Surface studies and photoluminescence of GaN epitaxial layers grown by MOCVD technique for applications of solar cells and light emitting devices

Abstract

This article presents theoretical and experimental results of n-type GaN semiconductor. In the first part, the variation of surface potential of GaN with varying carrier concentration in the range from 1015 to 1019 cm−3 is determined as well as the change in the electric field with variation of surface potential is also estimated. In addition, the change in the electric field (F) with effect of irradiation of laser beam on the GaN sample, while measuring photoluminescence, is found by developing appropriate theoretical models using known activation energies of traps, which are obtained by the experimental photoluminescence (PL) measurements. In the second part, the experimental results of n-type GaN such as growth by MOCVD, electrical, structural and optical properties are presented. The un-doped GaN thin films have been successfully deposited onto c-sapphire substrates by MOCVD technique with different recipes. It is observed that the carrier concentration of the films varies with varying deposition parameters. The PL studies are systematically carried out on these samples at low temperature. The correlation between PL results and the electrical data such as carrier concentration (n) or mobility (μ) is analyzed for GaN thin films. Based on the activation energies of donor–acceptor pair (DAP) and the yellow band emissions obtained from the PL measurements, the possible transition energy levels in the energy band are hypothetically proposed. The highly resolved phonon replicas in PL spectra are observed for high carrier density samples. The n and μ are found to be in the range of 1017–1018 cm−3 and 65–120 cm2/V−s, respectively. The XRD analysis reveals that the samples show preferred orientation and hexagonal structure. On the other hand, the nature of the samples changes from epitaxy to polycrystalline with increasing carrier concentration in the samples.