Journal of Crystal Growth 370 (2013) 217–220

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Journal of Crystal Growth

0022-02

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AlGaInAsPSb-based high-speed short-cavity VCSEL with single-modeemission at 1.3 mm grown by MOVPE on InP substrate

Christian Grasse a,n, Michael Mueller a, Tobias Gruendl a, Gerhard Boehm a,Enno Roenneberg b, Peter Wiecha a, Juergen Rosskopf b, Markus Ortsiefer b,Ralf Meyer a, Markus-Christian Amann a

a Walter Schottky Institut, Technische Universitaet Muenchen, 85748 Garching, Germanyb Vertilas GmbH, c/o Gate Garching, 85748 Garching, Germany

a r t i c l e i n f o

Available online 7 July 2012

Keywords:

A3. Metalorganic vapor phase epitaxy

B1. Antimonides

B1. Phosphides

B2. Semiconducting III–V materials

B3. Laser diodes

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jcrysgro.2012.06.051

esponding author. Tel.: þ49 89 28912789.

ail address: grasse@wsi.tum.de (C. Grasse).

a b s t r a c t

In this paper we present the first InP-based short-cavity Vertical-Cavity Surface-Emitting Laser with an

AlGaInAsP/GaInAsP active region and a re-grown and structured GaAs0.51Sb:C/Ga0.47InAs:Si buried

tunnel junction (BTJ), which serves as current aperture, grown by LP-MOVPE. We achieved over 1 mW

single-mode continuous-wave (cw) emission at around 1.3 mm wavelength and room-temperature.

The small-signal modulation bandwidth exceeds 7.5 GHz, which is appropriate for 10 Gb/s data

transmission, and the series resistance is as low as 24 O. The latter value indicates around three times

lower dissipated power consumption than comparable MOVPE grown InP-based VCSELs.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

InP-based buried tunnel junction (BTJ) Vertical-Cavity Surface-Emitting Lasers (VCSELs) are necessary to satisfy the rising band-width demand of access networks like fiber-to-the-home andpassive optical networks (PON), since they emit in the 1.3 mm and1.55 mm wavelength ranges, which corresponds to the O- andC-bands, important for telecommunications. In particular the1.3 mm wavelength range is important for data-transmission overfiber based networks, since this band serves as the upstreamchannel for PONs. The main advantage of implementing VCSELsinstead of standard DFB edge-emitters is, besides their cost effec-tiveness due to on-wafer testing, the very low power consumptionof these devices. Comparably low energy-per-bit values of 230 fJ/bitmake them also interesting for warehouse-scale data centers suchas optical cables and optical interconnects [1]. This energy effi-ciency is mainly achieved due to a structured and overgrown tunneljunction, which converts holes to electrons and serves as a currentaperture (see Fig. 1). Furthermore, p-doped material with its highohmic resistance due to the low hole-mobility and high optical losscan be exchanged with n-material. Hence, only a small amount ofthe lossy p-cladding between the active region and the tunneljunction remains and most of the material in the cavity consists ofn-InP (Fig. 1). Unfortunately, due to compensation effects, highp-doping over 1�1019 cm�3 with a segregation-stable dopant like

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carbon is hardly achievable with the standard alloy Ga0.47InAs byMOVPE, since carbon is an n-dopant in InAs [2,3]. Thus, therealization of a tunnel junction with low specific sheet-resistanceis the main challenge for growing such devices with a large scalegrowth technique like MOVPE. However, by exchanging this mate-rial with GaAs0.51Sb we could realize a sufficient p-doping of higherthan 5�1019 cm�3. Furthermore, the type-II band alignment incombination with Ga0.47InAs (see Fig. 1) decreases the tunnelbarrier [4]. But also the implementation of an AlGaInAsSb:C gradingis required to inject the holes into the active region. Here, we usedfive GaInAsP quantum wells in combination with AlGaInAsP insteadof just AlGaInAs to remove the Aluminum from the well materialand, hence, decrease the non-radiative SRH-recombination-rate.To enhance the modulation speed, dielectric top and bottommirrors have been evaporated, which reduce the cavity lengthdue to the higher refractive index difference [1]. The first realizedso called Short-Cavity VCSEL (SC-VCSEL) with AlGaInAsPSb-basedmaterial system grown by MOVPE exhibits a series resistance of24 O and an optical output of more than 1 mW at 1.3 mmwavelength. This means a factor of around 3 lower ohmic resistanceand, thus, dissipated power compared to AlGaInAs-based MOVPEgrown devices [5,6].

2. Experimental

The growth was carried out with an AIX 200/4 MOVPE systemin a temperature range from 650 1C to 500 1C at 150 mbar on

Fig. 1. Sketch of the SC-VCSEL. The type-II band alignment between GaAsSb and GaInAs (the material combination for the buried tunnel junction) and the standing field

patterns of the optical wave are also illustrated.

C. Grasse et al. / Journal of Crystal Growth 370 (2013) 217–220218

n-doped (001) InP substrates without miscut. As precursorsTMGa, TMAl, TMIn, TMSb, CBr4 (for p-doping), phosphine, arsineand silane (for n-doping) have been used. Characterization of thelayers was performed with X-ray diffraction (XRD), photolumi-nescence (PL), Hall effect and reflection measurements. For in-situreflectance a LayTec ‘‘EpiTT’’ was used providing a growthtemperature accuracy of better than 1 1C. PL-measurements werecarried out with a green frequency-doubled Nd:YAG laser, aBruker Vertex 70 FTIR spectrometer and an extended-InGaAsdetector collecting the luminescence light.

The growth of the device was separated into two parts: at firsta GaInAs etch-stop layer and a 500 nm thick InP buffer layer weregrown followed by the nþþ-GaInAs contact-layer and the n-dopedInP cladding layers with �700 nm thickness. The cladding wasgrown with a varying doping of 5�1018 cm�3 at the nodes and5�1017 cm�3 at the antinodes of the optical field (see Fig. 1) tominimize optical losses and maximize electrical conductivity.Then the growth of the active region took place with five 6 nmthick Ga0.10InAs0.52P quantum wells (QWs) and 6 nm quinternaryAl0.13Ga0.11InAs0.52P barrier layers. This active region is sand-wiched between 10 nm thick AlGaInAsP layers of the samecomposition as the barriers. All of these layers were grown at650 1C and with a V/III ratio of 179 (QW) and 168 (barrier).The growth temperature was then set to 550 1C to grow the40 nm thick AlGaInAsSb grading layers, around 2�1018 cm�3

p-doped with CBr4. Since growth of antimonides is very complexand quinternary material is not sufficiently determined by justusing XRD- and PL-measurements this grading was fabricated byusing an AlGaAsSb/AlInAs digital alloy (5 periods) with a varyingthickness ratio from 0.6 nm/1.0 nm to 1.5 nm/0.5 nm. A V/III ratioof 1.0 and 65, respectively, was used for these layers. To avoidantimony segregation and accumulation, 5 s growth interruptionswith only arsine stabilization have been implemented betweenthe AlGaAsSb and AlInAs interface [7,8]. Finally, the growthtemperature was reduced to 500 1C for the growth of theGaAs0.51Sb:C/Ga0.47InAs:Si tunnel junction with 15 nm and11 nm thicknesses, respectively. This reduction in temperaturewas necessary to increase the CBr4 carbon doping efficiency asdescribed in [9]. The V/III ratio was set to 1.2 for GaAsSb:C and to322 for GaInAs:Si. After the first growth step the nþþ-GaInAs andaround 2 nm of the pþþ-GaAsSb layer of the tunnel junction werepartly removed by using dry etching, so that cylindrical mesas

with different diameters were created. After a cleaning procedurethe n-InP overgrowth took place, starting at 500 1C growthtemperature to avoid degradation of the mesa structure [4].The temperature was then raised to 600 1C and the growth endedwith a 15 nm nþþ-GaInAs contact layer, n-doped as high as5�1019 cm�3.

During processing the bottom DBR, which consists of amor-phous dielectric material like CaF2 and ZnS, was evaporated andthe gold heatsink electroplated, after separating the VCSEL mesasby using dry etching and embedding them in the low-k plasticbenzo-cyclo-butene (BCB). Finally the n-InP substrate was wetchemically removed and a Ti/Pt/Au top contact and the top DBRwas evaporated. Hence, on the top and bottom a Ti/Pt/Au contacton an nþþ-Ga0.47InAs layer is used for current injection with acontact resistivity lower than 2�10�7 O cm2 [10].

3. Results and discussion

Before the growth of the entire VCSEL device was carried out,the key parts of the structure, more specifically the active regionand the tunnel junction, were evaluated separately.

The standard alloy for InP-based laser devices is AlGaInAs,since it has a higher conduction band offset and lower valenceband offset compared to GaInAsP and, hence, better high tem-perature stability due to less thermionic electron escape [11].Otherwise, GaInAsP has a higher radiative recombination rate andit is known that Aluminum tends to introduce deep impuritylevels and, thus, increase Shockley–Read–Hall (SRH) recombina-tion [12,13]. Therefore our aim was to combine the advantage ofboth materials systems by using GaInAsP QWs and AlGaInAsPbarriers. Furthermore, with this material combination the flow ofthe group V sources could be kept constant, ensuring abruptinterfaces, since no long growth interruption for the exchange ofthe arsine and phosphine gas flow are necessary. For the calcula-tion of the bandgap and band offsets the band parameters givenby Vurgaftman [14] and the interpolation scheme of Donati [15]were used. To determine the growth of AlGaInAsP the group IIIand group V incorporation were treated separately and werecalibrated with AlGaInAs and GaInAsP test structures.

The strong agreement between XRD-measurement and simu-lation of an active region test sample with five GaInAsP QWs and

Fig. 2. XRD- and PL-measurement of an AlGaInAsP/GaInAsP active region test sample. The good agreement between measurement and simulation and the low PL

linewidth (FWHM) of 12 meV at 4.2 K represent good material quality.

Fig. 3. GaAsSb/GaInAs tunnel junction test sample grown on n-doped InP with a

GaInAsP:Si grading layer. The low voltage drop even at high current densities

proves very low electrical resistance (see text).

C. Grasse et al. / Journal of Crystal Growth 370 (2013) 217–220 219

AlGaInAsP barriers, illustrated in Fig. 2, indicates good materialquality, which is confirmed by the low temperature PL linewidth(FWHM) of only 12 meV. Hence, this new material combinationwas proved to be suitable for use in a VCSEL device.

To investigate the second key element for the operation of theVCSEL, the GaAsSb:C and GaInAs:Si tunnel junction layers weregrown on highly n-doped (n¼5�1018 cm�3) InP substrate andwere etched into cylindrical mesa structures with gold top andbottom contacts as shown in Fig. 3. Even at a mesa diameter D ofonly 5 mm and a current density of 100 kA/cm2 (corresponding to20 mA) the voltage drop is only 70 meV, which yields a specificsheet resistance r lower than 1�10�6 O cm2 including thecontact resistance and the contribution of the substrate [16].Since state of the art MBE grown GaInAs-based tunnel junctionspossess r¼3�10�6 O cm2 [17], this is a clear evidence of thesuperior performance of the GaAsSb/GaInAs material combinationand strongly suggests its usage for InP-based VCSEL devices asdetailed in the next section.

The layer structure of the realized VCSEL device was designedfor emission at 1.31 mm by using the refractive indexes anddetermination method given in [18]. The highly doped BTJ wasplaced into a node of the optical wave to avoid absorption losses,whereas the active region was set into an antinode of the opticalfield as illustrated in Fig. 1. After the first epitaxial growth the

PL-emission of the active region was determined and a maximumat 1265 nm could be measured. This corresponds to a desireddetuning of the gain of the active region and the cavity laserwavelength of over 30 meV. This offset is intended to compensatethe bandgap shrinkage of the active region material due to internalheating and to allow high temperature operation. The performanceof the first fabricated device is shown in Fig. 4. A series resistance ofonly 24 O confirms the expected performance of the implementedGaAsSb/GaInAs tunnel junction. Furthermore, with test struc-tures, which are available on the fabricated VCSEL wafer, theratio between current flow through the unetched (tunneling) andetched (blocking) part of the BTJ was measured to be as high as2000 proving the BTJ’s operation as a real current aperture. Theoptical output power, which was collected with an InAs detector,is higher than 1 mW at 20 1C and the measured laser spectrumindicates single-mode operation with a side-mode suppressionratio of over 30 dB.

The relatively high threshold current of 4.2 mA that this deviceshowed, unfortunately, can be explained by the detuning of thecavity with respect to the designed wavelength of 1.31 mm. This isalso confirmed by taking the temperature dependence of the laserthreshold current (Ith) into account, shown in Fig. 4. The reductionof Ith with decreasing heatsink temperature implies that the gainmaximum of the active region matches better to the laserwavelength determined by the cavity. Hence, by implementinga phase matching layer between the top DBR and the semicon-ductor material, even increased device performance can beexpected.

Nevertheless, S21 small-signal measurements were also carriedout to determine the high-speed properties of the device (Fig. 5).The setup and measurement technique are described in [1]. Ameasured modulation bandwidth over 7.5 GHz indicates that10 Gb/s data transmission is possible with this laser [5]. There-fore, the achieved performance and the low power consumptionwith this AlGaInAsPSb-based device are promising for applica-tions in local area fiber networks.

4. Conclusions

The growth and performance of an InP-based VCSEL with5 GaInAsP QWs and AlGaInAsP barrier layers as active regionand a GaAsSb/GaInAs buried tunnel junction has been presented.The good crystal quality of the active region was evaluated withXRD- and PL-measurements and single-mode laser output powerof over 1 mW indicates that this material combination is suitable

Fig. 4. LIV-characteristics and laser spectrum of VCSEL with 8 mm BTJ diameter at 20 1C. The temperature dependence of the laser threshold is also illustrated.

Fig. 5. S21 measurements at different currents (dots) and fits (solid lines) after [1].

The achieved modulation bandwidth at 3 dB is higher than 7.5 GHz. The measure-

ments have been carried out at room-temperature.

C. Grasse et al. / Journal of Crystal Growth 370 (2013) 217–220220

for VCSEL devices. The tunnel junction has a specific sheetresistance lower than 1�10�6 O cm2 and the entire VCSEL devicewith AlGaInAsSb grading layers has a resistance as low as 24 O,which corresponds to the expectations due to test samples.Hence, antimony based alloys like GaAsSb can strongly enhancethe electrical properties of InP-based BTJ VCSELs due to theirtype-II band alignment. Due to the small cavity length of only 9/2l, the application of BCB with its low dielectric constant and theimplementation of dielectric mirrors with large refraction indexdifference, a modulation bandwidth over 7.5 GHz has beenachieved, which enables 10 Gb/s data transmission. Therefore,the presented sixternary material system, AlGaInAsPSb, is apromising alternative to standard AlGaInAs-based devices forapplications in passive optical networks.

Acknowledgments

The authors would like to thank E. Sckopke, G. Riedl and S.Grottenthaler for technical assistance. Financial support by the

European Union in the framework of the project SUBTUNE(Project no. 224259) is gratefully acknowledged.

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