A Comparison of Optical Detectors
Willlam E. L. Grossman Hunter College of CUNY, New York. NY 10021 Any instructor who talks about spectroscopy in Instrumental Analysis has to spend a good deal of time nowadays discussing detectors. The familiar photomultiplier tube, king of the mountain for many years, is being challenged by a number of devices, some not yet available in commercial instruments. The new detectors have in common the fact that they are fabricated from silicon using modem semiconductor technology and they they are array detectors: that is, they can detect differences in light intensity at different points on their photosensitive surfaces. They behave, in fact, like small electronic photographic plates, but their sensitivity rivals that of the photomultiplier. The advent of these sensitive semiconductor array detectors will be one of the major forces for change in the way spectroscopy is done in the visible and ultraviolet. Very little information about the latest and most promising devices can be found in Instrumental textbooks, or indeed in the chemical literature. Moreover, it is im~ortantin an advanced course like lnstrumental togo a lit& beyond the text, to eive the student a flavor of the situations faced by chemistswhen using the instruments being described. My purpose in writing this article is to give instructors enough data for a coherent presentation of the current state of UV-visible detector technology and to show them some sources of further information. I will introduce a discussion of the semiconductor-based detectors with a short description of their mechanisms of action. I also want to suggest a basis for a limited comparison of the various detectors with each other. Available manufacturers' data do not usually facilitate such comparisons, but thev are often necessarv in real life. The three most important characteristics of optical detectors are probably sensitivity, noise sources (from which a prediction of signal-tonoise ratio can be made), and dynamic range. Given a knowledge of these three factors, a student can attempt a comparative discussion of the different devices. I think that to demonstrate acompletegrasp . - of the material,a student must be able to make such comparisons, rather than simply being able to recite the characteristics of an instrument or comoonent. The comparison of dissimilar instruments or devices that do the same thine is often extremelv difficult. but it is nonetheless necessar;. While most sp&troscopists would not consider purchasing an instrument on whicb they had not run samples similar to those that the instrument would be used for, a preliminary paper comparison can be extremely helpful in reducing the number of alternative possibilities, and in focusing attention on the areas where performance differences may be critical. As a teaching tool, it is useful in that it forces students to realize that, in many instances, it is necessary to make defensible comparisons between apples and oranges, and it gives the instructor the opportunity to bring related issues, such as the array advantage (or disadvantage ( I ) ) , into the discussion in an unforced way. I will add the standard disclaimer, that any comparison of this kind will be incomplete; the weights of the various fac-
tors being considered will depend very much on the use to whicb the instrument is put. Because of their importance, I will confine myself to three types of detector: photomultiplier tubes (PM's), silicon photodiode arrays (PDA's), and charge transfer devices (CTD's), both charge injection devices (CID's) and chargecoupled devices (CCD's). I will mention silicon intensified target tubes (SIT'S) such as vidicons only briefly, and will not discuss image intensifiers, although they are quite commonly used with SIT'S and PDA's. Photomulilpller Tubes The mechanism bv which ohotomultiolier tubes detect. ~ Istandarh ~ ~ textson lnstrumental Anallight is W ~ I I C O V in vsis and need not he discussed here. Sensitivitv is characterbed in different ways by different manufacturers, which makes it difficult to compare different tubes directly by looking a t the data sheets. A reasonable comparison would be to find the output current per incident photon at a given wavelength for different tubes. Typically, data sheets or catalogs give the quantum efficiency andlor the cathode sensitivity at a given wavelength or for a range of wavelengths defined by the output of some standard light source. l'hese cquantities are related as follows: the cathode sensitivity or responsivity S = output current (or voltage) per input flux in watts or lumens. The units will he amperes1 watt (radiant sensitivity) or ampereshmen (luminous sensitivitv). S mav. be eiven as a response to "white" lirht (usually a iungsten filament lamp ohrated with a filam.kt temoeratureof 287001 2856 K)or as S(A)for a aiven wavelength. - The quantum efficiency (QE) of the pKotocathode is almost alwavs available. If the photocathode tvpe (Sl, S20, GaAs, etc.j is known, published curves of quantum efficiencies are readily available from manufacturers' literature. QE is defined as the number of photoelectrons emitted by the cathode per second divided by the number of incident photons at wavelength X per second. It is related to S as follows:
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where h is Planck's constant, r is the speed of light, e is the charge on the electron, and A is the wavelength. Since hcle =
where the units of A are micrometers. Thus from the QE one can always calculate S(X) for any wavelength. QE's are available for essentially every photocathode material. Now it is only necessary to know the gain of the P M to calculate the output current for a given input. This quantity is called the anode sensitivity and often has the units amoereslwatt. I t is the oroduct of the PM rain and the cathode sensitivity S(A).~ h d ~ acan i n ordinarilibe determined from the PM data sheet. If does not varv with wavelenath. Sometimes it is given directly for a given set of (usuafiy typical) Volume 66 Number 8 August 1989
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operating conditions. When it is not, it may be calculated as the ratio: gain = anode sensitivity/cathde sensitivity
(3)
Both anode and cathode sensitivities must be for the same wavelength or wavelength range (e.g., the standard tungsten lamp source). While this may appear to be simply a circular process, since we must use the anode sensitivity to calculate the gain to calculate the anode sensitivity, the end result will be an ahilitv to calculate the anode sensitivitv at anv wavelength, andenot simply the few given in the publishkd data for the device. To recapitulate, S(X) can be determined from the QE of the photocathode material, which is a readily available piece of information. The tube gain must be found. Then the anode sensitivity = gain X S(X) can be calculated for any wavelength. Anode sensitivities can he compared directly from one PM to another. Parenthetically, with the ahove information it becomes easy to make the useful conversion between the output of a PM in countslsecond and in amoeres. as is measured hv Lalog amplifiers. The two q.ua~;tities are related by the charge on the electron and the photocathode QE: anode m e n t = photonsls X QE (electrons/photon) X PM gain X coulombs/electron (4) For a system with a QE of 0.1 and a gain of 6 X lo5, an average flux of 10000 photonsls gives: 10000 photonsls X 0.1 electronslphoton X 6 X lo5 X 1.6 X lo-'' coulomh/electron = 9.6 x 10~"coulomb/s(amperes) or, rounding to one significant figure, 0.1 nanoampere. Array Detectors
All the alternative devices currently available for sensitive detection of light in the 200-1000-nm range are array detectors. The important ones are SiliconIntensifier Target (SIT) tubes, the silicon photodiode array (PDA), and two charge transfer devices (CTD's): the charge injection device (CID) and the charge-coupled device (CCD). All of these were originally conceived as television camera sensing elements, and information ahout them has only relatively recently become available in the general scientific literature. The scientific market is also a small one, and development of the devices in forms useful to soectroscooists has heen slow. The advantage an array detector gives over a single detector of the same kind isa factor ofW 2. where N is the numher of resolution elements in the spectru&, arising from the fact that all resolution elements are being observed for the full scan time of the spectrum (I,2). Operating Mechanisms Silicon Intensifier Target Tubes. SIT'S (3,4) are vacuum tubes. The photosensitive surface is a photocathode, similar to those in PM's. The photocurrent is amplified by an internal -- silicon tareet intensifier. and the sienal develooed is read out by meansif an electrun beam: a kiid of invertid cathode rav tube. If aSI'I' is couoled to an external image intensifier, it i s called an ISIT (intensified SIT) and canattain single ohoton detection caoabilitv. ISIT's have been used in spectrometers in the pa& and-have been particularly useful in instruments where a two-dimensional surface is scanned spectrometrically, as in the early models of the Raman microscope as developed by Delhaye and co-workers (5). In general, however, one can say that these tubes are not sensitive enough to compare with photomultiplier tubes, even given the advantage of being ahle to detect a multiplicity of wavelengths simultaneously, without the use of an image intensifier. Because of this, and because they do not seem to 696
Journal of Chemical Education
he as promising as the other possibilities, I will not discuss them further. They are now well covered in up-to-date texts on Instrumental Analvsis: a more comolete descrintion of them can be found in the ieview by ~ h & g(3). Silicon Photodiode Arravs. Silicon PDA chios are commercially available from at [east two sources, EG&G Reticon and Hamamatsu, and assembled detector systems can he purchased from several manufacturers. They are becoming auite common as detectors in Raman and fluorescence spec&oscopy as well as in absorption instruments. ~ 0 t h - t h e ahove manufacturers have built sensors that are optimized for spectroscopic use, in the sense that each diode in the array is rectangular, with a high aspect ratio (25 or 50 pm by 2.5 mm) that matches the shape of the slit that is imaged onto them. The arrays are linear, with up to 2048 diodes in a single row. In operation, each diode in the array is reverse-biased. In this confieuration it can store charae. like a caoacitor, and before behg exposed to the light tb ' e detected, the diodes are fullv chareed throuah a transistor switch. Light falling on the PDA wi~y~enerate'chargecarriers in the s';licon, which will combine with stored charges of opposite polarity and neutralize them. The amount of charge lost is proportional to the intensity of the light. It is measured during the readout phase by measuring the amount of current needed to recharge each diode. The recharging signal is commonly sent to a sample-and-hold amplifier and then digitized (3,6). Charge Transfer Deuices. Charge transfer devices have not been develooed for scientific ourooses to the same extent as PDA's (?, 8). Nearly all the available ones are twodimensional arravs, with the dimensions of the sensitive element being the same as the center-to-center spacing hetween elements, giving a square pixel shape. Typical pixel dimensions are 20 X 20 pm. Both CID's and CCD's accumulate photogenerated charges in similar ways, but they differ in the way the accumulated charge is detected. Charge InjectionDeuices. One can think of a CID sensing element as being made up of two electrodes side by side. In operation, one of the electrodes is biased so as to create a well near it. When an incident photon creates an electron-hole pair in the sensor region, one member of the oair will be attracted to the well and held there. Current 6 1 ~ 'use s n-doped Si as the charge storage region, and it is therefore the hole (the minority carrier) that is stored. After exposure to light, the accumul&ed charge is moved from one electrode of the sensing element to the other, and the potential change caused hfthe change in charge stored on the second electrode is measured. This potential change is proportional to the amount of stored charge, and therefore to the integrated light flux reaching the detector. At the cost of alittle time. the charm sensine mav be done nondestructively by passing the sto;ed charges back and forth between the electrodes. Thus it is possible to take several readings of the same accumulated charge, thereby reducing the random error associated with the readout and imorovina- the signal-tonoise ratio. Charge-Coupled Deuices. The CCD stores photogenerated charges in the same way: in a potential well formed hy an electrode. Normally CCD's are made using p-type material, so that the charges stored are electrons. To determine the amount of charge created after exposure to light, the charge packets are transferred along the row (or rows and columns, for a two-dimensional CCD structure) to a special low-capacitance readout diode. The passage of charge into this diode induces a voltage change proportional to the amount of charee. The low caoacitance of the readout circuit has two advantages: it increases the voltage change induced by a given amount of charge, since
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dV = dQIC and it permits CCD's to attain a very low read noise.
(5)
The small pixels of CCD's are not as well adapted to ordinm snectrosco~vas are the slit-shaned de- disnersive . tection elements o f ~ ~ ~A i3i qs u. e feature of CCD'S helps to minimize the loss of efficiency caused by the mismatch between the shape of the pixel and that of the entrance slit image in the exit nlane. The feature is called "binning", and i t i s t h e process b f aggregating charges formed in several detector elements into one element before being read out. This can give a substantial improvement in detector sensitivity a t a possible cost in spectral resolution. The cost depends on the area of the elements summed together relative to the slit image size. The fact that the individual elements are smallis an advantaee in this case. Since the summation is done on the chip, rathe; than in memory after readout, only one read oneration is reauired for all the nixels summed, and therefore lower readout noise per pixeiread is achieved.
a
Operating Characteristics In the following paragraphs I will discuss a few of the most important operating characteristics of these detectors. Sensitiuity. The quantum efficiency of silicon is inherently much higher than that of the cathode materials used in photomultiplier tubes. Typical curves of responsivity as a function of wavelength can be found in data sheets and manufacturers' handbooks. The peak response comes hetween 700 and 800 nm. For I'DA's, the renponsivity S(h) can be used directly. For examole: from the Reticon data book (9).the responsivitvat ~o&l/joule/~m2. If this 750 i m is found to be 2.8 X number is divided hv the area of a single diode, we will get the output sensitivii~in ampereslwatt;comparahle to what we previously calculated for a photomultiplier tube. AS an example, we can do a calculation that will let us compare thesensitiviry of a I'DA with that ofa PM. Thearea ofoneof thediodes in the Reticon device is2.5 mm high X 25 cm'. Output sensitivity per diode at pm wide = 6.25 X 750 nm = 2.8 X In-' coul/ioule/cm2 I 6.25 X 10-4cm2 = 0.45 amplwatt. By way of co&ast,consider a high-quality GaAs P M such as the RCA C31034. At 750 nm GaAs bas a auanturn efficiency of 11%. We can calculate the responsivity of the cathode by rearranging eq 2: ~
~
S(750) = (750 X 0.11)/1.23985 = 0.067 amplwatt The photodiode can be seen to have a substantially higher sensitivity than the photocathode. The photomultiplier's output signal is magnified by the high gain of the dynode chain. however. For the C31034. the aain is 6 X lo5. aivina an mod; sensitivity = 0.067 amp/wat
