Room-temperature CW operation of red vertical-cavity surface-emitting lasers grown by solid-source molecular beam epitaxy

Abstract

The authors present the first report of MBE-grown AlGaInP
vertical-cavity surface-emitting lasers. The lasers exhibit continuous
wave operation at 690 nm with a maximum output power of 0.56 mW. With a
10 μm optical aperture the threshold current is only 1.3 mA, and CW
lasing is achieved up to 45°C

Full text

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Journal of Crystal Growth 227–228 (2001) 324–328
Red vertical-cavity surface-emitting lasers grown
by solid-source molecular beam epitaxy
M. Saarinen*, N. Xiang, V. Vilokkinen, P. Melanen, S. Orsila,
P. Uusimaa, P. Savolainen, M. Toivonen1, M. Pessa
Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, 33101 Tampere, Finland
Abstract
Plastic optical fibres, which have a local attenuation minimum at 650nm, have attracted much interest for low-cost
short-haul communication systems. Red vertical-cavity surface-emitting lasers (VCSELs) provide a potential solution
as light sources for these systems. The operation of vertical cavity emitters is based on a Fabry–Perot microcavity,
which is formed by placing an optically active region inside of two parallel mirrors. These mirrors are usually formed
epitaxially. So far, metal organic chemical vapour deposition (MOCVD) has been the major technology used for
growing visible VCSELs. Recently, an alternative growth method}solid-source molecular beam epitaxy (SSMBE)}
has been introduced to be a viable solution to the fabrication of these structures. The authors present the first MBE-
grown visible AlGaInP vertical-cavity surface-emitting lasers. A laser with a 10mm emitting window has an external
quantum efficiency of 6.65% under continuous wave operation and it is still lasing at 458C. Furthermore, a threshold
current less than 1.0mA is obtained for a device, which has an 8mm emitting window. # 2001 Elsevier Science B.V. All
rights reserved.
PACS: 81.15.H; 42.55.P
Keywords: A3. Molecular beam epitaxy B2. Semiconducting III–V materials; B3. Laser diodes
1. Introduction
Redvertical-cavitysurface-emittinglasers
(VCSELs) are interesting components due to their
potential applications in plastic optical fibre (POF)
communications, local area networks (LAN),
chemical sensing, optical data storage, barcode
scanners, and laser printing. A surface normal
operation with potential for on-wafer testing, a
low angular divergence (6–108), and a natural
employment in two-dimensional arrays make
visible VCSELs superior to edge-emitting lasers
for many low-power applications. Since the
demonstration of the first optically pumped [1]
and electrically injected red VCSELs [2], practi-
cally all red VCSELs have been grown by metal
organic chemical vapour deposition (MOCVD). In
this paper, we present red (690nm) VCSELs which
are grown by solid-source molecular beam epitaxy
(SSMBE). In SSMBE, valved crackers, containing
solid-source materials, produce stable and closely
*Corresponding author. Tel. +358-3-365-2552; fax: +358-3-
365-3400.
E-mail address: mika.saarinen@orc.tut.fi (M. Saarinen).
1Present address: Coherent-Tutcore Ltd., Tampere, Finland.
0022-0248/01/$-see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0022 - 0248(01 )0 0714- X

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controlled group-V (As2/4, P2) fluxes [3]. Recently,
SSMBE has been used to prepare high-perfor-
mance red edge-emitting lasers [4] and resonant
cavity light-emitting diodes (RCLEDs) [5].
The first electrically injected visible VCSELs,
which operated between 639 and 661nm with
pulsed current and launched a peak power of
3.3mW from a 120mm device, were demonstrated
in 1992 [2]. The structure consisted of AlAs/
AlGaAs distributed Bragg reflectors (DBRs) and
an AlGaInP optical cavity with GaInP quantum
wells. During the same year, a 699nm phosphorus-
free VCSEL, having an AlAs/GaAs superlattice
active region, was demonstrated [6]. A room
temperature continuous wave operation of red
VCSEL was achieved in 1993 [7], when a
component with a 115mm mesa had a threshold
current of 5mA and a peak output power of
25mW at 670nm. Sandia National Laboratories
has fabricated a red monolithic AlGaInP VCSEL
with a record-high cw output power of 2.9mW at
l=676nm [8]. This device had a wall-plug
efficiency of 10%. For a VCSEL operating at
l=687nm, Sandia National Laboratories ob-
tained Pout¼ 8:2mW with a power conversion
efficiency of 11%. This VCSEL also had a single-
mode operation up to 1.9mW [9]. The significant
improvements in device performance are most
likely results of a better material quality and a
shorter optical cavity length (1?l), which tend to
reduce optical losses in the vicinity of the gain
region.
Although the results have been promising, the
reliability of the red VCSELs has not been
thoroughly investigated yet. It is generally known
that red VCSEL characteristics degrade quite
rapidly during the first 100h. With AlGaInP
structures, initial performance improves after
annealing [10], but the performance is not good
enough for a reliable cw-operation up to 808C,
whichisneededfor
packages.
uncooledoptical fibre
2. Material growth
The VCSEL structure was grown on an (100) n-
GaAs 200substrate using a MBE reactor equipped
with valved P- and As-cracking cells and hot-lip
dual-filament group-III effusion cells (Al, Ga, In).
The cells exhibited good effusion stability, which
enabled us to grow the devices without any in situ
monitoring system. Si and Be were used as n- and
p-type dopants, respectively. The reactor has very
low oxygen level, which is known to be important
when phosphorus-containing materials have to be
used in active layers [11]. Having optimized the
optical efficiency of the AlGaInP materials and the
active region [12], the growth of the epitaxial
DBRs stacks was investigated. Fig. 1 shows a
room-temperature photoluminescence spectrum of
an active QW structure and a reflectivity of a
calibrated 15-period-DBR structure as a function
of wavelength. The active region growth tempera-
ture was Tgr¼ 5108C, and Tgr¼ 5908C and 5308C
were used for the n- and p-mirrors, respectively.
Growth rates were ?1.5mm/h for the DBR layers
and ?1.8mm/h for the AlGaInP active layers.
TheVCSELconsisted
(44.5nm)/Al0.75Ga0.25As(10nm)/Al0.50Ga0.50As
(50.5nm) DBRs and an (Al0.3Ga0.7)0.51In0.49P
active region, which contained three compressively
strained (?0.86%) GaxIn1?xP (6nm) quantum
of Al0.95Ga0.05As
Fig. 1. A photoluminescence spectrum of an active QW
structure and a simulated and measured reflectivity spectrum
of a 15-period-DBR structure.
M. Saarinen et al. / Journal of Crystal Growth 227–228 (2001) 324–328325

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wells placed at the antinode of the cavity mode.
The bottom n-DBR had 55.5 pairs of l/4 layers,
and the top p-DBR had 38 pairs of l/4 layers. An
Al0.75Ga0.25As intermediate layer was grown to
reduce series resistance. To improve the current
confinement, an AlAs oxidation layer was inserted
close to the cavity. No composition grading was
applied anywhere in the structure. The doping
profile was graded for the DBRs from 1?1018to
5?1017cm?3near the cavity to reduce optical
losses to free carriers, which is known to be a
major loss mechanism in the visible-light VCSELs
[13]. No other modulation or interface doping was
used. The 1?l Fabry–Perot cavity length was
positively detuned (lcav>lqw) to compensate the
red-shift of the gain peak with an increasing
current injection, due to heating effects.
3. Device fabrication
Circular mesas with diameters ranging from 34
to 50mm were fabricated by reactive ion etching
using SiCl4. The top DBR was etched down to the
AlGaInP active region. Selective wet thermal
oxidation at 3708C was employed to form current
apertures of 4–20mm in diameter, and Si3N4was
used for passivation. Ti/Pt/Au top metal contacts
and bonding pads were formed by e-beam
evaporation and lift-off technique. Ni/Au/Ge/Au
n-contact metal was evaporated on the backside of
the thinned sample and annealed at 4208C for 30s.
Processed devices were separated and bonded on
TO-46 cans with silver-filled epoxy.
4. Results and discussion
The red VCSELs were tested under direct
current injection. Fig. 2 shows light power–for-
ward voltage/current (cw) curves for devices with
8, 10 and 15mm emitting windows, measured
without a temperature control. These devices have
diode turn-on voltages of approximately 2.0, 1.9
and 1.8V, and they exhibit maximum output
powers of 0.37, 0.56 and 0.64mW at room
temperature for 8, 10 and 15mm devices, respec-
tively. The threshold currents are 0.92, 1.3 and
2.5mA, which correspond threshold current den-
sities of approximately 1.8, 1.7 and 1.6kA/cm2.
The smaller devices have higher series resistance,
and thus the turn-on voltages are higher. On the
other hand, the smaller emitters also have lower
threshold currents, but lower maximum output
power.
External quantum efficiencies (EQEs) versus
current curves are presented in Fig. 3. An 8mm
device has a maximum external quantum efficiency
of 5.56%, while for the larger ones, 10 and 15mm
devices, these values are 6.65% and 5.56%, respec-
tively. The best MOCVD-grown red VCSELs
have achieved an EQE of 9.6% at I=5.6mA [9].
We believe that the lower quantum efficiency in
our devices is partly due to the too heavily doped
top DBR, which tends to absorb light.
Fig. 4, for its part, shows a typical electrolumi-
nescence spectrum of our red VCSELs with three
different currents. The lasing wavelength is about
690nm and FWHM clearly less than 1nm. Due to
internal heating the peak is slightly red-shifted
with increasing current.
Fig. 5 shows the temperature dependence of the
output power of a 10mm device. Lasing is observed
up to 458C with threshold currents changing from
Fig. 2. Light power and forward voltages as functions of DC
drive current for 8, 10 and 15mm devices.
M. Saarinen et al. / Journal of Crystal Growth 227–228 (2001) 324–328326

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0.9mA at 58C to 3.4mA at 458C, corresponding to
the characteristic temperature (T0) of 30K. In the
range of 5–358C, T0is about 40K. A faster shift of
the gain spectrum compared to that of the cavity
mode is mainly responsible for the poor values of
T0and a rapid decrease of efficiency.
The burn-in test, displayed in Fig. 6, is a
promising evidence of reliability of these VCSELs.
After 150h of a constant current operation the
output power is still gradually increasing and no
signs of degradation can be observed, even though
the output power is close to its maximum value.
The increase in power is mainly due to an
annealing effect, which removes undesired non-
radiative recombination centres in the vicinity of
the active region [12].
In summary, SSMBE-grown 690nm VCSELs
have been demonstrated. Less than 1mA thresh-
old current is observed for an 8mm device.
Furthermore, a 10mm device has an external
quantum efficiency of 6.65%, and the continuous
wave lasing action is detained up to 458C.
Fig. 3. External quantum efficiencies versus current for three
different devices.
Fig. 4. Electroluminescence spectra of a red VCSEL with
increasing current. The lasing wavelength is 690nm.
Fig. 5. The temperature dependence of the threshold current
for a 10mm VCSEL. Device shows a lasing action up to 458C.
M. Saarinen et al. / Journal of Crystal Growth 227–228 (2001) 324–328 327

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Acknowledgements
This work was supported by the Technology
Development Centre (TEKES) of Finland within
the Cost-268 B (# 40562/99) Project and the
Academy of Finland within the EMMA MACO-
MIO (# 46784) Project.
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