The donor and exciton states in ultra high-quality heteroepitaxial GaN grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) on sapphire substrates are investigated using low temperature photoluminescence (PL), reflectance, magnetospectroscopy in fields up to 12 T, and resonant electronic Raman scattering (RERS). The A free exciton is confirmed to have a binding energy of about 26.4 meV, independent of strain in the material. Bound n=2 exciton peaks are distinguished in the PL spectrum by their thermalization and sample dependence. The Si donor is shown to have a binding energy of about 21 meV using Si-doped HVPE samples grown at Epitronics. Up to five additional residual donor species are observed when comparing various HVPE and MOCVD samples. Pronounced temperaturedependence of the two-electron satellites is observed, suggesting the existence of unresolved excited rotator states of the neutral donor-bound exciton. Highly resolved magnetic splitting patterns are observed in the two-electron satellites. A nonperturbative theory of these donor splittings is developed, including anisotropy. Resonant electronic Raman scattering from residual donors is reported, and yields improved linewidths compared to PL.

Raman analysis of the E2 mode of AlxGal-xN in the composition range 0 ≤ x ≤ 1 is presented. The lineshape was observed to exhibit a significant asymmetry and broadening toward the high energy range. The spatial correlation model is discussed, and is shown to account for the lineshape. The model calculations also indicate the lack of a long-range order in the CVD (chemical vapor deposition) alloys. These results were confirmed by X-ray scattering: the relative intensity of the superlattice line was found to be negligible. The line broadening of the E2 mode was found to exhibit a maximum at a composition x∼0.5 indicative of a random disordered alloy system. The stress state of the alloys was found to be tensile and was attributed to the difference in the thermal expansion coefficients of the SiC substrate and the film.

Polycrystalline, wurtzitic indium nitride was synthesized by saturating indium metal with atomic nitrogen from a microwave plasma source. Plasma synthesis avoids the high equilibrium pressures required when molecular nitrogen is used as the nitrogen source. Two types of growth were observed: 1) small amounts of indium nitride crystallized from the melt during cooling and 2) hexagonal platelets formed adjacent to the In metal source on the crucible sides. The mechanism of this latter growth is not established, but may involve transport of indium as a liquid film. The crystals were characterized by electron diffraction, X-ray diffraction, elemental analysis, scanning electron microscopy, and Raman spectroscopy. Lattice parameter and Raman active phonon modes are reported and compared with calculations based on the full-potential linear muffin-tin orbital method (FP-LMTO).

We present a comprehensive study of the direct photoluminescence from excitonic states in GaN using polarization-dependent and magnetooptical measurements in fields up to 15 T. We measure and identify fine-structure splittings in the states n=l and n=2 of the A-exciton. The splitting between the energy levels of longitudinal and transverse states in the A-exciton is 0.96 meV. Emission from the spin triplet exciton in the state n=l is observed. We also identify emissions from three sublevels of the state n=2 of the A-exciton. From the analysis of our data we obtain the effective-mass parameters of electrons and holes, g-values and the Rydberg energy of the A-exciton.

Optically-detected electron-nuclear double resonance (ODENDOR) studies at 24 GHz on high-resistivity GaN films grown on Al2O3 have been combined with x-ray diffraction measurements to obtain information on the location of the residual shallow donors. Strong ODENDOR assigned to 69,71Ga lattice nuclei was detected on the g=1.951 effective-mass donor resonance found on the 2.2 eV emission bands. The x-ray studies reveal that the layers are under biaxial compression with high values of strain (∼ 2–3 × 10−3). The quadrupole splittings for 69Ga are smaller than those reported for strain-free samples by 15–25 %. The dominant sources of the local electric field gradient (EFG) responsible for the splittings are attributed to the wurtzite crystal structure and the strain fields that arise from the lattice constant mismatch and the difference in thermal expansion coefficients. An EFG/strain relationship of 3 × 1022 Vm−2 per unit strain at the 69,71Ga nuclei is deduced. The ODENDOR can be described with asymmetry parameter η=0. This provides evidence that the donors are in the crystallites rather than near grain boundaries.

We investigated structural, electrical and optical properties and their relationships for MOCVD grown Si-doped GaN with different doping levels. The changes in structural properties such as strain, crystallinity and strain relaxation as a function of carrier concentrations were measured by high-resolution triple-axis X-ray diffraction and reciprocal space mapping technique. As the carrier concentration of the samples increases from 1.6×1018 to 9.5×1018/cm3 Hall mobility decreases from 222 to 170cm2/Vsec, and peak intensity ratio of band edge to yellow luminescence in photoluminescence spectra decreases too. The variation of mosaic spread along [00.1] direction determined from full width at half maximum(FWHM) of GaN (00.2) peaks shows good agreement with the variation in Hall mobility. In addition, dislocation density determined from FWHM of (10.2) peaks is shown to be related to the variation in the intensity of yellow luminescence in PL spectra.

Using contactless electroreflectance and surface photovoltage spectroscopy at room temperature we have nondestructively evaluated the band bending (carrier type) at the surface of epitaxial n- and p-type GaN/sapphire samples as well as at both the InGaN surface and the GaN at the InGaN/GaN interface of samples of epitaxial InGaN grown on top of thick GaN epilayers/sapphire, having average n- and p-type character.

Strong room temperature Er-related photoluminescence (PL) and electroluminescence (EL) at 1539 nm was observed from Er and 0 implanted n-type GaN. Good device performance requires that the Er-related excitation and emission processes be efficient. Single exponential PL and EL time decays with l/e lifetimes of 2.33 ms and 1.74 ms indicates highly efficient radiative process. The Er excitation process in GaN was studied by comparing the efficiency of direct Erabsorption, electron-hole pair recombination, and hot electron (impact) excitation. The strongest Er luminescence and the lowest pump power was found using impact excitation.

We report on novel transport and optical data for GaN/AlGaN modulation doped heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Variable temperature galvanomagnetic, resistivity, photoluminescence and photoconductivity measurements have been performed. Evidence for potential fluctuations is provided by the observation of weakly localized transport at low temperatures, together with a negative magnetoresistance due to disorder in the interface region. Additional evidence for a built-in electric field caused by the fluctuations near the heterointerface region is given by the observation of photoconductivity dips resonant with free excitons, indicating free exciton ionization. It is concluded that interface roughness, dislocations, and similar structural defects, have a strong influence on the transport properties of the two-dimensional electron gas in these structures.

Exciton absorption peak has been clearly observed at room temperature by photovoltaic spectra, on a GaN layer grown on (0001)-plane Sapphire by metalorganic chemical vapor deposition. From the spectra, we obtained A and B exciton transition energies to be 3.401 eV, 3.408 eV, and the energy gap to be 3.426 eV in wurztite GaN, respectively. We have also performed photovoltaic measurements with various incidence angles of light, and observed the polarization behavior of exciton absorption in GaN. Finally, we used UV-polarizer to further confirm the polarization properties of GaN. In conjuction with previous room temperature photoreflectance measurements, this work provide direct and reliable assessment of the excitonic properties and crystal quality of GaN semiconductor layers.

We have studied the effects on the phonon spectrum and on the electron-longitudinal optical phonon scattering in GaN/AlN and GaAs/AlAs quantum wells. Phonon modes and potentials have been calculated for both systems. Results for emission due to electroninterface phonons interactions are presented. We will discuss the implications for relaxation times and electron mobility due to modified LO-phonon scattering in both systems.

The recombination dynamics of excitons in GaN / Ga0.93Al0.07N multiple quantum wells is studied versus lattice temperature. The average decay time of photoluminescence measured at 8K is of ∼330 ps, with a substantial variation of times within the emission line. This is interpreted in terms of carrier localization due to alloy disorder and to well width and depth variations. The radiative lifetime τr of excitons in the wells is found to increase linearly with temperature, with ∂τr / ∂T = 20.5 ± 0.7 ps.K−1. The radiative lifetime of free excitons in the low-temperature limit is deduced to be 2.4 ps, consistent with a longitudinal-transverse splitting ћωLT in GaN of 0.6 meV, in excellent agreement with recent estimations. The ratio of the lifetimes of localized and free excitons is found coherent with the picture of electrons and holes independently localized on short-range defects, instead of excitons localized as a whole on long-range potential fluctuations.

Emission mechanisms of the device-quality quantum well (QW) structure and bulk three dimensional (3D) InGaN materials grown on sapphire substrates without any epitaxial lateral overgrown GaN (ELOG) base layers were investigated. The InxGx1−xN layers showed various degree of spatial potential (bandgap) fluctuation, which is probably due to a compositional inhomogeneity or monolayer thickness fluctuation produced by some kinetic driving forces initiated by the threading dislocations (TDs) or growth steps during the growth. The degree of fluctuation changed remarkably around nominal InN molar fraction x=0.2, which changes to nearly 8–10 % for the strained InxGa1−xN. This potential fluctuation induces energy tail states both in QW and 3D InGaN, showing a large Stokes-like shift combined with the red shift due to quantum confined Stark effect (QCSE) induced by the piezoelectric field. The spontaneous emission from undoped InGaN single quantum well (SQW) light-emitting diodes (LED's), undoped 3D double heterostructure (DH) LED's, and multiple quantum well (MQW) laser diode (LD) wafers was assigned as being due to the recombination of excitons localized at the potential minima, whose area was determined by cathodoluminescence (CL) mapping to vary from less than 60 nm to 300 nm in lateral size in the case of QW's. The lasing mechanisms of the cw In0.15Gao.85N MQW LD's having small potential fluctuation, whose bandgap broadenings are less than about 50 meV, can be described by the well-known electron-hole-plasma (EIHP) picture with Coulomb enhancement. The inhomogenous MQW LD's are considered to lase by EHP in segmented QW's or Q-disks. It is desirable to use entire QW planes with small potential inhomogeneity as gain media for higher performance LD operation.

Low temperature cathodoluminescence has been used to investigate the spatial characteristics of light emission in InxGa1−xN single quantum wells. High spatial resolution, narrow band pass imaging shows the luminescence to be strongly inhomogeneous in wavelength as well as in intensity on a sub-micron scale. Cathodoluminescence spectra correlate favorably with photoluminescence spectra. However, when spectra are recorded from different areas in spot mode, the quantum emission varies significantly in wavelength. The observed variations are consistent with composition inhomogeneities in the quantum well.

A set of GaN/InGaN multiple quantum wells (QWs) with well thickness 30 Å and barrier thickness 60 Å were grown by MOCVD on sapphire substrates. The n-type Si doping of the InGaN QWs was varied, in order to produce a different electron concentration in the QWs for the different samples. Optical spectra were obtained by time resolved photoluminescence spectroscopy. The data show weak excitonic spectra from the QWs as well as a broad deeper emission with a much stronger intensity. The spectral shape becomes narrower and the energy position shifts to higher energies with increasing doping. The two different emissions are not easily separated in CW or time integrated spectra, but are clearly observed in a time resolved spectral measurement due to their different recombination rates. The deeper emission has a long and non-exponential decay, with an average decay time in the order of several hundred nanoseconds. The higher energy exciton emission has a much faster decay of about 1 ns. The lower energy band is tentatively explained as due to separately localized electron-hole (e-h) pairs in the QW.

We report on investigations of the excitonic quantum efficiency in GaN epilayers as function of buffer layer thickness, buffer layer material, sample thickness and residual oxygen content. These values are compared to that of GaN bulk material. The quantum efficiency of the free excitons rises with increasing buffer layer thickness, increasing sample thickness and decreasing residual oxygen content. The influence of oxygen on the quantum efficiency is stronger than that of the buffer layer thickness. Additionally, the homoepitaxial growth of GaN shows higher quantum efficiencies than the growth with an AIN buffer layer. In general, the observed quantum efficiencies in GaN epilayers are below 20% indicating the strong impact of nonradiative relaxation and recombination processes in the excitonic range. Only, GaN bulk material shows quantum efficiencies of 25 % for the free A-exciton XA and of 50 % for the donor-bound exciton complex D0,X).

Effects of well thickness and Si-doping on the photoluminescence (PL) emission properties of GaN/AlxGa1-xN (x ∼ 0.07) and InxGa1-xN/GaN (x ∼ 0.15–0.2) multiple quantum wells (MQWs) grown both by MOCVD and reactive MBE have been studied. The room temperature emission mechanisms have also been addressed.

An array of microdisks with diameter of about 9 μm and spacing of 50 μm has been fabricated by dry etching from a 50 Å/50 Å GaN/AlxGa1-xN (x∼ 0.07) multiple quantum well (MQW) structure grown by reactive molecular beam epitaxy. The as-grown MQWs and the microdisk structures have been studied by picosecond time-resolved photoluminescence (PL) spectroscopy. PL emission spectra and decay dynamics were measured at various temperatures and pump intensities. With respect to the original MQWs, we observe strong enhancement of the transition intensity and lifetime for both the intrinsic and barrier transitions. The intrinsic transition is excitonic at low temperatures and exhibits an approximate 10 fold increase in both lifetime and PL intensity upon formation of the microdisks. The exciton transition magnitude diminishes rapidly with increased temperature however, while the enhanced lifetime shows little change. At room temperature the dominant GaN well transition is found to be band-to-band in nature as evidenced by effective band gap shrinkage and band-filling effects seen within the PL spectrum. The implications of our results to III-Nitride microdisk lasers are discussed.

This paper addresses the broadening mechanism of the near band gap photoluminescence in GaN films doped n-type with silicon. The films were produced by plasma assisted MBE and their carrier concentration was varied systematically from 1015 to 1020 cm−3. The photoluminescence was excited with a 10 mW He-Cd laser at 77K. At low carrier conentration ( < 1017 cm−3 ) the photoluminescence peak has FWHM of about 18 meV, while at high carrier concentration (>1018 cm−3 ) the FWHM increase monotonically with the carrier concentration up to about 120 meV. The broadening of the line at high carrier concentration is attributed to impurity band broadening and tailing of the density of states. The data were quantitatively analyzed, as a function of carrier concentration and compensation ratio, using the impurity band broadening model of Morgan [1] and the agreement between model and experimental data supports the model's validity and suggest a potential method of determining the compensation in degenerate semiconductors.

We present a detailed investigation of the cathodoluminescence (CL) of MBEgrown cubic GaN (c-GaN) epitaxial layers. Under high excitation conditions the room temperature CL-spectrum is dominated by a single excitonic emission band at about 3.21 eV with a full width at half maximum of 55 meV. Under low electron beam excitation a second, deep emission band at 2.4 eV is observable. A strong variation of the ratio between the deep luminescence intensity and the near band edge luminescence intensity ratio is observed with increasing excitation intensity. Using a simple model based on bimolecular rate equations the concentration of defects involved in this transition is estimated to be about 1015 cm−3. CL-measurements with varying excitation intensity reveal that these recombination levels have only minor influence on the performance of high injection optoelectronic devices like laser diodes. Depth resolved measurements and CL-images from a cleaved c-GaN/GaAs reveal that the deep recombination centers are homogeneously distributed within the epilayer, however the concentration of nonradiative recombination centers is increasing towards the GaN/GaAs interface.