P. A. Maki

Laser bilayer etching of GaAs surfaces

A method of etching the surface of a substrate in a closed environment including the steps of contacting the surface of the substrate with a molecular chlorine etching gas, allowing a passivation layer to form on the surface of the substrate with atomic layer precision, and photochemically removing at least a portion of the passivation layer by irradiating the portion without destruction of the layer of material immediately underlying the passivation layer.

The Role of Impurities in Hydride Vapor Phase Epitaxially Grown Gallium Nitride

Gallium nitride (GaN) films grown by hydride vapor phase epitaxy on a variety of substrates have been investigated to study what role silicon and oxygen impurities play in determining the residual donor levels found in these films. Secondary ion mass spectroscopy analysis has been performed on these films and impurity levels have been normalized to ion implanted calibration standards. While oxygen appears to be a predominate impurity in all of the films, in many of them the sum of silicon and oxygen levels is insufficient to account for the donor concentration determined by Hall measurements. This suggests that either another impurity or a native defect is at least partly responsible for the autodoping of GaN. Additionally, the variation of impurity and carrier concentration with surface orientation and/or nucleation density suggests either a crystallographic or defect-related incorporation mechanism.

Optically pumped GaN/Al0.1Ga0.9N double‐heterostructure ultraviolet laser

Molecular‐beam epitaxy was used to grow a 100 nm Al0.1Ga0.9N/100 nm GaN/500 nm Al0.1Ga0.9N double heterostructure on a 10‐μm‐thick GaN buffer layer grown with hydride vapor phase epitaxy on (0001) sapphire. Lasing from the 100 nm GaN active layer has been obtained at ∼359 nm at liquid‐nitrogen temperature (77 K) and at ∼365 nm at room temperature (295 K), using transverse optical pumping at 337.1 nm with a 600 ps transversely excited atmospheric pressure pulsed nitrogen laser. Threshold pump fluences were measured to be 0.3 and 0.5 mJ/cm2 at 77 and 295 K, respectively, for a laser with 65 μm cavity length. In a laser of 23 μm cavity length, longitudinal cavity modes were observed with 0.56 nm spacing, corresponding to a group refractive index of 5.0 at the lasing wavelength.

Spectral and random telegraph noise characterizations of low-frequency fluctuations in GaAs/Al/sub 0.4/Ga/sub 0.6/As resonant tunneling diodes

The origin of low-frequency noise in resonant-tunneling diodes is investigated through spectral and time-domain characterizations over a wide range of temperatures and biasing conditions. The experiments were conducted on devices fabricated on GaAs/Al/sub 0.4/Ga/sub 0.6/As material system. Detailed analyses on the temperature and bias dependences of the random telegraph noise and Lorentzian structures in the noise power spectral densities showed that the low-frequency excess noise arises from hopping conduction of electrons from the emitter to the quasi-bound states in the quantum well. The capture of an electron by a trap in the energy barrier causes fluctuations in the transmission coefficient of the electron due to the modulation of the barrier potential. >

Gallium Nitride Thick Films Grown by Hydride Vapor Phase Epitaxy

Gallium nitride (GaN) thick films (to 150 μm) have been deposited by hydride vapor phase epitaxy (HVPE). These films are unintentionally doped n-type (n = 1–2 × 10 17 cm −3 at 300 K) and exhibit structural and electronic properties which are comparable with the best reported for GaN films grown by organometallic vapor phase epitaxy. Additionally, these properties are found to be uniform over 2-in diameter films grown on sapphire substrates. The use of either a GaCl or ZnO surface pretreatment has been found to substantially enhance the nucleation density, resulting in improved surface morphology and film properties, even though it appears that the ZnO film is thermochemically desorbed early on in the growth. Dislocation densities as low as ˜5×10 7 cm −2 have been attained for films 40 μtm thick. Homoepitaxial overgrowths both by electron-cyclotron-resonance plasma enhanced molecular beam epitaxy and OMVPE proceed in a straightforward manner, essentially replicating the defect structure of the HVPE GaN film.

Light scattering in high‐dislocation‐density GaN

Light scattering by edge dislocations and the resulting loss coefficient have been modeled for GaN layers. Phase‐front deformation caused by the refractive‐index variation in the dislocation’s strain field has been considered and the resulting scattering loss calculated. We show that the high dislocation densities observed in recent GaN layers can result in significant large loss coefficients. The present work also offers some insights for improved lasers.

Observation of random‐telegraph noise in resonant‐tunneling diodes

We report the observation of random‐telegraph noise in GaAs/Al0.4Ga0.6As resonant tunneling diodes. Measurements made on our devices from 57 to 70 K revealed discrete switching events with step heights ranging from 6 to 20 μV. Our studies indicated that the 20‐μV switching sequences correspond to two‐state thermally activated processes involving a single trap. At a bias of −0.4 V, the capture and emission activation energies of this trap are 81 and 51 meV, respectively, implying that the trap is located in the barrier. Our results suggest that the noise arises from transmission coefficient fluctuations due to hopping conduction of carriers through the barrier.

Comparison of a rate-equation model with experiment for the resonant tunneling diode in the scattering-dominated regime

A quasistatic rate-equation model for the resonant tunneling diode (RTD) is presented. In this model, the RTD is divided into three regions, each assumed in quasithermal equilibrium. The electron transfer between states in adjacent regions, assumed to be by elastic tunneling, is then proportional to the probability that the initial state is occupied and the final state is empty. Using this approach, we derive a small-signal equivalent circuit. The model parameters may be either calculated or measured in a fairly straightforward manner, as is demonstrated for an In0.53Ga0.47As/AlAs RTD. We find that the dc characteristic and ac behavior up to 4 GHz are modeled well; we also find that the measured and calculated parameters agree to approximately a factor of 2 except in the vicinity of the current valley. We show that this approach is accurate enough to provide insight into the operation of the device, yields results that are simple enough to be suitable for ac circuit simulation, and should be valid as long...

Diode‐laser‐pumped InGaAs/GaAs/AlGaAs heterostructure lasers with low internal loss and 4‐W average power

Optically pumped InGaAs/GaAs/AlGaAs heterostructure lasers exhibit differential quantum efficiencies that are not sensitive to output coupling or active length, indicating low free carrier absorption loss. A broad‐stripe 2.5‐mm‐long device yielded 4‐W average power and 20‐W peak power.

Resonant-tunneling-diode relaxation oscillator☆

Abstract Monolithic resonant-tunneling-diode (RTD) relaxation oscillators are fabricated. The highest repetition rate of this pulse generator is 6.7 GHz with a pulse width of approximately 60 ps. Oscillators with an RTD connected to an off-chip transmission line have been operated at a rate as low as 34 MHz while maintaining a similar pulse width. Characterization aided with simulations provides a better understanding of the RTD relaxation oscillator and the effects of the RTD characteristics on the performance of the oscillator.