New high-performance avalanche photodiodes based on the unique properties of dilute nitrides

为了满足互联网长距离传输大量数据的需求，信息以短脉冲光的形式发送。接收这些信息的光电探测器必须具有高灵敏度、快速响应和低水平的“噪声”（随机杂散信号）。光探测器甚至可以足够灵敏地探测到单个光子，“光子计数”在许多应用中都是一项重要的技术，包括人类基因组测序和量子计算。大多数高灵敏度光电探测器都是半导体雪崩光电二极管（apd）：半导体材料坚固、廉价、紧凑、高效，而apd利用了一种效应，即一个非常微弱的信号可以触发一个非常大的电流（就像一片雪花引发一场巨大的雪崩）。有许多不同的半导体材料，每一种都对不同颜色的光或波长敏感。虽然硅作为APD工作得很好，但它不能检测光通信和其他应用所需波长的红外光。我们可以使用多种材料的组合——一种材料吸收光，另一种材料进行雪崩倍增——但要将信号从一种材料传递到另一种材料可能会很棘手。因此，apd很难制造，因此价格昂贵。我们将制造具有硅性能但对红外光敏感的新型apd，这也比现有的红外探测器更容易/更便宜。首先，我们将使用一种相对较新的半导体（“稀氮化物”）作为吸收层。稀氮化物与其他材料完全不同：在像砷化镓这样的传统半导体中加入少量的氮会对其性能产生巨大影响，并使其对红外光敏感。与其他吸收层相比，稀氮化物的噪音似乎更小，因为它们的特殊性质抑制了量子力学隧道效应产生的噪声源（电子在稀氮化物中感觉“更重”，更难隧穿屏障）。其次，我们将取代由磷化铟或砷化镓制成的传统倍增层，这些倍增层与硅制成的倍增层相比噪音较大。噪声的出现是因为乘法是随机的：我们知道乘法在一定时间内发生的概率，但不知道它发生的确切时间。稀氮化物的特殊电子特性意味着，一个能带（价带）中的电子很容易引发雪崩，而另一个能带（导带）中的电子则很难引发雪崩。这种情况将导致非常低的乘法噪声，甚至可能与硅一样低，并且以前从未研究过。在稀氮半导体中的电子运动和薄层中雪崩倍增的统计中，有许多有趣的物理现象。我们将使用专门的技术来研究这些问题，包括在非常高的压力下挤压材料以改变其性质。这将使我们了解生产更好的高灵敏度光探测器所需的知识，这些探测器可用于通信、医学、污染监测以及影响我们日常生活的许多其他领域。

To meet the demands of the internet to transmit large volumes of data over long distances, information is sent as short pulses of light. The photodetector which receives this information must have high sensitivity, a fast response, and low levels of 'noise' (random spurious signals). Photodetectors can even be made sensitive enough to detect single photons, and 'photon counting' is an important technique in many applications including sequencing the human genome and quantum computing. Most high-sensitivity photodetectors are semiconductor avalanche photodiodes (APDs): semiconductor materials are robust, cheap, compact, and efficient, while APDs make use of an effect where a very weak signal can trigger a very large current flow (like a single snowflake setting off a massive avalanche of snow).There are many different semiconducting materials, and each is sensitive to a different colour of light or wavelength. While silicon works really well as an APD, it doesn't detect infrared light at the wavelengths needed for optical communications and other applications. We can use combinations of material - one to absorb the light and one to do the avalanche multiplication - but it can be tricky getting the signal across from one material to the other. So APDs are hard to make and therefore expensive. We are going to make new types of APDs with the performance of silicon but sensitive to infrared light, which are also easier/cheaper to make than existing infrared detectors. Firstly, we are going to use a relatively new type of semiconductor (a 'dilute nitride') as the absorbing layer. Dilute nitrides are completely different from other materials: adding a small amount of nitrogen to a conventional semiconductor like gallium arsenide has a huge effect on the properties and can make it sensitive to infrared light. Dilute nitrides even seem to be less noisy than other absorbing layers, since their special properties suppress a source of noise which comes from quantum mechanical tunneling (electrons feel 'heavier' in dilute nitrides and find it harder to tunnel through barriers).Secondly, we are going to replace the conventional multiplication layer made of indium phosphide or gallium arsenide, which compared to multiplication layers made of silicon are rather noisy. The noise comes because multiplication is random: we know the probability that multiplication will occur within a certain time, but not exactly when it will occur. The particular electronic properties of dilute nitrides means that electrons in one energy band (the valance band) can easily trigger avalanches, while electrons in another band (the conduction band) should find it very hard. This situation should lead to very low multiplication noise, perhaps even as low as silicon, and has never been studied before.There is a lot of interesting physics in the movement of electrons in dilute nitride semiconductors, and in the statistics of avalanche multiplication in thin layers. We will use specialized techniques to study these, including squeezing the material under very high pressures to change its properties. This will give us the understanding we need to produce better high-sensitivity light detectors, which are useful for communications, medicine, pollution monitoring, and many other areas that affect our daily lives.