Just as the alignment marks, the QD fabrication started with the deposition of 40nm
SiO2and the spinning of PMMA A4 at 6000rpm (Fig. 2.10 step 1) followed by the EBL
writing of an array of triangles. The written structures were developed by a 1 minute bath in MIBK:IPA 1:3 (Fig. 2.10 step 2). The PMMA mask was transferred to the SiO2
layer through five minutes of RIE 2.2.1.6 (Fig. 2.10 step 3). PMMA was then stripped away by five minutes in an ultrasound bath in acetone followed by a five minute O2
plasma (power: 50W , pressure: 60mTor r , Fig. 2.10 step 4). The inverted pyramids were etched in a process involving five steps (Fig. 2.10):
1. 30s water bath
2. 10s methanol bath,
3. 6s etching in a solution of Br:Me with 0.06% of brome prepared four hours earlier.
4. 10s rinsing in methanol
The SiO2layer was removed in a HF bath (Fig. 2.10 step 6). At this stage, the sample
takes the form of a GaAs substrate, patterned with an array of inverted pyramids in which the QD array can be grown. Before the growth, we followed a last deoxydation and cleaning procedure, crucial to ensure the cleanest possible surface and a high QD optical quality:
1. 5 minutes O2plasma (power: 50mW , pressure: 60mTor r )
2. 5 minutes in a buffered-HF bath.
3. 2 minutes rinsing in water (more recent results showed that a 10 minutes rinsing time improved significantly the growth quality).
4. Transport of the sample in a nitrogen filled glove to the growth chamber.
2.2.2.1 Metal organic chemical vapor deposition
Pyramidal QDs are then grown using the metal organic chemical vapor deposition technique (MOCVD). MOCVD is a broadly used epitaxial technique to produce mul- tilayer crystalline thin films. It can grow a wide variety of materials with monolayer precision, a very good uniformity over large areas and a low defect density. It is widely used both in research institutes and for industrial mass production of optoelec- tronic devices thanks to its uniform deposition capability permitting high throughput growth.
In MOCVD, metal-organic compounds combined with hydrides such as arsine (AsH3) are injected into a reactor through a carrier gas (here N2), where they are
decomposed in contact with the hot substrate, resulting in the growth of a pure crystalline layer as shown on Fig. 2.9.
The substrate is rotated to ensure the growth uniformity and needs to be kept at a temperature high enough to break the bond between the Ga or In adatom and its associated ethyl or methyl radicals. In contact with the surface, the precursors adsorb to the surface, where they flow over the surface and undergo a thermal decomposition until they reach their final adsorption (crystallization) site. The organic byproducts (H, methane, ethane, ethene) desorbs and are evacuated from the reactor.
Under normal growth conditions, the flow rate of group V components is larger than that of group III. This difference is measured by the V/III ratio. A fine control over the substrate temperature, the V/III ratio, the reactor pressure, the nature of precursors and the growth rate was used to achieve a high purity growth of semiconductor heterostructures.
In this thesis, the QDs were grown in an Aixtron 200 system operated by A. Rudra, using a combination of trimethylgallium (TMGa: G a(C H3)3), triethylgallium (T EG a:
(CH3)3-Ga AsH3
+
AsH3 CH4 N2 N2 H+
sample T=600KFigure 2.9 – Principle of MOCVD of GaAs
(TMIn: I n(C H3)3). The carrier gas was N2. The growths were performed at a tempera-
ture of 600◦C under a pressure of 20mB ar .
2.2.2.2 QD growth
The growth is carried out in the MOCVD chamber with the following steps (Fig. 2.10 step 7):
1. Stabilization of the sample at a thermocouple temperature of 600°C.
2. Deoxydation of the sample during 4 minutes at a thermocouple temperature of 600°C.
3. Buffer of GaAs: 5nm grown with TEGa and AsH3.
4. 0.8nm of I n0.25G a0.75As grown with TEGa, TMIn and AsH3.
5. 2nm of GaAs grown with TEGa and AsH3.
6. An 8nm thick cap of GaAs grown with TMGa and AsH3to planarize the surface.
During all these steps, the GaAs growth rates were stabilized in the 0.005 - 0.01nm/s range. All thicknesses and In mole fractions mentioned here are those that were measured on planar (100) GaAs. The growth rate and In mole fraction were calibrated by A. Rudra. (G a)I n As/G a As superlattice structures were grown. X-ray diffraction rocking curves were fitted with simulated curves. Actual thicknesses are much larger due to the larger growth rates inside the inverted pyramids.
The growth of pyramidal QDs results from the interplay between several phenom- ena: first there is a decomposition of precursors on the gallium terminated (111)A facets as opposed to the arsenic -terminated (111)B facets. After decomposition, adatoms undergo a surface diffusion over 200-300nm. As a consequence, the layers are grown predominantly on (111)A facets, inside the pyramids. Capillarity leads to a larger thickening of the layers at the pyramid edges where QWRs are created and at
1) SiO2 and PMMA deposition 2) EBL writing, MIBK developement 3) RIE etching 4) PMMA stripping 5) Br-Me wet etching 6) SiO2 removal (a) (b) (c)
Cap: 4nm GaAs (TMGa) Spacer: 2nm GaAs QD: 0.5nm In0.2Ga0.8As Buffer: 4.3nm GaAs
QD 7) Growth, side view
QWR (111B) (111A) 7) Growth, top (111B) (111A) 260nm 450nm GaAs SiO2 450nm
Figure 2.10 – Pyramidal QDs fabrication: (a) Step by step fabrication of the inverted pyramidal recesses; (b) SEM image of the SiO2mask of the array of triangles; (c) SEM
the apex of pyramids where the QD is formed [115] as shown on the sketch of Fig. 2.10 (step 7).
50nm
QD QWR barriers
Figure 2.11 – Transmission Electron Microscope (TEM) image of a pyramidal QD (TEM imaging realized on sample 5309, with a Tecnai Osiris FEI, with an electron beam accelerated at 200kV, courtesy of T. Lagrange from the Interdisciplinary Centre for Electron Microscopy, CIME, EPFL)
A Transmission Electron Microscope (TEM) image of a pyramidal QD is shown on Fig. 2.11. Contrary to the SEM technique, TEM requires thinning the sample down to a 100nm thickness with ion beam etching. The image was acquired by collecting the electrons transmitted through the sample. With this technique, the position of each individual atom can be measured.
The variations in contrast correspond to distortions of the crystal lattice. These distortions are caused by the presence of indium atoms because InAs has a smaller lattice parameter. Black areas are thus correlated to the presence of indium and can be used to image the QD and the surrounding QWRs. On Fig. 2.11 a black 20-30nm zone corresponding to the QD is observed at the apex of the pyramid, while the grey areas correspond to the QWR barriers. Note, however, that the thickness of the QD layer is only 0.564nm. The QDs studied in Chapters 3-4 and 5 were grown with a QD layer thickness of 0.75nm and are thus expected to be thicker along the growth direction. An analysis of these TEM measurements combined with k.p simulations and PL measurement may permit an in depth understanding of the confinement of electrons and holes in such QDs grown in small pyramids as was obtained for QDs grown in larger pyramids [133].