Dennis K. Wickenden

H, He, and N implant isolation of n‐type GaN

The effect of ion‐implantation‐induced damage on the resistivity of n‐type GaN has been investigated. H, He, and N ions were studied. The resistivity as a function of temperature, implant concentration, and post‐implant annealing temperature has been examined. Helium implantation produced material with an as‐implanted resistivity of 1010 Ω‐cm. He‐implanted material remained highly resistive after an 800 °C furnace anneal. The damage associated with H implantation had a significant anneal stage at 250 °C and the details of the as‐implanted resistivity were sample dependent. N implants had to be annealed at 400 °C to optimize the resulting resistivity but were then thermally stable to over 800 °C. The 300 °C resistivity of thermally stabilized He‐ and N‐ implanted layers was 104 Ω‐cm, whereas for H‐implanted layers the 300 °C resistivity was less than 10 Ω‐cm.

High quality self‐nucleated AlxGa1−x N layers on (00.1) sapphire by low‐pressure metalorganic chemical vapor deposition

High quality AlxGa1−xN alloy films with x<0.4 have been prepared on self‐nucleated (00.1) sapphire substrates by low‐pressure metalorganic chemical vapor deposition. It has been shown that the lattice constant of the films varies linearly with alloy composition x (Vegard’s law is obeyed) and that homogeneous and inhomogeneous strain and alloy clustering are minimized in these self‐nucleated AlxGa1−xN films. Consistent with their reduced strain and chemical uniformity, the derived optical band gaps of these AlxGa1−xN films also show a linear dependence on alloy composition x, yielding a bowing parameter b≊0 eV.

The effect of thermal annealing on GaN nucleation layers deposited on (0001) sapphire by metalorganic chemical vapor deposition

It has been shown by optical and x‐ray measurements that GaN nucleation layers deposited at 540 °C on (0001)‐oriented sapphire substrates have a measurable crystalline component, although the x‐ray data and the lack of absorbance features near the direct band gap of GaN suggest that the crystallite size is very small. Upon annealing to higher temperatures, the crystallite size increases and the crystal perfection improves markedly, until at temperatures near those empirically determined to be optimum for growth of an epitaxial overlayer, it approaches that of good quality single‐crystal material. Most of the recrystallization of the nucleation layer occurs during the ramp from its deposition temperature to the growth temperature of the GaN overlayer, and there appears to be no advantage to prolonged annealing at high temperatures prior to epitaxial growth. In fact, x‐ray diffractometer results suggest that the nucleation layer deteriorates after 20 min at temperatures above 1015 °C, under the conditions u...

Characterization of GaN using thermally stimulated current and photocurrent spectroscopies and its application to UV detectors

Deep levels in insulating GaN grown by metalorganic vapor phase epitaxy (MOVPE) have been studied using thermally stimulated current (TSC) and photocurrent (PC) spectroscopies. Five main levels (0.11, 0.24, 0.36, 0.53 and 0.62 eV) were observed by TSC measurements in the as-grown undoped GaN. PC measurements showed three deep levels located within bandgap at 1.32, 1.70 and 2.36 eV, respectively. We found that three of the levels, located at 0.24, 0.36 and 0.53 eV, were eliminated by annealing at 1000°C under N2 for six hours, whereas the 0.62 eV level density increased after annealing. In addition, both the responsivity and on/off times of GaN metal-semiconductor-metal (MSM) detectors degrade with increasing concentration of the 0.62 eV trap. We have also found that this trap can be effectively reduced by increasing the ammonia flow rate during the MOVPE growth. Accordingly, a high responsivity (∼ 3200 A/W) UV detector with an improved response time, from 8 to 0.4 ms, was fabricated on GaN grown under the optimized conditions.

Optical quenching of photoconductivity in GaN photoconductors

The observation of optical quenching of photoconductivity in GaN photoconductors at room temperature is reported on. Three prominent quenching bands were found at Ev+1.44, 1.58, and 2.20 eV, respectively. These levels are related to three hole traps in GaN materials based on a hole trap model to interpret the quenching mechanism. The responsivity was reduced about 12% with an additional He–Ne laser shining on the photoconductor.

Extended x-ray absorption fine structure study of AlxGa(1−x)N films

Extended x-ray absorption fine structure above the Ga–K edge has been used to study the local structure of AlxGa1−xN films grown by metal organic chemical vapor deposition. With increasing Al content, x, the Ga–N bond length decreases, but much less than the average bond length. On the other hand, the x dependence of the Ga–Ga and Ga–Al distances does follow the variation of the average cation–cation distance. We conclude that bond angle distortions accommodate the differences between the Ga–N and Al–N bond lengths.

A microelectromechanical‐based magnetostrictive magnetometer

The principles of operation of a microelectromechanical (MEMS)‐based magnetometer designed on the magnetoelastic effect are described. The active transduction element is a commercial (001) silicon microcantilever coated with an amorphous thin film of the giant magnetostrictive alloy Terfenol‐D [(Dy0.7Te0.3)Fe2]. In addition to the magnetostrictive transducer, basic components of the magnetometer include: (a) mechanical resonance of the coated‐microcantilever through coupling to an ac magnetic field; and (b) detection by optical beam deflection of the microcantilever motion utilizing a laser diode source and a position‐sensitive detector. Currently, the sensitivity of this MEMS‐based magnetostrictive magnetometer is ∼1μT.

A high sensitivity, wide dynamic range magnetometer designed on a xylophone resonator

A novel magnetometer based on a classical xylophone resonator is described. The device consists of an aluminum bar supported by two wires placed at the nodal points of the fundamental resonance frequency. The wires also supply current of this frequency to the bar. In the presence of a magnetic field, the Lorentz force causes the resonator to vibrate. The amplitude of this vibration is proportional to a vector component of the magnetic field. The device is intrinsically linear, and by altering the drive current the sensitivity can range from nanoteslas to teslas.

Native defects and dopants in GaN studied through photoluminescence and optically detected magnetic resonance

Native defects and dopants in GaN grown by organometallic chemical vapor deposition have been studied with photoluminescence and optically detected magnetic resonance. For undoped samples, the combined results indicate the presence of residual shallow donors and acceptors and deep donors. A model for the capture and recombination among these defects is developed. For Mg-doped samples, the experiments reveal shallow and perturbed acceptors and shallow and deep donors. Hence, shallow and deep states for the native donor or donors appear in all samples. The Mg-acceptor is perturbed from its effective-mass state by nearby point defects.

TRANSIENT PHOTOCURRENT INDUCED IN GALLIUM NITRIDE BY TWO-PHOTON ABSORPTION

We have studied the subband gap induced, transient photocurrent in an epitaxial GaN film immersed in an electrolyte solution. For photon energies near the midgap position, one‐ and two‐photon (TP) contributoins were observed in the photocurrent. The one‐photon term exhibited a sublinear intensity dependence and was attributed to carrier generation from traps in the gap. The TP current was negligible for energies below Egap/2. Above this energy, the dispersion was consistent with previous calculations of the TP absorption coefficient [β(ω)] in direct gap semiconductors. A relationship between the TP photocurrent and β(ω) determined a value for the latter of ∼1.5 cm/GW at photon energies above Egap/2.