Carrier concentrations devices

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

Friend et at. studied the influence of electrodes with different work-functions on the performance of PPV photodiodes 143). For ITO/PPV/Mg devices the fully saturated open circuit voltage was 1.2 V and 1.7 V for an ITO/PPV/Ca device. These values for the V c are almost equal to the difference in the work-function of Mg and Ca with respect to 1TO. The open circuit voltage of the ITO/PPV/A1 device observed at 1.2 V, however, is considerably higher than the difference of the work-function between ITO and Al. The Cambridge group references its PPV with a very low dark carrier concentration and consequently the formation of Schottky barriers at the PPV/Al interface is not expected. The mobility of the holes was measured at KT4 cm2 V-1 s l [62] and that for the electrons is expected to be clearly lower. 

Photoelectrochemical techniques have been utilized to determine the minority (electron) diffusion length (L) and other electrical parameters of p-ZnTe [125] and p-type Cdi-jcZnjcTe alloys [126]. In the latter case, the results for a series of single crystals with free carrier concentration in the range 10 " -10 cm (L = 2-4 xm, constant Urbach s parameter at ca. 125 eV ) were considered encouraging for the production of optical and electro-optical devices based on heterojunctions of these alloys. 

The net carrier concentration, shown in Fig. 7.8, was obtained at a frequency of 100 kHz. DLTS spectra were recorded using reverse- and forward-bias modes in the temperature range of 80-350 K. In the re verse-bias mode, the devices were reverse biased from -1.2V to -0.2V, with a pulse width of 1 ms. Two hole (majority-carrier) trap levels were found in all the devices. These levels were designated as Hi at I iv+0.26 and H2, for which an activation energy could not be resolved. Upon minority-carrier injection (forward-bias mode), DLTS showed two additional electron (minority-carrier) traps, which are labeled Ei (Ec-0.1eV) and E2 (Ec-0.83eV) in Table 7.1. The spectra were measured at an emission time of 465.2 s and the width of the... 

Figure 7.8. The net carrier concentration versus depth for the devices measured by C-V (DLTS measurements).The levels are designated for AP = autoplated (electroless deposition) EP = electroplated (electrodeposition) and PVD = physical vapor deposition. [Reproduced with permission from Ref. 95. Copyright 2001 Elsevier.]...

The relationship between 7c and the hole concentration indicates the possibility of control of magnetic properties isothermally by light irradiation, electric field, carrier injection, and all other means that change the carrier concentration in semiconductors. The concept of such devices was proposed already in 1960s in the context of work on rare-earth magnetic semiconductors (Methfessel 1965 Methfessel and Holtzberg 1966). 

Quantitative descriptions of MOS devices are available (2, 8, 9,12). This short summary of solid-state physics was intended to illustrate the importance of carrier concentrations, transport, generation, and recombination in device performance. These properties, in turn, depend critically on material parameters resulting from a large number of chemical process sequences. 

A p-type HgCdTe substrate 1 is exposed to Hg vapour at a high temperature. This treatment removes Hg atoms from the substrate and high carrier concentration layers 9 are formed. Then, windows W are formed in a mask 10. The device is subjected to Hg vapour at a low temperature to diffuse Hg atoms into the surface areas of the windows These areas return to the same carrier concentration as the one of the substrate while the areas covered by the mask will keep the high carrier concentration and consequently form the channel stops of the device. 

---