On the modulation mechanisms in photoreflectance of an ensemble of self-assembled InAs∕GaAs quantum dots

Abstract

We present the investigation of the modulation mechanisms in photoreflectance (PR) spectroscopy of an ensemble of self-assembled semiconductor quantum dots (QDs). In order to distinguish between possible factors contributing to the total modulation efficiency of QD transitions, a photoreflectance excitation experiment has been performed on an InAs∕GaAs quantum dot structure grown by solid-source molecular beam epitaxy. It has been observed that the intensity of PR features related to QDs changes in a function of the wavelength of the pumping laser, tuned from above-GaAs band gap down to below wetting layer ground state transition. Based on this dependence we have shown that most of the QD PR signal intensity originates from the modulation of the built-in electric field caused by carriers photogenerated in GaAs layers. We also conclude that the modulation of QD transitions related to a possible modification of the dot properties due to filling them with carriers is negligible in PR experiment on an ensemble of dots. An additional confirmation of the PR results has been obtained by using contactless electroreflectance (CER), demonstrating that the line shape of PR and CER QD resonances is almost identical in both spectra. Thus, the QD transitions can be analyzed by using the standard low field line shape functional form applicable in any electromodulation spectroscopy.

Temperature-dependent modulated reflectance of InAs/InGaAs/GaAs quantum dots-in-a-well infrared photodetectors

“…A more detailed analysis of the PR spectra shows that, at high temperatures, the lineshape is dominated by the QCSE. 10 Whereas at low temperatures, the PR lineshape indicates the presence of state-filling mechanism, 23 and this is supported by the trend of suppressing the QD transitions. Alongside this modulation mechanism, the PR spectra contain useful information about optical transitions in DWELL structures, and this is of the main interest in this study.…”

“…Such a photomodulation mechanism was predicted and is widely accepted for undoped QDs. 10 On the other hand, the photomodulated signal which originates from the QD layers located outside the surface space-charge region can be interpreted in terms of statefilling mechanism. This mechanism accounts for the Pauli blocking of interband transitions in the QDs and Coulomb carrier-carrier interaction effects.…”

“…To date, these methods have been successfully applied to characterize a number of low-dimensional systems and nanostructures. [8][9][10][11][12][13][14][15][16][17] But despite the previous extensive studies of quantum heterostructures, the temperature-dependent modulated reflectance spectra of multi-layer DWELL photodetector structures with doped QDs have yet to be investigated extensively, and further work is still needed to clarify the underlying mechanisms.…”

Verification of band offsets and electron effective masses in GaAsN/GaAs quantum wells: Spectroscopic experiment versus 10-band k·p modeling

“…This gives the derivative-like spectra with enhanced sensitivity to the optical transitions occurring in the investigated structure, of even a very small oscillator strength (as the nominally parity forbidden transitions, or those in quantum dots). [28][29][30] Photoreflectance was measured by using a standard setup in the so called bright configuration 31 when the tungsten halogen lamp and the 660 nm line of a semiconductor laser served as sources of the probe and pump beams, respectively. The photomodulated changes in the intensity of the reflected beam were detected by an InGaAs thermoelectrically cooled photodiode combined with a 0.5 m focal length monochromator dispersing the reflected light.…”

Contactless electroreflectance spectroscopy of optical transitions in low dimensional semiconductor structures

“…It is worth noting that the same photon density applied to PL measurements of a QW allows to detect the fundamental transition only because of larger volume of QW in compari− son to the QD volume and faster carrier thermalization and recombination. CER spectroscopy due to their high sensiti− vity and absorption−like character is able to probe the each allowed optical transition in QDs as well as optical transi− tions in the wetting layer [48][49][50]. A few examples of the application of CER spectroscopy to study QD structures is presented below.…”