JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 24, DECEMBER 15, 2009 5775 Effect of the Wetting Layer on the Output Power of a Double Tunneling-Injection Quantum-Dot Laser Dae-Seob Han and Levon V. Asryan, Senior Member, IEEE Abstract—To suppress bipolar population and hence elec- tron–hole recombination outside quantum dots (QDs), tun- neling-injection of electrons and holes into QDs from two separate quantum wells was proposed earlier. Close-to-ideal operating characteristics were predicted for such a double tunneling-in- jection (DTI) laser. In the Stranski–Krastanow growth mode, a two-dimensional wetting layer (WL) is initially grown followed by the formation of QDs. Due to thermal escape of carriers from QDs, there will be bipolar population and hence electron–hole recombination in the WL, even in a DTI structure. In this work, the light–current characteristic (LCC) of a DTI QD laser is studied in the presence of the WL. Since the opposite sides of a DTI structure are only connected by the current paths through QDs and the WL is located in the n-side of the structure, the only source of holes for the WL is provided by QDs. It is shown that, due to the zero-dimensional nature of QDs, the rate of the hole supply to the WL remains limited with increasing injection current. For this reason, as in the other parts of the structure outside QDs (quantum wells and optical confinement layer), the parasitic electron–hole recombination remains restricted in the WL. As a result, even in the presence of the WL, the LCC of a DTI QD laser becomes increasingly linear at high injection currents, which is a further demonstration of the potential of such a laser for high-power operation. Index Terms—Quantum-dot laser, semiconductor laser. I. INTRODUCTION S EMICONDUCTOR quantum dots (QDs) can be conve- niently used as an active medium for stimulated emission in injection lasers [1]–[7]. Conventionally, QDs are grown by the strain-induced island formation method, which is called as the Stranski–Krastanow growth mode [8]. In the Stranski–Kras- tanow growth mode, several monolayers of one material are grown first on a crystal surface of another material (substrate) having a different lattice constant. Beyond a critical thickness of the deposited layer, three-dimensional (3-D) islands (QDs) start forming from two-dimensional (2-D) monolayers thus partially relaxing the strain and reducing the elastic energy. The initially grown monolayers are called as the wetting layer (WL). Hence, the 2-D WL is inherently present in self-assembled Stranski–Krastanow grown QD structures [9]–[12]. In the conventional design of QD lasers, the carriers are first injected from the cladding layers into the optical confinement Manuscript received August 09, 2009; revised September 22, 2009. First published October 06, 2009; current version published November 30, 2009. This work was supported by the U.S. Army Research Office under Grant W911-NF-08-1-0462. The authors are with the Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 USA (e-mail: asryan@mse.vt.edu). Digital Object Identifier 10.1109/JLT.2009.2033716 layer (OCL), and then captured into the WL and QDs. A certain fraction of carriers thermally escapes back from QDs to the WL and OCL. Due to bipolar (both electron and hole) population in the OCL and WL, parasitic electron–hole recombination occurs there [13]–[15] in addition to recombination in QDs. The role of the WL in conventional QD lasers has been investigated both experimentally and theoretically (see, e.g., [15]–[19]). To suppress the parasitic recombination outside QDs, tun- neling-injection of both electrons and holes into QDs was pro- posed [20]–[22]. In such a double-tunneling injection (DTI) QD laser, the parasitic recombination rate remains restricted even if there is out-tunneling leakage of carriers from QDs [23]. As a result, the light–current characteristic (LCC) of a DTI QD laser is essentially linear. No WL was assumed in the struc- tures of [20]–[22]. If the Stranski–Krastanow mode is used for the growth of QDs, the WL should be properly taken into ac- count. As seen from Fig. 1, even if there is no tunneling be- tween the electron-injecting quantum well (QW) and the WL, there will be bipolar population in the WL. This is because there is such population in QDs (which is maintained to have stimulated emission) and (ii) the WL is coupled to QDs by the processes of thermal escape and capture. Besides, while QDs present the sole source for the hole supply to the WL, electrons can directly tunnel to the WL from the electron-injecting QW (Fig. 1). Hence, even in an ideal case of total suppression of parasitic recombination in the QWs and OCL, such recombina- tion will occur in the WL. In this work, we develop a theoretical model for the optical power of a DTI QD laser, which includes the WL and processes therein. II. THEORETICAL MODEL Fig. 1 shows the energy band diagram of a DTI QD laser with the WL, which follows the barrier separating the electron-in- jecting QW from QDs. As seen from the figure, the holes can only be supplied to the WL by thermal escapes from QDs. In contrast, in addition to thermal escapes from QDs, electrons can directly tunnel to the WL from the left-hand-side (electron-in- jecting) QW. We assume that the material separating QDs in the QD layer (it may be the same as the material of barriers) has high enough bandgap to suppress all tunneling other than via QDs, in par- ticular, tunneling between the QWs, and between the hole-in- jecting (right-hand side) QW and the WL. Hence, the opposite sides of the structure are only connected to each other by the current paths through QDs. 0733-8724/$26.00 © 2009 IEEE