Doping Assessment of Ga-Assisted MBE Grown Be-Doped GaAs and Te-Doped GaAsSb Nanowires for Infrared Photodetector Application

Abstract

Abstract Body: In recent years, III-V semiconductor nanowires (NWs) have significantly attracted researchers due to one-dimensional architecture, quantum confinement effects, and a higher tolerance for stress-strain mismatch that allow greater freedom in engineering combinations of material systems in a variety of various nanowire architectures to meet the demands of next-generation optoelectronic devices. Dopant incorporation in a well-controlled manner is essential to realize advanced devices in NW configuration successfully. Unfortunately, the knowledge obtained on dopant incorporation and carrier concentration from the thin film studies cannot be directly translated to NWs due to the dopant’s influence on the growth kinetics and different growth mechanisms along with the axial and radial directions. The commonly used measurement techniques in thin films for the determination of carrier concentration namely, Hall effect, field-effect transistor, and capacitance-voltage require highly sophisticated lithography steps. In the last decade, several characterization methods have evolved for the assessment of dopants in NWs namely, off-axis electron holography for which the sample preparation is complex, and it requires additional information on the NW thickness and homogeneity; secondary ion mass spectrometry and atom probe tomography requires a standard of known dopant concentration and is destructive. Therefore, a combination of conductive-atomic force microscopy (C-AFM), scanning Kelvin probe microscopy (SKPM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS), provide a set of excellent characterization techniques for doping assessment, as they do not involve any complex contact fabrications. In this work, we evaluate the carrier concentration and incorporation of Beryllium (Be) dopants in GaAs NWs and Tellurium (Te) dopants in GaAsSb NWs grown using Ga-assisted molecular beam epitaxy (MBE) with variation in Be cell temperatures (TBe) from 750 oC to 950 oC and Te cell temperatures (TGaTe) from 500 oC to 570 oC, respectively. The extensive research interest in GaAs and GaAsSb NWs stems from its narrow bandgap region covering the important optical telecommunication wavelengths of 1.3 µm and 1.55 µm. The current-voltage measurements using C-AFM of the vertical single NW showed a significant enhancement in current for GaAs NWs doped with TBe at 950 oC and GaAsSb NWs doped with TGaTe at 550 oC. The carrier concentration of Be-doped GaAs and Te-doped GaAsSb NWs are found using COMSOL Multiphysics fitting. From XPS and UPS, the atomic percent, work function, and carrier concentration of Be-doped GaAs NWs and Te-doped GaAsSb NWs are determined. A shift in Fermi level towards the valence band with increasing TBe for GaAs NWs, provided evidence towards higher Be incorporation with TBe - 950 oC doped NWs having a work function of 5.6 eV. Whereas the Fermi level is shifted towards the conduction band in Te–doped GaAsSb NWs. The values of electron density from UPS concur very well with the values of electron density determined from C-AFM. The change in surface potential observed in doped NWs in SKPM characterization also provided strong evidence of Be and Te incorporation. Hence, these surface analytical tools without any contact fabrication are found to be powerful characterization techniques for direct measurement of the dopant levels in NWs, which are critical for bandgap engineering design of optoelectronic devices. Acknowledgement This material is based upon research supported by the Air Force Office of Scientific Research (AFOSR) under grant number W911NF1910002.