Silicon carbide has been widely used as material for manufacturing yellow, red, green LED and optoelecronics devices (in¬dicators, screens). The silicon carbide LED technology has been in¬vestigated for improvement of their operational characteristics. This in¬cludes the influences of the surface processing (etching, annealing), the formation method for the p-n junctions and the contacts on the LED properties. Light-emitting devices used as light sources for optical-fiber communication lines. LED fabricated by Al⁺ ion-implanted in 6H-SiC and investigated their characteristics for an effective green LED. The brightness of the ion-implanted p-n junction was found to be two orders higher than that of diffusion p-n junction, and the best value was 2000-10000 cd/m² with passing current about 0.5 mA through area 50x50 mm and applied voltage about 2.6 ± 0.2 V. The ion-implanted structures showed a high stability of light in the temperature range of 77-600 K

Vlaskina, S.I.

Semiconductor Physics Quantum Electronics & Optoelectronics

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

71' 2002, Institute of Semiconductor Physics, National Academy of Sciences of UkraineSemiconductor  Physics, Quantum  Electronics & Optoelectronics. 2002. V. 5, N 1. P. 71-75.1. IntroductionSilicon carbide LED first became a significant commer-cial success when ÑREE (USA) got them on the market inthe early 1990s. Today CREEs CB series of super brightLED are a new generation of solid-state LED emitters,which combine highly efficient InGaN with Crees SiCsubstrate. Crees main LED products now have SiC as asubstrate for gallium nitride or indium gallium nitride asactive ingredients, and emit blue as well as green andbluegreen luminescence.  Yellow silicon carbide LEDshad been made around 1970  1975  in the former SovietUnion. Diffusion p-n junctions had been applied to manu-facturing this LED devices. Single crystalline SiC withND - NA=(1~5)•1018 cm-3 was suitable for manufacturingsuch devices. The diffusion factors depend on the con-centrations of the impurities in the crystals.  The processof p-n junction creation is rather  complicated becausethe usual impurity concentrations in crystals range from1017 to 1018 cm- 3.The optimal depth (1µm) of the p-n junc-tion had been obtained with industrial crystals [1].To  obtain a high manufacturing yield for LED withPACS: 77.55.+f, 77.80.Silicon carbide LEDS.I. VlaskinaInstitute of Semiconductor Physics, NAS of Ukraine, 45 prospect Nauky, 03028 Kyiv, UkrainePhone: +380 (44) 269 3792; fax +380 (44) 265 8342; e-mail: businkaa@mail.ruDong Seoul College, 461-714, 423, Bokjung-Dong, Sungnam-city, Kyonggi-do, KoreaPhone: 82 (031) 720 2141; fax: 82 (031) 720 2261; e-mail: svitlana@haksan.dsc.ac.krAbstract. Silicon carbide has been widely used as material for manufacturing yellow, red,green LED and optoelecronics devices (indicators, screens). The silicon carbide LED technol-ogy has been investigated for improvement of their operational characteristics. This includesthe influences of the surface processing (etching, annealing), the formation method for the p-njunctions and the contacts on the LED properties. Light-emitting devices used as light sourcesfor optical-fiber communication lines. LED fabricated by Al+ ion-implanted in 6H-SiC andinvestigated their characteristics for an effective green LED. The brightness of the ion-im-planted p-n junction was found to be two orders higher than that of diffusion p-n junction,and the best value was 2000-10000 cd/m2 with passing current about 0.5 mA through area50x50 µm and applied voltage about 2.6 – 0.2 V.  The ion-implanted structures showed a highstability of light in the temperature range of 77-600 K.Keywords: silicon carbide, LED, ion implantation.Paper received 05.02.02; revised manuscript received 08.02.02; accepted for publication 05.03.02.uniform light, the following technique had been applied.Diffusion of Al with O was carried out for 2 hr at 1700°C.Then, B and Al were introduced for 15 min at 1600°C.The sources for O, Al, and B were SiO, 99.99% Al andB2O3, respectively. After the diffusion processes, ap-preciable changes in the surface morphology or the car-bon traces were observed using optical microscopy. Thelow-temperature (77 K) and the high-temperature (300K) photoluminescence in the ultraviolet wavelength rangeshowed repeatable and clear spectra for the uniformlydoped samples. Diffusion of Al and O above 1700°C re-sulted in a deterioration of the surface conditions, anddiffusion of B and Al at higher temperatures made thephotoluminescence spectra less clear and the surface condi-tions even worse. The time at optimum temperature couldbe adjusted to change the color of electro-luminescence fromyellow-green to red (in the  wavelength range between 5600and 6000 Å). In the case of a yellow-green color (5600 Å),the time for the diffusion of B and Al turned out to be 20-25 min. A further increase in the diffusion time causedthe p-n junction to be less sharp as well as the voltage dropon the p-layer to increase, and, consequently, spreading of72 SQO, 5(1), 2002S. I. Vlaskina: Silicon carbide LED the current carriers on the p-layer and less clear light draw-ings on the displays. At the shortest diffusion time (5 min)for B and Al, the thickness of the p-n junction was too smallto assure an effective LED. The parameters of such LEDand indicators, which had been fabricated, are listed in Ta-ble 1. Usage of another SiC polytypes (4H, 3C, 15R) allowsto change the color of light, too.Because of the extreme stability of silicon carbide, it isnot necessary to dope the crystal by thermal diffusion. In-stead, dopants can be introduced by ion implantation. Onceimplanted into the crystal, the dopant atoms occupy inter-stitial positions in the lattice and must be transferred to sub-stitution sites to become electrically active. This activa-tion is accomplished by high temperature annealing in aninert ambient such as argon.Nitrogen and phosphorous are typical n-type dopantsin SiC.Aluminum and boron are p-type dopants. Implanta-tion is usually conducted with the sample at elevated tem-peratures (600 - 800 °C) to provide some in-situ anneal-ing of lattice damages caused by the implant. The im-planted sample is subsequently annealed at temperatures1000  1700 °C for times 5 and 90 minutes to activate thedopants. The dynamics of activation depends both on thedopant species (i.e. aluminum and boron, nitrogen andphosphorus) and upon the SiC polytype (i.e., 4H or 6H).Activation of nitrogen implants in 4H-SiC requires higherannealing temperatures than those in 6H-SiC.Phosphorus is an excellent n-type dopant in 4H-SiCwhen implanted at high doses, such as for source anddrain regions of MOSFET. p-type dopants, aluminumand boron, require much higher temperatures for efficientactivation and temperatures above of 1650 °C arenescessary for that.  Aluminum implants typically achievethe same degree of activation at annealing temperaturesabout 100 °C lower than boron. At any given annealingtemperature, there exists an optimum annealing time [2].This article reports results of SiC green LED electrical char-acterization. This LED had the same brightness as CREEsLED, but higher stability, more simple design (withoutAlGaN or GaN layers) and can be used in various usualapplications.2. ExperimentGreen LED had been prepared by the method of ion-implantation of impurities into 6H-SiC crystals.  Al+-im-planted for p-n junctions in 6H-SiC substrates and theircharacteristics were investigated as an effective LED. Theion-implantation was carried out on polished n-type 6H-SiC crystals and on epitaxial layers of 6H-SiC with an im-purity concentration of ND - NA = (2-8)•1018 cm-3. Substratewas doped by donor nitrogen. Implantation was performedwith 80 keV Al+ by using an ion accelerator. The im-planted depth was 0.5 µm, and the concentration of Alwas 1020 cm-3.The optimum implantation time and the substratetemperature were 2 min and 660°C,  respectively.Despite the low resistance of the p-layers, a thin high-resistive layer of C was formed on the surface of the crys-tals after implantation. This C film reduced the adhesionof the metals deposited for the contacts, increased theresistance of the contacts to the p-layers, and worsenedthe light uniformity. The crystals were annealed at 800 -1100 °C for 2-10 min to remove the C layer and to reducethe quantities of radioactive defects. Before the LED fab-rication process, the crystals were etched in an acid mix-ture of HF and HNO3.Contacts were prepared by deposition of Al layer onp-SiC heated to the temperature 550 °C in vacuum (im-planted and annealed). Al contact thickness was less orabout 4000 Å. Ni layers was deposited for protection ofAl layer from oxidation and for better contact with wireduring LED manufacturing. Thickness of Ni layer wasabout 2000 Å. Contacts to the n-SiC substrate were madeby  laser using Ti  and Ni wire. Contact of necessaryconfiguration were made by photolithography meth-ods. Photos of prepared LED are shown in Fig.1 (a, b, c).Device Light Light Working Brightness Size ofvoltage current voltage (cd/m2)           light area(V) (mA)    (V) (µm2)LED indicator(10 elements) 2.2   5  2.4 – 0.2 30 - 80 800 x 300LED matrix 120 x 120(64 elements) 2.5 – 0.1    0.5   3.0 – 0.2 60   step 100LED break with 40  x 40high resolution 2.4 – 0.2   2.5   3.1 – 0.1 70   step 60(64 elements)LED break 100 x 100(100 elements) 2.4 – 0.2  0.5    2.8 – 0.1 80   step 100LED break with 40 x 40face conclusion  step 60of light (step 100)Table 1. LED (diffusion p-n junction) on 6H-SiC.S. I. Vlaskina: Silicon carbide LED73SQO, 5(1), 2002               Fig. 1. Micro photo of ion-implanted LED:magnitude: a - x75; b - x380; c  x730.Just this LED was made on  n-epitaxial layer grown byTairovs method. The boundary between n-layer andimplanted p-layer is shown. The block structure ofepilayer and p-n junction is also shown. Single crys-tals grown by the Lely method had been used for such-10-9-8-7-6-5-4-3-2-10.0 1.0 1.8 2.4 3.4 5.0 8.0 13.0 18.0Voltage (V)lgI(A)Fig. 2.Voltage-current characteristic of ion implanted p-n junc-tion in 6H-SiC.012345678910111213140.0 0.8 1.8 2.3 3.2 4.5 7.0 12.0 17.0Voltage (V)dlgI/dV(r.u)Fig. 3. Derivation from voltage-current characteristic of ion im-planted LED.LEDs, too, but photos of epilayers with implantedaluminum are more interesting than these of single crys-tals with p-n junction.The voltage-current characteristics of ion-implantedp-n junctions are shown in Fig. 2.The differential degree: α = dlgI/dV is shown in Fig. 3.In the low voltage area (at a  voltages less then 0.08V) αis about unity. Determined from this linear site leakage is(1-2)•104 Ω. The reason of such resistance is carbon or sili-con oxide on the surface of LED.In the region of very small voltages:                I=I0 exp(V/VT).This circumstance, as well as independence of VT ontemperature permits to think about tunnel character ofabc74 SQO, 5(1), 2002S. I. Vlaskina: Silicon carbide LEDN about (1-4)•1020 cm-3 is indicative of a rather high dop-ing level of the junction. The ideality factor 1.9 to 3.2was obtained from the following exponential part of theforward bias current-voltage characteristics. The forwardsaturation current was (3-5)•10-16 to (3-5)•10-8 A/cm2 andforward turn-on voltage of 0.8 to 2 V (at current densityabout (5-8)•10-8 A/cm2. Reverse biasing produced averageleakage currents were that were of the order of 10-8to10-3 A/cm2 (at 10 to 20V reverse bias). The ideality factor had some trends. Namely, theideality factor is decreased and forward current densityis increased with increasing operation temperature in therange of 20 °C to 400 °C. The ideality factor is of theorder of 2 for most LED suggesting that recombination/generation current is dominant over diffusion current.The ion-implanted p-n junctions are sharp, and the injec-tion of carriers as well as their recombination in the p-layer prevail. The sharpness was maintained in the tem-perature range between 77 and 500 K. Recombinationhappens in the layer of the volume charge in the p-n junc-tion at low levels of injection, but the recombination pre-vails in either the p- or the n-area at high levels of injec-tion, depending on the doping degree.In the region of voltages changing from 2.2 V to 18 Vthe α  degree is reduced passing the flat minimum. Then αdegree (Figs 2, 3) is again increased and can be describedby the expression:++=∞VBIVAIV41021The square-law part of the voltage-current charac-teristics enables to calculate the major carrier lifetime nearthe cathode:sVLkn80210)2.18.0(2−⋅−==µτ(µn=100 cm2/Vs).That is in good agreement with the experimental data oftransitive characteristics, where ignition time is (2.5-3)•10-8 s and emission time is (1-1.5)•10-8 s.  The light-brightness characteristic of the ion-implantedp-n junction is shown in Fig. 4. The differential degreeβ=dB/dlgI is shown in Fig. 5. The linear region extendsfrom 7•10-4 A to 10-2 A/cm2 .The high stability of the light,even at 77-600 K, is another characteristic for the ion-im-planted structures, which is understood mainly by the roleof the radioactive defects in the emission. The concentra-tion of defects is constant in the temperature range of 77-600 K.The destructive features of the ion-implanted structuresproduced narrow lines in the electro-luminescence spectraat relatively small current (Fig. 6) A correlation between01000200030004000500060007000-6.1 -4.7 -4.6 -3.9 -3.2 -2.1 -1.5lgI (A)B(cd/m2)Fig. 4. Ion-implanted p-n junction light brightness characteristic0.00.51.01.52.02.53.0-5.80 -4.73 -4.05 -3.42 -1.94lgI (A)β(r.u.)Fig. 5. Derivation from brightness-current characteristic of ion-implanted p-n junction.the narrow lines in the spectra and the brightness of LEDhad been established; most of the effective LED had narrowlines at  low temperatures. It is a very convenient way toselect crystals for green highly bright LED. When thetemperature increases, the narrow lines disappear from thespectra. The fact that the sharp-line structures of theelectro-luminescence spectra coincide with those of thephotoluminescence spectra at low temperatures (77 K) isan additional indication to support the suggestion thatthe characteristics of the spectra are due to radioactivedefects.The ion-implanted structures have much faster re-sponse times than the diffused ones (Table 2). The switch-ing time is close to 10-8 s, which provides wide op-portunities for the applications of such LED to opticalcouples and optical-fiber communication lines. The22232)(1TnnVhmdnnneNnsεpiρ== ∫this part of the voltage-current characteristics. Concen-tration evaluated from a volume charge is as follows:S. I. Vlaskina: Silicon carbide LED75SQO, 5(1), 2002Fig.6. Ion-implanted p-n  junction electroluminescence spectrum.brightness of the diffused LED is 30-100 cd/m2 at10 mA/mm2, while for the ion-implanted it is 500-600 cd/m2 at 10 mA/mm2 and even (2-10)•1000 cd/m2 forthe best samples.On the other hand, ion-implanted p-n junctions can beapplied widely in optical-fiber communication, especially,short communication lines, because of their high speed(10-8 s), high stability of light for 10,000 hrs, linear bright-ness-current characteristics up to a current density of10 A/cm2, high brightness, absence of brightness deg-radation, and stability of characteristics over a wide in-terval of temperatures.Switching  Switching Working     Bright-      Size of     Switching  voltage     current     voltage     ness      light area     time      (V)          (mA)      (V)      (cd/m2)      (µm2)         (s)  2.6–0.2         0.5 3.2–0.1  2000-4000  50x50         0.15         80x50     0.01Table 2. LED for the fiber communication line3. ConclusionThe characteristics of silicon carbide LEDs were in-vestigated. These included the influence of surfaceprocessing, the formation method of the p-n junctions,and the contacts on the properties of the LED. Green SiCLEDs can be used as light sources for optical-fibercommunication lines, in the traffic lights, as indicators,in screens and so on. p-n junctions were fabricated by Al+ion-implantation 6H-SiC, and annealed at 800-1100°C.Characteristics for using as effective green LED had beeninvestigated. The brightness of the best ion-implantedp-n junction was found to be about 2000-10000 cd/m2.The ion-implanted structures showed a high stability oflight in the temperature range of 77-600K.The pulsing characteristics of these LEDs based onion-implanted structures had a response time close to10-8 s.References1. S.I.Vlaskina, K.W.Kim, Y.S.Kim, Y.P.Lee, G.S.Svechnikov,Optoelectronics Devices on Silicon Carbide // Journal of theKorean Physical Society, 30(1),pp.117-121(1997).2. M.A.Capano, R.Santhakumar, J.A.Cooper,Ir., andM.R.Meloch, SiC Ion Implantation Research at Purdue, USA(internet information), 2002., rel. un.0.01.0

Silicon carbide LED

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Silicon carbide LED

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)