This study has highlighted the formation of an amorphous-nano-oxide film on Si substrate dominating the preferred orientation of sputtered β-Ga2O3. After the rapid thermal annealing (RTA) process from 700 to 1000 °C for β-Ga2O3 (100)/SiO2/p-Si, all the amorphous transformed into a β-Ga2O3 structure. Meanwhile, the thermal-induced massive twin boundaries and stacking faults generation have been observed in the annealing process above 800 °C. Therefore, an optimum metal-semiconductor-metal photodetector performance is achieved for the 800°C-RTA-treated β-Ga2O3 samples with the photo/dark current ratio of 3.91×102 and responsivity of 0.702 A/W (at 5 V bias). Furthermore, the interface energies per area (Ei) by density functional theory between β-Ga2O3 films ((001), (010), (100), and (-201)) with various facets and amorphous SiO2 were determine to quantify the sequence of the preferred orientations.
The ZnGa2O4 (ZGO) complex oxide with a wide bandgap of 5.1 eV has become one of the promising materials for deep ultraviolet sensing applications. However, the sputtered ZGO films always showed the disordered nanocrystalline structure resulting in the relatively poor performance. In this study, the solid-phase epitaxy method is used for crystallizing the ZGO structure via rapid thermal annealing (RTA) process. The disordered crystal-grains as incubated seeds are obtained in the as-deposited film at the substrate temperature of 400℃. By employing RTA at 700℃, the ZGO film structure approaches the quasi-single-crystalline structure, which is evidenced by checking the transmission electron microscopy. Suppression of the Zn diffusion out under the annealing will be discussed. As a result, the spectral responsivity of RTA-treated ZGO photodetector can reach 2.53 A W-1 at 240 nm and 5 V bias, indicating a relative enhancement of 256% as compared with the as-deposited one.
Ultra-wide bandgap zinc gallium oxide (ZGO) and GO films were prepared on c-plane sapphire by conventional radio-frequency magnetron sputtering. In the current sputtered oxide studies, target composition or growth temperature is usually the main deposition variable, and the other growth conditions are fixed. This would make it difficult to fully understand the theory and characterization of ZGO films. In this study, several growth parameters as well as the post-thermal treatment were all modulated to realize and optimize the ZGO growth. From x-ray and TEM analyses, stabilization of stoichiometry and control of crystallinity transformation were confirmed to be important factors in determining the film quality. The optical bandgap of ZGO can reach 5.0-5.1 eV with a maximum responsivity peak at 240 nm. A metal-semiconductor-metal photodetector is demonstrated with a maximum responsivity over 2 A/W under a 5-V biased voltage. Furthermore, the photo/dark current ratio can be improved to be over ten thousand. As compared with those of the sputtered GO photodetector, the spectral response peak of ZGO showed a blue shift to 240 nm with higher responsivity. The data presented exhibit the ZGO material will become another potential candidate for ultra-wide bandgap semiconductor applications.
In this work, the AlxGa1-xN-on-sapphire template was successfully prepared by halide vapor phase epitaxy (HVPE) at 1100oC. The flow ratio (R) of the HCl flow rate through Al metal to the total HCl flow rates through Al and Ga metals was adjusted to control the Al content. The AlxGa1-xN films without phase separation can be obtained as the R value exceeds 0.67. The low R value strongly enhances the formation probability of GaN instead of AlxGa1-xN. This indicates that a sufficient concentration of the Al precursor is certainly vital for the formation of AlxGa1-xN compounds. The AlxGa1-xN prepared at the R value of 0.80 can achieve best crystallinity and the lowest surface roughness. Furthermore, the optical transmittance of the AlxGa1-xN under R=0.80 is above 70% between the wavelength of 225 nm and 400 nm. As a result, the AlxGa1-xN under R=0.80 can be regarded as a suitable growth template for the ultraviolet light-emitting diodes. The internal quantum efficiency of the 370 nm-quantum wells on the AlxGa1-xN/sapphire prepared under R=0.80 shows 127% improvement as compared with that on the undoped-GaN/sapphire.
In this study, the effects of chemical treatment on the properties of MOS capacitors and metal-oxide-semiconductor high electron mobility transistor (MOS-HEMT) were studied. The structure consist of Al2O3/u-GaN/AlN buffer/ Si substrate and Al2O3 (10 nm)/u-AlGaN (25 nm)/u-GaN (2μm)/AlN buffer/Si substrate for MOS capacitor and MOS-HEMT device, respectively. There are four chemical treatment recipes, which consist of organic solvents, oxygen plasma, BCl3 plasma, dilute acidic solvent, hydrofluoric acid and RCA-like clean process to remove the metal ions, organic contamination and native oxide. Four different chemical treatment recipes treated the surface of u-GaN before Al2O3 was grown on the treated surface to reduce the interface state trap densities (Dit). The Dit value was calculated from measurement of C-V curve with 1M Hz frequency. The formation of interface state trap of u-GaN surface is modified by different chemical solution of varied chemical treatment recipe, which further influence the breakdown voltage (Vbk), on-resistance (Ron), threshold voltage (Vth) and drain current (Id) of MOS-HEMT. The Vth of MOS-HEMT with organic solvents clean treatment is -11.00V. The MOS-HEMT after BCl3 plasma and organic solvents clean treatment shows the lowest Vth of -9.55V. The electronic characteristics of MOS HEMT device with four different chemical treatment recipes were investigated in this article.
A twice wafer-transfer technique can be used to fabricate high-brightness p-side-up thin-film AlGaInP-based light-emitting diodes (LEDs) with an aluminum-doped zinc oxide (AZO) thin films transparent conductive layer deposited on a GaP window layer. The GaP window layer consist of the two different doping profile, the carbon doped Gap (GaP:C) window layer of 50 nm is on the top of Mg doped GaP window layer of 8 μm. The GaP:C window layer is used to improved the ohmic contact properties of GaP:C/AZO. The AZO with different cycle ratio of Zn:Al (15:1, 20:1 and 25:1) is deposited on GaP:C window layer as current spreading layer by atomic layer deposition. The AZO layer can be used to improve light extraction, which enhances light output power. The output power of p-side-up thin-film AlGaInP LED with an AZO layer of 20:1 cycle ratio has improved up to 19.2 % at injection current of 350 mA, as compared with that of LED without AZO film. The p-side-up thin-film AlGaInP LED with AZO current spreading layer exhibited excellent performance stability, the emission wavelength shift of p-side-up thin-film AlGaInP LED without and with AZO thin film(Zn:Al=20:1) are 17 nm and 3 nm under the injection current increased from 20 mA to 1000mA, respectively. This stability can be attributed to the following factors: 1) Refractive index matching, performed by introducing AZO thin film between the epoxy and the GaP window layer enhances light extraction; and 2) the favorable thermal dissipation of the silicon substrate reduces thermal degradation.
Recently, near-ultraviolet light-emitting diodes (NUV-LEDs) have been used in many applications such as light sources for ultraviolet curing, environmental cleaning, biomedical instrumentation, counterfeit bill detection and phosphor-based white LEDs. However, it is difficult to fabricate NUV-LEDs with high emission efficiency. As the wavelength of NUVLEDs decreases, the most dominant emission will be photons with transverse-magnetic (TM) polarization. For LED structures grown on a c-plane substrate, TM-light propagates mainly in the lateral direction, and it suffers strong effects of total internal reflection (TIR) due to the large incident angle on the interface. Therefore, light extraction efficiency (LEE) of NUV-LEDs is still lower than that of visible LEDs. In this study, a spin coating process in which the grating structure comprises the metallic nanoparticle layer coated on a p-GaN top layer was developed. Various sizes of metallic nanoparticles forming a suspended nanoparticle layer (SNL) embedded in a transparent conductive layer were clearly observed after the deposition of indium tin oxide (ITO). The SNL enhanced the light extraction efficiency of NUVLEDs. Light output power was 1.4 times the magnitude of that of conventional NUV-LEDs operating at 350 mA, but retained nearly the same current-voltage characteristic. Unlike in previous research on surface-plasmon-enhanced LEDs, the metallic nanoparticles were consistently distributed over the surface area. Device performance can be improved substantially by using the three-dimensional distribution of metallic nanoparticles in the SNL, which scatters the propagating light randomly and is coupled between the localized surface plasmon and incident light internally trapped in the LED structure through TIR.
GaN-based light-emitting diode (LED) was successfully fabricated using surface plasmons (SPs) excited at the metal/semiconductor interface. Normally, SPs excited by the interaction between light and metal surfaces were known to enhance the internal quantum efficiency of LEDs via the quantum-wells (QWs) and SPs coupling. The penetration depth of SPs fringing field for the Ag film was calculated as 42.0 nm at an emission wavelength of 460 nm. Thus, the distance between the QWs and metal was critical for QWs-SP coupling. However, the p-GaN layer in blue emissive GaN-based LEDs, which is a sandwich layer between the active and p-type conducting layers, must has a thickness of over 120 nm to ensure the sufficient mobility of carriers. In this study, an Ag nanoparticle layer was coated on p-GaN top layer as the grating structure. The light extraction efficiency of lateral conducting blue LED is expected to be enhanced by the surface plasmon-TM light mode coupling during the localized surface plasmon resonance. Compared to the optoelectronic performance of the conventional LED, the SP-enhanced LED shows the superior performance, even though the silver nanoparticles were placed at least 200 nm away from the quantum-well active layer. The output power of the SP-enhanced LED exhibited over 1.1 times in magnitude as compared with that of the conventional LED at 350 mA, while still keeping nearly the same current-voltage characteristic.
The growth of hexagonal GaN-on-Si(100) sample prepared by pulsed laser deposition (PLD) was employed in the
development of GaN-on-Si technology. In contrast to common GaN-on-sapphire and GaN-on-Si(111) technologies, the
use of the GaN film on Si(100) by PLD provides low-cost and large-area single crystalline GaN template for GaN
applications, via a single growth process without any interlayer or interruption layer. The evolution of GaN growth
mechanism on Si(100) substrate with various growth times is established by SEM and TEM data, which indicated that
the growth mode of the GaN films gradually changes from island growth to layer growth when the growth time increases
up to 2hrs. Moreover, no significant GaN meltback was found on the GaN sample surface due to the high-temperature
operation of PLD. The GaN sample was subjected to MOCVD treatment to regrow a GaN layer. The results of X-ray
diffraction analysis and photoluminescence measurement show the reliability of the PLD-GaN sample and are promising
for the development of the GaN-on-Si technology using PLD technique.
High performance of GaN light-emitting diodes with a finger-type embedded contact (F-LEDs) was demonstrated on a Cu substrate for improved thermal management and increased efficiency. In contrast to common sapphire-based LEDs (C-LEDs) and the wing-type embedded LEDs (W-LEDs), the use of the finger-type embedded contact not only reduces the effect of the current crowding of W-LEDs to achieve a uniform current injection but also eliminates the problem of light shading of C-LEDs to obtain more output power. At 350 mA, the output power of the three LEDs was measured to be 329.39, 284.52, and 236.38 mW for the C-LED, W-LED, and F-LED, respectively; representing that the F-LED in output intensity was raised 39.3% and 20.3% higher than that in the C-LED and the W-LED. Similarly, a 63.61% increase of output power of F-LED was obtained as compared to the C-LED case at 700 mA current injection. At this point, the efficiency droop of F-LED is 33.7%, which is lower than that of 44.1% and 53.5% in W-LED and C-LED, respectively; results are promising for the development of high performance LEDs using the finger-type embedded contact.
High performance 365 nm vertical-type ultraviolet light-emitting diodes (UV-LEDs) were developed using an embedded
self-textured oxide (STO) structure using metal-organic chemical vapor deposition system. From etch-pit-density results,
the dislocation densities of LED epilayers were effectively reduced to 5.6×106 cm-2 by inserting the STO structures due
to the relaxation of residual stress. The vertical-type UV-LEDs are fabricated using a combination technique of metal
bonding and sapphire substrate separation. When the UV-LEDs (size: 45 × 45 mil2) were driven with a 20 mA injection
current, the output powers of the LEDs with and without STO were measured to be 10.2 and 5.51 mW, respectively. The
external quantum efficiency of LEDs with STO exhibits 32% higher than that of LED without STO. As increasing
injection current to 350 mA, a near 45 mW light output was measured from STO-LED sample. This benefit was
attributed to the introduction of STO structure which can not only block the propagation of threading dislocations but
also intensify the light extraction of LED.
High indium content InGaN films were grown on sapphire substrate using low temperature pulsed laser deposition (PLD)
with nitrogen plasma and a specific target. The controllable target consists of two separate sections: an indium sheet with
periodic rectangular-holes and a standard GaN wafer. By changing the rectangular-hole area, a modulated indium vapor
was excited by pulsed laser and introduced into the InGaN deposition reaction, contributing the increase in the
incorporation of indium into the InGaN film. The structural and optical stability of the 33 and 60% indium InGaN
revealed no differences in the line-shape and peak position even after annealing at 800°C for 75 min from x-ray
diffraction and luminescence results. Moreover, such high thermal stability of 60% InGaN film was put in metal organic
chemical vapor deposition (MOCVD) to regrow GaN layer, the peak position of 860 nm remained unchanged after
MOCVD regrowth. The flat and uniform of regrown sample indicates that the PLD method used in this study is indeed
promising for the development long wavelength of high indium content InGaN emitters.
Gold-indium metal bonding method was used in this study to increase the product yield of vertical light emitting diodes
(LEDs) during laser lift-off (LLO) process. The vertical GaN LED transferred onto Si substrate presented good electrical
and optical properties due to the existence of high reflective mirror and texture surface. The chip size and dominant
wavelength for vertical type LED are 40×40 mil2 and 450 nm. The optimal conditions of temperature and pressure for
2-inch wafer bonding are set of 200oC and 100 kg/inch2, respectively. The products yield of light output power, forward voltage and leakage current are 96 %, 96.4% and 61.2%, respectively. After aging test, the characteristics decay of light output power, forward voltage and leakage current are less than 4%. Summarization of optical and electrical properties,
the total yield of these LEDs products is about 60 %.
High indium composition InGaN films were co-deposited on u-GaN templates using low temperature (300°C)
pulsed laser deposition (PLD). The du-composition target consisted of a 3-inch indium sheet drilled with periodic
rectangular holes mounted on a normal GaN wafer. By changing the ratio of the holes areas to total sheet area, the
indium concentration in two InGaN films was set to 33% and 60%. The structural and optical characteristics of these
films are investigated through isochronal and isothermal annealing. X-ray diffraction (XRD) and cathodeluminescence
results for the 33% sample exhibited no significant differences in line-shape and peak position even after annealing at
800°C for 100 minutes. In contrast, the XRD peak of 60% sample became broadened under the same annealing condition.
This slight inhomogeneity in composition also resulted in two visible peaks in the photoluminescence spectrum.
Although the optical properties of the 60% sample can be considered merely acceptable, the advantages of applying PLD
to the growth of high thermal stability and high indium composition InGaN have been made clear. The PLD technique
shows promise for developing long wavelength devices.
The β-Ga2O3 films were grown on (0001) sapphire at 500 °C by metal organic chemical vapor deposition. In the
analysis of crystal structure, we found that the (-201) oriented single crystal β-Ga2O3 epilayer can be obtained under low
chamber pressure of 15 torr. Moreover, a metal-semiconductor-metal solar-blind deep ultraviolet photodetector was
fabricated with the β-Ga2O3 epilayer. As the bias voltage is 5 V, the photodetector exhibits a relatively low dark current
about 0.2 pA, which induced by the highly resistive nature of the β-Ga2O3 thin films. From the responsivity result, it can
be observed that photodetector shows a maximum responsivity at 260 nm, revealing the β-Ga2O3 photodetector was
really solar-blind. The responsivity of the photodetector was as high as 20.1 A/W with an applied bias of 5 V and an
incident light wavelength of 260 nm. The improved performance is attributed to the high quality of β-Ga2O3 epilayer.
High performance 375-nm ultraviolet (UV) InGaN/AlGaN light-emitting diodes (LEDs) was developed using a
heavy Si-doping technique with metalorganic chemical vapor deposition (MOCVD). From the transmission electron
microcopy (TEM) image, the dislocation density was reduced after inserting a heavily Si-doping growth mode transition
layer (GMTL) between un-doped GaN layer and Si-doped Al0.02Ga0.98N contact layer. The internal quantum efficiency
(IQE) of the sample with GMTL measured by power-dependent photoluminescence shows 39.4% improved compared
with the sample without GMTL. When the vertical type LED chips (size: 1mm×1mm) driving by a 350-mA current, the
output powers of the LEDs with and without GMTL were measured to be 286.7 mW and 204.2 mW, respectively. As
much as 40.4% increased light output power was achieved. Therefore, using the GMTL to reduce dislocation defects
would be a promising prospective for InGaN/AlGaN UV LEDs to achieve high internal quantum efficiency.
The characteristics of high voltage LED consisted of an array of 64 micro-cells GaN LEDs was investigated through
using different substrate. In this study, two kinds of high voltage LEDs are presented; one is grown on sapphire
substrate and the other one is on mirror/Si substrate. The output power of high voltage LEDs with sapphire and
mirror/Si substrate is 170 and 216 mW at an injection current of 24 mA, respectively. The LEDs on mirror/Si
substrate leads to the superior performance in output power as compared with one on sapphire substrate is attributed
to the improvement of thermal dissipation and light extraction.
GaN epilayer can be grown on sapphire substrate with a Ga2O3 sacrificial layer. It was employed for the epilayer
transferring from sapphire substrate to Cu substrate using the chemical lift-off process application. The (-201)
oriented β-Ga2O3 thin film was first deposited on the c-plane sapphire substrate, followed by the GaN growth
via metalorganic vapor phase epitaxy under N2 and H2 environment in sequence. The crystal quality of GaN
epilayer can be improved dramatically with the regrowth in a H2 ambient. A GaN epilayer with an electroplated
copper substrate was demonstrated using a chemical lift-off process where the Ga2O3 sacrificial layer can be
laterally etched out with a hydrofluoric solution. It is worthy to mention that the separated sapphire substrate can
be cleaned and reused for LED epitaxial growth next time. It is benefiting the cost down for the LED fabrication
and Green Photonics Development.
Al-doped ZnO can replace tin-doped indium oxide (ITO) as a good transparent conductive oxide (TCO) in LEDs and
optoelectronic applications. We investigate on nanometer scale AlZnO thin film materials epitaxied on sapphire
substrates in 350-650°C from pulsed laser deposition (PLD). Synchrotron radiation X-ray absorption fine-structure
spectroscopy on O K-edge indicates that Al-doped ZnO can not form alloy at growth temperature 350°C without Al-O
bonding feature. The Al-O transition of AZO550 is stronger than AZO650. These are correlated to Raman scattering
measurements and analyses. Al-doped ZnO grown at 350°C possesses weak/broad Raman signals indicating a poor
crystalline film. The E2 (high) mode is strong and narrow in AZO550. All these experimental results indicate that PLD
grown AlZnO film on sapphire could get a better crystalline quality at 550°C than 350°C and 650°C.
We have performed a comparative structural and optical investigation of InGaN/GaN MQW LED wafers grown on (1122) facet GaN/sapphire templates by Metalorganic Chemical Vapor Deposition. The effect of the growth time of epitaxial
lateral overgrowth GaN with (11 2 2) facets on the structural and optical properties were investigated via
photoluminescence (PL), PL excitation, time-resolved PL, Raman and SEM measurement on two typical InGaN MQW
samples. From temperature dependence PL measurement, we can find that these two samples both exhibit two
distinguish peaks attributed to quantum wells with wide range wavelength. Raman E2 (high) signals revealed a partially
relaxation of compressive stress in the facet GaN template. Experimental observations have revealed: (a) a Stokes shift
between the emission peak energy and absorption edge and (b) a red shift behavior of emission with decay time
(equivalently, a rise in decay time with decreasing emission energy). The large Stokes shift can be attributed to the
quantum confined Stark effect (QCSE). The lower-energy side of the InGaN main emission peak is governed mainly by
carrier generation in the GaN barriers and subsequent carrier transfer to the InGaN wells. Different amounts of Stokes
shift indicate the interface imperfection from longer growth time of epitaxial lateral overgrowth GaN with (1122) facets.
Temperature dependence of PL decay time τPL shows an interesting behavior of τPL with temperature.
A set of AlGaN epilayers were grown on sapphire (0001) substrate by MOCVD, with intermediate growths of low/high
temperature AlN nucleation layers. Variable flow rates of trimethylindium (TMIn), 0, 50 and 500 sccm were introduced
during growth. Three AlGaN samples were originally designed with similar Al composition of ~20%. Rutherford
backscattering (RBS), RT and 10-300K photoluminescence (PL) were used for analyzing the microstructure of thin
films. The Al content was calculated to decrease with increasing the In-flow rate. Main PL bands spread over 310-350
nm with peaks in 320-335 nm. PL (10-300K) exhibited anomalous temperature dependent emission behavior
(specifically an S-shaped shift, i.e. red-blue-red shifts) of the AlGaN related PL emission. Carriers transfer between
different luminescent centers. Abnormally high activation energy was obtained, which shows that the excitons are not in
the free states. Raman Scattering and spectral line shape analysis leaded to an optical determination of the electrical
property free carrier concentration of AlGaN. Our results on In-doped AlGaN provide useful information for designing
UV-LEDs.
Chemical lift-off (CLO) technique has been paid more attention since no damages will be
induced to GaN epi-layer during the epilayer lift-off process. In this study two novel CLO
approaches were used to separate GaN epilayer from sapphire substrate. One is using Ga2O3sacrificial layer deposited by pulsed laser deposition. The other is using a stripe patterned SiO2grown by PECVD. Afterwards, the CLO of GaN epilayers grown on these two templates via metal
organic chemical vapor deposition from sapphire substrate was successfully realized with a
hydrofluoric acid as an etchant.
In this paper we propose a promising die-bonding material consisted of diamond-added AgSnCu
solders for chip package, and combine it with our well-established cup-shaped copper technique for
chip heat dissipation. The composite solder was prepared by mixing the commercial Sn-3wt.%Ag-
0.5wt.%Cu (SAC305) solder paste with the diamond paste [0.25(W)475-MA, Engis, USA] in a
weight ratio of 10:1. Thermal resistance analysis shows that total thermal resistance of the LED
packaged using the composite solder is only 6.4 K/W, which is much lower than both the LED using
AgSnCu solder (9.2 K/W) and the LED using silver paste (10.4 K/W). As a result, the LED with the
composite solder exhibits larger light output power and smaller junction temperature than the other
two samples. The improved device performance is mainly due to enhanced heat dissipation of the
die-bonding materials used. These results suggest that the composite diamond-added AgSnCu solder
is promising in high-power LED application.
In this study, two approaches using various insertion structures are proposed for the near-UV LEDs. One is through a
single MOCVD process where a heavily Mg-doped GaN insertion layer (HD-IL) technique is employed to improve
crystalline quality of the GaN layer and followed by rest of required GaN-based LED structure. Another approach was
demonstrated by the near-UV LEDs with an embedded distributed SiO2-disk structure. The periodically spaced
hexagonal disk-shaped SiO2 mask array was deposited on the GaN/sapphire template and followed by the MOCVD regrowth
process. These improvements contribute the high-performance 380-nm LEDs with enhanced output powers by
20-40% in magnitude.
The p-side-up GaN light-emitting diode (LED) were fabricated using a combination of omni-directional reflector (ODR)
and double-sided textured surface (both p-GaN and undoped-GaN) techniques. An Essential Macleod program was used
to simulate the optimum thickness of the ODR structure. The reflectivity value of ODR structure used in work can reach
99%. On the top-side textured surface, the p-type GaN with hexagonal cavities was grown under low temperature
conditions using metalorganic chemical vapor deposition. The GaN LED with a suitable low-temperature p-GaN cap
layer thickness was also studied. Experimental results indicate that the LED sample with a 200-nm hexagonal cavity
GaN layer on the surface exhibits a 50% enhancement in luminance intensity. For a small chip size of 250 μm×500 μm,
the luminance efficiency can be improved from 23.2 to 28.2% at 20 mA. However, the luminance efficiency with a
larger chip size of 1mm×1mm can be improved from 19.8 to 28.9%. This indicates that the thin-film structure can
enhance the light extraction efficiency of GaN-based LEDs, especially for the large chip sizes.
AlGaInP light emitting diode (LED) with a mirror substrate has been successfully fabricated by wafer bonding. The bonding technique using a metallic interlayer has been developed to eliminate handling the fragile, free-standing epilayers. Various structures of the mirror substrate have been studied, and a suitable structure of Au/AuBe/SiO2/Si is proposed. From the observation of the chip fabrication process, it was found that the SiO2 layer could isolate the stress causing from the Si substrate. The device performance of bonded LED is obviously far superior to that of the standard absorb-substrate LED. It exhibits normal p-n diode behavior with a low series resistance. Moreover, the emission wavelength of the bonded LED was independent of the injection current. The low forward series resistance and a good heat sink provided by Si substrate solve the joule heating inhering in conventional LED problem. Furthermore, the bonded LED with high reliability has been demonstrated.
The role of temperature ramping rate during the two-step growth of GaN-on-sapphire by metalorganic chemical vapor deposition is explored. The surface morphology and crystalline properties of the GaN buffer layer annealed under various temperature ramping rates (20-60 degree(s)C/min) were investigated by atomic force microscopy and x-ray measurements. For the lower ramping rates employed, a dramatic re-evaporation of the GaN buffer layer was observed. This makes the buffer layer thinner, yielding the GaN epilayer of hexagonal morphology. However, as the higher ramping rates applied, the surface becomes rougher and exhibits hexagonal three-dimensional islands. It could be due to the fact that the grains of the GaN buffer layer have no enough time to course. Under a temperature ramping rate of 40 degree(s)C/min, a smooth buffer-layer surface can be maintained and result in a subsequent high-quality over- layer deposition. The mirror GaN epilayer shows a near-band- edge peak (25 K) centered at 3.477 eV with a full width at half maximum as narrow as 13.1 meV. The observed temperature-ramping-rate effects can be interpreted by the coalescence mechanism of the GaN buffer layer involving Ostwald ripening, sintering and cluster migration.
Amorphous, hydrogenated silicon carbide (a-SiC:H) deposited by plasma-enhanced-chemical-vapor deposition has been used as the most important film of passivation layers in a thermal ink-jet printhead. When the printhead was thermal- cycled from room temperature to about 400 degree(s)C, the a-SiC:H film is sustained by a variety of thermal and mechanical stresses that are detrimental to it's integrity. Thermal stress changes of a-SiC:H films were varied with different CH4/SiH4 gas ratios. Microstructure investigation was mainly achieved by FTIR technique. Less variation of the Si-H absorption bond causes less thermal stress change. Thin-film adhesion is an important problem in thermal ink- jet printhead between the Ta thin film and a-SiC:H films. A qualitative measure of film adhesion can be made with the scratch tester. The adhesive critical load and Ta coating failure modes on a-SiC:H were acquired to examine the film adhesion on these two investigated films. The adhesion depends on the nature of the interfacial region, which depends on the interactions between the depositing Ta thin film and the surface a-SiC:H films. An increased effective contact area in the interfacial region promotes a good adhesion.
We present a new method to fabricate silicon microfluidic channels by through-hole etching with subsequent planarization. The method is based on etching out the deep grooves through a perforated silicon carbide membrane, followed by sealing the membrane with plasma-enhanced chemical vapor deposition (PECVD). Low-pressure-chemical-vapor- deposited (LPCVD) polysilicon was used as a sacrificial layer to define the channel structure and only one etching step is required. This permits the realization of planarization after a very deep etching step in silicon and offers the possibility for film deposition, resist spinning and film patterning across deep grooves. The process technology was demonstrated on the fabrication of a monolithic silicon microchannel structure for thermal inkjet printing. The Ta-Al heater arrays are integrated on the top of each microchannel, which connect to a common on-chip front-end ink reservoir. The fabrication of this device requires six masks and no active nozzle-to-chip alignment. Moreover, the present micromachining process is compatible with the addition of on-chip circuitry for multiplexing the heater control signals. Heat transfer efficiency to the ink is enhanced by the high thermal conductivity of the silicon carbide in the channel ceiling, while the bulk silicon maintains high interchannel isolation. The fabricated inkjet devices show the droplet sizes of 20 - 50 micrometer in diameter with various channel dimensions and stable ejection of ink droplets more than 1 million.
In a thermal bubble inkjet printhead, thin film resistor plays the most important role. The thin film resistor must fire about 1 billion times during the device operation. During firing, temperatures are up to 400 degrees Celsius and the duration is on the order of microseconds. In this situation, the resistance stability and mechanical property of the thin film resistor are critical for fabrication. In this study, resistivity and stress behavior related to a wild range of Ta- Al compositions are examined.
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