G. E. Stillman

Electroabsorption in GaAs and its application to waveguide detectors and modulators

The electroabsorption coefficient of GaAs has been measured in uniform electric fields at wavelengths from 0.91 to 0.93 μm. These measurements were made using Schottky barrier contacts on low‐loss GaAs waveguides consisting of high‐purity epitaxial GaAs grown on heavily doped GaAs substrates. The experimental results are in good agreement with theoretical calculations of the Franz‐Keldysh effect. Electroabsorption detectors with subnanosecond response time and 100% internal quantum efficiency have been integrated into these waveguides. Small values of avalanche gain have been obtained without any intentional guard‐ring structure. Integrated electroabsorption modulators with greater than 20‐dB depth of modulation were also fabricated.

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.

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.

FAR‐INFRARED PHOTOCONDUCTIVITY IN HIGH‐PURITY EPITAXIAL GaAs

Extrinsic far‐infrared photoconductivity has been observed at 4.2°K in high‐purity n‐type epitaxial layers of GaAs grown on Cr‐doped semi‐insulating GaAs substrates. Measurements of the responsivity and noise in the detection system at 300 Hz in a 1‐Hz bandwidth yield an NEP of 1.2 × 10−11 W at 195 μ, 1.4 × 10−12 W at 337 μ and 6 × 10−11 W at 902 μ. The time constant of the detector has been determined to be shorter than 1 μsec using a Ge avalanche modulator to chop the incident radiation. A time constant of about 5 nsec has been measured using impact impurity ionization in the GaAs.

Electroabsorption avalanche photodiodes

Schottky barrier avalanche photodiodes have been fabricated on n‐type high‐purity epitaxial GaAs. These devices have their largest response at wavelengths beyond the usual absorption edge for high‐purity materials. The absorption mechanism involves the Franz‐Keldysh shift of the absorption edge, and the higher response at the longer wavelengths can be explained by a much higher ionization coefficient for holes than for electrons. The results indicate that the ratio of βp to αn is even larger than previous measurements have given.

Monolithic integrated InxGa1−xAs Schottky‐barrier waveguide photodetector

InxGa1−xAs Schottky‐barrier diodes for detection in the 0.9–1.06‐μm wavelength range have been incorporated in high‐purity GaAs planar waveguides using a selective epitaxial growth process. A quantum efficiency of 60% at 1.06 μm has been obtained for these detectors, and current gain has been observed.

ANOMALOUSLY HIGH ``MOBILITY'' IN SEMICONDUCTORS

In this paper we present a simple model for an inhomogeneous semiconductor which leads to an anomalously high apparent mobility. The model consists of a cylindrically symmetric van der Pauw measurement with a conducting inhomogeneity. This model is analyzed quantitatively and verified experimentally for several measurement configurations. The results cast doubt on the use of a high mobility as an indication of the quality of a semiconductor unless the homogeneity is unambiguously determined or the magnetic field dependence of the Hall constant is carefully examined.

Silicon as a residual donor in high‐purity GaAs

The question regarding the role of silicon as a residual impurity in high‐purity GaAs is resolved by the use of careful doping experiments and high‐resolution far‐infrared spectroscopy. The results indicate that silicon is readily incorporated into the material as a simple substitutional donor with a binding energy of 5.854 meV and show that silicon is a major, but not necessarily the dominant, residual donor in high‐purity vapor and liquid epitaxial GaAs.

Identification of donor species in high‐purity GaAs using optically pumped submillimeter lasers

An optically pumped NH3 laser has been used in conjunction with careful doping experiments in high‐purity GaAs to determine the ionization energy of isolated tin donors. The energy obtained is 5.820 meV, which corresponds to a central‐cell correction of 0.081 meV. This technique for impurity analysis is estimated to be sensitive to donor concentrations of less than 1011 cm−3.

Recombination radiation from landau states in impactionized GaAs

Abstract Recombination radiation from Landau states in impact ionized high-purity GaAs has been observed. The narrow band (∼ 3 cm−1) emission has been magnetically tuned from 80–120 cm−1, and used to observe rotational transitions in water vapor.