comparisons in Section 6.3. 100 Table A-1: List of InN samples grown by MOCVD, together with important growth parameters*. . 126 List of Figures Fig. 1-1: The group-III nitride material system, compared to other III-V and IIVI material systems. Image taken from Ref. [11]. . Fig. 1-2: Atomic force microscopy images of InN grown at 450oC on GaN templates by plasma-assisted MBE under (a) In-rich conditions (In flux = 13.5 nm/min, N flux = 10.5 nm/min), and under (b) N-rich conditions (In flux = 8.5 nm/min, N flux = 10.5 nm/min). In droplets were not visible in (a) as they were removed after growth using HCl etch. Images taken from Ref [18]. Fig. 1-3: Scanning electron microscopy images of InN grown by MBE on GaN/c-sapphire at 450oC under In-rich conditions (a) without and (b) with nitrogen radical beam irradiation which removed In droplets, resulting in a very smooth InN film of r.m.s. roughness < nm. Images taken from Ref [16]. Fig. 1-4: Scanning electron microscope images of InN grown on GaN/csapphire by MOCVD at 510oC at a V/III ratio of 12,460 (a) with constant TMI flow and (b) with pulses of TMI flow (36 sec pulse, followed by 18 sec interruption). The dark regions in (a) are In droplets. The InN sample in (b) is free of In droplets and has a r.m.s. roughness of nm. Images are taken from Ref. [26]. Fig. 1-5: Atomic force microscope images of InN grown by MOCVD on cGaN/sapphire at 450oC with increasing CBrCl3 flows: (a) Molar flow ratio: CBrCl3/TMI = 0, r.m.s. roughness = 40 nm. (b) Molar flow ratio: CBrCl3/TMI viii = 0.02, r.m.s. roughness = 25 nm. (a) Molar flow ratio: CBrCl3/TMI = 0.04, r.m.s. roughness = 6.8 nm. (a) Molar flow ratio: CBrCl3/TMI = 9.28, r.m.s. roughness = 1.6 nm. Images taken from Ref. [31] . Fig. 1-6: Trend of global mobile traffic between 2010 and 2018. Image taken from Ref. [32]. . 11 Fig. 1-7: Infrared emission from an InN-based LED [33], with its peak emission coinciding with point of lowest attenuation in silica fibers [34]. . 11 Fig. 1-8: Decomposition temperatures of InN at 100 mbar under different ambient: hydrogen (top), nitrogen (middle) and in mixed nitrogen ammonia ambient (75% ammonia, bottom). Image taken from Ref. [24] . 15 Fig. 1-9: (left) In droplets formed together with InN (when the growth temperature is too low, appearing as the In (101) peak in XRD ω-2θ rocking curve scans (right). The InN and In droplets were grown on a GaN/AlGaN/AlN/Si(111) template. 15 Fig. 1-10: Amorphous SiNx formed during an attempt to grow InN directly on Si (111) with preflow of TMI. . 17 Fig. 1-11: Melting points of Group III nitrides and equilibrium N2 pressures from high pressure experiments and theoretical calculations. Image taken from Ref. [56]. 20 Fig. 1-12: Calculated band diagram and associated carrier concentrations near the surface for n-type InN doped with Nd = x 1018 cm-3 in (a) and (c) respectively, and for p-type InN doped with Na = 2.3 x 1019 cm-3 in (b) and (d). For the n-type case, the free hole concentration (~108 cm-3) is negligible, hence it is not pictured. Figures taken from Ref. [57]. 21 Fig. 1-13: Hall mobility and carrier concentration of InN layers grown by both MBE and MOCVD techniques, as a function of film thickness. Mobilities are mostly comparable for both techniques for the same thickness while MOCVD layers exhibit higher free electron concentration. Image taken from Ref. [23]. 22 Fig. 2-1: (a) The Emcore D125 MOCVD system. (b) Samples are introduced into the growth chamber via a sample load lock to prevent contamination of the growth chamber. The gaseous reactants enter the chamber from the top via several inlets . 30 Fig. 2-2: Reflectivity of the wafer carrier (with GaN/c-sapphire substrates) at 700 nm during MOCVD growth of InN. Growth occurs when TMI flow is introduced, with ammonia and nitrogen gas flowing in the background. (a) Slow growth rate of InN nanoislands at V/III ratio of 55,000 – Sample C1. (b) Fast growth rate of InN film at V/III ratio of 15,000 – Sample C3. Further information about these samples is provided in Chapter and Appendix A1. 30 Fig. 2-3: Photoexcitation of charge carriers which subsequently relax energetically and undergo Auger, Shockley-Read-Hall (SRH), and radiative ix measurable by ω scans at a (00l) reflection. In these symmetric scans, χ = where χ is the inclination angle of the crystal planes relative to the surface normal (See Fig. 2-9 in Chapter for the geometry of the χ angle). Edge dislocations not broaden symmetric (00l) scans as their Burgers vectors lie within the (00l) plane. Fig. A2-2: Lattice twist Δωtwist of a GaN sample estimated by extrapolating Δωhkl values to χ = 90o. Note the large number of data points required for a reliable curve fit using Eq. A2.4. Figure obtained from Ref. [78]. On the other hand, edge dislocations broaden the (h00) reflection at χ = 90o, resulting in lattice twist Δωtwist, while both tilt and twist from screw and edge dislocations respectively contribute to the ω broadening of off-normal (h0l) reflections (0 < χ < 90o). Since the (h00) reflection is not accessible by most XRD systems, Δωtwist is usually estimated via ω scans of several (h0l) planes under the skew-symmetric geometry. Usually, the data is extrapolated using an appropriate curve fit to obtain the Δωtwist value at χ = 90o as shown in Fig. A2-2. Numerous data points are necessary for an accurate extrapolation. This tedious process is avoided in our linear fitting procedure which will be described as follows. 129 Δωtilt and Δωtwist are related to dislocation densities with a screw (Dscrew) or edge (Dedge) character respectively according the following expressions, assuming random distribution of dislocations [107-108]: (A2.2) (A2.3) where a and c are the in-plane and out-of-plane lattice parameters respectively and Δωtilt and Δωtwist have units in radians. For InN, a = 3.539 Å and c = 5.708 Å [108]. For a highly dislocated material like InN, Δωhkl is related to Δωtilt and Δωtwist by [108-109]: (A2.4) where the correlation length L is the statistical distance between dislocations, K = 2sinθ/λ is the magnitude of the reciprocal lattice vector, with θ and λ being the Bragg angle and X-ray wavelength respectively. n = + (1 – f)2 where f is the fraction of Lorentzian character calculated in the Pseudo-Voigt fit earlier. n assumes a value between (purely Lorentzian) and (purely Gaussian). For symmetric reflections about (00l) planes where χ = 0, the expression can be simplified into: (A2.5) This expression allows Δωtilt to be calculated from the slope in the linear plot of (K.Δω00l)n vs. Kn. The (002), (004), and (006) reflections are accessible in XRD and provide three data points for an accurate linear fit as shown in Fig. 130 A2-3a. This procedure is frequently described in literature as a WilliamsonHall plot [105-106]. Fig. A2-3: (a) Lattice tilt Δωtilt calculated using Eq. A2.5. (b) Lattice twist Δωtwist calculated using Eq. A2.6. Data obtained from Sample C3 (Δωtilt = 0.480o, Δωtwist = 0.559o) by performing XRD ω scans about the indicated InN crystal planes. Once Δωtilt is known, Eq. A2.5 can be rearranged into: (A2.6) where Δωtwist can be calculated from the slope in the linear plot of [(K.Δωhkl)n – (KΔωtiltcosχ)n] vs. (Ksinχ)n. The (101), (102), (103), (201), (202), (203), and (302) form a set of (h0l) planes suitable for skew-symmetric ω scans and their results are shown in Fig. A2-3b. Interestingly, the data points fall into three regions corresponding to (10l), (20l), and (302) planes. This is because Ksinχ = 1.15(h/a) (See Appendix A3), which is independent of l. For quick measurements of Δωtwist, we would recommend the (101), (201), and (302) planes due to their higher intensities compared to other (h0l) planes, resulting in only three data points required for the linear fit, as opposed to numerous data points required for a curve fit (Fig. A2-2). This adaption 131 retains the theoretical model originally proposed by Lee and West [78], but involves a more reliable and less tedious data fitting process. A.3. Angle and distance between hexagonal planes The angle χ between two hexagonal planes: (h1 k1 l1) and (h2 k2 l2) is given by the following expression [110]: (A3.1) where a and c are the in-plane and out-of-plane lattice parameters respectively. In the specific case of the angle between a (h0l) plane and the c-plane (00l’): (A3.2) (A3.3) The distance d between hexagonal planes (hkl) is given by the following expression from Ref. [111]: (A3.4) For (h0l) planes: (A3.5) We get, after combining and simplifying Eqs. (A3.3) and (A3.5), (A3.6) 132 A.4. Biography 1) 6/2009 – 6/2014: Singapore-MIT Alliance (National University of Singapore), PhD candidate, thesis advisors: Prof Chua Soo Jin (NUS) & Prof Eugene Fitzgerald (MIT) 2) 8/2009 – 5/2010: Massachusetts Institute of Technology, Master of Engineering (Materials Science and Engineering) 3) 8/2005 – 5/2009: National University of Singapore, Bachelor of Engineering (Chemical), 1st Class honors, specializing in microelectronics 4) 1/2008 – 5/2008: University of California at Berkeley, Overseas student exchange A.5. Publication list A.5.1. Academic theses 1) 2010: Massachusetts Institute of Technology, Master of Engineering (Materials Science and Engineering), “Commercialization of group III nitrides-on-silicon technologies.” 2) 2009: National University of Singapore, Bachelor of Engineering (Chemical), “Lead(II)-modified silicon nanowire-based field-effect transistor sensors for 2-in-1 detection of ammonia and hydrogen sulphide” A.5.2. Research journals 1) I. P. Seetoh, C. B. Soh, L. Zhang, K. H. Patrick Tung, E. A. Fitzgerald, and S. J. Chua. "Improvement in the internal quantum efficiency of InN grown 133 over nanoporous GaN by the reduction of Shockley-Read-Hall recombination centers." Appl. Phys. Lett. 103, 12 (2013). 2) I. P. Seetoh, C. B. Soh, E. A. Fitzgerald, and S. J. Chua. "Auger recombination as the dominant recombination process in indium nitride at low temperatures during steady-state photoluminescence." Appl. Phys. Lett. 102, 10 (2013). 3) I. P. Seetoh, C. B. Soh, E. A. Fitzgerald, and S. J. Chua. "Effects of valence band tails on the blue and red spectral shifts observed in the temperaturedependent photoluminescence of InN." Phys. Status Solidi B 250, (2013). 4) C. B. Soh, C. B. Tay, R. J. N. Tan, A. P. Vajpeyi, I. P. Seetoh, K. K. Ansah-Antwi, and S. J. Chua. "Nanopore morphology in porous GaN template and its effect on the LEDs emission." Journal of Physics D: Applied Physics 46, 36 (2013). A.5.3. 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Band gap (eV) Electron effective mass at band edge me*/mo Conduction band offset (eV) Valence band offset (eV) Refractive index at 1. 55 μm, nr InN 0.7 – 0.8 GaN 3.2 InGaAsP 0.8 InP 1. 35 0.042 0.20 0.056 0.076 2 .1 (InN/GaN) 0.5 (InN/GaN) 2.9 2.3 0.27 (InGaAsP/InP) 0.28 (InGaAsP/InP) 3.2 3.5 13 1. 2.2 Challenges in the MOCVD growth of InN While there has been encouraging developments in the growth of InN... resulting in the thermal decomposition of InN and very little growth (see Section 1. 2.2) At growth temperatures of around 450 to 550 oC, the steady N flux provided during plasma-assisted MBE enables fast growth rates of 1 – 3 μm per hour The V/III ratio during plasma-assisted MBE is controlled by the relative rates of In and N flux and is very important in determining the quality of the material Initially,... the corresponding value of 8.5% for the InGaAsP/InP system, enabling better optical confinement and waveguiding These advantages in terms of effective masses, conduction band offset, and refractive indices help lower the threshold current required for population inversion and lasing to as low as 51 A/cm2, as predicted for InN-based lasers [ 41] This compares well with other emerging 1. 55 μm semiconductor... signals into optical fibers and can stimulate further innovation resulting in significant savings in cost, energy, and device footprint Besides the integration of GaN-based electronics, another advantage of using InN for 1. 55 μm emission is that it is a binary compound, as opposed to quarternary InGaAsP and complex GaIn(N)AsSb materials, resulting in better compositional control during material growth and. .. diffraction imaging conditions with g = 1- 10InN that enables the observation of edge-type dislocations (yellow arrows) in the form of white lines (c) The corresponding selected area diffraction pattern with the zone axis of [11 0]InN where the diffraction imaging was performed 11 1 Fig 6-9: PL spectra of InN grown on nanoporous and planar samples The emission spectra at 300 K were compared to the normalized... diagram of InN growth experiments on various substrates, resulting in four sets of samples The structure of InGaN grown in Samples C1 to C3 will be elaborated in Chapter 5 A summary of growth conditions and InN’s morphology is provided in Appendix A1 48 Fig 3-3: Typical XRD scans obtained from InN samples (a) ω-2θ scan showing the c-axes of GaN and InN to be aligned The strain-free position of InN... as the carrier gas The situation mirrors that of InGaN, where better growth results were obtained using a N2 carrier gas In addition, as the pyrolysis of NH3 releases hydrogen, the etching of InN by hydrogen sets an upper limit to the V/III ratio [ 51] It was shown that reducing the V/III ratio led to faster growth rates [15 , 46], with the further lowering of V/III ratio leading to the formation of In. .. Fig 1- 1: The group-III nitride material system, compared to other III-V and II-VI material systems Image taken from Ref [11 ] 2 1. 1.2 Recent developments in the epitaxial growth of InN Early attempts in InN growth by radio-frequency sputtering produced polycrystalline films [12 -13 ] These materials have very high free electron concentrations exceeding 10 20 cm-3 and were unable to exhibit photoluminescence... when the growth temperature is too low (< 500oC), the pyrolysis rate of ammonia becomes too low to generate sufficient N atoms for reaction, resulting in the formation of In droplets (see Fig 1- 9) [47] In droplets interrupt the continuity of the InN material and can form short circuits in electronic devices Also, the In droplets appear to draw reactants away from 15 its surroundings, resulting in negligible... Samples A1 and A3 Growth was performed at a V/III ratio of 55, 000 for (a & c) 10 min and (b & d) 60 min 10 2 Fig 6-4: InN nanoislands grown on (a – b) nanoporous GaN – Samples B2 and B4, and on (c – d) InGaN-treated planar GaN – Samples C1 and C2 The V/III ratio is varied at (a & c) 55, 000 and (b & d) 30,000 10 3 Fig 6-5: InN grown on (a) InGaN-treated planar GaN for 40 min at V/III ratio of 30,000 . controversy 1 1. 1.2. Recent developments in the epitaxial growth of InN 3 1. 2. Motivation 10 1. 2 .1. InN for optical communications 10 1. 2.2. Challenges in the MOCVD growth of InN 14 1. 3. Objectives. GaN/AlGaN/AlN/Si (11 1) template. 15 Fig. 1- 10: Amorphous SiN x formed during an attempt to grow InN directly on Si (11 1) with preflow of TMI. 17 Fig. 1- 11: Melting points of Group III nitrides and equilibrium. GROWTH AND LUMINESCENCE CHARACTERISTICS OF INDIUM NITRIDE FOR OPTOELECTRONICS APPLICATIONS IN THE 1. 55- MICRON REGION SEETOH PEIYUAN, IAN (M.Eng. Massachusetts Institute of