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40501-AC10
Physics of Electron Injection-Induced Effects in III-Nitrides
In order to gain fundamental understanding of electron injection effects observed previously in GaN, we extended our studies to another wide band gap semiconductor, ZnO. The results of these studies confirmed that the influence of electron injection on the diffusion length (L) and lifetime (tau) of minority carriers is not unique to GaN and provided further insight into the role of deep acceptor states in electron injection-induced phenomena. Over the course of the past year, the injection-induced increase of minority carrier diffusion length and lifetime due to electron trapping was studied in bulk ZnO containing Li, in epitaxial p-type ZnO:Sb films, and ZnO-based homojunctions.
Electron trapping in bulk ZnO substrates containing Li:
The experiments were carried out on commercially available (Tokyo Denpa (TD)) bulk ZnO substrates grown by hydrothermal technique. Secondary Ion Mass Spectroscopy (SIMS) measurements performed on these substrates revealed the presence of lithium (Li) in the crystal on the level of ~ 4xE16 per cubic cm (Li is often added to ZnO to increase the resistivity of initially n-type samples). A linear increase of L as a function of duration (t) of irradiation with the beam of the scanning electron microscope (SEM) was observed. We have reported a similar behavior in (Al)GaN doped with Mg or Mn. Electron Beam Induced Current (EBIC) measurements carried out on bulk ZnO substrates containing no lithium did not reveal any noticeable increase of L with t (up to 3600 s), suggesting that the presence of Li is important for the observed behavior.
Electron trapping in ZnO doped with antimony (ZnO:Sb):
The effects of electron injection were also studied in MBE-grown p-type ZnO doped with antimony in a manner similar to that for bulk ZnO. The activation energy for the electron injection-induced increase of L of ~ 219 ± 8 meV was obtained. This value is in agreement with that for a (Sb on Zn site - Zn vacancy) acceptor complex.
The saturation and relaxation of irradiation-induced change of diffusion length was studied at room temperature. L reaches its maximum value after about 50 min of continuous exposure to the electron beam. Further monitoring revealed that irradiation-induced increase persists for at least one week. Annealing the sample at 175 C for about 30 minutes resulted in a decrease of the diffusion length. This behavior further supports the involvement of deep electron traps in the phenomenon of interest, since temperature-induced de-trapping of carriers re-activates the original recombination route, thus reducing carrier lifetime and diffusion length.
Electron trapping in ZnO p-n junctions under forward bias electron injection:
If a p-n junction is biased in forward direction, the junction potential barrier decreases and the electrons from the n-type region are injected into p-type region. This should result in increase of minority carrier diffusion length in p-type ZnO and, therefore, enhancement of p-n junction's photoresponse.
EBIC measurements on ZnO p-n junction (as well as spectral and temporal photoresponse measurements) were carried out at room temperature on the cleaved structures before and after forward bias electron injection. A significant increase of L with forward bias electron injection was observed, consistent with a pronounced and long-lasting enhancement of ZnO p-n junction spectral and temporal photoresponse.
The model for the effect of electron injection (see nugget and TOC) is as follows:
1. A non-equilibrium electron, generated by a Scanning Electron Microscope (SEM) beam (I) (or due to a forward bias of p-n junction), gets trapped by a neutral meta-stable level (II). The concentration of involved deep levels increases with the duration of electron injection. Trapping non-equilibrium electrons on these levels (they create a band in ZnO forbidden gap) prevents recombination of the conduction band electrons through these levels (II). This leads to an increase of lifetime for a non-equilibrium electron in the conduction band and, as a result, to an increase of L.
2. The level containing a trapped electron becomes available for recombination of a non-equilibrium conduction band electron as this level captures a hole. Capturing a hole means a transition of the trapped electron to the valence band (III). The rate of this transition increases with increasing temperature, and we note the existence of the activation energy, preventing the immediate hole capture by the ionized impurity.
3. As the rate of hole capture on the deep level increases, the conduction band electrons have more chance for recombination on this level. This results in a shorter non-equilibrium minority electron lifetime and a slower rate for L increase at higher temperatures.
