Low Resistivity Aluminum Nitride: Carbon (AIN:C) Films Grown by Metal Organic Chemical Vapor Deposition
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In this work, we
present the growth and characterization of AIN:C films with resistivity less than .1 ohm-cm. We believe that the resistivity data presented in this paper is the lowest ever reported for AIN. EXPERIMENT AIN layers were grown in a low pressure metal organic chemical vapor deposition (MOCVD) vertical reactor. This reactor has two separated chambers joined by a common load lock. One of the chambers is equipped with a reflection high energy electron diffraction (RHEED) system which allows a quick determination of film crystallinity after growth. The common load lock provides a way of transporting samples between chambers without exposing them to the ambient air. Sapphire substrates were used for most of the experiments. P-n junctions were fabricated on n-type 6H-SiC Lely platelets which are nominally oriented in the (0001) direction. Trimethylaluminum (TMA), ammonia (NH3) and hydrogen (H2) were used as precursors. Propane (C3H8) was utilized as a independent carbon (C) source. The propane flow rates were 28 scem. The growth temperature and pressure were 1200°C and 10 torr, respectively. The TMA was supplied by passing H2 through a TMA bubbler which was kept at 35°C. Growth was performed vertical MOCVD system. After growth, surface composition analysis was performed by Auger spectroscopy or secondary ion mass spectroscopy (SIMS). Prior the chemical measurement the surface was sputtered to remove surface carbon. Optical reflection and absorption measurement were made using a computer controlled spectrophotometer. The film thickness was determined by the interference fringes in the optical reflection data, typical sample thickness were about 2000 angstroms. Resistivity, mobility, carrier concentration, and carrier type 279 Mat. Res. Soc. Symp. Proc. Vol. 395 0 1996 Materials Research Society
were determined by Hall and Van der Paw techniques (In dots were used to form contacts to the Van der Paw pattern). A p-n junction was fabricated from an AIN film grown directly on a n-type 6H-SiC substrate. RESULTS Figure 1 shows the electrical and chemical data from as grown films of AIN on sapphire (on which RHEED measurements after growth indicated the films to be crystalline). It can be seen that the conductivity of the AIN:C films changes dramatically from no propane flow to a propane partial pressure of .5 xl0-3 torr. Unintentionally doped films grown on sapphire exhibited a resistivity 108 ohm-cm, while films grown at a propane partial pressure of greater than .5 x 10-3 torr had a resistivities less than .2 ohm-cm. The film resistivity continues to drop as the propane flow is increased. Also shown in Fig. 1 is the carbon concentration in the films (obtained from Auger studies) as a function of the partial pressure of propane during growth. The concentration of carbon in the as grown layer is a linear function of the partial pressure of propane during growth and changes from about 4% to 13%.
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