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 ensembl...

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

New methods of modulation reflectance spectroscopy have been proposed for investigation of semiconductor heterostructures. Current-, radio-frequency- and microwave-modulated reflectance are proposed and applied for determination of band structure features and band diagram of heterostructures of high electron mobility transistors and laser diodes. New physical contributions to formation of spectra of the modulation reflectance of light with quantum energy near the fundamental absorption edge are discussed. The main contributing mechanism, free carriers' heating, is associated with alternating current, radio-frequency and microwave electric field, or with radiation with photon energy below bandgap energy of semiconductor layers. Novel techniques of local excitation of the heterostructures by radio-frequency field or by pump light with scanning by probe signal are also proposed.

Novel modulation reflectance spectroscopy of semiconductor heterostructures

The spectrum of electron-hole states in a GaAs/In0.5Ga0.5As quantum well with a width graded in the range from 1.1 to 3.6 nm is studied by photoreflectance spectroscopy. The energies of the size-quantization levels of electrons and holes are calculated taking into account the strain-induced changes in the band structures of the quantum well. It is shown that the best fit of the experimental data to the results of calculations is attained if the ratio between the offset of the conduction band and that of the valence band at the heterojunction is Q = ΔEc/ΔEv = 0.62/0.38. A photoreflectance signal is detected in the region of the shadow of modulating radiation beam at a spacing between the spots produced by probing and modulating radiation shorter than 6 mm.

Photoreflectance spectroscopy of electron-hole states in a graded-width GaAs/InGaAs/GaAs quantum well

The AlGaAs/GaAs laser heterostructure with the InGaAs quantum well is studied by the contactless photoreflection method upon local optical pumping. Unlike the conventional photoreflection method, regions on the sample surface illuminated by the probe and pump light are spatially separated. The method allows one to separate photoluminescence and photoreflection signals and to construct the three-dimensional energy band diagram of the laser structure.

Photoreflection from a locally optically pumped semiconductor laser structure

Photoreflectance (PR) spectroscopy is a powerful and non-destructive experimental technique to explore interband transitions of semiconductors. In most PR systems, the photon energy of the pumping beam is usually chosen to be higher than the bandgap energy of the sample. To the best of our knowledge, the highest energy of pumping laser in reported PR systems is 5.08 eV (244 nm), not yet in the vacuum ultraviolet (VUV) region. In this work, we report the design and construction of a PR system pumped by VUV laser of 7.0 eV (177.3 nm). At the same time, dual-modulated technique is applied and a dual channel lock-in-amplifier is integrated into the system for efficient PR measurement. The system’s performance is verified by the PR spectroscopy measurement of well-studied semiconductors, which testifies its ability to probe critical-point energies of the electronic band in semiconductors from ultraviolet to near-infrared spectral region.

Photoreflectance system based on vacuum ultraviolet laser at 177.3 nm

A new method of modulated light reflection ? below-bandgap photoreflection, is considered. Unlike the conventional photoreflection method, the proposed method uses optical pumping by photons of energy smaller than the bandgap of any layer of a semiconductor structure under study. Such pumping allows one to obtain the modulated reflection spectrum for all layers of the structure without excitation of photoluminescence. This method is especially promising for the study of wide-gap semiconductors. The results of the study of semiconductor structures used in modern high-power multimode semiconductor lasers are presented.

P Trubenko

Papers

Below-bandgap photoreflection spectroscopy of semiconductor laser structures

Photoreflection from a locally optically pumped semiconductor laser structure