Nitride-based green light-emitting diodes with high temperature GaN barrier layers

High-quality InGaN-GaN multiquantum well (MQW) light-emitting diode (LED) structures were prepared by temperature ramping method during metalorganic chemical vapor deposition (MOCVD) growth. It was found that we could reduce the 20-mA forward voltage and increase the output intensity of the nitride-based green LEDs by increasing the growth temperature of GaN barrier layers from 700/spl deg/C to 950/spl deg/C. The 20-mA output power and maximum output power of the nitride-based green LEDs with high temperature GaN barrier layers was found to be 2.2 and 8.9 mW, respectively, which were more than 65% larger than those observed from conventional InGaN-GaN green LEDs. Such an observation could be attributed to the improved crystal quality of GaN barrier layers. The reliability of these LEDs was also found to be reasonably good.     

Influence of Si-doping on the characteristics of InGaN-GaN multiplequantum-well blue light emitting diodes

A detailed study on the effects of Si-doping in the GaN barrier layers of InGaN-GaN multiquantum well (MQW) light-emitting diodes (LEDs) has been performed. Compared with unintentionally doped samples, X-ray diffraction results indicate that Si-doping in barrier layers can improve the crystal and interfacial qualities of the InGaN-GaN MQW LEDs. It was also found that the forward voltage is 3.5 and 4.52 V, the 20-mA luminous intensity is 36.1 and 25.1 mcd for LEDs with a Si-doped barrier and an unintentionally doped barrier, respectively. These results suggests that one can significantly improve the performance of InGaN-GaN MQW LEDs by introducing Si doping in the GaN barrier layers     

Influence of GaN material characteristics on device performance for blue and ultraviolet light-emitting diodes

An analysis of blue and near-ultraviolet (UV) light-emitting diodes (LEDs) and material structures explores the dependence of device performance on material properties as measured by various analytical techniques. The method used for reducing dislocations in the epitaxial III-N films that is explored here is homoepitaxial growth on commercial hybride vapor-phase epitaxy (HVPE) GaN substrates. Blue and UV LED devices are demonstrated to offer superior performance when grown on GaN substrates as compared to the more conventional sapphire substrate. In particular, the optical analysis of the near-UV LEDs on GaN versus ones on sapphire show substantially higher light output over the entire current-injection regime and twice the internal quantum efficiency at low forward current. As the wavelength is further decreased to the deep-UV, the performance improvement of the homoepitaxially grown structure as compared to that grown on sapphire is enhanced.     

Influence of growth temperature and growth rate of p-GaN layers on the characteristics of green light emitting diodes

We investigated the electrical and structural qualities of Mg-doped p-type GaN layers grown under different growth conditions by metalorganic chemical vapor deposition (MOCVD). Lower 300 K free-hole concentrations and rough surfaces were observed by reducing the growth temperature from 1,040°C to 930°C. The hole concentration, mobility, and electrical resistivity were improved slightly for Mg-doped GaN layers grown at 930°C with a lower growth rate, and also an improved surface morphology was observed. In_0.25Ga_0.75N/GaN multiple-quantum-well light emitting diodes (LEDs) with p-GaN layers grown under different conditions were also studied. It was found from photoluminescence studies that the optical and structural properties of the multiple quantum wells in the LED structure were improved by reducing the growth temperature of the p-layer due to a reduced detrimental thermal annealing effect of the active region during the GaN:Mg p-layer growth. No significant difference in the photoluminescence intensity depending on the growth time of the p-GaN layer was observed. However, it was also found that the electroluminescence (EL) intensity was higher for LEDs having p-GaN layers with a lower growth rate. Further improvement of the p-GaN layer crystalline and structural quality may be required for the optimization of the EL properties of long-wavelength (∼540 nm) green LEDs.     

Improvement of near-ultraviolet InGaN-GaN light-emitting diodes with an AlGaN electron-blocking layer grown at low temperature

The 400-nm near-ultraviolet InGaN-GaN multiple quantum well light-emitting diodes (LEDs) with Mg-doped AlGaN electron-blocking (EB) layers of various configurations and grown under various conditions, were grown on sapphire substrates by metal-organic vapor phase epitaxy system. LEDs with AlGaN EB layers grown at low temperature (LT) were found more effectively to prevent electron overflow than conventional LEDs with an AlGaN one grown at high temperature (HT). The electroluminescent intensity of LEDs with an LT-grown AlGaN layer was nearly three times greater than that of LEDs with an HT-grown AlGaN. Additionally, the LEDs with an LT-grown AlGaN layer in H/sub 2/ ambient were found to increase the leakage current by three orders of magnitude and reduce the efficiency of emission.     

Improved Light Output of Nitride-Based Light-Emitting Diodes by Lattice-Matched AlInN Cladding Structure

We report the growth of AlInN nearly lattice-matched to GaN using metal-organic vapor phase epitaxy. The full-width at half-maximum of the AlInN peak measured by double crystal X-ray diffraction was 219.8 arcsec for the indium content of 20.8%. The effects of AlInN cladding layers on InGaN-GaN multiple-quantum-well light-emitting diodes (LEDs) were also investigated. From the room-temperature photoluminescence spectra, the shorter emission wavelength and the higher intensity were observed after employing AlInN cladding layers. Compared to the conventional LED, the light output intensity of the LED with AlInN cladding layers was increased due to the enhanced carrier confinement. Besides, we found the light output intensity could be saturated at higher injection current. Although the electrical property of the LED with AlInN cladding layers was slightly degraded, the experimental results in this study could explain the potential applicability of AlInN to the fabrication of cladding layers.     

Effect of Thickness of the p-AlGaN Electron Blocking Layer on the Improvement of ESD Characteristics in GaN-Based LEDs

The following letter presents a study regarding GaN-based light-emitting diodes (LEDs) with p-type AlGaN electron blocking layers (EBLs) of different thicknesses. The study revealed that the LEDs could endure higher electrostatic discharge (ESD) levels as the thickness of the AlGaN EBL increased. The observed improvement in the ESD endurance ability could be attributed to the fact that the thickened p-AlGaN EBL may partly fill the dislocation-related pits that occur on the surface of the InGaN-GaN multiple-quantum well (MQW) and that are due to the strain and the low-temperature-growth process. If these dislocation-related pits are not partly suppressed, they will eventually result in numerous surface pits associated with threading dislocations that intersect the InGaN-GaN (MQW), thereby reducing the ESD endurance ability. The results of the experiment show that the ESD endurance voltages could increase from 1500 to 6000 V when the thickness of the p-AlGaN EBL in the GaN LEDs is increased from 32.5 to 130 nm, while the forward voltages and light output powers remained almost the same.     

P 型氮化镓层生长压力对InGaN/GaN基发光二极管性能的影响

The advantages of In Ga N/Ga N light emitting diodes(LEDs) with p-Ga N grown under high pressures are studied.It is shown that the high growth pressure could lead to better electronic properties of p-Ga N layers due to the eliminated compensation effect.The contact resistivity of p-Ga N layers are decreased due to the reduced donor-like defects on the p-Ga N surface.The leakage current is also reduced,which may be induced by the better filling of V-defects with p-Ga N layers grown under high pressures.The LED efficiency thus could be enhanced with high pressure grown p-Ga N layers.     

Enhanced output power in an InGaN-GaN multiquantum-welllight-emitting diode with an InGaN current-spreading layer

InGaN-GaN multiple quantum-well (MQW) light-emitting diodes (LEDs) with InGaN current-spreading layer were grown by metal-organic vapor-phase epitaxy (MOVPE) and their characteristics were evaluated by current-voltage (I-V), as well as output power measurements. Experimental results indicate that the LEDs exhibited a higher output power and a lower operation voltage than that of conventional LEDs. The external quantum efficiency of InGaN-GaN MQW LEDs for bare chips operated at injection current of 20 mA with InGaN current spreading layer near 5%. This is two times higher than that of conventional LEDs. This could be tentatively attributed to the better current-spreading effect resulting from Si-doped In_0.18Ga_0.82N wide potential well in which electron states are not quantized     

InGaN–GaN MQW Metal–Semiconductor–Metal Photodiodes With Semi-Insulating Mg-Doped GaN Cap Layers

InGaN-GaN multiple-quantum-well metal-semiconductor-metal photodiodes (PDs) with in situ grown 40-nm-thick unactivated semi-insulating Mg-doped GaN cap layer were successfully fabricated. The dark leakage current of this PD was comparably much smaller than that of conventional PD without the semi-insulating layer, because of a thicker and higher potential barrier of semi-insulating cap layer, and also a smaller number of surface states involved. For the PDs with the semi-insulating Mg-doped GaN cap layers, the responsivity at 380nm was 0.372A/W when biasing at 5 V. In short, incorporating a semi-insulating Mg-doped GaN cap layer into the PDs beneficially leads to the suppression of dark current and a corresponding improvement in the ultraviolet-to-visible rejection ratio     

Nitride-based LEDs with 800/spl deg/C grown p-AlInGaN-GaN double-cap layers

GaN-based light-emitting diodes (LEDs) with various p-cap layers were prepared. It was found that surface morphologies of the LEDs with 800/spl deg/C grown cap layers were rough due to the low lateral growth rate of GaN. It was also found that 20-mA forward voltage of the LED with 800/spl deg/C grown p-AlInGaN-GaN double-cap layer was only 3.05 V. Furthermore, it was found that we could achieve a high output power and a long lifetime by using the 800/spl deg/C grown p-AlInGaN-GaN double-cap layer.     

Characteristics of Green Light-Emitting Diodes Using an InGaN:Mg/GaN:Mg Superlattice as p-Type Hole Injection and Contact Layers

Mg-doped InGaN/GaN p-type short-period superlattices (SPSLs) are developed for hole injection and contact layers of green light-emitting diodes (LEDs). V-defect-related pits, which are commonly found in an InGaN bulk layer, can be eliminated in an InGaN/GaN superlattice with thickness and average composition comparable to those of the bulk InGaN layer. Mg-doped InGaN/GaN SPSLs show significantly improved electrical properties with resistivity as low as ∼0.35 ohm-cm, which is lower than that of GaN:Mg and InGaN:Mg bulk layers grown under optimized growth conditions. Green LEDs employing Mg-doped InGaN/GaN SPSLs for hole injection and contact layers have significantly lower reverse leakage current, which is considered to be attributed to improved surface morphology. The peak electroluminescence intensity of LEDs with a SPSL is compared to that with InGaN:Mg bulk hole injection and contact layers.     

Growth of AlGaN epitaxial layers and AlGaN/GaN superlattices by metal-organic chemical vapor deposition

Special features of metal-organic chemical vapor deposition of AlGaN epitaxial layers and AlGaN/GaN superlattices either in an Epiquip VP-50 RP research and development reactor (for a single wafer 2 in. in diameter) or in an AIX2000HT production-scale reactor (for up to six wafers 2 in. in diameter) are stud-ied. It is found that the dependence of the aluminum content in the solid phase on the trimethylaluminum (TMA) flux in a reactor levels off; this effect hinders the growth of the layers with a high aluminum content in both types of reactors and is more pronounced in the larger reactor (AIX2000HT). Presumably, this effect is a consequence of spurious reactions in the vapor phase and depends on the partial pressure of TMA in the reactor. The aluminum content in the layers can be increased not only by reducing the total pressure in the reactor but also by increasing the total gas flow through the reactor and reducing the trimethylgallium flux. The approaches described above were used to grow layers with a mole fraction of AlN as large as 20% in the AIX2000HT production-scale reactor at a pressure of 400 mbar (this fraction was as large as 40% at 200 mbar). AlGaN layers with the entire range of composition were grown in the Epiquip VP-50 RP reactor.     

Inserting a low-temperature n-GaN underlying layer to separate nonradiative recombination centers improves the luminescence efficiency of blue InGaN/GaN LEDs

We have investigated the effects of nonradiative recombination centers (NRCs) on the device performance of InGaN/GaN multi-quantum-well (MQW) light-emitting diodes (LEDs) inserting low-temperature n-GaN (LT-GaN) underlying layers. Inserting an LT-GaN underlying layer prior to growing the MQWs is a successful means of separating the induced nonradiative recombination centers because a growth interrupt interface exists between the n-GaN template and the InGaN QW. We found that by introducing this technique would improve the external quantum efficiency of the as-grown conventional LEDs. The electroluminescence relative intensity of a blue LED incorporating a 70-nm-thick LT-GaN was 20.6% higher (at 20 mA current injection) than that of the corresponding as-grown blue LED in the best case.     

White-light emission from InGaN-GaN multiquantum-welllight-emitting diodes with Si and Zn codoped active well layer

Si and Zn codoped In_xGa_1-xN-GaN multiple-quantum-well (MQW) light-emitting diode (LED) structures were grown by metal-organic vapor phase epitaxy (MOVPE). It was found that we can observe a broad long-wavelength donor-acceptor (D-A) pair related emission at 500 nm~560 nm. White light can thus be achieved by the combination of such a long-wavelength D-A pair emission with the InGaN bandedge related blue emission. It was also found that the electroluminescence (EL) spectra of such Si and Zn codoped InGaN-GaN MQW LEDs are very similar to those measured from phosphor-converted white LEDs. That is, we can achieve white light emission without the use of phosphor by properly adjusting the indium composition and the concentrations of the codoped Si and Zn atoms in the active well layers and the amount of injection current     

Improving the Luminescence of InGaN–GaN Blue LEDs Through Selective Ring-Region Activation of the Mg-Doped GaN Layer

In this study, we used the selective ring-region activation technique to restrain the surface leakage current and to monitor the luminescence characteristics of InGaN-GaN multiple quantum-well blue light-emitting diodes (LEDs). To access the current blocking region after forming a periphery high-resistance ring-region of the Mg-doped GaN layer and to reduce the degree of carrier trapping by the surface recombination centers, we deposited a titanium film onto the Mg-doped GaN epitaxial layer to form a high-resistance current blocking region. To characterize their luminescence performance, we prepared LEDs incorporating titanium films of various widths of the highly resistive current blocking layer. The hole concentration in the Mg-doped GaN epitaxial layer decreased from 3.45times10¹⁷ cm^-3 to 3.31times10¹⁶cm^-3 after capping with a 250-nm-thick layer of titanium and annealing at 700 degC under a nitrogen atmosphere for 30 min. Furthermore, the luminescence characteristics could be improved by varying the width of the highly resistive region of the current blocking area; in our best result, the relative electroluminescence intensity was 30% (20 mA) and 50% (100 mA) higher than that of the as-grown blue LEDs     

InGaN/GaN多量子阱蓝光LED的p-GaN盖层的MOCVD生长研究

利用金属有机物化学气相淀积（MOCVD）生长了InGaN／GaN多量子阱（MQW）蓝光发光二极管（LED）,研究了不同Cp2Mg流量下生长的p-GaN盖层对器件电学特性的影响。结果表明,随着Cp2Mg流量的提高,漏电流升高,并且到达一临界点会迅速恶化;正向压降则先降低,后升高。进而研究相同生长条件下生长的p-GaN薄膜的电学特性、表面形貌及晶体质量,结果表明,生长p-GaN盖层时,Cp2Mg流量过低,盖层的空穴浓度低,电学特性不好;Cp2Mg流量过高,则会产生大量的缺陷,盖层晶体质量与表面形貌变差,使得空穴浓度降低,电学特性变差。因此,生长p-GaN盖层时,为使器件的正向压降与反向漏电流均达到要求,Cp2Mg流量应精确控制。     

Mg fluctuation in p-GaN layers and its effects on InGaN/GaN blue light-emitting diodes dependent on p-GaN growth temperature

Correlation between material properties of bulk p-GaN layers grown on undoped GaN and device performance of InGaN/GaN blue light-emitting diodes (LEDs) as a function of p-GaN growth temperature were investigated. The p-GaN layers of both structures grown by metal-organic chemical-vapor deposition were heavily doped with Mg. As the growth temperature of the bulk p-GaN layer increased up to 1,080°C, N_A-N_D increased. However, above 1,110°C, N_A-N_D sharply decreased, while the fluctuation of Mg concentration ([Mg]) increased. At this time, a peculiar surface, which originated from inversion domain boundaries (IDBs), was clearly observed in the bulk p-GaN layer. The IDBs were not found in all LEDs because the p-GaN contact layer was relatively thin. The change in photoluminescence emission from the ultraviolet band to blue band is found to be associated with the fluctuation of [Mg] and IDBs in bulk p-GaN layers. The LED operating voltage and reverse voltage improved gradually up to the p-GaN contact-layer growth temperature of 1,080°C. However, the high growth temperature of 1,110°C, which could favor the formation of IDBs in the bulk p-GaN layer, yielded poorer reverse voltage and saturated output power of the LEDs.     

Growth of GaN on porous SiC and GaN substrates

We have studied the growth of GaN on porous SiC and GaN substrates, employing both plasma-assisted molecular-beam epitaxy (PAMBE) and metal-organic chemical-vapor deposition (MOCVD). For growth on porous SiC, transmission electron microscopy (TEM) observations indicate that the epitaxial-GaN growth initiates primarily from surface areas between pores, and the exposed surface pores tend to extend into GaN as open tubes and trap Ga droplets. The dislocation density in the GaN layers is similar to, or slightly less than, that observed in layers grown on nonporous substrates. For the case of GaN growth on porous GaN, the overgrown layer replicates the underlying dislocation structure (although considerable dislocation reduction can occur as this overgrowth proceeds, independent of the presence of the porous layer). The GaN layers grown on a porous SiC substrate were found to be mechanically more relaxed than those grown on nonporous substrates; electron-diffraction patterns indicate that the former are free of misfit strain or are even in tension after cooling to room temperature.     

Enhanced luminescence from GaN-based blue LEDs grown on grooved sapphire substrates

Luminescence from GaN-based blue light-emitting diodes grown on grooved sapphire substrates was investigated using cathodoluminescence (CL) and electroluminescence (EL). The 60-nm-deep 2 (ridge) /spl times/4 /spl mu/m (trench) grooves along the <101~0> direction were created by BCl/sub 3/-Cl/sub 2/-based inductively coupled plasma reactive ion etching. Stronger CL and EL from the trench regions of the grooves in GaN and InGaN-GaN multiquantum-wells were observed, confirming its better crystalline quality over the trench regions, further supported by the EL mapping results. Epitaxial lateral growth was believed to initiate from the ridge regions to cover the trench regions at the foremost stage of GaN growth that is similar to the coalescence of islands.