Carrier Recombination Dynamics of Surface-Passivated Epitaxial (100)Ge, (110)Ge, and (111)Ge Layers by Atomic Layer Deposited Al2O3

Germanium (Ge) and its heterostructures with compoundsemiconductorsoffer a unique optoelectronic functionality due to its pseudo-bandgapnature, that can be transformed to a direct bandgap material by providingstrain and/or mixing with tin. Moreover, two crystal surfaces, (100)Geand (110)Ge, that are technologically important for ultralow powerfin or nanosheet transistors, could offer unprecedented propertieswith reduced surface defects after passivating these surfaces by atomiclayer deposited (ALD) dielectrics. In this work, the crystallographicallyoriented epitaxial Ge/AlAs heterostructures were grown and passivatedwith ALD Al2O3 dielectrics, and the microwavephotoconductive decay (mu-PCD) technique was employed to evaluatecarrier lifetimes at room temperature. The X-ray photoelectron spectroscopyanalysis reveals no role of orientation effect in the quality of theALD Al2O3 dielectric on oriented Ge layers.The carrier lifetimes measured using the mu-PCD technique werebenchmarked against unpassivated Ge/AlAs heterostructures. Excitationwavelengths of 1500 and 1800 nm with an estimated injection levelof similar to 10(13) cm(-3) were selected tomeasure the orientation-specific carrier lifetimes. The carrier lifetimewas increased from 390 ns to 565 ns for (100)Ge and from 260 ns to440 ns for (110)Ge orientations with passivation, whereas the carrierlifetime is almost unchanged for (111)Ge after passivation. This behaviorindicates a strong dependence of the measured lifetime on surfaceorientation and surface passivation. The observed increase (>1.5x)in lifetime with Al2O3-passivated (100)Ge and(110)Ge surfaces is due to the lower surface recombination velocitycompared to unpassivated Ge/AlAs heterostructures. The enhancementof carrier lifetime from passivated Ge/AlAs heterostructures with(100)Ge and (110)Ge surface orientations offers a path for the developmentof nanoscale transistors due to the reduced interface state density.