Session Z20: Invited Session: Optical Processes in Nitrides and Other Wide-Band-Gap Semiconductors

Electronic Properties of ZnO: Reconciling Multiple Techniques

Significant progress has been made recently in our understanding of the electronic properties of ZnO and their physical origins, and yet we are far from being in possession of a complete picture despite decades of effort. Given the ultraviolet bandgap of this semiconductor (essentially the same as GaN), its easy synthesis using a wide variety of crystal growth techniques, the ready availability of high-quality single crystals, and intrinsic highly efficient luminescence, it is a material of great interest for a wide variety of device applications. Still, reliable p-type doping remains a considerable obstacle to realizing junction devices, and luminescence features often attributed to acceptor related transitions in fact have alternative physical origins. The role of impurities is reasonably well-understood, but a detailed understanding of dominant defects is somewhat elusive, although several techniques based directly and indirectly on Schottky contacts have provided some illumination on the topic. In this talk, I will summarize recent results in the field, and outline some of the key issues to which definitive answers are desirable if ZnO is going to be commercially competitive with GaN.   

Loss mechanisms in nitrides

Indium gallium nitride alloys are successfully being used for light emitting diodes (LEDs) and laser diodes (LDs) in the green to ultraviolet part of the optical spectrum. These devices are the key enablers to Solid-State Lighting, which promises to significantly cut electricity consumption. Applications are still limited, however, by the declining efficiency of LEDs at high currents (``droop'') and by absorption losses of undetermined origin in LDs. Several mechanisms have been suggested as the cause of this efficiency loss, such as Auger recombination and free carrier absorption. Experimentally it is very difficult to discriminate between different nonradiative processes. We have therefore addressed the loss mechanisms based on state-of-the-art first-principles computational theory. We use ab initio wave functions and bands that are accurate throughout the entire Brillouin zone (as opposed to k.p band structures). For Auger recombination we find that both electron-electron-hole and hole-hole-electron processes contribute, and that indirect processes assisted by alloy scattering and by electron-phonon coupling dominate. The magnitude of the resulting Auger coefficient indicates that Auger recombination is indeed responsible for the efficiency reduction at high carrier densities. Strategies for overcoming this limitation will be discussed. For free-carrier absorption, the relevant optical processes are again indirect. We determine the optical absorption coefficient and the corresponding photon mean free path as a function of carrier concentration. The computed values indicate that the effect is weak in LEDs but constitutes an important loss mechanism in LDs.   

Carrier localisation mechanisms and efficiency droop in nitride quantum wells

A variety of experimental evidence indicates that the carriers in InGaN quantum wells (QWs) in InGaN/GaN QW structures are localised at room temperature, for example the S-shape temperature dependence of the peak photoluminescence (PL) energy with increasing excitation power density, a key fingerprint of carrier localisation. This localisation is believed to be responsible for the high efficiency of light emission from InGaN QWs since it prevents the carriers from moving to non-radiative recombination centers such as dislocations. In-rich clusters in the InGaN QWs were widely believed to be responsible for the localisation of the carriers. However, careful electron microscopy (EM) and atom probe tomography (APT) have shown that such clusters do not exist in InGaN QWs, at least for In contents less than 25{\%}, and hence such clusters cannot be responsible for the localisation of the carriers. So what mechanisms are localising the carriers? APT and EM have shown that the InGaN in the QWs is a random alloy, with the In atoms distributed at random on the Ga sites. They have also shown that the InGaN QWs have monolayer and bilayer thickness fluctuations. Quantum mechanical calculations show that the holes in the InGaN QWs are localised on a scale of 1-2nm by the random indium fluctuations, and the electrons are localised on a scale of about 5nm by the QW thickness fluctuations. This localisation prevents the carrier from diffusing to dislocations and hence results in a high efficiency of light emission at room temperature. At high carrier densities the localised states saturate with carriers and the additional non-localised carriers can then diffuse to defects and recombine non-radiatively. It is suggested that this is a significant contributory factor to the efficiency droop observed in InGaN/GaN QW structures at higher current densities.   

Photoluminescence as a tool for characterizing point defects in semiconductors

Photoluminescence is one of the most powerful tools used to study optically-active point defects in semiconductors, especially in wide-bandgap materials. Gallium nitride (GaN) and zinc oxide (ZnO) have attracted considerable attention in the last two decades due to their prospects in optoelectronics applications, including blue and ultraviolet light-emitting devices. However, in spite of many years of extensive studies and a great number of publications on photoluminescence from GaN and ZnO, only a few defect-related luminescence bands are reliably identified. Among them are the Zn-related blue band in GaN, Cu-related green band and Li-related orange band in ZnO. Numerous suggestions for the identification of other luminescence bands, such as the yellow band in GaN, or green and yellow bands in ZnO, do not stand up under scrutiny. In these conditions, it is important to classify the defect-related luminescence bands and find their unique characteristics. In this presentation, we will review the origin of the major luminescence bands in GaN and ZnO. Through simulations of the temperature and excitation intensity dependences of photoluminescence and by employing phenomenological models we are able to obtain important characteristics of point defects such as carrier capture cross-sections for defects, concentrations of defects, and their charge states. These models are also used to find the absolute internal quantum efficiency of photoluminescence and obtain information about nonradiative defects. Results from photoluminescence measurements will be compared with results of the first-principle calculations, as well as with the experimental data obtained by other techniques such as positron annihilation spectroscopy, deep-level transient spectroscopy, and secondary ion mass spectrometry.   

Numerical Simulation of III-Nitrides Materials and Light Emitting Devices

The versatility of III-Nitrides semiconductors has led to their use in an increasing number of technologically important applications. Their ability to operate over a wide spectral range from the infrared to the deep ultraviolet has propelled this material system into the field light emitters and detectors. Furthermore, their desirable high field transport properties make III-Nitrides an ideal platform for power semiconductor devices. Along with the experimental activity to fabricate and characterize optoelectronic and electronics devices, a number of significant theoretical efforts are underway to understand the novel properties of this material system. This presentation will discuss the unique characteristics that the III-Nitrides material presents from the point of view of the carrier transport and optical properties. The quantum mechanical processes that are responsible for breakdown at high fields will also be discussed. In particular it will be show that these quantum mechanical effects have to be taken in to account to reproduce a number of experimental results. Furthermore, an analysis of the non-radiative recombination processes that are relevant in analyzing the quantum efficiency data of III-Nitride based light emitters will be presented. Finally the model used and the results obtained for the direct and assisted (phonons and electrons) Auger recombination rates and their impact on the calculated quantum efficiency of III-Nitrides based LEDs will be presented.