Indium segregation during multilayer InAs/GaAs(0 0 1) quantum dot formation

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Abstract

Indium segregation during the growth of multilayer InAs/GaAs(0 0 1) quantum dot (QD) structures has been studied using reflection high-energy electron diffraction (RHEED) measurements of the critical coverage (θcrit) for second layer QD formation. A model bilayer structure was used in order to separate the effects of segregation and strain. The structure comprises an upper QD layer formed on top of a buried two-dimensional InAs layer. Growth temperature and the GaAs spacer layer thickness (S) are both found to have a significant effect on θcrit. Indium segregation during growth of the capping layer leads to the presence of a surface In adatom population prior to deposition of the second InAs layer. Segregation occurs for S up to 8 nm at 510 °C, this value being reduced by ∼50% at 450 °C.

Introduction

Low-dimensional semiconductor structures such as quantum dots (QDs) are attracting considerable interest due to their potential application in a wide range of optoelectronic devices. With lasers and other devices being produced at 1300 nm [1], the InAs/GaAs(0 0 1) QD system has been the most intensively studied, and longer wavelength QD emitters are now being demonstrated at ∼1500 nm [2]. Multilayer QD structures are required for these applications with the QD layers often separated by relatively small spacer layers (S∼10 nm). Several reports have shown that the growth of multilayer InAs/GaAs QDs can be complex with In/Ga intermixing, strain, segregation and spacer layer morphology all having a significant influence on the resulting structural and optical properties [3], [4], [5], [6], [7], [8].

The role of In segregation during multilayer InAs/GaAs QD growth has received surprisingly little attention, although segregation effects are known to be an important issue in the formation of InGaAs quantum wells [9]. One key problem is the difficulty in deconvoluting segregation and strain effects in multilayer QD growth. The strain fields from a buried QD layer extend through relatively large spacer layers (S<40 nm), the actual magnitude depending on factors such as the size of the initial QD layer [3], [4], [5]. By contrast, In segregation is likely to be limited to much lower values of S, typically <10 nm [10].

In this study we use a model bilayer structure to quantify In segregation during the growth of second layer InAs/GaAs QDs. The inhomogeneous effects of strain are eliminated by using a buried two-dimensional (2D) InAs layer (1.75 ML) instead of a QD layer. Reflection high-energy electron diffraction (RHEED) is used to monitor the 2D→3D growth mode transition (the critical thickness, θcrit) as a function of temperature (TS) and S. The results show that segregation during growth at 510 °C is significant for S<8 nm.

Section snippets

Experimental details

All samples were grown on epi-ready GaAs(0 0 1) substrates (n+, Si doped) in an MBE system (DCA Instruments) equipped with RHEED. The InAs growth rate (0.016 ML s−1) was carefully controlled by growing a single QD layer at the temperature of the (2×4)−c(4×4) reconstruction change (510 °C). The growth mode transition (monitored by the abrupt appearance of chevrons in the [1¯10] azimuth of the RHEED pattern) always occurred at 130 s±2 s (θcrit=2.08ML) under these conditions. More details of the growth

Results and discussion

Fig. 1 shows the influence of substrate temperature on θcrit for a single InAs/GaAs QD layer, and also for the bilayer structure in which θcrit was recorded for a QD layer formed on top of a buried 2D InAs layer of thickness 1.75 ML (∼0.5 nm). The GaAs spacer layer thickness was 4 nm. Temperature clearly has a pronounced effect in both cases and θcrit is reduced from 2.1 ML in the single layer and 1.9 ML in the bilayer structure to 1.8 and 1.75 ML, respectively, when TS is reduced from 510 to 450 °C.

Conclusions

Accurate measurement of the critical thickness during second layer InAs/GaAs QD formation has been used to provide quantitative information regarding the segregation of In through the GaAs spacer layer in a multilayer QD structure. Segregation during growth means that a surface population of In exists before the deposition of the second InAs layer, and the amount of InAs required for the 2D→3D growth mode change is therefore reduced. This effect only operates for GaAs spacer layers <8 nm for a

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