S. Rolfe

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 106 Ω cm 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/V s, and liquid-nitrogen-temperature mobilities up to 5660 cm2/V s. The carrier density was almost constant, with less than 3% change over the measured temperature range.

Growth kinetics and electronic properties of unintentionally doped semi-insulating GaN on SiC and high-resistivity GaN on sapphire grown by ammonia molecular-beam epitaxy

Growth of unintentionally doped (UID) semi-insulating GaN on SiC and highly resistive GaN on sapphire using the ammonia molecular-beam epitaxy technique is reported. The semi-insulating UID GaN on SiC shows room temperature (RT) resistivity of 1011 Ω cm and well defined activation energy of 1.0 eV. The balance of compensation of unintentional donors and acceptors is such that the Fermi level is lowered to midgap, and controlled by a 1.0 eV deep level defect, which is thought to be related to the nitrogen antisite NGa, similar to the “EL2” center (arsenic antisite) in unintentionally doped semi-insulating GaAs. The highly resistive GaN on sapphire shows RT resistivity in range of 106–109 Ω cm and activation energy varying from 0.25 to 0.9 eV. In this case, the compensation of shallow donors is incomplete, and the Fermi level is controlled by levels shallower than the 1.0 eV deep centers. The growth mechanisms for the resistive UID GaN materials were investigated by experimental studies of the surface kinet...

Growth of crack-free, carbon-doped GaN and AlGaN∕GaN high electron mobility transistor structures on Si (111) substrates by ammonia molecular beam epitaxy

The growth of C-doped GaN epilayers on p-Si (111) substrates by ammonia molecular beam epitaxy is reported. Highly insulating and crack-free 1.5-μm-thick C-doped GaN layers have been prepared using ionized methane as the dopant source. Using such a template, AlGaN∕GaN two-dimensional electron gas structures with a mobility of 1260cm2∕Vs for a sheet carrier density of 1.24×1013cm−2 have been achieved at room temperature. Fabricated devices demonstrated an excellent pinch-off characteristic as revealed by an on-to-off ratio higher than four orders of magnitudes and by very low leakage current (10μA∕mm at VDS=20V).

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 10−4 Ω cm2 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.

Characterization of very thin Ge epilayers on (100) Si by spectroscopic ellipsometry

A series of very thin Ge epilayers (1–12 monolayers) grown on Si (100) that had previously been characterized by transmission electron microscopy, X-ray reflection, Rutherford back-scattering, secondary-ion mass spectrometry, Raman spectroscopy and extended X-ray absorption fine structure analysis has been examined by spectroscopic ellipsometry (SE). It was found that the SE results significantly enhance the other findings. Analysis of the SE measurements was based on the nominal growth conditions rather than the results of the post mortem findings mentioned above. This analysis was able to provide the thicknesses and compositions of the layers comprising the heterostructures easily and quickly. The findings of the previous characterizations of these samples are compared with the results of the SE measurements. The main result is that not even in the thickest epilayer was there a phase that could be described as pure germanium. Intermingling of the Ge epilayer with the Si cap was substantial in every case. It is concluded that the minimum epilayer thickness that would guarantee at least some pure germanium under these conditions is greater than 12 monolayers.

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.

Interface effects on electrical properties of high purity InP grown by gas-source molecular beam epitaxy

Abstract The effect of the substrate-epilayer interface of InP on Hall mobilities and carrier concentrations at 300 and 77K is studied. It is shown that photoluminescence (PL) and Hall results are contradictory due to the highly conductive interface of InP, which falsifies the Hall results. After correcting mathematically for the interface effect in the Hall results, PL and Hall data show that our InP layers are of high purity and only slightly compensated. The SIMS profiles reveal an accumulation of carbon and silicon at the interface.

Molecular beam epitaxy synthesis of Si1−yCy and Si1−x−yGexCy alloys and Ge islands using an electron cyclotron resonance argon/methane plasma

Abstract We report the growth of Si 1− y C y and Si 1− x − y Ge x C y alloys on Si(001) by electron cyclotron resonance plasma-assisted Si molecular beam epitaxy using an argon/methane gas mixture. Various Si/Si 1− y C y and Si/Si 1− x − y Ge x C y multilayers have been grown and characterized principally by X-ray diffraction and Raman spectroscopy. The influence of growth parameters and electron cyclotron resonance plasma source operating conditions on the C substitutional incorporation was studied. Under optimum growth conditions the structures show good structural properties and sharp interfaces with carbon being essentially substitutionally incorporated up to concentrations of ∼1%. No significant carbon incorporation was measured in films grown under a high methane partial pressure without plasma excitation. Si 1− x − y Ge x C y layers grown with this technique exhibit the strain compensation and enhanced thermal stability expected for these ternary alloys. Carbon pre-deposition of Si through surface exposure to the argon/methane plasma is shown to act as an antisurfactant on the growth of Ge islands by suppressing the formation of a Ge wetting layer on the surface.

CONTROL OF ANOMALOUS BORON DIFFUSION IN THE BASE OF SI/SIGE/SI HETEROJUNCTION BIPOLAR TRANSISTORS USING PTSI

In Si/SiGe/Si heterojunction bipolar transistor structures, very shallow arsenic implant on the emitter has been found to cause anomalous boron diffusion in the base. This phenomenon imposes stringent constraints on the device fabrication processes. We discovered that by using platinum silicide, which also served as a self‐aligned low resistance contact material to the emitter and base, the anomalous diffusion in the base was significantly reduced. In this letter, we report the experiment results, and propose possible explanations.

Si1−x−yGexCy alloy growth by electron cyclotron resonance plasma-assisted Si molecular beam epitaxy

Abstract We report the growth of Si 1− y C y and Si 1− x − y Ge x C y alloys by electron cyclotron resonance (ECR) plasma-assisted Si MBE using an argon/methane gas mixture. No significant carbon incorporation was measured in films grown under a high methane partial pressure without plasma excitation, whereas C incorporation up to ≈2 at.% has been achieved upon plasma excitation. Various Si/Si 1− y C y and Si/Si 1− x − y Ge x C y multilayers have been grown and characterized. The structures show good structural properties and sharp interfaces, with carbon being essentially substitutionally incorporated up to concentrations of ≈1%. Raman scattering spectroscopy of Si 1− y C y revealed two bands near 480 and 606 cm −1 that are indicative of substitutional incorporation. Strong strain compensation and enhanced thermal stability were observed in Si 1− x − y Ge x C y alloys. These results suggest that ECR plasma-assisted Si MBE may be an interesting alternative to more conventional methods for producing lattice matched epitaxial Si 1− y C y and Si 1− x − y Ge x C y thin films on Si.