Chemically Assisted Ion Beam Etching Research Articles

Characterization of damage in InP dry etched using nitrogen containing chemistries

Etching of InP by different ion beam etching processes using chemistries containing nitrogen, such as nitrogen milling, N(CH3)3 and Ar/N(CH3)3 based reactive ion beam etching (RIBE), and N2/CH4/H2 based chemically assisted ion beam etching (CAIBE), is investigated. The residual surface damage is characterized using I–V characteristics of Au/InP metal–semiconductor (MS) contacts and photoluminescence (PL) yield measurements from near surface quantum wells and bulk InP. The contacts formed on as-etched surfaces, irrespective of the etch process, show ohmic behavior. On the other hand, although the PL yield is reduced for the different processes, the measured intensities show variations which are attributed to passivation of defects by hydrogen. Secondary ion mass spectroscopy (SIMS) was used to check (qualitatively) the presence of nitrogen in the etched and annealed InP samples. SIMS data show the presence of nitrogen in the near surface region of the etched samples, but nitrogen levels drop to background levels upon annealing at typical regrowth conditions (650 °C under phosphine flow). Interestingly, the annealing step results not only in a significant recovery of the PL yield, but also in a recovery of Schottky characteristics of the Au contacts formed on the annealed surfaces. These observations show that the etch-induced damage is significantly reduced by annealing and suggests recovery of the near-surface stoichiometry and possibly reduction of nitrogen-related defects.

Fabrication of photonic quantum ring laser using chemically assisted ion beam etching

This article presents an etching process to fabricate a high-quality vertical facet for the photonic quantum ring (PQR) laser. The presented process uses the chemically assisted ion beam etching (CAIBE) method with a gas mixture of Ar:Cl2:BCl3=5:2:3 sccm, and uses a single-layer photoresist etch mask to reduce the process complexity of etching GaAs/AlGaAs structures for the PQR laser. The angle θ between the incident beam and the normal direction of the substrate has been adjusted to achieve a high-quality vertical facet. Distortions on the mask pattern are not transferred to the etched side wall for θ⩾25°, and a vertical facet is obtained at θ∼25°. The fabricated PQR lasers with the CAIBE process in this article show a sharp linewidth of ∼0.06 nm.

Fabrication of quantum wires by in-situ ion etching and MBE overgrowth

We fabricate GaAs quantum wires (QW) that are completely embedded in epitaxial material. Here we report about an in-vacuo fabrication process, in which an ex-situ pre-patterned GaAlAs/GaAs heterostructure is in-situ etched and subsequently transferred under ultra high vacuum conditions into the growth chamber for epitaxial overgrowth. The in-situ etching step involves a chemical assisted ion beam etching with subsequent chemical gas etching. The aim of our investigations is to optimize the in-vacuo process for the reduction of interface states between the etched and overgrown material. We have studied structural properties of such processed samples with cross-sectional scanning-electron microscopy and transmission-electron microscopy. The optical and electronic properties of overgrown QW are investigated with magneto-transport measurements and far-infrared transmission spectroscopy. Overgrown QW show in comparison to conventional QW without overgrowth clearly increased carrier densities and a reduced depletion zone at the QW side-walls. These results indicate the successful reduction of surface states and establish the potential of the suggested in-situ technique for the fabrication of quantum structures.

High-power laser diodes with dry-etched mirror facets and integrated monitor photodiodes

High-power broad-area InGaAs-AlGaAs-GaAs single-quantum-well separate-confinement heterostructure (SQW-GRINSCH) lasers with dry-etched mirror facets and integrated monitor photodiodes have been investigated. A multilayer resist system has been employed as a mask for the chemically assisted ion-beam etching (CAIBE) process resulting in vertical and smooth laser facets. Thick electroplated gold layers on top of the ohmic contacts improve the heatsinking of the lasers leading to reasonable continuous-wave (CW) output powers even when the devices are mounted junction-side up. Monolithically integrated monitor photodiodes provide a linear response to the optical output powers of the laser diodes. The properties of broad-area lasers with dry-etched and cleaved facets are almost identical, Record values for the CW output powers of 2.59 W per uncoated facet and wall-plug efficiencies of more than 55% have been achieved with junction-side-down mounted devices.

Characterization of in situ etched and molecular beam epitaxy regrown GaAs interfaces using capacitance–voltage measurements, far infrared spectroscopy, and magnetotransport measurements

In this study, we characterize GaAs interfaces processed by ion beam etching (IBE), chemical assisted ion beam etching (CAIBE), and chemical gas etching (CGE). The etched interfaces are overgrown with GaAs or AlGaAs using molecular beam epitaxy. Interfaces in Si-doped GaAs are characterized with capacitance–voltage (C–V) profiling. Furthermore, we use a two-dimensional electron gas (2DEG), located directly at the etched interface in modulation doped AlGaAs/GaAs heterostructures, to probe the interface quality with far infrared (FIR) transmission spectroscopy and magnetotransport measurements. Applying a CGE process with suitable parameters we can remove more than 100 nm GaAs and generate clean surfaces with 2DEGs similar to those of nonetched reference samples. In addition we can reduce the interface state density of an IBE or CAIBE etched surface significantly applying a subsequent CGE process, which removes the ion damaged layer. However the two-dimensional electron systems at such interfaces show characteristic FIR absorption of dot-like potentials, which is a clear sign of a contaminated or disturbed surface.

Buried heterostructure complex-coupled distributed feedback 1.55 μm lasers fabricated using dry etching processes and quaternary layer overgrowth

An attractive process for fabricating buried heterostructure complex-coupled distributed feedback (BH-CC-DFB) lasers emitting at 1.55 μm has been developed. It combines low damage chemically assisted ion beam etching (CAIBE) for grating definition, subsequent overgrowth of a quaternary GaInAsP layer (λPL=1.22 μm) by low-pressure metalorganic vapor phase epitaxy, reactive ion etching for mesa definition and semi-insulating InP:Fe regrowth by hydride vapor phase epitaxy. CAIBE improves grating uniformity, GaInAsP layer overgrowth increases gain coupling and semi-insulating regrowth facilitates lateral mode confinement and thermal dissipation. The as-cleaved BH-CC-DFB lasers fabricated with the above combination show a statistical single mode yield of 87% and a very high side-mode suppression ratio up to 55 dB. A −3 dB bandwidth of 10 GHz is also demonstrated.

Facet formation of a GaN-based device using chemically assisted ion beam etching with a photoresist mask

The etch characteristics of GaN were investigated using chemically assisted ion beam etching (CAIBE) for Cl2 as a function of tilt angle. With increasing tilt angle the measured GaN etch rate showed a maximum at 30° similar to the effect of tilt angle on the sputter yield. The etch profiles of GaN etched by tilting the substrate less than 20° showed sidewall trenching. When the tilt angle was more than 30°, an etch tail was observed at the bottom of the etched structure. With the Cl2 CAIBE, the anisotropy of the GaN etch profile was enhanced as the tilt angle was increased, and a vertical etch profile could be obtained at a 50° tilt angle. The sidewall roughness of the etched GaN laser device varied with the Cl2 flow rate and ion beam voltage. A highly anisotropic etch profile with a smooth sidewall could be obtained by optimization of the ion beam voltage/current and Cl2 flow rate. The surface of GaN etched by the Cl2 CAIBE system showed a Ga-deficient surface due to the removal of Ga by the formation of GaClx.

Patterned substrate overgrowth for optoelectronic device integration using chemical beam epitaxy (CBE)

InGaAs/InP layers are grown on (1 0 0) substrates patterned using chemically assisted ion beam etching (CAIBE) to produce undercut mesas aligned along the 〈0 1 1〉 direction. Undercut mesas are used to avoid any interaction between growth on the etched and non-etched regions of the substrate. We show that optimised growth conditions for InP produce (1 1 1)B facets on the sides of the ridges which are atomically smooth along their full length. These ridges form an excellent template for the diffusion of the group III species onto the (1 0 0) plane in the ridge centre. The success of this technique relies heavily on the quality of the growth template used since the diffusion of group III species off the higher-order (n 1 1) planes onto the (1 0 0) is plane specific. The formation of this template by growth rather than wet chemical etching results in atomically smooth facets of a well-defined orientation and height, which is well suited to our application.

Comparison of the etching behavior of GaAs and GaN in a chemically-assisted ion-beam etching system

The characteristics of Cl 2 Ar chemically-assisted ion-beam etching processes for GaN and GaAs are reported. The etch rate and anisotropy have been investigated varying ion energy, tilt angle, substrate temperature, and Cl 2 flow. Vertical and smooth sidewalls, which fulfill the requirements on laser facets, have been demonstrated in GaAs and GaN.

Extremely smooth surface morphologies in N 2/H 2/CH 4 based low energy chemically assisted ion beam etching of InP/GaInAsP

We present investigations of N 2/H 2/CH 4 based chemically assisted ion beam etching (CAIBE) process For InP/GaInAsP. The novelty lies in using nitrogen as the inert ion species in the etch chemistry. The etch induced damage was evaluated by photoluminescence (PL) using near surface quantum wells as probes, and by roughness measurements by atomic force microscopy (AFM). The 75 eV N 2/H 2/CH 4 CAIBE process is shown to result in extremely smooth surfaces and low optical damage. Upon annealing in PH 3, enhancement in PL intensity is observed for samples with smooth surface morphology (rms < 1 nm) and degradation for rougher samples. The degradation is attributed to migration of surface disorder. Addition of AsH 3 during annealing forms island-like structures. Their size and density are shown to depend on the surface morphology as-etched.

Dry etching of GaN substrates for high-quality homoepitaxy

Chemically assisted ion-beam etching (CAIBE) was used to remove subsurface damage from polished GaN bulk substrates prior to growth. Subsequently, GaN layers were deposited by metalorganic vapor phase epitaxy. Only the CAIBE-treated areas reveal a mirror-like surface without trenches, scratches, or holes. A dramatic increase of crystal quality is determined by low-temperature cathodoluminescence (CL). Compared to not CAIBE-treated material, the CL intensity is improved by a factor of 1000 and the linewidth is ten times narrower.

Chemically-assisted ion-beam etching of (AlGa)As/GaAs: lattice damage and removal by in-situ Cl 2 treatment

We evaluated the lattice damage in Al 0.4Ga 0.6As/GaAs Single Quantum Well (SQW) structures caused by Chemically-Assisted Ion-Beam Etching (CAIBE) in comparison with Ion-Beam Etching (IBE) and wet etching. The damage was analyzed by measuring the Photoluminescence (PL) of the SQWs as a function of the etch depth. While IBE ( E kin=400 eV) causes a damaged region of 27 nm depth, BCl 3/Cl 2-CAIBE ( E kin=400 eV) damages to a depth of 10 nm. An in-situ Cl 2-treatment (C1 2-flow=6 sccm, T Substrate=120°C, p=3×10 −4 mbar, without plasma) allows a pure chemical removal of the surface layer which was damaged by CAIBE. This combined process facilitates anisotropic etching together with a lattice damage as low as with wet etching.

High Etch Rate and Smooth Morphology Using a Novel Chemistry in Reactive Ion Etching of GaN

GaN-based materials are commonly considered as potential candidates for light sources in the green and blue spectral range. The bandgap energy of AlGaInN varies between 1.95 and 6.2 eV. Significant research has been performed on high-brightness blue light emitting diodes (LEDs) and laser diodes (LDs). For example, the advantages of using LEDs over conventional electric incandescent lighting are faster response time, longer life, lower energy consumption, and higher luminescence efficiency. Also GaN, a wide-gap material, presents a potential use for high-temperature and high-power microelectronic applications. The high electron drift velocity, low thermal-generation rate, and high breakdown field make GaN and its related materials the most technologically interesting electronic materials for field-effect transistor applications. However, the intrinsic characteristics of this material system and, more specifically, its inertness, due to an atomic gallium-nitrogen bond energy of 8.92 eV/atom, make processing possible only with dry etching techniques. Dry etching of GaN has been extensively investigated using a wide range of plasma-based machines and sources. Most authors use electron cyclotron resonance (ECR) plasma based on CH 4 -H 2 -Ar, Cl 2 -H 2 , or CH 4 -H 2 -Ar-Cl 2 1-3 or chemically assisted ion beam etching (CAIBE).4,5 Only a few reports describe dry etching of GaN using various chemistries in a more conventional reactive ion etching (RIE) and the number of papers dealing with a SiCl 4 -based chemistry is even lower.6 Experimental In this paper, we present the results of RIE etching of metallorganic vapor-phase epitaxy (MOVPE) grown GaN on a (0001) sapphire substrate. The experiments were performed in a conventional parallel plate and load-locked Oxford Plasmalab System 100. Various combinations have been investigated using SiCl 4 :Ar, and eventually adding SF 6 . The mask used in these experiments is a plasma-deposited SiN x layer ~300 nm thick, annealed at 450°C for 10 min. Patterning the mask was carried out either by wet etching in a buffered HF solution or by using a dry etching process with SF 6 :Ar [10:10 standard cubic centimeters per minute (sccm)] at 150 W and 40 mTorr. The etch rate of the SiN x mask was ~200 nm/min. The influence of gas flow, pressure, and radio-frequency (rf) power were investigated. The basic gas flow rate used was SiCl 4 :Ar (10:10) sccm. Most gas combinations were investigated at three rf power values (70, 105, and 140 W) and at three pressure values (20, 40, and 60 mTorr). These parameters, combined together, determine the dc voltage which is related to the acceleration energy to the active species.

Etched cavity InGaAsP-InP waveguide Fabry-Perot filter tunable by current injection

A short cavity Fabry-Perot (FP) tunable filter was fabricated by chemically assisted ion beam etching (CAIBE) of an InGaAsP-InP waveguide. The FP cavity mirrors were formed by high reflectivity dielectric coatings deposited onto the two etched facets of the cavity. High-speed (less than 3 ns) wavelength-tuning of the filter was achieved by current injection. A super-linear dependence on current injection was observed in the wavelength tuning. The filter was tunable over a continuous range of 4 nm by up to 50 mA current injection. The device had a free-spectral range of 9 nm and gave a contrast ratio of about 10 dB. A theoretical simulation of the filter was developed using a beam propagation method calculation to estimate the reflection efficiency at different mirror tilt angles. The calculated contrast ratio of the filter was in good agreement with the measured result. The calculations showed that 20 dB contrast ratio should be possible by having a facet reflectivity of over 90% and ensuring that the etched facets are off-vertical by less than 1.8.

Characterization of InGaN/GaN-Based Multi-Quantum Well Distributed Feedback Lasers

We present a device fabrication technology and measurement results of both optically pumped and electrically injected InGaN/GaN-based distributed feedback (DFB) lasers operated at room temperature. For the optically pumped DFB laser, we demonstrate a complex coupling scheme for the first time, whereas the electrically injected device is based on normal index coupling. Threshold currents as low as 1.1 A were observed in 500 μm long and 10 μm wide devices. The 3rd order grating providing feedback was defined holographically and dry-etched into the upper waveguiding layer by chemically-assisted ion beam etching. Even when operating these lasers considerably above threshold, a spectrally narrow emission (3.5 Å) at wavelengths around 400 nm was seen.

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.

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.

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).

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 (&amp;lt;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.