Y. C. Wang

GaN-Based Schottky Barrier Photodetectors With a 12-Pair Mg

GaN-based ultraviolet (UV) photodetectors (PDs) separately prepared with a conventional single low-temperature (LT) GaN buffer layer and a 12-pair Mg_xN_y-GaN buffer layer were both fabricated. It was found that we could reduce threading dislocation (TD) density and thus improve crystal quality of the GaN-based UV PDs by using the 12-pair Mg_xN_y-GaN buffer layer. With a -2-V applied bias, it was found that the reverse leakage currents measured from PDs with a single LT GaN buffer layer and that with a 12-pair Mg_xN_y-GaN buffer layer were 4.57 times 10^-6 and 1.44 times 10^-12 A, respectively. It was also found that we could use the 12-pair Mg_xN_y-GaN buffer layer to suppress photoconductive gain, enhance UV-to-visible rejection ratio, reduce noise level, and enhance the detectivity.

_{\rm x}

We report the fabrication of GaN-based Schottky barrier diodes with multi-MgxNy/GaN buffer. Compared to conventional devices with a low-temperature GaN buffer, we achieved a six orders of magnitude smaller leakage current. It was also found that effective Schottky barrier height is larger for the proposed device due to the reduction in surface defect density by using the multi-MgxNy/GaN buffer.

N

AlGaN/GaN heterostructure Schottky barrier photodetector (PD) with multi-MgxNy/GaN buffer was proposed and fabricated. Compared with AlGaN/GaN heterostructure PD prepared on conventional low-temperature GaN buffer, it was found that we can reduce dark leakage current by more than three orders of magnitude. It was also found that we can use the multi-MgxNy/GaN buffer to suppress photoconductance gain, enhance UV-to-visible rejection ratio, reduce noise level and enhance the detectivity.

_{\rm y}

AlGaN/GaN metal-semiconductor-metal photodetectors (MSM PDs) with a low-temperature (LT) AlGaN interlayer (IL) were fabricated. Compared with the conventional AlGaN/GaN MSM PD, it was found that leakage current can be suppressed by insertion of a LT AlGaN IL due to the reduction of surface pits and improvement of crystalline quality. It was also found that larger photoresponsivity can be achieved due to the enhanced electric field strength as a result of inserting a LT AlGaN IL. Furthermore, suppressed photoconductive gain, lower noise level, and larger detectivity of MSM PD can also be achieved by using a LT AlGaN IL.

–GaN Buffer Layer

GaN UV metal-semiconductor-metal photodetectors (MSM PDs) with an unactivated Mg-doped cap layer and sputtered indium tin oxide (ITO) were fabricated. Compared with conventional MSM PDs without a cap layer, it was found that we could achieve a significantly much smaller dark current, larger UV to visible rejection ratio, and larger normalized detectivity by inserting an unactivated Mg-doped GaN cap layer. The dark leakage current for the MSM PDs with an unactivated Mg-doped GaN cap layer was shown to be about ten orders of magnitude smaller than that for the conventional MSM PDs. Under a 0.5 V bias, the measured responsivity and UV-to-visible rejection ratio were 0.017 A/W and 1.44 X 10 4 , respectively, for the MSM PDs with an unactivated Mg-doped GaN cap layer. This result could be attributed to the thicker and higher potential barrier and effective surface passivation after inserting this in situ grown cap layer. With a 1 V applied bias, it was also found that we could achieve a lower noise level and a higher normalized detectivity of 2.67 X 1010 cm Hz 0.5 W -1 by inserting an unactivated Mg-doped GaN cap layer into MSM PDs.

High quality GaN-based Schottky barrier diodes

We present the characteristics of an AlGaN/GaN Schottky barrier ultraviolet (UV) photodetectors (PDs) with and without a GaN sandwich layer. The effect of inserting this secondary GaN buffer layer on the growth mode and crystal properties of the whole epitaxial GaN layer was reported. It was found that we could reduce defect density and thus improve crystal quality of the AlGaN/GaN Schottky barrier UV PDs by using a GaN sandwich layer. It was also found that we could use the GaN sandwich structure to suppress photoconductive gain (PCG), enhance UV-to-visible rejection ratio (UTV-RR), reduce noise level, and enhance the detectivity of the fabricated PDs.

AlGaN/GaN Schottky Barrier Photodetector With Multi-

In chemical mechanical planarization, a rotating wafer is pressed facedown against a rotating pad, while a slurry is dragged into the pad-wafer interface to assist in planarizing the wafer surface. Due to stress concentration, the interfacial contact stress near the wafer edge generally is much higher than that near the wafer center, resulting in a spatially nonuniform material removal rate and hence an imperfect planarity of the wafer surface. Here, integrating theories of fluid film lubrication and two-dimensional contact mechanics, we calculate the interfacial contact stress and slurry pressure distributions. In particular, the possibility of using a multizone (i.e., piecewise constant) wafer-back pressure profile to improve the contact stress uniformity is examined by studying a particular case with realistic parameter settings. The numerical results indicate that using a two-zone wafer-back pressure profile with optimized zonal sizes and pressures can increase the usable wafer surface area (within which the average contact stress nonuniformity is below 0.1%) by as much as 12%. Using an optimized three-zone wafer-back pressure profile, however, does not increase the usable wafer surface area much further.

{\rm Mg}_{\rm x} {\rm N} _{\rm y}

AlGaN/GaN Schottky barrier diodes (SBDs) with multi-Mg x N Y /GaN buffer were fabricated and investigated. It was found that we can effectively suppress the formation of threading dislocation in the epitaxial layers and thus obtain better crystal quality using the multi-Mg x N y /GaN buffer. It was also found that we can achieve a larger effective Schottky barrier height and thus reduce leakage current of the SBDs by using the multi-Mg x N y /GaN buffer.

/GaN Buffer

AlGaN/GaN Schottky barrier photodetectors (PDs) with and without a low temperature (LT) AlGaN intermediate layer (IML) were both fabricated and characterized. It was found that we can reduce dark leakage current by around 4 orders of magnitude and enhance UV-to-visible rejection ratio by around 2 orders of magnitude by using an LT AlGaN IML. With ―5 V applied bias, we can achieve lower noise equivalent power and higher normalized detectivity (D * ) of the PD, which were 5.19 × 10 ―10 W and 2.8 × 10 9 cm Hz 0.5 W ―1 , respectively, with an LT AlGaN IML.

Characterization of AlGaN/GaN Metal- Semiconductor-Metal Photodetectors With a Low-Temperature AlGaN Interlayer

We present the characteristics of a nitride-based UV metal-semiconductor-metal photodetector (MSM PD) with an in situ low-temperature (LT) grown AIN cap layer. From atomic-force microscopy scan images, we could clearly see that surface pits on the sample surface are almost invisible with an LT AIN cap layer but can be observed in a conventional cap layer. Compared with conventional MSM PDs, it was found that we achieved smaller dark current and larger UV-to-visible rejection ratio for the PDs with an LT AIN cap layer. With a 5 V applied bias, the UV-to-visible rejection ratio between 360 and 400 nm was 7.26 X 10 2 for the MSM PDs with an LT AIN cap layer. The calculated noise equivalent power and normalized detectivity biased at 5 V for the MSM PDs with an LT AIN cap layer was 1.21 X 10 -11 W and 4.16 X 10 10 cm Hz 0.5 W -1 , respectively.