Transcript New results published by IOP

MIOMD-X, September 5-9, 2010, Shanghai, China
P-28
Diagnostic and characterization of VCSEL diodes
based on GaSb substrate
E. Hulicius1*, J. Pangrác1, J. Oswald1, T. Šimeček1, J. Vyskočil1, I. Matulková2, J. Cihelka2, Z. Zelinger2, S. Civiš2, Z. Chobola3
* Corresponding author email: [email protected]
1
Institute of Physics, Academy of Sciences of the Czech Republic, v. v. i., Prague, Czech Republic
2 J.
Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v. v. i., Prague, Czech Republic
3 Brno
University of Technology, Faculty of Civil Engineering, Brno, Czech Republic
Abstract
Experimental
Vertical-Cavity Surface-Emitting Laser (VCSEL) diodes are among the
youngest members of the semiconductor laser diode family. The aim of our
work focuses on the measurement of the properties (the spectral range of
the laser emission, temperature, current
tunability, far-field and lifetime) of
experimental VCSEL diode lasers based
on GaSb and operating in the wavelength
infrared region around 2.3 μm.
A new type of VCSEL GaSb-based diodes for the 4250 cm−1 (around 2.3 μm) wavelength region have been developed at WSI
of the TU München. The laser structures were prepared by MBE on n-type GaSb, with the n-contact at the bottom. The bottom
Bragg epitaxial mirror consists of twenty-four couples of AlAsSb/GaSb layers. The active zone of these lasers consists of five
11-nm-thick Ga0.65In0.35As0.1Sb0.97 quantum wells surrounded by Al0.35Ga0.65As0.03Sb0.97 barriers. The device employs a
p-GaSb/n-InAsSb-buried tunnel junction (5-8 μm in diameter) as a current aperture and replaces the high loss p-material by
n-type material. The Ti/Pt/Au ring contacts are sputtered on InAsSb contact layer. The dielectric mirror from four couples of
Si/SiO2 layers is placed at the top of the laser structure above the buried tunnel junction.
VCSEL structures and their preparation are described by our colleagues from WSI TU München here during MIOMD 10.
These lasers were prepared in frame of
EU project NEMIS at TU München.
A high-resolution spectrometry was
used for laser diagnostics.
Fine tuning of these lasers can be realized by changing the temperature or the injection current. Lasers operate in the CW
regime at room temperature with an output power around 50 μW. The threshold current is around 1.5 mA and its temperature
dependence is relatively weak.
AGEING
FAR FIELD MEASUREMENT
• We have measured the far field patterns of the NEMIS
TL=30°C
1:1 33Hz
PbSe1P
30V/1M
120
6.0
110
5.5
100
5.0
90
4.5
80
4.0
Power [a.u.]
Relative Power [%]
1-A12440 10mA
2-A12441 10mA
4-A12444 8mA
70
60
50
1.5
20
1.0
10
0.5
0
0.0
6000
8000 10000 12000 14000
Time [Hours]
1:1 33Hz
PbSe1P
30V/1M
• There are two main shapes of the laser beam: Either a
more or less circular symmetrical beam in the axis of the
laser or a twofold distribution of the power in two directions
both inclined from the axis of the laser some 11 degrees.
• Measuring of the two beams revealed no difference in
their spectra.
• The spectra of both beams contained all the modes.
2.5
30
4000
TL=30°C
3.0
2.0
2000
1-A12440 10mA
2-A12441 10mA
4-A12444 8mA
3.5
40
0
lasers in order to provide certain additional insight into the
modal behaviour.
Far fields of some lasers exhibits this double beam (double
mode) behaviour, but majority have nice simple ones.
0
2000
4000
6000
8000 10000 12000 14000
Time [Hours]
Ageing test equipment
Ageing box with holders for 10 individual lasers on the headers under CW current provided by ten
stabilized power sources and a temperature stabilized detector that can be regularly precisely
attached to these lasers to monitor their relative optical power output was used.
For special purposes also two positions have been prepared for pulsed pumping by 1μs pulses with
up to 100 kHz repetition frequency with pulsed detection using lock in amplifier. These positions
enable to measure lasers with higher currents and worse heat dissipation.
Increase of output power of the green-marked laser A1244 can be explained by changes of its far
field.
FF spectrum of laser A1244 (green-marked in the Ageing part, left Fig.)
after 1500 and 9020 hours ageing, respectively.
ELECTRICAL NOISE LASER MEASUREMENT
HIGH RESOLUTION MODE SPECTRA MEASUREMENT
• The last set of lasers is containing several devices, which exhibit greater side mode suppression ratio
reaching even 20 dB, but even these lasers have one or few side modes of lower intensity.
• A surprising feature of the side modes of some of the latest lasers is, that the side modes exhibit
greater line width while the older lasers did not behave this way. We have not found any convincing
explanation for the origin of these modes and their different behaviour in the complicated VCSEL
optical resonator.
Plot of log I versus lin U and log I versus log U for different laser diodes in the forward bias direction.
• We have observed short circuiting resistances for all samples
• Most of the lasers, especially those prepared earlier exhibit multimode emission with relatively
irregularly spaced modes as can be seen on the FTS spectra with the upper estimate of their line
widths being some 150 MHz.
at voltages below 0.3 V, being in the range 1×103 – 5×104 .
• For voltages exceeding 0.6 V we observed substantial voltage
drop on the series resistance and contact resistance leading to
the estimation of this resistance to be in the range 8 – 200 .
• In the case of sample No. 3 we observed two PN junctions in
series.
The noise spectral density as a
function of forward bias and load
resistor RL=1k Ω for different laser
diodes.
• From the measurement of spectral voltage density SU on
applied field in forward bias it was evident, that the contact
quality is poor, especially for sample No.5. From the
measurements at voltages in access of 0.5 V we observed in
case of samples 2 and 5 also pulse noise, indicating instability in
the tested structure.
The authors gratefully acknowledge the financial support by the European Union via NEMIS (contract no. FP6-2005-IST-5-031845), research center LC-510, and the
program MSM 6840770014 and K. Kashani, A. Bachmann and Prof. M.-C. Amann from the Walter Schottky Institut for providing the VCSEL samples and O. Petříček
for ageing measurements. This work is the part of the research programs of the Grant Agency of the Academy of Sciences of the Czech Republic (Grant No.
A400400705), the Ministry of Education, Youth and Sports (COST MP0702 and COST 729) and the research program of Institute of Physics AS CR 10100521.