TUP-=

UNIFORMITY OF 1 . 3 - p LASER DIODES WITH BUTT-JOINTED SELECTIVELY GROWN SPOT-SIZE CONVERTER

FABRICATED ON 2-INCH Id’ SUBSTRATES

, H. Okamoto, Y. Suzaki, Y. Tohmori, M. Okamoto, Y. Kondo, Y. Kadota, M, Yamamoto, K. Kishi, Y. Sakai, T. Sugie, and Y. Itaya

NTT Opto-electronics Laboratories 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa, 243-0 1 Japan

Introduction

A spot-size converted laser diode (SSC-LD) is an attractive device for low cost optical modules because it provides low-loss coupling to a single-mode fiber and to a Planar Lig?atwave Circuit (PLC) without using lenses as well as large alignment tolerance El]. Many intensive studies have been done in this field [2-121. We have recently developed 1.3 pm-LDs with butt-jointed selectively grown spot-size converters [13, 141. They have low threshold and good temperature characteristics. However, it has been uncertain whether the butt-joint selective growth process is suitable for mass production or not. The problem to be solved is how to improve the yield. A key process for the uniform fabrication of the SSC-LD is the uniform etching of the active region in preparation for butt-joint growth. In this report, we demonstrate SSC-LDs on 2-inch InP substrates with good uniform characteristics by using a combination of uniform epitaxial growth, wet etching and dry etching.

I. Device structure and fabrication Figure 1 shows a schematic ofthe SSC-LD. The

spot-size converter (SSC) region is butt-jointed to the MQW active region. Both the active and SSC regions are 300-pm long. For the SSC structure, a taper waveguide with changing thickness in the vertical direction is used.

I Spotsize I Active Region Converter I

ButtJoint

Fig. 1. Schematic structure of the 1.3 pm LD with butt-jointed selectively grown spot-size converter.

267

The SSC-LDs were formed by five-step MOVPE growth: MQW growth for an active layer, butt-joint selective growth, thin InP layer growth, buried heterostructure (BH) embedding growth, and cladding layer growth. The uniformity of film thickness in a 2-inch wafer was w i t h f 1.5% for the active layer growth, and the average uniformity of PL wavelength for a 1.3-pm strained MQW was f 5 nm [E].

For the butt-joint growth, a n active layer, which is composed of 8-well strained-MQW, except for the area covered by the SiNx-selective-growth mask was removed by selective wet chemical etching. Figure 2 shows a n example of the MQW- wet etching depth vs. time characteristics measured at 10 isolated points on a 2-inch wafer. In ths figure, the etching characteristics are divided into two regions: a high etching rate region, which shows the etching profile ofMQW layer, and a low etching rate region, which shows that of InP layer beneath the MQW. As clearly shown, all of the points where etching rate changes from high to

low localize a t one point, whch means that the thckness of the MQW layer is uniform, and also the variation of etching time to remove the MQW layer is very small a t each point on the 2-inch wafer. Because the etchng rate of the MQW layer was well controlled on whole wafer, the side etchng depth inside the active layer was maintained a t around 0.3 pm on the whole wafer. During the butt-joint taper waveguide layer (hg =1.1 pm) growth, the side etching region was suf6ciently buried by the epitaxially grown layer, and a planar surface was formed. A cross sectional SEM image of the butt-joint portion is shown in Fig. 3. The quality of the epitaxial layer around the butt-joint region was identical to that grown on the planar surface by micro-PL measurement. The vertical taper structure was formed by selective growth during the butt-joint growth. After removing the SiNx mask, a thin p-type InP layer was grown. Next, the mesa was formed by CH4/H2 reactive ion etching (RIE) using a stripe- Si02 mask. This was followed by conventional two-step pn-buried hetemstructure growth. Because the etching-depth uniformity of CH4/H2 RIE is very good (k 3.5%, except for the area 3 mm from the epitaxially grown wafer edge), d o r m pn-buried growth was achieved over the whole wafer. A p-type InP cladding layer and a contact layer were then grown. Finally, top and bottom electrode metals were formed. Figure 4 shows a completed 2-inch wafer. Each square region on the wafer is 12 x 12 mmz.

250

200

150

100

50

0

0 50 100 150 200 250 Etching Time (s)

Fig. 2. Typical MQW-selective-wet etchmg depth vs. time characteristics.

Inset shows the measured points.

Fig. 3. Cross sectional SEM image of butt-joint portion.

Fig. 4. Completed 2-inch wafer covered with SSC-LDs.

30

20 E Y L

n U

2 10

s CI

0 0 50 100 150

Current (mA) Fig. 5. Typical light to current characteristics.

11. Device performance Typical L-I characteristics (CW, cleave to 95%

HR) of the SSC-LD are shown in Fig. 5. The threshold current is 5.6 mAat 25 "c, and nokink is observed a t all of the measured temperatures.

268

Figure 6 shows the lasing spectrum of the SSC-LD. In these spectra, the mode interval is 0.4 nm, which is determined by the 600-pm-cavity Fabry- Perot oscillation, and no signaficant ripple in the lasing spectrum appears. This fact means tha t the internal reflection at the butt-joint portion is very small. Figure 7 shows the measured coupling losses between the SSC-LDs and flat-end dispersion-shift single mode fibers (4-pm spot size). The coupling losses were around 2 dB, which shows the vertical taper structure in the SSC was successfully formed. No large degradation has been measured in a preliminary life test [E].

. . . . . . .. . , , . , . , _. , , . . . . . . . . . . . . . . . .

The inset of Fig. 8(a) shows the measured area in a %inch wafer. Figures 9(a) and (b) show the distribution of the Ith and FFPs sampled from a larger area of the wafer (sampled 580 LDs). As clearly seen in these figures, the uniformity of both Ith and FFPs are very good. These uniform characteristics are considered to be achieved by uniform MQW growth, CH4IH2-RIE mesa formation, and pn-buried heterostructure growth. In addition, the uniform characteristics of the FFPs show that the butt-joint portion and vertical taper structure of the SSC were uniformly fabricated by using uniform wet etching and butt-joint selective growth.

Pulsed, CL-CL; 120 LDs 16 , I

3 14 E - 12 . I =

-20 -15 -10 -5 0 5 10 15 20 2 1.305 1.31 5 1.325 Wavelength (vm)

Fig. 6. Lasing spectra of the SSC-LD.

Distance from wafer center (mm)

(a) FFP-Horizontal

16 I I 14 12

s 8 $ 6

h p 10

2 4

2 0 -20 -15 -10 -5 0 5 10 15 20

Distance from wafer center (mm)

(b) 0 1 2 3 4 5

Coupling loss (dB) Fig 7. Measured coupling losses between the SSC-LDs and flat-end dispersion-shift

single mode fibers (4-pm spot size).

111. Device uniformity in a 2-inch wafer The distributions of the threshold current (I&)

and the far-field pattern (FFP) half width in a 2- inch wafer were measured with pulsed-current drive using cleave-to-cleave SSC-LDs. Figure S(a) shows the Ith distribution, and Figs. 8(b) and (c) show the distribution of horizontal and vertical FFP FWHM over the diameter of the 2-inch wafer.

269

. , ~~ , FFP-Vertral ! ii , 12

-20 -15 -10 -5 0 5 10 15 20 Distance from wafer center (mm)

(c> Fig. 8. Uniformity of threshold current (a),

and FFPs (b) and (c) over the diameter of the2-inch wafer.

99.99 99.9 99

88 c 70 $ 50 $2 30 2 1g

1 .1 .01

99.99 99.9 99

88

l!!

2 70 50 $ 30

1 .1 .01

-...........*....I .....................,..................... ; ..................- I 1 I l l , I 1 I I I I I 1 I l l 1 I .

0 5 10 15 20 Threshold current (mA)

(a)

....................

....................

.................. -......... ......... ....,.."""""'."'-

.................. ..................

.......................

-.........A..... o. ......................................... ................ I I I I 1 1 1 1 1 1 1 1 1 1 1 1

0 5 10 15 20 FWHM of FFP (deg)

(7.3)

Fig. 9. Distribution of the Ith (a), and FFPs (b), sampled from the 2-inch wafer.

IV. Conclusion 1.3-pm LDs with a spot-size converter using a

butt-joint and selective growth were uniformly fabricated on a 2-inch InP substrate. Highly uniform SSC-LD characteristics in the 2-inch wafer were obtained by a combination of d o r m MQW growth, selective wet e t h g of MQW, butt-joint selective growth, CH4/HZ-RIE mesa formation, and pn-buried heterostructure growth.

Acknowledgment We would like thank T. Maruyama and H.

Nakajima for their help in fabrication. Thanks are also due to 0. Mztomi, M. Wada, I. Kotaka, H. Oohashi, M. Nakao, H. Fukano, K. Kasaya, and J . Yoshida for technical discussions and encouragement.

References [l] J. Yoshida, M. Kawachi, T. Sugie, M. Horiguch, Y. Itaya, and M. Fukuda, Cod. proc. of VI1 Int. Workshop on Optical Access Networks (OAN),

VI-6.1, 1995. [2] T. L. Koch, U. Koren, G. Eisenstein, M. G. Young, M. Oron, C.R. Giles, and B.I. Miller, IEEE Photon. Technol. Lett., vol. 2, pp. 88-90, 1990. [3] K. Kasaya, 0. Mitomi, M. Naganuma, Y. Kondo, and Y. Noguchi, IEEE Photon. Technol. Lett., vol. 5 , pp. 345-347, 1993. [4] P. Doussiere, P. Garabedian, C. Graver, E. Derouin, E. Gaumont-Goarin, G. Michaud, and R. Meilleur, Appl. Phys. Lett., vol. 64, pp. 539-541, 1994. [5] I. F. Lealman, L. J. Rivers, M. J. Harlow, S. D. Perrin, and M. J. Robertson, Electron. Lett., vol.

[6] H. Kobayash, M. Ekawa, N. Okazaki, 0. Aoki, S. Ogita, and H. Soda, IEEE Photon. Technol. Lett., vol. 6, pp. 1080-1081, 1994. [7] I. Moerman, M. D' Hondt, W. Vanderbauwhede, J. Haes, L. Vanwassenhove, P. De Dobbelaere, R. Baets, P. Van Daele, P. Demeester, W. Hunziker, and C.Holtmann, Proc. ECOC '94, pp. 1027-1030, 1994. [8] A. Kasukawa, N. Iwai, N. Yamanaka, and N. Yokouchi, Electron. Lett., vol. 31, pp. 559-560, 1995. [9] R. Zengerle, 0. Leminger, W. Weiershausen, K. Faltin, and B. Hubner, IEEE Photon. Technol. Lett., vol. 7, pp. 532-534, 1995. [lo] H. Sato, M. Aoki, M. Takahashi, M. Komori, K. Uomi, and S. Tsuji, Electron. Lett., vol. 31, pp.

[ l l] H. Fukano, K. Yokoyama, Y. Kadota, Y. Kondo, M. Ueki, and J. Yoshida, Proc. ECOC '95 @russels, Belgium), vol. 3, Post-deadline papers,

1121 M. Wada, M. Kohtoku, K. Kawano, H. Okamoto, Y. Kadota, Y. Kondo, K. K i sh , S. Kondo, Y. Sakai, I. Kotaka, Y. Noguchi, and Y. Itaya, Electron. Lett., vol. 31, pp. 2102-2104, 1995. [13] T. Sugie, et al., Technical Digest ofIOOC-95,

[14] Y. Tohmori, Y. Suzaki, H. Oohashi, Y. Sakai, Y. Kondo, H. Okamoto, M. Okamoto, Y. Kadota, 0. Mitomi, Y. Itaya, and T. Sugie, Electron. Lett.,

[ E ] M. Yamamoto, Y. Itaya, and T. Sugie, to be presented a t IPRM '96, TUB 2-1, 1996.

30, pp. 857-859, 1994.

1241-1242, 1995.

pp. 1027-1030, 1995.

ThB2-6, 1995.

vol. 31, pp. 1838-1840, 1995.

270