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UV Light Emitting Diodes –Their Applications and Benefits

Tim Bettles,* Sandra Schujman, Joseph A. Smart, Wayne Liu and Leo SchowalterCrystal IS, Inc., Green Island, NY 12183

* Corresponding Author: Email: bettles@crystal-is.com

ABSTRACTThe field of ultraviolet (UV) processing and analytics is becoming more and more widespread. The light sources for theseapplications remain based on the old technology of mercury lamps, while, for some applications, there could be many advantagesof having a low voltage, small light source with tunable emission wavelengths.

We report on ultraviolet light emitting diodes (UV-LEDs). LEDs are becoming commonplace in the visible light applications, butthere remain many technical barriers when transferring this technology into the shorter UV light ranges. Recent advances haveproduced UV-LEDs fabricated on bulk native AlN substrates yielding devices emitting in the 280 nm to 340 nm range. Devicesare fabricated using standard semiconductor processing techniques in various geometries to accommodate specific imagingapplications. These UV-LEDs are compact, rugged and efficient, enabling new applications in existing markets as well as openingnew market areas.

KEYWORDS UV Light Emitting Diodes; Nitrides; Native Substrates;

INTRODUCTIONUltraviolet (UV) light, from 170 nm to 395 nm, isemployed in a wide range of applications as diverse asbiochemical analysis in laboratories, curing of epoxies inindustry, detection of watermarks used for counteringmoney forging, medical treatment of some skin conditions,water purification and generation of white light byphosphor excitation, as in fluorescent tubes. The lightsources for all these applications are still based on the oldtechnology of plasma emission, with most of the marketcovered by mercury based lamps. These light sourcesusually require the application of a high voltage, work athigh temperatures and the emission wavelength is fixed,determined by the composition of the excited gas in thelamp. In the case of mercury lamps, there is an additionaldisadvantage related to the toxicity of mercury that can bereleased to the environment in case of a breakage.

In the case of visible light, solid-state light emitting diodes(LEDs) are mainstream and ubiquitous. They are present in

practically every home appliance, traffic lights, publicitysigns, flashlights and even car lights. Light emitting diodesare cheap, lightweight, small and rugged. Red LEDs showefficiencies in the order of 80% with lifetimes exceeding100,000 hours. The availability of a similar light source forthe ultraviolet range would make possible applicationssuch as: in-home treatment of some skin conditions,elimination of the risk of mercury contamination inapplications such as water and air purification, reduction ofthe energy associated cost in applications that require UVillumination for prolonged periods of time, and it wouldmake possible applications that are not even inconsideration at the moment. Although the availability ofthese devices is hopefully just a matter of time, severaltechnical barriers need to be surmounted before efficient,long lifetime and inexpensive devices are introduced intothe market.

What is an LED?A Light Emitting Diode is a semiconductor device thatemits light when carriers of different polarity (electrons andholes) combine generating a photon. The wavelength ofthe photon (and thus the color of the light emitted by thedevice) depends on the energy difference the carriersovercome in order to combine. In Figure 1, different

possibilities for the active area of a light emitting diode areshown. Figures 1a and b show a schematic of ahomojunction, that is, a device based on a single materialin which two different types of impurities are added indifferent locations to cause two different regions withopposite polarity. The majority carriers in one region

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combine with the majority carriers of the other releasing photons with a wavelength related to the energy bandgap of thematerial. Figure 1c shows the case of a heterojunction, in which the carriers are confined in a region surrounded by materialof a different composition. This is the most common geometry used for LEDs because the carrier confinement implies a higherefficiency than in the case of a homojunction.

Figure 1. Schematic of photon generation geometries in semiconductor junctions, such as the ones found in the activeregion of a light emitting diode showing a p-n homojunction under (a) zero bias and (b) forward bias and a p-nhetrojunction under forward bias (c). (Courtesy of Prof. E. Fred Schubert, Rensselaer Polytechnic Institute, Troy, NY)

is possible to generate light with a wavelength rangevarying from around 210 nm to 365 nm. In Figure 2, weshown the photoluminescence emitted as a function ofcomposition for the AlGaN system.

Currently, UV LEDs (with emission wavelengths between210 nm and 340 nm) are commercially available atresearch grade and in limited quantities. Their lifetimesreach of the order of 200 hours and the efficiencies are inthe order of less than 1%. Compared to the visible lightLEDs, they are in their initial stage of development, andthe principal reason for this is the lack of an appropriatesubstrate.

The Importance of the SubstrateThe LEDs designed for any wavelength are fabricated in asimilar manner: very thin, monocrystalline layers thatcompose the active area of the device are epitaxiallygrown on top of a monocrystalline substrate that acts as atemplate for the growth of the layers. The substrate is afundamental component of the device. Chosen with care,the substrate not only provides the crystalline template forthe different layers deposition, but also can be a windowfor the LED, and a thermal sink for the generated heat.

In the case of visible light or infrared LEDs, the substrateused is a crystalline material that belongs to the samesystem used to build the active area of the device. Forexample, if the active area of the LED is composed byalloys of gallium arsenide (GaAs) and aluminum arsenide(AlAs), the substrate used is a GaAs wafer. This is referredto as “native” substrate.

The historical difference between the development of thelonger wavelength LEDs and the ones based on the nitridesystem is that until a few years ago, the nitride systemlacked a high quality native substrate. Most of thedevelopment of the blue, violet and UV LEDs that areavailable nowadays was carried out using foreignsubstrates such as sapphire or silicon carbide (SiC).

Sapphire is far from the best possible substrate for thenitride system because it has a large lattice mismatch withGaN (about 16%) and has a relatively low thermalconductivity. As an advantage, it is a low cost substrate

In order to have an efficient device, that is, in which mostof the power applied converts into light of the desiredwavelength, it is very important to have a material systemwith high crystal quality in which the charge carriers arenot trapped before recombining, and where thegenerated photons can escape the device without beingabsorbed. The material system to build the device ischosen specifically with the emission wavelength in mind.It has to have an appropriate bandgap and the differentcomponents in the system must have matching crystallinestructures and lattice parameters in order to reduce stressin the layers and generation of defects that can act ascarrier or photon sinks.

Typically, visible light LEDs (e.g., red, orange or yellow)are commercially available for pennies, with efficienciesover 75% and lifetimes in excess of 100,000 hours (over11 years). LEDs are robust, do not have hot filaments normoving parts, with a size around 5 mm in diameter.

ULTRAVIOLET LIGHTEMITTING DIODESThe system of choice for ultraviolet light emitting diodes(UV LEDs) operating between 210 nm and 365 nm is theone formed by aluminum nitride (AlN), gallium nitride(GaN) and intermediate alloys. Both materials share thesame wurtzite (hexagonal) crystal structure, and thelattice mismatch between the two, measured in acrystalline direction within the hexagonal face, is 2.4%.The two compounds are miscible in all the compositionalrange. Translated to emission wavelength of the device, it

Figure 2.Photoluminescence spectraof AlGaN structures withdifferent composition as afunction of wavelength. Thestructures were grown andcharacterized at the Centerfor Quantum Devices,Northwestern University.

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and is transparent for the UV range of interest, having abandgap of 9.9 eV. SiC has a smaller lattice mismatch withGaN than sapphire and a much higher thermalconductivity, but it is very expensive and its relatively smallbandgap precludes it from being used as a window for thedevice for wavelengths shorter than 400nm.

In the nitride system, AlN is the natural substrate forAlGaN/GaN optical devices because:

• It has a lattice mismatch with pure GaN of only 2.4%.This number is reduced further if instead of pure GaNone uses AlxGa(1-x)N alloys.

• The thermal expansion coefficient matches that ofGaN in the whole range from about 1,100° C down toroom temperature. This reduces the thermal stressthat can appear on the epilayers as the wafers aretaken down from growth to room temperature.

• It has a very large thermal conductivity of 3.4 W/cm-K, which means it can carry away the heat generatedon the device. Nishida et al. (2004) showed animprovement on the output power of an LED grownon bulk AlN substrates with respect to LEDs grown onsapphire.

• It has a bandgap of 6.2 eV, which allows its use as awindow in the wavelength range of interest.

The efficiency of light-emitting diodes (LED) depends on thereduction of carrier and photon traps. Dislocations anddefects on the crystalline structures can lead to photon loss,and thus to a device with a lower efficiency (Nishida et al.2004; Takaya et al. 2003). The use of foreign substrates forepitaxial growth promotes the formation of dislocations asthe epilayers release stress caused by lattice mismatch withthe substrate. Using a substrate from the same nitridesystem, such as aluminum nitride, provides a template forepitaxial growth with a similar lattice parameter.

In the past five years, Crystal IS, Inc. (CIS) has startedproviding high quality AlN native substrates with adislocation density in the order of 1000/cm2. In the past twoyears, it has also started the development of UV LED devices.

DEVICE DEVELOPMENTThe technical challenges for the design and developmentof UV LEDs range from learning to prepare high crystallinequality and low defect density epitaxial layers of therequired compounds on top of CIS AlN substrates,achieving low-resistance, good electric contacts, andpackaging the device.

In Figure 3 a device fabricated on top of CIS bulk AlNsubstrate is shown, where a 350 µm diameter active areaunderneath the p-type electric contact is uniformly lit up.This picture was taken of the LED still on the wafer, priorto dicing and packaging and is being tested by means ofa wafer probe system. After this probe test is complete,the individual LEDs are diced from the wafer into 750µm x500µm die and then packaged in TO46 cans.

Figure 3. AUV LED testedon a waferprobe system.

Figure 3 shows packaged LEDs without lenses. The diameterof the metal base is 5 mm. In order to package the diode, asubmount is used with evaporated metallic pads, ontowhich the diode is flipped and soldered. A close-up of theinside of the LED package is shown in (b). The light emittedby the LED comes out through the back of the device (thesubstrate). The metallic pads in the submount are largeenough so that wire bonds to it for the electricalconnections from the device to the pins in the packagingcan. This technique is known as “flip-chip” packaging.

In Figure 5, the light-voltage characteristic for identicalstructures grown on sapphire and AlN substrates are given.The upper dotted curve shows the higher forward voltagedrop of the device on sapphire compared to devices on AlNsubstrates (lower dotted curve). The light output powerprematurely saturates on devices fabricated on sapphiresubstrates while the light output power on AlN remains linearin the range tested due to the improved thermal conductivityof the AlN substrate. In the plot at the right side, acomparison between intensities measured devices grown onAlN substrates are shown to be substantially higher thancomparable structures grown on sapphire substrates.

Figure 5. Light-Current-Voltage characteristics of a 295nm LED on AlN (shown as the red curves) and sapphiresubstrates (shown as the blue curves). (a) Devices on AlNsubstrate exhibit sharper turn-on voltages and do not showa premature saturation in light output power due to thebetter thermal conductivity and lower defect levels of thesubstrate. (b) Devices grown on AlN substrates can bebrighter than those grown on sapphire.

Figure 4:PackagedLEDs

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APPLICATIONS AND BENEFITSUltraviolet lamps are used for many, diverse, applicationswith many, diverse, requirements for wavelength range,power, size and lifetime. These applications range frommedical, such as treatment of different skin conditions,water purification, homeland security, analyticalinstrumentation, etc. Other applications that preclude theuse of mercury lamps, such as air purification on aircraft,will open up when an effective replacement for mercurylamps is developed.

Homeland SecurityAs our world becomes more of a complex place and theneed for homeland security increases, there comes a newwave of security systems. Amongst these new defenses is abioagent detection system. These systems are designed todetect aerosol bioagent threats released into the air of criticalbuildings including government buildings, airports, subways,office buildings, shopping malls, sports arenas, hotels, andhospitals. Rapid detection of an aerosol release will enabletimely implementation of protective measures to protectoccupants and minimize the extent of contamination.

The measurement systems will make use of UV fluorescence;a bioagent such as anthrax, excited by ultraviolet lightcreates a unique color spectrum, or fluorescence, making itvisible. This fluorescence “fingerprint” is then detected, thethreat is confirmed and action is taken accordingly. Thesenew UV systems are designed to be small and themeasurement head looks similar to a regular smokedetector. Clearly a bulky mercury lamp cannot beincorporated into a small device such as this but a UV LEDoffers a perfect footprint for this application.

Germicidal PurificationDisinfection with short wavelength ultraviolet light (so-called UVC with wavelength shorter 290 nm) is one of thefew methods accepted by the US EnvironmentalProtection Agency (USEPA 2006) for inactivating chlorine-resistant organisms such as Cryptosporidium in drinkingwater and as a disinfectant for wastewater withoutintroducing potential carcinogens which result whenchemical disinfectants, such as chlorine, interact withorganic matter. All the systems utilized for this purpose,disregarding their scale, presently employ mercury-basedlamps. These lamps may have different power andbrightness specifications, but in general require high inputpower, work at high temperature, last between 5,000 and13,000 hours, and the systems require permanentmaintenance, such as lamp cleaning (especially in the caseof wastewater treatment).

Water and wastewater treatment systems can benefit fromthe use of UV-LEDs for UV purification on a number oflevels from ecological to economical. Advantages of UV-LEDs over mercury lamps, besides substantial energysavings, include:

• Mercury-free: there is no risk of toxic substancescontaminating the water flow. In the case of municipal

systems, the use of UV-LEDs eliminates the ever-present liability due to mercury spills in the case of alamp breakage.

• The emission wavelength can be tuned by design tomatch the most effective wavelength for disinfectionin a given environment. Better wavelength matchresults in higher system efficiency and thus, additionalenergy savings.

• Longer lifetimes: mercury lamps are rated for about8,000 – 12,000 hours. NYS regulations require thereplacement of the lamps after one year of use.Commercially available visible LEDs show lifetimes in theorder of several tens of thousands of hours, (typicallifetime for red LEDs is 100,000 h). Longer life also leadsto cost savings, both on replacement parts and on theregular maintenance needed for mercury lamp systems.

• No warm up time: UV-LEDs emit immediately as theyare biased so they can be connected to a flow sensorand be lighted only when needed. This may not seempractical for municipal drinking water or wastewatertreatment facilities, but it can help on cost savings inthe case of power outages. When using mercurylamps, they require at least several minutes of warm uptime after a power outage to be back on line. The useof UV-LEDs would reduce the cost of backup batteriesor generators needed to prevent loss of service duringor after a power shutdown.

In addition, the availability of a small UV light source wouldopen up markets that are extremely reduced nowadays,such as that for portable devices. It would also help buildmarkets that might eventually be considerable, such asthat for water purification in third world countries oremergency systems where powering with batteries or solarcells would be possible for low consumption lamps, butnot for mercury lamps.

UV CuringUV curing is the method of hardening a liquid film whenapplying ultraviolet light. The liquid film can vary incomposition depending on the end application whichspans from inks and coatings in the graphic arts industrythrough leather and leather and textiles in the footwearindustry. However, the film basically comprises of basepolymers, non-solvent diluents and photoinitiators. Themarket currently uses either medium pressure mercury UVlamps or electrodeless UV lamps, both of which are bulkylight sources which generate a significant amount of heat.LEDs are an ideal light source where heating is an issueduring the curing process and for applications where thelarge form factor of a lamp is not ideal.

Some LED systems are now becoming available for nicheapplications within this market, but these make use of UVALEDs operating between 365 nm and 390 nm. There arepotential benefits of going shorter in wavelength whereLEDs are not currently available for faster curing whenusing certain photoinitiators.

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Medical ApplicationsUltraviolet light sources can be used to treat psoriasissuccessfully and are promising for the treatment of otherskin diseases, including acne vularis, atopic dermatitis,vitiligo, pruritus, lichen planus, pityriasis rosea and earlycutaneous T-cell lymphoma. Broadband UVB, narrow bandUVB, broadband UVA, photochemotherapy (PUVA) andphotodynamic therapy (PDT) are among the applicationsin which UV sources are required. Roughly, 2 to 2.6% ofthe US population are affected by psoriasis, or between 5.8and 7.5 million people. Psoriasis is a skin disorder involvinga type of white blood cell within the skin, known as a T-cell.In the case of psoriasis, T-cells are mistakenly activated.They trigger other immune responses, leading toinflammation and rapid turnover (growth) of the skin cells(Parrish and Jaenicke 1981; Barbagallo et al. 2001).Existing treatments are costly and, time-consuming andcan interfere with employment or school schedules.

The availability of a small UV light source with little powerrequirements would make possible for many of theapplications to reach a much wider market. For example,in the case of medical treatment, it would be possible forthe patient to receive the treatment at home or even atwork instead of spending hours at the doctor’s office. Anadvantage of UV-LEDs is that it is possible to design thedevice so that the emission wavelength covers the preciserange of treatment while avoiding the erythema threshold.In the case of skin disease, the range is narrow, between305 and 315 nm.

Analytical InstrumentationUltraviolet light sources are ubiquitous in biomedicalinstrumentation. They can be found in microplate readers,gel transilluminators, fluorescence microscopes, liquidchromatography detectors, capillary electrophoresisdetectors, flow cytometers and numerous other specialpurpose instruments. In addition, the development ofanalytical techniques employing UV excited fluorescenceand UV absorbance is ongoing.

Light emitting diodes with emission wavelengths in the UVregion of the electromagnetic spectrum are amongst themost desired photonic devices in the biomedical imaging.Currently, the UV light requirements, in the range between260 and 340 nm, are covered by gas or arc lamps that arebulky, expensive and typically short-lived. In the last 10years, great strides have been made in developing highefficiency red, green, blue and white light emitting diodes(LEDs) made from semiconductors. These solid-state lightsources are compact, efficient (>50% of electric powerused converted into desired light), long lived (approaching100,000 h), and rugged. The availability of commercial UVLEDs could start a new paradigm in biomedical imaging.

As examples of applications, fluorescence microscopesusually make use of special mercury lamps (also called“burners”) that work at high pressure and hightemperature, that emit between around 250 nm and 700

nm. A complicated system of filters and special mirrors isused in order to isolate the fluorescence emitted by afluorophore excited by a particular wavelength from themercury lamp and distinguish it from the background. Theuse of an array of LEDs with different wavelengths (theemission wavelength of an LED can be selected by design),which can be individually controlled, would simplify theoverall instrument, reducing the need for filters. Theaverage lifetime of a mercury burner used in a fluorescencemicroscope varies between 200 and 400 h. The estimatedlifetime for a UV-LED is tens of thousands of hours. Abroken/burnt mercury lamp creates a hazardous wasteissue. UV-LEDs would eliminate this problem as well.

CONCLUSIONThe availability of low cost, small size, mercury free, energyefficient and long lifetime UV lamps is desired for a rangeof very different applications. Some of them are nowcovered by mercury lamps at high cost and with some riskdue to the liability presented by the presence of mercury.Others are not even possible precisely because of thepresence of mercury. In this paper, we have reported onthe development of UV-LEDs using native AlN substrates,and the comparative advantage of using native substratesas opposed to foreign substrates. It is very likely that in thenext two to five years, many of the applications that todaymake use of mercury lamps will be carried out by UV-LEDs,and that others that are dreamed of nowadays, such asportable purification systems, or portable medical devicesfor skin condition treatments, will be a reality.

REFERENCESBarbagallo, J., Spann, C., Tutrone, W. and Weinberg, J.

2001. Narrowband UVB Phototherapy for the Treatmentof Psoriasis: a Review and Update, Cutis, 68: 345-347.

Nishida, T. and Makimoto, T. 2004. AlGaN-basedultraviolet light-emitting diodes grown on bulk AlNsubstrates, Appl. Phys. Lett, 84(6): 1002-1003.

Parrish, J. and Jaenicke, K. 1981. Action Spectrum forPhototherapy of Psoriasis, J. Invest. Derm. 76(5): 359-362.

Takeya, M., Mizuno, T., Sasaki, T., Ikeda, S., Fujimoto, T.,Ohfuji, Y., Oikawa, K., Yabuki, Y., Uchida, S. and Ikeda.,M. 2003. Degradation in AlGaInN lasers, Phys. Stat. Sol.(C) 0 (7): 2292–2295.

Taniyasu, Y., Kasu, M. and Makimoto, T. 2006. Increasedelectron mobility in n-type Si-doped AlN by reducingdislocation density, Appl. Phys. Lett., 89: 1821121-1821123.

USEPA. 2006. Ultraviolet Disinfection Guidance Manual forthe Final Long Term 2 Enhanced Surface WaterTreatment Rule, United States Environmental ProtectionAgency, Office of Safe Water, November 2006.