J.B. Webb

Semi-insulating C-doped GaN and high-mobility AlGaN/GaN heterostructures grown by ammonia molecular beam epitaxy

A method of growing semi-insulating GaN epilayers by ammonia molecular beam epitaxy through intentional doping with carbon is reported. Thick GaN layers of high resistivity are an important element in GaN-based heterostructure field-effect transistors. A methane ion source was used as the carbon dopant source. The cracking of the methane gas by the ion source was found to be the key to the effective incorporation of carbon. High-quality C-doped GaN layers with resistivities greater than 106u200aΩu200acm have been grown with high reproducibility and reliability. AlGaN/GaN heterostructures grown on the C-doped semi-insulating GaN-based layers exhibited a high-mobility two-dimensional electron gas at the heterointerface, with room-temperature mobilities typically between 1000 and 1200 cm2/Vu200as, and liquid-nitrogen-temperature mobilities up to 5660 cm2/Vu200as. The carrier density was almost constant, with less than 3% change over the measured temperature range.

Ultraviolet photoenhanced wet etching of GaN in K2S2O8 solution

The mechanism of the UV photoenhanced wet etching of GaN is determined. The UV photoenhanced wet etching does not require an electrical contact to be made to the sample, and nitrides deposited on insulating substrates (such as sapphire) can be etched, unlike photoelectrochemical (PEC) wet etching. The present technique relies on adding an appropriate oxidizing agent, in this case, peroxydisulfate (S2O82−), to KOH solutions. In a similar mechanism to PEC wet etching, the regions of low defect density are preferentially etched, leaving regions of high electron recombination such as threading dislocations relatively intact. The threading dislocations may be physically broken off, either by stirring or by a postetch sonication of the sample in KOH solution. Smoothly etched surfaces can be obtained under the proper conditions. A noble metal mask acts in a catalytic manner, yielding etch rates approximately one order of magnitude greater than those observed using inert masks. The essential role of the free radi...

Smooth wet etching by ultraviolet-assisted photoetching and its application to the fabrication of AlGaN/GaN heterostructure field-effect transistors

We characterize a KOH-based ultraviolet (UV) photoassisted wet etching technique using K2S2O8 as the oxidizing agent. The solution provides a well-controlled etch rate and produces smooth high-quality etched surfaces with a minimal degradation in surface roughness as measured by atomic force microscopy. The evolution of the solution pH upon exposure to UV radiation is identified as key to obtaining smooth etched surfaces and a controlled etch rate: Unless steps are taken to maintain the pH above 12.0, the etch rate displays a sharp drop that coincides with a gross roughening of the etched surface. The applicability of the present technique is demonstrated by the fabrication of high-quality mesa-isolated AlGaN/GaN hetrostructure field-effect transistors. In addition, the etch presented here features a high selectivity to C-doped layers which should prove useful in the fabrication of AlGaN/GaN hetrostructure bipolar transistors. The method is well adapted to device processing applications because it does no...

On the Carrier Concentration and Hall Mobility in GaN Epilayers

The dependence of Hall mobility and carrier concentration in GaN epilayers on light illumination was examined. It was found that Hall mobility and electron concentration both increased after illumination with red laser (632.8 nm) and green laser (530 nm). However, no changes in Hall mobility and carrier concentration were found, if IR-laser (850 nm) was used. The results reveal that deep-level defects were excited and hence extra carriers were generated by light illumination. The influence is more pronounced for thinner films. These observations indicate that donor-like defect-related states were located 1.48 to 2.33 eV below the conduction band edge.

Bias Stress Measurements on High Performance AlGaN/GaN HFET Devices

High performance AlGaN/GaN heterojunction field effect transistor (HFET) devices have been fabricated, using material grown by MBE on sapphire substrates. The Hall effect 2DEG sheet charge densities and mobilities were as high as 1.68 × 10 13 cm -2 and 1000 cm 2 /Vs respectively. Peak dc currents in excess of 1 A/mm, with transconductances in excess of 220 mS/mm were common. f T and f max scaled linearly with the inverse of the gate length, with maximum values of 39 and 80 GHz recorded for T-gates of length 0.3 μm. The devices with optically defined gates were stressed at V DS = 10 V and V G = 1 V. for times up to 5000 s. The stress decreased both f T and f max , but this degradation mostly recovered after a few days. In contrast, the gate leakage current improved (decreased) with stressing, but did not return to the initial undesirable levels. Thus, we found that electrical bias stressing can have a positive effect on some of the characteristics of our devices.

Reproducibility of growing AlGaN/GaN high-electron-mobility-transistor heterostructures by molecular-beam epitaxy

Abstract High-quality GaN/AlGaN high-electron-mobility transistors (HEMT) characterized by room temperature mobilities of ∼1000 cm2xa0V−1xa0s−1 and sheet electron densities in the range of 3×1012–2×1013 cm−2 have been grown by reactive molecular-beam epitaxy on insulating C-doped GaN template layers. Growth data and mobility values resulting from over 50 HEMT growth experiments on 2 in. diameter sapphire wafers are presented to show the remarkable overall high yield and reproducibility of the HEMT structures grown by this method. The use of insulating C-doped GaN buffer layers has greatly increased reproducibility of the device structures by ensuring device isolation through controlled carbon doping. Moreover, an undoped GaN channel layer of remarkably low defect density and high mobility can be grown on the C-doped GaN template with high reproducibility. Precise control of the growth temperature was key to achieving the high quality and reproducibility of the structures.

In situ Mg surface treatment of p-type GaN grown by ammonia-molecular-beam epitaxy for efficient Ohmic contact formation

The pronounced effect of in situ Mg surface treatment of p-type GaN grown by ammonia-molecular-beam epitaxy on Ohmic contact formation is reported. The surface of the Mg-doped p-type GaN layer was held at the growth temperature following the growth, and exposed to a Mg beam under a NH3 flow for 15 min. With such a treatment, a specific contact resistance in the low 10−4u2009Ωu200acm2 was obtained with as-deposited Ni/Au contacts and without any ex situ treatment. In contrast, contacts on similar p-type layers without such an in situ treatment were highly rectifying even after annealing. A surface Fermi level shift as a result of the in situ treatment was observed by x-ray photoemission studies, and is ascribed, together with the high surface doping concentration, to be the mechanisms behind the effective Ohmic contact formation.

Selective growth of GaN on a SiC substrate patterned with an AlN seed layer by ammonia molecular-beam epitaxy

Highly selective growth of GaN on 4H–SiC using the SiC substrate as a pseudomask has been demonstrated using the ammonia molecular-beam-epitaxy technique. A total lack of nucleation on the bare SiC surface was observed under typical GaN growth conditions. The nucleation of the GaN layer occurred preferentially from a patterned thin (300 A) AlN seed layer, which had been predeposited on the SiC surface using the magnetron-sputter-epitaxy technique and patterned into parallel stripes by photolithography and chemically assisted ion-beam etching. Evidence of lateral overgrowth was observed by scanning electron microscopy and x-ray diffraction studies. The GaN stripes grown show extremely smooth side facets due to the lateral growth.

Effect of template morphology on the efficiency of InGaN∕GaN quantum wells and light-emitting diodes grown by molecular-beam epitaxy

The pronounced enhancement of indium incorporation efficiency for InGaN∕GaN quantum wells due to the rough, faceted surface of the GaN template grown in situ by ammonia-molecular-beam epitaxy is reported. The InGaN∕GaN quantum wells are grown by plasma-assisted molecular-beam epitaxy. Unlike the smooth (0002) surface of GaN template layers grown by metalorganic chemical vapor deposition, the surface of the template layers grown by ammonia-molecular-beam epitaxy is defined by {10-1m} pyramidal facets causing significant surface roughness. The drastically enhanced indium incorporation rate associated with the rough templates allows the InGaN∕GaN quantum wells to be grown at higher temperatures as it compensates for the increased thermal decomposition. High luminescence efficiency is achieved as a result. Using such efficient InGaN∕GaN quantum wells, light-emitting diodes have been grown entirely by molecular-beam epitaxy on sapphire substrates, demonstrating output power of 0.22mW for 20mA injection current.

Nitride-based 2DEG photodetectors with a large AC responsivity

Abstract Nitride-based AlGaN/GaN heterostructure two-dimensional electron gas photodetectors have been successfully fabricated by low-pressure metalorganic vapor phase epitaxy on sapphire substrate. By using such an AlGaN/GaN heterostructure, we could significantly reduce the recombination of photogenerated carriers and thus achieve extremely high photodetector responsivity due to the use of AlGaN/GaN heterostructure. With an incident light wavelength of 270 nm, it was found that the AC responsivity could reach 8.7xa0×xa0106 A/W.