During the four decades from 1930 to 1970, seven photocathodes having useful sensitivity to visible light were developed. The first six have two features in common: First, they were all based on lucky observations, rather than on scientific principles. Second, despite a great deal of systematic research it is still not understood why these particular combinations of chemical elements have the observed properties. By contrast, a seventh type of photocathode, the so- called negative-electron-affinity (NEA) cathode materials were based on the principles of solid-state physics. The NEA cathodes have two advantages over the earlier materials in that they have higher photoelastic sensitivity and are better understood theoretically. However, they have the practical disadvantage that they involve single-crystal growth, whereas the earlier cathodes can all be produced by simple evaporation processes. As a result, they are used only in special applications.
A theory of photoemission is presented which has proven over the last thirty years to provide a very useful description of this process, both for fundamental and practical applications. This is the Three Step Model of Spicer. By treating photoemission in terms of three successive steps-- optical absorption, electrons transport, and escape across the surface, it allows photoemission to be related to parameters of the emitter, such as optical absorption coefficient, electron scattering mechanisms, and the height of the potential barrier at the surface. Using simple equations and established parameters, this theory is very useful in predicting the performance of cathodes and in understanding in detail the unexpected phenomena appearing when photocathodes are pushed into new practical domains.
This paper reports on the status of a program to develop a streak tube compatible photocathode optimized for 1300-nm operation. Performance characteristics will be presented for a Transferred Electron photocathode with greater than 1% quantum efficiency at 1300-nm. Photocathode performance results will also be presented over the 950 - 1700 nm spectral range.
The SLAC Linear Collider (SLC) Polarized Electron Source (PES) photocathodes have shown a charge saturation when illuminated with a high intensity laser pulse. This charge limit in the cesium-activated GaAs crystal seems to be strongly dependent on its surface condition and on the incident light wavelength. Charge limit studies with highly polarized strained lattice GaAs materials are presented.
A new high intensity, short time electron source is now being used at the Stanford Lineal Accelerator Center (SLAC). By using a GaAs negative affinity semiconductor in the construction of the cathode, it was possible to fulfill the operation requirements, such as peak currents of tens of amperes, peak widths of the order of nanoseconds, hundreds of hours of operation stability, and electron spin polarization. The cathode is illuminated with high intensity laser pulses, and photoemitted electrons constitute the yield. Because of the high currents, some non-linear effects are present. Very noticeable is the so called Charge Limit (CL) effect, which consist of a limit on the total charge in each pulse, that is, the total bunch charge stops increasing as the light pulse total energy increases. In this paper we will explain the mechanism of the CL and how it is caused by the photovoltaic effect. The treatment is based on the Three Step model of photoemission. We will relate the CL to the characteristics of the surface and bulk of the semiconductor, such as doping, band bending, surface vacuum level, and density of surface states. We also discuss possible ways to prevent the CL to take place.
The ultrafast characteristics of crystalline-silicon metal-semiconductor-metal (MSM) photodiodes with finger widths and spacings down to 200 nm, subjected to femtosecond optical pulse excitations, was measured with a subpicosecond electro-optic sampling system. Electrical responses with fullwidth at half-maximum (FWHM) as short as 3.7 ps, at a corresponding 3 dB bandwidth of 110 GHz, were generated by violet-light excitation. These diodes are the fastest silicon photodetectors reported to date. Detailed bias and light-intensity dependence of the diode response has been measured. These results are used to obtain the velocity-field relation of electrons in silicon and to demonstrate the ideal transit-time-limited response of the diodes.
The transient photoresponse of a hydrogenated amorphous silicon (a-Si:H) p-i-n photodetector has been studied from the circuit analysis point of view. Three possible transient working modes--dc bias mode, continuous light wave mode, and charge-storage mode, were analyzed and the speed, resolution, and accuracy of an 'analog' sensor or sensor array were examined. It is concluded that dc bias mode is the best among the three.
Photodetection mechanisms can be quite complex in amorphous semiconductors due to the extensive trapping and electric field redistribution. When properly understood and exploited, this rich complexity can lead to enhanced photodetection. Using the AMPS computer model, we explore two such experimentally verified situations: one is an example of a primary photoconductivity type of effect which can yield quantum efficiencies greater than unity and the other is an example of a secondary photoconductivity type of effect which can yield gains of 103.
The purpose of this paper is to present a review of the basic issues implicit in the design of confined state photodetectors. The basic device structure consists of repeated unit cells each comprised of a narrow gap semiconductor layer sandwiched between barrier layers of wider band gap material. Gain in these structures is derived through carrier multiplication via impact excitation of confined electrons out of the narrow gap semiconductor layer. Different device designs are considered in an attempt to maximize the device gain at minimum dark current. In some implementations, the barrier layers are chosen to be graded such that the leading edge discontinuity is at least twice that at the trailing edge of the well forming an asymmetric well design. It is found that an asymmetric well design offers a much higher impact excitation of electrons confined within the well at lower operating voltage than a symmetric well design however at the expense of increased dark current. Quantum versus classical confinement of the electrons within the well is also investigated.
Using bevelled edge avalanche photodiode technology, we have built several two dimensional 64-element arrays. Each pixel measures 1.24 X 1.24 mm2, with no dead space in between . The array was built by thinning and segmenting the back side of a large area APD to form the pixels. Very good isolation is obtained, and the response from each pixel on the back side corresponds to the location and intensity of the input signal on the front side. Signal gain in each pixel is the same as the starting APD, approaching approximately 1000, with good uniformity. The crosstalk between pixels is proportional to the resistance between them, and is dependent on the bias applied to the detector, at high bias, the crosstalk is less than 1%. Because of the small pixel size, the response time to laser excitation approaches approximately 6 ns. These arrays have very good potential for imaging systems and optical fiber readout in high energy physics detectors.
A charge-accumulation-and-multiplication photodetector (CHAMP) has been developed. The unit cell contains three coupled MOS capacitors designated as detector, transfer, and avalanche gates. The photocurrent at the detector gate is first integrated with the transfer gate closed and the avalanche gate ramping toward avalanche voltages. The charge is then transferred to the avalanche gate where charge multiplication occurs. These processes induce a displacement current pulse for readout which is proportional to the photocurrent, the integration time, and the multiplication gain factor, and is inversely proportional to the transfer time. A tunable current gain capable of exceeding the avalanche gain is achieved. A cooled CHAMP device is suitable for detecting photons of extremely low flux. The internal current gain and noise issues will be discussed.
The capability of combining single photon counting with information on the spatial distribution of incoming photons is of great interest to many scientific disciplines such as astronomy and spectroscopy, where an accurate assessment of this information can be critical to the success of an experiment. Photomultiplier arrays and scintillators are usually used in these applications. However these electromechanical solutions need to be operated at high voltages (approximately kV), they tend to be very fragile, bulky and extremely expensive. A solution to this problem which utilizes silicon integrated circuit technology would alleviate a lot of these undesirable effects and would be low cost, small area and more robust. This makes the concept of a solid state photon counting array very attractive for a large number of applications. To date this has not been achieved because of the difficulties involved in integrating high voltage optical detectors into an integrated circuit. This paper shows how this can be achieved and indicates areas of future research which will enable the possibility of large area and high pixel count spatially resolved photon counting devices.
Liquid crystal (LC) optically addressed spatial light modulators (OASLMs) have been successfully addressed with semiconductor thin film photosensors. Nematic liquid crystals (NLCs), which are insensitive to the polarity of the applied voltage, are addressed with photoconductive thin film structures having symmetric current-voltage characteristics. Much faster ferroelectric liquid crystals (FLC), which are polarity sensitive, are addressed with asymmetric photodiodes or photoconductive diodes. The highest performance OASLMs have been fabricated using FLCs and a-Si:H photodiodes as the photosensor. In this paper we analyze the role of the thin film photosensor in a LC OASLM and present design criteria. From these criteria we discuss hydrogenated amorphous silicon (a-Si:H) photosensor structures for use in OASLMs. In particular we review the a-Si:H p-i-n photodiode, the transparent conducting oxide (TCO)/intrinsic a-Si:H photoconducting diode, and the metal/insulator/a-Si:H structures which have been successfully incorporated into FLC OASLMs, and the n-i-n photoconductor which has not.
This paper describes the present efforts at NIST to improve the accuracy of the electrically calibrated pyroelectric detector for measuring optical power by an order of magnitude. The principal limitation, the uniformity of the responsivity over the receiving aperture, has been significantly improved.
As optical sensor and source technologies advance, it is important to improve the ability to accurately calibrate these sensors and sources. A new type of power meter which has excellent absolute accuracy, long term stability, and linear output over its full dynamic range will be described. The Multispectral Power Meter (MSP) makes direct power measurements by the substitution of electrical power to balance the radiant power from a radiation source. The MSP can operate in air at ambient temperature, in a vacuum environment, or even in space. It monitors ultraviolet to far infrared wavelengths with equal responsivity and can thus measure laser output at any wavelength or blackbodies at any temperature. In addition to characterizing the output from spectral sources, the MSP can also calibrate sensors and act as a transfer standard from a cryogenic primary standard. A digital controller enables the MSP to be remotely operated. The various measurement techniques and the performance levels of the MSP will be described.
Lateral-effect position sensitive photodetectors (PSDs) have received a great deal of attention as an alternative to scanning and array techniques for deriving spatially dependent information from incident radiation, since they have the advantages of instantaneous and fast measurements, greater position resolution, simplicity in circuitry and inexpensive. Basic physics, operation and demanding applications are reviewed. In this paper different experimental techniques which are established to evaluate transfer characteristics, repeatability, sensitivity of a single axis detector UDT SL-76 are presented. The percentage of nonlinearity and the Root Mean Square error have been calculated. An analog processing circuitry for position measurement is designed and presented. The cause of bias variation to the detector, laser beam intensity fluctuation and laser drift during warm-up in position measurements and the minimum intensity required to produce stable position information have also been presented.
Photometric or radiometric characterization of detectors or detector arrays requires uniform, stable illuminance or irradiance. For the measurement of linearity or spectral linearity the illuminance/irradiance level should be variable without changing either the spectral or angular distribution of the beam. A realization of a set-up using a glass rod as homogenizer is shown. The achieved uniformity is better than +/- 2% over a 40 mm diameter area. The irradiance and illuminance are over 3 mW/cm2 and 20 klux respectively. Suitable choosing the lamp current the set-up produces the spectral distribution of a standard illuminant A recommended by CIE for the measurements of image sensors.
Glass filter packages were designed to cut out UV-A band (320 - 400 nm) using GaP photodiodes and UV-B band (280 - 320 nm) with solar blind phototubes. UV diffusor plates were applied in input optics for cosine correction. Probes are calibrated to equienergetic source. Spectral and directional errors are calculated, as well as the relative responsivity factors for a number of radiation sources. Systems for measuring the spectral characteristics of the photodetectors and the filters, and the directional characteristics of the probes, as well as a software optimizing the thicknesses of the filters in order to reach the minimal integral error are touched upon.
Photodetectors play a fundamental role in optical communication and measurement systems. To date, the fastest reported photodiodes have bandwidths around 100 GHz. Because presently available oscilloscopes have bandwidths up to only 50 GHz, electro-optic sampling was used to measure their responses. The speed of the actual devices was indirectly determined by a deconvolution of the system response from the measured data. To overcome this measurement limitation, we have devised two new circuits that allow high-speed photodetectors to be used to measure laser pulses without using expensive sampling oscilloscopes or complex laser systems. The first is a high-speed Schottky photodiode monolithically integrated with an electronic sampler. With this circuit we were able to measure an impulse response of 1.8 ps FWHM corresponding to 3-dB bandwidth of 200 GHz. The second circuit is a high-speed photodiode monolithically integrated with a microwave detector. This second circuit can replace the wavelength-selective nonlinear crystal in autocorrelation and crosscorrelation setups. With this circuit, we were able to measure a 1.4 ps FWHM. The main advantage that the photodetector circuit has over the nonlinear crystal is that it is not wavelength specific.
We describe both the high-speed response and the steady-state behavior of a novel high-speed graded superlattice photovoltaic detector. The 3-dB bandwidth of the detector is larger than 10 GHz. Its ultimate speed is on the order of picoseconds. The intrinsic speed does not depend on the device size. The differences between two different superlattice structures are compared.
We designed a simple heterodyne system for frequency domain photodetector characterization which is intended to avoid many of the disadvantages of earlier characterization systems and to minimize errors due to intensity fluctuation, frequency calibration, and source impedance mismatch without using vector network analysis. A detailed uncertainty analysis for the system indicates 95% confidence intervals between +/- 0.25 and +/- 0.6 dB at 25 GHz, depending on the photodetector output impedance.
Photodetectors with both high bandwidth and high quantum efficiencies are crucial components for a variety of key technologies such as telecommunications, information processing, storage and sensing. The most widely used devices today are p.. i-n photodetectors and avalanche photodetectors. However, conventional device structures suffer from inherent bandwidth- quantum efficiency tradeoff. It can be shown that for small values of absorption layer thickness d, the RC limited bandwidth, which is a function of the active area diameter and d, can be the determining factor governing the frequency response of the device. For the same absorption coefficient, but for higher vales of d, i.e. wider absorption regions, the transit-timelimited bandwidth governs the frequency response. This is due to the fact that for a wider absorption region, the photogenerated carriers have a longer path to travel in order to reach their respective contacts. Thus it would seem beneficial to design structures with thin absorption regions. This would however, limit the quantum efficiency of the device, as is evident from Fig. 1. Most llIV semiconductors of interest have absorption coefficients of approximately, 10' cm, for the wavelengths of interest for telecommunications. Thus, in order to have a quantum efficiency of approximately 80 %, a minimum absorption region thickness of 2 xm is required.
We review recent progress in the monolithic integration of optical waveguides and photodetectors on III-V semiconductors for applications in the 800 - 1600 nm wavelength range. Optical coupling mechanisms and integration techniques are compared in detail, and application to a variety of advanced lightwave receivers is described.
The operation of the inversion-channel Resonant-Cavity Enhanced (RCE) photodetector is demonstrated in a configuration compatible with the Vertical Cavity Surface Emitting Laser (VCSEL). The phototransistor used 3 strained InGaAs/GaAs quantum well's as the absorbing region and a post-growth dielectric top stack. A quantum efficiency of 41% was obtained at a resonant wavelength of 0.94 micrometers , thereby giving a resonant enhancement factor of 13.5. A bipolar transistor gain of 6.8 at a current density of 10 A/cm2 allowed the phototransistor responsivity to reach 2.1 A/W at the resonant wavelength. We also demonstrate the movement of the resonant peak through the use of Focussed Ion-Beam (FIB) etching which has potential applications in Wavelength Division Multiplexed (WDM) systems.
A CMOS charge-to-voltage converting circuit is designed. This circuit works with a hydrogenated amorphous silicon (a-SiH) p-i-n photodetector in either dc bias mode or charge- storage mode, converts the photo-charge of the detector into a voltage with a high linearity at 10 MHz, and eliminates the effect of the dark current. The modeling of the a-Si:H p-i-n photodetector in the two working modes is implemented using SPICE switches.
PtSi/silicon Schottky barrier detectors and focal plane arrays are important components for the near and medium infrared spectral range. Ir/Si silicon detectors have smaller barriers with response to the 10 - 12 micrometers range, but are difficult to manufacture reliably. An alternate way to extending the spectral response of these detectors is to use a heavier doped substrate, particularly if only a thin surface layer is heavily doped. We have modeled such surface doped Schottky barriers and have calculated the expected photoresponse. Considerable reductions in barrier height are possible while keeping dark currents to a minimum. Experimental results confirm the model.
A novel high performance intensified photodiode (IPD) intended for general use in most applications requiring photomultiplication is described. The IPD has high quantum efficiency and fast time response. The detector is stable and requires a single high voltage power supply to operate. This paper describes the device physics as well as preliminary measurements of D.C. gain, quantum efficiency, and impulse response.
The experiment found that 10.6 micrometers wavelength laser incident on a bulk germanium affects conductivity of germanium. The change of conductivity is ascribed to a ultrasonic field excited by change of laser power, which scatters carriers in semiconductor and results in a change of conductivity. Relative change of conductivity (Delta) (sigma) /(sigma) approximately equals 5 X 10-4 is observed when laser power changes about 1 Watt. Application in mid-infrared laser detection is proposed and discussed.