In this paper, we present a critical study of a multi-quantum barrier (MQB) structure fabricated using the Al0.5In0.5P/Ga0.5In0.5P material system. The structure was optimised theoretically based on a single Gamma band model. When an identical structure was placed in a quantum well laser device, no improvement in the threshold current was observed compared to QW laser structure without an MQB. Cross-sectional scanning tunnelling microscopy (STM) was used to assess the structural quality of the MQB structure for the first time. Factors affecting the limited barrier enhancement are discussed in terms of the interfacial quality of the Al0.5In0.5P/Ga0.5In0.5P interface in the superlattice of the MQB and the need to consider multi-valley transport. The paper highlights some of the fundamental problems that must be overcome for MQBs to be a viable method to improve red laser efficiency. (C) 2002 Elsevier Science B.V. All rights reserved.