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Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments., publishedVersion

The presence of hexagonal-pyramid-shaped hillocks (HPHs) in AlGaN epitaxial films affects device characteristics; this effect is significant in DUV laser diodes (LDs) on AlN substrates, where the presence of HPHs under the p-electrode increases the threshold current density and inhibits the lasing. In this study, we investigated the difference between the lasing characteristics of LDs with and without HPHs. It was found that in the presence of HPHs, the threshold excitation power density increased and the slope efficiency decreased by optical excitation. To investigate the cause of these phenomena, we performed structural, optical, and electrical analyses of the HPHs. Various imaging techniques were used to directly capture the characteristics of the HPHs. As a result, we concluded that HPHs cause the degradation of LD characteristics due to a combination of structural, optical, and electrical factors.

The electroluminescence (EL) uniformity of AlGaN-based deep UV laser diodes on AlN substrate was analyzed by using the EL imaging technique. Although nonuniform EL patterns were observed, the uniformity was improved by changing the position of the p-electrode. The threshold current density was also reduced by suppressing the inhomogeneity of the EL. Cathodoluminescence analysis revealed that the cause of the non-uniformity is the degradation of the active layer and the nonuniformity emission formed by rapid thermal annealing at high temperature after mesa structure formation.

The space charge density profile of the nondoped AlGaN-based p-type cladding layer for UV-C laser diodes realized by distributed polarization doping is examined theoretically and experimentally. The analysis of the capacitance-voltage measurement revealed that the average effective acceptor density of 4.2 × 1017 cm–3 is achieved even without impurity doping, and it is in good agreement with the theoretical prediction from the measured Al composition profile. This result suggests that the cladding layer is ideal for UV-C LDs because it provides sufficient hole injection while potentially avoiding internal losses due to impurity doping.

We have demonstrated an on-wafer fabrication process for AlGaN-based UV-C laser diodes (LDs) with etched mirrors and have achieved lasing for 100 ns pulsed current injection at room temperature. A combined process of dry and wet etching was employed to achieve smooth and vertical AlGaN (1 1 ¯00) facets. These etched facets were then uniformly coated with a distributed Bragg reflector by atomic layer deposition. A remarkable reduction of the lasing threshold current density to 19.6 kA / cm 2 was obtained owing to the high reflectivity of the etched and coated mirror facets. The entire laser diode fabrication process was carried out on a whole 2-in. wafer. We propose this mirror fabrication process as a viable low-cost AlGaN-based UV-C LD production method that is also compatible with highly integrated optoelectronics based on AlN substrates.

We demonstrate high output power UVC-LEDs from 230 to 237 nm on AlN single-crystal substrates. The UVC-LEDs show a single peak in the electroluminescence spectrum, from 20 to 300 mA. Forward voltages were typically ~7 V at 100 mA while measured initial output powers at 237 nm, 235 nm, 233 nm and 230 nm were 2.2 mW, 1.9 mW, 1.5 mW and 1.2 mW, respectively. At 20 mA, the measured wall-plug efficiencies were 0.37%, 0.32%, 0.25% and 0.19% at the same wavelengths, respectively. These devices have demonstrated over 3600 h of lifetime operating at 20 mA.

We present a deep-ultraviolet semiconductor laser diode that operates under current injection at room temperature and at a very short wavelength. The laser structure was grown on the (0001) face of a single-crystal aluminum nitride substrate. The measured lasing wavelength was 271.8 nm with a pulsed duration of 50 ns and a repetition frequency of 2 kHz. A polarization-induced doping cladding layer was employed to achieve hole conductivity and injection without intentional impurity doping. Even with this undoped layer, we were still able to achieve a low operation voltage of 13.8 V at a lasing threshold current of 0.4 A.

Mid-ultraviolet LEDs grown on AlN substrates with a range of quantum efficiencies and wavelengths spanning 250 to 280 nm have been investigated by time-resolved photoluminescence and electroluminescence. Through scaling of room temperature internal quantum efficiencies across all devices, radiative and nonradiative lifetimes are also estimated. General trends observed include an increase in PL lifetime for longer wavelength and higher external quantum efficiency devices, consistent with the increase in the estimated nonradiative life-time with increasing wavelength. Despite these trends, the external quantum efficiency of the devices increases only weakly with increasing wavelength from 258 to 279 nm, suggesting that optimization of radiative lifetime and injection efficiency also play an important role. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Water disinfection has always been a focal point of public discourse due to both the shortage of water and the abundance of water-transmitted diseases. According to the World Health Organization (WHO), there are over 3.4 million reported deaths annually due to water, sanitation, and hygiene-related issues [1]. There are a number of technologies being developed to fight against issues related to waterborne diseases. The use of UV light is gaining popularity over chlorine disinfection due to a lack of aftertaste and harmful by-products after treatment. Specifically, UV radiation in the wavelength range of 250–280 nm (UVC) has been shown to effectively disinfect water. Current UVC technology, which uses low and medium pressure mercury lamps, is hampered by the use of fragile quartz housings, long warm up times, and the toxicity of mercury [2]. There is tremendous development happening in the field of semiconductor-based UVC LED technology. This technology can be efficient, cost-effective, and an environmentally friendlier alternative to traditional UVC technology. Emerging Al x Ga1−x N and AlN -based UVC light-emitting diodes (LEDs) provide many advantages over mercury lamps—including design flexibility, low power consumption, and environmentally friendly construction [3]. Unlike low-pressure mercury lamp technology, which is limited to an emission wavelength near 254 nm, LEDs can be tailored to specific wavelengths throughout the UVC range. UVC LEDs are being developed for disinfection in the 265 nm wavelength range and are showing tremendous progress in power output and device lifetime [4]. This progress has been driven by the relatively recent development of high quality, single-crystal AlN substrates. These AlN substrates allow the growth of pseudomorphic Al x Ga1−x N device layers with very low defect densities. These low defect densities have resulted in improvements in efficiency and power as discussed in this chapter. In addition, UVC LEDs emit radiation in a very different pattern than mercury lamps or other UVC sources that they are replacing. For instance, the LEDs can be designed to emit in a “Lambertian” pattern, allowing the LED to be imaged as a near point source. Using UVC LEDs in water disinfection requires substantial rethinking in the arrangement of UVC radiation sources to achieve an efficient system. In this chapter, we will discuss some of the important parameters necessary for successful flow cell design. The chapter will show the design flexibility offered by UVC LEDs, with some examples of potential designs examined through optical modeling. We will also review some of the recent progress in improving UVC LEDs through pseudomorphic growth of Al x Ga1−x N on single crystal AlN substrates.

Low dislocation density pseudomorphic epitaxial layers of Al x Ga 1- x N have been grown on c -face AlN substrates prepared from high quality bulk crystals. As reported previously, pseudomorphic growth yields very low dislocation density layers with atomically smooth surfaces throughout the active region of a full LED device structure. An advantage of the low dislocation density is the ability to n -type dope the high aluminum content Al x Ga 1- x N (x ~ 70%) epitaxial layers required for UVLED devices to obtain sheet resistances less than 350 Ohm/square for 0.5 μm thick layers. Here, we report on the characterization of our pseudomorphic epitaxial AlGaN layers via cathodoluminescence (CL) and on-wafer and initial packaged level characterization of fully fabricated pseudomorphic ultraviolet LEDs (PUVLEDs) with an emission wavelength between 250 - 265 nm. An additional benefit of PUVLED devices is the ability to run these devices at high input powers and current densities. Further, the aforementioned low dislocation density of the epitaxial structure results in improved device performance over previously published data. Mean output powers of greater than 4 mW were obtained on-wafer prior to thinning and roughening while output powers as high as 45 mW were achieved for packaged devices.

High quality, bulk aluminum nitride substrates were used to obtain pseudomorphic AlxGa1-xN layers with low dislocation density, smooth surfaces, and high conductivity. These layers were fabricated into mid-ultraviolet light emitting diodes with peak wavelengths in the range of 240-260 nm. From transmission electron microscope images, it was confirmed that the dislocation density in the n-type Al0.7Ga0.3N layer is low with no dislocations in the field of view. The low dislocation density continued through the quantum wells and electron blocking layer. However, the lattice mismatch between the pseudomorphic AlxGa1-xN layers and the p-type GaN contact layer is high (approximately 2.4%) and the pseudomorphic growth could not be achieved for thick layers. Instead, a network of misfit dislocations was formed at the GaN/AlxGa1–xN interface, which relieved strain in the GaN layer. The threading dislocations did not appear to propagate into the active region and thus enable the achievement of high internal (IQE) and external quantum efficiencies (EQE) from the devices fabricated on low dislocation density pseudomorphic active layers. Values of EQE as high as 1.44% for a device emitting at 248 nm operated in pulsed mode were achieved. Devices were packaged in lead frame packages and achieved output powers of 45 mW at 1.5 A for a 257 nm device with a peak EQE of 0.8% at 200 mA when operated in pulsed mode. The thermal properties of the packaged devices limited high current performance in CW mode. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the growth of sub-300 nm ultraviolet light emitting diodes (UV LEDs) on bulk AlN substrates. Heteroepitaxial evolution study through interrupted growth experiments revealed that AlxGa1-xN (x > 0.5) epilayers can be grown pseudomorphically with well-defined step-flow growth mode below a certain critical thickness. The build-up of compressive strain energy eventually induces a morphological roughening followed by the admission of misfit dislocations. LEDs grown on bulk AlN substrates exhibit noticable improvement over those on sapphire in device impedance, efficiency and thermal characteristics under high-level injection, pointing to a promising substrate platform for high performance III-nitride ultraviolet optoelectronics. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reliability tests on packaged ultraviolet light-emitting diodes (LEDs) in the ultraviolet-C (UVC) range (< 280 nm) were performed under various temperatures and currents. At higher case temperature, a greater degradation in output power was observed over 1,000 hours. Degradation in output power was also observed at higher operating currents. Some of the output power degradation can be attributed to increased leakage current generated in a subset of devices tested. Physical analysis of a stressed LED indicated a connection between degradation in output power and defects in multi-quantum well (MQW), originating from crystal defects in substrate.

Recent advances in mid-ultraviolet light-emitting diodes grown pseudomorphically on bulk AlN substrates have led to improved efficiencies and lifetimes. For a 266 nm device an output power of 66 mW at 300 mA has been achieved with an external quantum efficiency of 4.5%. More importantly, the lifetimes of these devices have been increased substantially. Testing of LEDs in both surface mount design (SMD) and TO-39 packages show L50 lifetimes well in excess of 1,000 hours under a variety of case temperatures and currents. Package-related catastrophic failures are eliminated through encapsulation and hermetic sealing, further reducing failure rates and extending the lifetime.

Low dislocation density, epitaxial layers of AlxGa1–xN are grown pseudomorphically on native AlN substrates prepared from high quality, bulk crystals. In addition to low dislocation density, they are atomically smooth and can be doped n-type to obtain sheet resistances < 200 Ohms/sq/μm. These layers are used as templates for the growth of high quality multiple quantum well (MQW) structures and ultraviolet (UV) light emitting diodes (LED). The pseudomorphic growth and atomically smooth surfaces are achieved for a full device structure. Previously we have demonstrated the high optical quality of these structures through temperature dependant photoluminescence measurements and X-ray diffraction. Recently full device structures have been fabricated and packaged. These devices, with emission wavelength between 250 nm and 280 nm show linear increase in output power as a function of current up to 150 mA in thermal equilibrium. Additionally output powers up to 1.3 mW have been achieved at 258 nm at 400 mA drive current. Reliability of the devices has been measured for 1000 hours with expected lifetimes greater than 3000 hours for input currents up to 150 mA (with an average current density of approximately 150 A/cm2). (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Recently it has been discovered that when growing Al x Ga 1− x N on low-defect-density bulk AlN substrates pseudomorphic layers can be achieved with a thickness far exceeding the critical thickness as given by the Matthews and Blakeslee model. For instance, the critical thickness of an Al x Ga 1− x N layer (with x =0.6) is about 40 nm thick. However we have been able to grow layers with this composition that are pseudomorphic with a thickness exceeding the critical thickness by more than an order of magnitude. This work defines the limits of pseudomorphic growth on low defect density, bulk AlN substrates to obtain low defect density, high-power UV LEDs.

The hot electron transport properties and growth morphology of ultrathin Pt/CaF2/Si(111) metal-insulator-semiconductor structures have been characterized in situ by ballistic electron emission microscopy (BEEM) and scanning tunneling microscopy (STM). Platinum thicknesses from submonolayer to 50 A and CaF2 thicknesses from 2 to 10 ML have been characterized. The STM images of the Pt/CaF2/Si(111) structures show the atomic steps of the underlying CaF2 morphology, as well as the formation of Pt nodules, which nucleate at step edges and defect sites. Some BEEM spectra show an anomalous peak near 2 eV, which has not been observed in previous studies of other metal/CaF2/Si(111) structures. The localized nature of this peak indicates that it results from an interaction between the ballistic electrons and fluorine vacancies at the Pt–CaF2 interface.

A scanning tunneling microscope that performs scanning tunneling microscopy, scanning tunneling spectroscopy, and ballistic electron emission microscopy measurements on 2-in. wafers has been designed and constructed. The instrument is incorporated into the Si preparation chamber of a cryo-pumped Fisons V90H Si/III-V molecular beam epitaxy machine. Its design uses two commercial Burleigh inchworms: one for performing scanning tunneling microscopy measurements and a second for making a front contact which is necessary for the ballistic electron emission microscopy measurements. The substrate holder for the V90H system is designed to handle wafers up to 6 in. in diameter. Therefore, a custom 6-in.-diam. holder has been constructed which supports two 2-in. holders: one for performing reflection high energy electron diffraction measurements and a second which allows transfer of the wafer to the scanning tunneling microscope and also incorporates a removable shadow mask for growing metal/semiconductor Schottky ...

Mid ultraviolet light emitting diodes were pseudomorphically grown on bulk AlN substrates. Devices stressed at 100mA show minimal power decay for over 1000 hours. 66mW output power is achieved at continuous wave current of 300mA.

UVC light output of 66 mW at 300 mA CW has been achieved from LEDs on AlN substrates with extensive photon extraction. Proper vessel design allows for efficient irradiation of a water sample for purification.

Electrical transport through high-density arrays of carbon nanotubes grown within vertical pores of anodized alumina was measured. Individual nanotubes were studied using conductive tip atomic force microscopy, with bias applied between the tip and platinum back electrode. Multiwalled nanotubes of diameter about 50 nm, with 5 nm thick walls were found to have a resistivity lower than 1.4×10−5 Ω m. A potential barrier was found to exist between the sensing tip and nanotube, resulting in nonlinear current–voltage characteristics. Low-resistance contact was formed by breaking down this barrier, once the circuit was stressed beyond 1.5 V.

Improvements to 260 nm ultraviolet light emitting diodes has led to wall plug efficiencies greater than 5% and output powers of 50 mW for a single chip in continuous wave operation.

We designed and fabricated 260 nm ultraviolet light emitting diodes that were pseudomorphically grown on AlN substrates. Thinning the substrates, roughening the emission surfaces, and encapsulating contributed to increasing the photon extraction efficiency by more than one order of magnitude.

We designed and fabricated 260 nm ultraviolet LEDs which demonstrated 20mW with greater than 1.5% WPE. Through surface roughening, substrate thinning and encapsulation, a 6× improvement in photon extraction efficiency has been achieved.

We measured the surface electric potential distribution on individual, electrically contacted and biased, multiwall carbon nanotubes using scanning surface potential microscopy. The voltage varies linearly along the nanotube and the voltage drop is directly proportional to the bias applied between electrodes. Its resistance decreases four times when a 4.5 V bias is applied between the nanotube and the substrate. Under these conditions, we were able to resolve the voltage drop along the nanotube and at the contacts, providing a unique way of measuring contact resistance, which is observed in this case to be on the order of 50 kΩ.

Ultraviolet-C (UVC) disinfection is considered environmentally friendly since it does not create any chemical load on the environment and does not generate any hazardous byproducts. Mercury lamps (which are essentially fluorescent lights without the phosphors) are currently used by municipalities to generate UVC but mercury lamps are not an attractive solution for low volumes or non-continuous use due to high power consumption, fragility and disposal issues. UVC LEDs would be an attractive alternative but, unlike visible LEDs, the current generation of UVC LEDs, fabricated from nitride semiconductors grown on foreign substrates, is rather inefficient with relatively poor lifetimes. Crystal IS is developing UVC LEDs to solve these issues by utilizing very high quality pseudomorphic layers of Al x Ga 1−x N grown on native AlN substrates prepared from high quality, bulk crystals. These layers are fabricated into mid-ultraviolet light emitting diodes (referred to as pseudomorphic ultraviolet light emitting diodes or PUVLEDs) with peak wavelengths in the range of 240–275 nm. The low threading dislocation density in the active region (

Bulk AlN substrates are an ideal solution for the growth of ultraviolet-C light emitting diodes (UVCLED). They are optically transparent at this wavelength and closely lattice and thermally expansion matched to the layers required for the device structure. In addition pseudomorphic growth can be achieved allowing for low dislocation density in the active region of the device [1]. This approach has been used to achieve the best efficiency and output power [2], as well as lifetime [3], in the 265 nm wavelength range. However, currently these substrates are only available in limit quantities and sizes preventing large scale manufacturing such as is occurring with visible LEDs. Regardless, we have been able to succeed in setting up pilot production of UVCLEDs to enable low volume manufacturing and allow for rapid increase in volumes as supply and size of the AlN substrates are increased.

Light sources in the wavelength range below 300nm have attracted extensive attention due to their applications in instrumentation and in disinfection of water, air and surface. Nitride-semiconductor-based, light emitting diodes (LEDs) are of especially great interest due to numerous advantages compared to conventional mercury lamps. Although significant progress of more than 9mW of quasi-CW power around 260nm at room temperature has been achieved from devices with Al x Ga 1−x N heterostructures pseudomorphically grown on native AlN substrate [1], those devices still have relatively low photon extraction efficiencies (estimated to be around 4%). In particular, half of the generated photons are directed towards the p-contact and absorbed by the smaller bandgap p-GaN. The other half of the photons, which are directed toward the exit surface, will experience absorption in AlN substrate due to point defects which create states within the bandgap of the AlN material. In addition, only a small fraction of those photons actually reaching the exit surface will escape due to the relatively large refractive index difference between AlN and air resulting in a narrow escape cone.

The use of low defect density bulk AlN substrates has led to reliable pseudomorphic ultraviolet light emitting diodes capable of mW level power outputs from packaged devices emitting from 250–275 nm.

Ultra-violet light-emitting diodes (UV-LEDs) fabricated with integrated surface textures show an enhancement of the output power and more directionality of the emitted light, which is in agreement with the calculated results.

The objective of this project was to develop and then demonstrate the efficacy of a costeffective approach for a low defect density substrate on which AlInGaN LEDs can be fabricated. The efficacy of this “GaN-ready” substrate would then be tested by growing high efficiency, long lifetime InxGa1-xN blue LEDs. The approach used to meet the project objectives was to start with low dislocation density AlN single-crystal substrates and grow graded AlxGa1-xN layers on top. Pseudomorphic AlxGa1-xN epitaxial layers grown on bulk AlN substrates were used to fabricate light emitting diodes and demonstrate better device performance as a result of the low defect density in these layers when benched marked against state-of-the-art LEDs fabricated on sapphire substrates. The pseudomorphic LEDs showed excellent output powers compared to similar wavelength devices grown on sapphire substrates, with lifetimes exceeding 10,000 hours (which was the longest time that could reliably be estimated). In addition, high internal quantum efficiencies were demonstrated at high driving current densities even though the external quantum efficiencies were low due to poor photon extraction. Unfortunately, these pseudomorphic LEDs require high Al content so they emit in the ultraviolet. Sapphire based LEDs typically have threading dislocation densities (TDD) > 108 cm-2 while the pseudomorphic LEDs have TDD ≤ 105 cm-2. The resulting TDD, when grading the AlxGa1-xN layer all the way to pure GaN to produce a “GaN-ready” substrate, has varied between the mid 108 down to the 106 cm-2. These inconsistencies are not well understood. Finally, an approach to improve the LED structures on AlN substrates for light extraction efficiency was developed by thinning and roughening the substrate.

Photoluminescence lifetimes of nearly dislocation free high Al content AlGaN MQW LEDs on bulk AlN are presented as a function of temperature and excitation power for both direct photo-excitation of the wells and barriers.

Time-resolved photoluminescence and transmission electron microscopy results suggest that the density of point defects may have a more significant role than threading dislocations in the performance of UVLED AlGaN active regions emitting at shorter wavelengths.

Reliability and performance of ultraviolet light emitting diodes have suffered due to the high dislocation density of the AlN and high Al-content AlxGa1-xN layers when grown on foreign substrates such as sapphire. The development of pseudomorphic layers on low dislocation density AlN substrates is leading to improvements in reliability and performance of devices operating in the ultraviolet-C (UVC) range. One major improvement is the ability to operate devices at much higher current densities and input powers than devices on sapphire substrates. This is due to the better thermal properties and lower dislocation density of devices on AlN substrates. Devices with active area of 0.001 cm2 emitting at ∼265 nm have been measured for their reliability and change in power output over time at input currents of 20 mA (20 A/cm2), 100 mA (100A/cm2) and 150 mA (150 A/cm2). When operating at currents of 20 mA over 3500 hours of consecutive operation has been demonstrated with typical decay of ∼27% over the 3500 hours. Extrapolating the decay with a linear fit gives a L50 (time to 50% of initial power) of ∼5000 hrs. However it is desirable to be able to model the decay to better understand the kinetics and better understand the mechanisms. In order to do this, the lifetime at 20 mA and 100 mA were modeled using an exponential decay function, square root transformation and a log transformation to both be able to fit the experimental data and predict future performance.

Aluminum nitride (AlN) offers exceptional properties necessary to explore the development of large area substrates for nitride based electronics and photonics. Recent studies on AlN bulk growth using the sublimation-recondensation method developed at Crystal IS demonstrated high-quality crystals with low dislocation density and crystallographic uniformity. The diameter enlargement of these AlN boules is often associated with extensive defect generation. The goal of this study is to evaluate the origin and distribution of growth defects in AlN bulk crystals. AlN crystals were grown using the sublimation-recondensation technique and then they were sliced into wafers. The defect evaluation in this study was performed using x-ray topography, differential image contrast and polarized-light optical microscopy, atomic force microscopy (AFM) and etch pit pattern delineation. Special attention was paid to crack development and propagation, grain boundary distribution, micro-scale inhomogenities as well as to the origin and density of dislocations. The major cause of growth defect appears to be non-linearity of both axial and radial temperature gradients. Growth optimization results in lower defect density and improved crystallinity of the AlN crystals.

Carbon nanotubes have the potential of being used as interconnects and active semiconducting material in future electronic circuits. It is necessary to study such nano-scale circuits with probes that can make measurements with molecular precision. We describe results using two nanoprobe techniques, namely scanning surface potential microscopy (SSPM), and conductive tip atomic force microscopy (CT-AFM), in the investigation of electrical properties of nanotube circuits. Vertical arrays of multi-walled nanotubes, grown in a porous alumina template with a metal back contact were analyzed. Current mapping confirmed that the nanotubes were electrically connected to the back contact. Isolated single-walled nanotube bundles deposited on an oxidized silicon wafer, and contacted electrically through chromium electrodes were also studied. Contact potential differences between the metal and nanotubes, and the current in some connected nanotubes were measured. Measurements of contact potential with different metals, and the nature of microscopic transport is crucial. Contact potential measurements can also provide fast and reliable characterization of junctions between metallic and semiconducting nanotubes and metals electrodes.

Deep ultraviolet (DUV) photons interact strongly with a broad range of chemical and biological molecules; compact DUV light sources could enable a wide range of applications in chemi/bio-sensing, sterilization, agriculture, and industrial curing. The much shorter wavelength also results in useful characteristics related to optical diffraction (for lithography) and scattering (non-line-of-sight communication). The family of III–N (AlGaInN) compound semiconductors offers a tunable energy gap from infrared to DUV. While InGaN-based blue light emitters have been the primary focus for the obvious application of solid state lighting, there is a growing interest in the development of efficient UV and DUV light-emitting devices. In the past few years we have witnessed an increasing investment from both government and industry sectors to further the state of DUV light-emitting devices. The contributions in Semiconductor Science and Technology 's special issue on DUV devices provide an up-to-date snapshot covering many relevant topics in this field. Given the expected importance of bulk AlN substrate in DUV technology, we are pleased to include a review article by Hartmann et al on the growth of AlN bulk crystal by physical vapour transport. The issue of polarization field within the deep ultraviolet LEDs is examined in the article by Braut et al. Several commercial companies provide useful updates in their development of DUV emitters, including Nichia (Fujioka et al ), Nitride Semiconductors (Muramoto et al ) and Sensor Electronic Technology (Shatalov et al ). We believe these articles will provide an excellent overview of the state of technology. The growth of AlGaN heterostructures by molecular beam epitaxy, in contrast to the common organo-metallic vapour phase epitaxy, is discussed by Ivanov et al. Since hexagonal boron nitride (BN) has received much attention as both a UV and a two-dimensional electronic material, we believe it serves readers well to include the article by Jiang et al on using BN for UV devices; potentially as a p-type wide band gap semiconductor contact. Finally, an in-depth discussion of one DUV application in defense, the non-line-of-sight (NLOS) communication, is given by Drost and Sadler. Overall, we believe that this special issue of Semiconductor Science and Technology provides a useful overview of the state-of-art in the field on DUV materials and devices. In view of the rapidly growing interest in this field, the demonstrated enhanced device performance, and the wide range of applications, this special issue can be considered a very timely contribution. Finally, we would like to thank the IOP editorial staff, in particular Alice Malhador, for their support and also like to thank all contributors for their efforts to make this special issue possible.

In order to build electrical devices based on carbon nanotubes, it is crucial to understand the effects of the substrate, electrical contacts, defects and junctions on the transport properties of both multi-wall and single-wall carbon nanotubes (MWNT and SWNT). In the present study two different dispositions are analyzed. In one of them, isolated MWNT were deposited onto an insulating oxide layer over a conducting substrate and platinum electrical contacts were prepared via a focused ion beam (FIB) deposition method. An ohmic contact to the substrate allowed us to polarize it at a different potential than the nanotubes themselves. In the second one, a vertical array of MWNT was grown on a template of alumina in which one of the ends was coated with gold, to provide an electric contact. To investigate transport properties, we used an atomic force microscope (AFM) to determine the electric surface potential of MWNT. The technique employed, scanning surface potential microscopy (SSPM), applied a dc-voltage to the tip that equilibrates the local electrostatic potential on the sample so as to eliminate forces on the AFM tip caused by electric repulsion or attraction between tip and sample. There is no alteration of the sample potential caused by the tip. Surface potential measurements on the first system show that the electric transport on the MWNT has a diffusive character. The application of a (4.5V) bias between nanotube and substrate causes the resistance of the nanotube to drop by a factor of four. We can also resolve the voltage drop on the nanotube and at the contacts. The measurements in the second system (vertical arrangement) show that the nanotubes are uncapped and we can observe a contrast that varies with the application of different voltages to the back contact.

Magnesium is incorporated into the growth of Ca1-xMgxF2 to reduce the lattice constant of fluorite (CaF2) which is 0.6% larger than that of Si at room temperature. When grown epitaxially on Si(111) substrates at 300°C, the lattice constant of the alloy became smaller than that of Si by 1.5% when the Mg concentration was around 20%. At higher Mg concentrations, the lattice constant did not decrease any further. This invariability of the lattice constant was caused by a phase separation of the Ca1-xMgxF2 layer into a Mg-rich region and a Mg-deficient region. When the growth temperature was increased, the critical Mg concentration for the phase separation became smaller. When Ca1-xMgxF2 was grown on vicinal Si(111) substrates, the reflection high energy electron diffraction (RHEED) intensity oscillations reflected no change in the composition, suggesting segregation of a Mg-rich phase along the steps. Nevertheless, the oscillations in the intensity of the specular spot for Ca1-xMgxF2 lasted longer than those observed for pure CaF2, suggesting a flatter surface for the alloy. Scanning tunneling microscopy (STM) observations support this model.

High quality bulk aluminum nitride substrates were used to obtain pseudomorphic AlxGa1-xN layers with low dislocation density, smooth surfaces, and high conductivity. These layers were fabricated into mid-ultraviolet light emitting diodes with peak wavelengths in the range of 240–260 nm. The low dislocation density of the pseudomorphic quantum wells resulted in improved performance over previously published data. The output powers of the on-wafer measurements were greater than 5 mW in continuous wave operation, and 16 mW in pulsed operation. This was achieved utilizing single die (with an active area of 1×10-3 cm2).

Here, we report on our studies of ballistic electron transport across metal layers and metal/semiconductor interfaces using ballistic-electron-emission microscopy (BEEM). This new technique, which uses a scanning tunneling microscope to inject electrons with a controlled energy into a thin metal film, allows measurements (with spatial resolution approaching 1 nm) of (1) the local Schottky barrier height, (2) ballistic mean free paths of energetic electrons (or holes), and (3) transmission probability of hot carriers across the metal/semiconductor interface. We have measured the attenuation length of hot electrons (1.5 eV above the Fermi level) in PtSi to be approximately 4 nm. This should be compared with an attenuation length of 13 nm for similar energy electrons in Au layers. BEEM images of the Au/Si interface show features on a very small length scale suggesting that the inelastic mean free path of electrons in Au is close to the attenuation length. The SB height (as determined by BEEM) is 0.87 eV in good agreement with optical measurements. We have also used BEEM to observe the sharp onset of inelastic scattering mechanisms in Au/Si and in PtSi/Si. It is our belief that these studies of ballistic carrier transport will allow a fundamental determination of how to achieve higher quantum efficiencies in SBIRDs.

The authors report the growth of AlGaN epilayers and deep ultraviolet (UV) light emitting diodes (LEDs) on bulk AlN substrates. Heteroepitaxial nucleation and strain relaxation are studied through controlled growth interruptions. Due to a low density of preexisting dislocations in bulk AlN, the compressive strain during AlGaN heteroepitaxy cannot be relieved effectively. The built-up of strain energy eventually induces either an elastic surface roughening or plastic deformation via generation and inclination of dislocations, depending on the stressor interlayers and growth parameters used. AlGaN LEDs on bulk AlN exhibit noticeable improvements in performance over those on sapphire, pointing to a promising substrate platform for III-nitride UV optoelectronics.

Current-injection ultraviolet lasers are demonstrated on low-dislocation-density bulk AlN substrates. The AlGaInN heterostructures were grown by metalorganic chemical vapor deposition. Requisite smooth surface morphologies were obtained by growing on near-c-plane AlN substrates, with a nominal off-axis orientation of less than 0.5°. Lasing was obtained from gain-guided laser diodes with uncoated facets and cavity lengths ranging from 200 to 1500 μm. Threshold current densities as low as 13 kA/cm2 were achieved for laser emission wavelengths as short as 368 nm, under pulsed operation. The maximum light output power was near 300 mW with a differential quantum efficiency of 6.7%. This (first) demonstration of nitride laser diodes on bulk AlN substrates suggests the feasibility of using such substrates to realize nitride laser diodes emitting from the near to deep ultraviolet spectral regions.

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectra are obtained from a donor in aluminum nitride (AlN) crystals. Although observed in as-grown crystals, exposure to x rays significantly increases the concentration of this center. ENDOR identifies a strong hyperfine interaction with one aluminum neighbor along the c axis and weaker equivalent hyperfine interactions with three additional aluminum neighbors in the basal plane. These aluminum interactions indicate that the responsible center is a deep donor at a nitrogen site. The observed paramagnetic defect is either a neutral oxygen substituting for nitrogen (ON0) or a neutral nitrogen vacancy (VN0).

Resonant tunneling structures [M. Bockrath, W. Liang, D. Bozovic, J. H. Hafner, C. B. Lieber, M. Tinkham, and H. Park, Science 291, 283 (2001)], formed between the junction of two single walled nanotubes and the conductive atomic force microscopy tip contact were investigated using current sensing atomic force microscopy. Oscillations in the current voltage characteristics were measured at several positions of the investigated nanotube. The oscillatory behavior is shown to follow a simple quantum mechanical model, dependent on the energy separation in the quantum well formed within the two junctions. Our model shows that these observations seen over several hundreds of nanometers, are possible only if the scattering cross section at defects is small resulting in long phase coherence length, and if the effective mass of the carrier electrons is small. We have calculated the approximate mass of the conduction electrons to be 0.003me.