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     

阴极荧光联合分析系统在III族氮化物研究中的应用

利用高性能阴极荧光（CL）联合分析系统对几类典型的III族氮化物材料进行测试分析.在光谱研究中,利用CL紫外可见光谱系统,对c面蓝宝石衬底上生长的AlxGa1-xN薄膜进行阴极荧光单色谱测试分析,揭示了CL的激发强度与发光带之间的变化关系.进一步研究了掺Mg的Al0.5Ga0.5N薄膜的带边和杂质能级发光机理.利用CL近红外光谱系统对InN薄膜的阴极发光特性进行了研究,验证了InN实际光学带边Eg在0.77eV附近.利用微区分析（CLmapping）系统,可在紫外波段确切地给出材料不同波长的荧光发光区这一特点,对HVPE生长的自支撑GaN衬底进行了SEM和CL微区的对比分析,研究了GaN的位错类型和分布.     

Photoluminescence of V-doped GaN thin films grown by MOVPE technique

This work reports the photoluminescence (PL) study of vanadium-doped GaN (GaN: V) in the 9-300 K range. Samples have been successfully prepared on sapphire substrates by metalorganic vapour phase epitaxy technique (MOVPE). At room temperature (RT) the PL spectra of GaN: V are dominated by a blue band (BB) in the 2.6 eV range. This BB emission is very strong and its intensity increases with increasing V doping level. We also observed that the peak position of the blue luminescence shifted at lower energy with decreasing excitation density. Upon V-doping, the yellow luminescence band shows a drastic reduction in integrated intensity. This observation is explained by a reaction involving V and gallium vacancy (_√Ga). PL spectra at low temperature exhibited a series of peaks. The donor-acceptor (D-A) pair emission peak at 3.27 eV was strongly pronounced, as the temperature was decreased. On the other hand, the intensity of the BB emission decreased. This BB emission is due to a radiative transition from a shallow donor with a depth of 29 meV to a deep acceptor with a depth of 832 meV.     

Defect-related luminescence of GaN:Zn films thermally treated in a radio-frequency ammonia plasma

The effect of thermal annealing in nitrogen radicals obtained by the treatment of NH₃ in a radio-frequency discharge on the luminescence properties of GaN:Zn films grown by MOCVD/hydride epitaxy on sapphire (0001) substrates is investigated. As the thermal treatment temperature was increased, a steady weakening of the violet (2.88 eV) and near-edge (3.48 eV) photoluminescence bands was observed. As a result of the thermal treatment in nitrogen radicals at 500–750°C, new bands that peaked at 3.27 and 3.42 eV were detected; the intensities of these bands increased with increasing treatment temperature. The mechanism of formation and the origin of all the bands are analyzed comprehensively. It is found that the luminescence bands at 2.88, 3.42, and 3.27 eV are characteristic of the GaN films obtained by practically each technology and are associated with the simple structural defects. The participation of O in the formation of the band at 3.42 eV is proved experimentally.     

异质外延GaN中穿透位错对材料发光效率的影响

用阴极射线致发光(CL)法、透射电子显微镜(TEM)和X射线衍射(XRD)法研究了异质外延GaN材料的发光性质与结构特性的关系.结果表明,GaN外延层中的穿透位错是材料有效的非辐射复合中心,但GaN的CL带边峰强度并不随位错密度的增加而减少.两步法生长GaN形成的马赛克结构的亚晶粒尺寸和晶粒间合并产生的位错的弯曲程度是影响材料发光效率的关键.     

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。     

Effect of gas feeding methods on optical properties of GaN grown by rapid thermal chemical vapor deposition reactor

The origin of the radiative recombination leading to yellow luminescence (YL) has been elucidated by the study of luminescence properties of GaN films grown with two different gas feeding methods. GaN films were grown on a (0001) sapphire substrate in a rapid thermal chemical vapor deposition (RTCVD) reactor. GaN films emitted two different luminescence energies, 2.2 and 3.47 eV, depending on the introducing position of hydrogen gas in the growth reactor. The distribution of the TMGa flow and gas phase reactions in the reactor were investigated to understand the effect of the gas feeding methods on the optical properties of GaN films. The results suggest that YL is related to Ga vacancies in the grown films.     

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.     

氮杂质对直拉单晶硅中位错的作用

通过常温下压痕之后的高温热处理实验 ,研究了掺氮直拉硅单晶 (NCZSi)中氮杂质对位错滑移的钉扎作用 ,以及塑性变形能在热处理过程中通过位错的滑移释放的机理 .实验结果表明氮杂质对位错有着较强的钉扎作用 ,使掺氮直拉硅单晶中的位错在同一温度下热处理时的滑移距离均小于普通直拉硅单晶 (CZSi) .同时指出 ,NCZSi的位错激活能比 CZSi的要高 ,NCZSi中塑性变形能通过位错滑移释放较 CZSi快 ,并讨论了压痕造成的塑性变形能通过位错的滑移而释放的可能机理     

Investigations regarding the maskless pendeo-epitaxial growth of GaN films prior to coalescence

Pendeo-epitaxy employs lateral growth from etched seed forms to achieve a marked reduction in dislocation density in a material. In this research, high-resolution X-ray diffraction and atomic force microscopy of GaN stripes and the laterally grown wings confirmed transmission electron microscopy results regarding the reduction in dislocations in the latter regions. Micro-Raman and X-ray diffraction measurements showed the wings to be tilted /spl les/0.15/spl deg/ due to tensile stresses in the stripes induced primarily by the mismatch in the coefficients of thermal expansion between the GaN stripe and the SiC substrate. A strong, low-temperature D/spl deg/X peak at /spl ap/3.466 eV with a FWHM of /spl les/300 /spl mu/eV was measured in the wing material by micro-photoluminescence. Films grown at 1020/spl deg/C exhibited similar vertical [0001] and lateral [112~0] growth rates. Increasing the growth temperature increased the latter due to the higher thermal stability of the (112~0) GaN and initiated growth of spiral hillocks on the (0001) surface of the stripes. The latter were due to adatom diffusion to heterogeneous steps previously nucleated at the intersections of pure screw or mixed dislocations. The (112~0) surface was atomically smooth under all growth conditions with a root mean square roughness value of 0.17 nm.     

Electron levels and luminescence of dislocation networks formed by the hydrophilic bonding of silicon wafers

Local energy levels produced by dislocations at the interface between bonded n- and p-Si wafers are studied by deep level transient spectroscopy and by a new technique for the detection of impurity luminescence, induced by the occupation of electron states upon the application of electric pulses (the pulsed trap-refilling-enhanced luminescence technique). It is established that only the shallow levels of the dislocation network, with activation energies of about 0.1 eV, are responsible for the D1 dislocation-related luminescence band in both n- and p-type samples. The occupation of deep levels has no effect on the D1-band intensity. A model of coupled neutral trapping centers for charge carriers is proposed. In this model, the difference between the energy position of the D1 band (0.8 eV) and the corresponding interlevel energy spacing (0.97 eV) is attributed to the Coulomb interaction between charge carriers trapped at the levels.     

Low energy electron excited nanoscale luminescence spectroscopy of erbium doped AIN

We have investigated the optical activity of Er³⁺ ions in AlN via depth-resolved, (5 nm to 250 nm), low energy electron-excited nanoscale (LEEN) luminescence spectroscopy and compared it with the luminescence of an Er-free AlN film. For the Er-free film, there was no emission in the IR from the AlN at any depth, and at higher energies we measured only a broad, weak feature between 1.7–3.25 eV along with an O defect related feature at 3.8 eV, which is significantly enhanced toward the surface. We found strong emission in the AlN:Er films from the first excited → ground state transition of Er³⁺ at 0.80 eV along with many other excited state transitions, although the features are broad compared to those of GaN:Er. The AlN:Er luminescence saturates near a concentration of 10²¹ cm⁻³, at which point we also observe enhanced O defect related luminescence uniformly distributed throughout the film. This finding suggests a role for O in activating the Er at low Er concentrations, while inhibiting the Er activity at high O concentrations.     

Impact ionization luminescence of InGaN/AlGaN/GaN p-n-heterostructures

The luminescence spectra of InGaN/AlGaN/GaN p-n heterostructures with reverse bias sufficient for impact ionization are investigated. The injection luminescence of light-emitting diodes with such structures was examined earlier. A strong electric field is present in the InGaN active layer of the heterostructures, and for small reverse bias the tunneling component of the current predominates. Avalanche breakdown commences at voltages V _th>8–10 V, i.e., ∼3E _g , (E _g is the width of the band gap) in the absence of lightly doped structures. The luminescence spectra have a short-wavelength edge corresponding to the width of the GaN band gap (3.40 eV) and maxima in the region 2.60–2.80 eV corresponding to the maxima of the injection luminescence spectra in the active layer. The long-wavelength edge of the spectra in the region 1.7–1.8 eV may be associated with deep recombination levels. Mechanisms of recombination of the hot electron-hole plasma in the strong electric fields of the p-n heterostructures are discussed. Fiz. Tekh. Poluprovodn. 32, 63–67 (January 1998)     

Correlation of carbon doping variations with the vertical breakdown of GaN-on-Si for power electronics

In this work gallium nitride (GaN) grown on silicon substrates was investigated in order to determine critical defects responsible for differences in the vertical breakdown of HEMT structures. Cathodoluminescence studies at the SEM revealed a direct correlation between the intensity of the blue luminescence (BL) band and the carbon doping concentration. Observing this, carbon depletion zones were found around threading dislocations in the active GaN layer, as well as a deep depletion in growth columns concluded from a reduction of the BL intensity. Using the given results a model of a defect, here called the deep carbon depletion (DCD), is proposed to explain the correlation between carbon variations and the vertical breakdown.     

Cathodoluminescence Study on Spatial Luminescence Properties of InN/GaN Multiple Quantum Wells Consisting of 1-Monolayer-Thick InN Wells/GaN Matrix

Spatially resolved luminescence properties of InN/GaN multiple quantum wells (MQWs) consisting of nominally one monolayer (1-ML)-thick InN QWs embedded in a GaN matrix are studied by cross-sectional and plan-view cathodoluminescence measurements. First it is confirmed that the dominant emission peaks observed at around 390 nm to 430 nm in the MQWs samples are attributed to the effects of inserting ∼1-ML-thick InN wells in the GaN matrix, resulting in efficient localization of GaN excitons at InN QWs. Furthermore, it is revealed that the detailed structure of the MQWs, such as the thickness distribution and interface sharpness, is very sensitive to the presence of surface defects such as hillocks around screw-component threading dislocations, resulting in different emission wavelengths/energies. This is because the epitaxy process for depositing such thin InN wells is seriously affected by the atomic-level surface structures/properties of the growth front. It will be concluded that it is necessary to use lower dislocation density GaN bulk templates to obtain much higher structural quality InN/GaN MQWs good enough for characterizing their optical properties.     

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.     

Fe掺杂GaN中Fe相关缺陷发光特性研究

利用氢化物气相外延方法制备了 Fe掺杂GaN单晶.材料发光特性随掺杂浓度、温度和激发功率的变化规律表明,位于蓝光波段的发光与掺杂的Fe离子有关.随着Fe掺杂浓度的增加,红外波段发光强度将被抑制,而蓝光波段发光强度显著增加.在较高位错密度样品中,相比于蓝光,红外发光强度显著增加.掺杂样品的光致激发光谱进一步说明了 Fe离子相关的红外和蓝光发光能级跃迁特征及过程.结果表明GaN中的本征缺陷结构以及位错对Fe相关能级跃迁具有重要影响.     

The influence of nitrogen ion energy on the quality of GaN films grown with molecular beam epitaxy

Since the growth of GaN using molecular beam epitaxy (MBE) occurs under metastable growth conditions, activated nitrogen is required to drive the forward synthesis reaction. In the process of exciting the nitrogen using a plasma or ion-beam source, species with large kinetic energies are generated. Impingement on the growth surface by these species can result in subsurface damage to the growing film, as well as an enhancement of the reverse decomposition reaction rate. In this study, we investigate the effect of the kinetic energy of the impinging nitrogen ions during growth on the resulting optical and structural properties of GaN films. Strong band-edge photoluminescence and cathodoluminescence are found when a kinetic energy of ∼10 eV are used, while luminescence is not detectable when the kinetic energies exceeds 18 eV. Also, we find that the use of conductive SiC substrates results in more homogeneous luminescence than the use of insulating sapphire substrates. This is attributed to sample surface charging in the case of sapphire substrates and subsequent variation in the incident ion flux and kinetic energy across the growth surface. This study clearly shows that the quality of GaN films grown by MBE are presently limited by damage from the impingement of high energy species on the growth surface.     

不同厚度未掺杂GaN薄膜的特性分析

对厚度不同的样品进行了XRD和PL谱测量,由(0002)面、(30—32)面的摇摆曲线的半峰宽值和GaN(0002)衍射峰位置计算了样品的刃位错、螺旋位错的密度以及C轴应变,实验结果表明厚度增加后Gain薄膜中的刃位错、螺旋位错密度及C轴薄膜应力均得到减小,而PL谱带边峰和蓝带强度显著增强。分析认为：厚度增加后,位错减少是由材料生长过程中位错的合并和湮灭作用造成的;样品PL谱的带边峰和蓝带强度显著增强是因为位错引入的非辐射性复合中心数目减少。     

Electron and hole traps in GaN p-i-n photodetectors grown by reactive molecular beam epitaxy

GaN p-i-n photodetectors grown on sapphire by reactive molecular beam epitaxy have been characterized by measurements of room-temperature current-voltage (I-V), temperature-dependent capacitance (C-V-T), and deep level transient spectroscopy (DLTS) under both majority and minority carrier injection. Due to what we believe to be threading dislocations, the reverse I-V curves of p-i-n photodetectors show typical electric-field enhanced soft breakdown characteristics. A carrier freeze-out due to the de-ionization of Mg-related deep acceptors has been found by C-V-T measurements. Three electron traps, B (0.61 eV), D (0.23 eV), and E₁ (0.25 eV) and one hole trap, H₃ (0.79 eV) have been revealed by DLTS measurements. The photodetectors with lower leakage currents usually show higher responsivity and lower trap densities of D and E₁.