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Photon correlation

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The photon correlation experiment was performed in a Hanbury-Brown and Twiss setup [1] as illustrated in Fig.4.6.

The sample was excited with a continuous wave laser. The laser beam was focused on the sample surface by an immersion mirror objective [2] placed directly in the liquid helium inside the magnet bore. The system allows to focus the laser beam to a spot of approx. 1µm in diameter. The highest magnetic fields available in the system were of 8T.

The photoluminescence was sent through a beam splitter to two grating monochro- mators (spectral resolution of 100µeV each) set to pass a desired wavelength of light.

The filtered light was then detected on two avalanche photodiodes (APD’s). The electric output of the diodes was fed to the start and stop inputs of the correlation counting card Time Harp 200. Overall temporal setup resolution was determined as 1.5ns. Depending of the chosen wavelengths (equal or different) the auto- or cross- correlation between the same or different excitonic emission lines was mea- sured. The single correlation event is constituted by the detection of such a photon pair that one photon is detected on the ’start’ and the second one on the ’stop’

diode. The correlation measurement yields as the result a histogram plotting the number of correlated counts as a function of time separating the detection of the first and the second photon in the pair. The negative values on the time axis of the histogram refer to the events where the first photon in pair was detected on the

’stop’ and the second one on the ’start’ diode.

The experimental setup was available at the Institute of Solid State Physics at University of Warsaw, in Poland, in J.A.Gaj group with the collaboration of J.Suffczy´nski, A.Golnik and P.Kossacki.

Coincidence counting electronics

Cryostat with microscope and the sample

START STOP

Beamsplitters

CCD CCD Spectrometer

Avalanche photodiode

ττττ = t

START

– t

STOP Spectrometer

Avalanche photodiode

Immersion mirror objective

Laser

Lens Sample

Figure 4.6: Scheme of the photon correlation experimental setup.

Bibliography

[1] R. Hanbury-Brown and R. Q. Twiss, “The question of correlation between pho- tons in coherent light rays.,”Nature (London), vol. 178, p. 1447, 1956.

[2] J. Jasny and K. SepioÃl, “Single molecules observed by immersion mirror objec- tive. a novel method of finding the orientation of a radiating dipole.,” Chem.

Phys. Lett., vol. 273, p. 439, 1997.

Chapter 5

Review of the Optical

Properties of an Indirect

GaAs/AlAs DQW : Towards QD Emission

Dans ce chapitre, nous d´ecrivons les principales caract´eristiques des syst`emes

`

a double puits quantique GaAs/AlAs de type II en insistant sur les propri´et´es sp´ecifiques qui conf`erent `a ces mat´eriaux un int´eret particulier pour les ´etudes op- tiques. Le but de cette discussion est de clarifier le comportement surprenant de la macro-luminescence dans ces syst`emes, qui apparaˆıt sous forme de raies tr`es fines ce qui prouve qu’elle est due `a la formation de boˆıtes quantiques.

Nous discutons dans un premier temps les propri´et´es typiques des excitons indirects en deux dimensions, c’est-`a-dire leur temps de vie radiatif tr`es important (1 ms) et leur forte diffusion (jusqu’`a 100µm). Dans un deuxieme temps, nous d´ecrivons grˆace `a des exp´eriences de macro-photoluminescence, les propri´et´es optiques d’en- sembles de boˆıtes quantiques.

Investigations of a double quantum well system such as, for example, a GaAs/AlAs bilayer with a built-in type-II interface, started with the quest for the possibility of optical generation of cold, tunable-density, two-dimensional electron-hole gas. Lu- minescence spectra of such systems were known to exhibit a number of intriguing properties which were interpreted as the observation of a precursor of an exciton condensate [1]. Alternatively, they were assigned to effects of trapping of photoex- cited carriers in built-in quantum-dot like objects formed by potential fluctuations caused by interface roughness [2, 3].

The structures studied in this work show conventional spectra characteristic of their (intentional) 2D character, but unexpectedly also a number of sharp emission lines. The latter lines, their origin and properties, are the main object of this thesis.

As shown in this work, these lines can be attributed to quantum dots formed in the GaAs/AlAs type II bilayer. However, in contrast to previous studies on similar systems, the dots are not due to interface roughness but exhibit a strong 3D con-

finement.

In this chapter, the optical properties of the investigated structures, mostly as seen from macro-PL measurements, are discussed. The characteristic spectra re- lated to the 2D character, the appearance of sharp emission lines from ensembles of quantum dots and the diffusion process of photocreated carriers are presented [4, 5].

5.1 Indirect and pseudo - direct transitions

The intentional 2D character of the investigated structure is best recognized in macro-PL spectra measured under very low excitation power. Such spectra ob- tained for two different characteristic structures: indirect and pseudo-direct, are shown in Fig.5.1 a) and b), respectively. (See chapter 3.1 for layer sequences of those samples).

The spectrum of theindirect structure (Fig.5.1a) consists of a weak emis- sion peak which corresponds to the direct ΓGaAsGaAs recombination and four, more pronounced lines, associated with indirect XAlAsXYGaAs transitions: a zero- phonon line and its TA(X)GaAs, LO(X)GaAs, and LO(X)AlAs phonon replicas [3, 6, 7]. Such a spectrum is characteristic of type-II GaAs/AlAs indirect systems for which the lowest electronic state in the AlAs well is of XXY symmetry and the XZ

state is located a few meV higher.

This is in contrast to the type-II GaAs/AlAs pseudo-direct structure for which the lowest electronic state in the AlAs well is of XZ symmetry and the XXY

state is located a few meV higher. The dominant transition is therefore associated with the indirect XAlAsZGaAs transition (Fig.5.1b). A weak emission peak which corresponds to the direct ΓGaAsGaAsrecombination is also visible in the spectra.

Less pronounced phonon replicas of zero-phonon lines correspond to LA(X)AlAs

and LO(X)AlAs phonons.

The main difference in both spectra is the relative strength of the XAlAsXYGaAs and XAlAsZGaAs emission lines, in Fig.5.1a) and b), respectively. In both cases they correspond to the ground state emission. The selection rules which strictly forbid the XAlAsGaAs transitions, in the ideal case, are relaxed due to interface roughness. This relaxation of the selection rules is more efficient for the XAlAsZ - ΓGaAs transition as compared to XAlAsXYGaAs [7].

The indirect-type spectrum changes considerably under higher excitation in- tensities. As can be seen in Fig.5.2, for an excitation density of a few mW/cm2, the spectrum shows an ensemble of numerous sharp emission lines, with typical half-widths less than 0.1meV. These lines are observed over a very broad spectral range (1.56eV - 1.72eV). As it is proved in the following chapter, they correspond to ensembles of quantum dot emission. The very characteristic onset of photolu- minescence at 1.56meV is assigned to the situation when the dot is formed out of both GaAs and AlAs layers in the growth direction.

1.60 1.65 1.70 1.75 1.80 1.85 LOAlAs LAAlAs

λexc = 325nm T = 4.2K

Pseudo-direct

XZ− Γ

Γ − Γ

log (PL Intensity)

Energy (eV)

1.55 1.60 1.65 1.70 1.75 1.80

LOAlAs LOGaAs

TAAlAs

λexc = 325nm T = 4.2K

Indirect

XX Y − Γ Γ − Γ

PL Intensity

E nerg y (eV )

a)

b)

(J707)

(J709)

Figure 5.1: A representative macro-PL spectra of a) indirect (J707) and b) pseudo- direct (J709) structures taken with an excitation power of∼0.1mW/cm2.

1.48 1.52 1.56 1.60 1.64 1.68 1.72 1.76 0

500 1000 1500 2000 2500

-

bulk GaAs

PL Intensity (arb. units)

Energy (eV)

X XY

AlAs -

GaAs GaAs -

GaAs

LO(X) AlAS TA(X) GaAs

LO(X) GaAs

X Z

AlAs -

GaAs x 10

Figure 5.2: Photoluminescence spectrum of a 1mm size area of the indirect sample (J707) measured at T=4.2K with high power density.

The macro-photoluminescence spectra from the GaAs/AlAs type II bilayer are of two types, depending on whether the structure has an indirect or pseudo-direct band alignment. The indirect transitions are mostly associated with strong phonon replicas. Under high excitation power the number of sharp emission lines is visible in the energy region below the main quantum well transitions and spread over a very broad spectral range.

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