Chapter 4 Characteristics and measurement of indium-tin-oxide
4.3 Result and discussion
4.3.1 Specific contact resistance
In this thesis , we use four different ITO structure deposited by E-gun evaporation and sputter. The annealing parameter for sputter ITO have not yet be realized for our group , so we utilized the CTLM to measure specific contact resistance of different annealing condition. Deposited ITO thin films were annealed using a rapid thermal annealing (RTA) system in nitrogen atmosphere. The annealing temperature and time
shown in Fig4.6(a) and Fig4.6(b) . Fig4.6(a) demonstrates the specific contact resistance at fixed annealing temperature (600℃) , and Fig4.6(b) was measured for the same annealing time (10mins). As shown in these two figures , we can find that there is a minimum specific contact resistance(~0.2Ωcm2) at 600℃ with annealing time of 10mins , so we applied this parameter for ITO annealing before other measurement.
Fig 4.6 ITO specific contact resistance (a)with different annealing time and(b) with different annealing temperature
4.3.1 ITO surface morphology
In this section , we use scanning electron microscopy(SEM) to observe the surface morphology of ITO thin film. The top-view SEM image are shown from Fig 4.7(a) to Fig 4.7(d). Fig4.7(a) and Fig4.7(b) are the 30nm ITO layer deposited by sputter.
The difference of them is that the former is deposited at 150℃ , and the latter is
continue residual layer. While depositing at 150℃ , ITO thin film can be crystalline compared with at room temperature. Despite the slight difference in deposition temperature , the surface morphology of them are similar , and the grain size is as small as below 10nm diameter. As shown in Fig4.7(c) and Fig4.7(d) , the 30nm and 210nm ITO deposited by E-gun have lager grain size(~30nm in Fig4.7(c)) and amorphous structure.
Fig 4.7 (a) sputter 150℃ crystalline 30nm ITO (b) sputter 10nm amorphous + 20nm crystalline ITO
4.3.3 Refractive index and extinction coefficient
To realize the material absorption characteristics of ITO , we measure the refractive index and extinction coefficient by mean of n＆k analyzer 1280. The schematic diagrams of wavelength dependence of refractive index and extinction coefficient for four different ITO layer are shown from Fig 4.8 (a) to Fig 4.8 (d). Fig 4.8(a) and Fig 4.8(b) are the measurement result of sputter 30nm crystalline ITO and sputter 10nm α+ 20nm crystalline ITO. The curves of experimental data are nearly identical , and this appearance may be due to the same deposition source and thickness. The extinction coefficient has a maximum in ultraviolet region , it means the high absorption because of interband transition or atomic oscillation[39]. In visible region, the extinction coefficient is nearly zero . It represents the most of light is transparent and this material is almost colorless. The value of absorption in transparent region gradually increases as wavelength approach infrared region. This increase is owing to the vibrational absorption associated with lattice vibration. As shown in Fig 4.8 (c), the extinction coefficient of 30nm deposited by E-gun is especially lower than other three kind of ITO layers , and absorption curve does not raise near IR region. We suppose this phenomenon is due to larger spacing between lager grain size , so this ITO thin film has lowest absorption but current would spread more difficultly. Fig
4.8(d) shows the higher absorption in longer wavelength than others during visible region . That is why the one optical wavelength thickness usually exhibit green appearance.
Fig 4.8 the refractive index and extinction coefficient versus wavelength diagram for sputter (a) 30nm crystalline ITO (b) 10nm α+ 20nm crystalline ITO
200 400 600 800 1000
Fig 4.8 the refractive index and extinction coefficient versus wavelength diagram
4.3.4 Summary
Table4.1 is the summary of ITO characteristics. The 30nm ITO thin film show the larger sheet resistance and resistivity than 210nm ITO layer. The sheet resistance is associated with lateral current spreading , and it generally decreases as thin film thickness increase. The value of resistivity is affected by defects density within thin film layer. Due to thinner deposition thickness, 30nm ITO has still more defects than 210nm ITO. These defects would cause electron scattering and reduction of Hall mobility[40]. However , the specific contact resistance between ITO and p-GaN of 30nm ITO are slightly lower than 210nm ITO layer. This result reveals the specific contact resistance is related to annealing parameter , not thin film thickness.
Especially , the 30 nm ITO layer deposited by E-gun has the largest sheet resistance and resistivity but the lowest absorption among four different structure ITO layer. We suppose that it may be resulted from more defect of vacancies . The deposition quality of E-gun evaporation should be promoted when deposition thickness is too thin.
Table 4.1 the characteristics comparison of four different ITO thin film
Chapter 5
Fabrication and measurement of electrically driven GaN-based VCSELs
5.1 GaN-based VCSELs process
5.1.1 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).
2. Soft bake: hot plate 90oC, 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 90oC, 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
6.1.2 Process flowchart
The sample structure we use in our process is the same as Fig3.2 without depositing upper DBR. To fabricate the VCSELs device on chip , we need to experience six process steps. Fig 5.1 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 5.2 , 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 5.3. 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 5.4 , and Fig5.5 is the OM image of VCSEL device with 5um metal aperture when current injects.
Fig 5.1 The schematic diagram of overall VCSELs process flowchart
Fig 5.2 The defined mesa of first step was measured by probe station
Fig 5.3 The ITO transparent conduct layer deposited on current aperture
Fig 5.4 The schematic diagram of completed electrical pumped VCSEL
Fig 5.5 The OM image of completed VCSEL device when current injects
5.2 The electroluminescence (EL) measurement system
The electroluminescence (EL) characteristics of fabricated VCSELs were measured by the probe station system and evaluated by injecting different current density. The device is driven by Keithley 238 CW current source, and its light output power could be measured by optical power meter through a integrated sphere. Current-light output power (L-I) and current-voltage (I-V) measurements were performed by using the probe station, Keithley 238 CW Current Source, UV power detector, Newport 1835-C optical power meter. Figure 5.6 shows the electrical and optical measurement system.
The emission signal could be received by CCD and observed on the CRT screen. The direct emission from device is collected by 40X microscope and transmitted to Jobin-Yvon Triax 320 spectrometer by optical fiber with 25μm in diameter. All the data could be directly fee-backed to the computer from these facilities, including optical meter spectrometer and Keithley 238 current source by the GPIB connector.
Fig 5.6 The EL measurement system of electrically driven VCSELs
5.3 The characteristics of GaN-based VCSEL at Room temperature
In this thesis , we applied four different ITO thin film on our VCSEL devices , so we would introduce the optical and electrical characteristics of them , respectively.
5.3.1 VCSEL devices with 210nm ITO deposited by E-gun evaporation
The 210nm ITO layer , which has nearly one optical wavelength thickness , deposited on our VCSEL device is used as an reference compared with the devices with 30nm ITO thin film. Fig5.8(a) is the power and voltage versus current density diagram (LIV curve) . The series resistance and turn on voltage of devices are about 202Ω and 6.5V, respectively. The output power begin to roll over when current density reaches 11kA/cm2 . These electrically characteristics are better than the result we reported earlier [34] , as shown in Fig 5.7. This improvement of electrically performance reveals the promotion of our epitaxial quality in these years. We also observed the optical intensity distribution by mean of CCD and Beam-view program . The color on Fig 5.8(b) represents the relative optical intensity emitted from the observed device , and this figure shows the percent of optical intensity within the aperture region(circle of dotted line) is 53% . It means about 50% of emission escapes
from the edge of mesa due to poor lateral optical confinement in our devices. The reduction of emission from the aperture would promote the threshold condition for lasing. Fig 5.9 is the electroluminescence spectrum of VCSEL device with 30um metal aperture and the inset is the CCD image of current aperture. The cavity mode and quality factor estimated from this spectrum are 420nm and about 700, respectively. There are several bright spots within the current aperture , which maybe attributed to the inhomogengity of nitride DBR. These bright spots represent the high quality region on the sample surface , so we suppose the spectra of narrow linewidth are received from there .
Fig 5.7 The previous electrically characteristics of VCSEL device with 240nm ITO
Fig 5.8 (a) LIV curve and (b) optical intensity distribution of VCSEL with 210nm ITO
Fig 5.9 The EL spectrum of VCSEL device with 210nm ITO, and the inset figure is the CCD image of the current aperture
5.3.2 VCSEL devices with 30nm ITO deposited by E-gun evaporation
The electrically characteristics of VCSEL with 30nm ITO deposited by E-gun are shown in Fig 5.10 (a) and (b). The series resistance and turn on voltage of devices are about 182Ω and 7V, respectively. The output power reaches a maximum when the device is operated at 11.8kA/cm2. These performance can be comparable to the devices with 210nm ITO layer , but the percent of optical intensity within the current
highest sheet resistance mentioned at earlier chapter. The carriers do not spread uniformly before entering the p-GaN layer , so that the emission area is broader and inhomogeneous near current aperture. More emitted light can escape from cavity through neighbor edge of mesa. Although the worse current spreading observed in VCSEL device with 30nm ITO deposited by E-gun , the quality factor measured from Fig 5.11 is about two time lager than 210nm ITO . The improvement of quality is due to the lower optical absorption of thinner ITO layer , and it can confirm our expectation of using 30nm ITO to replace thicker ITO layer.
Fig 5.10 (a) LIV curve and (b) optical intensity distribution of VCSEL with 30nm ITO deposited by E-gun
Fig 5.11 The EL spectrum of VCSEL device with 30nm ITO deposited by E-gun , and the inset figure is the CCD image of the current aperture
5.3.3 VCSEL devices with 30nm crystalline ITO deposited by sputter
The electrically characteristics of VCSEL with crystalline 30nm ITO deposited by sputter are shown in Fig 5.12 (a) and (b). The series resistance and turn on voltage of devices are about 164Ω and 5V, respectively. The roll over current density of devices is 13 kA/cm2 , and the percent of optical intensity within the current aperture is 37% , slightly higher than E-gun 30nm ITO layer. The electrically characteristics of sputter ITO is better than which deposited by E-gun for the same thickness , but the total loss within resonant cavity obtained indirectly from quality factor showed in Fig 5.13 is similar . The equation , which associates the quality factor and total loss, is as follow：
Fig 5.12 (a) LIV curve and (b) optical intensity distribution of VCSEL with crystalline 30nm ITO deposited by sputter
Fig 5.13 The EL spectrum of VCSEL device with crystalline 30nm ITO deposited by sputter, and the inset figure is the CCD image of the current aperture
5.3.4 VCSEL devices with 10nmα+20nm crystalline ITO deposited by sputter
The electrically characteristics of VCSEL with 10nmα+20nm crystalline ITO deposited by sputter are shown in Fig 5.14 (a) and (b). The series resistance and turn on voltage of devices are about 165Ω and 5V, respectively. The roll over current density of devices is 14 kA/cm2 . The cavity mode and quality factor estimated from the spectrum showed in Fig 5.15 are 415.2nm and about 1600, respectively. These electrical and optical performance are comparable to the ITO thin film deposited by the same source. However , the percent of optical intensity within current aperture is as high as 94% , and this value is the largest compared with other three kinds of ITO thin film. The reason of this unique phenomenon is under investigation. We suppose that it is possible owing to the heterojunction between amorphous and crystalline ITO layer. But , most importantly , the characteristics of this kind of ITO thin film match our demand for GaN-based VCSEL very well.
Fig 5.14 (a) LIV curve and (b) optical intensity distribution of VCSEL with 10nmα+20nm crystalline ITO deposited by sputter
Fig 5.15 The EL spectrum of VCSEL device with 10nmα+20nm crystalline ITO deposited by sputter, and the inset figure is the CCD image of the current aperture
5.3.5 The reliably of GaN-based VCSEL with different ITO TCL
In order to realize the device reliably of operation , we also measure the damage current density of four kinds of VCSEL devices by mean of EL measurement system.
Fig5.16 are the voltage and output power versus current density diagram (LIV curve) , and we choose the current density when the LIV curve change unusually as the damage current density of the devices. Because of the same epitaxial quality of
Fig 5.16(a) to (d) , the damage current is higher for ITO layer deposited by sputter than by E-gun. Besides, the devices with 210nm ITO has the lowest damage current density(~17kA/cm2) . It maybe attributed to the blocking of heat dissipation for the thicker ITO layer.
Fig 5.16 Voltage and output power versus current density diagram of VCSEL with different ITO structures
5.4 Summary
Table 5.1 is the overall electrical and optical characteristics of GaN-based VCSEL with four different ITO structures. From this table , the VCSEL devices with 30nm ITO layer have better electrically characteristics , such as series resistance and roll over current density , than the devices with 210nm ITO layer . Owing to lower optical absorption for 30nm ITO , the quality factor is also higher . This result has reached the goal we set before the experiment. Next, we should find the best ITO structure for our VCSEL devices. Compared with different deposition methods for the same thickness , the electrically characteristics of VCSELs with ITO layer deposited by sputter are better than deposited by E-gun. The 30nm crystalline and 10nm amorphous + 20nm crystalline ITO thin film deposited by sputter have similar electrically and optical performance of VCSEL devices , but the former suffers from inhomogeneous current spreading and large amount of leaky light , so the 10nm amorphous + 20nm crystalline ITO thin film is the best choice for our VCSEL devices.
Table 5.1 The overall electrical and optical characteristics of GaN-based VCSEL with four different ITO structures
Chapter 6
Conclusions and future work
6.1 Conclusions
In this report, we have designed and fabricated the electrical pumping GaN-based VCSELs with hybrid mirrors and four different ITO structures. The VCSEL resonant cavity is including the high-reflectivity AlN/GaN bottom DBRs (99.3%), about 7λ GaN-cavity, transparent conduct layer (ITO), and SiO
2/Ta
2O
5 dielectric DBRs (99%).
The series resistance of VCSEL devices with four different ITO structures are between 150Ω and 202Ω, and this value is better than the devices we reported previously (530Ω). However, the quality factor of VCSEL devices with 30nm ITO layer is about 1600, slightly lower than the devices without ITO layer (1900), but higher than the devices with 210nm ITO layer of about 700 due to the optical absorption in ITO layer. Compared the results with the same thickness ITO film deposited by E-gun and sputter, the devices with 30nm ITO layer by sputter has better electrically characteristics, such as series resistance(~165Ω) , roll over current density(~13kA/cm2), and damage current density (~43kA/cm2). Among VCSEL devices with four different ITO structures, the devices with 10nm amorphous plus 20nm crystalline ITO film have the similar electrically and the best optical
performance than 30nm crystalline ITO caused less emitted light to leak from the cavity. In summary, the 10nm amorphous plus 20nm crystalline ITO deposited by sputter is the most suitable material as the transparent contact layer to improve our VCSEL performance and characteristic.
6.2 Future work
According to the study in this thesis , there are several work for our group to continue and improve. One is the heat dissipation problem due to worse thermal conductivity of sapphire substrate. We can package our devices into TO cans or use pulsed current source for current injection to avoid large thermal effect in our devices.
The other is to get better current and optical confinement. To solve this problem , we have design the new VCSELs structure shown in Fig6.1 (a) and (b). These isolation layers of lower refractive index can efficiently upgrade both the current and optical confinement. We expect these structures can finally reduce the threshold condition for the VCSEL devices.
Fig7.1 (a) the devices with AlN current blocking layer (b) the devices with ions
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