J. M. Van Hove

High‐responsivity photoconductive ultraviolet sensors based on insulating single‐crystal GaN epilayers

We report on the fabrication and characterization of photoconductive ultraviolet detectors based on insulating single‐crystal GaN. The active layer (GaN) was deposited over basal‐plane sapphire substrates using a unique switched atomic‐layer‐epitaxy process. The sensors were measured to have a responsivity of 2000 A/W at a wavelength of 365 nm under a 5‐V bias. The responsivity remained nearly constant for wavelengths from 200 to 365 nm and dropped by three orders of magnitude within 10 nm of the band edge (by 375 nm). We estimate our sensors to have a gain of 6×103 (for wavelength 365 nm) and a bandwidth in excess of 2 kHz. The photosignal exhibited a linear behavior over five orders of incident optical power, thereby implying a very large dynamic range for these GaN‐based ultraviolet sensors.

Damped oscillations in reflection high energy electron diffraction during GaAs MBE

Oscillations in the time evolution of electron diffraction during MBE growth of GaAs are shown to be related to periodic variations in the step distributions on GaAs surfaces during epitaxial growth. Unintentionally doped GaAs surfaces were first prepared by MBE. Then the Ga flux is interrupted until an instrument limited diffraction pattern was obtained. During this process the angular width of the specular beam was measured versus time. When the Ga flux there are intensity oscillations that are weak near the Bragg angle. At the Bragg angle, where the diffraction is insensitive to surface steps, the length of the specular RHEED streak does not change. At angles between the Bragg angles, where steps lengthen the streaks, there are periodic variations in the streak length. We interpret the results in terms of a model in which a partially completed surface has a step distribution with smaller average terrace lengths than a completed surface.

Growth of high optical and electrical quality GaN layers using low‐pressure metalorganic chemical vapor deposition

We report on the low‐pressure metalorganic chemical vapor deposition of high quality single‐crystal GaN layers over basal plane sapphire substrates. Optimization of growth conditions resulted in material with carrier densities of 1017 /cm3 at room temperature and corresponding mobilities around 350 cm2 /V s. The photoluminescence linewidths improved from 160 meV [full width at half maximum (FWHM)] to 25 meV (FWHM). With improved material quality we were able to observe the polar optical mode and the ionized impurity scattering regimes in the mobility versus temperature data. Good quality Schottky barriers were formed on the as‐grown material using a tungsten probe and an alloyed indium contact. Our observations indicate a direct correlation between electrical and optical characteristics of good material and strongly question nitrogen vacancies as the sole explanation for the high carrier densities observed in poor quality GaN growths.

Observation of a two‐dimensional electron gas in low pressure metalorganic chemical vapor deposited GaN‐AlxGa1−xN heterojunctions

We have confirmed the presence of a two‐dimensional electron gas (2DEG) in a wide band‐gap GaN‐AlxGa1−xN heterojunction by observing steplike features in the quantum Hall effect. The 2DEG mobility for a GaN‐Al0.13Ga0.87N heterojunction was measured to be 834 cm2/V s at room temperature. It monotonically increased and saturated at a value of 2626 cm2/V s at 77 K. The 2DEG mobility remained nearly constant for temperatures ranging from 77 to 4.2 K. Using Shubnikov–de Haas (SdH) measurements the two‐dimensional carrier concentration was estimated to be 1×1011 cm−2. The peak mobility for the 2DEG was found to decrease with the heterojunction aluminum compositions in excess of 13%.

ULTRAVIOLET-SENSITIVE, VISIBLE-BLIND GAN PHOTODIODES FABRICATED BY MOLECULAR BEAM EPITAXY

GaN p–i–n photovoltaic diode arrays were fabricated from epitaxial films deposited on sapphire by molecular beam epitaxy. Peak UV responsivity was 0.11 A/W at 360 nm, corresponding to 48% internal quantum efficiency. Visible rejection over 400–800 nm was 3–4 orders of magnitude. Typical pulsed time response was measured at 8.2 μs. Spectral response modeling was performed to analyze the photocurrent contributions from photogenerated carrier drift in the depletion region and from minority carrier diffusion in the p and n layers. With the model, a maximum internal quantum efficiency of 55% at 360 nm was calculated for the photovoltaic diode structure.

Electrical effects of plasma damage in p-GaN

The reverse breakdown voltage of p-GaN Schottky diodes was used to measure the electrical effects of high density Ar or H2 plasma exposure. The near surface of the p-GaN became more compensated through introduction of shallow donor states whose concentration depended on ion flux, ion energy, and ion mass. At high fluxes or energies, the donor concentration exceeded 1019 cm−3 and produced p-to-n surface conversion. The damage depth was established as ∼400 A based on electrical and wet etch rate measurements. Rapid thermal annealing at 900 °C under a N2 ambient restored the initial electrical properties of the p-GaN.

High electron mobility GaN/AlxGa1−xN heterostructures grown by low‐pressure metalorganic chemical vapor deposition

In this letter we report the first observation of enhanced electron mobility in GaN/AlxGa1−xN heterojunctions. These structures were deposited on basal plane sapphire using low‐pressure metalorganic chemical vapor deposition. The electron mobility of a single heterojunction composed of 500 A of Al0.09Ga0.91N deposited onto 0.3 μm of GaN was around 620 cm2/V s at room temperature as compared to 56 cm2/V s for bulk GaN of the same thickness deposited under identical conditions. The mobility for the single heterojunction increased to a value of 1600 cm2/V s at 77 K whereas the mobility of the 0.3 μm GaN layer alone peaked at 62 cm2/V s at 180 K and decreased to 19 cm2/V s at 77 K. A 18‐layer multiple heterojunction structure displayed a peak mobility of 1980 cm2/V s at 77 K.

Vertical‐cavity, room‐temperature stimulated emission from photopumped GaN films deposited over sapphire substrates using low‐pressure metalorganic chemical vapor deposition

We report the first observation of near‐UV vertical‐cavity stimulated emission from a photopumped GaN epilayer at room temperature. The epilayer was deposited over AIN‐coated basal plane sapphire substrate using low‐pressure metalorganic chemical vapor deposition. Epitaxy quality of a 1.5‐μm‐thick GaN layer was high enough to achieve stimulated emission at room temperature. The observed near‐UV optical emission power was a nonlinear function of the pump power density. At threshold power density, we also observed line narrowing and a shift of the peak UV emission towards longer wavelengths. Data comparing the UV emission for the vertical‐cavity and the edge emission geometry are also presented.

Depth and thermal stability of dry etch damage in GaN Schottky diodes

GaN Schottky diodes were exposed to N2 or H2 inductively coupled plasmas prior to deposition of the rectifying contact. Subsequent annealing, wet photochemical etching, or (NH4)2S surface passivation treatments were examined for their effect on diode current–voltage (I–V) characteristics. We found that either annealing at 750 °C under N2, or removal of ∼500–600 A of the surface essentially restored the initial I–V characteristics. There was no measurable improvement in the plasma-exposed diode behavior with (NH4)2S treatments.

Photoluminescence characteristics of AlGaN‐GaN‐AlGaN quantum wells

AlxGa1−xN‐GaN quantum wells were grown on basal plane sapphire by low‐pressure metalorganic vapor deposition. The photoluminescence spectra of samples of different well thicknesses and x values were measured. The experimental data were compared with the calculated solutions of the finite square quantum well and the bound states involved in the optical transition were identified.