Comparison of negative capacitance phenomenon in ordinary amorphous and electroformed nanocrystalline silicon based LEDs

Hydrogenated amorphous silicon p+in+ homojunction LED (Figure 1-a) can be electroformed under a sufficiently high, calibrated forward voltage leading to its instant nanocrystallization at room temperature. This... [ view full abstract ]

Hydrogenated amorphous silicon p+in+ homojunction LED (Figure 1-a) can be electroformed under a sufficiently high, calibrated forward voltage leading to its instant nanocrystallization at room temperature. This nanocrystallization process is of scientific and technological interest since the electroformed LED exhibits boosted electroluminescence together with a memory effect. Transport and luminescence phenomena in the electroformed LEDs were recently studied and explained by a modified band-tail hopping model where the charge carriers are mainly supplied from the silicon nanocrystallites [1]. In the present work, the electroformed nanocrystalline LED has been analyzed by capacitance spectroscopy and compared with the previous work [2] on the ordinary LED. The depletion regions of both ordinary and electroformed devices disappear above some particular forward bias, which leads to an interesting capacitance-voltage-frequency behavior. When the frequency is decreased at a constant forward voltage, the capacitance of the ordinary LED first increases, passes over a peak and then sharply decreases to relatively huge negative values (Figure 1-b). The peak points in the capacitance spectra are attributed to the condition where the injected charge carriers fill the relevant gap states around the demarcation level (Ed), and further decrease in the frequency at that forward bias results in recombination of the charges below Ed [2]. Compared to the ordinary LED, the capacitance-frequency behavior of the electroformed LED remains almost unaffected at forward voltages below 0.6 V and above 1.4 V (Figure 1-b); whereas, significant differences are observed around 1.0 V and at reverse bias regime (Figure 1-c). These similarities and differences in the capacitance spectra of both LEDs have been discussed within the frame of the modification of the gap states after the formation of Si nanocrystallites. From the circuit analysis point of view, the observed negative capacitance phenomenon has been regarded as an inductive effect since, similar to inductance, recombination also exhibits 180° phase difference with the traditional capacitance. Although this approach can be debated by means of the device physics, inclusion of an inductor in the equivalent circuit has been shown to be fruitful during simulations giving out reasonable parameters such as the frequency dependence of AC hopping conductivity.