GalnN Quantum Wells Optical Properties

In this Datareview, we will discuss the fundamental optical properties of GalnN/GaN quantum wells, the only exception being the optical gain, which is presented separately in Datareviews C2.4 and C5.3. 

B ELECTRONIC STATES AND OPTICAL PROPERTIES OF QUANTUM WELLS B1 Level Structure 

The electronic states in semiconductor quantum wells are determined by the width of the well, the effective masses of electrons and holes, and the height of the potential barriers in the conduction and valence bands. 

FIGURE 1 Calculated electron and hole quantisation energies in nitride quantum wells with and without piezoelectric field 

Interband optical transitions in quantum wells are governed by selection rules determined by the symmetry of the wavefimctions. In an ideal one-dimensional potential well, only transitions between levels of identical quantum number would be allowed. In a real quasi-two-dimensional quantum well, band mixing for finite wave vectors within the layer plane leads to a weakening of the selection rule, so that transitions with An 0 may show up in optical spectra. 

---

C2.1 GalnN quantum wells composition pulling effect C2.2 GalnN quantum wells microstructure C2.3 GalnN quantum wells optical properties C2.4 GalnN quantum wells effect of phase separation on lasing... 

It is now being widely discussed whether phase separation of GalnN into InN mid GaN due to their large differences in lattice constants plays a decisive role for the properties of GalnN/GaN quantum wells. Its influence on the optical properties and on lasing from such quantum wells is discussed in Datareview C2.5. 

The optical properties of GalnN/GaN quantum wells differ somewhat from the well-known behaviour of other III-V-based strained quantum well structures, partly due to the rather strong composition and well width fluctuations, possibly induced by a partial phase separation of InN and GaN. The even more dominant effect seems to be the piezoelectric field characteristic for strained wurtzite quantum wells, which strongly modifies the transition energies and the oscillator strengths. However, the relative influence of localisation and piezoelectric field effect is still subject to considerable controversy. 

It has been proposed recently that phase separation of GalnN into In-rich and Ga-rich phases has profound consequences on the optical properties and on the lasing properties of GalnN/GaN/AlGaN quantum well heterostructures [1,2]. The nanoscale compositional fluctuations resulting from phase separation are believed to lead not only to exciton localisation [2] but even to a quantum-dot-like behaviour [3], A more detailed discussion of the microscopic aspects of phase separation is given elsewhere in this volume. 

Low-excitation, low-temperature experiments like photoluminescence or photoluminescence excitation spectroscopy tend to indicate a considerable influence of localisation effects on the optical properties of GakiN/GaN quantum wells. Under high-excitation conditions typical for lasing, however, it is clearly seen that lasing from GalnN/GaN quantum well structures is due to a free-carrier plasma. 

---