Nitrogen Plasma Source Research Articles

Molecular beam epitaxy growth and properties of GaN films on GaN/SiC substrates

Two types of nitrogen plasma sources, an electron cyclotron resonance (ECR) plasma source and a radio frequency (rf) plasma source, were used for the growth of GaN by molecular beam epitaxy (MBE). GaN film quality was correlated with the optical emission characteristics of each type of nitrogen plasma source employed. The best quality GaN films were those grown using the rf nitrogen plasma source. This source was found to emit a much larger fraction of atomic nitrogen and 1st-positive series excited molecular nitrogen in contrast to the ECR plasma source which mainly produced 2nd-positive series excited molecular nitrogen and nitrogen molecular ions when operated under the same conditions. The benefit of homoepitaxial growth of GaN, using metalorganic vapor phase epitaxy grown GaN layers on basal plane 6H-SiC, was seen by the observation of surface reconstructions before, during, and after GaN film growth by MBE. In addition, the MBE-grown GaN films exhibited remarkably intense photoluminescence dominated by a sharp band-edge peak at 3.409 eV having a full-width at half-maximum (FWHM) of 29.7 meV at room temperature. At lower temperatures, splitting of the near-edge excition peaks was observed. Double-crystal x-ray rocking curve measurements of selected MBE-grown GaN films yielded (0002) diffraction peaks having FWHMs as narrow as 156 arcsec.

Growth and Properties of III-V Nitride Films, Quantum Well Structures and Integrated Heterostructure Devices

ABSTRACTGrowth of lll-V nitrides by molecular beam epitaxy (MBE) is being studied at NCSU using an rf nitrogen plasma source. GaN/SiC substrates consisting of ∼3 μm thick GaN buffer layers grown on 6H-SiC wafers by MOVPE at Cree Research, Inc. are being used as substrates in the MBE film growth experiments. The MBE-grown GaN films exhibit excellent structural and optical properties — comparable to the best GaN films grown by MOVPE—as determined from photoluminescence, x-ray diffraction, and vertical-cross-section TEM micrographs. Mg and Si have been used as dopants for p-type and n-type layers, respectively. AlxGa1−xN films (x∼0.06-0.08) and AlxGa1−xN/GaN multi-quantum-well structures have been grown which display good optical properties. Light-emitting diodes (LEDs) based on double-heterostructures of AlxGa1−xN/GaN which emit violet light at ∼400 nm have also been demonstrated. Key issues that must be addressed before lll-V nitride laser diodes can be demonstrated and commercialized are discussed. New integrated heterostructures are proposed for the development of a variety of vertical-transport devices such as light-emitting diodes, laser diodes, photocathodes, electron emitters based on the negative-electron-affinity of AIN, and certain transistor structures.

Auger investigation of group III nitride films grown by molecular beam epitaxy

We discuss the growth and properties of III-nitride films prepared by molecular beam epitaxy (MBE) using an RF activated nitrogen plasma source. Binary compounds of GaN and InN have been grown on (001) oriented GaAs substrates at 0.2 to 0.4 μm/h. A novel material system Ga(AsN), which can in principle be lattice matched to GaP (or Si), has been studied. Alloy films of In(AsN) have also been prepared. The properties of the films were investigated using in-situ reflection high energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). We have established that GaN and InN films grown at 620 and 300°C, respectively, can be prepared with relatively abrupt epilayer-substrate interfaces. However, InN grown at a higher substrate temperature (∼-350°C) has a much less abrupt interface. The nitrogen content of the alloy films of Ga(AsN) is estimated to be approximately 20% from AES studies. Oxygen, commonly observed in AES studies of nitrides, is present on the film surface and arises from rapid oxidation due to residual gases in the analysis chamber and/or from the argon used for sputtering. However, oxygen is not detected in AES sputtering studies and is therefore not present in the bulk of the films within our detection limit (∼ 0.1%).

Anti-corrosive surface modification of 6061 aluminum using plasma source ion implantation

Corrosion properties of 6061T aluminum alloy treated by nitrogen plasma source ion implantation (PSII) have been investigated. Results from corrosion tests demonstrate that the nitrogen PSII significantly increased the pitting resistance of implanted 6061T Al samples. Scanning electron microscopy of corrosion-tested samples found that both pit size and density of the PSII treated samples were smaller than those of untreated samples, although no significant changes in the corrosion potential or the corrosion current were observed. These results demonstrate PSII's potential for metal corrosion protection.

Cathodoluminescence characterization of nitrogen-doped ZnSe

Today nitrogen seems to be the most suitable acceptor for ZnSe-device applications. p-Type doping of ZnSe with nitrogen by metalorganic vapour phase epitaxy (MOVPE) is still not satisfactorily achieved. Stimulated by the success of the plasma doping technique in MBE, a nitrogen plasma source in used in order to overcome the well-known problem of low acceptor activation. In this work nitrogen-doped ZnSe layers were grown by low-pressure MOVPE at growth temperatures of T D = 320–480°C from diethylzinc (DEZn) and diisopropylselenide (DIPSe) or ditertiarybutylselenide (DTBSe) on (100) GaAs substrates. The layer morphology and luminescence properties of doped and undoped ZnSe layers were characterized by secondary electron (SE) imaging and panchromatic cathodoluminescence (CL) imaging in order to investigate the influence of the plasma source on the growth process. It is found that the layer morphology is not deteriorated by the nitrogen dopant source. For monitoring the dopant incorporation spectral cathodoluminescence measurements were performed at 14 K and at 77 K. Spectral CL measurements with focussed electron beam show that even with these fairly high excitation power densities (about 10 kW/cm 2), donor-acceptor pair (DAP) luminescence is observed, which indicates the incorporation of a shallow acceptor at low growth temperatures T D = 380°C. Although there is evidence for the incorporation of N by SIMS, it cannot be excluded that oxygen is also incorporated and contributes to the DAP luminescence. p-Type conductivity could not be demonstrated yet. The layers are highly resistive.

Photoluminescence and p-type conductivity in CdTe:N grown by molecular beam epitaxy

An rf nitrogen (N) plasma source has been used to achieve p-type conductivity in molecular beam epitaxy CdTe layers grown with a Cd overpressure. Photoluminescence and secondary ion mass spectrometry measurements have confirmed the incorporation of the N species, and evidence for the resulting p-type conductivity has been obtained using capacitance-voltage and current-voltage techniques. Net hole concentrations as high as 2×1017 cm−3 have so far been achieved, which contrasts with the normally n-type nature of our undoped CdTe layers.

Observations on the limits to p-type doping in ZnSe

In this letter secondary ion mass spectroscopy measurements of nitrogen concentrations in p-type ZnSe and ZnTe, doped using a nitrogen plasma source with molecular beam epitaxy, are correlated with transport data from the temperature-dependent Hall effect measurements. The results suggest that, at least for the growth conditions employed in this study, the nitrogen acceptor solubility is the controlling factor in determining that the acceptor concentration in ZnTe exceeds that in ZnSe by about one order of magnitude despite the similar growth conditions.

P-type CdSe grown by molecular beam epitaxy using a nitrogen plasma source

We report the growth of p-type CdSe having zinc blende structure with the hole concentration of 1×1017 cm−3 by molecular beam epitaxy using a nitrogen plasma source. The electrical and optical properties of CdSe:N epilayers are characterized by Hall and photoluminescence (PL) measurements. PL spectra from p-type CdSe:N epilayers at 14 K show dominant emission associated with a neutral donor-bound excitation at 1.745 eV and two kinds of donor-acceptor pair emission bands with zero phonon energies of 1.724 and 1.695 eV. An Au/p-CdSe:N electrode is examined as ohmic contact for p-ZnSe in terms of current-voltage characteristic. It is shown that the Au p-CdSe electrode works as an ohmic contact for p-ZnSe.

Doping of zinc-selenide-telluride

We investigate the doping behavior of ZnSe/ZnTe short period superlattices. p-type doping is achieved with a dc nitrogen plasma source, n-type doping with chlorine from a ZnCl2 Knudsen source. Even a small Te content has a strong positive effect on p doping: Doping levels in the upper 1019 cm−3 range are achieved, and ohmic contacts can be obtained even for low carrier concentrations. The data are in excellent agreement with a theory based on the amphoteric native defect model. The opposite is valid for n doping: At Te concentrations above 20% electron concentrations are below 1016 cm−3. As a possible way to get both good n- and p-type doping at the same lattice constant we propose the use of the quaternary compound Zn(1−y)Mg(y)Se(1−x)Te(x).

Atomic nitrogen production in nitrogen-plasma sources used for the growth of ZnSe : N and related alloys by molecular-beam epitaxy

A high resolution study of the optical emission from nitrogen-plasmas produced by an Oxford Applied Research radio frequency plasma source is reported. Strong atomic emission lines in the near-infrared spectral region provide unequivocal evidence that the nitrogen-plasma source generates an appreciable flux of nitrogen atoms. Nitrogen atoms, rather than N 2 molecules in the plasma, are the most likely nitrogen-plasma species responsible for the successful p-type doping of ZnSe : N and related alloys. Emission spectra from the radio frequency nitrogen-plasma source are compared with similar emission spectra obtained from an electron cyclotron resonance microwave nitrogen-plasma source.

Advances in the development of graded injector visible light emitters

Novel visible light emitters with low turn-on voltages and efficient room temperature operation have been demonstrated in the n-CdSe/p-ZnTe heterojunction separated by a graded Mg x Cd 1− x Se injecting region. The successful p-type doping of ZnTe up to 1 × 10 20 cm -3 using a nitrogen plasma source has allowed using higher quality GaSb substrates and growing of p-ZnTe epilayers. Diodes with ideality factors of 1.8 are realized with turn-on voltages near that of the bandgap of ZnTe. This deviation from an ideality of 1 may be due to using too small a peak Mg concentration in the Mg n Cd 1− x Se layer resulting in a barrier for electron injection and increasing recombination at the interface.

Nitrogen plasma source ion implantation of aluminum

Nitrogen plasma source ion implantation has been applied to pure aluminum and aluminum alloy 7075 (Al–Cu–Mg–Zn). Nitrogen implantation modified the surface properties through transformation of the surface to aluminum–nitride (AlN). A uniformly thick (∼0.15 μm), polycrystalline (∼30 nm grain size) and continuous AlN layer was formed on both pure aluminum and 7075. Knoop microhardness tests showed an increase in surface hardness at low loads. Pin-on-disk wear tests showed nitrogen implantation decreased the wear rates by a factor of ∼10 for alloy 7075. This study demonstrates significant modification of the tribological properties of an aluminum surface can be accomplished using nitrogen plasma source ion implantation.

Epitaxial growth of cubic and hexagonal GaN by gas source molecular beam epitaxy using a microwave plasma nitrogen source

Cubic and hexagonal GaN epilayers were successfully grown on GaAs substrates by gas source molecular beam epitaxy using a microwave plasma nitrogen source. These GaN epilayers showed quite strong cathodoluminescence emission. X-ray and electron diffraction analyses indicated the improved crystalline quality, compared with GaN epilayers grown using dimetylhydrazine. GaN epilayers were also grown, by the same method, on 3C-SiC substrates, which have a lattice constant much closer to GaN than GaAs.

Atomic-nitrogen production in a radio-frequency plasma source

The first high-resolution study, to our knowledge, of the optical emission from an Oxford Applied Research MPD21 nitrogen-plasma source is reported. The spectroscopic results clearly show that the MPD21 nitrogen-plasma source generates an appreciable flux of nitrogen atoms. This suggests that nitrogen atoms are the plasma species responsible for the successful p-type doping of ZnSe:N and related alloys.

Transport study of ZnSe:N employing Zn(Se,Te) graded contacts

Hall effect measurements for temperatures ranging from 77 to 320 K on a series of p-ZnSe films grown on GaAs substrates are reported. The ZnSe epilayers were doped using a nitrogen plasma source during growth by molecular beam epitaxy. A Zn(Se,Te) graded band-gap layer was used to provide ohmic contacts over the temperature range of the measurements. The activation energy of nitrogen in ZnSe at the infinite dilution limit was found to be 114 meV, and compensation ratios ranged from 6% to 11%.

Growth and characterization of ZnTe:N; p-ZnTe/n-AISb diodes

In recent months the successful p-doping of ZnSe and Zn(S,Se) using a nitrogen plasma source during growth by molecular beam epitaxy was one factor leading to the realization of diode lasers and light emitting diodes. This paper reports the results of the nitrogen doping of ZnTe using similar techniques. Doping levels exceeding the 1019 cm−3 range are reported along with electrical, optical, and microstructural characterization. The nitrogen-doped ZnTe is used to implement p-ZnTe/n-AISb diodes; the growth and characterization of these hetero-junction diodes are described.

Heavy p-doping of ZnTe by molecular beam epitaxy using a nitrogen plasma source

The successful p-doping of ZnSe and Zn(S,Se) using a nitrogen plasma source during growth by molecular beam epitaxy has been an important factor leading to the recent realization of blue-green diode lasers and light-emitting diodes. This letter reports the results of the nitrogen doping of ZnTe using similar techniques. Doping levels approaching the 1019 cm−3 range are reported along with electrical, optical, and microstructural characterization. Highly doped ZnTe provides an opportunity for forming an ohmic contact to p-ZnSe.