Borgström, M.

Bryllert, T.

Landi, S.

Pires, M.P.

Seifert, W.

Souza, P.L.

English

Università del Salento: ESE - Salento University Publishing

InAs quantum dots grown on InAlGaAs lattice matched to InPM. Borgstrom 2) ,  M. P. Pires 1) ,  T. Bryllert 2) ,  S. Landi 1) ,  W. Seifert 2) ,  P. L. Souza 1)1) CETUC/LabSem, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente 225, 22453, Rio de Janeiro, Brazil,  2) Solid State Physics, University of Lund, Box 118, S−221 00, Lund, SwedenThe self−assembled formation of three−dimensional islands is considered to be one of the most promising ways tomake quantum dots (QDs). Quantum dot lasers have been extensively studied due to the low threshold current, lowchirp, and temperature insensitive operation. Typical devices, realized with InAs QDs on GaAs substrate, operateat wavelengths as long as 1.2 µm with longer wavelengths achievable by the use of InGaAs capped InAs dots.There is great interest in being able to extend this wavelength to 1.55 µm, and beyond, for use in fiber optictelecommunication systems. Growth of InAs on InP allows for these wavelengths to be readily reached.For laser applications, it is desirable to achieve samples with a high density of QDs in a narrow size distribution,and it would be advantageous to have a large refractive index change between the active region and thesurrounding InP. For this purpose, we have grown InAs QDs on InAlGaAs, lattice matched to InP, in low−pressure(50 mBar) metal organic vapor phase epitaxy (MOVPE). We studied the effect of varying the Al content in thequaternary compound from 0 to 30% on dot formation. Atomic force microscopy (AFM) measurements onfreestanding QDs revealed that dots on material with higher Al content are smaller, and that the local densities onstep−bunches formed on the vicinal (001) surfaces increase.The samples were grown in MOVPE with phosphine (PH3 ), arsine (AsH3 ), trimethyl−indium (TMI),trimethyl−gallium (TMG), and trimethyl−aluminium (TMA) as precursors, and hydrogen as carrier gas. Theprocess was controlled by a flow and pressure balanced ventilation/run system. The total gas flow in the reactorcell was about 6000 cm3 /min. We used S doped (n+) InP (001) "epi−ready" wafers with a 2o misorientationtowards the nearest <011> direction for our experiments. A 150 nm thick InP buffer layer was grown, initially atan elevated growth temperature of 630o  C, after which the reactor was cooled down to 600o C. Then a 500 nmthick Inx Aly Ga1−x−y As layer was grown with a V/III ratio of 30. The growth rates for InP and Inx Aly Ga1−x−y Aswere 1.4 ML/s and 2.8 ML/s, respectively. The temperature was ramped down to 500o  C during a 5 min periodwithout growth, to keep the well−defined terrace structure before dot deposition. 2.5 ML InAs dot material wasdeposited on the surface at a growth rate of 0.5 ML/s and then the surface was annealed for 12 s under an arsineflow. At this point, the samples were either cooled down under arsine containing atmosphere, or capped with 50nm InP during which the temperature was ramped up to 600o  C. For capping the samples, the arsine was switchedoff and replaced by phosphine. Then TMI was reintroduced into the reactor, after a delay time of 1 s in order toreduce carry over effects. Photoluminescence (PL) of capped dot structures was measured by Fourier transforminfrared spectroscopy with the samples either immersed in liquid nitrogen, or in air at room temperature. Asexcitation source we used an Ar−ion laser with an excitation power of 50 mW. An InSb detector was used fordetection. For non−capped samples, the lattice matching of the quaternary layer was checked by a combination ofPL and X−ray diffraction measurements. The Al content in the Inx Aly Ga1−x−y As layers, lattice matched to InP,was varied between 0−30%. Surfaces were then studied with tapping−mode atomic force microscopy (AFM). Theheight distributions of the islands were evaluated by measuring the height of 324 islands from respective AFMimage, and from these data histograms were made. We take the height values as representative for the size of theislands. The dot densities were evaluated by counting the islands on 2 x 2 µm2  images.We start by looking at the effect on dot formation when Al is introduced into the layer onto which dot material isdeposited. Figure 1 and 2 shows 1 x 1 µm2  AFM images of InAs QDs on Inx Aly Ga1−x−y As, with 0 and 15 % Alcontent, respectively. On our samples, with increasing Al content, the local density of dots at the step bunchbecomes higher, and the dot sizes become smaller. This can be observed on the graph of figure 3.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.I.10M. Borgstrom et al Figure 4 depicts the change of the QD PL peak and the full width of half maximum (FWHM) of the PL spectrawith the Al concentration change. With increasing Al content, the emission energy of the dots increases for tworeasons: First, a higher density of dots (using the same amount of dot material) leads to a smaller dot size, due tomaterials distribution over the nuclei, and thereby to an increased quantum confinement of the dot. Second, thelarger the Al content in the barrier material, the larger its bandgap. Both effects influence the electronic levelstructure of the dots. Thus, the emission wavelength of the dots can, in principle, be tuned by adjusting the Alcontent in the barrier material. We find that these dots emit at very long wavelengths (2.1 µm) and that theemission wavelengths are blue−shifted with an increase of Al in the surface onto which the dots nucleate.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.I.10M. Borgstrom et al Analyzing the height histograms of the AFM images it is possible to measure the size homogeneity of the dots.The results indicate that the dots become more homogeneous with the insertion of Al into the layer immediatelybeneath the InAs. It is interesting to note that the PL measurements show the opposite trend, something that couldbe explained by the observed shift in dot size. A narrow apparent dot size only (as observed by AFM), will notcorrectly reflect the distribution of the electronic states within the dots since a change in dot size for a smaller dothas relatively larger impact on the change in quantization energy.Growth temperature, deposition rate, and V/III ratio are adjustable parameters that offer additional possibilities oftuning the electronic states of the dots, and consequently the energy of light emission in devices based on InAs/Inx Aly Ga1−x−y As quantum dot structures.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.I.10M. Borgstrom et al 

InAs quantum dots grown on InAlGaAs lattice matched to InP

ESE - Salento University Publishing

http://siba-ese.unile.it/index.php/ewmovpex/article/download/7027/6390

InAs quantum dots grown on InAlGaAs lattice matched to InP

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