Subnanometer Control of the Heteroepitaxial Growth of Multimicrometer-Thick Ge / Si - Ge Quantum Cascade Structures

Abstract

The fabrication of complex low-dimensional quantum devices requires the control of the heteroepitaxial growth at the subnanometer scale. This is particularly challenging when the total thickness of stacked layers of device-active material becomes extremely large and exceeds the multi-µm limit, as in the case of quantum cascade structures. Here, we use the ultrahigh-vacuum chemical vapor deposition technique for the growth of multi-µm-thick stacks of high Ge content strain-balanced Ge/SiGe tunneling heterostructures on Si substrates, designed to serve as the active material in a THz quantum cascade laser. By combining thorough structural investigation with THz spectroscopy absorption experiments and numerical simulations we show that the optimized deposition process can produce state-of-the-art threading dislocation density, ultrasharp interfaces, control of dopant atom position at the nanoscale, and reproducibility within 1% of the layer thickness and composition within the whole multilayer. We show that by using ultrahigh-vacuum chemical vapor deposition one achieves simultaneously a control of the epitaxy down to the sub-nm scale typical of the molecular beam epitaxy, and the high growth rate and technological relevance of chemical vapor deposition. Thus, this technique is a key enabler for the deposition of integrated THz devices and other complex quantum structures based on the Ge/SiGe material system.

Three-Dimensional Reconstruction of Interface Roughness and Alloy Disorder in Ge/GeSi Asymmetric Coupled Quantum Wells Using Electron Tomography

“…The cold-wall reactor has a differential pumping system allowing for a very short residential time of gases (<10 s) in the reactor itself, enabling the realization of sharp interfaces and a base pressure of 2 × 10 –10 Torr, while the typical reactant gas pressure during the growth is in the mTorr range. Detailed information on the CVD growth process can be found in ref , . The ACQW stack is deposited on top of a Si–Ge reverse-graded virtual substrate, ending with a 1.5 μm thick constant-composition fully relaxed Ge 0.87 Si 0.13 buffer.…”

“…The role of interfaces is even more crucial for QCLs operating in the THz far-infrared range (THz-QCL). At these operating wavelengths, the size of the laser optical mode reaches tens of μm and, consequently, these devices require active layers made of hundreds of cascade modules, each a few nm thick, and thus feature several hundreds of heterointerfaces. , It is worth mentioning that QCL technology has been developed almost uniquely based on III–V heterostructures. Only recently, the use of n-type Ge/Ge x Si 1– x MQW systems ( x ≥ 0.8) grown on Ge/Si virtual substrates has been proposed as a viable route to achieve cost-efficient and performing THz-QCLs, possibly operating at room temperature. , Indeed, in contrast to III–V-based QCLs, Ge/Si structures, being purely covalent, are not affected by the Frölich interaction, which strongly limits the optical gain at noncryogenic temperatures .…”

Ultrastrong Coupling of Si1–xGex Parabolic Quantum Wells to Terahertz Microcavities

“…N-type doping was achieved by codepositing phosphine. The active layer stack composed by 25 identical PQWs (thickness W = 61 nm) separated by Si 0.18 Ge 0.82 barriers (thickness b = 16 nm) was grown on top of relaxed Si 1– x Ge x VS having low threading dislocation densities (TDD = 3 × 10 6 cm –2 ) . The VS growth on a Si(100) substrate leverages a reverse grading technique in which a Ge film is deposited directly on the Si substrate using a multitemperature approach, followed by a compositional graded Si 1– x Ge x layer .…”

Interface Investigation on SiGe/Si Multilayer Structures: Influence of Different Epitaxial Process Conditions

“…3,6−9 The valley splitting is caused by the sharp potential barrier between the SiGe/Si interface and is very sensitive to the details of interface, such as, strain, mismatch, abruptness, etc. 10 The SiGe/Si stacked quantum well or superlattice structure can also be widely used in optoelectronic devices, 11 and the sharpness of the internal interface will be reflected in the absorption and reflection performance of electron and photon. 1,2,12 The group IV material epitaxy processes mainly consist of molecular-beam epitaxy (MBE) and chemical vapor deposition (CVD).…”

“…The interface between SiGe and Si layers must be as abrupt/sharp as possible in order to control etching and to further precisely control the final Si nanowire/nanosheet surface roughness and profile. − For electron spin qubits in a compressive strain Si quantum well, a sharp and flat interface can effectively reduce the scattering and noise during charge manipulation. ,− The valley splitting is caused by the sharp potential barrier between the SiGe/Si interface and is very sensitive to the details of interface, such as, strain, mismatch, abruptness, etc . The SiGe/Si stacked quantum well or superlattice structure can also be widely used in optoelectronic devices, and the sharpness of the internal interface will be reflected in the absorption and reflection performance of electron and photon. ,, …”