Enhanced photoluminescence of multilayer Ge quantum dots on Si(001) substrates by increased overgrowth temperature
© Liu et al.; licensee Springer. 2012
Received: 19 May 2012
Accepted: 11 July 2012
Published: 11 July 2012
Four-bilayer Ge quantum dots (QDs) with Si spacers were grown on Si(001) substrates by ultrahigh vacuum chemical vapor deposition. In three samples, all Ge QDs were grown at 520 °C, while Si spacers were grown at various temperatures (520 °C, 550 °C, and 580 °C). Enhancement and redshift of room temperature photoluminescence (PL) were observed from the samples in which Si spacers were grown at a higher temperature. The enhancement of PL is explained by higher effective electrons capturing in the larger size Ge QDs. Quantum confinement of the Ge QDs is responsible for the redshift of PL spectra. The Ge QDs’ size and content were investigated by atomic force microscopy and Raman scattering measurements.
KeywordsGe quantum dots Photoluminescence Ostwald ripening Overgrowth temperature
Si-based light emitter is one of the most important components for Si-based photonic integration. Although many progresses have been made for silicon-based light emitter in recent years [1–4], it is still a big challenge to overcome the inefficient band-to-band radiative recombination of silicon. With large band offset and strong quantum confinement, the self-assembled QDs are promising structure to enhance the optical characteristics . In the past two decades, the self-assembled Ge QDs on Si substrates, which are compatible with complementary metal-oxide semiconductor processes, have been widely studied for Si-based optoelectronic device applications [6, 7]. Unfortunately, the Ge QDs on Si can only provide a good confinement for the holes, which is hard to capture the electrons. Lacking of electrons for radiative recombination in Ge QDs limits its emission efficiency. A lot of efforts had been made to investigate luminescence of Ge QDs/Si(001) multilayer structure [8–10]. However, the radiative recombination in Ge QDs is still weak, even observed at low temperature [8, 9, 11]. How to increase the radiative recombination of Ge QDs is still a problem. Usually, carrier collection is a size-dependent behavior . Therefore, increasing Ge QDs’ probability of capturing the electrons by increasing Ge QDs’ size is a feasible way to improve the Ge QDs’ emission performance. However, many studies concentrated on lower temperature grown small-size Ge QDs [6, 13], which have stronger quantum confinement and lower Si-Ge interdiffusion.
In this work, we balance the advantages of small-size Ge QDs (strong quantum confinement and low Si-Ge interdiffusion) and the advantages of large-size Ge QDs (high electron capture probability). Ostwald ripening of Ge QDs induced by higher Si spaces’ overgrowth temperature was used to obtain large-size Ge QDs. The PL spectra obtained from the sample in which Si spacers were grown at higher temperature show a significant signal enhancement.
Three samples were grown by cold-wall UHV-CVD on Si(001) substrates with a resistivity of 2 to approximately 4 Ω cm, using pure disilane (Si2H6) and germane (GeH4). The Si substrates were first cleaned using an ex situ improved RCA wet-chemical cleaning recipe and then loaded into the pretreatment chamber. Before growing, the substrate was degassed at 300 °C for several hours in the pretreatment chamber and then was heated up to 920 °C for 5 min in the growth chamber with a background pressure lower than 1 × 10−7 Pa to deoxidize. Next, a 15-nm-thick Si layer was grown at 520 °C (sample A), 550 °C (sample B), and 580 °C (sample C), respectively, to obtain a flat starting surface. After a 240-s growth interruption to change the growth temperature, 5 monolayers (ML, 1 ML = 6.27 × 1014 Ge atom cm−2) of Ge was deposited at 520 °C with a rate of 0.04 Å/s. After a second growth interruption to change the growth temperature, the next three bilayer was grown in the same way. In order to study the morphology of Ge QDs, the top Ge QDs are not covered with Si cap. For all samples, the thickness of Ge QDs and Si spacer was 5 ML and 15 nm, respectively. All Si spacers were grown below 600 °C to prevent the Si-Ge interdiffusion [14, 15]. The reflection high-energy electron diffraction system was used to in situ monitor the growth of Ge QDs and Si spacers. The surface morphology of the samples was examined by the AFM, which was performed in contact mode. Scanning transmission electron microscopy (STEM) was used to study the Ge QDs growth behavior in the multilayer structure. PL (Raman) measurements were performed with LabRam HR 800 Raman instrumentation (HORIBA Jobin Yvon Inc., Paris, France) at room temperature, using a 488-nm-line Ar+ laser with the laser power of 15 (5) mW and an InGaAs photodetector within 1,150 to 1,600-nm range.
Results and discussion
Summary of the AFM statistic analysis of samples
Density (1010 m−2)
32 ± 5
2.5 ± 0.3
50 ± 8
4.4 ± 0.5
92 ± 10
8.1 ± 1
30 to 60
2.4 to 5
According to Fermi’s age equation  and Ge QDs’ sizes from AFM, we estimate the capture probability for electrons of Ge QDs in different samples. The probability in sample C is about 1.6 and 1.2 times higher than that of sample A and B, respectively. Further, the nonradiative recombination channels related to point defects that form at low growth temperature can decrease the PL intensity [11, 19]. The growth process of Ge QDs in which Si spacers were grown at higher temperature are similar to cyclic annealing. It may decrease the point defects and improve the crystal quality .
where Egap,Si is the bulk Si band gap; ΔEv, the valence band offset of Ge on Si which depended on the content of the Ge QDs; and ΔE(nmk), the confinement energy shift of the Ge QDs. Therefore, we calculate the average content and the energy shift of the Ge QDs in the samples.
where n, m, k = 1, 2,… are the quantum numbers for coordinates z, x and y, respectively. m* = 0.28 m0 is the effective mass of heavy holes of Ge (m0 is the mass of a free electron). According to Equation 3, the value of ΔE111 = 22 and 7 meV corresponding to samples B and C. In this way, the redshift of PL is 15 meV. It can be seen that the experimental data is in good agreement with the results of calculations based on the model used here. Therefore, the reason of redshift of PL spectra is the quantum confinement in Ge QDs.
Besides, we notice some interesting square nanopits’ morphology, with a depth of about 7 nm and contains small Ge QDs which are formed in the Si spacer layer (Figure 2A). Similar morphology was described in the literature . They believe that Si spacer grown at low temperature has higher strain. The Si atoms of high-strained Si mounds formed over the Ge QDs migrate to the surrounding area responsible for the nanopits.
In summary, we obtain large-size Ge QDs below 600 °C by Ostwald ripening of Ge QDs which is induced by higher Si spaces’ overgrowth temperature. Enhancement and redshift of room temperature PL were observed from the sample which have larger size Ge QDs. Large-size Ge QDs have more probability to capture the electrons for radiative recombination which is responsible for the PL intensity enhancement. Si spacers grown at higher temperature can improve the crystal quality. The redshift of PL peaks is attributed to the quantum confinement of Ge QDs.
This work was supported by the National Natural Science Foundation of China (grant nos. 61036003, 61176013, 60906035, and 61177038), the National High Technology Research and Development Program of China (grant no. 2011AA010302), and by the Tsinghua National Laboratory for Information Science and Technology (TNList) Cross-discipline Foundation.
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