Measurement of Semiconductor Surface Temperature
Using Raman Spectroscopy
by Takeharu Yamaguchi *, Michio Ohkubo *,Nariaki Ikeda * and Takehiko Nomura *Temperatures in minute regions of semiconductor devices have been measured

ABSTRACT using Raman spectroscopy. The temperature of the emitting portion of a 1480-nm
InGaAsP semiconductor laser manufactured by Furukawa Electric and having the world's highest
output power has been measured. By means of comparison with InGaAs/GaAs lasers, it has beenshown that InGaAsP lasers exhibit excellent resistance to catastrophic optical damage resulting
from heat.
In another application, the temperatures of MESFETs were also measured, demonstrating that
there is a temperature distribution even within the 5-µm distance between source and drain, andthat there is a marked temperature rise from the gate. In addition it has been demonstrated thattemperature measurement using Raman spectroscopy offers extremely high area resolution.

1.

OBJECTIVE

As electro-optical semiconductor devices become smaller,
the actual operating region within the device shrinks to
the micron or submicron order. Since the electrical power
consumed in the device is concentrated in this minutespace, there is considerable heat generated in the operating region.
Heat causes degradation of the inherent properties andreliability of the device. Devices that handle high power
are particularly subject to demands for high-power handling, and due to device miniaturization, confront the mostacute problem.
The direct means of clarifying the cause of heat generation is to measure temperature, but in micron-order areas, ordinary techniques of temperature measurement
cannot be used.
The present work aims at measuring temperatures near
the surface in minute regions of semiconductors, and forthis purpose adopts the technique of Raman spectroscopy, which not only makes possible the measurement of
temperatures in minute regions but also allows measurements to be made when devices are in operation, without
contact and without damage1).
As an example, the results of measuring temperaturesat the emitting facet of a semiconductor laser and in thevicinity of MESFET gates are reported. In the case of
MESFET temperature, measurement with area resolutionof the submicron order was achieved.
* Semiconductor R&D Center, Yokohama Research Lab.,R&D Div.

2.

MEASUREMENT METHODOLOGY

The heat-generating portion of the semiconductor laser
described here was several micrometers in diameter, andthat of the MESFET measured several micrometers byseveral hundred micrometers. Both had regions that could
be called minute--several micrometers in size. Thus measurement with an area resolution of 1µm or less was required.
Research on techniques for measuring temperatures inminute regions is on-going, and no specific method has
yet been established2)~4). The problem bedeviling the development of such technology is the need to measureminute amounts of heat in minute regions. Even though
the energy density in the heat generating region may behigh, the small size of the region means that the total heatproduced is small.In contact-type measurement of heat,as in a thermocouple, heat flows into the measurer (thethermocouple) damaging its actual operating state.
Noncontact methods of temperature measurement include measurement of black-body radiation in the infrared
range, but the most commonly used infrared wavelengths
are from several micrometers to several tens of micrometers. Area resolution is limited by wavelength and is thus
several micrometers at best, and accuracy is further limited by the optics, so that the limit of area resolution isaround 5µm.
Thus we see that the essential conditions for the measurement of the temperature of a semiconductor device inoperation are to be noncontact, to not damage the operating state, and to give high area resolution.

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Raman spectroscopy provides noncontact temperature
measurement, and when light in the visible short wavelength from an Ar or He-Ne laser is used as the stimulating light, area resolutions of 1µm or less can be obtained.
Today's measurement systems can effect measurement
even at a stimulating light intensity in the order of 1 mW,
and are unlikely to alter the operating state of the device.
For these reasons we have conducted experiments on
the use of Raman spectroscopy in measuring temperatures in minute areas of semiconductors.

3.

RAMAN SCATTERING AND TEMPERATURE
MEASUREMENT

Raman scattering is a phenomenon whereby, when aspecimen is irradiated with (monochrome) light, the energy of the photons of the irradiating light undergoes nonelastic scattering due to lattice vibration specific to thematerial of the specimens, so that the frequency of theirradiating light shifts by an amount equivalent to the lattice vibration frequency.
Figure 1 shows the Raman spectrum from the (110)
surface of GaAs. There are two types of Raman scattered
light: Stokes, with a Raman light frequency of (ω i-ωµ), andanti-Stokes, with a Raman light frequency of (ω i+ωµ), whereω i is the stimulating light frequency and ωµ is the lattice
vibration frequency. In Figure 1, the ωµ for GaAs is 268 cm-1.
To evaluate the temperature of a substance using Raman
scattering, we may use the formula5)
exp(–

hc
k BT

)=

I AS(
I S(

) / ( i–) / ( i+

)4
)

(1)

4

where: IAS and IS are the intensities of the anti- Stokes
and Stokes scattered light, and T is the temperatureof the specimen.

7 10

3

6 10

3

5 10

3

4 10

3

3 10

3

2 10

3

1 10

3

0 10

3

-1 10

3

TEMPERATURE MEASUREMENT

4.1 Temperature Calibration
Before measuring the temperature of a device, measurements were made using a GaAs substrate.
Since in Raman microscopy the stimulating light is focused on the specimen, heating will occur at the focal pointin materials that absorb the stimulating light. Since thisconstitutes measurement error at the point of measurement, the tendency must be investigated in advance.
Figure 2 is a graph showing the relationship between
the intensity of the stimulating light from an Ar laser andthe temperature as measured by Raman spectroscopy.
The specimens used were (100) GaAs substrates. The
substrate was connected to a Peltier device via a heatsink to maintain a temperature of 25°C. As can be seen,
the measured temperature was substantially constant at
25°C when stimulating light power was 5 mW or less. That
is to say, if the effect of temperature rise due to stimulating
light is to be ignored, stimulating light power should be set
at 5 mW or less.
45
Heat sink: 25˚C

40
Ar laser, 514.5-nm excitation
Stokes35
30
25

Anti-Stokes

20

-600

Figure 174

4.

Temperature (˚C)

Intensity (arb. units)

In substance, the specimen temperature determines theratio between the intensities of anti-Stokes and Stokes
scattered light. The lattice vibration frequency to be measured is specific to the substance, and does not dependon the wavelength of the stimulating light.
Measurement is done by means of an ordinary Raman
spectroscopy microscope. The stimulating light is from an
Ar laser with a wavelength of 514.5 nm. The laser light isirradiated onto the specimen through a half mirror and themicroscope objective lens. The diameter of the stimulating light on the sample is thus the same as the area resolution, and area resolution may be improved by focusing
the stimulating light using a higher-power objective lens.
The objective lens used was 100-power, and this yields a
focal diameter of approximately 0.5µm.
The Raman scattered light that is emitted from the specimen in the opposite direction to the stimulating light isonce more passed through the objective lens and the half
mirror before entering the spectroscope. The spectroscopeconsists of a triple monochrometer, with provision madeso that the weak Raman light is not blotted out by background light. Light emitted from the spectroscope goes toa detector, where the Raman light is measured.
To keep the specimen at constant temperature, the specimen stage is temperature-controlled by a Peltier device.
To measure the surface distribution of device temperature, a movable X-Y-Z stage is provided, the movement of
which can be set at the submicron level.

0
-400

-200
0
200
Wave number (cm-1)

400

600

Raman spectrum from the GaAs (110) surface

Figure 22

4
6
Laser power (mW)810Increase in measured temperature on thesurface of GaAs substrates irradiated by
Ar laser

Furukawa Review, No. 18. 1999

70

Measured temperature (˚C)

60

50

40

30

20

10

0
0

10

20

30

40

50

60

70

Substrate temperature (˚C)

Figure 3

GaAs substrate temperature vs. the value
measured by Raman spectroscopy

At 5 mW or more, the measured temperature rose indirect proportion to the increase in stimulating light power.
In this power region, the stimulating light injected exceeded
the heat-absorbing capacity of the Peltier device. The slopeof the graph is equivalent to the thermal resistance between the point of measurement and the heat sink, or approximately 4 K/mW.
In this example the boundary at which the effect of temperature rise due to stimulating light was at 5 mW, but thiswill vary somewhat according to the thermal resistance ofthe specimen. Actual measurements were made at a stimulating light power of several milliwatts.
Figure 3 shows the results of temperature measurements
made while the temperature of the specimen as a whole
was changed by means of the Peltier device. Substratetemperature shows good agreement with the measured
temperature, demonstrating the feasibility of temperature
measurement using Raman spectroscopy.
In Figure 3, the temperature measurement error was
not more than ±3°C. And as a result of long-term temperature calibration, a temperature measurement error of±5°C has been achieved.
4.2

340

Facet temperature (K)Measuring the Temperature of the Emitting
Surface of a Semiconductor Laser
The pumping light source used for the optical fiber amplifiers that have significantly increased the range of fiberoptic communications is a 1480-nm high-power semiconductor laser. Furukawa Electric manufactures the world's
highest-power 1480-nm semiconductor laser, providing anemitting power of 180 mW.
To improve emission efficiency and fiber coupling efficiency, the emitting region is only a few micrometers indiameter, and when the optical power is thus concentrated
in such a small region, it becomes hot, reducing emission
efficiency and reliability.
Temperature measurements were carried out by Raman
spectroscopy to investigate how much heat is actually
generated at the emitting facet.

Figure 4 shows the injection current Iinj and emitting region temperature of a 1480-nm semiconductor laser. It canbe seen that at an injection current of 400 mA there is atemperature rise of 35°C, and that the temperature of theemitting region increases substantially in proportion to Iinj.
The lasing threshold current Ith is 30 mA, and at Ith andabove, the light output is proportional to (Iinj-Ith). When Iinj isconsiderably greater than Ith (i.e., several hundred milliamperes), it may be taken that the light output is substantially proportional to Iinj. With an injection current of approximately 400 mA, the light output of the 1480-nm semiconductor laser is 140 mW, so that at a light output of 140
mW, the temperature of the emitting region increases inproportion to the light output. The gradient of the temperature rise of the emission region relative to light output may
be estimated as 0.2 K/mW for the chips measured.
Since this paper is the first to report on facet temperature measurements in 1480-nm semiconductor lasers,
there is no standard against which to judge whether the
results are superior or not. Accordingly a comparison ismade with reports on GaAs/AlGaAs and InGaAs/AlGaAs
semiconductor lasers6). The rates of temperature rise were
1.6 and 0.3 K/mW, respectively. In all cases a situation
conducive to rapid degradation was intentionally arranged
by not providing a protective film of the emitting facet.
Comparing Furukawa Electric's 1480-nm laser, its rateof temperature rise was not as high as the GaAs-basedlaser, but was about the same level as the InGaAs. The
InGaAs-based laser referred to above degraded rapidly
but the InGaAsP-based 1480-nm laser measured on thisoccasion did not exhibit rapid degradation, and reliability
was adequate to allow use in repeaters for submarine
optical fiber cables.
Thus we may say that the InGaAsP-based laser showedstronger resistance to emission region temperature risethan did InGaAs/GaAs-based lasers.
The degradation mode in the GaAs- and InGaAs-basedlasers referred to above occurred when operating underautomatic power control (APC), in which light power isheld constant. This is known as the catastrophic opticaldamage (COD) mode, in which the rise in emitting region
temperature leads to a drop in emission efficiency and theflow of injection current to the laser increases, until feed-

1.48µm-LD
25˚C

330
320
310
300
290
-100

0

100

200

300

400

500

Injection current (mA)

Figure 4

Facet temperature vs. injection current of a 1480nm high-power laser at a heat sink temperatureof 25°C75

back that causes further temperature increase results infusion of the emitting region. To analyze this phenomenon
requires analysis of emitting region temperature, and thetechnique of temperature measurement using Raman
spectroscopy that is introduced here will also prove usefulin COD analysis.
4.3 Measuring the Temperature of MESFETs
MESFETs are unipolar transistors consisting primarily ofGroup III-V compound semiconductors. Since the mobility of compound semiconductors is greater than that of Si,
MESFETs are used in such high-speed applications ascompanders in satellite broadcasting and transceivers incellular telephones.
Figure 5 shows a top view of a typical MESFET, in whichcurrent flows in a channel formed between the source (S)and drain (D) electrodes. The gate (G) electrode is between the semiconductors having a Schottky junction,
controlling the current in the channel beneath the gate.
MESFETs are high-speed devices, but speed may befurther increased by shortening gate length in Figure 5. To
reduce the parasitic resistance component, the distanceof the S-G and D-G gaps may also be reduced. These
distances are in the order of 1µm or less.
MESFETs are generally used with the source electrodereferenced to 0 V and drain positive and the gate in thevicinity of 0 V or negative bias. In this bias state, a highreverse-bias is applied between drain and gate. Thus thepotential gradient between drain and gate is largest in thechannel between source and drain, and virtually all of theinjected power is dissipated here. This, then, is consid-

Gate
Mesa
B'

Drain

Sourceered to be the point at which heat is generated.
This heat--what is known as the junction temperature-is an important factor in determining the reliability time inthe design of circuits using MESFETs.
In a report on the measurement of heat generated in
MESFETs using infrared thermography2), the heat generation point was identified as being near the gate electrode, but since area resolution was 5µm, it was impossible to specify whether it was at the drain or the source.
Accordingly we measured the temperature distribution in
MESFETs using Raman spectroscopy.
The MESFETs used consisted of a GaAs substrate on
which a GaAs channel was grown epitaxially. To define
the region of a single MESFET, the outside of the rectangular area on the top surface denoted as a mesa in Figure
5 was etched and the channel was removed from the mesa
area.
The electrode arrangement of the MESFETs measured
was similar to that shown in Figure 5, with an S-G gap of
2µm, gate length of 1µm and D-G gap of 2µm. Gate width
was 100µm, but by wiring another FET in parallel with thisone, a 2-finger gate structure was created, with an equivalent gate width of 200µm. Measurements were carried out
in the vicinity of one of these gate electrodes. The pointsof measurement were along line A-A' in Figure 5.
Electrically speaking, measurement conditions were Vds= 4 V and Vgs = 0 V, with a drain current Id of 40 mA. Measurements were also made at zero bias ( Vds = 0 V).
MESFET temperature was kept at 25°C by means of a
Peltier device.
Figure 6 shows the result of temperature measurements
across the gate (direction A-A' in Figure 5). The horizontal
axis represents the distance in the direction of the gate
referenced to the end of the electrode. The difference intemperature measurements immediately before and after
application of bias to the drain was defined as the temperature rise, and this is shown on the vertical axis in Figure 5.

100

A'

80

60
50
40

D-G
gap

Figure 576

Gate length

20
S-G
gap

Top view of MESFET (temperatures
were measured along the dotted lines)

10

G-D gap

G-S gap

Sourceelectrode30

Gateelectrode

B

70

Drain
electrodeA

Temperature rise ∆T (K)Gate width

90

0
-1

0

1

2

3

4

5

6

Position across gate (µm)

Figure 6

Temperature distribution across MESFET gateelectrode along line A-A' in Figure 5

Furukawa Review, No. 18. 1999

From Figure 6 it can be seen that the rise in temperature peaked at about 80°C. The temperature rise between
source and gate was about 40°C greater than the drainto-gate value. By means of the high-accuracy movablestage, temperature measurements were carried out with
0.4-µm pitch positional resolution, so that it was possible
to examine the D-G gap in detail and even to confirm thatthe peak in temperature rise was closer to the gate than tothe drain. In this device the source-to-drain distance wasonly 5µm so that measurements using apparatus with anarea resolution of about 5µm would average the temperatures in this area. In the example shown in Figure 6, forexample, a temperature rise of 50-60°C was measured,differing by 20-30°C from the results of 80°C obtained in
the present work. Thus the existence of a temperature distribution in the mere 5µm between source and drain, demonstrates the great utility of Raman spectroscopy used inthis study in the measurement of temperature rise peaks.
The results obtained were then compared with thosereported elsewhere. It is thought that the power dissipatedbetween drain and gate is distributed uniformly along thedirection toward the gate, or put another way, temperaturerise is a function of power dissipation per gate. If the power
dissipation per unit gate width is calculated for the
MESFETs measured in this work, the result is 0.8 mW/µm. Since this was accompanied by a temperature rise of
approximately 85°C, the temperature rise per unit gate
width is 110 K/(mW/µm).
Similarly examining the data from the literature2), there
was a temperature rise of approximately 50°C against a
power dissipation per unit gate width of 0.46 mW/µm, yielding a temperature rise rate of 110 K/(mW/µm)--the samevalue obtained in the present work. However since reference 2) uses infrared thermography, it is possible that themeasured temperatures have been averaged out. Thus it
was realized that the temperature rise rate in the MESFETs
measured in this work was the same as or lower than thatobtained in the literature cited.

5.

ture distribution even over the minute separation of 5µm
between source and drain. It was shown by actual measurement that this temperature rise was particularly marked
toward the gate. In addition it was demonstrated that temperature measurement using Raman spectroscopy had
extremely high area resolution.

Acknowledgments
The authors would like to thank Mr. Ninomiya and Mr.
Tokunaga, together with the staff of their Department, Mr.
Ijichi, Mr. Ikegami, Mr. Tsukiji and Mr. Shimizu as well as
the staff of the Optical Device Department for their cooperation in conducting this research.

REFERENCES(1) S. Todoroki, M. Sawai and K. Aiki: J. Appl. Phys, 58(1985),
1124.
(2) C. Canali, F. Chiussi, G. Donzelli, F. Magistrali and E. Zanoni;
Microelectron. Reliab., 29(1989), 117.
(3) R. Ostermeir, K. Brunner, G. Abstreiter and W. Weber; IEEE
Trans.Electron. Dev., 39(1992), 858(4) H. Brugger and P. W. Epperlein; Appl. Phys. Lett., 56(1990),
1049(5) S. Murata, M. Kitamura, M. Yamaguchi, I. Mito and K.
Kobayashi; Electron. Lett., 19(1983), 1084.
(6) P. W. Epperlein; Conf. Laser and Electro-Optic(CLEO96),
CTul4(1996), 108.
Manuscript received on August 10, 1998.

CONCLUSION

Raman spectroscopy was examined as a method of measuring the temperature of minute regions of semiconductor devices, and as a result of carrying out temperature
calibration, a measuring accuracy of ±5°C was achieved
over the long term.
The method was applied to actual devices. The temperature of the emitting region of Furukawa Electric's 1480nm InGaAsP lasers, which have achieved the highest output power in the world, was measured. The results showedthat the temperature rise per unit output power was approximately 0.2 K/mW, and it was demonstrated that the
InGaAsP lasers had higher resistance to catastrophic
optical damage (COD) than did devices based on InGaAs/
GaAs.
As another device application, MESFET temperatures
were also measured. As a result of linear analysis carriedout at 0.4-µm pitch, it was found that there was a tempera-

77