Spectroscopy in Polarized and Piezoelectric AlGaInN Heterostructures
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spontaneous polarization (GaN: Peq = -0.029 C/m2 [6]). In this context we shall experimentally quantify the associated electric fields and explore the role of such polarization fields in the optoelectronic properties of thin film and quantum well heterostructures. We present experimental results of pseudomorphic AlGaInN/GaN thin film and multiple quantum well (MQW) structures of various well width and composition near the performance optimum of laser diodes in the 430 - 400 nm range using combinations of modulated reflectivity and photoluminescence spectroscopy. BACKGROUND Among the challenges in group-III nitrides is the identification of the electronic bandstructure in AlGaInN/GaN heterostructures in order to identify the light emission processes, the threshold edges for detector devices and the channels of highest electron mobility. Especially the aspect of light emission in light emission diodes and laser diodes has been subject of substantial controversy in recent years [7,8]. Work has been hampered by a wide range of material qualities in this highly strained superlattice system exhibiting a strong tendency towards inhomogeneities [9]. To resolve this dilemma we concentrate on material that has been optimized for the highest homogeneity of the structural and optical properties [1]. In order to explain the processes within the wells we first develop a foundation by studying the conditions of the barriers that define the well. In this way the energy of binary GaN layers serves as an accurate point of reference. Here we will show that by such an approach we gain important insight into the conditions of the AlGaInN quantum well system in general and only thereafter endeavor into a more phenomenological description of the conditions within the depth of the wells. EXPERIMENTAL GaInN/GaN structures have been grown by MOVPE on basal plane sapphire using the technique of low temperature deposited buffer layers of AlN and GaN [3]. Layers have been optimized for microscopic homogeneity and smooth morphology. Typical growth temperatures for GaInN layers range from 680 -780 oC. A first set of GaInN layers was grown at thicknesses of 400 Å and variable composition x, (0 < x < 0.2) pseudomorphically on GaN. Details have been given in Refs. [10,11]. Next a set of GaInN/GaN MQWs with variable x, (0 < x < 0.2) and well width Lw = 30 Å, barrier width Lb = 60 Å were studied [12]. A third set comprises nominally fixed compositions and variable well width Lw, 23 Å ≤ Lw ≤ 70 Å. A fourth set consists of AlyGa1-yN/GaN MQWs on GaN (Lw = 34 Å, Lb = 100 Å, x ≈ 0.06). A fifth set was Si doped in the barriers at concentrations of 3x1018 cm-3 (Lw = 30 Å, Lb = 60 Å, x = 0.15). Time integrated photoluminescence (PL) was performed using a 325 nm HeCd laser. Stimulated emission was performed using
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