eV For GaN Research Articles

Photoluminescence from Be‐Doped GaN Grown by Metal‐Organic Chemical Vapor Deposition

A systematic photoluminescence study of Be‐doped GaN grown by metal‐organic chemical vapor deposition is presented. All Be‐doped samples show the ultraviolet luminescence (UVLBe) band with a maximum at 3.38 eV and the yellow luminescence (YLBe) band with a maximum at ≈2.15 eV in GaN:Be having various concentrations of Be. The UVLBe band is attributed to the shallow state of the BeGa acceptor with a delocalized hole. The YLBe band is caused by a Be‐related defect, possibly the polaronic state of the BeGa acceptor with the charge transition level at 0.3 eV above the valence band. This broad band exhibits unusual properties. In particular, it always shows two steps in its thermal quenching. The second step at T ≈ 200 K is attributed to the emission of holes from the 0.3 eV level to the conduction band. The origin of the first step remains unexplained.

Toward h-BN/GaN Schottky Diodes: Spectroscopic Study on the Electronic Phenomena at the Interface.

Hexagonal boron nitride (h-BN), together with other members of the van der Waals crystal family, has been studied for over a decade, both in terms of fundamental and applied research. Up to now, the spectrum of h-BN-based devices has broadened significantly, and systems containing the h-BN/III-V junctions have gained substantial interest as building blocks in, inter alia, light emitters, photodetectors, or transistor structures. Therefore, the understanding of electronic phenomena at the h-BN/III-V interfaces becomes a question of high importance regarding device engineering. In this study, we present the investigation of electronic phenomena at the h-BN/GaN interface by means of contactless electroreflectance (CER) spectroscopy. This nondestructive method enables precise determination of the Fermi level position at the h-BN/GaN interface and the investigation of carrier transport across the interface. CER results showed that h-BN induces an enlargement of the surface barrier height at the GaN surface. Such an effect translates to Fermi level pinning deeper inside the GaN band gap. As an explanation, we propose a mechanism based on electron transfer from GaN surface states to the native acceptor states in h-BN. We reinforced our findings by thorough structural characterization and demonstration of the h-BN/GaN Schottky diode. The surface barriers obtained from CER (0.60 ± 0.09 eV for GaN and 0.91 ± 0.12 eV for h-BN/GaN) and electrical measurements are consistent within the experimental accuracy, proving that CER is an excellent tool for interfacial studies of 2D/III–V hybrids.

Identification of origin of EC –0.6 eV electron trap level by correlation with iron concentration in n-type GaN grown on GaN freestanding substrate by metalorganic vapor phase epitaxy

The origin of E3 electron traps at EC –0.58 eV in GaN was investigated using Si-doped n-type GaN layers grown on freestanding GaN substrates using MOVPE. These layers contained impurity Fe at various concentrations depending on the growth conditions and the position within the wafer. Twenty E3 concentrations (NT,E3) determined by deep-level transient spectroscopy were plotted against the corresponding Fe concentrations ([Fe]) obtained from secondary ion mass spectrometry. A correlation was evident between NT,E3 and [Fe] in the range (0.4–12) × 1015 cm–3, suggesting that the E3 level in MOVPE-grown homoepitaxial GaN originates from the substitution of Fe atoms at Ga sites.

Giant shifts of photoluminescence bands in GaN

Unusually large shifts of photoluminescence (PL) bands with excitation intensity or with temperature are sometimes observed in GaN. In particular, the ultraviolet luminescence band with a maximum at 3.28 eV is attributed to transitions from the conduction band (or shallow donors) to the shallow MgGa acceptor. In semi-insulating GaN samples doped with Mg, enormous redshift (up to 0.6 eV) and broadening of this PL band can be observed with variation of temperature or excitation intensity. A similar behavior of PL was observed in semi-insulating GaN doped with Zn. The blue band with a maximum at 2.9 eV in GaN:Zn is attributed to electron transitions from the conduction band or from shallow donors to the ZnGa acceptor level located at 0.4 eV above the valence band. In conductive n-type GaN, this PL band redshifts by only 7 meV with decreasing excitation intensity by five orders of magnitude, and the shift is explained with the donor–acceptor pair model. However, in semi-insulating GaN:Zn samples, a gradual shift by up to 0.8 eV with decreasing excitation intensity has been observed. Finally, the yellow luminescence band shifts by not more than 7 meV in high-quality, undoped GaN samples, while a redshift by more than 0.1 eV can be observed in GaN heavily doped with Si. The giant shifts are attributed to the presence of electric fields in high-resistivity or heavily doped n-type GaN.

Extremely Slow Decay of Yellow Luminescence in Be‐Doped GaN and Its Identification

The extremely slowly decaying yellow luminescence band around 2.08 eV in GaN:Be:O is studied by continuous‐wave and time‐resolved photoluminescence, as well as photoluminescence excitation spectroscopy and absorption. The lifetime obtained for this process is around 2.2 s and is thermally quenched for temperatures between 250 and 400 K. From the photoluminescence excitation measurements, two major pumping thresholds at around 2.7 and 3.1 eV can be identified. We discuss the origin of the Be‐related 2.08 eV transition as well as the origin of the overlaying photoluminescence bands at 1.88 and 2.12 eV.

Solar Water Oxidation by an InGaN Nanowire Photoanode with a Bandgap of 1.7 eV

The performance of overall solar water splitting has been largely limited by the half-reaction of water oxidation. Here, we report a 1.7 eV bandgap InGaN nanowire photoanode for efficient solar water oxidation. It produces a low onset potential of 0.1 V versus a reversible hydrogen electrode (RHE) and a high photocurrent density of 5.2 mA/cm2 at a potential as low as 0.6 V versus RHE. The photoanode yields a half-cell solar energy conversion efficiency up to 3.6%, a record for a single-photon photoanode to our knowledge. Furthermore, in the presence of hole scavengers, the photocurrent density of the InGaN photoanode reaches 21.2 mA/cm2 at 1.23 V versus RHE, which approaches the theoretical limit for a 1.7 eV InGaN absorber. The InGaN nanowire photoanode may serve as an ideal top cell in a photoelectrochemical tandem device when stacked with a 0.9–1.2 eV bandgap bottom cell, which can potentially deliver solar-to-hydrogen efficiency over 25%.

Strain effect on the electronic properties of III-nitride nanosheets: Ab-initio study

In this study the structural and electronic properties of III-nitride monolayers XN (X=B, Al, Ga and In) under different percentages of homogeneous and shear strain are investigated using the full potential linearized augmented plane wave within the density functional theory. Geometry optimizations indicate that GaN and InN monolayers get buckled under compressive strain. Our calculations show that the free-strains of these four monolayers have an indirect band gap. By applying compressive biaxial strain, a transition from indirect to direct band gap occurs for GaN and InN, while the character of band gap for BN and AlN is not changed. Under tensile strain, only BN monolayer behaves as direct band gap semiconductor. In addition, when the shear strain is applied, only InN undergoes an indirect to direct band gap transition. Furthermore, the variations of band gap versus strain for III-nitride monolayers have been calculated. When a homogeneous uniform strain, in the range of [−10%, +10%], is applied to the monolayers, the band gap can be tuned for from 3.92 eV to 4.58 eV for BN, from 1.67 eV to 3.46 eV for AlN, from 0.24 eV to 2.79 eV for GaN and from 0.60 eV to 0.90 eV for InN.

Deep-level transient spectroscopy of low-free-carrier-concentration n-GaN layers grown on freestanding GaN substrates: Dependence on carbon compensation ratio

Electron traps in n-GaN layers with a relatively low-free-carrier-concentration of approximately 1 × 1016 cm−3 were characterized by deep-level transient spectroscopy. Sample layers were grown by metal organic chemical vapor deposition with a thickness of 12 µm on freestanding GaN substrates, and were doped with both silicon and carbon. The measurement results showed a reduction in the density of carbon-related electron traps at an energy level of EC −0.40 eV in GaN on GaN samples, compared with GaN on SiC samples. It was also observed that the doping of carbon significantly suppressed electron traps at EC −0.61 eV, which was associated with the nitrogen antisite. Consequently, the possibility of minimizing all of the electron traps located between EC −0.19 and −0.89 eV in n-GaN was demonstrated by controlling the carbon doping in the nitrogen site.

Tuning the work function of GaN with organic molecular acceptors

We demonstrate the capability of two molecular organic acceptors [1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile and 2,2\ensuremath{'}-(perfluoronaphthalene-2,6-diylidene)dimalononitrile] to tune the work function (\ensuremath{\Phi}) of intrinsically doped GaN(0001) and for comparison of intrinsically doped ZnO(0001). With ultraviolet photoelectron spectroscopy we determine the accessible \ensuremath{\Phi} range as 4.2--6.0 eV for GaN and 3.9--6.3 eV for ZnO. The contribution to the \ensuremath{\Phi} change (\ensuremath{\Delta}\ensuremath{\Phi}) of acceptor-induced surface band bending within GaN was significantly smaller than in ZnO (0.35 versus 1 eV), which we attribute to surface gap states. We introduce a model that takes these surface states into account and thus allows quantifying the impact of the semiconductor bulk doping level and surface state density on \ensuremath{\Delta}\ensuremath{\Phi}. The lower the doping level is, the lower the surface state density needs to be to reduce the band bending contribution to \ensuremath{\Delta}\ensuremath{\Phi}. This limits the maximum value of \ensuremath{\Phi} tuning for GaN with molecular acceptors, whereas, on the other hand, it renders the method robust against surface structural disorder.

Electronic and optical properties of advance semiconductor materials: BN, AlN and GaN nanosheets from first principles

We investigate the electronic and linear optical properties of BN, AlN and GaN hexagonal nanosheets using the band structure results obtained through the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT). The dielectric tensor and corresponding optical properties are derived within the random phase approximation (RPA). Specifically, the dielectric function, absorption coefficient, optical conductivity, extinction index, reflectivity, loss function and the refraction index of these above mentioned nanosheets are calculated for both parallel (E||x) and perpendicular (E||z) electric field polarizations. Calculated results show that the hexagonal XN (X=B, Al and Ga) nanosheets have semiconductor characters with wide band gap of about 4.96, 2.73 and 1.95eV respectively. The optical conductivity in E||x and E||z starts with a gap about 2.92eV and 6.73eV for BN, 2.73eV and 3.52eV for AlN and 1.61eV and 3.58eV for GaN, confirms that these nanosheets have semiconductor properties moreover the optical spectra are isotropic along these two polarizations. These results propose potential application for the development of BN, AlN and GaN nano structures in electronic and optoelectronic devices.

Evaluation of Dislocation Mobility in Wurtzite Semiconductors

ABSTRACTThe indentation hardness and yield strength of various wurtzite-structured semiconductors, such as AlN, GaN, InN, and ZnO, were summarized together with those of 6H-SiC. From analysis of the data, the activation energy for motion of an individual dislocation was deduced to be 2–2.7 and 0.7–1.2 eV in GaN and ZnO, respectively, and the evaluated activation energy for dislocation motion showed a dependence on the dislocation energy in the minimum length. The results were evaluated in terms of homology and the basic mechanism of the dislocation process. Dislocation motion is thought to be primarily controlled by the atomic bonding character of the semiconductors.

Ultraviolet photodiodes based on AlGaN solid solutions

We present the characteristics of ultraviolet photodiodes based on Schottky barriers to the Aluminum Gallium Nitride (AlGaN) solid solution. The paper is devoted to the creation of ohmic and rectifying contacts to AlxGa1−xN solid solutions with a high (x > 0.5) proportion of AlN. The most important property of (AlxGa1−xN) materials is their solar blindness: the wide bandgap (Eg = 3.4 eV for GaN; 6.2 eV for AlN) makes these materials insensitive to the visual and IR light. The study of solid solutions with various mole fractions of Al has created solar-blind and see-blind ultraviolet photodetectors.

Diffusion of Mg dopant in metal-organic vapor-phase epitaxy grown GaN and AlxGa1−xN

Diffusion of the p-type dopant Mg in GaN and AlxGa1−xN which is accompanied by segregation and affected by transient effects in metal-organic vapor-phase epitaxy reactors is investigated. We have grown 110 nm thick Mg doped GaN and Al0.1Ga0.9N layers on top of undoped GaN and Al0.1Ga0.9N layers, respectively, in a temperature range between 925 °C and 1050 °C where we placed special emphasis on the lower temperature limit without diffusion to allow separation of Mg transients, diffusion, and segregation. Hereby, AlxGa1−xN layers enable monitoring of the resolution limit by secondary ion mass spectrometry analyses for the respective samples; therefore, thin AlxGa1−xN marker layers are incorporated in the thick GaN layers. We found an upper limit of 1.25 × 1019 cm−3 for diffusing Mg atoms in both sample types. Owing to the marked influence of Mg segregation in Al0.1Ga0.9N, diffusion is only seen by using a GaN cap on top of the Al0.1Ga0.9N layer sequence. Diffusion in Al0.1Ga0.9N is shown to be increased by about 25%−30% compared to GaN. Post growth annealing experiments under conditions equivalent to those used for growth of the Mg doped samples showed negligible diffusion. Comparing the results to well established findings on other doped III-V compounds, diffusion is explained by an interstitial-substitutional mechanism with a diffusion coefficient, which is concentration dependent. Analysis of the temperature dependent diffusivity revealed an activation energy of 5.0 eV for GaN:Mg and 5.2 eV for Al0.1Ga0.9N:Mg.

Optical signature of Mg-doped GaN: Transfer processes

Mg doping of high quality, metal organic chemical vapor deposition grown GaN films results in distinct traces in their photoluminescence and photoluminescence excitation spectra. We analyze GaN:Mg grown on sapphire substrates and identify two Mg related acceptor states, one additional acceptor state and three donor states that are involved in the donor-acceptor pair band transitions situated at 3.26--3.29 eV in GaN:Mg. The presented determination of the donor-acceptor pair band excitation channels by photoluminescence excitation spectroscopy in conjunction with temperature-dependent photoluminescence measurements results in a direct determination of the donor and acceptor binding, localization, and activation energies, which is put into a broader context based on Haynes's rule. Furthermore, we analyze the biexponential decay dynamics of the photoluminescence signal of the acceptor and donor bound excitons. As all observed lifetimes scale with the localization energy of the donor and acceptor related bound excitons, defect and complex bound excitons can be excluded as their origin. Detailed analysis of the exciton transfer processes in the close energetic vicinity of the GaN band edge reveals excitation via free and bound excitonic channels but also via an excited state as resolved for the deepest localized Mg related acceptor bound exciton. For the two Mg acceptor states, we determine binding energies of 164 $\ifmmode\pm\else\textpm\fi{}$ 5 and 195 $\ifmmode\pm\else\textpm\fi{}$ 5 meV, which is in good agreement with recent density functional theory results. This observation confirms and quantifies the general dual nature of acceptor states in GaN based on the presented analysis of the photoluminescence and photoluminescence excitation spectra.

InGaN Solar Cells: Present State of the Art and Important Challenges

Solar cells are a promising renewable and carbon-free electric energy resource to address the fossil-fuel shortage and global warming. Energy conversion efficiencies over 40% have been recently achieved using conventional III–V semiconductor compounds as photovoltaic materials. The revision of InN bandgap to a much narrower value has extended the fundamental bandgap of the group III-nitride alloy system over a wider spectral region (from 0.64 eV for InN to 3.4 eV for GaN or 6.2 eV for AlN), raising the possibility of a variety of new applications. The tunable bandgap, predicted high radiation resistance, and strong absorption coefficient of the In $_x$ Ga $_{ 1-x}$ N material system are promising for high-efficiency photovoltaic systems. During the past few years, the interest in In $_x$ Ga $_{ 1-x}$ N solar cells has been remarkable. The development of high-performance solar cells using In $_x$ Ga $_{1-x}$ N materials is one of the most important goals when compared with the existing solar cells using Si and other III–V materials. Significant efforts and progress have been made toward this goal, while great opportunities and grand challenges coexist. In this paper, we present a review on the present state of the art of In $_x$ Ga $_{1-x}$ N-based solar cells. The most important challenges toward the high-efficiency In $_x$ Ga $_{ 1-x}$ N-based solar cells are discussed in the context of the recent results.

Optimized effective potential using the Hylleraas variational method

In electronic structure calculations the optimized effective potential (OEP) is a method that treats exchange interactions exactly using a local potential within density-functional theory. In the exchange-only case, this is commonly called exact exchange. We present a method using density-functional perturbation theory combined with the Hylleraas variational method for finding the OEP by direct minimization, which avoids any sum over unoccupied states. The method has been implemented within the plane-wave, pseudopotential formalism. Band structures for zinc-blende semiconductors Si, Ge, C, GaAs, CdTe, and ZnSe; wurtzite semiconductors InN, GaN, and ZnO; and the rocksalt insulators CaO and NaCl have been calculated using the OEP and compared to calculations using the local-density approximation (LDA), a selection of generalized gradient approximations (GGAs) and Hartree-Fock (HF) functionals. The band gaps found with the OEP improve on the calculated results for the LDA, GGAs, or HF, with calculated values of 1.16 eV for Si, 3.32 eV for GaN, and 3.48 eV for ZnO. The OEP energies of semicore $d$ states are also greatly improved compared to LDA.

GaNAs alloys over the whole composition range grown on crystalline and amorphous substrates

AbstractUsing low temperature molecular beam epitaxy (LT‐MBE) technique we have overcome the miscibility gap of GaAs and GaN alloys and successfully synthesized GaN1‐xAsx alloys in the whole composition range on crystalline (sapphire and silicon) and amorphous (Pyrex glass) substrates. On the N‐rich side we found an increased incorporation of As with decreasing growth temperature. At high enough As content the films lose their crystallinity and become amorphous. On sapphire substrate, the alloys are amorphous in the composition range of 0.17<x<0.75. For the films grown on glass substrates, the composition range for amorphous alloys extends to x∼0.1. However, films grown on silicon shows small fraction of As‐rich and N‐rich GaNAs nanocrystals for films with As content up to 36%. These amorphous GaNAs films have smooth morphology, homogeneous composition and sharp, well defined optical absorption edges. The bandgap energy of the GaN1‐xAsx alloys covers a broad energy range from ∼3.4 eV in GaN to∼0.8 eV at x∼0.85. This provides an almost perfect fit to the solar spectrum offering the opportunity to design high efficiency multijunction solar cells using a single ternary alloy system. The amorphous nature of this alloy over a wide alloy range can also be advantageous since they can be deposited on low‐cost glass substrate. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN1−xBix: Extremely mismatched semiconductor alloys

Through nonequilibrium low-temperature molecular beam epitaxy, we have grown GaN1−xBix alloys on sapphire substrates with x up to 0.11. The GaN1−xBix alloys are found to be amorphous with GaN crystals distributed throughout the film. A dramatic reduction in the optical band gap from 3.4 eV in GaN to as low as 1.2 eV for x∼0.11 was qualitatively explained by formation of a narrow band originating from anticrossing interaction between Bi localized states and the extended states of the GaN matrix.

Non‐equilibrium GaNAs alloys with band gap ranging from 0.8‐3.4 eV

AbstractA new alloy system, the GaN1‐xAsx alloys in the whole composition range was successfully synthesized using the non‐equilibrium low temperature molecular beam epitaxy method. The alloys are amorphous in the composition range of 0.17 < x < 0.75 and crystalline outside this region. The amorphous films have smooth morphology, homogeneous composition and sharp, well defined optical absorption edges. The bandgap energy varies in a broad energy range from ∼3.4 eV in GaN to∼0.8 eV at x∼0.85. The reduction of the band gap can be attributed primarily to the downward movement of the conduction band for alloys with x > 0.2, and to the upward movement of the valence band for alloys with x < 0.2. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Measurement of GaN/Ge(001) Heterojunction Valence Band Offset by X-Ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset (VBO) at the GaN/Ge heterostructure interface. The VBO is directly determined to be 1.13 ± 0.19 eV, according to the relationship between the conduction band offset ΔEC and the valence band offset ΔEV: ΔEC = E9Gan – E9Ge – ΔEV, and taking the room-temperature band-gaps as 3.4 and 0.67 eV for GaN and Ge, respectively. The conduction band offset is deduced to be 1.6 ± 0.19 eV, which indicates a type-I band alignment for GaN/Ge. Accurate determination of the valence and conduction band offsets is important for the use of GaN/Ge based devices.