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Evaluation of VCSEL Mounted on CVD DiamondSubstrates
Harry C. Shaw301-286-6616
Associate Branch HeadComponent Technology and Radiation Effects Branch
Goddard Space Flight Centerharry.c.shaw@gsfc.nasa.gov
Melanie N. Ott301-286-0127
Director, Advanced Photonic Interconnection ManufacturingLaboratory
Director, Technology Validation Laboratory for PhotonicsSigma Research and Engineering
Goddard Space Flight CenterMelanie.Ott@gsfc.nasa.gov
Evaluation of Vertical Cavity Surface Emitting Lasers(VCSEL) mounted on CVD Diamond Substrates
Part 1 of ReportPart 1B of ReportPart 2 of ReportConclusion of Report
NEPP Publication: Evaluation of VCSELS on Diamond
http://misspiggy.gsfc.nasa.gov/tva/meldoc/vcsel/index.htm [10/23/2001 2:19:02 PM]
Evaluation of Vertical Cavity Surface Emitting Lasers (VCSEL) mounted on CVD Diamond Substrates
NASA/GSFC Component Technology and Radiation Effects Branch
September 21, 2001
Contact: Melanie Ott, Sigma Research and Engineering, (301)-286-0127 Harry Shaw, NASA/GSFC, 301-286-6616
2
Introduction………………………………………………………………………………………………………………...3 VCSEL Optical output power vs bias…………………………………………………………………………………….6 Characteristics of the VCSEL-on-diamond………………………………………………………………………………7 Characteristics of the VCSELs mounted directly to Kovar headers………………………………………………….16 Wavelength shifting of the VCSELs……………………………………………………………………………………..24 Conclusions………………………………………………………………………………………………………………..29
3
Introduction This report summarizes the evaluation of 850 nm VCSELs manufactured by Emcore. The purpose of the evaluation was determine if the superior thermal properties of diamond as a substrate would improve the performance and reliability of the mounted VCSELs. Four VCSELs were mounted to diamond substrates and four more were mounted directly to Kovar headers and subjected accelerated voltage and current overdrive conditions. The diamond substrates provided a significant reliability advantage, however, there were unexplained shifts in the spectral properties of the devices. These mode shifts could be the subject of further investigation. Bare die were purchased and mounted onto gold plated CVD diamond substrates 500um in thickness. Die attach was performed using 84-1 conductive epoxy (Ablestik). 1 mil Au thermosonic wire bonding was used. The individual die were mounted on 250 um centers. The substrates were mounted onto TO-8 gold plated kovar headers. A MTP ribbon cable was used as a waveguide to the HP7095 Optical Spectrum Analyzer (OSA) and HP 8153 Lightwave Multimeters. 4 VCSELs were mounted directly on the TO-8 cans to permit comparison of the performance with and without diamond substrates. Due to alignment fiber-VCSEL alignment difficulties, mating and demating of the ribbon cable was kept to a minimum. Figures 1 and 2 depict the VCSELs unmounted and mounted on diamond. The devices used in the evaluation are no longer available as discrete parts. The next generation is preferred because the devices are available as monolithic arrays. The arrays are preferred because the optical alignment problems with the discrete devices can be eliminated. Figure 2a depicts the bare diamond substrate. Figure 2b is a block diagram of the test setup.
4
Figure 1. 850 nm VCSEL with 3 apertures Figure 2. VCSELs mounted to CVD diamond substrates.
Figure 2a. Bare diamond substrate. Metalization is Ti/Pt/Au. Dimensions: 0.256” x 0.06” x 0.017” (thickness)
8 µm 15 µm 20 µm
5
PowerSupply
HP 7095OSA
HP8153
DUTs
100 Ohms
Figure 2b. Electrical Test Block Diagram
6
1. VCSEL Optical output power vs bias Each of the 4 VCSELs are driven by single power supply through a 100 ohm resistor. Table 1 depicts the power output response of the a typical device under swept bias. This device is mounted directly to the TO-8 can
Table 1 PEAK_A is the power of the highest spectral output component. TOT_PO is the sum of the power of all the spectral components. The detected power is less than the actual power due to fiber-VCSEL misalignment. It is interesting to note that divergence of the two curves at the low and high ends of the bias conditions
TOT_PO (L)PEAK_A (R)
Output power vs swept bias0.275 v to 7.5 v, 0.5 v increments
VDC
Tot
al P
out(
dBm
)
Pea
k S
pect
ral C
ompo
nent
Pou
t (dB
m)
-65
-55
-45
-35
-25
-15
-48
-44
-40
-36
-32
-28
-24
-20
-16
-1 1 3 5 7 9
7
2. Characteristics of the VCSEL-on-diamond. The following charts summarize the 41 days of continuous monitoring of the VCSEL-on-diamond units. One of the 4 devices (VCSEL_1) was monitored by the OSA. The other 3 devices (VCSEL_2, _5, _8) were monitored the lightwave multimeter. For the latter devices, the only parameter recorded by the LABVIEW program is total power out. The summary of the raw power out numbers is shown in Table 2
8
Statistical Summary of Raw Power Output Data (dBm) for VCSEL_1 Table 2
5 V bias Observations Mean Pout Minimum Pout Maximum Pout Std.Dev. VCSEL_1 674 -17.6358 -18.5157 -16.9374 .284717 VCSEL_5 674 -7.7309 -8.3565 -7.4715 .084840 VCSEL_2 674 -6.4325 -6.8194 -6.0730 .194107 VCSEL_8 674 -8.4453 -8.6328 -8.2102 .114320
6.25 V bias Observations Mean Pout Minimum Pout Maximum Pout Std.Dev. VCSEL_1 913 -20.3849 -21.0891 -19.7740 .155107 VCSEL_5 913 -9.5464 -9.8297 -9.3930 .059760 VCSEL_2 913 -8.4815 -8.7290 -8.0134 .126077 VCSEL_8 913 -9.1052 -9.4310 -8.6967 .144907 7.5 V bias Observations Mean Pout Minimum Pout Maximum Pout Std.Dev. VCSEL_1 387 -23.5835 -24.8671 -22.7887 .498200 VCSEL_5 387 -11.8555 -12.2915 -11.5739 .123101 VCSEL_2 387 -12.3018 -12.9328 -11.9314 .156877 VCSEL_8 387 -11.4199 -12.1042 -10.9691 .199144
9
Figure 3. 5 V VCSEL data
VCSEL_1VCSEL_5VCSEL_2VCSEL_8
VCSEL characteristics at 25 C. 5V through 100 Ohm resistorVCSEL_1 = -17.911+0.001*x+eps
VCSEL_5 = -7.611-0*x+epsVCSEL_2 = -6.317-0*x+epsVCSEL_8 = -8.282-0*x+eps
14 days
Opt
ical
Pou
t (dB
m)
-20
-18
-16
-14
-12
-10
-8
-6
-4
10
Figure 4. 6.25V VCSEL data
VCSEL_1VCSEL_5VCSEL_2VCSEL_8
VCSEL characteristics at 25 C. 6.25V through 100 ohm resistorVCSEL_1 = -20.494+0*x+eps
VCSEL_5 = -9.576+0.0001*x+epsVCSEL_2 = -8.281-0*x+epsVCSEL_8 = -8.898-0*x+eps
19 days
Opt
ical
Pou
t (dB
m)
-24
-22
-20
-18
-16
-14
-12
-10
-8
-6
11
Figure 6. 7.5 V VCSEL data
VCSEL_1VCSEL_5VCSEL_2VCSEL_8
VCSEL characteristics at 25 C. 7.5 V through 100 ohm resistorVCSEL_1 = -24.27+0.004*x+eps
VCSEL_5 = -11.873+0.0001*x+epsVCSEL_2 = -12.284-0.0001*x+epsVCSEL_8 = -11.596+0.001*x+eps
8 days
Opt
ical
Pou
t (dB
m)
-26
-24
-22
-20
-18
-16
-14
-12
-10
-8
12
Figure 7. 3D Surface Plot of Power vs FHWM (5V)
-18.684 -18.569 -18.453 -18.337 -18.222 -18.106 -17.990 -17.874 -17.759 -17.643 -17.527 -17.412 -17.296 -17.180 -17.065 -16.949
3D Surface Plot of Power vs FWHM (5V)z = 0.086+1.002*x-6.543e7*y
13
Figure 8. 3D Surface Plot of Power vs FHWM (6.25V)
-21.287 -21.185 -21.083 -20.981 -20.879 -20.777 -20.675 -20.573 -20.471 -20.370 -20.268 -20.166 -20.064 -19.962 -19.860 -19.758
3D Surface Plot of Power vs FWHM (6.25V)z = 0.218+1.007*x-8.698e7*y
14
Figure 9. 3D Surface Plot of Power vs FHWM (7.5V)
-26.612 -26.424 -26.235 -26.047 -25.859 -25.671 -25.482 -25.294 -25.106 -24.918 -24.729 -24.541 -24.353 -24.165 -23.976 -23.788
3D Surface Plot of Power vs FWHM (7.5V)z = 4.319+1.231*x-8.063e7*y
15
Figure 10. Optical Power output summary
VCSEL_1VCSEL_5VCSEL_2VCSEL_8
Optical Power output summary
41 days
Opt
ical
Pou
t (dB
m)
-28
-24
-20
-16
-12
-8
-4
16
3. Characteristics of the VCSELs mounted directly to Kovar headers. The VCSELs mounted directly to the Kovar headers exhibited a power slump over time. After 2 days at 6.25 V, all 4 devices stopped exhibiting any power output at 850nm.
Figure 11. Comparison of Optical power out on diamond vs Kovar substrate
TOTAL_PO (L)TOT_PO (R)
Comparison of Optical power out on diamond vs Kovar5V bias
Opt
ical
Pou
t (dB
m)
w/o
dia
mon
d su
bstr
ate
Opt
ical
Pou
t (dB
m)
w/ d
iam
ond
subs
trat
e
-18.8
-18.4
-18
-17.6
-17.2
-16.8
-22
-21.8
-21.6
-21.4
-21.2
-21
-20.8
-20.6
-20.4
17
Table 3
Statistical Comparison of Output power data (dBm) for VCSEL mounted to diamond substrate vs Kovar header over the same number of observations
Observations Mean Pout Minimum Pout Maximum Pout Variance Std.Dev. PEAK_A 674 -21.2620 -22.1000 -20.8000 .058823 .242534 TOTAL_PO 674 -21.1051 -21.8966 -20.6775 .055431 .235438 PEAK_B 674 -17.7111 -18.6000 -17.0000 .084051 .289916 TOT_PO 674 -17.6358 -18.5157 -16.9374 .081063 .284717 PEAK_A = Peak power of primary spectral peak for Kovar mounted sample TOTAL_PO = Total power output for Kovar mounted sample PEAK_B = Peak power of primary spectral peak for diamond mounted sample TOT_PO = Total power out for diamond mounted sample
18
Figure 12. Optical power output for s/n 1 mounted on Kovar
PEAK_A (L)TOTAL_PO (R)
Optical power output for s/n1 mounted on Kovar
28 days
Pou
t (dB
m) P
eak
Spe
ctra
l Out
put
Pou
t (dB
m) T
otal
Pow
er
-25.5
-24.5
-23.5
-22.5
-21.5
-20.5
-19.5
-25.3-25.1-24.9-24.7
-22.3-22.1-21.9-21.7-21.5-21.3-21.1-20.9
19
Figure 13. Optical power for S/N 1 at 6.25 V mounted on Kovar
PEAK_A (L)TOTAL_PO (R)
Optical Power output at 6.25V bias through 100 ohm resistors/n 1 mounted on Kovar
2 days
Pou
t (dB
m) o
f pea
k sp
ectra
l com
pone
nt
Pou
t (dB
m) T
otal
pow
er
-25
-24.9
-24.8
-24.7
-24.6
-24.5
-24.4
-24.3
-25.3
-25.2
-25.1
-25
-24.9
-24.8
-24.7
20
Figure 14. Output power comparison at 6.25 V for diamond and Kovar mounting
TOTAL_PO (L)TOTPO2 (R)
Output power comparison at 6.25 V
2 days
Tot
al P
out (
dBm
) mou
nted
on
Kov
ar
Tot
al P
out (
dBm
) mou
nted
on
diam
ond
-20.9
-20.7
-20.5
-20.3
-20.1
-19.9
-19.7
-25
-24.9
-24.8
-24.7
-24.6
-24.5
-24.4
-24.3
21
Figure 15. Full Width at Half Max comparison between diamond mounted sample and kovar mounted sample. FWHM = 2.355 σ
FWHM Comparison
0.00E+00
5.00E-10
1.00E-09
1.50E-09
2.00E-09
2.50E-09
3.00E-09
3.50E-091 28 55 82
109
136
163
190
217
244
271
298
325
352
379
406
433
460
487
514
541
568
595
622
649
Observations
FWHM Diamond Substrate FWHM Kovar Header
22
Figure 16. Sigma comparison between diamond mounted sample and kovar mounted sample. σ = (Σ Pi, i=1,,n(λi −λmean)2/Ptot)0.5
Sigma (RMS spectral width)
0.00E+00
2.00E-10
4.00E-10
6.00E-10
8.00E-10
1.00E-09
1.20E-09
1.40E-091 25 49 73 97 121
145
169
193
217
241
265
289
313
337
361
385
409
433
457
481
505
529
553
577
601
625
649
673
Observations
Sigma diamond substrate Sigma Kovar Header
23
Figure 17. Mode Spacing comparison between diamond mounted sample and kovar header sample. Mode spacing is the average wavelength spacing between spectral components of the VCSEL 4. vvffd .000000000000000000 5.
Mode spacing
0.00E+00
1.50E-10
3.00E-101 23 45 67 89 111
133
155
177
199
221
243
265
287
309
331
353
375
397
419
441
463
485
507
529
551
573
595
617
639
661
Observations
wav
elen
gth
Mode Sp Diamond Substrate Mode Sp Kovar Header
24
4. Wavelength shifting of VCSELs. The comparison of the spectral performance of the VCSELs on diamond versus Kovar demonstrated provided two results of note. When observed over comparable period at 5V, the VCSELs on diamond had slightly better spectral stabilty. The VCSEL on Kovar had a small blue shift over time. Statisically, the variation between the two was small as shown in table 4. Diamond Diamond Kovar Kovar Mean Wavelength std dev Peak Wavelength std dev Mean Wavelength std dev Peak Wavelength std dev 3.31272E-11 3.23E-11 4.16378E-11 3.99E-11
Table 4. Standard deviation of wavelength stability of diamond and Kovar mounted units. The VCSEL mounted on diamond exhibited 6 abrupt shifts in mean wavelength during the 41 days of testing. Three of the shifts correlated to increases in the bias supplies. Three did not correspond to any change in the test conditions. The shifts are both higher and lower in wavelength. Incidentally, the peak wavelength exhibited only 3 shifts, each occuring at bias increase and each shift was a red shift. The VCSEL mounted on Kovar exhibited one wavelength shift, corresponding to the increase in bias voltage from 5V to 6.25 volts. There is insufficient data to draw any conclusions about the wavelength shifting over than the appearance of mode shifts within the VCLSEL. Additional characterization on a wider sample population and some knowledge of the VCSEL construction would be required.
25
Figure 18. Mean wavelength vs time at 5V
MEANWL1MEANWL2
Mean Wavelength vs time MEANWL1 = 8.537e-7-2.871e-14*x+epsMEANWL2 = 8.506e-7-1.174e-13*x+eps
5V bias
observations
Wav
elen
gth
ME
AN
WL1
= D
IAM
ON
D S
UB
STR
ATE
ME
AN
WL2
= K
OV
AR
HE
AD
ER
8.5e-7
8.505e-7
8.51e-7
8.515e-7
8.52e-7
8.525e-7
8.53e-7
8.535e-7
8.54e-7
8.545e-7
26
Figure 19. Peak wavelength vs. time at 5V
PEAKWL1PEAKWL2
Peak Wavelength vs timePEAKWL1 = 8.537e-7-2.849e-14*x+epsPEAKWL2 = 8.506e-7-1.162e-13*x+eps
5 V Bias
OBSERVATIONS
Wav
elen
gth
PE
AK
WL1
=DIA
MO
ND
SU
BS
TRA
TEP
EA
KW
L2=K
OV
AR
HE
AD
ER
8.5e-7
8.505e-7
8.51e-7
8.515e-7
8.52e-7
8.525e-7
8.53e-7
8.535e-7
8.54e-7
8.545e-7
27
Figure 20. Mean wavelength vs. time for the entire test
MEANWL1MEANWL2
Mean wavelength vs time
observations
wav
elen
gth
ME
AN
WL1
=DIA
MO
ND
SU
BS
TR
AT
E
ME
AN
WL2
=KO
VA
R H
EA
DE
R
8.49e-7
8.5e-7
8.51e-7
8.52e-7
8.53e-7
8.54e-7
8.55e-7
8.56e-7
8.57e-7
8.58e-7
037 74 111
148
185
222
259
296
333
370
407
444
481
518
555
592
629
666
703
740
777
814
851
888
925
962
999
1036
1073
1110
1147
1184
1221
1258
1295
1332
1369
1406
1443
1480
1517
1554
1591
1628
1665
1702
1739
1776
+5V TO +6.25V
+6.25V TO+7.5V
+5V TO +6,25V
28
Figure 21. Peak wavelength vs. time for the entire test
PEAKWL1PEAKWL2
Peak wavelength vs timew
avel
engt
h
PE
AK
WL1
=DIA
MO
ND
SU
BS
TRA
TEP
EA
KW
L2=K
OV
AR
HE
AD
ER
8.49e-7
8.51e-7
8.53e-7
8.55e-7
8.57e-7
8.59e-7
# 1
# 29
# 57
# 85
# 11
3#
141
# 16
9#
197
# 22
5#
253
# 28
1#
309
# 33
7#
365
# 39
3#
421
# 44
9#
477
# 50
5#
533
# 56
1#
589
# 61
7#
645
# 67
3#
701
# 72
9#
757
# 78
5#
813
# 84
1#
869
# 89
7#
925
# 95
3#
981
# 10
09#
1037
# 10
65#
1093
# 11
21#
1149
# 11
77#
1205
# 12
33#
1261
# 12
89#
1317
# 13
45#
1373
# 14
01#
1429
# 14
57#
1485
# 15
13#
1541
# 15
69#
1597
# 16
25#
1653
# 16
81#
1709
# 17
37#
1765
# 17
93
+5V TO+6.25V
+6.25V TO+7.5V
Figure 22 Thermal images at 5 V
bias
Diamond substrate
Kovar Header
28
Figure 23 Thermal images at 6.25 V
bias
Diamond Substrate
Kovar Header
29
Figure 24 Thermal images at 7.5 V
bias
Diamond substrate
Kovar Header
30
29
5.0 Conclusions The VCSELs mounted on diamond were capable of being overdriven at higher current levels without damage. The highly accelerated voltage and current stress demonstrated that the diamond substrate provided a significant margin of thermal mitigation. VCSELs were driven at up to 3X the manufacturers recommended maximum instantaneous operating current and 10X the typical threshold current. Key parameters from the data sheet are summarized below in Table 4.
Parameter Rating Min Typical Max Max Continous
Operating Current 20mA
Max Instantaneous Operating Current
25mA
Peak Wavelength 830nm 850nm 860nm Threshold Current 5mA 8mA 9mA Output Power at
20mA 3dBm 7.8dBm 10dBm
Operating Voltage at 9mA
1.6V 2.2V 2.3V
Table 4. Key Paramaters from Emcore VCSEL data sheet (p/n 8085-1000)
Additional testing is required to assess the effects of screening on the long term reliability of VCSEL mounted on diamond. Additional characterization of wavelength stability in current generation VCSEL arrays is also warranted.
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