Low-temperature GaN Research Articles

Influence of a low-temperature GaN cap layer on the electron concentration in AlGaN/GaN heterostructure

The influence of low-temperature passivating GaN cap layers on the electrophysical parameters of a 2D electron gas (2DEG) in heterostructure high-electron mobility transistors has been studied. It has been found that thin GaN layers deposited in situ at 550°C do not exhibit polar properties and do not change the carrier concentration in the 2DEG. However, GaN layers deposited at 830°C decrease the carrier concentration in the 2DEG, which is in agreement with theoretical calculations. Using the reflected high-energy electron diffraction technique, it has been established that this effect may be associated with different structures and morphologies of GaN layers deposited at different temperatures.

Advantages of InGaN/GaN multiple quantum wells with two-step grown low temperature GaN cap layers

Two-step grown low temperature GaN cap layers (LT-cap) are employed to improve the optical and structural properties of InGaN/GaN multiple quantum wells (MQWs). The first LT-cap layer is grown in nitrogen atmosphere, while a small hydrogen flow is added to the carrier gas during the growth of the second LT-cap layer. High-resolution X-ray diffraction results indicate that the two-step growth method can improve the interface quality of MQWs. Room temperature photoluminescence (PL) tests show about two-fold enhancement in integrated PL intensity, only 25 meV blue-shift in peak energy and almost unchanged line width. On the basis of temperature-dependent PL characteristics analysis, it is concluded that the first and the second LT-cap layer play a different role during the growth of MQWs. The first LT-cap layer acts as a protective layer, which protects quantum well from serious indium loss and interface roughening resulting from the hydrogen over-etching. The hydrogen gas employed in the second LT-cap layer is in favor of reducing defect density and indium segregation. Consequently, interface/surface and optical properties are improved by adopting the two-step growth method.

Growth conditions of semi and non-polar GaN on Si with Er2O3 buffer layer

The GaN was grown on Si(100) with Er2O3(110) buffer by metal organic chemical vapor deposition (MOCVD). The results showed the polycrystalline nature of the layers with dominant non-polar (11–20) and semi-polar (10–13) orientations. GaN was formed in a two step process regime: low temperature growth was followed by high temperature growth regime. At a low temperature GaN tends to crystalize with dominant non-polar (11–20) orientation, while high temperature leads to preferential semi-polar (10–13) orientation growth. Additionally, the effects of parameters such as growth temperature, V/III-ratio and type of carrier gases on structural, morphological and optical properties of GaN was discussed in the current work.

Origin of huge photoluminescence efficiency improvement in InGaN/GaN multiple quantum wells with low-temperature GaN cap layer grown in N2/H2 mixture gas

The nominal internal quantum efficiency of InGaN/GaN multiple quantum wells significantly increases from 5.6 to 26.8%, as a low-temperature GaN cap layer is grown in N2/H2 mixture gas. Meanwhile, the room-temperature photoluminescence (PL) peak energy shows a merely 73 meV blue shift. On the basis of temperature-dependent PL characteristics analysis, the huge improvement in PL efficiency arises mainly from the “etching effect” of hydrogen, which reduces the defect density and indium segregation at the upper well/barrier interface, and consequently contributes to the decrease in the number of nonradiative recombination centers and the enhancement of carrier localization.

Low-temperature growth of AlN and GaN by metal organic vapor phase epitaxy for polarization engineered water splitting photocathode

Crystal properties of low-temperature grown AlN (LT-AlN) combined with low temperature GaN (LT-GaN) grown by metal organic vapor phase epitaxy (MOVPE) were investigated to obtain a high quality GaN/AlN/GaN structure with a few-nm-thick AlN layer. LT-AlN suppresses unintentional Ga incorporation and can be pseudomorphically grown on GaN with a relatively smooth surface morphology. The lattice of LT-AlN coherent to GaN, however, was found to relax after reactor conditions were changed to grow the subsequent GaN layer at higher temperature. The top GaN layer grown on the relaxed LT-AlN, thus, exhibited a rough surface morphology and a threading dislocation density (TDD) higher than 109cm−2 estimated from an X-ray diffraction measurement. An LT-GaN capping layer was found to be highly effective for avoiding such lattice relaxation of LT-AlN. The combination of LT-AlN and LT-GaN enables us to obtain a GaN/AlN/GaN junction with high Al content, a low TDD, and abrupt interfaces. As a result, introducing an LT-GaN layer improved the photoelectrochemical (PEC) property of a polarization engineered un-doped GaN/AlN/n-type GaN (u-GaN/AlN/n-GaN) photocathode for water splitting.

V-shaped pits in HVPE-grown GaN associated with columnar inversion domains originating from foreign particles of α-Si3N4 and graphitic carbon

The v-shaped pits (so-called V-pits) observed in hydride-vapor-phase-epitaxy-grown GaN and associated with the columnar inversion domains originating from foreign particles were investigated. The inversion domains on the front and back surfaces of the test sample were recognized after chemical mechanical polishing. It was found that the V-pits originate from the columnar inversion domains. The inversion domains, in turn, arise from the particles that exist on a low-temperature GaN buffer layer on sapphire substrate. Using transmission electron microscopy, these particles were found to be of α-Si3N4 and graphitic carbon. Such particles are attributable to the components of the reactor and adhere to the low-temperature GaN buffer layer, which has a surface roughness of the order of several nanometers. Thus, an effective way of obtaining HVPE-grown thick GaN layers without the V-pits associated with columnar IDs is to maintain the parts of the HVPE chamber properly to prevent foreign particles from being generated.

High-quality III-nitride films on conductive, transparent (2̅01)-oriented β-Ga2O3 using a GaN buffer layer

We demonstrate the high structural and optical properties of InxGa1−xN epilayers (0 ≤ x ≤ 23) grown on conductive and transparent (01)-oriented β-Ga2O3 substrates using a low-temperature GaN buffer layer rather than AlN buffer layer, which enhances the quality and stability of the crystals compared to those grown on (100)-oriented β-Ga2O3. Raman maps show that the 2″ wafer is relaxed and uniform. Transmission electron microscopy (TEM) reveals that the dislocation density reduces considerably (~4.8 × 107 cm−2) at the grain centers. High-resolution TEM analysis demonstrates that most dislocations emerge at an angle with respect to the c-axis, whereas dislocations of the opposite phase form a loop and annihilate each other. The dislocation behavior is due to irregular (01) β-Ga2O3 surface at the interface and distorted buffer layer, followed by relaxed GaN epilayer. Photoluminescence results confirm high optical quality and time-resolved spectroscopy shows that the recombination is governed by bound excitons. We find that a low root-mean-square average (≤1.5 nm) of InxGa1−xN epilayers can be achieved with high optical quality of InxGa1−xN epilayers. We reveal that (01)-oriented β-Ga2O3 substrate has a strong potential for use in large-scale high-quality vertical light emitting device design.

Improved performance of GaN based light emitting diodes with ex-situ sputtered AlN nucleation layers

The crystal quality, electrical and optical properties of GaN based light emitting diodes (LEDs) with ex-situ sputtered physical vapor deposition (PVD) aluminum nitride (AlN) nucleation layers were investigated. It was found that the crystal quality in terms of defect density and x-ray diffraction linewidth was greatly improved in comparison to LEDs with in-situ low temperature GaN nucleation layer. The light output power was 3.7% increased and the reverse bias voltage of leakage current was twice on LEDs with ex-situ PVD AlN nucleation layers. However, larger compressive strain was discovered in LEDs with ex-situ PVD AlN nucleation layers. The study shows the potential and constrain in applying ex-situ PVD AlN nucleation layers to fabricate high quality GaN crystals in various optoelectronics.

Polarity control of GaN grown on pulsed-laser-deposited AlN/GaN template by metalorganic vapor phase epitaxy

We report on the polarity control of GaN regrown on pulsed-laser-deposition-grown N-polar AlN on a metalorganic-vapor-phase-epitaxy-grown Ga-polar GaN template. The polarity of the regrown GaN, which was confirmed using aqueous KOH solutions, can be inverted from that of AlN by inserting a low-temperature GaN (LT-GaN) buffer layer. We hypothetically ascribe the Ga-polarity selection of GaN on the LT-GaN buffer layer to the mixed polarity of LT-GaN grains and higher growth rate of the Ga-polar grain, which covers up the N-polar grain during the initial stage of the high-temperature growth. The X-ray rocking curve analysis revealed that the edge-dislocation density in the N-polar regrown GaN is 5 to 8 times smaller than that in the Ga-polar regrown GaN. N-polar GaN grows directly on N-polar AlN at higher temperatures. Therefore, nucleus islands grow larger than those of LT-GaN and the area fraction of coalescence boundaries between islands, where edge dislocations emerge, becomes smaller.

Effect of GaAs substrate orientation on the growth kinetic of GaN layer grown by MOVPE

We have investigated the kinetic growth of low temperature GaN nucleation layers (LT-GaN) grown on GaAs substrates with different crystalline orientations. GaN nucleation layers were grown by metal organic vapor phase epitaxy (MOVPE) in a temperature range of 500–600 °C on oriented (001), (113), (112) and (111) GaAs substrates. The growth was in-situ monitored by laser reflectometry (LR). Using an optical model, including time-dependent surface roughness and growth rate profiles, simulations were performed to best approach the experimental reflectivity curves. Results are discussed and correlated with ex-situ analyses, such as atomic force microscopy (AFM) and UV-visible reflectance (SR). We show that the GaN nucleation layers growth results the formation of GaN islands whose density and size vary greatly with both growth temperature and substrate orientation. Arrhenius plots of the growth rate for each substrate give values of activation energy varying from 0.20 eV for the (001) orientation to 0.35 eV for the (113) orientation. Using cathodoluminescence (CL), we also show that high temperature (800–900 °C) GaN layers grown on top of the low temperature (550 °C) GaN nucleation layers, grown themselves on the GaAs substrates with different orientations, exhibit cubic or hexagonal phase depending on both growth temperature and substrate orientation.

Dislocation analysis of InGaN/GaN quantum dots grown by metal organic chemical vapor deposition

The dislocations in InGaN/GaN quantum dots grown by metal organic chemical vapor deposition were studied by high-resolution transmission electron microscopy combining the Fourier filtering process. The misfit dislocations were observed in uncapped InGaN/GaN quantum dots. However, for the capped InGaN/GaN quantum dots, the GaN capping layer was found to suppress the generation of misfit dislocations and hence hindered the strain relaxation. Therefore, an overgrowth InGaN layer was used to relieve the strain in InGaN quantum dots and misfit dislocations were correspondingly found in these samples. In addition, defects were observed in low temperature GaN layers which suggested the existence of stacking faults.

Study of the structural quality of GaN epitaxial layers obtained by hydride vapor phase epitaxy using a low-temperature buffer layer

The structural quality and surface morphology of low-temperature (LT) buffer layers after deposition and high-temperature (HT) annealing and HT GaN layers grown on LT buffer layers by hydride vapor phase epitaxy have been investigated. The HCl flow rate through the Ga source varied from 0.3 to 2 L/h, and the carrier gas N2 flow rate was either 18 or 60 L/h. It is established that the structural quality of LT GaN buffer is determined to a great extent by the HCl and N2 flow rates; the best results are obtained at HCl and N2 flow rates of 0.3 and 18 L/h, respectively. These GaN layers are characterized by a mirror surface and a rocking curve half-width of 360". It is suggested that the layer structure is improved due to the increase in the lateral growth rate.

Investigations of in situ reflectance of GaN layers grown by MOVPE on GaAs (0 0 1)

The growth of low temperature GaN (LT-GaN) layers on GaAs (001) substrate was performed by metal organic vapor phase epitaxy (MOVPE) at growth temperature range of 500–800°C. Laser reflectometry (LR) was employed for in situ monitoring of all growth steps. The simulation of experimental time reflectance traces shows that at the first growth stage, the surface roughness increases to reach a limit value depending on growth temperature. Due to surface roughness profile the growth rate time-dependence was found non negligible at the first growth stage. The investigations of in situ reflectance give more precise measurement of growth rates that yields to thermal activation energy close to 0.12eV. The ex-situ analyses by spectral reflectance (SR) and Atomic Force Microscopy (AFM) showed that the better surface morphology was obtained when the GaN buffer layer is grown at lower temperature, while three dimensional (3D) growth mode was observed at higher temperature. A series of high temperature (800°C) GaN (HT-GaN) layers were grown on different thicknesses of low temperature (550°C) GaN buffer layer. The results showed that high density of nucleation sites enhances the initial growth rate and improves the morphological quality of GaN active layer.

The Blue LED Nobel Prize: Historical context, current scientific understanding, human benefit

The Blue LED Nobel Prize: Historical context, current scientific understanding, human benefit

Successive selective growth of semipolar (11-22) GaN on patterned sapphire substrate

Thanks to the use of two successive selective growths by metal organic chemical vapor deposition reactor, high quality semipolar (11-22) GaN with a homogenous defect repartition over the surface was achieved. The procedure starts with a first selective growth on a patterned sapphire substrate, leading to continuous stripes of three dimensional (3D) GaN crystals of low defect density. Then, a second selective growth step is achieved by depositing a SiNx nano-mask and a low temperature GaN nano-layer on the top of the GaN stripes. Hereby, we demonstrate an original way to obtain a homoepitaxial selective growth on 3D GaN crystals by taking advantage of the different crystallographic planes available. Basal stacking faults (BSFs) are generated during this second selective growth but could be eliminated by using a three-step growth method in which elongated voids are created above the defective area. For a fully coalesced sample grown using the 2 step method, dislocation density of 1.2 × 108 cm−2 and BSFs density of 154 cm−1 with a homogenous distribution have been measured by cathodoluminescence at 80 K. Consequently the material quality of this coalesced semipolar layer is comparable to the one of polar GaN on c-plane sapphire.

(Invited) Neutron Irradiation Effect on GaN-Based Materials

For the predominant property, GaN becomes a promising material for ultraviolet/ blue light-emitting diode, solar blind ultraviolet photodetector, radiation resistant particle detectors, and high power high temperature electronic devices. Because of the radiation hardness of GaN, those devices have the potential to be used in outer space and other radiation environment. Irradiation induced defects play the main role in the reliability of GaN based devices. Recently, a strong effort by several groups was devoted to the analyses of electrical and optical properties of defects and their interaction by irradiating GaN with electrons, ions, neutrons and protons [1–4], which is very important for the devices operating at extreme harsh environment. Positron annihilation spectroscopy (PAS) with a slow-positron beam is an efficient tool for the investigation of open-volume defects such as vacancies in semiconductors, at which they get trapped. In a vacancy the electron density is decreased which makes the positron lifetime longer and narrows the positron-electron momentum distribution. The latter can be observed by recording the Doppler broadening of the energy spectrum of the annihilation radiation. Especially the momentum distribution of the core electrons is sensitive to the chemical environment of an annihilation site. However, in the case of light elements such as O or N the annihilation probability with the core electrons is negligible. The samples used in the present work were grown on C-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD) with unintentionally doped and Si doped. A 20 nm low temperature GaN nucleation layer was initially grown, and 4 μm GaN film grown at 1050°C was followed. The sample was cut into small pieces with an area of about 10×10 mm2, and irradiated with 2MeV proton from heavy ion accelerator. The incident fluence is 1014, 1015, or 1016 cm-2, respectively. Positron annihilation spectroscopy, XRD, and Raman spectra were measured. Irradiation induced defect is vacancies which increased with incident proton. XRD result shows that the crystal-order was kept after proton irradiation. Raman spectra of the Si doped sample shows a “carrier removal” effect but there is not any change in the other spectra.

A new growth method of semi-insulating GaN layer for HEMT structure by eliminating degenerate layer at GaN/sapphire interface

A new method of growing a semi-insulating GaN layer for high electron mobility transistor (HEMT) structure by eliminating a degenerate layer located at the GaN/sapphire interface is proposed. In the process, during the temperature ramp-up after the growth of a low temperature GaN buffer layer, trimethylgallium (TMGa) was flowed into the reactor together with ammonia, leading to the growth of an additional GaN layer of high carbon concentration. We confirmed that the introduction of TMGa induced high carbon concentration in the degenerate layer. The incorporated carbon formed deep acceptors, CN, compensating donors such as ON, resulting in the elimination of the degenerate layer. A HEMT device made on the sample grown by the new growth process shows a good pinch-off characteristic and high off-state breakdown voltage over 800 V at a gate voltage of −4 V, indicating the new scheme is effective to grow a high quality semi-insulating GaN layer.

Influences of group-III source preflow on the polarity, optical, and structural properties of GaN grown on nitridated sapphire substrates by metal-organic chemical vapor deposition

We report the influences of group-III source preflow, which were introduced prior to the growth of the low temperature GaN on the polarity, photoluminescence (PL), and crystallographic properties of GaN epilayers grown on nitridated c-plane sapphire substrates by metal-organic chemical vapor deposition. By studying the surface morphology evolutions under chemical etching in KOH, we found that with increasing the trimethyl-gallium (TMGa) preflow duration (t), the polarity of the GaN film can be changed from a complete N-polarity to a mixture of N- and Ga-polarity and further to a complete Ga-polarity. PL and high-resolution X-ray diffraction studies revealed that the impurity incorporation and the edge- and screw-type threading dislocations are strongly polarity dependent. A further study at the optimized t (i.e., 30 s for TMGa) shows that the polarity inversion of GaN can be realized not only by TMGa preflow but also by trimethyl-aluminium preflow and by trimethyl-indium preflow. A two-monolayer model was employed to explain the polarity inversion mechanism.

Reflectance analysis on the MOCVD growth of AlN on Si(111) by the virtual interface model

AbstractThe reflectance‐time profile of AlN‐on‐Si(111) growth by MOCVD as acquired through an in‐situ optical reflectance monitor is analyzed. It is found that, similar to the case of GaN‐on‐sapphire nucleation and growth through the low temperature GaN or AlN buffer layer techniques, the reflectance profile for AlN‐on‐Si also contains information that relate to the growth mechanism, which can then be correlated to the as‐grown material quality. Based on the equations for the Virtual Interface model, a set of 6 parameters can be used to curve‐fit the reflectance traces of AlN growth as acquired by the in‐situ optical monitor. By analyzing a range of 405 nm AlN reflectance‐time data from single layer AlN growth to multilayer AlN/AlGaN/GaN stack grown on Si(111), the “goodness of fit” statistics are found to correlate with material quality metrics such as the XRD (002) rocking curve FWHM for AlN, microscopic morphological roughness of the AlN surface, as well as the resultant wafer bow and curvature for GaN/AlGaN grown on the AlN/Si(111) substrate. The correlations provide useful insight into the desirable mechanism and growth mode for the high temperature MOCVD growth of AlN and GaN on Si(111). When fully established, the technique has the potential of being used as an in‐situ pass/fail screen for the MOCVD growth of GaN‐on‐Si. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Mg Doping in GaN Interlayer on the Performance of Green Light-Emitting Diodes

A light-emitting diode (LED) structure containing a low-temperature (LT) GaN interlayer between active region and AlGaN electron blocking layer is proposed to improve the performance of InGaN-based green LEDs. The experimental and simulated results show that, as the Mg doping depth in the LT-GaN interlayer increases, the hole injection efficiency gets improved and electron current leakage decreases, while defect-related nonradiative recombination increases. With an optimized Mg doping depth in the LT-GaN interlayer, a substantial suppression of efficiency droop can be achieved compared with the conventional LED.