Short-wavelength intersubband transitions down to 1.6
m
in ZnSeÕBeTe type-II superlattices
R. Akimoto,
a)
Y. Kinpara, K. Akita, F. Sasaki, and S. Kobayashi
Electrotechnical Laboratory 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan
͑Received 19 October 2000; accepted for publication 4 December 2000͒
We report photoinduced electron intersubband absorption in ZnSe/BeTe type-II superlattices. The
wavelength of the intersubband transition as short as 1.6
m, covering the 1.55
m optical
communication wavelengths within its absorption band width (ϳ250nm), is achieved in the ZnSe/
BeTe SLs with 4.5 ML-thick ZnSe layers. The intensity in photoinduced intersubband absorption
increases sublinearly with pump intensity, reflecting the characteristic recombination processes of
electron-hole pairs in a heterostructure with type-II band alignment. © 2001 American Institute of
Physics. ͓DOI: 10.1063/1.1343843͔
Intersubband transitions in semiconductors quantum
wells ͑QWs͒ has attracted a lot of attention with rapid devel-
opments of quantum cascade lasers and novel infrared
photodetectors.
1
The ultrafast carrier relaxation ͑sub-ps to
ps͒ in intersubbands is attractive for other device applica-
tions such as ultrafast all-optical switches and modulators,
especially for use at 1.55
m optical communication sys-
tems. Most of the optical devices associated with intersub-
band transitions are, however, fabricated from InGaAs/
InAlAs and GaAs/AlGaAs and their operating wavelength is
limited in the range longer than ϳ3
m due to their insuffi-
cient conduction band offset. To shorten the intersubband
transition wavelength further, other materials with larger
conduction band offset are needed. So far, the short-
wavelength intersubband transition have been reported in the
materials such as InGaAs/AlAs ͑1.55
m͒,
2
InGaAs/AlAsSb
͑1.45
m͒,
3
and GaN/AlGaN ͑1.75
m͒.
4
In this letter, we propose another material system, ZnSe/
BeTe, based on II–VI semiconductors, and study its inter-
subband transitions through photoinduced absorption mea-
surements. The ZnSe/BeTe is a novel material system with a
type-II band alignment, originally adapted to extend a device
life time of II–VI blue-green diodes.
5
In this system, a huge
conduction band offset leads to localizing potentials of
ϳ2.3 eV for electrons in ZnSe layers,
6
by which we actually
demonstrate the intersubband transitions as short as 1.6
m
covering the 1.55
m wavelength within its spectral half
width. Besides the benefit from the huge conduction band
offset, the ZnSe/BeTe system takes the advantage of its
higher ionicity in ZnSe layers, regarding the ultrafast carrier
relaxation. That is, the intersubband relaxation process is
controlled mainly by the Fro
¨
hich interaction between elec-
trons and LO phonons, where a material with higher ionicity
gives faster relaxation. According to Ridley’s model,
7
ZnSe
QWs gives ϳ8 times shorter relaxation time than the one in
InGaAs QWs and also gives comparable time with GaN
QWs. As another point, the ZnSe/BeTe heterostructures can
be grown with high structural quality, since both ZnSe and
BeTe are closely lattice matched to GaAs substrate.
The samples of ZnSe/BeTe superlattices ͑SLs͒ were
grown on undoped ͑001͒-oriented GaAs substrates by mo-
lecular beam epitaxy in a dual-chamber system. After a 250-
nm-thick GaAs buffer layer was grown in a III–V chamber
to improve the quality of the GaAs surface, the sample was
transferred through ultrahigh-vacuum transfer modules into a
II–VI chamber for the growth of ZnSe/BeTe SLs. During the
growth, the termination of the ZnSe/BeTe interfaces was ad-
justed to have Zn–Te chemical bonds by controlling shutter
cycles of effusion cells. The each layer widths in the SLs
were determined by the observation of intensity oscillations
in reflection high-energy electron diffraction during the
growth. The ZnSe/BeTe SLs consists of 50 periods of alter-
nating a ZnSe and a 14-monolayer͑ML͒-thick BeTe layers. A
30 nm-thick ZnSe cap layer was grown on the top of the SLs.
All samples were grown in identical growth condition except
for their ZnSe layer width in SLs.
For the measurements of intersubband transitions,
samples with thickness of ϳ0.4mm were fabricated as a
multipass waveguide by polishing the two parallel ͑110͒ fac-
ets at an angle of 40°. This angle was chosen so as to pre-
vent total reflections of a probe light at the interface between
GaAs(n ϭ3.3) and ZnSe/BeTe layers(n ϭϳ2.3).
8
The
photoinduced absorption measurements were done employ-
ing a pump–probe technique. The 375 nm laser light from
frequency-doubled mode-locked Ti: sapphire laser with a
repetition rate of 76 MHz and a pulse duration of ϳ150 fs
was used as the pump, while a halogen lamp with a long-
pass filter ( Ͼ1
m) was used as the probe with a wire-grid
polarizer. The probe light transmitting from the sample was
dispersed by 0.275 m grating monochromator equipped with
cooled PbS detector. The pump laser beam was modulated
by an optical chopper at 190 Hz, and a modulated changes in
the transmission of the probe (Ϫ⌬T) were taken by a phase-
sensitive lock-in technique. The photoinduced absorption is
given by Ϫ⌬T/T in the case of Ϫ⌬T/T Ӷ1, where T is the
transmission without the pump. The sample was mounted on
a cryostat and kept at 4.6 K.
Figure 1͑a͒ shows a set of photoinduced absorption spec-
tra for the sample of ZnSe/BeTe ͑5.8/14 ML͒ SLs with the
pump laser intensity of 0.67 W/cm
2
͑averaged in time͒ at 4.6
K. Each spectrum was recorded with the incoming probe
a͒
Electronic mail: akimoto@etl.go.jp
APPLIED PHYSICS LETTERS VOLUME 78, NUMBER 5 29 JANUARY 2001
5800003-6951/2001/78(5)/580/3/$18.00 © 2001 American Institute of Physics