Feature: Semiconductors
Controlling impurities T e scientists prepared two samples of GaN layers grown on GaN substrates, one doped with silicon and the other with iron. Carbon impurities were doped during the silicon doping process. For the TR-PL measurements, the team recorded signals for temperatures to 350°C, whereas only to 250°C for µ-PCD. T ey used a 1ns-long UV laser pulse to excite the samples and measured the refl ection of microwaves from the samples for µ-PCD. T e TR-PL signals for both samples
showed a slower decay component of 0.2-0.4ms. Additionally, using a long-pass fi lter with a 461nm cutoff confi rmed that yellow light was indeed involved. In both samples and for both TR-PL and µ-PCD measurements the decay time declined when the temperature rose to 200°C, consistent with previous reports. To explain these fi ndings, the scientists
resorted to numerical calculations, which revealed that the deep levels essentially trapped holes (absence of electrons), that eventually recombined with free electrons but took so long due to the extremely small chance of an electron being captured by the deep levels. However, at high temperatures, the holes managed to escape from the trap and recombined with electrons through a much faster recombination channel, explaining the shorter decay time; see Figure 2. An analytical model based on energy-
band representation was proposed to explain the temperature dependence of the time constants in the TR-PL and µ-PCD decay curves. At low temperatures, holes are trapped in H1 and take long to recombine with electrons in EC due to diffi cult electron capture. At high temperatures, the holes escape to EV and recombine with electrons through the
recombination channel. To reduce the eff ects of the slow
decay component, either a low carbon concentration must be maintained, or device structures with suppressed hole injections must be adopted.
GaN future With these insights, it is perhaps only a matter of time before scientists fi gure out how to avoid these pitfalls. Uncovering the mechanism underlying the eff ect of carbon impurities on the charge carriers of gallium nitride helps control the devices better. Hence GaN’s rise to power and with it better electronics. GaN exhibits lower power losses in
electronic devices and therefore saves energy. As a semiconductor material, it can go a long way toward mitigating greenhouse effects and climate change, so findings on its impurities such as these may lead to a cleaner, greener future.
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