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Growth and characterization of SiC and GaN
Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

At present, focus of the SiC crystal growth development is on improving the crystalline quality without polytype inclusions, micropipes and the occurrence of extended defects. The purity of the grown material, as well as intentional doping must be well controlled and the processes understood. High-quality substrates will significantly improve device performance and yield. One of the aims of the thesis is further understanding of polytype inclusion formation as well as impurity control in SiC bulk crystals grown using PVT method also termed seeded sublimation method. Carbonization of the source was identified as a major reason behind the polytype inclusion occurrence during the growth. The aim of this work was further understanding of sublimation growth process of 4H-SiC bulk crystals in vacuum, in absence of an inert gas. For comparison growth in argon atmosphere (at 5 mbar) was performed. The effect of the ambient on the impurity incorporation was studied for different growth temperatures. For better control of the process in vacuum, tantalum as a carbon getter was utilized.

The focus of the SiC part of the thesis was put on further understanding of the PVT epitaxy with an emphasis on the high growth rate and purity of grown layers.

High resistivity 4H-SiC samples grown by sublimation with high growth rate were studied. The measurements show resistivity values up to high 104 cm. By correlation between the growth conditions and SIMS results, a model was applied in which it is proposed that an isolated carbon vacancy donor-like level is a possible candidate responsible for compensation of the shallow acceptors in p-type 4H-SiC. A relation between cathodoluminescence (CL) and DLTS data is taken into account to support the model.

To meet the requirements for high voltage blocking devices such as high voltage Schottky diodes and MOSFETs, 4H-SiC epitaxial layers have to exhibit low doping concentration in order to block reverse voltages up to few keV and at the same time have a low on-state resistance (Ron). High Ron leads to enhanced power consumption in the operation mode of the devices. In growth of thick layers for high voltage blocking devices, the conditions to achieve good on-state characteristics become more challenging due to the low doping and pronounced thicknesses needed, preferably in short growth periods. In case of high-speed epitaxy such as the sublimation, the need to apply higher growth temperature to yield the high growth rate, results in an increased concentration of background impurities in the layers as well as an influence on the intrinsic defects.

On-state resistance Ron estimated from current density-voltage characteristics of Schottky diodes on thick sublimation layers exhibits variations from tens of mΩ.cm2 to tens of Ω.cm2 for different doping levels. In order to understand the occurrence of high on-state resistance, Schottky barrier heights were first estimated for both forward and reverse bias with the application of thermionic emission theory and were in agreement with literature reported values. Decrease in mobility with increasing temperature was observed and its dependencies of T–1.3 and T–2.0 for moderately doped and low doped samples, respectively, were estimated. From deep level measurements by Minority Carrier Transient Spectroscopy (MCTS), an influence of shallow boron related levels and D-center on the on-state resistance was observed, being more pronounced in low doped samples. Similar tendency was observed in depth profiling of Ron. This suggests a major role of boron in a compensation mechanism.

In the second part of the thesis growth and characterization of GaN is presented. Excellent electron transport properties with high electron saturate drift velocity make GaN an excellent candidate for electronic devices. Especially, AlGaN/GaN based high electron mobility transistors (HEMT) have received an increased attention in last years due to their attractive properties. The presence of strong spontaneous and piezoelectric polarization due to the lattice mismatch between AlGaN and GaN is responsible for high free electrons concentrations present in the vicinity of the interface. Due to the spatial separation of electrons and ionized donors or surface states, 2DEG electron gas formed near the interface of the heterostructure exhibits high sheet carrier density and high mobility of electrons. Al0.23Ga0.77N/GaN based HEMT structures with an AlN exclusion layer on 100 mm semiinsulating 4H-SiC substrates have been grown by hot-wall MOCVD. The electrical properties of the two-dimensional electron gas (2DEG) such as electron mobility, sheet carrier density and sheet resistance were obtained from Hall measurements, capacitance-voltage and contact-less eddy-current techniques. The effect of different scattering mechanisms on the mobility have been taken into account and compared to the experimental data. Hall measurements were performed in the range of 80 to 600 K. Hall electron mobility is equal to 17140 cm2(Vs)-1 at 80 K, 2310 cm2(Vs)-1 at room temperature, and as high as 800 cm2(Vs)-1 at 450 K, while the sheet carrier density is 1.04x1013 cm-2 at room temperature and does not vary very much with temperature. Estimation of different electron scattering mechanisms reveals that at temperatures higher than room temperature, experimental mobility data is mainly limited by optical phonon scattering. At relevant high power device temperature (450 K) there is still an increase of mobility due to the AlN exclusion layer.

We have studied the behaviour of Ga-face GaN epilayers after in-situ thermal treatment in different gas mixtures in a hot-wall MOCVD reactor. Influence of N2, N2+NH3 and N2+NH3+H2 ambient on the morphology was investigated in this work. The most stable thermal treatment conditions were obtained in the case of N2+NH3 gas ambients. We have also studied the effect of the increased molar ratio of hydrogen in order to establish proper etching conditions for hot-wall MOCVD growth.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2007. , 50 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1151
Keyword [en]
SiC, GaN, Deep level transient spectroscopy, Minority Carrier Transient Spectroscopy, Hall effect, Cathodoluminescence, Scanning electron microscopy, Atomic Force microscopy, sublimation growth, MOCVD, heterostructures, High Electron Mobility transistor, point defects
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
URN: urn:nbn:se:liu:diva-10314ISBN: 978-91-85895-26-7 (print)OAI: oai:DiVA.org:liu-10314DiVA: diva2:17053
Public defence
2007-12-18, Planck, Fysikhuset, Campus Valla, Linköpings Universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2007-12-04 Created: 2007-12-04 Last updated: 2009-04-24Bibliographically approved
List of papers
1. Effect of Ambient on 4H-SiC Bulk Crystals grown by Sublimation
Open this publication in new window or tab >>Effect of Ambient on 4H-SiC Bulk Crystals grown by Sublimation
2003 (English)In: Materials Science Forum, Vols. 433-436 / [ed] Peder Bergman and Erik Janzén, 2003, Vol. 433-436, 75-78 p.Conference paper, Published paper (Refereed)
Abstract [en]

Sublimation bulk growth in vacuum using graphite crucibles and such with tantalum shielding of the crucible walls has been studied. Residual nitrogen, aluminum and boron doping in the material grown in vacuum is presented. Activation energies of growth rate in respect to growth temperature in vacuum are deduced. The estimated values are 21 kcallmole for growth temperatures below 2075°C and 128 kcal/mole in the range of growth temperatures between 2075°C and 2275°C. Cathodoluminescence spectra taken from samples grown in the graphite crucible in absence of tantalum under different pressures show nitrogen-alurninum DAP transition and strong luminescence from deep boron. This is not the case for samples grown in the tantalum environment.

Keyword
Bulk Growth, Doping, Boron, Tantalum, Cathodoluminescence
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12782 (URN)10.4028/www.scientific.net/MSF.433-436.75 (DOI)
Conference
ECSCRM 2002. Linköping, Sweden, September 1-5, 2002
Available from: 2007-12-04 Created: 2007-12-04 Last updated: 2013-11-07
2. Structural instabilities in growth of SiC crystals
Open this publication in new window or tab >>Structural instabilities in growth of SiC crystals
2005 (English)In: Journal of Crystal Growth, ISSN 0022-0248, Vol. 275, no 1-2, e461-e466 p.Article in journal (Refereed) Published
Abstract [en]

Misoriented grains, which may occur on the growth front of 6H–SiC boules have been studied in relation to their appearance during sublimation growth. The effect was obtained by applying growth conditions at which the source powder was gradually approaching graphitisation and the vapour becoming C-rich. The high off-orientation of the grains is demonstrated through etching in molten KOH and transmission light optical microscopy. Micropipes propagating in the single crystal area and facing the misoriented grain have been studied, and it is shown that they may either be terminated at the grain or their propagation is altered to be parallel with the grain boundary. It has been found that the polytype of the grains may switch from 6H to 4H, which is explained by the change of the Si/C ratio in the vapour.

Keyword
Crystal morphology; Volume defects; Growth from vapor; Single crystal growth; Semiconductor silicon compounds
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-12783 (URN)10.1016/j.jcrysgro.2004.11.020 (DOI)
Available from: 2007-12-04 Created: 2007-12-04 Last updated: 2013-11-07
3. Effect of boron on the resistivity of compensated 4H-SiC
Open this publication in new window or tab >>Effect of boron on the resistivity of compensated 4H-SiC
2003 (English)In: Journal of electronic materials, ISSN 0361-5235, Vol. 32, no 5, 452-457 p.Article in journal (Refereed) Published
Abstract [en]

High-resistivity 4H-SiC samples grown by sublimation with a high growth rate are studied. The measurements show resistivity values up to a high of 104 Ωcm. The secondary ion mass spectroscopy (SIMS) results revealed a presence of only common trace impurities such as nitrogen, aluminum, and boron. To understand the compensation mechanism in these samples, capacitance deep-level transient spectroscopy (DLTS) on the p-type epilayers has been performed. By correlation between the growth conditions and SIMS results, we apply a model in which it is proposed that an isolated carbon vacancy donorlike level is a possible candidate responsible for compensation of the shallow acceptors in p-type 4H-SiC. A relation between cathodoluminescence (CL) and DLTS data is taken into account to support the model.

Keyword
SiC sublimation epitaxy, DLTS, compensation, deep levels, carbon vacancy, high resistivity, semi-insulating
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12784 (URN)10.1007/s11664-003-0177-0 (DOI)
Available from: 2007-12-04 Created: 2007-12-04 Last updated: 2013-11-07
4. Evaluation of On-state Resistance and Boron-related Levels in n-type 4H-SiC
Open this publication in new window or tab >>Evaluation of On-state Resistance and Boron-related Levels in n-type 4H-SiC
2005 (English)In: Materials Science Forum, Vols. 483-485, 2005, Vol. 483-485, 425-428 p.Conference paper, Published paper (Refereed)
Abstract [en]

Specific on-resistance Ron estimated from current density-voltage characteristics of Schottky diodes on thick layers exhibits variations from tens of mΩ.cm2 to tens of Ω.cm2 for different doping levels. In order to understand the occurrence of high on-state resistance, Schottky barrier heights were first estimated for both forward and reverse bias with the application of thermionic emission theory and were in agreement with a literature reported values. Decrease in mobility with the temperature was observed and its dependencies of T–1.3 and T–2.0 for moderately doped and low doped samples respectively were estimated. From deep level measurements by Minority Carrier Transient Spectroscopy, an influence of shallow boron related levels and D-center on dependence of on-state resistance was observed, being more pronounced in low doped samples. Similar tendency was observed in depth profiling of Ron. This suggests a major role of boron in a compensation mechanism thus resulting in high Ron.

Keyword
On-state resistance, I-V, C-V, MCTS, Sublimation, High-speed epitaxy, Deep
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12785 (URN)
Conference
ECSCRM2004
Available from: 2007-12-04 Created: 2007-12-04 Last updated: 2013-11-07
5. Electrical Analysis and Interface States Evaluation of of Ni Schottky diodes on 4H-SiC thick epilayers
Open this publication in new window or tab >>Electrical Analysis and Interface States Evaluation of of Ni Schottky diodes on 4H-SiC thick epilayers
2005 (English)In: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319, Vol. 202, no 13, 2508-2514 p.Article in journal (Refereed) Published
Abstract [en]

This work has been focused on characterization of thick 4H-SiC layers produced by sublimation epitaxy. Nickel Schottky contacts have been fabricated in order to characterize the grown material and evaluate the interfacial layer between metal and semiconductor. The characterization study includes current-voltage and capacitance-voltage high temperature measurements, from which Schottky barrier, net donor concentration and on-state resistance values have been extracted. The diodes show a typical behavior of J-V and C-V curves with temperature, with Schottky barrier heights of 1.3 eV ÷ 1.4 eV and net donor concentration of 4 × 1015 cm-3 ÷ 1 × 1016 cm-3. From the Bardeen's model on reverse J-V, the density of states of the interfacial layer has been estimated to 7 × 1011 eV-1 cm-2 ÷ 8 × 1011 eV-1 cm-2, a value that is similar to the density of states of oxide layers in deliberated MOS structures realized on the same epilayers.

Place, publisher, year, edition, pages
John Wiley & Sons, 2005
Keyword
73.20.Hb, 73.30.+y, 73.40.Ns, 73.61.Le
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-12786 (URN)10.1002/pssa.200521147 (DOI)
Available from: 2007-12-04 Created: 2007-12-04 Last updated: 2017-05-11Bibliographically approved
6. High 2DEG mobility of HEMT structures grown on 100 mm SI 4H-SiC substrates by hot-wall MOCVD
Open this publication in new window or tab >>High 2DEG mobility of HEMT structures grown on 100 mm SI 4H-SiC substrates by hot-wall MOCVD
Show others...
2007 (English)In: Journal of Applied PhysicsArticle in journal (Refereed) Submitted
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12787 (URN)
Available from: 2007-12-04 Created: 2007-12-04 Last updated: 2015-03-11
7. In-situ treatment of GaN epilayers in hot-wall MOCVD
Open this publication in new window or tab >>In-situ treatment of GaN epilayers in hot-wall MOCVD
Show others...
(English)Manuscript (Other academic)
Identifiers
urn:nbn:se:liu:diva-12788 (URN)
Available from: 2007-12-04 Created: 2007-12-04 Last updated: 2015-03-11

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