238
/
CLE0'98
/
WEDNESDAY AFTERNOON
h 100- 4E
-
8 0 - b 6 0 -B
3
40-5
20 c'*'
1
(300K).-
c . - Deduced from AS 0.80.7
N.-
5: Measured from LI..
.'
0
0.5 1 1.5 2 2.5 3Pressure (GPa)
0.6'
CWF16 Fig. 3. Measured variation (symbols)
in the laser threshold current (Ith) with pressure from light-current curves to 2.5 GPa. The dashed line is a least square's fit. The solid line is the calculated variation in It,, with pressure including changes in dgldn, no, and losses with pressure.
sure. In Fig. 2, the optical losses are plotted as a function of pressure and corresponding bandgap energy. The losses decreased with pressure up to about 1.7 GPa, and showed a slight increase beyond and up to the maximum pressure of the experiments, 2.5 GPa. We be- lieve the initial decrease in the losses is the result of a reduced contribution of IVBA as the bandgap of the active layer is increased. The subsequent increase beyond 1.7 GPa arises due to small changes in the mirror 1 0 ~ s . ~
The differential gain (dg/dn) and transpar- ency carrier density (no) were also determined from the gain spectra. Taking into account the changes in dgldn, no, and the optical losses with pressure, we calculated the laser threshold current (Ith). This is shown by the solid line in Fig. 3, where we have also plotted I, deter- mined from light-current measurements also performed in these experiments. The initial decrease in Ith can be accounted for by the decrease in the optical losses, a small decrease in no and small increase in dgldn. I,, increases above
-
1.2 GPa due to a decrease in dg/dn and small increases in the losses and no.This work is supported by the National Sci- ence Foundation through Grants ECS 9408321 and Career Award ECS 9502888.
1. B.W. Hal&, T.L. Paoli, J. Appl. Phys. 46,
1299 (1975).
2. D. Patel et al., J. Appl. Phys. 74, 737 (1993).
3. C.S. Chang et. al., IEEE J. Sel. Topics Quantum Electron. 1, 1100 (1995). 4. D. Patel, C.S. Menoni, A.A. Bernussi, H.
Temkin, Phys. Status Solidi. B 198, 375 (1996).
CWF17
Nonuniform carrier distribution in multiple quantum well laser diodes
Bor-Lin Lee, Ching-Fuh Lin, Institute of
Electro-Optical Engineering and Department of
Electrical Engineering, National Taiwan University, Taipei, Taiwan, R.O. C.; E-mail: [email protected]
Carrier distribution in multiple quantum wells (MQWs) is an important factor that influences
LDG221
GaAs wells
GaAs wells
CWF17 Fig. 1. The layer structures of four
quantum wells of different widths.
0, h ' I
Wavelength (nm)
CWF17 Fig. 2. The measured spectra of the
laser diodes.
the performance of semiconductor lasers. In general, increasing the number of wells re- duces the threshold current density per well and increases the differential gain coefficient for QW laser diodes. However, there exists an optimum well number to maximize the differ- ential gain,' possibly due to the nonuniform carrier distribution. The nonuniform carrier distribution in MQWs has been proposed and studied the~retically.~.~ In contrast, experi- mental evidence is presented relatively less4 In this work, we demonstrate the evidence of nonuniform carrier distribution by measuring the lasing characteristics of the laser diodes fabricated on substrates with the designed MQWs.
The layer structures of the substrates with four quantum wells for the study are schema$- call~shown in Fig. 1.The wellwidths are 20 A, 33 A, 56
A,
and 125 A, respectively. Their n = 1 transitions separate for about 50 meV, which is large enough to distinguish the correspond- ing emission spectra. The LDG221 sample and the LDG231 sample have the opposite se- quence of the four wells. Fabry-Perot laser diodes with a 6-pm ridge waveguide were fab- ricated on the two types of substrates. Standard processing techniques were used for the device fabrication. No coatings were applied to the device facets. The measured devices are all 700 p m long.Figure 2 shows the measured spectra of the fabricated Fabry-Perot laser diodes. In spite of the different sequence of the four wells, they both oscillate at the transition wavelength of the widest well. This observation is different from that in Yamazaki et aL4 Because the emis- sion due to the narrow wells is absorbed by the wide wells, the emission of the widest well ex- periences the least loss. As a result, oscillation
O L
10 20 30 40 50
Heat sink temperature ("C)
CWF17 Fig. 3. The measured threshold cur-
rent versus temperature.
most likely happens at the transition wave- length of the widest well. However, the laser diodes fabricated on the LDG221 substrate have a threshold current around 30 mA, while those fabricated on the LDG23 1 substrate have an almost doubled threshold current. The dif- ference of the threshold current indicates that the injected carriers are not uniformly distrib- uted among the four wells and the nonuniform distribution of the injected holes plays the dominant role. A brief explanation is as fol- lows. Because the LDG221 sample has the 125-A well closer to the p-cladding layer than the LDG231 sample, it has many more holes accumulated in this wide well at the same in- jection current, leading to a significantly re- duced threshold current.
The temperature dependence of the thresh- old current is also measured and shown in Fig. 3. The LDG23 1 devices suffer the temperature effect more seriously because, at the high tem- perature, the mean free path of holes is re- duced and so holes therein have more diffi- culty in passing through other wells to reach the 125-A well.
Other evidence of the nonuniform carrier distribution is also observed in devices fabri- cated on substrates with asymmetric dual quantum wells.5 Detailed discussions on the carrier distribution among the MQWs will be given in the presentation.
1. K. Uomi,M. Aoki,T. Tsuchiya, M. Suzuki, N. Chinone, IEEE Photonics Technol. Lett. 3,493-495 (1991).
R. Nagarajan, M. Ishikawa, T. Fukushima,
R.S. Geels, J.E. Bowers, IEEE J. Quantum Electron. 28,1990-2008 (1992). N. Tessler, G. Eisenstein, IEEE J. Quantum Electron. 29, 1586-1595 (1993). H. Yamazaki, A. Tomita, M. Yamaguchi, Appl. Phys. Lett. 71,767-769 (1997). C.-F. Lin, B.-L. Lee, P.-C. Lin, IEEE Pho- tonicsTechno1. Lett. 8,1456-1458 (1996).
2.
3. 4. 5.
