A.G. Foyt

Isolation of junction devices in GaAs using proton bombardment

Abstract Proton bombardment has been used to convert both p- and n-type GaAs into high resistivity material. It will be shown that this technique is useful for isolating junction devices and fabricating arrays. The average carrier concentration in the bombarded layer is less than 1011/cm3, and the layer thickness is about one micron for every 100 keV of proton energy. These layers are apparently unaffected by a 16 hr anneal at 300°, and only slightly affected at 400°. Using this technique, we have isolated islands of n-type GaAs on a semi-insulating substrate, separated p-n junctions on an n-type substrate, and suppressed edge breakdown in Au-GaAs Schottky barrier diodes.

The Gunn effect in polar semiconductors

The main features of the Gunn effect can be accounted for by the transferred electron model of Ridley and Watkins, which predicts a bulk differential negative resistance and subsequent domain formation if electrons can be transferred sufficiently rapidly from the lowest conduction-band minimum to lower-mobility subsidiary minima. Experimental results for n -GaAs in verification of such a bulk negative resistance are presented. In the longer samples the current-time waveform consists of sharp spikes separated by flat valleys, as expected from the motion of domains. The voltage across the domains is found to scale with sample length as predicted; the value of the electric field inside the domain is estimated to be ≥60 000 V/cm, while the field outside is about 1500 V/cm. Gunn effect oscillations have also been observed in n -CdTe, but not in n -InSb or n -InAs. The absence of an instability in InSb and InAs is consistent with the transferred electron model, since the subsidiary minima in these materials are believed to be too high in energy to be populated before carrier multiplication occurs. Finally, it will be shown that unless the separation of the conduction-band minima is very small, the critical electric field at which the Gunn effect occurs is reasonably well predicted by the one-band polar optical mode runaway field.

Unequal electron and hole impact ionization coefficients in GaAs

GaAs Schottky barrier avalanche photodiodes have been fabricated in which it is possible to achieve nearly pure hole and pure electron injection in the same device. Measurements of the multiplication characteristics of these devices show that the ionization coefficients of electrons (α) and holes (β) are not equal and that β>α. This result is in agreement with the variation of multiplication with bias voltage at different wavelengths observed for standard GaAs Schottky barrier diodes but contrary to the generally accepted belief that α=β.

TYPE CONVERSION AND n‐p JUNCTION FORMATION IN Hg1−xCdxTe PRODUCED BY PROTON BOMBARDMENT

We have fabricated n‐p junction photovoltaic detectors in Hg1−xCdxTe with x=0.50, 0.31, and 0.25 using proton bombardment to create the n‐type layer. Peak detection sensitivities were in the wavelength range 1.6–6 μm. Although high‐sensitivity photodiodes were obtained with each composition, the best results were obtained with the x=0.31 material, which has a peak response at 3.8 μm. At 77°K, 15‐mil×15‐mil diodes made with this material had zero‐bias impedances of several megaohms. The peak detectivity at 3.8μ was 9×1011 cmHz1/2/W in reduced background and the quantum efficiency at the peak was 29%.

EFFICIENT DOPING OF GaAs BY Se+ ION IMPLANTATION

Efficient doping of GaAs by ion implantation has been obtained using Se+ ions. For a Se+ dose of 3 × 1012/cm2 implanted at 400 keV, a peak carrier concentration of 2 × 1017/cm3 occurred at a depth of 750 A, with a standard deviation of 500 A. Integration of the excess carrier concentration caused by the implantation indicates that for this ion dose at least 50% of the implanted ions are electrically active. For larger doses the doping efficiency decreases, and the carrier concentration approaches a limiting value of approximately 1019/cm3. The lowest observed sheet resistance for the implanted layer, about 250 Ω/square, occurred for a Se+ dose of 2 × 1014/cm2.

n‐p JUNCTION PHOTODETECTORS IN InSb FABRICATED BY PROTON BOMBARDMENT

We have fabricated n‐p junction photovoltaic detectors in InSb using proton bombardment to create the n‐type layer. At 77 °K, diodes which were 20 mils in diameter had zero‐bias resistances of several hundred thousand ohms. The peak detectivity at 4.9 μ of these diodes with a 2π, 300 °K background was typically greater than 3×1010 cm Hz1/2/W with the largest value observed being 1011 cm Hz1/2/W. Diode quantum efficiencies near 35% were observed.

Schottky barrier InxGa1−xAs alloy avalanche photodiodes for 1.06 μm

Uniform Schottky barrier avalanche photodiodes with gains greater than 250, rise times less than 200 psec, and good quantum efficiencies at 1.06 μm have been fabricated in InxGa1−xAs alloys. The material used for these devices was grown epitaxially on GaAs substrates using an AsCl3–H2–Ga–In vapor‐phase system which permitted grading the epitaxial layers from GsAs to the desired composition.

n‐p JUNCTION PHOTOVOLTAIC DETECTORS IN PbTe PRODUCED BY PROTON BOMBARDMENT

n‐p junction photovoltaic detectors in PbTe have been fabricated using proton bombardment to create the n‐type layer. At 77°K, zero‐bias resistance area products of 300 Ω cm2 were observed for diodes with dimensions as large as 15 mil square. Peak detectivities at 5 μm in reduced background as high as 3.3×1011 cm Hz1/2/W were observed. Diode quantum efficiencies were typically greater than 30% at 5 μm.

TYPE CONVERSION AND p‐n JUNCTIONS IN n‐CdTe PRODUCED BY ION IMPLANTATION

Type conversion and p‐n junctions have been produced by the implantation of 400‐keV As+ ions into n‐CdTe. In forward bias, the junctions emit bandgap infrared radiation at room temperature.

p‐n Junction Photodiodes in PbTe Prepared by Sb+ Ion Implantation

n‐p junction photovoltaic detectors in PbTe have been fabricated using Sb+ ion implantation to create the n‐type layer. At 77 °K, 15‐mil square diodes have had zero‐bias resistances as high as 15 MΩ for a resistance‐area product of 2.1×104 Ωcm2. Peak detectivities at 4.4 μm in reduced background as high as 1.6×1012 cmHz1/2/W were observed. Diode quantum efficiencies were typically 40% at 4.4 μm.