Undoped GaN Layers Research Articles

Performance improvement of GaN-based vertical cavity surface emitting laser by employing a hole storage layer and enhancing the carrier transport behavior

In this paper, the transport behavior of carriers between multiple quantum wells (vertical) and inside a single quantum well (radial) in a GaN-based Vertical Cavity Surface Emitting Laser (VCSEL) is investigated, and a new device structure with a hole storage layer is proposed to enhance the laser performance. The theoretical results indicate that the vertical transport capability of holes is the key factor for total stimulated recombination rate and hole leakage. The VCSEL with a double-QW structure exhibits better output performance compared to the devices with three or more QWs owing to the lower light absorption loss and the stronger optical field in the active region. Furthermore, a thicker undoped GaN layer is proposed to be added as the hole storage layer to mitigate hole leakage for the VCSEL with a double-QW structure. This new structure can increase the tilt of energy band in the region of the hole storage layer and effectively improve the uniformity of radial distribution of holes in QWs. Therefore, it is beneficial to reduce the threshold current and enhance the laser output power.

Study of stress/strain and structural defects at Cu/GaN interface

Cu films with different thicknesses were deposited on GaN wafers with different doping states. Several techniques were employed to study element, dislocation density and stress/strain at Cu/GaN interfaces. The stress decreased with the increase of Cu thickness in the three undoped GaN layers. In the undoped GaN, screw dislocations with a Burger's vector of 1/2<0001 > and edge dislocations with a Burger's vector of 1/3<112‾0 > were produced. The SFs in the Mg doped GaN were type I1 basal-plane SFs with a ABABCBC stacking in the [0002‾] direction and a total displacement vector 1/6<202‾3 >. The 50 nm Cu/Si-doped GaN had the biggest stress (0.48 GPa) and the largest screw dislocation density (7.01 × 107cm−2) among the five Cu/GaN samples. Dislocation movement and stress relaxation were restrained by Si atoms. In the Si-doped GaN, abundant positive and negative strain extremes in [0002‾] direction far from the border line were due to the generation of dislocations. One reason for the existence of interfacial stress was the difference of thermal expansion coefficients between Cu and GaN. The Cu/GaN interfaces were mainly semi-coherent. The lattice mismatch between Cu and GaN did not generate abundant dislocations near the interface line and thus the strain was preserved.

Plasma-Assisted Halide Vapor Phase Epitaxy for Low Temperature Growth of III-Nitrides

Developing growth techniques for the manufacture of wide band gap III-nitrides semiconductors is important for the further improvement of optoelectronic applications. A plasma-assisted halide phase vapor epitaxy (PA-HVPE) approach is demonstrated for the manufacture of undoped and In-doped GaN layers at ~600 °C. A dielectric barrier discharge (DBD) plasma source is utilized for the low-temperature activation of ammonia. The use of the plasma source at a growth temperature of ~600 °C increases the growth rate from ~1.2 to ~4–5 µm/h. Furthermore, the possibility for the growth of InGaN at ~600 °C has been studied. Precursors of GaCl and InCl/InCl3 are formed in situ in the reactor by flowing HCl gas over a melt of metallic Ga and In, respectively. The In concentration was low, in the order of a few percent, as the incorporation of In is reduced by plasma due to the activation of chlorine-containing species that etch the relatively poorly bonded In atoms. Nevertheless, the approach of using plasma for ammonia activation is a very promising approach to growing epitaxial III-nitrides at low temperatures.

Design of indium-free nitride-based tunnel junction by polarization engineering

Structural design and characterization of indium-free nitride-based tunnel junctions (TJs) are investigated numerically. Specifically, polarization-engineered n- and p-type compositional-graded AlGaN layers are employed to pursue superior tunneling characteristics in the tunnel junctions under typical doping conditions. The corresponding polarization field in the undoped GaN layer that is surrounded by graded AlGaN layers is with the same direction as the doping-induced built-in electric field, which is beneficial for carrier tunneling. Moreover, the detrimental polarization fields in AlGaN layers whose direction is opposite to the built-in electric field can be effectively suppressed with the compositional-graded configuration. Simulation results indicate that, by appropriately adjusting the thickness of undoped GaN layer and n- and p-type graded AlGaN layers, voltage drop and resistivity of the proposed TJ structure can be as low as 0.045 V and 4.48 × 10−5 Ω cm2, respectively, at 1 kA/cm2. The results of this study may be of great help for the development of indium-free nitride-based tunnel junctions.

Luminescence and scintillation properties of the Si doped InGaN/GaN multiple quantum wells

The InGaN multiple quantum wells (MQW) samples with the undoped and Si doped GaN barriers were grown by Metal Organic Vapour Phase Epitaxy (MOVPE). By comparing defects-related emission bands in the undoped GaN and InGaN layers, one may conclude that the band is complex in the InGaN layer, composed of at least two contributions peaking at 2.17 and 2.39 eV, respectively. In and Si affect the intensity of the defects-related band – the larger the In and/or Si concentration the stronger the band. The detailed investigation of the observed phenomena was conducted, and the observed peculiarities were explained.

Carbon and silicon background impurity control in undoped GaN layers grown with trimethylgallium and triethylgallium via metalorganic chemical vapor deposition

The unintentional impurity incorporation in GaN epitaxial layers impacts the electrical conductivity and optical properties of the films grown by metalorganic chemical vapor deposition (MOCVD). It is critical to control impurity-related states for device structure development. The aim of this work is to contribute to the understanding of the reasons for the presence of certain background impurities. In this paper, the unintentional impurity incorporation of carbon, hydrogen, oxygen, and silicon in undoped (u-) GaN films grown by low-pressure MOCVD were studied. Background impurity concentrations were evaluated by secondary ion mass spectroscopy (SIMS) to optimize growth parameters including V/III ratio, growth temperature (Tg), growth pressure (Pg), and gallium (Ga) precursors of trimethylgallium (TMG) or triethylgallium (TEG). Unintentional [C] and [Si] incorporations are found to be highly dependent on the growth parameters. For the same growth conditions, u-GaN films grown with a TEG precursor exhibited a lower background concentration of C compared to that of GaN grown with TMG. However, the lowest background [C] achieved was grown with TMG using optimized conditions and exhibited [C] < 4 × 1015 cm−3 which was at the detection limit of the SIMS measurement. These results were measured for a u-GaN layer grown with TMG at 200 Torr, 1030 °C, and a V/III of 3700. The lowest background [Si] of 4 × 1015 cm−3 was achieved by growing with TMG at 200 Torr, 1000 °C, and V/III of 650.

Origins of the Schottky Barrier to a 2DHG in a Au/Ni/GaN/AlGaN/GaN Heterostructure.

We report the influence of thickness of an undoped GaN (u-GaN) layer on current transport to a 2DHG through the metal/p++GaN contact in a GaN/AlGaN/GaN heterostructure. The current is dominated by an internal potential barrier of 0.2–0.27 eV at the p+ GaN/u-GaN, which increases with thickness from 5 to 15 nm and remains constant thereafter due to Fermi pinning by a defect at ∼0.6 eV from the top valence band. We also report a nonideality factor, n, between 6 and 12, for the combined tunneling current through the p+GaN/u-GaN to the 2DHG. Our contact resistivity of 5.3 × 10–4 Ω cm2 and hole mobility, μ, of ∼15.65 cm2/V s are the best-in-class for this metal stack on a GaN/AlGaN/GaN heterostructure, reported to date.

Design and Analysis of DC/DC Boost Converter Vertical GaN Power Device based on Epitaxially Grown GaN-on-sapphire

In this study, a high-performance vertical gallium nitride (GaN) power transistor was designed by using two-dimensional technology computer-aided design simulator. In addition, the vertical GaN transistor is used to analyze the DC/DC boost converter. Systems requiring high voltages of 1000 V or more, such as electric vehicles, need wide devices to achieve a high breakdown voltage when using conventional power devices. However, vertical GaN transistors can be fabricated with a small device area and a high breakdown voltage. The proposed device has an off-current of 413 pA/㎠, an on-current of 22 kA/㎠, and a high breakdown voltage of 1693 V due to good gate controllability and the undoped-GaN layer. The designed device was used to construct a boost converter that doubled the input voltage and its characteristics were examined. The boost converter produced an output voltage of 1955 V and the voltage conversion efficiency was high at 97.75 %.

Realization of both enhancement and depletion mode MOSFETs on GaN-on-Si LED epitaxial wafer

In this letter, we propose, fabricate, and characterize both an enhancement mode (E-mode) and a depletion mode (D-mode) MOSFET on a standard LED epitaxial wafer based on silicon substrate for the first time. The devices are fabricated on the n-GaN layer or undoped-GaN layer of the LED epitaxial wafer by semiconductor manufacturing techniques. The working mode (E-mode or D-mode) of the device is dependent on the etching depths of the gate recess. The experiments show that the E-mode and D-mode MOSFETs possess a maximum output current density of 0.15 mA mm−1 and 4.6 mA mm−1 under the gate-to-source voltage of 8 V and drain-to-source voltage of 8 V, respectively. It is expected that such E-mode and D-mode MOSFETs achieved on GaN-on-Si LED epitaxial wafer should have potential applications in monolithic optical electronic integrated circuits.

Carrier dynamics and excitation of Eu3+ ions in GaN

The carrier mobility and dynamics of nonequilibrium carriers in Eu-doped GaN have been determined by above-band-gap, optically excited, time-resolved terahertz spectroscopy and compared with that of undoped GaN. The carrier mobility of GaN:Eu was an order of magnitude smaller than that of a similarly grown undoped GaN layer. Two time constants were observed in the carrier relaxation dynamics for GaN:Eu. A fast one (2.2 ps), which we related to an efficient nonradiative carrier trap, and a slower one (\ensuremath{\sim}85 ps) related to Eu-related traps. Ultrafast photoluminescence spectroscopy shows that Eu-related emission is observed within 100 ps after a pulsed excitation. These values set an upper value to the energy-transfer time from the Eu-related trap to $\mathrm{E}{\mathrm{u}}^{3+}$ ion and underlie the high efficiency of the excitation of $\mathrm{E}{\mathrm{u}}^{3+}$ ions in GaN.

Extra half-plane shortening of dislocations as an origin of tensile strain in Si-doped (Al)GaN

Si doping of (Al,Ga)N layers grown by metalorganic chemical vapor deposition induces an inclination of threading dislocations (TDs). This inclination leads to a change of the extra half-plane size of edge and mixed type dislocations. Depending on the dislocation density and the doping concentration, these effects are accompanied by the generation of tensile strain, which can also lead to crack formation. Several models have been published in the past in order to explain this process. Different models result in opposite TD inclination directions with respect to the extra half-plane position. Therefore, this work examines the correlation between the extra half-plane position and the inclination direction to clarify the origin of the tensile strain increase using scanning transmission electron microscopy. With this approach, it can be unambiguously experimentally verified that Si doping leads to a shortening of the dislocations half-plane. An analysis of in situ wafer curvature measurement proves that the increase of tensile strain in GaN caused by Si doping can be explained by this process. Aside from the inclination caused by Si doping, a TD inclination in undoped GaN layers has been analyzed. Possible explanations for the inclination process are discussed.

1/f noise characteristics of AlGaN/GaN HEMTs with periodically carbon-doped GaN buffer layer

We investigate the DC and 1/f noise properties in Al0.25Ga0.75N/GaN high-electron mobility transistors (HEMTs) with two types of 2 μm-thick periodically carbon-doped GaN buffer layer (PC-doped GaN buffer) with and without inserting the 30 nm-thick Al0.05Ga0.95N back barrier layer between the GaN channel layer and the PC-doped GaN buffer. The PC-doped GaN buffer layer consists of multiple layers of 12 nm-thick C-doped GaN layer with doping concentration of 1 × 1018 cm−3 and 50 nm-thick undoped GaN layer with unintentional n-typing concentration of 2 × 1016 cm−3. A reference AlGaN/GaN HEMT with 2 μm-thick highly-resistive GaN buffer layer without C-doping is also fabricated for comparison. Similarly to the reference AlGaN/GaN HEMT, the AlGaN/GaN HEMTs with PC-doped GaN buffer show typical 1/f noise characteristics mainly due to the trapping effects at the AlGaN/GaN interface from subthreshold region to strong-accumulation region, which indicates that the deep trapping effects in the PC-doped GaN buffer layer is negligible, and experience the correlated mobility fluctuations (CMF), which is convinced from the drain current power spectral density (PSD) versus drain current. At off-state (deep-subthreshold region), on the other hand, the HEMTs with the PC-doped GaN buffer layer exhibit 1/f2 noise characteristics, which are closely related to the generation-recombination (g-r) noise caused by the spatial trapping/detrapping process between the deep acceptor in the C-doped layer and the shallow donor in the undoped layer in the PC-doped GaN buffer, while the reference HEMT still shows typical 1/f noise characteristics.

Structural and Optical Changes of Undoped GaN Layers Grown via Radio-frequency Magnetron Sputtering Obtained from GaN Powders

Undoped GaN layers were grown via radio-frequency magnetron sputtering, using a target manufactured with undoped GaN powders. Where the GaN powders were sintetized by nitridation of metallic gallium at 1000 °C in ammonia flow for two hours. X-ray diffraction patterns demonstrated that there are not a significant difference between the diffraction angles of the GaN powders and the GaN layers. The x-ray diffraction patterns for the GaN powders showed narrow peaks with a crystal size of 41 nm, while the GaN layers showed broad peaks with a crystal size of 7.7 nm. Scanning electron microscopy micrographs demonstrated the formation of crystals of irregular size with an average length of 1.56 μm for the GaN powders, while a homogeneous surface morphology with a thickness of 6.6 μm for the GaN layers was observed. Photoluminescence spectra showed a high emission at 3.49 eV (355.13 nm) for the GaN powders and an emission band energy located at 3.42 eV (361.54 nm) for the GaN layers, both emission bands were related to the band-to-band transition for the GaN. Raman spectra for the GaN powders showed the A1(TO), E1(TO), and E2(High) classical vibration modes. The GaN layers only showed the A1(TO) mode.

Structural and Optical Changes of Undoped GaN Layers Grown via Radio-frequency Magnetron Sputtering Obtained from GaN Powders

Undoped GaN layers were grown via radio-frequency magnetron sputtering, using a target manufactured with undoped GaN powders. Where the GaN powders were sintetized by nitridation of metallic gallium at 1000 °C in ammonia flow for two hours. X-ray diffraction patterns demonstrated that there are not a significant difference between the diffraction angles of the GaN powders and the GaN layers. The x-ray diffraction patterns for the GaN powders showed narrow peaks with a crystal size of 41 nm, while the GaN layers showed broad peaks with a crystal size of 7.7 nm. Scanning electron microscopy micrographs demonstrated the formation of crystals of irregular size with an average length of 1.56 ?m for the GaN powders, while a homogeneous surface morphology with a thickness of 6.6 ?m for the GaN layers was observed. Photoluminescence spectra showed a high emission at 3.49 eV (355.13 nm) for the GaN powders and an emission band energy located at 3.42 eV (361.54 nm) for the GaN layers, both emission bands were related to the band-to-band transition for the GaN. Raman spectra for the GaN powders showed the A1(TO), E1(TO), and E2(High) classical vibration modes. The GaN layers only showed the A1(TO) mode.

Kinetic limitations of stress relaxation and generation in GaN/AlN and AlGaN: Si/AlN heterostructures grown on c-sapphire by plasma-assisted molecular beam epitaxy

The paper describes experimental study of stress relaxation and generation in (1-2)-μm-thick GaN and AlGaN layers grown on AlN/c-Al2O3 buffer layers by low temperature (<720 °C) metal-rich plasma-assisted molecular beam epitaxy (PA MBE). The atomically smooth undoped GaN layers demonstrate only gradual relaxation of the compressive stress, which is probably related to thermodynamically driven inclination of threading dislocations (TDs).The slower stress relaxation at the lower growth temperature is explained by kinetic limitation of this process. The switch of compressive to tensile stress in the less-strained undoped Al0.7Ga0.3N layers, attributed mostly to the same effect of TD inclination, occurs in the low-temperature PA MBE conditions at much larger thickness (∼0.6 with) as compared to MOVPE ones. Introduction of high Si doping (n∼1019cm-3) reduces noticeably the initial compressive stress in the AlGaN film due to substitution of small Si atoms in the group-III sublattice. At larger thickness, Si atoms seem to effect the TD propagation and suppress generation of tensile stress related to TD inclination, which makes possible to grow ∼1μm-thick Al0.7Ga0.3N: Si films without cracking.

Suppression of Iron Memory Effect in GaN Epitaxial Layers

AlGaN/GaN High Electron Mobility Transistors (HEMTs) require a semi‐insulating buffer to compensate a high background donor concentration and to prevent parasitic effects, such as parallel conduction. Iron and carbon are typical impurities used for such purpose, since they can behave as deep acceptors in GaN layers. The former (Fe) brings as drawback a well‐known memory effect which consists in the segregation of Fe atoms through the GaN layers, requiring thick undoped layers to keep the two‐dimensional electron gas (2DEG) away from such electron traps. The latter (C), although easier to incorporate in the GaN layers and free of any memory effect, could cause current collapse. In this study we investigate the effect of differently strained epitaxial layers on Fe‐segregation. HEMT structures were grown on sapphire substrates by MOCVD. By growing the Fe‐doped layers under unusual growth conditions (low temperature, or as AlGaN), or by adding an interlayer (AlN, 5× AlN/GaN, or C‐doped GaN) between the doped and undoped epitaxial layers, we have succeeded in limiting the Fe segregation within 200 nm of undoped GaN layer instead of the typically required 800 nm, as proven by SIMS. While the morphology and the crystal quality of the HEMT structures have been affected to a very low extent, the electrical characteristics have benefitted from such interlayers. Higher carrier mobility and lower sheet resistance distinguish such epitaxial structures. Further benefits are expected by the device performance such as reduced soft‐subthreshold behavior (soft‐breakdown) and dispersion effects.

Simultaneous determination of defect distributions and energies near InGaN/GaN quantum wells by capacitance–voltage measurement

The distributions, densities and energies of defects near InGaN/GaN quantum wells in blue light-emitting diodes were simultaneously determined by utilizing capacitance–voltage (C–V) measurements. By combining the modulation frequency dependency, temperature dependency and C–V depth profiling with additional laser illumination, the densities and the locations of the defective layers could be determined. The relative defect densities of the devices were directly compared by monitoring the magnitude of the frequency dependence. This frequency dependency varies distinctly as the sample temperature changes. The activation energies of defects are then determined by analyzing the frequency dependency of C–V with temperature. We found that three different defects states were formed in a low-temperature-grown un-doped GaN (LT-GaN) layer inserted under the active layer. The activation energies of those defects were determined to be 3.96, 12.1 and 45.9 meV. The formation of additional defects states in the active layers induced by the insertion of LT-GaN layer was also observed.

Hydride-vapor-phase epitaxial growth of highly pure GaN layers with smooth as-grown surfaces on freestanding GaN substrates

Thick (20–30 µm) layers of highly pure GaN with device-quality smooth as-grown surfaces were prepared on freestanding GaN substrates by using our advanced hydride-vapor-phase epitaxy (HVPE) system. Removal of quartz parts from the HVPE system markedly reduced concentrations of residual impurities to below the limits of detection by secondary-ion mass spectrometry. Appropriate gas-flow management in the HVPE system realized device-quality, smooth, as-grown surfaces with an excellent uniformity of thickness. The undoped GaN layer showed insulating properties. By Si doping, the electron concentration could be controlled over a wide range, down to 2 × 1014 cm−3, with a maximum mobility of 1150 cm2·V−1·s−1. The concentration of residual deep levels in lightly Si-doped layers was in the 1014 cm−3 range or less throughout the entire 2-in. wafer surface. These achievements clearly demonstrate the potential of HVPE as a tool for epitaxial growth of power-device structures.

Capacitance-voltage characterization of Al2O3/GaN-on-insulator (GaNOI) structures with TMAH surface treatment

The capacitance-voltage (C-V) characterizations of Al2O3/GaN-on-insulator (GaNOI) structure, prepared with the Smart Cut™ technology, with and without tetramethylammonium hydroxide (TMAH) surface treatment have been investigated. The GaNOI structure consists of a 150nm-thick GaN layer and a 800μm-thick Si3N4/SiO2 buried insulating layer deposited on sapphire substrate. For fabrication of the MIS capacitor, an Al2O3 layer with thickness of 28nm as a gate dielectric was deposited on the GaNOI wafer by atomic layer deposition (ALD). The calculated carrier concentration of the GaN layer on the buried insulator was increased to 2.8×1017cm−3 from the value of ~5×1016cm−3 for the as-grown undoped GaN layer prior to the wafer transfer. The MIS capacitor with TMAH surface treatment showed a positive threshold voltage shift with negligible hysteresis and low interface trap density compared to the capacitor without TMAH surface treatment. Severe frequency dispersion was observed regardless of the TMAH treatment due to the crystal defects generated during the complicated wafer transfer process.

非故意掺杂GaN层厚度对蓝光LED波长均匀性的影响

非故意掺杂GaN层厚度对蓝光LED波长均匀性的影响