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Evaluation of On-state Resistance and Boron-related Levels in n-type 4H-SiC
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
Polytechnic of Turin, Physics Department, Torino, Italy.
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
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.

Place, publisher, year, edition, pages
2005. Vol. 483-485, 425-428 p.
Keyword [en]
On-state resistance, I-V, C-V, MCTS, Sublimation, High-speed epitaxy, Deep
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-12785OAI: oai:DiVA.org:liu-12785DiVA: diva2:17049
Conference
ECSCRM2004
Available from: 2007-12-04 Created: 2007-12-04 Last updated: 2013-11-07
In thesis
1. Growth and characterization of SiC and GaN
Open this publication in new window or tab >>Growth and characterization of SiC and GaN
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
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:nbn:se:liu:diva-10314 (URN)978-91-85895-26-7 (ISBN)
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
2. Device characteristics of sublimation grown 4H-SiC layers
Open this publication in new window or tab >>Device characteristics of sublimation grown 4H-SiC layers
2005 (English)Licentiate 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 licentiate 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. We have identified a carbonization of the source 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 major focus of the thesis was put on further understanding of the PVT epitaxy with an emphasis on the high growth rate and the 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 Ωcm. By correlation between the growth conditions and SIMS results, we apply a model 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 DL TS 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 a 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, 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.

Metal-Oxide-Semiconductor (MOS) capacitors were fabricated and characterized on 4H-SiC epilayers grown by PVT (sublimation) epitaxy and compared to the properties of similar structures on CVD grown layers. Detailed investigations of the PVT structures indicate a stable behaviour of the interface traps from roomtemperature up to 475 K. The amount of positive oxide charges Qo is 6.83x109 cm-2 at room temperature and decreases with temperature increase which suggests temperature stability of processed devices. The density of interface states Dit obtained by AC conductance method is lower in the case of PVT sample.

AI203 as an alternative gate dielectric was studied. The implementation of this high-k dielectric is required in the case of high electric fields at which the usage of SiO2 may result in the reliability problems. The electrical properties of AI2O3 as a gate dielectric in MOS structures based on n- and p-type 4H-SiC grown by sublimation method were investigated and compared to the properties of similar structures utilizing SiO2.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2005. 50 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1150
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-31210 (URN)16958 (Local ID)91-85297-50-X (ISBN)16958 (Archive number)16958 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-07

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Ciechonski, RafalSyväjärvi, MikaelYakimova, Rositsa

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