Raman scattering of InAs/AlAs quantum dot superlattices grown on (001) and (311)B GaAs surfaces
© Milekhin et al.; licensee Springer. 2012
Received: 18 July 2012
Accepted: 3 August 2012
Published: 23 August 2012
We present a comparative analysis of Raman scattering by acoustic and optical phonons in InAs/AlAs quantum dot superlattices grown on (001) and (311)B GaAs surfaces. Doublets of folded longitudinal acoustic phonons up to the fifth order were observed in the Raman spectra of (001)- and (311)B-oriented quantum dot superlattices measured in polarized scattering geometries. The energy positions of the folded acoustic phonons are well described by the elastic continuum model. Besides the acoustic phonons, the spectra display features related to confined transverse and longitudinal optical as well as interface phonons in quantum dots and spacer layers. Their frequency positions are discussed in terms of phonon confinement, elastic stress, and atomic intermixing.
Semiconductor nanostructures such as quantum dot superlattices (QD SLs) grown by molecular beam epitaxy (MBE) in the Stranski-Krastanov growth mode offer unique opportunity of engineering their electron and phonon spectra with the most appropriate properties for nanodevices. Among optical techniques, Raman spectroscopy is considered as the most informative method for determining phonon spectra of semiconductor nanostructures including QD SLs consisting of a variety of materials (Ge/Si, (In,Ga,Al)Sb/GaAs, In(As,Sb)/InP, InAs/(Al,Ga)As) [1–8].
The phonon spectrum of QD SLs presents a fundamental interest because of new physical phenomena (such as localization of optical phonons, interference of acoustic phonons, and spectrum renormalization of interface phonons in QD SLs). It also provides valuable information on structural parameters of QDs such as QD size and shape as well as strain and atomic intermixing in QD structures. Most of the published data related to phonons in QD SLs were obtained by Raman scattering experiments and refer mainly to the study of resonance effects on acoustic and optical phonons in Ge/Si QD SLs [1, 2], to establishing the phonon spectra in novel QD SLs [3, 4], and to investigation of the topology effects as well as strain and intermixing on the optical and interface phonons in InAs/AlGaAs QD SLs [5–7]. InAs/AlAs QDs are of special interest because they have electronic bandgap transitions in the visible spectral range (600 to 700 nm) making resonant Raman scattering experiments possible .
The majority of the reported Raman experiments were carried out for QD SLs grown along the principal crystallographic axis . For these structures, according to the Raman selection rules, only longitudinal optical (LO) phonons can be observed in backscattering from the planar surfaces of the SLs while rather sophisticated scattering geometries should be used for probing the transverse optical (TO) phonons . The QD SLs grown on high-index surfaces present significant interest for optoelectronic applications because they reveal intensive narrow linewidth bandgap photoluminescence. These structures are much less investigated by Raman spectroscopy despite Raman selection rules allow simultaneous observation of both LO and TO phonons in backscattering experiments from the planar surface. Here, we report the comparison of Raman spectra by acoustic and optical phonons in InAs/AlAs QD SL fabricated on (001) and (311)B GaAs surfaces in the same growth process.
InAs/AlAs QD SLs were grown by molecular beam epitaxy in a Riber 32P system simultaneously on (001)- and (113)B-oriented GaAs substrates utilizing Stranski-Krastanov growth mode. Samples are composed of 20 periods of InAs QD layers with a nominal thickness of 2.4 monolayers separated by AlAs spacer layers with thicknesses of 6, 8, 10, and 13 nm (samples A n , B n , C n , and D n , respectively). Here, the indexes n = 001 and 113 note the sample growth direction. The substrate temperature was 460°C during the growth of InAs QDs at an arsenic pressure of 8 × 10−6Torr. After the deposition of the nominal amount of island material, the growth was interrupted for 10 s for InAs QDs. The growth was monitored by reflection high-energy electron diffraction (RHEED). According to RHEED data, the transition from a two-dimensional to a three-dimensional growth mode (beginning of island formation) for all the samples occurs after the deposition of 1.8 monolayers of the QD material. After the dot formation, the first 5 nm of AlAs spacers were grown at the same temperature as the QDs (460°C). Then the temperature was increased to 610°C, and the rest of the AlAs spacer was deposited.
Raman spectra in acoustic and optical phonons were recorded at T = 20 and 300 K using a Dilor XY800 triple spectrometer (HORIBA JobinYvon Inc., Edison, NJ, USA). Different lines of Ar+ and Kr+ lasers (457.9 to 676.4 nm) were used for excitation. The spectra were measured in backscattering geometries parallel to the growth axes in both polarized (z(xx) − z, z′(x′x′) − z′) and depolarized configurations (z(yx) − z, and z′(y′x′) − z′) with x, y, z, x′, y′, z′ parallel to the , , , [−110], [33–2], and  directions, respectively. The spectral resolution was 2 cm−1 over the entire spectral range.
Results and discussion
According to , the In/Al atomic intermixing is relatively small for this growth temperature, and the Al content in InAs QDs is below 15%. As one can see from Figure 3, the behavior of LO phonons as a function of excitation energy in InAs QDs grown on (113)B-oriented GaAs surfaces (sample B113) is similar to that observed for InAs QDs on (001) GaAs (sample B001). However, the frequency position of the LO phonon in sample B113 is regularly lower (on 2 to 5 cm−1) than that for sample B001. This indicates enhanced In/Al atomic intermixing that leads to increasing Al content in InAs QDs on (113)B GaAs (up to 20%). Raman spectra of InAs QDs grown on (113)B-oriented GaAs measured in depolarized geometry reveal additional features near 230 cm−1 attributed TO phonons localized in InAs QDs. These frequency positions of features remain unchanged due to negligible dispersion of TO phonons in InAs [11, 13].
Raman spectra of InAs QDs measured in the polarized geometry with different excitation wavelengths show interesting behavior of interface phonons (Figure 4b). In this geometry, AlAs-like interface (IF) phonons localized in the vicinity of InAs QDs are observed. Their frequency positions are located between LO and TO mode frequencies and are shifted from 380 to 395 cm−1 with decreasing the excitation wavelength (increasing excitation energy) from 676.4 to 457.9 nm (from 1.83 to 2.71 eV). This behavior can be explained by Raman scattering of InAs QD array having not only different size but also different QD aspect ratio (QD height/base size). It was already shown  that the IF phonon frequencies depend on the aspect ratio in InAs/AlAs QDs having oblate shape. In the case of spherical QDs, InAs-like IF phonons have frequencies located in the middle between the frequencies of LO and TO phonons. With decreasing the aspect ratio, the IF phonon frequencies aspire to frequencies of LO phonons in InAs QDs. One can see from Figure 3b that IF phonons in sample B113 have higher frequencies at higher excitation energies than that in sample B001, thus, indicating lower QD aspect ratio for sample B001.
Thus, with increasing excitation energy, InAs QDs having smaller height and larger base size are selectively excited in the Raman process. This is accomplished by the decreasing frequencies of InAs phonons localized in InAs/AlAs QDs and increasing frequencies of IF AlAs-like phonons localized in the vicinity of InAs QDs having lower aspect ratio.
InAs/AlAs QD SLs were grown on (001)- and (113)B-oriented GaAs with excellent optical and crystalline quality confirmed by high-resolution transmission electron microscopy and Raman scattering by folded acoustical phonons observed up to the fifth order. The Raman spectra were interpreted within the elastic continuum model, and excellent agreement between the measured and calculated data was obtained. The dependences of optical and interface modes in the QD structure on the excitation energies were explained in terms of size and shape selective Raman scattering.
AM is a senior researcher and Dr. of Science, NY is a senior researcher and Ph.D. student, AT is a doctor and senior researcher, and DD is a researcher at the Institute of Semiconductor Physics, Novosibirsk, Russia. ES is a Ph.D. student and DRTZ is a professor at the Semiconductor Physics, Chemnitz University of Technology, Chemnitz, Germany.
The work was supported by the following projects: DFG project ZA146/22-1, RFBR-DFG 11-02-91348, RFBR 11-02-90427-Ukr_at, the Program of the Presidium of the Russian Academy of Sciences (project 24.27), and IRTG (grant GRK 1215).
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