Z. LILIENTAL-WEBER, J. JASINSKI, and D.N. ZAKHAROV
In this paper, defects formed in GaN grown by different methods are reviewed. Thin GaN films were grown on c-, m-, and a-planes on a number of substrates and typical defects as characterized by transmission electron microscopy are described. For polar epilayers grown on c-plane sapphire the typical defects are dislocations (edge, screw and mixed). The lowest dislocation density was obtained for homoepitaxial growth using molecular beam epitaxy (MBE) or hydride vapour phase epitaxy (HVPE). In these cases, the core structure of screw dislocations were studied in detail. In both cases, the cores are full. In the layers grown by HVPE the dislocations are decorated by pinholes stacked on top of each other. These pinholes are empty inside and their formation is attributed to impurities (oxygen) present in these layers. In these layers Ga-rich cores have been found. These were not observed in the layers grown by MBE on the top of the HVPE templates. Epilayers grown in non-polar directions (m- or a-plane) have a high density of planar defects (stacking faults) terminated by partial dislocations. Only low energy faults were found. The majority of these faults are formed at the interface with the substrate and propagate to the sample surface.
Keywords: GaN films, MBE growth, HVPE, dislocations, polar and non-polar growth.
J. ANTOSZEWSKI and L. FARAONE
For modern semiconductor heterostructures containing multiple populations of distinct carrier species, conventional Hall and resistivity data acquired at a single magnetic field provide far less information than measurements as a function of magnetic field. However, the extraction of reliable and accurate carrier densities and mobilities from the field-dependent data can present a number of difficult challenges, which were never fully overcome by earlier methods such as the multi-carrier fit, the mobility spectrum analysis of Beck and Anderson, and the hybrid mixed-conduction analysis. More recently, in order to overcome the limitations of those methods, several research groups have contributed to development of the quantitative mobility spectrum analysis (QMSA), which is now available as a commercial product. The algorithm is analogous to a fast Fourier transform, in that it transforms from the magnetic field B domain to the mobility µ domain. QMSA converts the field-dependent Hall and resistivity data into a visually-meaningful transformed output, comprising the conductivity density of electrons and holes in the mobility domain. In this article, we apply QMSA to both synthetic and real experimental data that are representative of modern semiconductor structures.
Keywords: resistivity, Hall effect, carrier mobility, carrier density, Fourier transform.
In-depth and in-plane profiling of light emission properties fromsemiconductor-based heterostructures
M. GODLEWSKI, T. WOJTOWICZ, E.M. GOLDYS, M.R. PHILLIPS, R. CZERNECKI, P. PRYSTAWKO, M. LESZCZYNSKI, P. PERLIN, I. GRZEGORY, S. POROWSKI, T. BOTTCHER, S. FIGGE, and D. HOMMEL
Cathodoluminescence (CL) technique is applied for evaluation of in-depth and in-plane variations of light emission from semiconductor heterostructures, including laser diode structures. Light emission properties of heteroepitaxial and homoepitaxial structures are studied. We demonstrate possibility of in-depth profiling of complicated multi quantum well structures, which allows us to evaluate light emission characteristics from different regions of, e.g., laser structures. Due to this property of the CL, we can evaluate interconnections between structural quality of the samples and light emission characteristics. Stimulated emission under electron beam pumping is achieved in a conventional CL set up for selected heterostructures. Threshold currents for stimulated emission are evaluated from the CL investigations. We demonstrate that potential fluctuations are not fully screened in the active regions of laser structures, even at large excitation densities.
Keywords: semiconductors, heterostructures, cathodoluminescence, depth-profiling, defect distribution, laser emission.
Surface emitting quantum cascade lasers
C. PFLUGL, M. AUSTERER, W. SCHRENK , S. GOLKA, and G. STRASSER
We present high power surface-emitting single mode GaAs-based quantum cascade lasers (QCLs) in the mid-infrared. By using an air-AlGaAs waveguide combined with second-order distributed feedback processing, we obtained optical output via the surface above 3 W. Surface-normal dual-lobe light emission exceeds the emission from one as-cleaved facet by a factor of six.
Keywords: optoelectronic devices, semiconductor laser.
Growth and scintillation properties of Ce-doped PrF3 single crystals
K. KAMADA, A. YOSHIKAWA, T. SATONAGA, H. SATO, A. BEITLEROVA, M. NIKL, N. SOLOVIEVA, and T. FUKUDA
The scintillation characteristics of the Ce-doped PrF3 single crystals have been studied to find out their potential as for heavy scintillators. Crystal growth was performed in a vacuum-tight micro-pulling down (µ-PD) system. PrF3 crystal was prepared with various concentrations (0, 0.1, 0.5, 1, 3, 5, 10, 20, 60, 80, 100%) of Ce3+. The crystals were transparent (CeF3) or of greenish colour, 3 mm in diameter and 15-50 mm in length. Neither visible inclusions nor cracks were observed. At room temperature, the radio- and photoluminescence spectra and the decay kinetics were measured for the sample set.
Keywords: micro-pulling-down method, fluoride, single crystal, luminescence, scintillator, Ce3+.
Spatial separation of recombining carriers within nitride GaN/(AlGa)N quantum wells induced by piezoelectric phenomena
J. GALCZAK, R.P. SARZALA and W. NAKWASKI
Nitride AIIIN materials manifest very strong piezoelectric effects. Their piezoelectric coefficients are one order of magnitude higher than those in similar AIIIBV semiconductors. Therefore stress fields, generated for example at nitride heterojunctions, because of different lattice constants of their both components, are followed by an additional piezoelectric polarization and some local curving of band edges. In quantum wells (QWs), on the other hand, this stress-related piezoelectric polarization and, additionally, the spontaneous polarization are sources of an electric field which causes the so-called quantum confined Stark effect, leading to an effective band-gap shrinkage and spatial separation of electrons and holes. Both the above phenomena influence considerably recombination processes in QW devices. The last effect, i.e. the stress-induced spatial separation of electrons and holes confined within nitride QWs, is investigated theoretically in the present paper. Electron transitions from its ground state to the second heavy-hole state have been found to play much more considerable role in recombination phenomena within wider nitride QWs than it has been expected.
Keywords: polarization phenomena, k°p method, nitride quantum-well devices, envelope functions.
Influence of defect traps and inhomogeneities of SiC crystals and radiation detectors on carrier transport
V. KAZUKAUSKAS and J.-V. VAITKUS
We present investigation of carrier traps and their transport in 4H-SiC single crystals and high energy radiation detectors. SiC detectors have been produced from bulk vanadium-compensated semi-insulating 4H-SiC single crystal. They were supplied with a nickel ohmic contact on the back surface and titanium Schottky contact on the front surface. The prevailing defect levels were revealed by means of thermally stimulated current (TSC) and thermally stimulated depolarization (TSD) methods and their advanced modification ń multiple heating technique. From I-V measurements of the samples a barrier height of ~ 1.9 eV was found. In 4H-SiC:Va, the following thermal activation values were deduced: 0.18-0.19 eV, 0.20-0.22 eV, 0.33-0.41 eV, and 0.63 eV. The maximum with activation energy of 0.33-0.41 eV appears below 125 K and most probably is caused by the thermal carrier generation from defect levels. In contrast, the first two maxima with the lowest activation energies, which nevertheless appear at higher temperatures, are likely associated with material inhomogeneities causing potential fluctuations of the band gap. The existence of different polarization sources in different temperature ranges is also demonstrated by TSD.
Keywords: SiC, defects, carrier transport, carrier trapping, thermally stimulated currents.
Microstructure of Czochralski silicon co-implanted with helium and hydrogen and treated at high temperature and pressure
A. MISIUK, A. BARCZ, B. SURMA, J. BAK-MISIUK and A. WNUK
The effect of high temperature (HT) up to 1400 K and high pressure (HP) up to 1.1 GPa on Cz-Si co-implanted with
He+ (energy,
E = 50 KeV, dose, DHe = 5x1016 cm-2) and H2
+ (E = 135 KeV, DH 5x1016 cm-2), with almost overlapping implantation-
disturbed layers, has been investigated. Numerous extended defects are created at HT and HP near the He and H concentration
peaks, the overall structural perfection of annealed Si:He,H improves with HP. Oxygen gettering in the implantation-
disturbed areas is much less pronounced under HP. The observed effects are related, among others, to decreased hydrogen
out-diffusion and lowered dimensions of gas filled defects in Si:He,H treated under HP. Qualitative explanation of
HP-mediated gettering of oxygen has been proposed.
Keywords: silicon, implantation, hydrogen, helium, high pressure, microstructure.
Anticipated performance characteristics of possible InP-based vertical-cavity surface-emitting diode lasers for the third-generation 1.55-µm optical fibre communication systems
S. MACEGONIUK, R.P. SARZALA, and W. NAKWASKI
Anticipated performance of possible InP-based vertical-cavity surface-emitting lasers (VCSELs) for the third generation 1.55-µm optical-fibre communication has been simulated with the aid of the comprehensive optical-electrical-thermal-gain self-consistent model. While the room-temperature (RT) pulse operation of the above VCSELs has been found to be reached without serious difficulties, their efficient RT continuous-wave operation does not seem to be possible because of inherently insufficient physical properties of the AIIIP phosphides. Therefore, as the final conclusion of this paper, we have to stress that, taking into consideration currently available technology, InP-based VCSELs cannot be recommended as laser sources for future long-wavelength optical-fibre communication systems.
Keywords: InP-based VCSELs, 1.55-µm optical-fibre communication, simulation of a VCSEL operation.
Carrier transport in GaN single crystals and radiation detectors investigated by thermally stimulated spectroscopy
V. KAZUKAUSKAS and J.-V. VAITKUS
We investigated single crystals of GaN and thin film GaN radiation detectors by thermally stimulated currents (TSCs) and thermally stimulated depolarization (TSD) methods in order to characterize carrier transport properties as influenced by material defect structure. In thick GaN, no expressed structure of the TSC spectra was observed in the temperature range from 100 K up to 350 K that could be characteristic for thermal carrier generation from trap levels. The experimental facts imply that TSC spectra might be caused not by carrier generation from traps, but it could be due to thermal mobility changes. Therefore we had applied the numerical analysis by taking into account carrier scattering by ionized impurities and by phonons. It was found that mobility limited by ionized impurities varies as ~ T2.8 and lattice scattering causes the dependence ~ T-3.5. The highest mobility values were up to 1550 cm2/Vs at 148-153 K. Such high values indicate relatively good quality of the single GaN thick crystals. In high resistivity GaN detectors irradiated by high doses of high-energy neutrons and X-rays current, the instabilities were observed that could be caused by the change of carrier drift paths in a highly disordered mater. A model of carrier percolation transport is presented.
Keywords: GaN, carrier transport and capture, mobility, carrier scattering.
Linear and nonlinear optical properties of Zn1-xMgxSe layers grown by MBE and PLD methods
B. DERKOWSKA, M. WOJDYLA, P. PLOCIENNIK, B. SAHRAOUI, and W. BALA
Ternary and quaternary AIIBVI mixing semiconductors are very attractive materials for various optical devices. Their optical properties such as the energy gap, linear refractive index, absorption coefficient and lattice constant can be changed with increasing component. For the practical application linear and nonlinear optical characterizations such as the two-photon absorption (TPA), linear and nonlinear refractive indexes are an important aspect. A practical motivation to measure the magnitude of TPA coefficient is that the performance of a device based on nonlinear refraction is strongly affected by the eventual nonlinear absorption. The refractive index and TPA coefficient of Zn1-xMgxSe compounds grown on glass substrates by molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) methods were systematically investigated as a function of Mg composition. The linear optical properties have been studied using transmission, reflection, and photoreflection spectroscopy. The nonlinear optical properties of these materials were investigated by the nonlinear transmission. The energy gap and linear refractive index of these materials change with Mg content, hence the nonlinear optical processes such as TPA and nonlinear refraction index can be modified.
Keywords: Zn1-xMgxSe layers, reflection, transmission, refractive index, two-photon absorption, photoreflection.
An attempt to design room-temperature-operated nitridediode VCSELs
P. MACKOWIAK, R.P. SARZALA, M. WASIAK, and W. NAKWASKI
The new modified structure of nitride diode vertical-cavity surface-emitting lasers (VCSELs) of anticipated high-performance low-threshold room-temperature operation is presented. Two essential structure modifications are proposed in this new design. First, tunnel junctions and a semi-transparent contact are used to enhance uniformity of the current injection into a VCSEL active region. Second, a new built-in radial waveguiding mechanism is introduced within the output distributed Bragg reflector. According to simulation results, the radius of the active region may be dramatically reduced in this new VCSEL design, even to just 1 µm, without a significant increase in the cavity optical losses. Hence, using this new proposed design, manufacturing low-threshold, electrically-driven, room-temperature-operated nitride VCSELs may become possible even at the current, still immature, development stage of nitride technology.
Keywords: nitride diode lasers, nitride vertical-cavity surface-emitting lasers, modelling of a room-temperature
operation.
Generation-recombination effects in high temperature HgCdTe heterostructure photodiodes
A. JOZWIKOWSKA, K. JOZWIKOWSKI, J. RUTKOWSKI, Z. ORMAN, and A. ROGALSKI
The effect of built-in electric fields and misfit dislocations on dark currents in high temperature MOCVD HgCdTe infrared heterostructure photodiodes has been investigated. From experimental data results that the current-voltage characteristics at 240 K and 300 K indicate significant contributions from tunnelling effects, which dominate the leakage current mechanism for reverse bias greater than a few tens of milivolts. Standard theoretical models show that Auger generation-recombination processes determine dark current in high temperature HgCdTe photodiodes. But taking into account only Auger mechanisms much overestimated theoretical results are obtained. To explain this fact, a two-dimensional model has been developed to investigate the dark current mechanisms in the vicinity of the junction termination at built-in electric fields. Calculated profiles of the energy bands and electric field along different cross-sections of the photodiode indicate that the electric field achieves a maximum value of the order of mid 105 V/cm in the area the junction termination at the HgCdTe heterointerface. In these regions the high density of misfit dislocations are observed too. The presence of high electric field in this area decreases the ionisation energies of trap levels located in region of dislocations core, and hence increases the efficiency of Shockley-Read-Hall generation-recombination process. In addition to diffusion, generation-recombination and trap assisted tunnelling mechanisms, our model include the Poole-Frankel and phonon-trap assisted tunnelling effects in calculations of dynamic resistance of the junctions. The best fit of experimental data with theoretical predictions for dynamic resistance versus temperature has been obtained for dislocation density in the bulk of HgCdTe layer equal to 5x10-7 cm-2.
Keywords: HgCdTe photodiodes, generation-recombination effects, Poole-Frankel effect, R0A product.
Photoluminescence, optical transmission and reflection of Alq3 layers obtained by thermal evaporation deposition
P. DALASINSKI, Z. LUKASIAK, M. R BARZ, M. WOJDYLA, A. BRATKOWSKI, and W. BALA
In this paper we present the photoluminescence emission spectra (PL), photoluminescence excitation spectra (PLE), transmission (TS) and reflection (RS) spectra of tris(8-hydroxyquinoline) aluminum(III) (Alq3) layers grown by thermal evaporation deposition method. All investigated samples exhibit strong luminescence in the wide temperature range from 13 K to room temperature and for different energies of excitation. In our experiments, we have focused on temperature dependence of photoluminescence. The thermal quenching of PL in the measured samples has been found. We have observed also distinct oscillations from reflection and transmission measurements. The energies of thermal activation estimated by means of configuration coordinate diagram and also the layer thickness and refractive index are presented.
Keywords: photoluminescence, transmission, reflection, refractive index, Alq3.
Photoreflectance and photoluminescence of thick GaN layers grown by HVPE
R. KUDRAWIEC , R. KORBUTOWICZ, R. PASZKIEWICZ, M. SYPEREK, and J. MISIEWICZ
Very thick (up to 100 µm) GaN layers grown by HVPE are investigated by photoreflectance (PR) and photoluminescence (PL) spectroscopies. The layers were deposited on a GaN buffer layer which was grown on a c-plane sapphire substrate by MOVPE. Both, N- and Ga-polar layers were selected to these investigations. We have observed a strong dependence of the optical properties on the polarity of GaN surface. We have obtained that the bandgap-related emission for Ga-polar layers is stronger and narrower than the emission for N-polar layers. Also, significant differences have been found in PR spectra of the two type layers. In the case of Ga-polar layer a broad PR resonance with Franz-Keldysh oscillation (FKO) related to the surface electric field (215 kV/cm) has been observed, while in the case of N-polar layer narrow resonances have been found as being predominant. No-FKO for N-polar layer indicates that the surface electric field for this layer is weak. It means that the surface barrier for N-polar GaN is much smaller than for Ga-polar GaN layer.
Keywords: GaN, HVPE, Photoreflectance, N-face, Ga-face.
Exciton energies and probability of their radiative decay in GaN/AlN quantum structures
T. TCHELIDZE and T. KERESELIDZE
Exciton energies and probabilities of radiative transition from exciton state are calculated in GaN/AlN quantum dots. Electric field conditioned by strain induced and spontaneous polarization is considered by means of tight bind approximation, electron-hole Coulomb interaction is calculated by means of perturbation theory. Despite the presence of an electrical field, the calculated Coulomb energy is significantly increased with respect to bulk material. In large dots, the Coulomb term as well as probability of radiation transition for higher energetic excitons was strongly increased.
Keywords: GaN, quantum dots, excitons.
Photoluminescence characterization of vacuum deposited PTCDA thin films
W. BALA, Z. LUKASIAK, M R BARZ, P. DALASINSKI, A. BRATKOWSKI, D. BAUMAN, and R. HERTMANOWSKI
We investigated photoluminescence (PL) under steady state excitation and photoluminescence excitation (PLE) spectra of thin 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) films deposited on (001)Si substrate with various layer thicknesses grown at different substrate temperatures. The PL and PLE spectra have been measured at various temperatures, ranging from 10 K to 325 K. The PTCDA films exhibit strong luminescence at all mentioned ranges of temperature and for different values of excitation energy. The vibronic structure of PL spectra is clearly resolved at different temperatures. The position of peaks on energy scale depends on the temperature. We have observed a blue-shift of peaks with the decrease in the temperature. In all the investigated samples, the thermal quenching of PL has been observed. Analysis of the temperature dependence of the intensity bands, their position and full width half maximum (FWHM) allowed to find the energy barriers between the excited state and defect state. We propose a schematic potential energy diagram which explains mechanism of PL recombination.
Keywords: PTCDA, photoluminescence, excitons.
Experimental investigation of the anisotropy of quadratic electrooptic effect in ADP
R. LEDZION, M. IZDEBSKI, K. BONDARCZUK, and W. KUCHARCZYK
Measurements of the quadratic electrooptic coefficient g1212 of ammonium dihydrogen phosphate (ADP) were made. The coefficient g* = g1111 - g2211 - 2g1212 related to the anisotropy of electrooptic effect is determined. The value obtained for the coefficient is g*= -15.8x10x20 m2V-2. This shows that the anisotropy of quadratic electrooptic effect in ADP is stronger than that estimated previously for potassium dihydrogen phosphate.
Keywords: quadratic electrooptic effect, NH4H2PO4.
MOCVD growth of Hg1-xCdxTe heterostructures for uncooled infrared photodetectors
A. PIOTROWSKI, P. MADEJCZYK, W. GAWRON, K. KLOS, M. ROMANIS, M. GRUDZIEN, J. PIOTROWSKI, and A. ROGALSKI
We report here recent progress at VIGO/WAT MOCVD Laboratory in the growth of
Hg1-xCdxTe (HgCdTe) multilayer
heterostructures on GaAs/CdTe and other composite substrates for uncooled infrared photodetectors. The optimum conditions
for the growth of single layers and complex multilayer heterostructures have been established.
One of the crucial stages of the technology was CdTe nucleation on GaAs substrate. Successful composite substrates were
obtained with suitable substrate preparation, liner and susceptor treatment, proper control of background fluxes and appropriate
nucleation conditions. The other critical stage is the interdiffused multilayer process (IMP). The growth of device-quality HgCdTe heterostructures
requires complete homogenization of CdTe-HgTe pairs preserving at the same time suitable sharpness of composition
and doping profiles. This requires for IMP pairs to be very thin and grown in a short time.
The practical implications for the IMP process are the CdTe/HgTe growth times that become comparable with transition
times between the phases, characteristic for the MOCVD machine. The growth during transition stages is characterized by
the non-optimum flow velocities and partial pressures that may induce poor morphology, reduce growth rate and cause other
problems. This became especially acute for doped layers when large Cd/Te ratio is required for efficient incorporation and
full activation of dopants. This has been solved by careful selection of hydrogen carrier gas and metaloorganics fluxes with
suitable switching on and off times. Arsenic and iodine has been used for acceptor and donor doping. Suitable growth conditions and post growth anneal is
essential for stable and reproducible doping. In-situ anneal seems to be sufficient for iodine doping at any required level. In
contrast, efficient As doping with near 100% activation requires ex situ anneal at near saturated mercury vapors.
As the result, we are able to grow multilayer fully doped (100) and (111) heterostructures for various infrared devices including
photoconductors, photoelectromagnetic and photovoltaic detectors.
Keywords: MOCVD growth, Hg1-xCdxTe infrared photodetectors.
Temperature dependence of the electrooptic coefficients r22 and m22 in LiNbO3
P. GORSKI, K. BONDARCZUK, and W. KUCHARCZYK
Measurements of temperature dependence of linear electrooptic coefficients defined in terms of the electric field r22 and the induced polarization m22 were made for LiNbO3 within the temperature range 25°C-200°C. After annealing, the coefficient r22 in samples with xc =49.3% is found to be nearly temperature independent, increasing at 125°C only 0.6%. Contrary to the commonly observed temperature behaviour of intrinsic coefficients, m22 instead of r22, changes significantly with temperature.
Keywords: electrooptic effect, LiNbO3.