Effects of surface treatments on the electrical and the microstructural changes of Pd contact on p-type GaN

We investigated the effects of surface treatments by aqua regia and (NH₄)₂S_x on the electrical and the microstructural changes of Pd contact on p-type GaN during annealing. The formation of a surface oxide was suppressed by the (NH₄)₂S_x treatment, and S-Ga and S-N bonds with binding energy of 162.1 eV and 163.6eV were formed, degrading the structural ordering of Pd. After 300°C annealing, the contact resistivity in the aqua regia-treated sample increased significantly. This could be attributed to the outdiffusion of N atoms leaving N vacancies below the contact, as confirmed by the increase of the Pd (111) plane spacing probably due to the dissolution of N atoms in Pd interstitial sites. Meanwhile, the contact resistivity in the (NH₄)₂S_x-treated sample was not degraded and no change was observed in the Pd (111) plane spacing. These results suggest that S-Ga and S-N bonds formed on (NH₄)₂S_x-treated GaN could act as a diffusion barrier for the outdiffusion of N atoms. The contact resistivity for the aqua regia-treated sample decreased again, probably due to the outdiffusion of Ga as well as N atoms at 500°C.     

Microstructural and electrical investigation of Ni/Au ohmic contact on p-type GaN

Electrical properties of Ni/Au ohmic contacts on p-type GaN were interpreted with the change of microstructure observed under transmission electron microscopy. The contact resistivity was decreased from 1.3×10⁻² to 6.1×10⁻⁴ Ωcm² after annealing at 600°C. The reduction is due to the dissolution of Ga atoms into Au−Ni solid solution formed during annealing, via the generation of Ga vacancies. Thus, net concentration of holes increased below the contact, resulting in the reduction of contact resistivity. At 800°C, N atoms decomposed; reacted with Ni, and forming cubic Ni₄N. Consequently, N vacancies, acting as donors in GaN, were generated below the contact, leading to the increase of contact resistivity to 3.8×10⁻² Ωcm².     

MOCVD growth of GaBN on 6H-SiC (0001) substrates

B_xGa_1−xN films were deposited on 6H-SiC (0001) substrates at 1000°C by low pressure MOVPE using diborane, trimethylgallium, and ammonia as precursors. The presence of boron was detected by Auger scanning microprobe, the shift of the (00.2) x-ray diffraction peak, and low-temperature photoluminescence. A single-phase B_xGa_1−xN alloy with x=1.5% was produced at the gas phase B/Ga ratio of 0.005. Phase separation into wurtzite BGaN and the B-rich phase occurred for a B/Ga ratio in the 0.01–0.2 range. Only BN was formed for B/Ga >0.2. The B-rich phase was identified as h-BN with sp² bonding based on the results of Fourier transform infrared spectroscopy. As the diborane flow exceeds the threshold concentration, the growth rate of BGaN decreases sharply, because the growth of GaN is poisoned by the formation of the slow growing BN phase. The bandedge emission of B_xGa_1−xN varies from 3.451 eV for x=0% with FWHM of 39.2 meV to 3.465 eV for x=1.5% with FWHM of 35.1 meV. The narrower FWHM indicates that the quality of GaN epilayer is improved with a small amount of boron incorporation. The PL linewidths become broader as more boron is introduced into the solid solution.     

Mechanism investigation of NiO<Subscript>x</Subscript> in Au/Ni/p-type GaN ohmic contacts annealed in air

Recently, Au/Ni/p-type GaN ohmic contacts annealed in an air ambient have been widely investigated. However, to obtain a low specific-contact resistance, the annealing window is limited. In this study, to understand the oxidation function of metallic Ni, the Au/Ni/p-type GaN structure was annealed in an air ambient for 10 min at various temperatures. Using x-ray photoelectron spectroscopy (XPS) analysis, the metallic Ni was oxidized into NiO and NiO_1.3 compositions at annealing temperatures of 500°C and 600°C, respectively. However, metallic Ni still existed on the interface of the Ni/p-type GaN annealed at 400°C. The associated barrier heights of 0.42 eV, 0.21 eV, and 0.31 eV were obtained with p-type GaN for the Ni, NiO, and NiO_1.3 contacts, respectively. The hole concentrations of p-type NiO and p-type NiO_1.3 were 2.6×10¹⁶ cm⁻³ and 2.0×10¹⁸ cm⁻³, respectively. The lower hole concentration of the p-type NiO would lead to reducing the valence-band bending of the p-type GaN, as well as the barrier height for holes crossing from the p-type NiO to the p-type GaN. The formation of NiO was thus an important issue for lowering the specific-contact resistance of the Au/Ni/p-type GaN ohmic contacts annealed in an air ambient.     

Annealing of AsGa-related defects in LT-GaAs: The role of gallium vacancies

We have studied the annealing properties of As_Ga-related defects in layers of GaAs grown at low substrate temperatures (300°C) by molecular beam epitaxy (low temperature[LTx]-GaAs). The concentration of neutral As_Ga-related native defects, estimated by infrared absorption measurements, ranges from 2×10¹⁹ to 1×10²⁰ cm⁻³. Slow positron annihilation results indicate an excess concentration of Ga vacancies in LT layers over bulk grown crystals. A sharp annealing stage at 450°C marks a rapid decrease in the As_Ga defect concentration. We propose that the defect removal mechanism is the diffusion of As_Ga to arsenic precipitates, which is enhanced by the presence of excess V_Ga. The supersaturated concentration of V_Ga must also decrease. Hence, the diffusivity of the As_Ga defects is time dependent. Analysis of isothermal annealing kinetics gives an enthalpy of migration of 2.0±0.3 eV for the photoquenchable As_Ga defects, 1.5±0.3 eV for the V_Ga, and 1.1±0.3 eV for the nonphotoquenchable defects. The difference in activation enthalpy represents difference energy between an As atom and Ga atom swapping sites with a V_Ga.     

2D Oxides Realized via Confinement Heteroepitaxy

Novel confinement techniques facilitate the formation of non-layered 2D materials. Here it is demonstrated that the formation and properties of 2D oxides (GaO_x, InO_x, SnO_x) at the epitaxial graphene (EG)/silicon carbide (SiC) interface is dependent on the EG buffer layer properties prior to element intercalation. Using 2D Ga, it is demonstrated that defects in the EG buffer layer lead to Ga transforming to GaO_x with non-periodic oxygen in a crystalline Ga matrix via air oxidation at room temperature. However, crystalline monolayer GaO₂ and bilayer Ga₂O₃ with ferroelectric wurtzite structure(FE-WZ') can then be formed via subsequent high-temperature O₂ annealing. Furthermore, the graphene/X/SiC (X = 2D Ga or Ga₂O₃) junction is tunable from Ohmic to a Schottky or tunnel barrier depending on the interface species. Finally, using vertical transport measurements and electron energy loss spectroscopy analysis, the bandgap of 2D gallium oxide is identified as 6.6 ± 0.6 eV, significantly larger than that of bulk β-Ga₂O₃ (≈4.8 eV), suggesting strong quantum confinement effects at the 2D limit. The study presented here is foundational for development of atomic-scale, vertical 2D/3D heterostructure for applications requiring short transit times, such as GHz and THz devices.     

Electronic near-surface defect states of bare and metal covered n-GaN films observed by cathodoluminescence spectroscopy

Depth-dependent low energy cathodoluminescence spectroscopy (CLS) has been used to investigate the near-surface optical properties of n-type GaN epelayers grown under various growth conditions. Both bare and reacted-Mg/n-GaN and Al/n-GaN (annealed to 1000°C) surfaces were investigated. We find enhanced emission at ∼1.4, 1.6, and 2.2eV from states within then-type GaN bandgap near the interface of the reacted Mg with the semiconductor, which correlates with previous measurements of Schottky barrier formation on the same specimens. No clear evidence for p-type doping at the reacted interfacial layer is apparent. For Al on n-type GaN, CLS emission is dominated before and after metallization by “yellow” emission, which correlates only weakly with the Fermi level stabilization energy. Instead, we observe emission above the GaN band edge emission at 3.85 eV, due either to deep level emission from AlN or to the formation of the alloy Al_xGa_1-xN (x≈0.2) in the reacted near-surface region.     

Effect of Se-doping on deep impurities in AlxGa1−xAs grown by metalorganic chemical vapor deposition

We have studied the effect of Se-doping on deep impurities in Al_xGa_1−xAs (x = 0.2∼0.3) grown by metalorganic chemical vapor deposition (MOCVD). Deep impurities in various Se-doped Al_xGa_1−xAs layers grown on GaAs substrates were measured by deep level transient spectroscopy and secondary ion mass spectroscopy. We have found that the commonly observed oxygen contamination-related deep levels at E_c-0.53 and 0.70 eV and germanium-related level at E_c-0.30 eV in MOCVD grown Al_xGa_1−xAs can be effectively eliminated by Se-doping. In addition, a deep hole level located at E_y + 0.65 eV was found for the first time in Se-doped Al_xGa_1-xAs when Se ≥2 × 10¹⁷ cm⁻³ or x ≥ 0.25. The concentration of this hole trap increases with increasing Se doping level and Al composition. Under optimized Se-doping conditions, an extremely low deep level density (N_t less than 5 × 10¹² cm⁻³, detection limit) Al_0.22Ga_0.78As layer was achieved. A p-type Al_0.2Ga_0.8As layer with a low deep level density was also obtained by a (Zn, Se) codoping technique.     

表面处理对p-GaN欧姆接触的影响

分别用稀盐酸、王水以及(NH4)2S溶液处理p-GaN表面,通过测试样品表面Ols的X射线光电子能谱(XPS),比较了这些溶液去除p-GaN表面氧化层的能力;在经不同溶液处理后的样品表面,以相同的条件制作Ni/Au电极,并测试其与p-GaN的比接触电阻,结果表明经稀盐酸处理后的样品表面,由于其氧含量较高,不能与Ni/Au形成良好的欧姆接触,而经王水和(NH4)2S溶液处理后的p-GaN表面,能与Ni/Au形成良好的欧姆接触;最后,通过比较样品表面的Ga/N原子浓度比,探讨了王水处理p-GaN表面能够形成良好欧姆接触的原因.     

Te and Ge — doping studies in Ga1−xAlxAs

The temperature dependence in the range 77–400 K of the carrier concentration, resistivity and mobility of a series of n and p-type single crystal, liquid-phase epitaxial layers of Ga_1−xAl_xAs are presented. These layers were doped, n-type with tellurium, and p-type with germanium to yield carrier concentrations in the range 10¹⁷ – 10¹⁸cm⁻³ at 295 K. Donor and acceptor ionization energies, ε_D and ε_A, are derived from the data. The dependence of ε_D on alloy composition is interpreted in terms of the known band structure variation in the alloy system.     

Photoluminescence of CuGaTeAs and CuGaSnGa complexes in n-GaAs under resonance polarized excitation

Photoluminescence (PL) of n-type GaAs:Te:Cu and GaAs:Sn:Cu with an electron density of about 10¹⁸ cm^?3 was studied at 77 K. A broad band with a peak at the photon energy near 1.30 eV (GaAs:Te:Cu) or 1.27 eV (GaAs:Sn:Cu) was dominant in the PL spectrum under interband excitation. This band arose from the recombination of electrons with holes trapped by Cu_GaTe_As or Cu_GaSn_Ga complexes. It has been found that the low-energy edge of the excitation spectrum of this PL band at photon energies below ～1.4 eV is controlled by the optical ejection of electrons from a complex into the conduction band or to a shallow excited state. The PL polarization factors upon excitation by polarized light from this spectral range suggest that the complexes have no additional distortions caused by an interaction of a hole bound at the center in the light-emitting state with local phonons of low symmetry. This feature makes Cu_GaTe_As and Cu_GaSn_Ga complexes different from those with the Ga vacancy (V _Ga) instead of Cu_Ga. The dissimilarity arises from the difference in the intensity of interaction of a hole localized at the orbital of an isolated deep-level acceptor in the state corresponding to its preemission state in the complex (Cu _Ga ^? and V _Ga ^2? ) with low-symmetry vibrations of atoms. The perturbation of the hole orbital induced by the donor in the complex practically does not affect this interaction.     

Acceptor and donor doping of AlxGa1−xN thin film alloys grown on 6H-SiC(0001) substrates via metalorganic vapor phase epitaxy

Thin films of Si-doped Al_xGa_1−xN (0.03≤x≤0.58) having smooth surfaces and strong near-band edge cathodoluminescence were deposited at 0.35–0.5 μm/h on on-axis 6H-SiC(0001) substrates at 1100°C using a 0.1 μm AlN buffer layer for electrical isolation. Alloy films having the compositions of Al_0.08Ga_0.92N and Al_0.48Ga_0.52N exhibited mobilities of 110 and 14 cm²/V·s at carrier concentrations of 9.6×10¹⁸ and 5.0×10¹⁷ cm⁻³, respectively. This marked change was due primarily to charge scattering as a result of the increasing Al concentration in these random alloys. Comparably doped GaN films grown under similar conditions had mobilities between 170 and ∼350 cm²/V·s. Acceptor doping of Al_xGa_1−xN for x≤0.13 was achieved for films deposited at 1100°C. No correlation between the O concentration and p-type electrical behavior was observed.     

Structure,optical property,and photo-catalytic activity of solid-solution β-AgAl1−xGaxO2 under visible light

Visible-light responsive β-AgAl_1−xGa_xO₂ photocatalysts were prepared through a sol-gel method combined with a cation exchange reaction. The samples were characterized by X-ray diffraction, scanning electron microscopy, UV–vis absorption spectrum, and photoluminescence spectrum. The results demonstrated that the solid-solution β-AgAl_1-xGa_xO₂ with delafossite structure showed high crystallinity. The microparticles with smooth surface were polycrystal and had polyhedron crystal morphologies. The optical band gap of the β-AgAl_1−xGa_xO₂ was modulated in the range of 2.31–2.70 eV; meanwhile the photoluminescence spectra showed a red shift by varying the molar ratio of Ga/Al from 0.25 to 4 attributed to the decrease of band gap. The photo-catalytic degradation of methyl orange was studied under visible light irradiation (wavelength>420 nm). The degradation process followed the first-order kinetics. The β-AgAl_0.2Ga_0.8O₂ with the reaction rate of 0.0402 min⁻¹ showed the highest photo-catalytic activity. This might be resulted from the narrow band gap of 2.31 eV for the β-AgAl_0.2Ga_0.8O₂ which could utilize visible light efficiently to photo-generate electrons and holes. The β-AgAl_1−xGa_xO₂ had potential applications in degrading organic dyes by using visible light efficiently.     

Growth characteristics of LPE InSb and InGaSb

Growth characteristics of InSb and In_l-xGa_xSb (x < 0.09) prepared by infinite solution epitaxy are investigated. Substrate surface contamination is found to be a controlling factor in epitaxial layer continuity and surface morphology; as the degree of contamination decreases, growth varies from discontinuous to terraced continuous to terrace-free continuous. The dependence of layer thickness on growth parameters is consistent with a diffusion-controlled process in which the Sb diffusion coefficient D is given by D = 1.1 × 10⁻³ exp[(-0.43 eV)/kT] cm² sec⁻¹. Electron microprobe analysis of the Ga distribution through epitaxial layers indicates that Ga diffuses fast enough in the solution under typical growth conditions to maintain a constant ternary composition.     

Microstructural analysis of a Au/Pt/Pd/Zn ohmic contact to an AlGaAs/GaAs heterojunction bipolar transistor

Gold-based ohmic contacts, incorporating Pt, Pd, and Zn layers, to AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have been characterized using transmission electron microscopy (TEM). The metallization was deposited onto a 30 nm graded emitter layer of n-type Al_xGa_1−xAs, which was on a 30 nm emitter layer of n-type Al_0.3Ga_0.7As, with the aim of contacting the underlying 80 nm thick graded base layer of p-type Al_xGa_1−xAs. Metal layers were deposited sequentially using electron beam evaporation and the resultant metallizations were annealed at temperatures ranging from 250-500°C for up to several minutes. A minimum contact resistance of ≈8.5 × 10⁻⁷ Ω-cm² was achieved, which corresponded to the decomposition of ternary phases at the metallization/semiconductor interface, to binary phases, i.e., PdGa and PtAs₂. Long term stability tests were done on the optimum contacts. Anneals at 270°C for up to four weeks in duration produced virtually no change in microstructure, with the exception of some outward diffusion of Ga and As.     

Growth of GaBN ternary solutions by organometallic vapor phase epitaxy

Layers of Ga_1-xB_xN with compositions from x = 0 to x = 0.07 were grown by organometallic vapor phase epitaxy on sapphire substrates using trimethylgallium, triethylboron (TEB) and NH₃ as precursors. Growth was done in the temperature range from 450 to 1000°C. The presence of boron was detected by the shift in the (0002) peak position in x-ray diffraction, by x-ray photoelectron spectroscopy, secondary ion mass spectrometry measurements, and by the changes in the band gap as measured by optical transmission. It was found that for the studied range of compositions the band gap varied from 3.4 eV for x = 0 to 3.63 eVfor x = 0.05. At certain TEB concentrations in the gas phase, the growth rate decreased abruptly, most likely because of a growth poisoning by the onset of growth of a very slow growing B-rich phase. The threshold TEB concentration for this growth poisoning decreased with increasing growth temperature; and at 1000°C, less than 1% of B could be incorporated as a result. GaBN alloys with about 7% substitutional boron were also produced by implantation of 5 × 10¹⁶ cm⁻² B ions at 60 keV into GaN, as evidenced by the shift of the band edge emission in cathodoluminescence spectra from 3.4 eV for GaN to 3.85 eV for GaBN.     

Selective dry etching of (Sc2O3)x(Ga2O3)1−x gate dielectrics and surface passivation films on GaN

(Sc₂O₃)_x(Ga₂O₃)_1?x films grown by molecular beam epitaxy show promise for use as surface passivation layers and gate dielectrics on GaN-based high electron mobility transistors. Completely selective, low-damage, dry etching of (Sc₂O₃)_x(Ga₂O₃)_1?x films with respect to GaN can be achieved with low-power inductively coupled plasmas of CH₄/H₂/Ar with etch rates in the range 200–300 Å/min. The incident ion energies are of order 100 eV, and no roughening of the underlying GaN was observed under these conditions. Similar etch rates were obtained with Cl₂/Ar discharges under the same conditions, but GaN showed rates almost an order of magnitude higher.     

Ordering and disordering of doped Ga0.5In0.5P

The band gap of Ga_0.5In_0.5P is reported as a function of doping level and growth rate. The lowest band gaps are obtained for hole concentrations of about 2 × 10¹⁷ cm⁻³. For samples doped p-type above 1 × 10¹⁸ cm⁻³, the band gap increases dramatically, regardless of growth rate. This effect is shown to be the result of disordering during growth rather than a change in the equilibrium surface structure with doping. The doping level dependence of the band gap of Ga_0.5In_0.5P samples grown at higher and lower growth rates differs for selenium and zinc doping even though the effects of high doping are the same for both dopants.     

P- and N-type doping of GaN and AlGaN epitaxial layers grown by metalorganic chemical vapor deposition

Mg- and Si-doped GaN and AlGaN films were grown by metalorganic chemical vapor deposition and characterized by room-temperature photoluminescence and Hall-effect measurements. We show that the p-type carrier concentration resulting from Mg incorporation in GaN:Mg films exhibits a nonlinear dependence both on growth temperature and growth pressure. For GaN and AlGaN, n-type doping due to Si incorporation was found to be a linear function of the silane molar flow. Mg-doped GaN layers with 300K hole concentrations p ∼2×10¹⁸ cm⁻³ and Si-doped GaN films with electron concentrations n∼1×10¹⁹ cm⁻³ have been grown. N-type Al_0.10Ga_0.90N:Si films with resistivities as low as p ∼6.6×10⁻³ Ω-cm have been measured.     

Electric and photoelectric properties of n-GaxIn1−x N/p-Si anisotypic heterojunctions

We have developed a technology for producing n-type Ga_xIn_1−x N/p-Si heterostructures by combined pyrolysis of indium and gallium monoammoniate chlorides, making it possible to obtain heterolayers with composition varying over wide limits (from GaN up to InN). The composition and basic electric and optical characteristics of nitride films were determined. The electric and photoelectric properties of the heterostructures with Ga_xIn_1−x N films of different composition were investigated. It was shown that the anisotypic heterojunction n-Ga_xIn_1−x N/p-Si is a promising photosensitive element for detecting visible-range radiation. The maximum values of the specific detectivity were D*=1.2×10¹¹ Hz^1/2·W⁻¹ at 290 K. A band diagram of the heterojunction was constructed. Fiz. Tekh. Poluprovodn. 32, 461–465 (April 1998)