Group-III Nitride Quantum Heterostructures Emitting in the whole Visible Range
- PDF / 165,356 Bytes
- 10 Pages / 596 x 842 pts (A4) Page_size
- 58 Downloads / 128 Views
Group-III Nitride Quantum Heterostructures Emitting in the whole Visible Range Nicolas Grandjean, Benjamin Damilano and Jean Massies CRHEA-CNRS, rue B. Grégory, Sophia Antipolis, 06560 Valbonne, France ABSTRACT Group-III nitride quantum wells (QWs) and quantum dots (QDs) have been grown by molecular beam epitaxy (MBE). Their optical properties are shown to be governed by the presence of a huge internal polarization field. For example, GaN/AlN QDs emit from the blue to the orange due to the giant quantum confined Stark effect (QCSE) induced by a builtin electric field of 4 MV/cm. Another consequence of the QCSE is to drastically reduces the oscillator strength of the ground state transition and thereby to increase by several orders of magnitude its radiative lifetime. Despite the very large density of dislocations in nitride layers, which induce non-radiative recombinations, carrier localization enhances the photoluminescence (PL) efficiency. This is demonstrated by GaN/AlN QDs grown on silicon substrates exhibiting strong PL intensity at room temperature. InGaN/GaN QWs with In composition of 20 % also display 300 K PL peaking through the whole visible spectrum (0.40.66 µm). This is achieved by varying the QW thickness from 1.5 to 5.5 nm, the red-shift resulting from an internal electric field of 2.5 MV/cm. For InGaN/GaN QWs emitting at 2.82.9 eV, the PL efficiency at 300 K is larger than 10 %. This is ascribed to carrier localization, which is not due to InGaN phase separation that would form QDs, as revealed by transmission electron microscopy. Another origin of the carrier localization in InGaN/GaN QWs is then discussed.
INTRODUCTION Group-III nitrides semiconductors are presently the subject of an intense activity due to their attractive properties for short wavelength optoelectronics and high temperature, high power, and high frequency electronics [1]. For instance, high brightness blue InGaN/GaN based light emitting diodes (LEDs) with external quantum efficiencies as high as 20 % have been reported [2]. This is quite surprising if one considers the large dislocation density (108 to 1010 cm-2 ) present in nitride layers grown on sapphire or silicon substrates. Narukawa et al. first proposed that the strong carrier localization in InGaN alloys is the origin of the high efficiency of LEDs [3]. This interpretation was based on transmission electron microscopy (TEM) observations of InGaN QWs showing high In content areas due to phase separation. Therefore, InGaN/GaN QWs would be considered as InGaN/GaN QDs from the point of view of their optical properties. The use of QDs for increasing the radiative efficiency in dislocated materials has been first experimentally demonstrated by Gérard et al. in the case of InAs/GaAs QDs grown on silicon substrate [4]. QDs can be fabricated by using the Stranski-Krastanov (SK) growth mode transition induced by the lattice mismatch: 3D islands form because the elastic energy is released by elastic deformation at their free-edges. As indicated above, QDs can also be self-formed by a
Data Loading...
