Mid-infrared intersubband absorptions in ZnO/ZnMgO multiple quantum wells

Abstract

The authors report the observation of intersubband absorptions in ZnO/ZnMgO multiple quantum wells (QWs). The multiple QW structures were grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The high structural and optical properties of the samples were demonstrated by high-resolution x-ray diffraction and photoluminescence measurements. At room temperature, intersubband absorptions at ∼3 μm were observed.

Multisubband Plasmons in Doped ZnO Quantum Wells

“…The FWHM of the measured MSP lies in the range of 800 cm −1 , i.e., from γ ISBT /ω ≈ 0.38 in the samples with the lowest n 2D to γ ISBT /ω ≈ 0.28 in the samples with the largest n 2D , whereω is the observed MSP peak frequency. This is similar to what was obtained by Belmoubarik et al [14] in ISB transition photocurrent experiments at 18 K on cplane ZnO QWs (γ ISBT ≈ 800 cm −1 , γ ISBT /ω ≈ 0.3), and lower than the values obtained by Zhao et al [15] in room-temperature absorption experiments, also in c-plane ZnO QWs (γ ISBT ≈ 1250-1450 cm −1 , γ ISBT /ω ≈ 0.4).…”

“…ZnO is a material with great potential for ISB transition-based detectors and emitters from the midinfrared to the THz, since it offers several singular characteristics that are not present in the commonly used GaAs-based systems. ISB transition-based absorption and detection have already been observed in ZnO quantum wells [13][14][15], and the development of quantum cascade lasers in the THz range would benefit from the large phonon energy of ZnO (approximately 72 meV) [16].…”

“…The ZnO/Mg x Zn 1−x O material system is also chosen in this work because of the availability of nonpolar, native ZnO substrates. To date, ISB transitions in wurtzite wideband-gap semiconductors, including the GaN and ZnO material systems, have mainly been observed in polar QWs grown along the c axis (mostly on foreign substrates, e.g., sapphire or Si), where the quantum confined Stark effect (QCSE) takes place [14,15,17]. Growing the QW structures on a nonpolar orientation (i.e., with the c axis parallel to the interface between layers) can suppress the QCSE [18] and should facilitate the ISB transitions due to the absence of internal electric fields that have a detrimental impact on the oscillator strength of the transitions [13].…”

Breaking the Intersubband Selection Rules for Absorption with ZnO Quantum Wells: Light Polarization Sensitivity under Normal Incidence

“…In order to overcome these inherent limitations to ISB transitions and to achieve polarization-sensitive ISB absorption at normal incidence without the need to use additional processing steps or external optical components, a metamaterial is needed that meets the following requirements: (1) high-quality-crystal QWs need to be grown in a highly controlled and reproducible fashion in order to obtain ISB transitions; (2) the active region must show a spontaneously generated morphology that facilitates the absorption of light under normal incidence; and (3) this morphology must be anisotropic, i.e., must appear only in one in-plane direction, but not in the perpendicular one, such that light polarization sensitivity can be achieved. Here, we demonstrate that these conditions are nicely met by the ZnO/(Mg,Zn)O alloy family: ISB transitions from multiple quantum wells (MQWs) have already been demonstrated with excellent characteristics [10][11][12][13] and the anisotropy of its wurztize crystal structure can be exploited by growing the QWs along the m axis with the c axis in-plane, i.e., by using a nonpolar orientation [10,13]. Moreover, the nonpolar orientation facilitates the ISB transitions owing to the absence of internal electric fields that would have an impact on the oscillator strengths via the quantum-confined Stark effect.…”

Tunable intersubband transitions in ZnO/ZnMgO multiple quantum wells in the mid infrared spectral range

“…This is despite the fact that n-doping of ZnO can be achieved easily enabling fabrication of plasmonic waveguides and the technique to fabricate high-quality ZnO/ZnMgO multiple quantum well (MQW) structures is well established. [10][11][12] So far only ISBT from the first to the second electronic energy state in ZnO/ZnMgO MQWs have been directly observed at about 400 meV, 13 and a weak photocurrent at low temperature due to the transition from the first to the third electronic energy state was reported. 14 In this letter, we demonstrate the ability to control ISBT energies between the first and the second electronic energy state in the ZnO/Zn 0. by the growth time with an estimated accuracy of 10%.…”