T. R. Fullowan

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 200 °C with a collector current of 2.5×104 A cm−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...

Implant isolation of GaAs‐AlGaAs heterojunction bipolar transistor structures

The formation of high‐resistivity (>107Ω/⧠) regions in GaAs‐AlGaAs heterojunction bipolar transistor (HBT) structures by oxygen and hydrogen ion implantation has been investigated as a function of ion dose and subsequent annealing temperature (400–700 °C). Isolation leakage currents as low as 8 μA mm−1 at 6 V can be achieved between 100‐μm‐wide ohmic contacts separated by a 16 μm spacing. The isolation of these 1.8‐μm‐thick heterojunctions requires up to six different energy oxygen implants (40–400 keV) and three different energy proton implants (100–200 keV) with doses in the mid 1012 cm−2 range for O+ and 5×1014 cm−2 for H+ ions. Similar results can be achieved by substituting a MeV energy oxygen implant for the proton implants. The optimum post‐implant annealing temperature depends on the ion dose but is in the range 500–600 °C. The evolution of the sheet resistance of the implanted GaAs‐AlGaAs material with annealing is consistent with a reduction in tunneling probabilities of trapped carriers between...

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...

Dry processed, through‐wafer via holes for GaAs power devices

The fabrication of through‐wafer via holes in GaAs substrates by plasma etching and laser drilling is reported. Using a low pressure (15–20 mTorr), low dc bias (−150 V) Cl2/BCl3 discharge with Cl2‐to‐BCl3 ratios ≤0.2, we are able to produce narrow (≤30 μm) via holes. This enables the use of a higher density of vias in closer proximity to the active GaAs power devices than can be achieved with normal diameter (100–150 μm) holes. Microwave enhancement of the plasma density using an electron cyclotron resonance source increases the GaAs vertical and lateral etch rates and requires use of low Cl2‐to‐BCl3 ratios in order to retain the anisotropic nature of the vias. Multiple pass (∼100 per hole) drilling of vias with a Q‐switched Nd‐YAG frequency doubled (532 nm), 30 mW laser has also successfully produced through‐wafer connections. This provides a maskless, versatile method for producting vias customized for a particular wafer, but is less developed than plasma etching.

Damage introduction in InP and InGaAs during Ar and H2 plasma exposure

Changes in the sheet resistance of doped epitaxial layers of InP and In0.53Ga0.47As exposed to microwave Ar or H2 discharges were measured as a function of the exposure time (1–20 min), plasma pressure (1–20 mTorr) and the additional rf‐induced negative dc bias (50–400 V) on the sample. Changes in sheet resistance of ≤10% are only obtained for low dc biases (≤−75 V) or short exposures for either type of discharge. Hydrogen plasmas led to more substantial resistance changes than argon plasmas under all conditions, with the amount of damage introduction or hydrogen passivation being strongly dependent on dc bias and exposure time, but weakly dependent on pressure. The results indicate that high density, low pressure plasmas with low dc biases are capable of causing minimal disruption to InP‐based materials during dry etching for device fabrication.

High-rate, anisotropic dry etching of InP in HI-based discharges

Electron cyclotron resonance HI/H2Ar discharges with additional rf‐induced dc biasing of the sample have been used to obtain extremely anisotropic dry etching of InP. At a fixed ratio of 10 HI/10 H2/5 Ar (total flow rate 25 sccm) and 1 mTorr pressure, both n+ and p+ InP have etch rates of ∼875 A×min−1 at −100 V bias and ∼3000 A min−1 at −400 V bias. The etch rates increase rapidly with total discharge pressure, reaching 4000 A min−1 at 20 mTorr and −100 V dc bias. Rates in excess of 1 μm min−1 are obtained with higher HI flow rates or higher biases. Features 0.5 μm wide and 13 μm high have been etched, demonstrating the promise of this gas chemistry for production of laser mesas on InP and related materials with substantially faster etch rates (typically a factor of 8–10) relative to the more conventional CH4/H2 mixtures. The etched surfaces are smooth, with no evidence for iodine‐containing residues or preferential loss of either In or P. Both photoresist and SiO2 masks show minimal erosion in this mixtu...

Self-aligned InGaP/GaAs heterojunction bipolar transistors for microwave power application

As an alternative to AlGaAs/GaAs heterojunction bipolar transistors (HBTs) for microwave applications, InGaP/GaAs HBTs with carbon-doped base layers grown by metal organic molecular beam epitaxy (MOMBE) with excellent DC, RF, and microwave performance are demonstrated. As previously reported, with a 700-AA-thick base layer (135- Omega /sq sheet resistance), a DC current gain of 25, and cutoff frequency and maximum frequency of oscillation above 70 GHz were measured for a 2- mu m*5- mu m emitter area device. A device with 12 cells, each consisting of a 2- mu m*15- mu m emitter area device for a total emitter area of 360 mu m/sup 2/, was power tested at 4 GHz under continuous-wave (CW) bias condition. The device delivered 0.6-W output power with 13-dB linear gain and a power-added efficiency of 50%.<<ETX>>

III-V semiconductor device dry etching using ECR discharges

Electron cyclotron resonance (ECR) discharges are characterized by high ion densities (<or approximately=1012 cm-3) at low pressure ( approximately 1 mTorr) and by low ion energies (<or=15 eV). They are ideally suited for dry etching of III-V semiconductor devices, particularly when combined with additional RF biasing of the sample. In this paper the authors give examples of the use of this hybrid ECR-RF approach for etching of InP based devices in CH4/H2 plasmas and GaAs-based devices in CCl2F2 or PCl3 plasmas. The systematics of damage introduction into HEMTs and HBTs using these mixtures have been studied and the authors slow that it is possible to dry etch such devices without creating ion-induced surface damage. A feature of ECR etching is the highly anisotropic nature of the pattern transfer. The role of the masking material in the subsequent morphology of the sidewall is also discussed.

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.

AlInAs/InGaAs based heterojunction bipolar transistors fabricated by electron cyclotron resonance etch

A dry etch fabrication technology for high‐speed AlInAs/InGaAs heterojunction bipolar transistors (HBTs) utilizing low‐damage electron cyclotron resonance (ECR) CH4/H2/Ar plasma etching is detailed. Small‐area (2×4 to 3×9 μm2 ) devices demonstrated current gains up to 160, unity gain cutoff frequency ( fT) of 57 GHz and a maximum oscillation frequency ( fmax) of 35 GHz. The dry etch process uses triple self‐alignment of the emitter and base metals and the base mesa, minimizing the base‐collector capacitance (CBC). These results represent the first report of a truly scalable process for In‐based HBTs and demonstrate the ability of ECR plasma etching to provide smooth, degradation‐free etching of III–V semiconductors.