Optical and structural properties of InP quantum dots embedded in ${\left({\text{Al}}_x{\text{Ga}}_{1-x}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$

Within this work we present optical and structural properties of InP quantum dots embedded in ${\left({\text{Al}}_x{\text{Ga}}_{1-x}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$ barriers. Atomic force microscopy measurements show a mainly bimodal height distribution with aspect ratios (ratio of width to height) of about 10:1 and quantum dot heights of around 2 nm for the smaller quantum dot class (type A) and around 4 nm for the larger quantum dot class (type B). From ensemble-photoluminescence measurements we estimated thermal activation energies of up to 270 meV for the type-A quantum dots, resulting in a 300 times higher luminescence intensity at 200 K in comparison to our InP quantum dots in ${\text{Ga}}_{0.51}{\text{In}}_{0.49}\text{P}$ at the same emission wavelength. Photon statistic measurements clearly display that InP quantum dots in ${\left({\text{Al}}_{0.20}{\text{Ga}}_{0.80}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$ emit single photons up to 80 K, making them promising candidates for high-temperature single-photon emitters.

Figure 1

(Color online) Atomic force microscope scan results from areas of $1\times 1\mu {\text{m}}^2$ of standard 2.1 ML uncapped InP QDs deposited [(a)–(c)] at $710°\text{C}$ and [(d)–(f)] at $650°\text{C}$ on different aluminum-containing ${\left({\text{Al}}_x{\text{Ga}}_{1-x}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$ barriers. The aluminum concentration $x$ and the height scale $\Delta z$ are given in the corresponding insets.

Figure 2

(Color online) Integration-normalized height histograms of standard 2.1 ML InP QDs on ${\left({\text{Al}}_x{\text{Ga}}_{1-x}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$ barriers. For low aluminum contents and a growth temperature of $710°\text{C}$, the distribution is monomodal. At around $x=0.15$ the distribution gets bimodal, exhibiting a smaller QD class (type A) and a larger QD class (type B). For $650°\text{C}$ deposition temperature a bimodal distribution for the whole composition range was found. The solid curves are Gaussian fits, taking into account two occurrence emphases.

Figure 3

(Color online) Peak heights of the smaller type-A (circles) and the larger type-B (boxes) InP QDs on ${\left({\text{Al}}_x{\text{Ga}}_{1-x}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$ barriers. The QDs were manufactured at (a) 710 and (b) $650°\text{C}$ by depositing 2.1 ML of InP. In addition the overall QD density (stars) is provided for both growth temperatures.

Figure 4

(Color online) (a) Ensemble-PL spectra of standard 2.1 ML InP QDs in ${\left({\text{Al}}_x{\text{Ga}}_{1-x}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$ with different aluminum contents grown at $710°\text{C}$. The red lines correspond to the calculated band-gap energies of the $\Gamma$ and $X$ bands of the barrier. (b) Ensemble-PL spectra of standard 2.1 ML InP QDs in ${\left({\text{Al}}_{0.2}{\text{Ga}}_{0.8}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$ deposited at different growth temperatures. The topmost spectrum corresponds to a deposition of 4.2 ML of InP, showing the evolution of the type-B emission.

Figure 5

(Color online) (a) Confinement energies and (b) relative PL intensities per dot of standard 2.1 ML InP QDs in ${\left({\text{Al}}_x{\text{Ga}}_{1-x}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$ with different aluminum contents grown at $710°\text{C}$ (triangles) and at $650°\text{C}$ (boxes). The relative intensities per dot of the structures were scaled with respect to the normalized intensity per dot of the InP QDs in ${\text{Ga}}_{0.51}{\text{In}}_{0.49}\text{P}$ grown at $710°\text{C}$.

Figure 6

(Color online) Temperature-dependent ensemble-PL spectra of 4.2 ML InP QDs in ${\left({\text{Al}}_{0.5}{\text{Ga}}_{0.5}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$ grown at $650°\text{C}$. The dashed vertical lines are guides for the eyes and indicate the emission features from the two QD classes. At elevated temperatures the larger, lower-energy type-B QDs dominate the luminescence spectra.

Figure 7

(Color online) Temperature-dependent ratio of the integrated intensity of the two dot classes type A and type B (see Fig. 6). The solid curve is the fit using the model, while the inset shows the integrated PL intensity of both QD types and the corresponding Arrhenius fits which were applied separately. The two dashed vertical lines indicate the temperatures at which key changes in intensity ratio occur. The associated thermal energies $k_{B}T$ are also displayed.

Figure 8

(Color online) Temperature-dependent decay times obtained from type-A QDs (circles) and type-B QDs (triangles) of Fig. 6. The inset shows the monoexponential transients with their corresponding fits (solid curves) according to Ref. 30. For elevated temperatures the decay times decrease strongly due to additionally activated nonradiative decay channels.

Figure 9

(Color online) Relative PL intensities of type-A QDs of standard 2.1 ML InP in ${\text{Ga}}_{0.51}{\text{In}}_{0.49}\text{P}$ deposited at $710°\text{C}$ (stars) and 4.2 ML InP in ${\left({\text{Al}}_{0.2}{\text{Ga}}_{0.8}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$ (boxes) and in ${\left({\text{Al}}_{0.5}{\text{Ga}}_{0.5}\right)}_{0.51}{\text{In}}_{0.49}\text{P}$ (triangles) both grown at $650°\text{C}$. The intensities are normalized with respect to the low-temperature intensity (5 K) of the InP/GaInP QDs.

Figure 10

(Color online) (a) Temperature-dependent single-QD $\mu \text{-PL}$ measurements and second-order autocorrelation measurements performed (b) at 4 K and (c) at 80 K. The autocorrelation measurements were performed on the emission line indicated by the (red) arrow. Growth conditions: 4.2 ML of InP deposited at $650°\text{C}$.

Figure 11

(Color online) Temperature-dependent (a) excitonic transition energies and (b) FWHMs corresponding to the QD in Fig. 10. The solid curves are fits according to the models described in Eqs. (4, 5) (Refs. 35, 39, 40).