MOCVD growth of highly strained 1.3 mu m InGaAs : Sb/GaAs vertical cavity surface emitting laser

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Journal of Crystal Growth 287 (2006) 550–553

MOCVD growth of highly strained 1.3 mm InGaAs:Sb/GaAs vertical

cavity surface emitting laser

Y.A. Chang

a,

, J.T. Chu

a

, C.T. Ko

a

, H.C. Kuo

a

, C.F. Lin

b

, S.C. Wang

a

a_{Institute of Electro-Optical Engineering, National Chiao-Tung University, 1001 Ta-Hsieh Rd, Hsin-Tsu, Taiwan, ROC} b_{Department of Material Science and Engineering, National Chung-Hsing University, 250 Kuo-Kuang Rd, Tai-Chung, Taiwan, ROC}

Available online 28 November 2005

Abstract

In0.45Ga0.55As:Sb/GaAs (1.3 mm) vertical cavity surface emitting lasers (VCSELs) were grown by metalorganic chemical vapor deposition (MOCVD). A In0.45Ga0.55As:Sb/GaAs quantum well (QW) exhibited an emission wavelength of 1.26 mm with narrow full-width at half-maximum (FWHM) of 35.9 meV. For the fabricated VCSEL with 5 mm diameter oxide-conﬁned aperture, a room-temperature (RT) threshold current of 2.1 mA and an output power of 0.9 mW with 1.3 mm emission were obtained. The 3 dB modulation frequency was measured to be 7.6 GHz.

Keywords: A3. Metalorganic chemical vapor deposition; B1. InGaAs:Sb; B2. Semiconductor; B3. Laser diodes; B3. Optical ﬁber devices; B3. VCSELs

1. Introduction

Semiconductor material with emission wavelength range 1.3–1.55 mm is required for potential applications in infrared lasers, integrated optoelectronic and local-area-network systems. Long-wavelength vertical cavity surface emitting lasers (VCSELs) are key devices in the optical fiber metropolitan-area networks (MAN)[1]. To date, the most promising results on low-cost long-wavelength lasers or VCSELs have been obtained using InGaAsN quantum wells (QWs) grown on GaAs substrates [2–5]. The large conduction band offset leads to the improvement of temperature performance compared to conventional InP-based materials. The GaAs system can provide high-performance AlGaAs/GaAs distributed Bragg reflector (DBR) mirrors and allows the use of well-established oxide-confined GaAs-based VCSEL manufacturing infra-structure. However, the incorporation of N beyond about

1% into InGaAs QWs has also been found to cause strong quenching of the luminescence and broaden the lumines-cence. While postgrowth rapid thermal annealing can enhance the photoluminescence (PL) intensity by a factor of 10–100, a blueshift of the PL peak is found simulta-neously[6,7].

InGaAs/GaAs QW structure has received much atten-tion as a potential candidate in semiconductor laser due to its relatively uncomplicated and mature techniques. Recently, highly strained InGaAs VCSELs with PL peak at 1.205 mm and laser emission wavelength around 1.26–1.27 mm have demonstrated very promising perfor-mance and continuous-wave (CW) operation up to 120 1C as well as 10 Gb/s operation [8]. Previously, the Sb surfactant-mediated growth had been investigated in group-IV Si/SiC superlattices, and for group III–V hetero-structure materials. This technology had also been utilized in InGaP bandgap control, and during the lateral epitaxial overgrowth of GaN to alter the predominant facet formation [9–11]. More recently, Sb has been shown to be an efﬁcient surfactant, serving as an isoelectronic impurity, to provide the delay of lateral relaxation of the highly strained InGaAs QW. The Sb-assisted growth of

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E-mail addresses: [email protected] (Y.A. Chang), [email protected] (H.C. Kuo).

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InGaAs has demonstrated a spectral full-width at half-maximum (FWHM) of 30 meV with PL emission at 1.19 mm[12]. A PL emission of 1.27 mm was also achieved by a 9-nm-thick In0.41Ga0.59As0.992Sb0.008 QW [13].

How-ever, the PL characteristics of the Sb-assisted InGaAs QW structure with emission wavelength above 1.25 mm still need to be improved. In this study, the crystalline quality of In0.45Ga0.55As:Sb/GaAs QW active region is optimized and

investigated by temperature-dependent PL. With the aim of extending the PL emission wavelength and narrowing the FWHM value, an indium-graded intermediate layer is inserted on both sides of the In0.45Ga0.55As:Sb/GaAs QW

active region. Finally, a 1.3 mm In0.45Ga0.55As:Sb/GaAs

double quantum well (DQW) VCSEL is fabricated and found to provide better laser performance.

2. Experiments

All structures were grown on n-type GaAs (1 0 0) substrates by low-pressure metalorganic chemical vapor deposition (MOCVD). AsH3 and PH3 hydride sources

were used as group-V precursors. Trimethyl alkyls of gallium (Ga), aluminum (Al), indium (In) and Antimony (Sb) were the group-III precursors. The epitaxial structure was formed by an n+-GaAs buffer, a 40.5-pair n+ -Al0.9Ga0.1As/n+-GaAs (Si-doped) DBR, undoped active

region, a p-Al0.98Ga0.02As oxidation layer, a 25-pair p +

-Al0.9Ga0.1As/p+-GaAs DBR (carbon-doped) and a p+

-GaAs (carbon-doped) contact layer. The graded-index separate conﬁnement heterostructure (GRINSCH) active region mainly consisted of a DQW active region In

0.45-Ga0.55As:Sb/GaAs/GaAs0.85P0.15 embedded between two

linear-graded AlxGa1xAs (x ¼ 0–0.6) conﬁnement layers.

The AsH3/III ratio was as low as 20 and Sb/V ratio was

0.005. The thickness of the cavity active region was one wavelength. Carbon was used as the p-type dopant in the DBR to obtain higher carrier concentration (2–3 1018cm3). The interfaces of both the p-type and n-type Al0.9Ga0.1As/GaAs DBR layers were linearly

graded to reduce the series resistance. The optical proper-ties of QWs were optimized through PL measurement and structural analysis, as described below. The Fabry– Periot dip wavelength was determined by reﬂection measurement.

To realize a 1.3 mm In0.45Ga0.55As:Sb/GaAs VCSEL with

better output characteristics, the QW active region is undoubtedly required to be optimized. However, the higher In content InGaAs QW, leading to the degradation of crystal quality, has been found to possess poorer optical characteristics. To extend the emission wavelength and obtain better crystalline quality, we insert an indium-graded intermediate layer on both sides of the In

0.45-Ga0.55As:Sb/GaAs QW. A schematic plot of the In

0.45-Ga0.55As:Sb/GaAs single QW grown by inserting the

2.5-nm-thick indium-graded intermediate layers is shown inFig. 1. The indium composition in the intermediate layer on top of the QW is graded from 0.45 to 0 and with the

opposite grading below the QW. The full layer thickness of the In0.45Ga0.55As:Sb QW is 10 nm. The Sb composition in

In0.45Ga0.55As:Sb is 0.5%, which is determined by

second-ary ion mass spectroscopy (SIMS). The growth rate is about 0.5 nm/s and the growth temperature is 500 1C. Other growth conditions are similar to our prior work[14]. The thickness of the simple In0.45Ga0.55As QW is 7 nm and

the thickness of the GaAs barrier for both structures is 10 nm.

Fig. 2shows the room-temperature (RT) PL spectra of the In0.45Ga0.55As and indium-graded In0.45Ga0.55As:Sb

single QWs. The low-PL peak intensity and the broadened FWHM of the high In content In0.45Ga0.55As QW may be

attributed to the degraded crystalline quality. Nevertheless,

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GaAs template GaAs0.85P0.15 (10 nm) GaAs cap In_0.45Ga_0.55AsSb QW (5 nm) In-graded layer (2.5 nm)

Fig. 1. A schematic plot of the In0.45Ga0.55As:Sb/GaAs single QW grown

with trapezoid-potential structure by inserting the 2.5-nm-thick indium-graded intermediate layers.

0 0.2 0.4 0.6 0.8 1 1.1 1.15 1.2 1.25 1.3 In-graded InGaAsSb InGaAs Wavelength (µm)

PL intensity (a. u.)

×5

Fig. 2. RT PL spectra of the In0.45Ga0.55As and indium-graded

In0.45Ga0.55As:Sb single QWs.

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the PL characteristics of the InGaAs/GaAs QW are improved with the assistance of Sb and the use of the indium-graded intermediate layers. An increased peak intensity factor of 20 is found with the help of Sb and indium-graded intermediate layers, while a reduction of FWHM value from 60.4 to 35.9 meV is observed. It needs to be mentioned that the PL characteristics are only slightly enhanced when the InGaAs/GaAs QW is with the indium-graded intermediate layers and without the Sb surfactant.

Variable temperature spectra for the In0.45Ga0.55As and

indium-graded In0.45Ga0.55As:Sb single-QW structures are

shown in Figs. 3a and b, respectively. Smooth Gaussian peaks with strong luminescence are observed for both structures. For Sb-assisted indium-graded In0.45Ga0.55As

QW structure, the peaks are sharper, while the peak emission wavelength and the corresponding FWHM value change from 1.055 to 0.984 eV and 23.9 to 35.9 meV when the temperature is increased from 17 to 300 K. Thus, the quality of the Sb-assisted indium-graded In0.45Ga0.55As

single QW is considered to be excellent, and is suitable for the fabrication of 1.3 mm VCSEL. After ﬁtting the experimental data to the Varshni semiempirical relation-ship, the bandgap energy at 0 K, a and b coefﬁcients are 1.039 eV, 4.470.7 104eV/K and 5477121 K for the In0.45Ga0.55As, and 1.054 eV, 6.371.1 104eV/K and

4857124 K for the Sb-assisted indium-graded single-QW structure, respectively. Fitting our data to the classical Arrhenius law I ¼ I0=f1 þ c expðEa=KT Þg,

ac-cording to the thermal carrier transfer mechanism, the activation energies (Ea) of our simple and Sb-assisted

indium-graded In0.45Ga0.55As single-QW structures are

20.87 and 27.09 meV, respectively.

3. Device characteristics

The processing sequence began with the deposition of a 1.3-mm-thick SiNxlayer, which acted as a hard mask in the

following process, onto the wafer by plasma enhanced chemical vapor deposition (PECVD) at 300 1C. Standard photolithography and reactive ion etching (RIE) using SF6

with a ﬂow rate of 20 sccm as etching gas were then performed to deﬁne the etching pattern on the hard mask. The mesa diameter was 22 mm and the oxide aperture was 7 mm. After oxidation, the residual dielectric was removed, and a second 150-nm-thick SiO2by PECVD was deposited

for passivation, followed by a partial etching process for contact window. Ti (30 nm)/Pt (50 nm)/Au (200 nm) were deposited onto the heavily p-doped GaAs contact layer for p-contact, and AuGe (50 nm)/Ni (20 nm)/Au (350 nm) were deposited for n-contact. Multiple proton implantations with a dose of 1015cm2and four different proton energies between 300 and 420 keV were adopted according to simulation results of the stopping and range of ions in matter (TRIM). The implantation region was kept apart the mesa to prevent the damage which was caused by ion bombardment from destroying the active region.

The direct-current characteristic of the completed VCSEL was measured using a probe station, an Agilent 4145A semiconductor parameter analyzer and an InGaAs photodiode. The spectrum of the In0.45Ga0.55As:Sb/GaAs

VCSEL was measured using an Advantest Q8381A optical spectrum analyzer. Fig. 4 plots the RT light output and voltage versus current (L–I–V) characteristics of the In0.45Ga0.55As:Sb/GaAs VCSEL. The RT lasing mode

spectrum is shown in the inset. The VCSEL exhibits single transverse mode characteristic at a lasing wavelength of

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0 1 2 3 4 0.95 1 1.05 1.1 17 K 20 K 30 K 40 K 50 K 60 K 70 K 80 K 90 K 100 K 120 K 140 K 160 K 180 K 300 K 260 K 200 K 230 K (a) 0.95 1 1.05 1.1 1.15 17 K 20 K 30 K 40 K 50 K 60 K 70 K 80 K 90 K 100 K 120 K 140 K 160 K 180 K 300 K 260 K 200 K 230 K (b)

PL intensity (a. u.)

Photon energy (eV) Photon energy (eV)

Fig. 3. Variable temperature spectra for: (a) In0.45Ga0.55As and (b) indium-graded In0.45Ga0.55As:Sb single-QW structures.

Y.A. Chang et al. / Journal of Crystal Growth 287 (2006) 550–553 552

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1.3 mm, with a side mode suppression ratio (SMSR) of 430 dB. The maximum single mode output power exceeds 0.9 mW with a slope efﬁciency of 0.15 W/A at RT and the threshold current is about 2 mA. The series resistance of the VCSEL is 116 O.

The small signal response of VCSELs as a function of bias current was measured using a calibrated vector network analyzer (Agilent 8720) with wafer probing and a 50 mm multimode optical ﬁber connected to a New Focus 25 GHz photodetector.Fig. 5plots the 3-dB bandwidth as a function of the bias current. At a bias current of 7.6 mA,

the maximum 3-dB modulation frequency is measured as 7.6 GHz, which is suitable for 10 Gb/s operation.

4. Conclusion

A high-performance In0.45Ga0.55As:Sb/GaAs VCSEL

was successfully grown by MOCVD. The VCSEL shows a low threshold current of 2 mA with 1.3 mm emission and a maximum output power of 0.9 mW with 0.15 W/A slope efﬁciency was obtained. The maximum 3-dB modulation frequency is measured as 7.6 GHz. The results of In

0.45-Ga0.55As:Sb VCSELs can reach a performance level

comparable to GaInAsN VCSELs with better thermal stability and should be considered as a very promising candidate for 1.3 mm commercial applications.

Acknowledgments

The authors would like to thank Prof. Nelson Tansu of Leigh University, Dr. Chih Ping Kuo of LuxNet Corpora-tion, and Drs. C.P. Sung and Jim Chi of ITRI for useful discussion and technical support. This work is supported by the National Science Council, Republic of China, under grant NSC-93-2120-M009-006 and by the Academic Excellence Program of the Ministry of Educa-tion of ROC under the contract NSC-93-2752-E009-008. References

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-70 -65 -60 -55 -50 -45 -40 -35 1290 1297 1305 1312 Intensity (dB) Wavelength (nm) 0 0.5 1 1.5 2 2.5 3 0 0.2 0.4 0.6 0.8 1 0 ₁₀ ₁₂ Current (mA) Voltage (V) _{Power (mW)} 2 4 6 8

Fig. 4. RT light output and voltage versus current (L–I–V) characteristics of the In0.45Ga0.55As:Sb/GaAs VCSEL.

-10 -8 -6 -4 -2 0 2 4 0 10 Frequency (GHz) Response (dB) 2 4 6 8

Fig. 5. Three-decibel bandwidth as a function of the biased current of the In0.45Ga0.55As:Sb/GaAs VCSEL.