Deep Levels in n-Type Schottky and p + -n Homojunction GaN Diodes
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ABSTRACT The deep level spectra in both p+-n homojunction and n-type Schottky GaN diodes are studied by deep level transient spectroscopy (DLTS) in order to compare the role of the junction configuration on the defects found within the n-GaN layer. Both majority and minority carrier DLTS measurements are performed on the diodes allowing the observation of both electron and hole traps in n-GaN. An electron level at Ec-Et=0.58 and 0.62 V is observed in the p+-n and Schottky diodes, respectively, with a concentration of ~3-4×1014 cm-3 and a capture cross section of ~1-5×10-15 cm2. The similar Arrhenius behavior indicates that both emissions are related to the same defect. The shift in activation energy is correlated to the electric field enhanced-emission in the p+-n diode, where the junction barrier is much larger. The p+-n diode configuration allows the observation of a hole trap at Et-Ev=0.87 eV in the n-GaN which is very likely related to the yellow luminescence band.
INTRODUCTION GaN is a material of great interest due to its wide applicability in optoelectronics and high temperature electronics. However, the role of electrically active defects and their sources in GaN are still not well understood [1-5]. Thus, further studies regarding the deep levels found within the n-GaN bandgap which are associated with such defects are needed. While p+-n diode configurations are used for GaN LED’s and lasers applications, n-Schottky diodes are of interest for GaN FET’s. Thus, the role of the device configuration on the deep level spectrum is also of great interest. In this article deep level transient spectroscopy (DLTS) is used to study the deep level spectra in p+-n and n-Schottky GaN diodes and the fundamental properties of specific deep levels are explored. Both majority and minority carrier injection conditions are used for the DLTS measurements which allows the observation of both electron and hole traps in the n-GaN layer found up to ~0.9 eV from the conduction and valence bands, respectively.
EXPERIMENT GaN test devices were grown by MOCVD using a horizontal flow reactor on a c-sapphire substrate. A 0.5 µm-thick unintentionally doped n-type layer (n=3x1016 cm-3)
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V (Volts) Figure 1: I-V curves measured at 300K for the n-Schottky and p+-n GaN diodes. As expected, a much larger built-in voltage is observed for the p+-n diode.
was first deposited followed by 0.5 µm-thick Si-doped (n=3x1018 cm-3) and 1 µm-thick Si-doped (n=8x1018 cm-3) GaN layers [6]. A 1.2 µm-thick unintentionally doped n-GaN layer (n=3.5x1016 cm-3) was next deposited for the n-Schottky and p+-n diodes, followed by a 0.16 µm-thick Mg-doped GaN layer (p=0.8-1x1018 cm-3) for the p+-n structure. Pd/Au and Ni were next deposited providing ohmic and Schottky contacts to the GaN. Under these conditions the depletion region is fully contained in the n-GaN layer in both types of diodes. Back contact to the 1 µm-thick n-GaN layer was prov
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