Effects of Reactive Ion Etching on Phonon-Electron Interactions in Inalas-Ingaas Modulation-Doped Field-Effect Transisto
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BSTRACT Raman Scattering by coupled longitudinal optic phonons and two-dimensional electron gas electrons in In0.52A10.48As-In0.53Ga0.47As 8-doped heterostructures provides a powerful probe of electronic properties in these In-based structures. The two highest frequency modes, of the three coupled electron-phonon modes expected in this system, were observed, with the highest frequency mode being identified for the first time in InGaAs-based systems. The large dispersion of this mode makes it a particularly sensitive probe for changes in such properties as carrier concentration and subband energy. For structures with higher carrier concentrations coupling of the longitudinal optic phonon to multiple electron intersubband transitions is resolved. These measurements are particularly useful for heavily-doped structures for which room-temperature Hall measurements cannot distinguish channel electrons from those in parallel conduction paths. In addition HBr-based reactive ion etching has been performed on these structures, and the effects have been correlated with etch time and bias voltage for structures with different cap layer thicknesses.
INTRODUCTION It has been demonstrated that InGaAs/InAlAs heterostructures can be utilized to produce both electronic and photonic devices which exhibit excellent operating characteristics. Dry etching is an important tool in the fabrication of these devices because of the high resolution, selectivity, and anisotropy possible with this type of processing. However, dry etching can result in unwanted material modification which degrades device performance. The type and extent of modification which can be tolerated depends on the material and device to be fabricated. For example, creation of the gate recess, for which selective removal of InGaAs over InAlAs is necessary, is critical in the fabrication of InGaAs/InAlAs modulation-doped field-effect transistors. It has been demonstrated that HBr-based reactive ion etching (RIE) provides selectivity as high as 160:1 for etching of InGaAs over InAlAs 1 . Unfortunately, while InAlAs etch rates in HBr-based plasmas are smalll, 2 , plasma exposure can result in the creation of carrier traps and/or passivation of dopants in InAlAs layers. HBr-based discharges also have been proposed as a possible alternative to other classes of plasmas for etching III-V materials. Thus, it is important to characterize the electrical and structural modifications which result from HBr RIE. Burstein and coworkers first proposed that resonant light scattering measurements should be sufficiently sensitive to observe excitations in two-dimensional electron gas (2DEG) systems 3 . Since that time the observation of light scattering by single particle spin-density intersubband excitations and collective charge-density intersubband excitations has been used to probe intersubband energies, carrier interactions, concentrations, and mobilities in the 2DEG of a wide variety of semiconductor heterostructures. Initial investigations involved GaAs-AlxGalixAs systems 4 . More rec
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