Chapter 2 Operation principle of VCSEL
2.2 Electrical characteristics of VCSELs
2.2.3 Output Beam Characteristics
In a conventional edge-emitting diode laser, thin epitaxial layers must be used, which has the consequence that the width of optical mode in the direction perpendicular to the layers is always quite small (typically about two free-space wavelength). As a result, the emission angle in that direction is large (often of the order 30o or more). In the direction parallel to the layers, the width of the aperture is usually wider, thus yielding a noncircular beam. As the width of the aperture is widened beyond a few wavelengths, multiple transverse modes are excited and the beam become highly nonsymmetric and not longer diffraction limited. In contrast, a VCSEL with a circular aperture put out a circle beam, which simplifies the coupling into subsequent optical components (e.g., lenses and fibers). Moreover, a VCSEL with a single lateral mode can have an aperture several wavelengths across, thus yielding a narrower output beam than a conventional diode laser.
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Chapter3
Experimental principles and methods
3.1 Four point probe
The 4-point probe setup consists of four equally spaced tungsten metal tips with finite radius. Each tip is supported by springs on the other end to minimize sample damage during probing. The four metal tips are part of an auto-mechanical stage which travels up and down during measurements. A high impedance current source is used to supply current through the outer two probes; a voltmeter measures the voltage across the inner two probes (See Fig3.1) to determine the sample resistivity. Typical probe spacing s ~ 1 mm. Fig 3.2 is the four point probe we used to sheet resistance measurement in NCTU NFC.
Fig. 3.1 four points probe
Fig 5.2 Schematic of 4-point probe configuration
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Fig 3.2 four point probe in NCTU Nano Facility Center 3.2 AFM
The atomic force microscope (AFM) or scanning force microscope (SFM) was invented in 1986 by Binnig, Quate and Gerber. Like all other scanning probe microscopes, the AFM utilizes a sharp probe moving over the surface of a sample in a raster scan. In the case of the AFM, the probe is a tip on the end of a cantilever which bends in response to the force between the tip and the sample. The small probe-sample separation (on the order of the instrument's resolution) makes it possible to take measurements over a small area. To acquire an image the microscope raster-scans the probe over the sample while measuring the local property in question.
The resulting image resembles an image on a television screen in that both consist of many rows or lines of information placed one above the other. Unlike traditional microscopes, scanned-probe systems do not use lenses, so the size of the probe rather than diffraction effect generally limits their resolution. As shown in Fig. 3.3. AFM operates by measuring attractive or repulsive forces between a tip and the sample. In its repulsive "contact" mode, the instrument lightly touches a tip at the end of a leaf
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spring or "cantilever" to the sample. As a raster-scan drags the tip over the sample, some sort of detection apparatus measures the vertical deflection of the cantilever, which indicates the local sample height. Thus, in contact mode the AFM measures hard-sphere repulsion forces between the tip and sample. In tapping mode, the AFM derives topographic images from measurements of attractive forces; the tip does not touch the sample.
The presence of a feedback loop is one of the subtler differences between AFMs and older stylus-based instruments such as record players and stylus profilometers.
The AFM not only measures the force on the sample but also regulates it, allowing acquisition of images at very low forces. As shown in Fig. 3.4, the feedback loop consists of the tube scanner that controls the height of the entire sample; the cantilever and optical lever, which measures the local height of the sample; and a feedback circuit that attempts to keep the cantilever deflection constant by adjusting the voltage applied to the scanner. One point of interest: the faster the feedback loop can correct deviations of the cantilever deflection, the faster the AFM can acquire images;
therefore, a well-constructed feedback loop is essential to microscope performance.
AFM feedback loops tend to have a bandwidth of about 10 kHz, resulting in image acquisition time of about one minute.
Fig. 3.3 Concept of AFM and the optical lever: (a) a cantilever touching a sample,
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(b) illustration of the meaning of "spring constant" as applied to cantilevers, (c) the optical lever. Scale drawing; the tube scanner measures 24 mm in diameter, while the cantilever is 100 μm long typically.
Fig. 3.4 The AFM feedback loop. A compensation network monitors the cantilever deflection and keeps it constant by adjusting the height of the sample (or cantilever).
3.3Photoluminescence spectroscopy(PL)
Photoluminescence characterization equipment generally uses the PL method to obtain the wavelength and intensity of the semiconductor material being analyzed. PL is the process of optical absorption of electrons in solids between an initial energy state Ei and a final energy state Ef. Excitation of an electron to Ef will leave Ei
unoccupied creating a hole. Absorption creates electron-hole pairs while luminescence is the process which occurs when electrons in excited states drop to a lower level emitting a photon ħw as shown in Fig. 3.5 The electron–hole recombination creates a photon which is also known as a radiative transition. Direct
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gap materials are good light emitters and their optical properties are analyzed using this technique.
Fig. 3.5 Interband transitions in photoluminescence system
Photons are absorbed using an excitation source which is typically a laser. The frequency of the source ħwL must be greater than the energy gap Eg. The result is that electrons are injected into the conduction band and holes into the valence band.
Electrons and holes are initially created in higher states within these bands but will rapidly relax to the bottom of their respective bands reaching their lowest energy state.
Relaxation occurs by emitting phonons, for energy loses from the higher states, which obeys the conservation laws. The difference in energy between the two bands is Eg which is the energy gap, also known as the band gap. Luminescence occurs close to the band gap Eg, near k = 0. After excitation, both electrons and holes relax to their lowest energy states by emitting phonons.
3.4 Electroluminescence spectroscopy (EL)
As shown in Fig. 3.6, Electroluminescence (EL) is an optical phenomenon and electrical phenomenon in which a material emits light in response to an electric current passed through it, or to a strong electric field. This is distinct from light
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emission resulting from heat (incandescence), chemical reaction (chemiluminescence), sound (sonoluminescence), or other mechanical action (mechanoluminescence).
Electroluminescence is the result of radiative recombination of electrons and holes in a material (usually a semiconductor). The excited electrons release their energy as photons - light. Prior to recombination, electrons and holes are separated either as a result of doping of the material to form a p-n junction (in semiconductor electroluminescent devices such as LEDs), or through excitation by impact of high-energy electrons accelerated by a strong electric field (as with the phosphors in electroluminescent displays).
Fig. 3.6 Spectrum of a blue electroluminescent light source. Peak wavelength is at 454 nanometers and the FWHM spectral bandwidth is quite wide at about 9 nm.
400 450 500
1000 2000 3000 4000
Intensity
Wavelength (nm)
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Chapter 4
Characteristics of electrical pumped GaN-based VCSELs
4.1 Previous work of our group 4.1.1Optical pumped VCSEL
In our previous work, the characteristics of an optical pumped GaN-based VCSEL was successfully fabricated and investigated. The schematic diagram of the overall VCSEL structure are shown in the Fig. 4.1 . The hybrid DBR VCSELs structure is composed of 2-inch diameter c-plane sapphire , 2um thickness GaN buffer layer , 29 pairs AlN/GaN DBR , 5λ GaN cavity and 8 pairs Ta2O5/SiO2 upper DBR. The reflectivity of lower and upper DBR are all about 99%. A narrow PL emission with full width at half maximum of 0.21nm corresponds to the cavity resonant mode at 454.3nm was observed, as shown in Figure 4.2. The cavity quality factor, estimated from the emission linewidth of 0.21nm, was about 2200. Figure 4.3 shows the laser action was achieved under the optical pumping at room temperature with a threshold pumping energy (Eth) of about 55 nJ corresponding to an energy density of 7.8 mJ/cm2. A dominant laser emission line at 448.9 nm appearing above the threshold pumping energy is shown in the Fig.4.4.
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Fig. 4.1 The schematic diagram of the optical pumped VCSEL structure
Fig. 4.2 PL emission of the optical pumped VCSEL structure.
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Fig. 4.3 The excitation energy - emission intensity curve (L-I)
Fig. 4.4 The laser emission spectrum with the increasing pumping energy.
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4.1.2 CW lasing of current injected GaN-based VCSEL at 77k
With the achievement of optically pumped GaN-based VCSEL, the realization of electrically-injected GaN-based VCSEL has become promising. Fig. 4.5 is the overall current injected VCSEL structure. The resonant cavity structure has an 0ne optical wavelength thickness ITO layer deposited on the p-type GaN layer compared with optical pumped cavity. The ITO layer can be used as a transparent conduct layer(TCL) and improve the current spreading problem resulting from low conductivity of p-GaN.
In addition ,One optical wavelength thickness can match the resonance phase condition of microcavity and reach high transmittance(~98%) for ITO layer.
However , we can find that the quality factor of electrically pumped cavity is about 900 from PL spectrum , as shown in Fig. 4.6. The value of quality factor is about half of the optical pumped result due to additional ITO absorption. We consider the loss of ITO maybe one of the main challenges for us to reach CW lasing in current injected VCSELs at room temperature.
In order to observe the lasing behavior in current injected VCSEL, we packaged our devices into TO can. The packaged VCSEL device was mounted inside a cryogenic chamber for testing under cw current injection condition using a cw current source at 77 K. Fig. 4.7 shows the light output power versus cw injection current and current-voltage characteristics of the VCSEL sample at 77 K. The laser light
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output power showed a distinct threshold characteristic at the threshold current (Ith) of about 1.4 mA and then was linearly increased with the injection current beyond the threshold. The threshold current density is estimated to be about 1.8 kA/cm2 for a current injection aperture of 10 um in diameter, assuming the current is uniformly injected within the aperture. The lasing wavelength is 462.8nm with 0.15nm line width shown in Fig. 4.8. The inset of Fig3.8 is the CCD image of the spatial l emission pattern slightly below threshold. We believe the nonuniformity in the emission intensity across the aperture could be due to the In nonuniformity that creates a nonuniform spatial gain distribution in the emitting aperture as reported earlier[36]
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Fig. 4.5 The schematic diagram of the current injected VCSEL structure
Fig. 4.6 Emission spectrum of the current injected VCSEL structure.
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Fig. 4.7 The light output intensity vs injection current and current–voltage characteristics of GaN VCSEL
Fig. 4.8 The laser emission spectrum at different injection current levels measured at 77 K.
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4.2 The design of electrical pumped VCSEL
4.2.1The reflectance and quality factor simulation with different ITO thickness
So far, our group have fabricated and demonstrated the CW operation of an electrically pumped GaN-based VCSEL at 77 K. Next step, we should work toward the cw lasing of current injected devices at room temperature. In order to reach the goal, we try to reduce the loss of ITO so that the threshold current can also be lower at room temperature. In general, the thinner layer has the smaller absorption for the same material, so we would like to replace previous one optical wavelength thickness ITO with thinner ITO layer.
Figure show the simulation cavity structure with different ITO layer thickness from 0 nm, 30 nm, 120 nm, 210 nm, and 225 nm. Here, the 225nm-thick ITO layer stands for one optical wavelength thickness at 440nm. Owing to DBR reflectivity symmetry, the we chosen 18-pair AlN/GaN DBR. Figure is the simulated reflectance spectra under different ITO thickness. The dip positions in the reflectance spectra represent the cavity modes with different ITO thickness and the quality factor can be estimated from the linewidth of the dip. In
Figure, the cavity mode wavelength is the function of different ITO thickness. The cavity mode wavelength shifts to longer wavelength because of the longer cavity length, but the cavity mode wavelength would turn back to shorter wavelength when ITO thickness is thicker than 120nm due to the exceeding of the stop band region of the lower DBR. In this case, the cavity mode would jump to the (m+1)th mode from the mth mode. Furthermore, the cavity mode also changes to multimode owing to longer cavity length and smaller mode spacing when ITO thickness is larger than 30nm. In Figure , the quality factor is about 700 using the cavity with a 225nm-thick
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ITO layer, but this value is much lower than that without ITO layer of about 3300. If we consider the qualify factor of the cavity with a 30nm-thick ITO layer, the value of about 3100 is a little smaller than that with a 0nm-thick ITO layer but the structure with a ITO layer can be efficiently injected current in our electrical pumped VCSEL devices. Base on the simulation results and reality device requests, we can expect the 30nm ITO layer can efficiently reduce the loss and threshold current density of our VCSEL devices.
Figure 4.9 The simulation cavity structure
Figure 4.10The simulated reflectance spectra with different thickness of ITO
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Figure4.11the simulated cavity mode with different thickness of ITO
Figure 4.12 The simulated quality factor with different thickness of ITO
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4.2.2Design structure of electrical pumped device
For a typical VCSEL, the cavity length is designed to be 1 lambda or 3/2 lambda, 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, a 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. Figure 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 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.
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Figure 4.13 Electric field intensity (red line) and refractive index (black line) as a function of the distance from top layer.
4.3 Fabrication process of GaN-based VCSELs
To fabricate the VCSELs device on chip , we need to experience six process steps.
Fig 4.14 is the schematic diagram of overall VCSELs process flowchart. In the beginning , SiNx mesa etching mask and ICP dry etching machine were used to define the mesa region . As shown in Fig 4.15, we check the electrically properties by probe station after mesa was finished. Then, 200nm SiNx layer was deposited by PECVD and patterned to form current confinement layer with effective current aperture varying from 15um to 40um. The four different structure ITO , which was mentioned earlier , were deposited individually by E-gun or sputter , and annealed at RTA system under nitrogen ambient. The ITO transparent conduct layer on a 40um current aperture after etching is shown in Fig 4.16. The Ti/Al/Ni/Au and Ni/Au contacts was deposited to serve as n-type and p-type electrode, respectively. The final step is deposition of 10pairs SiO2/Ta2O5 dielectric DBR as upper mirror. The schematic diagram of completed electrical pumped VCSEL is shown in Fig 4.17 , and Fig4.18 is the OM image of VCSEL device with 5um metal aperture when current
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injects.
Fig 4.14 The schematic diagram of overall VCSELs process flowchart
Fig 4.15 The defined mesa of first step was measured by probe station
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Fig 4.16 The ITO transparent conduct layer deposited on current aperture
Fig 4.17 The schematic diagram of completed electrical pumped VCSEL
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Fig 4.18 The OM image of completed VCSEL device when current injects
4-4 Characteristics of optically pumped GaN-based VCSELs
The overall optical pumped GaN VCSEL structure is shown in Fig 4.19 The mirocavity and bottom DBR structure are grown in a vertical-type MOCVD system (EMCORE D75), which can hold one 2-inch sapphire wafer. The nitride-based DBR used in the experiment is the stacks of 29-pair AlN/GaN 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. Fig4.20 is the reflectance spectrum of bottom DBR, and there is a high reflectivity (~99.3%) at 440nm. The stop band of bottom DBR is as wide as about 20nm.
Then, a micro-cavity formed by a p-n junction was grown following the growth of the DBR structure. The micro-cavity composed of about 860-nm-thick n-type GaN, a
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ten pairs In0.2Ga0.8N/GaN (2.5 nm/7.5 nm) MQW, 24nm AlGaN electron blocking layer, a 115-nm-thick p-type GaN and 2nm InGaN layer. The cavity length is about 7-λ in optical length. The 2nm InGaN layer is used for reduction the Schottky barrier height between ITO and p type GaN layer[37], and it can also improve current spreading when current inject. The SEM cross-section image is shown in Fig4.21 , and Fig4.22 are the PL spectrum and reflectance spectrum of VCSEL sample without upper DBR. Obviously, the cavity mode is at about 435nm.
The final process to complete a 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 about 150℃. The reflectivity spectrum of the dielectric mirror was measured as shown in Fig 4.23 The Ta2O5/SiO2 DBR shows a very high reflectivity as high as 99% centered at 450 nm with a wide stop-band of about 100 nm.
Fig4.24 is the PL spectrum of GaN-based VCSEL with bottom and upper DBR.
The cavity mode wavelength is about 440nm with a 0.24nm linewidth , and the quality factor estimated from PL spectrum is as high as 1900. The lasing behavior can be observed under optical pumping at room temperature in Fig4.25 . The threshold pumping energy density is about 3.3mJ/cm2, and this lower value compared with the previous result of our group represents the improvement of epitaxial quality, so that the internal loss and threshold energy density is reduced. Fig4.26 shows the lasing spectrum above the threshold condition, and the lasing wavelength is 438nm with a 0.15nm linewidth.
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Fig 4.19 The schematic diagram of the optical pumped VCSEL structure
Fig 4.19 The schematic diagram of the optical pumped VCSEL structure