research review Corning simplifies semi-polar green lasers
Semi-polar lasers don’t need electron-blocking layers to deliver high output powers in the green devices.
REMOVING the electron-blocking layer (EBL) has little impact on the performance of green semi-polar lasers, according to experimental efforts by researchers at Corning. This US team has shown that even when the EBL is omitted, semi-polar lasers can still have good injection efficiencies and deliver in a relatively similar perforance over a wide temperature range.
These beneficial characteristics strengthen the case that the semi-polar green laser is the best candidate for providing a single- chip green source for various ‘RGB’ displays, including pico-projectors.
Lead-author Dmitry Sizov argues that the team’s findings should not raise any eyebrows, given that EBLs are rarely used in semiconductor systems that are free from polarization fields. “Semi-polar structures with reduced polarization fields are closer to those systems, which may be part of the reason why the EBL is not critical,” says Sizov. He and his co-workers also believe that the carrier injection mechanism in semi- polar laser-diodes differs from those in their conventional cousins.
Removing the EBL from semi-polar lasers
Driven in pulsed mode, all of these lasers show no significant change in slope efficiency with temperature, indicating that variations in design had little impact on injection efficiency.
Temperature insensitivity, which was determined by values for the ‘characteristic temperature of lasing threshold’, T0
, did
not depend on the inclusion or absence of the EBL. However, the value of T0
for all
Corning’s semi-polar lasers deliver a 60 mW continuous-wave output at 519 nm
could make the semi-polar lasers more attractive, because it enables the devices to combine superior injection architectures with lower operating voltages.
Corning’s engineers exposed the lack of importance of the EBL by comparing the performance of 11 semi-polar devices with ridge widths of 1.0 µm, 1.5 µm and 2.0
µm and emission wavelengths ranging from 508 nm to 522 nm. Four of these MOCVD-grown lasers had an Al0.28
Ga0.78 N
EBL positioned 10 nm above the top-most QW; the other seven were EBL-free
D. Sivoz et al. Appl. Phys. Express 4 102103 (2011)
Flipping 60 GHz transistors InGaAs HEMTs form two-stage gain blocks delivering 9 dB of gain while consuming just 20 mW
A TEAM of Taiwanese engineers has used flip-chip packaging to build an InGaAs HEMT delivering up to 6.5 dB of gain at 60 GHz.
The researchers argue that one of the strengths of flip-chip technology is its simplicity: The circuit needs no passive components, such as metal-insulator-metal capacitors and thin-film resistors.
“The main advantage of this approach is the inclusion of matching circuits on the carrier for flip-chip packaging. This provides a cost effective solution for seamless integration of the device onto the circuit,” claims team-member Edward Chang from Yuan Ze University.
Silicon CMOS can also yield circuits operating at 60 GHz, which can find deployment in wireless personal area networks, wireless high-definition
multimedia interfaces and wireless docking stations. However, III-Vs have the upper hand in several key areas: A lower noise figure, lower DC power consumption and superior linearity.
Construction of 60 GHz devices began with MBE growth of HEMT epistructures featuring a 15 nm-thick In0.6
Ga0.4 As channel. Epiwafers were thinned to
100 µm and diced into die that were screened for DC and RF characteristics. Good die were then bonded to sapphire substrates, which were coated with a thin film of metal that had been processed to create transmission lines.
Flip-chip packaging made no impact on the DC performance of the InGaAs HEMT, which has a drain-source current of 350 mA/mm and a transconductance of 600 mS/mm at a drain-source voltage
50
www.compoundsemiconductor.net November/December 2011
Cheng claims that the simplicity and low- cost of the process makes the flip-chip approach suitable for high-volume manufacturing: “It’s based on the very mature PCB printing technology.”
The team is aiming to make its process even cheaper with flip-chip-on-board technology that replaces substrates with organic dielectrics.
C.-Y Chiang et. al. Appl. Phys Express 4 104105 (2011)
of 0.5 V. Minimal parasitic effects at interconnections lead to a 0.7 dB reduction in the maximum achievable gain to 6.5 dB.
Two-stage gain blocks were also built with the team’s flip-chip approach. At 60 GHz these devices delivered 9 dB of small signal gain when drawing just 20 mW.
these lasers, which ranged from 161 K to 246K, is significantly higher than the typical value for conventional green laser
diodes.The team attribute’s the superior temperature insensitivity of the semi-polar laser to intrinsic quantum well properties, such as a higher recombination rate. The team’s most impressive EBL-free laser emits at 519.4 nm and delivers a CW output of 60 mW at 10 o
C, falling to 35 mW at 60 o
Sivoz and his co-workers plan to continue working on the development of novel approaches for creating efficient laser devices, including GaN-based lasers.
C.
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