Assisted Ion Beam Research Articles

Etch Properties of Gallium Nitride Using Chemically Assisted Ion Beam Etching (CAIBE)

Dry etching properties of GaN were investigated using chemically assisted ion beam etching (CAIBE) with Cl2, BCl3, and HCl gases. The measured GaN etch rate with increasing tilt angle showed the maximum at a 30° tilt angle, similar to the effect of tilt angle on sputter yield. The GaN etch rate was the highest with Cl2 and the lowest with BCl3. In the case of GaN etching with BCl3, the GaN etch rate was even lower than that in the case of GaN etched with Ar ion source only. The lower etch rate appears to be related to the higher binding energy of BCl3 and high degree of condensation on the substrate maintained at 0°C. A highly anisotropic etch profile with smooth sidewalls could be obtained with Cl2. The addition of Cl2 to BCl3 did not improve GaN etch profile. The lowest roughness value of about 10 Å was obtained for the etching with Cl2. The GaN surface etched by Cl2 and BCl3 showed an N-rich surface possibly due to the removal of Ga preferentially by forming GaClx, and GaN surface etched by HCl showed near-stoichiometric surface possibly due to the removal of Ga by forming GaClx and N by forming NHx.

Room-temperature pulsed operation of an electrically injected InGaN/GaN multi-quantum well distributed feedback laser

We demonstrate room-temperature pulsed operation of an electrically injected InGaN/GaN-based distributed feedback laser with an emission wavelength of 403 nm. The threshold current of a 1000-μm-long and 20-μm-wide device was 3.2 A; corresponding to a threshold current density of 16 kA/cm2. The 3rd order grating providing feedback was defined holographically and dry etched into the upper waveguiding layer by chemically assisted ion beam etching. We observed single mode operation of the laser with a side mode suppression ratio of 15 dB over a temperature range of about 30 K.

Carbon nanotube tipped atomic force microscopy for measurement of <100 nm etch morphology on semiconductors

The use of carbon nanotubes as tips in atomic force microscopy for a systematic study of dry etching pattern transfer in GaAs is described. The GaAs samples are patterned via electron beam lithography and then etched using magnetron reactive ion or chemically assisted ion beam processing. The technique allows diagnosis, in air, of etched features with scale sizes of <100 nm.

Sidewall slope control of chemically assisted ion-beam etched structures in InP-based materials

Chemically assisted ion beam etching (CAIBE) of InP-based materials has been newly developed with BCl3/Ar in comparison to Cl2/Ar and IBr3/Ar. Using halogen gases and an argon ion beam at 400 V a very good surface morphology was obtained at a low substrate temperature of −5 °C in any case; with BCl3/Ar a surface roughness of 0.2 nm was observed. The etch rates were in the range of 40–75 nm/min depending on the reactive gas. Mixing the reactive gases or tilting the substrate with respect to the impinging ion beam allowed us an excellent control of the etched sidewall slope. By mixing BCl3 and IBr3 we were able to tune the sidewall slopes between 15° (measured to the surface normal) for pure BCl3 and 38° for the pure IBr3, respectively. Tilting the substrate allowed us to adjust the slope angle between 0° and 60°. In addition we have analyzed the etched surfaces by energy dispersive x-ray measurements. The low temperature processes yielded stoichiometric InP surfaces, etching at higher substrate temperatures results nonstoichiometric surfaces. These low temperature halogen CAIBE processes were successfully applied for the fabrication of gratings, ridge waveguides and facets for long wavelength (InGa)(AsP) and (AlInGa)(AsP) laser diodes. In this article we present ridge waveguide (InGa)(AsP)/InP lasers (1.55 μm) with direct CW modulation bandwidths of 9.5 GHz.

Dry-etching and characterization of mirrors on III-nitride laser diodes from chemically assisted ion beam etching

Vertical mirrors have been fabricated with chemically assisted ion beam etching (CAIBE) on OMVPE grown InGaN/AlGaN laser diode structures. AFM measurements show that smooth vertical sidewalls are obtained which exhibit a root mean squared (rms) roughness of only 40–60Å. The inclination angle of the etched mirrors is within ±2° of vertical, as SEM studies indicate. Photopumping measurements reveal that the reflectivity of the etched mirrors corresponds to 60–70% of the value for an ideal GaN/air interface. The reduced reflectivity may be due to surface roughness, a slight tilt in the facet angle, and the excitation of higher-order transverse waveguide modes in the laser structure.

Low energy ion beam etching of InP using methane chemistry

Reactive ion beam etching (RIBE) and chemically assisted ion beam etching (CAIBE) of InP at room temperature have been performed, using an inductively coupled plasma source. Two types of chemistries were used: N2/CH4/H2 and Ar/CH4/H2. The etch rate, surface roughness and anisotropy were investigated for different process parameters. In hydrocarbon chemistry polymer buildup is a commonly encountered problem and therefore efforts were also devoted to minimize the polymer growth. Comparing the two chemistries it was found that the Ar based processes always lead to rougher surfaces, while the N2 based chemistry at low ion energies (<100 eV) results in very smooth surfaces for both types of ion beam processes. However, the CAIBE process not only provides smooth surfaces, but also generates much less polymers than its RIBE counterpart. CAIBE, using N2/CH4/H2 chemistry, is then identified as a promising candidate for fabrication of nanometer sized structures. In addition, the low polymer growth is also advantageous from an equipment service point of view.

Plasma Characterization in Chlorine-Based Reactive Ion Beam Etching and Chemically Assisted Ion Beam Etching

We measured the chlorine plasma characteristics in reactive ion beam etching (RIBE) and chemically assisted ion beam etching (CAIBE) process. In the CAIBE process, the energy peak of Cl+ ionized by Ar+ appears in the energy range of 50–70 eV and this Cl+ energy peak increases with an increase of Ar+ energy. We observed that the excited state Cl2 density in CAIBE is lower than that in RIBE as measured by appearance mass spectrometry (AMS).

Characterization of AlGaInN diode lasers with mirrors from chemically assisted ion beam etching

Current-injection InGaAlN heterostructure laser diodes grown by metalorganic chemical vapor deposition on sapphire substrates are demonstrated with mirrors fabricated by chemically assisted ion beam etching. Due to the independent control of physical and chemical etching, smooth vertical sidewalls with a root-mean-squared roughness of 4–6 nm have been achieved. The diodes lased under pulsed current-injection conditions at wavelengths in the range from 419 to 423 nm. The lowest threshold current density was 25 kA/cm2. Lasing was observed in both gain-guided and ridge-waveguide test diodes, with cavity lengths from 300 to 1000 μm; and output powers of 10–20 mW were achieved. Laser performance is illustrated with light output-current and current–voltage characteristics and with a high-resolution optical spectrum.

Chemically assisted ion beam etching of submicron features in GaSb

We describe the use of chemically assisted ion beam etching to realize smooth, highly anisotropic features in GaSb. The measured etch rates are comparable to those of GaAs, and are fitted to a model that assumes the formation of trichloride etch product species.

Ambient temperature synthesis of polycrystalline thin films of lithium cobalt oxide with controlled crystallites† orientations

ABSTRACTPolycrystalline thin films of lithium cobalt oxide (nominally LiCo02) have been deposited by a process which we have named, ion beam directed assembly (IBDA). It is similar to ion assisted deposition (IAD) or ion beam assisted deposition (IBAD), but the major difference is that in IBDA the ion beam directs, via the vector momenta of the ions, rather than merely assists, the growth. Hence, we also term the process directed assembly to distinguish it from self assembly. Also, in directed assembly the ions may or may not become an integral part of the film's composition.In this paper we wish to show that the IBDA process may be capable of controlling microstructure, e.g., in the case of polycrystalline films, control the orientations of the crystallites. For LiCoO2 we have observed this in two ways - by X-ray analysis, and electrochemically, by lithium diffusivity measurements. In particular, we have observed increased lithium diffusivities (≥ 10−8 cm 2/s) in IBDA deposited films as compared to that found for rf sputter deposited films (z 10−10 cm2/s). In addition, the coulometric titration characteristics of an IBDA deposited Lil−yCoO2 are very similar to those of sputtered deposited films (for lithium extraction/insertion, y ≤0.5, and for quasi-equilibrium voltages up to 4.2 volts).

Flow Modulation Epitaxial Lateral Overgrowth Of Gallium Nitride On Masked 6H-Silicon Carbide And Sapphire Surfaces

ABSTRACTSelective Area Flow Modulation Epitaxial growth of GaN is carried out in a low pressure Organometallic Vapor Phase Epitaxy reactor. This process is known to enhance reactant surface migration lengths on patterned group III-arsenide and phosphide growth surfaces. With this process, high quality laterally overgrown GaN epitaxial materials result. Under the ammonia rich growth conditions used, enhanced migration (by flux modulation) across masked regions of the substrate has not been observed. The mask materials were silicon dioxide and silicon nitride, both deposited on GaN/AlGaN buffer structures on sapphire and SiC substrates. Window stripes were patterned parallel and perpendicular to the (1100) crystal directions to observe the orientation dependence of the lateral growth rate. Structures exhibited heights above the mask surface as large as 30 microns and atomically smooth surfaces. With a periodic array of stripe window openings in the mask, planarized laterally overgrown surfaces are achieved after roughly 4 microns of overgrowth. Chemical assisted ion beam etching with chlorine gas was used to delineate defects in the selectively grown layers. Additional evidence on the defect reduction is given by Atomic Force and Scanning Transmission Electron Microscopies.

Characteristics of InGaN-AlGaN multiple-quantum-well laser diodes

We demonstrate room-temperature pulsed current-injected operation of InGaAlN heterostructure laser diodes with mirrors fabricated by chemically assisted ion beam etching. The multiple-quantum-well devices were grown by organometallic vapor phase epitaxy on c-face sapphire substrates. The emission wavelengths of the gain-guided laser diodes were in the range from 419 to 432 nm. The lowest threshold current density obtained was 20 kA/cm/sup 2/ with maximum output powers of 50 mW. Longitudinal Fabry-Perot modes are clearly resolved in the high-resolution optical spectrum of the lasers, with a spacing consistent with the cavity length. Cavity length studies on a set of samples indicate that the distributed losses in the structure are on the order of 30-40 cm/sup -1/.

Control of the YSZ biaxial alignment on polycrystalline Ni[sbnd]Cr substrates by ion beam selective resputtering

(001)-oriented and in-plane aligned yttria-stabilized zirconia (YSZ) films were synthesized on polycrystalline NiCr substrates by ion beam assisted deposition (IBAD). The YSZ biaxial alignment were found to be controlled by the assisted argon ion beam bombardment. The incident angle and energy of the assisted bombarding ion beam strongly influence the alignment. The optimal alignment was obtained with the assisted ion beam bombarding at 55° incident angle and 250–500 energy. The Monte Carlo method was used to simulate the alignment formation with argon ions bombarding the zirconia crystals. Results indicated that when the energy of the bombarding ions is less than 50 eV, there is no obvious difference in the sputtering yields and the radiation damage of zirconia between different incident angles. However with the ion energy increasing, the variation increases rapidly. When the ion energy reaches 250 eV, the sputtering yields and the radiation damage of zirconia induced by Ar★ bombarding along [111] direction (54.7° from the [001]) of the zirconia are much lower than those produced by Ar★ bombarding along other directions. It can be concluded that the (001)-oriented grains are resputtered less than other grains when the assisted ions bombarding at 55° incident angle, which causes YSZ grains with (001) orientation to prevail over initial random oriented grains and form the biaxial alignment.

Optical pumping in nitride cavities with etched mirror facets

We present the fabrication of mirror facets by chemically assisted ion beam etching. Optical pumping experiments are performed to characterize the facets. The reflection coefficient is measured and its value confirms the high quality of the etched mirrors. Optical resonators are then realized in a double heterostructure composed of InGaN and GaN. Stimulated and laser emissions are investigated in this heterostructure and we show that population inversion can occur in both materials.

Nanolithography with metastable neon atoms: Enhanced rate of contamination resist formation for nanostructure fabrication

We report a sevenfold improvement in the rate of contamination resist formation over previous experiments by using metastable neon atoms for nanolithography. Chemically assisted ion beam etching was used to transfer the resist pattern into the substrate. We demonstrate the fabrication of 50-nm-wide features in GaAs with well-defined edges and an aspect ratio >2:1. These are the best resolution and highest aspect ratio features that have been achieved with metastable atom lithography. The resist formation rate by the metastable neon atoms and the etch selectivity of the contamination resist with GaAs were measured.

Organic–inorganic dielectric multilayer systems as high reflectivity distributed Bragg reflectors

Unprecedentedly high reflectivity distributed Bragg reflectors consisting of fluorocarbon polymer (CFx) and high refractive index inorganic oxide multilayers have been fabricated by means of assisted ion-beam sputtering at room temperature. Multilayer stacks consisting of a CFx/TiOx and CFx/HfOx pairs exhibit reflectivities larger than 98% in the infrared and ultraviolet spectral region, respectively. The superior wide tunability and the high efficiency, with respect to inorganic–inorganic multilayers is due to the exceptionally low refractive index of the fluorcarbon polymer (n≃1.35) on a very large spectral region (300–2000 nm).

InGaAs/InP pin photodiode arrays on AlGaAs/GaAswaveguide films by solid source molecular beam epitaxy

The first solid-source molecular beam epitaxial growth of infrared-sensitive In0.53Ga0.47As on top of an AlGaAs/GaAs waveguide for monolithic 1.55 µm receiver applications has been demonstrated using a single, thin InP buffer layer to suppress dislocations. Mesa-type detector arrays were made by chemically assisted ion beam etching. Using a novel dense array fabrication technique, the integration of InGaAs optoelectronic devices with AlGaAs/GaAs waveguide films grown on a GaAs substrate has been demonstrated.

Ultrahigh vacuum chemically assisted ion beam etching system with a three grid ion source

A chemically assisted ion beam etching system has been developed which performs high quality, highly anisotropic etching of AlGaAs/GaAs at relatively low ion energies (200 eV). The use of three grid ion optics in a Kaufman ion source allows etching at low energies with reasonable rates without loss of profile verticality. All ultrahigh vacuum etching chamber construction with a high throughput turbomolecular pump and high vacuum loadlock provide routine high quality etching of AlGaAs without the use of etch chamber cryo panels or cryo pumps. Low ion energy and a clean vacuum environment permit the use of a single level, non hard baked photoresist mask. Benefits include high pattern resolution and fidelity, low mask erosion rate, good dimensional control, and easy, complete mask stripping as well as reduced substrate damage.

Influence of material properties on TEM specimen preparation of thin films.

Several factors must be taken into account when deciding which specimen preparation technique(s) to use. These factors include the amount of material available, ease of preparing this material due to its properties and familiarity, location and size of the region of interest, amount of information sought, facilities accessible, and time available by the researcher to devote to the preparation of the specimen. The more popular specimen preparations for thin films, namely, electropolishing, cleaving, crushing, mechanical thinning followed by ion milling, and ultramicrotomy are discussed and the more unusual techniques such as extraction/replication, photochemical etching, lithography and reactive ion etching (RIE), chemically assisted ion beam etching (CAIBE), and precision polishing-based techniques are described. Their advantages and disadvantages in the context of the above factors are discussed. Suggestions for increasing one's success rate in preparing specimens are given. The role of transmission electron microscopy (TEM) analysis is considered since it rarely stands in isolation from other physical analytical techniques, nor is it often used as a quick diagnostic tool. Conservation of material by the minimization of the amount of material used (or destroyed) by TEM specimen preparation, and conservation of one's time by performing TEM analysis only on "worthy" samples should be given maximum consideration.

Photoluminescence of Wet- and Dry-Etched Gallium Nitride

AbstractThe photoluminescence (PL) of GaN grown on SiC is studied as a function of etch depth for two types of etches, photoelectrochemical and chemically assisted ion beam. It is found that as the etch proceeds deeper toward the substrate, the PL exhibits an increasing blue-shift and an increase in emission intensity of the donor acceptor pair band, which indicates increasing biaxial compressive stress and increasing impurity concentration near the substrate. The PL spectra of the dry etched GaN tended to have slightly higher intensities than comparably wet etched GaN.