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We investigated by Raman spectroscopy (RS) the crystalline quality of CeO 2 thin films radio frequency magnetron sputtered on n-type (111) Si substrates from CeO 2 target. The deposition temperature was in the range of 200-800 • C. We also realized structural investigations on CeO 2 layers after Rapid Thermal Annealing (RTA) performed in the range of 750-1000 • C for 30 s under nitrogen atmosphere. So this study displays that a high-growth temperature and a high post-growth-RTA temperature improves the crystalline structure of the film.In fact, the best crystalline quality, which is close to the CeO 2 target taken as a reference, is obtained for a CeO 2 layer deposited at 800 • C and post-annealed at 1000 • C for 30 s.

Mg + ions were implanted at room temperature in n-type hexagonal GaN for the device isolation purposes. The implantation dose varied from 7.5 × 10 12 to 10 16 ions cm −2 . We performed resonance Raman spectroscopy and DC electrical measurements in order to monitor the structural and electrical changes of non-annealed and annealed implanted GaN samples. Annealing was carried out at 900 • C for 30 s, these conditions being used to achieve good Ohmic contacts. The aim was to determine, on the one hand, the influence of ion doses on the device isolation and, on the other, to establish the order of the technological steps which should be made between ion implantation and Ohmic contact annealing. On increasing the implantation dose from 7.5 × 10 12 to 2 × 10 14 ions cm −2 , an increase in the electrical isolation and a decrease in the photoluminescence (PL) were observed. For the highest dose, the implanted layer became conductive owing to a hopping mechanism and only the first-order phonon lines remained observable. After annealing, the implanted samples became conductive and the PL reappeared or increased compared with the nonannealed samples at same implantation doses, except for the sample implanted at the highest dose, which became insulating. Then, it is possible to achieve device electrical isolation by using a lower ion dose without thermal annealing or using a higher ion dose with thermal annealing.

Ar + ions were implanted at room temperature in n-type hexagonal GaN for device isolation purpose. We performed electrical measurements and resonance Raman spectroscopy in order to monitor the electrical and structural changes of non-annealed and annealed implanted GaN samples. On increasing the implantation dose from 3.4 × 10 12 to 3.4 × 10 14 ions cm −2 , an increase in the electrical isolation and a decrease in the photoluminescence were observed. For a 10 16 ions cm −2 dose, the implanted layer became conductive owing to a hopping mechanism and only the first-order phonon lines remained observable. After annealing at 900 • C for 30 s, the implanted samples became conductive and the photoluminescence reappeared or increased compared with the non-annealed samples at same implantation doses, except for the sample implanted at the highest dose, which became insulating.

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AlGaN/GaN HFETs up to 800°C in vacuum [3]. These devices operate up to 600°C with the use of ohmic contacts with a high specific contact resistivity (3 ϫ 10 Ϫ3 ⍀ cm 2 ). Above 600°C, an irreversible degradation of the device performance was observed [3]. In this Letter the high-temperature dc performance of AlGaN/GaN high electron mobility transistors (HEMTs) is reported. The measurement is carried out under exposure to air, which constitutes realistic but drastic conditions. DEVICE TECHNOLOGYAlGaN/GaN HEMTs are processed on epilayers grown by metal organic chemical vapor deposition (MOCVD) on (0001) sapphire substrates. It consists of a 3-m GaN undoped layer and a 300-Å Si-doped Al X Ga 1ϪX N layer.Then, Ti/Al/Ni/Au (150 Å/2200 Å/400 Å/500 Å) metallization layers are evaporated to make ohmic contacts. These contacts are annealed under nitrogen atmosphere at 900°C for 40 s [4]. The mesa isolation is made by reactive ion etching (RIE) using 4 sccm of SiCl 4 gas, a rf power of 200 W, and a pressure of 20 mTorr, resulting in an etch rate of 180 Å/min. The gate length is defined by electron beam lithography and varies from 0.3 to 2 m. The metallization layers used for the Schottky contact are Pt/Au (100 Å/1000 Å). The gate width varies from 2 ϫ 25 to 2 ϫ 75 m, and the gate-to-drain and gate-to-source spacings are 1 m. The devices are not passivated. DEVICE MEASUREMENT AND PHYSICAL UNDERSTANDINGHigh-temperature transistor performances require a high temperature stability of the ohmic contacts. It has previously been shown that the ohmic contacts using Ti/Al/Ni/Au metallization layers are stable on GaN epilayer up to 600°C during at least 5 days of exposure to air [5]. Figure 1 shows the evolution of this resistance, versus aging time at 550°C for ohmic contacts deposited on Al 0.3 Ga 0.7 N with the same metallization schemes as described above. These contacts have a low specific contact resistivity (9.6 ϫ 10 Ϫ6 ⍀ cm 2 ) and a small contact resistance (0.46 ⍀ mm) in comparison with literature results [4]. They are stable in air at least up to 500°C for 50 h and for several hours at 600°C. It has also been observed that the sheet resistance of the AlGaN material increases after a long exposure to air at 500°C and 600°C, even when the specific contact resistivity stays constant. This phenomenon can be explained by thermal oxidation of the AlGaN cap layer due to a high Al content of 30%. The passivation of the AlGaN cap layer should circumvent this problem.After the ohmic contact technology validation for high-temperature applications, the high-temperature effect on the dc electrical behavior of AlGaN/GaN HEMTs has also been studied. The devices are heated in air from room temperature up to 700°C with a step of 50°C. dc electrical measurements are performed for each temperature step. Figure 2 shows that the AlGaN/GaN HEMTs operate up to 550°C. This result confirms that the current HEMT technology is developed enough for use in high-temperature applications. The operating temperature limit is due to the Schottky contact d...