0 50 100 150 200 250
430 440 450 460
C av it y m od e w a ve le n gt h (n m )
ITO thickness(nm)
Figure 2. 9 the simulated cavity modes with different thickness of ITO
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Figure 2. 10 The simulated quality factors with different thickness of ITO
For a typical VCSEL, the cavity length is designed to be 1 λ or 3/2 λ, so the antinode of optical field and active region could be easily designed to match each other.
However, in our design, the cavity length has been determined to be seven-λ (optical length), which correspond to a geometric thickness of about 1.1 μm, due to the consideration of device fabrication. The structure should compose of a top dielectric reflector, an ITO layer, a p-type GaN, MQWs, an n-type GaN, and a bottom nitride-based reflector. The optimal thickness of the ITO layer to be about 30 nm, the p-type InGaN to be about 2 nm, the p-type GaN to be about 110 nm, of the p-type AlGaN as a electron blocking layer to be about 24 nm, of 10-pair In0.2Ga0.8N(2.5 nm)/GaN(12.5nm) multi-quantum wells (MQWs) to be about 150 nm, and of the n-type GaN to be about 860 nm, and slightly modified these thicknesses to make the center of MQWs and the ITO layer at the anti-node and node of optical field, respectively.
The design of electrical pumped VCSELs
Figure 2. 11 shows the electric field intensity (EFI) and the refractive index as the functions of the distance from top layer. From the figure, it can be observed that a pronounced resonant enhancement of
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the electric field was built up in the active region. It suggests that the light could be amplified inside the resonant cavity and the more opportunity could be obtained to achieve laser operation.
Figure 2. 11 Electric field intensity and refractive index as a function of the distance from top layer.
2-4 Fabrication of GaN-based VCSELs
The nitride-based structures including micro-cavity and bottom reflector in the experiments were grown in a vertical-type metal-organic chemical vapor deposition (MOCVD) system (EMCORE D-75) with a fast rotating disk, which can hold one 2-inch wafer. The polished optical-grade C-face (0001) 2-inch-diameter sapphire was used as substrate. Trimethylindium (TMIn), Trimethylgallium (TMGa), Trimethylaluminum (TMAl), and ammonia (NH3) were used as the In, Ga, Al, and N sources, respectively. In this section, the fabrication of the nitride-based VCSEL is divided into two parts: (1) Growth of nitride-based reflectors and micro-cavity. (2) Deposition of dielectric mirror.
2-4.1 Growth of nitride-based reflectors and micro-cavity
The nitride-based DBR used in the experiment is the stacks of 29-pair AlN/GaN
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layers with insertion of the AlN/GaN super-lattice (SL). The super-lattice in structure is inserted for releasing strain during the growth of AlN/GaN DBR to further improve interface and raise reflectivity of the DBR. The growths and characteristics of the DBR and micro-cavity are described as following:
First, the substrate was thermally cleaned in the hydrogen ambient for 5 min at 1100
°C, and then a 30 nm-thick GaN nucleation layer was grown at 500°C. The growth temperature was raised up to 1100 °C for the growth of a 2 µm-thick GaN buffer layer.
The subsequent epitaxial structure consisted of a 29-pair of quarter-wave AlN/GaN DBR grown at 1100 °C, a 7-lamda cavity (λ = 410 nm) including a 860 nm-thick Si-doped n-GaN layer, 10 pairs In0.2Ga0.8N/GaN (2.5 nm/12.5 nm) MQWs, a 24 nm-thick AlGaN layer as the electron blocking layer, a 110 nm-thick Mg-doped p-GaN layer, and a 2 nm-thick p+ InGaN layer as the contact layer. The AlGaN electron blocking layer was served to reduce the electron overflow to the p-GaN layer.
For the DBR structure, in order to reduce the crack problems encountered in the AlN/GaN DBRs, we inserted one AlN/GaN superlattice into each five DBR periods at first twenty pairs of DBR. Then the superlattice was inserted into each three DBR periods for the remaining nine pairs of DBR to reduce the tensile strain. The thicknesses of AlN and GaN layers are ~3–5 nm in SL. The ambient gas was changed from hydrogen into nitrogen before the DBR layers were grown. The center wavelength of these DBRs was designed to be around 410 nm. The detail of the growth was reported elsewhere [14].
Figure 2. 12(a) shows cross-sectional transmission electron microscopy (TEM) images of the SL sample. The lighter layers represent AlN layers while the darker layers represent GaN layers. In Figure 2. 12 (a), no cracks can be observed in the TEM image.
However, some V-shaped defects dark spots were still observed on the interfaces of GaN or AlN layers in Figure 2. 12 (a). These V-shaped defects have been reported earlier to be due to various origins such as stacking mismatch boundaries and surface undulation [23]. Figure 2. 12 (b) shows the cross section of one set of 5.5 pairs of GaN/AlN SL insertion layers. The interface between GaN and AlN is sharp and abrupt. Figure 2. 13 shows the AFM image of the DBR. The surface is lumpy, and the drop in height is within the range of 10-30 nm. The reflectivity spectrum of the AlN/GaN DBR is shown in Figure 2. 14. It shows the highest reflectivity of the DBR is about 99% at 416 nm. The stop-band of the
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DBR is as wide as about 25 nm. Figure 2. 15 is (a) the OM and (b) cross-sectional TEM images of the as-grown micro-cavity sample.
SLs SLs GaN/AlN DBR × 5
GaN/AlN DBR × 5
GaN/AlN DBR × 5
(a)
GaN GaN
SL (AlN/GaN)5
(b)
Figure 2. 12 Cross-sectional TEM images of (a) the DBR sample and (b) one set of 5.5 pairs of GaN/AlN SL insertion layers.
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Figure 2. 13 AFM image of the DBR sample
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350 400 450 500
0 20 40 60 80 100
R ef lec tan c e( % )
Wavelength(nm)
25nm
99%@416nm
Figure 2. 14 The reflectivity spectrum of the AlN/GaN DBR
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1μm AlN/GaN
DBR
SLs GaN
Cavity
(b)
As-grown 2-inch wafer
(a)
Figure 2. 15 (A) OM and (B) cross-sectional TEM images of the as-grown micro-cavity sample.
2-4.2 Deposition of dielectric mirrors
The final process to complete an optical pumped VCSEL is the deposition of a dielectric mirror. The dielectric mirror in the experiment, an eight pairs Ta2O5/SiO2 DBR, was deposited using the electron beam evaporation. The dielectric mirror was coated onto as-grown sample surface in an O2 ambient at the controlled temperature below 170oC.
The reflectivity spectrum of the dielectric mirror was measured as shown in Figure 2. 16.
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The Ta2O5/SiO2 DBR shows a very high reflectivity as high as 99% centered at 451 nm with a wide stop-band of about 130 nm. The schematic diagram of the overall VCSEL structure is shown in Figure 2. 17(a). The scanning electron microscopy (SEM) and OM images of the overall VCSEL structure are also shown in the Figure 2. 17(b) and (c), respectively.
451nm
130nm
Figure 2. 16 The reflectivity spectrum of the Ta2O5/SiO2 DBR
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(a)
VCSEL sample