Be-doped GaN Research Articles

The Origin of the Yellow Luminescence Band in Be-Doped Bulk GaN

Photoluminescence (PL) from Be-doped bulk GaN crystals grown by the High Nitrogen Pressure Solution method was studied and compared with PL from GaN:Be layers on sapphire grown by molecular beam epitaxy and metalorganic chemical vapor deposition techniques. The yellow luminescence band in the latter is caused by the isolated BeGa acceptor (the YLBe band), while the broad yellow band in bulk GaN:Be crystals is a superposition of the YLBe band and another band, most likely the CN-related YL1 band. The attribution of the yellow band in bulk GaN:Be crystals to the BeGaON complex (a deep donor) is questioned.

MOCVD Growth and Characterization of Be-Doped GaN.

Beryllium has been considered a potential alternative to magnesium as a p-type dopant in GaN, but attempts to produce conductive p-GaN:Be have not been successful. Photoluminescence studies have repeatedly shown Be to have an acceptor level shallower than that of Mg, but deep Be defects and other compensating defects render most GaN:Be materials n-type or semi-insulating at best. Previous reports use molecular beam epitaxy or ion implantation to dope GaN with Be, almost exclusively. Due to the high toxicity of Be organometallics, reports of GaN:Be by metal-organic chemical vapor deposition (MOCVD) have been largely absent. Here, we report a systematic study of growth of GaN:Be by MOCVD. All doped samples show the established UV band and yellow luminescence signature of GaN:Be, and growth conditions resulting in high-quality GaN with stable Be incorporation were established. Our results show that the MOCVD growth technique allows for Be incorporation pathways that have not been possible with previous growth methodologies and is highly promising in achieving p-type conductivity in GaN:Be.

Point defects in beryllium-doped GaN

Detailed analysis of the defect-related photoluminescence (PL) in semi-insulating and $n$-type Be-doped GaN samples grown by molecular beam epitaxy is presented. The dominant PL bands in these samples are the ultraviolet $({\mathrm{UVL}}_{\mathrm{Be}})$, yellow $({\mathrm{YL}}_{\mathrm{Be}})$, red $({\mathrm{RL}}_{\mathrm{Be}})$, and green (GL2) bands. The ${\mathrm{UVL}}_{\mathrm{Be}}$ band is caused by electron transitions via the $0/\ensuremath{-}$ level of the ${\mathrm{Be}}_{\mathrm{Ga}}$ acceptor located at 113 meV above the valence band. The ${\mathrm{UVL}}_{\mathrm{Be}}$ band is quenched above 60 K, and the quenching is tunable by excitation intensity. The quenching of the ${\mathrm{YL}}_{\mathrm{Be}}$ above 200 K is likely related to an acceptor level at 0.3 eV. In semi-insulating samples, the quenching temperature is tunable with the intensity of the excitation laser. Based on the first-principles calculations, the ${\mathrm{YL}}_{\mathrm{Be}}$ band is preliminarily attributed to the ${\mathrm{Be}}_{\mathrm{Ga}}\text{\ensuremath{-}}{\mathrm{V}}_{\mathrm{N}}\text{\ensuremath{-}}{\mathrm{Be}}_{\mathrm{Ga}}$ complex. The new ${\mathrm{RL}}_{\mathrm{Be}}$ band is attributed to the internal transition in the ${\mathrm{Be}}_{\mathrm{Ga}}\text{\ensuremath{-}}{\mathrm{V}}_{\mathrm{N}}$ complex with a deep $+/0$ transition level at about 0.95 eV.

Passivation of the beryllium acceptor in GaN and a possible route for p-type doping

Using theory and experiment, we address a possible p-type doping route of GaN using the Be acceptor. Hybrid functional calculations suggest that straightforward incorporation of Be in GaN will likely lead to compensated high resistivity samples. In this case, beryllium interstitials and nitrogen vacancies are the most likely compensating donors. However, calculations also show that simultaneous incorporation of beryllium and hydrogen in the nitrogen-rich growth regime is likely to lead to the formation of neutral BeGa-Hi complexes. These complexes are energetically favorable and do not exhibit any transition levels in the bandgap. Subsequent annealing in hydrogen-free or nitrogen ambient will dissociate these complexes and activate shallow BeGa acceptors in GaN. Experimental measurements, where an ultraviolet photoluminescence band related to the shallow BeGa acceptor emerges after annealing the Be-doped GaN, confirm these findings.

Photo-EPR study of compensated defects in Be-doped GaN substrates

Time-dependent photo-induced electron paramagnetic resonance measurements were performed on GaN crystals grown by the high nitrogen pressure solution method. A resonance was induced in Be-doped samples by illuminating with photon energies greater than 2.7 eV. The EPR was thermally stable at T=4K and was quenched with photon energies between 0.5 and 2.7 eV. Optical cross sections for EPR excitation and quenching processes were extracted from time-dependent measurements using a model for defect charge state populations. A fit to the cross sections was performed using a lineshape for photon absorption by deep centers in GaN. The threshold for photo-excitation of a compensated acceptor (A−+hν→A0+eCB−) was 2.5 eV followed by a lattice relaxation of 0.3 eV. The threshold for the complimentary process (A0+hν→A−+hVB+) was 0.75 eV with a relaxation energy of 0.3 eV. These values are consistent with CN being the EPR-active acceptor. Thermal quenching is attributed to delocalization of carriers from a beryllium-related electron trap, which are then captured by neutral CN.

Amphoteric Be in GaN: Experimental Evidence for Switching between Substitutional and Interstitial Lattice Sites.

We show that Be exhibits amphoteric behavior in GaN, involving switching between substitutional and interstitial positions in the lattice. This behavior is observed through the dominance of Be_{Ga} in the positron annihilation signals in Be-doped GaN, while the emergence of V_{Ga} at high temperatures is a consequence of the Be impurities being driven to interstitial positions. The similarity of this behavior to that found for Na and Li in ZnO suggests that this could be a universal property of light dopants substituting for heavy cations in compound semiconductors.

Identification of yellow luminescence centers in Be-doped GaN through pressure-dependent studies

Effective acceptor doping of wide-band-gap semiconductors is still an outstanding problem. Beryllium has been suggested as a shallow acceptor in GaN, but despite sporadic announcements, Be-induced p-type doping has never been practically realized. Be-doped GaN possesses two luminescence bands; one at 3.38 eV and a second near 2.2 eV at an energy close to that of the parasitic yellow luminescence often found in undoped GaN crystals. We have performed high hydrostatic pressure studies of bulk, Be-doped gallium nitride crystals using the diamond anvil cell technique. We observed a splitting of the yellow luminescence line under hydrostatic pressure into two components, one which is strongly dependent on applied pressure and another whose pressure dependence is more modest. Together with hybrid functional calculations, we attribute the strongly-varying component to the beryllium–oxygen complex. The second component of the yellow luminescence possesses very similar pressure behavior to the yellow luminescence observed in undoped samples grown by the same method, behavior which we find consistent with the CN acceptor. At higher pressure, we observe the vanishing of yellow luminescence and a rapid increase in luminescence intensity of the UV line. We explain this as the pressure-induced transformation of the Be–O complex from a highly localized state with large lattice relaxation to a delocalized state with limited lattice relaxation.

Optical and magnetic resonance studies of Be-doped GaN bulk crystals

Low temperature photoluminescence (PL) and optically-detected magnetic resonance (ODMR) at 24GHz have been performed on a set of Be-doped GaN bulk crystals grown by the high pressure nitrogen solution method. Most notably, these crystals exhibit an intense 2.25eV “yellow” PL band that is currently of high interest for use as a converter in white light-emitting-diode applications. In addition, the samples investigated in this work are found to emit a broad “violet” PL band of varying intensity with peak energy near 3.16eV that is shown to be associated with a high concentration of residual Mg acceptor impurities as supported by secondary ion mass spectroscopy measurements. ODMR angular rotation studies obtained on the “yellow” and “violet” emission bands revealed two strong signals in each case and helped to provide information on the nature of these recombination processes. One feature, common to the ODMR of each PL band, has Zeeman splitting g-values of ~1.95 and is ascribed to residual Si and O shallow donor impurities. The second ODMR signal observed on the 2.25eV “yellow” PL exhibits nearly isotropic g-values of ~2.0 characteristic of deep centers in GaN and is tentatively associated with Be-related deep acceptors. In contrast, the second ODMR feature found on the 3.16eV “violet” PL has a larger degree of anisotropy with g║~2.1 and g⊥~2.0 and are similar to the g-values assigned previously to Mg shallow acceptors from ODMR studies of highly Mg-doped GaN heteroepitaxial layers with large degree (≥10%) of compensation.

Homoepitaxial N-polar GaN layers and HEMT structures grown by rf-plasma assisted molecular beam epitaxy

The authors have investigated the growth and structural and electrical properties of homoepitaxial GaN layers and GaN/AlGaN heterostructures grown on free-standing, hydride vapor phase epitaxy grown, N-polar GaN:Fe substrates by rf-plasma molecular beam epitaxy. Secondary-ion mass spectroscopic analysis of unintentionally doped and Be-doped N-polar GaN layers indicate that oxygen is the dominant impurity in all layers and is largely insensitive to growth temperature in the range investigated (675 °C < TS < 760 °C). Transmission electron microscopy (TEM) indicates that threading dislocations are generated at the regrowth interface in these samples; in contrast to homoepitaxial growth on Ga-polar GaN, and that the density of threading dislocations diminishes as the growth temperature increases. However, examination by TEM indicates that threading dislocations are not generated at the regrowth interface of samples subjected to pregrowth substrate surface cleaning by gallium deposition and desorption and subsequent growth of ultrathin (15 Å) initial AlN layers. N-polar GaN/AlGaN heterostructures grown on Be-doped homoepitaxial N-polar GaN buffers exhibit low buffer leakage and Hall mobilities up to 1680 cm2/Vs at sheet densities of 1.3 × 1013 cm−2. High electron mobility transistors have been fabricated on these structures; drain current densities over 700 mA/mm and breakdown voltages as high as 70 V have been measured.

Microwave annealing of Mg-implanted and in situ Be-doped GaN

An ultrafast microwave annealing method, different from conventional thermal annealing, is used to activate Mg-implants in GaN layer. The x-ray diffraction measurements indicated complete disappearance of the defect sublattice peak, introduced by the implantation process for single-energy Mg-implantation, when the annealing was performed at ≥1400 °C for 15 s. An increase in the intensity of Mg-acceptor related luminescence peak (at 3.26 eV) in the photoluminescence spectra confirms the Mg-acceptor activation in single-energy Mg-implanted GaN. In case of multiple-energy implantation, the implant generated defects persisted even after 1500 °C/15 s annealing, resulting in no net Mg-acceptor activation of the Mg-implant. The Mg-implant is relatively thermally stable and the sample surface roughness is 6 nm after 1500 °C/15 s annealing, using a 600 nm thick AlN cap. In situ Be-doped GaN films, after 1300 °C/5 s annealing have shown Be out-diffusion into the AlN layer and also in-diffusion toward the GaN/SiC interface. The in-diffusion and out-diffusion of the Be increased with increasing annealing temperature. In fact, after 1500 °C/5 s annealing, only a small fraction of in situ doped Be remained in the GaN layer, revealing the inadequateness of using Be-implantation for forming p-type doped layers in the GaN.

Stress in polycrystalline GaN films prepared by r.f sputtering

Undoped, Be-doped and Si-doped polycrystalline GaN films were deposited by R.F. sputtering onto fused silica substrates. The films were deposited at various deposition temperatures ranging from 300 K to 623 K and characterized by optical measurements while the microstructural information was obtained from SEM and XRD studies. The compositional study for the GaN film was carried out using SIMS. Residual stresses in these films were evaluated from the band tail of the absorption spectra as well as from direct measurements of hardness by commercially available depth sensing indentometer. It was observed that undoped GaN films had the highest hardness followed by that for Be-doped and Si-doped films. The values of hardness obtained form the above optical measurement tallied quite well with those obtained from direct indentation measurement.

Beryllium-doped polycrystalline GaN films: Optical and grain boundary properties

Be-doped polycrystalline GaN films were deposited by radio frequency sputtering of a GaN/Be composite target onto fused silica substrates. The films were characterized by optical measurements, while the microstructural information was obtained from scanning electron microscopy, atomic force microscopy and X-ray diffraction studies. Grain boundary parameters like density of trap states ( Q t) and the barrier height ( E b) at the grain boundaries were estimated from the broadening of the absorption tail. Photoluminescence measurement at 80 K exhibited two strong transitions located ∼ 2.1 eV and ∼ 2.7 eV along with lower intensity peaks for band edge luminescence at ∼ 3.47 eV and 3.28 eV for films deposited at T = 423 K and 623 K, respectively.

C and N co-implantation in Be-doped GaN

Carbon and nitrogen co-implantation characteristics in Be-doped GaN with different dopant concentration ratios have been systematically investigated. It was found that the conductive characteristics of Be-doped GaN convert to p-type by co-implantation of C or N atoms and subsequent annealing, which are essentially related to the dopant concentration ratio and annealing conditions. The conversion could be attributed to the reduction of self-compensation and the shift of the surface Fermi level towards the valance band edge. The outcome was reasonably in agreement with the surface stoichiometric switching as determined by x-ray photoelectron spectroscopy measurements. From photoluminescence data, the activation energy of the Be acceptor level was evaluated to be about 145–155 meV, which is shallower than that of Mg acceptors. These experimental results indicated that co-implanting C or N with Be atoms into the selective area of GaN is an effective method to enhance the electrical activation efficiency of Be acceptors and to reduce the surface barrier height, which can help to decrease the metal contact resistivity to p-type GaN.

Thermal activation of beryllium‐related photoluminescence by annealing of GaN grown by molecular beam epitaxy

Surface polarity related differences in the optical activity of Be in GaN epilayers grown by rf-plasma molecular beam epitaxy are investigated. N-polar GaN doped with Be exhibits a significantly higher intensity of donor-acceptor pair (DAP) photoluminescence (PL) than similarly doped Ga-polar GaN, indicating the Be is incorporating at microscopically different sites, or possibly is forming different compensating complexes. Highly Be-doped Ga-polar GaN forms isolated polarity-inverted regions which then incorporate Be via the N-polar mechanism resulting in the DAP PL. High temperature annealing of the Ga-polar layers both under nitrogen/hydrogen mixtures and under pure nitrogen atmospheres activates the DAP PL. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Pressure behavior of beryllium-acceptor level in gallium nitride

Beryllium is a promising dopant for obtaining an efficient p-type conductivity of GaN. A high-pressure study of the donor-acceptor-pair (DAP) photoluminescence line at 3.39eV in Be-doped GaN samples is reported, together with ab initio calculations of the electronic structure of GaN:Be under pressure. The analysis confirms the formation of a Be-acceptor state which is involved in the DAP photoluminescence. Different properties of the Mg- and the Be-acceptor states in GaN are found. The Be level is located ∼100–140meV lower in the band gap than that of Mg, and its pressure coefficient is lower by approximately 3–4meV∕GPa.

Growth of Be-doped p-type GaN under Invariant Polarity Conditions

We have successfully grown Be-doped p-type GaN on a sapphire (0001) substrate by radio frequency plasma assisted molecular beam epitaxy (RF-MBE) by conserving the surface polarity during growth. It is found that while the surface polarity of undoped GaN grown on a sapphire (0001) substrate is N-terminated, it changes from N- to Ga-terminated during the growth of Be-doped GaN. When this polarity change occurs, the resistivity of the grown layer is extremely high. Moreover, a broad deep level photoluminescence distinctly appears around 2.0 eV. When an AlN buffer layer is used, a GaN layer on this buffer layer can be grown under the Ga-terminated condition. By growing Be-doped GaN on the Ga-terminated GaN, we have successfully grown Be-doped p-type GaN without the polarity change. In addition, the optical property of the sample is markedly improved. We have confirmed that the Be acceptor level is shallower than the Mg acceptor level by approximately 100 meV.

Calculated properties of nitrogen-vacancy complexes in beryllium- and magnesium-doped GaN

The properties of defect complexes consisting of a nitrogen vacancy with a substitutional beryllium or magnesium atom on neighboring lattice sites in hexagonal GaN are calculated using the AIMPRO local-densityfunctional theory method. Both types of defects VN-BeGa and VN-MgGa are bound with respect to their isolated constituents. They do not appear to have any electronic levels in the bandgap, and are expected to be neutral defects. Important structural differences are found. In its minimum energy configuration, the Be atom in the VN-BeGa complex lies nearly in the same plane as the three equivalent N atoms nearest to it. Thus, it has shorter Be-N bonds than the Ga-N distance in the bulk crystal, while the Mg atom in the VN-MgGa complex occupies a position closer the lattice site of the Ga atom it replaces. Hence, the VN-BeGa complex has a larger open volume than the VN-MgGa complex. This is consistent with positron annihilation experiments @Saarinen et al., J. Cryst. Growth 246, 281 ~2002!; Hautakangas et al., Phys. Rev. Lett. 90, 137402 ~2003!#. The frequency of the highest local vibrational mode of the VN-BeGa center is calculated to be within 3‐ 4 % of an infrared absorption line detected in Be-doped GaN @Clerjaud ~private communication!#.

Calculated properties of point defects in Be-doped GaN

The properties of several point defects in hexagonal gallium nitride that can compensate beryllium shallow acceptors $({\mathrm{Be}}_{\mathrm{Ga}})$ are calculated using the AIMPRO method based on local density functional theory. ${\mathrm{Be}}_{\mathrm{Ga}}$ itself is predicted to have local vibrational modes (LVM's) very similar to magnesium acceptors. The highest frequency is about $663{\mathrm{cm}}^{\ensuremath{-}1}.$ Consistent with other recent studies, we find that interstitial beryllium double donors and single-donor beryllium split interstitial pairs at gallium sites are very likely causes of compensation. The calculations predict that the split interstitial pairs possess three main LVM's at about 1041, 789, and $738{\mathrm{cm}}^{\ensuremath{-}1}.$ Of these, the highest is very close to the experimental observation in Be-doped GaN. Although an oxygen donor at the nearest-neighboring site to a beryllium acceptor $({\mathrm{Be}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{O}}_{\mathrm{N}})$ is also a prime suspect among defects that are possibly responsible for compensation, its highest frequency is calculated to be about $699{\mathrm{cm}}^{\ensuremath{-}1}$ and hence is not related in any way to the observed center. Another mode for this defect is estimated to be about $523{\mathrm{cm}}^{\ensuremath{-}1}$ and is localized on the ${\mathrm{O}}_{\mathrm{N}}$ atom. These two vibrations of ${\mathrm{Be}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{O}}_{\mathrm{N}}$ are thus equivalent to those for the isolated substitutional centers perturbed by the presence of their impurity partners.

Thermal Activation of Beryllium in GaN Grown by RF-Plasma Molecular Beam Epitaxy

ABSTRACTDifferences in the optical activity of Be in GaN epilayers grown on different surface polarities by rf-plasma molecular beam epitaxy are investigated. Nitrogen-polar GaN doped with Be exhibits a significantly higher intensity of donor-acceptor pair (DAP) photoluminescence (PL) than similarly doped Ga-polar GaN, indicating the Be is incorporating at microscopically different sites, or possibly is forming different compensating complexes. Highly Be-doped Ga-polar GaN apparently forms isolated polarity-inverted regions which then incorporate Be via the N-polar mechanism resulting in the DAP PL. High temperature annealing of the Ga-polar layers both under nitrogen/hydrogen mixtures and under pure nitrogen atmospheres activates the DAP PL.

極性制御によるBeドープp型GaNの成長

極性制御によるBeドープp型GaNの成長