Anomalous temperature and excitation power dependence of cathodoluminescence from $\mathrm{InAs}$ quantum dots

Self-assembled $\mathrm{InAs}$ quantum dots (QDs) in $\mathrm{GaAs}$ layers were studied with a cathodoluminescense (CL) detection system combined with a transmission electron microscope. Three distinct peaks were observed to appear in the CL spectrum collected from a $1\mu {\mathrm{m}}^2$ region. The excitation power dependence of the CL spectra and monochromatic CL image observations identified those peaks that are the emissions associated with the ground state and excited state of the QDs in different size groups. Anomalous temperature dependence of those QD emission peaks was observed in the temperature range from 20 to around $100\mathrm{K}$, where the emission intensities increase with temperature. Steady-state rate equations for the recombination processes of holes and excitons are proposed with introduction of a potential barrier at the interface between the $\mathrm{GaAs}$ layer and the wetting layer (WL). This model can explain the temperature dependence of the emission intensities from the QDs and WL in a wide temperature range.

Figure 1

TEM dark field image of a plan view sample of $\mathrm{InAs}∕\mathrm{GaAs}$.

Figure 2

CL spectra acquired (a) and (b) from scanning areas of (a) $10\mu \mathrm{m}\times 10\mu \mathrm{m}$ and (b) $1\mu \mathrm{m}\times 1\mu \mathrm{m}$, and (c) with stationary beam illumination. Accelerating voltage is $80\mathrm{kV}$, electron beam current $0.9\mathrm{nA}$, and a sample temperature $40\mathrm{K}$.

Figure 3

Excitation power dependence of CL spectra taken for various electron beam currents $I_b$ ranging from 0.14 to $5.01\mathrm{nA}$. Accelerating voltage is $80\mathrm{kV}$ and sample temperature $80\mathrm{K}$.

Figure 4

CL intensities of the $\mathrm{P}1$, $\mathrm{P}2$, and $\mathrm{P}3$ peaks as a function of beam current.

Figure 5

CL spectra taken at various temperatures ranging from 20 to $220\mathrm{K}$. Accelerating voltage is $80\mathrm{kV}$ and electron beam current is $0.9\mathrm{nA}$.

Figure 6

Photon energies of the $\mathrm{P}1$, $\mathrm{P}2$, $\mathrm{P}3$, and WL peaks vs temperature. The peak energy variations expected from the temperature dependence of the $\mathrm{InAs}$ band gap (Varshni’s law) are shown by dashed lines.

Figure 7

Arrhenius plot for the CL intensities of the $\mathrm{P}1$, $\mathrm{P}2$, $\mathrm{P}3$, and WL peaks.

Figure 8

Monochromatic CL images of the $\mathrm{InAs}∕\mathrm{GaAs}$ thin sample taken at $40\mathrm{K}$ using photon energies of (a) $1.144\mathrm{eV}$ $\left(\mathrm{P}1\right)$, (b) $1.197\mathrm{eV}$ $\left(\mathrm{P}2\right)$, (c) $1.247\mathrm{eV}$ $\left(\mathrm{P}3\right)$, (d) $1.449\mathrm{eV}$ (WL), (e) $1.181\mathrm{eV}$, and (f) $1.211\mathrm{eV}$. The scan area is $10\mu \mathrm{m}\times 10\mu \mathrm{m}$.

Figure 9

Energy diagrams of the $\mathrm{InAs}$ $\mathrm{QDs}–\mathrm{GaAs}$ system for the QD groups with large and small sizes. A potential barrier of energy $E_a$ for heavy holes (HH) is formed at the interface between the $\mathrm{GaAs}$ layer and the WL.

Figure 10

Schematic of the rate equation model. Carrier density is indicated by $n$ and $p$ for electrons and holes in $\mathrm{GaAs}$, respectively, and $n_w$ and $p_w$ in the WL. Exciton density is indicated by $n_{WX}$ in the WL and $n_{Xi}$ in the $i$-th QD.