Hole spins in Ge quantum wells have shown success in both spintronic and quantum applications, thereby increasing the demand for high-quality material. We performed material analysis and device characterization of commercially grown shallow and undoped Ge/SiGe quantum well heterostructures on 8-in. (100) Si wafers. Material analysis reveals the high crystalline quality, sharp interfaces, and uniformity of the material. We demonstrate a high mobility (1.7 × 105 cm2 V–1 s–1) 2D hole gas in a device with a conduction threshold density of 9.2 × 1010 cm–2. We study the use of surface preparation as a tool to control barrier thickness, density, mobility, and interface trap density. We report interface trap densities of 6 × 1012 eV–1. Our results validate the material’s high quality and show that further investigation into improving device performance is needed. We conclude that surface preparations which include weak Ge etchants, such as dilute H2O2, can be used for postgrowth control of quantum well depth in Ge-rich SiGe while still providing a relatively smooth oxide–semiconductor interface. Our results show that interface state density is mostly independent of our surface preparations, thereby implying that a Si cap layer is not necessary for device performance. Transport in our devices is instead limited by the quantum well depth. Commercially sourced Ge/SiGe, such as studied here, will provide accessibility for future investigations.
Spin–orbit effects, inherent to electrons confined in quantum dots at a silicon heterointerface, provide a means to control electron spin qubits without the added complexity of on-chip, nanofabricated micromagnets or nearby coplanar striplines. Here, we demonstrate a singlet–triplet qubit operating mode that can drive qubit evolution at frequencies in excess of 200 MHz. This approach offers a means to electrically turn on and off fast control, while providing high logic gate orthogonality and long qubit dephasing times. We utilize this operational mode for dynamical decoupling experiments to probe the charge noise power spectrum in a silicon metal-oxide-semiconductor double quantum dot. In addition, we assess qubit frequency drift over longer timescales to capture low-frequency noise. We present the charge noise power spectral density up to 3 MHz, which exhibits a 1/fα dependence consistent with α ~ 0.7, over 9 orders of magnitude in noise frequency.
Hole spin qubits confined to lithographically - defined lateral quantum dots in Ge/SiGe heterostructures show great promise. On reason for this is the intrinsic spin - orbit coupling that allows all - electric control of the qubit. That same feature can be exploited as a coupling mechanism to coherently link spin qubits to a photon field in a superconducting resonator, which could, in principle, be used as a quantum bus to distribute quantum information. The work reported here advances the knowledge and technology required for such a demonstration. We discuss the device fabrication and characterization of different quantum dot designs and the demonstration of single hole occupation in multiple devices. Superconductor resonators fabricated using an outside vendor were found to have adequate performance and a path toward flip-chip integration with quantum devices is discussed. The results of an optical study exploring aspects of using implanted Ga as quantum memory in a Ge system are presented.
Even as today's most prominent spin-based qubit technologies are maturing in terms of capability and sophistication, there is growing interest in exploring alternate material platforms that may provide advantages, such as enhanced qubit control, longer coherence times, and improved extensibility. Recent advances in heterostructure material growth have opened new possibilities for employing hole spins in semiconductors for qubit applications. Undoped, strained Ge/SiGe quantum wells are promising candidate hosts for hole spin-based qubits due to their low disorder, large intrinsic spin-orbit coupling strength, and absence of valley states. Here, we use a simple one-layer gated device structure to demonstrate both a single quantum dot as well as coupling between two adjacent quantum dots. The hole effective mass in these undoped structures, m∗ ∼ 0.08 m 0, is significantly lower than for electrons in Si/SiGe, pointing to the possibility of enhanced tunnel couplings in quantum dots and favorable qubit-qubit interactions in an industry-compatible semiconductor platform.
In the field of semiconductor quantum dot spin qubits, there is growing interest in leveraging the unique properties of hole-carrier systems and their intrinsically strong spin-orbit coupling to engineer novel qubits. Recent advances in semiconductor heterostructure growth have made available high quality, undoped Ge/SiGe quantum wells, consisting of a pure strained Ge layer flanked by Ge-rich SiGe layers above and below. These quantum wells feature heavy hole carriers and a cubic Rashba-type spin-orbit interaction. Here, we describe progress toward realizing spin qubits in this platform, including development of multi-metal-layer gated device architectures, device tuning protocols, and charge-sensing capabilities. Iterative improvement of a three-layer metal gate architecture has significantly enhanced device performance over that achieved using an earlier single-layer gate design. We discuss ongoing, simulation-informed work to fine-tune the device geometry, as well as efforts toward a single-spin qubit demonstration.
Here, we demonstrate coupled triple dot operation and charge sensing capability for the recently introduced quantum dot technology employing undoped Si/Si0.8Ge0.2 hetero-structures which also incorporate a single metal-gate layer to simplify fabrication. Si/SiGe hetero-structures with a Ge concentration of 20% rather than the more usual 30% typically encountered offer higher electron mobility. The devices consist of two in-plane parallel electron channels that host a double dot in one channel and a single dot in the other channel. In a device where the channels are sufficiently close a triple dot in a triangular configuration is induced leading to regions in the charge stability diagram where three charge-addition lines of different slope approach each other and anti-cross. In a device where the channels are further apart, the single dot charge-senses the double dot with relative change of ~2% in the sensor current.
We demonstrate a capability of deterministic doping at the single atom level using a combination of direct write focused ion beam and solid-state ion detectors. The focused ion beam system can position a single ion to within 35 nm of a targeted location and the detection system is sensitive to single low energy heavy ions. This platform can be used to deterministically fabricate single atom devices in materials where the nanostructure and ion detectors can be integrated, including donor-based qubits in Si and color centers in diamond.
We use a cryogenic high-electron-mobility transistor circuit to amplify the current from a single electron transistor, allowing for demonstration of single shot readout of an electron spin on a single P donor in Si with 100 kHz bandwidth and a signal to noise ratio of ∼9. In order to reduce the impact of cable capacitance, the amplifier is located adjacent to the Si sample, at the mixing chamber stage of a dilution refrigerator. For a current gain of ∼ 2.7 × 10 3, the power dissipation of the amplifier is 13 μW, the bandwidth is ∼ 1.3 MHz, and for frequencies above 300 kHz the current noise referred to input is ≤ 70 fA/ Hz. With this amplification scheme, we are able to observe coherent oscillations of a P donor electron spin in isotopically enriched 28Si with 96% visibility.
Deterministic control over the location and number of donors is crucial to donor spin quantum bits (qubits) in semiconductor based quantum computing. A focused ion beam is used to implant close to quantum dots. Ion detectors are integrated next to the quantum dots to sense the implants. The numbers of ions implanted can be counted to a precision of a single ion. Regular coulomb blockade is observed from the quantum dots. Charge offsets indicative of donor ionization, are observed in devices with counted implants.