Roumen Kakanakov

Nickel based ohmic contacts on SiC

We have compared the chemical and structural properties of Ni/SiC and Ni2Si/SiC interfaces. In the case of Ni/SiC, the contact formation is initiated by the dissociation of SiC, due to the strong reactivity of nickel at 950 °C. Ni2Si is formed and carbon accumulates, both at the interface and throughout the metal layer. At the interface, many Kirkendall voids are observed by TEM. Despite this poor interface morphology, low contact resistances have been measured. But the presence of carbon in the contact layer and at the interface is a potential source of contact degradation at high temperature. In the case of Ni/Si multilayers evaporated on SiC instead of pure Ni, the contact formation is preceded by Ni and Si mutual diffusion in the deposited layer yielding Ni2Si. Therefore, a smaller amount of carbon is released from SiC. Low carbon segregation, abrupt interface and low contact resistance characterize this contact. The thermal stability of Ni2Si contacts is illustrated with ageing experiments carried out at 500 °C.

Temperature stable Pd ohmic contacts to p-type 4H-SiC formed at low temperatures

The formation of low resistivity Pd-based ohmic contacts to p-type 4H-SiC below 750/spl deg/C are reported herein. The electrical properties of the contacts were examined using I-V measurements and the transmission-line model (TLM) technique. Contact resistivity as a function of annealing was investigated over the temperature range of 600/spl deg/C-700/spl deg/C. The lowest contact resistivity (5.5/spl times/10/sup -5/ /spl Omega/cm/sup 2/) was obtained after annealing at 700/spl deg/C for 5 min. Atomic force microscopy of the as-deposited Pd layer showed a root-mean-square roughness of /spl sim/8 nm, while after annealing at 700/spl deg/C, agglomeration occurred, increasing the roughness to 111 nm. Auger electron spectroscopy depth profiles revealed that with annealing, interdiffusion had resulted in the formation of Pd-rich silicides. However, X-ray diffraction and Rutherford backscattering showed that the majority of the film was still (unreacted) Pd. The thermal stability and reliability of the Pd contacts were examined by aging and temperature dependence electrical tests. The contacts annealed at 700/spl deg/C were stable at prolonged heating at a constant temperature of 500/spl deg/C and they showed thermal stability in air at operating temperatures up to 450/spl deg/C. This stability was not found for contacts formed at lower temperatures of 600/spl deg/C or 650/spl deg/C.

Improved Al/Si ohmic contacts to p-type 4H-SiC

Abstract An AlSi-based ohmic contact with a new composition is reported in this paper. AlSi(2%)Ti(0.15%) contacts are formed by evaporation on p-type 4H-SiC grown by liquid phase epitaxy (LPE) and annealed in the temperature range from 700 to 950°C. The ohmic behaviour has been checked by I–V characteristics and the contact resistivity has been measured by the linear transmission-line-model (TLM) method. The dependence of the contact resistivity on the annealing conditions has been studied. An ohmic behaviour has been established at 700°C while the lowest contact resistivity value of 9.6×10 −5 Ω cm 2 has been obtained after annealing at 950°C. The thermal stability of both Al/Si/SiC and AlSiTi/SiC contacts at a temperature of 600°C has been studied. It has been found that the AlSiTi/SiC contacts are stable for 100 h at this ageing temperature while the Al/Si/SiC contacts deteriorate after 24 h.

Interface chemistry of WN/4H-SiC structures

The interface chemistry of WN/4H–SiC structures has been studied by means of X-ray photoelectron spectroscopy (XPS). XPS investigations have been performed on as deposited, 800°C and 1200°C annealed (4 min) samples. The as deposited and 800°C annealed samples are characterized by chemically inert interfaces. Complete nitrogen out-diffusion from the WN layer, significant carbon diffusion into the contact layer, tungsten carbide and tungsten silicide formation occur during the 1200°C annealing process. The 800°C annealed WN/4H–SiC contacts are found to be of a Schottky type with a barrier height of 0.91 eV. The Schottky barrier height and the ideality factor show no significant changes during 100 h storage at 500°C under nitrogen and during operation at increasing temperature up to 350°C in air.

Ohmic Contacts for High Power and High Temperature Microelectronics

The increased requirements to the microelectronics regarding the device potential for work at high temperatures, high powers, and high frequencies and in harsh environments engendered the increased interest to the wide band-gap semiconductors. They are considered as a third generation materials in the semiconductor industry, after Si and Ge, and A3B5 compounds and their solid solutions. Several materials of the wide band-gap semiconductor group such as SiC, III-V nitrides (GaN, AlN, c-BN), ZnSe, and diamond are very important for the device industry. The unique combination of physical properties in these materials allows development of devices, which could be applied in fields where the devices of the first and second generations cannot be used. Whereas Si and GaAs are chemically stable at 400 0C and 650 0C, respectively, SiC and III-V nitrides are stable up to 1000 0C (Meyer & Metzger, 1996). This high thermal stability allows development of new class high temperature and high power devices with maximal working temperature of 600 0C, which is three and four times higher than this one of GaAs and Si devices, respectively. Among the wide band-gap semiconductors, SiC and GaN have been most successfully applied in the device fabrication. These semiconductors offer a higher electric breakdown field (4-20 times), a higher thermal conductivity (3-13 times), and a larger saturated electron drift velocity (2-2.5 times) in comparison with silicon. These features make them very useful materials in development of high temperature and high power devices. The advantages of SiC and III-V nitrides technologies allowed manufacture of SiC-based and GaN-based devices such as unipolar high-voltage power FETs (MOSFET, JFET and HEMT), bipolar power diodes (p-n and p-i-n) and transistors (BJT, IGBT and HBT). The existing applications present many challenges in obtaining high-performance ohmic contacts because they are limiting for device functioning. The ohmic contacts are a critical factor that could restrict the high power and high temperature device application. The high operating temperatures may cause diffusion processes in the contact layer and reactions between the contact components, which could lead to changes of the contact properties during operation at high temperatures, and deterioration of the devices. If the contact resistivity is not sufficiently low inadmissible high voltage drop could arise due to the high current density in the contact of the high power devices. Hence, the following requirements to the ohmic contacts are decisive for application in high power and high temperature microelectronics:

High Temperature and High Power Stability Investigation of Al-Based Ohmic Contacts to p-Type 4H-SiC

Three types of low resistivity Al-based ohmic contacts to p-type 4H-SiC, Au/Al/Si, Au/AlSiTi and Au/Ti/Al have been studied. Reproducible contact resistivity of 8.30x10 -5 Ωcm 2 , 6.42x10 -5 Ωcm 2 and 1.42x10 -5 Ωcm 2 has been obtained for Al/Si, AlSiTi and Al/Ti contacts, respectively. The investigation of the thermal properties of Al-based contacts shows that addition of Ti to the contact composition improves its stability at ageing temperatures as high as 700 o C, at operating temperatures up to 450 o C and current densities of 10 3 A/cm 2 . XPS analysis of as-deposited and annealed contacts has been performed to explain the observed thermal properties.

Study of the electrical, thermal and chemical properties of Pd ohmic contacts to p-type 4H-SiC: dependence on annealing conditions

The electrical and chemical properties of Pd ohmic contacts to p-type 4H-SiC, together with their thermal stability, have been studied in the annealing temperature range 600–700°C. The ohmic behaviour of as-deposited and annealed contacts has been checked from I–V characteristics and the contact resistivity has been determined by the linear TLM method in order to determine the electrical properties and the thermal stability. An ohmic behaviour was established after annealing at 600°C, while the lowest contact resistivity 5.5×10−5 Ω.cm2 was obtained at 700°C. The contact structure, before and after annealing, was investigated using X-ray photoelectron spectroscopy depth analysis. As-deposited Pd films form an abrupt and chemically inert Pd/SiC interface. Annealing causes the formation of palladium silicide. After formation at 600°C the contact structure consists of unreacted Pd and Pd3Si. During annealing at 700°C, Pd and SiC react completely and a mixture of Pd3Si, Pd2Si and C in a graphite state is found in the contact layer. The examination of the thermal stability shows that after a 100 h heating at 500°C, only the contacts annealed at 700°C did not suffer from a change in resistivity. This can be explained by a more complete reaction between the Pd contact layer and the SiC substrate at this higher annealing temperature.

Au/Ti/Al contacts to SiC for power applications: electrical, chemical and thermal properties

A new Au/Ti(70%)/Al(30%) composition has been proposed and investigated as an ohmic contact to p-type SiC. The dependence of the resistivity on the annealing conditions has been studied at temperatures ranging from 700 /spl deg/C to 950 /spl deg/C. The lowest contact resistivity of 1.40 /spl times/ 10/sup -5/ /spl Omega/.cm/sup 2/ is achieved after annealing at 900 /spl deg/C. The study of the thermal properties determines the contact stability during the ageing at temperatures of 500-700 /spl deg/C, operating temperatures up to 450 /spl deg/C and current densities of 10/sup 3/ A/cm/sup 2/. XPS depth analysis of annealed and aged contacts has been performed to understand the origin of the ohmic properties and the thermal and power stability observed.

Thermally stable low resistivity ohmic contacts for high power and high temperature SiC device applications

Ti/Al/p-SiC and Ni/n-SiC ohmic contacts with improved electrical and thermal properties in respect to their application in high power and high temperature SiC devices are reported in this work. Contact resistivity as a function of annealing was investigated over the temperature range of 700/spl deg/C -950/spl deg/C. The lowest resistivity of 1.42/spl times/10/sup -5/ /spl Omega/.cm/sup 2/ for the Ti/Al contact was obtained after annealing at 900/spl deg/C while for the Ni contact the lowest resistivity of 4.9/spl times/10/sup -6/ /spl Omega/ cm/sup 2/ was achieved at 950/spl deg/C. The contact stability during prolonged ageing and at high operating temperatures and current density was studied as a criterion for their reliability. It was found that both contacts were thermally stable during ageing in an inert ambient (N/sub 2/) at high temperature of 600/spl deg/C for 100 hours as well as at operating temperatures up to 450/spl deg/C in air and at current density of 10/sup 3/ A/cm/sup 2/ passed through the contacts during the heating. The improved electrical and thermal properties of the Ti/Al/p-SiC and Ni/n-SiC ohmic contacts were demonstrated in the power p-n SiC diode developed.

Thermostable Ti/Au/Pt/Ti Schottky contacts to n-type 4H-SiC

The electrical properties and interface chemistry of Ti/Au/Pt/Ti Schottky contacts to n-type 4H-SiC have been investigated with respect to their utilization for MESFETs operated at high temperatures. The electrical properties of these contacts were studied at room temperature as well as during thermal treatment. The barrier height determined from I-V characteristics was calculated to be 1.17 eV with an ideality factor of 1.09. These parameters were examined by ageing and temperature dependence tests as criteria for the thermal stability and reliability of the contacts. The barrier height and ideality factor did not change after prolonged heating at a constant temperature of and operating temperatures up to , which confirmed the contact stability. Diodes used in the measurements showed a low leakage current at 100 V reverse voltage and room temperature ( A) as well as at ( A) and breakdown voltage above 400 V. The chemical interface properties were studied by x-ray photoelectron spectroscopy for as-deposited, annealed and heated contacts. Annealing at for 10 min led to formation of TiC and in a restricted region close to the SiC interface. The data revealed a chemically stable Ti/SiC interface after annealing, which is of importance for stable rectifying characteristics during long-term operation.