(Ga,In)N/GaN light emitting diodes with a tunnel junction and a rough n-contact layer grown by metalorganic chemical vapor deposition

V Fan Arcara,B Damilano,J.-Y Duboz,S Chenot,G Feuillet,A Courville

doi:10.1063/1.5092693

Abstract

Tunnel junctions (TJs) are envisaged as potential solutions to improve the electrical injection efficiency of nitride emitters in the visible as well as in the UV range. Indeed TJs would solve the issues related to the poor contact with the top p type nitride layer, replacing it by an n type one. But if metal-organic chemical vapor deposition (MOCVD) is chosen to grow the n side of the TJ on a LED, one faces the problem of a potential re-passivation by hydrogen of the underlying p type layer. We propose a TJ epitaxial process whereby low growth temperatures, high growth rates and the type of carrier gas will minimize hydrogen incorporation in the underlying layers. In this view, n++/p++ GaN TJs with and without an (Ga,In)N intermediate layer are grown by MOCVD at varying temperatures (800°C and 1080°C), using N2 as a carrier gas under a very high growth rate of 2.5μm/h on top of blue (Ga,In)N/GaN LEDs. The LEDs made under N2 carrier gas and lower temperature growth conditions are operational without the need for further thermal activation of the Mg acceptors. The light emission intensity from the top surface of the TJ-LEDs is improved compared to the reference LED without TJ: besides the more efficient carrier injection this is also attributable to the larger photon extraction efficiency because of the rough surface of the low temperature grown n-GaN contact layer of the TJ-LEDs.

Highlights

Gallium nitride-based devices have been developed for the last decades to become one of the most important technologies in electronics and optoelectronics, being able to produce in particular efficient laser diodes and light-emitting-diodes (LEDs).1 The much higher resistivity (at least a factor 100 larger) of p-type GaN layers compared to n-type layers imposes specific metallic contact geometries: to have a good current spreading in the LED it is mandatory that the quasi totality of the p-type layer surface is covered by a metal (acting as a reflector) for bottom emission or a transparent conductive electrode such as indium tin oxide (ITO) for top emission.2,3 For standard LEDs, these solutions are perfectly acceptable as shown by the record performance achieved for blue LEDs4 – but for other applications such as vertical cavity surface emitting lasers, which require intracavity metallic contacts,5 the limited current spreading in the p-GaN layer induces a serious limitation to the device performance
The much higher resistivity of p-type GaN layers compared to n-type layers imposes specific metallic contact geometries: to have a good current spreading in the LED it is mandatory that the quasi totality of the p-type layer surface is covered by a metal for bottom emission or a transparent conductive electrode such as indium tin oxide (ITO) for top emission
Adding a p++/n++ tunnel junction (TJ) on top of this kind of devices can be of great help regarding this issue

Summary

Introduction

Gallium nitride-based devices have been developed for the last decades to become one of the most important technologies in electronics and optoelectronics, being able to produce in particular efficient laser diodes and light-emitting-diodes (LEDs).1 The much higher resistivity (at least a factor 100 larger) of p-type GaN layers compared to n-type layers imposes specific metallic contact geometries: to have a good current spreading in the LED it is mandatory that the quasi totality of the p-type layer surface is covered by a metal (acting as a reflector) for bottom emission or a transparent conductive electrode such as indium tin oxide (ITO) for top emission.2,3 For standard LEDs, these solutions are perfectly acceptable as shown by the record performance achieved for blue LEDs4 – but for other applications such as vertical cavity surface emitting lasers, which require intracavity metallic contacts,5 the limited current spreading in the p-GaN layer induces a serious limitation to the device performance. Polarizationenhanced tunnel junctions have been developed, profiting from a high electric field caused by an (Ga,In)N intermediate layer that results in a large band bending and, an increase in tunneling probability.8–10 it has been shown that the efficiency droop could be reduced by using structures including several active regions linked by tunnel junction.11 Such stacked LEDs would be much easier to grow if a single growth method is used, as discussed later.

Results

Conclusion