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Epitaxial Growth and Characterization of SiC for High Power Devices
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Silicon Carbide (SiC) is a semiconductor with a set of superior properties, including wide bandgap, high thermal conductivity, high critical electric field and high electron mobility. This makes it an excellent material for unipolar and bipolar electronic device applications that can operate under high temperature and high power conditions. Despite major advancements in SiC bulk growth technology, during last decade, the crystalline quality of bulk grown material is still not good enough to be used as the active device structure. Also, doping of the material through high temperature diffusion is not possible while ion implantation leads to severe damage to the crystalline quality of the material. Therefore, to exploit the superior quality of the material, epitaxial growth is a preferred technology for the active layers in SiC-based devices. Horizontal Hot-wall chemical vapor deposition is probably the best way to produce high quality epitaxial layers where complete device structure with different doping type or concentrations can be grown during a single growth run.

SiC exists in many different polytypes and to maintain the polytype stability during epitaxial growth, off-cut substrates are required to utilize step-flow growth. The major disadvantage of growth on off-cut substrates is the replication of basal plane dislocations from the substrate into the epilayer. These are known to be the main source of degradation of bipolar devices during forward current injection. The bipolar degradation is caused by expanding stacking faults which increases the resistance and leads to fatal damage to the device. Structural defects replicated from the substrate are also important for the formation of defects in the epitaxial layer. In this thesis we have developed an epitaxial growth process to reduce the basal plane dislocations and the bipolar degradation. We have further studied the properties of the epitaxial layer with a focus on morphological defects and structural defects in the epitaxial layer.

The approach to avoid basal plane dislocation penetration from the substrate is to grow on nominally on-axis substrate. The main obstacle with on-axis growth is to avoid the formation of parasitic 3C polytype inclusions. The first results (Paper 1) on epitaxial growth on nominally on-axis Si-face substrates showed that the 3C inclusions nucleated at the beginning of the growth and expand laterally without following any particular crystallographic direction. Also, the extended defects in the substrate like micropipes, clusters of threading screw and edge dislocations do not give rise to 3C inclusion. The substrate surface damage was instead found to be the main source. To improve the starting surface different in-situ etching conditions were studied (Paper 2) and Si-rich conditions were found to effectively remove the substrate surface damages with lowest roughness and more importantly uniform distribution of steps on the surface. Therefore, in-situ etching under Si-rich conditions was performed before epitaxial growth. Using this 100 % 4H polytype was obtained in the epilayer on full 2” wafer (Paper 3) using an improved set of growth parameters with Si-rich conditions at the beginning of the growth. Simple PiN diodes were processed on the on-axis material, and tested for bipolar degradation. More than 70 % of these (Paper 4) showed a stable forward voltage drop during constant high current injection.

High voltage power devices require thick epitaxial layers with low doping. In addition, the high current needs large area devices with a reduced number of defects. Growth and properties of thick epilayers have been studied in details (Paper 5) and the process parameters in Horizontal Hotwall chemical vapor deposition reactor are found to be stable during the growth of over 100 µm thick epilayers.

An extensive study of epitaxial defect known as the carrot defect has been conducted to investigate the structure of the defect and its probable relation to the extended defects in the substrate (Paper 6). Other epitaxial defects observed and studied were different in-grown stacking faults which frequently occur in as-grown epilayers (Paper 7) and also play an important role in the device performance. Minority carrier lifetime is an important property especially for high power bipolar devices. The influence of structural defects on minority carrier lifetime has been studied (Paper 8) in several epilayers, using a unique high resolution photoluminescence decay mapping. The technique has shown the influence on carrier lifetime from different structural defects, and also revealed the presence of non-visible structural defects such as dislocations and stacking faults, normally not observed with standard techniques.

Abstract [sv]

Kiselkarbid (SiC) är en halvledare med överlägsna materialegenskaper, stort bandgap, hög termisk konduktivitet, hög kritisk fältstyrka och hög elektron mobilitet. Dessa gör den till ett utmärkt material för unipolära och bipolära komponenter som kan användas vid höga temperaturer, höga spänningar och höga strömmar. Trots stora framsteg under de senaste åren inom SiC bulk tillväxt, är material kvalitén hos bulk material fortfarande inte tillräckligt bra för att användas för aktiva skikt i komponenterna. Dessutom är dopning av materialet genom diffusion vid höga temperaturer inte möjligt, medan dopning via jonimplantation ger upphov till stora skador i kristallstrukturen. Därför behövs epitaxiell tillväxt av de aktive skikten i SiC baserade komponenter, för att fullt kunna utnyttja materialets egenskaper. Horisontell CVD (Hot-Wall Chemical Vapor Deposition) är en av de bästa tekniker att producera epitaxiella skikt med hög kvalité, där kompletta komponent strukturer med olika dopnings typ och koncentrationer kan växas i samma körning.

SiC existerar i många polytyper och för att bibehålla polytype stabiliteten under tillväxt, används substrat med lutande kristallplan för använda s.k. step-flow tillväxt. En stor nackdel med substrat med lutande kristallplan är dock att dislokationer i basalplanet kommer att propagera från substratet in i det epitaxiella skiktet under tillväxten. Dessa dislokationer är den huvudsakliga orsaken till den degradering av bipolära komponenter som uppstår då höga strömmar går igenom komponenten. Den bipolära degraderingen orsakas av expanderade staplingsfel, som successivt ökar resistansen och slutligen förstörs komponenten. Strukturella defekter som replikeras från substratet är ofta även orsaken till kritiska defekter som skapas i det epitaxiella skiktet under tillväxt. I den här avhandlingen har vi utvecklat en epitaxiell som minskar problemet med basalplans dislokationer och bipolär degradering. Vi har även studerat egenskaper hos de epitaxiella skikten med fokus på morfologiska och strukturella defekter.

Tekniken att hindra dislokationerna att replikeras in i de epitaxiella skikten bygger på att använda substrat utan lutning hos kristallplanen, s.k. on-axis substrat. Det hittills stora problemet med att växa på on-axis substrat har varit svårigheterna att bibehålla polytyp stabiliteten och undvika framförallt 3C polytyp inklusioner. Första försöken (Papper 1) försöken att växa epitaxi på on-axis substrat på Si sidan visade att 3C inklusionerna alltid startade i början av tillväxten för att sedan sprida sig lateralt under den fortsatta tillväxten. Vi kunde också visa att strukturella defekter som mikropipor, eller kluster av skruv- eller kant- dislokationer inte orsakade 3C inklusionerna. Den dominerande orsaken till 3C inklusionerna var istället skador eller repor på substratets yta. För att förbättra ytan innan den epitaxiella tillväxten studerade vi olika in-situ etsningar av ytan (Papper 2), och vi fann att etsning under Si dominerande förhållanden effektivast tog bort de flesta skador på substratets yta och gav en yta med minst ojämnheter. Dessutom skapades en homogen fördelning av atomära steg på ytan, och denna förbehandling användes sedan inför den epitaxiella tillväxten. Genom att dessutom optimera tillväxt förhållandena i inledningen av tillväxten kunde vi till 100% bibehålla samma polytyp från substratet in i det epitaxiella skiktet för hela 2” substrat (Papper 3). Enkla bipolära PiN dioder tillverkades och testades med avseende på bipolär degradering och mer än 70% av dioderna (Papper 4) visade ett stabilt framspänningsfall vid höga strömtätheter.

Kraftkomponenter för höga spänningar kräver tjocka epitaxiella skikt med låg dopning. Dessutom, för höga strömmar krävs komponenter med stor aktiv area där kravet på lägre defekt täthet blir allt viktigare. Vi har i detalj studerat tillväxt och egenskaper av tjocka skikt (Papper 5), och funnit att de flesta material egenskaperna är stabila vid tillväxt av över 100 mm tjocka skikt i vår horisontella CVD reaktor. Vi har även i detalj studerat uppkomst och egenskaper av en av de mest kritiska epitaxiella defekterna, dem s.k. moroten (Papper 6). Speciellt har vi studerat dess uppkomst i relation till strukturella defekter i substratet. Vi har även studerat ända epitaxiella defekter i form av olika typer av staplingsfel (Papper 7), som även dessa har stor inverkan på komponenter. Livstiden för minoritetsladdningsbärarna är en viktig egenskap hos speciellt bipolära komponenter. I (Papper 8) har vi studerat hur denna påverkas av strukturella defekter i de epitaxiella skikten. Vi har använt en unik mätmetod för att optiskt kunna mäta över hela skivor, med hög upplösning. Mätningarna har lyckats påvisa hur olika strukturella defekter påverkar livstiden, och även kunnat visa på förekomsten av defekter som inte har upptäckts med andra mätmetoder.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2009. , 75 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1243
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-17440ISBN: 978-91-7393-686-6 (print)OAI: oai:DiVA.org:liu-17440DiVA: diva2:209404
Public defence
2009-03-26, Plank, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2009-03-24 Created: 2009-03-24 Last updated: 2012-01-31Bibliographically approved
List of papers
1. 4H-SiC Epitaxial Layers Grown on on-axis Si-face Substrate
Open this publication in new window or tab >>4H-SiC Epitaxial Layers Grown on on-axis Si-face Substrate
Show others...
2007 (English)In: Materials Science Forum, Vols. 556-557, Trans Tech Publications , 2007, Vol. 556-557, 53-56 p.Conference paper, Published paper (Refereed)
Abstract [en]

We report on the growth of 4H-SiC epitaxial layer on Si-face polished nominally on-axis 2” full wafer, using Hot-Wall CVD epitaxy. The polytype stability has been maintained over the larger part of the wafer, but 3C inclusions have not been possible to avoid. Special attention has given to the mechanism of generation and propagation of 3C polytype in 4H-SiC epilayer. Different optical and structural techniques were used to characterize the material and to understand the growth mechanisms. It was found that all 3C inclusions were generated at the interface between the substrate and the epitaxial layer, and no 3C inclusions were initiated at later stages of the growth.

Place, publisher, year, edition, pages
Trans Tech Publications, 2007
Keyword
Chemical vapor deposition, Hot-Wall CVD reactor, On-axis, X-ray topography, AFM, SWBXT
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17434 (URN)10.4028/www.scientific.net/MSF.556-557 (DOI)
Conference
ECSCRM2006
Available from: 2009-03-24 Created: 2009-03-24 Last updated: 2015-03-11Bibliographically approved
2. In-situ surface preparation of nominally on-axis 4H-SiC substrates
Open this publication in new window or tab >>In-situ surface preparation of nominally on-axis 4H-SiC substrates
2008 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 310, no 20, 4430-4437 p.Article in journal (Refereed) Published
Abstract [en]

A study of the in-situ surface preparation has been performed for both Si- and C-face 4H-SiC nominally on-axis samples. The surface was studied after etching under C-rich, Si-rich and under pure hydrogen ambient conditions at the same temperature, pressure and time interval using a hot-wall chemical vapor deposition reactor. The surfaces of all the samples were analyzed using optical microscopy with Normarski diffractional interference contrast and atomic force microscopy with tapping mode before and after in-situ etching. Polishing related damages were found to be removed under all etching conditions, also the surface step structure was uncovered and a few defect-selective etch pits were observed. For the Si-face sample, the best surface morphology was obtained after Si-rich etching conditions with more uniform and small macro-step height which resulted in the lowest surface roughness. For the C-face sample the surface morphology was comparable under all etching conditions and not much difference was found except for the etching rate. Si droplets were not observed under any etching conditions.

Keyword
Atomic force microscopy, Etching, Chemical vapor deposition processes
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-15195 (URN)10.1016/j.jcrysgro.2008.06.083 (DOI)
Note
Original publication: J. Hassan, J.P. Bergman, A. Henry and E. Janzén, In-situ surface preparation of nominally on-axis 4H-SiC substrates, 2008, Journal of Crystal Growth, (310), 20, 4430-4437.http://dx.doi.org/10.1016/j.jcrysgro.2008.06.083. Copyright: Elsevier B.V., http://www.elsevier.com/Available from: 2008-10-23 Created: 2008-10-23 Last updated: 2017-12-14Bibliographically approved
3. On-axis homoepitaxial growth on Si-face 4H–SiC substrates
Open this publication in new window or tab >>On-axis homoepitaxial growth on Si-face 4H–SiC substrates
2008 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 310, no 20, 4424-4429 p.Article in journal (Refereed) Published
Abstract [en]

Homoepitaxial growth has been performed on Si-face nominally on-axis 4H–SiC substrates using horizontal Hot-wall chemical vapor deposition system. Special attention was paid to the surface preparation before starting the growth. In-situ surface preparation, starting growth parameters and growth temperature are found to play a vital role to maintain the polytype stability in the epilayer. High quality epilayers with 100% 4H–SiC were obtained on full 2″ substrates. Different optical and structural techniques were used to characterize the material and to understand the growth mechanisms. It was found that the replication of the basal plane dislocation from the substrate into the epilayer can be completely eliminated. The on-axis grown epitaxial layers were of high quality and did not show surface morphological defects, typically seen in off-axis grown layers, but had a high surface roughness.

Keyword
Atomic force microscopy, Etching, Hot wall epitaxy, Semiconducting materials, Bipolar transistors
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-15197 (URN)10.1016/j.jcrysgro.2008.06.081 (DOI)
Note
Original publication: J. Hassan, J.P. Bergman, A. Henry and E. Janzén, On-axis homoepitaxial growth on Si-face 4H–SiC substrates, 2008, Journal of Crystal Growth, (310), 20, 4424-4429.http://dx.doi.org/10.1016/j.jcrysgro.2008.06.081. Copyright: Elsevier B.V., http://www.elsevier.com/Available from: 2008-10-23 Created: 2008-10-23 Last updated: 2017-12-14Bibliographically approved
4. Non Degrading PiN Diodes Grown On On-axis 4H-SiC Substrates
Open this publication in new window or tab >>Non Degrading PiN Diodes Grown On On-axis 4H-SiC Substrates
Show others...
(English)Manuscript (Other academic)
Abstract [en]

Bipolar PiN diodes have been fabricated using epitaxial layers grown on nominally onaxis Si-face 4H-SiC substrates. Top metal contacts were processed with windows to observe the electroluminescence and any formation of stacking faults during forward current injection. The diodes were tested for voltage stability using a forward current density of 120 A/cm2 for 30 min. More than 70 % of the diodes showed a stable behavior and change in the forward voltage was less than 0.1 V. No movement of basal plane dislocations or the formation of stacking faults was observed using electroluminescence. Most of the remaining diodes failed completely due to contact breakdown.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17435 (URN)
Available from: 2009-03-24 Created: 2009-03-24 Last updated: 2014-10-08Bibliographically approved
5. Properties of Thick n- and p-type Epitaxial Layers of 4H-SiC Grown by Hot-Wall CVD on off- and on-axis Substrates
Open this publication in new window or tab >>Properties of Thick n- and p-type Epitaxial Layers of 4H-SiC Grown by Hot-Wall CVD on off- and on-axis Substrates
2006 (English)In: Materials Science Forum, Vols. 527-529, 2006, Vol. 527-529, 183-186 p.Conference paper, Published paper (Refereed)
Abstract [en]

Thick epitaxial layers of 4H-SiC both n- and p-type were grown using horizontal Hot- Wall CVD (HWCVD). No large difference in the carrier lifetime was observed for the layers grown on n- and p-type substrates. The carrier lifetime usually increases with the increasing thickness of the epilayer. To investigate if the growth conditions and material properties are changing during the longer growth time a sample was prepared with uniformly varying epilayer thickness from 20μm on one side to 110μm on other side. Results of optical and electrical measurements, the variation in background impurities and other deep levels are discussed. Furthermore, the properties of thick layers grown on on-axis substrates are presented.

Keyword
Epitaxial growth, Hot-Wall CVD, Thick layers, Carrier Lifetime, Photoluminescence, DLTS
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17436 (URN)10.4028/www.scientific.net/MSF.527-529 (DOI)
Conference
ICSCRM2005
Available from: 2009-03-24 Created: 2009-03-24 Last updated: 2010-12-07Bibliographically approved
6. Characterization of the Carrot Defect in 4H-SiC Epitaxial Layers
Open this publication in new window or tab >>Characterization of the Carrot Defect in 4H-SiC Epitaxial Layers
2010 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 313, 1828-1837 p.Article in journal (Other academic) Published
Abstract [en]

Characterization of the epitaxial defect known as the carrot was performed in thick 4HSiC epilayers. A large number of the carrot defects have been studied using different experimental techniques such as optical microscopy, KOH etching, cathodoluminescence and synchrotron white beam x-ray topography. This has revealed that the carrot defects appear in many different shapes and structures in the layers. Our results support the previous assignment of the defect as related to a prismatic stacking fault. However, we have observed the carrot defects with and without a visible threading dislocation related etch pit in the head region, after KOH etching. The carrot defects have found to be originated both at epi-substrate interface and during the epitaxial growth. Also, different sources of the carrot defect have been observed which resulted in different structure of the defect inside the epilayer.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17437 (URN)10.1016/j.jcrysgro.2010.02.037 (DOI)
Available from: 2009-03-24 Created: 2009-03-24 Last updated: 2017-12-13Bibliographically approved
7. In-grown stacking faults in 4H-SiC epilayer grown on off-cut substrates
Open this publication in new window or tab >>In-grown stacking faults in 4H-SiC epilayer grown on off-cut substrates
2009 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 105, no 12, 123513- p.Article in journal (Other academic) Published
Abstract [en]

Different and novel in-grown stacking faults have been observed and characterized in 4H-SiC epitaxial layers grown on 4 or 8o off-cut substrates. Two different kinds of triangular stacking faults were observed in the epilayers grown on 4o off-cut substrates. The faults were formed during epitaxial growth close to the epi-substrate interface and increased continuously in size during growth. Their structural and optical properties were however different as seen from both synchrotron white beam topography and low temperature photoluminescence. The luminescence spectra were similar but appeared in different energy regions 2.85 – 2.95 eV and 2.48 – 2.64 eV, respectively which have not been observed for previously reported stacking faults. A third stacking fault was observed in 8o off-cut as-grown epilayers, sometime with higher density. A combination of back polishing, etching in molten KOH and optical microscopy revealed the geometrical structure of the stacking fault inside the epilayer. Also this fault started close to the epi-substrate interface, expanded rapidly but changed geometry after some time and reduced in size during further growth. The optical spectrum from this fault is identical to the emission from the stacking faults previously only observed and formed in the bipolar diodes during forward voltage operation.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17438 (URN)10.1063/1.3139268 (DOI)
Available from: 2009-03-24 Created: 2009-03-24 Last updated: 2017-12-13Bibliographically approved
8. Influence of Structural Defects on Minority Carrier Lifetime in 4H-SiC Epitaxial Layers: Optical Lifetime Mapping
Open this publication in new window or tab >>Influence of Structural Defects on Minority Carrier Lifetime in 4H-SiC Epitaxial Layers: Optical Lifetime Mapping
2009 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 105, no 12Article in journal (Refereed) Published
Abstract [en]

The influence of structural defects on the minority carrier lifetime in 4H-SiC epilayers has been studied using high spatial resolution optically detected lifetime measurements. Full wafers mappings with 200 µm spatial resolution revealed the carrier lifetime variations that can be associated with structural defects replicated from the substrate and variations in the epitaxial growth conditions due to the susceptor design. High resolution mappings over smaller regions with lateral step size down to 20 µm, revealed local carrier lifetime reductions associated with different structural defects in the epitaxial layers. Identified defects that influence the carrier lifetime are the carrot defects, different types of in-grown stacking faults. Also clusters of threading screw dislocations in the epilayer, probably originating from the dissociation of micropipe in the substrate, are found to effectively reduce the carrier lifetime. Furthermore, optically detected lifetime mapping has been demonstrated as a non-destructive technique which allows non-visible structural defects to be detected in as-grown epilayers.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2009
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17439 (URN)10.1063/1.3147903 (DOI)000267599600051 ()
Available from: 2009-03-24 Created: 2009-03-24 Last updated: 2017-12-13Bibliographically approved

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