Long-wavelength room-temperature luminescence from InAs/GaAs quantum dots with an optimized GaAsSbN capping layer
© Utrilla et al.; licensee Springer. 2014
Received: 15 November 2013
Accepted: 9 January 2014
Published: 17 January 2014
An extensive study on molecular beam epitaxy growth conditions of quaternary GaAsSbN as a capping layer (CL) for InAs/GaAs quantum dots (QD) was carried out. In particular, CL thickness, growth temperature, and growth rate were optimized. Problems related to the simultaneous presence of Sb and N, responsible for a significant degradation of photoluminescence (PL), are thereby solved allowing the achievement of room-temperature (RT) emission. A particularly strong improvement on the PL is obtained when the growth rate of the CL is increased. This is likely due to an improvement in the structural quality of the quaternary alloy that resulted from reduced strain and composition inhomogeneities. Nevertheless, a significant reduction of Sb and N incorporation was found when the growth rate was increased. Indeed, the incorporation of N is intrinsically limited to a maximum value of approximately 1.6% when the growth rate is at 2.0 ML s−1. Therefore, achieving RT emission and extending it somewhat beyond 1.3 μm were possible by means of a compromise among the growth conditions. This opens the possibility of exploiting the versatility on band structure engineering offered by this QD-CL structure in devices working at RT.
81.15.Hi (molecular beam epitaxy); 78.55.Cr (III-V semiconductors); 73.21.La (quantum dots)
KeywordsInAs Quantum dots GaAsSbN Capping layer Growth rate Luminescence
Tailoring the band structure and optical properties of the technologically mastered InAs/GaAs quantum dots (QDs) has been the focus of many efforts in the last decade. The use of a GaAsSb strain-reducing capping layer (CL) has been widely studied for that purpose [1–4]. The presence of Sb raises the valence band (VB) of GaAs  allowing the extending of QD emission over a wide wavelength range. Moreover, Sb suppresses the decomposition of GaAs-capped QDs  and has been shown to provide devices with improved characteristics [7–10]. Within this approach, the rise of the VB induced by the presence of Sb makes the band alignment structure become type II for contents of Sb above structures 14% to 16% [2–4]. A further step forward which has been recently proposed is the addition of N to the ternary GaAsSb CL. The incorporation of N in GaAs, according to the band anticrossing model , reduces only the conduction band (CB) of GaAs the same way Sb raises only its VB. Therefore, the use of the quaternary GaAsSbN CL on InAs/GaAs QDs allows tuning independently the electron and hole confinement potentials, as it has already been demonstrated . Moreover, this approach allows modifying the band alignment from type I to type II in both the CB and the VB. Thus, the versatility in band structure engineering makes this system a promising candidate for optoelectronic device applications of InAs/GaAs QDs requiring different band alignments. For instance, type-II InAs/GaAs QDs with a larger carrier lifetime could enhance the carrier extraction efficiency in photodetectors or QD solar cells, as proposed for the GaSb/GaAs system . Moreover, the strongly improved responsivity recently demonstrated in GaAsSb-capped InAs/GaAs QD infrared photodetectors (QDIPs)  could be spectrally tuned by controlling the N content in the quaternary CL. Light-emitting devices, such as laser diodes (LD), could also benefit from this approach. As an example, the difficulty to achieve long lasing wavelengths in GaAsSb-capped InAs/GaAs QD LDs due to the inhibition of type-II transitions  could be overcome by adding N, which allows reaching longer wavelengths while keeping the type-I band alignment.
However, significant structural changes of the capping layer due to the addition of N have been found to take place . Strain and compositional inhomogeneities are induced during the CL growth, yielding a degradation of the luminescence such that, as far as we know, no room-temperature (RT) emission has been reported to date using such a CL. Nevertheless, the resulting morphology of the CL could be modified through the growth conditions. Growth parameters such as growth temperature or growth rate could significantly influence the mass transport phenomena and composition modulation. Therefore, a need arises to find the optimal growth conditions in order to exploit the promising properties of this QD-CL system in optoelectronic applications. In this work, we study the effect of modifying the CL growth temperature, thickness, and growth rate on QD luminescence. RT photoluminescence (PL) is shown to be achievable through different growth conditions, and extending the emission to 1.3 μm is possible by means of the appropriate combination of the growth parameters.
All of the analyzed samples were grown by solid source molecular beam epitaxy on n+-doped GaAs (001) substrates. The QD layers were always grown under the same conditions by depositing 2.8 monolayers (ML) of InAs at 450°C and 0.04 ML s−1 on an intrinsic 0.5-μm-thick GaAs buffer layer. The GaAsSbN CL was grown under the reference conditions discussed below, modifying only one of the growth parameters for each series of samples. A 250-nm-thick GaAs layer was grown on top of the GaAsSbN capping. Sb was supplied from an effusion cell, while active N was generated from a radio-frequency (RF) plasma source with a 0.1-sccmflow of pure N2. The samples were characterized by PL measurements at 15 K and RT. A He-Ne laser was used as the excitation source, and low-temperature (LT) measurements were done using a closed-cycle He cryostat. The emitted light from the samples was dispersed by a 1-m spectrometer and detected with a liquid nitrogen-cooled Ge detector through standard lock-in techniques.
Results and discussion
First, it is necessary to establish the reference growth conditions for the GaAsSbN CL as a starting point from which one of the parameters will be modified in each series of samples. Thus, as reference conditions for the CL growth, those used in previous studies are considered , i.e., a 470°C growth temperature, a ratio of As4/Ga beam equivalent pressure of 32, a thickness of 5 nm, and a growth rate of 1 ML s−1. Regarding the N and Sb contents, a power of 140 W for the RF plasma source and a temperature of 335°C for the Sb effusion cell were chosen as reference source conditions. These conditions correspond in our system to nominal contents of 2.5% of N and 15% of Sb. In order to support the accuracy of these values, three samples consisting of single GaAsSb, GaAsN, and GaAsSbN quantum wells (QWs) were grown under the same reference conditions. For the GaAsSb QW sample, an emission peak of 1.242 eV at RT was found, corresponding to an Sb content of approximately 15% according to theoretical and experimental results for such a GaAsSb QW thickness . Regarding the GaAsN QW, a content of N around 2.3% can be estimated when comparing with similar reported QWs . The LT PL from the quaternary QW sample shifted from the GaAs gap energy a higher value (527 meV) than the addition of shifts in the GaAsSb (216 meV) and GaAsN (255 meV) QW samples. This is in agreement with studies reporting a facilitated incorporation of N by the presence of Sb [17, 18]. Indeed, the difference of 56 mV points to a higher N content corresponding to approximately 2.8%. For these N and Sb contents, the system will still be in the type-I band alignment region . Furthermore, since the Sb/N ratio is larger than 2.6 (the condition for lattice matching to GaAs) it can be assumed that the GaAsSbN layer grows under compressive strain on GaAs and will act as a strain-reducing CL.
Capping layer growth temperature
Capping layer thickness
Capping layer growth rate
The GaAsSbN CL
The GaAsSb and GaAsN CLs
In order to decouple the effect of the N concentration on the PL properties from that of the growth rate, a third sample was grown at 1 ML s−1 (D3, dashed black line in Figure 4a). The N RF plasma power was decreased until the PL peak energy matched that of D2, i.e., until the N concentration was the same. A comparison of the PL from samples D2 and D3 (equal N concentration and 2/1-ML s−1 growth rates, respectively) now clearly shows that the PL improvement at higher growth rates is not only due to a reduced N incorporation but also due to an improved structural quality of the CL.
In the case of the GaAsSb CL, a blueshift and a moderate PL enhancement is observed with increasing growth rate (Figure 4b), also indicative of a lower Sb incorporation. This behavior contradicts that reported for GaAsSb QWs grown at growth rates below 1 ML s−1, but no reports for higher growth rates are available in the literature. Like in the case of the GaAsN CL, a third sample was grown to decouple the effect of the growth rate and the Sb concentration. This sample (E3, dashed black line in Figure 4b) had a lower Sb content to match that of E2 (similar PL peak energy) and a 1-ML s−1 CL growth rate. Contrary to the case of GaAsN, increasing the growth rate while maintaining the Sb content constant seems to produce a minimum improvement of the PL (see the PLs from E2 and E3 in Figure 4b). Thus, we can conclude the sole increase of the growth rate (samples E1 and E2) leads to a decreased Sb content that is entirely responsible for the improved PL. Indeed, it has been shown that the PL of GaAsSb-capped InAs QDs is degraded for Sb contents above 12% , so reducing the initial content (approximately 15%) should result in an improved PL.
Quantification of the Sb/N content reduction
Comparison among the three CL materials
Extending the emission wavelength
A similar study was carried out also for a lower growth rate of 1.5 ML s−1. The three samples described in the previous paragraph, with the same parameters for the Sb and N sources, were reproduced with a CL growth rate of 1.5 ML s−1 (G1, G2 and G3, respectively). The PL spectra are shown in Figure 7b. The PL peak redshift in sample G2 is now 97 meV, as compared to 40 meV at 2 ML s−1. This means that a higher amount of Sb is now incorporated for the same Sb flux than at 2 ML s−1. Moreover, adding higher N contents is still possible at this lower growth rate, resulting in a long wavelength peak close to 1.4 μm at 15 K (sample G3).
This result shows that a strict limitation exists related to N incorporation in the GaAsSbN CL at high growth rates. N contents above approximately 1.6% cannot be incorporated into the lattice when growing at 2 ML s−1. This forces us to limit ourselves to lower growth rates in order to achieve long emission wavelengths.
Results at RT
The effect of modifying the growth conditions of the quaternary GaAsSbN CL on the PL properties of the InAs/GaAs QDs has been analyzed. Regarding growth temperature, 470°C was found to be the optimum value. A clear tendency was found when the CL thickness was modified, whereby the peak is red-shifted and the PL is degraded as the CL thickness increased. The best results were found when the CL growth rate was increased. The strong PL improvement at high growth rates up to 2 ML s−1 is shown to be specific for N-containing structures and likely related to a reduced composition modulation and plasma ion-induced defect density. Nevertheless, a strict limitation regarding N incorporation is found when the CL is grown at 2 ML s−1, which forces one to remain at lower values in order to reach longer wavelengths. RT PL is obtained through different growth conditions, some of them leading to 1.3-μm emission. The best luminescence properties were found for the highest CL growth rate, being still possible to extend the emission wavelength by adding higher Sb contents. The obtained outcomes from the growth optimization of this system could represent a starting point from which the versatility of the GaAsSbN CL might be exploited for real device applications.
This work has been supported by Comunidad de Madrid through project P2009/ESP-1503 and by the EU (COST ActionMP0805). Jose M Ulloa was supported by the Spanish MICINN through the ‘Ramón y Cajal’ program.
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