Coexistence of deep levels with optically active InAs quantum dots

We present direct experimental evidence of the coexistence of deep levels with intrinsic quantum confinement states in large, self-assembled InAs quantum dots embedded in a GaAs matrix. The InAs quantum dots show very good optical properties, as evidenced by the strong photoluminescence at room temperature at $\sim 1.3\mu \mathrm{m}$. Deep levels 160 and 484 meV below the GaAs conduction band edge have been identified at large reverse biases and high temperatures using deep level transient spectroscopy (DLTS) measurements. The reverse-bias dependence of the DLTS signal together with experimental results from the reference samples, containing thin InAs layers but no quantum dots, confirms that the deep levels coexist in the same layer as the InAs dots, and are most likely caused by the strain field during the lattice mismatched growth process. The densities of the deep levels in the structure are comparable to the density of the optically active quantum dots.

Figure 1

(a) Schematics of the InAs QD material and device structures. (b) PL signals at room temperature for both the QD sample and reference sample A with a 1.2 ML InAs wetting layer.

Figure 2

(a) $C\text{−}V$ characteristics and (b) the corresponding carrier concentration profiles taken at $T=120\mathrm{K}$ and 1 MHz for both the QD sample and reference sample A with a 1.2 ML InAs wetting layer.

Figure 3

(Color online) DLTS spectra for the QD sample (a) and Ref. A (b). The filling pulse bias is set to $-0.5\mathrm{V}$. The reverse bias is increased in steps of 0.5 V from $-2.5$ to $-5.0\mathrm{V}$ and from $-3.0$ to $-5.5\mathrm{V}$ for the QD sample and Ref. A, respectively. Referring to the DLTS peak positions, the two pronounced peaks observed in both samples are named as 115 K peak and 250 K peak, respectively. Traps 1 and 2 denote bulk defects in the QD sample. Arrow P in (a) shows the point at $T=35\mathrm{K}$ and $V_r=-3.0\mathrm{V}$ where the tunneling rate directly from the QD ground state to GaAs conduction band is approximately equal to the thermal excitation rate from the ground state to the excited state.

Figure 4

Arrhenius plots of the emission rates determined from the maximum positions of the DLTS spectra at different rate windows. The activation energies are determined from the slopes of a linear fit to the data (straight lines). The apparent electron capture cross sections are determined from the intercepts of the linear fit on the vertical axes.

Figure 5

Bias dependence of normalized DLTS peak height, $\Delta C∕\Delta C_{\max }$, at 115 K (squares) and 250 K (dots) in the DLTS spectra of the QD sample (a) and Ref. A (b). The filling pulse bias is fixed at $-0.5\mathrm{V}$. Because of the presence of traps 1 and 2, the relative height of the 250 K peak can be determined reliably only when the bias is below $-3.0\mathrm{V}$ in (a).