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A top-illuminated deep-ultraviolet Al0.6Ga0.4N p-i-n avalanche photodiode (APD) structure was designed and grown by metalorganic chemical vapor deposition on an AlN bulk substrate and on two different quality AlN/sapphire templates, and APDs were fabricated and tested. The APD devices with a circular diameter of 20 μm have demonstrated a distinctive reverse-bias breakdown behavior. The reverse breakdown voltage of the APDs is approximately −140 V, which corresponds to a breakdown electric field of 6–6.2 MV/cm for the Al0.6Ga0.4N material as estimated by Silvaco TCAD simulation. The APDs grown on the AlN bulk substrate show the lowest leakage current density of −8 A/cm2 (at low reverse bias) compared to that of the devices grown on the AlN templates. From the photocurrent measurement, a maximum gain (current limited) of 1.2 × 104 is calculated. The average temperature coefficients of the breakdown voltage are negative for APD devices fabricated from both the AlN bulk substrate and the AlN templates, but these data show that the coefficient is the least negative for the APD devices grown on the low-dislocation-density AlN bulk substrate.

The current-voltage characteristics and metastability in GaN p++/n++ homojunction tunnel diodes and n++/p++/i/n tunnel-contacted diodes grown via metal modulated epitaxy have been investigated. The room temperature negative differential resistance (NDR) beginning at ∼1.35 V is reported for GaN homojunction devices grown on sapphire. The NDR vanishes, and the conductivity increases as multiple I-V sweeps are performed, thus suggesting that charge trapping states with long trap lifetimes exist at defect sites, and these traps play a crucial role in the tunneling mechanism. Additionally, the use of extremely high n-type (ND ∼ 4.6 × 1020 cm−3) and p-type (NA ∼ 7.7× 1020 cm−3) doping results in a near linear characteristic with minimal rectification at current densities less than 200 A/cm2 and soft rectification above this current density. Forward-bias tunneling and NDR are still present at 77 K. The highest silicon-doped n++/p++/i/n tunnel-contacted pin diode demonstrates a turn-on voltage of 3.12 V, only 0.14 V higher than that of the pin control diode, and an improved specific on-resistance of 3.24 × 10−4 Ω cm2, which is 13% lower than that of the control pin diode.

Deep-UV distributed Bragg reflectors (DBRs) operating at λ = 220–250 nm with reflectivity close to unity were produced using epitaxial AlxGa1-xN/AlN superlattice structures grown on AlN/sapphire templates via metalorganic chemical vapor deposition. Owing to the near-bandedge excitonic resonance in the AlxGa1-xN layers, the AlN mole fractions, x, were regulated to keep the reflective plateau within the enhanced refractive index contrast region between AlGaN and AlN of approximately 7%–11%. For DBRs incorporating high-index layers of AlGaN grown via a flow-rate modulated epitaxy technique, a reflectivity of 97% was achieved with a total pair number of 30.5 which was much smaller than number of pairs needed for the DBRs with conventionally grown AlGaN layers. The stopbands of these DBRs were about 6–9 nm.

We report the crack-free growth of a 45-pair Al0.30Ga0.70N/Al0.04Ga0.96N distributed Bragg reflector (DBR) on 2 in. diameter AlN/sapphire template by metalorganic chemical vapor deposition. To mitigate the cracking issue originating from the tensile strain of Al0.30Ga0.70N on GaN, an AlN template was employed in this work. On the other hand, strong compressive strain experienced by Al0.04Ga0.96N favors 3D island growth, which is undesired. We found that inserting an 11 nm thick GaN interlayer upon the completion of AlN template layer properly managed the strain such that the Al0.30Ga0.70N/Al0.04Ga0.96N DBR was able to be grown with an atomically smooth surface morphology. Smooth surfaces and sharp interfaces were observed throughout the structure using high-angle annular dark-field imaging in the STEM. The 45-pair AlGaN-based DBR provided a peak reflectivity of 95.4% at λ = 368 nm with a bandwidth of 15 nm.

We report an electroluminescence (EL) study on npn GaN/InGaN heterojunction bipolar transistors (HBTs). Three radiative recombination paths are resolved in the HBTs, corresponding to the band-to-band transition (3.3 eV), conduction-band-to-acceptor-level transition (3.15 eV), and yellow luminescence (YL) with the emission peak at 2.2 eV. We further study possible light emission paths by operating the HBTs under different biasing conditions. The band-to-band and the conduction-band-to-acceptor-level transitions mostly arise from the intrinsic base region, while a defect-related YL band could likely originate from the quasi-neutral base region of a GaN/InGaN HBT. The IB-dependent EL intensities for these three recombination paths are discussed. The results also show the radiative emission under the forward-active transistor mode operation is more effective than that using a diode-based emitter due to the enhanced excess electron concentration in the base region as increasing the collector current increases.

We demonstrate transverse-magnetic (TM) dominant deep-ultraviolet (DUV) stimulated emission from photo-pumped AlGaN multiple-quantum-well lasers grown pseudomorphically on an AlN/sapphire template by means of photoluminescence at room temperature. The TM-dominant stimulated emission was observed at wavelengths of 239, 242, and 243 nm with low thresholds of 280, 250, and 290 kW/cm2, respectively. In particular, the lasing wavelength of 239 nm is shorter compared to other reports for AlGaN lasers grown on foreign substrates including sapphire and SiC. The peak wavelength difference between the transverse-electric (TE)-polarized emission and TM-polarized emission was approximately zero for the lasers in this study, indicating the crossover of crystal-field split-off hole and heavy-hole valence bands. The rapid variation of polarization between TE- and TM-dominance versus the change in lasing wavelength from 243 to 249 nm can be attributed to a dramatic change in the TE-to-TM gain coefficient ratio for the sa...

Optically pumped deep-ultraviolet (DUV) lasing with low threshold was demonstrated from AlGaN-based multiple-quantum-well (MQW) heterostructures grown on sapphire substrates. The epitaxial layers were grown pseudomorphically by metalorganic chemical vapor deposition on (0001) sapphire substrates. Stimulated emission was observed at wavelengths of 256 nm and 249 nm with thresholds of 61 kW/cm2 and 95 kW/cm2 at room temperature, respectively. The thresholds are comparable to the reported state-of-the-art AlGaN-based MQW DUV lasers grown on bulk AlN substrates emitting at 266 nm. These low thresholds are attributed to the optimization of active region and waveguide layer as well as the use of high-quality AlN/sapphire templates. The stimulated emission above threshold was dominated by transverse-electric polarization. This work demonstrates the potential candidacy of sapphire substrates for DUV diode lasers.

Deep-ultraviolet lasing was achieved at 243.5 nm from an AlxGa1−xN-based multi-quantum-well structure using a pulsed excimer laser for optical pumping. The threshold pump power density at room-temperature was 427 kW/cm2 with transverse electric (TE)-polarization-dominant emission. The structure was epitaxially grown by metalorganic chemical vapor deposition on an Al-polar free-standing AlN (0001) substrate. Stimulated emission is achieved by design of the active region, optimizing the growth, and the reduction in defect density afforded by homoepitaxial growth of AlN buffer layers on AlN substrates, demonstrating the feasibility of deep-ultraviolet diode lasers on free-standing AlN.

Data and analysis are presented for NpN-GaN/InGaN/GaN double-heterojunction bipolar transistors (HBTs) grown and fabricated on a free-standing GaN (FS-GaN) substrate in comparison to that on a sapphire substrate to investigate the effect of dislocations in III-nitride HBT epitaxial structures. The performance characteristics of HBTs on FS-GaN exhibit a maximum collector current density of ∼12.3 kA/cm2, dc current gain of ∼90, and maximum differential gain of ∼120 without surface passivation, representing a substantial improvement over similar devices grown on sapphire. This is attributed to the reduction in threading dislocation density afforded by using a homoepitaxial growth on a high-crystalline-quality substrate. The minority carrier diffusion length increases significantly owing to not only a mitigated carrier trap effect via fewer dislocations, but also possibly reduced microscopic localized states.

A step-graded AlxGa1-xN electron blocking layer (EBL) is studied on InGaN-based laser diodes (LDs). Its efficacy on device performance is investigated with respect to stimulated emission properties, internal quantum efficiency, internal loss, and temperature-dependent characteristics. When compared to a LD structure with an abrupt Al0.18Ga0.82N EBL design, the LD with the step-graded AlxGa1-xN EBL design demonstrates lower threshold current density and higher slope efficiency. The threshold current density is reduced from 4.6 kA/cm2 to 2.5 kA/cm2 under pulsed-current operation and the corresponding slope efficiency is increased from 0.72 W/A to 1.03 W/A. The insertion of the step-graded AlxGa1−xN EBL leads to a dramatic enhancement in internal quantum efficiency from 0.60 to 0.92, while internal loss keeps at 9 ∼ 10 cm−1. The temperature-dependent measurement also shows that the step-graded AlxGa1−xN EBL can improve the thermal stability with reduced red-shift from 0.05 nm/K to 0.034 nm/K. This simple yet...

We describe a method to change the threshold voltage of heterostructure field-effect transistors (HFETs) using InxAl1−xN/GaN heterostructures by using polarization and strain modification in the InAlN barrier layer to realize enhancement-mode operation. The threshold voltage and electronic band structure of the heterostructures were calculated for different indium compositions in the InAlN layer. Band structure calculations predict the enhancement-mode operation of compressively strained InAlN/GaN HFETs with an indium composition higher than 0.25. Studies of InAlN/GaN HFETs with different In alloy compositions show that the sheet resistance increases and the carrier concentration decreases for the heterostructures with increasing indium composition due to changes in the compressive strain and polarization in the InAlN barrier layer. Fabricated HFETs show threshold voltages of −2.5, −0.75, and +0.2 V for In∼0.18Al∼0.82N/GaN, In∼0.22Al∼0.78N/GaN, and In∼0.25Al∼0.75N/GaN HFETs, respectively, corresponding to...

We report low-noise GaN visible-blind homojunction p-i-n photodiodes. The devices are grown on a freestanding bulk GaN substrate and are fabricated using a “ledged” surface depletion technique to suppress the mesa sidewall leakage. For an 80-μm-diameter photodetector, the dark current density is lower than 40 pA/cm2. A room-temperature noise equivalent power of 4.27×10−17 W Hz−0.5 and a detectivity of 1.66×1014 cm Hz0.5 W−1 are achieved at a reverse bias of 20 V. The noise performance of the reverse-biased GaN p-i-n photodiodes are among the best values reported to date and demonstrate the potential of GaN photodiodes for low-noise high-speed UV detection.

Charge is observed at the regrowth interface for heterostructure field-effect transistors (HFETs) grown on semi-insulating (SI) bulk GaN substrates, even with Fe doping in the regrown buffer layer for reduction of the interface charge. Ultraviolet photoenhanced chemical (PEC) etching is used to treat the surface of SI bulk GaN substrates. Employing optimized etching conditions, a very smooth surface is achieved for the bulk GaN substrate after the etching. The charge at the regrowth interface is eliminated for HFETs grown on etched SI GaN substrates. Secondary ion mass spectrometry measurements show that the Si impurity concentration at the regrowth interface for HFETs grown on etched SI GaN substrates is much lower than that for HFETs grown on unetched SI GaN substrates, which suggests that the charge-containing layer on the SI substrate is removed by PEC etching and that the effects of the reduced charge layer near the regrowth interface can be eliminated by Fe doping for HFETs grown on etched SI substr...

The authors report radiative recombination from a graded-base InGaN∕GaN heterojunction bipolar transistor (HBT) grown by metal-organic chemical vapor deposition on sapphire. For a device with a 40×40μm2 emitter area, a differential dc current gain of 15 is measured from the common-emitter current-voltage characteristics, with the HBT breakdown voltage BVCEO>65V. The heterojunction bipolar light-emitting transistor exhibits a base-region recombination radiation peak in the visible spectral range with a dominant peak at λ=385nm (blue emission).

We report the performance of GaN p-i-n ultraviolet avalanche photodiodes grown on bulk GaN substrates by metal-organic chemical vapor deposition. The low dislocation density in the devices enables low reverse-bias dark currents prior to avalanche breakdown for ∼30μm diameter mesa photodetectors. The photoresponse is relatively independent of the bias voltage prior to the onset of avalanche gain which occurs at an electric field of ∼2.8MV∕cm. The magnitude of the reverse-bias breakdown voltage shows a positive temperature coefficient of ∼0.05V∕K, confirming that the avalanche breakdown mechanism dominates. With ultraviolet illumination at λ∼360nm, devices with mesa diameters of ∼50μm achieve stable maximum optical gains greater than 1000. To the best of our knowledge, this is the highest optical gain achieved for GaN-based avalanche photodiodes and the largest area III-N avalance photodetectors yet reported.

The device operation of InGaN heterojunction bipolar transistors with a graded InGaN emitter-base design grown by metal organic chemical vapor deposition on sapphire substrates is demonstrated. The Gummel plot, current gain, and common-emitter current-voltage characteristics of the device are presented. The dc common-emitter current gain of a 25×25μm2 (emitter size) device increases with collector current and the current gain reaches a high value of 13 at IC=10mA with a base width of 100nm and a hole concentration of p=2×1018cm−3. The improved device performance is attributed to the graded emitter-base design as well as the high quality of the material made possible by the indium composition grading that enables the epitaxial growth of InGaN layers with a low density of pits or defects. The results demonstrate the potential of the graded InGaN emitter-base junction design in nitride heterojunction bipolar transistors.