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1.3 pm Wavelength GaInAsP/InP Distributed Feedback Lasers Grown Directly on Grating Substrates by Solid Source Molecular Beam Epitaxy

W.-Y. Hwang, J. N. Baillargeon, S. N. G. Chu, P. F. Sciortino, and A. Y. Cho Bell Laboratories, Lucent Technologies, 600 Mountain Ave., Murray Hill, NJ 07974

Abstract. Successful growth of GaInAsP/InF' multi-quantum well lasers directly on a distributed feedback (DFB) grating substrate using all solid source molecular beam epitaxy (MBE) was demonstrated. A 500 A thick 1.12 pm wavelength GaInAsP planarization layer was first grown on the DFB gratings at an elevated temperature to create a smooth surface for subsequent layer growth. Transmission electron micrograph showed smooth interfaces after the growth of this GaInAsP planarization layer. Low threshold current density and high quantum efficiency were obtained from these index-coupled DFB lasers grown by solid source MBE.

1. Introduction

Distributed feedback (DFB) lasers operating at 1.3 and 1.55 pm wavelengths employing GaInAsPAnP materials are crucial components for wide-band single-mode optical fiber communications. This is because of their closely controlled and stable single longitudinal mode operation even at high output power range. Fabrication of a high quality DFB laser structure requires growth on a corrugated crystal surface with precise control of material composition and layer thickness. Although all presently utilized growth techniques can achieve high quality layers on planar surfaces, growth on grated or patterned substrates is far more Typically, a DFB laser can be constructed by growing a laser structure directly on a grated substrate or by making gratings on a grown laser structure and followed by a second step overgrowth. When growing a DFB laser structure directly on a grated InP substrate, the growth surface must be mechanically smooth and dislocation free after only a few hundred angstroms of the quaternary is deposited. Metalorganic chemical vapor deposition (MOCVD) is presently the most successful and dominate growth technique for fabricating DFB lasers. Recently, MBE growth of high quality GaInAsP layers and high performance GaInAsPAnP lasers have been achieved by using all solid source^.^-^ This technology subsequently enables the growth investigation of DFB lasers on InP grating surfaces.

2. Experiments

The growth of GaInAsPAnP index-coupled DFB lasers were carried out with a Riber 2300 MBE system using elemental In, Ga, Si, and Be, and P2 and As2 supplied via solid phosphorous and arsenic valved sources. The valved P2 cell used is a three temperature zone Riber model KPC40 with a modified cracker head. The As2 flux is supplied via an EPI VC-IV valved cell with two temperature

CCC Code 0-7803-3883-9/98/$10.00 0 1998 IEEE

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zones. Growth temperatures were measured with an IRCON 6000 series optical pyrometer that was calibrated to a surface oxide desorption temperature of 460 "C with S-doped InP. First order gratings,

with a periodicity of 0.202 pm, were optically patterned parallel to the [ 0 i l l direction using holographic photolithography for the 1.3 pm wavelength DFB lasers. Wet chemically etching in WBr:HN03:HzO (1:1:20) for one minute was then used to define the grating on the InP surface. The etched depth of the V-grooves with (1 1 l)A side-walls was about 600 8,. A planar InP substrate was also placed adjacent to the grated InP substrate for each growth. Samples were affixed to the molybdenum block using indium. Both of the grated and planar InP wafers were etched in H$S04:H202:HzO (10: 1 : 1) solution for 40 seconds before loading into the MBE system.

Before initiating the growth, the S-doped InP substrates were heated at 470°C for 10 min under a P2 flux of 1 .Ox 1 0-5 Torr to desorb the native oxide. The substrate temperature was then ramped from 470 "C to 510 "C in two minutes for growth of the Gao.ljIn0.8jAs0.32P0.68 ( k p ~ = 1.12 pm, denoted as 1.124) quaternary planarization layer, Si-doped at 8 ~ 1 0 ' ~ ~ m - ~ . An As2 flux of 8 . 0 ~ 1 0 - ~ Torr was used in addition to the P2 flux during this substrate temperature ramping in order to preserve a proper grating depth. The higher growth temperature for the 1.12Q planarization layer is crucial for producing a flat surface for the subsequent growth o f the MQW laser structure.6 After the planarization layer, a 1200 8, thick InP lower cladding layer (Si-doped at 8 ~ 1 0 ' ~ ~ m - ~ ) was then grown, followed by a 800 8, thick undoped 1.124 separate confmement layer, an undoped active region, a 800 8, thick undoped 1.12Q separate confinement layer, a 1.2 pm thick InP (Be doped at 1 ~ 1 0 ' ~ ~ m - ~ ) upper cladding layer and fmally a 500 8, thick Ga0,47In,~.s3As (Be doped at 4x 1019 ~ m - ~ ) contact layer. The active region consists of six (nine), 75 8, thick GaInAsP quantum wells and five (eight), 100 8, thick l.12Q barrier layers. A growth interruption of 10 seconds was used at the interfaces between barriers and wells. The growth temperatures of the InP cladding layers and the active regions were 460 O C and 490 'C, respectively. Between the growth of the planarization layer and the InP lower cladding layer, a growth intenuption of 90 seconds was employed to reduce the substrate temperature from 510 OC to 460 OC. The growth rates for InP, 1.124, and GaInAsP quantum wells were 1.20, 1.41, and 0.71 pm/h, respectively. A P2 flux of I . O X ~ O - ~ Torr was used for the growth of all layers except the GaInAs p- contact layer. The As2 beam fluxes used for the growth of 1.12Q and quantum wells were 8 .0~10-~ and 5 . 8 ~ 1 0 ~ Torr, respectively. These group V fluxes were measured by an ionization gauge located in the beam path. Broad area stripe lasers (80 pm wide) were fabricated without any facet coating or lateral current Confinement.

3. Results and Discussion The transmission electron micrograph of a 6 quantum well DFB laser is shown in Fig. 1. The

preserved grating depth is about 300 8, and smooth interfaces were obtained between all epitaxial layers. When a deeper grating depth is required, a larger initial grating depth and a lower oxide desorption temperature can be used. For example, when performing the oxide desorption at 465°C for 10 minutes from a 600 8, deep grating, the preserved grating depth is about 550 8,. The threshold current densities per-well as a fimction of inverse cavity length under pulsed operation are plotted in Fig. 2(a) for both lasers grown simultaneously on the grated and planar substrates. The transparency current density of the lasers grown simultaneously on grated and planar substrates are 68 and 94 A/cm2 per well, respectively. From Fig. 2(a), one can observe that lasers grown on grated and planar

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Fig.1 sample is about 300 A. Smooth interfaces are observed after the growth of a 1.124 planarization layer.

The transmission electron micrograph of a 6-quantum-well 1.3 pm DFB laser. The preserved grating depth in this

0 10 20 30 40

inverse Cavity Length ( cm-' )

Fig. 2 The threshold current density per-well as a function of the inverse cavity length, (a) and the inverse extemal quantum efficiency as a function of the cavity length (b) for 1.3 pm lasers grown simultaneously on a DFB grating substrate and a planar substrate. Each data point represents the average of 3 lasers with the same cavity length.

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substrates with the same cavity length showed similar threshold current densities. The inverse extemal quantum efficiency as a fimction of the laser cavity length for these DFB lasers are shown in Fig. 2(b). The intemal quantum efficiency, qi, and internal loss, a;, for these 6-quantum-well lasers are 0.65 and 13.4 cm-* for the grated substrate and 0.59 and 7.5 cm-l for the planar substrate, respectively. The transparency current density, q; and ai of the 9-quantum-well DFB lasers are 61 A/cm2 per well, 0.67 and 15.2 cm-', respectively. These results indicate that the properties of the lasers grown on the grated substrate are similar to that of the !asers grown on the planar substrate. The emission spectra of an uncoated DFB laser is shown in Fig. 3 with a side mode suppression ratio of more thzn 20.

1.33 x It,, ...... 1.50 x Ith

9 - 1.67 x Ith .- 3 .

E 1

U) C 0 ; ...,....... ~ ......,.. >.. - .... ....

1310 1312 1314 1316 1318 1320

Wavelength (nm)

Fig. 3 three different current injection levels are shown.

The emission spectra of an uncoated DFB laser at

4. Conclusion

Successful growth of GaInAsPilnP multi-quantum well (MQW) lasers directly on DFB grating substrates using all solid source MBE was achieved. After desorbing the surface oxide at 470 "C, a 500 8, thick 1.124 planarization layer was first deposited on the DFB gratings at an elevated temperature to create a smooth surface for subsequent layer growth. Transmission electron micrographs show smooth interfaces after growth of the GaInAsP planarization layer. Low threshold current density and high quantum efficiency were obtained from these index-coupled MQW DFB lasers. Broad area lasers show room-temperature transparency current densities as low as 61 and 68 A/cm2 per well for 9-well and 6-well DFB lasers, respectively. The measured internal quantum efficiency and internal loss for 9-well (6-well) DFB lasers are 0.67 (0.65) and 15.2 (13.4) cm-l, respectively.

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Molecular Beam Epitaxy Conference, August, 1996 (Malibu, CA)