Search

A self-aligned fabrication process for small emitter contact area (2×4 μm2) GaN/AlGaN heterojunction bipolar transistor is described. The process features dielectric-spacer sidewalls, low damage dry etching, and selected-area regrowth of GaAs(C) on the base contact. The junction current–voltage (I–V) characteristics were evaluated at various stages of the process sequence and provided an excellent diagnostic for monitoring the effect of plasma processes such as chemical vapor deposition or etching. A comparison is given with large emitter-area (1.96×103 μm2) devices fabricated on the same material. The small-area devices are attractive for microwave power switching applications provided a high-yield process can be developed.

The use of carbon tetrachloride was examined as a means of suppressing deposition on the dielectric mask during the selective regrowth of III–V epitaxial layers by organometallic vapor phase epitaxy. Highly anisotropic dry etching was used to form narrow (∼1 μm wide) trenches and mesas. Reentrant mesas were formed by wet chemical etching. Under conventional growth conditions, polycrystalline GaAs was deposited over the mask adjacent to the trenches. By adding CCl4, selectivity could be greatly enhanced; however the growth rate in the trenches was significantly increased due to lateral diffusion of the reactants upon desorption from the mask thereby making acceptable control of the growth rate difficult. Addition of CCl4 during the growth of InP and AlGaAs significantly improved the selectivity in the case of growth around etched mesas.

Etch rates for InP, InAlP, and InGaP can be controlled between ∼1500 A/min and ≳1 μm/min by varying the microwave power, rf power, or N2 composition in BCl3N2 discharges under electron cyclotron resonance conditions. The surface morphology of InGaP is poor at low microwave power (250 W), but becomes very smooth at high powers (1000 W). InP has exactly the opposite dependence on microwave power, while InAlP shows little change in morphology over a wide range of powers. The high ion current under electron cyclotron resonance conditions enables etching of In‐based semiconductors without the need for high sample temperatures to enhance desorption of InClX species.

Plasma‐enhanced CVD requires close control of the plasma power density for reproducibility of both deposition rate and spatial uniformity in a radial‐flow reactor. Frequently the low rf power levels needed are not adequately controlled by the output power control of the relatively high power rf generator found in most commercial multipurpose plasma etch/deposition systems. This paper describes very simple and inexpensive broadband optical monitoring of the plasma emission for improved control sensitivity. The technique has the advantage of directly monitoring the plasma intensity, avoiding effects of variable power transmission losses between the rf generator and the plasma. Application to the control of low power plasma‐enhanced CVD of ‘‘SiO2’’ from Ar:SiH4:N2O plasmas, where the monitor signal is almost entirely due to near infrared Ar emission, is described.