Monte Carlo Modeling of Phonon-assisted Carrier Transport in Cubic and Hexagonal Gallium Nitride

Monte Carlo method is employed for the calculations of electron and hole transport characteristics of cubic and hexagonal GaN at T = 300 K in the fields of E ≤ 1000 kV/cm⁻¹. It is shown that electron drift velocity and mobility is heavily reduced in hexagonal crystals due to additional phonon modes (~ 26 meV) and by fast electron scattering between the lowest Γ₁ valley and the minimally (~ 400 meV) up-shifted Γ₃ valley. Intervalley scattering is mediated most efficiently by the low-energy (~ 2 meV) acoustic phonons. The randomizing scattering is even more pronounced in p-type crystals where the sub-bands of light and heavy holes merge at the Γ-point of Brillouin zone. Cubic phase crystals are concluded to be advantageous for ultrafast electronic and photonics device performance because electron drift mobility is higher by an order of magnitude, and the hole mobility is several times higher than those in hexagonal phase.     

GaN和GaN:Mg外延膜低温拉曼谱的对比研究

对液氮温度下六方相GaN和掺Mg的P型GaN薄膜的拉曼谱进行了对比研究。除对两个样品中主晶格振动模进行了对比分析外,着重讨论了位于247 cm-1的散射峰的产生机制。结果表明GaN:Mg的谱中该峰的散射强度随温度升高先增大再减小,在500K以上消失且对样品重新降温到78K观察此峰不再出现,因此认为它是缺陷产生的振动模。而GaN样品中经同样加热降温的过程此峰仍然存在,说明两个样品中该峰的产生机制不同。此外,在GaN:Mg的谱中还观察到Mg诱导的局域振动模。     

Spectral measurement of the caesium D2 line with a tunable heterodyne interferometer

The optical properties of a caesium atomic beam driven on a resonant hyperfine transition in the D₂ line were studied as a function of the probe laser frequency. Using a third off-resonant laser system, a heterodyne interferometer allowed simultaneous absorption and phase shift measurements of either the probe or the coupling laser. The signal features of the probe and coupling laser transmitted intensities showed strong differences in the vicinity of the hyperfine transitions excited by the probe laser. Regular absorption signals and electromagnetically induced transparency were found in either transmitted intensities. Furthermore, light induced birefringence of the probe laser was measured.     

Mg+注入对GaN晶体辐射损伤的研究

在蓝宝石衬底上通过金属有机物化学气相沉积(metal-organic chemical vapor deposition,MOCVD)方法外延生长的GaN薄膜具有良好的结晶品质,xmin达到2.00%.结合卢瑟福背散射/沟道(Rutherford backscattering/channeling,RBS/C)和高分辨X射线衍射(high-resolution X-ray diffraction,HXRD)的实验测量,研究了不同剂量和不同角度Mg+注入GaN所造成的辐射损伤.实验结果表明,随注入剂量的增大,晶体的辐射损伤也增大,注入剂量在1×1015atom/cm2以下,xmix小于4.78%,1×1016atom/cm2是Mg+注入GaN的剂量阈值,超过这个阈值,结晶品质急剧变差,xmin达到29.5%;随机注入比沟道注入的辐射损伤大,且在一定范围内随注入角度的增大,损伤也增大,在4×1015atom/cm2剂量下偏离〈0001〉沟道0°,4°,6°,9°时的xmin(%)分别为6.28,8.46,10.06,10.85;经过700℃/10min+1050℃/20s两步退火和1000℃/30s高温快速退火后,晶体的辐射损伤都有一定程度的恢复,而且1000℃/30s高温快速退火的效果更好,晶体的辐射损伤可以得到更好的恢复.     

CW mode locked Nd:YVO4 laser pumped by 20-W laser diode bar

The efficient cw mode locking (cw-ML) regime was demonstrated in Nd:YVO₄ laser by means of saturable absorber mirror (SAM). The 0.3-at.% Nd³⁺ doped 10-mm-long YVO₄ crystal end pumped by 20-W diode module with a beam shaper was applied as a gain medium located in the close vicinity to the rear flat mirror of the first arm of Z-type resonator of 316 cm total length with two curved mirrors of 100-cm curvature radii. The SAM of 2%-saturable absorptance and saturation fluence of 50 μJ/cm² was mounted at the opposite end of a resonator. The developed “dynamically stable” cavity design mitigates detrimental role of thermal aberration in gain medium, enforcing clean perfect mode locking even for the highest pump densities. The cw-ML pulses with 47.5 MHz repetition rate and pulse durations in the range of 15–20 ps were observed for a wide range of pump powers and output coupler losses. In the best case, for 32% of output coupler transmission, up to 6.2 W of average power with near 35% slope efficiency was achieved. The thresholds for Q-switched ML, cw-ML regimes were 2.67 W and 6.13 W of pump power, respectively. For the maximum pump power of 20 W we obtained 133 nJ of pulse energy with 16-ps pulse duration, resulting in a peak power higher than 8 kW. The threshold energy density at SAM giving the QML regime was estimated to be about 30 μJ/cm², threshold of cw-ML regime was 220 μJ/cm².     

Synthesis and light-emitting properties of 2-(N-phenyl-α-naphthylamino) and 2-dimesitylboron-7-(N-phenyl-α-naphthylamino)-9,9-diethylfluorene

A fluorescent organic triarylamine with a symmetric structure, 2,7-bis(N-α-naphthyl-phenylamino)-9,9- diethylfluorene (NPAEF) was synthesized using two methods, modified Ullmann coupling and modified palladium-catalyzed amination. An activated copper and a combination of Pd(OAc)2/P(t-Bu)3 and Pd(dba)2/P(t-Bu)3 were selected as catalysts to improve yields of reactions. These synthetic procedures were also successfully applied to an asymmetric 2-dimesitylboron-7-(N-phenyl-α-naphthylamino)-9,9-diethylfluorene ...     

半导体激光器的电流调制特性研究

本介绍了半导体激光器注入电流调制特性，研究了半导体激光器的输出光频与注入电流的关系，并给出了其测量结果。     

Observation of the transition from diffusive regime to ballistic regime of the 2DEG transport property in AlxGa1−xN/GaN heterostructures

Electron–electron interaction effect of the two-dimensional electron gas (2DEG) in Al_xGa_1−xN/GaN heterostructures has been investigated by means of magnetotransport measurements at low temperatures. From the temperature dependence of the longitudinal conductivity of the heterostructures, a clear transition region has been observed. Based on the theoretical analysis, we conclude that this region corresponds to the transition from the diffusive regime to the ballistic regime of the 2DEG transport property. The interaction constant is determined to be −0.423, which is consistent with the theoretical prediction. However, the critical temperature for the transition, which is 8 K in Al_xGa_1−xN/GaN heterostructures, is much higher than the theoretical prediction.     

Activation of Mg-doped P-GaN by using two-step annealing

One- and two-step rapid thermal annealing (RTA) for activating Mg-doped p-type GaN films had been performed to compare with conventional furnace annealing (CFA). The two-step annealing process consists of two annealing steps: the first step is performed at 750 °C for 1 min and the second step is performed at 600 °C for 5 min in pure O₂ or air ambient. It is found that the samples annealed in air ambient exhibit poor electrical properties as compared to those annealed in pure O₂. Compared to one-step RTA annealing and CFA annealing, the samples with two-step annealing exhibit higher hole concentration and lower resistivity. This means that the two-step annealing is a powerful method to enhance the electrical performance of Mg-doped p-type GaN films. Similar results were also evidenced by photoluminescence (PL) measurement. Possible mechanism was confirmed by secondary ion mass spectrometry analysis.     

Control of nucleation site density of GaN nanowires

The control of nucleation site size and density for Au catalyst-driven growth of GaN nanowires is reported. By using initial Au film thicknesses of 15-50 Å we have shown that annealing between 300 and 900 °C creates Au cluster size in the range 30-100 nm diameter with a cluster density from 300 to 3500 μm⁻².Conventional optical lithography to create parallel Au stripes shoes that a minimum separation of ∼15 μm is needed to avoid overlap of wires onto neighboring lines with our growth conditions that yield wires of this same length. The GaN nanowires exhibit strong band-edge photoluminescence and total resistances of 1.2 × 10⁸-5.5 × 10⁶ Ω in the temperature range from 240 to 400 K, as determined for the temperature-dependent current-voltage characteristics.