Fabrication and characteristics of optical pumped VCSELs…

Chapter 3 Characteristics of an Optical Pumped GaN-Based VCSELs…

3.2 Fabrication and characteristics of optical pumped VCSELs…

The overall optical pumped GaN VCSEL structure is shown in Fig 3.2. 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. Fig3.3 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 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 Fig3.4 , and Fig3.5 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 3.6 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.

Fig3.7 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 Fig3.8 . 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. Fig3.9 shows the lasing spectrum above the threshold condition, and the lasing wavelength is 438nm with a 0.15nm linewidth.

Fig 3.2 The schematic diagram of the optical pumped VCSEL structure

Fig 3.3 The 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 3.4 The cross section SEM image of VCSEL cavity without upper DBR

Fig 3.5 the PL and reflectance spectrum of VCSEL without upper DBR

380 400 420 440 460 480 500

-0.5

Fig 3.6 The reflectance spectrum of upper DBR

350 400 450 500 550 600

0

Fig 3.8 The pumping energy density versus normalized intensity diagram

Pumping energy density (mJ/cm

)

Threshold energy density~3.3mJ/cm2

Chapter 4 Characteristics and measurement of indium-tin-oxide

4.1 Introduction to indium-tin-oxide

4.1.1 Transparent conduct layer (TCL)

Gallium nitride (GaN) is an attractive material which has a wide direct band gap, GaN-based optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs) have been researched and commercially available . Most commercial GaN-based devices use an Mg-doped GaN layer as a p-type semiconductor . However, the high activation energy (~180 meV) of Mg atoms leads to only 1% of Mg atoms can be ionized, and therefore the carrier concentration of p-type GaN is quite low and the poor conductivity will limit current spreading laterally. It is necessary to deposit a conductive layer on p-GaN for current spreading. This layer should not only form good ohmic contacts to p-GaN but also be transparent to the emitted light from the active layer.

Because of the high work function of p-GaN , metal of higher work function such as Ni , Pd and Pt were used to form ohmic contact for p-GaN. In gereral, Ni/Au contact is widely applied in commercially GaN-based LED. The specific contact

resistance can reach 10^-2~10^-6Ω·cm² after annealing. However , the Ni/Au layer is a semitransparency layer , and it has been reported that the transmittance of Ni/Au is only around 60% to 85% in the 450-550 nm wavelength ^[38]. The absorption of contact layer is important issue for semiconductor laser , because the optical absorption would raise the threshold gain for laser emission. To improve the transmittance of current spreading layer, it is feasible to replace the conventional Ni/Au contact by a better transparent conductive contact. Nowadays , the ITO film has been widely used as a transparent conductor due to its high transparency (~90%) in the visible spectrum and its low electrical resistivity (< 10³Ω·cm).

4.1.2 Characteristics of ITO

Indium tin oxide (ITO) thin film have been studied extensively for optoelectronic device application owing to its unique transparency and conduction properties . It is a mixture of indium (III) oxide (In

2 by weight) and highly degenerate n-type semiconductor , which has a wide band gap (~3.5–4.3 eV) and low electrical resistivity (2~4×10^-4Ωcm). The oxygen vacancies accompanied with the Sn donor , which are responsible for high conductivity, can also lead to non-stoichiometric ITO. The optical characteristics of

ITO shows high absorption in UV region , high transmittance in visible region and high reflectance in IR region. In addition, ITO thin film is thermal stable and has better device reliability compared with Au-based contact. Due to these unique properties, ITO has been used in a wide range application, such as transparent electrode for display and solar cell , IR reflective mirror for building and transparent conduct layer for nitride-base devices.

There are many method to deposit ITO thin film including sputtering , e-gun evaporation , chemical vapor deposition , pulsed laser deposition and sol-gel process.

In this study , we use e-gun evaporation and sputter deposition to deposit four different structure ITO layer(e-gun 210nm and 30nm amorphous ITO , sputter 30nm crystalline ITO and sputter 10nm amorphous + 20nm crystalline ITO ).

4.2 Measurement method and equipments

4.2.1 Scanning electron microscopy (SEM)

Scanning electron microscope (SEM) is a type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity. The signals result from interactions of the electron beam with atoms at or near the surface of the sample. In the most common or standard detection mode, secondary electron imaging or SEI, the SEM can produce very high-resolution images of a sample surface, revealing details about 1 to 5 nm in size. Due to the way these images are created, SEM micrographs have a very large depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample. Fig 4.1 is the SEM equipment of our group.

Fig 4.1 the scanning electron microscopy (SEM) system of our lab

4.2.2 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 Fig4.2) to determine the sample resistivity. Typical probe spacing s ~ 1

mm. Fig 4.3 is the four point probe we used to sheet resistance measurement in NCTU

NFC.

Fig 4.2 Schematic of 4-point probe configuration

Fig 4.3 four point probe in NCTU Nano Facility Center

4.2.3 Circular Transmission Line Model (CTLM) measurement

In order to realize the interfacial characteristics between ITO and p type GaN, we make use of CTLM (Circular Transmission Line Model) to measure the specific contact resistance. First, we clean the LED sample and deposit the ITO thin film on p-GaN surface. Then , we etch the ITO to form CTLM pattern after Photolithography Process. Fig 4.4 is the CTLM pattern after ITO etching and removing the photo resist.

The CTLM pattern is a series of ring with the identical inner circle and different gap distance. The gap region is only p-GaN surface and the other is all ITO layer. By

and plot RT. versus gap distance, as shown in Fig5.5. If the voltage is impressed two of these contacts , separated by d , then the equation of total resistance can be derived as follow：

Where ρs is the sheet resistance of semiconductor material between the contacts . LT

is the transfer length that donates the 1/e distance , which the voltage curve decay nearly exponentially with distance . After fitting the linear curve of the Fig 5.5 , we can get the value of ρs and LT . Finally , take these value into the equation 5.2 , and then we calculate the specific contact resistance( ρc ) of between ITO and p-GaN layer.

ρ

= ρ

× ( L

)

2 (5.2)

The potential distribution under the contact can be determine by both ρs and ρc

according equation 5.3.

Where L is the contact length , WC the contact width , and I the current flowing into the contact.

Fig 4.4 the ITO CTLM pattern after etching

Fig 4.5 the schematic diagram of total resistance versus gap distance

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 120^oC, 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 90^oC, 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 120^oC, 4min

IR exposure technique

1.Spin coating by photoresist (1000rpm/10s, 3500rpm/30s).

2. Soft bake: hot plate 90^oC, 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 120^oC, 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

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