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Abstract
The optical properties are investigated by spectroscopic characterizations for bilayer InGaAs/GaAs quantum dot (QD) structures consisting of a layer of surface quantum dots (SQDs) separated from a layer of buried quantum dots (BQDs) by different GaAs spacers with thicknesses of 7 nm, 10.5 nm and 70 nm. The coupling from the BQDs to SQDs leads to carrier transfer for the two samples with thin spacers, 7 nm and 10.5 nm, in which QD pairs are obtained while not for the 70 nm spacer sample. The carrier tunneling time is measured to be 0.145 ns and 0.275 ns from BQDs to SQD through the 7 nm and 10.5 nm spacers, respectively. A weak emission band can be observed at the wavelength of ∼ 960 nm, while the excitation intensity dependent PL and PLE spectra show that this is from the wetting layer (WL) of the SQDs. This WL is very important for carrier dynamics in bilayer structures of BQDs and SQDs, including for carrier generation, capture, relaxation, tunneling, and recombination. These results provide useful information for understanding the optical properties of InGaAs SQDs and for using such hybrid structures as building blocks for surface sensing devices.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Epitaxial, self-assembled In(Ga)As semiconductor quantum dots (QDs) in a Ga(Al)As matrix have been well investigated for almost thirty years due to their potential applications for versatile optoelectronics devices [1–6]. Usually, these devices consist of buried In(Ga)As QDs (BQDs) that are buried in a wider band gap Ga(Al)As material [7–9]. In contrast, surface QDs (SQDs) are grown on the wafer surface and are directly expose to air [10–11]. It is well known that the size and shape as well as the properties of the In(Ga)As QDs are quite different before and after burying with a Ga(Al)As capping layer [12–14]. In particular, without the capping layer, the optical and electronic performances of SQDs become very sensitive to the fluctuations of surface potential [15–16]. Such surface-sensitive properties of SQDs are predicted to play an important role in sensor applications and high sensitive humidity sensors based on self-assembled In(Ga)As SQDs [17–19]. As a result, research efforts to study and control In(Ga)As SQDs have greatly intensified.
In general, the optical and electronic response of In(Ga)As SQD devices are reported to be very weak due to the presence of surface states, which not only enable surface sensitivity, but also can act as non-radiative recombination centers to trap carriers. In order to improve the performance of SQD devices, hybrid stacking structures have been studied [20–21]. In such hybrid stacking structures, the surface sensitivity is measured by its impact on SQD emission or sub-surface transport signals and it strongly depends on inter-layer coupling or carrier transfer with the SQDs. Therefore, it is very important to understand how the SQDs are coupled, and how their carrier transfer is affected. Yet studies of the carrier transfer properties are indispensable when attempting to understand the performance of optoelectronic devices based on stacking SQDs on buried structures. In this work, we investigate the hybrid bilayer QD structures with SQDs stacked on one layer of In(Ga)As/GaAs BQDs through a GaAs spacer. The coupling from BQDs to SQDs with respect to the GaAs spacer thickness is studied using spectroscopic measurements. In particular, the features of the wetting layer for the SQDs and its influence on carrier dynamics in such vertically aligned QD-pair structures are evaluated.
2. Experiments and methods
The samples were grown on GaAs (001) semi-insulating substrates in a solid-source molecular beam epitaxy (MBE) reactor. As shown in Fig. 1, after desorption of the oxide layer at 600 °C and growth of a 100 nm GaAs buffer at 580 °C, 12 monolayers (MLs) of In0.35Ga0.65As were deposited to form the BQD layer [22]. The GaAs spacer was deposited next at a substrate temperature of 510 °C with a thickness of either 7 nm, 10.5 nm, or 70 nm. On top of the spacer, 14 MLs of In0.35Ga0.65As were deposited to form the SQDs at 510 °C. Finally, the samples were cooled down to 325 °C under dimeric arsenic (As2) flux and removed from the growth chamber for atomic force microscopy (AFM) and spectroscopic characterizations. Samples A, B, and C have GaAs spacer thicknesses of 7 nm, 10.5 nm, or 70 nm, respectively between the SQDs and BQDs. Additionally, two reference samples, D and E, were grown with structures similar to sample A, but having the 70nm GaAs capped BQDs only or SQDs only, respectively. Throughout all the growths, the As2 beam equivalent pressure was maintained at 5 × 10−6 Torr, while the QDs were grown with an InGaAs deposition rate of 0.5 ML/s.
3. Results and discussion
The morphologies of the InGaAs SQDs were studied by AFM in tapping mode at room temperature, as shown in Fig. 2(a)-(c), with the QD height distributions shown in Figs. 2(d)-(f) for sample A, B, and C, respectively. We measured that the SQDs for sample A have an areal density of 3.4×1010 cm−2 and an average height of 12.1 ± 2.4 nm. For sample B, the areal density is 3.4×1010 cm−2, and the average height is 7.7 ± 1.7 nm. For the sample C, the areal density is 3.5×1010 cm−2, and the average height is 7.2 ± 1.2 nm.
Fig. 2. AFM and TEM characterizations. The 1 µm ×1 µm AFM image for (a) sample A of 7 nm spacer, (b) sample B of 10.5 nm spacer, and (c) sample C of 70 nm spacer, respectively; (d), (e) and (f) are the column chart of QDs height distribution extracted from the AFM images of the three samples; (g), (h) and (i) are the cross-section TEM images to show the structures with BQDs and SQDs, the inset gives one example of the QD pair for sample A and sample B, respectively.
Clearly, three samples have almost the same SQD density, but the SQDs have a larger average height for the sample with a thinner GaAs spacer. This is likely due to the strain transfer from the BQDs through the thin GaAs spacer layer, resulting in the transition from two-dimensional to three-dimensional growth to happen earlier in the subsequent SQD layer growth for the sample with the thinner spacer layer. The strain transfer also causes vertically aligned QD pairs to form [23–24]. The vertical alignment is verified by the cross-section Transmission Electron Microscopy (TEM) images in Figs. 2(g)-(h) for samples A and B. For such QD pair structures we expect that there is strong electronic coupling and thereafter carrier tunneling from the BQD to the SQD in each pair [21]. Because, the maximum spacer thickness to result in vertically aligned QD pairs due to strain interaction is ∼10 nm, sample C, with the 70 nm GaAs spacer layer is neither structurally or electrically coupled between the BQDs and the SQDs, as is shown in Fig. 2(i). Here, sample C should have carrier dynamics different from samples A and B, which have coupled QD pair structures.
The carrier dynamics were carefully studied by spectroscopic measurements including photoluminescence (PL), photoluminescence excitation (PLE), and time-resolved photoluminescence (TRPL). For PL measurements, the samples were mounted into a closed-cycle, variable temperature cryostat with a variable temperature from 10 K to 300 K and the samples were excited by a continuous-wave laser operated at 532 nm. The PL signal is collected by a 20× objective lens and then sent to a 0.5-m spectrometer to be measured by a liquid nitrogen-cooled CCD detector array. For PLE and TRPL measurements, the samples were excited by a NKT super-continuum pulse laser (78 MHz, 20 ps). The TRPL spectra were measured by a PicoHarp-300 time-correlated-single-photon-counting (TCSPC) system. It is worth noting that for the PL, PLE, and TRPL measurements for these hybrid structures, the SQDs may absorb light emitted from the BQDs and subsequently emit light at the SQD energy. However, we assume this absorption is negligible as the effective volume and therefore absorption cross section of the SQDs is very small. As a result, it should not impact the results and conclusions here.
First, low temperature (10 K) PL spectra excited with a laser intensity of 3 W/cm2 are displayed in Figs. 3(a)-(d). In Fig. 3(d), one prominent PL peak is observed at ∼1000 nm that is from the BQDs emission for the reference sample D, while for the reference sample E one broad PL peak is found at ∼1225 nm from the SQDs emission. For sample C, two prominent PL peaks appear, with the long wavelength emission peak at ∼1242 nm attributed to the SQDs by comparison with sample E, and the short wavelength peak at ∼1000 nm is assigned to the BQDs by comparison with sample D. This is consistent with our previous results from similar QD pair structures [25–26]. It is surprising, then that sample B shows three PL peaks. After comparing with sample C and the reference samples, we attributed the peak at 1218 nm to the SQDs, the peak at 1028 nm to BQDs, while a third peak can be clearly observed at ∼956 nm. For sample A, the PL emission peak at 1213 nm is assigned to the SQD emission, while a weak discrete PL peak can be observed at ∼965 nm, similar to that of sample B. Guassian fitting reveals, additionally, a small peak at ∼1063nm, which is identified as that from the BQDs.
Fig. 3. The PL spectra measured at 10 K with a laser excitation intensity of 3 W/cm2 for (a) sample A, (b) sample B, (c) sample C, and (d) BQD reference sample D and SQD reference sample E.
Figure 3 reveals that the PL spectrum for the BQDs shifts toward longer wavelength as the BQDs approach the sample surface. This is likely due to the increased coupling between the BQDs and the surface states, the change of residual strain, or change in indium composition within the QDs [21–22]. Unfortunately, we cannot identify a similar shift for the SQD PL band as the GaAs spacer decreases, although the AFM indicates an increase in the SQD height from sample C to sample A. Therefore, we believe that the emission wavelength for the SQDs is not mainly determined by the SQD height, but by the residual strain of the SQDs together with the interaction between the SQDs and the surface states [23–24]. Finally, the PL spectra in Fig. 3 also show a weak PL peak at ∼ 960 nm for both coupled QD pair structures. It is likely from either an excited state or from the wetting layer emission of the QDs.
Next, in Fig. 4 the excitation intensity dependent PL spectra are measured at 10 K in order to investigate the origin of this weak PL signal. Looking at Fig. 4(c), clearly, there is a state filling behavior for the SQD reference sample E with this weak peak appearing in the spectra under strong excitation power. Therefore, this weak signal at ∼960 nm is attributed to the SQDs, and not BQDs. For sample A and sample B, as shown in Fig. 4(a) and 4(b), it can be found that this weak peak exists even at relative low excitation intensity. Furthermore, this PL peak is narrow (FWHM ∼ 25nm) and significantly separated (∼ 258 nm) from the SQDs emission band, supporting the claim that it comes from the wetting layer (WL), but not from the excited state emission of the SQDs. In comparison, sample A, B and also E show a broad PL band at about ∼1110 nm It is likely from the excited state emission of the SQDs, although we need to give more samples and measurements to verify this hypothesis. Additionally, the PL spectra measured as functions of temperature for samples A, B, and C enforce the assignment as the 960nm PL band quenches fast starting from a lower temperature than the BQD and SQD PL band in Figs. 4(d)-(f). This quenching behavior matches with typical characteristics for 1D quantum confined structures like QWs or WLs [25].
Fig. 4. PL spectra measured at 10 K with excitation intensity from I0=30 mW/cm2 to 105I0 for (a) sample A, (b) sample B, and (c) SQD reference sample E. The PL spectra measured with a laser excitation intensity of 30 W/cm2 with respect to temperature from 10 K to 295 K for (d) sample A, (e) sample B, and (f) sample C.
In order to further explore the WL features for SQDs, PLE spectra were measured at 10 K with the detection wavelength fixed at the maximum intensity position of the SQDs or BQDs PL band, as indicated in Fig. 5. All PLE spectra reveal an excitation resonance at ∼820 nm due to the contribution from GaAs matrix. For both sample C and sample D, the PLE spectrum from the BQDs reveals an absorption band between 835 nm and 865 nm, which is attributed to the WL absorption of the InGaAs BQDs [26–27]. For both sample C and sample E, the PLE spectrum from the SQDs shows a broad, pronounced absorption band between 830 nm and ∼970 nm. This broad excitation band in PLE spectrum is attributed to the SQD WL from our previous study [27]. However, very interestingly, the coupled QD-pair structures of sample A and sample B demonstrate very different PLE features from the uncouple QD layers in sample C. Their PLE spectra exhibit strong absorption features from 830 nm to ∼970 nm and can be divided into three subbands as shown in Fig. 5(a) and 5(b). In comparison with the PLE from the BQDs in samples C and E, we conclude that the first and the second subbands between 835nm and 880nm are correlated with the BQDs, due to the carriers transfer from the BQDs to the SQDs through the thin spacer. The third subband between 890 nm and 970 nm corresponds well with the SQD WL emission as discussed in Fig. 3, above. We attribute this resonant subband to the carrier transfer from the WL to the discrete energy states inside the SQDs.
Fig. 5. The PLE spectra were measured at 10 K for (a) sample A, (b) sample B, (c) sample C, and (d) BQD reference sample D as well as SQD reference sample E, while setting the detection at the SQD or BQD PL peak wavelength, in together with the PL spectra obtained with a laser excitation intensity of 3 mW; The TRPL spectra were measured at 10 K for (e) sample A, (f) sample B, (g) sample C, and for (h) SQDs for four samples; (i) and (j) give the schematic diagram to show the carrier dynamics, including carrier’s generation, relaxation, and tunneling.
Finally, TRPL were conducted at low temperature, 10 K, to investigate the coupling and carrier tunneling from the BQDs to the SQDs. The samples were excited by the NKT super-continuum pulse laser (3 mW, 78 MHz, 20 ps) operated at 532 nm, and the results are plotted in Figs. 5(e)-(h). We take the mono-exponential decay approximation to fit the experimental data and obtain the carrier lifetime for SQDs as well as for the BQDs of sample D, but we use a bi-exponential decay function to obtain the carrier lifetimes for the BQDs and the WL for samples A and B. The SQDs are determined to have lifetimes of 0.998 ns, 0.968 ns, 0.945 ns and 0.943 ns for samples A, B, C, and the reference sample E, respectively. In Fig. 5(h) a clear trend can be observed from the change of lifetime for these SQDs, although further measurements are needed to confirm it in consideration of the resolution (∼100ps) of the TRPL system. Figure 5(h) indicates that the carrier tunneling from BQDs to the SQDs can provide additional carriers for the SQDs, but does not significantly affect their lifetime. In contrast, we observed a significant impact on the carrier lifetime for the BQDs as they approach the SQDs. For samples C and D, we find lifetimes of 1.166 ns and 1.105 ns for BQDs, respectively. However, there is a significant reduction to 0.147 ns and 0.355 ns in samples A and B, respectively. This is the result of carrier tunneling from the BQDs to the SQDs in these two samples. Based on the TRPL data in Fig. 5, we estimated the tunneling time to be 0.145 ns for sample A and 0.275 ns for sample B by using a modified form of the semi-classical Wentzel–Kramers–Brillouin (WKB) approximation [28]. The estimated carrier tunneling time is at the same level as previously reported results for QD pair structures [28–29]. Another interesting result is that both coupled structures have a very short lifetime of ∼0.110 ns for the SQD WL, which is also limited by the resolution of our TRPL setup. Better temporal resolution would be required to understand the details further.
From the above spectroscopic results, we can get a clear picture of the carrier dynamics, including excitation, relaxation, tunneling, and recombination of the carriers in these SQD samples. While we use the 532 nm excitation laser for PL and TRPL, the laser can penetrate for ∼120 nm from sample surface. As indicated in Fig. 5(i) and 5(j), the photo-generated carriers mainly occur in the GaAs matrix, i.e., either in the GaAs buffer layer for the coupled samples in Fig. 5(i), or in both the thick GaAs spacer and the GaAs buffer for the uncoupled sample C in Fig. 5(j). For sample C, after carrier generation most of the carriers are captured by the WL and then relax into the QDs. Only a small portion of the carriers can be captured directly by the QDs as the absorption cross-section is much smaller for QDs than that for the WL. However, for sample A and sample B, due to the thin GaAs spacer, BQDs and SQDs form vertically-aligned QD pair structures. The photo-excited carriers are mainly generated in the GaAs buffer layers. In addition to the above mentioned carrier capture and relaxation channels, there is also carrier tunneling from BQDs to SQDs, and carrier tunneling from the BQD WL to the SQD WL [29–30]. The SQDs receive additional carriers that impact the SQD emission intensity, but not SQD PL lifetime. We also observed WL emission for SQDs in both samples A and B, indicating enhancement for carrier collection and carrier localization in the WL of the SQDs for these two samples. The WL plays a very important role for QD structures in the collection of carriers from the GaAs matrix, which then relax efficiently to the excitonic states of the QDs [31–33].
As we have mentioned, hybrid stacking structures are generally adapted for SQD device applications in order to improve their performance. In these stacked structures, the surface sensitivity is measured by conversion into transport signals such as sheet resistivity or photovoltaic voltage underneath the SQDs [18,34–35]. This conversion strongly depends on inter-layer coupling or carrier transfer between the SQDs and the layer below. It is therefore very important to understand how the SQDs are coupled, and how their carrier transfer is affected. As a good example, we have observed WL emission from SQDs in both samples A and B, indicating carrier localization in the WL of the SQDs for these two coupled structures. If we want to reduce the carrier localization effect in the WL and efficiently correlate the surface sensitivity with resistivity or voltage change of the layer below, a possible choice is to tailor the SQD WL into resonance with the BQDs discrete energy level in the coupled SQD structures. Such resonant alignment of energy levels will increase any response resulting from surface environment variation. Therefore, this work sheds light on the InGaAs SQDs as building blocks for surface sensing devices.
4. Conclusions
In conclusion, the hybrid, bi-layer structures of BQDs and SQDs with different spacer thicknesses are carefully investigated by spectroscopic characterizations. The experimental results of PL and TRPL at low temperature indicate that there is coupling and carrier transfer in the QD pair structures of the 7 nm and 10.5 nm spacers, while not in the 70 nm spacer sample. The carrier tunneling time is measured to be 0.145 ns and 0.275 ns from the BQDs to the SQDs via the 7 nm and 10.5 nm spacers, respectively. A weak emission band can be observed at ∼960 nm, while the excitation intensity dependent PL and PLE spectra show that this is attributed to the WL of the SQDs. The WL of the SQDs plays an important role for understanding the carrier behaviors in the hybrid structures of bi-layers of BQDs and SQDs. Carrier dynamics including generation, capture, relaxation, tunneling, and recombination processes are discussed for the hybrid structures of bi-layered QDs. These results provide very useful information for understanding the optical properties of InGaAs SQDs and for applying the SQD hybrid structures as building blocks for surface sensing devices.
Funding
National Science Foundation (EPSCoR OIA-1457888); Advanced Talents Incubation Program of the Hebei University (8012605); National Natural Science Foundation of China (61774053); Natural Science Foundation of Hebei Province (F2019201446).
Acknowledgments
The authors acknowledge the useful discussion with Dr. Dingkun Ren at University of California – Los Angeles.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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