Session B28: Focus Session: Dopants and Defects in Semiconductors - Nitrides

Optical signatures of defects in nitride semiconductors

Despite successful commercialization of GaN-based light emitting diodes (LEDs) and high frequency transistors, crystal defects continue to have a strong and often undesired impact on the opto-electronic properties of the III-Nitride family of materials. Fully realizing the potential of this fascinating materials system requires a better understanding of the physical origin of defects, their dependence on both substrate quality and epitaxial growth conditions, and their influence on electrical and optical properties. This talk will discuss the use of deep level optical spectroscopy (DLOS) to quantitatively study defect states in GaN-based materials. As a photocapacitance technique, DLOS is able to probe defect levels that are otherwise inaccessible to thermally-stimulated defect spectroscopies in wide band gap materials, such as the III-Nitrides. DLOS quantifies both the energy level and density of defects and probes defect states with nanoscale depth resolution. Beyond the canonical application of DLOS to thin films, this talk will describe new developments of DLOS to quantitatively study defect states in a wide variety of structures with nanoscale dimensionality, including InGaN/GaN multi-quantum wells and AlGaN/GaN core-shell nanowires. The microscopic origin of observed defect states and their influence on the electrical and optical properties of GaN-based LEDs and nanowire devices will be discussed. The reported studies establish DLOS as a critical technique for nanoscale dimensional defect metrology that is able to advance the development of conventional and emerging opto-electronic devices.   

Shallow versus deep nature of Mg acceptors in nitride semiconductors

Although Mg doping is the only known method for achieving p-type conductivity in nitride semiconductors, Mg is not a perfect acceptor. Hydrogen is known to passivate the Mg acceptor, necessitating a post-growth anneal for acceptor activation. Furthermore, the acceptor ionization energy of Mg is relatively large (200 meV) in GaN, thus only a few percent of Mg acceptors are ionized at room temperature. Surprisingly, despite the importance of this impurity, open questions remain regarding the nature of the acceptor. Optical and magnetic resonance measurements on Mg-doped GaN indicate intriguing and complex behavior that depends on the growth, doping level, and thermal treatment of the samples. Motivated by these studies, we have revisited this topic by performing first-principles calculations based on a hybrid functional. We investigate the electrical and optical properties of the isolated Mg acceptor and its complexes with hydrogen in GaN, InN, and AlN. With the help of these advanced techniques we explain the deep or shallow nature of the Mg acceptor and its relation to optical signals often seen in Mg-doped GaN. We also explore the properties of the Mg acceptor in InN and AlN, allowing predictions of the behavior of the Mg dopant in ternary nitride alloys.   

Effect of Doping Profile and Concentration on the Near-Infrared Optical Properties of AlGaN/GaN and AlInN/GaN Heterostructures

Intersubband (ISB) devices utilizing III-nitrides have recently attracted attention for near- and far- infrared optoelectronic applications. In order to achieve efficient ISB transitions, large doping densities are typically required ($>$1E18 cm$^{-3}$). The large impurity density has significant effects on the band structure and material quality, effects that are reflected in important device parameters such as transition energies and linewidths. To determine the optimal doping concentration and profile for III-N intersubband devices, we carried out a systematic study of optical and structural properties of strained AlGaN/GaN and lattice-matched AlInN/GaN heterostructures grown by MBE on quasi-bulk GaN substrates. The lattice-matched AlInN/GaN system is targeted because it allows growth of thick strain-free materials. However, it also presents some considerable growth challenges due to the vastly different optimal growth conditions for Al and In containing nitrides. The transition energy and line profile were determined by direct and photoinduced absorption measurements, while the material quality was assessed using TEM and high resolution x-ray diffraction. The FWHM of the ISB transition at 1.9 $\mu$m was found to vary up to 60\% with the position of delta doping in the quantum well.   

Field-enhanced vacancy diffusion in AlGaN

Room-temperature (RT) native defect diffusion does not generally occur in semiconductors because of high activation energies ($>$1.5 eV). However, recent observations of plastic deformation in AlGaN/GaN High Electron Mobility Transistors (HEMTs) have been attributed to diffusive processes. Here we report first-principles density-functional calculations of the formation and migration energies of vacancies, including the effect of strain and electric fields. We find that triply-negatively charged cation vacancies are the enablers of self-diffusion, as follows: though strain alone is insufficient, we find significant activation barrier lowering due to the applied electric field acting on charged vacancies, reducing cation vacancy barriers in AlGaN to $\sim $1 eV or lower where RT diffusion becomes significant. The described mechanism of electric field enhanced vacancy diffusion is relevant for other materials, including several oxides that also feature charged vacancies with low formation energy.   

Role of native defects and related complexes in absorption and luminescence of AlN

AlN is a wide-band-gap material that has being considered as a substrate for GaN-based optoelectronic devices, or in its own right for deep ultraviolet light-emitting diodes and laser diodes. The band gap of 6.2 eV in principle allows transparency in the visible to UV range, but in practice AlN crystals exhibit several sub-band-gap absorption and emission bands, likely due to the presence of native defects and impurities (the most common being oxygen). Using first-principles calculations with the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE), we investigate the structural, electronic, and optical, properties of N and Al vacancies, and their complexes with O impurities in AlN. Defect charge transition levels and stable charge states are determined from the calculated formation energies, and absorption and emission energies are obtained by constructing configuration coordinate diagrams. Our results indicate that Al vacancies and O impurities are responsible for several absorption/emission lines observed experimentally, in particular for the absorption band around 2.85 eV (which gives AlN crystals a yellowish color) and the emission around 3.20 eV emission (375 nm), often observed in O-doped AlN. Mechanisms for each of these processes will be discussed in detail.   

Effective mass calculations for shallow acceptors in nitrides

In the effective mass approximation for shallow acceptors in semiconductors, the defect eigenstates are written as a product of a slowly varying envelope function and the band extrema Bloch functions. The Kohn-Luttinger Hamiltonian describing the valence band manifold in zincblende, or its generalization for other crystals structures, then becomes a set of coupled differential equations for the envelope function. These can be solved by a variational approach with hydrogenic type basis functions. We have implemented this approach for the appropriate Hamiltonians for zincblende, wurtzite and an orthorhombic crystal structure occurring for II-IV-N2 semiconductors. The Hamiltonian parameters used were extracted from first-principles GW calculations. The central cell correction to the Coulomb potential was added based on pseudopotential differences as proposed by Mireles and Ulloa (Phys. Rev. B 58, 3879 (1998)). Results are presented for various acceptors in GaN, AlN, InN, ZnGeN2 and ZnSnS2. The effects of varying the crystal field splitting parameter, and the type of pseudopotentials (including or not semicore d-states) were investigated.   

Evidence for mobile electrons in p-type GaN:Mg

Although Mg-doping is the only successful means of achieving p-type conductivity in GaN, little is known about the local environment of the impurity. Our work focuses on a unique phenomena revealed in the electron paramagnetic resonance (EPR) spectrum attributed to Mg: an angular dependent line-shape suggesting the presence of free carriers. 10 GHz EPR measurements were made at 4 K with the magnetic field in the plane of the c-axis. Samples included 0.5 -- 1.5 um thick Mg-doped GaN films grown on sapphire by molecular beam epitaxy or chemical vapor deposition. As expected, the angular dependence of the g value reflects axial symmetry. Unexpected is the line shape change from pure Lorentzian with the magnetic field 30$^{o}$ from the c-axis to increasingly Dysonian upon rotation. The latter reflects the presence of mobile carriers due to, for instance, an interfacial conducting layer, polarization charge, or loosely bound electrons from Ga near neighbors. The new EPR analysis suggests local fields surround the Mg impurity which influence the acceptor's response.   

A Combined Excitation Experiment and the Emission Nature of Eu in GaN

We have developed a fiber based confocal optical microscope that operates inside of a commercial SEM instrument (JEOL 6400) enabling the excitation of a sample either by a laser or by electron beam, and hence combining the complimentary techniques of photoluminescence (PL) and cathodoluminescence (CL). The capabilities of the instrument are demonstrated by experiments involving the excitation of europium ions in-situ doped in Mg doped GaN thin films. The combination of the Eu and Mg defect create new optically active centers (Eu/Mg centers) that absorb energy from electron-hole pairs (EHP) more efficiently than the normal Eu centers in non co-doped GaN (Eu centers). However, the luminescence from these centers decrease as a function of EHP exposure time. The instrument used enables us to perform PL studies before and after electron beam exposure to investigate the nature of this effect. We use a below band gap PL excitation source tuned to resonantly excite the Eu centers using their $^7$F$_0$ to $^5$D$_0$ transitions. This allows us to determine their relative numbers and monitor changes of each of the relevant centers. Uitilizing these data, the nature of the decrease in emission is investigated with our new and unique experimental apparatus.   

Magneto-Optical Studies of Rare Earth Doped III-V Nitrides

We investigated the site selective optical and magneto-optical properties of Neodymium doped Gallium and Aluminum Nitride and Erbium doped Gallium Nitride. For our current study, we applied magnetic fields parallel and antiparallel to the C-axis of the crystals and observed the resulting Zeeman splitting both in excitation and emission transitions. On the basis of these measurements, we determined the effective g-factors of all the states involved in the Nd$^{3+}$ transitions. For erbium doping, we observed the Zeeman splitting of the $^{4}$I$_{13/2 }$and $^{4}$I$_{15/2 }$levels. Due to small crystal field splitting and large Zeeman splitting, the assignment of levels and corresponding g-factors is very complex. In addition, we observed unexpected asymmetries in the emission intensities when we compared the spectra obtained for fields parallel and antiparallel to the growth direction. The degree of this asymmetry depends on the substrate material and is unambiguously related to the strain and resulting internal fields that are induced by lattice mismatch. ~The asymmetry behavior parallels the ferromagnetic behavior that is induced by the rare earth ions in GaN and hence our observation suggests that magnetization can be controlled by strain.   

Origins of Persistent Photoconductivity in Highly Mismatched Semiconductor Alloys

Persistent photoconductivity (PPC) is a nonequilibrium phenomenon in which an illumination-induced increase in conductivity of a semiconductor persists following the termination of illumination. The PPC effect has been explained in terms of the large-lattice-relaxation (LLR) model, in which photoexcited carriers are unable to relax to equilibrium due to an energy barrier between shallow donor and deep donor complex (DX) center configurations. To date, an experimental identification of atomistic configurations in support of a model for the PPC effect has yet to be reported. Here, we examine the origins of the PPC effect in GaAsN alloy films, providing the first direct correlation between the concentration of interstitials and the strength of the PPC effect. Thus, the PPC effect in GaAsN is attributed to a change in the bond orientation or a shift in the center of mass of either N-N or N-As pairs. Similar investigations of GaAsBi alloy films will be discussed.   

Influence of excitation frequency on A$_{1}$(LO) and E$_{2}$ Raman modes in In rich In$_{1-x}$Ga$_{x}$N thin films

MBE grown In$_{1-x}$Ga$_{x}$N ($x$ = 0, 0.1, 0.3 and 0.54) thin films with bandgap energies varying from 0.77 to 1.85 eV have been investigated using Raman spectroscopy with 1.58 and 2.41eV excitation energies. The carrier mobility for InN film is 900 cm$^{2}$/V$\cdot $s and decreases with increasing $x$ with its value being 20 cm$^{2}$/V$\cdot $s for In$_{0.46}$Ga$_{0.54}$N . We observe a one-mode behavior of the A$_{1}$(LO) and E$_{2}$ modes. An enhancement in intensity of A$_{1}$(LO) and 2A$_{1}$(LO) replica modes in In$_{1-x}$Ga$_{x}$N films with bandgap energies close to the excitation energy is observed. For samples with $x \quad >$ 0, the A$_{1}$(LO) mode shows a higher intensity relative to E$_{2 }$mode which indicates a resonant enhancement of the A$_{1}$(LO) mode due to Fr\"{o}lich interaction. We find that the energies of longitudinal optical modes (A$_{1}$(LO) and 2A$_{1}$(LO)) vary nonlinearly, unlike the E$_{2}$ mode, with increasing Ga fraction. The width and asymmetry of the A$_{1}$(LO) band is higher for the lower excitation energy (1.58 eV). This is perhaps due to the structural disorder in the deeper regions of the films or due to the distribution of regions with different indium fractions. This may explain the lower carrier mobilities observed in In$_{1-x}$Ga$_{x}$N films with higher values of $x$.   

Defect and Impurity Properties of Hexagonal Boron-nitride

By using both GGA and hybrid functional calculations, we have systematically calculated the properties of defects and impurity in hexagonal boron-nitride ($h$-BN). Our calculations show that the defect configurations and the local bond lengths around defects are sensitive to their charge states. The possible highest negative charge states of defects are largely determined by the nearly- free-electron state at the conduction band minimum. The in-gap defect levels got from hybrid functional calculations are much deeper than those got from GGA calculations. The formation energies of neutral defects calculated by hybrid functional and GGA are close to each other, but the defect transition energy level between charge states and neutral state respect to valence band maximum are quite different in GGA and hybrid functional calculations. Finally, we show that the charged defect configurations as well as the transition energy levels exhibit interesting layer effects.   

Non-linear piezoelectric polarization in III-V and nitride semiconductors

Piezoelectricity can have a large impact on the electronic and optical properties of quantum well and quantum dots based devices such as lasers, light emitting diodes, infrared photodetectors. In particular it has been shown to be important in III-V and nitride semiconductors. The piezoelectric effect in quantum Well or in Quantum Dots is usually taken into account by neglecting the non linear term in the piezoelectric tensor. We have calculated the second order piezoelectric tensor for all the III-V (including the nitrides) semiconductors in the Wurtzite and Zincblende structure. And we have derived a relation between the proper and the improper second order piezoelectric coefficients. This relation is used to calculate the proper coefficients which are the experimentally measurable ones. We have calculated the piezoelectric field in several Quantum wells and compare our values to experiment. We show that the second order can be so large for Zinc-Blende materials that it cancels the first order term, we demonstrate also that for Nitrides this effect is much lower. However we show that for severely strained structure such as quantum dots or thin films, the second order piezoelectric effect can even exceed the spontaneous polarization in the nitrides.