GaN-based violet laser diodes grown on free-standing GaN substrate

A violet laser diode (LD) structure is grown on a free-standing c-plane GaN substrate and 4~μ m× 800~μ m ridge waveguide LDs are fabricated. The electrical and the optical characteristics of LDs under different facet-coating and chip-mounting conditions are investigated under pulse mode operation. The active region temperatures of p-side up and p-side down mounted LDs are calculated with different injection currents. The calculated thermal resistances of p-side up and p-side down mounted LDs are 4.6~K/W and 3~K/W, respectively. The threshold current of the p-side down mounted LD is much lower than that of the p-side up mounted LD. The blue shift of the emission wavelength with increasing injection current is observed only for the LD with p-side down mounting configuration, due to the more efficient heat dissipation.     

InGaN/GaN量子阱垒层和阱层厚度对GaN基激光器性能的影响及机理

采用LASTIP软件研究了InGaN/GaN(In组分为15%)量子阱垒层和阱层厚度对GaN基蓝紫光激光器性能的影响及机理. 模拟计算结果表明, 当阱层太薄或太厚时, GaN基激光器的阈值电流增加、输出功率下降, 最优的阱层厚度为4.0 nm左右; 当阱层厚度太薄时, 载流子很容易泄漏, 而当阱层厚度太厚时, 极化效应导致发光效率降低, 研究还发现, 与垒层厚度为7 nm 相比, 垒层厚度为15 nm时激光器的阈值电流更低、输出功率更高, 因此适当地增加垒层厚度能显著抑制载流子泄漏, 从而改善激光器性能.     

激光诱导p-GaN掺杂对发光二极管性能改善的分析

采用激光诱导掺锌的方法提高了常规GaN基外延片p-GaN层的空穴浓度,并将它制备成小功率白光发光二极管(LED).对其光电性能做了详细的测量并进行了加速老化实验和分析.结果表明,与常规LED相比,经过激光诱导p-GaN层掺锌LED的光电性能获得了明显改善:正向工作电压VF从3.33V降到3.13V,串联电阻从30.27Ω降到20.27Ω,室温下衰退系数从1.68×10-4降到1.34×10-4,老化1600h后的反向漏电流从超过0.2μA降为不超过0.025μA,器件的预测寿命延长了41%.器件光电性能改善的主要原因是激光诱导掺锌使LED的p-型欧姆接触改善和热阻降低所致.     

Breakdown characteristics of AlGaN/GaN Schottky barrier diodes fabricated on a silicon substrate

In this work, the breakdown characteristics of AlGaN/GaN planar Schottky barrier diodes(SBDs) fabricated on the silicon substrate are investigated. The breakdown voltage(BV) of the SBDs first increases as a function of the anodeto-cathode distance and then tends to saturate at larger inter-electrode spacing. The saturation behavior of the BV is likely caused by the vertical breakdown through the intrinsic GaN buffer layer on silicon, which is supported by the postbreakdown primary leakage path analysis with the emission microscopy. Surface passivation and field plate termination are found effective to suppress the leakage current and enhance the BV of the SBDs. A high BV of 601 V is obtained with a low on-resistance of 3.15 mΩ·cm2.     

Breakdown characteristics of AIGaN/GaN Schottky barrier diodes fabricated on a silicon substrate

In this work, the breakdown characteristics of AlGaN/GaN planar Schottky barrier diodes （SBDs） fabricated on the silicon substrate are investigated. The breakdown voltage （BV） of the SBDs first increases as a function of the anodeto-cathode distance and then tends to saturate at larger inter-electrode spacing. The saturation behavior of the BV is likely caused by the vertical breakdown through the intrinsic GaN buffer layer on silicon, which is supported by the post-breakdown primary leakage path analysis with the emission microscopy. Surface passivation and field plate termination are found effective to suppress the leakage current and enhance the BV of the SBDs. A high BV of 601 V is obtained with a low on-resistance of 3.15 mΩ·cm^2.     

Droop improvement in blue InGaN light-emitting diodes with GaN/InGaN superlattice barriers

GaN/InGaN superlattice barriers are used in InGaN-based light-emitting diodes （LEDs）. The electrostatic field in the quantum wells, electron hole wavefunction overlap, carrier concentration, spontaneous emission spectrum, light-current performance curve, and internal quantum efficiency are numerically investigated using the APSYS simulation software. It is found that the structure with GaN/InGaN superlattice barriers shows improved light output power, and lower current leakage and efficiency droop. According to our numerical simulation and analysis, these improvements in the electrical and optical characteristics are mainly attributed to the alleviation of the electrostatic field in the active region.     

Proposals for a standard for the safe use of light-emitting diodes, laser diodes, and optical fibre transmission systems

A new safety standard for light emitting diodes, laser diodes and fibre optical transmission systems is needed because of the difficulties in applying existing standards to these products. New proposals, generated by a BSI working group, include a new classification system which takes into account the inherent beam divergence of most of these products, and the need to classify at different locations within an extended optical system such as a fibre-optic link.     

Chirp reduction in three-cavity semiconductor laser diodes

The oscillation frequency shift ratio (OFSR) of a three-cavity coupled laser diode, which is caused by refractive index changes due to current modulation and temperature variation, has been investigated. It is found that the OFSR is significantly reduced by using a laser with an AR coated facet and a high external-mirror reflectivity, in addition to large external cavity lengths. Also, compared with an external cavity laser, the three-cavity laser has a smaller OFSR provided that the external cavities' reflectivities r₀ and r₃, are not identical (that is, r₃ > r₀). The analysis indicates that the structural parameters affect the OFSR of the three-cavity laser, which was shown to have a high longitudinal mode discrimination¹.     

A polariton laser based on a bulk GaN microcavity

Simulations of polariton relaxation in a bulk GaN microcavity are performed by solving semi-classical Boltzmann equations. The structure which is considered is that of a recently grown microcavity showing strong coupling at 300 K. A kinetic phase diagram is calculated. We show that such a cavity can operate as a low-threshold polariton laser at room temperature.     

GaN薄膜的热导率模型研究

准确预测GaN半导体材料的热导率对GaN基功率电子器件的热设计具有重要意义.本文基于第一性原理计算和经典Debye-Callaway模型,通过分析和完善Debye-Callaway模型中关于声子散射率的子模型,建立了用于预测温度、同位素、点缺陷、位错、薄膜厚度、应力等因素影响的GaN薄膜热导率的理论模型.具体来说,对声...     

Instability of the steady-state regime of generation in laser diodes with a broad gain spectrum

The laser-diode parameters at which the steady-state regime of generation becomes unstable are analyzed within the framework of the mode-locking model. The crucial role of the transverse inhomogeneity of the field, pumping intensity, and spectrum width in developing the instabilities of the steady-state regime of generation is demonstrated. The calculated values of the instability threshold are shown to be consistent with the experimental results. Talk presented at the oral issue of J. Russ. Laser Res. dedicated to the memory of Professor Vladimir A. Isakov, Professor Alexander S. Shumovsky, and Professor Andrei V. Vinogradov held in Moscow February 21–22, 2008.     

GaN基蓝光半导体激光器的发展

文章介绍了下一代光存储用半导体激光器——GaN蓝光激光器的发展状况．对衬底材料的发展现状、外延片的生长技术以及激光器的制作工艺都作了论述．阐明了GaN激光器的一些技术路线，如GaN同质生长衬底的发展，侧向外延生长技术的采用以及湿法腐蚀腔面等．另外还介绍了GaN半导体激光器数字多功能光盘(DVD)的实用化进程．     

Effects of a prestrained InGaN interlayer on the emission properties of InGaN/GaN multiple quantum wells in a laser diode structure

The electroluminescence (EL) and photoluminescence (PL) spectra of InGaN/GaN multiple quantum wells (MQWs) with a prestrained InGaN interlayer in a laser diode structure are investigated. When the injection current increases from 5 mA to 50 mA, the blueshift of the EL emission peak is 1 meV for the prestrained sample and 23 meV for a control sample with the conventional structure. Also, the internal quantum efficiency and the EL intensity at the injection current of 20 mA are increased by 71% and 65% respectively by inserting the prestrained InGaN interlayer. The reduced blueshift and the enhanced emission are attributed mainly to the reduced quantum-confined Stark effect (QCSE) in the prestrained sample. Such attributions are supported by the theoretical simulation results, which reveal the smaller piezoelectric field and the enhanced overlap of electron and hole wave functions in the prestrained sample. Therefore, the prestrained InGaN interlayer contributes to strain relaxation in the MQW layer and enhancement of light emission due to the reduction of QCSE.     

Gamma-ray irradiation effects on distributed-feedback laser diodes

Since laser diodes are increasingly used in harsh environments, the effect of irradiation on their performance attracts a lot of attention. We perform experiments for investigating the irradiation effects on laser diodes with distributed feedback operating at 1550 nm output wavelength with 2 mW output power. The radiation source is Co⁶⁰ gamma ray with a dose rate of 0.5 Gy/s and the dose range within 10² – 8 ∙ 10³ Gy. We study experimentally the threshold current, slope efficiency, and spectrum versus the variations in total dose. The results show that the threshold current increases exponentially and the slope efficiency decreases with increase in total dose. In addition, some sharp peaks appear in the spectrum at small driving current, and the spectrum broadens when the driving current increases and, meanwhile, the peak blue shift is observed. The spectrum can be recovered after annealing for 12 hours and when a greater driving current is applied.     

Optically pumped GaN/AlGaN quantum well intersubband terahertz laser

We propose an optically pumped nonpolar GaN/AlGaN quantum well (QW) active region design for terahertz (THz) lasing in the wavelength range of 30 μm～ 40 μm and operating at room temperature. The fast longitudinal optical (LO) phonon scattering in GaN/AlGaN QWs is used to depopulate the lower laser state, and more importantly, the large LO phonon energy is utilized to reduce the thermal population of the lasing states at high temperatures. The influences of temperature and pump intensity on gain and electron densities are investigated. Based on our simulations, we predict that with a sufficiently high pump intensity, a room temperature operated THz laser using a nonpolar GaN/AlGaN structure is realizable.     

Simulation of current spreading for GaN-based light-emitting diodes

Uniform current spreading is crucial for the performance of light-emitting diodes (LEDs). It has been reported that the reliability and light distribution are affected by non-uniform current spreading. In this paper, the impact of different electrode patterns on the performance of LED chips is investigated. A hybrid modeling method is employed to analyze the electrical and thermal characteristics of LEDs with two different electrode mesa structures. Corresponding experiments are also carried out to validate the calculation results. It is found that increasing amount of p-electrodes in interdigitated electrode patterns is effective in improving the performance of LEDs.     

Si(110)和Si(111)衬底上制备InGaN/GaN蓝光发光二极管

分别在Si(110)和Si(111)衬底上制备了In Ga N/Ga N多量子阱结构蓝光发光二极管(LED)器件.利用高分辨X射线衍射、原子力显微镜、室温拉曼光谱和变温光致发光谱对生长的LED结构进行了结构表征.结果表明,相对于Si(111)上生长LED样品,Si(110)上生长的LED结构晶体质量较好,样品中存在较小的张应力,具有较高的内量子效率.对制备的LED芯片进行光电特性分析测试表明,两种衬底上制备的LED芯片等效串联电阻相差不大,在大电流注入下内量子效率下降较小;但是,相比于Si(111)上制备LED芯片,Si(110)上LED芯片具有较小的开启电压和更优异的发光特性.对LED器件电致发光(EL)发光峰随驱动电流的变化研究发现,由于Si(110)衬底上LED结构中阱层和垒层存在较小的应力/应变而在器件中产生较弱的量子限制斯塔克效应,致使Si(110)上LED芯片EL发光峰随驱动电流的蓝移量更小.     

Determination of the series resistance under the Schottky contacts of AlGaN/AlN/GaN Schottky barrier diodes

Rectangular AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) were fabricated,and the gate and the source of the HFETs consisted of AlGaN/AlN/GaN Schottky barrier diodes (SBDs).Based on the measured forward current-voltage and the capacitance-voltage characteristics of the AlGaN/AlN/GaN SBDs,the series resistance under the Schottky contacts (RS) was calculated using the method of power consumption,which has been proved to be valid.Finally,the method of power consumption for calculating R S was successfully used to study the two-dimensional electron gas electron mobility for a series of circular AlGaN/AlN/GaN SBDs.It is shown that the series resistance under the Schottky contacts cannot be neglected and is important for analysing and characterizing the AlGaN/AlN/GaN SBDs and the AlGaN/AlN/GaN HFETs.     

An optically pumped GaN/AlGaN quantum well intersubband terahertz laser

We propose an optically pumped nonpolar GaN/AlGaN quantum well(QW) active region design for terahertz(THz) lasing in the wavelength range of 30 μm～ 40 μm and operating at room temperature.The fast longitudinal optical(LO) phonon scattering in GaN/AlGaN QWs is used to depopulate the lower laser state,and more importantly,the large LO phonon energy is utilized to reduce the thermal population of the lasing states at high temperatures.The influences of temperature and pump intensity on gain and electron densities are investigated.Based on our simulations,we predict that with a sufficiently high pump intensity,a room temperature operated THz laser using a nonpolar GaN/AlGaN structure is realizable.