Materials Science Forum Vol. 711

Abstract: Focusing on the change in aluminium-related photoluminescence lines in 4H-SiC versus doping concentration, we have used a combination of LTPL (Low Temperature PhotoLuminescence) and secondary ion mass spectrometry measurements to set new calibration curves. In this way, one can probe the change in aluminum concentration in the range 10¹⁷ to 10¹⁹ cm^-3. When applied to LTPL maps collected on full 3-inch wafers, we show that such abacuses constitute a powerful tool to control efficiently the doping level of as-grown p⁺ (emitters) and p⁺⁺ (contact) layers for power device applications.

Abstract: Transfer Length Method (TLM) based-structures were fabricated on 0.8 µm-thick epitaxial p-type Silicon Carbide (4H-SiC) layers. TLM mesas were defined by a 2 µm height using an SF₆/O₂ reactive ion etching. TLM metal patterns were obtained by a lift-off procedure and electron beam deposition of Ni, Ti, Al and Pt. The patterned samples were annealed in Argon ambient at temperature ranging from 700°C up to 1000°C in a RTA furnace with a rapid heating ramp (up to 50°C/s) to complete the ohmic contact with the p-type SiC layer. Specific contact resistances were extracted from current/voltage measurements. To identify and follow the profile evolution of constituting element in the contacts and at the SiC/contact interface, the ohmic contacts were characterized using Secondary Ion Mass Spectrometry and Energy-Dispersive X-Ray spectroscopy before and after annealing. Ohmic contacts are obtained only for the Ni/Ti/Al and Ni/Ti/Al/Ni stacking layers and not for the Ti/Al/Ti/Ni and Ti/Al/Ti/Pt/Ni compositions. The specific contact resistance of Ni/Ti/Al/Ni stacking layers was observed to decrease from 2.7×10^-4 Ω.cm² at 700°C and 6.3×10^-5 Ω.cm² at 750°C to a minimal value of 1.5×10^-5 Ω.cm² at 800°C. Ohmic contacts are obtained with a reproducibility of 80 %.

Abstract: Various attempts have been made to evaluate the correct value (A*=146 A/cm2.K2) ofRichardson's constant. In 2005 S. Ferrero et al. published their research in which they performedan analysis of electrical characterizations of twenty Ti/4H-SiC(titanium on silicon carbide) Schottkydiodes with the help of thermionic emission theory and evaluated the value of Richardson's constantto be 17±8 A/cm2.K2; which is very low as compared to the theoretical value of 146 A/cm2.K2.Wehave tried in this paper to evaluate the Richardson's constant's value by nearly same experimental tech-niques followed by S. Ferrero et al. and additionally, have applied Tung's theoretical approach whichdeals with the incorrect value of A* in the perspective of Schottky barrier inhomogeneities caused bythe presence of nanometer size low barrier patches present in the uniform high barrier of the Schottkydiode.We have fabricated two Ti/4H-SiC (titanium on silicon carbide) Schottky diodes with differentareas and oneMo/4H-SiC (molybdenumon silicon carbide) Schottky diode. In this paper we have pre-sented a comparative analysis of forward current-voltage characteristics of all three Schottky diodes.In all three cases we were successful in the evaluation of nearly correct value of Richardson's constant.This work emphasizes the effects of differentmetal-SiC combinations and laboratory environments onthe evaluation of Richardson's constant and the effective area involved in the current transport. As pre-dicted by Tung's model the effective area is seen to be substantially different from the geometric areaof the Schottky diode. Evaluated values of A*, with an error of ±2, come out to be 145.39, 148.33and 148.33 A/cm2.K2for Ti/4H-SiC(large area), Mo/4H-SiC and Ti/4H-SiC(small area) Schottkydiodes, respectively.

Abstract: We report on the influence of titanium thickness on the structural and electrical properties of annealed Ti/Ni ohmic contacts on highly doped n-type 3C-SiC. Electrical analysis by means of circular transfer length method demonstrate that an interlayer of titanium with thickness in the range of 25-150 nm has no significant influence on specific contact resistance. However, from a structural point of view, the formation of nickel silicides as well as Ti₃SiC₂ is severely affected by the titanium thickness. Moreover, the Kirkendall effect due to the reaction between Ni and SiC is influenced by the titanium thickness. In fact, Scanning Electron Microscopy analysis demonstrates that the adjunction of titanium affects the distribution of Kirkendall voids in the contact. Current maps determined by conductive Atomic Force Microscopy reveal significant variation of uniformity according to the titanium thickness.

Abstract: Metal/semiconductor contacts have a great impact on device performances. Contact properties to wide band gap semiconductors, in particular, are more difficult to control due to the large potential barrier which arises when the metal is deposited on the semiconductor’s surface. Moreover, intrinsic interface states also lead to deviation of the Schottky-Mott limit and the barrier height is no more dependent of the work function of the metal. The contact property has also become very important with the race for miniaturisation toward the nanoscale. Contacts must also be adherent, able to resist to the temperatures for which SiC based-devices are intended, and also they should be compatible with conventional device processing techniques (die attachment). Ohmic contacts to SiC have thus been investigated for decades. The difficulties of controlling the interface properties between the metal and SiC to obtain low resistive ohmic contact have not been overcome yet; the specific contact resistance being proportional to the exponential of the barrier height for a given doping concentration. For example, nickel has been studied for the ohmic contacts on n and p-type, however the presence of voids at the interface has been reported leading to the degradation of the contact properties [1]. More recently low ohmic contact resistance has been reported of Au/Ti/Al/n-type-4H-SiC contact [2]. The formation of TiSi, TiSi₂ and Ti₃SiC₂ has been reported according to x-ray diffraction experiments after annealing. The formation of Ti₃SiC₂ (or MAX phase) has also been reported in TiAl-based contacts to both n-and p-type [3-6]. This ternary carbide layer is supposed to reduce the barrier height at the contact and thus leads to low contact resistances. The addition of Ge also leads to the formation of Ti₃SiC₂ at lower temperature of annealing [7]. However, other compounds are frequently observed at the interface showing that the control of the interfacial structure must be optimized. The objective of our work is to obtain uniform epitaxial Ti₃SiC₂ thin film on n-type 4H-SiC to form ohmic contact with low resistance by studying the influence of different parameters such as the role of Aluminium on the formation mechanisms, the polarity and doping dependence. The temperature and the annealing time are also parameters to be optimized for the improvement of the ohmic contact.

Abstract: Forward current-voltage characteristics of a medium sized (3.05mm2)Mo/4H-SiC (molyb-denum on silicon carbide) Schottky diode|fabricated for high power applications | are analysedwithin a temperature range of 125-450 K. Accurate theoretical modeling is carried out using Tung'smodel in which it is considered that numerous low barrier nanometer size patches, present in uniformhigh barrier, are responsible for the inhomogeneities in the Schottky barrier of SiC-based electronicdevices. A significant difference is observed between the effective area involved in the current trans-port and the geometric area of the Schottky contact along with a dependence of the ideality factor andhe barrier height on temperature. The obtained values of uniform Schottky barrier and Richardson'sconstant are seen to be in accordance with previous works. It is concluded that the above mentionedmodel can be used to describe the electrical behaviour of Mo/4H-SiC Schottky diodes.

Abstract: An extensive study on the use of Si as a substrate for the growth of AlGaN/GaN layers for High-Electron-Mobility Transistor (HEMT) were studied and reported in this article. We have used thick buffers to grow high resistive i-GaN by MOCVD which offers a high breakdown voltage. While the leakage through buffer and substrate can be controlled by thick buffer, the leakage through gate is controlled using a thin 2-nm in-situ grown i-GaN cap layer. We have evidenced a high figure of merit (BV²/R_ON) of 2.6 x 10⁸ V²Ω^-1cm^-2 for AlGaN/GaN HEMTs grown on 4-inch Si substrate. The challenges before the MOCVD growth of GaN on Si is also discussed in detail.

Abstract: The formation of good Ohmic contacts to p-type silicon carbide (SiC) and gallium nitride (GaN) is an important physical and technological concern, because of the difficulty to find metals with low Schottky barriers to p-type wide band gap materials, and due to the high ionization energies of p-type dopant impurities. Typically, to overcome these issues, alloyed metallic compounds are used. In this work, the electrical properties of alloyed Ohmic contacts to p-type (Al-implanted) 4H-SiC and p-type (Mg-doped epilayers) GaN are presented and correlated with their microstructure. The impact of the surface preparation and annealing conditions are discussed, reporting the cases of Al/Ti contacts to p-SiC and Au/Ni contacts to p-GaN. The electrical characterization as a function of temperature allowed to define the dominant transport mechanism and to determine the barrier heights.

Abstract: This work reports of investigation on the origin of the ohmic behavior for Ti/Al basedcontacts on n-GaN. Indeed, last publications tend to prove that the ''nitrogen vacancy'' theory is notconfirmed by the experimental ground available. To find new answers, we first made electrical charac-terizations on some Al/Ti/n+-GaN contacts with different annealing conditions. Then, we performedSIMS depth profiles of annealed TiN/Ti/n+-GaN samples. Results developed in this paper tend todemonstrate that the ohmic behavior of Ti/Al based contact is mainly due to an increase of the dop-ing concentration. Moreover, the high doping level can be explained by an in-diffusion of Ti and anexo-diffusion of Si elements close to the metal / semiconductor interface.

Abstract: In this paper, we evaluated gallium nitride heteroepitaxially grown on sapphire (GaN/Sa) and grown on silicon (GaN/Si) faced to implantation doping. Si⁺ was implanted on low doped n-type epilayers in order to create a plateau around 10²⁰at.cm^-3. All the samples were capped with a silicon oxide and annealed between 1000°C and 1150°C. The surface quality was evaluated in terms of roughness, pit density and maximum pit diameter using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Finally, the dopant electrical activation was studied with Ti-Al contacts using the circular Transfert Length Method (c-TLM). This study shows that low Specific Contact Resistance (SCR) values of 8x10^-5Ω.cm² and 6x10^-6Ω.cm² are respectively obtained on GaN/Sa sample annealed at 1150°C-2min and on GaN/Si sample annealed at 1150°C-30s, proving that good ohmic contacts are obtained on both materials. Nevertheless, a compromise has to be done between the low SCR values obtained and the GaN surface degradation, observed by AFM and SEM after the different annealing treatments and which could affect the good behaviour of the GaN devices.