GaInNAsSb=GaAs vertical cavity surface
emitting lasers at 1534 nm
M.A. Wistey, S.R. Bank, H.P. Bae, H.B. Yuen, E.R. Pickett,
L.L. Goddard and J.S. Harris
Electrically pumped, C-band vertical cavity surface emitting lasers
(VCSELs) grown on GaAs are reported for the first time. The
VCSELs employed three GaInNAsSb quantum wells separated by
GaNAs barriers. Pulsed lasing was observed at 1534 nm, in the ITU
C-band, when cooled. These lasers exhibit the longest wavelength
reported to date for electrically pumped VCSELs grown on GaAs
substrates.
Introduction: The transition from copper wire to optical fibre in local
and me tro area networks has long bee n hampered by the lack o f
inexpensive, thermally stable lasers in the C-band, at wavelengths of
1530–1560 nm, which is the minimum loss window in optical fibre.
Vertical cavity surface emitting lasers (VCSELs) promise stable
wavelength, economies of scale for arrays, fabrication and testing,
and inexpensive, lens-free packaging with optical fibre. Although
VCSELs previously have been demonstrated on InP with quaternary,
distributed Bragg reflector (DBR) mir rors [1, 2], it has been difficult
to grow lasers at long wavelengths using inexpensive GaAs substrates,
with electrically injected VC SELs reported only at 1300 nm [3] and
1460 nm [4]. GaAs-based VCSELs offer additional advantages,
including higher ther mal and electri cal conductivity using Al(Ga)As=
GaAs DBRs, as well as selective wet oxidation of AlGaAs for
electrical and opt ical confinement. There has been one recent repor t
of an optically-pumped VCSEL with dielectric DBRs [5] and several
edge emitting las ers in t he C-band [6–8], but to the best of our
knowledge, no electrically-pumped VCSELs on GaAs have been
reported in the literature to date. We report monolithically g rown,
electrically pumped GaInNAsSb VCSELs g rown on GaAs, operating
in pulsed mode a t 153 4 nm.
Growth and fab ricatio n: The VCSELs were grown on an n-doped
GaAs wafer by plasma assisted, solid source mole cular beam epitaxy
(MBE). Arsenic and antimony were supplied by valved a nd unvalved
crackers, respectively. Nitrogen was supplied by a modified SVTA
plasma source. The ion flux from the plasma was minimised by using
ion deflection p lates biased at 40 V and g r ound [9]. The bottom,
silicon-doped DBR was composed of 31 pairs of AlAs=GaAs,
followed by four pairs with a lower Al composition ( 91%) to prevent
oxidation after the dry et ch. The top, carbon-doped DBR was 21 pairs
of Al
0.91
Ga
0.09
As=GaAs, with a 40 nm 98% AlGaA s layer for
selective oxidation embedded in the first AlGaAs layer. The doping
andstep-gradingprofilesforthetopDBRwerebasedonthoseof
Yechuri et al. [10], modified for MBE growth using t wo aluminum
and two gallium cel ls at fixed temperatures. Each p-doped hetero-
junction was g raded in six s teps over 30 nm.
The nominally undoped, one-lambda cavity had three 7.5 nm
Ga
0.62
In
0.38
N
0.03
As
0.94
Sb
0.03
quantum wells (QWs) sur rounded by
21 nm GaN
0.04
As
0.96
barriers. A high nitrogen content had previously
been shown to be advantageous for low-threshold lasers at long
wavelengths [11]. Owing to a shortage of ports on our MBE machine,
the DBRs were grown in one machine, and the cavity in another,
transferred under ultra high vacuum and an arsenic cap. Further growth
details have been reported elsewhere [11, 12].
After growth, one-quarter of the wafer was annealed for 1 min at
680
C. A second GaAs wafer was used as a proximity cap to minimise
arsenic evaporation during the anneal. Mesas were defined by a dry etch
into the cavity, followed by selective wet oxidation for current confine-
ment. Annular top contacts were defined by evaporation and liftoff of
Ti=Pt=Au. Au=Ge=Ni=Au was evaporated onto the backside for the
n-contact, then annealed at 410
C for 1 min.
Results: Fig. 1 shows the spectrum from a VCSEL with a 14 mm
diameter current aperture operating at 48
C and 110 mA (nominal),
with a 0.67% duty cycle, at 1.6 times threshold. Limitations of the
pulsed laser driver prevented operation at higher currents. Fig. 2
shows th e light i ntensit y against c urrent. The nominal curren t
reported here is g reater than the actual peak cur rent. Each 0.1 m s
pulse was fol lowed by a long, exponential decay, up to 1 msin
duration, which is believed to be due to parasitic capacitance in the
testing stage. The extended pulse widt h meant that the actual duty
cycle was greater than that reported above, and the time-averaged
current was higher, although only the peak cur rent contributed to
lasing. As a consequence, the th reshold cur rent repor ted here, 70 mA
( j
th
¼45 kA=cm
2
), represents an upper bound on the a ctual threshold
curre nt.
10
-5
10
-6
10
-7
10
-8
10
-9
log intensity, a.u.
l
,
nm
1530 1532 1534 1536 153
8
Fig. 1 Pulsed lasing spectrum at 1.6 threshold for 14 mm aperture
VCSEL
0.030
0.025
0.020
0.015
0.010
0.005
0
peak power, mW
current
,
mA
0 20 40 60 80 100 12
0
Fig. 2 Light intensity against current in pulsed operatio n
Cooling was required owing to a gain-cavity misalignment. The
cavity was designed for 1540 nm lasing at 5
C, using quantum wells
designed for 1550 nm edge emitters [7], and refractive index data from
Leibiger et al. [13]. However, the spontaneous emission from this
sample peaked near 1585 nm at room temperature. This 35 nm redshift
was consistent with several other samples grown on the same day. The
peak gain wavelength therefore had to be temperature tuned to match
the cavity. Lasing was obser ved at temperatures below 25
C, with a
peak output power at 50
C, which was the limit of the cooling
apparatus. Assuming a relative gain=cavity shift of 0.5 nm=
C, the
35 nm redshift would suggest that the peak output power would have
been between 70 and 60
C.
We believe that the redshift is related to an anomaly observed in the
nitrogen plasma conditions. The anomaly was marked by unusually low
reflected RF power from the cell, higher backpressure in the gas
foreline, and a decrease in the ratio of atomic to molecular lines in
the optical emission spectrum. It remains unclear whether this changed
the energy or type of nitrogen species reaching the wafer, or whether it
was merely symptomatic of another problem, such as gas contamina-
tion. A redshift associated with several kinds of surface damage has
previously been observed, but we believe that the present case can be
explained by an excessively high nitrogen content. Secondary ion mass
spectroscopy (SIMS) showed that the nitrogen concentration was up to
15% higher than intended. Further experiments to identify the causes of
the anomaly are underway. Whatever the cause, the cavity was designed
ELECTRONICS LETTERS 2nd March 2006 Vol. 42 No. 5
to be resonant at only 1540 nm, so the peak gain needed to be thermally
tuned to much shorter wavelengths.
Conclusions: We have demonstrated elec trically pumped, GaIn-
NAsSb VCSELs operating in the ITU fi bre communication C-band
at a wavelength of 1534 nm. These are the first such V CSELs on
GaAs, to the best of our knowledge. These VCSELs required
substantial cooling owing to a large gain=cavity misalignment, but a
simple shift in cavity resonance is expected to be sufficient to produce
lasing at room temperature. These monolithic VCSELs represent a
significant st ep toward inexpensive l asers fo r fibre communication.
Acknowledgments: The aut hors tha nk A. Moto of Su mitomo Electric
Industries for SIMS and donation of subs trates, and R. Wheel er for
expert technical as sistanc e. Thi s work was supported by the DARPA
Chip-Scale WDM program, the MARCO Interconnect Re search
Center, and the Stanford Network Research Center (SNRC).
# IEE 2006 27 December 2005
Electronics Letters o nline no: 20064455
doi: 10.10 49/el:2 00 64455
M.A. Wistey, S.R. Bank, H.P. Bae, H.B. Yuen, E.R. Pickett, L.L.
Goddard and J.S. Harris (Solid State and Phot onics Laboratory,
Stanford University, Stanford, CA 94305, USA)
E-mail: wistey @snowmass.stanford.edu
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