Observation of Large, Anomalous Pyroelectric Response in AlN Thin Films
- PDF / 517,057 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 58 Downloads / 271 Views
Observation of Large, Anomalous Pyroelectric Response in AlN Thin Films Everett Crisman 1, Vladimir Vasilyev2, Alvin Drehman2, and Richard Webster2 1 2
Dept. of Chemical Engineering, University of Rhode Island, Kingston, RI 02881, U.S.A Air Force Research Laboratory/RYHA, Hanscom AFB, MA 01731, U.S.A.
ABSTRACT We have observed a pyroelectric effect (PE) in reactively sputtered aluminum nitride (AlN) thin films that is typically a factor of twenty greater than commercial pyroelectric materials such as triglycine sulfate (TGS). This is most likely due to an extrinsic effect since the known crystalline structures of AlN are too symmetric to allow such high values for the PE response. Preliminary annealing studies support the assumption that residual strains remaining from the AlN thin film deposition are the most likely source of the anomalously high PE response. The results of these studies are presented along with some measurements that indicate a still higher PE response might be obtainable. INTRODUCTION In 1996, Bykhovski et al. reported observations of gallium nitride as a pyroelectric material with pyroelectric voltage coefficient Pv ~ 104V/mK° [1]. Since AlN has a greater piezoelectric coefficient than GaN, it was reasonable to expect, as suggested in [1], that the pyroelectric effect in AlN might be even higher. Fuflyigin et al [2] reported pyroelectric charge coefficient values of PC ~ 6-8μC/m2K° for “high quality” AlN films grown on 0.1 Ω-cm (111) Si substrates. The values reported in [2] are about two orders lower than the values for typical pyroelectric materials currently in use [3]. Subsequently, our group at the Air Force Research Laboratory, using a different preparation method, demonstrated a larger PE voltage output in AlN [4], which was of the same order as commercial PE materials [3]. At that time, we measured the pyroelectric voltage coefficient to be Pv = 0.3x106V/m·K°. Because of the high symmetry of group-III nitrides in general, one would expect that the Pv in AlN (and GaN) would be low on such compounds; more like the observation of [2] rather than [1] or [4]. This suggests that there may be an extrinsic effect resulting from the specimen preparation that is giving the high value Pv. EXPERIMENT Aluminum nitride films were reactively sputtered from a 6N’s Al target using ultra-high pure Ar/N2 gas mixture (50/50). The 50mm diameter n-type Si substrates were either (111) or (100) orientation with resistivity about 0.1Ω-cm. The chamber was first pumped to below 3x10-7 Torr. Then the substrate was heated in vacuum to 50C° above the deposition temperature to drive off any solvents, and held for 30 minutes before cooling to the deposition temperature. Although we initially deposited over a large temperature range (RT to 750°C) there was no consistent correlation between the deposition temperature and the pyroelectric voltage output. So the specimens we report on here where all prepared in the range 200°C to 400°C. Correlations
between the PE output and sputter gas composition, deposition rate and
Data Loading...
