Electrical and optical characterization studies of lower dose Si-implanted AlxGa1−xN

Electrical and optical activation studies of lower dose Si-implanted Al_xGa_1?xN (x=0.14 and 0.24) have been made systematically as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1×10¹³ cm^?2 to 1×10¹⁴ cm^?2 at room temperature. The samples were proximity cap annealed from 1,100°C to 1,350°C with a 500-Å-thick AlN cap in a nitrogen environment. Nearly 100% electrical activation efficiency was obtained for Al_0.24Ga_0.76N implanted with a dose of 1 × 10¹⁴ cm^?2 after annealing at an optimum temperature around 1,300°C, whereas for lower dose (≤5×10¹³ cm^?2) implanted Al_0.24Ga_0.76N samples, the electrical activation efficiencies continue to increase with anneal temperature up through 1,350°C. Seventy-six percent electrical activation efficiency was obtained for Al_0.14Ga_0.86N implanted with a dose of 1 × 10¹⁴ cm^?2 at an optimum anneal temperature of around 1,250°C. The highest mobilities obtained were 89 cm²/Vs and 76 cm²/Vs for the Al_0.14Ga_0.86N and Al_0.24Ga_0.76N, respectively. Consistent with the electrical results, the photoluminescence (PL) intensity of the donor-bound exciton peak increases as the anneal temperature increases from 1,100°C to 1,250°C, indicating an increased implantation damage recovery with anneal temperature.     

Annealing studies of photoluminescence spectra from multiple Er3+ centers in er-implanted GaN

A study of selectively excited photoluminescence (PL) at ∼ 6K in Er-im planted GaN as a function of annealing temperature (400–1000°C) has detected nine different Er³⁺ centers with distinct ∼ 1540 nm ⁴I_13/2 → ⁴I_15/2 Er³⁺ PL spectra and different activation temperatures. However, most of the optically active implanted Er atoms are incorporated at annealing temperatures as low as 400°C on a single type of center for which PL can only be excited efficiently by direct intra-4f shell absorption and is not strongly pumped by either above-gap or broad-band below-gap absorption. This strongly suggests that this high-concentration Er³⁺ center is an isolated, isoelectronic center consistent with Er³⁺ substituted on a Ga site. In contrast, a very small fraction of the Er atoms that form a variety of Er-defect/impurity complexes dominate the Er³⁺ emission spectra excited by above-gap and broad-band below-gap absorption because of their larger cross sections for both carrier capture and optical absorption.     

Electrical and optical properties of oxygen doped GaN grown by MOCVD using N2O

Oxygen doped GaN has been grown by metalorganic chemical vapor deposition using N₂O as oxygen dopant source. The layers were deposited on 2″ sapphire substrates from trimethylgallium and especially dried ammonia using nitrogen (N₂) as carrier gas. Prior to the growth of the films, an AIN nucleation layer with a thickness of about 300? was grown using trimethylaluminum. The films were deposited at 1085°C at a growth rate of 1.0 μm/h and showed a specular, mirrorlike surface. Not intentionally doped layers have high resistivity (>20 kW/square). The gas phase concentration of the N₂O was varied between 25 and 400 ppm with respect to the total gas volume. The doped layers were n-type with carrier concentrations in the range of 4×10¹⁶ cm⁻³ to 4×10¹⁸ cm⁻³ as measured by Hall effect. The observed carrier concentration increased with increasing N₂O concentration. Low temperature photoluminescence experiments performed on the doped layers revealed besides free A and B exciton emission an exciton bound to a shallow donor. With increasing N₂O concentration in the gas phase, the intensity of the donor bound exciton increased relative to that of the free excitons. These observations indicate that oxygen behaves as a shallow donor in GaN. This interpretation is supported by covalent radius and electronegativity arguments.     

Nearly Perfect Electrical Activation Efficiencies from Silicon-Implanted Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N with High Aluminum Mole Fraction

Electrical activation studies of Al _x Ga_1−x N (x = 0.45 and 0.51) implanted with Si for n-type conductivity have been made as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1 × 10¹⁴ cm⁻² to 1 × 10¹⁵ cm⁻² at room temperature. The samples were subsequently annealed from 1150°C to 1350°C for 20 min in a nitrogen environment. Nearly 100% electrical activation efficiency was successfully obtained for the Si-implanted Al_0.45Ga_0.55N samples after annealing at 1350°C for doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² and at 1200°C for a dose of 1 × 10¹⁵ cm⁻², and for the Al_0.51Ga_0.49N implanted with silicon doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² after annealing at 1300°C. The highest room-temperature mobility obtained was 61 cm²/V s and 55 cm²/V s for the low-dose implanted Al_0.45Ga_0.55N and Al_0.51Ga_0.49N, respectively, after annealing at 1350°C for 20 min. These results show unprecedented activation efficiencies for Al _x Ga_1−x N with high Al mole fractions and provide suitable annealing conditions for Al _x Ga_1−x N-based device applications.     

Optical properties of thick-film GaN grown by hydride vapor phase epitaxy on MgAl2O4 substrate

A hydride vapor phase epitaxy was employed to grow the 10∼240 μm thick GaN films on a (111) MgAl₂O₄ substrate. The GaN films on a MgAl₂O₄ substrate revealed characteristics of photoluminescence (PL) in impurity doped GaN, which may be due to the out-diffusion and auto-doping of Mg from the MgAl₂O₄ substrate during GaN growth. The PL peak energy of neutral donor bound exciton emission and the frequency of Raman E₂ mode were decreased by increasing the GaN thickness, due to the residual strain relaxation in the epilayers. The dependence of Raman E₂ mode of GaN films on residual strain can be estimated as Δ ω/Δ σ=3.93 (cm⁻¹/GPa).     

生长压力对GaN材料光学与电学性能的影响

研究了采用MOCVD技术分别在100与500Torr反应室压力下生长的非故意掺杂GaN薄膜的光学与电学性能。研究表明,低压100Torr外延生长条件可以有效地降低Ga与NH3气相反应造成GaN薄膜的碳杂质沾污,从而抑制造成光致发光中黄光峰与蓝光峰的深受主的形成,所制备的材料表现出较好的光学性能。同时,不同生长压力下的GaN薄膜表现出相异的电学性能,即在500Torr下生长的样品通常表现出更高的载流子浓度（（4.6-6.4）×1016 cm-3）与更高的迁移率（446-561cm2/（V.s））,而100Torr下生长的样品通常表现为更低的载流子浓度（1.56-3.99）×1016 cm-3与更低迁移率（22.9-202cm2/（V.s））。     

生长停顿改善MOCVD生长InGaN/GaN多量子阱的特性

利用金属有机物化学气相淀积(MOCVD)生长了InGaN/GaN多量子阱(MQWs)结构,研究了生长停顿对InGaN/GaN MQWs特性的影响.结果表明,采用生长停顿,可以改善MQWs界面质量,提高MQWs的光致发光(PL)与电致发光(EL)强度;但生长停顿的时间过长,阱的厚度会变薄,界面质量变差,不仅In组分变低,富In的发光中心减少,而且会引入杂质,致使EL强度下降.     

Metalorganic chemical vapor deposition growth of high optical quality and high mobility GaN

Hall mobilities as high as 702 and 1230 cm²/Vs at 300 and 160K along with low dislocation densities of 4.0 × 10⁸ cm^-2 have been achieved in GaN films grown on sapphire by metalorganic chemical vapor deposition. High growth temperatures have been established to be crucial for optimal GaN film quality. Photoluminescence measurements revealed a low intensity of the deep defect band around 550 nm in films grown under optimized conditions.     

Effects of H2/NH3 flow-rate ratio on the luminescent, structural, and electrical properties of GaN epitaxial layers grown by MOCVD

GaN epitaxial layers were grown on sapphire substrates in a separate-flow reactor by metalorganic chemical vapor deposition. The flow-rate ratio of H₂ on the upper stream to NH₃ on the bottom stream is varied from 0.5 to 2. The growth condition and characterization of the GaN epitaxial layers are investigated in detail. The H₂ flow rate of the upper stream strongly affects the reactant gas flow pattern near the substrate surface and thus influences the quality of epitaxial layers. At the optimum H₂/NH₃ flow ratio of 1.0, we can obtain a good quality of GaN epitaxial layers which exhibit a strong near band-edge emis-sion in the 20 K photoluminescence (PL), a full width at half maximum of 66 meV for the 300 K PL, an electron mobility of 266 cm²/V-s and concentration of 1 × 10¹⁸ cm⁻³ at 300 K.     

Photoluminescence characteristics of GaN/lnGaN/GaN quantum wells

Photoluminescence (PL) characteristics of GaN/lnGaN/GaN single quantum wells (QWs) and an InGaN/GaN single heterojunction were studied using continuous wave (CW) and pulsed photoluminescence in both edge and surface emitting configurations. Samples were grown on c-plane sapphire substrates by atmospheric pressure metalorganic chemical vapor deposition (MOCVD). Room temperature and 77K PL measurements were performed using a CW Ar-ion laser (305 nm) and a frequency tripled (280 nm), pulsed, mode-locked Ti: sapphire laser. CW PL emission spectra from the quantum wells (24, 30, 80Å) were all blue shifted with respect to the reference sample. The difference (i. e., the blue shift) between the measured value of peak emission energy from the QW and the band-edge emission from the reference sample was attributed to quantum size effects, and to strain arising due to a significant lattice mismatch between InGaN and GaN. In addition, stimulated emission was observed from an InGaN/GaN single heterojunction in the edge and surface emitting configu-ration at 77K. The narrowing of emission spectra, the nonlinear dependence of output emission intensity on input power density, and the observation of a strongly polarized output are presented.     

Maskless pendeo-epitaxial growth of GaN films

High-resolution x-ray diffraction (XRD) and atomic force microscopy (AFM) of pendeo-epitaxial (PE) GaN films confirmed transmission electron microscopy (TEM) results regarding the reduction in dislocations in the wings. Wing tilt ≤0.15° was due to tensile stresses in the stripes induced by thermal expansion mismatch between the GaN and the SiC substrate. A strong D°X peak at ≈3.466 eV (full-width half-maximum (FWHM) ≤300 μeV) was measured in the wing material. Films grown at 1020°C exhibited similar vertical [0001] and lateral [11 0] growth rates. Increasing the temperature increased the latter due to the higher thermal stability of the GaN(11 0). The (11 0) surface was atomically smooth under all growth conditions with a root mean square (RMS)=0.17 nm.     

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.     

红橙光InGaN/GaN量子阱的结构与光学性质研究

采用金属有机物化学气相沉积(MOCVD)技术生长了具有高In组分InGaN阱层的InGaN/GaN多量子阱(MQW)结构,高分辨X射线衍射(HRXRD)ω-2θ扫描拟合得到阱层In含量28%。比较大的表面粗糙度表明有很大的位错密度。室温下光致荧光(PL)研究发现该量子阱发射可见的红橙光,峰位波长在610 nm附近。变温PL(15～300 K)进一步揭示量子阱在低温下有两个发光机制,对应的发射峰波长分别为538 nm和610 nm。由于In分凝和载流子的局域化导致的载流子动力改变,使得量子阱PL发光峰值随温度增加呈明显的"S"变化趋势。     

Growth of GaN Films on Si (100) Buffered with ZnO by Ion-Beam-Assisted Filtered Cathodic Vacuum Arc Technique

GaN films have been fabricated on Si (100) substrates with ZnO buffer layers by an ion-beam-assisted filtered cathodic vacuum arc (I-FCVA) technique at␣450°C. GaN films are highly (002)-oriented with a hexagonal structure examined by X-ray diffraction. The room-temperature photoluminescence spectrum of the GaN film exhibits a strong and sharp band-edge emission peak at 3.36 eV. The obtained results demonstrate the potential of the I-FCVA technique for the fabrication of high-quality GaN layers on Si substrates.     

载气流量对HVPE外延生长GaN膜光学性质的影响

研究了利用水平氢化物气相外延 (HVPE)系统在蓝宝石衬底上外延氮化镓 (Ga N)的生长规律 ,重点研究了作为载气的氮气流量对 Ga N膜的结构及光学性质的影响。观察到载气流量对预反应的强弱有很大影响 ,外延膜的质量和生长速度对载气流量极为敏感。当载气流量较小时 ,样品的 X射线衍射谱 (XRD)中出现了杂峰(1 0 -1 1 )和 (1 1 -2 0 ) ,相应的光致发光谱 (PL)中出现了黄带 (YL) ,靠近带边有杂质态。而当载气流量增大时 ,样品质量改善。Ga N外延膜的结构和光学性质的相关性表明深能级的黄带与生长过程中产生的非 c轴方向晶面有关 ,据此我们推测 :Ga空位与束缚在 (1 0 -1 1 )和 (1 1 -2 0 )等原子面上的杂质构成复合结构 ,这些复合结构所产生的深能级对黄带的发射有贡献 ;由于预反应使生长过程中混入的附加产物及杂质对带边发射有重要影响     

Analysis of reactor geometry and diluent gas flow effects on the metalorganic vapor phase epitaxy of AIN and GaN thin films on α(6H)-SiC substrates

The influence of diluent gas on the metalorganic vapor phase epitaxy of AlN and GaN thin films has been investigated. A computational fluid dynamics model using the finite element method was employed to improve film uniformity and to analyze transport phenomena. The properties of AlN and GaN thin films grown on α(6H)-SiC(0001) substrates in H₂ and N₂ diluent gas environments were evaluated. Thin films of AlN grown in H₂ and N₂ had root mean square (rms) roughness values of 1.5 and 1.8 nm, respectively. The surface and defect microstructures of the GaN thin films, observed by scanning and transmission electron microscopy, respectively, were very similar for both diluents. Low temperature (12K) photoluminescence measurements of GaN films grown in N₂ had peak intensities and full widths at half maximum equal to or better than those films grown in H₂. A room temperature Hall mobility of 275 cm²/V·s was measured on 1 μm thick, Si-doped, n-type (1×10¹⁷ cm⁻³) GaN films grown in N₂. Acceptor-type behavior of Mg-doped GaN films deposited in N₂ was repeatably obtained without post-growth annealing, in contrast to similar films grown in H₂. The GaN growth rates were ∼30% higher when H₂ was used as the diluent. The measured differences in the growth rates of AlN and GaN films in H₂ and N₂ was attributed to the different transport properties of these mixtures, and agreed well with the computer model predictions. Nitrogen is shown to be a feasible alternative diluent to hydrogen for the growth of AlN and GaN thin films.     

Comparative Study on MOCVD Growth of a-Plane GaN Films on r-Plane Sapphire Substrates Using GaN, AlGaN, and AlN Buffer Layers

In this work, we have comparatively investigated the effects of the GaN, AlGaN, and AlN low-temperature buffer layers (BL) on the crystal quality of a-plane GaN thin films grown on r-plane sapphire substrates. Scanning electron microscopy images of the a-plane GaN epilayers show that using an AlGaN BL can significantly reduce the density of surface pits. The full-width at half-maximum values of the x-ray rocking curve (XRC) are 0.19°, 0.36°, and 0.48° for the films grown using Al_0.15Ga_0.85N, GaN, and AlN BLs, respectively, indicating that an AlGaN BL can effectively reduce the mosaicity of the films. Room-temperature photoluminescence shows that the AlGaN BL results in lower impurity incorporation in the subsequent a-plane GaN films, as compared with the case of GaN and AlN BLs. The higher crystal quality of a-plane GaN films produced by the Al_0.15Ga_0.85N BL could be due to improvement of BL quality by reducing the lattice mismatch between the BL and r-sapphire substrates, while still keeping the lattice mismatch between the BL and epitaxial a-plane GaN films relatively small.     

Compositional changes in erbium-implanted GaN films due to annealing

We have conducted a study of the material and infrared-luminescence properties of Er-implanted GaN thin films as a function of annealing. The GaN films, grown by metal-organic chemical-vapor deposition, were coimplanted with Er and O ions. After implantation, the films were furnace annealed at temperatures up to 1,100°C. Following each annealing stage, the samples were examined by photoluminescence (PL) measurements and secondary ion-mass spectrometry (SIMS) analysis. In the as-implanted samples, no PL signal near 1,540 nm could be detected with either above-bandgap or below-bandgap excitation. Only after annealing at temperatures above 900°C was the 1,540-nm luminescence detectable. Annealing at higher temperatures resulted first in an increase and then a decrease in the PL-signal intensities. The SIMS data showed that large concentrations of Al, O, and C atoms entered into the GaN films with high-temperature annealing. The stoichiometric changes in the GaN appear responsible for the changes in the Er-related luminescence.     

The effect of substrate misorientation on the optical, structural, and electrical properties of GaN grown on sapphire by MOCVD

We report the growth and characterization of unintentionally doped GaN on both exact and vicinal (0001) sapphire substrates. The GaN heteroepitaxial layers are grown by metalorganic chemical vapor deposition on c-plane A1₂O₃ substrates either on-axis or intentionally misoriented 2° toward the a-plane (1120) or 5 or 9° toward the m-plane (10 10). The samples are characterized by 300K photoluminescence, cathodoluminescence, and Hall-effect measurements as well as by triple-axis x-ray diffractometry to determine the effect of the misorientation on the optical, electrical, and structural properties of heteroepitaxial undoped GaN. Ten different sample sets are studied. The data reveal enhanced photo-luminescence properties, increased electron mobility, a reduced n-type background carrier concentration, and a somewhat degraded surface morphology and crystalline quality for the misoriented samples compared to the on-axis samples.     

Electrical and optical properties of lodine doped CdZnTe layers grown by metalorganic vapor phase epitaxy

Electrical and photoluminescence properties of iodine doped CdZnTe (CZT) layers grown by metalorganic vapor phase epitaxy have been studied. Doped layers showed an n-type conductivity from the Zn composition x=0 (CdTe) to 0.07. Above x=0.07, resistivities of doped layers increased steeply up to 10⁶ Ω-cm. Resistivities of doped CZT layers were higher than those of undoped layers above x=0.6. Photoluminescence intensity of doped layers increased compared to undoped layers. Doped CdTe and ZnTe layers showed neutral donor bound exciton emission lines at the exciton related region. Also, these layers showed an increase in emission intensity at the donor acceptor pair recombination bands. Sharp emission lines were observed in doped CZT layers at around 1.49 eV. These emission lines were considered to be originated from GaAs substrates which were optically excited by the intense emission from doped CZT layers.