J.K. Kim

Epitaxially-stacked multiple-active-region 1.55μm lasers for increased differential efficiency

Semiconductor lasers emitting at 1.55 μm with external differential efficiencies >1 have been created by monolithically connecting several active regions in series within a single optical waveguide. This is accomplished by epitaxially stacking a number of p–i–n multiquantum well active regions with intermediate n++–p++ back diodes, which enable the entire terminal current to flow through each active region stages in series. Such lasers should also improve the impedance match as well as provide for low-noise, high-efficiency microwave links.

88 °C, continuous-wave operation of apertured, intracavity contacted, 1.55 μm vertical-cavity surface-emitting lasers

We demonstrate a lattice-matched 1.55 μm vertical-cavity surface-emitting laser operating continuous wave up to 88 °C. The laser employs AlAsSb-based mirrors, which provide high reflectivity and lattice matching to InP. The poor electrical and thermal conductivity of these mirrors is circumvented by utilizing an InP double-intracavity contacted structure. Benefits of the intracavity contacts are addressed by comparing the characteristics with the alternative contact scheme where current is injected through the Sb-based mirrors. Current and optical confinement is provided by an undercut aperture. The device shows a threshold current of 800 μA, a differential efficiency of 23%, and a maximum output power of over 1 mW at 20 °C.

Near-room-temperature continuous-wave operation of multiple-active-region 1.55 μm vertical-cavity lasers with high differential efficiency

We present completely monolithic, single-step grown, bipolar cascade vertical-cavity surface-emitting lasers at 1.55 μm with a greater-than-unity differential quantum efficiency. A typical device had a threshold current density of 1 kA/cm2, a threshold voltage of 3.2 V, and demonstrated continuous wave operation up to 8 °C. Devices smaller than 10 μm in diameter lased single mode. Active regions in our device were epitaxially stacked in three stages. This technique of multiple-active regions enabled the greater-than-unity differential quantum efficiency operation, which is essential in constructing high-efficiency microwave optical links with gain. We report the device characteristics and a model on the scaling properties of active region stacking in multiple-active-region vertical-cavity lasers.

Design parameters for lateral carrier confinement in quantum-dot lasers

Quantum-dot (QD) lasers have fallen short of their promise of ultralow threshold and high characteristic temperature. Here, we report that QDs show great promise for controlling lateral carrier leakage. While oxide apertures continue to enable improved performance in vertical cavity surface emitting lasers (VCSELs) by reducing optical losses and current spreading, lateral carrier losses remain uncontrolled. We investigate QD active material in which lateral diffusion is intentionally reduced. Cathodoluminescence results demonstrate reduced lateral diffusion in the material with which we expect >50% reduction in the threshold current for 1-μm-wide edge emitters or 5-μm-diam VCSELs. However, initial edge-emitter results demonstrated 10% reduction due to unintended current spreading and lasing from higher states.

Dielectric stress tests and capacitance-voltage analysis to evaluate the effect of post deposition annealing on Al2O3 films deposited on GaN

Systematic stress tests that help to evaluate the stability and dielectric performance of Al2O3 films under DC bias conditions are reported. Capacitance-voltage (C-V) curves were monitored for changes after subjecting the dielectric film to constant forward and reverse bias stress. Stress tests, along with C-V analysis, are used to evaluate the effect of post deposition annealing on Metal-Organic Chemical Vapor Deposition) Al2O3 films deposited on GaN. The individual benefits and drawbacks of each film and anneal condition were identified. These suggest that the anneals can be tailored to the unannealed film characteristics to achieve desired improvements in performance. It is found that post deposition annealing in forming gas improves performance under reverse bias stress by reducing the fixed charge and the field in the oxide but does not improve performance under forward bias.

Wafer-Bonded p-n Heterojunction of GaAs and Chemomechanically Polished N-Polar GaN

This letter reports wafer-bonded p-n heterojunction diodes, which consist of GaAs and chemomechanically polished N-polar GaN. The measured I-V and C-V show well-behaved p-n junction characteristics. The built-in voltage extrapolated from the C-V is 0.2 V less than the theoretical, suggesting the existence of interface states. However, this offset is much less than that (1.14 V) reported of wafer-bonded GaAs/Ga-polar GaN p-n diodes. The limited maximum current suggests pinning of the Fermi level at interface traps near the conduction band accessed under forward bias. Yet, this junction shows promise as a collector junction for wafer-bonded devices to achieve higher breakdown voltages.

1.55-/spl mu/m, InP-lattice-matched VCSELs operating at RT under CW

We demonstrate the first room temperature, continuous-wave (CW) operation of a 1.55-/spl mu/m InGaAs vertical-cavity surface emitting laser (VCSEL) that is completely lattice-matched to InP and produced in one epitaxial growth. The structure employed intracavity contacts with an air-gap aperture. The threshold current and threshold current density were 6.2mA and 1.97kA/cm/sup 2/, respectively, at 200 C with a 20-/spl mu/m-diameter current aperture.

Vertical electron transistors with In 0.53 Ga 0.47 As channel and N-polar In 0.1 Ga 0.9 N/GaN drain achieved by direct wafer-bonding

The design space of conventional semiconductor devices - from the materials point of view - is currently set by heteroepitaxy, whose capability is strongly limited by lattice parameters and structures of materials of interest. Heterostructures of substantially different semiconductors may offer significant advantages in device design, but many of them are likely heteroepitaxy-incompatible. In such cases, direct wafer-bonding can be exploited, in which materials of interest are separately grown and then their heterostructures formed by thermo-compression. InGaAs, with its superior electron mobility and injection velocity, is considered as the best candidate material for achieving electronic devices in THz applications, whereas III-N has proven its promise in high-power applications. Thus, transistors consisting of InGaAs channel and III-N drain may potentially attain both the high-speed and high-power performances. The first bonded aperture vertical electron transistor (BAVET) with In0.53Ga0.47As channel and Ga-polar InGaN/GaN drain was demonstrated in 2009 [1], and further improvements on it have also been reported [2], [3]. Here, we demonstrate the first BAVET consisting of In0.53Ga0.47As channel and N-polar In0.1Ga0.9N/GaN drain, which has an inherent advantage over Ga-polar InGaN/GaN drain, as discussed below.

Bipolar cascade 1.55 /spl mu/m VCSELs with >1 differential quantum efficiency and CW operation

We present completely monolithic, single-step grown, bipolar cascade MQW vertical-cavity surface-emitting lasers (VCSELs) at 1.55 /spl mu/m with greater-than-unity differential quantum efficiency. A typical device had a threshold current density of 1 kA/cm/sup 2/, a threshold voltage of 3.2 V, and demonstrated continuous wave (CW) operation up to 8 C.

Barrier reduction via implementation of InGaN interlayer in wafer-bonded current aperture vertical electron transistors consisting of InGaAs channel and N-polar GaN drain

This letter reports the influence of the added InGaN interlayer on reducing the inherent interfacial barrier and hence improving the electrical characteristics of wafer-bonded current aperture vertical electron transistors consisting of an InGaAs channel and N-polar GaN drain. The current-voltage characteristics of the transistors show that the implementation of N-polar InGaN interlayer effectively reduces the barrier to electron transport across the wafer-bonded interface most likely due to its polarization induced downward band bending, which increases the electron tunneling probability. Fully functional wafer-bonded transistors with nearly 600 mA/mm of drain current at VGS = 0 V and Lgo = 2 μm have been achieved, and thus demonstrate the feasibility of using wafer-bonded heterostructures for applications that require active carrier transport through both materials.