2.3.3.1 Advantages of TEGa
One major improvement in the QD fabrication realized during this thesis is the use of TEGa as the precursor for the Ga atoms [111]. TEGa has ethyl radicals, which has deep impacts on the growth mechanism, helpful for the growth of high quality QDs:
1. TEGa is advantageous first because during TMGa growth, the carbon byproducts of the reactions leading to GaAs growth can be incorporated in the GaAs lattice
leading to doping impurities. This effect is largely mitigated by the use of ethyl radicals, which thanks to their larger size are less likely to be incorporated [136, 137]. High purity growth is a critical parameter to achieve narrow excitonic lines. Indeed, the spectral width of single excitonic lines at 10K is broadened by the pure dephasing arising as a consequence of the dynamic Stark effect caused by flickering charges, which are captured by impurities.
2. Besides, TEGa can decompose at lower temperature [138] since its larger radicals have weaker bonds. This is advantageous because the integration of QDs in a 250nm thick photonic crystal membrane limits the maximum pyramid height below 300nm, which imposes an upper limit to the growth temperature around 620°C to keep a sharp profile of the pyramid and take full advantage of the capillarity effect. On the contrary, the optimal growth temperature for TMGa based growth of GaAs on (100) oriented substrate lies around 650°C [139]. There is a tradeoff between growing high quality material and keeping a sharp profile of the pyramids when using TMGa, which is improved by the use of TEGa.
3. The growth rate of TMGa at T=650°C range varies rapidly with temperature, while it is quite stable for TEGa because of the lower temperature decomposition of TEGa [138]. Thus, TEGa is likely to improve the reproducibility of the growth. Secondly, TMGa decomposes on (111)A, but only very slowly on (111)B [140]. As a consequence, at the border of the pyramids array, the flux of precursor is higher. This leads to larger nanostructure and more confined, red-shifted QDs at the edges. On the contrary, TEGa decomposes on both (111)A and (111)B oriented surfaces [141] which may help to reduce this edge effect.
2.3.3.2 TEGa versus TMGa
The results of two growths using TEGa and TMGa are compared in Fig. 2.19(a-b), with the layer sequence indicated on the right side of each sketch. One notable difference between the two precursors is caused by growth on the (111)B facet, which is negligible with TMGa but not with TEGa.
These two growths were carried out on a pattern design with a 2.4mm square array of inverted pyramids similar to the one presented previously. The array was a triangular lattice of inverted pyramids with a 450nm pitch and a pyramid size of 250nm. PL spectra measured for different positions of the excitation spot across ensembles of pyramidal QDs grown with TMGa and TEGa are shown in Fig. 2.19(c-d). The position of the excitation spot with respect to the edge of the QD square array was scanned over the QD array as is indicated for each spectrum. The exact parameters of each growths are given in Appendix B.
Cap: 4nm GaAs (TMGa) Spacer: 2nm GaAs QW: 0.5nm In0.2Ga0.8As Buffer: 4.3nm GaAs QD (b) TEGa 0 1 2 1.3 1.34 1.38 Energy [eV]
Position from edge [mm]
954 939 925 912 898 886 :DYHOHQJWKѤ>QP@ 69meV 37meV TMGa TEGa P=20W T=10K (e) (C2H5)3-Ga: Cap: 5nm GaAs QW: 0.75nm In0.2Ga0.8As Buffer: 5nm GaAs QD
(a) TMGa (CH3)3-Ga:Ga:
1.3 1.35 1.4 1.45 1.5 0 mm 0.24 mm 0.48 mm 0.72 mm 0.96 mm 1.2 mm 1.44 mm 1.68 mm 1.92 mm 2.16 mm 2.4 mm Energy [eV] 954 918 886 855 827 Wavelength [nm] 1.3 1.35 1.4 1.45 1.5 0 mm 0.24 mm 0.48 mm 0.72 mm 0.96 mm 1.2 mm 1.44 mm 1.68 mm 1.92 mm 2.16 mm 2.4 mm Energy [eV] 954 918 886 855 827 Wavelength [nm] Doping impurity Doping impurity s states excited states QWR excited states (c) (d)
Figure 2.19 – (a) Side view sketch of a TMGa QD growth; (b) Side view sketch of a TEGa QD growth; (c-d) Photoluminescence spectra scan across an ensemble of pyramidal patterns grown with (c) TMGa and (d) TEGa (Excitation power: P= 20μW ,
T= 10K ); (e) Central emission energy of s-states transitions of the QD ensembles as a
function of the excitation spot distance measured from the edge of the QD array. The sample grown with TMGa precursors was number 5199; The sample grown with TEGa precursors was number 5255; (c-d-e) Reprinted from Publication [111], Copyright (2014), with permission from Elsevier.
The typical spectral signatures of QDs obtained with TMGa growth can be seen in Fig. 2.19(c). Emission from the QD s and excited states is observed, as well as the spectral signature of the acceptors incorporated in the bulk GaAs. The sample grown with TEGa, presented in Fig. 2.19(d), shows spectral features strikingly similar to those of the sample grown with TMGa: distinct QD s-state and excited states emission as well as a pronounced impurity-related emission. The energy shift between the
s-transition lines of both samples originates from the different precursors, which leads to a different InGaAs QDs thickness and composition. However, it is notable that the QD emission of the TEGa sample exhibits sharp lines up to 960nm. Indeed, the growth of high quality QDs beyond 900nm using TMGa had been a major challenge. A possible cause for this effect may be the higher purity of TEGa grown GaAs material at such low temperatures.
Fig. 2.19(e) displays the mean emission energy of the QD s-state transition as a function of the excitation laser beam position across the QD array for samples grown with TMGa and TEGa precursors. For both samples, the pyramidal arrays used for growing the QDs had similar pyramid sizes. The use of TEGa precursors reduced by 46% the spectral redshift observed at the edge of the QD array with respect to TMGa samples. This reduction of edge effects is consistent with the more efficient decomposition of TEGa on the {111}B wafer surface, causing a smaller gradient of precursors near the edge [141]. The two samples exhibited similar emission intensities with a maximum of excitonic emission peaks up to≈ 300cts/s.
Prior trials to grow TEGa QDs were realized at 650°C. However, no thin lines were observed probably as a consequence of the temperature related planarization of pyra- mids. On the contrary growths around 600°C were shown to lead to high quality, much more reproducible growths of QD ensembles. A large number of QD growths demon- strated that QD ensembles with narrow lines are obtained over a large parameter space (with varying indium concentration, buffer and QD thickness) opening the possibility to tailor the electron and hole confinement for specific applications, and eventually to optimize the electron hole overlap. However, when the size or indium content were tuned to reach QD emission beyond 1000nm, the quality of QD ensem- bles degraded notably and no narrow linewidths (i.e., <100μeV) were observed. This limitation suggests a transition to a different growth mode for high In-content, maybe dominated by random nucleation similar to the Stranski-Krastanov method.