Visible emission from ZnCdO/ZnO multiple quantum wells

Abstract

Due to the large demand for optoelectronic devices covering the spectral range from green to ultraviolet, extensive research has been triggered into wide-bandgap group III-nitride semiconductors. Since InGaN-based devices exhibit a strong decrease in efficiency advancing the green spectral region [1], ZnO-based devices are a promising alternative.An emission energy in the visible spectral range was obtained in e.g. Zn 1-x Cd x O/ZnO quantum wells (QWs) or double heterostructures [2,3]. Large Cd contents up to ≈0.50 with emission energies down to 1.8 eV were obtained in wurtzite Zn 1-x Cd x O thin films [4], making a wide spectral range accessible. However, up to now only few publications exist that report on Zn 1-x Cd x O/ZnO QWs exhibiting emission energies far below theZnO bandgap resulting from difficulties to obtain large Cd contents in the QWs due to the low solubility of Cd in ZnO.For PLD-grown samples the lowest reported QW emission energy is as high as 2.76 eV. QW energies down to 2.2 eV are accessible for molecular beem epitaxy-grown samples, supported by the occurrence of the Quantum Confined Stark Effect (QCSE) [1], that is expected to occur in samples with a polar growth direction. Sadofev et al. [2] have up to now exclusively proved the occurrence of the QCSE, by obtaining emission significantly below the respective thin film emission energies. Matsui et al. concluded the occurrence of the QCSE as they obtain excitation-power dependent QW energies and a decrease of the degree of localization and the thermal activation energy of the PL-intensity quenching with increasing QW thickness [5]. However, they were not able to tune the emission energy significantly below that of the thin film. The absence of the QCSE in the other studies can be understood following the argumentation of Brandt et al. [6], who explained it for PLD-grown ZnO/Mg x Zn 1-x O QWs with reduced interface abruptness.In this Letter, optical properties of Zn 0.75 Cd 0.25 O/ZnO MQWs with small QW thicknesses are studied. They were deposited at low temperatures in order to achieve a high Cd content. MQWs were deposited to enhance the luminescence intensity and precisely determine the QW thickness by using X-ray diffraction (XRD). Due to the high Cd content, the MQW emission energy was tuned from 2.5 eV to 3.1 eV by varying the QW thickness.The MQW structures were grown on a-plane sapphire substrates using PLD. Sintered ceramic targets of highly Polar c-axis oriented Zn 0.75 Cd 0.25 O/ZnO multiple quantum wells (MQWs), grown by pulsed-laser deposition (PLD), emitting in the visible spectral range are reported. By applying a low growth temperature of ≈ 300 °C a large Cd content of 0.25 and abrupt interfaces could be achieved using PLD. The emission energy was tuned from the green to the violet spectral range (2.5 eV to 3.1 eV) by tuning the quantum well thickness. It is determined by the quantum confinement effect and the quantum-confined Stark effect.Intensity (arb. units) 2.0 2.5 3.0 Energy (eV) increasing well width

Excitonic and Optical Confinement in Microwire Heterostructures with Nonpolar (Zn,Cd)O/(Mg,Zn)O Multiple Quantum Wells

“…(See Figure a.) The energy of the QW-related emission was tuned in almost the entire range between the emission energy of the barrier material and the bulk emission energy of the QW material of 2.46 eV . The luminescence of the (Mg,Zn)O barrier layer is observed at an energy of ∼3.45 eV corresponding to a Mg content of 0.075.…”

“…The NWs direct toward the plasma plume, and the substrate rotates for a better homogeneity of the coating process. To obtain a large Cd content in the QWs, the NWs were held at a temperature of ∼300 °C . Oxygen partial pressures of 2.6 Pa for (Zn,Cd)O and 0.2 Pa for (Mg,Zn)O were applied, respectively.…”

“…ZnO exhibits a large oscillator strength and enhanced exciton stability up to temperatures well above room temperature, making it a perfect material for the realization of a room-temperature polariton laser. Additionally, the fabrication of naturally built, perfectly shaped resonators with distinct cavity properties and quantum-well structures with emission energies in the visible spectral range , is possible with ZnO and its ternary alloys.…”

Theoretical study of O- and Zn-face polarity effect on the optical properties of the conventional and staggered ZnO/Zn1−xCdxO/ZnO quantum wells

“…Yamamoto et al [12] obtained the band-edge green electroluminescence (EL) at 500 nm from Zn 0.80 Cd 0.20 O/Zn 0.92 Cd 0.08 O multiple quantum-well light-emitting diodes (LEDs) on p-type 4H-SiC substrates. The change of the emission energy from the green to the violet spectral range (2.5 eV to 3.1 eV) by tuning the quantum well thickness was observed by Lange et al [13]. Thus, the optical performance of ZnO-based devices and heterostructures is strongly governed by the phenomena occurring in the Zn 1-x Cd x O layer.…”

“…Recently, several groups have studied the properties of Zn 1-x Cd x O/ZnO QWs [10][11][12][13]. Liu et al [10] observed the increase of the spontaneous emission rate from Zn 1-x Cd x O QWs by surface plasmon coupling.…”