Nichia claims GaN VCSEL breakthrough
Nichia has produced the first electrically pumped GaN VCSEL to emit continuous-wave light at room temperature.
The 410 nm laser improves on existing low-temperature operation GaN VCSELs thanks to improved orientation of its electrodes and removal of its sapphire substrate.
Researchers from the Japanese GaN pioneer reported the approximately 13.9 kA/cm2 threshold current density device in Applied Physics Express online on December 5.
The best previous continuous-wave (CW) GaN VCSEL, deposited on a sapphire substrate with contacts located next to each other, operated only at temperatures up to 77 K.
Nichia says that this device was limited by the fact that sapphire is a poor thermal conductor and the close proximity of the contacts.
“In side-by-side configuration for the n-type and p-type contacts current crowding occurs near the edge of the p-type mesa,” Yu Higuchi wrote.
"Current crowding induces an increase in operating voltage and heat generation, especially in VCSELs, which require a high current density.”
Higuchi and his colleagues grew an InGaN/GaN region by MOCVD on a sapphire wafer, starting with n-type GaN. Above this they deposited an active region consisting of multiple quantum wells made from alternating 9 nm-thick InGaN and 13 nm-thick GaN layers.
The Nichia team grew a p-type GaN layer above the active region, followed by a SiO2 layer, into which they patterned an 8 µm diameter current aperture. Depositing 50 nm of indium-tin-oxide (ITO) provided a p-type ohmic contact and current spreading layer.
Above this the team used niobium oxide as the basis of a layer that performed the combined role of electrode and backside mirror. The lack of suitable distributed Bragg reflector (DBR) mirrors with suitable reflectivity and conductivity has been one of the key challenges for GaN VCSELs.
In addressing this challenge, Nichia deposited a niobium dioxide electrode onto the ITO layer, around the current aperture. Above the aperture 11.5 pairs of alternating SiO2 and niobium dioxide layers were grown as a DBR.
The researchers then bonded a silicon substrate on top of this structure, inverted it, and removed the sapphire substrate by laser-induced lift off. They also polished back the n-type GaN layer so that the optical cavity was 1.1 µm thick, before making a 7-pair SiO2/niobium dioxide DBR and n-type contact on top of it.
Although the final device s threshold current density is higher than the 77 K VCSEL, at 4.3 V its threshold voltage is lower. Higuchi says that this is “probably due to vertical current injection without current crowding”.
The GaN VCSEL s final output power was 0.14 mW at 12 mA injection current. The Nichia team concedes that so far “the VCSEL was easily degraded under CW operation”.
