We have demonstrated defect-induced color-tunable monolithic GaN-based vertical light-emitting diodes (VLEDs). With Ag nanorod arrays embedded in p-GaN, large numbers of Ga vacancies (VGa) were produced during the thermal bonding process in VLED fabrication. VGa-related donor–acceptor pair (DAP) transitions in p-GaN resulted in red emission in photoluminescence (PL) measurements as well as a broad electroluminescence (EL) emission spectrum extending from green to red. In combination with high-emission-efficiency blue InGaN/GaN multiple quantum wells (MQWs), the emission color of VLEDs can be changed from red to white by increasing the injection current.

http://iopscience.iop.org/article/10.7567/APEX.7.102102/pdf

Defect-induced color-tunable monolithic GaN-based light-emitting diodes

The influence of a deep-level-defect (DLD) band formed in a heavily Mg-doped GaN contact layer on the performance of Ni/Au contact to p-GaN is investigated. The thin heavily Mg-doped GaN (p(++)-GaN) contact layer with DLD band can effectively improve the performance of Ni/Au ohmic contact to p-GaN. The temperature-dependent I-V measurement shows that the variable-range hopping (VRH) transportation through the DLD band plays a dominant role in the ohmic contact. The thickness and Mg/Ga flow ratio of p(++)-GaN contact layer have a significant effect on ohmic contact by controlling the Mg impurity doping and the formation of a proper DLD band. When the thickness of the p(++)-GaN contact layer is 25 nm thick and the Mg/Ga flow rate ratio is 10.29%, an ohmic contact with low specific contact resistivity of 6.97 x 10(-4) Omega.cm(2) is achieved.

http://iopscience.iop.org/article/10.1088/1674-1056/24/9/096804/pdf

Influence of a deep-level-defect band formed in a heavily Mg-doped GaN contact layer on the Ni/Au contact to p-GaN

Thin heavily Mg-doped InGaN and GaN compound contact layer is used to form Ni/Au Ohmic contact to p-GaN. The growth conditions of the compound contact layer and its effect on the performance of Ni/Au Ohmic contact to p-GaN are investigated. It is confirmed that the specific contact resistivity can be lowered nearly two orders by optimizing the growth conditions of compound contact layer. When the flow rate ratio between Mg and Ga gas sources of p++-InGaN layer is 10.6% and the thickness of p++-InGaN layer is 3 nm, the lowest specific contact resistivity of 3.98 × 10−5 Ωcm2 is achieved. In addition, the experimental results indicate that the specific contact resistivity can be further lowered to 1.07 × 10−7 Ωcm2 by optimizing the alloying annealing temperature to 520 °C.

Low contact resistivity between Ni/Au and p-GaN through thin heavily Mg-doped p-GaN and p-InGaN compound contact layer*

Residual stress conditions in GaN-based LEDs will have a significant influence on device performance and reliability. In this paper, GaN-based vertical LEDs under different stress conditions are fabricated by bonding with three types of submounts (Al2O3 submount, CuW submount and Si submount), changing the soak temperature (290 ℃, 320 ℃, 350 ℃ and 380 ℃) and using different laser energy densities (875, 945 and 1015 mJ·cm-2). The warpage and Raman scattering spectra of those GaN-based LEDs are measured. The experimental results show that the residual stress conditions in GaN-based vertical LEDs are a consequence of the bonded submounts and bonded metal, and the soak temperature is the primary factor that determines the degree of residual stress in LED chips. In the laser lift-off process, changing laser energy density in an appropriate range has little influence on residual strain of LED chips, and the micro-cracks in GaN layer caused by LLO process will play a role in releasing the residual stress. The warpage of epitaxial sapphire substrate becomes large after boding with Si submount, the residual stress in GaN-based vertical LEDs is tensile stress and becomes larger with the soak temperature rising. When GaN epi wafer bonds with Al2O3 submount and CuW submount, the warpages becomes small and large respectively and the residual stress in chips is compressive stress. Because of the mismatch of coefficient of thermal expansion, the compressive stress in GaN-based LED chips increases for Al2O3 submount and drops for CuW submount with the soak temperature rising.

Effect of wafer bonding and laser liftoff process on residual stress of GaN-based vertical light emitting diode chips

研究了Ag的厚度、退火时间、沉积温度对于Ni/Ag/Ti/Au电极的反射率及与p-GaN欧姆接触性能的影响. 利用分光光度计测量反射率, 采用圆形传输线模型计算比接触电阻率. 结果表明: 随着Ag厚度的增加, Ni/Ag/Ti/Au电极的反射率逐渐增大; 在氧气氛围中, 随着退火时间从1 min增至10 min, 300 ℃退火时, 比接触电阻率持续下降, 而对于400-600 ℃退火, 比接触电阻率先减小后增大; 在300和400 ℃氧气中进行1-10 min 的退火后, Ni/Ag/Ti/Au的反射率变化较小, 退火温度高于400 ℃时, 随着退火时间的增加, 反射率急剧下降; 在400 ℃氧气中3 min退火后, 比接触电阻率可以达到3.6×10-3 Ω·cm2. 此外, 适当提高沉积温度可以增加Ni/Ag/Ti/Au的反射率并降低比接触电阻率, 沉积温度为120 ℃条件下的Ni/Ag/Ti/Au电极在450 nm处反射率达到90.1%, 比接触电阻率为6.4×10-3 Ω·cm2. 综合考虑电学和光学性能, 在沉积温度为120 ℃下蒸镀Ni/Ag/Ti/Au (1/200/100/100 nm)并在400 ℃氧气中进行3 min退火可以得到较优化的电极. 利用此电极制作的垂直结构发光二极管在350 mA电流下的工作电压为2.95 V, 输出光功率为387.1 mW, 电光转换效率达到37.5%.

低接触电阻率Ni/Ag/Ti/Au反射镜电极的研究

研究了不同Ni厚度的Ni/Ag/Ti/Au电极在不同退火温度和退火气氛下与p-GaN之间的欧姆接触性能以及电极的光反射率的变化. 采用矩形传输线模型对各电极的比接触电阻率进行测算，利用分光光度计对电极在不同波长下的反射率进行测量. 结果表明，Ni金属层的厚度越小，电极的光反射率越高，而Ni层厚度对比接触电阻率的影响较小；当退火温度高于400 ℃后，电极的光反射率降低，在氧气氛围中退火后光反射率比在氮气中退火后下降更加明显. 但在氧气氛围中退火有利于减小比接触电阻率. 综合考虑接触电阻和光反射率，电极Ni（1 nm）/Ag/Ti/Au在400 ℃氧气中快速退火后得到了较好的结果，其比接触电阻率为5.5×10-3 Ω·cm2，在450 nm 处反射率为85%. 利用此电极制作了垂直结构发光二极管（LED）器件，LED在350 mA注入电流下，工作电压为3.2 V，发光功率为270 mW，电光转换效率达到24%.

Ni/Ag/Ti/Au与p-GaN的欧姆接触性能及光反射率

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Ni/Ag/Ti/Au与p-GaN的欧姆接触性能及光反射率