C.W. Chin

BARRIER HEIGHT ENHANCED GaN SCHOTTKY DIODES USING A THIN AlN SURFACE LAYER

Gallium nitride (GaN) is a highly promising wide band gap semiconductor with applications in high power electronic and optoelectronic devices. Thin films of GaN are most commonly grown in the hexagonal wurtzite structure on sapphire substrates. Growth of GaN onto silicon substrates offers a very attractive opportunity to incorporate GaN devices onto silicon-based integrated circuits. Although direct epitaxial growth of GaN films on Si substrates is a difficult task (mainly due to the 17% lattice mismatch present), substantial progress in the crystal quality can be achieved using a buffer layer. A full characterization of the quality of the material needs to be assessed by a combination of different techniques. In this work, a thin AlN cap layer of 50 nm was incorporated in GaN Schottky diode to enhance the effective Schottky barrier height and reduces the dark current. A barrier height of 0.52 eV for normal GaN Schottky diode was increased to the effective barrier height of 0.63 eV. The resulting Schottky diodes show a dark current of as low as 6.3×10-5 A at 5 V bias, which is about two orders of magnitude lower than that of normal GaN (5.2×10-3 A at 5 V bias) Schottky diode.

LARGE AREA GaN METAL SEMICONDUCTOR METAL (MSM) PHOTODIODE USING A THIN LOW TEMPERATURE GaN CAP LAYER

Small area metal semiconductor metal (MSM) photodiode (PD) has been one of the most favoured detector choices for high speed optoelectronics integrated circuits due to their low parasitic capacitance and simple planar device structure, which is compatible with FETs. Large MSM PDs, on the other hand, can also be useful in many network and interconnect applications such as fibre distributed data interfaces. An MBE grown GaN metal semiconductor metal photodiode with a thin low temperature GaN (50nm) barrier enhancement layer is reported, which has low dark current. The detector using Nickel (Ni) Schottky metal fingers with 400 μm spacing on a large active area exhibit a low dark current of 1.23 mA at 10 V bias, which is about three orders of magnitude lower than that of the normal GaN Schottky photodiode.

THE STUDIES OF THERMAL ANNEALING ON Pt/AlGaN GROWN ON Si(111) BY PLASMA-ASSISTED MOLECULAR BEAM EPITAXY (PA-MBE)

The application of thermal annealing at various annealing temperatures (473–1073 K) has been shown to significantly modify surface morphology of platinum (Pt) metal contacts on AlGaN/GaN/AlN heterostructure grown on silicon by plasma-assisted molecular beam epitaxy (PA-MBE). Structural analysis of the AlGaN/GaN samples used for the Pt Schottky contacts fabrication were performed by using high resolution X-ray diffraction (HR-XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Pt metal contacts were then deposited on the samples followed by current–voltage (I–V) characterization. Thermally-treated samples showed significant decrease in current compared with untreated samples. From the I–V measurements, the Schottky barrier height (SBH) and ideality factor (n) were calculated. We found that the lowest value of SBH obtained was 0.526 eV at 873 K annealing temperature. Unfortunately, there are no values for the SBH and ideality factor at 1073 K annealing temperature. The SEM analysis has shown some island formation at high annealing temperature due to the difference of surface energies between thin metal films and AlGaN that causes dewetting. We suggest that the reason for the barrier height reduction is due to the metal island formation on the samples.

Porous Silicon Dioxide Synthesized using Photoelectrochemical (PEC) Wet Etching

Porous SiO2 can be used as a template to reduce substrate-induced stress, similar to porous GaN. Such a regrowth method may reduce the defect density in the epitaxial layer leading to high quality stress free layer on porous template. The samples were prepared on silicon (Si) wafers, (111)-oriented, with n- doping. After standard cleaning steps, SiO2 of 1200 Aring thickness was prepared by thermal oxidation of the Si at 1000deg C for 1.50 hours. The wafer was then cleaved into few pieces. To prepare porous structures by photoelectrochemical (PEC) method, the samples were dipped into a mixture of hydrofluoric acid (HF): water: ethanol under different etching durations. Structural properties of porous SiO2 have been investigated by scanning electron microscope (SEM). Elemental composition of the sample was identified using energy dispersive X-ray (EDX) analysis. Fourier transform infrared reflectance (FTIR) spectroscopy was used to characterize the chemical species and chemical bonding state.

THE STUDY OF Al0.29Ga0.71N-BASED SCHOTTKY PHOTODIODES GROWN ON SILICON BY PLASMA-ASSISTED MOLECULAR BEAM EPITAXY

In this paper, the growth and characterization of epitaxial Al0.29Ga0.71N grown on Si(111) by RF-plasma assisted molecular beam epitaxy (MBE) are described. The Al mole fraction was derived from the HR-XRD symmetric rocking curve (RC) ω/2θ scans of (0002) plane as x = 0.29. PL spectrum of sample has shown sharp and intense band edge emission of GaN without the existence of yellow emission band, showing that it is comparable in crystal quality of the sample when compared with previous reports. From the Raman measurement of as-grown Al0.29Ga0.71N layer on GaN/AlN/Si sample. We found that the dominant E2 (high) phonon mode of GaN appears at 572.7 cm-1. The E2 (high) mode of AlN appears at 656.7 cm-1 and deviates from the standard value of 655 cm-1 for unstrained AlN. Finally, AlGaN Schottky photodiode have been fabricated and analyzed by mean of electrical characterization, using current–voltage (I–V) measurement to evaluate the performance of this device.

FABRICATION OF GaN HOMO-JUNCTION ON Si (111) SUBSTRATE FOR SENSOR APPLICATIONS

In this paper, we investigated growth of GaN pn-homo junction layers on silicon (111) by plasma-assisted molecular beam epitaxy (PA-MBE) system and its application for photo-sensors. Silicon (Si) and magnesium (Mg) were used in this study as n- and p-dopants, respectively. Structural and optical analysis of the GaN homo-junction samples were performed by using reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), Raman spectroscopy and photoluminescence (PL) to analyze the crystalline quality of the samples. Surprisingly, the Raman analysis has revealed that there is no quenching of the A1 (LO) peak, with the presence of Si- and Mg-dopants in sample. The pn-junctions sample has good optical quality as measured by the PL system. Electrical characterization of the photo-sensors was carried out by using current–voltage (I–V) measurements.

Silicon Schottky barrier photodiodes with a thin AlN nucleation layer

Purpose – The purpose of this paper is to present the characteristics of novel silicon Schottky barrier (SB) photodiodes (PDs) with aluminium nitride (AlN) (100 nm) nucleation layer.Design/methodology/approach – Comparison was made with conventional silicon SB PDs.Findings – It was found that smaller dark current could be achieved with AlN nucleation layer. It was also found that effective SB height increased from 0.65 to 0.71 eV with the insertion of the AlN layer. The dark leakage current for the Schottky PDs with the AlN layer was shown to be about two orders of magnitude smaller than that for the conventional silicon SB PDs.Research limitations/implications – It is possible that the detrimental effect of interface states situated near the metal semiconductor interface was less pronounced for the sample owing to the insertion of the AlN nucleation layer.Originality/value – There is believed to be no other report on silicon SB PDs capped with an AlN layer in the literature. This paper describes the fabr...

RF-MBE GROWTH OF GaN ON SAPPHIRE FOR GAS SENSING APPLICATION

Besides SiC, a group III-V nitrides are also suitable large-band gap semiconductor materials for high-temperature gas sensor devices. In this paper, we present the study of the H2 sensitive device fabricated based on n-type GaN wafer. The GaN thin film with AlN buffer layer was grown on sapphire by RF plasma-assisted molecular beam epitaxy (RF-MBE). A few monolayers of AlN were deposited using high flux Al before the growth of the AlN buffer layer. This step is speculated to be able to form a better relaxed layer for the subsequent growth of the AlN buffer layer. Pt contacts with thickness of about 150 nm were then deposited on the GaN/AlN/Al2O3 using the sputtering system. Gas detection was carried out at room temperature. Prior to the current–voltage (I–V) measurements, the samples were annealed at temperatures ranging from 200°C to 600°C in N2 ambience. A significant change of current in the Pt/ GaN gas sensor was observed for the 600°C annealed sample when exposed to 0.5% H2 in N2 gas.

Structural, Optical and Electrical Properties of n‐type GaN on Si (111) Grown by RF‐plasma assisted Molecular Beam Epitaxy

In this paper, we present the study of the structural, optical and electrical of n‐type GaN grown on silicon (111) by RF plasma‐assisted molecular beam epitaxy (RF‐MBE). X‐ray diffraction (XRD) measurement reveals that the GaN was epitaxially grown on silicon. For the photoluminescence (PL) measurement, a sharp and intense peak at 364.5 nm indicates that the sample is of high optical quality. Hall effect measurement shows that the film has a carrier concentration of 3.28×1019 cm−3. The surface of the n‐type GaN was smooth and no any cracks and pits.

Characteristics of Thermally Treated Contacts on Porous Silicon Based Metal-Semiconductor-Metal (MSM) Photodetector Structures

To date, little work has been done on porous silicon-based MSM photodetectors. Porous silicon (PS) was obtained on n-type silicon (111) using photoelectrochemical etching in HF. Microstructural investigation has been done by scanning electron microscope (SEM) and X-ray diffraction (XRD) measurements. We have found that the PS consists of a regular silicon microarray with triangular geometry. From the X-ray diffraction scan, PS shows a broadening of the full width at half maximum with respect to the as-grown epilayer. In this work, PS-based MSM photodetectors (photodiodes) with nickel (Ni) Schottky contacts were fabricated and characterized. The application of thermal treatment to the contacts at various annealing temperatures (500-700degC) was investigated. Electrical characterization was performed by current-voltage (I-V) measurements. Morphological characterization was performed by atomic force microscopy (AFM) measurements.