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Although the pulsed operation of AlGaN-based laser diodes at UV-C wavelengths has been confirmed in the previous studies, continuous oscillation without cooling is difficult because of the high operating voltage. In this study, the temperature dependence of key parameters was investigated and their impact on achieving continuous-wave lasing was discussed. A reduction in the threshold voltage was achieved by tapering the sides of the laser diode mesa and reducing the lateral distance between the n- and p-electrodes. As a result, continuous-wave lasing at room temperature was demonstrated at a threshold current density of [Formula: see text] and a threshold voltage of [Formula: see text].

Previously reported UV-C laser diode (LD) structures have been subject to design constraints owing to dark line defects at the edge of the mesa stripe after device fabrication. To address this issue, a detailed analysis revealed that the dark line defects were dislocations generated by local residual shear stresses associated with mesa formation on highly strained epitaxial layers. A technique for controlling the local concentration of shear stress using a sloped mesa geometry was proposed based on insight gained by modeling the stress distribution at the edge of the mesa stripe. Experimental results showed that this technique succeeded in completely suppressing the emergence of dark-line defects. This technique will be useful in improving the performance of pseudomorphic AlGaN/AlN-based optoelectronic devices, including UV-C LDs.

UV-C laser diodes (LDs) have not been realized for many years owing to the problems of crystal quality and p-type conductivity control. In our group, AlGaN-based LD structures with low dislocation density were fabricated using AlN single-crystal substrates, and a p-type cladding layer with sufficient hole concentration was realized without impurities doped by distributed polarization doping (DPD). As a result, we have demonstrated pulsed current injection UV-C LDs at room temperature. We have also developed an on-wafer process technology to solve the problems of LDs manufactured by the conventional cleavage method. The key points of this method are the flatness of mirror facets, their angle to the cavity, and the coating of distributed Bragg reflector (DBR) on the mirror facets formed perpendicular to the wafer. The method is a combination of dry etching and TMAH wet etching to selectively expose the m-plane, and the DBR fabrication by atomic layer deposition (ALD) which provides good coverage. The LDs fabricated by the on-wafer method were observed to lase in the UV-C region when a pulsed current was injected similarly to the LDs fabricated by the cleavage method. These results indicate the potential of not only the realization of UV-C LDs but also for the fabrication of devices using high-Al-composition AlGaN with p-type conductivity and optical integrated circuits.

We demonstrated continuous-wave lasing of an AlGaN-based ultraviolet laser diode, fabricated on a single-crystal AlN substrate when operating at 5 °C. The threshold current density and device series resistance were reduced by improvements to the epitaxial structure and electrode arrangement. A peak wavelength of 274.8 nm was observed for lasing at a drive current over 110 mA, which corresponded to a threshold current density of 3.7 kA cm−2. The operating voltage at the threshold current was as low as 9.6 V.

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.

Single-crystal Aluminum Nitride (AlN) crystals enable the epitaxial growth of ultrawide bandgap Al(Ga)N alloys with drastically lower extended defect densities. Here, we report the plasma-MBE growth conditions for high Al-composition AlGaN alloys on single-crystal AlN substrates. An AlGaN growth guideline map is developed, leading to pseudomorphic AlxGa1−xN epitaxial layers with x ∼0.6–1.0 Al contents at a growth rate of ∼0.3 μm/h. These epitaxial layers exhibit atomic steps, indicating step flow epitaxial growth, and room-temperature band edge emission from ∼4.5 to 5.9 eV. Growth conditions are identified in which the background impurity concentrations of O, C, Si, and H in the MBE layers are found to be very near or below detection limits. An interesting Si segregation and gettering behavior is observed at the epitaxial AlGaN/AlN heterojunction with significant implications for the formation and transport of 2D electron or hole gases. Well-controlled intentional Si doping ranging from ∼2 × 1017 to 3 × 1019 atoms/cm3 is obtained, with sharp dopant density transition profiles. In Si-doped Al0.6Ga0.4N epilayers, a room-temperature free electron concentration of ∼3 × 1019/cm3, an electron mobility of ∼27 cm2/V s, and an n-type resistivity of ∼7.5 m Ω cm are obtained. The implications of these findings on electronic and photonic devices on single-crystal AlN substrates are discussed.

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.

The evolution of surface morphology for single-crystal bulk Al-polar aluminum nitride substrates during ex situ cleaning, in situ cleaning, and subsequent homoepitaxy is investigated. Ex situ acid treatment is found to reveal atomic steps on the bulk AlN substrates. After in situ Al-assisted cleaning at high temperatures in a high vacuum environment monitored with reflection high-energy electron diffraction, cleaner atomic step edges are observed. Subsequent growth on the cleaned bulk AlN by molecular beam epitaxy is used to develop a phase-diagram for homoepitaxy on AlN single crystals. Secondary ion mass spectrometry profiles reveal high-purity epitaxial layers with undesired chemical impurity densities of Si, O, and C to be below detection limits. The grown homoepitaxial films are observed to oxidize in the ambient environment, but repeating the ex situ acid treatment again reveals atomic steps.

We compare the effectiveness of in situ thermal cleaning with that of Al-assisted cleaning of native surface oxides of bulk AlN for homoepitaxial growth by molecular beam epitaxy. Thermal deoxidation performed at 1450 ° C in vacuum results in voids in the AlN substrate. On the other hand, Al-assisted deoxidation at ≈ 900 ° C results in high-quality AlN homoepitaxy, evidenced by clean and wide atomic terraces on the surface and no extended defects at the growth interface. This study shows that Al-assisted in situ deoxidation is effective in removing native oxides on AlN, providing a clean surface to enable homoepitaxial growth of AlN and its heterostructures; furthermore, it is more attractive over thermal deoxidation, which needs to be conducted at much higher temperatures due to the strong bonding strength of native oxides on AlN.

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)

The optical and structural properties of AlGaN active regions containing nanoscale compositional inhomogeneities (NCI) grown on low dislocation density bulk AlN substrates are reported. These substrates are found to improve the internal quantum efficiency and structural quality of NCI-AlGaN active regions for high Al content alloys, as well as the interfaces of the NCI with the surrounding wider bandgap matrix, as manifested in the absence of any significant long decay component of the low temperature radiative lifetime, which is well characterized by a single exponential photoluminescence decay with a 330 ps time constant. However, room temperature results indicate that non-radiative recombination associated with the high point defect density becomes a limiting factor in these films even at low dislocation densities for larger AlN mole fractions.

We report on the progress of high quality AlN bulk crystal growth by the sublimation-recondensation technique and present a theoretical model for optimizing the growth conditions. The theoretical model is consistent with our experimental findings and projects a path to maximize the growth rate by adjusting the growth temperature, external nitrogen pressure and source-to-seed distance. The growth of large AlN boules has resulted in the demonstration of crack-free AlN wafers up to 2-inch diameter. The crystallinity of these AlN boules and wafers has been characterized by X-ray techniques and etch pit density (EPD) measurements. The AIN wafers exhibited X-ray rocking curves with a full width at half maximum (FWHM) close to 30 arcsec for both symmetric and asymmetric curves with a corresponding EPD of < 10 4 cm ―2 . High quality homoepitaxial and graded AlGaN layers have been grown on these AIN substrates by organometallic vapor phase epitaxy (OMVPE). We have demonstrated pseudomorphic growth of graded Al 1―x Ga x N layers with a thickness of one order of magnitude higher than the expected critical thickness from the classical Matthews―Blakeslee theory. This achievement has resulted in low dislocation density AlGaN epi-layers with both symmetric and asymmetric rocking curves routinely below 100 arcsec.

We report on the status of the vapor growth of high-quality AlN bulk crystals by the sublimation-recondensation technique for the commercial production of AlN wafers up to 2 in diameter. AlN boules and wafers have been characterized by X-ray Laue backscattering, diffraction, and rocking curves measured in double-axis configuration, demonstrating a full-width at half-maximum (FWHM) of 28 and 32 arcsec for the symmetric and asymmetric rocking curves, respectively. The etch pit density (EPD) for different AlN substrate orientations was found to be less than 10 4 cm -2 . Impurity levels have been measured by glow discharge mass-spectrometry (GDMS) and secondary ion mass-spectroscopy (SIMS). The oxygen content as measured by SIMS shows concentrations

In order to meet the need for higher quality nitride substrates, 2 in diameter boules of AlN have been developed and, from them, 2 in diameter substrates have been prepared with high crystalline quality. Double-crystal X-ray rocking curves indicate a full-width at half-maximum (FWHM) of 65 and 83 in on the symmetric (0002) and the asymmetric (1014) lines, respectively. Etch pit density (EPD) measurements of dislocations are consistent with ∼10 3 cm -2 in the substrate. EPD measurements after homoepitaxial growth are typically ∼10 4 cm -2 . Growth of Al x Ga 1-x N layers will increase the number of threading dislocations but graded buffer layers have been used to produce GaN layers with EPD of order 10 5 cm -2 .

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)

Ultra-low dislocation density aluminum nitride is a very promising substrate for many device structures based on the III-V nitride system. A better lattice and thermal expansion match than foreign substrates such as SiC or sapphire make AlN the substrate of choice for heteroepitaxial growth of AlGaN alloys, especially those with high aluminum concentration. In this paper, we show crystalline and chemical characterization of native aluminum nitride substrates, as well as characterization of AlGaN epilayers with 40 and 50% concentration of aluminum. The observation of atomic steps in atomic force microscope scans of the bare substrates and epilayers, as well as the narrow Full Width at Half Maximum (FWHM) measured on X-ray diffraction, are an indication of both the good surface preparation of the substrates and quality of the epilayers. An estimation of defect density on epilayers grown by Migration Enhanced Metal Organic Chemical Vapour Deposition (MEMOCVD) resulted in mid 10 6 /cm 2 .

We report on the velocity V and the electromechanical coupling coefficient K2 of the first and the second leaky surface acoustic waves in various propagation directions in the a-plane AlN single-crystal. For c-propagation direction, the second leaky wave exhibited the velocity of 11016 m/s and K2 of 0.45%. For this direction, the temperature coefficient of frequency was found to be -30 ppm/°C. A near match of the velocities of the plane and leaky waves in the a-plane AlN allowed us to establish analytical relationships between the piezoelectric and elastic constants. A full set of elastic and piezoelectric constants of AlN has been evaluated by fitting the measured and calculated dependencies of velocities and electromechanical coupling coefficients on the propagation direction for both Rayleigh and leaky waves.

The optical properties of high quality large bulk AlN crystals have been tested by cathodoluminescence measurements carried out at different temperatures, excitation densities and electron beam energies. Both c-plane (0001) and a-plane () wafers were cut from bulk, single-crystals of AlN which were fabricated by the sublimation-recondensation technique. The wafer surfaces were subsequently prepared by chemical-mechanical polishing. The low-temperature near-band-edge cathodoluminescence spectra of a typical a-plane oriented AlN crystal show up to five transitions, which are tentatively assigned to free- and bound exciton recombination processes. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Oxygen is a common substitutional impurity in both AlN and GaN crystals. In the wurtzite 2H phase it can be present in AlN up to concentrations of 1 x 10 21 /cm 3 while in GaN it can reach concentrations of 3 × 10 22 /cm 3 . These high concentrations of oxygen affect the luminescence, the optical absorption, the thermal conductivity, and the crystal perfection. The effects are somewhat similar in AlN and GaN. Representative experimental data will be presented to demonstrate the similarities, and to show how the oxygen content may be estimated from these property measurements.

A comparative study of epitaxy of AlN, GaN and their alloys, grown on c -axis and off-axis substrates of single-crystal aluminum nitride has been carried out. Growth on off-axis (>30°) substrates appears to result in rough surfaces and the absence of two-dimensional electron gas (2DEG). However, smooth morphologies were demonstrated for both homoepitaxial and heteroepitaxial growth on on-axis ( 2 /V s and a sheet density of 8.5×10 12 cm −2 at room temperature, was also demonstrated for the first time.

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.

High-quality, bulk aluminum nitride crystal grains exceeding 1 cm in dimension have been obtained using a self-seeded sublimation–recondensation growth technique at 0.9 mm/h driving rate. X-ray double crystal diffraction and topography show a full-width-at-half-maximum of around 100 arcsec and extensive areas with a density of dislocations less than 10 4 cm −2 , respectively. These substrates have been prepared by chemical mechanical polishing techniques to obtain a surface roughness of 1.4–1.6 nm. The size, structural quality, and surface roughness prove these substrates to be adequate for III-nitride device fabrication.

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)

Large (up to 10mm diameter) aluminum nitride (AlN) boules have been grown by the sublimation-recondensation method to study the preparation of high-quality single crystal substrates. The growth mechanism of the boules has been studied using AFM. It has been determined that large single crystal grains in those boules grow with a density of screw dislocations below 5×104 cm−3 while edge dislocations are at lower density (none were observed). High-quality AlN single crystal substrates for epitaxial growth have been prepared and characterized using Chemical Mechanical Polishing (CMP) and AFM imaging, respectively. Also, the differential etching effect of KOH solutions on the N and Al-terminated faces of AlN on vicinal c-faces has been investigated. In order to identify the N or Al-terminated face, convergent beam electron diffraction has been used.

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.

Characterization of InAs self-assembled islands, grown on GaAs(111)B by molecular beam epitaxy, is carried out by atomic force microscopy and photoluminescence spectroscopy for different growth conditions. Islands show lateral sizes ca 40 to 300 nm long by 30 nm wide. Narrow height distributions are achieved centered ca 4 or 20 nm, depending on the growth conditions. Samples with the taller islands show the stronger luminescence. The position of the peaks is surprisingly similar to those reported for self-assembled islands on GaAs(100), despite the differences on crystal orientation and island sizes. This results suggests that the photoluminescence observed for both (100) and (111) orientation is not determined by quantum size confinement in the islands.

We have measured strain relaxation and clustering in Ge films grown by molecular beam epitaxy on Si(111) at substrate temperatures between 450 and 700 °C in real time with reflection high energy electron diffraction (RHEED). At 450 °C, we observe an oscillation of the surface lattice constant for the first 3.5 bilayers [(BLs) thicknesses were calibrated by Rutherford backscattering spectrometry], followed by a sharp two-dimensional–three-dimensional (2D–3D) growth mode transition, when transmission diffraction features appear in RHEED. The surface lattice constant then begins to relax at an initial rate of about 0.5%/BL. The mechanisms of island growth and strain relaxation change with growth temperature. At 500 °C, the surface lattice constant begins to relax after only 1 BL, and at 550 °C relaxation begins immediately. At both temperatures, however, 3D spots do not appear until after 3.5 BL. The initial rate of strain relaxation decreases with increasing temperature until, at 700 °C (when 3D spots never...

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.

Strain relaxation and clustering of Ge thin films grown on Si(111) and Si(100) by molecular beam epitaxy (MBE) have been studied in situ with reflection high energy electron diffraction (RHEED) and analyzed by atomic force microscopy (AFM) and Rutherford backscattering spectrometry (RBS). At low temperature, growth is dominated by island nucleation and by strain relief through island formation. The cluster size distribution (measured by AFM) just after the 2D–3D growth mode transition is broader than that for ‘late-stage’ growth (when diffusion gradients dominate cluster growth) and is well fit by a model in which the surface diffusion of adatoms is described by a random walk. At high temperature, growth is dominated by dislocation formation and the aspect ratio of the islands changes. The barrier to dislocation formation is reduced at step bunches; thus, islands nucleate preferentially at the step bunches and grow along them. By understanding Ge island nucleation and evolution, we hope to grow a population of uniformly-sized nanocrystals exhibiting quantum confinement effects.

Ballistic electron emission microscopy (BEEM) measurements have been performed on n-type Au Si (100) interfaces as a function of STM tip bias, Au film thickness, and temperature. From these measurements, the attenuation length, λa, of the BEEM electrons in the metal overlayer has been determined to be 133 ± 2 A at room temperature (RT) and 147 ± 6 A at 77 K for tip biases from −1.20 V to −0.92 V. The ratio of the zero thickness BEEM transmittances at 77 K to that at RT, I 0 (77 K ) I 0 ( RT ) , was determined to be 1.79 ± 0.09. Within the experimental uncertainties of these measurements, no energy dependence of λa or I 0 (77 K ) I 0 ( RT ) was observed. The large increase in the BEEM transmittance and the relatively small increase in λa at 77 K indicate that the primary temperature dependent scattering mode affecting BEEM electron transport is phonon absorption in the Si substrate. Images with large reductions in the BEEM current at topographic locations which have a large surface gradient have been obtained at RT. Our calculations, which assume that the probability of transmission across the interface is independent of the electron's transverse momentum, correlate well with the experimentally observed reductions. This result indicates that the BEEM electrons remain forward focused with very little broadening as they pass through the Au overlayer, and also implies that strong scattering must occur at the Au Si interface to explain the previously documented non-conservation of transverse momentum at Au Si interfaces.

The advantage that epitaxy offers the electronics and optoelectronics industries is that it allows the possibility of producing precisely controlled layers of very high crystal quality. Heteroepitaxy of different materials offers the promise of tailoring device layers in clever ways that nature did not intend. However unlike fruit juices, nature has made it difficult to epitaxially combine different materials. As the preceding articles have clearly pointed out, it is very difficult to obtain smooth epitaxial layers that are free both of defects and strain when there is a lattice mismatch between the layers and their substrates.As already discussed in this issue, a uniform network of dislocations at the interface between a flat, uniform epitaxial layer and its substrate can completely relieve strain in the majority of the epitaxial layer. This would be a satisfactory situation for many devices so long as the active region of the device could be kept away from the interface. The problem is how to introduce the dislocations in an appropriate way. When an epitaxial layer has a larger lattice parameter than the underlying substrate, a misfit dislocation running along the interface represents a plane of atoms that has been removed from the epitaxial layer. (One would insert a plane of atoms if the epitaxial lattice parameter was smaller. For simplicity however we will continue to assume that the epitaxial layer has a larger lattice parameter.) It is not possible for a whole half plane of atoms, bounded by the dislocation at the interface and the substrate edges along the two sides, to be removed at once. The boundary between where the extra plane of atoms has been removed and where the epitaxial layer has not relaxed yet will represent a threading dislocation. This threading dislocation would continue to move as the size of the misfit dislocation along the interface grows. Ideally it moves all the way out to the substrate edge and vanishes there while the misfit dislocation along the interface would end up extending from one side of the substrate to the other. However other dislocations and other kinds of defects can effectively pin the threading dislocation resulting in an epitaxial layer with many threading dislocations. Unfortunately these threading dislocations are generally detrimental to most kinds of devices. It is precisely this high density of threading dislocations that limits applications of many heteroepitaxial layers.

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.