Metamorphic Tunnel Junctions Grown Via MOCVD Designed for GaAs0.75P0.25/Si Tandem Solar Cells

A high-performance metamorphic Al
_0.2
Ga
_0.8
As
_0.75
P
_0.25
/GaAs
_0.75
P
_0.25
heterojunction tunnel junction structure was developed for application to monolithic epitaxial GaAs
_0.75
P
_0.25
/Si 1.7 eV/1.1 eV bandgap tandem solar cells produced via metal-organic chemical vapor deposition. Doping optimization focused on both bulk doping concentrations and mitigating the transient incorporation effects that otherwise causes nonabrupt doping profiles in the Te doped layer. These efforts resulted in a fully relaxed metamorphic tunnel diode at the target lattice constant having a peak tunneling current density (JP) of 279.1 A·cm
^-2
and a zero-bias resistance-area product (RA) of 3.0 × 10
^-4
Ω·cm
²
. After postgrowth annealing to emulate the thermal load of the GaAs
_0.75
P
_0.25
top cell growth and elimination of the performance-enhancing carbon activation anneal, the device achieved JP = 13.1 A·cm
^-2
and RA = 1.5 × 10
^-3
Ω·cm
²
. These worst-case values are very promising as they enable the operation of GaAs
_0.75
P
_0.25
/Si tandems under one-sun AM1.5G illumination with negligible series resistance and at AM1.5D concentrations up to 240 suns with only 0.1% absolute efficiency loss.