In this paper, we discuss the issue of fabricating reliable and reproducible ohmic contacts on AlGaN HFET structures. During the course of our investigation of fabricating contacts to HFETs, we found that the contact properties could vary significantly from one sample to another, even though they were nominally the same. This problem was prominently manifested in the ohmic contact behavior. The origin of this problem was traced back to the variation of the HFET structure during growth. In this paper, we report an attempt to fabricate reproducible ohmic contacts of these structures.

We review the state of the art and our perspectives on silicon and hybrid silicon photonic devices for optical interconnects in datacenters. After a brief discussion of the key requirements for intra-datacenter optical interconnects, we propose a wavelength-division-multiplexing (WDM)-based optical interconnect for intra-datacenter applications. Following our proposed interconnects configuration, the bulk of the review emphasizes recent developments concerning on-chip hybrid silicon microlasers and WDM transmitters, and silicon photonic switch fabrics for intra-datacenters. For hybrid silicon microlasers and WDM transmitters, we outline the remaining challenges and key issues toward realizing low power consumption, direct modulation, and integration of multiwavelength microlaser arrays. For silicon photonic switch fabrics, we review various topologies and configurations of high-port-count N-by-N switch fabrics using Mach–Zehnder interferometers and microring resonators as switch elements, and discuss their prospects toward practical implementations with active reconfiguration. For the microring-based switch fabrics, we review recent developments of active stabilization schemes at the subsystem level. Last, we outline several large challenges and problems for silicon and hybrid silicon photonics to meet for intra-datacenter applications and propose potential solutions.

We have studied the temperature-dependent carrier transfer processes in InGaN∕GaN multi-quantum-well light-emitting devices using various optical techniques such as photoluminescence, electroluminescence, and photoluminescence excitation spectra. The role of the defects in the GaN barrier neighboring to the InGaN region was demonstrated clearly in capturing carriers only at low temperatures. The physical origin of the defects was most possibly attributed to the stacking faults at the interface according to the high-resolution transmission electron spectroscopy pictures.

Temperature dependent electroluminescence (EL) and photoluminescence (PL) spectra were studied for blue, green and yellow-green InGaN/GaN light emitting devices (LEDs). Selective excitation has unambiguously proven that the origin of the 3.27 eV defect peak was radiative recombination at the defects in the GaN barriers adjacent to the QDs. Temperature dependent photoluminescence excitation (PLE) spectra demonstrated a still clearer evidence that at low temperatures the carrier transfer efficiency to the luminescent InGaN QDs is poor, due to carriers being captured by the defects in the GaN barrier. At high temperatures however those carriers generated inside the GaN barriers contribute more effectively to and finally dominate the PL emission of the InGaN QDs, showing a thermally activated carrier transfer process from the GaN layer to the QDs. High resolution transmission electron microscopy (HRTEM) pictures were used to reveal that the origin of the defects is most probably due to stacking faults in the GaN barrier.

A Reply to the Comment by M. Schott.Received 29 October 2003DOI:https://doi.org/10.1103/PhysRevLett.92.059702©2004 American Physical Society

Oxidation of Ni∕Au (5nm∕10nm) contact to p-GaN layer was performed by O2 plasma in a reactive ion etching system. The structural characteristics of the Ni∕Au p-GaN for different oxidation time were investigated by x-ray diffraction (XRD) measurements, using an intense synchrotron x-ray source. The XRD measurements indicated that the grains of nickel oxide polycrystalline in the contact were grown continually when the oxidation time increased in 10min. However, Au showed amorphouslike and the intensities of Bragg diffraction peaks were hardly changed when oxidation time increased to 10min. The nickel oxide formed by O2 plasma without sequent thermal annealing did not reduce the specific contact resistance (ρc) to p-GaN, but it took an important role in lowering ρc followed by thermal annealing in N2 at 500°C for 10min. Optical transmission spectra confirmed that the nickel was easy to be oxidized and few interdiffusions occurred at the metal interface in O2 plasma ambient. Finally, the mechanism of oxidation on the formation of low ρc ohmic contact was also discussed.

We report a simple and nondestructive photocurrent method for characterizing the bonding interface between two hydrophobically bonded Si wafers. The relationship of photocurrent versus wavelength was measured and analyzed. The direction of the photocurrent indicates the band-bending direction, and thus, the donor or acceptor nature of the interface states. The photocurrent is proportional to the electric field at the interface induced by band bending. Our results showed that the Si pairs bonded in air have much larger band bending at the interface than those bonded in dry nitrogen, and that both have donor-like interface states.

The possible origins of the leaky characteristics of a Schottky barrier on p-GaN have been investigated. The as-grown samples did not show any electrical activity using Hall measurements. Ni diodes made on as-activated samples, either at 950/spl deg/C for 5 s or at 750/spl deg/C for 5 min exhibited quasiohmic behavior. Upon sequential etching of the sample to remove a surface layer of 150 /spl Aring/, 1200 /spl Aring/, and 5000 /spl Aring/ from the sample, the I-V behavior became rectifying. I-V-T measurements showed that the slopes of the lnI-V curves were independent of the temperature, indicative of a prominent component of carrier tunneling across the Schottky junction. C-V measurements at each etch-depth indicated a decreasing acceptor concentration from the surface. The highly doped (>1.7 /spl times/ 10/sup 19/ cm/sup -3/) and defective surface region (within the top 150 /spl Aring/ from surface) rendered the as-activated Schottky diodes quasiohmic in their I-V characteristics. The leaky I-V characteristics, often reported in the literature, were likely to originated from the surface layer, which gives rise to carrier tunneling across the Schottky barrier. This highly doped/defective surface region, however, can play an important role in ohmic contact formation on p-GaN.

Light emitting spatial inhomogeneity in InGaN/GaN multiple quantum well (MQW) blue and green LEDs were observed directly with a microscope. Electroluminescence (EL) spectra of InGaN/GaN QW blue and green LEDs were studied in the temperature range of 10-300 K at an injection current of 5 mA. Some anomalous behavior was observed. Intensity of the EL main peak increased monotonically with temperature from 10 to 200 K and slightly decreased with further temperature increase in the 200 K range. This is in contrast with the monotonic decrease of EL with increasing temperature for conventional AlGaInP QW red LEDs. The anomalous behavior can only be observed on InGaN/GaN systems. The origin of such behaviors was discussed using a model of quantum dot clusters in the InGaN/GaN pseudo-quantum wells, with a small potential barrier at the boundary of the quantum dots.

The transport properties of the advancing interface ohmic contact to AlGaN/GaN heterostructure field-effect transistors have been investigated. We found that carrier transport across the AlGaN barrier layer is dominated by the tunneling of electrons that originate from the two-dimensional electron gas located at the AlGaN/GaN interface. The observed temperature dependence of the specific contact resistivity is different from that of the contact on highly doped bulk semiconductors, although tunneling current dominates the carrier transport in both cases.

The process of ion cutting was used to integrate single crystalline Si layers on glass for potential active matrix flat panel display and other applications. It was found that p-Si wafers implanted at 100–150 °C with H with a dose in the order of a few times 1016 cm−2 could be readily bonded to glass substrates when both of the surfaces were properly treated and activated. The as-implanted Si wafer surface was converted from p type to n type. Upon bonding at room temperature, annealing (300 °C) and exfoliation (450 °C), the transferred Si layer on glass and the as-exfoliated surface of the implanted Si wafer remained n type. A highly defective region was observed near the top of the Si layer on glass, however the crystalline quality was nearly defect free in the deeper region of the layer. Annealing at sequentially higher temperatures led to the recovery of p type conductivity at ∼600–650 °C. The type conversion and the subsequent annealing behavior observed on the samples were rationalized in terms of ion enhanced oxygen diffusion and the presence of H-related shallow donors in the Si.

The Schottky barrier characteristics of Ni on p-GaN have been investigated using current–voltage–temperature (I–V–T) and capacitance–voltage characteristics (C–V) measurements. Barrier height values ranging from 2.68 to 2.87 eV were obtained from C–V measurements. The temperature dependence of I–V characteristics clearly indicated the dominance of tunneling current in the transport mechanism of the diodes, therefore, barrier height determination using I–V measurements can lead to erroneous results, as indicated by the wide range of barrier heights reported in the literature. Acceptor concentration, deduced from C–V measurements, was found to be of 1019/cm3 within 200 Å of the sample surface, and tapered off to be ∼1018/cm3. These values are 10–100 times higher than the hole concentration of ∼1017/cm3 obtained from Hall measurements.

Al/Ti based metallization is commonly used for ohmic contacts to n-GaN and related compounds. We have previously reported an ohmic contact scheme specially designed for AlGaN/GaN heterostructure field-effect transistors (HFETs) [D. Qiao et al., Appl. Phys. Lett. 74, 2652 (1999)]. This scheme, referred to as the “advancing interface” contact, takes advantage of the interfacial reactions between the metal layers and the AlGaN barrier layer in the HFET structure. These reactions consume a portion of the barrier, thus facilitating carrier tunneling from the source/drain regions to the channel region. The advancing interface approach has led to consistently low contact resistance on Al0.25Ga0.75N/GaN HFETs. There are two drawbacks of the Al/Ti based advancing interface scheme, (i) it requires a capping layer for the ohmic formation annealing since Ti is too reactive and is easily oxidized when annealing is performed in pure N2 or even in forming gas, and (ii) the atomic number of Al and that of Ti are too low to yield efficient backscattered electron emission for e-beam lithographic alignment purposes. In this work, we investigated a Ta based advancing interface contact scheme for the HFET structures. We found that the presence of Ta in this ohmic scheme leads to (1) a specific contact resistivity as low as 5×10−7 Ω cm2, (2) efficient electron emission for e-beam lithographic alignment, and (3) elimination of the capping layer for the ohmic annealing.

The dependence of the Schottky barrier height of Ni/AlxGa1−xN contact on the Al mole fraction up to x=0.23 was studied. The barrier heights were measured by I–V, capacitance–voltage, and the internal photoemission method. The Al mole fractions were estimated from the AlGaN band gap energies measured by photoluminescence. In the range of x<0.2 a linear relationship between the barrier height and Al mole fraction was obtained. This was consistent with the slope predicted by the Schottky rule. For x=0.23, the measured barrier height was lower than predicted. We believed this was due to crystalline defects at the Ni/AlGaN interface.

The formation mechanism of the ohmic Au/Ni/p-GaN contact has been investigated. We found that it is essential to (i) deposit a structure of Au and Ni in the proper deposition sequence, and (ii) anneal the bilayer structure in an oxygen containing ambient. Our findings indicated that oxygen assists the layer-reversal reactions of the metallized layers to form a structure of NiO/Au/p-GaN. The presence of oxygen during annealing appears to increase the conductivity of the p-GaN. It is further suggested that Ni removes or reduces the surface contamination of the GaN sample before or during layer reversal. In the final contact structure, an Au layer, which has a large work function, is in contact with the p-GaN substrate. The presence of Au in the entire contacting layer improves the conductivity of the contact. An ohmic formation mechanism based on our experimental results is proposed and discussed in this work.

Acceptor (Mg)-doped AlGaN/GaN heterostructures were grown via MOVPE and compared to similarly doped GaN standard films grown in the same reactor. Chemical analysis of the films, via secondary ion mass spectrometry (SIMS), revealed comparable Mg concentrations of ∼2×1019 atoms/cm3 in all films. The Mg-doped GaN standard sample had a sheet conductance of 7-μS compared to a sheet conductance of 20-μS for an AlGaN/GaN heterostructure. The sheet conductance of the AlGaN/GaN heterostructures was higher due to piezoelectric acceptor doping and modulation doping effects in addition to conventional Mg acceptor doping.

Employing photoelastic effect with thermally stable and controllable metal stressor stripes for low propagation loss (on the order of 1 dB/cm) optical waveguide has been achieved in both InP and GaAs based planar waveguides. The study of stressors is based on Ni and WNi stripes. Planar processes, involving both photoelastic WNi stressor and He-implantation, have been used in the fabrication of single-quantum-well photoelastic GaAs/AlGaAs lasers and of the InGaAsP/InP Franz-Keldysh effect electroabsorption waveguide modulators. Get high performance phtoelastic semiconductor laser and electroabsorption modulator.

A photoconductance method was used to determine the band-gap energy and, therefore, the Al mole fraction of bulk AlxGa1−xN and AlxGa1−xN/GaN heterostructures. The results are compared with those obtained by a more elaborate photoluminescence method.

The electrical characteristics of Ni and Ti Schottky barriers on n-Al0.15Ga0.85N on SiC were investigated. We report that the barrier height for Ni on n-Al0.15Ga0.85N was about 1.26 eV and about 1 eV or less for Ti. These barrier heights are about 0.3–0.4 eV larger than those for Ni and Ti on n-GaN, which are in good agreement with Schottky model predictions.

A method for enhancing effective Schottky barrier heights in III–V nitride heterostructures based on the piezoelectric effect is proposed, demonstrated, and analyzed. Two-layer GaN/AlxGa1−xN barriers within heterostructure field-effect transistor epitaxial layer structures are shown to possess significantly larger effective barrier heights than those for AlxGa1−xN, and the influence of composition, doping, and layer thicknesses is assessed. A GaN/Al0.25Ga0.75N barrier structure optimized for heterojunction field-effect transistors is shown to yield a barrier height enhancement of 0.37 V over that for Al0.25Ga0.75N. Corresponding reductions in forward-bias current and reverse-bias leakage are observed in current–voltage measurements performed on Schottky diodes.