Nanoscale characterization of electrical transport at metal/3C-SiC interfaces

In this work, the transport properties of metal/3C-SiC interfaces were monitored employing a nanoscale characterization approach in combination with conventional electrical measurements. In particular, using conductive atomic force microscopy allowed demonstrating that the stacking fault is the most pervasive, electrically active extended defect at 3C-SiC(111) surfaces, and it can be electrically passivated by an ultraviolet irradiation treatment. For the Au/3C-SiC Schottky interface, a contact area dependence of the Schottky barrier height (ΦB) was found even after this passivation, indicating that there are still some electrically active defects at the interface. Improved electrical properties were observed in the case of the Pt/3C-SiC system. In this case, annealing at 500°C resulted in a reduction of the leakage current and an increase of the Schottky barrier height (from 0.77 to 1.12 eV). A structural analysis of the reaction zone carried out by transmission electron microscopy [TEM] and X-ray diffraction showed that the improved electrical properties can be attributed to a consumption of the surface layer of SiC due to silicide (Pt2Si) formation. The degradation of Schottky characteristics at higher temperatures (up to 900°C) could be ascribed to the out-diffusion and aggregation of carbon into clusters, observed by TEM analysis

Bauelement-relevante Defektzentren und Minoritätsträgerlebensdauer in 3C-, 4H- und 6H-SiC

1. Erbium in SiC Two Er-related centers termed Er(p)1/Er(p)2 are observed in the DLTS spectra of p-type 4H- and 6H-SiC. The following values of thermal ionization energy were found out for dominating Er-related defect Er(p)1: Eth(Er(p)1) = 0,58eV in 4H-SiC, Eth(Er(p)1) = 0,62eV in 6H-SiC. These centers are donor-like. Hole capturing correspond to multiphonon capture mechanism. Capture barrier for holes Ec(Er(p)1) = 0.1eV for both 4H- and 6H-SiC. The observed Er-related centers can be presumably identified as ErSi or ErSi-NC complex. The energy transfer process to the Er3+ 4f-shell electron likely occurs via the detected Er-related levels. The recombination energy corresponds to the energy difference between the N-donor level and the Er-related levels, which is 2.5eV and 2.3eV in 4H- and 6H-SiC, respectively. 2. Defects in Al-doped SiC A series of shallow defect centers in Al-doped 4H-, 6H- and 3C-SiC epilayers, which can be generated by processing steps like implantation, annealing or oxidation, was observed. These defects possess the following values of activation energy: 3C-SiC: Ea(RE) = 175meV, 6H-SiC: Ea(RE1) = 165meV, Ea(RE2) = 250meV, 4H-SiC: Ea(RE3) = 250meV. REi-centers (i = 1 to 3) are assumed to be of the same origin and to consist of one Al atom and, in addition, of one or several intrinsic defects. RE1 and RE2 in 6H-SiC are assumed to be metastable states belonging to the same defect complex. REi-centers are thermally stable up to 1700°C; their microscopic structure is not identified yet. 3. Sulfur in SiC Sulfur acts as a double donor in 3C-, 6H- and 4H-SiC. It likely occupies a substitutional position in the host SiC lattice. The following values of thermal ionization energy were found out for S-donors by taking into account the Poole-Frenkel correction: S0/+ 160meV(3C-SiC) 310meV, 375meV, 395meV(6H-SiC) 350meV, 520meV(4H-SiC) S+/++ 330meV(3C-SiC) 485meV, 615meV, 635meV(6H-SiC) 560meV, 570meV(4H-SiC) Electron capturing by S-donors correspond to cascade capture mechanism. Post-implantation annealing at 1700°C for 30min results in the degree of electrical activation close to 1 for [S]=(1x1015-1x1017)cm-3. 4. Midgap defects in 6H-SiC The electrical DLOS technique is demonstrated to be effective for the investigations of midgap defects in 6H-SiC. The modified Chantre et al. model for the optical capture cross-section is applied to extract the optical ionization energies and the Frank-Condon shift from the optical absorption spectra. The semi-classical approximation of the Franck-Condon factor is required for an appropriate analysis of the thermally broadened optical cross section in SiC. The effective phonon energy of 6H-SiC is found to be 60meV. A comparison of the defect parameters obtained by DLOS and DLTS results in the identification of observed defects. The following dominating midgap defects were detected by DLOS in quenched n-type 6H-SiC: Z1/Z2(6H): Eo=1,1eV, dFC=0,4eV, Eth=0,7eV, R-center: Eo=1,64eV, dFC=0,31eV, Eth=1,33eV, M1: Eo=2,2eV, dFC=0,3eV, Eth=1,9eV. The configuration coordinate diagram for the R-center is obtained. The defect M1 located at Ec -1.9eV is supposed to be largely responsible for the limitation of the minority carrier lifetime in n-type 6H-SiC. From the analysis of the photoconductivity spectra taken on V/Al-codoped SI 6H-SiC, the thermal ionization energy of vanadium donor levels is found to be 6H-SiC: Eth(V4+/V5+(k1,k2))=1,52eV, Eth(V4+/V5+(h))=1,67eV. 5. Minority carrier lifetime measurements The comparison of the hole lifetime determined by device relevant electrical methods (CRT, OCVD) and TRPL has shown that it strongly depends on the particular analysis method. These discrepancies are based on physical reasons and are not due to deficiencies of the measurement. Regarding the electrical methods, the value of the hole lifetime depends on the device region, in which the recombination of minority carriers occurs. The CRT method determines the hole lifetime at the edge of the space charge region, where implantation-induced defect centers reduce the lifetime (to 10-8s), while the OCVD method is dominated by the recombination in the high-quality n-base epilayer. This method results in the highest lifetime values (10-6s). The optical TRPL technique strongly depends on the experimental conditions. The observed time constants are affected by surface recombination and are due to emitted photons originating from different recombination processes. The determined optical hole lifetimes are settled between the values obtained from the CRT and OCVD method. The lifetime, therefore, more likely describes the property of an electron or hole under particular environmental conditions than a property of the semiconductor itself.1. Erbium in SiC Zwei Er-korrelierte Defektzentren Er(p)1/Er(p)2 wurden in DLTS Spektren von p-Typ 4H- bzw. 6H-SiC beobachtet. Die folgenden thermischen Ionisationsenergien wurden für den dominierenden Er-korrelierten Defekt Er(p)1 bestimmt: Eth(Er(p)1) = 0,58eV in 4H-SiC, Eth(Er(p)1) = 0,62eV in 6H-SiC. Diese Defektzentren sind donatorartig. Für den Löchereinfang ist der Multiphonon-Einfangmechanismus verantwortlich. Die Einfangbarriere beträgt für Löcher Ec(Er(p)1) = 0.1eV in 4H- und 6H-SiC. Für Er-korrelierte Defektzentren werden ErSi oder ein (ErSi-NC)-Komplex vorgeschlagen. Der Energietransport zum Er3+ 4f-Elektron erfolgt höchstwahrscheinlich durch die Er-korrelierte Defektzentren. Die Rekombinationsenergie entspricht der Energiedifferenz zwischen dem N-Donatorniveau und den Er-korrelierten Defekten, die 2,5eV bzw. 2,3eV für 4H-SiC bzw. 6H-SiC beträgt. 2. Defekte in Al-dotiertem SiC In Al-dotierten 4H-, 6H- und 3C-SiC Epitaxieschichten wurde eine Reihe von flachen Defektzentren beobachtet, die durch Prozessschritte wie Implantation, Ausheilung oder Oxidation generiert werden können. Diese Defektzentren haben folgende Aktivierungsenergie: 3C-SiC: Ea(RE) = 175meV, 6H-SiC: Ea(RE1) = 165meV, Ea(RE2) = 250meV, 4H-SiC: Ea(RE3) = 250meV. Die REi-Defektzentren (i = 1 bis 3) sind wahrscheinlich chemisch identisch und bestehen aus einem Al-Atom in Verbindung mit einem oder mehreren intrinsischen Defekten. RE1 und RE2 in 6H-SiC sind wahrscheinlich metastabile Zustände, die ein und demselben Defektkomplex angehören. REi-Defektzentren sind thermisch stabil bis zu 1700°C. 3. Schwefel in SiC Schwefel ist ein Doppeldonator in 3C-, 6H- and 4H-SiC. Er wird höchstwahrscheinlich substitutionell in das SiC Gitter eingebaut. Die folgenden thermischen Ionisierungsenergien wurden für S-Doppeldonatoren (einschließlich der Poole-Frenkel Korrektur) bestimmt: S0/+ 160meV(3C-SiC) 310meV, 375meV, 395meV(6H-SiC) 350meV, 520meV(4H-SiC) S+/++ 330meV(3C-SiC) 485meV, 615meV, 635meV(6H-SiC) 560meV, 570meV(4H-SiC) Die Einfang des Elektrons am S-Donator erfolgt durch den Kaskadeneinfang. Der elektrische Aktivierungsgrad von S-Doppeldonatoren liegt bis nahezu 100% nach einem Ausheilschritt bei 1700°C für 30min für eine implantierte S Konzentration von (1x1015-1x1017)cm-3. 4. Midgap-Defekte in 6H-SiC Die elektrische DLOS-Methode wurde als ein effektives Verfahren für die Untersuchung von Midgap-Defekten in 6H-SiC vorgestellt. Das modifizierte Modell von Chantre et al. für den optischen Einfangquerschnitt wurde für die Bestimmung der optischen Ionisationsenergie und der Franck-Condon-Energie aus den optischen Absorptionsspektren verwendet. Eine semi-klassische Annäherung für das Franck-Condon-Moment ist nötig um eine adäquate Analyse des thermisch verbreiterten optischen Querschnitts in SiC durchzuführen. Die effektive Phononenenergie von 6H-SiC wurde bestimmt zu 60meV. Folgende dominierende Midgap-Defekte wurden mit DLOS in quenched n-Typ 6H-SiC beobachtet: Z1/Z2(6H): Eo=1,1eV, dFC=0,4eV, Eth=0,7eV, R-center: Eo=1,64eV, dFC=0,31eV, Eth=1,33eV, M1: Eo=2,2eV, dFC=0,3eV, Eth=1,9eV. Das Konfigurationskoordinatendiagramm für das R-Defektzentrum wurde aus DLOS-Ergebnissen erstellt. M1 (Ec-1,9eV) wird als verantwortlicher Defekt für die Limitierung der Minoritätsladungsträgerlebensdauer in n-Typ 6H-SiC vorgeschlagen. Die thermische Ionisierungsenergie des Vanadium-Donatorniveaus wurde aus der Analyse der Photoleitungsspektren, gemessen an V/Al-codotiertem SI 6H-SiC, bestimmt 6H-SiC: Eth(V4+/V5+(k1,k2))=1,52eV, Eth(V4+/V5+(h))=1,67eV. 5. Minoritätsträgerlebensdauer-Messungen Aus dem Vergleich der Löcherlebensdauer, die mit bauelementrelevanten elektrischen Messmethoden (CRT, OCVD) bzw. optischen Messmethoden (TRPL) bestimmt wurde, wird gefolgert, dass die erhaltenen Lebensdauerwerte stark von der verwendeten Analysemethode abhängen. Diese Unterschiede beruhen auf physikalischen Gründen und nicht auf systematischen Fehlern der Messmethoden. In Hinblick auf elektrische Messmethoden, hängt die Löcherlebensdauer vom jeweiligen Bauelementbereich ab, in dem die Minoritätsträgerrekombination stattfindet. Die CRT Methode bestimmt die Löcherlebensdauer am Rand der Raumladungszone, wo implantationsinduzierte Defektzentren die Lebensdauer (bis zu 10-8s) absenken, während die OCVD Methode die Rekombination in der n-Basisbereichepischicht detektiert und die Kristallqualität dieses Bereichs charakterisiert. Diese Methode führt zu den höchsten Lebensdauerwerten (10-6s). Die optische TRPL Methode hängt stark von experimentellen Bedingungen ab. Die beobachteten Zeitkonstanten werden durch Oberflächenrekombination beeinflusst und sind durch emittierte Photonen bestimmt, die aus verschiedenen Rekombinationsprozessen stammen

Nanoscale characterization of electrical transport at metal/3C-SiC interfaces

Abstract In this work, the transport properties of metal/3C-SiC interfaces were monitored employing a nanoscale characterization approach in combination with conventional electrical measurements. In particular, using conductive atomic force microscopy allowed demonstrating that the stacking fault is the most pervasive, electrically active extended defect at 3C-SiC(111) surfaces, and it can be electrically passivated by an ultraviolet irradiation treatment. For the Au/3C-SiC Schottky interface, a contact area dependence of the Schottky barrier height (&#934;B) was found even after this passivation, indicating that there are still some electrically active defects at the interface. Improved electrical properties were observed in the case of the Pt/3C-SiC system. In this case, annealing at 500&#176;C resulted in a reduction of the leakage current and an increase of the Schottky barrier height (from 0.77 to 1.12 eV). A structural analysis of the reaction zone carried out by transmission electron microscopy [TEM] and X-ray diffraction showed that the improved electrical properties can be attributed to a consumption of the surface layer of SiC due to silicide (Pt2Si) formation. The degradation of Schottky characteristics at higher temperatures (up to 900&#176;C) could be ascribed to the out-diffusion and aggregation of carbon into clusters, observed by TEM analysis.</p

Large area scanning probe microscope in ultra-high vacuum demonstrated for electrostatic force measurements on high-voltage devices

Background: The resolution in electrostatic force microscopy (EFM), a descendant of atomic force microscopy (AFM), has reached nanometre dimensions, necessary to investigate integrated circuits in modern electronic devices. However, the characterization of conducting or semiconducting power devices with EFM methods requires an accurate and reliable technique from the nanometre up to the micrometre scale. For high force sensitivity it is indispensable to operate the microscope under high to ultra-high vacuum (UHV) conditions to suppress viscous damping of the sensor. Furthermore, UHV environment allows for the analysis of clean surfaces under controlled environmental conditions. Because of these requirements we built a large area scanning probe microscope operating under UHV conditions at room temperature allowing to perform various electrical measurements, such as Kelvin probe force microscopy, scanning capacitance force microscopy, scanning spreading resistance microscopy, and also electrostatic force microscopy at higher harmonics. The instrument incorporates beside a standard beam deflection detection system a closed loop scanner with a scan range of 100 μm in lateral and 25 μm in vertical direction as well as an additional fibre optics. This enables the illumination of the tip–sample interface for optically excited measurements such as local surface photo voltage detection. Results: We present Kelvin probe force microscopy (KPFM) measurements before and after sputtering of a copper alloy with chromium grains used as electrical contact surface in ultra-high power switches. In addition, we discuss KPFM measurements on cross sections of cleaved silicon carbide structures: a calibration layer sample and a power rectifier. To demonstrate the benefit of surface photo voltage measurements, we analysed the contact potential difference of a silicon carbide p/n-junction under illumination

Isotopic Enriched and Natural SiC Junction Barrier Schottky Diodes under Heavy Ion Irradiation.

The radiation tolerance of isotopic enriched and natural silicon carbide junction barrier Schottky diodes are compared under heavy ion irradiation. Both types of devices experience leakage current degradation as well as single event burnout events. The results were comparable, although the data may indicate a marginally lower thresholds for the isotopic enriched devices at lower LET. Slightly higher reverse bias threshold values for leakage current degradation was also observed compared to previously published work.peerReviewe

Two-Dimensional Carrier Profiling on Lightly Doped n-Type 4H-SiC Epitaxially Grown Layers

Electronically active dopant profiles of epitaxially grown n-type 4H-SiC calibration layer structures with concentrations ranging from 3.1015 cm-3 to 1·1019 cm-3 have been investigated by non-contact Scanning Probe Microscopy (SPM) methods. We have shown that Kelvin Probe Force Microscopy (KPFM) and Electrostatic Force Microscopy (EFM) are capable of resolving two-dimensional carrier maps in the low doping concentration regime with nanoscale spatial resolution. Furthermore, different information depths of this wide band gap semiconductor material could be assessed due to the inherent properties of each profiling method. We additionally observed a resolution enhancement under laser illumination which we explain by reduced band-bending conditions. To gauge our SPM signals, we utilized epitaxially grown layers which were calibrated, in terms of dopant concentration, by C-V measurements

Localized Lifetime Control of 10 kV 4H-SiC PiN Diodes by MeV Proton Implantation

In this paper, proton implantation with different combinations of MeV energies and doses from 2x109 to 1x1011 cm-2 is used to create defects in the drift region of 10 kV 4H-SiC PiN diodes to obtain a localized drop in the SRH lifetime. On-state and reverse recovery behaviors are measured to observe how MeV proton implantation influences these devices and values of reverse recovery charge Qrr are extracted. These measurements are carried out under different temperatures, showing that the reverse recovery behavior is sensitive to temperature due to the activation of incompletely ionized p-type acceptors. The results also show that increasing proton implantation energies and fluencies can have a strong effect on diodes and cause lower Qrr and switching losses, but also higher on-state voltage drop and forward conduction losses. The trade-off between static and dynamic performance is evaluated using Qrr and forward voltage drop. Higher fluencies, or energies, help to improve the turn-off performance, but at a cost of the static performance