The infrared dielectric function of wurtzite AlN has been determined by fitting an analytic Lorentzian dielectric function to experimentally observed interference fringes in infrared transmission. The analytic model is scaled to agree with recent measurements of the visible refractive index, and the experiment and model extend to the submillimeter range of the infrared. A complete, experimentally verified dielectric function is found from the visible to the submillimeter spectral region for radiation with E⊥c axis, and an analytic model is produced for E‖c axis. Refractive indices and extinction coefficients from the visible to zero frequency are presented.

High‐resolution, variable temperature PL experiments were performed in the spectral region associated with recombination processes involving the ground and excited states of the neutral donor bound excitons. High‐resolution infrared measurements in combination with high‐sensitive SIMS unambiguously identified Si and O shallow donors and yield their ground state binding energies. These binding energies are in excellent agreement with values obtained by the analysis of the two‐electron‐satellite PL spectra considering the participation of ground and excited state donor bound excitons. This work clarifies conflicting aspects existing in donor identification and the binding energies of the impurities and excitons. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) experiments have been performed on a set of GaN epitaxial layers doped with Mg from 2.5 x 10(18) to 5.0 x 10(19) cm(-3). The samples were also characterized by secondary-ion-mass spectroscopy (SIMS), temperature-dependent Hall effect, and low-temperature photoluminescence (PL) measurements. EPR at 9 QHz on the conductive films reveals a single line with g(parallel to)similar to2.1 and g(perpendicular to)similar to2 and is assigned to shallow Mg acceptors based on, The similarity of the spin density with that found for the number of uncompensated Mg shallow acceptors from Hall effect and the total Mg concentration by SIMS. PL bands of different character are observed from these layers,, including shallow-donor-shallow-acceptor recombination at 3.27 eV from the lowest,doped sample, and, in most cases, broad emission bands with peak energy between 2.9 and 3.2 eV from the more heavily doped films. In addition, several of the films exhibit a weal, broad emission band between 1.4 and 1.9 eV. ODMR at 24 GHz on the "blue" PL bands reveals two dominant features. The first is characterized by g(parallel to),g(perpendicular to)similar to1.95-1.96 and is assigned to. shallow effective-mass donors. The second line is described by similar g tensors as found by the EPR experiments and, thus, is also attributed to shallow Mg acceptors. Although several groups have related the 2.8 eV PL in heavily Mg-doped GaN with the formation of deep donors, no clear evidence was found from the ODMR on this emission for such centers. However, based on the near-midgap PL energy and the observation of the feature assigned to shallow Mg acceptors, the strongest case from magnetic resonance for the existence of deep donors in these films is the isotropic ODMR signal with g = 2.003 found on emission, <19 eV. Possible recombination mechanisms to account for-the ODMR on these "blue" and near-IR PL bands are discussed.

We will highlight our recent work on the properties of residual defects and dopants in GaN heteroepitaxial layers and on the nature of recombination from InGaN single quantum well (SQW) light emitting diodes (LEDs) through magnetic resonance techniques. Electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) were performed on undoped (highly resistive and n-type) and intentionally doped (Si, Mg, or Be) GaN films grown by a variety of techniques (MOCVD, MBE, and HVPE) in order to obtain general trends and behavior. Through the spin-Hamiltonian parameters, these methods can reveal symmetry information, the character of the wave function and (ideally) the chemical identity of the defect. In addition, low temperature EPR intensities can be used to determine the neutral acceptor or donor concentrations without the need for contacts or the high temperatures required for Hall effect measurements. The ODMR was performed on both bandedge (mainly shallow donor–shallow acceptor recombination) and deep (visible and near-IR) PL bands. In spite of the radically different (non-equilibrium) growth techniques, many of the same defects were found in the various samples. Finally, earlier ODMR studies of recombination from Nichia InGaN 'green' and 'blue' LEDs were extended to include shorter ('violet') and longer ('amber') wavelength LEDs and an undoped 30 Å In0.3Ga0.7N/GaN heterostructure. The results provide evidence for spatially separated electrons and holes in the optically-active 30 Å InGaN layers under low photoexcitation conditions, likely due to localization at different potential minima in the x–y planes and/or the large strain-induced piezoelectric fields parallel to the growth direction.

Magneto-optical studies of donor excitation in, hydride-vapor-phase epitaxial GaN are reported. Donor ground-to-excited state transitions are observed in the infrared for Si, O, and a third unidentified donor as a. function of magnetic field to 11 T. Transitions from the ground state to 2p(+/-) and 3p(+/-) excited states are studied. Values for effective mass ground and excited state binding energies are determined from the observed excited-state separation. We find a value of 29.1+/-0.5 meV for the effective-mass donor binding energy in GaN. Ground state binding energies for Si-Ga and O-N are 30.18+/-0.1 meV and 33.20+/-0.1 meV, respectively. Separation rates for the 2p(+) and 2p(-) excited states with magnetic field are consistent with an electron, effective mass of 0.22m(0).

Although considerable progress has been made in reducing background impurities and controlling optical and electronic properties of heteroepitaxial GaN layers over the last three decades, strong experimental evidence that clarifies the chemical identity of the shallow donors has been presented only recently. Variable temperature photoluminescence studies of recombination processes associated with excitons bound to donors and Fourier transform infrared (FTIR) measurements of donor absorption in hydride-vapor-phase epitaxial GaN are presented. PL studies of undoped and Si-doped homoepitaxial layers are consistent with and support the identification of Si and O as the dominants pervasive shallow donors in GaN, as measured by FTIR and secondary ion mass spectroscopy. Analyses of the two-electron satellite (2ES) transitions reveal the observation of a number of transitions associated with ground and excited states of both the donors and the donor-bound exciton complexes. All the 2ES lines observed in this work can be accounted for by recombination of excitons bound to Si and O donors and line positions are in excellent agreement with the energies of donor intraimpurity transitions measured previously by infrared absorption.

High-resolution, variable-temperature photoluminescence studies of recombination processes associated with excitons bound to donors in hydride-vapor-phase epitaxial GaN are presented. Detailed analyses of the two-electron satellite (2ES) region identify transitions associated with ground and excited states of both the donor-bound exciton complexes and of the donor itself. All of the 2ES transitions observed in this work can be accounted for by the recombination of excitons bound to Si and O substitutional impurities and the line positions are in excellent agreement with the energies of donor intraimpurity transitions measured previously by infrared absorption. Conflicting aspects of donor identification and the binding energies of impurities and excitons are clarified.

Semi-insulating (SI) silicon carbide is important for applications in high-power, high-frequency electronics, such as SiC MESFETs and GaN FETs. In this work, we discuss the use of low-temperature electron paramagnetic resonance (EPR), room- and low-temperature FTIR and photoluminescence as potential screening probes. In addition, the improved materials quality enhances the resolution of such spectroscopic measurements to better understand the material. The EPR spectra reveal the expected V4+ as well as the shallow boron center, suggesting inhomogeneities in the B and/or V distribution. We observe significant reduction in the free-carrier absorption compared with n-type material and the intra-3d-shell E2–2T2 IR absorption. In addition, we observe an N-related gap mode which may serve as a quantitative probe of the N content. The photoluminescence from the V-doped sample exhibits weak donor-bound excitons and shows a broad structured band near 3.0eV related to recombination between photoneutralized N-donors and photoneutralized acceptors.

Donor impurity excitation spectra in the infrared from two high-quality, not-intentionally doped, hydride-vapor-phase epitaxial GaN wafers are reported. Two previously observed shallow donors which we designate N1 and N2 were observed in both wafers. However, spectra of one wafer are dominated by N1 and spectra of the other by N2. A comparison of infrared and secondary ion mass spectroscopic data allows identification of N1 as Si and N2 as O. Silicon is the shallowest uncompensated donor in these samples with an activation energy of 30.18±0.1 meV in the freestanding Samsung wafer. The activation energy of O is found to be 33.20±0.1 meV. An unidentified third donor with an activation energy of 31.23±0.1 meV also was observed. Integrated absorption cross sections are found to be 8.5×10−14 cm for Si and 8.6×10−14 cm for O.

Structural and optical properties of thick (larger than 160 μm) freestanding hydride vapor phase epitaxy GaN templates have been investigated. AFM measurements showed that flat and smooth surface could be fabricated. High-resolution X-ray diffraction studies carried out with different spectrometer slit for the symmetric and asymmetric diffractions show that the linewidth increases with increasing slits width, indicating that a considerable degree of tilting and twisting of the individual grains are still present in these thick samples. Raman scattering measurements performed in a few samples indicate good crystalline quality and reduced strain. Very sharp and intense exciton related lines (FWHM less than 1 meV) have been observed in the low temperature photoluminescence spectra. Variable-temperature photoluminescence experiments were performed on both the growth surface and interface to identify the nature of the recombination processes observed in the luminescence spectra. FTIR absorption measurements show the presence of at least two donors with binding energy of 30.5 and 33.6 meV.

Low temperature photoluminescence experiments carried out on HVPE GaN samples produced in two different reactors are consistent with a low background level of donors and an extremely low concentration of compensating shallow acceptors. Infrared absorption experiments on these samples indicated that different shallow donors are incorporated at different rates in each reactor. High sensitivity SIMS was employed to measure the concentration of background impurities of a number of samples produced in both reactors. The comparison between the infrared absorption cross-sections with the SIMS results allows the identification of Si, the shallower, and O as the two dominant residual donors in these examples of HVPE GaN.

Growth pressure has a dramatic influence on the grain size, transport characteristics, optical recombination processes, and alloy composition of GaN and AlGaN films. We report on systematic studies which have been performed in a close spaced showerhead reactor and a vertical quartz tube reactor, which demonstrate increased grain size with increased growth pressure. Data suggesting the compensating nature of grain boundaries in GaN films is presented, and the impact of grain size on high mobility silicon-doped GaN and highly resistive unintentionally doped GaN films is discussed. We detail the influence of pressure on AlGaN film growth, and show how AlGaN must be grown at pressures which are lower than those used for the growth of optimized GaN films. By controlling growth pressure, we have grown high electron mobility transistor (HEMT) device structures having highly resistive (10 5 Ω-cm) isolation layers, room temperature sheet carrier concentrations of 1.2×10 13 cm −2 and mobilities of 1500 cm 2 /Vs, and reduced trapping effects in fabricated devices.

Infrared photoluminescence has been used to study the band-gap energy of InAs1−xSbx digital superlattices and band alignment of InAs1−xSbx/AlSb quantum wells at 5 K. It is found that the InAs1−xSbx digital alloys have a smaller effective band gap than InAs1−xSbx random alloys. In addition, the valence band offset between type-II InAs/AlSb is determined to be 130 meV. This number reduces as the Sb mole fraction in InAs1−xSbx is increased, and the alignment between InAs1−xSbx/AlSb becomes type I when x>0.15.

We present experimental flux-temperature phase diagrams for surface reconstruction transitions on the 6.1 Å compound semiconductors. The phase transitions occur within or near typical substrate temperature ranges for growth of these materials by molecular beam epitaxy and therefore provide a convenient temperature standard for optimizing growth conditions. Phase boundaries for InAs (0 0 1) [(2×4)→(4×2)], AlSb (0 0 1) [c(4×4)→(1×3)], and GaSb (0 0 1) [(2×5)→(1×3)] are presented as a function of substrate temperature and Group V-limited growth rate (proportional to flux), for both cracked and uncracked Group V species. We discuss differences between materials in the slopes and offsets of the phase boundaries for both types of Group V species.

Thick epitaxial layers of AlSb(Si) and AlSb(Be) were grown by molecular beam epitaxy and characterized by variable-temperature Hall/van der Pauw measurements. Si is shown to be predominantly an acceptor in AlSb, with an energy level 33±4 meV above the top of the valence band. Be is also an acceptor, with an energy level 38±4 meV above the top of the valence band. Be is a robust doping source for p-AlSb for carrier densities ranging from 1015 to 1019 cm−3. Background impurity levels in AlSb can be assessed by measuring the transport properties of lightly doped AlSb(Be) layers.

Heterostructures formed from III-V semiconductors with the 6.1 angstroms lattice spacing (InAs, GaSb, AlSb and related alloys) have attracted significant interest because of their potential to define a new `state of the art' in applications including 100 GHz high-speed logic circuits, terahertz transistors, sensitive infrared detectors and mid-infrared semiconductor lasers. In this paper, we describe the ongoing work at the Naval Research Laboratory to develop the materials growth and fabrication technology for a variety of 6.1 angstroms-based devices which have the potential to revolutionize infrared optoelectronics and low-power, high- speed electronics.

Transmission spectra of GaSb have been obtained over a temperature range from 10 to 470 °C. Using this information, transmission thermometry is applied to obtain accurate measurements of sample temperature during molecular beam epitaxy growth on GaSb substrates. A GaSb surface reconstruction transition is determined as a function of Sb flux and substrate temperature, establishing a laboratory-independent temperature standard.

The surface, structural, and optical properties of InAs/InGaSb/AlSb mid-infrared lasers grown by molecular beam epitaxy have been systematically studied, respectively, by Nomarski differential interference contrast, high-resolution x-ray diffraction, and variable-temperature photoluminescence. It is found that the optimum growth temperature is between 400 and 450 °C, based on the calibrated transmission thermometry. In addition, the impact of interfacial bond type and Sb sources has been investigated. A 5.91 μm laser, grown with the optimal growth parameters, exhibits a maximum cw operating temperature of 210 K.

The wide bandgap semiconductors Silicon carbide and gallium nitride have been studied by Raman scattering, photoluminescence, and infrared absorption. Raman scattering studies determined crystalline quality and polytype of bulk and thin film samples at room temperature. Photoluminescence studies were carried out as a function of temperature to detect and/or identify impurity centers and structure related defects and their role on the electronic properties of the materials. Infrared absorption spectroscopy was used to measure the binding energy of donors and their concentration.

The authors have studied the molecular-beam epitaxial growth of type-II heterostructures for mid-wavelength infrared lasers. Based on their photoluminescence spectra and X-ray diffraction patterns, it is found that the quality of these heterostructures is highly sensitive to the growth temperature and the interfacial bond type.