Xiaoguang He

The significant effect of the thickness of Ni film on the performance of the Ni/Au Ohmic contact to p-GaN

The significant effect of the thickness of Ni film on the performance of the Ohmic contact of Ni/Au to p-GaN is studied. The Ni/Au metal films with thickness of 15/50 nm on p-GaN led to better electrical characteristics, showing a lower specific contact resistivity after annealing in the presence of oxygen. Both the formation of a NiO layer and the evolution of metal structure on the sample surface and at the interface with p-GaN were checked by transmission electron microscopy and energy-dispersive x-ray spectroscopy. The experimental results indicate that a too thin Ni film cannot form enough NiO to decrease the barrier height and get Ohmic contact to p-GaN, while a too thick Ni film will transform into too thick NiO cover on the sample surface and thus will also deteriorate the electrical conductivity of sample.

Unintentionally doped semi-insulating GaN with a low dislocation density grown by metalorganic chemical vapor deposition

Unintentionally doped semi-insulating GaN films possessing a low dislocation density are grown via metalorganic vapor phase epitaxy on a sapphire substrate. The obtained electrical resistivity reaches 4.8 × .a8 Ω·cm, and the full width at half maximum of the x-ray diffraction rocking curves of the (002) and (102) planes is as low as 201.6 and 378.0 arcsec, respectively. The GaN films are grown at low pressure (LP) to introduce residual carbon and to compensate background donors. The dislocation density in these samples is significantly reduced by using a low V/III ratio in the initial epitaxial growth stage. It is also found that the resistivity of the LP-grown samples decrease when the residual carbon concentration is excessive, which is attributed to the self-compensation of carbon impurities. Therefore, the carbon concentration of LP-grown GaN should be carefully modulated to maintain its high resistivity.

Different variation behaviors of resistivity for high-temperature-grown and low-temperature-grown p-GaN films*

Two series of p-GaN films grown at different temperatures are obtained by metal organic chemical vapor deposition (MOCVD). And the different variation behaviors of resistivity with growth condition for high- temperature(HT)-grown and low-temperature(LT)-grown p-GaN films are investigated. It is found that the resistivity of HT-grown p-GaN film is nearly unchanged when the NH3 flow rate or reactor pressure increases. However, it decreases largely for LT-grown p-GaN film. These different variations may be attributed to the fact that carbon impurities are easy to incorporate into p-GaN film when the growth temperature is low. It results in a relatively high carbon concentration in LT-grown p-GaN film compared with HT-grown one. Therefore, carbon concentration is more sensitive to the growth condition in these samples, ultimately, leading to the different variation behaviors of resistivity for HT- and LT-grown ones.

Utilization of polarization-inverted AlInGaN or relatively thinner AlGaN electron blocking layer in InGaN-based blue–violet laser diodes

Polarization-induced downward band-bending at the interface between the last quantum barrier (QB) and electron blocking layer (EBL) potentially reduce the effective barrier height and thus may increase the electron leakage in InGaN-based blue–violet laser diodes (LDs). In this work, LD structures with a specially designed polarization-inverted AlInGaN EBL or with a relatively thin ternary Al0.2Ga0.8N EBL are proposed, and their influences on device characteristics are evaluated numerically by using the lastip simulation program. The results indicate that for LDs with proposed EBLs, the problem induced by electric field and the downward band-bending at the interface between the last QB and EBL is alleviated, which results in a reduction of the electron leakage and an improvement of the performances of LDs.

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

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.

Influence of residual carbon impurities in i-GaN layer on the performance of GaN-based p-i-n photodetectors

The influence of unintentionally doped carbon impurities of i-GaN layer on the performance of GaN-based p-i-n photodetectors is investigated. The photoluminescence spectra exhibits that the carbon impurities are strongly involved in deep trap level-related yellow luminescence band. The results of secondary ion mass spectroscopy suggest that the residual carbon impurities in the i-layer have great effect on the generation of deep trap levels, and have a strong influence on the spectral responsivity and dark current of photodetectors. Thus, the way to decrease the residual carbon impurity concentration of the i-GaN layer, such as enlarging the growth pressure, can improve the performance of p-i-n photodetectors.

Comparative study of the differential resistance of GaAs- and GaN-based laser diodes

The differential resistance curves of GaAs- and GaN-based laser diodes (LDs) are experimentally and numerically investigated. It is found that the dependence of the differential resistance upon the injection current differs in the GaAs- and GaN-based LDs mainly in two aspects. The first is the kink polarity of the differential resistance in the vicinity of the threshold current, and the second is the behavior of the differential resistance curve beyond the threshold current. Self-consistent calculation results suggest that the LD kink and its polarity are determined by the superposition effects of the n-side, active and p-side regions of LDs. It is found that this kink mainly differs in the differential resistance curves of the active region, while the difference in the behavior of the differential resistance curve after the lasing threshold is ascribed to a resistance change in the p-side region caused by a reduced ideality factor.

Influence of hydrogen impurities on p-type resistivity in Mg-doped GaN films

The effects of hydrogen impurities on p-type resistivity in Mg-doped GaN films were investigated. It was found that hydrogen impurities may have the dual role of passivating MgGa acceptors and passivating donor defects. A decrease in p-type resistivity when O2 is introduced during the postannealing process is attributed to the fact that annealing in an O2-containing environment can enhance the dissociation of MgGa-H complexes as well as the outdiffusion of H atoms from p-GaN films. However, low H concentrations are not necessarily beneficial in Mg-doped GaN films, as H atoms may also be bound at donor species and passivate them, leading to the positive effect of reduced compensation.

Performance comparison of front- and back-illuminated modes of the AlGaN-based p-i-n solar-blind ultraviolet photodetectors

The authors report the comparison of front- and back-illuminated mode operations of Al0.4Ga0.6N positive-intrinsic-negative solar-blind photodetectors (PDs) grown on the double-side polished sapphire substrates by metalorganic chemical vapor deposition. It is shown that the responsivity in back-illumination mode of fabricated PDs can be almost three times as that in front-illumination mode under the same reverse bias. In addition, a wide spectral response between 300 nm and 370 nm is observed, which is not expected for solar-blind PDs in both illumination modes, while the PDs in back-illumination mode have a stronger ability to restrain the long-wavelength response, showing larger solar-blind/ultraviolet rejection ratio than front-illumination mode. The reasons for the performance differences are discussed.

Effects of polarization and p-type GaN resistivity on the spectral response of InGaN/GaN multiple quantum well solar cells

Effects of polarization and p-type GaN resistivity on the spectral response of InGaN/GaN multiple quantum well (MQW) solar cells are investigated. It is found that due to the reduction of piezoelectric polarization and the enhancement of tunneling transport of photo-generated carriers in MQWs, the external quantum efficiency (EQE) of the solar cells increases in a low energy spectral range (λ > 370 nm) when the barrier thickness value decreases from 15 nm to 7.5 nm. But the EQE decreases abruptly when the barrier thickness value decreases down to 3.75 nm. The reasons for these experimental results are analyzed. We are aware that the reduction of depletion width in MQW region, caused by the high resistivity of the p-type GaN layer may be the main reason for the abnormally low EQE value at long wavelengths (λ > 370 nm).