GaN外延材料中持续光电导的光淬灭

研究了非故意掺杂和掺Si的n型GaN外延材料持续光电导的光淬灭。实验发现非故意掺杂GaN的持续光电导淬灭程度远大于掺Si的n型GaN;撤去淬灭光后前者的持续光电导几乎没有变化,后者的却有明显减小;稍后再次加淬灭光前者的持续光电导无变化,而后者的有明显增加。我们认为两者持续光电导的形成都与空穴陷阱有关,用空穴陷阱模型解释了非故意掺杂GaN持续光电导的形成以及淬灭;认为掺Si的n型GaN的持续光电导是电子陷阱(杂质能级)和空穴陷阱共同作用的结果,并且在持续光电导发生的不同阶段其中一种陷阱的作用占主要地位。     

立方相GaN的持续光电导

研究了金属有机物化学气相外延 (MOVPE)方法生长的非故意掺杂的立方相 Ga N的持续光电导效应 .在六方相 Ga N中普遍认为持续光电导效应与黄光发射有关 ,而实验则显示在立方 Ga N中 ,持续光电导效应与其中的六方相 Ga N夹杂有关系 ,而与黄光发射没有关系 .文中提出 ,立方相 Ga N与其中的六方相 Ga N夹杂之间的势垒引起的空间载流子分离是导致持续光电导现象的物理原因 .通过建立势垒限制复合模型 ,解释了立方相 Ga N的持续光电导现象的物理过程 ,并对光电导衰减过程的动力学作了分析 .对实验数据拟合的结果证明以上的模型和推导是与实验相符的 .     

Effect of Pt barrier on thermal stability of Ti/Al/Pt/Au in ohmic contact with Si-implanted n-type GaN layers

We report the effect of the Pt barrier on the thermal stability of Ti/Al/Pt/Au in ohmic contact with Si-implanted n-type GaN layers. Ti/Al/Au (25/100/200 nm) and Ti/Al/Pt/Au (25/100/50/200 nm) multilayers were, respectively, deposited on as-implanted and recovered Si-implanted n-type GaN samples. The associated dependence of the specific contact resistance on the annealing time at various temperatures was compared. The long-term ohmic stability of a Ti/Al/Pt/Au multilayer in contact with a Si-implanted n-type GaN layer was much better than that of the Ti/Al/Au multilayer. This superior stability is attributed to the barrier function of the Pt interlayer. The Pt/Au bilayer can also passivate the propensity of oxidation for the conventional Ti/Al bilayer in contact with n-type GaN layers at elevated temperatures.     

Dopant-selective photoenhanced wet etching of GaN

A dopant-selective photoelectrochemical wet etching process for GaN is described. The process utilizes KOH solution and Hg arc lamp illumination to achieve selective etching of n-type GaN with respect to intrinsic and p-type GaN. An intrinsic GaN etch-stop layer as well as the selective undercutting of a buried n-type layer within a p-n GaN homostructure are demonstrated.     

低温生长n型氮化镓插入层提高LED抗静电能力

在这篇论文里,我们通过在InGaN/GaN 多量子阱和n型氮化镓层中间插入一层低温生长的n型氮化镓显著提高了LED的抗静电能力。通过引入低温生长的氮化镓插入层使得LED抗击穿电压超过4000V的良品率从9.9%提升到74.7%。低温生长的氮化镓插入层作为后续生长的多量子阱的缓冲层,释放了量子阱中的应力并且改善了量子阱的界面质量。另外,我们证明了在氮气气氛下生长低温氮化镓插入层对于LED抗静电能力的改善要强于氢气气氛,同时也进一步证明低温插入层对量子阱中应力的释放有利于提高LED的抗静电能力。光电测试结果表明,在引入低温nGaN缓冲层后,LED的电学特性并没有衰退,并且LED的光输出功率提高了13.9%。     

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.     

The hall mobility and its relationship with persistent photoconductivity of undoped GaN

Temperature-variable Hall effect measurements have been used to investigate the electrical properties of undoped GaN, which have the electron densities on the order of mid-10¹⁶ cm⁻³ and a Hall mobility varying from <50 cm²/sV to >500 cm²/sV. We found that very strong ionized impurity scattering limits the Hall mobility of GaN. Illumination even at 77 K has very little effect on the electron density but can lead to a noticeable persistent increase of the Hall mobility. The induced persistent photoconductivity (PPC) effect is therefore related to the Hall mobility through intrinsic electrically active defects. The properties of those defects were further investigated by monitoring a transient change of resistivity after removal of illumination at different temperatures. It reveals that the recapturing process of excited electrons into illumination-neutralized defects is the mechanism responsible for the PPC effect of undoped GaN.     

非故意掺杂n型GaN的负持续光电导现象

研究了MOCVD方法制备的非故意掺杂n型GaN薄膜的持续光电导现象.实验发现样品的光电导与入射光强有密切的关系,当入射光强由弱到强变化时,样品会依次出现正常持续光电导(PPC)、负光电导(NPC)和负持续光电导(NPPC)现象.据知,这是首次在一个样品中仅仅通过改变入射光强就可以依次产生以上现象的实验报道.通过系统的实验分析和理论研究认为,该现象形成的主要原因是材料中深能级电子陷阱和空穴陷阱共同作用的结果.     

非故意掺杂n型GaN的负持续光电导现象

研究了MOCVD方法制备的非故意掺杂n型GaN薄膜的持续光电导现象.实验发现样品的光电导与入射光强有密切的关系,当入射光强由弱到强变化时,样品会依次出现正常持续光电导(PPC)、负光电导(NPC)和负持续光电导(NPPC)现象.据知,这是首次在一个样品中仅仅通过改变入射光强就可以依次产生以上现象的实验报道.通过系统的实验分析和理论研究认为,该现象形成的主要原因是材料中深能级电子陷阱和空穴陷阱共同作用的结果.     

Schottky barriers on n-GaN grown on SiC

Characteristics of Schottky barriers fabricated on n-type GaN were investigated. The barriers were formed by vacuum thermal evaporation of Cr, Au, and Ni. Current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the barriers were measured in a wide temperature and current density range. Fundamental parameters (barrier height and built-in potential) of the Schottky barriers were determined. The dependence of the barrier ideality factor on doping concentration in GaN was measured. Correlation between the barrier height and metal work function was observed. The electron affinity for GaN was determined using both C-V and I-V characteristics. The current flow mechanism through the barriers is discussed.     

Enhanced light output in nitride-based light-emitting diodes by roughening the mesa sidewall

In this letter, we will report on a nitride-based light emitting diode with a mesa sidewall roughening process that increases light output power. The fabricated GaN-based light-emitting diode (LED) wafers were first treated through a photoelectrochemical (PEC) process. The Ga/sub 2/O/sub 3/ layers then formed around the GaN : Si n-type mesa sidewalls and the bottoms mesa etching regions. Selective wet oxidation occurred at the mesa sidewall between the p- and the n-type GaN interface. The light output power of the PEC treated LED was seen to increase by about 82% which was caused by a reduced index reflectance of GaN-Ga/sub 2/O/sub 3/-air layers, by a rough Ga/sub 2/O/sub 3/ surface, by a microroughening of the GaN sidewall surface, and by a selective oxidation step profile of the mesa sidewall that increases the light-extraction efficiency from the mesa sidewall direction. Consequently, this wet PEC treated process is suitable for high powered nitride-based LEDs lighting applications.     

Contactless electroreflectance study of band bending for undoped, Si- and Mg-doped GaN layers and AlGaN/GaN transistor heterostructures

The surface band bending in undoped, Si-doped and Mg-doped GaN layers with Ga-face polarity as well as AlGaN/GaN heterostructures with Ga(Al)-face polarity has been investigated by room temperature contactless electroreflectance (CER) spectroscopy. The opposite phase of CER resonance (i.e., opposite band bendings) has been observed for n-type (undoped and Si-doped) and p-type (Mg-doped) GaN layers. It means that for thick GaN layers the surface band bending results not from crystal polarity but from the Fermi-level pinning at the surface and carrier type/concentration inside the layer. The crystal polarity can influence the surface band bending for thin (Al)GaN layers for which the screening phenomena can be neglected or are very weak. Such a situation is typical of AlGaN/GaN transistor structures where the thickness of AlGaN layer is below 40 nm. In this case, the strong internal electric field in AlGaN layer is manifested in CER spectra by a resonance with a long period Franz-Keldysh oscillation.     

Electrical characterisation of Ti Schottky barriers can n-type GaN

The Schottky barrier height of Ti on n-type GaN has been measured to be 0.58 and 0.59 eV by capacitance-voltage and current-voltage techniques, respectively. This work is of particular interest because it is the first measure of the Schottky barrier height on GaN for a metal other than Au. The barrier height of Ti on GaN is significantly less than that of Au. This supports the prediction that the Fermi level is not pinned at the GaN surface     

氧化物缓冲层制备Si(111)基GaN纳米线

采用氧化物缓冲层,通过射频磁控溅射系统依次在n型Si(111)衬底上沉积Ga2O3/ZnO(Ga2O3/MgO)薄膜,然后将薄膜于950℃氨化合成GaN纳米结构,氨化时间为15min。采用X射线衍射(XRD)、傅里叶红外吸收谱(FTIR)和高分辨透射电镜(HRTEM)对样品的结构进行了分析,结果显示两种缓冲层下制备的样品均为六方纤锌矿单晶GaN纳米结构,且缓冲层的取向对纳米线的生长方向有很大影响;采用扫描电镜(SEM)对样品的形貌进行了测试,发现纳米线表面光滑,长度可达几十微米,表明采用氧化物缓冲层制备了高质量的GaN线。同时对GaN纳米线的生长机理进行了简单讨论。     

Phosphor-Free GaN-Based Transverse Junction White-Light Light-Emitting Diodes With Regrown n-Type Regions

In this study, we demonstrate a GaN-based phosphor-free white-light light-emitting diode (LED), which is composed of GaN-based dual-wavelength (blue and yellow-green) multiple-quantum-wells (MQWs) and a transverse p-n junction. The device was realized by the regrowth of n-type GaN layers on the sidewall of p-type GaN and undoped MQWs. The problems related to the bias-dependent shape of the electroluminescence spectra that occur in traditional phosphor-free white-light LEDs (with vertical p-n junctions) are greatly minimized. The current-voltage performance of our device is comparable to that of the commercially available phosphor white-light LEDs. In addition, the dynamic measurement results indicate that we can attain a much higher modulation bandwidth (22 versus 3 MHz) with this device than with the currently available commercial ones.     

Effect of O2/CHF3 plasma treatment on n-type GaN grown on sapphire by MOCVD

Plasma-induced damage of n-type GaN in Cl₂/CH₄/Ar reactants and its recovery by the O₂/CHF₃ plasma treatment in reactive ion etching (RIE) system were studied by etching rate, self-bias voltage and Hall measurement. RIE of n-type GaN was performed at a radio frequency power of 250 W in Cl₂/CH₄/Ar ambient prior to in the O₂/CHF₃ plasma treatment. The effect of O₂/CHF₃ plasma treatment on electrical characteristics of n-type GaN was investigated by changing the ratio of O₂/CHF₃ flow rate. It is found that the damage caused by conventional RIE processing could be partly recovered by CHF₃/O₂ plasma treatment.     

Doping of gallium nitride using disilane

The silicon doping of n-type GaN using disilane has been demonstrated for films grown on sapphire substrates by low pressure organometallic vapor phase epitaxy. The binding energy of an exciton bound to a neutral Si donor has been determined from low temperature (6K) photoluminescence spectra to be 8.6 meV. Nearly complete activation of the Si impurity atom in the GaN lattice has been observed.     

GaN MOS device using SiO/sub 2/-Ga/sub 2/O/sub 3/ insulator grown by photoelectrochemical oxidation method

We report on a SiO/sub 2/-Ga/sub 2/O/sub 3/ gate insulator stack directly grown on n-type GaN by the photoelectrochemical oxidation method. The resultant MOS devices are fabricated using standard photolithography and liftoff techniques. The effect of annealing temperature on the SiO/sub 2/-Ga/sub 2/O/sub 3//n-type GaN MOS devices is investigated. The properties of high breakdown field, low gate leakage current, and low interface state density are investigated for the MOS devices.     

Investigation of Ga oxide films directly grown on n-type GaN by photoelectrochemical oxidation using He-Cd laser

By using a He-Cd laser in a chemical solution of H₃PO₄ with a pH value of 3.5, Ga oxide films were directly grown on n-type GaN. From the energy-dispersive spectrometer (EDS) measurement and x-ray diffraction (XRD) measurement, the grown Ga oxide film was identified as (104) α-Ga₂O₃ structure. A small amount of phosphors existed and bonded with oxygen on the grown films. The as-grown films were amorphous. From the XRD analysis, it is evident that annealing of the α-Ga₂O₃ films led to a change in the microstructure from an amorphous to a polycrystalline phase. In addition, the as-grown low-density films gradually became dense films during the annealing process. Furthermore, the surface roughness of the annealed films also gradually decreased. Hexagonal pinholes on the grown films were observed. The density of the hexagonal pinholes was similar to the defect density of the n-type GaN. From the cross-sectional transmission electron microscopy (TEM) micrographs, it is evident that the hexagonal pinholes originated from defects in the n-type GaN.     

Forming Low-Resistance p-Type Layers in Ga1 – xAlxN/GaN Heterostructures

Semiconductors - A technique for fabricating a GaN-based functional heterostructure is proposed, which allows the growth of n-type GaN and InxGa1&nbsp;– xN layers and р-type GaN and...