Photoreflectance and photoreflectance-excitation spectroscopy of a GaAs/Ga0.67Al0.33<

Abstract

We have studied the pump-beam-wavelength (Xr) dependence of the 77-K photoreflectance (PR) of a GaAs/Gap 67Alp33As multiple-quantum-well structure. The PR signal as a function of kr is a new spectroscopy denoted as photoreflectance-excitation spectroscopy (PRE). Both PR and PRE exhibit features from the 1S exciton states as we11 as continuum transitions of the fundamental heavyand light-hole to conduction-band states, thus yielding exciton binding energies. The advantages of PRE are discussed. Optical techniques are extremely powerful tools for studying the fundamental nature of the quantum transitions in semiconductor quantum-well structures. These methods include absorption, ' photoconductivity, ' resonance Raman scattering, photoluminescence (PL), ' photoluminescence-excitation (PLE), ' ' and modulation spectroscopy. " Until recently, the predominant optical techniques were PL and PLE. Modulation spectroscopy, particularly the contactless method of photoreflectance (PR), ' ' is proving to be very useful since it produces sharp, derivativelike spectral features. Even at room temperature it is possible to observe all the allowed, confined transitions as well as symmetryforbidden features and unconfined states. ' ' ' In general, modulation spectroscopy can be performed over a wide range of temperatures (except for electrolyte electroreflectance"). Although features due to groundstate (1S) excitonic transitions have been reported in modulation spectroscopy, there has been, as yet, no direct evidence for excited exciton (or continuum) transitions in order to obtain exciton binding energies. Recently, low-temperature PLE studies have produced structures related to the 2S exciton (or continuum) states of the fundamental heavyand light-hole to conduction-band transitions, ' thus making it possible to deduce exciton binding energies. From a study of PR and photoreflectance-excitation spectroscopy (PRE) at 77 K from a GaAs/Gao 67Alo 33As multiple-quantum-well (MQW) structure we have found evidence for the continuum transitions. This has allowed us to determine, for the first time using modulation spectroscopy, the binding energies of the fundamental heavyand light-hole excitons. Also, in the process we have developed the new spectroscopy of PRE. In this article, we report a new spectroscopy based on an investigation of the pump-wavelength (X~) dependence of the 77-K PR from a GaAs/Ga067Alo 33As MQW. We denote the intensity of the PR spectrum as a function of k~( or the pump photon energy, hco~) as PRE. The PRE spectrum exhibits very strong resonances when Aco~c or-responds to the 1S exciton of 11H and 11L, where mnH(L) denote the rrtth conduction and nth valence subband of heavy-(H) or light-(L) hole character. The energies of these PRE peaks are found to be in excellent agreement with corresponding PR features. In addition, there are small high-energy features in PRE on each side of the fundamental exciton-creation peaks somewhat similar to recent PLE observations, ' which have been identified as the 2S (or continuum a...

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

“…The laser light was chopped by a mechanical chopper at a frequency of 275 Hz. PRE measurements 17 have been performed using the same setup, in which the detection is set at a constant energy ͑at QD ground state transition͒ and the wavelength of the pump beam is changed ͑light from tunable Ti: sapphire laser͒ from 800 to 1000 nm. In the case of CER measurements the sample was mounted in a capacitor with the top electrode made of a copper mesh and the bottom electrode made of a copper solid block.…”

“…This method has been already successfully applied to quantum well structures or bulklike materials a few times, and it has been proved that it probes the optical transitions in a similar way as PLE and can be used to determine the modulation mechanisms. 17,18 Here, we demonstrate its applicability for investigation of QD structures as well and we show that the main factor responsible for PR response from self-assembled QDs is the photomodulation of the surface electric field.…”

Stretched exponential profiles of photoluminescence decays related to localized states in InGaAsN/GaAs single-quantum wells

“…In order to investigate localized states, it is essential to evaluate the intrinsic band-edge energy. For that purpose, we adopted photoreflectance (PR) spectroscopy that is sensitive to the optical transitions at critical points of density of sates [7]. The nitrogen-content dependence of the band-edge PR signal indicates that the incorporation of nitrogen produces considerable disorders in the band-edge states, which results in a Stokes shift of the PL energy reflecting the localization properties.…”

Line Shape Analysis of Photoreflectance Excitation Spectra of GaN Films on 6H-SiC(0001)

“…Introduction Photoreflectance excitation spectroscopy (PRE) was first introduced by Shen et al in 1987 [1] for investigating an (Al,Ga)As/GaAs single quantum well at low temperature. Since then this method was applied only once to study the interface properties of ZnSe/GaAs [2].…”

“…We used an experimental PRE set-up, which is described in detail in Ref. [3], and is very similar to the set-up used by Shen et al [1]. For the generation of both the pump and the probe light, a combination of a xenon arc lamp and a monochromator is used.…”