There has been a significant need in many fields for high quantum efficiency, high speed response light detectors, particularly in the blue and near ultraviolet, (200 to 500 nm) as well as for particle detection, and low-energy X-rays. In many applications there is a further need for miniature, portable, rugged, field ready devices, which drives requirements away from photomultiplier tubes and towards solid state devices. In many applications requiring such solid state devices there is a need for relatively large surface area devices. Larger area devices intrinsically have lower signal to noise (SNR) and longer response time than smaller area devices.
Many nuclear detection devices utilize scintillators that convert nuclear energy to light. Most scintillators emit light in the region of 200 to 500 nm. This light must be read or monitored with an appropriate light detector. In many applications a scintillating crystal or ceramic is mounted directly onto a solid state detector such as an avalanche photodiode. This method of direct coupling significantly improves the efficiency of detection. In medical applications such as PET and CT scanning high sensitivity and short response time are required to reduce the patient's exposure to radiation as much as possible. Thus, there is a critical need in nuclear medicine for high quantum efficiency, high speed response, and often large surface area devices.
In other medical applications where detectors are in direct contact with the patient there is the further requirement that the bias voltage that drives the device be as low as possible for reasons of patient safety.
There is also a need for high quantum efficiency, high speed response, and often large surface area light detectors in military applications, such as tracking, targeting, and ranging devices. This is particularly the case for daylight applications where military operations are conducted in the presence of large amounts of visible and infrared light. The human eye has little sensitivity at wavelengths below 400 nm. For laser exposure, there is an “eye safe” UVA region between 315 and 400 nm (ideal, for instance, for operation with tripled Nd:YAG lasers at 355 nm or with solid state lasers emitting in this spectral band region). Solar radiation is minimal in the UVA band and can be significantly further reduced by use of appropriate band pass or short pass filters. These filters can, in many instances, be directly coated onto the detector or an appropriate filter can be placed in the path of incoming light. For tracking and targeting applications it may also be advantageous to use positional sensitive detectors, based on arrays of detectors and/or proportional detectors.
Scientific research instrumentation often requires high quantum efficiency, high speed response, and often large surface area devices in the spectral region between 200 and 1050 nm. Such applications include, but are not limited to, radiation detectors employing scintillators, radiation detectors based upon Cherenkov radiation, radiation detectors for X-ray detection, radiation detectors for ionizing particle detection and astronomical measurements in this spectral region.
Over the past two decades, free-space optical communication (FSOC) has proven to be a viable way to transfer large packets of digital information. Capable of achieving Gbit/s transfer rates over several km, relatively compact systems for local area networks are commercially available. However, the technology required to deploy robust system architectures, such as those used on temporary or mobile platforms operating in a range of daylight and temperature conditions, has yet to be fully realized.
A significant operational barrier for FSOC is the need to maintain eye-safety, which limits the permissible signal power and thus system ranging capabilities. Poor weather conditions, the need for direct line-of-sight, and interference with solar radiation within the receiver's line-of-sight are also factors that can significantly degrade the system performance. Despite these factors, FSOC offers significant advantages over commonly used microwave links. These advantages include: smaller size, weight and power requirements for the transmitter and receiver; higher immunity to electromagnetic interference; faster data transfer rates; and higher security due to directionality.
Such optical communication systems consist of two primary components; namely the transmitter and the receiver. Medium-range FSOC systems have been realized thanks to advances in laser transmitter and LED technology in the ultraviolet (UV). Compared to systems that utilize red or near-IR sources, the advantages of using a UV source include higher permissible transmission levels and reduced interference from solar radiation. Strong absorption and Rayleigh scattering in the atmosphere greatly reduces the solar background in the deep UV (wavelengths<280 nm). However, this same scattering and absorption may also reduce the distance over which the communication signal can be relayed, particularly at sea level where water content is high. As a result, the use of deep UV has been proposed for short range non-line of sight (NLOS) operations where the scattered light is used to relay the signal around objects [2]. For medium to long range communications, therefore, longer wavelengths near the blue-UV region of the spectrum (350 to 380 nm) are preferred.
The advantage created by recent advances in transmitter technology for UV FSOC has been dampened by the lag in the development of matching receiver technology for the UV. In particular, the highest quantum efficiency (QE) (the ratio between the number of photon-induced electrons collected and the number of incident photons) is approximately 50% for detectors at UN wavelengths, compared to >90% at the longer wavelengths. Table 1 compares the QE of various large-area UV-blue sensitive detectors when operated in proportional mode at the common laser wavelength of 355 nm (Nd:YAG).
TABLE 1Detector typeGainQE (at 355 nm)Silicon detector 150%Avalanche Photodiode20050%Photomultiplier Tube  10625%Silicon Carbide 1<10% 
Silicon detectors have the highest QEs at 355 nm, which is two to three times higher than photomultiplier tubes (PMTs) and six to seven times higher than silicon carbide detectors, which have a peak response at shorter wavelengths. Silicon p-i-n photodiodes are compact, rugged high-speed optical receivers that are relatively inexpensive and require low operating bias. However, their signal-to-noise ratio (SNR) is much lower than detectors with internal gain structures, such as APD's or PMTs. As a result, post-amplification circuits are often needed to generate appropriate signal outputs, further reducing the SNR.
Avalanche photodiodes (APD's), like the PMTs, exhibit internal gain created by impact ionization, when a strong electric field is present in the detector. However, APD's are much thinner than PMTs, which increases design flexibility, enabling improved light collection geometries. Unlike the PMT, the APD is very rugged, does not require a recovery period (several to tens of minutes) following intense illumination from an unexpected signal or background source, can be operated in magnetic fields, and can be mass produced at low costs. As a result, APD's are often the detector of choice for field-ready instrumentation such as FSOC.
An exemplary APD is depicted in FIG. 1. The substrate contains different regions that correspond to differing amounts of dopant present in that region. For example, an APD may include a photosensitive neutral drift p-region 100 located at, or adjacent to, the top surface of the APD. A depleted p-region 102 may be located beneath the neutral drift region. A depleted n-region 104 may be located beneath the depleted p-region. A neutral n-region 106 may be located beneath the depleted n-region. In some cases the APD may also include a passivation layer 108 disposed on top of the neutral drift p-region. During use an electrical potential may be applied to the APD such that when a photon is absorbed by the photosensitive neutral drift p-region, an electron-hole pair is formed at photon absorption site 110. Due to the applied electrical potential the charge carriers, i.e. the electron and hole, move in opposite directions within the APD. This initial migration of a charge carrier towards the bottom of the APD is indicated by arrow 112 which corresponds to minority carrier transport. As the charge carrier is accelerated due to the applied electrical potential it causes additional electron hole pairs to be formed within the depleted p-region and depleted n-region due to impact ionization which then exhibit similar behavior. This process is referred to as avalanche breakdown and is indicated by enlarged arrow 114. This phenomenon is responsible for the signal gain exhibited by avalanche photodiodes. After passing through the depleted p-region and depleted n-region the generated charge carriers undergo majority carrier transport to the neutral n-region which includes electrical contacts for outputting the generated electrical signal.