Epitaxial Growth Of ?-? Materials On Step-free Ge Surface

The prominent optoelectronic properties of ?-? materials,which arise from the direct-gap features,make them act as the candidates for high-efficiency optical detectors and emitters.IV materials such as Si and Ge,are basic materials of microelectronics.Over the past decades,there are significant research interests in direct epitaxial growth of ?-? optoelectronic materials on group ? platform for hybrid photonics integration.Ga As/Ge heterostructure is also predicted to exhibit novel properties in specific systems,such as topological band in Ga As/Ge/Ga As quantum well.In order to achieve high performance in opotonics devices and robust topological states,high quality heterostructure are demanded.The material quality of epitaxial Ga As layer on Ge is limited by issues such as antiphase domain(APD)and stacking-fault pyramids(SFP).Previous growth methods,such as epitaxy on offcut Ge and epitaxy on zigzag patterned substrate,are focused on annihilating the defects near the interface.For novel devices that demanded for thin Ga As epi-layer,those techniques are no longer available.In this work,we demonstrate a new method to grow ultra-thin high-quality Ga As/Ge heterostructure by implement step-free interface.APD and SFP are totally avoided during growth.We integrate high-quality ?-? quantum dots and quantum well photonics materials on such platform by direct growth.Except for integration purpose,such growth technique is also expected to apply in Si-based quantum computing and 2D topological insulator,high-efficiency solar cells and high-performance CMOS transistors.The main research details are as below:1.We firstly fabricate periodic mesa-array on Ge(001)substrate by micromachining technology.Then we systematically study the morphology of Ge epi-layer(deposited by molecular beam epitaxy)on Ge mesa in different growth condition.Atomically step-free surface is obtained on different size of Ge mesa,up to5×5?m~2as optimized.2.Buffer-less high-quality Ga As layer is directly deposited on step-free Ge mesa.Anti-phase domians and“wadge-shape stacking faults”are totally avoided.Surface roughness RMS are less than 0.22 nm for Ga As layer from 40 nm to 120 nm.The morphology of Ga As epi-layer on defects of“Ge dimer domain boundary”and“single layer pits”are also studied.A forming mechanism of“wedge-shape staching faults”is proposed.We also observe the surface RMS of Ga As film with As pre-layer is one order of magnitude less than Ga pre-layer and MEE layer results.3.We utilize the defect-free Ga As/Ge heterostructure as substrate to grow In As quantum dots and Ga As/Al Ga As quantum well photonics materials.Both of them exhibit high-performance photoluminescence,which are comparable to materials grown on Ga As(001)substrate.For In As quantum dots,1280 nm wavelength main peak photoluminescence is achieved at room temperature,which is desirable in silicon photonics.4.At last,we investigate the interface property in Ga As/Ge/Ga As(111)quantum well structure by scanning tunneling microscope,which is expected to become 2D topological insulator when ideal interface is achieved.We deposit single As pre-layer on Ge(111)for upper interface characterization.Then we deposit 1?4 bi-layer Ge on Ga As(111)B substrate for lower interface and Ge layer characterization.After systematic morphology and local-electron-density-of-states characterization,we infer the atom intermixing in the quantum well is strong,mean while,ideal interface with strong charge accumulation is hardly to achieve.