Vertical-cavity surface-emitting lasers are generating much interest due to their geometric suitability for two-dimensional array fabrication and their potential for achieving ultra-low thresholds. Here we report on optically- and electrically-pumped microlaser devices. having transverse dimensions of a few microns and active material lengths of a few hundred A. The very small volumes are a key factor in achieving low thresholds. So far however surface recombination has prevented us from achieving thresholds much below 1 mA

Burrus, C. A.

Florez, L. T.

Harbison, J. P.

Jewell, J. L.

Lee, Y. H.

McCall, S. L.

Olsson, N. A.

Sandroff, C. J.

Scherer, A.

Tucker, R. S.

Caltech Authors - Main

PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spie
Vertical cavity lasers for optical
interconnects
Jack L. Jewell, Yong H. Lee, Samuel L. McCall, Axel
Scherer, James P. Harbison, et al.
Jack L. Jewell, Yong H. Lee, Samuel L. McCall, Axel  Scherer, James P.
Harbison, Leigh T. Florez, N. Anders Olsson, Rodney S. Tucker, Charles A.
Burrus, Claude J. Sandroff, "Vertical cavity lasers for optical interconnects,"
Proc. SPIE 1389, Microelectronic Interconnects and Packages: Optical and
Electrical Technologies,  (1 April 1991); doi: 10.1117/12.25542
Event: Advances in Intelligent Robotics Systems, 1990, Boston, MA, United
States
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Vertical cavity lasers for optical interconnects
J.L. Jewella, Y.H. Leea, S.L. McCallb, A. Schererc,
J.P. Harbisonc, L.T. Florez', N.A Olssonb, R.S. Tuckera,
C.A. Burrus', and C.J. Sandroffc
a) AT&T Bell Laboratories, Rm. 4G-520, Holrndel, NJ 07733 USA 1-201-949-7666
b) AT&T Bell Laboratories, Murray Hill, NJ 07974 USA
c) Bell Communications Research, Red Bank, NJ 07701 USA
ABSTRACT
Vertical-cavity surface-emitting lasers1 are generating much interest due to their geometric suitability for
two-dimensional array fabrication and their potential for achieving ultra-low thresholds. Here, we report on
optically- and electrically-pumped microlaser devices. having transverse dimensions of a few microns and
active material lengths of a few hundred A. The very small volumes are a key factor in achieving low
thresholds. So far however, surface recombination has prevented us from achieving thresholds much below 1
mA.
1. ELECTRICALLY PUMPED MICROLASERS
We have constructed more than one million electrically-pumped vertical-cavity surface-emitting plasers with
dimensions of a few pm on a single GaAs chip3. Cylindrical plasers have diameters 1, 1.5, 2, 3, 4, and 5 pm
with heights about 5.5 pm (Fig. 1). Device density is greater than two million per square cm. Cylindrical and
rectangular-shaped devices of various sizes up to 100 pm across were also tested. The lowest thresholds were
achieved in a single quantum well (SQW) structure and were 1.1 mA pulsed in a 4-pm diameter p-laser, and
1.5 mA CW in a 5-pm square device4. The latter device, when treated to reduce surface recombination,
showed a CW threshold of 1.0 mA. A 5-pm diameter three-quantum-well (3QW) device, similarly treated, had
a 0.8 mA pulsed threshold. In most of the chips tested the 5-pm diameter devices had yields of 95-100%.
Lasing was observed in 3QW plasers as small as 1.5-pm in diameter with an active volume <0.05 pm3. In
SQW plasers lOx 10 pm square, modulation of pseudorandom data streams at rates of 1 Gb/sec were
demonstrated with < 10 10 bit error rates measured by the receiver. Sinusoidal modulation of 3QW lasers of
the same size showed a -3 dB cutoff at >8 GHz.
Molecular beam epitaxy (MIRE) was used to grow the two samples. Both substrates were GaAs Si-
doped at 3x1018 cm3 on each of which alternate layers of AlAs and GaAs were grown, forming an
interference mirror. These layers, each nominally a 1/4 wave optical thickness at 980 nm, were Si-doped, also
at 3X 1018 cm3. In the three-quantum-well (3QW) sample the undoped active region consists of three 80 A
In02Ga08As quantum wells interleaved with four 100 A GaAs barriers. The SQW sample had an undoped 100
A In02Ga08As active layer. Surrounding each active region are 200 A compositionally graded undoped
A1Ga1_As layers. Above and below the graded layer active region complex are spacer regions of doped
A105Ga05As of thicknesses so that a full wave exists between mirrors. The second mirror was constructed
similarly to the first except that Be was used as the dopant at 5x1018 cm3 density. In the 3QW (SQW)
sample the Si-doped mirror had 20 1/2 (23 1/2) half-wave periods while the Be-doped mirror had 12 (15). The
well/barrier nature of the heterostructure of the mirrors introduces high electrical resistivity. For this reason
both mirrors, the Be-mirror in particular, had superlattices at the interfaces where the carriers had to climb
SPIE Vol. 1389 International Conference on Advances in Interconnection and Packaging (1990) / 401
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up the potential barriers. The non-abrupt interfaces help reduce the electrical resistance without ubstantial1y
degrading the reflectivity. Above the Be-doped mirror were grown a -'0.2 wave optical thickness of
A1As/superlattice and a 30-A GaAs cap, both Be-doped at 5x1018 cm3. For low contact resistance, the cap
was also delta-doped with i0' cm2 of Be. The cap layer was covered with unalloyed Au 1500 A thick.
Standard contact photolithography with liftoff was used to create spots of Ni 1 to 5 jim diameter
and 5-200 cm square on the Au. Chemically-assisted ion beam etching (CATBE) was used to etch through the
Au and the 5-6 pm heterostructure, leaving the Ni covered areas6. The Ni (etch mask) thickness was about
1500 A and it barely eroded away. Residual Ni should have no significant effect on laser operation. The other
side of substrate was polished and a wire serving as ground was spot welded to it. Positive voltage pulses were
applied to the Au caps using a small probe tip of radius ''5 pm. Laser outputs went through the 500-pm-
thick substrate and out the polished side. The configuration is shown is Fig. 2.
', Y V
r
A A & I A ..
f
r
4r
, S
r
.
4
. I Zi : : t : i1rl$ Pb4$$
\LLAAAAAA
\'
S
*i kE- 10 iim
Fig.1 - Small portion of an array of plasers.
1.1 Microlaser Results
All experiments were performed at room temperature. For all lasers below threshold the luminescence was
unpolarized, while above threshold the laser light was linearly polarized. Slight asymmetry in the structures,
rather than crystal orientation appears to cause the polarization as with previous optically-pumped
microlasers8. Figure 3 displays output light at 958 nm vs. driving current for a 3-pm 3QW plaser on thefirst
chip tested. Voltage pulses were 50 ns long at low duty cycle. The measured single-facet differential quantum
efficiency was about 16% despite some absorption of the laser output in the doped substrate. At threshold the
voltage was about 15 V rising to 20 V at 3.5 mA. The 5-pm 3QW plasers required about S V and 2 mA for
threshold. 4-pm diameter SQW plasers had typical pulsed thresholds of 1.1 mA with --' 7 % differential
quantum efficiency. Other devices were etched to a level slightly below the active layer, resulting in lower
electrical resistance and higher thermal conductivity4. Diffractive losses in the non-waveguiding part of the
402 / SPIE Vol. 1389 International Conference on Advances in Interconnection and Packaging (1990)
ACTIVE
Fig. 2 - Schematic of the electrical pumping and
optical output. Devices are enlarged to show detail.
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bottom mirror were sufficiently small in 5 pm devices to be negligible. Room-temperature CW operation
was achieved in these devices with no heat-sinking applied. The main heat flow was conduction through the
bottom mirror into the substrate. Fig. 4 shows the input/output of a 5-pm square device having a 1.5 mA
threshold and 5 volts applied. The wavelength was 983 nm with a measured width of 3.5 A limited by the
spectrometer resolution. A 7-pm diameter 3QW laser had a CW threshold 2.5 mA at 4 V and output over 0.3
mW with 6 mA current. The same device, at 1% duty cycle, had 18 mW peak output power and >28 %
differential quantum efficiency (Fig. 5).
Fig. 3 - Pulsed output light vs. current in a 3-pm
diameter 3QW plaser. Differential quantum
efficiency is --' 16 %.
Fig. 4 - Room-temperature CW input/output for a
5-pm diameter SQW plaser with no applied
heatsinking.
Modulation speeds of the OW-operating SQW lasers were tested using a 3 GHz coaxial probe tip
and a pseudorandom pulse generator. A 50 resistor and 0.1 pf chip capacitor wired onto the probe tip
supressed large reflections from the highly resistive devices. The data was continuously incident on the plaser
with no gating. Despite electrical limitations due to detector speed and long wire lengths we obtained bit error
rates <10_b at a modulation speed of 1 Gb/sec from a 10 pm square laser. Fig. 6 shows the wide open eye
diagram and the direct laser output. We saw a clear trend of smaller plasers showing faster modulation
capability presumably due to their lower capacitance. Due to the probe tip size and spacing of the devices we
were unable to contact the 5-pm lasers without also touching the larger ones. Sinusoidal modulation
experiments were performed on 10 pm square 3QW lasers with very low inductance mountings. To a OW bias
current was added a small sinusoidal modulation. Modulation of the laser output was then monitored as a
function of the current modulation frequency. A typical result is shown in Fig. 7 with the output modulation
falling to -3 dB at >8 GHz. The same devices showed gain switching times of about 25 p5 produced by a 2
GHz sinusoidal current pump and observed with a streak camera. Micron-diameter plasers packaged in an
integrated circuit driver should be capable of many Gb/sec operation.
SPIE Vol. 1389 International Conference on Advances in Interconnection and Packaging (1990) / 403
I-
0I-
0
I-
0I-
0
2
INPUT (mA)
2
IN PUT (mA)
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I—
:D 10
:,0
Fig 5 - Peak input/output for a 7-pm diameter
3QW j1aser run at 1 % duty cycle. Differential
quantum efficiency is '- 28 %.
Fig. 8 (Upper trace) Eye diagram of 1.0 Gb/sec
modulation of a 1O-pmX 10-pm SQW 1ulaser. Time
scale is 200 ps/div. (Lower trace) Direct laser output
of the continuous stream of pseudorandom bit-
generated data. (5 ns/div)
404 / SPIE Vol. 1389 International Conference on Advances in Interconnection and Packaging (1990)
20
INPUT (mA)
5
z0 5
a0
0 3 6 9 12 15
FREQUENCY (GHz)
Fig. 7 - Modulation vs. frequency for a
GHz.
continuously-driven 1O-im square 3QW laser. The -3 dB cutoff is '3
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We saw lasing in plasers as small as 1.5 pm diameter with an active material volume less than
O.05pm3 in the 3QW sample. They typically required 2.3 mA and 27 volts for threshold. The current is
higher than that required for the 3- and 5-zm devices because surface recombination'° on the sidewalls greatly
reduces the carrier lifetimes. The expected 1/e lifetime in 1.5 pm diameter devices is <100 ps if the
recombination velocity in 1n02Ga08As is as large as that of GaAs. The high current density coupled with the
high resistance of this small diameter causes the 27 V requirement.
Both the current thresholds and voltages can be greatly reduced in lasers. Techniques are
developing which can supress surface recombination9. We have tried a Se-based chemical passivation on some
deeply etched 3QW lasers. Figure 8 shows the characteristic for pulsed operation with submilliamp threshold
in a 5-pm diameter plaser. The 0.8 mA threshold is only about a factor of two less than the threshold before
the chemical treatment, a much smaller improvement than hoped for. The improvement appears to be fairly
permanent since it has not degraded for more than 6 months. True passivation of the sidewalls should have
produced a larger improvement; also the solution used was not expected to last more than a few days. Most
likely the chemical simply removed part of the outer sidewall material which had been damaged by the CAIBE
processing, and any true passivation did not last long enough for us to observe. 2-pm diameter devices do still
lase with 1.0 mA pulsed threshold. Another group7 using SQW structures similar to ours has achieved 0.7 mA
CW threshold at room temperature without passivation.
E
I-
0
I—
0
2
INPUT (mA)
Fig. 8 - Pulsed light/current response with 0.8 mA threshold for a 5-pm diameter 3QW plaser with some
damaged sidewall material removed by chemical etching. Threshold before treatment was 1.5 mA.
The ability to pass the laser output through the substrate allows us to use the geometry of Fig. 2,
which is well-suited for micro-optic integration11. The output collecting lens can be replaced by lenslets with
diameters on the order of 100 pm formed on the substrate back side by etching processes. These processes are
non-labor-intensive and will help minimize packaging costs. Furthermore the high numerical apertures, made
possible by the very high refractive index of semiconductors, are ideal for coupling in or out of ultra-small
devices. Extremely compact optical systems to image data from one array of devices to another with transit
times '--' 10 ps or less11 can be built with this technology.
SPIE Vol. 1389 International Conference on Advances in Interconnection and Packaging (1990) / 405
0.
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2. OPTICALLY PUMPED MICROLASERS
We previously reported the characteristics of optically pumped microlasers8 with thick (1.6 pm) bulk GaAs
active regions and diameters as small as 1.5 pm. Now we have preliminary results of structures with a single
In02Ga08As quantum well and diameters down to about 0.5 m. Even in these smallest structures, lasing is
observed in a single transverse and longitudinal mode. Nearby 1- and 2-am diameter devices showed a
multitude of transverse modes, as expected from previous work on micron and submicron diameter
microresonators12. It is encouraging that in such small cavities (0.5 pm diameter is smaller than the light's
wavelength in air) the losses are still low enough to allow lasing with a single quantum well active layer. In
half-micron and smaller-diameter cavities the fraction of photons spontaneously emitted into the laser mode
can be orders of magnitude larger than for conventional lasers'3 and even approach unity. This effect can
result in extremely low thresholds with speeds much higher than could be achieved in larger cavities.
3. CONCLUSION
We have shown that it is possible to fabricate plasers in quantities of > 106 on centimeter size chips. The
yields for devices about 5 m diameter is very high, thresholds are in the mA range and speeds > 1 Gb/sec
have been demonstrated. Passivation should result in -10 pA thresholds. Series resistance in the mirrors
needs to be reduced to realize practical devices. The -- 0.96 pm wavelength is well-suited for microoptic
integration with GaAs lenslets.
4. REFERENCES
[1] F. Koyama, S. Kinoshita, and K. Iga, Appl. Phys. Lett., vol. 55, pp. 221-223 (1989).
[2] A. Ibaraki, K. Kawashima, K. Furusawa, T. Ishikawa, T. Yamaguchi, and T. Niina, Jpn. J. Appi. Phys.,
vol. 28, pp. L667 (1989).
[3] J.L. Jewell, A. Scherer, S.L. McCall, Y.H. Lee, S.J. Walker, J.P. Harbison, and L.T. Florez, Electron.
Lett., vol. 25, pp. 1123-1124 (1989).
[4] Y.H. Lee, J.L. Jewell, A. Scherer, S.L. McCall, J.P. Harbison, and L.T. Florez, Electron. Lett., vol. 25,
pp. 1377 (1989)
[5] K. Tai, R.J. Fischer, C.W. Seabury, N.A. Olsson, T-C. D. Huo, Y. Ota, and A.Y. Cho, Appl. Phys.
Lett., vol 55, pp. 2473-2475 (1989).
[6] A. Scherer, J.L. Jewell, Y.H. Lee, J.P. Harbison, and L.T. Florez, Appi. Phys. Lett., vol. 55, pp.
2724-2726 (1989).
[7] R. Geels, S.W. Corzine, J.W. Scott, D.B. Young, and L.A. Coldren, Photon. Tech. Lett., vol. 2, pp. 234-
236.
[8] J.L. Jewell, S.L. McCall, Y.H. Lee, A. Scherer, A.C. Gossard, and J.H. English, Appl. Phys.
Lett., vol.
54, pp. 1400-1402 (1989).
406 / SPIE Vol. 1389 International Conference on Advances in Interconnection and Packaging (1990)
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[9] C.J. Sandroff, M.S. Hegde, L.A. Farrow, C.C. Chang, and J.P. Harbison, App!. Phys. Lett., vo!. 54,
362-364 (1989).
[10] J. L. Jewel!, A. Scherer, S. L. McCall, A. C. Gossard, and J. H. English, Appi. Phys. Lett., vol. 51, pp.
94-96 (1987).
[11] J.L. Jewell, S.L. McCall, Y.H. Lee, A. Scherer, A.C. Gossard, and J.H. English, Tech. Dig., MOO/GRIN
'89 (The Japan Society of Applied Physics, Tokyo, Japan, July 24-26 1989) paper Ji; App!. Opt., to be
published.
[12] J.L. Jewel!, S.L. McCall, A. Scherer, H.H. Houh, N.A. Whitaker, A.C. Gossard, and J.H. English, App!.
Phys. Lett., vol. 55, pp. 22-24, 1989.
[13] T. Baba, T. Hamano, F. Koyama, and K. Iga, 16th European Conference on Optical Communication,
postdeadline paper ThG1.3, pp. 995-998.
SPIE Vol. 1389 International Conference on Advances in Interconnection and Packaging (1990) / 407
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Vertical cavity lasers for optical interconnects

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Vertical cavity lasers for optical interconnects

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