外文翻译---用于量子密钥的单光子APD探测器设计.doc

design and characterization of single photon apd detector for qkd applicationabstractmodeling and design of a single photon detector and its various characteristics are presented. it is a type of avalanche photo diode (apd) designed to suit the requirements of a quantum key distribution (qkd) detection system. the device is modeled to operate in a gated mode at liquid nitrogen temperature for minimum noise and maximum gain. different types of apds are compared for best performance. the apd is part of an optical communication link, which is a private channel to transmit the key signal. the encrypted message is sent via a public channel. the optical link operates at a wavelength of 1.55m. the design is based on ingaas with a quantum efficiency of more than 75% and a multiplication factor of 1000. the calculated dark current is below 10-12a with an overall signal to noise ratio better than 18db. the device sensitivity is better than -40dbm, which is more than an order of magnitude higher than the dark current, corresponding to a detection sensitivity of two photons in pico-second pulses. i. introductionphoton detectors sensitive to extremely low light levels are needed in a variety of applications. it was not possible to introduce these devices commercially several years ago because of the stringent requirements of qkd. research efforts however resulted in photon detectors with reasonably good performance characteristics. the objective here is to model a single photon detector of high sensitivity, suitable for a qkd system. the detector is basically an apd, which needs cooling to very low temperature (77k) for the dark current to be low. the wavelength of interest is 1.55m. different applications may impose different requirements, and hence the dependence of the various parameters on wavelength, temperature, responsivity, dark current, noise etc, are modeled. comparison of the results from calculations based on a suitable model provides amenable grounds to determine the suitability of each type of apd for a specific application. attacks on communication systems in recent years have become a main concern accompanying the technological advances. the measures and counter measures against attacks have driven research effort towards security techniques that aim at absolute infallibility. quantum mechanics is considered one of the answers, due to inherent physical phenomena. qkd systems which depend on entangled pairs or polarization states will inevitably require the usage of apds in photon detection systems. how suitable these detectors may be, depends on their ability to detect low light level signals, in other words “photon counting”. it is therefore anticipated that the application of high security systems will be in high demand in a variety of fields such as banking sector, military, medical care, e-commerce, e-government etc. avalanche photo diodea. structure of the apd fig. 1 shows a schematic diagram of the structure of an apd. the apd is a photodiode with a built-in amplification mechanism. the applied reverse potential difference causes accelerates photo-generated carriers to very high speeds so that a transfer of momentum occurs upon collisions, which liberates other electrons. secondary electrons are accelerated in turn and the result is an avalanche process. the photo generated carriers traverse the high electric field region causing further ionization by releasing bound electrons in the valence band upon collision. this carrier generation mechanism is known as impact ionization. when carriers collide with the crystal lattice, they lose some energy to the crystal. if the kinetic energy of a carrier is greater than the band-gap, the collision will free a bound electron. the free electrons and holes so created also acquire enough energy to cause further impact ionization. the result is an avalanche, where the number of free carriers grows exponentially as the process continues.the number of ionization collisions per unit length for holes and electrons is designated ionization coefficients n and p, respectively. the type of materials and their band structures are responsible for the variation in n and p. ionization coefficients also depend on the applied electric field according to the following relationship: (1) for n = p = , the multiplication factor, m takes the form (2) w is the width of the depletion region. it can be observed that m tends to when w 1, which signifies the condition for junction breakdown. therefore, the high values of m can be obtained when the apd is biased close to the breakdown region. the thickness of the multiplication region for m = 1000, has been calculated and compared with those found by other workers and the results are shown in table 1. the layer thickness for undoped inp is 10m, for a substrate thickness of 100m. the photon-generated electron-hole pairs in the absorption layer are accelerated under the influence of an electric field of 3.105v/cm. the acceleration process is constantly interrupted by random collisions with the lattice. the two competing processes will continue until eventually an average saturation velocity is reached. secondary electron-hole pairs are generated at any time during the process, when they acquire energy larger than the band gap eg. the electrons are then accelerated and may cause further impact ionization. impact ionization of holes due to bound electrons is not as effective as that due to free electrons. hence, most of the ionization is achieved by free electrons. the avalanche process then proceeds principally from the p to the n side of the device. it terminates after a certain time, when the electrons arrive at the n side of the depletion layer. holes moving to the left create electrons that move to the right, which in turn generate further holes moving to the left in a possibly unending circulation. although this feedback process increases the gain of the device, it is nevertheless undesirable for several reasons. first, it is time consuming and reduces the device bandwidth. second, it is a random process and therefore increases the noise in the device. third, it is unstable, which may cause avalanche breakdown.it may be desirable to fabricate apds from materials that permit impact ionization by only one type of carriers either electrons or holes. photo detector materials generally exhibit different ionization rates for electrons and holes. the ratio ofthe two ionization rates k = i/i is a measure of the photodiode performance. if for example, electrons have higher ionization coefficient, optimal behavior is achieved by injecting electrons of photo-carrier pairs at the p-type edge of the depletion layer and by using a material with k value as small as possible. if holes are injected, they should be injected at the n-type edge of the depletion layer and k should be as large as possible. single-carrier multiplication is achieved ideally, when k = 0 with electrons or with k = for holes. b. geiger modegeiger mode (gm) operation means that the diode is operated slightly above the breakdown threshold voltage, where a single electronhole pair can trigger a strong avalanche. in the case of such an event, the electronics reduce the diode voltage to below the threshold value for a short time called “dead time”, during which the avalanche is stopped and the detector is made ready to detect the next batch of photons. gm operation is one of the basic of quantum counting techniques when utilizing an avalanche process (apd) that increases the detector efficiency significantly. there are a number of parameters related to geiger mode. the general idea however is to temporarily disturb the equilibrium inside the apd.the geiger mode is placing the apd in a gated regime and the bias is raised above the breakdown voltage for a short period of time. fig. 2 shows the parameters characterizing the geiger operation. the rise and fall times of the edges are neglected because they are made fast. detection of single photons occurs during the gate window.作者：khalid a. s. al-khateeb, nazmus shaker nafi, khalid hasan国籍：美国出处：computer and communication engineering (iccce), 2010 international conference on11-12 may 2010用于量子密钥的单光子apd探测器设计 摘要本文提出的是单光子探测器及其各种特性的建模与设计。它是利用雪崩光电二极管（apd）的一种特性，以适应量子密钥分配（qkd）检测系统的要求。该设备是在液氮温度下按门控模式运行，以便使其噪声最小和增益最大。通过不同类型的apd相比，以获得性能最佳的探测器。apd是用来传输关键信号的私人光通信号通路的一部分。通过一个公共的通道发送加密的消息。光电通路的工作波为1.55m。这是在ingaas的量子效率超过75，和倍增因子为1000基础上设计的。计算所得的暗电流低于10-12a，且整体信噪声比高于18分贝。该器件的灵敏度高于-40dbm的，比暗电流多一个量级，相当于在微微秒脉冲时检测到两个光子的灵敏度。这里的目标是，以单光子探测器灵敏度高，适合的量子密码系统的建模。1. 引言 在各种应用中，有些需要对微弱光敏感的光子探测器。在几年前是不可能引进这些设备的，因为量子密钥技术的要求严格。然而随着研究发展，发明了有合理性能等特点的光子探测器的。这里的目标是建一个灵敏度高的单光子探测器，来适应量子密码系统。apd探测器需要把它冷却到非常低的温度（77k），以便其暗电流很小。探测器的探测波长为1.55m。不同的应用设计可能会提出不同的要求，因此需参照波长，温度，响应度，暗电流，噪声等各种参数。比较来自计算的结果，在此基础上找到并提供适合特殊应用的合适的apd探测器。近年来伴随着科技进步，通信系统的研究已成为一个主要的关注方向。随着避免受攻击措施和对策的提出，推动着研究工作朝安全技术发展，旨在不发生错误。由于其固有的物理现象，量子力学被认为是一种方法。依赖于空穴-电子对或偏振态的量子密码系统，将不可避免地需要使用的apd的光子探测系统。如何找到合适的些探测器，取决于它们对微光信号检测的能力，换句话说就是“光子计数” 。因此，预计高安全性系统的将应用在多种领域，如银行业，军事，医疗，电子商务，电子政务等。2. 雪崩光电二极管a. apd的结构apd结构如图1所示。apd是一个带有一个内置的放大机制的光电二极管。在其上施加反向的电位差使光生载流子加速度，使其转移的时与原子发生碰撞，从而解放其他电子。新产生的电子再次加速，重复上述过程，导致发生雪崩。生成的光生载子转移到高电场区域与价带中束缚中释放出来的电子碰撞后，发生进一步的电离。这个电子-空穴对的生成过程被称为碰撞电离过程。当载流子与原子发生碰撞时，他们就给原子一些能量。如果载流子的动能大于带隙，碰撞时就会释放出一个束缚着的电子。获得足够的能量的电子-空穴对，还能引起进一步的碰撞电离。其结果就是发生雪崩，自由载流子的数量呈指数增长的进程继续下去。图1 apd结构如图电离碰撞电离系数n和p分别代表每单位长度内电离碰撞产生的电子和空穴的数目。材料及其能带结构影响n和p的参数变化。依赖于外加电场的电离系数有以下关系式： (1)其中n=p=，而倍增因子m有以下形式： (2)w是耗尽区的宽度。可以观察到当w1时，的m趋于无穷，这是发生击穿的条件。因此，当apd偏置接近击穿区条件时，就可以得到高的m值。m = 1000的倍增区厚度的计算，并与其他已知的材料比较，结果表1所示。在厚度为100m的衬底中掺杂inp层的厚度为10微米。表1 apd的各层厚度和特性物质材料厚度（nm）参杂物（cm-3）功能作用inp10000无锌元素扩散/倍增inp300n:61016场效应控制ingaasp1