Beyond modulation doping

当半导体晶体中的杂质原子比它所取代的原子具有更多（或更少）的价电子时，它可以向晶格提供一个或多个电子（或从晶格中接受电子）。故意添加这种杂质，称为掺杂剂，是在半导体器件中产生移动电荷载流子（带负电荷的电子或带正电荷的空穴）的传统方法，包括现代电子中普遍存在的硅基金属氧化物半导体场效应晶体管（mosfet）和化合物半导体高电子迁移率晶体管（hemt）。高迁移率，特别是基于砷化镓的hemt，可以由分子束外延（MBE）生长的超高纯度晶圆制成，也有助于新物理学的发现，包括分数量子霍尔（FQH）效应，微波诱导电阻振荡，磁场中的Wigner固相，一维通道中的弹道输运和电导量子化，单电子量子点，近田物理，基于自旋的固态量子比特，双量子阱结构中可能的激子超流动性，以及某些新FQH态中可能的非阿贝尔统计。即使采用调制掺杂技术，将掺杂剂放置在远离导电通道的地方，在高纯度晶圆中的载流子仍然可以感觉到电离掺杂剂造成的混乱，并且这种混乱会干扰正在研究的现象。然而，如果使用外电场在半导体异质界面静电诱导二维电子气体（2DEG）或空穴气体（2DHG），则不需要这些有意掺杂剂。该电场可以应用于设备正面和/或背面的静电门。尽管美国贝尔实验室（Bell Labs）在18年前就报道了未掺杂设备（只需要一个工作设备）的原理验证演示，但复杂的洁净室制造过程和随之而来的非常低的工作设备产量阻碍了未掺杂设备的使用成为主流。在过去的三年中，我们的团队取得了许多技术突破，使工作器件的成品率达到90%以上，包括霍尔棒和纳米结构（如量子点）。现在的产量高到足以让研究项目依赖于稳定、可靠的高质量样品供应。为了利用这一成功，我们建议将我们按需制造此类器件的能力与我们在MBE半导体晶圆生长和毫开尔文温度测量方面的专业知识结合起来，以进一步推进上述两个主题，分数量子霍尔效应和基于自旋的固态量子比特。第二朗道水平中存在的许多“外来”FQH状态不符合描述“传统”FQH状态的Laughlin/Jain理论，并且对掺杂剂诱导的紊乱特别敏感。我们的实验程序将阐明这些状态的性质，特别是著名的填充因子为5/2的状态及其可能的非阿贝尔性质。GaAs中的门定义电子自旋量子位曾经是实现量子计算机的先驱系统之一。然而，这个系统受到超精细相互作用和电荷噪声的影响，这两者都会导致自旋退相干，时间尺度太短，无法用于实际的量子计算机。我们的实验计划将证明，当由未掺杂的2dhg制备门定义双量子点时，如何显著降低超精细相互作用和电荷噪声。我们提出的工作将对即使使用最高质量的掺杂异质结构也不容易获得的物理现象产生基本的见解。

When an impurity atom in a semiconductor crystal has more (or fewer) valence electrons than the atom it replaces, it can donate one or more electrons to (or accept them from) the crystal lattice. The deliberate addition of such impurities, called dopants, is the traditional means of generating mobile charge carriers (negatively-charged electrons or positively-charged holes) within semiconductor devices, including the silicon-based metal-oxide-semiconductor field-effect transistors (MOSFETs) and compound semiconductor high-electron-mobility transistors (HEMTs) ubiquitous in modern electronics.High-mobility, gallium-arsenide-based HEMTs in particular, which can be made from ultrahigh-purity wafers grown by molecular beam epitaxy (MBE), have also been instrumental in the discoveries of new physics, including the fractional quantum Hall (FQH) effect, microwave-induced resistance oscillations, Wigner solid phases in magnetic field, ballistic transport and conductance quantisation in one-dimensional channels, single-electron quantum dots, Kondo physics, spin-based solid-state qubits, possible excitonic superfluidity in double-quantum-well structures, and possible non-Abelian statistics in certain novel FQH states.Even with the technique of modulation doping, where dopants are placed far away from the conducting channel, disorder due to the ionised dopants can still be felt by the carriers in a high-purity wafer, and this disorder can interfere with phenomena being studied. However, these intentional dopants are not necessary if one uses instead an external electric field to electrostatically induce a two-dimensional electron gas (2DEG) or hole gas (2DHG) at the semiconductor heterointerface. This electric field can be applied with electrostatic gates on the front and/or back side of devices. Although the proof-of-principle demonstration of undoped devices (which required only one working device) was reported more than eighteen years ago by Bell Labs (USA), the complex cleanroom fabrication process and the ensuing very low yield of working devices have prevented the use of undoped devices from becoming mainstream. Over the last three years, our group has made a number of technological breakthroughs which allow a 90+% yield of working devices, including Hall bars and nanostructures (e.g., quantum dots). This yield is now high enough to have research projects depend on a steady, reliable supply of high-quality samples.To capitalise on this success, we propose to combine our ability to fabricate such devices on demand with our expertise in MBE semiconductor wafer growth and millikelvin temperature measurements to further progress on two of the topics listed above, the fractional quantum hall effect and spin-based solid-state qubits. Many "exotic" FQH states present in the second Landau level do not fit the Laughlin/Jain theory which describes "conventional" FQH states, and are particularly sensitive to dopant-induced disorder. Our experimental programme will shed light on the nature of these states, particularly the famous state at filling factor 5/2 and its possible non-Abelian properties. Gate-defined electron spin qubits in GaAs were once amongst the forerunner systems for the realisation of a quantum computer. However, this system suffers from the presence of hyperfine interactions and charge noise, both of which cause spin decoherence on timescales too short for a practical quantum computer. Our experimental programme will demonstrate how both hyperfine interactions and charge noise are significantly reduced when gate-defined double quantum dots are fabricated from undoped 2DHGs.Our proposed work will yield fundamental insights into physical phenomena not easily accessible using even the highest quality doped heterostructures.