阶梯AlGaN外延新型Al_(0.25)Ga_(0.75)N/GaNHEMTs击穿特性分析

为了优化AlGaN/GaN HEMTs器件表面电场,提高击穿电压,本文首次提出了一种新型阶梯AlGaN/GaN HEMTs结构.新结构利用AlGaN/GaN异质结形成的2DEG浓度随外延AlGaN层厚度降低而减小的规律,通过减薄靠近栅边缘外延的AlGaN层,使沟道2DEG浓度分区,形成栅边缘低浓度2DEG区,低的2DEG使阶梯AlGaN交界出现新的电场峰,新电场峰的出现有效降低了栅边缘的高峰电场,优化了AlGaN/GaN HEMTs器件的表面电场分布,使器件击穿电压从传统结构的446 V,提高到新结构的640 V.为了获得与实际测试结果一致的击穿曲线,本文在GaN缓冲层中设定了一定浓度的受主型缺陷,通过仿真分析验证了国际上外延GaN缓冲层时掺入受主型离子的原因,并通过仿真分析获得了与实际测试结果一致的击穿曲线.     

高压下Al_(0.86)Ga_(0.14)N半导体合金“双模”拉曼声子行为研究

氮化铝镓(AlxGa_(1-x)N)合金是重要的半导体材料,在发光二极管和紫外探测器等领域都有广阔的应用前景。对高铝成分AlxGa_(1-x)N合金的高压行为研究还较为稀少。利用原位拉曼光谱对高铝成分的AlxGa_(1-x)N合金与本征的GaN和AlN进行了对比研究。结果表明,Al原子掺杂引起GaN亚晶格的收缩效应与对本征GaN施加外部正压力的作用效果等效,引起AlN亚晶格的拉伸效应与对本征AlN施加外部负压力的作用效果相似。Al原子掺杂引起Al0.86Ga0.14N晶格的收缩效应在晶胞c轴方向上更显著。研究还发现,Al原子掺杂对类AlN拉曼声子振动模高压行为的影响比类GaN更明显。     

Gd_3Ga_(4.5)Al_(0.5)O_(12)单晶的低温磁性

采用提拉法制备了Gd_3Ga_(4.5)Al_(0.5)O_(12)单晶。经X射线粉末照相证实为石榴石型单相结构。在T=1.5—77K温区内,测量了磁化强度和磁化率。并与GGG的数据作了比较。实验结果表明:在场强H<500Oe时,磁化率x与温度T的关系遵守居里——外斯定律,其有序温度T_N远低于1.5 K.H>10000Oe时,该材料的磁化强度(或磁化率)比GGG高3一4％。在特定温度下,允许使用的临界场H_临比GGG高约4000 Oe     

Al_(0.17)Ga_(0.83)As/GaAs(001)薄膜退火过程的热力学分析

在As_4束流等效压强为1.2×10~(-3)Pa、退火60 min条件下改变退火温度,对Al_(0.17)Ga_(0.83)As/GaAs薄膜表面平坦化的条件进行了探讨.定量分析了薄膜表面坑、岛与平台的覆盖率和台阶-平台间薄膜粗糙度随退火温度变化的规律,得到最合适的退火温度为545℃(±1℃);根据退火模型发现退火温度的改变会影响参与熟化的原子的数量,熟化原子比θ正比于退火温度,即θ∝Τ.退火温度540℃条件下退火约60 min,薄膜表面达到基本平坦,推测此时0.20θ0.25;退火温度为545℃时,推测退火时间约为55-60 min.本实验得到的结论可以为生长平坦的Al_(0.17)Ga_(0.83)As/GaAs薄膜提供理论与实验指导.     

衬底温度对Ga液滴在Al_(0.4)Ga_(0.6)As表面形成纳米结构的影响

本文研究了不同衬底温度对Ga液滴在Al_(0.4)Ga_(0.6)As表面形成纳米结构的影响,当300℃≤T≤380℃时,Ga液滴演化成纳米孔(Nanohole)和盘状结构(diffusion halo),纳米结构的尺寸随温度升高而增大.当T≥385℃时,盘状结构消失,形成一定平坦的Al_xGa_(1-x)As薄膜,Ga液滴在界面处继续向下刻蚀直至耗尽,形成平均直径为75 nm,平均孔深为5.52 nm的纳米孔.本文还通过盘状结构测出平均扩散长度△R,并拟合出Ga原子在Al_(0.4)Ga_(0.6)As表面的激活能E_A=0.78(±0.01) eV和扩散前因子D_0=0.15(×4.1~(±1))10~(-2 ) cm~2s~(-1).     

GaAs/Al_(0.23)Ga_(0.77)As双量子阱的带边不连续性和阱间耦合

本文报道在300和77K对一组具有不同垒宽Lb的GaAs/Al_(0.23)Ga_(0.77)As双量子阱样品的光调制反射谱(PR)的研究结果。除观察到11H,11L和22H等容许跃迁外,同时还识别一个从Al_(0.23)Ga_(0.77)As价带顶至量子阱第一电子束缚能级的跃迁,另一个从量子阱第一轻空穴束缚能级至Al_(0.23)Ga_(0.77)As导带底的跃迁。利用这些跃迁确定导带边不连续性为0.63。对Lb≤40实验观察到11H和11L跃迁都明显地分裂成对称和反对称分量,对分裂大小的实验值与理论计算作了比较。在77K的11H的PR谱中在高能端明显地出现一个肩形峰,其形状不能用PR标准线形来拟合。如认为它对应于带间跃迁,并由此估计激子的束缚能约为8meV。     

温度对Al_(0.5)Ga_(0.5)As/AlAs分布布喇格反射器的反射谱影响

采用光学传输矩阵理论对Al0.5Ga0.5As/AlAs材料分布布喇格反射器(DBR)进行理论研究,分析了-10℃到100℃的范围内,温度变化对不同DBR结构的反射光谱影响.结果表明:随着温度的升高,传统20周期DBR的反射光谱向长波长方向移动,速率约0.05 nm/℃,其中由线热膨胀系数带来的影响小于0.001 nm/℃.当传统DBR的周期数增大时,温度对DBR光谱反射率的影响在减小,同时DBR的反射谱峰值波长发生红移.为了降低温度对DBR反射光谱的影响,提出一种新型的复式DBR结构.分析指出:该复式DBR比传统DBR有更大的反射光谱半峰宽,基本能覆盖同温度的AlGaInP LED电致发光光谱,这对提高LED的出光效率有现实意义.     

晶格匹配In_(0.17)Al_(0.83)N/GaN异质结电容散射机制

制备了晶格匹配In_(0.17)Al_(0.83)N/GaN异质结圆形平面结构肖特基二极管,通过测试和拟合器件的电容-频率曲线,研究了电容的频率散射机制.结果表明:在频率高于200 kHz后,积累区电容随频率出现增加现象,而传统的电容模型无法解释该现象.通过考虑漏电流、界面态和串联电阻等影响对传统模型进行修正,修正后的电容频率散射模型与实验结果很好地符合,表明晶格匹配In_(0.17)Al_(0.83)N/GaN异质结电容随频率散射是漏电流、界面态和串联电阻共同作用的结果.     

InAlAs浓度对In_(0.83)Al_(0.17)As/In_(0.83)Ga_(0.17)As红外探测器特性的影响北大核心CSCD

红外探测器的性能受内部结构各层掺杂浓度的影响,而倍增层掺杂浓度会明显改变器件的性能。为了降低暗电流,提高器件性能,采用三元化合物In_(0.83)Al_(0.17)As作为倍增层材料,借助仿真软件Silvaco详细研究了In_(0.83)Al_(0.17)As/In_(0.83)Ga_(0.17)As红外探测器的倍增层掺杂浓度对器件电场强度、电流特性和光响应度的影响规律。结果表明,随着倍增层掺杂浓度的增加,器件倍增层内的电场强度峰值增加,同时,器件的暗电流与光响应度减小。进一步研究发现,当倍增层掺杂浓度为2×10^(16) cm^(−3)时,器件获得最优性能,暗电流密度为0.62144 A/cm^(2),在波长为1.5μm时,光响应度和比探测率分别为0.9544 A/W和1.9475×10^(9) cmHz^(1/2)W^(−1)。     

(GaAs)_1(AlAs)_1(001)超晶格与闪锌矿结构Ga_(0.5)Al_(0.5)As合金虚晶能带的对应性

基于线性丸盒轨道原子球近似(LMTO-ASA)数值计算,比较(GaAs)_1(AlAs)_1(001)超晶格与闪锌矿结构Ga_(0.5)Al_(0.5)As合金虚晶能带本征态,发现它们可以用Ⅲ价和Ⅴ价原子平面的分波态进行统一描述,用这种方法详细分析超晶格与闪锌矿结构合金在布里渊区Γ,M(X)和R(L)诸点主要能带本征态之间的对应关系,讨论了超晶格布里渊区能带折叠对本征态的影响。     

An improved design for Al Ga N solar-blind avalanche photodiodes with enhanced avalanche ionization

To enhance the avalanche ionization, we designed a new separate absorption and multiplication AlGaN solarblind avalanche photodiode(APD) by using a high/low-Al-content AlGaN heterostructure as the multiplication region instead of the conventional AlGaN homogeneous layer. The calculated results show that the designed APD with Al_(0.3)Ga_(0.7)N/Al_(0.45)Ga_(0.55)N heterostructure multiplication region exhibits a 60% higher gain than the conventional APD and a smaller avalanche breakdown voltage due to the use of the low-Al-content Al_(0.3)Ga_(0.7)N which has about a six times higher hole ionization coefficient than the high-Al-content Al_(0.45)Ga_(0.55)N. Meanwhile, the designed APD still remains a good solar-blind characteristic by introducing a quarter-wave AlGaN/AlN distributed Bragg reflectors structure at the bottom of the device.     

Excitonic polaron and phonon assisted photoluminescence of ZnO nanowires

The coupling strength of the radiative transition of hexagonal ZnO nanowires to the longitudinal optic (LO) phonon polarization field is deduced from temperature dependent photoluminescence spectra. An excitonic polaron formation is discussed to explain why the interaction of free excitons with LO phonons in ZnO nanowires is much stronger than that of bound excitons with LO phonons. The strong exciton-phonon coupling in ZnO nanowires affects not only the Haung-Ray S factor but also the FXA-1LO phonon energy spacing, which can be explained by the excitonic polaron formation.     

高线性度Al_xGa_(1-x)N/Al_yGa_(1-y)N/GaN高电子迁移率晶体管优化设计

对新型复合沟道AlxGa1-xN/AlyGa1-yN/GaN高电子迁移率晶体管(HEMT)进行了优化设计.从半导体能带理论与量子阱理论出发,自洽求解了器件层结构参数对器件导带能级以及二维电子气(2DEG)中载流子浓度和横向电场的影响.用TCAD软件仿真得到了器件的层结构参数对器件性能的影响.结合理论分析和仿真结果确定了器件的最佳外延层结构Al0.31Ga0.69N/Al0.04Ga0.96N/GaNHEMT.对栅长1μm,栅宽100μm的器件仿真表明,器件的最大跨导为300mS/mm,且在栅极电压-2—1V的宽范围内跨导变化很小,表明器件具有较好的线性度;器件的最大电流密度为1300mA/mm,特征频率为11.5GHz,最大振荡频率为32.5GHz.     

Characteristics of high Al content AlGaN grown by pulsed atomic layer epitaxy

Al_0.91Ga_0.09N epilayers have been obtained by pulsed atomic layer epitaxy (PALE) technique on sapphire (0 0 0 1) substrates. Deep ultraviolet (DUV) photoluminescence (PL) spectroscopy and Raman scattering spectrum have been employed to study the optical transitions in Al_0.91Ga_0.09N epilayers. We found the exciton-phonon interaction by fitting the asymmetric PL peak, in which the transverse optical phonon (TO) and the longitudinal optical (LO) phonon are the main contributor. The abnormal S-shaped temperature dependence of the PL band peak is less pronounced or has disappeared. Further analysis shows that there possibly exists a high density of deeper localized state (∼90 meV) in Al_0.91Ga_0.09N. The formation of these localized states provides a favorable condition for efficient light emission.     

Al12N、Al12B团簇结构和电子性质的密度泛函计算

利用密度泛函理论研究了Al12N和Al12B团簇的原子结构和电子性质,通过各种异构体的比较,发现两种掺杂团簇的最低能量结构都是完好的二十面体(Ih)结构,N(B)原子占据在二十面体的中心.高对称性团簇形成稀疏离散的电子态密度和大的电子能隙.在Al-N之间发生较大的电荷转移.因此我们建议把Al12N团簇看作是碱金属超原子,Al12B团簇看作是卤素超原子,用来构造团簇组装固体.     

Enhanced performance of AlGaN-based ultraviolet light-emitting diodes with linearly graded AlGaN inserting layer in electron blocking layer

The conventional stationary Al content Al GaN electron blocking layer(EBL) in ultraviolet light-emitting diode(UV LED) is optimized by employing a linearly graded Al Ga N inserting layer which is 2.0 nm Al_(0.3) Ga_(0.7) N/5.0 nm Alx Ga_(1-x) N/8.0 nm Al_(0.3) Ga_(0.7) N with decreasing value of x. The results indicate that the internal quantum efficiency is significantly improved and the efficiency droop is mitigated by using the proposed structure. These improvements are attributed to the increase of the effective barrier height for electrons and the reduction of the effective barrier height for holes,which result in an increased hole injection efficiency and a decreased electron leakage into the p-type region. In addition,the linearly graded AlGaN inserting layer can generate more holes in EBL due to the polarization-induced hole doping and a tunneling effect probably occurs to enhance the hole transportation to the active regions, which will be beneficial to the radiative recombination.     

Fang–Howard wave function modelling of electron mobility in AlInGaN/AlN/InGaN/GaN double heterostructures

To study the electron transport properties in InGaN channel-based heterostructures,a revised Fang-Howard wave function is proposed by combining the effect of GaN back barrier.Various scattering mechanisms,such as dislocation impurity(DIS) scattering,polar optical phonon(POP) scattering,piezoelectric field(PE) scattering,interface roughness(IFR) scattering,deformation potential(DP) scattering,alloy disorder(ADO) scattering from InGaN channel layer,and temperature-dependent energy bandgaps are considered in the calculation model.A contrast of AlInGaN/AlN/InGaN/GaN double heterostructure(DH) to the theoretical AlInGaN/AlN/InGaN single heterostructure(SH) is made and analyzed with a full range of barrier alloy composition.The effect of channel alloy composition on InGaN channel-based DH with technologically important Al(In,Ga)N barrier is estimated and optimal indium mole fraction is 0.04 for higher mobility in DH with Al_(0.4)In_(0.07)Ga_(0.53)N barrier.Finally,the temperature-dependent two-dimensional electron gas(2 DEG) density and mobility in InGaN channel-based DH with Al_(0.83)In_(0.13)Ga_(0.0)4 N and Al_(0.4)In_(0.07)Ga_(0.53)N barrier are investigated.Our results are expected to conduce to the practical application of InGaN channel-based heterostructures.     

Exciton-phonon coupling strength in single wall carbon nanotubes

The coupling strength of the exciton and different phonons, including the radial breathing mode (RBM), longitudinal (LO) and transverse (TO) optical phonons, are calculated for different diameter single wall carbon nanotubes (SWNTs) in the framework of tight-binding model. It is found that the exciton-phonon coupling strength with the LO mode or RBM shows a clear (2n+m)-family behavior due to the trigonal warping effect, but it with the TO mode remains to be zero. In the same SWNT, the E₂₂ exciton-phonon coupling strength is found to be slightly smaller than that of E₁₁ exciton. Finally, the exciton-RBM-phonon coupling strengths for several SWNTs are found to be in good agreement with the recent experimental data [Phys. Rev. Lett. 98, 037405 (2007)].     

The optical properties of high quality ZnO/ZnMgO quantum wells on Si(111) substrates

The ZnO/Zn_0.85Mg_0.15O multiple quantum wells (MQWs) were fabricated on Si(111) substrates by plasma-assisted molecular beam epitaxy (P-MBE) using ZnMgO as buffer layers. The RHEED images indicated that the MQWs were of high quality. The free exciton (FE) emission line originated from the well region and its phonon replicas (FE-1LO, FE-2LO and FE-3LO) were observed in the 86 K photoluminescence (PL) spectrum. Blueshift of the FE line in the MQW sample as compared to that in the ZnO bulk was found at temperatures as high as room temperature. Time-resolved PL study on the FE line showed a fast lifetime of 140 ps. The high quality of the MQW structure was revealed by the observation of the quantum-confinement-induced blueshift and shortened lifetime of the FE emission.     

Exciton-phonon coupling in indirect gap AgBr nanocrystals

The resonance Raman scattering observed in the exciton region of indirect AgBr nanocrystals with mean particle radius R ≈ 5 nm is analyzed. Considering the processes as absorption followed by emission and calculating the size dependent exciton energies by the donor-like exciton model of Ekimov et al. [1], the 1TO(L) and 2TO(L)&LO(Γ) resonance behavior can be well-reproduced. The analysis of the Raman spectral lineshape based on linear exciton-phonon coupling shows a substantial increase of coupling strength with excitation photon energy. It is attributed to enhanced acoustic and surface phonon interaction of higher exciton states.