David Gotthold

V/Al/Pt/Au Ohmic contact to n-AlGaN/GaN heterostructures

A study of V/Al/Pt/Au and Ti/Al/Pt/Au contacts to n-Al0.3Ga0.7N/GaN heterostructures is presented. Vanadium was chosen as a potential replacement for Ti because V is expected to form a thermally stable nitride with a low work function. Low-resistance Ohmic contacts are achieved with V/Al/Pt/Au layers after annealing at 650 °C, which represents a decrease of 150 °C compared to the Ti/Al/Pt/Au counterpart. This contact exhibits two orders of magnitude lower specific contact resistance compared to the Ti/Al/Pt/Au contacts studied in this work when annealed at temperatures less than 800 °C, although the specific contact resistance of the Ti/Al/Pt/Au contacts is lower when annealed at higher temperatures. A contact resistance and specific contact resistance of 0.8±0.1 Ω mm and (1.4±0.3)×10−5 Ω cm2, respectively, are obtained after annealing at 650 °C for 45 s.

Growth of GaNAs by molecular beam expitaxy using a N2/Ar rf plasma

A high efficiency nitrogen rf plasma source has been used to grow GaNAs by diluting the N2 gas with Ar. This source (an EPI UniBulb™ source) was originally designed for use in the growth of pure nitrides at high growth rates. For growth of As-rich GaNAs, high concentrations of active nitrogen lead to the growth of GaN instead of a random alloy. In this work we demonstrate that a dilute N2/Ar mixture leads to GaNAs films where the amount of nitrogen incorporation varies directly with the percentage of N2 in the gas mixture. Films with high structural quality were grown, thus validating the use of this approach.A high efficiency nitrogen rf plasma source has been used to grow GaNAs by diluting the N2 gas with Ar. This source (an EPI UniBulb™ source) was originally designed for use in the growth of pure nitrides at high growth rates. For growth of As-rich GaNAs, high concentrations of active nitrogen lead to the growth of GaN instead of a random alloy. In this work we demonstrate that a dilute N2/Ar mixture leads to GaNAs films where the amount of nitrogen incorporation varies directly with the percentage of N2 in the gas mixture. Films with high structural quality were grown, thus validating the use of this approach.

GaNAs resonant-cavity avalanche photodiode operating at 1.064 μm

A resonant-cavity avalanche photodiode using a GaNAs absorption region grown on GaAs has been demonstrated. The peak quantum efficiency was 59% at 1.064 μm with a full width at half maximum of 11 nm. The absorption coefficient was determined to be α=9×103/cm at this wavelength. The devices exhibited gain up to 100 at a low breakdown voltage of 13 V.

Improved quality GaN films grown by molecular beam epitaxy on sapphire

We report on the epitaxial growth and characterization of gallium nitride by molecular beam epitaxy (MBE). The samples were grown on (0002) sapphire using solid metal group III sources and an EPI UniBulb™ rf plasma source to produce atomic neutral nitrogen species. The films were characterized optically using reflectance and photoluminescence, van der Pauw Hall effect measurements, and x-ray rocking curves for the (0002), (1014), and (2204) directions. Nominally undoped films were grown with a (0002) rocking curve (ω) full width at half maximum (FWHM) from 700 to 125 arcsec, electron mobilities of up to 180 cm2/V s, and background carrier concentrations from the mid- 1017–2×1016/cm3. The room temperature photoluminescence FWHM for these films ranged from 38 up to 62 meV, with peak to yellow ratios from 7 to 70. These values represent an improvement in the quality of nominally undoped heteroepitaxially grown GaN by MBE.

GaNAs avalanche photodiode operating at 0.94 μm

A p-i-n avalanche photodiode (APD) using GaNAs grown on GaAs has been demonstrated. Characterization of the excess noise in the material was used to determine that the ratio of ionization coefficients (k=β/α) is k=0.4. The quantum efficiency was above 25% at 0.94 μm for 0.75% nitrogen incorporation. The APDs exhibited low dark currents (<60 nA/mm2 at 90% of breakdown) and a gain-bandwidth product of 42 GHz. GaNAs therefore shows promise for extending the operation of GaAs-based APDs to longer wavelengths.

Molecular-beam epitaxy growth of Ga(In)NAs/GaAs heterostructures for photodiodes

The incorporation of nitrogen into GaInAs/GaAs heterostructures has received intense interest recently due to the large negative bowing parameter of the resultant alloy. This change in the band gap for GaInAsN makes it useful for near-infrared optoelectronic devices on the GaAs substrate. However, the effect of adding nitrogen into GaInAs is an important issue in the use of these alloys. This article focuses on the growth of GaInNAs/GaAs alloys for use in photodiodes. Under our growth conditions, we show that the incorporation of nitrogen into GaAs and Ga0.8In0.2As is linear with nitrogen in the growth chemistry up to approximately 3.5%. Photodiodes using GaNAs absorption regions show low dark currents and high quantum efficiencies with nitrogen values up to 1.75%.

Construction of a variable aperture cell for source flux control in a molecular-beam epitaxy environment

A molecular-beam source utilizing instant flux adjustment for growth rate control is presented. The design uses a two-filament pyrolytic boron nitride (pBN) heater constructed with eight heated exit aperture holes, masked by an aperture-mating pBN closed end cylinder. The rotatable pBN mask opens and closes the effusion cell apertures to provide mechanical control of the source flux. This adjustment is provided by a rotational manipulator that translates rotary motion through the vacuum environment to the mounting journal of the pBN mask cylinder. RHEED oscillation changes in GaAs homoepitaxial growth shows an effectively instantaneous change of nearly an order of magnitude in the growth rate.

Contacts to High Aluminum Fraction p -type Aluminum Gallium Nitride

Gold, palladium, platinum or nickel ohmic contacts on Mg doped p -type Al x Ga 1-x N with x = 0.4 and x = 0.45 have been examined. The Au contact provided the lowest contact resistivity with p c = 1.8 (± 1.1) x 10 −3 Ωcm 2 , but only following annealing at 850°C. For the Pd, Au, and Pt contacts annealed at greater than 700°C, a rapid degradation in the current-voltage curves was observed upon testing. The degradation was induced by exposure to sub-bandgap light and was reversed with a mild anneal at 500°C. Possible mechanisms for the degradation are discussed.

Ohmic contacts to n-type AlGaN and nitride HEMT epilayers

The authors examine the influence of a variety of processing variables on the specific contact resistance of Ti/Al/Pt/Au ohmic contacts to n-type AlGaN, and we study the effect of replacing Ti with V when fabricating ohmic contacts to n-type AlGaN/GaN HEMT epilayers. The replacement of Ti with V allows the annealing temperature for formation of low resistance ohmic contacts to be decreased by 150 /spl deg/C.

Production Scale Growth of AlGaN/GaN Field Effect Transistors

This paper addresses issues with the manufacturability of AlGaN/GaN FET structures. A robust Metalorganic Chemical Vapor Deposition growth process has been developed that will now allow reliability measurements to be obtained on the resulting devices. During a small scale production run mobilities in excess of 1600 cm 2 /V.s, sheet charge (N s ) between 0.8x10 13 and 1.2x10 13 cm -2 , and R s