A Comparative Study Of GaN Diodes Grown by MBE on Sapphire and HVPE-GaN /Sapphire Substrates
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RODUCTION GaN photodiodes have recently been reported by a number of laboratories [13]. Some studies suggest that threading dislocations in these devices act as leakage paths resulting in increased dark current [4-6]. The Santa-Barbara group has demonstrated that p-n junctions fabricated by the deposition of p-GaN by the MOCVD method on epitaxially laterally overgrown (ELO) GaN stripes show orders of magnitude reduction in the reverse-bias dark current [7]. However, such devices are small and impractical as photodetectors because of the narrow width of the ELO-stripes (~ 8 µm).An alternative approach is to use the HVPE method for the growth of GaN substrates. Due to its high growth rates, this method is capable of producing thick GaN films with significantly reduced concentration in threading defects at the free surface [8]. Furthermore, significantly broader ELO GaN stripes can be produced by this method [ 9]. Also, Smith et. al. have recently demonstrated that photoconductive detectors grown on HVPE GaN/ sapphire have improved optical response and sharpness than those grown directly on sapphire [10]. In this paper we report on the fabrication and characterization of GaN Schottky diodes grown by molecular beam epitaxy on c-plane sapphire as well as 5-6 µm thick HVPE grown GaN films. Furthermore, the fabrication and characterization of p-n junctions formed by depositing p-GaN by MBE on n+ GaN grown by HVPE is presented.
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Presently at Lucent Technology, Bell Labs
F99W11.1
EXPERIMENTAL METHODS The 5 –6 µm thick n+- GaN /sapphire substrates employed in this study were grown by the HVPE method. Briefly, the films were grown in a horizontal quartz reactor by flowing HCl over Ga melt that produces GaCl. Using N2 as a carrier gas, the GaCl is transported to the substrate where it reacts with NH3 and forms GaN. The films grown by this method are typically unintentionally doped n-type with carrier concentration between 5x1018 to 1x1019 cm-3. The density of dislocations in these relatively thin GaN substrates was estimated from photoelectrochemical etching to be on the order of ~ 109 cm-2. The active layers for both types of diodes were grown by plasma assisted MBE. Active nitrogen for the MBE growth was produced from molecular nitrogen by a microwave plasma-assisted electron cyclotron resonance source. The active layers (n+/n-, or p) were grown at 700- 800 °C using procedures described in our recent papers [11]. The films were doped n-type with silicon by varying the cell temperature from 1025 to 1100 °C for the n- and n+ layers respectively. The p-type films were doped with Mg by varying the cell temperature from 300 – 350 °C. For the devices grown directly on (0001) sapphire, the substrates were first subjected to a nitridation step at 800 °C (conversion of the surface of Al2O3 to AlN) using procedures developed first in our laboratory [12]. This step was followed by the deposition of a 20-30 nm thick AlN buffer grown at 700-750 °C. The growth of the active layers on the HVPE grown GaN/sapphire proceeded directly with
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