review research
Electroluminescence exposes phase separation in AlInGaN
Scientists at West Virginia University have obtained experimental evidence of phase separation in AlInGaN layers with a few percent aluminum and indium.
Their findings contradict earlier theoretical studies by a team of Brazilian researchers that suggested that InAlGaN films - which form the active region in UV LEDs - are random alloys when aluminum and indium concentrations are very low.
Xian-An Cao and Yi Yang identified nanoscale phase separation in the AlInGaN material system through a series of electroluminescence measurements.
This duo studied an MOCVD-grown LED that comprised a sapphire substrate, a 3 µm-thick AlGaN template, a silicon-doped Al0.15
Ga0.85 with 3 nm thick Al0.06 Ga0.92
wells and 10 nm-thick Al0.1 and magnesium-doped Al0.25 and GaN capping layers.
Ga0.9
N quantum N barriers,
Ga0.75
temperatures and currents. Heat up the device, or driver it harder, and the electroluminescence profile switches to a single peak at 3.39 eV. Cao and Yang claim that these results indicate that the active region is composed of GaN and aluminum- rich and indium-rich nanoclusters (see figure). Indium-rich quantum dot-like clusters form potential wells that are 0.2 eV deep, which are each surrounded by an aluminum- rich region that acts as a raised rim.
N cladding layer, an active region In0.02
N cladding
Electroluminescence measurements were recorded at temperatures ranging from 5K to 300K, and various drive currents. These studies revealed that emission is dominated by peaks at 3.47 eV and 3.59 eV at low
At low temperatures and low currents luminescence from the GaN and AlGaN phases dominates, because most injected carriers fall into the energy bands of GaN and AlGaN. Emission from InGaN is negligible, because this phase accounts for just a fraction of the active region. Either increasing the temperature of the LED beyond 150K or cranking up the current leads to the injection of carriers that overcome the energy associated with the aluminum-rich rim and reach the InGaN phases. There they recombine radiatively. This switch in the distribution of carriers accounts for the red-shift from 3.47 eV and 3.59 eV emission to a peak at 3.39 eV.
The phase separation in ultraviolet LEDs that Cao and Yang have uncovered is
The electroluminescence profile of UV LEDs with an AlInGaN active region varies with the temperature and drive current. According to researchers at West Virginia University, this shift in the electroluminescence peak is caused by the interplay of injected carriers with GaN, Al-rich and In-rich nanoclusters in the active region.
undesirable, because it reduces the device’s internal quantum efficiency. But this can be suppressed through strain reduction and optimization of the growth recipe.
“We plan to work with LED growers to improve the performance of UV LEDs based on quaternary alloys,” says Cao. “Two specific tasks are: to design and grow lattice-matched templates and heterostructures by tuning quaternary compositions; and determining the optimal growth temperature to allow for aluminum/indium incorporation, while maintaining good structural quality.”
X. A. Cao et al. Appl. Phys .Lett. 96 151109 (2010)
GaAs-based detectors extend to the far infrared
A team of French researchers claims that it has fabricated the first GaAs/AlGaAs quantum cascade detector (QCD) capable of operating at very long infrared wavelengths. Development of this 15 µm detector could provide a stepping stone towards the manufacture of focal plane arrays operating in this spectral range that could be used for meteorology, atmospheric chemistry studies, and Earth observation missions.
Corresponding author Amandine Buffaz from the University of Paris, Diderot-Paris 7, says that the performance of the team’s detectors are comparable to those of the incumbent technology, quantum well infrared photodetectors. However, the cascading detectors have one distinct advantage - very low dark currents that enable long
integration times. The team, which also includes researchers from the Alcatel-Thales 3-5 lab, produced their detectors via MBE growth on a semi-insulating GaAs (001) substrate. The detector’s epitaxial layers consist of 30 identical periods of 4 coupled quantum wells that feature AlGaAs barriers with a 232 meV conduction band offset.
Square shaped mesas with 50 µm and 100 µm sides were created with dry-etching techniques, and Au/Ge/Ni ohmic contacts were deposited onto these pixels.
The detector has a responsitivity peak of 14.3 µm, and its detectivity at 25 K and an applied bias of –0.6V is 1 x 1012 Jones.
The detector’s performance can be taken to a new level by cutting the tunneling current.
“To reach that aim we will use two theoretical models of electronic transport in QCDs: a ‘thermalized subbands’ approach that models transport based on diffusion mechanisms; and a resonant tunneling model.”
Comparing the results of each of these calculations should uncover a structure that has carrier transport dominated by diffusion rather than tunneling. Another of the team’s goals is to develop detectors operating in other regions of the infrared spectrum. “The first QCD detecting in the terahertz is under study, and in the immediate future the first thermal imager based on QCD detectors should be fabricated.”
A. Buffaz et al. Appl. Phys. Lett 96 172101 (2010)
June 2010
www.compoundsemiconductor.net 41
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127