1774 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 33, NO. 10, OCTOBER 1997 Effects of Electric Field on the Exciton Linewidth Broadening Due to Scattering by Free Carriers in Semiconducting Quantum-Well Structures Tong San Koh, Yuan Ping Feng, and Harold N. Spector Abstract—The effects of an applied electric field perpendicular to the well layers on the broadening of the exciton linewidth due to scattering by free carriers in semiconducting quantum- well (QW) structures is theoretically investigated based on a finite confining potential model. The dependence of the free carrier-exciton linewidth broadening on carrier concentration, temperature, and well width are calculated and discussed for various electric field strengths. It is found that the influence of the electric field on the linewidth broadening is appreciable in wide wells, while in narrow wells little change is shown even in the presence of a strong electric field. Index Terms—Electric field, exciton linewidth, quantum wells. I. INTRODUCTION T HE STUDY of excitonic properties in quantum wells in the presence of a transverse electric field perpendicular to the well layers has attracted much interest for the past decade, as the basic properties of excitons are extremely important for the application of quantum wells (QW’s) in optical modulation and switching devices. In particular, the linewidth of the excitons play an important role in determining the shapes of optical absorption edges and of photoluminescence features in direct-bandgap semiconductor systems. In the quantum- confined Stark effect (QCSE) [1], [2], the magnitude and energy at which absorption due to excitons occur may be controlled by the application of a transverse electric field. As this phenomenon can be an important consideration in the design of optical switches, it is hence, of interest to understand how the exciton linewidth broadens as a function of the applied electric field and, more importantly, how the effects of the electric field differ for QW’s of different widths. Excitons exist up to relatively high transverse electric fields in the QW’s, due to both the confinement of the well [1], [2] and the electron-hole Coulomb interaction [3], which inhibit field ionization. One of the mechanisms which lead to the finite lifetime for the exciton states, and hence, to the broadening of the exciton linewidth, is the scattering due to free carriers. It has been found that the contribution to the exciton linewidth due to scattering by free carriers can be significant in situations where high densities of free carriers are generated and when the scattering of excitons by optical Manuscript received February 6, 1997; revised May 5, 1997. T. S. Koh and Y. P. Feng are with the Department of Physics, National University of Singapore, Singapore 119260. H. N. Spector is with the Department of Physics, Illinois Institute of Technology, Chicago, IL 60616 USA. Publisher Item Identifier S 0018-9197(97)07126-1. and acoustic phonons is reduced [4]–[10]. Recently [11], we have also performed calculations on the exciton linewidth broadening due to scattering by free carriers in the absence of the electric field using a quasi-two-dimensional scattering model with a finite confining potential and discussed the well width dependence of the linewidth broadening. It was found that by considering a finite well width, the calculated linewidth is larger than that obtained using the exact two-dimensional (2-D) model [4], [5] and is closer to the experimental results of Honold et al. [6]. Good agreement between our calculations and the experimental results of Koch et al. [9] was also found. In [11], we have also included the contribution to the linewidth due to ionization scattering of the exciton to the continuum electron-hole states, which is absent in previous works [4], [5] and which further enhance the importance of free-carrier scattering in the broadening of the exciton linewidth. In fact, this is also the case for phonon-exciton scattering [12], [13], where it is well known that the major contribution to the linewidth is due to scattering of the excitons to the continuum electron-hole states, whereby the exciton is ionized into a free electron-hole pair. Interesting features in [11] that arise due to the finite confinement include the enhancement of the linewidth as a result of the emergence of three-dimensional (3-D) exciton scattering due to the increasing penetration of the exciton into the barrier regions in the very narrow QW’s. Such features were attributed to the quasi-3-D nature of the excitons due to substantial smearing of the excitonic wavefunctions into the barrier regions when the well width is below a certain narrow well limit. The existence of quasi- 3-D excitons has been proposed by several authors [14]–[16] in their calculations of the exciton binding energy, and the narrow well limit is generally estimated by the maximum of the exciton binding energy in a finite potential well. In the presence of an applied transverse electric field, the importance of using the finite confining potential should be further emphasized. The penetration of the exciton wavefunc- tion into the barrier regions also leads to a greater polarization of the exciton wavefunction, which is important when consid- ering the effect of the electric field. In the previous work on the cross sections due to free-carrier exciton scattering in the presence of a transverse electric field using a finite confining potential model [17], we showed that the main effects of the electric field are on the polarization of the exciton together with the enhancement of the interaction between the free carrier and the particle of similar charge in the exciton. In 0018–9197/97$10.00  1997 IEEE