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Graphene is considered to be a promising candidate to replace indium tin oxide (ITO) as transparent conductive electrodes in optoelectronics applications. We use graphene films as transparent electrodes for THz applications such as phase shifters, reflector, and intensity modulators, leading to a low voltage operation and very small insertion loss. First, we experimentally demonstrate the excellent performance of THz modulators based on graphene/ionic liquid/graphene sandwich structures. The modulation covers a broadband frequency range from 0.1 to 2.5 THz with the modulation depth of up to 99% by applying a small gate voltage of 3 V. To our knowledge, this is the highest modulation ratio from graphene based THz devices to date. We also report a highly efficient tunable THz reflector in graphene. By applying a small gate voltage (up to ± 3 V), the reflectance of graphene is modulated from a minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures at room temperature, and the reflection tuning is uniform within a wide spectral range (0.1 – 1.5 THz). In addition, we design and fabricate terahertz phase shifters based on thin liquid crystal cells sandwiched by two graphene layers. A maximum 10.8 degree phase shift is obtained with 5 V voltage. The proposed phase shifters are fully electrical controllable, continuous tunable, and require very low DC voltages for operation. Finally, we show a high performance THz emitter based on ferromagnetic/nonmagnetic heterostructures. Our THz emitter based on nonmagnetic (NM) and ferromagnetic (FM) heterostructures has a peak intensity exceeding 500 μm thick ZnTe crystals (standard THz emitters). We have also fabricated the devices on flexible substrates with a great performance, and demonstrated that the devices can be driven by low power lasers.

In this paper we present results of photoluminescence (PL), photoluminescence excitation (PLE), and time resolved PL spectroscopy of the 4I13/2 → 4I15/2 transition in Er optical centers in GaN epilayers grown by metal-organic chemical vapor deposition. Under resonance excitation via the higher-lying inner 4f shell transitions and band-to-band excitation of the semiconductor host, the PL and PLE spectra reveal an existence of two types of Er optical centers from isolated and the defect-related Er centers in GaN epilayers. These centers have different PL spectra, local defect environments, decay dynamics, and excitation cross-sections. The isolated Er optical center, which can be excited by either excitation mechanism, has the same decay dynamics, but possesses a much higher cross-section under band-to-band excitation. In contrast, the defect-related Er center can only be observed through band-to-band excitation but has the largest crosssection. Our results indicate pathways for efficient optical excitation of Er-doped GaN semiconductors.

Details are presented concerning the growth of GaN and InGaN epilayers by metal-organic chemical vapor deposition and their in situ doping with erbium (Er). Structural properties of the Er-doped epilayers were examined using X-ray diffraction, atomic force microscopy, and secondary ion mass spectrometry. The photoluminescence characteristics of these films were measured and strain-induced effects were identified. Multilayer structures were processed into prototype light-emitting diodes that yielded an infrared emission at 1540 nm. Magnetic measurements of the Er-doped films were made at room temperature and hysteretic behavior was observed. Resonant excitation spectroscopy combined with an applied magnetic field, showed that the Er ions enter into a defect-related complex with critical impact on the optical and magnetic properties.

Doping of GaN and InGaN epilayers with Mn and FE has been achieved by metal organic chemical vapor deposition (MOCVD) and solid-state diffusion. Hysteretic behavior was observed at room temperature and structural measurements, X-ray diffraction, and atomic force microscopy gave no evidence of second phases in the transition metal-doped epilayers. Multilayer structures grown by MOCVD were used to determine the dependence of magnetic ordering in GaMnN epilayers upon the availability of free holes. Based on these results a prototype spin device was designed and fabricated in which holes, adjacent to a GaMnN layer, could be modulated by an applied electric field. The device demonstrated that the transition from paramagnetic to ferromagnetic behavior could be switched on and off electrically at room temperature.

Spin-dependent phenomena in ZnO may lead to devices with new or enhanced functionality, such as polarized solid-state light sources and sensitive biological and chemical sensors. In this paper, we review the experimental results on transition metal doping of ZnO and show that the material can be made with a single phase at high levels of Co incorporation (~15 at.%) and exhibits the anomalous Hall effect. ZnO is expected to be one of the most promising materials for room-temperature polarized light emission; but to date, we have been unable to detect the optical spin polarization in ZnO. The short spin relaxation time observed likely results from the Rashba effect. Possible solutions involve either cubic phase ZnO or the use of additional stressor layers to create a larger spin splitting in order to get a polarized light emission from these structures or to look at alternative semiconductors and fresh device approaches

Silicon photonics is rapidly growing and a number of Si-based active and passive components have recently been demonstrated. We demonstrate new functionality of Er-doped silicon: a memory effect in electroluminescence. This finding opens a prospect of necessary, and thus far not available, component for Si optoelectronics-a fully complimentary metal-oxide-semiconductor-compatible electro-optical converter with a memory function, operating in the technologically important 1.5-mum band. When developed and optimized, prospect applications could include optical intraand inter-chip connectors and volatile flash memory elements

Fundamental electrical and optical properties of strained wurtzite InGaN/GaN-based quantum-well light-emitting diodes are calculated based on the Rashba–Sheka–Pikus Hamiltonian in the vicinity of the Γ point. It is found that the strain and the strain-induced piezoelectric field significantly alter the subband structure and determines the output intensity of the nitride quantum well light emitting diodes. For the case with high In composition (≳0.2), the calculation also supports the possibility of strain relaxation in the quantum well. Coupled with an optimized set of parameters, our theoretical model provides an excellent agreement with the available experimental data over a wide range of In composition (0–0.5).

Electrical and optical properties and the spectra of deep hole traps in p-GaN films implanted with high doses (3 × 1016 cm−3) of Co,Mn, Fe and Cr and annealed at 700 °C are reported. The dominant deep traps generated by this implantation are of the same type as observed in similar films heavily implanted with protons. The magnitude of the changes in the conductance and transmission of the GaN correlates with the atomic mass of the transition metal ions and the density of primary radiation defects. For fabrication of spintronic devices such as spin-LEDs using implantation of the TM species into the top p-GaN contact layer, the best results should be expected using Cr implantation since this produces both room temperature ferromagnetism and the smallest reduction in conductivity of the p-GaN.

Recent results on achieving ferromagnetism in transition-metal-doped GaN, A1N and related materials are discussed. The field of semiconductor spintronics seeks to exploit the spin of charge carrier ...

Both ion implantation and epitaxial crystal growth provide convenient methods of introducing transition metals such as Mn,Cr,Fe,Ni and Co into GaN, GaP, SiC and ZnO for creating dilute magnetic semiconductors exhibiting room temperature ferromagnetism. In this paper we review progress in wide band gap ferromagnetic semiconductors and the role of defects and doping on the resulting magnetic properties.

Electrical properties, room temperature optical absorption bands and 300 and 90 K microcathodoluminescence (MCL) bands were studied in heavily Mn and Co implanted ZnO samples annealed at 700 °C. Samples processed under the same conditions have previously shown ferromagnetic behavior. The Mn and Co implanted and annealed regions remain n-type and have a higher electron concentration than in control samples. Therefore, the spin alignment in ZnMnO and ZnCoO is not mediated by the classical mechanism involving free holes. Optical absorption bands near 1.9 eV (Co) and 2 eV (Mn) and the MCL bands near 1.84 eV (Co) and 1.89 eV (Mn) are found to be associated with transition metal (TM) ions. These bands are tentatively attributed to internal transitions between the levels of the substitutional TM ions. In addition to those TM ions related bands we also observed strong absorption bands at 0.75, 1.4, 2.35 and 2.5 eV and MCL bands near 2.3 and 2.5 eV. All these bands are related to radiation damage defects.

We present experimental evidence of an optical memory effect in crystalline silicon doped with Er3+ ions. It is observed at low temperature using two-color experiments in the visible and the mid-infrared (with a free-electron laser). Based on the physical mechanism governing the effect, possibilities for improvement of thermal stability and increase of archival time are discussed. An all-optical all-silicon memory element for use in photonic circuits is proposed.

Structural and magnetic characteristics of Ni-implanted GaP were measured for doses in the range 3/5/10 16 cm 2 . After subsequent annealing at 700 8C, transmission electron microscopy (TEM) showed residual lattice damage which was more significant in highly carbon-doped epi layers as compared to bulk GaP substrates. The magnetization measurements showed two different contributions, one present at 5/75 K and the other below � /225 K. No secondary phase formation was detected in either type of GaP. # 2002 Elsevier Science B.V. All rights reserved.

Standard variable angle spectroscopic ellipsometry (VASE) has been employed to study the ordinary optical dielectric response of hexagonal gallium nitride (GaN) thin films—an important material for blue and ultraviolet light emitting device applications. The GaN films were grown by molecular beam epitaxy on c-plane sapphire substrates (α-Al2O3). Room temperature isotropic and anisotropic mode VASE measurements were made at angles of incidence between of 20° and 80°. Evidence of anisotropy was observed from the anisotropic mode measurements, reflecting the nature of wurtzite crystal structure of GaN. The sizable off-diagonal elements (Aps and Asp) of the Jones matrix indicate that the optical axis 〈c〉 of the c-plane sample are slightly off from the surface normal due to a small miscut of substrates. VASE data simulations by isotropic and anisotropic models indicate that the anisotropic effect on both diagonal and off-diagonal elements of the Jones matrix can be minimized to a negligible level at small angl...

Semiconductors doped with rare earth atoms have been studied for more than a decade because of the potential of using them to develop compact and efficient electroluminescence (EL) devices. Trivalent erbium ions (Er 3+ ) are of special interest because they exhibit atomic-like transitions centered at 1540 nm, which corresponds to the low-loss window of silica-based optical fibers. While EL devices, based on Er-doped Si and GaAs materials, have been fabricated, their efficiency remains too low for practical applications. Several years ago an important observation was made that there was less detrimental temperature quenching of Er luminescence intensity for larger bandgap host materials. Therefore, Er-doping of wide gap semiconductors, such as the III–V nitrides, appears to be a promising approach to overcoming the thermal quenching of Er luminescence found in Si and GaAs. In particular, GaN epilayers doped with Er ions have shown a highly reduced thermal quenching of the intensity of the Er luminescence from cryogenic to elevated temperatures. The remarkable thermal stability of the light emission may be due to the large energy bandgap of the material, as well as to the optical inactivity of the material defects in the GaN films. In this paper, recent data concerning the luminescence characteristics of Er-doped GaN thin films are presented. Two different methods have been used for Er-doping of the GaN films: ion implantation and in situ doping during epitaxial growth. Both methods have proven successful for incorporation and optical activation of Er 3+ ions. Infrared photoluminescence spectra, centered at 1540 nm, have been measured for various Er-doped III–N films. Considerably different emission spectra, with different thermal quenching characteristics, have been observed, depending upon the wavelength of the optical pump and the Er-doping method. Defect-related absorption centers permit excitation of the Er ions using below-bandgap optical sources. Elemental impurities, such as O and C, in the thin films have also been shown to influence the emission spectra and to lead to different optical characteristics.

Erbium (Er) doped semiconductors are of interest for light-emitting device applications operating at around 1.55 μm and for the potential integration with other semiconductor devices. However, the optical emission of Er3+ ions in semiconductors has not been as efficient as in dielectric materials, particularly at room temperature. This may be because ionic bonds, which are characteristic of dielectrics, are better suited for forming the required Er3+ energy levels than are covalent bonds, which are present in most III-V semiconductors. In this paper, we report 1.55 μm emission from an Er-doped GaN LED. We also discuss the effect of the measurement temperature on the emission spectrum as well as the effect of sample annealing on the emission spectrum.

GaN, AIGaN and InGaN films have been grown by molecular beam epitaxy (MBE) using RF plasma sources for the generation of active nitrogen. These films have been deposited homoepitaxially onto GaN/SiC substrates and hetero-epitaxially onto LiGaO 2 substrates. LiGaO 2 is an ordered and closely-lattice-matched orthorhombic variant of the wurtzite crystal structure of GaN. A low-temperature AIN buffer layer is necessary in order to nucleate GaN on LiGaO2. Thick GaN and AIGaN layers may then be grown once deposition is initiated. InGaN has been grown by MBE at mole fractions of up to 20% as a quantum well between GaN cladding layers. The indium containing structures were deposited onto GaN/SiC substrates to focus the development effort on the InGaN growth process rather than on heteroepitaxial nucleation. A modulated beam technique, with alternating short periods of (In, Ga)N and (Ga)N, was used to grow high-quality InGaN. The modulated beam limits the nucleation of metal droplets on the growth surface, which form due to thermodynamic limitations. A narrow PL dominated by band edge luminescence at 421 nm results from this growth technique. Growth of GaN at high temperatures is also reported.

Several kinds of nitride-based micro-resonators have been fabricated. Firstly, a microdisk laser structure comprising three InGaN GaN quantum wells on a thick AlN buffer has been grown by metal-organic molecular beam epitaxy and fabricated using a combination of non-selective Cl2/CH4/H2/Ar dry etching and selective KOH-based wet etching of the AlN. These structures are of potential use in short wavelength photonic or optoelectronic circuits. In a second structure Er was implanted into a GaN layer to produce strong emission at 1.54 μm. Similar results have been obtained in Er-implanted AlN, and AlN doped during epitaxial growth.

AlN epilayers grown by metalorganic chemical vapor deposition were implanted with cobalt ions and studied by deep UV photoluminescence (PL). A PL emission peak at 5.87 eV (at 10 K) was observed for the Co-implanted AlN epilayers, which was absent in as-grown AlN epilayers. Temperature dependence of the PL intensity of the 5.87 eV emission line revealed an ion-implantation induced defect with energy level of about 260 meV below the conduction band. The 5.87 eV emission line is believed due to a band-to-impurity transition involving the nitrogen vacancy (VN) in ion-implanted AlN. The experimentally determined energy level of the nitrogen vacancy is in reasonable agreement with the calculated value of 300 meV. From the band-to-impurity transition involving VN, we have deduced the energy band gap of AlN to be 6.13 eV, which is consistent with our previous result. Our results suggest that nitrogen vacancies in AlN cannot make any significant contribution to the n-type conductivity due to the large binding ener...

We report on the synthesis of Er-doped III–N double heterostructure light-emitting diodes (LEDs) and their electroluminescence (EL) properties. The device structures were grown through a combination of metalorganic chemical vapor deposition (MOCVD) and molecular-beam epitaxy (MBE) on c-plane sapphire substrates. The AlGaN layers, with an Al concentration of ∼12%, were prepared by MOCVD and doped with Si or Mg to achieve n- and p-type conductivity, respectively. The Er+O-doped GaN active region was grown by MBE and had a thickness of 50 nm. The Er concentration was estimated to be ∼1018 cm−3. The multilayer n-AlGaN/GaN:Er/p-AlGaN structures were processed into LEDs using standard etching and contacting methods. Several different LEDs were produced and EL spectra were recorded with both forward and reverse bias conditions. Typically, the EL under reverse bias was five to ten times more intense than that under forward bias. The LEDs displayed a number of narrow emission lines representative of the GaN:Er sys...

Optoelectronic materials doped with Er atoms are receiving widespread attentions due to their impact on optical communication systems operating at 1.54 μm. Optical amplifiers based on Er-doped fibers have demonstrated major improvements in link distance, data rates and reduced needs for signal regeneration. III–V semiconductors doped with Er offer the prospect of very stable, temperature-insensitive, laser diodes emitting at 1.54 μm. This paper provides a review of the luminescence characteristics of III–V semiconductors doped with Er atoms. Aspects of Er incorporation in the III–V crystal host, photoluminescence properties, and prototype electroluminescent devices are addressed. Details of some of the first observations of photoluminescence of Er atoms in III–V nitride semiconductors, in particular GaN epilayers, are discussed. The GaN epilayers were optically excited using an argon-ion laser and spectra, centered at 1.54 μm, were observed at 6, 77 and 300 K. The spectra display many of the allowed transitions typical of the Er 3+ configuration and are nearly as intense at room temperature as at 77K. This result indicates that the wide bandgap III–V semiconductors may be ideal host materials for Er-doped electroluminescent devices.

We report on the photoluminescence (PL) properties of in situ Tm-doped AlxGa1−xN films (0⩽x⩽1) grown by solid-source molecular-beam epitaxy. It was found that the blue PL properties of AlxGa1−xN:Tm greatly change as a function of Al content. Under above-gap pumping, GaN:Tm exhibited a weak blue emission at ∼478 nm from the 1G4→3H6 transition of Tm3+. Upon increasing Al content, an enhancement of the blue PL at 478 nm was observed. In addition, an intense blue PL line appeared at ∼465 nm, which is assigned to the 1D2→3F4 transition of Tm3+. The overall blue PL intensity reached a maximum for x=0.62, with the 465 nm line dominating the visible PL spectrum. Under below-gap pumping, AlN:Tm also exhibited intense blue PL at 465 and 478 nm, as well as several other PL lines ranging from the ultraviolet to near-infrared. The Tm3+ PL from AlN:Tm was most likely excited through defect-related complexes in the AlN host.

Ion implantation of Mn or Cr at concentrations of 1–5 at. % were performed in AlxGa1−xP (x=0.24,0.38) epilayers grown by gas source molecular beam epitaxy. Ferromagnetic-like ordering above 100 K for Cr and 300 K for Mn was observed in superconducting quantum interference device measurements. Structural characterization revealed no second phases that could influence the measured magnetic properties. As the AIP mole fraction in the AlxGa1−xP layers increased, the magnetic ordering temperatures were generally observed to increase, while the calculated magnetic moment decreased. Mn appears to be a more promising choice than Cr for high temperature ferromagnetism in AlGaP.

Precision reflection measurements were performed on GaAs/AlAs superlattices of the same composition but different layer spacings. Nonlinear-least-squares fits to the data were performed to a single layer. Measurements were extracted for the superlattice thickness, thickness of a disturbed interface layer between the superlattice and substrate, the uniformity in composition and/or spacing and the composition. It was demonstrated that these nondestructive measurements in the infrared region (3000 to 12 000 cm−1) in conjunction with a simple single layer model are capable of accurately yielding the above quantities with high precision.

Solid-state lasers operating in the mid-IR wavelength region (3-5 mum) continue to be of great current interest for applications in laser remote sensing of chemical and biological agents, IR countermeasures, and bio-medical procedures. The development of mid-IR solid-state lasers based on oxide and fluoride crystals is hampered by large non-radiative decay rates through multi-phonon emission. On the contrary, rare earth (RE) doped crystals with low maximum phonon energies have shown efficient mid-IR emission as well as lasing at room temperature [1-9]. Examples of emission transitions in the 3-5 mum spectral region are indicated in Fig. 1 for the rare earth ions Pr3+, Nd3+, Dy3+ and Er3+.

A complete structural phase diagram of annealed proton-exchanged X-cut LiTaO3 is presented, to relate refractive index change to proton-induced lattice deformation. From this diagram, explanations for the previously observed phenomena of index increase upon annealing and index decrease with increasing proton concentration (above a certain value) are derived. We have analyzed the evolution of index profiles during postexchange annealing, describing how the presence of multiple phases within a waveguide can lead to large index variations and buried index profiles. Finally, we identify at least two high-concentration phases that exhibit large temporal instabilities.

1.54 μm photoluminescence has been observed from GaN doped with Er during growth by metalorganic molecular beam epitaxy. Strong Er3+-related photoluminescence (PL) was measured at room temperature for GaN:Er doped during growth on c-plane Al2O3 and Si. Experiments to evaluate the effects C and O on the optical activity of Er indicated that these impurities dramatically enhance Er PL in GaN. GaN films doped with Er to a concentration of 3×1018 cm−3 with [O]∼1020 cm−3 and [C]∼1021 cm−3 luminesce at 1.54 μm with an intensity ∼2 orders of magnitude greater than films with oxygen and carbon backgrounds of less than 1019 cm−3. The thermal PL quenching behavior was also markedly different for samples of varying O and C content. Er3+ luminescence from samples with high O and C concentrations quenched by only 10% between 15 and 300 K while the integrated PL signal from the samples with lower [O] and [C] quenched ∼85% over the same temperature range.

Experimental results on a new type of light-emitting device, the light-emitting triode (LET), are presented. The LET is a three-terminal p-n junction device that accelerates carriers in the lateral direction, i.e. parallel to the p-n junction plane, by means of an electric field between two anodes. The lateral field provides additional energy to carriers thereby allowing them to overcome barriers and increasing the carrier injection efficiency into the active region. LETs were fabricated using a ultraviolet LED structure that has an AlGaN/GaN superlattice in the p-type confinement region for high-conductivity 2 dimensional hole gas. LET mesa structures were obtained by standard photolithographic patterning followed by chemically-assisted ion-beam etching using Cl2 and Ar to expose the n-type cladding layer. The n-type contact was fabricated by electron-beam evaporation of Ti/Al/Ni/Au. Ni/Au (50/50 A) metallization was deposited for both anodes, Anode 1 and Anode 2, and subsequently annealed at 500 °C in an O2 ambient. It is shown that both the current between Anode 1 and the cathode, and the light-output power increase with increasing negative bias to the Anode 2. This is consistent with the expectation that a negative bias to the second anode allows carriers to acquire a high kinetic energy thereby enabling them to overcome the barrier for holes, resulting in high injection efficiency into the active region that lies beyond the barrier.

Publisher Summary This chapter reviews the experimental results on transition metal doping of zinc oxide (ZnO) and the current state of theories for ferromagnetism. It is important to re-examine some of the earlier concepts for spintronics devices, such as the spin field-effect transistor, to account for the presence of the strong magnetic field, which has deleterious effects. In some of these cases, the spin device appears to have no advantage relative to the conventional charge-control electronic analog. The most promising materials for room temperature polarized light emission are expected to be gallium nitride (GaN) and ZnO, but progress on realizing devices has been disappointing. To date it has been difficult to detect optical spin polarization in ZnO. The short spin-relaxation time observed likely results from the Rashba effect. Possible solutions involve either cubic phase ZnO or use of additional stressor layers to create a larger spin-splitting in order to get polarized light emission from these structures or to look at alternative semiconductors and fresh device approaches.

Results of a photoluminescence (PL) and photoluminescence excitation (PLE) study of Er implanted GaN are presented. Upon optical excitation at 325 and 488 nm, we observed strong 1.54 μm Er3+ PL which remained temperature stable from 15 to 550 K. At 550 K, the integrated PL intensity decreased by ∼10% for above gap excitation (λex=325 nm) and ∼50% for below gap excitation (λex=488 nm) relative to its value at 15 K. The excellent temperature stability makes GaN:Er very attractive for high temperature optoelectronic device applications. PLE measurements were conducted to gain insight into the Er3+ excitation mechanisms in the GaN host. The PLE results show that Er3+ can be excited continuously over a broad wavelength region spanning from 425 to 680 nm. In addition, sharp PLE features were observed at approximately 495, 525, 553, 651, and 980 nm. The PLE spectrum suggests that optically active Er3+ ions can be excited either through carrier-mediated processes involving defects in the host or through resonant ...

We present a photoluminescence (PL) study of Er-doped AlN epilayer on sapphire substrate. The AlN : Er film was grown by metalorganic molecular beam epitaxy and an Er concentration of 2–5 × 1019Er/cm3 was obtained. Following the excitation of an argon ion laser at 488 nm, we observed a strong 1.54 μm Er3+ luminescence, which is quenched by only a factor of two between 15 K and room temperature. The photoluminescence excitation (PLE) spectrum, as well as PL lifetime measurement suggest that at 488 nm, Er3+ is excited through a photo-carrier mediated process. In contrast, exciting AlN : Er at ~525 nm seems to result in the direct excitation of an intra-4f transition of Er3+.

The aim of this chapter is to discuss ferromagnetism in gallium nitride (GaN) and related materials. This chapter elucidates recent results on achieving ferromagnetism in transition metal-doped GaN, AlN, and related materials. While current generations of semiconductor electronic and photonic devices utilize the charge on electrons and holes to perform their specific functionality such as signal processing or light emission, the field of semiconductor spintronics seeks to exploit the spin of charge carriers in new generations of transistors, lasers, and integrated magnetic sensors. There is strong potential for new classes of ultralow power, high-speed memory, logic, and photonic devices. The utility of such devices depends on the availability of materials with practical magnetic ordering temperatures and most theories predict that the Curie temperature will be a strong function of bandgap. This chapter discusses the current state-of-the-art in producing ferromagnetism in GaN and related materials, exhibiting room temperature ferromagnetism, the origins of the magnetism, and its potential applications.

Measurement of the room temperature forward bias current‐voltage behavior of InGaN/AlGaN double heterostructure blue light‐emitting diodes demonstrates a significant departure from the usual Is exp(qV/ nkT) behavior where n is the ideality factor which varies between 1 and 2. The observed current‐voltage behavior at room temperature may be represented as I=2.7×10−11 exp(5.7V) which suggests a tunneling mechanism. Measurement of the electroluminescence for currents from 0.5 to 100 mA demonstrates that the emission peak shifts to higher energy while increasing in intensity. The shifting peak spectra is due to band filling, a process which results from the injection of holes via tunneling into an empty acceptor impurity band and vacant valence band tails. At currents near 100 mA, a non‐shifting band‐to‐band emission approaches the intensity of the shifting peak spectra. The active layer of these diodes is codoped with both the donor Si and the acceptor Zn.

In this paper, we report on the growth of in-situ Tm-doped Al/sub x/Ga/sub 1-x/N films and the corresponding effect of Al composition on the EL emission. PL and CL show almost same trend as EL with various Al compositions. The 465-nm emission is barely present at x=0.16, it becomes very clear for x/spl ges/0.39, and it dominates for x/spl ges/0.81. The EL emission at 802 nm experienced the opposite trend, decreasing with Al composition. We have confirmed that blue EL emission becomes dominant over IR emission with increasing Al composition in the Al/sub x/Ga/sub 1-x/N host.

Some examples of recent advances in enhancing or adding functionality to GaN and SiC devices through the use of novel processing techniques are discussed. The first example is the use of ion implantation to incorporate transition metals such as Mn, Cr and Co at atomic percent levels in the wide bandgap semiconductors to produce room temperature ferromagnetism. A discussion is given of the phase space within which single-phase material can be obtained and the requirements for demonstrating the presence of a true dilute magnetic semiconductor. The ability to make GaN and SiC ferromagnetic leads to the possibility of magnetic devices with gain, spin FETs operating at low voltages and spin polarized light emitters. The second example is the use of novel oxides such as Sc 2 O 3 and MgO as gate dielectrics or surface passivants on GaN. True inversion behavior has been demonstrated in gated MOS-GaN diodes with implanted n -regions supplying the minority carriers need for inversion. These oxide layers also effectively mitigate current collapse in AlGaN/GaN HEMTs through their passivation of surface states in the gate–drain region. The third example is the use of laser drilling to make through-wafer via holes in SiC, sapphire and GaN. The ablation rate is sufficiently high that this maskless, serial process appears capable of achieving similar throughput to the more conventional approach of plasma etching of vias. The fourth example is the use of either ungated AlGaN/GaN HEMTs or simple GaN and SiC Schottky diodes as sensors for chemicals, biogens, radiation, combustion gases or strain. The sensitivity of either the channel carrier density or the barrier height to changes in surface condition make these materials systems ideal for compact robust sensors capable of operating at elevated temperatures.

GaN:Er prepared by metalorganic molecular beam epitaxy shows intense 1.54 /spl mu/m photoluminescence (PL) at room temperature. Efficient below-gap absorption bands have been identified using photoluminescence excitation spectroscopy. An infrared hybrid InGaN-GaN:Er LED operating at 1.54 /spl mu/m has been demonstrated. Even though the resulting emission at 1.54 /spl mu/m was weak, it shows that the electroluminescence from InGaN can be used to excited Er ions in GaN.

Light emission characteristics from silicon nanoparticles consisting of a crystalline core encased in an amorphous oxide shell are presented. The particles were thermally oxidized in the open atmosphere at 800 °C for times from 5 to 160 min in order to decrease the Si core dimensions. Photoluminescence spectra, at low excitation levels, reveal that the light emission shifts to shorter wavelengths as the oxidation time is increased. At high excitation levels, photoluminescence spectra show little or no shift. These results indicate that there are at least two mechanisms involved with light emission from Si nanoparticles, one associated with quantum size effects and another which is independent of size distribution.

New developments in dry and wet etching, ohmic contacts and epitaxial growth of III-V nitrides are reported. These make possible devices such as microdisk laser structures and GaAs/AlGaAs heterojunction bipolar transistors with improved InN ohmic contacts.

We have studied the behavior of the extraordinary refractive index in a set of annealed z‐cut proton‐exchanged lithium niobate (LiNbO3) substrates by means of optical prism‐coupling measurements and numerical simulations. Values of the index as a function of depth have been determined and fitted with a single Gaussian diffusion expression. At 400 °C, the effective index diffusivity Dn of 4.6×10−12 cm−2/s is nearly identical to the diffusivity of substitutional H in these samples. Furthermore, we find a linear relationship between the increase in the extraordinary refractive index and the substitutional H concentration. At a wavelength of 0.6238 μm, the coefficient for this relationship is ∼1.5×10−23 (H cm−3)−1.

Publisher Summary This chapter presents a review of the luminescence characteristics of wide gap compound semiconductors doped with rare earth atoms. In particular, aspects of rare earth atom incorporation in the semiconductor crystal, photoluminescence properties, and prototype electroluminescent devices are addressed in the chapter. Intra-subshell transitions of 4f electrons in rare earth ions lead to narrow absorption peaks in the ultra-violet, visible, and near-infrared regions of the electromagnetic spectrum. Because it appears that the intensity of the room temperature light emission of these ions depends upon the energy bandgap of the host material, wide gap semiconductors may prove to be the best materials for optoelectronic device applications. Prototype electroluminescent (EL) devices have been fabricated based on both low gap semiconductors and wide gap semiconductors. In general, the emission efficiencies need to be improved. The use of wide gap semiconductors leads to less thermal quenching of the electroluminescence. However, O doped Si does lead to state-of-the-art EL devices. Because of the intense current research in SiC and III–V nitride semiconductors, improvements in the crystal quality and in the processing technology of these materials are likely to occur. Such advances will assist efforts to develop EL devices based on wide gap semiconductors doped with RE ions.

Optical anisotropy of sapphire have been studied by both polarized Raman scattering and transmission variable angle spectroscopic ellipsometry (TVASE). The polarized Raman effect was measured on both c-plane (0001) and a-plane (2110) sapphire substrates. Two TO phonons (379 and 431 cm−l) and four LO phonons (418, 450, 577, and 750 cm−1) were observed when the optical axis is perpendicular to the polarization of the incident laser beam (for both c-plane and a-plane). While only two TO phonons (379 and 431cm−1) and two LO phonons (418 and 645 cm−1) can be seen when the optical axis is parallel to the polarization for an a-plane sapphire. The optical axis of the a-plane sapphire can then be quickly determined by either maximizing or minimizing the 645 LO peak intensity with about ±10° error. TVASE measurements were carried out in the energy range of 0.75eV to 5.8eV at room temperature. Sizable off-diagonal Jones matrix elements Apst, and Aspt can be detected even with the optical axis 1° off the X-axis. This indicates that TVASE has a high sensitivity to optical anisotropy. By minimizing these off-diagonal elements and combining the Raman analysis, the optical axis orientation of an a-plane or m-plane (0110) sapphire can therefore be fully determined with an error of less than 1°.

AIN(Er) doped with Er during Metal Organic Molecular Beam Epitaxy has been plasma hydrogenated in-situ at 200–250°C using an Electron Cyclotron Resonance source. By isotopic substitution of 2H for 1H, we have found from Secondary Ion Mass Spectrometry profiling that a 30 min hydrogenation treatment can incorporate ˜2×1019cm−3 deuterium atoms to depths ≥1μm. The intensity of the 1.54μm Er3+ luminescence is increased by a factor of ˜5 by the 200°C hydrogenation, and this effect is thermally stable to 300°C, indicating a binding energy of >1.5eV for hydrogen at defects in the AIN. These defects would normally either be recombination centers or provide an alternative de-excitation path for the Er. We have previously found that AIN provides the best resistance to thermal quenching of Er luminescence of any semiconductor due to its wide bandgap. Together, these results suggest that AIN(Er) may be a promising material for optical control of devices such as light-triggered SiC or GaN thyristors for power switching applications, where a fiber-transmitted signal from temperaturetolerant material is necessary in controlling power distribution grids. Hydrogen does not leave the AIN until ˜800°C and presumably forms an intermediate state such as H2 or larger clusters prior to evolution from the surface, and again this stability is among the best for any semiconductor.