N-doped GaN Research Articles

WSe2/n-GaN and WSe2/p-GaN heterojunction band alignment

Abstract In this work, we measured the band alignment of 2D/3D heterostructures of WSe2 on n-doped, pdoped, and intrinsic GaN by X-ray photoelectron spectroscopy (XPS). The WSe2/n-GaN heterojunction exhibits type-II band alignment, with measured valence band offset (VBO) and conduction band offset (CBO) values of 2.28±0.15 eV and 0.96±0.15 eV, respectively. On the other hand, the WSe2/p-GaN heterojunction shows type-I band alignment, with measured VBO and CBO values of 0.1±0.15 eV and 1.22±0.15 eV, respectively. The results show that doping has a significant effect on the arrangement of the energy bands of the heterostructure, and provides a reference for device applications based on heterojunctions.

Characterization of the p-NiO/n-GaN heterojunction and development of ultraviolet photodiode

Nickel oxide semiconductor is a good p-type material for fabricating a p-n-heterojunction with n-type GaN semiconductor. The p-NiO/n-GaN heterojunction, which produces a type II band bias, has significant prospects for creating photosensitive devices. Herein, it is shown that a selective ultraviolet photodetector can be created based on this heterojunction. p-NiO films were formed using magnetron sputtering onto n-doped GaN epitaxial layers grown by plasma assisted molecular beam epitaxy. The structural, optical, and electronic properties of the formed nickel oxide films were studied. A mechanism for reducing dislocation density and conductivity and increasing transparency due to film annealing is proposed. It has been established that the current–voltage characteristics of the p-NiO/n-GaN heterojunction are typical for diode structures and show selective sensitivity to ultraviolet radiation. The band model and sensitivity of the fabricated ultraviolet photodiode were evaluated. The photosensitivity at zero voltage was 3.11 mA/W, the quantum yield was 0.011, and the detection ability was 8.69 × 109 Jones. The work shows the potential of using the p-NiO/n-GaN heterojunction to create ultraviolet photodetectors.

Impact of eliminating ungated access regions on DC and thermal performance of GaN-based MIS-HEMT

The impacts of eliminating access regions (AR) on DC and thermal performance of GaN-based MIS-HEMT have been studied using a device of 4 µm gate length. Nominal absence of ARs was achieved by a metal-organic chemical vapor deposition-regrown heavily n-doped GaN contact region extended to the two-dimensional electron gas (2DEG) channel (with sheet resistance as low as 14 Ω/ ◻ and ohmic contact resistance of 0.20 Ω⋅ mm) and 22-nm-thick AlN/Al2O3/HfO2 insulator layers acting as both gate dielectrics and sidewall spacers. As a result, a low knee voltage (2.5 V @ VGS = +2 V) comparable to deeply-scaled devices was attained, revealing the dominant role of ARs in knee voltages. High linearity at lower supply voltages (gate voltage swing of 7.3 V @ VDS = 5 V) and a faster GVS saturation trend with V DS increasing were observed, benefiting from the improved utility of applied lateral voltage. Moreover, a much lower thermal resistance compared to that of the conventional MIS-HEMT structure (146 vs. 202 K W−1) was extracted by a static-pulsed I-V measurement method. Simplified TCAD simulations were conducted to explain the underlying mechanisms, demonstrating that the enhanced surface heat flow covered by the gate metal as well as the more uniform electric field along the 2DEG channel accounts for the better capability of heat management in the device free of ARs. Our results indicate the size of DC and thermal performance enhancements obtained by eliminating ARs in a GaN-based MIS-HEMT structure.

Growth and Properties of Ultra-Thin PTCDI-C8 Films on GaN(0001)

Ultra-thin PTCDI-C8 films are vapor-deposited under ultra-high vacuum (UHV) conditions onto surfaces of p- or n-doped GaN(0001) samples. The X-ray photoelectron spectroscopy (XPS) results reveal a lack of strong chemical interaction between the PTCDI-C8 molecule and the substrate. Changes in the electronic structure of the substrate or the adsorbed molecules due to adsorption are not noticed at the XPS spectra. Work function changes have been measured as a function of the film thickness. The position of the HOMO level for films of thicknesses 3.2–5.5 nm has been determined. Energy diagrams of the interface between p- and n-type GaN(0001) substates and the PTCDI-C8 films are proposed. The fundamental molecular building blocks of the PTCDI-C8 films on GaN(0001), assembled by self-organization, have been identified. They are rows of PTCDI-C8 molecules stacked in “stand-up” positions in reference to the substrate, supported by the π–π bonds which are formed between the molecular cores of the molecules and monomolecular layers constituted by rows which are tilted in reference to the layer plane. The layers are epitaxially oriented. The epitaxial relation between the rows and the crystallographic directions of the substrate are determined. A model of the PTCDI-C8 film’s growth on GaN(0001) substrate is proposed. The 3D islands of PTCDI-C8 molecules formed on the substrate surface during film deposition are thermodynamically unstable. The Volmer–Weber type of growth observed here is a kinetic effect. Rewetting processes are noticeable after film aging at room temperature or annealing at up to 100 °C.

Mid-infrared thermal radiation resonating with longitudinal-optical like phonon from n++-doped GaN–semi-insulating GaN grating structure

The mid-infrared emission mechanism of line-and-space structures of metallic plates on dielectric materials is substantiated using high conductive n-doped (n++-) GaN–semi-insulating (SI-) GaN microstripe structures on an SI-GaN epitaxial layer, which was veiled when using line-and-space structures of Au plates. The present structure exhibits a few thermal emission lines originating from electric dipoles resonating with the coherent longitudinal optical (LO) phonon-like lattice vibration, which are formed by the local depolarization electric field in the surface n++-GaN/SI-GaN/n++-GaN regions. The energies of the LO-phonon-like modes shift from the original LO-phonon energy of GaN to the lower energy region, which contrasts with the LO-phonon resonant emission from the microstructures on GaAs. These emission lines have another notable feature, i.e. the observed peak energies are independent of the polar emission angle for both s- and p-polarizations, unlike the emissions by surface phonon polaritons showing a significant directive nature of peak energies. The results show that each peak energy of the present emission lines is positioned at the zero-point of the real part of the electric permittivity comprising the components of the transverse optical phonon and other electric dipoles induced by the LO-like modes, excluding the target mode. The significant peak-energy shift of the LO-like phonons is applicable to materials with wide Reststrahlen bands, which contrasts with that of the nearly LO-phonon resonating feature of materials with narrow Reststrahlen bands, such as GaAs. The peak energy shift depending on the emission direction is observed for Au–GaN stripe structures. This property is ascribed to the imperfect Au/GaN interface with surface states through the theoretical analysis of the modified electric permittivity in the surface region, numerical simulation of the local electric field via finite-difference time-domain calculation, and experimental studies on a Ti–GaN structure and emission peaks originating from an LO-like phonon of the α-Al2O3 substrate.

Spin injection into heavily-doped n-GaN via Schottky barrier

Spin injection and detection in bulk GaN were investigated by performing magnetotransport measurements at low temperatures. A non-local four-terminal lateral spin valve device was fabricated with Co/GaN Schottky contacts. The spin injection efficiency of 21% was achieved at 1.7 K. It was confirmed that the thin Schottky barrier formed between the heavily n-doped GaN and Co was conducive to the direct spin tunneling, by reducing the spin scattering relaxation through the interface states.

Ultra-small low-threshold mid-infrared plasmonic nanowire lasers based on n-doped GaN

An ultra-small mid-infrared plasmonic nanowire laser based on n-doped GaN metallic material is proposed and studied by the finite-difference time-domain method. In comparison with the noble metals, nGaN is found to possess superior permittivity characteristics in the mid-infrared range, beneficial for generating low-loss surface plasmon polaritons and achieving strong subwavelength optical confinement. The results show that at a wavelength of 4.2 µm, the penetration depth into the dielectric is substantially decreased from 1384 to 163 nm by replacing Au with nGaN, and the cutoff diameter of nGaN-based laser is as small as 265 nm, only 65% that of the Au-based one. To suppress the relatively large propagation loss induced by nGaN, an nGaN/Au-based laser structure is designed, whose threshold gain has been reduced by nearly half. This work may pave the way for the development of miniaturized low-consumption mid-infrared lasers.

Determination of optical properties of quantum wells with a structure of AlGaN/GaN resonant tunneling diodes (RTDs)

Two-dimensional multiple quantum well (MQW) and modified symmetric quantum well (MSQW) carrier confinement play an important role in AlGaN/GaN resonant tunneling diodes (RTDs) efficiency improvement. Therefore, AlGaN-based materials are suitable for development of ultraviolet-light sources. Hence, an attempt is made to investigate the optical properties of AlGaN/GaN. Asymmetric quantum well structure consisting of N-doped AlGaN wells, GaN steps, and undoped AlGaAs barriers is developed. The significant absorption is observed due to the strong coupling effect of the electronic state wave functions. Asymmetric potential well proposed for AlGaN/GaN/AlGaN RTDs. The barriers and the thin layer in the quantum well are theoretically determined. For GaN/AlGaN QW of width L, the frequencies of the symmetric interface modes satisfy the condition. The increase in well width increases the full-width at half maxima (FWHM) from 0 to 100 nm, while increased Al concentration decreases FWHM from 6 to 1 nm.

Effect of doping on Al2O3/GaN MOS capacitance

This paper investigates the turning-on-voltage (VFB/VTH) of Al2O3/GaN MOS stacks with n-doped GaN, p-doped GaN and not intentionally doped (NID) GaN by exploiting capacitance measurements on large gate area test structures with systematic variation of Al2O3 thickness (tox). Measurements are compared with 1D Schrödinger-Poisson simulations including incomplete ionization model. The necessity of using a quantum description of electron density is demonstrated especially for thinner gate oxides. We found that, contrary to what is expected, p-doping below the channel barely increases the VTH and the VTH is independent of tox, even if the density of activated acceptors is demonstrated to be sufficiently high. Our results highly suggest that the negative charge induced by p-doping is compensated at the oxide level.

Micro-Raman characterization of homo-epitaxial n doped GaN layers for vertical device applications

N-doped homo-epitaxial GaN samples grown on freestanding GaN substrates have been investigated by micro-Raman spectroscopy. Quantitative analysis of the E2h and the A1(LO) modes’ behavior has been performed while intentionally increasing the carrier density using silicon doping. We noticed that as the carrier concentration increases up to 1.8 × 1018 cm−3, the E2h mode remains unchanged. On the other hand, when the doping gets higher, the A1(LO) position shifts to a higher frequency range, its width becomes larger, and its intensity drastically diminishes. This change in the A1(LO) behavior is due to its interaction and its coupling with the free negative charge carriers. Furthermore, we calibrated the A1(LO) frequency position shift as a function of the n-carrier concentration. We found out that for low n doping, the change in the A1(LO) position can be considered as a linear variation while in the overall doping range, a sigmoid growth trend with a Boltzmann fit can be tentatively applied to describe the A1(LO) position shift. This calibration curve can also be used to describe the coupling strength between the carriers and the A1(LO) phonon. Eventually, this study shows that micro-Raman spectroscopy is a powerful non-destructive tool to probe the doping concentration and the crystalline quality of GaN material with a microscopic spatial resolution.

Mechanism leading to semi-insulating property of carbon-doped GaN: Analysis of donor acceptor ratio and method for its determination

Carbon impurities in GaN form both acceptors and donors. Donor-to-acceptor ratios (DARs) determine the semi-insulating behavior of carbon-doped GaN (GaN:C) layers and are still debated. Two models are discussed; both can theoretically achieve semi-insulating behavior: the dominant acceptor model (DAM, DAR<1) and the auto-compensation model (ACM, DAR=1). We perform a capacitance–voltage analysis on metal/GaN:C/nGaN (n-doped GaN) structures, exhibiting Fermi-level pinning in GaN:C, 0.7 eV above the valence band maximum. This observation coupled with further interpretation clearly supports the DAM and contradicts the ACM. Furthermore, we reveal a finite depletion width of a transition region in GaN:C next to nGaN, where carbon acceptors drop below the Fermi level becoming fully ionized. Calculation of the potential drop in this region exhibits DAR values of 0.5–0.67 for GaN:C with total carbon concentrations of 1018 cm−3 and 1019 cm−3. Based on those results, we re-evaluate formerly published density functional theory (DFT)-calculated formation energies of point defects in GaN. Unexpectedly, growth in thermodynamic equilibrium with the bulk carbon phase contradicts our experimental analysis. Therefore, we propose the consideration of extreme carbon-rich growth conditions. As bulk carbon and carbon cluster formation are not reported to date, we consider a metastable GaN:C solid solution with the competing carbon bulk phase being kinetically hindered. DFT and experimental results agree, confirming the role of carbon at nitrogen sites as dominant acceptors. Under N-rich conditions, carbon at gallium sites is the dominant donor, whereas additional nitrogen vacancies are generated under Ga-rich conditions.

Selective-area growth study of GaN micropillars for quasi-vertical Schottky diodes

In this work, metal organic vapor phase epitaxy (MOVPE) is employed for selective-area growth (SAG) of undoped and lightly n-doped GaN micropillars on masked GaN-on-sapphire templates. In micropillar geometry, the limits of GaN drift layer thickness in hetereoepitaxial Schottky diodes are expected to be significantly pushed upwards. This is an important step towards the realization of GaN-based quasi-vertical power devices with high breakdown voltage (V br) on low-cost foreign substrates. Micropillar growth evolution and the impact of growth rate and V/III ratio on micropillar morphology and parasitic GaN deposition on AlOx hard masks are investigated. By using a combination of low growth rate and high V/III ratio, 12–22 µm high micropillars with planar pillar tops are grown in circular mask openings with radii of 35–100 µm. The threading dislocation density of the highest pillars is (1.4 ± 0.3) × 107 cm−2. Schottky diodes on micropillars with lowest net doping concentration (N D − N A) of 1.3 × 1016 cm−3 exhibit excellent forward characteristics with ideality factors , on/off-ratios up to 1010, and on-resistances R on 1 m cm−2.

Ge doped GaN and Al0.5Ga0.5N-based tunnel junctions on top of visible and UV light emitting diodes

The use of tunnel junctions (TJs) is a potential solution in blue light-emitting diodes (LEDs) to poor p-contacts, replacing it with another n-contact. TJs are even more advantageous for UV-emitting structures, which suffer from considerably low injection efficiency in high Al concentration UV LEDs. In this article, we report our work on Ge n-doped GaN and AlGaN TJs grown on top of blue and UV LEDs, respectively, by a hybrid growth method. We have achieved state-of-the-art mobility (67 cm2/V s) and resistivity (1.7 × 10−4 Ω cm) at a free electron concentration of 5.5 × 1020 cm−3 in Ge-doped GaN. With an emission wavelength of 436 nm, the GaN TJ slightly increased the optical power of the blue LED. The AlGaN TJs, on the other hand, improved the optical power of the UV LED (304 nm) by at least a factor of 3, suggesting the enhancement of the hole injection efficiency by the use of TJs in UV-emitting structures.

Improvement of luminescence properties of n-GaN using TEGa precursor

Abstract The aim of this work is to compare and improve optical and structural properties of GaN layers prepared using TMGa or TEGa precursors. MOVPE grown GaN buffer layers on sapphire substrates are usually grown from TMGa precursor at the temperatures above 1000 °C. These layers contain deep and shallow acceptor levels which are responsible for blue and yellow defect bands in luminescent spectra. Both defect bands are detrimental for all major nitride device applications. Especially n-doped GaN layers suffer from strong yellow defect bands. In this work, it is shown that yellow band photoluminescence intensity can be suppressed by using TEGa precursor during the growth of n–doped GaN layers. Different kinds of growth parameters, such as growth temperature or growth rate, have been studied. It is also shown that the change of carrier gas (H2 or N2) has very strong influence on the layer quality. H2 carrier gas increased intensity of yellow band in sample grown from TEGa precursor while N2 carrier gas had the same effect for sample grown from TMGa precursor. Variable energy positron annihilation spectroscopy showed creation of single VGa in H2 atmosphere and clustering of VGa to big complexes ((VGa)3(VN)n) in N2 atmosphere.

Study of electronic properties on the n-GaN (0001) surface with points defects

The influence of defects on the surface of the semiconductor is irreversible. The influence of intrinsic point defects on the electronic properties of n-doped GaN (0001) surface is studied based on the first principles. The results show that, the N interstitial defect (Ni) and Ga Vacancy (VGa) are the more easily formed in the case of Si doping. The defect level generated by an appropriate amount of defects contributes to the transition of electrons, thereby improving the n-type conductivity characteristics. In particular, the Ga vacancy makes the work function drop significantly, which promotes the emission of electrons. However, once the defects inside the material exceed a certain level, any defects will play a counterproductive role. This paper could provide some guidance for the preparation of n-GaN optoelectronic devices.

Accurate surface band bending determination on Ga-polar n-type GaN films by fitting x-ray valence band photoemission spectrum

The surface band bending in Ga-polar n-type GaN surfaces, as well as the effect of Si doping levels and in situ Ar+ ion processing on band bending, was systematically investigated. To precisely determine the valence band maximum (VBM) of GaN beyond instrumental and material surface environments by XPS, a valence band feature fitting procedure based on photoemission spectra and theoretical densities of states has been developed. Poisson calculation with quadratic depletion approximation on surface potential has been used to model the band bending and further correct the VBM energy. Then, the actual surface band bending was correctly evaluated. Upward band bending of 1.55 ± 0.03 eV with highly Si doped n-GaN, which is about 0.88 eV higher than that of the moderately doped sample, was found. After in situ Ar+ plasma treatment, the varying degree of band bending was observed distinctly depending on the Si doping density. The surface components associated with the Ga/N ratio and Ga–O bonding concentration on the n-GaN surface have been used to evaluate the contribution to surface band bending.

71Ga NMR characterization of an n-doped free-standing gallium nitride wafer

The crystallinity of an n-doped free-standing GaN wafer sliced from a GaN crystal grown on a GaN template with a mask pattern was characterized by 71Ga NMR. The quadrupolar frequency determined from the quadrupolar-split 71Ga spectra agreed with that of free-standing GaN single crystals reported in the literature. On the other hand, the broad satellite transition lines compared to the central transition line manifested structural deviation. The satellite linewidths were notably anisotropic with respect to the angle between the magnetic field and the crystalline axes; this phenomenon was attributed to the strain distribution inside the crystal. Assuming uniform deformation of the crystal as the main origin of the satellite linewidths, the longitudinal strain distribution was estimated to be on the order of 10−5. The strain distribution was also found to be slightly larger at the center of the wafer than at the edge.

Vertical GaN nanocolumns grown on graphene intermediated with a thin AlN buffer layer

We report on the self-assembled growth of high-density and vertically-oriented n-doped GaN nanocolumns on graphene by radio-frequency plasma-assisted molecular beam epitaxy. Graphene was transferred to silica glass, which was used as the substrate carrier. Using a migration enhanced epitaxy grown AlN buffer layer for the nucleation is found to enable a high density of vertical GaN nanocolumns with c-axis growth orientation on graphene. Furthermore, micro-Raman spectroscopy indicates that the AlN buffer reduces damage on the graphene caused by impinging active N species generated by the radio-frequency plasma source during the initial growth stage and nucleation of GaN. In addition, the grown GaN nanocolumns on graphene are found to be virtually stress-free. Micro-photoluminescence measurements show near band-edge emission from wurtzite GaN, exhibiting higher GaN bandgap related photoluminescence intensity relative to a reference GaN bulk substrate and the absence of both yellow luminescence and excitonic defect emission. Transmission electron microscopy reveals the interface of GaN nanocolumns on graphene via a thin AlN buffer layer. Even though the first few monolayers of AlN on top of graphene are strained due to in-plane lattice mismatch between AlN and graphene, the grown GaN nanocolumns have a wurtzite crystal structure without observable defects. The results of this initial work pave the way towards realizing low-cost and high-performance electronic and optoelectronic devices based on III-N semiconductors grown on graphene.

Raman spectroscopy as a depth sensor in cubic phase n-GaN

In this work, we investigate the elementary excitations of epitaxial n-doped GaN grown in its meta-stable cubic phase on GaAs substrate. The Raman spectra were obtained in backscattering geometry with the incoming and outgoing light polarizations parallel. We covered the whole spectrum of wavelengths of an argon laser (457–514.5 nm) in a wide range of Raman energy shift, with a minimum temperature of 10 K. We focused our attention on the nature of the Raman structures, in particular, the ones which Stokes shifts are between 550 and 4000 cm−1 whose natures were identified. The control of laser light penetration in the GaN samples together with electronic Raman cross-sections calculations lead us to assign the structures as plasmon-LO phonon coupled modes and density of states of phonons resulting from structural disorder.

Effect of electron avalanche breakdown on a high-purity semi-insulating 4H-SiC photoconductive semiconductor switch under intrinsic absorption.

High-power photoconductive semiconductor switching devices were fabricated from a high-purity, semi-insulating 4H-SiC wafer. A highly n-doped GaN subcontact layer was inserted between the contact metal and the high-resistivity SiC wafer. The minimum ON-state resistance of the device was less than 1ohm when the energy of a 355nm laser was 10.5mJ with a bias voltage of 6kV. The maximum device lifetime is 3151 pulses, after which the device completely fails. The failure mechanisms are determined using several analysis methods. Under a strong electric field, the failure mechanism differs for the two electrodes. Near the edge of the anode electrode, the switch is damaged due to the thermal stress caused by impact ionization. At the edge of the cathode electrode, the electrode erosion is the main reason for the failure to operate for long periods of time. These two different damage mechanisms are both important factors influencing the device performance. The electron avalanche breakdown at the edge of the anode electrode causes the formation of cracks between the electrodes, which is the root cause of the switch failure.