L. Green

Narrow-gap semiconductor magnetic-field sensors and applications

Narrow-gap semiconductors have been used for decades in the fabrication of magnetic field sensors, such as magnetoresistors and Hall sensors. Magnetic field sensors are, in turn, used in conjunction with permanent magnets to make contactless potentiometers and rotary encoders. This sensing technology offers the most reliable way to convert a mechanical movement into an electrical signal, and is widespread in automotive applications. Recent developments in the growth of thin epitaxial layers of InAs and InSb on semiinsulating GaAs or InP substrates have resulted in the development of magnetoresistors with excellent sensitivity and operating temperatures up to 285 degrees C. Magnetoresistors and Hall sensors require a very thin active semiconductor region, a high carrier density and a high room-temperature mobility. The best materials are narrow-gap III-V compounds. 2DEG layers in InSb and InAs would be ideally suited for these devices. The accumulation layer at the surface of InAs has been used to make magnetoresistors, Hall sensors and magnetotransistors. n-type doped thin InSb films are used to make magnetoresistors that outperform Si-based Hall sensors, even with integrated amplification. The authors describe device design criteria, materials requirements and a direct comparison of the three types of galvanomagnetic devices, magnetoresistors, Hall sensors and magnetotransistors, made from the same material. They compare the output of different magnetic field sensing technologies, such as Si and GaAs Hall sensors, and NiFe-based magnetoresistors, with InSb magnetoresistors.

Growth of high mobility InSb by metalorganic chemical vapor deposition

Thin films of InSb have been grown on insulating GaAs substrates using the metalorganic chemical vapor deposition technique with trimethyl indium and trimethyl antimony as reactants. We find that the mobilities obtained are usually low unless indium is predeposited onto the substrate. This indium predeposition technique greatly improves the yield of InSb films with mobilities of ~50000 crn2V−1S−1 at room temperature and a typical thickness of 2 microns. With this predeposition technique, the electron mobilities of these films become relatively independent of the vapor stroichiometry during growth and of the growth temperature. The electron mobilities are also very uniform across a wafer. These properties are obtained even when the film growth rate exceeds 2 μm/h.

The influence of stoichiometry on the growth of tellurium-doped indium antimonide for magnetic field sensors

Indium antimonide magnetoresistors are used as magnetic position sensors in very demanding automotive environments such as crankshaft and camshaft sensors for engine control. The use of tellurium as an n-type dopant was studied using Hall effect measurements up to 200°C, Hall depth profiling, and secondary ion mass spectroscopy. The films were grown by metal organic chemical vapor deposition using trimethyl indium, trisdimethylamino antimony, and diethyl telluride. It was found that the incorporation of tellurium strongly depends upon the V/III ratio during growth, implying that it is influenced by the availability of antimony vacancies. Thus, our results show that the reproducibility of tellurium doping is not limited by memory effects in a well-designed reactor, but by the control of stoichiometry. It is now possible to grow films with optimum doping profile and with good uniformity and reproducibility over hundreds of growth runs. These films can be used to make magnetoresistors that have good sensitivity to a magnetic field and good stability over a wide temperature range.

Role of a nucleation layer in suppressing interfacial pitting in InAs films grown on (100) InP substrates

In this work, we investigate the role of a low temperature nucleation layer on the interfacial properties of InAs epilayers grown on (100) semi-insulating InP substrates using a two-step metalorganic chemical vapor deposition method. Cross-sectional and plan-view transmission electron microscopy studies were carried out on InAs films of nearly equal total film thicknesses but for different thicknesses of a nucleation layer of InAs deposited at low temperature on the substrate. Our studies show that thermal etchpits are created at the interface between the InAs film, and the InP substrate for thin nucleation layer thicknesses. This is because the low temperature nucleation layer of InAs does not cover completely the surface of the InP substrate. Hence, when the temperature is raised to deposit the bulk of the InAs film, severe thermal pitting is observed at the interface. These thermal etchpits are sources of threading dislocations. To obtain high quality InAs films and suppress interfacial pitting there is an optimum thickness of the nucleation layer. Also, our studies show that there is a relationship between the density of defects in the film and the thickness of the nucleation layer. This in turn relates to the variation of the electronic properties of the InAs films. We have observed that for all nucleation layer thicknesses, the density of threading dislocations is higher close to the interface than at the free surface of the film.