Abstract
The GaP/AlP/GaP heterostructure has an indirect gap both in real as well as momentum space, making the first order radiative recombination doubly forbidden. Nevertheless, we have observed relatively efficient emission from these structures. This paper comprehensively studies the origin of this improved light emission through a detailed analysis of the photoluminescence (PL) spectra. Our observations suggest that localized excitons within the acceptor states in GaP close to the heterostructure interface are enough for efficient light emission in these structures, doing away with the need for more complicated structures (superlattices or neighboring confinement structures). This real space localization of holes, close to the interface, apart from increasing the wave function overlap, also relaxes the delta-function momentum selection rule. Independent experimental evidence for this assertion comes from (i) the PL spectrum at high excitation power where transitions from both the localized as well as extended states are independently observed, (ii) the observation that extended states emission has the expected band-bending-induced blue-shift with increase in excitation power, whereas the localized states do not, (iii) observation of phonon replicas for PL from localized states, and (iv) observation of persistent photoconductivity at low temperature. Finally, we propose a simple analytical model that accounts for both the type-II nature as well as the indirect bandgap to explain the improvement of radiative recombination efficiency with increased localization. The experimental observations are reproduced within an order of magnitude. The model is very general and it also provides a framework to study the optical properties of other such (type-II and/or indirect gap) heterostructures.
Highlights
Despite the near lattice match, AlP/GaP is among the least studied of the group III–V heterojunction pairs
Low temperature (15 K) PL spectra were measured by nonresonantly exciting the samples with a 405 nm diode laser with the power incident on the sample varied between 20 lW and 20 mW
Based on experimental observations stated above, we shall argue in the following that at low excitation densities, the PL emission
Summary
Despite the near lattice match, AlP/GaP is among the least studied of the group III–V heterojunction pairs. This is because both AlP and GaP have an indirect gap. To make things bleaker for optoelectronics, the band alignment between AlP and GaP is type-II; the X-valley electrons (Cvalley holes) see a lower potential in AlP (GaP) [Fig. 1]. The overlap of electron and hole wave functions is not expected to be optimal. Non-optimal overlap would lead to poor radiative efficiency. These MQWs, we have attempted to elucidate the primary mechanism responsible for the relatively efficient light emission
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