Characterization of p-GaN_1−xAs_x/n-GaN PN junction diodes

Abstract

The structural properties and electrical conduction mechanisms of p-type amorphous GaN 1-x As x /n-type crystalline GaN PN junction diodes are presented. A hole concentration of 8.5×1019 cm -3 is achieved which allows a specific contact resistance of 1.3×10 -4 Ω-cm 2 . An increased gallium beam equivalent pressure during growth allows reduced resistivity but can result in the formation of a polycrystalline structure. Temperature dependent current voltage characteristics at low forward bias (<0.35 V)show that conduction is recombination dominated in the amorphous structure whereas a transition from tunneling to recombination is observed in the polycrystalline structure. At higher bias, the currents are space charge limited due to the low carrier density in the n-type region. In reverse bias, tunneling current dominates at low bias (<0.3 V) and becomes recombination dominated at higher reverse bias.GaN-based PN diodes are attractive for power electronic applications due to the high breakdown field and low power loss and are likely to play an important part in supporting the required large voltages in vertical power devices. However, achieving a high hole concentration (>10 18 cm -3 ) remains difficult due to the high activation energy of Mg dopants which can result in high resistivity and poor ohmic contacts. Mg-doped p-type amorphous GaN 1-x As x (0.17<x<0.8) with band gap varying from 0.8-3.4 eV has been demonstrated in an effort to cover a wide range of the solar spectrum for full spectrum solar cells. 1 A hole concentration up to 1×10 20 cm -3 was achieved and the electrical properties of the free carriers were studied. 2 The high hole concentration can be used to improve the performance of electronic devices such as PN diodes, pGaN gated HFETs and JFETs. However, reports on the characteristics of the material in electronic devices is still lacking. In this paper, for the first time, we present a study of the electrical characteristics of p-GaN 1-x As x /n-GaN junction diodes and include the structural properties and transport mechanisms. 2The diode structure is shown in Fig. 1 with the lowest resistivity which, as will be discussed later, is also close to the conditions where transition between amorphous and polycrystalline is possible. The As 2 and Mg BEPs were kept the same at 6.6×10 -6 and 6×10 -9 Torr and the substrate temperature was held at 245 ˚C during growth for both samples. In addition, a set of calibration layers were grown to study the dependence of resistivity and contact resistance on Ga BEP which was specified between 1.3×10 -7 to 2.2×10 -7Torr.The mesa diode was patterned by standard lithography with an active area of 1.2×10 -3 cm 2 and dry-etched in an inductively coupled plasma system with Cl 2 -based gases to access the n+GaN current spreading layer. Ti/Al/Ni/Au and Ni/Au contacts were deposited using a thermal evaporator as the cathode and anode, respectively. It is worth mentioning that most Ni-based ohmic contacts formed on p-GaN in the literature required a post-deposition annealing i...