Chapter 2 Motivation
2.1 Previous work of our group
2.1.2 CW lasing of current injected GaN-based VCSEL at
With the achievement of optically pumped GaN-based VCSEL, the realization of electrically-injected GaN-based VCSEL has become promising. Fig 2.5 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 2.6. 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. Fig2.7 shows the light output power versus cw injection current
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 Fig2.8. 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 2.5 The schematic diagram of the current injected VCSEL structure
Fig2.6 emission spectrum of the current injected VCSEL structure.
Fig2.7 The light output intensity vs injection current and current–voltage
Fig 2.8 The laser emission spectrum at different injection current levels measured at 77 K.
2.2 Reflectance spectra simulation
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 should 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 0ne optical wavelength thickness ITO with thinner ITO layer. Before proceeding with the experiment , we first simulate the reflectance spectra with different thickness of ITO layer. Fig2.9 shows the cavity structure of simulation, and ITO layer thickness are 0nm、30nm、
120nm、210nm and 225nm (about one optical wavelength thickness for 440nm).
Owing to reflectivity symmetry of DBR , the AlN/GaN DBR are chosen to 18 pairs.
Fig 2.10 are the simulated reflectance spectra of different ITO thickness. The dip positions in the reflectance spectra represent the cavity mode of these cavities , and the quality factor can be estimated form the linewidth of the dip. Fig 2.11 shows the cavity mode wavelength versus different ITO thickness. The cavity mode wavelength shift to longer wavelength because of the increase of the cavity length, but the cavity mode wavelength return to shorter wavelength when the ITO thickness is thicker than
120nm. This phenomenon is due to the exceeding the stop band region of the lower DBR , so that the cavity mode jumps to the (m+1)th mode from the mth mode.
Furthermore, the cavity mode also change to multimode owing to longer cavity length and smaller mode spacing , when ITO thickness is lager than 30nm. In Fig2.12 , we can observe that the quality factor is about 700 in 225nm ITO cavity , but this value is much lower than the quality factor of the cavity without ITO layer(~3300). However, the quality factor of 30nm ITO cavity(~3100) is a little smaller than 0nm ITO. Base on the simulation result, we can expect the 30nm ITO layer can efficiently reduce the loss and threshold current density of our VCSEL devices.
Fig 2.9 the cavity structure of simulation
Fig 2.10 the simulated reflectance spectra with different thickness of ITO
Fig 2.11 the simulated cavity mode with different thickness of ITO
Fig 2.12 the simulated quality factor with different thickness of ITO
Chapter 3
Characteristics of an Optical Pumped GaN-Based VCSEL
3.1 optical measurement system
Figure 3.1 shows the optical pumping scheme for our VCSELs. As shown in Fig.
3.1 the emission spectrum of the GaN-based VCSEL structure was measured using a microscopy system (WITec, alpha snom) at room temperature. The optical pumping of the samples was performed using a frequency-tripled Nd:YVO4 355-nm pulsed laser with a pulse width of 0:5 ns at a repetition rate of 1 kHz and a He-Cd 325nm cw laser. The pumping laser beam with an about 50um spot size was incident normal to the VCSEL sample surface. The light emission from the VCSEL sample was collected using an imaging optic into a spectrometer/charged coupled device (Jobin-Yvon Triax 320 Spectrometer) with a spectral resolution of 0:1 nm for spectral output measurement.
Fig3.1 The setup of optical measurement system
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
2)
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 10-2~10-6 Ω·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
2O
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
C= ρS × ( L
T )
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.
During process of GaN-based VCSELs, two basic skills will be frequently used.