D. J. H. Lambert

GaN avalanche photodiodes

We report the electrical and optical characteristics of avalanche photodiodes fabricated in GaN grown by metalorganic chemical vapor deposition. The current–voltage characteristics indicate a multiplication of >25. Experiment indicates and simulation verifies that the magnitude of the electric field at the onset of avalanche gain is ⩾3 MV/cm. Small-area devices exhibit stable gain with no evidence of microplasmas.

Back illuminated AlGaN solar-blind photodetectors

We report the growth, fabrication, and characterization of AlxGa1−xN (0⩽x⩽0.60) heteroepitaxial back-illuminated solar-blind p-i-n photodiodes on (0001) sapphire substrates. The group III-nitride heteroepitaxial layers are grown by low-pressure metalorganic chemical vapor deposition on double polished sapphire substrates using various growth conditions. The back-illuminated devices exhibit very low dark current densities. Furthermore, they exhibit external quantum efficiencies up to 35% at the peak of the photoresponse (λ∼280 nm). Improvements were made to the growth technique in order to achieve crack-free Al0.4Ga0.6N active regions on a thick Al0.6Ga0.4N window layer and to obtain activated p-type Al0.4Ga0.6N layers.

Mg-doped GaN: Similar defects in bulk crystals and layers grown on Al2O3 by metal–organic chemical-vapor deposition

Defects were observed in GaN:Mg grown on sapphire substrates using metal–organic chemical-vapor deposition (MOCVD) with Mg-delta doping similar to those previously observed in bulk GaN:Mg grown from Ga solution under high hydrostatic pressure of nitrogen. Pyramidal defects (pinholes) with (1100) hexagonal facets on the (0001) base plane and six {1122} side facets, and defects with a rectangular shape also delineated by planar facets on the basal (0001) planes, were observed for growth with Ga polarity for both of these very different growth methods. The Mg dopant is apparently responsible for their formation since the oxygen concentration in the MOCVD-grown samples was orders of magnitude lower than in the bulk samples. Mg segregation on these planes apparently does not allow uniform continuous growth on these planes leading to these hollow defects. Some defects in the heterolayers also develop into longer nanotubes elongated along the c axis. Change of polarity from Ga to N followed by a change back to...

High-voltage mesa-structure GaN Schottky rectifiers processed by dry and wet etching

We have fabricated and investigated high-voltage GaN vertical Schottky-barrier rectifiers grown by metalorganic chemical vapor deposition. A mesageometry Schottky-barrier rectifier having a 5-μm-thick i region, and processed using reactive-ion etching, exhibited a reverse breakdown voltage of −450 V (at 10 mA/cm2) and an on-resistance of 23 mΩ cm2. For comparison, we have also applied wet chemical etching for the fabrication of mesageometry Schottky-barrier rectifiers. The 2-μm-thick i-region GaN mesa-Schottky rectifiers showed a breakdown voltage of −310 and −280 V for wet-etched and dry-etched devices, respectively, and an on-resistance of 8.2 and 6.4 mΩ cm2, respectively. These results indicate that the performance of the wet-etched rectifiers is comparable to or better than that of comparable dry-etched devices.

Selective regrowth of Al0.30Ga0.70N p–i–n photodiodes

We report on the device performance of selective-area regrown Al0.30Ga0.70N p–i–n photodiodes. Tensile strain, induced by the lattice mismatch between AlxGa1−xN and GaN, leads to cracking above the critical thickness in layers with high aluminum concentration. Selective-area regrown devices with ⩽70 μm diameters were fabricated without signs of cracking. These devices show low dark current densities with flat photoresponse and a forward turn-on current of ∼25 A/cm2 at 7 V. A quantum efficiency greater than 20% was achieved at zero bias with a peak wavelength of λ=315 nm. A differential resistance of R0=3.46×1014 Ω and a detectivity of D*=4.85×1013 cm Hz1/2 W−1 was demonstrated.

Properties of InGaN quantum-well heterostructures grown on sapphire by metalorganic chemical vapor deposition

We report the growth and characterization of InGaN heteroepitaxial thin films and quantum-well heterostructures on (0001) sapphire substrates. The III-N heteroepitaxial layers are grown by metalorganic chemical vapor deposition on sapphire substrates using various growth conditions. A comparison of the 300 K photoluminescence (PL) spectra of the samples indicates that a higher PL intensity is measured for the quantum-well structures having an intentional n-type Si-doping concentration. Furthermore, three-, five-, and eight-period InGaN quantum-well structures exhibit similar narrow PL spectra.

Observation of long photoluminescence decay times for high-quality GaN grown by metalorganic chemical vapor deposition

GaN epitaxial layers with different crystalline quality grown on sapphire substrates by metalorganic chemical vapor deposition are investigated using time-resolved photoluminescence at 300 K. It is found that the time-dependent photoluminescence of low-quality GaN decays faster than that of the high-quality GaN films. The time constants for the dual-exponential decay of the photoluminescence are calculated to be 50 and 250 ps for high-quality undoped GaN and 30 ps for low-quality undoped GaN. For high-quality Si-doped GaN, time constants of 150 and 740 ps are extracted while corresponding time constants of 40 and 200 ps are measured for low-quality Si-doped GaN. We believe that the time constant of 740 ps measured for our high-quality Si-doped GaN is the longest ever reported for thin GaN/sapphire films.

Radiative recombination of two-dimensional electrons in a modulation-doped Al0.37Ga0.63N/GaN single heterostructure

We have studied the low-temperature (4 K) photoluminescence (PL) of a modulation-doped Al0.37Ga0.63N/GaN single heterostructure. Radiative recombination is observed between electrons in the two-dimensional quantum states at the heterointerface and holes in either the flatband region or bound to residual acceptors. This PL emission disappears when the top AlGaN layer is removed by reactive ion etching, indicating that the luminescence is from the two-dimensional electron states. From the peak separation of the luminescence related to the two-dimensional electrons, the maximum electric field at the heterointerface is estimated to be 3.7×104 V/cm.

Simulation of the electrical characteristics of AlGaN/GaN heterojunction bipolar transistors

Abstract Heterojunction bipolar transistors fabricated from the III–V nitrides offer improved linearity for high-power and high-frequency amplifiers. We report the results of two-dimensional simulations of the characteristics of AlGaN/GaN heterojunction bipolar transistors (HBTs) using the MEDICI® simulator. The simulated performance of single-heterojunction devices predict that it is possible to realize an amplification coefficient β =1130, an emitter-base turn-on voltage V BCO =3 V , a collector saturation current I Csat =3.5 kA / cm 2 , and a collector breakdown voltage BV CEO =55 V . The simulated frequency response of these devices indicate a maximum cutoff frequency f T =18 GHz .

Time‐Resolved Reflectivity Studies of Carrier Dynamics as a Function of Al Content in AlGaN Alloys

We have used intensity dependent pump-probe reflectivity measurements to investigate the carrier dynamics in AlGaN alloys with Al content ranging from ∼0.15 to 0.4. For the Al 0.15 Ga 0.85 N sample the intensity dependence of the ΔR decay suggests that at high intensity the shallow traps are saturated and ultrafast nonradiative recombination dominates the carrier dynamics. For the Al 0.25 Ga 0.75 N and Al 0.4 Ga 0.6 N samples AR decays faster with decreasing intensity and changes sign. Moreover, the decays are faster for a given intensity in the higher Al content sample. This behavior suggests that in these cases the dynamics are governed by trapping at localized states that become deeper and more numerous as the Al content increases. Within this context the sign change in AR may reflect the onset of photoinduced absorption associated with the excitation of carriers from the localized states to the bands.