Abstract
The performance characteristics of green light-emitting diodes (LEDs)
grown by metal–organic chemical-vapor deposition were investigated to
study the dependence of the device performance on the materials and the growth
conditions of p-type layer grown after the InGaN multiple-quantum-well active
region. The electrical and structural qualities of Mg-doped p-In<sub>0.04</sub> Ga<sub>0.96</sub> N and p-GaN layers grown under different growth conditions were studied to optimize the growth conditions of p-type hole injection layers of green LEDs. A free-hole concentration of p = 1.6×10<sup>18</sup> cm<sup>-3</sup> of with a resistivity of 0.33 Ω · cm was achieved for p-GaN:Mg layers grown at 1040°C. Lower hole concentrations and mobilities
and rough surfaces were obtained when the growth temperature was decreased
to 930°C in H<sub>2</sub> ambient. In the case of p - In<sub>0.04</sub>Ga<sub>0.96</sub>N grown at 840 °C N<sub>2</sub>,
a significant improvement of the hole concentration was achieved due to the
reduced ionization activation energy of Mg acceptors in InGaN. Also we observed
that as-grown p-GaN layers grown in N<sub>2</sub> ambient showed p-type properties without Mg dopant activation.
The electrical and optical properties of In<sub>0.25</sub> Ga<sub>0.75</sub> N/GaN
multiple-quantum-well green LEDs with such different
p-layers were investigated. The electroluminescence intensity was improved
for the LEDs with p-In<sub>0.04</sub> layers grown at 840°C as compared to the LEDs with p-GaN layers grown at higher temperatures
due to the reduced thermal damage to the active region, high hole injection,
and low piezoelectric field induced in the active region. p-InGaN layers are
very attractive candidates for the p-layer in green LED structures. The low
temperature and N<sub>2</sub> ambient used during the growth of InGaN layers are beneficial to
protect the InGaN active region containing high-indium composition quantum-well
layers in addition to the advantage of providing a higher hole concentration.
However, the LEDs with p-In<sub>0.04</sub> Ga<sub>0.96</sub> N layer showed a slightly higher turn on voltage which could originate from the potential barrier for hole transport at the interface of the p-InGaN
layer and the last GaN quantum-well barrier. to reduce this problem, we designed
and characterized an LED structure having a graded indium composition in the p-In<sub>0.04</sub> Ga<sub>0.96</sub>N layer in order to improve hole transport into the active region. Optimized LEDs with p-InGaN layers grown in a N<sub>2</sub> ambient showed much brighter electroluminescence due to low damage to the active region during p-InGaN layer growth.
© 2007 IEEE
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