Doped GaN Research Articles

Germanium - the superior dopant in n-type GaN

We present an experimental study about the influence of Si and Ge doping in GaN with focus on the occurring strain levels and overall crystalline quality. Extremely high quality samples were examined by means of Raman spectroscopy, demonstrating effective, n-type doping concentrations up to the 5 × 1019 cm–3 regime. By studying the full width at half maximum (FWHM) of the E2(high) Raman mode with rising doping concentration, Ge is approved as the by far superior dopant if compared to Si. Even elevated nominal Ge concentrations yield corresponding FWHM values of just 3 cm–1, a most competitive value even for bare bulk GaN samples. At the same time, the biaxial, compressive stress that is introduced by such high Ge doping amounts to just 0.2 GPa, in clear contrast to the particular case of silicon. Here, even moderate doping levels lead to tensile stress up to 1 GPa and consequently to a serious degeneration of the overall crystal quality as approved by our Raman analysis. Additionally, the examined high doping concentrations enable the observation of longitudinal optical phonon plasmon (LPP) modes in the Raman spectra, which serve as a direct tool for the determination of the effective doping concentration. A careful analysis of the LPP coupling at cryogenic and room temperature yields within the error interval identical free carrier concentrations in all germanium doped samples, pointing towards an energetically shallow nature of the dopant. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Impact of acceptor concentration on the resistivity of Ni/Au p‐contacts on semipolar (20–21) GaN:Mg

The p‐type doping of GaN with Mg, in particular doping of p++ cap layers and its influence on the resistivity of Ni/Au contacts on semipolar (20–21) GaN, has been investigated. For this purpose, we have compared GaN:Mg grown on several semipolar and polar orientations with respect to the acceptor concentration NA measured by electrochemical capacitance voltage techniques. For the same Mg precursor flow and Mg/III ratio, we observe very similar acceptor densities NA of up to 1 × 1019 cm−3 for (0001), (20–21), (20–2–1), and (11–22) GaN:Mg. Furthermore, the impact of the II/III ratio for the p++ cap layer (NA > 1 × 1019 cm−3) on I–V characteristics of Ni/Au (20 nm/30 nm) contacts on (20–21) oriented GaN:Mg has been investigated. Ohmic I–V characteristics were observed for Mg/III ratios >1 × 10−2. Specific contact resistivities as low as 2.4 × 10−3 Ω cm2 could be achieved. Inclusion of this p++ cap layer resulted in reduced turn on voltages in light emitting diodes with 450 nm emission wavelength.

The influence of aluminum content on the surface morphology of heavily doped (Al)GaN mesastrip structures grown by selective metalorganic vapor phase epitaxy

Heavily Si-doped Al x Ga1–x N mesastrip structures were grown by selective MOVPE technology. Al x Ga1–x N:Si mesastructures with x ~ 0.01–0.07 possess a smoother top and more even side surfaces as compared to those in analogous GaN:Si structures. During the growth of mesastructures with x ~ 0.03–0.07, a thin nanocrystalline AlN deposit appears on the Si3N4 mask. This deposit is not formed during the growth of structures with sufficiently low aluminum content.

(Invited) From MRTA to SMRTA: Improvements in Activating Implanted Dopants in GaN

The implantation and activation of dopants in GaN is a key enabling step for future devices requiring selective area doping. High temperatures (over 1300 ˚C) are required to activate p-type dopants in GaN. These high temperatures are above the decomposition temperature of GaN, which necessitates advanced annealing processes. One such process, the multicycle rapid thermal annealing (MRTA) process, has enabled the implantation and activation of p-type dopants in GaN. In this research, we demonstrate the symmetrical multicycle rapid thermal annealing (SMRTA) annealing process. The SMRTA annealing process includes an extra conventional annealing step after the rapid heating and cooling cycles and is shown to improve the crystalline structure of annealed GaN compared to the MRTA process.

Principles of electroless photoetching of non-uniformly doped GaN: Kinetics and defect revealing

When using electroless etching of semiconductors for quality control or device applications one must be aware of the possibility of galvanic interaction in the system. Here we show how highly doped (n++) GaN in a bilayer heterostructure dramatically increases the photoetch rate of the low doped (n) GaN layer in alkaline peroxydisulphate solution (the n++ material does not etch). Contacting the bilayer to a platinum sheet in solution further increases the photoetch rate. We show how previous electrochemical studies on GaN can be used to understand such “galvanic” effects. The present results offer an explanation for unexpected features in the photoetching of compound semiconductors which, during growth, become non-uniformly doped either by accident or by design. The work describes an example of the former case: non-uniformity due to the formation of extended defects, e.g. overgrown pinholes in GaN. The relevance of this study for practical applications, including the revealing of crystallographic defects for quality control, is considered.

(Invited) Vertical GaN p-i-n Diodes Formed by Mg Ion Implantation

GaN is an ideal material for power applications due to its high breakdown field, high switching speed, and low switching losses. The ability to implant and activate dopants, particularly p-type dopants, in GaN still remains a challenge. Dopant implantation and activation is crucial for many devices since it is the most straightforward method of selective lateral doping in GaN. Devices such as the current aperture vertical electron transistor (CAVET), Schottky diodes with guard ring termination or grayscale junction termination extension, and vertical p-i-n diodes will be enhanced and enabled by the ability to selectively dope GaN without the need for etching or regrowth. Herein, we report on the first implanted p-i-n diode fabricated on a bulk GaN substrate. A 8 μm thick unintentionally doped GaN layer was grown by metal organic chemical vapor deposition (MOCVD) on a n+ Ga-face c-oriented GaN substrate. The p-GaN anode and termination regions were formed by Mg implantation to a concentration of 2x1019 cm-3 following a box profile to a depth of 500nm. The implanted dopants were activated using the multicycle rapid thermal annealing (MRTA) technique described elsewhere [1]. After activation and removal of the protective cap structure [2], a 1 μm SiO2 field oxide layer was deposited by plasma enhanced chemical vapor deposition (PECVD). Contact windows were opened by reactive ion etching in a CHF3-based plasma. The anode metal was formed by lift-off of Pd/Au (20/100nm), followed by rapid thermal annealing in a N2atmosphere, and the cathode metal was a blanket film of e-beam deposited Al on the back side of the sample. Rectifying behavior was observed, a first for a p-i-n structure formed by Mg ion implantation in GaN. The turn-on voltage is consistent with a p-n junction, and the ideality factor of 2.6 is consistent with a recombination/generation-based transport mechanism, confirmed by temperature-dependent I-V. Additional non-ideal behavior is associated with recombination in the drift layer due to crystalline defects. The devices suffer from high ON-resistance due to high contact resistance and recombination in the drift layer, which limited the forward current to 1 A/cm2. Capacitance-voltage measurements indicated the presence of a p-GaN layer, with a depletion layer thickness ~500nm and a doping density of 2x1018 cm-3, corresponding to an activation ratio of 10%. 1. B.N. Feigelson, et. al. J. Cryst. Growth. 350, 21-26 (2012) 2. J.D. Greenlee, et. al. Appl. Phys. Express 8 036501 (2015)

(Invited) From MRTA to SMRTA: Improvements in Activating Implanted Dopants in GaN

GaN and related compounds have received a great deal of attention from the research community due to their tunable direct bandgap, radiation hardness, and a favorable Baliga figure of merit compared to SiC and Si. P-type GaN is a challenging material and has been the focus of many studies because it has many potential applications in power electronics and optoelectronics. P-type dopant implantation and activation adds additional complexity to the synthesis of p-type GaN. The ability to implant and activate p-type species in GaN is crucial for devices that require selective area doping, such as many vertical devices. The implantation and activation of Mg in GaN is difficult because of the high temperatures required to activate the implanted dopants. Temperatures of at least 1200 ˚C are necessary to achieve Mg activation. At these high temperatures, GaN decomposes, and N vacancies, which are donors, form. Thus, for p-type dopant activation, it is necessary to avoid any compensating decomposition or defects. Advanced annealing processes, such as the Multicycle Rapid Thermal Annealing (MRTA) are required for Mg activation. In this research, we compare the previously reported MRTA technique with a new method termed Symmetrical Multicycle Rapid Thermal Annealing (SMRTA). The MRTA technique includes a conventional anneal followed by rapid heating and cooling pulses (1). MRTA has previously been shown to activate ~8% of Mg implanted GaN (2). The SMRTA technique differs from the MRTA process by introducing an additional conventional anneal after the rapid heating and cooling pulses to improve the crystal quality and thus repeatability of the process. GaN films grown on sapphire substrates using MOCVD were implanted with a previously described implantation profile using a three-layer AlN capping structure (3, 4). One of the implanted samples was annealed using the MRTA technique, which included a conventional anneal at 1000 ˚C for 30 minutes followed by 40 rapid heating and cooling pulses with a peak temperature of 1350 ˚C. A second sample was annealed using the same MRTA process with the exception of an additional conventional anneal at 1000 ˚C for 30 minutes after the rapid heating and cooling pulses. Characterization performed on the samples includes atomic force microscopy for assessing the surface morphology and Raman spectroscopy for monitoring the crystal quality. AFM measurements before and after the SMRTA treatment revealed that the surface morphology and surface roughness were unchanged after the annealing process. The RMS roughnesses before and after annealing were 1.55 and 1.65 nm, respectively. Thus, the three-layer cap which has been previously shown to protect the underlying GaN film during the MRTA process also adequately protects the surface during the SMRTA process (3). To determine crystal structure changes during the annealing processes, the full widths at half maximum (FWHM) of the Raman E2 modes were compared before and after the SMRTA process. After the MRTA process, the E2 Raman mode FWHM decreases from 6.19 cm-1 (as implanted) to 5.4 cm-1 (MRTA annealed). The SMRTA process results in a further decrease in the E2 FWHM to 5.2 cm-1. This decrease in the FWHM indicates an improvement in the crystalline quality of the implanted GaN after the MRTA process and an even further improvement after the SMRTA process. The additional crystalline improvement afforded by the conventional anneal that distinguishes the MRTA and SMRTA processes is attributed to the removal of stresses and associated defects that form in the GaN layer during the rapid heating and cooling pulses. Electrical data for SMRTA annealed devices will also be presented. The additional improvement in crystal quality provided by the SMRTA process will be a key enabling step for future Mg-implanted devices.

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

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

Defect-engineered GaN:Mg nanowire arrays for overall water splitting under violet light

We report that by engineering the intra-gap defect related energy states in GaN nanowire arrays using Mg dopants, efficient and stable overall neutral water splitting can be achieved under violet light. Overall neutral water splitting on Rh/Cr2O3 co-catalyst decorated Mg doped GaN nanowires is demonstrated with intra-gap excitation up to 450 nm. Through optimized Mg doping, the absorbed photon conversion efficiency of GaN nanowires reaches ∼43% at 375–450 nm, providing a viable approach to extend the solar absorption of oxide and non-oxide photocatalysts.

Precursors for carbon doping of GaN in chemical vapor deposition

Methane (CH4), ethylene (C2H4), acetylene (C2H2), propane (C3H8), iso-butane (i-C4H10), and trimethylamine [N(CH3)3] have been investigated as precursors for intentional carbon doping of (0001) GaN in chemical vapor deposition. The carbon precursors were studied by comparing the efficiency of carbon incorporation in GaN together with their influence on morphology and structural quality of carbon doped GaN. The unsaturated hydrocarbons C2H4 and C2H2 were found to be more suitable for carbon doping than the saturated ones, with higher carbon incorporation efficiency and a reduced effect on the quality of the GaN epitaxial layers. The results indicate that the C2H2 molecule as a direct precursor, or formed by the gas phase chemistry, is a key species for carbon doping without degrading the GaN quality; however, the CH3 species should be avoided in the carbon doping chemistry.

Enhanced ferromagnetic properties of Cu doped two-dimensional GaN monolayer

We demonstrate enhanced ferromagnetism in copper doped two-dimensional GaN monolayer ( GaN -ML). Our first principle calculation based on density functional theory predicted that nonmagnetic Cu -dopant with concentration of 6.25% to be ferromagnetic (FM) in 2D GaN layer which carries a magnetic moment of 2.0 μB per Cu atom and it is found to be long range magnetic coupling among the Cu -dopant. The Cu-dopant in 2D GaN -ML which can be explained in terms of p-d hybridization at Curie temperature and this dopant prefer the FM behavior in 2D GaN layer. Hence Cu doped 2D GaN layer shows strong magnetic properties so that it is a promising material in the field of spintronics.

Study of the optical-electrical characteristics of InxGa1−xN alloy with low in doping

The electronic structures and optical properties of In doped GaN were calculated with different doping concentration, from first-principles using density function theory with the plane-wave ultrasoft pseudopotential method. The influence of In doping on the volume, interactions among atoms, density of states, electron density difference, and optical properties of GaN was analyzed. The results show that the interactions among atoms are reduced, band gap decreases, and absorption spectra have red shift along with the increase of In doping concentration.

GaN:Co epitaxial layers grown by MOVPE

We present a growth of GaN layers doped by cobalt using low pressure metalorganic vapor phase epitaxy on c-plane sapphire substrates. The in situ doping of GaN by Co was performed by the decomposition of bis(cyclopentadienyl)cobalt precursor. Three parameters, the temperature and pressure of the deposition and the Ga/Co ratio in the gas phase, influencing cobalt concentration were investigated. The obtained results were confronted with the thermodynamic predictions of Co solubility within GaN lattice and electronic structure calculations of GaN:Co. The magnetic properties of GaN:Co thin films were investigated using superconducting quantum interference device magnetometer. In addition, the layers were characterized by Raman and photoluminescence spectroscopy and atomic force microscopy. The concentration of Co was measured using electron microprobe and depth profile was measured using secondary ion mass spectroscopy. Room temperature ferromagnetic ordering was observed on the Co doped GaN layers.

Anisotropic polaron localization and spontaneous symmetry breaking: Comparison of cation-site acceptors in GaN and ZnO

The behavior of cation substitutional hole doping in GaN and ZnO is investigated using hybrid density functional calculations. Our results reveal that Mg substitution for Ga $({\mathrm{Mg}}_{\mathrm{Ga}})$ in GaN can assume three different configurations. Two of the configurations are characterized by the formation of defect-bound small polaron (i.e., a large structural distortion accompanied by hole localization on one of the neighboring N atoms). The third one has a relatively small but significant distortion that is characterized by highly anisotropic polaron localization. In this third configuration, ${\mathrm{Mg}}_{\mathrm{Ga}}$ exhibits both effective-mass-like and noneffective-mass-like characters. In contrast, a similar defect in ZnO, ${\mathrm{Li}}_{\mathrm{Zn}}$, cannot sustain the anisotropic polaron in the hybrid functional calculation, but undergoes spontaneous breaking of a mirror symmetry through a mechanism driven by the hole localization. Finally, using ${\mathrm{Na}}_{\mathrm{Zn}}$ in ZnO as an example, we show that the deep acceptor levels of the small-polaron defects could be made shallower by applying compressive strain to the material.

Growth and doping of semipolar GaN grown on patterned sapphire substrates

In order to achieve large area semipolar GaN layers with high crystal quality, we have etched trenches into n-plane and r-plane sapphire wafers exposing c-plane-like side-walls, from which GaN stripes can be grown by metalorganic vapor phase epitaxy mainly in c-direction, forming semipolar {101¯1} or {112¯2} surfaces after coalescence. Here, we describe how to improve such layers by optimizing the side-facet orientation and by including a SiN nanomask interlayer in situ into the growth process, eventually resulting in a basal plane stacking fault density below 5×103cm−1. Moreover, doping experiments have revealed a substantially lower Mg incorporation efficiency on the {112¯2} surface as compared to the c-plane, whereas Si does not show such differences.

Stability and Bandgap Analysis of Fe Doped GaN Nanotubes

Stability and Bandgap Analysis of Fe Doped GaN Nanotubes

(Invited) Impurity Co-doping of Gallium Nitride Materials for Enhanced Light Emission

Impurity doping of silicon and other semiconductor materials has been an active research area for more than 20 years. There have been two main thrusts of this work, erbium doping to provide infrared light emission in silicon for telecommunications, and doping with other rare earths for full color displays and solid state lighting. Major advances in both of these areas have been achieved through metal-organic chemical vapor deposition synthesis of rare earth doped GaN thin films [1, 2]. However, the low efficiency emission in prototype devices has prevented commercial applications. In this talk, recent results concerning light emission from erbium doped and from europium doped GaN films will be discussed. Experimental data showing enhanced light emission through co-doping of the GaN films will be presented. The role of strain fields and theoretical simulations of location of rare earth ions in the GaN lattice will be included. These results indicate that significant ligh emission can be achieved through co-doping of GaN films.[1] C. Ugolini, N. Nepal, J. Y. Lin, H. X. Jiang, and J. M. Zavada, Appl. Phys. Lett. 89 151903 (2006).[2] A. Nishikawa, N. Furukawa, T. Kawasaki, Y. Terai, and Y. Fujiwara, Appl. Phys. Lett. 97, 051113 (2010).[3] P. Kaur, S. S. Sekhon, J.M. Zavada, and Vijay Kumar, Phys. Rev. B (2012).

(Invited) Impurity Co-doping of Gallium Nitride Materials for Enhanced Light Emission

Impurity doping of silicon and other semiconductor materials has been an active research area for more than 20 years. There have been two main thrusts of this work, erbium doping to provide infrared light emission in silicon for telecommunications, and doping with other rare earths for full color displays and solid state lighting. Major advances in both of these areas have been achieved through metal-organic chemical vapor deposition synthesis of rare earth doped GaN thin films. However, the low emission efficiency in prototype devices has prevented commercial applications. Recent advances concerning co-doping GaN films with a rare earth and an impurity element are addressed. Experimental data showing enhanced light emission through co-doping of the GaN films are presented. Theoretical simulations of europium and silicon atoms in the GaN lattice are described. These results indicate that significant light emission can be achieved via co-doping of GaN.

Emission channeling studies on transition-metal doped GaN and ZnO: Cation versus anion substitution

The magnetic and electric properties of impurities in semiconductors are strongly dependent on the lattice sites which they occupy. While the majority site can often be predicted based on chemical similarities with the host elements and is usually simple to confirm experimentally, minority sites are far more complicated to predict, detect and identify. We have carried out extensive β− emission channeling studies on the lattice location of transition metal impurities in wide-gap dilute magnetic semiconductors, namely Co and Mn in GaN and ZnO, making use of radioactive 61Co and 56Mn implanted at the ISOLDE facility at CERN. In addition to the majority occupation of cation (Ga, Zn) sites, we located significant fractions (of the order of 20%) of the Co and Mn impurities in anion (N, O) sites, which are virtually unaffected by thermal annealing up to 900°C. Here, we present the β− emission channeling experiments on 61Co-implanted GaN. We discuss these results in the context of our recent reports of minority anion substitution in Mn-implanted GaN Pereira et al. (2012) [19] and Mn/Co-implanted ZnO Pereira et al. (2011) [20], particularly in terms of the advantages of the emission channeling technique in such cases of multi-site occupancy.

Dramatic reduction in process temperature of InGaN-based light-emitting diodes by pulsed sputtering growth technique

P-type doping of GaN by pulsed sputtering deposition (PSD) at a low growth temperature of 480 °C and dramatic reduction in the growth process temperature for InGaN-based light-emitting diodes (LEDs) were achieved. Mg-doped GaN layers grown on semi-insulating GaN at 480 °C exhibited clear p-type conductivity with a hole concentration and mobility of 3.0 × 1017 cm−3 and 3.1 cm2 V−1 s−1, respectively. GaN/In0.33Ga0.67N/GaN LEDs fabricated at 480 °C showed clear rectifying characteristics and a bright electroluminescence emission near 640 nm. These results indicate that this low temperature PSD growth technique is quite promising for the production of nitride-based light-emitting devices on large-area glass substrates.