Heavily Ge-doped GaN as transparent current spreading layer for blue tunnel junction light emitting diodes

Abstract

We report on metalorganic vapor phase epitaxy of highly conductive germanium-doped GaN layers and their application for blue tunnel-junction light emitting diodes (TJ-LEDs). Using Ge as donor, carrier densities of up to 2 × 1020 cm−3 and low bulk resistivities down to 3 × 10−4 Ωcm are achieved. Under optimum growth conditions, no degradation of the crystalline quality is observed and layers exhibit high transparency making GaN:Ge very attractive as current spreading layer in light emitting devices. We have realized GaN-based TJ-LEDs by capping conventional InGaN LED structures with highly doped GaN:Ge. Such TJ-LEDs withstand operation currents up to 20 kA/cm2 in continuous and up to 60 kA/cm2 in pulsed operation conditions. Moreover, TJ-LEDs exhibit homogeneous electroluminescence and light output through the top surface that is increased by more than 60% as compared to conventional LEDs with transparent indium tin oxide contacts. The impact of the doping profile, carrier gas conditions, and acceptor activation by annealing for low-resistive TJ characteristics is discussed. Light output and current voltage characteristics of blue-light emitting devices with GaN-TJ are presented at low and high-current densities.

Recent Progress in III‐Nitride Tunnel Junction Light‐Emitting Diodes

“…For micro‐LEDs treated with in situ activation, the differential resistance of device was down to 2.8 × 10 −4 Ω cm 2 at 2 kA cm −2 . [ 43 ] The in situ Mg activation not only avoids sidewall damage caused by dry etching but also simplifies device fabrication, making it a more efficient Mg‐acceptor activation method.…”

“…The critical thickness decreases with increasing In composition due to the Reproduced with permission. [43] Copyright 2022, AIP Publishing. increase in polarization charge and the decrease in bandgap.…”

“…[ 42 ] Figure illustrates the Hall mobility and resistivity as a function of carrier concentration for highly doped GaN:Si and GaN:Ge films. [ 43 ] It can be seen that the mobility and resistivity of GaN:Si and GaN:Ge films both decrease with increasing carrier concentration. In addition, the dependence between carrier concentration and mobility or resistivity is similar for both donor types.…”

“…Moreover, the resistivity of GaN:Ge film is down to 3 × 10 −4 Ω cm, which is nearly one order of magnitude lower than the lowest resistance of MOCVD‐grown GaN:Si film. [ 43 ] Arcara et al achieved state‐of‐the‐art mobility (67 cm 2 V −1 s −1 ) and resistivity (1.7 × 10 −4 Ω cm) at a free electron concentration of 5.5 × 10 20 cm −3 in Ge‐doped GaN. [ 45 ] However, Ueno et al found that degenerate Ge‐doped GaN films exhibited slightly lower electron mobility compared to Si‐doped GaN films with an identical carrier concentration, due to the formation of a small number of compensation centers in Ge‐doped GaN films.…”

“…Dependence of a) mobility and b) resistivity of GaN:Si and GaN:Ge films on carrier concentration. Reproduced with permission [43]. Copyright 2022, AIP Publishing.…”