ELECTRONICS

High-Power Short-Cavity AlGaInP Laser Diodes
Hiroyuki SUMITOMO*, Satoshi KAJIYAMA, Hiroyuki OGURI, Makoto UEDA, Syu GOTO,Toshihiro KITA, Hidenori AMANO, Hirotada SATOYOSHI, Toru YAMAMOTO, Kensei NAKAO
and Shigekazu IZUMIAlGaInP laser diodes (LDs) are used in a wide range of digital equipment, such as MO/CD/DVD optical pick-up units,laser printers and barcode scanners. These lasers are expected to be used for the next-generation hard disk drives(HDDs) using thermally-assisted magnetic recording systems, mobile devices and other digital products. To keep up
with the increasing demand, we have undertaken the development of higher-output and lower-power-consumption LDs.
By optimizing our 4-inch-wafer process, we have succeeded in the development of a 660 nm short-cavity ridge-waveguide laser (L = 300 µm) that can demonstrate the output power per unit cavity length as high as 100 mW/mm (CW operation, 25°C to 80°C). This output power far surpasses 64 mW/mm by a high-power laser (L = 2200 µm) designed forhigh-speed recordable DVD systems.
Keywords: laser diode, AlGaInP, short cavity, high power, HDD

1. Introduction

2. Mass Production Process of AlGaInP LD

AlGaInP laser diodes (LDs) with wavelengths 630 690 nm are used in a variety of digital equipment, such as
MO/CD/DVD optical pickup units, laser printers andbarcode scanners. These lasers are currently expected toexpand the application to projectors, displays, and nextgeneration hard disk drives (HDDs), and therefore, it isvital to improve their characteristics and cost efficiency.
Furthermore, downsizing of the laser chips is also required especially for HDDs and other commercial devices
such as micro projectors and mobile barcode scanners.
More specifically, heat-assisted magnetic recording (HAMR)
technique, which is highly expected to realize the next
generation high density HDD, will use laser beams as theenergy source to heat the recording media (1). In this case,the laser needs to be installed inside the HDD and the incorporation of the laser chips into the magnetic recording head is also being considered (2).
To respond to the market demands of productive LD
design, future development needs to be based on a market-oriented concept rather than merely extending conventional technology of DVD optical pickup units andother existing products. Take the laser chip assembly as
an example, not only conventional techniques for general
CAN packages but also flexible tailor-made design need
to be considered for customer convenience. We haveproactively worked to develop lasers, sticking to the market-oriented production concept.
In this paper, we report the study of a 660 nm shortcavity (L = 300 µm) ridge-waveguide laser, which demonstrates a high output power of 100 mW/mm in continuous
wave (CW) operation at the temperature range from 25°C
to 80°C. This output power is far higher than 64 mW/mm
by a high-power laser (L = 2200 µm) designed for highspeed recordable DVD systems.

For the commercial production of devices, not only
their performance improvement but also reduction in
production costs needs to be considered at the early stageof designing the devices. In our study, we adopted a laserthat has a simple ridge-waveguide structure with a silicone
nitride (SiNx) current blocking layer. This layer was fabricated by one-step metal-organic chemical vapor deposition (MOCVD) growth and mesa-etching procedure forthe ridge waveguide formation. The ridge formation plays
the key role in the LD production process (3) - (5).
In order to maintain the single-transverse-mode operation and stabilize the beam divergence angle, the sizeand shape of the ridge need to be uniformly controlled
over the entire wafer. We adopted inductively coupledplasma (ICP) dry etching techniques to realize the excellent controllability of ridge formation process. By using
this novel etching technique, we succeeded in the mass
production of the LD with a small beam aspect ratio,
which had previously been impossible with conventionalridge lasers (6).
Furthermore, the optimization of ICP etching process
eliminated the damage to the dry etching layer, thereby enabling to curtail the wet etching process. In conventionalridge formation, dry process needed to be followed by wet
etching process, in which residual plasma-induced damage
caused by dry process were removed. However, because of
the poor reproducibility of ridge fabrication in the wetprocess, only dry etching was required for mass production.
The deposit condition and properties of SiNx dielectric film, which act as a current blocking layer, determine
the reliability of the laser. Photo 1 shows the transmission
electron microscope (TEM) image of the interface between the dry etched AlGaInP surface and the SiNx film.
By controlling the formation and structure of the SiNxfilm, the distortion of the AlGaInP surface became as
small as approximately 10 nm, and the lattice stress was

SEI TECHNICAL REVIEW · NUMBER 70 · APRIL 2010 · 79

kept about 0.3% or lower.
In addition, to achieve the high-power operation while
preventing the catastrophic optical damage (COD) *1, weapplied Zn-diffused window structure *2 to the proximity ofthe edge facets of the laser. Figure 1 shows the photoluminescence (PL) spectra profiles of the window region
measured by micro-PL spectroscopy. The vertical axis indicates the distance from the edge facet (0 µm position)in the cavity direction of the laser. Figure 1 (a) shows thePL profile of an almost optically transparent structure forthe 660 nm lasing wavelength, in which PL peak wavelengths in the window region (about 0 - 10 µm position)
are substantially shifted towards the shorter wavelength.
On the other hand, Fig. 1 (b) shows the PL profile of animperfect window structure, in which a substantial opticalabsorption exists. We optimized the Zn-diffusion processby controlling the PL profiles uniformly over the wafer toobtain a proper window structure as shown in Fig. 1 (a).
To reduce the fabrication costs of LD chips, we havedeveloped new techniques of epitaxial growth and wafer
process for large-scale GaAs substrates (3 to 4 inch in diameter (3), (4)). Moreover, we employed Mg as a p-type
dopant, which has a lower diffusion coefficient and morestable doping characteristics compared with Zn (5), so as toobtain the better temperature characteristics of the laser.
Although 6 inch GaAs substrates are widely used for electricdevices, such as field effect transistors and high electron
mobility transistors, enlarging LD substrates is not easy due

Semiconductor :AlGaInP

Dielectric film:
SiNx

5 nm

Photo 1. Transmission electron microscope (TEM) image of interface
between dry etched AlGaInP surface and SiNx film

to a strict requirement for a low dislocation density (a hundred times lower than that of electric device wafers).
Most of the 4 inch LD wafer processing has been wellestablished on our GaAs-IC manufacturing technology.

3. Shortening Laser Cavity
We have examined the feasibility of shortening thecavity length of the high power laser by using aforementioned mass-production process. The target power was set
at 30 mW at 80°C to meet the requirement of mobile device application.
AlGaInP-based LD has several inherent disadvantages, compared to AlGaAs-based LD. More specifically,because of the small conduction band offset in theGaInP/AlGaInP heterostructure, AlGaInP LD induces apoor electron confinement, which increases the electroncarrier leakage from the active layer to the p-cladding at
a high temperature, resulting in a decrease in the output
power. Nevertheless, the AlGaInP LD for commercial use
needs to be capable of operating at 80°C or higher. To
stabilize the operation at high temperature, electron overflow from an active layer needed to be prevented by reducing the threshold carrier density. Our high-power LD
which have been optimized to be used for high-speedrecordable DVD drivers even in high temperature conditions achieved industry-leading levels of performance (5):kink-free high-power operation of over 400 mW, the lasing wavelength of 660 nm, and the cavity length of L =2200 µm under the pulsed operation.
3-1 Output Power Per Unit Cavity Length
As the first step to develop the high output power LDwith a short cavity length, we considered shortening thecavity length of the above mentioned high power laser (L= 2200 µm). Meanwhile, the threshold current density Jth
was remained constant through the optimization of thefront and rear facet reflectivity, R f and R r, so as for thegood temperature characteristics of the shortened cavity
length to be maintained. Here, the external quantum efficiency hd for the total optical output stays constant. And
the optical output power Pout from the front facet is described as follows with the constant injection current density Jo and the output power ratio of the front to the rear
facet h f.

Pout =ηf ·ηd · W · L · (Jo – Jth) = a·ηf · L
(a)

(b)

20
151050570

Window Reagion

25590

610630650670

PL Wavelength (nm)690 570

590

610

630

650670

Cavity Direction

30Window Reagion

Position (µm)

35

CleavedEdge

690

PL Wavelength (nm)Fig. 1. PL spectra profiles of Zn-diffused window region(a) Transparent window for 660 nm lasing wavelength and(b) Imperfect window structure

80 · High-Power Short-Cavity AlGaInP Laser Diodes

where W is the stripe width of the injection current
and a is a constant value,—ηf = 1/ 1+ (1 – Rr) / (1 – Rf) √ Rf / Rr{}

To simplify the calculation, the influence of the spontaneous emission and the loss in the window region andfacet coating film were ignored in our study. In general,
the rear fact reflectivity R r is set high in order to extract
light efficiently from the front facet. When R r = 1, Pout = a’L(a’ : a constant value) and the output power per unit length
stays constant regardless of the value of L. In addition, on

30

100CW258025˚C40˚C50˚C60˚C70˚C201510604080˚C

205001020304050060Output Power per Unit Length (mW/mm)

can be shortened to no more than approximately 300 µm
when the manageability of the laser chip in a production
phase is considered.
Figure 4 shows the temperature dependence of L-I
characteristics of the L = 300 µm LD under CW operation.
The front facet reflectivity R f is set at 58%. The thermalrollover is observed around 18 mW, and thus Pout/L wasestimated to be no higher than 60 mW/mm at 80°C. Thisresult indicates that simply shortening the long cavity highpower laser can not generate 30 mW-class output.

Output Power (mW)a realistic heat sink which has a large thermal capacity, theself-heating effect of the LD is not affected by the value of
L in the current injection. This is because the thermal resistance of the laser is proportional to 1/L(7), while the injection power to the laser is proportional to L under theconstant current density J o. Consequently, it is clear thatthe output power per unit cavity length (Pout/L) remains
constant with changing cavity length and constant current
density, even after the thermal dependence is considered.
Thus, we may conclude that Pout/L should be widely used
as a parameter to compare laser characteristics.
3-2 Test Results of Laser Characteristics
Figure 2 shows the temperature dependence of thelight output power-current (L-I ) characteristics of thehigh power L = 2200 µm LD under CW operation. As thethermal rollover is observed around 140 mW, Pout/L at
80°C is approximately 64 mW/mm at the highest.
The facet reflectivity of the L = 2200 µm LD is designed to be R f = 3% and R r = 95%. When the R f is adjusted depending on the cavity length L with the thresholdcurrent density Jth remaining constant, the dependenceof R f on the value of L is as shown in Fig. 3. Furthermore,
we have fabricated a 660 nm short cavity LD with L = 300µm by optimizing the reflectivity R f based on the resulting
data in Fig. 3. We have concluded that the cavity length

Current (mA)

140CW

Output Power (mW)

1201008025˚C40˚C50˚C60˚C70˚C

60504075˚C80˚C

30602040102000100200

3000400

Output Power per Unit Length (mW/mm)

Fig. 4. Temperature dependence of L-I characteristics(L = 300 µm LD, CW operation)

Current (mA)

FrontFacet Reflectivity, R f (%)

Fig. 2. Temperature dependence of L-I characteristics(L = 2200 µm LD, CW operation)

1009080706050403020100R r = 95%0500

1000

1500

2000

2500

Cavity Length, L (µm)

Fig. 3. Dependence of front facet reflectivity Rf on cavity length L withconstant Jth

4. High Power and Short Cavity LDIn order to improve the high temperature characteristics of the short cavity laser, we optimized the thicknessof barrier and guide layers to enlarge the optical confinement. Furthermore, we controlled the Al composition in
the barrier and cladding layers to reduce the carrier leakage. The optimization of the window structure was also performed along with the regulation of the epitaxial structure.
Compared with the long-cavity laser, the short-cavity
laser has lower output power, however, its optical density
near the laser facet is not necessarily low due to the facet
reflectivity which has been designed to become higher
than that of the long-cavity laser, and therefore the window structure is necessary even for a 30 mW-class shortcavity LD. However, the window structure optimized forthe high-power long-cavity LD do not always work well for
short-cavity LD, namely, the window can fail to improve
the temperature characteristics of the short-cavity laser.
Hence, a specially-designed window optimization method
is vital for the short-cavity laser.
Figure 5 shows the temperature dependence of L-I
characteristics of a 300 µm LD whose epitaxial and window structures have been optimized for the short cavity.
The lasing wavelength was adjusted to 660 nm, which isequivalent to the basic LD. As shown Fig. 5, its output
power reached over 30 mW at 80°C and the Pout/L rose
above 100 mW/mm.

SEI TECHNICAL REVIEW · NUMBER 70 · APRIL 2010 · 81

The result of reliability test for the improved short-cavity
laser under a CW operation (20 mW) at 80°C is shown inFig. 6. The laser chips were intentionally mounted on a heatsink with the junction side up, which resulted in higher thermal impedances than junction-down mounting, and thus,
the temperature of the laser chip was remarkably increased
by its self-heating effect. Despite such difficult conditions,the laser exhibited stable operation for over 1000 hours.

25˚C
40˚C
50˚C
60˚C
70˚C

CW

25

Output Power (mW)

1002080
75˚C
80˚C

1560
40

1020

500102030

40

50

60

0

Current (mA)

Operating Current (mA)

(5)

(6)

References
K. Matsumoto, A. Inomata, S. Hasegawa, “Thermally Assisted Magnetic Recording,” FUJITSU Sci. Tech. J., vol. 42, No.1,pp.158-167,Jan., 2006
K. Hongo and T. Watanabe, “Lensless Surface Plasmon Head with
1 Tbit/in.2 Recording Density,” Jpn. J. of Appl. Phys., Vol. 47, No.
7, pp.6000-6006, 2008
H. Sumitomo, S. Kajiyama, H. Oguri, T. Sakashita, S. Domoto, K.
Nakao, T. Yamamoto, T. Komatani, H. Kawakubo, M. Ono, T. Maejima, S. Izumi, “High-Power Laser Diodes by 4-inch Full-Wafer
Process Technology for DVD-R/RW,” IEIC Technical Report,OPE2006-5, pp.23-28, (2006-04) (in Japanese)
H. Sumitomo, S. Kajiyama, H. Oguri, T. Sakashita, K. Nakao, Y. Yamamoto, T. Kita, T. Komatani, H. Kawakubo, M. Ono, and S. Izumi,“Uniform and High-Power Characteristics of AlGaInP-based Laser
Diodes by 4-inch Wafer Process Technology for DVD-R/RW/RAM,”
Proc. International Semiconductor Laser Conference 2006, Kohala
Coast, Hawaii, pp. 53-54
H. Sumitomo, S. Kajiyama, H. Oguri, T. Sakashita, T. Yamamoto, K.
Nakao, S. Domoto, M. Ueda, H. Amano, H. Satoyoshi, T. Kita, andS. Izumi, “Uniform and High-Power Characteristics of AlGaInP-BasedLaser Diodes with 4-Inch-Wafer Process Technology,” IEEE J. Sel.
Topics in Quantum Electron., vol. 13, No. 5, pp. 1170-1175, (2007)
M. Ueda, H. Sumitomo, S. Goto, T. Yamamoto, H. Oguri, S. Kajiyama, K. Nakao, H. Amano, H. Satoyoshi, T. Kita, S. Izumi, “LowPower Consumption and Low-Aspect Ratio Laser Diodes,” IEIC
Technical Report, OPE2008-4, pp.17-22, (2008-04) (in Japanese)
O.J.F. Martin, G.-L. Bona, P. Wolf, “Thermal behavior of visible AlGaInP-GaInP ridge laser diodes,” IEEE, J. of Quantum Electron.,vol. 28, No. 11, pp. 2582-2588, (1992)

CW, Po = 20 mW, Ta = 80˚C
N=57060

Contributors (The lead author is indicated by an asterisk (*)).

50

H. SUMITOMO*40
30
20100(3)(7)

Fig. 5. Temperature dependence of L-I characteristics(Improved L = 300 µm LD, CW operation)

80

(2)

(4)
Output Power per Unit Length (mW/mm)

30(1)

0200

400

600

800

1000

1200Aging Time (h)Fig. 6. Reliability test result under 20-mW CW operation at 80°C

• Doctor of Engineering,
Sumitomo Electric Device Innovations, Inc.
He is engaged in the research anddevelopment of visible semiconductor
lasers.

S. KAJIYAMA• Technical Training Center, Plant and Production
Systems Engineering Division

H. OGURI
5. Conclusion
We have successfully developed a high power andshort cavity (L = 300 µm) 660 nm AlGaInP LD by applying
the 4-inch full-wafer process so as to meet the market
needs. An output power of over 30 mW was obtained
under CW operation at 25°C to 80°C, and output power
per unit length was over 100 mW/mm, which is muchhigher than that of the high-power laser designed forhigh-speed recordable DVD systems (64 mW/mm).
*1 Catastrophic optical damage (COD): A phenomenon of an LD factsuddenly melting due to a large absorption of produced laser light energy. In general, it causes an irreparable and catastrophic failure to asemiconductor laser.
*2 Window structure: A structure fabricated near an LD facet to minimize
the optical absorption at the laser facet.

82 · High-Power Short-Cavity AlGaInP Laser Diodes• Sumitomo Electric Device Innovations, Inc.

M. UEDA• Sumitomo Electric Device Innovations, Inc.

S. GOTO• Sumitomo Electric Device Innovations, Inc.

T. KITA• Sumitomo Electric Device Innovations, Inc.

H. AMANO• Sumitomo Electric Device Innovations, Inc.

H. SATOYOSHI• Sumitomo Electric Device Innovations, Inc.

T. YAMAMOTO• Integrated Device Process R&D Department,
Transmission Devices R&D Laboratories

K. NAKAO• Integrated Device Process R&D Department,
Transmission Devices R&D Laboratories

S. IZUMI• Doctor of Engineering,
Deputy General Manager, Device Production Department,
Sumitomo Electric Device Innovations, Inc.

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