S. H. Hsu

Studies of band alignment and two-dimensional electron gas in InGaPN/GaAs heterostructures

Room-temperature photoreflectance (PR) and photoluminescence (PL) spectra are measured for a series of In0.54Ga0.46P1−yNy∕GaAs heterostructures grown on GaAs (100) substrate. Redshifts of the PR and PL peaks indicate that the band gap of In0.54Ga0.46P1−yNy is dramatically reduced as nitrogen is incorporated. The emergence of additional peaks in PR spectra as nitrogen is incorporated indicates that the band alignment switches from type I to type II, due to the lowering of the conduction band, thus forming a two-dimensional electron gas (2DEG) in the interface region between In0.54Ga0.46P1−yNy and GaAs. The band gap energy and transition energies between the confined levels in the 2DEG are determined for samples with various nitrogen concentrations y. The number of confined levels in the 2DEG is found to increase with y; the composition-dependent bowing parameter is determined.

InGaAsN metal-semiconductor-metal photodetectors with Modulation-doped heterostructures

InGaAsN metal-semiconductor-metal (MSM) photodetectors (PDs) with modulation-doped heterostructures have been successfully fabricated. It was found from capacitance-voltage measurements that carriers were well confined. Using thermionic emission theory and measured dark currents, it was found that effective Schottky barrier heights were 0.61, 0.72, 0.50, and 0.23 eV for the MSM-PDs with Al_0.2Ga_0.8As cap layer doping N_d=5times10¹⁶, 9times10¹⁶, 2times10¹⁷, and 6times10¹⁷ cm^-3, respectively. Furthermore, it was found that measured responsivities were 0.02, 0.14, and 0.22 A/W and for the MSM-PDs with N_d=5times10¹⁶, 9times10¹⁶, and 2times10¹⁷ cm^-3, respectively

Effects of weak ordering of InGaPN

The effect of weak ordering on InGaPN∕GaAs heterostructure grown by gas source molecular-beam epitaxy is quantitatively studied by room-temperature Raman, photoluminescence (PL), and photoreflectance spectroscopy in this work. The PL intensity decreases rapidly as the nitrogen concentration increases, implying that more nonradiative centers are generated by the ordering effect and the degradation of the samples. The band gap of InGaPN decreases dramatically as the nitrogen is incorporated. The Raman modes of InGaPN between 130 and 1000cm−1 are analyzed. Polarized Raman spectra reveal that the InGaPN layers become more ordered as more nitrogen is incorporated. A broad Raman structure that appeared around 730cm−1 is attributed to an InGaN-like LO-phonon mode. The transition of the crystal structure from the zinc blende to CuPt structure and the formation of GaN clusters are responsible for the ordering effect in the InGaPN layer.

Observation of spontaneous ordering in the optoelectronic material GaInNP

We report a detailed structural and optical characterization of high-quality GaInNP films. These films were grown by gas-source molecular-beam epitaxy on GaAs (100) substrates. These epitaxial layers were then characterized by a high-resolution x-ray rocking curve (HRXRC) and photoluminescence (PL) measurements. With nitrogen incorporation, the PL peak redshifts, indicating bandgap reduction and the line-width broadening increases due to alloy scattering. The anisotropic properties of the polarized HRXRC and polarized piezoreflectance spectra are used to prove the spontaneous ordering in GaInP incorporating nitrogen. Furthermore, ordering-induced superlattice-like microstructure shown in high-resolution transmission electron microscope images is used to confirm the spontaneous ordering in Ga0.44In0.56NxP1−x epitaxial layers.

Improvement in Linearity of Novel InGaAsN-Based High Electron Mobility Transistors

We have fabricated InGaAsN-based high electron mobility transistors (HEMTs) using InGaAsN as the channel layer. An extremely large gate-voltage swing (GVS) up to 4.2 V can be achieved by utilizing the large conduction band offset between the GaAs spacer layer and the InGaAsN channel layer. However, the poor channel mobility and current density as a result of nitrogen-induced electrically active defects limit the transconductance (gm) performance. Attempts using various annealing temperatures have demonstrated that better device characteristics can be obtained via rapid thermal annealing at 700 °C. In this study, we investigate the effect of nitrogen-induced traps on the basis of Hall measurements and device characterizations of HEMTs. The improvement in GVS in the annealed samples is also discussed. Despite the relatively poor gain, InGaAsN HEMTs with excellent linearity performance after proper thermal annealing are expected to be compatible for novel InGaAsN-based optoelectronics integral circuits (OEICs).

Investigation of the optical properties of InGaAs(N):(Sb) quantum wells grown by metal organic vapor phase epitaxy

Dilute-nitride materials have attracted much attention due to their capability of operating in the wavelength range of optical communication. However, their optical properties degrade with the nitrogen incorporation into the host material such as InGaAs or GaAs, which can be recovered slightly by performing thermal treatment. More recently, the surfactant effect of Sb was verified by molecular beam epitaxy grown samples. Adding Sb into dilute-nitride materials can help to maintain the luminescence efficiency while increasing the emission wavelength toward 1.55μm. But its effect on the metal organic vapor phase epitaxy grown samples is not very clear. In this article, we performed a series of experiments on the InGaAsN(Sb) quantum wells to clarify the role of Sb. The photoluminescence (PL) intensities of InGaAsN:Sb were higher than those of the undoped samples and the extent of blueshift after annealing was slighter than that of the InGaAsN quantum wells. However, the PL intensities were still low and thus...

Study of Electronic Properties by Persistent Photoconductivity Measurement in GaxIn1-xNyAs1-y Grown by MOCVD

Electronic properties of GaInNAs thin films have been investigated by the temperature-dependent Hall-effect and persistent photoconductivity (PPC) measurements. The GaxIn1-xNyAs1-y thin films were grown by metal-organic chemical vapor deposition (MOCVD) on GaAs(100) substrates. High resolution X-ray diffraction (HR-XRD) and photoluminescence (PL) were used to determine the crystal quality. Compared with the N-free sample, the mobility of the GaInNAs sample quenched significantly due to the lattice defects induced by the small amount of nitrogen atoms incorporated during growth. The free carrier concentration of the GaInNAs samples was not affected by the increasing temperature ranging from 120 K to 400 K. Persistent photoconductivity (PPC) was also observed in this material, and the properties of PPC were found to depend on the temperature and nitrogen content. The relationship between trend toward saturation of the free carrier concentration of the temperature-dependent Hall measurement and PPC is also discussed.

Triple Luminescence Peaks Observed in the InGaAsN/GaAs Single Quantum Well Grown by Metalorganic Vapor Phase Epitaxy

In this paper, we report the observed triple luminescence peaks of InGaAsN/GaAs single quantum well (SQW) grown by metal organic vapor phase epitaxy (MOVPE). The triplet-peak was discovered by the photoluminescence (PL) measurement performed under low temperature. The saturation of PL-peak intensities at high excitation power suggests the transition of the lower-energy peak was dominated by fewer numbers of available states as compared to the higher-energy peak. The position of the higher-energy peaks remained invariant after rapid thermal annealing (RTA) treatments revealed that the responsible transition was attributed to effects not associated with incorporated nitrogen. In addition, a large blue-shift was observed for the lower-energy peak and was likely attributed to local nitrogen bonding transformation. As results of our observation, the PL spectrum of InGaAsN SQW is mainly composed of a cluster-induced emission peak and two planar-feature peaks.

InGaAsN Metal–Semiconductor–Metal Photodetectors with Transparent Indium Tin Oxide Schottky Contacts

GaAs-based photodetectors (PDs) draw much attention owing to several advantages, such as low fabrication cost and the mature process techniques. However, fabricating them is a difficult task for the lack of suitable absorption materials that can be coherently grown on the GaAs substrate. In this report, we provide another approach to fabricate GaAs-based PDs, in which the novel InGaAsN alloy is used as the absorption layer. Three structures of planar PDs were demonstrated using transparent indium tin oxide (ITO) contacts. The dark current of PDs were suppressed by applying the SiO2 Schottky-barrier enhancement layer, but the rough interface between the SiO2 and InGaAsN layer limited the device characteristics. Depositing a thin layer of ITO between the SiO2 and InGaAsN layers greatly improved the interface quality and the photo current-to-dark current ratio was also increased from 17.7 to 45.29 under 0.5 V bias.

Investigation of InGaAsN MSM photodetectors with transparent ITO Schottky contacts

Metal–semiconductor–metal photodetectors (MSM-PDs) with transparent ITO contacts were studied. The RF-sputtered ITO layers were formed under various ambient gases Ar, Ar/N2, and Ar/O2. The ITO film fabricated under the Ar/O2 ambient has highest Schottky barrier-height, but the high resistivity limited the photocurrent of the photodetectors. Consequently, using Ar/N2 as the plasma gas would be a suitable choice for MSM-PD application. The photo/dark current ratios of the MSM-PDs were 5, 25 and 12 (measured under 0.2 V) using Ar, Ar/N2 and Ar/O2 as the plasma gases. To further improve the photo/dark current ratio, we fabricated the InGaAN-PDs using metal–insulator–metal–semiconductor (MIMS) structures. The dark current was greatly suppressed by the SiO2 layer, and the highest photo/dark current ratio was 66 under 0.2 V bias.