MBE growth and properties of ZnO on sapphire and SiC substrates

Molecular beam epitaxy (MBE) of ZnO on both sapphire and SiC substrates has been demonstrated. ZnO was used as a buffer layer for the epitaxial growth of GaN. ZnO is a würtzite crystal with a close lattice match (<2% mismatch) to GaN, an energy gap of 3.3 eV at room temperature, a low predicted conduction band offset to both GaN and SiC, and high electron conductivity. ZnO is relatively soft compared to the nitride semiconductors and is expected to act as a compliant buffer layer. Inductively coupled radio frequency plasma sources were used to generate active beams of nitrogen and oxygen for MBE growth. Characterization of the oxygen plasma by optical emission spectroscopy clearly indicated significant dissociation of O₂ into atomic oxygen. Reflected high energy electron diffraction (RHEED) of the ZnO growth surface showed a two-dimensional growth. ZnO layers had n-type carrier concentration of 9 × 10¹⁸ cm⁻³ with an electron mobility of 260 cm²/V-s. Initial I-V measurements displayed ohmic behavior across the SiC/ZnO and the ZnO/GaN heterointerfaces. RHEED of GaN growth by MBE on the ZnO buffer layers also exhibited a two-dimensional growth. We have demonstrated the viability of using ZnO as a buffer layer for the MBE growth of GaN.     

镍纳米岛模板制备氮化镓纳米柱研究

Uniform GaN nanopillar arrays have been successfully fabricated by inductively coupled plasma etching using self-organized nickel nano-islands as the masks on GaN/sapphire. GaN nanopillars with diameters of 350 nm and densities of 2.6 × 10^8 cm^-2 were demonstrated and controlled by the thickness of Ni film and the NH3 annealing time. These GaN nanopillars show improved optical properties and strain change compared to that of GaN film before ICP etching. Such structures with large-area uniformity and high density could provide additional advantages for light emission of light-emitting diodes, quality improvement of ELO regrowth, etc.     

Influence of an increase in the implantation dose of erbium ions and annealing temperature on photoluminescence in AlGaN/GaN superlattices and GaN epitaxial layers

Room-temperature photoluminescence (PL) has been studied in AlGaN/GaN superlattices and GaN epitaxial layers implanted with 1-MeV erbium at a dose of 3 × 10¹⁵ cm^?2 and annealed in argon. The intensity of PL from Er³⁺ ions in the superlattices exceeds that for the epitaxial layers at annealing temperatures of 700–1000°C. The strongest difference (by a factor of ～2.8) in PL intensity between the epitaxial layers and the superlattices and the highest PL intensity for the superlattices are observed upon annealing at 900°C. On raising the annealing temperature to 1050°C, the intensity of the erbium emission from the superlattices decreases substantially. This circumstance may be due to their thermal destruction.     

Si衬底上侧向外延生长GaN的研究

采用条形Al掩模在Si(111)衬底上进行了GaN薄膜侧向外延的研究.结果显示,当掩模条垂直于Si衬底[11-2]方向,也即GaN[10-10]方向时,GaN无法通过侧向生长合并得到表面平整的薄膜;当掩模条平行于Si衬底[11-2]方向,也即GaN[10-10]方向时,GaN侧向外延速度较快,有利于合并得到平整的薄膜.同时,研究表明,升高温度和降低生长气压都有利于侧向生长.通过优化生长工艺,在条形Al掩模Si(111)衬底上得到了连续完整的GaN薄膜.原子力显微镜测试显示,窗口区域生长的GaN薄膜位错密度约为1×109/cm2,而侧向生长的GaN薄膜位错密度降低到了5×107/cm2以下.     

Transport and Mobility Properties of Bulk Indium Nitride (InN) and a Two-Dimensional Electron Gas in an InGaN/GaN Quantum Well

We studied the transport and low-field mobility properties of bulk InN and a two-dimensional electron gas confined in an InGaN/GaN quantum well with regard to various parameters such as well width and interface roughness as a function of temperature. Since new material parameters for InN have been suggested by recent studies, the traditionally accepted and recently published parameter values for InN are used in our simulations and the results are compared. Mobility values in two and three dimensions are found from the steady-state drift velocities of carriers calculated using an ensemble Monte Carlo technique. Electron transport properties of bulk GaN and AlN are also presented and compared with bulk InN and InGaN/GaN quantum wells. The mobility of carriers in two dimensions is about 10,000 cm²/V s for low temperatures and in bulk InN increases significantly to a value of about 6,450 cm²/V s at room temperature when recently established material parameters are used.     

Thermal-field forward current in GaN-based surface-barrier structures

The voltage and temperature dependences of the capacitance and forward current in surface-barrier Ni-n-GaN structures are experimentally studied. The results are compared with the Padovani-Stratton thermofield emission theory. It is established that, in a temperature range of 250–410 K, the forward current of the Ni-n-GaN surface-barrier structures (the electron density in GaN is ～10¹⁷ cm^?3) is caused by a thermofield emission of electrons, whose energy is ～0.1 eV below the potential-barrier top.     

The impact of nitridation and nucleation layer process conditions on morphology and electron transport in GaN epitaxial films

A systematic study has been performed to determine the characteristics of an optimized nucleation layer for GaN growth on sapphire. The films were grown during GaN process development in a vertical close-spaced showerhead metalorganic chemical vapor deposition reactor. The relationship between growth process parameters and the resultant properties of low temperature GaN nucleation layers and high temperature epitaxial GaN films is detailed. In particular, we discuss the combined influence of nitridation conditions, V/III ratio, temperature and pressure on optimized nucleation layer formation required to achieve reproducible high mobility GaN epitaxy in this reactor geometry. Atomic force microscopy and transmission electron microscopy have been used to study improvements in grain size and orientation of initial epitaxial film growth as a function of varied nitridation and nucleation layer process parameters. Improvements in film morphology and structure are directly related to Hall transport measurements of silicon-doped GaN films. Reproducible growth of silicon-doped GaN films having mobilities of 550 cm²/Vs with electron concentrations of 3 × 10¹⁷ cm⁻³, and defect densities less than 10⁸ cm⁻² is reported. These represent the best reported results to date for GaN growth using a standard two-step process in this reactor geometry.     

Luminescence of stepped quantum wells in GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures

Luminescence spectra of doped and undoped GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures containing several tens of stepped quantum wells (QW) are investigated. The emission bands related to free and bound excitons and impurity states are observed in QW spectra. The luminescence excitation spectra indicate that the relaxation of free excitons to the e1hh1 state proceeds via the exciton mechanism, whereas an independent relaxation of electrons and holes is specific to bound excitons and impurity states. The energy levels for electrons and holes in stepped QWs, calculated in terms of Kane’s model, are compared with the data obtained from the luminescence excitation spectra. The analysis of the relative intensities of emission bands related to e1hh1 excitons and exciton states of higher energy shows that, as the optical excitation intensity increases, the e1hh1 transition is more readily saturated at higher temperature, because the lifetime of excitons increases. Under stronger excitation, the emission band of electron-hole plasma arises and increases in intensity superlinearly. At an excitation level of ～10⁵ W/cm², excitons are screened and the plasma emission band dominates in the QW emission. Nonequilibrium luminescence spectra obtained in a picosecond excitation and recording mode show that the e1hh1 and e2hh2 radiative transitions are 100% polarized in the plane of QWs.     

MOCVD-grown AlGaN/GaN heterostructures with high electron mobility

Specific features of MOCVD growth of AlGaN/GaN heterostructures have been studied. In the structures obtained, the 2D electron gas in the channel had a density of 1.2×10¹³ cm^?2 and a mobility of 1290 cm²/(V s) at room temperature. The effect of the purity of starting components on the properties of the structure is studied.     

Optical Properties of Nanocrystalline GaN Films Prepared by Annealing Amorphous GaN in Ammonia

Nanocrystalline GaN films were prepared by thermal treatment of amorphous GaN films under flowing NH₃ at a temperature of 600°C to 950°C for 1 h to 2 h. X-ray diffraction and field-emission scanning electron microscopy confirmed the formation of high-crystal-quality hexagonal GaN films with preferential (002) orientation. The photoluminescence spectrum showed a sharp peak near the band gap emission located at 368 nm and a broad blue peak centered at 430 nm. Five first-order Raman modes near ∼143 cm⁻¹, 535 cm⁻¹, 555 cm⁻¹, 568 cm⁻¹, and 731 cm⁻¹ with two new additional Raman peaks at 257 cm⁻¹ and 423 cm⁻¹ were observed. The origin of these new Raman peaks is discussed briefly.     

Reflection spectra of doped bismuth-antimony crystals in the far-infrared region of the spectrum

The plasma reflection spectra of the doped bismuth and bismuth-antimony alloyed crystals were studied experimentally at liquid-nitrogen temperature within a range of 30–600 cm^?1. Specificities in the optical functions of the samples are revealed within the infrared range that lies on the low-frequency side of the plasma edge and coincides with the optical-phonon frequencies in bismuth. When the plasma frequency is approached, the character of the interaction of radiation with anisotropic electron-hole plasma changes considerably.     

Electron mobility in two-dimensional electron gas in AIGaN/GaN heterostructures and in bulk GaN

We report on temperature dependencies of the electron mobility in the two-dimensional electron gas (2DEG) in AIGaN/GaN heterostructures and in doped bulk GaN. Calculations and experimental data show that the polar optical scattering and ionized impurity scattering are the two dominant scattering mechanisms in bulk GaN for temperatures between 77 and 500K. In the 2DEG in AIGaN/GaN heterostructures, the piezoelectric scattering also plays an important role. Even for doped GaN, with a significant concentration of ionized impurities, a large volume electron concentration in the 2DEG significantly enhances the electron mobility, and the mobility values close to 1700 cm²/Vs may be obtained in the GaN 2DEG at room temperature. The maximum measured Hall mobility at 80K is nearly 5000 cm²/Vs compared to approximately 1200 cm²/Vs in a bulk GaN layer. With a change in temperature from 300 to 80K, the 2DEG in our samples changes from nondegenerate and weakly degenerate to degenerate. Therefore, in order to interpret the experimental data, we propose a new interpolation formula for low field mobility limited by the ionized impurity scattering. This formula is valid for an arbitrary degree of the electron gas degeneracy. Based on our theory, we show that the mobility enhancement in the 2DEG is related to a much higher volume electron concentration in the 2DEG, and, hence, to a more effective screening.     

Electrical transport properties of single GaN and InN nanowires

The transport properties of single GaN and InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of temperature, annealing condition (for GaN) and length/square of radius ratio (for InN). The as-grown GaN nanowires were insulating and exhibited n-type conductivity (n ≈ 2×10¹⁷ cm⁻³, mobility of 30 cm²/V s) after annealing at 700°C. A simple fabrication process for GaN nanowire field-effect transistors on Si substrates was employed to measure the temperature dependence of resistance. The transport was dominated by tunneling in these annealed nanowires. InN nanowires showed resistivity on the order of 4×10⁻⁴ Ω cm and the specific contact resistivity for unalloyed Pd/Ti/Pt/Au ohmic contacts was near 1.09×10⁻⁷ Ω cm². For In N nanowires with diameters <100 nm, the total resistance did not increase linearly with length/square of radius ratio but decreased exponentially, presumably due to more pronounced surface effect. The temperature dependence of resistance showed a positive temperature coefficient and a functional form characteristic of metallic conduction in the InN nanowires.     

Metastable states in InGaN/GaN MQW structures doped with Sm, Eu, and Eu + Sm

Measurements of the microphotoluminescence (microPL) spectra of InGaN/GaN:Sm and InGaN/GaN:Eu quantum well (QW) structures show that the action of a magnetic field gives rise to Van Vleck paramagnetism for Eu³⁺ and Sm³⁺. The macrophotoluminescence (macroPL) spectra recorded after measuring the microPL spectra of InGaN/GaN QW structures doped with Sm or Eu + Sm at a high excitation level (>10²³ photons cm^?2 s^?1) in magnetic fields contain no QW emission lines which are present in the macroPL spectra recorded before these microPL measurements. This is indicative of the presence of photoinduced defects. Annealing of the InGaN/GaN:Sm and InGaN/GaN:(Eu + Sm) structures reduces the concentration of photoinduced defects.     

Photoluminescence of electron-hole plasma in semi-insulating undoped GaAs

The dependence of the photoluminescence spectrum of electron-hole plasma in semi-insulating undoped GaAs on the concentration of the background carbon impurity N_C(3×10¹⁵ cm^?3≤N_C≤4×10¹⁶ cm^?3) is studied at 77 K. It is established that the density of the electron-hole plasma, which is equal to n _e?h ≈1.1×10¹⁶ cm^?3 in crystals with the lowest impurity concentration at an excitation intensity of 6×10²² photons/(cm² s), decreases considerably as the value of N_C increases in the range mentioned above. A decrease in the density of the electron-hole plasma with increasing N_C is attributed to the effect of fluctuations in the carbon concentration N_C, which give rise to a nonuniform distribution of interacting charge carriers and to localization of holes in the tails of the density of states of the valence band.     

纤锌矿GaN量子点中相关电子空穴的光跃迁：应变和流体静压力效应

在有效质量和有限高势垒近似下,变分研究了应变和流体静压力对纤锌矿GaN/AlxGa1-xN柱形量子点中激子发光波长和电子空穴复合率的影响。计算结果表明,在量子点高度较大情况下( >3.8 nm ),考虑应变后的发光波长比不考虑应变的发光波长大。而在量子点高度较小情况下( <3.8 nm )则相反。由于应变效应,为了获得有效的电子、空穴复合过程,GaN量子点的高度必须小于5.5nm。发光波长随流体静压力的增大而线性减小,电子空穴复合率随流体静压力的增大而近线性增大。在量子点尺寸较大的情况下,流体静压力对发光波长的影响比较显著,而在量子点尺寸较小的情况下,流体静压力对电子空穴复合率的影响比较显著。此外,我们将零流体静压力下光跃迁能的理论计算值和实验值进行了比较,理论值和实验值相符合。     

Unintentional incorporation of B, As, and O impurities in GaN grown by molecular beam epitaxy

We have investigated systematically the effects of growth parameters upon the unintentional incorporation of B, As, and O impurities in GaN grown by molecular beam epitaxy with an RF-plasma activated nitrogen source. The prepared samples were analyzed using secondary ion mass spectrometry to determine the absolute concentration of the impurities. The boron background concentration in the unintentionally doped GaN was found to strongly correlate with the nitrogen plasma power used during the growth, indicating a decomposition of the pBN crucible in the plasma source. Due to previous GaAs growth in the same chamber, a considerably large amount of As contamination (≈3×10¹⁸ at/cm³) was also observed in the grown layer. The presence of Al in GaN is found to facilitate the incorporation of oxygen impurities in the layer. We determined an empirical formula, C_o ^t/C_o ^b 3.8×(C_Al/C_Al)^0.27, representing the correlation between O concentration and Al mole fraction (%) in the small range of Al content, 0.03≈1%, in the layer. The residual oxygen level was substantially reduced from 3.4×10¹⁹ to mid-10¹⁸ at/cm³ in the GaN layer when the buffer layer structure was changed from low temperature grown GaN single buffer to GaN/AlN double buffer layer. We ascribe this significantly lowered oxygen impurity level to improved crystalline quality of the layer due to the double buffer layer structure.     

Long-wavelength edge of the spectrum of hot electron-hole plasma radiation in photoexcited indium arsenide

The long-wavelength edge of the radiation spectrum of hot electron-hole plasma induced in indium arsenide by neodymium laser pulses is studied. The shape of the long-wavelength edge is nearly exponential and is independent of the pump power in the range 1–2 MW/cm². The constancy of the exponential factor indicates that the filling of LO-phonon states only slightly depends on the excitation power; i.e., the ensemble of LO phonons still does not heat up. The phonon temperature is determined and is shown to coincide with that of the crystal lattice. The absence of phonon heating is explained by strong e-h interaction and the screening of the electron energy scattering by LO phonons.     

Observation of localized centers with anomalous behavior in light-emitting heterostructures with multiple InGaN/GaN quantum wells

For the first time, using a complex of admittance spectroscopy, light-emitting heterostructures with InGaN/GaN multiple quantum wells were studied in a wide temperature range of 6–300 K. Three peaks are found in the conductance spectra; these peaks correspond to emission of charge carriers from the quantum wells and point defects distributed in the semiconductor bulk. Two low-temperature peaks possess an anomalous behavior, specifically, the peak with a low value of apparent activation energy (17 meV) is shifted to higher temperatures compared with the higher-energy peak (30 meV). The latter is attributed to a bulk defect having anomalously large capture cross section σ _n = 1.5 × 10⁻¹¹ cm².     

Electron scattering in AlGaN/GaN heterostructures with a two-dimensional electron gas

The temperature and concentration dependences of electron mobility in AlGaN/GaN hetero-structures are studied. The mobility for the samples under study at T = 300 K lies in the range of 450–1740 cm²/(V s). It is established that scattering at charged centers is dominant for samples with low mobility (lower than 1000 cm²/(V s) right up to room temperature. These centers are associated with a disordered piezoelectric charge at the heterointerface because of its roughness or with a piezoelectric charge similarly to the Al-GaN barrier because of alloy disorder, as well as with the deformation field around dislocations. Scattering at optical phonons is dominant for samples with mobility exceeding 1000 cm²/(V s) at T = 300 K. Scattering at alloy disorders, heterointerface roughness, and dislocations are dominant at temperatures lower than 200 K. A decrease in the influence of scattering at roughness with improvement of the heterointerface morphology increases room-temperature mobility from 1400 cm²/(V s) to 1700 cm²/(V s).