A. L. Holmes

Wafer fusion: materials issues and device results

A large number of novel devices have been recently demonstrated using wafer fusion to integrate materials with different lattice constants. In many cases, devices created using this technique have shown dramatic improvements over those which maintain a single lattice constant. We present device results and characterizations of the fused interface between several groups of materials.

Avalanche photodiodes with an impact-ionization-engineered multiplication region

We demonstrate an impact-ionization-engineered structure for the multiplication region of avalanche photodiodes. By enhancing the control of the impact-ionization position, the structure achieved high gain, low dark current, and very low noise.

A long-wavelength photodiode on InP using lattice-matched GaInAs-GaAsSb type-II quantum wells

We report a photodiode on InP substrate with a cutoff wavelength of 2.39 /spl mu/m and peak room-temperature external quantum efficiency of 43% at 2.23 /spl mu/m. Type-II GaInAs-GaAsSb quantum wells lattice-matched to InP were placed in the absorption region for long wavelength absorption. The device showed a peak detectivity of 5.6/spl times/10/sup 10/ cm/spl radic/HzW/sup -1/ at 200 K.

Unpinned metal gate/high-κ GaAs capacitors: Fabrication and characterization

Fabrication of GaAs metal-oxide-semiconductor capacitors (MOSCAPs) with an unpinned interface is reported. The MOSCAP structure consists of a few monolayers of germanium grown in a molecular beam epitaxy (MBE) system in order to terminate an MBE-grown silicon-doped (100) GaAs layer. An ex situ HfO2 high-κ dielectric with an equivalent oxide thickness of 12A was deposited by using a dc magnetron sputtering system. A midgap interface state density (Dit) of 5×1011eV−1cm−2 was measured using the high-frequency conductance technique. A rapid thermal annealing study was performed in order to examine the integrity of the gate stack at different temperatures. In addition, a forming gas anneal at 400°C appears to significantly reduce the midgap Dit revealed by probing the frequency dispersion behavior of the MOSCAPs.

Thin multiplication region InAlAs homojunction avalanche photodiodes

Low excess noise in avalanche photodetectors (APDs) is desired for improved sensitivity and high-frequency performance. Gain and noise characteristics are measured for InAlAs p-i-n homojunction APDs that were grown with varying i-region widths on InP by molecular beam epitaxy. The effective ionization ratio k (β/α) determined by noise measurements shows a dependence on multiplication region width, reducing from 0.31 to 0.18 for multiplication region thicknesses of 1600–200 nm. This trend follows previously shown results in AlGaAs-based APDs, which exhibit reduced excess noise due to nonlocal multiplication effects. These results show that this effect is a characteristic of thin avalanche regions and is not a material-specific phenomenon.

The effect of substrate misorientation on the photoluminescence properties of GaN grown on sapphire by metalorganic chemical vapor deposition

We report the growth and photoluminescence (300 and 4.2 K) characterization of unintentionally doped GaN on both exact and vicinal (0001) sapphire substrates. The GaN heteroepitaxial layers are grown by metalorganic chemical vapor deposition on sapphire substrates using various growth conditions. The (0001) Al2O3 c‐plane substrates are oriented exactly (0001) or misoriented either 2° towards the a plane (1120), 5° towards the m plane (1010), or 9° toward the m plane. A comparison of the 300 and 4.2 K optical characteristics of the samples grown on the different substrates indicates that a higher photoluminescence intensity is measured for the films on misoriented substrates.

Monte Carlo simulation of low-noise avalanche photodiodes with heterojunctions

A Monte Carlo model is developed to simulate avalanche photodiodes with AlGaAs/GaAs heterojunctions. The experimentally observed ultralow-noise behavior of a center-well avalanche photodiode is successfully reproduced in the model. It is found that the arrangement of different materials in the intrinsic region can modulate the positional dependence of impact ionization events, and hence the gain distribution. Consequently, the noise is sensitive to the structural parameters such as well thickness. Hot and energetic electrons are distinguished by their distribution in k space. This distinction is used to explain why the noise behavior is sensitive to the initial carrier excess energy from photogeneration but relatively insensitive to carrier energy gained from the electric field.

Optimal excess noise reduction in thin heterojunction Al/sub 0.6/Ga/sub 0.4/As-GaAs avalanche photodiodes

It has been recently found that the initial-energy effect, which is associated with the finite initial energy of carriers entering the multiplication region of an avalanche photodiode (APD), can be tailored to reduce the excess noise well beyond the previously known limits for thin APDs. However, the control of the initial energy of injected carriers can be difficult in practice for an APD with a single multiplication layer. In this paper, the dead-space multiplication recurrence theory is used to show that the low noise characteristics associated with the initial-energy effect can be achieved by utilizing a two-layer multiplication region. As an example, a high bandgap Al/sub 0.6/Ga/sub 0.4/As material, termed the energy-buildup layer, is used to elevate the energy of injected carriers without incurring significant multiplication events, while a second GaAs layer with a lower bandgap energy is used as the primary carrier multiplication layer. Computations show that devices can be optimally designed through judicious choice of the charge-layer width to produce excess noise factor levels that are comparable to those corresponding to homojunction APDs benefiting from a maximal initial-energy effect. A structure is presented to achieve precisely that.

Cleaved GaN facets by wafer fusion of GaN to InP

Basal plane sapphire is a common substrate for the heteroepitaxy of GaN. This presents a challenge for fabrication of cleaved‐facet GaN lasers because the natural cleavage planes in (0001) α‐Al2O3 are not perpendicular to the wafer surface. This letter describes a method for achieving perpendicular cleaved facets through wafer fusion that can potentially be used to fabricate GaN based in‐plane lasers. We demonstrate successful fusion of GaN to InP without voids or oxide at the interface and fabricate optically flat cleaved GaN facets that are parallel to the crystallographic planes of the host InP. I–V measurements have been performed across the n‐N fused interface. These results show that the fused junction exhibits a barrier of several electron volts for electrons passing from the InP to the GaN and ohmic conduction of electrons moving in the opposite direction.

High‐quality GaN heteroepitaxial films grown by metalorganic chemical vapor deposition

We report the growth of high‐quality GaN heteroepitaxial films on (0001) sapphire substrates by low‐pressure metalorganic chemical vapor deposition. These films have exhibited narrow x‐ray‐diffraction rocking curves with full‐width‐at‐half‐maximum values as low as ΔΘ∼38 arc sec. These are the narrowest x‐ray‐diffraction rocking curve linewidths reported to date for any III‐V nitride film. Electrical and optical measurements further indicate the samples to be of high quality.