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The study of high-brightness and high-power InGaAIP red LED
Abstract
It is reported that the design and fabrication of high-brightness and high-power InGaAlP ring-shaped interdigitated red LED. High-brightness and high-power InGaAlP LED is a new kind of visible light LED developed in recent years, which is driven by large current capacity, high luminous efficiency and excellent heat resistance. It has been used in various fields, such as large area displays, traffic lights, back-lighting, aviation lighting and so on. As compared with the conventional LED chip, the ring-shaped interdigitated LED chip is more flexible to integrate with other devices, and it leads the more uniform current spreading. The InGaAIP LED epitaxial layer has six layers. From top to bottom, they are the window layer (p-GaP), the p clad layer(p-InGaAlP), the active layer between clad layers (i-InGaAlP), the n clad layer(n-InGaAlP), distributed Bragg reflectors (DBR) and the n-Substrate(n-GaAs). The epitaxial wafer is tested by scanning electron microscope (SEM) and X-ray double crystal diffraction. The results show the interface of materials is flat, and the integrality and quality of the epitaxial wafer are optimum. The size of chip is 1 mm2. The fabrication of ring-shaped interdigitated LED chip, essentially, is the same as conventional LED chip, involving photolithography, PECVD SiO2. wet etching, evaporation, lift off and rapid thermal annealing using four masks. To control the widths of mesa and n area precisely, the selecting etch technique has been adopted, using HCL;H2Oand H2O2 as an InGaAlP etching solution, and the chip is protected by SiO2 and single layer photoresist during the etching. The fabrication of III-V compound semiconductor p-type ohmic contact is more difficult than the fabrication of n-type Ohmic contact. So how to fabricate p-type ohmic contact is a second important technology. AuZn/Au is used to be the p-contact metal in this study, and the chips are sintering for 20 s at 400 °C in N2. The I-V characteristics, light emission spectrum, luminous flux, luminous intensity of this LED have been measured. A good characteristic is obtained with turn-on voltage of 1.5 V and forward current of 500 mA at its forward voltage of 3 V. The peak wavelength is 635 nm, which corresponds to red light, and the full width of half maximum is 16.4 nm at injection current of 350 mA. The luminous intensity is 830 med. The color coordinates is x=0.694 3,γ=0.305 6 and the color index is 18.4. So we will conclude that the high-brightness and high-power InGaAlP red LED is the first step for a wide scope of general illumination with LED in the future, and it will become new focus in both scientific research and industrial investment for its wide application.
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Al x Ga y In 1-x-y P with x+y=0.51, lattice matched to the GaAs substrate, has been grown by organometallic vapor phase epitaxy. The simple, horizontal, IR heated system operates at atmospheric pressure using the reactants TMAl, TMGa, TMIn, PH 3 , and H 2 . Alloys giving room temperature, band edge photoluminescence (PL) have been grown in the temperature range 625–780 °C. Emission wavelengths as short as 5820 Å (2.13 eV) are reported. High growth temperatures are found to give layers with the best surface morphology and PL intensity. The use of high temperatures is especially important for high Al content alloys. PL at x=0.2 is obtained only for temperatures above 740 °C.
We present the weighted-sum expression of quaternary parameter estimations via balancing surface bowing estimation errors. Expressions have been derived for both quaternary alloys AxByCzD and AxB1-xCyD1-y and applied to the band gaps of AlGaInAs, AlGaInP, InAsPSb, GaInAsP, and GaInAsSb. Approximation can be improved by fitting known data of quaternary alloys and maximum deviations are less than 20 meV with respect to the lattice-matched polynomial expressions.
The relation of contact resistance to heat treatment and microstructure was examined for the three following ohmic contact metallizations on n-GaAs: 200 Å Ge/200 Å Ni/2000 Å Au, 200 Å AuGe/200 Å Ni/2000 Å Au and 1000 Å AuGe/200 Å Ni. After an initial anneal at 440°C, the Ge/Ni/Au metallization had the lowest contact resistance and the highest proportion of the NiGeAs phase at the GaAs/contact interface. It was also the most stable with subsequent ageing at 330°C, this stability being attributed to the NiGeAs phase acting as a diffusion barrier at the GaAs/contact interface.
Electroluminescence efficiency of GaAlAs heterostructure light‐emitting diodes (LED’s) fabricated by the temperature difference method is discussed by measuring electroluminescence, carrier lifetime, and absorption coefficient of the active layer. Deep levels, distribution of carriers in the indirect valley and the reabsorption of the emitted light in the active layer contribute to the reduction of the efficiency of the red LED’s. The measured absorption coefficient of Ga 0.65 Al 0.35 As active layer is about 103 cm-1 at 6500 Å; therefore the active layer thickness should be designed to be less than about 10 μm, to minimize the reabsorption region of the emitted light. The maximum luminescence efficiency is calculated for GaAlAs double heterostructure LED’s, taking the active layer thickness and the potential barrier height into consideration.
Efficient red GaP LED's have been fabricated by compensating the p side of the junction with a donor. The observed efficiency in the diodes fabricated remains invariant over a wide range of net acceptor concentration. Utilizing the recombination kinetic analysis of Jayson, Bhargava, and Dixon, it is shown that better injection efficiencies and higher ZnO complex concentrations are the cause for this invariance. The compensation technique potentially offers a commercial LPE growth process from which high‐efficiency low‐cost LED's could be fabricated.
As part of the investigation of the use of AuNiGe as the ohmic contact to n‐type GaAs at a high integration level, cross‐sectional transmission electron microscopy was used to explore the uniformity at the metal/GaAs interface and the thermal stability of the AuNiGe contact after the ohmic contact formation. A close relation between spread of the contact resistance and nonuniformity of the interfacial microstructure of the contact was found. Deposition of 5‐nm‐thick Ni as the first layer of the AuNiGe ohmic contact significantly reduced the spread of the contact resistance and led to the formation of a uniform interface without large protrusions. The improvement in uniformity of compound distribution and the reduction of interface roughness are believed to be due to a change in the sequence of alloying reactions, compared to those in the contact without a Ni first layer. This suggests an ideal interface structure for a low resistance AuNiGe ohmic contact after alloying to be a uniform two layer structure: a high density of the NiAs(Ge) grains contacting the GaAs substrate, and a homogeneous β‐AuGa phase close to the top surface. However, due to the existence of β‐AuGa phases with a low melting point of around 375 °C, the thermal stability of the contact at 400 °C is of serious concern. Segregation of the NiAs(Ge) grains was observed after annealing at 400 °C for 10 h, which reduced the contact areas between the NiAs(Ge) grains and GaAs. During subsequent annealing at this temperature for up to 90 h, liquidlike flow of the β‐AuGa phase was observed which deteriorated the interface uniformity, causing an increase in contact resistance. A typical contact edge slide distance after contact alloying at 440 °C for 2 min was measured to be 0.2 μm and the longest distance among specimens examined was 0.47 μm. This edge deteriorati-
on could limit the use of the AuNiGe contact in GaAs submicron devices.
were determined for the LED and MCLED, respectively. In order to explain this difference, we measured h QE for devices with different sizes. From a fitting procedure based on a simple model taking into account the device size, we found that the radiative efficiencies of LEDs and MCLEDs were close to 90%. We concluded that the low h int app of MCLED was due to a bad current injection, and especially to electron leakage current, as confirmed by numerical simulations. © 2002 American Institute of Physics. @DOI: 10.1063/1.1433938#