10 Gb/s single-mode vertical-cavity surface-emitting laser with large aperture and oxygen implantation

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10 Gb/s single-mode vertical-cavity surface-emitting laser with large aperture and oxygen

implantation

View the table of contents for this issue, or go to the journal homepage for more 2004 Semicond. Sci. Technol. 19 L86

(http://iopscience.iop.org/0268-1242/19/8/L02)

Semicond. Sci. Technol. 19 (2004) L86–L89 PII: S0268-1242(04)79330-X

LETTER TO THE EDITOR

10 Gb/s single-mode vertical-cavity

surface-emitting laser with large aperture

and oxygen implantation

Fang-I Lai

, Tao-Hung Hsueh

, Ya-Hsien Chang

,

Hao-Chung Kuo

, S C Wang

, Li-Hong Laih

, C P Song

and

H P Yang

1_{Institute of Electro-optical Engineering, National Chiao-Tung University, Hsin-Chu,} Taiwan, Republic of China

2_{Millennium Communication Co., Ltd, Hsinchu Industrial Park, Hsinchu Hsien, Taiwan} 3_{Opto-Electronics & Systems Laboratories of the Industrial Technology Research Institute,} Hsin-Chu, Taiwan, Republic of China

E-mail: [email protected] Received 20 April 2004 Published 21 June 2004 Online atstacks.iop.org/SST/19/L86 doi:10.1088/0268-1242/19/8/L02 Abstract

High-speed single transverse mode 850 nm vertical cavity surface emitting lasers (VCSELs) with large emission aperture with a diameter of 8 µm were fabricated. These VCSELs exhibit good performance with threshold currents of 1.5 mA, a single transverse mode emission within the full operational range and a maximum output power of 3.8 mW. The large aperture is advantageous to these VCSELs with a smaller dynamic resistance (60
) than that of conventional single-mode VCSEL. These single-mode VCSELs also demonstrate superior high-speed performance up to 10 Gb s−1. (Some figures in this article are in colour only in the electronic version)

1. Introduction

Vertical cavity surface emitting lasers (VCSELs) have become a standard applied technology in local area networks (LANs) from 1.25 Gb s−1 to 10 Gb s−1. The main advantages of VCSELs are their low threshold current, low divergent angle and circular beam, which simplify packaging and reduce electrical power consumption. The surface emission from the VCSELs also facilitates the integration of a two-dimensional array and the testing of wafer levels, reducing the cost of fabrication [1]. High-power single transverse mode operation is preferred for many applications, including laser printing, optical storage and long-wavelength telecommunications. Two major approaches were previously developed for fabricating single-mode VCSELs. The first is to make the current-confined aperture sufficiently small to support only the fundamental mode; the best results using this

method have been achieved using oxide-confined VCSELs [2]. Typically, the current-confined aperture must be less than 3 µm in diameter to ensure stable single-mode operation. However, the large resistance inherited from the small aperture limits the modulation bandwidth and degrades the high-speed performance. Furthermore, the lifetime of the oxide VCSEL decreases proportionally as the diameter of the oxide aperture declines, even when the device is operated at a reduced current [3].

The second approach for fabricating a single-mode VCSEL is to suppress the high-order mode in a multimode VCSEL by surface-relief etching [4, 5], or by extending the optical cavity [6]. Young et al proposed hybrid implant/ oxide VCSELs that support single-mode operation, based on ‘cold-cavity’ considerations. The basic concept is to increase the optical losses of higher order modes [7]. However, the threshold current (Ith) of this VCSEL is rather high, at

Letter to the Editor

5 x Al0.26Ga0.74As/GaAs QWs

Semi-insulating GaAs substrate

Regrowth interface Oxidation layer

O+ implant Ti/Pt/Au n-DBR p-DBR Au/Ge/Ni/Au 5 x Al0.26Ga0.74As/GaAs QWs

Semi-insulating GaAs substrate

Regrowth interface Oxidation layer

O+ implant Ti/P Au

n-DBR p-DBR

Au/Ge/Ni/Au

Figure 1. The schematic structure of a single-mode VCSEL. The

O+_{-implanted aperture is 8 µm in diameter and the oxidation} aperture is 10 µm in diameter.

∼5.8 mA. This study presents a low Ith, high-power

single-mode VCSEL fabricated using oxygen (O+) implantation, MOCVD regrowth and selective oxidation. Two types of apertures in this device were designed to reduce the overlap of the higher order modes with the current-confined profile while at the same time keeping a sufficient fundamental mode/current-confined overlap [8]. The current flow is confined by an oxygen-implanted aperture with a diameter of 8 µm and the optical mode is confined by an oxide aperture with a diameter of 10 µm. These VCSELs emit a single transverse mode within the full range of the operated current and can operate up to 10 Gb s−1.

2. Device structure and fabrication

Figure 1 schematically depicts the cross-sectional O+ -implanted VCSEL structure. The epitaxial structure was grown by metal organic chemical vapour deposition (MOCVD) on a semi-insulated GaAs (100) at 6◦to (111A) substrate. Prior to O+_{implantation, the wafer structure consists}

of an n+_{-GaAs (Si-doped) buffer layer and 39 pairs of n-type}

(Si-doped) Al0.19Ga0.81As/Al0.9Ga0.1As bottom-distributed

Bragg reflectors (DBRs), an Al0.26Ga0.74As/GaAs five period

quantum-well (QW) active region in a one-wavelength cavity for 850 nm emission and two pairs of p-type (C-doped) Al0.19Ga0.81As/Al0.9Ga0.1As top-DBRs. Then, a 50 ˚A GaAs

cap layer was grown at the null position of the optical field to avoid optical absorption; the GaAs layer was used to prevent oxidation of the surface before regrowth. O+_{was implanted}

(dosage∼2 × 1014_{) at 120 keV to define a current aperture}

with a diameter of 8 µm by photolithography, as indicated in figures2(a) and (b). After implantation, 24 pairs of top-DBRs were regrown with a 30 nm Al0.98Ga0.02As oxide layer which is

located at the third null node position. This position is one pair away from the QW region, compared to conventional VCSELs, to reduce the index guiding effect. During the regrowth period, the implanted layer became an effective insulator as the current-confined aperture because of the activation process. The processing sequence included six photomasks to fabricate oxide-confined polyimide-planarized VCSELs with coplanar wave-guide probe pads. This process was designed to minimize capacitance while keeping reasonably

Active region

39 pairs n-DBR 2 pairs p-DBR

Semi-insulating substrate GaAs cap layer

Active region

39 pairs n-DBR 2 pairs p-DBR

Semi-insulating substrate GaAs cap layer

PR O+ _O+ n-DBR p-DBR Semi-insulating substrate 8 _µm PR O+ _O+ PR O+ _O+ n-DBR Semi-insulating substrate 8 _µm ICP etching (BCl ICP etching (BCl33, Cl, Cl22)) p-DBR 20 m ICP etching (BCl ICP etching (BCl33, Cl, Cl22)) n-DBR p-DBR 20µm (a) (b) (c) (d ) Semi-insulating substrate Semi-insulating substrate P-contact n-contact n-contact Si3N4

Figure 2. Device process flowchart. (a) The VCSEL with 39 pairs

of n-type DBRs, 5-QW, two pairs of p-type DBRs and a 50 ˚A GaAs cap layer. (b) The O+_{-implanted aperture was defined by}

photolithography. (c) The surface relief pattern with a diameter of 20 µm was etched by chemically assisted RIE (reactive ion etching). (d) Ti/Au were deposited for metal interconnects and coplanar waveguide probe-bond pads.

low resistance. Device fabrication began with the formation of cylindrical mesas 20 µm in diameter by etching the surrounding semiconductor to a depth of 5 µm into the bottom n-type mirror using a reactive ion etching (RIE) system as shown in figure 2(c). The gas used in the ion source was Cl2, and the gas that flowed on the sample was BCl3. The

samples were then selectively oxidized to form circular optical confined apertures with a diameter of 10 µm. Following this Si3N4was deposited for passivation. Ti/Au was evaporated

for the p-type contact ring, and AuGeNi/Au was evaporated onto the etched n-buffer layer etched bottom mirror to form the n-type contact which is connected to the semi-insulating substrate. Contacts were alloyed for 30 s at 420 ◦C using RTA. After contact formation, photosensitive polyimide was spun on the sample for field insulation and planarization. Ti/Au with thicknesses of 200/3000 ˚A were deposited for metal interconnects and coplanar waveguide probe-bond pads (figure2(d)). Heat treatment after the metal deposition was utilized to improve metal-to-polyimide adhesion strength. For comparison, a conventional VCSEL was also processed with only an oxide-confined aperture with a diameter of 8 µm. The same processes as above were performed but without O+

implantation and regrowth process.

3. Results and discussion

Figure3(a) plots the continuous-wave (CW) power–current– voltage (L–I–V) curves and the spectral characteristics (insertion) of the O+_{-implanted VCSELs. The VCSEL emits}

near 3.8 mW peak power at a 12.3 mA drive current and exhibits a low threshold current of 1.5 mA with a threshold voltage of 1.7 V and slope efficiencies of∼ 0.35 W A−1. The insets of figure 3(a) present the emission spectrum and the near-field pattern of the device at 10 mA. The figure reveals that only the fundamental mode is present over the full range of operational current. Over 90% of the series resistances L87

(a) 0 5 10 15 0 1 2 3 4 5 6 Current (mA) Voltage (V) 0 1 2 3 4 5 6 Power (mW) 840 842 844 846 848 850 852 854 -70 -60 -50 -40 -30 -20 Si ng al (dB m ) Wavelength (nm) I=10mA I=5mA 0 5 10 15 0 1 2 3 4 5 6 0 1 2 3 4 5 6 840 842 844 846 848 850 852 854 -70 -60 -50 -40 -30 -20 Si ng al (dB m ) Wavelength (nm) I=10mA 0 5 10 15 0 1 2 3 4 5 6 0 1 2 3 4 5 6 840 842 844 846 848 850 852 854 -70 -60 -50 -40 -30 -20 Si ng al (dB m ) Wavelength (nm) I=10mA 0 5 10 0 1 2 3 4 5 6 0 1 2 3 4 5 6 840 842 844 846 848 850 852 854 -70 -60 -50 -40 -30 -20 Si ng al (dB m ) Wavelength (nm) I=10mA 0 5 10 0 1 2 3 4 5 6 0 1 2 3 4 5 6 840 842 844 846 848 850 852 854 -70 -60 -50 -40 -30 -20 Si ng al (dB m ) Wavelength (nm) I=10mA I=5mA (b) 0 2 4 6 8 10 12 14 0 1 2 3 4 5 0 1 2 3 4 5 Power (mW) Voltage (V) Current (mA)

Figure 3. (a) L–I–V curves, spectral characteristics and the

near-field pattern of the O+_{-implanted VCSEL. The emission} spectrum is obtained at 10 mA and the near-field pattern is obtained at 5 mA. They demonstrate that the O+_{-implanted VCSEL has a} fundamental mode only over the full operating range. (b) L–I–V curves of the comparable conventional VCSEL.

of the VCSELs are 60–65
. This range of resistance is almost the same as that for comparable conventional VCSELs (the L–I–V characteristics are shown in figure 3(b)), but lower than for single-mode VCSELs with a small aperture, which typically have around 250
of series resistance [9]. Additionally, the roll-over current is about 12 mA, which exceeds that of the single-mode VCSEL with a small aperture and so the proposed VCSELs should be more reliable [2]. Figure3(b) plots the L–I–V characteristics of the comparable conventional VCSEL. The threshold current (Ith) of the

VCSEL is around 1 mA with a threshold voltage of 1.5 V and the slope efficiency of ∼0.4 W A−1. The peak output power is 4.7 mW at a drive current of 12.3 mA. Although the O+_{-implanted VCSELs have a lower peak power and a higher}

Iththan comparable conventional VCSELs, perhaps because of

the regrowth interface and the losses of higher order modes, the O+_{-implanted VCSELs can operate in the fundamental single}

mode within the full operating range exhibiting a better high-speed modulation response than conventional VCSELs [10]. To measure the high-speed characteristic of the O+ -implanted and comparable conventional VCSELs under large signal modulation, microwave and light wave probes were used in conjunction with a 10 Gb s−1(223_{− 1 long pseudorandom}

bit sequence generated) pattern generator (MP1763 Anritsu). (a)

(b)

Figure 4. Characteristic eye diagram of (a) large-aperture

single-mode VCSEL with a diameter of 8 µm; (b) conventional multi-mode VCSELs with a diameter of 8 µm, transmitted at 10 Gb s−1with a bias of 5 mA and an extinct ratio of 6 dB. A 12.5 GHz New Focus photoreceiver with an OC-192 low pass Bessel Thompson filter was used for detection. The eye diagrams were obtained for back-to-back (BTB) transmission of VCSELs. Figures4(a) and 4(b) demonstrate the eye patterns of O+-implanted and comparable conventional VCSELs, respectively. Figure4(a) shows the wide-open eye pattern, indicating good performance of the O+_-implanted

single-mode VCSEL. The rise time (Tr) and the fall time (Tf)

are estimated to be 28 ps and 38 ps, respectively, with jitter (p–p)= 14 ps. The comparable conventional oxide-confined multiple-mode VCSEL yields a more noisy eye pattern, with jitter= 17 ps (see figure4(b)).

4. Summary

This study reports a new technique for fabricating a single-mode VCSEL with a large emission aperture with a diameter of 8 µm, using oxygen implantation, MOCVD regrowth and selective oxidation. The single-mode VCSELs have a low Ith of about 1.5 mA and a high output power of 3.8 mW.

Although this approach involves regrowth, it provides the single transverse mode with large aperture with a low Ith

and a high output power. These single-mode VCSELs also demonstrate superior high-speed performance up to 10 Gb s−1. The authors believe that the concept can be applied to fabricate long-wavelength single-mode VCSELs.

Letter to the Editor Acknowledgments

The authors would like to thank Drs C P Kuo of LuxNet Corporation, and Professor S L Chuang of University of Illinois for useful discussions and technical support. This work was partially supported by the National Science Council of the Republic of China (ROC) in Taiwan under contract no NSC 90-2215-E-009-102 and by the Academic Excellence Program of the ROC Ministry of Education under contract no 88-FA06-AB. The support by the Institute of Nuclear Energy under contract no 922001InER015 of the proton implantation is also gratefully acknowledged.

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