Near-infrared photoluminescence of V-doped GaN

The visible and infrared luminescence of vanadium-doped GaN (GaN:V) grown by metalorganic vapour phase epitaxy technique (MOVPE) on SiN-treated sapphire substrate were examined. Growth process was in-situ monitored by laser reflectometry. At room temperature and in the visible spectral range, photoluminescence (PL) shows a strong blue emission band. At 10 K, the near-infrared PL spectra exhibit several emissions dominated by a zero-phonon line (ZPL) at 0.821 eV with a full-width at half-maximum (FWHM) of 8.8 meV. Other peaks emerge in the low- and high-energy side of ZPL, which can be assigned to the fine structure of the charge state or the satellite lines. By increasing the temperature, the peaks’ intensities decrease and disappear above 150 K. The red-shift and the FWHM of the 0.821 eV line increase versus temperature, indicating a high contribution of the photonic Raman processes. This emission was assigned to be a vanadium intracenter emission.     

金属有机物化学气相沉积GaAs／Si外延层中1.13eV发光带特性研究

用不同温度和激发强度下的近红外光致发光研究了金属有机物化学气相沉积方法在Ｓｉ衬底上生长的ＧａＡｓ外延层中的１．１３ｅＶ发光带的发光特性，表明此发光带为施主－受主对复合发光．根据１．１３ｅＶ发光带的峰值能量和发光强度随温度和激发强度的变化关系，确定施主和受主的束缚能分别为５和２９５ｍｅＶ，并证实ＧａＡｓ／Ｓｉ外延层中的１．１３ｅＶ发光为硅施主－镓空位受主对的复合发光．     

未掺杂GaN外延膜的结晶特性与蓝带发光关系的研究

采用卢瑟福背散射／沟道技术，X射线双晶衍射技术和光致发光技术对几个以MOCVD技术生长的蓝带发光差异明显的未掺杂GaN外延膜和GaN：Mg外延膜进行了测试。结果表明，未掺杂GaN薄膜中出现的2．9eV左右的蓝带发光与薄膜的结晶品质密切相关。随未掺杂GaN的蓝带强度与带边强度之比增大，GaN的卢瑟福背散射／沟道谱最低产额增大，X射线双晶衍射峰半高宽增大。未掺杂GaN薄膜的蓝带发光与薄膜中的某种本征缺陷有关。研究还表明，未掺杂GaN中出现的蓝带与GaN：Mg外延膜中出现的2．9eV左右的发光峰的发光机理不同。     

Structural and Optical Properties of ZnO Nanotips Grown on GaN Using Metalorganic Chemical Vapor Deposition

ZnO nanotips are grown on epitaxial GaN/c-sapphire templates by metalorganic chemical vapor deposition. X-ray diffraction (XRD) studies indicate that the epitaxial relationship between ZnO nanotips and the GaN layer is (0002)ZnO||(0002)GaN and (101̄0)ZnO||(101̄0)GaN. Temperature-dependent photoluminescence (PL) spectra have been measured. Sharp free exciton and donor-bound exciton peaks are observed at 4.4 K with photon energies of 3.380 eV, 3.369 eV, and 3.364 eV, confirming high optical quality of ZnO nanotips. Free exciton emission dominates at temperatures above 50 K. The thermal dissociation of these bound excitons forms free excitons and neutral donors. The thermal activation energies of the bound excitons at 3.369 eV and 3.364 eV are 11 meV and 16 meV, respectively. Temperature-dependent free A exciton peak emission is fitted to the Varshni’s equation to study the variation of energy bandgap versus temperature.     

Analysis of defect related optical transitions in biased AlGaN/GaN heterostructures

The optical transitions in AlGaN/GaN heterostructures that are grown by metalorganic chemical vapor deposition (MOCVD) have been investigated in detail by using Hall and room temperature (RT) photoluminescence (PL) measurements. The Hall measurements show that there is two-dimensional electron gas (2DEG) conduction at the AlGaN/GaN heterointerface. PL measurements show that in addition to the characteristic near-band edge (BE) transition, there are blue (BL) and yellow luminescence (YL) bands, free-exciton transition (FE), and a neighboring emission band (NEB). To analyze these transitions in detail, the PL measurements were taken under bias where the applied electric field changed from 0 to 50 V/cm. Due to the applied electric field, band bending occurs and NEB separates into two different peaks as an ultraviolet luminescence (UVL) and Y₄ band. Among these bands, only the yellow band is unaffected with the applied electric field. The luminescence intensity change of these bands with an electric field is investigated in detail. As a result, the most probable candidate of the intensity decrease with an increasing electric field is the reduction in the radiative lifetime.     

The role of stacking faults and their associated 0.13 ev acceptor state in doped and undoped ZnO layers and nanostructures

An emission band at 3.31 eV is frequently observed in low-temperature photoluminescence (PL) measurements on ZnO p-doped with group-V elements, and also on nominally undoped ZnO layers and nanostructures. It has alternatively been ascribed to LO- or TO-phonon replicas of free excitons, to acceptor-bound excitons, to donor-acceptor pair transitions, to two-electron satellites of donor-bound excitons, or to free-to-bound transitions. This band frequently dominates the PL of ZnO nanostructures and layers at room temperature. Annealing leads to drastic changes in its intensity.We report on low-temperature cathodoluminescence measurements with very high spatial resolution and high-resolution transmission electron microscope investigations carried out on the same pieces of hetero-epitaxial ZnO samples with unusual layer orientation. These data allow us to correlate this emission unambiguously with c-plane stacking faults. The emission is found to be due to the recombination of a free electron with a hole bound to a relatively shallow acceptor state ≈130 meV above the valence band edge. Locally, these acceptor states occur in high concentrations of up to some 10¹⁸ cm⁻³, and thus lead to strong two-dimensional perturbations of the free carrier concentration. They have severe implications for the conductivity of layers and nanostructures in general, and on the interpretation of Hall and luminescence data in particular. Literature data are critically reviewed in the light of these findings.     

氢化对ZnO发光性质的影响

通过低温光致发光(PL)谱研究氢化对ZnO发光性质的影响.氢通过一个直流等离子体发生装置引入到ZnO晶体.研究发现氢的引入影响了束缚激子的相对发光强度,特别是氢化以后I4峰(3.363eV)的强度增加.与未氢化样品相比较,氢化样品显示出不同的温度依赖PL谱.在4.2K温度下,测量了氢化样品表面以下不同深度处的PL谱.研究发现I4峰的强度和I4峰与I8峰强度比随深度变化而变化,说明了在引入的氢和浅施主之间的直接联系.一般而言,氢化会增强带边发射并钝化绿光发射.     

Characterization of undoped and silicon-doped InGaN/GaN single quantum wells

We investigated the influence of doping and InGaN layer thickness on the emission wavelength and full width at half maximum (FWHM) of InGaN/GaN single quantum wells (SQW) of thicknesses between 1 nm and 5 nm by temperature and intensity resolved photoluminescence (PL). The crystalline quality of the GaN claddings was assessed by low temperature PL. The emission energy of 5 nm Si doped SQW could be tuned from 3.24 eV to 2.98 eV by reducing the deposition temperature. An increase of piezoelectric (PE) field screening with increasing deposition temperature is attributed to an increase of the SiH₄ decomposition efficiency. Piezoelectric (PE) fields between 0.5 MV/cm and 1.2 MV/cm in undoped structures of varying SQW thicknesses were calculated. Two activation energies of 15 meV and 46 meV of the SQW emission could be observed in temperature resolved measurements. The higher value was assigned to the confined exciton binding energy, whereas the activation energy of 15 meV is probably due to a decrease in carrier supply from the absorption zone in the GaN cladding into the SQW.     

Elementary blue-emission bands in the luminescence spectrum of undoped gallium nitride films

Inhomogeneous broadening of the blue-emission band in the luminescence spectrum of nominally undoped gallium nitride films grown on substrates of sapphire with orientation (0001) and silicon with orientation (001) using chemical vapor deposition is observed. Studies of the emission spectra under different conditions of excitation of GaN films made it possible to detect three elementary bands with peaks at 2.65, 2.84, and 3.01 eV in the blue region of luminescence of these films at room temperature. Assumptions are made about the types of intrinsic and impurity defects involved in the formation of various centers in GaN as sources of blue emission.     

Influence of AlGaN deep level defects on AlGaN/GaN 2-DEG carrierconfinement

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures and to correlate their effect on two-dimensional electron gas (2-DEG) confinement. We investigated AlGaN/GaN heterostructures with different electrical properties using incident electron beam energies of 0.5 to 15 keV to probe electronic state transitions within each of the heterostructure layers. AlGaN heterostructures of 25 nm thickness and nominal 30% Al concentration grown on GaN buffer layers on sapphire substrates by plasma-assisted molecular beam epitaxy exhibited a range of polarization-induced electron densities and room temperature mobilities. In general, the spectra exhibit AlGaN band edge emission at ~3.8 eV or ~4.0 eV, GaN band edge emission at ~3.4 eV, yellow luminescence (YL) features at 2.18 eV and 2.34 eV, and a large emission in the infrared (<1.6 eV) from the GaN cap layer used to passivate the AlGaN outer surface. These heterostructures also show high strain in the 2 nm-thick GaN layer with evidence for a Franz-Keldysh red shift due to piezoelectric charging. The LEEN depth profiles reveal differences between the structures with and without 2-DEG confinement and highlight the importance of AlGaN defects in the near 2-DEG region     

Characterization of defects in colloidal CdSe nanocrystals by the modified thermostimulated luminescence technique

The temperature dependencies of the luminescence spectra of 5-nm-diameter CdSe semiconductor nanocrystals synthesized by colloidal-chemistry methods are investigated. The two bands observed in these spectra around 2.01 and 1.37 eV correspond to band-to-band transitions and luminescence of defect states, respectively. A model explaining the temperature behavior of the luminescence band intensities both upon cooling and heating is put forward. A new modification of spectrally resolved thermostimulated luminescence technique making it possible to determine the activation energies and the character of traps responsible for the temperature dependence of the luminescence intensities is suggested. This technique is used to obtain the activation energies of the emission and capture of electrons at traps (190 and 205 meV, respectively) and to determine the depth of the electron level (57 meV) responsible for luminescence in the 1.37-eV region.     

HVPE生长GaN厚膜光致发光特性研究

利用吸收光谱和光致发光(PL)光谱研究了氢化物气相外延(HVPE)法生长的GaN厚膜材料发光特性。研究发现当激发脉冲光源的重复频率较低时,PL光谱中仅能观察到带边发光峰,当重复频率增加时,PL光谱中不仅出现带边发光峰,还可观察到蓝带发光峰和黄带发光峰;随着光源重复频率的增加,带边发光峰与黄带发光峰、蓝带发光峰的光强之比也随着增大。分析认为蓝带发光起源于材料中碳杂质缺陷而黄带发光可能与位错等结构缺陷有关。     

Long-wavelength emission from single InAs quantum dots layer grown on porous GaAs substrate

In this paper, we present the growth and photoluminescence (PL) results of InAs quantum dots (QDs) on a p-type porous GaAs (001) substrate. It has been shown that critical layer thickness of InAs overgrowth on porous GaAs has been enhanced compared to that deposited on nominal GaAs. Using porous GaAs substrate, growth interruption and depositing 10 atomic monolayer (ML) In_0.4Ga_0.6As on InAs QDs, photoluminescence measured at 10 K exhibits an emission at 0.739 eV (∼1.67 μm) with an ultranarrow full width at half maximum (FWHM) of 16 meV. This emission represents the longer wavelength obtained up to date to our knowledge and has been attributed to the radiative transition in the InAs QDs.     

非掺杂n型氮化镓外延层的光致发光

研究了热处理对非掺杂 n型氮化镓外延层光致发光谱的影响和光谱中各发光带强度与温度之间的关系 .热处理后 ,光谱中的带边峰和黄光峰的强度较热处理前都有明显降低 .黄光峰强度随温度升高的衰减速度要比带边峰慢得多 .由这些实验结果得出结论 :光谱中的带边峰是由自由激子和束缚在一浅施主能级的束缚激子的谱线重合而成 ,这个浅施主能级很有可能是由氮空位产生 ;黄色荧光的机制应为自由电子或施主能级向深受主能级的跃迁 ,并且黄色荧光肯定和氮化镓中的一内部缺陷产生的深受主能级有关 ,该内部缺陷很有可能是镓空位 .     

Increase in photoluminescence of Zn-doped p-type InP after hydrogenation

The authors report on the effect of hydrogenation on the low-temperature (5.5 K) photoluminescence properties of Zn-doped p-type (p approximately 3*10/sup 18/ cm/sup -3/) InP substrates. The photoluminescence spectrum of the as-grown sample shows a Zn/sub In/ acceptor-related transition near the band-edge at 1.386 eV, a Zn-related PL band at 1.214 eV and a phosphorus vacancy V/sub P/-related PL band at 1.01 eV. After hydrogenation of the samples by exposure to hydrogen plasma, which completely passivates the Zn/sub In/ acceptors over a depth of more than 1 mu m, the deep luminescence bands (1.214 and 1.01 eV) disappeared, with a concomitant approximately 2000-fold increase in the intensity of the near-band-edge emission. Such a large increase in radiative efficiency together with the elimination of the deep luminescence bands indicates hydrogen passivation of deep nonradiative centers in addition to passivation of shallow acceptors.<>     

New photoluminescence lines in Vanadium doped GaAs grown by MOVPE

We examined the electrical and optical properties of vanadium-doped GaAs grown by metalorganic vapour phase epitaxy using vanadium tetrachloride (VCl₄) as a novel dopant source. Samples with various vanadium incorporations were investigated. All samples were n type. The electron concentration dependence on the VCl₄ flow rate was established. At 15 K, by comparison with undoped layers grown in the same conditions, photoluminescence spectra of V-doped exhibited three new emission bands: at 1.41, 1 and 0.72 eV. The 1 and 0.72 eV band emissions were attributed to V²⁺ and V³⁺ intracenter emission, while the 1.41 eV band was suggested to be a donor-bound transition. The identity of the donor is tentatively attributed to a donor complex that associates vanadium to an arsenic vacancy. From Hall effect as function of temperature, the donor ionisation energy was estimated to be about 102±5 meV.     

Luminescence spectra of blue and green light-emitting diodes based on multilayer InGaN/AlGaN/GaN heterostructures with quantum wells

The luminescence spectra of blue and green light-emitting diodes based on In_xGa_1−x N/Al_yGa_1−y N/GaN heterostructures with a thin (2–3 nm) In_xGa_1−x N active layer have been investigated in the temperature and current intervals 100–300 K and J=0.01–20 mA, respectively. The spectra of the blue and green light-emitting diodes have maxima in the interavals ℏω_max=2.55–2.75 eV and ℏω_max=2.38–2.50 eV, respectively, depending on the In content in the active layer. The spectral intensity of the principal band decreases exponentially in the long-wavelength region with energy constant E ₀=45–70 meV; this is described by a model that takes into account the tails of the density of states in the two-dimensional active region and the degree of filling of the tails near the band edges. At low currents radiative tunneling recombination with a voltage-dependent maximum in the spectrum is observed in the spectra of the blue diodes. A model of the energy diagram of the heterostructures is discussed. Fiz. Tekh. Poluprovodn. 31, 1055–1061 (September 1997)     

Basic studies of gallium nitride growth on sapphire by metalorganic chemical vapor deposition and optical properties of deposited layers

We have studied the growth of gallium nitride on c-plane sapphire substrates. The layers were grown in a horizontal metalorganic chemical vapor deposition reactor at atmospheric pressure using trimethylgallium (TMG) and ammonia (NH₃). Variation of the V/III ratio (150–2500) shows a distinct effect on the growth rate. With decreasing V/III ratio, we find an increasing growth rate. Variation of the growth temperature (700–1000°C) shows a weak increase in growth rate with temperature. Furthermore, we performed secondary ion mass spectroscopy measurements and find an increasing carbon incorporation in the GaN films with decreasing ammonia partial pressure and a growing accumulation of carbon at the substrate interface. Photoluminescence measurements show that samples with high carbon content show a strong yellow luminescence peaking at 2.2 eV and a near band gap emission at 3.31 eV. With increasing carbon content, the intensity of the 3.31 eV line increases suggesting that a carbon related center is involved.     

氮化镓注镁(Mg:GaN)的光致发光

利用低压MOCVD在蓝宝石衬底上外延生长了GaN,用离子注入法掺入Mg杂质,退火后,进行光致发光测量,观察到显著的蓝光发射和黄带发射.光谱分析给出了与注入Mg离子相关的GaN禁带中能级的精细结构,其中: 间位Mg(Mgi)能级(导带下170meV)到替位Mg(MgGa)受主能级(价带上250meV)的跃迁产生了415nm发光峰; 该能级到价带上390meV能级的跃迁,以及带有紧邻N空位的替位Mg(MgGaVN)能级(导带下310meV)到 MgGa受主能级的跃迁,均产生了438nm发光峰.另外,退火使GaN晶格结构部分恢复,再现了黄带发射.     

Visible photoluminescence from Ge nanoclusters implanted in nanoporous aluminum oxide films

Nanoporous aluminum oxide (Al₂O₃) films with uniform porous size of 45 nm prepared by the electrochemical process in inorganic acid medium were implanted at room temperature (RT) with 120 keV Ge⁺ ions with a fluence of 1.2×10¹⁶ cm⁻². The nucleation and growths of Ge nanoparticles, were obtained by thermal annealing of the implanted samples at the temperature range of 200-600 °C. The size and distribution of the nanoparticles were characterized by photoluminescence (PL) measurements. The photoluminescence measurements as a function of the annealing temperature shows that at low annealing temperature (200 °C), the sample presents a low intensity and broad emission band centered at 5456 Å consistent with emission band characteristics of nanocluster of Ge with diameter in the range of 4-8 nm, as the annealing temperature increases to 400 °C the PL intensity increases by a factor of almost 20 and the emission band suffers a small red shift. The intensity increases can be related to the increase of the number of Ge nanocluster. At the annealing temperature of 600 °C, the emission band is considerably red shifted by almost 172 Å and the emission intensity decreases significantly, strongly suggesting that nanocrystalline Ge having a character of direct optical transitions exhibits the visible photoluminescence.