Liliana Kassamakova

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.

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.

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.

4H-SiC pn Diode Grown by LPE Method for High-Power Applications

The current-voltage (I-V) characteristics of large area (5 mm) 4H-SiC pn diodes fabricated by liquid phase epitaxy (LPE) were studied up to 2 kA/cm 2 in the temperature range from 20 to 300 C. At 200 A/cm and room temperature, the forward voltage drop (V F) was measured to be 3.7 V. The specific on-state resistance (R on) was found to be (2.3-3.4) m Ω⋅cm . The VF showed a negative temperature coefficient in the investigated region. The lon g-term stability testing of the pn diodes during 100 hr at forward current density of 200 A/cm 2 was performed. The V F increased by 0.2 V with time. Deep traps were investigated before and after l ong-term testing. A deep trap with thermal activation energy of E C-1.43±0.03 eV was detected after diode operation during 20 hr at 200 A/cm. It is speculated that this deep trap determines the degradation of electrical characteristics.

Reliability of 4H-SiC p-n Diodes on LPE Grown Layers

In this paper we present for the first time the results from the long-term testing of power p-n diodes, grown on 4H-SiC substrates by liquid phase epitaxy (LPE) . A comparison with 4H-SiC p-n diodes, made on commercially available chemical vapor deposi tion (CVD) ppnon + epitaxial wafers within the same device processing, is given. The incre ase of the forward voltage drop, ∆VF, as a function of the time for LPE and CVD diodes was observed. The s triking feature was that the forward degradation in LPE diodes is expressed in less extent tha n he CVD ones. The observed lower degradation of the LPE diodes is explained by the lower recombi nation centers density (<10 13 cm) in LPE layers and consequently of the lower defects propagation w hen the diodes are subjected to a current load. At the same time, an increase of the specific on-state resistance is observed, which is explained by the presence of thin transition layer. Introduction The recent investigation of the long-term reliability of CVD 4H-Si C p-n diodes at pulsed or constant forward current (100 A/cm 2 or more), revealed an increase of forward voltage drop with the time [1, 2, 3]. This phenomenon was interpreted as a consequence of re ombination-enhanced stacking faults formation, which lowers the forward current [4, 5]. Ot her material defects such as screw and edge dislocations, impurities and grain boundaries were al so discussed as a reason for the device degradation. As a result, the current density of the individual l arge area SiC chips is limited to about 200 A/cm at operation temperature of 125oC – too far from the potential of SiC material. That is why the degradation phenomenon is becoming a key issue in powe r SiC device technology. Meanwhile, it was demonstrated [6] that the SiC layers grown by LPE have reduced density of deep traps (<10 cm ) and micropipes in comparison to the CVD ones. In this paper we present for the first time the results from the long-term testing of power p-n diodes, grown on 4H-SiC substrates by LPE. A comparison with 4H-SiC p-n diodes, made on commercially available CVD p non + epitaxial wafers from Cree Inc. within the same device processing, is given. Experimental details The LPE grown p pnon + structure had an undoped (drift) n o layer with thickness of 10 m and concentration of 3 x 10 16 cm, while the values of the counterpart CVD drift layer were 6 m and 5x10 cm, respectively. Both kinds of diodes were prepared with the same active area of 5 mm. The p-n junctions of the devices were terminated by deep mesa et ching, made by RIE in RF SF6 plasma. The SiC chips were attached to the Mo-Cu base plates of metal-ceram ic packages using Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 929-932 doi:10.4028/www.scientific.net/MSF.433-436.929