Progress on and challenges of p-type formation for GaN power devices

Abstract

The fabrication processes of p-type regions for vertical GaN power devices are investigated. A p-type body layer in a trench gate metal-oxide-semiconductor field-effect transistor requires precise control of the effective acceptor concentration, which is equal to the difference between the Mg acceptor concentration (Na) and the compensating donor concentration (Nd). The carbon atoms incorporated during growth via metalorganic vapor phase epitaxy substitute nitrogen sites (CN) and function as donor sources in a p-type GaN layer. Since interstitial H atoms (Hi) also compensate holes, their removal from an Mg-doped layer is crucial. Extended anneals to release H atoms cause the formation of extra hole traps. The p+ capping layer allows effective and rapid removal of H atoms from a p-type body layer owing to the electric field across the p+/p– junction. On the other hand, selective area p-type doping via Mg ion implantation is needed to control the electrical field distribution at the device edge. Ultrahigh-pressure annealing (UHPA) under a nitrogen pressure of 1 GPa enables post-implantation annealing up to 1753 K without thermal decomposition. Cathodoluminescence spectra and Hall-effect measurements suggest that the acceptor activation ratio improves dramatically by annealing above 1673 K as compared to annealing at up to 1573 K. High-temperature UHPA also induces Mg atom diffusion. We demonstrate that vacancy diffusion and the introduction of H atoms from the UHPA ambient play a key role in the redistribution of Mg atoms.

Lattice Location Studies of the Amphoteric Nature of Implanted Mg in GaN

“…Our estimate for EM falls close to the range of values EM||=2.01 eV and EM⊥=2.20 eV recently predicted by theory [32] for migration of interstitial Mgi 2+ in GaN parallel and perpendicular to the c-axis, respectively. As we have previously outlined, [33] activation energies for interstitial migration EM estimated from emission channeling experiments of 8 Li, 11 Be, 24 Na and 27 Mg in GaN and 24 Na and 27 Mg in AlN were found to be correlated with the ionic radii of Li + , Be 2+ , Na + , and Mg 2+ and in agreement with most theoretical predictions. The data presented here further confirm this finding.…”

“…[7] The vast majority of GaN-based devices require besides ntype also p-type doping, and it is well-known that the group-II element Mg is the only technologically feasible acceptor dopant in GaN. [3][4][5]8] Yet, Mg doping suffers from a number of limitations, most notably the deep nature of the acceptor level at ≈245 meV, [9] which means that high dopant concentrations have to be incorporated in order to realize the hole concentrations required in the devices. Hydrogen passivation related to Mg-H complex formation is another issue, [10][11][12] as well as compensation by native donors, [10,13] the tendency of Mg to form clusters, [14] and its amphoteric nature.…”

“…Many of the power devices also require selective area doping, [3][4][5]8] which has greatly renewed the interest in ion implantation as a technique to introduce dopants into GaN since it allows precise transverse and lateral control of dopant concentrations. Already in the early days of GaN research, ion implantation of Mg has been investigated in order to achieve p-type doping, [15][16][17][18] however, with limited success.…”

“…Recently, a number of novel approaches using Mg ion implantation and annealing have achieved very promising results regarding p-type doping, e.g. multicycle rapid thermal annealing (RTA), [19] high-temperature implantation around 500°C [19][20][21] or 1000°C, [22] ultra-high-pressure annealing, [8,[23][24] microwave annealing, [21] pulsed laser annealing, [25] or co-implantation of N. [26] However, also conventional RTA [20,27,28] or furnace annealing [29,30] has been reported successful and suitable for creating devices such as visible-blind photodiodes [29] or power rectifiers. [30] A crucial step for the electrical activation of the group II element Mg is its incorporation on substitutional Ga sites, since only in this lattice position it can act as an acceptor.…”

Triple‐Site Dopant–Defect Complexes in Mg–H‐Codoped GaN: First‐Principles Identification

“…The calculated (þ/0) transition level of Mg Ga -H i is only 0.08 eV above the VBM, consistent with a previous HSE-calculated result (0.13 eV) [36] and a recent experimental measured result (0.051 eV). [38] The formation energy of Mg Ga -H N is also low and becomes even negative when the Fermi level is close to the valence band (in p-type sample), indicating that the donor-acceptor pair Mg Ga -H N can form spontaneously if the sample is doped into p-type GaN. Mg Ga -H N becomes more stable in þ1 charge state than in the neutral state and thus acts as a donor in p-type samples, consistent with the finding of Lee et al and Freysoldt et al [17,18] The spontaneous formation of the þ1 charged Mg Ga -H N will shift the Fermi level up and decrease the hole carrier concentration, which causes the sample to change back into n-type and thus limits the p-type doping thermodynamically.…”