ZnO/MgZnO Quantum Wells

Abstract

In this review, we discuss first some of the recent works to reveal properties of conventional ZnO/ZnMgO QWs grown c-axis oriented. This will include the properties of the quantum confined Stark effect (QCSE) that results from the internal electric field in the unit cell. We will then discuss various unconventional QW growths, including non-polar ZnO QWs, graded barrier QWs and double QWs. We finish with a review of current progress towards light emitting devices based on ZnO/ZnMgO QWs. ZnO has been a material of interest for over 50 years; however, the ability to grow high-quality epilayers of ZnO and ZnO-based ternary systems[1–4 has led to renewed interest over the past decade in ZnO for device applications. The demand for optoelectronic devices in the blue-UV region of the electromagnetic spectrum has been well established and ZnO possesses several properties that are superior to GaN for many applications [5, 6]. The large exciton binding energy of ∼ 60 meV suggests excitonic emission that is very efficient above room temperature, leading to great potential for light emitting devices. The large piezoelectric and pyroelectric coefficients suggest potential for applications as piezoelectric sensors or actuators. Other advantages of ZnO are its comparatively low growth temperatures [5], low optical power threshold for lasing [7], radiation hardness [8, 9] and biocompatibility [10]. ZnO may also find application as a transparent conductive oxide [11] to replace indium-tin-oxide (ITO) in photovoltaic applications because it remains transparent even when doped above the level of degeneracy [12]. This ability to grow high-quality epilayers and thin films rapidly led to the development of ZnO-based quantum wells (QWs). The two main attractions of developing QWs are the tunability offered for the transition energy, and the increased exciton binding energy, which typically leads to increased oscillator strength and greater efficiency for light emitting devices. The two most common types of ZnO-based QWs are Zno/ZnMgo and ZnO/ZnCdO-based systems, where alloying with Mg leads to an increase in the band gap and with Cd leads to a reduction in the band gap. In this chapter, we will focus on the more common ZnO/ZnMgO QWs. The band gap of Zn_{x − 1}Mg _x O is given by 3. 37 + 2. 51x eV [13], which together with a conduction to valence band offset ratio of approximately 70:30 leads to type I confinement in ${\mathrm{ZnO/Zn}}_{x-1}{\mathrm{Mg}}_{x}\mathrm{O}$ QWs. The value of x is limited to less than 0.43 [13] in these systems as above this value phase separation tends to occur. Nevertheless, even with values significantly less than 0.43, strong confinement is obtained. In such ZnO/ZnMgO QWs, exciton binding energies up to 120 meV have been reported [14]. The biexciton binding energy is also enhanced in QWs going from 15 meV in bulk ZnO to ∼ 25 meV in ZnO/ZnMgO QWs [15–17]. With biexciton binding energies greater than the thermal energy, it is possible that biexcitons may play a major role in the optical properties at room temperature. Since the first ZnO epitaxial layers were grown, high-quality ZnO quantum wells have been grown by several different methods, including molecular beam epitaxy (MBE), metal-organic vapour phase epitaxy (MOVPE) and pulsed laser deposition (PLD). The growth in almost all cases is c-axis oriented despite growth on a range of single crystal substrates, including sapphire, ZnO and Si oriented along various crystal planes.