Singlemode 1.1 µm InGaAs Quantum Well microstructured Photonic Crystal VCSEL

Renaud Stevens, Philippe Gilet, Alexandre Larrue, Laurent Grenouillet, Nicolas Olivier, Philippe

Grosse, Karen Gilbert, Raphael Teysseyre, Alexei Chelnokov

CEA-LETI MINATEC, Laboratoire de Photonique sur Silicium, 17 rue des Martyrs, 38054 Grenoble cedex 09, France.

renaud.stevens@cea.fr

ABSTRACT

In this article, we present our results on long wavelength (1.1 µm) single-mode micro-structured photonic crystal strained InGaAs quantum wells VCSELs for optical interconnection applications. Single fundamental mode room-temperature continuous-wave lasing operation was demonstrated for devices designed and processed with a number of different two-dimensional etched patterns. The conventional epitaxial structure was grown by Molecular Beam Epitaxy (MBE) and contains fully doped GaAs/AlGaAs DBRs, one oxidation layer and three strained InGaAs quantum wells. The holes were etched half-way through the top-mirror following various designs (triangular and square lattices) and with varying hole’s diameters and pitches.

At room temperature and in continuous wave operation, micro-structured 50 µm diameter mesa VCSELs with 10 µm oxidation aperture exhibited more than 1 mW optical power, 2 to 5 mA threshold currents and more than 30 dB side mode suppression ratio at a wavelength of 1090 nm. These structures show slight power reduction but similar electrical performances than unstructured devices. Systematic static electrical, optical and spectral characterization was performed on wafer using an automated probe station. Numerical modeling using the MIT Photonic-Bands (MPB [1]) package of the transverse modal behaviors in the photonic crystal was performed using the plane wave method in order to understand the index-guiding effects of the chosen patterns, and to further optimize the design structures for mode selection at extended wavelength range.

Keywords: long-wavelength, VCSEL, singlemode, photonic crystals, strained InGaAs quantum well.

1. INTRODUCTION

Vertical Cavity Surface Emitting Lasers (VCSELs) are widely used as low cost light sources for datacom applications, due their inherent properties such as high coupling efficiency, low power consumption, low cost mass production, and potential for highly integrated packaged solution. However, as the market continues to grow, so does the need for extended reaches and higher transmission speeds. In order to address these issues there is a strong need for long wavelength, single mode and polarization stable devices. A number of options have been investigated to overcome this twofold challenge: find a material system with high reflectivity / low loss Distributed Bragg Reflectors (DBR) and high material gain [2-4], and provide an efficient mode selection scheme to enable single fundamental mode operation of the laser [5-7].

The aim of this work is to explore the control of the emitted transverse modes through the structuring of the VCSEL top-mirror [8]. In this architecture, a micro-patterning of the top-mirror is applied after the epitaxial growth of the VCSEL

Vertical-Cavity Surface-Emitting Lasers XII, edited by Chun Lei, James K. Guenter, Proc. of SPIE Vol. 6908, 69080D, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.760052

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p-contact

oxide

structure, at the step of the definition of the VCSEL transverse structure. The growth is thus done in a single step and both VCSEL mirrors can be made of semiconductor material.

For a sufficiently large emitting area, the essential modal properties of the Photonic Crystal (PhC) VCSEL can be understood by the decoupling of the vertical and lateral confinements. Guided photons are vertically confined by the distributed Bragg reflection, but the transverse mode profile is determined by the lateral modulation of the refractive index. In the same way as micro-structured fibres can be single-mode even with a core diameter much larger than the wavelength, large-emitting-area PhC VCSEL has been designed to support the propagation of only the fundamental mode of radically larger sizes (greater than 10 µm in diameter). Typical periodicity of the mirror patterning is of the order of several microns as in micro-structured optical fibres. These dimensions are accessible with a reasonably good optical lithography, while the patterns should not necessarily be etched through the whole thickness of the VCSEL mirrors [9]. The mode structure stability brought in by the micro-patterning and higher single-mode injection currents would also improve the modulation speed of the VCSELs.

In this work VCSELs based on AlGaAs/GaAs DBRs and MBE grown Quantum Wells (QWs) have been fabricated and single mode operation at 1.1 µm was achieved. The effect of these structures is evaluated both numerically and experimentally.

2. DEVICE FABRICATION AND DESIGN 2.1 VCSEL fabrication

The GaInAs/GaAs MQW VCSEL structure is grown by Molecular Beam Epitaxy (MBE) at CEA-LETI [10]. For series resistance reduction purposes, 10 nm intermediate composition layers are inserted between the gallium-rich and aluminium-rich layers, and carbon δ-doping was introduced at the interfaces. P type DBR mirror consists of 29 Al0.92Ga0.08As/GaAs C-doped, doping level is lower (5e17 cm-3) in the five first pairs, where the optical field is strongest, and higher (above 1e18 cm-3) in the remaining periods. Lower DBR consists of a 35.5 Al0.92Ga0.08As/GaAs n type Silicon-doped. The cavity layer is one wavelength thick and contains three InGaAs quantum wells. Just above the cavity, in the first pair of the p-side DBR, one λ/4 Al0.92Ga0.1As layer contains a 40 nm thick AlGaAs layer for subsequent selective wet oxidation. The structure fabricated in this study is shown in Figure 1. The VCSELs are designed to operate around 1.1 µm.

Fig. 1. Schematic of the photonic crystal VCSEL (left) and scanning electronic microscopic image of the cross-section of a PhC VCSEL

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The VCSELs consist of circular mesas defined with reactive ion etching through the top DBR. Their diameters vary from 48 to 54 µm. A Ti/Pt/Au p-type contact ring is deposited on top of the upper mirror. The p-contact is thermally annealed during selective oxidation process. The selective wet oxidation provides electrical confinement close to the active region. In the case of this study, optical confinement would mainly be achieved by micro-structuring the top Bragg mirror. No back-side n-contact was deposited.

2.2 Microstructure

The structuring of the top-mirror is performed during the first step (i.e. the definition of the VCSEL mesa structure) on a planar structure for high resolution optical lithography purposes. 2D air holes with triangular and square lattice configurations were investigated, with hole diameters (d) ranging from 1 to 3 µm and pitch (a) between 2.5 and 4 µm, photonic crystals being defined by a single central defect. The optical microscope images are presented on the Figure 2.

Fig. 2. Optical microscopic pictures of the triangular and square lattices

3. NUMERICAL APPROACH

An out-of-plane photonic band diagram is computed using the plane-wave expansion method. First, considering a perfect periodic lattice composed by infinite holes, the propagation of light parallel to their axes is studied (vertical direction z). The dispersion relations curves relate the variations of the normalized frequency of modes against k-vector along the direction of propagation (so called kz). After adding a defect inside the structure (for instance by missing a hole) the dispersion relations curves related to this defect are calculated by the super cells method. The band diagram presented in Figure 3 concerns a square lattice 2D photonic crystal with infinite holes, with a hole radius to lattice constant ratio r = d/2a = 0.4, for both the TE and the TM modes. We use this very raw approximation to describe the optical behaviour of the sole upper mirror were the lattices of holes are etched. The grey area (lower right half) is defined by a line which corresponds to the lowest order out-of- plane mode. Schematically, this line defines an “equivalent index” of the overall structure composed by the perfect periodic lattice and corresponds to the conventional index guiding effect. With a good specific design so called defect modes could appear inside the index guiding zone. The central area where the hole is missing presents a higher index and can act as a waveguide. In the case presented here 3 modes can co-exist in the photonic crystal.

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(-n 0)

cx,

Nor

mal

ised

fr

(-n

0000000000oo•oo0000000000

00000000000012000000000000

0000000000(Th (The —_) J. Q.J0000000000

Fig. 3. Calculated photonic band diagram of a 2D square PhC structure for out-of-plane propagation

The horizontal dashed line on Figure 3 shows the normalised frequency chosen for the VCSEL structure, with a pitch of 3 µm and a wavelength of 1.09 µm ω = a / λ = 2.75. Similar calculations with smaller values r of 0.3 and 0.2 (smaller hole diameters) show that the allowed number of propagating modes is reduced to 2 and 1 respectively. Figure 4 shows the transverse mode profiles for each of the modes in the single defect square lattice structure with r = 0.4.

Fig. 4. Calculated modal power distribution for the fundamental (left), 1st order (central) and 2nd order modes

The normalized frequency or V-parameter [11] is commonly used to evaluate the number of confined modes in a step-index cylindrical waveguide: namely the waveguide is expected to be singlemode if V < 2.405. Yokouchi et al demonstrated that it is important to take into account the fact that the photonic crystal has a finite depth into the top DBR and only overlaps part of the total optical field in the cavity [12]. Veff is defined as follow:

Veff = (π D / λ ) √ (n12 – n2

2)

Where D is the waveguide diameter D = 2 a – d

λ the operating wavelength, n1 the index of the un-etched DBR, and n2 the index of the cladding area.

In order to take into account the fact that the holes have a finite depth in the top DBR, it is convenient to introduce the etch depth dependence factor γ [13] as follow:

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n2 = γ nPhC + (1 - γ ) n1

Where nPhC is the refractive index of the photonic crystal without defect and is calculated from the band diagram.

Applying this theory to the test device used in the calculation, and considering that the holes were etched through 17 Bragg pairs γ ≅ 0.1 [12]

d = 2.4 µm, a = 3 µm, r = 0.4, λ = 1.090 µm, n1 = 3.4, nPhC = 3.3798, n2 = 3.39798

The resulting calculation gives Veff = 1.22

The square lattice structure with the lattice ratio of r =0.4 calculated above which exhibits 3 modes with infinite air holes therefore becomes single-mode with a finite etch of about 17 pairs of the top mirror. The square lattice with smaller lattice ratio and the triangular lattices with equivalent lattice ratio are also expected to be single-mode.

Ultimately, the propagating modal characteristics of the VCSEL would be determined principally by the design parameters of the photonic crystal lattice. An increase in hole diameter with a given lattice constant, an increase in the size of the central defect (by removing extra etched holes) or an increase in the etched depth, will increase the index difference between the defect and photonic crystal region. These will increase the likelihood of laser operation in multiple higher order transverse modes, rather than the lowest order single transverse mode operating point. Nevertheless, increasing the size of the central defect would also decrease losses and therefore sustain single mode operation with the maximum output power.

4. EXPERIMENTAL RESULTS

In order to evaluate the effects of the various designs (square and triangular) on the performance of the micro-patterned VCSELs emitting at 1.1 µm, systematic electro optical characterizations tests have been performed on wafer at room temperature. The backside of the wafer was connected to the ground whereas the p-contact was provided by direct probing onto the contact pad at the top of each component. The emitted light was measured either by an integrating sphere (for overall optical power measurements) or coupled into a multimode fibre connected to an optical spectrum analyser for spectrum measurements. For each component type a large set of measurements was performed: light-current-voltage characteristics, optical spectra at various currents and infrared pictures below and above threshold.

The various types of VCSELs measured include unstructured components (for reference), triangular and square lattices. Single mode behaviours have been obtained for different designs (triangular and square lattices). Threshold currents ranging from 2 to 5 mA with maximum output powers between 1 and 1.5 mW at around 1090 nm and SMSR better than 30 dB have been obtained.

4.1 Unstructured VCSEL

The mesa diameter was 50 µm. The threshold was slightly below 1 mA and a maximum optical power of 2.5 mW at 20 mA. The voltage above threshold is high (>6 V) because the backside of the wafer was not metallised. Figure 5 shows the Light-Current and Current-Voltage characteristics of the component, together with spectrum emitted at 10 mA and a microscopic picture of the emitted light at the surface of the component. The optical spectrum at 10 mA exhibits many transverse modes between 1086 and 1089 nm. The misaligned emitting zone with respect to the contact pad can be

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explained by both a non-uniform oxidation area (as explained in section 5) and a possible misalignment of different masks (namely mesa etching and metal deposition) during processing of the wafer.

LI curve VCSEL unstructured

0

0,5

1

1,5

2

2,5

3

0 5 10 15 20

Current (mA)

Opt

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W)

IV curve VCSEL unstructured

0123456789

0 5 10 15 20

Current (mA)

Volta

ge (V

)

Spectrum VCSEL unstructured @ 10mA

-80

-70

-60

-50

-40

-30

1080 1082 1084 1086 1088 1090 1092 1094

Wavelength (nm)

Am

plitu

de (d

B)

Fig. 5: Standard VCSEL. LI curve (top right). IV curve (top left). Emission spectrum at 10 mA (bottom left). Optical microscopic image of the VCSEL near threshold (at 1 mA) (bottom right)

4.2 Triangular VCSEL

The single mode VCSEL measured had a hole diameter of 1.8 µm and a pitch of 3 µm (lattice constant r = 0.3). It exhibits a high threshold (5 mA) and optical power (1 mW at 12 mA) at a wavelength around 1085 nm (Figure 6).

LI curve VCSEL triangular lattice

0

0,2

0,4

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0 2 4 6 8 10 12 14

Current (mA)

Opt

ical

pow

er (m

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Spectra VCSEL triangular lattice

-80

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1081 1083 1085 1087

Wavelength (nm)

Am

plitu

de (d

B)

6mA8mA10mA12mA13mA

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Fig. 6: VCSEL triangular lattice. Light-current Characteristics (top left), spectra at various currents (top right), optical microscopic

image of the emitted light around threshold (bottom left) and surface plot (bottom right) The side mode suppression ratios (SMSR) were measured at various currents and are shown in the following table:

I (mA) 6 8 10 12 13

SMSR (dB) 28 29.3 31.6 31.6 30.4

The optical microscopic image and its surface plot of the emitted light slightly below threshold show the photonic crystal mode appearing in the central defect region.

4.3 Square VCSEL

With a hole diameter of 2.4 µm and a pitch of 3 µm (lattice constant r = 0.4) the device described here shown single mode behaviour over the full range of current with wavelength between 1090 and 1095 nm. The threshold was about 2 mA with a maximum power of 1.5 mW at 10 mA and a quick thermal roll over at higher currents (Figure 7). The optical microscopic image and its surface plot of the emitted light slightly below threshold show the fundamental photonic crystal mode appearing in the central defect region.

LI curve VCSEL square lattice

0

0,2

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Spectra VCSEL square lattice

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1085 1090 1095 1100

Wavelength (nm)

Am

plitu

de (d

B) 2mA

4mA6mA8mA10mA12mA

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Fig. 7: VCSEL square lattice. Light-current Characteristics (top left), spectra at various currents (top right), optical microscopic image

of the emitted light around threshold (bottom left) and surface plot (bottom right) SMSR is well above 30 dB at each current above threshold.

I (mA) 2 4 6 8 10 12

SMSR (dB) 26.6 31.8 34.8 36.1 35.8 34.6

5. DISCUSSIONS

The different mode selection schemes described here acted at filters, only allowing one mode to operate and preventing others. When comparing the 3 spectra for the unstructured, the triangular and the square lattices at 10mA, one can see that the square lattice structure VCSEL filters out the lower wavelength peaks of the original spectrum and emits in the longest available wavelength (or lowest energy) and is therefore expected to be the fundamental central mode, due to the index guiding effect. On the other hand the triangular structure has filtered the longer wavelength resonance peaks in the cavity, only allowing the lower wavelength peak to emit at 1085nm, and is therefore expected to operate at a higher order mode. The presence of the micro-patterned structures increases losses in the cavity which therefore increases the threshold current.

Although all designed and fabricated structures (square and triangular lattices) were supposed to be single-mode according to the model depicted above, a number of measured devices were found to be multimode. The reasons for that would be a combined effect of a non-uniform oxidation process across the wafer resulting in varying oxidation apertures for similar devices, and the oxidation of the etched holes of photonic crystal altering the index profile of the structure.

Firstly, the oxidation opening diameter was measured on a test wafer (without backside metal contact) using infrared transmission microscope on the oxidation test mesas, varying from 40 to 65 µm (steps of 5 µm) with special attention brought to the 50 µm mesa, being the closest to the working devices (diameters ranging from 48 to 54 µm). Measurements were done on five points across the wafer (the 4 corners of the wafer, tests points 1 to 4, and at the centre, test point 5) in order to evaluate the uniformity and reproducibility of the process.

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0'a0'

0

Fig. 8: Optical microscopic images of test mesa, the bottom left mesa is 50 µm diameter

Figure 8 shows the oxidation opening on mesas ranging from 40 to 65 µm (in steps of 5 µm). As expected it shows that the oxidation diameters increase with the size of the mesas, but also that they are not perfectly circular and exhibit some kind of “almond” shape, which would imply an off-centred current confinement.

The diameters were measured on the 50 µm mesa and are depicted in the following table for the 5 areas described above

Diameter (µm) Note

Test 1 14/17 Rectangular shape, un-centred Test 2 (Fig 8) 21/23 Almond shape

Test 3 27/29 Rectangular shape, un-centred Test 4 13 Square shape, centred

Test 5 (centre) 15 Square shape, centred

The results are not uniform, neither on the shape nor on the opening diameters and could be greatly improved. The overall performances of the tested VCSELs are greatly dependent on the size and shape of the oxidation aperture due to their effects on the carrier injection and the transverse index profiles. As explained previously, it has to be noticed that the effect of the oxide ring on the modal behavior has not been taken into account in the modeling described in the work and would need a more detailed study of the VCSEL structure using a 3D model. Unfortunately this effect seems to have a strong effect on the modal properties. A deeper hole etching may increase the effect of the microstructure and then overcome the confinement of the oxide aperture.

Secondly, the scanning electronic microscopic image of a cross-section of a Photonic Crystal VCSEL (Fig.1) shows that the VCSEL mesa is clearly defined, together with the photonic crystal holes and the oxidation aperture. The holes are etched through roughly 17 mirror pairs of the top DBR. A number of different oxidized areas (darker areas) can be identified: the oxidation layer, the mesa side walls but also the photonic crystal etched holes’ side walls. Changing the refractive index of the material surrounding the hole (from 3.4 to 1.6) also changes the optical confinement, making it harder to predict the transverse mode profile of the fabricated structure without modifying the model used.

In this study, single mode 1.1 µm VCSEL were designed and fabricated through the process of micro-structured 2D lattices etched in the top DBR. This method will ultimately be applied to longer wavelength VCSEL structures in order to demonstrate the validity of the micro-structured photonic crystal approach to 1.3 µm single mode VCSELs.

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ACKNOWLEDGMENT

This work is supported by EU IST-FP6 and is part of the MOSEL project (“MOnomode Surface Emitting Lasers”) [14].

REFERENCES

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