Low Background Carrier Concentration Research Articles

Electrical and optical characterisation of InGaAsSb-based photodetectors for SWIR applications

Abstract The In x Ga 1 − x A s y Sb 1 − y alloy was studied in epilayers and photodiodes grown lattice matched to GaSb across a comprehensive composition range, 0 ⩽ x ⩽ 0.3 , as a promising technology to support the extended short-wave infrared region ∼ 1.7 − 3 µm. Low background carrier concentrations between 6 × 10 14 and 1 × 10 15 cm − 3 were achieved in all samples, reducing with In fraction. Both the absorption coefficient and external quantum efficiency were found to increase with indium fraction, up to ∼ 10 4 cm − 1 and 70% without an AR coating, respectively. Counter to the fundamental bandgap dependence, leakage current density initially reduced with the addition of low In and As fractions, before rising as the fractions increased and the bandgap reduced further. These properties resulted in specific detectivity reaching a maximum in the sample with x = 0.043, before decreasing towards higher alloy fractions. It is concluded that for moderate In fractions, the InGaAsSb alloy offers clear potential to improve on the disappointing material and photodiode properties of GaSb and support emerging SWIR sensing applications. However, development of fabrication and passivation technology is required to fully exploit this potential.

Ultralow Dark Current (6 fA at 1 V) in Quasi-Two-Dimensional CsGaGeSe4 Single Crystals for High Signal-to-Noise Ratio Photodetectors under Strong Electron Localization.

To satisfy the demands of photodetectors for weak-light detection, materials selected for device fabrication should have an extremely low background carrier concentration to suppress the dark current of devices. In this work, a new quasi-two-dimensional CsGaGeSe4 single crystal with an extremely low background carrier concentration was synthesized by a co-solvent reaction based on which a photoconductive detector was prepared with an ultralow dark current density (6 fA at 1 V and ∼10-10 A cm-2) and a high response speed (∼0.74 s) was achieved, presenting a great potential of being applied to the field of weak-light detection. The ultralow dark current density originates from both the good crystal quality and the strongly asymmetric band structure of CsGaGeSe4. In the darkness, electrons locally bound in the valence band bring an ultralow dark current density; after illumination, the electrons transiting to the conduction band will participate in the conduction in a non-localized state, resulting in a high signal-to-noise ratio. This work not only provides a new choice of potential materials for weak-light detection but also proposes an effective strategy for material selection.

Enhanced crystalline quality of non-polar a-plane AlGaN epitaxial film grown with Al-composition-graded AlGaN intermediate layer

The non-polar a-AlGaN epitaxial film was successfully grown on the semi-polar r-sapphire substrate by metal-organic chemical vapor deposition technique. An Al-composition-graded AlxGa1−xN (x = 0.0 to 1.0) intermediate layer with varying film thickness from 260 to 695 nm was deposited between the high-temperature AlN layer and the non-polar a-AlGaN epitaxial film to enhance the morphological and crystalline quality. The non-polar a-AlGaN epitaxial films were investigated by using atomic force microscopy (AFM), high-resolution x-ray diffraction, photoluminescence (PL) spectroscopy and the Hall effect measurement techniques. The characterisation results indicate substantial improvements in surface morphology and crystalline quality for the non-polar a- AlGaN epitaxial film grown by adding an Al-composition-graded AlGaN intermediate layer. The surface roughness measured from AFM and the defect-related emission (yellow band) relative to the near-band-edge emission from PL spectra were decreased significantly by optimizing the layer thickness of the Al-composition-graded AlGaN layer. A relatively low background carrier concentration down to −4.4 × cm−3 was achieved from Hall effect measurement for the non-polar a-AlGaN epitaxial film.

Ultra-wide spectral range (0.4–8 μm) transparent conductive ZnO bulk single crystals: a leading runner for mid-infrared optoelectronics

Transparent conductive oxides have been widely applied to solar cells, liquid crystal displays, and other optoelectronic devices because of their excellent transparent conductive properties in the visible-near-infrared (Vis-NIR) region. At present, the transparent conductive oxide films (TCOFs) such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and aluminum-doped zinc oxide (AZO) are widely noticed and used. However, a high carrier concentration (≥1020 cm−3) of these TCOFs leads to a result that their transparency will be affected by the plasmons effect, and thus their transparent range will generally end around 2–3 μm. As such, they are more applicable in the Vis-NIR region but unable to meet the requirements of mid-infrared (MIR) optoelectronic devices. Faced with this situation, herein, we reported the native ZnO bulk single crystals with low background carrier concentration (1015–1016 cm−3) and high carrier mobility (μ > 300 cm2 V−1 s−1). These single crystals boast excellent transparent conductive properties (IR cutoff edge: 6–8 μm, Rsheet: 3.7–28.5 Ω/sq) with an advantage that they can be massively produced by a hydrothermal method at a low cost. Based on these merits, the native ZnO bulk single crystals bring a new possibility of breaking the bottleneck of MIR transparent conductivity.

Current transport mechanism of Schottky contact of Pt/Au/n-InGaN

The Pt/Au Schottky contacts to InGaN samples with different background carrier concentrations are fabricated. The crystal qualities of InGaN samples are characterized by X-ray diffraction (XRD) and atomic force microscope (AFM), and the correlation between threading dislocation density of InGaN and growth temperature is further clarified. The full width at half maximum (FWHM) values of the InGaN (0002) XRD rocking curves show that the density of threading dislocations in InGaN, which can seriously deteriorate InGaN crystal quality and surface morphology, decreases rapidly with increasing growth temperature. The Hall measurements show that the background carrier concentration of InGaN increases by two orders of magnitude as growth temperature decreases from 750 to 700℃, which is due to a reduced ammonia decomposition efficiency leading to the presence of high-density donor-type nitrogen vacancy (VN) defects at lower temperature. Therefore, combining the studies of XRD, AFM and Hall, it can be concluded that the higher growth temperature is favorable for realizing the InGaN film with low density of VN defects and threading dislocations for fabricating high-quality Schottky contacts, and then the barrier characteristics and current transport mechanism of Pt/Au/n-InGaN Schottky contact are investigated by current-voltage measurements and theory analysis based on the thermionic emission (TE) model and thermionic field emission (TFE) model. The results show that Schottky characteristics for InGaN with different carrier concentrations manifest obvious differences. It is noted that the high carrier concentration leads to the Schottky barrier height and the ideality factor obtained by TE model are quite different from that by TFE model due to the presence of high density of VN defects. This discrepancy suggests that the VN defects lead to the formation of the tunneling current and further reduced Schottky barrier height. Consequently, the presence of tunneling current results in the increasing of total transport current, which means that the defects-assisted tunneling transport and TE constitute the current transport mechanism in the Schottky. However, the fitted results obtained by TE and TFE models are almost identical for the InGaN with lower carrier concentration, indicating that TE is the dominant current transport mechanism. The above studies prove that the Pt/Au/n-InGaN Schottky contact fabricated using low background carrier concentration shows better Schottky characteristics. Thus, the properly designed growth parameters can effectively suppress defects-assisted tunneling transport, which is crucial to fabricating high-quality Schottky devices.

Current transport mechanisms in Pt/Au Schottky contacts to AlInGaN using AlGaN/InGaN short-period superlattices

High-quality AlInGaN film grown by metal organic chemical vapor deposition is realized by fabricating the AlGaN/InGaN short-period superlattices, and then, Schottky contacts are fabricated on the AlInGaN superlattices. The crystal quality of AlInGaN superlattice samples is characterized by X-ray diffraction and scanning electron microscopy, and the correlation between the compositions of AlInGaN and growth condition is further clarified. The current transport mechanisms of the Pt/Au Schottky contacts to AlInGaN superlattice samples with different background carrier concentrations are studied by current–voltage (I–V) characteristics and theory analysis based on the thermionic emission (TE) and thermionic field emission (TFE) models. It is found that the results obtained from the TE and TFE models are almost identical for the Schottky contact to strain-balanced AlInGaN sample with low background carrier concentration, indicating that the thermionic emission is a dominant current transport mechanism. However, the Schottky contacts fabricated using high background carrier concentration AlInGaN samples demonstrate degraded barrier characteristics due to an occurrence of donor-like defects which result in the defect-assisted tunneling current, and thus, the combination of tunneling transport and thermionic emission constitutes the current transport mechanisms in Schottky contacts.

Enhancement-mode Ga2O3 MOSFETs with Si-ion-implanted source and drain

Enhancement-mode β-Ga2O3 metal–oxide–semiconductor field-effect transistors with low series resistance were achieved by Si-ion implantation doping of the source/drain contacts and access regions. An unintentionally doped Ga2O3 channel with low background carrier concentration that was fully depleted at a gate bias of 0 V gave rise to a positive threshold voltage without additional constraints on the channel dimensions or device architecture. Transistors with a channel length of 4 µm delivered a maximum drain current density (IDS) of 1.4 mA/mm and an IDS on/off ratio near 106. Nonidealities associated with the Al2O3 gate dielectric as well as their impact on enhancement-mode device performance are discussed.

High breakdown voltage p-n diodes on GaN on sapphire by MOCVD

In order to achieve high breakdown voltage in GaN vertical power devices, low threading dislocation density and low background carrier concentration is required. This work demonstrates a decrease in the background carrier concentration and threading dislocation density (TDD) with an increase in the thickness of un-intentionally doped (UID) GaN grown on sapphire. p–n diodes grown and fabricated on this epi, using 4.8 µm UID GaN, showed a breakdown voltage of 730 V, breakdown field of 1.75 MV cm−1 and an on-resistance of 5.1 m Ω cm2. The figure of merit (FOM), VBR2/RON, thus obtained is approximately 105 MW cm−2. This is the highest reported FOM value for p–n diodes on GaN on sapphire or Si. Lowering the carrier concentration and dislocation density is thus shown to be critical for achieving high breakdown voltages on GaN on sapphire.

(Invited) Dilute-Nitride-Antimonide Materials Grown by MOVPE for Multi-Junction Solar Cell Application

Multinary compounds, such as quaternary or quinternary materials, are attractive for solar cell implementation, since the fourth and fifth element in the alloy permits an additional degree of freedom for tailoring the energy band structure (i.e. bandgap and offsets), while fixing the lattice parameter. We have investigated the growth by metalorganic vapor phase epitaxy (MOVPE) of multinary (four- and five- element) dilute-nitride-antimonide materials on GaAs substrates. Bulk films of dilute-nitride-antimonide materials with a narrow energy bandgap (Eg ~ 1.0-1.2 eV), InGaAsN, GaAsSbN, and InGaAsSbN, are attractive for multi-junction solar cell applications owing to the ease of bandgap tuning and lattice-matching while requiring only a small amount of N. Dilute-nitride materials grown by molecular beam epitaxy (MBE) generally have very low unintentional background carrier concentrations (<1x1015 cm-3). By contrast, a high background carbon concentration, which correlates with poor luminescence properties and short minority carrier diffusion length, is a challenging issue for MOVPE-grown dilute-nitride-antimonide materials. In the MOVPE growth of dilute-nitride-antimonide materials, the background carbon concentration is highly dependent on the gas-phase growth conditions and selection of the specific metalorganic sources. Sb-containing materials are found to exhibit an order of magnitude higher background carbon concentration than dilute-nitride films without Sb. We have employed an unconventional antimony precursor, TrisSb, which results in a significant decrease in the carbon background levels in (In)GaAsSbN materials. Employing relatively high growth temperatures (> 600 oC) also leads to a further reduction of the background carbon impurity concentration. However, the presence of Sb is found to also significantly inhibit N incorporation, making it challenging to achieve (In)GaAsSbN grown at 600 oC which contains sufficient N for producing a 1 eV band gap energy. The lowest background carbon concentration (~ 5 × 1016 cm-3) is observed in dilute-nitride materials grown at high temperature which do not contain Sb (i.e. InGaAsN). An increased depletion region width significantly improves the solar cell performance over that found from dilute-nitride cells grown at lower growth temperatures (~525oC). The device performance of the solar cells with the low carbon background InGaAsN base region exhibit short-circuit current density, open-circuit voltage, fill factor, and efficiency values of 26.05 mA/cm2, 0.67 V, 75.85 %, and 13.2 %, with anti-reflecting coating (ARC), respectively. This measured short-circuit current density value is sufficient for the current-matching condition (~ 15 mA/cm2) of a triple-junction solar cell on Ge substrate. These results demonstrate MOVPE-grown cells are viable with performance comparable to solar cell structures employing a similar band gap dilute-nitride material grown by MBE.

Impact of growth temperature and substrate orientation on dilute-nitride-antimonide materials grown by MOVPE for multi-junction solar cell application

Nitrogen incorporation in bulk films of GaAsN, InGaAsN, and GaAsSbN films grown by metalorganic vapor phase epitaxy (MOVPE) on (100) and (311)B GaAs substrates was investigated. These films, nominally lattice-matched to a GaAs substrate, were deposited at relatively higher growth temperature (600°C) than typically used for MOVPE-grown dilute-nitride materials (~500–530°C), in order to reduce the background carbon impurity concentration. Even at these higher growth temperatures, sufficient N incorporation is achieved for targeting Eg~1eV InGaAsN and GaAsN with low background carrier concentration (1–2×1017cm−3). The presence of Sb is found to significantly inhibit N incorporation, making it challenging to achieve films of GaAsSbN grown at 600°C with a sufficient N concentration to achieve a 1eV band gap energy. For GaAsN and InGaAsN on (311)B GaAs substrates, increased N incorporation with lower background carbon concentration is observed, relative to films on (100) GaAs. By contrast, GaAsSbN on (311)B GaAs substrate exhibit lower-N incorporation relative to films on (100) GaAs, presumably due to surface site competition between Sb and N. The background hole carrier concentrations of thermally annealed InGaAsN and GaAsSbN on (311)B are about a factor of two lower than those on (100) GaAs substrate.

Large area single crystal (0001) oriented MoS2

Layered metal dichalcogenide materials are a family of semiconductors with a wide range of energy band gaps and properties, the potential for exciting physics and technology applications. However, obtaining high crystal quality thin films over a large area remains a challenge. Here we show that chemical vapor deposition (CVD) can be used to achieve large area single crystal Molybdenum Disulfide (MoS2) thin films. Growth temperature and choice of substrate were found to critically impact the quality of film grown, and high temperature growth on (0001) oriented sapphire yielded highly oriented single crystal MoS2 films. Films grown under optimal conditions were found to be of high structural quality from high-resolution X-ray diffraction, transmission electron microscopy, and Raman measurements, approaching the quality of reference geological MoS2. Photoluminescence and electrical measurements confirmed the growth of optically active MoS2 with a low background carrier concentration, and high mobility. The CVD method reported here for the growth of high quality MoS2 thin films paves the way towards growth of a variety of layered 2D chalcogenide semiconductors and their heterostructures.

Development of AlGaN/GaN HEMTs with efficiencies above 60% up to 100 V for next generation mobile communication 100 W power bars

AbstractWe present a systematic study of epitaxial growth, processing technology, device performance and reliability of our GaN HEMTs manufactured on 3‐inch SiC substrates. Epitaxy and processing are optimized for both performance and reliability. The deposition of the AlGaN/GaN HEMT epitaxial structures is designed for both low background carrier concentration and a low trap density in order to simultaneously achieve a high buffer isolation and low DC to RF dispersion. Device fabrication is performed using standard processing techniques involving both electron‐beam and stepper lithography. The developed HEMTs demonstrate excellent high‐voltage stability, high power performance and large power added efficiencies. Devices exhibit two‐terminal gate‐drain breakdown voltages in excess of 160 V (current criterion 1 mA/mm) across the entire 3‐inch wafer with parasitic gate and drain currents well below 1 mA/mm when biased up to 80 V drain bias under pinch‐off conditions. Load‐Pull measurements at 2 GHz on 800 μm gate periphery devices return both a well‐behaved relationship between bias‐voltage and output‐power as well as power‐added‐efficiencies beyond 60% up to UDS = 100 V. For a drain bias of 100 V an output‐power‐density around 22 W/mm with 26 dB linear gain is obtained. On large periphery devices (32 mm gate width packaged in industry‐standard ceramic packages) an output power beyond 100 W is achieved with a PAE above 50% and a linear gain around 15 dB. Reliability is tested on devices having a gate periphery of 8×60 μm at an operating bias of 50 V under both DC and RF conditions. About 10% drain‐current change under DC‐stress (50 mA/mm) is observed after more than 1000 h of operation with an extrapolated drain‐current degradation below 20% after 200,000 h (more than 20 years) of operation. Under RF stress (2 GHz, 1 dB compression) the observed change in output power density is below 0.2 dB after more than 1000 h. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

GaN HEMT and MMIC development at Fraunhofer IAF: performance and reliability

AbstractWe present a systematic study of epitaxial growth, processing technology, device performance and reliability of our GaN HEMTs and MMICs manufactured on 3 inch SiC substrates. Epitaxy and processing are optimized for both performance and reliability. The deposition of the AlGaN/GaN HEMT epitaxial structures is designed for low background carrier concentration and a low trap density in order to simultaneously achieve a high buffer isolation and low DC to RF dispersion. Device fabrication is performed using standard processing techniques involving both electron‐beam and stepper lithography. Gate lengths of 250 nm and 500 nm are employed for 10 GHz and 2 GHz applications, respectively. The developed HEMTs demonstrate excellent high‐voltage stability, high power performance and large power added efficiencies. Devices exhibit two‐terminal gate–drain breakdown voltages in excess of 160 V (current criterion 1 mA/mm) across the entire 3 inch wafer with parasitic gate and drain currents well below 1 mA/mm when biased up to 80 V drain bias under pinch‐off conditions. Load‐Pull measurements at 2 GHz on 800 μm gate width devices return a well‐behaved relationship between bias‐voltage and output‐power as well as power‐added‐efficiencies beyond 60% up to UDS = 100 V. For a drain bias of 100 V an output‐power‐density around 22 W/mm with 26 dB linear gain is obtained. On large devices (32 mm gate width packaged in industry‐standard ceramic packages) an output power beyond 100 W is achieved with a PAE above 50% and a linear gain around 15 dB. Dual‐stage MMICs in microstrip transmission line technology yield a power added efficiency of 40% at 8.56 GHz for a power level of 11 W. A single‐stage MMIC yields a PAE of 46% with 7 W of output power at VDS = 28 V. Reliability is tested on HEMT devices having a gate periphery of 8 × 60 μm at an operating bias of 50 V under both DC and RF conditions. About 10% drain‐current change under DC‐stress (50 mA/mm) is observed after more than 1000 h of operation with an extrapolated drain‐current degradation below 20% after 200000 h (more than 20 years) of operation. Under RF stress (2 GHz, 1 dB compression) the observed change in output power density is below 0.2 dB after more than 1000 h. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High-quality Schottky contacts to n-InGaN alloys prepared for photovoltaic devices

Large-area Au∕Pt∕n-In0.2Ga0.8N Schottky contacts have been fabricated for photovoltaic devices. The current transport mechanisms of the Schottky contacts to n-In0.2Ga0.8N with different background carrier concentrations are investigated. The thermionic emission is a dominating current transport mechanism at the Pt∕n-InGaN interface in a low background carrier concentration sample, while the defect-assisted tunneling current and trap-related recombination current play important roles in high background carrier concentration samples. The Schottky diode fabricated using the low background carrier concentration sample gives much better Schottky barrier characteristics and exhibits a three to four order of magnitude higher spectral responsivity and a larger rejection ratio in comparison with those fabricated using the high background carrier concentration samples.

Electronic Properties of GaN Films Grown on TiC Substrates

Crystalline GaN films have been successfully deposited on titanium carbide substrates by low-pressure organometallic vapor-phase epitaxy technique. Low background carrier concentration was verified by capacitance-voltage measurements. Low-temperature photoluminescence measurements indicate that the heteroepitaxial layer has high crystalline quality, as verified by X-ray measurements, and is consistent with low background donor concentration near the film surface.

High-quality and crack-free Al xGa 1−xN ( x∼0.2) grown on sapphire by a two-step growth method

We report a two-step growth of Al x Ga 1− x N ( x∼0.2) by metal-organic vapor-phase epitaxy without a pretreatment of the substrate. High-quality, crack-free and near GaN-free AlGaN was achieved with a thickness much exceeding the theoretical critical thickness of the growth of AlGaN on GaN. The method was compared with the conventional ones such as AlGaN on AlN nucleation layer and AlGaN on GaN buffer layer. In situ reflectometry monitoring indicated different behaviors of the high temperature grown AlGaN over different underlayers. The double crystal X-ray rocking curve showed a full-width at half-maximum of only 500 arcsec for the 1.7 μm Al 0.2Ga 0.8N grown. The Hall measurement showed a low background carrier concentration of 7×10 17 cm −3. The electron mobility was 150 cm 2 V −1 s −1 for the Al 0.2Ga 0.8N film doped to 2×10 18 cm −3. The surface morphology and crystal quality were systematically examined at different growth stages. The likely mechanism for the strain relaxation and high-quality AlGaN films grown were also suggested.

Optical properties of GaN grown on Si (111) by gas source molecular beam epitaxy with ammonia

We report a study of the optical properties of GaN grown on Si(111) by gas source molecular beam epitaxy with ammonia. Temperature dependence of edge luminescence was studied in the range of 77–495 K for samples with low background carrier concentrations, as determined by capacitance voltage profiling and Raman spectroscopy, and the results were fitted using Passler’s and Varshni’s models. We also demonstrate strong correlation between electron concentration in GaN and relative Raman intensity of A1 (longitudinal optical) and E22 modes. The binding energy of free excitons is estimated to be 29±2 meV. The contributions of different mechanisms to free exciton line broadening are discussed.

Plasma-assisted molecular beam epitaxy of ZnO thin films on sapphire substrates with an MgO buffer

We report on plasma-assisted molecular beam epitaxy of ZnO on Al 2O 3 (0 0 0 1) substrates with an MgO buffer layer. We found a thin MgO buffer greatly influence the initial growth of ZnO by facilitating the adhesion and promoting the lateral epitaxy of ZnO. Consequently, a flat surface with a (3×3) reconstruction of ZnO is observed and reflection high-energy electron diffraction intensity oscillations are recorded. Both in situ investigations and structural analysis revealed that the MgO buffer has a rocksalt structure and consists of a 2D wetting layer and 3D islands above. The change in surface energy by covering the substrate with an MgO wetting layer results in a drastic suppression of columnar growth and layer-by-layer epitaxy of ZnO is achieved. The resulting improvement in crystal quality of the ZnO epilayers is confirmed by a large decrease in the X-ray rocking curve widths of both symmetrical and asymmetrical diffraction peaks, as well as the presence of free exciton recombinations in low temperature photoluminescence and the low n-type background carrier concentration.

Thin-film-induced index change and channel waveguiding in epitaxial GaN films

We report on optical waveguiding in the channel region of an epitaxial GaN film defined by a SiN cladding layer of a stripe window pattern. We carried out numerical analyses on the various possible effects that might contribute to the overcompensation of the negative loading effect of a SiN cladding window. This includes the photoelastic, piezoelectric, and electro-optic effects in GaN induced by a SiN window layer. The analysis result suggests that the observed phenomenon can be ascribed to a combination of both the photoelastic and electro-optic effects, and especially that the spontaneous polarization field in undoped GaN with a low background carrier concentration might play an important role in forming a channel waveguide in the window region.

Oxygen pressure-tuned epitaxy and optoelectronic properties of laser-deposited ZnO films on sapphire

Influence of oxygen pressure on the epitaxy, surface morphology, and optoelectronic properties has been studied in the case of ZnO thin films grown on sapphire (0001) by pulsed-laser deposition. Results of Rutherford backscattering and ion channeling in conjunction with atomic force microscopy clearly indicate that the growth mode, degree of epitaxy, and the defect density strongly depend on the oxygen background pressure during growth. It is also found that the growth mode and the defects strongly influence the electron mobility, free-electron concentration, and the luminescence properties of the ZnO films. By tuning the oxygen pressure during the initial and the final growth stages, smooth and epitaxial ZnO films with high optical quality, high electron mobility, and low background carrier concentration have been obtained. The implication of these results towards the fabrication of superlattices and controlled n- and p-type doping is discussed.