Deep-Level Defects in MBE-Grown GaN-Based Laser Structure

Abstract

In this paper we present results of deep-level transient spectroscopy (DLTS) investigations of defects in a GaN-based heterostructure of a blue-violet laser diode, grown by plasma-assisted molecular beam epitaxy on n + -type, dislocation free, bulk GaN substrate synthesized by a high-pressure high-temperature method. The heterostructure contained multiple InGaN quantum well as the active region, which was embedded between two, n-type (Si-doped) and p-type (Mg-doped), GaN wave-guiding layers and two, nand p-type, AlGaN cladding layers, and was covered with a highly Mg-doped GaN contact layer; see [1] for the exact sequence of the layers. The use of a p-n junction of the diode allowed for investigation of both majorityand minority-carrier traps with DLTS. In addition, the kinetics for capture of charge carriers into the defect states, measured by means of recording the dependence of the DLTS-signal amplitude on the filling-pulse duration, provided information on the defect geometry, which allowed distinguishing point defects from extended defects, such as dislocations. Two majority-carrier traps, called T1 and T2, with the activation energies of 0.28 eV and 0.33 eV, respectively, have been revealed in the DLTS spectra measured in the temperature range 80–400 K under typical bias conditions, i.e. under reverse quiescent bias, which was decreased during the filling pulse. On the other hand, DLTS measurements performed under injection conditions, i.e. under zero quiescent voltage and forward-bias filling pulse, revealed one minority-carrier trap, T3, with the activation energy of 0.20 eV. The two majority-carrier traps have been revealed in the DLTS spectra measured under different reverse bias conditions, suggesting that they are present in different parts of the laser-diode heterostructure. In addition, the two traps represent different charge-carrier capture behaviours. The T1 trap, which exhibits the logarithmic capture kinetics, is tentatively attributed to electron states of dislocations in the n-type wave-guiding layer of the structure. In contrast, the T2 trap displays the exponential capture kinetics and is assigned to point defects in the p-type wave-guiding layer of the structure. Additional information on the nature of electronic states of deep levels related to dislocations has been obtained from analysis of the dependence of DLTS-line shape of the T1 trap on the filling-pulse duration. This analysis allowed us to specify the type of electronic states associated with dislocations, which, owing to translational symmetry along dislocation lines, are expected to form one-dimensional energy bands rather than isolated localized electron states. According to the model recently proposed by Schroter et al. [2], which takes into account the rate at which the states reach their internal electron equilibrium within the band, we classify the revealed dislocation-related T1 trap to so-called band-like states in the cores of dislocations.