Electroluminescence spectroscopy (EL) - Operation principle of VCSEL

Chapter 2 Operation principle of VCSEL

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

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)

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.

Fig. 4.1 The schematic diagram of the optical pumped VCSEL structure

Fig. 4.2 PL emission of the optical pumped VCSEL structure.

Fig. 4.3 The excitation energy - emission intensity curve (L-I)

Fig. 4.4 The laser emission spectrum with the increasing pumping energy.

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

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]

Fig. 4.5 The schematic diagram of the current injected VCSEL structure

Fig. 4.6 Emission spectrum of the current injected VCSEL structure.

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.

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 m_thmode. 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

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

Figure4.11the simulated cavity mode with different thickness of ITO

Figure 4.12 The simulated quality factor with different thickness of ITO

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.

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

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

Fig 4.16 The ITO transparent conduct layer deposited on current aperture

Fig 4.17 The schematic diagram of completed electrical pumped VCSEL

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

ten pairs In_0.2Ga_0.8N/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.

Fig 4.19 The schematic diagram of the optical pumped VCSEL structure

Fig 4.20The reflectance spectrum of bottom DBR 380 400 420 440 460 480 500 0

20 40 60 80 100

Reflectance(%)

Wavelength(nm)

R ~99.3%

Fig 4.21 The cross section SEM image of VCSEL cavity without upper DBR

Fig 4.22 the PL and reflectance spectrum of VCSEL without upper DBR

380 400 420 440 460 480 500

Fig 4.23 The reflectance spectrum of upper DBR

Fig 4.24 The PL spectrum of optical pumped VCSEL with upper DBR

350 400 450 500 550 600

Fig 4.25 The pumping energy density versus normalized intensity diagram

Fig 4.26 The lasing spectrum of optical pumping VCSEL 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

4-5 Characteristics of electrically pumped GaN-based VCSELs

We report the demonstration of the CW laser action on GaN-based vertical cavity surface emitting lasers (VCSEL) at room temperature. The laser structure consists of a 10-pair Ta₂O₅/SiO₂ distributed Bragg reflector (DBfR), a 7λ-thick optical cavity, 10 pairs InGaN/GaN multi-quantum wells with an AlGaN electron blocking layer, and a 29-pair AlN/GaN DBR. The laser has a threshold current of about 9.7 mA corresponding to the current density of about 12.4 kA/cm² and a turn-on voltage about 4.3 V at 300K. The lasing wavelength was 412 nm with a linewidth of about 0.5 nm.

A spontaneous emission coupling efficiency factor of about 5×10^-3 and the degree of polarization of about 55% were measured, respectively. The laser beam has a narrow divergence angle of about 8^o.

GaN-based materials have attracted a great attention since the early 1990s due to the wide direct band gap and the promising potential for the optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs) ^[38-39]. So far, GaN-based edge emitting lasers have been demonstrated and applied in commercial products for high density optical storage applications. However, the vertical cavity surface emitting lasers (VCSELs), with superior characteristics such as the single longitudinal mode emission, low divergence angle, and array capability, are still under development and currently gaining much attention. Optically pumped GaN-based VCSELs have been reported by using different kinds of optical cavity structures, such as dielectric distributed Bragg reflectors (DBR) VCSELs with cavities consisting of dielectric top and bottom DBRs ^[40], and hybrid DBR VCSELs with cavities consisting of epitaxially grown nitride bottom DBRs and dielectric top DBRs^[41].We have recently demonstrated the CW current injection of GaN-based VCSEL with

hybrid mirrors at 77K in 2008 ^[42]. Subsequently, the room temperature operation of GaN-based VCSEL devices was reported using optical cavities sandwiched by double dielectric DBRs ^[43,44]. The major improvements of their devices to achieve room temperature operation are by using a thinner transparent conducting layer of about 50 nm to reduce the internal optical loss and by using the GaN substrate to ensure the good crystal quality of active layers. However, to form VCSELs with double dielectric DBRs required complex fabrication process, such as laser lift-off or elaborated polishing and bonding process ^[45]. In this paper, we report the achievement of CW room temperature lasing with hybrid DBR cavity and a thin Indium-Tin-Oxide (ITO) layer of 30 nm as the transparent conducting layer combining with a thin heavily doped p-type InGaN contact layer to reduce the optical loss while maintaining good current spreading capability. Moreover, we inserted an AlGaN electric blocking layer on the top of the InGaN multiple quantum well (MQW) to prevent the carrier overflow ^[46]. The lasing characteristics such as laser output power and device voltage versus injected current characteristics, degree of polarization, divergence angle, and spontaneous emission coupling factor have been measured and investigated.

Fig. 4-27(a) shows the schematic diagram of the whole GaN-based VCSEL structure. In the structure, the positions of the ITO layer and MQWs region are located at the node and anti-node positions of the electric field, respectively to reduce the absorption from the ITO layer and to further increase the coupling between the electric field and MQWs region. The VCSEL structure was grown on a 2-inch sapphire substrate by the metal-organic chemical vapor deposition (MOCVD) system.

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 AlN/GaN DBR, a 7 cavity ( = 410 nm) including a 860 nm-thick n-GaN layer, 10 pairs InGaN/GaN (2.5 nm/12.5 nm) MQWs, a 24 nm-thick AlGaN layer as the electron blocking layer, a 110 nm-thick 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. 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 ^[47]. In the fabrication process, a 200 nm-thick SiNx layer was deposited by the plasma enhanced chemical vapor deposition as a current confined layer. By this way, the current injection aperture of VCSEL devices was about 10 m in diameter. Then, a 30 nm-thick ITO layer was deposited as the current spreading layer due to the poor conductivity of the p-GaN layer and annealed at 600 ^oC for 10 min by rapid thermal annealing. The 2 nm-thick p⁺ InGaN layer on the p-GaN surface can further reduce the series resistance between the thin ITO layer and the p-GaN layer with a slight increase of absorption.

Then, the p-contact and n-contact were deposited with Ni/Au of about 20 nm/150 nm and Ti/Al/Ni/Au of about 20 nm/150 nm/20 nm/150 nm by the e-gun system, respectively. Finally, 10 pairs Ta₂O₅/SiO₂ of the top dielectric DBR were deposited by the ion-assisted e-gun system to complete the whole GaN-based VCSEL devices.

Both of the 29-pair AlN/GaN DBR and the 10-pair Ta₂O₅ DBR show a high reflectivity of over 99 % at the peak wavelength at 410nm in the n-k measurement system. Fig. 4-15(b) shows the charge-coupled device (CCD) image of a VCSEL device injected at 2 mA under CW current injection at room temperature

. Fig. 4-27(a)VCSEL full structure (b)turn on image at 2mA

The GaN-based VCSEL devices with current injection apertures of about 10 μm

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