Room temperature photodetectors

Because of its indirect bandgap, bulk crystalline silicon shows only a very weak PL signal at 1100 nm, as shown for RT and 77 K in Fig. 7.9. Therefore optoelectronic applications of bulk silicon are so far limited to devices that convert light to electricity, such as solar cells or photodetectors. The observation of red PL from PS layers at room temperature in 1990 [Cal] initiated vigorous research in this field, because efficient EL, the conversion of electricity into light, seemed to be within reach. Soon it was found that in addition to the red band, luminescence in the IR as well as in the blue-green region can be observed from PS. 

If there is no fluctuation of laser intensity, we have to measure /q only once. Actually, the envelope of laser pulses changes in a relatively long time range (typically from several minutes to a few tens of minutes) because of the change of environmental factors such as room temperature and coolant temperature. There is also an intensity jitter caused by factors such as the mechanical vibration of mirrors and the timing jitter of electronics. Furthermore, in our system, the laser system is located about 15 m from the beam port to prevent radiation damage to the laser system. (Later, it was moved into a clean room, which was installed in the control room to keep the room temperature constant and to keep the laser system clean. The distance is about 10 m.) Therefore it is predicted that a slight tilt of a mirror placed upstream will cause a displacement of the laser pulse at the downstream position where the photodetector is placed. 

Other detectors that are useful in the near- and mid-infrared regions are bolometers and pyroelectric detectors. Both these detectors have very large bandwidths and can operate at room temperature however, they have long response times compared to the photodetectors and they have low D s. Pyroelectric detectors are useful in the far-infrared region with rapid-scanning spectrometers whereas Golay cell detectors are often used with slow scanning far-infrared interferometers. These cells are modulated at or below 20 Hz. 

We will focus our attention to the last item, noise decrease in photodetectors. We start from (1.95), assuming that the given parameters are temperature (must be as near to the room temperature as possible) and photoelectric gain (must furnish maximum sensitivity and basically is given by the chosen detection mechanism). We conclude that the following two conditions are to be met... 

There is a single concept behind all of the existing nonequilibrium methods for IR detector performance improvement A modification of charge carrier transport within a photodetector is done with the aim to cause a local equilibrium dismrbance between the carriers and the crystal lattice. This is used to suppress minority carrier concentration below its equilibrium value. As a result, carrier concentration may reach values several orders of magnitude below the equilibrium one at near-room temperatures. As far as the carrier generation-recombination is concerned, the effects of nonequilibrium concentration decrease are equivalent to photodetector cooling. 

Elhott et al. [339] analyzed noise mechanisms in MWIR and LWIR infrared detectors operating in the range from 3-13 om and proved that there is no fundamental obstacle that would prevent room temperature operation of photodetectors in background hmited performance, even if the field of view is reduced. 

Figure 3.69 shows a measured U1 characteristics of an InSb detector at room temperature. As expected, an increase of magnetic induction results in a corresponding increase of the dynamic resistance of the photodetector. 

Z. Djuric, J. Piotrowski, Room temperature IR photodetector with electromagnetic carrier depletion. Electron. Lett. 26(20), 1689-1691 (1990)... 

Device response time is another important characteristic of frozen jimction PLEG devices. To check the respxmse time, devices were driven with a 6.3V pulse train at 152Hz with 43% duty cycle at room temperature, and the light output was measured with a photodetector (peak brightness approximately lOOOcd/m ). The response time was less than 2ms and consistent with the RG time constant of the device (data not shown). 

Figure 7.16 Photocurrent responsivity versus wavelength A for different in-plane polarization angles (/>, under normal incidence at room temperature for a photodetector realized with sample VII.

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