[an error occurred while processing this directive]

[an error occurred while processing this directive]

Some practical considerations about the use of an increasingly popular detector.

Optical spectroscopy has rapidly evolved to meet the increasingly demanding needs of its users as one of the preeminent measurement tools in scientific research. In its most basic form, the individual components of an optical spectroscopy system include a light source, a light-dispersive medium, and a detector. Advances in all three components have led to tremendous gains in the ability to obtain data. Arc lamps and lasers have replaced simple blackbody elements as light sources. As fabrication methods of gratings and mirrors have improved, dispersive elements, specifically spectrometers, have become more efficient.

As the semiconductor and telecommunications industries continue to grow more sophisticated, the near-infrared (NIR) region of the spectrum is coming in for increasing interest for the characterization of optical fibers, filters, light sources, semiconductors and other related materials. Over the past decade the most dramatic improvements in spectroscopic tools have been the advances of detector technology.

Optical spectroscopy detectors convert measured radiant power into an electrical signal that can be processed, recorded, and displayed. There are two main categories of measurement systems, based on either single-channel or multichannel detectors. Single-channel detector systems contain a single element that accepts light through the exit slit of the monochromator. In this system, a spectral measurement is made by rotating the grating of the monochromator and recording one data point for each grating position. The entire spectral bandpass through that slit contributes to the generated signal. This bandpass defines the achievable spectral resolution of the system.

By contrast, a multichannel or array detector system allows a user to simultaneously collect many data points without scanning the monochromator grating. This provides for much more efficient data collection than single-channel counterparts do, because large amounts of spectral data can be collected in a single exposure. First developed mainly for the visible region of the spectrum, multichannel detectors enable collection of hundreds, and in some cases thousands, of data points in a single exposure. No exit slit is used on the spectrometer. Instead, the dispersed spectrum is incident on the detector, which consists of several small and evenly spaced individual detection elements. The most popular multichannel detectors are charge-coupled devices (CCDs). CCDs are typically two-dimensional arrays made of silicon with several hundred elements, or pixels, arranged in a rectangle. For spectroscopy, the longer side of the rectangular array is arranged to coincide with the dispersed spectrum.

Scientific-grade CCDs exhibit high responsivity from the near-ultraviolet to the near-infrared region of the spectrum: 200 nm to 1.1 micrometers. At longer wavelengths, the photon energy is lower than the silicon bandgap, and the silicon thus becomes transparent to the incident photons. Three-five materials, however, like indium gallium arsenide (InGaAs), have a lower bandgap and can absorb the NIR photons. For this reason, InGaAs arrays are quickly becoming the array detector of choice between 0.9 to 1.7 micrometers because of their excellent quantum efficiency in this spectral range. By changing the alloy composition of the detector material, the wavelength can be extended up to 2.2 micrometers, although this extension comes at the expense of significantly higher noise levels. Figure 1 displays the quantum efficiency for regular and extended InGaAs arrays, compared to that of a back-illuminated scientific-grade CCD sensor.

Understanding Signal-to-Noise

The InGaAs arrays used in spectroscopy are constructed from individual photodiodes arranged in a linear array with a silicon CMOS readout multiplexer circuit. These arrays typically consist of 128, 256, or even as many as 512 pixels, in which active pixel areas of 50 ´ 500 micrometers provide very high sensitivity. In high-precision low-light-level spectroscopic applications it is important to understand the various noise sources which contribute to a measurement with an InGaAs array and thus affect the signal-to-noise ratio.

Each individual InGaAs photodiode pixel in an array is connected to its own capacitive transimpedance preamplifier circuit. As a result, the bias voltages are often slightly different from pixel to pixel. These minute differences lead to a predictable and repeatable noise source known as fixed pattern noise. This noise source is a strong function of both the integration time and the array operating temperature, and can be reduced by cooling the array with either thermoelectric or liquid nitrogen cooling. Fortunately, the fixed pattern noise is highly repeatable and can be almost completely eliminated by subtracting a dark acquisition of the same integration time as the illuminated spectrum of interest.

In order to accomplish the dark acquisition, it is mandatory to optical-block the signal to the detector, using a mechanical shutter. Some InGaAs arrays have an electronic sink circuit that dumps the photocurrent and essentially flushes the detector in a manner analogous to a CCD detector. This so-called Òelectronic shutterÓ does not permit a dark acquisition to be made, but it does allow the user to set a fast integration time. Other sources of noise in an InGaAs array that contribute to the measurement include read noise, which is electronic noise that occurs when the elements are read out, and dark signal, which is the unilluminated response of the detector.

The total noise in the measurement contains contributions from all sources of dark, readout, fixed-pattern, and shot noise. Shot noise, the intrinsic noise distribution in the photon signal, is equal to the square root of the total signal. A clear understanding of the noise process and the technical specifications of the detector, and how these figures of merit are defined and influence the measurement, are of critical importance in comparing InGaAs array detector performances. For example, is the readout noise equal to the total readout noise, including contributions from the photodiode, amplifier, and CMOS multiplexer components, or is it simply but incorrectly the multiplexer noise.

Thus, the total noise signal is:

Reducing the detector temperature can reduce the total noise signal of a detector. When an InGaAs array is cooled, however, the long-wavelength response cutoff changes at a rate of 1 nm/K. From a practical point of view, the measurements with an InGaAs array are often shot-noise limited. In this case the reproducibility of the complete system is of importance in ensuring repeatable measurements. Figure 2 represents ten individual spectra of a Xe pen lamp source using a spectrometer with a 320-nm focal length and two-stage thermoelectrically cooled (-25 degrees C) regular InGaAs array. As demonstrated by this figure, the spectra overlay one another nearly perfectly, indicating the very high reproducibility of the detection system.

The primary advantage of using an array detector such as the Jobin Yvon IGA3000 InGaAs array is the fast data acquisition rate. For example, a measurement containing 500 data points obtained with a one-second integration time will take one second to acquire, whereas with a scanning detection system it will take more than 500 seconds. Thus there is little penalty in using the speed advantage of the array to take multiple spectra in order to improve the signal-to-noise ratio under low-light-level conditions. The signal-to-noise ratio of the resultant averaged spectrum is improved by a factor of Ön (called the multiplex or Fellgett advantage). For example, if 100 spectra are acquired and averaged, the signal-to-noise ratio of the resultant spectrum will improve by a factor of ten.

Spectroscopic Systems and Software

For optimum use in spectroscopic applications, InGaAs array detectors must be mounted onto an imaging spectrograph. Such spectrographs, like the Jobin Yvon Triax 320 (Figure 3), have correcting optics that disperse the spectral information across a flat field in the exit plane of the instrument. This allows the array detector, mounted at the exit plane, to collect large amounts of spectral information in a single exposure. Note that, in general, InGaAs arrays are between 250 and 500 micrometers in height. Accurate imaging onto the detector is critical to ensure maximum optical coupling from sample to detector, hence maximization of the signal-to-noise ratio, and to ensure uniform illumination across the array.

It is also very important for spectroscopic applications that the detector and the spectrograph be controlled by a single dedicated spectroscopic software package. In these applications, the data from the InGaAs array needs to be specified as a function of wavelength. By itself, however, the array cannot differentiate between the wavelengths of light incident upon it. The imaging spectrograph is responsible for that function. Because different spectrometer and grating combinations have different spectral dispersions, the spectrum over the IR array changes with the varying conditions. If the dispersion and central wavelength are known, the software can calculate wavelength-dependent data from the NIR array.

Proper spectroscopic software should be able not only to read the data and convert it into a useful format (e.g., wavelength versus counts), but also to change the state of the devices to make the appropriate measurements. For a movable-grating spectrometer, the software must be able to change the center wavelength to allow measurement of the area of interest. For the InGaAs array, gain selection via the software is essential. The most complete spectroscopic packages not only read the data in the desired format, but also have extensive provision for data analysis and display. In the applications examples that follow, integrated systems consisting of an InGaAs linear array, spectrometer, and complete spectroscopic software provide data to satisfy the varied areas of spectroscopy.

Photoluminescence Spectroscopy

Photoluminescence (PL) spectroscopy is a simple and powerful technique that is widely used in the semiconductor industry for material characterization. In PL measurements, a sample is irradiated with a visible light source, which stimulates the solid-state material to an excited electronic state. As this excited state relaxes to the ground state, it emits light at a wavelength dependent on the material properties. Thus the PL technique can provide information about the material bandgap, impurity content, alloy mix, homogeneity, and thickness of the epitaxial layers.

In conventional PL systems, a scanning monochromator equipped with a single-element InGaAs detector is used for analyzing the emission from a sample. The drawback with this technique is that a significant amount of time is required to collect a single spectrum. With an InGaAs array detector mounted onto an imaging spectrograph, however, a large amount of spectral information can be collected quickly and simultaneously. Figure 4 shows PL spectra from GaAs, InGaAs (unknown composition), and silicon obtained with a 256-element liquid-nitrogen-cooled Jobin Yvon InGaAs array detector mounted on a Jobin Yvon LabRam imaging spectrograph. The spectra were collected from room-temperature samples excited by a 532-nm laser with a one-second integration time. Note the excellent signal-to-noise ratio obtained with these measurements even for a one-second measurement time.

Other applications of NIR optical spectroscopy using an InGaAs array detector include Raman spectroscopy, singlet oxygen fluorescence monitoring, IR laser diode characterization, and photoreflectance. The number of applications is growing rapidly, as InGaAs IR arrays make previously difficult applications more feasible. As part of an integrated system including spectrometer and software, InGaAs arrays will become an increasingly important instrument for the infrared spectroscopist.

For further information , please contact the authors of this article, Dr. John R. Gilchrist, director of the optical spectroscopy division, and Dr. Linda M. Casson, an applications engineer in the same division at Jobin Yvon Horiba Inc., 3880 Park Avenue, Edison, NJ 08820-3012; (732) 494-8660 (Gilchrist ext. 131, Casson ext. 153); www.jyhoriba.com.