Effects of MBE growth on the optical properties of AlGaN quantum wells
- PDF / 446,808 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 60 Downloads / 267 Views
Effects of MBE growth on the optical properties of AlGaN quantum wells W. Feng,1,2 G. Rajanna,1,2 S. Sohal,1,3 S. A. Nikishin,1,2 A. A. Bernussi,1,2 and M. Holtz1,3 1 Nano Tech Center, Texas Tech University, Lubbock, TX 79409, U.S.A. 2 Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, TX 79409, U.S.A. 3 Department of Physics, Texas Tech University, Lubbock, TX 79409, U.S.A. ABSTRACT Optical studies are reported of multiple quantum wells, based on AlGaN for emission in the deep ultraviolet. The materials are grown using gas source molecular beam epitaxy in a growth regime which transitions from purely two-dimensional to mixed two- and three-dimensional well formation. Low temperature photoluminescence and absorption measurements are used to obtain the Stokes shift, and temperature dependence is used to estimate the thermal activation energy associated with photoluminescence intensity decrease. Variations in these energies are attributed to the well morphologies. INTRODUCTION The internal efficiency of quantum wells of InGaN is known to be enhanced by formation of InN-rich quantum dots (QDs). These QDs are formed by self-assembly because of poor miscibility between GaN and InN, leading to phase separation. QDs of GaN can be formed by inducing a growth mode change, from two dimensional (2D) to three dimensional (3D), by applying Si anti-surfactant [1, 2] or by residual strain in the well material [3, 4]. Much less is known about QDs in AlxGa1-xN. Previous work has investigated growth of AlxGa1-xN (0.01 ≤ x ≤ 0.05) QDs on Al0.38Ga0.62N using Si anti-surfactant [2]. However, they observed decreasing luminescence efficiency with increasing AlN content in the well. Increased luminescence has been reported for AlxGa1-xN layers having various compositions [5-7]. The efficiency enhancement was attributed to the formation of nanoscale compositional inhomogeneities with slightly higher Ga content, corresponding to locally lower energy gaps. These regions serve to localize excitons, thereby increasing the radiative recombination. We have also reported luminescence enhancement in multiple quantum well (MQW) structures grown by gas source molecular beam epitaxy (GSMBE) [8-10]. The formation of quantum structures in the well regions was associated with ammonia flux and temperature to achieve primarily 2D, mixed 2D and 3D, and 3D growth, as confirmed by in situ reflection high energy electron diffraction (RHEED) studies. The mixed 2D and 3D growth produces wells with planar morphology surrounding regions having slightly higher thickness. The highest luminescence was obtained from these samples. We describe here photoluminescence (PL) and optical absorption studies of AlGaN MQW active regions grown at different temperatures but under otherwise identical conditions. Lateral confinement within the well layers is described. This localization mechanism is fundamentally different from what is described for alloys incorporating composition variations.
EXPERIMENT All samples studied here were grown using G
Data Loading...
