Background Electron Concentration Research Articles

Growth of nonpolar a-plane AlGaN epilayer on Al-composition graded-AlGaN buffer layer and characterization of its surface morphology and crystalline quality

Metal-organic chemical vapor deposition was successfully used to grow un-doped nonpolar a-plane AlGaN on an r-plane sapphire substrate. High-resolution x-ray (HR-XRD), atomic force microscopy, Polarization Indirect Microscopic Imaging, and Hall effect measurement were used to investigate the effect of the nonpolar a-plane AlGaN layer on the graded AlGaN buffer layer. The results reveal that inserting the AlGaN-graded layer improves the crystalline quality and morphology of the surface. The root means the square value was less than 1.47 nm, while the background electron concentration was In addition, the HR-XRD full width at half maximum indicates improved crystalline quality.

Influences of nitridation treatment on the properties of semi-polar (11 [formula omitted]) plane AlGaN films

The characteristics of the semi-polar (11 2‾ 2) plane Al0.34Ga0.66N films grown with various nitridation time for m-plane substrate were investigated. The characterization results demonstrated that the surface flatness, crystalline quality as well as the PL full width at half maximum values were sensitive to the nitridation time, indicating that nitridation time is a key factor to achieve (11 2‾ 2) plane AlGaN films with high quality. Meanwhile, the background electron concentration in the epi-layer reduced by nearly two orders of magnitude via optimizing the nitridation time. These results made remarkably positive contributions to obtaining (112‾ 2)-orientated p-AlGaN films with high concentration, which is of great importance in fabricating high efficiency (112‾ 2)-orientated AlGaN-based light emitting diodes.

Особенности структурных и оптических свойств InGaN-слоев, полученных методом МПЭ ПА с импульсной подачей потоков металлов

This paper presents the results of studying the properties of InGaN layers with a high InN content (80-90%) obtained by molecular beam epitaxy with plasma activation of nitrogen on sapphire substrates with AlN / GaN buffer layers. The InGaN layers were formed using the metal modulated epitaxy (MME) method, as well as in nitrogen and metal rich conditions. It was found that the use of the MME method leads to a decrease in the density of threading dislocations in the InGaN layers. Nevertheless, despite the higher dislocation density, the smallest threshold of stimulated emission of ~ 20 kW / cm2 at 77 K was obtained for the In0.8Ga0.2N layer grown under nitrogen rich conditions, which is associated with the lowest background electron concentration in this sample (1.6•1019 cm-3).

Performance deterioration of GaN-based laser diode by V-pits in the upper waveguide layer

Abstract The effect of V-pits in the upper waveguide (UWG) on device performance of GaN-based laser diodes (LDs) has been studied. Experimental results demonstrate that in comparison with the LDs with u-In0.017Ga0.983N/u-GaN multiple UWG or u-In0.017Ga0.983N one, the LDs with a single u-GaN UWG has the best device performance. They have a smaller threshold current density, and a larger and more stable output optical power. The lowest threshold current density is as low as 1.3 kA/cm2, and the optical power reaches to 2.77 W. Furthermore, atomic force microscopy suggests that the deterioration of device performance of former kinds of devices may be attributed to the increase of V-pits’ size and quantity in the undoped-In0.017Ga0.983N UWG layer, and these V-pits could introduce more nonradiative recombination centers and exacerbate the inhomogeneity of injection current. Moreover, theoretical calculation results indicate that the increase of leakage current and optical loss are additional reasons for the device performance deterioration, which may be caused by a reduction of the potential barrier height for electrons in the quantum wells and by an increased background electron concentration in UWG.

Observation and mitigation of RF-plasma-induced damage to III-nitrides grown by molecular beam epitaxy

In this work, radio-frequency (RF) plasma-induced damage to III-nitride surfaces and bulk defects is observed and mitigated. It is shown that for InN films, the surface is more sensitive to plasma-induced damage than GaN films, as observed via atomic force microscopy and reflection high energy electron diffraction. In order to isolate any possible plasma-induced damage, a growth window for InN is established, and temperature ranges are determined for other damaging effects which include roughening due to low adatom mobility, InN decomposition, and indium desorption. In situ plasma monitoring and optimization are accomplished with a combination of optical emission spectroscopy as well as a remote Langmuir probe. It is shown that by increasing the plasma nitrogen flow, the positive ion content increases; however, the ion acceleration potential reduces. Additionally, a reduced RF plasma power results in a reduction of atomic nitrogen species. These plasma species and energetic variations result in variations in the bulk unintentional background electron concentrations observed by room temperature Hall effect measurements of ∼1 μm thick InN films. By increasing the nitrogen flow from 2.5 to 7.5 sccm for a constant RF power of 350 W, the background electron concentration decreases by 74% from 1.36 × 1019 cm−3 to 3.54 × 1018 cm−3, while maintaining a smooth surface morphology. Additionally, photoluminescence spectra indicate optical emission energies shift from ∼0.81 to 0.71 eV (closer to the fundamental bandgap of InN) by limiting the damaging plasma species. Finally, conditions are presented to further minimize plasma-induced damage in III-nitride devices.

Optical Properties of MBE-Grown Hg1−xCdxSe

In this work, we present a study on the temperature-dependent infrared absorption spectra of Hg1−xCdxSe grown by molecular beam epitaxy (MBE) on GaSb (211) substrate, having a nominal x-value of 0.21. For temperatures below 200 K, the observed optical bandgap is found to correspond to the Fermi energy level rather than the intrinsic bandgap, which is quantitatively explained by the Burstein–Moss shift due to the presence of a background electron concentration of 3.5 × 1016 cm−3. In addition, empirical formulae for calculating both the absorption edge and intrinsic absorption coefficient in the Kane region have been derived from the previously reported absorption spectra of Bridgeman-grown Hg1−xCdxSe samples (0.15 ≤ x ≤ 0.3). By employing the empirical expressions, the infrared transmission spectra have been modeled based on the characteristic matrix method, and the x value profile of the sample along the growth direction has been determined, which is in good agreement with the experimental results obtained from secondary ion mass spectrometry (SIMS) depth profiling combined with Rutherford backscattering spectrometry (RBS).

Crucial influential factor on background electron concentration in semi-polar (11[formula omitted]2) plane AlGaN epi-layers

The semi-polar (112¯2) plane Al0.3Ga0.7N epi-layers with different background electron concentrations were deposited on (101¯0) m-plane sapphire substrates with metal-organic chemical vapor deposition technology, and characterized with Hall effect measurement, photoluminescence spectroscopy, high-resolution X-ray diffraction, and atomic force microscopy. It was found that there is a strong positive correlation between the background electron concentrations and the anisotropy in two-dimensional (2D)-growth rate of the triangular structures on the surface of semi-polar Al0.3Ga0.7N epi-layers. That is to say, the anisotropy in the 2D-growth rate is indeed the crucial influential factor on the background electron concentration. In fact, by carefully suppressing the anisotropy in 2D-growth rate, the background electron concentration in semi-polar Al0.3Ga0.7N epi-layers was reduced successfully from 1.38×1019 cm−3 to 4.20×1015 cm−3. This work should open a new way to realize the p-type semi-polar AlGaN epi-layers, which is of great significance for the manufacturing of semi-polar AlGaN-based DUV-LEDs.

MBE growth of high quality HgCdSe on GaSb substrates

This paper demonstrates MBE growth of high quality HgCdSe infrared materials on GaSb (211)B substrates. The as-grown Hg1−xCdxSe samples have a range of x-values (x = 0.37–0.18) and cut-off wavelengths (λc = 3.9–10.4 μm at 80 K), and show typical n-type semiconductor behaviour. At a measurement temperature of 80 K, the as-grown HgCdSe samples with x = 0.18 present a cut-off wavelength of 10.4 μm, an electron mobility as high as 1.3 × 105 cm2 V−1 s−1, a background electron concentration as low as 1.6 × 1016 cm−3, and a minority carrier lifetime as long as 2.2 μs. These values of electron mobility and minority carrier lifetime represent a significant improvement on previous studies of MBE-grown HgCdSe reported in the open literature, and are comparable to those of corresponding HgCdTe materials grown on lattice-matched CdZnTe substrates. This high material quality is primarily due to the nearly lattice-matched epitaxial growth of HgCdSe on GaSb, as well as an optimised growth temperature. These preliminary results indicate that HgCdSe materials grown on GaSb can meet the basic material quality requirements for the fabrication of high performance infrared detectors, although further effort is required in order to reduce the background electron concentration to <1015 cm−3. Furthermore, even higher quality HgCdSe materials on GaSb are expected by further optimization of the growth conditions, using higher purity Se source material, and implementing post-growth thermal annealing in a Se environment. The results of this study demonstrate the great potential of HgCdSe infrared materials grown on large-area commercially-available substrates for meeting the requirements of next generation infrared imaging focal plane arrays with features of lower cost and larger array format size.

Sulfurization-induced growth of single-crystalline high-mobility β-In2S3 films on InP

Metalorganic chemical vapor deposition was used to grow single-crystalline tetragonal β-In2S3 films on InP to afford covalently bonded In2S3/InP heterostructures, with the crystal structure of these films identified by high-resolution scanning transmission electron microscopy, X-ray diffraction, and Raman spectroscopy analyses, and the corresponding bandgap energies determined by photoluminescence measurements at room (300 K) and low temperatures (40 K). RT-PL measurements reveal the three peaks spectral emission at 464.3, 574.7, and 648.5 nm associated with luminescence from band-edge and two above conduction band-edge, respectively, although the LT-PL (40K) measurements of β-In2S3 film found two dominant peaks. Moreover, the above films exhibited n-type conductivity, with background electron concentration = 4.9 × 1015 cm–3, electron mobility = 1810.9 cm2 V–1 s–1, and resistivity = 0.704 Ω cm. Thus, single-crystalline β-In2S3 films deposited on InP are promising constituents of high-performance next-generation electronic, optoelectronic, and photovoltaic devices.

Influence of Indium Content on the Unintentional Background Doping and Device Performance of InGaN/GaN Multiple-Quantum-Well Solar Cells

We experimentally investigate the influence of indium composition on the photovoltaic performance of InGaN/GaN multiple-quantum-well (MQW) solar cells. Three MQW structures with different In content are grown via metal-organic chemical vapor deposition, and then fabricated into solar cell devices. The solar cells with lower In content show stronger photovoltaic response, which may be ascribed to both smaller piezoelectric fields in InGaN well layers and better material quality of MQW active region. In addition, based on the capacitance–voltage measurements by which the profiles of apparent carrier concentration can be determined, we find that in the higher-In-content solar cells the background electron concentration in unintentionally doped InGaN QWs is higher. Consequently, the effective volume of active region, or the width of depletion region, may become smaller at zero bias and correspondingly the light absorption by InGaN active layers is reduced. This may be attributed to the increased nitrogen vacancy-related defects which are easier induced into In-rich InGaN alloys as a source of shallow donors.

Influence of homo-buffer layers and post-deposition rapid thermal annealing upon atomic layer deposition grown ZnO at 100 °C with three-pulsed precursors per growth cycle

ZnO main epilayers are deposited with three-pulsed precursors in every growth cycle at 100°C on various thicknesses of 300°C-grown homo-buffer layers by atomic layer deposition (ALD) on sapphire substrate. Samples are treated without and with post-deposition rapid thermal annealing (RTA). Two different annealing temperatures 300 and 1000°C are utilized in the ambience of oxygen for 5 min. Extremely low background electron concentration 8.4×1014cm−3, high electron mobility 62.1cm2/V s, and pronounced enhancement of near bandgap edge photoluminescence (PL) are achieved for ZnO main epilayer with sufficient thickness of buffer layer (200 ALD cycles) and post-deposition RTA at 1000°C. Effective block and remove of thermally unstable mobile defects and other crystal lattice imperfections are the agents of quality promotion of ZnO thin film.

Growth and electrical properties of high-quality InGaAsBi thin films using gas source molecular beam epitaxy

The effects of Bi flux and In/Ga ratio on Bi incorporation and electrical properties of InGaAsBi grown by gas source molecular beam epitaxy were systematically studied. It is found that use of a low In/Ga ratio has an enhancement effect on the incorporation of Bi and its content increases linearly with Bi flux until reach a saturation. Incorporation of Bi induces p-type dopant that compensates the background electron concentration but does not degrade the electron mobility for the Bi content up to 6.2%. Up to 7.5% of Bi incorporation has been confirmed by Rutherford backscattering spectroscopy (RBS) and a maximum electron mobility of 5600 cm2·V−1·s−1 at room temperature was achieved in InGaAsBi with xBi = 6.2%, which is the highest value reported in InGaAsBi with xBi > 5%.

Structural and electronic properties of InN epitaxial layer grown on c-plane sapphire by chemical vapor deposition technique

Growth of InN epilayers on c-plane sapphire substrate by chemical vapor deposition technique using pure indium metal and ammonia as precursors has been systematically explored. It has been found that [0001] oriented indium nitride epitaxial layers with smooth surface morphology can be grown on c-plane sapphire substrates by optimizing the growth conditions. Bandgap of the film is observed to be Burstein–Moss shifted likely to be due to high background electron concentration. It has been found that the concentration of this unintentional doping decreases with the increase in the growth temperature and the ammonia flux. Epitaxial quality on the other hand deteriorates as the growth temperature increases. Moreover, the morphology of the deposited layer has been found to change from flat top islands to faceted mounds as the flow rate of ammonia increases. This phenomenon is expected to be related to the difference in surface termination character at low and high ammonia flow rates.

Effect of cross-focusing of two q-Gaussian laser beams on excitation of electron plasma wave in collisional plasma

This paper presents theoretical investigation of the effect of cross-focusing of two q-Gaussian laser beams on excitation of electron plasma wave (EPW) in an underdense plasma where collisional nonlinearity is operative. The EPW is generated on account of beating of the two laser beams of slightly different frequencies ω1 and ω2. Starting from Maxwell's equations, the coupled differential equations governing the evolution of spot size of the two laser beams with distance of propagation have been derived with the help of moment theory approach in Wentzel–Kramers–Brillouin (W.K.B.) approximation. The collisional nonlinearity depends not only on the intensity of first laser beam but also on that of second laser beam. Therefore, the dynamics of one laser beam affects that of other and hence, cross-focusing of the two laser beams takes place. Due to nonuniform intensity distribution of the laser beams, the background electron concentration is modified. The amplitude of EPW, that depends on the background electron concentration, is thus nonlinearly coupled with the laser beams. The effects of collisional nonlinearity and cross-focusing of the laser beams on excitation of EPW have been incorporated. Numerical simulations have been carried out to investigate the effect of laser and plasma parameters on cross-focusing of the two laser beams and further its effect on EPW excitation.

P-Type Conductivity Control in ZnO Films Grown by Molecular-Beam Epitaxy

We report on the p-type conductivity control using N and Te codoping and thermal annealing in the ZnO films, heteroepitaxially grown on Al2O3 substrates and homoepitaxially grown on ZnO substrates by molecular-beam epitaxy, respectively. The N and Te codoping and the homoepitaxy effectively reduce the background electron concentration in ZnO films due to the suppression of various defect generation, and the thermal annealing causes the conductivity conversion from n-type to p-type due to the activation of N-related defects and the annihilation of donor-type defects. The p-type conductivity with a hole concentration of 1.61016 cm-3 and a hole mobility of 16 cm2/Vsec is obtained in the ZnO:N+T film grown on the Al2O3 substrate and the p-type conductivity with a hole concentration of 4.01016 cm-3 and a hole mobility of 11 cm2/Vsec is obtained in the ZnO:N+Te film grown on the ZnO substrate.

Electron plasma wave excitation by beating of two q-Gaussian laser beams in collisionless plasma

AbstractThis paper presents a scheme for excitation of an electron-plasma wave (EPW) by beating two q-Gaussian laser beams in an underdense plasma where ponderomotive nonlinearity is operative. Starting from nonlinear Schrödinger-type wave equation in Wentzel–Kramers–Brillouin (WKB) approximation, the coupled differential equations governing the evolution of spot size of laser beams with distance of propagation have been derived. The ponderomotive nonlinearity depends not only on the intensity of first laser beam, but also on that of second laser beam. Therefore, the dynamics of one laser beam affects that of other and hence, cross-focusing of the two laser beams takes place. Due to nonuniform intensity distribution along the wavefronts of the laser beams, the background electron concentration is modified. The amplitude of EPW, which depends on the background electron concentration, is thus nonlinearly coupled with the laser beams. The effects of ponderomotive nonlinearity and cross-focusing of the laser beams on excitation of EPW have been incorporated. Numerical simulations have been carried out to investigate the effect of laser and plasma parameters on cross-focusing of the two laser beams and further its effect on EPW excitation.

Optical investigations of Be doped ZnO films grown by molecular beam epitaxy

In this article, the optical properties of ZnO:Be films grown by plasma-assisted molecular beam epitaxy were investigated by the excitation density-dependent and temperature-dependent photoluminescence measurements. The low temperature photoluminescence spectra showed a dominant excitons bound to neutral donors (D°X) emission centered at 3.3540eV and strong donor-acceptor pair (DAP) transitions at 3.3000eV. In addition, it showed that the intensity ratio of the DAP and D°X peaks changed with background electron concentration. Furthermore, a shallow acceptor state with ionization energy of 116meV was found and attributed to Zn vacancy. The present study further suggests that Be and N codoping ZnO might be suitable for fabricating reliable p-type ZnO materials.

InGaN Visible Photodiodes With Improved Performance Using Oxidized Ir Schottky Contact

Oxidized iridium (Ir) was employed as the Schottky contact of In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.18</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.82</sub> N photodiodes. The Schottky barrier height (SBH) increases from 0.70 eV for the as-deposited Ir contacts to 0.91 eV for the contacts annealed at 500 ° C in O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ambient, while the dark current at -3 V bias decreases from 4.65×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> to 2.29×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> A. A zero-bias responsivity of 32 mA/W was obtained at 410 nm in the devices with the oxidized contacts, corresponding to an external quantum efficiency of 10%. Analysis results indicate that the increased SBH and decreased leakage current can be mainly ascribed to two factors: 1) the higher work function of the IrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> contacts formed by oxidation annealing and 2) the reduction in the background electron concentration near the IrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.18</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.82</sub> N interface caused by the generation of compensating group-III vacancies during the oxidation process. Besides, the oxidation of group-III elements may also play a minor role in the performance improvement.

Structural and optical characterization of nonpolar (10–10) m-InN/m-GaN epilayers grown by PAMBE

Plasma-assisted molecular beam epitaxy growth of (10–10) m-InN/(10–10) m-GaN was carried out on bare (10–10) m-sapphire substrate. The high resolution X-ray diffraction studies confirmed the orientation of the as-grown films. Nonpolar InN layer was grown at different growth temperatures ranging from 390°C to 440°C and the FWHM of rocking curve revealed good quality film at low temperatures. An in-plane relationship was established for the hetrostructures using phi-scan and a perfect alignment was found for the epilayers. Change of morphology of the films grown at different temperatures was observed using an atomic force microscopy technique showing the smoothest film grown at 400°C. InN optical band gap was found to be vary from 0.79–0.83eV from absorption spectra. The blue-shift of absorption edge was found to be induced by excess background electron concentration.

Control of ion content and nitrogen species using a mixed chemistry plasma for GaN grown at extremely high growth rates &amp;gt;9 μm/h by plasma-assisted molecular beam epitaxy

Utilizing a modified nitrogen plasma source, plasma assisted molecular beam epitaxy (PAMBE) has been used to achieve higher growth rates in GaN. A higher conductance aperture plate, combined with higher nitrogen flow and added pumping capacity, resulted in dramatically increased growth rates up to 8.4 μm/h using 34 sccm of N2 while still maintaining acceptably low operating pressure. It was further discovered that argon could be added to the plasma gas to enhance growth rates up to 9.8 μm/h, which was achieved using 20 sccm of N2 and 7.7 sccm Ar flows at 600 W radio frequency power, for which the standard deviation of thickness was just 2% over a full 2 in. diameter wafer. A remote Langmuir style probe employing the flux gauge was used to indirectly measure the relative ion content in the plasma. The use of argon dilution at low plasma pressures resulted in a dramatic reduction of the plasma ion current by more than half, while high plasma pressures suppressed ion content regardless of plasma gas chemistry. Moreover, different trends are apparent for the molecular and atomic nitrogen species generated by varying pressure and nitrogen composition in the plasma. Argon dilution resulted in nearly an order of magnitude achievable growth rate range from 1 μm/h to nearly 10 μm/h. Even for films grown at more than 6 μm/h, the surface morphology remained smooth showing clear atomic steps with root mean square roughness less than 1 nm. Due to the low vapor pressure of Si, Ge was explored as an alternative n-type dopant for high growth rate applications. Electron concentrations from 2.2 × 1016 to 3.8 × 1019 cm−3 were achieved in GaN using Ge doping, and unintentionally doped GaN films exhibited low background electron concentrations of just 1–2 × 1015 cm−3. The highest growth rates resulted in macroscopic surface features due to Ga cell spitting, which is an engineering challenge still to be addressed. Nonetheless, the dramatically enhanced growth rates demonstrate great promise for the future of III-nitride devices grown by PAMBE.