TU4G-(
6)-3
FEMTOSECOND PUMP-PROBE STUDIES ON CARRIER DYNAMICS
IN InGaN/GaN QUANTUM
WELLS
WITH
INDIUM
AGGREGATED
QUANTUM DOT STRUCTURES
Hsiang-Chen Wang, Cheng-Yeh Tsai, Yung-Chen Cheng, En-Chiang Lin, Shih-Wei Feng,
and
C . C.
Yang
Graduate Institute
of Electro-Optical Engineering, Graduate Institute
of Electronics
Engineering, and Department
of Electrical Engineering, National Taiwan University,
1,
Roosevelt Road, Sec.
4,
Taipei, Taiwan, R.O.C.
@hone)
886-2-23657624
(fax)
886-2-2365263 (E-mail) [email protected]
Kung-Jen Ma
Department of Mechanical Engineering, Chung Hua University, Hsinchu, Taiwan, R.O.C.
Cheng-Ta Kuo and Jian-Shihn Tsang
Advanced Epitaxy Technology Inc., Hsinchu Industrial
Park, Taiwan, R.O.C.
Abstract-Temperature-dependent pump-probe measurements were conducted f o r observing the process of carrier relaxation into localized states of quantum dots, which were formed through indium aggregation in InGaN/GaN quantum well structures of various parameters.
Because of the large lattice mismatch between GaN and InN (up to 13 %), their miscibility is quite low,
leading to the phenomena of indium aggregation and phase separation through the process of spinodal
decomposition. The indium-rich cluster formation in InGaN compounds results in potential fluctuations and hence the effect of carrier localization. In this paper, we report the experimental results of fsec carrier dynamics based on pump-probe measurements of various InGaN/GaN quantum well samples with various quantum well widths, nominal indium contents, and under various thermal annealing conditions. Quantum dot structures in some of the samples have been confirmed with material analyses. Five samples of the same nominal indium
content but different well widths at 2,2.5,3,3,5, and 4 nm were prepared and referred to as samples w20, w25,
w30, w35, and w40, respectively. The growth temperatures were I010 and 720
"C
for GaN and InGaN, respectively. As-grown samples were thermally annealed in a quartz tube furnace at different temperatures ranging from 800 to 900 "C in nitrogen ambient for 30 min. From the room-temperature probe intensity profilesof sample w25 annealed with temperature 800'C for 30 min under pumping at different wavelengths, one can
see that at a wavelength (418 nm) within the PL spechum, the transmission intensity drops slowly from its peak.
The early-stage faster decay (with calibrated decay times shown in the figure) is supposed to be due to camer
relaxation through phonon interaction such that quasi-equilibrium condition is reached. The extended slower decay is attributed to carrier recombination under the quasi- equilibrium condition and its decay time is in the psec-nsec range. During this process, the interaction
with acoustic phonons also occurs. At shorter wavelengths for pumping, carrier relaxation to lower energy states becomes faster and more significant. In these situations, the residual camers become fewer. Fig. 1 shows the relative transmission intensity profiles of probe of the same sample pumped at 395
nm at various sample temperatures. It,is interesting
to see that the oscillations due to acoustic phonon interaction occur only in the temperature range from
100 to 150 K. Also, the LO phonon effect can be obselved only when the sample temperature is below
around 200 K. This phenomenon deserves further
investigation. Furthermore, from Fig. 1 one can see that the level of extended slow decay decreases with increasing temperature. This trend can be attributed to the effective thermal relaxation of carriers into lower-energy states as the pump photon energy is
much higher than the PL emission levels. T i m e (pa) Fig. 1
0-7803-7766-4/03/$l7.00 02003 IEEE
