The density of the InAs QDs is too high for the application of a single-photon source if the deposition of InAs is equal to θc
adjusted by the transition of the RHEED pattern from reconstruction streaks to a spotty pattern. According to the kinetic model, the formation of QDs is divided into four steps: atom deposition on the growth surface, adatom diffusion over the surface, attachment and detachment, and 2D-3D growth transition
]. When the deposited InAs layer was below the critical thickness, as shown in Figure
a, both main and reconstruction streaky patterns disappeared as described in
]. Meanwhile, several spots at a fixed position were caused by the transmitted beam. When the spotty pattern appears (Figure
b), the transformation of the 2D-3D growth has occurred, and the deposition of InAs is defined as the critical thickness (θc
). For sample 9 (Table
), the critical thickness (θc
) of InAs was grown, but the micro-PL and Fourier-PL were envelop curves at 80 K (Figure
a,b), which demonstrated that the density of QDs was too high for single-photon source devices.
RHEED patterns of InAs deposition. (a) After deposition of InAs and before 3D growth and (b) when 2D-3D growth transition appears.
Micro-PL and Fourier spectrum. (a) Micro-PL of sample 9 at 80 K, (b) Fourier spectrum of sample 9 at 80 K, and (c) schematic illustration of sample 9.
By growing a reference sample to obtain the critical growth parameters, then increasing growth interruption and growth temperature, and decreasing deposition of InAs, a very low density of QDs can be realized
]. However, the repeatability is very low if the critical conditions were obtained from samples in different batches because of the accidental error and system error, such as differences caused by different molybdenum sample holder blocks, ambience in the growth chamber, measurement of growth rate and temperature, and so on. For our samples used in this method, the repeatability is less than 47%.
To resolve this problem, the critical growth parameters were obtained in situ
. A SQD layer was grown to obtain the θc
of InAs QDs and then annealed for the desorption of InAs. After growing a 50-nm GaAs barrier layer to separate the SQD layer, the InAs QD layer was grown to investigate the best condition of low density. Samples 1 to 6 (Table
) were grown to study the effects of the deposition of InAs. The deposition of the SQD layer was in the critical condition when a spotty pattern just appears. The growth temperature of the QD layer is 5°C higher than that of the SQD layer to achieve lower-density QDs and obtain a better micro-PL spectrum. The spotty pattern in the RHEED did not appear after the growth of the InAs QD layer, which implies that the actual deposition (total deposition − desorption) is slightly less than θc
a show a series of micro-PL of decreasing
from samples 1 to 6. We can find that the micro-PL spectra are multiple lines when
> 0 and become a sharp single line when
≤ 0. As shown in Figure
a,b, under the same pumping energy, micro-PL transfers from a single narrow peak to double narrow peaks, and the intensity of the spectra decreases sharply. Moreover, blue shift occurs when
< 0. This can be explained by the fact that QDs are not nucleated completely when deposition is less than the critical condition. In this case, the so-called quantum dots are similar to interface fluctuations. This can also be demonstrated in Figure
< 0, an additional wetting layer peak appears at 870 nm, and the intensity of the peak increases with the decrease of
. We can also find that the micro-PL is sharp and that the peak intensity is highest when
is equal to 0. Therefore, the best condition of low density is 5°C higher than the growth temperature of the SQD layer, and the deposition of InAs is the same as the SQD layer.
Another reason for the low repeatability is that the condition of the low-density InAs QD for single-photon source devices is strict, so a small deviation of deposition may affect the micro-PL seriously. The micro-PL spectra of samples 3 and 4 at 80 K are shown in Figure
c,d. The sharp single peak indicates that sample 4 has a good single-photon characteristic. The multiple peaks of sample 3 demonstrate that a slight change (0.025 ML) of deposition may determine the optical characteristic, so the critical growth parameters obtained from the reference sample ex situ make the repeatability low.
The annealing temperature of the SQD layer was also studied. Figure
a shows the TEM result of sample 10 annealed at 580°C. The green dot line stands at the position of the SQD layer, and the black line is the InAs QD layer. Comparing the InAs QD layer and the SQD layer, it is found that almost all the InAs in the SQD layer desorbed after annealing. However, the micro-PL shows other interesting phenomena in Figure
b. Firstly, when the annealing temperature decreases, the wavelength increases inversely. This indicates that the InAs SQD layer may be not completely desorbed after annealing. After growth of the 50-nm GaAs barrier layer, the interface roughness of the three samples is different. This results in the larger size of the QD and longer wavelength if the interface is much rougher for samples 7 and 8. Secondly, an additional exciton appears at the shorter wavelength when the annealing temperature of sample 7 decreases. A slight change of the pump laser beam position dramatically restrains the main peak and increases the neighboring multiple peak intensity. This phenomenon is attributed to multiple quantum dots, which demonstrates that the density increases when the annealing temperature decreases. When annealing temperature decreases to 580°C for sample 8, micro-PL becomes a broad emission spectrum. This trend confirms that the interface roughness becomes worse. Therefore, the annealing temperature should not be less than 610°C.
TEM and micro-PL. (a) TEM of sample 10. (b) Micro-PL of samples 4, 7, and 8 annealed respectively at 650°C, 630°C, and 620°C.