J. M. Hinckley

Direct measurement of auger recombination in In0.1Ga0.9N/GaN quantum wells and its impact on the efficiency of In0.1Ga0.9N/GaN multiple quantum well light emitting diodes

The Auger recombination coefficient in In0.1Ga0.9N/GaN quantum wells, emitting at 407 nm has been determined from large signal modulation measurements on lasers in which these quantum wells form the gain region. A value of 1.5×10−30 cm6 s−1 is determined for the Auger coefficient at room temperature, which is used to analyze the reported efficiency characteristics of 410 nm In0.1Ga0.9N/GaN quantum wells light emitting diodes. The calculated efficiencies agree remarkably well with the measured ones. It is apparent that Auger recombination is largely responsible for limiting device efficiencies at high injection currents.

A InGaN/GaN quantum dot green (λ=524 nm) laser

The characteristics of self-organized InGaN/GaN quantum dot lasers are reported. The laser heterostructures were grown on c-plane GaN substrates by plasma-assisted molecular beam epitaxy and the laser facets were formed by focused ion beam etching with gallium. Emission above threshold is characterized by a peak at 524 nm (green) and linewidth of 0.7 nm. The lowest measured threshold current density is 1.2 kA/cm2 at 278 K. The slope and wall plug efficiencies are 0.74 W/A and ∼1.1%, respectively, at 1.3 kA/cm2. The value of T0=233 K in the temperature range of 260–300 K.

Charged carrier transport in Si1−xGex pseudomorphic alloys matched to Si—strain‐related transport improvements

Charge carrier transport studies are reported for Si1−xGex pseudomorphic alloy layers matched to the (001) Si substrate lattice constant. The effect of biaxial compressive strain on transport is studied by first examining the band structure changes via deformation potential theory and then studying the transport via a generalized Monte Carlo approach. Marked improvements in in‐plane hole transport are obtained while significant improvements also occur in the out‐of‐plane electron transport. These changes are ideally suited for use in n(Si)‐p(Si1−xGex)‐n(Si) heterojunction bipolar transistors.

A comparison of optoelectronic properties of lattice-matched and strained quantum-well and quantum-wire structures

The k/spl middot/p formalism is used to study the absorption spectra, material and differential gain in quantum wires as a function of orientation, built-in strain, and wire dimensions. The results for material and differential gain are compared with those for an optimized quantum-well structure. We find that for quantum wires at 300 K, the gain becomes positive at a carrier density of 1.27/spl middot/10/sup 18/ cm/sup /spl minus/3/, while in quantum wells this density is calculated to be 1.82/spl middot/10/sup 18/ cm/sup /spl minus/3/. Incorporating tensile strain in the wires reduces the transparency carrier concentration to 0.96/spl middot/10/sup 18/ cm/sup /spl minus/3/ while compressive strain allows one to obtain positive gain for densities greater than 1.08/spl middot/10/sup 18/ cm/sup /spl minus/3/. Orienting the wire along the /spl lsqb/111/spl rsqb/ direction reduces the transparency carrier density to 0.60/spl middot/10/sup 18/ cm/sup /spl minus/3/. The differential gain in quantum-well structures for injections near the threshold is on the order of 10/sup /spl minus/14/ cm/sup /spl minus/4/, while for 50 /spl Aring//spl middot/100-/spl Aring/ quantum wires the differential gain near the threshold is found to be on the order of 10/sup /spl minus/13/ cm/sup /spl minus/4/. The differential gain in wires whose wire axis is parallel to the /spl lsqb/111/spl rsqb/ direction has also been found to be on the order of 10/sup /spl minus/13/ cm/sup /spl minus/4/ for carrier injections close to the threshold. >

Carrier velocity‐field characteristics and alloy scattering potential in Si1−xGex/Si

The alloy scattering potential is an important parameter in SiGe alloys since it not only affects the velocity‐field characteristics for carrier transport, but also allows increased optical transitions by relaxing k‐selection rules. In this letter, we report on the velocity‐field measurements for relaxed and coherently strained SiGe alloys. The alloy scattering potential is obtained from a careful fit to the data. The hole velocity at any field is found to have a bowing behavior as a function of alloy composition. This reflects a strong alloy scattering potential which is calculated to be 0.6 eV for the valence band.

Calculation of electron and hole impact ionization coefficients in SiGe alloys

Silicon–germanium alloys offer a system where the ratio of the electron impact ionization coefficient (α) and hole impact ionization coefficient (β) varies from a value larger than unity (in high silicon content alloys), to a value smaller than unity (in high germanium content alloys). We report results for α and β for this alloy system. The electron results are based on a multivalley nonparabolic band structure. The hole results are based on a six‐band k⋅p model for low energies coupled to an eight‐band model for high energies. We find that for the alloy Si0.4Ge0.6, α∼β. Alloy scattering is found to play an important role in determining the impact ionization coefficient. For compositions around Si0.5Ge0.5, the strong alloy scattering is found to suppress the impact ionization coefficient.

Tunnel injection In0.25Ga0.75N/GaN quantum dot light-emitting diodes

Hole tunnel injection is incorporated in the design of In0.25Ga0.75N/GaN quantum dot light-emitting diodes with peak emission at λ∼500 nm. Calculations show that cold holes are uniformly injected into all five quantum dot layers in the active region. Measurements were made on devices having different thicknesses, teff, of the In0.43Al0.57N hole tunnel barrier. The best performance is exhibited by a device with teff=1.5 nm. The maximum external quantum efficiency is 0.66% at 220 A/cm2, and an efficiency droop of 20% at 360 A/cm2 is tentatively attributed to reduced Auger recombination and leakage of hot carriers.

Gate Bias Dependence of Defect-Mediated Hot-Carrier Degradation in GaN HEMTs

Monte Carlo analysis of hot-electron degradation in AlGaN/GaN high-electron mobility transistors shows that, for gate voltages corresponding to semi-ON bias conditions, the average electron energy has a spatial peak with (EAVE) ~ 1.5 eV. The peak is located at the edge of the gate. At this location, the carrier versus energy distribution has a large tail extending over 3 eV. When transferred to the lattice, this energy can cause defect dehydrogenation and device degradation. These results are consistent with the experimental data indicating maximum degradation in the semi-ON bias condition.

Theoretical study on threshold energy and impact ionization coefficient for electrons in Si1−xGex

Threshold energy and electron impact ionization coefficients (α) are calculated for unstrained and strained Si1−xGex on {100} silicon substrate using nonparabolic and ellipsoidal band structure for conduction band and k⋅p method for valence band. The threshold energy in the unstrained Si1−xGex is smaller than that in pure silicon due to the reduced band‐gap energy. The strain causes band degeneracy lifting for both the conduction band and valence band. It gives an additional band‐gap narrowing which leads to a much smaller threshold energy. On the basis of these results, the electron impact ionization coefficient is estimated up to 30% germanium using a Monte Carlo simulation. The reduced threshold energy is found to be the most dominant factor in determining α in the strained Si1−xGex. As a result, the strained Si1−xGex has much larger α than pure silicon while the unstrained Si1−xGex does not due to the effect of alloy scattering and the relatively small change of the threshold energy.

Theoretical investigation of hole transport in strained III‐V semiconductors: Application to GaAs

A Monte Carlo method has been developed and applied to study the anisotropic transport of holes in unstrained and strained bulk III‐V compound semiconductors. In this letter, we present the results for the prototypical GaAs, T=300 K material system. We find that the hole mobility can be significantly increased by the presence of biaxial compressive strain in the system. This arises from strain‐induced modifications in the densities of states and the overlap functions and from a separation of the heavy and light hole bands at k=0 which decreases the heavy to light hole interband scattering. For a 1.5% biaxial compressive strain, the hole mobilities are increased by up to a factor of 2 over the unstrained values. This improvement is sustained up to the highest field in our simulations which was 20 kV/cm.