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Schottky diodes on AlGaN/GaN heterostructures with Pt, IrPt, and PdAg catalytic metals are fabricated and characterized from 200 °C to 800 °C for H 2 sensing. Over this large range of temperature, the forward current of all the diodes increases with exposure to H 2 gas, which is attributed to Schottky barrier height reduction caused by the atomic hydrogen absorption on the metal–oxide interface. The results indicate that AlGaN/GaN heterostructure Schottky diodes are capable of high-temperature H 2 sensor operation up to 800 °C. As temperature increases, the hydrogen detection sensitivity of Pt and IrPt diodes improves due to the more effective H 2 dissociation. However, the sensitivity of PdAg diodes degrades with the increase of temperature due to thermal instability of PdAg. At a range of temperature from 200 °C to 300 °C, PdAg diodes exhibit significant higher sensitivity compared with Pt and IrPt diodes. IrPt and Pt diodes show higher sensitivity at temperatures above 400 °C.

Subpicosecond time-resolved photoluminescence (TRPL) has been used to compare the room temperature carrier dynamics in Al0.1Ga0.9N/Al0.3Ga0.7N multiple quantum well (MQW) structures simultaneously deposited on a high quality free standing HVPE GaN substrate (dislocation density ∼1 × 107cm–2) and 1 µm MOCVD GaN template on sapphire. The PL lifetime of ∼500 ps in the MQW on GaN substrate is about 5 times longer than that for the MQW on GaN template, with a concomitant increase in CW PL intensity. This behavior is attributed primarily to an increase in nonradiative lifetime associated with a 100 times reduction in dislocation density in the GaN substrate. The observation that the PL lifetime in the MQW falls short of the ∼900 ps dominant decay time in the GaN substrate may be indicative of generation of additional defects and dislocations due to substrate surface preparation, strain relaxation, and nonoptimal growth temperature associated with the difference in heating of the thin GaN template on sapphire and the thick GaN substrate. An extended PL rise time of greater than 20 ps for the MQW emission when above barrier pumping is employed implies that both wells and barriers are of high quality. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-aligned AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated and the direct current and radio frequency small signal performance of self-aligned devices is characterized in comparison with non-self-aligned devices. An ultra-thin Ti/Al/Ti/Au ohmic metal scheme is used for gate to source and drain self-alignment. To suppress the gate leakage current, the ohmic contact annealing of self-aligned devices is performed in a furnace. The self-aligned devices with 0.25 μm gate-length and 100 μm gate-width exhibit good pinch-off characteristics. The maximum drain current at a gate bias of 1 V is 620 mA/mm for self-aligned HEMTs, and 400 mA/mm for non-self-aligned devices, respectively. A maximum extrinsic transconductance of 146 mS/mm is measured in self-aligned devices, while non-self-aligned HEMTs show only a peak g m of 92 mS/mm. The self-aligned devices exhibit an extrinsic f T of 39 GHz and an f MAX of 130 GHz, whereas non-self-aligned HEMTs show an f T of 15 GHz and an f MAX of 35 GHz.

2 Experimental Thick delta doped Al0.25Ga0.75N layers and Al0.25Ga0.75N/GaN HEMT layers were produced in an Aixtron 200/4 MOCVD system using NH3, TMG, TMA and Si2H6 precursors. The mate- rial was grown on 2-inch, c-plane sapphire at 1220 °C and 100 mbar in a hydrogen carrier gas. Delta doping was accomplished by halting deposition for 10 s under NH3 and H2, exposing the material to Si2H6 for 75 s under NH3 and H2, and then continuing nitride material growth. Initial growths consisted of growing thick (2.0 µm) AlGaN layers containing a single delta doped layer. Delta doped AlGaN /GaN HEMTs consisting of a 250 A delta doped Al0.25Ga0.75N layer on 3 µm GaN layers on sapphire were also grown; the position and dopant level of the delta doped layer was varied by adjusting location of the delta doped layer and Si2H6 flux. Two sets of HEMT samples were produced: For the first set of samples, the position of the delta doped layer was fixed at 60 A from the AlGaN/GaN interface and the Si2H6 flux was varied. In the second set of samples, the Si2H6 flux was fixed and the delta doped layer position in the AlGaN barrier was adjusted. The samples were characterized using secondary ion mass spectroscopy (SIMS) to determine dopant concentration and depth and C-V, Lehighton sheet resistance mapping and 300 °K Hall effect measurements to determine the electrical properties.

In this study, quasi-enhancement mode AlGaN/GaN HEMT devices with the gate length of 1-μm are fabricated using a conventional method (i.e. without gate-recessing process) and the direct current and radio frequency characteristics of these devices are investigated. The threshold voltages are in the range of −0.3 to −0.5 V. The devices exhibit a maximum drain current of 280 mA/mm at a gate bias of 2 V. The pinch off voltage is about −1.0 V. At the gate bias of 1.5 V and the drain bias of 6 V, the devices exhibit an extrinsic transconductance of 140 mS/mm, a unity current gain cutoff frequency (fT) of 4.3 GHz, and a maximum oscillation frequency (fMAX) of 13.3 GHz, respectively.

Metalorganic vapor-phase epitaxially grown GaN films have been studied using beam positron annihilation spectroscopy. Positron lifetime and Doppler broadening of annihilation radiation measurements indicate that defects, probably Ga vacancies, are reduced in concentration when films are doped with Mg, as previously observed by other workers. In lightly doped films a homogeneous layer with a minimum defect concentration is present from just below the surface, to a depth of around 0.1 μm. For heavily doped films, there is evidence of a further substantial decrease in the defect concentration following an anneal to 900 °C. The effects of impurities and defects are discussed.

Hot phosphoric-acid etching and atomic force microscopy (AFM) were used to etch and characterize various GaN materials, including freestanding GaN grown by hydride vapor-phase epitaxy (HVPE), metal-organic chemical-vapor deposition (MOCVD) GaN films on sapphire and silicon carbide, and homoepitaxial GaN films on polished freestanding-GaN wafers. It was found that etching at optimal conditions can accurately reveal the dislocations in GaN; however, the optimal etch conditions were different for samples grown by different techniques. The as-grown HVPE samples were most easily etched, while the MOCVD homoepitaxial films were most difficult to etch. Etch-pit density (EPD) ranging from 4 × 106 cm2 to 5 × 109 cm-2 was measured in close agreement with the respective dislocation density determined from transmission electron microscopy (TEM).

We report on the effect of strain induced polarization fields in AlGaN/GaN heterostructures due to the incorporation of Si dopant ions in the lattice. By Si-doping (Al)GaN, a contraction of the wurtzite unit cell can occur leading to strain in doped AlGaN/GaN heterostructures such as high electron mobility transistors (HEMTs). In a typical modulation doped AlGaN/GaN HEMT structure, the Si-doped AlGaN supply layer is separated from the two-dimensional electron gas channel by an undoped AlGaN spacer layer. This dopant-induced strain, which is tensile, can create an additional source of charge at the AlGaN:Si/AlGaN interface. The magnitude of this strain increases as the Si doping concentration increases and the AlN mole fraction in the AlGaN decreases. Consideration of this strain should be given in AlGaN/GaN HEMT structure design.

We have experimentally determined the effective mass (m*) of GaN, the classical (τc), and quantum (τq) scattering times for a two-dimensional electron gas residing at the interface of an AlGaN/GaN heterostructure, using the Shubnikovde Haas effect. The ratio of the two scattering times, τc/τq, suggests that, at low temperatures, the scattering mechanism limiting the mobility is due to remote ionized impurities located in AlGaN. This study should provide sample growers with information useful for improving the quality of the nitride heterostuctures.

Ir, Ni, and Re Schottky contacts on strained Al0.25Ga0.75N/GaN heterostructures are characterized using capacitance–voltage (C–V) and I–V techniques. Based on the measured C–V characteristics, two dimensional electron gas sheet carrier concentrations at the AlGaN/GaN interface and barrier heights of Ir, Ni, and Re Schottky contacts are calculated. The barrier heights of 1.12, 1.27, and 1.68 eV are obtained for Ir, Ni, and Re Schottky contacts, respectively. The results show that the barrier heights of Schottky contacts on strained AlGaN/GaN heterostructures are strongly dependent on the metal work functions. However, contrary to Schottky contacts on bulk AlGaN or GaN, the barrier height on strained AlGaN/GaN heterostructures is lower for a Schottky contact with a higher metal work function. This is attributed to the stronger wave function coupling between electrons in the Schottky metal and surface donor electrons. The I–V characteristics for Ir, Ni, and Re Schottky contacts confirm the results obtained b...

Self-aligned AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated and the direct current and radio frequency small signal performance of self-aligned devices is characterized in comparison with non-self-aligned devices. An ultra-thin Ti/Al/Ti/Au ohmic metal scheme is used for gate to source and drain self-alignment. To suppress the gate leakage current, the ohmic contact annealing of self-aligned devices is performed in a furnace. The self-aligned devices with 0.25 /spl mu/m gate-length and 100 /spl mu/m gate-width exhibit good pinch-off characteristics. The maximum drain current at a gate bias of 1 V is 620 mA/mm for self-aligned HEMTs, and 400 mA/mm for non-self-aligned devices, respectively. A maximum extrinsic transconductance of 146 mS/mm is measured in self-aligned devices, while non-self-aligned HEMTs show only a peak gm of 92 mS/mm. The self-aligned devices exhibit an extrinsic f/sub T/ of 39 GHz and an f/sub MAX/ of 130 GHz, whereas non-self-aligned HEMTs show an f/sub T/ of 15 GHz and an f/sub MAX/ of 35 GHz.

High quality Al0.15Ga0.85N/GaN heterostructures have been fabricated on 6H–SiC and sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). A temperature independent mobility, indicative of the presence of a two‐dimensional electron gas (2DEG), was observed in all samples below 80 K. The highest low temperature 2DEG mobility, 7500 cm2/V s, was measured in AlGaN/GaN grown on 6H–SiC; the sheet carrier density was 6×1012 cm−2. Strong, well resolved, Shubnikov–de Haas oscillations were observed in fields as low as 3 T and persisted to temperatures as high as 15 K. Hall effect measurements also revealed the presence of well‐defined plateaus in the Hall resistance. The high quality 2DEG properties of the AlGaN/GaN heterostructures grown on 6H–SiC are attributed to the absence of significant parallel conduction paths in the material.

An optically pumped GaN‐based vertical cavity surface emitting laser (VCSEL) is demonstrated. Laser emission near 363 nm is observed at room temperature from the surface of a VCSEL structure optically pumped along a cleaved sample edge by focused light from a nitrogen laser. The VCSEL structure, which was grown on a sapphire substrate by metalorganic vapor phase epitaxy, consists of a 10 μm GaN active region sandwiched between 30‐period Al0.40Ga0.60N–Al0.12Ga0.88N Bragg reflectors. At optical pump intensities above ∼2.0 MW/cm2, a narrow (

In this paper, we report the electrical and optical characteristics of Si delta-doped AlGaN cladding layers, p-cladding structure optimization and the impact on the efficiency of 340nm AlGaN UV LEDs. Compared to the uniformly doped n-AlGaN layer, adding Si Δ-doping layers reduced the sheet resistance by improving both the Hall mobility and carrier concentration. Increasing the number of Si Δ-doped layers further lowered the sheet resistance without cracking the material. The Δ-doped layers in n-Al0.3Ga0.7N improved the optical properties by enhancing near band edge emission as much as 2-fold relative to deep level emission. Additionally, Δ-doping in n-AlGaN layers had no detrimental effect on the optical transparency of the LEDs. The p-cladding layer was found to have a strong absorption at 340nm. Reducing the p-GaN cap layer from 35nm to 10nm tripled the light emission intensity. By optimizing the n- and p-AlGaN cladding layers, a highly efficient UV LED at 340nm was achieved with 1mW output under 800mA/mm2 DC drive current.

Delta doping (paused growth doping) was investigated as an alternative to uniformly distributing the dopant in the nitride semiconductor layer. In this work, delta doped layers were produced in MOVPE-grown AlGaN and GaN layers at a susceptor temperature of 1220°C by turning off the group III precursors (TMG and TMA) and introducing into the reactor a silicon precursor Si2H6 (disilane) for a fixed period (pause time) before growth was restarted. The compositional and electrical properties as a function of aluminum content and dopant flux were investigated for nitride layers on 2 inch c-plane sapphire substrates. Secondary ion mass spectroscopy (SIMS) measurements revealed a sharp silicon peak with a FWHM of 5.7 ± 0.6 nm for an Al0.25Ga0.75N sample and 10.0 ± 0.6 nm for a GaN sample with sheet charges of 7.9×1012 cm−2 and 9.9×1012 cm−2,respectively. Room temperature Hall mobility as high as 265 cm2 V−1s−1 for a sheet charge 7.9×1012 cm−2 was demonstrated for delta doped Al0.25Ga0.75N layers, but the mobility enhancement saturated and then decreased with increasing sheet charge. Room temperature sheet charge increased with increasing dopant flux for delta-doped AlGaN and GaN layers. Sheet charge density as high as 2.2×1013 cm−2 and 1.3×1013 cm−2 was measured at room temperature for Al0.25Ga0.75N and GaN delta doped layers, respectively. Under identical doping conditions, the Hall sheet charge of the delta doped Al0.25Ga0.75N layer was approximately half as large as GaN layers. The impurity and electrical characteristics of the delta doped layers are further discussed.

A comparative study, using time-resolved and CW photoluminescence spectroscopy, of MOVPE grown InGaN/GaN multiple quantum wells deposited on HVPE GaN/Sapphire at different growth temperatures was undertaken. It was found that the PL linewidth increased and the peak emission energy decreased as the growth temperature was reduced. Moreover, the sample grown at an intermediate growth temperature exhibited total integrated luminescence intensity much greater than the samples grown at higher or lower growth temperatures. A phenomenological carrier recombination dynamics model based on the competition of quantum well-like radative recombination, spatially localized radiative recombination in potential minima and non-radiative recombination through defects is presented to provide an explanation of the observed emission dynamics and efficiency. In this model, the emission efficiency is determined by the relative area of defects and the number density of localized states in the potential minima, both of which are influenced by the growth temperature. Furthermore, the photon energy dependent lifetimes are well fitted with this model by assuming a Gaussian shape localized states distribution. The localized potential minima are consistent with nanoscale indium rich regions due to indium aggregation.

Laser action is demonstrated in InGaN/GaN double heterostructures with cleaved facets. Hydride vapor phase epitaxy is used to grow a 10-micrometer-thick buffer layer of GaN on (0001) sapphire, and metal-organic vapor phase epitaxy is used to subsequently grow a GaN/In 0.09 Ga 0.91 N/GaN double heterostructure. One-mm-long cavities are produced by cleaving the structure along the (1010) plane of the sapphire substrate. A pulsed Nitrogen laser is used for optical excitation. At room temperature, the laser threshold occurs at an incident power density of 1.3 MW/cm 2 . Above threshold, the differential quantum efficiency increases by a factor of 34, the emission linewidth decreases to 13.5 meV, and the output becomes highly TE polarized.

We report on fabrication and characterization of cleaved laser facets and photoelectrochemically wet etched laser facets in III-Nitrides grown by MOVPE on c-plane sapphire. The roughness of the cleaved facets in the InGaN/GaN double heterostructure (DH) laser cavities with a 1000-Å-thick active region is ≈25 rim, while that of the wet etched GaN facets is ≈100 nm. A theoretical model is developed for the maximum allowable laser facet roughness, which yields a value of 18 nrm for uncoated GaN and 22 rm for the uncoated DH. Optically pumped laser action at room temperature is demonstrated in the cleaved DH laser cavities. 2 Above the incident threshold pumping power of 1.3 MW/cm2, the differential quantum efficiency increases by a factor of 34, the emission linewidth decreases to 13.5 meV, and the output becomes highly TE polarized. Wet chemical etching a 1-mm-long laser cavity into the GaN homostructure is found to increase the differential quantum efficiency by a factor of 2.

We have fabricated AlxGa1−xN/GaN heterostructures with high two-dimensional electron gas (2DEG) mobilities and high sheet carrier densities by metalorganic vapor phase epitaxy (MOVPE). The 2DEG sheet density and mobility exhibit a compositional dependence on the Al fraction of the electron donor layer. The highest mobility (5750 cm2/Vs at 16K) was measured in a sample with x=0.15 that had a sheet carrier density of 8.5×1012 cm−2. The undoped AlxGa1−xN layers have low background carrier concentrations and can be intentionally doped n-type using SiH4. The effect of intentional n-type doping of the AlxGa1−xN donor layer on the electrical properties of the 2DEG was studied in structures that included an undoped AlxGa1−xN spacer layer of varying thickness. Higher 2DEG mobilities were obtained when a 100Å thick undoped layer was included in the structure due to spatial separation of the 2DEG from ionized impurities in the doped AlxGa1−xN. These initial results demonstrate that the electrical properties of AlxGa1−xN/GaN heterostructures can be controlled by intentional doping and appropriate layer design.

Schottky diodes on AlGaN∕GaN heterostructures with Pt catalytic metal are fabricated and characterized from 200to800°C for H2 sensing. Over this large range of temperature, the forward current of Schottky diodes increases with exposure to H2 gas, which is attributed to the reduction of Schottky barrier heights resulting from hydrogen absorption in the catalytic metal. The results indicate that AlGaN∕GaN heterostructure Schottky diodes are capable of high-temperature operation for H2 sensing up to 800°C. As temperature increases, the hydrogen detection sensitivity of Pt–AlGaN∕GaN Schottky diodes improves due to the more effective H2 dissociation. When the testing ambient is changed from N2 to 5% H2/95% N2, the value of the Schottky barrier height decreases by 11 and 120meV at 200 and 800°C, respectively.

The influence of annealed ohmic contact metals on the polarisation of the AlGaN barrier layer has been investigated by the Schottky contacts on the AlGaN/GaN HFET structure. The analysed result shows that annealed ohmic contact metals weaken the polarisation of the AlGaN barrier layer. When ohmic contact metals are close to Schottky contact metals, the weakened polarisation decreases the 2DEG sheet carrier concentration in the channel.

Someya et al.. recently reported [Jpn. J. Appl. Phys. 37 (1998) L1424] optically pumped laser operation from an InGaN-based vertical cavity surface emitting laser, which they characterize as the “the first unambiguous observation of laser emission by a III–V nitride VCSEL.” They claim that our previous report of lasing in a GaN-based vertical cavity laser is attributable to phenomena other than vertical lasing. We argue against the plausibility the authors' unsubstantiated alternative explanation for our results, which forms the basis for their claim.

Optically pumped laser action is demonstrated in an InGaN/GaN double heterostructure with a 1000 /spl Aring/ thick InGaN active region. Hydride vapour phase epitaxy (HVPE) is used to grow a 10 /spl mu/m thick GaN buffer layer on (0001) sapphire, and the GaN/In/sub 0.09/Ga/sub 0.91/N/GaN double heterostructure is subsequently grown by metal organic vapour phase epitaxy (MOVPE). 1 mm long cavities are produced by cleaving the structure along the (1010) plane of the sapphire substrate. Optical pumping at room temperature with a pulsed nitrogen laser yields an incident threshold power density of 1.3 MW/cm/sup 2/. Above threshold, the differential quantum efficiency increases by a factor of 34, the emission linewidth decreases to 13.5 meV, and the output becomes highly TE polarised.