Exciton ionization induced by intersubband absorption in nonpolar ZnO-ZnMgO quantum wells at room temperature

We present a systematic investigation of intersubband transitions in nonintentionally doped $m\text{−}\mathrm{plane}$ ZnO/ZnMgO quantum wells (QWs). The investigation is performed using photoinduced absorption spectroscopy at room temperature under optical pumping by a UV laser to generate electron-hole pairs. All samples exhibit TM-polarized intersubbandlike absorption resonances. However, the peak transition energy is largely blueshifted (>100 meV) with expectations from electronic quantum confinement simulations. Based on calculations of the exciton binding energies, we attribute the photoinduced absorption at room temperature to the dissociation of $h{h}_1\text{−}{e}_1$ excitons towards free carriers in the $e_2$ state and not to $h{h}_1\text{−}{e}_1$ to $h{h}_1\text{−}{e}_2$ excitonic transitions induced by the intersubband absorption as previously stated by Olszakier et al. [Phys. Rev. Lett. 62, 2997 (1989)]. This effect is a consequence of the huge binding energy of excitons in the ZnO material system, which is further enhanced in QWs due to the quantum confinement. This may pave the way for a better understanding of semiconductors’ excitonic processes as well as for developing intersubband devices with a blueshifted operating range.

Figure 1

(a) Photoluminescence spectra of the samples at room temperature. (b) The red dots are peak PL energy of the ZnO QWs versus the well thickness; the red curve corresponds to the simulated gap energy.

Figure 2

PIA spectra of samples A (left), B (center), and C (right) at room temperature for TM- and TE-polarized light.

Figure 3

PIA spectra of sample E at room temperature for TM-polarized light at different UV pump power 40 mW (black), 114 mW (red), and 350 mW (blue) along (a) the $c$ axis and (b) the $a$ axis.

Figure 4

Calculated CB ISB energy versus well thickness for Mg content in the barrier of 33% (orange curve), 40% (purple curve), and 48% (green curve). The full black squares are the experimental peak energies measured by PIA. The blue curve is the $X_1-X_2$ ( $h{h}_1\text{−}{e}_1$ to $h{h}_1\text{−}{e}_2$) excitonic transition energy for 40% Mg content, the blue filled area delimited by dots corresponds to the $X_1-X_2$ transition error accounting for a ±1 ML fluctuation of the well thickness. The red curve is the ISB energy corrected by the $h{h}_1\text{−}{e}_1$ exciton binding energy, which corresponds to a transition $X_1\text{−}{e}_2$ implying a fundamental excitonic state and an excited $e_2$ state with unbound electrons and holes and the red filled area delimited by dots corresponds to the $X_1\text{−}{e}_2$ transition error accounting for a ±1 ML fluctuation of the well thickness.

Figure 5

Scheme of the $X_1\left(h{h}_1\text{−}{e}_1\right)$ exciton ionization to free carriers in the $e_2$ state induced by the $e_1-e_2$ intersubband absorption. The black curved lines represent the electronic states confined in a QW in the conduction and valence bands. The black dotted line is the Fermi energy. The orange dashed lines simulate the excitonic states. The blue full (hollow) dots correspond to electron (hole) particles; their electron-hole Coulomb interaction, symbolized by the green dotted curve, forms an exciton quasiparticle represented by the green dot. The purple (red) arrows illustrate the electron excitation inducing the exciton generation (exciton ionization) by the ultraviolet pump (infrared probe).