Doped GaN Research Articles

Using ground state and excited state density functional theory to decipher 3d dopant defects in GaN

Using ground state density functional theory (DFT) and implementing an occupation-constrained DFT (occ-DFT) for self-consistent excited state calculations, we decipher the electronic structure of the Mn dopant and other 3ddefects in GaN across the band gap. Our analysis, validated with broad agreement with defect levels (ground-state calculations) and photoluminescence data (excited-state calculations), mandates reinterpretation and reassignment of 3ddefect data in GaN. The MnGadefect is determined to span stable charge states from (1-) inn-type GaN through (2+) inp-type GaN. The Mn(2+) is predicted to be ad2ground state spin triplet defect with a singlet excited state, isoelectronic with the defect associated with the 1.19 eV photoluminescence inn-type GaN. The combined analysis of defect levels and excited states invites reassessment of alld2-capable dopants in GaN. We demonstrate that the 1.19 eV defect, a candidate defect for optically controlled quantum applications, cannot be the Cr(1+) assumed in literature and instead must be the V(0). The combined ground-state/excited-state DFT analysis is shown to be able to chemically fingerprint defects.

Relevance of Substrate Temperature and Ga Kinetics on Mg Doping in GaN by Plasma-Assisted Molecular Beam Epitaxy

Relevance of Substrate Temperature and Ga Kinetics on Mg Doping in GaN by Plasma-Assisted Molecular Beam Epitaxy

Electronic structure, optical properties, and thermoelectric properties of Mg-doped GaN materials

This work was based on first principles calculations and investigated the electronic structure, optical properties, and thermoelectric properties of Mg doped GaN bulk materials. It analyzed the effect of Mg impurities defects on the properties of GaN bulk materials. Mg doping can transform GaN materials into p-type semiconductors, which is crucial for achieving p-n junctions or p-type layers in GaN based electronic devices. The calculation results showed that MgN-GaN and MgInterstitial-GaN(MgI-GaN) belong to n-type doping, while MgGa-GaN belongs to p-type doping. In addition, the optical properties and thermoelectric properties of different GaN doping models were calculated. It was found that the maximum ZT values of the MgN-GaN, MgGa-GaN, MgI-GaN doping models reached 4.46, 5.09 and 5.35, respectively. The Mg impurities can help improve the ZT value of semiconductors compared to intrinsic GaN material. This research result has significance for the application of GaN based semiconductor materials in thermoelectric fields.

Origin of giant enhancement of phase contrast in electron holography of modulation-doped [formula omitted]-type GaN

The electron optical phase contrast probed by electron holography at n-n+ GaN doping steps is found to exhibit a giant enhancement, in sharp contrast to the always smaller than expected phase contrast reported for p-n junctions. We unravel the physical origin of the giant enhancement by combining off-axis electron holography data with self-consistent electrostatic potential calculations. The predominant contribution to the phase contrast is shown to arise from the doping dependent screening length of the surface Fermi-level pinning, which is induced by FIB-implanted carbon point defects below the outer amorphous shell. The contribution of the built-in potential is negligible for modulation doping and only relevant for large built-in potentials at e.g. p-n junctions. This work provides a quantitative approach to so-called dead layers at TEM lamellas.

Impact of Sequential N Ion Implantation on Extended Defects and Mg Distribution in Mg Ion‐Implanted GaN

In Mg ion implantation doping of GaN, sequential N ion implantation reportedly changes Mg concentrations in the Mg ion‐implanted region and the underlying region after activation annealing. The impact of sequential N ion implantation on defects and Mg distribution after postimplantation annealing is investigated. The atomic‐resolution analyses show that, in the Mg ion‐implanted region, the N ion implantation increases the concentration of MgGa. It is thus concluded that the Mg soluble in GaN by Mg ion implantation is increased by N ion implantation. The rest of the Mg atoms agglomerate to form clusters on the extended defects, and their concentration is also increased by the N implantation. The coarsening of extended defects is suppressed by the N ion implantation: the defects in the Mg+N‐implanted sample are nanoscale interstitial‐type defects, and they do not grow or annihilate after annealing. This indicates that the N implantation changes the concentrations of interstitials.

Heavy Ge Doping in GaN via Pulsed Sputtering to Tailor the Optical Bandgap Energy

The epitaxial growth of heavily Ge‐doped GaN films using pulsed sputtering deposition (PSD) on AlN (0001)/sapphire substrates is presented and the correlations among their structural, electrical, and optical properties are investigated. High‐quality Ge‐doped PSD‐GaN films are grown with electron concentrations of 0.33–4.4 × 1020 cm−3. Notably, cathode luminescence spectra reveal the absence of defect‐induced emissions associated with Ga vacancies (2.3–2.4 eV), consistent with the high electron mobilities of the films. As the electron concentration increases, the optical bandgap also increases. This correlation can be explained by combining the theoretical formulas for the Burstein–Moss shift and the bandgap renormalization effects. Remarkably, the optical bandgap reaches 3.84 eV at 4.4 × 1020 cm−3 carrier concentration, marking the highest value reported for the optical bandgap of GaN. These results indicate the potential of heavy Ge doping in GaN via PSD for applications in highly transparent conductive layers and tunneling junctions within GaN‐based optoelectronic devices.

Nitrogen vacancy–acceptor complexes in gallium nitride

We used photoluminescence (PL) spectroscopy and first-principles calculations to investigate GaN doped with Mg, Be, and implanted with Ca. The PL spectra revealed distinct red emission bands (RLA, where A = Be, Mg, and Ca) with maxima between 1.68 and 1.82 eV, each associated with a specific impurity. These bands consistently appeared alongside the green GL2 PL band at 2.33 eV, attributed to nitrogen vacancy (VN). Our calculations suggest that these bands result from recombination via defect complexes of group-II acceptors substituting for Ga with VN (AGaVN, A = Be, Mg, and Ca). The experimental +/0 transition levels for these complexes were estimated to be 0.6, 0.8, and 1.0 eV above the valence band maximum for Mg-, Be-, and Ca-containing complexes, respectively. The radiative recombination is facilitated by excited donor states located close to the conduction band minimum. Furthermore, our theory predicts that ZnGaVN and CdGaVN are stable and possess similar properties, although, no PL was detected from these defect complexes. The presented findings shed light on the identity of compensating donor complexes that impede the efficiency of p-type doping in GaN.

Comparison of Mg-based liquid metal ion sources for scalable focused-ion-implantation doping of GaN

We compare the suitability of various magnesium-based liquid metal alloy ion sources (LMAISs) for scalable focused-ion-beam (FIB) implantation doping of GaN. We consider GaMg, MgSO4●7H2O, MgZn, AlMg, and AuMgSi alloys. Although issues of oxidation (GaMg), decomposition (MgSO4●7H2O), and excessive vapor pressure (MgZn and AlMg) were encountered, the AuMgSi alloy LMAIS operating in a Wien-filtered FIB column emits all Mg isotopes in singly and doubly charged ionization states. We discuss the operating conditions to achieve <20 nm spot size Mg FIB implantation and present Mg depth profile data from time-of-flight secondary ion mass spectrometry. We also provide insight into implantation damage and recovery based on cathodoluminescence spectroscopy before and after rapid thermal processing. Prospects for incorporating the Mg LMAIS into high-power electronic device fabrication are also discussed.

High performance few-layered h-BN-based MIS blocks by Fowler-Nordheim tunneling for infrared photodetection

Hexagonal Boron Nitride (h-BN) is a promising material for a wide range of applications. Since its discovery, it has been used in nanoelectronic and optoelectronic devices as an optimal substrate for various two-dimensional materials. In addition, h-BN is a natural hyperbolic material in the mid-infrared region, where there are few options for photonic materials. To understand the relationships between structure and properties, it is essential to assess the number of layers in h-BN at the nanoscale. Here, using a combination of simulation and experiment, we systematically studied on the Fowler-Nordheim tunneling effect in few-layer h-BN, and basic physical parameters such as the layer-dependent effective mass are accurately obtained and verified. This work proceeds with a systematic investigation on the design of GaN-based metal/insulator/semiconductor (MIS) blocks with few-layered h-BN as insulating layers. It is found that the structural and material properties such as the number of h-BN layers, the GaN doping concentration, and the work function of contact metals exert dominant influence on the electrical characteristics of these MIS blocks, while the ideal heterogeneous interfaces of the 2D h-BN films mitigate the suffering by non-ideal factors such as interfacial traps and SRH recombination. By comprehensively balancing the mutual constraints of the key factors, this work achieves GaN-based MIS blocks capable of operating under higher voltages, currents, or power conditions compared to their counterparts. This paper aims to provide the fundamental physics of h-BN devices and help develop related h-BN-based infrared optoelectronics.

Highly Linear and Low Noise Shell Doped GaN Junctionless Nanotube TeraFET for the Design of Ultra-Wideband LNA in 6G Communications

Highly Linear and Low Noise Shell Doped GaN Junctionless Nanotube TeraFET for the Design of Ultra-Wideband LNA in 6G Communications

Oxygen-doped colloidal GaN quantum dots with blue emission

Gallium nitride quantum dots (GaN QDs), as a third-generation semiconductor material, are usually synthesized through a vapor deposition method at an elevated temperature (typically ≥800 °C). Thermal injection method is rarely reported for the synthesis of GaN QDs due to the lack of active precursors and the require of harsh reaction conditions. Here, we report the fabrication of oxygen doped colloidal GaN QDs via rapid thermal injection. By screening the reaction temperature and solvent, well-dispersed GaN QDs with a size of 3.8 nm were obtained in octadecylamine (ODA) at 360 °C. The synthesized GaN QDs show blue emission, the maximum emission wavelength is about 440 nm, and the photoluminescence quantum yield is 14.3%. Furthermore, we reveal the donor-acceptor mechanism of blue emission from GaN QDs. During the growth of GaN QDs, oxygen atoms occupy some nitrogen sites and produce a shallow oxygen donor energy level (ON). A part of the ON can be combined with the adjacent gallium vacancy (VGa) to form a VGa-ON acceptor, whose energy level is about 0.98 eV above the valence band of GaN QDs. The transition of excited electrons from the donor (ON) to the acceptor (VGa-ON) is responsible for the blue emission of GaN QDs.

(Invited) Effect of Oxygen on Activation Annealing of Mg-Doped GaN

The effects of N2 and O2:N2 (1:1) as ambient gases during activation annealing of Mg as p-type doping of GaN grown by MOCVD are investigated. The purpose is to understand the mechanisms involved and especially the impact of O2 on the resulting free hole concentration and hole mobility. The addition of O2 to the ambient gas during annealing is very effective in reducing the H level of the Mg-doped GaN layer, especially at lower activation temperatures, but the maximum achievable hole concentration and mobility, as determined by Hall characterization, is still higher with pure N2.The difference between activation in pure N2 or O2:N2 (1:1) ambient a is explained by an in-diffusion of O to the GaN layer acting as n-dopant and thus giving rise to a compensation effect. It is found that to a large degree only the Mg-H complexes at substitutional (MgGa), i.e., the electrically active acceptor sites that provide free holes, are activated by annealing with N2 only as ambient gas, while annealing with O2:N2 (1:1) also dissociates electrically inactive Mg-H complexes resulting in much less residual H. Thus, the residual H level in relation to the Mg level after activation annealing with N2 only may provide a representative measure of the resulting free hole concentration of the Mg-doped GaN layer.An additional factor to be taken into account for in order to maximize the hole concentration is the growth pressure. At the lower growth pressure of 150 mbar compared to higher growth pressure of 300 mbar, an increasing carbon incorporation leads to a compensation effect drastically reducing the free hole concentration while the mobility is unaffected. Conclusions Addition of O2 during RTP is very effective in reducing the H level however, the maximum hole concentration and mobility are still higher with pure N2.Mg-H complexes at substitutional (MgGa) electrically active acceptor are preferentially dissociated by annealing with N2 as ambient gas.By employing a higher growth pressure, carbon incorporation that leads to a compensation effect drastically reducing the free hole concentration can be minimized.By activation annealing with N2 the residual H level provides a measure of the free hole concentration. Figure 1

Electrochemistry in Photonic Devices Processing: a Disruptive Approach in GaN Technology

Electrochemistry, except the electrodeposition of metal for making contacts, is rarely represented in microelectronics, despite the many possibilities it may offer like “soft” surface treatments (no surface bombardment, low damage), very sensitive methods (femtoampers sensitivity) and its large compatibility with high aspect ratio structures (TSV Cu filling). Nowadays, the More than Moore law compels researchers to look for other solutions than standard microelectronic processes to reach disruptive solutions: electrochemical processes may provide this new pathway. In that context, our study focuses on III-N material, specifically on GaN porosification through electrochemical anodization for photonic applications. After a study to understand GaN electrochemistry, we will see how new photonic device architectures may arise from the controlled porosification of GaN.Before going towards photonic applications, the understanding of the GaN electrochemical system is required. First we show that the variation of the main electrochemical and material parameters like GaN doping level, anodization potential, and electrolyte composition enables the fine-tuning of the porosity level, pore density, pore size and pore morphology. Then nano-indentation, ellipsometry and reflectometry measurements give us access to a good knowledge of the variation of the Young modulus and refractive index versus porosity level and pore morphology. The measured physico-chemical properties, mechanical as well as optical, of porous GaN eventually paves the way for new device developments.In our talk, we will show how electrochemistry requirements have an impact on the design of the final structure. We will focus on two porous GaN structures built from the previous electrochemical systemic study on GaN thin layers: i) a LED on porous GaN Bragg mirror (DBR) and ii) a relaxed InGaN pseudo-substrate for RGB micro-display applications. LED on DBR: Specifically designed epitaxial stack and technology allow the fabrication of a blue GaN/InGaN based LED structure on a DBR made of alternating porous and dense GaN layers. The optimized one-step electrochemical process results in a very efficient DBR with 99,99% reflectance and a large stop band centered on 460-520 nm. The presence of the porous GaN DBR below the LED stack exalts the photoluminescence and the cathodoluminescence signals compared with standard LEDs grown on bulk substrates. These results open a new field of research regarding the capability of tuning GaN refractive index. Fully relaxed InGaN structure for red light emission: Results on partially or totally relaxed InGaN pseudo-substrate will be presented. The InGaN pseudo-substrate relaxation helps indium incorporation in the overgrown quantum wells and enables light emission in long wavelength towards red. The control of the electrochemical and semi-conductor parameters shows that the relaxation of the III-N structure measured by HR-DRX clearly depends on the porosity level and pore morphology. This means that relaxation can be fine-tuned: that will allow an epitaxial regrowth of micro-LED structures compatible with high indium concentration in the quantum wells and possibly RGB micro-LED fabrication for micro-display applications. In that part, we will demonstrate how electrochemistry associated with microelectronic technology brings a new way of controlling relaxation of InGaN on GaN and how the obtained experimental results are promising for the fabrication of high indium content relaxed structure. Through these two examples, we put in evidence how electrochemistry allows inventing solutions unimaginable with the standard microelectronics processes.

Annealing‐Free Ohmic Contacts to n‐Type GaN via Hydrogen Plasma‐Assisted Atomic Layer Deposition of Sub‐Nanometer AlOx

AbstractA plasma‐assisted atomic layer deposition (PE‐ALD) process is reported for creating ohmic contacts to n‐type GaN that combines native oxide reduction, near‐surface doping, and encapsulation of GaN in a single processing step, thereby eliminating the need for both wet chemical etching of the native oxide before metallization and thermal annealing after contact formation. Repeated ALD cycling of trimethyl aluminum (TMA) and high‐intensity hydrogen (H2) plasma results in the deposition of a sub‐nanometer‐thin (≈8 Å) AlOx layer via the partial transformation of the GaN surface oxide into AlOx. Hydrogen plasma‐induced nitrogen vacancies in the near‐surface region of GaN serve as shallow donors, promoting efficient out‐of‐plane electrical transport. Subsequent metallization with a Ti/Al/Ti/Au stack results in low contact resistance, ohmic behavior, and smooth morphology without requiring annealing. This electrical contracting approach thus meets the thermal budget requirements for Si‐based complementary metal–oxide–semiconductor structures and can facilitate the design and fabrication of advanced GaN‐on‐Si heterodevices.

(Invited) Selective Area Growth, Etching, and Doping of GaN By MOCVD for Power Electronics

The lack of selective-area doping technique in GaN greatly hinders the demonstration of sophisticated device structures for high power application. We explored the possibility of using selective-area etching followed by selective area growth to achieve SAD. For the etching step, we explored the in-situ TBCl etching, where we achieved controlled etch rate and smooth trenches. The TBCl etching was proven to be a low-damage etching process compared to the conventional dry etching. Moreover, selective area growth of p-GaN in a patterned trench was analyzed by atom probe tomography (APT). We found Mg atoms cannot diffuse on the GaN surface, which led to an observed inverse proportionality between Mg concentration and local growth rate. However, it was discovered that Mg precursors, like Ga, exhibit inter-trench diffusion through the gas phase.

Atomic Structures and Chemical states of active and inactive dopants in GaN

We investigated atomic structures and chemical states of active and inactive dopant sites for Mg- and Si- doped in GaN using photoelectron holography and X-ray absorption near edge structure. In the case of Mg-doped GaN, we found that a Mg atom substituting a Ga atom (MgGa) is an active dopant site in GaN whereas MgGa with two H atoms is an inactive dopant site in GaN. We found that a Si atom substituting a Ga atom is an active dopant site in Si-doped GaN whereas Si3N4 is an inactive dopant site in GaN.

High-density 2D hole gas in p-GaN/AlN/AlGaN on a silicon substrate with polarization-enhanced Mg ionization

Improving the hole concentration in p-GaN specimens has posed a major challenge due to the high activation energy of Mg doping in GaN. Recently, a delta doping technique for modulating the valence band and increasing the Mg ionization efficiency in GaN has been proposed. However, the memory effect of Mg in the reaction chamber greatly increases the difficulty of epitaxial growth; additionally, the Mg ionization efficiency remains too low. In this study, we demonstrate a novel p-GaN/AlN/AlGaN structure with an increased Mg ionization efficiency, from 5.16% to 14.22%, relative to the conventional p-GaN/AlGaN structure. The inserted AlN layer effectively modulates the valence band of p-GaN near the p-GaN/AlN interface and decreases the activation energy, as confirmed by density functional theory and the 1D Poisson–Schrodinger equation. Moreover, a high-performance p-channel GaN FinFET with a high output current of 114 mA/mm is fabricated, and the polarization-enhanced epitaxial layer shows great promise for next-generation digital ICs.

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.

Effect of Carrier Gas on Silicon Doped GaN Epilayer Characteristics

Effect of Carrier Gas on Silicon Doped GaN Epilayer Characteristics

Nanostars in Highly Si-Doped GaN.

Understanding the relation between surface morphology during epitaxy of GaN:Si and its electrical properties is important from both the fundamental and application perspectives. This work evidences the formation of nanostars in highly doped GaN:Si layers with doping level ranging from 5 × 1019 to 1 × 1020 cm-3 grown by plasma-assisted molecular beam epitaxy (PAMBE). Nanostars are 50-nm-wide platelets arranged in six-fold symmetry around the [0001] axis and have different electrical properties from the surrounding layer. Nanostars are formed in highly doped GaN:Si layers due to the enhanced growth rate along the a-direction ⟨112̅0⟩. Then, the hexagonal-shaped growth spirals, typically observed in GaN grown on GaN/sapphire templates, develop distinct arms that extend in the a-direction ⟨112̅0⟩. The nanostar surface morphology is reflected in the inhomogeneity of electrical properties at the nanoscale as evidenced in this work. Complementary techniques such as electrochemical etching (ECE), atomic force microscopy (AFM), and scanning spreading resistance microscopy (SSRM) are used to link the morphology and conductivity variations across the surface. Additionally, transmission electron microscopy (TEM) studies with high spatial resolution composition mapping by energy-dispersive X-ray spectroscopy (EDX) confirmed about 10% lower incorporation of Si in the hillock arms than in the layer. However, the lower Si content in the nanostars cannot solely be responsible for the fact that they are not etched in ECE. The compensation mechanism in the nanostars observed in GaN:Si is discussed to be an additional contribution to the local decrease in conductivity at the nanoscale.