Hole migration and optically induced charge depletion in GaSb/GaAs wetting layers and quantum rings

Abstract

We present the results of photoluminescence (PL) measurements on a type-II GaSb/GaAs quantum dot/ring sample as a function of temperature (2 to 400 K) and over six orders of magnitude of incident laser excitation power. Optically induced charge depletion (OICD) was seen in both the wetting layer (WL) and quantum dots/rings but with remarkably different temperature dependent behavior. Holes originating from background acceptors migrate out of the WL as the sample temperature is raised to 30 K, while the onset of a blueshift in the PL from quantum rings, signaling their thermally induced charging with holes, is only observed at temperatures above 300 K. The presence of dark dots as a hidden reservoir for acceptor holes at the intermediate temperatures is proposed to explain this anomalous behavior. Due to the deep localization potential of GaSb/GaAs, thermalization of acceptor holes between dark dots and bright rings only occurs above room temperature. A rate equation model is presented which successfully replicates the main features of OICD observed here and in previous reports.

Hydrogenation of GaSb/GaAs quantum rings

“…The formation of QRs in capped GaSb layers is common and results from Sb migration away from the highly strained QD centre during capping. 37,38 The sample studied in this paper was previously shown to exhibit OICD in the wetting layer (WL) emission at temperatures, T, <30 K and in the QR emission at T > 300 K. 23 This dependence on temperature was interpreted as the thermalisation of acceptor holes from the WL, where they are located at low temperatures, into the QRs at high temperatures. Highly strained "dark" QD states were used to account for the location of the acceptor holes at intermediate temperatures.…”

“…21 In a few cases, redshifts of the emission energy with increasing excitation power (which precedes the blueshift) have also been observed in these nanostructures. 22,23 These redshifts are the result of optically induced charge depletion (OICD). 24 OICD is a dynamic process involving interactions between photogenerated carriers, a nanostructure and donor or acceptor states in the surrounding semiconductor material (Fig.…”

“…25 In this paper, we will discuss the effect of hydrogenation on a GaSb/GaAs sample which previously exhibited OICD. 23 Post-growth hydrogenation is widely used to engineer the electronic and optical properties of many semiconductors, such as Si, 26 ZnSe, 27 GaAs, 28 and graphene. 29 But the effects of hydrogenation are complex: 30 Hydrogen behaves exclusively as a donor in ZnO 31 and as an acceptor in GaSb.…”

“…We propose that small dissimilarities in the homogeneity of the individual hydrogenated samples can lead to differences in the photo-induced charging of nanostructures at low temperatures. Indeed, fluctuations in the WL valence band edge were used to explain the localisation of carriers in the WL of this sample at temperatures below 20 K 23 and have also been used to explain similar behaviour in other systems. 41,42 This explains the range of emission energies seen in the 4.2 K data of the hydrogenated samples and, more specifically, indicates that the inverse relationship between change in emission energy and hydrogen dose seen for samples A and D in Fig.…”