Study of Ultra High Frequency Surface Acoustic Wave Filters by the use of Diamond-cubic AIN Structure

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The research subject we studied in the fiscal year 2000 was to evaluate the p-type diamond as the SAW substrate and the deposition method of a thin film AlN grown on the diamond. The conduction mechanism of the heavily doped p-type diamond was analyzed. The diamond is a semiconductor with a wide band gap energy of 5.5 eV, and has a different conduction mechanism from that of Si and GaAs having a band gap energy of about 1 eV.When the impurity concentration increases, a new and shallower impurity level is formed due to the interaction between the impurities, and the carriers moves via this level. This mechanism was demonstrated by the use of a sandwich structure of lightly doped p-type diamond in between the heavily doped p-type diamonds with impurity band conduction (see the references (1), (5) and (6)).As for the deposition method for the AlN, we investigated two methods, one was the deposition at the resonance point of the nitrogen electron cyclotron plasma and the other was the conv … More entional metal organic chemical vapor deposition (MOCVD). By the use of MOCVD, we got good quality AlN at the deposition temperature of 1150℃. By the use of the former method, we succeeded in growing good quality AlN at the temperature lower than 600℃. This method enabled us to grow high quality AlN on metal Al hetero-epitaxially. By the use of this method, we fabricated AlN SAW filters onto the Al inter-digital-transducer (IDT) and we got the quality factor of 3 million at the fabricated SAW filter. This high quality factor is understood to be due to the Lamb wave excitation in the AlN thin film of uniform thickness. This result was reported at the International Workshop of Nitride (IWN 2000) at Nagoya (see reference (2,3)).To evaluate our computer simulation of the SAW filter, we made several SAW filters by the use of AlN grown on sapphire C-plane and on R-plane. The line and space of IDT was 1.3 μm. The fabricated SAW filters had a band-pass frequency of 1.1 GHz and SAW velocity of 5,620 km/s. These values were within 1.8% error of the estimated values (see reference (5)). From these investigations, we obtained plenty of data on how to grow cubic AlN onto p-type diamond. Less