Single-dot Spectroscopy of GaAs Quantum Dots Fabricated by Filling of Self-assembled Nanoholes
© The Author(s) 2010
Received: 15 June 2010
Accepted: 1 July 2010
Published: 14 July 2010
We study the optical emission of single GaAs quantum dots (QDs). The QDs are fabricated by filling of nanoholes in AlGaAs and AlAs which are generated in a self-assembled fashion by local droplet etching with Al droplets. Using suitable process parameters, we create either uniform QDs in partially filled deep holes or QDs with very broad size distribution in completely filled shallow holes. Micro photoluminescence measurements of single QDs of both types establish sharp excitonic peaks. We measure a fine-structure splitting in the range of 22–40μeV and no dependence on QD size. Furthermore, we find a decrease in exciton–biexciton splitting with increasing QD size.
Semiconductor quantum dots (QDs) are very attractive for advanced applications for instance in the field of quantum computing  and as single photon sources [2, 3] for quantum cryptography. In this context, the detailed knowledge of the symmetries and level-structure inside these artificial atoms and its correlation to the QD structural properties is essential. A prominent example is the excitonic fine-structure splitting (FSS) which is a crucial parameter for the generation of entangled photons for cryptography [3, 4]. The FSS is related to the exchange interaction between electrons and holes in the strong QD confinement [5–7] and can be measured for instance by using micro photoluminescence (PL) spectroscopy [8, 9].
Most studies on the fine structure in single-dot PL have been performed on self-assembled InAs QDs [7, 9–11] grown on (001) GaAs. The FSS in InAs QDs is caused by at least three different effects: dominantly by the strain-induced piezoelectricity, furthermore, by the QD elongation, and by atomistic anisotropy . Unintentional intermixing with substrate material [12, 13] causes a poorly known QD composition and strain distribution , and, thus, significantly complicates studies of the relation between the FSS and the structural properties of the strain-induced QDs.
A more clear situation is found in the case of strain-free GaAs QDs, where piezoelectricity is expected to have negligible contribution. The excitonic fine structure of several types of GaAs QDs has been studied so far [8, 15–17]. First studies have been performed on so-called natural QDs . These are formed by quantum-well interface-fluctuations and have relatively small lateral quantization energies. Larger GaAs QDs have been fabricated with droplet epitaxy , or filling of nanoholes which are generated either by pre-patterning with lithography  or by in situ gas etching .
The samples discussed in the following are fabricated using solid-source MBE. After thermal oxide desorption, a 200-nm-thick Al0.35Ga0.65 As barrier layer was grown on a (001) GaAs substrate. We have fabricated two types of samples denoted in the following as type I and type II. For the type II samples, an additional 5-nm-thick AlAs layer was grown on top of the AlGaAs layer. Now, the As shutter and valve were closed and Al droplet formation was initiated by opening the Al shutter for 6 s. The temperature was T = 620°C for type I and T = 650°C for type II samples. The Al flux corresponded to a growth speed of 0.47 ML/s and droplet material was deposited onto the surface with coverage of 2.8 ML. After droplet deposition, a thermal annealing step of 180 s was applied. During this time, the etching process takes place, liquid etching residues are removed, and a wall surrounding the nanoholes is formed .
The nanoholes were filled with GaAs at a substrate temperature of T = 600°C in a pulsed mode by applying n P pulses with 0.5 s growth and 30 s pause, respectively. We present results of two type I samples, one with n P = 3 and one with n P = 7, which corresponds to GaAs layers with thickness d f = 0.34 nm and 0.79 nm, respectively. The additional type II sample was filled with d f = 0.57 nm resulting in uniform QDs with height of 7.6 nm . Finally, the QDs were capped by a 120-nm-thick Al0.35Ga0.65 As barrier.
Types of GaAs Quantum Dots
Figure 1a shows an atomic force microscopy (AFM) image of the AlGaAs surface of a typical type I sample after Al LDE. Clearly visible on this surface is the coexistence of shallow holes (up to 7 nm depth) and deep holes (deeper than 7 nm). We have already observed this effect earlier for Ga LDE on AlGaAs at low temperatures . On the other hand, type II samples have no such bimodal depth distribution and the resulting surfaces show only deep holes [24, 25]. Since the AlAs surfaces of type II samples oxidate very fast and, thus, are not accessible to AFM measurements under air, for illustration we provide a sample where Ga LDE has been performed on AlGaAs at T = 620°C. The corresponding surface (Fig. 1b) shows only deep holes, similar to the type II samples.
These surfaces act as a template for the QD formation by filling of the nanoholes with GaAs. Both types of samples with the different nanoholes result in different types of QDs. The deep nanoholes in type II samples are only partially filled (Fig. 1f) and yield highly uniform QDs with size precisely controlled by the filling level . Photoluminescence (PL) measurements of QD ensembles formed only in deep holes demonstrate extremely narrow linewidths of less then 10 meV . Furthermore, we assume that the deep-hole QDs are not in contact with the GaAs quantum well used for filling.
On the other hand, shallow holes in type I samples are completely filled (Fig. 1e) and the QD size is given by the hole depth with broad distribution . As a consequence, ensembles of shallow-hole QDs show a very broad optical emission band and no systematic influence of the filling level d f . This provides the interesting advantage that a large range of QD sizes can be studied on a single type I sample. In contrast to the deep-hole QDs, QDs formed in shallow holes are in direct contact with the GaAs quantum well used for filling.
Polarization-dependent measurements of the neutral exciton and biexciton peaks reveal a polarization angle α-dependent shift of the peak maxima that is related to the FSS. The deviation of the peak maxima E from the average peak energy E av is fitted by the expression ). Figures 3b, c show examples with an exciton FSS of 22 μeV and a bieciton splitting of 28 μeV. These data demonstrate the state of the art optical quality of LDE GaAs QDs being comparable to the established InAs dots.
Regarding the trend of our data in Fig. 4a, over a wide range of peak energies (QD sizes) the values of the FSS are nearly constant. In contrast, similar experiments on InAs QDs yield a strong decrease of the FSS from 500 μeV at E = 1.05 eV down to −80 μeV at E = 1.33 eV . The large FSS versus size effect in InAs QDs is probably related to the additional influence of strain in that material system. For strain-free droplet epitaxial (DE) GaAs QDs, also a decrease of the FSS with dot size is reported . However, the decrease is smaller than for the InAs QDs and the FSS values range from 90 μeV at E = 1.72 eV down to 18 μeV at 1.89 eV. The authors explain the influence of dot size on the FSS with a size-dependent dot shape. A reduction in the QD asymmetry is found when the size is reduced. The present LDE dots are in average larger than the DE dots. We associate our observation of an only negligible influence of dot size on FSS to the shape of the LDE QDs which here does not vary with size.
Finally, Fig. 4b illustrates that the separation between the exciton and biexciton peaks increases with increasing peak energy (decreasing QD size). The exciton binding energy is given by the electron-hole-binding state, whereas the biexciton-binding energy reflects in addition electron–electron and hole–hole interactions. This complex interplay depends sensitively on details of the QD morphology . QDs with low exciton–biexciton splitting are highly interesting since they represent a novel path for entangled photon generation using the time reordering scheme .
In conclusion, we have studied a novel type of strain-free GaAs quantum dots which are fabricated by filling of self-assembled nanoholes generated by local droplet etching. Using appropriate process conditions, the resulting QDs have either a very narrow or a broad size distribution which allows to study the single-dot excitonic fine structure over a wide range of QD sizes. The experiments establish sharp excitonic lines for both shallow-hole and deep-hole QDs. For shallow-hole QDs, the measurements reveal values of the fine-structure splitting of 22–40 μeV that do not significantly depend on QD size. In addition, we find a decrease in the exciton–biexciton separation with increasing dot size.
The authors would like to thank the “Deutsche Forschungsgemeinschaft” for financial support via GrK 1286 and HA 2042/6-1.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
- Knill E, Laflamme R, Milburn GJ: Nature. 2001, 409: 46. COI number [1:CAS:528:DC%2BD3MXkt1WrtQ%3D%3D]; Bibcode number [2001Natur.409...46K] COI number [1:CAS:528:DC%2BD3MXkt1WrtQ%3D%3D]; Bibcode number [2001Natur.409...46K] 10.1038/35051009View ArticleGoogle Scholar
- Michler P, Kiraz A, Becher C, Schoenfeld WV, Petroff PM, Zhang L, Hu E, Imamoglu A: Science. 2000, 290: 2282. COI number [1:CAS:528:DC%2BD3cXptFSnu7s%3D]; Bibcode number [2000Sci...290.2282M] COI number [1:CAS:528:DC%2BD3cXptFSnu7s%3D]; Bibcode number [2000Sci...290.2282M] 10.1126/science.290.5500.2282View ArticleGoogle Scholar
- Benson O, Santori C, Pelton M, Yamamoto Y: Phys. Rev. Lett.. 2000, 84: 2513. COI number [1:CAS:528:DC%2BD3cXhs1CmsLk%3D]; Bibcode number [2000PhRvL..84.2513B] COI number [1:CAS:528:DC%2BD3cXhs1CmsLk%3D]; Bibcode number [2000PhRvL..84.2513B] 10.1103/PhysRevLett.84.2513View ArticleGoogle Scholar
- Shields AJ: Nat. Photonics. 2007, 1: 215. COI number [1:CAS:528:DC%2BD2sXksFSmur8%3D]; Bibcode number [2007NaPho...1..215S] COI number [1:CAS:528:DC%2BD2sXksFSmur8%3D]; Bibcode number [2007NaPho...1..215S] 10.1038/nphoton.2007.46View ArticleGoogle Scholar
- Bayer M, Ortner G, Stern O, Kuther A, Gorbunov AA, Forchel A, Hawrylak P, Fafard S, Hinzer K, Reinecke TL, Walck SN, Reithmaier JP, Klopf F, Schäfer F: Phys. Rev. B. 2002, 65: 195315. Bibcode number [2002PhRvB..65s5315B] Bibcode number [2002PhRvB..65s5315B] 10.1103/PhysRevB.65.195315View ArticleGoogle Scholar
- Bester G, Nair S, Zunger A: Phys. Rev. B. 2003, 67: 161306(R). Bibcode number [2003PhRvB..67p1306B] Bibcode number [2003PhRvB..67p1306B] 10.1103/PhysRevB.67.161306View ArticleGoogle Scholar
- Seguin R, Schliwa A, Rodt S, Pötschke K, Pohl UW, Bimberg D: Phys. Rev. Lett.. 2005, 95: 257401. Bibcode number [2005PhRvL..95y7401S] Bibcode number [2005PhRvL..95y7401S] 10.1103/PhysRevLett.95.257402View ArticleGoogle Scholar
- Gammon D, Snow ES, Shanabrook BV, Katzer DS, Park D: Phys. Rev. Lett.. 1996, 76: 3005. COI number [1:CAS:528:DyaK28Xit1Ortbg%3D]; Bibcode number [1996PhRvL..76.3005G] COI number [1:CAS:528:DyaK28Xit1Ortbg%3D]; Bibcode number [1996PhRvL..76.3005G] 10.1103/PhysRevLett.76.3005View ArticleGoogle Scholar
- Bayer M, Kuther A, Forchel A, Gorbunov AA, Timofeev VB, Schäfer F, Reithmaier JP: Phys. Rev. Lett.. 1999, 82: 1748. COI number [1:CAS:528:DyaK1MXhtF2ltbk%3D]; Bibcode number [1999PhRvL..82.1748B] COI number [1:CAS:528:DyaK1MXhtF2ltbk%3D]; Bibcode number [1999PhRvL..82.1748B] 10.1103/PhysRevLett.82.1748View ArticleGoogle Scholar
- Finley JJ, Mowbray DJ, Skolnick MS, Ashmore AD, Baker C, Monte AFG, Hopkinson M: Phys. Rev. B. 2002, 66: 153316. Bibcode number [2002PhRvB..66o3316F] Bibcode number [2002PhRvB..66o3316F] 10.1103/PhysRevB.66.153316View ArticleGoogle Scholar
- Kowalik K, Krebs O, Lemaitre A, Laurent S, Senellart P, Voisin P, Gaj JA: Appl. Phys. Lett.. 2005, 86: 041907. Bibcode number [2005ApPhL..86d1907K] Bibcode number [2005ApPhL..86d1907K] 10.1063/1.1855409View ArticleGoogle Scholar
- Joyce PB, Krzyzewski TJ, Bell GR, Joyce BA, Jones TS: Phys. Rev. B. 1998, 58: R15981. COI number [1:CAS:528:DyaK1cXotVentLs%3D]; Bibcode number [1998PhRvB..5815981J] COI number [1:CAS:528:DyaK1cXotVentLs%3D]; Bibcode number [1998PhRvB..5815981J] 10.1103/PhysRevB.58.R15981View ArticleGoogle Scholar
- Heyn Ch: Phys. Rev. B. 2001, 64: 165306. Bibcode number [2001PhRvB..64p5306H] Bibcode number [2001PhRvB..64p5306H] 10.1103/PhysRevB.64.165306View ArticleGoogle Scholar
- Zhang K, Heyn Ch, Hansen W, Schmidt T, Falta J: Appl. Phys. Lett.. 2000, 77: 1295. COI number [1:CAS:528:DC%2BD3cXlvFKlu70%3D]; Bibcode number [2000ApPhL..77.1295Z] COI number [1:CAS:528:DC%2BD3cXlvFKlu70%3D]; Bibcode number [2000ApPhL..77.1295Z] 10.1063/1.1290152View ArticleGoogle Scholar
- Abbarchi M, Mastrandrea CA, Kuroda T, Mano T, Sakoda K, Koguchi N, Sanguinetti S, Vinattieri A, Gurioli M: Phys. Rev. B. 2008, 78: 125321. Bibcode number [2008PhRvB..78l5321A] Bibcode number [2008PhRvB..78l5321A] 10.1103/PhysRevB.78.125321View ArticleGoogle Scholar
- Kiravittaya S, Benyoucef M, Zapf-Gottwick R, Rastelli A, Schmidt OG: Appl. Phys. Lett.. 2006, 89: 233102. Bibcode number [2006ApPhL..89w3102K] Bibcode number [2006ApPhL..89w3102K] 10.1063/1.2399354View ArticleGoogle Scholar
- Wang L, Krapek V, Ding F, Horton F, Schliwa A, Bimberg D, Rastelli A, Schmidt OG: Phys. Rev. B. 2009, 80: 085309. Bibcode number [2009PhRvB..80h5309W] Bibcode number [2009PhRvB..80h5309W] 10.1103/PhysRevB.80.085309View ArticleGoogle Scholar
- Wang ZhM, Liang BL, Sablon KA, Salamo GJ: Appl. Phys. Lett.. 2007, 90: 113120. Bibcode number [2007ApPhL..90k3120W] Bibcode number [2007ApPhL..90k3120W] 10.1063/1.2713745View ArticleGoogle Scholar
- Stemmann A, Heyn Ch, Köppen T, Kipp T, Hansen W: Appl. Phys. Lett.. 2008, 93: 123108. Bibcode number [2008ApPhL..93l3108S] Bibcode number [2008ApPhL..93l3108S] 10.1063/1.2981517View ArticleGoogle Scholar
- Heyn Ch, Stemmann A, Hansen W: J. Cryst. Growth. 2009, 311: 1839. COI number [1:CAS:528:DC%2BD1MXktFSgsL0%3D]; Bibcode number [2009JCrGr.311.1839H] COI number [1:CAS:528:DC%2BD1MXktFSgsL0%3D]; Bibcode number [2009JCrGr.311.1839H] 10.1016/j.jcrysgro.2008.11.001View ArticleGoogle Scholar
- Heyn Ch, Stemmann A, Eiselt R, Hansen W: J. Appl. Phys.. 2009, 105: 054316. Bibcode number [2009JAP...105e4316H] Bibcode number [2009JAP...105e4316H] 10.1063/1.3079789View ArticleGoogle Scholar
- Heyn Ch, Stemmann A, Hansen W: Appl. Phys. Lett.. 2009, 95: 173110. Bibcode number [2009ApPhL..95q3110H] Bibcode number [2009ApPhL..95q3110H] 10.1063/1.3254216View ArticleGoogle Scholar
- Stemmann A, Heyn Ch, Hansen W: J. Appl. Phys.. 2009, 106: 064315. Bibcode number [2009JAP...106f4315S] Bibcode number [2009JAP...106f4315S] 10.1063/1.3225759View ArticleGoogle Scholar
- Heyn Ch, Stemmann A, Köppen T, Strelow Ch, Kipp T, Grave M, Mendach S, Hansen W: Nanoscale Res. Lett.. 2010, 5: 576. COI number [1:CAS:528:DC%2BC3cXkslels74%3D]; Bibcode number [2010NRL.....5..576H] COI number [1:CAS:528:DC%2BC3cXkslels74%3D]; Bibcode number [2010NRL.....5..576H] 10.1007/s11671-009-9507-3View ArticleGoogle Scholar
- Heyn Ch, Stemmann A, Köppen T, Strelow Ch, Kipp T, Mendach S, Hansen W: Appl. Phys. Lett.. 2009, 94: 183113. Bibcode number [2009ApPhL..94r3113H] Bibcode number [2009ApPhL..94r3113H] 10.1063/1.3133338View ArticleGoogle Scholar
- Avron JE, Bisker G, Gershoni D, Lindner NH, Meirom EA, Warburton RJ: Phys. Rev. Lett.. 2009, 103: 048902. Bibcode number [2009PhRvL.103d8902A] Bibcode number [2009PhRvL.103d8902A] 10.1103/PhysRevLett.103.048902View ArticleGoogle Scholar