11.1 Quantum dot diode lasers

Abstract

To realize lasing operation in a semiconductor structure it is necessary to provide a sufficient carrier concentration within a very limited energy interval of available electronic states. However, a distribution of the charge carriers over energy states is governed by a specific shape of the Density Of States (DOS). In most cases DOS is quite broad leading to additional contribution to the threshold current density. Quantum confinement within a sufficiently narrow region of a semiconductor material can significantly change the energy spectrum of the charge carriers. A structure, where energy barriers exist in three directions of propagation is now known as a Quantum Dot (QD). As compared to other types of the active region, quantum dots are much more favorable for laser applications. In this ultimate case the only allowed energy states correspond to discrete quantum levels. This results in steep dependence of the optical gain on the current density. Moreover, a quantum dot array, as compared to a Quantum Well (QW), may provide much wider flexibility in varying the surface density of states as it is controlled by the surface density of the QD array, which can be arbitrary low. Another important aspect of a quantum dot laser is a possibility to achieve a temperature-independent threshold because DOS may be much narrower than the thermal energy. As quantum dots are small and separated from each other, in-plane electron transport may be suppressed. Therefore, QD structures may be less sensitive to threading dislocations and other types of crystalline defect. As a result, quantum dots may noticeably expand an emission range available for laser structures grown on a certain type of semiconductor substrate. All the aforementioned features of ideal quantum dots have motivated research and efforts, which were undertaken by different research teams worldwide to develop a fabrication technology of semiconductor quantum dots suitable for device applications. Below we describe basic aspects of practical realizations of quantum dots and diode lasers on their basis. This chapter is organized as follows: In Sect. 11.1.2 basic principles of QD formation using self-organization phenomena, their structural and optical properties are discussed. We pay special attention to inhomogeneous broadening of QD optical transitions and methods of controlling the emission wavelength. In Sect. 11.1.3 we describe theoretical and experimental results on gain and threshold characteristics of QD lasers and correlate them to the specific structure of the active region (inhomogeneous broadening, excited states, multiple stacking of QD planes). Various peculiarities of QD lasers are discussed in Sect. 11.1.4. In particular, we address issues related to the long-wavelength lasers on a GaAs substrate, low-threshold QD lasers, temperature stability of device characteristics, high-frequency characteristics of QD lasers. We also touch upon reliability of QD lasers and quantum dot structures grown on metamorphic buffers. Section 11.1.5 is focused on quantum dots capable of providing extremely broad gain and lasing spectra with a high power level and low noise. Possible use of such lasers for optical communication is argued. We deliberately restrict the content to only lasers of edge-emitting type. Use of quantum dots in microcavity lasers, e.g., Vertical-Cavity Surface-Emitting Lasers (VCSELs), is quite specific due mostly to their limited optical gain. The In(Ga)As quantum dot material system, the best studied and practically exploited, is considered in this chapter.