Nonpolar GaN Research Articles

The determination of WSe2/GaN heterojunction band offsets in the semipolar (11–22) and nonpolar (11–20) planes by x-ray photoelectron spectroscopy

Wurtzite structured GaN has a severe polarization effect in the c (0001) plane, compared to which the polarization effect is small in the semipolar (11–22) plane, and there is no polarization effect in the nonpolar (11–20) plane GaN. To investigate the influence of the polarization effect on the band bending at the heterojunction interface, we fabricated tungsten diselenide (WSe2)/(0001) GaN, WSe2/(11–22) GaN, and WSe2/(11–20) GaN heterojunctions. We measured the heterojunction valence band offsets (VBOs) by x-ray photoelectron spectroscopy. The VBOs of the three WSe2/GaN heterojunctions were measured to be 2.43 ± 0.15, 2.51 ± 0.15, and 2.23 ± 0.15 eV, and the conduction band offsets were calculated to be 1.11 ± 0.15, 1.19 ± 0.15, and 0.91 ± 0.15 eV, showing the type II energy band alignment of the three heterojunctions. The results demonstrate that WSe2/(11–22) GaN-faced heterojunction band bending is the largest. This provides theoretical insights for two-dimensional WSe2 and three-dimensional semipolar (11–22) GaN and nonpolar (11–20) GaN heterojunction device preparation.

Effects of gamma irradiation on non-polar GaN films deposited on sapphire using pulsed laser deposition

The GaN films/layers exposed to γ-radiations is known to harvest defects and vacancies in the crystals producing donor, acceptor and recombination centers within the bandgap. Therefore it is important to investigate and study the γ- ray irradiation effects on various physical and chemical properties of a material before any optoelectronic and/or electronic devices are being fabricated. To avoid Stark effect which is observed in most of the optoelectronic devices fabricated using GaN films grew along polar face, use of non-polar GaN films is suggested by researchers. To address such issues the article reports the investigations of physical and chemical properties of non-polar GaN films grown on polar substrate using pulsed laser deposition, which were exposed to the 60Co gamma rays varying dose values. Resistive nature against the impairment of the films caused by γ-rays observed herewith is highly encouraging, suggesting the use of non-polar GaN films as radiations harden material suitable for fabricating new generation γ-ray detectors. To our knowledge very limited information is available that report such investigations.

Impacts of specific element-treated three-dimensional GaN layer on characteristics of nonpolar a-plane GaN film

Nonpolar a-plane GaN films with three-dimensional (3D) GaN layers have successfully grown on r-plane sapphire substrates by metal organic chemical vapor deposition. The inserted 3D GaN layers were grown accompanied with the treatment by various kinds of elements such as Si, Mg, and In to further improve the characteristics of the subsequently lateral overgrown nonpolar a-plane GaN films. Scanning electron microscopy, x-ray rocking curve, and room temperature photoluminescence spectroscopy were used to examine the effects of the introduction of the specific element-treated 3D GaN layer on the crystalline quality, the anisotropy, and the optical property of the nonpolar a-plane GaN film. It was found that significant improvements in the crystalline quality and optical property as well as a remarkable reduction in anisotropy have been achieved for the nonpolar a-plane GaN film grown on the r-plane sapphire substrate by inserting a 3D GaN layer treated with Si. In fact, evident reduction in full width at half maximum of x-ray rocking curves from 972 to 651 arcsec along the c-axis (φ = 0°) and from 1234 to 752 arcsec along the m-axis (φ = 90°), and a notable decrease in anisotropy from 27.0% to 15.6% were obtained with the employment of Si treatment to the 3D GaN layer.

External catalyst-free InGaN photoelectrode for highly efficient energy conversion and H2 generation

GaN is a well-known material whose energy band edges can straddle the redox potentials deep in the visible and infrared wavelengths, thereby promising a drastically improved photon-to-current efficiency under applied bias. However, the material is still limited by the half-reactions of water splitting due to its high defect density, low light absorption, small reaction area, and large energy band bending. Here, our study provides a turn-key solution to all these issues. The synergistic effect of InGaN/GaN quantum pyramids on nanowires (QPs-NWs) directly addresses the performance degradation of the photocathodes (PCs). New InGaN QP structures on non-polar GaN nanowire show a unique tunable energy band (Eg: ∼2 eV to ∼1.36 eV) by quantum-sliding interface recombination effect. Without the use of external catalysts, the photoelectrochemical water splitting (PEC-WS) of QPs-NWs PC demonstrated enhanced performance with a current density of 34.36 mA cm−2 and a photon-to-current efficiency of 13.75 % under the −0.8 to 0 V applied biasing condition, which is much higher than in previous reports. The current density and the H2 production were measured to be ∼61.81 mA cm−2 and 11.5 mmol cm−2 for 10 h. The external catalyst-free electrode and the metal organic chemical vapor deposition (MOCVD) process will open a new platform for the commercialization of III-nitride based water splitting hydrogen technology.

Effects of Buffer Layer on Structural Properties of Nonpolar (112¯0)-Plane GaN Film

Nonpolar (112¯0) a-plane GaN films were grown on semipolar (11¯02) r-plane sapphire substrates using various buffer layers within a low-pressure metal organic chemical vapor deposition system. The structural properties of nonpolar a-plane GaN films were intensively investigated by X-ray diffraction and Raman spectra measurements. A set of buffer layers were adopted from a GaN layer to a composite layer containing a multiple AlN layers and a gradually varied-Al-content AlGaN layer, the full width at half maximum of the X-ray rocking curves measured along the [0001] and [101¯0] directions of a-plane GaN were reduced by 35% and 37%, respectively. It was also found that the basal-plane stacking faults (BSFs) density can be effectively reduced by the heterogeneous interface introduced together with the composite buffer layer. An order of magnitude reduction in BSFs density, as low as 2.95 × 104 cm−1, and a pit-free surface morphology were achieved for the a-plane GaN film grown with the composite buffer layer, which is promising for the development of nonpolar GaN-based devices in the future.

M-plane GaN terahertz quantum cascade laser structure design and doping effect for resonant-phonon and phonon-scattering-injection schemes

Non-polar m-plane GaN terahertz quantum cascade laser (THz-QCL) structures have been studied. One is traditional three-well resonant-phonon (RP) design scheme. The other is two-well phonon scattering injection (PSI) design scheme. The peak gains of 41.8 and 44.2 cm−1 have been obtained at 8.2 and 7.7 THz respectively at 300 K according to the self-consistent non-equilibrium Green’s function calculation. Different from the usual GaAs two-well design, the upper and lower lasing levels are both ground states in the GaN quantum wells for the PSI scheme, mitigating the severe broadening effect for the excited states in GaN. To guide the fabrication of such devices, the doping effect on the peak gain has been analyzed. The two designs have demonstrated distinct doping density dependence and it is mainly attributed to the very different doping dependent broadening behaviors. The results reveal the possibility of GaN based THz-QCL lasing at room temperature.

Effects of Mg-doping temperature on the structural and electrical properties of nonpolar a-plane p-type GaN films

Nonpolar (11–20) a-plane p-type GaN films were successfully grown on r-plane sapphire substrate with the metal–organic chemical vapor deposition (MOCVD) system. The effects of Mg-doping temperature on the structural and electrical properties of nonpolar p-type GaN films were investigated in detail. It is found that all the surface morphology, crystalline quality, strains, and electrical properties of nonpolar a-plane p-type GaN films are interconnected, and are closely related to the Mg-doping temperature. This means that a proper performance of nonpolar p-type GaN can be expected by optimizing the Mg-doping temperature. In fact, a hole concentration of 1.3 × 1018 cm−3, a high Mg activation efficiency of 6.5%, an activation energy of 114 meV for Mg acceptor, and a low anisotropy of 8.3% in crystalline quality were achieved with a growth temperature of 990 °C. This approach to optimizing the Mg-doping temperature of the nonpolar a-plane p-type GaN film provides an effective way to fabricate high-efficiency optoelectronic devices in the future.

Growth of non-polar m-plane GaN pseudo-substrates by Molecular beam epitaxy

Non-polar m-plane GaN films were grown by Plasma Assisted Molecular Beam Epitaxy on γ-LiAlO2 (100) substrates by a controlled coalescence of GaN nanocolumns obtained by a two-step process including a top-down nanopillars etching from a GaN buffer and a subsequent bottom-up overgrowth. Transmission electron microscopy data show a significant reduction of extended defects density in the coalesced film as compared to the initial GaN buffer, most likely due to a filter effect by the regrowth process on the nanopillars inclined walls. Low temperature photoluminescence spectra back this reduction by a strong intensity decrease of the stacking faults fingerprint emission peaks, while a very intense donor-bound excitonic emission at 3.472 eV, 2.8 meV wide, becomes dominant.

Joint effect of miscut r-plane sapphire substrate and different nucleation layers on structural characteristics of non-polar a-plane GaN films

High-quality non-polar a-plane GaN films are achieved with optimized miscut r-plane sapphire substrate and nucleation layers.

Self-Nucleated Nonpolar GaN Nanowires with Strong and Enhanced UV Luminescence

Self-Nucleated Nonpolar GaN Nanowires with Strong and Enhanced UV Luminescence

The spectroscopic ellipsometry measurement of non-polar freestanding GaN: comparison between isotropic and anisotropic models

For anisotropic materials, the pseudo-dielectric function (DF) from isotropic model is often applied in spectroscopic ellipsometry data analysis instead of the DF from anisotropic model because of its simplicity. Whether this approximation is applicable depends on the discrepancy between the pseudo-DF and the DF, which we find to be related to the illumination configuration greatly. For the ellipsometry spectra obtained from a non-polar freestanding GaN in two configurations with the c-axis perpendicular and parallel to the incident plane respectively, both anisotropic model and isotropic model are used to compare the difference between the fitted absorption coefficient and pseudo-absorption coefficient. As a consequence, the discrepancy between the two coefficients is less than 10 even at the absorption edge when electric field is perpendicular to the c-axis, while the discrepancy exceeds 40 at the absorption edge when electric field is parallel to the c-axis. These results indicate that the approximation of using isotropic model instead of anisotropic model is reasonable in the former configuration, while it is not applicable in the latter configuration and will render an overestimation of bandgap. The above two different discrepancies could be explained by the first-order term approximation of pseudodielectric function. We further demonstrate the necessity for anisotropic model by comparing the deviations of the crystal-field splitting obtained from anisotropic and isotropic model with the theoretical value respectively.

Plasma-Assisted MOCVD Growth of Non-Polar GaN and AlGaN on Si(111) Substrates Utilizing GaN-AlN Buffer Layer

We report the growth of non-polar GaN and AlGaN films on Si(111) substrates by plasma-assisted metal-organic chemical vapor deposition (PA-MOCVD). Low-temperature growth of GaN or AlN was used as a buffer layer to overcome the lattice mismatch and thermal expansion coefficient between GaN and Si(111) and GaN’s poor wetting on Si(111). As grown, the buffer layer is amorphous, and it crystalizes during annealing to the growth temperature and then serves as a template for the growth of GaN or AlGaN. We used scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) characterization to investigate the influence of the buffer layer on crystal structure, orientation, and the morphology of GaN. We found that the GaN buffer layer is superior to the AlN buffer layer. The thickness of the GaN buffer layer played a critical role in the crystal quality and plane orientation and in reducing the cracks during the growth of GaN/Si(111) layers. The optimum GaN buffer layer thickness is around 50 nm, and by using the optimized GaN buffer layer, we investigated the growth of AlGaN with varying Al compositions. The morphology of the AlGaN films is flat and homogenous, with less than 1 nm surface roughness, and has preferred orientation in a-axis.

Simultaneous Nucleation and Growth of Quaternary Polar and Nonpolar GaN–ZnO Solid Solution Nanowires and Nanorods

Simultaneous Nucleation and Growth of Quaternary Polar and Nonpolar GaN–ZnO Solid Solution Nanowires and Nanorods

Influence of current conduction paths and native defects on gas sensing properties of polar and non-polar GaN

The CO gas sensing characteristics of polar GaN (P–GaN) and non-polar GaN (NP–GaN) thin-films grown by RF–plasma assisted molecular beam epitaxy on c–Al2O3 and r–Al2O3 substrate is analyzed. The temperature-dependent (27–300 °C) current-voltage (I–V) measurements were performed for both (P–GaN & NP–GaN) Schottky barrier diodes having identical device dimensions with Au as the metal contact. The Schottky barrier height increases with the increment in temperature, while vice versa was perceived for the ideality factor and series resistance. The I–V characteristics dictated that the terminal current for P–GaN is 25 times that of NP-GaN, further corroborated by simulation results. The I-V curve fitting suggests the initial emission of charge carrier from a trapped state to a continuum of electronic state, following the Frenkel-Poole emission model for P-GaN. The NP–GaN demonstrated a higher surface-to-volume ratio and native (shallow and deep level) defects corresponding to VGa and ON as compared to the P–GaN. The sensing response obtained for the NP–GaN for 100 ppm CO gas at 300 °C is ~ 33%, which is about seven times the response in the case of P–GaN. The role of native defects and the potentiality of the fabricated NP–GaN over P–GaN films in providing a better sensing characteristic by reducing the current conduction paths are elaborated.

Engineering of electron–longitudinal optical phonon coupling strength in m-plane GaN terahertz quantum cascade lasers

In this work nonpolar (m-plane) GaN terahertz quantum cascade lasers are designed based on the non-equilibrium Green’s function method. The lasing quantum-level broadening that arises from electron–longitudinal optical (LO) phonon coupling is studied by tuning the coupling strength, and the changes of optical gain due to this broadening are demonstrated in a self-consistent way. It is found this coupling process largely widens the linewidth of intersubband transition and then quenches the lasing even at a low temperature of 10 K. The electron–LO phonon coupling strength, however, can be manually engineered by tailoring the quantum well structure. As a result, the optical gain will be recovered above the cavity threshold, indicating a potential for terahertz lasing from GaN semiconductors.

Non-polar true-lateral GaN power diodes on foreign substrates

We have demonstrated non-polar GaN power diodes (Schottky barrier diode and p–n junction diode) on foreign substrates featuring the true-lateral p–n and metal–semiconductor junctions. The diodes were fabricated on GaN islands laterally overgrown on the mask-patterned sapphire and Si substrates by metalorganic vapor phase epitaxy. The anode and cathode were formed on the opposed a-plane sidewalls of the island, making the device architecture essentially like the 90° rotation of the desired true-vertical power diodes. The ideality factor of the Schottky barrier diode remained 1.0 (from 1.00 to 1.05) over 7 decades in current. Specifically, a high critical electric field of 3.3 MV/cm was demonstrated on the p–n junction diode with avalanche capability. These performances reveal a strong potential of non-polar GaN with the true-lateral junctions for high power applications.

The optimization of n-type and p-type m-plane GaN grown on m-plane sapphire substrate by metal organic chemical vapor deposition

The properties of non-polar m-plane GaN layers grown on m-plane sapphire substrate using metal organic chemical vapor deposition at different disilane (Si2H6) and bis(cyclopentadienyl)magnesium (Cp2Mg) flow rates were investigated. The surface morphologies for Si- and Mg-doped of m-plane GaN exhibit root mean square of 1.22 and 2.95 nm, respectively. Using x-ray diffraction method, the epi-layer properties of doped m-GaN layers were studied emphasizing on the disilane and Cp2Mg flow rates towards the effect on threading dislocations and basal stacking faults. Room temperature Hall measurement indicate the maximum respective electron and hole concentration of ~1.88 × 1018 cm−3 and 6.50 × 1017 cm−3 were achieved at higher flow rate as well as the mobility of the electron. However, the mobility of hole was recorded to be low of ~5.2 cm2/V at higher Cp2Mg flow rates. The implementation of optimized n- and p-type contact layer demonstrate a 483 nm non-polar LED with light output power ~0.2 mW and forward voltage of 4.8 V at 20 mA current injection.

The growth behaviors and high controllability of GaN nanostructures on stripe-patterned sapphire substrates

The growth behaviors of multi-dimensional GaN nanostructures including simultaneous epitaxial growth of films and horizontal nanowires on the sapphire substrates were observed by making substrates per-patterned, leading to some interesting phenomena, thereby helping us understand the characteristics of GaN growth more comprehensively and clearly. The nonpolar a-plane GaN films grown on the Au-coated areas were formed by many triangular pyramids. And there are two kinds of GaN horizontal nanowires in the Au-free areas containing straight-growing nanowires and U-shaped nanowires. The detailed mechanism of those growth phenomena is proposed: when Au catalysts and Ga are sufficient, the growth of GaN is mainly controlled by vapor-solid (VS) mechanism, gradually growing into films. While when Au catalysts are insufficient, the growth of GaN mainly follows vapor-liquid-solid (VLS) mechanism. The difference between the growth of straight and U-shaped nanowires reflects that ±polarity has an important impact on the growth direction of nanowires, making the nanowires take an U-turn during the growth. In addition, the GaN nanostructures show a high degree of controllability. The width and length of nanowires, the continuity of films, even the presence of Au particles at the end of nanowires can be controlled, which provides a simple and convenient synthesis strategy.

Band alignment between NiO x and nonpolar/semipolar GaN planes for selective-area-doped termination structure* *Project supported by the Fund from the Open Project Key Laboratory of Microelectronic Devices and Integrated Technology, China (Grant No. 202006), the Doctoral Research Support Foundation of Shenyang Ligong University, China (Grant No. 1010147000914), and the Science and Technology Program of Ningbo, China (Grant No. 2019B10129).

Band alignment between NiO x and nonpolar GaN plane and between NiO x and semipolar GaN plane are measured by x-ray photoelectron spectroscopy. They demonstrate that the maximum value of the valence band in the unintentional-doped a-plane, m-plane, and r-plane GaN are comparable to each other, which means that all the substrates are of n-type with similar background carrier concentrations. However, the band offset at the NiO x /GaN interface presents obvious crystalline plane dependency although they are coated with the same NiO x films. By fitting the Ga 3d spectrum obtained from the NiO x /GaN interface, we find that relatively high Ga–O content at the interface corresponds to a small band offset. On the one hand, the high Ga–O content on the GaN surface will change the growth mode of NiO x . On the other hand, the affinity difference between Ga and O forms a dipole which will introduce an extra energy band bending.

GaN基多量子阱红外探测器研究进展（特邀）

Quantum well infrared photodetector (QWIP) is a new device utilizing the intersubband transition in conduction band or valance band, which has a very high free degree of device design. Due to the large conduction band-offset, the ultrafast electron relax time, the ultra-wide infrared transparency and the high energy LO-phonon, the GaN/Al(Ga)N multi-quantum wells (MQWs) has become a potential candidate for the infrared detector since the GaAs based MQWs. In this paper, the research progresses of intersubband transition absorption (ISBT) and corresponding photoresponse of GaN based MQWs were systematically reviewed. First, the operation principle and the selection rule of the quantum well infrared photodetector was explained. Then, the main research work was introduced including the ISBT absorption of polar, nonpolar and nanowire GaN based MQWs, from the near infrared to far infrared, even the THz range. Finally, the progress of GaN based QWIP and quantum cascade detectors (QCD) was reviewed including the photofresponse and the frequency response of the device. A conclusion and perspective was presented for the future research in GaN based QWIP and QCDs.