III Nitride UV Devices

Transcription

1 Japanese Journal of Applied Physics Vol. 44, No. 10, 2005, pp #2005 The Japan Society of Applied Physics Invited Paper SELECTED TOPICS in APPLIED PHYSICS III Physics of UV Materials and Devices and Their Applications III Nitride UV Devices M. ASIF KHAN,M.SHATALOV, H.P.MARUSKA, H.M.WANG and E. KUOKSTIS Department of Electrical Engineering, University of South Carolina, Columbia, SC U.S.A. (Received March 8, 2005; accepted June 30, 2005; published October 11, 2005) The need for efficient, compact and robust solid-state UV optical sources and sensors had stimulated the development of optical devices based on III nitride material system. Rapid progress in material growth, device fabrication and packaging enabled demonstration of high efficiency visible-blind and solar-blind photodetectors, deep-uv light-emitting diodes with emission from 400 to 250 nm, and UV laser diodes with operation wavelengths ranging from 340 to 350 nm. Applications of these UV optical devices include flame sensing; fluorescence-based biochemical sensing; covert communications; air, water and food purification and disinfection; and biomedical instrumentation. This paper provides a review of recent advances in the development of UV optical devices. Performance of state-of-the-art devices as well as future prospects and challenges are discussed. [DOI: /JJAP ] KEYWORDS: MOCVD, AlGaN, AlInGaN, ultraviolet, epitaxy, growth, light-emitting diode 1. Introduction Tremendous progress in the development of III nitride high brightness visible-light-emitting diodes (LED) and violet laser diodes (LD) have led to a number of new applications. Among them are next-generation high-density optical storage and solid-state lighting, where the market reached $3.7 billion for 2004 and is expected to nearly double by ) Although visible technology is approaching its maturity, many groups have shifted their research focus towards short-wavelength ultraviolet devices. Visible and solar blind nitride detectors have reached outstanding performance at a very short cutoff wavelength. The quantum efficiency of near-uv LED received a huge boost with advanced growth, metallization and packaging techniques. The emission wavelengths of LEDs now cover the majority of the UV spectrum ranging from 400 to 250 nm. Finally, UV LD emission has shifted towards shorter wavelengths with viable operation demonstrated at nm. In this paper, we will discuss recent progress in the area of III nitride UV optical devices. As device operation in the deep-uv region requires primarily AlGaN-based active layers with a very high Al molar fraction in the alloy, we discuss some pertinent material issues and challenges in material growth in 2. We include here problems of strain, cracking and defects in AlGaN layers heteroepitaxially grown over sapphire substrates. Problems of doping AlGaN with Si and Mg are also covered along with the discussion of AlGaN band gap bowing. Advances in the developments of visible- ( 400 nm) and solar-blind ( 280nm) III nitride photodetectors are covered in 3, where we give a historical overview of research on photoconductive and photovoltaic detectors as well as describe the performance of recent state-of-the-art devices. Recent progress in III nitride surface acoustic wave oscillator-based sensors for operation in the solar blind spectral region is also included. In 4 we describe the progress in the area of UV LED starting from a brief overview of advanced near-uv ( 370{ 400 nm) light emitters, in which devices with excellent performance address: asif@engr.sc.edu 7191 have recently been reported. This is followed by an overview of research work performed in deep UV-LEDs emitting in the UV-A, UV-B and UV-C regions of the electromagnetic spectrum. We discuss various technological approaches that led to a tremendous progress in short-wavelength emitters, and remarkable achievements in deep UV-LED operation by several research groups. Research efforts in the area of UV lasers are described in 5, where we discuss recent advances in UV LD operation and prospects for deep-uv lasers that follow from optical pumping experiments. Finally, in 6 we summarize our efforts by discussing perspectives on further improvements of III nitride UV devices and the potential areas of application of such devices. 2. AlGaN Materials High-quality AlGaN films are the key element in obtaining efficient UV photo-detectors and light emitters. Compared with that of GaN, the growth of AlGaN has proved to be significantly difficult. This problem lies partially in the fact that Al adatoms have a much larger sticking coefficient than Ga adatoms. The layer-by-layer growth of films with attachment of adatoms at steps or kinks gives the smoothest surface features, as opposed to three-dimensional island growth. Because Al adatoms are much less mobile on the surface, they are less likely to be able to move from their point of impact from the vapor; rather than incorporating at the most energetically favorable lattice sites such as a step, they tend to cause islands to nucleate, thereby disturbing epitaxial growth. As a result, higher densities of extended defects, such as dislocations and grain boundaries, are much easier to generate as these growth islands coalesce from different nucleation sites coalesce. Furthermore, the commonly used Al metalorganic precursors (trimethylaluminum, TMA, and triethylaluminum, TEA) are more reactive than their Ga-based counterparts. Hence, gas-phase reactions and related adduct formations tend to interfere with the growth process. Figure 1 shows an experimental plot for Al incorporation in a solid film as a function of Al fraction in the gas phase. 2) Under such growth conditions, Al incorporation is very efficient when the gas phase fraction is smaller than 50% [molar fraction: TMA=ðTMA þ TMGÞ]. Above 50%, a reduced level of incorporation has been observed due

2 7192 Jpn. J. Appl. Phys., Vol. 44, No. 10 (2005) Invited Paper M. ASIF KHAN et al. Fig. 1. Al content in AlGaN films as function of Al fraction in gas phase. Note that under the growth conditions used, Al incorporation is efficient when the Al gas fraction is less than 50%. Above 50%, a reduced Al content in solids compared with that in gas phase indicates that a pronounced gas-phase reaction has occurred. High gas velocity (including lower growth pressure and high push gas) could enhance Al incorporation. to the gas-phase reaction between TMA and NH 3. This reaction could interrupt the normal epitaxial growth process by generating nano- or micro-particles that fall onto the growing surfaces. As a result, for high-al-content AlGaN, to avoid surface roughening and the formation of pits and particles and assorted structural defects, a lower growth rate must be employed. Other critical problems affecting the epitaxial growth of high-quality AlGaN films include film cracking and poor electrical conductivity. These factors present great challenges for device development efforts, and require a full exploration of the material deposition domain for AlGaN, including the introduction of extensive innovative approaches. Recent attempts to resolve these issues will now be presented. 2.1 Cracking and defects Despite extensive recent efforts to develop native substrates including GaN, AlN, and their alloys as free-standing wafers using a variety of growth techniques, none has yet reached the stage of widespread commercial availability. 3 6) Due to the lack of a native substrates, nitride thin films tend to be grown on foreign substrates, a process known as heteroepitaxy. Commonly used substrates include sapphire (Al 2 O 3 ), SiC, Si, and GaAs. All foreign substrates differ in lattice parameters and thermal expansion coefficients from nitride materials. In addition to the formation of threading dislocations, nitride films tend to bow and even crack. The usefulness of low-temperature buffer layers for improving the quality of heteroepitaxial GaN films has been recognized since ) Because the problems with AlGaN films are even more severe, researchers have developed novel approaches on the basis of the idea of compliant-layer insertion. Kamiyama et al. 8) and Han et al. 9) individually adopted low-temperature (LT) AlN and AlGaN insertions to avoid cracking of AlGaN grown on GaN. The periodic insertion of LT-grown AlGaN layers was shown to effectively reduce biaxial tensile strain in the AlGaN films, thus reducing cracking. 9) LT-grown films tend to exhibit poor structural quality and hence low stiffness. LT films are more elastic, making them more compliant and able to accommodate more strain. Unfortunately, according to Akasaki and coworkers report, 8) this method could introduce dislocations after the insertion of each LT-layer. Besides, there are too many thermal cyclings during one growth which also makes the technique less attractive. The cracking of AlGaN films grown on basal plane sapphire substrates is actually surprising because from the lattice and thermal mismatch point of view, AlGaN grown on sapphire should experience biaxial compressive strain, which should not cause cracking. The strain responsible for this type of cracking is rooted in a different mechanism and is called intrinsic tensile strain, often observed in imperfectmaterial (polycrystalline) growth. It arises from the coalescence of islands where the film consists of discrete mosaic blocks. During their growth, when two islands (mosaic blocks) approach each other at a certain distance, they start to coalesce through atomic rearrangement to allow a minimized system free energy by reducing two surface boundaries to one. This process however generates tensile strain in the vicinity of the zipped area; this strain is inversely proportional to the square root of island dimensions. 10) The energy from the tensile strain will therefore increase with film thickness, to a point where it will be released by either dislocation generation or cracking. Cracking is likely to happen in material systems where dislocations have no slip systems, such as in AlGaN films. AlGaN films grown on sapphire consist of many fine mosaic blocks, the coalescence of which generates intrinsic tensile strain, eventually resulting in film cracking. This type of cracking can be avoided by improving material quality, i.e., by decreasing material mosaicity (increasing mosaic block dimensions). Khan et al. suggested using the pulse atomic layer deposition (PALE) approach for growing high-quality GaN, AlN and AlGaN films ) In PALE the flow rates of group III and group V precursors are sequentially modulated, thereby enhancing the surface migration of Al and Ga adatoms, which results in better crystalline quality and better surface morphology of epitaxial layers. Another approach was suggested by Bykhovsky et al. who theoretically predicted elastic strain relaxation in GaN/AlN and GaN/ AlGaN superlattices. 14) Zhang et al. 15) have reported on the use of AlN/AlGaN superlattices for alleviating the crack problem of thick AlGaN films over sapphire substrates, and Wang et al. 16) in their detailed x-ray measurement indicated that these superlattices are efficient in increasing AlGaN mosaic block dimensions. Recently, a modified PALE approach known as migration-enhanced metalorganic chemical vapor deposition (MEMOCVD) was employed to further enlarge the AlGaN mosaic block dimensions. 17) In MEMOCVD, the duration of group III and group V precursor pulses is further optimized in a pattern that allows a beneficial surface migration of reacting species on the growing film. This allows us to change the V/III ratio from almost zero to several thousands, which greatly increases the probability that the Al and Ga adatoms migrate to their appropriate sites in the lattice. Very high quality AlN and AlN/AlGaN superlattices were grown by this method (see Fig. 2). AlGaN device structures deposited over MEM- OCVD grown AlN and AlN/AlGaN superlattices were also determined to be of superior quality, in terms of structural quality and minority carrier lifetime. 18) Subsequently, these

3 Jpn. J. Appl. Phys., Vol. 44, No. 10 (2005) Invited Paper M. ASIF KHAN et al Fig. 2. (a) Experimental (top) and simulated (bottom) (002) XRD 2! scans for AlN/Al 0:85 Ga 0:15 N SL sample. (b) The (002) XRD rocking curve of the 0.3 mm AlN buffer used for the SL growth (cited from ref. 17). MEMOCVD buffers were used for the fabrication of highpower deep-uv light-emitting diodes (LEDs). 19,20) In the literature, there are several other approaches that have been explored to improve AlGaN quality. Kamiyama et al. used grooved GaN templates for AlGaN growth with the aim of bending threading dislocations away from the growth direction. 21) A similar approach that uses GaN templates and the air-bridged lateral growth of AlGaN had also been demonstrated. 22) One limitation of such approaches is that they are only efficient for low Al-content AlGaN growth. These approaches encounter difficulties in case of high-al-content materials because of the very low lateral growth rate. Pioneering work on the epitaxial growth and characterization of AlN and AlGaN layers on bulk AlN substrates was reported by several groups ) Numerous advantages of bulk AlN substrates such as possibility of homoepitaxial growth, potential transparency in the UV region, and high thermal conductivity, have been addressed. The widespread commercial availability of these bulk substrates can be expected to provide the ultimate solution for the growth of crack-free high quality AlGaN films. 2.2 Doping The electrical conductivity of semiconductor films is always a critical issue for developing high-performance device structures. Although the successful n- and p-type dopings of GaN and InGaN are well established, the successful doping of AlGaN has been much more problematic. There is a rapid decrease in the conductivity of epilayers for both doping types with increasing Al composition. This is due to the continuous increase in the donor and acceptor ionization energies which lowers the carrier concentrations as well as to the reduction in material quality which reduces the carrier mobility. 28,29) As mentioned above, MEMOCVD has been shown to produce high-quality AlN and AlN/AlGaN superlattices for defect reduction in AlGaN and such defect reduction can greatly improve doping efficiency by reducing compensating and scattering centers. As a result, room-temperature electron Hall mobilities exceeding 85 cm 2 /(V s) for n-algan with an Al molar fraction of 50% have been obtained. 30) Using molecular beam epitaxy (MBE) Si doping in high-al-mole-fraction AlGaN alloys has been achieved with very high n-type carrier concentrations of cm 3, but with a low mobility. 31) It was also reported that there is no significant degradation of materials properties. Si -doping, in which basically an atomic plane of Si atoms is introduced as the electron source, has been introduced for a 340 nm UV LED structure for which improved performance with output powers of 150 mw at a dc driving current of 100 ma has been achieved. 32) Mg is the commonly used acceptor for p-gan, but its room-temperature activation energy is as high as 250 mev. This value increases almost linearly as Al is added to form AlGaN ternaries. Several approaches have been suggested to enhance Mg-doped AlGaN p-type conductivity. One method is to use either a short-period or a large-period superlattice (SPSL or LPSL) composed of Mg-doped AlGaN/GaN layers to replace conventional p-algan ) The SPSL has a very small period, typically below 4 nm; hence minibands are formed, and vertical conduction of the p- SPSL should not be degraded. In a large-period case, the period is typically larger than 15 nm, but valence band discontinuity as well as polarization fields can enhance the ionization of acceptors in the AlGaN barriers and transfer holes into GaN wells. However, since the period is large, wavefunction coupling between neighboring wells is forbidden, which greatly reduces vertical conductivity. As a result, this LPSL approach can only realize a good horizontal p-conductivity. Several groups have used Mg-doped Al- GaN/GaN SPSL in nm UV LED growth ) The second approach was proposed by Shur et al., namely, the use of a p-gan/p-algan single heterostructure to achieve hole accumulation at the heterointerface. 40,41) The mechanism of this approach is similar to that of the LPSL approach. However, since in the p-gan/p-algan single heterostructure only one barrier exists for hole transport, vertical conductivity can be greatly enhanced due to the high-density hole accumulation at the interface and field assisted tunneling as well as thermionic emission. Most of the reported deep UV LEDs use this approach for hole injection into the active layers and get reasonably good powers ) The third approach, -doping, has been investigated by Nakarmi et al. 49) It was demonstrated that Mg -doping improves not only p-type conduction, but also the overall quality of p-type GaN and AlGaN. It was observed that Mg -doping increases hole concentration while inducing no changes in hole mobility. 2.3 Bowing of energy gap When designing a UV device, it is important to determine the fundamental band gap of AlGaN over a large range of Al mole fractions. The results of absorption measurements performed at atmospheric pressure yielded a variation in band-gap energy which was found to be E g ðxþ ¼3:43 þ 1:44x þ 1:33x 2 ev for the Al x Ga 1 x N system. 50) There is a large nonlinear variation in fundamental band gap with Al concentration. If one uses the experimental bowing parameter of 1.0, then one obtains the relationship E g ðxþ ¼6:2x þ SELECTED TOPICS in APPLIED PHYSICS III

4 7194 Jpn. J. Appl. Phys., Vol. 44, No. 10 (2005) Invited Paper M. ASIF KHAN et al. 3:43ð1 xþ xð1 xþ, which has been adopted by most groups working on this material system. 51) 3. Ultraviolet Photodetectors Along with light emitters, light detectors are equally important elements of many optoelectronic systems. Traditionally, the detection of light in the UV part of the electromagnetic spectrum has been realized using photomultiplier tubes (PMTs). These devices offer unprecedented sensitivity in the UV range, low dark currents and high speeds. The high internal gain (>10 6 ) achieved through photomultiplication makes PMTs very effective for the detection of low intensity signals. However, very high operating voltages (typically > 1 kv) requiring special (and sometimes very bulky) power supplies make PMTs much less attractive for portable applications. In addition, PMTs are very fragile vacuum tube devices that are poorly suited for many space and field applications. An alternative approach uses UV-enhanced Si detectors that are typically p i n diodes with a special anti-reflection coating designed for the UV wavelength range. Si has a direct band gap at 3.1 ev; thus, photons in the UV range are absorbed efficiently near the front surface of the device. These semiconductor detectors can be miniaturized and made highly reliable, robust, and well integrated with other semiconductor components. Although UV-enhanced Si detectors have many important advantages, their dark currents are high (typically in the na range) and quantum efficiencies are poor. Because Si has a very high absorption constant beyond 3 ev, carriers are generated very near the front surface, and minority carriers may be lost through surface state recombination. For operation in the deep UV, the efficiency of these detectors may suffer from degradation of the SiO 2 /Si interface after prolonged UV exposures, due to the generation of surface states. Both PMT and Si detectors are sensitive throughout the UV and visible wavelengths, thus requiring costly UV filters to achieve visible-blind or solar-blind operation. Certain outdoor detector applications would benefit from operation in total darkness to reduce or eliminate background currents, and since the solar spectrum terminates at 280 nm, they need to be nonresponsive to wavelengths greater than 280 nm (hence the solar blind ). Other applications in the near UV (below 300 nm) would also benefit from being nonresponsive to visible light, be it sunlight or indoor lighting. Thus, an alternative to PMTs and Si is desirable. Other semiconductor detector materials have been also pursued by researchers including Ge, GaAs, and SiC, in order to combine the sensitivity of a PMT and the robustness of Si with blindness to longer wavelengths. Ge and GaAs offer no obvious advantages over Si, but 4H SiC is certainly visible-blind. However, 4H SiC has only one fixed band edge and thus cannot be tailored to the application. Wideband-gap III nitride semiconductors, in particular, the Al x In y Ga 1 x y N material system, have emerged as the most promising material for the realization of solar-blind detectors. The initial development of III nitride photodetectors has been started in the early 1990s 52) due to the fact that photodetector structures are relatively simple to grow and fabricate. Over the last decade, significant progress had been made in the development of AlGaN-based photodetectors, which are able to detect light in part of the UV-C wavelength range (200{ 280 nm). Very high quantum efficiencies, low dark currents and high speeds have been achieved along with several orders of magnitude rejection of sensitivity beyond the solar-blind region. Numerous applications for such devices include flame detection, furnace control, engine monitoring, UV radiation dosimetry, pollution monitoring, and early missile threat warnings. 53,54) Recent progress in the development of LEDs emitting in the wavelength range from 240 to 365 nm and LDs emitting at approximately 340 nm has stimulated the prototyping of chemical/biological battlefield reagent detectors, space communications and non-lineof-sight covert communications, where AlGaN-based photodetectors may also play a key role. 55) Numerous research studies devoted to the development of GaN-based visible-blind and AlGaN-based solar-blind detectors have been published over the past 10 years. Material properties of the AlGaN system, growth and fabrication details, and operating principles of photodetectors have already been discussed in several review papers 56 59) and book chapters. 60) Thus, in this paper, we limit our discussion to a brief historical overview and to the review of the stateof-the-art of III nitride-based photodetectors. 3.1 Photoconductors Studies of photoconductivity in Zn-doped GaN films were reported by Pankove and Berkeyheizer in ) In the 1990s, several groups studied photoconductivity of GaN and reported on photoconductive detectors using GaN, 52,62 64) AlGaN 65 67) and AlN. 68) The normalized responsivities of AlGaN photoconductive detectors with different Al mole fractions showed a sharp peak at the cut-off wavelength (see Fig. 3) corresponding to the intrinsic photoconductivity. 65) Photoconductive devices cannot operate at a zero bias and, therefore, have extra noise coming from the dark current. Also, such detectors had often showed a photoconductive gain due to the trapping of minority carriers (holes) and thus slow response times. 67) The wavelength selectivity of AlGaN photoconductive detectors and absorption below the band gap absorption indicated that compositional fluctuations in Fig. 3. Normalized spectral response of Al x Ga 1 x N photoconductors (cited from ref. 65).

5 Jpn. J. Appl. Phys., Vol. 44, No. 10 (2005) Invited Paper M. ASIF KHAN et al Fig. 4. Schematic representation of photodetectors realized using III N material system (cited from ref. 71). Al molar fraction and deep centers, such as Al/Ga vacancies, O or C impurities and crystalline defects, contribute to trapping processes. 69) The comparison of the dc and frequency-dependent responsivities of GaN and AlGaN/GaN/AlGaN heterostructure photoconductive detectors points to a strong effect of a polarization-induced electric field on detector speed. 70) 3.2 Photovoltaic detectors Photovoltaic detectors such as Schottky barrier detectors, metal-semiconductor-metal (MSM) photodetectors, p n junction detectors and p i n photodetectors have also been demonstrated using III nitride material system (see Fig. 4). 71) The first photovoltaic detector was realized using a Ti/Au Schottky contact to p-gan doped with Mg up to about cm 3. 72) The detector had absorbing electrodes on the top surface and was illuminated from the bottom (through the sapphire substrate). Later, Schottky barrier detectors with transparent contacts were also realized using both GaN 73) and AlGaN ) This approach enables top surface illumination through the transparent Schottky electrode with ohmic contacts made to the base layer which is accessed through etching. However, the doping of the bottom Al x Ga 1 x N layers becomes difficult for Al composition greater than 20%. An In Si co-doping approach was suggested to obtain n-al 0:4 Ga 0:6 N active layers with low resistivities. 77) Besides a significantly increased doping efficiency, the introduction of a small concentration of In also allows the direct deposition of a crack-free 0.5-mm-thick Si-doped Al 0:4 Ga 0:6 N layer over the sapphire substrates. Another important Schottky barrier photodetector is the MSM detector. This photodetector cannot operate at a zero bias, which increases noise. However, it potentially can offer a very high speed of operation. However, at moderate bias voltages, the photoresponse of MSM diodes typically has a significant slow photoconductive component because the space charge width in the AlGaN layer is smaller than the electrode spacing. Early GaN-based MSM detectors 78 80) demonstrated high quantum efficiencies for an 350 nm cutoff wavelength. Solar-blind AlGaN-based MSM detectors have also been reported with quantum efficiencies up to 47% at 262 nm ) Recently, an MSM detector based on magnetron sputtered AlN with a high internal gain due to carrier trapping has also been demonstrated. 85) Various configurations of visible-blind GaN 86 92) and solar-blind AlGaN 93 96) p n junction based photodetectors have been reported by many groups. These p i n detectors offer low bias voltage, low dark currents (due to large hetero-barriers), and high speed. The major disadvantages of p i n detectors are related to the difficulties in the p-type doping of GaN and, especially, AlGaN layers with a high Al content 95) and to a high resistance of ohmic contacts to p-type AlGaN layers. This resistance can be decreased using p-gan as the contact layer with i-al x Ga 1 x N(x > 0:4) as the active layer. 93,94,96) However, the GaN contact layer absorbs a significant fraction of the optical beam, thereby reducing the device responsivity and, hence, UV/visible selectivity. Therefore, AlGaN/GaN heterostructure-based devices with transparent bottom window layers have attracted significant attention, and such devices with very low dark currents and very high zero-bias external quantum efficiencies have recently been reported ) Avalanche detectors potentially offer a high internal gain via avalanche multiplication. As opposed to photoconductive and MSM detectors, which, in some cases, show a slow response speed associated with carrier trapping, avalanche detectors demonstrate very high speed operation. The realization of avalanche detectors is very challenging due to issues related to the nonuniform breakdown via material defects. Nevertheless, GaN avalanche detectors with multiplication ratios of have been successfully demonstrated without microplasma formation ) Realization of solar-blind AlGaN-based avalanche detectors will likely require more efforts in growth of high-quality AlGaN layers and their p-type doping. Tremendous progress in the area of III nitride visibleblind and solar-blind photodetectors has led to the developments of phototransistors, imaging detectors, cameras and sensors. High speed p i n and Schottky detectors with a 3- db bandwidth of about 1 GHz as well as MSM detectors with an approximately 5 GHz bandwidth have been shown recently. 106) An 8 8 GaN Schottky photodiode array was described as early as ) Recently large area focal plane arrays 107,108) and cameras 109) have been demonstrated owing SELECTED TOPICS in APPLIED PHYSICS III

6 7196 Jpn. J. Appl. Phys., Vol. 44, No. 10 (2005) Invited Paper M. ASIF KHAN et al. to development of back-illuminated structures. A flame sensor using a solar-blind AlGaN p i n photodiode has recently been reported. 110) Despite all these technological advances the widespread commercialization of III nitridebased photodetectors is still being delayed by two major research challenges: the requirement of native substrates for the growth of low-defect-density films and effective p- doping of Al x Ga 1 x N layers with a high Al content. 3.3 Surface acoustic wave detectors The strong piezoelectric properties of nitride semiconductors enable their applications in surface acoustic wave (SAW) and acoustooptic devices. 111) SAW propagation in these materials is very sensitive to UV radiation, enabling the implementation of SAW-based UV and solar-blind sensors. A remote sensor for wavelengths 365 nm using a GaN-based SAW oscillator was first demonstrated by Ciplys et al. 112) The output signal of this sensor is in the radiofrequency range enabling remote wireless signal pickup. Similar SAW-oscillator-based sensors for wavelengths both above and below 300 nm have also been reported recently. 113) A perturbation by the SAW of a photogenerated charge carrier system in III nitride semiconductors leads to a variety of interesting effects, which have potential applications in new types of UV sensors. 4. UV Light-Emitting Diodes It is generally expected that life as we know it will be significantly changed by the emergence of compact solidstate devices that emit UV light. At present, the most common source of UV light is the mercury lamp, which is generally used in curing of various materials (photopolymerization), screen printing, water disinfection, air purification, medical procedures, and particularly in industrial, commercial and domestic lighting (fluorescent lamps). Low-pressure mercury tubes emit a narrow line at 254 nm which can be converted to visible light using appropriate phosphors. Higher pressures in the lamp lead to the transfer of the emission line to 365 nm. Mercury lamps require the application of a high voltage arc to initiate discharge, and they are typically rated for about 1000 h of life. Issues include size, weight, limited effective lifetime, high-voltage operation, lack of wavelength choices or tunability (there are only two lines possible), difficulty in power modulation, and ultimately environmental pollution. Therefore, the replacement of mercury lamps with semiconductor devices is a worthwhile goal. In contrast, light-emitting devices based on III nitride semiconductors offer many advantages including miniaturization, reliability, reduced costs, low power consumption, and ultimately a choice of wavelength of operation between 365 and 200 nm. Over the last decade, major efforts in the development of III nitride optoelectronic devices have been focused on blue and green LEDs and violet LDs. 114) These devices typically employ AlGaN as a wide-band-gap confinement layers while InGaN alloys are used as active layers to shift the emission into the visible spectrum range ( >400 nm). In early reports on homojunction GaN p n diodes grown by halide vapor phase epitaxy (HVPE) or MOCVD, ) the emission is often dominated by a blue emission peak ( 430 nm) at low injection levels due to carrier recombination involving Mg acceptors. Much improved performance of blue LEDs came with the introduction of p-gan/n-ingan/n-gan double-heterostructure designs. 118) Near band (375 nm) edge emission was already successfully achieved in 1994 in a double heterostructure AlGaN/ GaN design. 119,120) More recently, shorter-emission nearultraviolet (355 <<400 nm) high-power LEDs with GaN, ternary InGaN or quaternary AlInGaN active layer have been actively studied mostly due to their use as pump sources for phosphor-converted white LEDs or as substitutes for the i-line (365 nm) of high-pressure Hg-lamps ) Along with this progress in the near UV, the rapid development of III nitride deep ultraviolet LEDs and LDs with wavelengths ranging from 350 nm down to 250 nm has been strongly stimulated by a desire to replace mercury lamps in various applications listed above. In addition, interesting military applications including homeland security (biochemical detection), LIDARs, and covert communications can become available. 4.1 UV-A ( nm) LEDs Several approaches were explored in the late 1990s to reduce the LED emission wavelength below 365 nm (the band edge of GaN) and obtain LEDs with emission wavelengths in the UV-C spectral range. Han et al. used an Al 0:2 Ga 0:8 N/GaN multiple quantum well (MQW) structure to achieve emission at 353 nm by changing the transition energy through quantum confinement in GaN quantum wells. 127) They reported an output power of 13 mw at 20 ma, which corresponds to an external quantum efficiency much less than 1%. To shift the emission wavelength below 350 nm, several groups used Al x Ga 1 x N/ Al y Ga 1 y NMQWs 128,129) or a double-heterostructure (DH) design 130) of the LED active layer. Otsuka et al. 130) employed an LED structure with a DH configuration of the active region. In their study, an Al 0:1 Ga 0:9 N active layer was incorporated between two Al 0:13 Ga 0:87 N cladding layers. The LED structure was grown on a 1-mm-thick GaN layer over a sapphire substrate by MOCVD. The electroluminescence emission peak at 339 nm was reported under low-current injection and the role of the doping profile of cladding layers on device performance was discussed. Nishida and Kobayashi 128) used an LED structure grown by MOCVD over a SiC substrate with an active layer consisting of two Al 0:08 Ga 0:92 N quantum wells separated by Al 0:12 Ga 0:88 N barriers, and reported the electroluminescence peak at 346 nm. In their design, the top and bottom cladding layers were p- and n-al 0:12 Ga 0:88 N doped with Mg and Si, respectively. Kinoshita et al. 129) have reported an LED structure with emission at 333 nm featuring five Al 0:03 Ga 0:97 N quantum wells and Al 0:25 Ga 0:75 N barriers grown by MOCVD over SiC substrates. In their structure, a top cladding layer consisting of a Mg-doped Al 0:25 Ga 0:75 N and a Mg-doped Al 0:25 Ga 0:75 N/GaN superlattice (SL) was claimed to improve hole injection into the active region. For all these early reports, the emission intensity was relatively low and no output power measurements were presented. Several key factors that were judged to be responsible for the low output intensities were discussed. Strain due to the difference in Al molar fraction between the well layer and

7 Jpn. J. Appl. Phys., Vol. 44, No. 10 (2005) Invited Paper M. ASIF KHAN et al barrier layer and the difference in spontaneous polarization between these layers causes a significant electric field to appear across the quantum wells, leading to the quantum confined Stark effect. 131) This field tends to physically separate an electron and a hole localized in a quantum well, leading to spatially indirect transitions and hence a significant decrease in radiative efficiency. 128,130) For visible LEDs based on InGaN active layers, the formation of In-rich clusters due to the miscibility gap allows the capture of holes and electrons in localized centers, whereby radiative recombination efficiency markedly increases. InN has a very small band gap, so the significant inclusion of In in the active region would preclude emission in the UV. The lack of In in AlGaN active layers increases the probability of nonradiative recombination in the active layer. 128,129) In addition, poor carrier injection into the active layers, especially for holes, and not enough carrier confinement lead to the appearance of parasitic peaks at longer wavelengths due to transitions through deep levels in barrier regions. 130) Finally, in all the above designs the LED structures were grown on either conducting SiC substrates or thick GaN layers deposited on sapphire substrates, making significant light extraction from the substrate side virtually impossible. The output emission was typically extracted through the top p-gan contact layer which also strongly absorbed the UV emission. All the above issues, which were and still are critical for the development of UV LEDs, have been addressed in later developments, and many improvements have been made. To improve the doping efficiency of AlGaN cladding layers while maintaining their transparency, Nishida et al. used a short-period-super-lattice (SPSL) design adopted from studies of visible LEDs. 132) An increased doping efficiency in p-algan/algan excluded the use of an absorbing p-gan contact layer and improved the overall transmission of the top layers. With these improvements, they reported a submilliwatt (0.11 mw at 950 ma) cw operation of a UV LED emitting at 350 nm. 132) Further optimization of the active region design, i.e., replacement of the MQW active region with a single quantum well surrounded with an asymmetric barrier and blocking layers, resulted in better carrier confinement in the active region and increased the output power to 1 mw at 420 ma. 133) Estimations of an internal quantum efficiency of 7% were given indicating that further reduction in defect density and improvements in carrier injection and confinement should be made. The importance of the reduced number of nonradiative defects (primarily the density of threading dislocations) was illustrated through the growth of a UV LED over a free-standing 500-mm-thick GaN substrate 36) originally grown by HVPE on a removable GaAs substrate. 134) These free-standing GaN wafers typically have threading dislocation densities below cm 2, and feature excellent n-type conductivity. Even though this UVopaque substrate still absorbed approximately half of the UV emission, an external quantum efficiency of 1% was measured, while an internal quantum efficiency close to 80% was estimated for a UV LED with an emission wavelength of approximately 350 nm. Because the presence of In in the active regions of visible LEDs leads to greatly improved efficiencies, attempts have Intensity (arb. units) EL been made to include small amounts of In in UV LEDs as well. The use of an MQW structure with quaternary AlInGaN well and barrier layers in the active region of a UV LED with emission at 340 nm was discussed by Adivarahan et al. 135) The layer structure of the UV LED and the emission spectra are shown in Fig. 5. The quantum well contained 2% In and the barrier 1% In. Previously, several other groups demonstrated that the introduction of In into ternary AlGaN alloys could improve the optical quality of AlGaN layers ) and thus the optical and electrical properties of the LEDs ) The quaternary AlInGaN MQW active region was grown using PALE which has been shown to result in better-quality materials for the quaternary layers because PALE gives better control of the composition and surface morphology. 145,146) As seen from Fig. 5, Sidoped and Mg-doped Al x Ga 1 x N/Al y Ga 1 y N SLs were also used as n- and p-cladding layers. To analyze the efficiency of the light extraction, two similar layer structures were grown on sapphire and n-sic substrates using transparent Al 0:26 Ga 0:74 N buffer layers. In Fig. 6, the output power vs current and I V curves for both structures are shown. As seen from the difference in the I V curves, the SiC substrate offers vertical conduction and very good heat dissipation, reducing series resistance and improving the dc power saturation, while sapphire significantly improves the light extraction, thereby improving external quantum efficiency. More recently, numerous reports on the development of UV LED with emission at nm have been published. Several efforts geared at optimizing the buffer and active layer growths to induce stronger carrier localization in AlGaN MQW and/or reduce defect density have been suggested. 21,34, ) 100 ma dc RT PL 500 Å p + -GaN contact layer p-algan/algan SL 0-10 AlInGaN QWs n-algan/algan SL 0.8 µm n + AlGaN buffer Sapphire / n + - SiC Wavelength (nm) Fig. 5. Room-temperature electroluminescence spectrum of UV LED under 100 ma dc drive current. The RT PL spectrum of a corresponding structure is shown for comparison. The inset presents a schematic drawing of the quaternary UV LED structure (cited from ref. 135). The advantages of using thick-film GaN substrates prepared by HVPE for 350-nm-emission UV LEDs have been discussed and external quantum efficiencies of up to 1.2% have been reported on these low-dislocationdensity templates ) UV LEDs on SiC substrates emitting at nm have recently been reported by Edmond et al. demonstrating external quantum efficiencies of up to 4% for packaged devices. 153) Due to marked improvements in the operating characteristics of UV LEDs emitting at approximately nm, such devices are starting to find applications in the field of fluorescence spectroscopy. 37,154) SELECTED TOPICS in APPLIED PHYSICS III

8 7198 Jpn. J. Appl. Phys., Vol. 44, No. 10 (2005) Invited Paper M. ASIF KHAN et al. Power (µw) sapphire dc sapphire pulse SiC dc SiC pulse sapphire Voltage (V) Current (ma) 4.2 UV-B ( nm) LEDs Along with developmental efforts in achieving highpower LEDs in the nm range, another major research thrust has been to further shift the emission wavelength into the UV-B and even UV-C regions of the spectrum. Since the reduction in emission wavelength can only be achieved by increasing the Al molar fraction in the AlGaN or AlInGaN active region, this inevitably leads to problems with material quality and activating the dopants. Basically, pure AlN appears to be extremely difficult if not impossible to make electrically conducting. The closer the alloy compositions approach that of pure AlN, the larger the ionization energies for the dopants become. Therefore, both n-type and p-type AlGaN films with high Al concentrations exhibit high resistivities. Most deep-uv LEDs are prepared on transparent sapphire substrates, which are completely insulating. The current applied to deep UV LEDs must be spread laterally through an n-type AlGaN layer near the substrate, and a high resistivity in such a film limits the extent of current spreading, leading to crowding along the perimeter. 155) A passage of the current through highresistance films generates heat. There are even worse problems with p-type AlGaN. Since it is very difficult to fabricate an ohmic contact to p-algan films, most researchers apply a simple p-gan film to the top surface to serve as a contact layer, and this film can absorb UV emission. Even if one can inject holes effectively from the metal into the p-gan material, such holes are then faced with a potential barrier before they can be transported across the subsequent AlGaN barrier layers down to the quantum well. Holes that are trapped at the first interface set up an electric field that can attract electrons to bypass the quantum wells and hence recombine nonradiatively in the p-gan layer, which has a major impact on quantum efficiency. Hence, in terms of device performance, these issues create major challenges: uniformity of current spreading, carrier injection, light extraction and thermal management. The first UV-B LED with an emission wavelength at 305 nm was reported by Khan et al. 156) As before, the quaternary AlInGaN MQW active region was grown by PALE. Limitations in material quality, doping problems and insufficient thickness of the bottom n-algan layers put 20 SiC Current (ma) Power (µw) Fig. 6. Output power vs bias current for an AlInGaN MQW UV LED on sapphire (squares) and on SiC (circles) under dc (closed symbols) and pulsed (open symbols) pumping. The dc current vs voltage characteristics of sapphire- (squares) and SiC- (circles) based devices are shown in the inset (cited from ref. 135). 1 severe limits on performance due to the high sheet resistance of n-algan, which immediately created a problem of poor current spreading in mesa geometry devices on sapphire. To overcome the current spreading problem, a stripe geometry configuration was suggested. The growth of thick (up to 2.5 mm) highly doped n-algan was enabled after the development of novel AlGaN/AlN SL buffer layers 15,16) which resulted in a rapid development of improved UV-B LED devices. The submilliwatt operation for an LED with emission at 315 nm and the milliwatt operation for an LED with emission at 325 nm were next reported by Chitnis et al. 157,158) This improvement was achieved by implementing the innovative SL buffer layer approach for the growth of low-sheet-resistance n-algan, by optimizing the device geometry for better current spreading, and by introducing flip-chip packaging for improvements in thermal management and light extraction. The problem of lateral current crowding in sapphire-based deep UV LEDs has been analyzed and the optimization of device geometry through the use of an interdigitated multifinger geometry was suggested by Shatalov et al. 159,160) The optimization of growth approaches and the optimization of the LED structural design along with advanced packaging techniques resulted in milliwatt dc operation of LEDs with emission at 325 nm as reported by Chitnis et al. 44,161,162) 4.3 UV-C ( nm) LEDs As noted above there are strong reasons for desiring the development of solid-state devices emitting at wavelengths below 290 nm, such as the replacement of mercury-based bulbs relying on emission at 254 nm. With rapid developments in the preparation of high-quality AlGaN layers with higher Al concentrations and advances in the fabrication of LEDs emitting in the UV-B range, it became conceivable to move forward and develop even more advanced devices with emission in the UV-C spectral range. The initial submilliwatt operation of a nitride-based LED emitting in the UV-C band was demonstrated by Adivarahan et al. in 2002, a device that operated at 285 nm with a cw output power of 10 mw at 60 ma drive current. 163) Under pulsed driving conditions, this device yielded 0.15 mw with 400 ma applied. Relying on further advances in contact processing technology, they were able to achieve output powers as high as 0.25 mw at 650 ma under pulse pumping conditions with the next-generation devices. The output power was shown to be thermally quenched by at least 10 times as the operating temperature was raised from 100 K to room temperature under pulsed conditions. 164,165) This data indicated that nonradiative recombination at defects rather than the lack of hole transport to the quantum well is the key contribution to the low quantum efficiency of 285 nm devices. Further optimization of the AlN buffer layer quality by PALE has been reported by Zhang et al. 17,30) which enabled improvements in the material quality of n-algan bottom cladding layer as well as MQW layers; and these advances resulted in a rapid increase in LED output power up to milliwatt levels. 42) By optimization of the quality of the AlGaN layers grown on sapphire substrates, several groups have reported improved LED devices with emission at approximately 280 nm ) Although the majority of

9 Jpn. J. Appl. Phys., Vol. 44, No. 10 (2005) Invited Paper M. ASIF KHAN et al researchers described the optimization of AlGaN growth by MOCVD, others have presented the use of MBE with an ammonia source as an alternative approach, along with the use of innovative SPSL cladding layers ) In spite of tremendous progress, the quantum efficiency of these UV-C LEDs remained much lower than 1% and worse yet, the emission spectrum consisted of several peaks: a near band-edge emission from the quantum well and longwavelength peaks associated with deep-level transitions in barrier layers. 43,174) This stimulated a second round of device structure and growth optimizations, and recently many groups have reported a significant increase in device output power and spectral purity. 45,48, ) Fisher et al. 48) reported dc power levels as high as 1.34 mw at 300 ma for large-area 290 nm devices, while achieving an external quantum efficiency as high as 0.18% for devices with smaller areas. Recently, Sun et al. 19) and Zhang et al. 20) have demonstrated high-power UV-C LEDs with emission at around 280 nm with a dc output power of approximately 1 mw at 20 ma and a corresponding external quantum efficiency of 1:1%. UV-C LEDs with and emission wavelength at nm have also been reported by several groups. Remarkable device performance was achieved by Yasan et al. who reported submilliwatt dc and pulsed output powers as high as 4.5 mw at 267 nm corresponding to a quantum efficiency of 0:1%. 47) These powers have been further increased by Adivarahan et al. 46) and Bilenko et al., 178) who have recently reported quantum efficiencies of 0.4% and 0.2% at 269 nm and 265 nm, respectively. Submilliwatt pulsed operation of deep-uv-c LEDs with emission wavelength as short as 250 nm was reported in 2004 by Adivarahan et al. 179) Recently, Allerman et al. 176) have obtained an electroluminescence peak from a UV-C LED structure at a wavelength as short as 237 nm, demonstrating the possibility for further reduction in emission wavelength towards 200 nm. The dc operation of LEDs with a wavelength shorter than 260 nm becomes severely limited by the lack of conductivity of the bottom Si: doped AlGaN cladding layer since the Al molar fraction required for transparency increases above 70%. To improve current spreading in LEDs with high Al molar fractions in AlGaN cladding layers, an interconnected micropixel design has been adopted. 180,181) This use of micropixels was first introduced in III nitrides by Mair et al. 182) to improve light extraction from a AlGaN/GaN slab via the formation of microcavities, and the approach was then used in blue 183) and UV-A LEDs. 32) The micropixel LEDs design was further extended to the formation of photonic crystals 184) by the reduction in the size and the period of array elements. 185,186) An LED design with interconnected micropixels separated by the n-algan contact metal was also shown to be very efficient in achieving the desired uniform current pumping for deep UV-C LEDs, and devices with emission at 255 nm with 1 mw dc and 3.4 mw pulse powers and corresponding maximum quantum efficiencies of 0.14 and 0.3% (in dc and pulse pumping, respectively) have recently been demonstrated by Khan et al. 187) The normalized electroluminescence spectra of deep-uv LEDs are shown in Fig. 7. All the devices exhibit a distinct peak corresponding to near band-edge emission from the MQW active region with a full width at half maximum of Fig. 7. Electroluminescence spectra of deep UV LEDs under 20 ma dc pump current. Fig. 8. Output power vs dc pump current characteristics of packaged deep-uv LEDs. Data for mm 2 square device for 280 nm (squares) and 265 nm (circles) emission LEDs and micropixel array LED for 255 nm (triangles) emission LED are shown. about 10 nm. As can be seen, by proper choice of the AlGaN MQW active region composition the emission wavelength can be effectively tuned from 255 to 280 nm. The intensity of the main emission peak is stronger than that of longwavelength emission peaks (at nm) by at least 300 times (not shown). The output power vs pump current characteristics of these deep-uv LEDs are plotted in Fig. 8. From Fig. 8, the power (and quantum efficiency) strongly reduces with emission wavelength. This is a consequence of a lower doping efficiency and reduced material quality of AlGaN layers with very high Al contents. Increasing operation voltage also leads to early power saturation in dc pumping for square-geometry devices. Thus, as described above, interconnected micropixel array geometry was shown to be an effective solution for devices with shorter emission (i.e., 255 nm). Great efforts have been made by many groups around the world in developing novel advanced structures for III nitride deep-uv LEDs resulting now in the successful demonstration of compact and robust solid-state light sources, much needed for a variety of applications. To summarize the achievements that have been made in this area over the last 5 years, we present a graph showing the maximum quantum efficiency achieved for these devices vs the emission SELECTED TOPICS in APPLIED PHYSICS III