5.1 Introduction
Vertical-cavity surface-emitting lasers (VCSELs) have attracted a lot of attention in recent years. Single-mode VCSELs are necessary for a number of applications, including high-speed laser printing, optical storage and long-wavelength telecommunications. Small oxide aperture VCSELs below about 4 μm diameter operate in the fundamental transverse mode. However, the large resistance inherited from the small aperture limits the modulation bandwidth and degrades the high-speed performance. The lifetime of the oxide VCSEL also decreases proportionally to the diameter of the oxide aperture, even when the device is operated at a reduced current.[1] When the aperture diameter is increased to obtain higher output power, however, multiple higher-order transverse modes oscillate, causing increased noise, a broadened spectrum, and a strong increase of the far-field angle. Techniques used to solve the problem include the increase of higher order mode loss by surface-relief etching [2], hybrid oxide-implanted VCSELs [3,4] and two-dimensional triangular holey structure.[5] Recently, two-dimensional photonic crystal (2D PC) structure formed on the VCSEL surface has been used as a control method of transverse modes.
Single-mode output was realized from larger aperture photonic crystal VCSELs (PC-VCSELs) [6,7]. However, those PC-VCSELs exhibit relatively high threshold currents (Ith) due to large oxide confined apertures. For long-wavelength applications, InAs quantum-dot (QD) VCSELs[8] and QD PhC-VCSELs[9] achieved laser emission at 1300 nm. For shorter wavelength emission, InGaAs/GaAs sub-monolayer (SML) quantum dot (QD), embedded in a GaAs matrix shows luminescence peaks and high power lasing performance in the 0.92 – 1 μm range [10].
Recently single-mode InGaAs SML QD VCSELs with room-temperature output power as high as 4 mW have been demonstrated [11]. However singlemode operation of the InGaAs SML QD VCSEL with PC is still yet to be realized. In this paper, we report our results on the InGaAs QD PC-VCSELs in the 990 nm range.
Single-transverse-mode operation with very high side-mode suppression ratio (SMSR) is demonstrated for the first time. The beam profile characteristics of the devices were also measured and analyzed.
5.2 Device fabrication
The epitaxial layers of the InGaAs SML QD PhC-VCSEL wafers were grown on 3 inch n+-GaAs (001) substrates by molecular beam epitaxy (MBE) in Riber 49 chamber. The bottom distributed Bragg reflector (DBR) consists of a 33-pair n-type (Si-doped) quarter-wave stack (λ/4) of Al0.9Ga0.1As/GaAs. The top DBR consists of 20-period p-type (carbon-doped) Al0.9Ga0.1As/GaAs quarter-wave stack. Above that is a heavily doped p-type GaAs contact layer. The undoped 1λ-cavity contains 3 InGaAs SML QD layers, separated by GaAs barrier layers. Each of the InGaAs SML QD layer is formed by alternate deposition of InAs (<1 ML) and GaAs. The current confinement of the device was done by the selectively oxidized AlOx tapered-aperture.
Firstly, mesas with diameters varied from 50 to 68 μm was defined by reactive ion etch (RIE). The p-contact ring with an inner diameter 4 μm larger than the oxide aperture was formed on the top of p-contact layer. The AlAs layer within the Al0.9Ga0.1As confinement layers was selectively oxidized to AlOx. The oxidation depth was about 15 μm toward the center from the mesa edge so that the resulting oxide aperture varied from 20 to 38 μm in diameter. The oxide aperture was introduced in minimum of optical field in order to reduce the lateral optical loss and the leakage current. The n-contact was formed at the bottom of the n+-GaAs substrate.
After that, triangular lattice patterns of photonic crystal with a single-point defect in the center were defined within the p-contact ring using photo-lithography and etched through the p-type DBR using RIE. The lateral index around a single defect can be
controlled by the hole diameter (α) -to-lattice constant (Λ) ratio and etching depth [6].
This ratio (α/Λ) is 0.5; the lattice constant Λ is 5 μm in the PC-VCSEL and the etching depth of the holes is about 16-pair thick into the 20-pair top DBR layer. The device structure is shown in Fig. 5-2. By using two types of apertures in this device, we decouple the effects of the current confinement from the optical confinement. The AlOx layer is used to confine the current flow, while the single-point defect (approximately ≥ 10 µm in diameter) photonic crystal is used to confine the optical mode. In order to clarify the effect of photonic crystal index-guiding layer, an oxide-confined VCSEL (oxide aperture = 20 µm in diameter without PC) was also fabricated for comparison. The relation of the normalize lattice constant and Veff were calculated by using the Rsoft-BandSOLVE software and shown in Figure 5-1. The ratios (α/Λ) are changed from 0.2 to 0.7 and the defects are 1 and 7 point defects. γ is an etching depth dependence factor. By using the γ-factor, the effective index ( ) of the VCSEL structure can be written as
clad to 1 depending on the etching depth and structure. In our structure, the etching depth of the holes is about 17 pairs and the γ is 0.06. As shown in Figure 5-1, when the ratios (α/Λ) change from 0.2 to 0.7 with one point defect, the Veff parameter all under single-mode guiding condition of 2.405.
0 2 4 6 8 10
Normalzed Lattice Constant,Λ/λ
Figure 5-1 Veff parameters forγ=0.06, which correspond to etching depths of 17 pairs, are calculated.
Figure 5-2 Schematic of PC-VCSEL. The hole depth of PC is 16 pairs out of 20 pairs of top DBR been etched off.
5.3 Results and Discussions
Fig. 5-3 shows continuous-wave (CW) light-current-voltage (L-I-V) output of the InGaAs SML QD VCSEL without photonic crystal (PC). The mesa of the VCSEL is 50 μm in diameter. The VCSEL shows a peak power of 12.2 mW at 30 mA, with a threshold current (Ith) of 1 mA. The differential series resistance is approximately 100 Ω at 12 mA.
Fig. 5-4 shows CW L-I-V output of the PC-VCSEL. The near-field image of the PC-VCSEL operated at 4 mA is also shown (inset). The mesa of the VCSEL is also 50 μm. The PC-VCSEL emits 3.8 mW maximum power at 28 mA and exhibits single modes operation throughout the current range of operation. The near-field image of the lasing output remains to be fundamental TEM00 mode at the center of the PC structure throughout the operating current range. From the near-field image, laser
emission from photonic crystal holes surrounding the single-defect area was also observed. This strayed laser emission from the photonic crystal holes corresponds to off-axis laser beam deviated from the optical axis perpendicular to the top-emitting surface of the VCSEL.12) The optical axis is defined to be the main axis of resonance cavity of the VCSEL, which is perpendicular to the top-emitting surface. The Ith of the PC-VCSEL is 0.9 mA. The differential series resistance of the PC-VCSEL is approximately 125 Ω at 12 mA. The I-V characteristics exhibit slightly higher series resistance for the PC-VCSEL, which should be mainly due to blocking of the current flow in the region by photonic crystal holes.
Lasing spectra of the PC-VCSEL is shown in Fig. 5-5, confirming single-mode operation within overall operation current. The peak lasing wavelengths are 991, 992, and 996 nm at 3 mA, 9 mA, and 20 mA, respectively. The PC-VCSEL exhibits a SMSR > 35dB throughout the current range.
For comparison, lasing spectra of the InGaAs SML QD VCSEL without photonic crystal holes shows multiple mode operation as the driving current is increased above Ith (Fig. 5-6). The InGaAs SML QD VCSEL without PC shows multiple transverse mode characteristics with a broader wavelength span. Fig. 5-7(a) is the photograph of the near-field image at 10 mA, with light illumination in order to show the laser emission pattern and the photonic crystal structure simultaneously. The near-field image at 14mA is shown in Fig. 5-7(b), and its three-dimensional near-field pattern shown in Fig. 5-7(c). The ratio α/Λ of photonic crystal is 0.5 and the lattice constant Λ is 5 μm. The laser beam emits not only from the single-point defect area at the center of the photonic crystal structure, but also from the six nearby photonic crystal holes. The three-dimensional near-field profile in Fig. 5-7(c) shows the laser output consists of one TEM00 mode emission at the center and six smaller beams with lower intensity. The laser beam intensity increases with increasing biasing current
before thermal rollover occurs. The strayed laser emission from the photonic crystal holes is originated from the off-axis laser resonance within the laser cavity. Most of the laser resonance within the laser cavity of the device is parallel to the optical axis perpendicular to the top-emitting surface[12]. The laser resonance that is not parallel to the optical axis is referred to as the off-axis resonance. The reflectivity of the remaining p-DBR layers under the photonic crystal hole region is significantly reduced due to RIE etch. The resonance under the photonic crystal hole region cannot achieve laser emission because of the insufficient gain within that portion of the cavity. Therefore the laser emission from the photonic crystal holes must be contributed from off-axis laser resonance and diffraction within the laser cavity. The measured spectra in Fig. 5-5 indicate that all the laser emission beams, including those from the photonic crystal holes, are of the same peak lasing wavelength. The spectral single-mode characteristics of the PC-VCSEL are not affected by the laser emission from the photonic crystal holes.
The beam profile results obtained from the far-field beam pattern measurements are shown in Figs. 5-8~5-10. The beam profiles of the PC-VCSEL are shown in Fig. 5-8. The beam divergence angle obtained from the beam profile of the device remains to be 6.7 to 6.9°. The beam intensity increases with increasing current.
This very small divergence angle indicates that the laser beam is well confined by the photonic structure of the device. Fig. 5-9 shows the beam profiles of the VCSEL without PC at 3 mA, 7 mA, and 10 mA. The beam intensity increases with increasing current. The beam divergence angle increases from 17.2° at 3 mA to 21.5° at 10 mA.
The two laser emission lobes correspond to the multi-mode laser emission, as observed by the multi-mode spectra in Fig. 5-6. Fig. 5-10 shows the current-dependent divergence angle of the InGaAs SML QD PC-VCSEL and InGaAs SML QD VCSEL without PC. For the InGaAs SML QD PC-VCSEL, the beam
divergence angle remains to be very small and almost unchanged within 6.7° to 6.9°, with respect to current increase. For the InGaAs SML QD VCSEL without PC, the divergence angle increases monotonically with increasing current.
Figure 5-3 CW L-I-V characteristics without Photonic Crystal
Figure 5-4 CW L-I-V characteristics and near-field image of PC-VCSEL at 4 mA.
The ratio (α/Λ) is 0.5 and the lattice constant Λ is 5 um.
Figure 5-5 Spectra of PC VCSEL
Figure 5-6 Spectra of VCSEL without PC holes
(a)
(b)
(c)
Figure 5-7 Near Field image and the ratio (α/Λ) is 0.5 and the lattice constant Λ is 5 um.
Figure 5-8 Normalized intensity against far-field angle at various current with Photonic Crystal
Figure 5-9 Normalized intensity against far-field angle at various current without Photonic Crystal
Figure 5-10 Far-field angle against CW current at various with PC and without PC
5.4 Conclusion
We report a single-mode InGaAs SML QD PC-VCSEL with SMSR > 35 dB throughout the operation current range. A maximum single-mode output power of 3.8 mW has been demonstrated. The present results indicate that a VCSEL using an oxide layer for current confinement and photonic crystal for optical confinement is a promising approach to achieve single-mode operation of VCSEL. The beam profile and near-field image study of the PC-VCSEL indicates that the laser beam is well confined by the photonic crystal structure of the device.
Reference
[1] B. M. Hawkins, R. A. Hawthorne III, J. K. Guenter, J. A. Tatum and J. R. Biard:
52nd Electron. Comps. & Technol. Conf., Proc. (2002) p.540.
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[12] A. E. Siegman: Lasers, (University Science Books), Chap. 15.
Chapter VI Singlemode InAs quantum dot photonic crystal VCSELs
6.1 Introduction
Vertical-cavity surface-emitting lasers (VCSELs) have attracted much attention in recent years. Singlemode VCSELs are needed for a number of applications, including high-speed laser printing, optical storage and long-wavelength telecommunications. For oxide-confined VCSELs, the current-confined aperture must be less than 3 um in diameter to ensure stable singlemode operation [1].
However, the large resistance inherited from the small aperture limits the modulation bandwidth and degrades the high-speed performance. The lifetime of the oxide VCSEL also decreases proportionally as the diameter of the oxide aperture shrinks, even when the device is operated at a reduced current [1]. Methods reported to solve the problem include the increase of higher-order mode loss by surfacerelief etching [2]
and hybrid oxide-implanted VCSELs [3, 4]. Recently, a two-dimensional photonic crystal (2-D PC) structure formed on the VCSEL surface has been investigated as a control method of lateral mode. Singlemode output was realised from larger aperture photonic crystal VCSELs (PC-VCSELs) [5, 6]. However, those PC-VCSELs exhibit relatively high threshold currents (Ith) owing to large oxide-confined apertures. For long-wavelength applications, InGaNAs [7, 8] and InAs-InGaAs quantum dot (QD) VCSELs [9] achieved laser emission at 1300 nm. Singlemode operation of the QD VCSEL with PC is yet to be realized. In this Letter, we report our results on the QD PC-VCSELs in the 1300 nm range. Single-lateral-mode operation with very high side-mode suppression ratio (SMSR) is demonstrated for the first time.
6.2 Device fabrication
The epitaxial layers of the QD PC-VCSEL wafers were grown on n+-GaAs substrates by molecular beam epitaxy (MBE). The bottom distributed Bragg reflector (DBR) consists of a 33.5-pair n-type (Si-doped) quarter-wave stack ( λ /4) of Al0.9Ga0.1As/GaAs. The top DBR consists of a 23-period p-type (carbon-doped) Al0.9Ga0.1As /GaAs quarter-wave stack. Above that, is a heavily doped p-type GaAs contact layer. The active region contains 17 undoped InAs-InGaAs QD layers, separated by GaAs barrier layers. Each of the InAs-InGaAs QD layers consists of 2.5 monolayer-thick InAs pyramidal islands and a 5 nm-thick In0.15Ga0.85As quantum well overgrowth layer. The current confinement of the device was achieved by combining selective oxidation with proton (H+)-implantation. First, mesas with diameters varied from 44 to 68 um were defined by reactive ion etch (RIE). The AlAs layer within the Al0.9Ga0.1As confinement layers was selectively oxidised to AlOx. The oxidation depth was about 12 mm towards the centre from the mesa edge so that
the resulting oxide aperture varied from 20 to 44 mm in diameter. The oxide aperture was introduced to reduce the lateral optical loss and the leakage current. The p-contact ring with an inner diameter 2 mm larger than the oxide aperture was formed on the top of the p-contact layer. The n-contact was formed at the bottom of the n+-GaAs substrate. After that, triangular lattice patterns of photonic crystal with a singlepoint defect in the centre were defined within the p-contact ring using photolithography and etched through the p-type DBR using RIE. The lateral index around a single defect can be controlled by the hole diameter (α)-to-lattice constant (Λ) ratio and etching depth [5]. This ratio (α/Λ) is 0.5; the lattice constantΛis 5 um in the PC-VCSEL and the etching depth of the holes is about 18 pairs thick into the 23-pair top DBR layer. To ensure better current confinement of the device, proton implantation was
carried out with a diameter of 12 um, followed by an annealing at 430oC under N2 ambient. The implantation energy was 240 keV, with a dose of 6×1014 cm2, to form an insulating region laying 10 DBR pairs above the active region. Higher implantation energy may introduce more defects in the lateral direction. The device structure is shown in Fig. 6-2. By using two types of apertures in this device, we decouple the effects of the current confinement from the optical confinement. The Ht implant aperture (12 um) and the AlOx layer are used to confine the current flow, while the single-point defect (≧12 um in diameter) photonic crystal is used to confine the optical mode. To clarify the effect of the photonic crystal index-guiding layer, a VCSEL with H+ implant aperture (12 um in diameter without PC) was also fabricated for comparison. The relation of the normalize lattice constant and Veff were calculated by software of Rsoft-BandSOLVE and shown in Figure 6-1. The ratios (α/Λ) are changed from 0.2 to 0.7 and the defects are 1 and 7 point defects. γ is an etching depth dependence factor. By using the γ-factor, the effective index ( ) of the VCSEL
structure can be written as
clad depending on the etching depth and structure. In our structure, the etching depth of the holes is about 17 pairs and the γ is 0.06. As shown in Figure 6-1, when the ratios (α/Λ) change from 0.2 to 0.7 with one point defect, the Veff parameter all under single-mode guiding condition of 2.405.
0 2 4 6 8 10 0.0
0.5 1.0 1.5 2.0 2.5 3.0
2.406
r=0.06
a/Λ=0.2 defect=1 a/Λ=0.3 defect=1 a/Λ=0.4 defect=1 a/Λ=0.5 defect=1 a/Λ=0.7 defect=1
V eff
Normalzed Lattice Constant,Λ/λ
Figure 6-1 Veff parameters forγ=0.06, which correspond to etching depths of 18 pairs, are calculated.
Figure 6-2 Schematic of QD PC-VCSEL. Hole etching depth of PC is 18 pairs out of the 23-pair top DBR having been etched off. The proton implantation position is 10 pairs of DBR layers above active region.
6.3 Results and Conclusions
Fig. 6-3 shows the CW light-current-voltage (L–I–V) output and near-field image operated at 6 mA (inset) of the PC-VCSEL. The VCSEL emits 0.2 mW peak power and exhibits single modes throughout the current range of operation. The threshold current (Ith) of the PC-VCSEL is 4.75mA. The I–V characteristics exhibit higher series resistance for the PC-VCSEL, which should be mainly due to proton implantation through the p-ohmic contact of the device and blocking of the current flow in the region by photonic crystal holes. The differential series resistance is 170Ω at 12 mA. The output power could be improved by reducing the series resistance of the PC-VCSEL. Lasing spectra of the PC-VCSEL is shown in Fig. 6-4(a), confirming singlemode operation within the overall operation current. The peak lasing wavelengths are 1268 and 1272 nm at 6 and 22 mA, respectively. The PC-VCSEL exhibits an SMSR > 40 dB throughout the current range. For comparison, a lasing spectra of a QD VCSEL without photonic crystal holes shows multiple mode operation as the driving current increased above 5 mA (Fig. 6-4b). The QD VCSEL showed multiple transverse mode characteristics over a broader wavelength span.
Figure 6-3 CW L-I-V characteristics and near-field image (inset) of PC-VCSEL (ratio (α/Λ) is 0.5 and lattice constant L is 5 um
Figure 6-4 Spectra of QD a PhC-VCSEL b VCSEL without PhC holes
6.4 Conclusions
We report a singlemode QD PhC-VCSEL with SMSR > 40 dB throughout the operation current range. The present results indicate that a VCSEL using a combined oxide layer with proton implantation for current confinement and photonic crystal for optical confinement is a promising approach to achieve singlemode operation of VCSELs.
Reference
[1] Hawkins, B.M., Hawthorne III, R.A., Guenter, J.K., Tatum, J.A., and Biard, J.R.:
‘Reliability of various size oxide aperture VCSELs’. Proc. 52nd Electronic Components and Technology Conf., 2002, pp. 540–550
[2] Haglund, A., Gustavsson, J.S., Vukusic, J., Modh, P., and Larsson, A.: ‘Single fundamental mode output power exceeding 6 mW from VCSELs with a shallow
[2] Haglund, A., Gustavsson, J.S., Vukusic, J., Modh, P., and Larsson, A.: ‘Single fundamental mode output power exceeding 6 mW from VCSELs with a shallow