InGaN/GaN p-i-n Photodiodes Fabricated with Mg-Doped p-InGaN Layer

Mg-doped p-InGaN layers with In composition of about 10% are grown by metalorganic chemical vapor deposition (MOCVD). The effect of the annealing temperature on the p-type behavior of Mg-doped InGaN is studied. It is found that the hole concentration in p-InGaN increases with a rising annealing temperature in the range of 600-850°C, while the hole mobility remains nearly unchanged until the annealing temperature increases up to 750°C, after which it decreases. On the basis of conductive p-type InGaN growth, the p-In_0.1Ga_0.9N/i-In_0.1Ga_0.9N/n-GaN junction structure is grown and fabricated into photodiodes. The spectral responsivity of the InGaN/GaN p-i-n photodiodes shows that the peak responsivity at zero bias is in the wavelength range 350-400nm.     

SiGe (Ge-dot) heterojunction phototransistors for efficient light detection at 1.3–1.55 μm

The aim of this work is to develop a Si/SiGe HBT-type phototransistor with several Ge dot layers incorporated in the collector, in order to obtain improved light detectivity at 1.3–1.55 μm. The MBE grown HBT detectors are of n–p–n type and based on a multilayer structure containing 10 Ge-dot layers (8 ML in each layer, separated by 60 nm Si spacer) in the base-collector junction. The transistors were processed for normal incidence or with waveguide geometry where the light is coupled through the edge of the sample. The measured breakdown voltage, BV_ceo, was about 6 V. Compared to a p–i–n reference photodiode with the same dot layer structure, photoconductivity measurements show that the responsivity is improved by a factor of 60 for normal incidence at 1.3 μm. When the light is coupled through the edge of the device, the detectivity is even further enhanced. The measured photo-responsivity is more than 100 and 5 mA/W at 1.3 and 1.55 μm, respectively.     

An electrical characterization of a hetero-junction nanowire (NW) PN diode (n-GaN NW/p-Si) formed by dielectrophoresis alignment

This is a report on the electrical characterization of gallium nitride (GaN) nanowire (NW) p–n junction diodes. These diodes were formed by assembling n-GaN NWs on p-Si (1 0 0) substrates using alternating current (AC) dielectrophoresis (DEP). The AC DEP was optimized with a bias voltage of 15 V_p−p at a frequency of 1 kHz. The hetero-junction single GaN nanowire p-n diode (n-GaN NW/p-Si) showed well-defined current rectifying behavior with a forward voltage drop of 1.2–2.0 V at a current density of 10–60 A/cm². The GaN nanowire p–n diodes had a high parasite resistance in the range of >470 kΩ. We observed that these high resistances were mostly the result of the metal contacts to the n-GaN NWs. We also found that these parasite resistances were reduced by the formation of an additional capping layer on the top of the n-GaN NW as well as high temperature annealing.     

GaN基MIS紫外探测器的电学及光电特性

制备了GaN基金属-绝缘层-半导体(MIS)结构紫外探测器,并测量了其暗电流和光谱响应。通过分析其暗电流,发现在反偏情况下,其主要电流输运机制为隧穿复合机制;在正偏情况下,随着偏压的增大,电流输运机制从隧穿机制变为空间电荷限制电流机制。光谱响应测试结果显示,该探测器在-5 V的偏压下,在315 nm处获得了最大响应度170 mA/W,探测度为2.3×10¹² cm·Hz^1/2·W^-1。此外,还研究了不同厚度I层对器件光电压的影响,结果表明,光电压受隧穿机制与漏电流机制的共同制约。     

Performance improvement of InGaN/GaN multiple quantum well visible-light photodiodes by optimizing TEGa flow

The performance of an InGaN/GaN multiple quantum well(MQW) based visible-light Schottky photodiode(PD)is improved by optimizing the source flow of TEGa during In Ga N QW growth. The samples with five-pair InGaN/GaN MQWs are grown on sapphire substrates by metal organic chemical vapor deposition. From the fabricated Schottky-barrier PDs, it is found that the smaller the TEGa flow, the lower the reverse-bias leakage is. The photocurrent can also be enhanced by depositing the In GaN QWs with using lower TEGa flow. A high responsivity of 1.94 A/W is obtained at 470 nm and -3-V bias in the PD grown with optimized TEGa flow. Analysis results show that the lower TEGa flow used for depositing In Ga N may lead to superior crystalline quality with improved InGaN/GaN interface, and less structural defects related non-radiative recombination centers formed in the MQWs.     

Metal-semiconductor-metal photodiodes based on ZnCdS/GaP wide-gap heterostructures

Good epitaxial ZnCdS layers are grown on GaP semiconductor substrates by metal-organic chemical vapor deposition. The respective photodiode structures are fabricated by the metal-semiconductor-metal method, and their characteristics are studied. The diodes feature low dark currents. The bias dependence of the spectral response of the detector is determined. The long-wavelength edge of the ZnCdS/GaP diodes shifts from 355 to 440 nm as the bias voltage varies from 40 to 80 V. At the maximal photosensitivity wavelength (355 nm), the ampere/watt sensitivity of the detector is 0.1 A/W.     

High responsivity 4H-SiC based metal-semiconductor-metal ultraviolet photodetectors

4H-SiC based metal-semiconductor-metal (MSM) photodetectors with diverse spacing were designed, fabricated, and characterized, in which nickel Schottky contacts were needed. Current-voltage and spectral responsivity measurements were carried out at room temperature. The fabricated 4 μm-spacing device showed a very low dark current (0.25 pA at 5 V bias voltage), a typical responsivity of 0.103 A/W at 20 V, and a peak response wavelength at 290 nm. The fabricated devices held a high DUV to visible rejection ratio of >10³.     

Photocurrent properties of high—sensitivity GaN ultraviolet photodetectors

GaN epilayers were grown on sapphire substrates by metal-organic chemical vapour deposition. Metal－semiconductor－metal photoconductive detectors were fabricated using this material. The photocurrent properties of the detectors were measured and analysed. The spectrum response shows a high sensitivity in the wavelength region from 330 to 360nm, with a peak at 358nm and a sharp cutoff near 360nm. The maximum responsivities at 358nm were 700A/W (2V) and 7000A/W (30V). The relationship between responsivity and bias indicates that the responsivity increases linearly with bias until 30V. The influence of the spacing between two electrodes on the detector responsivity was also studied.     

Metal-semiconductor-metal ultraviolet photodetector based on GaN

A metal-semiconductor-metal (MSM) ultraviolet photodetector has been fabricated using unintentionally doped n-GaN films grown on sapphire substrates. Its dark current, photocurrent under the illumination with λ = 360 nm light, responsivity, and the dependence of responsivity on bias voltage were measured at room temperature. The dark current of the photodetector is 1.03 Na under 5 V bias, and is 15.3 Na under 10 V bias. A maximum responsivity of 0.166 A/W has been achieved under the illumination with λ= 366 nm light and 15 V bias. It exhibits a typical sharp band-edge cutoff at the wavelength of 366 nm, and a high responsivity at the wavelength from 320 nm to 366 nm. Its responsivity under the illumination with λ= 360 nm light increases when the bias voltage increases.     

ITON Schottky contacts for GaN based UV photodetectors

Lateral Schottky ultraviolet detectors were fabricated in GaN using indium-tin-oxynitride (ITON) as a contact metal. The GaN semiconductor material was grown on 2 in. sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The Schottky contact has been realized using ITON that has been deposited using sputter techniques. I-V characteristics have been measured with and without UV illumination. The device shows photo-to-dark current ratios of 10³ at −1 V bias. The spectral responsivity of the UV detectors has been determined. The high spectral responsivity of more than 30 A/W at 240 nm is explained by a high internal gain caused by generation-recombination centers at the ITON/GaN interface. Persistent photocurrent effect has been observed in UV light (on-off) switching operation, time constant and electron capture coefficient of the transition has been determined.     

Electroreflectance spectroscopy of compressively strained InGaN/GaN multi-quantum well structures

The built-in piezoelectric field induced by compressive stress in InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) was investigated using the electric field dependent electroreflectance (ER) spectroscopic method. InGaN/GaN MQW structures were prepared on sapphire substrates of different thicknesses. Thinning the sapphire substrate enables control of the compressive stress by changing the curvature of the wafer bowing. The wafer bowing-induced mechanical stress alters the piezoelectric field in the InGaN/GaN MQW. The flat band voltage, estimated by measuring the applied reverse bias voltage that induces a 180° phase shift in the ER spectra, was decreased from −11.21 V to −10.51 V by thinning the sapphire substrate thickness from 200 to 60 μm. To calculate the piezoelectric field (F_pz) from the compensation voltage, the depletion width was obtained from the capacitance–voltage measurement. The F_pz estimated from the energy shift in ER peak in a bias range from 0 to −12 V was changed by 110 kV/cm.     

Fabrication of ZnO nanorod-based p–n heterojunction on SiC substrate

Due to the special properties of ZnO and numerous envisaged areas of application of its nanostructures, much effort has been made in fabricating ZnO nanostructures. The next challenging step seems to be the processing and hence realisation of devices based on the nanostructure. We have grown ZnO nanorods of high crystal quality and good optical properties on 6H-SiC and 4H-SiC substrates. Considering the p–n junction as a basis for electronic and optoelectronic devices, we realised ohmic contacts on p-type 4H-SiC and fabricated ZnO nanorod-based p–n heterojunctions with the p-type 4H-SiC serving as the hole-conducting region. Nanorod-based p–n diodes with a turn-on voltage of 1.8 V and relatively large reverse-bias breakdown voltage were obtained, thus suggesting both the possibility of ZnO nanorod-based ultraviolet photodetectors and light-emitting devices, and the miniaturisation of device scales.     

Properties of Schottky barrier photodiodes based on InGaN/GaN MQW structures

The characterization and fabrication of Schottky barrier photodiodes based on InGaN/GaN multiple-quantum well structures in the active region are presented. These devices allow photodetection based on nitrides from the visible (VIS) to the ultraviolet (UV) ranges to be covered, and offer an alternative to InGaN bulk devices. Indium concentrations in the 8 to 14% range have been used. It has also been shown that in these devices the envelope average electric field in the depletion region can be tailored as a result of the huge polarization fields present in wells and barriers. As a consequence, two different device operation modes, as a function of voltage bias, are possible. By proper well and barrier design, charge accumulation can be produced at the boundaries of the active region and a large responsivity for photons with energies close to the barrier bandgap is found. Photodetectors show a rather sharp detection edge with a contrast of more than four orders of magnitude, reaching peak responsivities in the 0.1–1.0 A/W range. Self-consistent simulations and a discussion on the electric fields in the active regions are also presented.     

MOCVD生长InGaN/GaN MQW紫光LED

利用LP－MOCVD系统生长了InGaN/GaN MQW紫光LED外延片，双晶X射线衍射测试获得了2级卫星峰，室温光致发光谱的峰值波长为399.5nm，FWHM为15.5nm，波长均匀性良好。制成的LED管芯，正向电流20mA时，工作电压在4V以下。     

AlGaN-based high-performance metal–semiconductor–metal photodetectors

Design, structure growth, fabrication, and characterization of high performance AlGaN-based metal–semiconductor–metal (MSM) photodetectors (PD) are reported. By incorporating AlN nucleation and buffer layers, the leakage current density of GaN MSM PD was reduced to 1.96 × 10⁻¹⁰ A/cm² at a 50 V bias, which is four orders of magnitude lower compared to control devices. A 229 nm cut-off wavelength, a peak responsivity of 0.53 A/W at 222 nm, and seven orders of magnitude visible rejection was obtained from Al_0.75Ga_0.25N MSM PD. Two-color monolithic AlGaN MSM PD with excellent dark current characteristics were demonstrated, where both detectors reject the other detector spectral band with more than three orders of magnitude. High-speed measurements of Al_0.38Ga_0.62N MSM PD resulted in fast responses with greater than gigahertz bandwidths, where the fastest devices had a 3-dB bandwidth of 5.4 GHz.     

图形化Silicon-on-Insulator衬底上分子束外延生长可动GaN微光栅的研究

GaN材料作为第三代半导体材料,具有宽禁带、直接带隙、耐腐蚀等优点,是一种非常有前景的MOEMS材料。由于GaN的刻蚀目前尚未成熟,因此图形化外延生长法是一种较好的选择。本文基于SOI(silicon-on-insulator)基片,利用硅的微加工技术和图形化GaN分子束外延生长工艺,设计并加工了工作在太赫兹波段的、可以在二维方向上运动的SOI基GaN光栅。光栅周期为16μm,光栅宽度为6μm,峰值位置为25.901μm。通过仿真优化,设计的微驱动器在水平电压220V时,水平方向上的位移为±7.26μm;垂直方向加200V电压时,垂直位移2.5μm。为了研究在图形化SOI衬底上外延生长的InGaN/GaN量子阱薄膜的光学性能,用激光拉曼光谱仪对薄膜进行了光致发光光谱实验。实验结果表明,InGaN/GaN量子阱薄膜具有良好的发光性能,其发光范围为350~500nm,覆盖了紫外光到黄绿光。由于局域态效应与禁带收缩的作用,随着环境温度由10K升高至室温,薄膜的PL光谱的峰位呈现"S"形变化趋势。     

低压MOCVD生长GaN p—n结蓝光LED

简要报道了采用一种改进的低压金属有机化物化学气相沉积（ＬＰ－ＭＯＣＶＤ）方法制备ＧａＮｐ－ｎ结蓝光光发射二极管（ＬＥＤ），介绍了ＬＥＤ的基本特性，这种ＬＥＤ具有良好的Ｉ－Ｖ特性和光谱特性，室温下，在正向电压５Ｖ，正向电流３－２０ｍＡ的条件下，峰值波长为依Ｍｇ的掺杂浓度和退火条件的不同而不同，分别在４２５ｎｍ，４３５ｎｍ和４８０ｎｍ附近，发射谱的半峰宽约为５０ｎｍ。     

InGaN/GaN multiple quantum well solar cells with an enhanced open-circuit voltage

In this paper,InGaN/GaN multiple quantum well solar cells (MQWSCs) with an In content of 0.15 are fabricated and studied.The short-circuit density,fill factor and open-circuit voltage (V oc) of the device are 0.7 mA/cm 2,0.40 and 2.22 V,respectively.The results exhibit a significant enhancement of V oc compared with those of InGaN-based hetero and homojunction cells.This enhancement indicates that the InGaN/GaN MQWSC offers an effective way for increasing V oc of an In-rich In x Ga 1 x N solar cell.The device exhibits an external quantum efficiency (EQE) of 36% (7%) at 388 nm (430 nm).The photovoltaic performance of the device can be improved by optimizing the structure of the InGaN/GaN multiple quantum well.     

Performance characteristics of deep violet InGaN DQW laser diodes with InGaN/GaN superlattice waveguide layers

The effect of the indium (In) composition of In_xGa_1−xN (GaN) waveguide layers on the performance of deep violet In_0.082Ga_0.918N/GaN double quantum well (DQW) laser diodes (LDs) emitting at 390 nm output emission wavelength has been numerically investigated. Simulation results indicated that by increasing In composition of the In_xGa_1−xN waveguide layers, the threshold current decreases, the slope efficiency, and differential quantum efficiency (DQE) increase, whereas the output power decreases. The increase in the In composition of the InGaN waveguide layers increases the refractive index and consequently increases the optical confinement factor (OCF) which result in the increase in the slope efficiency and DQE and the decrease in the threshold current. The decreasing movement of electron and hole carriers from the bulk waveguide layers to the active regions also causes to decrease the output power. A new LD structure with InGaN/GaN superlattice (SL) waveguide layers has been proposed to exploit the increased OCF of InGaN waveguide structures, and the enhanced electron and hole mobilities and the tunneling effect of the periodic structure of the SL structures. The results also showed that the use of InGaN/GaN SL waveguide structures effectively improves the output power, slope efficiency and DQE and decreases the threshold current of the LD compared with (In)GaN bulk waveguide structure.     

Near-field optical characterization of GaN and InxGa1−xN/GaN heterostructures grown on freestanding GaN substrates

Using near-field scanning optical microscopy (NSOM), we report the spatial distribution of photoluminescence (PL) intensity in III-nitride-based semiconductor layers grown on GaN substrates. Undoped GaN, In_0.11Ga_0.89N, and In_0.13Ga_0.87N/GaN multi-quantum wells (MQWs) were grown by metal organic chemical vapor deposition (MOCVD) on freestanding GaN substrates. Micro-Raman spectroscopy has been used to evaluate the crystalline properties of the GaN homoepitaxial layers. The variation of the PL intensity from the NSOM imaging indicates that the external PL efficiency fluctuates from 20% to 40% in the 200 nm InGaN single layer on freestanding GaN, whereas it fluctuates from 20% to 60% in InGaN/GaN MQWs. In the NSOM-PL images, bright island-like features are observed. After deconvolution with the spatial resolution of the NSOM, the size of these features is estimated to be in the range of 150–250 nm.