Chapter 4 Characteristics of electrical pumped GaN-based VCSELs
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 8o.
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
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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.
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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
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. 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 in diameter were tested by using a Keithely 238 CW current source. The emission light was collected by a 100 m diameter multimode fiber and fed into the spectrometer using a grating of 1800 g/mm with a spectral resolution of about 0.15 nm. The output from the spectrometer was detected by a CCD to record the emission
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spectrum. The VCSEL devices were then measured at environment temperature at 300 K. Fig. 4-28(a) shows L-I-V curves at 300 K. The dash line is the linear fitting curve of the laser intensity versus injection current. A clear lasing transition from spontaneous emission to stimulated emission can be observed at room temperature.
From the linear fitting curve, the laser threshold current is around 9.7 mA corresponding to the current density of about 12.4 kA/cm2. The relative low threshold at room temperature operation could be due in part to the successful prevention of carrier overflow by using the electron blocking layer on top of the MQWs and the lower internal absorption loss of the thinner ITO layer. The turn-on voltage is about 4.3 V indicating the good electrical contact of the 30 nm ITO transparent layer and the 2 nm-thick InGaN layers. The output laser intensity from the sample increased linearly with current injection beyond the threshold current. However, the laser intensity started to roll over at higher injection current beyond 15 mA due to the thermal effect. We estimated the spontaneous emisson coupling factor from the log-log plot of L-I curve as shown in Fig. 4-28(b). The data points are matched well to the solid fitting line calculated from microcavity laser rate equations [48]. From the curve, we obtained an estmated β value of about 510-3.
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Fig. 4-28(a)LIV curve at 300k (b) coupling factor from the log-log plot of L-I curve
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Fig. 4-29(a) shows the emission spectra of our GaN-based VCSEL devices at current injection of 0.6 Ith, 1 Ith and 1.2 Ith, respectively. The laser emission wavelength was measured to be 412 nm with a linewidth of about 0.5 nm. The inset of Fig. 4-29(b) shows the CCD image of a lasing spot size of about 2 m in diameter.
Finally, using the angular-resolved measurement system, the laser intensity at different angles emitted from the GaN-based VCSELs was collected by using a 600
m fiber. Fig. 4-29(b) shows the measurement data at different angles and the solid curve is the fitting curve. We obtained a laser beam divergence angle of about 8o. Fig.
4-29(c) shows the degree of polarization (DOP) of the laser beam. The solid line is the fitting curve. The DOP value was estimated to be about 55o.
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Fig.4-29(a) emission spectra of our GaN-based VCSEL devices at current injection of 0.6 Ith, 1 Ith and 1.2 Ith, respectively (b) the measurement data at different angles and the solid curve is the fitting curve, (insert)the CCD image of a lasing spot size of about 2 m in diameter.(c) the degree of polarization (DOP) of the laser beam
In summary, we have demonstrated the CW RT operation of GaN-based vertical cavity surface emitting lasers with hybrid mirrors. The laser has a thin Indium-Tin-Oxide 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. An AlGaN electric blocking layer on the top of the InGaN multiple quantum well is also inserted to prevent the carrier overflow. At 300K, the laser has a threshold current at 9.7 mA corresponding to 12.4 kA/cm2. The laser emission wavelength is 412 nm with a linewidth of about 0.5 nm.
The laser has an estimated spontaneous emission coupling factor of about 510-3 . The degree of polarization, and divergence angle of the laser are measured to be 55% and 8o, respectively.
52 occurs above 700°C, and even at room temperature, surface oxide layers of 5-10 nm have been detected. This oxide layer protects the material up to 1370°C. Above this temperature bulk oxidation occurs. Aluminum nitride is stable in hydrogen and carbon dioxide atmospheres up to 980°C.[3]The material dissolves slowly in mineral acids through grain boundary attack, and in strong alkalies through attack on the aluminum nitride grains. The material hydrolyzes slowly in water. Aluminum nitride is resistant to attack from most molten salts, including chlorides and cryolite. Aluminum nitride (AlN) is a nitride of aluminium. Its wurtzite phase (w-AlN) is a wide band gap (6.2 eV) semiconductor material, giving it potential application for deep ultraviolet optoelectronics.
Among the prominent nitride semiconductors such as, GaN, AlN, InN and their alloys, with the notable exception of AlN and its alloys, layers of high-quality most of the materials can be grown at temperatures of 1200 °C or less. The crystalline quality of AlN layers is always inferior to its counterpart GaN grown at much lower temperatures. The high temperature growth of AlN films is expected to be effective in
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improving crystalline quality and surface morphology because surface migration of Al-species would increase at high temperatures.
Although the high temperature re-growth condition contribute the better quality of AlN film, but it may probably damage the quantum well. As the result, we choose lower temperature conditions to re-growth the AlN current blocking layer.
The re-growth temperatures various from 850oC to 1080oC. We used four probe and AFM to measure the sheet resistance and roughness of AlN. Fig.5.1 shows the AFM of three re-growth conditions, as the temperature rise the morphology of film become more flatness. Table5.1 shows the comparisons of three temperatures in different re-growth condition.
Fig. 5.1 (a) AFM of three AlN re-growth conditions.
Table 5.1 comparison of AlN film
850
oC 1020
oC 1080
oC
Sheetresistance 3~5KΩ 6~8KΩ 6~13KΩ
R.M.S. 3.627nm 2.935nm 2.485nm
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As the result, we choose t he 1020oC as the AlN re-growth condition, due to the sheet resistance and surface roughness is close to 1080oC but higher than 850oC.
After two steps of re-growth we use PL spectrum and NK reflection spectrum to check the quantum well signal .Fig. 5.1(b) shows the after two steps re-growth the quantum well PL spectrum, which still in the center of bottom DBR reflectance. The dip curve in the DBR reflectance indicates the quantum well’s absorption through NK measurement.
Fig.5.1 (b) PL after two steps re-growth and bottom DBR reflectance.
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5.2 Fabrication of GaN-based VCSELs with AlN current blocking layer process 5.2.1 Initial clean and photolithography technique
During process of GaN-based VCSELs, two basic skills will be frequently used.
One is the initial clean (I.C.), and another is photolithography technique. The purpose of the I.C. is to remove the small particle, and organism on the sample surface, before we start any process procedure. The steps of I.C. are described as below ：
1. Degreasing by ultrasonic baths in acetone (ACE) 5min.
2. Dipping by ultrasonic baths in isopropyl alcohol (IPA) 5min for organism removed.
3. Rinsing in de-ionized water (D.I. water) 5min for surface clean.
4. Blowing with N
2 gas for surface drying.
5. Baking by hot plate 120oC, 5min, for wafer drying.
The purpose of the photolithography is to transfer the pattern of the mask to the photoresist (PR) on the wafer. In the process of photolithography, a positive photoresist AZ 5214E was used. Although it is positive photoresist , it is capable of image reversal (IR) resulting in the effect of negative photoresist. In fact AZ 5214E is almost exclusively used in the IR-mode which is proper to be used in the lift-off process. Both positive exposure and IR exposure photolithography technique were employed in our VCSEL process. These photolithography techniques are described as below：
Positive exposure technique
1.Spin coating by photoresist (1000rpm/10s, 3500rpm/30s).
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2. Soft bake: hot plate 90
o
C, 90sec.
3. Alignment and exposure
4. Development: dipping in AZ-300 for 30sec.
5. check exposure PR pattern by OM.
6. Hard bake: hot plate 120oC, 4min
IR exposure technique
1.Spin coating by photoresist (1000rpm/10s, 3500rpm/30s).
2. Soft bake: hot plate 90
o
C, 90sec.
3. Alignment and exposure (about half time of positive exposure) 4.Hard bake 120℃ 110sec
5.Exposure without mask 57sec
6. Development: dipping in AZ-300 for 30sec.
7. check exposure PR pattern by OM.
8. Hard bake: hot plate 120oC, 4min 5.2.2 Epitaxial flowchart
The overall GaN VCSEL structure is shown in Fig. 5.1. 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. Fig. 5.2 is the reflectance spectrum of bottom DBR, and there is a high reflectivity (~97%) at
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447nm. 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 900-nm-thick n-type GaN, a ten pairs In0.2Ga0.8N/GaN (2.5 nm/10.5 nm) MQW, 24nm AlGaN electron blocking layer , 20nm p-GaN, 30nm AlN current blocking layer and 155nm p-GaN as shown in Fig.
5.3.
The AlN VCSEL epitaxial flowchart was fabricated by five steps. In the beginning, SiO2 hard mask was used to define current aperture. The SiO2 was grown by PECVD patterned to define the current confinement layer with the effective current aperture varying from 3μm to 10μm, as shown in Fig. 5.4.
The 30nm AlN layer process is epitaxial by a vertical-type MOCVD system (EMCORE D75) and the epitaxial environment at 1020oC and 100 torr. After liftoff SiO2 by using BOE(as shown in Fig.5.5), the surface is clear . In this step, we must check the surface is clear and without SiO2, the remain SiO2 will to impede the wavelength and FWHM are 447 and 7nm respectively.
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Fig.5.1.1 The overall diagram of VCSEL structure
Fig. 5.2 The reflection spectrum of bottom DBR
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Fig.5.3 The cross section SEM image of VCSEL cavity without upper DBR, the insert figure is the thickness of AlN layer.
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Fig. 5.4 current aperture defined by SiO2.varying from 3um to 10um
Fig. 5.5 The p-GaN surface after liftoff SiO2 and the insert figure point out the pits on p-GaN
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5.2.3 Process flowchart
The AlN- VCSEL was fabricated by six process steps. In the beginning, mesa was etched by ICP dry etching, as shown in Fig. 5.7. Then 200nm SiNx layer was deposited by PECVD and patterned to defined current injection aperture as shown in Fig. 5.8. The 30nm ITO were deposited by sputter and annealed at oven 2hr and nitrogen ambient. The inner diameter is 25um and the outer diameter is 80um as shown in Fug. 5.9 .Through the ring shape ITO the VCSEL cavity will become an ideally cavity without ITO absorption. At current injection, the current will laterally spreading into aperture (mark in Fig.5.9.2) by high sheet resistance of AlN current blocking layer and the turn on image is shown in Fig. 5.9.1. The Ti/Al/Ni/Au and Ni/Au contacts were deposited to serve as n-type and p-type electrode respectively, as shown in Fig. 5.10. The VCSEL was completed by capping the structure with 10 periods of SiO2/Ta2O5 DBR stack (R~99%), as shown in Fig. 5.11
Fig. 5.7 Mesa etching step and the device turn on image
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Fig. 5.8 Deposited SiNx as the passivation layer
Fig.5.9.1 The ring shape ITO
Fig. 5.9.2 The ring shape ITO and its CCD image.
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Fig. 5.10 deposited n-contact and p-contact
Fig. 5.11 Deposited 10-pair Ta2O5/SiO2 DBR
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5.3Characteristics of electrically pumped GaN-based VCSELs with AlN current blocking layer(CBL)
5.3.1 The emission images of AlN VCSEL
Figure 5.12 shows the light emission photograph of AlN-VCSEL operated at 0.03mA.
The bright apertures show AlN layer can effective block the current. The sizes of aperture were defined 3um, 5um, 7um, 10um.. We can clearly find the light emission is concentrate in the current aperture defined by AlN.
In order to observe more detail in the current aperture, we also observed the optical intensity distribution by means of CCD and Beam-view program. The color on Fig 5.12(c) represents the relative optical intensity emitted from observed device, and this figure shows the current injection uniformly into the aperture. We can observe the light beam showing a Guass distribution in Fig. 5.12(d).Fig. 5.8 shows the VCSEL before contact process image, the ring shape ITO with 25um and 80 um, inner and outer diameter respectively. Fig. 5.9 shows the turn on image of VCSEL without ITO layer, which injection current laterally flow through p-GaN layer. The concentrated emission light not only confirms that current is uniformity injection into the current aperture but also shows that the AlN plays a role of current blocking layer successfully.
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Fig. 5.12 Emission images of AlN VCSEL
Fig. 5.13 (a)The emission image of aperture size of 3μm under high magnification CCD image (b)(c)(d)Beam-view on 3um aperture emission
Fig. 5.8 The ring shape ITO and CCD image.
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Fig. 5.9 Current lateral spreading by AlN current blocking, the white mark point out the 10um current aperture.
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5.3.2The L- I-V curves of AlN blocking layer and conventional VCSEL
The current-voltage (I-V) characteristics of AlN and conventional device are both shown in Figure 5.14. The turn on voltage and resistance of the conventional VCSEL which is 10um current aperture define by SiNx passivation was about 7.14V and 385Ω respectively. The 10um current aperture of VCSEL with AlN CBL shows 7.3V and 389Ω respectively. The slightly higher voltage and resistance compared with the conventional device because the current path could be longer than the conventional VCSEL, such results are realizable and acceptable.
Fig.5.14 I-V curve
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5.3.3The optical characteristics of AlN VCSEL
Fig.5.16 shows the AlN VCSEL device structure and turn on image. Fig. 5.17(
a) shows the EL spectrum of AlN VCSEL. The equation, which associates the quality factor and total loss, is as follow:
Δλ
λδ
By the equation, the quality factor estimated about 1100, which is at the same order
By the equation, the quality factor estimated about 1100, which is at the same order