A. B. Emerson

Stability of carbon and beryllium‐doped base GaAs/AlGaAs heterojunction bipolar transistors

GaAs/AlGaAs heterojunction bipolar transitors (HBTs) utilizing highly Be‐doped base layers display a rapid degradation of dc current gain and junction ideality factors during bias application at elevated temperature. For example, the gain of a 2×10 μm2 device with a 4×1019 cm−3 Be‐doped base layer operated at 200u2009°C with a collector current of 2.5×104 Au2009cm−2 falls from 16 to 1.5 within 2 h. Both the base emitter and base collector junction ideality factors also rise rapidly during device operation, and this current‐induced degradation is consistent with recombination‐enhanced diffusion of Be interstitials producing graded junctions. By sharp contrast, devices with highly C‐doped (p=7×1019 cm−3) base layers operated under the same conditions show no measurable degradation over much longer periods (12 h). This high degree of stability is most likely a result of the fact that C occupies the As sublattice, rather than the Ga sublattice as in the case of Be, and also has a higher solubility than Be. The effect...

Plasma etching of III–V semiconductors in CH4/H2/Ar electron cyclotron resonance discharges

We have investigated the etch rates, residual lattice damage, surface morphologies, and chemistries of InP, InGaAs, AlInAs, and GaAs plasma etched in electron cyclotron resonance (ECR) CH4/H2/Ar discharges. The etch rates of InP and InGaAs increase linearly with additional rf biasing of the substrate, and are approximately a factor of 2 faster than for GaAs. Under our conditions the etch rate of Al0.52Ga0.48As is very low (∼25 Au2009min−1) even for the addition of 100 V rf bias. In all of these materials the residual damage layer remaining after dry etching is very shallow (∼20 A) as evidenced from Schottky barrier height and photoluminescence measurements combined with wet chemical etching. InP shows significant P depletion with the addition of rf biasing during the ECR etching while GaAs retains a near‐stoichiometric surface. Hydrogen passivation of shallow donors in n‐type GaAs occurs to a depth of ∼3000 A during exposure to the CH4/H2/Ar discharge for long periods (60 min). The surface morphologies in the...

Damage to InP and InGaAsP surfaces resulting from CH4/H2 reactive ion etching

Structural and electrical damage imparted to InP and In0.72Ga0.28As0.6P0.4 (λg≂1.3 μm) surfaces during CH4/H2 reactive ion etching (RIE) have been examined. X‐ray photoelectron spectroscopy was used to monitor changes in the surface chemistry, Rutherford backscattering spectrometry was used to measure crystallographic damage, and current‐voltage and capacitance‐voltage measurements were made to examine electrically active damage and its depth. Two classes of damage are observed: crystallographic damage originating from preferential loss of P (As) and/or ion bombardment‐induced collision cascade mixing and, for p‐type material, hydrogen passivation of Zn acceptors. Etching at 13.6 MHz, 60–90 mTorr, 10% CH4/H2, and bias voltages of ∼300 V contains gross (≳1%) damage as measured by RBS to within 40 A and electrically active damage to within 200 A of the surface. This is a factor of 3–6 shallower than other RIE processes operated below 10 mT with comparable or higher bias voltages. Acceptor passivation of bot...

Reactive ion etching of InP, InGaAs, InAlAs: comparison of C2H6/H2 with CCl2F2/O2

The reactive ion etching of InP, InGaAs, and InAlAs in CCl2F2/O2 or C2H6/H2 discharges was investigated as a function of the plasma parameters pressure, power density, flow rate, and relative composition. The etch rates of these materials are a factor of 3–5× faster in CCl2F2/O2 (∼600–1000 Au2009min−1) compared to C2H6/H2 (160–320 Au2009min−1). Significantly smoother morphologies are obtained with C2H6/H2 etching provided the composition of the plasma is no more than 10%–20% by volume of C2H6. At higher ethane compositions, polymer formation increases leading to micromasking and rough surface morphologies. Subsurface disorder is limited to <300 A deep for both gas chemistries for plasma power densities of 0.85 Wu2009cm−2.The C2H6/H2 mixture leaves an In‐rich surface in all cases, but this surface is free of any residual contamination, whereas the CCl2F2/O2 chemistry leaves chlorofluorocarbon residues ∼20–50 A thick on the surface of all three In‐based materials.

Dry etch processing of GaAs/AlGaAs high electron mobility transistor structures

Damage introduction into GaAs/AlGaAs high electron mobility transistor (HEMT) structures during either pattern transfer or gate mesa etching steps has been characterized. For O2 reactive ion etching of the polydimethylglutarimide (PMGI) planarizing layer in a trilevel resist mask, the threshold dc bias for observable damage introduction in the AlGaAs donor layer is ∼200 V. This threshold bias for damage is a function of the PMGI overetch time and for extended times (>10 min), a decrease in saturated drain‐source current (IDSS) of the HEMTs can be detected for oxygen ions accelerated through a bias of ∼150 V. The use of combined electron cyclotron resonance (ECR)/radio frequency (rf) O2 discharges enhances the PMGI etch rate without creating additional damage to the device, and 0.25‐μm gate widths have been demonstrated. Gate mesa formation by etching the GaAs cap with CCl2F2/O2 or CH4/H2/Ar discharges is shown to produce damage in the underlying AlGaAs at dc negative biases of 125–150 V. In addition, subs...

Improvement of ohmic contacts on GaAs with in situ cleaning

An in situ argon ion mill clean step prior to ohmic metal deposition has been demonstrated to improve the uniformity of the contact parameters and reduce the contact resistance. After ion mill cleaning, the native oxide regrowth of molecular beam epitaxy grown GaAs and AlGaAs layers in vacuum chamber was also studied to optimize the processing. These oxide layers were identified as the cause of problems in the formation of good ohmic contacts to the GaAs or AlGaAs.

Temperature dependence of reactive ion etching of GaAs with CCl2F2:O2

The etch rate of GaAs during reactive ion etching (RIE) in a CCl2F2:O2 discharge (4 mTorr, 0.56 Wu2009cm−2) shows a strong temperature dependence, increasing from ∼500 Au2009min−1 at 50u2009°C to 2800 Au2009min−1 at 400u2009°C. Arrhenius plots of the etch rate show two activation energies (0.17 eV from 50 to 150u2009°C and 0.11 eV from 150 to 400u2009°C). There is no significant plasma power density dependence of the etch rate at elevated temperatures (≥100u2009°C) in contrast to the strong dependence at 50u2009°C. The surface morphology undergoes smooth‐to‐rough‐to‐smooth‐to‐rough transitions at ∼150, 250, and 400u2009°C, respectively, although TiPtAu Schottky diodes exhibit near‐ideal behavior on GaAs etched at 150u2009°C. The As‐to‐Ga ratio in the first 100 A from the surface increases with increasing RIE temperature, with chloride residues absent above 150u2009°C. Fluorocarbon residues were present on all samples, but were limited to the first 10–15 A. As determined by x‐ray photoelectron spectroscopy, fluorine was present almost exclusively as met...

Self‐aligned TiN barrier formation by rapid thermal nitridation of TiSi2 in ammonia

Partial or full nitridation of TiSi2 layers has been achieved by rapid thermal nitridation in pure ammonia. Several analytical techniques such as transmission electron microscopy, x‐ray photoemission spectroscopy, and Rutherford backscattering, have been used to study the nitridation process. It was found that a 60‐s anneal at 900u2009°C was sufficient to yield a 20–30 nm TiN layer on top of the TiSi2 layer, provided that the starting TiSi2 surface was free of Si or Ti oxides. Annealing at 1000u2009°C for 60 s, on the other hand, nitrided the entire 95‐nm TiSi2 layer. Detailed structural and chemical analysis of the samples before and after nitridation showed that the nitridation process takes place by nitrogen atoms bonding to Ti atoms and displacing the Si atoms. The dissociated Si atoms diffuse towards the TiSi2/Si interface forming an epitaxial layer of nonuniform thickness on the Si substrate. The resulting TiN layer is substantially thinner than the starting TiSi2 layer. It has a somewhat rough top surface ...

Aluminum composition dependence of reactive ion etching of AlGaAs with CCl2F2:O2

The etch rate and surface chemistry of AlxGa1−xAs after reactive ion etching (RIE) in CCl2F2:O2 was examined as a function of etch time (1–22 min), plasma power density (0.3–1.3 Wu2009cm−2), pressure (1–30 mTorr), gas composition (0%–80% O2), gas flow rate (10–50 sccm), sample temperature (50–350u2009°C), and Al composition (x=0.15–1). The etch rate is nonlinear with time, and decreases rapidly with increasing AlAs mole fraction. Essentially no temperature dependence of the etch rate is observed under our conditions, and there are no major differences in the surface chemistries of AlGaAs etched at different temperatures. The formation of a thin layer (50–90 A) of AlF3 during the RIE treatment appears to control the etch rate, and the surface morphology becomes progressively smoother for increasing Al composition. No residual lattice disorder is detected by cross‐sectional transmission electron microscopy under any of our conditions, although current‐voltage measurements on Schottky barrier diodes fabricated after...

Improved n -type GaAs ohmic contacts compatible with a chlorine-based dry-etch process

We report the use of a Mo barrier layer within Ni/Au-Ge based ohmic contacts to GaAs for eliminating an etch stop reaction that occurs during Cl-based dry etching of heterojunction bipolar transistors. With conventional Ni/Au-Ge/Ag ohmic contacts, chlorinecontaining discharges produce a passivating layer of AgCl on the semiconductor surface, preventing further etching. This layer is absent when the Ag in the contact is replaced with Mo. The Mo has several advantages over other diffusion barrier layers and yields contacts with excellent adhesion, smooth morphology, and sharp edge definition. The average contact resistivity of these contacts ton+-GaAs(n = 6 × 1018 cm-3) was 0.074 ohm-mm, which is lower than the typical contact resistivity of conventional Ni/Au-Ge/ Ag metallization (0.11 ohm-mm).