| 1. |
- Lilja, Louise, 1985-
(författare)
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4H-SiC epitaxy investigating carrier lifetime and substrate off-axis dependence
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Silicon carbide (SiC) is a wide bandgap semiconductor with unique material properties making it useful for various device applications using high power, high frequency and high temperature. Compared to Si-based electronics, SiC based electronics have an improved energy efficiency. One of the most critical problems is to reduce this planets power consumption, where large improvements can be made enhancing the energy efficiency. Independent on how the electrical power is generated, power conversion is needed and about 10% of the electrical power is lost for every power conversion step using Si-based electronics. Since the efficiency is related to the performance of the semiconductor device, SiC can make contributions to the efficiency. Compared to Si, SiC has three times larger bandgap, about ten times higher breakdown electric field strength and about three times higher thermal conductivity. The wide bandgap together with the chemical stability of SiC makes it possible for SiC electronic devices to operate at much higher temperatures (>250°C) compared to Si-based devices and do not require large cooling units as with Si power converters.The current status for 4H-SiC devices regard unipolar devices (≤ 1700 V), such as metal-oxide-semiconductor field-effect transistors (MOSFETs) and Schottky barrier diodes (SBDs), are now on the market for mass production. The research focus is now on high-voltage (>10 kV) bipolar devices, such as, bipolar junction transistors (BJTs), p‑i‑n diodes and insulated-gate bipolar transistors (IGBTs).The focus of this thesis are material improvements relevant for the development of 4H-SiC high-voltage bipolar devices. A key parameter for such devices is the minority carrier lifetime, where long carrier lifetimes reduce the on-resistance through conductivity modulation. However, too long carrier lifetimes give long reverse recovery times leading to large switching losses. Thus, a tailored carrier lifetime is needed for the specific application. Carrier lifetimes of the epilayers can both be controlled by the CVD growth conditions and by post-growth processing, such as thermal oxidation and carbon implantation followed by thermal annealing. Emphasis in this thesis (Paper 1‑2) is to find optimal CVD growth conditions (growth temperature, C/Si ratio, growth rate, doping) improving the carrier lifetime. Since the main lifetime limiting defect has shown to be the Z1/2 center, identified as isolated carbon vacancies, growth conditions minimizing the Z1/2 concentration are strived for.To achieve high-voltage bipolar devices, thick epilayers of high quality is needed. An important factor is then the growth rate that needs to be relatively high in order to reduce the fabrication time, and thus the cost of the final device. In this thesis the growth process has been optimized for high growth rates (30 µm/h) using standard silane and propane chemistry (Paper 3), compared to other chemistries that includes chlorine, which results in corroded reactor parts and new defects in the epitaxial layers.Another important parameter for 4H-SiC bipolar devices is the basal plane dislocations (BPDs) in the substrate and epilayers, since the BPDs can act as source of nucleation and expansion of Shockley stacking faults (SSFs). The expanded SSFs give a lowered carrier lifetime and form a potential barrier for carrier transport leading to an increased forward voltage drop which in turn leads to bipolar degradation. The bipolar degradation is detrimental for 4H-SiC bipolar devices. Several strategies are developed to reduce the density of BPDs including buffer layers, growth interrupts and decreasing the substrates off-cut angle. Paper 4‑6 is focused on developing a CVD growth process for low substrate off-cut angles (1° and 2°) compared to the today’s standard off-cut angle of 4°. By reducing the substrate off-cut angle the number of BPDs intersecting the substrate surface is reduced. In addition, the conversion from BPDs to threading edge dislocations (TEDs) during epitaxial growth is increased with lower off-cut angles.
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| 2. |
- Lilja, Louise, 1985-, et al.
(författare)
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Influence of n-Type Doping Levels on Carrier Lifetime in 4H-SiC Epitaxial Layers
- 2017
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Ingår i: Silicon Carbide and Related Materials 2016. - : Trans Tech Publications Ltd. ; , s. 238-241
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Konferensbidrag (refereegranskat)abstract
- In this study we have grown thick 4H-SiC epitaxial layers with different n-type doping levels in the range 1E15 cm-3 to mid 1E18 cm-3, in order to investigate the influence on carrier lifetime. The epilayers were grown with identical growth conditions except the doping level on comparable substrates, in order to minimize the influence of other parameters than the n-type doping level. We have found a drastic decrease in carrier lifetime with increasing n-type doping level. Epilayers were further characterized with low temperature photoluminescence and deep level transient spectroscopy.
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| 3. |
- Ayedh, H. M., et al.
(författare)
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Controlling the carbon vacancy in 4H-SiC by thermal processing
- 2018
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Ingår i: ECS Transactions. - : Electrochemical Society Inc.. - 1938-6737 .- 1938-5862. ; , s. 91-97
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Konferensbidrag (refereegranskat)abstract
- The carbon vacancy (Vc) is perhaps the most prominent point defect in silicon carbide (SiC) and it is an efficient charge carrier lifetime killer in high-purity epitaxial layers of 4H-SÌC. The Vc concentration needs to be controlled and minimized for optimum materials and device performance, and an approach based on post-growth thermal processing under C-rich ambient conditions is presented. It utilizes thermodynamic equilibration and after heat treatment at 1500 °C for 1 h, the Vc concentration is shown to be reduced by a factor-25 relative to that in as-grown state-of-the-art epi-layers. Concurrently, a considerable enhancement of the carrier lifetime occurs throughout the whole of >40 urn thick epi-layers.
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| 4. |
- Ghezellou, Misagh, 1988-, et al.
(författare)
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The role of boron related defects in limiting charge carrier lifetime in 4H–SiC epitaxial layers
- 2023
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Ingår i: APL Materials. - : American Institute of Physics (AIP). - 2166-532X. ; 11:3
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Tidskriftsartikel (refereegranskat)abstract
- One of the main challenges in realizing 4H–SiC (silicon carbide)-based bipolar devices is the improvement of minority carrier lifetime in as-grown epitaxial layers. Although Z1/2 has been identified as the dominant carrier lifetime limiting defect, we report on B-related centers being another dominant source of recombination and acting as lifetime limiting defects in 4H–SiC epitaxial layers. Combining time-resolved photoluminescence (TRPL) measurement in near band edge emission and 530 nm, deep level transient spectroscopy, and minority carrier transient spectroscopy (MCTS), it was found that B related deep levels in the lower half of the bandgap are responsible for killing the minority carriers in n-type, 4H–SiC epitaxial layers when the concentration of Z1/2 is already low. The impact of these centers on the charge carrier dynamics is investigated by correlating the MCTS results with temperature-dependent TRPL decay measurements. It is shown that the influence of shallow B acceptors on the minority carrier lifetime becomes neutralized at temperatures above ∼422 K. Instead, the deep B related acceptor level, known as the D-center, remains active until temperatures above ∼570 K. Moreover, a correlation between the deep level concentrations, minority carrier lifetimes, and growth parameters indicates that intentional nitrogen doping hinders the formation of deep B acceptor levels. Furthermore, tuning growth parameters, including growth temperature and C/Si ratio, is shown to be crucial for improving the minority carrier lifetime in as-grown 4H–SiC epitaxial layers.
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